Reduction of Delayed Fracture Susceptibility Through Increased Si Content and Surface-Softening of Tempered Martensitic Steel

摘要

High-strength steel with a lower susceptibility to delayed fracture needs to be developed, as steel with tensile strength above 1,200 MPa generally has higher susceptibility. In the present study, an attempt was made to reduce the delayed fracture susceptibility of tempered martensitic steel with tensile strength of 1,450 MPa through higher-temperature tempering by adding Si and surface-softening by induction heating. The tensile strength of the surface-softened steel was 1,150 MPa at the surface, 1,500 MPa at the center and 1,450 MPa on average. Increasing the Si content reduces delayed fracture susceptibility for two reasons. The first is carbide particles both on prior-γ grain boundaries and in matrix is smaller. The second is that the relaxation value is smaller, which implies that Si stabilizes the dislocation structure. Surface-softening also reduces delayed fracture susceptibility, because the hydrogen concentration at the surface, where a crack initiates, is lower than that at the center.