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  • 1.
    Bäcke, Linda
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Modeling the Effect of Solute Drag on Recovery and Recrystallization during Hot Deformation of Nb Microalloyed Steels2010In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 50, no 2, p. 239-247Article in journal (Refereed)
    Abstract [en]

    The effect of solute drag on recovery and recrystallization during hot deformation of Nb microalloyed steels has been modeled using a newly developed microstructure model. The model is based on dislocation theory and the calculated dislocation density determines the driving force for recrystallization. Subgrains act as nuclei for recrystallization and have to reach a critical size and configuration in order for recrystallization to start. In the model, the solute drag effect of Nb in solution is described. Nb retards both dislocation and grain boundary movement giving retardation in both recovery and recrystallization. Calculations were compared to experimental results from axisymmetric compression tests combined with stress relaxation. In order to model the effect of solute drag, the experiments were carried out at temperatures where precipitation of Nb(C, N) should not occur. The calculated flow stresses for the compression tests show good fit with experimental data. Also, the calculated results of the relaxation tests show good agreement with experimental data.

  • 2.
    Bäcke, Linda
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Modeling the Microstructural Evolution during Hot Deformation of Microalloyed Steels2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

     

    This thesis contains the development of a physically-based model describing the microstructural evolution during hot deformation of microalloyed steels. The work is mainly focused on the recrystallization kinetics. During hot rolling, the repeated deformation and recrystallization provides progressively refined recrystallized grains. Also, recrystallization enables the material to be deformed more easily and knowledge of the recrystallization kinetics is important in order to predict the required roll forces. Hot strip rolling is generally conducted in a reversing roughing mill followed by a continuous finishing mill. During rolling in the roughing mill the temperature is high and complete recrystallization should occur between passes. In the finishing mill the temperature is lower which means slower recrystallization kinetics and partial or no recrystallization often occurs. If microalloying elements such as Nb, Ti or V are present, the recrystallization can be further retarded by either solute drag or particle pinning. When recrystallization is completely retarded and strain is accumulated between passes, the austenite grains will be severely deformed, i.e. pancaking occurs. Pancaking of the grains provides larger amount of nucleation sites for ferrite grains upon transformation and hence a finer ferrite grain size is achieved.

    In this work a physically-based model has been used to describe the microstructural evolution of austenite. The model is built-up by several sub-models describing dislocation density evolution, recrystallization, grain growth and precipitation. It is based on dislocation density theory where the generated dislocations during deformation provide the driving force for recrystallization. In the model, subgrains act as nuclei for recrystallization and the condition for recrystallization to start is that the subgrains reach a critical size and configuration. The retarding effect due to elements in solution and as precipitated particles is accounted for in the model.

    To verify and validate the model axisymmetric compression tests combined with relaxation were modeled and the results were compared with experimental data. The precipitation sub-model was verified by the use of literature data. In addition, rolling in the hot strip mill was modeled using process data from the hot strip mill at SSAB Strip Products Division. The materials investigated were plain C-Mn steels and Nb microalloyed steels. The results from the model show good agreement with measured data.

     

  • 3.
    Bäcke, Linda
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Borggren, Ulrika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Engberg, Göran
    Högskolan i Dalarna.
    Modeling the kinetics of strain induced precipitation and its effect on recovery and recrystallization in Nb microalloyed steelsManuscript (preprint) (Other academic)
  • 4.
    Bäcke, Linda
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Engberg, Göran
    Högskolan i Dalarna.
    Physical based model for predicting the microstructural evolution at hot rolling2009In: Invited paper to the International Conference MEFORM 2009, Freiberg, Germany , 2009Conference paper (Other academic)
  • 5.
    Engberg, Goran
    et al.
    Högskolan i Dalarna.
    Lissel, Linda
    A physically based microstructure model for predicting the microstructural evolution of a C-Mn steel during and after hot deformation2008In: STEEL RES INT, ISSN 1611-3683, Vol. 79, no 1, p. 47-58Article in journal (Refereed)
    Abstract [en]

    A physically based model for predicting microstructural evolution has been developed. The model is based on a physical description of dislocation density evolution, where the generation and recovery of dislocations determine the flow stress and also the driving force for recrystallization. In the model, abnormally growing subgrains are assumed to be nuclei of recrystallized grains and recrystallization starts when the subgrains reach a critical size and configuration. To verify that the model is able to describe dynamic, static and metadynamic recrystallization of C-Mn steels, hot compression tests combined with relaxation were performed at various temperatures, strains and strain rates. The model showed reasonable agreement with the experimental data for the compression tests performed at temperatures ranging from 850 degrees C to 1200 degrees C and strain rates ranging from 0.1 to 10 s(-1). Similarly, the calculations of the stress relaxation tests were in accordance with experimental data. A validation of the model was done by calculating a multi-step test where good agreement with both flow-stress values and grain sizes was obtained. The main purpose of the model is to predict the microstructural evolution during hot rolling and this investigation presents very promising results.

