METHOD AND APPARATUS FOR ELECTROMAGNETIC CONFINEMENT OF MOLTEN METAL

Abstract: A method and apparatus for electromagnetic confinement of molten metal during strip casting of molten metal between two belts that wrap around entry pulleys to create a molding zone. The molten metal is delivered to a curved surface of each belt to form strip metal. The molten metal is contained within the molding zone by a pair of edge containment devices. The edge containment devices have a C-shaped magnetic member with an upper pole having an upper pole face and a lower pole face. Current flows through a coil wrapped around a core region of the magnetic member to create magnetic lines of force that flow between the upper pole face and the lower pole face. The C-shaped magnetic member is positioned such that magnetic lines of force pass through the edges of the two belts and the molten metal, and keep the molten metal within the molding zone. According to the present invention, the pole faces can have a positive angle, a negative angle, no angle relative to a horizontal, or a combination thereof. The pole faces have a curved shape to maintain a constant distance between the pole faces and the edges of the two belts. The magnetic member can be formed from a series of bonded elements, a series of elements mechanically held together, or from a solid core. In a particular embodiment, the magnetic member comprises two halves which are removably attached to the core to facilitate replacement of the coil wrapped around the core region.

Description: METHOD AND APPARATUS FOR ELECTROMAGNETIC CONFINEMENT OF MOLTEN METAL Technical Field The present invention relates to the continuous casting of metals, and more particularly, to the electromagnetic confinement of molten metal in a twin belt casting system. Background Art The continuous casting of thin metal strip has been employed in the prior art but with only limited success. Prior techniques for the continuous casting of metal strip have been limited to a relatively small number of alloys; it has been found that if the alloy content of various metals such as aluminum alloy are increased, the as-cast surface quality of the strip deteriorates. One approach employed by the prior art in the strip cast of metals has been what has become known in the art as the twin drum caster. Such devices include a source of molten metal supplied to the space between a pair of counter-rotating, internally cooled drums. The molten metal thus solidifies when it comes into contact with the drums to form a cast metal strip; the drums also assert a compressive force on the solidified metal, and effect hot reduction of the cast alloy immediately after freezing. Such twin drum casters have enjoyed the greatest extent of commercial utilization in the strip casting of metals. Nonetheless, they offer some serious disadvantages arising from the fact that the output of such casting systems is substantially lower than that for other strip casting techniques. In addition, twin drum casting, while providing acceptable surface quality in the casting of high purity aluminum (e.g., foil), suffers from the disadvantages of poor surface quality when used in the casting of aluminum having a high alloy content. In addition, twin drum casters also suffer from the problem of centerline segregation of the alloy due to deformation during solidification. Another conventional technique is the strip casting of metals, and particularly aluminum, involving a twin belt strip casting technique in which two endless moving belts are positioned adjacent each to the other to define a moving molding zone between them. Cooling of the belts is typically effected by contacting a cooling fluid with the side of the belt opposite of the side in contact with the molten metal. Such twin belt strip casting machines have the advantage over twin drum casting machines of providing significant output of cast metal. However, because cooling is typically effected by having a cooling fluid in contact with one side of the belt while either molten metal or a hot cast metal strip is in contact with the other side of the belt, such casting systems give rise to high thermal gradients over the thickness of the belt. Those thermal gradients, dynamically unstable, cause distortion in the belt and, as a result, neither the upper nor the lower belt is substantially flat. The result is that the product thus produced has areas of segregation and porosity. Substantial improvements in the strip cast of metals have been achieved as described in U.S. Patent No. 5,515,908 and 5,564,491. Those patents describe a substantially improved method and apparatus for use in the strip casting of metals using a twin belt technique in which use is made of two endless belts positioned adjacent each to the other to define a molding zone therebetween. At one end of the machine, the belts each pass over an entry pulley which defines a curved surface as each belt passes around the entry pulleys. The molten metal, as disclosed in U.S. Patent No. 5,515,908, is supplied to the curved surface of both of the belts in the molding zone at a time when the belts are supported by the pulleys over which they are advanced, thus, preventing any initial warping of the belts as molten metal is supplied thereto. Thereafter, the cast metal strip is carried between the belts, and supported by the lower belt whereby both the molten metal and the hot cast strip transfer heat to the belts. That heat is then removed from the belts at a time when the belts are not in contact with either the molten metal or the hot cast metal strip to substantially prevent, minimize or eliminate the high thermal gradients which have plagued the prior art. Further improvements in the techniques described in U.S. Patent No. 5,515,908 are disclosed in U.S. Patent No. 5,564,491 in which there is provided a