Steelmaking
Steelmaking is the process of producing steel from iron ore and/or scrap. Steel has been made for millennia, and was commercialized on a massive scale in the 1850s and 1860s, using the Bessemer and Siemens-Martin processes.
Currently, two major commercial processes are used. Basic oxygen steelmaking uses liquid pig-iron from a blast furnace and scrap steel as the main feed materials. Electric arc furnace steelmaking uses scrap steel or direct reduced iron. Oxygen steelmaking has become more popular over time.
Steelmaking is one of the most carbon emission-intensive industries. In 2020, the steelmaking industry was reported to be responsible for 7% of energy sector greenhouse gas emissions. The industry is seeking significant emission reductions.
Steel
Steel is made from iron and carbon. Cast iron is a hard, brittle material that is difficult to work, whereas steel is malleable, relatively easily formed and versatile. On its own, iron is not strong, but a low concentration of carbon – less than 1 percent, depending on the kind of steel – gives steel strength and other important properties. Impurities such as nitrogen, silicon, phosphorus, sulfur, and excess carbon are removed, and alloying elements such as manganese, nickel, chromium, carbon, and vanadium are added to produce different grades of steel.History
Early history
Early processes evolved during the classical era in China, India, Rome and among hunter-foragers in northern Sweden. The earliest means of producing steel was in a bloomery.For much of human history, steel was made only in small quantities. Early modern methods of producing steel were often labor-intensive and highly skilled arts. The Bessemer process and subsequent developments allowed steel to become integral to the global economy.
China
A system akin to the Bessemer process originated in the 11th century in East Asia. Hartwell wrote that the Song dynasty innovated a "partial decarbonization" method of repeated forging of cast iron under a cold blast. Needham and Wertime described the method as a predecessor to the Bessemer process. This process was first described by government official Shen Kuo in 1075, when he visited Cizhou. Hartwell stated that the earliest center where this was practiced was perhaps the great iron-production district along the Henan–Hebei border during the 11th century.Europe
In the 15th century, the finery process, which shares the air-blowing principle with the Bessemer process, was developed in Europe.High-quality steel was also made by the reverse process of adding carbon to carbon-free wrought iron, usually imported from Sweden. The manufacturing process, called the cementation process, consisted of heating bars of wrought iron together with charcoal for periods of up to a week in a long stone box. This produced blister steel. The blister steel was put in a crucible with wrought iron and melted, producing crucible steel. Up to 3 tons of coke was burnt for each ton of steel produced. When rolled into bars such steel was sold at £50 to £60 a long ton. The most difficult and laborious part of the process was the production of wrought iron in finery forges in Sweden.
In 1740, Benjamin Huntsman developed the crucible technique for steel manufacture at his workshop in Handsworth, England. This process greatly improved the quantity and quality of steel production. It added three hours firing time and required large quantities of coke. In making crucible steel, the blister steel bars were broken into pieces and melted in small crucibles, each containing 20 kg or so. This produced higher quality metal, but increased the cost.
The Bessemer process reduced the time needed to make lower-grade steel to about half an hour while requiring only enough coke needed to melt the pig iron. The earliest Bessemer converters produced steel for £7 a long ton, although it initially sold for around £40 a ton.
Japan
The Japanese may have made use of a Bessemer-type process, as observed by 17th century European travellers. Adventurer Johan Albrecht de Mandelslo described the process in a book published in English in 1669. He wrote, "They have, among others, particular invention for the melting of iron, without the using of fire, casting it into a tun done about on the inside without about half a foot of earth, where they keep it with continual blowing, take it out by ladles full, to give it what form they please." Wagner stated that Mandelslo did not visit Japan, so his description of the process is likely derived from other accounts. Wagner stated that the Japanese process may have been similar to the Bessemer process, but cautions that alternative explanations are plausible.By the early 19th century the puddling process was widespread. At the time, process heat was too low to entirely remove slag impurities, but the reverberatory furnace made it possible to heat iron without placing it directly in the fire, offering some protection from impurities in the fuel source. Coal then began to replace charcoal as fuel.
The Bessemer process allowed steel to be produced without fuel, using the iron's impurities to create the necessary heat. This drastically reduced costs, but raw materials with the required characteristics were not always easy to find.
