Patent Publication Number: US-7594950-B2

Title: Method and device for removing slag

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and a device for removing slag, which is generated on a surface of molten metal in a metal furnace. 
     2. Description of the Related Art 
     When a metal material is liquidized in a liquidizing furnace by heating in a vacuum atmosphere so as to form a molten metal and cast into a mold for precise casing, slag is generated on a surface of the molten metal. Since it is difficult to remove the slag, the molten metal is cast in the mold with the slag, the slag affects the quality of the produced cast products. Particularly in producing precise cast products (for example, parts for air and space industry), any slag is not allowed to be contained. Thus, any contaminant of the slag significantly affects the yield of the cast products particularly in the precise cast products field. 
     Heretofore, when the molten metal is cast into the mold, slag is conventionally prevented from entering into the mold by the following methods: One method is to remove the slag by tilting the furnace to the other side of the mold for removing the slag before pouring the molten metal into the mold. Another method is to remove the slag by installing a filter at an entrance of the mold for filtering the slag. 
     However, even though the slag can be removed by tilting the furnace to the other side of the mold before pouring the hot melt into the mold, it is difficult to remove the slag perfectly. By installing a filter at entrance of the mold for filtering the slag, it is easy to remove a larger size of the slag, but it is difficult to remove a smaller size of the slag. If trying to remove the smaller size of the slag by the filter, it is required to make the filter finer, however when making the filter finer, it causes slower of flowing speed of the molten metal into the mold and generates bad products because of lack of the molten metal flowing into the mold. Further, the filter chips easily, and when chip of the filter is entered into the mold with hot melt, it generates bad products because of contaminants of filter chip. 
     Further, a method of removing slag, which is floating on a surface of the molten metal, in air atmosphere is known as Japanese laid-open patent publication No. H5-240588. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method and a device for removing slag, which is capable of removing slag efficiently in a liquidizing furnace for precise-casting, in which any contaminant of slag is not allowed. 
     To achieve the above object, there is provided in accordance with the present invention, a method and a device for removing slag comprises traveling a filter of heat-resistant porous ceramics along a surface of the molten metal and removing the slag, which is generated by liquidizing metal material by heating and floating on the surface of the molten metal in the furnace. 
     According to the present invention, since the filter passes the molten metal and catches slag only, therefore, by traveling the filter along a surface of the molten metal, on which slag is floating, the filter can catch slag and remove slag from the molten metal effectively. Thus, by pouring the molten metal without any contaminants of slag into a mold directly, precise cast products can be produced with high yield. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, which illustrate a preferred embodiment of the present invention by way of example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of essential portion of a vacuum liquidizing and casting apparatus and relating to a step of liquidizing metal and removing slag; 
         FIG. 2  is a perspective view showing a step of traveling a filter along a surface of molten metal and removing the slag according to an embodiment of the present invention; 
         FIG. 3  is a view of essential portion of a vacuum liquidizing and casting apparatus and relating to a step of pouring the molten metal into a mold for casing; 
         FIGS. 4A and 4B  is a perspective view of examples of filters of the present invention; 
         FIG. 5  is a view of an example of a driving device for traveling the filter of the present invention; 
         FIGS. 6A and 6B  is a plan view of examples of traveling paths of filters in a crucible of the present invention 
     
    
    
