Patent Publication Number: US-2007095499-A1

Title: Method and apparatus for electromagnetic confinement of molten metal in horizontal casting systems

Description:
FIELD OF THE INVENTION  
      The present invention relates to the continuous casting of metal strip, and more particularly, to the electromagnetic confinement of molten metal in a continuous casting system.  
     BACKGROUND OF THE INVENTION  
      Continuous casting of metals is performed in twin-roll casters and belt casters or combinations thereof. Methods are available for casting both in the horizontal and in the vertical direction. In particular, the steel industry has recently developed high speed twin roll strip casters which operate in the vertically down direction.  
      Up to the present, the mechanical edge dams have been employed to provide containment of the molten metal in the casting zone. Such devices have included the caterpillar type edge dams that move with the strip (as in the Hazelett casters) or fixed edge dams that are pressed against the surface of the rolls. The latter is used in the twin-roll steel strip casting industry. Such fixed mechanical edge dams have a short service life as they get eroded by contact with the cold sidewall of the rolls. In addition, such mechanical edge dams provide sites for the formation of skulls that have a tendency to be sheared off and thus enter the cast strip to render the microstructure metallurgically undesirable. Caterpillar edge dams, while well proven for the thicker slab castings (10-25 mm thick), become impractical for thin strip casters or twin drum casters of the steel industry where the cross section to be contained changes sharply along the casting zone.  
      Electromagnetic edge dams have been employed in the prior art in the strip casting of metals in vertical twin drum (roller) casting systems. Electromagnetic edge dams of a magnetic system type use a combination of a magnet assembly and an AC coil to generate confinement forces. Electromagnetic edge dams of an induction system type rely solely on an AC coil to generate the containment forces.  
      The magnetic system electromagnetic edge dams use a magnetic member which comprises a yoke or core connecting two pole faces disposed on either side of the gap on which the molten metal is to be confined. The magnetic member is made of a ferromagnetic material and is surrounded over a given length of the yoke by a coil carrying an AC current. The magnetic flux generated by the flow of the current into the coil is transmitted to the poles of the magnet through the yoke and establishes containment forces at the metal surface in the gap.  
      Typically, in magnetic systems, part of the magnetic member is covered with an electrically conductive shield to minimize leakage of flux in a direction away from the gap. Such magnetic confinement systems have the advantage that the confinement current need not be as high as compared to those systems using solely an induction coil. If a stronger magnetic field is required, it can be achieved with the same current level by reducing the area of the pole faces to concentrate the field. However, such systems are not without disadvantages. For example, such systems typically have poor operating efficiency resulting from core losses and losses due to magnetic hysterisis when an alternating magnetic field is applied to the magnetic material. Additionally, high temperatures are typically generated that need to be dissipated by cooling in order to prevent damage to the magnetic system.  
      Induction confinement systems typically employ a shaped inductor positioned close to the gap in which the molten metal is to be contained. The AC current flowing in the inductor generates induced currents as well as a time-varying magnetic field on the surface of the molten metal to be contained. The interaction between the current and the magnetic field provide containment forces. To improve efficiency, a magnetic member is built around the inductor to focus the current to the inductor surface facing the molten metal. Induction coil systems are generally simpler in design than magnetic systems. However, induction systems are disadvantageously limited in terms of the maximum metallostatic head that can be contained by the system. The maximum metallostatic head that can be supported in induction coil systems is limited, because induction coil systems require very high inductor currents to provide adequate containment forces, wherein such high currents are accompanied by increased heat generation, which in turn hinders or slows the solidification process during casting.  
      Referring to  FIG. 1 , in vertical twin roll casters, the molten metal head against which containment must be provided tends to be very high. For typical operating condition, the metal head height H 1  is about 65% the radius of the casting rolls. Therefore, electromagnetic edge dam apparatus used in vertical twin roll casters must provide a magnetic field strong enough to contain a metal pool having a head height H 1  that is 65% the radius of the casting rolls. Such electromagnetic edge dams have not been successfully commercialized for two reasons. First, the high current required to contain the molten metal pool creates standing waves on the top surface of the metal pool that are too large in magnitude for the casting process. Second, the large electromagnetic forces needed to contain the molten metal head formed atop vertical roller caster systems create induction heating on the metal pool&#39;s sidewall, which interferes with the solidification process.  
      U.S. Pat. No. 4,936,374 describes a vertical casting system and electromagnetic confinement apparatus having the disadvantages described above. Further, U.S. Pat. No. 4,936,374 describes casting rollers having a rim portion, in which the containment magnetic field is conducted through the rim portion of the casting roll. In addition to induction heating and wave generation, the rim portions of the casting rolls disclosed in U.S. Pat. No. 4,936,374 produce a ridge in the cast product and therefore fail to provide a casting strip having uniform sidewalls (edges). The ridge formed in the casting strip produced using the apparatus and method disclosed in U.S. Pat. No. 4,936,374 must be machined prior to rolling of the casting strip. Additional machining disadvantageously adds to the cost of the production.  
      Accordingly, a need remains for a method of high-speed continuous casting of metals and alloys, which achieves uniformity in the cast strip surface, provides good molten metal containment in the casting zone, and results in strip edges which can be rolled without needing to be machined by trimming.  
     SUMMARY OF THE INVENTION  
      The present invention overcomes the above-described obstacles and disadvantages by providing an electromagnetic confinement apparatus incorporated into a horizontal casting apparatus, wherein the positioning of the electromagnetic confinement apparatus and a magnetic field that is produced by an alternating current provides a cast metal strip having substantially uniform edges (sidewalls). The present invention further provides a method and apparatus for producing a cast metal strip, which provides a means for adjusting the profile of the cast metal strip&#39;s sidewall.  
