Abstract:
A method for the application of an aqueous treatment solution to the surface of a strip which is moving at a preset trip speed, includes the following steps: spraying the aqueous treatment solution onto at least one rotating roll of a roll coater with a rotary sprayer having spray rotors disposed side by side in the longitudinal direction of the roll, to which the aqueous treatment solution is supplied and which are made to rotate by a spray rotor drive so that the treatment solution, driven by the centrifugal force, is sprayed in the form of a spray jet onto the surface of the roll; and applying the aqueous treatment solution to at least one surface of the strip by moving the strip past a rotating application roll of the roll coater, with the application roll transferring the aqueous treatment solution in the form of a wet film onto the surface of the strip. Additionally, related to the method, there are an application device having a roll coater and a rotary sprayer as well as to a system for carrying out the method.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for the application of an aqueous treatment solution to the surface of a strip, in particular a steel strip, which moves at a preset strip speed. In addition, the present invention also relates to an application device for the application of an aqueous treatment solution to the surface of a strip as well as to a system for carrying out the method which comprises such an application device. 
       BACKGROUND 
       [0002]    It is known from the prior art that an optionally coated surface of a steel metal sheet can be treated with an aqueous treatment solution so as to render the steel metal sheet resistant to oxidation and to lower the coefficient of friction to ensure that the steel metal sheet can be more easily processed and shaped during a subsequent processing step, for example, during the production of packaging containers. The publications DE 10 2005 045 034 A1 and DE 10 2012 102 082 B3, for example, disclose methods for passivating the surface of metal-coated steel strips in which an aqueous solution of a treatment agent is sprayed onto a steel strip which moves at a certain strip speed. 
         [0003]    An alternative to spraying an aqueous treatment solution to the surface of the strip is the application of the treatment agent by means of a dipping method. In order to ensure a homogeneous distribution of the treatment agent across the entire surface area of the strip, it is necessary, both when using the prior-art spray methods and when using the dipping methods, to apply the aqueous solution in excess of the amount required to the surface of the strip and to remove the excess portion of the treatment solution, for example, by means of squeeze rollers. As a result, both the conventional spray methods and the dipping methods have the disadvantage that relatively large amounts of the aqueous treatment solution are required and that an excess portion of the treatment solution, which is removed from the surface of the strip, for example, by means of squeeze rollers, must be collected in receiving tanks and transported to a recycling station. In addition, the prior-art spray and dipping methods frequently lead to an inhomogeneous application of the aqueous treatment solution to the surface of the strip. This is especially the case when the strip is moved at a high strip speed, for example, of more than 400 m/min through a dipping bath containing the treatment solution or when an aqueous treatment solution is applied by spraying it onto a strip which is moved at such strip speeds. 
         [0004]    To avoid these drawbacks, DE 10 2013 107 505 A1 proposes a method for the application of an aqueous treatment solution to the surface of a steel strip which moves at a strip speed of 200 to 700 m/min, in which, after the steel strip has been dried with a gas stream, the aqueous treatment solution is applied to at least one surface of the steel strip by means of a rotary sprayer, said rotary sprayer having a plurality of spray rotors disposed side by side at a right angle to the travel direction of the strip, to which the aqueous treatment solution is supplied and which are made to rotate by a drive so that the treatment solution, driven by the centrifugal force, is sprayed in the form of a spray jet onto the surface of the steel strip, thereby coating said surface with a wet film of the aqueous treatment solution. The wet film of the aqueous treatment solution which has been applied by means of the rotary sprayer is then homogeneously distributed on the surface by means of driven smoothing rollers and is subsequently dried. However, when spraying the aqueous treatment solution by means of a rotary sprayer, it was found that, because of the droplet-like application, it is possible for inhomogeneities to form on the surface of the steel strip, which, after the sprayed-on wet film has dried, create a spot-like appearance. 
