Cold crucible melting furnace

Abstract

To provide a cold crucible melting furnace that can heat a solidified layer present at a crucible bottom part more efficiently while having a water cooling function at the crucible bottom part.SOLUTION: A cold crucible melting furnace comprises a cylindrical crucible 1 and a tapping coil 21 arranged in a bottom part side of the crucible 1. The bottom part of the crucible 1 is closed with the tapping coil 21, and a segment integrated type induction heating coil 2 is constituted which has both a function of forming a tapping hole in a part and a function of a water cooling segment, so a magnetic flux Φ from the tapping coil 21 can be applied directly to a molten material A, so that heating efficiency can be improved.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a cold crucible melting furnace that enables more efficient heating of the solidified layer present at the bottom of a crucible. [Background technology]

[0002] Unlike refractory crucibles, cold crucible melting furnaces form a solidified layer of melting material inside the furnace, allowing the melting material to be melted and poured by induction heating without coming into contact with the crucible. This prevents the inclusion of impurities and makes them particularly suitable for melting and refining active metals.

[0003] There are various conventional methods for casting using cold crucible melting, such as solidification in a crucible, pouring the molten metal into a mold by tilting the crucible, and solidifying it. Among these, in order to pour the molten metal into a mold or to tap the molten metal from the bottom for various atomization methods from the bottom, the solidified layer that exists at the bottom of the crucible must be melted or made as thin as possible in accordance with the principles of the melting method, in order to achieve bottom tapping.

[0004] However, because the cold crucible melting furnace is cooled to form a solidified layer, it is technically difficult to thin the solidified layer at the bottom of the crucible and achieve bottom tapping by heating only with a heating coil. Therefore, it is important to strike a balance between the heat transfer from the melting material to the cold crucible melting furnace and the melting of the solidified layer by induction heating.

[0005] 6(a) is an example of a conventional bottom-tap type cold crucible melting furnace using segments without a tapping nozzle. Side segments 101 have a cooling structure, and when melting material A is melted by heating coils 103, a solidified layer 102 forms on the bottom segment 104, closing the tapping port 105. When the melt is to be tapped, the vicinity of the tapping port 105 is heated by a tapping coil 106, which opens the tapping port 105 and the melt is tapped through a tapping nozzle 107. In this melting furnace, the required tapping diameter is, for example, 10 mm or more, so the conditions for forming the solidified layer 102 and the melting conditions are not strict, and precise control of the tapping diameter is not necessary.

[0006] However, in recent years, there has been an increase in demand for high-purity metal powders, such as those used in metal 3D printers, and the powders required for higher precision are also being required to be finer (for example, 45 μm or less). For example, in gas atomization, a thin outlet flow is an important factor for finer powder, and the outlet diameter needs to be as thin as possible to increase powder yield.

[0007] As a method for realizing bottom tapping with such a fine grain size, for example, a cold crucible melting furnace using water-cooled segments and an electromagnetic tapping nozzle, as shown in FIG. 6(b), is known (see Patent Document 1).

[0008] This melting furnace is configured so that the hearth and the tapping section are made up of water-cooled segments 201, and a solidified layer is formed at the bottom 202 by cooling, while the heating coil 303 for the tapping nozzle acts to thin the solidified layer at the bottom 202, achieving a balance between these two and achieving bottom tapping. The sidewall of the crucible 200 is also made up of water-cooled segments 204. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent Publication No. 2005-2114438 Summary of the Invention [Problem to be solved by the invention]

[0010] However, a cold crucible melting furnace with a water-cooled segment structure has the disadvantage that a large amount of power is consumed in the water-cooled segments (in the figure, the bottom segment 201 and the side segments 204 of the upper crucible 200) located between the induction heating coil 303 and the object to be melted A. As a result, the melting efficiency relative to the input power is poor compared to refractory crucible melting, and it is generally said that about 30% of the input power is consumed in the segments. This results in a problem of low energy efficiency contributing to melting relative to the input power.