      In one embodiment of the present invention, the current applied through the electromagnetic confinement apparatus, as well as, the positioning of the electromagnetic confinement apparatus to the molding zone of the horizontal casting apparatus is selected to provide a cast metal strip having substantially uniform edges, in which the sidewall of the cast metal strip edges may be substantially flat, or concave or convex in relation to the cast metal strip&#39;s centerline. The cast metal strip&#39;s substantially uniform edges allows for the cast metal strip to be rolled without further machining. Broadly, one embodiment of an apparatus of the present invention comprises: 
          (a) a pair of casting rollers adapted to receive molten metal along a horizontal axis, wherein a vertical distance separating the pair of casting rollers defines a molding zone;     (b) an electromagnetic edge containment apparatus positioned on each side of the molding zone, comprising an induction coil wound about a portion of a magnetic member to generate magnetic lines of force upon application of a current, wherein said magnetic member comprises a first and second pole positioned distal from and aligned to a sidewall of said pair of casting rollers and the current provides magnetic lines of force perpendicular to said horizontal axis that contain the molten metal in contact to the casting rollers with substantially no increase in temperature to the molten metal; and     (c) a means for supplying the molten metal to the molding zone along said horizontal axis from a tundish while ensuring said molten metal remains substantially non-oxidized, wherein the tundish is separated from the molding zone by a distance to substantially eliminate wave generation within the tundish by the magnetic lines of force.        

      In another embodiment of the apparatus of the present invention, a horizontal roller casting apparatus is provided in which containment of the metal through the apparatus is provided by the combination of a mechanical edge dam and an electromagnetic edge dam. Broadly, the inventive casting apparatus comprises: 
          (a) a pair of casting rollers adapted to receive molten metal along a horizontal axis, wherein a vertical distance separating the pair of casting rollers defines a molding zone;     (b) a tip delivery structure positioned to supply the molten metal to the molding zone along said horizontal axis from a tundish while ensuring said molten metal remains substantially non-oxidized; and     (c) an edge containment apparatus positioned on each side of the molding zone, said edge containment apparatus comprising: 
            a mechanical edge dam positioned overlying at least an end portion of said tip delivery structure and partially extending towards said molding zone, and     an electromagnetic edge dam comprises a first and second magnetic pole positioned distal from and aligned to a sidewall of said pair of casting rollers and overlying a portion of said mechanical edge dam partially extending towards said molding zone, wherein said electromagnetic edge dam provides magnetic lines of force perpendicular to said horizontal axis that contain the molten metal in contact to the casting rollers.    
               

      In each embodiment, the vertical distance separating the horizontally disposed pair of casting rollers provides a metal head height that allows for containment of the molten metal by magnetic lines of force that are provided by an electromagnetic containment device without a substantial increase in the temperature of the molten metal. For the purposes of this disclosure, the term “positioned distal from and aligned to a sidewall of said pair of casting rollers” is intended to denote that the poles of the electromagnetic edge dam do not extend towards the casting apparatuses centerline beyond a plane defined by the sidewall of the casting rollers, but are positioned within close enough proximity to the castings roller&#39;s sidewall to provide a sufficient magnetic field to contain molten metal within the molding zone. It is noted that the poles of the electromagnetic edge dam may be adjusted from adjacent to the casting rollers sidewall to any distance from the sidewall, so long as sufficient containment forces are provided by the poles to the molding zone. In one embodiment, the sidewall of the casting roller may be substantially planar. The term “substantially planar” with respect to the casting roller&#39;s sidewall denotes that the casting roller does not incorporate a lip portion. In one embodiment, the electromagnetic lines of force are produced by an alternating current having a frequency ranging from 40 Hz to 10,000 Hz through the electromagnetic edge containment device.  
      In another embodiment of the present invention, a belt casting system is provided that employs electromagnetic edge containment and produces a metal strip having substantially uniform edges, wherein the substantially uniform edges allows for the cast metal strip to be rolled without further machining. Broadly, the inventive belt casting system for strip casting of molten metal comprising: 
          (a) a pair of opposing endless metal belts, each of the pair of opposing endless metal belts passing over a roller and having a periphery substantially aligned to a periphery of the roller, said each of said opposing endless metal belts having a surface for accepting molten metal, wherein a vertical dimension separating the pair of opposing endless metal belts defines a molding zone;     (b) an electromagnetic edge containment apparatus positioned on each side of the molding zone comprising an induction coil wound about a portion of a magnetic member to generate magnetic lines of force upon application of a current, wherein the current provides magnetic lines of force that contain the molten metal within a width and in contact to at least a portion of said pair of opposing endless metal belts with substantially no increase in temperature to the molten metal; and     (c) a means for supplying said molten metal to the molding zone along a horizontal axis from a tundish, the tundish separated from said molding zone by a distance to substantially eliminate wave generation within the tundish by the magnetic lines of force.        

      In another aspect of the present invention, a casting strip is provided that may be formed by the above casting apparatus. Broadly, the cast strip comprises: 
          (a) a first shell;     (b) a second shell; and     (c) a central portion between said first shell and said second shell, said central portion comprising grains having an equiaxed structure, wherein said cast metal strip has sidewall edges being substantially uniform.        

      In another aspect of the present invention, a method is provided for casting a metal strip in which a magnetic field is utilized to control the geometry of the metal strip&#39;s sidewall. Broadly, the inventive method comprises: 
          providing molten metal to a molding zone along a horizontal axis;     containing said molten metal within said molding zone with a magnetic containment means; and     casting said molten metal into a cast metal strip, wherein sidewall geometry of said cast metal strip is configured by adjusting said magnetic containment means.        

      The magnetic field may be adjusted to provide a metal casting strip sidewall geometry that is flat or is concave or convex relative to the centerline of the cast metal strip. In one embodiment, the magnetic containment means may include an induction coil wound about a magnetic member to generate magnetic lines of force upon application of a current. The magnetic member having a first and second magnetic pole positioned distal from to adjacent to the molding zone.  
      The magnetic lines of force produced by the magnetic containment means may be adjusted by increasing or decreasing the current through the induction coil or by changing the positioning of the magnetic containment means relative to the molding zone. Positioning the first and second magnetic poles of the magnetic containment means adjacent to-the molding zone may produce a cast metal strip having a concave sidewall and positioning the first and second magnetic poles of the magnetic containment means distal from the molding zone may produce a cast metal strip having a convex sidewall. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  (side cross sectional view) is a schematic of a portion of a vertical roller caster casting apparatus depicting a molten metal head and a pair of rolls operated according to the prior art.  
