Device for hydrodynamic stabilization of a continuously travelling metal strip

Abstract

A facility for dip-coating a metal strip in continuous motion includes: a liquid coating metal bath from which the strip exits in a vertical strand; a bottom roller, a decambering roller, and, optionally, a stabilizing roller, all immersed in the liquid-metal bath; drying blades at an exit of the bath, for injecting compressed gas in order to remove excess coating that has not yet solidified so as to create a drying wave having a downward return stream of liquid metal; and a dissipating hydrodynamic-stabilization device placed between the drying blades and a last immersed roller, the dissipating hydrodynamic-stabilization device including a plurality of hydrodynamic pads for applying a load to at least one side of the metal strip and mounted so as to pivot around hinges so as to self-align the pads, the plurality of hydrodynamic pads extending transversely across a width of the strip.

Claims

1. A facility for dip-coating a metal strip with a coating of a liquid-metal in a continuous motion, comprising: a liquid-metal bath from which the metal strip exits as a vertical strand; a bottom roll and a decambering roll all immersed in the liquid-metal bath; gas knives at an exit of the liquid-metal bath, the gas knives being configured to inject a compressed gas in order to remove an excess coating of the liquid-metal that has not yet solidified so as to create a downward return stream of the liquid-metal; and a dissipating hydrodynamic-stabilization device placed between the gas knives and a last immersed roll from a top of the liquid-metal bath, the dissipating hydrodynamic-stabilization device comprising a plurality of pads, the plurality of pads comprising hydrodynamic pads configured to apply a load to at least one side of the metal strip and mounted so as to pivot around hinges so as to self-align and accommodate the plurality of pads to a local shape of the metal strip as the metal strip exits the liquid-metal bath, the plurality of pads extending transversely across a width of the metal strip, each of the plurality of pads having a front which faces the metal strip exiting the liquid-metal bath and a back opposite the front, the back facing away from the metal strip exiting the liquid-metal bath, and positioned such that, when in use, the downward return stream of the liquid-metal flows at least in part over backs of the pads, wherein distal ends of the plurality of pads relative to the liquid-metal bath are in a wiping zone, are tapered, and are configured to provide pre-wiping of the excess coating, and wherein the hinges are arranged such that the distal ends of the plurality of pads are stationary.

2. The facility according to claim 1, wherein the back of each of the plurality of pads is non-wetting for the liquid metal or is provided with a non-wetting coating.

3. The facility according to claim 1, further comprising a channel or grooves configured to channel the flow of the downward return stream of the liquid-metal on the back of each of the plurality of pads.

4. The facility according to claim 1, wherein each of the plurality of pads has a part that is partially immersed in the liquid-metal bath.

5. The facility according to claim 1, where at least a portion of the plurality of pads are located on a same side of the metal strip and are parallel to one another and separated by an interval in a direction transverse to the motion of the metal strip.

6. The facility according to claim 5, wherein the portion of the plurality of pads located on the same side of the metal strip are in lateral contact via a ceramic felt placed in the interval.

7. The facility according to claim 5, wherein the portion of the plurality of pads located on the same side of the metal strip are in interleaved lateral contact.

8. The facility according to claim 1, further comprising a pneumatic jack configured to independently load each of the plurality of pads.

9. The facility according to claim 8, wherein the pneumatic jack is assisted by a spring-dashpot system.

10. The facility according to claim 1, wherein the plurality of pads are arranged on each side of the metal strip while facing one another in pairs.

11. The facility according to claim 1, wherein the plurality of pads are arranged on each side of the metal strip and in staggered rows.

12. The facility according to claim 10, wherein the plurality of pads are configured to be controlled in groups or individually by a programmable logic control that provides at least one measurement of a camber of the metal strip, an analysis of a defect, and a closed-loop correction of forces applied on the pads.

