Process for deposing metal or metal alloys on a metal strip and guide pieces for implementing this process

Abstract: Process for the deposition on a metal strip (1) of a deposit of metal or metal alloys involving a stage of immersion of the metal strip in a molten bath (5) of this metal or these alloys, in which the strip (1) passes on at least one guide piece (9, 19). The guide piece is produced from a material based on vitreous silica. The material based on vitreous silica has a content that can range up to 20 % of one or more of the materials included in the group comprised of : refractory earths, metal oxides and compounds of oxides, carbides, borides, oxynitrides, the SiAlON's, the thermal decomposition products of carbosilanes, CaO, MgO, ZrSiO4, ZrO2, Cr203 and SiC.

Description: The invention concerns guide pieces such as fixed guides or rollers, solid, hollow , fixed, in free or driven rotation, immersed in a bath of metal or metal alloys for the hot deposition of a protective layer against corrosion on a strip of sheet metal, e. g., by galvanization or by the deposition of a zinc, aluminum or tin alloy. It also concerns a deposition process using these guides or these rollers. The deposition of a protective layer, e. g., of zinc on a strip of sheet metal protects the latter against oxidation. This operation is carried out industrially by passing the sheet through a bath of metal or metal alloys (molten), making sure that as regular as possible a deposition of a protective layer is obtained. According to a known technique, in order to guide the sheet in the bath, it is placed in contact with rollers. These rollers are quite frequently metallic, e. g., stainless steel. This gives rise to numerous shortcomings. In effect, the rollers operate in a highly aggressive medium, due notably to the elevated temperature of the bath, but primarily due to the corrosive reactions between the metals of the bath and the metal of the rollers. Consequently, it is necessary to disassemble them at frequent intervals, e. g., every week, descale and/or remachine them to remove the corroded layer, and finishgrind them to restore acceptable surface state and geometry. The result is frequent shutdowns of the installation and high maintenance costs. In order to remedy these disadvantages of metal rollers, an attempt is made to coat them with a protective layer. For example, the document EP 0 339 338 describes cast iron rollers coated with a layer of carbon fibers enveloped in a carbon matrix. The document JP-A-61- 37 955 (OSAKA FUJI KOGYO) describes a metal roller coated with a ceramic layer obtained by plasma deposition. However, these rollers present numerous disadvantages. Adhesion problems between the coating layer and the metal roller appears rapidly due to the difference in expansion between the metal and the deposit. The force opposing the phenomenon of differential expansion is the force of cohesion and bonding of the deposit on the metal support. This force is not sufficient to prevent debonding of the deposit. The result is a rapid localized or generalized disappearance of the protective layer, which causes the reappearance of corrosion problems and deterioration of the surface state of the roller. These phenomena necessitate reapplying the coating of the rollers at short intervals, hence a high maintenance cost. These rollers thus present a reduced reliability and service life. The present invention is for the precise purpose of offering a process for treating a strip of sheet metal and guide pieces for the bath that remedy these shortcomings. According to the invention process, the strip of sheet metal is introduced into a bath of the metal or alloys to be deposited, in which it is guided by at least one guide piece. This piece is characterized by the fact that it is produced from a base material of vitreous silica. Contact with the sheet is obtained by one of more pieces. The use of a component of vitreous silica for contact with the sheet makes it possible to remedy the principal shortcomings of the coated or uncoated metal rollers. The effects of corrosion and oxidation are suppressed, the problems of differential expansion also. Consequently, the service life is greatly increased. Moreover, the surface state of the guide piece is preserved over time so that the roller does not mark the sheet. The result is a better quality of surface state of the coated product. The ceramic material based on vitreous silica preferably has a content that can range up to 20 % of one or more of the materials pertaining to the group comprised of refractory earths, metal oxides and compounds of oxides, carbides, borides, oxynitrides, the SiAlON's, the thermal decomposition products of carbosilanes, CaO, MgO, ZrSiO4, ZrO2, Cr203 and SiC. Another advantage of using vitreous silica lies in the fact that the latter has a low wettability by the bath of molten metal. Moreover, the invention concerns a ceramic guide piece designed in particular for the galvanization of a sheet metal strip. According to an initial embodiment of the invention, the guide piece is a roller as defined in claim 5. A fixed guide immersed in the bath has the advantage of suppressing the need for rotatings bearings that are immersed and work under delicate conditions. They are therefore highly subject to the danger of failure such as seizing, but also to increased wear due to the phenomena of corrosion and erosion. The use of fixed guides thus facilitates an improvement in the reliability. Other characteristics and advantages of the invention will show up in a reading of the description of an implementation example given solely for the sake of illustration with references to the attached Figures : Figure 1 is a schematic view of a galvanization installation according to the invention. Figures 2 and 3 represent a roller according to the invention, designed for an installation for galvanization such as that of Fig. 1. Figure 4 shows a guide piece consisting of a fixed guide. The treatment installation shown in Fig. 1 makes it possible to galvanize a strip of sheet metal 1. This strip, unwound from a reel (not shown), passes through a preheating zone in which the temperature is of the order of generally 1150-1300°C, then a treatment (or annealing) zone 4 at a temperature of 900-950°C. In the example described, the rate of displacement of the sheet is ca. 140 m/min. The strip then passes through a zinc bath 5 contained in a vat 7. The strip is guided in the vat by a roller 9 of relatively substantial diameter so as to minimize the radius of curvature of the sheet. The sheet is then trued up by a roller 11 of smaller diameter. The rollers 9 and 11 work in a very aggressive environment. In effect, the bath is at an elevated temperature. It can range from 450°C for a bath of pure zinc up to 600°C for a bath of Galvalum (containing 50 % aluminum by weight). The use of vitreous silica for the rollers 9 and 11 offers important advantages : absence of corrosion by the strip, absence of corrosion by the molten metal, excellent resistance to thermal shock, notably during immersion of the roller in the zinc bath. In addition, the molten silica is a material that is not wettable by the molten metal. Upon leaving the zinc bath, the strip of sheet metal passes through a blowing zone 17 that cools the zinc, notably to solidify the deposit. Figures 2 and 3 respectively show a view in longitudinal section and a cross section of a roller according to the invention and comprising part of the installation such as the treatment installation shown in Fig. 1. This roller 9 is entirely of cast silica. It is traversed by a longitudinal hole 11. A metal shaft 13 passes through the longitudinal hole 11. It is noted that the diameter of the hole 11 is larger with respect to the roller 9 than the outside diameter of the metal shaft 13. The purpose of this arrangement is notably to permit the expansion of the metal shaft. The shaft 13 is mounted on two arms 15 that serve to immerse the whole unit and prevent the longitudinal displacement of the roller 9. Figure 4 shows a variant of implementing the invention, in which the guide piece is comprised of a fixed guide 19. In a manner similar to that described with reference to Fig. 1, the installation is comprised of a vat 7 containing a bath 5 of molten metal or metal alloys. The strip of sheet 1 passes through a heating zone 4. The difference with the implementation mode shown in Figures 1 to 3 resides in that fact that the roller 9 was replaced with a fixed guide 19. This avoids having to use rotating bearings. The reliability of the installation is thus increased.

