Effect of TiN particle on the bending behaivor of the martensitic steels using high-resolution digital image correlation

Abstract

The passenger’s safety is one of the important factors in the automobile industry, in this reason, the diverse safety devices are increasing, and the studies of the vehicle’s stiffness have been conducted. Especially, for securing the safe space for passengers, safety zone, the advanced high strength steels (AHSS) are currently used as an automobile structural materials and are studied. The one of the AHSS, martensitic steel has features of the excellent energy absorption at high strain rate condition and high strength, therefore, this material is used for safety zone. However, because of the low ductility, it is made to the limited-shaped products through the roll forming process. Therefore, the bendability of the martensitic steel is an important and interesting factor for bending deformation. For eliminating the N element due to the clearness of the steel and forming the TiC and TiN precipitation for improving the mechanical properties through the precipitation strengthening, the Ti element is used during the steelmaking. TiN formation is decided by the Ti/N ratio, and formed from the solid solution of molten steel directly or nuclei such as the oxide and sulfide. The coarse TiN particle affects adversely the mechanical properties of the martensitic steel, and many studies proved this results through the many testing. Therefore, for improving or maintaining the bendability, the formation of the coarse TiN particle must be restricted during the steelmaking, and the study for how the TiN particle affects the bendability of the martensitic steel must be conducted. In this study, we discussed the effect of the TiN particle on the bendability of the martensitic steel. We analyzed the microstructure of the martensitic steel using the electron backscatter diffraction (EBSD) analysis and the TiN particle through the optical microscopy (OM) analysis and scanning electron microscopy (SEM) + energy dispersive X-ray spectroscopy (EDS) analysis. We confirmed the tensile properties and bending property of the martensitic steel through the tensile test and the three-point bending test. For calculating the strain distribution on the matrix and TiN particle during the bending deformation, we used the high-resolution digital image correlation (HRDIC) method with nanoscale Ag patterns. We observed the strain localization on the lath martensite and the formation of the void caused by the crack in the TiN particle and debonding between the TiN particle and matrix.