13. The facility according to claim 1, further comprising a stabilizing roll immersed in the liquid-metal bath.

14. A facility for dip-coating a metal strip with a coating of a liquid-metal in a continuous motion, comprising: a liquid-metal bath from which the metal strip exits with an upward movement; a bottom roll and a decambering roll all immersed in the liquid-metal bath; gas knives at an exit of the liquid-metal bath, the gas knives being configured to inject a compressed gas in order to remove an excess coating of the liquid-metal that has not yet solidified so as to create a downward return stream of the liquid-metal; and a dissipating hydrodynamic-stabilization device placed between the gas knives and a last immersed roll from a top of the liquid-metal bath, the dissipating hydrodynamic-stabilization device comprising a plurality of pads, the plurality of pads comprising hydrodynamic pads configured to apply a load to at least one side of the metal strip and mounted so as to pivot around hinges so as to self-align and accommodate the plurality of pads to a local shape of the metal strip as the metal strip exits the liquid-metal bath, the plurality of pads extending transversely across a width of the metal strip, each of the plurality of pads having a front which faces the metal strip exiting the liquid-metal bath and a back opposite the front, the back facing away from the metal strip exiting the liquid-metal bath, and positioned such that, when in use, the downward return stream of the liquid-metal flows at least in part over backs of the pads, where at least a portion of the plurality of pads are located on a same side of the metal strip and are parallel to one another and separated by an interval in a direction transverse to the upward movement of the metal strip, and wherein the portion of the plurality of pads located on the same side of the metal strip are in interleaved lateral contact.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. Other features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:

(2) FIG. 1 shows a vertical section view of the hydrodynamic stabilization device of a metal strip according to the present invention.

(3) FIG. 2 shows a top view of the strip between the gas knives, schematically showing the distance Z between the knives and the ideal reference plane of the strip, the camber defect z)c and the movement z)v corresponding to the vibrations.

(4) FIG. 3 respectively shows a section view of the wiping wave schematically showing the splashing phenomenon, on the one hand, and the wiping wave in the presence of the end of the hydrodynamic pad, on the other hand.

(5) FIG. 4 shows an elevation view of three preferred embodiments of the present invention, relative to the channels present on the back of each pad, on the one hand, and relative to the interface between adjacent pads, on the other hand.

(6) FIG. 5 shows a planar view of two preferred embodiments of the present invention, showing the relative arrangement of the pads on either sides of the strip, according to its camber defect relative to a reference plane.

(7) FIG. 6 shows a vertical section view of the hydrodynamic stabilization device of a metal strip according to the present invention, but without the stabilizing roll.

DETAILED DESCRIPTION

(8) Embodiments of the present invention provide a solution to the problem of stabilizing a metal strip in continuous motion, that allows to overcome the drawbacks of the state of the art.

(9) Embodiments of the present invention stabilize and/or damp the vibrations of the strip upon leaving a liquid-metal bath owing to hydrodynamic means that allow to dissipate the vibration energy generated in the strip by the facility.

(10) Embodiments of the present invention avoid the implementation, as in the prior art, of additional gas jets in the immediate vicinity of the wipers that could affect the appearance of the final product.

(11) Embodiments of the present invention decamber the strip, and more generally improve the flatness of the strip in the very vicinity of the location where the final thickness of the coating is achieved, i.e. at the wipers, as well as guarantee a uniform coating thickness in the plane of the strip.

(12) Embodiments of the present invention solve the splashing problem encountered at a high motion speed.

(13) The present invention relates to a facility for dip-coating a metal strip in continuous motion, comprising a liquid-metal bath, from which the strip exits in a vertical strand, a sink roll, a decambering roll and, where necessary, a stabilizing roll, all immersed in the liquid-metal bath, gas knives placed at the exit from the bath and injecting compressed gas in order to remove the excess coating that has not yet solidified, creating a wiping wave with a return stream of liquid metal that is oriented downwards, as well as a dissipating hydrodynamic-stabilization device placed between the gas knives and the last immersed roll, comprising a plurality of hydrodynamic pads intended for applying a load to at least one side of the metal strip and mounted so as to pivot around hinges for self-aligning and accommodating said hydrodynamic pads to the local strip shape, also extending transversely across the width of the strip, and positioned such that, when in use, the return stream of liquid metal from the wiping wave flows at least in part over the back of the pads, i.e. over the face thereof that is not facing the metal strip in continuous motion.

