THE FORMING OF METAL WORKPIECES FROM MOLTEN METAL

Abstract: A method of, and apparatus for, forming metal workpieces (9) includes the provision of a metal or alloy, e.g., tin or tin alloy, on the surface of at least one cooled shaping body (1, 3) which is engaged by the molten metal to be formed. This is particularly applicable to molten aluminium. In a two-roll caster, a layer of tin is formed on the periphery of each roll either before or during casting.

Description: THE FORMING OF METAL WORKPIECES FROM MOLTEN METAL This invention relates to the forming of metal workpieces from molten metal in which the molten metal is brought into engagement with at least one cooled shaping body. A particular, but not sole, application of the invention is to the casting of strip in aluminium or aluminium alloy by the two-roll casting process. Henceforth in this specification the term aluminium is to be interpreted to include aluminium alloys. One of the problems which is encountered with these processes is that the workpiece may stick to the or each shaping body. Even if the workpiece can be removed, the surface finish of the workpiece is damaged. If the workpiece cannot readily be removed, the process has to be shut down. In an attempt to reduce sticking, it is known to continuously apply a coating of lubricant to the or each cooled shaping body which may take the form of a mould or rotatable roll. When the process is a casting operation employing a two-roll caster, the two most common lubricants are suspension of colloidal graphite or magnesium hydroxide in water. The suspension is sprayed on to the hot caster rolls and care has to be taken to prevent excess liquid from being carried around the rolls into the region of the roll bite where it comes into contact with the molten metal. It is important that the spray coating is uniform because irregular coatings can generate other casting defects and it has been found that, even when a suspension is sprayed on to the rolls, sticking can still occur, particularly when casting thin strip, typically strip of less than 6 mm. Sticking of the workpiece to a shaping body is particularly likely to occur during start-up of the manufacturing process and for a short time thereafter. Once the process is underway, sticking is unlikely to occur unless there is a sudden change, such as operating temperature, in the process, in which case it may cause the workpiece to stick to one of the shaping bodies. It is one of the objects of the present invention to lessen the chance of sticking occurring during the start-up of a process for manufacturing workpieces from molten metal. The steps taken to lessen the chance of sticking may cease when start-up has occurred. Alternatively, the steps may be taken continuously throughout the manufacturing process. According to a first aspect of the present invention, in a method of forming a metal workpiece, molten metal is brought into engagement with at least one cooled shaping body and the surface of the or each shaping body which is engaged by the molten metal has in contact with it a metal or alloy having a melting point which is lower than that of the molten metal in order to reduce sticking between the solidified metal workpiece and the shaping body. The shaping bodies conveniently comprise two rotating rolls and the molten metal is introduced into the bite between the rolls. When the molten metal is aluminium, tin or an alloy thereof is preferably employed. Tin and alloys containing tin has very limited solubility in aluminium and so the tin/tin alloy may be added to the molten aluminium. The amount of metal or alloy is very small, less than 0.1% and, consequently, the alloy being cast remains within normal composition limits. Because of its limited solubility, the tin forms a surface film on the aluminium and thus the film is present between the aluminium and the rolls as casting commences. In another embodiment of the invention, the rolls may be provided with a surface coating of the tin or tin alloy prior to casting. The coating will periodically have to be replaced. This may be between every cast or after a predetermined number of casts. In a still further embodiment of the invention, at least during start-up of the process, the tin or tin alloy is provided continuously in the form of a coating on to a portion of the peripheral surface of each roll prior to that portion of the peripheral surface engaging the molten aluminium. A powder or suspension of the tin or its alloy or a compound of tin may be sprayed on to the rolls during casting. When a tin compound, such as stannous oxide or stannous chloride, is used, it reacts with the heated roll to form a coating of tin on the surface of the roll. In an alternative arrangement, each roll may be caused to pass through a bath containing liquid tin or alloy in order to provide a thin film of the tin or alloy on the surface of the roll. Alternatively, the roll may pass through an electrolytic bath with a tin or tin alloy electrode so that a film of tin or alloy is deposited on the surface of the roll. In a still further emdodiment, a fluxed paste of tin or tin alloy is applied as a layer to the surface of the roll and a heat source, such as an electric heater, is used to fuse the paste to a coating of tin or tin alloy on the