Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to its high elastic modulus, yield strength, fracture strength and low raw material cost, steel is one of the most commonly manufactured materials in the world. Steel is used in structures (as concrete reinforcing rods or steel beams), in bridges, infrastructure, tools, ships, trains, cars, bicycles, machines, electrical appliances, furniture, and weapons.
Steel is defined as an alloy of iron and carbon and often other elements, with a carbon content up to 2.14%. Iron and carbon are always the main elements in steel, but other elements are used to produce various grades of steel, demonstrating altered material, mechanical, and microstructural properties. Stainless steels, for example, typically contain 18% chromium and exhibit improved corrosion and oxidation resistance versus their carbon steel counterpart. Galvanized steel is coated in a layer of zinc to achieve a similar effect. Under atmospheric pressures, steels generally take on two crystalline forms: body-centered cubic and face-centered cubic; however, depending on the thermal history and alloying, the microstructure may contain the distorted martensite phase or the carbon-rich cementite phase, which are tetragonal and orthorhombic, respectively. In the case of alloyed iron, the strengthening is primarily due to the introduction of carbon in the primarily-iron lattice, inhibiting deformation under mechanical stress. Alloying may also induce additional phases that affect the mechanical properties. In most cases, the engineered mechanical properties are at the expense of the ductility and elongation of the pure iron state, which decrease upon the addition of carbon.
Steel was produced in bloomery furnaces for thousands of years, but its large-scale, industrial use began only after more efficient production methods were devised in the 17th century, with the introduction of the blast furnace and production of crucible steel. This was followed by the Bessemer process in England in the mid-19th century, and then by the open-hearth furnace. With the invention of the Bessemer process, a new era of mass-produced steel began. Mild steel replaced wrought iron. The German states were the major steel producers in Europe in the 19th century. American steel production was centred in Pittsburgh, Bethlehem, Pennsylvania, and Cleveland, Ohio until the late 20th century. Currently, world steel production is centered in China, which produced 54% of the world's steel in 2023.
Further refinements in the process, such as basic oxygen steelmaking (BOS), largely replaced earlier methods by further lowering the cost of production and increasing the quality of the final product. Today, more than 1.6 billion tons of steel are produced annually. Modern steel is generally identified by various grades defined by assorted standards organizations. The modern steel industry is one of the largest manufacturing industries in the world, but also one of the most energy and greenhouse gas emission intense industries, contributing 8% of global emissions. However, steel is also very reusable: it is one of the world's most-recycled materials, with a recycling rate of over 60% globally.
Definitions and related materials
The noun steel originates from the Proto-Germanic adjective or 'made of steel', which is related to or 'standing firm'.
The carbon content of steel is between 0.02% and 2.14% by weight for plain carbon steel (iron-carbon alloys). Alloy steel is steel to which other alloying elements have been intentionally added to modify the characteristics of steel. Common alloying elements include: manganese, nickel, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, and niobium. Additional elements, most frequently considered undesirable, are also important in steel: phosphorus, sulphur, silicon, and traces of oxygen, nitrogen, and copper.
Plain iron–carbon alloys with a higher than 2.1% carbon content are known as cast iron. With modern steelmaking techniques such as powder metal forming, it is possible to make very high-carbon (and other alloy material) steels, but such are not common. Cast iron is not malleable even when hot, but it can be formed by casting as it has a lower melting point than steel and good castability properties. Certain compositions of cast iron, while retaining the economies of melting and casting, can be heat treated after casting to make malleable iron or ductile iron objects. Steel is distinguishable from wrought iron (now largely obsolete), which may contain a small amount of carbon A (<0.1 %) but large amounts of slag (around 1–2%).
Production
When iron is smelted from its ore with coke or charcoal, the result has a high amount of carbon and is known as pig iron. To become steel, it must be reprocessed to reduce the carbon to the correct amount, at which point other elements may be added. This process is called refinement, where oxygen is injected into the molten iron to remove excess carbon; oxygen reacts with the carbon to create carbon dioxide gas.
