From Halopedia, the Halo wiki
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 incorporated steel as a decorative element. The doors and ceiling braces on Rapid Conversion's feasting hall were constructed of forged steel.
Despite normally relying on more exotic materials in their warships, the Covenant are nonetheless known to use steel on certain occasions. For example, the asteroid-based ship manufacturing plant K7-49 had pools of molten steel maintained by Huragok.