JP4403129B2 - Vacuum arc melting method for refractory metals - Google Patents

Description

  The present invention relates to a method for melting a refractory metal, and more particularly to a method for efficiently cooling a formed metal ingot.

  Titanium metal has been widely used for aircraft materials and parts in the past, and in recent years, application development has progressed and it is widely used for building materials, roads, sports equipment, and the like. For such titanium metal, sponge titanium is produced by a crawl method in which titanium tetrachloride is reduced with molten magnesium, and after the sponge titanium is crushed and sized, it is pressure-molded into briquettes, and this briquette is combined into an electrode. Can be obtained by dissolving.

  In order to meet the demand for titanium metal as seen in recent years, it is important to increase the productivity of existing facilities as well as to enhance facilities. Efforts have been made to increase productivity in the above-described sponge titanium manufacturing process, but similar measures are also required in the melting process.

  In the sponge titanium melting step, after completion of the melting, it is necessary to cool the generated ingot to near room temperature. This operation has been conventionally performed in a vacuum atmosphere. However, since cooling under vacuum atmosphere is mainly cooling by radiation, the cooling efficiency is low and improvement is required.

  To solve this problem, there are several technologies that improve the cooling efficiency and shorten the cooling time by using helium gas in the melting process of metal ingots by utilizing the properties of helium gas with excellent cooling ability. Are known.

  Specifically, a technique of increasing the cooling rate of the ingot by flowing helium gas between the mold and the ingot is disclosed (for example, see Patent Document 1). However, although this technique is a technique for circulating helium gas in the gap between the ingot and the mold, there is no description as to how much He gas is required, and depending on this disclosure, it may actually be implemented. Seems to be difficult. In addition, this technique has a difficulty in terms of cost because helium gas is maintained at a pressure higher than atmospheric pressure.

  In addition, a technique has been disclosed in which the casting surface is improved by supplying helium gas to the space between the steel ingot and the mold when the steel ingot is melted in an ESR (electroslag remelting furnace) ( For example, see Patent Document 2).

BE1008246 JP 09-029420 A

  However, all of these technologies are performed under atmospheric pressure, and in order to enjoy the effects of helium gas with a large heat transfer coefficient, a large amount of helium gas is required, and there is room for improvement in terms of economy. Is left.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a melting method of a refractory metal that is excellent in economic efficiency by increasing cooling efficiency in the melting method of a refractory metal.

  As a result of intensive investigations in view of such circumstances, after completion of the melting of the high melting point metal in the vacuum arc melting of the metal, it was melted by the above method by holding the generated ingot in a small amount of helium gas atmosphere. It has been found that the ingot can be efficiently cooled, and the present invention has been completed.

That is, the vacuum arc melting method of the refractory metal of the present invention is a refractory metal vacuum which is performed by applying a voltage between the refractory metal and the refractory metal melting electrode in a vacuum melting furnace. In arc melting, after melting the melting electrode, helium gas is introduced into a vacuum melting furnace and held in a helium atmosphere depressurized from atmospheric pressure, then evacuated to hold the inside of the furnace at a reduced pressure. After that, helium gas is introduced again , and the generated high melting point metal ingot is cooled.

  According to the present invention having the above configuration, the ingot melted in vacuum can be efficiently cooled. In addition, since the introduced helium gas is in a very small amount, the cost can be reduced as compared with the method of cooling by forming a helium gas atmosphere as in the prior art.

Moreover, in this invention, after completion | finish of melt | dissolution of a melting electrode, after shutting off and sealing off the atmosphere in a melting furnace from a vacuum exhaust system, helium gas is introduced into a melting furnace atmosphere as a preferable form. Further, the pressure in the melting furnace after the introduction of the helium gas is characterized by being 0.01 to 0.10 MPa.

  According to the present invention, cooling can be efficiently performed by placing the generated ingot in a helium atmosphere according to the present invention, as compared with cooling in a vacuum atmosphere that has been conventionally performed immediately after the end of dissolution. As a result, it is possible to increase the productivity of the vacuum arc melting furnace.

