Strain Rate Sensitivity of Microstructural Damage Evolution in a Dual-Phase Steel Pre-Charged with Hydrogen

We evaluated the strain rate sensitivity of the micro-damage evolution behavior in a ferrite/martensite dual-phase steel. The micro-damage evolution behavior can be divided into three regimes: damage incubation, damage arrest, and damage growth. All regimes are associated with local deformability. Thus, the total elongation of DP steels is determined by a combination of plastic damage initiation resistance and damage growth arrestability. This fact implies that hydrogen must have a critical effect on the damage evolution, because hydrogen enhances strain localization and lowers crack resistance. In this context, the strain rate must be an important factor because it affects the time for microstructural hydrogen diffusion/segregation at a specific microstructural location or at the damage tip. In this study, tensile tests were carried out on a DP steel with different strain rates of 10 ^{- 2} and 10 ^{- 4} s ^-1 . We performed the damage quantification, microstructure characterization and fractography. Specifically, the quantitative data of the damage evolution was analyzed using the classification of the damage evolution regimes in order to separately elucidate the effects of the hydrogen on damage initiation resistance and damage arrestability. In this study, we obtained the following conclusions with respect to the strain rate. Lowering the strain rate increased the damage nucleation rate at martensite and reduced the critical strain for fracture through shortening the damage arrest regime. However, the failure occurred via ductile modes, regardless of strain rate.