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Steelmaking
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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.

Currently, two major commercial processes are used. Basic oxygen steelmaking (BOS) 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.[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 sector greenhouse gas emissions.[2] The industry is seeking significant emission reductions.[3]

Steel

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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

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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

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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.[4]

China

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A system akin to the Bessemer process originated in the 11th century in East Asia.[5][6] Hartwell wrote that the Song dynasty (960–1279 CE) innovated a "partial decarbonization" method of repeated forging of cast 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 official Shen 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 the Henan–Hebei border during the 11th century.[7]

Europe

[edit]
Johan Albrecht de Mandelslo described the Japanese use of the Bessemer process.[10]

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)[11] 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

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The Japanese may have made use of a Bessemer-type process, as observed by 17th century European travellers.[10] 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.[10]

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.[12]

Industrialization

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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

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Distribution of world steel production by methods

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]

Primary steelmaking

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Basic oxygen

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Main article: Basic oxygen steelmaking

Basic oxygen steelmaking (BOS) 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 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.[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 an open hearth furnace.[15]

Electric arc

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Main article: Electric arc furnace

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]

HIsarna

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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 the intermediary production of pig iron pellets required for BOS. Skipping this preparatory step makes the HIsarna process more energy-efficient and reduces the CO
2 emissions by around 20%.[17]

Hydrogen reduction

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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.[18][19][20]

Secondary steelmaking

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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.[13]

Tertiary steelmaking

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Tertiary steelmaking is a term given to a variety of processes used for shaping the liquid steel.

Carbon dioxide emissions

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As of 2021, steelmaking was estimated to be responsible for around 11% of global CO
2 emissions and around 7% of greenhouse gas emissions.[21][22] Making 1 ton of steel emits about 1.8 tons of CO
2.[23] 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.[24]

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.[25]

Mining and extraction

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

Blast furnace

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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%.[26]

Decarburization

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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

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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.[27] It acts as a chemical flux, removing impurities (such as sulfur or phosphorus (e.g. apatite or fluorapatite)[28]) 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

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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.[29]

Strategies for reducing carbon emissions

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The steel industry produces 7-8% of anthropogenic CO
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 without fossil fuels.[32] Some companies that claim to produce green steel reduce, but do not eliminate, emissions.[33]

Australia

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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.[34]

The Whyalla Hydrogen Project, part of South Australian Premier Malinauskas’ State Prosperity Project, aims to produce green steel. However, the project has been placed on hold due to financial and operational challenges of GFG Alliance. Both the federal and state governments have intervened in an effort to address these issues with the steelworks.[35]

Europe

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European projects from HYBRIT, LKAB, Voestalpine, and ThyssenKrupp are pursuing strategies to reduce emissions.[36] HYBRIT claims to produce green steel.[33]

Top gas recovery in BF/BOF

[edit]

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.[citation needed] A 2012 study suggested that this process can reduce blast furnace CO
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, 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

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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.[40]

As of 2021, ArcelorMittal, Voestalpine, and TATA 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]

Iron ore electrolysis

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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 2022 Boston 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-use in BF/BOF

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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.[48] 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,[clarification needed] and the recycling processes, using arc furnaces, use electricity.[30]

H2 enrichment in BF/BOF

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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.[49] This process can reduce emissions by an estimated 20%.[citation needed]

Other strategies

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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]

China

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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]

See also

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References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Steelmaking

View on Grokipedia
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Steelmaking is the metallurgical process of refining iron from pig iron or scrap into steel, an alloy of iron and carbon with typically less than 2% carbon content, by removing impurities such as excess carbon, silicon, phosphorus, and sulfur while adding alloying elements to achieve desired mechanical properties. The dominant modern techniques are the basic oxygen furnace (BOF) process, which blows high-purity oxygen onto molten pig iron to oxidize impurities, and the electric arc furnace (EAF) method, which melts scrap steel using electric arcs for recycling-based production. Historically, the Bessemer process, invented in the 1850s, enabled the first large-scale steel production by blowing air through molten iron to reduce carbon content rapidly, paving the way for the Industrial Revolution's expansion in railways, ships, and machinery. Subsequent innovations like the open-hearth furnace and BOF in the mid-20th century improved efficiency and scale, while EAF has grown due to scrap availability and lower energy needs relative to primary routes. Global crude steel output in 2024 totaled 1.886 billion tonnes, supporting infrastructure, automotive, and construction sectors, though the process remains highly energy-intensive, accounting for about 7-9% of anthropogenic CO2 emissions due to reliance on carbon-based reductants in ironmaking precursors.

Fundamentals of Steelmaking

Definition and Chemical Composition

Steelmaking is the metallurgical process of producing , an , from raw materials such as , , or scrap metal, primarily through the removal of impurities and precise control of carbon and alloying elements to achieve desired mechanical properties. This involves converting high-carbon , which contains about 4% carbon and other impurities like , , and , into by oxidation and refinement in furnaces such as basic oxygen or types. The process distinguishes from by reducing carbon below the eutectoid point, enabling for enhanced strength, , and . Steel is fundamentally an of iron and carbon, with carbon content ranging from approximately 0.02% to 2.1% by weight; contents below 0.02% classify the material as low-carbon or mild steel, while exceeding 2.1% shifts it toward , which is more brittle due to higher carbide formation. This carbon threshold arises from the iron-carbon , where the solubility limit at room temperature defines the boundary between steel and at the eutectoid composition of 0.76% carbon, though practical steelmaking targets up to 2.14% for hypereutectoid grades. Beyond carbon, steel typically includes (0.3–2.0%) to improve and deoxidize the melt, silicon (0.1–0.6%) for strength and fluidity, and minimal phosphorus (<0.05%) and sulfur (<0.05%) to avoid brittleness. Alloying elements beyond these base components define steel grades: low-alloy steels add up to 10% of elements like chromium or nickel for corrosion resistance or toughness, while high-alloy variants, such as , exceed 10% with at least 10.5% chromium to form a passive oxide layer. Standards like SAE J403 specify heat chemical compositions for carbon steels, ensuring ranges such as 0.08–1.00% carbon for automotive and structural applications, with tolerances for manganese and other residuals to maintain consistency. These compositions are verified through spectrometry during production to meet empirical performance criteria, as higher carbon increases hardness but reduces weldability and ductility via pearlite and martensite microstructures.

