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Steel is an alloy consisting mostly of iron, with a carbon contentbetween 0.02% and 1.7 or 2.04% by weight (C:1000–10,8.67Fe),depending on grade. Carbon is the most cost-effective alloyingmaterial for iron, but various other alloying elements are used such asmanganese and tungsten.[1] Carbon and other elements act as a

hardening agent, preventing dislocations in the iron atom crystallattice from sliding past one another. Varying the amount of alloyingelements and form of their presence in the steel (solute elements,precipitated phase) controls qualities such as the hardness, ductility,and tensile strength of the resulting steel. Steel with increased carboncontent can be made harder and stronger than iron, but is also morebrittle. The maximum solubility of carbon in iron (in austenite region) is2.14% by weight, occurring at 1149 °C; higher concentrations of carbon or lower temperatures will produce cementite. Alloys withhigher carbon content than this are known as cast iron because of theirlower melting point.[1] Steel is also to be distinguished from wrought

iron containing only a very small amount of other elements, butcontaining 1–3% by weight of slag in the form of particles elongated inone direction, giving the iron a characteristic grain. It is more rust-resistant than steel and welds more easily. It is common today to talkabout 'the iron and steel industry' as if it were a single entity, buthistorically they were separate products.

 Though steel had been produced by various inefficient methods longbefore the Renaissance, its use became more common after moreefficient production methods were devised in the 17th century. Withthe invention of the Bessemer process in the mid-19th century, steel

became a relatively inexpensive mass-produced good. Furtherrefinements in the process, such as basic oxygen steelmaking, furtherlowered the cost of production while increasing the quality of themetal. Today, steel is one of the most common materials in the worldand is a major component in buildings, tools, automobiles, andappliances. Modern steel is generally identified by various grades of steel defined by various standards organizations.

Contents

[hide]• 1 Material properties

• 2 History of steelmaking

o 2.1 Ancient steel

o 2.2 Early modern steel

2.2.1 Blister steel 2.2.2 Crucible steel

o 2.3 Modern steelmaking

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• 3 Steel industry

• 4 Recycling

• 5 Contemporary steel

• 6 Modern production methods

• 7 Uses of steel

o 7.1 Historicallyo 7.2 Since 1850

7.2.1 Long steel

7.2.2 Flat carbon steel

7.2.3 Stainless steel

• 8 See also

• 9 References

• 10 Further reading

• 11 External links

Material properties

v • d • e

Iron alloy phases

Austenite (γ-iron; hard)BainiteMartensiteCementite (iron carbide; Fe3C)Ledeburite (ferrite - cementite eutectic, 4.3%carbon)Ferrite (α-iron, δ-iron; soft)

Pearlite (88% ferrite, 12% cementite)Spheroidite

Types of Steel

Plain-carbon steel (up to 2.1% carbon)Stainless steel (alloy with chromium)HSLA steel (high strength low alloy)

 Tool steel (very hard; heat-treated)

Other Iron-based materials

Cast iron (>2.1% carbon)Wrought iron (almost no carbon)

Ductile ironIron, like most metals, is not usually found in the Earth's crust in anelemental state.[2] Iron can be found in the crust only in combinationwith oxygen or sulfur. Typical iron-containing minerals include Fe2O3—the form of iron oxide found as the mineral hematite, and FeS2—pyrite (fool's gold).[3] Iron is extracted from ore by removing the oxygen bycombining it with a preferred chemical partner such as carbon. Thisprocess, known as smelting, was first applied to metals with lower

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melting points. Copper melts at just over 1000 °C, while tin meltsaround 250 °C. Cast iron—iron alloyed with greater than 1.7% carbon—melts at around 1370 °C. All of these temperatures could be reachedwith ancient methods that have been used for at least 6000 years(since the Bronze Age). Since the oxidation rate itself increases rapidly

beyond 800 °C, it is important that smelting take place in a low-oxygenenvironment. Unlike copper and tin, liquid iron dissolves carbon quitereadily, so that smelting results in an alloy containing too much carbonto be called steel.[4]

