Modeling the microstructural evolution during hot working of C-Mn and of Nb microalloyed steels using a physically based model

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Keyword: Austenite, Modeling, Hot deformation, Microstructure, Evolution, Static recrystallization, Dynamic recrystallization, Metadynamic recrystallization, Modellering av mikrostrukturutvecklingen hos mikrolegerade stål under varmdeformation
Publication year: 2006

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, recrystalizations enables the material to be deformed more easily and knowledge of the recrystallization kinetics is important in order to predict the SSAB´s hot strip mill, rolling is conducted in a reversing roughing mill followed by a continuous finishing mill. In the reverse roughing mill the temperature is high and the inter-pass times are long. this allowa for full recrystallization to occur during the inter-pass times. Due to high the high temperature, the rather low strain rates and the large strains there is also a possibility for dynamic 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 recrystalization. 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.


Linda Lissel

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