Steel is the most common metal alloy in the world. In its simplest form, it consists of iron and varying amounts of carbon. Because both iron and carbon have been well-known since ancient times, the alloy has been produced in one form or another since well before the birth of Christ. Some of the very earliest steel was made in the eastern regions of Africa, around 1400 BCE.

Iron is the major component of steel, with carbon being a distant second, at between 0.2% and 2.14%, depending on the grade. Carbon is the most cost-effective way to alloy iron, but other metals can be used to augment the element and to give the metal certain properties. The metal known as cast iron is actually steel that has a very high carbon content, which gives it a lower melting point and greater castability. Steel is well-known for its strength, which is given to it precisely by the alloying elements.

When iron is in its solid metal state, the atoms form a crystal lattice structure. While this structure is fairly rigid, there can be imperfections in it, which create tiny weak points in the metal. Atoms of the alloying elements can fill in these microscopic weak spots in the lattice, giving the alloy the flexibility and tensile strength it is known for.

The modern steel industry produces the metal through the use of what is called the basic oxygen furnace. In these furnaces, molten iron has pure oxygen blown through it, lowering the levels of impurities. Cleaning agents called fluxes are also added for this same purpose. The main advantage to this process, other than a high-quality product, is its speed.

Previous processes included the Bessemer process, where air was forced through molten iron to oxidize impurities. Carbon monoxide is one by-product of this process, with the other impurities forming slag. The invention of the Bessemer process was especially noteworthy because it made steel a mass-produced, cheap commodity. The alloy had been produced by various methods in the Middle Ages and before, but none of these were especially efficient, nor could they be employed on a large scale.

As steelmaking techniques have improved, a wider variety of alloys have become available. The use of several different metals, such as tungsten and chromium, can make metal that is tailored to very specific applications. The array of properties that steel can be given by the addition of different elements is seen, for example, in the fact that two things as different as samurai swords and automobiles are both made from iron alloyed by other metals.