AU710657B2

AU710657B2 - Component of a mould for the continuous casting of metals, comprising a cooled copper or copper-alloy wall having a metallic coating on its external surface, and process for coating it

Info

Publication number: AU710657B2
Application number: AU34488/97A
Authority: AU
Inventors: Christian Allely; Jean-Claude Catonne; Jean-Michel Damasse; Guido Stebner
Original assignee: Thyssen Stahl AG; USINOR SA
Current assignee: Thyssen Stahl AG; USINOR SA
Priority date: 1996-07-11
Filing date: 1997-06-26
Publication date: 1999-09-23
Anticipated expiration: 2017-06-26
Also published as: TW438911B; WO1998002263A1; EP0910489B1; FR2750903A1; CZ6499A3; KR20000022396A; RO119994B1; PT910489E; CA2258927A1; BR9710229A; CN1225046A; FR2750903B1; JP2000514361A; EP0910489A1; PL331180A1; ATE192951T1; GR3034001T3; ZA975970B; TR199900041T2; DE69702064T2

Abstract

The invention concerns an element of a continuous metal casting ingot mould with a copper or copper alloy cooled wall to be contacted with liquid metal and comprising on its external surface a metal coating, characterised in that the said coating consists of a silver plating. In a preferred embodiment, this wall is a cylinder hoop for a continuous casting machine of thin metal strips between two cylinders or on one single cylinder. The invention also concerns a method for coating with a metal plating the external surface of a copper or copper alloy cooled wall of an element of a continuous metal casting ingot mould, characterised in that a coating is effected by the deposit of a silver plating on said surface preferably by electrolysis. Preferably, the restoration of said silver plating is done by allowing a residual silver plating to subsist on said wall, and by effecting a re-silvering of said plating by placing said wall in cathode in an electrolysis consisting, for instance, of an aqueous silver cyanide solution, of an alkaline metal cyanide and of an alkaline metal carbonate.

Description

WO 98/02263 PCT/FR97/0113 9 COMPONENT OF A MOULD FOR THE CONTINUOUS CASTING

OF

METALS, COMPRISING A COOLED COPPER OR COPPER-ALLOY

WALL

HAVING A METALLIC COATING ON ITS EXTERNAL SURFACE,

AND

PROCESS FOR COATING

IT

The invention relates to the continuous casting of metals. More precisely, it relates to the coating of the external surface of the copper or copper-alloy walls of the moulds in which the solidification of the metals such as steel is initiated.

The continuous casting of metals such as steel is carried out in bottomless moulds, at the walls which are vigorously cooled by the internal circulation of a coolant, such as water. The metal in the liquid state is brought into contact with the external surfaces of these walls and starts to solidify thereon. These walls must be made of a material which is an excellent heat conductor so that they can remove sufficient heat from the metal in a short time. Generally, copper or one of its alloys, containing for example chromium and zirconium is adopted for this purpose.

Generally, the faces of these walls which are intended to be in contact with the liquid metal are coated with a layer of nickel, the initial thickness of which may be as high as 3 mm. It forms a protective layer for the copper, protecting it from being excessively stressed thermally and mechanically.

This nickel layer wears out in the course of use of the mould. It must therefore be restored periodically by complete removal of the remaining thickness, followed by deposition of a new layer, but such restoration obviously costs much less than complete replacement of worn copper walls.

Conventionally, the nickel layer is restored as soon as its thickness has dropped to approximately 0.6 mm.

