Induction heating furnace and bottom tapping mechanism thereof

Abstract: An induction heating furnace includes a furnace body (1) having a side wall (3) extending so obliquely as to increase in radius from the bottom to the top edge portion and formed by a plurality of longitudinally split, conductive segments (4) being arrayed circumferentially with their being insulated from each other; a first induction heating coil (5), arranged at an outer periphery side of the side wall (3), for subjecting a to-be-heated material accommodated in the furnace body to induction heating; and a melt-use (7) power source for supplying AC power to the first induction heating coil (5).

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION: The present invention relates to an induction heating furnace for melting metals through induction heating and a bottom tapping mechanism thereof. DESCRIPTION OF THE PRIOR ART: In the case of producing a high purity metal or a metal alloy of desired components through the operation of melting a high reactive metal, attention has been attracted to an induction heating furnace which is capable of ensuring an uniform temperature over the entirety of a molten metal through the operation of induction heating and agitation to prevent variations in quality, and also suppressing the mixing of impurities into the molten metal to a low level to prevent reduction in quality. A conventional type induction heating furnace has a side wall extending so obliquely as to increase an aperture from a bottom having a tapping portion to a certain point and then rising up vertically therefrom to an upper edge with the aperture kept at a constant diameter, as disclosed by, for example, Japanese Laid-open Patent No. Hei 4(1992)-327342. The side wall is formed by a plurality of longitudinally split, conductive segments being arrayed circumferentially with their being insulated from each other. At the outer periphery side of the side wall, an induction coil is arranged so that a metal accommodating in the inside of the side wall can be heated through the induction heating. The tapping portion is provided with a mold to which a tapping passageway is communicated vertically. With the induction heating furnace thus constructed, the metal is melted into a molten metal through the induction heating and then the molten metal is flown into the tapping passageway of the mold, so as to be taken out with being solidified. Also, Japanese Laid-open Patent No. Hei 8(1996) -145571 discloses an induction heating furnace including: a side wall rising up vertically from a flat bottom having a tapping portion to an upper end, with an aperture kept at a constant diameter; and a bottom lid for closing the tapping portion. This induction heating furnace is so designed that when a metal is melted into a molten metal through the induction heating, the bottom lid can be melted to open the tapping portion, so as to take out the molten metal. With the former arrangement in which the mold is provided at the tapping portion, a solidified layer in the tapping passageway in the mold and a solidified layer on the side wall come into a state of being connected with each other. Due to this, taking out the metal from the mold requires a very large drawing force, thus causing difficulties in taking it out. Also, with the latter arrangement in which the tapping portion is closed with the bottom lid, once the bottom lid is melted to open the tapping portion, the tapping portion cannot be closed until all molten metal has completely been taken out. Due to this, the switching between the melt of metal and the taking out the molten metal cannot be made smoothly. In short, the conventional type arrangements have the first problem that the melt of the metal and the task of taking out the molten metal cannot be made with ease and the switching operation between the melt of metal and the taking out the molten metal cannot be made smoothly. Further, in the case of the side wall rising up with the aperture kept at a constant diameter, as in the above-described arrangement, when metallic vapor evaporates from the molten metal surface or the components of the gas produced in the molten metal dissipates from the molten metal surface, the evaporating direction of the metallic vapor or the rising direction of the gas come to be in parallel to a wall surface of the side wall. Thus, the conventional type arrangements have the second problem that the metal is easy to adhere to the side wall, thus causing the labor on the cleaning of the side wall, while also the gas is easy to contact with the side wall to increase the resistance to the exhaust of gas and resultantly hinder the gas from being fully eliminated, thus causing reduction of quality. GB-A-2 279 593 discloses a device for melting down a solid layer of electrically conductive material and for producing a homogeneous melt stream coming out of a ceramic-free crucible, comprising at least two separately controllable induction coils and at least one slotted, liquid-cooled central base part in the area of the upper induction coil which forms a vessel or a tank, which surrounds the melt and which comprises electrically conductive material at least in the area of the depth of penetration of the electromagnetic field or completely. Accordingly, the present invention aims to provide an induction heating furnace capable to solve at least one of the first and second problems described above and a bottom tapping mechanism thereof. SUMMARY OF THE INVENTION The present invention is defined by the appended claims. The invention is directed to a novel induction heating furnace, which comprises accommodating means, having a bottom, a tapping portion formed at the bottom, and a side wall formed by a plurality of longitudinally split, conductive segments being arrayed circumferentially with their being insulated from each other, for accommodating a to-be-melted material therein while cooling it; coil means, arranged at an outer periphery side of the tapping portion and the side wall, for subjecting the to-be-melted material accommodated in the accommodating means to induction heating; power source means for supplying power to the coil means; and power source control means for controlling the power source means so that the tapping portion can be selectively switched between open and close by the melt and solidification of the to-be-melted material. This construction can provide the following results. When the to-be-melted material accommodated in the accommodating means is subjected to the induction heating, the to-be-melted material is melted into the molten material by the heating, while on the other hand, the molten material at the part contacting with the side wall and a bottom wall of the accommodating means and the wall surface of the tapping portion is cooled down into a solidified state. Thus, the power source control means controlling the induction heating by the power source means enables the tapping portion to be closed by the solidified material when the to-be-melted material is melted and be opened by melting the solidified material when the to-be-melted material as molten is taken out. This enables the melt of the to-be-melted material and the take-out