| 1. |
- Akbarnejad, Shahin, 1978-
(författare)
-
Mathematical and Experimental Study on Filtration of Solid Inclusions from Molten Aluminium and Steel
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aluminum and steel have been the most produced metal and alloy, respectively, for many years. Their extensive use in various industries, their fundamental role in our everyday life, and their excellent recycling characteristics are the major driving forces for development of their production towards more sustainable processes. A successful integration and application of molten metal filtration from unwanted inclusions in production processes could result in reducing scrap, rework and would provide a cleaner molten metal which could lead to production of metallic materials with enhanced mechanical properties. Filtration of aluminum melts by ceramic foam filters is an established process in aluminum industry. Ceramic filters are also used in steel foundries to remove inclusions from the melt prior to casting to the mold. However, the use of ceramic filters is either limited to specific types of alloys or casts or to specific filters with large pores and openings. As a result, utilization of ceramic filters in the steel industry has limitations in capturing inclusions, where specifically small size inclusions may not be captured. This research work aims at contributing to the global effort in developing the molten metal production processes to become more sustainable and to increase the quality of the final product. To be specific, it is aimed at shedding more light into filtration applications and the use of ceramic filters for removal of solid non-metallic inclusions from molten aluminum and steel. Thus, permeability characteristics of single 30, 50, and 80 Pore Per Inch (PPI) alumina Ceramic Foam Filter (CFF) grades as well as stacks of three 30, three 50, and three 80 PPI alumina CFF grades were both experimentally and numerically obtained and studied. This provides the information needed to estimate the pressure required to prime and/or push the molten aluminum through the filters. The pressure could either be built up by gravitational or other forces. It has been shown recently that it is possible to prime such filters with electromagnetic forces and filter solid inclusions from molten aluminum. Lastly, physical refining of molten steel from solid alumina inclusions through monolithic extruded square-celled alumina ceramic filter was investigated and studied with a developed mathematical Computational Fluid Dynamics (CFD) model as well as the particle trajectories of inclusions in the size range of 1 to 100 [µm]. The experimentally obtained permeability characteristics as well as the obtained pressure gradient profiles of the single 30, 50, and 80 PPI CFFs were compared to previous research findings from the literature. Overall, a good agreement between the current and previous findings was found. It was also shown that fluid bypassing should be avoided during permeability experiments, otherwise deviations as high as 60% may occur. It was also revealed that similar permeability characteristics for the stacked filters, compared to single filters, could be achieved. However, an about three times higher pressure gradient or pressure needs to be applied when using a stack of three identical PPI filters compared to using single filters. The numerical simulations also validated the experimental findings of the permeability experiments. The CFD simulations and particle trajectories of the solid alumina inclusions in molten steel through the monolithic alumina filter revealed that it was possible to capture all particles larger than 50 [µm]. However, it was not possible to capture all particles smaller than 50 [µm] due to the applied simulation approach as well as current simulation limitations in the software.
|
|
| 2. |
- Al-Saadi, Munir, 1965-
(författare)
-
Thermomechanical Processing of Nickel-Base Alloy 825
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Alloy 825 material was studied using a Gleeble-3800 thermosimulatorby performing single-hot compression experiments.Optical microscopy and electron backscatter diffraction wereutilized to characterize the microstructure. Dynamicrecrystallization is not considerable in the as-cast alloy anddislocation recovery is deemed to be dominant. Based on thisfinding, the effect of adding trace amounts of alloying additionson the mechanical properties of cast alloy 825 was studied, withemphasis on whether or not dynamic recrystallization occurred.The results show that dynamic recrystallization was moreprevalent under all test conditions in samples containing a traceamount of magnesium, but not for the conventional alloy.However, alloying with trace magnesium did not lead to animprovement of the mechanical properties. Instead, processingmaps for hot forging of conventional Alloy 825 were required toidentify optimal working parameters and to achieve dynamicrecrystallization. The hot deformation behavior of cast Alloy 825was characterized by using dynamic materials modelling of hotcompression data. The results show that the maximum powerdissipation efficiency is over 35%. The highest efficiency isachieved in the temperature range of 1100 ℃ - 1250 ℃ and instrain rates in the range of 0.01 ≤ strain rate / s ≤ 0.1. The optimumprocessing parameters for good strain hardening are obtained inthe temperature range between 950 ℃ and 1100 ℃ with strainrates of 0.3 ≤ strain rate/ s ≤ 10.0. In addition, the influence of thedeformation level on the recrystallization and microstructuralchanges in Alloy 825 during hot forging operations attemperatures between 950 °C and 1200 °C was studied. Themaximum yield strength and ultimate tensile strength wereobtained after forging to achieve a true strain of 0.9 were 413 MPa and 622 MPa , respectively, with a ductility of 40%.However, Alloy 825 is often supplied as annealed bars.Therefore, the effect of the forging strain magnitude andsubsequent annealing on the microstructure, strengtheningmechanisms and room temperature mechanical properties wereinvestigated to assess the suitability of current industrialpractice. The results showed that the majority of strengtheningwas attributed to grain refinement, the dislocation densities thatarise due to the large forging strain, and due to solid solutionstrengthening. The results of calculations are in excellentagreement with experimental data, with less than 1% difference.These results can be used by future researchers and industry topredict the strength of Alloy 825 and similar alloys, especially inmaterial after a completed hot forging operation.
|
|
| 3. |
- Alevanau, Aliaksandr, 1965-
(författare)
-
Study of pyrolysis and gasification of biomass from the self-organization perspective
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on the analysis of kinetics of i) low-temperature pyrolysis of gaseous hydrocarbons, ii) high-temperature steam gasification of char of wood pellets (>700oC), iii) high temperature pyrolysis of straw pellets in an atmosphere of argon and steam, and iv) high temperature pyrolysis of slices of transversally cut wooden sticks. The results of the kinetic measurements in the high-temperature cases are approximated using a least-square based optimization software, which was specially developed to analyse kinetics prone for deviation from the Arrhenius law.In the thesis a general analysis of the researched materials and kinetics of their pyrolysis and gasification is presented from the self-organization perspective. The energy transfer phenomena in both the pyrolysis and gasification processes of biomass are discussed with an emphasis on an analysis of basic phenomena involving the self-organized dynamics on fractal structures in the chosen biomass samples.
|
|
| 4. |
- Bai, Haitong, 1986-
(författare)
-
A Study of the Swirling Flow Pattern when Using TurboSwirl in the Casting Process
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of a swirling flow can provide a more uniform velocity distribution and a calmer filling condition according to previous studies of both ingot and continuous casting processes of steel. However, the existing swirling flow generation methods developed in last decades all have some limitations. Recently, a new swirling flow generator, the TurboSwirl device, was proposed. In this work, the convergent nozzle was studied with different angles. The maximum wall shear stress can be reduced by changing the convergent angle between 40º and 60º to obtain a higher swirl intensity. Also, a lower maximum axial velocity can be obtained with a smaller convergent angle. Furthermore, the maximum axial velocity and wall shear stress can also be affected by moving the location of the vertical runner. A water model experiment was carried out to verify the simulation results of the effect of the convergent angle on the swirling flow pattern. The shape of the air-core vortex in the water model experiment could only be accurately simulated by using the Reynolds Stress Model (RSM). The simulation results were also validated by the measured radial velocity in the vertical runner by the ultrasonic velocity profiler (UVP). The TurboSwirl was reversed and connected to a traditional SEN to generate the swirling flow. The periodic characteristic of the swirling flow and asymmetry flow pattern were observed in both the simulated and measured results. The detached eddy simulation (DES) turbulence model was used to catch the time-dependent flow pattern and the predicted results agree well with measured axial and tangential velocities. This new design of the SEN with the reverse TurboSwirl could provide an almost equivalent strength of the swirling flow generated by an electromagnetic swirling flow generator. It can also reduce the downward axial velocities in the center of the SEN outlet and obtain a calmer meniscus and internal flow in the mold.
|
|
| 5. |
- Basirat, Mitra, 1977-
(författare)
-
Microstructural Evolution In As-cast Alloys during Plastic Deformation
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The effect of deformation on microstructural changes in metals and alloys is the subject of considerable practical interest. The ultimate goal is to control, improve and optimize the microstructure and texture of the finished products produced by metal forming operations. The development in the subject field is remarkable but a more in-depth study could lead us to the better understanding of the phenomena. In the present work microstructural evolution during the plastic deformation of as-cast pure metals and alloys is studied. An experimental method was developed to study the material behavior under the hot compression testing. This method was applied on the as-cast structure of copper, bearing steel, Incoloy 825 and β brass at different temperatures and strain rates. The temperature of the samples was measured during and after the deformation process. The microstructure of the samples was examined by optical microscopy and scanning electron microscopy (SEM). The microstructural evolution during deformation process was investigated by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The samples were subsequently subjected to electron microprobe analysis (EMPA) to investigate the effect of the deformation on the microsegregation of Mo, Cr, Si, and Mn. It was observed that the temperature of the samples deformed at strain rates of 5 and 10 s-1 increases abruptly after the deformation stops. However, compression test at the lower strain rates of 1 and 0.5 s-1 revealed that a constant temperature was maintained in the early stage of deformation, followed by an increase until the maximum temperature was obtained. This temperature behavior can be explained by the microstructural evolution during the deformation process. Micrograph analysis revealed the formation of deformation bands (DBs) in highly strained regions. The DBs are highly effective sites for recrystallization. The interdendritic regions are suitable sites for the formation of DBs due to the high internal energy in these regions. EMPA indicated a tendency towards uphill diffusion of Mo in the DBs with increasing strain. The effect of strain on the dissolution of carbides in the band structure of bearing steel was investigated by measuring the volume fraction of carbides inside the band structure at different strain levels. The results indicate that carbide dissolution is influenced by strain. The microstructural evolution inside the DBs was studied as a function of several properties: temperature, internal energy, and microsegregation. Compression of β brass revealed that twinning is the most prominent feature in the microstructure. EBSD analysis and energy calculations demonstrated that the twinning is not due to a martensitic process but rather the order/disorder transition during the deformation process. The effect of heat treatment at Tc (650°C) prior to deformation on the microstructure of β brass was also investigated, which revealed a relationship between twin formation and the anti-phase domain boundaries
|
|
| 6. |
- Bu, Junfu, 1982-
(författare)
-
Advanced BaZrO3-BaCeO3 Based Proton Conductors Used for Intermediate Temperature Solid Oxide Fuel Cells (ITSOFCs)
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, the focus is on studying BaZrO3-BaCeO3 based proton conductors due to that they represent very promising proton conductors to be used for Intermediate Temperature Solid Oxide Fuel Cells (ITSOFCs). Here, dense BaZr0.5Ce0.3Y0.2O3-δ (BZCY532) ceramics were selected as the major studied materials. These ceramics were prepared by different sintering methods and doping strategies. Based on achieved results, the thesis work can simply be divided into the following parts:1) An improved synthesis method, which included a water-based milling procedure followed by a freeze-drying post-processing, was presented. A lowered calcination and sintering temperature for a Hf0.7Y0.3O2-δ (YSH) compound was achieved. The value of the relative density in this work was higher than previously reported data. It is also concluded that this improved method can be used for mass-production of ceramics.2) As the solid-state reactive sintering (SSRS) represent a cost-effective sintering method, the sintering behaviors of proton conductors BaZrxCe0.8-xLn0.2O3-δ (x = 0.8, 0.5, 0.1; Ln = Y, Sm, Gd, Dy) during the SSRS process were investigated. According to the obtained results, it was found that the sintering temperature will decrease, when the Ce content increases from 0 (BZCLn802) to 0.3 (BZCLn532) and 0.7 (BZCLn172). Moreover, the radii of the dopant ions similar to the radii of Zr4+ or Ce4+ ions show a better sinterability. This means that it is possible to obtain dense ceramics at a lower temperature. Moreover, the conductivities of dense BZCLn532 ceramics were determined. The conductivity data indicate that dense BZCY532 ceramics are good candidates as either oxygen ion conductors or proton conductors used for ITSOFCs.3) The effect of NiO on the sintering behaviors, morphologies and conductivities of BZCY532 based electrolytes were systematically investigated. According to the achieved results, it can be concluded that the dense BZCY532B ceramics (NiO was added during ball-milling before a powder mixture calcination) show an enhanced oxygen and proton conductivity. Also, that BZCY532A (NiO was added after a powder mixture calcination) and BZCY532N (No NiO was added in the whole preparation procedures) showed lower values. In addition, dense BZCY532B and BZCY532N ceramics showed only small electronic conductivities, when the testing temperature was lower than 800 ℃. However, the BZCY532A ceramics revealed an obvious electronic conduction, when they were tested in the range of 600 ℃ to 800 ℃. Therefore, it is preferable to add the NiO powder during the BZCY532 powder preparation, which can lower the sintering temperature and also increase the conductivity.4) Dense BZCY532 ceramics were successfully prepared by using the Spark Plasma Sintering (SPS) method at a temperature of 1350 ℃ with a holding time of 5 min. It was found that a lower sintering temperature (< 1400 ℃) and a very fast cooling rate (> 200 ℃/min) are two key parameters to prepare dense BZCY532 ceramics. These results confirm that the SPS technique represents a feasible and cost-effective sintering method to prepare dense Ce-containing BaZrO3-BaCeO3 based proton conductors.5) Finally, a preliminary study for preparation of Ce0.8Sm0.2O2-δ (SDC) and BZCY532 basedcomposite electrolytes was carried out. The novel SDC-BZCY532 based composite electrolytes were prepared by using the powder mixing and co-sintering method. The sintering behaviors, morphologies and ionic conductivities of the composite electrolytes were investigated. The obtained results show that the composite electrolyte with a composition of 60SDC-40BZCY532 has the highest conductivity. In contrast, the composite electrolyte with a composition of 40SDC-60BZCY532 shows the lowest conductivity.In summary, the results show that BaZrO3-BaCeO3 based proton-conducting ceramic materials represent very promising materials for future ITSOFCs electrolyte applications.
