Field of the invention

The invention relates to a gas wiping device for controlling the thickness of a metallic coating applied on a metal strip by means of hot dip coating.

Background of the invention

Gas wiping devices are used to control the thickness of a metallic coating applied on a moving metal strip, such as a steel strip. The metallic coating is applied by means of hot dip coating wherein in a continuous or semi-continuous process a metal strip passes as part of the process through a bath of molten metal of for instance Zn, Zn + Fe alloy, Zn + Al or Zn + Mg + Al. The metal strip leaves the bath in an about vertical direction after which the excess of the applied metallic coating is blown of with a high pressure air/gas device also known as "air knife". The removal of excess metallic coating from the moving strip is in fact the control of the thickness of the applied metallic coating.

Since the metallic coating is applied before the final manufacturing from the coated steel strip of for instance outer parts for the automotive industry the applied coating should fulfil requirements like an exact predefined thickness and uniform thickness over the complete coated steel strip. This is important not only to be able to subject the coated steel strip to forming operations but also for the final appearance of the final steel product formed from the coated steel strip.

These requirements mean that the gas wiping device should discharge a uniform gas jet over the total width of the gas nozzle of the gas wiping device which means that the gas jet should be of uniform velocity and pressure. This implies high standards for the construction of the gas wiping device and its gas discharge nozzle.

The gas flow from the gas inlet through the construction of the gas wiping device should be such that turbulences/vortices in the gas flow and any other regular variations, for example resonances, are prevented. Another prerequisite is that the gas is evenly distributed over the length of the gas discharge nozzle which length corresponds to the width of the strip. A known solution as disclosed in US2008/0245903 is to provide a gas feed chamber with a gas inlet and multiple gas outlets, wherein each gas outlet is provided with a controllable outlet valve.

The gas discharge slot of the gas discharge nozzle needs to be set to tight tolerances over the total length of the gas discharge slot. This discharge slot is in fact a narrow slot with a large length / width ratio which makes if difficult to provide or adjust to a predefined width over the total length of the gas discharge slot. In known systems, see for instance US2010/0224120, the opening is formed by an lower and upper lip that can be adjusted with respect to each other. With such an adjustable system it is difficult, if possible at all to adjust the width to a predefined width over the total length of the gas discharge slot, which will result in an uneven coating thickness across the strip or imperfections on the surface of the coated strip.

Further the stiffness of the gas discharge nozzle or the lips of the nozzle of the current designs is often insufficient, leading to vibrations in the gas discharge nozzle and gas discharge slot and consequently to irregularities in the applied coating such as an uneven coating thickness across the width and/or length of the strip and/or to coating surface defects.

Objectives of the invention

It is an objective of the present invention to provide a gas wiping device which provides an even gas discharge along the total length of the gas discharge slot.

It is another objective of the present invention to provide a gas wiping device wherein disturbances / vortices in the gas flow are suppressed before the gas is discharged.

It is another objective of the present invention to provide a gas wiping device with a gas feed chamber designed to suppress resonances in the gas feed chamber.

It is another objective of the present invention to provide a gas wiping device with a gas discharge nozzle that allows tight tolerances of the gas discharge slot.

It is another objective of the present invention to provide a gas wiping device with a gas discharge nozzle of sufficient stiffness to prevent vibrations in the gas discharge nozzle.

It is still another objective of the present invention to provide a demountable gas discharge nozzle for the gas wiping device.

Description of the invention

The invention relates to a gas wiping device as defined in claims 1 -10 and a gas discharge nozzle as defined in claims 1 1 -14.

One or more of the objectives are realized by providing a gas wiping device for controlling the thickness of a metallic coating applied on a metal strip by means of hot dip coating comprising a gas feed chamber with a gas inlet and multiple gas outlets, a gas discharge nozzle with an elongated gas discharge slot, wherein the gas outlets of the gas feed chamber are in fluid connection with the gas discharge nozzle, wherein the gas discharge nozzle is divided in multiple gas discharge channels, wherein the gas discharge channels are in fluid connection with the elongated gas discharge slot and wherein each of the multiple gas outlets of the gas feed chamber is in fluid connection with a gas discharge channel of the gas discharge nozzle.

The term "gas" used in the description comprises any gas or gas composition which is suitable to be used for the gas wiping of a metallic coating applied on a metal strip. Most commonly nitrogen, ambient air or a mixture of nitrogen and ambient air are used for this purpose.

