Effects of Tempering on Shear Banding and Crack Formation during Bending in Low-carbon Martensitic Steel

To expand the application of high-strength martensitic steel in automotive body components, improving the bendability and understanding the factors affecting it are essential. In this study, we investigated the formation of shear bands and surface cracks in Fe-0.24C-1.0Mn (mass%) lath martensite during three-point bending tests using scanning electron microscopy and electron backscatter diffraction, with a focus on the effects of tempering up to 400 °C. During bending, fine striated steps and elongated notches formed on the surface, which were attributed to block boundary sliding along the {110}α in-habit-plane slip system and the formation of shear bands, respectively. Boundary sliding dominated subsurface deformation in as-quenched martensite, and shear-band formation was enhanced by tempering at ≥200 °C, with a more pronounced effect at higher tempering temperatures. In as-quenched martensite, most surface cracks initiated near less-deformed hard martensite packets, and tempering at ≥200 °C retarded the initiation and growth of these cracks. The limiting strain for crack initiation, as evaluated through digital image correlation analysis, increased with an increase in tempering temperature at ≥200 °C and was found to correlate with the critical fracture strain determined via tensile tests. These findings suggest that the inhibition of crack initiation and growth by tempering is primarily caused by the improvement in local deformability within shear bands and at crack tips, which is primarily due to the transition from boundary sliding to the enhanced activation of multiple slip systems. The reduction in surface local stress at the bend apex could also be a factor induced by tempering.