  • 6.
    Lissel, Linda
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Modeling the microstructural evolution during hot working of C-Mn and Nb microalloyed steels using a physically based model2006Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Recrystallization kinetics, during and after hot deformation, has been investigated for decades. From these investigations several equations have been derived for describing it. The equations are often empirical or semi-empirical, i.e. they are derived for certain steel grades and are consequently only applicable to steel grades similar to these. To be able to describe the recrystallization kinetics for a variety of steel grades, more physically based models are necessary.

    During rolling in hot strip mills, recrystallization enables the material to be deformed more easily and knowledge of the recrystallization kinetics is important in order to predict the required roll forces. SSAB Tunnplåt in Borlänge is a producer of low-carbon steel strips. In SSAB’s hot strip mill, rolling is conducted in a reversing roughing mill followed by a continuous finishing mill. In the reversing roughing mill the temperature is high and the inter-pass times are long. This allows for full recrystallization to occur during the inter-pass times. Due to the high temperature, the rather low strain rates and the large strains there is also a possibility for dynamic recrystallization to occur during deformation, which in turn leads to metadynamic recrystallization after deformation. In the finishing mill the temperature is lower and the inter-pass times are shorter. The lower temperature means slower recrystallization kinetics and the shorter inter-pass times could mean that there is not enough time for full recrystallization to occur. Hence, partial or no recrystallization occurs in the finishing mill, but the accumulated strain from pass to pass could lead to dynamic recrystallization and subsequently to metadynamic recrystallization.

    In this work a newly developed physically based model has been used to describe the microstructural evolution of austenite. The model is based on dislocation theory where the generated dislocations during deformation provide the driving force for recrystallization. The model is built up by several submodels where the recrystallization model is one of them. The recrystallization model is based on the unified theory of continuous and discontinuous recovery, recrystallization and grain growth by Humphreys.

    To verify and validate the model, rolling in the hot strip mill was modeled using process data from SSAB’s hot strip mill. In addition axisymmetric compression tests combined with relaxation was modeled using experimental results from tests conducted on a Gleeble 1500 thermomechanical simulator at Oulu University, Finland. The results show good agreement with measured data.

  • 7.
    Lissel, Linda
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Modeling the microstructural evolution of a Nb microalloyed steel during and after hot deformationArticle in journal (Refereed)
  • 8. Lissel, Linda
    Prediction of microstructural behavior during hot rolling2005In: Conference proceedings of the 8th International Conference on Technology of Plasticity (ICTP), Verona, Italy, 2005Conference paper (Refereed)
  • 9. Lissel, Linda
    et al.
    Engberg, Göran
    Högskolan i Dalarna.
    Prediction of the microstructural evolution during hot strip rolling of Nb microalloyed steels2007Conference paper (Other academic)
    Abstract [en]

    A physically based model is used to describe the microstructural evolution of Nb microalloyed steels during hot rolling. The model is based on a physical description of dislocation density evolution, where the generation and recovery of dislocations determines the flow stress and also the driving force for recrystallization. In the model, abnormally growing subgrains are assumed to be the nuclei of recrystallized grains and recrystallization starts when the subgrains reach a critical size and configuration. The model is used to predict the flow stress during rolling in SSAB Tunnplat's hot strip mill. The predicted flow stress in each stand was compared to the stresses calculated by a friction-hill roll-force model. Good fit is obtained between the predicted values by the microstructure model and the measured mill data, with an agreement generally within the interval +/-15%.

  • 10. Lissel, Linda
    et al.
    Engberg, Göran
    Borggren, Ulrika
    Modeling precipitation and its effect on recrystallization during hot strip rolling of niobium steels2008In: Conference proceeding of the 3rd International Conference on TMP; Padua; Italy , 2008Conference paper (Other academic)
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