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.

The current study investigates the microscale mechanical properties of the constituent phases and their influence on the macroscale properties of multi-phase steels with different microstructural constituents. Two steels, a Transformation Induced Plasticity (TRIP) and an enhanced ductility Dual Phase (DH) steel were produced in a continuous annealing line (CAL). Optical microscopy and EBSD results were utilized for segmentation of microstructural phase constituents. Macroscale mechanical properties were obtained using tensile testing and microscale properties with nanoindentation mapping. The hardness of each constituent is extracted from the hardness maps using a clustering algorithm. TRIP steel shows a homogeneous distribution of retained austenite in ferrite matrix and a small amount of bainite/martensite which is non-banded. In contrast, DH steel shows heterogeneous microstructures where martensite/bainite/retained austenite is found to be banded in the ferrite matrix. Both steels exhibit notable variations in hardness across their constituent phases, which are associated with the resulting microstructural characteristics. Nanoindentation results show that overall hardness/strength in TRIP steel is contributed from ferrite (66 %), retained austenite (33 %) and martensite/bainite (1 %). Whereas in DH steel, it is contributed from ferrite (55 %), mixture of RA, martensite/bainite (∼40 %) and martensite (∼5 %). The macroscopic behavior of TRIP and DH steels was explained and discussed using the rule of mixtures in conjunction with the microscopic properties of individual phases.