Steel is an alloy consisting mostly iron. Depending on the grade it contains between 0.2% and 2.1% of carbon that is the most cost-effective alloying material.

Apart of carbon, there are various other alloying elements used (manganese, chromium, vanadium, and tungsten). Carbon and other elements act as a hardening agent. Varying the amount of alloying elements in the steel controls the hardness, ductility, and tensile strength of the resulting steel. Steel with higher content of carbon is less ductile but can be made harder and stronger than iron. These alloys are known as cast iron for their lower melting point and castability.
Two distinguishing factors for steel with small amount of carbon are steel's increased rust-resistance and better weldability.

Steel is one of the most common materials and is a major component in buildings, infrastructure, tools, ships, automobiles, machines, and appliances.