The best embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 shows a configuration example of a suitable vacuum arc melting furnace M (hereinafter sometimes simply referred to as “melting furnace M”) for use in carrying out the present invention. In the present embodiment, the case where the refractory metal is pure titanium will be described below as an example, but the present invention can also be suitably applied to the dissolution of other zirconium and niobium.

  In the melting furnace M, reference numeral 1 is a water-cooled copper mold, and a metal titanium ingot 2 is held in the water-cooled copper mold 1. An electrode elevating device 3 is provided at the upper end of the water-cooled copper mold 1, and a stub 4 is engaged with the tip thereof, and an electrode 5 is engaged with the tip of the stub 4. An air supply valve 6 and an exhaust valve 7 are attached to the side surface of the water-cooled copper mold 1.

  When the electrode elevating device 3 is used to move and hold the stub 4 and the electrode 5 to a predetermined position and a voltage is applied between the bottom of the mold 1 and the electrode 5 engaged and held on the stub 4, An arc discharge is generated between the bottom and the electrode 5, and the electrode 5 is consumed and dissolved by the heat generated by the arc discharge, falls into the mold 1 and solidifies to form a titanium ingot 2. The upper end portion of the titanium ingot 2 is maintained at a high temperature by arc discharge with the electrode 5 to form a molten titanium pool 8. In this way, by proceeding with melting by arc discharge, all the electrodes 5 can be melted and the titanium ingot 2 can be melted.

  While the electrode 5 engaged with the stub 4 is being melted, the air supply valve 6 is closed, the exhaust valve 7 is opened, and a vacuum exhaust device (not shown) disposed downstream of the exhaust valve 7 is operated. It is required to always maintain the pressure in the melting furnace in a reduced pressure state.

  As soon as the dissolution of the electrode 5 joined to the stub 4 is completed, the stub 4 is turned off to stop the generation of the arc, and the operation of cooling the titanium ingot 2 is started. After the stub 4 is turned off, the exhaust valve 7 is closed in a state where the vacuum exhaust device is operated, and the inside of the melting furnace M is sealed in a vacuum state. As soon as the pressure in the melting furnace M is stabilized, the vacuum exhaust device connected downstream of the exhaust valve 7 is stopped.

  Next, the air supply valve 6 is opened, and helium gas is introduced into the melting furnace M from outside the system. Since the melting furnace M is maintained at a reduced pressure, the helium gas supplied into the melting furnace M is quickly filled into the upper space of the titanium ingot 2 and the space between the titanium ingot 2 and the water-cooled copper mold 1. The

  The timing of supplying the helium gas into the melting furnace M is such that the peripheral portion of the molten titanium pool 8 generated at the top of the titanium ingot 2 is solidified to form a shell, and a gap is formed between the shell and the mold. It is preferable to carry out immediately after it is determined that this has been done. This is because immediately after the energization of the stub 4 is stopped, the molten titanium pool 8 formed at the top of the titanium ingot 2 is still in a molten state, and the pool and the water-cooled copper mold 1 are in contact with or in close proximity to each other. This is because the helium gas introduced into the melting furnace M does not easily penetrate into the space between the titanium ingot 2 and the water-cooled copper mold 1.

  The titanium ingot 2 is cooled in the above-described helium gas atmosphere, and when the surface temperature of the ingot drops to near room temperature, the air release valve at the tip of the exhaust valve 7 is closed to release the air. After returning to pressure, the melted ingot is extracted.

Next, a preferred embodiment of the present invention will be described.
In the melting furnace M, the pressure in the melting furnace M when the electrodes are melted is preferably controlled in the range of 1.0 to 10 Pa. By setting to such a pressure range, the arc generated between the electrode 5 and the molten titanium pool 8 can be stably continued.

