Patent Application
20240271264
The present invention relates to a device for cooling the coated surface of a traveling strip, preferably a coated metal strip at the outlet of a liquid metal bath. The invention particularly relates to a device intended to “freeze” the liquid skin of a thick coating laid on a relatively “thick” metal strip (thickness>2 mm), just after wiping, while avoiding complete solidification of the coating, especially in case of coatings with a thickness greater than 25-30 microns. In this context, “freeze” shall be understood as selectively solidifying the skin (external) layer of the coating, at the exclusion of the bulk layer thereof.
Driven by the market of galvanized steel sheets for extended corrosion protection, steel producers are requiring increased zinc coating layer thickness, preferably above 25 microns and up to about 60 microns, while a standard zinc coating layer is typically between 7 to 20 microns.
The market is also asking for products without a defect called spangle which appears as the molten zinc coating on the galvanized steel begins to cool below the melting point of zinc (419° C.). It is known that, for avoiding such defects, the molten zinc bath composition should be Pb and/or Sb-free
As shown in
After wiping, the coating is usually cooled through forced air cooling 4 in an upward cooling tower before being deflected at a top roll 12 and further cooled by forced air cooling and/or water cooling 4′. Between the nozzles and the first cooler, natural cooling takes place. The heavier the coating weight and the thicker the strip, that means a strip with a thickness over 2 mm, the longer the cooling required. The cooling speed by the forced air cooler 4 shall be adjusted according to the solidification state of the coating. Specifically, when the surface of the coating is still liquid, forced cooling must be limited. Water trapped inside liquid zinc can burst due to its transformation to gaseous form and this may lead to safety issue.
When the coating thickness increases at values typically higher than 25 microns, and due to the fact that it takes a certain time between the coating adjustment and solidification, the coating tends to flow downward under the gravity and also owing to its low viscosity. Because of the existence of a significant time between coating adjustment and solidification, the length along which the coating can flow becomes high. The thicker the coating, the longer will be the flowing length. This gravity-caused flow defect is also called “angel wing”.
The inventors have computed that the mass of the liquid flowing down varies with the cubic power of the coating thickness. In some situations, this flow downward is disturbed either by the oxide formed on the surface of the liquid or by the roughness of the substrates or finally by intermetallic particles that are inevitably entrained by the strip going out of the coating pot. Therefore, according to prior art methods, the coating becomes non-uniform and strong waves are formed.
Very strangely, the inventors have found that the disturbance on the down flow and so the attached coating thickness non-uniformity is more pronounced close to the strip edges and that the attached defects, looking like drips, most usually form an angle of about 45° with the running direction of the strip (see
Document KR20110064506A discloses a device and method for manufacturing a zero spangle of a Zn—Al alloy hot-dip plated steel sheet that are provided to prevent spangle difference due to the cooling difference of a transition surface using a magnetic strip stabilizing unit when mist is sprayed. The water supply pipe is connected to the spraying nozzle in order to supply water to the spraying nozzle. The air supply pipe is connected to the spraying nozzle in order to supply air to the spraying nozzle. The water control valve is installed on the water supply pipe and controls the amount of water to be supplied to the spraying nozzle. This application refers to cooling in the tower to improve the cooling rate especially on a thick strip. The control of solidification and/or the viscosity of the coating when cooling is not the target. It uses a mixture or air and water at the nozzles. In addition devices must be implemented to avoid that water droplets fall down due to gravity. In this document the amount of coating is adjusted at a thickness of 14 μm.
Document KR102004971B1 has the objective of providing a method and an apparatus for manufacturing a hot-dip galvanized steel sheet which efficiently cool a plating layer in a manufacturing process of a hot-dip steel sheet to stably obtain a hot-dip galvanized steel sheet with an elegant surface without fitting defects, drop mark defects, and linear Moiré fringe defects. The apparatus for manufacturing a hot-dip galvanized steel sheet comprises a plating port, a gas wiping device, and a cooling chamber. The cooling chamber is vertically driven by a cooling chamber driving device, and includes an ionic wind generation device generating ionic wind and a spray unit spraying a solution. In this document the amount of coating is adjusted at a thickness of 20 μm.
