Microstructure model for a dual-phase steel

The microstructural evolution has been studied for hot rolling of a dual-phase steel with a lean C-Mn-Si chemistry. This study includes the investigation of austenite grain growth during reheating, constitutive behaviour and static recrystallization kinetics of austenite, and austenite decomposition during simulated run-out table cooling conditions. To develop and validate the microstructure models for these phenomena, experimental studies have been carried out in the laboratory using a Gleeble 3500 thermornechanical simulator. The hyperbolic sine relationship between flow stress and Zener-Hollomon parameter is employed to describe the constitutive behaviour. The Johnson-Mehi-Avrami-Kolmogorov (JMAK) theory is used to predict the static recrystallization kinetics. Ferrite transformation start is described with an approach that considers early growth of corner nucleated ferrite. The fraction of ferrite transformed from austenite during continuous 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 overall microstructure model has been validated based on a number of laboratory simulations of the entire hot strip rolling and controlled cooling process with an emphasis on industrially relevant run-out table cooling strategies.