Thermodynamic and Metallurgical Principles

Steelmaking relies on thermodynamic principles to determine the feasibility of reduction and oxidation reactions, primarily through the Gibbs free energy change (ΔG = ΔH - TΔS), where reactions proceed spontaneously when ΔG < 0 under given temperature (T) and composition conditions. In the blast furnace, indirect reduction of iron oxides by carbon monoxide predominates, as exemplified by the reaction FeO + CO → Fe + CO₂, which becomes thermodynamically favorable above approximately 700°C due to the entropy-driven decrease in ΔG with increasing temperature. The Ellingham diagram quantifies this by plotting ΔG° versus temperature for oxide formation reactions normalized to oxygen consumption, revealing that the 2CO → 2C + O₂ line intersects the Fe/FeO line around 670°C, enabling CO to reduce iron oxides while more stable oxides like SiO₂ and Al₂O₃ remain unreduced, forming slag components. During primary steelmaking in the basic oxygen furnace (BOF), pure oxygen oxidizes dissolved carbon, silicon, manganese, and in molten pig iron, with reactions such as [C] + O₂ → CO₂ exhibiting strongly negative ΔG values (e.g., ΔG° ≈ -395 kJ/mol at 1600°C), driving exothermic decarburization and providing for the process. formation, involving lime (CaO) to flux silica and (e.g., 2[P] + 5O + 3CaO → 3CaO·P₂O₅), is thermodynamically controlled by the activity of oxygen and basicity ratio (CaO/SiO₂ > 2 for effective phosphorus removal), ensuring impurity partitioning into the slag phase per distribution coefficients derived from equilibrium constants. balances must account for endothermic and mixing, with typical BOF oxygen blows supplying 12-16 GJ/t through reaction enthalpies, minimizing external energy input. Metallurgically, steel's properties stem from controlled phase transformations in the iron-carbon system, where carbon varies markedly: up to 2.1 wt% in (γ-Fe) at 1147°C but only 0.02 wt% in ferrite (α-Fe) at , necessitating precise to avoid brittle high-carbon phases like (Fe₃C). During solidification from molten (typically 0.05-1.5 wt% C), a peritectic reaction at 1493°C transforms δ-ferrite + liquid to , promoting microsegregation of solutes like and , which can form low-melting inclusions if not diluted below 0.01 wt% via deoxidation with aluminum or (e.g., 2Al + 3O → Al₂O₃, ΔG° ≈ -1200 kJ/mol). Alloying elements such as (up to 1.5 wt%) stabilize and refine (α + Fe₃C eutectoid at 0.77 wt% C, 727°C), enhancing strength via solid-solution and , while removes to prevent cracking from its low (Henry's constant ~0.0003 wt% atm⁻¹ at 1600°C). These principles ensure defect-free microstructures, with non-metallic inclusions limited to <50 ppm for high-quality steels through kinetic control of nucleation and growth during cooling.

Historical Development

Pre-Industrial Techniques

Pre-industrial steelmaking encompassed small-scale, labor-intensive methods that produced steel in limited quantities, primarily for high-value applications such as swords, tools, and armor, rather than bulk production. These techniques predated large-scale industrialization and relied on direct reduction of iron ore or refinement of cast iron, often achieving inconsistent carbon contents between 0.2% and 2% to distinguish steel from low-carbon wrought iron or high-carbon cast iron. The bloomery process, originating around 1200 BCE in the Near East and , represented the earliest widespread method for producing iron and rudimentary steel. Iron ore, typically or , was reduced in a clay furnace using charcoal as both fuel and reductant, with air blown in via bellows to reach temperatures of about 1100–1200°C; this yielded a spongy bloom of metallic iron intermixed with slag, which was hammered to expel impurities and consolidate the metal. Carbon content varied due to absorption from charcoal, sometimes resulting in steel-like material if forging controlled carburization, though most output was wrought iron requiring secondary pack carburizing in charcoal to elevate carbon levels for steel. This direct smelting avoided melting, limiting scale to blooms of 5–20 kg per smelt. In South Asia, particularly India, crucible steel production emerged independently by the 6th century BCE or earlier, exemplified by . This involved co-melting low-carbon wrought iron or sponge iron with organic carbon sources like charcoal or plant residues in closed clay-graphite crucibles heated to 1200–1500°C for several hours, homogenizing the alloy into ingots with 1–1.5% carbon and trace elements like vanadium that enabled distinctive microstructures upon forging. Archaeological evidence from sites in Telangana and Karnataka confirms production sites operational until the mid-19th century, with ingots exported widely for pattern-welded blades. The sealed crucible environment prevented oxidation, allowing liquid-phase formation absent in bloomeries. European pre-industrial steelmaking from the late medieval period onward incorporated blast furnaces for pig iron production—first documented in Sweden around 1300 CE—and subsequent refinement. The finery forge decarburized molten pig iron (4–5% carbon) in a hearth with air blasts and slag-forming fluxes like limestone, stirring to oxidize excess carbon and silicon, yielding wrought iron blooms; partial fining could retain 0.5–1.5% carbon for steel, though consistency was low without precise control. Complementing this, the cementation process, practiced by the 17th century and possibly earlier in Germany, carburized wrought iron bars packed in charcoal within sealed clay chests heated to 1000–1200°C for 7–12 days, diffusing carbon to form surface-hardened blister steel bars up to 1–2% carbon overall. This method scaled better for bar steel but produced heterogeneous material requiring further forging or crucible remelting for uniformity.

Industrial Revolution Advances

The puddling process, patented by Henry Cort in 1784, transformed pig iron into wrought iron on an industrial scale using reverberatory furnaces fueled by coke, where workers stirred the melt to oxidize carbon and impurities, yielding balls of malleable iron that were then rolled into bars. This method superseded charcoal-based finery forges, enabling Britain to produce over 30,000 tons of wrought iron annually by the early 1800s and supporting mechanized rolling mills for bars and sheets essential to machinery and infrastructure. Though primarily yielding low-carbon wrought iron rather than true steel, variants like puddled steel—produced by controlled recarburization—provided a precursor for higher-strength applications amid rising demand. The pivotal advance for mass steel production arrived with Henry Bessemer's converter process, patented in 1856, which blasted compressed air through molten pig iron in a pear-shaped vessel to rapidly oxidize excess carbon and impurities, completing conversion in approximately 20-25 minutes for batches up to 25 tons. This innovation slashed steel costs from £50-60 per ton to £6-7 per ton by eliminating labor-intensive stirring and fuel-heavy melting, propelling output from artisanal levels to millions of tons yearly and enabling widespread use in railways, ships, and construction. Bessemer's method, independently paralleled by William Kelly in the U.S. around 1851 though with disputed priority, initially struggled with phosphorus impurities in British ores but spurred global adoption after refinements like the Gilchrist-Thomas basic lining in 1878. Complementing Bessemer, the open-hearth process—developed by William Siemens's regenerative furnace design in the 1850s and adapted for steel by Pierre-Émile Martin in France in 1864—melted pig iron and scrap in shallow, open basins heated indirectly to 1,600-1,700°C, allowing precise alloying and dephosphorization over 8-12 hour cycles for batches exceeding 100 tons. This flexibility addressed Bessemer's limitations in quality control and scrap incorporation, dominating production by the 1890s as steel output in major nations rose from under 1 million tons in 1870 to over 28 million tons by 1913, fueling urbanization and heavy engineering. These processes collectively shifted steelmaking from empirical craftsmanship to chemical engineering principles, prioritizing oxidation kinetics and heat efficiency for scalable, cost-driven output.

