Carbon steels are steels having carbon as the primary alloying element. They are designated by AISI four-digit numbers. They also contain 0.4% silicon and 1.2% manganese. Some of the residual elements such as molybdenum, chromium, aluminium, copper, and nickel are present in small quantities. In addition to all these, impurities such as phosphorous and sulfur are also found in these steels.