Patent Publication Number: US-6984357-B2

Title: Apparatus and method for holding molten metal in continuous hot dip coating of metal strip

Description:
BACKGROUND OF THE INVENTION 
   (a) Field of the Invention 
   The present invention relates to an apparatus for holding molten metal in continuous hot dip coating of a metal strip. More particularly, the present invention relates to a molten metal holding apparatus for the continuous hot dip coating of a metal strip, in which a metal strip is passed through a vessel filled with a molten coating metal and an electromagnetic field is used during the coating process to stably float the molten metal. 
   (b) Description of the Related Art 
   In continuous hot dip coating of metal strips, metal strips are continuously passed through a vessel filled with a molten metal, which is used as a coating solution. As shown in  FIG. 15 , in the conventional continuous hot dip coating method, a vessel  83  is filled with a molten metal  81 , which is obtained by melting a metal by using as a metal solution aluminum, zinc, or an alloy of these metals, and a metal strip  89  that is continuously supplied to the vessel  83  using a sink roll  85  and a stabilizing roll  86  is dipped in the molten metal  81 , after which the metal strip  89  is removed from the vessel  83 . 
   The sink roll  85  acts to change a direction at which the metal strip  89  travels, and the stabilizing roll  86  acts to adjust the conveying state of the metal strip  89 . The sink roll  85  and the stabilizing roll  86  are submerged in the molten metal  81  in the vessel  83 , and axis members of the sink roll  85  and the stabilizing roll  86  are supported by a sleeve-bush configuration and without the use of lubrication as a result of the high temperature environment of inside the vessel  83 . 
   At this time, parts forming the sink and stabilizing rolls  85  and  86  react with the molten metal  81  to generate metal compounds. If impurities created as a result adhere to a surface of the metal strip  89 , the metal strip  89  is compressed in this state to reduce the quality of the metal strip  89 . 
   Further, the rotation of the axis members of the sink and stabilizing rolls  85  and  86  without the use of lubricant results in wear of the axis members. This causes the metal strip  89  to vibrate to thereby result in defects such as a streaked pattern formed on the metal strip  89  or differences in the amount of coating. 
   To solve such problems, it is necessary to use a vessel structure in which such rolls are not submerged in the molten metal. In this regard, a molten metal process is disclosed that eliminates the use of metal strip support rolls that are submerged in the molten metal. In such a process, an opening through which the metal strip is supplied is formed in a lower section of a vessel. A metal strip to be plated is supplied to a lower portion of the molten metal through the opening then removed from the vessel through an upper section thereof. A configuration for preventing the molten metal from exiting through the opening is provided. 
   With regard to the configuration for preventing the molten metal from exiting through the opening in such a process where rolls submerged in molten metal are not used, Japanese Patent Laid-Open No. 63-109148 discloses a method in which gas pressure obtained by a gas pressure chamber mounted in the vicinity of the opening of the vessel is used to support the weight of the molten metal so that it floats. Also, Japanese Patent Laid-Open No. 63-303045 discloses a method in which a direct-current (DC) magnet is mounted in the area of the opening to supply a direct current to the molten metal such that it floats by the generated electromagnetic force. 
   In addition, U.S. Pat. No. 5,665,437 and Japanese Patent Laid-Open No. 63-310949 mount a linear induction motor in the area of the opening of the vessel to form a traveling magnetic field. The electromagnetic force formed as a result floats the molten metal. U.S. Pat. No. 5,897,683 discloses a holding method that uses an electromagnetic force generated by an alternating-current (AC) electromagnet mounted in the vicinity of the opening of the vessel and a conducting block in a specific area of the vessel, and uses a gas pressure obtained by providing a gas pressure chamber below the opening so that the molten metal does not exit the opening. 
   However, among the configurations and processes disclosed as described above, in the methods using gas pressure to float the molten metal, it is difficult to maintain a uniform pressure of the gas pressure chamber and a significant noise is generated. Also, if the gas permeates the molten metal, bubbles may form within the molten metal. 
