Influence Of Oxygen Contamination From Crucible Material On Deep Deoxidation Of Molten Steel During Vacuum Induction Melting

As an important method for melting and purifying molten steel, vacuum induction melting(VIM) can provide a superior circumstance of oxidization avoidance under vacuum and good electromagnetic stirring for degassing treatment including dehydrogenation, denitrification, deoxidization, etc.. However, during VIM, to some extent, crucible material may contaminate the molten steel by heat decomposition for contact.Through thermodynamic study on two types of basic crucibles—magnesia crucible and calcia crucible—used generally in vacuum induction furnace(VIF), it is concluded that these two types of crucibles can contaminate the molten steel by heat decomposition to increase the oxygen content under the condition of some melting temperature, melting vacuity and oxygen content of the molten steel and that the trend towards heat decomposition of the crucibles will rise with the increasing of temperature and vacuity and with the decreasing of the dissolved oxygen content of the molten steel. Under high temperature and vacuum, calcia crucible has better resistance to thermal chock and better chemical denudation than magnesia crucible.During the actual production, that the oxygen contamination from the crucible material by heat composition increases the oxygen content of molten steel will result in melting losses of the alloys in the molten steel, and will results in more heavier melting losses of the alloys, incontrollable exact chemical content of molten steel, increment of oxide inclusions and bad processability and serviceability, especially in the high strength(σb≥1600MPa) steels and alloys with special performance used in aerospace industry, manufacturing industry for precision instruments and meters, military industry, nuclear industry etc. because these steels usually have a high content of some active alloys such as Ti, V, Nb, Al, RE, etc. and a nearly zero content of dissolved oxygen. So, using magnesia and calcia crucible in VIF respectively, the experiments on deep deoxidation by carbon and the phenomenon of oxygen contamination from crucible materials in the molten steel under some temperature and vacuity and oxygen content of the molten steel were made and it were found that properly low melting temperature and vacuity helped to lighten and even avoid oxygen contamination from crucible material and the refining effect of final oxygen content of 5×10-4% in the molten steel was achieved using calcia crucible with better resistance to thermal shock under higher vacuity.