working to be facilitated and also enables the switching operation between the melt and the take-out to be made with much ease. The induction heating furnace according to the invention may comprise accommodating means having a bottom, a top edge portion, and a side wall extending so obliquely as to increase in radius from the bottom to the top edge portion and formed by a plurality of longitudinally split, conductive segments being arrayed circumferentially with their being insulated from each other; coil means, arranged at an outer periphery side of the side wall, for subjecting a to-be-melted material accommodated in the accommodating means to induction heating; and power source means for supplying AC power to the coil means. This construction can provide the following results. When the AC power is supplied to the coil means from the power source means, the alternating magnetic field is generated by the coil means, whereby the to-be-melted material accommodated in the accommodating means is subjected to the induction heating to be melted. When the to-be-melted material is melted into a molten material, the to-be-melted material evaporates from the molten material surface and also components of gas produced in the molten material dissipates therefrom in the form of gas. At that time, the rise of the evaporated material and of the vaporized gas is not obstructed by the side wall, because the side wall of the accommodating means extends so obliquely as to increase in radius from the bottom to the top edge portion. Thus, almost no evaporated material contacts with the side wall above the molten material surface, so that the drawbacks caused by the to-be-melted material adhering to the side wall are reduced. In addition, since almost no gas contacts with the side wall, the resistance to the exhaust of gas can be reduced and resultantly the gas can be fully eliminated. Also, the induction heating furnace according to the present invention may comprise accommodating means having a bottom, a top edge portion, a tapping portion formed at the bottom, and a side wall extending so obliquely as to increase in radius from the bottom to the top edge portion and formed by a plurality of longitudinally split, conductive segments being arrayed circumferentially with their being insulated from each other; coil means, arranged at an outer periphery side of the tapping portion and the side wall, for subjecting a to-be-heated material accommodated in the accommodating means to induction heating; power source means for supplying AC power to the coil means; and power source control means for controlling the power source means so that the tapping portion can be selectively switched between open and close by melt and solidification of the to-be-melted material. This construction can provide the following results. When the AC power is supplied to the coil means from the power source means, the alternating magnetic field is generated by the coil means, whereby the to-be-melted material accommodated in the accommodating means is subjected to the induction heating to be melted. When the to-be-melted material is melted into a molten material, the to-be-melted material evaporates from the molten material surface and also components of gas produced in the molten material dissipates therefrom in the form of gas. At that time, the rise of the evaporated material and of the vaporized gas is not obstructed by the side wall of the accommodating means, because the side wall extends so obliquely as to increase in radius from the bottom to the top edge portion. Thus, almost no evaporated material contacts with the side wall above the molten material surface, so that the drawbacks caused by the to-be-melted material adhering to the side wall are reduced. In addition, since almost no gas produced from the molten material contacts with the side wall, the resistance to the exhaust of gas can be reduced and resultantly the gas in the molten material can be fully eliminated. Further, the control of the induction heating by the power source means can provide the result that when the to-be-melted material is melted, the tapping portion is closed by the to-be-melted material as solidified, while on the other hand, when the to-be-melted material as melted is taken out, the tapping portion is opened by melting the to-be-melted material as solidified. This enables the melt of the to-be-melted material and the take-out working to be facilitated and also enables the switching operation between the melt and the take-out to be made with much ease. The tapping portion of the above-described induction heating furnace has an inlet portion which is joined to the bottom of the accommodating means and so formed that an aperture of the inlet portion is gradually reduced in diameter from a top toward a bottom; and a hollow cylinder-like outlet portion which is integrally formed with the inlet portion and located below the inlet portion. This construction can provide the result that the solidification of the to-be-melted material progresses along the wall surface of the tapping portion and then runs into the inner periphery. Accordingly, the closing operation of the tapping portion starts from the bottom of the inlet portion having a smallest aperture and progresses in sequence toward the top. Due to this, the entirety of the tapping portion can be prevented from being abruptly closed by a great force caused by solidification of the to-be-melted material, and as such can allow the opening degree of the tapping portion to increase and decrease with ease. As a result of this, the molten material can be taken out while the tapping amount of the molten material is finely adjusted. Also, the coil means of the induction heating furnace has an integral form comprising a first coil means arranged at an outer periphery side of the side wall and a second coil means arranged at an outer periphery side of the tapping portion, and the power source control means controls the power source means so that when the to-be-melted material is melted, the tapping portion is closed by part of the to-be-melted material as solidified, while on the other hand, when a molten material of the to-be-melted material is taken out, the part of the to-be-melted material is allowed to melt to open the tapping portion. This construction can provide the result that the first and second coil means can be continuously formed by a single coil, to form the coil means. Also, in the induction heating furnace, the coil means may be separated into the first coil means arranged at the outer periphery side of the side wall and the second coil means arranged at the outer periphery side of the tapping portion; the power source means may comprise a first power source means for supplying power to the first coil means and a second power source means for supplying power to the second coil means; and the power source control means may control the first power source means and the second power source means independently. This construction can provide the result that the melt of the to-be-melted material and the take-out of the molten material can be done independently to provide improved productivity. The second power source means comprises a melt-use power source portion