EP0735931B1

EP0735931B1 - Method and apparatus for twin belt casting

Info

Publication number: EP0735931B1
Application number: EP95907962A
Authority: EP
Inventors: Donald G. Harrington
Original assignee: Alcoa Inc
Current assignee: Howmet Aerospace Inc
Priority date: 1993-12-23
Filing date: 1994-12-22
Publication date: 2000-11-29
Anticipated expiration: 2014-12-22
Also published as: DE69426362D1; WO1995017274A1; AU692236B2; EP0735931A1; JP3497170B2; EP0735931A4; AU1860495A; DE69426362T2; CA2178587A1; JPH09507793A; DK0735931T3; BR9408373A; ES2151953T3

Description

Background Of The Invention

This invention relates to a method and apparatus for the continuous casting of metals, and particularly the casting of metal strip.

The continuous casting of thin metal strip has been employed with only limited success. By and large, prior processes for the continuous casting of metal strip have been limited to a relatively small number of alloys and products. It has been found that as the alloy content of various metals are increased, as-cast surface quality deteriorates. As a result, many alloys must be fabricated using ingot methods.

In the case of aluminum, relatively pure aluminum product such as foil can be continuously strip cast on a commercial basis. Building products can likewise be continuously strip cast, principally because surface quality in the case of such building products is less critical than in other aluminum products, such as can stock. However, as the alloy content of aluminum is increased, surface quality problems appear, and strip casting has generally been unsuitable for use in making many aluminum alloy products.

A number of strip casting machines have been proposed in the prior art. One conventional device is a twin belt strip casting machine, but such machines have not achieved widespread acceptance in the casting of many metals, and particularly metal alloys with wide freezing ranges. In such twin belt strip casting equipment, two moving belts are provided which define between them a moving mold for the metal to be cast. Cooling of the belts is typically effected by contacting a cooling fluid with the side of the belt opposite the side in contact with .the molten metal. As a result, the belt is subjected to extremely high thermal gradients, with molten metal in contact with the belt on one side and a water coolant, for example, in contact with the belt on the other side. The dynamically unstable thermal gradients cause distortion in the belt, and consequently neither the upper nor the lower belt is flat. The product thus produced has areas of segregation and porosity as described below.

Leone, in the Proceedings Of The Aluminum Association, Ingot and Continuous Casting Process Technology Seminar For Flat Rolled Products, Vol. II, May 10, 1989, said that severe problems develop if belt stability and reasonable heat flow are not achieved. In the first place, if any area of the belt distorts after solidification of the molten metal has begun and strip shell coherency has been reached, the resulting increase in the gap between the belt and the strip in the distorted region will cause strip shell reheating, or, at least, a locally reduced shell growth rate. That, in turn, gives rise to inverse segregation in the strip which generates interdendritic eutectic exudates at the surface Moreover, in severe cases with medium and long freezing range alloys, liquid metal is drawn away from a distorted region to feed adjacent, faster solidifying portions of the strip. That in turn causes the surface of the strip to collapse and forms massive areas of shrinkage porosity in the strip which can crack on subsequent rolling or produce severe surface streaks on the rolled surface.

As a result, twin belt casting processes have not generally achieved acceptance in the casting of alloys for surface-critical applications, such as the manufacturing of can stock. Various improvements have been proposed in the prior art, including preheating of the belts as described in US-A-3937270 and US-A-4002197, continuously applied and removed parting layers as described in US-A-3795269, moving endless side dams as described in US-A-4586559 and improved belt cooling as described in US-A-4061177, US-A-4061178 and US-A-4193440. None of those techniques has achieved widespread acceptance either.

An additional approach to continuous belt casting of steel is described in US-A-4561487 utilizing a pair of counter-rotating belts in which one is chilled while it is not in contact with the metal being cast. Thereafter, a supply of molten steel is supplied to the surface of the belt just before the belt passes downwardly and around a supporting pulley and the metal being cast is passed between the belts. While the approach taken in that patent may avoid the thermal distortion affects caused by large temperature gradients when a cooling fluid is supplied to one side of the belt and the other side of the belt is in contact with hot metal, it presents other problems. The supply of the molten metal to the belt just as it passes around a supporting pulley means that the molten metal must be cooled very quickly; otherwise, molten metal will flow off the belt into the area surrounding the equipment, representing a hazard to workers. In addition, US-A-4561487 casts the molten metal on a single belt, and uses the second belt only as a "hugger" belt to maintain the cast ribbon in contact with the chilled belt.

Other attempts at belt casting approaches are described in US-A-3432293 and EP-A-0181566. In the techniques described by both publications, a cooling liquid is applied to the opposite side of a belt on which a metal is cast both while the belt is not in contact with the metal and while it is in contact with the metal. Thus, neither recognizes the concept that the heat transmitted to the belt from the molten metal is substantially removed by application of a cooling fluid at a time when the belt is out of contact with the metal being cast to avoid formation of large thermal gradients.

