How is Steel Made?
Iron ore, limestone and coke (made from coal) are combined in the blast furnace and heated to temperatures of over 3000° F by blasts of hot air. Once in a molten state, the hot air removes oxygen and other properties to produce iron.
Steel used to be made in open-hearth furnaces, where regenerative heating of gases, alternately reversed to raise the temperature, melted the iron and scrap while removing impurities to form steel. The combination of blast furnace and open hearth once represented the major means of producing steel in Ohio, but these have largely been replaced by the basic oxygen furnace and the electric arc furnace.
At the basic oxygen furnace (BOF), an oxygen lance blows pure oxygen into a mixture of molten iron and scrap, combining the oxygen with carbon to reduce impurities. The BOF is the most efficient process for large-scale conversion of iron to steel, being able to produce as much steel in an hour as the open hearth would produce in eight hours. However, the BOF cannot utilize as much scrap as the electric arc furnace.
At the electric arc furnace (EAF), scrap is the principal raw material. Three carbon electrodes are lowered into the furnace until they meet the cold scrap. Electric arcs produce intense heat, transforming the scrap into molten steel.
A ladle metallurgy center, where the steel’s composition is analyzed, enables the properties of the steel to be fine-tuned and customized to customer and government specifications. This enables the steel produced by modern Ohio mills to be the most efficiently and precisely made, high-quality product available in the world.
Steel destined for plate, sheet, coil, bars or welded pipe and tubing is produced on a continuous caster, then hot rolled. The continuous caster eliminates the necessity of pouring liquid steel into ingots and eliminates the formerly used soaking pit and blooming mill steps, and thus is much faster than older methods. The caster can literally accept pours on a continuous basis, limited only by the melting capacity and the speed of the roll apron.
After being hot rolled, the steel can be sold as "hot bands" or continue to the cold rolling operation for more demanding applications. It may be heat treated or receive a corrosive-resistant coating, or be welded into pipe or tubing for mechanical and other applications.
Steel for bar is cast into blooms or billets, then hot rolled into many kinds of cross sections. Further processing may include cold drawing — required for sophisticated automotive and aerospace applications. Seamless tubing for applications like oil wells and steam boilers is produced from billets that are first pierced and then drawn over a mandrel to reach the desired diameter and wall thickness.
In the last decade, the term mini-mill has become common in the steel industry. There is no separate SIC code for mini-mills — they are grouped with the primary producers. However, they do not make iron into steel, as do the integrated mills, but rather melt steel scrap in electric arc furnaces as their source of product.
Charging aisle of a Basic Oxygen Steelmaking Plant showing scrap being charged into the BOF vessel. A ladle full of hot metal is seen to the right.
A ladle of molten steel leaving for the ladle metallurgical facility or the caster.