[0011] The present invention has been made with a focus on these problems, and aims to realize a cold crucible melting furnace that has a water-cooling function at the bottom of the crucible while enabling more efficient heating of the solidified layer at the bottom of the crucible. [Means for solving the problem]

[0012] In order to achieve the above object, the present invention takes the following measures.

[0013] That is, the cold crucible melting furnace according to the present invention is characterized in that it comprises a cylindrical crucible and a tapping coil arranged on the bottom side of the crucible, and the tapping coil is configured as a segment-integrated induction heating coil that combines the function of sealing the bottom of the crucible and forming a tapping outlet in part of it, as well as the function of a water-cooled segment.

[0014] In this way, the water-cooled segment, which would otherwise cause heating loss, is not located between the tapping coil and the object to be melted, and the magnetic flux from the tapping coil is applied directly to the material to be melted, thereby improving heating efficiency.

[0015] The segment-integrated induction heating coil is preferably a discharge coil having a cooling passage inside, formed in a spiral shape when viewed from the top and bottom of the crucible, with an insulating material that allows magnetic flux to pass between the inner and outer periphery.

[0016] In this way, the bottom of the cylindrical crucible can be appropriately configured, the insulating material prevents leakage of the molten metal, and magnetic flux can be applied directly and uniformly to the solidified layer over the entire bottom area.

[0017] In particular, it is preferable that a coating having insulating and heat-insulating properties is formed on the surface of the segment-integrated induction heating coil that comes into contact with the molten metal.

[0018] This increases the magnetic flux that reaches the solidified layer along the coating, allowing for efficient heating and a thinner solidified layer. Furthermore, the insulating properties of the coating reduce heat transfer from the molten metal to the furnace during melting, enabling stable bottom tapping even with a small bottom tapping diameter.

[0019] It is also preferable that the segment-integrated induction heating coil is configured to be concavely inclined toward the tapping port.

[0020] In this way, when melting a small amount of material, the segment-integrated induction heating coil can also serve as a crucible (a container for holding the initial raw materials). Also, when melting, the molten metal can easily flow into the tapping hole along the slope. [Effects of the Invention]

[0021] According to the present invention as described above, it is possible to provide a cold crucible melting furnace that has a water-cooling function at the bottom of the crucible and is capable of more efficiently heating the solidified layer present at the bottom of the crucible. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a schematic diagram showing a cold crucible melting furnace according to an embodiment of the present invention; [Figure 2]FIG. 10 is a diagram showing a comparative example of the same embodiment. [Figure 3] FIG. [Figure 4] FIG. 2 is a schematic diagram showing the configuration of the segment-integrated induction heating coil according to the embodiment; [Figure 5] FIG. [Figure 6] FIG. 10 is a schematic diagram showing a conventional example. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0024] As shown in FIG. 1, the cold crucible melting furnace X of this embodiment comprises a cylindrical crucible 1 and a discharge coil 21, which is an induction heating coil arranged on the bottom side of the crucible 1. The discharge coil 21 constitutes a segment-integrated induction heating coil 2 that seals the bottom of the crucible 1 and has the function of forming a discharge section (discharge port) 23 in part, as well as the function of a water-cooled segment.

[0025] The crucible 1 is formed by joining some or all of the vertically elongated water-cooled copper segments 11, each with a partially arcuate cross section, in the circumferential direction while being spaced apart by slits 11a. Insulation between adjacent water-cooled copper segments 11 in the circumferential direction can be ensured by interposing an insulating member between the water-cooled copper segments 11 or by separating the water-cooled copper segments 11. This configuration ensures that the water-cooled copper segments 11 constituting the crucible 1 are electrically insulated from each other, allowing the magnetic flux generated by the melting coil 12, an induction heating coil arranged around the crucible 1, to be efficiently introduced into the crucible 1 and act on the material A to be melted. The melting coil 12 is connected to a melting power supply (not shown) that outputs an AC current to control the melting. Examples of insulating materials to be placed between the water-cooled copper segments 11 include ceramic refractory materials such as alumina, zirconia, and yttria.

