The carbon content of steel is between 0.002% and 2.1% by weight. Too little carbon content leaves (pure) iron quite soft, ductile, and weak. Carbon contents higher than those of steel make an alloy commonly called pig iron that is brittle and not malleable. Alloy steel is steel to which additional alloying elements have been intentionally added to modify the characteristics of steel. Common alloying elements include: manganese, nickel, chromium, molybdenum, boron, titanium, vanadium, and niobium. Additional elements may be present in steel: manganese, phosphorus, sulfur, silicon, and traces of oxygen, nitrogen, and aluminium.
Alloys with a higher than 2.1% carbon content, depending on other element content and possibly on processing, are known as cast iron. Cast iron is not malleable even when hot, but it can be formed by casting as it has a lower melting point than steel and good castability properties. Steel is also distinguishable from wrought iron (now largely obsolete), which may contain a small amount of carbon but large amounts of slag. Note that the percentages of carbon and other elements quoted are on a weight basis.