How does iron ore become steel?

Iron ore usually contains iron chemically bonded to oxygen in minerals such as iron oxides. Heating the ore until it melts is not enough to turn all that oxide into metal: the oxygen must be removed through chemical reduction. In a conventional blast-furnace route, carbon-based materials provide energy and generate reducing gases, including carbon monoxide.

One representative overall reaction is Fe₂O₃ + 3CO → 2Fe + 3CO₂. Oxygen transfers from iron oxide to carbon monoxide, leaving metallic iron. Each element has the same number of atoms before and after the reaction. An actual furnace contains several linked reactions, and its iron product contains substantial dissolved carbon and other elements.

Steelmaking then changes that composition. In a basic oxygen furnace, oxygen reacts with excess carbon and some other elements in the molten iron. Carbon leaves largely in gases, while some other unwanted materials enter a separate slag. Refining and later additions set the remaining carbon and alloying elements for the intended steel. Steel is therefore not made merely by mixing carbon into an ore.

The liquid steel is cast into solid forms and then shaped by processes such as rolling. Another major route melts recycled steel in an electric arc furnace, often with additional metallic iron. That route starts with metal already separated from much of its original oxygen, although its composition still needs adjustment.