Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
  • B22D11/0637—Accessories therefor
  • B22D11/0665—Accessories therefor for treating the casting surfaces, e.g. calibrating, cleaning, dressing, preheating

Definitions

• This invention relates to continuous casting of metal sheet or strip (hereinafter referred to as strip) , and more particularly to an improved method of high speed direct casting of thin metal strip by withdrawing the strip from a supply of molten metal on a chilled casting surface.
• Patent 993,904 and Japanese published application 58-41656 flowing the melt into the nip of a pair of spaced counter-rotating chill rolls to withdraw a strip which is rolled and chilled on both surf ces as shown for example in U.S. Patent 4,212,344; partially submerging a driven cylindrical chill into the melt to withdraw a strip or filaments as disclosed in U.S. Patents 3,540,517 and 3,817,901; and, partially submerging a pair of counter-rotating cylindrical chills in a melt to withdraw a continuous strip chilled and rolled on both sides as shown for example in U.S. Patent 3,823,762.
• melt drag process molten metal from a melt
• the metal is solidified by extracting heat through the chill so that a thin skin is solidified immediately upon contact with the chill. This skin grows in thickness as the chill moves progressively through or past the melt until the strip is formed.
• the production of high quality directly cast strip product depends to a large degree upon a controlled heat transfer rate and uniform release of the cast product from the chilled surface. For example, when the heat transfer rate is too high, release can result before the solidifying skin has attained the desired or necessary thickness and remelting or breakage may result. Non-uniform release will result in gauge variations which may be so great as to make subsequent cold rolling operation difficult or impossible. Also, surface cracks may be formed in the top surface of the product, i.e., the product surface not in contact with the chill.
• a thin coating of a particulate refractory material to the chill prior to its contact with the melt.
• the particulate refractory is applied in a water slurry or other fast drying solution on a continuous basis as by spraying.
• the slurry may be applied with or without a binder.
• particulate material may be alumina, silicate oxide, magnesia or ground fire clay particles, for example, and is intended to break up the contact area between the solidifying melt and the chill.
• This technique necessarily results in a substantial number of the refractory particles adhering to and/or becoming embedded in the surface of the product and a uniform casting thickness or particle distribution cannot always be achieved.
• the chill moves in the open atmosphere for a substantial distance after release of the cast strip and before the chill surface is again presented in contact with the melt.
• the natural oxide layer or coating which builds up on the chill may include at least a small percentage of oxides of the melt.
• this coating is not a coherent, dense coating throughout but rather it builds as a dense, compact inner layer and a less dense outer portion comprising loose particles which can flake off or adhere
• Another object is to provide an improved process for controlling the gauge thickness of metal strip produced by direct casting from a melt on a continuous chill moving in contact with the melt to withdraw the cast strip.
• Another object is to provide an improved melt drag strip casting process employing a continuous chill surface to solidify and withdraw the strip from a melt of the metal to be cast and in which a thin compact natural oxide interface is established and maintained between the base metal of the chill and the melt to thereby control the rate of transfer of heat between the melt and the chill, the metal oxide interface being a compact,
• a melt of the metal to be cast is brought into contact with a continuously moving chill to solidify a strip of uniform thickness on the surface of the chill and the strip removed after solidification is substantially complete.
• the chill may be an internally cooled cylin ⁇ drical casting wheel and the process will be described herein with specific reference to a process using such a chill, it being understood that other chill con- figurations may also be employed.
• a melt of the metal to be cast is brought into contact with the chill which is driven at a predetermined rate to solidify and withdraw the strip from the melt at a substantially constant velocity.
• the invention will be described herein with reference to a process wherein the chill is positioned to effectively form one wall of a container for the melt, it being understood that other arrangements could be employed as suggested above.
• the chill could be partially submerged into the top surface of the melt, or the chill may be positioned adjacent the edge surface of the melt container and the molten metal caused to overflow to contact the chill.
• a conduit or nozzle could be employed to conduct the melt from a supply into contact with the moving surface of the chill.
• the uniform interface is achieved by continuously wiping or brushing the chill surface following release of a cast strip and prior to the surface re-entering the melt, whereby loose oxide particles and any adhering particles of the melt are continuously removed from the chill.
• the brushing or wiping action acts to lightly polish the oxide coating, producing a smooth surface on the oxide interface, with the oxide being of substantially uniform density over the entire chill surface.
• this light polishing action can maintain the oxide thickness within a range which will produce a substantially uniform heat transfer rate. It is critical, however, that the polishing action not be sufficiently abrasive to re- move the dense adhering oxide interface and thereby expose and abrade or damage the base metal of the chill surface. By controlling the polishing action to increase or decrease the oxide coating thickness and to maintain the desired iickness, the heat transfer rate can be controlled and thereby an effective strip gauge control is achieved.
