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.
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.
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.
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.
Smelting, using carbon to reduce iron oxides, results in an alloy (pig iron) that retains too much carbon to be called steel.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.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 able to take on two crystalline forms (allotropic forms), body centered cubic (BCC) and face centered cubic (FCC), depending on its temperature.