Iron and Steel Manufacture

I

Introduction

Iron and Steel Manufacture, technology related to the production of iron and its alloys, particularly those containing a small percentage of carbon. The differences between the various types of iron and steel are sometimes confusing because of the nomenclature used. Steel in general is an alloy of iron and carbon, often with an admixture of other elements. Some alloys that are commercially called irons contain more carbon than commercial steels. Open-hearth iron and wrought iron contain only a few hundredths of 1 percent of carbon. Steels of various types contain from 0.04 percent to 2.25 percent of carbon. Cast iron, malleable cast iron, and pig iron contain amounts of carbon varying from 2 to 4 percent. A special form of malleable iron, containing virtually no carbon, is known as white-heart malleable iron. A special group of iron alloys, known as ferroalloys, is used in the manufacture of iron and steel alloys; they contain from 20 to 80 percent of an alloying element, such as manganese, silicon, or chromium.

II

History

The exact date at which people discovered the technique of smelting iron ore to produce usable metal is not known. The earliest iron implements discovered by archaeologists in Egypt date from about 3000 bc, and iron ornaments were used even earlier; the comparatively advanced technique of hardening iron weapons by heat treatment was known to the Greeks about 1000 BC.

The alloys produced by early iron workers, and, indeed, all the iron alloys made until about the 14th century ad, would be classified today as wrought iron. They were made by heating a mass of iron ore and charcoal in a forge or furnace having a forced draft. Under this treatment the ore was reduced to the sponge of metallic iron filled with a slag composed of metallic impurities and charcoal ash. This sponge of iron was removed from the furnace while still incandescent and beaten with heavy sledges to drive out the slag and to weld and consolidate the iron. The iron produced under these conditions usually contained about 3 percent of slag particles and 0.1 percent of other impurities. Occasionally this technique of ironmaking produced, by accident, a true steel rather than wrought iron. Ironworkers learned to make steel by heating wrought iron and charcoal in clay boxes for a period of several days. By this process the iron absorbed enough carbon to become a true steel.

After the 14th century the furnaces used in smelting were increased in size, and increased draft was used to force the combustion gases through the “charge,” the mixture of raw materials. In these larger furnaces, the iron ore in the upper part of the furnace was first reduced to metallic iron and then took on more carbon as a result of the gases forced through it by the blast. The product of these furnaces was pig iron, an alloy that melts at a lower temperature than steel or wrought iron. Pig iron (so called because it was usually cast in stubby, round ingots known as pigs) was then further refined to make steel.

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Modern steelmaking employs blast furnaces that are merely refinements of the furnaces used by the old ironworkers. The process of refining molten iron with blasts of air was accomplished by the British inventor Sir Henry Bessemer who developed the Bessemer furnace, or converter, in 1855. Since the 1960s, several so-called minimills have been producing steel from scrap metal in electric furnaces. Such mills are an important component of total U.S. steel production. The giant steel mills remain essential for the production of steel from iron ore.

III

Pig-Iron Production

The basic materials used for the manufacture of pig iron are iron ore, coke, and limestone. The coke is burned as a fuel to heat the furnace; as it burns, the coke gives off carbon monoxide, which combines with the iron oxides in the ore, reducing them to metallic iron. This is the basic chemical reaction in the blast furnace; it has the equation: Fe₂O₃ + 3CO = 3CO₂ + 2Fe. The limestone in the furnace charge is used as an additional source of carbon monoxide and as a “flux” to combine with the infusible silica present in the ore to form fusible calcium silicate. Without the limestone, iron silicate would be formed, with a resulting loss of metallic iron. Calcium silicate plus other impurities form a slag that floats on top of the molten metal at the bottom of the furnace. Ordinary pig iron as produced by blast furnaces contains iron, about 92 percent; carbon, 3 or 4 percent; silicon, 0.5 to 3 percent; manganese, 0.25 to 2.5 percent; phosphorus, 0.04 to 2 percent; and a trace of sulfur.

A typical blast furnace consists of a cylindrical steel shell lined with a refractory, which is any nonmetallic substance such as firebrick. The shell is tapered at the top and at the bottom and is widest at a point about one-quarter of the distance from the bottom. The lower portion of the furnace, called the bosh, is equipped with several tubular openings or tuyeres through which the air blast is forced. Near the bottom of the bosh is a hole through which the molten pig iron flows when the furnace is tapped, and above this hole, but below the tuyeres, is another hole for draining the slag. The top of the furnace, which is about 27 m (about 90 ft) in height, contains vents for the escaping gases, and a pair of round hoppers closed with bell-shaped valves through which the charge is introduced into the furnace. The materials are brought up to the hoppers in small dump cars or skips that are hauled up an inclined external skip hoist.

Blast furnaces operate continuously. The raw material to be fed into the furnace is divided into a number of small charges that are introduced into the furnace at 10- to 15-min intervals. Slag is drawn off from the top of the melt about once every 2 hr, and the iron itself is drawn off or tapped about five times a day.

The air used to supply the blast in a blast furnace is preheated to temperatures between approximately 540° and 870° C (approximately 1,000° and 1,600° F). The heating is performed in stoves, cylinders containing networks of firebrick. The bricks in the stoves are heated for several hours by burning blast-furnace gas, the waste gases from the top of the furnace. Then the flame is turned off and the air for the blast is blown through the stove. The weight of air used in the operation of a blast furnace exceeds the total weight of the other raw materials employed.

An important development in blast furnace technology, the pressurizing of furnaces, was introduced after World War II. By “throttling” the flow of gas from the furnace vents, the pressure within the furnace may be built up to 1.7 atm or more. The pressurizing technique makes possible better combustion of the coke and higher output of pig iron. The output of many blast furnaces can be increased 25 percent by pressurizing. Experimental installations have also shown that the output of blast furnaces can be increased by enriching the air blast with oxygen.

The process of tapping consists of knocking out a clay plug from the iron hole near the bottom of the bosh and allowing the molten metal to flow into a clay-lined runner and then into a large, brick-lined metal container, which may be either a ladle or a rail car capable of holding as much as 100 tons of metal. Any slag that may flow from the furnace with the metal is skimmed off before it reaches the container. The container of molten pig iron is then transported to the steelmaking shop.

Modern-day blast furnaces are operated in conjunction with basic oxygen furnaces and sometimes the older open-hearth furnaces as part of a single steel-producing plant. In such plants the molten pig iron is used to charge the steel furnaces. The molten metal from several blast furnaces may be mixed in a large ladle before it is converted to steel, to minimize any irregularities in the composition of the individual melts.

IV

Other Methods of Iron Refining

Although almost all the iron and steel manufactured in the world is made from pig iron produced by the blast-furnace process, other methods of iron refining are possible and have been practiced to a limited extent. One such method is the so-called direct method of making iron and steel from ore, without making pig iron. In this process iron ore and coke are mixed in a revolving kiln and heated to a temperature of about 950° C (about 1,740° F). Carbon monoxide is given off from the heated coke just as in the blast furnace and reduces the oxides of the ore to metallic iron. The secondary reactions that occur in a blast furnace, however, do not occur, and the kiln produces so-called sponge iron of much higher purity than pig iron. Virtually pure iron is also produced by means of electrolysis (see Electrochemistry), by passing an electric current through a solution of ferrous chloride. Neither the direct nor the electrolytic processes has yet achieved any great commercial significance.

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