Effects of Nb and Mo alloying on resistance to hydrogen embrittlement in 1.9 GPa-grade hot-stamping steels

Ultra-high-strength (over 1.8 GPa) of hot-stamping steels can be achieved by increasing C content of conventional hot stamping steel (1.5 GPa) or controlling precipitation behavior, but the enhanced strength often leads to the deteriorated resistance to hydrogen embrittlement. The complex alloying of Nb and Mo improves the resistance; however, the underlying microstructural evolutions and synergistic mechanisms are not understood yet. In this study, Nb- or (Nb + Mo)-alloyed 1.9 GPa-grade hot-stamping steels were fabricated in the laboratory, and their resistance to hydrogen embrittlement was evaluated by slow strain-rate tensile (SSRT) tests without and after hydrogen charging. Most of Nb consumed to form Nb carbides, hindering the migration of prior austenite grain boundaries mainly by a common Zener's pinning effect. The reduced grain size resulted in the decreased amount of diffusible hydrogen per unit grain boundary area. In addition, coherent or semi-coherent precipitates provided stable hydrogen trapping sites, leading to the low diffusivity and the consequently high resistance to hydrogen embrittlement. In the (Nb + Mo)-alloyed steel, Mo carbides preferred to nucleate at the surface of the pre-existing Nb carbides or to form needle-shaped isolated Mo2C carbide, but a considerably large amount of Mo remained inside the matrix as a solute. The prior austenite grain size was thus further reduced mainly by an additional solute drag effect. The solute Mo also enhanced the grain boundary cohesion, thereby leading to the absence of intergranular fracture and the sufficient post elongation even after the hydrogen charging.