45# round steel (GB/T 699) is a medium-carbon structural steel with a classic composition: 0.42–0.50% carbon (C), 0.17–0.37% silicon (Si), 0.50–0.80% manganese (Mn), ≤0.035% phosphorus (P), ≤0.035% sulfur (S). This carbon content strikes a perfect balance between strength and machinability, making it the "workhorse" of mechanical manufacturing.
Its tensile strength ranges from 600–700 MPa, and elongation is ≥16%, allowing it to be easily turned, milled, and heat-treated. After quenching and tempering, its hardness can reach 220–250 HB, suitable for making gears, shafts, and connecting rods. International equivalents include AISI 1045 (USA), DIN C45 (Germany), and JIS S45C (Japan). 45# is a staple in Chinese factories, with Made-in-China listings often labeling it as "45# Carbon Steel Round Bar for General Machinery Parts."