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Asteel mill orsteelworks is anindustrial plant for themanufacture ofsteel. It may be an integrated steel works carrying out all steps ofsteelmaking from smeltingiron ore to rolled product, but may also be a plant where steelsemi-finished casting products are made from moltenpig iron or fromscrap.
Since the invention of theBessemer process, steel mills have replacedironworks, based onpuddling orfining methods. New ways to produce steel appeared later: from scrap melted in anelectric arc furnace and, more recently, fromdirect reduced iron processes.
In the late 19th and early 20th centuries the world's largest steel mill was theBarrow Hematite Steel Company steelworks located inBarrow-in-Furness,United Kingdom. Today, the world's largest steel mill is inGwangyang,South Korea.[1][2]
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 ablast 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 intopig iron or directed to other vessels for further steel making operations. Historically theBessemer process was a major advancement in the production of economical steel, but it has now been entirely replaced by other processes such as thebasic oxygen furnace.
Molten steel is cast into large blocks calledblooms. During the casting process various methods are used, such as addition ofaluminum, 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 steel making 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 occasionallylong products such asbars andpipe.
A majorenvironmental hazard associated with integrated steel mills is the pollution produced in the manufacture ofcoke, 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.
A minimill is traditionally a secondary steel producer; however,Nucor (one of the world's largest steel producers) and Commercial Metals Company (CMC) use minimills exclusively. Usually it obtains most of its iron fromscrap 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 anelectric arc furnace for scrap melting, aladle furnace orvacuum furnace for precision control of chemistry, a strip or billetcontinuous caster for converting molten steel to solid form, areheat furnace and arolling mill.
Originally the minimill was adapted to production of bar products only, such asconcretereinforcing bar, flats, angles, channels, pipe, and light rails. Since the late 1980s, successful introduction of thedirect strip casting process has made minimill production of strip feasible. Often a minimill will be constructed in an area with no other steel production, to take advantage of local markets, resources, or lower-cost labour. Minimill plants may specialize, for example, in making coils of rod forwire-drawing use, or pipe, or in special sections for transportation and agriculture.
Capacities of minimills 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 four sheet mills, 6,700,000 tons of bar steel from its 10 bar mills and 2,100,000 tons of plate steel from its two plate mills.
Since the electric arc furnace can be easily started and stopped on a regular basis, minimills 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.
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