By providing multiple gas discharge channels in the gas discharge nozzle the discharge of the gas is controlled such that lateral flow of the gas in the gas discharge nozzle and therewith in the gas discharge slot is prevented. As a result uneven coating of the strip is prevented or suppressed to a great extent which also applies for surface defects caused by an uneven gas discharge.

According to a first embodiment the multiple gas outlets of the gas feed chamber are in direct fluid connection with the gas discharge channels. This provides a construction with relatively few parts which can be realized easily and against relatively low costs.

In a second embodiment multiple gas channels are provided for the fluid connection between the gas outlets of the gas feed chamber and the multiple gas discharge channels of the gas discharge nozzle.

The advantage of this embodiment is that the flow of the gas from the gas feed chamber to the gas discharge channels is better divided over the length of the gas discharge nozzle and therewith over the gas discharge channels and over the length of the gas discharge slot. This will result in a further improvement of the quality of the applied coating.

The supplied gas enters the feed chamber under pressure and will have a turbulent flow over at least part of the path to the gas discharge nozzle. With the extra length of the gas feed channels the turbulent flow can be easier suppressed and changed into a laminar flow. The pressure under which the supplied gas enters the feed chamber is mostly in the order of up to 1 bar overpressure but could also be higher.

Another advantage is that the gas feed chamber and the gas discharge nozzle can be positioned at a distance from each other. This is favourable because of the short distance between the gas discharge nozzle and the moving strip and gives a larger freedom where it comes to positioning of the gas discharge nozzle and more in particular to the positioning of the gas discharge slot with respect to the moving strip. The positioning of the gas discharge nozzle with respect of the moving strip concerns the distance with respect of the moving strip as well as the angle with respect to the moving strip.

According to a further aspect it is provided that one or more of the gas feed channels are separated in two or more sub-channels over at least part of the length of the one or more gas feed channels. By separating the gas feed channels in subchannels the flow of the gas can be better controlled, more in particular at the location of a bend in the gas feed channels. Since straight gas feed channels would require more space for the gas wiping device, which in many hot dip installation will not be available, it is almost unavoidable to have one or more bends in the gas feed channels. It was found that a construction with an about U-shape of the gas feed channels with sub-channels in the bends gave good results where it comes to the control of turbulent flow. Moreover, a relatively compact construction of the gas wiping device could be realized with these U-shaped gas feed channels.

The sub-channels can have various shapes such as round, square or polygonal. Preferably the sub-channels have an elongated shape seen in cross section. In order to have an optimal fluid connection with the gas discharge nozzle it is further provided that the elongated shape of the sub-channels run parallel or about parallel to the elongated gas discharge slot of the gas discharge nozzle.

According to a further aspect one or more of the gas feed channels are provided with a diverging section at the outer end thereof seen in the feed direction of the gas. With this feature the gas will expand therewith further reducing any vortices in the gas and changing to a laminar flow.

Another feature is that in the fluid connection with the gas discharge nozzle a flow chamber is provided between the multiple gas outlets of the gas feed chamber or between the outer ends of the gas feed channels and the gas discharge nozzle. The flow chamber is an undivided chamber to which the multiple gas outlets or gas feed channels connect and which provides for levelling of any pressure differences that there might be between any of the incoming gas flows. At the same time the flow chamber allows for expansion or further expansion of the gas.

In the control of the gas flow, more in particular where it comes to preventing and suppressing turbulent flow, it is important to start already as far upstream of the gas flow as possible, preferably at or directly after the gas inlet of the gas feed chamber of the gas wiping device. Since the gas has to be divided evenly over the width of the gas discharge nozzle it is provided that the gas feed chamber comprises an elongated pipe with the gas inlet at an outer end of the pipe and with the multiple gas outlets along the length of the elongated pipe. The pipe can have any cylindrical shape, including more in particular a round or oval shape seen in cross-section.

For an even distribution of the gas over the gas outlets along the length of the gas feed chamber and thus over the gas discharge nozzle it is provided that the elongated pipe has a cross-sectional area which is equal or larger than the cross- sectional area of all gas feed channels together, and preferably of all gas outlets together. In this respect the cross-sectional area of a gas feed channel means the smallest cross-sectional area of a gas feed channel.

A further feature is that the elongated pipe is provided at the opposite end of the gas inlet with an end closure plate provided with a wedge shaped protrusion projecting in the direction of the gas inlet. Since the gas enters the gas feed chamber at high pressure and given the length of the elongated pipe forming the gas feed chamber resonance in the gas feed chamber could easily occur which would result in an increase in turbulent flow. The wedge shaped protrusion provides that the length of the gas feed chamber differs over the cross-sectional area of the pipe therewith preventing the occurrence of resonance in the gas feed chamber to a great extent.

