Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/04—Continuous casting of metals, i.e. casting in indefinite lengths into open-ended moulds
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/04—Continuous casting of metals, i.e. casting in indefinite lengths into open-ended moulds
  • B22D11/059—Mould materials or platings
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/46—Electroplating: Baths therefor from solutions of silver
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/10—Electroplating with more than one layer of the same or of different metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/60—Electroplating characterised by the structure or texture of the layers
  • C25D5/615—Microstructure of the layers, e.g. mixed structure
  • C25D5/617—Crystalline layers
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/67—Electroplating to repair workpiece

Definitions

• the invention relates to the continuous casting of metals. More specifically, it relates to the coating of the external surface of the copper or copper alloy walls of the molds in which the solidification of metals such as steel is initiated.
• the continuous casting of metals such as steel is carried out in molds bottomless, with walls energetically cooled by an internal circulation of a cooling liquid such as water
• the metal in the liquid state is brought into contact with the external surfaces of these walls and initiates its solidification there.
• These walls must be made of an excellent heat conducting material, so that they * can evacuate enough calories from the metal in a reduced time.
• copper or one of its alloys, for example containing chromium and zirconium, is adopted for this purpose.
• the faces of these walls which are intended to be in contact with the liquid metal are coated with a layer of nickel whose initial thickness can reach up to 3 mm. It constitutes a protective layer for copper which prevents it from being overheated thermally and mechanically.
• This nickel layer wears out over the use of the mold. It must therefore be restored periodically by total removal of the remaining thickness and then depositing a new layer, but such restoration obviously costs much less than a complete replacement of the worn copper walls. Conventionally, the nickel layer is restored as soon as its thickness has dropped to about 0.6 mm.
• this layer of nickel on the walls of the mold is therefore a fundamental step in the preparation of the casting machine, and it is important to optimize both the cost, the properties of use and the qualities of adhesion. This is, in particular, the case on machines intended for casting steel products in the form of strips a few mm thick which do not need to be then hot rolled.
• These machines comprise an ingot mold constituted by two cylinders rotating in opposite directions around their axes which are kept horizontal, and two refractory side plates pressed against the edges of the cylinders. These cylinders have a diameter of up to 1500 mm, and a width which, on current experimental installations, is approximately 600 to 1300 mm.
• the complete nickel removal operation of the shell which must precede the restoration of the nickel layer is also fundamental.
• its successful completion largely conditions the quality of the nickel layer which will then be deposited, in particular its adhesion to the shell, since it turns out to be very difficult to deposit a new layer of strongly adherent nickel on a older nickel layer.
• this nickel removal operation must be carried out without very significant consumption of the copper from the ferrule which is an extremely expensive part, and the duration of use of which must be extended as much as possible.
• This last requirement in particular, practically excludes the use of a purely mechanical method for this nickel-plating, since its precision would not be sufficient to guarantee both a total elimination of the nickel and a safeguard of the copper over the entire surface. of the shell.
• the object of the invention is to propose a method of coating the external surface of the copper or copper alloy wall of a continuous casting mold generally more economical than the usual methods where a layer of nickel is deposited on this surface.
• This method should also provide the walls of the mold with characteristics and quality at least comparable to those obtained. by depositing a layer of nickel. It should also include a step of periodic regeneration of this surface.
• This method should be particularly suitable for the coating of cylinder ferrules for a casting machine between cylinders or on a single cylinder.
• the subject of the invention is an element of a mold for the continuous casting of metals, comprising a cooled wall of copper or copper alloy intended to be brought into contact with liquid metal and comprising on its external surface a metallic coating, characterized in that said coating consists of a silver layer.
• this wall is a cylinder shell for a machine for continuously casting thin metal strips between two cylinders or on a single cylinder.
• the invention also relates to a method of coating with a metallic layer the external surface of a wall cooled in copper or copper alloy of a mold element for continuous casting of metals, characterized in that this is carried out coating by depositing a layer of silver on said surface, preferably by electrolytic means.
• the restoration of said silver layer is carried out by leaving a residual silver layer on said wall, and by resilvering said layer by placing said cathode wall in an electrolysis bath constituted, by for example, with an aqueous solution of silver cyanide, cyanide of an alkali metal and carbonate of an alkali metal.
• the invention consists first of all in replacing with silver the nickel traditionally used to form the external coating of the walls of copper ingot molds for continuous casting of metals such as steel. Contrary to what you might think at first glance since solid silver is considered a precious metal, this solution has multiple economic advantages, and it is perfectly technically viable. This is particularly the case when the silvering is carried out by an electrolytic method using a bath with alkaline cyanides. It has been found that such baths are suitable for producing silver deposits on copper having properties of use well suited to the protection of the walls of continuous casting molds.