|
|
| 7. |
- Carlsson, Leo, 1992-
(författare)
-
Applied Machine Learning in Steel Process Engineering : Using Supervised Machine Learning Models to Predict the Electrical Energy Consumption of Electric Arc Furnaces
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The steel industry is in constant need of improving its production processes. This is partly due to increasing competition and partly due to environmental concerns. One commonly used method for improving these processes is through the act of modeling. Models are representations of the reality that can be used to study and test new processes and strategies without costly interventions. In recent years, Machine Learning (ML) has emerged as a promising modeling approach for the steel industry. This has partly been driven by the Industry 4.0 development, which highlights ML as one of the key technologies for its realization. However, these models are often difficult to interpret, which makes it impractical to validate if the model accurately represents reality. This can lead to a lack of trust in ML models by domain practitioners in the steel industry. Thus, the present work investigates the practical usefulness of ML models in the context steel process engineering. The chosen application to answer this research question is the prediction of the Electrical Energy (EE) consumption of Electric Arc Furnaces (EAF). The EAF process was chosen due to its widespread use in the steel industry and due to the difficulty to accurately model the EE consumption using physical modeling. In the present literature, the use of linear statistical models are commonly used even though the EE consumption is non-linearly dependant on multiple important EAF process variables. In addition, the literature does neither investigate the correlations between input variables nor attempts to find the most optimal model with respect to model complexity, predictive performance, stability, and generalizability. Furthermore, a consistent reporting of predictive performance metrics and interpreting the non-transparent models is lacking. These shortcomings motivated the development of a Model Construction methodology and a Model Evaluation methodology that eliminate these shortcomings by considering both the domain-specific (metallurgical) aspects as well as the challenges imposed by ML modeling. By using the developed methodologies, several important findings originated from the resulting ML models predicting the EE consumption of two disparate EAF. A high model complexity, governed by an elevated number of input variables and model coefficients, is not necessary to achieve a state-of-the-art predictive performance on test data. This was confirmed both by the extensive number of produced models and by the comparison of the selected models with the models reported in the literature. To improve the predictive performance of the models, the main focus should instead be on data quality improvements. Experts in both process metallurgy and the specific process under study must be utilized when developing practically useful ML models. They support both in the selection of input variables and in the evaluation of the contribution of the input variables on the EE consumption prediction in relation to established physico-chemical laws and experiences with the specific EAF under study. In addition, a data cleaning strategy performed by an expert at one of the two EAF resulted in the best performing model. The scrap melting process in the EAF is complex and therefore challenging to accurately model using physico-chemical modeling. Using ML modeling, it was demonstrated that a scrap categorization based on the surface-area-to-volume ratio of scrap produced ML models with the highest predictive performance. This agrees well with the physico-chemical phenomena that governs the melting of scrap; temperature gradients, alloying gradients, stirring velocity, and the freezing effect. Multiple different practical use cases of ML models were exemplified in the present work, since the model evaluation methodology demonstrated the possibility to reveal the true contributions by each input variable on the EE consumption. The most prominent example was the analysis of the contribution by various scrap categories on the EE consumption. Three of these scrap categories were confirmed by the steel plant engineers to be accurately interpreted by the model. However, to be able to draw specific conclusions, a higher model predictive performance is required. This can only be realized after significant data quality improvements. Lastly, the developed methodology is not limited to the case used in the present work. It can be used to develop supervised ML models for other processes in the steel industry. This is valuable for the steel industry moving forward in the Industry 4.0 development.
|
|
| 8. |
- Chen, Chao, 1989-
(författare)
-
Some Aspects on Macroscopic Mixing in a Tundish
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two aspects on macroscopic mixing in a continuous flow system – metallurgical tundish were studied. Specifically, 1) the first focus was on salt solution tracer mixing, which is important for tundish design from perspectives of tracer technology and Residence Time Distributions (RTD) as well as for the understanding of the macroscopic mixing in tundishes. The different amounts of salt solution tracer mixing in a tundish were studied by using both physical models and mathematical models. The disturbance of KCl salt tracer on the flow in the tundish with respect to different amounts is like the “butterfly effect”, i.e. only a slight increase of the amount of tracer, the flow field might be disturbed. This, in turn, will result in a shifted RTD curve. 2) The second focus was on Eulerian modeling of inclusions macroscopic transport and removal, which is important for tundish design from perspectives of inclusions removal and to provide information of macroscopic removal of inclusions. In the study, an approach that combined the meso-scale inclusions deposition at turbulent boundary layers of steel-slag interface and the macroscopic transport of inclusions in the tundish was used. The theoretical calculation results showed that the effect of the roughness on the deposition velocity of small inclusions (radius of 1 μm) were more pronounced than that for the big inclusions (up to the radius of 9 μm). The dynamic inclusions removal studies showed that the tundish with a weir and a dam exhibited a better performance with respect to the removal of bigger inclusions (radii of 5 μm, 7 μm and 9 μm) than that of the case without weirs and dams. However, the tundish without weirs and dams showed a higher removal ratio of smaller inclusions (radius of 1 μm).
|
|
| 9. |
- Hessling, Oscar, 1988-
(författare)
-
Some aspects of hydrogen reduction of iron ore
- 2024
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fines of hematite and magnetite were studied in the fluidized bed using a pure hydrogen atmosphere in the temperature range of 768 to 888 K. Hematite pellets were studied based on Thermogravimetric Analysis (TGA) experiments using hydrogen atmospheres containing 0–15 % pH2O, in the temperature range of 873 to 1173 K. Thermocouples in contact with the pellet’s surface and embedded in the pellet’s center recorded the thermal gradient in the pellet during reduction. The fluidized bed and the TGA experiments used an alternative method to start the reaction. The furnace was pre-heated with a reactive atmosphere. After this, the reaction was initiated by introducing the sample to the hot furnace, to eliminate the effect of gas dilution during gas switching. After the experiments, Scanning Electron Microscopy (SEM)analyses were employed to study the reduction microstructures. Both types of fines showed similar reduction rates. Fines and pellets showed high initial reduction rates, which increased with increasing temperatures. The reduction rate in the last reduction stage was low for both fines and pellets. An increasing pH2O content in the atmosphere lowered the reaction rate, and theeffect decreased with increased temperature. A difference between the pellet's surface and center temperatures was observed during reduction. The pellet'smacro-pore structure was seen to be unaltered by changes in temperature or atmosphere. However, at 873 K, the iron product microstructure was found to be highly porous. Furthermore, increasing temperatures caused dense iron to form. In addition, when porous iron or iron oxides were observed, increasing pH2O contentsincreased the pore diameter but decreased the pore amount. Pellet properties with varied pellet compositions were also investigated using Cold Crushing Strength (CCS), reduction in a TGA setup, and melting experiments. The composition was not found to influence the mechanical or reduction properties but significantly affected the phosphorus refining during melting.The results showed that a mixed reaction rate control occurred during the early reduction stage for both the fines and the pellets. The temperature differences observed during this reduction stage resulted from a combined effect of heat transfer and an endothermic chemical reaction. The impact of water in the atmosphere influenced the reaction rate through the backward reaction and mass transfer. At 873 K, the retarding effect is mainly caused by the backwardreaction. The results show the late stage of reduction to be primarily diffusioncontrolled. In addition, it should be possible to alter the pellet composition while maintaining pellet properties to increase the usefulness of the pellet.
|
|
| 10. |
- Huss, Joar, 1993-
(författare)
-
A study of the autogenous Hydrogen-DRI slag and its impact on the dephosphorization of fossil-free steel at different oxygen potentials
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present study comprises aspects related to the phosphate capacity, the dephosphorization of fossil-free steel, and the utilization of potential by-products. The focus is mainly given to the functions and impact of the autogenous H-DRI slag in the dephosphorization process and the possibility for future slag valorization.At the outset, the applicability of the phosphate capacity concept on systems containing multivalent species is critically examined. For the examination, the variation in the slag structure depending on the oxygen potential was considered theoretically. To support the theoretical consideration, experiments were conducted to illustrate the dependence and to show the effect on the phosphate capacity. The results demonstrated a significant effect of the oxygen potential on the phosphate capacity. Consequently, the use of the concept for multivalent slags falls under serious question.To better orientate the future steelmaking process, the dephosphorization power of slags related to the autogenous H-DRI slag was investigated experimentally. The CaO-MgO-SiO2-FeO system constituted the fully liquid slags, which were equilibrated with liquid iron at 1873 K. Further, the oxygen potential was fixed by closing the system. The dephosphorization power of the autogenous slag was found to be theoretically sufficient to refine the steel made from H-DRI from phosphorus adequately. Thus, it was concluded that the H-DRI slag could be used as a base for the EAF slag to save energy and material.Due to the industrial novelty of the H-DRI material, little is known about the dephosphorization mechanism. Therefore, to facilitate a more efficient process design, the dephosphorization mechanism for H-DRI with different reduction degrees was studied under two different heat transfer conditions. Firstly, by heating and melting H-DRI in a poor heat transfer situation, i.e., in a gas phase at 1873 K, and secondly, under better conditions where the heat transfer is still insufficient for direct melting, i.e., by heating in a liquid slag at 1923 K. The melting process was found to influence the dephosphorization mechanism significantly. In the poor heat transfer situation, the dissolution of the phosphorus-bearing apatite phase was facilitated by the bulk movement of the autogenous slag, which occurred during the melting of the metal phase. In the better heat transfer situation, the bulk slag penetrated the pore network of the H-DRI, a process that was enhanced by the autogenous slag. Since a greater slag mass was available for dissolution, the steel made from H-DRI was dephosphorized already prior to melting.Lastly, the possibility for vanadium extraction from an especially engineered autogenous H-DRI slag was investigated experimentally at 1873 K. For the production of high-quality ferrovanadium alloy, a feasible vanadium extraction requires the fulfillment of two demands. Phosphorus should be predominantly partitioned to the metal and vanadium to the slag. Thus, the experiments featured an acidic slag of the Al2O3-SiO2-FeOx-VzOy system and liquid iron as the metal phase. Also, to fix the oxygen potential, the system was closed. The dephosphorization power of the acidic slags was very low, within the investigated range, while vanadium was mostly partitioned to the slag. The proposed slag system could, therefore, provide an opportunity to utilize an especially engineered autogenous slag for vanadium extraction.
|
|
| 11. |
- Janis, Jesper, 1979-
(författare)
-
Inclusions and/or Particles Engineering for Grain Refining Purposes in Ferritic Fe-20mass%Cr alloys
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Compared to more common used austenitic stainless steels, ferritic stainless steels contain very low amounts of the expensive alloying element Ni. In addition, they have good corrosion properties, but are sometimes suffering from poor weldability and bad mechanical properties. This is mainly due to the presence of large grains after casting and large grain growth during heat treatment or welding. Processes for reducing the grain size (grain refining) of metal alloys are widely known and proven before to be suitable for many alloys. A successful grain refining process can increase the strength of an alloy without decreasing the ductility. This can be achieved by different methods, such as rolling or cooling. In this work, the focus has been on studying the aspect from a metallurgist point of view, to analyse the possibilities to create small particles in the liquid stage to enhance the solidification. The focus has been on oxide and nitride formation for nucleation of smaller grains during solidification. This study was made by forming particles, develop the analysis methods and thereafter to study the effect of particles on the structure of a ferritic stainless steel. The particles were formed by additions of Ti, Ce and Zr in to a liquid Fe-20mass% Cr alloy containing different amounts of oxygen and nitrogen. The electrolytic extraction technique was used to achieve a good understanding of the reaction processes and the particles size, number, composition and morphology. The grain sizes and the particles were then studied in as-cast samples as well as in specimens heat treated for 60 minutes at 1200, 1300 and 1400°C in a Confocal Scanning Laser Microscope (CSLM). Also, based on Scanning Electron Microscope (SEM) determinations and Thermo-Calc calculations, the precipitated particles were divided as primary and secondary particles. Thereafter, the grain refining potential was studied for each of these types. In this work, particles engineering by using small particles have been proven suitable for the Fe-20mass% Cr ferritic stainless steel alloys. Although the work has been based on small-scale samples and experiments, the results show good tendencies with respect to grain refining as well as a clear relationship between the particles in the steels and the microstructures. It was found that Ti-Ce additions in Fe-20mass% Cr alloys resulted in complex oxides including Ti, Ce and Cr. These oxides were observed to have high agglomeration tendencies and therefore to form larger particles or clusters. The grain refining potential on the solidification structure was insignificant, despite a relatively large amount of particles. However, Ti-Zr additions in Fe- 20mass% Cr alloys on the other hand resulted in primary precipitated particles such as ZrO2 and ZrO2+ZrN. Furthermore, ZrN nucleated ferrite during solidification, which lead to a clearly observed grain refining effect. This effect was also increased with an increased number of particles. The amount of particles (primary and secondary) was also found to increase with an increased amount of nitrogen. Secondary particles (mostly TiN) were precipitated near grain boundaries, which lead to a location based pinning effect of the grain growth during heat treatment at 1200 °C. This pinning effect was increased by an increased nitrogen content and thereby an increased number of particles. Finally, the pinning effect was clearly reduced during heat treatment at 1400 °C.
|
|
| 12. |
- Kasedde, Hillary, 1984-
(författare)
-
Towards the Improvement of Salt Extraction from Lake Katwe Raw Materials in Uganda
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Uganda is well endowed with economic quantities of mineral salts present in the interstitial brines and evaporite deposits of Lake Katwe, a closed (endorheic) saline lake located in the western branch of the great East African rift valley. Currently, rudimentally and artisanal methods continue to be used for salt extraction from the lake raw materials. These have proved to be risky and unsustainable to the salt miners and the environment and they have a low productivity and poor product quality. This work involves the investigation of the salt raw materials that naturally occur in the brines and evaporites of Lake Katwe. The purpose is to propose strategies for the extraction of improved salt products for the domestic and commercial industry in Uganda.The literature concerning the occurrence of salt and the most common available technologies for salt extraction was documented. Also, field investigations were undertaken to characterize the salt lake deposits and to assess the salt processing methods and practices. The mineral salt raw materials (brines and evaporites) were characterized to assess their quality in terms of the physical, chemical, mineralogical, and morphological composition through field and laboratory analyses. An evaluation of the potential of future sustainable salt extraction from the lake deposits was done through field, experimental, and modeling methods. Moreover, the mineral solubilities in the lake brine systems and dissolution kinetics aspects were investigated.The results reveal that the salt lake raw materials contain substantial amounts of salt, which can be commercialized to enable an optimum production. The brines are highly alkaline and rich in Na+, K+, Cl-, SO42-, CO32-, and HCO3-. Moreover, they contain trace amounts of Mg2+, Ca2+, Br-, and F-. The lake is hydro-chemically of a carbonate type with the brines showing an intermediate transition between Na-Cl and Na-HCO3 water types. Also, the evaporation-crystallization is the main mechanism controlling the lake brine chemistry. These evaporites are composed of halite mixed with other salts such as hanksite, burkeite, trona etc, but with a composition that varies considerably within the same grades. The laboratory isothermal extraction experiments indicate that various types of economic salts such as thenardite, anhydrite, mirabilite, burkeite, hanksite, gypsum, trona, halite, nahcolite, soda ash, and thermonatrite exist in the brine of Lake Katwe. In addition, the salts were found to crystallize in the following the sequence: sulfates, chlorides, and carbonates.A combination of results from the Pitzer’s ion-interaction model in PHREEQC and experimental data provided a valuable insight into the thermodynamic conditions of the brine and the sequence of salt precipitation during an isothermal evaporation. A good agreement between the theoretical and experimental results of the mineral solubilities in the lake brine systems was observed with an average deviation ranging between 8-28%. The understanding of the mineral solubility and sequence of salt precipitation from the brine helps to control its evolution during concentration. Hence, it will lead to an improved operating design scheme of the current extraction processes. The dissolution rate of the salt raw materials was found to increase with an increased temperature, agitation speed and to decrease with an increased particle size and solid-to-liquid ratio. Moreover, the Avrami model provided the best agreement with the obtained experimental data (R2 = 0.9127-0.9731). In addition, the dissolution process was found to be controlled by a diffusion mechanism, with an activation energy of 33.3 kJ/mol.Under natural field conditions, the evaporative-crystallization process at Lake Katwe is influenced by in-situ weather conditions. Especially, the depth of the brine layer in the salt pans and the temperature play a significant role on the brine evaporation rates. With the optimal use of solar energy, it was established that the brine evaporation flux can be speeded up in the salt pans, which could increase the production rates. Moreover, recrystallization can be a viable technique to improve the salt product purity. Overall, it is believed that the current work provides useful information on how to exploit the mineral salts from the salt lake resources in the future.
