The growing demands of weight saving, emission reduction and passenger safety in the automotive industry have promoted new concepts for medium-Mn steels (MMnS) design to achieve an excellent combination of high strength and superior ductility. Towards the novel alloy-lean design of MMS with relatively low Mn and low C contents, in most of the cases, the athermal martensitic formation is inevitable and often results in increased strength. The mechanical responses of triplex microstructure have not been unrevealed, especially the yielding mechanism.
In this study, we report the influence of cooling processes (air cooling versus water quenching) on the yielding behavior of a medium Mn steel (MMS) with nanostructured triplex microstructure, i.e. austenite (γ), ferrite (α) and as-quenched martensite (α'). The microstructures of both air-cooled (AC) and water-quenched (WQ) triplex MMS were studied by the multi-scale characterization techniques down to nano scale, e.g. high-energy synchrotron X-ray diffraction (SynXRD), electron backscattering diffraction (EBSD) and atom probe tomography (APT). It revealed the discontinuous yielding after the AC following the intercritical annealing while the continuous yielding after the WQ. Considering the complexity of nanostructure, the presence of martensite with decisive influences on yielding was systematically analysed by APT. The APT results directly revealed the C/Mn co-segregations at the α'/α interfaces in the AC samples but not in WQ samples nor at the α'/γ interfaces in AC samples. It is concluded that the yielding behavior of triplex MMS is determined by both the density and the mobile ability of GNDs generated near the α/α' interfaces when the presence of martensite is sufficient. Since the rapid cooling can promote more GNDs generation and prevent the segregation of C/Mn at the interfaces, it is beneficial to suppress the discontinuous yielding in nanocrystalline MMS.