Patent Application
20100243194
(1) Field of the Invention
This invention relates to the casting of metal strip articles by means of continuous strip casting apparatus of the kind that employ continuously moving elongated casting surfaces and side dams that confine the molten and semi-solid metal to the casting cavity formed between the moving casting surfaces. More particularly, the invention relates to the side dams themselves, and particularly, but not exclusively, to those intended for the casting of aluminum and alloys thereof.
(2) Description of the Related Art
Metal strip articles (such as metal strip, slab and plate), particularly those made of aluminum and aluminum alloys, are commonly produced in continuous strip casting apparatus. In such apparatus, molten metal is introduced between two closely spaced (usually actively cooled) elongated moving casting surfaces forming a casting cavity, and is confined within the casting cavity until the metal solidifies (at least sufficiently to form an outer solid shell). The solidified strip article, which may be produced in indefinite length, is continuously ejected from the casting cavity by the moving casting surfaces. One form of such apparatus is a twin-belt caster in which two confronting belts are rotated continuously and molten metal is introduced by a launder or injector into a thin casting cavity or mold formed between the confronting regions of the belts. An alternative is a rotating block caster in which the casting surfaces are formed by blocks that move around fixed paths and align with each other within the casting cavity. In both kinds of apparatus, the molten metal is introduced at one end of the apparatus, conveyed by the moving belts or blocks for a distance effective to solidify the metal, and then the solidified strip emerges from between the belts or blocks at the opposite end of the apparatus.
In order to confine the molten and semi-solid metal within the casting cavity, i.e. to prevent the metal escaping laterally from between the casting surfaces, it is usual to provide metal dams at each side of the apparatus. For twin-belt and rotating block casters, side dams of this kind can be formed by a series of metal blocks joined together to form a continuous line or chain extending in the casting direction at each side of the casting cavity. These blocks, normally referred to as side dam blocks, are trapped between and move along with the casting surfaces and are recirculated so that blocks emerging from the casting cavity exit move around a guided circuit and are fed back into the entrance of the casting cavity. The blocks are guided around this circuit by means of a metal track, or similar guide, on which the blocks can slide in a loose fashion that allows for limited movement between the blocks, especially as they move around curved parts of the circuit outside the casting cavity.
A problem with side dams made of blocks of this kind is that it is sometimes desired to change the through-thickness convergence of the belts, i.e. to make the casting cavity thinner at its exit than at its entrance (referred to as convergent) in order to extract more heat from the metal slab, or alternatively, to make the casting cavity thicker at the exit (referred to as divergent) in order to extract less heat from the metal slab. A requirement that the belts also drive the side dam blocks through the casting cavity may limit the extent to which the casting belts can be changed in this way.
The casting belts or blocks extract heat from the molten metal passing through the casting cavity, but heat is also extracted at the sides of the cavity where the molten metal contacts the side dam blocks which are usually made of a heat conductive material such as cast iron or mild steel. This heat extraction at the sides of the cavity often changes the microstructure and thickness of the slab in those areas, resulting in undesirable side-to-center non-uniformity of the cast metal slab.
U.S. Pat. No. 4,869,310 issued to Yanagi et al. on Sep. 26, 1989 discloses a twin-belt casting apparatus having side dams provided by moving side dam blocks as explained above. For comparison with the moving side dam blocks, however, this patent also shows the use of fixed side dams in
There is therefore a need to address the problems mentioned above.
According to one exemplary embodiment, there is provided a side dam for a continuous metal casting apparatus having elongated opposed casting surfaces forming a casting cavity therebetween. The side dam comprises an elongated upstream part and an elongated downstream part that are mutually laterally pivotable, and a smooth metal-contacting side surface extending continuously from an upstream end to a downstream end of the side dam. The side surface has regions thereof formed on the upstream part and the downstream part, whereby mutual pivoting of the upstream part and the downstream part of the side dam enables the regions of the smooth metal-contacting side surface to be moved out of mutual coplanar alignment.