Deposition of this nickel layer on the walls of the mould is therefore a fundamental step in preparing the casting machine, and it is important to optimize, 2 at the same time, the cost, the use properties and the adhesion qualities thereof. This is, in particular, the case on machines intended to cast ferrous-metal products, in the form of strip a few mm in thickness, which do not need subsequently to be hot rolled. These machines, the development of which is currently in progress, include a mould consisting of two rolls rotating in opposite directions about their axes, which are maintained horizontal, and of two refractory side plates pressed against the ends of the rolls. These rolls have a diameter which may be as high as 1500 mm and a width which, on the current experimental plants, is approximately 600 to 1300 mm. However, long term, this width will have to be as high as 1300 to 1900 mm in order to meet the productivity requirements of an industrial plant. These rolls consist of a steel core around which is fixed a copper or copper-alloy sleeve, the sleeve being cooled by circulating water between the core and the sleeve or, more generally, by circulating water inside the sleeve. It is the external face of this sleeve which must be covered with nickel, and it may easily be imagined that, because of the shape and size of this sleeve, its coating is more complex than that of the walls of conventional continuous-casting moulds which are formed from tubular elements or from an assemblage of flat plates, and which are of much smaller size. Optimization of the way in which the nickel is deposited is more especially important in the case of the sleeves for casting rolls since: because there is no subsequent hot rolling, the surface defects on the strip, which would result from a mediocre quality of the nickel coating, further run the risk of proving to be redhibitory in respect of the quality of the end-product; as the quantities of nickel to be deposited on the sleeves before they are used, and to be removed at the start of the operation of regeneration of the layer, are relatively large, this means that large 3 quantities of electricity are consumed and that a very considerable amount of time is spent, especially in the nickel-plating operation, typically several days.

The operation of complete removal of the nickel from the sleeve, which must precede restoration of the nickel layer, is also very important. On the one hand, its proper completion largely determines the quality of the nickel layer which will be subsequently deposited, especially its adhesion to the sleeve, as it proves very difficult to deposit a new nickel layer which is highly adherent to an older nickel layer. On the other hand, this nickel-removal operation must be carried out without consuming a very large amount of the copper of the sleeve, which is an extremely expensive component 15 and the duration of its use must be extended as long as p. ossible. This last requirement, in particular, ego• virtually excludes the use of a purely mechanical o o method for this nickel removal, since its precision OS 00 would not be sufficient to guarantee both the complete removal of the nickel and the safeguarding of the copper over the entire surface of the sleeve.

Other casting processes are intended to cast oo06 even thinner metal strip by depositing the liquid metal onto the periphery of a single rotating roll, which may 25 also consist of a steel core and a cooled copper sleeve. The problems of coating the surface of the sleeve which have just been described apply thereto in exactly the same way.

It would be desirable to provide a S. 30 method of coating the external surface of the copper or copper-alloy wall of a continuous-casting mould which is overall more economic than the usual methods in which a nickel layer is deposited on this surface. This method should also give the walls of the mould properties and a quality which are at least comparable to those obtained by depositing a nickel layer. It should also include a step of periodic regeneration of this surface. This method should be particularly suited I!l to the case of coating the sleeves of rolls for twin-roll or single-roll casting machines.

According to one aspect of the present invention there is provided a component of a mould for the continuous casting of metals, including a cooled copper or copper-alloy wall intended to be brought into contact with the liquid metal and having a metallic coating on its external surface, wherein the said coating consists of a silver layer. In a preferred application of the invention, this wall is a roll sleeve for a machine for the twin-roll or single-roll continuous casting of thin metal strip.

According to another aspect of the invention there is provided a process for coating the external surface of a cooled copper or copper-alloy wall of a S component of a continuous-metal-casting mould with a metallic layer, wherein 0@ this coating is produced by depositing a silver layer on the said surface, preferably electrolytically.

15 Preferably, restoration of the said silver layer is effected by leaving a residual silver layer on the said wall and by replating the said layer with silver by placing the said wall as the cathode in an electrolysis bath containing, for example, an aqueous solution of silver cyanide, of a cyanide of an alkali metal and a carbonate of an alkali metal.

As will have been understood, the invention consists in the first place of replacing the nickel conventionally used to form the external coating of the copper walls of moulds for the continuous casting of metals, such as steel, with silver. Contrary to what might be believed at first sight, since solid silver is regarded as a precious metal, this solution has many economic advantages S: 25 and is completely viable technically. This is particularly so when the silver plating is carried out using an electrolytic method employing a bath containing alkaline cyanides. It has proved to be the case that such baths are able to produce silver coatings on copper which have use I_ C:\WINWORD\VOLETVPHfL\NODELETE\3 4 4 8 8.97.DOC "iQi 5 properties well suited to protecting the walls of continuous-casting moulds.