|
|
| 13. |
- Liu, Qiang, 1985-
(författare)
-
Microstructure Evaluation and Wear-Resistant Properties of Ti-alloyed Hypereutectic High Chromium Cast Iron
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High chromium cast iron (HCCI) is considered as one of the most useful wear resistance materials and their usage are widely spread in industry. The mechanical properties of HCCI mainly depend on type, size, number, morphology of hard carbides and the matrix structure (γ or α). The hypereutectic HCCI with large volume fractions of hard carbides is preferred to apply in wear applications. However, the coarser and larger primary M7C3 carbides will be precipitated during the solidification of the hypereutectic alloy and these will have a negative influence on the wear resistance.In this thesis, the Ti-alloyed hypereutectic HCCI with a main composition of Fe-17mass%Cr-4mass%C is studied based on the experimental results and calculation results. The type, size distribution, composition and morphology of hard carbides and martensite units are discussed quantitatively. For a as-cast condition, a 11.2μm border size is suggested to classify the primary M7C3 carbides and eutectic M7C3 carbides. Thereafter, the change of the solidification structure and especially the refinement of carbides (M7C3 and TiC) size by changing the cooling rates and Ti addition is determined and discussed. Furthermore, the mechanical properties of hypereutectic HCCI related to the solidification structure are discussed.Mechanical properties of HCCI can normally be improved by a heat treatment process. The size distribution and the volume fraction of carbides (M7C3 and TiC) as well as the matrix structure (martensite) were examined by means of scanning electron microscopy (SEM), in-situ observation by using Confocal Laser Scanning Microscope (CLSM), Transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). Especially for the matrix structure and secondary M7C3 carbides, EBSD and CLSM are useful tools to classify the fcc (γ) and bcc (α) phases and to study the dynamic behavior of secondary M7C3 carbides. In conclusion, low holding temperatures close to the eutectic temperature and long holding times are the best heat treatment strategies in order to improve wear resistance and hardness of Ti-alloyed hypereutectic HCCI.Finally, the maximum carbides size is estimated by using statistics of extreme values (SEV) method in order to complete the size distribution results. Meanwhile, the characteristic of different carbides types will be summarized and classified based on the shape factor.
|
|
| 14. |
- Ni, Peiyuan
(författare)
-
A Study on Particle Motion and Deposition Rate : Application in Steel Flows
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Non-metallic inclusions in molten steel have received worldwide attention due to their serious influence on both the steel product quality and the steel production process. These inclusions may come from the de-oxidation process, the re-oxidation by air and/or slag due to an entrainment during steel transfer, and so on. The presence of some inclusion types can cause a termination of a casting process by clogging a nozzle. Thus, a good knowledge of the inclusion behavior and deposition rate in steel flows is really important to understand phenomena such as nozzle clogging. In this thesis, inclusion behaviors and deposition rates in steel flows were investigated by using mathematical simulations and validation by experiments.A ladle teeming process was simulated and Ce2O3 inclusion behavior during a teeming stage was studied. A Lagrangian method was used to track the inclusions in a steel flow and to compare the behaviors of inclusions of different sizes. In addition, a statistical analysis was conducted by the use of a stochastic turbulence model to investigate the behaviors of different-sized inclusions in different nozzle regions. The results show that inclusions with a diameter smaller than 20 μm were found to have similar trajectories and velocity distributions in the nozzle. The inertia force and buoyancy force were found to play an important role for the behavior of large-size inclusions or clusters. The statistical analysis results indicate that the region close to the connection region of the straight pipe and the expanding part of the nozzle seems to be very sensitive for an inclusion deposition.In order to know the deposition rate of non-metallic inclusions, an improved Eulerian particle deposition model was developed and subsequently used to predict the deposition rate of inclusions. It accounts for the differences in properties between air and liquid metals and considers Brownian and turbulent diffusion, turbophoresis and thermophoresis as transport mechanisms. A CFD model was firstly built up to obtain the friction velocity caused by a fluid flow. Then, the friction velocity was put into the deposition model to calculate the deposition rate.For the case of inclusion/particle deposition in vertical steel flows, effects on the deposition rate of parameters such as steel flow rate, particle diameter, particle density, wall roughness and temperature gradient near a wall were investigated. The results show that the steel flow rate/friction velocity has a very important influence on the rate of the deposition of large particles, for which turbophoresis is the main deposition mechanism. For small particles, both the wall roughness and thermophoresis have a significant influence on the particle deposition rate. The extended Eulerian model was thereafter used to predict the inclusion deposition rate in a submerged entry nozzle (SEN). Deposition rates of different-size inclusions in the SEN were obtained. The result shows that the steel flow is non-uniform in the SEN of the tundish. This leads to an uneven distribution of the inclusion deposition rates at different locations of the inner wall of the SEN. A large deposition rate was found to occur at the regions near the SEN inlet, the SEN bottom and the upper region of two SEN ports.For the case of an inclusion/particle deposition in horizontal straight channel flows, the deposition rates of particles at different locations of a horizontal straight pipe cross- section were found to be different due to the influence of gravity and buoyancy. For small particles with a small particle relaxation time, the gravity separation is important for their deposition behaviors at high and low parts of the horizontal pipe compared to the turbophoresis. For large particles with a large particle relaxation time, turbophoresis is the dominating deposition mechanism.
|
|
| 15. |
- Safavi Nick, Reza, 1977-
(författare)
-
Modelling Approaches of Multiphase Phenomena Focusing on an Integrated Steelmaking Route
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Improvements in numerical methods and developments of computer hardware brought attention to the modelling of metallurgical processes. This has been a critical step in understanding in-furnace conditions throughout the ironmaking and steelmaking process since it is, if not impossible, extremely difficult to conduct measurements during the process.With the introduction of mathematical models, attention has been on the computational fluid dynamics (CFD) modelling of each process to try to shed light on the furnaces’ interior operations. Moreover, most of these processes fall into the category of a multiphase system where different phases of material interact with each other. For example, in a blast furnace shaft, solid particles descend into the furnace while the blast pushes gas through the packed bed; during the desulphurization process, a powder material is injected into the hot metal by means of inert gas; in the ladle station, the steel cleanness is achieved by means of gas and/or induction stirring.Each of these processes has been the focus of a number of studies where the processes have been modelled using different methods namely, fully Eulerian or Eulerian-Lagrangian methods mostly for solid-fluid systems and Volume-of-Fluid (VOF) or VOF-Lagrangian for liquid-liquid or liquid-gas systems. However, there have been fewer studies where the investigations focused on which of these methods is best fitted for a specific process and why. Thus, the current study has been trying to fill this gap to give an overview of the integrated steelmaking route modelling approaches with a focus on the usability of models in the frame of results and computational efforts for the industry.This study goes through the integrated steelmaking route by looking at solid particles flow in the blast furnace, desulphurization of the hot metal produced by blast furnaces and the ladle refining process where a novel stirring strategy is introduced. Then, the focus turns on one of the most common by-products of the steelmaking process namely, the slag. This is due to the fact that the steelmaking slag contains a substantial amount of energy. However, there has been lesser effort in the utilization of such energy. The recent global greenhouse gas reduction initiative has brought significant attention to different aspects of steelmaking processes with respect to socio-economic development in combination with governmental policies. Hence, the study investigates the utilization of deposited energy within the steelmaking slag by means of a heat exchanger to answer the calls for a greener industry.In the end, the current study recommended to use the Eulerian modelling approach to model solid-fluid interactions in a blast furnace and powder injection into a liquid bath in a sulphur refining station. Furthermore, it is suggested that it can be advantageous to use the Volume-of-Fluid modelling approach to model liquid-gas interactions during the ladle refining process. Finally, it is recommended to use the reduced single-phase model for the conjugate heat transfer mdoelling to predict the static liquid energy dissipation in a slag heat recovery station.
|
|
| 16. |
- Vickerfält, Amanda
(författare)
-
A study of an autogenous slag for steel production with consideration of possible vanadium extraction
- 2024
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis discusses the use of the autogenous slag that forms from the residual oxides present in hydrogen reduced iron (H-DRI) pellets during melting. The studies are motivated by a better understanding of how H-DRI affect the steelmaking operations. A possible optimization of the steelmaking process is to recover the vanadium that is contained in the iron ore raw material. Therefore, understanding the role of vanadium during melting of H-DRI is given an extra focus. Taking advantage of the autogenous slag by utilizing its dephosphorization power to a maximum, or using it to extract vanadium, could make an important contribution to the process economics. To assist the developments in these directions, the phosphorus and vanadium partitions between slag and metal (LP and LV) as well as the phase relationship of the autogenous slag were investigated. The partitions were studied by melting H-DRI with reduction degrees between 91 and 99% in closed systems at 1873 K. The obtained LP and LV were in the ranges 8-26 and 501-1994, respectively. The LV values increased with decreased reduction degrees. The values for LP increased with decreasing reduction degrees until a 97% degree of reduction. Further lowered reduction degrees correlated with decreased LP values. The lowest phosphorus levels encountered in the iron (130 ppm) were obtained after melting of H-DRI with degrees of reduction between 94 and 98%. This indicates that the autogenous slag has a potential to make a significant contribution to the phosphorus refining. To find out about the phase relationship in the autogenous slag at 1873 K, small (5 g) samples of synthetic slag were equilibrated with 1 g iron in closed systems. Composition-wise, these slags corresponded to the autogenous slags from H-DRI with 98.4-99.7% reduction degrees. Preservation of the high temperature phase relationship required fast cooling; therefore, the samples were quenched in oil. This was also the reason for using small samples. Spinel, magnesiowüstite and liquid phase were identified as the stable phases at 1873 K. The spinel and magnesiowüstite phases were high in V, while the liquid contained almost no V. Increased FeO-contents (decreased degrees of reduction) correlated with a decreased amount of spinel, an increased amount of magnesiowüsite, as well as a decreased content of V in both phases. To increase the understanding about the phases in the autogenous slag, a sub-system containing MgO and V2O3 was investigated under conditions relevant to H-DRI melting, namely temperatures between 1661-1873 K and pO2 values between 1.75×10-11 and 1.75×10-10 atm. The phase boundaries for the three stable phases MgO-halite, spinel and V2O3-corundum were established. The oxygen potential and the temperature had limited impacts on the phase boundaries for the spinel and V2O3-corundum phases, while the maximum solubility of V2O3 in MgO-halite was affected to a somewhat larger extent. As earlier research has shown that an acid slag could be suitable for V extraction, the pseudo-ternary phase diagram between Al2O3, SiO2 and V2O3 at 1873 K and pO2=3.4×10-11-3.4×10-9 atm was also investigated. 5 different phases were identified, namely mullite, Al2O3-corundum, V2O3-corundum, cristobalite, and a liquid phase. The most significant effect of the oxygen potential was on the invariant point representing double Al2O3 and V2O3 saturation of the liquid. The multivalent nature of vanadium is suggested as the reason for the slight impact of the oxygen potential on the phase diagrams. To understand how the autogenous slag forms from the residual oxides, individual pellets with 90 and 99% reduction degrees were studied during heating to either 1773 or 1873 K. It was observed that the autogenous slag forms before iron melts. The slag likely forms as FeO melts and dissolves the other remaining oxides. Thereby, vanadium is transferred to the autogenous slag. Before iron melts, the movement of the autogenous slag is restricted to the pellet’s pore network. Thereafter, when iron melts, the slag starts to coalesce as well as to floatate. As the autogenous slag may contain solid phases, the effect of the fraction of solid phase on the slags foamability was finally investigated. This was done by measuring the maximum foaming heights of slags containing Al2O3, CaO, FeO and SiO2, reminiscent in their compositions to the autogenous slag. The slag compositions were chosen so that the fraction of precipitated magnesiowüstite phase was the main variable. It was found that some amount of solid phase (1.6 vol%) increased the foaming height by approximately 7%, while ≥8.7 vol% more than halved the foaming height.
|
|
| 17. |
- Doostmohammadi, Hamid, 1980-
(författare)
-
A Study of Slag/Metal Equilibrium and Inclusion Characteristics during Ladle Treatment and after Ingot Casting
- 2009
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Today, there is a high demand on clean steel for high performance materialproperties. Thus, steel producers try to deliver a steel product with the highestquality and cleanliness to the market. The number of parameters that affect thesteel cleanliness may vary depending on the required material properties of thefinal product. However, the non-metallic inclusion characteristics represent one ofthe most important parameters. More specifically, the composition, size, numberand morphology affect steel cleanliness. In this work, selected parameters affectingthe inclusion characteristics were studied using the following methods: i)thermodynamic calculations (including computational thermodynamiccalculations), ii) inclusion determinations using a cross sectional (CS) method (2Dinvestigations) and iii) inclusion determinations using an electrolytic extraction(EE) method (3D investigations). The computational thermodynamic calculations of the slag-steel and inclusion-steelequilibriums were carried out using the Thermo-Calc software. With the help ofthese calculations, the influence of the slag carryover on the top slag, aluminumcontent in steel and sulfur distribution ratio as well as predictions of stable phasesof inclusions were studied. In addition, inclusion determinations of tool steel gradesamples collected during various stages of the ladle treatment in a scrap-based steelplant were carried out using both 2D and 3D methods. Furthermore, inclusiondeterminations of bearing steel grade samples from a runner system after ingotcasting were performed using a 2D metallographic method (CS-method). Also, theINCAFeature software was used, when using cross sectional method, in order tocollect more statistics of the inclusion characteristics. It was found that slag carryover has a large influence on the composition of theactual top slag as well as the aluminum content in the steel as well as the sulfurdistribution ratio. In addition, steel and slag were found to be in “near”-equilibriumconditions, after the completion of the vacuum degassing operation. Furthermore,the composition of small-size inclusions in samples taken from tool steel was foundto be very scattered. Moreover, the composition of the large-size inclusions wasfound to be less scattered. Furthermore, closer to the top slag composition insamples collected after vacuum degassing. Finally, the accuracy of the inclusioncomposition determinations of tool steel samples using the electrolytic extractionmethod was found to be better than for the cross sectional method. The worseaccuracy of the CS-method is due to a considerable effect of matrix elements oninclusion composition.
