Steel

Material properties

v • d • e alloy phases
Austenite (γ-iron; hard) Bainite Martensite Cementite (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 iron

tin melts around 250 °C. Cast iron—iron alloyed with greater than 1.7% carbon—melts at around 1370 °C. All of these temperatures could be reached with 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-oxygen environment. 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.^[4]

Even in the narrow range of concentrations that make up steel, mixtures of carbon and iron can form into a number of different structures, with very different properties; understanding these is essential to making quality steel. At room temperature, the most stable form of iron is the body-centered cubic (BCC) structure pearl-like appearance, or the similar but less beautiful bainite.

  Perhaps the most important 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. The transformation into martensite, by contrast, occurs almost immediately, due to a lower activation energy.

Martensite has a lower density than austenite, so that transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of work hardening and other microscopic imperfections. It is common for quench cracks to form when water quenched, although they may not always be visible.^[6]

  At this point, if the carbon content is high enough to produce a significant concentration of martensite, the result is an extremely hard but very brittle material. Often, steel undergoes further heat treatment at a lower temperature to destroy some of the martensite (by allowing enough time for cementite etc. to form) and help settle the internal stresses and defects. This softens the steel, producing a more ductile and fracture-resistant metal. Because time is so critical to the end result, this process is known as tempering, which forms tempered steel.^[7]

Other materials are often added to the iron/carbon mixture to tailor the resulting properties. phosphorus make steel more brittle, so these commonly found elements must be removed from the ore during processing.^[8]

When iron is smelted from its ore by commercial processes, it contains more carbon than is desirable. To become steel, it must be melted and reprocessed to remove the correct amount of carbon, at which point other elements can be added. Once this liquid is cast into ingots, it usually must be "worked" at high temperature to remove any cracks or poorly mixed regions from the solidification process, and to produce shapes such as plate, sheet, wire, etc. It is then heat-treated to produce a desirable crystal structure, and often "cold worked" to produce the final shape. In modern steel making these processes are often combined, with ore going in one end of the assembly line and finished steel coming out the other. These can be streamlined by a deft control of the interaction between work hardening and tempering.

History of steelmaking

 

Main article: History of ferrous metallurgy

Ancient steel

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) created steel by melting together 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 produced inferior, inhomogeneous steel and a precursor to the modern Bessemer process that utilized partial decarbonization via repeated forging under a cold blast.^[16]

Early modern steel

 

Blister steel

Main article: Cementation process

Blister steel, produced by the oregrounds iron from a region of Sweden, north of Stockholm. This was still the usual raw material in the 19th century, almost as long as the process was used.^[17][18]

Crucible steel

Main article: Crucible steel

Crucible steel is steel that has been melted in a crucible rather than being 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 in a furnace, and cast (usually) into ingots.^[18]

Modern steelmaking

 

Main article: Steelmaking

See also: History of the modern steel industry

The modern era in Siemens-Martin process of open hearth steelmaking, which like the Gilchrist-Thomas process complemented, rather than replaced, the original Bessemer process.^[18]

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

Steel industry

    Because of the critical role played by steel in infrastructural and overall economic development, the steel industry is often considered to be an indicator of economic prowess.

The economic boom in China and India has caused a massive increase in the demand for steel in recent years. Between 2000 and 2005, world steel demand increased by 6%. Since 2000, several Indian^[21] and Chinese steel firms have risen to prominence like Tata Steel (which bought Corus Group in 2007), Shanghai Baosteel Group Corporation and Shagang Group. Arcelor-Mittal is however the world's largest steel producer.

The British Geological Survey reports that in 2005, China was the top producer of steel with about one-third world share followed by Japan, Russia and the USA.

In 2008, steel will be traded as a commodity in the London Metal Exchange.