       SUMMARY 
       [0005]    Taking this as a starting point, the problem to be addressed by at least some embodiments of the present invention is to devise a method for the application of an aqueous treatment solution to the surface of a moving strip, by means of which the most homogeneous possible distribution of the treatment solution on the surface of the strip is made possible, without risking the development of spot-like appearance on the surface of the strip after the treatment solution has dried. In addition, another problem to be addressed by at least some embodiments of the present invention is to create a system for carrying out the method, by means of which an aqueous treatment solution can be applied as efficiently as possible and with little waste to the surface of a moving steel strip while ensuring the most homogeneous distribution. 
         [0006]    These problems are addressed by means of a method with the features as disclosed herein, with an application device with the features as disclosed herein, and with a system for carrying out the method with the features as disclosed herein. Preferred embodiments of the method and of the devices according to this invention are also disclosed. 
         [0007]    In the method according to the present invention, using a rotary sprayer, an aqueous treatment solution is first sprayed onto at least one rotating roll of a roll coater, said rotary sprayer having a plurality of spray rotors disposed side by side in the longitudinal direction of the roll, to which the aqueous treatment solution is supplied and which are made to rotate by a spray rotor drive so that the treatment solution, driven by the centrifugal force, is sprayed in the form of a spray jet onto the surface of the roll. The aqueous treatment solution is subsequently applied to at least one surface of the strip which moves at a preset strip speed, by moving the strip past a rotating application roll of the roll coater, with the application roll transferring the aqueous treatment solution in the form of a wet film to the surface of the strip. 
         [0008]    In a first embodiment of the present invention, the rotating roll of the roll coater, onto which the treatment solution is first sprayed by means of the rotary sprayer, can be the application roll itself. However, in an alternative embodiment of the invention, the rotating roll, onto which the treatment solution is sprayed by means of the rotary sprayer, can also be a transfer roll which, relative to the application roll of the roll coater, is disposed in such a manner that their axes run parallel to each other and their surfaces are in contact with each other. In this embodiment of the present invention, the aqueous treatment solution is first sprayed by means of the rotary sprayer onto the rotating transfer roll and transferred by this transfer roll to the application roll in rolling contact with said transfer roll. The application roll applies the aqueous treatment solution to at least one surface of the strip by moving the strip at the strip speed past the rotating application roll. In the process, the transfer roll and the application roll rotate in opposite directions to each other and preferably at the same or at least approximately the same tangential speed. 
         [0009]    In both embodiments of the present invention, the surface of the application roll in a transfer zone is in contact with the surface of the strip, with the direction of the tangential speed of the surface of the rotating application roll in this transfer zone corresponding to the travel direction of the strip which is (linearly) moving at the strip speed. 
         [0010]    As a rule, the strip speed is greater than 200 m/min and corresponds, for example, to the strip speed at which the steel strips in strip coating lines (such as strip tinning lines) are moved and which typically is in a range between 200 and 700 m/min. The value of the tangential speed of the rotating application roll corresponds substantially to at least the value of the strip speed. This allows an efficient transfer of the aqueous treatment solution from the surface of the application roll to a surface of the strip, without the possibility of friction or slippage developing between the moving strip and the rotating application roll. 
         [0011]    Prior to applying the aqueous treatment solution, it is advisable to dry the strip with a gas stream before the strip comes into contact with the surface of the rotating application roll in the transfer zone. Drying the strip preferably takes place by means of an “air knife” which blows a laminar stream of hot air onto the surface of the moving strip. 
         [0012]    To coat the surface of the strip as homogeneously as possible with the treatment solution, it may be useful to homogeneously distribute the wet film of the aqueous treatment solution, which has been applied to the surface of the strip by means of the rotary sprayer, using driven smoothing rollers. To this end, driven rotating smoothing rollers are disposed in the travel direction of the strip downstream of the transfer zone. Either immediately after application of the aqueous treatment solution to the surface of the strip by means of the application roll or after homogeneously distributing the wet film, which has been applied by means of the application roll, using the smoothing rollers, the wet film of the treatment solution is dried so as to obtain a dry film coating. Drying can take place in a furnace or by exposing the treatment solution-coated surface of the strip to infrared radiation. 
