How is iron converted to steel?

　　The element iron is converted to the alloy commonly called steel by controlling the percentage of carbon in the product, which is usually 0.25 percent to 1.5 percent of the final alloy content. The first economical method for producing steel in large quantities was the Bessemer furnace, which was invented in the mid 1850s and introduced a stream of air to the molten metal that enabled better control over the carbon content. In the modern process, high-purity oxygen is pumped into the furnace instead, which avoids adding the impurities found in ordinary air.

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