The possibility of enhanced hydrogen embrittlement resistance of medium-Mn steels by addition of micro-alloying elements

In the present study, we elucidated the possibility of improvement in H embrittlement (HE) resistance of microalloyed medium-Mn steels by MC carbides formation. The Fe-6Mn-0.08C (wt%) steel was used as a base material, and the different amount of Nb, Ti, and V was added to form MC carbides. All steels exhibited the nano-sized globular morphology with dual-phase of alpha ferrite plus gamma(R) retained austenite, and the phase fraction was almost identical regardless of steel compositions. The steels showed an almost similar grain size of gamma(R) phase, regardless of composition. However, the micro-alloyed steel revealed the smaller size of alpha phase relative to base steel owing to MC carbides precipitation in the alpha phase. The micro-alloyed steel exhibited higher strength and lower ductility compared to the base material due to precipitation hardening and fine grain size of alpha. Considering HE resistance, we realized that the addition of the micro-alloying element resulted in a trivial effect on the medium-Mn steels, because MC carbides in the a phase could not act as influential trap sites of diffusible H. Most of the H atoms were preferentially trapped in the gamma(R) phase, and the H-enriched gamma(R) phase caused H-assisted nano crack when it transformed to strain-induced martensite. After that, the nano crack developed to the intergranular crack, leading to premature fracture.