         [0013]    To apply a wet film coating to the surface of the strip as efficiently as possible, the amount of the aqueous treatment solution supplied per unit of time to the spray rotors can be adjusted to the speed of the strip. As a result of such an adjustment, only the amount of aqueous treatment solution actually required to produce a desired dry film thickness of the treatment solution is applied by means of the spray rotors to the rotating rolls of the roll coater (possibly directly to the application roll) to the application roll nd from the application roll to the surface of the strip that is to be treated. In addition, this measure also makes it possible to change the strip speed during operation of the system without applying an excessively high or excessively low amount of the treatment solution to the application roll of the roll coater. In this manner, it is possible, on the one hand, to maintain a desired dry film thickness of the treatment solution and, on the other hand, to avoid the build-up of excess treatment solution on the surface of the strip. 
         [0014]    To supply the rotary sprayer with the required amount of aqueous treatment solution, it is useful to link the rotary sprayers to a supply system which comprises a supply line which is connected to the spray rotors of the rotary sprayers, on the one hand, and to a storage tank for the treatment solution, on the other hand. The supply system is preferably automatically controllable via a control system, with the control system also controlling the strip speed of the strip and adjusting the amount of the treatment solution, which is supplied per unit of time to the spray rotors, to the strip speed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    These and other features and advantages of the method disclosed by the present invention and of the devices according to the present invention follow from the practical example described in greater detail below with reference to the accompanying drawings. The drawings include: 
           [0016]      FIG. 1 : A schematic representation of a first practical example of a system for carrying out the method disclosed by the present invention; 
           [0017]      FIG. 2 : A detail view of a section of the system shown in  FIG. 1  in the area of the rotary sprayer and the application roll of the roll coater; 
           [0018]      FIG. 3 : A lateral view of a second embodiment of a system for carrying out the method disclosed by the present invention, which comprises a first application device according to the present invention on one side of the strip to be treated and a second application device according to the present invention on the other side of the strip. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  is a schematic representation of a system for carrying out the method disclosed by the present invention for applying an aqueous treatment solution to the surface of a moving strip in the form of a steel strip. The steel strip  1  is guided over a plurality of deflection rollers U and moves at a preset strip speed v into the travel direction of the strip which in  FIG. 1  is designated by an arrow. As a rule, the strip speed is greater than 200 m/min and can be as high as 750 m/min. The steel strip  1  can be a cold-rolled steel strip which is coated with a metal coating, for example, a tinned sheet iron strip or a galvanized steel strip. However, it can also be an uncoated steel sheet strip, for example, a black sheet iron strip. 
         [0020]    The steel strip  1  is moved by a transporting system (not shown) at a preset strip speed v in the travel direction of the strip and in the course of this process is guided over deflection rollers U. First, the steel strip  1  is passed through a first drying unit  4  where the surfaces of the steel strip  1  are dried and cleaned. The first drying unit  4  can be, for example, an “air knife” which blows a laminar stream of hot air onto the surfaces of the steel strip  1  moving at the strip speed v in order to dry the surfaces of the steel strip and to blow off undesirable foreign particles. 
         [0021]    Downstream of the first drying unit  4  is an application device  10  for the application of an aqueous treatment solution to the surface of the strip  1  which is moved at the preset strip speed in the travel direction v of the strip. The application device  10  shown in detail in  FIG. 2  comprises a rotary sprayer  2  and a roll coater  20 . The rotary sprayer  2  has a plurality of spray rotors  3  which are disposed side by side at a distance from one another at a right angle to the travel direction of the strip. The spray rotors  3  are connected to a storage tank  9  via a central supply line  6  and branch lines  6   a ,  6   b ,  6   c , etc., branching off said central supply line. The storage tank  9  holds the aqueous treatment solution which is to be applied to the surface of the steel strip. The aqueous treatment solution is preferably pumped by means of a pump  8  into the supply line  6  from where it is conducted into the branch lines  6   a ,  6   b ,  6   c , with each branch line being connected to one of the spray rotors  3 . To measure the amount of the aqueous treatment solution being pumped into the supply line  6 , a flowmeter  11  is preferably provided. 
