Towards the understanding of fracture resistance of an ultrahigh-strength martensitic press-hardened steel

Abstract

Identifying the fracture resistance capabilities and gaining a deeper understanding of the controlling damage mechanisms are important to the development of press-hardened steels (PHS) for automotive applications. In this study, the fracture properties of a novel PHS alloyed with Cr and Si (CrSi-PHS) were assessed by using uniaxial tensile and double-edge notched tensile (DENT) tests. Under uniaxial tension, the fracture resistance is characterized by the fracture strain and work of fracture, and the CrSi-PHS presents a desirable compromise with strength when comparing with other grades of advance high-strength steels (AHSS). The large fracture strain is attributed to the high resistance to damage nucleation even with a high density of grain boundaries. Fracture toughness is characterized by the DENT tests. The fracture toughness parameters of CrSi-PHS are lower than that of the commercial PHS 22MnB5 but comparable to the multiphase AHSSs with lower strength levels. The stress triaxiality near the fatigue pre-crack in DENT tests leads to a change of damage mechanism in CrSi-PHS. The crack propagates discontinuously by joining the micro-cracks and subsequently by the ductile fracture of micro-ligaments. The local plasticity at the crack tip region is suggested to contribute significantly to the energy dissipation. The observations demonstrate a strong dependence of the fracture mode and fracture resistance on the loading condition in the ultrahigh-strength martensitic steels.