Mild steel is magnetic and can be referred to as a low carbon steel as it contains small amount of carbon and a high amount iron and ferrite. Because of its composition it has high tensile strength for impacts and will bend or deform rather than crack.
If you were to add additional carbon to mild steel, it would in essence become cast iron. There is little chromium or molybdenum in mild steel or other alloy elements such as those in stainless steel. The result is that the iron element it contains is subject to rusting (oxidation) if not treated with a hot dipped Galvanising to ensure corrosion resistance.