Before we talking about this topic, we need to know what are the mild steels? Below shows the definition of mild steels:

Carbon steel is sometimes referred to as “mild steel” or “plain carbon steel”. The American Iron and Steel Institute defines carbon steel as having no more than 2 % carbon and no other appreciable alloying element. Carbon steel makes up the largest part of steel production and is used in a vast range of applications.

Typically carbon steels are stiff and strong. They also exhibit ferromagnetism (i.e. they are magnetic). This means they are extensively used in motors and electrical appliances. Welding carbon steels with a carbon content greater than 0.3 % requires that special precautions be taken. However, welding carbon steel presents far fewer problems than welding stainless steels. The corrosion resistance of carbon steels is poor (i.e. they rust) and so they should not be used in a corrosive environment unless some form of protective coating is used.

If you aren’t going to see the steel surface, i.e.: the final piece is going to be painted or coated.

If a piece requires a large volume of metal, where stainless isn’t cost-effective.

If you need steel for structural components that are internal to the piece and not visible or effecting the overall visual concept.

If you are bending or welding the metal.