20th-Century Modernization and Scale-Up

The open-hearth process, which dominated steelmaking from the late 19th century, began yielding to more efficient methods in the early 20th century, particularly after World War II, as demand for steel surged for reconstruction and industrial expansion. By the 1950s, the basic oxygen process (BOP), also known as the Linz-Donawitz or LD process, emerged as a pivotal innovation, enabling rapid oxidation of impurities in molten pig iron using a supersonic lance delivering pure oxygen. Invented in 1948 by Swiss engineer Robert Durrer and commercially implemented in 1952 at Voestalpine's Linz and Donawitz plants in Austria, the BOP reduced refining time from 8-12 hours in open-hearth furnaces to about 40 minutes, increased furnace capacity, and lowered energy consumption, facilitating larger-scale production. Its adoption accelerated globally; by the 1970s, BOP accounted for over 50% of steel production in major economies like the United States and Japan, displacing open-hearth operations and contributing to cost reductions of up to 30%. Parallel to BOP advancements, electric arc furnace (EAF) steelmaking gained traction for scrap-based production, leveraging abundant post-war scrap supplies and declining electricity costs. Adapted for steel from crucible processes by the early 1900s, EAFs were initially suited for specialty alloys but scaled for commodity steel after World War II, with U.S. minimills pioneering decentralized, low-capital facilities from the 1960s onward. These furnaces melt scrap via electric arcs reaching 3,000°C, offering flexibility and lower emissions compared to integrated blast furnace routes, and by the late 20th century, EAFs produced over 30% of global steel, particularly in regions with high scrap availability like North America. Continuous casting further modernized the industry by solidifying molten steel directly into billets, blooms, or slabs, bypassing energy-intensive ingot molding and rolling. Commercialized in the 1950s following earlier patents, it achieved widespread use by the 1980s, improving yield by 10-15% through reduced cropping losses, enhancing surface quality, and enabling thinner sections for downstream processing. By 2000, over 90% of global steel was continuously cast, driving efficiency gains and supporting the shift to high-volume, customized output. These innovations underpinned explosive production growth, with global crude steel output rising from approximately 35 million metric tons in 1900 to 784 million metric tons by 2000, fueled by post-war economic booms, urbanization, and infrastructure demands in Europe, North America, and emerging Asia. Integrated plants expanded to gigascale capacities, exemplified by U.S. facilities like those of , which peaked at over 30 million tons annually in the mid-20th century before market shifts. The combined effect of faster processes, recycling integration, and automation not only scaled output but also improved energy efficiency, with modern plants requiring 20-30% less input per ton than early-20th-century counterparts.

Raw Materials and Preparation

Iron Ore Extraction and Beneficiation

Iron ore, the primary source of iron for steelmaking, consists mainly of oxides such as hematite (Fe₂O₃, containing up to 70% iron) and magnetite (Fe₃O₄, up to 72% iron), with lower-grade deposits including taconite, a banded iron formation requiring extensive processing. These ores are found in sedimentary layers or magmatic deposits, often intermixed with gangue minerals like silica and alumina that must be removed to achieve viable iron content for blast furnace feed, typically above 60-65% Fe. Extraction predominantly employs open-pit mining for shallow, large-volume deposits, which account for over 90% of global production due to lower costs and higher output rates. The process begins with overburden removal, followed by drilling and blasting to fragment the ore, then loading and hauling via trucks or conveyor systems to crushers at the mine site. Underground methods, used for deeper or structurally complex ores, involve sublevel caving or block caving but represent a minority, as seen historically in the where they supplemented open-pit operations until the mid-20th century. Major producers include , with 960 million metric tons (Mt) in 2023, and at 440 Mt, leveraging vast high-grade hematite reserves in the Carajás and Minas-Rio regions. Global output exceeded 2.5 billion tons in 2023, driven by demand for direct-shipping ores that minimize downstream processing. Beneficiation upgrades raw ore by concentrating iron minerals and rejecting impurities, essential for low-grade taconite (20-30% Fe) which dominates in regions like Minnesota and demands magnetic separation to yield pellets of 65%+ Fe. Initial stages involve multi-phase crushing to reduce particle size below 30 mm, followed by screening to separate fines and oversize material for recirculation. Grinding in ball or rod mills then liberates minerals, enabling separation techniques: wet low-intensity magnetic separation for magnetite recovery (effective at 1,000-3,000 gauss fields), gravity spirals for dense hematite, and flotation for silica removal using reagents like amines. These steps recover 90-95% of iron values while reducing gangue to under 5%, directly lowering blast furnace slag volume by 20-30% and energy consumption in steelmaking. Without beneficiation, impurities elevate phosphorus and sulfur levels, compromising steel quality and increasing refining costs.

Carbon Sources and Reductants

Metallurgical coke, derived from bituminous coking coals through high-temperature carbonization in coke ovens at 900–1,100°C, functions as the principal carbon source and reductant in the blast furnace-basic oxygen furnace (BF-BOF) route, which produced about 1.85 billion tonnes of crude steel in 2023. This process yields a strong, porous carbon-rich material with a fixed carbon content exceeding 85%, low volatile matter (typically under 1.5%), and minimal ash (less than 12% for high-quality grades), enabling it to sustain the furnace burden while facilitating the reduction of iron oxides via the Boudouard reaction (C + CO₂ → 2CO) and providing carburization to molten pig iron. Coke's dual role as fuel and reductant requires specific mechanical properties, including a Coke Reactivity Index (CRI) below 30 and Coke Strength after Reaction (CSR) above 60, to resist degradation under the high temperatures (up to 2,000°C) and reducing gases in the blast furnace, ensuring permeability for ascending gases and preventing channeling that could impair efficiency. Approximately 400–600 kg of coke is consumed per tonne of hot metal, though pulverized coal injection (PCI) of non-coking coals (e.g., anthracite or bituminous fines) at rates up to 200–250 kg/tonne can substitute 30–40% of coke, lowering costs and emissions while maintaining reduction via injected carbon gasification. In electric arc furnace (EAF) steelmaking, which relies on scrap, solid carbon sources like anthracite, calcined petroleum coke, or synthetic graphite are added for slag foaming, recarburization, and alloying, typically at 5–20 kg/tonne to adjust carbon levels to 0.05–1.5% in the final steel. In direct reduction processes for sponge iron, used in about 8% of global steel production, gaseous reductants predominate: reformed natural gas provides CO and H₂ (in a 1:1 ratio) in shaft furnaces like Midrex, reducing iron ore pellets at 800–1,050°C with H₂ + 3FeO → Fe + H₂O and CO + FeO → Fe + CO₂ reactions, yielding direct reduced iron (DRI) with 90–95% metallization and minimal carbon (under 2%). Coal-based direct reduction, as in rotary kilns, employs thermal coal or lignite char as both heat source and reductant at 1,000–1,200°C, achieving similar metallization but with higher impurities and emissions. Emerging alternatives include biomass-derived chars (e.g., from wood or agricultural waste) and hydrogen from electrolysis, tested in pilot plants to replace fossil carbons; for instance, hydrogen-based DRI can cut CO₂ emissions by over 90% versus coal routes, though scalability remains limited by energy costs and infrastructure as of 2024. These non-fossil options, while reducing reliance on coking coal reserves concentrated in , the U.S., and Russia, face challenges in matching coke's strength and purity for large-scale blast furnace integration.