Even in the narrow range of concentrations that make up steel,mixtures of carbon and iron can form into a number of differentstructures, with very different properties; understanding these isessential to making quality steel. At room temperature, the moststable form of iron is the body-centered cubic (BCC) structure ferrite orα-iron, a fairly soft metallic material that can dissolve only a smallconcentration of carbon (no more than 0.021 wt% at 910 °C). Above

910 °C ferrite undergoes a phase transition from body-centered cubicto a face-centered cubic (FCC) structure, called austenite or γ-iron,which is similarly soft and metallic but can dissolve considerably morecarbon (as much as 2.03 wt% carbon at 1154 °C).[5] As carbon-richaustenite cools, the mixture attempts to revert to the ferrite phase,resulting in an excess of carbon. One way for carbon to leave theaustenite is for cementite to precipitate out of the mix, leaving behindiron that is pure enough to take the form of ferrite, resulting in acementite-ferrite mixture. Cementite is a stoichiometric phase with thechemical formula of Fe3C. Cementite forms in regions of higher carboncontent while other areas revert to ferrite around it. Self-reinforcing

patterns often emerge during this process, leading to a patternedlayering known as pearlite (Fe3C:6.33Fe) due to its pearl-likeappearance, or the similar but less beautiful bainite.

Iron-carbon phase diagram, showing the conditions necessary to form different phases.Perhaps the most important allotrope is martensite, a chemicallymetastable substance with about four to five times the strength of ferrite. A minimum of 0.4 wt% of carbon (C:50Fe) is needed in order toform martensite. When the austenite is quenched to form martensite,the carbon is "frozen" in place when the cell structure changes from

FCC to BCC. The carbon atoms are much too large to fit in theinterstitial vacancies and thus distort the cell structure into a body-centered tetragonal (BCT) structure. Martensite and austenite have anidentical chemical composition. As such, it requires extremely littlethermal activation energy to form. The heat treatment process for most steels involves heating the alloyuntil austenite forms, then quenching the hot metal in water or oil,cooling it so rapidly that the transformation to ferrite or pearlite does

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not have time to take place. The transformation into martensite, bycontrast, occurs almost immediately, due to a lower activation energy.Martensite has a lower density than austenite, so that thetransformation between them results in a change of volume. In thiscase, expansion occurs. 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 bothconstituents. If quenching is done improperly, these internal stressescan cause a part to shatter as it cools; at the very least, they causeinternal work hardening and other microscopic imperfections. It iscommon for quench cracks to form when water quenched, althoughthey may not always be visible.[6]

Iron ore pellets for the production of steel.At this point, if the carbon content is high enough to produce asignificant concentration of martensite, the result is an extremely hardbut very brittle material. Often, steel undergoes further heat treatmentat a lower temperature to destroy some of the martensite (by allowing

enough time for cementite, etc., to form) and help settle the internalstresses and defects. This softens the steel, producing a more ductileand fracture-resistant metal. Because time is so critical to the endresult, this process is known as tempering, which forms temperedsteel.[7]

Other materials are often added to the iron-carbon mixture to tailor theresulting properties. Nickel and manganese in steel add to its tensilestrength and make austenite more chemically stable, chromium increases the hardness and melting temperature, and vanadium alsoincreases 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 onthe metal surface to inhibit corrosion. Tungsten interferes with theformation of cementite, allowing martensite to form with slowerquench rates, resulting in high speed steel. On the other hand sulfur,nitrogen, and phosphorus make steel more brittle, so these commonlyfound elements must be removed from the ore during processing.[8]

When iron is smelted from its ore by commercial processes, it containsmore carbon than is desirable. To become steel, it must be melted and

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reprocessed to remove the correct amount of carbon, at which pointother elements can be added. Once this liquid is cast into ingots, itusually must be "worked" at high temperature to remove any cracks orpoorly mixed regions from the solidification process, and to produceshapes such as plate, sheet, wire, etc. It is then heat-treated to

produce a desirable crystal structure, and often "cold worked" toproduce the final shape. In modern steelmaking these processes areoften combined, with ore going in one end of the assembly line andfinished steel coming out the other. These can be streamlined by a deftcontrol of the interaction between work hardening and tempering.

