The Effect of Ti and Mo Microalloying on Hydrogen Embrittlement Resistance of Ultra-High Strength Medium Mn Steel

This study elucidated the effect of Ti–Mo microalloying on the hydrogen embrittlement (HE) resistance and fracture behavior of warm-rolled Fe-5.6Mn-0.16C-1Al (wt%) steel. After intercritical annealing, both steels, i.e., without and with Ti–Mo microalloying, showed ultrafine ferrite (α) and austenite (γR) duplex microstructure. The addition of Ti–Mo to 5.6Mn steel reduces the volume fraction of γR, facilitating the formation of (Ti, Mo)C carbides in α phase and further refining the final microstructure. The product of ultimate tensile strength (UTS) and total elongation (TEL) of 5.6MnTiMo can be as high as 35 GPa·% with an ultra-high yield strength of above 1.2 GPa. Furthermore, the addition of Ti–Mo also had a significant effect on the resistance to HE of medium Mn steels. Firstly, the limited (Ti, Mo)C carbides precipitated in γR could act as irreversibly trap sites to capture a considerable amount of H, effectively increasing the CH (Diffusible Hydrogen Content). Additionally, 5.6MnTiMo displayed higher γR stability, resulting in a reduced susceptibility to HE. The H-assisted microcracks mainly formed inside γ(α′) and extended along γ(α′) grain boundaries, leading to intergranular cracking and premature fracture.