The purpose is to control the grain size and especially the spangle size. It targets a coating thickness below 20 μm and has the purpose to remove the spangles that appear during solidification. The system is using a propellant such air, under electrical field acceleration, which is a technology quite difficult to implement in coating lines
Both technologies above are dedicated either to control the solidification and/or improve the total cooling rate in the tower
In an embodiment, the present invention provides an installation for adjusting a thickness of a hot liquid coating on a traveling strip with suppression of “angel wing” flow defects, the traveling strip having a thickness equal to or greater than 2 mm, and for cooling the coating, the coating thickness being in a range above 25 μm, the installation successively comprising, from a bottom upwards, on either side of the traveling strip: a wiping device comprising gas knives configured to wipe excess liquid from the coated strip at an outlet of a liquid bath: at least one conventional gas cooling header with gas blowers configured to solidify the coating; and a water cooling and freezing header configured to produce airless spraying of demineralized water for skin freezing of the coating, the header being located as close as possible to the wiping device and between the wiping device and the at least one conventional gas cooling header, the water cooling and freezing header comprising one or more spraying nozzles, the one or more spraying nozzles being at a distance of between 50 and 300 mm from the traveling strip in use, the water cooling and freezing header being configured to spray water droplets with a size between 50 and 500 μm, with a water flow at a pressure of between 2 and 5 bar being between 0.1 and 5 m3/h, so as to increase a viscosity of an external surface or skin of the liquid coating, and not an entire bulk thickness thereof, before coating solidification is completed in the at least one conventional gas cooling header.
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:
A problem solved in the present invention is totally different from those addressed by the above-mentioned technologies of prior art because it addresses quality of a coating not making spangles due to well-chosen specific pot composition but also of a coating which is much thicker. The present invention does not therefore target the total solidification of the coating as targeted above.
In an embodiment, the present invention provides a cooling device preferably used as close as possible to the wiping device, allowing to “freeze” the skin or external surface of a coating while avoiding the complete solidification of this coating.
A purpose of the invention is to freeze the surface of the coating quickly while avoiding its full solidification in order to avoid wave effects and further flow defects such as “angel wing” defects for example, when high thickness of coating is applied on a strip, especially with a thickness greater than 25-30 microns.
In an embodiment, the invention deals with a cooling method that only uses water spray without addition of air in the nozzle or some electrical device as proposed by the prior art technologies.
In an embodiment, the invention allows to obtain an improved coating uniformity of the traveling strip, due to the freezing of the skin of the coating after the wiping by local change of its viscosity, and before performing the complete solidification for example by forced cooling. The solidification of the skin of the coating would then be performed without damaging the coating surface.
The present invention firstly relates to an installation for adjusting the thickness of a hot liquid coating on a traveling metal strip without flow defects due to thick coating, said traveling strip having a thickness equal to or higher than 2 mm, and for cooling said coating, the coating thickness being in a range above 25 μm, said traveling strip being preferably a metal strip dip-coated in a bath of liquid metal, said installation successively comprising, from the bottom upwards, on either side of the traveling strip in use, a wiping device comprising gas knives for wiping excess liquid from the coated strip at the outlet of a liquid bath and at least one conventional gas cooling header with gas blowers for completely solidifying said coating, wherein a water cooling and freezing header producing spraying of demineralized water, without air addition (airless spraying), for skin freezing of said coating is located on the strip path as close as possible of the wiping device and between the wiping device and the conventional gas cooling header, said water cooling and freezing header comprising one or more spraying nozzles, the spraying nozzles being at a distance between 50 and 300 mm from the traveling strip in use, the water cooling and freezing header being configured to spray water droplets with size between 50 and 500 μm, with a water flow at a pressure between 2 and 5 bar being between 0.1 and 5 m3/h, to give the droplets hitting the coated strip sufficient impulse without need of another accelerating device such as electrostatic or similar device, so as to increase the viscosity of an external surface or skin of the liquid coating, and not the entire bulk thickness thereof, before coating solidification is completed in the gas cooling header containing gas blowers. The said increase of viscosity contributes to erase the small pits induced by the droplet flow and thus to “self-repair” of the coating.
According to preferred embodiments of the invention, the installation is further limited by one of the following features or by a suitable combination thereof:
The present invention also relates to a coating control process for adjusting the thickness of a hot liquid coating on a traveling strip, and for cooling said coating, said traveling strip being preferably a metal strip dip-coated in a bath of liquid metal, using the installation described herein, comprising the following successive steps:
According to preferred embodiments of the invention, the coating control process is further limited by one of the following features or by a suitable combination thereof:
In the drawings above, the metal strip is travelling in a plane perpendicular to the plane of the figure.
After detailed simulations and analyses, the inventors discovered that the problem of non-uniformity of thick coatings was due to the long time elapsed between the thickness adjustment and solidification of the coating as explained above. Usually, the strip passes firstly through a wiping device for the coating adjustment, and secondly through a forced cooling device, such as gas blowing device for example, for cooling and solidifying of the coating. The coating tends then to flow down under the gravity and also due to its low viscosity.
For example, with a zinc coating, the wiping process is usually done at 460° C., whereas it is well-known that full solidification occurs at 420° C. The inventors have observed that in the classical industrial process where cooling is done by natural convection, a typical time to solidification of a 2 mm strip with is about 12 to 14 seconds and, as expected, double when the strip is 4 mm thick. This cooling rate is even much faster that what would be predicted by the well-known natural convection coefficient and this is most probably due to the fact that the strip is running.
