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The Modern Steel Manufacturing Process

Steel is the world's most popular construction material because of its unique combination of durability, workability, and cost. It's an iron alloy that contains between 0.2 and 2 percent carbon by weight.

According to the World Steel Association, in 2016 global crude steel production reached a high of 1, 628.5 million tonnes. Of this, approximately three-quarters were produced using BOS plants, while EAF facilities accounted for the remaining quarter.

Some of the largest steel producing countries are China, Japan, US, and India. China accounts for roughly 50 percent of this production.

The world's largest steel producers include ArcelorMittal, Hebei Steel Group, Baosteel, POSCO and Nippon Steel.

The 6 Step Modern Production Process

Methods for manufacturing steel have evolved significantly since industrial production began in the late 19th century. Modern methods, however, are still based the same premise as the original Bessemer Process, which uses oxygen to lower the carbon content in iron.

Today, steel production makes use of both recycled materials, as well as the traditional raw materials, such as iron ore, coal, and limestone. Two processes; basic oxygen steelmaking (BOS) and electric arc furnaces (EAF) account for virtually all steel production.

 

Modern steelmaking can be broken down into six steps:

1. Ironmaking: In the first step, the raw inputs iron ore, coke, and lime are melted in a blast furnace.

The resulting molten iron, also referred to as 'hot metal,' still contains 4-4.5% carbon and other impurities that make it brittle.

2. Primary Steelmaking: There are two primary steelmaking methods: BOS (Basic Oxygen Furnace) and the more modern EAF (Electric Arc Furnace) methods. BOS methods add recycled scrap steel to the molten iron in a converter.

At high temperatures, oxygen is blown through the metal, which reduces the carbon content to between 0-1.5%. EAF methods, however,  feed recycled steel scrap through use high power electric arcs (temperatures up to 1650 °C) to melt the metal and convert it to high-quality steel.

3. Secondary Steelmaking: Secondary steelmaking involves treating the molten steel produced from both BOS and EAF routes to adjust the steel composition. This is done by adding or removing certain elements and/or manipulating the temperature and production environment.Depending on the types of steel required, the following secondary steelmaking processes can be used:

  •     stirring
  •     ladle furnace
  •     ladle injection
  •     degassing
  •     CAS-OB (Composition Adjustment by Sealed argon bubbling with Oxygen Blowing).

4. Continuous Casting: In this step, the molten steel is cast into a cooled mold causing a thin steel shell to solidify. The shell strand is withdrawn using guided rolls and fully cooled and solidified. The strand is cut into desired lengths depending on application; slabs for flat products (plate and strip), blooms for sections (beams), billets for long products (wires) or thin strips.

5. Primary Forming: The steel that is cast is then formed into various shapes, often by hot rolling, a process that eliminates cast defects and achieves the required shape and surface quality.

Hot rolled products are divided into flat products, long products, seamless tubes, and specialty products.

6. Manufacturing, Fabrication, and Finishing: Finally, secondary forming techniques give the steel its final shape and properties. These techniques include:

    shaping (e.g. cold rolling)
    machining (e.g. drilling)
    joining (e.g. welding)
    coating (e.g. galvanizing)
    heat treatment (e.g. tempering)
    surface treatment   (e.g. carburizing)

steel

Steel is an alloy of iron and carbon and 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 (BCC) and face centered cubic (FCC), depending on its temperature. In the body-centred cubic arrangement, there is an iron atom in the centre of each cube, and in the face-centred cubic, there is one at the center of each of the six faces of the cube. 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 only possible by reducing iron's ductility.

Steel was produced in bloomery furnaces for thousands of years, but its large-scale, industrial use only began 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 man-made 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.

 

Definitions and related materials

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, iron, tungsten, carbon 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 are also important in steel: phosphorus, sulfur, silicon, and traces of oxygen, nitrogen, and copper, that are most frequently considered undesirable.

