JPH07207357A - Filtration method of molten al or al alloy - Google Patents

Abstract

PURPOSE:To extend the life of an expensive filter by adding a crystal grain size micronizing agent to molten metal in between a first and second stage of a refractory porous filter and filtering. CONSTITUTION:Molten Al or Al alloy is filtered by using a filter prior to casting. In this case, a refractory porous filter is fitted in two stages. A crystal grain size micronizing agent is added to molten metal in between the first porous filter and second porous filter to filter the molten metal. Also, when each filtration is executed by two stage filters, each molten metal is oxidized to form Al oxide in molten metal and is filtered, at the early stage of filtration, an oxide film is formed on the filtration front side of each filter, a removing efficiency of the inclusions in molten metal is improved. By this method, the adverse impact due to adding a crystal grain micronizing agent is suppressed to a minimum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for extending the life of a filter used for filtering molten aluminum or molten aluminum alloy.

[0002]

2. Description of the Related Art Al or Al alloys are used for various purposes because they have characteristics such as light weight, workability and surface beauty. However, the inclusion of impurity elements causes inconveniences such as coarsening of the eutectic compound, resulting in strength, toughness,
There is a problem that the surface treatability is extremely poor. In particular, in recent years, there is a tendency that demands for thinning and thinning of products tend to be strict, and to provide Al or an Al alloy (hereinafter sometimes represented by Al) which shows stable and high quality without generation of defects. Is needed.

The manufacturing process of Al is constituted by the basic processes of melting raw materials, refining in a melting furnace, refining outside the furnace, and casting. In order to prevent the occurrence of defects, among the above-mentioned processes, The refining process with, occupies an important position. The present invention relates to a filtration technique which constitutes the final step in the out-of-furnace refining, in other words the important finishing step provided immediately before casting.

One of the biggest causes of defects in Al products is inclusion of inclusions. The origin of inclusions is oxides and various foreign substances brought into the raw material of, and further molten metal in refining. An oxide or the like formed by the reaction of oxygen with oxygen in the atmosphere is considered. Inclusions derived from these are virtually unavoidable, and therefore, as described above, it becomes an indispensable important step to perform filtration at the final stage immediately before casting.

However, since all the inclusions that have been mixed in in the above step etc. will be removed in the filtration step,
There is a problem that a large amount of inclusions are trapped and accumulated on the filtration front side of the filter, which causes clogging with a few charges. Here, as a filter, a plate filter has been used for a long time, but in recent years, a technique of efficiently filtering from the outside of the tube to the inside while increasing the filtration area by combining multiple tube filters has been developed. Widely used. However, the tubular filter has the disadvantage that it is expensive, and as mentioned above, if it seems that clogging progresses due to the use of several charges and the filtration capacity decreases, it must be frequently discarded and replaced with a new one. Of course, this raises the problem of increasing the Al production cost.

From such a point, the life of the filter is extended.
The length is an issue and various studies have been made. Out of these
In Sho 52-41726, on the filtration front side of the filter
During the process of molten metal filtration
A technology to remove it has been proposed.
Of a cartridge type that combines a tube-shaped filter
Inert against the filter set from below to the molten metal
Blow in the gas and spray fine bubbles of inert gas to
Forcibly peeling off the key and redispersing it in the molten metal
H dissolved in the molten metal₂ Gas in an inert gas bubble
To disperse and remove H ₂ Those who try to carry out gas in parallel
The law is shown.

Since the cake layer adhering to the filtration front side of the filter is extremely strong, it is not possible to completely remove the cake by spraying fine bubbles of an inert gas. There is a desire to develop new means. On the other hand, according to recent researches by the present inventors, some of the reasons why the cake layer becomes stronger and becomes more difficult to peel off have been clarified.

[0008]

SUMMARY OF THE INVENTION Accordingly, the object of the present invention is to eliminate the cause of the cake layer becoming stronger,
By making it easier to remove the cake from the filter, it is intended to limit the formation of the cake to the extent that it can be substantially easily peeled off even by the known method as described above. is there.

