EP1728888A1 - Device for the pneumatic stabilisation of a continuously running metallic strip - Google Patents

Abstract

The pneumatic stabiliser, designed to deliver a gas such as air, nitrogen or carbon dioxide under pressure to at least one side of a continuously-running metal strip (1), especially a steel strip leaving a hot galvanising bath of molten zinc (2), consists of a housing (5) with at least two tranverse gas outlet slits (6) covering the width of the strip with the space between the slits covered by a perforated plate (8). The plate covers a housing cavity (7) with side walls (17) and a back (27), containing a moving plate (9) connected to a passive damper (10) essentially in the form of a damped harmonic oscillater. The gas flows from the slits can be delivered perpendicular or at an oblique angle to the moving strip's surface.

Description

Object of the invention

The present invention relates to a dissipative pneumatic device for stabilizing a metal strip in continuous scrolling through wipers after a dip coating operation.

The invention relates more particularly to the field of hot dip galvanizing of a steel strip in continuous motion. The pneumatic stabilization of the strip is effected at the outlet of the bath of liquid metal, in the vicinity of the wiper device.

Technological background and state of the art

The technique known as "dip coating" is known, which constitutes a method that is both simple and effective for depositing a coating on the surface of an object. According to this technique, after a possible preparation of the surface, the object to be coated is immersed in a bath comprising the product that is to be deposited on said object. The object is then removed from the bath with removal of the excess liquid 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 means of a metal such as zinc which will then serve as protection against corrosion.

After passing through the liquid metal bath, the coated strip undergoes the spinning operation. This operation is one of the most important in the dip coating process because it allows control of the final coating thickness. On the one hand, the spin must be homogeneous over the entire "section", that is to say the width for a band and the circumference for a wire, and the entire length of the product to be coated. At the same time, this operation must strictly limit the deposit to the target value, which is usually expressed either in terms of thickness deposited - typically from 3 to 20 μm - or by weight of the deposited layer per unit surface area - typically in gr / m ² .

Currently, the spin is generally performed by means of blades or linear gas jets in the case of strips and circular in the case of son from slots and usually directed perpendicular to the surface to be treated. The gas blades act as "pneumatic scrapers" and have the advantage of operating without mechanical contact and therefore without the risk of scratching the treated object. Such blades are called "gas wipers" or "spin knives". The pressurized gas used 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 the automobile bodywork. .

The thickness of the coating depends in particular on the one hand of the distance between the band and the spinning knives and, on the other hand, the pressure exerted by the jet of compressed gas on the band.

However, it is known that a number of characteristics of the installation such as the presence of cooling devices, the eccentricity of certain rollers, the wear of bearings, etc., induce vibrations of the band passing through the wipers. These vibrations eventually cause variations in the thickness of the coating which affect the quality of the product and require overconsumption of zinc to guarantee a minimum thickness of coating to the customer.

The document JP-A-56 153136 proposes to have at least one pair of pneumatic stabilizers or dampers at positions such that the distance between the two rolls, that is to say the bottom roll and the top roll, which are fixed points for the band, in a number 1 / (n + 1), the number n being an integer ranging from 1 to 6 or 0, or to several positions separated by more than one meter from the two aforementioned fixed points. The static pressure between the pneumatic stabilizer and the belt is maintained between 30 and 50 mm of water column.

The document JP-A-56 084452 proposes the use of a pneumatic stabilizer in which a part of the injected fluid flows along the strip in the direction opposite to that coming from the wipers. The collision of the two jets makes it possible to increase the static pressure of the stabilizer as well as of its application surface.

The main disadvantage of this type of system is that the stabilizer acts essentially as a compression spring, which tends to cause the band to pass through a fixed point. It follows that the vibration spectrum of the band, and in particular its natural frequency, can be modified without, however, significantly reducing the overall vibrational energy involved. We can notably observe an increase in the amplitude of the modes of vibration of which the nodes are located at the level of the stabilizer. This type of stabilizer is thus not able to prevent the appearance of resonance phenomena.

In addition, the objective being to stabilize the band in the wipers, it is necessary that this type of stabilizer is in their vicinity, involving blowing a gas under pressure on a coating not yet solidified, which may affect the aspect of the final product.

Moreover, a number of methods for controlling or eliminating vibrations affecting a metal strip in continuous scrolling based on the use of electromagnetic means are also known (see, for example, the documents JP-A-10 298728 , JP-A-5 001362 , JP-A-9 143652 , JP-A-10 87755 , JP-A-8 010847 ).

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 in the band currents in phase opposition, the eddy currents. The interaction between the inductive currents and the induced eddy currents generates a magnetic pressure tending to stabilize the steel strip. Another solution is to use electromagnets. However, methods of this type imply additional control because of the magnetic attraction force, which tends to make the band unstable. Furthermore, it is known that the high frequency currents used cause a rise in temperature in the band, which is contrary to what is sought in this step of the process.

