METHOD AND APPARATUS FOR THE CLEANING AND COATING OF METAL STRIP

Abstract

A method and an apparatus for cleaning and coating a metal strip wherein the metal strip is cleaned in a cleaning chamber connected to a deposition chamber and wherein the vacuum pressure in the cleaning chamber is kept in the range of 0.01-100 mbar and the vacuum pressure in the deposition chamber in the range of 0.01-10 mbar.

Claims

1. A method for cleaning and coating a metal strip comprising the steps of: cleaning the metal strip prior to applying a coating, generating a coating vapour by heating a material in a vapour chamber, applying the coating vapour in a deposition chamber via a vapour distribution section connected to the vapour chamber on the metal strip, and wherein the coating is applied in a heated enclosure, wherein the metal strip is cleaned in a cleaning chamber connected to the deposition chamber and wherein the pressure in the cleaning chamber is kept in the range of 0.01-100 mbar and the pressure in the deposition chamber in the range of 0.01-10 mbar.

2. A method according to claim 1, wherein the metal strip is cleaned by using a plasma cleaning technique.

3. A method according to claim 1, wherein a gas stream is maintained to remove residues resulting from the cleaning of the metal strip before the metal strip enters the deposition chamber.

4. A method according to claim 3, wherein the gas stream is maintained by using a gas bearing lock between the cleaning chamber and the deposition chamber.

5. A method according to claim 1, wherein an intermediate chamber is provided between the cleaning chamber and the deposition chamber with gas bearing locks connecting the intermediate chamber to the cleaning chamber and the deposition chamber.

6. A method according to claim 1, wherein the metal strip is mechanically cleaned before entering the deposition chamber.

7. A method according to claim 1, wherein the metal strip is subjected to a stream of pressurised gas inside the intermediate chamber.

8. A method according to claim 1, wherein the strip is activated before applying the coating.

9. An apparatus for cleaning and coating a metal strip provided with: a deposition chamber, air locks at the entry and exit sections for the metal strip to enter and exit the apparatus, a vapour chamber to heat a metal and generate a coating vapour, a vapour distribution section with one or more orifices to direct the coating vapour to the metal strip, a hood at least partially enclosing a space which connects to the distribution section with an open side directed at the metal strip which is to be coated, heating means to heat the hood, and connecting means to connect the vapour chamber to the distribution section in the deposition chamber, wherein the apparatus includes a cleaning chamber provided with a plasma cleaning device to clean the metal strip and wherein the cleaning chamber is connected to the deposition chamber wherein the coating vapour is applied to the metal strip.

10. The apparatus according claim 9, wherein the cleaning chamber is provided with means to provide a gas stream through the cleaning chamber and maintaining a vacuum pressure in the range of 0.01-100 mbar.

11. The apparatus according to claim 9, wherein the connection between cleaning chamber and deposition chamber includes an air lock.

12. The apparatus according to claim 9, wherein the connection between cleaning chamber and deposition chamber includes an intermediate chamber provided with air locks on opposite sides.

13. The apparatus according to claim 12, wherein at least one of the air locks is a gas bearing lock with at least one gas permeable bearing surface with gas supply means and one or more grooves connected to gas pumping means.

14. The apparatus according to claim 13, wherein at least the gas bearing lock at the side of the cleaning chamber is provided with one or more grooves preceding a gas permeable surface.

15. The apparatus according to claim 13, wherein the grooves and gas permeable bearing surfaces are provided in opposite pairs of grooves and bearing surfaces.

16. The apparatus according to claim 12, wherein a pressurised gas supply is provided and inside the intermediate chamber means to guide a stream of pressurised gas at the surface of the metal strip.

17. The apparatus according to claim 9, wherein a plasma activation device is provided in intermediate chamber or deposition chamber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The invention is further explained on hand of the figures in which:

[0054] FIG. 1 shows a diagram of deposition chamber contamination as function of vapour yield and coating thickness,

[0055] FIG. 2 shows a diagram of deposition chamber contamination as function of the oxide layer thickness,

[0056] FIG. 3a shows schematically a lay-out of a cleaning chamber and a deposition chamber,

[0057] FIG. 3b shows schematically a lay-out of a cleaning chamber, intermediate chamber and deposition chamber with different air locks at opposite sides of the intermediate chamber,

[0058] FIG. 3c shows schematically a lay-out of a cleaning chamber, intermediate chamber and deposition chamber with gas bearing locks at opposite sides of the intermediate chamber,

[0059] FIG. 4a shows schematically a section through the gas bearing lock between cleaning chamber and deposition chamber,

[0060] FIG. 4b shows schematically a section through opposite gas bearing locks of the intermediate chamber,

[0061] FIG. 4c shows schematically a section through a gas bearing lock provided with a brush in the intermediate chamber,

[0062] FIG. 5 shows schematically a lay-out of a cleaning chamber, intermediate chamber and deposition chamber with means to activate the metal strip in the deposition chamber,

[0063] FIG. 6 shows schematically a lay-out of a cleaning chamber, intermediate chamber and deposition chamber with gas bearing locks and double coating system, and

[0064] FIG. 7 shows schematically a lay-out of a cleaning chamber, intermediate chamber and deposition chamber wherein heating means to heat the space wherein the coating is applied to the metal strip is integrated in the wall of the deposition chamber.