   In the methods of holding the molten metal using a DC magnet and a DC source, DC current may pass through the metal strip to affect peripheral equipment. This poses safety risks to users. 
   Further, in the method of mounting a linear induction motor in the area of the vessel opening to float the molten metal, the metal strip passing through the opening may be deformed. 
   Finally, in the method of simultaneously using the AC electromagnet and the gas pressure chamber to float the molten metal, significant costs are involved by using both these configurations and gas may permeate the molten metal to form bubbles therein. Also, not only is it difficult to maintain the original shape of the conductor dipped in the molten metal, but also it is difficult to maintain the chemical composition of the molten metal itself. 
   SUMMARY OF THE INVENTION 
   It is one object of the present invention to provide a molten metal holding apparatus for the continuous hot dip coating of a metal strip, in which an electromagnetic force generating apparatus, which is made of an electromagnet core and an electromagnetic coil, is mounted in proximity to a lower portion of a vessel so that molten metal does not escape through an opening of a bottom surface of the vessel. 
   It is another object of the present invention to provide a molten metal holding apparatus for the continuous hot dip coating of a metal strip, in which a molten metal in a vessel is circulated through an external path to re-supply the molten metal into the vessel from a lower portion thereof, thereby maintaining a more stable molten metal floating state in an opening area of a bottom surface of the vessel. 
   It is yet another object of the present invention to provide a molten metal holding apparatus for the continuous hot dip coating of a metal strip, in which molten metal solidification layers are artificially formed within lower portions of short sides of a vessel such that a floating state of the molten metal is more stably maintained. 
   The molten metal holding apparatus for the continuous hot dip coating of a metal strip includes a vessel that is substantially rectangular in cross section having long sides and short sides and has formed a slot-shaped opening in a bottom surface, the vessel containing molten metal; subsidiary vessels formed in a bucket-shape following an outer circumference of an upper end of the vessel and for temporarily storing molten metal that overflows from the upper end of the vessel; chambers formed outwardly following long sides of a lower end of the vessel and that communicate with the vessel via slit-shaped branch openings that are formed at a predetermined slant toward the vessel; a plurality of subsidiary tubes communicating the chambers with the subsidiary vessels; and alternating current electromagnets including a core mounted adjacent to outside side surfaces of the vessel and between the subsidiary vessels and the chambers and a coil wound around the core and to which an alternating current is supplied. 
   Exhaust openings are formed in upper long sides of the vessel such that the molten metal may be exhausted from the vessel to the subsidiary vessels. 
   At least one subsidiary tube is formed in each corner portion of the vessels. 
   The subsidiary tubes are provided outwardly adjacent to a pair of opposing poles of the cores of the electromagnets. Also, the subsidiary tubes are provided between opposing poles of the cores of the electromagnets. The subsidiary tubes are provided external to yokes of the cores of the electromagnets. 
   Molten metal supplied through the branch openings have an angle in the range of 30° to 45° with a metal strip supplied through the opening formed in the bottom surface of the vessel. 
   In an alternative preferred embodiment of the present invention, the molten metal holding apparatus for continuously plating a metal strip includes a vessel that is substantially rectangular in cross section having long sides and short sides and has formed a slot-shaped opening in a bottom, surface, the vessel containing molten metal; alternating current electromagnets mounted adjacent to outside, lower long side surfaces of the vessel; and molten metal coolers mounted adjacent to outside, lower short side surfaces of the vessel for forming solidification layers inside the vessel at a lower end of the short sides thereof. 
   The metal holding apparatus further includes a temperature sensor provided at each an inner lower surface of the short sides of the vessel where the solidification layers are formed and an outer lower surface of the short sides of the vessel; a coolant supply valve connected to the molten metal coolers and controlled to regulate the amount of coolant supplied to the molten metal coolers; and a controller connected to the temperature sensors and the coolant supply valve to control the supply amount of coolant according to the detected temperatures to thereby control a thickness of the solidification layers formed inside the vessel. 
