The steel cable of a colliery winding tower
Steel is an alloy of iron and carbon, and sometimes other elements. Because of its high tensile strength and low cost, it is a major component used in buildings, infrastructure, tools, ships, automobiles, machines, appliances, and weapons.
Iron is the base metal of steel. Iron is able to take on two crystalline forms (allotropic forms), body centered cubic and face centered cubic,
depending on its temperature. In the body-centered cubic arrangement,
there is an iron atom in the center and eight atoms at the vertices of
each cubic unit cell; in the face-centered cubic, there is one atom at
the center of each of the six faces of the cubic unit cell and eight
atoms at its vertices. It is the interaction of the allotropes of iron with the alloying elements, primarily carbon, that gives steel and cast iron their range of unique properties.
In pure iron, the crystal structure has relatively little
resistance to the iron atoms slipping past one another, and so pure iron
is quite ductile,
or soft and easily formed. In steel, small amounts of carbon, other
elements, and inclusions within the iron act as hardening agents that
prevent the movement of dislocations that are common in the crystal lattices of iron atoms.
The carbon in typical steel alloys may contribute up to 2.14% of
its weight. Varying the amount of carbon and many other alloying
elements, as well as controlling their chemical and physical makeup in
the final steel (either as solute elements, or as precipitated phases),
slows the movement of those dislocations that make pure iron ductile,
and thus controls and enhances its qualities. These qualities include
such things as the hardness, quenching behavior, need for annealing, tempering behavior, yield strength, and tensile strength of the resulting steel. The increase in steel's strength compared to pure iron is possible only by reducing iron's ductility.
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 production of blister steel and then crucible steel. With the invention of the Bessemer process in the mid-19th century, a new era of mass-produced steel began. This was followed by the Siemens–Martin process and then the Gilchrist–Thomas process that refined the quality of steel. With their introductions, mild steel replaced wrought iron.
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,
steel is one of the most common manmade materials in the world, with
more than 1.6 billion tons produced annually. Modern steel is generally
identified by various grades defined by assorted standards organizations.
The noun steel originates from the Proto-Germanic adjective stahliją or stakhlijan (made of steel), which is related to stahlaz or stahliją (standing firm).
The carbon content of steel is between 0.002% and 2.14% by weight for plain iron–carbon alloys. These values vary depending on alloying elements such as manganese, chromium, nickel, tungsten, and so on. Basically, steel is an iron-carbon alloy that does not undergo eutectic reaction. In contrast, cast iron
does undergo eutectic reaction. Too little carbon content leaves (pure)
iron quite soft, ductile, and weak. Carbon contents higher than those
of steel make a brittle alloy commonly called pig iron. While iron alloyed with carbon is called carbon steel, 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, sulfur, silicon, and traces of oxygen, nitrogen, and copper.
Plain carbon-iron 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 but large amounts of slag.
Material properties
Iron-carbon equilibrium phase diagram, showing the conditions necessary to form different phases
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
by removing the oxygen through its combination with a preferred
chemical partner such as carbon which is then lost to the atmosphere as
carbon dioxide. This process, known as smelting, was first applied to metals with lower melting points, such as tin, which melts at about 250 °C (482 °F), and copper,
which melts at about 1,100 °C (2,010 °F), and the combination, bronze,
which has a melting point lower than 1,083 °C (1,981 °F). In comparison,
cast iron melts at about 1,375 °C (2,507 °F). Small quantities of iron were smelted in ancient times, in the solid state, by heating the ore in a charcoal
fire and then welding the clumps together with a hammer and in the
process squeezing out the impurities. With care, the carbon content
could be controlled by moving it around in the fire. 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 800 °C
(1,470 °F), it is important that smelting take 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 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 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 is 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. On the other hand, sulfur, nitrogen, and phosphorus are considered contaminants that make steel more brittle and are removed from the steel melt during processing.