DETAILED DESCRIPTION OF THE DRAWINGS

[0065] In FIG. 1 the deposition chamber contamination is shown as function of vapour yield and coating thickness. The coating contamination is proportional to the mass flow of generated vapour, the campaign length and the vapour yield. The contour plot of FIG. 1 is calculated assuming a strip of 1.0 m wide running at 120 m/min which is coated on both sides with a zinc coating that varies from 0 to 2.5 micron. It is further assumed that the vapour yield lies somewhere between 99 and 100%. The contamination of the deposition chamber is given in litres/week. The contour plot clearly shows that the vapour yield has to be very high to obtain a low contamination value that makes the equipment suitable for continuous or semi-continuous operation.

[0066] One advantage of the method according to the invention is the application of the vapour distribution box, with which a vapour yield of 99% could be realised and which has further been improved with the use of a heat channel. With such a channel the metal coating atoms that initially bounce off the strip are given a second chance to condense on the metal strip. In the description the term heat channel, is used interchangeably with the terms hood, heated hood and heat box and will all mean an envelope forming a space connecting the vapour distribution section and the metal strip which is to be coated. With such an envelope the vapour flow from the vapour distribution section is largely restricted to flow directions directly to the strip surface. Where the metal strip is coated on both sides the hoods on either side of the metal strip are preferable connected to form a box, wherein the box is provided with an entry and exit slit for the metal strip.

[0067] In FIG. 2 a diagram is shown of deposition chamber contamination as a function of the thickness of the oxide layer that needs to be etched away for proper metallic coating adhesion. The oxide layer on the metal strip can have a thickness in the range from 1 to 50 nm for a low alloyed steel. In the case of Advanced and Ultra High Strength Steels this can be several tens to several hundreds of nanometres. The chamber contamination in FIG. 2 shows that with an oxide layer of 10 nm the contamination due to plasma cleaning is already in the order of 20-30 litres/week. From this it will be clear that controlling deposition chamber contamination by use of a heat box and higher vacuum pressure has only limited effect if the residue resulting from cleaning of the metal strip is not sufficiently controlled.

[0068] In FIG. 3a a lay-out of an apparatus 1 is shown schematically, comprising a cleaning chamber 2 and a deposition chamber 4. At the entry of the cleaning chamber 2 and the exit of the deposition chamber 4 air locks 5,6 are provided and between the cleaning chamber 2 and the deposition chamber 4 a gas bearing lock 7 is provided.

[0069] In the cleaning chamber 2 plasma cleaning means 9, 10 are provided such as electric arc discharge means or dielectric barrier discharge means. Cleaning by magnetron sputtering is also possible, but the other cleaning techniques are preferably used since these can be operated at much higher vacuum pressures than magnetron sputtering. These techniques can be used at pressures in the range of 0.01-100 mbar and at even higher pressures.

[0070] The cleaning chamber 2 is provided with a gas inlet 15 to supply gas into the cleaning chamber 2 and with pumping means to remove the gas from the cleaning chamber 2, by means of which a sufficient gas flow through the cleaning chamber 2 is realized to remove residue resulting from the cleaning operation from the cleaning chamber 2. The supply of gas is limited such that a certain pressure is maintained inside cleaning chamber 2. The gas used could be dry air, nitrogen or argon. In the set-up of FIG. 3a the pumping means to remove gas with the cleaning residue are the pumping means associated with gas bearing lock 7.

[0071] In the deposition chamber 4 on both sides of the metal strip 11, for instance a steel strip, a vapour distribution section 12 is provided. The vapour distribution section 12 is provided with nozzles and/or slits as to cover the total width of the metal strip 11. A hood 13 is connected to the vapour distribution section 12 and heating means 14 are provided to heat the hood. If the metal strip 11 is to be coated on both sides the hoods 13 on both sides are preferably connected to each other to form a box, wherein the box is provided with entry and exit slits for the metal strip.

[0072] A vapour chamber (not shown in the drawing) wherein a metal is heated to generate a coating vapour is connected to the vapour distribution section 12. The vapour distribution section 12 is operated such that the vapour leaves the vapour distribution section 12 through the nozzles at sonic speed. The hood or heat box is heated to reduce the deposition of the vapour on the heat box. To assure that no contamination is occurring the hood or heat box is heated to a temperature that is equal to or larger than the saturation vapour temperature of the deposited material that corresponds to the pressure of the vapour in the head box or hood. For a Zn-coating the hood or heat box is heated in the temperature interval between 330 and 580? C., this is roughly the Zn temperature range that coincides with a vapour pressure between 0.01 and 10 mbar. The exact temperature of the heat box will also be determined by the maximum allowable strip temperature.

[0073] In FIG. 3b a lay-out of an apparatus 1 is shown schematically, comprising a cleaning chamber 2, an intermediate chamber 3 and a deposition chamber 4. The entry and exit of the intermediate chamber 3 are provided with respectively a gas bearing lock 7 and an air lock 8. By providing an intermediate chamber 3 the removal of residue coming from the cleaning operation can be improved therewith limiting the chance that any residue will enter the deposition chamber 4. Since most if not all of the residue is removed before the strip arrives at the exit of the intermediate chamber 3 a conventional air lock 8 may be used at the exit.

