A method and apparatus for the continuous etching and aluminum plating of stainless steel strips

Abstract: This method combines etching of the strip by passing through reduced pressure electric plasma discharge zones and the direct off-line dip-coating of the etched strip in a bath of molten aluminum. The strip brought to cathode potential defilades conti­ nuously in registration with the magnet elements and anodes of a plurality of consecutively disposed magnetron devices and, therefore, directly into said molten metal bath.

Description: The present invention relates to a method for continuously hot dip coating a stainless steel strip with aluminum, comprising the steps of alternatively submitting each side of said strip to an ion magnetically-enhanced sputtering process for cleaning said strip; dipping said cleaned strip into a molten aluminum bath to deposit a coating layer on each side of said strip; and cooling said coated strip. It also relates to an apparatus for continuously dip coating both sides of a stainless steel strip with aluminum having a plurality of reciprocally acting magnetically-enhanced sputtering and heating devices alternatively placed on both sides of said strip, said devices comprising i) a magnet element on one side of the strip and, in registration therewith, ii) a counter-electrode on the other side of the strip, and iii) means to apply a positive voltage thereto relative to the strip to generate a low pressure argon plasma discharge which will be concentrated by the magnetic field of the magnet element to at least one confinement zone between the strip and said counter-electrode. The continuous cleaning or etching of defilading elongated substrates such as wire, strips, bands and the like by ion bombardment prior to coating with another material or metal is known. This technique is indeed considered much more effective in the case of high chromium content alloys than the more conventional high temperature reductive cleaning treatments because chromium oxide is difficult to reduce and poor reduction efficiency is likely to cause problems of adhesion of the final aluminum layer. Some pertinent prior art in this field is summarized below. The prior art DDR-120.474 (HEISIG et al.) discloses an installation for the precleaning by sputtering before plating under vacuum of a stainless strip. The precleaning unit can be integral with or separated from the plating unit itself. The precleaning unit comprises a plurality of magnetron elements arranged consecutively along the defilading strip (see the drawing). The strip is narrowly confined in the discharge region of the magnetrons by means of rolls (7) which prevent it from touching the pole-pieces of the magnet or the anode on the other side of the strip. The strip is grounded as well as the remainder of the apparatus; only the anode is insulated and held at positive voltage relative to the strip. Seventy % of the energy fed to the magnetrons is used up to heat the strip. The document does not specify how the sputter-cleaned strip is vacuum-plated afterwards. GB-A-926.619 discloses a process consisting of pretreating the steel strip by passing it into a chamber having a hydrogen atmosphere in which the strip is heated to a temperature of 540° to 650°C at which the hydrogen is effective to reduce surface oxides. Afterwards the strip if heated further and then enters the bath of molten aluminum. EP-A-134.143 relates to a continuous cleaning and hot dip coating process according to which a steel sheet is preheated and introduced into a reducing furnace in which a hydrogen atmosphere is maintained, so that the oxidation layer on the surface to be coated is reduced while the steel sheet is annealed. The annealed steel sheet is then fed to a cooling furnace in which the temperature of the steel is adjusted optionally for the hot dipping. The steel sheets processed according to these two last documents are not stainless steel sheets so that no chromium oxides have to be reduced. As already stated here above the chromium oxides are particularly difficult to reduce and request temperatures substantially higher than the melting point of the aluminum bath so that the reduced sheet has to be cooled before to be dipped into the molten bath of aluminum. During the cooling step there is a risk that the cleaned sheet be reoxidised. Moreover these processes are costly since a lot of energy has to be spent to heat the sheet well over the desired pre-heating temperature for carrying out the hot dipping process. Summary of the invention The present invention proposes, as summarized in claim 1, to alternately submit each side of a strip to be hot dip coated to low argon pressure plasmatron discharges for cleaning said strip, and simultaneously heating it to the desired temperature for dip-coating. Many advantages result from the application of the present method including very high etching efficiency even for hard to remove oxides like chromium oxide, well adhering aluminum films, easy control of protective film thickness and relatively low production costs due to compactness of the apparatus for achieving the method, and high production rates. The apparatus is disclosed in annexed claim 3. Brief description of the drawings Fig. 1 is a schematic view of an apparatus for carrying out the process according to the invention, but this apparatus as such does not illustrate the apparatus of the invention. Fig. 2 is an enlarged schematic view of an etching magnetron device used in the apparatus of Fig. 1. Fig. 3 is a schematic view of another embodiment of the apparatus