SK299A3 - Element of a continuous metal casting ingot mould with a copper or copper alloy cooled wall comprising on its external surface a metal coating, and method of coating - Google Patents

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

Technical field

The invention relates to the continuous casting of metals. More specifically, it relates to an outer surface coating of copper or copper alloy for ingot molds in which solidification of metals such as steel begins.

BACKGROUND OF THE INVENTION

The continuous casting of metals such as steel is carried out in molds without a bottom, with walls intensively cooled by the internal circulation of a coolant such as water. The metal in the liquid state is brought into contact with the outer surface of these walls and begins to solidify thereon. These walls must be made of a material that conducts heat well enough to dissipate heat from the metal in a short time. Usually, copper or one of its alloys, for example chromium and zirconium, is used for this purpose.

The surfaces of these walls, which are intended to come into contact with the liquid metal, are usually covered with a layer of nickel, the initial thickness of which can be up to 3 mm. This forms a protective layer for copper, protecting it from excessive thermal and mechanical stress.

This nickel layer wears out during use of the ingot mold. It must therefore be repeatedly renewed, completely removing the remaining thickness and then applying a new layer, but such a reconstruction is clearly less expensive than a complete replacement of worn copper walls. Typically, the nickel layer is renewed when its thickness falls below about 0.6 mm.

The application of this nickel layer to the walls of the mold thus constitutes a substantial step in the preparation of the casting machine and is important for the simultaneous optimization of costs, user properties and adhesion quality. This is particularly the case with machines for casting metallurgical products in the form of strips of several mm thickness, which do not subsequently have to be hot rolled. These machines, which are currently under development, include a mold consisting of two cylinders rotating in the opposite direction about their axes, which are held horizontally, and two refractory side plates, pressed against the sides of the cylinders. The diameter of these rollers can be 1,500 mm and their width is about 600 to 1,300 mm in current experimental equipment. Visually, however, this width should be between 1,300 and 1,900 mm in order to meet the productivity requirements of the industrial plant. These rollers consist of a steel core around which a ring of copper or copper alloy is fastened, cooled by the circulation of water between the core and the ring, or generally by the internal circulation of water in the ring. It is the outer surface of the ring that has to be nickel-plated, and it is understandable that due to the shape and dimensions of the ring, it is more difficult to coat than the walls of conventional continuous casting molds consisting of tubular elements or planar plate assemblies and much smaller in size. Optimizing the method of nickel deposition is all the more important for these casting ring rings:

- there is a greater risk that surface defects in the strip, which may be due to poor nickel coating quality, may become essential to the quality of the end product due to non-final hot rolling,

- since the amount of nickel deposited on the rings prior to their use and removed at the beginning of the coating regeneration operation is relatively large, this involves a considerable amount of energy and time consumption, especially for the nickel plating operation, usually for several days.

The operation of completely removing the nickel from the ring, which must precede the recovery of the nickel layer, is also essential. On the one hand, its successful implementation largely determines the quality of the nickel layer which will then be deposited, especially its adherence to the ring, since it has proved very difficult to apply a new nickel layer strongly adhering to the older nickel layer. On the other hand, this nickel removal operation has to be carried out without much copper consumption in the ring, since it is a very expensive component whose shelf life should be extended as much as possible. In particular, this latter requirement virtually eliminates the removal of nickel by pure mechanical means, since its accuracy is not sufficient to ensure complete removal of the nickel and preservation of copper on the entire surface of the ring.

Other casting processes are designed to cast even thinner metal strips by applying liquid metal to the periphery of a single rotating cylinder, which may also be a steel core and a copper cooled ring. The problems of coating the surface of the ring just described also occur here.

The purpose of the invention is to propose a generally more economical method of coating the outer surface of a copper or copper alloy wall in a continuous casting mold than conventional nickel coating processes. This method should also impart to the mold wall walls characteristics and quality that are at least comparable to those of nickel coating. It should also include a periodic regeneration of this surface. This method should be particularly adapted in the case of coating the roller rings for the casting machine between rollers or on one roll.

SUMMARY OF THE INVENTION

For this purpose, the invention relates to a continuous metal casting mold element comprising a cooled copper or copper alloy wall to be contacted with a liquid metal and having a metallic coating on its outer surface, the principle of the invention being that the coating it consists of a layer of silver. In a preferred embodiment of the invention, this wall is a roller ring for machines for continuously casting thin metal strips between two rollers or on one roller.

