Patent Publication Number: US-2009223646-A1

Title: Apparatus for melting metal and method for manufacturing metal

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for melting metal materials and relates to a method for producing metals using the apparatus. Specifically, the present invention relates to a technique for preventing melting failure and surface defects, which may form on an ingot portion at the initial melting stage for the production of an ingot of the metal material in an electron-beam melting furnace or a plasma-arc melting furnace. 
     BACKGROUND ART 
     It is known, as a technique for producing an ingot, that a melting furnace provided with a water-cooled copper crucible may be used, and that a perpendicularly movable water-cooled base unit may be disposed at the bottom of the crucible. While an electron beam is emitted on a metal material in a vacuum atmosphere, and a molten metal material is poured or is dropped into the water-cooled base unit, the water-cooled base unit is continuously or intermittently pulled down so as to produce an ingot of the metal material. 
     The molten metal contacts the water-cooled base unit and is solidified from the portion at the start of melting of the metal material. The molten metal is piled up and is solidified on the approximately entire surface of the water-cooled base unit after the molten metal is fed for a certain time. Then, an electron beam is emitted on the metal material piled up on the water-cooled base unit so as to melt the entire surface, and the water-cooled base unit is pulled down. Since the level of the molten metal is lowered according to the pulling down of the water-cooled base unit, a molten metal melted by an electron beam is further fed into the water-cooled cooper crucible. Thus, by pulling down the water-cooled base unit and continuously feeding a molten metal, ingots may be successively produced. 
     The water-cooled base unit has a flat surface, and the molten metal that is poured thereinto is solidified in a short time at the location at which the molten metal falls. When more molten metal subsequently falls on the solidified metal, the molten metal flows in any direction and is solidified in a short time. In the initial stage of the melting, the falling of the molten metal into the water-cooled base unit and solidification of the molten metal as described above occur repeatedly. Therefore, a bottom portion of the ingot produced in the initial stage of the melting (hereinafter called an “initial molten metal portion”), specifically, an ingot portion contacting the water-cooled base unit, is formed with portions that are insufficiently melted and have surface defects. Since there may be a case in which the portions that are insufficiently melted and have surface defects will be an obstacle in the subsequent working process, the portions that are insufficiently melted and that have surface defects are removed in advance by machining or cutting. This process reduces the process yield of the ingot and thereby requires improvement. 
     A technique for preventing the melting failure and the surface defects is known. In this technique, an electron beam is emitted on a molten metal immediately after the molten metal falls on the water-cooled base unit so as to maintain the initial molten metal portion in a melted state. In this case, the electron beam is emitted on a location in which the molten metal fell on the water-cooled base unit so as to form a molten metal and to maintain the melted state. Therefore, the fallen molten metal solidifies while an irradiation location of the electron beam thereto is adjusted. 
     Regarding this problem, a technique for preventing the solidification of the initial molten metal portion has been proposed. In this technique, an electron beam is emitted on a wide surface including the fallen molten metal on the water-cooled base unit, thereby temporarily preventing a part of the initial molten metal from solidifying. In this case, the electron beam is emitted on the surface of the water-cooled base unit which is not covered with the molten metal, and there may be a case in which the surface of the water-cooled base unit is damaged by melting. Therefore, this technique requires improvement. 
     As a method for solving the above problems, a technique is disclosed in Japanese Unexamined Patent Application Publication No. 2000-274957, for example. In this method, a metal block material having the same grade as that of a metal to be melted is disposed on the water-cooled base unit, and melting is started after the metal block material is irradiated with an electron beam so as to form sufficient molten metal surface. According to this method, the charged molten metal does not solidify before an electron beam is emitted thereon, and the water-cooled base unit is not damaged even when the electron beam is emitted on an area including the fallen molten metal. The patent document discloses only a method in which feeding of a molten metal into a mold is started after a molten metal surface is formed on the top of the metal block that is previously disposed on the water-cooled base unit. On the other hand, the patent document does not disclose a method for forming an initial molten metal portion. Accordingly, the portion corresponding to the metal block disposed on the water-cooled base unit should be removed, whereby the resultant yield is decreased, and the additional costs are incurred for the removal. 