The smooth continuous surface is preferably an outer surface of an elongated strip of flexible refractory material extending continuously from the upstream end to the downstream end of the side dam, and the strip is preferably made of a material that has a coefficient of friction with molten metal such that the metal does not build up on the surface as the metal solidifies during casting. For example, the elongated strip may be made of flexible graphite composition. Preferably, the elongated strip stands proud (e.g. by a distance of up to about 1 mm) of the remainder of the upstream and downstream parts of the side dam at the surfaces thereof that, in use, confront the casting surfaces of the continuous casting apparatus. Ideally, the remainder of the surfaces of the side dam that, in use, confront the casting surfaces have a coating of a refractory low friction wear-resistant material (e.g. a metal nitride, such as boron nitride).
The side dam may have a layer of heat insulating material (e.g. refractory insulating board) adjacent to the elongated flexible strip. This reduces heat loss from the metal being cast into the fabric of the side dam. The side dam may also have an elongated backing element made of rigid material (preferably a metal such as steel) along a side of the upstream and/or downstream parts opposite to the metal-contacting side surface of the side dam.
The side dam preferably also has at least one anchor point (which may be a hold for a bolt, a region for application of adhesive, an attachment bracket, or the like) adjacent to the upstream end for rigid attachment of the side dam to an element of the continuous metal casting apparatus. This prevents the side dams from being dragged in the casting direction by the casting surfaces.
The side dam preferably has a hinge acting between the upstream and downstream parts thereof, the hinge enabling and guiding the mutual pivoting of the parts. The hinge may be a door-type hinge made of the material of the backing element, or it may simply be a web of flexible material adhered or otherwise attached to each part of the side dam.
The side dam preferably has a length from the upstream end to the downstream end that is less than the length of a casting cavity of a continuous casting apparatus with which the side dam is used, but greater than the downstream extent of molten and semi-solid metal cast in the apparatus. The side dam therefore merely covers the distance over which metal may leak or flow from the casting cavity.
Another exemplary embodiment provides a continuous metal casting apparatus comprising opposed rotating casting surfaces forming a casting cavity therebetween, a metal inlet for introducing molten metal into the cavity, and two side dams for confining molten metal to the casting cavity. At least one of the two side dams (and preferably both) comprises an elongated upstream part and an elongated downstream part that are mutually laterally pivotable, and a smooth metal-contacting side surface extending continuously from an upstream end to a downstream end of the side dam and having regions thereof formed on the upstream part and the downstream part, whereby mutual pivoting of the upstream part and the downstream part of the side dam enables the regions of the smooth metal-contacting side surface to be moved out of mutual coplanar alignment.
In the casting apparatus, the casting surfaces are preferably surfaces of a pair of opposed rotating casting belts or, alternatively, surfaces of a series of rotating casting blocks. The metal inlet is preferably a molten metal injector having a nozzle projecting between the opposed casting surfaces, and wherein at least one of the side dams is attached to the nozzle, either to the outer surface of the nozzle or the inner surface thereof.
In the casting apparatus, the upstream and downstream part of the side dam is preferably arranged at a convergent angle, or a divergent angle, and most preferably the latter, relative to a casting direction of the metal. This angle is preferably 10° or less.
Another exemplary embodiment provides a continuous metal casting apparatus comprising opposed rotating casting surfaces forming a casting cavity therebetween, a metal inlet for introducing molten metal into the cavity, and two side dams for confining molten metal to the casting cavity, wherein at least one of the two side dams comprises a flexible elongated strip of low friction refractory material that is resistant to attack by molten metal, the flexible elongated strip having a metal-contacting side and an opposed side, an elongated block of heat insulating material contacting the opposed side of the flexible elongated strip, the elongated block having a surface remote from the flexible elongated strip, and a backing element of rigid material contacting the remote surface of the elongated block, wherein the flexible elongated strip, the elongated block and the backing element fit between the opposed casting surfaces adjacent to the metal inlet thereof in contact with both of the opposed casting surfaces.
While the exemplary embodiments are particularly suited for use with, or the casting of, aluminum or aluminum alloys, it is also possible to cast other metals in the same way, e.g. copper, lead and zinc, and even magnesium and steel.
