WO2005010229A1

WO2005010229A1 - Method and device for hot-dip coating a metal strip

Info

Publication number: WO2005010229A1
Application number: PCT/EP2004/008170
Authority: WO
Inventors: Heinrich Pannenbecker; Hermann Erwig; Wilhelm DÜRR; Joachim HÜLSTRUNG; Bert-Heiner Willeke
Original assignee: Thyssenkrupp Steel Ag; Duma Maschinen- Und Anlagenbau Gmbh
Priority date: 2003-07-23
Filing date: 2004-07-22
Publication date: 2005-02-03
Also published as: DE10333766A1; DE10333766B4

Abstract

The invention relates to a method and device for hot-dip coating a metal strip (2). The metal strip (2) to be coated passes through a molten bath (5). Once it has left the molten bath (5), the still molten metal coating which is present on a surface (2',2') of the metal strip is blown off the metal strip (2) by means of at least one gas flow (G',G') emanating from a stripping nozzle (9',9') to achieve a specific coating strength for the final remaining coating on the surface (2',2') which is respectively impinged upon by the gas flow (G',G'). The aim of the invention is to maintain the desired stripping effect during the above-mentioned process to such an extent that as little as possible slack arises and to keep errors occurring on the surface of the coated strip and which are caused by the stripping, to a minimum. According to the invention, the gas flow (G',G') flowing off the respective surface (2',2') of the metal strip (2) is suctioned off by means of a suctioning device (8',8';10',10') which is arranged in the vicinity of the stripping nozzle (9',9') and the surface (2',2') of the metal strip (2).

Description

METHOD AND DEVICE FOR HOT-DIP COATING METAL STRIP

The invention relates to a method for hot-dip coating of metal strip, in which the metal strip to be coated is moved through a melt bath and the still molten metallic coating present on a surface of the metal strip after leaving the melt bath by means of a gas stream applied from at least one stripping nozzle to a desired thickness the coating finally remaining on the surface hit by the gas stream is blown off the metal strip. The invention also relates to a device for hot-dip coating of metal strip, with a melting vessel for holding molten coating metal and with at least one stripping nozzle which directs a gas stream onto the metal strip which emerges from the melting vessel and is coated with the coating metal, in order to remove the metal strip present on the surface of the metal strip to blow off still molten coating metal from the metal strip up to a desired thickness of the coating finally remaining on the surface hit by the gas stream.

In a continuous pass, hot-dip finishing of steel strip with zinc, aluminum or their alloys, the strip to be coated is annealed in a continuous furnace and then passed through a melt bath. When leaving the melt pool liquid coating metal taken from the tape adheres to the tape surface.

The desired thickness of the coating metal is then usually set by means of a pair of gas nozzles that extend linearly across the width of the strip. A gas stream, usually an air stream, is directed from the gas nozzles onto the surfaces of the metal strip, through which the excess, still liquid layer is deliberately reduced by blowing the coating material that is not required from the strip. As a result, only coating metal with the required layer thickness remains on the belt. Only then does the coating layer solidify.

In order to achieve the momentum of the flow medium necessary for stripping off the excess coating metal, high pressures and large volume flows of the stripping gas applied are required. In the conventional stripping process, the gas spreads uncontrollably along the strip surface after the stripping process. The gas flow caused thereby and running over the coated strip can lead to atmospheric oxygen coming into intensive contact with the still liquid coating material, so that the surface of the stripped liquid coating material is oxidized to a greater extent. Furthermore, as a result of the gas flowing near the surface, wave-like drainage structures can also be formed, which can be seen to a greater or lesser extent after the solidification of the coating material and which are disadvantageously noticeable as unevenness in the further processing. The latter is particularly undesirable for high-quality surfaces that meet the highest demands, such as those required for exterior parts of automobile bodies. Another problem in the operational practice of hot dip coating is that the strong gas flow reaches the surface of the melt pool. This causes a wave movement on the bath surface, in the vicinity of the emerging strip, which in turn has an unfavorable effect on the uniformity of the coating. Furthermore, an increase in the oxidation of the coating material can be caused by the gas stream hitting the strip surface. The oxidized liquid coating material then floats as slag on the bath surface, interferes with the coating process and has to be removed manually on a regular basis. This leads to material losses and is uneconomical.

