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The chief alloy of iron, and the most used of all metals. It consists of iron hardened by the presence of a small proportion of carbon. It was made in small amounts in ancient times by heating cast-iron to reduce surface carbon, and was later made in crucibles in small quantities for tools. Steel production began in China before the 6th-c AD. Western large-scale manufacture for constructional purposes began with the Bessemer process (1856 onwards). Most steel used today is a simple carbon steel, but there exist many special steels formed by the addition of other metals, such as high alloy steels for tools, and stainless steel (with nickel and chromium).

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Steel is a metal alloy whose major component is iron, with carbon content between 0.02% and 1.7% by weight. Carbon is the most cost effective alloying material for iron, but many other alloying elements are also used. Carbon and other elements act as a hardening agent, preventing dislocations in the iron atom crystal lattice from sliding past one another. Varying the amount of alloying elements and their distribution in the steel controls qualities such as the hardness, elasticity, ductility, and tensile strength of the resulting steel. Steel with increased carbon content can be made harder and stronger than iron, but is also more brittle. Alloys with higher carbon content than this are known as cast iron because of their lower melting point. Steel is also to be distinguished from wrought iron with little or no carbon, usually less than 0.035%.

Iron and steel

Iron, like most metals, is not found in the Earth's crust in an elemental state. Iron can be found in the crust only in combination with oxygen or sulfur. Typically Fe₂O₃—the form of iron oxide (rust) found as the mineral hematite, and FeS₂—Pyrite (fool's gold). This process, known as smelting, was first applied to metals with lower melting points. Copper melts at just over 1000 °C, while tin melts around 250 °C. Unlike copper and tin, liquid iron dissolves carbon quite readily, so that smelting results in an alloy containing too much carbon to be called steel.

Even in the narrow range of concentrations that make up steel, mixtures of carbon and iron can form into a number of different structures, or allotropes, with very different properties; At room temperature, the most stable form of iron is the body-centered cubic (BCC) structure ferrite or ?-iron, a fairly soft metallic material that can dissolve only a small concentration of carbon (no more than 0.021 wt% at 910 °C). Above 910 °C ferrite undergoes a phase transition from body-centered cubic to a face-centered cubic (FCC) structure, called austenite or ?-iron, which is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.03 wt% carbon at 1154 °C). One way for carbon to leave the austenite is for cementite to precipitate out of the mix, leaving behind iron that is pure enough to take the form of ferrite, and resulting in a cementite-ferrite mixture. Self-reinforcing patterns often emerge during this process, leading to a patterned layering known as pearlite due to its pearl-like appearance, or the similar but less beautiful bainite.

Perhaps the most important allotrope is martensite, a chemically metastable substance with about four to five times the strength of ferrite. As such, it requires extremely little thermal activation energy to form.

The heat treatment process for most steels involves heating the alloy until austenite forms, then quenching the hot metal in water or oil, cooling it so rapidly that the transformation to ferrite or pearlite does not have time to take place. Internal stresses from this expansion generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. Nickel and manganese in steel add to its tensile strength and make austenite more chemically stable, chromium increases the hardness and melting temperature, and vanadium also increases the hardness while reducing the effects of metal fatigue. Large amounts of chromium and nickel (often 18% and 8%, respectively) are added to stainless steel so that a hard oxide forms on the metal surface to inhibit corrosion. Tungsten interferes with the formation of cementite, allowing martensite to form with slower quench rates, resulting in high speed steel. On the other hand sulfur, nitrogen, and phosphorus make steel more brittle, so these commonly found elements must be removed from the ore during processing.

When iron is smelted from its ore by commercial processes, it contains more carbon than is desirable.

History of iron and steelmaking

Iron was in limited use long before it became possible to smelt it. The first signs of iron use come from Ancient Egypt and Sumer, where around 4000 BC small items, such as the tips of spears and ornaments, were being fashioned from iron recovered from meteorites (see Iron: History). About 6% of meteorites are composed of an iron-nickel alloy, and iron recovered from meteorite falls allowed ancient peoples to manufacture small numbers of iron artifacts.