Scrap Utilization and Recycling

Steel scrap, consisting of ferrous metal remnants from manufacturing, fabrication, and end-of-life products, serves as a critical input in modern steelmaking, particularly in electric arc furnace (EAF) processes where it can constitute up to 100% of the charge. Scrap utilization avoids the energy-intensive reduction of iron ore, substituting direct melting of pre-reduced iron, and globally accounted for approximately 460.6 million metric tons consumed in steel production in 2024, representing about 24% of total crude steel output when adjusted for partial use in basic oxygen furnaces (BOF). In EAF routes, which produced 29.1% of global steel in 2024 (around 548 million tons out of 1.885 billion tons total), scrap forms the primary feedstock, enabling higher recycling rates compared to the BF-BOF route's typical 10-30% scrap charge. Scrap is categorized into three main types based on origin: home or internal scrap generated directly in steel mills during production (e.g., crop ends, rejects); prompt or industrial scrap from manufacturing processes like automotive stamping; and obsolete or post-consumer scrap from demolished structures, vehicles, and appliances, which dominates supply but poses quality variability due to unknown compositions. Preparation involves shredding large pieces into fist-sized fragments, magnetic separation to remove non-ferrous contaminants, and sometimes eddy current or density sorting to isolate steel from alloys like copper or aluminum; these steps ensure charge uniformity and minimize tramp elements such as copper (up to 0.2-0.3% tolerable in most grades) that accumulate over recycles and degrade steel properties. Heavy melting steel (HMS) grades, like HMS 1&2, are common inputs, baled or shredded for density, with specifications set by standards from organizations like the Bureau of International Recycling to maintain market quality. The environmental advantages stem from scrap's lower processing demands: melting one ton requires 74% less energy than ore-based routes, yielding 0.4-0.6 tons CO2 versus 1.8-2.0 tons for BF-BOF, and conserving 1.1 tons of iron ore plus associated reductants per ton recycled. Economically, it reduces raw material costs and landfill waste, with global recycling rates for steel scrap exceeding 80% in developed economies, though supply constraints arise from urban mining inefficiencies and export restrictions, as seen in China's 2024 usage at 23.1% amid domestic shortages. Challenges include residual element buildup—copper and tin from obsolete scrap can exceed limits after 5-10 cycles, necessitating dilution with primary iron or specialized refining—and inconsistent supply tied to economic cycles, with 2024 consumption flat at prior levels due to stagnant steel demand. Quality control relies on chemical analysis and sorting technologies, but tramp elements still drive premium pricing for low-residual "shred" scrap, limiting scalability in high-alloy steels. Despite these, scrap's role is expanding, projected to reach 40% EAF share by 2030 in decarbonization efforts, contingent on improved collection and policy support for open markets.

Primary Steelmaking Processes

Blast Furnace-Basic Oxygen Furnace Route

The blast furnace-basic oxygen furnace (BF-BOF) route represents the predominant pathway for primary steel production, utilizing iron ore as the primary feedstock to yield approximately 71% of global crude steel output as of 2023. In this integrated process, the blast furnace reduces iron ore to molten pig iron, or hot metal, containing 4-5% carbon, which is subsequently refined in the basic oxygen furnace to remove excess carbon and impurities, producing liquid steel suitable for further processing. This method relies on carbon-based reductants like coke for smelting and high-purity oxygen for oxidation, enabling high-volume production but generating significant CO₂ emissions due to the combustion of carbon in both stages. In the blast furnace stage, iron-bearing materials such as sinter, pellets, or lump ore—typically 60-70% iron content—are charged from the top alongside metallurgical coke and fluxes like limestone. Preheated air, enriched with oxygen or hydrocarbons, is blasted in at the bottom through tuyeres at temperatures exceeding 1000°C, driving the countercurrent reduction reactions where coke combusts to carbon monoxide, which reduces iron oxides to molten iron while limestone forms slag to capture gangue minerals like silica and alumina. The process operates continuously at internal temperatures up to 2000°C in the hearth, yielding hot metal at 1400-1500°C and liquid slag, with modern coke consumption rates averaging 300-400 kg per tonne of hot metal under optimized conditions influenced by ore quality, blast temperature, and pulverized coal injection. Tap-to-tap times range from 6-10 hours, with furnace volumes scaled to 4000-6000 cubic meters for capacities over 10,000 tonnes daily. The hot metal is then transported via torpedo ladle to the basic oxygen furnace, a pear-shaped vessel lined with basic refractories like magnesia-carbon bricks, where it is charged with 20-30% steel scrap to dilute heat and provide alloying elements. High-purity oxygen, delivered via a water-cooled lance submerged into the bath, is blown at supersonic speeds for 15-25 minutes, oxidizing carbon to CO and CO₂, silicon to SiO₂, manganese to MnO, and phosphorus to P₂O₅, with the exothermic reactions raising the temperature to 1650-1700°C. Lime or dolomitic flux is added to form slag that absorbs these oxides, enabling phosphorus removal efficiency above 90% in basic environments; carbon content is reduced from 4-5% to 0.05-1.5%, depending on steel grade specifications. Cycle times total 30-50 minutes, including charging and tapping, with BOF vessels handling 200-400 tonne heats and yielding steel with yields of 90-95% after slag separation. This route's efficiency stems from the self-sustaining heat from oxidation in the BOF, minimizing external energy inputs beyond the blast furnace's coke-derived heat, but it demands large-scale integration with sintering plants and coke ovens, contributing to its dominance in regions with abundant ore and coal resources. Variations include top- and bottom-blowing lances for improved mixing and reduced refractory wear, with process control via off-gas analysis ensuring precise endpoint carbon levels within 0.01% accuracy. Despite optimizations like hot metal desulfurization pretreatment, the BF-BOF pathway remains carbon-intensive, emitting roughly 1.8-2.0 tonnes CO₂ per tonne of steel due to coke reduction and off-gas losses.

Electric Arc Furnace Process

The electric arc furnace (EAF) is a steelmaking process that utilizes high-voltage electric arcs generated between graphite electrodes and a conductive charge to melt ferrous scrap, direct reduced iron (DRI), or pig iron into molten steel. The furnace consists of a cylindrical, refractory-lined steel shell with a retractable roof, typically water-cooled walls, and three carbon electrodes suspended vertically, capable of producing arcs exceeding 3,000°C while the steel melt is maintained at approximately 1,600–1,700°C. This method accounts for about 28.6% of global crude steel production as of 2023, predominantly in regions with abundant scrap supplies and electricity. The EAF process originated in the late 19th century with experimental arcs for melting iron, but commercial viability for steelmaking emerged in 1900 when Paul Héroult developed the first successful furnace, followed by adoption for steel production around 1906 initially for alloys and specialties. U.S. EAF steel output grew nearly 90% in the 1920s due to scrap availability post-World War I, reaching over 1 million tons by 1940, though it remained secondary to blast furnace routes until post-1960s miniaturization and oxygen lancing enabled mass production of commodity steels. Operation begins with charging the furnace—typically 100% scrap steel sorted for composition, though up to 30% DRI or hot metal may be added for consistency—via overhead baskets after swinging off the roof. Electrodes lower to strike an arc, generating resistive and radiant heat to melt the charge in 45–60 minutes, consuming 350–500 kWh per metric ton of liquid steel; foaming slag from injected carbon and oxygen enhances efficiency by shielding walls and improving energy transfer. Refining follows via oxygen lancing to oxidize impurities like carbon (to <0.05%), phosphorus, and sulfur, forming slag with lime additions for desulfurization; alloying elements are adjusted, and stirring via argon or electromagnetic fields homogenizes the bath. Tapping pours molten steel through a taphole into a ladle for secondary refining, while slag is skimmed; cycle times average 50–90 minutes per heat, yielding 80–150 tons. Compared to blast furnace-basic oxygen furnace (BF-BOF) routes, EAF offers lower capital costs, rapid startup (hours vs. weeks), and flexibility for small-batch production, enabling 100% scrap recycling that cuts energy use by 60–75% and direct CO2 emissions to 0.2–0.5 tons per ton of steel (versus 1.8–2.0 for BF-BOF), though total emissions depend on grid electricity carbon intensity. Drawbacks include scrap quality variability introducing tramp elements like copper (limited to <0.3% for most grades), higher electricity demand, and electrode wear costing 2–3 kg per ton. Modern optimizations, such as DC arcs reducing flicker and oxy-fuel burners cutting power needs by 10–20%, have boosted yields to 95% and expanded EAF to 29.1% of global output by 2024.

