Developed during the early 1850s, the Bessemer process was the first method for making steel cheaply and in large quantities. Named for its inventor, British engineer Henry Bessemer (1813–1898), the process was also developed independently by William Kelly (who patented the process in 1857) in the United States. Bessemer and Kelly experimented with injecting (blowing) air into molten pig iron (crude iron); the oxygen in the air helps rid the iron of its impurities (such as manganese, silicon, and carbon), converting the iron to molten steel, which is then poured into molds. The process was introduced in the U.S. steel manufacturing industry in 1864. Alloys were also added to the refining process to help purify the metal. Within two decades, the method was used to produce more than 90 percent of the nation’s steel; it was also implemented throughout the industrialized world.

In the mid-1800s rich iron ore deposits had been discovered in the Upper Peninsula of Michigan, along Lake Superior. The discovery of the minerals and the innovation of the Bessemer process combined to create a thriving steel industry in the United States. At the same time there was a growing market for the material: Railroads needed iron to make rail gauges, while the new auto manufacturing industry used steel to make cars. As a result, between 1880 and 1910 annual U.S. steel production increased by a factor of 20. One of the early industry leaders was Andrew Carnegie (1835–1919), who in 1873 founded the nation’s first large-scale steel plant, at Braddock, Pennsylvania. In 1901 Carnegie sold this and other steel mills to the United States Steel Corporation (today the USX Corporation, which is the largest steel producer in the United States). The Bessemer process continued to be used until after World War II (1939–45). The open-hearth method of purification gradually replaced it.