Microstructure evolution and the corresponding mechanical properties of Fe-Mn-Al-C-Nb low-density steel under aging treatment

To develop high-strength, low-density steels applicable for automotive field, the study systematically investigated Fe-28Mn-10Al-C-0.5Nb steel after aging at 450 °C-550 °C in terms of its microstructure evolution, mechanical properties, and deformation and strengthening mechanisms. Electron backscatter diffraction (EBSD) method served for examining the austenite grain morphology and the orientation of annealing twins at different aging temperatures. Transmission electron microscopy (TEM) served for elucidating the precipitation behavior and spatial distribution of NbC, κ-carbides, and other secondary phases. Furthermore, the deformation mechanisms under different tensile strains were explored using TEM and EBSD, with particular attention to the evolution of dislocations and other substructures in the deformed specimens. Quantitative evaluation was conducted on the yield strength variation under varying strengthening mechanisms through theoretical modeling. According to relevant results, with rising aging temperature, the finely dispersed spherical κ-carbides gradually transform into a uniformly distributed rectangular morphology. The strength and toughness of the experimental steel both increase with aging temperature, and the steel aged at 500 °C exhibits an outstanding overall property, with a tensile strength of 1199  MPa and an elongation of 37 %. Planar dislocation slip is the primary deformation mode, and the favorable strength-ductility balance results from the microband-induced plasticity. Calculations confirm dislocation strengthening as the primary strengthening mechanism in the experimental steel.