Hot dip coating apparatus, device and method for wiping a coated metal strip

Abstract

The invention relates to a hot dip coating apparatus for coating a metal strip. The invention also relates to a device for wiping excess coating metal from a coated metal strip in a hot dip coating apparatus, and a method for wiping excess coating metal from a coated metal strip in a hot dip coating apparatus.

Description

[0001] The invention relates to a hot dip coating apparatus for coating a metal strip. The invention also relates to a device for wiping excess coating metal from a coated metal strip in a hot dip coating apparatus, and a method for wiping excess coating metal from a coated metal strip in a hot dip coating apparatus.

[0002] Metal strip, such as steel strip, is nowadays often coated using a hot dip coating apparatus. The coating is provided as a metal corrosion protection, and usually is a zinc or zinc alloy layer, or an aluminium or aluminium alloy layer. To coat the metal strip, the moving strip is introduced in a bath containing liquid metal; when the metal strip leaves the bath with liquid metal, a coating of the liquid metal adheres to the metal strip. This coating layer is much thicker than is needed. Usually, the excess coating metal is removed using air knives.

[0003] A hot dip coating apparatus for coating a metal strip usually comprises a bath for liquid metal, a snout for introducing the metal strip into the liquid metal, a sink roll for deflecting the metal strip out of the liquid metal, and air knives for wiping excess coating metal from the coated metal strip. The metal strip has two main faces and two side edges. In most cases a stabilising roll and a corrector roll are present in the liquid metal below the air knives to correct the transverse curvature or crossbow of the metal strip that is introduced by the bending of the metal strip over the sink roll, such that the transverse curvature is minimal and the thickness of the coating after the wiping by the air knives is even. However, it is also possible that one or more stabilising rolls are present in the snout instead of the stabilising and corrector rolls in the liquid metal.

[0004] Although the hot dip coating apparatus and the use of it are improved over the years, such that also coated metal strip for demanding purposes can be produced, for instance for automotive purposes, the use of the air knives for wiping the excess liquid coating metal gives a limitation to the thickness of the coatings. At coating speeds of the metal strip above 120 meters per minute, which is a usual production speed, the coating cannot be thinner than 35 grams per square meter, single sided, for zinc coatings. This means that the zinc coating can not be thinner than about 7 micrometres. For thinner coatings a higher pressure of the air leaving the air knives is needed, but this results in splashing, which is detrimental to the quality of the coating.

[0005] Several attempts have been made to use electromagnetic wiping to provide thin coatings at high velocity of the strip. This has been done by generating a varying magnetic field, by using permanent magnets, by using rotating magnets or by having an electromagnet generated by an alternating current that generates a varying magnetic field, that again induces a current in the coating. For most of these solutions the force is too small to produce a coating with the correct gauge. Using electromagnets leads to inductive heating by the induced current, which leads to overheating.

[0006] JP61-227158A shows the use of electromagnetic wiping in combination with the use of air knives.

[0007] US2011 / 0177258 shows the use of a static magnetic field using electromagnets, wherein the strip can be kept dynamically in a given position between the two electromagnets by controlling the magnetic capacity of at least one of the electromagnets.

[0008] It is an object of the invention to provide a hot dip coating apparatus that can be used to produce thin coatings at high velocity of the strip.

[0009] It is another object of the invention to provide a device and method for wiping excess coating metal from a coated metal strip in a hot dip coating apparatus with which thin coatings can be produces at high velocity of the strip.

[0010] It is a further object of the invention to provide a hot dip coating apparatus that is simple, and that is stable in use.

[0011] It is moreover an object of the invention to provide a device and method for wiping excess coating metal from a coated metal strip in a hot dip coating apparatus that makes the production of thin coatings at high velocity of the strip easy and stable.

[0012] According to a first aspect of the invention a hot dip coating apparatus is provided for coating a metal strip, comprising a bath for liquid metal, comprising a snout for introducing the metal strip into the liquid metal, wherein the metal strip has two main faces and two side edges, comprising a sink roll for deflecting the metal strip out of the liquid metal, and comprising a wiping device for wiping excess coating metal from the coated metal strip, wherein the wiping device comprises at least one magnet for providing a magnetic field (B) near a main face of the coated metal strip, means for providing a direct current (I) in the coated metal strip from one side edge to the other side edge of the metal strip, and a stabilizer.

[0013] In the hot dip coating apparatus according to the invention the wiping of the excess coating material from the strip is not performed by air knives, but by using at least one magnet that provides a magnetic field, in combination with a stabilizer. So as to generate a high pressure on the liquid coating on the strip, an electric current must be present in the coated metal strip, which current has to run from one side edge to the other side edge of the coated strip. The pressure on the coating of liquid metal on the strip when it passes the at least one magnet reduces the thickness of the coating, in the same way as the pressure of the air (or nitrogen) from the air knives reduces the thickness of the coating of liquid metal on the strip. By choosing the right combination of the strength of the magnetic field (B) and the direct current (I) the thickness of the coating after the electromagnetic wiping can be set, in conjunction with the velocity of the strip. It has been found that in this way it is possible to realise a thickness of the coating on the strip that is thinner than the minimal thickness that can be produced using air knives. A stabilizer has to be present to stabilize the strip such that a certain distance between the magnet and the strip is maintained.

