A plain carbon steel to which another element, other than iron and carbon, has been added in a percentage large enough to alter the characteristics and properties of the steel. These elements may be chromium, manganese, nickel, tungsten, or vanadium, and are added to produce or increase certain specific physical properties, such as hardness, toughness, ductility, strength, resistance to corrosion or resistance to wear.

The various kinds of steel are most often identified by a type designation or a specification number.  SAE (formerly Society for Automotive Engineers) and the American Society for Testing Materials developed a unified system for designating metals and alloys. See UNIFIED NUMBERING SYSTEM.

The American Iron and Steel Institute and SAE published a system for designating carbon and alloy steels using the UNS identifications.

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