In a further embodiment it is provided that the gas discharge nozzle is constructed as a separate part that allows for mounting the gas discharge nozzle to and demounting the gas discharge nozzle from the gas wiping device. The gas discharge nozzle is constructed as a fixed part and does not need to be adjusted.

This is a large advantage over the commonly used gas discharge nozzles which form an integral part of the known gas wiping devices. This known gas discharge nozzle has lips that are adjustable over the length of the gas discharge nozzle and consequently also have to be adjusted over the whole length to have a predefined dimension of the gas discharge slot. Since these adjustable lips may depart from an initial setting due to vibrations during operation these need to be checked and adjusted on a regular basis. To that end the complete gas wiper device has to be taken out of the hot dip installation, checked and if need be adjusted and built into the installation again.

It is further provided that the gas discharge nozzle is either an assembly of separate parts or is a single additive manufactured or cast part. With a gas discharge nozzle assembled from separate parts these parts are manufactured and where necessary machined to very close tolerances. If need be the assembled gas discharge nozzle is machined once more which will mainly concern the exact dimensions of the gas discharge slot. With a gas discharge nozzle that is manufactured as a single part, machining will be necessary to get smooth surfaces were necessary and also in this embodiment to get the gas discharge slot within predefined specifications.

With an assembly of parts the gas discharge nozzle comprises a first and a second plate enclosing a sharp angle, which are connected at the outer ends by end plates and are further connected at spaced intervals between the end plates by separator vanes dividing at least part of the gas discharge nozzle in multiple gas discharge channels.

In order to prevent any disturbances in the gas discharge in the gas discharge nozzle the separator vanes are bevelled of at one or both ends seen in the gas discharge direction at an angle between 10 - 20°, preferably between 12 - 18°.

Brief description of the drawings

The invention will be further explained on basis of the example shown in the drawing, in which:

fig .1 shows an exploded view of the gas wiping device,

fig.2 shows a view of a gas feed channel with sub-channels,

fig.3A shows a connection flange provided at the end of the gas feed channels, fig.3B shows a detail of the connection flange,

fig.4A shows the gas discharge nozzle,

fig.4B shows a cross section through the gas discharge slot, and

fig.4C shows a separator vane of the gas discharge nozzle in detail.

Detailed description of the drawings

In fig.1 an exploded view of the gas wiping device 1 is shown comprising a gas feed chamber 2 with a gas inlet 3 and multiple gas outlets 4 of which only one can be seen in the drawing. The multiple gas outlets 4 connect to multiple gas feed channels 5, which at their outer ends connect to a connection flange 6. The connection flange 6 connects to a flow chamber 7 and at the opposite side of the flow chamber 7 the gas discharge nozzle 8 is mounted.

The gas inlet 3 of the gas feed chamber 2 is connected to a high pressure source of a gas, such as nitrogen, air or a mixture thereof. Since the gas enters the gas feed chamber 2 at high velocity resonance may occur in the gas feed chamber 2. In order to suppress the occurrence of resonance an end plate 9 is mounted at the opposite end of the gas feed chamber 2, wherein the end plate 9 is provided with a wedge 10. The wedge 10 provides that seen in the length direction of the gas feed chamber 2 the length of the gas feed chamber 2 is different over the cross-sectional area, by means of which resonance is suppressed effectively.

The cross sectional area of the gas feed chamber 2 is larger, preferably much larger than the total area of all the multiple gas outlets 4 taken together as a result of which the gas is evenly distributed over all gas outlets 4 and therewith through all gas feed channels 5.

In fig. 2 a single gas feed channel 5 is shown provided with a converging section part 1 1 at the upstream side where the gas feed channel 5 is to be connected to the gas feed chamber and with a diverging section 12 at the other end of the gas feed channel 5 where it is to be connected to connection flange 6. The gas feed channels 5 have two bends 13, 14 with a straight part 15 between the bends 13, 14.

With the construction of the gas feed chamber 2 on top of the flow chamber 7, which is done to save space in horizontal direction, another option would be to connect the gas outlets 4 directly or through short vertical channels with the flow chamber 7. However, such a construction would imply an about 90° bend of the incoming gas in the flow chamber 7 resulting in much turbulence directly upstream of the gas discharge nozzle 8.

