Martensite transformation of retained austenite with diverse stability and strain partitioning during tensile deformation of a carbide-free Bainitic steel

Abstract

A carbide-free bainitic steel characterized by an excellent combination of mechanical properties is presented here. The transformation behavior of austenite to martensite with different morphologies during tensile straining was quantitatively characterized by combination of X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) at different engineering strains. It was observed that the unstable blocky austenite started to transform at an early stage, which was associated with strain partitioning. The lath (thick-film) austenite transformed at larger strain, while the film-like (thin-film) austenite was regarded stable during the entire deformation process. The transformation of lath austenite altered strain partitioning in blocky austenite, leading to variation in transformation of blocky austenite. The combined contribution of TRIP effect of blocky and lath austenite accounted for the increased work hardening rate at the latter stage of deformation, and consequently good combination of strength and ductility. Given that the exponential model for austenite transformation was not appropriate for multiscale austenite, the reduction curves of gradual transformation behavior for austenite are presented for the experiment steel. • Good ductility of bainitic steel attributed to gradual transformation of austenite. • TRIP effect of blocky austenite worked entire deformation process. • Lath austenite transformation altered the decreasing trend of blocky austenite. • Deviation with previous model was caused by strain partition and diverse morphology.