peripheral surface of the roll. According to a second aspect of the invention, a forming machine has at least one cooled shaping body, means for bringing molten metal into engagement with the or each shaping body and wherein the molten metal engaging surface of the or each shaping body has in contact with it a metal or alloy having a melting point lower than that of the molten metal. In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figures 1 and 2 are diagrammatic side elevations of a two-roll caster showing alternative embodiments of the invention; and Figures 3, 4 and 5 are diagrammatic side elevations of one roll of a two-roll caster showing still further embodiments of the invention. Referring particularly to Figures 1 and 2 , the two rolls 1, 3 are arranged with their axes of rotation horizontal with one roll above the other. A nozzle 5 projects into the bite 7 between the rolls. In use, the rolls are rotated by means (not shown) in the directions of the arrows and the interior of each roll is water cooled. A metal to be cast, conveniently aluminium, is directed through the nozzle 5 into the bite. The metal is cooled sufficiently in contact with the rolls for it to commence to solidify and form a thin metal strip 9. Care has to be taken that the strip 9 pulls away from both the rolls and does not tend to stick to either of them. It has been found that, when casting aluminium, the chance of the aluminium sticking to either of the rolls- is lessened considerably if there is tin present between the rolls and the aluminium. The tin may be applied to each roll before casting commences; only during start-up of the casting process; or continuously throughout the casting process. Referring to Figure 1, blocks of tin 11 are mounted in holders (not shown) and are positioned on the sides of the rolls which are opposite to the nozzle 5. The holders enable the blocks to be urged into contact with the periphery of the rolls 1, 3 so that a film of tin is rubbed off the blocks on to the rolls. If the blocks do not extend for the full lengths of the rolls the holders are oscillated along the length of the rolls to provide a coating along the length of the rolls. In this way, there is a film of tin between the rolls and the aluminium and this prevents sticking of the aluminium to the rolls. In the arrangement shown in Figure 2, a rotary brush 13 is positioned between each block 11 and the associated roll 1, 3. The brushes extend for the full width of the rolls and as they are rotated they wipe a small quantity of the metal from the blocks and deposit it on the surface of the rolls. By adjusting the speed of rotation of the brushes 13 and the pressure exerted by the blocks 11 against the brushes, the quantity of metal transferred from the blocks to the rolls can be adjusted. In a further embodiment which is not illustrated, each of the rolls of the caster has a thin band extending around its periphery. Each band also passes around a tensioning roll. The bands are held in tight contact with the surfaces of the rolls, particularly in the vicinity of the roll bite. The molten metal is introduced between the bands and each band is conveniently of steel with a coating of tin on its outer surface. In use, a layer of tin is present between the molten metal and the rolls. When the coating of tin on the band is worn away, the band can readily be replaced. Referring to Figure 3, the lower roll 3 has a tray 15 located beneath it. A quantity 16 of molten tin or tin alloy is present in the tray and a part of the periphery of the roll passes through the tray in contact with the molten tin as the roll rotates. A heater (not shown) serves to keep the tin in molten form and a wiper (not shown) may be provided to ensure that a uniform thin film of tin is applied to the roll. In Figure 4, an alternative embodiment is illustrated in which the tray 15 contains a liquid electrolyte with a tin or tin alloy electrode 17 dipping into the electrolyte. . A source of direct current 18 has its negative terminal in contact with the rotating roll by way of a wiper 19 and its positive terminal is in contact with the electrode 17. As the roll rotates, a thin film of tin is deposited on the surface of the roll which is wetted by the electrolyte. In the arrangement illustrated in Figure 5, the tray 15 contains a fluxed paste 20 of tin or tin alloy. As the roll 3 rotates, a layer of paste is applied to the part of the periphery of the roll as it comes into contact with the paste. Means (not shown) may be provided to smooth out the paste into a thin film. To one side of the roll 3, in advance of the portion of the roll in the tray 15, there is an electric heater 21 which, when energised, heats the fluxed paste as it passes by the heater causing the paste to fuse into a tin film of tin or tin alloy on the surface of the roll. In the embodiment of the invention where tin/tin alloy is added to the molten aluminium, the tin/tin alloy is initially added to the molten aluminium but, when casting is operating satisfactorily without sticking, the addition of tin/tin alloy may be stopped. As alternatives to a two-roll caster, the forming machine may be a belt caster or a drag caster. Other metals which may be cast include lead and zinc. The cooled shaping body may be a continuous casting mould.

