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Steel

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For other uses, see Steel (disambiguation).Steel is an alloy consisting mostly ofiron, with a carbon content between 0.2% and 2.1%

by weight, depending on the grade. Carbon is the most cost-effective alloying materialfor iron, but various other alloying elements are used, such as manganese, chromium,

vanadium, and tungsten.[1] 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 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 carbon content can be made harderand stronger than iron, but is also less ductile.

Alloys with a higher carbon content are known as cast iron because of their lowermelting

point and castability.[1] Steel is also distinguished from wrought iron, which can contain a

small amount of carbon, but it is included in the form ofslaginclusions. Twodistinguishing factors are steel's increased rust resistance and betterweldability.

Though steel had been produced by various inefficient methods long before the

Renaissance, its use became more common after more efficient production methods weredevised in the 17th century. With the invention of the Bessemer process in the mid-19th

century, steel became an inexpensive mass-produced material. Further refinements in the

process, such asbasic oxygen steelmaking, further lowered the cost of production while

increasing the quality of the metal. Today, steel is one of the most common materials inthe world, with more than 1300 million tons produced annually. It is a major component

in buildings, infrastructure, tools, ships, automobiles, machines,appliances, and weapons.

Modern steel is generally identified by various grades of steel defined by variousstandards organizations.

The steel cable of a collierywinding tower

Contents [hide]

1 Material properties

o 1.1 Heat treatment
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2 Steel production 3 History of steelmaking

o 3.1 Ancient steel

o 3.2 Wootz steel and Damascus steel

o 3.3 Modern steelmaking

3.3.1 Processes starting from bar iron 3.3.2 Processes starting from pig iron

4 Steel industry

5 Recycling 6 Contemporary steel

7 Uses

o 7.1 Historical

7.1.1 Long steel 7.1.2 Flat carbon steel

7.1.3 Stainless steel

8 See also

9 References

o 9.1 Bibliography

10 Further reading

11 External links

[edit] Material properties

Iron-carbonphase diagram, showing the conditions necessary to form different phases

Iron, like most metals, is found in the Earth's crust only in the form of an ore, ie.combined with other elements such as oxygen orsulfur.[2] Typical iron-containing

minerals include Fe2O3the form ofiron oxide found as the mineral hematite, and FeS2pyrite (fool's gold).[3] Iron is extracted from oreby removing oxygen and combiningthe ore with a preferred chemical partner such as carbon. This process, known as
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smelting, was first applied to metals with lowermelting points, such as tin, which meltsat approximately 250 C (482 F) and copper, which melts at approximately 1,000 C

(1,830 F). In comparison, cast iron melts at approximately 1,370 C (2,500 F). All of

these temperatures could be reached with ancient methods that have been used since theBronze Age. Since the oxidation rate itself increases rapidly beyond 800 C, it is

important that smelting take place in a low-oxygen environment. Unlike copper and tin,

liquid iron dissolves carbon quite readily. Smelting results in an alloy (pig iron)containing too much carbon to be called steel.[4] The excess carbon and other impurities

are removed in a subsequent step.

Other materials are often added to the iron/carbon mixture to produce steel with desired

properties.Nickel and manganese in steel add to its tensile strength and make austenitemore chemically stable, chromium increases hardness and melting temperature, and

vanadium also increases hardness while reducing the effects ofmetal fatigue. To prevent

corrosion, at least 11% chromium is added to steel so that a hard oxide forms on the

metal surface; this is known as stainless steel.Tungsten interferes with the formation ofcementite, allowing martensite to form with slower quench rates, resulting in high speed

steel. On the other hand, sulfur,nitrogen, andphosphorus make steel more brittle, so

these commonly found elements must be removed from the ore during processing. [5]

The density of steel varies based on the alloying constituents, but usually ranges between

7.75 and 8.05 g/cm3 (0.2800.291 lb/in3).[6]