[0026] Each water-cooled copper segment 11 is formed from a material that has excellent electrical and thermal conductivity, resistance to thermal shock, and the necessary mechanical strength, as well as the high thermal conductivity required to form a solidified layer by circulating a cooling medium through the water-cooled passages, such as copper, chromium copper, beryllium copper, zirconium copper, chromium zirconium copper, or tellurium copper. Copper-based materials are particularly advantageous over refractory materials such as metal oxides because they do not form oxides when the crucible 1 is placed inside a vacuum chamber and melting material A is performed in a vacuum or reduced-pressure atmosphere. Examples of melting material A include titanium, zirconium, hafnium, chromium, niobium, tantalum, molybdenum, uranium, rare earth metals, thorium, and alloys thereof. The crucible 1 is cooled by circulating a coolant such as water through the cooling passages 11b formed inside the water-cooled copper segments 11.

[0027] 2 is a comparative example showing a conventional furnace bottom structure that does not employ the above-described segment-integrated induction heating coil 2. In the figure, the furnace includes a cylindrical crucible 301, a bottom plate 302 that is positioned to close the bottom side of the crucible 301 and has a cooling passage 320 and a tapping port 321 at predetermined locations, a melting coil 303 that is arranged around the crucible 301, and a tapping coil 304 that is arranged around the tapping port 321. The bottom plate 302 is configured by circumferentially arranging multiple electrically insulated water-cooled copper bottom segments 321. The solidified layer formed on the bottom plate 302 is melted by the tapping coil 304, and the tapping port 321 is opened to discharge the molten metal.

[0028] This structure also generates heat loss because the water-cooled segment 321 is present between the discharge coil 304 and the molten material A, similar to the structure shown in FIG. 6(b).

[0029] Therefore, the segment-integrated induction heating coil 2 in Fig. 1 does not have the bottom segment 321 in Fig. 2, and is configured so that the bottom discharge coil 21 also functions as a segment. Specifically, as shown in Fig. 4, this segment-integrated induction heating coil 2 is formed by spirally forming an induction heating coil to form the discharge coil 21, and the gap between the inner and outer spirals of the discharge coil 21 is filled with insulating material 22 to form a strong flat disk, and the surface is coated with the coating 5 shown in Fig. 3. The insulating material 22 prevents leakage of the molten metal and short-circuits between the discharge coils 21, 21, thereby facilitating the transmission of magnetic flux Φ, and is made of a ceramic refractory material such as alumina, zirconia, or yttria, similar to the insulating material provided in the slit 11a of the crucible 1 described above.

[0030] One end and the other end of the inner and outer periphery of the spiral-shaped discharge coil 21 shown in Fig. 4 are connected to the high-frequency power supply 3 shown in Fig. 1. Because the discharge coil 21 is spiral-shaped, it can be easily connected to the high-frequency power supply 3.

[0031] As shown in FIG. 3, the cross section of the discharge coil 21 is generally rectangular, forming a hollow cooling passage 21a through which cooling water flows. While cooling water can be introduced into the cooling passage 21a from either the inner or outer periphery of the discharge coil 21 and discharged from the other, to ensure sufficient water volume and improve heat dissipation efficiency, the interior may be divided into two sections, with cooling water introduced from two locations and discharged from appropriate positions. For example, one embodiment of the cooling passage 21a is shown in FIG. 4, in which cooling water is introduced from one end of the inner or outer periphery along the spiral discharge coil 21 and discharged midway, and another embodiment in which cooling water is introduced from the other end of the inner or outer periphery and discharged midway. Of course, creating multiple cooling water flows within the interior can be achieved without dividing the interior into two sections, or by dividing the interior into three or more sections.

