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- Fredriksson, Hasse, 1939-, et al.
(författare)
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Thoughts about the Initial Solidification Process during Continuous Casting of Steel
- 2002
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Ingår i: Scandinavian journal of metallurgy. - : Wiley. - 0371-0459 .- 1600-0692. ; 31, s. 292-297
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The initial solidification during continuous casting is very important for the surface quality of the strand. Oscillation marks are formed by the movement of the mould. One mainly distinguishes between two different types of marks, namely folding marks and overflow marks. The formation of these can be described by the heat flow and the surface tension balance. A theoretical analysis outlining the most important parameters controlling the formation of these marks is presented. A metallographic analysis of the formation was also performed. The theory and observations have been used to analyse the effect of casting speed and oscillation frequency on the formation of different types of marks. It is shown that a meniscus is formed at the top of the growing shell. The maximum height of the meniscus determines the ideal distance between the marks. The mould frequency is related to this height. A frequency that is far from ideal causes overflow marks and folding marks with macrosegregation and cracks. The distance between the marks is, in such cases, not constant. The depth of the marks is calculated from the heat flow and the surface tension balance.
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- Adolfi, Sofia
(författare)
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Slag inclusion formation during solidification of steel alloys and in cast iron
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis explores the formation of segregation and inclusions during solidification of steel and cast iron. A better understanding of the formation mechanism should result in decreasing fraction of defects during solidification of ingot and strand material. Density driven macrosegregation was studied both experimentally and theoretically to see the effect of channel segregation on the total segregation. Formation of these pencil-like segregations is due to natural convection in the solidifying metal caused by liquid enrichment of elements with lower density compared to the bulk. It is suggested to change the composition to compensate for this density difference. Inclusion precipitation can be finite by limitations in segregation. Saturated liquid is found in the last solidified areas, often between dendrites. Here the enrichment of the liquid is possible due to microsegregation. Meanwhile crystals form and solidify the elements with low solubility in the solid is pushed out in the remaining liquid. Soon the liquid is saturated to the level where spontaneous formation of inclusions occurs. Microstructure studies by aid of SEM and micro-probe measurements are analysed to find at what point during solidification process the inclusions start to form. In steel making this formation has a detrimental effect on the mechanical properties in contrary to the production of nodular cast iron where the inclusions have a beneficial effect on the graphite formation. Inoculation of cast iron aims at reaching higher number density of graphite nodules, nodule morphology modification and control of nodule distribution during solidification. Late precipitation of nucleation sites has shown to have a positive impact on preventing chill. To find the most potent inoculation agent different additives were tested. Special effort has been made to analyse the effect of oxides and sulphides as nucleation sites.
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- Lagerstedt, Anders, et al.
(författare)
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Crack formation during continuous casting of tool steel
- 2005
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Ingår i: Transactions of the Indian Institute of Metals. - 0019-493X. ; 58:4, s. 671-678
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Tidskriftsartikel (refereegranskat)abstract
- The formation of cracks has been studied during continuous casting of a tool steel. Experimental work was performed in a continuous slab caster and the position of the cracks was determined as a function of distance from the cooling surface. Numerically calculated temperatures and elastic stresses were combined with experimentally determined thermomechanical data of the cast material in order to predict crack positions. The model was extended to include the effect on the strain from both thermal shrinkage and the shrinkage caused by condensation of vacancies during the solidification process.
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- Makaya, Advenit, 1977-
(författare)
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On the effect of nitrogen, hydrogen and cooling rate on the solidification and pore formation in Fe-base and Al-base alloys
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Experiments on the production of porous metallic materials were performed on Fe-base and Al-base alloys. The method involves dissolution of gases in the liquid state and solidification at various cooling rates. The alloy compositions were selected to induce solidification of primary particles intended to control the pore distribution. For the Fe-base alloys, nitrogen was introduced into the melt by dissolution of chromium nitride powder. Fe-Cr-Mn-Si-C alloys featuring M7C3 carbide particles were selected. For the Al-base alloys, hydrogen gas was dissolved into the melt by decomposition of water vapor. Al-Ti and Al-Fe alloys featuring primary Al3Ti and Al3Fe intermetallic particles, respectively, were considered. In the Fe-base alloys, a homogeneous distribution of gas pores through the specimens’ volume was obtained at high cooling rate (water quenching) and after introduction of external nucleating agents. In the case of the Al-base alloys, a good pore distribution was observed at all cooling rates and without addition of nucleating agents. Calculations of the variation of nitrogen (respectively hydrogen) solubility based on Wagner interaction parameters suggest that pore nucleation and growth occur during precipitation of the primary particles (M7C3 carbides, Al3Ti or Al3Fe intermetallics), due to composition changes in the melt and resultant supersaturation with gas atoms. Microscopic analyses revealed that the primary particles control the pore growth in the melt and act as barriers between adjacent pores, thereby preventing pore coalescence and promoting a fine pore distribution. Uniaxial compression testing of the porous Al-Ti and Al-Fe materials showed the typical compressive behavior of cellular metals. Further work is needed to improve the quality and reproducibility of the porous structures which can possibly be used in energy absorption or load-bearing applications. As a corollary result of the quenching of hypereutectic Fe-Cr-Mn-Si-C alloys in the experiments of synthesis of porous metals, a homogeneous featureless structure was observed in some parts of the samples, instead of the equilibrium structure of M7C3 and eutectic phases. Subsequent investigations on rapid solidification of Fe-base alloys at various alloy compositions and cooling rates led to the formation of a single-phase structure for the composition Fe-8Cr-6Mn-5Mo-5Si-3.2C (wt.%), at relatively low cooling rates (≈103 K/s) and for large sample dimensions (2.85 mm). The single phase, which is likely to be the hcp ɛ-phase, was found to decompose into a finely distributed structure of bainite and carbides at ≈600 °C. The annealed structure showed very high hardness values (850 to 1200 HV), which could be exploited in the development of high-strength Fe-base materials.
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- Makaya, Advenit, et al.
(författare)
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Study on the production of Fe–Cr–Mn–C–Si foam by nitrogen solubility difference between the liquid and solid phases
- 2005
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Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 413-414, s. 533-537
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Tidskriftsartikel (refereegranskat)abstract
- The feasibility of the production of Fe-base metallic foam by using the nitrogen solubility difference between the liquid and austenite phases has been studied in the Fe-Cr-Mn-C-Si system. Compositions showing a suitable solubility gap for precipitation of gas pores upon solidification have been derived by thermodynamic calculations of the nitrogen solubility in the liquid and solid phases, using the interaction parameters of nitrogen. Small amount of foams were produced for different compositions. The foaming involved dissolution of chromium nitrides into the melt and subsequent quenching. Four different compositions were tested: by varying the C content between 2 and 6 wt.%, the effect of the primary carbides on the foam microstructure could be studied. The presence of those carbides appears as an important element for the promotion of the pore nucleation and the prevention of pore coalescence. The addition of SiO2 powder in some experiments illustrated the beneficial effect of a nucleating agent to reach a homogeneous distribution of the gas pores.
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