Claims: Process for the deposition on a metal strip (1) of a deposit of metal or metal alloys involving a stage of immersion of the metal strip in a molten bath (5) of this metal or these alloys, in which the strip (1) passes on at least one guide piece (9, 19), characterized in that the said guide piece is produced from a material based on vitreous silica. Process according to claim 1, characterized in that the material based on vitreous silica has a content that can range up to 20 % of one or more of the materials included in the group comprised of : refractory earths, metal oxides and compounds of oxides, carbides, borides, oxynitrides, the SiAlON's, the thermal decomposition products of carbosilanes, CaO, MgO, ZrSiO4, ZrO2, Cr203 and SiC. Process according to claims 1 or 2, characterized in that the guide piece is a roller (9). Process according to claims 1 or 2, characterized in that the guide piece is a fixed guide (19). Device for the treatment of a strip of sheet (1) in a bath of molten metal or alloys (5), characterized in that it is comprised of a roller made of a material based on vitreous silica, said roller having a hole (11) designed for the passage of a shaft (13), the diameter of the hole (11) being larger than the outside diameter of said shaft (13). Device according to claim 5, characterized in that the ceramic material of the roller has a content that can range up to 20 % of one or more of the materials contained in the group comprised of : refractory earths, metal oxides and compounds of oxides, carbides, borides, oxynitrides, SiAlON's, the thermal decomposition products of carbosilanes, CaO, MgO, ZrSiO4, ZrO2, Cr203 and SiC.

Inventors: ABELS HUBERT, VIGNOT JEAN-MARIE, ABELS, HUBERT, VIGNOT, JEAN-MARIE
Applicants: VESUVIUS FRANCE SA, VESUVIUS FRANCE S.A.
Application year: 1992
Main classification: C23C2
IPC codes: C23C2, F16C13