• Fig. 1 is a schematic elevation view, partially in section, of an apparatus for direct strip casting of metal by a melt drag process embodying the present invention
• Fig. 2 is a sectional view taken on the line 2-2 of Fig. 1;
• FIG. 3 is an enlarged fragmentary view, in section, of a portion of the apparatus shown in Fig. 1;
• Fig. 4 is a sectional view taken on line 4-4 of Fig. 3;
• Fig. 5 is a view similar to Fig. 4 and showing an alternate construction of a portion of the apparatus employed in the practice of ihe invention. Description of the Preferred Embodiment Referring now to the drawings in detail, a melt drag strip casting apparatus suitable for use in the practice of the present invention is illustrated schematically in Fig. 1 and is designated generally by the reference numeral 10.
• the apparatus includes a chill 12 in the form of a casting wheel or drum having a cooled cylindrical outer surface 14 upon which the metallic strip 16 is cast.
• a tundish assembly 18 is supported in close proximity to the casting wheel 12 in position to supply molten metal 20 contained therein at a uniform depth into contact with the chill surface 14.
• the casting wheel 12 is internally cooled with circulating water or other cooling fluid to rapidly extract heat through surface 14 to thereby quench and solidify liquid metal from the melt 20 which contacts the peripheral casting surface 14 as it rotates upwardly through the melt in tundish 18.
• Internally cooled casting wheels are known for example from U.S. Patent 2,348,178, and as schematically illustrated in Fig. 2 may comprise a hollow drum made from a pair of end flanges 22, 24 and an outer peripheral rim 26, the outer surface of which rim defines the chill or casting surface 14.
• a central hub 28 supported within the hollow drum has axially and radially extending, connect- ing inlet passages 30, 32, respectively communicating with the annular space 34 between hub 28 and the outer rim 26, and radially and axially extending connecting passages 36, 38 communicating with the annular space 34 provide an outlet for the cooling fluid.
• the cylindrical rim 26 is formed from any suitable metal having the desired thermal conductivity and wear characteristics, copper, steel and aluminum, and alloys of these metals, being examples of materials which have been successfully employed in the high speed casting of strip metal in apparatus of the type illustrated in Fig. 1.
• the chill surface 14 may be substantially smooth, but preferably has a pattern of closely spaced shallow grooves 39 formed therein- as more fully described hereinbelow with respect to Fig. 5 and as illustrated for example in U.S. Patent Nos. 4,250,950 and 3,345,738 and in French Patent 1,364,717.
• Suitable means such as bearings 40 support the casting wheel 12 for rotation about a fixed horizontal axis on a rigid support frame adjacent tundish 18, and suitable drive means such as a variable speed motor and reduction gear mechanism 42 and drive chain 44 illustrated schematically in Fig. 1 drive the chill about its fixed axis.
• tundish 18 has an open end positioned adjacent the chill, with transverse walls or dams being provided for controlling the flow of molten metal through the tundish to maintain a substantially uniform depth of the melt in contact with the rotating chill surface 14.
• Suitable means such as a ceramic trough or runner indicated generally at 46 provides molten metal
• an air knife assembly 48 may be provided to direct
• the jet 50 assists in control of the shape of the top surface of the strip 16 and prevents dross, oxides, slag or the like floating on the top of the melt 20 from coming into contact with and adhering to the top surface of the emerging strip in the manner disclosed and described in detail in copending application Serial No. 838,659, assigned to the assignee of this application.
• a heated top roll (not shown) may be em ⁇ ployed to contact the top liquid surface portion only of the emerging strip moving with the chill surface 14.
• an inert atmosphere above the free surface of the melt 20 in tundish 18. This may be accomplished by providing a cover or shield for the tundish and injecting an inert atmosphere under the shield to there ⁇ by purge air from the tundish enclosure.
• an air knife 48 When an air knife 48 is employed, the inert atmosphere may be supplied by the gas jet 50 discharged onto the emerging strip surface.
• the outer surface 14 is heated to a relatively high temperature when in contact with the melt 20 and this sur- face tends to oxidize rapidly in the presence of oxygen in the open atmosphere.
• this oxide formed on sur ⁇ face 14 has been removed by an abrasion or wire brushing process to prevent buildup on the external surface of the chill at least in the area contacted by the melt during the strip casting operation. For example, when casting 3000 series aluminum alloy on a steel chill surface, the steel surface will quickly turn a dark blue as it commences to oxidize following contact with the molten metal while cast ⁇ ing 1000 series alloy will produce a more gray-black oxide.
• an oxide coating developed in this manner is not a homogenous coating and does not have uniform surface characteristics, at least in part because of the non-uniform flaking and peeling of the outer portion of the coating during casting. Further, flakes or particles of the less dense portion of the coating tend to adhere to or become embedded in the surface of the cast strip, thereby producing non-uniform surface characteristics on the strip.
