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steel,alloy of iron, carbon, and small proportions of other elements. Iron contains impurities in the form of silicon, phosphorus, sulfur, and manganese; steelmaking involves the removal of these impurities, known as slag, and the addition of desirable alloying elements.
Steel was first made by cementation, a process of heating bars of iron with charcoal in a closed furnace so that the surface of the iron acquired a high carbon content. The crucible method, originally developed to remove the slag from cementation steel, melts iron and other substances together in a fire-clay and graphite crucible. The famous blades of Damascus and of Toledo, Spain, were made by the cementation and crucible techniques.
The Bessemer processBessemer process
[for Sir Henry Bessemer], industrial process for the manufacture of steel from molten pig iron. The principle involved is that of oxidation of the impurities in the iron by the oxygen of air that is blown through the molten iron; the heat of oxidation raises the
..... Click the link for more information. , the open-hearth process, and the basic oxygen processbasic oxygen process,
method of producing steel from a charge consisting mostly of pig iron. The charge is placed in a furnace similar to the one used in the Bessemer process of steelmaking except that pure oxygen instead of air is blown into the charge to oxidize the impurities
..... Click the link for more information. are more widely used in modern steelmaking. The open-hearth uses a type of furnace called a regenerative furnace; instead of a firebox at one end and a flue at the other, it has devices at each end for the intake and outflow of both fuel and air. The air is preheated by a system of current reversals that causes very high temperatures. This process, developed c.1866 by Sir William Siemens, uses iron ore and pig iron. In the basic oxygen process, or Linz-Donawitz process, developed in the 1950s, the design of the furnace is changed, and oxygen added to the air intake permits more rapid refining of the charge (material in the furnace). The electric-arc furnace is another modern development; it provides a means of making large quantities of high-grade steel, with the advantages of positive temperature control, freedom from contamination of the product by the fuel, and simultaneous deoxidation and desulfurization actions.
Steel is shaped for commercial use in rolling mills, where successive passages of the red-hot ingot between variously shaped rollers give it the desired form. Pittsburgh, one of the world's great steel centers, built its first rolling mill in 1811; Bessemer steel rails were rolled in Chicago as early as 1865.
Types and Uses
Steel is often classified by its carbon content: a high-carbon steel is serviceable for dies and cutting tools because of its great hardness and brittleness; low- or medium-carbon steel is used for sheeting and structural forms because of its amenability to welding and tooling. Alloy steels, now most widely used, contain one or more other elements to give them specific qualities. Aluminum steel is smooth and has a high tensile strength. Chromium steel finds wide use in automobile and airplane parts on account of its hardness, strength, and elasticity, as does the chromium-vanadium variety. Nickel steel is the most widely used of the alloys; it is nonmagnetic and has the tensile properties of high-carbon steel without the brittleness. Nickel-chromium steel possesses a shock resistant quality that makes it suitable for armor plate. Wolfram (tungsten), molybdenum, and high-manganese steel are other alloys. Stainless steel, which was developed in England, has a high tensile strength and resists abrasion and corrosion because of its high chromium content.
See R. M. Brick, Structure and Properties of Alloys (1965); K. Warren, The American Steel Industry, 1850–1970 (1973).
(in Russian, ognivo or kresalo), a piece of iron or steel usually with an oval or rectangular-oval shape, struck against a piece of flint in order to kindle a fire. The steel was widely used from the time iron came into use until matches were invented in the 19th century, although it survived in a number of rural areas in Europe until the early 20th century. The steel, together with the flint and tinder, was carried in a leather pouch attached to the belt.
a workable (malleable) alloy of iron with carbon (up to 2 percent) and other elements. Steel is the most important product of ferrous metallurgy and is the basic material for almost all branches of industry. The scale of steel production characterizes to a considerable extent the technological and economic level of development of a state.
History. Steel as a material used by man has a very long history. The oldest method for production of steel in a spongy state is blooming. Here, iron was reduced from ores by charcoal in hearths (later, small shaft furnaces). To obtain cast steel, early masters used a crucible process, which involved melting small pieces of steel and cast iron in refractory crucibles. Crucible steel was of extremely high quality, but the process was expensive and unproductive. The crucible process, which was used for producing Damascus steels, continued in existence until the beginning of the 20th century, when it was replaced altogether by electric-furnace steelmaking processes. The 14th century saw the development of a process known as bloomery conversion, in which previously produced cast iron was refined in a refinery hearth. (The two-stage process of obtaining cast iron and then converting the iron into steel forms the basis of modern steelmaking processes.) Puddling was first used in the late 18th century; just as in bloomery conversion, the starting material was cast iron, and the product a spongy metal (bloom), but the quality of the metal produced was higher, and the process itself was more efficient. Although puddling played an important role in the development of technology, it was not able to meet the growing demand for steel. Mass production of cast steel became possible only with the development of the Bessemer process and the open-hearth process in the second half of the 19th century and then of the Thomas-Gilchrist process. Electric furnaces for smelting steel were introduced in the late 19th century. The open-hearth process predominated until the mid-20th century, accounting for approximately 80 percent of world output. In the 1950’s, the basic oxygen process was introduced, and its role grew sharply in the years that followed. In addition to these methods for the mass production of steel, other more expensive and less efficient methods are being developed that permit the production of especially pure, high-quality metal. Among them are melting in arc vacuum furnaces, vacuum induction melting, electroslag melting, electron-beam melting, and melting in plasma arc furnaces.