Another continuous casting process that has been proposed in the prior art is that known as block casting. In that technique, a number of chilling blocks are mounted adjacent to each other on a pair of opposing tracks. Each set of chilling blocks rotates in the opposite direction to form therebetween a casting cavity into which a molten metal such as an aluminum alloy is introduced. The liquid metal in contact with the chilling blocks is cooled and solidified by the heat capacity of the chilling blocks themselves. Block casting thus differs both in concept and in execution from continuous belt casting. Block casting depends on the heat transfer which can be effected by the chilling blocks. Thus, heat is transferred from the molten metal to the chilling blocks in the casting section of the equipment and then extracted on the return loop. Block casters require precise dimensional control to prevent flash (i.e. transverse metal fins) caused by small gaps between the blocks. Such flash causes sliver defects when the strip is hot rolled. As a result, good surface quality is difficult to maintain. Examples of such block casting processes are set forth in US-A-4235646 and US-A-4238248.

Another technique which has been proposed in continuous strip casting is the single drum caster. In single drum casters, a supply of molten metal is delivered to the surface of a rotating drum, which is internally water cooled, and the molten metal is dragged onto the surface of the drum to form a thin strip of metal which is cooled on contact with the surface of the drum. The strip is frequently too thin for many applications, and the free surface has poor quality by reason of slow cooling and micro-shrinkage cracks. Various improvements in such drum casters have been proposed. For example, US-A-4793400 and US-A-4945974 suggest grooving of the drums to improve surface quality; US-A-4934443 recommends a metal oxide on the drum surface to improve surface quality. Various other techniques are proposed in US-A-4771819, US-A-4979557, US-A-4828012, US-A-4940077 and US-A-4955429.

Another approach which has been employed in the prior art has been the use of twin drum casters, such as in US-A-3790216, US-A-4054173, US-A-4303181, or US-A-4751958. Such devices include a source of molten metal supplied to the space between a pair of counter-rotating, internally cooled drums. The twin drum casting approach differs from the other techniques described above in that the drums exert a compressive force on the solidified metal, and thus effect hot reduction of the alloy immediately after freezing. While twin drum casters have enjoyed the greatest extent of commercial utilization, they nonetheless suffer from serious disadvantages, not the least of which is an output substantially lower than that achieved in many prior art devices described above. Once again, the twin drum casting approach, while providing acceptable surface quality in the casting of high purity aluminum (e.g. foil), suffers from poor surface quality when used in the casting of aluminum with high alloy content and wide freezing range. Another problem encountered in the use of twin drum casters is center-line segregation of the alloy due to deformation during solidification.

There is thus a need to provide an apparatus and method for continuously casting thin metallic strip at high speeds and improved surface quality as compared to methods currently employed.

In EP-A-058867, there is described a method and apparatus where the continuous casting of metal strip, and particularly metal strip formed form highly alloyed aluminum, which. overcomes many limitations of the prior art disclosed above. In the method and apparatus there described, uses made of the heat sink capabilities of the belts in a substantially horizontal molding zone in which substantially all of the heat transmitted to the belts from the metal being cast is removed from the belts while the belts are out of contact with the metal being cast. In that way, the method and apparatus described in the foregoing application substantially minimizes the formation of thermal gradients over the thickness of the belts which caused distortion of belts used in the prior art. This invention provides even further improvements over the method and apparatus to improve both heat transfer characteristics and the reduction of cracking in the metal.

It is a specific objective of this invention to provide an improvement which is to supply the molten metal on the curvature of the belts at the entry pulleys rather than on the straight section, thereby further reducing belt distortion and improving heat transfer and the surface quality of the strip when processing metals such as aluminium with high alloy content.

It is another object of the present invention to provide further improvements in which the molten metal is supplied on the curvature of the belts and solidifies substantially before the nip of the entry pulleys while restraining the nip of the pulleys from being displaced by solidified metal. It has been found that the positive control of the gap between the nip of the entry pulleys, combined with solidification prior to the nip, cause a compressive force to be exerted on the cast strip at the nip which in turn serves to even further minimize distortion of the belts and to reduce cracking of the metal.

It is a more specific object of the invention to provide an apparatus and method for the continuous casting of thin metallic strip which provides improved surface quality even when processing metals such as aluminum with high alloy content.

These and other objects and advantages of the invention appear more fully hereinafter from a detailed description of the invention.

Summary Of The Invention

The present invention is defined in the claims, to which reference should now be made. On account of EP-A-0583867, the claims consist of two sets, for the contracting states BE, CH, DK, ES, LU and NL and for the contracting states DE, FR, GB and IT. The division of the independent claims of both sets into precharacterising and characterising parts is on the basis of US-A-4561487 referred to above.