[0009]

According to the present invention, which has been able to achieve the above object, a refractory porous filter is used for filtering molten aluminum or aluminum alloy prior to casting.
The basic constitution is that it is provided in a stage and filtration is performed by adding a crystal grain refining agent to the molten metal between the first stage refractory porous filter and the second stage refractory porous filter. Is. The first stage fire-resistant porous filter and the second stage
When performing the filtration with the multi-stage refractory porous filter, the molten metal to be filtered is oxidized to form an Al oxide in the molten metal, and then the filtration is performed. Since an oxide film is formed on the front side of the filtration of (1) to reduce the apparent openings, the efficiency of removing inclusions from the molten metal is increased, and thus a stable filtration effect can be obtained. According to another preferred embodiment, during the filtration with the second stage refractory porous filter, the molten metal is oxidized at the same time as or before or after the addition of the grain refining agent, thereby oxidizing Al in the molten metal. It is possible to perform filtration after forming the thing. According to yet another preferred embodiment, it is recommended to use a tube-shaped one as the first stage refractory porous filter and a plate-shaped one as the second stage refractory porous filter. .

[0010]

[Function] First, the clogging condition of the filter in the molten Al filtration will be described. Among the inclusions contained in the Al melt, those to be removed by the filter are roughly classified into two. One is a refractory oxide that is extremely larger than the filter mesh and mainly has a film shape, and the other is a refractory oxide that is extremely smaller than the filter mesh and mainly has a fine particle shape. In the filtration, first, the film-like inclusions settle on the filtration front side of the filter (hereinafter may be simply referred to as the surface) and are entangled with the skeleton constituting the filter to cause clogging within a short time, It is a mesh filter with a very small opening apparently. Therefore, the fine particulate inclusions that would normally pass through the filter will be captured by the filter having a small opening. That is, the Al molten metal is filtered by the "surface filtration" mechanism. In this way, after the filtration of one charge is completed, a film oxide is entangled and a state in which granular oxide is scatteredly adhered to the film oxide is formed.

The cake formed in this manner has a base that grows as a fairly solid material as understood from the above-mentioned formation mode, but it is further enhanced by the addition of the above-mentioned factors based on the investigation by the present inventors. A strong cake is formed. This point will be described below.

Strength is one of the properties required for Al products. In order to satisfy this characteristic, it is necessary to refine the crystal grains, and T as a grain refiner is used.
It is common practice to add i or Ti-B as lump or wire-shaped Al-Ti or Al-Ti-B. Therefore, for example, when Al scrap is used as a melting raw material, Ti and Ti-B mixed in the scrap are mixed.
Will be brought into the molten metal.
B may grow as a considerably large inclusion in the Al product depending on the melting conditions at the time of addition or depending on the cooling rate during the casting process. Naturally, it is also known that even if it does not grow and is finely dispersed, it will no longer exert the grain refining effect in the Al melt after remelting. Therefore, when it is intended to re-melt and use the Al scrap of the present invention, it is essential to newly add a grain refiner. However, what is commercially available as this crystal grain refining agent is a wire or a lump of Al-Ti or Al-Ti-B as described above, and even when these are put into a remelted Al melt. It is the actual situation in the current manufacturing process that coarse insoluble inclusions are often mixed in the molten metal. Therefore, in the molten aluminum, coarse Ti and Ti-B derived from Al scrap, and further coarse Ti and Ti-B in the newly added crystal grain refining agent substantially reduce the grain refinement action. It is mixed in as impurities that do not exhibit in particular, and these impurities must also be passed through the furnace together with the oxide inclusions.

When filtration is carried out in the state where such a crystal grain refining agent is mixed, as described above, the particulate inclusions and the coarse Al-- particles are formed on the surface of the filter by the surface filtration mechanism accompanying the adhesion of the film-like inclusions. Ti and Al-Ti-B are captured. The fact that they are trapped in itself is a purported purpose for the purpose of filtration, but such mixed inclusions of large and small sizes grow and grow one after another as the filtration charge progresses, and finally, it becomes impossible to filter. It leads to a blockage. Therefore, as a result of examining the progress of such a surface filtration mechanism from a free angle, the present inventors found that the crystal grain refining agent to be added after re-dissolution was not sufficiently finely dispersed in the Al melt and partially coarsened. Being dispersed as it is and not exhibiting the grain refining effect, it is trapped on the filter surface together with the oxide inclusions, which further increases the firmness of the cake and is large against peeling from the filter surface. We came to the suggestion that it might be a resistance.