Direct Reduction and Emerging Alternatives

Direct reduction produces metallic iron by reducing iron oxides in iron ore to direct reduced iron (DRI) or hot briquetted iron (HBI) at temperatures below iron's melting point, avoiding the coke-dependent blast furnace and enabling integration with electric arc furnaces for steelmaking. The process utilizes shaft furnaces or fluidized beds where ore pellets or lumps react with reducing gases, primarily a mixture of hydrogen (H₂) and carbon monoxide (CO) generated via reforming of natural gas, achieving metallization rates of 90-97% and carbon contents of 1-4%. This route accounted for 140.8 million metric tons of global DRI production in 2024, a 3.8% increase from 2023 and equivalent to about 7% of virgin iron inputs for steel, with major capacity in regions like the Middle East and India benefiting from abundant natural gas. The Midrex process dominates, employing countercurrent flow in a vertical shaft furnace: ore descends while hot reformed gas (typically 800-900°C, 55-65% H₂ and 25-35% CO) ascends, reducing Fe₂O₃ to Fe via sequential reactions like 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂, with top-gas recycled after CO₂ removal to minimize energy use of around 2.5-3.0 GJ per ton of DRI. Alternative gas-based methods like HYL Energia use similar principles but incorporate higher recycle rates and natural gas pre-reforming for flexibility with ore fines. Coal-based DR variants exist but are less common due to higher impurities and emissions, limited mostly to smaller-scale operations in regions with cheap coal. DRI's advantages include modular plant scalability (capacities from 0.2 to 2.5 million tons annually) and lower CO₂ intensity—about 0.5-1.0 tons per ton of steel when paired with EAF—compared to 1.8-2.0 tons for blast furnace-basic oxygen furnace routes, though natural gas reforming still generates process CO₂. Emerging alternatives prioritize hydrogen to eliminate fossil carbon, with pure H₂-DRI producing only water as byproduct: Fe₂O₃ + 3H₂ → 2Fe + 3H₂O, requiring ore preheating to 700-900°C and hydrogen supply costs below $2/kg for viability. Pilot and demonstration plants, such as Sweden's HYBRIT, produced the first fossil-free steel briquettes from H₂-DRI in 2021 using intermittent renewable-powered electrolysis for hydrogen, and in 2025 completed large-scale underground hydrogen storage trials to address intermittency, enabling potential industrial rollout by 2026-2030 with capacities targeting 2.5 million tons annually. Complementary technologies include fluidized-bed H₂-DR for finer ores and hydrogen plasma smelting reduction, which combines reduction and melting in electric arcs for hotter operation (up to 1600°C) and higher energy efficiency but remains at technology readiness level 4-6 due to electrode wear and scale-up challenges. These pathways depend on green hydrogen scalability, with projections indicating H₂-DRI could capture 10-20% of new primary steel capacity by 2035 if electrolysis costs fall to $300-500/kW.

Refining and Shaping

Secondary Steelmaking Operations

Secondary steelmaking operations, conducted primarily in the transfer ladle following primary steel production, refine molten steel by adjusting its chemical composition, temperature, and inclusion content to meet specifications for high-quality grades. These processes, often termed ladle metallurgy or ladle refining, enable precise control over elements like carbon, sulfur, and gases, which is essential for producing steels with enhanced mechanical properties and cleanliness. Operations typically include deoxidation, desulfurization, alloying, homogenization via gas stirring, and vacuum degassing, with treatment times ranging from 20 to 90 minutes depending on steel grade and plant configuration. Deoxidation removes excess oxygen dissolved in the steel, preventing defects such as porosity or brittleness, by adding agents like aluminum or that form stable oxides which float to the slag. This step occurs early in ladle treatment to stabilize the melt before further refinements, with oxygen levels reduced to below 20 ppm in many modern operations for ultra-clean steels. Alloying follows, involving additions of ferroalloys or elements such as and to achieve target compositions, often under controlled stirring to ensure uniformity. Argon gas bubbling through porous plugs in the ladle bottom promotes mixing, inclusion flotation, and slag-metal reactions, improving homogeneity without excessive refractory wear. Desulfurization targets sulfur removal to levels as low as 0.001% or below, critical for ductility and toughness, as sulfur forms brittle inclusions like MnS. This is achieved by injecting calcium-based powders (e.g., lime or calcium carbide) into the ladle, which react with sulfur in the presence of low oxygen activity to form calcium sulfide inclusions that enter the slag; efficiency depends on slag basicity, temperature (typically 1550–1600°C), and initial sulfur concentration, often reducing it by 50–80% in a single treatment. Ladle furnaces (LF) provide electric arc heating via graphite electrodes to maintain temperature and facilitate these reactions, with power inputs up to 50 kW/ton for reslagging and alloy recovery. Vacuum degassing, a key operation for gas removal and decarburization, employs processes like Ruhrstahl-Heraeus (RH) or vacuum tank degassing (VD), where the ladle or a snorkel vessel is evacuated to 1–10 mbar, allowing hydrogen, nitrogen, and hydrogen to evolve from the melt. In RH systems, steel is recirculated between ladle and vacuum vessel using inert gas lift, enabling carbon reduction to below 0.03% via oxygen blowing while minimizing chromium loss in stainless grades; treatment durations are 15–30 minutes, yielding steels with hydrogen under 2 ppm. These operations collectively enable over 90% of global steel production to meet stringent quality demands, though they increase energy use by 10–20 kWh/ton compared to primary routes alone.