   A molten metal holding method in a process for the continuous hot dip coating of a metal strip includes supplying an alternating current to a coil of an alternating current electromagnet, which is mounted adjacent to an outer lower surface of long sides of a vessel to thereby generate electromagnetic force in the vessel in a direction opposite that of the gravitational force; and supplying a coolant to molten metal coolers to cool lower short sides of the vessel, thereby resulting in the formation of molten metal solidification layers within the vessel at lower short side areas thereof. 
   The formation of the molten metal solidification layers in the method includes measuring temperatures within and outside the lower short sides of the vessel; calculating a desired thickness of the solidification layers according to a difference in the temperatures within and outside the lower short sides of the vessel, and determining an amount of coolant to be supplied to the molten metal coolers; and supplying coolant to the molten metal coolers in the determined amount. 
   In yet another alternative preferred embodiment of the present invention, the molten metal holding apparatus for the continuous hot dip coating of a metal strip includes a vessel that is substantially rectangular in cross section having long sides and short sides and has formed a slot-shaped opening in a bottom surface, the vessel containing molten metal; subsidiary vessels formed in a bucket-shape following an outer circumference of an upper end of the vessel and for temporarily storing molten metal that overflows from the upper end of the vessel; chambers formed outwardly following long sides of a lower end of the vessel and that communicate with the vessel via slit-shaped branch openings that are formed at a predetermined slant toward the vessel; a plurality of subsidiary tubes communicating the chambers with the subsidiary vessels; alternating current electromagnets including a core mounted adjacent to outside side surfaces of the vessel and between the subsidiary vessels and the chambers and a coil wound around the core and to which an alternating current is supplied; and molten metal coolers mounted adjacent to outside, lower short side surfaces of the vessel for forming solidification layers inside the vessel at a lower end of the short sides thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention: 
       FIG. 1  is a schematic longitudinal sectional view of a molten metal holding apparatus according to a first preferred embodiment of the present invention; 
       FIG. 2  is a partial plan view of the molten metal holding apparatus of  FIG. 1 ; 
       FIG. 3  is a sectional view taken along line III—III of  FIG. 2 ; 
       FIG. 4  is a sectional view taken along line IV—IV of  FIG. 2 ; 
       FIG. 5  is a sectional view taken along line V—V of  FIG. 1 ; 
       FIG. 6  is a transverse sectional view of a molten metal holding apparatus according to an alternate preferred embodiment of the present invention; 
       FIG. 7  is a schematic view for interpreting an electromagnetic field formed in a molten metal holding apparatus according to the present invention; 
       FIG. 8  is a schematic view for schematically illustrating induced current and electromagnetic force generated in a vessel of a molten metal holding apparatus according to the present invention; 
       FIG. 9  is a schematic view showing numerical analysis results of flow fields of molten metal in the vicinity of a vessel lower opening portion of a molten metal holding apparatus according to the present invention; 
       FIG. 10  is a side sectional view of a molten metal holding apparatus according to a second preferred embodiment of the present invention; 
       FIG. 11  is a front sectional view of the molten metal holding apparatus of  FIG. 10 ; 
       FIG. 12  is a schematic view for describing molten metal coolers of the molten metal holding apparatus of  FIG. 10 ; 
       FIG. 13  is a schematic view of an inducement current and an electromagnetic force in a vessel of the holding apparatus of  FIG. 10  prior to the formation of a solidification layer; 
       FIG. 14  is a schematic view of an inducement current and an electromagnetic-force in a vessel of the holding apparature of  FIG. 10  after the formation of a solidification layer; and 
       FIG. 15  is a schematic view of a conventional plating apparatus for performing molten plating processes. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 1  is a schematic longitudinal sectional view of a molten metal holding apparatus according to a first preferred embodiment of the present invention. 