Goals of the invention

The object of the present invention is to propose a solution to the problem of stabilizing a metal strip in continuous scrolling which makes it possible to overcome the drawbacks of the state of the art.

In particular, the present invention aims to stabilize the strip at the exit of the liquid metal bath through pneumatic means, passive or active, which allow to dissipate the vibration energy generated in the band by the installation.

In addition, the invention also aims to avoid the implementation of additional gas jets in the immediate vicinity of the wipers that could affect the appearance of the final product.

Finally, the invention also pursues the goal of being compatible with the cooling of the band required at the exit of the coating bath by hot dipping.

Main characteristic elements of the invention

A first object of the present invention, set forth in claim 1, relates to a dissipative device for pneumatically stabilizing a metal strip in continuous scrolling, in the form of a box comprising at least two slots, for blowing a tube. pressurized gas on at least one side of the strip, which extend transversely substantially across the width of the strip, separated by a distance in the longitudinal direction and substantially parallel to each other, characterized in that the box further comprises in the space between two successive slots, a cavity closed by a rear wall and side walls, open at the front, facing the strip and limited in the longitudinal direction at least by said slots.

Preferred embodiments of this device are detailed in claims 2 to 16.

Claims 17 to 27 relate to methods for carrying out the various preferred embodiments of the above-mentioned stabilizing device, including preferred methods of these different methods.

Brief description of the figures

Figure 1 shows a vertical sectional view of the pneumatic stabilization device of a passive damping metal strip according to the present invention.

FIG. 2 represents a corresponding elevational view of the device of FIG. 1.

FIG. 3 represents a vertical sectional view of the pneumatic stabilization device of an active damping metal strip according to the present invention.

Figures 4 and 5 show a sectional view of two preferred embodiments of the invention as an alternative to the pneumatic stabilization device of a passive damping metal strip shown in Figure 1.

Description of preferred embodiments of the invention

For the sake of clarity, FIG. 1 shows a preferred embodiment of the device of the invention arranged in front of the steel strip 1 in continuous upward movement, after passage through the bottom roller 4 of the liquid zinc bath. 2 and after spin 3.

The device of the invention is essentially in the form of a box 5 having on its front face at least two slots 6 oriented towards the band 1, by which is blown a compressed gas for maintaining a pressure greater than ambient atmospheric pressure between the front face of said device and the strip. Between these two slots is arranged a cavity 7 in the box, said cavity being either completely open or partially obstructed on the side of the strip 1 by a grid, that is to say a plate 8 provided with openings 8 '. This cavity 7 serves to dissipate the vibration energy of the strip essentially by a viscosity effect of the gas. The number and shape of the openings 8 'makes it possible to modulate the dissipation of energy. The objective pursued by the installation of the grid 8, 8 'is to create a controlled loss of local load.

The width of the slots 6 must be significantly smaller than the width of the cavity 7 (for example 10 times smaller).

Advantageously, the slots 6 will have a trumpet-shaped end making it possible to exploit the Coanda effect or gluing effect, making it possible to eject the fluid over a larger strip surface than a usual slot with sharp edges. In addition, this type of shape makes it possible to very greatly reduce the pressure gradient at the level of the strip, in front of each slot, where a very large pressure gradient is observed with the slots that are usually used. Thus, it is possible to place the device of the invention in the vicinity of the wipers, where the zinc is not yet solidified.

Two more preferred embodiments of the invention may be envisaged, differing in the nature of the damping achieved.

Passive amortization

To increase the energy dissipation in the cavity 7, the latter can be equipped with a wall movable rear 9. In the case of the use of a passive damping system, the wall 9 is connected to a dissipative energy system 10 consisting essentially of a spring and a damper (of the dash-pot type ).

Advantageously, the damping system is adjusted so as to damp the vibrations of the band in a frequency range between 1 and n times the natural frequency of the band (n integer).

The natural frequency of the band depends on its dimensions, the size of the strand, the line voltage as well as the nature and position of the different supports on the band. In the most usual cases, the first natural frequency of the band is between 0.5 and 1 Hz. It is observed that the vibration spectrum has harmonics of significant amplitude between this frequency and n times this frequency (n = 2 , 3, etc.), that is to say typically from 0.5 Hz to 5 Hz or even from 0.5 Hz to 10 Hz.