The concepts of the present invention reside in a method and apparatus for strip casting of metals by continuous belt casting utilizing a pair of continuous belts formed of a heat conductive material positioned adjacent to each other to define a molding zone therebetween. The belts are each mounted on at least two pulley means and each passes around pulley means whereby each belt defines a curved surface about the pulley means and a substantially flat, and preferably horizontal, surface after the belt passes around the pulley means. The system also employs means for supplying to the curved surfaces of the belts a molten metal whereby the molten metal solidifies on the surface of the belts in the molding zone to form a cast strip of metal, thereby transferring heat from the molten metal and the cast metal to the belts. Substantially all of the heat transfer to the belts from the molten metal and the cast strip is thereafter removed from the belts while they are out of contact with either the molten metal or the cast strip.

Thus, in the practice of the invention, the molten metal is supplied to the belt on the curved section around the pulley means. In conventional belt casters of the prior art, the metal is supplied to the belt in the straight section of the belt after it passes around the entry pulley and cooled concurrently from the backside as solidification occurs. It has been found that the supply of molten metal to the curved section of the belt has the advantage of increased mechanical stability to resist thermal distortions of the casting belt and thereby maintaining more uniform thickness and better thermal contact between the strip and belt and consequent improvements in the quality of the surface of the cast strip.

In the most preferred practice of the invention, the apparatus includes a pair of belts, each substantially horizontally disposed, with one being positioned above the other to define a substantially horizontal molding zone between the belts. As used herein, the term "horizontally" is intended to refer to the disposition of the belts at angles plus or minus 30°. In some instances, it may be desirable to orient the belts at an angle within the range. The supply of molten metal comes from a conventional tundish provided with nozzle means through which the molten metal flows in a substantially horizontal stream. Thus, molten metal from the nozzle means flows in a substantially horizontal stream into the space defined between the belts preceding the nip of the pulleys for solidification in the molding zone defined by the nozzle means and the belts passing around each of the pulleys. In the typical practice of the invention, the cast strip is substantially solidified by the time it reaches the nip of the pulleys on which the belts are mounted. The horizontal stream of molten metal flowing into the space between the belts preceding the nip insures that the molten metal is always maintained in contact with the surface of both belts as the metal is being cast.

It has also been discovered that the use of floating backup rolls on the top side of the straight portion of the mold section enhances heat transfer and isolates the mechanical disturbances of downstream equipment from the critical solidification zone at the entry of the caster.

The concepts of the present invention also reside in a method and apparatus for strip casting of metals for continuous belt casting in which the molten metal is cast on curved surfaces of a pair of opposing belts so that solidification of the metal occurs prior to the nip between the entry pulleys in the molding zone, and then the entry pulleys at the nip exert a compressive force on the cast metal strip to effect elongation of that strip. It has been found that the features of solidification prior to the nip followed by the exertion of a compressive force by the nip serve to improve the surface quality of the cast metal strip and to reduce the tendency of the strip to crack.

In accordance with the practice of the invention, use is made of a pair of belts mounted adjacent to each other, with each belt being carried on at least two pulleys to define a molding zone therebetween. Each belt passes around an entry pulley whereby each belt defines a curved surface about that pulley and also a substantially flat, and preferably horizontal, surface after the belt passes around the pulley.

In the practice of this embodiment of the invention, there is provided means to supply to the curved surfaces of the belts a molten metal whereby the molten metal solidifies on those opposing curved surfaces in the molding zone prior to the nip of the entry pulleys to form a cast strip of metal having a thickness greater than the thickness of the nip. As a result, the cast metal strip is advanced to the nip where it is subjected to compressive forces to effect elongation of the cast metal strip to thereby improve the surface characteristics of the strip as well as reduce its tendency to crack.

In a preferred embodiment of the invention, each of the belts is heated by heat transfer from the cast metal to the belt. The heat thus transferred to the belt is substantially all removed from the belts while they are out of contact with either the molten metal or the cast strip.

In the practice of this embodiment of the invention, the method and apparatus employed in the practice of the invention preferably utilize a positive control means to control the gap in the molding zone at the nip between the entry pulleys for the twin belts. Such control can be achieved by a variety of mechanisms. For example, it is possible to employ means to exert tension between the axis of the entry pulleys, and by preventing those axes from being displaced one from the other. Such means can be in the form of a hydraulic cylinder to control the gap between the axes of the entry pulleys or like mechanical means such as a mechanical screw jack to control the relative position of the axes of the entry pulleys with respect to each other. Alternatively, use can be made of a spacer block to establish the desired space between the axis of the entry pulleys and a tension member mounted on those axes to prevent displacement of the axes relative to each other.