       FIG. 2   a  (side cross sectional view) is a schematic of one embodiment of a horizontal casting apparatus having electromagnetic edge dams in accordance with the present invention.  
       FIG. 2   b  (side cross sectional view) depicts one embodiment of a twin belt caster equipped with an electromagnetic edge dam apparatus in accordance with the present invention.  
       FIG. 3  (side cross sectional view) depicts the molding zone of the inventive horizontal casting device.  
       FIG. 4  depicts a table summarizing the magnetic field density that is required to contain a molten pool of aluminum at different head heights.  
       FIG. 5  depicts a plot of the magnetic field strength produced by an electromagnetic containment device in accordance with the present invention at varying currents and distances wherein the distance is measured from the sidewall of the caster roll.  
       FIG. 6  (side cross sectional views) depicts a sectional view taken along the lines  2 - 2  in  FIG. 2   a , and illustrate the positioning of the electromagnetic edge dams in relationship to the sidewall of the roller casters.  
       FIGS. 7   a - 7   d  provide a sectional view of the electromagnetic edge dam apparatus of the present invention illustrating the path of the magnetic lines of force in relation to the roller casters of the horizontal roller caster casting apparatus.  
       FIGS. 8   a - c  (side view) illustrate different pole face angles and orientations in accordance with the present invention.  
       FIG. 9  illustrates an exemplary embodiment of the present invention wherein a magnetic member has a split core design.  
       FIG. 10  illustrates an exemplary embodiment of the present invention wherein the magnetic member has a laminate design.  
       FIG. 11  illustrates an exemplary embodiment of the present invention wherein a mechanical edge dam is used in conjunction with an electromagnetic edge dam.  
       FIG. 12  depicts a table summarizing the push of the electromagnetic edge dam.  
       FIGS. 13   a - c  are pictorial representations of sidewall of a casting strip.  
       FIGS. 14   a - b  are photographic representations of the edges of the strip made with a high magnetic force in the electromagnetic dam.  
       FIG. 15  is a pictorial representation of a casting strip having a flat edge profile (straight edge).  
       FIG. 16  is a pictorial representation of a casting strip following an 87% reduction (acceptable degree of edge cracking). 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      The present invention provides an electromagnetic edge dam that confines molten metal to the molding zone of a horizontally disposed roller casting or belt casting system with a magnetic field that is produced by a lower AC current than was previously possible. By providing sufficient electromagnetic containment means at lower AC currents, the present invention utilizes electromagnetic confinement without creating a substantial increase in the temperature of the molten metal or producing wave generation effects.  
      As discussed above, in prior vertical casting methods with larger molten metal head height, larger magnetic forces are required in order to contain the greater pressure produced by the molten metal, wherein larger magnetic forces typically require higher currents that generate heat. For example, to contain molten aluminum against a 300 mm height, as representative of typical vertical casting methods, a minimum magnetic field intensity of 0.24 T would be needed. In the present invention, the metal head height is kept low, as achieved by a horizontally disposed casting system, so that the required containment can be met with relatively low magnetic field density. For example, a 50 mm head height in a horizontal casting apparatus consistent with the present invention requires a magnetic field density of only 0.055 T to contain molten aluminum in the horizontal position while casting. The present invention is now discussed in more detail referring to the drawings that accompany the present application. In the accompanying drawings, like and/or corresponding elements are referred to by like reference numbers.  
      Referring to  FIG. 2   a , in one embodiment of the present invention, a horizontal roller casting apparatus  10  is provided having an electromagnetic edge dam  15  positioned to provide magnetic lines of force to confine molten metal M within the molding zone  20  of the apparatus  10 , wherein the magnetic lines of force extend along a plane perpendicular to the plane on which the casting is drawn. The horizontal roller casting apparatus  10  is practiced using a pair of counter-rotating cooled rolls R 1  and R 2  rotating in the directions of the arrows A 1  and A 2 , respectively. By the term horizontal, it is meant to denote that the cast strip is produced along a horizontal plane, in which the horizontal plane is parallel to section line  2 - 2 , or at an angle of plus or minus about 30° from the horizontal plane.  
      Referring to  FIG. 2   b , in one embodiment of the present invention, a horizontal belt casting apparatus  10 ′ is provided having an electromagnetic edge dam  15  positioned to provide ma genetic lines of force to confine molten metal M within the molding zone  20  of the apparatus  10 , wherein the magnetic lines of force extend along a plane perpendicular to the plane  2 - 2  on which the casting is drawn. The horizontal belt casting apparatus  10 ′ is practiced using a pair of counter-rotating belts B 1  and B 2  rotating in the directions of the arrows A 1  and A 2 , respectively. It is noted that although the following figures are directed towards the horizontal roller caster  10  depicted in  FIG. 2   a , the following description is equally applicable to the horizontal belt caster  10 ′ disclosed in  FIG. 2   b  with the exception that instead of the molten metal contacting the roller casters R 1 , R 2  the molten metal is contacting the counter-rotating belts B 1 , B 2 . It is further noted, that further differences between the horizontal roller casting apparatus  10  and the belt casting apparatus  10 ′ in accordance with the present invention are noted when relevant throughout the following portions of the specification.  
      Referring to  FIG. 3 , molten metal M is transported to the molding zone  20  by a feed tip T, which may be made from a suitable ceramic material. The feed tip T distributes molten metal M in the direction of arrow B directly onto the casting rolls R 1  and R 2  rotating in the direction of the arrows A 1  and A 2 , respectively. Gaps G 1  and G 2  between the feed tip T and the respective rolls R 1  and R 2  are maintained as small as possible to prevent molten metal from leaking out and to minimize the exposure of the molten metal to the atmosphere. A suitable dimension of the gaps G 1  and G 2  is about 0.01 inch (0.25 mm). A plane L through the centerline of the rolls R 1  and R 2  passes through a region of minimum clearance between the rolls R 1  and R 2  referred to as the roll nip N.  