Exemplary embodiments of the invention are described in detail in the following with reference to the accompanying drawings, in which:
The exemplary embodiments of this invention described in the following are directed in particular for use with twin belt casters, e.g. of the kind disclosed in U.S. Pat. No. 4,061,178 issued to Sivilotti et al. on Dec. 6, 1977 (the disclosure of which is incorporated herein by reference). However, other exemplary embodiments may be used with casters of other kinds, e.g. rotating block casters. Twin belt casters have an upper flexible belt and a lower flexible belt that rotate about rollers and/or stationary guides. The belts confront each other for part of their length to form a thin casting cavity or mold having an entrance and an exit. Molten metal is fed into the entrance and a cast metal slab emerges from the exit. Cooling water sprays are directed onto the interior surfaces of the belts in the region of the casting cavity for the purpose of cooling the metal. The molten metal may be introduced into the casting cavity by means of a launder, but it is more usual to provide an injector that projects partially into the casting cavity between the belts at the entrance. Exemplary embodiments may be used most preferably with a type of metal injector having a flexible nozzle as disclosed in U.S. Pat. No. 5,671,800 issued to Sulzer et al. on Sep. 30, 1997 (the disclosure of which is incorporated herein by reference).
The injector 18 has a metal-conveying channel 36 formed between upper and lower walls 38, 39 (only the upper wall 38 is visible in
One of the side dams 46 is shown in isolation in
The side dams 46 remain stationary in the casting apparatus and the low friction property of the flexible elongated strip 54 resists any tendency of the moving metal to stick or jam against the side dam 46 as it solidifies and is carried forwards by the belts. The elongated strip 54 is dimensioned to contact both of the casting belts and the flexible nature of the strip allows it to yield to the shape of the belt and to form a good seal against molten metal outflow. The low friction properties of the strip reduce frictional drag from the belts as they move over the side dam. To facilitate the formation of the seal, the strip may stand proud of the remainder of upper and lower surfaces 66 and 68 of the side dam by a small amount (e.g. up to about 1 mm). This is shown in
It should be mentioned here that, although the previous description refers to the formation of a good seal between the strip 54 and the casting belts (which is preferred), there may in fact be a gap of up to about 1 mm between the strip 54 (or the highest part of surfaces 66, 68) and the adjacent surfaces of the casting belts without loss of metal. This is because the molten metal has a degree of surface tension that creates a meniscus that bridges gaps up to about 1 mm without penetration through such gaps. Direct and firm contact between the side dam and the metal surfaces is therefore not essential. The provision of a gap in this way makes it possible, for example, to accommodate a convergence of the casting belts between the entrance and the exit. That is to say, the side dam 46 may not quite touch the casting belts in the region of the nozzle 44 but may gently touch the belts adjacent to the downstream end 49 due to convergence of the belts. The flexibility of the strip 54 may accommodate further belt convergence because the parts that stand proud may compress, thus decreasing the distances X. If even further convergence of the belts is to be accommodated, the side dam 46 may be made to taper down in height from the upstream end 47 to the downstream end 49. In contrast, it may be desirable in some cases to arrange the casting cavity to diverge in the casting direction, and this can correspondingly be accommodated by providing a slight spacing between side wall and belts at the downstream end, and/or by making the sidewall taper up in height from the upstream to the downstream ends.
The elongated flexible strip 54 and the insulating block 56 are preferably made of heat insulating material and thus have low thermal mass and low thermal conductivity (much lower than the metal of conventional side dam blocks) so that very little heat is withdrawn from the metal slab at the sides allowing the metal to cool uniformly across the slab width to provide more uniform solid microstructure and thickness. Furthermore, the heat insulating property means that the metal tends not to freeze on the elongated flexible layer 54 as little heat is withdrawn through this layer. Any metal that does freeze directly onto the flexible strip is easily carried away by the remainder of the moving slab because of the low friction properties of the strip. Therefore, solid metal tends not to build up on the stationary side dams.
