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Medium manganese (Mn) steel has excellent strength and ductility and is widely used in the automobile industry and elsewhere; however, hydrogen embrittlement (HE) is considered as one of the most serious issues of its application. In this paper, in situ transmission electron microscopy was used to study the microstructural evolution and fracture micromechanisms of medium Mn steel under the influence of hydrogen and annealing. The results indicated that two specimens with different annealing processes showed transformation-induced plasticity and twinning-induced plasticity, albeit to a varying degree due to different microstructures with distinct mechanical properties. It was shown that hydrogen affected the fracture micromechanisms, which promoted the initiation of microcracks at the boundaries of martensite transformed from austenite. Moreover, the main cracks propagate in a zigzag fashion, following hydrogen charging, due to the hardness of the martensite. These findings provide directions for improving HE resistance through microstructural regulation of medium Mn steel.

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 &deg, 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 (&lsquo, M&rsquo, referring to Mn, &lsquo, 7&rsquo, meaning the content of Mn, &lsquo, B&rsquo, denoting batch annealing) steel maintains a steady value of 0.82 &sigma, b (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 &sigma, b to 0.52 &sigma, b. We define 80% relative humidity as the &lsquo, threshold humidity&rsquo, for M10B.