|
|
| 18. |
- Ekengård, Johan, 1976-
(författare)
-
Slag/Metal Metallurgy in Iron and Steel Melts
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this work, the metal and slag phase mixing in three steps of a ladle refining operation of steel melts and for an oxygen balance during cooling of cast iron melts have been studied at two Swedish steel plants and at two Swedish cast iron foundries, respectively. In order to predict the oxygen activity in the steel bulk in equilibrium with the top slag as well as in metal droplets in the top slag in equilibrium with the top slag, three slag models were used. In addition, the assumptions of a sulphur-oxygen equilibrium between steel and slag and the dilute solution model for the liquid steel phase were utilized in the calculations. Measured oxygen activities in steel bulk, which varied between 3.5-6 ppm, were compared to predicted oxygen activities. The differences between the predicted and measured oxygen activities were found to be significant (0-500%) and the reasons for the differences are discussed in the thesis. Slag samples have been evaluated to determine the distribution of the metal droplets. The results show that the relatively largest numbers of metal droplets are present in the slag samples taken before vacuum degassing. Also, the projected interfacial area between steel bulk and top slag has been compared to the interfacial area between the metal droplets and slag.The results show that the droplet-slag interfacial area is 3 to 14 times larger than the flat projected interfacial area between the steel and top slag. Furthermore, the effect of the reactions between top slag and steel and the slag viscosity on the metal droplet formation is discussed. The results show significant differences between the steel bulk and steel droplet compositions and the reasons for the differences are discussed in the thesis. The oxygen activity in different cast irons was studied. Plant trials were performed at three occasions for lamellar, compacted and nodular iron melts. The results show that at temperatures close to the liquidus temperature the oxygen activities were 0.03-0.1 ppm for LGI, around 0.02 ppm for CGI, and 0.001ppm for SGI. In addition, it was found that as the oxygen activities increased with time after an Mg treatment, the ability to form a compact graphite or a nodular graphite in Mg-treated iron melts was decreased. Also, extrapolated oxygen activity differences up to 0.07 ppm were found for different hypoeutectic iron compositions for lamellar graphite iron at the liquidus temperature. Overall, the observed differences in the dissolved oxygen levels were believed to influence how graphite particles are incorporated into the austenite matrix and how the graphite morphology will be in the cast product.
|
|
| 19. |
- Ericsson, Ola
(författare)
-
An Experimental Study of a Liquid Steel Sampling Process
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During the steelmaking process samples are taken from the liquid steel, mainly to assess the chemical composition of the steel. Recently, methods for rapid determination of inclusion characteristics (size and composition) have progressed to the level where they can be implemented in process control. Inclusions in steel can have either good or detrimental effects depending on their characteristics (size, number, composition and morphology). Thereby, by determination of the inclusion characteristics during the steelmaking process it is possible to steer the inclusion characteristics in order to increase the quality of the steel. However, in order to successfully implement these methods it is critical that the samples taken from the liquid steel represent the inclusion characteristics in the liquid steel at the sampling moment. The purpose of this study is to investigate the changes in inclusion characteristics during the liquid steel sampling process. Experimental studies were carried out at steel plants to measure filling velocity and solidification rate in real industrial samples. The sampling conditions for three sample geometries and two slag protection types were determined. Furthermore, the dispersion of the total oxygen content in the samples was evaluated as a function of sample geometry and type of slag protection. In addition, the effects of cooling rate as well as oxygen and sulfur content on the inclusion characteristics were investigated in laboratory and industrial samples. Possibilities to separate primary (existing in the liquid steel at sampling moment) and secondary (formed during cooling and solidification) inclusions depending on size and composition were investigated. Finally, in order to evaluate the homogeneity and representative of the industrial samples the dispersion of inclusion characteristics in different zones and layers of the samples were investigated. It was concluded that the type of slag protection has a significant effect on the filling velocity and the sampling repeatability. Furthermore, that the thickness of the samples is the main controlling factor for the solidification rate. It was shown that top slag can contaminate the samples. Therefore, the choice of slag protection type is critical to obtain representative samples. It was shown that the cooling rate has a significant effect on the number of secondary precipitated inclusions. However, the number of primary inclusions was almost constant and independent on the cooling rate. In most cases it is possible to roughly separate the secondary and primary oxide inclusions based on the particle size distributions. However, in high-sulfur steels a significant amount of sulfides precipitate heterogeneously during cooling and solidification. This makes separation of secondary and primary inclusions very difficult. Moreover, the secondary sulfides which precipitate heterogeneously significantly change the characteristics (size, composition and morphology) of primary inclusions. The study revealed that both secondary and primary inclusions are heterogeneously dispersed in the industrial samples. In general, the middle zone of the surface layer is recommended for investigation of primary inclusions.
|
|
| 20. |
- Hallgren, Line, 1978-
(författare)
-
A Study of the Initial Mould Filling during Up-hill Ingot Casting
- 2012
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The fluid flow in the mould during up-hill teeming is of great importance for the quality of the cast ingot and thereby the quality of the final steel products. At the early stage of the filling of an up-hill teeming mould, liquid steel flows with a high velocity from the runner into the mould. The resulting turbulence on the meniscus can lead to entrainment of mould flux. The entrained mould flux might subsequently end up as defects in the final product. It is therefore very important to get a less turbulent and more stable inlet flow in the entrance region of the mould, to minimize the interactions between the steel and the mould flux. It has been acknowledged recently that a swirling motion, induced using a helix shaped swirl blade in the submerged entry nozzle, is remarkably effective to control the fluid flow pattern in both the slab and billet type continuous casting moulds. The focus in this thesis is the effect of a swirling flow, generated through a helix shaped swirl blade inserted into the runner system of the up-hill casting system, as a new method of reducing the deformation of the rising surface and the unevenness of the flow during filling of the up-hill teeming mould. The study covers a theoretical part, including mathematical modelling and water modelling experiments. Moreover, a part with some novel plant trials to test the swirling technology at the steel plants of Ovako bar in Hofors and Scana steel in Björneborg. The swirl blade has two features: (1) to generate a swirling flow in the entrance nozzle and (2) to suppress the uneven flow developed after flowing through a bend. Water modelling was used to assert the effect of the swirl blade on rectifying of tangential and axial velocities in the filling tube for the up-hill teeming and also to verify the results from the numerical calculations. The effect of swirl in combination with a diverged nozzle was also investigated in a similar way, i. e. with water model trials and numerical calculations. The earlier studies, using water as a fluid, have shown that the placement of a swirl blade in the vertical part of the runner channel, at the entrance to the mould, will be beneficial for a calm initial filling of the ingot during casting. Later, the plant trials were carried out to test if it would be possible to place a ceramic swirl blade in the runner channel during filling of ingots and to investigate the effect of a widening of the entry nozzle. Because of constrains of the current design of the runner systems at the steel plants it was not possible to place the swirl blade at the entrance. Instead, the swirl blade was placed in the horizontal part of the runner system. The initial experiments with swirl blade in the runner showed that no production disturbances were found. More specifically, no problems with unusual refractory wear or cracks in the refractory were found. Also, the mould powder consumption was found to decrease when using the swirl blade in combination with an inlet with a 6° diverged nozzle. Thus, it was concluded that the use of swirl blade has a potential in the future to be used to influence the initial filling conditions in a positive way. In addition to the plant trials with swirl blade, separate trials of changing the inlet angle from 1.7° to 6° or 10° showed that the widened inlet area resulted in a decrease in mould powder consumption. However, the decrease was not to the same extent as for the case with swirl blade.
|
|
| 21. |
- Janis, Diana, 1984-
(författare)
-
A Study of Different Methods for Inclusion Characterization towards On-line use during Steelmaking
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The interest of gaining on-line information related to non-metallic inclusions during the steelmaking process has recently increased due to the development as well as the promising results of the Pulse Distribution Analysis with Optical Emission Spectroscopy method (PDA/OES). Even though, the time from sampling to presented results on inclusions is only about 5-10 minutes, the method has also shown limitations with respect to the determination of some inclusion characteristics.Therefore, a first step was to perform a study on other methods such as the cross-section method (CS) on a polished sample surface, the cross-section after etching method (CSE), the bromine-methanol extraction method (BME), and the electrolytic extraction method (EE). This study focused on the evaluation of these methods with respect to the time consumption for preparation and analysis of a sample, the analyzed volume and the determination of inclusion and cluster characteristics such as size, number, particle size distribution (PSD) and composition. The CS and CSE methods were found to be suitable in the determination of the largest cluster in a sample which can be recommended in order to select proper extraction parameters for further studies. The BME method was considered to be fast with the possibility of analyzing a large volume. However, the used solution is chemically stronger compared to electrolytic extraction solutions, which can affect the results. In most aspects, the EE method was found to be the most stable, reliable and accurate method with some limitations regarding the time aspect.Based on this conclusion, the EE method was selected for a comparative study with the PDA/OES method. Reliably detected size ranges by using the PDA/OES method were defined for two low-alloyed steel grades. These are 2.0-5.7 μm and 1.4-5.7 μm for steel samples taken before and after a Ca-addition during the secondary steelmaking, respectively. Moreover, agreements between the EE and PDA/OES methods were observed in the average size and number of detected inclusions when only inclusions with the size > 2 μm were considered. Also, a theoretical minimum size and a maximum number ofinclusions present in the steel sample, which can be detected by using the PDA/OES method, were estimated.The work continued by successfully applying the EE method to study correlations between inclusions observed in the liquid steel samples and in a clogged nozzle (clogging material). It was found that the average sizes of spherical and non-spherical inclusions observed in the steel corresponded well with those observed in the clogging material. However, there were some differences in the frequencies of these inclusions. This was explained by a possible transformation of the present inclusions due to a reoxidation and a reaction with the nozzle refractory of the steel melt. The results of this study may contribute in the selection of proper process parameters or inclusion characteristics for future studies on the improvement and application of on-line methods.Finally, suggestions on how to present and interpret data obtained by the PDA/OES method during production of stainless steels were given in the present thesis. More specifically, the possibilities of defining operating windows with respect to inclusion composition and the use of a B-factor for Al (the total content of Al in inclusions detected by using the PDA/OES method) during the secondary steelmaking were discussed. In addition, a correlation study between B-factors for Al and numbers of inclusions (dV > 4 μm) obtained by using the PDA/OES method on process samples, and corresponding slivers indices from plate products was performed. The results showed a moderate correlation between these parameters as well as an increase of the slivers index with increased values of the chosen PDA/OES data. This indicates that it could be possible to predict when there is an increased risk of having slivers on the final rolled product at an early stage of the steelmaking process.
|
|
| 22. |
- Kohlstädt, Sebastian, 1986-
(författare)
-
On determining lost core viability in high-pressure die casting using Computational Continuum Mechanics
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The subject of this thesis work is to investigate whether Computational ContinuumMechanics (CCM) can serve as a valuable tool for the casting engineerto determine a priori whether a housing concept with inlying geometries thatso far only exists in Computer Aided Design (CAD) will have the desired coolingperformance and will be manufacturable with an acceptable number ofrejects.As of spring 2019, no application in serial production of lost cores, i.e.cores that are destroyed during deforming, in high-pressure die casting isknown. The reason for this is believed to be the absence of an engineeringtool that can tell upfront whether a concept of casting and process combinedwill be viable. This thesis aims to ll precisely that void by presenting, implementingand testing a CCM model inside the OpenFOAM toolbox in order todetermine upfront whether a design of a housing will be manufacturable withlost cores. The two-phase ow of air and melt is modeled with the volumeof-uid-concept. Turbulence modeling is done via the Reynolds-Averaged-Navier-Stokes (RANS) approach, mostly using the Menter SST k-omega-model.An isotropic linear elastic model was assumed for the solid mechanics.Industrial operators and managers like short and easy to grasp conclusions. As it, however, turned out during the process of this research project, there is no clear and easy answer to the question whether salt cores in high-pressuredie casting are a viable concept and will lead to sound castings. First of all, it was proven that housings made with lost cores can improve the heat transfer capabilities of castings. It was possible to produce castingswith cores up to an impact velocity of 30 ms^-1. The impact velocity wasfound to be the most decisive parameter. But the reader should bear in mindthat this limit is only valid for the given setup. Each conguration has to betested with the introduced model separately. The slamming events at rstimpact of the melt were found to be not failure-critical if crack-free coresare used. It was also found that the approach of evaluating only the peakforce does not go far enough. Eects later in the process may have a moreimportant impact due to larger force-time integrals. Also, dierent from theoriginal assumptions, the heat transferred from the melt to the core maynot be neglected even though filling times are below 0.1 s. Dening generalnumerical constraints for conditions under which salt cores are a viabletechnology is very dicult as geometry alterations play an important roletoo. This underscores the power and usefulness of the presented model evenIfurther as the engineer is now capable of testing each setup individually.It soon became clear that a fully comprehensive model is still for futureresearchers to develop. It was found that it is not benecial to attach the shotsleeve to the casting model with currently available open-source CFD technology.The presented strategy in this thesis together with the developed CCMtools can therefore provide a powerful tool for the casting or CAD-engineerto decide case by case whether a concept for a casting will be producible ornot. The tools range from a limited CFD approach for evaluating only theforces to a fully coupled FSI methodology describing the core deformationover time. All models have been tested and validated with high-pressure diecasting experiments and are in line with previously published ndings withdeviations of 5-10 % at maximum.
|
|
| 23. |
|
|
| 24. |
- Liu, Hailong
(författare)
-
A Study of the Particle Transport Behavior in Enclosed Environments
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main purpose of the present work is to increase the fundamental understanding of the particle transport behavior in an enclosed environment and to provide knowledge to the estimate and measure the particle emission from pellets during a steel production process.A laboratory study focused on the effect of the high sliding velocity on the particle generation from dry sliding wheel-rail contacts has been conducted. The particle concentration and the size distribution were acquired online by using particle number counters during the tests. After the completion of each test, the characteristics of pin worn surfaces and collected particles were analyzed with the aid of SEM (scanning electron microscopy) combined with EDS (energy disperse X-ray analysis). The results show that the amount of the particle generation increases significantly as the sliding velocity increases from 0.1 to 3.4 m/s. Moreover, the particle size distribution results indicate that the majority of the generated particles are submicron (ultrafine and fine) particles in the case of a high sliding velocity (1.2 and 3.4 m/s). The observations of iron oxide layers within the pin worn surface and the collected iron-oxide containing particles reveal that these substantial small particles can be attributed to an oxidative wear between the dry sliding wheel-rail contacts under high sliding velocities.The effect of the particle transport behavior with respect to submicron particles in the test chamber on the measurements taken at the outlet was studied by a three dimensional mathematical model. With the assistance of CFD (computational fluid dynamics) simulations, the airflow pattern was found to have a major effect on the particle transport during the tests. By estimating the particle loss rate, 30% of generated particles failed to be captured at the outlet. The reason for that could be a temporary suspension and a deposition onto the surfaces. It should be noted that the particles were assumed to follow the air stream as a result of the small particle size. In addition, the Lagrangian tracking results reveal that the limiting size for particles to become airborne during tests is around 10 µm. However, the computational cost is found to be significant high when the Lagrangian method is adopted.To consider the measurements of micron particles and to reduce the computational time, a coupled drift flux and Eulerian deposition model was developed. In this model, the effects of the gravitational sedimentation and deposition on the particle dispersion were included. The simulation results are in a good agreement with the available experimental data. The value of APD (average percentage deviation) is in the range of 7.7% to 21.2%. Therefore, a set of simulation cases have been carried out to investigate the influential factors (particle size, wall roughness, source location and duration). The results show that the homogeneity of the particle concentration distribution in the model room declines with an increased particle size (0.01 to 10 µm). An almost uniform particle concentration field is formed for submicron particles (0.01 and 0.1 µm) and for fine particles (1 and 2 µm). However, a clear concentration gradient is obtained for coarse particles (4, 6, 8 and 10 µm). This is due to that the gravitational settling dominates the motion of coarse particles. As a result, a large deposited amount and a high deposition fraction was predicted for coarse particles. Moreover, the surface roughness was found to enhance the deposition of submicron particles (0.1 and 0.01 µm) for a given friction velocity. On the contrary, the deposition of micron particles is much less sensitive to the variation of the surface roughness. For a case of an internal source in the room, where a release over a long duration is considered, the particle dispersion strongly depends on the release location. However, this is not the case for a short release time.The dispersions and depositions of micron particles were explored in a laboratory test focused on the particle emission from the wear between the pellets. The simulation results were compared to the measured data with respect to the particle flux at the outlet. A good agreement (4.92% < APD < 12.02%) is obtained. In addition, the influence of the air flow rate at the inlet and the particle size on the sampling results at the outlet was investigated carefully. The results show that a stronger air supply at the inlet can push more particles to the outlet for any given particle sizes. However, the resulted increase of the measurable fraction is more significant for 4, 6, 8 10 µm particles compared to 1, 2 and 20 µm particles. Moreover, it is apparent that 20 µm particles are unable to be measured in such a measurement system.
