35# round steel (GB/T 699) has a composition of 0.32–0.40% carbon (C), 0.17–0.37% silicon (Si), 0.50–0.80% manganese (Mn), ≤0.035% phosphorus (P), ≤0.035% sulfur (S), and trace chromium (Cr ≤0.25%)/nickel (Ni ≤0.25%).
Carbon and manganese work synergistically to enhance chemical strength: Carbon (0.32–0.40%) is the primary strengthener, as it dissolves in the iron matrix to form a solid solution-this creates lattice distortions that resist the movement of dislocations (the atomic defects responsible for deformation). Manganese (0.50–0.80%) amplifies this effect by increasing carbon's solubility in iron, allowing more carbon atoms to contribute to solid-solution strengthening. Additionally, manganese slows the diffusion of carbon atoms, preventing the formation of large, brittle iron carbide (Fe₃C) particles that could weaken the matrix. Silicon acts as a deoxidizer to reduce oxide inclusions, which would otherwise create microcracks and undermine strength. Phosphorus and sulfur are limited to ≤0.035% because they form brittle compounds (Fe₃P, FeS) that reduce the steel's ductility and overall structural integrity.