The invention also relates to a method for coating the outer surface of a cooled wall of copper or copper alloy in a continuous metal ingot mold by a metal layer according to the invention, wherein the coating is carried out by depositing a silver layer on said surface, preferably by electrolysis.

Preferably, the restoration of the silver layer is carried out by leaving a residual silver layer on the wall and silvering the layer, the wall being placed as a cathode in an electrolytic bath, such as an aqueous solution of silver cyanide, alkali metal cyanide and alkali metal carbonate .

Accordingly, the invention consists primarily of replacing nickel, traditionally used for the outer coating of the mold walls of copper molds for the continuous casting of metals, such as steel, with silver. Although it may not seem at first sight, because solid silver is considered a precious metal, this solution has numerous economic advantages and is technically feasible. This is particularly the case when silvering is produced electrolytically using an al4 calic cyanide bath. These baths have been shown to be suitable for forming silver copper coatings having utility properties adapted to protect the walls of the ingot molds for continuous casting.

A particular method of coating the ingot mold surface, which is also described and claimed, includes the step of silvering and, optionally, the degree of stripping of this surface if it is desired to renew the coating of the worn ingot mold. This stripping may be only partial, whereas in the case of a nickel coating, the copper sputtering must be essentially complete, with the risk of consuming part of the copper from the wall. Both silvering and silvering can be carried out in an electrolytic manner. The silver removed from the ring is obtained in a metallic state on a silver cathode in a stripping reactor. This cathode can then be regenerated as an anode in the silvering reactor. Alternatively, the stripping may be performed at least in part chemically or mechanically.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in detail in one embodiment applied to the coating of a copper or copper alloy ring in a roll of a continuous steel casting machine between two rolls or on a single roll. However, the described example can, of course, be readily adapted to other types of ingot molds with copper walls or copper alloy, such as solid wall molds for continuous casting of slabs, blocks or billets. It will also be appreciated that the silvering or stripping process may employ various other electrolytic processes, such as buffering or spray coating, as well as electrolytes different from those exemplified. Full immersion of the copper wall into the silver bath may also be used, and under these conditions, the invention may be applied to a permanently or intermittently rotating ring or to a ring which is held stationary in a forced circulation electrolyte.

The new ring is generally in the form of a hollow cylinder of copper or copper alloy, such as copper-chromium (1%) -zirconium (0.1%) alloy. Its outer diameter is for example of the order of 1500 mm and its length is equal to the width of the strips to be cast, for example of the order of 600 to 1500 mm. Its thickness can be approximately 180 mm, but it varies in places depending mainly on the method of fixing the ring on the cylinder core. Through the ring passageways are provided for the passage of a coolant such as water during use of the casting machine.

To facilitate handling of the ring during the operations to be described, the ring is first mounted on the shaft and transported there between workstations prior to mounting on the core of the cylinder. The individual workstations of the silvering and deburring workshop consist of a tank containing a solution for carrying out the working stage above which the shaft with the horizontal axis can be placed and rotated about this axis. Thus, the lower part of the ring is soaked in solution and by rotating the shaft / ring assembly it is possible to treat the entire ring (of course the ring normally performs several turns during this treatment, for example at a rate of about 10 min ^-1 ). To prevent contamination or passivation of the emerging part of the ring by the surrounding atmosphere, a device for spraying the emerging part with the working solution can also be used at these workstations. For this purpose, the ambient atmosphere may also be inerted with a neutral gas such as argon and / or a ring cathodic protection system may be installed. However, if possible, the tanks may allow complete immersion of the ring, in which the spraying or inerting will be devoid of purpose.

The exposed ring (in the case of first silvering of a new ring or silvering of a worn ring in which the copper surface has been exposed) is preferably first subjected to mechanical preparation by polishing the surface. Chemical degreasing is then carried out in an alkaline medium, the purpose of which is to free the surface of the ring of organic substances which could contaminate it. It is carried out warm, at a temperature of about 40 to 70 ° C, for a period of 15 min, followed by rinsing with water. It can be replaced or supplemented with a degree of electrolytic degreasing to provide even better surface quality.