|
|
| 25. |
- Liu, Qiang, 1985-
(författare)
-
Control of Wear-Resistance Properties in Ti-added Hypereutectic High Chromium Cast Iron
- 2012
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High chromium cast iron (HCCI) is considered as one of the most useful wear resistance materials and their usage are widely spread in industry. The wear resistance and mechanical properties of HCCI mainly depend on type, size, number, morphology of hard carbides and the matrix structure (γ or α). The Hypereutectic HCCI with large volume fractions of hard carbides is preferred to apply in wear applications. However, the coarser and larger primary M7C3 carbides will be precipitated during the solidification of the hypereutectic alloy and these will have a negative influence on the wear resistance.In this thesis, the Ti-added hypereutectic HCCI with a main composition of Fe-17mass%Cr-4mass%C is quantitatively studied based on the type, size distribution, composition and morphology of hard carbides and martensite units. A 11.2μm border size is suggested to classify the primary M7C3 carbides and eutectic M7C3 carbides. Thereafter, the change of the solidification structure and especially the refinement of carbides (M7C3 and TiC) size by changing the cooling rates and Ti addition is determined and discussed. Furthermore, the mechanical properties of hypereutectic HCCI related to the solidification structure are discussed.Mechanical properties of HCCI can normally be improved by a heat treatment process. The size distribution and the volume fraction of carbides (M7C3 and TiC) as well as the matrix structure (martensite) were examined by means of scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Especially for the matrix structure, EBSD is a useful tool to classify the fcc (γ) and bcc (α) phases. In conclusion, low holding temperatures close to the eutectic temperature and long holding times are the best heat treatment strategies in order to improve wear resistance and hardness of Ti-alloyed hypereutectic HCCI.
|
|
| 26. |
- Liu, Yu, 1990-
(författare)
-
Thermodynamics and Kinetics in Metallurgical Processes - with a Special Focus on Bubble Dynamics
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gas stirring is commonly used in the steelmaking processes to reinforce chemical reactions, kinetic transfer, and inclusion removal, etc. This dissertation concentrates on multiphase flows with gas bubbling to study fluid dynamics and thermodynamics in metallurgical processes. A study of bubble behavior has been carried out using a multiscale approach as follows: Prototype scale (macro) → Plume scale → Single bubble scale → Reaction scale (micro).Initially, previous works on physical modeling and mathematical modeling in relation to the gas bubbling in the ladle have been reviewed. From that, several aspects that can be improved were found:For physical modeling, such as mixing and homogenization in ladles, the general empirical rules have not been analyzed sufficiently;The mathematical models focusing on inclusion behaviors at the steel-slag interface need to be improved;The phenomena governing the transfer of elements, vacuum degassing, and the combination of fluid dynamics and thermodynamics, such as in desulfurization, need to be investigated further.The kinetics transfer with regards to temperature and element homogenization is one of the most extensive research fields in steel metallurgy. For the analysis on prototype scale, the optimal plug configuration has been studied for a 50t ladle. For stirring using bottom-blowing, a separation angle between dual plugs of 160 degree is mostly recommended, and the optimal dual-plug radial position is around 0.65R. Moreover, the influence of the tracer’s natural convection on its homogenization pattern cannot be neglected, especially for ‘soft bubbling’ conditions using low gas flow rates.Subsequently, in studies of the statistical behavior of gas bubbling in the plume, mathematical modeling using an Euler-Euler approach and an Euler-Lagrange approach have been compared. With respect to the bubble coalescence and breakup, the Euler-Lagrange approach is more accurate in predicting the flow pattern for gas injection using a porous plug. With regards to the effect of plug design on the statistical behavior of gas bubbling, gas injection using a slot plug promotes kinetic reactions close to the open eye due to the concentrated plume structure, and gas bubbling using a porous plug promotes a good inclusion removal because of the increased amount of bubbles.Focusing on single bubble behavior, under the same flow rate, as the top gauge pressure is reduced, the bubble diameter increases and the bubble generation frequency decreases. During the bubble ascent, a large bubble gradually reaches stable conditions by means of shedding several small bubbles. In a steel-argon system, under a flow rate in the range of 5.0(mL‧min-1)STP to 2000(mL‧min-1)STP, the bubble diameter is in the range of 6.0mm to 20.0mm. Under laminar conditions, the maximum bubble width is 65mm when the surrounding pressure is 0.2bar, and the steady bubble width is around 58mm under a pressure of 2.0bar.Finally, a coupling method, named Multi-zone Reaction Model, has been developed to predict the conditions in the EAF refining process. Using a combined injection of O2 and argon, and the same injected mass of O2, the decarburization rate increases due to an efficient kinetic mass transfer of carbon in the molten steel. Furthermore, using CO2 to replace argon, as the ratio of the CO2 content in the injection increases, the maximum hot spot temperature, the increment rate of average temperature, and the decarburization rate decrease dramatically.The research step from multiphase fluid dynamics to its coupling with high temperature thermodynamics is a large advancement in this study. Moreover, the research process using open source software to replace the commercial software is also an important technical route. This can help the transparent development of future modules for reacting flow in metallurgical processes.
|
|
| 27. |
- Mu, Wangzhong, 1985-
(författare)
-
Microstructure and Inclusion Characteristics in Steels with Ti-oxide and TiN Additions
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Non-metallic inclusions in steels are generally considered to be detrimental for mechanical properties. However, it has been recognized that certain inclusions, such as Ti-oxide and TiN, can serve as potent nucleation sites for the formation of intragranular ferrite (IGF) in low-alloy steels. The formation of IGF could improve the toughness of the coarse grained heat affected zone (CGHAZ) of weld metals. Thus, the present thesis mainly focuses on the effect of size of nucleation sites on the IGF formation. Quantitative studies on the composition, size distribution and nucleation probability for each size of the inclusions as well as the area fraction, starting temperature and morphology of an IGF have been carried out.In the present work, the Ti-oxide and TiN powders were mixed with metallic powders. The mixed powders were heated up to the liquid state and cooled with a slow cooling rate of 3.6 ºC/min. These as-cast steels with Ti-oxide and TiN additions were used to simulate the IGF formation in the CGHAZ of weld metals. Specifically, the inclusion and microstructure characteristics in as-cast steels have been investigated. The results show that the nucleant inclusion was identified as a TiOx+MnS phase in steels with Ti2O3 additions and as a TiN+Mn-Al-Si-Ti-O+MnS phase in steels with TiN additions. In addition, the TiOx and TiN phases are detected to be the effective nucleation sites for IGF formation. It is clearly shown that an increased inclusion size leads to an increased probability of IGF nucleation. This probability of IGF nucleation for each inclusion size of the TiOx+MnS inclusions is clearly higher than that of the complex TiN+Mn-Al-Si-Ti-O+MnS inclusions. In addition, the area fraction of IGF in the steels with Ti2O3 additions is larger than that of the steels with TiN additions. This result agrees with the predicted tendency of the probability of IGF nucleation for each inclusion size in the steels with Ti2O3 and TiN additions.In order to predict the effective inclusion size for IGF formation, the critical diameters of the TiO, TiN and VN inclusions, which acted as the nucleation sites of IGF formation, were also calculated based on the classical nucleation theory. The critical diameters of TiO, TiN and VN inclusions for IGF formation were found to be 0.192, 0.355 and 0.810 μm in the present steels. The calculation results were found to be in agreement with the experiment data of an effective inclusion size. Moreover, the effects of the S, Mn and C contents on the critical diameters of inclusions were also calculated. It was found that the critical diameter of the TiO, TiN and VN inclusions increases with an increased content of Mn or C. However, the S content doesn’t have a direct effect on the critical diameter of the inclusions for IGF formation. The probability of IGF nucleation for each inclusion size slightly decreases in the steel containing a higher S content. This fact is due to that an increased amount of MnS precipitation covers the nucleant inclusion surface.In the as-cast experiment, it was noted that an IGF can be formed in steels with Ti2O3 and TiN additions with a cooling rate of 3.6 ºC/min. In order to control the microstructure characteristics, such as the area fraction and the morphology of an IGF, and to investigate the starting temperature of IGF and grain boundary ferrite (GBF) formation, the dynamic transformation behavior of IGF and GBF was studied in-situ by a high temperature confocal laser scanning microscope (CLSM). Furthermore, the chemical compositions of the inclusions and the morphology of IGF after the in-situ observations were investigated by using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) which equipped wavelength dispersive spectrometer (WDS). The results show that the area fraction of IGF is larger in the steels with Ti2O3 additions compared to the steels with TiN additions, after the same thermal cycle has been imposed. This is due to that the TiOx phase provides more potent nucleation sites for IGF than the TiN phase does. Also, the area fraction of IGF in the steels is highest after at an intermediate cooling rate of 70 ºC/min, since the competing phase transformations are avoided. This fact has been detected by using a hybrid methodology in combination with CLSM and differential scanning calorimetry (DSC). In addition, it is noted that the morphology of an IGF is refined with an increased cooling rate.
|
|
| 28. |
- Ni, Peiyuan
(författare)
-
Numerical Study on Steel Flow and Inclusion Behavior during a Ladle Teeming Process
- 2013
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Inclusions in molten steel have received worldwide concern due to their serious influence on both the steel product quality and the steel production process. These inclusions may come from the deoxidation process, reoxidation by air and/or slag due to an entrainment during steel transfer, and so on. They can break up a casting process by clogging a nozzle. A good knowledge on both steel flow and inclusion behavior is really important to understand nozzle clogging, as well as to take some possible measures to alleviate clogging. In this thesis, steel flow and inclusion behavior during a teeming process were investigated by mathematical simulations with verification by pilot-plant experiments.Firstly, steel flow phenomena during a ladle teeming process were studied. Different turbulence models, including the low Reynolds number k-ɛ model and the realizable k-ɛ model both with an enhanced wall treatment (EWT) and a standard wall function (SWF), were used to simulate this process. All of these turbulence model predictions generally agreed well with the experimental results. The velocity distributions in the nozzle were also predicted by these turbulence models. A large difference of the boundary-layer velocity predicted with these two near wall treatment methods was found. At the late stage of the teeming process, the drain sink flow phenomena were studied. The combination of an inclined ladle bottom and a gradually expanding nozzle was found to be an effective way to alleviate a drain sink flow during teeming.Then, inclusion behavior during a teeming stage was studied. A Lagranian method was used to track the inclusions in steel flow and compare the behaviors of different-size inclusions. In addition, a statistical analysis was conducted by the use of a stochastic turbulence model to investigate the behaviors of different-size inclusions in different nozzle regions. Inclusions with a diameter smaller than 20μm were found to have a similar trajectory and velocity distribution in the nozzle. However, inertia force and buoyancy force were found to play an important role for the behavior of large-size inclusions or clusters. The statistical analysis results indicate that the regions close to the connection between different angled nozzle parts seem to be very sensitive for an inclusion deposition.
