Patent Application
20060191664
I. Field of the Invention
This invention relates to the continuous casting of molten metals, preferably aluminum and aluminum alloys. More particularly, the invention relates to a method of introducing the molten metal into the casting cavity of a continuous caster and the design of a metal feeder used for this purpose.
II. Background Art
Continuous casting of metals has been carried out for many years, e.g. by using a twin belt caster, twin roll caster or rotating block caster. Continuous casters of this kind usually have a horizontal, or slightly downwardly-sloping, casting cavity formed between two confronting and continuously rotating casting surfaces. The molten metal is introduced into one end of the casting cavity and it is cooled and solidified as it is drawn through the casting cavity by the rotating casting surfaces. A cast ingot, slab or strip of solidified metal emerges from the casting cavity at the opposite end.
Molten metal is introduced into the casting cavity by some form of molten metal feeder that introduces a stream of molten metal between the casting surfaces. The feeder may be in the form of an open topped trough, in which molten metal is directed by means of an open spout or channel into the casting cavity (referred to as “pool feeding”), or more preferably by means of a nozzle which encloses and confines the molten metal until it emerges from a tip at the extreme end of the nozzle.
As-cast ingots produced by both DC (direct chill) and continuous strip casters produce metal slabs or strips having surface defects of various kinds. In DC casting such surface defects are often removed by means of “scalping” (i.e. removing a thin surface layer from the cast article). However, in continuous strip casting, scalping may not be practical or economical and it is desirable to provide an article at the outset having a minimum of surface defects.
Surface defects may be produced by a variety of mechanisms, including reaction with the refractory materials of the metal delivery system and localized cooling non-uniformities, and many improvements have been made to reduce the size and number of such defects.
Another common mechanism involves entrainment of surface oxides that form “cold shuts”. Such defects arise from the inevitable surface oxides that form on the meniscus surface of the molten metal where it exits the metal feeder to contact the moving casting surface. As the meniscus is dragged along by the moving casting surface, the oxide film becomes strained and breaks causing relatively large and visible surface defects of an irregular nature. This not only affects the appearance of the cast article, but also can introduce structural weaknesses that cause rollability problems. The defects are particularly critical in surface critical applications such as foil stock, can stock and automotive sheet and can limit the speed of casting.
There are various prior references disclosing feeder design and methods of metal introduction into a continuous casting cavity. For example, the assignee of the present application is also the assign e of U.S. Pat. No. 5,636,681 which issued on Jun. 10, 1997 and U.S. Pat. No. 6,725,904 which issued on Apr. 27, 2004. These patents disclose feeder designs that are intended to produce non-turbulent metal flow into the casting cavity.
European Patent No. EP 0 962 271 B1, which was granted on Dec. 17, 2003 to Hazelett Strip-Casting Corporation (inventor Valerie G. Kagan) discloses a belt casting apparatus with a metal delivery device that “pours” the metal onto a belt. The tip of the delivery device is spaced a distance away from the surface of the belt and it terminates at an end surface disposed at right angles to the metal-contacting inner surface of the delivery device.
U.S. Pat. No. 4,648,438 which issued on Mar. 10, 1987 to Hazelett Strip-Casting Corporation (inventor Robert W. Hazelett, et al.) discloses a belt caster and metal delivery device in which the end of the tip is “squared” and is arranged at right angles to the casting surface.
The following are examples of strip casters having tips in which the interior of the tip is tapered in the direction of the tip:
U.S. Pat. No. 3,774,670 which issused on Nov. 27, 1973 to Prolizenz AG (inventor Ivan Gyöngyös); U.S. Pat. No. 5,660,757 which issued on Aug. 26, 1997 to Hunter Engineering Co., Inc. (inventor Denis M. Smith); and U.S. Pat. No. 6,173,755 which issued on Jan. 16, 2001 to Aluminum Company of America (inventor Nai-Yi Li, et al.).
An object of the present invention is to improve the continuous casting of molten metal, particularly molten aluminum and its alloys, particularly with a view to reducing surface defects of the cast article and more particularly to reduce the incidence of oxide incorporation into the cast surface.
