Effect of w(Mn)/w(S) ratio on inclusion precipitation behavior of 20MnCr5 gear steel hot-rolled bar

MnS inclusions significantly affect the microstructure homogeneity and abnormal grain growth behavior during annealing of Mn-Cr high-quality gear steels. This study investigated the effect of manganese-to-sulfur ratio on the number, size, and aspect ratio of MnS inclusions in hot-rolled bars of 20MnCr5 gear steel. Inclusion analysis results showed that increasing the manganese-to-sulfur ratio w(Mn)/w(S) significantly improved the morphology of MnS inclusions, reduced their size and aspect ratio, and increased the number density. In low manganese-to-sulfur ratio steel MS54 (w(Mn)/w(S)=54), the number density of MnS inclusions per unit area was 152.5, accounting for 77.4% of all types of inclusions, among which sub-micron inclusions accounted for 26.7%, and inclusions smaller than 5 μm accounted for 92.1%. In high manganese-to-sulfur ratio steel MS103 (w(Mn)/w(S)=103), the number density of MnS inclusions per unit area increased to 211.3, accounting for 86.4% of all types of inclusions, among which sub-micron inclusions accounted for 32.7%, and inclusions smaller than 5 μm accounted for 96.2%. The particle size distribution was fitted with Gaussian and Boltzmann curves. In MS54 steel, the average and median values of inclusion size were (1.64±0.06) μm and 1.36 μm, respectively, while in MS103 steel, the average and median values of inclusion size were (0.73±0.03) μm and 1.01 μm, respectively. In MS54 steel, 20.6% of inclusions had an aspect ratio of 1-2, and 63.6% of inclusions had an aspect ratio below 5. In MS103 steel, 50.3% of inclusions had an aspect ratio of 1-2, and 76.3% of inclusions had an aspect ratio below 5. The research results provide a scientific basis for elucidating the abnormal grain growth behavior during annealing of Mn-Cr high-quality gear steels.