         [0022]    The aqueous treatment solution is supplied to the spray rotors  3  of the rotary sprayer  2  via the supply line  6  and the branch lines disposed thereon. Each of the spray rotors  3  has a plate which is made to rotate by a drive. Because of the rotation of the plate of the spray rotors  3 , the aqueous treatment solution supplied is moved by the centrifugal force outwardly to the rim of the plate. The rim of the plate is shaped in such a manner that the aqueous treatment solution in the form of minute droplets is propelled from the rim of the rotating plate. As a rule, the diameter of the droplets, as a function of the viscosity and the surface tension of the treatment solution used, measures between 30 and 70 micrometers. The droplets of the treatment solution being propelled from the rim of the plate are sprayed all around the rotating plate of the spray rotors  3 . 
         [0023]    The spray rotors  3  of the rotary sprayer  2  are disposed at a distance from the roll coater  20 . The roll coater  20  comprises a driven rotating application roll  21  which is linked to a drive motor (not shown) which causes the application roll to rotate. The rotating spray rotors  3  of the rotary sprayer  2  spray the aqueous treatment solution onto the surface of the application roll  21  of the roll coater  20 . To this end, the spray rotors  3  are disposed at a right angle to the travel direction of the strip and along the longitudinal axis of the application roll  21  in such a manner that the spray jets  12  of the adjacent spray rotors  3  overlap one another on the surface of the application roll  21 , thereby ensuring a uniform application of the aqueous treatment solution across the entire length of the application roll  21 . 
         [0024]    The rotating application roll  21  applies the treatment solution adhering to its outer circumference to a surface of the strip  1  which is linearly moving at the strip speed along the travel direction v. To this end, in a transfer zone  23 , the moving strip  1  is in contact with the surface of the rotating application roll  21  of the roll coater  20 . As a result of the contact between the surface of the strip that faces the application roll  21  and the surface of the application roll  21 , the treatment solution adhering to the surface of the application roll  21  is transferred to the surface of the strip. The direction of rotation and the speed of rotation are preferably adjusted to the travel direction v of the strip and to the strip speed so that, in the transfer zone  23 , the direction of the tangential speed on the surface of the rotating application roll  21  corresponds to the travel direction v of the strip and the value of the tangential speed is at least substantially equivalent to the strip speed. 
         [0025]    The amount of aqueous treatment solution supplied per unit of time to the spray rotors  3  is preferably adjusted to the strip speed v at which the steel strip  1  is moving. There is a linear correlation between the amount of the treatment solution supplied per unit of time to the spray rotors and the strip speed v. As a rule, the amount M the of treatment solution supplied per unit of time Δt to the spray rotors and relative to the width B of the steel strip  1  varies between M/Δt·B=0.4 and 5.5 L per minute and meter and preferably is in a range between M/Δt·B=1.0 and 3.5 L per minute and meter. At a typical strip speed of 200 to 700 m/min, the amount of the wet film of the treatment solution sprayed by means of the spray rotors  3  onto the surface of the steel strip  1  measures between  1  and 8 mL/m 2 , preferably between 1 and 3 mL/m 2 , and most preferably approximately 2 mL/m 2 . 
