STEEL DRUM

tuned gong made from the unstoppered end and part of the wall of a metal shipping drum. The end surface is hammered concave, and several areas are outlined by acoustically important chiseled grooves. It is heated and tempered, and bosses, or domes, are hammered into the outlined areas. The depth, curvature, and size of each boss determine its pitch. The drums are struck with rubber-tipped hammers. Steel drums originated in Trinidad, in the West Indies, in the 20th century and are played in ensembles, or steel bands, of about 4 to 100 performers. Drums are commonly made in four sizes from bass to treble, called boom, cellopan, guitar pan, and ping pong. Forming of steel Principles Forming processes convert solidified steel into products useful for the fabricating and construction industries. The objectives are to obtain a desired shape, to improve cast steel's physical properties (which are not suitable for most applications), and to produce a surface suitable for a specific use. During plastic forming, the large crystals in cast steel are converted into many small, long crystals, transforming the usually brittle cast into a ductile and tough steel. In order to accomplish this, it is often necessary to reduce the cross section of a cast structure to one-eighth or even less of its original. The major forming processes are carried out hot, at about 1,200 C (2,200 F), because of steel's low resistance to plastic deformation at this temperature. This requires the use of reheating furnaces of different designs. Cold forming is often applied as a secondary process for making special steel products such as sheet or wire. Gap between two rolls, showing reduction and elongation of workpiece (see text). There are a number of steel-forming processesincluding forging, pressing, piercing, drawing, and extrudingbut by far the most important one is rolling. In this process, the rolls, working always in pairs, are driven in opposite directions with the same peripheral velocity and are held at a specific distance from each other by heavy bearings and mill housings. The steel workpiece is pulled by friction into the roll gap, which is smaller than the cross section of the workpiece, so that both rolls exert a pressure and continuously form the piece until it leaves the roll gap with a smaller section and increased length. As shown in the figure, the reduction in cross section is calculated by subtracting the out-section (S2) from the in-section (S1) and then dividing by S1. Assuming the workpiece maintains its original volume as it is formed, the elongation (L2) divided by the original length (L1) equals S1 divided by S2. When rolling flat products, there is not much change in width, so that the thickness alone can be used to calculate reduction. Two basic rolling-mill designs. Two basic rolling-mill designs. Two-high, three-high, four-high, six-high, cluster, and planetary roll arrangements. The basic principles of a rolling-mill design are shown in B in the figure. Two heavy bearings mounted on each side of a roll sit in chocks, which slide in a mill housing for adjusting the roll gap with a screw. The two housings are connected to each other and to the foundation, and the complete assembly is called a roll stand. There are also compact rolling units (C in the figure), which do not have housings; often used in the tandem rolling of long products, they can be exchanged quickly for repair or for a change in the rolling program. Rolls are driven through spindles and couplings, either directly or via a gear, by one or several electric motors. Depending on the product rolled, there are stands that have two, three, four, and more rolls; accordingly, they are given the names two-high, three-high, four-high, six-high, cluster mill, and planetary mill (schematically shown in the figure). For rolling strip, heavy backup rolls support the smaller work rolls, because thin rolls form flat material better than do large-diameter rolls. Two rolling-mill arrangements. Two rolling-mill arrangements. In a rolling shop, stands are arranged according to three layout principles. One is called the open train (G in the figure), in which the stands are arranged side by side, often driven by the same motor and linked by spindles. This arrangement is applied only to the rolling of long products, with guides or cross-transfers being used to move the workpiece from stand to stand. A tandem mill arrangement (H in the figure) has one stand behind the other and is used for high-production rolling of almost all products. This continuous arrangement requires the construction of long rolling trains and buildings, but layouts can be shortened by a so-called semicontinuous mill, in which the workpiece is passed back and forth through a reversing mill before being sent through the rest of the line. When