The particular process for coating the surface of the mould, which is also described and claimed, includes a silver-plating step and also, optionally, a step of removal of silver from the said surface when it is desired to restore the coating of a worn mould. This silver removal may only be partial, while in the case of a nickel coating, the removal of nickel from the copper must virtually necessarily be complete, with the risk of consuming some of the copper of the wall. The silver plating and silver removal may both be carried out by electrolytic means. The silver removed from the sleeve is recovered in the metallic state on the silver cathode in the silver removal reactor. The said cathode may in turn be recycled as the anode in the silver plating reactor. As a variant, the silver removal may be carried out, at least in part, using chemical or mechanical means.

The invention will now be described in detail in one of its embodiments, this being applied to the coating of a copper or copper-alloy roll sleeve for a machine for the twin-roll or single-roll continuous casting of steel. However, it is clear that the example described may easily be adapted to the cases of other types of moulds having copper or copper-alloy walls, such as the moulds having fixed walls for the continuous casting of slabs, blooms or billets. It is also clear that the silver plating or silver removal method may employ various other electrolytic processes, such as buffer or spray coatings, as well as electrolytes other than those given by way of example.

It is also possible to provide complete immersion of the copper wall in a silver-plating bath and, under these conditions, the invention may be applied to a sleeve which is rotating continuously or intermittently, or else on a sleeve which is maintained stationary in a forced-circulation electrolyte.

6 Conventionally, the new sleeve has overall the shape of a hollow cylinder, made of copper or copper alloy, such as a copper-(l%)chromium-(0.1%)zirconium alloy. Its outside diameter is, for example, about 1500 mm and its length is equal to the width of the strip which it is desired to cast, i.e. about 600 to 1500 mm. By way of indication, its thickness may be about 180-mm, but it varies locally depending, in particular, on the method adopted for fixing the sleeve to the core of the roll. The sleeve is penetrated by channels through which a coolant, such as water, is intended to flow while the casting machine is in use.

In order to make it easier to handle the sleeve during the operations which will be described, it is firstly mounted on an arbor, and it is in this way that it will be transported from one treatment station to another before it is mounted on the core of the roll.

The treatment stations in the silver plating/silverremoval shop each consist of a tank containing a solution suitable for carrying out a given step in the treatment, above which tank it is possible to place the said arbor, with its axis horizontal, and to rotate it about its axis. Thus the lower part of the sleeve is dipped into the solution, and rotating the arbor/sleeve assembly enables the treatment of the entire sleeve to be carried out (it being understood that the sleeve normally performs several revolutions on itself during the same treatment, at a speed of approximately revolutions/min for example). It may also be useful, in order to avoid contamination or passivation by the ambient atmosphere of that part of the sleeve which has emerged, to provide on these treatment stations a device for spraying this part which has emerged with the treatment solution. For this purpose, it is also possible to envisage inerting the ambient atmosphere by means of an inert gas, such as argon, and/or to install a system for the cathodic protection of the sleeve.

However, although this is possible, provision may be made for these tanks to allow total immersion of the 7 sleeve, thereby making such spraying or inerting unnecessary.

The bared sleeve (in the case of the first silver plating of a new sleeve, or of the silver plating of a worn sleeve whose copper surface would have been bared) firstly undergoes, preferably, a mechanical preparation by polishing its surface. Next, it undergoes chemical cleaning in an alkaline medium, which has the purpose of ridding the surface of the sleeve of organic matter which may contaminate it.

Cleaning is carried out hot, at a temperature of approximately 40 to 70°C for fifteen minutes, and is followed by rinsing in water. It may be substituted with, or even supplemented by, an electrolytic cleaning step which would provide an even better surface quality.