The density of steel varies based on the alloying constituents but usually ranges between 7,750 and 8,050 kg/m3 (484 and 503 lb/cu ft), or 7.75 and 8.05 g/cm3 (4.48 and 4.65 oz/cu in).
Even in a narrow range of concentrations of mixtures of carbon
and iron that make a steel, a number of 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-centered 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 0 °C (32 °F) and 0.021 wt% at 723 °C (1,333 °F). The
inclusion of carbon in alpha iron is called ferrite. At 910 °C, pure iron transforms into a face-centered 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) carbon at 1,148 °C (2,098 °F), 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).
When steels with exactly 0.8% carbon (known as a eutectoid steel), are cooled, the austenitic
phase (FCC) of the mixture attempts to revert to the ferrite phase
(BCC). The carbon no longer fits within the FCC austenite structure,
resulting in an excess of carbon. One way for carbon to leave the
austenite is for it to precipitate out of solution as cementite,
leaving behind a surrounding phase of BCC iron called ferrite with a
small percentage of carbon in solution. The two, ferrite and cementite,
precipitate simultaneously producing a layered structure called pearlite, named for its resemblance to mother of pearl.
In a hypereutectoid composition (greater than 0.8% carbon), the carbon
will first precipitate out as large inclusions of cementite at the
austenite grain boundaries until the percentage of carbon in the grains
has decreased to the eutectoid composition (0.8% carbon), at which point
the pearlite structure forms. For steels that have less than 0.8%
carbon (hypoeutectoid), ferrite will first form within the grains until
the remaining composition rises to 0.8% of carbon, at which point the
pearlite structure will form. No large inclusions of cementite will form
at the boundaries in hypoeuctoid steel. The above assumes that the cooling process is very slow, allowing enough time for the carbon to migrate.
As the rate of cooling is increased the carbon will have less
time to migrate to form carbide at the grain boundaries but will have
increasingly large amounts of pearlite of a finer and finer structure
within the grains; hence the carbide is more widely dispersed and acts
to prevent slip of defects within those grains, resulting in hardening
of the steel. At the very high cooling rates produced by quenching, the
carbon has no time to migrate but is locked within the face-centered
austenite and forms martensite.
Martensite is a highly strained and stressed, supersaturated form of
carbon and iron and is exceedingly hard but brittle. Depending on the
carbon content, the martensitic phase takes different forms. Below 0.2%
carbon, it takes on a ferrite BCC crystal form, but at higher carbon
content it takes a body-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.
Martensite has a lower density (it expands during the cooling)
than does austenite, so that the transformation between them results in a
change of volume. In this case, 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 both constituents. If quenching 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 steel is water quenched, although they may not always be
visible.
Heat treatment
There are many types of heat treating processes available to steel. The most common are annealing, quenching, and tempering.
Heat treatment is effective on compositions above the eutectoid
composition (hypereutectoid) of 0.8% carbon. Hypoeutectoid steel does
not benefit from heat treatment.
Annealing is the process of heating the steel to a sufficiently
high temperature to relieve local internal stresses. It does not create a
general softening of the product but only locally relieves strains and
stresses locked up within the material. Annealing goes through three
phases: recovery, recrystallization, and grain growth.
The temperature required to anneal a particular steel depends on the
type of annealing to be achieved and the alloying constituents.
Quenching involves heating the steel to create the austenite phase then quenching it in water or oil. This rapid cooling results in a hard but brittle martensitic structure.
The steel is then tempered, which is just a specialized type of
annealing, to reduce brittleness. In this application the annealing
(tempering) process transforms some of the martensite into cementite, or
spheroidite and hence it reduces the internal stresses and defects. The result is a more ductile and fracture-resistant steel.
Steel production
Iron ore pellets for the production of steel
When iron is smelted from its ore, it contains more carbon than is
desirable. To become steel, it must be reprocessed to reduce the carbon
to the correct amount, at which point other elements can be added. 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 only a small fraction is cast into ingots. 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.
History of steelmaking
Bloomery smelting during the Middle Ages
Ancient steel
Steel was known in antiquity and was produced in bloomeries and crucibles.
