Abstract: The microstructure characteristics and mechanical properties evolution of 45 steel under different annealing temperatures and heating rates were studied. The orientation, grain boundaries, grain types, geometrically necessary dislocations, and precipitates of samples with different annealing parameterswere analyzed. Combined with tensile mechanical properties testing, the mechanical properties of different samples were explained. The results indicated that the microstructure and mechanical properties of 45 steel have significant differences under different annealing parameters. As the annealing temperature increases, the orientation gradient, low-angle grain boundaries, deformed grains, geometrically necessary dislocations, and cementite content of 45 steel decrease. The effect of annealing heating rate on microstructure is complex. At low temperatures, the slower the heating rate, the greater the degree of recrystallization. However, at high temperatures, the phase transition participates in the static recrystallization nucleation and growth during the annealing process. Under the annealing parameters of 0.08 ℃/s-690 ℃, the tensile mechanical strength and toughness of 45 steel are the best, with the tensile fracture surface exhibiting a plastic fracture mode.

Key words: 45 steel, annealing parameters, microstructure, mechanical properties