Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten.
Carbon and other elements act as a hardening agent, preventing dislocations in the iron atom crystal lattice from sliding past one another. Varying the amount of alloying elements and the form of their presence in the steel (solute elements, precipitated phase) controls qualities such as the hardness, ductility, and tensile strength of the resulting steel. Steel with increased carbon content can be made harder and stronger than iron, but such steel is also less ductile than iron.
Alloys with a higher than 2.1% carbon content are known as cast iron because of their lower melting point and castability. Steel is also distinguishable from wrought iron, which can contain a small amount of carbon, but it is included in the form of slag inclusions. Two distinguishing factors are steel’s increased rust resistance and better weldability.
Though steel had been produced by various inefficient methods long before the Renaissance, its use became more common after more-efficient production methods were devised in the 17th century. With the invention of the Bessemer process in the mid-19th century, steel became an inexpensive mass-produced material. Further refinements in the process, such as basic oxygen steelmaking (BOS), lowered the cost of production while increasing the quality of the metal. Today, steel is one of the most common materials in the world, with more than 1.3 billion tons produced annually. It is a major component in buildings, infrastructure, tools, ships, automobiles, machines, appliances, and weapons. Modern steel is generally identified by various grades defined by assorted standards organizations.
Iron, like most metals, is found in the Earth’s crust only in the form of an ore, i.e., combined with other elements such as oxygen or sulfur. Typical iron-containing minerals include Fe2O3—the form of iron oxide found as the mineral hematite, and FeS2—pyrite (fool’s gold).
Iron is extracted from ore by removing oxygen and combining the ore with a preferred chemical partner such as carbon. This process, known as smelting, was first applied to metals with lower melting points, such as tin, which melts at approximately 250 °C (482 °F) and copper, which melts at approximately 1,100 °C (2,010 °F). In comparison, cast iron melts at approximately 1,375 °C (2,507 °F). All of these temperatures could be reached with ancient methods that have been used 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.
Smelting results in an alloy (pig iron) containing 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 austenite more chemically stable, chromium increases hardness and melting temperature, and vanadium also increases hardness while reducing the effects of metal fatigue. To prevent corrosion, at least 11% chromium is added to steel so that a hard oxide forms on the metal surface; this is known as stainless steel. Tungsten interferes with the formation of cementite, allowing martensite to form with slower quench rates, resulting in high speed steel. On the other hand, sulfur, nitrogen, and phosphorus make steel more brittle, so these commonly found elements must be removed from the ore during processing.
The density of steel varies based on the alloying constituents, but usually ranges between 7750 and 8050 kg/m3 (0.280–۰٫۲۹۱ lb/in3).
Even in the narrow range of concentrations which make up steel, mixtures of carbon and iron can form a number of different structures, with very different properties. Understanding such properties is essential to making quality steel. At room temperature, the most stable form of iron is the body-centered cubic (BCC) structure α-ferrite. It is a fairly soft metallic material that can dissolve only a small concentration of carbon, no more than 0.021 wt% at 723 °C (1,333 °F), and only 0.005% at 0 °C (32 °F). If the steel contains more than 0.021% carbon then it transforms into a face-centered cubic (FCC) structure, called austenite or γ-iron. It is also soft and metallic but can dissolve considerably more carbon, as much as 2.1% carbon at 1,148 °C (2,098 °F), which reflects the upper carbon content of steel.
One way for carbon to leave the austenite is for cementite to precipitate out of the mix, leaving behind iron that is pure enough to take the form of ferrite, resulting in a cementite-ferrite mixture. Cementite is a hard and brittle intermetallic compound with the chemical formula of Fe3C.
There are many types of heat treating processes available to steel. The most common are annealing and quenching and tempering. Annealing is the process of heating the steel to a sufficiently high temperature to soften it. This process occurs through three phases: recovery, recrystallization, and grain growth. The temperature required to anneal steel depends on the type of annealing and the constituents of the alloy.
Quenching and tempering first involves heating the steel to the austenite phase, then quenching it in water or oil. This rapid cooling results in a hard and brittle martensitic structure. The steel is then tempered, which is just a specialized type of annealing. In this application the annealing (tempering) process transforms some of the martensite into cementite, or spheroidite to reduce internal stresses and defects, which ultimately results in a more ductile and fracture-resistant metal.
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 reduce the carbon to the correct amount, at which point other elements can be added. This liquid is then continuously cast into long slabs or cast into ingots. Approximately 96% of steel is continuously cast, while only 4% is produced as cast steel ingots. The ingots are then heated in a soaking pit and hot rolled into slabs, blooms, or billets. 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 foundries these processes often occur in one assembly line, with ore coming in and finished steel coming out. Sometimes after a steel’s final rolling it is heat treated for strength, however this is relatively rare.
History of steelmaking
Steel was known in antiquity, and may have been produced by managing bloomeries, iron-smelting facilities, where the bloom contained carbon.
The earliest known production of steel is a piece of ironware excavated from an archaeological site in Anatolia (Kaman-Kalehoyuk) and is about 4,000 years old. Other ancient steel comes from East Africa, dating back to 1400 BC. In the 4th century BC steel weapons like the Falcata were produced in the Iberian Peninsula, while Noric steel was used by the Roman military. The Chinese of the Warring States (403–۲۲۱ BC) had quench-hardened steel, while Chinese of the Han Dynasty (202 BC – ۲۲۰ 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 discovered a type of high-heat blast furnace which allowed them to forge carbon steel at 1,802 °C (3,276 °F) nearly 2,000 years ago. This ability was not duplicated until centuries later in Europe during the Industrial Revolution.
Evidence of the earliest production of high carbon steel in the Indian Subcontinent was found in Samanalawewa area in Sri Lanka.
Recent studies have suggested that carbon nanotubes were included in its structure, which might explain some of its legendary qualities, though given the technology available at that time, they were produced by chance rather than by design.