Steel

Steel is used in buildings, as concrete reinforcing rods, in bridges, infrastructure, tools, ships, trains, cars, bicycles, machines, electrical appliances, furniture, and weapons.

In steel, small amounts of carbon, other elements, and inclusions within the iron act as hardening agents that prevent the movement of dislocations.

Carbon contents higher than those of steel make a brittle alloy commonly called pig iron.

Common alloying elements include: manganese, nickel, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, and niobium.

[5] Additional elements, most frequently considered undesirable, are also important in steel: phosphorus, sulphur, silicon, and traces of oxygen, nitrogen, and copper.

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.

[6] 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.

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.

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.

The addition of lead and sulphur decrease grain size, thereby making the steel easier to turn, but also more brittle and prone to corrosion.

Such alloys are nevertheless frequently used for components such as nuts, bolts, and washers in applications where toughness and corrosion resistance are not paramount.

[8] Even in a narrow range of concentrations of mixtures of carbon and iron that make steel, several different metallurgical structures, with very different properties can form.

The two, cementite and ferrite, precipitate simultaneously producing a layered structure called pearlite, named for its resemblance to mother of pearl.

At the very high cooling rates produced by quenching, the carbon has no time to migrate but is locked within the face-centred austenite and forms martensite.

Martensite is a highly strained and stressed, supersaturated form of carbon and iron and is exceedingly hard but brittle.

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.

Annealing is the process of heating the steel to a sufficiently high temperature to relieve local internal stresses.

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.

[19][20] The earliest known production of steel is seen in pieces of ironware excavated from an archaeological site in Anatolia (Kaman-Kalehöyük) which are nearly 4,000 years old, dating from 1800 BC.

[28][29] There is evidence that carbon steel was made in Western Tanzania by the ancestors of the Haya people as early as 2,000 years ago by a complex process of "pre-heating" allowing temperatures inside a furnace to reach 1300 to 1400 °C.

[39] 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.

[40][41][42] 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.

[citation needed] In 327 BC, Alexander the Great was rewarded by the defeated King Porus, not with gold or silver but with 30 pounds of steel.

[58][59][page needed] The modern era in steelmaking began with the introduction of Henry Bessemer's process in 1855, the raw material for which was pig iron.

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,[63] and because, with respect to the open hearth process, the same quantity of steel from a BOS process is manufactured in one-twelfth the time.

[62] Today, electric arc furnaces (EAF) are a common method of reprocessing scrap metal to create new steel.

[72][73] Reduction of these emissions are expected to come from a shift in the main production route using cokes, more recycling of steel and the application of carbon capture and storage technology.

[78] Twinning Induced Plasticity (TWIP) steel uses a specific type of strain to increase the effectiveness of work hardening on the alloy.

Steel is used in a variety of other construction materials, such as bolts, nails and screws, and other household products and cooking utensils.

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.