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The next step is a pickling operation in an acid oxidizing medium, the purpose of which is to remove oxides from the surface by dissolving only a very small thickness of the ring. For example, an aqueous solution of 100 ml / l sulfuric acid to which 50 ml / l of a 30% hydrogen peroxide solution or a solution of another peroxy compound is added prior to each operation is used for this purpose. It is also possible to use a solution of chromic acid which has both acidic and oxidative properties. This pickling operation in an acid oxidizing environment achieves maximum efficiency when the electrolyte temperature is in the range of 40-55 ° C. It is advantageous to maintain this temperature at the interface by circulating hot water within the channels of the rotating ring. The operation takes about 5 minutes, followed by rinsing with water.

Furthermore, it is advantageous to carry out polishing, preferably with a 10 g / l sulfuric acid solution, in order to prevent passivation of the ring surface.

The total time of all the pre-silvering operations just described does not in principle exceed 30 min.

The pre-silvering operation, carried out prior to the silvering itself, is intended to establish chemical conditions to prevent silver being displaced by the copper during silvering, which would prevent the silver coating from adhering. This operation is suitable even if the ring is not of pure copper but of Cu-Cr-Zr alloy. It takes 4 to 5 minutes and is preferably carried out at ambient temperature, the ring being inserted as a cathode into an electrolyte consisting of an aqueous solution of sodium cyanide (about 50 to 90 g / l) and silver cyanide sufficiently diluted with dissolved metal (30 to 50 g / l) . Sodium cyanide can also be replaced with potassium cyanide (65-100 g / l). If an electrolyte is used for this pre-silvering, the composition of which, as will be seen, is qualitatively comparable to the silver-plating bath, rinsing can be omitted in the meantime. In addition, it is possible to utilize a rinse outlet after silvering which can be recycled to the pre-silvering bath. The cathode current density is 4 to 5 A / dm ² . One or more soluble (silver) or insoluble (e.g. Ti / PtO ₂ or Ti / RuCh) anodes may be used. In the case of insoluble anodes, the free cyanide decomposes, which is converted to carbonate and liberates ammonia. This electrolyte must therefore be repeatedly replenished by the addition of free cyanide, which can advantageously be taken from the rinsing effluent following the silvering itself. This pre-silvering operation makes it possible to deposit on the ring surface a layer of silver of several gm thickness (for example 1 to 2 gm) while removing acidic deposits that may persist after polishing. The ring is then transferred to the silvering station as quickly as possible, without rinsing, using the presence of a cyanide film on its surface, which protects it from passivation.

The self-silvering operation is carried out in an electrolyte essentially based on an aqueous solution of sodium and silver cyanide, to which an excess of free sodium hydroxide is added, but may also consist of a mixture of potassium and silver cyanide in excess of free potassium hydroxide. Potassium carbonate is also added. The typical composition of this bath is:

AgCN 115 to 150 g / l KCN 215 to 250 g / l KOH 30 to 40 g / l

K ₂ CO ₃ 10 to 15 g / l

The optimum working temperature is 40 to 50 ° C.

Potassium carbonate is necessary to achieve uniform corrosion of the anodes. It may be replaced by sodium carbonate, but has the disadvantage that it has less solubility. Potassium hydroxide may be replaced by sodium hydroxide. Both provide for the conductivity of the electrolyte, as well as the stability of the anion complex, which is silver (Ag (CN) ² ₄ '). The silvering operation is generally carried out by means of a direct current source, which may advantageously be replaced by a transient current source, which makes it possible to increase the fineness of crystallization. The crystallization can also advantageously be modified by lowering the annulus / electrolyte interface temperature, for example by the flow of cold water through the ducts in the annulus. Under these conditions, the silvering electrolyte is a hot source and the ring is a cold source. A temperature gradient is established, and the interface thus offers greater activation overvoltage, favorable to increase the hardness of the coating.

As mentioned, a soluble anode or anodes consisting of one or more titanium anode baskets containing silver spheres or silver metal in any other form, for example in the form of pellets, are used in the described example (which is not limiting in this respect). These titanium anodes are used as dimensionally stable electrodes. Their shape fits into the shape of the ring in its submerged part, which makes it possible to homogenize the distribution of cathode current densities on the ring. Since the anode-cathode distance does not change under these conditions, the anodes keep the current densities constant at the cathode.