system to control the spacing between the entry pulleys whereby the entry pulleys exert a compression force on the substantially frozen cast strip at the nip of the entry pulleys, that compressive force being sufficient to cause elongation of the cast strip to ensure that the cast strip is in compression in the direction of travel after exiting from the nip to minimize cracking of the cast strip. The strip casting techniques as described above and other strip casting techniques require the use of a containment system to maintain molten metal in the gap between the rolls of the caster. Up to the present, the casting system has employed mechanical edge dams to provide containment of the molten metal. One of the principal advantages of the strip casting system as described in the foregoing patents is that the system is capable of operation at tremendously high speeds. As a result, however, the mechanical edge dams heretofore employed have a short life. They tend to be worn out rapidly by erosion of the hot metal flowing at high velocities in contact with such mechanical edge dams. In addition, such mechanical edge dams provide sites for the formation of skull which has a tendency to be sheared off and thus enters the cast to render the microstructure metallurgically undesirable. Electromagnetic edge dams have been employed in the prior art in the strip casting of metals such as aluminum in twin drum casting systems. Such systems have been generally categorized into one of two distinct systems. The first are those systems that use a combination of a magnet assembly and an AC coil to generate confinement forces and those systems that rely solely on an AC coil to generate the containment forces. The magnetic systems 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 surrounded over the given length of the yoke by a coil carrying an AC current. The magnetic flux generated by the flow of the current in the coil is transmitted to the poles of the magnet through the yoke, and when the flux lines pass through the molten metal, they cause the flow of induced currents in the molten metal. The interaction between the applied field and these induced currents establishes containment forces at the metal surface in the gap. Typically, in systems of that type, 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. Such systems are not without disadvantages, however. Such systems typically have poor operating efficiency by reason of core losses and losses due to magnetic hysterisis when an AC current is applied to the magnetic material. In addition, high amounts of heat are generated, necessitating the need for cooling systems to avoid damage to the magnetic system. The induction coil type confinement system typically employs 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 in the molten metal as well as a time- varying magnetic field on the surface of the molten metal. The current-magnetic field interaction provides the containment forces. Such induction coil systems are generally simpler in design than the magnetic systems. However, the induction coil type systems are limited in terms of maximum metallostatic head which can be supported. That is because very high inductor currents are needed to provide the necessary containment forces, and such high currents are accompanied by extremely high heat which in turn hinders or slows the solidification process. One such magnetic confinement system is disclosed in U.S. Patent No. 4,936,374, illustrating a magnetic type confinement system to support molten metal in the gap between the rolls of a vertical twin roll caster. Other electromagnetic confinement systems are disclosed in U.S. Patent Nos. 4,974,661, 5,197,534, 4,986,339 and 5,487,421. Each of the systems disclosed in the foregoing patents is directed to an electromagnetic containment system for use with twin drum casting apparatus as distinguished from twin belt casting apparatus. Thus, the containment forces need only be great enough to support the metallostatic head in the reservoir of molten aluminum maintained above the nip between the twin drums of the twin drum casting system. In contrast, twin belt casters of the type described in U.S. Patent Nos. 5,515,908 and 5,564,491 present different types of containment problems because of their unique configuration. For example, electromagnetic edge containment systems for use in U.S. Patent Nos. 5,515,908 and 5,564,491 must be capable of addressing the unique problem arising from the squeeze pressure arising from belts positioned in a generally horizontal plane. Thus, there is a need to provide electromagnetic containment systems capable of use with twin belt casting systems of the type described in the foregoing patents. It is accordingly an object of the invention to provide an alternative technique for edge containment which overcomes the foregoing disadvantages. It is a more specific object of the invention to provide a technique for edge containment which is electromagnetic to produce containment involving no physical contact between the molten metal and the containment element. It is yet another object of the invention to provide apparatus for use in the strip casting of molten metal utilizing a pair of endless belts equipped with edge containment apparatus to efficiently provide edge containment of the molten metal. Disclosure of the Invention The concepts of the present invention reside in electromagnetic edge containment apparatus for use in the strip casting of molten metal by means of a pair of endless belts. In the practice of the invention, a pair of endless belts is positioned adjacent each to the other to define a molding zone therebetween. Each belt passes over an entry pulley and defines a curved surface on the belt adapted to receive molten metal. The apparatus also includes means for supplying molten metal to the curved surfaces