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.[3] 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 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).[4] 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%[7] (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 percenage 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.[clarification needed] Moreover, there is no compositional change so the atoms generally retain their same neighbors.[10]

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

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 inand 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 possibly was produced in bloomeries and crucibles

The earliest known production of steel are pieces of ironware excavated from an archaeological site in Anatolia (Kaman-Kalehoyuk) 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) amongst the rest of the world grew considerably.[18] 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.

The Haya people of East Africa invented a type of furnace they used to make carbon steel at 1,802 °C (3,276 °F) nearly 2,000 years ago. East African steel has been suggested by Richard Hooker to date back to 1400 BC.

 

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. 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,[citation needed] 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 BCE, 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
 

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
Main articles: Blister steel and Crucible steel

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 armour 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.[50] 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.[51] 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.[48] 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
See also: History of the modern steel industry, Global steel industry trends, Steel production by country, and List of steel producers
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Steel production (in million tons) by country in 2007
A steel plant in the United Kingdom.

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 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 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[56] and Chinese steel firms have risen to prominence,[according to whom?] such as Tata Steel (which bought Corus Group in 2007), Baosteel Group and Shagang Group. ArcelorMittal is however 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.

The world steel industry peaked in 2007. That year, ThyssenKrupp spent $12 billion to build the two most modern mills in the world, in Calvert, Alabama and Sepetiba, Rio de Janeiro, Brazil. The worldwide Great Recession starting in 2008, however, sharply lowered demand and new construction, and so prices fell. ThyssenKrupp lost $11 billion on its two new plants, which sold steel below the cost of production.
Recycling
Main article: Ferrous metal recycling

Steel is one of the world's most-recycled materials, with a recycling rate of over 60% globally;[59] in the United States alone, over 82,000,000 metric tons (81,000,000 long tons) were recycled in the year 2008, for an overall recycling rate of 83%.
Contemporary steel
Bethlehem Steel (Bethlehem, Pennsylvania facility pictured) was one of the world's largest manufacturers of steel before its closure in 2003
See also: Steel grades
Carbon steels

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.[3] 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.[3] 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 price increase.[61]

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.[63] 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.[66] 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.[3] 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.[67] 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.

In 2016, a breakthrough in creating a strong light aluminium steel alloy which might be suitable in applications such as aircraft was announced by researchers at Pohang University of Science and Technology. Adding small amounts of nickel was found to result in precipitation as nano particles of brittle B2 intermetallic compounds which had previously resulted in weakness. The result was a cheap strong light steel alloy—nearly as strong as titanium at 10% of the cost[70]—which is slated for trial production[when?] at industrial scale by POSCO, a Korean steelmaker.
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 third world 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)
Main article: Weathering steel

    Intermodal containers
    Outdoor sculptures
    Architecture
    Highliner train cars

 

Stainless steel
A stainless steel gravy boat
Main article: Stainless steel

    Cutlery

  •     Rulers
  •     Surgical instruments
  •     Watches
  •     Guns
  •     Rail passenger vehicles
  •     Tablets
  •     Trash Cans
  •     Body piercing jewellery


 

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.

source: wikipedia

Galvanizing process

The hot dip galvanizing process is relatively simple. It involves cleaning steel and immersing it in molten zinc to obtain a coating.


Cleaning cycle

Galvanizing ProcessHot dip galvanizing is the process of coating iron or steel with a layer of zinc by immersing the metal in a bath of molten zinc at a temperature of around 842 °F (450 °C). During the process, a metallurgically bonded coating is formed which protects the steel from harsh environments, whether they be external or internal. Galvanized steel is widely used in applications where corrosion resistance is needed without the cost of stainless steel and can be identified by the crystallised pattern on the surface (often called a ‘spangle’). Galvanizing is probably the most environmentally friendly process available to prevent corrosion.

