Abstract: Effect of annealing temperature on the microstructure and mechanical properties of a medium-Mn low density medium thick plate for bogie was designed and investigated by means of Thermo-Calc calculations, scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), tensile tests and machine learning from the perspectives of phase transformation, densities of geometrically necessary dislocations, grain boundary types and grain boundary density. The results show that the hot-rolled microstructure of the low-density steel at room temperature consists of δ-ferrite, austenite, and martensite. The matrix undergoes reverse transformation and the growth of prior austenite during annealing treatment, and the stability of austenite decreases with the increase of annealing temperature. The densities of geometrically necessary dislocations of the tested steel decrease with the increase of annealing temperature. The growth of the grains and the merging of martensite laths during annealing lead to a decrease in the density of prior austenite grain boundaries and martensite lath boundaries. The annealed tested steel shows significantly higher elongation than that of the hot-rolled state, with yield strength decreasing as the annealing temperature increasing. The tested steel under both hot-rolled and annealed states shows uniform deformation and cleavage fracture during tensile test. After annealing at 820 and 880 ℃, the yield strength of the tested steel reaches 491 and 413 MPa, respectively.

Key words: low-density steel, medium plate, bogie frame, annealing, microstructure, mechanical properties