of each of the belts, the molten metal being confined on each of the lateral edges of the belts by electromagnetic apparatus positioned on each side of the belts adjacent to the molding zone. Thus, the electromagnetic apparatus serves to electromagnetically confine the molten metal in the molding zone in contact with the curved surfaces. In the practice of the invention, the electromagnetic apparatus includes a magnetic member having an upper pole and a lower pole and an induction coil wound about a core portion of the magnetic member whereby magnetic lines of force pass from one of the upper and lower poles to the other. The magnetic member is positioned adjacent each lateral edge of the belt in the area of the molding zone so that the upper and lower pole faces allow the magnetic lines of force to pass through the lateral edges of each of the belts to establish containment forces at the edges of the belts to contain the molten metal in the molding zone between the edges of the belts. It has been found that by positioning the magnetic member adjacent to the edges of each of a ferromagnetic member such as the belts in the molding zone, the magnetic lines of force from the poles of the magnetic member are attracted to the feromagnetic member in the form of upper and lower belts and thus serve to focus containment forces on the molten metal maintained in the molding zone between the upper and lower belt. The attraction of the ferromagnetic member, that is the magnetic belts, thus serves to direct and conserve the energy and hence the confinement forces, focusing those forces on the liquid metal in the molding zone at the edges of the belt. In contrast, such electromagnetic edge containment apparatus as used in the prior art with twin drum casters typically involve loss of substantial energy because the magnetic flux could become dissipated to the drum. In this invention, the belts, formed of a magnetic metal, serve to focus the magnetic lines of force at the edge of the belts to ensure adequate containment forces without substantial loss of energy. According to the present invention, the pole faces can have (1) a positive angle relative to the horizontal so that the inter-pole-face gap is larger at the outside edge of each pole face than at the inside edge of each pole face, (2) no angle relative to the horizontal so that the inter-pole-face gap is the same at the outside edges of each pole face as at the inside edge of each pole face, (3) a negative angle relative to the horizontal so that the inter-pole-face gap is smaller at the outside edge of each pole face that at the inside edge of each pole face, or (4) a combination thereof. The configuration of the pole faces should be chosen so as to create a strong magnetic field and a steep magnetic field gradient across each pole face. In some embodiments of the present invention, a mechanical edge dam precedes the electromagnetic edge dam so that both dams confine the molten metal in the molding zone. In other embodiments, the electromagnetic edge dam is extended to completely replace the mechanical edge dam. According to the present invention, the ferromagnetic member can be formed from layers that are bonded together, layers that are mechanically held together, or a solid core that is cut to form the inter-pole-face gap. In addition, the ferromagnetic member can be formed so that the upper and lower halves of the ferromagnetic member are removably attached so as to facilitate the repair and maintenance of the containment system. Brief Description of the Drawings Fig. 1 is a side view in elevation of a twin belt caster equipped with an electromagnetic edge dam apparatus in accordance with the practice of the invention. Fig. 2 is a sectional view taken along the lines 2-2 in Fig. 1. Fig. 3 is 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 belts of the caster and the molten metal therebetween. Fig. 4 is a view in perspective of one magnetic member suitable for use in the practice of the present invention. Fig. 5 is a sectional view of an electromagnetic edge containment apparatus in accordance with another embodiment of the invention. Fig. 6 is a sectional view of another embodiment of the invention illustrating the use of contoured belts. Fig. 7 is a perspective view of an alternative embodiment illustrating a magnetic member having stepped pole faces. Fig. 8 is a cross-sectional view like Fig. 3 illustrating another embodiment of the invention utilizing cooling channels within the core winding. Figs. 9, 10, 11, and 12 illustrate different pole face angles and orientations in accordance with the present invention. Fig. 13 illustrates an exemplary embodiment of the present invention wherein a mechanical edge dam is used in conjunction with an electromagnetic edge dam. Fig. 14 illustrates a solid structure from which a magnetic member in accordance with the present invention can be formed. Fig. 15 illustrates an exemplary embodiment of the present invention wherein a magnetic member has a split core design. Fig. 16 illustrates an exemplary embodiment of the present invention wherein a magnetic member has two regions. Mode for Carrying Out the Invention The apparatus embodied in the practice of the invention is illustrated overall in Figs. 1 and 2 of the drawings illustrating a strip caster employing the concepts of U.S. Patent No. 5,515,908 and 5,564,491 equipped with electromagnetic edge containment apparatus in accordance with the practice of this invention. As there shown, the apparatus includes a pair of endless belts 10 and 12 carried by upper pulley 14 and lower pulley 16. As will be appreciated by those skilled in the art, the belts 10 and 12 are likewise supported by additional pulleys as disclosed in the foregoing patents which are not illustrated in the present drawings for purposes of simplicity. The pulleys are of a