Effect of cold rolling reduction ratio on microstructure and mechanical properties of Fe–10Mn–4Al-0.4C steel containing δ ferrite

A new low-cost low-density high-strength steel was produced by a combination of hot rolling, cold rolling and a short-time annealing process. The effect of cold rolling reduction on the microstructure and mechanical properties of Fe–10Mn–4Al-0.4C steel after annealing was studied. The microstructure of short-time annealing (750 °C/30 min) after cold rolling with different reduction consists of equiaxed α-ferrite, austenite and banded δ-ferrite. As the reduction increased, the grain size gradually decreased. After annealing, the distribution of austenite in the sample is uniform. When the reduction is 60%, the content of austenite reaches the maximum of 61%. The annealed sample with 60% reduction exhibited the highest tensile strength and elongation with a product of strength and elongation (PSE) of 40.23 GPa·%. The ferrite and martensite were distinguished in the electron backscatter diffraction (EBSD) map by the Nishiyama-Wassermann orientation relationship. After the tensile test, part of the austenite was transformed into martensite, which is banded. The strengthening mechanism was calculated according to the rule of mixtures (ROM), and the contributions of grain boundaries strengthening, dislocation strengthening, transformation induced plasticity (TRIP) effect strengthening and additional strengthening were investigated. The annealed sample with 60% reduction has favorable TRIP effect with a strengthening value of 211 MPa. EBSD analysis of the tensile sample showed that the banded δ-ferrite can accommodate a large number of dislocations.