Effect of Mo and Nb on mechanical properties and hydrogen embrittlement of hot-rolled medium-Mn steels

Abstract

The effects of Mo and Nb on the mechanical properties and hydrogen embrittlement (HE) of hot-rolled medium-Mn steel were investigated. The diffusion behavior of hydrogen was examined using an electrochemical hydrogen permeation test. The mechanical properties and HE were studied using a slow strain rate test. The results show that most Mo and Nb served as solid-solution atoms in the hot-rolled steels, and little carbide content was observed. Both Mo and Nb increased the volume fraction of retained austenite (VA), decreased the starting temperature of martensitic transformation (MS), and retarded the martensite transformation. The effect of Nb was more pronounced. Simultaneous addition of Mo and Nb had a synergistic effect on increasing VA, decreasing MS, and retarding martensite transformation. The elongation of the samples increased with an increase in VA. The strength exhibited the opposite trend. A small amount of Nb (0.04 wt%) significantly improved the HE resistance of steels to a much greater extent than Mo (1 wt%). Filmy retained austenite formed along the primary austenite grain boundaries because of the Nb addition; it inhibited hydrogen-induced cracks, leading to better HE resistance. Blocky retained austenite, formed by the simultaneous addition of Mo and Nb, deteriorated the HE resistance.