JPH0531568A - Plasma melting/casting method - Google Patents

Abstract

PURPOSE:To produce various kinds of metals of cast ingot obtaining good surface quality at active metal, high m.p. metal or ultra high cleanness and to execute casting having no fear of break-out with plasma melting/casting method. CONSTITUTION:A non-bottom water-cooled mold 3 is set so as to develop gas stream between a melting chamber 1 and an ingot making chamber 2 in inside of this mold 3, and also gas pressure is controlled so that absolute value of pressure difference ¦Pc-Pm¦ between atmospheric pressure (Pm) in the melting chamber 1 and atmospheric pressure (Pc) in the ingot chamber 2 satisfies in the range of 0.5<=¦Pc-Pm¦<0.5 atm. As the cast ingot having good surface quality can be produced, cleaning, etc., of the surface on the cast ingot in the following process is reduced, and the yield and the productivity can be improved. Further, it is no fear to develop the break-out and stable casting is executed.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to various metals that require high cleanliness and high purity, active metals that require oxidation prevention during melting (titanium (Ti), zirconium (Zr), rare earth metals and their Alloy, etc.), or chromium (Cr), niobium (N
b) relates to a plasma melting casting method for refractory metals such as molybdenum (Mo).

[0002]

2. Description of the Related Art Vacuum arc remelting method (hereinafter referred to as VAR method) has been widely used as a melting method for obtaining ingots of the above metals and alloys. . In this VAR method, an electrode made of a molten metal is manufactured in advance, an arc is generated between the electrode and a molten metal in a water-cooled crucible under high vacuum (about 10 ^-2 to 10 ^-3 torr), and the electrode is melted by arc heat. Is the way. However, this VA
In the R method, as described above, it is necessary to manufacture an electrode made of a molten metal prior to melting, and the manufacturing process of the electrode is complicated and the productivity of the ingot is low.

On the other hand, in recent years, with the progress of vacuum technology and the development of high energy processing technology, a plasma melting casting method utilizing a plasma melting method has been proposed and attracted attention. This method uses an inert gas as a plasma arc generation gas, irradiates plasma to the melting raw material under an atmosphere pressure of the melting furnace of about 0.01 to 1 atmosphere to melt, and sequentially supplies the obtained molten metal to a water-cooled mold. Is a method of obtaining an ingot.

In this method, since a bottomless water-cooled mold is used as the mold, the ingot solidified in the mold is sequentially drawn downward, that is, a so-called continuous casting method can be used. Although there is a feature that the ingot is easy to manufacture, when the ingot is pulled out from the mold, a frictional force is generated between the mold and the high temperature solidified shell, which easily causes defects on the ingot surface. However, the surface of the ingot must be chamfered before the subsequent step, and the yield is likely to be reduced. In addition, cracks may occur in the high temperature solidified shell portion, and sometimes breakout may occur from that portion, making casting impossible.

To solve the above problems, it is possible to use a flux for lubrication in continuous casting of steel.
In the plasma melting method, the raw material to be melted is an active metal such as Ti or Zr, a high melting point metal such as Cr, Nb, Mo, or a Ni-based or Co-based alloy that requires ultra-high cleanliness. It was not possible to use the lubricating flux that could react with, and it was necessary to take alternative measures.

The present invention has been made based on the above circumstances, and an object thereof is to provide an active metal having a good surface quality,
It is an object of the present invention to provide a plasma melting casting method capable of producing an ingot of a high melting point metal or various metals required to have an extremely high cleanliness and at the same time capable of performing casting without fear of breakout.

[0007]

In order to achieve the above object, the plasma melting and casting method according to the present invention melts a raw material using a plasma arc as a heat source in an inert gas atmosphere, and melts the molten metal without bottom water cooling. Solidify in the mold,
In the plasma melting and casting method for producing a relatively long ingot while pulling out the solidified ingot downward, so that the gas flow between the melting chamber and the agglomeration chamber can occur inside the bottomless water-cooled mold. A bottomless water-cooled mold is placed, and the absolute value | Pc-Pm | of the pressure difference between the atmospheric pressure (Pm) in the melting chamber and the atmospheric pressure (Pc) in the agglomeration chamber is 0.05≤ | Pc-Pm | <0.5. The gas pressure is controlled so as to satisfy the atmospheric pressure range.

