MOST IMPORTANT METERIAL
Iron is most widely found in the crust of the earth, in the form of various minerals (oxides, hydrated ores, carbonates, sulphides, silicates and so on). Since prehistoric times, humans have learned to prepare and process these minerals by various washing, crushing and screening operations, by separating the gangue, calcining, sintering and pelletizing, in order to render the ores smeltable and to obtain iron and steel.
In historic times, a prosperous iron industry developed in many countries, based on local supplies of ore and the proximity of forests to supply the charcoal for fuel. Early in the 18th century, the discovery that coke could be used in place of charcoal revolutionized the industry, making possible its rapid development as the base on which all other developments of the Industrial Revolution rested. Great advantages accrued to those countries where natural deposits of coal and iron ore lay close together.Steel making was largely a development of the 19th century, with the invention of melting processes; the Bessemer (1855), the open hearth, usually fired by producer gas (1864); and the electric furnace (1900). Since the middle of the 20th century, oxygen conversion, pre-eminently the Linz-Donowitz (LD) process by oxygen lance, has made it possible to manufacture high quality steel with relatively low production costs.
Today, steel production is an index of national prosperity and the basis of mass production in many other industries such as shipbuilding, automobiles, construction, machinery, tools, and industrial and domestic equipment. The development of transport, in particular by sea, has made the international exchange of the raw materials required (iron ores, coal, fuel oil, scrap and additives) economically profitable. Therefore, the countries possessing iron ore deposits near coal fields are no longer privileged, and large smelting plants and steelworks have been built in the coastal regions of major industrialised countries and are supplied with raw materials from exporting countries which are able to meet the present-day requirements for high-grade materials.During the past decades, so-called direct-reduction processes have been developed and have met with success. The iron ores, in particular high-grade or upgraded ores, are reduced to sponge iron by extracting the oxygen they contain, thus obtaining a ferrous material that replaces scrap.During the past decades, so-called direct-reduction processes have been developed and have met with success. The iron ores, in particular high-grade or upgraded ores, are reduced to sponge iron by extracting the oxygen they contain, thus obtaining a ferrous material that replaces scrap.
The steel industry has been undergoing a technological revolution, and the trend in building new production capacity has been towards the recycled steel-scrap-using electric arc furnace (EAF) by smaller mills (see figure 73.3). Although integrated steel works where steel is made from iron ore are operating at record levels of efficiency, EAF steel works with production capacities in the order of less than 1 million tonnes a year are becoming more common in the main steel-producing countries of the world.
IRON MAKING
For iron making, the essential feature is the blast furnace, where iron ore is melted (reduced) to produce pig iron. The furnace is charged from the top with iron ore, coke and limestone; hot air, frequently enriched with oxygen, is blown in from the bottom; and the carbon monoxide produced from the coke transforms the iron ore into pig iron containing carbon. The limestone acts as a flux. At a temperature of 1,600°C (see figure 73.5) the pig iron melts and collects at the bottom of the furnace, and the limestone combines with the earth to form slagThe furnace is tapped (i.e., the pig iron is removed) periodically, and the pig iron may then be poured into pigs for later use (e.g., in foundries), or into ladles where it is transferred, still molten, to the steel-making plant.Some large plants have coke ovens on the same site. The iron ores are generally subjected to special preparatory processes before being charged into the blast furnace (washing, reduction to ideal lump size by crushing and screening, separation of fine ore for sintering and pelletizing, mechanized sorting to separate the gangue, calcining, sintering and pelletizing). The slag that is removed from the furnace may be converted on the premises for other uses, in particular for making cement
Steel making
Pig iron contains large amounts of carbon as well as other impurities (mainly sulphur and phosphorus). It must, therefore, be refined. The carbon content must be reduced, the impurities oxidized and removed, and the iron converted into a highly elastic metal which can be forged and fabricated. This is the purpose of the steel-making operations. There are three types of steel-making furnaces: the open-hearth furnace, the basic-oxygen process converter (see figure 73.6) and the electric arc furnace (see figure 73.7). Open-hearth furnaces for the most part have been replaced by basic-oxygen converters (where steel is made by blowing air or oxygen into molten iron) and electric arc furnaces (where steel is made from scrap iron and sponge-iron pellets).Special steels are alloys in which other metallic elements are incorporated to produce steels with special qualities and for special purposes, (e.g., chromium to prevent rusting, tungsten to give hardness and toughness at high temperatures, nickel to increase strength, ductility and corrosion resistance). These alloying constituents may be added either to the blast-furnace charge (see figure 73.8) or to the molten steel (in the furnace or ladle) (see figure 73.9). Molten metal from the steel-making process is poured into continuous-casting machines to form billets (see figure 73.10), blooms (see figure 73.11) or slabs. The molten metal can also be poured into moulds to form ingots. The majority of steel is produced by the casting method (see figure 73.12). The benefits of continuous casting are increased yield, higher quality, energy savings and a reduction in both capital and operating costs. Ingot-poured moulds are stored in soaking pits (i.e. underground ovens with doors), where ingots can be reheated before passing to the rolling mills or other subsequent processing (figure 73.4). Recently, companies have begun making steel with continuous casters. Rolling mills are discussed elsewhere in this chapter; foundries, forging and pressing are discussed in the chapter