History of steelmaking

Steel was known in antiquity, and may have been produced by managing the bloomery so

that the bloom contained carbon.[9] Some of the first steel comes from East Africa, dating back to 1400 BCE.[10] In the 4th century BCE steel weapons like the Falcata were produced

in the Iberian peninsula. The Chinese of the Han Dynasty (202 BCE – 220 CE) createdsteel by melting together wrought iron with cast iron, gaining ultimate product of acarbon intermediate—steel—by the 1st century CE.[11][12] Along with their original

methods of forging steel, the Chinese had also adopted the production methods of 

creating Wootz steel, an idea imported from India to China by the 5th century CE.[13] 

Wootz steel was produced in India and Sri Lanka from around 300 BCE. This early steel-making method employed the use of a wind furnace, blown by the monsoon winds.[14] 

Also known as Damascus steel, wootz is famous for its durability and ability to hold an

edge. It was originally created from a number of different materials including varioustrace elements. It was essentially a complicated alloy with iron as its main component.

Recent studies have suggested that carbon nanotubes were included in its structure, which

might explain some of its legendary qualities, though given the technology available atthat time, they were probably produced more by chance than by design.[15] Crucible steel 

was produced in Merv by 9th to 10th century CE.In the 11th century, there is evidence of the production of steel in Song China using two techniques: a "berganesque" method that producedinferior, inhomogeneous steel and a precursor to the modern Bessemerprocess that utilized partial decarbonization via repeated forging undera cold blast.[16]

Early modern steel

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A Bessemer converter in Sheffield, England.

[edit] Blister steel

 Main article: Cementation processBlister steel, produced by the cementation process was first made inItaly in the early 17th century CE and soon after introduced to England.It was probably produced by Sir Basil Brooke at Coalbrookdale duringthe 1610s. The raw material for this was bars of wrought iron. Duringthe 17th century it was realised that the best steel came from

oregrounds iron from a region of Sweden, north of Stockholm. This wasstill the usual raw material in the 19th century, almost as long as theprocess was used.[17][18]

[edit] Crucible steel

 Main article: Crucible steel Crucible steel is steel that has been melted in a crucible rather thanbeing forged, with the result that it is more homogeneous. Mostprevious furnaces could not reach high enough temperatures to meltthe steel. The early modern crucible steel industry resulted from the

invention of Benjamin Huntsman in the 1740s. Blister steel (made asabove) was melted in a crucible in a furnace, and cast (usually) intoingots.[18]

[edit] Modern steelmaking

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A Siemens-Martin steel oven from the Brandenburg Museum of Industry.

See also History of the modern steel industry.  The modern era in steelmaking began with the introduction of HenryBessemer's Bessemer process in the late 1850s. This enabled steel tobe produced in large quantities cheaply, so that mild steel is now usedfor most purposes for which wrought iron was formerly used.[19] Thiswas only the first of a number of methods of steel production. The

Gilchrist-Thomas process (or basic Bessemer process) was animprovement to the Bessemer process, lining the converter with abasic material to remove phosphorus. Another was the Siemens-Martinprocess of open hearth steelmaking, which like the Gilchrist-Thomasprocess complemented, rather than replaced, the original Bessemerprocess.[18]

 These were rendered obsolete by the Linz-Donawitz process of basicoxygen steelmaking, developed in the 1950s, and other oxygensteelmaking processes.[20]

[edit] Steel industry

Tata Steel  plant in the United Kingdom.

Steel output in 2005Because of the critical role played by steel in infrastructural and overalleconomic development, the steel industry is often considered to be anindicative for economic prowess.

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 The economic boom in China and India has caused a massive increasein the demand for steel in recent years. Between 2000 and 2005, worldsteel demand increased by 6%. Since 2000, several Indian[21] andChinese steel firms have rose to prominence like Tata Steel (whichbought Corus Group in 2007), Shanghai Baosteel Group Corporation 

and Shagang Group. Arcelor-Mittal is however the world's largest steelproducer. The British Geological Survey reports that in 2005, China was the topproducer of steel with about one-third world share followed by Japan,Russia and the USA.

