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Medium Mn steels are a class of the new-generation ultra-high-strength materials used in automotives. However, despite excellent ductility, they may suffer from delayed cracking and thus cause serious concerns. In this study, several medium Mn steels were tested with different prestrain and hydrogen charging conditions. The interaction and synergistic effects of prestrain and hydrogen content on hydrogen-induced delayed cracking behavior are investigated. The threshold stress of hydrogen-induced cracking (HIC) decreased during dynamic hydrogen charging under a constant load. In the process of dynamic hydrogen charging, for M7B and M10B steels, the normalized stress intensity factor σ/σb and the corresponding threshold stress rHIC decreased sharply as prestrain increased. This is because the volume fraction of retained austenite decreased with an increase in prestrain. Similarly, rHIC was reduced and the critical hydrogen content dropped drastically with increasing prestrain. For M7C, the influence of prestrain on threshold stress and hydrogen concentration was less than that of M7B. This is because the different treatment processes leads to a different stability of the retained austenite. By observing the SEM fractographs, the fracture surface of medium Mn steels showed different fracture characteristics, such as dimple fractures and intergranular and transgranular modes.

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Medium Mn steels have been considered as the next-generation materials for use in the automotive industry due to their excellent strength and ductility balance. To reduce the total weight and improve the safety of vehicles, medium Mn steels look forward to a highly promising future. However, hydrogen-induced delayed cracking is a concern for the use of high strength steels. This work is focused on the service characteristics of two kinds of medium Mn steels under different relative humidity conditions (40%, 60%, 80% and 100%). Under normal relative humidity (about 40%) at 25 °C, the hydrogen concentration in steel is 0.4 ppm. When exposed to higher relative humidity, the hydrogen concentration in steel increases slowly and reaches a stable value, about 0.8 ppm. In slow strain rate tensile tests under different relative humidity conditions, the tensile strength changed, the hydrogen concentration increased and the elongation decreased as well, thereby increasing the hydrogen embrittlement sensitivity. In other words, the smaller the tensile rate applied, the greater the hydrogen embrittlement sensitivity. In constant load tests under different relative humidity conditions, the threshold value of the delayed cracking of M7B (‘M’ referring to Mn, ‘7’ meaning the content of Mn, ‘B’ denoting batch annealing) steel maintains a steady value of 0.82 σb (tensile strength). The threshold value of the delayed cracking of M10B significantly changed along with relative humidity. When relative humidity increased from 60% to 80%, the threshold dropped sharply from 0.63 σb to 0.52 σb. We define 80% relative humidity as the ‘threshold humidity’ for M10B.