   As shown in  FIG. 1 , a molten metal holding apparatus  20  is used for the continuous hot dip coating of a metal strip, and includes main elements of a vessel  21  containing molten metal  22  and having formed in a bottom surface a slot-shaped opening, and an alternating current (AC) electromagnet  30  mounted adjacent to outer side surfaces of the vessel  21 . The AC electromagnet  30  provides buoyancy to the molten metal  22  so that it does not exit through the opening of the vessel  21 . 
   The vessel  21  is substantially rectangular in cross section such that it has long sides and short sides. A metal strip  33  is supplied through the slot-shaped opening formed in the bottom surface of the vessel  21 . Bucket-shaped subsidiary vessels  24  are formed on an upper end of the vessel  21  following an outer circumference of an upper end of the same. The subsidiary vessels  24  temporarily store the molten metal  22  that flows out from the upper end of the vessel  21 . A pair of the subsidiary vessels  24  may be provided, with the subsidiary vessels  24  being provided adjacent to the long sides of the vessel  21  and symmetrically about the metal strip  33  that passes through the vessel  21 . 
     FIG. 2  is a partial plan view of the molten metal holding apparatus of  FIG. 1  showing one of the subsidiary vessels  24 . 
   As shown in the drawing, an exhaust opening  23  is formed in an upper side surface of a long side of the vessel  21 , the long side of the vessel  21  forming one side wall of the subsidiary vessel  24 . The exhaust opening  23  allows the molten metal  22  to easily spill over into the subsidiary vessel  24 . 
   A chamber  26  is formed at a bottom end of the vessel  21 . Also, a slit-shaped branch opening  38  is formed upwardly at a predetermined angle extending from the chamber  26  to the vessel  21  such that the chamber  26  is communicated with the inside of the vessel  21 . 
   It is preferable that each of the chambers  26  includes a tube-shaped configuration following the long side of the vessel  21  for communication with the corresponding subsidiary vessel  24 . Further, it is preferable that the branch openings  38  have a long slit shape that is formed at a predetermined angle to the long side of the vessel  21 . 
     FIG. 3  is a sectional view taken along line III—III of  FIG. 2 , and  FIG. 4  is a sectional view taken along line IV—IV of  FIG. 2 . 
   As shown in  FIGS. 3 and 4 , the subsidiary vessel  24  and the chamber  26  (the drawings show one of a pair of each element) are communicated through a plurality of subsidiary tubes  28 . The subsidiary tubes  28  extend downward following the side wall of the vessel  21  starting from a bottom surface of the subsidiary vessel  24  and continuing until reaching an upper surface of the chamber  26 . 
   Further, the subsidiary tubes  28 , with reference to  FIG. 5 , may start their formation in each corner of the vessel  21 , which is substantially rectangular in cross section as described above. The molten metal  22  temporarily stored in the subsidiary vessels  24  after flowing out of the vessel  21  flows to the chambers  26  through the subsidiary tubes  28 . 
   As described above, the AC electromagnet  30  is mounted adjacent to the outer side surfaces of the vessel  21 . The AC electromagnet  30  includes a core  31  mounted adjacent to the long walls of the vessel  21  between the subsidiary vessels  24  and the chambers  26 , and a coil  32  wound around the core  31 . The core  31  includes poles opposing one another with the vessel  21  therebetween, and a yoke connecting the poles. The coil  32  is wound around the poles of the core  31 , with AC current being supplied through the coil  32  during operation. It is preferable that the poles of the core  31  have a width at least as great as a width of the long sides of the vessel  21 . 
   The subsidiary tubes  28 , with reference again to  FIG. 5 , may be formed outwardly from a pair of the opposing poles  31   a  of the core  31 . As shown in  FIG. 6 , it is possible for the subsidiary tubes  28  to be formed between the pair of the poles  31   a.    
