Comparative investigation of the stretch-flange cracking mechanism of ultrahigh-strength steels with different microstructures and hole-expansion ratios

Ultrahigh-strength steel sheets (UHSSs) have been widely adopted for reducing the weight of automobile body structures in order to combine fuel efficiency with crashworthiness. One of the issues in the forming of UHSSs is to prevent stretch-flange cracking on the sheared edges of the sheets. Although countermeasures have been developed in terms of both materials and processes, the fundamental picture of stretch-flange cracking in diverse types of UHSSs was unclear. In this study, we investigated the mechanism and material factors of stretch-flange cracking in UHSSs with a tensile strength of 980 MPa, comparing dual-phase (DP), transformation-induced-plasticity (TRIP), and full-martensite (FM) microstructures. The material sheets were pierced by punching or machining, and subsequently, hole-expansion-tested and observed. Macroscopic observation in the tests revealed that the hole-expansion limit was determined by the earliness of crack propagation relative to the hole-expansion ratio. Scanning electron microscope (SEM) analysis of the expanded edge interior showed that void formation occurred exclusively around the crack tip area, thus contributing to ductile crack growth. Microscopy analysis of the expanded edge surfaces revealed the details of stretch-flange cracking. The analysis results suggested that the flange cracks more easily proceeded and cleaved in the order of TRIP, DP, and FM. It was concluded that the crack growth behavior and the hole-expansion limit were dominated by the microstructural resistance to crack growth generated from work-hardened layer.