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An alloy of iron and up to 1.7% carbon, with small amounts of manganese, phosphorus, sulfur, and silicon. These are termed carbon steels; those with other metals are termed alloy steels; low-alloy steels if they have less than 5% of the alloying metal, high-alloy steels if more than 5%. Carbon steels are far stronger than iron, and their properties can be tailored to their uses by adjusting composition and treatment. Alloy steels – including stainless steel – are used for their special properties.

Steel was first mass-produced in the mid-19th century (see crucible process), and steel production is now one of the chief world industries, being basic to all industrial economies. Steel's innumerable uses include automobile manufacture, shipbuilding, skyscraper frames, reinforced concrete, and machinery.

All steel making processes remove the impurities in the raw material – pig iron, scrap steel, and reduced iron ore – by oxidizing them with an air or oxygen blast. Thus most of the carbon, silicon, manganese, phosphorus, and sulfur are converted to their oxides, and together with added flux and other waste matter present, form the slag.

The main processes used for manufacturing steel are the Bessemer process (now obsolete), the Siemens-Martin open-hearth process, and the Linz-Donawitz process and the similar electric-arc process used for the highest-quality steel. Most modern processes use a basic slag and a basic refractory furnace lining: acidic processes are incapable of removing phosphorus. When the impurities have been removed, desired elements are added in calculated proportions. The molten steel is cast as ingots which are shaped while still red-hot in rolling mills, or it may be cast as a continuous bar ("strand casting").

The properties of medium-carbon (0.25% to 0.45% carbon) and high-carbon (up to 1.7% carbon) steel may be greatly improved by heat treatment: annealing, casehardening, and tempering. Steel metallurgy is somewhat complex: unharded steel may contain combinations of three phases – austenite, ferrite, and cementite – differing in structure and carbon content; hardened steel contains martensite, which may be thought of as ferrite supersaturated with carbon.

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