(14) According to preferred embodiments of the invention, the facility further comprises at least one of the following features, or even an appropriate combination of several thereof: the back of each pad is non-wetting for the liquid metal or is provided with a non-wetting coating; at the back of each pad, there is further a channel or grooves channeling the flow of the return stream; the distal end of the pads relative to the liquid-metal bath is in the wiping zone, is tapered and can provide pre-wiping of the coating by limiting the risk of splashing; the hinges are arranged such that the tapered distal ends of the pads are quasi-stationary; the pads are either completely emerged, or are partially or completely immersed in the liquid metal; the facility comprises an outside heating device for preheating the pads; the pads located on the same side of the strip are essentially parallel to one another and separated by an interval in the direction that is transverse to the motion of the strip; the pads located on the same side of the strip are in lateral contact via a ceramic felt placed in this interval; the pads located on the same side of the strip are in nested lateral contact via a labyrinth joint; the facility comprises a pneumatic jack for independently loading each pad; the pneumatic jack is assisted by a spring-dashpot system; the pads are arranged on each side of the strip while essentially facing one another in pairs; the pads are arranged on each side of the strip and in staggered rows; the pads are controlled in groups or individually by a programmable logic control that provides at least a measurement of the camber of the strip, an analysis of the defect and a closed-loop correction of the forces applied on the pads.

(15) The facility of the invention will find a preferred application in the context of an industrial method for the continuous hot-dip coating of a metal strip having a motion speed preferably comprised between 0.5 and >3 meters/second (30 and >180 meters/minute), more preferably up to 10 meters/second (600 meters/minute). In the context of this method, the metal strip will preferably be made from steel, aluminum, zinc, copper, or one of their alloys. The thickness of the metal strip will preferably be comprised between 0.15 and 5 millimeters. The molten coating metal will preferably comprise zinc, aluminum, tin, magnesium, silicon or an alloy of at least two of these elements. The thickness of the metal coating layer obtained after wiping will preferably be comprised between 3 and 50 micrometers. The pressurized gas injected by the gas wipers will preferably be air, nitrogen or carbon dioxide.

(16) To make things clear, FIG. 1 schematically shows one preferred embodiment of the hydrodynamic stabilization device of the invention arranged across from the steel strip 1 driven in a continuous upward movement (i.e., in a vertical strand) after passing by the sink roll 4, the decambering roll 5a and optionally by the stabilizing roll 5b of the liquid-metal bath 2 and before it passes at the gas knives 3.

(17) The device according to the invention essentially assumes the form of at least one, but generally several, self-aligned (or self-aligning) hydrodynamic pads 6, pivotingly mounted around a hinge 7. Pads refer to rigid planar devices such as plates. They may either be arranged outside the bath 2, or have a partially immersed part 8, or even be completely immersed. The loading of the pads 6 aims to balance the hydrodynamic lift generated within the film of liquid metal at the strip-pad interface, and also to flatten the strip 1 upon its exit from the bath 2.

(18) More specifically, completely emerged or completely immersed pads 6 advantageously allow to avoid trapping scum located at the surface of the bath primarily upon starting up the line, while completely emerged pads favor stabilization as close as possible to the wipers. In addition, partially or completely immersed pads 6 allow to favor preheating and temperature maintenance of the pad by heat conduction via direct contact with the bath. This also allows to take advantage of the speed profile in the vicinity of the strip, just before it leaves the bath, and thus to significantly improve the hydrodynamic lift (R.sub.hydrodyn.), the thicknesses at the interface, and therefore the operating safety with respect to a risk of contact between the pads and the strip.

(19) It can be seen from FIG. 2 that variations in coating thickness will correspond to the camber defects z)c and to the movements z)v due to the vibrations. Where the strip is closer to a gas knife than the reference plane 12, which is by definition at an equal distance Z from the gas knives, the final coating thickness will be lower, and inversely. More particularly, the camber leads to a continuous variation in thickness over the width of the strip. The vibrations in rigid or string mode lead to alternating thickness variations in the motion direction, while higher-order vibrations (twisting or flapping) lead to variations affecting both the longitudinal direction and the transverse direction. The device presented here therefore aims to eliminate these different variations in order to obtain a flat and stable strip at the gas knives and consequently to guarantee uniform coating thickness in both directions of the plane of the strip.