Continuous Casting and Initial Forming

Continuous casting represents the predominant method for solidifying molten steel into semi-finished shapes following primary or secondary steelmaking, replacing traditional ingot casting for the majority of global production due to superior efficiency and product uniformity. In this process, molten steel is transferred from a ladle into a tundish, which regulates flow and distributes the metal into a water-cooled oscillatory mold, typically made of copper, where an outer shell rapidly forms via primary cooling. The partially solidified strand is then withdrawn vertically or curved horizontally through secondary cooling zones using water sprays, achieving full solidification before being straightened, cut to length via torches or shears, and discharged as slabs, blooms, or billets. The technology originated from early concepts in the 19th century but achieved practical implementation in the mid-20th century, with the first industrial-scale machines developed by Concast in the 1950s through collaboration with European steelmakers, enabling continuous operation and reducing material losses inherent in batch ingot processes. By the 1960s, adoption accelerated, particularly in where the initial caster commenced operations in 1955, leading to widespread replacement of ingot teeming by the 1980s as mills upgraded for cost and quality gains. Key equipment includes submerged entry nozzles to minimize turbulence and oxidation at the mold meniscus, electromagnetic stirrers for homogenizing the internal structure, and automated control systems for casting speed, typically 1-3 meters per minute depending on section size. The resulting products—slabs (rectangular, 150-300 mm thick for plate rolling), blooms (square, >200 mm side for structural shapes), and billets (square or rectangular, <150 mm for long products)—exhibit reduced macrosegregation and centerline porosity compared to ingots, as the directional solidification minimizes remelting and cropping needs. Initial forming follows directly or after brief cooling, where cast strands undergo hot charging into rolling mills for deformation into final geometries, bypassing extensive reheating to conserve energy—yielding up to 10-15% higher overall process efficiency versus ingot routes. This integration enhances metallurgical quality by limiting exposure to atmospheric contamination and enables thinner slabs (via thin-slab casters) for near-net-shape rolling, reducing subsequent passes and surface defects. Yield improvements stem from lower discard rates, often achieving 95-98% metal recovery against 80-90% in ingot casting, driven by precise control over solidification gradients. Despite these benefits, the process demands rigorous defect mitigation, such as oscillation marks or internal cracks from uneven cooling, addressed through optimized mold powders and dynamic soft reduction techniques to compress the unsolidified core. Economic analyses confirm continuous casting's role in lowering energy use by eliminating ingot soaking pits, cutting pollution via reduced scrap handling, and boosting labor productivity through automation.

Economic and Industrial Landscape

Global Production Statistics and Capacity

In 2024, global crude steel production reached 1,886 million tonnes, marking a 0.9% decline from the 1,904 million tonnes produced in 2023, reflecting subdued demand in key markets amid economic slowdowns and high energy costs. This figure encompasses data from 71 countries reporting to the , covering approximately 98% of worldwide output. Production via the blast furnace-basic oxygen furnace route dominated, though shares have grown due to scrap availability and energy transitions in developed economies. China led production with 1,005 million tonnes, comprising 53.3% of the global total, followed by India at 149 million tonnes (7.9%), Japan at 84 million tonnes (4.5%), and the United States at around 80 million tonnes (4.2%). This concentration underscores China's role as the dominant producer, driven by state-supported expansions and exports exceeding 110 million tonnes annually, which have pressured prices in importing regions. Global steelmaking capacity in 2024 totaled approximately 2,207 million tonnes per annum in operating plants, implying an average utilization rate of about 85.5%. However, excess capacity remains pronounced, estimated at over 602 million tonnes globally, with China contributing the majority through permitted expansions reaching 1.25-1.3 billion tonnes annually despite production caps and domestic demand peaking below 900 million tonnes. Such overcapacity, often subsidized via low-interest loans and environmental leniency, sustains exports that distort markets, as utilization in non-Chinese regions hovers near 75-80% while Chinese mills operate below efficient thresholds during off-peak periods. Planned additions of 68 million tonnes through 2026, predominantly in Asia, signal continued pressure unless offset by retirements.

Major Producers and Competitive Dynamics

China produces over half of the world's crude steel, with output reaching 1.005 billion metric tons in 2024, compared to a global total of approximately 1.88 billion metric tons. This dominance stems from state-supported consolidation into massive enterprises, such as China Baowu Group, which led global company production at 130.09 million tons in 2024. Other major Chinese firms include (around 55 million tons) and HBIS Group (41 million tons), enabling economies of scale but also contributing to persistent overcapacity estimated at 50 million tons surplus in 2025.
RankCompanyCountry2024 Production (million tons)
1China Baowu Group130.09
2Luxembourg (global ops)65
3~55
443.66
5HBIS Group41.34
Outside China, ArcelorMittal operates as the largest multinational with facilities across Europe, North America, and Asia, producing 65 million tons in 2024 through a mix of integrated and mini-mills. In the United States, Nucor Corporation leads with 20.66 million tons, emphasizing electric arc furnace (EAF) recycling to leverage scrap availability and lower emissions. India's Tata Steel and JSW Steel are expanding capacity to meet domestic infrastructure demand, with national output rising 6.3% to 149.6 million tons in 2024. Competitive pressures arise primarily from China's subsidized overproduction, which has driven exports to 90 million tons in 2023—a 35% increase year-over-year—often at prices below production costs, eroding margins for unsubsidized producers elsewhere. Government interventions, including low-interest loans and energy subsidies, sustain this excess capacity, leading to global price plunges and factory idlings in regions like Europe and the US. In response, the US imposed 25% tariffs on steel imports under Section 232 in 2018, expanded in 2025 to counter surges, while the EU applies safeguards and anti-dumping duties. Mergers, such as Nippon Steel's attempted acquisition of US Steel in 2024, reflect efforts at consolidation for resilience, though regulatory scrutiny highlights national security concerns over foreign control. Shifts toward EAF in developed markets enhance agility via scrap-based production, contrasting China's reliance on coal-intensive blast furnaces, but vulnerability to raw material import costs limits scalability without protectionism. Rising producers like India benefit from lower labor costs and domestic demand growth, projecting 6-8% annual increases, yet face import competition that prompts reciprocal tariffs. Overall, dynamics favor state-backed scale in China at the expense of market-driven efficiency elsewhere, perpetuating trade frictions and calls for multilateral capacity cuts that Beijing has historically resisted.

Trade Disputes, Tariffs, and Overcapacity Issues

Global steel overcapacity, estimated at 602 million metric tons in 2024 and projected to reach 721 million metric tons by 2027, has intensified trade tensions, with China accounting for the majority due to state-supported production exceeding domestic demand. China's steel output surpassed 1 billion metric tons annually for the fifth consecutive year in 2024, representing over half of global production, while its exports doubled to 118 million metric tons since 2020, often at prices below production costs due to subsidies and excess capacity. This surplus has depressed international prices, eroded profit margins for efficient producers, and prompted accusations of dumping, where steel is sold abroad cheaper than domestically to offload inventory amid weakening internal consumption. In response, the United States invoked Section 232 of the Trade Expansion Act of 1962 in March 2018 to impose 25% tariffs on steel imports, citing national security threats from import reliance and overcapacity distortions. These measures were expanded under President Trump in June 2025, raising tariffs to 50% effective June 4, 2025, for most countries to counter persistent surges in low-priced imports, particularly from China, which had previously retaliated with tariffs on U.S. goods. The tariffs aimed to revive domestic capacity utilization, which had fallen below viable levels, and protect against subsidized competition, though critics argue they raise costs for downstream industries like automotive manufacturing. The European Union has similarly escalated protections, proposing in October 2025 to halve tariff-free steel import quotas from 2024 levels—a 47% reduction—and double out-of-quota tariffs to 50% to shield its industry from overcapacity floods. The EU has also imposed anti-dumping duties on specific Chinese steel products, such as track shoes, following investigations revealing unfair pricing advantages from non-market practices. These actions, coordinated with global partners via forums like the OECD, seek to address how Chinese excess—equivalent to over four times EU annual consumption—undermines market stability, employment, and investments in low-carbon technologies by undercutting higher-cost, regulated producers. Retaliatory measures from China, including duties on EU exports like pork, have further strained relations, highlighting the causal link between state-driven overproduction and escalating bilateral disputes.

