Iron and Steel Manufacture

X

Tin Plate

By far the most important coated product of the steel mill is tin plate for the manufacture of containers. The “tin” can is actually more than 99 percent steel. In some mills steel sheets that have been hot-rolled and then cold-rolled are coated by passing them through a bath of molten tin. The most common method of coating is by the electrolytic process. Sheet steel is slowly unrolled from its coil and passed through a chemical solution. Meanwhile, a current of electricity is passing through a piece of pure tin into the same solution, causing the tin to dissolve slowly and to be deposited on the steel. In electrolytic processing, less than half a kilogram of tin will coat more than 18.6 sq m (more than 200 sq ft) of steel. For the product known as thin tin, sheet and strip are given a second cold rolling before being coated with tin, a treatment that makes the steel plate extra tough as well as extra thin. Cans made of thin tin are about as strong as ordinary tin cans, yet they contain less steel, with a resultant saving in weight and cost. Lightweight packaging containers are also being made of tin-plated steel foil that has been laminated to paper or cardboard.

Other processes of steel fabrication include forging, founding, and drawing the steel through dies (see Die).

XI

Wrought Iron

The process of making the tough, malleable alloy known as wrought iron differs markedly from other forms of steel making. Because this process, known as puddling, required a great deal of hand labor, production of wrought iron in tonnage quantities was impossible. The development of new processes using Bessemer converters and open-hearth furnaces allowed the production of larger quantities of wrought iron.

Wrought iron is no longer produced commercially, however, because it can be effectively replaced in nearly all applications by low-carbon steel, which is less expensive to produce and is typically of more uniform quality than wrought iron.

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The puddling furnace used in the older process has a low, arched roof and a depressed hearth on which the crude metal lies, separated by a wall from the combustion chamber in which bituminous coal is burned. The flame in the combustion chamber surmounts the wall, strikes the arched roof, and “reverberates” upon the contents of the hearth. After the furnace is lit and has become moderately heated, the puddler, or furnace operator, “fettles” it by plastering the hearth and walls with a paste of iron oxide, usually hematite ore. The furnace is then charged with about 270 kg (about 600 lb) of pig iron and the door is closed. After about 30 min the iron is melted and the puddler adds more iron oxide or mill scale to the charge, working the oxide into the iron with a bent iron bar called a raddle. The silicon and most of the manganese in the iron are oxidized and some sulfur and phosphorus are eliminated. The temperature of the furnace is then raised slightly, and the carbon starts to burn out as carbon-oxide gases. As the gas is evolved the slag puffs up and the level of the charge rises. As the carbon is burned away the melting temperature of the alloy increases and the charge becomes more and more pasty, and finally the bath drops to its former level. As the iron increases in purity, the puddler stirs the charge with the raddle to ensure uniform composition and proper cohesion of the particles. The resulting pasty, spongelike mass is separated into lumps, called balls, of about 80 to 90 kg (about 180 to 200 lb) each. The balls are withdrawn from the furnace with tongs and are placed directly in a squeezer, a machine in which the greater part of the intermingled siliceous slag is expelled from the ball and the grains of pure iron are thoroughly welded together. The iron is then cut into flat pieces that are piled on one another, heated to welding temperature, and then rolled into a single piece. This rolling process is sometimes repeated to improve the quality of the product.

The modern technique of making wrought iron uses molten iron from a Bessemer converter and molten slag, which is usually prepared by melting iron ore, mill scale, and sand in an open-hearth furnace. The molten slag is maintained in a ladle at a temperature several hundred degrees below the temperature of the molten iron. When the molten iron, which carries a large amount of gas in solution, is poured into the ladle containing the molten slag, the metal solidifies almost instantly, releasing the dissolved gas. The force exerted by the gas shatters the metal into minute particles that are heavier than the slag and that accumulate in the bottom of the ladle, agglomerating into a spongy mass similar to the balls produced in a puddling furnace. After the slag has been poured off the top of the ladle, the ball of iron is removed and squeezed and rolled like the product of the puddling furnace.

XII

Classifications of Steel

Steels are grouped into five main classifications.

A

Carbon Steels

More than 90 percent of all steels are carbon steels. They contain varying amounts of carbon and not more than 1.65 percent manganese, 0.60 percent silicon, and 0.60 percent copper. Machines, automobile bodies, most structural steel for buildings, ship hulls, bedsprings, and bobby pins are among the products made of carbon steels.

B

Alloy Steels

These steels have a specified composition, containing certain percentages of vanadium, molybdenum, or other elements, as well as larger amounts of manganese, silicon, and copper than do the regular carbon steels. Automobile gears and axles, roller skates, and carving knives are some of the many things that are made of alloy steels.

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