Thus, in the practice of the invention, the molten metal is supplied to the belt on the curved section around the pulley means. In conventional belt casters of the prior art, the metal is supplied to the belt in the straight section of the belt after it passes around the entry pulley and cooled concurrently from the backside as solidification occurs. As already noted, it has been found that the supply of molten metal to the curved section of the belt has the advantage increased mechanical stability to resist thermal distortions of the casting belt and thereby maintaining more uniform thickness and better thermal contact between the strip and belt and consequent improvements in the quality of the surface of the cast strip.

The positive control of the nip between the entry pulleys in the molding zone provides improved surface quality in the cast strip. Without limiting the invention as to theory, it is believed that the positive control of the nip between the entry pulleys serves to enhance heat transfer as the molten metal is solidified, thereby minimizing the tendency to form interdendritic eutectic exudater.

In addition, the positive control of the nip between the entry pulleys substantially eliminates cracking of the cast metal strip. Once again, without limiting the present invention as to theory, it is believed that the control of the molding zone at the nip between the entry pulleys also causes the cast metal strip, formed prior to the nip, to be subjected to compressive forces by the entry pulleys, thereby causing the cast metal strip to be elongated. That in turn insures that the cast metal strip is always in compression as distinguished from tension in the molding zone, thus minimizing cracking of the strip due to tension.

The concepts of the present invention can be employed in the strip casting of most metals, including steel, copper, zinc and lead, but are particularly well suited to the casting of thin aluminum alloy strip, while overcoming the problems of the prior art.

Brief Description Of The Drawings

Fig. 1 is a schematic illustration of the casting method and apparatus embodying the present invention.

Fig. 2 is a perspective view of one casting apparatus embodying the invention.

Fig. 3 is a cross-sectional view of the entry of molten metal to the apparatus illustrated in Figs. 1 and 2.

Fig. 4 is a detailed view of the mechanism supporting the belts in the apparatus of Figs. 1 and 2.

Fig. 5 is a top view illustrating one embodiment of the edge containment means employed in the practice of the invention.

Fig. 6 is a schematic illustration of the casting method and apparatus embodying the concept of another feature of the present invention.

Fig. 7 is a perspective view of the apparatus schematically illustrated in Fig. 6.

Fig. 8 is a cross-sectional view of entry of molten metal of the embodiment illustrated in Figs. 6 and 7.

Detailed Description Of The Invention

The apparatus employed in the practice of the present invention is perhaps best illustrated in Figs. 1, 2 and 3 of the drawings. As there shown, the apparatus includes a pair of

endless belts

10 and 12 carried by a pair of

upper pulleys

14 and 16 and a pair of corresponding

lower pulleys

18 and 20 of Fig. 1. Each pulley is mounted for rotation about an

axis

21, 22, 24, and 26 respectively of Fig. 2. The pulleys are of a suitable heat resistant type, and either or both of the

upper pulleys

14 and 16 is driven by a suitable motor means not illustrated in the drawing for purposes of simplicity. The same is equally true for the

lower pulleys

18 and 20. Each of the

belts

10 and 12 is an endless belt, and is preferably formed of a metal which has low reactivity or is non-reactive with the metal being cast. Quite a number of suitable metal alloys may be employed as well known by those skilled in the art. Good results have been achieved using steel and copper alloy belts.

The pulleys are positioned, as illustrated in Figs. 1 and 2, one above the other with a molding gap therebetween. In the preferred practice of the invention, the gap is dimensioned to correspond to the desired thickness of the metal strip being cast. Thus, the thickness of the metal strip being cast is thus determined by the dimensions of the nip between

belts

10 and 12 passing over pulleys 14 and 18 along a line passing through the axis of

pulleys

14 and 18 which is perpendicular to the

belts

10 and 12. As is described in the earlier co-pending application, the thickness of the strip being cast is limited by the heat capacity of the belts between which the molding takes place.

Molten metal to be cast is supplied to the molding zone through suitable metal supply means 28 such as a tundish. The inside of tundish 28 corresponds in width to the width of the product to be cast, and can have a width up to the width of the narrower of the

belts

10 and 12. The tundish 28 includes a metal supply delivery casting nozzle 30 to deliver a horizontal stream of molten metal to the molding zone between the

belts

10 and 12. Such tundishes are conventional in strip casting.