     Moreover, a technique for continuously casting an ingot is disclosed in Japanese Unexamined Patent Application Publication No. 2000-153345, for example. In this technique, a water-cooled base unit having a wedge-shaped portion that is engageable with an ingot is provided on the bottom of a mold. The wedge-shaped portion and an initial molten metal portion of an ingot are engaged when the molten metal falls on the water-cooled base unit, and the water-cooled base unit is pulled down after the ingot is solidified. 
     Japanese Unexamined Patent Application Publication No. 2000-153345 does not disclose a method for forming an initial molten metal portion and a method for preventing portions that are insufficiently melted and have surface defects occurring on an initial molten metal portion. As described above, there have been no effective methods for solving the problem relating to the formation of an initial molten metal portion in the production of ingots. 
     DISCLOSURE OF THE INVENTION 
     The present invention has been completed in view of the above circumstances. An object of the present invention is to provide a method for solving the above problem remaining in the conventional techniques and to produce ingots on the more superior conditions of the process yields. That is, the present invention provides a method for melting a metal material while preventing melting failure and surface defects, which will form on an initial molten metal portion at the start of melting. 
     The inventors performed intensive research so as to solve the problem remaining in the above conventional techniques. As a result, the inventors found the following, and the present invention has thereby been completed. That is, a melting furnace comprising a water-cooled copper mold is used, and the bottom of the water-cooled copper mold is disposed with a water-cooled base unit for pulling down an ingot that is produced. The water-cooled base unit is provided with a concave portion on the surface thereof and is provided with an incline on the surface surrounding the concave portion of the base unit. Therefore, molten metal that falls or drops into the mold can be collected at the concave portion provided at the water-cooled base unit. 
     That is, the present invention provides an apparatus for melting metals comprising a hearth for melting a metal raw material and a mold into which a molten metal is poured so as to form an ingot. The mold has a bottom and is provided with a base unit for pulling down the ingot at the bottom. The base unit has a surface and is provided with a concave portion at an optional location of the surface, and the surface surrounding the concave portion of the base unit is inclined toward the concave portion. 
     Moreover, in the present invention, another concave portion is provided to the concave portion at the bottom on the surface of the base unit. The base unit to be pulled down the ingot has a separable structure so that the ingot formed thereon can be pulled out. 
     According to the apparatus for melting metals of the present invention having the above structure, the surface of the base unit arranged at the bottom of the mold is provided with a concave portion, and the surface of the base unit other than the concave portion is inclined toward the concave portion. Therefore, a molten metal, which is poured from the hearth and reaches the base unit at the beginning, flows into the concave portion first according to the incline. Then, the following molten metals fed are filled in the mold in order from the concave portion and are solidified by cooling. As a result, surface defects and the portions caused by the insufficient melting conditions are effectively reduced and may not occur on the initial molten metal portion of an ingot that is produced. 
     The molten metal can be collected at the bottom of the concave portion on the base unit surface, thereby further which reduces the surface defects and the portions caused by the insufficient melting conditions. 
     Moreover, since the base unit for pulling down the ingot has a separable structure, the ingot produced on the base unit can be easily pulled out after melting of the metal raw material is completed. 
     In the conventional methods, the melting failure and the surface defects of the initial molten metal portion are removed by cutting. In contrast, in the present invention, the surface defects and the portions that are insufficiently melted hardly occur on the initial molten metal portion as described above. Therefore, the process yield of the ingot can be improved compared to the conventional methods. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a schematic sectional view of an electron-beam melting apparatus of the present invention. 
         FIG. 2  is a schematic sectional view showing a modification example of a water-cooled base unit of the present invention. 
         FIG. 3  is a schematic sectional view showing another modification example of a water-cooled base unit of the present invention. 
         FIG. 4  is a schematic sectional view showing another modification example of a water-cooled base unit of the present invention. 
         FIG. 5  is a schematic sectional view showing another modification example of a water-cooled base unit of the present invention. 
