|There is more information available on this subject at Steel on the English Wikipedia.|
Steel is an alloy consisting mostly of iron, with a carbon content between 0.2 and 1.7 or 2.04% by weight (C:1000-10,8.67Fe), depending on grade.
Carbon is the most cost-effective alloying material for iron, but various other alloy elements are used such as manganese, chromium, vanadium, and tungsten. Carbon and other elements act as a hardening agent, preventing dislocations in the iron atom crystal lattice from sliding past one another. Varying the amount of alloying elements and form of their presence in the steel (solute elements, precipitated phase) controls qualities such as the hardness, ductility and tensile strength of the resulting steel. Steel with increased carbon content can be made harder and stronger than iron, but is also more brittle. The maximum solubility of carbon in iron (in austenite region) is 2.14% by weight, occurring at 1149 °C; higher concentrations of carbon or lower temperatures will produce cementite. Alloys with higher carbon content than this are known as cast iron because of their lower melting point. Steel is also to be distinguished from wrought iron containing only a very small amount of other elements, but containing 1-3% by weight of slag in the form of particles elongated in one direction, giving the iron a characteristic grain. It is more rust-resistant than steel and welds more easily.
Steel is an unusual metal for a Covenant vessel, but some Jiralhanae ships such as Rapid Conversion have steel incorporated into their superstructures. The doors and ceiling braces on Rapid Conversion were constructed of forged steel.
Steel can be used as an armor color in Halo 3 and Halo: Reach's player model setup.