Effect of annealing treatment on microstructure and mechanical properties of lightweight steels micro-alloyed with vanadium and niobium

The microstructure evolution and tensile properties of a newly designed Fe-27Mn-10Al-1.4C-0.1V-0.1Nb (wt.%) lightweight steel were investigated at the annealing temperature of 900 C for 1min, 3min and 10min. The steel's microstructure revealed non-recrystallized austenite grains containing dislocations and fine-sized recrystallized grains and the dense nano-sized (V,Nb)C particles and kappa-carbides observed in austenite matrix after annealing for 3 min. The experimental steel achieved ultra-high tensile strength (yield and ultimate tensile strengths of 1152 MPa and 1305 MPa, respectively) because of precipitation strengthening, grain boundary and dislocation strengthening, and excellent ductility (44% of the total elongation). The deformation microstructure of the experimental steel was observed through tensile interruption experiments and TEM characterization. As a result, the pinning effect of (V,Nb)C particles on dislocations and the refinement of the spacing of planar slip bands increase the material's work hardening rate during the first stage of deformation. By activating various dislocation processes involved in the arrangement of slip bands into double dislocation walls and the subsequent evolution of dislocation walls into microbands, the planar slip bands evolve into a microband structure in the later stages of deformation. At later stages of deformation, microband refinement dominates the development of the deformation microstructure of steel.