Enhancing the Strength of Medium Mn Steel by Flash Treatment

Abstract

As a representative of the third generation advanced high-strength steels (AHSSs), medium Mn steels (MMS) have broad development prospects in the field of automobile manufacturing. MMS with typical austenite reversion treatment have a soft duplex microstructure, i.e. ferrite + austenite, presenting a high ductility but a low yield strength. Here we show that a flash heating and cooling after austenite reversion treatment can replace the ferrite with strong martensite, which greatly enhances the yield strength of a 0.25C-4Mn steel by about 461–886 MPa. By adjusting the reversion temperature before the flash treatment, the C and Mn concentrations of reverted austenite can be altered, which determine the fraction of reverted austenite surviving the flash treatment. In addition, the mechanical stability of final retained austenite is also linked to the reversion temperature, resulting different work hardening behaviors due to transformation induced plasticity (TRIP) effect. By tweaking the reversion temperature before the flash treatment, an optimized combination of strength and ductility can be achieved. The micromechanical differences caused by the replacement of the matrix are also investigated via in-situ digital image correlation method.