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A physically based microstructure model for predicting the microstructural evolution of a C-Mn steel during and after hot deformation
Högskolan i Dalarna.
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.

Place, publisher, year, edition, pages
2008. Vol. 79, no 1, 47-58 p.
Keyword [en]
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: urn:nbn:se:kth:diva-13984DOI: 10.2374/SRI07SP033ISI: 000253276600007OAI: oai:DiVA.org:kth-13984DiVA: diva2:328801
Note
QC 20100706Available from: 2010-07-06 Created: 2010-07-06 Last updated: 2010-11-18Bibliographically approved
In thesis
1. Modeling the Microstructural Evolution during Hot Deformation of Microalloyed Steels
Open this publication in new window or tab >>Modeling the Microstructural Evolution during Hot Deformation of Microalloyed Steels
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
modeling, austenite, microalloyed steels, hot deformation, microstructure evolution, static recrystallization, dynamic recrystallization, metadynamic recrystallization<
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-10390 (URN)978-91-7415-267-8 (ISBN)
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
2. Modeling the microstructural evolution during hot working of C-Mn and Nb microalloyed steels using a physically based model
Open this publication in new window or tab >>Modeling the microstructural evolution during hot working of C-Mn and Nb microalloyed steels using a physically based model
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
austenite, modeling, hot deformation, microstructure evolution, static recrystallization, dynamic recrystallization, metadynamic recrystallization
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-4118 (URN)
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

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