for producing a first frequency of AC power to the extent that the to-be-melted material can be allowed to melt; and a solidification-use power source portion for producing a second frequency of AC power to the extent that the to-be-melted material is allowed to solidify, and the power source control means functions such that when the tapping portion is opened, the AC power can be produced from the melt-use power source portion, while on the other hand, when the tapping portion is closed, the AC power is produced from said solidification-use power source portion. This construction can provide the result that the tapping portion can be easily switched between open and close by switching between the melt-use power source portion and the solidification-use power source portion and also the tapping amounts can be easily adjusted by adjusting the time for supplying the high frequency power and the low frequency power. Desirably, the induction heating furnace according to the invention may further comprise a drawing means for forcibly drawing the to-be-melted material out from the tapping portion. This construction can provide the result that even when solidification of the melt is in progress, the to-be-melted material can be forcibly drawn out from the tapping portion, to obtain the to-be-melted material in a desired solidification state. The induction heating furnace enables the to-be-melted material to be melted under a reduced pressure. This construction enables a proper use under a reduced pressure under which a large amount of gas is produced. Also, a bottom tapping mechanism of an induction heating furnace includes: an inverted-hollow-cone-shaped aperture bored in a bottom of an accommodating means for accommodating therein a molten material of a to-be-melted material; a funnel-shaped tapping portion comprising an inlet portion formed in an inside of the aperture with contacting with an inner periphery thereof and a hollow-pipe-like outlet portion integrally formed with and located below the inlet portion, the tapping portion being divided into a plurality of segments by a plurality of slits which are continuous to each other and being connected to cooling water feed/discharge pipes; induction heating coils arranged around the tapping portion at the inlet portion and the outlet portion, respectively; and a solidification-use power source portion and a melt-use power source portion which are selectively connected to the induction heating coils arbitrarily. This construction can provide the result that the time for the melt and the tapping of the molten material and the amount of the molten material can be controlled with a relatively simple structure. Preferably, the above-described tapping portion comprises an inlet portion being wide at a top end thereof and gradually narrowing toward a bottom end thereof; and a hollow-pipe-like outlet portion extending downward in continuation to the inlet portion. This construction enables the opening degree of the tapping portion to increase and decrease with ease, so that the molten material is taken out while the tapping amount of the molten material is finely adjusted. Further, the bottom tapping mechanism of the induction heating furnace is so constructed that when the tapping of the molten material is done, high-frequency power is supplied to the induction heating coils arranged around the tapping portion at the inlet portion and at the outlet portion, respectively, while on the other hand, when the tapping of the molten material is stopped, low-frequency power is supplied thereto. This construction enables the bottom tapping mechanism to have a further simplified construction. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described with reference to the accompanying drawing wherein: FIG. 1 is an illustration showing a diagrammatic arrangement of an induction heating furnace of the first embodiment; FIG. 2 is a perspective view of the induction heating furnace; FIG. 3 is an illustration showing a to-be-melted material which is in the process of being melted; FIG. 4 is an illustration showing the to-be-melted material which has been melted; FIG. 5 is an illustration showing a thickness of a layer of skull in the relationship between the distance from surface and induction heating power; FIG. 6 is a diagrammatic construction view of the induction heating furnace; FIG. 7 is a perspective view of the induction heating furnace; FIG. 8 is a diagrammatic construction view of an induction heating furnace; FIG. 9 is a perspective view of an induction heating furnace; FIG. 10 is an illustration showing a to-be-melted material which is in the process of being melted; FIG. 11 is an illustration showing the to-be-melted material which has been melted; FIG. 12 is a diagrammatic construction view of an induction heating furnace of the third embodiment: (A) is a side view in section of the induction heating furnace; (B) is an enlarged sectional view of a tapping portion; and (C) is a perspective view of the tapping portion; and FIG. 13 is a diagrammatic construction view of a bottom tapping mechanism. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS: The first embodiment of the present invention will be described below with reference to FIGS. 1 to 7. The induction heating furnace of this embodied form has, as shown in FIG. 2, a furnace body 1, made of copper, for accommodating therein a to-be-melted material 13 such as titanium. The furnace body 1 may be made of gold or silver low in electrical resistivity, or stainless steel in some cases, in addition to copper including pure copper and copper alloy. The to-be-melted materials 13, which may be used in addition to titanium, include zirconium, hafnium, chrome, niobium, tantalum, molybdenum, uranium, rare earth metal, thorium, and reactive metals consisting of metals selected from the alloys of such materials. The furnace body 1 is arranged in a vacuum chamber, not shown, capable of being reduced to any selected pressure between high vacuum and atmospheric pressure. The furnace body 1 has a tapping portion 2 located at the bottom and an inverted-circular-cone-shaped side wall 3 extending so obliquely as to increase in radius from the bottom to the top edge portion. The tapping portion 2 opens at the bottom of the furnace body 1, as shown in FIG. 1, and has a communicating hole 2a for forming a vertical communication for the opening. The tapping portion 2 and the side wall 3 are formed by a plurality of (eight) longitudinally split, conductive segments 4 being arrayed circumferentially with their being insulated from each other. The insulation is provided by an insulating member being interposed between neighboring conductive segments 4, 4 or by the conductive segments 4, 4 being kept apart from each other. Each of the conductive segments 4 has, in an inside thereof, a cooling water channel 4a for allowing cooling water to flow therethrough. Each cooling water channel 4a is formed to extend from an upper end of the conductive segment 4 (an upper end of the side wall 3) to a lower end portion (a lower end portion of the tapping portion 2). The cooling water channels 4a at the upper ends of the segments 4 are connected to a cooling water supplying apparatus, not