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Steel, alloy of iron and up to 1.7% carbon, with small amounts of manganese, phosphorous, 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 steels—are used for their special properties. Steel was first mass-produced in the mid-19th century and is now basic to all industrial economies. The United States, the former USSR, and Japan are the major producers. All steelmaking processes remove the impurities in the raw materials—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 slag. The main processes are the Bessemer process; the Linz-Donawitz, or basic oxygen, process, and the similar electric-arc process, used for highest-quality steel; and the open-hearth process. When the impurities have been removed, desired elements are added in calculated proportions. The molten steel is cast as ingots that are shaped while still red-hot in rolling mills, or it may be cast as a continuous bar (strand casting). The properties of carbon steels may be greatly improved by heat treatment: annealing, casehardening, and tempering.

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