Modeling of microstructure evolution during hot strip rolling of dual phase steels

Abstract

Microstructure models for hot strip rolling of CMnSi and CMnMo dual-phase steels have been proposed. The constitutive behaviour of austenite prior to the onset of dynamic recrystallization has been described by the physically based Kocks-Mecking model. The static recrystallization kinetics has been simulated by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. The recrystalllzed austenite grain size is described using an empirical equation considering the effect of strain, prior austenite grain size and deformation temperature. Ferrite transformation start is modeled with an approach that considers early growth of corner nucleated ferrite. The fraction of ferrite transformed from austenite during continuous and/or stepped cooling Is described using the JMAK approach in combination with the additivity rule. The ferrite grain size is quantified as a function of the transformation start temperature. The critical conditions for the onset of bainite and martensite transformations in the remaining austenite have been empirically evaluated as a function of ferrite fraction transformed. The microstructure models for these metallurgical phenomena have been validated with experimental studies in the laboratory emphasizing industrially relevant hot strip rolling conditions and run-out table cooling strategies. © 2007 ISIJ.