open-train and tandem arrangements are combined for rolling long products in more compact layouts, it is called a cross-country mill. Slabs and blooms Cast ingots, sometimes still hot, arrive at slabbing and blooming mills on railroad cars and are charged upright by a special crane into under-floor soaking pits. These are gas-fired rectangular chambers, about 5 metres deep, in which four to eight ingots are simultaneously heated to about 1,250 C (2,300 F). An ingot used for conversion into a slab can be 1.5 metres wide, 0.8 metre thick, and 2.5 metres high and can weigh 23 tons. The soaking pits are highly computerized for scheduling, firing rates, heating times (which can last 8 to 18 hours), and rolling programs. After heating, a tiltable transfer buggy brings a hot ingot to a two-high reversing mill, which takes one pass after another, reversing the rolls and roller table each time the ingot has passed through. Because each pass reduces the slab by only about 50 millimetres, it may take 21 passes, including several edge passes with the slab standing upright on its edges, to obtain a slab measuring 0.2 metre thick, 1.5 metres wide, and 10 metres long. The rolls usually have a diameter of about 1.2 metres; each is driven by one or two electric motors totaling 7,000 to 12,000 horsepower. The two roller tables, situated in front and in back of the stand, have movable manipulators that guide the slab into the rolls and turn it onto its edges when required. High-pressure water nozzles remove surface scale, and a crop-shear discards the ends and cuts the slab into proper length. Some slabbing mills place a pair of heavy vertical rolls next to the horizontal rolls for edge rolling; this avoids the time-consuming turning of the slab into an upright position. Such an arrangement is called a universal mill. For making long products, blooms some 250 millimetres square are rolled from ingots in a similar fashion on the same type of mill. History The steel industry has grown from ancient times, when a few men may have operated, periodically, a small furnace producing 10 kilograms, to the modern integrated iron- and steelworks, with annual steel production of about 1 million tons. The largest commercial steelmaking enterprise, Nippon Steel in Japan, was responsible for producing 26 million tons in 1987, and 11 other companies generally distributed throughout the world each had outputs of more than 10 million tons. Excluding the Eastern-bloc countries, for which employment data are not available, some 1.7 million people were employed in 1987 in producing 430 million tons of steel. That is equivalent to about 250 tons of steel per person employed per yeara remarkably efficient use of human endeavour. Primary steelmaking Early iron and steel Iron production began in Anatolia about 2000 BC, and the Iron Age was well established by 1000 BC. The technology of iron making then spread widely; by 500 BC it had reached the western limits of Europe, and by 400 BC it had reached China. Iron ores are widely distributed, and the other raw material, charcoal, was readily available. The iron was produced in small shaft furnaces as solid lumps, called blooms, and these were then hot forged into bars of wrought iron, a malleable material containing bits of slag and charcoal. The carbon contents of the early irons ranged from very low (0.07 percent) to high (0.8 percent), the latter constituting a genuine steel. When the carbon content of steel is above 0.3 percent, the material will become very hard and brittle if it is quenched in water from a temperature of about 850 to 900 C (1,550 to 1,650 F). The brittleness can be decreased by reheating the steel within the range of 350 to 500 C (660 to 930 F), in a process known as tempering. This type of heat treatment was known to the Egyptians by 900 BC, as can be judged by the microstructure of remaining artifacts, and formed the basis of a steel industry for producing a material that was ideally suited to the fabrication of swords and knives. The Chinese made a rapid transition from the production of low-carbon iron to high-carbon cast iron, and there is evidence that they could produce heat-treated steel during the early Han dynasty (206 BCAD 25). The Japanese acquired the art of metalworking from the Chinese, but there is little evidence of a specifically Japanese steel industry until a much later date. The Romans, who have never been looked upon as innovators but more as organizers, helped to spread the knowledge of iron making, so that the output of wrought iron in the Roman world greatly increased. With the decline of Roman influence, iron making continued much as before in Europe, and there is little evidence of any change for many centuries in the rest of the world. However, by the beginning of the 15th century, waterpower was used to blow air