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Steel is an alloy of iron that contains carbon. Typically the carbon content ranges from 0.002% and 2.1% by weight. Carbon makes steel harder than pure iron. The carbon atoms make it more difficult for dislocations in the iron crystal lattice to slide past each other.
There are many different types of steel. Steel contains additional elements, either as impurities or added to confer desirable properties. Most steel contains manganese, phosphorus, sulfur, silicon, and trace amounts of aluminum, oxygen, and nitrogen. Intentionally addition of nickel, chromium, manganese, titanium, molybdenum, boron, niobium and other metals influence the hardness, ductility, strength, and other properties of steel. Addition of at least 11% chromium adds corrosion resistance to make stainless steel. Another way to add corrosion resistance is to galvanize steel (usually carbon steel) by electroplating or hot-dipping the metal in zinc.
The oldest piece of steel is a piece of ironware that was recovered from an archaeological site in Anatolia, dating back to about 2000 BC. Steel from ancient Africa dates back to 1400 BC.
How Steel Is Made
Steel contains iron and carbon, but when iron ore is smelted, it contains too much carbon to confer desirable properties for steel. Iron ore pellets are remelted and processed to reduce the amount of carbon. Then, additional elements are added and the steel is either continuously cast or made into ingots.
Modern steel is made from pig iron using one of two processes. About 40% of steel is made using the basic oxygen furnace (BOF) process. In this process, pure oxygen is blown into melted iron, reducing the amounts of carbon, manganese, silicon, and phosphorus. Chemicals called fluxes further reduce levels of sulfur and phosphorus in the metal. In the United States, the BOF process recycles 25-35% scrap steel to make new steel. In the U.S., the electric arc furnace (EAF) process is used to make about 60% of steel, consisting nearly entirely of recycled scrap steel.
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