Meteoric iron was also fashioned into tools in precontact North America. Beginning around the year 1000, the Thule people of Greenland began making harpoons and other edged tools from pieces of the Cape York meteorite. When the American polar explorer Robert Peary shipped the largest piece of the meteorite to the American Museum of Natural History in New York City in 1897, it still weighed over 33 tons.

The name for iron in several ancient languages means "sky metal" or something similar. In distant antiquity, iron was regarded as a precious metal, suitable for royal ornaments.

The Iron Age

Beginning between 3000 BC to 2000 BC increasing numbers of smelted iron objects (distinguishable from meteoric iron by their lack of nickel) appear in Anatolia, Egypt and Mesopotamia (see Iron: History). The oldest known samples of iron that appear to have been smelted from iron oxides are small lumps found at copper-smelting sites on the Sinai Peninsula, dated to about 3000 BC.

In Anatolia, smelted iron was occasionally used for ornamental weapons: an iron-bladed dagger with a bronze hilt has been recovered from a Hattic tomb dating from 2500 BC. Also, the Egyptian ruler Tutankhamun died in 1323 BC and was buried with an iron dagger with a golden hilt. An Ancient Egyptian sword bearing the name of pharaoh Merneptah as well as a battle axe with an iron blade and gold-decorated bronze haft were both found in the excavation of Ugarit (see Ugarit). The early Hittites are known to have bartered iron for silver, at a rate of 40 times the iron's weight, with Assyria.

Iron did not, however, replace bronze as the chief metal used for weapons and tools for several centuries, despite some attempts. Then, between 1200 and 1000 BC, iron tools and weapons displaced bronze ones throughout the near east. This process appears to have begun in the Hittite Empire around 1300 BC, or in Cyprus and southern Greece, where iron artifacts dominate the archaeological record after 1050 BC. Mesopotamia was fully into the Iron Age by 900 BC, central Europe by 800 BC. Egypt, on the other hand, did not experience such a rapid transition from the bronze to iron ages: although Egyptian smiths did produce iron artifacts, bronze remained in widespread use there until after Egypt's conquest by Assyria in 663 BC.

Iron smelting at this time was based on the bloomery, a furnace where bellows were used to force air through a pile of iron ore and burning charcoal. The carbon monoxide produced by the charcoal reduced the iron oxides to metallic iron, but the bloomery was not hot enough to melt the iron. The result of this time-consuming and laborious process was wrought iron, a malleable but fairly soft alloy containing little carbon.

Wrought iron can be carburized into a mild steel by holding it in a charcoal fire for prolonged periods of time. The oldest quench-hardened steel artifact is a knife found on Cyprus at a site dated to 1100 BC. Around 500 BC, however, metalworkers in the southern state of Wu developed an iron smelting technology that would not be practiced in Europe until late medieval times. As a liquid, iron can be cast into molds, a method far less laborious than individually forging each piece of iron from a bloom.

Cast iron is rather brittle and unsuitable for striking implements. The artifacts recovered from this grave are variously made of wrought iron, cast iron, malleabilized cast iron, and quench-hardened steel, with only a few, probably ornamental, bronze weapons.

During the Han Dynasty (202 BC窶鄭D 220), Chinese ironworking achieved a scale and sophistication not reached in the West until the eighteenth century. (In Chinese, the process was called chao, literally, stir frying.)

Also during this time, Chinese metallurgists had found that wrought iron and cast iron could be melted together to yield an alloy of intermediate carbon content, that is, steel.

Steelmaking in India and Sri Lanka

Perhaps as early as 300 BC, although certainly by AD 200, high quality steel was being produced in southern India also by what Europeans would later call the crucible technique. forged or cast in the 4th century AD, and which has stood for many centuries next to the Qutab Minar in the Qutb complex in Delhi, is a testimony of the metallurgical skills of Indian artisans.

Steelmaking in the Middle East

By the 9th century, smiths in the Abbasid caliphate had developed techniques for forging wootz to produce steel blades of unusual flexibility and sharpness (Damascus steel).

Ironworking in medieval Europe

The middle ages in Europe saw the construction of progressively larger bloomeries. By the 8th century, smiths in northern Spain had developed a style that become known as a Catalan forge, a furnace about 1 meter (3 feet) tall, capable of smelting up to 150 kg (350 lb) of iron in each batch. In succeeding centuries, smiths in the Frankish empire and later the Holy Roman Empire scaled up this basic design, increasing the height of the flue to as tall as 5 meters (16 feet) and smelting as much as 350 kg (750 lb) of iron in each batch. To this end, waterwheels were employed to power the bellows and hammers.