The next step is an operation of pickling in an oxidizing acid medium, which has the purpose of stripping off the surface oxides, ensuring that only a very minute thickness of the sleeve is dissolved. For this purpose, use is made, for example, of a 100 ml/l aqueous sulphuric acid solution to which is added, before each operation, 50 ml/l of a 30% hydrogen peroxide solution or of a solution of another peroxy compound. It is also possible to use a chromic acid solution, this compound having both acidic and oxidizing properties. This operation of pickling in oxidizing acid medium is most effective when the temperature of the electrolyte is between 40 and 55 0

C.

It is advantageous to maintain this temperature at the interface by circulating hot water inside the channels in the rotating sleeve. The operation lasts approximately 5 minutes and is followed by rinsing in water.

Next, an operation of brightening the surface of the sleeve is advantageously carried out, preferably using a 10 g/l sulphuric acid solution for the purpose of avoiding passivation of the surface of the sleeve.

I

8 The total duration of all the operations preparatory to silver plating which have just been described does not, in principle, exceed 30 minutes.

The pre-silver-plating operation, carried out prior to the silver plating proper, has the purpose of establishing chemical conditions intended to prevent displacement of silver by copper during the silver plating, which would be prejudicial to the adhesion of the silver coating. It is particularly useful even when the sleeve is not made of pure copper but of a Cu-Cr-Zr alloy. It lasts 4 to 5 minutes and is preferably carried out at ambient temperature, the sleeve being placed as the cathode in an electrolyte containing an aqueous solution of sodium cyanide (approximately 50 to 90 g/l) and of silver cyanide sufficiently diluted with dissolved metal (30 to 50 It is also possible to replace the sodium cyanide by potassium cyanide (65 to 100 The fact of using for this pre-silver-plating operation an electrolyte whose composition, as will be seen, is qualitatively comparable to that of the silver-plating bath makes it possible to dispense with an intermediate rinsing step. Moreover, it also makes it possible to utilize the effluent resulting from the rinsing after silver plating, it being possible for this effluent advantageously to be recycled in the presilver-plating bath. The cathode current density is 4 to 5 A/dm 2 It is possible to use one or more soluble anodes (made of silver) or insoluble anodes (for example made of Ti/PtO 2 or Ti/RuO 2 In the case of insoluble anodes, the free cyanide is destroyed while being converted into carbonate with the evolution of ammonia. It is therefore necessary to recharge this electrolyte periodically with additions of free cyanide which may advantageously be removed from the effluent coming from the rinsing operation which follows the silver-plating operation proper. This pre-silverplating operation makes it possible to deposit a silver layer of a few pm in thickness (for example, from 1 to S2 Pm) on the surface of the sleeve, while at the same

I

9 time removing the acid deposits which might remain after the brightening operation. Next, the sleeve is transferred as rapidly as possible to the silverplating station without undergoing rinsing, so as to profit from the presence on its surface of a cyanide film which protects it from passivation.

The silver-plating operation proper is conducted -in an electrolyte essentially based on an aqueous solution of sodium and silver cyanides to which an excess of free sodium hydroxide is added, but it may also consist of a mixture of potassium and silver cyanides in an excess of free potassium hydroxide.

Potassium carbonate may also be added. A typical composition for this bath is: AgCN: 115 to 150 g/l; KCN: 215 to 250 g/l; KOH: 30 to 40 g/l; K2C03: 10 to 15 g/l.

The optimum operating temperature is 40 to 45°0C.

The potassium carbonate is necessary in order to obtain uniform corrosion of the anodes. It may be replaced by sodium carbonate, with the drawback that sodium carbonate has a lower solubility. The potassium hydroxide may be replaced by sodium hydroxide. They ensure the conductivity of the electrolyte as well as the stability of the anionic complex (Ag(CN) 4 2 in which the silver is found. The silver-plating operation is generally carried out using a DC source, which may advantageously be replaced by a source of transient currents which enable the fineness of the crystallization to be increased. The crystallization may also be advantageously modified by lowering the temperature of the sleeve/electrolyte interface, for example by circulating cold water through the channels in the sleeve. Under these conditions, the silverplating electrolyte is the hot source and the sleeve is the cold source. A temperature gradient is established and the interface then provides greater activation

I

10 overpotential favourable to increasing the hardness of the coating.

