JPH0429723B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD This invention relates to a vacuum arc melting method for obtaining an ingot with excellent homogeneity in vacuum arc melting of high melting point active metals such as Ti and Zr and their alloys. Conventional technology High melting point active metals are usually melted by vacuum arc melting, but this method involves constant melting with a constant electric current.
In the latter stage of melting, the melting current is reduced within a relatively short period of about 20 to 60 minutes, a so-called hot-top process. In the solidification of molten metal in consumable electrode vacuum arc melting, solidification from the bottom of the mold is predominant in the initial stage, but after a certain period of time, solidification from the mold wall becomes significantly large. At this time,
The depth of the molten metal pool is constant if the melting rate is constant, and solidification progresses sequentially from the bottom to the top, and as the metal is supplied from the consumable electrode, the remaining portion is relatively uniform, and solidification progresses from the bottom to the top. There is little component segregation and shrinkage pores. However, if the molten metal pool is deep when the power is turned off at the end of melting, component segregation and shrinkage pores will become larger. Therefore, hot-topping is carried out in the late stage of melting, but the homogeneity cannot be improved unless the current is properly controlled. However, the current hot-top process, which is generally carried out at the final stage of melting, takes a relatively short time and finishes melting while the molten metal pool is still quite deep, so component segregation and shrinkage pores are sufficiently reduced. It has not yet been achieved. Therefore, various methods have been devised in the past, and as a method for reducing the occurrence of shrinkage pores, for example, there is a method disclosed in Japanese Patent Publication No. 8321/1983 in which the melting current is gradually lowered by a constant deceleration coefficient from the initial stage of melting. Problems to be Solved by the Invention The above method for reducing the occurrence of shrinkage holes is to prevent the occurrence of shrinkage holes by gradually reducing the depth of the molten metal pool while preventing solidification from the upper surface of the molten metal pool. However, in this case, since the hot-top time is increased, the melting rate is reduced, productivity is deteriorated, and the surface quality of the ingot is also deteriorated. In view of the current situation, this invention performs high-current melting until the start of hot-topping, gradually makes the molten metal pool shallower after starting hot-topping, and maintains a state close to uniform solidification while supplying appropriate hot water, thereby improving homogeneity and surface quality. This paper proposes a vacuum arc melting method for high melting point active metals and their alloys to obtain excellent ingots. Means for Solving the Problems This invention is a consumable electrode type vacuum arc melting method for high melting point active metals and their alloys, in which the melting current is reduced in several stages at the hot top in the latter stage of melting, and each stage is maintained for a certain period of time. The gist is to hold the molten metal, gradually reduce the depth of the molten metal pool, and solidify in a state close to uniform solidification. Operation Next, the details of this invention will be explained based on FIG. 1. A method according to the present invention is shown in C, and for comparison, conventional method A involves hot-topping T _A for a short time after steady melting, and decelerated melting method B, which gradually reduces the melting current with a constant deceleration coefficient from the beginning. was raised. In the method of the present invention, in a steady state, melting is performed with a steady current I _O determined by the electrode and mold dimensions, and when the hot-top start time comes, the current is lowered to I ₁ and melting is performed with this current for a time T ₁ . Further, the current is reduced to I ₂ and melted for a time T ₂ and the power is turned off, or the current is further reduced to I ₃ and melted for a time T ₃ and the power is turned off. As mentioned above, this invention gradually makes the molten metal pool shallower, and in order to ensure the surface quality of the ingot while supplying the molten metal appropriately, the hot top is heated in several stages (stages 2 and 3 are shown in the drawing). This method employs a melting current pattern that is carried out over several periods. At this time, the hot-top currents I ₁ , I ₂ , and I ₃ are
Determined by mold dimensions and material, i.e. level of added elements. As a result of various experiments from the viewpoint of component segregation, surface quality, productivity, etc., the inventors found that I ₁ =0.60 to 0.80 I ₀ I ₂ = 0.40 to 0.60 _{I 0} I ₃ = 0.15 to 0.40 _{I 0} are desirable. Also, the time varies depending on the dissolution situation, but according to the experimental results, in the case of 2 stages T ₁ = 20 to 40 minutes T ₂ ≧ 20 minutes (including adjustment) In the case of 3 stages T ₁ = 20 to 40 minutes T ₂ = 20 to 40 minutes T ₃ ≧10 minutes (including adjustment) is desirable. The implementation of hot top in this invention is determined by taking into account the depth of the molten metal pool, retained heat, and melting situation when reducing the current, and the depth of the molten metal pool when the power is turned off is made as shallow as possible. to do,
By properly supplying molten metal during the process of lowering the current, a state close to uniform solidification is achieved. Furthermore, even if the molten metal pool is deep, as long as the molten metal is supplied properly, it can be assumed that there will be no segregation of components or formation of shrinkage pores. We should aim to improve the quality of life. Examples Example 1 Using a compact formed by a hydraulic press, a primary electrode of 788 mm in diameter, 6300 mm in length, and 7200 kg in weight was produced by plasma beam welding, and this was then melted into a secondary electrode using a consumable electrode type vacuum arc melting furnace. A pure titanium ingot with a diameter of 980 mm, a length of 2100 mm, and a weight of 7150 kg was produced by heavy melting. At this time, all primary melting was performed under the same conditions,
Production was carried out by changing the hot-top conditions for secondary melting as shown in FIG. That is, A is a conventional method with a short hot-top time, B is a deceleration melting method in which the melting current is reduced by a constant deceleration coefficient from the beginning to the end of melting, and C is an embodiment of the present invention. In this case, the hot-top time (T _A ) of conventional method A is 30 minutes,
The hot-top time (T _C ) of invention method C was 100 minutes. At the same time, each ingot was longitudinally cut into two, and the component distribution was adjusted for Fe and O, which were significantly segregated.
Productivity and surface quality were observed. The results are shown in Table 1.