         [0026]    After application of the aqueous treatment solution in the form of a wet film to the surface or to each surface of the steel strip  1 , the steel strip  1  can optionally be passed between driven rotating smoothing rollers  5   a ,  5   b . The smoothing rollers  5  serve to homogeneously distribute the applied wet film of the aqueous solution. To this end, preferably a pair of smoothing rollers  5  is used, having two smoothing rollers  5  an and  5   b  that are offset with respect to each other. The offset arrangement of the smoothing rollers  5   a ,  5   b  is illustrated in  FIGS. 1 and 2 . As  FIGS. 1 and 2  show, the smoothing rollers  5   a ,  5   b  are disposed with respect to each other in such a way that the line connecting the axes of rotation of the smoothing rollers, which run parallel to each other and parallel to the surface of the steel strip, in the cross section with the steel strip  1  passing between the two smoothing rollers encloses an angle of approximately 30° to 60°, in particular an angle of approximately 45°. In the process, the smoothing rollers do not exert substantial contact pressure onto the surface of the strip. This means that, if at all, only a very small portion of the treatment solution is squeezed off the surface of the strip. The pair of smoothing rollers  5  basically only serves to homogeneously distribute the wet film of the treatment solution across the entire surface of the strip. In this manner, a uniform application of a wet film of the treatment solution, with a homogeneous layer thickness across the entire surface of the strip, is ensured, and the accumulation of excess treatment solution that needs to be collected and transported to a recycling station is prevented. 
         [0027]    After the steel strip  1  has passed through the smoothing rollers  5 , it is transported to a second drying unit  7 . The second drying unit  7  can be, for example, a drying furnace or an infrared or hot air dryer. 
         [0028]    After the drying step, a uniform dry film coating of the treatment solution remains on the surface or on each surface of the steel strip  1 , with the dry film weight after drying, as a rule, measuring between 1 and 50 mg/m 2  and preferably between 2 and 30 mg/m 2 . The dry film weight of the treatment solution most preferably measures approximately 10 mg/m 2 . 
         [0029]      FIG. 3  shows a lateral view of another embodiment of a system for carrying out the method according to the present invention. This embodiment of the system for carrying out the method according to the present invention comprises a first application device  10  and a second application device  10 ′, with the first application device  10  being disposed on one side of the strip  1  to be treated and with the second application device  10 ′ being disposed on the other side of strip  1 . The first application device  10  comprises a roll coater  20  having a single rotating roll, i.e., an application roll  21 . As in the practical example of  FIG. 2 , the aqueous treatment solution is sprayed onto this single roll  21  by means of a rotary sprayer  2  disposed at a distance from roll  21 . 
         [0030]    The rotating application roll  21  which, in the practical example of  FIG. 3 , rotates in the counterclockwise direction applies the aqueous treatment solution to a surface of strip  1  which, at the strip speed, is moving linearly along the travel direction v of the strip. To this end, the moving strip  1 , in a transfer zone  23 , is in contact with the surface of the rotating application roll  21  of the roll coater  20 . As a result of the contact between the surface of the application roll  21  and surface O of the strip which faces the application roll  21 , the treatment solution adhering to the surface of the application roll  21  is transferred to surface O of the strip. 
         [0031]    On the other side of strip  1 , i.e., on the side facing the second surface O′ of the strip, a second application device  10 ′ is disposed. This second application device serves to apply the aqueous treatment solution to the second surface O′ of strip  1  and also comprises a roll coater  20 ′ as well as a rotary sprayer  2 ′. In contrast to the first application device  10 , the roll coater  20 ′ of the second application device  10 ′ comprises two rotating rolls, i.e., a first rotating roll  21 ′ in the form of an application roll and a second rotating roll  22  in the form of a transfer roll. The application roll  21 ′ and the transfer roll  22  are driven to rotate, and the axes of the two rotating rolls  21 ′,  22  run parallel to each other. In a contact zone  24 , the surfaces of the two rotating rolls  21 ′,  22  are in contact with each other. The two rotating rolls  21 ′ and  22  preferably rotate at the same tangential speed on their respective surfaces and in opposite directions to each other. 