Even in the narrow range of concentrations which make up steel, mixtures of carbon and

iron can form a number of different structures, with very different properties.Understanding such properties is essential to making quality steel. At room temperature,

the most stable form of iron is thebody-centered cubic (BCC) structure -ferrite. It is a

fairly soft metallic material that can dissolve only a small concentration of carbon, no

more than 0.021 wt% at 723 C (1,333 F), and only 0.005% at 0 C (32 F). If the steelcontains more than 0.021% carbon then it transforms into a face-centered cubic (FCC)

structure, called austenite or -iron. It is also soft and metallic but can dissolve

considerably more carbon, as much as 2.1%[7] carbon at 1,148 C (2,098 F)), whichreflects the upper carbon content of steel.[8]

When steels with less than 0.8% carbon, known as a hypoeutectoid steel, are cooled from

an austenitic phase the mixture attempts to revert to the ferrite phase, resulting in an

excess of carbon. One way for carbon to leave the austenite is forcementite to precipitateout of the mix, leaving behind iron that is pure enough to take the form of ferrite,

resulting in a cementite-ferrite mixture. Cementite is a hard and brittle intermetallic

compound with the chemical formula of Fe3C. At the eutectoid, 0.8% carbon, the cooledstructure takes the form ofpearlite, named after its resemblance to mother of pearl. For

steels that have more than 0.8% carbon the cooled structure takes the form of pearlite and

cementite.[9]

Perhaps the most importantpolymorphic form is martensite, a metastable phase which issignificantly stronger than other steel phases. When the steel is in an austenitic phase and

then quenched it forms into martensite, because the atoms "freeze" in place when the cell

structure changes from FCC to BCC. Depending on the carbon content the martensiticphase takes different forms. Below approximately 0.2% carbon it takes an ferrite BCC

crystal form, but higher carbon contents take abody-centered tetragonal (BCT) structure.

There is no thermal activation energy for the transformation from austenite to martensite.Moreover, there is no compositional change so the atoms generally retain their same

neighbors.[10]
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Martensite has a lower density than austenite does, so that transformation between themresults in a change of volume. In this case, expansion occurs. Internal stresses from this

expansion generally take the form ofcompression on the crystals of martensite and

tension on the remaining ferrite, with a fair amount ofshearon both constituents. Ifquenching is done improperly, the internal stresses can cause a part to shatter as it cools.

At the very least, they cause internal work hardening and other microscopic

imperfections. It is common for quench cracks to form when water quenched, althoughthey may not always be visible.[11]

[edit] Heat treatment

Main article: Heat treating carbon steel

There are many types ofheat treatingprocesses available to steel. The most common are

annealing and quenching and tempering. Annealing is the process of heating the steel to a

sufficiently high temperature to soften it. This process occurs through three phases:recovery, recrystallization, and grain growth. The temperature required to anneal steel

depends on the type of annealing and the constituents of the alloy.[12]

Quenching and tempering first involves heating the steel to the austenite phase, then

quenching it in wateroroil. This rapid cooling results in a hard and brittle martensiticstructure.[10] The steel is then tempered, which is just a specialized type of annealing. In

this application the annealing (tempering) process transforms some of the martensite into

cementite orspheroidite to reduce internal stresses and defects, which ultimately resultsin a more ductile and fracture-resistant metal.[13][edit] Steel production

Iron ore pellets for the production of steel

Main article: Steelmaking

See also: Steel production by country

When iron is smelted from its ore by commercial processes, it contains more carbon thanis desirable. To become steel, it must be melted and reprocessed to reduce the carbon to

the correct amount, at which point other elements can be added. This liquid is then

continuously cast into long slabs orcast into ingots. Approximately 96% of steel iscontinuously cast, while only 4% is produced as cast steel ingots per year.[citation needed] The

ingots are then heated in a soaking pit and hot rolled into slabs,blooms, orbillets. Slabs

are hot orcold 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 foundries these processes often occur in one assembly line, with ore

coming in and finished steel coming out.[14]Sometimes after a steel's final rolling it is

heat treated for strength, however this is relatively rare.[15]
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[edit] History of steelmaking

Bloomery smelting during the Middle Ages

Main article: History of ferrous metallurgy

[edit] Ancient steel

Steel was known in antiquity, and may have been produced by managing bloomeries

iron-smelting facilities so that the bloom contained carbon.[16] Steel is mentioned in the

Bible: Jeremiah 15:12 of the Authorized King James Version, it reads: "Shall iron breakthe northern iron and the steel?". However, it seems the Hebrews had no word for "steel"

but used instead (istoma).[17]

