Patent Application
20190032188
The present invention relates to a dissipating hydrodynamic device allowing to stabilize a metal strip in continuous motion passing through dryers at the end of a dip-coating operation.
The invention more particularly relates to the field of hot-dip galvanization of a steel strip in continuous motion. The hydrodynamic stabilization of the strip is achieved upon exit from the liquid-metal bath, in the vicinity of the drying device.
The so-called “dip-coating” technique is known, which method is both simple and effective for depositing a coating on the surface of an object. According to this technique, after any preparation of the surface, the object to be coated is immersed in a bath comprising the product to be deposited on said object. The object is next extracted from the bath with the excess liquid being removed, and the coating is made solid, for example by drying, solidification, polymerization, etc.
One of the most widespread applications of this technique is the coating of steel parts such as strips or wires using a metal such as zinc that will next be used for protection against corrosion.
After passing in the liquid-metal bath, the coated part undergoes the drying operation. This operation is one of the most important operations in the dip-coating method, since it allows to control the final thickness of the coating. On the one hand, the drying must be homogeneous over the entire surface of the product, i.e., the width for a strip and the circumference for a wire, and over the entire length of the product to be coated. At the same time, this operation must strictly limit the deposition to the target value, which is typically expressed either in terms of deposited thickness—typically from 3 to 50 μm—, or by weight of the deposited layer per surface unit—typically in gr/m2.
Currently, drying is generally achieved using gas gaps or jets, linear in the case of strips and circular in the case of wires, released from slits and most often oriented perpendicular to the surface to be treated. The gas gaps act as “pneumatic scrapers” and have the advantage of operating without mechanical contact and therefore without any risk of scratching the treated object. Such gaps are called “gas dryers” or “drying blades”. The compressed gas implemented is either air, or a neutral gas such as nitrogen in the most delicate applications, such as the treatment of steel strips intended for the manufacture of visible parts for motor vehicle bodies.
The final thickness of the coating in particular depends on the speed of motion of the strip, on the distance between the strip and the drying blades, and lastly, on the action exerted by the compressed gas jet on the strip.
Yet, it is known that when the strip passes over the bottom roller, it assumes the form of a tile. This plastic deformation must be corrected by means of a second roller, called decambering roller, which imparts a reverse plastic deformation to the strip. Secondarily, a third roller, called stabilizing roller, allows to fix the pass line independently of the decambering. However, poor control of the nesting of the rollers causes residual deformation and therefore deteriorated flatness.
Other phenomena can also alter the flatness of the strip. This may involve heterogeneous quality of the base steel, deteriorated rolling conditions or heating conditions, non-homogeneous cooling and temperature maintenance during the annealing cycle of the strip, before it enters the liquid-metal bath.
Furthermore, certain characteristics of the facility, such as the presence of cooling devices before the upper roller, the off-centered nature of certain rollers, the wear of the rollers or that of the bearings of the immersed rollers, etc., cause vibrations of the strip passing in the dryers.
Ultimately, these flatness defects and these vibrations cause variations in the thickness of the coating that affect the quality of the product and entail zinc overconsumption in order to guarantee a minimal coating thickness for the client.
Furthermore, for a given coating thickness, it is necessary to increase the drying pressure when the speed of the strip increases. Yet, it is known that the motion of the strip cannot exceed a critical speed, beyond which splashing occurs: droplets are torn from the drying wave and are projected on the surface of the bath and on the equipment. This results in significant deterioration of the quality of the product as well as considerable increase in the volume of foam at the surface of the bath.
To solve these problems, builders have proposed to use pneumatic or electromagnetic devices for decambering and stabilizing the strip or still other devices allowing to avoid splashing. It has also been proposed to mount immersed rollers on ceramic bearings or rolling bearings.
Document JP 56 153136 A proposes to arrange at least one pair of pneumatic stabilizers or shock absorbers in positions such that the vibrating length decreases between the bottom roller and the upper roller, which are fixed points for the strip.
Document JP 56 084452 A proposes to use a pneumatic stabilizer in which part of the injected fluid flows along the strip in the direction opposite that coming from the dryers.
Document JP 2005298908 A proposes to avoid splashing by combining a pneumatic cushion with a scraper, where the gas mixes with the liquid to pass under the scraper.