suitable heat resistant type, with one of the pulleys in each pair being driven by suitable motor means likewise not illustrated in the drawings for purposes of simplicity. Each of the belts 10 and 12 is an endless belt and is preferably formed of a magnetic metal which has low reactivity or is nonreactive with the metal being cast. A number of suitable metal alloys, as is understood by those skilled in the art, may be employed, including steel belts. As illustrated in Fig. 1, pulleys 14 and 16 are positioned one above the other whereby the belts 10 and 12 passing over those pulleys define curved surface 18 and 20 as the belts 10 and 12 pass over the pulleys 14 and 16, respectively. Thus, the belts, and particularly the curved surface 18 and 20, define a molding zone therebetween, with the minimum gap between them dimensioned to correspond to the desired thickness of the metal strip being cast. As will be appreciated by those skilled in the art, the thickness of the metal strip being cast is determined by the dimensions of the nip between belts 10 and 12 passing over pulleys 14 and 16 along a line through the axis of pulleys 14 and 16 perpendicular to belts 10 and 12. While it is not illustrated in Fig. 1 for purposes of simplicity, it is some times desirable to include, as disclosed in U.S. Patent No. 5,564,491, the disclosure of which is incorporated herein by reference, an apparatus to rigidly fix the relative positions of pulleys 14 and 16. As described in the foregoing patent, means to fix the relative positions of those pulleys may be a tension member, which is either fixed or adjustable, such as a turn buckle, a pillow block or a hydraulic cylinder as disclosed in Patent No. 5,564,491. As described in that patent, the means controlling the spacing between entry pulleys 14 and 16 serves to effect a compressive force on the substantially frozen cast strip at the nip between pulleys 14 and 16 to cause elongation thereof so that the cast strip is in compression in the direction of travel after exiting from the nip to minimize cracking of the cast strip. Molten metal to be cast is supplied to the molding zone through suitable metal supply means 22 such as a tundish, substantially corresponding in width to the width of the narrower of the belts 10 and 12. As will be appreciated by those skilled in the art, the tundish is a conventional means of supplying molten metal to strip casters. Thus, the molten metal flows into the molding zone defined between the curved surface 18 and 20 of belts 10 and 12, respectively, passing substantially horizontally from the tundish to fill the molding zone between the curved surface 18 and 20. As is described in U.S. Patent No. 5,515,908, the molten metal begins to solidify and is substantially solidified by the point at which the cast strip reaches the nip of pulleys 14 and 16. As is described in U.S. Patent No. 5,515,908, the molten metal flows substantially horizontally to provide a flowing stream of molten metal to the molding zone where it is in contact with the curved surface 18 and 20 of belts 10 and 12, respectively, as those belts pass around pulleys 14 and 16. That serves to limit distortion of the belts and maintain better thermal contact between the molten metal and each of the belts as well as improving the quality of the top and bottom surfaces of the cast strip. In accordance with the concepts of the invention, the casting apparatus is equipped with a pair of electromagnetic edge dams 24 and 26 positioned immediately adjacent the molding zone defined by the curved surface 18 and 20 on each lateral edge of the belts 10 and 12 as illustrated in Fig. 2. Electrical current to the electromagnetic edge dam is supplied by means of electrical conduits 26 as illustrated in Fig. 1. The details of the electromagnetic edge dam apparatus are illustrated in detail in Fig. 3 of the drawing representing a sectional view of the edge dam apparatus 24 illustrated in Fig. 2. In the preferred practice of the invention, the electromagnetic edge containment apparatus employed in the practice of the invention is a magnet type of confinement system as described above and includes a generally C-shaped magnetic member 30 as illustrated in Fig. 3. 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. As illustrated in Fig. 3, an induction coil winding 38, composed of a plurality of turns 40, is wound around the core 32 of the magnetic member 30. As is also disclosed in Fig. 3, the upper arm 34 terminates in a pole face 42 where as the lower arm 36 terminates in a pole face 44 positioned adjacent belts 10 and 12, respectively, with the molten metal 46 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 as show in Fig. 3. It is not important in the practice of the present invention whether the magnetic lines of force pass from the upper pole face 42 to the lower pole face 44 or vice versa. In one practice of the 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 a series of laminated elements machined and secured together using mechanical means, an adhesive or like means to yield the desired configuration. In many instances, the use of such laminates is preferable since such laminates 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. 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. Particularly good results are obtained when 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 52 can be employed to pass a cooling fluid through the outer shield for removal from a discharge port 54 as illustrated in Fig. 3 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 as illustrated in Fig. 3. 