Synergistic improvement in the strength and ductility of a medium Mn steel by single-step warm rolling and intercritical annealing

Abstract

In this study, we successfully fabricated a medium Mn steel (8Mn-6Al-2Cu-0.6C) with a heterogeneous microstructure by applying single-step warm rolling and intercritical annealing (IA) treatment. Compared to the homogeneous microstructure, the heterogeneous microstructure significantly improved the mechanical properties of the designed steel, reflected by the values of yield strength from 624.3 to 1036.1 MPa, ultimate tensile strength from 812.6 to 1208 MPa, and total elongation from 33.8 % to 54 %. According to quantitative analyses, yield strength increased primarily due to hetero-deformation-induced (HDI) strengthening, which was produced by the extensive geometrically necessary dislocations (GNDs) generated in the soft domain of the inhomogeneous microstructure during deformation. Moreover, HDI strengthening reduced the mechanical mismatch between the soft (austenite) and hard (martensite/ferrite) domains, thus improving phase compatibility. As the strain levels increased, GND accumulations further promoted the occurrence of dynamic strain partitioning between the phases and a sufficient transformation-induced plasticity (TRIP) effect. On the other hand, the heterogeneous austenite with different morphologies and dimensions exhibited various stabilities, leading to a stepwise TRIP effect and the excellent ductility of the steel.