Effects of Nb or (Nb + Mo) alloying on Charpy impact, bending, and delayed fracture properties in 1.9-GPa-grade press hardening steels

In this study, effects of Nb or (Nb + Mo) alloying into a 1.9-GPa-grade 32MnB5 press hardening steel on Charpy impact, bending, and delayed fracture properties were investigated. All the properties were enhanced in the Nb- or (Nb + Mo)-alloyed steels, especially in the (Nb + Mo)-alloyed steel. The Nb alloying worked for the precipitation of nano-sized complex carbides, thereby leading to the refinement of prior austenite grains by a Zener pinning effect. Mo also contributed to the grain refinement, while Mo affected it mostly in a solid-solution state by a solute drag effect. This grain refinement resulted in the more deviated crack propagation path and consequently in the more improved Charpy impact energy. The size reduction of Ti(C,N) inclusion due to the attachment of a small amount of Nb and Mo also improved the impact toughness. As well as microstructural effects, the lower residual stress due to the lower peak bending load by the Nb or (Nb + Mo) alloying contributed to the increase in H-induced delayed cracking time. Therefore, this study suggests that synergic interactions of Nb and Mo alloying would provide promising solutions for improving various properties, which should be more importantly considered under actually using environments such as mechanically forming and crashing for press hardening components, while satisfying the 1.9-GPa-strength 32MnB5 specification.