         FIG. 6  is a schematic sectional view showing a conventional water-cooled base unit. 
     
    
    
     EXPLANATION OF REFERENCE NUMERALS 
       1  denotes a device for feeding raw materials,  2  denotes a titanium sponge,  3  denotes a cold hearth,  4  denotes a molten metal,  5  denotes a water-cooled copper mold,  6  denotes an electron beam gun,  7   a  to  7   e  denote water-cooled base units (examples of the present invention),  7   f  denotes a water-cooled base unit (example used in conventional techniques),  71  denotes an inclined portion of a base unit,  72  denotes a concave portion of a base unit,  73  denotes an inverse tapered portion,  74  denotes a bottom of a base unit,  75  denotes an inclined bottom portion,  76  denotes a horizontal bottom portion, and  77  denotes an inclined bottom portion. 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Preferred embodiments of the present invention will be described in detail with reference to the figures. 
       FIG. 1  shows a preferred embodiment for producing a titanium ingot from a titanium sponge, which is a raw material to be melted, by using an electron-beam cold hearth melting furnace. The reference numeral  1  indicates a device for feeding raw materials, by which a titanium sponge  2  that is a raw material is fed. In downstream of the device  1  for feeding raw materials, there is provided a cold hearth  3  which is made of a water-cooled copper and contains a molten metal  4  including molten titanium. A water-cooled copper mold  5  is provided downstream of the cold hearth  3 , and the whole of the cold hearth  3  is obliquely arranged, whereby the molten metal  4  can be poured from the cold hearth  3  into the water-cooled copper mold  5 . An electron beam gun  6  is provided over the cold hearth  3  and the water-cooled copper mold  5 , and an electron beam is emitted therefrom so as to melt the titanium sponge  2 . 
     The water-cooled copper mold  5  is provided with a water-cooled base unit  7   a  at the bottom. As shown in  FIG. 1 , a concave portion  72  of the base unit is formed at the center of the water cooled base unit  7   a , and the periphery thereof is formed with an inclined portion  71  of the base unit that is inclined toward the concave portion  72  of the base unit. The concave portion  72  of the base unit comprises an inverse tapered portion  73 , which forms the side wall of the concave portion  72  of the base unit, and a bottom  74  of the base unit. As shown in  FIG. 1 , the inverse tapered portion  73  is inclined from the perpendicular direction so that the inverse tapered portion  73  and the bottom  74  of the base unit form an acute angle. 
     Then, operation methods of the hearth electron-beam melting furnace will be described as follows. The bottom of the cold hearth  3  is disposed with a solid layer of a titanium called a “skull” (not shown in the figure) before the melting of the titanium is started, and an electron beam is emitted on the skull so as to form a molten metal  4 . Then, a titanium sponge  2  is fed into the cold hearth  3  by the device  1  for feeding raw materials, and the electron beam is emitted on the titanium sponge  2  so that the titanium sponge  2  is melted and is mixed with the molten metal  4 . After the molten metal  4  is flown and is refined in the cold hearth  3 , the molten metal  4  is poured into the water-cooled copper mold  5 . 
     The molten metal  4  poured into the water-cooled copper mold  5  reaches the water-cooled base unit  7   a  that is disposed at the bottom of the water-cooled copper mold  5 . A portion of the molten metal  4 , which reached the water-cooled base unit  7   a , reaches the concave portion  72  of the base unit and is solidified after the molten metal  4  is cooled by the bottom portion  74  of the base unit for a while. The other portion of the molten metal  4 , which reached the water-cooled base unit  7   a , reaches the inclined portion  71  of the base unit and rapidly flows into the concave portion  72  of the base unit according to the incline of the inclined portion  71  of the base unit. Then, the molten metal  4  is solidified in the same way as that of the above molten metal  4 . Thus, the molten metal that reaches any portion of the base unit flows into the concave portion  72  of the base unit, thereby being solidified. The molten metal  4  is further fed into the water-cooled copper mold  5  until the molten metal  4  approximately covers the inclined portion  71  of the base unit, and the molten metal  4  is solidified so as to form an initial molten metal portion of titanium ingot. In this case, the molten metal may be solidified in a short time immediately after the molten metal is fed into the concave portion  72  of the base unit at the beginning, because the water-cooled base unit is not sufficiently heated by the heat of the molten metal itself and is in a low-temperature condition. Therefore, the output level of the electron beam for irradiation is preferably increased. 
