Steelmaking is the process of producingsteel from iron ore and/orscrap. Steel has been made for millennia, and was commercialized on a massive scale in the 1850s and 1860s, using theBessemer andSiemens-Martin processes.
Currently, two major commercial processes are used.Basic oxygen steelmaking (BOS) uses liquidpig-iron from ablast furnace and scrap steel as the main feed materials.Electric arc furnace (EAF) steelmaking uses scrap steel ordirect reduced iron (DRI). Oxygen steelmaking has become more popular over time.[1]
Steelmaking is one of the most carbon emission-intensive industries. In 2020, the steelmaking industry was reported to be responsible for 7% of energy sectorgreenhouse gas emissions.[2] The industry is seeking significant emission reductions.[3]
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 asnitrogen,silicon,phosphorus,sulfur, and excesscarbon (the most important impurity) are removed, and alloying elements such asmanganese,nickel,chromium, carbon, andvanadium are added to produce differentgrades of steel.
Early processes evolved during the classical era inChina,India,Rome and among hunter-foragers in northern Sweden. The earliest means of producing steel was in abloomery.
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. TheBessemer process and subsequent developments allowed steel to become integral to the global economy.[4]
A system akin to the Bessemer process originated in the 11th century in East Asia.[5][6] Hartwell wrote that theSong dynasty (960–1279 CE) innovated a "partial decarbonization" method of repeated forging ofcast iron under a cold blast.[7]Needham and Wertime described the method as a predecessor to the Bessemer process.[5][8][9] This process was first described by government officialShen Kuo (1031–1095) in 1075, when he visited Cizhou.[7] Hartwell stated that the earliest center where this was practiced was perhaps the great iron-production district along theHenan–Hebei border during the 11th century.[7]
In the 15th century, thefinery 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-freewrought iron, usually imported fromSweden. The manufacturing process, called thecementation process, consisted of heating bars of wrought iron together withcharcoal for periods of up to a week in a long stone box. This producedblister steel. The blister steel was put in a crucible with wrought iron and melted, producingcrucible steel. Up to 3 tons of (then expensive)coke was burnt for each ton of steel produced. When rolled into bars such steel was sold at £50 to £60 (approximately £3,390 to £4,070 in 2008)[11] along ton. The most difficult and laborious part of the process was the production of wrought iron infinery forges in Sweden.
In 1740,Benjamin Huntsman developed thecrucible technique for steel manufacture at his workshop inHandsworth,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 along ton, although it initially sold for around £40 a ton.
The Japanese may have made use of a Bessemer-type process, as observed by 17th century European travellers.[10] AdventurerJohan 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.[10]
By the early 19th century the puddling process was widespread. At the time, process heat was too low to entirely removeslag impurities, but thereverberatory 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 replacecharcoal 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, butraw materials with the required characteristics were not always easy to find.[12]
Modern steelmaking began at the end of the 1850s when theBessemer process became the first successful method of steelmaking in high quantity, followed by theopen-hearth furnace.
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.[13]
Basic oxygen steelmaking (BOS) involves melting carbon-richpig iron that has been developed bysmelting iron ore in ablast furnace, and converting it into steel. Blowing oxygen through molten pig iron oxidizes some of the carbon intoCO−
andCO
2, turning the iron into steel.Refractories (materials resistant to decomposition under high temperatures)—calcium oxide andmagnesium oxide—line the smelting vessel to withstand the heat, corrosive molten metal, andslag. The chemistry is controlled to remove impurities such as silicon and phosphorus.
The basic oxygen process was developed in 1948 byRobert Durrer, as a refinement of theBessemer converter that replaced air with (more efficient) pureoxygen. 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.[citation needed] In 2013, 70% of global steel output came from the basic oxygen furnace.[14] Furnaces can convert up to 350 tons of iron into steel in less than 40 minutes, compared to 10–12 hours in anopen hearth furnace.[15]
Electric arc furnaces make steel from scrap or direct reduced iron. A "heat" (batch) of iron is loaded into the furnace, sometimes with a "hot heel" (molten steel from a previous heat). 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.[15] This process allows larger alloy additions than the basic oxygen method.[16]
In HIsarna ironmaking, iron ore is processed almost directly into liquid iron orhot metal. The process is based around acyclone 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 theCO
2 emissions by around 20%.[17]
Direct-reduced iron can be produced from iron ore as it reacts with atomic hydrogen.Renewable hydrogen allows steelmaking withoutfossil fuels. Direct reduction occurs at 1,500 °F (820 °C). The iron is infused with carbon (from coal) in an electric arc furnace. Hydrogenelectrolysis requires approximately 2600kWh per ton of steel. Hydrogen production raises costs by an estimated 20–30% over conventional methods.[18][19][20]
The next step commonly usesladles. Ladle operations include de-oxidation (or "killing"), 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.[13]
Tertiary steelmaking is a term given to a variety of processes used for shaping the liquid steel.