Thus, the nozzle 30, as is best shown in Fig. 3 of the drawings, defines, along with the

belts

10 and 12 immediately adjacent to nozzle 30, a molding zone into which the horizontal stream of molten metal flows. Thus, the stream of molten metal flowing substantially horizontally from the nozzle fills the molding zone between the curvature of each

belt

10 and 12 to the nip of the

pulleys

14 and 18. It begins to solidify and is substantially solidified by the point at which the cast strip reaches the nip of

pulleys

14 and 18. Supplying the horizontally flowing stream of molten metal to the molding zone where it is in contact with a curved section of the

belts

10 and 12 passing about pulleys 14 and 18 serves to limit distortion and thereby 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 of the invention includes a pair of cooling means 32 and 34 positioned opposite that portion of the endless belt in contact with the metal being cast in the molding gap between

belts

10 and 12. The cooling means 32 and 34 thus serve to cool the

belts

10 and 12 just after they pass over

pulleys

16 and 20, respectively, and before they come into contact with the molten metal. In the most preferred embodiment as illustrated in Figs. 1 and 2, the

coolers

32 and 34 are positioned as shown on the return run of

belts

10 and 12, respectively. In that embodiment, the cooling means 32 and 34 can be conventional cooling means such as fluid cooling nozzles positioned to spray a cooling fluid directly on the inside and/or outside of

belts

10 and 12 to cool the belts through their thicknesses. In that preferred embodiment, it is sometimes desirable to employ scratch brush means 36 and 38 which frictionally engage the

endless belts

10 and 12, respectively, as they pass over

pulleys

14 and 18 to clean any metal or other forms of debris from the surface of the

endless belts

10 and 12 before they receive molten metal from the tundish 28.

Thus, in the practice of this invention, molten metal flows horizontally from the tundish through the casting nozzle 30 into the casting or molding zone defined between the

belts

10 and 12 where the

belts

10 and 12 are heated by heat transfer from the cast strip to the

belts

10 and 12. The cast metal strip remains between and conveyed by the casting

belts

10 and 12 until each of them is turned past the centerline of

pulleys

16 and 20. Thereafter, in the return loop, the cooling means 32 and 34 cool the

belts

10 and 12, respectively, and remove therefrom substantially all of the heat transferred to the belts in the molding zone. After the belts are cleaned by the scratch brush means 36 and 38 while passing over pulleys 14 and 18, they approach each other to once again define a molding zone.

The most preferred supply of molten metal from the tundish through the casting nozzle 30 is shown in greater detail in Fig. 3 of the drawings. As is shown in that figure, the casting nozzle 30 is formed of an upper wall 40 and a lower wall 42 defining a central opening 44 therebetween whose width may extend substantially over the width of the

belts

10 and 12 as they pass around pulleys 14 and 18, respectively.

The distal ends of the

walls

40 and 42 of the casting nozzle 30 are in substantial proximity of the surface of the casting

belts

10 and 12, respectively, and define with the belts 10 and 12 a casting cavity or molding zone 46 into which the molten metal flows through the central opening 44. As the molten metal in the casting cavity 46 flows between the

belts

10 and 12, it transfers its heat to the

belts

10 and 12, simultaneously cooling the molten metal to form a solid strip 50 maintained between casting

belts

10 and 12.

In the preferred practice of the invention, sufficient setback (defined as the distance between first contact 47 of the molten metal 46 and the nip 48 defined as the closet approach of the entry pulleys 14 and 18) should be provided to allow substantially complete solidification prior to the nip 48. In prior art belt casters, the molten metal contacts the belt after the nip 48 in the straight section. Hence, in the present invention solidification is substantially complete prior to the nip 48, and in prior art belt caster solidification does not begin until after the nip 48.

The importance of freezing before the nip 48 in the present invention is that the

belts

10 and 12 are much more stable when held in tension on the curved surface of the pulley and distort much less than if the molten metal 46 first contacts the

belts

10 and 12 in the straight section as in prior art. Moreover, in the practice of the present invention, there is a momentary high thermal gradient over the

belts

10 and 12 when first contacted by molten metal 46. Because each belt is in tension and is well supported prior to the nip by the

pulleys

14 and 18, the belts are more stable against distortion arising from that momentary thermal gradient. In addition, the space between the belts at the time that they first come into contact with the molten metal is substantially larger then the gap between the belts corresponding to the thickness of the cast strip. As a result, any distortion in the belts have little effect on the metal being cast at that location. The high thermal gradient largely dissipates before the

belts

10 and 12 reach the nip 48, and thus any distortions that do occur diminish as the belts approach the nip.

The thickness of the strip that can be cast is, as those skilled in the art will appreciate, related to the thickness of the

belts

10 and 12, the return temperature of the casting belts and the exit temperature of the strip and belts. In addition, the thickness of the strip depends also on the metal being cast. It has been found that aluminum strip having a thickness of 0.100 inches (2.54 mm) using steel belts having a thickness of 0.08 inches (2.03 mm) provides a return temperature of 300°F (149°C) and an exit temperature of 800°F (427°C). The interrelationship of the exit temperature with belt and strip thickness is described in detail in EP-A-0583867. For example, for casting aluminum strip for a thickness of 0.100 inches (2.54 mm) using a steel belt having a thickness of 0.06 inches (1.52 mm), the exit temperature is 900°F (482°C) when the return temperature is 300° F (149°C) and the exit temperature is 960°F (516°C) when the return temperature is 400°F (204°C).

