Heterogeneous Fe-Mn-Al-C lightweight steel breaking the strength-ductility trade-off via high-temperature warm rolling process

In this study, a lightweight heterogeneous Fe-6Mn-4.1Al-0.41C-0.62Si-0.2 V (wt%) steel with the synergy of high tensile strength and ductility was achieved by warm-rolling at 850 °C with a 93 % thickness reduction, namely, an ultimate tensile strength of 1.18 GPa, a total elongation of 35.4 %, and the product of strength and elongation (PSE) of 41.9 GPa%. The microstructure of the investigated steel consists of δ-ferrite (δ-F), retained austenite (RA), inter-critical ferrite (IF) and Fe₃C precipitates. The moderate stability of RA, induced by its different morphologies (i.e., lath, block shape), provides a discontinuous and local phase transformation-induced plasticity (TRIP) effect that contributes to the excellent work hardening ability. Meanwhile, the isomeric hetero deformation induced (HDI) strengthening between RA and δ-F, and between RA and IF, further enhances the investigated steel's high strength and ductility. In addition, our results indicate that increasing the rolling temperature results in the increment of RA, while increasing the rolling reduction (i.e., 5 % at 800 °C) has a limited effect on the formation of RA.