The gas pressure may be controlled by supplying gas to the ingot making chamber or discharging gas from the ingot making chamber.

[0009]

The present invention will be described in more detail below. The conventional plasma melting casting method uses a water-cooled container 18 called a water-cooled hearth as shown in FIG.
2b, the molten metal is poured into the bottomless water-cooled mold 3 in a form of overflowing from the water-cooled container 18, and FIG.
As shown in FIG. 2, a raw material 9 is directly charged into the bottomless water-cooled mold 3, or a rod-shaped raw material (not shown) is supplied directly above the bottomless water-cooled mold 3 to melt by a plasma arc. Broadly divided.

In either method, the bottomless water-cooled mold 3 is used.
Plasma heating must be provided to hold the molten metal bath 11 formed therein. The ingot 12 is a bottomless water-cooled mold 3
The molten metal bath 11 is drawn out according to the charging speed of the raw material 9 into the bottomless water-cooled mold 3 so that the bath surface (meniscus) of the molten metal bath 11 formed therein is substantially constant.

In the figure, 19 is a melting and casting chamber, 10 is a raw material feeder, 8 is a plasma torch, and 13 is a rod of a drawing device.

The surface defects of the ingot produced by the above-mentioned plasma melting casting method are (a) unevenness caused by insufficient plasma heating output and insufficient formation of a molten metal bath in a bottomless water-cooled mold. , (B) The plasma heating output is too large, and the molten metal covers the surface of the ingot that solidified first. Double crust defect, (c) the inner wall of the mold and the solidified shell when the ingot is drawn downward. There are defects such as horizontal minute cracks that occur in the solidified shell due to frictional force.

Among these defects, the defect (a) can be relatively easily resolved by selecting an appropriate plasma heating output. However, the defects (b) and (c) depend on the contact condition between the molten metal bath and the solidified shell and the inner wall of the mold. It is necessary to perform the casting while relaxing the contact pressure with the water-cooled mold. As such a casting method, for example, in the field of aluminum, electromagnetic field casting in which molten metal is held by electromagnetic force is known. However, the cooling method in the case of this electromagnetic field casting method is a method in which water is directly applied to the surface of the ingot, and it is easy to apply to aluminum that can be melted at a relatively low temperature in the air atmosphere, but it is active and has a high melting point. In the plasma melting casting method for titanium, chromium, etc., it is often difficult to immediately apply the electromagnetic field casting method because water cannot be directly used for cooling. Further, the device for generating the electromagnetic force is expensive and the cost is high.

As another casting method, there has been proposed a casting method in which the inner wall of the mold is made porous and pressurized gas is ejected from the hole to hold the molten metal or the solidified shell. However, it seems that this casting method has not been put to practical use because of the difficulty in manufacturing the porous body.

On the other hand, in the plasma melting casting method, a high-purity inert gas (argon, helium, etc.) is used as the plasma gas, and the atmospheric pressure during melting is usually around 1 atm. However, the operation is performed in the range of 0.01 to 2 atm.

Therefore, the inventors of the present invention have proposed, as means for relaxing the contact pressure between the bottomless water-cooled mold and the molten metal or the solidified shell,
Focusing on the atmosphere gas, it was considered to form a gas flow by the atmosphere gas between the mold and the solidified shell.

That is, according to the present invention, the bottomless water-cooled mold is arranged so that the gas flow between the melting chamber and the ingot-making chamber can occur inside the bottomless water-cooled mold, and the atmospheric pressure (Pm ) And the atmospheric pressure (Pc) in the ingot making room
The gas pressure is controlled so that Pc-Pm | satisfies the range of 0.05≤ | Pc-Pm | <0.5 atm.

