50Mn round steel (GB/T 699) has a composition of 0.48–0.56% carbon (C), 0.17–0.37% silicon (Si), 0.70–1.00% manganese (Mn), ≤0.035% phosphorus (P), ≤0.035% sulfur (S), and trace elements (Cr ≤0.25%, Ni ≤0.25%).
Manganese (0.70–1.00%) is the defining element here, with three key chemical effects: first, it enhances hardenability by increasing the steel's critical cooling rate, allowing it to form martensite (a hard, strong microstructure) during quenching-this is more pronounced than in lower-manganese steels like 45# (0.50–0.80% Mn). Second, manganese reacts with sulfur to form manganese sulfides (MnS) instead of iron sulfides (FeS); FeS is brittle and causes hot shortness (cracking during hot working), while MnS is more ductile and less harmful to the steel's integrity. Third, manganese contributes to solid-solution strengthening by dissolving in the iron matrix, which increases the steel's strength without excessive carbide formation. Carbon (0.48–0.56%) is higher than 45#, providing additional strengthening, but the higher manganese ensures this does not lead to excessive brittleness. Silicon acts as a deoxidizer, and phosphorus/sulfur are limited to reduce impurities.