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A basic oxygen furnace is a refractory-lined converter used for basic oxygen steelmaking. It holds molten pig iron, recycled steel and fluxes while an oxygen supply drives refining reactions. The vessel must contain the bath and slag, accommodate vigorous mixing and allow the finished steel and reaction products to be removed.^[ref]

Its thermal balance couples composition and charge selection: oxidation releases heat, while heating and melting additions consume it. This makes the furnace a useful example of simultaneous mass balance and energy balance. The furnace is the physical reactor; the steelmaking process includes its operation, measurements and composition adjustments.

Basic oxygen steelmaking refines molten pig iron into steel by blowing high-purity oxygen into the bath. Oxidation lowers the carbon content and removes or redistributes other constituents. The reactions release heat, helping melt added recycled steel. The basic oxygen furnace is the vessel in which the operation takes place.^[ref]

The word basic refers to the chemical character of the lining and slag, not to the simplicity of the process. A basic slag helps capture acidic oxides formed during refining. A useful process description must therefore track both the metallic product and the nonmetallic reaction products; a low final carbon content alone does not specify a steel's full chemical composition.

An electric arc furnace supplies heat through an electric arc between electrodes and the charge or molten bath. In steelmaking, it melts and refines material that can include recycled steel and direct reduced iron. It provides a different heating route from the internal oxidation heat central to basic oxygen steelmaking.^[ref]

The electrical input does not by itself determine the product's chemical composition. The charge, additives, slag and refining reactions must also be controlled. Similarly, the route's carbon dioxide emissions depend on how its electricity and metallic feed were produced; melting scrap and reducing fresh iron ore are different system boundaries.

Steelmaking produces steel by controlling the carbon content, other alloying elements and unwanted constituents of iron. A blast furnace first produces carbon-rich pig iron; a basic oxygen furnace then refines the molten material by oxidation and composition adjustment. An electric arc furnace instead supplies heat electrically to melt a charge that can include recycled steel, direct reduced iron and pig iron.^[ref]

The distinction between iron extraction and steelmaking matters: converting an iron oxide to metallic iron and adjusting an existing iron-based alloy to the required composition are different tasks. The Bessemer process is an important historical example of refining pig iron by an air blast; modern basic oxygen steelmaking uses an oxygen supply rather than ordinary air.