Active depreciation

In the case of the use of an active damping system, a device for measuring the movement of the strip drives a displacement of the movable wall 9. Like any active damping system, it is composed of at least one sensor 11 and an actuator 12 controlled by a control system 13. The displacement, the speed or the acceleration of the band 1 are measured by the (or) sensor (s) 11. The signal of the sensor is transmitted to the controller input 13. The output of the controller 13 is connected to the actuator 12. The latter converts the control signal which it receives, preferably according to an open or closed loop control law, into a mechanical force applied to the actuator 12. movable wall 9. Thus the active element reacts to the initial vibration by generating a reaction force so as to cancel or attenuate this vibration. The regulation is parameterized so as to optimize the damping at the mobile wall 9.

For example, the sensor 11 is a non-contact distance sensor of the laser triangulation type, the actuator 12 being an electromagnetic jack.

Other forms of execution

Figures 4 and 5 show other advantageous embodiments of the pneumatic stabilization device of a metal strip according to the present invention. These methods of execution can be used both in the context of a passive damping system and in that of an active damping system.

In Figure 4, the cavity 7 in the box 5 and the gas supply ducts compressed by the slots 6 do not form two separate elements. In the cavity 7 is a second inner box (or shoe) 5 'having a front face in the form of a plate 18 full, that is to say without holes such as the orifices 8' shown on the Figures 1 to 3. The shoe 5 'can move in the direction perpendicular to the strip 1. To fix the ideas, in Figure 4, this box 5' was connected at the rear to a dissipative energy system 10 (passive depreciation). However, it can also be used in an active damping system as described above. The gas injection ducts 6 'are formed in part by the side walls 17' of the box 5 ', which makes it possible to modulate the passage section of these ducts when the box 5' moves relative to the strip. In this embodiment, in use, the entire box 5 'is movable and not only a rear wall located in the cavity 7 as in Figures 1 to 3.

In a variant shown in Figure 5, the box 5 'is fixed and has a conduit 7' passing right through its center and opening at an opening 18 'on the front face 18 of the box 5' . This duct may be obstructed at the rear by an element such as a sphere 37 integral with the dissipative system 10. The increase in the pressure of compressed gas in this duct under the effect of the movements of the band 1 may cause the separation shutter 37 of the rear end of the duct, which allows the injection of gas to the band also through the central duct 7 'which is then connected to the supply system.

The operation of this device shown in Figure 4 or 5 is as follows. As the band approaches the front face 18 of the box 5 ', the pressure drop between the cushion and its environment increases. The pressure in the cushion therefore increases. The element 37 of the central part 5 'of the box 5 (FIG. 5) or the whole of it (FIG. 4) moves backwards, so that the section of clean passage of the compressed gas supply ducts increases. As a result, the pressure drop between the environment and the cushion decreases and the gas flow increases. The pressure in the cushion increases and repels the band. A balance is thus established between the different forces in presence.

Claims (27)

