Abstract:
A twin-belt casting machine for casting metal strip. The machine is provided with a casting cavity which includes an upstream fixed casting region, in which the belts are in fixed convergent paths in contact with the cast slab, and an adjacent downstream portion in which the belts are adjustable between alignment with the fixed convergent paths and non-alignment therewith (being less convergent or divergent). When the adjustable portions of the paths are moved outwardly relative to the fixed convergent paths, the belts separate from the cast slab at differing predetermined points within the casting cavity. By adjusting the downstream portion of the casting cavity in this manner, the casting machine can operate at essentially constant throughput for a wide range of alloys while ensuring that the cast slab exiting the caster has a temperature within a predetermined range suitable for further rolling to produce sheet product.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority right of our prior U.S. provisional patent application Ser. No. 60/783,767 filed Mar. 16, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to a process and apparatus for the continuous belt casting of metal strips and, particularly, to the twin-belt casting of metal strips from a variety of molten metals having different cooling requirements and characteristics. 
     (2) Description of the Related Art 
     Twin-belt casting of metal strips typically involves the use of a pair of endless belts, usually made of flexible, resilient steel bands or the like, which are driven over suitable rollers and other path defining means, so that they travel together along opposite sides of an elongated narrow space, typically downward-sloping or horizontal, which forms a casting cavity. Molten metal is introduced between the belts in the vicinity of the upstream entry end of the casting cavity and the metal is discharged as a solidified strip or slab from the downstream exit end of the cavity. 
     An example of a twin-belt casting system can be found in Rochester et al. U.S. Pat. No. 3,163,896, issued Jan. 5, 1965. That patent describes a casting machine in which each belt is circulated, in turn, around a tension roll, a guide roll, at least a pair of sizing rolls and a power roll. The belts are maintained in position to form a casting cavity by the guide rolls and the sizing rolls, such that the cavity after the last sizing roll diverges before feeding onto the power rolls. The sizing rolls, in combination the guide rolls, press against the opposite sides of the belts throughout the cooling and solidification region, and serve to maintain (adjustably, if desired) the selected, predetermined distance between the belts, depending on the thickness desired in the resulting cast strip. 
     In Hazelett et al. U.S. Pat. No. 3,167,830, issued Feb. 2, 1965, a twin-belt casting apparatus is described in which the upper and lower belt assemblies can be moved with respect to each other so as to affect the cavity length/position. This is used to permit flexibility in the type of operation, e.g. pool vs. direct nozzle feed, and thickness. The flexibility does not affect the cavity length when measured as the total length in which the belt actually confines the slab. 
     Wood et al. U.S. Pat. No. 4,367,783, issued Jan. 11, 1983, describes a further twin-belt casting system in which load cells are used to measure the pressure applied to a shrinking metal slab and are the results are then used to apply a corrective taper to the cavity. This adjustment to the taper does not affect the length of the cavity. 
     A still further design is described in Braun et al. WO 97/18049 published May 22, 1997 . This document describes a block caster which can be adapted to have a belt-type liner, and hence behave as a belt caster backed up by a series of connected blocks. The taper of the cavity can be adjusted to meet various metallurgical needs, but there is no description of a system for varying the contact length with the cast strip. 
     Different alloys, e.g. foil alloys versus can-end or automotive alloys, have remarkably different heat flux requirements, i.e. they require very different heat extraction rates to ensure that a good quality cast slab is obtained. As a result, a caster designed to cast foil alloys, requiring a relatively low heat extraction, will have a relatively long cavity. If the same caster is used with a high heat flux suitable for can-end or similar alloys, the amount of slab cooling that occurs along the cavity is too high and the exit temperature of the slab is too low for subsequent processing (e.g. rolling). If the overall convergence of the cavity is lessened to compensate, the surface quality of the slab deteriorates. Thus, there remains a need for a twin-belt caster that, for a wide range of aluminum alloys, can operate at essentially constant throughput yet ensure that the cast slab exiting the caster has a temperature lying within a predetermined temperature range suitable for further rolling to produce a desired sheet product. 