[0074] With a conventional air lock the strip 11 has to pass one or more sections with rolls and if there is still residue on the strip or adhering to the strip, the residue will be rolled into the strip which may give rise to certain surface defects in the final product. With the set-up according to FIG. 3c wherein gas bearing locks 7,8 are provided at the entry and exit of the intermediate chamber 2 this can be avoided.

[0075] FIG. 4a shows a section through gas bearing lock 7 between cleaning chamber 2 and deposition chamber 4. In the drawing only the upper half of the gas bearing lock is shown for the sake of simplicity. Gas bearing lock 7 is provided with number of opposite pairs of grooves 16,21 at both sides of a central gas bearing part 17. Through grooves 16 gas from the cleaning chamber 2 is removed to maintain a certain pressure inside the cleaning chamber and at the same time part of the gas coming through gas bearing part 17 is removed. The grooves 21 at the other side of gas bearing part 17 are also used to remove part of the gas coming through gas bearing part 17 and to maintain a certain vacuum pressure in the deposition chamber 4.

[0076] In FIG. 4b a section through gas bearing lock 7,8 at the entry and exit of the intermediate chamber 3 is shown. Gas bearing lock 7 has a number of opposite pairs of grooves 16 through which gas from the cleaning chamber 2 is removed as well as part of the gas coming through gas bearing part 17. The gas removal through grooves 16 at least part of the residue from the cleaning operation is removed while maintaining a certain pressure inside the cleaning chamber. Gas bearing lock 8 at the exit of the intermediate chamber 3 is of a similar design with opposite gas bearing parts 20 and a number of opposite grooves 21. These grooves 21 are to remove part of the gas coming through gas bearing part 20 and to regulate the pressure in the deposition chamber 4. After passing the final groove 21 the metal strip enters the deposition chamber 4.

[0077] Inside the intermediate chamber a pressurised gas supply 19 is provided which is used to remove any residue still on the surface of strip 11. The gas supply may comprise a long slit extending over the width of the strip or a number of nozzles. This manner to remove residue from the strip brings a lot of gas into the intermediate chamber and in order to remove that gas outlet channels 18, 18 are provided at both sides of the pressurised gas supply 19. As far as necessary pumping means may be provided to improve the removal of used supply of pressurised gas. Together with the used gas the residue that is blown of the strip is removed. Through these outlet channels 18,18 also part of the gas coming through the gas bearing parts 17,20 is removed. The amount of residue removed through outlet channel 18 may be larger than the amount that is removed through outlet channel 18 because of the location with respect to the cleaning chamber 2.

[0078] The grooves 16, 21, outlet channels 18,18 and gas bearing parts 17,20 continue across the width of the metal strip 11.

[0079] The gas supplied through the pressurised gas supply 19 can also be used to cool the strip. Cooling of the strip will be necessary if the temperature of the strip as a result of the cleaning operation has risen above a value that is still acceptable to apply a coating on the strip or where the temperature has risen such that the properties of the strip are altered.

[0080] FIG. 4c shows the upper part of the gas bearing lock 7 of FIG. 4b in an embodiment wherein a brush is provided at the location of the pairs of opposite outlet channels 18 following the opposite gas permeable surfaces 17. With the brush 22 any residue still present on or adhering to the surface of metal strip 11 can be removed. The side 23 of groove 18 is used as a scraper to remove residue from brush 22 to keep the brush clean.

[0081] FIG. 5 shows another embodiment wherein in the deposition chamber 4 activating means 24 are provided to activate the surface of the metal strip. This is necessary for some metallic coatings such as ZnMg coatings to get sufficient adherence of the coating to the metal strip. For activation of the strip magnetron sputtering, glow discharge or dielectric barrier discharge can be used. Electric arc discharge could also be used but the energy level provided is far more than necessary for the purpose.

[0082] In FIG. 6 a set-up is shown wherein all entry and exit sections are provided with gas bearing locks 5, 7, 8, 6. Further two vapour distribution sections 12, 12 are provided which allow to apply coating layers over each other, for instance a Zn coating layer followed by a ZnMg coating layer. In the intermediate chamber 3 activating means 24 are provided, which in this set-up are near enough to the first vapour distribution section 12.

[0083] FIG. 7 shows schematically a set-up wherein the walls of the deposition chamber 4 form the hood or heat box 13 with the heating means 14 integrated in the walls of the deposition chamber 4. This integrated set-up allows to form a compact deposition chamber 4/heat box 13. With such combined deposition chamber 4/heat box 13 the activating means 24, if necessary, are provided in the intermediate chamber 3 which will be close enough to the vapour distribution box 12.

[0084] Instead of providing that the complete deposition chamber is formed as a heat box, it is also possible to provide that only part of the deposition chamber facing one side of the metal strip is formed as a heated hood. This would be a suitable embodiment for metal strip that is to be coated on one side only.