How Steel Is Made

How Steel Is Made

The iron and steel industry has been this nation’s industrial backbone for over 100 years. Great cities like Pittsburgh in the north and Birmingham, AL in the south, “boomed” because of the busy iron and steel industry. Making iron and steel is an interesting process, and requires some knowledge of metallurgy to do it right.

Iron comes from iron ore. Iron ore comes from rocks such as hematite, magnetite, limonite, and siderite. These are minerals with high concentrations of iron ore. Iron ore must be processed to have all the oxygen removed from it to make pure iron.

The mined ore is taken to a blast furnace which is heated, usually with charcoal or coke, and charged with the ore and limestone. The furnace is heated to about 3,000 degrees F and huge amounts of air blast into the furnace. The oxygen mixes with the carbon from the charcoal to bind with the iron ore and carry it away, leaving the metal iron. The limestone charge then combines with silicates and other impurities in the liquid iron to form slag. Slag is lighter than iron and floats to the top, where it is drained off.

The iron then drains into a bed of sand with indentations. The iron cubes formed in the indentations are called pig iron. This is the basis for wrought iron and steel.

For wrought iron, the manufacturers re-melt the pig iron, mix it with slag and hammer it to eliminate the carbon. This creates a tough metal that can be heated and worked. This is the metal that blacksmiths use for their products.

Steel is, essentially, tempered, very high-grade iron. It is strong, not brittle, like pig iron, and useful for structures such as buildings and automobiles. Steel-making has come a long way in the past 100 years. The first methods for steel-making included open hearth furnaces or the Bessemer method.

Open hearth steel-making means the pig iron, more iron ore and limestone go into an open hearth furnace. The charge is heated to about 1,600 degrees F. The limestone and ore form another type of slag, which floats to the surface, carrying with it other impurities (including carbon) from the pig iron. When the carbon content is correct throughout, steel is formed.

The Bessemer process takes its name from its inventor, Sir Henry Bessemer. It involves a huge metal, egg-shaped Bessemer converter, which is made of steel and lined with silica and clay, or dolomite. The bottom of the converter is perforated and hot air is blown up through these holes. The hot air mixes with the pig iron and impurities are either burned off or form a slag, which is removed. The molten steel is then poured out of the top of the converter.

The modern method of making steel involves the basic oxygen furnace. This method is much faster and much safer than the open hearth or Bessemer methods. The molten iron from the blast furnace and scrap iron are all loaded into a furnace lined with refractory bricks. A lance is lowered into the furnace and blows a high-pressure stream of oxygen through the mixture.

This causes the necessary chemical reactions, as in the open hearth or Bessemer method, and the steel is refined in this way. These furnaces are superior because they can refine a batch of steel in about 45 minutes, as opposed to the five or six hours needed for an open hearth furnace. They came into vogue in the 1950s.

The steel may be further processed by galvanizing it or making it into various forms, depending upon what it will be used for.

Building materials are growing more high-tech every day, but steel still has a long future in the building industry.

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