COATING OF CONTINUOUS CASTING MACHINE COMPONENTS

Abstract: A coating consisting of ceramo-metallic material, e.g. WC-Co, is applied to an internal surface of a mould wall component or a grid plate by flame spraying. On the mould wall component the coating thickness increases from an upper region to a lower region. On the grid plate the coating is in the form of an array of islands. A ceramic slurry is applied and fired to produce a ceramic coating which is partially removed to produce a composite ceramo-metallic and ceramic surface which is then bonded and densified.

Description: COATING OF CONTINUOUS CASTING MACHINE COMPONENTS This invention relates to the coating of components used in the continuous casting of ferrous and non-ferrous materials. The topmost stage of a continuous casting machine is composed of a hollow box made up of side plates and end plates (and sometimes a central divider), usually of copper, which are water cooled. At the initial stage of casting a plug or dummy bar is positioned at the bottom of the mould and molten metal is poured into the mould from a ladle via a tundish. The water-cooled copper walls of the mould and the cold surface of the plug create a thin solidified skin which contains the molten material. As the molten metal continuous to solidify, the plug is slowly withdrawn downwards from the mould zone whilst the level of molten material at the top of the mould is maintained by continuously adding further molten material. A mould powder may be added simultaneously, to form a layer on the surface of the molten metal. As the plug or dummy bar and the solidifying slab or strand to which it is now attached leaves the mould zone, it then enters a grid plate zone. The grid plates are usually made of cast iron or steel. Each grid plate comprises several raised "fingers" with surfaces which are in contact with the chilled skin of the slab and which continue to constrain the solidifying slab to the requisite cross-sectional shape whilst cooling water is continuously sprayed onto the surface of the slab in between the fingers. During the time of this transfer of the slab from the mould and through the grid plate zone, the thickness of the chilled skin is increasing. The slab then passes further on down through the machine through a series of guides and rolls, which continue to constrain the slab to the designed shape. Whilst passing down the casting machine the plane of the slab is gradually changed from a vertical direction to a horizontal direction until the fully solidified but still hot slab emerges on a conveyor table at the bottom of the machine, where it is cut to the requisite length The surfaces of both the mould components and the grid plates are subject to severe wear due to the abrasive, adhesive, and erosive wear mechanisms induced by the high ferrostatic pressures, the high temperatures, and the formation of oxide skins and exacerbated by the entrainment of extraneous third body particles In a relatively short time, these components become so worn that they are unable to constrain the slab to the correct size and shape The casting campaign then has to be terminated whilst new or refurbished parts are fitted Thus the severe wear of these critical components limits the casting time and therefore the efficiency of the continuous casting process There is, therefore, a need for a coating that is capable of withstanding the aggressive high temperature wear environment thus extending the length of the normal continuous casting campaign The present invention provides a method of coating the surfaces of such components, comprising the following step (a) applying a coating, consisting of one or more layers, by flame spraying, and also the following sequential steps. (b) applying a ceramic slurry to the first coating, (c) heating the ceramic slurry to form a second ceramic coating (d) removing excess ceramic coating to produce a composite surface, (e) impregnating the composite surface with a solution of at least one soluble compound capable of being converted, on heating, into an insoluble substance which bonds the coatings, and (f) heating the coatings to cause bonding and densifi cation of the coatings by the said substance In one aspect, the coatings are applied to an internal surface of a continuous casting mould, i e a surface which confines the metal being cast. In another aspect, the coatings are applied to strand-constraining surfaces of a grid plate Step (e) preferably comprises impregnating the composite surface with a solution of at least one chromia or phosphate forming compound The final ceramic coating is preferably a so-called Monitox coating, which is achieved by applying to a substrate a slurry containing a chromium compound capable of being converted into a chromium oxide and/or a chromium phosphate at temperatures of at least 250°C, heating the slurry (e g at about 260°C) to produce a porous ceramic coating, and then densifying and bonding the coating by one or more process cycles comprising impregnating the porous coating with at least one chromia or phosphate forming solution, removing excess impregnant, and heating Such a technique is described in GB-A-1 466 074, for example The first coating may be any cermet (ceramo-metallic) coating but is preferably a tungsten carbide cobalt coating and is preferably applied in several successive layers The flame spraying process is preferably a high velocity oxygen-liquid fuel process which is often referred to as an HVOF process This process is a commercially available flame spraying process in which fuel and oxygen are combusted in a specially designed chamber which is connected to a water-cooled tube or nozzle at the combustion chamber exit The combustion products are accelerated down the nozzle, constituting a gun or torch. Powdered materials of closely controlled sizes are metered into the gun and are thus accelerated and heated as they pass along the hot high-velocity gas stream Upon impacting with the substrate at a specific distance from the gun the particles splat and build up upon each other and the substrate to form a highly bonded and dense coating Various coatings can be produced in this manner but a tungsten carbide 17% cobalt coating is the preferred coating for applying