EP0023472B1 - Process for continuously coating, at least a part of one side at least, of a metallic substrate and apparatus used therefor - Google Patents

Description

Technical area

The invention relates to the coating of sheets with another metal and in particular to a process which allows the partial coating of the sheet.

State of the art

The production of galvanized sheet generally relates to the coating of the two faces of the sheet immersed in a bath of molten zinc. This manufacturing method is perfectly developed and allows the production of sheet metal protected against corrosion for many years.

For some time now, a new market has opened up outlets for galvanized sheet on one and no longer on two sides. This market is that of the automobile, an area in which the increasingly frequent use of salt spilled on the roads in winter, causes accelerated degradation of bodywork by corrosion. If, in the building, the galvanized two-sided sheet is tolerable in many cases, it is however not for the automotive industry since on a layer of zinc, it is not possible to achieve a layer sufficiently smooth paint. This zinc layer also constitutes a hindrance for spot welding of the sheets.

Various solutions have already been proposed for coating sheets only on one of their faces. Patent application FR 2 344 640 thus relates to a process according to which a bath of molten zinc is formed, the strip is passed close to the free surface of the zinc bath so that the surface tension and the wettability of the molten metal allow the formation of a meniscus on the free surface of this bath in contact with the face of the strip facing towards it.

According to another solution described in patent application FR-A 2 348 278, the strip to be coated is passed through a bath of molten zinc by simultaneously placing the face of the sheet which is not to be coated, in contact with a cylinder. rotatable around an axis parallel to the level of the bath, the contact between this face and the cylinder being maintained as long as the sheet is soaked in the bath.

It has also been proposed in patent application JP 77-151 638 to mask the face which must not be coated.

BE-A 859 420 relates to a process according to which a thin jet of molten zinc is projected against the face of the strip of sheet metal to be coated.

The first two solutions mentioned do not allow the thickness and regularity of the deposited layer to be controlled. In addition, the second of these solutions poses problems with regard to the accumulation of zinc on the parts of the cylinder which project beyond the longitudinal edges of the metal strip, as well as to preserve the other uncoated face of the sheet.

The solution using masking generally requires the subsequent removal of the coating intended to mask the non-galvanized face.

The zinc jet process is difficult to implement as it is true that it is difficult to form such a very thin and perfectly laminar jet, especially if one wishes to produce a coating of the order of a few tens of microns .

It is also possible to point out other solutions relating to the uniform coating of the substrates.

FR-A 1 153 715 relates to a device for the metallic coating of metallic ribbons on one or two faces in which the molten metal is contained in a crucible having outlet lips pressed against the ribbon so that the width of the the orifice formed between the two lips is so small that any metal applied to the tape is entrained out of the applicator by the displacement of the tape under the effect of capillary forces resulting from the surface tension between the liquid metal and the tape to be coated. Therefore, the movement of the wet ribbon by the molten metal drives the metal out of the crucible as it goes. Such a device has several drawbacks such as the necessary contact between the crucible distribution lips and the ribbon to be coated, causing the wear of these lips and the fact that only the capillary forces resulting from the surface tensions between the liquid metal and the ribbon are used to pump liquid out of the crucible, thereby limiting the coating speed. Finally, the possibilities of adjusting the layer thickness of such a device are also limited.

Another device based on a pumping action generated by capillary forces is also described in US-A 3,201,275 as well as in DE-A 1 080 373.

Finally, according to US-A 1 973 431, a stream of liquid metal is formed through a passage whose cross section corresponds to that of the layer to be deposited, the outlet end of this passage being itself at a distance of the substrate very exactly equal to the thickness of the layer to be deposited. This process is similar to an extrusion process and is very difficult to implement without significant pressure exerted on the metal. In addition, given the capillarity phenomena, this process is limited to relatively large coating thicknesses.

Statement of the invention

The object of the present invention is to remedy, at least in part, the various drawbacks mentioned above.

To this end, the present invention firstly relates to a continuous coating process according to claim 1.

The invention then relates to a device for implementing this method, according to claim 2.

• La fig. 1 est une vue en élévation latérale de cette forme d'exécution.
• La fig. 2 est une vue de détail agrandie d'une partie de cette forme d'exécution.
• La fig. 3 est une vue de détail selon III-III de la fig. 2.
• La fig. est une vue partielle de face d'une variante d'un détail du dispositif des figs 1 à 3. La fig. 5 est une vue en coupe selon V-V de la fig. 4.
• La fig. 6 est une vue en élévation latérale d'une variante de la fig. 1.
• La fig. 7 est une vue de détail agrandie de cette variante.