The galvanizing reaction will only occur on a chemically clean surface. In common with most coating processes, the secret to achieving a good quality coating lies in the preparation of the surface. It is essential that this is free of grease, dirt and scale before galvanizing. These types of contamination are removed by a variety of processes and common practice is to degrease first using an alkaline or acidic solution into which the component is dipped. The article is then rinsed in cold water to avoid contaminating the rest of the process.

The article is then dipped in hydrochloric acid at ambient temperature to remove rust and mill scale. Welding slag, paint and heavy grease will not be removed by these cleaning steps and should be removed by the fabricator before the work is sent to the galvanizer. After further rinsing, the components will then commonly undergo a fluxing procedure. This is normally applied by dipping in a flux solution – usually about 30% zinc ammonium chloride at around 65-80°C. Alternatively, some galvanizing plants may operate using a flux blanket on top of the galvanizing bath. The fluxing operation removes the last traces of oxide from the surface and allows the molten zinc to wet the steel.


Galvanizing

When the clean iron or steel component is dipped into the molten zinc (which is commonly at around 450°C) a series of zinc-iron alloy layers are formed by a metallurgical reaction between the iron and zinc. The rate of reaction between the steel and the zinc is normally parabolic with time and so the initial rate of reaction is very rapid and considerable agitation can be seen in the zinc bath. The main thickness of coating is formed during this period. Subsequently, the reaction slows down and the coating thickness is not increased significantly even if the article is in the bath for a longer period of time.

A typical time of immersion is about four or five minutes but it can be longer for heavy articles that have high thermal inertia or where the zinc is required to penetrate internal spaces. Upon withdrawal from the galvanizing bath, a layer of molten zinc will be taken out on top of the alloy layer. Often this cools to exhibit the bright shiny appearance associated with galvanized products.


Post treatment

Post galvanizing treatment can include quenching into water or air cooling. Conditions in the galvanizing plant such as temperature, humidity and air quality do not affect the quality of the galvanized coating. By contrast, these are critically important for good quality painting. No post treatment of galvanized articles is necessary and a paint or a powder coating may be applied for enhanced aesthetics or for additional protection where the environment is extremely aggressive. Chemical conversion coatings and other barrier systems may be applied to minimise the occurrence of wet storage stain.


How long does hot dip galvanizing take?

Provided reasonable notice is given, most articles can be hot dip galvanized and returned to the fabricator within a week. A typical turnaround, depending on size of the order, is three days. Galvanized bolts and nuts are now widely stocked but it is advisable that orders for galvanized fasteners should be placed as early as possible.

Hot Dip Galvanizing 101

Hot Dip Galvanizing 101

Important Things About Galvanizing You Should Understand...Made Simple

    The Hot Dip Galvanizing Process
    The Thickness and Appearance of Our Coating
    The Expected Outdoor Service Life of Galvanized Coating
    Designing Products for Galvanizing   Typical types of Products that are Galvanized

THE HOT DIP GALVANIZING PROCESS


At Ohio Galvanizing, the galvanizing process consists of three basic steps:

    Surface Preparation
    Galvanizing
    Inspection

Surface Preparation
This is the critical initial step. When a coating fails before the end of its expected service life, it is most often due to incorrect or inadequate surface preparation.

In the galvanizing process, zinc simply will not adhere with a steel surface that is not perfectly clean. At Ohio Galvanizing, we cannot take responsibility for properly galvanizing your product unless we correctly clean it first.

Surface preparation for galvanizing typically consists of these three steps:

Caustic Cleaning. Your product is dipped in a hot alkali solution to remove organic contaminants such as dirt, paint markings, grease, and oil from the metal surface. It is then dipped in a rinsing tank. Note: epoxies, vinyl, asphalt, or welding slag must be removed before galvanizing by grit blasting, sand blasting, or other mechanical means.

Pickling. Your product is then dipped in a dilute solution of hydrochloric acid.  Called pickling, this step removes scale and rust from the steel surface. It is then dipped in a second rinsing tank.

