Produced more than any other type of steel on the planet, mild steel is by far the most common type of steel used in commercial goods and day to day objects. One of the many reasons for its popularity is its inherent properties, which are well balanced, making it affordable as well as the ideal all-round material.
Steel is essentially a mixture of iron, carbon and small amounts of other elements, such as manganese, nickel or chromium. For the most part however it is the carbon content that dictates the properties that a type of steel possesses and mild steel has a carbon content that can range anywhere between 0.16% and 0.29%.Iron, in its purest form, is quite soft and able to conduct both heat and electricity whilst also maintaining the high melting point of 1535oC. The problem is iron on its own reacts with pretty much anything, causing it to bond with any number of other elements. It is this reactivity that causes iron to rust when in contact with oxygen; but it is also what allows iron to absorb carbon so readily, and become steel.
As said, the level of carbon determines the properties of each type of steel. The higher the carbon content, the harder and stronger the material becomes; conversely the melting point decreases and it becomes more brittle. 0.05% is considered a very low carbon content, but it would be easy to shape and cheap to produce; whereas 0.54% is at the high end of medium and would boast a high level of wear resistance and an extremely high tensile strength.
When the necessity arises the strength of mild steel can be enhanced by putting it through the carburising process. Carburising is basically a heat treatment that you can put iron or steel through which allows it to absorb additional carbon from a high-carbon source present during the process. Charcoal is normally used as the carbon source during the carburising process, but the carbon can also be taken from gasses, such as Carbon Monoxide.