Microstructure-mechanical properties evolution of hot-rolled medium Mn steel: Effects of short-time holding and low-temperature tempering following ultrafast heating

In this work, the mechanical properties of hot-rolled 7Mn steel are significantly sensitive to the short-time isothermal holding after ultrafast heating (UFH). These samples also exhibited significant disparities in sensitivity of mechanical properties to subsequent low-temperature tempering. Through a combination of multi-scale experimental characterization, calculations of yield strength, and numerical simulations of elemental diffusion, we discussed the sensitivities described above. The results indicate that mechanical properties of the samples subjected to ultra-fast heating is not sensitive to subsequent low-temperature tempering. This due to the preferential segregation of C atoms at dislocation sites during the ultra-fast heating, leading to less noticeable C atoms segregation during low-temperature tempering. Short-time isothermal holding after ultrafast heating results in the precipitation of numerous nanoscale carbides within the martensite, thereby reducing solid solution strengthening and significantly decreasing the yield strength. Simultaneously, the low-temperature tempering process induces the segregation of C atoms towards dislocations, grain boundaries, or phase interfaces. This significantly mitigates the Portevin-Le Chatelier (PLC) effect in the sample, increasing the elongation rate of the steel from about 10% to 19%. Additionally, the combined effects of strengthening induced by Cottrell atmosphere and phase boundary segregation contribute to an elevation in yield strength.