(20) The splashing phenomenon which occurs beyond a critical motion speed of the strip can be schematically seen from FIG. 3: for a given final thickness, when the speed of the strip increases, the upward stream 13 and the return stream 14 will inflate the thickness of the wiping wave 11. To retain constant final thickness of coating, it is necessary to increase the wiping pressure and therefore the pressure gradient and the shearing of the surface of the fluid film in the wiping zone 20. Past a critical value of the speed-thickness pair, the shearing rate leads to the projection of liquid-metal droplets 15 (splashing). The present invention therefore proposes to limit the thickness of the wiping wave 11 by placing the end of the pad 6, which will preferably be tapered, within the wiping zone 20. The effectiveness will be even better when the back of the pad 6, i.e., its face opposite the strip, is made non-wetting, by nature or by depositing an appropriate coating. In fact, part of the return stream will flow in the back of the pads 6, and it should be avoided that the liquid metal ends up solidifying in this location.

(21) For strips to be coated generally reaching up to 2 meters wide, it is necessary to arrange several pads side by side if the entire width of the strip should be covered. In FIG. 4, the pads 6 are placed on at least one side of the strip 1, and extend transversely essentially over the entire width of the strip 1. For the same reason explained above, the back of each pad 6 advantageously has at least one channel or grooves 17 allowing the return stream to be channeled outside the supports of the hinges. The pads 6 are optionally separated by some distance in the transverse direction and are essentially parallel to one another. Otherwise, they may optionally be in contact via a ceramic felt 18 or be interleaved owing to an assembled labyrinth joint 19 at the level of their adjacent sides opposing the upward stream, which limits the risk of having an overthickness of coating in this location, after wiping.

(22) In a first embodiment shown in FIG. 5 (A), the pads 6 are placed in staggered rows on either sides of the strip 1 shown with its camber defect relative to the reference plane 12. Each pad 6 can be subjected to a same force via its bearing jack or to a particular force (Fi) (i=1, 2, 3, . . . , N). Still according to the invention, a programmable logic control (PLC) can be added to the device for better control of the result while advantageously allowing a measurement of the camber, an analysis of the defect and a closed-loop correction of the forces (Fi).

(23) In the second embodiment shown in FIG. 5 (B), the pads 6 face one another on either sides of the strip 1. Each pair of pads can be subjected to a same force via its bearing jack or to a force differential (Fi)1, (Fi)2 (i=1, 2, . . . , N). Here also, the use of a measurement, analysis and closed-loop correction PLC system can advantageously be considered.

(24) The invention allows, at least under certain operating conditions, to do without the decambering roll 5a and stabilizing roll 5b, which is even more advantageous given that both generate additional vibrations given the wear of their immersed bearings, that they also generate dross and that their upkeep and replacement require line shutdowns affecting the plant's productivity. An embodiment of the invention without the stabilizing roll 5b is shown in FIG. 6.

(25) Other preferred embodiments of the invention may also be considered, differing here by the nature of the dissipation of vibrations achieved. For example, the spring-dashpot system 10 could simply be replaced by the compressed air-internal friction assembly of the jack.

(26) While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below. Additionally, statements made herein characterizing the invention refer to an embodiment of the invention and not necessarily all embodiments.

(27) The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article a or the in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation ofor should be interpreted as being inclusive, such that the recitation of A or B is not exclusive of A and B, unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of at least one of A, B and C should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of A, B and/or C or at least one of A, B or C should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.

LIST OF REFERENCE SYMBOLS

(28) 1 Steel strip 2 Liquid-metal bath 3 Gas knives 4 Sink roll 5a Decambering roll 5b Stabilizing roll 6 Hydrodynamic pads 7 Pad hinge 8 Part of immersed pad 9 Pneumatic jack 10 Spring/damper 11 Wiping wave 12 Reference plane 13 Upward stream 14 Return stream 15 Droplets (splashing) 16 Tapered end of pad 17 Channel (groove) 18 Ceramic felt 19 Interleaved pads (baffle) 20 Wiping zone 21 Programmable logic component (PLC)