  The timing of supplying the helium gas into the melting furnace M is performed immediately after a shell is formed around the molten titanium pool 8 generated at the top of the ingot 2 and a gap is formed between the shell and the mold. As described above, specifically, it is preferable that helium gas is introduced into the furnace within 3 minutes after the stub 4 is turned off and the arc discharge between the electrode 5 and the molten titanium pool 8 is turned off. It is more preferable to introduce helium gas into the furnace within 1 minute. In actual operation, since the shell has already been formed around the titanium pool when the sealing operation is completed, it is preferable that helium gas be circulated at this timing. Although helium gas can be introduced over the above-mentioned time, the cooling efficiency is low and it is not economical.

  The helium gas introduced into the melting furnace M is preferably supplied at a rate of 40 to 60 mmHg / min. When the supply rate of the helium gas is smaller than the lower limit value in this range, the heat removal rate of the cooled ingot is also slow, and a remarkable ingot cooling rate cannot be obtained. On the other hand, if the supply rate of helium gas exceeds the upper limit of the above range, the cooling rate of the ingot increases, but it is not preferable because the molten titanium pool 8 is rapidly cooled to easily form shrinkage holes inside.

  Although the pressure in the melting furnace M increases according to the amount of helium gas supplied into the melting furnace M, in the present invention, this pressure is preferably maintained in the range of 0.01 to 0.15 MPa. A range of 0.05 to 0.1 MPa is particularly preferable. Although the heat removal ability to the water-cooled copper mold 1 is improved in proportion to the partial pressure of the helium gas supplied into the melting furnace M, the heat removal ability approaches an equilibrium state in the region exceeding the upper limit of 0.15 MPa of the pressure range. Therefore, a remarkable improvement effect is not seen. On the other hand, when the lower limit of 0.01 MPa is not reached, the cooling effect is insufficient and the effect of the present invention cannot be fully enjoyed.

Next, another preferred modification of the present invention will be described below.
In the process of introducing helium gas into the melting furnace M and cooling to room temperature, the helium gas in the melting furnace M may be once exhausted out of the system, and then cooled and supplied to the melting furnace M again. By performing such an operation, the titanium ingot 2 held in the water-cooled copper mold 1 can be efficiently cooled, and the cost can be reduced because helium gas is reused.

  In this case, as a more preferable aspect, the helium gas held in the melting furnace M may be cooled and circulated through a tank with a heat exchanger provided outside the system. At this time, after the helium gas is supplied from the tank into the melting furnace M, the inside of the tank is preferably kept at a reduced pressure. By holding in such a reduced pressure state, the helium gas held in the melting furnace M can be efficiently exhausted.

  The helium gas introduced into the melting furnace M may be supplied from the bottom of the mold. By supplying to such a position, the titanium ingot 2 can be efficiently cooled by moving the helium gas between the titanium ingot 2 and the water-cooled copper mold 1.

  When the present invention is implemented with the above-described configuration, the cooling time can be shortened by about 5 to 20% compared to the case where helium gas is not introduced, and the titanium production efficiency can be improved.

  In addition, although embodiment of this invention was described taking titanium as an example, this invention is applicable also to melt | dissolution of other refractory metals, such as a zirconium or niobium.

  As described above, the cooling time of the titanium ingot can be shortened by setting the cooling atmosphere of the titanium ingot to a helium gas atmosphere. As a result, the productivity of the dissolution process can be improved.

[Example 1]
Using the melting furnace M shown in FIG. 1, an electrode (15t) made of pure titanium sponge titanium is disposed on the stub 4, and a pure titanium ingot having a diameter of 1.3 m and a length of 2.7 m is placed in the water-cooled copper mold 1. And dissolved. The pressure during dissolution was maintained in the range of 1.0 to 5.0 Pa. After melting the titanium electrode, the exhaust valve 7 was closed and the inside of the furnace was sealed in a reduced pressure state. The reduced pressure state was maintained until it was estimated that a sufficient gap was formed between the molten titanium pool 8 and the water-cooled copper mold 1. In this example, it was about 20 minutes. Thereafter, the air supply valve 6 was opened, and helium gas was introduced from here until the pressure in the melting furnace M reached 0.079 MPa. The temperature of the ingot surface after helium gas introduction was continuously measured with an optical pyrometer. Further, the titanium ingot was melted and cooled in the same manner except that helium gas was not introduced, and the temperature of the ingot surface was continuously measured with an optical pyrometer. Table 1 shows the results.