One of the advantages of the method and apparatus of the present invention is that there is no need to employ a thermal barrier coating on the belts to reduce heat flow arid thermal stress, as is typically employed in the prior art. The absence of fluid cooling on the back side of the belt while the belt is in contact with hot metal in the molding zone significantly reduces thermal gradients and eliminates problems of film boiling occurring when the critical heat flux is exceeded. The method and apparatus of the present invention also minimizes cold framing, a condition where cold belt sections exist in three locations of (1) before metal entry and (2) on each of the two sides of mold zone of the belt. Those conditions can cause severe belt distortion.

In addition, the concepts of the present invention also obviate the need to employ parting agents as have been used in the prior art to prevent sticking of the cast metal strip to either of the belts.

For some applications, it can be desirable to employ one or more belts having longitudinal grooves on the surface of the belt in contact with the metal being cast. Such grooves have been used in single drum casters as described in US-A-4934443.

In the preferred practice of the present invention, the

belts

10 and 12 are supported at least in the first portion of the molding zone by a plurality of pulleys positioned to maintain both belts in a manner to ensure that the belts are substantially flat. That is illustrated in Fig. 4 of the drawings which illustrates the pulley 18 and the

belts

10 and 12 as they face each other to define a mold cavity defining the solid strip 50. The lower pulleys 52 thus support the belt 12 as it passes over pulley 18. As shown in Fig. 4, each of those pulleys is mounted for rotation about an axis parallel to and extending transversely beneath belt 12 to maintain the belt in a substantially flat configuration, and thus assist in supporting both the weight of the belt and the weight of the metal strip 50 being cast.

A corresponding set of backup rolls 54 are mounted in tangential contact with the upper belt 10 and thus serve to exert sufficient pressure on the belt 10 to maintain the belt 10 in contact with the strip 50 as it is transformed from molten metal to a solid strip. In the preferred practice of this invention, the backup rolls in contact with the upper belt are not fixed, but rather float, although it is possible to utilize a system in which some of the backup rolls 54 float while others are fixed, depending on the application.

In the preferred embodiment, the upper set of backup rolls 54 are set in vertical slots so that gravity acts to close the gap and retain some thermal contact between the

belts

10 and 12 and the cast strip 50. These backup rolls serve to isolate the solidifying metal from mechanical vibrations of downstream equipment and to improve heat transfer, thereby cooling the strip 50 and making it stronger.

In accordance with the practice of another embodiment of the invention, there is provided means associated with the entry pulleys 14 and 18 to prevent displacement of those pulleys relative to each other. Any suitable apparatus to rigidly fix the relative positions of

pulleys

14 and 18 may be used. Figures 5 and 6 illustrate a simple mechanism including a

pillow block

45 and 47 on each of the

axes

21 and 24 of the entry pulleys 14 and 18, respectively, secured to each other by means of a tension member 49. The tension member may be either fixed or adjustable; it has been found that good results are obtained by simply using a turnbuckle 49 as the tension member to prevent relative displacement of

axes

21 and 24 relative to each other. As will be appreciated by those skilled in the art, various other and more sophisticated tension members may likewise be used. For example, use can be made of a hydraulic cylinder as the tension member to prevent relative displacement of the

axes

21 and 24 relative to each other. The use of such a hydraulic cylinder has the further advantage that it is adjustable, and thus the tension can be conveniently changed depending on the application and the metal being cast.

As in the preceding embodiment, the importance of freezing before the nip 48 in the present invention is that the

belts

10 and 12 are much mare stable when held in tension on the curved surface of the pulley and distort much less than if the molten metal 46 first contacts the

belts

pulleys

14 and 18, the belts are mare stable against distortion arising from that momentary thermal gradient. In addition, the space between the belts at the time that they first come into contact with the molten metal is substantially larger then the gap between the belts corresponding to the thickness of the cast strip. As a result, any distortion in the belts have little effect on the metal being cast at that location. The high thermal gradient largely dissipates before the

belts

The importance of freezing or solidification before the nip 48 also arises from the fact that as shown in Fig. 8 of the drawings, the metal solidifying between the curved surfaces in the molding zone prior to the nip has a dimension or thickness greater than the corresponding dimension or thickness of the nip itself. That insures that when the solidified cast metal is advanced to the nip 48, it has a larger dimension than that of the nip, thereby insuring that the nip 48 exerts a compressive force on the cast metal strip and thereby cause elongation to improve not only surface characteristics but also to reduce the tendency of the strip to crack. In addition, the compressive force exerted on the cast metal strip after solidification between the point of solidification and the nip itself insures good thermal contact between the cast metal strip and the belts.