Environmental and Sustainability Aspects

Actual Emission Footprints by Method

The blast furnace-basic oxygen furnace (BF-BOF) route, which relies on coke for iron reduction and produces about 70% of global crude , generates an average of 2.33 tonnes of CO2 per tonne of crude steel cast, encompassing direct process emissions and indirect emissions from purchased . This intensity stems primarily from the carbon-intensive reduction of , where coke provides both heat and the reducing agent, releasing CO2 via the and combustion. In regions like , where sub-bituminous coal often substitutes for higher-quality coking coal, BF-BOF intensities can exceed 2.5 tonnes CO2 per tonne due to lower and higher ash content. The (EAF) route, utilizing steel and for melting, yields significantly lower emissions, averaging 0.70 s CO2 per for -based operations globally. This figure varies substantially by grid decarbonization; for instance, EAF plants in coal-heavy grids like India's emit up to 1.5 s CO2 per , while those in hydro- or nuclear-dominant regions like parts of or approach 0.2-0.4 s. Direct emissions in EAF arise mainly from consumption and residual impurities in , but indirect grid emissions dominate the footprint, highlighting source as the primary lever for reduction. Direct reduced iron (DRI) combined with EAF, an alternative using or for ore reduction, averages 1.43 tonnes CO2 per tonne, bridging BF-BOF and scrap-EAF levels; gas-based DRI contributes about 1.0 tonne direct CO2 from reforming and reforming, plus 0.4 tonne indirect. These footprints reflect 2021-2023 data, with global steel industry CO2 intensity at 1.91 tonnes per tonne, weighted by BF-BOF's dominance (86% of total emissions despite 70-73% production share).
Production RouteCO2 Intensity (tonnes per tonne crude )Scope IncludedReference Year
BF-BOF2.33Direct + indirect2022
Scrap-based EAF0.70Direct + indirectRecent average
DRI-EAF1.43Direct + indirectRecent average
Global Weighted Average1.91Direct + indirect2022
These metrics exclude upstream mining emissions and downstream use, focusing on cradle-to-gate at the ; actual full life-cycle footprints can add 0.1-0.3 s per from extraction. BF-BOF's higher baseline reflects thermodynamic necessities of carbothermic reduction, limiting abatement without route shifts, whereas EAF's scalability depends on availability and grid emissions, which have declined modestly in (e.g., 5-10% per decade via renewables).

Efficiency Gains and Pollution Controls Over Time

The steel industry's , measured as energy input per tonne of crude steel produced, has declined substantially since the early , primarily through process innovations and material efficiency improvements. Globally, intensity fell by approximately 67% from 1900 to recent decades, with the most rapid reductions occurring after 1950 due to the widespread adoption of the basic oxygen furnace (BOF), which replaced the less efficient open-hearth process and cut energy use by enabling faster refining with lower coke consumption. In the United States, energy consumption per of steel dropped from over 25 million Btu in the immediate post-World War II era to about 17 million Btu by the late 1990s, reflecting shifts toward electric arc furnaces (EAFs) for scrap-based production and the implementation of , which now accounts for over 90% of global output and saves 5-15% in energy by minimizing reheating needs. Pollution controls advanced significantly from the 1970s onward, driven by regulatory pressures such as the U.S. Clean Air Act amendments, which prompted investments exceeding $7 billion in abatement technologies by the early 2000s. These included electrostatic precipitators and baghouse filters for capturing particulate matter (PM) from gas cleaning and operations, achieving removal efficiencies above 99% for PM2.5 and leading to consistent global declines in PM emissions intensity from 1970 to 2000. For (SO2) and nitrogen oxides (), wet and low-NOx burners were deployed in developed economies post-1970, resulting in gradual emission reductions per tonne of steel despite rising production volumes; for instance, U.S. steel sector SO2 emissions fell by over 90% from 1970 levels through fuel switching to low-sulfur coal and . Further efficiency gains in the late 20th and early 21st centuries incorporated waste heat recovery systems, such as top-gas recycling in blast furnaces, which improved fuel efficiency by 10-20% in modern integrated plants, while pollution controls evolved to include selective catalytic reduction for NOx and advanced dedusting for coke oven emissions. Despite these advancements, global energy intensity has stabilized at 19-20 gigajoules per tonne since the 2000s, limited by the dominance of carbon-intensive blast furnace-basic oxygen furnace routes in emerging markets, though scrap recycling via EAFs continues to yield lower per-tonne emissions and energy use.

Decarbonization Strategies: Technologies and Trade-offs

Decarbonization of steelmaking primarily targets the carbon-intensive iron reduction and refining stages, where traditional blast furnace-basic oxygen furnace (BF-BOF) routes emit approximately 1.8-2.0 tonnes of CO2 per tonne of steel due to coal-based coke use. Key strategies include retrofitting existing BF-BOF plants with carbon capture, utilization, and storage (CCUS), shifting to hydrogen direct reduced iron (H2-DRI) combined with electric arc furnaces (EAF), and enhancing scrap-based EAF recycling, though the latter is limited by global scrap availability of about 600-700 million tonnes annually against 1.9 billion tonnes total steel demand. These approaches aim for net-zero emissions by 2050, but face trade-offs in cost, energy demand, and deployment scale, with CCUS offering nearer-term feasibility while H2-DRI requires breakthroughs in low-cost green hydrogen production. CCUS integration with BF-BOF captures up to 90-95% of CO2 emissions, primarily from gases, enabling retention of 70% of global primary capacity without full process overhaul. Pilot projects, such as those by and others, demonstrate capture rates exceeding 80% at scales of 1-2 million s CO2 per year, but full deployment incurs a 10-20% penalty from compression and separation, raising production costs by 20-50% to $600-800 per tonne depending on carbon and storage . Trade-offs include dependency on geological storage sites, which are unevenly distributed (e.g., limited in Asia's high-production regions), and residual emissions from non-captured sources like , potentially limiting net reductions to 0.2-0.5 s CO2 per tonne unless paired with renewables. Economic models indicate CCUS achieves abatement at $50-100 per tonne CO2 avoided, lower than alternatives in regions with cheap , though long-term viability hinges on incentives amid variable capture efficiencies reported in field trials. Hydrogen-based DRI replaces fossil reductants with H2, producing iron for EAF melting with emissions as low as 0.05 CO2 per if using electrolytic "" from renewables, theoretically enabling 95%+ reduction versus BF-BOF. Commercial pilots, including Sweden's HYBRIT and Germany's initiatives, targeted 2025-2030 operations at 1-2 million capacity, but as of mid-2025, several projects faced delays or cancellations due to constraints and costs exceeding $2-3 per kg for green H2. Scalability challenges arise from electrolyzer needs: one DRI requires 50-60 kg H2, equivalent to 2,500-3,000 kWh , demanding 10-15% of a country's renewable output for nationwide adoption, with global low-emissions H2 output projected at under 1 million in 2025 against steel's 100+ million demand. Cost trade-offs are stark, with H2-DRI levelized expenses 1.5-2.5 times BF-BOF without subsidies ($800-1,200 per ), sensitive to prices below $20/MWh, while for H2 transport adds 20-30% premiums; models favor CCUS for cost-effective abatement until 2040s. Hybrid and alternative paths, such as substitution or plasma , offer niche reductions (20-50% emissions cuts) but lack scalability for , constrained by feedstock limits and higher energy inputs. Overall, no single resolves all emissions without trade-offs: CCUS preserves incumbents but perpetuates fossil dependencies, while H2-DRI promises deeper cuts yet amplifies demands amid grid limitations, with real-world progress lagging optimistic roadmaps due to exceeding $10-20 billion per large plant and uncertainties. Deployment scenarios project CCUS capturing 40-60% of residual emissions by 2050 in cost-optimized paths, with H2-DRI viable only in hydrogen-abundant regions like or .