In addition to the prior art explained above. Processes are known in which, in order to improve the coating surface, all units used for stripping off the coating material are housed and gaseous nitrogen is applied. The oxidation of the coating is thus inhibited. However, this procedure is disadvantageous because the housing represents a serious handicap for the operating personnel both during operation and during maintenance of the coating system. Also, the housing may be the origin ^'of the undesirable, can not avoid the irregularities noticeable solubilizing flow structures in the coating.

On the basis of the prior art explained above, the invention was based on the object of creating a method and a device, the operation of which produces as little slag as possible while maintaining the desired wiping action, while at the same time the surface of the coated strip detectable, streak-related errors should be reduced to a minimum.

Starting from a method of the type specified at the outset, this object is achieved in that the gas stream flowing off the respective surface of the metal strip is sucked off by means of a suction device arranged near the wiping nozzle and near the surface of the metal strip. In the same way, the invention provides for the elimination of the disadvantages existing in the prior art, a device of the type described at the outset, in which, according to the invention, a suction device is assigned to the stripping nozzle, which is arranged near the stripping nozzle and near the metal strip by one of the respective ones Extract the surface of the gas stream flowing out of the metal strip.

According to the invention, the gas stream striking the strip surface is sucked off near the strip surface. In this way is reliably prevented, ^'itself that forms a flowing parallel to the respective strip surface gas flow, on the one hand promote the oxidation of the applied coating metal on the strip surface and would otherwise promote the formation of undesirable as runtime structures. In the procedure according to the invention, the gas stream is instead discharged in a controlled manner, if possible immediately after the gas stream has impinged on the strip surface assigned to it. The occurrence of surface defects and the risk of excessive oxidation of the coating material are thus reduced to a minimum.

The distance between the suction device and the assigned belt surface should be set in such a way that the distance between the belt and the suction is large enough to safely prevent liquid metal particles from being sucked into the suction device. Sucking in solidified dust particles, as they arise in the conventional method, however, is desirable. These solid particles are deposited on the coated tape in a conventional manner and reduce the quality of the coated surface. They also cause a certain environmental impact in the area of the coating bath.

In this context, a “close arrangement” is understood according to the invention to mean a positioning of the suction device which is so close to the belt surface that the gas jet emerging from the stripping nozzle and deflected on the belt surface in and against the direction of the belt is reliably prevented from the suction of the belt Suction nozzle is detected before it enters the more distant general environment of the wiper nozzle as an undirected, diffuse gas flow.

Practical tests have shown that an optimal effect of the suction is achieved when the suction device is arranged in front of the stripping nozzle in the conveying direction if the distance between the respectively assigned belt surface and the respective nozzle opening determined as solder on the belt surface is not more than 100 mm. Such a close arrangement of the suction device ensures that the stripping gas flow flowing against the belt conveying direction and the particles carried by it are completely detected before they can lead to faults on the belt surface.

If the suction device is arranged in the conveying direction of the belt behind the stripping nozzle, it has proven to be advantageous proven if the distance between the nozzle opening and the strip, which is also determined as solder on the strip surface, is not more than 300 mm. The somewhat further spaced arrangement of a suction device positioned behind the stripping nozzle in the conveying direction has proven to be expedient, because in practice this can cause the stripping gas flow to swirl in the immediate vicinity of the stripping nozzle. The swirled gas stream increasingly tends to drag coating particles with it. At the same time, it takes up a larger volume due to the turbulence. It has therefore proven to be expedient to position the suction device at a distance of up to 300 mm from the belt surface in order to also suck off the swirled stripping gas flow as completely as possible before it and the particles transported by it impair the quality of the surface of the coated belt.