The amount of compressive force is not critical to the practice of the invention. It has been found that the compressive force should be sufficiently high as to insure good thermal contact between the cast metal strip and the belt as well as sufficiently high so as to cause elongation. The elongation is preferably sufficient to insure that the cast metal strip, while it is conveyed from the nip 48 through the remainder of the molding zone, is in a state of compression as distinguished from tension. As is described herein above, it has been found that maintaining the cast strip under compressive force serves to minimize cracking that would otherwise occur if the cast strip were maintained under tension. In general, it is desirable that the percent elongation be relatively low, generally below 15 percent, and most preferably below 10 percent. Good results have been achieved by the practice of the invention when the percent elongation is less than 5 percent.

belts

10 and 12, the return temperature of the casting belts and the exit temperature of the strip and belts. In addition, the thickness of the strip depends also on the metal being cast. It has been found that aluminum strip having a thickness of 0.100 inches (2.54 mm) using steel belts having a thickness of 0.08 inches (2.03 mm) provides a return temperature of 300°F (149°C) and an exit temperature of 800°F (427°C). The interrelationship of the exit temperature with belt and strip thickness is described in detail in EP-A-0583867. For example, for casting aluminum strip for a thickness of 0.100 using a steel belt having a thickness of 0.06 inches (1.52 mm), the exit temperature is 900°F (482°C) when the return temperature is 300°F (149°C) and the exit temperature is 960°F (516°C) when the return temperature is 400°F (204°C).

In accordance with another embodiment of the invention, it is sometimes desirable to provide means along the respective edges of the belts to contain the metal and prevent it from flowing outwardly in a transverse direction from the belt. It is accordingly possible to use a conventional edge dam for that purpose such as used on twin drum casting machines. A suitable edge dam is illustrated in Fig. 5 of the drawings showing a pair of edge dam members 56 which are positioned adjacent to the edge of

belts

10 and 12. The edge dam members 56 are composed of a pair of walls extending substantially perpendicularly from the surfaces of the

belts

10 and 12 to prevent the flow of molten metal outwardly from the molding zone defined between the belts. For that purpose, the edge dam elements 56 have a leading edge 58 which is mounted forward of the casting nozzle 30 so that molten metal supplied by the casting nozzle 30 is confined between the

belts

10 and 20 and the opposing edge dam elements 56. As will be appreciated by those skilled in the art, other edge dams can likewise be used in the practice of the invention.

It will be understood that various changes and modifications can be made in the details of structure configuration and use without departing from the invention as defined in the claims.

Claims

Apparatus for strip casting of metals by continuous belt casting, comprising:

(a) a pair of continuous, endless belts (10, 12) formed of heat conductive material, said belts being positioned adjacent each other to define a zone (46) therebetween;

(b) each of said belts being mounted on at least a pair of pulley means (14, 16; 18, 20) whereby each belt defines a curved surface where it passes about one pulley means (14, 18) of the respective pair and a substantially flat surface thereafter;

(c) means (28) for supplying a molten metal; and

(d) cooling means (32) positioned adjacent to one of the belts (10) for cooling that belt;
characterised in that:

(e) the molten metal supply means (28) is positioned to supply molten metal to said curved surfaces of said belts (10, 12), whereby the said zone (46) is a molding zone in which molten metal solidifies to form a cast strip of metal (50), thereby transferring heat from the molten metal and the cast metal to the belts; and

(f) the apparatus includes further cooling means (34) positioned adjacent to the other belt (12) for cooling that belt, the cooling means (32) and the further cooling means (34) serving to reduce the temperature of the belts (10, 12) by removing, when the belts are not in contact with either the molten metal or the cast strip (50), substantially all of the heat transferred by the molten metal and the cast strip to the belts;
whereby the molten metal is deposited substantially on the curved surfaces of the belts (10, 12) about the said pulley means (14, 18) to transfer heat thereto while the belts are supported by the said pulley means and then the heat thus transferred to the belt is removed when the belt is not in contact with the molten metal or the cast strip to thereby minimize distortion of the belts so as to improve surface quality of the cast strip.
Apparatus according to claim 1, wherein each belt (10, 12) defines a substantially flat horizontal surface after the belt passes around the said pulley means (14, 18).
Apparatus for strip casting of metals by continuous belt casting comprising:

(a) a pair of continuous, endless belts (10,12) formed of heat conductive material, said belts being positioned adjacent each other to define a zone (46) therebetween;

(b) each of said belts being mounted on at least a pair of pulley means (14, 16; 18, 20) whereby each belt defines a curved surface where it passes about a respective entry pulley means (14; 18) of the respective pair and a substantially flat surface thereafter, the belts passing around the respective entry pulley means defining a nip (48) therebetween, said nip lying in a plane defined by the axes (21, 24) of the entry pulley means and extending substantially perpendicular to the belts; and