• the thickness of a natural, undisturbed oxide coating which develops in a direct strip casting process will. of course, vary both with the metal being cast and with the base metal of the chill. Also, the maximum thickness of the compact interface which can be maintained will vary in a like manner.
• the casting of steel strip for example, will result in a more rapid oxidation of the chill surface than casting aluminum which has a much lower melting tem ⁇ perature.
• the gradual buildup in thickness of the natural oxide coating which develops on a chill surface during strip casting will result in a gradual decrease in gauge thickness of the strip being cast until the coating stabi ⁇ lizes in thickness.
• variations in the surface characteristics and thickness of such an undisturbed oxide coating over the chill surface, and the consequent vari ⁇ ations in thermal conductivity of the coating will be evidenced by variations in strip gauge and surface characteristics.
• the driven rotary brush device illustrated in Figs. 1 and 2 has proven highly effective in removing the loose outer layer of oxide par ⁇ ticles and in producing a smooth clean surface on the dense natural oxide interface remaining on the chill surface.
• the rotation of the cylindrical brush produces a natural cleaning action preventing the buildup of oxide particles on the polishing medium, i.e., the brush bristles.
• Fur ⁇ ther by adjusting the position of the rotary brush 60 to produce an increased or decreased contact, or pressure, on the rotating chill surface, the thickness of the oxide interface can be increased or decreased, as desired, to thereby provide an effective gauge control for the strip.
• the speed of operation of brush 60 will also effect its efficiency in removing oxide and preferably a variable speed brush drive is employed to thereby provide an easy, readily adjustable control of oxide thickness during operation.
• the variable speed drive can be used in con ⁇ junction with power means for adjusting the brush bearing position to provide a wide range of control for prolonged or continuous operations.
• a similar brush containing 320- grit particles has also been used and may be desirable for longer continuous casting runs.
• the rotary brush bristles 62 are trimmed so that their free ends accurately define a cylindrical brushing outer surface, and the brush position may be adjusted so that the end portions only of the bristles lightly touch, or "kiss" the chill surface during operation. Preferably, however, contact is sufficient to cause slight bending at the end of the bristles, and in this respect, it is noted that contact between the brush 60 and chill 12 is exaggerated in Fig. 1 for illustration purposes only.
• the groove spacing measured axially along the sur ⁇ face of the cylindrical chill may be such as to provide from about ten to about forty grooves per centimeter and the grooves may have a depth of from about 0.025 to about 0.60 millimeters.
• the nylon-silicon carbide brush described above is employed with such a grooved chill surface, contact between the ends of the brush bristles and the chill is maintained so as to produce a polished oxide surface on the outer or land portion of the grooves but the bristles do not penetrate into the root of the respective grooves with sufficient pressure to remove all
• such a chill surface will have the visual appearance of alternate light and dark lines, with the light lines being smooth polished oxide areas on the lands and the darker areas being in the root regions of the grooves.
• surface tension normally prevents the liquid metal from penetrating into the root area of the grooves. Since the strip 16 does not contact the unpolished oxide coating near the roots of the grooves, it is not necessary to polish these areas.
• a smooth chill surface is employed, however, it is important that the smooth lightly polished natural oxide interface extend over the entire area of the chill surface which will be contacted by the melt.
• the position of the polishing brush rela ⁇ tive to the chill surface can be adjusted to remove more or less of the natural oxide coating. It has been found that a natural oxide interface will immediately start to build on the steel chill surface and will continue to grow in thickness until gauge control and strip surface qualities quickly commence to deteriorate if the oxide thickness and surface condition are not controlled in accordance with this invention.
• the oxide interface thickness can be controlled to prevent excessive buildup, however, by accurately positioning and driving the polishing brush so that the loose outer oxide particles are removed. Since the oxide interface thickness directly affects the heat transfer rate between the molten metal and the chill, and thereby directly affects the thickness of the strip being formed, control of the thickness of the oxide interface produces an effective strip shape and gauge control.
• a smooth polished oxide interface acts as a release agent, resulting in a uniform release of the formed strip from the chill surface. This greatly improves the strip quality both from the standpoint of top surface charac ⁇ teristics and from the standpoint of eliminating or greatly reducing variations in strip thickness. Further, extensive testing has shown that this process substantially eliminates the problem of longitudinal cracking of the top strip surface of direct cast aluminum strip.
• the cast strip be of substantially uniform thickness throughout a production run which may involve
• the present invention makes this uniform gauge possible by making it practical to maintain a substantially uniform oxide coating on the chill surface for sustained periods.
• the use of a simple, commercially available variable speed drive for the rotary brush provides easy and accurate control of the oxide thickness to compensate for brush wear or other changing conditions to maintain the desired substantially uniform oxide thickness throughout a run.
