Steel is an alloy of iron and carbon. Carbon is the primary alloying element, and its content in the steel is between 0.002% and 2.1% by weight. Additional elements are also present in steel: manganese, phosphorus, sulfur, silicon, and traces of oxygen, nitrogen and aluminum. Alloying elements intentionally added to modify the characteristics of steel include: manganese, nickel, chromium, molybdenum, boron, titanium, vanadium and niobium.

Carbon and other elements act as a hardening agent, preventing dislocations in the iron atom crystal lattice from sliding past one another. Varying the amount of alloying elements and the form of their presence in the steel controls qualities such as the hardness, ductility, and tensile strength of the resulting steel.