      The molten metal M delivered from the feeding tip T directly contacts the cooled rolls R 1  and R 2  at regions  18  and  19 , respectively. Upon contact with the rolls R 1  and R 2 , the metal M begins to cool and solidify. The cooling metal produces an upper shell  16  of solidified metal adjacent the roll R 1  and a lower shell  17  of solidified metal adjacent to the roll R 2 . The thickness of the shells  16  and  17  increases as the metal M advances towards the nip N. Large dendrites  21  of solidified metal (not shown to scale) are produced at the interfaces between each of the upper and lower shells  16  and  17  and the molten metal M. The large dendrites  21  are broken and dragged into a center portion  12  of the slower moving flow of the molten metal M and are carried in the direction of arrows C 1  and C 2 .  
      The dragging action of the flow can cause the large dendrites  21  to be broken further into smaller dendrites  22  (not shown to scale). In the central portion  12  upstream of the nip N, the metal M is semi-solid including a solid component including solidified small dendrites  22  and a molten metal component. The metal M in the region  23  has a mushy consistency due in part to the dispersion of the small dendrites  22  therein. At the location of the nip N, some of the molten metal is squeezed backwards in a direction opposite to the arrows C 1  and C 2 . The forward rotation of the rolls R 1  and R 2  at the nip N advances substantially only the solid portion of the metal (the upper and lower shells  16  and  17  and the small dendrites  22  in the central portion  12 ) while forcing molten metal in the central portion  12  upstream from the nip N such that the metal is completely solid as it leaves the point of the nip N.  
      Downstream of the nip N, the central portion  13  is a solid central layer  13  containing the small dendrites  22  sandwiched between the upper shell  16  and the lower shell  17 . In the central layer  13 , the small dendrites  22  may be about  20  to about  50  microns in size and have a generally equaixed (globular) shape, as opposed to having a columnar shape. The three layers of the upper and lower shells  16  and  17  and the solidified central layer  13  constitute a solid cast strip.  
      The rolls R 1  and R 2  serve as heat sinks for the heat of the molten metal M. In the present invention, heat is transferred from the molten metal M to the rolls R 1  and R 2  in a uniform manner to ensure uniformity in the surface of the cast strip. Surfaces D 1  and D 2  of the respective rolls R 1  and R 2  may be made from a material of good thermal conductivity such as steel or copper or other metallic materials and are textured and include surface irregularities (not shown) which contact the molten metal M. The surface irregularities may serve to increase the heat transfer from the surfaces D 1  and D 2 . The rolls R 1  and R 2  may be coated with a material to enhance separation of the cast strip from the rolls R 1  and R 2  such as chromium or nickel. In a preferred embodiment, the rolls R 1  and R 2 , including surfaces D 1  and D 2 , comprise a ferromagnetic material. In the embodiments of the present invention, in which the rolls R 1  and R 2  do not comprise a ferromagnetic material, the casting surfaces D 1 , D 2  of the roller as well as the roller&#39;s sidewall may be coated with a ferromagnetic materials.  
      The control, maintenance and selection of the appropriate speed of the rolls R 1  and R 2  may impact the operability of the present invention. The roll speed determines the speed that the molten metal M advances towards the nip N. If the speed is too slow, the large dendrites  21  will not experience sufficient forces to become entrained in the central portion  12  and break into the small dendrites  22 . Accordingly, the present invention is suited for operation at high speeds such as about 25 to about 400 feet per minute or about 100 to about 400 feet per minute or about 150 to about 300 feet per minute. The linear speed that molten aluminum is delivered to the rolls R 1  and R 2  may be less than the speed of the rolls R 1  and R 2  or about one quarter of the roll speed. High-speed continuous casting according to the present invention may be achievable in part because the textured surfaces D 1  and D 2  ensure uniform heat transfer from the molten metal M.  
      The roll separating force may be a parameter in practicing the present invention. The roll separating force is the force present between the rolls due to the presence of the strip within the roll gap. The roll force is particularly high when the strip is being plastically deformed by the rolls during roll casting. A significant benefit of the present invention is that solid strip is not produced until the metal reaches the nip N. The thickness is determined by the dimension of the nip N between the rolls R 1  and R 2 . The roll separating force may be sufficiently great to squeeze molten metal upstream and away from the nip N. Excessive molten metal passing through the nip N may cause the layers of the upper and lower shells  16  and  17  and the solid central portion  13  to fall away from each other and become misaligned. Insufficient molten metal reaching the nip N causes the strip to form prematurely as occurs in conventional roll casting processes. A prematurely formed strip  20  may be deformed by the rolls R 1  and R 2  and experience centerline segregation. Suitable roll separating forces are about 25 to about 300 pounds per inch of width cast or about 100 pounds per inch of width cast. In general, slower casting speeds may be needed when casting thicker gauge aluminum alloy in order to remove the heat from the thick alloy. Unlike conventional roll casting, such slower casting speeds do not result in excessive roll separating forces in the present invention because fully solid aluminum strip is not produced upstream of the nip.  
      In prior applications, roll separating force has been a limiting factor in producing low gauge aluminum alloy strip product but the present invention is not so limited because the roll separating forces are orders of magnitude less than in conventional processes. Aluminum alloy strip may be produced at thicknesses of about 0.1 inch or less at casting speeds of 25 to about 400 feet per minute. Thicker gauge aluminum alloy strip may also be produced using the method of the present invention, for example at a thickness of about ¼ inch.  
      The aluminum alloy strip  20  continuously cast according to the present invention includes a first layer of an aluminum alloy and a second layer of the aluminum alloy (corresponding to the shells  16  and  17 ) with an intermediate layer (the solidified central layer  13 ) therebetween. The grains in the aluminum alloy strip of the present invention are substantially undeformed because the force applied by the rolls is low (300 pounds per inch of width or less). The strip is not solid until it reaches the nip N; hence it is not hot rolled in the manner of conventional twin roll casting and does not receive typical thermo-mechanical treatment. In the absence of conventional hot rolling in the caster, the grains in the strip  20  are substantially undeformed and retain their initial structure achieved upon solidification, i.e. an equiaxed structure, such as globular.  
