Steel manufacture

A sequence of operations in which pig iron and scrap steel are processed to remove impurities and are separated into the refined metal and slag.

Reduction of iron ores by carbonaceous fuel directly to a steel composition was practiced in ancient times, but liquid processing was unknown until development of the crucible process, in which iron ore, coal, and flux materials were melted in a crucible to produce small quantities of liquid steel. Modern steelmaking processes began with the invention of the airblown converter by H. Bessemer in 1856. The Thomas process was developed in 1878; it modified the Bessemer process to permit treatment of high-phosphorus pig iron. The Siemens-Martin process, also known as the open-hearth process, was developed at about the same time. The open-hearth process utilizes regenerative heat transfer to preheat air used with a burner; it can generate sufficient heat to refine solid steel scrap and pig iron in a reverberatory furnace. After World War II, various oxygen steelmaking processes were developed.

Steelmaking can be divided into acid and basic processes depending upon whether the slag is high in silica (acid) or high in lime (basic). The furnace lining in contact with the slag should be a compatible material. A silica or siliceous material is used in acid processes, and a basic material such as burned dolomite or magnesite is used in basic processes. Carbon, manganese, and silicon, the principal impurities in pig iron, are easily oxidized and separated; the manganese and silicon oxides go into the slag, and the carbon is removed as carbon monoxide and carbon dioxide in the off-gases. Phosphorus is also oxidized but does not separate from the metal unless the slag is basic. Removal of sulfur occurs to some extent by absorption in a basic slag. Thus, the basic steelmaking processes are more versatile in terms of the raw materials they can handle, and have become the predominant steelmaking processes.

A typical pig iron charged to the steelmaking process might contain roughly 4% carbon, 1% manganese, and 1% silicon. The phosphorus and sulfur levels in the pig iron vary. The composition of the steel tapped from the steelmaking furnace generally ranges from 0.04 to 0.80% carbon, 0.06 to 0.30% manganese, 0.01 to 0.05% phosphorus, and 0.01 to 0.05% sulfur, with negligible amounts of silicon.

Electric arc furnace technology began late in the nineteenth century with the original design of P. L. T. Heroult. The three-graphite electrode furnace with a swinging roof for top charging and a rocker base for tilting to tap the finished molten steel has been continuously improved and developed further.

The rapid development of steelmaking technology using the electric arc furnace, not only for alloy and stainless steels but especially for carbon steel production, has increased its share of production capacity to about 20% of the steel industry.

Remelting and refining of special alloys are carried out in duplex or secondary processes; the principal ones are argon-oxygen decarburization, electroslag refining, vacuum arc remelting, and vacuum induction melting.

Ladle metallurgy was used first to produce high-quality steels, but has been extended to producing many grades of steel because of the economic advantages of higher productivity. The purpose of these ladle treatments is to produce clean steel; introduce reactive additions, such as calcium or rare earths; add alloying additions, as for microalloyed steels, with high recovery; and increase furnace utilization, allowing higher-productivity smelting operations of the blast furnace, and melting and refining operations in steelmaking. Ladle treatments in steel production generally are classified as synthetic slag systems; gas stirring or purging; direct immersion of reactants, such as rare earths; lance injection of reactants; and wire feeding of reactants. These are often used in combination to produce synergistic effects, for example, synthetic slag and gas stirring for desulfurization followed by direct immersion, injection, or wire feeding for inclusion shape control.