shown, and the cooling water channels 4a at the lower ends of the adjoining conductive segments 4 are connected to each other through communication channels 4b. The cooling water channels 4a in each set of two adjoining conductive segments 4, 4 form a cooling system. With this formed cooling system, cooling water is first introduced from the upper end of one of the two adjoining conductive segments 4 and flown down to the lower end of the one conductive segment 4, so as to cool the one conductive segment 4. Thereafter, the cooling water is flown into the cooling water channel 4a of the other of the two adjoining conductive segments 4 through the communication channel 4b at the lower end and flown up to the upper end from the lower end, so as to cool the other conductive segment 4. At the outer periphery side of the furnace body 1 thus formed, an induction heating coil 16 separated into a first induction heating coil 5 and a second induction heating coil 6 is arranged. The first induction heating coil 5 is wound around the side wall 3 from the bottom to the top end portion thereof, while on the other hand, the second induction heating coil 6 is wound around the tapping portion 2 from the bottom to the top end thereof. The first and second induction heating coils 5, 6 are connected to a melt-use power source 7 and a tapping-use power source 8 of a power unit 17, respectively, so that, when AC power is supplied from these power sources 7, 8, an alternating magnetic field 9 is produced along the side wall 3 and a wall surface of the tapping portion 2. The melt-use power source 7 is so set as to produce the first frequency of AC power to the extent that the to-be-melted material 13 is allowed to melt and also is so constructed as to change the frequency to any selected frequency. On the other hand, the tapping-use power source 8 has a melt-use power source portion 10 for producing the first frequency of AC power to the extent that the to-be-melted material 13 is allowed to melt and a solidification-use power source portion 11 for producing the second frequency of AC power to the extent that the to-be-melted material 13 is allowed to solidify. The power source portions 10, 11 are both constructed as to change the frequencies to any selected frequencies, as in the case with the above-described melt-use power source 7. The power source portions 10, 11 and the melt-use power source 7 are connected to a power source control unit 12. The first frequencies of the melt-use power source 7 and the melt-use power source portion 10 are set at a high frequency of the order of 2 kHz. The second frequency of the solidification-use power portion 11 is usually set at the commercial power frequency (at a low frequency of the order of 100-200 Hz). The power source control unit 12 enables the AC power output from each of the power sources 7, 8 to be selectively switched between on and off by outputting operation signals thereto and also enables the operation of the melt-use power source portion 10 and the operation of the solidification-use power source portion 11 to be selectively switched. Operation of the induction heating furnace constructed as mentioned above will be described below. First, the to-be-melted material 13 is thrown into the furnace body 1 from the above of the same. The furnace body 1 is formed into an inverted circular cone shape, with its side body 3 extending so obliquely as to increase in radius from the bottom to the top edge portion, and accordingly has the largest aperture at the top edge portion. Therefore, even when the to-be-melted material 13 is thrown into a somewhat deviated position or the to-be-melted material 13 is thrown in with accompany with significantly different sized ones, the whole amount of materials 13 is surely introduced into the furnace body 1. Thereafter, the furnace body 1 is cooled down by flowing the cooling water through the cooling water channel 4a, for completion of the preparation of melt. When an operator enters a melt starting command into the power source control unit 12, the power source control unit 12 puts the melt-use power source 7 into on mode, so as to output the first frequency (high frequency) of AC power to the first induction heating coil 5. When the first induction heating coil 5 is supplied with the AC power, the alternating magnetic field 9 is produced from the first induction heating coil 5 along the side wall 3. The to-be-melted material 13, which is a block-like piece, is subjected to induction heating by the alternating magnetic field 9, and thereby is melted from its surfaces. When contacting with the surface of the side wall 3, the melted materials 13 are solidified again by the cooling action of the side wall 3, and thereby are formed into a container-shaped skull 14 along the side wall 3. Thus, in the early stage of the melting, there presents the condition of the block-like, to-be-melted material 13 and the molten material 13 being mixed and placed on a large layer of skull 14, as shown in FIG. 3. Thereafter, when the induction heating continues to melt the entirety of the to-be-melted material 13, the material 13 as melted is brought into a state of being accommodated in the containing portion defined by the skull 14, as shown in FIG. 4. Thereupon, the second induction heating coil 6 not yet energized does not produce the alternating magnetic field 9, and accordingly a small alternating magnetic field 9 produced by the first induction heating coil 5 merely occurs around the tapping portion 2. Thus, the tapping portion 2 is in the condition of being closed by the large layer of skull 14 formed by the cooling of the side wall 3. Thereafter, heating power by the induction heating is applied in such a manner as to be decayed from the side of the molten material contacting with the surface of the side wall 3 toward the inside thereof, as shown in FIG. 5. When the layer of the skull 14 comes to have a thickness for allowing heat dissipation of the to-be-melted material 13 and heat input caused by the induction heating of the alternating magnetic field 9 to be in equilibrium with each other, the to-be-melted material 13 is subjected to the induction heating by a part of the alternating magnetic field 9 penetrating the skull 14, thereby maintaining the molten condition, as shown in FIG. 1. It is to be noted that though the factors for the heat dissipation of the to-be-melted material 13 include emission of the to-be-melted material 13 from the molten material surface 13a, convection of gas in the molten material surface 13a and the cooling by the side wall 3, the thickness of the layer of the skull 14 is determined mainly by the cooling by the side wall 3 and the induction heating of the alternating magnetic field 9. When the to-be-melted material 13 melts into the molten material as mentioned above, the to-be-melted material 13 evaporates from the molten material surface 13a and also components of gas produced in the molten material dissipates therefrom. At that time, the rise of the evaporated material 13 and of the components of vaporized gas (in the direction indicated by an arrow) is not obstructed by the side wall 3, because the side wall 3 extends so obliquely as to increase in radius from the bottom