Steel is an alloy made from iron mixed with carbon and other elements, which make the steel harder than pure iron. Where the main alloying element is carbon, the finished steel is called carbon steel or plain-carbon steel. Carbon steel may also contain up to 1.65 per cent manganese and up to 0.5 per cent silicon, sulphur and phosphorus.
Low-Carbon Steel and Mild Steel
Low-carbon steel typically contains 0.05 to 0.15 per cent carbon. Mild steel contains 0.16 to 0.29 per cent. Such steels are not brittle, but not ductile (easily stretched) either. They are less hard but more malleable (easily deformed under compressive stress) than higher-carbon steels. Their malleability and lower price make them useful in construction. They are also used for wires and automobile body panels.
Medium-carbon steels contain 0.30 to 0.59 per cent carbon. They can also contain 0.60 to 1.65 per cent manganese. Medium-carbon steels are more ductile than high-carbon steels, and harder and less malleable than low-carbon steels. Medium-carbon steels have good wear resistance, so they are often used for automobile parts and similar applications. They are also used in forging and for large parts.
High-carbon steels contain around 0.6 to 0.99 per cent carbon. They may also contain up to 0.90 per cent manganese. They are very hard and strong, but less ductile and malleable than medium- and low-carbon steels. They respond well to heat treatment, becoming harder. They have excellent wear resistance. Because of their strength and resistance to deformation under compression, they can be used to make springs and high-strength wires.
Ultra-high-carbon steels are used for speciality non-industrial applications such as knives, punches and axles. They can be tempered (heat- and pressure-treated) to become exceptionally hard. This means they can be given a very sharp edge, making them a good choice for cutting tools. Ultra-high-carbon steels may contain between 1.25 and 2.0 per cent carbon. They are mostly made using a process called "powder metallurgy." Iron alloys with more than 2 per cent carbon are considered to be cast iron, rather than steel.