Abstract: This investigation examines the impact of tempering parameters on the mechanical properties and microstructure evolution of ultra-high-strength medium manganese steel containing 7 wt.% Mn intended for hot stamping applications. Tempering experiments were conducted based on the low-temperature tempering conditions typical of automobile parts baking processes, at four distinct temperatures and holding times. Mechanical properties were evaluated using tensile testing, while scanning electron microscope, Electron Probe X-ray Microanalysis, and electron backscatter diffraction analyses were employed to characterize the microstructural features. Results indicate that tempering effectively mitigates the risk of brittle fracture in quenched samples. However, the mechanical properties do not exhibit a linear relationship with tempering temperature or duration. Consequently, in this study, based on the mechanical properties under different tempering parameters, the concepts of appropriate-tempering, over-tempering, and under-tempering are proposed. Over-tempering leads to excessive stability of austenite, whereas under-tempering results in its instability, both scenarios hindering the optimization of the transformation-induced plasticity effect. Based on the optimal tempering parameters and paint-baking conditions, a novel two-step low-temperature tempering process is proposed, which enhances the mechanical performance of the steel during service and reduces the potential damages that may occur between hot stamping and paint-baking processes, including both localized high-strain forming-induced cracks and hydrogen embrittlement-related deterioration.

Key words: Medium-Mn steel, Hot stamping, Low-temperature tempering, Residual austenite