Steelmaking

Steelmaking is the industrial process of producing steel from iron ore, iron-bearing materials, and/or recycled scrap. Steel has been manufactured in small quantities for several millennia, but large-scale commercial production only became possible in the mid-nineteenth century with the development of modern industrial processes. Today, steelmaking underpins global infrastructure, manufacturing, and construction, while also posing significant technological, economic, and environmental challenges.
Steelmaking is among the most carbon emission-intensive industries worldwide. In 2020, it was estimated to account for approximately 7 per cent of global energy-sector greenhouse gas emissions, prompting major international efforts to decarbonise production while maintaining output and material quality.

Steel as a material

Steel is primarily an alloy of iron and carbon, with carbon content generally below 1 per cent by mass, depending on the grade. This distinguishes steel from cast iron, which contains higher carbon levels and is hard, brittle, and difficult to shape. By contrast, steel is malleable, ductile, and versatile, making it suitable for a wide range of structural and mechanical applications.
In addition to carbon, steel composition is carefully controlled through the removal of impurities such as sulfur, phosphorus, silicon, nitrogen, and excess carbon. Alloying elements—including manganese, nickel, chromium, vanadium, and others—are added to produce steels with specific mechanical, chemical, and thermal properties. These compositional variations result in a wide spectrum of steel grades, each optimised for particular uses.

Historical development

Early steel production

The earliest steelmaking methods emerged independently in several regions, including Ancient China, India, Rome, and parts of northern Europe. One of the earliest techniques was the bloomery, in which iron ore was reduced in a small furnace to produce a spongy mass of iron and slag that could be worked into steel-like material.
For much of human history, steel was produced only in small quantities due to the labour-intensive and highly skilled nature of the processes involved. Steel was therefore expensive and reserved for weapons, tools, and high-status objects.

Developments in China

In East Asia, methods resembling later industrial steelmaking appeared as early as the 11th century. During the Song dynasty, metallurgists developed techniques involving the repeated forging of cast iron under controlled air blasts, partially decarburising the metal. These methods have been described by historians of science as important precursors to the modern Bessemer process. Written descriptions from the period indicate that large-scale iron and steel production centres existed in northern China well before comparable developments in Europe.

Europe and Japan

In Europe, the finery forge emerged in the fifteenth century, using air-blowing principles to refine pig iron into wrought iron. High-quality steel was later produced through the cementation process, in which wrought iron bars were heated with charcoal for extended periods, producing blister steel. Further refinement in crucibles led to crucible steel, a high-quality but expensive product.
In 1740, Benjamin Huntsman significantly improved crucible steel production in England, increasing both quality and consistency. However, these processes remained fuel-intensive and costly.
Historical accounts also suggest that Japan may have employed a process with similarities to the Bessemer method, involving air-blown molten iron, though interpretations of these descriptions remain debated among historians.

Industrialisation and the Bessemer process

Modern steelmaking began in the late 1850s with the introduction of the Bessemer process, which enabled the rapid conversion of pig iron into steel by blowing air through molten metal. This dramatically reduced production time and cost, allowing steel to become a cornerstone of industrial economies. The Bessemer process was soon followed by the open-hearth furnace, which offered greater control over composition and quality.

Modern steelmaking processes

Contemporary steelmaking is conventionally divided into three stages: primary, secondary, and tertiary steelmaking. Each stage employs multiple techniques depending on desired output and plant configuration.

Primary steelmaking

Primary steelmaking involves converting iron-bearing materials into molten steel.
Basic oxygen steelmaking (BOS)Basic oxygen steelmaking uses molten pig iron produced in a blast furnace, combined with scrap steel. High-purity oxygen is blown through the molten metal, oxidising carbon to carbon monoxide and carbon dioxide and removing impurities such as silicon and phosphorus. The vessel is lined with basic refractory materials, typically calcium oxide and magnesium oxide, to withstand extreme temperatures and corrosive slag.
Developed in 1948 as a refinement of the Bessemer process, BOS dramatically increased productivity and reduced labour requirements. By 2013, approximately 70 per cent of global steel output was produced using this method. A single basic oxygen furnace can convert up to 350 tonnes of iron into steel in under 40 minutes, compared with many hours in older open-hearth furnaces.
Electric arc furnace (EAF) steelmakingElectric arc furnaces produce steel primarily from scrap steel or direct reduced iron (DRI). An electric arc generated between graphite electrodes melts the charge, sometimes assisted by gas burners. Fluxes are added to remove impurities and protect the furnace lining.
Typical EAFs have capacities around 100 tonnes and complete a heat in 40 to 50 minutes. This process allows greater flexibility in alloying and is especially important for recycling steel. Many modern plants operate with two arc furnaces, enabling continuous or semi-continuous production.
Emerging processesInnovative processes such as HIsarna aim to reduce emissions by processing iron ore directly into molten iron without the need for intermediate pelletisation. This approach improves energy efficiency and can reduce carbon dioxide emissions by around 20 per cent.
Another major area of development is hydrogen-based reduction, where iron ore is reduced using hydrogen instead of carbon. When combined with renewable electricity, this offers the potential for near-zero-emission steelmaking, although current costs remain significantly higher than conventional methods.

Secondary steelmaking

Secondary steelmaking focuses on refining molten steel after primary conversion. This stage is often carried out using ladle metallurgy, which allows precise control over chemical composition and cleanliness.
Common secondary operations include:

• Deoxidation or killing
• Vacuum degassing to remove dissolved gases
• Alloy addition
• Inclusion removal and modification
• Desulphurisation
• Thermal and chemical homogenisation

These processes are frequently performed in gas-stirred ladles with electric arc heating. Tight control at this stage is essential for producing high-grade steels with narrow compositional tolerances.

Tertiary steelmaking

Tertiary steelmaking refers to the shaping and solidification of molten steel into semi-finished or finished products. This includes casting steel into slabs, billets, or blooms, followed by rolling, forging, or other forming processes. Continuous casting has largely replaced older ingot-based methods due to improved efficiency and material quality.

Environmental impact and decarbonisation

Steelmaking’s reliance on carbon-based reduction and high-temperature processes makes it a major contributor to greenhouse gas emissions. Reducing these emissions is a central challenge for the industry, with strategies including increased recycling via electric arc furnaces, improved energy efficiency, carbon capture technologies, and the transition to hydrogen-based reduction.
International roadmaps and policy frameworks emphasise the need for deep emission reductions while maintaining the supply of steel essential to modern economies.