to the top edge portion. Accordingly, almost no evaporated material 13 contacts with the side wall 3 above the molten material surface 13a, so that the adherence of the material 13 to the side wall 3 is reduced. In addition, since almost no gas rising from the molten material surface 13a also contacts with the side wall 3, the resistance to the exhaust of gas is reduced and thereby gas in the molten material is fully eliminated. Next, when the molten material of the to-be-melted material 13 is taken out, the melt-use power source portion 10 is put into on mode, so as to output the first frequency (high frequency) of AC power to the second induction heating coil 6. When the second induction heating coil 6 is supplied with the AC power, the alternating magnetic field 9 is produced around the tapping portion 2 by the second induction heating coil 6. This allows the skull 14 existing at an upper part of the tapping portion 2 to be melted by the induction heating, and thereby the tapping portion 2 is put into an open state and then the molten material of the to-be-melted material 13 is removed out by gravity through the tapping portion 2. When the take-out of the molten material is interrupted or the amount of the molten material to be taken out is regulated, the power supply to the second induction heating coil 6 is switched from the melt-use power source portion 10 to the solidification-use power source portion 11. When the switching to the solidification-use power source portion 11 is done, the second frequency (low frequency) of alternating magnetic field 9 is produced around the tapping portion 2, so that eddy currents are induced, running considerably deep into the molten material from the surface thereof. Then, the electric power density at that part is reduced, and resultantly the molten material is lifted up solely by the magnetic pressure, rather than by the heating. As a result of this, the pressure applied to the tapping portion 2 by the molten material's own weight is reduced, and thereby the amount of molten material is reduced. Thus, as the amount of the molten material reduces, the amount of heat supplied from the molten material reduces, so that solidification of the molten material begins from its part contacting with the tapping portion 2 to allow the amount of molten material to further reduce and in turn allow the aperture at the tapping portion 2 to gradually reduce in diameter. By allowing the solidification of the material 13 to progress thoroughly, the tapping portion 2 can be closed completely to stop the tapping of the molten material. On the other hand, when the aperture at the tapping portion 2 reaches a predetermined diameter, the power supply to the second induction heating coil 6 may be switched from the solidification-use power source portion 11 to the melt-use power source portion 10. This can produce the result that after the reduction in diameter of the aperture at the tapping portion 2 is caused to stop, the diameter of the aperture is caused to increase in reverse. Thus, the control of the switching between the melt-use power source portion 10 and the solidification-use power source portion 11 can allow the aperture at the tapping portion 2 to be kept at a constant diameter, so as to take out a specified amount of the to-be-melted material 13. As mentioned above, the induction heating furnace of the embodied form has the first construction comprising the furnace body 1 (accommodating means) having the side wall 3 which extends so obliquely as to increase in radius from the bottom to the top edge portion and which is formed by a plurality of longitudinally split, conductive segments 4 being arrayed circumferentially with their being insulated from each other; the first induction heating coil 5 (the first coil means), arranged at the outer periphery side of the side wall 3, for subjecting the to-be-heated material 13 accommodated in the furnace body 1 to the induction heating; and the melt-use power source 7 (the first power means) for supplying AC power for the first induction heating coil 5. It is noted that as long as the induction heating furnace has the first construction, the furnace body 1 may be provided at its bottom with the tapping portion 2 for taking out the material 13 therefrom, as in the embodied form, or may be so modified that the material 13 can be taken out by tilting the furnace body 1, without providing the tapping portion 2. Also, as long as the side wall 3 extends so obliquely as to increase in radius from the bottom to the top edge portion, the side wall may have a linear form or a curved form. With the first construction, when the AC power is supplied to the first induction heating coil 5 from the melt-use power source 7, the alternating magnetic field 9 is produced by the induction heating coil 5, so that the material 13 accommodated in the furnace body 1 is subjected to the induction heating, to be melted. When the to-be-melted material 13 is melted into a molten material, the to-be-melted material 13 evaporates from the molten material surface 13a and also components of gas produced in the molten material dissipates therefrom. At that time, the rise of the evaporated material 13 and of the vaporized gas components is not obstructed by the side wall 3, because the side wall 3 extends so obliquely as to increase in radius from the bottom to the top edge portion. Accordingly, almost no evaporated material 13 contacts with the side wall 3 above the molten material surface 13a, so that the drawbacks caused by large amounts of material 13 adhering to the side wall 3 are reduced. Specifically, reduction in purity of the material 13 and impurities in component ratio, which are caused by a large amount of impurities containing deposits being dropped into the molten material, can be reduced and also labors required for the deposits to be eliminated can be reduced. In addition, since almost no gas evaporating on the molten material surface 13a and rising therefrom also contacts with the side wall 3, the resistance to the exhaust of gas can be reduced and resultantly the gas components in the molten material can be fully eliminated. The induction heating furnace of the embodied form has, in addition to the above-described first construction, the second construction comprising the tapping portion 2 formed at the bottom of the side wall 3; the second induction heating coil 6 (the second coil means), arranged at the outer periphery side of the tapping portion 2, for subjecting the to-be-heated material 13 to the induction heating; a tapping-use power source (the second power source means) 8 for supplying AC power to the second induction heating coil 6; and the power source control unit 12 (the power source control means) for controlling the tapping-use power source 8 so that the tapping portion 2 can be selectively switched between open and close by the melt and solidification of the to-be-melted material 13. It is noted that as long as the induction heating furnace has the second construction, the side wall 3 of the furnace body 1 may be so modified as to extend so obliquely as to increase in radius from the bottom until a certain point and then rise up vertically therefrom, as shown in FIG. 6. According