|
|
| 29. |
- Persson, Fredrik, 1971-
(författare)
-
A Study of Parameters and Properties Influencing the Size, Morphology and Oxygen Content of Water Atomized Metal Powders
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The production of metal powders by water atomization is a well-established process, which can be used to produce a wide range of particle sizes for different applications. In general, there is a lack of detailed knowledge about what process parameters that affect the powder properties for water atomized metal powders. More specifically, this thesis focuses on the particle size, morphology and oxygen content of water atomized iron powders. A careful control of the particle size distribution is necessary to atomize powders with a high quality and at a low production cost. Demands on the particle morphologies vary depending on the application for the final product. It is important to control both the melt properties and atomizing parameters, to produce powders with an even particle shape and sintered steel components with tight tolerances. The oxidation of the liquid metal should also be as low as possible during the water atomization, to avoid a large amount of harmful oxide forming in the final powder. Pores are generally considered as defects in metal powders. Therefore, the powder porosity should be as low as possible.The main objective of this thesis is to obtain a more in-depth knowledge of water atomization of metal powders, by investigating some fundamental parts of the process. The study investigates how the median particle size (d50 value) for iron powders is influenced by the water pressure, the melt stream diameter, the jet angle, the water level in the atomizing tank, changed configurations of the water jets, superheat of the melt, and the carbon and sulfur content in the liquid steel. Similarly, the thesis also investigates factors that influence the particle shape, porosity and oxidation of water atomized iron powders.Laboratory and pilot experiments show that the effect on the d50 value was large for the water pressure, medium for the viscosity, surface tension and water to metal ratio, and small for the melt stream diameter. Calculations indicate that the water jet angle has a large effect on the d50 value. In practice, this effect cannot be exploited beyond certain limits caused by instabilities in the atomizing system, which occur if the jet angle is too large.The particle size decreases when the carbon and sulfur contents in the liquid iron are increased. This is attributed to decreased viscosities and surface tensions, respectively. An alternative explanation could be that the superheats at increased carbon contents result in a longer time spent in the molten state before the atomization is completed. This may also lead to a decrease in the particle size. Calculations using a developed d50 model estimate that a decreased viscosity from 6.8 mPa s to 4.3 mPa s leads to a reduction in the d50 value by 33%. Similarly, a decreased surface tension from 1840 mN/m to 900 mN m-1 reduces the d50 value by 27%.The distribution of oxides in pilot water atomized Fe-Mn-C powders was determined by using optical and scanning electron microscopy, combined with energy dispersive X-ray microanalysis. The oxygen in the atomized powders was mainly present as thin surface oxide layers, which increase in thickness from 10 nm to 50 nm as the particle sizes increase from 10 microns to 750 microns. Manganese oxides were observed to be unevenly distributed at the surface of several particles, when the alloy contained 0.3 wt.% manganese. Experimental data indicate that between 10 - 20% of the manganese was present as oxides in the powders. However, equilibrium calculations at 1550 °C estimate that only 4% of the initial manganese content remained in the steel after a completed atomization.The sphericity of the atomized powders decreases as the particle size increases. One feasible explanation is that some larger particles are irregular, since they are formed by collisions of smaller particles. Conversely, smaller particles are formed directly from breakups of the melt and are not the product of collisions between droplets. The sphericity of the size fraction 20-45 microns increases as the carbon content in the iron increases from 0.2 wt.% to 4.2 wt.%. The atomized droplets with larger carbon contents spend a longer time in the molten state, which allows them more time to form a spherical shape during the atomization process. The porosity of iron-carbon powders increases with increasing carbon contents in the melt. Dissociation of steam to hydrogen at the melt surface and precipitation of hydrogen pores in the melt were the most likely mechanisms to cause a pore formation in the powders.Keywords: water atomization; metal powder: particle size; oxygen content; particle shape; porosity; steelmaking
|
|
| 30. |
- Ratnasari, Devy Kartika, 1990-
(författare)
-
Enhanced Catalytic Pyrolysis of Biomass for High-Quality Biofuel Production
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The rapid increase in energy demand, the extensive use of fossil fuels, and the urgent need to reduce carbon dioxide emissions have raised concerns in the transportation sector, since transportation has been primarily dependent on fossil fuels. Biofuel from biomass can make significant contributions to overcome the expected depletion of fossil fuels and reduce carbon emissions. The availability and wide diversity of biomass resources have made them an attractive and promising source of fuels. Biomass can be converted into biofuel by thermochemical pyrolysis process. Improvements on the pyrolysis process of biomass fuels are needed to obtain a high-quality of bio-oil. Pre-treatment by acid leaching prior to the pyrolysis process is considered to remove Alkali and Alkaline Earth Metal (AAEM) from the biomass, since AAEM adversely affect the catalytic pyrolysis process. Information about biomass pyrolysis kinetics is also important to evaluate biomass as a feedstock for fuel or chemical production as well as efficient design and control of thermochemical processes. Further, the use of H-ZSM-5 and Al-MCM-41 as a mesoporous and a microporous catalyst has been proved to improve the quality of bio-oil. The influence of a catalyst regeneration on the chemical composition of the upgraded oil is also one of the factors pertaining to the catalytic process.In this study, the catalytic pyrolysis kinetics of lignocellulose biomass with a mixed catalyst of H-ZSM-5 and Al-MCM-41 at different ratios for both, un-leached and leached biomass, is analyzed. The derived activation energies are determined based on the solid-state reaction mechanism. Bench-scale experiments have also been investigated to improve the quality of bio-oil, in terms of Organic Fraction (OF), water content, acidity, favorable fractions, as well as gasoline-range chemicals. The effect of a mixed-catalysts and staged-catalysts consisting of H-ZSM-5 and Al-MCM-41 at different ratios in a lignocellulose biomass pyrolysis has been compared. The ratio of H-ZSM-5 and Al-MCM-41 in the catalyst mixtures for lignocellulose biomass catalytic pyrolysis has also been optimized. Further, the effect of sequential catalyst regenerations of H-ZSM-5 and Al-MCM-41 catalyst mixtures on the obtained catalytic pyrolysis products has been analysed.The bench-scale experiments of lignocellulosic biomass pyrolysis and catalytic pyrolysis were performed using a fixed bed reactor equipped with oil condensers and a gas collection sample bag. The quality of bio-oil produced from the thermal pyrolysis of lignocellulosic biomass, catalytic pyrolysis with single catalyst, catalytic pyrolysis with staged catalyst system, as well as catalytic pyrolysis with mixed catalyst system were studied. Later, the catalyst was regenerated several times and the regenerated catalyst was reloaded in the reactor to proceed with the next run. The composition of the derived upgraded pyrolysis oils in relation to the catalyst regeneration was determined.The results from the acid leaching treatment showed that the optimum leaching process was set to 30 minutes, 30°C and 5 wt.% acetic acid in the leaching liquid. This resulted in 59%, 95%, 99%, and 96% reduction degree of Calcium (Ca), Magnesium (Mg), Potassium or Kalium (K), and Natrium (Na), respectively. The use of the acid leaching process as a treatment prior to catalytic pyrolysis is positive, since it resulted in high devolatilization and reaction rate. For the kinetic studies, the second order (F2) mechanism was able to illustrate the catalytic pyrolysis process, proven by the result that the coefficient of determination (R2) was higher than 0.99, which was high compared to other mechanisms.The bench-scale experiments show that that Al-MCM-41 with H-ZSM-5 in the staged catalyst system enhanced the production of favorable compounds: hydrocarbons, phenols, furans, and alcohols. The favorable compounds yield that boosted 5.25-6.43% of that with single H-ZSM-5 catalyst was produced with H-ZSM-5:Al-MCM-41 mass ratio of 3:1 and 7:1. The pyrolysis and catalysis temperature of 500°C with H-ZSM-5:Al-MCM-41 ratio of 3:1 obtained the optimum quality of bio-oil with 11.08 wt.% of Organic Fraction (OF), 76.20% of favorable fractions, 41.97 wt.% of water content, low TAN of 43.01 mg-KOH/g, high deoxygenation, as well as high gasoline-range production of 97.89%.The catalyst mixture of H-ZSM-5 and Al-MCM-41 with a ratio of 7:1 resulted in a 65.75% deoxygenation degree. An organic-rich bio-oil was obtained with 74.90 wt.% of carbon content, 8 wt.% of hydrogen content, 15 wt.% oxygen content, a 0.39 wt.% water content, and a high heating value of 34.15 MJ/kg. The highest amount of favorable compounds among the studied catalytic experiments was obtained with a value of 95.89%. The significant improvement in the quality of bio-oil with the utilisation of H-ZSM-5 and Al-MCM-41 catalyst mixtures was the rise of favorable compounds in bio-oil.The experiments of sequential catalyst regenerations of H-ZSM-5 and Al-MCM-41 catalyst mixtures show that the catalytic activity decreased as the number of reaction cycles increased, albeit an increase yield of Organic Fraction (OF) and a decrease in water as well as coke yields. The HHV of bio-oils decreased. However, the minimum value of HHV (22.42 MJ/kg) after 6 sequential usage was still higher than the value for the non-catalytic experiment (19.55 MJ/kg). The favorable compounds yield, which includes hydrocarbons, phenols, furans, and alcohol, decreased. The dominant components contributed to the yield of favorable compounds were hydrocarbon aromatics and phenols.
|
|
| 31. |
- Safavi Nick, Arash, 1985-
(författare)
-
Pores, inclusions and electromagnetic stirring : Topics from the continuous casting of steel
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with two topics of relevance to the continuous casting of steel,in view of their importance as regards the quality of the final solidified structure.The first concerns the precipitation of gas pores and inclusions in the interden-dritic region of the solidifying metal. Motivated by experimental results thatindicate the formation of pore-inclusion clusters in the final cast structure, a the-oretical model is developed to describe how thus might occur; the model makesuse of the basic principles of fluid mechanics and heat transfer, with asymptoticmethods then being used in order to obtain solutions. In particular, it is foundthat soluto-thermocapillary drift in a direction perpendicular to the direction ofcasting, as a consequence of the dependence of surface tension at the pore-metalinterface on temperature and sulphur concentration, could explain cluster forma-tion. The second is a theoretical study concerning longitudinal electromagneticstirring (EMS), which is often used in the continuous casting of blooms in order toimprove product quality. Via an analysis of the three-dimensional (3D) Maxwellequations for the components of the magnetic flux density, a flaw is found inthe way that the components of the stirring Lorentz force have previously beencalculated; this is corrected and the new results are confirmed by comparison ofsolutions obtained from asymptotic analysis and time-dependent 3D computa-tions using finite-element methods. The analysis identifies the importance of theproduct of the bloom width and the wave vector of the applied field as a keydimensionless parameter.
|
|
| 32. |
- Safavi Nick, Reza, 1977-
(författare)
-
Mathematical Model of the Solid Flow Behavior in a Real Dimension Blast Furnace
- 2012
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A mathematical model based on the continuum mechanic concept has been developed to describe the profile of solid particles in an industrial scale blast furnace. The focus is the in-furnace conditions and its characteristics such as the shape and size of the deadman. The Navier-Stokes differential equation for multi-phase multi-dimensional space has been used to describe the behavior of existing phases. The equation has been modified to make it possible to describe the dual nature of the solid phase in the system. This has been done by applying the concept of the solid surface stress to describe the intergranular surface interactions between particles. More specifically, this term is added as an extra term to the Navier-Stokes equation to describe the particle-particle interactions. This extra term in behave as a breaking force when the particles are sliding down in the furnace. During the descending movement in the furnace it is shown that the particles change their profile from a V-shape to a Wshape, due to the characteristics of the deadman. Moreover, the velocity magnitude is higher at the outer surface of the deadman for higher grid-slabs in this region than the near-wall cells. However, the situation changes as solid particles moving to even lower levels of the grid-slabs at the outer surface of the deadman in comparison to near-wall cells. It has also been shown that an increase in the magnitude of the effective pressure reduces the velocity magnitude of descending particles. Furthermore, since different phases co-exist in a blast furnace, the volume fraction plays an important role in the blast furnace. Therefore, the influence of three different packing densities (0.68, 0.71 and 0.74 respectively) on the profile of the flow through the upper part of the blast furnace from the throat to the tuyeres level has been studied. It is shown that an increase in the volume fraction of the solid phase lead to a decrease in the velocity magnitude. This decrease is due to an increase in the solid volume fraction, which will increase the resident time of the particles inside a blast furnace. In addition, it is shown that the velocity magnitude of the solid phase decreases from the throat to the belly of the furnace, for the studied conditions. However, after belly the velocity magnitude increases again.
|
|
| 33. |
- Samuelsson, Peter
(författare)
-
Management of technology in the process industries: Matching market and machine
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The process industries span multiple industrial sectors and constitute a substantial part of the entire manufacturing industry. Since companies belonging to this family of industries are often very asset intensive, their ability to respond to changes is often limited in the short term.The adaptation of the capabilities of existing processes, and conversely finding products and market segments to match the production system capabilities, are an important part of product- and market development activities in the process industry. The importance to companies in the process industry of having a well-articulated manufacturing strategy congruent with the business strategy is second to none. However, to facilitate manufacturing strategy developments, it is essential to start with an improved characterization and understanding of the material transformation system.To that end an extensive set of variables was developed and related measures and scales were defined. The resulting configuration model, focusing on company generic process capabilities in the process industries, is to be regarded as a conceptual taxonomy and as a proposition available for further testing. The usability of the model was subsequently assessed using “mini-cases” in the forestry industry, where the respondents confirmed that the company’s overall strategy could benefit from this kind of platform as a possible avenue to follow.The model was deployed as an instrument in the profiling of company material transformation systems to facilitate the further development of companies' functional and business strategies. The use of company-generic production capabilities was studied in three case companies representing the mineral, food and steel industries. The model was found by the respondents to be usable as a knowledge platform to develop production strategies. In the final analysis of the research results, a new concept emerged called “production capability configuration":A process-industrial company’s alignment of its generic production capabilities in the areas of raw materials, process technology and products to improve the consistency among the variable elements that define operations and improve the congruence between operations and its environment.From the perspective of value creation and capture, firms must be able to manufacture products in a competitive cost structure within the framework of a proper business model. By using the configuration model, the relationship between manufacturing and innovation activities has been studied in the previously mentioned three case studies.In many cases the gap in capability appears as a limitation in the production system, requiring development efforts and sometimes investments to overcome. This is illustrated with two examples from the steel industry, where development efforts of the production system capabilities are initiated to better match the market demands. One example is the increase the volume- and product flexibility of an existing stainless steel melt shop, resulting in a proposed oblong Argon Oxygen Decarburisation (AOD) converter configuration that was subsequently verified using water modelling. The second example is from a carbon steel mill, where the target was to increase the raw material- and volume flexibility of another melt shop, by modifying the capabilities of the Electric Arc Furnace (EAF). Enabling EAF technologies are further described and evaluated using operational data and engineering type of estimates.
|
|
| 34. |
- Sidorova, Elena, 1991-
(författare)
-
Non-metallic inclusions in pipeline steels and their effect on the corrosion resistance
- 2022
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focus on investigations of the influence of non-metallic inclusions on corrosion in steel samples taken at different stages of the steel production when producing pipeline steels. The electrolytic extraction technique was used to extract inclusions from these steel samples and thereafter studying them using scanning electron microscopy in combination with energy dispersive microscopy. This approach enabled three-dimensional evaluations of different non-metallic inclusions present in two metal samples of low-carbon Ca-treated pipeline steels. The modification of the inclusion characteristics was investigated and compared for steel samples taken from various stages of the smelting production process. Thereafter, the corrosion resistance of these steels was discussed depending on the characteristics of non-metallic inclusions present in the steel. In addition, this study also presents a new method of soft chemical extraction for qualitative and quantitative evaluations of the initial corrosion processes of a steel matrix surrounding various non-metallic inclusions. The results showed that the initial dissolution of the steel matrix started in areas surrounding CaS inclusions or inclusion phases containing a CaS phase. Furthermore, the results showed that no dissolution of the metal matrix surrounding CaO·Al2O3 and TiN inclusions could be detected. Key words: pipeline steels, Ca-treatment, non-metallic inclusions, electrolytic extraction, corrosion, chemical extraction.
