A lamellar structured ultrafine grain ferrite-martensite dual-phase steel and its resistance to hydrogen embrittlement

A lamellar structured ultrafine grain dual-phase (UFG DP) steel was prepared by intercritical annealing and subsequent warm rolling of a low-carbon martensite steel. The ultrafine structure is composed of alternate ferrite and martensite strips (i.e., lamellar structure) parallel to the rolling direction, and the strips are composted of very fine grains in an average size of 0.96 μm. Hydrogen embrittlement (HE) of the UFG DP steel was investigated by a slow strain rate tensile (SSRT) of hydrogen charged specimens. Compared with uniform structured steel obtained in normal quenching and tempering (QT) technology, the UFG DP steel exhibits markedly high resistance to HE at a tensile strength level of 1300 MPa. A fracture model based on plastic zone and stress distribution was proposed to explain the fracture process. The lamellar structure leaves weaker ferrite/martensite interface parallel to the longitudinal direction that leads to delamination, which relaxes stress concentration and makes crack deflection, resulting in the higher HE resistance of the lamellar structured UFG DP steel. Moreover, the UFG structure increases fracture strength that also increases HE resistance.