Steel is one of the most important and extensive classes of engineering alloys. The most important alloying element in steels is carbon, and very significant differences in properties are obtained in low, medium and high carbon steels.

Heat treatments can be used to change the microstructure significantly, and so improve the properties of the steel. There are many types of steel, and subtle differences in microstructure (such as precipitates, inclusions and texture) are very important. Alloying, with elements such as Chromium (Cr), Nickel (Ni), Vanadium (V) and Molybdenum (Mo) can be used to modify the structure of steels and the precipitates within them. High levels of alloying can also be used to obtain stainless steels.