     After an initial molten metal portion is formed so as to cover the concave portion  72  of the base unit and the inclined portion  71  of the base unit, the water-cooled base unit  7   a  is pulled down so as to expand the space over the water-cooled copper mold  5 , and the molten metal  4  is further fed to the space. Thus, the water-cooled base unit  7   a  is pulled down while the molten metal  4  is fed into the water-cooled copper mold  5 , whereby the molten metal is cooled and is solidified in order from the lower portion to the upper portion of the mold. As a result, titanium ingots can be successively produced. When the water-cooled base unit  7   a  is pulled down, since the inverse tapered portion  73  in the concave portion  72  of the base unit engages with the initial molten metal portion of titanium ingot, the initial molten metal portion and the water-cooled base unit  72  will not be separated, and the titanium ingot can be pulled down. 
     Other preferred modification examples of the structural member of the present invention will be described. 
     The inclined portion of the base unit on the water-cooled base unit of the present invention is preferably inclined from the periphery toward the center thereof. 
     The inclined portion of the base unit is preferably formed to have an inclination angle of 2 to 10° with respect to the horizontal surface when a titanium is used as a molten metal. By forming the inclined portion of the base unit to have an inclination angle of the above range, the molten metal which is fed can be uniformly poured into the center portion of the water-cooled base unit. When the inclination angle is less than 2°, it is difficult to rapidly pour the molten metal to optional locations of the water-cooled base unit due to the viscosity of the molten titanium. When the inclination angle is more than 10°, the ratio of the initial molten metal portion to the ingot produced is increased, whereby the process yield may be decreased. It should be noted that the lower limit of 2° of the inclination angle range is effective when titanium is to be ingoted, and the lower limit is selected according to the viscosity of a metal to be ingoted. 
     As another preferred embodiment of the present invention, the following steps may be used. Titanium sponge is previously disposed on the water-cooled base unit, and an electron beam is emitted thereon so as to melt the titanium sponge. Then, the molten titanium sponge is poured into the concave portion of the base unit so as to form an initial molten metal portion. According to this embodiment, the titanium sponge covers the water-cooled base unit, whereby the water-cooled base unit is not damaged by irradiation by the electron beam in forming an initial molten metal portion. 
       FIG. 2  shows another preferred embodiment relating to the present invention. That is,  FIG. 2  shows a modification example of the water-cooled base unit  7   a  of  FIG. 1 . In the water-cooled base unit  7   b , the concave portion  72  of the base unit is not provided with a horizontal bottom, but is instead provided with an inclined portion  75  that is inclined toward the center portion. According to the water-cooled base unit  7   b  having such a structure, the molten metal poured thereinto first is led to the center of the concave portion of the base unit and can be solidified in sequence while preventing the molten metal from solidifying at random portions. 
       FIG. 3  shows another preferred embodiment of the water-cooled base unit  7   a . The water-cooled base unit  7   c  further comprises an inclined portion  75  and a horizontal portion  76 . By forming such a concave portion, the bottom of the initial molten metal portion of an ingot that is produced can be formed with a convex portion having a gentle curve. Therefore, the initial molten metal portion produced by using the water-cooled base unit  7   c  can be handled more easily than that produced by using the water-cooled base unit  7   b  shown in  FIG. 2 . 
       FIG. 4  shows a further preferable embodiment of the water-cooled base unit  7   a . The water-cooled base unit  7   d  comprises a concave portion  72  of the base unit having a bottom  77 , and the bottom  77  is flat and is inclined toward the right of the paper surface. According to the water-cooled base unit  7   d  having such a structure, the molten metal that is poured from the hearth into the base unit can be led along the inclination direction of the bottom of the concave portion  72  of the base unit. As a result, the molten metal is solidified in order from the lowest end portion of the bottom  77 , whereby macroscopic defects do not form on the solidified portion of the molten metal, and an ingot with a good quality can be produced. 
