Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating moving substrate
Reexamination Certificate
1998-12-09
2001-05-08
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Coating moving substrate
C205S137000, C205S096000, C205S151000, C204S212000, C204S272000, C204S218000, C204S22400M
Reexamination Certificate
active
06228242
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the continuous casting of metals. More specifically, it relates to the conditioning of the external surface of the roll or rolls which constitutes or constitute the moving wall or walls of the molds for the continuous casting of thin strip made of metals such as steel.
PRIOR ART
The molds of machines for the twin-roll continuous casting of steel strip a few mm in thickness directly from liquid metal comprise a casting space defined by the lateral surfaces of two rolls counter-rotating about their axes, which are maintained horizontal, and by 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 800 mm. However, long term, this width will have to be as high as 1300 to 1500 mm in order to meet the productivity requirements of an industrial plant. These rolls usually 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 inside the sleeve.
Just like the surfaces of the molds for conventional continuous casting of blooms, billets or slabs, the surface of the sleeve which is to come into contact with the liquid metal may be coated with a metal layer, usually nickel, the thickness of which is in general as high as 1 to 2 mm. This nickel layer enables the heat transfer coefficient of the sleeve to be adjusted to an optimum value (this being lower than if the metal were brought directly into contact with the copper) so that the metal solidifies under proper metallurgical conditions: too rapid a solidification would cause defects on the surface of the product. This adjustment is carried out by varying the thickness and the structure of the nickel layer. On the other hand, it forms a protective layer for the copper, preventing it from being excessively stressed thermally and mechanically. This nickel layer wears out in the course of use of the roll, and it must be restored periodically by partial or complete removal of the remaining thickness, followed by deposition of a new layer, but such restoration obviously costs less than complete replacement of a worn bare copper sleeve.
The deposition of nickel is preferably carried out electrolytically, in the following manner. The new sleeve (a sleeve from which the nickel has been either partially or completely removed), which has overall the shape of a hollow cylinder made of copper or copper alloy, such as a copper—(1%) chromium—(0.1%) zirconium alloy, is mounted on an arbor, by means of which it can be readily transported from one treatment station to another in the nickel plating
ickel removal workshop. After having undergone various preparatory surface treatments (polishing, degreasing, acid pickling, etc.) for the purpose of improving the adhesion of the nickel to the copper, the sleeve is brought to the nickel electroplating station. This station consists of a tank containing the nickel-plating solution, above which the arbor may be placed in a horizontal position and made to rotate about its axis. Thus the lower part of the sleeve is dipped into the tank, and rotating the arbor/sleeve assembly at a speed of approximately 10 revolutions/min, for example, enables the treatment of the entire sleeve to be carried out. During the electroplating with nickel, the sleeve constitutes the cathode and the anode may consist of one or more titanium anode baskets immersed in the tank, which are closed by thin membranes, made to face the surface of the sleeve and contain nickel balls. If it is also desired to coat a major portion of the ends of the sleeve (which, during casting, rub against the refractory side plates and are therefore liable to wear out) with nickel, other anode baskets are arranged so as to face these ends. Other types of anodes (soluble or insoluble) may also be used.
As a variant, provision may be made for the sleeve to remain stationary and for it to be the electrolyte which moves past it. What is essential is therefore to create a relative movement, between the sleeve and their electrolyte, which ensures continuous renewal of their interface.
During casting runs, the nickel coating is exposed to very high stresses, both mechanical and thermal. The appearance of cracks in the nickel coating is often observed, near the edges of the rolls, after only a few casting runs. These cracks relate to regions a few cm in width starting from the arrises of the sleeve. They may lead to the formation of defects on the surface of the cast product since they cause the latter to be cooled non-uniformly. Above all, they constitute weak points from which very rapid degradation of the entire nickel coating may initiate. There may even be propagation of cracks beyond the nickel coating, which would lead to damage of the entire sleeve. These cracks therefore necessitate immediately and prematurely stopping the use of the roll and completely regenerating the coating on the sleeve. As this operation is lengthy (several days), an industrial application of the twin-roll steel casting process would require there to be a large number of sleeves ready to use, in order to ensure regular operation of the casting machine. Since the sleeve is a very expensive component, because of the materials used and the difficulty of machining it, this would lead to a high cost in the use of the plant.
SUMMARY OF THE INVENTION
The object of the invention is to improve the behavior of the metal coating of the sleeve with respect to its resistance to thermomechanical stresses, by slowing down as much as possible, or even preventing, the appearance of cracks in the edge regions so as to extend the average use time of the sleeve between two restorations of its coating.
The subject of the invention is a process for electrolytically coating the casting surface of a roll, for the twin-roll or single-roll continuous casting of thin metal strip, with a metal layer, in which process said casting surface is at least partially immersed in an electrolyte solution, containing a salt of the metal to be deposited, so as to face at least one anode, said surface is placed as the cathode and a relative movement is created between said casting surface and said electrolyte solution, wherein insulating masks are interposed between said anode or anodes and the arrises of said casting surface, said insulating masks preventing a concentration of the lines of current on said arrises and in their vicinity.
The subject of the invention is also a plant for electrolytically coating the casting surface of a roll, for the twin-roll or single-roll continuous casting of thin metal strip, with a metal layer, of the type comprising a tank which contains an electrolyte containing a salt of the metal to be deposited, means for immersing said casting surface at least partially in said tank and for creating a relative movement between said casting surface and said electrolyte, at least one anode arranged in the tank so as to face said casting surface, and means for raising said casting surface to a cathode potential, which plant includes masks, made of an insulating material, which are interposed between the arrises of said casting surface and said anode or anodes, said masks preventing a concentration of the lines of current on said arrises.
Preferably, said masks have a general shape in the form of a circular arc, the center of curvature of which is the same as that of the arris of the casting surface which they face, and have two parallel sides each placed in the extension of said arris at the same distance “d” from the latter and connected by a corner-shaped cut-out, the sides of which are perpendicular to each other.
As will be understood, the invention consists in carrying out the electrodeposition of the metal coating by arranging insulating masks near the edges of the sleeves. These masks, a preferred example of which is described, are designed to obtain a uniform distribution of the lines of current in t
Allely Christian
Breviere Yann
Catonne Jean-Claude
Jolivet Eric
Lavelaine Hervé
Cole Thomas W.
Nixon & Peabody LLP
Thyssen Stahl Aktiengesellschaft
Wong Edna
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