Industrialization
Modern steelmaking began at the end of the 1850s when the Bessemer process became the first successful method of steelmaking in high quantity, followed by the open-hearth furnace.Processes
Modern steelmaking consists of three steps: primary, secondary, and tertiary.Primary steelmaking involves melting iron into steel. Secondary steelmaking involves adding or removing other elements such as alloying agents and dissolved gases. Tertiary steelmaking casts molten metal into sheets, rolls or other forms. Multiple techniques are available for each step.
Primary steelmaking
Basic oxygen
Basic oxygen steelmaking involves melting carbon-rich pig iron that has been developed by smelting iron ore in a blast furnace, and converting it into steel. Blowing oxygen through molten pig iron oxidizes some of the carbon into Carbon monoxide| and Carbon dioxide|, turning the iron into steel. Refractories —calcium oxide and magnesium oxide—line the smelting vessel to withstand the heat, corrosive molten metal, and slag. The chemistry is controlled to remove impurities such as silicon and phosphorus.The basic oxygen process was developed in 1948 by Robert Durrer, as a refinement of the Bessemer converter that replaced air with pure oxygen. It reduced plant capital costs and smelting time, and increased labor productivity. Between 1920 and 2000, labour requirements decreased by a factor of 1000, to 3 man-hours per thousand tonnes. In 2013, 70% of global steel output came from the basic oxygen furnace. Furnaces can convert up to 350 tons of iron into steel in less than 40 minutes, compared to 10–12 hours in an open hearth furnace.
Electric arc
Electric arc furnaces make steel from scrap or direct reduced iron. A "heat" of iron is loaded into the furnace, sometimes with a "hot heel". Gas burners may assist with the melt. As with BOS, fluxes are added to protect the vessel lining and aid the removal of impurities. The furnaces are typically 100 tonne-capacity that produce steel every 40 to 50 minutes. This process allows larger alloy additions than the basic oxygen method.HIsarna
In HIsarna ironmaking, iron ore is processed almost directly into liquid iron or hot metal. The process is based around a cyclone converter blast furnace, which makes it possible to skip the intermediary production of pig iron pellets required for BOS. Skipping this preparatory step makes the HIsarna process more energy-efficient and reduces the emissions by around 20%.Hydrogen reduction
Direct-reduced iron can be produced from iron ore as it reacts with atomic hydrogen. Renewable hydrogen allows steelmaking without fossil fuels. Direct reduction occurs at. The iron is infused with carbon in an electric arc furnace. Hydrogen electrolysis requires approximately 2600 kWh per ton of steel. Hydrogen production raises costs by an estimated 20–30% over conventional methods.Secondary steelmaking
The next step commonly uses ladles. Ladle operations include de-oxidation, vacuum degassing, alloy addition, inclusion removal, inclusion chemistry modification, de-sulphurisation, and homogenisation. It is common to perform ladle operations in gas-stirred ladles with electric arc heating in the furnace lid. Tight control of ladle metallurgy produces high grades of steel with narrow tolerances.Tertiary steelmaking
Tertiary steelmaking refers to the final stage of the manufacturing process where molten steel is solidified and shaped into semi-finished or finished products. This stage is distinct from the chemical refining of the primary and secondary stages, focusing instead on physical form and surface properties.Casting
Before steel can be rolled, it must be solidified.- Continuous casting: This is the dominant method used for over 95% of global production. Molten steel flows from a ladle into a tundish and then through a water-cooled copper mold. The steel emerges as a continuous red-hot strand, which is straightened and cut. This produces standardized shapes such as slabs, blooms, and billets.
- Ingot casting: In this older batch process, steel is poured into stationary molds. It is still used today for specific high-alloy steels or for extremely large components that exceed the physical capacity of continuous casters.
Forming and finishing
- Hot rolling: The steel is heated above its recrystallization temperature and passed through heavy rollers. This reduces the thickness of the metal and refines the internal grain structure, improving toughness.
- Cold rolling: Performed at room temperature, this process further reduces thickness, improves surface finish, and increases tensile strength through strain hardening.
- Coating: To prevent corrosion, finished steel is often coated. Common methods include hot-dip galvanizing for construction materials and electrolytic tinning for packaging.