Alloy of iron and about 2% or less carbon .
Pure iron is soft, but carbon greatly hardens it. Several iron-carbon constituents with different compositions and/or crystal structures exist: austenite, ferrite, pearlite, cementite, and martensite can coexist in complex mixtures and combinations, depending on temperature and carbon content. Each microstructure differs in hardness, strength, toughness, corrosion resistance, and electrical resistivity, so adjusting the carbon content changes the properties. Heat treating , mechanical working at cold or hot temperatures, or addition of alloying elements may also give superior properties. The three major classes are carbon steels , low-alloy steels, and high-alloy steels. Low-alloy steels (with up to 8% alloying elements) are exceptionally strong and are used for machine parts, aircraft landing gear, shafts, hand tools, and gears, and in buildings and bridges. High-alloy steels, with more than 8% alloying elements (e.g., stainless steels ) offer unusual properties. Making steel involves melting, purifying (refining), and alloying, carried out at about 2,900°F (1,600°C). Steel is obtained by refining iron (from a blast furnace ) or scrap steel by the basic oxygen process , the open-hearth process , or in an electric furnace , then by removing excess carbon and impurities and adding alloying elements. Molten steel can be poured into molds and solidified into ingots ; these are reheated and rolled into semifinished shapes which are worked into finished products. Some steps in ingot pouring can be saved by continuous casting . Forming semifinished steel into finished shapes may be done by two major methods: hot-working consists primarily of hammering and pressing (together called forging ), extrusion , and rolling the steel under high heat; cold-working, which includes rolling, extrusion, and drawing (see wire drawing), is generally used to make bars, wire, tubes, sheets, and strips. Molten steel can also be cast directly into products. Certain products, particularly of sheet steel, are protected from corrosion by electroplating , galvanizing , or tinplating .