[0014] Preferably, the wiping device comprises at least one magnet providing a magnetic field (B) near each main face of the metal strip. In this way the hot dip coating apparatus will wipe both sides of the coated metal strip, such that both sides of the strip obtain a reduced thickness of the coating.

[0015] According to a preferred embodiment the means for providing a direct current (I) comprise rolls for contacting each side edge of the metal strip, or the means for providing a direct current (I) comprise sliding contacts for contacting each side edge of the metal strip. The rolls or sliding contacts press against both sides of the strip, and thus make it possible that the direct current (I) can run from one side edge of the strip to the other side edge. It will be clear that the rolls or sliding contacts are present at the same height above the liquid metal as the one or more magnets when the hot dip coating apparatus is in use. When coating a strip that changes in width during coating, the transition from one width to the next width of the strip is preferably a smooth curvature, such that the rolls or sliding contacts can remain in contact with the strip. If that is not possible, the rolls or sliding contacts will have to be removed from the side edges of the strip during the transition period.

[0016] Preferably, the stabilizer is an air stabilizer, a zinc-pad or a magnetic stabilizer. The air stabilizer, zinc-pad and magnetic stabilizer are known to the person skilled in the art and need not be discussed here.

[0017] According to a preferred embodiment the air stabilizer or the magnetic stabilizer is positioned above the at least one magnet. This is done to prevent that the coating metal that is wiped off the strip can foul the air stabilizer or magnetic stabilizer.

[0018] According to another preferred embodiment the zinc-pad is positioned below the at least one magnet. The zinc-pad thus acts on the coating metal on the strip before it is wiped off by the at least one magnet.

[0019] According to a second aspect of the invention a device for wiping excess coating metal from a coated metal strip in a hot dip coating apparatus is provided, wherein the metal strip has two main faces and two side edges, the device comprising at least one magnet for providing a magnetic field (B) near a main face of the coated metal strip, means for providing a direct current (I) in the coated metal strip from one side edge to the other side edge of the metal strip, and a stabilizer.

[0020] This device for wiping excess coating metal from a coated metal strip is the device used in the hot dip coating apparatus according to the first aspect of the invention. This device is elucidated above and need not be discussed here once more.

[0021] Preferably, the device for wiping a metal strip in a hot dip coating apparatus comprising at least one magnet for providing a magnetic field (B) near each main face of the metal strip. This has been elucidated above.

[0022] According to a preferred embodiment the means for providing a direct current (I) comprise rolls for contacting each side edge of the metal strip, or wherein the means for providing a direct current (I) comprise sliding contacts for contacting each side edge of the metal strip. The rolls and sliding contacts for contacting each side edge of the metal strip have been elucidated already when discussing the hot dip coating apparatus above.

[0023] Preferably, the stabilizer is an air stabilizer, a zinc-pad or a magnetic stabilizer. This has been elucidated above for the hot dip apparatus.

[0024] According to a third aspect of the invention a method for wiping excess coating metal from a coated metal strip in a hot dip coating apparatus is provided, wherein the metal strip has two main faces and two side edges, wherein a magnetic field (B) is provided above a metal bath in the hot dip coating apparatus near a main face of the coated metal strip leaving the metal bath, wherein a direct current (I) is provided from one side edge to the other side edge of the coated metal strip, and wherein the coated metal strip is stabilized above or below the magnetic field.

[0025] This method according to the third aspect of the invention is the method implemented when the hot dip coating apparatus according to the first aspect of the invention or the device for wiping excess coating metal from a coated metal strip in a hot dip coating apparatus according to the second aspect of the invention is used. The magnetic field, the direct current and the stabilization are provided by the hardware of the apparatus or the device. The elucidation of the hot dip apparatus is sufficient to understand the method according to the third aspect of the invention.

[0026] According to the invention the magnetic field (B) and the direct current (I) provide a Lorenz force (F L ) in the coated metal strip which is determined by the strength of the magnetic field (B) and the strength of the direct current (I). The Lorenz force F L generates a pressure over the thickness of the coating, and thus provides a very efficient wiping force to wipe liquid coating metal from the strip. The usual air knives generate a pressure that interacts only on the surface.

[0027] Preferably, the strength of the magnetic field (B) and / or the strength of the direct current (I) is based on the thickness of the metal strip, the thickness of the coating to be wiped, and the velocity of the metal strip. The thickness of the metal coating after wiping when using the method of the invention is thus influenced by the properties of the strip that has to be wiped, also in view of the changes in these properties during the wiping in the transition from one part of the strip to the next, or when the velocity of the strip is changed.

[0028] According to a preferred embodiment a magnetic field (B) is provided at each main face of the coated metal strip. In this way both sides of the metal strip will be wiped. It is possible to use a magnetic field that is different at each side of the strip, so the thickness of the metal coating after wiping will differ at both sides of the strip.

