Carbon steel is

Most-common type of steel that contains about 0.1 to 0.3 percent carbon.
In general, increase in the amount of carbon reduces ductility but increases tensile strength and the ability to harden through tempering.
As an industry-wide practice, steel that does not contain any specified or standard amount of one or more alloying elements (such as chromium, molybdenum, nickel, titanium, vanadium) to be classified as alloy steel is called carbon steel