The principal grades of steel are made by the Bessemer, Siemens-Martin, and open-hearth processes. Bessemer steel is manufactured from gray pig iron, which is free from phosphorus, sulphur, and manganese. In this process there are two separations - the conversion of the molten cast iron into pure iron, and the changing of the pure iron into steel by the addition of a small and definite quantity of alloy of manganese and carbon.

Steel may be made directly from pig iron by allowing the latter to run from the cupola into a converter lined with fire-brick and by blowing air into the metal through an opening in the bottom for about twenty minutes. This treatment removes all the carbon, since the oxygen in the air combines with the carbon of the iron to form carbon dioxide. From 5 to 10% manganese is then added and the blowing is resumed long enough to incorporate the mixture. The molten mass is ladled into iron molds to form ingots. As these are more or less porous they are reheated and run through a clogging mill or steam hammer and are finally rolled or forged into shape. This kind of steel is named from Bessemer, the inventor of the process, and is largely used for structural purposes.

The disadvantage of this process is that no sulphur or phosphorus is removed from the metal, so that it is necessary to use a grade of iron very nearly free from these impurities, if the steel is to have the required properties for tools, etc.