GB2320034A

GB2320034A - Coating of continuous casting machine grid plates

Info

Publication number: GB2320034A
Application number: GB9714452A
Authority: GB
Inventors: Philip Anthony Lavin
Original assignee: Monitor Coatings and Engineers Ltd
Current assignee: LWN Ltd
Priority date: 1996-11-08
Filing date: 1996-11-08
Publication date: 1998-06-10
Anticipated expiration: 2016-11-08
Also published as: GB2320034B; GB2319042B; ATE204615T1; PT946778E; GB9623344D0; EP0946778A1; WO1998021379A1; ES2163139T3; DK0946778T3; JP2001504162A; EP0946778B1; GB2320034A8; DE69706317T2; AU4877597A; GB9714452D0; GB2319042A; DE69706317D1

Abstract

A coating consisting of ceramo-metallic material, e.g. WC-Co, is applied to an internal surface of a grid plate by flame spraying. 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. The coating is in the form of an array of islands. A ceramic slurry may then be applied and fixed to produce a ceramic coating which is partially removed to produce a composite ceramic-metallic and ceramic surface which is then bonded and densified. The ceramic slurry may contain a chromium compound capable of being converted into chromium oxide at temperatures of at least 300{C. The composite surface may then be impregnated with a solution of at least one soluble compound capable of being converted on heating, into an insoluble substance which bonds the coatings. The soluble compound may be selected from chromia and phosphate forming compounds.

Description

1 2320034 COATING OF CON7LNUOUS CASTING MACHENE GRED PLATES Document.;:

163729 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 As the molten metal continuous to solidify, the plug is slowly withdrawn downwards from the mould zone whilst the level of molten material at the top of the mould is maintained by continuously adding further molten material.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 "fmgers" 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 avertical 2 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 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 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.

In particular the invention provides a method of protecting a grid plate for a continuous casting machine against wear, 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 ceraino-metallic material. (b) applying a ceramic slurry to the islands of ceramometallic 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 3 (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.

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 250T, heating the slurry 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.

4 All flame sprayed coatings to a greater or lesser degree contain microcracks 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 (up to 500OC) and 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 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 min in diameter and 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: Process for coating grid plates (1) Degrease the grid plates.

(2) Mask where 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. 450 to 500'C.

(8) Cool.

(9) Remove excess ceramic.

(10) Impregnate the coatings with oxide or phosphate forming compounds.

(11) Fire at approx. 450 to 5000C. (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).

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Claims

CLAIMS:

1. 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 cerarno-metallic material.

2. 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 ceraino-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.

3. A method as claimed in claim 1 or 2, wherein the islands have a thickness of 0.3 - 0.5 mm.

7 4, A method as claimed in any preceding claim, wherein the islands have transverse dimensions in the range

4 - 8 mm.

5. A method as claimed in any preceding claim, wherein the islands are spaced apart by 1 - 3 min.

6. A method as claimed in any preceding claim, wherein the islands are substantially square.

7. A method as claimed in claim 2, wherein the ceramic slurry contains a chromium compound capable of being converted into chromium oxide at temperatures of at least 3000C.

8. A method as claimed in claim 2, wherein the soluble compound is selected from chromia and phosphate forming compounds capable of being converted into oxides and phosphates on heating.

9. A method as claimed in claim 2, wherein the impregnation/heat treatment cycle of steps (e) and (f) is repeated.

10. A method as claimed in any preceding claim, wherein the cerarnometallic material is applied by a high-velocity oxygen - liquid fuel process.

11. A method as claimed in any preceding claim, wherein the ceramometailic material contains a carbide, preferably tungsten carbide.

12. A method as claimed in claim 11, wherein the said material contains cobalt.

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13. A method of protecting a grid plate for a continuous casting machine, substantially as described herein.

14. A grid plate produced by a method as claimed in any preceding claim.

1C7 Amendments to the claims have been fNed as follows 1. 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.

2. 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-metaflic 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.

/0 4. A method as claimed in any preceding claim, wherein the islands have transverse dimensions in the range 4 - 8 mm.

5. A method as claimed in any preceding claim, wherein the islands are spaced apart by 1 - 3 mTn.

7. A method as claimed in claim 2, wherein the ceramic slurry contains ' a chromium compound capable of being converted into chromium oxide at temperatures of at least 2STC.

8. A method as claimed in claim 2, wherein the soluble compound is selected from chromia and phosphate forming compounds capable of being converted into oxides and phosphates on heating Cr 9. A method as claimed M" claim 2, wherein the impregnation/heat treatment cycle of steps (e) and (f) is repeated.

10. A method as claimed in any preceding claim, wherein the ceramometallic material is applied by a high-velocity oxygen - liquid fuel process.

I- 11. A method as claimed in any preceding claim, wherein the ceramometallic material contains a carbide.

12. A method as claimed in claim 11, wherein the said carbide is rmgsten carbide.

13. A method as claimed in claim 11 or 12, wherein the said mateTial contains cobalt.

11 14. A method of protecting a grid plate for a continuous casting machine, substantially as described herein.

15. A grid plate produced by a method as claimed in any preceding claim.

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