【table】

[Table] Maximum segregation value = Maximum analysis value / Target value Also, based on the above results, calculate the maximum variation from the target value (Figure a), the ingot surface quality index (Figure b), and the production efficiency (Figure c). Shown in Figure 2. From the above results, this invention method C has significantly improved component segregation compared to the conventional method A, which has a short hot-top time, and can obtain the same homogeneity as the decelerated melting method B, which has hot-topped for a long time. It can be seen that an ingot with excellent productivity and surface quality can be obtained. The macrophotograph in Figure 4 shows the structure of the top part of pure titanium ingots produced by each of the methods A, B, and C described above. It can be seen that the item is more homogeneous than the other items A and B. Example 2 Using a compact molded using a hydraulic press, a diameter of 560 mm and a length of 560 mm were obtained by plasma beam welding.
A primary electrode with a diameter of 6250 mm and a weight of 4860 Kg was prepared, and this was double melted using a consumable electrode type vacuum arc melting furnace.
A 6Al-4V alloy was produced. At this time, all primary melting was performed under the same conditions,
By changing the hot top conditions for secondary melting as shown in Figure 1, the hot top time for conventional method A was 43
minutes, and invention method C was set at 103 minutes. At the same time, each ingot was longitudinally cut into two to remove Al, V, Fe, which have problems with segregation.
The component distribution of O was investigated, and the surface quality was also observed. The results are shown in Table 2.

【table】

[Table] However, the maximum variation from the target value and the maximum degree of segregation are the same as the notes in Table 1.
Furthermore, based on the above results, the maximum variation from the target value (Figure a), the ingot surface quality index (Figure B), and the production efficiency (Figure C) are shown in Figure 3. From the above results, this invention method C has significantly improved component segregation compared to conventional method A in which component adjustment is not performed, and it also achieves homogeneity equivalent to slow dissolution method B, which uses hot top for a long time. At the same time,
It can be seen that an ingot with excellent productivity and surface quality can be obtained. Effects of the Invention This invention reduces the melting current in several stages at the hot top in the latter half of the final melting process when high melting point active metals and their alloys are melted in a vacuum arc, thereby solidifying the metal in a state close to uniform solidification. It is possible to obtain ingots with excellent homogeneity, surface quality, and productivity.

[Brief explanation of drawings]

Figure 1 is a diagram showing the secondary melting current pattern in consumable electrode type vacuum arc melting, Figure 2 is a diagram showing the ingot homogeneity, surface quality, and production in Example 1, and Figure 3 is in Example 2. FIG. 4 is a photograph showing the macrostructure of the top part of the pure titanium ingot according to Example 1. A... Conventional method, B... Decelerated dissolution method, C... Invention method, I ₀
...steady melting current, I ₁ ... first stage hot top current, I ₂ ... second stage hot top current, I ₃ ... third stage hot top current, T ₁ ... first stage hot top time, T ₂ ... second stage hot top time, _T3 ...
3rd stage hot top time, T _A ... total hot top time in A pattern, T _B ... total hot top time in B pattern, T _C ... total hot top time in C pattern.

Claims (1)

[Claims] 1. In the consumable electrode vacuum arc melting method for high melting point active metals and their alloys, the melting current is reduced in several stages at the hot top in the latter stage of melting, and each stage is held for a certain period of time to gradually increase the depth of the molten metal pool. A vacuum arc melting method for high melting point active metals and their alloys, which is characterized by reducing the size and solidifying in a state close to uniform solidification.