Many classifications of mild steel exist. Some classify any steel with less than .3 per cent carbon content as mild steel. Others classify any steel formed into bars by cold rolling as mild steel. No matter the classification used, mild steel is one of the most common types of steels used in many industries including the automotive industry and structural applications due to its strength and low cost.
Mild steels are typically simple steels characterised by a chemical composition made up primarily of iron and carbon. Most industry sources maintain that the carbon content be less than .3 per cent. The two last numbers in the 1005 through 1025 series of steels relate to the carbon content. So 1005 has roughly .05 per cent carbon.
Other steels such as A36 also fall into the general category of mild steel since it also contains less than .3 per cent carbon. Because mild steel has such a low carbon content, it cannot typically be hardened or tempered since the lack of carbon eliminates the steel's ability to form a crystalline structure.
A low carbon content makes mild steel more malleable than higher carbon steels. Malleability is the steel's ability to be formed and rolled much like modelling clay is able to be formed. This malleability allows mild steel to be rolled into bars without the heating required in higher carbon steels. Mild steel also has a relatively high degree of ductility, meaning it can be bent without breaking.
The low cost, malleability, ductility, and ability to be cold formed makes mild steel an attractive option for a number of different applications. It can be found in structural industries such as concrete reinforcement and steel support structures. It is also found in the automotive industry in sheet metal and components such as axles and frames.