• the particular method of coating the surface of the mold which is also described and claimed includes a silvering step, and also optionally a step of silvering said surface when it is desired to restore the coating of a used mold.
• This silvering may be only partial, whereas in the case of a nickel coating, the nickel removal of the copper must almost imperatively be total, at the risk of consuming a part of the copper of the wall.
• Both silvering and silvering can be done by electrolytic means.
• the silver removed from the ferrule is recovered in metallic form on the silver cathode in the deticianing reactor. Said cathode can in turn be recycled as an anode in the silvering reactor.
• the silver removal can be carried out at least in part by chemical or mechanical means.
• the invention will now be described in detail in one of its embodiments, applied to the coating of a copper ferrule or copper alloy of a cylinder for a machine for continuously casting steel between two cylinders or on a single cylinder.
• the example described could easily be adapted to the cases of other types of ingot molds with copper or copper alloy walls, such as ingot molds with fixed walls for the continuous casting of slabs, biooms or billets.
• the silvering or de-silvering method can implement various other electrolytic processes such as pad or spray coatings, as well as electrolytes different from those given in example.
• the new ferrule is generally in the form of a hollow cylinder made of copper or a copper alloy, such as a copper-chromium alloy (1%) - zirconium (0.1%).
• Its outside diameter is, for example, of the order of 1500 mm and its length is equal to the width of the strips which it is desired to pour, that is to say of the order of 600 to 1500 mm.
• Its thickness may be, for information, of the order of 180 mm, but varies locally depending, in particular, on the method of fixing the ferrule to the core of the cylinder which has been adopted.
• the ferrule is crossed by channels intended to be traversed by a cooling fluid such as water, when using the casting machine.
• the treatment stations in the silvering / de-silvering workshop each consist of a tank containing a solution suitable for carrying out a given stage of treatment, above which the said tree can be placed with its horizontal axis and the rotate around its axis.
• the lower part of the ferrule is thus soaked in the solution, and the rotation of the shaft / ferrule assembly makes it possible to carry out the treatment of the entire ferrule (it being understood that the ferrule normally performs several turns on it - even during the same treatment, at a speed of about 10 revolutions / min, for example).
• the bare ferrule (in the case of the first silver plating of a new ferrule, or the silver plating of a used ferrule whose copper surface has been exposed), is preferably first subjected to mechanical preparation by polishing its surface. Then a chemical degreasing is carried out in an alkaline medium, which has the function of ridding the surface of the shell of organic materials which can pollute it. It is carried out hot, at a temperature of about 40 to 70 ° C for about fifteen minutes, and followed by rinsing with water. It can be substituted for, or even added to, an electrolytic degreasing step which would provide an even better surface quality.
• the next step is a pickling operation in an oxidizing acid medium, which has the function of removing surface oxides, taking care to dissolve only a very minimal thickness of the shell.
• an aqueous solution of sulfuric acid at 100 ml / 1 is used, to which 50 ml / 1 of a 30% solution of hydrogen peroxide or a another compound solution.
• a solution of chromic acid this compound having both acidic and oxidizing properties.
• This pickling operation in an oxidizing acid medium has maximum efficiency when the temperature of the electrolyte is between 40 and 55 ° C. It is advantageous to maintain this temperature at the interface by circulating hot water inside the channels of the rotating shell. The operation lasts approximately 5 minutes and is followed by rinsing with water.
• the set of preparatory operations for silvering which have just been described has a total duration which, in principle, does not exceed 30 minutes.
• the purpose of the pre-silvering operation, carried out prior to the actual silvering, is intended to be placed under chemical conditions intended to prevent a displacement of silver by the copper during the silvering, which would be detrimental to the adhesion deposit money. It is particularly useful even if the ferrule is not pure copper, but a Cu-Cr-Zr alloy. It lasts 4 to 5 minutes and is preferably done at room temperature, the ferrule being placed as a cathode in an electrolyte consisting of an aqueous solution of sodium cyanide (50 to 90 g / l approximately) and sufficiently silver cyanide diluted in dissolved metal (30 to 50 g / 1). We can also replace sodium cyanide with potassium cyanide (65 to 100 g / 1).
• One or more soluble (silver) or insoluble anodes can be used -
• insoluble anodes there is destruction of the free cyanide which transforms into carbonate and releases ammonium. It is therefore necessary to periodically recharge this electrolyte with free cyanide additions which can advantageously be taken from the rinsing effluents which follows the silvering operation proper.
• This pre-silvering operation makes it possible to deposit on the surface of the ferrule a layer of silver a few ⁇ m thick (1 to 2 ⁇ m for example) while removing the acid deposits which could remain after brightening.