   Separate ports are formed externally to the yoke  31   b  of the core  31 . Also, subsidiary tubes connecting the subsidiary vessels  24  and the ports, and subsidiary tubes connecting the ports and the chamber  26  are formed to enable the transmission of the molten metal. At this time, the ports may move upwardly and downwardly to adjust the amount of the molten metal that is circulated. 
   An operation of the molten metal holding apparatus according to the first preferred embodiment of the present invention will now be described. 
   First, the vessel  21  and the subsidiary tubes  28  are filled with molten metal  22 . If an AC current is then supplied to the coil  32  of the AC electromagnet  30 , an electromagnetic field is formed in the vessel  21  by the AC electromagnet  30  as shown in  FIG. 7 . At this time, an induced current is formed in the molten metal  22  filled in the vessel  21  such that a single current flow path  41  is formed as shown in  FIG. 8 . By the induced current and the electromagnetic field, a Lorentz force expressed by the vector product of the induced current and the electromagnetic field, that is, the electromagnetic force operates toward a center direction of the current flow path  41 , the intensity of which is proportional to the product of the induced current and the electromagnetic field. Accordingly, an electromagnetic force acts  43  in a direction opposite to the direction of the gravitational force at the bottom portion of the vessel  21 , while an electromagnetic force  42  acts in a direction corresponding to the direction of the gravitational force at the top portion of the vessel  21 . 
   In the molten metal holding apparatus of the first preferred embodiment of the present invention, the AC electromagnet  30  is in close proximity to the opening of the vessel  21  by the increasingly narrowly formed outer circumference of the vessel  21  at the bottom portion thereof. As a result, with reference again to  FIG. 8 , the electromagnetic force  43  acting in a direction opposite that of the force of gravity at the bottom portion of the vessel  21  is increased in strength, while the electromagnetic force  42  acting at the upper portion of the vessel  21  is relatively weak. Therefore, the total electromagnetic force acting on the molten metal  22  in the vessel  21  acts in a direction opposite the direction of the gravitational force such that the molten metal  22  in the vessel  21  floats. 
   The molten metal  22  floating in this manner within the vessel  21  spills over into the subsidiary vessels  24  through the exhaust openings  23  formed in the upper portions of the vessel  21 , then this molten metal  22  flows through the subsidiary tubes  28 , upper ends of which are formed starting from the bottom of the subsidiary vessels  24 . The molten metal  22  then flows through the subsidiary tubes  28  from the subsidiary vessels  24  into the chambers  26 . Next, the molten metal  22  that enters the chambers  26  is sprayed into the vessel in a free flat jet form through the branch openings  38  by hydrostatic pressure depending on the height of the subsidiary tubes  28  and the electromagnetic force generated by the AC electromagnet  30 . 
     FIG. 9  is a schematic view showing numerical analysis results of flow fields of the molten metal in the lower portion area of the vessel  21  in the molten metal holding apparatus according to the present invention. 
   As shown in the drawing, the free flat jet flows through the branch openings  38  having a predetermined angle (θ) with the supplied metal strip  33 , that is, inner most lines formed by the flow of the molten metal  22  have the predetermined angle (θ) with the metal strip  33  that is supplied to the molten metal holding apparatus. The angle (θ) is preferably between 30° and 45° in order to ensure the most stable floating of the molten metal  22 . If the angle (θ) is less than 30°, the free flat jet flow meeting the metal strip  33  excessively slows, and if the angle (θ) is greater than 45°, the free flat jet flow strikes the metal strip  33  and splashes downwardly away from the intended flow direction. 
   The molten metal  22  sprayed in this manner enters into the vessel  21  at a location close to the metal strip  33  in the vicinity of the lower opening portion of the vessel  21 . Also, this molten metal  22  not only has a velocity in a direction opposite that of the force of gravity, but an induced current path generated by the electromagnetic field is always ensured by the molten metal already in this area. Therefore, a free surface of the molten metal floating by the electromagnetic force in the lower opening portion of the vessel  21  is kinetically stabilized such that the floating of the molten metal  22  is stably maintained. 