[0032] From the perspective of magnetic field and thermal analysis, the center of the segment-integrated induction heating coil 2 has a tapered cross section 2a around the furnace axis m to form the tapping section (tap port) 23, and the remaining portion has a roughly rectangular outer cross section 2b. The tapered cross section 2a is for bottom tapping, and a straight section 2a' continues below it, forming a funnel-shaped tapping section 23 as a whole. The inside of the tapping section 23 is cooled through the tapered cross section 2a, forming a solidified layer of molten metal (skull) W inside, which closes the nozzle. The roughly rectangular outer cross section 2b, combined with the sealing effect of the insulating material 22, is suitable for preventing molten metal leakage from gaps between the adjacent inner and outer peripheries of the tapping coils 21.

[0033] Then, a sprayed film 5 is formed on the molten metal contact surface (the surface forming the solidified layer) of the segment-integrated induction heating coil 2 as a coating that combines electrical insulation and heat insulation. The sprayed film 5 is formed by a deposition process such as plasma PVD after the integrated coil is produced. The sprayed film thickness here is, for example, about 50 μm to 500 μm. In this example, yttria (Y2O3) is used as the material for the sprayed film 5. Of course, other than yttria, any refractory material that does not react with the molten material, such as alumina or zirconia, can be used.

[0034] First, by making the discharge coil 21, which is an induction heating coil, function as a water-cooled segment, the magnetic flux Φ generated by the discharge coil 21 connected to the high-frequency power source 3 reaches the solidified layer W directly without passing through the segment, thereby causing induction heating.

[0035] In addition, by providing the sprayed film 5, when current is applied to the discharge coil 21, the magnetic flux Φ generated at a position circling the discharge coil 21 as shown in Figure 3(b) becomes an active passageway, increasing the magnetic flux Φ that reaches the solidified layer W formed on the sprayed film 5 and efficiently melting the solidified layer W.

[0036] In addition, because the sprayed film 5 forms an insulating layer, during the induction heating stage before pouring shown in Figure 3(a), it reduces heat transfer from the molten material A to the water-cooled segment-integrated induction heating coil 2, prevents the solidified layer from becoming too thick, and makes it easier to pour the molten material from the bottom.

[0037] During this process, because the sprayed coating 5 is in contact with the water-cooled discharge coil 21, the surface temperature on the molten metal side does not become higher than in the refractory crucible, suppressing the reaction between the sprayed coating 5 of the integrated induction heating coil 2 and the molten material A and preventing the inclusion of impurities. In particular, by using an appropriate method and coating thickness for the sprayed coating 5, it is possible to achieve melting and refining without peeling or the inclusion of inclusions.

[0038] As described above, the cold crucible melting furnace of this embodiment comprises a cylindrical crucible 1 and a discharge coil 21 arranged on the bottom side of the crucible 1, and the discharge coil 21 is configured as a segment-integrated induction heating coil 2 that combines the function of sealing the bottom of the crucible 1 and forming a discharge section (discharge port) 23 in part, as well as the function of a water-cooled segment.

[0039] In this way, the water-cooled segment, which would otherwise cause heating loss, is no longer located inside the tapping coil 21, and the magnetic flux Φ from the tapping coil 21, which constitutes the segment-integrated induction heating coil 2, can be applied directly to the molten material A (particularly the solidified layer W), improving heating efficiency. In other words, segment heating, which accounts for 30% of the input power, is no longer necessary, making it possible to significantly improve energy efficiency. This also allows the rating of the high-frequency power source 3 to be reduced, leading to reduced initial and running costs.

[0040] Specifically, the segment-integrated induction heating coil 2 has a discharge coil 21 with a cooling passage 21a inside that is formed in a spiral shape when viewed from the top and bottom of the crucible 1, and is filled with insulating material 22 that allows magnetic flux Φ to pass through between the inner and outer periphery.This allows the bottom of the cylindrical crucible 1 to be appropriately configured, the insulating material 22 to prevent leakage of molten metal, and the magnetic flux Φ to act directly and uniformly on the solidified layer W over the entire bottom area in accordance with the arrangement of the insulating material 22, making it possible to appropriately achieve both the efficiency of induction heating and water cooling.

