Carbon steel is steel in which the main interstitial alloying constituent is carbon in the range of 0.12–2.0%.

The definition of carbon steel stated in the American Iron and Steel Institute (AISI) is as the following:

“Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, molybdenum, nickel, niobium, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.”

The term “carbon steel” may also be used in reference to steel which is not stainless steel; in this use carbon steel may include alloy steels.

As the carbon percentage content rises, steel has the ability to become harder and stronger through heat treating; however, it becomes less ductile. Regardless of the heat treatment, higher carbon content reduces weld-ability. In carbon steels, the higher carbon content lowers the melting point.

The majority of the nearly 3, 500 different types of steel produced and available in the global market today is carbon steel. Carbon steel is formed when two elements, iron, and carbon, is combined with carbon being used as the alloying element. The carbon is used as a hardening agent to prevent iron atoms in the crystal lattice from sliding around. The carbon steel structure also has ferrite, pearlite and cementite present in varying quantities, depending on the carbon quantity of the steel.

The percentage of carbon in the steel affects the hardness, strength elasticity and ductility of the steel. Low carbon content steel or mild steel has similar properties to iron but it is softer and easy to form.

As the carbon content rises, the steel is harder and stronger but it is less ductile.