|
|
| 35. |
- Sjöqvist Persson, Eva, 1962-
(författare)
-
On the Origin and Distributions of the Inclusions in Production-scale ESR and PESR Remelted Ingots and Materials from Different Ingot Sizes and Solidification Structures
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The study was carried out with the aim to evaluate the origin, morphology, and distribution of the non-metallic inclusions (NMI) in electro-slag remelted (ESR) steels and in electro-slag remelted steels using a pressured controlled inert atmosphere (PESR). In addition to the NMI studies, the solidification structure in different ingot sizes were studied in order to define the influence of the solidification on the NMI characteristics. The steel grade chosen for the studies was a common martensitic stainless steel. The focus is on the origin and the distribution of oxide inclusions with the assumption that sulfides and nitrides are secondary inclusions in remelted material.In order to get a good statistical basis, a large number of SEM samples from different axial positions were taken from both an electrode and several ESR and PESR remelted ingots as well as processed (rolling/forging) materials. The inclusions were investigated by using both two-dimensional (2-D) and three-dimensional (3-D) methods. Especially for steels with a higher cleanliness, as for example remelted steels, a large analyzed area is important in order to get a true picture of the inclusion morphology. As an attempt to localize the origin of the inclusions, a pilot trial using a La2O3 as a tracer in the ESR process slag was performed. To study the influence of the solidification structure on the inclusions, horizontal slice/slices were cut from different positions from the electrode as well as from ESR and PESR remelted ingots of different sizes. Beside inclusions and chemical composition determinations across the diameter of the slices, also the second dendrite arm spacing (SDAS) and the angles of the dendrites towards the axial plane were measured.The result gave rise to a new classification of the inclusions present in ESR or PESR remelted steels, i) Primary Inclusions. They survive from the electrode because they were trapped inside a steel drop or a fallen steel fragment, without having contact with the ESR/PESR process slag. The size depends on the size of the inclusions in the electrode and the size of the steel droplets. ii) Semi-Secondary Inclusions, primary Al-Mg oxides covered by process slag. Normal size class is ≈ < 30 µm. iii) Secondary Inclusions, precipitated during solidification of the liquid steel as a result of the reactions between alloying elements and the dissolved oxygen. Normal size class is < 10 µm.The structure study showed that the transition from a columnar-dendritic to an equiaxial structure (CET) in the center of the ingot have a strong effect on the number and size of the inclusions. As long as the center of the ingot solidifies in a columnar-dendritic manner, the increase of the inclusion number and size is almost linear with an increasing ingot size. However, after the CET transition in the center, the inclusion number and sizes are much larger. For this steel grade, the transition from a columnar-dendritic to an equiaxial is between the 800 mm in diameter (PESR-800) ingot and the 1050 mm in diameter (PESR-1050) ingot. The primary arms growth rate needed for the CET transition is less than 4 x 10-7m/s. In order to undertake the transition, the temperature gradient must be less than approximately 103 °C/m.On the whole, the results illustrated that the overall cleanliness of the electrode (as well as the composition of the inclusions in the electrode) has an extremely large influence on the cleanliness in ESR and PESR remelted steels. The majority of the failure critical inclusions originates direct or indirect from the inclusions in the electrode. Moreover, the solidification structure (ingot size) also has a direct bearing on the inclusion sizes and contents present in ESR and PESR ingots.
|
|
| 36. |
- Svensson, Jennie, 1987-
(författare)
-
The Submerged Entry Nozzle : A Study of how to Reduce Decarburization and Clogging
- 2014
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During continuous casting the submerged entry nozzle (SEN) is one of the critical points. This since sequence casting is favourable, clogging of the SEN results in fever sequences or aborted castings. In this study the preheating of SEN has been evaluated at different steel plants. This since it earlier has been found, preheating resulted in decarburisation of the refractory base material (RBM) which in turn resulted in clogging of the SEN. During the industrial preheating trials it was found that decarburisation of the SEN is possible.The decarburisation results in a higher oxygen potential near the SEN wall. This in turn makes it possible for aluminum in the molten steel to form alumina particles. For low carbon aluminum killed steel grades, accretion of alumina clusters on the SEN internal wall result in clogging which hinder or reduces the steel flow through the SEN. For this reason, laboratory trials were performed to establish the formation of a liquid phase. Thus simulating the formation of liquid inclusions when alumina clusters attaches to the SEN wall, and which can be washed away by the steel flow. Formation of a liquid phase was indicated at the temperatures 1550-1600°C, and plasma coated nozzles were tried in pilot plant trials. The coating contained of 5-9 pct, 99.9 pct pure, calcium titanate (CaTiO3) mixed with yttria stabilized zirconia (YSZ).
|
|
| 37. |
|
|
| 38. |
- Swartling, Maria, 1983-
(författare)
-
A Study of the Heat Flow in the Blast Furnace Hearth Lining
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of the present thesis was to study the heat flows in the blast furnace hearth lining by experimental measurements and numerical modeling. Thermocouple data from an operating furnace have been used throughout the work, to verify results and to develop methodologies to use the results in further studies. The hearth lining were divided into two zones based on the thermocouple readings: a region with regular temperature variations due to the tapping of the furnace, and another region with slow temperature variations. In an experimental study, the temperatures of the outer surfaces of the wall and bottom were measured and compared with lining temperature measured by thermocouples. Expressions to describe the outer surface temperature profiles were derived and used as input in a two-dimensional steady state heat transfer model. The aim of the study was to predict the lining temperature profiles in the region subjected to slow temperature variations. The methodology to calculate a steady state lining temperature profile was used as input to a three-dimensional model. The aim of the three-dimensional model was primarily to study the region with dynamic lining temperature variations caused by regular tappings. The study revealed that the replacement of original lining with tap clay has an effect when simulating the quasi-stationary temperature variations in the lining. The study initiated a more detailed study of the taphole region and the size and shape of the tap clay layer profile. It was concluded, that in order to make a more accurate heat transfer model of the blast furnace hearth, the presence of a skull build-up below the taphole, erosion above the taphole and the bath level variations must to be taken into consideration.
|
|
| 39. |
- Swartling, Maria, 1983-
(författare)
-
An Experimental and Numerical Study of the Heat Flow in the Blast Furnace Hearth
- 2008
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This study has focused on determining the heat flows in a production blast furnace hearth. This part of the blast furnace is exposed to high temperatures. In order to increase the campaign length of the lining an improved knowledge of heat flows are necessary. Thus, it has been studied both experimentally and numerically by heat transfer modeling. Measurements of outer surface temperatures in the lower part of a production blast furnace were carried out. In the experimental study, relations were established between lining temperatures and outer surface temperatures. These relations were used as boundary conditions in a mathematical model, in which the temperature profiles in the hearth lining are calculated. The predictions show that the corner between the wall and the bottom is the most sensitive part of the hearth. Furthermore, the predictions show that no studied part of the lining had an inner temperature higher than the critical temperature 1150°C, where the iron melt can be in contact with the lining.
|
|
| 40. |
- Tan, Zhe
(författare)
-
Some Aspects of Improving Initial Filling Conditions and Steel Cleanliness by Flow Pattern Control Using a Swirling Flow in the Uphill Teeming Process
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The flow pattern has widely been recognized to have an impact on the exogenous non-metallic inclusion generation in the gating system and mold flux entrapment in the uphill teeming process. Thus, a well-controlled flow pattern during the teeming process can improve the quality of ingots and further increase the yield during steel production. The current study focused on investigating and optimizing the flow pattern of steel in the gating system and molds to improve steel cleanliness during the initial filling moment. A mathematical model considering a trumpet was initially compared to a reduced model only considering part of the runner channel. Thereafter, the influence of swirl blades implemented at the bottom of the vertical runner on the improvement of initial filling conditions in the molds was investigated in a model considering the entire mold system including a trumpet. The effects of a swirl blade orientation on a swirling flow were further discussed. The simulation results, when utilizing swirl blades, were also verified by plant trials performed at Scana Steel. In addition, a new novel swirling flow generation component, TurboSwirl, was studied in a model considering the entire mold system including a trumpet. The model was based on modifications of the refractory geometry at the elbow of the runners near the mold without the usage of an inserted flow control device in the gating system. Owing to its great potential for improving the flow pattern of steel during the initial filling moment, the effect of TurboSwirl on steel cleanliness was also studied. The results showed that the initial filling conditions during the uphill teeming process can be improved by using a swirl blade or a TurboSwirl in the gating system. This makes it possible to further decrease the initial position of mold powder bags. In addition, it reduces the possibilities of exogenous non-metallic inclusion generation in the gating system as well as mold flux entrapment in the mold during the uphill teeming process. However, the utilization of swirl blades created a considerable amount of droplets when steel entered the molds during the first couple of seconds, which also was verified by the plant trials. The introduction of TurboSwirl showed a greater potential than a swirl blade due to a more evenly distributed swirling flow. The DPM model adopted in the simulations revealed that the TurboSwirl can improve steel cleanliness by increasing the non-metallic inclusion collision rate both with respect to Stokes and turbulent collisions.
|
|
| 41. |
- Wang, Jing, 1985-
(författare)
-
Development of Graphitic Carbon Nitride based Semiconductor Photocatalysts for Organic Pollutant Degradation
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As a potential solution to the global energy and environmental pollution, design and synthesis of artificial photocatalysts with high activities have attracted increasing scientific interests worldwide. In recent years, the graphitic carbon nitride (g-C3N4) has shown new possible applications in the photocatalytic field due to its unique properties. However, the photocatalytic efficiency of the pristine g-C3N4 is greatly limited by the high recombination rate of the photo-induced electron-hole pairs. In this thesis, the aim is to design and fabricate efficient g-C3N4 based photocatalysts with enhanced photocatalytic activities under a visible light irradiation.In order to achieve this goal, two strategies have been employed in the present thesis. First, the as-obtained g-C3N4 was used as the host material to construct staggered-aligned composite photocatalysts by selecting semiconductors with suitable band positions. By this method, three kinds of g-C3N4-based composite photocatalysts such as g-C3N4/ZnS nanocage, g-C3N4/m-Ag2Mo2O7 and g-C3N4/MIL-88A were successfully fabricated. Second, the microstructure of the g-C3N4 was modified by the H2O2-treatment at an elevated temperature and ambient pressure.In this study, the g-C3N4 was prepared by a simple pyrolysis of urea. As for all the as-synthesized phtocatalysts, the structures, morphologies and the optical properties were carefully characterized by the following techniques: XRD, SEM, TEM, FT-IR and DRS. Also, the band edge positions of m-Ag2Mo2O7 and MIL-88A were studied by the Mott-Schottky methods. Thereafter, the photocatalytic activities were evaluated by using a solution of rhodamine B (RhB) as a target pollutant for the photodegradation experiments performed under a visible light irradiation. The results showed that all the aforementioned g-C3N4-based photocatalysts exhibited enhanced photocatalytic activities in comparison with the pristine g-C3N4. For the case of the g-C3N4-based composite photocatalysts, the enhancement factor over the pristine g-C3N4 can achieve values ranging from 2.6 to 3.4. As for the H2O2-treated g-C3N4, the degradation rate constant can be 4.6 times higher than that of the pristine g-C3N4.To understand the key factors in new materials design, we also devote a lot of efforts to elucidate the basic mechanisms during the photocatalytic degradation of organic pollutant. Based on the results of the active species trapping (AST) experiments, the main active species in each photocatalytic system were determined. In the g-C3N4/m-Ag2Mo2O7 and the g-C3N4/MIL-88A system, three kinds of active species of ·O2-, h+ and ·OH were found to be involved in the photocatalytic reaction. Among them, the ·O2- and h+ were the main active species. In the g-C3N4/ZnS and H2O2-treated g-C3N4 photocatalytic systems, the main active species was determined as the ·O2-. The reaction pathways of these active species were also demonstrated by comparing the band edge positions with the potentials of the redox couple. In addition, the relationship between the active species and the photocatalytic behaviors of N-de-ethylation and conjugated structure cleavage were studied. Finally, possible mechanisms to explain the enhanced photocatalytic activities were proposed for each photocatalytic system.The results in this thesis clearly confirm that the photocatalytic activity of the g-C3N4 based photocatalyst can efficiently be enhanced by constructions of staggered-aligned composites and by modification of the microstructure of the g-C3N4. The enhanced photocatalytic performance can mainly be ascribed to the efficient separation of the photo-induced electron-hole pairs and the increase of the active sites for the photocatalytic reaction.
|
|
| 42. |
- Wang, Yong, 1990-
(författare)
-
Characterization of Impurities in Different Ferroalloys and Their Effects on the Inclusion Characteristics of Steels
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ferroalloys have become increasingly important due to their indispensable role in steelmaking. As the performance requirements of steel materials increase, it is necessary to have a better understanding of the impact of impurities in ferroalloys on the steel cleanliness. The quality of the ferroalloy will directly affect the quality of the steel. This is especially important when ferroalloys are added during the late stage of the ladle metallurgy process. The goal of the present work is to gain knowledge about various ferroalloy impurities added in the steel production process and to study the influence of ferroalloy impurities on inclusions in the steel. The research work is divided into four main parts.In the first part, previous works on impurities present in different ferroalloys as well as how these impurities can influence the steel cleanliness have been reviewed. The applications of different ferroalloys and their production trends were discussed. The possible harmful inclusions in different ferroalloys were identified. The results showed that: 1) MnO, MnSand MnO-SiO2-MnS inclusions from FeMn and SiMn alloys have a temporary influence on the steel quality; 2) The effect of trace elements, such as Al, Ca contents, should be considered before the addition of FeSi alloys to steel. Also, Al2O3 inclusions and relatively high Al contents are commonly found in FeTi, FeNb and FeV alloys due to their production process. This information should be paid more attention to when these ferroalloys are added to steel; and 3) specific alloys containing REM oxides, Cr(C,N), Cr-Mn-O, Al2O3, Al-Ti-O, and Ti(C,N) have not been studied enough to enable a judgement on their influence on the steel cleanliness. Moreover, the effect of large size SiO2 inclusions in FeSi and FeMoalloys on the steel cleanliness is not fully understood.In the second part, the impurity assessment of 10 different ferroalloys (FeSi, FeCr, FeMo,FeV, FeTi, FeNb, FeW, FeB, MnN, FeCrN) was carried out by using various characterization techniques. The inclusions obtained in these ferroalloys were mostly silica or alumina, or the oxides of the base elements. Also, the main elemental impurities and inclusions were closely related to their manufacturing route. The advantages and disadvantages of different methods were compared, and the detection technology of ferroalloy inclusions was optimized. The results showed that the traditional two-dimensional method on a polished surface cannot always be applied for the investigation of inclusions in some specific ferroalloys. Moreover, the investigations of inclusions on metal surface after electrolytic extraction showed a big potential to use to detect larger sized inclusions. The results on both the film filter and metal surface should be grouped together to obtain more comprehensive information on the inclusion characteristics. Among these ferroalloys, FeCr and FeNb were found to be relatively less studied ferroalloys. Thus, they were selected for further studies.In the third part, the early melting behaviours of FeNb, HCFeCr and LCFeCr alloys during additions in liquid iron was studied. The experiments were carried out by using the"liquid metal suction" technique. Here, the ferroalloy was contacted with liquid iron for a predetermined time and then quenched. The obtained samples were further studied to determine the microstructure and the formation of inclusions. It was found that the mutual diffusion between solid ferroalloy and liquid iron formed a reaction zone. Also, the initial dissolution mechanism of FeNbs alloy in liquid iron was proposed, and the mechanism was controlled by diffusion. The TiOx inclusions in FeNb alloy will partially or completely be reduced due to a reaction with Nb in the reaction zone. The original stable inclusions, such as Al2O3 in FeNb alloys and MnCr2O4 inclusions in LCFeCr alloys can move in this zone and keep their original forms without experiencing any changes. Under the same conditions, the melting speed of LCFeCr alloy is faster than that of HCFeCr alloy. The addition of FeNband FeCr alloys in steel certainly introduces inclusions to steel.In the fourth part, the influence of the addition of LCFeCr alloys on the inclusions in Ti containing ferritic stainless steel was studied on a laboratory scale. It was found that the theMnCr2O4 inclusions in the LCFeCr alloy would react with TiN and dissolved Ti in the Ti containing steel to form TiOx-Cr2O3 system inclusions. In addition, the removal effect of slag on such inclusions was also studied. The results found that the slag addition can modify their-rich inclusions, but that the Ti content in the steel was significantly reduced. Therefore, a proper amount of TiO2 content should be added into the slag to get a low Ti loss in the steel melt, which should be studied further. Therefore, the composition of the steel directly affects the behaviour of the inclusions from ferroalloys in steel.
