| 1. |
- Bergström, Yngve, et al.
(författare)
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A dislocation based theory for the deformation hardening behavior of DP-steels : Impact of martensite content and ferrite grain size
- 2010
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Ingår i: Journal of Metallurgy. - : Hindawi Limited. - 1687-9465 .- 1687-9473. ; 2010:647198
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Tidskriftsartikel (refereegranskat)abstract
- A dislocation model, accurately describing the uniaxial plastic stress-strain behavior of dual phase (DP) steels, is proposed and the impact of martensite content and ferrite grain size in four commercially produced DP-steels is analyzed. It is assumed that the plastic deformation process is localized to the ferrite. This is taken into account by introducing a non-homogeneity parameter, f(ε), that specifies the volume fraction of ferrite taking active part in the plastic deformation process. It is found that the larger the martensite content the smaller the initial volume fraction of active ferrite which yields a higher initial deformation hardening rate. This explains the high energy absorbing capacity of DP-steels with high volume fractions of martensite. Further, the effect of ferrite grain size strengthening in DP-steels is important. The flow stress grain size sensitivity for DP-steels is observed to be 7 times larger than that for single phase ferrite.
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| 2. |
- Bergström, Yngve, et al.
(författare)
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A dislocation model for the stress-strain behavior of dual phase steels
- 2008
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Ingår i: IDDRG 2008 International Conference. - Olofström. ; , s. 173-184
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Konferensbidrag (refereegranskat)abstract
- The Bergström dislocation model is used to study the influence of coiling temperature after hot rolling on the mechanical properties of the cold rolled SSAB-grade Docol 800DP. Further, the anisotropy-effect for this type of steel, caused for instance by banding, is investigated. The physical dislocation model is based on the assumption that the work hardening behaviour is controlled by the creation, the immobilisation and the remobilisation of dislocations and it is demonstrated that all these processes are strongly influenced by the amount of hard martensite particles in DP-steels. The true strain to necking is a widely used tool for estimating the stretch formability of DP-steels. A detailed discussion regarding the influence of for instance friction stress, rate of dislocation remobilisation and mean free path of dislocation motion is presented. It is also indicated that the best stretch formability of DP-steel is obtained by minimising the quotient between the rate of work hardening at smaller strains with that at larger strains, close to the strain at necking.
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| 3. |
- Granbom, Ylva
(författare)
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Effects of process parameters prior to annealing on the formability of two cold rolled dual phase steels
- 2008
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Ingår i: STEEL RES INT. - : Wiley. - 1611-3683 .- 1869-344X. ; 79:4, s. 297-305
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Tidskriftsartikel (refereegranskat)abstract
- This study concerns the effects of coiling temperature after hot rolling and the degree of reduction during cold rolling on formability-related properties of high strength cold rolled dual phase (DP) steels. The effect of coiling temperature on the final structure and properties of two cold rolled and annealed DIP-steels is investigated. Further, the effect of cold rolling reduction and its impact on the final properties of the material is studied. Aspects of the impact of the different process parameters on the ferrite to austenite and austenite to martensite transformation are discussed based on results from production scale experiments, tensile testing and metallographic examinations of the materials.
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| 4. |
- Granbom, Ylva
(författare)
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Influence of niobium and coiling temperature on the mechanical properties of a cold rolled dual phase steel
- 2007
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Ingår i: Revue de Metallurgie. - : Revue de Metallurgie. - 1156-3141. ; 104:4, s. 191-197
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Tidskriftsartikel (refereegranskat)abstract
- Dual-phase (ferrite-martensite) low-carbon sheet steels are produced by annealing in the intercritical temperature range to produce ferrite-austenite mixtures, followed by accelerated cooling to transform the austenite phase into martensite. Dual-phase steels have gained considerable importance over the conventional HSLA steels due to the high strength and formability achieved. A good formability is achieved by high deformation hardening, due to a fine distribution of martensite. A cold rolled DP-grade (TS 600 MPa), coiled at 550 °C, was compared with a steel of approximately the same analysis but with the addition of 0.015 wt% Nb, coiled at 520 and 600 °C respectively. The Nb-addition refines the ferrite grain size substantially and a more uniform grain shape is achieved. The smaller grain size of the ferrite- and martensite phases, results in a material with better formability. It appears that coiling at 520°C results in a slightly better formability than coiling at 600°C. The material shows a lower a?? start temperature, probably due to easier dissolution of finer carbides, compared to the strip coiled at 600°C. The recrystallization and phase transformation behaviour was studied using laser ultrasonics (LUS). Developing the LUS-technique for use in-line during annealing would open new ways of optimising and controlling the mechanical properties of the final strip.