  In Table 1, in the example using helium gas, the time required for the surface temperature of the ingot to drop to 400 ° C. was 420 minutes. On the other hand, in the comparative example which does not use helium gas, the time when the ingot surface temperature dropped to 400 ° C. was 540 minutes. Thus, by introducing helium gas, the cooling time of the ingot could be shortened by about 25%. As a result, the cycle time for melting one ingot could be reduced by 10%, and the ingot productivity could be improved by 5%.

[Example 2]
In Example 1, the pressure in the melting furnace M after introducing helium gas was changed in the range of 0.01 to 0.20 MPa, and the time until the ingot was cooled to a predetermined temperature was measured. For the cooling time, the cooling time achieved when the pressure in the melting furnace M was 0.10 MPa was indicated as 100. In addition, it was also investigated whether shrinkage holes were formed on the top of the ingot after melting. The results are shown in Table 2.

  When the pressure in the melting furnace M is in the range of 0.05 to 0.15 MPa, the cooling time of the ingot is shortened as the pressure in the melting furnace M increases. However, when the pressure in the melting furnace M exceeds 0.15 MPa, the cooling time is shortened as the pressure in the melting furnace M increases, but a shrinkage hole is seen at the top of the melted ingot, which is not preferable. Presented. On the other hand, if the pressure in the melting furnace M is lower than 0.05 MPa, it is not preferable because it acts in the direction of extending the cooling time. This tendency was less effective in reducing the cooling time than the tendency to increase the cooling time measured in a range exceeding 0.10 MPa, and the effect of reducing the cooling time could not be enjoyed unexpectedly.

[Example 3]
In Example 1, the time required for the ingot to reach room temperature was examined by changing the waiting time until the helium gas was introduced after turning off the power to the electrode. The results are shown in Table 3.

  The cooling time was 520 to 550 minutes in the range of 1 to 3 minutes of standby time from turning off the power supply to introduction of helium gas into the furnace. However, when the waiting time was outside the above range, 5 minutes and 10 minutes, the cooling time of the ingot was significantly delayed to 670 minutes and 730 minutes.

  ADVANTAGE OF THE INVENTION According to this invention, metal ingot manufacture can be implemented efficiently by shortening the cooling process of a metal ingot.

It is a schematic cross section of the titanium melting furnace of this invention.

Explanation of symbols

M Melting furnace 1 Water-cooled copper mold 2 Titanium ingot 3 Electrode lifting device 4 Stub 5 Electrode 6 Air supply valve 7 Exhaust valve 8 Molten titanium pool

Claims (5)

In vacuum arc melting of refractory metals, after the above melting is completed, helium gas is introduced into the melting furnace and held in a helium atmosphere depressurized from atmospheric pressure, and then the vacuum is pulled to hold the inside of the furnace under reduced pressure. Then, helium gas is again introduced to cool the refractory metal ingot, and the refractory metal vacuum arc melting method is characterized in that it is cooled.   2. The method of claim 1, wherein after the melting is completed, the atmosphere in the melting furnace is shut off from the vacuum exhaust system and sealed, and then helium gas is introduced into the atmosphere. .   After the melting is finished, the atmosphere in the melting furnace is shut off from the vacuum exhaust system and sealed, and a space is formed between the molten pool formed in the upper part of the ingot and the mold wall of the melting furnace, and then helium. The method of vacuum arc melting of refractory metal according to claim 1 or 2, wherein gas is introduced into the atmosphere. The vacuum arc melting of a refractory metal according to any one of claims 1 to 3 , wherein the pressure in the melting furnace after the introduction of the helium gas is 0.01 to 0.10 MPa. Method. The method for vacuum arc melting of a refractory metal according to any one of claims 1 to 4 , wherein the refractory metal is titanium, zirconium or niobium.

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