Carbon is the principle hardening element in steel. With iron, carbon combines to form carbide, which is hard and brittle. The increment in carbon percentage within steel, increases the hardness of steel.
Manganese is deoxidizer. It is important to free cutting steels, as the risk of hot shortness is reduced. It adds to the strength and hardness of carbon steels. Steels containing more than 18% Mn will remain non-magnetic even after important cold working operations.
Phosphorus increases strength and hardness of steel. It also increases the susceptibility to temper brittleness. Phosphorus embrittlement increases with rising carbon content, higher hardening temperatures grain growth and with decreasing ratio of reduction by forging.
Silicon is a deoxidizing agent. It improves tensile strength and abrasion resistance (Si-Mn quenched temper steel), as it markedly raises the yield point, it is used in spring steel. Silicon enhances resistance and therefore is used in high temperature steels.
Sulphur decreases ductility and notched impact toughness but has only a slight effect on longitudinal properties. It is beneficial to machinability. It is used in free cutting steels. The improvement in machining characteristics is only reason for adding sulphur to steel.