      Continuous casting of aluminum alloys according to the present invention is achieved by initially selecting the desired dimension of the nip N corresponding to the desired gauge of the strip S. The speed of the rolls R 1  and R 2  may be increased to a desired production rate, or to a speed that is less than the speed at which the roll separating force increases to a level that indicates that plastic deformation of the casting strip is occurring between the rolls R 1  and R 2 . Casting at the rates contemplated by the present invention (i.e. about 25 to about 400 feet per minute) solidifies the aluminum alloy strip about 1000 times faster than aluminum alloy cast as an ingot cast and improves the properties of the strip over aluminum alloys cast as an ingot.  
      The molten metal M being delivered from the feed tip T is confined within the molding zone  20  by at least an electromagnetic edge dam  15  that is positioned to direct magnetic lines of force perpendicular to the plane  2 - 2  on which the casting is being drawn. In one embodiment, an electromagnetic edge dam  15  is positioned on each side of the casting apparatus. In a preferred embodiment, the molten metal M is confined within the molding zone  20  during casting by a mechanical edge dam  55  in combination with an electromagnetic edge dam  15 , wherein the mechanical edge dam  55  is positioned proximate to the feed tip T and the electromagnetic edge dam  15  is positioned overlying the terminating end of the mechanical edge dam  55  and provides confinement forces along the entire length of the molding zone  20 , as depicted in  FIGS. 6 and 11 .  
      The current and/or frequency utilized by the electromagnetic edge dam  15  to maintain the molten metal M within the molding zone  20  is substantially less than typically required in prior casting apparatuses using electromagnetic edge dams. In prior casting apparatus employing electromagnetic edge dams, high magnetic force fields where required to contain the molten metal, which resulted in induction heating within the molten metal that disadvantageously effected the solidification process. In the present invention, by reducing the magnitude of the required electromagnetic force, the current and/or frequency conducted through the electromagnetic edge dam is also reduced, which in turn advantageously reduces the incidence of induction heating on the sidewall of the molten metal in the molding zone.  
      Without wishing to be bound, but in the interest of further describing the present invention, applicants&#39; believe that the reduction in the electromagnetic force that is required to contain the metal within the molding zone is related to the decreased head height H 2  of the molten metal from the feed tip T, as depicted in  FIG. 3 , as opposed to the greater height H 1  of the molten metal pool disposed atop the roller caster in prior vertical casting apparatuses, as depicted in  FIG. 1 . As discussed above, the height H 1  (or depth) of the molten pool atop the vertically disposed casting rollers is approximately 65% the height of the casting roller R 1 , R 2  and can range from 8 inches to 20 inches, as depicted in  FIG. 1 . Referring to  FIG. 3 , in the present invention, the height H 2  of the molten metal as delivered from the tip feed T to the molding zone  20  can be on the order of about 1 inch, and in some examples may be further reduced to 0.5 inches. Hereafter, the difference in vertical location of the metal level in the tundish and that of the center of the strip being cast is referred to as a “molten metal head”.  
      The relationship between the height of the molten metal head H 2  and the magnetic field density required for containing molten aluminum at different head levels is best described through the following equations. First, the pressure exterted by the molten metal head, which the magnetic field must contain within the molding zone  20  is calculated from:
 
 p=ρgH   2 
          where p is is the magnetic pressure in Pa, ρ is the density of the metal, g is the acceleration of gravity and H 2  is the height of the molten metal head. The pressure produced by the molten metal head in turn determines the strength of the magnetic field that must be produced by electromagnetic edge containment device  15  to contain the molten metal head within the molding zone  20 . In the present invention, the height of the molten metal head H 2  that is being horizontally delivered to the molding zone  20  by the feed tip T may be as low as 0.5 inches. The pressure that is produced by the molten metal head of varying height H 2 , from the feed tip T of the present horizontal roller casting apparatus  10 , was determined using the above equation and is listed in the Table depicted in  FIG. 4 . To summarize the pressure ranged from about 125 Pa for a metal head height H 2  of approximately 0.5 inches (12.7 mm) to about 2,492 Pa for a metal head height H 2  of approximately 10 inches (254 mm).        

      The pressure required to contain the molten metal head H 2  within the molding zone  20  is then used in the following equation to determine the required magnetic field density (B):
 
 p=B   2 /2μ o 
          where p is the magnetic pressure in Pa (Pascals), B is the magnetic field density in T (Tesla) and μ o  is the permeability of air (= 4 π× 10   −7  H/m). Referring to  FIG. 4 , from the above equation, it is calculated that for a relatively high molten metal head height H 2  for feed tip T delivery of approximately 254 mm (10 inch), the magnetic field density needed is 0.079 T (790 Gauss) and a molten metal head height H 2  of approximately 12.7 mm (0.5 inch), the magnetic field density needed is approximately 0.0177 T. As illustrated in  FIG. 4 , reducing the molten metal head height H 2  decreases the magnetic field density that is needed to contain the molten metal M within the molding zone  20 . The magnetic field density required to contain metal head heights consistent with the present invention can be obtained with electromagnets at relatively low current levels. In one embodiment, the electromagnetic edge dam operates at approximately 2000 ampere turns (i.e. a coil of 10 turns drawing  200  A).        

      In another aspect of the present invention, the physical positioning of the electromagnetic edge dam, the molten metal head height and the strength of the magnetic field can be varied to control the positioning of the edge of the molten metal within the molding zone with respect to the roller casters R 1 , R 2  sidewall. The strength of the magnetic field at different distances from the face (edge) of the roller casters may be calculated by the following equation:
 
B L =(μ o    nI/ 1)/{(2 D/H )sin  h ( L/ 1)+( w/ 1)cos  h ( L/ 1)}
          where:     B L =magnetic field intensity at a distance L (m) in the gap from the roll face.     nI=coil turns and current.     w=roll gap 1=√(μ r  δw/2) in which μ r =relative permeability of steel caster roll (taken as 600), δ=skin depth for steel (material of the caster roll), and w is the roll gap.     D=distance between electromagnet pole and the roll face.     H=height of magnet pole.        