|
|
| 43. |
- Xuan, Changji
(författare)
-
Wettability and Agglomeration Characteristics of Non-Metallic Inclusions
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, both the wettability and the agglomeration characteristics of non-metallic inclusions in liquid iron/steel were studied by using both experimental results and thermodynamic considerations. The mechanisms of the wettability of different types of inclusions were discussed. Also, the agglomeration behaviors of the inclusions were analyzed.Firstly, the wettability of different types of inclusions (including Al2O3, MgO, Ti2O3, TiO2 and TiN) in contact with the liquid iron/steel was studied. For the TiNcase, there is no reaction formation at the interface between TiN and pureiron/steel. In the caseo f pure Fe, the oxygen increase is the main factor for a contact angle decrease. As for the steel case, a sharp decrease of the contact angle is due to the effects of both an increased oxygen content in the liquid steel and a formation of a Ti(N,C,O) phase at the interface. For the Al2O3 and MgO cases, the formation of a FeAl2O4 and a MgO-FeO reaction layer at the interface, respectively, lead to a contact angle decrease. In the case of the Ti2O3/pure Fe case, the reaction at the interface cannot be identified. For the Ti2O3/steel case, the formation of an Al2TiO5 reaction layer is the main reason for a steep decrease of the contact angle. In the TiO2 case, the melting region appears at the temperature below the melting point of pure iron. This is due to the strong formation of a solid solution TiOx-FeO. The main source of the oxygen for the solid solution formationis due to a TiO2 substrate decomposition and a low partial pressure of oxygen in the chamber.Regarding to the non-metallic particle additions (TiO2 and TiN) into the molten steel, the steel composition should be controlled to have a small Al content (<0.005mass%) and a high Ti content (>0.035mass%), so as togeta high number of Ti-rich oxide inclusions with a small size. This conclusionis supported from the view point of the van der Waals force, liquid-capillary force and wettability.Regarding the Ti/Al complex deoxidation in the melt, the “mainly occupied clustered inclusions” with spherical shape is due to a TiOx-FeO liquid inclusion precipitation after an addition of Ti as a pre-deoxidizer. The much lower cluster number in the Ti/Al case than that in the Al case is mainly due to a coagulation of single TiOx-FeO liquid inclusions. Also, the cluster formation in a complex Ti/Al deoxidation is started after an Al addition rather than after a Ti addition.Regarding the Al2O3 cluster formation in deoxidation, the cavity bridge forceis larger than the van der Waals force. However, the difference between them is smaller than 7 times. In the reoxidation process, the influence of the cavity bridge force due to the wettability decreased, and became similar to that of the liquid-capillary force.
|
|
| 44. |
- Yang, Ying, 1982-
(författare)
-
Inclusion Motion under a Swirl Flow in the Continuous Casting Process and Wire Feeding in the Induction Furnace
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis includes the studies of two phenomena related to continuous casting. One subject is the inclusion motion in the submerged entry nozzle (SEN) and mold when using a swirl flow. The swirl flow is generated in a SEN by using an electromagnetic swirl flow generator (EMSFG). The other subject is focused on FeSiRE particles (powder) that are added into the molten steel by using a wire feeding method, which in the future could be used in a continuous casting mold.Firstly, two kinds of a full type and a half type EMSFG were designed based on mathematical modeling. Then, the distributions of the magnetic flux intensity and the Lorentz force were obtained for two types of EMSFG devices. Based on the results of the Lorentz force, the flow field, temperature field and inclusion motion in the SEN and the mold were studied by using a full type and a half type EMSFG. Moreover, a comparison from the above aspects was investigated between a full type EMSFG and a half type EMSFG. In addition, the effect of different inclusion parameters such as the densities, sizes and boundary conditions, on the inclusion behavior was studied. It was found that light Al2O3 inclusion moves towards the rotational center by a centrifugal force and that a swirl flow prevents nozzle clogging. The heavy CeO2 inclusion more or less moves outwards towards the SEN wall and they may stick to the wall. It was also found that the inclusion separation to a mold meniscus increased and that the inclusions being trapped into a solidified shell decreased when using a swirl flow compared to when not using a swirl flow.A study of a wire feeding into molten steel in an induction furnace was done to determine the potential to implement wire feeding into a mold in the future. A major difference compared to a traditional solid wire is that the wire contains particles (powder). Firstly of all, the wire melting behavior in the molten steel was studied. More specially, the influence of the different wire materials (aluminum, copper and 316L stainless steel wires) on the dissolution time in the molten steel was modeled. Moreover, these simulation results were compared to experimental results. In addition, the FeSiRE particle (powder) motion in the induction furnace was also simulated. The results showed that the copper wire is more suitable to apply in the continuous casting mold when a small addition and a lower wire feeding speed are used. Conversely, if a large amount and a higher feeding speed are needed, the aluminum wire may be recommended for injections in the mold. The results also indicated that the thermal properties of particle (powder) have an influence on the wire melting behavior in the molten steel.
|
|
| 45. |
- Zaini, Ilman Nuran, 1990-
(författare)
-
Enhancing the circular economy: Resource recovery through thermochemical conversion processes of landfill waste and biomass
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Currently, the global economy looses a considerable amount of potential secondary raw materials from the disposed waste streams. Furthermore, the existing landfill sites that often do not have proper environmental protection technologies pose a long-lasting risk for the environment, which urge immediate actions for landfill remediations. At the same time, the energy recovery from waste through conventional incinerators has been criticized for its CO2 emissions. Alternatively, pyrolysis and gasification offer the potential to recover secondary resources from waste and biomass streams, which can increase the circularity of the material resources and limit the CO2 emissions.This thesis aims to realize feasible thermochemical processes to enhance the material resources' circularity by treating landfill waste and biomass. Correspondingly, fundamental studies involving experimental works and process developments through lab-scale experiments and process simulations are carried out. The thesis is written based on the results from five different studies that cover the investigation regarding the effect of waste/biomass fuel properties on the performance of the pyrolysis and gasification processes, as well as the process development and improvement of thermochemical conversion processes of waste and biomass.The first study investigates the primary fragmentation behaviour of waste fuel pellets during the pyrolysis stage of thermochemical conversion processes. This study shows that the fragmentation degree of waste pellets correlates well with their volatile matter contents. Meanwhile, there is no clear relation between the fragmentation degree and the pellets’ mechanical strength. Generally, due to the high volatile matter content from plastic, fuel pellets from waste tend to fragment into a high number of smaller particles than typical biomass or coal pellets during thermochemical processes. Hence, for some processes, improving the thermal stability of waste pellets is more relevant than improving their mechanical strength. Subsequently, the second study examines the reactivity and kinetics behaviour of waste-derived char during gasification. In general, it is found that the char reactivity is a function of the ash amount and the ratio of inorganic catalytic elements (K, Ca, Na, Mg, and Fe) to the inhibitor elements (Si, Al, and Cl). More importantly, the char gasification test results demonstrated the significance of the waste sorting processes' operating conditions on the thermal behaviour of the waste fuel, especially during the gasification process.Meanwhile, the third study investigates the syngas and tar formations resulting from different interactions between plastic and paper fractions of solid waste. The results show that the interaction between plastic and paper significantly depends on the hydrocarbon chain structures of the plastic polymer. Specifically, the interactions of aliphatic-structured plastic polymers (represented by PE) and paper cause synergistic effects that reduce the tar and increase the syngas yields. Meanwhile, the synergistic effects tend to be less evident in the case of co-gasification between paper and an aromatic hydrocarbon polymer, represented by PS. Based on the results of the previous studies, a co-gasification process of waste with biomass or biochar is proposed in the fourth study. It is found that adding biochar during the gasification of waste could significantly increase the syngas and H2 production to become higher than that of when adding biomass. Synergistic effects are observed in the form of an extensive syngas yield increment and a tar yield reduction, due to the tar reforming reactions over biochar particles. In general, both biochar and biomass additions result in a higher energy yield ratio, suggesting that it could improve the efficiency of the waste gasification.Finally, the fifth study focuses on process simulations and operational cost assessments of co-production of H2, biochar, and bio-oil from biomass. The process simulation study is carried out to evaluate different scenarios for producing biochar, bio-oil, and H2 based on a biomass pyrolysis process coupled with a steam reforming and a WGS process. Based on the calculations of the total operating cost and the potential revenue, it is found that the production of bio-oil is more economically beneficial than the production of H2. The estimated minimum selling price for biochar and bio-oil based on the operating cost alone is within the price ranges of related commodities in Sweden (i.e., charcoal, coal, coke and oil crude). Nevertheless, capital and operating costs for post-processing of bio-oil should also be considered in the future to obtain a more complete economic judgement.
|
|
| 46. |
- Ånmark, Niclas, 1987-
(författare)
-
Inclusion Characteristics and Their Link to Tool wear in Metal Cutting of Clean Steels Suitable for Automotive Applications
- 2015
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis covers some aspects of hard part turning of carburised steels using a poly‑crystalline cubic boron nitride (PCBN) cutting tool during fine machining. The emphasis is on the influence of the steel cleanliness and the characteristics of non‑metallic inclusions in the workpiece on the active wear mechanisms of the cutting tool. Four carburising steel grades suitable for automotive applications were included, including one that was Ca‑treated.A superior tool life was obtained when turning the Ca-treated steel. The superior machinability is associated with the deposition of lubricating (Mn,Ca)S and (CaO)x-Al2O3-S slag layers, which are formed on the rake face of the cutting tool during machining. Moreover, the transfer of work material to the rake face crater is characteristic in hard part turning of clean steels. It can be because of the lack of sulfides that protect the cutting edge when turning machinability treated steels. This corresponds to the more pronounced crater wear caused by the low‑sulfur steels than that of the steels with higher sulfur contents. It was also concluded that the composition of the non‑metallic inclusions in the Ca‑treated steel is a more important factor than the inclusion number and size, in hard part turning using a PCBN cutting tool. Also, a 3D analysis after electrolytic extraction was found to give a more precise characterisation of non‑metallic inclusions than the conventional 2D analysis by SEM‑EDS. In turn, better correlations to machinability and mechanical properties can be obtained. Hence, the use of this technique is beneficial for future material development.Finally, the challenge for future metallurgy is to manufacture high‑performance steels with improved combined properties of mechanical strength and machinability.
|
|
| 47. |
- Ånmark, Niclas, 1987-
(författare)
-
Steel characteristics and their link to chip breaking and tool wear in metal cutting
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The vision of this thesis is to study how it is possible to obtain optimised workpieces during metal cutting processes in industry. Specifically, the work is aimed to increase the understanding between the steel characteristics and their link to the chip breaking and tool wear during metal cutting. The emphasis is on the influence of the cleanliness and the characteristics of non-metallic inclusions in the workpiece on the machinability of carburising steel grades. The machinability of a case hardening steel is improved by a M-treatment (additions of Ca). Also, the improved machinability of the M-steels offers an attractive potential to save money which makes it possible to reduce the tooling costs with up to 50%. The improved machinability of Ca-treated steels is correlated to the formation of lubricating slag layers consisting of Ca-enriched sulfide inclusions and oxy-sulfide inclusions, which are formed on the rake face during the machining operation. It is proposed that the formations of slag layers from the workpiece constituents are essential to minimise the chemical degradation of the tool edge due to a contact with the chip. During this process, sulfur minimises the material transfer from the chip flow, whilst Ca-treated impurities have a stabilising effect on the protective deposits made of slag layers.Since there is a remaining industrial need to increase the production rate, whilst maintaining a high quality of the finished parts, the future production will continue to require extreme demands on the quality of workpieces. If the emphasis is focused on the workpiece, it should be possible to obtain a robust manufacturing process. Therefore, the challenge for future steel metallurgists is to develop high performance grades with optimised combined properties.
|
|
| 48. |
|
|
| 49. |
- Arzpeyma, Niloofar
(författare)
-
Numerical Study of the Gas Flow and Heat Transfer during Electromagnetic Stirring and Post Combustion in EAFs
- 2014
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The effect of electromagnetic stirring on scrap melting and post combustion in two different electric arc furnaces (EAFs) have been studied using numerical modelling. The effect of electromagnetic stirring on melting of a piece of scrap located at the eccentric bottom tapping (EBT) region of an EAF has been studied. The results were compared to a condition in which the only source term for momentum transfer was buoyancy. It was shown that the use of electromagnetic stirring can contribute to a better heat transfer rate at the melt–scrap interface. The Grashof and Nusselt number for both electromagnetic stirring and natural convection were estimated, and were compared with those estimated in previous studies. The post combustion (PC) inside a duct system of an EAF has been studied considering combustion of hydrogen and carbon monoxide. The aim was to study the effect of air leakage through the airgap and fan power on post combustion of the off–gas leaving the furnace. Furthermore, to see how much uncombusted can be captured after the air gap. It was shown a higher off-gas mass flow rate and a higher power of the outlet fan led to a higher combustion of CO and H2. Moreover, a backward modeling of the flow was done to estimate the off–gas composition at the furnace outlet. The post combustion inside the same EAF used in the previous part has been studied. The domain consists of the fluid region above the melt bath. The oxy fuel reactions and combustion of CO were taken into account. The oxy-fuel reactions in a both simplified and a more accurate form (JC) were studied. The results showed less combustion of CO in the latter form. Using the results of the off–gas composition at the outlet, an attempt was made to estimate the flow rate of CO arising from the bath. Different flow rates of CO from the melt and air flow rate through the roof were assumed. In both burner mode and burner + lancing mode, the calculated concentration of CO2 was higher than that calculated in the previous part. The reason could be that the dissociation of CO2 to CO and O2 at high temperatures, and the de-post combustion of CO2 due to the reaction with carbon in the melt and in the electrodes were not considered.
|
|
| 50. |
- Björklund, Johan, 1979-
(författare)
-
A study of slag-steel-inclusion interaction during Ladle treatment
- 2006
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis is based on two supplements with two major objectives. In the first supplement equilibrium top slag-steel bulk and inclusions-steel bulk were investigated by comparison between calculated and measured oxygen activity values. This was done by applying different oxide activity models for slags combined with thermodynamic calculations. In the second supplement the inclusion composition is studied during the ladle refining process. The inclusion composition is related to top slag composition and other parameters during ladle treatment. The work was carried out by collecting data during well controlled sampling procedures at two different steel plants. Extensive inclusion analyses in Scanning Electron Microscope, SEM, were done. The data was used together with thermodynamics for a description of the interaction between slag-steel-inclusion interaction during ladle treatment. Evaluation of inclusion composition during the ladle refining have revealed that the majority of the inclusions belonged to the system Al2O3-CaO-MgO-SiO2 and showed a continuous composition change throughout the ladle refining process, from high Al2O3, via MgO-spinel to finally complex types rich in CaO and Al2O3. The final composition after vacuum treatment was found to be close to the top slag composition. Small process parameter changes and practical variations during ladle refining were proven to give large differences of the inclusion composition. Finally, it was concluded that equilibrium does not exist between top slag and steel bulk, with respect to oxygen, for the studied conditions. However, the equilibrium does exist between the steel bulk and inclusion.
|
|
