Microstructure and tensile behaviors for a medium Mn steel with δ-ferrite phase under different annealing temperatures

In this experiment, we systematically investigated the microstructure evolution, mechanical properties, and deformation behaviors as functions of annealing temperature using cold-rolled Fe-0.05C–6Mn–1Al-1.5Si steel. Almost all annealed specimens are composed of the equiaxed and granular α-ferrite (α) grains due to recrystallization, reversed austenite (γ) grains because of reversed transformation from deformed martensite, and fibrous δ-ferrite (δ) grains only undergoing recovery during intercritical annealing except for 760 °C. It is interesting that several annealing twins are formed within γ grains. The outstanding combination of strength and ductility is obtained for annealing at 740 °C. The tensile strength and total elongation are 980 MPa and 32.1%, which are attributed to the ultrafine grains and optimal TRIP effect associated with rational γ content and stability. A phenomenon that yield point elongation (YPE) gradually shortens with increased γ content and decreased γ stability is observed. What's more, the mechanism that active α′ formation can promote the improvement of work hardening rate and further curtail YPE is verified using deep-cryogenic treatment. Based on SEM observation, microscopic strain of δ grains appears nearly consistent with matrix (α and γ grains) during tensile process, although δ phase presents as fibrous morphology with coarse size.