Chemical compositions

Steel for automotive purposes is made up of iron (generally more than 99 per cent) and a range of other alloying elements, the most important of which is carbon.

Under a microscope, at x1000 magnification it can be seen that steel is actually made up of tiny crystals known as grains. These grains of steel are formed when liquid steel cools to a solid, the atoms of iron within each grain, aligning in a precise crystalline array. The size, shape and composition of these grains has a major effect on the strength and formability of the steel.

A carbon atom is smaller than an iron atom, and provides a strengthening mechanism by sitting between the iron atoms, preventing the rows of atoms sliding over one another. At carbon levels below 0.001 per cent, the steel is known as interstitial free (IF) and therefore has a low yield strength.

Other alloying elements, such as phosphorous or vanadium, have larger atoms that strengthen by substitution for an iron atom. This is known as solid-solution strengthening. Steel manufacturers combine this with other techniques to produce steel with an optimum balance of properties.