Structure and properties of steel. As the most important material of modern technology, steel must satisfy a wide range of requirements; there is a large number of steel types, which differ in chemical composition, structure, and properties. The principal component of steel is iron. Polymorphism, the capacity of the crystal lattice to undergo a change in structure upon heating and cooling, is characteristic of both iron and steel. Two crystal lattices are known for pure iron, namely, body-centered cubic (α-iron; at higher temperatures, δ-iron) and face-centered cubic (γ-iron). The temperatures at which the allotropic forms undergo reversible transitions (910°C and 1400°C) are referred to as transformation points, and carbon and other components and admixtures of steel can alter these points. The interaction between carbon and the allotropic forms of iron leads to the formation of solid solutions. The solubility of carbon in α-iron is very low, and the solution formed is called ferrite. Practically all the carbon contained in steel dissolves in γ-iron, which exists at higher temperatures (solubility limit of carbon in γ-iron, 2.01 percent); this solution is called austenite. The content of carbon in steel always exceeds carbon’s solubility in α-iron; the excess carbon forms a chemical compound with iron, namely, iron carbide, or cementite (Fe3C). Thus at room temperature, the structure of steel consists of particles of ferrite and cementite, which are present either in the form of individual inclusions (free ferrite and cementite) or as a fine mechanical mixture called pearlite. Information on the temperature and concentration boundaries defining the various phases (ferrite, cementite, pearlite, austenite) constitutes a phase diagram of the Fe-C alloys.
A relatively low strength and hardness and a high ductility and impact strength are characteristic of ferrite. Cementite is brittle but very hard and strong. Pearlite possesses a valuable combination of strength, hardness, ductility, and toughness. The ratio between these phases in the structure of steel is determined mainly by the content of carbon; it is the different properties of these phases also that account for such a great variety of properties of steel. Thus, steel containing ∼0.1 percent carbon (ferrite predominating in the structure) is characterized by high ductility; steel of this type is used for the production of thin sheets, from which parts of automobile bodies and other parts with complex shapes are formed. Steel with a carbon content of ∼0.6 percent usually has a pearlite structure; possessing high strength and hardness with sufficient ductility and toughness, it finds use in, for example, railroad rails, wheels, and axles. If steel contains approximately 1 percent carbon, particles of free cementite are present in addition to pearlite; the great hardness possessed by this steel after the hardening process makes the steel suitable for use in tools. The range of properties of steel is broadened through alloying, as well as through heat treatment, thermo-chemical treatment, and thermomechanical treatment. Thus, in the hardening of steel, the metastable martensite phase forms, which is a supersaturated solid solution of carbon in α-iron characterized by high hardness but also by high brittleness. By combining hardening with tempering the required combination of hardness and ductility may be imparted to steel.
Classification. Modern metallurgy involves the smelting of cast iron and steel scrap. Depending on the type of furnace vessel (basic oxygen furnace, open-hearth furnace, electricarc furnace), steel can be classified as, for example, basic oxygen steel, open-hearth steel, or electric steel. In addition, a distinction is made between metals smelted in furnaces having basic or acid linings; steels are correspondingly classified as basic or acid, for example, acid open-hearth steel.
On the basis of chemical composition, steels are divided into carbon and alloy steels. Carbon steels contain, in addition to Fe and C, Mn (0.1–1.0 percent) and Si (up to 0.4 percent), as well as the harmful impurities S and P. These elements, which are introduced into the steel during the production process, come mainly from the charge materials. Depending on the carbon content, distinctions are made between low-carbon (up to 0.25 percent C), medium-carbon (0.25–0.6 percent) and high-carbon (more than 0.6 percent) steels. Alloy steels contain, in addition to the components given above, such alloying elements as Cr, Ni, Mo, W, V, Ti, Nb, Zr, and Co, which are added to improve the steel’s fabricating and performance characteristics or to impart special properties. Mn (content of more than 1 percent) and Si (more than 0.8 percent) may also serve as alloying elements. Based on the extent of alloying, that is, on the total content of the alloying elements, distinctions are made between low-alloy (less than 2.5 percent), medium-alloy (2.5–10 percent), and high-alloy (more than 10 percent) steels. Alloy steels are often named according to the predominant component, for example, tungsten, high-chromium, chromium-molybdenum, chromium-nickel-manganese, and chromium-nickel-molybdenum-vanadium steels.