The earliest known production of steel is a piece of ironware excavated from an

archaeological site in Anatolia and is about 4,000 years old.[18] Other ancient steel comesfrom East Africa, dating back to 1400 BC.[19]In the 4th century BC steel weapons like the

Falcata were produced in the Iberian Peninsula, whileNoric steel was used by the Roman

military.[20] The Chinese of the Warring States (403221 BC) had quench-hardened steel,[21] while Chinese of the Han Dynasty (202 BC 220 AD) created steel by melting

togetherwrought iron with cast iron, gaining an ultimate product of a carbon-

intermediate steel by the 1st century AD.[22][23]

[edit] Wootz steel and Damascus steel

Main articles: Wootz steel and Damascus steel

Evidence of the earliest production of high carbon steel in the Indian Subcontinent wasfound in Samanalawewa area in Sri Lanka.[24] Wootz steel was produced in India by about

300 BC.[25] Along with their original methods of forging steel, the Chinese had also

adopted the production methods of creating Wootz steel, an idea imported into Chinafrom India by the 5th century AD.[26] This early steel-making method in Sri Lanka

employed the unique use of a wind furnace, blown by the monsoon winds and producing

almost pure steel.[27] 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 materialsincluding various trace elements. It was essentially a complicated alloy with iron as its

main component. Recent studies have suggested that carbon nanotubes were included in
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its structure, which might explain some of its legendary qualities, though given thetechnology available at that time, they were produced by chance rather than by design.[28]

Natural wind was used where the soil containing iron was heated up with the use of

wood. The ancient Sinhalese managed to extract a ton of steel for every 2 tons ofsoil[citation needed], a remarkable feat at the time. One such furnace was found in

Samanalawewa and archaeologists were able to produce steel as the ancients did long

ago.[27][29]

Crucible steel, formed by slowly heating and cooling pure iron and carbon (typically inthe form of charcoal) in a crucible, was produced in Merv by the 9th to 10th century AD.[25] In the 11th century, there is evidence of the production of steel in Song China using

two techniques: a "berganesque" method that produced inferior, inhomogeneous steel anda precursor to the modern Bessemer process that utilized partial decarbonization via

repeated forging under a cold blast.[30][edit] Modern steelmaking

A Bessemer converter in Sheffield, England

In Europe since 1600s, the first step in producing steel has been the smelting iron ore into

pig iron in ablast furnace from ore, charcoal, and air.[31] Modern methods use cokeinstead of charcoal, which has proven to be a great deal cheaper.[32][33][34]

[edit] Processes starting from bar iron

Main articles: Blister steel and Crucible steel

In these processes pig iron was "fined" in a finery forge to producebar iron (wrought

iron), which was then used in steel-making.[31]

The production of steel by the cementation process was described in a treatise publishedin Prague in 1574 and was in use inNuremberg from 1601. A similar process forcase

hardening armour and files was described in a book published inNaples in 1589. The

process was introduced to England in about 1614.[35] It was produced by SirBasil Brookeat Coalbrookdale during the 1610s. The raw material for this were bars of wrought iron.

During the 17th century it was realised that the best steel came from oregrounds iron

from a region ofSweden, north ofStockholm. This was still the usual raw material in the19th century, almost as long as the process was used.[36][37]
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Crucible steel is steel that has been melted in a crucible rather than being forged, with theresult that it is more homogeneous. Most previous furnaces could not reach high enough

temperatures to melt the steel. The early modern crucible steel industry resulted from the

invention ofBenjamin Huntsman in the 1740s. Blister steel (made as above) was meltedin a crucible or in a furnace, and cast (usually) into ingots .[37][38][edit] Processes starting

from pig iron

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

White-hot steel pouring out of an electric arc furnace

The modern era in steelmakingbegan with the introduction ofHenry Bessemer's

Bessemer process in 1858. His raw material was pig iron.[39]

This enabled steel to beproduced in large quantities cheaply, so that mild steel is now used for most purposes forwhich wrought iron was formerly used.[40] The Gilchrist-Thomas process (orbasic

Bessemer process) was an improvement to the Bessemer process, because it lined the

converter with abasic material to remove phosphorus. Another improvement insteelmaking was the Siemens-Martin process, which complemented the Bessemer

process.[37]

These were rendered obsolete by the Linz-Donawitz process ofbasic oxygen steelmaking

(BOS), developed in the 1950s, and other oxygen steelmaking processes. Basic oxygensteelmaking is superior to previous steelmaking methods because the oxygen pumped

into the furnace limits impurities.

