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Prediction of the microstructural evolution during hot strip rolling of Nb microalloyed steels
Högskolan i Dalarna.
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%.

Place, publisher, year, edition, pages
2007. Vol. 558-559, 1127-1132 p.
Series
Materials science forum, ISSN 0255-5476
Keyword [en]
modeling, recrystallization, hot rolling, C-MN, PRECIPITATION, DEFORMATION, AUSTENITE
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-13985ISI: 000250408000179OAI: oai:DiVA.org:kth-13985DiVA: diva2:328807
Note
QC 20100706Available from: 2010-07-06 Created: 2010-07-06 Last updated: 2010-07-06Bibliographically 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

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