Investigation on Hydrogen-Induced Delayed Fracture of Cold-Rolled DP980 Steels

In this study, the phenomenon of hydrogen-induced delayed fracture of two cold-rolled DP980 steels with different chemical compositions was studied. The results show that the microstructure of both steels is composed of ferrite matrix, martensite-austenite islands and small amount of bainite. DP980-1 having higher contents of C and Si exhibits higher tensile strength, lower yield strength and higher elongation in comparison with DP980-2 having lower contents of C and Si. According to the results of slow strain rate tensile tests, the tensile strength of DP980-1 after hydrogen charging is reduced by 20.8%, while it is just 5.4% for DP980-2. Moreover, very fine dimples can still be observed in the fracture surface of DP980-2 after hydrogen charging, which indicates a good ductile. The main reasons leading to the better delayed fracture resistance of DP980-2 are the lower volume fraction of martensite-austenite islands, lower content of diffusible hydrogen and the grain refinement effects.