[41]

Now, electric arc furnaces (EAF) are a commonmethod of reprocessing scrap metal to create new steel. They can also be used forconverting pig iron to steel, but they use a great deal of electricity (about 440 kWh per

metric ton), and are thus generally only economical when there is a plentiful supply of

cheap electricity.[42]

[edit] Steel industry
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A Corus Group plant in the United Kingdom

Steel production by country in 2007

See also: History of the modern steel industry, Global steel industry trends, Steel

production by country, and List of steel producers

It is common today to talk about "the iron and steel industry" as if it were a single entity,but historically they were separate products. The steel industry is often considered to be

an indicator of economic progress, because of the critical role played by steel in

infrastructural and overall economic development.[43]

The economic boom in China and India has caused a massive increase in the demand forsteel in recent years. Between 2000 and 2005, world steel demand increased by 6%.

Since 2000, several Indian [44] and Chinese steel firms have risen to prominence like Tata

Steel (which bought Corus Group in 2007), Shanghai Baosteel Group Corporation andShagang Group. ArcelorMittal is however the world's largest steel producer.

In 2005, the British Geological Survey stated China was the top steel producer with aboutone-third of the world share; Japan, Russia, and the US followed respectively.[45]

In 2008, steel started to be traded as a commodity in the London Metal Exchange. At the

end of 2008, the steel industry faced a sharp downturn that led to many cut-backs.[46]

[edit] Recycling

A pile of steel scrap in Brussels, waiting to be recycled
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Steel is one of the most recycled materials in the world,[47] and, as of 2008, more than83% of steel was recycled in the United States.[48]In the United States it is the most

widely recycled material; in 2000, more than 60 million metric tons were recycled.[47][49]

The most commonly recycled items are containers, automobiles, appliances, and

construction materials. For example, in 2008, more than 97% of structural steel and 106%of automobiles were recycled, comparing the current steel consumption for each industry

with the amount of recycled steel being produced.[48] A typical appliance is about 75%

steel by weight[50]and automobiles are about 65% steel and iron.[51]

The steel industry has been actively recycling for more than 150 years, in large partbecause it is economically advantageous to do so. It is cheaper to recycle steel than to

mine iron ore and manipulate it through the production process to form new steel. Steel

does not lose any of its inherentphysical properties during the recycling process, and hasdrastically reduced energy and material requirements compared with refinement from

iron ore. The energy saved by recycling reduces the annual energy consumption of the

industry by about 75%, which is enough to power eighteen million homes for one year.[52]

Steel from the World Trade Centeris poured for construction ofUSSNew York(LPD-21)

The BOS steelmaking uses between 25 and 35% recycled steel to make new steel. BOS

steel usually has less residual elements in it, such as copper, nickel and molybdenum and

is therefore more malleable than EAF steel so it is often used to make automotivefenders,soup cans,industrial drums or any product with a large degree of cold working.EAF steelmaking uses almost 100% recycled steel. This steel contains more residual

elements that cannot be removed through the application of oxygen and lime so it is used

to make structural beams,plates,reinforcing barand other products that require little coldworking.[53] Recycling one ton of steel saves 1,100 kilograms ofiron ore, 630 kilograms

ofcoal, and 55 kilograms oflimestone.[54]

Because steel beams are manufactured to standardized dimensions, there is often very

little waste produced during construction, and any waste that is produced may berecycled. For a typical 2,000-square-foot (200 m2) two-story house, a steel frame is

equivalent to about six recycled cars, while a comparable wooden frame house mayrequire as many as 4050 trees.[55]

