Synergistic enhancement mechanism of mechanical properties and hydrogen embrittlement resistance in medium Mn steels by coupling warm/cold rolling and delta ferrite

In the present research, Fe-0.2C–6Mn–3Al-0/0.6Si steels were labeled as 0Si (without δ-ferrite) and 6Si samples (with abundant δ-ferrite). The hot-rolled annealed 0Si samples (i.e. 0SiHRA), and 6Si samples subjected to the warm-rolling, warm-rolled annealing and cold-rolled annealing (i.e. 6SiWR, 6SiWRA and 6SiCRA) were investigated. 0SiHRA shows the lath-type, 6SiWR shows the elongated and 6SiWRA and 6SiCRA reveal the equiaxed microstructure. Compared to 0SiHRA, the mechanical properties and the hydrogen embrittlement (HE) resistance are generally enhanced in 6Si samples. H-induced cracks (HICs) in 0SiHRA mainly nucleate at γ(α′)/α interfaces and propagate along the prior austenite grain boundaries with little propagation resistance, thus showing the low HE resistance. HICs in 6SiWR and 6SiWRA nucleate at γ(α′)/α and (γ(α′)+α)/δ interfaces and propagate longitudinally. Subsequently, the elongated microstructure tearing in 6WR and the equiaxed grains intergranular fracture in 6SiWRA result in HICs transverse propagation. The fracture behaviors in 6SiWR and 6SiWRA consume the vast energy, thus enhancing HE resistance. This work provides a vital insight for the rolling technology and the microstructural design to improve the mechanical properties and HE resistance for the high-strength medium Mn steels.