for carrying out the process according to the invention and also illustrates the apparatus of the invention. Detailed description of the invention The apparatus represented on Fig. 1 comprises an enclosure consisting of four successive tubular compartments 1a - 1d connected to each other by reduced diameter apertures and terminated by a snout 2 which penetrates into a bath 3 of molten aluminum 4. A continuous stainless strip 5 is circulated within the installation starting from a feed-spool 6 up to a take-up spool 7 at the end of the line. The strip is guided by main rollers 8, 9, 10 and 11, and by seal-roll chambers 12a to 12e which also provide gas pressure isolation between compartments and from the outside. Seal-roll chambers are detailed in document EP-A-176 109 incorporated by reference. The components 1a to 1d of the present installation are provided with input ducts 13a to 13d, respectively, and output ducts 14a to 14d, respectively. The output ducts are used in connection with one or more suitable pumps to establish a reduced pressure within the enclosure. The input ducts are used to introduce a gas at low pressure to sustain the plasmatron discharges in the compartments; this gas is usually argon. In an embodiment of the present installation, the seal-roll chambers 12b, 12c and 12d can be omitted, whereby only one input duct, for instance 13d, and only one output duct, for instance 14a, are still necessary to maintain the full enclosure under the required low pressure of argon and all the other input and output ducts can be suppressed as well as the reduced diameter section between the compartments; in this case, the overall shape of the enclosure along its length remains approximately constant. Each compartment of the present enclosure 1 contains a plasmatron device 24 (individual plasmatron are given the reference numbers 24a to 24d) which is represented on an enlarged scale in fig. 2. A plasmatron device of the kind used in the present embodiment comprises a magnet frame 15 carrying three magnets, respectively 161, 162 and 163 arranged in order of alternating polarity, so that the magnetic field created by said magnets is closed in a confinement space between the magnets and an anode 18, as represented by reference 17 on the drawing. The magnets are placed very close to the path of the circulating strip 5 so that the strip will circulate within the confinement space 17 while being prevented from rubbing against the magnets by means of rolls 19 made of a non-magnetic material, for instance bronze or austenitic steel. The anode 18 is connected to a positive terminal of an electric generator (not shown) by a lead passing through an insulator 20 (for instance of steatite). When the strip is at ground potential (as is the enclosure as shown in the drawing) and the cathode 18 is at a positive voltage of a few hundred volts, for argon pressures of a few microbars, a luminescent discharge is generated in the confinement zone 17, as shown by the darkened area in fig. 2. Therefore the strip which passes through the luminescent discharge in zone 17 is etched by the impact of the gaseous ions formed in this region. Reference 22 designates cooling passages through which coolant fluids can be passed in case refrigeration is needed. The several successive magnetron devices housed within successive compartments 1a to 1d are identical with that represented in fig. 2, however they are arranged in successive alternate head-to-foot orientation, so that both sides of the strip can be etched as the strip 5 progresses along its path in the enclosure. Under operation, the strip 5 moves along its path in the enclosure 1 and each portion thereof passes successively in the discharge zones 17 of each successive plasmatron device 24a to 24d. of course, if desired, the number of compartments with respective plasmatron can be more than 4, for instance 6, 8 or more. After passing the last discharge zone, the etched strip is guided through seal-roll chamber 12e and snout 2 into the bath of molten aluminum 4, whereby it becomes coated with a film of aluminum. The coating weight (thickness) is controlled by means of a conventional wiping apparatus W or an equivalent, after which the aluminium solidifies by cooling. Then the plated strip is stored over take-up spool 7. Under normal operation, the energy developped in the plasmatron discharge is sufficient to heat up the strip to the desired temperature before it enters the molten aluminum bath. Fig. 3 represents schematically another apparatus for the continuous etching and subsequent immediate plating of a stainless strip. This apparatus consists of a double-sided enclosure 31, made for instance of high grade steel, one side being for the entrance of unplated strip and the other side for the removal of the plated strip. The entrance side comprises a succession of reduced size openings 32a to 32d of very narrow diameter to provide a pressure tight passage to a strip 33 supplied by a spool 34 which circulates vertically in the enclosure 31. Normally, the clearance between the strip and the edges in the passages 32a to 32d should be in the order of a few tens of µm (e.g. 30-100 µm) to be sealingly effective. Then, the entrance side of the enclosure comprises a series of magnetron devices 34a to 34d each of which corresponds to that illustrated in fig. 2 and comprising a magnet unit 35a to 35d and an anode (38a to 38d). The magnet units and the corresponding anodes are in registration with the moving strip 33 exactly as disclosed in the previous embodiment so that the strip becomes etched on both