See also: List of steel producers and Global steel industry trends 

[edit] Recycling

Steel is the most widely recycled material in North America. The steelindustry has been actively recycling for more than 150 years, in large

part because it is economically advantageous to do so. It is cheaper torecycle steel than to mine iron ore and manipulate it through theproduction process to form 'new' steel. Steel does not lose any of itsinherent physical properties during the recycling process, and hasdrastically reduced energy and material requirements than refinementfrom iron ore. The energy saved by recycling reduces the annualenergy consumption of the industry by about 75%, which is enough topower eighteen million homes for one year.[22] Recycling one ton of steel saves 1,100 kilograms of iron ore, 630 kilograms of coal, and55 kilograms of limestone.[23] 76 million tons of steel were recycled in2005.[22]

A pile of steel scrap in Brussels, waiting to be recycled.In recent years, about three quarters of the steel produced annuallyhas been recycled. However, the numbers are much higher for certaintypes of products. For example, in both 2004 and 2005, 97.5% of structural steel beams and plates were recycled.[24] Other steelconstruction elements such as reinforcement bars are recycled at arate of about 65%. Indeed, structural steel typically contains around

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95% recycled steel content, whereas lighter gauge, flat rolled steelcontains about 30% reused material.Because steel beams are manufactured to standardized dimensions,there is often very little waste produced during construction, and anywaste that is produced may be recycled. For a typical 2000-square-foot

two-story house, a steel frame is equivalent to about six recycled cars,while a comparable wooden frame house may require as many as 40–50 trees.[22]

Global demand for steel continues to grow, and though there are largeamounts of steel existing, much of it is actively in use. As such,recycled steel must be augmented by some first-use metal, derivedfrom raw materials. Commonly recycled steel products include cans,automobiles, appliances, and debris from demolished buildings. Atypical appliance is about 65% steel by weight and automobiles areabout 66% steel and iron.While some recycling takes place through the integrated steel mills 

and the basic oxygen process, most of the recycled steel is meltedelectrically, either using an electric arc furnace (for production of low-carbon steel) or an induction furnace (for production of some highly-alloyed ferrous products).

[edit] Contemporary steel

Modern steels are made with varying combinations of alloy metals tofulfill many purposes.[8] Carbon steel, composed simply of iron andcarbon, accounts for 90% of steel production.[1] High strength low alloysteel has small additions (usually < 2% by weight) of other elements,typically 1.5% manganese, to provide additional strength for a modestprice increase.[25] Low alloy steel is alloyed with other elements, usuallymolybdenum, manganese, chromium, or nickel, in amounts of up to10% by weight to improve the hardenability of thick sections.[1] Stainless steels and surgical stainless steels contain a minimum of 10%chromium, often combined with nickel, to resist corrosion (rust). Somestainless steels are magnetic, while others are nonmagnetic.[26]

Some more modern steels include tool steels, which are alloyed withlarge amounts of tungsten and cobalt or other elements to maximizesolution hardening. This also allows the use of precipitation hardening and improves the alloy's temperature resistance.[1] Tool steel isgenerally used in axes, drills, and other devices that need a sharp,long-lasting cutting edge. Other special-purpose alloys includeweathering steels such as Cor-ten, which weather by acquiring astable, rusted surface, and so can be used un-painted.[27]

Many other high-strength alloys exist, such as dual-phase steel, whichis heat treated to contain both a ferrite and martensic microstructurefor extra strength.[28] Transformation Induced Plasticity (TRIP) steel