The earliest known production of steel is seen in pieces of ironware excavated from an archaeological site in Anatolia (Kaman-Kalehöyük) and are nearly 4,000 years old, dating from 1800 BC. Horace identifies steel weapons such as the falcata in the Iberian Peninsula, while Noric steel was used by the Roman military.
The reputation of Seric iron of South India (wootz steel) grew considerably in the rest of the world. Metal production sites in Sri Lanka employed wind furnaces driven by the monsoon winds, capable of producing high-carbon steel. Large-scale Wootz steel production in Tamilakam using crucibles and carbon sources such as the plant Avāram occurred by the sixth century BC, the pioneering precursor to modern steel production and metallurgy.
The Chinese of the Warring States period (403–221 BC) had quench-hardened steel, while Chinese of the Han dynasty
(202 BC – 220 AD) created steel by melting together wrought iron with
cast iron, gaining an ultimate product of a carbon-intermediate steel by
the 1st century AD.
Wootz steel and Damascus steel
Evidence of the earliest production of high carbon steel in the Indian Subcontinent are found in Kodumanal in Tamil Nadu area, Golconda in Andhra Pradesh area and Karnataka, and in Samanalawewa areas of Sri Lanka. This came to be known as Wootz steel, produced in South India by about sixth century BC and exported globally. The steel technology existed prior to 326 BC in the region as they are mentioned in literature of Sangam Tamil,
Arabic and Latin as the finest steel in the world exported to the
Romans, Egyptian, Chinese and Arab worlds at that time – what they
called Seric Iron. A 200 BC Tamil trade guild in Tissamaharama,
in the South East of Sri Lanka, brought with them some of the oldest
iron and steel artifacts and production processes to the island from the
classical period. The Chinese and locals in Anuradhapura, Sri Lanka had also adopted the production methods of creating Wootz steel from the Chera Dynasty Tamils of South India by the 5th century AD.[32][33]
In Sri Lanka, this early steel-making method employed a unique wind
furnace, driven by the monsoon winds, capable of producing high-carbon
steel. Since the technology was acquired from the Tamilians from South India, the origin of steel technology in India can be conservatively estimated at 400–500 BC.
The manufacture of what came to be called Wootz, or Damascus steel,
famous for its durability and ability to hold an edge, may have been
taken by the Arabs from Persia, who took it from India. It was
originally created from a number of different materials including
various trace elements, apparently ultimately from the writings of Zosimos of Panopolis. In 327 BC, Alexander the Great was rewarded by the defeated King Porus, not with gold or silver but with 30 pounds of steel. Recent studies have suggested that carbon nanotubes
were included in its structure, which might explain some of its
legendary qualities, though given the technology of that time, such
qualities were produced by chance rather than by design. Natural wind was used where the soil containing iron was heated by the use of wood. The ancient Sinhalese managed to extract a ton of steel for every 2 tons of soil,
a remarkable feat at the time. One such furnace was found in
Samanalawewa and archaeologists were able to produce steel as the
ancients did.
Crucible steel, formed by slowly heating and cooling pure iron and carbon (typically in the form of charcoal) in a crucible, was produced in Merv by the 9th to 10th century AD. 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, and a precursor to the modern Bessemer process that used partial decarbonization via repeated forging under a cold blast.
Modern steelmaking
A Bessemer converter in Sheffield, England
Since the 17th century, the first step in European steel production has been the smelting of iron ore into pig iron in a blast furnace. Originally employing charcoal, modern methods use coke, which has proven more economical.
Processes starting from bar iron
In these processes pig iron was refined (fined) in a finery forge to produce bar iron, which was then used in steel-making.
The production of steel by the cementation process was described in a treatise published in Prague in 1574 and was in use in Nuremberg from 1601. A similar process for case hardening armor and files was described in a book published in Naples in 1589. The process was introduced to England in about 1614 and used to produce such steel by Sir Basil Brooke at Coalbrookdale during the 1610s.
