Steel is an alloy consisting mainly of iron, carbon and other elements. According to its carbon content, iron can be divided into pig iron and wrought iron. Pig iron contains between 2.11 and 4.5 percent carbon, wrought iron less than 0.0218 percent and steel between (0.0218 and 2.11 percent). Thus, it is possible to make steel by purifying pig iron by removing some of the carbon, or by adding carbon to wrought-iron.
Iron is found mainly in iron ore in the form of oxides. To reduce iron ore to pig iron, modern technology adopts coke, coal, heavy oil, natural gas and other fuels in the blast furnace combustion, generating a lot of heat and reducing gas, creating the necessary conditions for iron ore reduction. In the past, pig iron was usually made at relatively low temperatures. The method is to place iron ore and charcoal in a furnace layer by layer and roast them slowly at a low temperature. Under these conditions, carbon monoxide produced by incomplete combustion reaction of charcoal can reduce the iron oxide in iron ore to iron elements, which can be formed into iron blocks after cooling, and pig iron can be obtained. This method of making iron is called lump iron.
Modern steelmaking mainly uses pig iron as raw material. In the second half of the 19th century, people kept trying and invented a variety of steelmaking methods, such as Bessemer steelmaking process and Siemens open-hearth steelmaking process. Its production process is basically similar, melting pig iron into hot metal and then adding some substances in the hot metal, such as oxygen, and iron ore, which makes it react with the carbon in the hot metal so as to reduce the carbon content in the hot metal. Then pour the purified hot metal into the mold and make it into steel after cooling.
Modern steelmaking relies on standardized operation to accurately control the whole process, making the steelmaking process more efficient.