     SUMMARY OF THE INVENTION 
     An exemplary embodiment of the present invention relates to a twin-belt casting system for continuously casting a metal slab in strip form directly from molten metal in which the molten metal is confined and solidified in a parallel, or more usually convergent, casting cavity defined by upper and lower cooled, endless, flexible travelling casting belts supported by respective upper and lower belt supporting mechanisms. In such an embodiment, the portion of the casting belts in direct contact with the cast slab can be mechanically changed within the casting cavity so as to ensure that the slab exit temperature lies within a desired predetermined range, and yet the casting cavity characteristics (e.g. convergence) can be maintained sufficiently high in the upstream end to ensure that good slab quality is achieved for all alloys. This is achieved according to the exemplary embodiment by providing supporting mechanisms for the belts which permit adjustment between one position, in which the cavity is parallel or uniformly convergent and the belts are in contact with the slab substantially along its entire length, and one or more other positions in which the cavity is adapted to switch from parallel or convergent to a different slope, e.g. a less convergent or divergent angle, at a mid-region of the cavity sufficient to break contact between the belts and the cast slab. The sections of different slope may include belts in parallel or divergent paths. With such an arrangement, the first section of the belt remains in contact with the slab over its entire length, whereas the section of different slope (e.g. the less convergent or divergent section) is taken out of contact with the slab and so does not extract heat from the slab. 
     In one illustrative embodiment, the belt is carried by supporting blocks which are typically cooling blocks. One or more of these supporting blocks are mounted on a tiltable assembly whereby they can be adjusted to a position which forces the section of the belts travelling over the tilted supporting blocks from a parallel or convergent path, in which the belts are in contact with the cast slab, to a path in which contact between the belts and the cast slab is broken. 
     Embodiments of the invention also apply to twin-belt casters which use a series of supporting rollers for the belts. In a similar manner as described for the supporting blocks, groups of support rollers may be mounted on tiltable assemblies adapted tilt the belts out of contact with the cast slab at a predetermined location within the casting cavity. 
     Reducing the portion of the cavity in contact with the slab in the above manner significantly reduces the amount of heat being removed from the slab and therefore prevents any over-cooling effect. Where an alloy requiring a lower heat flux for casting is being processed, the tilt mechanism is pivoted so as to bring a greater portion of the casting cavity in contact with the slab, and thus ensure that the slab leaves the casting cavity at substantially the same exit temperature as other metals requiring a higher heat flux. This may require having the entire length of the casting cavity in contact with the slab. 
     Thus, embodiments of the present invention provide a casting machine that, for a wide range of metal alloys (e.g. aluminum alloys), can operate at essentially constant throughput while ensuring that the cast slab exiting the caster has a temperature lying within a predetermined range suitable for further rolling to produce a sheet product. This means that parameters can be established for different alloys and exit temperature requirement so that, depending on those requirements, the position of the adjustable portion of the casting region can be set prior to a casting run. 
     The fixed portion of the casting cavity preferably converges, most preferably with a convergence of about 0.015% to 0.025% (corresponding to the linear shrinkage of the solidified slab), while the adjustable portion may be moved between a position having the same convergence as the fixed portion, and another position having a divergence of as much as 1.0% to significantly reduce the rate of heat extraction through the belts once solidification is appreciably complete. 