      Referring to  FIG. 5 , using the above equation, the magnetic field strength was calculated and plotted as a function of the frequency of current (Hz) conducted through the electromagnetic edge dam  15 , in which the distance at which the magnetic field strength was calculated ranged from 10 mm to 80 mm inward from the sidewall of steel casting rolls (reference line 1=10 mm, reference line 2=20 mm, reference line 3=30 m, reference line 4=40 mm, reference line 5=60 mm, and reference line 6=60 mm). In each of the calculations, the height (H) of the magnetic pole was set at 8 mm, the distance (D) between the electromagnetic pole and the roll face was set at 4 mm, and the roll gap (w) was set at 4 mm. Additionally, reference lines where plotted to indicate the minimum the field strength required to contain a metal head having a height H 2  equal to 250 mm (reference line  7 ), 150 mm (reference line  8 ), 100 mm (reference line  9 ), and 50 mm (reference line  11 ). The plot depicted in  FIG. 5  illustrates that the 0.079 T field density required for the 250 mm metal head  8  could be created by this electromagnet in distances as far as 20 mm into the roll gap.  
      The edge of the casting strip can therefore be contained inwards from the casting roll R 1 , R 2  face (sidewall), if desired, by increasing the current in the edge dam. It is noted that the field density decreases rapidly at longer distances from the roll face and only small metal head heights, on the order of 50 mm, can be contained in distances 40 mm or greater by the operation of this edge dam at 2000 amp turns. The range of containment can be extended further, if needed, by increasing the magnetomotive force (nI) on the edge dam. When increasing the electromagnetic force, due consideration need to be given to the heating effect of the edge dam.  
      It is further noted that the plot depicted in  FIG. 5  also illustrates that the electromagnetic edge dam as utilized in the present invention would operate effectively at any chosen frequency. The loss in magnetic field becomes noticeable only for operation at frequencies greater than 10 kHz.  
      In addition to the height of the molten metal head and the magnetic field density, the positioning of the electromagnetic edge dam with respect to casting rollers may also be adjusted to provide electromagnetic force lines to confine the molten metal M within the molding zone  20 . Referring to  FIG. 6 , the electromagnetic edge dam  15  may be positioned wherein the poles of the magnetic member are aligned to the sidewalls  13  of the casting rollers R 1 , R 2 . In one embodiment, the electromagnetic edge dam may be positioned wherein the poles of the magnetic member are distal from the sidewalls of each casting roller R 1 , R 2 . In the embodiments of the present invention in which a horizontal belt casting apparatus is employed as depicted in  FIG. 2   a , the electromagnetic edge dam  15  may be positioned wherein each pole of the magnetic member is distal from to aligned to the adjacent sidewall of the casting belts B 1 , B 2 . For the purposes of this disclosure the term “distal from to aligned to the adjacent sidewall of the casting belts” is intended to denote that the poles of the electromagnetic edge dam do not extend towards the casting apparatuses centerline beyond a plane defined by the sidewall of the casting belts, but are positioned within close enough proximity to the sidewall of the castings belts to provide a sufficient magnetic field to contain molten metal within the molding zone.  
      The inventive electromagnetic edge dam will also perform in casters with rolls made from a non-magnetic (non-ferromagnetic) material, such as copper. However, when the rollers comprise a non-magnetic material, the penetration of the magnetic field into the roll gap may be limited and thus containment will typically occur on a plane close to the end of of the rolls. It may be possible to obtain penetration into the gap by coating with a ferromagnetic material (such as iron, nickel or cobalt) the end faces and casting surfaces  200  of such rolls to the required depth of containment, as depicted in  FIG. 8   d.    
      It is noted that prior casting apparatuses typically shape the magnetic poles of the electromagnetic devices and the casting rolls to focus the magnetic field towards the molding zone. In one example, prior casting rollers employ lips extending from the sidewall of each roller and may have further included magnetic poles having a geometry corresponding to the extending lips of prior casting rollers. Contrary to prior casting apparatuses, the present invention does not require specially configured casting rollers to facilitate the focus of the magnetic field produced by the electromagnetic edge dam. In one embodiment of the present invention, the sidewalls  113  of the casting rollers R 1 , R 2  may be substantially planar. Further, the electromagnetic edge dam  15  of the present invention may be positioned so that the face of the electromagnetic edge containment device is aligned to the face of the casting roller&#39;s planar sidewall  113 , wherein the electromagnetic edge dam  15  is in close proximity to the casting rollers R 1 , R 2 . The electromagnetic edge dam  15  may also be positioned distal from the casting roller&#39;s sidewall  113 . Regardless of the positioning of the electromagnetic edge dam  15 , the electromagnetic edge dam  15  is positioned to provide sufficient electromagnetic force to contain the molten metal M within the molding zone  20 .  
      The positioning of the edge dams  15  may be dependent on the current or frequency utilized in the edge dam. For example, lower currents may provide lower magnitude electromagnetic force line and therefore be more likely to require that the edge dam  15  be positioned in closer proximity to the molding zone  20 . The higher the current conducted through the electromagnetic edge dam the greater the magnitude of the electromagnetic force lines and hence the father away the electromagnetic edge dams may be positioned from the molding zone.  
      Referring to  FIGS. 7   a - 7   c,  in one embodiment, the positioning of the electromagnetic edge dam  15  and the magnitude of the electromagnetic force lines are selected to form a substantially flat sidewall ( FIG. 7   a ), a convex sidewall ( FIG. 7   b ), or concave sidewall ( FIG. 7   c ) in the molten metal M within the molding zone  20 . In one example, a current of 2200 Amp/turns produces a casting strip having a concave sidewall; a current of 1200 Amp/turns produces a casting strip having a substantially flat or straight sidewall; and a current of on the order of 1200 Amp/turns produces casting strip having a substantially convex sidewall. It is noted that the above examples are provided for illustrative purposes only and are not intended to limit the present invention, as any current is applicable to the present invention, so long as the current provides sufficient containment forces to the molding zone  20  and does not result in excessive induction heating. In some of the preferred embodiments of the present invention, in which the casting strip&#39;s sidewall is concave or convex, the curvature of the sidewall may be defined by a radius that is approximately half the molten head height.  
