Effect of heating temperature on the hydrogen embrittlement susceptibility in hot stamped medium-Mn steel

Medium-Mn steel with M3 microstructural characteristics (multiphase, multiscale, and metastable) is a promising third-generation automotive steel owing to its remarkable combination of ultrahigh strength and ductility. Hot stamping of medium-Mn steel is a new approach that integrates steel stamping and quenching, during which the formability and mechanical properties of the steel are enhanced. However, there exists an increasing danger of hydrogen-induced degradation of mechanical properties with increasing steel strength grade, and minimal research is available on the hydrogen embrittlement (HE) susceptibility of hot stamped medium-Mn steel. With this aim, the susceptibility to HE of a widely used medium-Mn steel 0.1C–5Mn subjected to hot stamping at 850, 950, and 1000 ℃ was examined using electrochemical hydrogen charging, hydrogen permeation test, and slow strain rate tensile test. The results of microstructural evaluation reveal that all hot stamped samples have a full martensite microstructure with a certain amount of needle-like ε-carbide precipitates owing to self-tempering during the hot stamping process. With increasing heating temperature, the prior austenite grain size increased, whereas the strength and ductility of the tested steel gradually decreased. At 850 ℃, a good combination of strength and ductility was obtained, and the product of the ultimate tensile strength and total elongation was 22 GPa·%. With increasing heating temperature, the content of diffusible hydrogen remarkably decreased, and that of nondiffusible hydrogen increased, whereas the HE index (HEI) expressed by the relative notch tensile strength loss and effective hydrogen diffusion coefficient initially increased and then decreased. The HE susceptibility of the sample heating to 1000 ℃ with an HEI value of ～65% was the lowest among the three samples examined. Further fracture analysis demonstrated that the prehydrogen-charged samples were fractured by intergranular cracking along the boundaries of prior austenite grains in the crack initiation region. It is concluded that the variations in the susceptibility to HE of the tested medium-Mn steel with heating temperature are primarily owing to the changes in strength and self-tempered ε-carbide precipitates during the hot stamping process. Compared with the currently widely used hot stamping steel 22MnB5 with an HEI value of ～62%, the susceptibility to HE of the hot stamped medium-Mn steel of interest remains a little higher than the steel 22MnB5, which is primarily owing to its relatively low martensite transformation start temperature (M_s) and thus low self-tempering degree.