Specialized metallurgical processes – compositions for use therei – Compositions – Solid treating composition for liquid metal or charge
Reexamination Certificate
1999-12-22
2001-01-09
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Solid treating composition for liquid metal or charge
C075S327000, C075S329000, C164S056100, C164S473000
Reexamination Certificate
active
06171362
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for refining a melt of pure aluminum or an aluminum alloy (referred to as an Al alloy hereinafter), and a flux for refining a melt of an Al alloy (i.e., a molten Al alloy).
RELATED ART
As is well known, Al alloy products such as sheets and plates, shapes, wire rods or rods are produced by subjecting cast Al alloy ingots to plastic working such as rolling (hot rolling or cold rolling), extrusion or forging.
In the melting or casting step for the Al alloy ingots, an Al raw material (a Al base metal, scrap of Al alloy products, or the like) is usually melted in a melting furnace, and then components of the melt are adjusted to refine the molten Al alloy. The refining of this molten Al alloy (which may be referred to as the molten Al hereinafter) is a process for cleaning the melt, which comprises, for example, the steps of injecting chlorine gas, or a chlorine-based flux together with an inert gas as a carrier gas, into the melt; removing gas components from the melt or making inclusions therein to slag; and removing the slag from the surface of the molten Al. The refined molten Al from the melting furnace is, with or without passage thereof through a holding furnace, supplied through respective launders to a mold. When the refined Al melt flows down through the launders, the inclusions are further removed with a filter fitted to the launder or a filter box immediately before the mold. In such a way, the refined Al melt is supplied to the mold and cast into an Al alloy ingot.
The gas such as H
2
gas in the melt is removed by chlorine gas according to the following mechanism. That is, Cl
2
injected into the melt is reacted with the molten Al to produce AlCl
3
. This produced AlCl
3
is sublimated from solid to gas to turn into gas bubbles which are smaller than the injected Cl
2
gas bubbles. The partial pressure of H
2
in the gas bubbles is substantially zero. Therefore, H
2
gas in the melt transfers from the melt to the AlCl
3
gas bubbles by diffusion and particle pressure equilibrium. The fine AlCl
3
gas bubbles float up to the surface of the melt and volatilize to remove H
2
from the melt.
The removal of the inclusions in the melt is performed by mutual adhesion between the AlCl
3
gas bubbles and the inclusion. The mutual adhesion is based on the phenomenon that the AlCl3 gas bubbles adhere to the inclusions and smaller inclusions adhere to the AlCl
3
gas bubbles.
Recently, however, in the light of harm of chlorine gas or contribution thereof to generation of dioxin, the following method has been adopted: the method of injecting a chlorine-based flux together with an inert gas as a carrier gas, instead of chlorine gas, into the molten Al to promote the removal of gas and slag from the melt.
In the case that the main substance for the refining treatment for removing H
2
gas and the inclusions from the melt was chlorine gas until then, the flux used in the refining was used as a substitute for chlorine gas and was mainly used in order to promote the removal of the gas or the removal of the slag from the melt surface after the removal of the inclusions from the melt. The slag-removing step is performed in the light of the following. Oxides containing impurities produced by the refining float up to the melt surface and are present as slag. As the refining advances, the amount of the slag increases. If the slag is allowed to stand as it is, the slag is redissolved into the melt or taken into the melt so that the slag may pollute the melt. The slag-removing step is performed to remove this slag from the melt or the melting furnace. As such a flux for moving the slag, Japanese Patent Application Laid-Open (JP-A) No. 61-243136 discloses a mixture flux which comprises, as main components, a chloride such as KCl and a fluoride such as AlF
3
; and, as combustion improvers for heating, sulfate such as potassium sulfate, carbonate or nitrate. In this flux, 20-50 parts by weight of the combustion improvers are added to 100 parts by weight of the main components. Japanese Patent Application Laid-Open No. 1-123035 discloses a mixture flux comprising, as a main component, KC1; and, as combustion improvers for heating, potassium sulfate, potassium nitrate and Al atomizing powder.
The reason why these fluxes are mixtures or composites comprising the above-mentioned chloride is as follows. The melting point (decomposing point) of the chlorides that are main components for refining treatment for removing gas such as H
2
and inclusions from molten Al is higher than that of the molten Al. Thus, even if chlorides alone are injected into the molten Al, they are not easily decomposed so that the molten Al cannot be efficiently refined. For this reason, there is adopted a manner of adding the above-mentioned fluoride or the like to the chloride so as to produce mixture or composite compounds having a lower melting point. According to this manner, the mixture or composite compounds are easily decomposed in the melt. There is also adopted a manner of adding Al powder or the like thereto, so as to generate heat for easy decomposition.
These fluxes containing the above-mentioned chloride have less serious problems than chlorine gas. However, there arises a problem that the chloride is decomposed to produce chlorine gas. Thus, a non-halogen based flux for refining has been demanded. As this non-halogen based flux, Japanese Patent Application Laid-Open No. 7-207358 discloses a mixture flux comprising as a main component potassium sulfate (K
2
SO
4
) to which a lithium (Li) or magnesium (Mg) compound is added to lower the melting point of the sulfate.
In Japanese Patent Application Laid-Open No. 7-207358, potassium sulfate, lithiumborate or the like for dehydrogenation is used. However, the melting point of potassium sulfate or lithium borate is higher than that of Al; therefore, it is recognized that reaction for the dehydrogenation advances as gas-solid reaction so that the efficiency of the dehydrogenation reaction drops. Therefore, in order to lower the melting point of potassium sulfate or lithium borate, the mixture flux to which lithium sulfate, magnesium sulfate or the like is added is used. The mixture flux having a lowered melting point is molten in the melt, so as to make the flux into a liquid state. In this way, the reaction of this liquid with hydrogen in the melt is advanced as gas-liquid reaction. The produced hydrogen is gasified or removed as slag to perform dehydrogenation. In this prior art, the potassium sulfate content is preferably set to 60-99 wt. % (% by weight). This is based on the following reason. At the time of injecting the mixture flux whose melting point falls to about the temperature of molten Al, together with an inert gas as a carrier, into the melt, it is necessary to prevent melting of the flux at the tip of an injecting nozzle and filling in the nozzle.
This non-halogen based flux for refining makes it possible to prevent the above-mentioned problems based on use of chlorine or the chloride. However, there remains a problem that according to this flux the important efficiency of the refining, such as removing H
2
gas or inclusion from the melt, is poorer than according to the chlorine or chloride based flux.
Incidentally, in the field of Al alloy products, demands for the following are increasingly becoming strict: surface properties (such as surface flatness and surface roughness) of Al alloy products for use as electric/electrical parts, such as a disc substrate of a magnetic disc, a printing plate, or A photosensitive drum; and qualities (such as strength, formability and corrosion resistance) of Al alloy products for use as packaging containers such as a can, transporting means such as an automobile, or fabrics. With this, therefore, it is increasingly becoming necessary to reduce, to a greater extent, impurities such as H
2
and inclusions in Al alloy ingots.
Melting Al raw materials are changing from conventional Al base material to scraps of Al alloy products from the viewpoint of a socia
Fujisawa Kazuhisa
Kitano Takayuki
Osumi Kenji
Combs Morillo Janelle
Kobe Steel Ltd
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wyszomierski George
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