In the production of high-grade steel, refining under vacuum was initially introduced to remove such gas components as hydrogen before casting the molten steel tapped from the converter. This is called vacuum degassing because the gas components in the molten steel are removed by reducing the balanced partial pressures during and after pouring the molten steel into a reduced-pressure vessel. The functions of temperature control, final refining, and composition control were subsequently added to the secondary refining equipment because the function of the converter is increasingly concentrated on decarburization, and further reductions in impurity elements and nonmetallic inclusions should therefore be performed by other means. The allowable ranges of target temperature and composition have also become tighter requiring fine tuning. Thus, secondary refining has recently become the standard process for producing high-grade steels. The most important functions of secondary refining are final desulfurization, degassing of oxygen, nitrogen, hydrogen, etc., removal of inclusions, and final decarburization for ultra-low carbon steel. Desulfurization is conducted by adding CaO, Na2CO3, CaF2, etc. in a similar manner to that used in the hot metal pretreatment process. Denitrification and dehydrogenation are achieved by treating the molten steel under reduced pressure in a vacuum vessel. Deoxidation is conducted by adding silicon and aluminum to the molten steel to form nonmetallic inclusions of silica (SiO2) and alumina (Al2O3), which are coagulated by stirring the molten steel for enhanced flotation. These are then absorbed into the top slag and removed. Additional decarburization, if required, is carried out by blowing pure oxygen gas onto or into the molten steel in the vacuum vessel to remove the carbon as carbon monoxide. Secondary refining equipment typically used in the mass production of high-purity steel at integrated steel mills includes the RH (Ruhrstahl-Hausen) vacuum degasser and LF (ladle furnace). The RH equipment injects argon gas into one (suction tube) of the two tubes (snorkels) immersed in the molten steel in the ladle, and the molten steel in the ladle is drawn through the suction tube into the vacuum vessel by the operation of air-lift pumping. After being exposed to the vacuum in the vessel, the molten steel flows back into the ladle through the down snorkel. Since the recirculation rate is relatively high, the RH process is suitable for rapid degassing of a large amount of molten steel. The refining functions of the RH process have also been expanded. For example, decarburization and heating-up are conducted by injecting pure oxygen gas, while the desulfurization and deoxidation rates are increased by adding fluxes, both onto or into the melt in the vacuum vessel. On the other hand, the LF equipment offers strong heating functions, permits the addition of a large amount of alloys, and enables precise temperature control. It also provides outstanding desulfurization by high-temperature treatment with reducing fluxes and the removal of deoxidation products. The LF process is therefore often used for the secondary refining of alloy steel. Secondary refining equipment used mainly in the final refining step for stainless steel includes the AOD furnace and the VOD furnace. Stainless steel contains a large amount of chromium as a basic component. Since chromium is a strong oxide-forming element, during normal refining it is difficult to decarburize stainless steel to a sufficiently low carbon level while preventing loss of chromium through oxidation to the slag phase. Thus, low carbon levels are achieved by decreasing the partial pressure of carbon monoxide in the refining atmosphere to ensure preferential decarburization in the presence of chromium. In practice, this is done in the AOD furnace by dilution with argon and in the VOD furnace by reducing the pressure.