The difference is the composition of carbon and breaks down as follows:
* Mild (low carbon) steel: approximately 0.05–0.15% carbon content for low carbon steel and 0.16-0.29% carbon content for mild steel (e.g. AISI 1018 steel). Mild steel has a relatively low tensile strength, but it is cheap and malleable; surface hardness can be increased through carburizing.
* Medium carbon steel: approximately 0.30–0.59% carbon content(e.g. AISI 1040 steel). Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components.
* High carbon steel: approximately 0.6–0.99% carbon content . Very strong, used for springs and high-strength wires.
* Ultra-high carbon steel: approximately 1.0–2.0% carbon content . Steels that can be tempered to great hardness. Used for special purposes like (non-industrial-purpose) knives, axles or punches. Most steels with more than 1.2% carbon content are made using powder metallurgy and usually fall in the category of high alloy carbon steels.