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Abstract

A comprehensive study on the microstructural evolution of a new generation Nb-Mo microalloyed model complex-phase (CP) steel under hot strip rolling conditions has been conducted. The experimental investigation includes the austenite conditioning during reheating, work hardening and static softening of austenite during hot deformation, austenite decomposition to multiphase structure during run out table cooling operation and finally precipitation strengthening during coiling at downcoiler. The flow stress and static softening behaviour of austenite is modeled by the physically based approaches of Kocks-Mecking and Zurob et al., respectively, whereas empirical approaches are employed to model recrystallized austenite grain size and grain growth after recrystallization. The start of ferrite formation is described by the early growth of comer nucleated ferrite. A limiting carbon concentration concept is postulated above which ferrite formation ceases. A semi-empirical approach based on the Johnson- Mehl-Avrami-Koknogorov (JMAK) theory adopting additivity is employed to describe ferrite as well as bainite growth with individual parameters for each reaction. The present ferrite model includes the formation of the transformation stasis regime, where a critical driving pressure approach is adopted to describe the stasis initiation. Present research concludes that the same driving pressure approach is applicable to describe bainite start and the transition from stasis to bainite start occurs at 620°C. The effect of carbon enrichment in the remaining austenite after ferrite formation is included to describe bainite growth. Martensite + retained austenite volume fraction is calculated empirically as a function of carbon enrichment resulting from the ferrite formation. The isothermal aging kinetics is modeled by a modified Shercliff-Ashby approach, which is then extended for coil cooling path to predict the optimum coiling temperature range (580- 610°C) to maximize the precipitation strengthening of microalloying elements. Finally the hardness of the material is expressed as a function of the volume fractions of various transformation products and the precipitation strength contribution. The overall model prediction is validated successfully by torsion simulation of the entire hot rolling and controlled cooling schedule. Current research suggests that fine multiphase structure is possible to achieve in the present steel through proper austenite conditioning and adopting complex cooling strategies.