Design concept and phase transformation study of advanced bainitic-austenitic medium-Mn steel

The isothermal heat treatment processing routes are designed for a novel 0.17 C-3.1Mn-1Si-0.55Al-0.22Mo-0.034Ti-0.073 V steel, aiming to obtain a homogeneous fine lath-type microstructure consisting of bainitic ferrite and retained austenite (RA). The phase transformation of the investigated steel is studied employing high-resolution dilatometry. Isothermal heat treatment cycles below and above the martensite start (M_s) temperature are performed to analyse the kinetics of bainitic transformation as well as the thermal stability of retained austenite. Microstructural characterization is carried out using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The quantitative analysis of individual microstructural constituents is performed based on the differences in diffraction pattern quality using the deep learning (DL) evaluation method and compared with the results of X-ray diffraction (XRD) measurements. The thickness of bainitic ferrite laths (BFL) is predicted using an approach integrating thermodynamic and kinetic parameters, achieving overall good agreement with the experimental observations. It is demonstrated that the isothermal holding temperature of 400 °C is the most beneficial material state in terms of the kinetics of bainitic transformation and stabilization of RA. The microstructure is consisted of fine BFLs with a mean thickness of 124 nm, the lath-type RA (10.3 vol%), and a small fraction of martensitic-austenitic islands.