According to one aspect of the present invention, there is provided a feeder for delivery of molten metal into a mold formed between confronting casting surfaces of a continuous casting machine, the feeder comprising a projecting nozzle having at least one wall provided with a molten metal-contacting inner surface, a generally flat outer surface and an end surface at a tip of the nozzle with the end surface extending between the inner and outer surfaces. The end surface is generally flat and extends from the inner surface to the outer surface such that the angle between the inner surface and the end surface is between 15 and 80 degrees.
According to another aspect of the present invention, there is provided a process of continuous metal strip casting in which molten metal is fed from a nozzle feeder having a projecting nozzle onto at least one casting surface such that the metal maintains a meniscus having a surface coating of metal oxide between an extremity of the tip and a casting surface, and generating free oscillations in the meniscus at a frequency of at least 50 Hertz to cause the oxide layer to break at that frequency adjacent to the extremity of the tip, thereby minimizing the extent of surface defects due to oxide breaks on a surface of the cast metal strip.
The present invention is concerned with obtaining a continuously cast strip article of good surface quality. As the inventors obtained improvements in surface quality by making general improvements to the casting technique and apparatus, they noticed the presence of periodic surface striations extending across the cast article at right angles to the direction of casting. The inventors found that these striations were due, at least in part, to oscillations of the meniscus formed between the casting tip and the casting surface. When casting reactive metals, such as aluminum, the meniscus is coated with a layer of metal oxide and the oscillation of the meniscus can cause this to break. The underlying metal thus exposed rapidly grows a new layer of oxide upon reaction with air, but the break forms a visible defect in the surface of the cast product as the oxide layer is drawn onto the casting surface. It is theorized that meniscus oscillations are inherent in continuous casters, at least in belt casters used for casting aluminum and its alloys, as well as other reactive metals, and that they cannot be entirely eliminated. The inventors therefore took another approach, i.e. of increasing the frequency and uniformity of the oscillations to produce a cast article having small, regularly spaced striations that do not manifest themselves as surface defects because of their regular and fine appearance. In particular, it was found that an oscillation of the meniscus of at least 50 Hertz was required to impart an acceptable appearance to the cast product.
The meniscus tends to oscillate at right angles to its surface, i.e. it tends to become more rounded and then less rounded in the region extending from the nozzle to the casting surface, each such cycle representing one oscillation.
The inventors found that the frequency of the meniscus oscillations can be affected by various parameters, e.g. the application of external forces, such as pneumatic pressure in the small gap between the nozzle and the casting belt, and the application of a varying magnetic field in the area of the meniscus. However, the most effective ways to increase the frequency of oscillation include (a) improving the design of the nozzle used for injecting the molten metal onto the casting surface, and (b) adjusting the metallostatic head of the molten casting metal in advance of the nozzle.
With regard to the design of the casting nozzle, the conventional nozzle employs a pair of projecting walls that define a molten metal channel between confronting inner metal-contacting surfaces. The channel has an exit at the tip of the nozzle where the projecting walls terminate at a flat end surface that extends at right angles to the inner metal-contacting surfaces. The walls also have outer surfaces that in use extend along the casting surfaces with a small gap. Using this type of nozzle, it was noticed that not only was the frequency of oscillation of the meniscus slow and erratic, but the oscillations bring the metal into contact with the end surfaces of the nozzle walls, causing oxide whiskers to form and build up, thereby causing further sticking and interference with the oscillations. The inventors found that this effect could be minimized or eliminated by “cutting-back” the end surface by 20 degrees or more. This means that the end surface is caused to slope in a rearward direction from the line of contact with the inner metal-contacting surface to form an included angle (the angle within the material of the nozzle wall) of 80 degrees or less. The minimum angle is found to be 15 degrees, because a smaller angle is currently impractical for constructing the nozzle (although the desirable effect would still be apparent if the constructional limitations could be overcome). A more desirable lower limit for the cut-back angle is 30 degrees. Even more preferred lower limit is 45 degrees. Most preferably, the end surface of the nozzle wall is flat along its full length, i.e. from the intersections with the inner and outer walls, and from side to side across the nozzle. This has the advantage of making the striations more regular, particularly across the width of the casting cavity.