to the casting machine components All flame sprayed coatings to a greater or lesser degree contain micro-cracks or fissures which are the result of the quench stresses generated as the hot high velocity particles impact upon the substrate. Sometimes these micro-cracks can be extremely small, to such an extent that they are very difficult to discern using normal metallographic techniques. However, these very small cracks can propagate under the superimposed thermally and mechanically generated stresses and unless they can be effectively 'locked' or 'keyed' the coating will eventually fail by progressive spallation. The Monitox coating process induces the growth of oxides within these very small fissures and thus creates a resistance to crack propagation. The Monitox coating is applied in an aqueous slurry form containing various oxides and chemicals. This layer is then dried and heated (250 to 500°C). At this stage is 'soft' in that it is a collection of hard particles which are not very well bonded to each other or to the substrate. Thus, at this stage, excess coating can easily be removed by scraping or sanding operations so as to reveal the peaks of the underlying tungsten carbide coating which, in the 'as sprayed' condition, exhibits a coarse surface somewhat like a coarse sandpaper. Thus, a relatively smooth composite surface can be produced which comprises areas of ceramic and carbide material. This composite surface is then further densified and hardened by a cyclic process of impregnation and further heat treatment; for example from 3 to 10 cycles of impregnation and heat treatment can be required. When applying the composite coating to the mould components it has been found to be surprisingly beneficial to deposit the first coating to a tapered thickness profile from the top of each component to the bottom. The hottest region of the end and side plates is at the top at the meniscus zone. The inventor believes that making the coating thin in this area accommodates and withstands the high thermal stresses imposed in this area. The inventor also believes that making the coating thicker towards the lower end of the walls resists ferrostatic pressures and prevents the underlying ductile material from deforming under the applied load; if the coating is too thin in this area, deformation of the material allows the now unsupported and brittle tungsten carbide material to crack, and then these cracks can be propagated along any layers of weakness within the coating or along the coating/substrate interface, eventually causing the coating to spall from the copper surface A suitable variable coating thickness profile has been found to be a substantially linear increase from 0 08 mm (0 003 inch) at the top to 0 46 mm (0 018 inch) at the bottom, although other thickness variations may be preferred depending upon the temperature and pressure characteristics of the casting process It is preferable for the coating to extend over the whole height of each mould component However, the coating may terminate below the upper edge of the mould wall, provided it extends above the highest level of the molten metal The coating may have a substantially constant thickness in an upper region adjacent the upper edge of the mould wall component, it may have a substantially constant thickness in a lower region adherent the lower edge of the mould wall component, and the thickness may increase progressively in a linear manner, an exponential manner, or any other suitable manner between the upper and lower regions The minimum thickness is chosen primarily having regard to heat transfer, preferably in the range from 0 05 to 0 1 mm The maximum thickness is chosen having regard to mechanical strength as well as heat transfer, preferably in the range from 0 4 to 1 0 mm When coating the grid plates, it has been found to be surprisingly beneficial to form a pattern of separate islands It appears that any particular thickness may be applied, but a thickness of approx 0 38 mm (0 015 inch) is presently preferred It has been found to be particularly beneficial to deposit the coating in an island pattern in which each island is approx 5 mm square or 5 mm in diameter, each island is defined at its perimeter by an uncoated band approx 2 mm wide The inventor believes that the island pattern allows the coating to accommodate the high thermal stresses which are imposed on the grid plates during service, thus preventing premature coating failure by spallation A suitable thickness range may be from 0 3 to 0 5 mm The transverse dimensions of the islands may conveniently be in the range from 4 to 8 mm, the islands being spaced apart by 1 to 3 mm, for example The invention will be described further, by way of example with reference to the accompanying drawing, whose sole Figure schematically shows the internal surface of a grid plate Example 1 Process for coating grid plates (1) Degrease the grid plates (2) Mask where no coating is required (3) Grit-blast (4) Apply WC - 17% Co coating to the fingers of the grid plates by HVOF using masking devices to ensure the requisite 'island pattern' (5) Re-mask (6) Apply Monitox slurry (7) Fire at approx 250 to 300°C (8) Cool (9) Remove excess ceramic (10) Impregnate the coatings with oxide or phosphate forming compounds (11) Fire at approx 250 to 300°C (12) Repeat operations (10) and (11) for 3 to 10 cycles The drawing shows a cast iron grid plate 1 with gaps 2, through which cooling water is sprayed, and raised fingers 3 provided with the composite coating 4 made up of a pattern of rectangular islands 5 (approx 5 mm square, separated by at least 1 mm) Example 2 Process for coating copper mould components (1) Degrease the components (2) Mask where no coating is required (3) Grit-blast (4) Apply WC - 17% Co coating using computer-controlled robotic manipulation so as to ensure that the required tapered thickness profile (linearly increasing from 0 08 mm to 0 46 mm) is obtained from top to bottom on each component (5) Re-mask. (6) Apply Monitox slurry. (7) Fire at approx. 250 to 300°C. (8) Cool. (9) Remove excess ceramic. (10) Impregnate the coatings with oxide or phosphate forming compounds. (11) Fire at approx. 250 to 300°C, (12) Repeat operations (10) and (11) for 3 to 10 cycles.

