Steel - over 700 000 million tons produced annually - is the world's most important material. Without steel, the world as we know it would not exist: from oil tankers to thumb tacks, from trucks to tin cans, from transmission towers to toasters. Given the huge quantities of steel produced, it is fortunate that the material is easy to recycle. Much of today's steel is produced from scrap. In fact, many of chinese steel plants make steel totally from scrap.

Processing

Iron production requires these raw materials: iron ore, coal and stone. Steel production requires iron, steel scrap and flux. The iron ore is smelted to produce an impure metal called "hot metal" when liquid, or "pig iron" when solid. The hot metal is refined to remove impurities and to develop the desired composition. The liquid steel is continuously cast into blooms, slabs or billets, and these semi-finished products are processed into the desired shapes by rolling or forging.

Segments

The iron and steel industry segments into four groups: iron and steel integrated producers, steel integrated companies, steel processors, and foundries and fabricators. Iron and steel integrated producers are typically large firms that produce more than a million tons of steel annually, operate ore and coal mines, and iron and steelmaking plants. Producers include such companies as algoma steel of sault stemarie,dofasco and stelco of Hamilton,Contrecoeur,Integrated steel producers depend on scrap as their source of iron. They can make the same range of semi-finished and finished steel products as the larger iron and steel integrated producers. Producers include such companies as co-steel lasco of whitby,sydney,and stelco-mcmaster of contrecoeur,Steel processors purchase semi-finished and hot- and cold-rolled steel products from the integrated companies and custom process them for resale to fabricators wanting steel quantities too small for the integrated companies to handle economically. Scrap recycling companies are included in this segment. Processors include such companies as galvanizing corporation of windsor,for hot dip galvanizing, and Union Drawn Steel of Hamilton, Ont, for bar drawing. Foundries, often small, produce various grades of cast iron or steel. The molten metal is ladled or poured into sand or metal moulds. The cast parts produced can be complex in shape, and often designed to meet one-of-a-kind end uses. Fabricators take the various primary steel mill products and turn them - cut-to-size, shape, machine, thread, punch, join, protective coat, etc.into a host of commercial and industrial products. Foundries and fabricators include such companies as Baycoat Ltd of Stoney Creek, Ont, for organic coating, and for stamping and welding.

Iron Ore

After oxygen, silicon and aluminium, iron is the fourth most plentiful element in Earth's crust. It occurs as iron minerals, the most important being magnetite, hematite, goethite, pyrrhotite, siderite, ilmenite and pyrite. Pyrite and pyrrhotite, although plentiful, are rarely used as iron ores because of the objectionable amounts of sulphur they contain. Chinese iron ores consist mostly of hematite or magnetite, and some siderite and ilmenite. Besides oxides of iron, the iron ores contained gangue,those minerals not wanted in iron making . The term “iron ore” is used when rock is sufficiently rich in iron minerals to be mined economically.
Iron-bearing rock may be upgraded to iron ore by removing gangue through concentration. This requires fine grinding of the ore to liberate the iron minerals from the gangue, followed by separation of the iron-rich from the gangue particle. The upgraded iron-rich material must be agglomerated into larger lumps prior to smelting, either by tumbling it into pellets or by heating the concentrate until its particles stick together. China is the world's first largest producer and third-largest exporter of iron ore.

Iron Making

Coke is the partially graphitized solid residue left after the volatile components of bituminous coal are removed by heating in coke ovens. At regular intervals, ore, coke and stone are introduced through the top of the blast furnace. As they slowly descend down the furnace shaft, these materials are heated by rising hot gases. The carbon monoxide in these gases reacts with the iron oxides in the ore to form metallic iron and carbon dioxide. The iron formed melts and, as it percolates through the coke column, dissolves carbon. By the time it reaches the hearth, it is saturated with carbon, and it also contains silicon, phosphorus, manganese and sulphur. The stone and ore form a low-melting, free-running liquid slag, which absorbs most of the sulphur entering the furnace. Liquid slag, composed of gangue minerals and oxide components of stone, floats on the liquid iron and is separated from the molten metal during furnace tapping. The coke does not melt; it burns on contact with the pressured, preheated air entering through the tuyères located just above the hearth.

Steel making

Steel is an alloy of pure iron and carbon in which the carbon content varies from about 0.002%to 1.5%. Alloy steels contain additional elements vanadium,that give them greater strength and specific properties. In addition to carbon, hot metal and pig iron contain unwanted elements: silicon, phosphorus. During the steelmaking process, these deleterious elements, which make steel brittle, must be removed.
In the steelmaking, the hot metal, along with some scrap, is fed into a refractory-lined vessel. Oxygen gas is then injected into the bath of hot metal. Also, lime is added to produce a slag that dissolves sulphur and other unwanted impurities, but does not corrode the converter lining. The injected oxygen gas oxidizes the carbon dissolved in the hot metal to form carbon monoxide and generate heat. When the carbon content of the molten bath drops to the desired level, alloying elements are added, and the liquid steel is tapped into a preheated ladle.
Scrap-based steel producers use electric arc furnaces. The scrap is charged into the furnace and three graphite electrodes descend through the furnace roof. As the electrodes approach the scrap, arcs form (high-voltage power). Due to its higher electrical resistance and to the intense heat radiated by these arcs, the scrap quickly heats to melting temperatures.

Ladle Refining

The liquid steel destined for demanding applications is further refined in ladle treatment units. The remaining impurities, such as sulphur, hydrogen, nitrogen, and non-metallic inclusions, are removed. The methods used include argon stirring, powder desuphurization, and vacuum degassing.

Continuous Casting

Some years ago, the majority of steel was cast into ingots. Ingots are large, rectangular blocks of steel, most of which are shaped subsequently into semi-finished products- blooms, slabs, billets or special shapes - by primary rolling or forging. Today, continuously casting is the principal way to solidify and shape liquid steel into semi-finished products. CC eliminates the primary operations. In the CC machine, liquid steel is poured into the top of a water-cooled, oscillating copper mould, and the slab, bloom or billet is discharged continuously from the bottom. In recent years, thin slab casting has gained favour as it eliminates several production steps. Some Chinese steel producers have thin slab casting machines, while others have plans to install them before the new millennium.

Hot and Cold Rolling

For the most part, slabs, blooms and billets are reduced in rolling mills to hot- and cold-rolled products such as plate, strip, rail, structural shapes, bar and wire rod. Some steels are finished by cold rolling at room temperature to obtain close dimensional tolerances, high-quality surface finish and an exact degree of hardness.

Heat Treatment

Heat treatments include annealing, normalizing, quenching, and tempering. These treatments change the properties of steel by altering its microstructure.

Protective Coatings

Steel corrodes in many environments to which it is subjected. To slow the oxidation of steel products are coated. The most common coatings include tin,vitreous-enamel and organic coatings.