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

Recycling

Steel is the most widely recycled material in North America. The steel industry has been actively limestone.^[23] 76 million tons of steel were recycled in 2005.^[22]

 

In recent years, about three quarters of the steel produced annually has been recycled. However, the numbers are much higher for certain types of products. For example, in both 2004 and 2005, 97.5% of structural steel beams and plates were recycled.^[24] Other steel construction elements such as reinforcement bars are recycled at a rate of about 65%. Indeed, structural steel typically contains around 95% recycled steel content, whereas lighter gauge, flat rolled steel contains about 30% reused material.

Because steel beams are manufactured to standardized dimensions, there is often very little waste produced during construction, and any waste that is produced may be recycled. 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 may require as many as 40–50 trees.^[22]

Global demand for steel continues to grow, and though there are large amounts of steel existing, much of it is actively in use. As such, recycled steel must be augmented by some first-use metal, derived from raw materials. Commonly recycled steel products include cans, automobiles, appliances, and debris from demolished buildings. A typical appliance is about 65% steel by weight and automobiles are about 66% steel and iron.

While some recycling takes place through the integrated steel mills and the induction furnace (for production of some highly-alloyed ferrous products).

Contemporary steel

Modern steels are made with varying combinations of alloy metals to fulfill many purposes.^[8] 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% nonmagnetic.^[26]

Some more modern steels include tool steels, which are alloyed with large amounts of tungsten and cobalt or other elements to maximize precipitation hardening and improves the alloy's temperature resistance.^[1] 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.^[27]

Many other high-strength alloys exist, such as dual-phase steel, which is heat treated to contain both a ferrite and martensic microstructure for extra strength.^[28] 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 ferritic steels. By applying strain to the metal, the austentite undergoes 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 very strong but still malleable metal.^[30] Twinning Induced Plasticity (TWIP) steel uses a specific type of strain to increase the effectiveness of work hardening on the alloy.^[31] 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 buster weapons. Hadfield steel (after Sir creep. These are commonly used in applications such as jet engine blades where temperatures can reach levels at which most other alloys would become weak.^[33]

Most of the more commonly used steel alloys are categorized into various grades by standards organizations. For example, 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.^[35]

Though not an alloy, zinc for protection against corrosion (rust).^[36]

Modern production methods

  Electric arc furnaces 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 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.^[38]

Uses of steel

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 airports, are supported by a steel skeleton. Even those with a concrete structure will employ steel for reinforcing. In addition to 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-related applications, such as bolts, nails, and screws.^[39] Other common applications include shipbuilding, pipeline transport, mining, aerospace, white goods (eg. washing machines), heavy equipment (eg. 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).  

Historically

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.^[18]  

Since 1850

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

Long steel

 

• As supports in reinforced concrete
• Wires
• railroad tracks
• Structural steel in modern buildings and bridges

Flat carbon steel

 

• The inside and outside body of automobiles, trains, and ships.
• Major appliances
• Magnetic cores

Stainless steel

 

Main article: Stainless steel

• Cutlery
• Rulers
• Wrist watches
• Surgical equipment

See also

References

Further reading

• Duncan Burn; The Economic History of Steelmaking, 1867-1939: A Study in Competition. Cambridge University Press, 1961 online version
• J. C. Carr and W. Taplin; History of the British Steel Industry Harvard University Press, 1962 online version
• Gernet, Jacques (1982). A History of Chinese Civilization. Cambridge: Cambridge University Press.
• Harukiyu Hasegawa; The Steel Industry in Japan: A Comparison with Britain 1996 online version
• Needham, Joseph (1986). Science and Civilization in China: Volume 4, Part 1 & Part 3. Taipei: Caves Books, Ltd.
• H. Lee Scamehorn; Mill & Mine: The Cf&I in the Twentieth Century University of Nebrasa Press, 1992 online version
• Warren, Kenneth, Big Steel: The First Century of the United States Steel Corporation, 1901-2001. (University of Pittsburgh Press, 2001) online review

• Extensive picture gallery of iron and steel production methods in North America and Europe. In German and English.
• International Iron & Steel Institute (IISI)
• Iron & steel industry news and information portal
• steeluniversity.org: Online steel education ressources from IISI and the University of Liverpool