• the ferrule is then transferred as quickly as possible to the silver plating station without undergoing rinsing, in order to take advantage of the presence on its surface of a cyanide film which protects it from passivation.
• the actual silvering operation is carried out 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 may also consist of a mixture of cyanides of potassium and silver in excess free potash. Potassium carbonate is also added.
• a typical composition for this bath is: - AgCN: 115 to 150 g / 1; - KCN: 215 to 250 g / 1; - KOH: 30 to 40 g / 1;
• the optimal operating temperature is 40 to 45 ° C.
• the potassium carbonate is necessary to obtain homogeneous corrosion of the anodes. It can be replaced by sodium carbonate, with the disadvantage that sodium carbonate has a lower solubility. Potash can be replaced by soda. They ensure the conductivity of the electrolyte, as well as the stability of the anionic complex under which the silver is found (Ag (CN) 4 • ** • -).
• the silver plating operation is generally carried out using a direct current source, which can advantageously be replaced by a source of transient currents, which make it possible to increase the fineness of the crystallization.
• the crystallization can also be advantageously modified by lowering the temperature of the shell / electrolyte interface, for example by circulating cold water through the channels of the shell. Under these conditions, the silver electrolyte is a hot source and the ferrule is cold source A temperature gradient is established and the interface then offers a greater activation overvoltage, favorable to increasing the hardness of the coating
• the anode or anodes are soluble anodes constituted by one or more anodic baskets of titanium containing beads of silver or metallic silver in any other form, for example cartons
• These titanium panodes are used as dimensionally stable electrodes
• Their shape follows that of the ferrule in its submerged part, which makes it possible to homogenize the distribution of the densities of cathodic current on the ferrule As the anode-cathode distance does not vary under these conditions, the panodes keep the current densities constant on the cathode
• the silver layer undergoes attacks and mechanical wear which lead to its progressive consumption. Between two castings, the surface of the shell must be cleaned, and the silver layer can, at least from time to time. , to undergo a light machining intended to compensate for the possible heterogeneities of its wear which could compromise the homogeneity of the thermomechanical behavior of the ferrule on all of its surface II is also important to restore the initial roughness of the ferrule whenever this is necessary.
• a predetermined value which is generally estimated at around 1 mm
• the use of the cylinder is interrupted, the ferrule is disassembled and can undergo a treatment for complete or only partial desilvering, which must precede the restoration of the silver layer of the shell. For this purpose, the ferrule can be again mounted on the axis which supported it during the silvering operations. If the silver removal is complete, the silver layer is then restored according to the whole process which has just been described.
• the silver is dissolved by placing the ferrule as an anode in an appropriate electrolyte, generally based on nitric acid and containing a copper inhibitor, such as phosphate ions.
• an appropriate electrolyte generally based on nitric acid and containing a copper inhibitor, such as phosphate ions.
• a copper inhibitor such as phosphate ions.
• One way to shorten the silver reduction operation would be to precede it with a mechanical silver removal operation which would aim to reduce its residual thickness without however reaching the copper.
• This operation would also have the advantage of homogenizing this thickness and removing the various surface impurities (in particular metal residues) which could locally slow down the start of dissolution. We would still avoid being still dissolving the silver in certain areas of the shell even when in other areas the copper has already been exposed.
• the method of silver plating by electrolytic route has the drawback of requiring for its implementation a special solution, incompatible for reasons of toxicity with the other operations carried out in the silver plating-silver plating workshop where ferrules are used. elsewhere cyanide solutions.
• the inventors therefore recommend practicing the restoration of the silver coating of the shell by direct recharging in a silver plating bath (advantageously that which served for the first silver plating previously described), without obligation to completely or almost completely remove the coating. residual money.
• Such a procedure is possible, because it is easy to electrochemically deposit a new layer of silver on an older layer of silver and to obtain good adhesion of the new layer to the old, whereas this does not is not an option for nickel.
• the silver refill does not have the defects generally attributed to other forms of demetallization in general and of nickel removal in particular, due to the natural alkalinity of the silver plating bath. .
• This alkalinity can, in fact, be used as a means of natural passivation of the infrastructure of the silvering station if it is made of uncoated steel.
• Another advantage of the invention is that it never needs to wear said steel infrastructures in an anodic situation, which would promote their corrosion and would be detrimental to their durability.
• Another advantage of direct refill silvering compared to almost total electrochemical silvering followed by resilvering is to avoid the total dissolution of silver in certain preferential areas (such as the edges of the shell) during silvering, which would lead to localized exposures of copper. In addition, it makes the renewal of the pre-silvering stage unnecessary. Finally, the refill silvering carried out under conditions avoiding any dissolution of the copper of the ferrule makes it possible not to attack the surface of the ferrule, therefore to prolong its duration of use.