The attached drawing illustrates schematically and by way of example, an embodiment and variants of the device for implementing the method which is the subject of the present invention.

• Fig. 1 is a side elevational view of this embodiment.
• Fig. 2 is an enlarged detail view of part of this embodiment.
• Fig. 3 is a detail view along III-III of FIG. 2.
• Fig. is a partial front view of a variant of a detail of the device of FIGS. 1 to 3. FIG. 5 is a sectional view along VV of FIG. 4.
• Fig. 6 is a side elevational view of a variant of FIG. 1.
• Fig. 7 is an enlarged detail view of this variant.

Best way to realize the invention

The device illustrated in FIG. 1 comprises a furnace 1 of known design intended for producing molten zinc. This furnace has a lateral distribution duct 3 which opens at one end into a recess 2 open towards the outside, while its other end leads to a crucible 4 containing molten metal. A level regulator of the molten metal contained in this crucible, constituted by a vertically movable piston 5, allows the molten zinc to flow into the conduit 3 to supply the clearance 2. An adjustment valve 3a placed in the conduit 3 allows '' adjust the flow rate with greater precision.

A ribbon 6, for example formed from a steel sheet, wound at one end on a supply reel 7 and at the other end on a storage reel 8 and guided by three rollers 9, 10 and 11 , scrolls past the opening of the clearance 2, driven by a motor 12. The distance between the strip 6 and the edge of the clearance 2 is adjustable. To this end, each end of one of the rollers 10 used for positioning is fixed to the end of an arm 13 mounted oscillating around a fixed axis 14, which bears against an adjustable stop screw 15. A gas burner rail 16, supplied by a source of gaseous fuel 17, is placed opposite the face of the ribbon 6 opposite to that facing the opening of the clearance 2 and in the portion of the path of the ribbon, the direction of travel of which is indicated by the arrow F, located just upstream of the clearance 2. This burner bank 16 is intended to raise the temperature of the strip 6 to around the temperature of the molten zinc, in order to ensure good wetting of the liquid zinc on the tape 6.

The portion of the installation surrounded by a dashed line is placed under a controlled atmosphere of H ₂ + N _z intended to prevent oxidation of the substrate. As can be seen by observing Figs 2 and 3, the clearance 2 has a particular conformation which plays an important role in the success of the process since it serves to form a distribution zone for the molten metal over the width of the strip. To this end, this clearance 2 is formed by an elongated groove disposed transversely to the longitudinal axis of the ribbon 6 and the length of which is very slightly less than the width of the ribbon 6 to be coated. This groove is formed in a plate 19 which extends through the outlet opening of the distribution duct 3. In this example, the depth of this clearance is of the order of 2 to 3 mm and its width is 3 mm. A series of connecting conduits 20 open into the bottom of this clearance 2 and make it communicate with the conduit 3. These connecting conduits 20 are formed by holes of 0.8 mm in diameter spaced 10 mm apart from each other in this example. They are intended to create a certain pressure drop at the outlet of the conduit 3 and to promote the distribution of the molten zinc over the entire length of the clearance 2. In this example, the connecting conduits are spaced 10 mm apart. the other, but this spacing is not critical. In practice, the section and the number of connecting conduits will be chosen according to the desired flow rate and so as to create a pressure drop such that the molten zinc does not come out in the form of a jet, the upstream pressure resulting only from the height of liquid zinc located above the level of the inlet of the conduit 3. This pressure drop is only intended to distribute the flow regularly over the entire length of the clearance 2 so that it is constantly filled with molten zinc during the coating process. The outer face of the plate 19 is bevelled on either side of the clearance 2 to prevent the molten zinc from spreading on this face when the strip 6 is placed near the edge of the opening of the clearance 2 and moves longitudinally. The tape 6 also makes an angle of about 10 ° with respect to the vertical on either side of the roll 10. It should also be noted that the supply of liquid zinc by a horizontal conduit while the tape to be coated is substantially vertical and moves opposite the clearance 2 from bottom to top, constitutes an advantageous arrangement. Indeed, the flow of zinc can be easily controlled, and above all, the direction of movement of the tape causes a entrainment of the liquid zinc due to the wettability of the heated tape which opposes the flow of zinc by gravity, so that it is possible to dynamically balance these two opposing forces.

Tests have been successfully carried out on a laboratory scale using the installation described. To carry out these tests, a 10 cm wide stainless steel strip was used. Stainless steel was chosen to carry out these tests so as to be able to work without a controlled atmosphere for the purpose of simplification. Since the wettability of an ordinary steel strip heated under a non-oxidizing atmosphere is better than that of an oxidizable steel, it is certain that the tests carried out in this way are in any case transferable to an industrial installation using ordinary steel sheets.

