Effects of Shape and Orientation of MnS on Charpy Impact and Bending Properties in Hot-Press-Forming (HPF) Steels

The hot-press-forming (HPF) steels might contain a few MnS inclusions, and their brittle characteristics and stringer shape can deteriorate the impact toughness and formability. In this study, effects of shape and orientation of MnS particles on Charpy impact and bending properties of 1470-MPa-grade HPF steel sheets were investigated by conducting ex situ three-point bending or tension tests. According to direct observations of sequential cracking processes of MnS particles in the longitudinal-oriented specimen, largely elongated (high-aspect-ratio-valued) MnS particles were prone to form cracks at low strains. In the transverse-oriented specimen, MnS/matrix interfaces were debonded at the lower strains, and were liable to develop into cracks along the longitudinal direction. These observed results in this study indicated that the MnS cracking and MnS/matrix interfacial debonding were major fracture-initiation sites in the longitudinal- and transverse-oriented specimens, respectively, thereby leading to the decreases in both Charpy impact and bending properties in the MnS-containing HPF steel sheet.