Fluxing. This is the final surface preparation step in the galvanizing process. Fluxing removes oxides and prevents further oxides from forming on the sur­face of the metal prior to galvanizing and promotes bonding of the zinc to the steel or iron surface. At Ohio Galvanizing your product is dipped or pre-fluxed in an aqueous solution of zinc ammonium chloride. The material is then air dried.

 

Galvanizing
In this step, your product is completely immersed in a bath consisting of a min­imum of 98% pure molten zinc. The bath chemistry is specified by the American Society for Testing and Materials (ASTM) in Specification B6.

Your material is immersed in the bath long enough to reach bath temperature, or about 850°F (454°C), then withdrawn slowly from the galvanizing bath whereby the excess zinc is removed by draining.

The chemical reactions that result in the formation and structure of the galvanized coating continue after the articles are withdrawn from the bath as long as the product is near the bath temperature. Your product is then cooled in a quench tank or ambi­ent air immediately after withdrawal from the bath.

 

Inspection
The two properties of the hot dip galvanized coating that are closely scrutinized after galvanizing are coating thickness and coating appearance. A variety of simple physical and laboratory tests may be performed to determine thickness, uniformity, adherence, and appearance.

Products are galvanized according to long-established, well-accepted, and approved standards of the ASTM. These standards cover everything from the minimum required coating thicknesses for various categories of galvanized items to the composition of the zinc metal used in the process.

Testing methods and interpretation of results are covered in the publication, The Inspection of Products Hot Dip Galvanized after Fabrication, published by the American Galvanizing Association (AGA). Because we are a member, this publication is available from Ohio Galvanizing.

According to numerous national and international studies, hot dip galvanizing produces no significant changes in the mechanical properties of the structur­al steels or welds commonly used throughout the world. The galvanized product's underlying steel is chemically and metallurgically equivalent to the uncoated steel.

THE THICKNESS AND APPEARANCE OF OUR COATING
ASTM specifications establish minimum standards for the thickness of galvanized coatings on various categories of items. These minimum standards are routinely exceeded by galvanizers due to the nature of the galvanizing process.

Some factors influencing the thickness and appearance of the galvanized coating Ohio Galvanizing can control-steel surface condition, bath temperature, bath immersion time, bath withdrawal rate, and steel cooling rate. Some factors are inherent in the product, such as the chemical composition of the steel and any cold working of the steel prior to galvanizing.

The chemical composition of the steel being galvanized strongly influences the thickness and appearance of the galvanized coating. For example, silicon, phosphorous, or combinations of these two elements can cause thick, brittle galvanized coatings. The carbon, sulfur, and manganese content of the steel also may have a minor effect on the galvanized coating thickness.

The combination of the elements mentioned above, known as "reactive steel" to the galvanizing industry, tend to accelerate the growth of zinc-iron alloy layers. This may result in a finished galvanized coating consisting entirely of zinc-iron alloy. Instead of a shiny appearance, the galvanized coating will have a dark gray matte finish that provides just as much corrosion protection as a galvanized coating having the common bright appearance.

THE EXPECTED OUTDOOR SERVICE
LIFE OF GALVANIZED COATING

The graph below is a plot of the thickness of the galvanized coating against the expected service life of the galvanized coating under outdoor exposure conditions. Most galvanized applications are 4 mils of thickness minimum per surface.

The expected service life is defined as the life until 5% of the surface is showing iron oxide (rust). At this stage, it is unlikely that the underlying steel or iron has been weakened or the integrity of the structures protected by the galvanized coating otherwise compromised through corrosion.

DESIGNING PRODUCTS FOR GALVANIZING
The best way to ensure the safe, effective, and economical galvanizing of steel products is for the designer, fabricator, and galvanizer to work together before the product is manufactured. Ohio Galvanizing can assist you with the practices that should be followed in designing products for effective and safe galvanizing. These practices are easily applied and in most cases are routine methods used to ensure maximum corrosion protection...and the most cost effective galvanizing process for you.