         [0032]    The rotary sprayer  2 ′ with a plurality of spray rotors  3  disposed side by side in the longitudinal direction of the transfer roll  22  is disposed at a distance from the transfer roll  22  and running parallel thereto. By means of the rotary sprayer  2 ′, the aqueous treatment solution is first sprayed onto the transfer roll  22  and, in the contact zone  24 , transferred from said transfer roll to the application roll  21 ′. In a transfer zone  23 , the surface of the rotating application roll  21 ′ is in contact with surface O′ of strip  1  so as to transfer the aqueous solution located on the surface of the application roll  21  to surface O′ of strip  1 . The direction of rotation and the speed of rotation are adjusted to the travel direction of the strip and to the speed of the strip in such a manner that in the transfer zone  23 , the direction of the tangential speed on the surface of the rotating application roll  21  is equivalent to the travel direction v of the strip and the value of the tangential speed coincides at least approximately with the strip speed. 
         [0033]    Using the system shown in  FIG. 3 , both surfaces O, O′ of strip  1  can be coated with a wet film of the aqueous treatment solution. As in the practical example shown in  FIG. 1 , a device for homogeneously distributing or smoothing the wet film, in particular two smoothing rollers  5   a ,  5   b  which are offset with respect to each other, can follow the two application devices  10 ,  10 ′ downstream in the travel direction of the strip. 
         [0034]    To make it possible to replace the rolls  21 ;  21 ′,  22  of the roll coaters  20 ;  20 ′ of the system shown in  FIG. 3 , it is recommended that the roll coaters  20 ,  20 ′ and the associated rotary sprayers  2 ,  2 ′ be disposed on sliding carriages  25 ,  25 ′. The carriages  25 ,  25 ′ are able to slide at a right angle to the travel direction v of the strip so that the first application device  10  as well as the second application device  10 ′ can be moved away from strip  1  when the rotating rolls of the roll coaters  20 ,  20 ′ need to be replaced. Via the sliding carriages  25 ,  25 ′, it is also possible to set the contact pressure of the application rolls  21 ,  21 ′ of the two roll coaters  20 ,  20 ′ on the surfaces O and O′, respectively, of strip  1 . To this end, when carrying out the method, it is recommended that the carriages  25 ,  25 ′ can be locked in any position so as to be able to set a constant and consistent contact pressure of the application rolls  21 ,  21 ′ on strip  1  that is passing between the rolls. 
         [0035]    The present invention is not limited to the embodiments that are illustrated in the drawings and that have been described in detail. Thus, if only one surface of strip  1  is to be coated with the aqueous treatment solution, it is possible to dispose an application device  10  or  10 ′ only on one side so as to coat the surface O or O′ facing the application device  10  or  10 ′, respectively, with the aqueous treatment solution. In addition, apart from the application roll  21  and an optionally provided transfer roll  22 , it is also possible to dispose additional rotating rolls, the axes of which run parallel to the axis of the application roll  21 , in the roll coater  20 . If a plurality of such transfer rolls  22  are provided in the roll coater, it is recommended that a rotary sprayer  2  be dedicated to each transfer roll, by means of which rotary sprayer the aqueous treatment solution can be sprayed to the respective transfer roll. In this case, in a contact zone, each of the transfer rolls is in contact with the surface of the application roll  21  so as to transfer the aqueous treatment solution from the respective transfer roll  22  to the application roll  21 . 
         [0036]    The application device  10  according to the present invention can essentially be used to apply any aqueous treatment solutions to one or both surfaces of moving strips and is especially suitable for use for the treatment of steel strips. The steel strips may be coated with an anti-corrosion metal coating, for example, a tin coating. The aqueous treatment solutions can, for example, be passivating solutions, such as are known, for example, for passivating tinned sheet iron or black sheet iron. Suitable treatment solutions are listed, for example, in DE 10 2013 107 505 A1. If necessary, prior to applying the aqueous treatment solution, the strip can be pretreated, for example, by means of anodic oxidation or cleaning and/or drying the surface(s) of the strip.