45# round steel (carbon structural steel, GB/T 699) relies on key elements for its properties:
1) Carbon (C, 0.42-0.50%): The main strengthener-higher carbon increases hardness (after quenching, hardness reaches HRC 55-60) but reduces weldability;
2) Manganese (Mn, 0.50-0.80%): Improves tensile strength and hardenability, preventing brittleness from high carbon;
3) Silicon (Si, 0.17-0.37%): Enhances yield strength by forming solid solutions with iron;
4) Phosphorus (P, ≤0.035%) and Sulfur (S, ≤0.035%): Harmful impurities-P causes cold brittleness (cracking at low temperatures), S leads to hot brittleness (fracture during forging);
5) Copper (Cu, ≤0.25%): A trace element that slightly improves corrosion resistance but is controlled to avoid reducing toughness.
These elements make 45# ideal for quenched/tempered parts like gears and connecting rods.