In order to reliably prevent the suction device from colliding with the belt, the minimum distance between the suction device and the assigned belt surface should not be less than 10 mm.

The suction device used according to the invention is preferably designed in such a way that it sucks over the entire bandwidth. By means of a control by means of which the distance between the suction device and the melt vessel containing the melt bath is set, and by means of a control by which the distance between the belt and the suction nozzle of the suction device can be set, the suction device can be adjusted in accordance with the respective mode of operation of the coating system be optimally positioned. The adjustability can thereby be facilitated that the suction device is coupled to the stripping nozzle in such a way that when the position of the stripping nozzle changes, the position of the suction device also changes.

A particularly practical embodiment of the invention is characterized in that the nozzle of the suction device is designed in the manner of a slot nozzle. With such a nozzle, the stripping gas flowing over the belt surface can be reliably detected over the entire width of the belt. In this context, an embodiment of the invention that is particularly advantageous in terms of adaptability to a large product range is characterized in that the width of the slot nozzle opening is adjustable.

A further embodiment of the invention which contributes to the respectively optimized adaptation of the effect of the suction device used according to the invention is characterized in that at least the nozzle of the suction device can be pivoted about an axis arranged parallel to the surface of the belt conveyed in the area of the suction device. In this variant of the invention, the angle at which the ^'suction of the suction device detects the Abstreifgasströmung can be easily adjusted so that, taking into account the geometry and flow patterns in the overall device an optimal effect at all times of the suction i-st.

A further particularly practical embodiment of the invention provides that the extracted gas volume flow is dimensioned such that a residual gas flow remains from the gas flow inflated onto the metal strip, which remains above the surface of the metal strip struck by the gas stream is painted. The suction power can be automatically adjusted to the performance of the wiping nozzles by means of a controlled suction fan. According to a further advantageous embodiment of the invention, the suction power can be adjusted so that enough residual gas still flows onto the metal bath surface to keep the remaining coating slag away from the emerging strip.

In order to be able to remove dusts and other particles from the extracted gas stream, the gas stream is passed according to a further advantageous variant of the invention through a filter in which the foreign bodies entrained by the gas stream are filtered out. They can then be sent for recycling.

The suction device according to the invention is also designed so that it can be easily removed. The stripping nozzles can also be operated without a suction device if required or in the event of maintenance. The suction device can be arranged in such a way that optimal suction is achieved taking into account the special features of the system technology available in each case. The ease of assembly and disassembly can be supported in that the suction device is arranged loosely hanging on a holding element.

Of course, depending on the respective need, there may be a plurality of stripping devices as well as a plurality of suction devices. It may be appropriate to the gas flow in the conveying direction of the Metal strip seen before, behind or both in front of and behind the wiping nozzle.

The invention is explained in more detail below on the basis of a drawing illustrating an exemplary embodiment. They show schematically:

Figure 1 shows a device for hot dip coating of metal strip in a side view.

Fig. 2 in the device shown in Fig. 1 scraper nozzles and suction devices.

The device 1 is used for hot-dip coating of metal strip 2, for example a ^'steel strip which passes through the apparatus 1 in a continuous flow. Before entering the device 1, the metal strip 2 is annealed in a continuous furnace 3 in order to bring it to the optimum bath entry temperature for optimal wetting with the molten coating metal to be applied to the metal strip 2.

The device 1 comprises a melt vessel 4, into which the metal strip 2 comes in coming from the continuous furnace 3. The metal strip 2 is guided through a channel 5 which seals off the metal strip 2 from the surroundings.

A melt bath 5 formed from molten coating metal is contained in the melt vessel 4. The coating metal can be, for example, a zinc alloy, as is usually used for the coating of steel strips. In the melt bath 5, the metal strip 2 is deflected on a roller 6 in such a way that it emerges from the melt bath 5 in a vertical conveying direction F directed from bottom to top.