• the refill silvering can be preceded by a light machining of the used silver layer, to homogenize its thickness and remove impurities which would be detrimental to the adhesion of the new silver layer on the old.
• a silver-plating workshop for ferrules would therefore be distinguished in that it would not necessarily include an installation for dissolving a used coating by chemical or electrochemical means. It would therefore be more economical to build. It would also be more economical to exploit, because it would consume less electricity: silver deposits three times faster than nickel with an equal current density, in particular because it is monovalent while nickel is bivalent. This advantage is however partially offset in that, to obtain equivalent thermal protection of the shell with a silver deposit and a nickel deposit, a silver layer must be deposited approximately twice as thick as the corresponding nickel layer. . On the other hand, this layer of silver offers mechanical protection of the shell that is superior to the thinner layer of nickel.
• the cost of the silver salts used is, in fact, not very different from that of the nickel salts used for the traditional nickel plating of the walls of the mold. Overall, the cost of a silver coating is therefore not much higher than that of a nickel coating, and above all the repair of a used ferrule of a casting cylinder is much faster and economical.
• Cyanide effluents from the workshop can be treated with bleach to destroy the cyanides. Since bleach is easily produced electrolytically, these effluents can be treated slightly chlorinated by continuous electrolysis: the metallic silver is recovered at the cathode and the cyanides in ammonium carbonate are directly destroyed on anionally stable anodes. Simple and economical solutions can therefore be found to the environmental problems that can arise from the use of cyanide salts.
• the invention particularly finds its application to the conditioning of the ferrules of cylinders of continuous casting installations of steel between cylinders or on a single cylinder, because of the large dimensions and the high cost of manufacturing these parts, of which it is important. to extend life as much as possible. But it goes without saying that one can envisage its transposition to the treatments of walls of ingot molds of copper or copper alloy of all shapes and formats, intended for the casting of all metals supporting being put in the liquid state. , in contact with silver under the casting conditions.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Metallurgy (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Organic Chemistry (AREA)
• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Continuous Casting (AREA)
• Electroplating Methods And Accessories (AREA)
• Moulds For Moulding Plastics Or The Like (AREA)
• Coating With Molten Metal (AREA)
• Electroplating And Plating Baths Therefor (AREA)

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Citations (4)

• 1996
  • 1996-07-11 FR FR9608658A patent/FR2750903B1/fr not_active Expired - Fee Related
• 1997
  • 1997-06-26 KR KR1019980710827A patent/KR20000022396A/ko not_active Application Discontinuation
  • 1997-06-26 TR TR1999/00041T patent/TR199900041T2/xx unknown
  • 1997-06-26 CN CN97196281A patent/CN1072047C/zh not_active Expired - Fee Related
  • 1997-06-26 PT PT97930592T patent/PT910489E/pt unknown
  • 1997-06-26 SK SK2-99A patent/SK299A3/sk unknown
  • 1997-06-26 PL PL97331180A patent/PL331180A1/xx unknown
  • 1997-06-26 DK DK97930592T patent/DK0910489T3/da active
  • 1997-06-26 DE DE69702064T patent/DE69702064T2/de not_active Expired - Fee Related
  • 1997-06-26 CZ CZ9964A patent/CZ6499A3/cs unknown
  • 1997-06-26 AU AU34488/97A patent/AU710657B2/en not_active Ceased
  • 1997-06-26 BR BR9710229A patent/BR9710229A/pt active Search and Examination
  • 1997-06-26 CA CA002258927A patent/CA2258927A1/fr not_active Abandoned
  • 1997-06-26 AT AT97930592T patent/ATE192951T1/de not_active IP Right Cessation
  • 1997-06-26 JP JP10505653A patent/JP2000514361A/ja active Pending
  • 1997-06-26 ES ES97930592T patent/ES2148994T3/es not_active Expired - Lifetime
  • 1997-06-26 WO PCT/FR1997/001139 patent/WO1998002263A1/fr not_active Application Discontinuation
  • 1997-06-26 RU RU99102726/02A patent/RU2181315C2/ru active
  • 1997-06-26 RO RO99-00014A patent/RO119994B1/ro unknown
  • 1997-06-26 EP EP97930592A patent/EP0910489B1/de not_active Expired - Lifetime
  • 1997-07-03 ZA ZA9705970A patent/ZA975970B/xx unknown
  • 1997-08-27 TW TW086112299A patent/TW438911B/zh not_active IP Right Cessation
• 2000
  • 2000-07-24 GR GR20000401687T patent/GR3034001T3/el not_active IP Right Cessation

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