       FIG. 5  is a plan view showing a water-cooled base unit that relates to the present invention and is observed from the top, and the water-cooled base unit has a rectangular shape. That is, an ingot that is produced in this embodiment has a rectangular shape in cross section. In this embodiment, the concave portion  72  of the base unit is formed into a trapezoidal shape. The line L-L′ indicates a separation line of the mold. The water-cooled base unit  7   e  has a structure that can be separated into two portions ( 7   e - a  and  7   e - b ) at the separation line. Specifically, the lower base unit of the trapezoidal shape portion is preferably arranged at the side of the separation line. In this case, the lower base unit is longer than the upper base unit, and each side connecting the upper base unit and the lower base unit preferably has an angle in a range of 30 to 60° with respect to the horizontal line from a practical point of view. 
     According to such an arrangement, after a predetermined amount of an ingot is produced, the water-cooled base unit  7   e - a  is separated from the water-cooled base unit  7   e - b , and the ingot piled up on the upper surface of the water-cooled base unit  7   e  is slid in a direction perpendicular to the line L-L′. As a result, the ingot formed with a fitting portion that corresponds to the concave portion can be separated from the water-cooled base unit  7   e.    
     The separable structure of the water-cooled base unit as described above is preferably applied to the above water-cooled base units  7   a  to  7   d . Such a separable structure facilitates pulling out of the ingot, which is produced, from the water-cooled base unit. 
     The horizontal cross section of the water-cooled base units  7   a  to  7   e  may be formed into a circular shape as well as the above rectangular shape so as to form an ingot having a circular shape. 
     One of the water-cooled base units  7   a  to  7   e  as described above is disposed in the water-cooled copper mold  5  before melting of the metal raw material. Therefore, a molten metal poured from the hearth is appropriately led to the concave portion, whereby the initial molten metal portion and the water-cooled base unit can be strongly engaged. Moreover, the formed ingot can be reliably pulled out from the water-cooled base unit after the melting is completed. 
     EMBODIMENTS 
     Hereinafter, the present invention will be specifically described with reference to embodiments. The embodiments are example of the preferred embodiment of the present invention, and the present invention is not limited thereto. 
     First Embodiment 
     The water-cooled base unit  7   c  shown in  FIG. 3  was mounted to the water-cooled copper mold  5  of the electron-beam melting apparatus in  FIG. 1 . Then, the cold hearth  3  was irradiated with an electron beam and was fed with a titanium sponge  2  so as to form a molten metal  4 . The molten metal  4  was fed into the water-cooled copper mold  5 , and a titanium ingot was produced. The ingot produced was cooled and was separated from the water-cooled base unit  7   c  by cutting. The structure at the bottom of the ingot that was cut had a good quality and had the same quality as that of a material that can be used for a hot forging. The process yield of the collected titanium ingot was 98% with respect to the theoretical yield that was calculated from the input amount of the raw material. 
     Second Embodiment 
     An ingot was produced under the same conditions as those of the First Embodiment, except that the water-cooled base unit  7   d  was used instead of the water-cooled base unit  7   c . As a result, the process yield of the collected titanium ingot was 98% with respect to the theoretical yield that was calculated from the input amount of the raw material. 
     Comparative Embodiment 
     An ingot was produced under the same conditions as those of the First Embodiment, except that the conventional water-cooled base unit  7   f  shown in  FIG. 6  was used. The titanium ingot that was produced was separated from the water-cooled base unit by cutting. The cut surface of the separated titanium ingot was examined, and there were portions which were not sufficiently melted, and the portions were thereby cut off. As a result, the net process yield of the titanium ingot was only 95%. 
     INDUSTRIAL APPLICABILITY 
     According to the method for melting a metal material by using an electron beam of the present invention, melting failure and surface defects of the initial molten metal portion of an ingot can be reduced. Therefore, the process yield in producing an ingot can be further improved.