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| 5. |
- Granbom, Ylva
(författare)
-
Influence of niobium and coiling temperature on the mechanical properties of a cold rolled dual phase steel
- 2007
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Ingår i: Revue de métallurgie (Imprimé). - : EDP Sciences. - 0035-1563 .- 1156-3141. ; 104:4, s. 191-197
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Tidskriftsartikel (refereegranskat)abstract
- Dual-phase (ferrite-martensite) low-carbon sheet steels are produced by annealing in the intercritical temperature range to produce ferrite-austenite mixtures, followed by accelerated cooling to transform the austenite phase into martensite. Dual-phase steels have gained considerable importance over the conventional HSLA steels due to the high strength and formability achieved. A good formability is achieved by high deformation hardening, due to a fine distribution of martensite. A cold rolled DP-grade (TS 600 Wa), coiled at 550 degrees C, was compared with a steel of approximately the same analysis but with the addition of 0.015 wt% Nb, coiled at 520 and 600 degrees C respectively. The Nb-addition refines the ferrite grain size substantially and a more uniform grain shape is achieved. The smaller grain size of the ferrite and martensite phases, results in a material with better formability. It appears that coiling at 520 degrees C results in a slightly better formability than coiling at 600 degrees C. The material shows a lower (alpha ->gamma start temperature, probably due to easier dissolution of thinner carbides, compared to the strip coiled at 600 degrees C. The recrystallization and phase transformation behavior was studied using the laser ultrasonic technique (LUS). Developing the LUS-technique for use in-line during annealing would open new ways of optimizing and controlling the mechanical properties of the final strip.
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| 6. |
- Granbom, Ylva, et al.
(författare)
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Simulation of the Soaking and Gas Jet Cooling in a Continuous Annealing Line using Dilatometry
- 2010
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Ingår i: Steel Research International. - : Wiley. - 1611-3683 .- 1869-344X. ; 81:2, s. 158-167
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Tidskriftsartikel (refereegranskat)abstract
- The present study concerns the simulation of a continuous annealing line (CAL), using dilatometry. Simulations of CAL have been performed on four commercial steel grades with different chemical compositions in order to investigate how the alloying elements C, Mn, Si and B affect the microstructure and hardness of dual phase (DP) and martensitic steels. Three annealing cycles corresponding to those used in a CAL have been applied. When annealing intercritically, as is the case in DP-steel production, the materials do not reach equilibrium during soaking. Mn and C increase the austenite content and consequently the hardness of the materials. Higher levels of Si (0.4 wt %) are required to retard the formation of new ferrite during cooling in the gas jet section, prior to quenching. 6 increases hardenability effectively when annealing in the austenite region but is not as efficient during intercritical annealing, which implies that boron restrains ferrite nucleation rather than impeding ferrite growth. Results from DICTRA calculations show that it is possible to simulate the phase transformations during soaking, gasjet cooling and quenching.
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| 7. |
- Granbom, Ylva
(författare)
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Structure and mechanical properties of dual phase steels : An experimental and theoretical analysis
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The key to the understanding of the mechanical behavior of dual phase (DP) steels is to a large extent to be found in the microstructure. The microstructure is in its turn a result of the chemical composition and the process parameters during its production. In this thesis the connection between microstructure and mechanical properties is studied, with focus on the microstructure development during annealing in a continuous annealing line. In-line trials as well as the lab simulations have been carried out in order to investigate the impact of alloying elements and process parameters on the microstructure. Further, a dislocation model has been developed in order to analyze the work hardening behavior of DP steels during plastic deformation. From the in-line trials it was concluded that there is an inheritance from the hot rolling process both on the microstructure and properties of the cold rolled and annealed product. Despite large cold rolling reductions, recrystallization and phase transformations, the final dual phase steel is still effected by process parameters far back in the production chain, such as the coiling temperature following the hot rolling. Lab simulations showed that the microstructure and consequently the mechanical properties are impacted not only by the chemical composition of the steel but also by a large number of process parameters such as soaking temperature, cooling rate prior to quenching, quench and temper annealing temperature.
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