steel
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What is steel?
Steel is an alloy consisting primarily of iron and carbon, with the carbon content ranging up to 2%. The addition of carbon enhances iron's hardness and strength, making steel a versatile material for various applications, including construction and manufacturing.
Steelmaking involves melting, purifying, and alloying iron at high temperatures, around 1,600° C (2,900° F). This process involves chemical reactions to achieve specific chemical compositions and temperatures. The major raw materials used are blast-furnace iron, steel scrap, and direct-reduced iron.
There are various steel grades with different chemical compositions, classified into carbon steels, low-alloy steels, and high-alloy steels. Carbon steels, the most common, contain varying carbon levels. Low-alloy steels have up to 8% alloying elements, while high-alloy steels have higher concentrations.
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How is steel different from pure iron?
Steel is an alloy consisting primarily of iron and carbon, with the carbon content typically up to 2 percent. In contrast, iron (Fe) is a chemical element found in the Earth's core and minerals. The addition of carbon enhances iron's hardness and strength, making steel more versatile for various applications, including construction and manufacturing tools. In its pure form, iron is soft and not particularly useful for engineering.
Steelmaking involves melting, purifying, and alloying iron at high temperatures, around 1,600° C (2,900° F). Steel's versatility is evident in its numerous applications, from construction to tools. This is due to its relatively low production cost, the abundance of raw materials, and its range of mechanical properties.
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What basic ingredients are used to make steel?
Steelmaking involves melting, purifying, and alloying raw materials like blast-furnace iron, steel scrap, and direct-reduced iron (DRI) at approximately 1,600° C (2,900° F).
The strengthening of iron to convert it into steel is achieved by adding small amounts of carbon. Alloying elements such as chromium, titanium, and vanadium are added to improve specific properties like strength, wear, and corrosion resistance. Most alloys are added as ferroalloys (iron-based alloys) because they are cheaper to produce than pure metals.
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What is the process of extracting steel from iron ore?
The process of extracting steel from iron ore involves melting, purifying, and alloying at high temperatures, around 1,600° C (2,900° F). This process occurs in molten conditions, where chemical reactions are initiated to achieve specific chemical compositions.
The major raw materials include blast-furnace iron, steel scrap, and direct-reduced iron (DRI). The most important chemical reactions are the oxidation of carbon, silicon, manganese, and phosphorus.
The blast furnace is a vertical shaft furnace that produces liquid metals from a mixture of metallic ore, coke, and flux, with air introduced under pressure. The iron oxides in the ore are chemically reduced to molten iron by carbon and carbon monoxide from the coke.
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Why do metallurgists add other elements to steel, like chromium or nickel?
Metallurgists add elements like chromium and nickel to steel to improve its properties. Alloying elements can modify the temperature and rate at which microstructures change, alter transformation points between ferrite and austenite, modify solution and diffusion rates, and form intermetallic compounds.
Adding elements can improve strength, wear resistance, and corrosion resistance. For example, chromium increases resistance to oxidation and atmospheric corrosion by forming a protective oxide film on the steel's surface. Nickel toughens steel and slows the hardening process, which allows larger sections to be heat-treated.
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How does the structure of steel affect its strength and uses?
The structure of steel significantly affects its strength and uses. Steel is strengthened by adding carbon, which can exist in solid solution or as a carbide. The amount of carbon present affects the steel's hardness; more carbon typically increases hardness.
Heat treatments and the addition of other elements also modify steel's properties. For example, quenching and tempering can produce martensite, an extremely hard form of steel. Alloying elements influence heat treatment by slowing atom diffusion, allowing martensite to form at lower cooling rates and improving hardenability. The yield strength can be increased by adding impurities, work hardening, or grain refinement.
Different steel grades and treatments result in a wide array of properties, making steel versatile for many applications.
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steel, alloy of iron and carbon in which the carbon content ranges up to 2 percent (with a higher carbon content, the material is defined as cast iron). By far the most widely used material for building the worldâs infrastructure and industries, it is used to fabricate everything from sewing needles to oil tankers. In addition, the tools required to build and manufacture such articles are also made of steel. As an indication of the relative importance of this material, in 2013 the worldâs raw steel production was about 1.6 billion tons, while production of the next most important engineering metal, aluminum, was about 47 million tons. (For a list of steel production by country, see below World steel production.) The main reasons for the popularity of steel are the relatively low cost of making, forming, and processing it, the abundance of its two raw materials (iron ore and scrap), and its unparalleled range of mechanical properties.