The raw material for this process were bars of iron. During the 17th century it was realized that the best steel came from oregrounds iron of a region north of Stockholm, Sweden. This was still the usual raw material source in the 19th century, almost as long as the process was used.
Crucible steel is steel that has been melted in a crucible rather than having been forged,
with the result 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 of Benjamin Huntsman in the 1740s. Blister steel (made as above) was melted in a crucible or in a furnace, and cast (usually) into ingots.
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 steelmaking began with the introduction of Henry Bessemer's Bessemer process in 1855, the raw material for which was pig iron. His method let him produce steel in large quantities cheaply, thus mild steel came to be used for most purposes for which wrought iron was formerly used. The Gilchrist-Thomas process (or basic Bessemer process) was an improvement to the Bessemer process, made by lining the converter with a basic material to remove phosphorus.
Another 19th-century steelmaking process was the Siemens-Martin process, which complemented the Bessemer process. It consisted of co-melting bar iron (or steel scrap) with pig iron.
These methods of steel production were rendered obsolete by the Linz-Donawitz process of basic oxygen steelmaking
(BOS), developed in the 1950s, and other oxygen steel making methods.
Basic oxygen steelmaking is superior to previous steelmaking methods
because the oxygen pumped into the furnace limited impurities, primarily
nitrogen, that previously had entered from the air used. Today, electric arc furnaces (EAF) are a common method of reprocessing scrap metal
to create new steel. They can also be used for converting pig iron to
steel, but they use a lot of electrical energy (about 440 kWh per metric
ton), and are thus generally only economical when there is a plentiful
supply of cheap electricity.
Steel industry
Steel production (in million tons) by country in 2007
The steel industry is often considered an indicator of economic
progress, because of the critical role played by steel in
infrastructural and overall economic development. In 1980, there were more than 500,000 U.S. steelworkers. By 2000, the number of steelworkers fell to 224,000.
The economic boom
in China and India caused a massive increase in the demand for steel.
Between 2000 and 2005, world steel demand increased by 6%. Since 2000,
several Indian and Chinese steel firms have risen to prominence, such as Tata Steel (which bought Corus Group in 2007), Baosteel Group and Shagang Group. As of 2017, though, ArcelorMittal is the world's largest steel producer. In 2005, the British Geological Survey
stated China was the top steel producer with about one-third of the
world share; Japan, Russia, and the US followed respectively.
In 2008, steel began trading as a commodity on the London Metal Exchange. At the end of 2008, the steel industry faced a sharp downturn that led to many cut-backs.
Recycling
Steel is one of the world's most-recycled materials, with a recycling rate of over 60% globally;
in the United States alone, over 82,000,000 metric tons (81,000,000
long tons; 90,000,000 short tons) were recycled in the year 2008, for an
overall recycling rate of 83%.
As more steel is produced than is scrapped, the amount of
recycled raw materials is about 40% of the total of steel produced - in
2016, 1,628,000,000 tonnes (1.602×109 long tons; 1.795×109
short tons) of crude steel was produced globally, with 630,000,000
tonnes (620,000,000 long tons; 690,000,000 short tons) recycled.
Contemporary steel
Bethlehem Steel (Bethlehem, Pennsylvania facility pictured) was one of the world's largest manufacturers of steel before its closure in 2003
Carbon steels
Modern steels are made with varying combinations of alloy metals to fulfill many purposes. Carbon steel, composed simply of iron and carbon, accounts for 90% of steel production. 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. High strength low alloy steel
has small additions (usually l.t. 2% by weight) of other elements,
typically 1.5% manganese, to provide additional strength for a modest
price increase.