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involves special alloying and heat treatments to stabilize amounts of austentite at room temperature in normally austentite-free low-alloyferritic steels. By applying strain to the metal, the austentiteundergoes a phase transition to martensite without the addition of heat.[29] Maraging steel is alloyed with nickel and other elements, but

unlike most steel contains almost no carbon at all. This creates a verystrong but still malleable metal.[30] Twinning Induced Plasticity (TWIP)steel uses a specific type of strain to increase the effectiveness of workhardening on the alloy.[31] Eglin Steel uses a combination of over adozen different elements in varying amounts to create a relatively low-cost metal for use in bunker buster weapons. Hadfield steel (after SirRobert Hadfield) or manganese steel contains 12–14% manganesewhich when abraded forms an incredibly hard skin which resistswearing. Examples include tank tracks, bulldozer blade edges andcutting blades on the jaws of life.[32] A special class of high-strengthalloy, the superalloys, retain their mechanical properties at extreme

temperatures while minimizing creep. These are commonly used inapplications such as jet engine blades where temperatures can reachlevels at which most other alloys would become weak.[33]

Most of the more commonly used steel alloys are categorized intovarious grades by standards organizations. For example, the AmericanIron and Steel Institute has a series of grades defining many types of steel ranging from standard carbon steel to HSLA and stainless steel.[34]

 The American Society for Testing and Materials has a separate set of standards, which define alloys such as A36 steel, the most commonlyused structural steel in the United States.[35]

 Though not an alloy, galvanized steel is a commonly used variety of 

steel which has been hot-dipped or electroplated in zinc for protectionagainst rust.[36]

[edit] Modern production methods

White-hot steel pouring out of an electric arc furnace.

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Blast furnaces have been used for two millennia to produce pig iron, acrucial step in the steel production process, from iron ore by combiningfuel, charcoal, and air. Modern methods use coke instead of charcoal,which has proven to be a great deal more efficient and is credited withcontributing to the British Industrial Revolution.[37] Once the iron is

refined, converters are used to create steel from the iron. During thelate 19th and early 20th century there were many widely usedmethods such as the Bessemer process and the Siemens-Martinprocess. However, basic oxygen steelmaking, in which pure oxygen isfed to the furnace to limit impurities, has generally replaced theseolder systems. Electric arc furnaces are a common method of reprocessing scrap metal to create new steel. They can also be usedfor converting pig iron to steel, but they use a great deal of electricity(about 440 kWh per metric ton), and are thus generally onlyeconomical when there is a plentiful supply of cheap electricity.[38]

[edit] Uses of steel

Iron and steel are used widely in the construction of roads, railways,infrastructure and buildings. Most large modern structures, such asstadiums and skyscrapers, are supported by a steel skeleton. Eventhose with a concrete structure will employ steel for reinforcing. Inaddition to widespread use, in electrical appliances and motor vehicles(despite growth in usage of aluminium, it is still the main material forcar bodies), steel is used in a variety of other construction-relatedapplications, such as bolts, nails, and screws.[39] Other commonapplications include shipbuilding, oil and gas pipelines, mining,aerospace, office furniture, steel wool, tools, and armour in the form of personal vests or vehicle armour (better known as rolled homogeneousarmour in this role).

A piece of steel wool

[edit] Historically

Before the introduction of the Bessemer process and other modernproduction techniques, steel was expensive and was only used whereno cheaper alternative existed, particularly for the cutting edge of knives, razors, swords, and other items where a hard, sharp edge wasneeded. It was also used for springs, including those used in clocks andwatches.[18]

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A carbon steel knife

[edit] Since 1850

With the advent of faster and more efficient steel production methods,steel has been easier to obtain and much cheaper. It has replacedwrought iron for a multitude of purposes. However, the availability of plastics during the later 20th century allowed these materials toreplace steel in many products due to their lower cost and weight.[40]

[edit] Long steel

A stainless steel sauce boat.

• As supports in reinforced concrete 

• Wires

• railroad tracks

• Structural steel in modern buildings and bridges 

[edit] Flat carbon steel

A steel pylon suspending overhead powerlines.

• The inside and outside body of automobiles, trains, and ships.

• Major appliances

• Magnetic cores

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[edit] Stainless steel

A steel roller coaster .

 Main article: Stainless steel 

• Cutlery

• Rulers

• Wrist watches

• Surgical equipment

[edit] See also

• Cold rolling

• Foundry

• Global steel industry trends

• Hot rolling

• Iron in mythology

• Pelletizing

• Rolling

• Rolling mill

•

Silicon steel• Steel producers

• Steel mill

• Tinplate

• The Riddle of Steel