The line of contact mentioned above (the apex of the angle) forms a so-called “take-off point” for the meniscus, i.e. the point at which the molten metal loses contact with the nozzle and is briefly supported by surface tension before contacting the casting surface. Without limitation as to the theory of operation of the invention, it appears that the acute angle at this position fixes the point of metal departure so that it does not wander onto the end face of the nozzle wall. Oscillations of the metal instead appear to break the oxide layer on a regular and frequent basis at or adjacent to the take-off point, causing regular and fine striations.
Again without limitation as to theory, it seems that the amplitude of the meniscus oscillations increases with casting speed (i.e. the speed of the casting belt). A greater amplitude of the oscillations increases the risk of meniscus “wander” onto the end face of the nozzle, so the “cut-back” angle may desirably be decreased as the casting speed increases (i.e. the angle of the end surface with the inner surface should desirably be made smaller within the stated range of 15 to 80 degrees). For fast casting, the angle should preferably be made no larger than 75 degrees, and 70 degrees or even 65 degrees is a more preferred upper limit.
As noted above, the metallostatic head of the metal can also affect the frequency of oscillation of the meniscus. In casting operations, molten metal is usually conveyed from a furnace to a casting machine via a launder or trough. At the casting machine, the metal is normally fed into a tun-dish or head box that allows the metallostatic head to be adjusted and controlled by varying the depth of metal in the device. By varying the head, it is possible to vary the frequency of meniscus oscillation from a low end of about 50 herz (at minimum metal head) to about 200 Herz (at the maximum practical metal head) at least for aluminum and aluminum alloys. The range of frequencies depends on physical properties of the metal, e.g. density, viscosity and surface tension, but only for significant changes in these properties. The variation among aluminum alloys is quite minor, but a change of base metal (e.g. aluminum to copper) may produce significant changes that affect the oscillations more noticeably.
At increased casting speeds, the striations caused by oscillations at any given frequency are more widely spaced and hence less desirable. Therefore it is desirable to increase the frequency of oscillation as the casting speed is increased, e.g. by increasing the metallostatic head.
Generally, the effects of the invention are not affected by the thickness of the nozzle walls or the stand-off of the walls from the casting surfaces. However, if the spacing between the “take-off point” and the casting surface is too large (e.g. more than about mm), it becomes difficult or impossible to maintain a stable meniscus and the metal may run back under the tip of the nozzle. The flow characteristics change to become more like pouring a liquid rather than casting at speed. However, the spacing should be large enough to allow a meniscus to form between the take-off point and the casting surface. The minimum distance is controlled by restrictions placed on the tip by methods of construction and the need for the tip to be spaced slightly from the casting surface. However, the invention is effective with normal nozzle wall thicknesses (usually about 1 mm or 1/32 inch) and spacing (normally about 10 mm).
From such a head box or tundish, the metal can be fed into a closed nozzle or to an open topped nozzle. The present invention may be used with both types of nozzle, but a closed nozzle is preferred.
When the nozzle is of the closed type, the two walls forming the nozzle may be flat and parallel throughout their length, or they may be “flared” or “divergent” at the end, i.e. with the walls adjacent to the metal delivery end bending outwardly at an angle of usually no greater than about 8 degrees. This allows the walls to converge towards the casting surfaces by a small angle at the extreme end of the tip.
The present invention may be used with both horizontal and vertical continuous casting machines, e.g. twin-belt casters, revolving block casters and even twin-roll casters (twin-roll casters are preferably operated at high speed when the invention is employed).
As noted above, continuous casting of metals to form strip articles (often referred to as sheets, plates, slabs, ingots, billets, layers, etc.) has been carried out for many years and in many different types of continuous casting machines. For example, a twin-belt caster is disclosed in detail in U.S. Pat. No. 4,061,177 which issued on Dec. 6, 1977 to Alcan Research and Development Limited (inventor Olivo Giuseppe Sivilotti), and casting machines of this kind (as well as others) are suitable for carrying out the present invention. The disclosure of this patent is incorporated herein by reference, and a brief and simplified description is provided below.
