The Basic Oxygen Furnace Steel Foundations
The basic oxygen furnace (BOF) process is a method of primary steelmaking in which carbon-rich hot metal or pig iron, produced by a blast furnace, is made into steel. It was originally developed in the early 1950s in the Austrian cities Linz and Donawitz, which is why it can also be referred to as the LD process.
In this process, either hot metal or pig iron, steel scrap, and flux (burnt lime) are charged, or loaded, into a furnace commonly called a converter or vessel. Next, pure oxygen is blown into the mixture using a water-cooled metal tube called a lance. In addition to mixing the materials in the converter, the oxygen reacts with the silicon and carbon dissolved in the hot metal, generating heat, carbon dioxide (CO2), silicon dioxide (SiO2), and other impurities. The CO2 is expelled as gas, while the SiO2 and other impurities are absorbed into waste matter, called slag, created from the burnt lime addition. The BOF process reduces the carbon via oxidation from 4.0% to approximately 0.04%.
Once the carbon level has been sufficiently reduced in the steel, the lance is removed, and the vessel is tapped, or emptied, by rotating it to pour the refined steel into a ladle. Then, the vessel is rotated back in the opposite direction to pour out and discard the slag along with all the detrimental impurities it absorbed. The vessel is now ready for the next load of material. Most steel producing mills increase efficiency by having two BOF vessels, so they can rotate being loaded and blown down with oxygen.
Typically, the steel delivered by the BOF still does not meet the customer’s metallurgical specifications, so it requires additional refinement in a secondary metallurgical station and/or a vacuum degasser prior to moving on to the next process.