into bloomery furnaces; as a consequence, the temperature in the furnace increased to above 1,200 C (2,200 F), so that, instead of forming a solid bloom of iron, a liquid was produced rich in carboni.e., cast iron. In order to make this into wrought iron by reducing the carbon content, solidified cast iron was passed through a finery, where it was melted in an oxidizing atmosphere with charcoal as the fuel. This removed the carbon to give a semisolid bloom, which, after cooling, was hammered into shape. Primary steelmaking Principles In principle, steelmaking is a melting, purifying, and alloying process carried out at approximately 1,600 C (2,900 F) in molten conditions. Various chemical reactions are initiated, either in sequence or simultaneously, in order to arrive at specified chemical compositions and temperatures. Indeed, many of the reactions interfere with one another, requiring the use of process models to help in analyzing options, optimizing competing reactions, and designing efficient commercial practices. Raw materials The major iron-bearing raw materials for steelmaking are blast-furnace iron, steel scrap, and direct-reduced iron (DRI). Liquid blast-furnace iron typically contains 3.8 to 4.5 percent carbon (C), 0.4 to 1.2 percent silicon (Si), 0.6 to 1.2 percent manganese (Mn), up to 0.2 percent phosphorus (P), and 0.04 percent sulfur (S). Its temperature is usually 1,400 to 1,500 C (2,550 to 2,700 F). The phosphorus content depends on the ore used, since phosphorus is not removed in the blast-furnace process, whereas sulfur is usually picked up during iron making from coke and other fuels. DRI is reduced from iron ore in the solid state by carbon monoxide (CO) and hydrogen (H2). It frequently contains about 3 percent unreduced iron ore and 4 percent gangue, depending on the ore used. It is normally shipped in briquettes and charged into the steelmaking furnace like scrap. Steel scrap is metallic iron containing residuals, such as copper, tin, and chromium, that vary with its origin. Of the three major steelmaking processesbasic oxygen, open hearth, and electric arcthe first two, with few exceptions, use liquid blast-furnace iron and scrap as raw material and the latter uses a solid charge of scrap and DRI. Secondary steelmaking The ladle An open-topped cylindrical container made of heavy steel plates and lined with refractory, the ladle is used for holding and transporting liquid steel. Here all secondary metallurgical work takes place, including deslagging and reslagging, electrical heating, chemical heating or cooling with scrap, powder injection or wire feeding, and stirring with gas or with electromagnetic fields. The ladle receives liquid steel during tapping while sitting on a stand beneath the primary steelmaking furnace. It is moved by cranes, ladle cars, turntables, or turrets. A ladle turret has two liftable forks, usually 180 apart, that revolve around a tower, each fork capable of holding a ladle. Ladles have two heavy trunnions on each side for crane pickup. Support plates under each trunnion are used for setting the ladles onto stands or ladle cars. The shell The side wall of a ladle is slightly cone-shaped, with the larger diameter on top for easy removal of a skulli.e., solidified steel and slag. A ladle capable of holding 200 tons of steel has an outside diameter of approximately four metres and is about five metres high. Inside the ladle there is usually a 60-millimetre-thick refractory safety lining next to the shell. The working lining, that part contacting the steel and slag, is 180 to 300 millimetres thick, depending on ladle size and location in the ladle. The lining thickness and type of brick in one ladle are often different to counteract increased wear at certain locationsfor example, at the impact area of the tapping stream or at the slag line. This results in more equal wear on the ladle lining and an extended ladle service life. Sometimes, fired clay bricks are used because they bloatthat is, they expand during heating and seal the joints between them. Their thermal shock resistance is high, but their resistance to slag corrosion is low, so that the working lining has to be replaced every 6 to 12 heats. Because ladle rebricking takes about eight hours, up to 12 ladles are sometimes in use in large steelmaking shops in order to assure availability. For ladle operations requiring longer holding times, higher-grade refractory linings are made of high alumina or magnesia bricks. These give greater slag resistance, but they do not bloat and are less resistant to thermal shock. For these reasons, they are kept hot at special preheating stations. Ladles that use these bricks have service lives of up to 80 heats, so that fewer ladles are required. Preheating also decreases the heat loss of liquid steel during tapping and holding.