Steels of the American Iron and Steel Institute. Common and alloy steels have been numbered in a system essentially the same as the SAE. The AISI system is more elaborate than the SAE in that all numbers are preceded by letters: “A” represents basic open-hearth alloy steel, “B” acid Bessemer carbon steel, “C” basic open-hearth carbon steel, “CB” either acid Bessemer or basicopen hearth carbon steel, “E” electric furnace alloy steel.