STEEL

an alloy of iron and carbon in which the carbon content amounts to about 2 percent or less. The addition of this tiny amount of carbon results in a substance that exhibits great strength, hardness, and other valuable mechanical properties, and these, together with its low manufacturing cost and abundant source materials (iron ore and scrap), make steel the most widely used of structural metals. It is used, for instance, in the construction of high-rise buildings and bridges, in automobiles and other vehicles, and in most household products ranging from large appliances and hardware to flatware and cooking utensils. A brief treatment of steel follows. For full treatment of the production and commercial applications of steel, see Industries, Extraction and Processing: Steel. Steel owes its diverse properties to the allotropy, or changeable crystalline structure, of iron. As iron cools below the melting temperature of 1,538 C (2,800 F), its atoms align themselves into a body-centred-cubic (bcc) structurei.e., they form cubes with an atom at each corner and one in the cube's centre. In this bcc form, iron is known as ferrite. When the iron cools below 1,394 C (2,541 F), it becomes austenite, which has a face-centred-cubic (fcc) structure characterized by an iron atom at each corner and one at the centre of each of the six faces of the cube. Below 912 C (1,674 F), the atoms revert to the ferritic structure. Pure iron is about as soft as copper and is of little structural value, but it is greatly hardened by the addition of small amounts of carbon. (Steel contains up to 2 percent carbon; higher carbon content creates cast iron, a metal of very different properties.) Carbon remains dissolved more readily in austenite than in ferrite because there is more room in the former's crystalline structure for carbon atoms to lodge in the interstices, or gaps, between the iron atoms. As steel with a carbon content of about 0.77 percent is cooled past the austenite zone and assumes the ferritic form, the excess carbon atoms in it may precipitate as iron carbide (also known as cementite) and form pearlite, a harder steel that consists of alternating platelets of ferrite and cementite. The use of a somewhat higher carbon contente.g., 1.05 percentresults in an even harder steel that consists of islands of pearlite in a continuous network of cementite. Adjusting the carbon content is thus the simplest way to change the hardness of steel, since the many variations of steel's basic microstructures depend on that content, as well as on the rate at which the steel is cooled. Each particular microstructure has its specific degrees of hardness, strength, toughness, corrosion resistance, and electrical resistivity. The properties of a steel may be further modified by heat treating, by mechanically working it at cold or hot temperatures, or by adding other alloying elements besides carbon. These alloys include manganese, silicon, nickel, chromium, molybdenum, vanadium, tungsten, niobium, and zirconium. Such elements are added during the refining process to give special properties such as superior strength, hardness, durability, or corrosion resistance. Based on the quantity of alloying elements, steels can be grouped into three major classes: the carbon steels, low-alloy steels, and high-alloy steels. Carbon steels are the most widely used and account for about 90 percent of the world's steel production. They contain from less than 0.015 to slightly more than 2 percent carbon. Carbon steels are used extensively for automobile bodies, appliances, machinery, ships, containers, and the structures of buildings. Low-alloy steels, which contain up to 8 percent alloying elements, are exceptionally strong and are used for machine parts, aircraft landing gear, shafts, hand tools, gears, and the structural members of buildings and bridges. High-alloy steels, which contain more than 8 percent alloying elements, are prized for their unusual properties. Typical of these are the stainless steels, which are used when resistance to corrosion and oxidation are required, as in jet-engine parts, chemical equipment, tableware and cooking utensils, and cutting tools. Stainless steels contain from 16 to 26 percent chromium and up to 35 percent nickel. In principle, steelmaking is a melting, purifying (refining), and alloying process carried out at approximately 1,600 C (2,900 F) under molten conditions. Steel is obtained by refining pig iron or scrap steel in a furnace, and then using further refining in a number of steps to produce a metal with desired specific characteristics. Steel can be produced in basic-oxygen, open-hearth, or electric-arc furnaces. Basic-oxygen and open-hearth furnaces make steel from liquid (i.e., molten) blast-furnace iron and varying quantities of scrap steel, while electric-arc furnaces remelt scrap steel or refine briquettes of direct-reduced iron ore. Refining consists essentially of burning off excess carbon and such impurities as silicon, manganese, and phosphorus, which separate from the molten steel as gases or as molten slag. Refining is also the stage at which desired alloying elements are added. The molten steel is tapped into a ladle, where