Properties of steel
The base metal: iron
The major component of steel is iron, a metal that in its pure state is not much harder than copper. Omitting very extreme cases, iron in its solid state is, like all other metals, polycrystallineâthat is, it consists of many crystals that join one another on their boundaries. A crystal is a well-ordered arrangement of atoms that can best be pictured as spheres touching one another. They are ordered in planes, called lattices, which penetrate one another in specific ways. For iron, the lattice arrangement can best be visualized by a unit cube with eight iron atoms at its corners. Important for the uniqueness of steel is the allotropy of ironâthat is, its existence in two crystalline forms. In the body-centred cubic (bcc) arrangement, there is an additional iron atom in the centre of each cube. In the face-centred cubic (fcc) arrangement, there is one additional iron atom at the centre of each of the six faces of the unit cube. It is significant that the sides of the face-centred cube, or the distances between neighbouring lattices in the fcc arrangement, are about 25 percent larger than in the bcc arrangement; this means that there is more space in the fcc than in the bcc structure to keep foreign (i.e., alloying) atoms in solid solution.
Iron has its bcc allotropy below 912° C (1,674° F) and from 1,394° C (2,541° F) up to its melting point of 1,538° C (2,800° F). Referred to as ferrite, iron in its bcc formation is also called alpha iron in the lower temperature range and delta iron in the higher temperature zone. Between 912° and 1,394° C iron is in its fcc order, which is called austenite or gamma iron. The allotropic behaviour of iron is retained with few exceptions in steel, even when the alloy contains considerable amounts of other elements.
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There is also the term beta iron, which refers not to mechanical properties but rather to the strong magnetic characteristics of iron. Below 770° C (1,420° F), iron is ferromagnetic; the temperature above which it loses this property is often called the Curie point.
Effects of carbon
In its pure form, iron is soft and generally not useful as an engineering material; the principal method of strengthening it and converting it into steel is by adding small amounts of carbon. In solid steel, carbon is generally found in two forms. Either it is in solid solution in austenite and ferrite or it is found as a carbide. The carbide form can be iron carbide (Fe3C, known as cementite), or it can be a carbide of an alloying element such as titanium. (On the other hand, in gray iron, carbon appears as flakes or clusters of graphite, owing to the presence of silicon, which suppresses carbide formation.)
The effects of carbon are best illustrated by an iron-carbon equilibrium diagram. The A-B-C line represents the liquidus points (i.e., the temperatures at which molten iron begins to solidify), and the H-J-E-C line represents the solidus points (at which solidification is completed). The A-B-C line indicates that solidification temperatures decrease as the carbon content of an iron melt is increased. (This explains why gray iron, which contains more than 2 percent carbon, is processed at much lower temperatures than steel.) Molten steel containing, for example, a carbon content of 0.77 percent (shown by the vertical dashed line in the figure) begins to solidify at about 1,475° C (2,660° F) and is completely solid at about 1,400° C (2,550° F). From this point down, the iron crystals are all in an austeniticâi.e., fccâarrangement and contain all of the carbon in solid solution. Cooling further, a dramatic change takes place at about 727° C (1,341° F) when the austenite crystals transform into a fine lamellar structure consisting of alternating platelets of ferrite and iron carbide. This microstructure is called pearlite, and the change is called the eutectoidic transformation. Pearlite has a diamond pyramid hardness (DPH) of approximately 200 kilograms-force per square millimetre (285,000 pounds per square inch), compared with a DPH of 70 kilograms-force per square millimetre for pure iron. Cooling steel with a lower carbon content (e.g., 0.25 percent) results in a microstructure containing about 50 percent pearlite and 50 percent ferrite; this is softer than pearlite, with a DPH of about 130. Steel with more than 0.77 percent carbonâfor instance, 1.05 percentâcontains in its microstructure pearlite and cementite; it is harder than pearlite and may have a DPH of 250.