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Process for producing steel
Steel mill with two arc furnaces

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.

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 (EAF) steelmaking uses scrap steel or direct reduced iron (DRI). Oxygen steelmaking has become more popular over time.

Steelmaking is one of the most carbon emission-intensive industries. As of 2020, steelmaking was responsible for about 10% of 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 (the most important impurity) are removed, and alloying elements such as manganese, nickel, chromium, carbon, and vanadium are added to produce different grades of steel.

History

Bethlehem Steel in Bethlehem, Pennsylvania, was one of the world's largest manufacturers of steel before its 2003 closure.
Main article: Ferrous metallurgy

Early history

Early processes evolved during the classical era in China, India, and Rome. 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 (960–1279 CE) 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 government official Shen Kuo (1031–1095) 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

Johan Albrecht de Mandelslo described the Japanese use of the Bessemer process.

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 (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) 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.

Bessemer converter at Högbo Bruk, Sandviken.

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

Distribution of world steel production by methods

Modern steelmaking consists of three steps: primary, secondary, and tertiary.

Primary steelmaking involves smelting 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 step

Main articles: Basic oxygen steelmaking and Electric arc furnace

Basic oxygen

Basic oxygen steelmaking (BOS)involves melting carbon-rich pig iron and converting it into steel. Blowing oxygen through molten pig iron oxidizes some of the carbon into CO
 and CO
2, turning the iron into steel. Refractories (materials resistant to decomposition under high temperatures)—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 (more efficient) 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" (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 in BOS, fluxes are added to protect the vessel lining and help impurity removal. 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

Main article: HIsarna ironmaking process

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 making the BOS-required pig iron pellets. Skipping this preparatory step makes the HIsarna process more energy-efficient and lowers the carbon footprint.

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 1,500 °F (820 °C). The iron is infused with carbon (from coal) 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.

Second step

The next step commonly uses ladles. 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.

Tertiary step

This section needs expansion. You can help by adding to it. (December 2024)

Carbon dioxide emissions

As of 2021, steelmaking was estimated to be responsible for around 11% of global CO
2 emissions and around 7% of greenhouse gas emissions. Making 1 ton of steel emits about 1.8 tons of CO
2. The bulk of these emissions are from the industrial process in which coal provides the carbon that binds with the oxygen from the iron ore in a blast furnace in:

Additional CO
2 emissions result from mining, refining and shipping ore, basic oxygen steelmaking, calcination, and the hot blast. Proposed techniques to reduce CO
2 emissions in the steel industry include reduction of iron ore using green hydrogen rather than carbon, and carbon capture and storage.

Mining and extraction

Coal and iron ore mining are energy intensive, and damage their surroundings, leaving pollution, biodiversity loss, deforestation, and greenhouse gas emissions behind.

Blast furnace

Main article: Blast furnace

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 and coke (a type of coal). The oxygen from the ore is carried away by the carbon from the coke in the form of CO
2. The reaction:

Fe
2O
3(s) + 3 CO(g) → 2 Fe(s) + 3 CO
2(g)

The reaction occurs due to the lower (favorable) energy state of CO
2 compared to iron oxide, and the high temperatures are needed to achieve the reaction's activation energy. A small amount of carbon bonds with the iron, forming pig iron, which is an intermediary before steel, as its carbon content is too high – around 4%.

Decarburization

Main articles: Decarburization and Basic oxygen steelmaking

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 in basic oxygen steelmaking. In this step, the oxygen binds with the undesired carbon, carrying it away in the form of CO
2 gas, an additional emission source. After this step, the carbon content in the pig iron is lowered sufficiently to obtain steel.

Calcination

Main articles: Calcination and Slag

Further CO
2 emissions result from the use of limestone, which is melted at high temperatures in a reaction called calcination, according to:

CaCO
3(s) → CaO(s) + CO
2(g)

The resulting CO
2 is an additional source of emissions. Calcium oxide (CaO, quicklime) can be used as a replacement to reduce emissions. It acts as a chemical flux, removing impurities (such as sulfur or phosphorus (e.g. apatite or fluorapatite)) in the form of slag and lowers CO
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 the basic oxygen steel making (to obtain steel).

Hot blast

Main article: Hot blast

CO
2 emissions result from the hot 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 and oxygen 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.

Strategies for reducing carbon emissions

The steel industry produces 7-8% of anthropogenic CO
2 emissions and is one of the most energy-intensive industries. 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.

"Green steel" describes steelmaking without fossil fuels. Some companies that claim to produce green steel reduce, but do not eliminate, emissions.

Australia

Australia produces nearly 40% of the world's iron ore. The Australian Renewable Energy Agency (ARENA) is funding research projects involving direct reduced ironmaking (DRI) to reduce emissions. Companies such as Rio Tinto, BHP, and BlueScope are developing green steel projects.

Europe

European projects from HYBRIT, LKAB, Voestalpine, and ThyssenKrupp are pursuing strategies to reduce emissions. HYBRIT claims to produce green steel.

Top gas recovery in BF/BOF

Top gas from the blast furnace is normally expelled into the air. This gas contains CO
2, H2, and CO. The top gas can be captured, the CO
2 removed, and the reducing agents reinjected into the blast furnace. A 2012 study suggested that this process can reduce blast furnace CO
2 emissions by 75%, 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. 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. Carbon could also be captured from coke oven gases. As of 2022, 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%.

Hydrogen direct reduction

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 between iron oxide and hydrogen.

As of 2021, ArcelorMittal, Voestalpine, and TATA had committed to using green hydrogen to smelt iron. In 2024 the HYBRIT project in Sweden was using HDR.

For the European Union, it is estimated that the hydrogen demand for HDR would require 180 GW of renewable capacity.

Iron ore electrolysis

Another developing possible technology is iron ore electrolysis, where the reducing agent is electrons. 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 2022 Boston Metal was at the semi-industrial stage for this process, with plans to commercialize by 2026.

The Siderwin research project involved Arcelormittal was testing a different type of electrolysis. It operates at around 110 °C.

Scrap-use in BF/BOF

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 of CO
2 for every ton. As of 2023, 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, and the recycling processes, using arc furnaces, use electricity.

H2 enrichment in BF/BOF

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 less CO
2. This process can reduce emissions by an estimated 20%.

Other strategies

The HIsarna ironmaking process is a way of producing iron in a cyclone converter furnace without the pre-processing steps of choking/agglomeration, which reduces the CO
2 emissions by around 20%.

One speculative idea is a project by SuSteel to develop a hydrogen plasma technology that reduces the ore with hydrogen at high operating temperatures.

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%.

See also

References

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