(c) means (28) for supplying a molten metal;
characterised in that

(d) the molten metal supply means (28) is positioned to supply molten metal to said curved surfaces of said belts, whereby the said zone is a molding zone in which molten metal substantially solidifies prior to the nip between the entry pulley means; and

(e) the apparatus includes means (45, 47, 49) associated with the entry pulley means to control the spacing therebetween and effect a compressive force on the substantially solidified cast strip (50) at the nip sufficient to cause elongation of the cast strip so that it is in compression in the direction of its travel after exiting from the nip to minimize cracking of the cast strip.
Apparatus according to claim 3, including cooling means (32, 34) positioned adjacent to each of said belts (10, 12) when said belts are not in contact with either the molten metal or the cast metal (50), said cooling means serving to reduce the temperature of the belts by removing, when the belts are not in contact with either the molten metal or the cast strip, substantially all of the heat transferred by the molten metal and the cast metal to the belts.
Apparatus according to claim 3 or 4, including tension means (49) connected to the axis of each entry pulley means (14, 18) to prevent displacement of the entry pulley means away from each other.
Apparatus according to claim 1, 2 or 4 or claim 5 when dependent on claim 4, wherein the cooling means (32, 34) includes means for applying a cooling fluid on the endless belts (10, 12).
Apparatus according to any preceding claim, wherein each belt (10, 12) is carried on a pair of pulleys (14, 16; 18, 20), each mounted for rotation.
Apparatus according to any preceding claim, including means for advancing each of said belts (10, 12) about the pulley means.
Apparatus according to any preceding claim, wherein the means for supplying molten metal includes tundish means (28) having a substantially horizontal nozzle (30) positioned to deposit molten metal on the curved surfaces of the belts.
Apparatus according to any preceding claim, wherein the belts are formed of a heat conductive metal.
Apparatus according to any preceding claim, further comprising edge containment means (56) to prevent flow of molten metal beyond the edge of the belts.
A method for casting of metals by continuous belt casting comprising the steps of:

(a) moving a pair of endless belts (10, 12) formed of heat conductive material around a pair of pulley means (14, 18) whereby the belts define a zone (46) therebetween and, each belt, as it passes over one of the pulley means, defines a curved surface;
characterised by:

(b) supplying to the curved surfaces of each of the belts a molten metal whereby the said zone is a molding zone in which the molten metal solidifies to form a strip of cast metal (50) while transferring heat to the belts; and

(c) cooling the belts to remove the heat transferred to the belt from the molten metal and the cast metal when the belts are not in contact with the molten metal or the cast metal and before the belts receive additional molten metal,
whereby the molten metal is deposited substantially on the curved surfaces of the belts about the pulley means to transfer heat thereto while the belts are supported by the pulley means and then the heat thus transferred to the belt is removed when the belt is not in contact with the molten metal or the cast strip to thereby minimize distortion of the belts so as to improve surface quality of the cast strip.
A method for casting of metals by continuous belt casting comprising the steps of:

(a) moving a pair of endless belts (10, 12) formed of heat conductive material around a pair of entry pulley means (14, 18) defining a nip (48) therebetween, with the nip lying in a plane defined by the axes (21, 24) of the entry pulley means and extending substantially perpendicular to the belts, whereby the belts define a zone (46) therebetween and, each belt, as it passes over the entry pulley means, defines a curved surface;
characterised by:

(b) supplying to the curved surfaces of each of the belts a molten metal whereby the said zone is a molding zone in which the molten metal solidifies to form a strip of cast metal (50) having a thickness greater than the thickness of the nip; and

(c) advancing the cast strip, to the nip to effect a compressive force on the substantially solidified cast strip at the nip sufficient to cause elongation of the cast strip so that it is in compression in the direction of its travel after exiting from the nip to minimize cracking of the cast strip.
A method according to claim 13, including the step of cooling the belt to remove the heat transferred to the belts from the molten metal and the cast metal when the belts are not in contact with the molten metal or the cast metal and before the belts receive additional molten metal.
A method according to claim 13 or 14, including the step of controlling the space between the entry pulley means whereby the strip of cast metal is subject to the compressive force to effect elongation thereof.
A method according to claim 15, wherein the compressive force is sufficient to effect less than 15% elongation of the cast strip.
A method according to any of claims 12 to 16, wherein the molten metal is supplied to a substantially horizonal molding zone.
A method according to any of claims 12 to 17, wherein the molten metal is supplied in a substantially horizontal stream to the molding zone.
A method according to any of claims 12 to 18, wherein said metal is an aluminum alloy.
A method according to any of claims 12 to 19, including the step of conveying the strip of cast metal from the molding zone between the surfaces of the belts.