Steel is a solid solution, a subtype of chemical mixtures that involve two or more molecules in a solid state that combine to form a single substance while retaining their individual identities. The component metals making up steel are not chemically bonded together.
Steel is an alloy composed chiefly of iron, with some carbon added. Many steels add to the iron-steel mix other metals that give the entire substance desirable properties such as strength, heat resistance, or flexibility. Steel is considered a mixture rather than a compound because the iron does not chemically bond with the carbon and other components. Instead, in simple iron-carbon alloys, the carbon atoms are small enough to intersperse themselves within the open spaces of the iron crystal. In steel alloys involving heavier elements such as nickel, the nickel atoms replace some of the iron atoms in the crystal structure.