Carbon (C): The most significant element. It increases tensile strength and hardness but decreases ductility and weldability. Content is carefully controlled based on the grade.
Manganese (Mn): Increases strength and hardness through solid solution strengthening. It also improves hardenability and helps to mitigate the brittleness caused by sulfur.
Silicon (Si): Primarily used as a deoxidizer during steelmaking (killed steel). It increases strength and hardness.
Phosphorus (P) and Sulfur (S): Generally considered residual impurities. Phosphorus increases strength but severely reduces toughness (cold brittleness). Sulfur forms sulfide inclusions which can reduce ductility and cause "hot shortness" during welding or hot working. Their content is strictly limited in specifications.