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As of 2021[update], steelmaking was estimated to be responsible for around 11% of globalCO
2 emissions and around 7% of greenhouse gas emissions.[21][22] Making 1 ton of steel emits about 1.8 tons ofCO
2.[23] The bulk of these emissions are from theindustrial process in which coal provides the carbon that binds with the oxygen from the iron ore in ablast furnace in.[24]
AdditionalCO
2 emissions result from mining, refining and shipping ore,basic oxygen steelmaking,calcination, and thehot blast. Proposed techniques to reduceCO
2 emissions in the steel industry include reduction of iron ore usinggreen hydrogen rather than carbon, andcarbon capture and storage.[25]
Coal and iron ore mining are energy intensive, and damage their surroundings, leaving pollution, biodiversity loss, deforestation, and greenhouse gas emissions behind.
Blast furnaces remove oxygen and trace elements from iron and add a tiny amount of carbon by melting the iron ore at 1,700 °C (3,090 °F) in the presence of ambient oxygen andcoke (a type of coal). The oxygen from the ore is carried away by the carbon from the coke in the form ofCO
2. The reaction:
Fe
2O
3(s) + 3 CO(g) → 2 Fe(s) + 3CO
2(g)
The reaction occurs due to the lower (favorable) energy state ofCO
2 compared to iron oxide, and the high temperatures are needed to achieve the reaction'sactivation energy. A small amount of carbon bonds with the iron, formingpig iron, which is an intermediary before steel, as its carbon content is too high – around 4%.[26]
To reduce the carbon content in pig iron and obtain the desired carbon content of steel, it is re-melted and oxygen is blown through inbasic oxygen steelmaking. In this step, the oxygen binds with the undesired carbon, carrying it away in the form ofCO
2 gas, an additional emission source. After this step, the carbon content in the pig iron is lowered sufficiently to obtain steel.
FurtherCO
2 emissions result from the use oflimestone, which is melted at high temperatures in a reaction calledcalcination, according to:
CaCO
3(s) → CaO(s) +CO
2(g)
The resultingCO
2 is an additional source of emissions.Calcium oxide (CaO,quicklime) can be used as a replacement to reduce emissions.[27] It acts as a chemicalflux, removing impurities (such assulfur orphosphorus (e.g.apatite orfluorapatite)[28]) in the form ofslag and lowersCO
2 emissions according to reactions such as:
SiO2 + CaO → CaSiO3
This use of limestone to provide a flux occurs both in the blast furnace (to obtain pig iron) and in thebasic oxygen steel making (to obtain steel).
