Improvement of strength-ductility combination in ultra-high-strength medium-Mn Q&P steel by tailoring the characteristics of martensite/ retained austenite constituents

In this study, a low carbon medium-Mn steel was subjected to different quenching and partitioning (Q&P) processes to investigate fraction and morphology of fresh martensite and retained austenite constituents (M/RA) on microstructure characteristics and mechanical properties by EPMA, EBSD, TEM and tensile test, etc. By increasing the annealing time, the fraction of M/RA decreases, which gradually evolves from large-sized blocky islands to fine granular or lath-like shape in morphology. This is attributed to the larger-grained prior austenite and the uniform distribution of Mn which reduces the thermal stability of austenite and hence increases the volume fraction of tempered martensite (TM) as well as effectively refining the resultant microstructure. Furthermore, a new CCE-PAG model has been proposed by taking into account the effect of the prior austenite grain size. Compared with the traditional CCE model, the prediction of RA fraction increases first and then decreases with the annealing time, being in better agreement with the experimental results. The long-time annealed samples show a better combination of strength and ductility, i.e., their tensile strength reaches ultra-high 1312–1391 MPa, yield strength up to 730–765 MPa, and the elongation is higher than 16%, which results from the existence of large-fractioned TM structure and the sustained TRIP effect. In short-time annealed samples, the crack tends to be initiated and propagated at the edge of coarse M/RA islands, leading to the final brittle fracture.