<Original Papers>Fundamental Studies on the Vacuum Melting of Steel

Some fundamentals of vacuum melting of steel, such as carbon-oxygen reaction, removal of impurities and liquid-crucible reaction, were discussed from the thermodynamic and kinetic points of view. The followings were observed from the results of previous investigators on the vacuum melting of steels in magnesia crucibles : (1) In the case of high carbon steel, oxygen content was reduced rapidly. (2) In the case of low carbon steel, oxygen was decreased, at first, by carbon-oxygen reaction, but increased gradually with the time of vacuum meltig. (3) In the case of low carbon and high oxygen steel, oxygen content was not reduced. These results could be explained as follows : The oxygen content of liquid steel under vacuum is determined by the dynamic equilibrium between the rate of oxygen removal by carbon-oxygen reaction and the rate of oxygen supply from magnesia crucible. When an alumina crucible was used, the increase of oxygen content in low carbon steel was not observed, because the dissociation pressure of alumina was lower than magnesia, and therefore, alumina was better for crucible or lining material. Possibility of removal of impurities in liquid steel by vacuum distillation may be estimated by the values of vapor pressure and activity coefficient. It is easy to remove manganese, copper and tin, but difficult to remove sulphur and arsenic and it is impossible to remove phosphorus, nickel and cobalt. Peculiar behavior of silicon and aluminium in vacuum melting may be attributed to the reaction between these elements and crucible materials. The process of removal by vacuum distillation is that of diffusion controlled, that is, the rate is determined by the diffusion of impurities through thin boundary layer of liquid steel adjacent to the surface