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 relationship between the electromagnetic edge containment apparatus and the belts is also illustrated in Fig. 3 of the drawings. As can be seen there, the upper belt 10 and the lower belt 12 define a molding zone with the molten or liquid metal 46 being positioned therebetween. The lateral edges of the belts 10 and 12 are positioned adjacent to the inner shield so that the magnetic lines of force passing out of one of the upper pole face 42 and the lower pole face 44 pass through the upper and lower belts 10 and 12 and the liquid metal 46. The magnetic field lines thus establish the containment force preventing the liquid metal from spilling over the edges of the upper and lower belts 10 and 12, respectively. As indicated above, the belts employed in the practice of the present invention have ferromagnetic characteristics, causing them to attract flux lines to themselves and thereby minimizing field losses between the poles 42 and 44. That in turn maximizes the field available at the space between the belts. The magnetic nature of the belts also minimizes the gradient of the magnetic field reducing the magnetic pressure that can be generated. In accordance with the preferred practice of the invention, it is important to embody a high gradient magnetic field on the belt to produce sufficient magnetic pressure after allowing for the reduction of the field gradient due to the magnetic nature of the belts. To produce that desired high magnetic field gradient at the belts, it is preferred to orient the poles at an angle relative to the belts. The lines of force illustrated in Fig. 3 illustrate the effects of the belts in modifying the field line distribution in the vicinity of the molding gap. The exact placement of the lateral edges of the belts relative to the electromagnetic dam is not critical to the practice of the invention, and can be varied. It is generally sufficient that the lateral edges of belts 10 and 12 be positioned such that most, if not all, the magnetic lines of force pass substantially immediately through belts 10 and 12. That ensures that sufficient containment forces will be generated to prevent the liquid metal from spilling over the lateral edges of the belt adjacent to the electromagnetic containment apparatus. The configuration of one embodiment of the magnetic member 30 is shown in Fig. 4 of the drawing which includes the base portion or core 32 about which the coil is wound, the coil winding having been omitted from Fig. 4 for purposes of simplicity. The magnetic element 30 also includes upper and lower arms or poles 34 and 36, respectively, which are formed integrally with the core 32, whether the magnetic element 30 is a solid piece of metal or a laminated structure as described above. Formed in the leading edges of the arms or poles 34 and 36 are the pole faces 42 and 44. The magnetic member 30 is preferably formed from a ferromagnetic material such as silicon steel. The magnetic member 30 can be formed from layers that are bonded together, layers that are mechanically held together, or from a solid core. In some instances, the use of laminates is preferred because there is a more uniform distribution of flux lines in the magnetic member and less loss due to saturation. If the laminate has good thermal conductivity, the heat is more evenly distributed and easier to remove. In other instances, it is better to mechanically hold the layers together. As shown in Fig. 4, the pole faces 42 and 44 have a curvature corresponding substantially to the curvature of the belts 10 and 12 adjacent to the electromagnetic containment apparatus. It has been found that best results are generally obtained when the pole faces have an arcuate or curved configuration so as to maintain a constant distance between the belts 10 and 12 as they pass around their respective pulleys and the pole faces themselves. As shown in Fig. 4, the pole faces of the magnetic member 30 have a substantially smooth surface corresponding to the curvature of the belts passing around the respective entry pulleys. As will be appreciated by those skilled in the art, however, it is also possible, and sometimes desirable from the standpoint of economies in manufacture, to approximate a smooth curved surface with a surface formed of a series of stepped elements. A magnetic member utilizing such stepped pole faces is illustrated in Fig. 7 of the drawing. As shown in that figure, the pole faces 42 and 44 are formed of a series of discrete substantially rectangular faces 45 which approximate the smooth curved pole faces as illustrated in Fig. 4. As will be appreciated by those skilled in the art, the use of such stepped pole faces represents a distinct advantage in economies in manufacture of the magnetic member 30. As shown in that figure, the core 32 may be equipped with a cooling conduit 47 extending therethrough; in that way, a cooling fluid can be passed through the cooling conduit 47 to aid in the dissipation of heat generated by the electromagnetic field. In the preferred embodiment as shown in Fig. 3, the positioning of the lateral edges of the belts 10 and 12 relative to the pole faces 42 and 44 can conveniently be controlled by utilizing pulleys 14 and 16 having a belt engaging surface 58 which is axially of diminished width as compared to the remainder of the pulley 14. In that way, the belts 10 and 12 can be positioned in closer proximity to the electromagnetic containment apparatus by dimensioning the belts to extend beyond the belt supporting surface 58 of the pulleys 14 and 16 as illustrated in Fig. 3. In an alternative embodiment, it is sometimes preferable as shown in Fig. 5 to provide an annular copper ring 60 between the pulleys 14 and 16 and the electromagnetic containment apparatus. That copper ring modifies the magnetic field in the vicinity of the molding zone to prevent the magnetic lines of force from entering the rolls 14 and 16. In accordance with another variation of the present invention as shown in Fig. 6, it is sometimes desirable to configure the belts 10 and 12 with lips 62 and 64, respectively. Those