Eventually, the scaling up of the bloomery reached a point where the furnace was hot enough to produce cast iron. Although the brittle cast iron may initially have been a nuisance to the smith, as it was too brittle to be forged, the spread of cannons to Europe in the 1300s provided an application for iron casting: cast iron cannonballs.

The oldest known blast furnace in Europe was constructed at Lapphyttan in Sweden, sometime between 1150 and 1350. Other early European blast furnaces were built throughout the Rhine valley: blast furnaces were in operation near Liティge (a city in modern-day Belgium) in the 1340s, and at Massevaux in France by 1409.

The first English blast furnace was not built until 1491, when Queenstock furnace was built at Buxted, followed by one commissioned Henry VII at Newbridge, in 1496 in a part of Sussex known as the Weald. In 1543, William Levett, an English rector who doubled as a Wealden ironmaster , and Peter Baude, a French craftsman in Henry VIII's employ, cast the Weald's first one-piece iron cannon. The superiority of English cannons over Spanish ones has been credited as one factor in England's 1588 defeat of the Spanish Armada.

In 1619, Jan Andries Moerbeck, a Dutch ironmaster, began importing Wealden iron ore for comparison to the ore available on the Continent.

Soon after that it was found that the best steel could only be produced by buying expensive テカrgrund (or oregrounds) iron from Sweden. This Swedish iron provided the main basis for English steelmaking until the 1850s

Benjamin Huntsman in the 1740s found a method of producing a more homogeneous steel. He made this discovery at Handsworth in England. Sheffield's Abbeydale Industrial Hamlet has preserved a waterwheel powered, scythe-making works dating from Huntsman's times. King, 'The cartel in oregrounds iron' Journal of Industrial History 6 (2003), 25-48.

Ironmaking in early modern Europe

From the 16th century to the 18th century, most iron was made by a two-stage process involving a blast furnace and finery forge, using charcoal as fuel. In 1709 Abraham Darby began smelting iron using coke, a refined coal product, in place of charcoal at his ironworks at Coalbrookdale in England. It was not until the 1750s, when Darby's son, also called Abraham, managed to start selling coke-smelted pig iron for the production of wrought iron in finery forges. In particular, the form of coal-fired puddling furnace developed by the British ironmaster Henry Cort in 1784 made it possible to convert cast iron into wrought iron in large batches (without charcoal), rendering the ancient finery forge obsolescent.

Industrial steelmaking

The problem of mass-producing steel was solved in 1855 by Henry Bessemer, with the introduction of the Bessemer converter at his steelworks in Sheffield, England. In the Bessemer process, molten pig iron from the blast furnace was charged into a large crucible, and then air was blown through the molten iron from below, igniting the dissolved carbon from the coke. After the carbon content in the melt had dropped to the desired level, the air draft was cut off: a typical Bessemer converter could convert a 25-ton batch of pig iron to steel in half an hour.

In 1867, the German-British engineer Sir William Siemens introduced an improved puddling furnace – The next year Pierre and Émile Martin, French ironmasters who had licensed Siemens' furnace design, developed a method for measuring the carbon content of molten iron. Reasons for this include its ability to recycle scrap metal in addition to fresh pig iron, its greater scalability (up to hundreds of tons per batch, compared to tens of tons for the Bessemer process), and the more precise quality control it permitted.

Initially, only ores low in phosphorus and sulfur could be used for quality steelmaking; This problem was solved in 1878 by Percy Carlyle Gilchrist and his cousin Sidney Gilchrist Thomas at the ironworks at Blaenavon in Wales. Their modified Bessemer process used a converter lined with limestone or dolomite, and additional lime was added to the molten metal as a Flux. This development expanded the range of iron ores that could be used to make steel, especially in France and Germany, where high-phosphorus ores abounded.

Finally, the basic oxygen process was introduced at the Voest-Alpine works in 1952; The top three steel-producing countries were China (349.4 mmt), Japan (112.5 mmt) and the United States (93.9 mmt) (see chart below).