With gas feed channels 5 provided with bends 13, 14 and straight part 15 the gas is guided into the flow chamber 7 in a direction which is in line with the gas discharge nozzle 8, avoiding that turbulences arise in this area. The gas feed channels 5 are provided with sub-channels 16 in bends 13, 14. With these subchannels 16 the flow of the gas is guided through a narrower space which decreases turbulence in the gas flow considerably compared to a gas flow through the same gas feed channel 15 without these sub-channels 16.

With the diverging section 12 the gas expands before entering the flow chamber

7. The connection flange 6 for the multiple gas feed channels 5 is provided with openings 17 for each of the gas feed channels 5. The sides 19 of the openings 17 are bevelled such that the bevelling is in line with the sides of the diverging section 12.

The diverging section 12 of the one or more gas feed channels 5 enclose an angle between 10 - 20° in at least one direction, preferably between 12 - 18° in order to stabilise the gas flow. Good results have been realized with angles of 16° or only a bit smaller, that is in the order of 1 -2° smaller. In the example given in the drawing the diverging section 12 expands within the given range of angles in two directions, which are about a right angle to each other. Instead of rectangular channels and rectangular diverging sections seen in cross-section also round channels and cone shaped diverging sections could be used. Important is that such other shaped diverging sections have a diverging angle as described above.

The connection flange 6 is demountable attached to the flow chamber 7 for which it is provided with means to allow for a gas tight connection, which in the example are holes 18 for bolts, screws or the like.

The flow chamber 7 is an undivided chamber allowing the levelling of any pressure differences that there might be between any of the gas flows entering the chamber through each of the gas feed channels 5.

Fig.4A shows a perspective view of gas discharge nozzle 8. The gas discharge nozzle 8 comprises an upper and lower part 20, 21 , end plates 22, 23 and separator vanes 24 between the upper and lower part 20, 21 . The separator vanes 24 provide a very rigid construction of the gas discharge nozzle 8 and considerably reduce any lateral gas flow in the gas discharge nozzle 8 and in the elongated gas discharge slot

25, see fig.4B.

The separator vanes 24 are bevelled at the upstream and downstream sides

26, 27 thereof, see fig.4C, to smoothly guide the gas flow to the elongated gas discharge slot 25. The angle enclosed by the bevelled sides corresponds to the angle of diverging section 12 and is between 10 - 20°, preferably between 12 - 18°. Good results have been realized with angles of 16° or only a bit smaller, that is in the order of 1 -2° smaller.

The separator vanes 24 are an important feature of the gas discharge nozzle 8 because these separator vanes 24 provide a rigid construction of the gas discharge nozzle 8 and control the gas flow through the gas discharge nozzle 8 and the elongated gas discharge slot 25. This results in an even gas flow over the length of the elongated gas discharge slot 25 resulting in an even coating on a metal strip without any or as good as any surface defects. Moreover, because of the rigidity of the gas discharge nozzle 8 there is no drift in slot width of the elongated gas discharge slot 25 and therefore no adjustment is needed.

The gas discharge nozzle 8 is demountable attached to the flow chamber 7 to which end it has provisions to allow to screw, bolt or the like the gas discharge nozzle 8 to the flow chamber 7 in a gas tight manner. With these mounting means the gas discharge nozzle 8 can easily be changed for a new gas discharge nozzle 8 or a gas discharge nozzle 8 with a different slot width if necessary.

Fig. 4C shows that the elongated gas discharge slot 25 has a considerable slot depth with regard to the slot width. The bevelled ends 28, 29 of the upper and lower part 20, 21 of the gas discharge nozzle 8 are machined to get a predefined slot width. Machining can be done either before or after the gas discharge nozzle is assembled or manufactured.

The ratio between the slot depth and the slot width is taken such that the slot depth is more than 10 - 15 times the slot width to even out any turbulence that might still be present in the gas at the entry of the elongated gas discharge slot 25. For instance, with a slot width between 1.00 and 1 .50mm the slot depth is taken between 15 and 25mm, thus ensuring a tightly controlled wiping gas jet. The thickness of the upper and lower part 20, 21 and the angle between the upper and lower part 20, 21 are to be taken in consideration in defining the slot width and the slot depth.

The gas discharge nozzle 8 can be assembled from discrete parts 20, 21 , 22, 23, 24 by fixing the parts to each other with screws, bolts, glue or by any suitable welding method. Another method is to cast the gas discharge nozzle 8 as a single part or manufacture the gas discharge nozzle 8 by additive manufacturing.