[edit] Contemporary steel

See also: Steel grades

Modern steels are made with varying combinations of alloy metals to fulfill many

purposes.[5]Carbon steel, composed simply of iron and carbon, accounts for 90% of steel

production.[1]High strength low alloy steel has small additions (usually < 2% by weight)of other elements, typically 1.5% manganese, to provide additional strength for a modest
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price increase.[56]Low alloy steel is alloyed with other elements, usually molybdenum,manganese, chromium, ornickel, in amounts of up to 10% by weight to improve the

hardenability of thick sections.[1]Stainless steels and surgical stainless steels contain a

minimum of 11% chromium, often combined with nickel, to resist corrosion (rust). Somestainless steels are magnetic, while others are nonmagnetic.[57]

Some more modern steels include tool steels, which are alloyed with large amounts of

tungsten and cobalt or other elements to maximize solution hardening. This also allows

the use ofprecipitation hardening and improves the alloy's temperature resistance.[1] Toolsteel is generally used in axes, drills, and other devices that need a sharp, long-lasting

cutting edge. Other special-purpose alloys include weathering steels such as Cor-ten,

which weather by acquiring a stable, rusted surface, and so can be used un-painted.[58]

Many other high-strength alloys exist, such as dual-phase steel, which is heat treated tocontain both a ferritic and martensitic microstructure for extra strength.[59] Transformation

Induced Plasticity (TRIP) steel involves special alloying and heat treatments to stabilize

amounts of austentite at room temperature in normally austentite-free low-alloy ferriticsteels. By applying strain to the metal, the austentite undergoes aphase transition to

martensite without the addition of heat.[60]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.[61]Twinning Induced Plasticity (TWIP) steel uses a

specific type of strain to increase the effectiveness of work hardening on the alloy.[62]

Eglin Steel uses a combination of over a dozen different elements in varying amounts to

create a relatively low-cost metal for use in bunker busterweapons. Hadfield steel (afterSirRobert Hadfield) ormanganese steel contains 1214% manganese which when

abraded forms an incredibly hard skin which resists wearing. Examples include tank

tracks,bulldozer blade edges and cutting blades on thejaws of life.[63]

Most of the more commonly used steel alloys are categorized into various grades bystandards organizations. For example, the Society of Automotive Engineers has a series

ofgrades defining many types of steel.[64] The American Society for Testing and

Materials has a separate set of standards, which define alloys such as A36 steel, the mostcommonly used structural steel in the United States.[65]

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

hot-dipped or electroplated in zinc for protection against rust.[66]

[edit] Uses

A roll ofsteel wool

Iron and steel are used widely in the construction of roads, railways, infrastructure, and

buildings. Most large modern structures, such as stadiums and skyscrapers, bridges, and
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airports, are supported by a steel skeleton. Even those with a concrete structure willemploy steel for reinforcing. In addition to widespread use in major appliances and cars.

Despite growth in usage ofaluminium, it is still the main material for car bodies. Steel is

used in a variety of otherconstruction materials, such as bolts, nails, and screws.[67] Othercommon applications include shipbuilding,pipeline transport, mining, offshore

construction,aerospace, white goods (e.g. washing machines), heavy equipment such as

bulldozers, office furniture, steel wool, tools, and armourin the form of personal vests orvehicle armour(better known as rolled homogeneous armourin this role).

[edit] Historical

A carbon steel knife

Before the introduction of the Bessemer process and other modern production techniques,steel was expensive and was only used where no cheaper alternative existed, particularly

for the cutting edge of knives, razors, swords, and other items where a hard, sharp edge

was needed. It was also used forsprings, including those used in clocks and watches.[37]

With the advent of speedier and thriftier production methods, steel has been easier to

obtain and much cheaper. It has replaced wrought iron for a multitude of purposes.

However, the availability ofplastics in the latter part of the 20th century allowed thesematerials to replace steel due to their lower cost and weight.[68][edit] Long steel

A steel pylon suspending overhead powerlines

As reinforcing bars and mesh in reinforced concrete

Railroad tracksStructural steel in modernbuildings andbridges

Wires

[edit] Flat carbon steelMajor appliances

Magnetic cores

The inside and outside body ofautomobiles, trains, and ships.[edit] Stainless steel
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A stainless steel gravy boatMain article: Stainless steel

Cutlery

Rulers

Surgical equipment

Wrist watches
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Steel Industries

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