lips can simply be formed by increasing the thickness of the lateral edges 66 and 68 of belts 10 and 12, respectively, so as to provide a lip area of increased belt thickness at the lateral edges of the belts. As shown in Fig. 6, providing belts with increased thickness toward lateral edges improves the magnetic field strength as well as the field distribution region between the belts 10 and 12, thereby maximizing the containment forces near the molding zone. In designing the electromagnetic containment apparatus employed in the practice of this invention, a number of different techniques can be employed in dissipating heat generated by the electromagnetic field. In the embodiment illustrated in Fig. 3, the core windings 40 about the core 32 can be, as illustrated in Fig. 3, made of solid metal such as copper wire. Alternatively, as shown in Fig. 8, 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. Figs. 9, 10, 11, and 12 illustrate different pole face angles and orientations in accordance with the present invention. Fig. 9 illustrates a cross section of a magnetic member 30 wherein the pole faces 42 and 44 have a positive angle relative to the horizontal. The positive angle means that the inter-pole-face gap 43 increases as the distance from the core increases, i.e. the inter-pole-face gap 43 is greater at the outside edge of each pole face than at the inside edge of each pole face. The angle of the pole faces should be chosen so as to create a strong magnetic field and a steep magnetic field gradient across the pole face. A pole face can have any desired angle such as 5, 10, 15, 20, 25, or 30 degrees relative to the horizontal. 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. As a result, the containment forces created by the magnetic member shown in Fig. 9 are stronger at the inside edge of each pole face than at the outside edge of each pole face. Fig. 10 illustrates a cross section of a magnetic member 30 wherein the pole faces 42 and 44 have no angle relative to the horizontal. 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. 10 is relatively uniform across each pole face. Fig. 11 illustrates a cross section of a magnetic member 30 wherein the pole faces 42 and 44 have a negative angle relative to the horizontal. 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. 11 are stronger at the outside edge of each pole face than at the inside edge of each pole face. Fig. 12 illustrates a cross section of a magnetic member 30 having a reverse angle configuration. The pole faces 42 and 44 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. The magnetic member shown in Fig. 12 and the magnetic member shown in Fig. 11 both result in a narrower strip than the magnetic member shown in Fig. 9. When the magnetic member shown in Fig. 9 is used to contain molten metal, the magnetic forces are at a maximum at the very edge of the belt. When the magnetic member shown in Figs. 11 or 12 are used to contain molten metal, the magnetic forces are at a maximum farther into the belt. As a result, the strip width would be lower when using magnets with a design as shown in Fig. 1 1 and 12 as compared to the designs shown in Fig. 9 and 10. Fig. 13 illustrates an exemplary embodiment of the present invention wherein a mechanical edge dam 55 is used in conjunction with an electromagnetic edge dam having a magnetic member 30. The magnetic member 30 is preceded by the mechanical edge dam 55. It is possible to replace the mechanical edge dam 55 completely. However, as mentioned above, the electromagnetic edge dam should curve to mirror the curved surface of the belts. When the pole faces 42 and 44 spread apart to mirror the curved surface of the belts, the inter-pole-face gap 43 increases and the strength of the magnetic field across the gap decreases. For this reason, the mechanical edge dam 55 is used in conjunction with the extended magnetic member 30 at the point where the magnetic field is the weakest. 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. Fig. 14 illustrates a solid structure 70 from which a magnetic member 30 in accordance with the present invention can be formed. As mentioned above, the magnetic member 30 can be formed from layers that are bonded together, layers that are bolted together, or a solid core that is cut to form the inter-pole-face gap. It will be appreciated by those skilled in the art that currents can be induced in the individual laminates and that the currents can lead to the generation of heat. For this reason, it may be advantageous to cut the magnetic member from a solid core. If, for example, a silicon steel sheet is wound into a racetrack-like shape 70 that corresponds to the external shape of the magnet 30, the pole faces 42 and 44 can be formed from the block 70 by removing a piece of metal 71 from the region where the inter-pole-face gap should be located, see the open region or gap in figure 15. This method of manufacturing the magnetic member 30 is usually less expensive than other methods of manufacturing. More importantly, when the silicon steel is very thin (approximately 1 mil thick) this method minimizes the heat induced in a stacked design and facilitate the cooling of the magnetic member 30. Fig. 15 and 16 illustrate an exemplary embodiment of the present invention wherein a magnetic member 30 has a split core design. It will be appreciated by those skilled in the art that as the containment system is used, insulation around the induction coil 40 begins to deteriorate and has to be replaced. In conventional systems, the coil 40 is replaced by dismounting the magnet 30 from the belt caster, cutting the coil 40, removing the coil 40, and winding a new coil (not shown) around the magnet 30. In addition, if the magnet 30 is damaged by one of the endless belts, it is necessary to replace the entire magnet 30. According to