to the second construction, when the to-be-melted material 13 accommodated in the furnace body 1 is subjected to the induction heating, the to-be-melted material 13 is melted into the molten material by the heating, while on the other hand, the molten material at a part contacting with the side wall 3 and a bottom wall of the furnace body 1 and the wall surface of the tapping portion 2 is cooled down into a solidified state. Thus, the control of the induction heating caused by the tapping-use power source 8 by the power source control unit 12 enables the tapping portion 2 to be closed by the solidified material 13 (the skull 14) when the to-be-melted material 13 is melted, but be opened by melting the skull 14 when the molten material of the to-be-melted material 13 is taken out. This enables the melt and take-out working of the to-be-melted material 13 to be facilitated and also enables the switching operation between the melt of the same and the take-out to be made with much ease. Also, the induction heating furnace of this embodied form includes the induction heating coil 16 separated into first induction heating coil 5 (the first coil means) and second induction heating coil 6 (the second coil means); and the power unit 17 having the melt-use power source 7 (the first power means) for supplying AC power to the first induction heating coil 5 and the tapping-use power source 8 (the second power means) for supplying AC power to the second induction heating coil 6. The melt-use power source 7 and the tapping-use power source 8 are separately controlled by the power control unit 12. This construction can provide the result that the melt of the to-be-melted material 13 caused by the induction heating by the first induction heating coil 5 and the take-out of the molten material produced by the induction heating by the second induction heating coil 6 can be made separately, thus providing an improved productivity. Further, the induction heating furnace of this embodied form includes the tapping-use power source 8 having the melt-use power source portion 10 for producing the first frequency of AC power to the extent that the to-be-melted material 13 is allowed to melt and the solidification-use power source portion 11 for producing the second frequency of AC power to the extent that the to-be-melted material 13 is allowed to solidify. The power source control unit 12 can allow the AC power to be output from the melt-use power source portion 10 when the tapping portion 2 is opened, while on the other hand, can allow the AC power to be output from the solidification-use power source portion 11 when the tapping portion 2 is closed. This enables the tapping portion 2 to be easily switched between open and close by switching between the melt-use power source portion 10 and the solidification-use power source portion 11 and also enables the tapping amounts to be easily adjusted by adjusting the time for supplying the first frequency of and the second frequency of AC power. In this embodied form of the invention, the induction heating coil 16 is separated into first induction heating coil 5 and second induction heating coil 6 so that the respective coils 5, 6 can be allowed to operate separately from each other, but this construction is not of restrictive. The induction heating furnace may be so modified, as shown in FIG. 7, as to comprise: an integrally formed induction heating coil 16 including the first induction heating coil 5 and the second induction heating coil 6; a melt-use/tapping-use power source 18 (power source means) capable to supply the AC power to the coil 16 at any selected frequency; and a power source control unit 12 capable to control the melt-use/tapping-use power source 18 such that the tapping portion 2 can be allowed to be closed by the solidified material 13 when the to-be-melted material 13 is melted and be opened by melting the solidified material 13 when the to-be-melted material 13 as molten is taken out. Next, a comparative arrangement will be described with reference to FIGS. 8 to 11. The same functional members as those in the first embodiment are given the same reference numerals and the description thereon will be omitted. As shown in FIG. 9, the induction heating furnace has the furnace body 1 (accommodating means) including the tapping portion 2 at the bottom and the side wall 3 extending so obliquely as to increase in radius from the bottom to the top edge portion and formed by a plurality of longitudinally split, conductive segments 4 being arrayed circumferentially with their being insulated from each other. At the outer periphery side of the furnace body 1 is provided the induction heating coil 16 through which the to-be-melted material 13 accommodated in the furnace body 1 is subjected to the induction heating, as shown in FIG. 8. The induction heating coil 16 is separated into first induction heating coil 5 disposed around the periphery of the side wall 3 and second induction heating coil 6 disposed around the periphery of the tapping portion 2. These coils 5, 6 are connected to the melt-use power source 7 and the tapping-use power source 8, respectively. The power unit 17 comprising the both coils 5, 6 is connected to the power source control unit 12. The above-described tapping portion 2 has a communication hole 2c extending vertically through the tapping portion with a constant diameter and an inductive short-circuit portion 2b at the bottom. The short-circuit portion 2b is electrically connected with each of the conductive segments 4 to suppress penetration of the alternating magnetic field 9 to the communication hole 2c, so as to allow the solidification of the to-be-melted material 13 to be accelerated. Also, a rod-like starting block 19 as cooled by cooling water and the like is movably inserted in the communication hole 2c of the tapping portion 2. The starting block 19 is provided, on its top surface, with an engaging portion 19a having an aperture progressively increasing in diameter from a top end to a bottom end. The engaging portion 19a is adapted to be engaged with the solidified material 13 to surely apply a drawing power to the to-be-melted material 13. The starting block 19 is connected with a drawing device 20 capable to move the starting block 19 up and down at any speed and timing. The remaining construction is identical to that in the first embodiment, so the description thereon is omitted. The operation of the induction heating furnace constructed as described above will be described below. The to-be-melted material 13 is thrown into the furnace body 1 and the furnace body 1 is cooled down by flowing the cooling water through the cooling water channel 4a, for completion of the preparation of melt. Then, the AC power is output to the first induction heating coil 5 and the second induction heating coil 6 by putting the melt-use power source 7 and the tapping-use power source 8 into on mode. When the coils 5, 6 are supplied with the AC power, the alternating magnetic field 9 is produced along the surface of the side wall 3 and the communication hole 2c at the tapping portion 2. The to-be-melted material 13, which is a block-like piece, is subjected to the induction heating by the alternating magnetic