A first pair of suction devices 8 ′, 8 ″, one of which suction device 8 ′ on one surface 2 ′ and the other suction device 8 ″ on the other, is positioned adjacent to the area 7 of the exit of the metal strip 2 from the melt bath 5 The surface 2 "of the metal strip 2 is assigned to the side of the metal strip 2. The respective distance h between the suction devices 8 ', 8" and the upper edge of the melt vessel 4 or the region 7 of the exit of the metal strip 2 from the melt bath 5 can be shown with the aid of a not shown Setting device can be set. Likewise, the respective distance v between the suction nozzle opening of the suction devices 8 ', 8 "and the band surface 2' or 2" assigned to them can be adjusted.

In the conveying direction F behind the suction devices 8 ', 8 "and at a short distance from them, a pair of wiping nozzles 9', 9" is arranged, of which one is also assigned to one surface 2 'and the other to the other surface 2 "of the metal strip 2 The ^' stripping nozzles 9', 9 "each direct a gas stream G ', G" against the metal strip 2 such that the respective gas stream G', G "is essentially perpendicular to the surface 2 assigned to the stripping nozzles 9 ', 9"'or 2 "hits. The gas applied to the surface 2 'or 2 "by the stripping nozzles 9', 9" can be, for example, air or nitrogen or a mixture of these two gases. If necessary, a further pair of suction devices 10 ', 10 "is arranged behind the stripping nozzles 9', 9" in the conveying direction F in the same way as the suction devices 8 ', 8 ". The suction devices 10', 10" are also spaced from the melt vessel 4 and to the surfaces 2 ', 2 "of the metal strip 2 assigned to them in each case are adjustable.

As can be seen in detail from FIG. 2, the wiping nozzles 9 ', 9 "are each carried by a crossmember 11', 11" which extends across the width of the metal strip 2. A second crossmember 12 ', 12 "extends parallel to the crossmember 11', 11" and is made from a hollow profile with a rectangular cross section.

On their side facing away from the stripping nozzles 9 ', 9 ", the cross members 12', 12" have openings 13 ', 13 "distributed at regular intervals across the width of the cross members 12', 12". The free end of a substantially horizontally extending bolt 20 ', 20 ", the other end of which is firmly connected to a support profile 14', 14", sits with play in the openings 13 ', 13 ". 20 "pushed on, in the assembled state between the respective support profile 12 ', 12" and the associated cross member 12', 12 ". Arranged spacer rings 15 ', 15" is the horizontal distance of the support profile 1', 14 "from the respective cross member 12 ', 12 "set.

The support profiles 1 ', 14 "each carry a suction pipe 16', 16" extending in the horizontal direction parallel to the metal strip 2. The suction pipes 16 ', 16 "are part of the respective suction device 8', 8", which in Delivery direction F are arranged in front of the stripping nozzles 9 ', 9 ".

The suction pipes 16 ′, 16 ″ form a pivot axis A, which likewise extends parallel to the strip surface, for the nozzle section 17 ′, 17 ″ of the respective suction device 8 ′, 8 ″, each designed in the manner of a slot nozzle and extending over the width of the metal strip 2 Swiveling around the axis A, the angle at which the suction directed into the respective nozzle opening 18 ', 18 "of the suction devices 8', 8" can be aligned so that an optimal effect is ensured.

The open width of the nozzle opening 18 ', 18 "of the nozzle section 17', 17" can be adjusted to the width of the strip being processed by means of a slide (not shown). The distance v measured horizontally as a plumb line on the strip surface is 10 mm to 100 mm. The gas stream G _a sucked off by the suction devices 8 ′, 8 ″ is fed to the preparation via further pipes connected to the respective suction pipe 16 ′, 16 ″.