Recent Corporate Average Fuel Economy
(CAFE) regulations have given rise to a new variety of steel known as
Advanced High Strength Steel (AHSS). This material is both strong and
ductile so that vehicle structures can maintain their current safety
levels while using less material. There are several commercially
available grades of AHSS, such as dual-phase steel, which is heat treated to contain both a ferritic and martensitic microstructure to produce a formable, high strength steel. Transformation Induced Plasticity (TRIP) steel involves special alloying and heat treatments to stabilize amounts of austenite at room temperature in normally austenite-free low-alloy ferritic steels. By applying strain, the austenite undergoes a phase transition to martensite without the addition of heat.
Twinning Induced Plasticity (TWIP) steel uses a specific type of strain
to increase the effectiveness of work hardening on the alloy.
Carbon Steels are often galvanized, through hot-dip or electroplating in zinc for protection against rust.
Alloy steels
Stainless steels contain a minimum of 11% chromium, often combined with nickel, to resist corrosion. Some stainless steels, such as the ferritic stainless steels are magnetic, while others, such as the austenitic, are nonmagnetic. Corrosion-resistant steels are abbreviated as CRES.
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 of precipitation hardening and improves the alloy's temperature resistance.
Tool steel 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. Maraging steel
is alloyed with nickel and other elements, but unlike most steel
contains little carbon (0.01%). This creates a very strong but still malleable steel.
Eglin steel uses a combination of over a dozen different elements in varying amounts to create a relatively low-cost steel for use in bunker buster weapons. Hadfield steel (after Sir Robert Hadfield)
or manganese steel contains 12–14% manganese which when abraded
strain-hardens to form an incredibly hard skin which resists wearing.
Examples include tank tracks, bulldozer blade edges and cutting blades on the jaws of life.
Standards
Most of the more commonly used steel alloys are categorized into various grades by standards organizations. For example, the Society of Automotive Engineers has a series of grades defining many types of steel. The American Society for Testing and Materials has a separate set of standards, which define alloys such as A36 steel, the most commonly used structural steel in the United States. The JIS also define series of steel grades that are being used extensively in Japan as well as in developing countries.
Uses
A roll of steel wool
Iron and steel are used widely in the construction of roads,
railways, other infrastructure, appliances, and buildings. Most large
modern structures, such as stadiums
and skyscrapers, bridges, and airports, are supported by a steel
skeleton. Even those with a concrete structure employ steel for
reinforcing. In addition, it sees widespread use in major appliances and cars. Despite growth in usage of aluminium, it is still the main material for car bodies. Steel is used in a variety of other construction materials, such as bolts, nails, and screws and other household products and cooking utensils.
Other common applications include shipbuilding, pipelines, mining, offshore construction, aerospace, white goods (e.g. washing machines), heavy equipment such as bulldozers, office furniture, steel wool, tools, and armour in the form of personal vests or vehicle armour (better known as rolled homogeneous armour in this role).
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 for springs, including those used in clocks and watches.
With the advent of speedier and thriftier production methods,
steel has become easier to obtain and much cheaper. It has replaced
wrought iron for a multitude of purposes. However, the availability of
plastics in the latter part of the 20th century allowed these materials
to replace steel in some applications due to their lower fabrication
cost and weight. Carbon fiber is replacing steel in some cost insensitive applications such as aircraft, sports equipment and high end automobiles.
Long steel
A steel bridge
A steel pylon suspending overhead power lines
- As reinforcing bars and mesh in reinforced concrete
- Railroad tracks
- Structural steel in modern buildings and bridges
- Wires
- Input to reforging applications
Flat carbon steel
- Major appliances
- Magnetic cores
- The inside and outside body of automobiles, trains, and ships.
Weathering steel (COR-TEN)
- Intermodal containers
- Outdoor sculptures
- Architecture
- Highliner train cars
Stainless steel
A stainless steel gravy boat
- Cutlery
- Rulers
- Surgical instruments
- Watches
- Guns
- Rail passenger vehicles
- Tablets
- Trash Cans
- Body piercing jewellery
Low-background steel
Steel manufactured after World War II became contaminated with radionuclides by nuclear weapons testing. Low-background steel, steel manufactured prior to 1945, is used for certain radiation-sensitive applications such as Geiger counters and radiation shielding.