The rigid backing element 58 serves to protect and support the other elements of the side dam since these other parts may be rather delicate and easily damaged. This element 58 also forms a solid base that allows the side dam to be anchored rigidly in place on the casting apparatus and, due to its relatively high heat capacity, serves to freeze and contain molten metal in the event of failure of the remainder of the side dam.
In the embodiment of
The distance along the casting cavity that the side dams 46 are required to extend beyond the injector 18 depends on the length of the region 30 of molten metal and the region 32 of semi-solid metal (referred to, in combination, as the molten metal “sump”). This, in turn, depends on the characteristics of the alloy being cast, the casting speed and the thickness of the slab being cast. Table 1 below provides typical working and preferred ranges for common aluminum alloys.
As noted above, the side dams 46 are each provided with a hinge 60 that permits articulation between an upstream part 46A of the side dam and a downstream part 46B. The upstream parts 46A are securely attached to the (normally parallel) sides of the injector 18 and are thus parallel and extend in the casting direction without sideways divergence or convergence. However, the downstream parts 46B can be rotated about hinge 60 as shown by arrows D in
The angle of the downstream part 46B of the side dam 46 relative to the casting direction may be set before casting commences or may be adjusted during casting when the effect of the adjustment or the need for it (e.g. molten metal leakage around the slab) can be observed. The low friction characteristics of the elongated strip 54 and the low friction coating (if any) provided on the remainder of the upper and lower surfaces 66, 68 of the side dam allow the downstream part to be moved as the casting apparatus is in operation. This can be done in a precise manner by means of rods 80 attached to the backing elements 58 near the downstream ends thereof. The rods are precisely moved axially forwards or backwards by desired amounts either manually or by electric or hydraulic/pneumatic motors 82 (which may be under computer control).
In the arrangement of
In the embodiment of
In the above embodiments, the side dams comprise three elements, namely the flexible strip 54, the insulating block 56 and the backing element 58. However, it is not always necessary to provide all these elements. The metal-contacting surface of the side dam should preferably be made of or coated with a material that has low friction and good heat resistance. The friction properties should preferably be low enough to prevent solid metal build up on the side dam and wear that reduces the operational life of the side dam. The metal-contacting surface should also preferably be capable of flexing or bending to allow the downstream part of the side dam to be pivoted laterally relative to the upstream part without causing a break that could result in leakage of metal or solid metal build-up. The side dam should also preferably be heat insulating to reduce heat flux from the molten metal at the sides of the casting cavity. The degree of heat insulation should preferably be sufficient to avoid the formation of problematic micro-structural defects in the cast strip article and significant variations of thickness across the cast article. This heat insulation may be provided by an insulating block or by the material of the flexible strip itself (or both). The backing element 58 may be omitted if the other elements are sufficiently structurally rigid and durable to avoid undue damage during use and to allow secure attachment to the injector or other parts of the apparatus. The hinge 60 may be replaced by a flexible web of material attached to the upstream and downstream elements of the side wall, or may be omitted entirely if the flexible member is sufficiently strong to prevent tearing or fracture at the junction.
The illustrated embodiments provide longitudinally fixed but bendable (pivotable) side dams at both sides of the casting cavity. This is preferred to ensure that both sides of the cast slab are subjected to the same casting conditions. However, if desired, one of the fixed side dams may be non-bendable or, alternatively, one side of the cavity may be closed by movable blocks of the conventional kind, although then the benefits of convergence/divergence of the casting cavity would be unavailable because the moving blocks must necessarily extend for the full length of the casting cavity.
It is also to be noted that some casting machines do not have a molten metal injector 18 but are instead fed with molten metal via a launder (metal feeding trough) or similar no-tip, drag-out style metal feeding arrangement. In such a case, the stationary side dam is fixed to the caster frame or to the metal feeding trough as there can be no anchorage to the injector itself.
This application claims the priority right of prior co-pending provisional application Ser. No. 61/211,277 filed Mar. 27, 2009 by applicants named herein. The entire contents of application Ser. No. 61/211,277 are specifically incorporated herein by this reference.
| Number | Date | Country | |
|---|---|---|---|
| 61211277 | Mar 2009 | US |