Unlike the nozzle openings 18 ', 18 ", in the suction devices 10', 10" arranged in the conveying direction F behind the stripping nozzles 9 ', 9 ", the respective nozzle opening 19', 19" is not at right angles with a slight angular deviation to the respectively assigned surface of the Metal strip 2 directed, but aligned with greater angular deviation so that the suction generated by them also those in the space above the stripping nozzles 9 ', _. 9 "swirled gases G and particles are reliably detected. The distance v between the respective nozzle openings 19 ', 19 "is between 10 mm and 300 mm. At the same time, the nozzle openings 19', 19" are designed further than the nozzle openings 18 ', 19 "in order to reliably detect larger, less rapidly flowing gas volumes G _a ,

The gas streams G ', G "striking the surface 2', 2" of the metal strip 2 are deflected on the surface 2 ', 2 "and are distributed close to the surface over the respective surface 2' or 2". Gas flowing downward in the direction of the melt vessel 4 is sucked off by the suction devices 8 ', 8 ". Their performance and their orientation in relation to the metal strip 2 are set such that the suction devices 8', 8" do not fully flow in the direction of the Aspirate melt bath 5 flowing gas stream, but leave a small fraction G _r on the metal strip 2. This fraction G _{r of} the gas flow G flows to the region 7 of the exit of the metal strip 2 from the melt bath 5 and prevents there, on the bath surface, from floating slag on the surfaces 2 ', 2 "of the metal strip 2 as it exits the melt bath 5 ,

The part of the gas streams G ', G "which flows upward in the direction of the suction devices 10', 10" which may be present is sucked off by them. The power of the suction devices 10 ′, 10 ″ is dimensioned such that the part of the gas stream G flowing in their direction is completely suctioned off.

The suctioned gas streams G _a take away the solid metal particles which are inevitably present above the surfaces 2 ', 2 "and which are formed by coating metal detached from the metal strip 2. These metal particles to recover, the extracted gas flows G _{a are passed} through filters or separators, not shown here.

The modular structure of the device 1 makes it possible to maintain or replace the suction devices 8 ', 8 ", 10', 10" and the scraping nozzles 9 ', 9 "in a particularly simple manner. For example, the suction devices 8' can easily be replaced. , 8 ", 10 ', 10" during operation or to optimize their setting by changing their height h above the melt bath 5 or their distance v from the surface 2' or 2 ". The distance v is adjusted in such a way that on the one hand the gas flow G _{a to} be discharged by the suction devices 8 ', 8 ", 10', 10" is reliably detected and on the other hand it is ensured that the suctioned gas flow G _a does not contain any of the still molten, on the metal band 2 adhering coating material entrains.

REFERENCE NUMBERS

Device 1 for hot dip coating of metal strip 2,

2 metal band

2 ', 2 "surfaces of the metal strip 2

3 continuous furnace

4 melt vessel

4a channel

5 melt bath

6 roll

7 area of the exit of the metal strip 2

8 ', 8 "suction devices

9'ι 9 "scraper nozzles

10, 10 "suction device ', 11" traverse

12, 12 "traverse

13, 13 "openings

14, 14 "support profile

15, 15 "spacer rings

16, 16 "suction pipe

17, 17 "nozzle section

18, 18 "nozzle openings of the nozzle sections 17 ', 17"

19, 19 "nozzle openings of the suction devices 10 ', 10"

20, 20 "pin swivel axis

F direction of conveyance

G ', G "gas flows

G _r fraction of the gas flowing on the metal strip 2

G _a suctioned gas streams h distance between the suction devices 8 ', 8 "and the upper edge of the melt vessel 4 or the region 7 of the exit of the metal strip 2 from the melt bath 5

V Distance between the suction nozzle opening of the suction devices 8 ', 8 "and the band surface 2' or 2" respectively assigned to them

Claims

P AT ENT AN SPOKEN

1. A method for hot-dip coating of metal strip (2), in which the metal strip (2) to be coated is moved through a melt bath (5) and which after leaving the melt bath (5) on a surface (2 ', 2 ") of the metal strip ( 2) Existing, still molten metallic coating by means of a gas flow (G ', 9 ") which is applied from at least one stripping nozzle (9-', 9") to a desired strength of that on the gas flow (G ', G ") surface (2 ', 2 ") of the finally remaining coating is blown off the metal strip (2), characterized in that the gas stream (G', G") flowing off the respective surface (2 ', 2 ") of the metal strip (2) is sucked off by means of a suction device (8 ', 8 ", - 10', 10") arranged near the stripping nozzle (9 ', 9 ") and near the surface (2', 2") of the metal strip (2).

