Hydrogen embrittlement (HE) has become an important issue in ultra-strong automotive steel applications. The addition of Mo to commercial 32MnB5 hot-stamping steel is preferred to enhance the strength levels with little ductility loss. However, the effects of solute Mo on HE have been rarely studied for developing 32MnB5 steel, and most studies on the alloying of Mo for interfacial cohesion have been conducted theoretically by calculating the cohesive energies in the Fe lattice. In this study, 0.15 wt.% Mo was added to the 32MnB5 steel and the resistance to HE was evaluated experimentally via electrochemical H-charging. The H-charged reference steel shows a large ductility loss (50–79%) after H-charging, while the addition of Mo significantly reduces the loss (17–26%) with sufficient post-elongation, indicating a higher resistance to HE. This is because the solute Mo decreases the H diffusivity, resulted from the high H affinity and repulsive strain field owing to the large atomic size of Mo. The direct observation of crack propagation reveals that the H-induced crack path changes from the prior austenite grain boundaries (PAGBs) to the grain interiors of H-enhanced slip planes. This is attributed to the reduced H- and strain-localization on the PAGBs by the solute Mo and the enhanced grain-boundary cohesion by Mo segregation. This work thus demonstrates the beneficial effects of the addition of Mo on the tensile properties and the intrinsic resistance to HE for the development of ultra-high-strength steels.

氢脆（HE）已成为超高强度汽车钢应用中的重要问题。优选将钼添加到市售32MnB5热冲压钢中，以提高强度水平而几乎没有延展性损失。然而，开发32MnB5钢时，很少研究溶质Mo对HE的影响，并且理论上已经通过计算Fe晶格中的内聚能来进行Mo合金化以实现界面内聚的大多数研究。在这项研究中，向32MnB5钢中添加了0.15 wt。％的Mo，并通过电化学H充电。充氢后的H参比钢显示出较大的延展性损失（50％至79％），而添加Mo则显着降低了损失（17％至26％），并具有足够的后延伸率，表明对HE的抵抗力更高。这是因为，由于Mo的原子大，由于高的H亲和力和排斥应变场，溶质Mo降低了H的扩散性。对裂纹扩展的直接观察表明，H引起的裂纹路径与原始奥氏体晶粒不同H增强滑移面的晶粒内部的边界（PAGB）。这归因于溶质Mo降低了PAGB的H-和应变局部化，以及Mo偏析提高了晶界的内聚力。