Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Patent
1996-01-24
1998-05-19
Andrews, Melvyn
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
75 1015, 75 1019, 75 1022, 75 1064, 266208, 266211, 373 7, 373 18, C21B 1110
Patent
active
057530041
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for refining a molten metal and apparatus therefor, in which the molten metal is refined in a vacuum or in a low oxygen partial pressure atmosphere such as an inert gas atmosphere up to several hundreds of torr, then the refined molten metal is further refined substantially under a surrounding atmosphere.
BACKGROUND ART
A method for refining a molten metal in a vacuum or in a low oxygen partial pressure atmosphere (hereinafter called "reduced-pressure-refining") has widely been used because a high grade steel can be obtained easily, and a vacuum induction furnace (VIF) is one of means for this method.
In the reduced-pressure-refining, some impurities are separated while being evaporated from, scattered above or floated onto the molten metal as themselves or as their compounds such as oxides, so that the refining proceeds. In this case the amount of impurities in the molten metal can be lowered very much. (In the present invention, the term "impurities" used herein generally mean impurity elements or substances causing non-metallic inclusions.)
In the reduced-pressure-refining, a part of the evaporated and/or scattered components is condensed on the free-board part of the furnace wall, and adheres thereon as scum. Further, on the surface of the molten metal in a refining furnace, other impurities that cannot evaporate are floated and separated as dross. Thus, when the molten metal is poured by tilting the refining furnace, the adhered substance and floating substances are washed away by the melt flow, and mixed in the molten metal again.
In a case where the refractory lining of the ladle or tundish used as a receiving vessel is not heated sufficiently before receiving molten metal, there occurs such a problem that the molten metal is polluted by active gas components absorbed in the refractory lining. Further, the refractory lining of the molten metal-receiving vessel partially reacts with the molten metal that have been activated by the reduced-pressure-refining, or is eroded by the molten metal, with the result that there occurs such a case as the molten metal is polluted by these reaction products and erosion products.
Further, in a case where the molten metal is directly exposed to atmospheric pressure after the reduced-pressure-refining, the impurities that cannot evaporate, such as dross, dissolve in the molten metal again to contaminate the molten metal.
In order to prevent the adhered substance and the floating substance from being mixed into a molten metal, there have been proposed a method in which the molten metal is discharged from the bottom of the refining furnace while leaving a part of the molten metal in the furnace for preventing the mixing of floating impurities, and another proposed method in which the refining and discharging of the molten metal is effected by use of an induction skull furnace so that the molten metal is not in contact with furnace wall.
Further, after the discharging of molten metal, there has been proposed a method in which metallic inclusions-causing substances are removed in the ladle or tundish by using a ceramic filter. Although some of them are practically used, these methods have not yet been adopted widely due to various limitations. Since these proposed methods are not always effective to all substances and contaminants to be mixed, these methods must be combined to make the methods effective to all of the substances to be mixed. However, the combination of these methods causes such problems as to make the cost thereof higher.
On the other hand, in order to remove the re-mixed substances etc., it may be possible to effect re-refining after vacuum refining by use of a ladle refining furnace of a graphite electrode arc heating type or a ASEA-SKF furnace. However, these methods cause such a problem as to pick up carbon into the molten metal.
JP-A-4-318118 discloses a method for producing ultra-low carbon, ultra-low sulfur steel in which method, after the decarburization of molten metal by a vacuum dega
REFERENCES:
patent: 4773079 (1988-09-01), Bruckmann et al.
Hanai Noboru
Kanamoto Katsumi
Kashiwagi Iwao
Kishida Tamiya
Mikutsu Kokichi
Andrews Melvyn
Hitachi Metals Ltd.
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