   The molten metal  22  circulated as described above is reduced in amount as it coats the metal strip  33  passing through the vessel  21  such that it is necessary to continuously or periodically replenish the supply of the molten metal  22 . 
   The intensity of the electromagnetic force generated by the AC electromagnet  30  is proportional to the square of the amount of current supplied to the coil  32 . As a result, prevention of the exiting of the molten metal  22  by the free flat jet flow sprayed through the branch openings  38  may be stably realized by adjusting the amount of current supplied to the coil  32  and adjusting the vertical height of the molten metal  22  in the subsidiary vessels  24 . 
     FIG. 10  is a side sectional view of a molten metal holding apparatus according to a second preferred embodiment of the present invention, and  FIG. 11  is a front sectional view of the molten metal holding apparatus of  FIG. 10 . 
   With reference to the drawings, a molten metal holding apparatus  50  according to the second preferred embodiment of the present invention includes main elements of a vessel  51  containing molten metal  22 , AC electromagnets  60  mounted adjacent to outer side surfaces of the vessel  51  for providing buoyancy to the molten, metal  22  in the vessel  51 , and molten metal coolers  53  for forming solidification layers  55  of the molten metal  22  in lower portions within the vessel  51  corresponding to where the molten metal coolers  53  are provided. The vessel  51  is substantially rectangular in cross section having long sides and short sides. A slot-shaped opening is formed in a bottom surface of the vessel  51  through which a metal strip  33  is supplied. 
   A pair of the AC electromagnets  60  is provided and they are mounted adjacent to a lower outer surface of the long sides of the vessel  51 . The AC electromagnets  60  oppose one another symmetrically about the metal strip  33  when the same is supplied to the vessel  51 . The molten metal coolers  53  are mounted to a lower outer surface of the short sides of the vessel  51 . When operated, the molten metal coolers  53  form solidification layers  55  of the molten metal  22  at lower areas within the vessel  51  next to the short sides of the same. 
     FIG. 12  is a schematic view for describing the molten metal coolers  53  of the molten metal holding apparatus of  FIG. 10 . 
   With reference to the drawing, a configuration for the supply and exhaust of coolant to and from the molten metal coolers  53  is provided thereon. With respect to the supply of coolant to the molten metal coolers  53 , there are provided temperature sensors  57   a  and  57   b  respectively inside and outside the vessel  51 , a coolant supply valve  63  controlled to regulate the amount of coolant supplied to the molten metal coolers  53 , and a controller  61  for controlling the supply of the coolant according to the sensed temperatures so that a thickness of the solidification layers  55  may be adjusted. 
   The temperature sensors  57   a  and  57   b  are provided at a height respectively inside and outside the vessel  51  corresponding to where the solidification layers  55  are formed. The temperatures detected by the temperature sensors  57   a  and  57   b  are transmitted to the controller  61 . The coolant supply valve  63  is connected to each of the molten metal coolers  53 , and is also connected to the controller  61 . The controller  61  then is connected to the coolant supply valve  63  as well as to the temperature sensors  57   a  and  57   b . Depending on the temperatures detected by the temperature sensors  57   a  and  57   b , the controller  61  outputs signals to the coolant supply valve  63  to adjust the amount of coolant that is supplied to the molten metal coolers  53 . The thickness of the solidification layers  55  in the vessel  51  is controlled by this process. 
     FIG. 13  is a schematic view of an inducement current and an electromagnetic force in the vessel  51  prior to the formation of solidification layers  55 . 
   An electromagnetic field formed by the AC electromagnet  60  generates an induced current within the molten metal  22  filled in the vessel  51 . This induced current forms a single current flow path  71 . A Lorentz force expressed by the vector product of the induced current and the electromagnetic field, that is, electromagnetic forces  72 ,  73 , and  75  operates toward a center direction of the current flow path  71 , the intensity of which is proportional to the product of the induced current and the electromagnetic field. 
