JPH0299263A - Method for cleaning molten metal under reduced pressure - Google Patents

Abstract

PURPOSE:To remove the inclusions in a molten metal and to reduce the man- hours of a treatment by dissolving a gas into the molten metal, reducing the pressure of the molten metal to generate the fine gaseous bubbles therein and subjecting the molten metal to degassing thereby trapping the inclusions into the fine gaseous bubbles and removing the same. CONSTITUTION:The molten metal is bubbled by the gas soluble therein under the atm. pressure or the state below the same to dissolve the gas into the molten metal. The molten metal is thereafter subjected to the rapid pressure reduction to generate the fine gaseous bubbles therein. The degassing of the bubbling gases remaining dissolved in the molten metal is executed simultaneously by this pressure reduction. The inclusions suspended in the molten metal are trapped into the gaseous bubbles generated by bubbling and the fine gaseous bubbles generated by the pressure reduction. The inclusions are removed after floating. Since the pressurization treatment is not executed in this way, the need for a pressurized vessel is eliminated and the inclusions in the molten metal are sufficiently removed.

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for vacuum cleaning molten metal for removing inclusions floating in molten metal. [Prior Art] Inclusions floating in molten metal (for example, alumina inclusions in molten steel) cause product quality defects, and various methods have been proposed to reduce and remove them. Among these methods, one method that is widely used as it is relatively efficient is to bubble inert gas into the molten metal from the bottom of the container under normal pressure to trap inclusions in the gas bubbles.
There is a way to remove this after surfacing. However, if the purpose is to manufacture high-grade materials, the total amount of oxygen in molten steel must be kept to 15 ppm or less, but there is a problem that such ultra-cleaning of molten metal cannot be achieved depending on the method described above. , it was hoped that a new method would be developed. That is, in the conventional gas bubbling method, the bubbling area is limited to the area that spreads upward from the gas inlet at the bottom of the container, and it is difficult to bubble from the entire area of the container due to restrictions on the bubbling method. was there. Another cause is that the bubbles created by bubbling are large, and when the bubbles float up, the molten metal flows around them, and along with the flow, fine inclusions also avoid the bubbles. There is also the problem that fine inclusions are difficult to be trapped by air bubbles because they move. Therefore, the present inventors made the following proposal. The proposal involves bubbling pressurized molten metal with a soluble gas to dissolve the gas in the molten metal, and then rapidly reducing the pressure to generate fine gas bubbles in the molten metal. Inclusions floating in molten metal are trapped in gas bubbles caused by bubbling and fine gas bubbles generated by reduced pressure, and are removed after floating. Normal inclusions in the molten metal will be trapped and brought to the surface by the initial bubbling. On the other hand, since this bubbling is performed on pressurized molten metal, a large amount of bubbling gas dissolves into the molten metal. Due to the subsequent rapid depressurization, the gas dissolved in the molten metal becomes fine gas bubbles and is generated from the entire area of the molten metal. At this time, fine inclusions are trapped by the gas bubbles and float up. [Problems to be solved by the invention] Although this is an excellent and extremely efficient method for removing inclusions from molten metal, it is difficult to pressurize the molten metal at the first stage of the treatment process. Therefore, if the standing time after depressurization is short, some of the bubbling gas that has dissolved into the molten metal will appear as fine gas bubbles when the pressure is decompressed, but the rest will remain dissolved in the molten metal. It will remain as it is. Therefore, after the above-mentioned treatment, a separate treatment step for further degassing is required, which poses a problem of equipment reinforcement and an increase in the number of processing steps for the degassing. In addition, there is another problem that equipment costs are high because a pressurized container is used. The present invention was created in view of the above problems, and
The present invention aims to improve the above-mentioned bubbling method involving pressure adjustment so that a pressurized container is not used and the degassing step as described above can be omitted. [Means for Solving the Problems] Therefore, the present invention stops the pressure treatment of the molten metal that was initially performed in the previously proposed molten metal cleaning methods, and applies atmospheric pressure or a lower pressure to the molten metal. In the following conditions, a soluble gas is bubbled therein, and then a reduced pressure treatment is performed. In particular, this depressurization process not only generates fine gas bubbles, but also degasses the bubbling gas remaining dissolved in the molten metal. However, the deeper the bath depth of the molten metal bath, the more static pressure is applied to the molten metal, making it difficult to degas the deep part of the bath during depressurization (this tendency is especially strong when the bath depth is 1.5 m or more). ). In such a case, it is preferable to bubble an inert gas into the molten metal and stir the molten metal to efficiently degas the metal under reduced pressure. The attached drawing shows the pressure inside the container (P 5tart ) during the first bubbling process when the method of the present invention is implemented.
By taking the pressure inside the container (P end ) in the treatment step after performing the depressurization treatment as coordinates, the amount of inclusions T・(0) and the final [N] in the molten metal obtained after the treatment is investigated and shown. It is something that In the figure, the area indicated by the intersecting line A indicates the area where the inclusions are reduced and the final product (N) has little influence on the final product and does not pose a problem using the method of the present invention. Also, the area indicated by diagonal line B is. Although the inclusions are reduced, the final (N) is a problem area, and depending on the steel type, a new de-N process is required after this treatment. It should be noted that the method of the present invention carried out in the examples shown below was carried out at the points indicated by (black dots) in the same graph. [Example] Specific examples of the method of the present invention will be described below. First, the inventors developed a 50 ton VOD equipment
0 ton of molten steel was maintained at 1660° C. and 300 torr, and 6 Nm' of N2 gas was blown into it from the bottom of the ladle using a porous plug for 10 minutes. After that, the molten steel was transferred into the VAD equipment and the pressure was rapidly reduced to 1 torr while performing heat compensation.
Hold for minutes. At this time, Ar gas was simultaneously bubbled from the bottom of the ladle at 15 ONn/min. In addition, the conventional Ar gas bubbling method and the previously proposed pressurization and depressurization method, in which the N2 gas is brought into a pressurized state during bubbling, were carried out together with the above experiments. Of these, the conventional method is 50 tons
While maintaining the molten steel at 0.5 to 1 torr in a vacuum ladle, Ar gas was bubbled through the bottom of the ladle at a blowing rate of 15 ON4/++in for 20 minutes. The pressurization and depressurization method was carried out by increasing the pressure to 3 atm when blowing N2 gas, and reducing the pressure to 0.5 torr during the subsequent depressurization treatment, and other conditions were the same as in the method of the present invention. However, when the pressure was reduced, stirring by blowing Ar gas was not performed, and no degassing treatment was performed after the pressure reduction treatment. Molten steel composition and T・