further refining is carried outmost importantly, the removal of dissolved gases and the addition of further alloying elements. From there the molten steel can be poured into molds for solidification into ingots; these can later be reheated and rolled into semifinished shapes such as blooms or slabs, which are in turn worked into finished products such as bars, sheets, tubes, rods, or wire. Some of the steps taken in ingot pouring can be saved by continuous casting, in which molten steel is tapped from the ladle into a mold, extracted as a long strand, and immediately cut, worked, and treated in one continuous operation. Forming semifinished steel into the finished shapes that are used in manufacturing and construction is done by two major methods, called hot- and cold-working. Hot-working consists primarily of hammering, pressing, rolling, and extruding the steel under conditions of high heat. Hammering and pressing are together called forging, which dates back to the earliest age of steelmaking. Hammering was first done by hand, later by water power, and now by steam-powered hammers. Pressing is accomplished by a hydraulic forging press that relies on pressure to force a die down on a plate of steel. Extrusion relies on forcing the hot steel through a chamber at one end of which is a die with an opening that is the desired shape. A very large force is required to force the steel through the opening. Rolling (q.v.) is the most widely used method for shaping steel. Basically, it consists of passing the steel between two rollers revolving at the same speed but in opposite directions. The temperature of the rolling steel, the distance between the rollers and the pressure exerted by them, and the velocity of the rolling process determines the result. Cold-working, which includes rolling, extrusion, and drawing (pulling steel through a die rather than pushing it, as in extrusion), is generally used to make bars, wire, tubes, sheets, and strips where better machinability, particular size accuracy, and a bright surface are desired. Thinner gauges of steel can be attained more economically by cold-working. Cold-rolling normally follows hot-rolling to yield a more highly refined product. Molten steel can also be cast directly into products as diverse as small valves or huge turbine blades. A number of steel products, particularly those made from sheets, are given a corrosion-resistant coating through chrome-plating, galvanizing (coating with zinc), or tinplating. alloy of iron and carbon in which the carbon content ranges up to 2 percent (with a higher carbon content, the material is defined as cast iron). By far the most widely used material for building the world's infrastructure and industries, it is used to fabricate everything from sewing needles to oil tankers. In addition, the tools required to build and manufacture such articles are also made of steel. As an indication of the relative importance of this material, in 1997 the world's steel production was about 795 million tons, while production of the next most important engineering metal, aluminum, was about 21 million tons. The main reasons for the popularity of steel are the relatively low cost of making, forming, and processing it, the abundance of its two raw materials (iron ore and scrap), and its unparalleled range of mechanical properties. Additional reading Comprehensive and up-to-date information on many aspects of metallurgy, individual metals, and alloys can be found in convenient reference-form arrangement in the following works: Metals Handbook, 9th ed., 17 vol. (197889), a massive and detailed source prepared under the direction of the American Society for Metals, with a 10th edition that began publication in 1990; Herman F. Mark et al. (eds.), Encyclopedia of Chemical Technology, 3rd ed., 31 vol. (197884), formerly known as Kirk-Othmer Encyclopedia of Chemical Technology, with a 4th edition begun in 1991; and its European counterpart, the first English-language edition of a monumental German work, Ullmann's Encyclopedia of Industrial Chemistry, 5th, completely rev. ed., edited by Wolfgang Gerhartz et al. (1985 ). The Editors of the Encyclopdia Britannica Information on the properties of a wide range of steels is given in R.W.K. Honeycombe, Steels: Microstructures and Properties (1981). William T. Lankford, Jr., et al. (eds.), The Making, Shaping, and Treating of Steel, 10th ed. (1985), provides a comprehensive survey of all steelmaking technologies; for descriptions of the important liquid steelmaking processes in a more compact form, see C. Moore and R.I. Marshall, Modern Steelmaking Methods (1980). The theory, design, and operation of basic oxygen methods are discussed in R.D. Pehlke et al. (eds.), BOF Steelmaking, 5 vol. (197477). Clarence E. Sims (ed.), Electric Furnace Steelmaking, 2 vol. (196263), discusses all aspects of this method. Charles R. Taylor (ed.), Electric Furnace Steelmaking (1985); and International Iron And Steel Institute Committee On Technology, The Electric Arc Furnace, 1990 (1990), cover more recent developments in the field. Secondary steelmaking methods are discussed in R.J. Fruehan, Ladle Metallurgy Principles and Practices (1985). For information