According to the present invention, in a first step, the coating is frozen on its external surface and, in a second step, cooled and it is usually done with the conventional technology to make full solidification of the coating. Concretely, the first step is performed by water spraying, preferably under the form of demineralized water, and consists in “freezing” as explained above. The second step is performed by classical air blowing in order to make the strip coating solidification as well as final cooling. Device 1 of the present invention advantageously allows to freeze the coating quickly in order to avoid wave effects and defects when high thickness of coating is applied on the strip. Also, it allows to use forced cooling at an earlier stage in the cooling process and thus prevents non uniform movement of coating along the width.
The present invention intends to avoid the above-mentioned non-uniformity in case of thick coatings. To this end, and as illustrated in
The inventors have also observed that the freezing of the surface cannot be obtained by simple air cooling because, given the required heat transfer coefficient, it would be needed to blow the cooling gas so strongly that this would damage the liquid coating. There is then a risk of explosion or craters.
The inventors have also found that mixture or air and water at the nozzle, in addition to complexity of the system, does not allow to reach the objective due to improper size of the droplets as well as the amount of water required. Similarly, using electrostatic acceleration of the droplets is not effective in that process because of the thick coating and the impulse required to be given to the droplets.
The device 1 according to the present invention comprises a water spraying device 3 without addition of air, having a plurality of nozzles 6, provided in a casing or plenum or header supplied with water.
According to one embodiment, the water cooling system 3 comprises several ramps located inside a header. Two headers are provided in the water spraying system 3 and located at equal distance on either side of the strip 2. Each ramp is equipped with specific nozzles 6, for example attached approximately every 100 mm and fed with demineralized water. The inventors found out that the mixture of entrained air and water inside the header located right above the wiping causes the solidification of the skin of the coating to be performed without damaging the coating surface. Only water is supplied by the nozzles, but ambient air is entrained by the droplets of water, and takes part to the freezing effect.
According to some embodiments, the excess of water sprayed on the liquid metal and that inevitably escapes is collected by specific devices located at the top and bottom of the water cooling system 3. As all the sprayed water is not vaporized, dedicated collectors are implemented to collect most of the residual liquid water. The collecting system is preferably based on mechanical devices, such as grids and baffle plates, plates, honeycombs, perforated sheets or similar, instead of vacuum systems that are never easy to adjust properly. For example, pans can be located under the header 3 in order to collect the overflow of demineralized water. This overflow is then rejected to the waste water system. A dedicated device can be also be located on the upper part of the header 3 in order to limit the quantity of water/vapor going outside of the casing due to the spraying on the strip 2. The demineralized water flow can also be adjusted depending of the strip speed and process requests.
According to a preferred embodiment, the water spraying device 3 and the lower air cooling system 4 (or the air cooling system which is the closest to the water spraying device 3) are mechanically connected to each other. This allows providing a common vertical movement of approximately 1500 mm for example. The purpose of this vertical movement is to adapt the position of the water spraying device 1 to the exact position of the air knives of the wiping device 5 during process. Further, water cooling system 3 is implemented in a box or header preferably made of stainless steel that can be adjusted more or less close to the wiping system 5, possibly on demand during production. This box is provided with special accessories, such as hoses and fast connections resisting to hot areas and allowing an easy connection by an operator.
The inventors have also found that the amount of water to be used cannot be too high and there is an optimized droplet size to avoid the formation of pits in the coating. The size of the droplets, very important to avoid defects on the coating, has to be adapted in function of different parameters (distance between the water nozzles and the strip, type of coating, line speed, etc.). The water flow at a pressure comprised between 2 and 5 bar is advantageously between 0.2 and 5 m3/h depending on the strip width and line speed, and still preferably between 0.3 and 1 m3/h for a 1500 mm strip width and with droplet size between 50 and 500 μm, and still preferably between 100 and 300 μm. Preferably, the water cooling system has a water consumption in the range of 0.1 to 1 m3/h per meter of width of steel sheet.
Preferably, the water cooling header has a length smaller than two meters and preferably close to one meter as the objective is not to solidify the coating totally but only to freeze its surface.
In the embodiments of the present invention, the nozzles 6 are preferably distributed along all the width of the strip 2 in which the air water mixture is made.
Preferably, the coating is made of Pb- and Sb-free zinc-aluminium alloy containing optionally Sn, Mg. Fe and inevitable impurities, and has a thickness between 15 and 60 μm.
Preferably, the water temperature is set between room temperature and 90° C. depending on the target aspect to obtain.
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 |
|---|---|---|---|
| 21190098.0 | Aug 2021 | EP | regional |
This application is a U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2022/057748, filed on Mar. 24, 2022, and claims benefit to European Patent Application No. EP 21190098.0, filed on Aug. 6, 2021. The International Application was published in English on Feb. 9, 2023 as WO/2023/011767 under PCT Article 21(2).
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/057748 | 3/24/2022 | WO |