[0] and T
・The table below shows the changes in the components of (N). As is clear from the table above, the method of the present invention removes inclusions from molten steel to the same extent as the pressurization and depressurization method, and moreover, the N2 gas in the molten steel is removed even without subsequent degassing treatment. It can be seen that this has been sufficiently reduced. [Effects of the Invention] As detailed above, according to the vacuum cleaning method of the present invention,
Inclusions in the molten metal are sufficiently removed, and since no pressure treatment is performed, no pressurized container is required, and sufficient degassing occurs during depressurization, so subsequent degassing is not necessary. It is no longer necessary to carry out this process as a treatment step, making it unnecessary to install additional processing equipment, and it has excellent effects such as being able to reduce the number of processing steps accordingly.

[Brief explanation of drawings]

The drawing shows the coordinates of the pressure inside the container during the first treatment performed when the method of the present invention is carried out, and the pressure inside the container during the subsequent depressurization treatment, and shows the inclusions in the molten metal obtained after the treatment and the final It is a graph diagram showing the amount of (N) investigated. P end (torr) Procedural amendment (voluntary) December 28, 1989

Claims (1)

[Claims] 1. The gas is dissolved in the molten metal by bubbling the molten metal with a gas soluble in the molten metal at atmospheric pressure or lower, and then the pressure is rapidly reduced to form a fine gas in the molten metal. Generating bubbles and degassing the bubbling gas remaining dissolved in the molten metal using this reduced pressure,
A method for cleaning molten metal under reduced pressure, which comprises trapping inclusions floating in molten metal in gas bubbles caused by bubbling and fine gas bubbles generated by reduced pressure, and removing the inclusions after floating. 2. In carrying out the vacuum cleaning method for molten metal described in the preceding paragraph, vacuum cleaning of molten metal is characterized by bubbling an inert gas into the molten metal and rapidly reducing the pressure while stirring the molten metal. method.