on all aspects of an important solidification method, see Continuous Casting (1983 ), published by the Iron & Steel Society of the American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME). All important rolling methods are covered in Vladimir B. Ginzburg, Steel-Rolling Technology: Theory and Practice (1989). The best English-language books covering all aspects of surface treating and heat treating are relevant volumes of Metals Handbook, cited above.Major products of the industry are covered in the Steel Products Manual (irregular), published by the American Iron and Steel Institite of the AIME. Conference proceedings and periodical publications of societies and institutions all over the world concerned specifically with steelmaking, steel products, and use of steel provide detailed state-of-the-art informationfor example, Steel Technology International (annual); Steel Today and Tomorrow (five times a year); and Steel Times International (bimonthly). Edward F. WenteK.C. Barraclough, Steelmaking Before Bessemer, 2 vol. (1984), is a history of steelmaking prior to 1850, focusing on blister steel and crucible steel, and Steelmaking: 18501900 (1990), is an account of the development of the Bessemer and open-hearth processes. Henry Bessemer, Sir Henry Bessemer, F.R.S.: An Autobiography (1905, reprinted 1989), provides relevant information in Bessemer's own account of his life and work. There is no specific historical work on post-1950 developments in oxygen steelmaking, but a concise account of these newer processes is offered in C. Moore and R.I. Marshall, Steelmaking (1991). Relevant information is found in the materials of Historical Metallurgy (semiannual), the journal of the British Historical Metallurgy Society. Jack Nutting Casting of steel Ingot pouring Liquid steel must be cooled and shaped into forms suitable for handling and further processing. The simplest way to solidify liquid steel is to pour it into heavy, thick-walled iron ingot molds, which stand on stout iron plates called stools. Solidification processes Ingot solidification. Ingot solidification. During and after pouring, the walls and bottom of the mold extract heat from the melt, and a solid shell forms, growing approximately with the square root of time multiplied by a constant. The value of the constant depends on the heat flux between the already solidified shell and the cooling media surrounding it and is actually equivalent to the solidified shell's thickness after one minutenamely, about 20 millimetres when solidifying steel. Accordingly, the ingot shell is about 40 millimetres thick after four minutes and 60 millimetres after nine minutes. As the shell thickens, the level of the liquid melt in the centre of the mold drops, because solidified steel has a higher density than liquid steeli.e., 7.86 versus 7.06 grams per cubic centimetre (4.5 versus 4.1 ounces per cubic inch). This creates a cavity on top of the ingot, as shown in A in the figure by a schematic presentation of solidifying layers. Since an open cavity oxidizes, it does not weld during hot rolling and must be cut off, resulting in a loss of steel. The cavity can be made shallower by keeping the top of the ingot hot and liquid longer. This is done by inserting insulating refractory heads (as shown in C in the figure) and by adding exothermic powders; more liquid steel can also be added after a good-sized shell has formed. Ingot solidification. Ingot solidification. The solidification pattern described above can be observed in well-deoxidized steel, which shows no evolution of gas as it solidifies. For this reason, it is called a killed steel. A different solidification pattern is applied to certain other steels to which fewer deoxidizers have been added. These contain a controlled amount of dissolved oxygen, which, during solidification, reacts with carbon and generates a mild carbon monoxide boil. The rising carbon monoxide bubbles stir the melt, lift inclusions, and cause the formation of a very clean shell about 50 millimetres thick, called the rim. After the rim has formed, a cooling plate is placed on top of the ingot, freezing a layer of liquid steel and trapping the gas bubbles inside the solidifying ingot, as shown in B in the figure. This ingot has no open cavity, but there are many blowholes in the centre that normally weld together during hot-rolling. Low-carbon steel, because of its higher dissolved oxygen content, is often cast this way and is called rimmed steel. Normally, rimmed steel is cast into a big-end-down mold, as shown in B in the figure, for easier mold stripping and ingot handling. An important characteristic of all solidification processes is segregation. This takes place when crystals grow in a multicomponent melt, because crystals are always purer than the liquid melt from which they solidify. Therefore, as steel solidifies, the levels of carbon, phosphorus, and sulfur grow in the remaining liquid, resulting in an enrichment of these elements in the centre of the ingot. Segregation can be minimized by keeping segregating elements at low levels or by solidifying at a fast ratei.e., by not providing the time for separation. It is also impaired by stirring the melt.