Most ferrous materials are suitable for galvanizing. These include cast iron, malleable iron, cast steels, and hot and cold rolled steels.

Sizes, Shapes and Dimensions     
Iron and steel products to be galvanized after fabrication range from small pieces of hardware to large welded steel assemblies. Ohio Galvanizing utilizes kettles that are 30 ½ feet long by 5 feet wide by 7 ½ feet deep.

If your product(s) is too deep or too long to fit into the kettle, it is often possible for us to utilize double-dipping to galvanize products that exceed the dimensions of our kettle.  Should you have questions about a product's galvanizability, contact Ohio Galvanizing today for a no-cost consolation.  

Filling and Vent Holes
Galvanizing requires that cleaning solutions and molten zinc flow into, over, through, and out of fabricated steel products. Designs that promote the flow of zinc are optimal.

Filling and vent holes must be provided to prevent pickling or other cleaning bath fluids from becoming trapped in an article. It is best to avoid narrow gaps between plates, overlapping surfaces, and back-to-back angles and channels. When overlapping or contacting surfaces cannot be avoided, all edges should be completely sealed by welding but provided with a small hole or a short gap in the welding to relieve pressure build-up in overlapping areas that exceed 16 square inches.

Galvanized and Type Coatings

Galvanized Coatings - Types of Coatings, Their Application and Characteristics

Background

There are a number of methods of applying zinc coatings and each will determine the coating’s thickness and its ultimate durability in a specific environments The most commonly encountered types of zinc coatings are:

•         Zinc electroplating

•         Mechanical plating

•         Sherardising

•         Continuously galvanized sheet

•         Continuously galvanized wire

•         Galvanized pipe and tube

•         General or hot dip galvanizing

•         Zinc metal spraying

A brief description of each application process and the characteristics of the coating formed are provided in the following sections.


Zinc Electroplating

Zinc electroplating involves immersion of the items to be coated in a solution containing zinc ions and applying an electric current to uniformly coat the surface.

Coating characteristics: Zinc electroplated coatings are bright coatings that are thin - typically around 5-10 microns and are not suitable for exterior use where durability is required. Heavy chromate coatings are frequently applied to zinc platings to improve their durability, especially for fastener applications. The coating is all pure zinc and lacks the hard alloy layers of the hot dipped coatings.


Mechanical Plating

Mechanical plating involves tumbling the items to be coated in zinc powder with glass beads and special reducing agents to bond the zinc particles to the steel surface.

Coating characteristics: The mechanical plating process is used to apply zinc or alloy coatings to fasteners and small parts. The zinc particles are in lamellar form and durability equivalent to hot dip coatings can be achieved in a uniform coating that is particularly suited to threaded fasteners and hardened TEK type screws that are unsuitable for hot dip galvanizing. These coatings are typically 15 - 20 microns thick.


Sherardizing

Sherardizing involves heating the articles to be coated in zinc powder to approximately 400 400oC at which temperature diffusion bonding of the zinc with the steel occurs.

Coating characteristics: Sherardised coatings are diffusion coatings whose thickness can be varied considerably up to over 300 microns and whose constituents can be modified by adding other metal or inorganic compounds to the zinc powder. The sherardized coatings are almost entirely made up of iron-zinc alloy phases. The long cycle times for the process make application costly. It is now rarely used.


Continuous Strip Galvanizing

Continuous strip galvanizing involves passing coil steel through a bath of molten zinc in a controlled reducing atmosphere at high speed (180 m/min).

Coating characteristics: The zinc coating thickness is closely controlled in the manufacturing process by air wiping of the sheet as it emerges from the galvanizing bath. The coating thickness varies from an average of 7 microns on ZI00 sheet to 42 microns on the heaviest Z600 sheet. The coating has a very thin zinc-iron alloy layer which gives it its flexibility for pressing and forming.


Continuously Galvanized Wire

Continuously galvanized Wire is produced by passing cleaned steel wire through a lead/zinc bath at high speed ( 180 m/min).

