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Modeling the Microstructural Evolution during Hot Deformation of Microalloyed Steels
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2009 (English)Doctoral 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.

 

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , x, 47 p.
Keyword [en]
modeling, austenite, microalloyed steels, hot deformation, microstructure evolution, static recrystallization, dynamic recrystallization, metadynamic recrystallization<
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-10390ISBN: 978-91-7415-267-8 (print)OAI: oai:DiVA.org:kth-10390DiVA: diva2:216454
Public defence
2009-04-20, B3, KTH, Brinellvägen 23, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
QC 20100706Available from: 2009-05-08 Created: 2009-05-08 Last updated: 2010-07-21Bibliographically approved
List of papers
1. Prediction of microstructural behavior during hot rolling
Open this publication in new window or tab >>Prediction of microstructural behavior during hot rolling
2005 (English)In: Conference proceedings of the 8th International Conference on Technology of Plasticity (ICTP), Verona, Italy, 2005Conference paper, Published paper (Refereed)
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-6179 (URN)
Conference
8th International Conference on Technology of Plasticity (ICTP), Verona, Italy
Note

QC 20100706

Available from: 2006-09-26 Created: 2006-09-26 Last updated: 2017-04-13Bibliographically approved
2. A physically based microstructure model for predicting the microstructural evolution of a C-Mn steel during and after hot deformation
Open this publication in new window or tab >>A physically based microstructure model for predicting the microstructural evolution of a C-Mn steel during and after hot deformation
2008 (English)In: STEEL RES INT, ISSN 1611-3683, Vol. 79, no 1, 47-58 p.Article in journal (Refereed) Published
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.

Keyword
modelling, microstructural evolution, recrystallization, flow stress, C-Mn steel, CELLULAR MICROSTRUCTURES, UNIFIED THEORY, GRAIN-GROWTH, AUSTENITE, STABILITY, RECOVERY, WORKING, STRESS
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13984 (URN)10.2374/SRI07SP033 (DOI)000253276600007 ()
Note
QC 20100706Available from: 2010-07-06 Created: 2010-07-06 Last updated: 2010-11-18Bibliographically approved
3. Prediction of the microstructural evolution during hot strip rolling of Nb microalloyed steels
Open this publication in new window or tab >>Prediction of the microstructural evolution during hot strip rolling of Nb microalloyed steels
2007 (English)Conference paper, Published 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%.

Series
Materials science forum, ISSN 0255-5476
Keyword
modeling, recrystallization, hot rolling, C-MN, PRECIPITATION, DEFORMATION, AUSTENITE
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13985 (URN)000250408000179 ()
Note
QC 20100706Available from: 2010-07-06 Created: 2010-07-06 Last updated: 2010-07-06Bibliographically approved
4. Modeling precipitation and its effect on recrystallization during hot strip rolling of niobium steels
Open this publication in new window or tab >>Modeling precipitation and its effect on recrystallization during hot strip rolling of niobium steels
2008 (English)In: Conference proceeding of the 3rd International Conference on TMP; Padua; Italy , 2008Conference paper, Published paper (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13987 (URN)
Note
QC 20100706Available from: 2010-07-06 Created: 2010-07-06 Last updated: 2010-07-06Bibliographically approved
5. Physical based model for predicting the microstructural evolution at hot rolling
Open this publication in new window or tab >>Physical based model for predicting the microstructural evolution at hot rolling
2009 (English)In: Invited paper to the International Conference MEFORM 2009, Freiberg, Germany , 2009Conference paper, Published paper (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13989 (URN)
Note
QC 20100706Available from: 2010-07-06 Created: 2010-07-06 Last updated: 2010-07-21Bibliographically approved
6. Modeling the Effect of Solute Drag on Recovery and Recrystallization during Hot Deformation of Nb Microalloyed Steels
Open this publication in new window or tab >>Modeling the Effect of Solute Drag on Recovery and Recrystallization during Hot Deformation of Nb Microalloyed Steels
2010 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 50, no 2, 239-247 p.Article in journal (Refereed) Published
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.

Keyword
modeling, flow stress, recrystallization, recovery, solute drag, Nb microalloyed steel, DEFORMED AUSTENITE, PRECIPITATION, EVOLUTION
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13992 (URN)10.2355/isijinternational.50.239 (DOI)000275434300009 ()
Note
QC 20100706Available from: 2010-07-06 Created: 2010-07-06 Last updated: 2017-12-12Bibliographically approved
7. Modeling the kinetics of strain induced precipitation and its effect on recovery and recrystallization in Nb microalloyed steels
Open this publication in new window or tab >>Modeling the kinetics of strain induced precipitation and its effect on recovery and recrystallization in Nb microalloyed steels
(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13993 (URN)
Note
QC 20100706Available from: 2010-07-06 Created: 2010-07-06 Last updated: 2010-07-21Bibliographically approved

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