Effects of Ti alloying on resistance to hydrogen embrittlement in (Nb plus Mo)-alloyed ultra-high-strength hot-stamping steels

A boron-containing 1.8-2.0-GPa-grade hot-stamping steel, which has been attracting great attentions as reinforcing automotive parts, often leads to the deteriorated resistance to hydrogen embrittlement due to its high strength level. The micro-alloying elements such as Nb and Mo have been known to improve the resistance to hydrogen embrittlement by refining grains and providing interfaces of precipitates. Ti is also utilized for the mentioned purposes; however, the effect of Ti on hydrogen embrittlement, particularly in (Nb + Mo) multi-alloyed system, has not been revealed clearly yet. In this study, therefore, the alloying effects of Ti on resistance to hydrogen embrittlement were investigated via controlling Ti content and conducting slow-strain-rate tensile (SSRT) tests and thermal desorption analyses (TDA) after the hydrogen charging. The complex addition of Nb, Mo, and Ti promotes the formation of nanoscale (Nb,Ti)C and (Nb,Mo,Ti)C complex precipitates along with coarse Ti(C,N) particles. The increased Ti content to 0.03 wt% increases the volume fraction of nanoscale precipitates, which effectively refines the prior austenite grain size and interfacial incoherency, thereby providing stable hydrogen trapping sites with the higher activation energy for hydrogen desorption. Although the increased Ti also promotes the formation of brittle coarse Ti(C,N) particles, this negative effect of the particles can be minimized or prevented by the decrease in particle size due to the interaction with Nb and Mo.