the embodiment shown in FIG. 15, the magnetic member 30 has a split core design wherein the magnetic member 30 has an upper region 81 and/or a lower region 82 that is removably attached to the central region 80 to form the C-shaped magnetic member 30. As a result, it is significantly easier to replace the induction coil 40. If the upper region 81 or the bottom region 82 is removed from the central region 80, it is possible to remove the induction coil 81 and insert a new pre-wound coil (not shown). Furthermore, if the upper region 81 or bottom region 82 is damaged by one of the belts, it is not necessary to replace the entire magnet 30. The top and bottom regions 81 and 82 can be replaced by connecting a new top or bottom region (not shown) to the central region 80. In the embodiment shown in FIG. 16, the magnetic member 30 has a split core design wherein the magnetic member 30 is formed of laminates and has an upper region 83 and/or a lower region 84 that attach to form a C-shaped magnetic member 30. It is easier to replace the induction coil in this design; the magnetic member is first disassembled, and a new pre-wound coil inserted before the magnetic member is re-assembled. As stated above, if the upper region 83 or bottom region 84 is damaged by one of the belts, it will not be necessary to replace the entire magnet 30. The damaged half of the magnetic member can be replaced by connecting a new top 83 or bottom region 84. The upper 83 and lower 84 regions are mechanically held together to maintain the proper orientation. For example, bolts 85, 86 are used to hold the laminates of each half together. Thereafter, plates 87 on each end attach to the two bolts 85, 86 and keep the two regions together. It is preferred that the bolts and plates be made of non-magnetic material, preferably stainless steel. It will be understood that various changes and modifications can be made in the details of construction and use without departing from the spirit of the invention, especially as defined in the following claims.

Claims: WHAT IS CLAIMED IS: 1. An apparatus for strip casting of molten metal using electromagnetic edge containment comprising: (a) a pair of endless metal belts positioned adjacent each to the other to define a molding zone therebetween, with each belt passing over an entry pulley to define a curved surface adapted to receive molten metal thereon, (b) means for supplying molten metal to the curved surfaces of each of the belts to supply molten metal thereto, and (c) edge containment apparatus positioned on each side of the molding zone to contain the molten metal in the molding zone in contact with the curved surfaces, said apparatus including a magnetic member having an upper pole and a lower pole, an induction coil wound about a portion of the magnetic member to generate magnetic lines of force passing from one of the upper and lower poles to the other, with the magnetic member being positioned such that the upper and lower poles direct magnetic lines of force through the edges of each of the belts to establish containment forces at the edges of the belts to contain the molten metal therebetween. 2. An apparatus as defined in claim 1 which includes shield means positioned about the magnetic member to contain the magnetic lines of force within the magnetic member. 3. An apparatus as defined in claim 1 which includes an inner shield positioned within the magnetic member to contain the magnetic lines of force to the magnetic member. 4. An apparatus as defined in claim 1, wherein the magnetic member has a generally C-shaped configuration, including a core portion and parallel poles integral with and extending therefrom. 5. An apparatus as defined in claim 4, wherein the poles of the magnetic member terminate in pole faces positioned adjacent the endless belts whereby the magnetic lines of force pass from one of the pole faces through the belts and to the other of the pole face to thereby establish containment forces for molten metal maintained in the molding zone between the endless belts. 6. An apparatus as defined in claim 4, wherein the induction coil is wound about the core of the magnetic member. 7. An apparatus as defined in claim 1, wherein the magnetic member is formed of a ferromagnetic material from a series of laminated elements secured together. 8. An apparatus as defined in claim 1 which includes shield means positioned between the magnetic member and the entry pulleys to shield the entry pulleys from the magnetic lines of force. 9. An apparatus as defined in claim 5, wherein the pole faces are positioned adjacent to and at an angle with respect to the endless belts to ensure a gradient of magnetic flux at the lateral edges of the endless belts. 10. An apparatus as defined in claim 1, wherein the belts each have a lip formed by gradually increasing the thickness of the belts at their lateral edges adjacent the edge containment apparatus, said lips serving to modify the strength and distribution of the magnetic lines of force to confine liquid metal between the belts. 11. An apparatus as defined in claim 1, wherein the magnetic member is formed of a ferromagnetic material from a stack of bonded elements. 12. An apparatus as defined in claim 1, wherein the magnetic member is formed of a ferromagnetic material from a plurality of sections that are mechanically held together. 13. An apparatus as defined in claim 1, wherein the magnetic member is formed from a solid core of ferromagnetic material. 14. An apparatus as defined in claim 1, wherein the pole faces are parallel. 15. An apparatus as defined in claim 1, wherein the pole faces are not parallel. 16. An apparatus as defined in claim 1, wherein the pole faces have a first opposed surface parallel to each other and a second opposed surface not parallel to each other. 