Historically this process would involve continuous trial and error testing of the cooled product to assess whether the correct amount of carbon was present for the material to possess the properties of steel. Today chemical analysis can be performed to determine the exact composition of the alloy, in addition to practical testing of the material's properties such as the Rockwell hardness test, Ultrasonic testing, the Charpy impact test, and the Three-point flexural test. In the past, steel facilities would cast the raw steel product into ingots which would be stored until use in further refinement processes that resulted in the finished product. In modern facilities, the initial product is close to the final composition and is continuously cast into long slabs, cut and shaped into bars and extrusions and heat treated to produce a final product. Today, approximately 96% of steel is continuously cast, while only 4% is produced as ingots. The ingots are then heated in a soaking pit and hot rolled into slabs, billets, or blooms. Slabs are hot or cold rolled into sheet metal or plates. Billets are hot or cold rolled into bars, rods, and wire. Blooms are hot or cold rolled into structural steel, such as I-beams and rails. In modern steel mills these processes often occur in one assembly line, with ore coming in and finished steel products coming out.. Sometimes after a steel's final rolling, it is heat treated for strength; however, this is relatively rare.
Material properties Origins and production
Iron is commonly found in the Earth's crust in the form of an ore, usually an iron oxide, such as magnetite or hematite. Iron is extracted from iron ore under reductive conditions, where oxygen reacts with carbon in the fuel to produce carbon monoxide, which then reduces the iron oxide into metallic iron. This process, known as smelting, was first applied to metals with lower melting points, such as tin, which melts at about , and copper, which melts at about , and the combination, bronze, which has a melting point lower than . In comparison, iron melts at about , a temperature not attainable at the start of the Iron Age. Small quantities of iron were smelted in ancient times in a semi-liquid state by repeatedly heating the ore in a charcoal fire and then welding the resulting clumps together with a hammer. The process eliminated much of the impurities, resulting in the production of wrought iron. As furnaces reached higher temperatures due to bellows improvements leading to increased airflow, iron with higher carbon contents were able to be produced. Unlike copper and tin, liquid or solid iron dissolves carbon quite readily.
All of these temperatures could be reached with ancient methods used since the Bronze Age. Since the oxidation rate of iron increases rapidly beyond , it is important that smelting takes place in a low-oxygen environment. Smelting, using carbon to reduce iron oxides, results in an alloy (pig iron) that retains too much carbon to be called steel. The excess carbon and other impurities are removed via further processing.
Other materials are often added to the iron/carbon mixture to produce steel with the desired properties. Nickel and manganese in steel add to its tensile strength and make the austenite form of the iron-carbon solution more stable, chromium increases hardness and melting temperature, and vanadium also increases hardness while making it less prone to metal fatigue.
To inhibit corrosion, at least 11% chromium can be added to steel so that a hard oxide forms on the metal surface; this is known as stainless steel. Tungsten slows the formation of cementite, keeping carbon in the iron matrix and allowing martensite to preferentially form at slower quench rates, resulting in high-speed steel. The addition of lead and sulphur decrease grain size, thereby making the steel easier to turn, but also more brittle and prone to corrosion. Such alloys are nevertheless frequently used for components such as nuts, bolts, and washers in applications where toughness and corrosion resistance are not paramount. For the most part, however, p-block elements such as sulphur, nitrogen, phosphorus, and lead are considered contaminants that make steel more brittle and are therefore removed from steel during the melting processing.
Properties
The density of steel varies based on the alloying constituents but usually ranges between , or .
Even in a narrow range of concentrations of mixtures of carbon and iron that make steel, several different metallurgical structures, with very different properties can form. Understanding such properties is essential to making quality steel. At room temperature, the most stable form of pure iron is the body-centred cubic (BCC) structure called alpha iron or α-iron. It is a fairly soft metal that can dissolve only a small concentration of carbon, no more than 0.005% at and 0.021 wt% at . The inclusion of carbon in alpha iron is called ferrite. At 910 °C, pure iron transforms into a face-centred cubic (FCC) structure, called gamma iron or γ-iron. The inclusion of carbon in gamma iron is called austenite. The more open FCC structure of austenite can dissolve considerably more carbon, as much as 2.1% (38 times that of ferrite),Sources differ on this value so it has been rounded to 2.1%; however, the exact value is rather academic because plain-carbon steel is very rarely made with this level of carbon. See: —2.08%. —2.11%. —2.14%. at , which reflects the upper carbon content of steel, beyond which is cast iron.. When carbon moves out of solution with iron, it forms a very hard, but brittle material called cementite (Fe3C).