As was stated in the example described (which, from this point of view, is not limiting), the anode or anodes are soluble anodes consisting of one or more titanium anode baskets containing silver balls or metallic silver in any other form, for example pellets.

These titanium anode baskets are used as dimensionally stable electrodes. Their shape matches that of the sleeve in its immersed part, thereby enabling the distribution of the cathode current densities on the sleeve to be made uniform. Since the anode-cathode distance does not vary under these conditions, the anode baskets keep the current densities on the cathode constant.

Unless it is not possible to immerse the sleeve in the electrolyte completely, it is highly recommended to spray the surface of the non-immersed part of the sleeve continuously with this same electrolyte, or to render this same part inert using an inert gas. In this way, the risks of passivation of the freshly silverplated surface are avoided, which passivation would be prejudicial to good adhesion and to good cohesion of the coating. For this same reason, spraying the sleeve or inerting its surface while it is being transferred between the pre-silver-plating station and the silverplating station is also recommended. Providing cathodic protection of the sleeve may also be envisaged. This transfer must, in any case be carried out as quickly as possible.

It is possible to work either at a set voltage or at a set current density. When the electrolysis is carried out at a voltage of about 10 V with a current density of approximately 4 A/dm 2 a duration of approximately 5 to 8 days (depending also on the depth of immersion of the sleeve in the bath) enables a silver deposit up to 3 mm in thickness to be obtained.

Next, the sleeve is unfastened from its support shaft and is ready to be joined onto the core in order to 11 form a roll which will be used in the casting machine, after a possible final conditioning of the surface of the silver layer, such as imprinting a defined roughness using a shot-peening process, a laser machining process or any other process. As is known, such conditioning is aimed at optimizing the conditions for heat transfer between the sleeve and the solidifying metal.

During this use, the silver layer is subjected to attack and to mechanical wear which result in its progressive disappearance. Between two casting runs, the surface of the sleeve must be cleaned and the silver layer may, at least from time to time, be lightly machined for the purpose of compensating for any heterogeneities in its wear which could compromise the uniformity in the thermomechanical behaviour of the sleeve over its entire surface. It is also important to restore the initial roughness of the sleeve each time this is necessary. When the average thickness of the silver layer on the sleeve reaches a predetermined value, which is generally estimated to be approximately 1 mm, the use of the roll is interrupted and the sleeve removed, the sleeve possibly undergoing a complete or only partial silver-removal treatment which must precede restoration of the silver layer on the sleeve.

For this purpose, the sleeve may again be mounted on the shaft which supported it during the silver-plating operations. If silver removal is complete, the next step is the restoration of the silver layer using the entire process which has just been described.

Several options are available to the user for accomplishing this silver removal. Purely chemical silver removal is conceivable. However, the reagent used should dissolve the silver without significantly attacking the copper substrate, and it would be difficult to carry out only partial silver removal in a well-controlled manner. Another conceivable way of removing silver completely or partially is the electrolytic route, because of the perceptible 12 differences between the standard potentials of copper and silver (respectively 0.3 V and -0.8 V with respect to the standard hydrogen electrode). It is also applicable for copper-chromium-zirconium alloys of which the sleeve may be made. In this case, silver dissolution occurs by placing the sleeve as the anode in an appropriate electrolyte, generally an electrolyte based on nitric acid and containing a copper inhibitor, such as phosphate ions. A means of shortening the silver-removal operation would consist in preceding it with a mechanical silver-removal operation which would aim to decrease its residual thickness without, however, touching the copper. This operation would also have the advantage of making this thickness uniform and of removing the various surface impurities (especially metallic residues) which could locally slow down the onset of dissolution. This would thus avoid a situation in which silver is still being dissolved in certain regions of the sleeve when in other regions the copper would have already been bared.

