Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:no minimum content is specified or required for chromium, cobalt, molybdenum, nickel, niobium, titanium, tungsten, vanadium, zirconium, or any other element to be added to obtain a desired alloying effect;the specified minimum for copper does not exceed 0.40%;or 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%.

Carbon steel is broken down into four classes based on carbon content:

Low-carbon steel: 0.05 to 0.15% carbon (plain carbon steel) content.
Medium-carbon steel: Approximately 0.3–0.5% carbon content.Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components.
High-carbon steel: Approximately 0.6 to 1.0% carbon content. Very strong, used for springs, edged tools, and high-strength wires.
Ultra-high-carbon steel: Approximately 1.25–2.0% carbon content. Steels that can be tempered to great hardness. Used for special purposes like (non-industrial-purpose) knives, axles, and punches. Most steels with more than 2.5% carbon content are made using powder metallurgy.