• a silicon carbide impregnated nylon bristle rotary brush has proven to be highly effective and reliable in establishing and controlling the natural oxide interface on the chill surface in a melt drag strip casting operation
• other materials and techniques may be employed to maintain this interface.
• other synthetic materials may be employed for the bristles in a rotary brush and the synthetic bristles may be impregnated
• both natural bristles and synthetic bristles which are not impregnated or coated with hard polishing agents may be employed and may be preferred when employed with a chill surface formed from a relatively soft material such as copper or aluminum, or alloys of these materials.
• the polishing material used it is important that the polishing material used be capable of removing the loose particles of natural oxide formed on the chill surface during the direct casting process while leaving the packed, more dense natural interface. Further, this interface should present a substantially smooth surface for contact with the molten metal to be cast.
• Examples of materials and techniques that have been successfully tested to establish and maintain the natural oxide interface on the chill surface in accordance with the invention have included both stationary and rotary brushes, brushes having horse hair bristles and bristles of other natural materials, and stationary felt pads. While these various materials and devices produce readily visible improvements in the cast strip over the prior art practices described above, reliable control of the process while employing such other devices and materials for longer or sustained runs have proven more difficult in operation than with the impregnated synthetic resin bristle brushes.
• An internally cooled casting wheel of the type illustrated in the drawings was used to directly cast aluminum strip from numerous heats of molten aluminum alloy.
• This casting wheel had an outer peripheral rim 26 made of alloy 1020 carbon steel and the outer casting surface had 44 generally circumferentially extending grooves per inch, a diameter of 27.635 inches, and a width of 42 inches.
• This used casting wheel was removed from the casting apparatus and sample sections were cut from the carbon steel rim for examination. The samples were removed from areas transversely spanning the casting surface from the edge to approximately the center of the casting surface. The actual casting track on the surface of the casting wheel was defined by a visibly oxidized surface. The samples were degreased and nickel plated to preserve the oxidized surface during metallagraphic preparation.
• Oxide thicknesses actually measured were substantially greater than original calculations indicated would be developed under the casting conditions. Further, localized variations in oxide thickness were observed which were not anticipated, with these variations being principally in the form of relatively thin spots at the base of small crevices in the land surfaces which possibly had minimum contact with the solidifying metal during casting. These minute crevices were found on careful examination although the casting wheel had been prepared to present a highly polished, uniformly smooth casting surface. The thicker oxide coating extended over most of the land surfaces except for the minute crevice areas mentioned. Maximum and minimum thickness measurements were recorded and the average of the maximum oxide thickness measurements was 12,400 angstroms while the average of the minimum oxide thickness measurements in the crevice areas was 1,450 angstroms. Again, it should be understood that the term "substantially uniform thickness" as used herein is in- , tended to embrace an oxide coating having such unavoidable localized thickness variations.
• a subsequent test conducted on a single 4 inch section from the casting band of the same casting wheel rim also included a survey of the oxide composition and thickness in the grooves as well as on the lands.
• the land width for this sample ranged from 0.009 to
• results of this test confirmed that the oxide contained within the grooves was essentially a single phase coating of iron oxide. Also, this test indicated that the thickness of the multiphase oxide on the land portion of the surface was approximately 1/3 the thickness and about four times more uniform than the oxide coating in the grooves.
• a second casting wheel similar to that described above has been used to make a number of runs to produce directly cast strip aluminum alloy 3105.
• a heated top roll was positioned in contact with the molten metal on the top surface of the strip emerging from the tundish, and a silicon carbide impregnated nylon brush of the type described above was employed to lightly polish the chill surface.
• a silicon carbide impregnated nylon brush of the type described above was employed to lightly polish the chill surface.
• the shape measurements were taken at one foot intervals and also showed a deviation in gauge from the mean of only about 0.002 inches. Casting speed during this run varied from 250 to 205 feet per minute. Approximately one half of the coil of 3105 aluminum strip alloy produced was slit to remove one inch from each side of the strip, then rolled in a cold mill at speeds of up to 500 feet per minute.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Continuous Casting (AREA)
• Manufacture Of Alloys Or Alloy Compounds (AREA)

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• 1988
  • 1988-12-29 WO PCT/US1988/004642 patent/WO1989007498A1/en not_active Ceased
  • 1988-12-29 DE DE3887518T patent/DE3887518T2/de not_active Expired - Fee Related
  • 1988-12-29 JP JP89501612A patent/JPH02503170A/ja active Pending
• 1989
  • 1989-01-31 CA CA000589653A patent/CA1325097C/en not_active Expired - Fee Related
  • 1989-08-29 EP EP89901700A patent/EP0362305B1/de not_active Expired - Lifetime
  • 1989-08-29 AT AT89901700T patent/ATE100743T1/de not_active IP Right Cessation

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