Fabrication of bimodal-grained Al-free medium Mn steel by double intercritical annealing and its tensile properties

The objectives of the present study were to develop the bimodal-grained microstructure in a medium Mn steel without the addition of Al only through a thermo-mechanical process, and to systematically investigate the relationship between the bimodal-grained microstructure and yielding behavior. The cold-rolled Fe-7Mn-0.05C (wt.%) steel with deformed α′ martensite was annealed twice with different temperatures and holding times. When the cold-rolled specimens were first-annealed at 660 °C for 4 h-22 h, the specimens showed a dual-phase microstructure consisting of globular-shaped ferrite (α_G) and thermally induced α′ martensite at room temperature. Whereas the sizes of both α_G grains and α′ martensite constituents increased with increasing the first annealing time, both the volume fraction and the chemical concentration of each phase were insignificantly changed. When the first-annealed specimens were annealed again at 640 °C for 5 min, the specimens exhibited bimodal-grained structures with coarse α_G and ultrafine lath-shaped ferrite (α_L) and retained austenite (γ_L). The coarse α_G came from α_G pre-existing in the first-annealed specimen; the fine α_L and γ_L formed by the α′ martensite to γ diffusive reverse transformation. The second-annealed specimens exhibited a transition of yielding behavior from discontinuous yielding with yield-point elongation (YEL) to continuous yielding without YEL due to the coarsening of α_G grains with increasing the first annealing time up to 22 h.