According to the present invention described above, the apparatus itself used in the basic plasma melting and casting method does not need to be significantly changed and can be applied relatively easily.

Although it is difficult to directly control the gas flow formed between the inner wall of the mold and the solidification shell, it is difficult to directly control the gas flow so that only the gap between the inner wall of the mold and the solidification shell serves as a gas passage. It is possible to form a stable gas flow between the inner wall of the mold and the solidification shell by providing a partition between the melting chamber and the ingot making chamber and controlling the gas pressure difference between the melting chamber and the ingot making chamber. .

The absolute value | Pc-Pm | of the gas pressure difference between the melting chamber and the agglomeration chamber is preferably controlled within the range of 0.05≤ | Pc-Pm | <0.5 atm. The reason is that the gas pressure difference is
If the pressure is less than 0.05 atm, the gas flow is less likely to be formed, and the effect of relaxing the contact pressure between the bottomless water-cooled mold and the molten metal or solidified shell cannot be expected. On the other hand, when the gas pressure difference is 0.5 atm or more, the molten metal near the upper surface of the mold becomes droplets and scatters, which is not practical.

In addition, a slightly better result can be obtained by making the gas flow flow from the ingot making chamber to the melting chamber.
This is because in the flow from the melting chamber to the agglomeration chamber side, the molten metal in the molten metal bath may be pushed into the gap between the inner wall of the mold and the solidification shell (formed by solidification shrinkage of the ingot). It is thought that it will increase.

[0022]

FIG. 1 shows an example of an apparatus applied to the plasma melting and casting method according to the present invention. In the apparatus shown in this figure, the melting chamber 1 and the ingot-making chamber 2 are bottomless water-cooled molds. 3, the melting chamber 1 and the ingot-making chamber 2 are provided with inert gas introduction pipes 4 and 5 and exhaust pipes 6 and 7 connected to a vacuum pump, respectively. A plasma torch 8 is provided above the mold.

In plasma melting and casting by the above apparatus, after the atmosphere in the melting chamber 1 and the ingot-making chamber 2 is set to a predetermined inert gas atmosphere pressure, the raw material 9 is fed into the bottomless water-cooled mold 3 by the raw material feeder 10. While melting with the plasma torch 8,
While forming the molten metal bath 11 in the bottomless water-cooled mold 3,
The ingot 12 cooled and solidified by the inner wall of the mold is drawn out from the bottomless water-cooled mold 3 by pulling down the rod 13 of the drawing device at a predetermined speed. During the melting and casting, the inert gas atmosphere pressure in the melting chamber 1 and the ingot-making chamber 2 is controlled by the valve 1 provided in the introduction pipes 4, 5 and the exhaust pipes 6, 7.
It is controlled by the opening of 4, 15, 16, 17.

Next, using the apparatus having the above-mentioned structure, the inner diameter 1
20mm bottomless water-cooled mold 3, 100kW plasma torch 8
And plasma power of melting Ti-6Al-4V titanium alloy, using argon (Ar) as plasma torch gas 70
kW, dissolution rate 15 kg / hr, plasma torch gas flow rate 30 l
/ Min under the condition of / min and by changing the pressure difference ΔP between the inert gas atmosphere pressure (Pm) in the melting chamber 1 and the inert gas atmosphere pressure (Pc) in the ingot making chamber 2 The surface quality of the obtained ingot 12 was investigated. The results of this investigation are shown in Table 1 in correspondence with the pressure difference ΔP. The pressure difference ΔP at this time was measured both when the melting chamber 1 side was pressurized and when the ingot forming chamber 2 side was pressurized.

Further, the operation of changing the pressure difference ΔP between the melting chamber 1 and the ingot forming chamber 2 at this time is performed by introducing pipes 4, 5 and exhaust pipes 6, 7.
This was done by controlling the opening of valves 14, 15, 16 and 17 provided in the. The atmospheric pressure at the beginning of melting was set to 1 atm.