Claims: Claims: 1. A method of forming a metal workpiece in which molten metal is brought into engagement with at least one cooled shaping body and the surface of the or each shaping body which is engaged by the molten metal has in contact with it a metal or alloy having a melting point which is lower than that of the molten metal in order to reduce sticking between the solidified metal workpiece and the shaping body. 2. A method as claimed in claim 1, in which the molten metal is aluminium or an alloy thereof and the metal or alloy having a melting point lower than that of the aluminium or aluminium alloy is tin or an alloy thereof. 3. A method of casting strip in which molten aluminium or an alloy therof is introduced into the bite between a pair of rotating cooled rolls and at least during commencing of casting the peripheral surface of each roll has tin or tin alloy in contact with it to reduce sticking between the strip and the rolls. 4. A method as claimed in claim 3, in which the tin or tin alloy is provided in the form of a coating on the peripheral surface of each roll prior to casting. 5. A method as claimed in claim 3, in which the tin or tin alloy is provided continuously in the form of a coating on to a portion of the peripheral surface of the roll prior to that portion of the peripheral surface engaging the molten aluminium or aluminium alloy. 6. A method as claimed in claim 5, in which the coating is applied to each roll by rubbing the periphery of the roll with either a body of tin or tin alloy or a member to which tin or tin alloy has been applied. 7. A method as claimed in claim 5, in which each roll is passed through a bath of liquid tin or tin alloy to provide the coating thereon. 8. A method as claimed in claim 4 or 5, in which the coating of tin or tin alloy is applied electrolytically. 9. A method as claimed in claim 4 or 5, in which a tin or tin alloy fluxed paste is applied to the roll and heated to fuse the tin or tin alloy to the roll. 10. A method as claimed in claim 3, in which the tin or tin alloy is in the molten aluminium or aluminium alloy and, due to its limited solubility, forms a surface film thereon. 11. A forming machine having at least one cooled shaping body, means for bringing molten metal into engagement with the or each shaping body and wherein the molten metal engaging surface of the or each shaping body has in contact with it a metal or alloy having a melting point lower than that of the molten metal. 12. A twin-roll caster comprising two spaced apart rotatable rolls, means for cooling the rolls; drive means for the rolls, a nozzle for introducing molten aluminium or an alloy thereof into the bite between the rolls and wherein the periphery of each roll has applied thereto a coating of tin or tin alloy. 13. A twin-roll caster as claimed in claim 12, in which the coating of tin or tin alloy is applied continuously to a portion of the peripheral surface of each roll prior to that portion of the peripheral surface engaging the molten aluminium or aluminiun alloy. 14. A twin-roll caster as claimed in claim 13, including means for urging a solid body of tin or tin alloy or a member to which tin or tin alloy has been applied against the peripheral surface of each roll.

Inventors: COOK RAYMOND, EDMONDS DAVID VERNON, GROCOCK PETER GEORGE, HUNT JOHN DAVID, THOMAS PHILIP MILROY, COOK, RAYMOND, EDMONDS, DAVID, VERNON, GROCOCK, PETER, GEORGE, HUNT, JOHN, DAVID, THOMAS, PHILIP, MILROY
Applicants: DAVY MCKEE POOLE, DAVY MCKEE (POOLE) LIMITED
Application year: 1992
Main classification: B22C3
IPC codes: B22C3, B22D11