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Steel - Manufacturing Processes

oxygen ingot rolls hearth

Most steel is produced using one of four methods: Bessemer converters, open-hearth furnaces, basic oxygen furnaces, and electric furnaces. The basic oxygen process is the most efficient, while the Bessemer and open-hearth methods have become obsolete. Electric furnaces are used to produce high quality steels from selected steel scrap, lime, and mill scale (an iron oxide that forms on the surface of hot steel and falls off as black scale).

Until 1909, most steel made in the United States came from Bessemer converters. A Bessemer converter looks like a huge pear-shaped pot and can hold anywhere from 5-25 tons. It is balanced on axles so that its open top can be tilted one way to take a charge and the other way to pour out steel. After the converter is charged with hot metal, it is swung to the upright position. Air is then blown through holes in its bottom at a typical rate of 30,000 cubic feet per minute. Sparks and thick, brown smoke pour from the converter's mouth as the oxygen in the blow combines with the iron, silicon, and manganese to form slag. Then 30-ft (9-m) flames replace the smoke as the oxygen combines with the carbon fuel and burns. The whole process took less than 15 minutes. Unfortunately, the blowing air contained contaminants (such as nitrogen) and also removed some desirable elements such as carbon and manganese. This was solved by adding the necessary elements back into the converter after the blow. Because of stricter air pollution regulations and more efficient processes, the Bessemer converter is no longer used.

From 1909 until the 1960s, the open-hearth process was the most popular method of steel production. Open-hearth furnaces got their name from a shallow area called a hearth that is exposed to a blast of flames that alternately sweeps across the hearth from one side for a period of time and then to the side of the furnace. To make a "heat," or one batch of steel, pig iron, limestone, and scrap steel, are initially "charged," or loaded, into the hearth. These materials are heated for about two hours at temperatures 2,700–3,000°F (1,482–1,649°C) until they begin to fuse. Then the furnace is charged with many tons of molten pig iron. Scrap is placed in the furnace with a charging machine which usually serves a series of open hearth furnaces in a single building. Other elements, such as fluxing agents, carbon (usually in the form of anthracite coal pellets), and alloying materials, are then added to improve the steel. These elements can be added either in the furnace charge, the melt or "bath," ladle, or the ingot molds to meet the desired chemical composition of the finished steel or to eliminate or counteract the effect of oxides or other impurities. Fluxing agents (primarily lime, added in the form of either limestone or burnt lime and supplemented by magnesia, MgO, and lime from the furnace bottom and sides) melt and combine with the impurities to form slag at the top of the melt which is poured off into a separate slag pot. Mill scale, a form of iron oxide (Fe3O4), is used to reduce carbon content. Aluminum ferrosilicon is added if the steel is to be "killed." A killed steel is one that has been deoxidized to prevent gas evolution in the ingot mold, making a more uniform steel. "Rimmed" steel is steel that has not been deoxidized and gas pockets and holes from free oxygen form in the center of the ingot while the rim near the surface of the ingot is free of defects. Rolling processes are used in later operations to remove these defects. Semikilled steels are a compromise between rimmed and killed steels and are mainly limited to steels to be rolled into sheet bar, plate, and structural sections. The quantity of deoxidizers used must be closely controlled to allow a limited evolution of gas from the carbon-oxygen reaction.

When the contents of the heat are acceptable and the temperature is right, the furnace is tapped and the molten metal is poured into a ladle. An open-hearth furnace is tapped through a hole in the furnace's bottom. A heat is refined into steel during an 8-12 hour time period. Oxygen released from the ore and additional injected oxygen combine with carbon in the molten pig iron to form carbon gases. These, along with any additional gases from the burned fuel, are used to heat incoming air and this is why the open-hearth process is sometimes called the regenerative open-hearth.

The basic oxygen converter resembles a Bessemer converter. It receives materials from the top and tips to pour off the finished steel into ladles. The main element is a water-cooled oxygen lance, which is placed into the top of the converter after it is charged with scrap steel, molten pig iron, and fluxing agents. The lance, lowered to within a few feet of the charge, directs high-purity oxygen at supersonic speeds into the molten metal. This burns out the impurities and also enables the making of steel with a minimum amount of nitrogen, which can make steel brittle. The oxidation of the carbon and impurities causes a violent rolling agitation which brings all the metal into contact with the oxygen stream. The furnace ladle is first tipped to remove slag and then rotated to pour molten steel into a ladle. The speed and efficiency of the oxygen process has had a significant impact on the steel industry. An oxygen converter can produce a heat of quality steel in 30-45 minutes. An open-hearth furnace without an oxygen lance requires as much as eight hours to produce steel of a similar quality. Recent advances in refractory "brick," the insulating ceramics that protect vessels from the hot steel, have allowed injection of oxygen from the bottom of a vessel without a large complicated lance. This allows for a much more efficient use of the oxygen and can lower the capital costs in constructing a basic oxygen facility, especially if the building and cranes of a retired open-hearth facility is used.

High-quality carbon and alloy steels, such as tool and stainless steels, are produced in electric arc furnaces. These furnaces can make 150-200 tons in a single heat in as little as 90 minutes. The charge is melted by the arcing between carbon electrodes and high quality scrap steel. Some of the electrodes can be 2 ft (0.6 m) in diameter and 24 ft (7.2 m) long. The entire electric furnace is tilted during a tapping operation in which molten steel flows into a waiting ladle. Electric furnaces are the most competitive where low-cost electricity is available and where very little coal or iron ore is found.