Therefore, (A) the case where the grain refining agent is added and filtered before the Al molten metal filtration step as before, and (B)
An experiment was performed for each case where the addition was omitted and filtration was performed, and the filtration was performed for several times, and the changes in the head value on the hot water side of the filters were compared. Here, the head value means that when the filter box is divided into a hot water side and a hot water side by a plate-like filter and the hot water side is an overflow system and the hot water level is constant, the molten aluminum is poured into the hot water side. It means the difference in height between the hot water surface (high level) and the hot water surface (low level) in a state in which the amount is controlled and overflow on the hot water side is constantly performed. Therefore, the head value increases as the amount of cake on the filter surface increases and clogging becomes stronger.

Comparison of changes in the head value in such an experimental system shows that in the conventional method of (A), the head value at the time when one filtration charge is completed and the first head value of the next filtration charge are almost the same. It didn't change. That is, the head value increases as the filtration progresses within a certain one of the filtration charges, but the head value reached by the previous charge is used as a starting point and the increase in the head value due to the filtration progress of the next charge is simply added. I knew it would go. Therefore, the head value is increasing, and as a whole, it reaches the limit range where filtration is impossible with an extremely small number of charges. On the other hand, in the system in which the crystal grain refining agent is not added as shown in (B), the head value increases with the progress of filtration within a certain one filtration charge, but at the time when the previous charge is completed. It was found that the head value at the start of the next filtration charge was significantly lower than the head value. This is a surprising finding, and the residual molten metal of the pre-charge, which was retained for the purpose of protecting the filter during the waiting time after the completion of the filtration of the pre-charge and before the next charge, adhered to the cake on the filter surface. Is spontaneously peeled off, which is interpreted as causing a decrease in the head value during the waiting time. Such a phenomenon has not been known at all in the past, and it has been found that the crystal grain refining agent, which has been commonly regarded as a new addition, is one of the causes of inhibiting the peeling of the cake. Is a big thing.

On the basis of such knowledge, the basic constitution was reached in which the crystal grain refining agent was not added before the filter. However, from the viewpoint of enhancing the strength characteristics of Al products, the crystal grain refining agent will eventually be used. It must be added at some point and the coarse grain refining agent mixed in during the addition must be removed by some means. Therefore, as a result of various studies, it was decided that the filter had a two-stage structure and the crystal grain refining agent was added before the second-stage filter. With such a configuration, the first stage filter has a longer filter life as can be seen from the comparison of (A) and (B), and therefore an expensive filter, for example, a tube filter is used as the first filter. It is also possible to use the plate-shaped filter as the second stage filter, which is used as the second stage filter and is considered to be inexpensive and disposable. The multi-stage filtration itself is described in, for example, Japanese Patent Application Laid-Open No.
It is known from JP-A-211937, etc., but this is intended to make a molten metal into a laminar flow, and is completely different from the present invention in both object and constitution.

On the other hand, as can be understood from the above description of the surface filtration mechanism, the film-like oxide inclusions are trapped by the first-stage filter, and the second-stage filter performs filtration. At the moment, it is not present in the melt. Therefore, it is considered to be a useful means to positively oxidize the molten Al just before the second stage filter to form a film-like inclusion so as to contribute to the performance of the surface filtration mechanism in the second stage filter. The following method is exemplified as a specific means for performing such positive oxidation.

(1) Method of Blowing Air into Molten Metal The inside of the molten metal is directly oxidized by air, and the molten metal surface is subjected to atmospheric oxidation due to the stirring effect of the molten air to form a film oxide. Since this can be attached to the surface of the filter, stable surface filtration starts from the initial stage of the next charge filtration. (2) Method of blowing an inert gas into the molten metal The surface of the molten metal is susceptible to atmospheric oxidation due to the stirring action of the inert gas, and although it is slightly behind the air blowing,
A film oxide is rapidly formed. (3) Method of mechanically stirring or electromagnetically stirring the molten metal Substantially the same action and effect as in (2) above are exhibited, and a film oxide is rapidly formed.