Innovations and Future Trajectories

Hydrogen-Based Reduction Developments

Hydrogen-based direct reduction (H2-DRI) processes use hydrogen gas, typically produced via with renewable electricity, to reduce pellets or lumps into (DRI), which is then melted in furnaces (EAF) to produce . Unlike traditional blast furnace-basic oxygen furnace (BF-BOF) routes that emit approximately 1.8-2.0 tons of CO2 per ton of , H2-DRI-EAF emits as the primary byproduct from reduction, potentially achieving near-zero process emissions if paired with . The HYBRIT demonstration project in , led by , , and since 2016, produced the first commercial batch of fossil-free from hydrogen-reduced in August 2021 using a in Luleå. By February 2025, HYBRIT validated large-scale underground storage of fossil-free , enabling seasonal balancing for intermittent inputs. The initiative plans industrial-scale production by 2030, with a demonstration sponge iron operational since and the first fossil-free structure slated for completion in 2025. Stegra (rebranded from H2 Green Steel in 2024) is developing a 5 million ton per year green steel facility in , integrating on-site production via a 1 GW electrolyzer powered by and . Construction advanced to over 60% completion by October 2025, with initial scrap-based EAF output in 2026 followed by full H2-DRI integration, supported by €6.5 billion in financing including debt and equity. The project targets export markets under EU carbon border adjustment mechanisms but faces delays from issues. Thyssenkrupp Steel broke ground in 2024 on a 2.5 million ton per year H2-DRI plant in , , designed to replace coal-based reduction and cut emissions by 3.5 million tons annually upon commissioning in 2027. The facility, funded partly by €2.2 billion in German subsidies, employs shaft furnace technology adaptable to 100% . However, escalating costs prompted a pause in tenders in March 2025 and a strategic review in October 2024, questioning final investment amid energy price volatility. ArcelorMittal advanced H2-DRI pilots, including a 2021 partnership with Marex for process optimization, but shelved two German projects in June 2025 despite €1.3 billion in subsidies, attributing decisions to prohibitive and costs rendering the route uneconomic before 2030 without expanded support. The company continues R&D in and , focusing on hybrid natural gas- transitions scalable to full . Globally, data indicates 164 million metric tons of DRI capacity in planning or construction through 2030, with only 15 million tons (9.2%) designed for , concentrated in and limited by feedstock constraints for direct reduction-grade pellets, which comprise under 10% of current seaborne trade. Established technologies like Midrex and Energiron, operational in variants since the 1970s, have demonstrated up to 100% operability in pilots but require retrofits for commercial H2 use. Scalability faces economic hurdles, with green H2-DRI-EAF production costs in Europe exceeding conventional BF-BOF by 20-40% as of 2024, driven by electrolysis electricity demands (50-60 kWh per kg H2) and capital for gigawatt-scale renewables. Additional challenges include hydrogen embrittlement risks in equipment, inefficient ore reduction kinetics at scale requiring higher temperatures, and infrastructure gaps for H2 transport and storage. Projects often rely on subsidies, with viability hinging on carbon prices above €100 per ton CO2 equivalent and renewable energy below €20 per MWh. Delays in Europe underscore that, despite technical feasibility in controlled settings, systemic integration remains constrained by energy market realities as of 2025.

Automation, AI, and Process Optimization

Automation in steelmaking has progressed significantly since the , incorporating sensors, , and control systems to enhance safety, efficiency, and precision in high-temperature environments like blast furnaces, furnaces (EAFs), and rolling mills. By 2025, industrial robots equipped with advanced sensors for force, vision, and temperature monitoring have become standard for tasks such as , slab , and ladle manipulation, reducing human exposure to hazardous areas and minimizing errors in repetitive operations. For instance, deployed ' Spot robot in 2025 for autonomous inspections in its steel , enabling real-time data collection on equipment conditions without manned intervention. Artificial intelligence augments these automated systems by processing vast datasets from IoT sensors to optimize processes in real time, particularly in predictive maintenance and yield maximization. AI algorithms, often based on machine learning models like neural networks and boosted decision trees, analyze vibration, temperature, and acoustic data to forecast equipment failures, potentially cutting unplanned downtime by up to 50% in components such as EAF electrodes or rolling mill bearings. In one implementation, a steel producer using C3 AI's Production Schedule Optimization software automated caster scheduling, slashing planning time by 98% and boosting scheduled yield through dynamic adjustments to production variables like alloy composition and throughput rates. Process optimization via AI extends to metallurgical control, where integration into Level 2 systems refines models for chemistry, temperature profiles, and use in converters and . Group's AI-enhanced models, introduced around 2025, improve process stability by predicting deviations in real-time, leading to reduced rates and savings of 2-5% per in basic oxygen furnaces. AI further aids by detecting surface defects on hot-rolled coils with over 95% accuracy, surpassing traditional manual inspections and enabling immediate adjustments to rolling parameters. These advancements, grounded in empirical sensor data rather than unverified simulations, address inherent variability in raw materials and furnace dynamics, though adoption remains uneven due to high upfront costs and the need for plant-specific data training.

Realistic Barriers to Radical Transformation

The transition to low-carbon steelmaking processes, such as direct reduced iron combined with furnaces (H2-DRI-EAF), faces substantial economic hurdles due to the elevated costs of production, which remains 2-5 times more expensive than gray hydrogen derived from in most regions as of 2024. Capital investments for or constructing new facilities can exceed $1 billion per plant, with payback periods extending beyond a decade under current energy prices, deterring widespread adoption without subsidies or carbon pricing mechanisms. Green steel premiums, estimated at 20-50% higher than conventional output, further strain competitiveness, particularly in price-sensitive markets where blast furnace-basic oxygen furnace (BF-BOF) methods dominate with established low-cost supply chains. Technical challenges compound these issues, as hydrogen-based reduction struggles with scalability for high-quality steel grades; managing residual elements like and tin from impurities or recycled inputs leads to defects in specialty alloys, requiring unproven purification steps that increase demands by up to 15%. furnaces (EAFs), while efficient for , cannot directly process raw without prior metallization via DRI, limiting their role in to about 30% of global capacity and necessitating reliable direct reduction-grade pellets, whose supply is constrained to fewer than 100 million tons annually worldwide. risks and the need for furnace redesigns to handle pure H2 atmospheres add operational complexities, with pilot plants reporting yield losses of 5-10% compared to carbon-based reductants. Infrastructure and resource barriers persist, including intermittent renewable electricity availability critical for , which could require doubling global capacity to 500 GW by 2050 yet faces grid integration delays in regions like and . Securing vast quantities of low-impurity —projected at 1.5 billion tons yearly for full decarbonization—exacerbates bottlenecks, while the nascent hydrogen network lacks the scale of existing natural gas pipelines, inflating logistics costs by 30-50%. In developing economies, where BF-BOF accounts for over 70% of output, these factors intersect with policy inertia, as low energy prices favor fossil fuels and international overcapacity from subsidized producers undermines incentives for local transformation.

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