JIS G4051 classifies round steel into carbon steels (S10C to S55C) and alloy steels (SCM435, SNCM439). Carbon content determines the grade (e.g., S45C = 0.45% C), with mechanical properties specified by tensile strength (S45C: 600 MPa min) and elongation (17% min). Alloy grades include chromium, molybdenum, or nickel for enhanced strength. The standard requires heat treatment options (annealed, normalized) and defines dimensional tolerances (±0.2 mm for 10-30 mm diameter bars). It is widely used in Japanese automotive and machinery industries.