The illustrated twin belt caster 10 has upper and lower endless rotating metal belts 12 and 14 arranged so that closely spaced moving confronting casting surfaces 16, 18 of the belts are disposed essentially parallel to each other through a region where they define a casting cavity (casting mold) 20 from a cavity entrance 21 to a cavity exit 22. The belts are guided as they rotate through suitable oval or otherwise looped return paths between the entrance and the exit of the casting cavity. The upper belt 12 passes around a cylindrical driving roll 24 and then travels along an upper path where it may be further supported, if desired, by rows of idler rollers or the like (not shown), and then around a semi-cylindrical bearing 25. The lower belt follows an essentially identical but mirror image path including a drive roll 26 and a semi-cylindrical bearing 27 similar to the bearing located immediately above. Molten metal is introduced into the casting cavity by a feeder 30 (not shown in
The present invention is primarily concerned with the delivery of molten metal into the casting cavity in the region of the nozzle tip 52. This is explained in more detail with reference to
In particular, the inventors have found that by causing the meniscus to freely oscillate at a controllable frequency of at least 50 Hz, the effect of oxide layer ruptures is controlled and reduced. It has been found that a nozzle tip designed in accordance with the present invention, e.g. as shown in
If the cut-back angle a is more than 80 degrees (e.g. 90 degrees as in a conventional nozzle tip), the meniscus 58 can touch the end surface 56 of the tip during oscillation. This rapidly forms oxide whiskers on the end surface of the tip and this in turn causes adherence of the meniscus to the end surface 56 below the line of intersection 65. This adhesion is variable and prevents regular and free oscillations of the meniscus from occurring. The breaks in the oxide layer are consequently irregular and delayed and the resulting surface defects are larger.
The amplitude of the meniscus oscillations appears to be somewhat casting speed related, i.e. larger amplitudes are encountered at higher casting speeds. As larger amplitudes can result in the meniscus being more difficult to fix on the nozzle, it is desirable to reduce the angle α at higher casting speeds and an angle no larger than 75 degrees is usually desirable for high casting speeds. On the other hand, if the cut back angle is less than 15 degrees, it becomes difficult to construct a nozzle tip having the requisite stiffness and strength properties.
The frequency of oscillation of the meniscus can be varied by altering the metallostatic head (in the tundish or feed-box) and typically for aluminum and its alloys can be varied between about 50 Hz and 200 Hz. The lower frequency generally occurs at the lowest metallostatic heads useable in a strip caster, and the upper frequency generally occurs at the highest metallostatic heads that can practically be achieved in typical commercial casting machines.
Different frequencies of oscillation can also be achieved by causing “forced” oscillations, for example, by applying regular mechanical vibrations to the nozzle tip or using electromagnetic forces to act directly on the meniscus.
The invention is illustrated in further detail with reference to the following Examples, which are not intended to be limiting of the scope of the invention.
The effects of different feeder tip angles were investigated on a single mold plane (water cooled) utilizing an open-topped box with a sliding bottom, to simulate the metal flow conditions from a stationary feeder tip onto a moving water cooled belt. The apparatus employed is shown schematically in
The metal was poured into a box 70 and a bottom plate 71 was pulled horizontally at predetermined speeds and molten metal temperatures, allowing the metal 75 to flow from an end 72 of the moving bottom plate onto a sheet steel mold 73, where it solidified progressively towards the moving bottom plate. The moving bottom plate (forming a thin slide) was made of the same material as the feeder tips used for continuous casting, and the right hand end was changed in geometry as shown in
A series of casts were performed in a pilot scale belt caster using metal delivery nozzles having various cut-back angles. Casts were made on copper belts using aluminum alloy AA5754 cast 10 mm thick cast at a speed of 8 to 10 meters/minute. The surface of the as cast strip was observed and photographed. The results are shown in FIGS. 8 to 12 and are summarized in Table 1 below.
Cut back angles of 93 and 88 degrees are outside the range of angles of the invention and the sheet cast using tips at such angles exhibit unacceptable oxide folds or banding. A spacing of 30 mm, typical of such bands, corresponds to a frequency of about 4 to 6 Hz. Cut back angles less than 80 degrees show an absence of such heavy bands, but display finer more regular surface marks with spacings of about 1 mm, corresponding to a frequency of over 100 Hz.