CO
2 emissions result from thehot blast, which increases blast furnace temperatures. The hot blast pumps hot air into the blast furnace. The hot blast temperature ranges from 900 to 1,300 °C (1,650 to 2,370 °F) depending on the design and condition. Oil,tar, natural gas, powdered coal andoxygen can be injected to combine with the coke to release additional energy and increase the percentage of reducing gases present, increasing productivity. Hot blast air is typically heated by burning fossil fuels, an additional emission source.[29]
The steel industry produces 7-8% of anthropogenicCO
2 emissions and is one of the most energy-intensive industries.[30][31] Emissions abatement and decarbonization strategies vary by manufacturing process. Options fall into three general categories: using a non-fossil energy source; increasing processing efficiency; and evolving the manufacturing process. They may be used individually or in combination.[citation needed]
"Green steel" describes steelmaking withoutfossil fuels.[32] Some companies that claim to produce green steel reduce, but do not eliminate, emissions.[33]
Australia produces nearly 40% of the world's iron ore. TheAustralian Renewable Energy Agency (ARENA) is funding research projects involving direct reduced ironmaking (DRI) to reduce emissions. Companies such asRio Tinto,BHP, andBlueScope are developing green steel projects.[34]
TheWhyalla Hydrogen Project, part ofSouth Australian PremierMalinauskas’State Prosperity Project, aims to producegreen steel. However, the project has been placed on hold due to financial and operational challenges ofGFG Alliance. Both the federal and state governments have intervened in an effort to address these issues with the steelworks.[35]
European projects from HYBRIT,LKAB,Voestalpine, andThyssenKrupp are pursuing strategies to reduce emissions.[36] HYBRIT claims to produce green steel.[33]
Top gas from the blast furnace is normally expelled into the air. This gas containsCO
2, H2, and CO. The top gas can be captured, theCO
2 removed, and the reducing agents reinjected into the blast furnace.[citation needed] A 2012 study suggested that this process can reduce blast furnaceCO
2 emissions by 75%,[37] while a 2017 study showed that emissions are reduced by 56.5% with carbon capture and storage, and reduced by 26.2% if only the recycling of the reducing agents is used.[38] To keep the carbon captured from entering the atmosphere, a method of storing it or using it would have to be found.
Another way to use the top gas is in a top recovery turbine which generates electricity, which thereby reduces external energy needs if electric arc smelting is used.[36] Carbon could also be captured from coke oven gases. As of 2022[update], separating the CO2 from other gases and components in the system, and the high cost of the equipment and infrastructure changes needed, have prevented adoption, but the emission reduction potential has been estimated to be up to 65% to 80%.[39][36]
Hydrogen direct reduction (HDR) using hydrogen produced from emission-free power (green hydrogen) offers emission-free iron-making, because water is the only by-product of the reaction betweeniron oxide and hydrogen.[40]
As of 2021,ArcelorMittal,Voestalpine, andTATA had committed to using green hydrogen to smelt iron.[41] In 2024 the HYBRIT project in Sweden was using HDR.[42]
For the European Union, it is estimated that the hydrogen demand for HDR would require 180 GW of renewable capacity.[43]
Another developing possible technology is iron ore electrolysis, where the reducing agent is electrons.[36] One method is molten oxide electrolysis. The cell consists of an inert anode, a liquid oxide electrolyte (CaO, MgO, etc.), and molten ore. When heated to ~1.600 °C, the ore is reduced to iron and oxygen. As of 2022Boston Metal was at the semi-industrial stage for this process, with plans to commercialize by 2026.[44][45]
The Siderwin research project involved Arcelormittal was testing a different type of electrolysis.[46] It operates at around 110 °C.[47]
Scrap steelmaking refers to steel that has either reached its end-of-life use, or is excess metal from the manufacture of steel components. Steel is easy to separate and recycle due to its magnetism. Using scrap avoids the emissions of 1.5 tons ofCO
2 for every ton.[48] As of 2023[update], steel had one of the highest recycling rates of any material, with around 30% of the world's steel coming from recycled components. However, steel cannot be recycled endlessly,[clarification needed] and the recycling processes, using arc furnaces, use electricity.[30]
In a blast furnace, iron oxides are reduced by a combination of CO, H2, and carbon. Only around 10% of the iron oxides are reduced by H2. With H2 enrichment, the proportion of iron oxides reduced by H2 is increased, consuming less carbon is consumed and emitting lessCO
2.[49] This process can reduce emissions by an estimated 20%.[citation needed]
One speculative idea is a project by SuSteel to develop a hydrogen plasma technology that reduces the ore with hydrogen at high operating temperatures.[36]
Biomass such as charcoal or wood pellets are a potential alternative blast furnace fuel, that does not involve fossil fuels, but still emits carbon. Emissions are reduced by 5% to 28%.[36]
German plantmaker SMS Group claims that Baosteel Desheng Stainless Steel Co., Ltd., a subsidiary of China Baowu Steel Group, has successfully completed the installation of a new vacuum oxygen decarburization (VOD) system at its Fuzhou plant with annual capacity of 417,000 metric tons of low carbon steel. Ultra-fine iron ore powder is injected into a superheated furnace using a specialized high-speed lance. The molten iron collects at the bottom of the furnace, creating a stream of high-purity iron.[50]
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