Influence of microstructure on the mechanical properties and hydrogen embrittlement characteristics of 1800 MPa grade hot-stamped 22MnB5 steel

The influence of the microstructural characteristics on the mechanical and hydrogen embrittlement properties of 1800 MPa grade hot-stamped 22MnB5 steel was experimentally investigated using samples processed under various hot-stamping conditions, i.e., different heating temperatures and strain levels. The tensile strength increased with increasing hot-stamping temperature up to approximately 920 °C and subsequently decreased owing to the increasing sizes of the lath martensite and prior austenite phases. Some degree of internal strain was introduced into the 22MnB5 steel specimen during hot stamping at 920 °C, which led to a slightly higher hardness although no clear microstructural change was observed. The severity of hydrogen embrittlement of the hot-stamped 22MnB5 steel samples was investigated after immersion in a NH₄SCN solution, and the degree of hydrogen embrittlement was found to be directly associated with the amount of hydrogen that penetrated into the grain boundary and lath martensite boundary. The high-strength 22MnB5 steel with a very small lath martensite phase exhibited severe hydrogen embrittlement due to the large amount of hydrogen in the sample, and the high internal strain (or high dislocation density) could lead to accelerated hydrogen embrittlement. Severe hydrogen embrittlement occurred upon charging with more than approximately 0.8 ppm hydrogen. Based on the obtained results, models are proposed for the hydrogen embrittlement characteristics of hot-stamped 22MnB5 steel.