   Accordingly, with the mounting of the AC electromagnet  60  at the bottom portion of the vessel  51 , the electromagnetic force  72  acting on the molten metal  22  in the vicinity of the opening operates in a direction opposite the direction of the gravitational force, and the electromagnetic force  73  acting on the molten metal  22  at an upper end of the vessel  51  operates corresponding to the direction of the gravitational force. Since the strength of the electromagnetic force  72  at the bottom portion of the vessel  51  and close to the AC electromagnet  60  is greater than that of the electromagnetic force  73  in the upper portion of the vessel  51  and relatively far from the AC electromagnet  60 , the direction of the overall electromagnetic force in the vessel  51  is opposite the direction of the force of gravity, thereby providing buoyancy to the molten metal  22  in the vessel  51 . 
   In corner areas at the bottom of the vessel  51 , the direction of the induced current  71  is changed such that the direction of the electromagnetic force is also changed. In more detail, the electromagnetic force  75  in the bottom corner portions of the vessel  51  includes components  75   a  perpendicular to the gravitational force direction and components  75   b  corresponding to the gravitational force direction. 
   Past the corner portions in the short side areas, the component  75   b  in the gravitational force direction is no longer a factor and only the component  75   a  perpendicular to the direction of the force of gravity is present. Accordingly, the electromagnetic force opposite the gravitational force direction in the lower corner portions at the short sides of the vessel  51  is substantially weaker than at the center portion of the long sides of the vessel such that a stable floating effect is obtained. This floating effect is even more stably realized with the operation of the molten metal coolers  53  to form the solidification layers  55 . 
     FIG. 14  is a schematic view of an inducement current and an electromagnetic force in the vessel after the formation of the solidification layers  55 . 
   As shown in the drawing, the flow path  71  of the induced current is identical to before the formation of the solidification layers  55 . However, at the bottom portion of the vessel  51 , only components of the electromagnetic force acting on the molten metal that are opposite the gravitational force direction are present. Further, with the formation of the solidification layers  55  at the bottom corner portions and short sides of the vessel  51 , only the desired forces are present such that the molten metal  22  is provided with sufficient buoyancy and does not exit through the opening. 
   The solidification layers  55  are formed in the vessel  51  such that they are attached to inside lower ends of the short sides of the vessel  51 . It is preferable that a thickness of the solidification layers  55  is such that the solidification layers  55  extend from the lower ends of the short sides of the vessel  51  to where the electromagnetic components perpendicular to the gravitational force start to be generated. 
   The method of determining the thickness of the solidification layers  55  will be described in more detail. A distance from the lower ends of the short sides of the vessel  51  to where the electromagnetic components perpendicular to the gravitational force start to be generated is almost identical to a skin depth (δ) of the AC electric field. Accordingly, it is preferable that the solidification layers  55  are formed thicker than the skin depth (δ), which is determined by the molten metal  22  that provides for the thickness of the solidification layers  55  and the frequency of the AC electric field. 
   The skin depth (δ) is obtained by Equation 1 below. 
             δ   =     1       2   ⁢   π   ⁢           ⁢   f   ⁢           ⁢   σμ                 [     Equation   ⁢           ⁢   1     ]             
 
   where f is the frequency of the AC electromagnetic field, σ is the electric conductivity of the molten metal, and μ is the magnetic permeability. 
   If the temperatures inside and outside the vessel  51  are known, the thickness of the solidification layers  55  may be determined from Equation 2 below. 
                 k   pot     ⁢       (       T   Pi     -     T   Po       )       t   pot         =       k   solid     ⁢       (       T   in     -     T   Pi       )       t   solid                 [     Equation   ⁢           ⁢   2     ]             
 
   where t pot  is the wall thickness of the short side of the vessel  51 , t solid  is the thickness of the molten metal solidification layers  55 , k pot  is the thermal conductivity of the vessel  51 , k solid  is the thermal conductivity of the solidified molten metal, T Po  is the outside wall temperature of the vessel  51 , T Pi  is the inside wall temperature of the vessel  51 , and T m  is the temperature at the boundary between the solidification layers  55  and the molten metal  22  and is the solidification point temperature of the metal. 
