Iron and Steel

Iron and Steel, materials essential to all modern industrialized economies. Iron is a common element throughout nature and is even an important component of our blood. This discussion focuses on iron as a useful metal. The highest natural concentrations of iron are found in meteors. A common element in the earth's crust, iron is combined in varying concentrations with oxygen, carbon, sulfur, and silicon. In its raw state it is iron ore. Steel does not occur in nature; it is man-made and requires refined iron for its manufacture. Steel is made by melting, purifying, and blending iron with other metals.

There are several kinds of iron ore; they are graded according to the percentage of iron they contain. The richest ores, hematite and magnetite, consist of about 70% iron. Other ores include limonite, pyrite, siderite, and taconite. Taconite is about 30% iron. When ore is found close to the earth's surface, it is gathered by open-pit mining. If it is necessary to reach an especially rich but deep seam of ore, shaft mining is used, but it is more expensive and more dangerous than open-pit mining. Though vast quantities of iron ore have been mined, including most of the top-grade ores, supplies are still abundant. The former USSR, Brazil, and Australia are major producers of iron ore. In the US, the Mesabi Range in Minnesota is a major source of ore.

Once it has been mined, the ore must be processed to separate the iron. The particular process used will depend upon the purity of the ore. The raw iron is then shaped into pellets; it is this iron that is further refined in furnaces, where it is mixed with various elements and melted to remove oxygen. The most common method for purifying iron is the blast furnace, so called because of the super heated air forced into the furnace through pipes located in its lower section. The blast of hot air melts the iron and burns off impurities. The purified molten iron is then poured into molds, where it hardens into pig iron. Pig iron can then be melted and combined with other elements to yield cast iron or wrought iron, but most pig iron is used to manufacture steel. Huge furnaces are also used to make steel, but the processes are more varied and sophisticated. There are basically 4 kinds of steel. Carbon steel is the most common, with a carbon content of less than 1%. Stainless steel, with an ad mixture of chromium, is more resistant to corrosion. Tool steel is used to make special tools and is extremely hard. Finally, there is a wide variety of alloy steels whose composition may include aluminum, manganese, titanium, nickel, or vanadium.

Steel is made by further purifying or refining pig iron. The principal task in making steel is to reduce the carbon content of the iron to less than 2% and to blend the resultant metal with other metals or alloys to produce a steel of the appropriate hardness, tensile strength, lightness, malleability, or resistance to corrosion. Depending upon the application, the steel used may be fairly commonplace and inexpensive to produce or exotic and manufactured only for special purposes. Steel can be produced in huge open hearths, in electric furnaces, or in furnaces that use an oxygen process in which oxygen is blown into the furnace to purify the metal. Operating an open hearth furnace requires large amounts of fuel oil and causes considerable air pollution. As a result, the open hearth method has been largely replaced by the oxygen process or by cheaper and more efficient electric furnaces.

Molten steel is cast into various forms, one of the more common being ingots. Steel is then shaped or finished by means of rolling, forging, or extruding. Rolling literally presses or squeezes steel into sheets or strips. Forging requires that the steel be reheated and then hammered or pressed into the desired shape. Extrusion is a process whereby hot steel is forced through an opening that shapes it into forms like I-beams or rails. Rolling, forging, and extruding are done in huge mills equipped with powerful machinery. Other common forms of finished steel include tubing and wire. At this point, the steel may be given its final form or it may be shipped to manufacturers who will use it in their own plants to produce everything from forks and spoons to automobiles, aircraft, machines, or appliances.

Iron, principally from meteorites, was used by our ancestors as early as 4000 B.C., but iron made from ore by means of systematic refinement dates to the Hittites about 1400 B.C. Over the centuries, various improvements were made in furnaces, but it was not until early modern times, about 1500 A.D., that large amounts of iron could be made for commercial purposes.

Small amounts of steel had been made from as early as 300 B.C., but even as late as 1740 steel could be made only in small quantities. The Englishman Henry Bessemer developed a process that revolutionized the making of steel by forcing hot air into the molten iron to purify it. After Bessemer's breakthrough in the mid-19th century, further developments in steel manufacture rapidly followed, giving birth to the steel industry and providing the material and tools that have made possible the world as we know it.