Initially, the temperature of the zinc in the crucible 4 is brought between 450 and 470 ° C. The steel strip 6 is brought by the roller 10 controlled by the adjusting screw 15 acting on the oscillating arm 13, in contact with the edge of the opening 2. The piston 5 is introduced into the zinc bath contained in the crucible 4 and the zinc level exceeds the level of the conduit 3 and flows in the direction of the clearance 2. It crosses the channels 20 and spreads in the clearance which it fills. At this moment the roller 10 is moved back using the screw 15 and the engine 12 is started while the gas burner ramp 16 has been lit, so that the ribbon, arriving opposite the clearance 2, or brought to a temperature close to that of the molten zinc, in this example the temperature of the strip can be of the order of 400 ° C. so that the zinc is not cooled as soon as it touches the sheet, which would prevent it from forming a regular coating and would eventually block the tape by an accumulation of zinc at clearance level 2.

The distance between the edge of the opening of the clearance 2 and the surface of the tape to be coated must be sufficient so that the tape does not in any case touch the wafer 19. During the tests carried out and with a tape speed of 10 m / min and a deposit thickness of the order of 30 μm, the spacing between the ribbon and the wafer 19 could reach 0.8 mm. Preferably, this distance is 0.5 to 0.6 mm. By reducing or increasing this distance we notice streaks or discontinuities in the coating. Beyond 1 mm and whatever the speed of the tape, the meniscus between the clearance 2 and the tape no longer forms and the metal flows by gravity. It has already been noted that the tests carried out on a stainless steel strip for reasons of simplification of the installation and its implementation do not give optimum wetting conditions and that the coating of a steel strip in a reducing atmosphere will give better wetting. Consequently, the passage from laboratory tests to the industrial stage does not pose any problem of principle and may make it possible to slightly increase the distance between the edge of clearance 2 and the strip. Anyway, this distance is already sufficient to ensure the running of the tape without contact with the edge of the clearance 2. The zinc flow rate must be fixed according to the thickness of the desired coating and the running speed, so that the mass of liquid zinc bathing the portion of the tape adjacent to the clearance 2 is constant. Maintaining this liquid mass in contact with a portion of the ribbon 6 over its entire width is the guarantee of a continuous coating, of constant thickness, over the entire width of the ribbon. The increase in the width of the covering from 10 cm to 1.50 m, which constitutes the current width in industrial applications, does not present any particular principle difficulty. One can either use a series of modules formed from the plates 19 arranged side by side, or lengthen the plate 19 and the clearance 2 to cover the entire width or else juxtapose side by side modules longer than that used during the tests, but shorter than the total width of the sheet to be coated. It is also possible to cover only part of the width of the ribbon and thus obtain savings. It suffices to dimension and distribute the clearances 2 properly so that only certain parts of the width of the strip are coated.

Tests of variation of the coating thickness by a modification of the flow made it possible to go from a coating of 80 pm by 10 cm wide with a flow of 1.33 cm ³ / s and a running speed of 10 m / min at a coating of 30 μm with a flow rate of 0.5 cm ³ / s. This coating could be lowered to 20 µm still under laboratory conditions. The scrolling speed seems limited upwards only by questions of vibrations and not by the nature of the process and downwards, it can be lowered to approximately 3 m / min for a flow rate of approximately 1 cm ³ / s. This shows the great flexibility of the method according to the invention.

Various variants, one of which is illustrated by FIGS. 4 and 5 can be envisaged. It is possible to provide a slot 21 in place of the clearance 2 and of the connecting conduits 20. It is also possible to envisage a series of conduits 20 in place of the slot 21 without these conduits leading to a clearance 2. It is also possible to reduce the number of conduits 20 and to spread them further by increasing or not their passage section. The advantage of the flowing zinc bath, produced adjacent to the face of the tape 6 to be coated lies in the fact that by its position, which tends to cause the zinc to flow by gravity and the combined action of the wettability of the tape and the drive effect consecutive to its upward movement, we find substantially the same conditions as with the conventional technique of coating on two sides, with a regular thickness of coating and the possibility of modifying it at will by acting on the flow or on the scrolling speed. In all the tests carried out, no trace of zinc appeared on the reverse of the ribbon. In an industrial installation in which the coating speed must be increased, provision should be made for a cooling station, for example cooling with nitrogen by creating a circulation of nitrogen in a conduit 18 from a source of 'nitrogen S, as illustrated in FIG. 1. It is obviously possible to provide a regulation system intended to create a control between the running speed of the ribbon 6 and the flow of zinc through the valve 3a as a function of a given coating thickness.