Claims: CLAIMS: 1 A method of protecting a wall component for a continuous casting mould against wear, the method comprising applying to an internal surface of the mould wall component, by flame spraying, a coating consisting of ceramo-metallic material, the coating having a thickness which increases from an upper region to a lower region of the internal surface 2 A method as claimed in claim 1, wherein the minimum thickness of the coating is 0 05 to 0 1 mm and the maximum thickness of the coating is 0 4 - 1 0 mm 3 A method of protecting a wall component for a continuous casting mould against wear, the method comprising a) applying to an internal surface of the mould wall component, by flame spraying, a rough first coating consisting of ceramo-metallic material, the first coating having a thickness which increases from an upper region to a lower region of the internal surface, b) applying a ceramic slurry to the first coating, c) heating the ceramic slurry to form a second coating consisting of ceramic material, d) removing excess ceramic material from the second coating to produce a composite ceramo-metallic and ceramic surface, e) impregnating the composite surface with a solution of at least one soluble compound capable of being converted, on heating, into an insoluble substance which bonds the coatings, and f) heating the coatings to cause bonding and densification of the coatings by the said substance, whereby a composite coating is produced having a thickness which increases from an upper region to a lower region of the internal surface of the mould wall component 4 A method as claimed in claim 3, wherein the minimum thickness of the composite coating is 0 05 - 0 1 mm and the maximum thickness of the composite coating is 0 4 - 1 0 mm 5 A method as claimed in any of claims 1 to 4, wherein the wall component is a copper end plate or side plate for a continuous casting mould. 6 A method of protecting a grid plate for a continuous casting machine against wear, the method comprising applying to a strand-confining surface of the grid plate, by flame spraying, a coating consisting of ceramo-metallic material, the coating being in the form of an array of mutually isolated islands of the ceramo-metallic material 7 A method of protecting a grid plate for a continuous casting machine against wear, the method comprising- a) applying to a strand-confining surface of the grid plate, by flame spraying, a rough first coating consisting of ceramo-metallic material, the first coating being in the form of an array of mutually isolated islands of the ceramo-metallic material, b) applying a ceramic slurry to the islands of ceramo-metallic material, c) heating the ceramic slurry to form a second coating consisting of ceramic material, d) removing excess ceramic material from the second coating to produce a composite ceramo-metallic and ceramic surface, e) impregnating the composite surface with a solution of at least one soluble compound capable of being converted, on heating, into an insoluble substance which bonds the coatings, and f) heating the coatings to cause bonding and densification of the coatings by the said substance, whereby a composite coating is produced which is in the form of an array of mutually isolated islands 8 A method as claimed in claim 6 or 7, wherein the islands have a thickness of 0 3 - 0 5 mm 9 A method as claimed in any of claims 6 to 8, wherein the islands have transverse dimensions in the range 4 - 8 mm 10 A method as claimed in any of claims 6 to 9, wherein the islands are spaced apart by 1 - 3 mm 11 A method as claimed in any of claims 6 to 10, wherein the islands are substantially square 12 A method as claimed in claim 3 or claim 7, wherein the ceramic slurry contains a chromium compound capable of being converted into chromium oxide at a temperatures of at least 250°C 13 A method as claimed in claim 3 or claim 7, wherein the soluble compound is selected from chromia and phosphate forming compounds capable of being converted into oxides and phosphates on heating 14 A method as claimed in claim 3 or claim 7, wherein the impregnation/heat treatment cycle of steps (e) and (f) is repeated 15 A method as claimed in any preceding claim, wherein the ceramo-metallic material is applied by a high-velocity oxygen-liquid fuel process 16 A method as claimed in any preceding claim, wherein the ceramo-metallic material contains a carbide, preferably tungsten carbide 17 A method as claimed in claim 16, wherein the ceramo-metallic material contains cobalt 18 A casting mould wall component produced by a method as claimed in any of claims 1 to 5 or any claim dependent thereon 19 A grid plate produced by a method as claimed in any of claims 6 to 11 or any claim dependent thereon

Inventors: LAVIN PHILIP ANTHONY, LAVIN, PHILIP, ANTHONY
Applicants: LAVIN PHILIP ANTHONY, MONITOR COATINGS & ENG, LAVIN, PHILIP, ANTHONY, MONITOR COATINGS AND ENGINEERS LIMITED
Application year: 1997
Main classification: C23C4
IPC codes: B22D11, C23C4