      In another embodiment, the electromagnetic edge dam  15  may be configured to provide molten metal within the molding zone having a convex sidewall relative to the centerline of the molten metal M within the molding zone  20 . Preferably, the sidewall of the molten metal within the molding zone is substantially aligned to the planar surface of the roller casters, as depicted in  FIGS. 8   a  and  8   c.  Alternatively, the electromagnetic edge dam  15  may be configured to project magnetic lines of force beyond the sidewall  113  of the casting rollers, wherein the molten metal is confined interior to the edge of the roller casters, as depicted in  FIGS. 8   b  and  8   d.    
      The electromagnetic edge dam&#39;s  15  structure is illustrated in detail in  FIG. 8   a,  representing a sectional view of the edge dam apparatus  15  illustrated in  FIG. 2   a.  In the preferred embodiment of the invention, the electromagnetic edge dam  15  is a magnet type of confinement system and includes a generally C-shaped magnetic member. The magnetic member  30  thus includes a core  32  having an upper arm or pole  34  and a lower arm or pole  36  extending therefrom to define a generally C-shaped cross section. The core  32 , includes an induction coil winding  38  comprising a coil wound about the core  32  of the magnetic member  30  to establish an induction coil. Thus, the winding is composed of a plurality of conductors wound about the core  32  of the magnetic member  30 . The core windings  38  about the core  32  can be, made of solid metal such as copper wire.  
      Still referring to  FIG. 8   a,  the upper arm  34  terminates in a pole face  42  where as the lower arm  36  terminates in a pole face  44 , respectively, with the molten metal M being maintained therebetween. The pole faces  42  and  44  thus define the surface from which the magnetic lines of force generated by the magnetic element  30  with its induction coil  38  pass from one of the pole faces  42  to the other pole face  44 , as illustrated by the magnetic lines of force  48 .  
       FIGS. 9   a - 9   c  illustrate different pole face  44  angles and orientations in accordance with the present invention. It will be appreciated by those skilled in the art that as the inter-pole-face gap  43  increases, the strength of the field across the gap decreases.  FIG. 9   a  illustrates a cross section of a magnetic member  30  wherein the pole faces  42  and  44  have a negative angle relative to the vertical plane substantially perpendicular to the plane on which the casting is being drawn. The negative angle means that the inter-pole-face gap  43  is less at the outside edge of each pole than at the inside edge of each pole face. As a result, the containment forces created by the magnetic member shown in  FIG. 9   c  are stronger at the outside edge of each pole face than at the inside edge of each pole face.  FIG. 9   b  illustrates a cross section of a magnetic member  30  wherein the pole faces  42  and  44  have no angle relative to the vertical plane substantially perpendicular to the plane on which the casting is being drawn. The zero angle means that the inter-pole-face gap  43  is the same at the inside edge of each pole face and the outside edge of each pole face. As a result, the magnetic field created by the magnetic member shown in  FIG. 9   b  is relatively uniform across each pole face.  FIG. 9   c  illustrates a cross section of a magnetic member  30  having pole faces  42  and  44  that are parallel in part and not parallel in part. The inside region of the pole faces  42  and  44  have a negative angle relative to the horizontal.  
      In one embodiment of the present invention, the magnetic member  30  is formed from a ferromagnetic material such as silicon steel, and can be formed from a solid piece of such ferromagnetic material. Alternatively, the magnetic member  30  can be formed from multiple ferromagnetic materials, such as the split core design depicted in  Figure 10 . In another embodiment, the magnetic member  30  can be formed from a series of laminated elements machined and secured together using mechanical means, an adhesive or like means to yield the desired configuration, as depicted in  FIG. 1l . In many instances, the use of such laminates is preferable, because laminates may serve to more uniformly distribute the flux lines in the magnetic member and reduce loss due to saturation of the magnetic member. In addition, for a magnetic member made of laminated ferromagnetic material, the electrical energy dissipated as heat is also more evenly distributed and more easily removed, particularly where the adhesive employed to hold the laminate elements together has good thermal conductivity.  
      Referring back to  FIGS. 8   a - 8   d , surrounding the magnetic member  30  is an outer shield  50 , which is preferably made of a material, and most preferably a metal, having structural rigidity and extremely high electrical and thermal conductivities. Preferably, the outer shield  50  is fabricated of copper, although other metals such as silver and gold can likewise be used. The high electrical conductivity of the outer shield  50  aids in containing the magnetic lines of force within the magnetic member while the good thermal conductivity aids in the dissipation of heat from the overall apparatus. As will be appreciated by those skilled in the art, the outer shield  50  may be provided with cooling channels therein or brazed tubes thereon to distribute cooling fluid through or at the surface of the outer shield to further aid in the removal of heat generated by the electromagnetic field. For example, an inlet can be employed to pass a cooling fluid through the outer shield for removal from a discharge port when additional cooling capability is required. Thus, the cooling fluid can be passed through a conduit within the outer shield to remove heat generated by the electromagnetic field.  
      The electromagnetic edge dam employed in the practice of the present invention also includes an inner shield  56  dimensioned to fit within the C-shaped configuration of the magnetic member  30 . The inner shield  56  likewise serves to contain the magnetic lines of force generated by the coil  38  of the magnetic member  30 , insuring that the magnetic lines of force are maintained within the magnetic member  30 . In addition, it is also possible, and some times desirable, to include within the inner shield conduit means for the passage of a cooling fluid therethrough where it is desired to increase the ability to dissipate heat from the magnet. It is also possible to do away with the inner shield; especially so when using grain oriented silicon steel laminates where the field lines prefer to flow within the laminates.  