The variant of the device illustrated in FIGS. 6 and 7 represents an embodiment in which an oven and its distribution duct 3 ′, one end of which opens into a clearance 2 ′ similar to the clearance 2 of FIGS. 1 to 3, are mounted in an industrial galvanizing installation essentially comprising an enclosure with a reducing atmosphere 22, which encloses a heating station 23 either electric or gas. One end of the enclosure 22 has an inlet 24 for the passage of the ribbon 6 ′ supplied by a roller 7 ′. A guide roller 25 is disposed near the clearance 2 'after which the tape 6' goes towards the outlet of the enclosure 22 towards another guide roller 26 located between the enclosure 22 and a cooling station 27. On partly notes culier in fig. 7 that the ribbon 6 'arrives horizontally on the guide roller 24 but that it envelops this roller at an angle greater than 90 °, so that it leaves it with a slight inclination of a few degrees to promote the delimitation of the meniscus in the direction of movement F of the ribbon 6 ', as already indicated in the embodiment of FIG. 1. The rest of the installation is well known to those skilled in the art and is outside the scope of the invention, so that it is not necessary to describe it in more detail.

The method described, although it was designed for coating on one side only, can be used for coating on two sides by providing a second coating device opposite the other side. These coatings could for example be of different thickness on either side of the tape. It is also possible to use this process to coat the two sides of a tape with savings on one or both sides.

Among the advantages of the method described, it should also be noted that the wringing necessary to adjust the thickness with the conventional methods is made unnecessary. Although the examples given above all relate to zinc, it is obviously possible to use this same process to form coatings of lead or tin or even aluminum by adapting in the latter case the material used for the plate 19 taking into account the melting temperature of aluminum and its corrosive action at this temperature.

Claims (5)

1. Process for continuously coating at least a portion of at least one surface of a metal substrate with another metal, according to which one forms a stream under pressure of said other metal though the outlet opening of a distribution channel (3, 3'), he guides the substrate previously heated to a temperature close to the melting point of the coating metal along a path passing near to said distribution opening, and he entrains this substrate along its path in order to successively bring its heated portions in facing relationship with the distribution opening, characterized by the fact that he provides a surface (2, 2') adjacent the distribution channel outlet opening and oriented according to a substantially vertical plane for enabling the molten metal dispensed by said outlet opening to flow under gravity, he orients said path in a vertical plane so that the horizontal dimension of said surface (2, 2') defines the width of said portion, and he displaces the substrate in regard to this surface in a direction opposite to the flow under gravity of said metal, he adjusts the distance between this surface (2, 2') and the substrate (6) so that the surface tension of the molten coating metal, the wettability of the substrate, the direction and the rate of displacement of said substrate generate forces capable to oppose the flow under gravity of said metal and enable to dynamically maintain a meniscus of this molten metal between the substrate and said metal and he adjusts the flow of metal supply to this meniscus as a function of a given rate of displacement of the substrate and of the required coating thickness.

2. Device for carrying out the process according to claim 1, characterized by the fact that it comports a distribution channel (3, 3') connecting a surface (2, 2') adjacent an outlet opening of said channel to a crucible (4) for melting the coating metal, according to a horizontal axis, the crucible being provided with means (5) for adjusting the level of molten metal with respect to the position of the distribution channel outlet opening (3, 3'), adjustable guiding means of a substrate to be coated (9, 10, 11, 25, 26) defining a path for this substrate in facing relationship to said surface adjacent the distribution channel outlet opening, said guiding means being arranged so that the portion of the substrate on level with the surface (2, 2') adjacent the distribution channel outlet opening forms at least an acute angle causing that part of this substrate leaving said surface (2, 2') to diverge therefrom, means for adjusting the spacing between said surface (2, 2') and the substrate's surface to be coated, means to heat that substrate's portion immediately upstream of said surface (2, 2') and means to control the flow rate of the molten metal and the substrate's displacement rate as a function of the desired coating thickness.

3. Device according to claim 2, characterized by the fact that said surface (2, 2') constitutes the bottom of at least one clearance whose depth defines the limiting distance between the substrate and said surface (2, 2') and whose edge forms meniscus accroaching means.

4. Device according to claim 3, characterized by the fact that the edge wall forming said clearance is perpendicular to its bottom which forms said surface (2, 2').

5. Device according to claim 3, characterized by the fact that the horizontal dimension of said clearance defines the width of the coating to be formed by this clearance.

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