field 9, and thereby is melted from its surface. When contacting with the side wall 3, the tapping portion 2 and the starting block 19, the molten material 13 is solidified again by the cooling action of the side wall 3 and others, and thereby are formed into the skull 14. Thus, in the early stage of the melting, there presents a condition of the block-like, to-be-melted material 13 and the molten material 13 being mixed and placed on a large layer of skull 14, as shown in FIG. 10. Thereafter, when the induction heating continues to melt the entirety of the to-be-melted material 13, the to-be-melted material 13 as melted evaporates from the molten material surface 13a and also gas components containing impurities produced in the molten material rises up and dissipates from the molten material surface 13a, as shown in FIG. 8. At that time, the evaporation of the material 13 and the rise of the components of gas are not obstructed by the side wall 3, because the side wall 3 extends so obliquely as to increase in radius from the bottom to the top edge portion. Accordingly, almost no evaporating material 13 contacts with the side wall 3 above the molten material surface 13a, so that the adherence of the to-be-melted material 13 to the side wall 3 is reduced. In addition, since almost no gas components vaporizing on the molten material surface 13a also contact with the side wall 3, the resistance to the exhaust of gas is reduced and thereby gas in the molten material is fully eliminated. Next, when the molten material of the to-be-melted material 13 is taken out, the power supply from the tapping-use power source 8 to the second induction heating coils 6 is increased to the extent that the skull 14 can be allowed to be melt. Thereafter, the drawing device 20 is actuated to lower the starting block 19. When the starting block 19 is lowered, the drawing force of the starting block 19 is surely applied to the solidified material 13 engaged with the engaging portion 19a of the starting block 19, and thus the material 13 is lowered together with the starting block 19. The solidification of the to-be-melted material 13 is further accelerated in the short-circuit portion 2b of the tapping portion 2, and thereafter the to-be-melted material 13 developing into a desired solidification state is drawn out from the tapping portion 2, as shown in FIG. 11. As discussed above, the induction heating furnace has the structure comprising the starting block 19 for enabling the to-be-melted material 13 to be forcibly drawn out from the tapping portion 2; and the drawing device (drawing means) 20. This enables the to-be-melted material 13 to be forcibly drawn out from the tapping portion 2, to obtain the material 13 in a desired solidification state. The induction heating coil 16 may be composed of a single coil, rather than of the first induction coil 5 and the second induction heating coil 6. Also, the power unit 17 may be composed of a single power source, rather than of the melt-use power source 7 and the tapping-use power source 8. Next, the third embodiment of the invention will be described with reference to FIGS. 12 and 13. As shown in FIG. 12(A), the induction heating furnace of the third embodiment has a furnace body 31 comprising a cylindrical side wall 33 around which an induction heating coil 38 is wound and a flat plate-like bottom wall 34 forming the bottom of the side wall 33, and is formed by a plurality of longitudinally split, conductive segments being arrayed circumferentially with their being insulated from each other. On a lower surface of the bottom wall 34 is provided a bottom tapping mechanism 30 having an inverted-hollow-cone-shaped aperture 25 bored in the bottom wall 34 of the furnace body 31 and a tapping portion 21 provided in the aperture 25. As shown in FIG. 12(B) as well, an upper end portion of the tapping portion 21 is joined to the aperture 25. The tapping portion 21 comprises a funnel-shaped inlet portion 21a being wide at the top end and progressively narrowing toward the interior to a given width; and a hollow-pipe-like outlet portion 21b extending downward in continuation to the inlet portion 21a. The tapping portion is L-like in section and is formed into a funnel shaped as a whole. Also, as shown in FIG. 12(C), the tapping portion 21 is divided into a plurality of conductive segments 21s by a plurality of axially extending slits 22. Each of the segments 21s is provided at an inside thereof with a hollow portion 21c forming a cooling water passageway. To the end of the hollow portion 21c are connected a cooling water inlet pipe 21e and a cooling water outlet pipe 21f, as shown in FIG. 13. Around the outlet portion 21b and the inlet portion 21a of the tapping portion 21, induction heating coils 26b, 26a are respectively arranged along the outer surfaces thereof. These induction heating coils 26a, 26b are connected to an tapping-use power source 28 for producing AC power. The tapping-use power source 28 has a solidification-use power source portion 23 for producing the second frequency of AC power to the extent that the to-be-melted material 13 can be allowed to solidify and the melt-use power source 24 for producing the first frequency of AC power to the extent that the to-be-melted material 13 is allowed to melt. The first frequency of the melt-use power source portion 24 is set to be higher than the second frequency of the solidification-use power source portion 23. The tapping-use power source 28 is connected to a power source control unit 29 which is adapted to control the tapping-use power source 28 to be selectively switched between the operation of the solidification-use power source portion 23 and the operation of the melt-use power source portion 24. In the above-described construction, when the melt and the tapping are performed, the melt-use induction heating coil 38 arranged around the side wall 33 is energized to melt the to-be-melted material 13, as shown in FIG. 12(A). At the point in time at which the material 13 being progressively molten in the furnace body 31 develops into a specified melted condition, the tapping is started. Specifically, as shown in FIG. 13, the first frequency of high-frequency power is supplied from the melt-use power source portion 24 to the induction heating coils 26a, 26b. When the first frequency of high-frequency power is supplied to the lower induction heating coil 26a, the high-frequency alternating magnetic field is produced by the high-frequency power. The high-frequency alternating magnetic field 9 thus produced feeds eddy currents through only a thin solidification layer (penetration depth) on an inner surface of the outlet portion 21b. As a result of this, due to increasing electric power density in the thin solidification layer, the material 13 solidified on the inner surface of the outlet portion 21b of the tapping portion 21 melts from its surface and eventually the solidification layer drops down, and thereby the state of the tapping being enabled is brought about. On the other hand, the upper induction heating coil 26b induces the eddy currents for a thin layer of the solidification layer which is in contact with the conductive segments 21s of the inlet portion 21a. As a result of