      The path of the magnetic field of the present invention is indicated in  FIGS. 8   a  thorough  8   d.  In  FIG. 8   a , magnetic field flows from one pole of the edge dam to the other in a plane essentially parallel to the side faces of the rolls. It is applicable to metallic rolls which are non-ferromagnetic (such as copper). The field creates the containment forces on the end faces of the rolls.  FIG. 8   b  illustrates the case when the field penetrates into the gap and contains the molten metal inwards from the roll faces. This will be the case for ferromagnetic rolls and strong fields. It can also be achieved by the application of a ferromagnetic coating  200  of sufficient depth to the end faces and end of the casting surface of a non-ferromagnetic roll material, as depicted in  FIG. 8   d.    
      In designing the electromagnetic containment apparatus employed in the practice of this invention, a number of different techniques can be used in dissipating heat generated by the electromagnetic field. As shown in  FIG. 8   c , the windings  40  may be formed of an annular conductor having a central opening  41  extending therethrough. Thus, cooled water can be passed through the central opening of the windings  40  to aid in the dissipation of heat generated by the electromagnetic field. As shown in  FIG. 12 , the core  30  may also be equipped with a cooling conduit  47  extending therethrough; in that way, a cooling fluid can be  370044 - 00038  passed through the cooling conduit  47  to aid in the dissipation of heat generated by the electromagnetic field.  
       FIG. 12  illustrates one preferred embodiment of the present invention, wherein a mechanical edge dam  55  is used in conjunction with an electromagnetic edge dam  15  having a magnetic member  30 . The magnetic member  30  is preceded by the mechanical edge dam  55 . The mechanical edge dam  55  shown should ideally have a ceramic-less surface and comprise magnetic material to reduce the reluctance at the mouth of the molding zone. A ceramic material may also be used to make mechanical edge dam  55  if process conditions preclude the use of a metallic material. In one embodiment of the present invention, the mechanical edge dam  55  is positioned to ensure that the molten metal is contained within the casting apparatus while being delivered from the tundish H to the feed tip T. Once the molten metal M reaches the feed tip T, containment forces are provided by the electromagnetic edge dam  15 . In this arrangement, the service life of the mechanical edge dam  55  is increased by the electromagnetic edge dam  15 , since the electromagnetic edge dam  15  is positioned in the most hostile portion of the casting apparatus.  
      The following examples are provided to further illustrate the present invention and demonstrate some advantages that arise therefrom. It is not intended that the invention be limited to the specific examples disclosed.  
     EXAMPLE 1  
     Confirmation of Electromagnetic Push  
      Aluminum strip was cast in accordance with the present invention using a caster with steel rolls. The strip was then metallographically tested to confirm the effect of the electromagnetic force on the molten metal within the molding zone. Test specimens were formed using a horizontal roller caster and a combination of electromagnetic and mechanical edge dams consistent with the present disclosure. Casting strips of three different thicknesses (2.44 mm, 2.29 mm, and 2.16 mm) were then cast operating the electromagnetic edge dam at 2180 A turns. Samples were then cut from the edges of the strips and were prepared for metallographic examination. It was observed that the center part of the casting strip was pushed inwards as compared to the outer surfaces of the strip, as shown in  FIGS. 14   a  and  14   b.  This observation confirms the confinement effect of the electromagnetic edge dam during casting, since the central portion of the strip is the last to solidify.  
      The depth of the confinement effect into the roll gap was estimated by first measuring the width of the casting strip at room temperature, wherein the width of the casting strip was approximately 400.5 mm. From this measurement, the width of the strip within the molding zone can be estimated as 406 mm by adding the contraction that occurred during solidification and cooling to room temperature.  
      Taking into account that the width of the casting roll is approximately 432 mm, it is evident that the magnetic field pushed the molten center of the casting strip a distance of approximately 13 mm (13 mm=(432(width of roller caster)−406)/2) from the casting roll face on each side of the casting roll. More specifically, by subtracting the calculated width of the casting strip in the molding zone from the width of the casting roller the total displacement produced by the electromagnetic edge dams is calculated. The amount of displacement produced by a single edge dam is calculated by the number of edge dams employed, which in this experiment consisted of two electromagnetic edge dams positioned at opposing ends of the casting rollers.  
      Similar electromagnetic push effects were observed for all three different strip thickness (strip gauge), as summarized in the Table depicted in  FIG. 13 . The degree of magnetic push was measured as the depth of the center portion of the strip with respect to the edges. The magnetic push was somewhat higher for thinner gauge strip, since the narrower roll gap would create a higher field density at any given distance. It is believed that the difference in the magnetic push between the two sides (drive side and operator side) of the caster rolls, as summarized in  FIG. 13 , is attributed to variations in the location of the electromagnets and the mechanical edge dams.  
     EXAMPLE 2  
     Control of Cast Strip Edge Profile  
      The edge profile of the as-cast strip was checked for operation at different magnetomotive force levels in the electromagnet. The edge profile obtained at 2180 A turn operation shown in  FIG. 14  were considered unsuitable for subsequent rolling of the strip unless the edges were trimmed prior to rolling. In order to provide cast strip having edge profiles suitable for rolling without additional machining, the magnetomotive force of the electromagnet was reduced to decrease the push on the central portion of the casting strip so that the edge profile of the strip would be flat or slightly convex.  
      A flat edge profile was obtained in the casting strip at a current level of 180 A (or 1620 A turns) being applied to the electromagnet. To obtain a flat edge profile, the magnetic field should be selected to just offset the pressure produced by the molten metal in the molding zone, which is produced by the metal head with a minor contribution small roll pressure. Referring to  FIG. 15 , the edge of the casting strip made under these conditions was flat and highly straight indicating that it could be rolled without trimming the edges of the casting strip or other additional machining.  
      This strip was rolled in-line successfully through four stands of rolling mills. The casting strip was rolled from a 2.7 mm (0.107 inch) as-cast thickness to a thickness of approximately 0.36 mm (0.014 inch), which corresponded to an 87% reduction in thickness. Referring to  FIG. 16 , the sheet made by this method showed only minor cracks at the edges, which could be removed by trimming prior to coiling.  
      Following proper adjustment of the electromagnetic edge dam, high quality edges are obtained in the as-cast strip which permits rolling to high reduction ratios saving materials and improving the efficiency of the process.  
      While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.