Achieving a superior combination of strength and ductility by adjusting heterogeneous structure in a Fe–Mn–Al–Mo–C lightweight steel

A combination of high strength and good ductility was achieved by adjusting heterogeneous structure through varying annealing temperatures ranging from 750 to 900 °C after cold rolling in a Fe–Mn–Al–Mo–C lightweight steel. The microstructure consists of heterogeneous recrystallized, unrecrystallized grains containing nanoscale Mo₂C precipitates, and intergranular Mo-enriched carbides. As annealing temperature increases, the fraction of recrystallization, and the average Schmid factor increase, while the average grain sizes and the volume fraction of Mo₂C decrease. Annealing at 825 °C forms a desirable heterogeneous structure characterized by a normal bimodal grain distribution and diffuse precipitation of intragranular Mo₂C particles. Tensile test results indicated that higher annealing temperatures decrease strength but enhance ductility. However, the yield strength of the 825 °C annealed sample only slightly decreases compared to the 800 °C annealed sample owing to the synergistic contributions of various strengthening mechanisms, including grain boundary, solid solution, precipitation, and heterogeneous deformation-induced strengthening. Furthermore, its ductility significantly improves, approaching that of the 850 °C annealed sample, facilitated by sustained heterogeneous deformation-induced strengthening effects and significant refinement in grain structure. The high strain hardening rate of the C825 sample is attributed to dynamic slip band refinement and local stress adjustments during later deformation stages. The heterogeneous grain structure induces a strong HDI strengthening effect due to uneven deformation, which also contributes to the high strain hardening rate. These findings highlight the potential of Fe–26Mn–8Al-1.2C–3Mo lightweight steel for applications requiring a balance of strength and ductility.