However, the electrolytic silver-removal method has the disadvantage of requiring for its implementation a special solution which is incompatible, for toxicity reasons, with the other operations carried out in the sleeve silverplating/silver-removal shop where, moreover, cyanidecontaining solutions are used.

The inventors therefore recommend restoring the silver coating on the sleeve by direct recharging in a silver-plating bath (advantageously the one used in the first silver plating described above), without the requirement of completely or almost completely removing the residual silver coating. Such a procedure is possible since it is easy to deposit a new silver layer electrochemically on an older silver layer and obtain good adhesion of the new layer to the old, whereas this is not conceivable in respect of nickel. On the one hand, this considerably simplifies the materials management in the sleeve-conditioning shop and, on the 13 other hand, this shortens the maintenance time on the sleeves, and therefore the time during which they are out of commission. What is more, silver recharging, as proposed by the inventors, does not have the drawbacks generally attributed to the other forms of demetallization in general and of nickel removal in particular, because of the natural alkalinity of the silver-plating bath. This alkalinity may, in fact, be used as a means of natural passivation of the infrastructure of the silver-plating station if this is made of uncoated steel. Another advantage of the invention is never having to make the said steel infrastructures anodic, which would promote their corrosion and be prejudicial to their longevity.

Another advantage of direct recharging silver plating, compared with almost complete electrochemical silver removal followed by silver plating again, is that it avoids complete dissolution of the silver in certain preferred regions (such as the edges of the sleeve) during the silver-removal operation, which total dissolution would lead to localized baring of the copper. In addition, it makes superfluous the renewal of the pre-silver-plating step. Finally, recharging silver plating carried out under conditions avoiding complete dissolution of copper on the sleeve prevents the surface of the sleeve being attacked and therefore of prolonging its period of use. Recharging silver plating may be preceded by lightly machining the worn silver layer in order to make its thickness uniform and to remove the impurities which would be prejudicial to adhesion of the new silver layer to the old one.

Compared with a sleeve nickel-plating/nickelremoval shop, a sleeve silver-plating shop would therefore be distinguished in that it would not necessarily include equipment for chemically or electrochemically dissolving a worn coating. It would therefore be less expensive to construct. It would also be more economical to run since it would consume less S.electricity silver is deposited three times more 14 quickly than nickel for the same current density especially because of the fact that it is monovalent while nickel is divalent. This advantage is, however, partially compensated for in that, in order to obtain equivalent thermal protection of the sleeve with a silver coating and with a nickel coating, it is necessary to deposit a silver layer approximately twice as thick as the corresponding nickel layer. However, on the other hand, this silver layer provides superior mechanical protection of the sleeve compared to the thinner nickel sleeve. As regards the reagents, the cost of the silver salts used is, in fact, not greatly different from that of the nickel salts employed for the conventional nickel plating of mould walls.

Overall, the cost of a silver coating is therefore not very much greater than that of a nickel coating, and above all the repair of a worn casting roll sleeve is much quicker and more economic.

The cyanide-containing effluent from the shop, especially the rinsing water, may be treated using Javel water in order to destroy the cyanides. Since Javel water is easily manufactured electrolytically, it is possible to treat this lightly chlorinated effluent by electrolysis continuously: the metallic silver is recovered at the cathode and the cyanides are destroyed directly, into ammonium carbonate, on dimensionally stable anodes. Simple and inexpensive solutions may therefore be found to the environmental problems which may arise in the use of cyanide salts.

The invention is particularly applicable to the conditioning of the sleeves of rolls in plants for the twin-roll or single-roll continuous casting of steel, because of the large dimensions and high manufacturing cost of these components, for which it is important to extend their life as long as possible. However, it goes without saying that its transposition to treatments of copper or copper-alloy casting mould walls of any shape and size, which are intended for casting any metal which, in the liquid state, is able to be brought into v'

N)

{9

C

15 contact with silver under the casting conditions, is conceivable.