sides as it progressively passes through the discharge zones generated between the strip surface (at cathode potential) and the respective anodes. As the strip leaves the last magnetron element (35d, 38d) it passes over a turning roller 39 which is partly immersed in a molten aluminum bath 40, this bath being replenished as necessary with molten metal by syphoning means 41 represented schematically by a reservoir 42 of molten aluminum and a bent tube 43, the molten metal of reservoir 42 being raised to the level of the bath 40 by the atmospheric pressure working against the reduced pressure of argon within the enclosure 31; therefore the level of molten metal of bath 40 is maintained under control. After being plated with Al by its passage in bath 40, the coating weight being conventionally controlled by wiping (see W in the drawing) the strip 5 leaves the enclosure through gas sealed passage means 44a to 44d which are of similar construction an the aforementioned passages 32a to 32d, and is stored over a take-up spool 45. The enclosure 31 is provided with a series of opening ducts referenced P₁, P₂, P₃, P₄ and Ar. The P labelled ducts are for build up of progressively reduced pressure within the enclosure, i.e. they are connected to respective vacuum pumps (not represented), while duct labelled Ar is for the arrival of a plasma sustaining gas, usually argon. The operation of this apparatus practically duplicates that of the previous embodiment. The strip supplied by the feed spool 34 penetrates into the enclosure through the successive gas tight openings 32a to 32d; it gets etched by passing through the discharge zones in the plasmatron devices 35a - 38a to 35d - 38d; then it is plated with aluminum by passing through bath 40 and, finally, it exits from the enclosure by passages 44a to 44d and is stored over take-up spool 45. The following example illustrates the invention in detail. Example An apparatus of the kind illustrated in fig. 3 was used. The strip was a 0.5 mm thick and 1 m wide stainless strip; therefore the width of each magnetron (10 units) was in correspondence. The distance between the strip and the magnet elements was set to 8 mm (see rolls 19 in fig. 1) and the discharge confinement zone between the strip and the anodes 38 (made of tantalum) was 25 mm thick 20 x 30cm (surface about 600 cm² for each magnetron). The magnets were made of samarium-cobalt alloy giving a magnetic field in the working surface. The pumps connected to outputs P₁ to P₄ gave, respectively, 10³, 10, 10⁻¹ and 10⁻³ Pa (10, 10⁻¹, 10⁻³, and 10⁻⁵ mbar) and the Argon input was adjusted to give about 3'-5 x 10⁻¹ Pa (3-5 x 10⁻³ mbar) argon pressure in the discharge areas. The molten aluminum was maintained at 640-680°C. The strip was grounded through the enclosure and under 500-600 V DC, the discharge current was about 20-40 mA/cm² which means an energy consumption of 2-5 kW per magnetron. With strip delivery rates of 20-60 m/min, homogenenous unpitted, well adherent Al plating of 3-100 µm thick were recorded.

Claims: A method for continuously hot dip-coating a stainless steel strip with aluminum, comprising the steps of: continuously circulating the strip in an argon swept enclosure along a path very close to a series of magnetron devices, and in registration therewith; alternatively submitting each side of said strip to low argon pressure plasmatron discharges for cleaning said strip; and simultaneously heating it to the desired temperature for dip-coating; dipping said cleaned strip into a molten aluminum bath to deposit a coating layer on each side of said strip; and cooling said coated strip. The method of claim 1, in which the molten aluminum bath is located within the same enclosure in which the strip is subjected to the ion magnetically-enhanced sputtering process, and in which the strip leaves the enclosure after being plated with aluminum to be stored on a take-up spool. Apparatus for continuously dip-coating both sides of a stainless steel strip with aluminum having: a plurality of reciprocally acting magnetically-enhanced sputtering and heating devices alternatively placed on both sides of said strip, said devices comprising i) a magnet element on one side of the strip and, in registration therewith, ii) a counter-electrode on the other side of the strip, and iii) means to apply a positive voltage thereto relative to the strip to generate a low pressure argon plasma discharge which will be concentrated by the magnetic field of the magnet element to at least one confinement zone between the strip and said counter-electrode, characterised in that it comprises an elongated vertically oriented vacuum enclosure connected on one hand to vacuum means and, on the other hand, to a source of argon for feeding this enclosure with a plasma sustaining gas under about 10⁻² -1 Pa(10⁻⁴-10⁻² mbar) a bath of molten aluminum located at bottom of said enclosure, connected by syphoning means to a supply of said molten metal submitted to the atmospheric pressure, maintaining the level of said bath under control, feeding and circulating means of said strip, downwards from a top input of said enclosure to said bath, into the bath around a turning guide and upwards to a top output of said enclosure, gastight means at said input and output of said enclosure, said sputtering and heating devices being disposed along the strip which displaces downwards.

Inventors: BOSWELL PETER, ZEGA BOGDAN, BOSWELL, PETER, ZEGA, BOGDAN
Applicants: NISSHIN STEEL CO LTD, NISSHIN STEEL CO., LTD.
Application year: 1989
Main classification: C23C2
IPC codes: C23F4, C23C2