If it is not possible to fully immerse the ring in the electrolyte, it is highly advisable to permanently spray the immersed part of the ring with the same electrolyte or by inerting the part with neutral gas. This avoids the risk of passivation of the freshly silvered surface, which would prevent good adhesion and good cohesiveness of the coating. For the same reason, it is further recommended that the ring be sprayed or inerted during its transport between the pre-silvering station and the silvering station. Cathodic protection of the ring is also suitable. In any case, this movement should be carried out as quickly as possible.

It is possible to work either at a given voltage or at a given current density. When the electrolysis is carried out at a voltage of the order of 10 V with a current density of about 4 A / dm ² , a duration of about 5-8 days (which also depends on the depth of immersion of the ring in the bath) allows to obtain a silver deposit of 3 mm. The ring is then released from the support shaft and is ready to be fitted onto the core and form a roll to be used for the casting machine, optionally after finishing the surface of the silver layer, such as forming a specified roughness by shot blasting, laser machining or other process. As is known, the purpose of such treatment is to optimize the heat transfer conditions between the annulus and the metal during solidification.

During use, the silver layer is attacked and is subject to mechanical wear and tear, as a result of which it is gradually consumed. Between the two melts it is necessary to clean the ring surface and the silver layer may at least occasionally be subjected to slight machining to compensate for any uneven wear of the ring that could impair the regularity of the thermomechanical behavior of the ring over its entire surface. It is also important to restore the ring's original roughness whenever necessary. If the average thickness of the silver layer on the ring reaches a predetermined value, which is usually estimated to be about 1 mm, the use of the cylinder is discontinued, the ring is collapsed and may undergo full or partial silvering treatment prior to restoring the silver layer on the ring. For this purpose, the ring can be reattached to the axis that supported it during the silvering operations. After the silvering is complete, the silver layer is restored as described above.

The user has several options for performing the silvering. It is preferred to purge by pure chemical means. However, the reagent used must dissolve the silver without significantly attacking the copper substrate and making it difficult to safely perform only partial silvering. Another suitable method of total or partial silvering is electrolysis due to the significant differences between the standard copper and silver potentials (0.3 V and -0.8 V, respectively, against the standard hydrogen electrode). It is also useful when the ring is made of copper-chromium-zirconium alloys. In this case, the silver is dissolved by placing the ring as an anode in a suitable electrolyte, usually based on nitric acid and containing a copper inhibitor, such as phosphate ions. It is possible to shorten the silvering operation by placing in front of it a mechanical silver removal operation intended to reduce its thickness, but without attacking the copper. The advantage of this operation is also the unification of this thickness and the removal of various surface impurities (especially metal residues), which could in some cases delay the start of dissolution. This avoids situations where silver is still melting in some zones while copper is already exposed in others.

However, the disadvantage of electrolytic stripping is the necessity to use a special solution in this process, because of its toxicity, incompatible with other operations carried out in the silvering and stripping shop, where cyanide solutions are used in addition.

The inventors therefore recommend restoring the silver coating on the ring directly by re-depositing in a silvering bath (preferably one that served for the first silvering described above) without having to completely or almost completely remove the residual silver coating. Such a procedure is possible because it is easy to electrochemically deposit a new silver layer on the older silver layer and achieve a good adhesion of the new layer to the old one, while in the case of nickel this is not an option. On the one hand, this greatly simplifies material handling in the ring treatment workshop, and on the other hand, it shortens the length of their maintenance and thus decommissioning. In addition, silver repositioning, as suggested by the inventors, does not have the drawbacks usually attributed to other methods of de-plating in general and de-plating in particular, due to the natural alkalinity of the silvering bath. Indeed, this alkalinity can be used as a means of natural passivation of the support structure of the silvering station if it is of uncoated steel. Another advantage of the invention is that these steel structures need never be in the anode position, which would promote their corrosion and compromise their service life. Another advantage of direct re-silvering over almost complete electrochemical silvering followed by silvering again is that silvering may not occur in certain preferred zones (such as ring edges) during silvering, which would result in local copper exposure. Finally, direct re-silvering, carried out under conditions which preclude any dissolution of copper in the ring, does not allow the ring surface to be attacked and thus prolong its life. Prior to this direct re-silvering, it is possible to include a slight machining of the worn silver layer to unify its thickness and to remove impurities that would prevent the new silver layer from adhering to the old one.