Claims

1. Component of a mould for the continuous casting of metal, including a cooled copper or copper-alloy wall intended to be brought into contact with the liquid metal and having a metallic coating on its external surface, wherein the said coating consists of a silver layer.

2. Mould component according to claim 1, wherein the said wall is a roll sleeve for a machine for the twin-roll or single-roll continuous casting of thin metal strip.

3. Mould component according to claim 1 or 2, wherein the said silver layer S•has been deposited using an electrolytic method.

4. Process for coating the external surface of a cooled copper or copper- 15 alloy wall of a component of a continuous-metal-casting mould with a metallic layer, wherein this coating is produced by depositing a silver layer on the said 00 0~ surface.

Process according to claim 4, wherein the said silver layer is deposited using an electrolytic method.

00006. Process according to claim 5, wherein it is applied to a bare copper or copper-alloy wall and wherein it includes, in succession, the following steps: an operation of cleaning the wall; 0 25 an operation of pickling the wall in an oxidizing acid medium; S° an operation of pre-silver-plating the wall, the latter being placed as the cathode in an electrolysis bath containing an aqueous solution of silver cyanide and of a cyanide of an alkali metal, so as to deposit a silver layer a few pm in thickness; an operation of silver plating the wall, the latter being placed as the cathode in an electrolysis bath containing an aqueous solution of silver C:\WINWORD\VIOLETPHIL\NODEIETE\34488-97.DOC +2 L 17 cyanide, a cyanide of an alkali metal, a hydroxide of an alkali metal and a carbonate of an alkali metal.

7. Process according to claim 6, wherein it also includes an operation of brightening the wall between the pickling and pre-silver-plating operations.

8. Process for restoring a silver coating deposited on the external surface of a copper or copper-alloy wall of a component of a continuous-metal-casting mould, wherein a residual silver layer is left on the said wall and wherein the said layer is replated with silver by placing the said wall as the cathode in an electrolysis bath containing a silver salt.

9. Process according to claim 8, wherein the said electrolysis bath contains an aqueous solution of silver cyanide, a cyanide of an alkali metal and a 15 carbonate of a alkali metal.

10. Process according to claim 8 or 9, wherein prior to replating the silver, the residual silver layer is lightly machined without removing it in its entirety. 20

11. Process for restoring a silver coating deposited on the external surface of a copper or copper-alloy wall of a component of a continuous-metal-casting mould, wherein a partial or complete silver removal operation is carried out by placing the said wall as the anode in an electrolysis bath based on nitric acid and containing a copper inhibitor and wherein the said wall or the residual silver 25 layer is replated with silver by placing the said wall as the cathode in an electrolysis bath consisting of an aqueous solution of silver cyanide, of a cyanide of an alkali metal and of a carbonate of an alkali metal.

12. Process according to any one of claims 6 to 11, wherein during the silver-plating or silver-replating operation, a temperature gradient is created between the wall and the electrolysis bath, cooling the wall. C\WINWORDVIOLET\PH ODELETE4488- i; C:\WINWORDWIOLETPHILINODELETE\34488-97.DOC I 18

13. Process according to any one of claims 6 to 12, wherein during the silver-plating or silver-replating operation, a transient-current source is used.

14. Component of a mould for the continuous casting of metal substantially as herein described with reference to the accompanying drawings. Process for coating the external surface of a cooled copper or copper- alloy wall of a component substantially as herein described with reference to the accompanying drawings. DATED: 20 April, 1999 9 b S d* S SS S *00S S 55S0 S C 0 PHILLIPS ORMONDE FITZPATRICK Attorneys for: USINOR and THYSSEN STAHL AKTIENGESELLSCHAFT 0000 S S S -0 S 0OO 0* 00 0 S 0OS* S C:\WINWORD\VIOLETIPHILlNODELETE\34488-97DOC