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

Abstract

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 (αG) and thermally induced α′ martensite at room temperature. Whereas the sizes of both αG 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 αG and ultrafine lath-shaped ferrite (αL) and retained austenite (γL). The coarse αG came from αG pre-existing in the first-annealed specimen; the fine αL and γL 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 αG grains with increasing the first annealing time up to 22 h.