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Modeling the microstructural evolution during hot working of C-Mn and Nb microalloyed steels using a physically based model
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2006 (English)Licentiate 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.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , x, 32 p.
Series
ISRN KTH/MSE--06/54--SE+MEK/AVH
Keyword [en]
austenite, modeling, hot deformation, microstructure evolution, static recrystallization, dynamic recrystallization, metadynamic recrystallization
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4118OAI: oai:DiVA.org:kth-4118DiVA: diva2:10817
Presentation
2006-10-04, Sal K408, KTH, Brinellvägen 23, Stockholm, 11:00
Opponent
Supervisors
Note
QC 20101118Available from: 2006-09-26 Created: 2006-09-26 Last updated: 2010-11-18Bibliographically 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. Modeling the microstructural evolution of a Nb microalloyed steel during and after hot deformation
Open this publication in new window or tab >>Modeling the microstructural evolution of a Nb microalloyed steel during and after hot deformation
(English)Article in journal (Refereed) Submitted
Identifiers
urn:nbn:se:kth:diva-6181 (URN)
Note
QS 20120327Available from: 2006-09-26 Created: 2006-09-26 Last updated: 2012-03-27Bibliographically approved

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