After the steel in the ladle has cooled to the desired temperature, the ladle is moved by a traveling crane to either a pouring platform for ingot production or to a continuous caster. Ingots may be square, rectangular, or round and weigh anywhere from a few hundred pounds to 40 tons. A small amount of steel is cast directly into the desired shape in molds of fine sand and fireclay. Small rail cars carrying a series of heavy cast iron ingot molds wait alongside the pouring platform. The steel is "teemed" or poured into the molds through a fire-clay nozzle in the bottom of the ladle. After the steel in the molds has solidified, the cars are pulled under a stripping crane. The crane's plunger holds down the ingot top as its jaws lift the mold from the glowing hot ingot. The ingots are then taken to soaking pits for further processing.

An underground soaking pit is used to heat the steel ingots to a uniform temperature throughout. The ingots must be the same temperature throughout so that they can be easily plastically deformed and to prevent damage to the heavy machinery of the mills. The jaws of the crane clamp onto the ingots and lower them into the open soaking pits. The roof of the pits is then closed and burning oil or gas heats the ingots to about 2,200°F (1,204°C). After "soaking" in the pits for several hours, the ingots are then lifted out by crane and transported by rail to the blooming and slabbing mills.

The mechanical working of steel, such as rolling, forging, hammering, or squeezing, improves it in several ways. Cavities and voids are closed, harmful concentrations of nonmetallic impurities are broken up and more evenly disbursed, and the grain structure is refined to produce a more homogeneous or uniform product. Some ingots are sent directly to a universal plate mill for immediate rolling of steel plates. Most ingots, however, are sent to semifinishing mills (also known as slabbing or blooming mills) for reduction and shaping into slabs, blooms, or billets. A slab is generally a large flat length of steel wider than a bloom, a bloom is a length of steel either square or rectangular with a cross-sectional area larger than 36 in (90 cm), and a billet is generally two to five inches square, although some billets can be round or rectangular. The exact sizes of slabs, blooms, and billets depend on the requirements of further processing.

In slabbing and blooming mills, the steel ingot is gradually squeezed between heavy rolls. To make billets, the steel is first shaped into blooms, then further reduced in a billet mill. Each time the ingot is forced through the rolls, it is further reduced in one dimension. Blooming mills can be classified as either two-high or three-high, depending on the number of rolls used. The two rolls of the two-high mill can be reversed so that the ingot is flattened and lengthened as it passes back and forth between the rolls. The top and bottom rolls of the three-high mill turn in one direction while the middle roll turns in the opposite direction. The ingot is flattened first between the bottom and middle rolls and ends up on a runout table. The table rises and the steel is then fed through the top and middle rolls. The continuous, or cross-country, mill is a third type of blooming mill. This mill has a series of two-high rolls. As many as 15 passes may be required to reduce an ingot 21 in2 (135 cm2) in cross section to a bloom 8 in2 (52 cm2) in cross section. The twoand three-high blooming mills roll the top and bottom of the steel in every pass. After one or two passes, mechanical manipulators on the runout table turn the steel to bring the side surfaces under the rolls for a more uniform material. After the steel is rolled, the uneven ends are sheared off, and the single long piece is cut into shorter lengths. The sheared off ends are reused as scrap. Most of the rolls used in these mills are horizontal, but there are also vertical rolls which squeeze the blooms or slabs from the sides. High-pressure water jets are used to remove mill scale which forms on the surface. Surface defects on the finished blooms and slabs are burned off, or scarfed, with an oxygen flame. The hot lengths of steel are moved from one station to another on a series of roller conveyors. The mill operations are automatically controlled by workers in an overhead glass-enclosed room called a "pulpit." The slabs, blooms, and billets are then taken to finishing mills where they are formed into special shapes and forms such as bars, beams, plates, and sheets. The steel is still not completely "finished" but it is closer to the form in which it will eventually be used in manufactured goods. Blooms and billets are finished into rails, wire rods, wires, bars, tubes, seamless pipe, and structural shapes such as I and H beams. Slabs are converted into plates, sheets, strips, and welded pipe.

After they are hot rolled, steel plates or shapes undergo further processing such as cleaning and pickling by chemicals to remove surface oxides, cold rolling to improve strength and surface finish, annealing (also known as stress relieving), and coating (galvanizing or aluminizing) for corrosion resistance.

Continuous or "strand" casting of steel eliminates the need to produce ingots and the use of soaking pits. In addition to costing less, continuously cast steels have more uniform compositions and properties than ingot cast steels. Continuous casting of steel produces an endless length of steel which is cut into long slabs or blooms that are ready for shaping in rolling mills. Molten steel is poured into the top of a continuous casting machine and is cooled by passing through a water-cooled mold. Pinch rolls draw the steel downward as it solidifies. Additional cooling is provided by water sprays along the travel path of the solidifying metal. The thickness of the steel "strand" is typically 10 in (25 cm) but new developments have reduced this thickness to 1 in (2.5 cm) or less. The thinner strand reduces the number of rolling operations required and improves the economy of the overall process. Some continuous cast machines bend the steel while it is still hot and flexible so that it comes out of the bottom in a horizontal position. Other machines cut the steel into sections while it is still in a vertical position. The continuous cast process has become the most economical method to produce large quantities of conventional steels. Small heats of alloy and specialty steels are still cast in ingots because the small size makes the continuous cast process impractical.

Some steel shapes are produced from powder. There are several chemical, electrochemical, and mechanical ways to make steel powder. One method involves improving the ore by magnetically separating the iron. A ball mill is then used to grind the ore into a powder that is then purified with hot hydrogen. This powder, under heat and pressure, is pressed into molds to form irregularly shaped objects; objects that would be hard to form any other way.


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over 2 years ago

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