Until these 19th century developments, steel was an expensive commodity and only used for a limited number of purposes where a particularly hard or flexible metal was needed, as in the cutting edges of tools and springs. Mild steel ultimately replaced wrought iron for almost all purposes, and wrought iron is not now (or is hardly now) made. Stainless steel was only developed on the eve of the First World War and only began to come into widespread use in the 1920s.

Types of steel

Alloy steels were known from antiquity, being nickel-rich iron from meteorites hot-worked into useful products.

Historic types

• Damascus steel, which was famous in ancient times for its durability and ability to hold an edge, was created from a number of different materials (some only in traces), essentially a complicated alloy with iron as main component.
  
• Blister steel - steel produced by the cementation process
  
• Crucible steel - steel produced by Benjamin Huntsman's crucible technique
  
• Styrian Steel, also called 'German steel' or 'Cullen steel' (being traded through Cologne) was made in Styria in Austria by fining cast iron from certain manganese-rich ores.
  
• Shear steel was blister steel that was broken up, faggotted, heated and welded to produce a more homogeneous product
  

Contemporary steel

• Carbon steel, composed simply of iron and carbon accounts for 90% of steel production.
  
• HSLA steel (high strength, low alloy) have small additions (usually <2% by weight) of other elements, typically 1.5% manganese, to provide additional strength for a modest price increase.
  
• Low alloy steel is alloyed with other elements, usually molybdenum, manganese, chromium, or nickel, in amounts of up to 10% by weight to improve the hardenability of thick sections.
  
• Stainless steels and surgical stainless steels contain a minimum of 10% chromium, often combined with nickel, to resist corrosion (rust).
  
• Tool steels are alloyed with large amounts of tungsten and cobalt or other elements to maximize solution hardening, allow precipitation hardening and improve temperature resistance.
  
• Advanced High Strength Steels
  
  • Complex Phase Steel
    
  • Dual Phase Steel
    
  • TRIP steel
    
  • TWIP steel
    
  • Maraging steel
    
  • Eglin Steel
    
• Ferrous superalloys
  
• Hadfield steel (after Sir Robert Hadfield) or Manganese steel, this contains 12-14% manganese which when abraded forms an incredibly hard skin which resists wearing. Some examples are tank tracks, bulldozer blade edges and cutting blades on the jaws of life.
  

Though not an alloy, there exists also galvanized steel, which is steel that has gone through the chemical process of being hot-dipped or electroplated in zinc for protection against rust. The relatively soft core helps in ductility of the steel while treated skin has good weldability to suit to the construction requirements.

Production methods

Historical methods

• bloomery
  
• pattern welding
  
• catalan forge
  
• wootz steel (crucible technique): developed in India, used in the Middle East where it was known as Damascus steel.
  
• Cementation process used to convert bars of wrought iron into blister steel.
  
• crucible technique, similar to the wootz steel, independently redeveloped in Sheffield by Benjamin Huntsman in c.1740, and Pavel Anosov in Russia in 1837.
  
• Puddling
  

Modern methods

• Electric arc furnace a form of secondary steelmaking from scrap, though the process can also use direct-reduced iron
  
• Production of pig iron using blast furnace
  
• Converters (steel from pig iron):
  

1. Bessemer process, the first large-scale steel production process for mild steel.
   
2. The Siemens-Martin process, using an Open hearth furnace
   
3. Basic oxygen steelmaking
   

Uses of steel

Historically

Steel was expensive and was only used where nothing else would do, particularly for the cutting edge of knives, razors, swords, and other tools where a hard sharp edged was needed. It continues to be used in many situations, though the new availabilty of plastics during the 20th century has meant that it has ceased to be used for some.

Long steel

• Wires
  
• Rail tracks
  
• As girders in building modern tall buildings, bridges
  

Flat carbon steel

• For the inside and outside body of cars, trains
  
• Major appliances
  

Stainless steel

• Cutlery
  
• Rulers
  

See also

• Structural steel
  
• Rolling (metalworking)
  
• Cold rolling
  
• Hot rolling
  
• Steel producers
  
• Steel mill
  
• Rolling mill
  
• Foundry
  
• Tinplate
  
• Global steel industry trends