     DETAILS DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments will be described with referring to the attached drawings. Like or corresponding parts are denoted by the same reference characters throughout views, and will not repetitively be described. 
       FIG. 1  shows an essential portion of a vacuum liquidizing and casting apparatus. The vacuum liquidizing and casting apparatus  11  is provided with a furnace  14  for liquidizing metal such as iron/nickel alloy by heating and mold  15  for casting precise cast products by pouring molten metal generated by the furnace  14 . The furnace  14  is provided with crucible  12  and coil  13  for liquidizing metal material loaded in crucible  12  by induction heating. Inside of vacuum liquidizing and casting apparatus  11  is evacuated to vacuum atmosphere. Metal material is loaded into crucible  12  from loading gate (not shown) and heated up to liquidizing temperature by induction heating of applying high frequency current to coil  13 , and then liquidized to be a molten metal A. Since the molten metal A is formed in vacuum atmosphere, oxidation of the molten metal A is relatively less, and contamination of the molten metal A is quite less, and then pure molten metal A is formed. However, slag is generated in process of liquidizing the metal material by reactions of the molten metal A and crucible  12 , and so on. 
     When temperature of the molten metal A reaches to a predetermined temperature, induction heating is stopped and molten metal A becomes settled down to be a quiet state. Then, since the specific gravity of the slag is lower, the slag becomes floating on the surface of the molten metal A. As shown in  FIG. 2 , by traveling the filter  21  for filtering slag along the surface of molten metal A, filter  21  catch slag floating on the surface of molten metal A, and then slag is removed from the molten metal A. Since filter  21  for filtering slag passes the molten metal and catches the slag only, filter  21  effectively catches and removes slag floating on the surface of molten metal A. 
     Molten metal A, from which slag has been removed, is poured directly (without passing through filter) to mold  15  as shown in  FIG. 3  and cast product  16  is produced. Since filter is not provided at entrance of mold  15  for filtering slag, molten metal A flows fast inside of mold  15  and fluidity of hot melt A in mold  15  is excellent. For example, very thin blade for air and space industry parts can be cast with excellent precise-formability. Further, since filter is not provided at entrance of mold  15  for filtering slag, a problem that the filter at the entrance of the mold easily chips and the fragment of the filter enters into the mold  15  with the molten metal A never happens. Therefore, since the molten metal A without slag can be poured directly to mold  15 , excellent fluidity of the molten metal A in mold  15  can be obtained and precise cast products such as a thin blade for air and space industry parts can be produced at high production yield. 
       FIGS. 4A and 4B  show examples of filter structures of the present invention. Material of filter  21  comprises highly heat-resistant porous ceramics and has been used for the filter, which is installed at entrance of the mold for removing slag as stated in “BACKGROUND OF THE INVENTION”.  FIG. 4A  shows an example of disk-shaped filter and  FIG. 4B  shows an example of plate-shaped filter having concave portion, which can catch the slag easily. These filters  21  travel along the surface of the molten metal A such that its lower half portion of filter  21  is immersed in the molten metal A and its upper half portion of filter  21  is shown above the surface of the molten metal A. Filter  21  is provided with an arm  22 , and arm  22  is fixed to filter  21  by fixing element  23  such as pin. Arm  22  drives filter  21  to travel along the surface of molten metal A. Further, the shape and filtering performance of filter  21  should be determined in correspondence with kinds of the molten metal and so on. 
       FIG. 5  shows an example of a device for traveling filter  21  and removing slag on the surface of the molten metal A. The device comprises: a rod  31  disposed above central, portion of crucible  12 , which accommodates the molten metal A; an outer cylinder  32  coaxially disposed with rod  31 , wherein cylinder  32  is rotatable and vertically movable with rod  31 , and also rod  31  is vertically movable relatively against cylinder  32 ; a driving device  33  for moving rod  31  vertically relatively against cylinder  32 ; and a driving device  34  for moving and rotating rod  31  and cylinder  32  as one unit. O-ring  36  seals rod  31  and cylinder  32  is sealed by O-ring  37 , and then vacuum atmosphere can be maintained in vacuum liquidizing and casting apparatus  11 . 
     One end of arm  22  is rotatably fixed to the lower end of cylinder  32 . Arm  22  is provided with a long-width hole  26 , and a pin  24  disposed at the lower end of rod  31  is engaged to long-width hole  26  and then arm  22  is slidably fixed to rod  31 . Another end of arm  22  is fixed to filter  21  by fixing element  23 . Therefore, by moving rod  31  vertically relatively against cylinder  32 , arm  22  can be rotated around pin  26  and radial position of filter  21  in crucible  12  is determined. Moving rod  31  and cylinder  32  vertically as one unit, vertical position of filter  21  in crucible  12  is determined. Rotating rod  31  and cylinder  32  around rod  31  as one unit, filter  21  travels rotating (in circumference direction) around rod  31  on the surface of hot melt A in crucible  12 . 
       FIGS. 6A and 6B  show examples of traveling paths of the filters for removing the slag floating on a surface of the molten metal in a crucible.  FIG. 6A  shows that filter  21  travels spirally in crucible  12  from start position Sa at center of crucible  12  to stop position Sb at peripheral portion of crucible  12 . According to this traveling path pattern, filter  21  catches and removes the slag all over the surface of the molten metal A in crucible  12 .  FIG. 6B  shows that filter  21  moves in radial direction from start position Sa at center of crucible  12  to circumference path C 1  and travels along path C 1 , next moves in radial direction from path C 1  to path C 2  and travels along path C 2 , and next moves in radial direction from path C 2  to path C 3  and travels along path C 3  to stop position Sb at outer portion of crucible  12 . Also according to this traveling path pattern, filter  21  catches and removes slag all over the surface of molten metal A in crucible  12 . 
     The device for traveling filter and removing slag can be operated at air atmosphere or at vacuum atmosphere. Also, the device can be operated manually or automatically by pre-inputting total quantity of metal to be liquidized and size of crucible, for example. In vacuum liquidizing and casting process, the process roughly comprises liquidizing metal, measuring temperature of the molten metal, settling down of the molten metal, removal of slag, measuring temperature of the molten metal, and casting the molten metal into a mold to produce cast products. The device can be operated alone or in combination with a temperature measuring equipment (thermo-couple thermometer, radiation thermometer) and switching each process in several seconds while keeping vacuum atmosphere. 
     Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.