     Another exemplary embodiment provides a method of operating a twin-belt caster having rotating belts provided with confronting sections of fixed length to form cast metal strip products from at least two molten metals having different cooling requirements in different casting operations. The method involves establishing for each metal the length and convergence (which may include parallel casting surfaces) of a casting cavity within the caster required to produce a cast product of predetermined characteristics, and, prior to casting each one of the metals, adjusting the paths of at least one of the twin belts in the confronting sections to form an upstream casting cavity having a length and convergence corresponding to those established for the metal to be cast, and a downstream region where the belts loose contact with the metal and cease to exert a significant cooling effect. This makes the casting apparatus more versatile in that many different metals may be cast in a caster having belts provided with confronting sections of fixed length without compromising the desired characteristics, as well as the desired exit temperatures, of the cast products. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a general side view in very simplified form of a twin-belt casting apparatus in which the present invention may be utilized; 
         FIG. 2  is a simplified sectional view of the belt support mechanism of a belt caster showing an embodiment of the invention; 
         FIG. 3  is a perspective view of a pivoting or tilting section; and 
         FIGS. 4A and 4B  are plan views showing details of the connection of the pivoting section. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, an example of a basic belt casting machine to which the present invention may be applied is shown in  FIG. 1 . It includes a pair of resiliently flexible, heat conducing metal bands, forming upper and lower endless belts  10  and  11 . These belts travel in looped paths in the directions of arrows A and B so that, in traversing a region where they are close together (i.e. a confronting section of fixed length), the belts define a casting cavity  12  (parallel or slightly converging) extending from a liquid metal entrance end  13  to a solid strip discharge exit end  14 . The belts  10  and  11  are respectively driven and carried around by large drive rollers  15  and  16 , to return toward the entrance end  13 , after passing around curved, liquid-layer bearing structures, respectively shown at  17  and  18 . Supporting carriage structures  19  and  20  are provided for the respective belts  10  and  11 , while the drive rolls  15  and  16  are appropriately carried and connected for suitable motor drive, all by well known means. 
     The molten metal is fed to the casting cavity  12  by any suitable means, e.g. from a continuously supplied trough or launder  21 . As the liquid metal in the cavity  12  moves along with the belts, it is continuously cooled and solidified, from the outside to the inside, from its contact with the belts, so that a solid, cast strip (not shown) is continuously discharged from exit end  14 . Convenient means for cooling the belts may typically be in the form of a series of cooling “pads” which contain chambers for coolant, e.g. water, and a multiplicity of outlet nozzles arranged so as to cover the area facing the reverse surface of each belt, with a slight spacing from the belt so that jet streams of liquid coolant projected perpendicular against the belt through the nozzle faces flow outwardly over the face, returning to the appropriate discharge means. The preferred nozzles for this purpose are those having a flat guiding face of hexagonal contour as described in Thorburn et al. U.S. Pat. No. 4,193,440, issued Mar. 18, 1980, and incorporated herein by reference. 
     As can be seen in  FIG. 2 , which shows a lower belt support forming part of the apparatus of  FIG. 1  (but modified according to an exemplary embodiment of the present invention), a series of cooling pads  25   a,    25   b,    25   c,    25   d  and  25   e  are supported from support carriage  20  via a series of bulkheads  26   a,    26   b,    26   c,    26   d  and  26   e.  The spaces between the bulkheads  26   a,    26   b,    26   c,    26   d  and  26   e  allow for the coolant to be removed from the space formed between the casting belts  10 ,  11  and the cooling nozzles (shown in more detail in  FIGS. 4A  and  4 B). The cooling pads  25   a,    25   b,    25   c  and  25   d  are all supported directly by the bulkheads, while the end cooling pad  25   e  is partially supported by a cantilever support  27  to ensure rigidity. 
     In this particular embodiment, three support bulkheads  26   a,    26   b  and  26   c  are all rigidly fixed between support carriage  20  and cooling pads  25   a  and  25   b.  However, bulkheads  26   d  and  26   e  are connected at their bottom ends to a pivotable subframe  28  supported by a bracket  29  and a pivot  30 . An additional bulkhead  31  is also connected to subframe  28  and bracket  29  and this serves to support one end of cooling pad  25   c.  A small gap  32  is provided between bulkheads  26   c  and  31  to permit mechanical assembly. Thus, it will be seen from  FIG. 2  that cooling pads  25   c,    25   d  and  25   e  are able to tilt together around pivot  30  (as indicated by arrow C) while being supported by subframe  28 . The tilting of pads  25   c,    25   d  and  25   e  is accomplished by means of a tapered wedge, screw jack or hydraulic ram  33  mounted at one end of the fixed carriage  20  and at the other end on the pivotable subframe  28 . The pivot  30  is preferably located about mid-length of the casting cavity  12 , i.e. at a point where the cast strip is normally solid (or sufficiently solid for self-support). In a typical installation, the upstream region of the casting cavity  12  is convergent, with a basic convergence of about 0.02%, while the downstream tilting region can move from alignment with the upstream region, to non-alignment causing a lesser convergence of the downstream region of the casting cavity, or even a divergence of as much as about 0.4 to 1.0%. 