   Accordingly, the temperature sensors  57   a  and  57   b  detect T Pi  and T Po , respectively, so that the thickness (t solid ) of the solidification layers  55  may be determined. The thickness (t solid ) of the solidification layers  55  must satisfy Equation 3 below to ensure the stable floating of the molten metal  22 .
 
t solid ≧δ  [Equation 3]
 
   The following experiment was performed to determine the effects of the molten metal holding apparatus according to the second preferred embodiment of the present invention. 
   First, the vessel  51  was made of stainless steel at a thickness of 10 mm and a 60 Hz AC magnetic field (B rms ) was applied at 0.3 T to the opening of the lower portion of the vessel  51 . A difference in the temperatures of the inside wall and outside wall of the vessel  51  was maintained at 100° C. or higher, and a lowermost thickness (t solid ) of the solidification layers  55  of the short sides of the vessel  51  was formed at greater than 55 mm, which is the skin depth (δ) of the molten zinc calculated from Equation 1. Accordingly, the molten zinc  22  filled in the vessel  51  is stably floated to a height of 500 mm from the opening. 
   At this time, if the difference in the inside wall temperature and the outside wall temperature of the short sides of the vessel  51  is less than 100° C., the thickness (t solid ) of the solidification layers becomes less than the skin depth (δ) and the exiting of the molten zinc at the short side area occurs. Therefore, the inside wall temperature and the outside wall temperature were detected respectively by the temperature sensors  57   a  and  57   b , and the controller  61  adjusted the supply valve  63  based on this information such that the temperature difference in the inside wall temperature and the outside wall temperature was maintained at 100° C. or greater, thereby realizing a thickness (t solid ) of the solidification layers  55  that is greater than the skin depth (δ). 
   A molten metal holding apparatus according to a third preferred embodiment of the present invention incorporates all the features of the molten metal holding apparatuses of both the first and second preferred embodiments of the present invention. 
   In particular, the molten metal holding apparatus according to the third preferred embodiment of the present invention includes a vessel that contains molten metal and has formed a slot in a bottom surface, subsidiary vessels for temporarily storing molten metal that overflows from the upper end of the vessel, chambers positioned at a lower end of the vessel and that communicate with the subsidiary vessels via subsidiary tubes and with the vessel via branch openings, AC electromagnets mounted adjacent to outside side surfaces of the vessel and provides buoyancy the molten metal so that the same does not exit the opening of the vessel, and molten metal coolers for forming solidification layers inside the vessel at a lower end of short sides thereof. 
   The vessel is substantially rectangular in cross section having long sides and short sides. The auxiliary vessels are bucket-shaped and follow an outer circumference of the upper end of the vessel. 
   Further, the chambers are formed following long side surface of the lower portion of the vessel, and communicates with the vessel through the branch openings that slit-shaped and upwardly slanted toward inside the vessel. A plurality of the subsidiary tubes are provided to communicate the subsidiary vessels with the chambers. 
   The AC electromagnet includes a core mounted adjacent to outside the long sides of the vessel between the subsidiary vessels and the chambers, and a coil wound around the core and through which an AC current flows. The molten metal coolers are mounted to lower outside surfaces of the short sides of the vessel. When operated, the molten metal coolers form solidification layers inside the vessel at a lower end of the short sides of the same. 
   The above molten metal holding apparatus further includes a temperature sensor provided at each an inner lower surface of the short sides of the vessel where the solidification layers are formed and an outer lower surface of the short sides of the vessel, a coolant supply valve controlled to regulate the amount of coolant supplied to the molten metal coolers, and a controller connected to the temperature sensors and the coolant supply valve to control the supply amount of coolant according to the detected temperatures to thereby control the thickness of the solidification layers formed inside the vessel. 
   Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.