The goal being to stabilize the strip in the dryers, it is necessary for this type of stabilizer to be located in their vicinity, which involves blowing a compressed gas over a coating having a definitive thickness, but not yet solidified, which risks affecting the appearance of the final product. Moreover, these devices do not guarantee the flatness of the strip at the dryers.
Still other devices for hydrodynamic stabilization have been proposed, like in document WO 03/054244 A1. However, this method requires injecting liquid metal into a pipe using a pump. Furthermore, the width of the pipe by which the strip is engaged does not necessarily adapt to the format of the strip, to the coating rate or to the motion speed of the strip.
Furthermore, a certain number of methods are also known for controlling or suppressing vibrations affecting a metal strip in continuous motion based on the implementation of electromagnetic means (see e.g. documents JP 10 298728 A, JP 5 001362 A, JP 9 143652 A, JP 10 87755 A, JP 8 010847 A).
The electromagnetic methods are based on the following principle. Conductors in which a high-frequency current flows are installed on both sides of the steel strip. They induce currents in phase opposition in the strip, Foucault currents. The interaction between the inducing currents and the induced Foucault currents generates a magnetic pressure tending to stabilize the steel strip. Another solution consists in using electromagnets. However, methods of this type involve additional control due to the magnetic attraction force, which tends to make the strip unstable. Moreover, it is known that the high-frequency currents implemented cause a temperature increase in the strip, which is contrary to what is intended in this step of the method.
The teaching of these various techniques does not allow to completely eliminate vibrations or lack of flatness of the strip, which, even if lessened, generally remain at the drying blades. It is therefore in this location that action should be taken, without altering the formation of the coating.
In an embodiment, the present invention provides a facility for dip-coating a metal strip in continuous motion, comprising: 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, which drying blades are configured to inject 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 comprising a plurality of 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 the pads, the plurality of hydrodynamic pads extending transversely across a width of the strip, and positioned such that, when in use, the liquid-metal return stream of the drying wave flows at least in part over backs of the pads.
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:
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.
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.
Embodiments of the present invention avoid the implementation, as in the prior art, of additional gas jets in the immediate vicinity of the dryers that could affect the appearance of the final product.
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 dryers, as well as guarantee a uniform coating thickness in the plane of the strip.
Embodiments of the present invention solve the splashing problem encountered at a high motion speed.
The present invention relates to a facility for dip-coating a metal strip in continuous motion, comprising a liquid coating metal bath, from which the strip exits in a vertical strand, a bottom roller, a decambering roller and, where necessary, a stabilizing roller, all immersed in the liquid-metal bath, drying blades 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 drying wave with a return stream of liquid metal that is oriented downwards, as well as a dissipating hydrodynamic-stabilization device placed between the drying blades and the last immersed roller, 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 said pads, also extending transversely across the width of the strip, and positioned such that, when in use, the return stream of liquid metal from the drying 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.
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 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 m/s (30 and >180 m/min), more preferably up to 10 m/s (600 m/min). 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 mm. 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 drying will preferably be comprised between 3 and 50 μm. The pressurized gas injected by the gas dryers will preferably be air, nitrogen or carbon dioxide.
To make things clear,
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.
More specifically, completely emerged or completely immersed pads 6 advantageously allow to avoid trapping foam 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 dryers. 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 (Rhydrodyn), the thicknesses at the interface, and therefore the operating safety with respect to a risk of contact between the pads and the strip.
It can be seen from
The splashing phenomenon which occurs beyond a critical motion speed of the strip can be schematically seen from
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
In a first embodiment shown in
In the second embodiment shown in
The invention allows, at least under certain operating conditions, to do without the decambering roller 5a and stabilizing roller 5b, which is even more advantageous given that both generate additional vibrations given the wear of their immersed bearings, that they also generate mattes and that their upkeep and replacement require line shutdowns affecting the plant's productivity.
Other preferred embodiments of the invention may also be considered, differing here by the nature of the shock absorption achieved. For example, the spring-shock absorber assembly 10 could simply be replaced by the “compressed air-internal friction” assembly of the jack.
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.
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 of “or” 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016/5073 | Jan 2016 | BE | national |
This application is a U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2017/050379, filed on Jan. 10, 2017, and claims benefit to Belgian Patent Application No. BE 2016/5073, filed on Jan. 29, 2016. The International Application was published in French on Aug. 3, 2017 as WO 2017/129391 under PCT Article 21(2).
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/050379 | 1/10/2017 | WO | 00 |