Depending on their use, steels are classified as structural, tool, or special. Structural steels are used in construction, as well as in the manufacture of machine and mechanism parts, boilers, and bodies of ships and railroad cars. They may be carbon steels (up to 0.7 percent C) or alloy steels (with Cr and Ni the principal alloying elements). The names of structural steels sometimes reflect the intended use, as seen in boiler, valve, spring, shipbuilding, gun, and artillery-shell steels and in armor plate. Tool steel is used in the production of cutters, milling cutters, punches, gauges, and other metal-cutting, impact, and measuring tools and instruments. Tool steels may also be carbon steels (usually 0.8–1.3 percent C) or alloy steels (chiefly with Cr, Mn, Si, W, Mo, V). Highspeed steel is one of the most widely used tool steels. Special steels include electrical steels, stainless steels, acid-resistant steels, scale-resistant steels, heat-resistant steels, and the steels used in permanent magnets. Many steels of this group have a low carbon content and a high degree of alloying.
Based on quality, distinctions are made between commercial, quality, high-quality, and very high quality steels. The differences between these grades derive from the amounts of harmful impurities (S and P) and nonmetallic inclusions. Thus, in certain commercial steels, the S content may reach 0.055–0.06 percent, and the P content 0.05–0.07 percent (with free-cutting steel an exception, containing up to 0.3 percent S and up to 0.16 percent P). In quality steels, no more than 0.035 percent of each of these elements is permitted, and in high quality steels, no more than 0.025 percent. Very high quality steels must contain less than 0.015 percent S. Sulfur adversely affects the mechanical properties of steel and is the cause of red-shortness, that is, brittleness in the heated state, while phosphorus increases the tendency of the metal to become cold-short, that is, brittle at reduced temperatures.
Depending on the mode of cooling the metal in the mold, distinctions are made between killed, semikilled, and rimmed steels. The behavior of the metal during crystallization is explained by the degree of deoxidation; the more completely oxygen is removed from the steel, the more quietly the solidification proceeds. Upon pouring steel that is only slightly deoxidized, there is a marked evolution of carbon monoxide gas, and the steel appears to be boiling in the mold; solidification results in rimmed steel. Semikilled steel occupies a position intermediate between killed and rimmed steels. Each of these types of metal has advantages and disadvantages; the selection of the method of deoxidation and pouring is dictated by the steel’s intended use and the engineering and economic factors governing the production process.
Marking of steel. There is no single worldwide system for marking steel. In the USSR, much work has been done on standardizing the designations used for the various types of steel, work reflected in state standards and specifications. The grades of commercial carbon steel are indicated by the letters “St” and a number (StO, ST1, ST2). Quality carbon steels are marked with a number that indicates the mean content of carbon in hundredths of a percent (05, 08, 10, 25, 40). Killed steels are sometimes marked with the letters “sp,” semikilled steels with “ps,” and rimmed steels with “kp” (St3sp, St5ps, 08kp). The letter “G” indicates a high content of Mn (14G, 18G). Free-cutting steels are marked with the letter “A” (A12, A30), and carbon tool steels with the letter “U” (U8, U10, U12, with the numbers indicating the content of carbon in tenths of a percent).
Alloy steels are marked with letters indicating the components present and numbers giving the mean content of these components. In the USSR, an arbitrary system of designations for the chemical composition of steel has been adopted. Here, Iu designates aluminum, R boron, F vanadium, V tungsten, K cobalt, S silicon, G manganese, D copper, M molybdenum, N nickel, B niobium, T titanium, U carbon, P phosphorus, Kh chromium, and Ts zirconium. The first integers in the marking give the mean content of carbon (in hundredths of a percent for structural steels and in tenths of a percent for tool and stainless steels); a letter or letters indicating the alloying elment then follows, and the numbers appearing thereafter give the mean content of this element. For example, steel marked 3Khl3 contains 0.3 percent carbon and 13 percent chromium, while that marked 2Khl7N2 contains 0.2 percent carbon, 17 percent chromium, and 2 percent nickel. When the content of an alloying element is less than 1.5 percent, numbers do not appear after the letter or letters designating the element. Thus, steel marked 12KhN3A contains less than 1.5 percent chromium. The letter “A” at the end of the marking indicates that the steel is high-quality; the letters “Sh” indicate very high quality. The markings of certain alloy steels include a letter or letters indicating the steel’s intended use. For example, ShKh9 is a steel with 0.9–1.2 percent chromium, designed for use in ball bearings, and E3 is an electrical steel with 3 percent silicon. Steels that have undergone industrial testing are often marked with the letters “El” or “EP” (ElektrostaP Works), “DI” (Dne-prospetsstal’ Works), and “ZI” (Zlatoust Works), with the corresponding serial number (EI268).
REFERENCESStaleplaviinoe proizvodstvo: Spravochnik, vols. 1–2. Edited by A. M. Samarin. Moscow, 1964.
Mes’kin, V. S. Osnovy legirovaniia stali, 2nd ed. Moscow, 1964.
Houdremont, E. Spetsial’nyestali, 2nd ed., vols. 1–2. Moscow, 1966. (Translated from German.)
Dröge, V. Stal’ kak konstruktsionnyi material. Moscow, 1967. (Translated from German.)
Guliaev, A. P. Chistaia stal’. Moscow, 1975.