The film-like oxide formed in the molten metal by such a forced oxidation method is generally m as a result of various experiments.
It is confirmed that the particles have a size of m units to several hundreds of μm and that minute particulate oxides are hardly formed. On the other hand, the openings of commonly used filters are usually several hundreds of μm or less, so the above film-like oxide is large enough to clog the filter surface, and is converted into a filter with a small opening by short-time oxidation. However, it is possible to prepare for the filtration of the next charge. It is recommended to know in advance how much film oxide is produced by the above-mentioned forced oxidation method by conducting a preliminary test while varying the conditions of the molten metal and the filter.

The positive oxidation of the molten Al as described above is
It can be carried out not only before the stage filter but also before the first stage filter, if necessary, to contribute to enhancing the surface filtration function. In addition, the second step is the active oxidation of the molten aluminum.
When it is carried out before the staged filter, it may be carried out at the same time as the addition of the crystal grain refining agent or before or after.

[0021]

[Examples] Ingots were obtained by performing the steps after melting the Al raw material under the following conditions. Raw material: Variety JIS 3004 alloy, waste mixture 70% + metal (99.7% Al) 30% Melting: Melting furnace / heavy oil fired reverberatory furnace (30t), atmospheric melting,
740 ° C. Scouring: Cl ₂ gas in the melt of the melting furnace, 200 Nl / min ×
Blow for 20 minutes, remove slag Hold: Hold molten metal in heavy oil-fired reverberatory furnace (same as above) Filtration: 1st stage Mitsui Metal tube filter (H
D) 1 set of 18 pieces is used 2nd stage ACT THERMIX filter made by Kobe Steel Co., Ltd. System diameter 2 mmφ Size 500 mm × 500 mm × 50 mm 1 piece used Installed vertically to the molten metal flow Addition of crystal grain refining agent (A ) = Al-Ti-B wire added so that Ti = 0.01% and B = 0.002% in the molten metal Active oxidation of the molten metal (B) = Lance (10 mmφ against the molten metal before the second stage filter) And blowing Ar gas at 5 liter / min for 10 minutes. The experimental results are shown in Table 1.

[0022]

[Table 1]

As can be seen in Table 1, the two-stage filtration configuration, in which the addition of the grain refiner and the positive oxidation of the molten metal were used together before the second stage filter gave the best results. Especially the first
The life of the tube filter used for stage filtration is about 1.5.
I could double it.

[0024]

EFFECTS OF THE INVENTION Since the present invention is constructed as described above, it has become possible to extend the life of an expensive filter while minimizing the adverse effects of the addition of a crystal grain refining agent.

 ─────────────────────────────────────────────────── ─── Continued front page (72) Inventor Kiyomasa Oga 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo Prefecture Kobe Steel Research Institute, Kobe Steel Co., Ltd. (72) Inventor Motohiro Arai Takatsuka, Nishi-ku, Kobe-shi, Hyogo Prefecture No. 1-5-5 Kobe Steel Works, Ltd., Kobe Research Institute (72) Inventor Minji Masuda 14-1, Chofu Minatomachi, Shimonoseki City, Yamaguchi Prefecture Kobe Steel Co., Ltd., Chofu Factory (72) Inventor Takayuki Kitano Yamaguchi 14-1 Chofu Minatomachi, Shimonoseki City, Shizuoka Prefecture Kobe Steel Co., Ltd. Chofu Works

Claims (4)

[Claims] 1. When filtering molten Al or molten Al alloy prior to casting, a refractory porous filter is provided in two stages, and between the first stage refractory porous filter and the second stage refractory porous filter. 2. A method for filtering molten Al or molten Al alloy, which comprises adding a crystal grain refining agent to the molten metal and filtering. 2. When each of the first-stage refractory porous filter and the second-stage refractory porous filter is subjected to filtration, the molten metal is oxidized to form an Al oxide in the molten metal. The filtration method according to claim 1, wherein filtration is performed, and an oxide film is formed on a filtration front surface side of each filter at an initial stage of filtration to enhance removal efficiency of inclusions in the molten metal. 3. When the second-stage refractory porous filter is used for filtration, the molten metal is oxidized by oxidizing the molten metal at the same time as or before or after the addition of the crystal grain refining agent.
The filtration method according to claim 2, wherein the filtration is performed after forming the oxide. 4. The tubular fireproof porous filter is used as the first-stage refractory porous filter, and the plate-like fireproof porous filter is used as the second-stage fireproof porous filter. Filtration method.