this, due to pseudo heat insulating function, the skull 35 at the inlet portion 21a is melted from its solidification interface contacting with the molten material, as shown in FIG. 12(A). In other words, the part of the material which is in contact with the conductive segments 21s is subjected to the induction heating to produce a pseudo heat insulating layer, by which heat absorption into the conductive segments 21s is suppressed to cause the melt to progress from the solidification interface 35". In addition, the flow V of the molten material at that part also encourages the reduction of the skull 35 at the inlet portion 21a, and eventually the skull 35 is reduced in thickness not only at the inlet portion 21a but also at the outlet portion 21b and is tapped by the pressure of the molten material. Next, when the tapping of the molten material is stopped, low-frequency power of, for example, a commercial frequency is supplied from the melt-use power source 24 to the induction heating coil 26a at the outlet portion 21b and the induction heating coil 26b at the inlet portion 21a, as shown in FIG. 13. A low-frequency magnetic field caused by the low-frequency power induces eddy currents which run considerably deep into the molten material layer from the surface thereof. As a result of this, the electric power density is reduced, solely by which the magnetic pressure is brought about in the molten material, rather than by the induction heating. Due to this phenomenon, the flow of the molten material is narrowed at and thus the flow rate is suppressed at the outlet portion 21b, while on the other hand, the effect of raising the molten material upward is produced at the inlet portion 21a. As a result of this, the downward pressure is reduced and thereby the tapping amount of the molten material is reduced. Thereafter, as the amount of the molten material passing through the tapping portion 21 reduces, the amount of heat supplied from the molten material reduces, so that the molten material begins to solidify from its part contacting with the conductive segments 21s at the inlet portion 21a. This causes further reduction of the amount of molten material, and eventually the tapping is stopped. In addition, the similar effect is produced by simply stopping the high-frequency power supplied from the melt-use power source 24. In this case, the skull around the inlet portion 21a layers increasingly, so that the aperture 25 to the outlet portion 21b comes to be blocked with the skull 35 to reduce the outflow of the molten material. As a result, the skull 35 increases further, so that the aperture 25 is eventually closed by the skull to stop the tapping, as in the case with the above. In the third embodiment, the side wall 33 is so provided as to extend vertically, but this is not of restrictive. The side wall may extend so obliquely as to increase in radius from the bottom 34 to the top edge portion. In this case, the adherence of the to-be-melted material 13 to the side wall 33 can be reduced, while also the gas in the molten material can be fully eliminated, as in the case of the first embodiment.

Claims: An induction heating furnace comprising: accommodating means (31) having a bottom (34), a tapping portion (21) formed at said bottom and being selectively switchable between open and close by melt and solidification of said to-be-melted material, and a side wall (33) formed by a plurality of longitudinally split conductive segments being arrayed circumferentially and being insulated from each other, for accommodating a to-be-melted material therein while cooling it; first coil means (38), arranged at an outer periphery side of said side wall (33), for subjecting said to-be-heated material accommodated in said accommodating means (31) to induction heating; second coil means (26a, 26b) arranged at an outer periphery side of said tapping portion (21) for subjecting said to-be-heated material to induction heating; first power source means for supplying AC power to said first coil means (38); second power source means (28) for supplying AC power to said second coil means (26a, 26b); and power source control means (29) for controlling said first power source means and said second power source means (26a, 26b) independently, characterised in that said second power source means (28) comprises a melt-use power source portion (24) for producing a high-frequency of AC power to the extent that the to-be-melted material can be allowed to melt and a solidification-use power source portion (23) for producing a low-frequency of AC power to the extent that the to-be-melted material is allowed to solidify, and in that said power source control means (29) controls said second power source means (28) such that, when said tapping portion (21) is opened, the AC power is produced from said melt-use power source portion (24), while on the other hand, when said tapping portion (21) is closed, the AC power is produced from said solidification-use power source portion (23). An induction heating furnace according to Claim 1, wherein said tapping portion (21) comprises an inlet portion (21a) being wide at a top end thereof and gradually narrowing toward a bottom end thereof; and a hollow-pipe-like outlet portion (21b) extending downward in continuation to said inlet portion (21a). An induction heating furnace according to claim 2, wherein said second coil means (26a, 26b) comprises an upper induction heating coil (26b) arranged around said inlet portion (21a) and a lower induction heating coil (26a) arranged around said outlet portion (21b). A furnace tapping method for tapping a bottom tapping mechanism (30) of an induction heating furnace , said bottom tapping mechanism comprising a funnel-shaped tapping portion (21) arranged at a bottom of an accommodating means (31) for accommodating therein a molten material of a to-be-melted material and comprising an inlet portion (21a) and a hollow-pipe-like outlet portion (21b) integrally formed with and located below said inlet portion, said tapping portion (21) being divided into a plurality of segments (21s) by a plurality of slits (22) which are continuous to each other and being connected to cooling water feed/discharge pipes (21e, 21f), induction heating coil means (26a, 26b) arranged around said tapping portion (21) at the inlet portion (21a) and the outlet portion (21b), respectively; and a power source (28) for supplying power to said coil means (26a, 26b), said method being characterized in that , when the tapping of the molten material is done, high-frequency power is supplied to said induction heating coil means (26a, 26b) arranged around said tapping portion (21) at said inlet portion (21a) and at said outlet portion (21b), respectively, and when the tapping of the molten material is stopped, low-frequency power is supplied thereto.

Inventors: NAKAI YASUHIRO, OKUNO ATSUSHI, TSUDA MASANORI, NAKAI, YASUHIRO, OKUNO, ATSUSHI, TSUDA, MASANORI, Nakai, Yasuhiro, c/o Ise Works of Shinko El. Co L., Okuno, Atsushi, c/o Ise Works of Shinko El. Co Lt., Tsuda, Masanori, c/o Ise Works of Shinko El. C.L.
Applicants: SHINKO ELECTRIC CO LTD, SHINKO ELECTRIC CO. LTD.
Application year: 1998
Main classification: F27B14
IPC codes: C21C5, F27B14, F27D3