[0041] Furthermore, because the sprayed film 5 is formed on the surface of the segment-integrated induction heating coil 2 that comes into contact with the molten metal, it serves as a coating with insulating and heat-insulating properties, which facilitates transmission of magnetic flux Φ to the solidified layer W along the sprayed film 5, efficiently heating the solidified layer W and resulting in a thinner solidified layer W. Furthermore, the insulating properties of the sprayed film 5 reduce heat transfer from the molten metal to the furnace during melting, making it possible to stably achieve bottom tapping even with a small bottom tapping diameter of, for example, about φ5mm. Therefore, when applied to gas atomization, for example, powder of the required diameter can be produced with a good yield.

[0042] Although one embodiment of the present invention has been described above, the specific configuration of each part is not limited to the above-described embodiment.

[0043] For example, in the above embodiment, yttria was used for the sprayed coating that is the coating of the segment-integrated induction heating coil, but other non-metallic materials (other ceramics such as zirconia and alumina) may also be suitable as long as they have insulating and / or heat-insulating properties.

[0044] In addition, in the above embodiment, a thermal spray coating is used to form a coating that provides insulation and heat insulation to the segment-integrated induction heating coil, but the coating may be formed by other methods such as aerosol deposition (AD) or CVD (chemical vapor deposition).

[0045] The coating on the segment-integrated induction heating coil is formed on the surface side that comes into contact with the molten metal, but it is also effective to form it on the entire outer periphery of the coil in order to form a magnetic path.

[0046] The spiral coil that makes up the segment-integrated induction heating coil can be produced by winding a coil made in a straight line, but if it is produced using a metal 3D printer, it is easier to create a flat surface that will allow it to function as a segment.

[0047] Furthermore, although the segment-integrated induction heating coil in the above embodiment has a flat disk shape, this is not limiting. For example, as shown in Fig. 5, the segment-integrated induction heating coil may be inclined concavely (tapered or bowl-shaped) toward the outlet 23, as in the segment-integrated induction heating coil 20. In this way, if a small amount of material is to be melted, the segment-integrated induction heating coil can also serve as a crucible (a container for holding the initial raw materials). Furthermore, this makes it easier for the molten metal to flow along the inclination toward the outlet 23.

[0048] Furthermore, the tapping section 23 may be formed with a separate tapping nozzle having a funnel-shaped portion that is caught and held by the tapered cross section 2a of the tapping section (tap opening) and a straight portion that extends downward from the funnel-shaped portion.

[0049] Other configurations can also be modified in various ways without departing from the spirit of the present invention. [Explanation of symbols]

[0050] 1...Crucible 2...Segment integrated induction heating coil 5...Coating (thermal spray coating) 21...Water outlet coil 21a...Cooling path 22...Insulating material 23...Water outlet (water outlet) Φ...Magnetic flux

Claims

1. A cylindrical crucible, a tapping coil disposed on the bottom side of the crucible, The tapping coil is configured as a segment-integrated induction heating coil that has both the function of closing the bottom of the crucible and forming a tapping port in a part of it, and the function of a water-cooled segment. A cold crucible melting furnace characterized by:

2. 2. The cold crucible melting furnace according to claim 1, wherein the segment-integrated induction heating coil is a discharge coil having a cooling passage therein, formed in a spiral shape when viewed from above and below the crucible, and filled with an insulating material that allows magnetic flux to pass between its inner and outer peripheries.

3. 3. The cold crucible melting furnace according to claim 1, wherein a coating having insulating and heat-insulating properties is formed on a surface of said segment-integrated induction heating coil that comes into contact with the molten metal.

4. 3. The cold crucible melting furnace according to claim 1, wherein the segment-integrated induction heating coil is concavely inclined toward the tapping hole.

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