Delayed Fracture of Hole-Expanded High-Strength Steel Sheet

High-strength steel sheets for automobiles with a tensile strength of more than 980 MPa have been required to reduce the car weight and improve collision safety. Delayed fracture or hydrogen embrittlement becomes a serious problem for the high-strength steel sheets. In this study, delayed fracture behavior of TRIP-aided martensitic (TM) steel was investigated, and the effects of residual stress and plastic strain on the occurrence of delayed fracture of a hole-expanded specimen of the TM steel were discussed. Delayed fracture cracks were initiated and propagated concentrically with the punched hole in the TM steel. A finite element analysis revealed that the high tensile residual stress in the radial direction was applied in the portion where the delayed fracture crack occurred although the equivalent plastic strain was not so high in comparison with that at the punched hole edge. It is considered that the high tensile residual stress was preferential factor for the occurrence of delayed fracture in the TM steel specimen.