Correlative characterization to understand ferrite recrystallization in dual phase steels

Abstract

A systematic experimental study has been carried out to understand ferrite recrystallization during isothermal annealing just below Ac1 in dual phase steels. Three different dual phase microstructures – ferrite-pearlite (FP), ferrite-bainite (FB) and ferrite-martensite (FM) were produced with an identical chemical composition. These samples were subjected to 80 % cold work and subsequently annealed at 725 °C for different soaking durations. The complex interaction between ferrite and secondary constituent/phase during deformation lead to differences in strain partitioning which influenced the kinetics of ferrite recrystallization. The sample with ferrite-martensite (FM) microstructure exhibited faster recrystallization kinetics followed by ferrite-bainite (FB) and ferrite-pearlite (FP). The microstructure and associated hardness evolution starting from cold rolling to annealing for different durations was carefully captured with electron back scattered diffraction (EBSD) and high-speed nanoindentation mapping. Excellent one-to-one correlation between hardness and KAM was observed by coupling EBSD-KAM and nanoindentation mapping. The effect of the secondary constituent/phase on ferrite recrystallization is presented and differences in the recrystallization kinetics are reconciled by correlative characterization. This work lays a foundation to link microstructure to the local mechanical response in dual phase steels and can be gainfully used to characterize multiphase steels and ultimately fine tune the processing.