A steel mill or steelworks is an industrial plant for the manufacture of steel.
Steel is an alloy of iron and carbon. It is produced in a two-stage process. First, iron ore is reduced or smelted with coke and limestone in a blast furnace, producing molten iron which is either cast into pig iron or carried to the next stage as molten iron. In the second stage, known as steelmaking, impurities such as sulfur, phosphorus, and excess carbon are removed and alloying elements such as manganese, nickel, chromium and vanadium are added to produce the exact steel required. In the late 19th Century and early 20th Century the world's largest steel mill was located in Barrow-in-Furness, UK. Today, the world's largest steel mill is in Gwangyang, South Korea.[1][2]
Steel mills turn molten steel into blooms, ingots, slabs and sheet through casting, hot rolling and cold rolling.
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An integrated steel mill has all the functions for primary steel production:
The principal raw materials for an integrated mill are iron ore, limestone, and coal (or coke). These materials are charged in batches into a blast furnace where the iron compounds in the ore give up excess oxygen and become liquid iron. At intervals of a few hours, the accumulated liquid iron is tapped from the blast furnace and either cast into pig iron or directed to other vessels for further steelmaking operations. Historically the Bessemer process was a major advancement in the production of economical steel, but it has now been entirely replaced by other processes such as the basic oxygen furnace.
Molten steel is cast into large blocks called "blooms". During the casting process various methods are used, such as addition of aluminum, so that impurities in the steel float to the surface where they can be cut off the finished bloom.
Because of the energy cost and structural stress associated with heating and cooling a blast furnace, typically these primary steelmaking vessels will operate on a continuous production campaign of several years duration. Even during periods of low steel demand, it may not be feasible to let the blast furnace grow cold, though some adjustment of the production rate is possible.
Integrated mills are large facilities that are typically only economical to build in 2,000,000 ton per year annual capacity and up. Final products made by an integrated plant are usually large structural sections, heavy plate, strip, wire rod, railway rails, and occasionally long products such as bars and pipe.
A major environmental hazard associated with integrated steel mills is the pollution produced in the manufacture of coke, which is an essential intermediate product in the reduction of iron ore in a blast furnace.
Integrated mills may also adopt some of the processes used in mini-mills, such as arc furnaces and direct casting, to reduce production costs.
World integrated steel production capacity is at or close to world demand, so competition between suppliers results in only the most efficient producers remaining viable. However, due to the large employment of integrated plants, often governments will financially assist an obsolescent facility rather than take the risk of having thousands of workers thrown out of jobs.
A mini-mill is traditionally a secondary steel producer; however, Nucor (one of the world's largest steel producers), as well as one of its competitors, Commercial Metals Company (CMC) use mini-mills exclusively. Usually it obtains most of its iron from scrap steel, recycled from used automobiles and equipment or byproducts of manufacturing. Direct reduced iron (DRI) is sometimes used with scrap, to help maintain desired chemistry of the steel, though usually DRI is too expensive to use as the primary raw steelmaking material. A typical mini-mill will have an electric arc furnace for scrap melting, a ladle furnace or vacuum furnace for precision control of chemistry, a strip or billet continuous caster for converting molten steel to solid form, a reheat furnace and a rolling mill.
Originally the mini-mill concept was adapted to production of bar products only, such as concrete reinforcing bar, flats, angles, channels, pipe, and light rails. Since the late 1980s, successful introduction of the direct strip casting process has made mini-mill production of strip feasible. Often a mini-mill will be constructed in an area with no other steel production, to take advantage of local resources and lower-cost labour. Mini-mill plants may specialize, for example, making coils of rod for wire-drawing use, or pipe, or in special sections for transportation and agriculture.
Capacities of mini-mills vary; some plants may make as much as 3,000,000 tons per year, a typical size is in the range 200,000 to 400,000 tons per year, and some old or specialty plants may make as little as 50,000 tons per year of finished product. Nucor Corporation, for example, annually produces around 9,100,000 tons of sheet steel from its 4 sheet mills, 6,700,000 tons of bar steel from its 10 bar mills and 2,100,000 tons of plate steel from its 2 plate mills.
Since the electric arc furnace can be easily started and stopped on a regular basis, mini-mills can follow the market demand for their products easily, operating on 24 hour schedules when demand is high and cutting back production when sales are lower.
McGannon, Harold E. editor (1971). The Making, Shaping and Treating of Steel: Ninth Edition. Pittsburgh, Pennsylvania: United States Steel Corporation