2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t, that the gas stream (G ', G ") seen in the conveying direction (F) of the metal strip (2) is suctioned off in front of the stripping nozzle (9', 9").

Method according to one of the preceding claims, characterized in that the gas stream (G ', G ") seen in the conveying direction (F) of the metal strip (2) is suctioned off behind the stripping nozzle (9', 9").

Method according to one of the preceding claims, characterized in that the extracted gas volume flow (G ', G ") is dimensioned such that a residual gas flow (G _r ) remains from the gas flow (G', G") inflated onto the metal strip (2), which sweeps over the surface (2 ', 2 ") of the metal strip (2) hit by the gas flow (G', G").

5. The method according to any one of the preceding claims, characterized in that the extracted gas volume flow (G _a ) undergoes filtering.

Device for the hot dip coating of metal strip (2), with a melting vessel (4) for holding molten coating metal and with at least one stripping nozzle (9 ', 9 ^' ") which flows a gas stream (G ', G") from the melting vessel ( 4) • Exiting metal band (2) coated with the coating metal aligns the still molten coating metal present on the surface (2 ', 2 ") of the metal band (2) to a desired thickness of the gas flow (G ', G ") hit the surface (2', 2") of the finally remaining coating from the metal band (2), thereby characterized in that the stripping nozzle (9 ', 9 ") is assigned a suction device (8', 8"; 10 ', 10 ") which is arranged near the stripping nozzle (9', 9") and near the metal strip (2) in order to suck off a gas stream (G ', G ") flowing off the respective surface (2', 2") of the metal strip (2).

7. The device according to claim 6, d a d u r c h g e k e n n z e i c h n e t, that the suction device (8 ', 8 ") in the conveying direction (F) of the metal strip (2) is arranged in front of the stripping nozzle (9', 9").

8. The device according to claim 6 or 7, d a d u r c h g e k e n n z e i c h n e t, that the suction device (10 ', 10 ") in the conveying direction (F) of the metal strip (2) is arranged behind the stripping nozzle (9', 9").

9. Device according to one of claims 6 to 8, characterized in that 'the suction device (8', 8 "; 10 ', 10") is connected to a filter for filtering the extracted gas stream (G _a ).

10. Device according to one of claims 6 to 9, characterized in that the distance (h) of the suction device • (8 ', 8 ";10',10") to the melt vessel (4) is adjustable.

11. Device according to one of claims 6 to 10, d a d u r c h g e k e n n z e i c h n e t, d a s s the distance (v) of the suction device (8 ', 8 "; 10', 10"). To its respective assigned belt surface (2 ', 2 ") is adjustable.

12. Device according to one of claims 10 or 11, characterized in that the suction device (8 ', 8 "; 10', 10") is coupled to the stripping nozzle (9 ', 9 ") such that when the position of the Scraper nozzle (9 ', 9 ") the position of the suction device (8', 8"; 10 ', 10 ") also changes.

13. Device according to one of claims 6 to 12, d a d u r c h g e k e n n e e c e n c e, that the nozzle of the suction device (8 ', 8 "; 10', 10") is designed as a slot nozzle.

14. The apparatus of claim 13, d a d u r c h g e k e n n z e i c h n e t, that the width of the slot nozzle opening is adjustable.

15. The device according to one of claims 6 to 14, characterized in that at least the nozzle of the suction device (8 ', 8 ") is arranged around an axis parallel to the surface of the belt conveyed in the area of the suction device (8', 8") is pivotable.

6. Device according to one of claims 6 to 15, d a d u r c h g e k e n n z e i c h n e t, that the suction device (8 ', 8 "; 10', 10") is arranged loosely hanging on a holding element.