Microstructural aspect of hydrogen embrittlement in sheared edge of Zn-coated advanced high-strength steels

The hydrogen embrittlement (HE) of sheared edges is a concern in advanced high-strength steel owing to their localized susceptibility to HE. This study investigated the HE of sheared edges from a microstructural perspective. Two types of 1.5 GPa-grade steel were prepared: complex-phase (CP) and single-phase (SP) steels. The CP steel comprises tempered martensite and bainite with island-shaped bainite, whereas the SP steel comprises only tempered martensite. The HE sensitivities of the steel matrix and sheared edge were evaluated using slow strain rate tensile tests (SSRT) after electrochemical H-charging and immersion with four-point bending tests, respectively. The CP steel had a significantly lower yield strength, higher tensile strength, and higher elongation than the SP steel, resulting in a higher hardening ability. Nevertheless, the SP steel exhibits a significantly higher fracture toughness than the CP steel. Furthermore, although CP steel shows higher hardening ability, lower intrinsic HE susceptibility in the steel matrix, and lower H content absorbed from the galvannealed (GA) coating, the SP steel exhibited significantly better resistance to the HE of the sheared edge during the immersion test. The low resistance to crack propagation in CP steel is attributed to microstructural vulnerabilities, specifically, the stress concentration at the matrix and island bainite phase interface. Consequently, the HE of the sheared edge demonstrates a close correlation with the fracture toughness originating from the microstructural features and local properties.