17. An apparatus as defined in claim 1, wherein a distance between the pole faces is greater at the outside edge of each pole face than at the inside edge of each pole face. 18. An apparatus as defined in claim 1, wherein a distance between the pole faces is greater at the inside edge of each pole face than at the outside edge of each pole face. 19. An apparatus as defined in claim 1, further comprising a mechanical edge dam that is positioned in a region along the belts where the magnetic lines of force are the weakest. 20. An apparatus as defined in claim 1, wherein either the upper or the lower half of the magnetic member is removable so as to facilitate replacement of the induction coil. 21. An electromagnetic edge containment device for a belt casting system for casting molten metal, the containment device comprising: a magnetic member having a first pole, a first pole face, a second pole, and a second pole face; and a coil that generates magnetic lines of force in the magnetic member when current is supplied to the coil, the magnetic lines of force passing from the first pole face through an edge of the belt casting system to the second pole face. 22. An apparatus in accordance with claim 21, further comprising the belt casting system, the belt casting system having two belts that mold the molten metal. 23. An apparatus in accordance with claim 22, wherein the magnetic lines of force contain the molten metal in an edge region between the two belts. 24. An apparatus in accordance with claim 22, wherein the two belts have a curved region and a linear region. 25. An apparatus in accordance with claim 24, wherein the magnetic lines of force contain the molten metal in a region between the curved region of the two belts. 26. An apparatus in accordance with claim 22, wherein each belt has an edge that passes within a region between the first pole face and the second pole face. 27. An apparatus in accordance with claim 24, further comprising a pulley defining the curved region and a shield positioned between the magnetic member and the pulley to shield the pulley from the magnetic lines of force. 28. An apparatus in accordance with claim 26, wherein the belts have a curved region in a gap between the first pole face and the second pole face. 29. An apparatus in accordance with claim 28, wherein the first pole face and the second pole face become farther apart along the curved region. 30. An apparatus in accordance with claim 29, wherein a mechanical edge dam is located in a region along the belts where a gap between the first pole face and the second pole face is at a maximum. 31. An apparatus in accordance with claim 21, further comprising an outer shield surrounding the magnetic member so as to contain the magnetic lines of force within the magnetic member. 32. An apparatus in accordance with claim 21, further comprising an inner shield positioned within the magnetic member so as to contain the magnetic lines of force within the magnetic member. 33. An apparatus in accordance with claim 21, wherein the magnetic member includes a cooling passage for cooling the magnetic member. 34. An apparatus in accordance with claim 21, wherein the belts have a thickness which increases towards an outer edge thereof. 35. An apparatus in accordance with claim 23, wherein the first pole face is not parallel to the second pole face. 36. An apparatus in accordance with claim 23, wherein the first pole face is parallel to the second pole face. 37. An apparatus in accordance with claim 23, wherein the first pole face and the second pole face have a first opposed surface parallel to each other and a second opposed surface not parallel to each other. 38. An apparatus in accordance with claim 23, wherein a distance between the first pole face and the second pole face is greater at an outside edge of each pole face than at an inside edge of each pole face. 39. An apparatus in accordance with claim 23, wherein a distance between the first pole face and the second pole face is greater at the inside edge of each pole face than at an outside edge of each pole face. 40. An apparatus in accordance with claim 21, wherein the magnetic member is formed of a ferromagnetic material from a stack of bonded elements. 41. An apparatus in accordance with claim 21 , wherein the magnetic member is formed of a ferromagnetic material from a plurality of sections that are mechanically held together. 42. An apparatus in accordance with claim 21, wherein the magnetic member is formed from a solid core of ferromagnetic material. 43. An apparatus in accordance with claim 21, wherein the first pole and/or the second pole is removably attached to the magnetic member so as to facilitate replacement of the coil. 44. A method of containing molten metal between two belts in a strip casting system, said method comprising the steps of: delivering molten metal to a molding zone at an upstream end of the belts; solidifying the molten metal into a strip between the belts and conveying the strip between the belts in a downstream direction; containing molten metal at later edges of the belts in the molding zone by providing electrical current to a coil around a magnetic member to generate magnetic lines of force which contain the molten metal. winding a coil around a magnetic member having an upper pole face and a lower pole face; positioning the magnetic member so that the upper pole face and the lower pole face overlay a lateral edge of each belt; and, providing electrical current to the coil to generate magnetic lines of force. 45. An apparatus in accordance with claim 1 wherein the pole faces are not smooth. 46. An apparatus in accordance with claim 21 wherein the pole faces are not smooth.

Inventors: BHAMIDIPATI JAGAN, BHAMIDIPATI, JAGAN
Applicants: KAISER ALUMINIUM CHEM CORP, KAISER ALUMINUM & CHEMICAL CORPORATION
Application year: 1998
Main classification: B22D11
IPC codes: B22D11