Further, in order to investigate the surface quality of the ingot, the ingot was cut in the longitudinal direction and the flaw depth in the cross section was evaluated.

[0027]

[Table 1]

As is clear from Table 1 above, when the atmospheric pressure in the melting chamber is increased and the gas flow in the mold is made to flow from the melting chamber to the ingot forming side, if the pressure difference ΔP is 0.025 atm or less, Since there was almost no gas flow, the surface properties of the ingot obtained by the conventional plasma melting casting method were almost the same. On the other hand, if the pressure difference ΔP becomes higher than 0.6 atm,
The gas flow in the mold becomes fast, the casting becomes unstable, and the surface of the obtained ingot has a deep flaw. Furthermore, the pressure difference ΔP
Above 0.8 atm, the gas flow in the mold became faster and flowed from top to bottom, so the molten metal in the outer peripheral portion of the molten metal bath was pushed between the molds, resulting in double skin defects.

On the contrary, when the atmosphere pressure in the ingot making chamber is increased and the gas flow in the mold is made to flow from the ingot forming chamber to the melting chamber side, if the pressure difference ΔP is 0.025 atm or less, Similarly, there was almost no gas flow in the mold, which was the same as the surface texture of the ingot obtained by the conventional plasma melting casting method, but when the pressure difference ΔP was 0.6 atm or 0.8 atm, it was different from the above. Since the gas flow in the inside flowed from bottom to top, there was no indentation of the molten metal bath between the molds, and the flaw depth on the surface of the obtained ingot was relatively shallow and was good to normal. However, a phenomenon was observed in which molten metal was blown and scattered from between the molds, and stable operation could not be performed.

When the pressure difference ΔP was 0.05 to 0.5 atm, an ingot having a good surface quality with a shallow flaw depth on the surface of the ingot was obtained. Further, in order to obtain a more preferable ingot, the pressure difference Δ
As is clear from Table 1, P is preferably around 0.1 atm.

[0031]

As described above, according to the plasma melting casting method according to the present invention, ingots of active metal having a good surface quality, refractory metal, or various metals required to have an extremely high cleanliness are produced. In addition, the maintenance of the surface of the ingot in the subsequent steps can be reduced, and the yield and productivity can be improved. Also, stable casting can be performed without fear of breakout.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of an apparatus applied to a plasma melting and casting method according to the present invention.

FIG. 2 is a schematic view of an apparatus applied to a conventional plasma melting and casting method, (a) is an example in which a molten raw material is melted in a water-cooled container and then injected into a mold, and (b) is a molten raw material. It is an example in the case of directly charging into a mold.

[Explanation of symbols]

1: melting chamber 2: ingot chamber
3: Bottomless water-cooled mold 4,5: Inert gas inlet pipe 6, 7: Exhaust pipe 8: Plasma torch
9: Melting raw material 10: Raw material feeder 11: Molten metal bath 1
2: Ingot 13: Drawing device rod 14, 15, 16, 17: Valve

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI Technical display location // B22D 21/00 Z 8926-4E

Claims (2)

[Claims] 1. A relatively long ingot while melting a raw material by using a plasma arc as a heat source in an inert gas atmosphere, solidifying the molten metal in a bottomless water-cooled mold, and pulling the solidified ingot downward. In the plasma melting and casting method for producing, the bottomless water-cooled mold is arranged so that the flow of gas between the melting chamber and the ingot molding chamber may occur inside the bottomless water-cooled mold, and the atmospheric pressure (Pm ) And the atmospheric pressure (Pc) in the ingot-making chamber, the absolute value | Pc-Pm |
A plasma melting casting method characterized in that the gas pressure is controlled so as to satisfy the range of Pc-Pm | <0.5 atm. 2. The plasma melting casting method according to claim 1, wherein the gas pressure is controlled by supplying gas to the ingot making chamber or discharging gas from the ingot making chamber.