High strength and ductility combination in nano-/ultrafine-grained medium-Mn steel by tuning the stability of reverted austenite involving intercritical annealing

The structure–property relationship in 0.06C–5.5Mn steel subjected to different annealing temperatures and time was studied. Mn played a stronger effect on stabilizing austenite in comparison with Ni, and low-C medium-Mn steel possessed excellent hardenability. The reverse transformation of martensite to austenite occurred during intercritical annealing, and the volume fraction was first increased and then decreased on increasing annealing temperature or prolonging annealing time, indicative of change in thermal stability by element partitioning and coarsening of grain size. Correspondingly, the elongation was first increased and then decreased, consistent with the variation in the stability of reverted austenite. The yield strength was gradually decreased because of several factors, including recrystallization of α′ martensite, decreased stability of reverted austenite, and coarse grain size. The maximum product of strength and ductility was obtained on annealing at 650 °C for 10 min, which was attributed to the optimal stability of reverted austenite rather than the highest volume fraction, and tensile strength and elongation were 1120 MPa and 23.3%. The strain partitioning behavior of two phases was elucidated by analyzing Luders straining and continuous work hardening after yield point elongation, and the deformation mechanism was strongly related to the stability of reverted austenite.