Dissipative device for the pneumatic stabilization of a metal strip (1) in continuous movement, in the form of a box (5) comprising at least two slots (6), for blowing a gas under pressure on at least one side of the strip (1), which extend transversely substantially across the width of the strip (1), separated by a certain distance in the longitudinal direction and substantially parallel to each other, characterized in that the box (5) further comprises in the space between two successive slits, a cavity (7,7 ') closed by a rear wall (27,37) and side walls (17), open at the front, facing the strip (1) and limited in the longitudinal direction at least by said slots (6). Device according to claim 1,
characterized in that the front opening of the cavity (7) is obstructed by a plate (8) provided with orifices (8 '). Device according to claim 1 or 2, characterized in that it is adapted to be arranged on both sides of the strip, symmetrically or otherwise with respect to the band pass line. Device according to any one of claims 1 to 3, characterized in that the slots (6) are associated with pressurized gas injection pipes (6 ') oriented substantially perpendicular to the direction of movement of the strip (1 ). Device according to any one of claims 1 to 3, characterized in that the slots (6) are associated with injection pipes (6 ') of the pressurized gas oriented obliquely with respect to the perpendicular to the direction of movement of the the band (1). Device according to claim 5,
characterized in that the injection pipes (6 ') of the pressurized gas are oriented towards the inside of the zone of the strip facing the device. Device according to claim 5 or 6, characterized in that the angle of orientation of the injection ducts relative to said perpendicular, not necessarily identical for all said ducts, is between 10 and 80 °, and preferably between 30 and 80 °. and 60 °. Device according to any one of the preceding claims, characterized in that the edges of the slots (6) are outwardly flared or are trumpet-shaped so as to have a pressure gradient at the flattened outlet by gluing effect or Coanda effect. Device according to any one of claims 1 to 8, characterized in that the cavity (7) comprises, in front of its rear wall (27), a movable plate (9) in the transverse direction making the volume of the cavity (7) variable. and connected to a passive damping system (10), essentially modelable as a damped harmonic oscillator. Device according to any one of claims 1 to 8, characterized in that the cavity (7) comprises, in front of its rear wall (27), a movable plate (9) in the transverse direction making the volume of the cavity (7) variable. and connected to an active damping system (11,12,13). Device according to claim 9,
characterized in that the passive damping system (10) is configured to damp the vibrations of the band in a frequency range of between 1 and n times (n integer) the natural frequency of the band, preferably between 0.5 Hz and 5 Hz and more preferably between 0.5 Hz and 10 Hz. Device according to claim 10,
characterized in that the active damping system comprises at least one sensor (11) measuring a position, speed or acceleration of the strip (1) and an actuator (12) controlled by a control system (13) and is configured for the signal of the sensor (11) to be transmitted to the input of the controller (13) whose output is connected to the actuator (12) and for the actuator (12) to convert the signal it receives from the controller (13), according to an open or closed loop control law, a mechanical force applied to the movable plate (9). Device according to claim 12,
characterized in that the sensor (11) is a non-contact distance sensor of the laser triangulation type and the actuator (12) is an electromagnetic actuator. Device according to any one of claims 1 to 8, characterized in that said cavity (7) comprises a closed internal box (5 ') whose side walls (17') form with the side walls (17) of the cavity ( 7) injection ducts (6 ') of the gas under pressure through the slots (6) to the strip, the inner box (5') being movable in the direction transverse to the strip and connected at a rear face to a passive (10) or active (11,12,13) damping system. Device according to claim 14, characterized in that it is configured so that the passage section of the injection ducts (6 ') varies with the relative position of the inner box (5') with respect to the strip (1) . Device according to any one of claims 1 to 8, characterized in that said cavity (7) comprises an inner box (5 ') whose walls sides (17 ') form with the side walls (17) of the cavity (7) injection pipes (6') of the pressurized gas through the slots (6) to the strip, the inner box (5 ') being fixed and comprising a central duct (7 ') passing therethrough transversely, opening at an opening (18') on the front face (18) of the inner box (5 ') and on its rear face at the level of an opening (27 ') that can be obstructed by a movable element (37) integral with a passive (10) or active (11, 12, 13) damping system, said central duct (7') being communication with the supply of compressed gas, when the opening (27 ') is not obstructed by the movable member (37). Method for implementing the device according to Claim 9, characterized in that : a pressurized gas is injected onto the strip (1) via the slots (6); a cushion for damping the vibrations experienced by the strip, situated between the strip and the device, is created and adjusted using the cavity (7) included therein, possibly provided with a plate (8) perforated with orifices (8 ') located on the front face of the cavity (7) and / or a movable rear plate (9) in the transverse direction connected to a passive damper (10). Process according to Claim 17, for the implementation of the device according to Claim 10, characterized in that the passive damping cushion is replaced by an active damping cushion, by using a cavity (7) provided with a moving rear plate (9) in the transverse direction whose movement is controlled by an actuator (12) controlled by a regulation law in open or closed loop via a controller (13) which continuously receives a sensor (11) physical information characteristic of the vibrations of the strip. Process according to claim 18,
characterized in that said physical information is a displacement, a speed or an acceleration of the band. Method for implementing the device according to Claim 14 or 15, characterized in that : a pressurized gas is injected onto the strip (1) via the slots (6); a cushion for damping the vibrations experienced by the strip, located between the strip and the device, is created and adjusted by using transverse movements of the inner box (5 ') cooperating with the passive damping system (10) or active (11,12,13) to which said box (5 ') is connected at its rear face. Method for implementing the device according to Claim 16, characterized in that : a pressurized gas is injected onto the strip (1) via the slots (6); a cushion for damping the vibrations experienced by the strip, situated between the strip and the device, is created and adjusted by using transverse movements of the movable element (37) for obstruction of the opening (27 ') the central duct of the inner box (5 '), on the rear face thereof, the fixed inner box (5') cooperating with the shutter (37) connected to the passive damping system (10) or active ( 11, 12, 13), for injecting compressed gas from the supply system to the strip via said central duct (7 '). Process according to any one of claims 17 to 21, characterized in that the metal strip (1) comprises steel, aluminum, zinc, copper or one of their alloys. Process according to any one of claims 17 to 22, characterized in that the thickness of the metal strip (1) is between 0.15 and 5 mm. A method according to any one of the preceding claims 17 to 23, characterized in that the metal strip is subjected to a continuous heat-dip coating process with a web speed of between 2 and 10 m / s. Method according to Claim 24,
characterized in that the molten coating metal comprises zinc, aluminum, tin or an alloy of at least two of these metals. Process according to Claim 24 or 25, characterized in that the thickness of the metal coating layer obtained after dewatering is between 3 and 20 μm. Process according to one of Claims 17 to 26, characterized in that the gas under pressure is air, nitrogen or carbon dioxide.

Images