     Further details of the tilting support portion are shown in  FIG. 3 , which is a perspective view of the subframe  28  in isolation showing more clearly the bulkheads  26   e,    26   d  and  31 . It will be seen that there is bracing  34  provided between the ribs for rigidity. In this illustration, the cooling pads  25   c,    25   d  and  25   e  have been omitted, but in use they are mounted between the top ends of the illustrated bulkheads as shown in  FIG. 2 . 
     The attachment of the cooling pads to bulkhead  31  and bulkhead  26   c  requires some special consideration. The cooling pad  25   b  ( FIG. 2 ) and is attached to bulkheads  26   b  and  26   c,  and cooling pad  25   c  is attached to bulkheads  31  and  26   d.  This means that the adjacent cooling pads  25   b  and  25   c  are free to separate as the pivotable subframe  28  moves with respect to the fixed portion of the carriage  20 . 
       FIGS. 4A and 4B  are plan views of the top surfaces of the cooling pads  25   b  and  25   c  showing hexagonal cooling nozzles  40  that cover the top surfaces, e.g. as described in U.S. Pat. No. 4,193,440 mentioned above. The nozzles  40  are mounted in a staggered manner to achieve a close-packed arrangement that is extended over the junctions between adjacent cooling pads. Thus, at the junction between cooling pads  25   b  and  25   c,  edge parts of the nozzles overhang the slight gap X between the pads in a staggered pattern, i.e. an edge part from a nozzle on one side of the gap projects between two adjacent edge parts of nozzles on the other side of the gap, and vice versa. 
       FIG. 4A  represents the arrangement before rotation of the subframe  28  in direction C takes place, and  FIG. 4B  represents the arrangement after such rotation, and it will be seen that the gap X′ in  FIG. 4B  is slightly wider then the gap X in  FIG. 4A  (but not by much, i.e. usually less than 1 mm). Although the gap between the pads increases when the rotation occurs, the gap  41  that opens between the nozzles has a zig-zag form, as shown. This means that the belt (not shown in these views) overlying the junction between the pads does not encounter a continuous straight line transverse gap that could cause the belt to sag between the pads. Instead, the zig-zag form of the gap provides support for the belt such that, considered transversely, various points on the belt remain supported from below at times when other points are unsupported due to passage over the gap. The supported and unsupported points alternate across the width of the belt as the belt passes over the junction. When the pivotable subframe  28  is rotated so as to create a more divergent cavity from the junction on, and the spaces between adjacent nozzles at the interface between these two pads begin to open up, the surfaces of the nozzles  40  become non-planar on opposite sides of the junction. In order to minimize any tendency for the edges of the nozzles to interfere with the movement of the belt passing over them, the pivot axis  30  is placed as far from the casting surface as practically possible (i.e. adjacent the lower end of the carriage, as shown). 
     During the rotation of subframe  28 , the roller  16  remains in place with respect to the remainder of the carriage. The rotation of the subframe causes a slight decrease in the total length of the path followed by the belt, but the decrease is less than 1 mm compared to a typical total belt length of 5 m or more. Such a change is easily accommodated by the kind of belt tensioners (not shown) provided in this kind of casting apparatus. For example, the roller  16  may be mounted on horizontally slidable bearings and urged by spring means or the like to the right as seen in  FIG. 2 , resisted only by the tension of the belt. 
     The apparatus configured in this way may be used for casting a variety of different metals having different heat flux requirements by varying the rotation of the subframe  28  prior to casting in order to suit the cooling and heat flux characteristics of the metal to be cast. Whether or not tilting is required, and the degree of such tilting, for any particular metal may be determined empirically or by calculation from known metal cooling properties and casting conditions. 
     It will be appreciated that, while  FIGS. 2 and 3  show a tiltable support mechanism for the lower belt of the apparatus of  FIG. 1 , the same arrangement could be provided for the upper belt either as well as, or alternatively instead of, providing the tiltable support for the lower belt. Therefore, just one, or alternatively both belts, may be made tiltable in the downstream region. It is generally found sufficient to make just one belt tiltable, and preferably just the lower belt as shown in the drawings.