Coating characteristics: Similar to those of continuously galvanized sheet. Coating thickness varies depending on the diameter and coating grade of the wire from 3 microns in the thinnest standard gauge to 43 microns in the thickest (8 mm) heavy galvanized grade.


Galvanized Pipe and Tube

Galvanized pipe and tube is produced by two methods; one is semi-continuous where stock lengths of tube are cleaned and passed continuously through a bath of molten zinc at 450 degrees centigrade.

The other method is continuous where strip is formed into tube from coil and the tube then passed through a bath of molten zinc at 450 degrees centigrade. This second method coats the exterior of the tube only.

Coating characteristics: The semi-continuously applied coating is a conventional galvanized coating having a coating thickness typically around 65 microns which consists largely of zinc-iron alloy layers as the free zinc layer is largely removed through air wiping during the process. The continuous tube galvanizing process produces a bright coating which is almost all free zinc with very thin alloy layers, giving the product good forming properties. Coating thickness is typically 12-25 microns on the exterior of the tube only.


General or Hot Dip Galvanizing

General or hot dip galvanizing involves preparing work by acid pickling in batches or on jigs and then dipping the work into a bath of molten zinc.

Coating characteristics: The typical general galvanized coating ranges from 65 microns to over 300 microns depending on the steel analysis, thickness of material and immersion time in the galvanizing bath. Typical coating thickness on most general galvanized products is 80-100 microns.


Zinc Metal Spraying

Zinc met metal spraying requires the steel surface to be cleaned to a Class 3 level and then zinc wire or zinc powder is sprayed onto the surface with an oxy-acetylene or plasma flame gun.

Coating characteristics: Zinc metal spraying produces a relatively porous coating that is able to be applied in any desired thickness but is typically 75-200 microns.

It is used where the size or shape of the article make it unsuitable for hot dip galvanizing. The availability of larger galvanizing baths has resulted in it being little used for other than repairs to galvanized coatings.

Source: Industrial Galvanizers Corp

Differences between Carbon Steel and Stainless Steel Pipe

Carbon steel and stainless steel are both metals that are used in a wide array of commercial and consumer applications. The main difference between the two is in the components that are added to the steel to make it useful for its intended purposes.

 
Carbon steel vs Stainless steel

 
Alloying Materials

Steel is an alloy made out of iron and carbon. The carbon percentage can vary depending on the grade, and mostly it is between 0.2% and 2.1% by weight. Though carbon is the main alloying material for iron some other elements like Tungsten, chromium, manganese can also be used for the purpose. Different types and amounts of alloying element used determine the hardness, ductility and tensile strength of steel. While in Carbon Steel, Carbon as the main alloying element. In carbon steel, the properties are mainly defined by the amount of carbon it has. For this alloy, the amounts of other alloying elements like chromium, manganese, cobalt, tungsten are not defined.

Stainless steel has a high chromium content that forms an invisible layer on the steel to prevent corrosion and staining. Carbon steel has a higher carbon content, which gives the steel a lower melting point, more malleability and durability, and better heat distribution.

 
How to Distinguish Carbon and Stainless Steel ?

Stainless steel is lustrous and comes in various grades that can increase the chromium in the alloy until the steel finish is as reflective as a mirror. To the casual observer, carbon steel and stainless steel are easy to distinguish. Carbon steel is dull, with a matte finish that is comparable to a cast iron pot or wrought iron fencing.

 
In a nutshell, main differences between Carbon Steel & Stainless Steel

There is an in built chromium oxide layer in stainless steel, which is not present in carbon steel.

Carbon steel can corrode whereas stainless steel is protected from corrosion

Stainless steel is preferred for many consumer products and can be used decoratively in construction, while carbon steel is often preferred in manufacturing, production and in projects where the steel is mostly hidden from view.

Stainless Steel has lower thermal conductivity than Carbon steel

 

Related Article

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