Thus, compared to the nickel-nickel-nickel-nickel-plating workshop, the silver-plating workshop differs in that it does not necessarily include a device for dissolving the worn coating by chemical or electrochemical means. Its construction is therefore more economical. It is also more economical to use because it consumes less electricity - silver is stored three times faster than nickel at the same current density, mainly because it is monovalent, while nickel is divalent. However, this advantage is partially compensated by the fact that in order to achieve equivalent thermal protection of the ring, it is necessary to apply an approximately twice as thick layer of silver as the corresponding nickel layer. On the other hand, this silver layer provides the ring with higher mechanical protection than a thinner nickel layer. As regards the substances used, the price of the silver salts used is in fact not too different from that of the nickel salts used for the traditional nickel plating of the chill walls. The cost of a silver coating is generally not much higher than that of a nickel coating, and in particular, the renewal of a worn casting ring is faster and more economical.

Cyanide waste from the workshop, especially rinsing water, can be treated with a Javelin liquor to destroy the cyanide, i. potassium hypochlorite solution. Because the Javasic liquor is readily recovered by electrolysis, these mild chloride effluents can be treated continuously by electrolysis - metallic silver is obtained on the cathode and cyanides are directly disposed of to ammonium carbonate on dimensionally stable anodes. Thus, simple and economical solutions can be found for the environmental problems that could cause the use of cyanide salts.

Industrial usability

The invention finds particular application in the treatment of the roll rings of a continuous steel casting machine between or on a single roll because of the large dimensions and high production costs of these components, which require a long service life. However, it can also be used to treat the walls of casting molds of copper or copper alloy of all shapes and forms intended for casting all metals which can be brought into contact with silver under casting conditions.

Claims (13)

CLAIMS 1. A continuous casting metal element comprising a cooled copper or copper alloy wall for contacting a liquid metal and having a metallic coating on its outer surface, characterized in that the coating is a silver layer. The ingot mold element according to claim 1, characterized in that said wall is a roller ring of a machine for continuously casting thin metal strips between two rollers or on one roller. The ingot mold element according to claim 1 or 2, characterized in that said silver layer has been deposited in an electrolytic manner. 4. A method of coating the outer surface of a cooled copper or copper alloy wall in a continuous metal molding mold element, characterized in that it is carried out by depositing a silver layer on said surface. Method according to claim 4, characterized in that said silver layer is applied by an electrolytic method. A method according to claim 5, characterized in that it is applied to a exposed copper or copper alloy wall and comprises the following steps successively: - degreasing the wall, - pickling the wall in an acidic oxidizing environment, - pre-silvering the wall, the wall being placed as a cathode in an electrolytic bath consisting of an aqueous solution of silver cyanide and an alkali metal cyanide to deposit a layer of silver with a thickness of several pm, silvering the wall, the wall being placed as a cathode in an electrolytic bath consisting of an aqueous solution of silver cyanide, alkali metal cyanide, alkali metal hydroxide and alkali metal carbonate. Method according to claim 6, characterized in that a wall polishing operation is further included between the pickling and the pre-silvering. 8. A method of recovering a silver coating applied to the outer surface of a copper or copper alloy wall at a continuous metal mold ingot mold element, characterized in that a residual silver layer is left on said wall and this layer is re-silvered, placing the wall as a cathode into an electrolytic bath containing a silver salt. The method of claim 8, wherein the electrolytic bath comprises an aqueous solution of silver cyanide, alkali metal cyanide, and alkali metal carbonate. Method according to claim 8 or 9, characterized in that, before re-silvering, a slight machining of the remaining silver layer is carried out without being removed as a whole. 11. A method for recovering a silver coating applied to an outer surface of a copper or copper alloy wall at a continuous metal molding mold element, characterized in that the wall is partially or completely stripped, wherein the wall is placed as an anode in an acid-based electrolytic bath. and the silver or silver layer is silver-plated, the wall being placed as a cathode in an electrolytic bath consisting of an aqueous solution of silver cyanide, alkali metal cyanide and alkali metal carbonate. Method according to one of Claims 6 to 11, characterized in that during the blasting or stripping operation a temperature gradient is formed between the wall and the electrolysis bath while cooling the wall. 13. The method according to any one of claims 6 to 12, characterized in that, during operation, silver-replating operation _of a source of the transient currents.