Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Patent
1996-07-24
1998-12-22
Ryan, Patrick
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
75 101, 75309, 75306, 75303, 75319, C21C 700
Patent
active
058512624
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of refining molten metal wherein the molten metal is refined in a vacuum or a low oxygen partial pressure atmosphere and subsequently further refined at a pressure about the atmospheric pressure.
BACKGROUND ART
A method of refining a molten metal in a vacuum or a low oxygen partial pressure atmosphere (herein referred to as "RPR": Reduced Pressure Refining) have been widely used due to their capability of allowing highly pure steel to be easily obtained. A vacuum induction furnace (referred to as "VIF") is utilized for such methods.
In the refining process under a reduced pressure, refining proceeds in such a way that some kinds of impurity turn into a compound of themselves or their oxides and are separated from the molten metal through vaporization, splattering or floating. In this case, as refining proceeds, an amount level of impurities in the molten metal becomes very low (the term "impurities" herein refers generally to materials that are sources of impure elements or nonmetallic inclusions).
In refining under a reduced pressure, vaporized or splattered components partially condenses to stick as scum on the free board zone of the wall of a refining furnace. Other drossy impurities that do not vaporize are separated and float on the surface of the molten metal. When the furnace is tilted to discharge the molten metal, the molten metal flow washes away a part of these adhesions and floating substances, which returns to the melt. Furthermore, if the lining refractory or liner of a ladle or tundish that receives the molten metal is not sufficiently heated before receiving the molten metal, there is a possibility that the molten metal is contaminated by active gaseous components adsorbed in the liner. The liner partially reacts with the molten metal activated by refining under a reduced pressure, or is eroded by the molten metal. The molten metal may be contaminated by the active gases or erosion products.
In order to prevent such adherents and floating substances, there have been proposed some ways which are a method of discharging molten metal from the bottom of a refining vessel and leaving a part of the molten metal in the vessel to prevent floating impurities to again mix into the molten metal, a method of without contact with the molten metal with utilization of an induction scull furnace. After discharging molten metal from the vessel, there has been proposed a method of removing substances, which can become to fine nonmetallic inclusions, with utilization of a ceramic filter. Some of those methods have been put into practice, but they are not yet usual due to various constraints. In addition, since the methods are not effective for all the re-entered substances or contaminants, they must be combined to have an effect on each re-entered substance listed above. However, costs increase with increasing number of combinations.
It is conceived that re-entered substances can be removed by, after vacuum refining, re-refining by a ladle refining furnace of a graphite electrode arc heating method or by ASEA-SKF furnace. However, since those methods use graphite electrodes that are dipped into slag on the surface of molten steel, there is a problem that carbon enters into the molten metal. JP-A-4-318118 proposes a producing method of an ultra-low-carbon/sulfur steel by decarburizing melt by vacuum degassing processing, then plasma-heating the melt containing not less than 0.2 wt % of solute aluminum, and stirring the melt under the presence of slag a basicity of 8 or more to desulfurize it.
According to the method of the JP-A-4-318118, there is a problem that solute aluminum must be increased and also the basicity of slag must be increased to 8 or more during plasma-refining because the slag generated before the plasma-refining is introduced into a plasma-refining furnace.
That is, since the method introduces a large amount of slag into a plasma-refining furnace, large amounts of deoxidizers and reducing agents are required to improve the slag, re
REFERENCES:
patent: 4484946 (1984-11-01), Ototani
patent: 4584015 (1986-04-01), Hartwig et al.
patent: 5160531 (1992-11-01), Koizumi
Elve M. Alexandra
Hitachi Metals Ltd.
Ryan Patrick
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