[0029] Preferably the metal strip is stabilized using an air stabilizer, a zinc-pad or a magnetic stabilizer. This has been elucidated for the hot dip coating apparatus according to the first aspect of the invention, above.

[0030] The invention will be elucidated referring to the accompanying drawings. Fig. 1 shows a schematic side view of the conventional wiping using air knives. Fig. 2 shows a schematic side view of the invention, using magnets for wiping. Fig. 3 shows a schematic front view of the side view of Fig. 2.

[0031] Figure 1 schematically shows the conventional way of wiping a metal coating on a metal strip using air knives, as it is nowadays commonly used. After a steel strip has entered a bath of liquid metal through a snout, its direction is deflected around a sink roll in the bath of liquid metal. All this is known to the skilled person and not shown in Figure 1. Figure 1 shows the metal strip 1 when it is pulled out of the bath 2 of liquid metal with a velocity V. The liquid metal adheres to the steel strip and forms a metal coating 3 on both sides of the metal strip. Since the coating 3 when leaving the batch of liquid metal is much thicker than needed for e.g. corrosion protection, the excess liquid coating metal has to be removed, which is performed by blowing air or nitrogen under high pressure against the strip using air knives 4. The excess liquid coating metal that is wiped off the metal strip falls back into the bath 2 of liquid metal.

[0032] Figure 2 schematically shows a side view of an exemplary embodiment of the wiping of a metal coating on a metal strip using the invention. As in Figure 1, Figure 2 shows the steel strip 1 when it is pulled out of the bath 2 of liquid metal with a velocity V. Of course here as well, the liquid metal adheres to the steel strip and forms a metal coating 3 on both sides of the metal strip 1. However, instead of the use of air knives as conventionally used, now permanent magnets 5 are used to wipe the liquid metal from the metal strip 1, so as to provide thin metal coatings 3 on both sides of the metal strip 1. The permanent magnets 5 provide a magnetic field B on both sides of the metal strip 1 (the magnetic field B is not shown in Figure 2).

[0033] Figure 3 schematically shows the exemplary embodiment of Figure 2 in front view. Here, the metal coating 3 on one of the main faces of the steel strip is shown when pulled out of the bath 2 of liquid metal. Also shown is one of the permanent magnets 5 that is located in front of the metal strip. Furthermore, shown is that means 7 for providing a direct current I are provided at both side edges of the metal strip. These means 7 are for instance rolls that can roll along the side edges of the moving metal strip, or sliding contacts that slide against the side edges of the moving metal strip. The rolls or sliding contacts are pressed against the side edges of the moving metal strip, such that the direct current I will run from one side edge to the other side edge.

[0034] When coating a strip that changes in width during coating, the transition from one width to the next width of the strip is preferably a smooth curvature, such that the rolls or sliding contacts can remain in contact with the strip. If that is not possible, the rolls or sliding contacts will have to be removed from the side edges of the strip during the transition period.

[0035] Figure 2 shows that above the permanent magnets 5 a strip stabilisation device 6 is present (the strip stabilisation device is not shown in Figure 3). Due to the magnetic fields that are provided by the permanent magnets 5, the steel strip 1 could be forced in the direction of one of the magnets 5. So as to stabilise the metal strip 1, the strip stabilisation device 6 is required.

[0036] The strip stabilisation device 6 that is schematically shown in Figure 2 is for instance an air stabilizer, that uses air or another gas that is blown towards both main sides of the metal strip 1. Thus, the movement of the teel strip towards one of the permanent magnets is minimised. Such air stabilizers are known in the art and need not be elucidated here.

[0037] The strip stabilisation device 6 that is schematically shown in Figure 2 can instead of an air stabilizer also be a magnetic stabilizer, that uses electromagnetic forces to stabilise the steel strip. The magnetic stabilizer in principle works in an analogous way as the air stabilizer. The magnetic stabilizer is known in the art and need not be elucidated here.

[0038] Instead of the strip stabilisation device 6 in the form of an air stabilizer of magnetic stabilizer it is also possible to use a zinc-pad as a strip stabilisation device. A zinc-pad can be placed below the permanent magnets 5 and acts as a hydrodynamic stabiliser by keeping the steel strip centred in the liquid metal in the zinc-pad. Though called a zinc-pad, it can also be used for other liquid metals such as aluminium (alloy).

[0039] It will be clear that the invention can be used for the wiping of a zinc (alloy) coating or an aluminium (alloy) coating, but that the invention is also suitable for the wiping of other coating metals.

[0040] The invention has been elucidated using the exemplary embodiment of the figures. It will be clear that all types of alterations can be made that are covered by the scope of the claims.

Claims

1. Hot dip coating apparatus for coating a metal strip, comprising a bath for liquid metal, comprising a snout for introducing the metal strip into the liquid metal, wherein the metal strip has two main faces and two side edges, comprising a sink roll for deflecting the metal strip out of the liquid metal, and comprising a wiping device for wiping excess coating metal from the coated metal strip, wherein the wiping device comprises at least one magnet for providing a magnetic field (B) near a main face of the coated metal strip, means for providing a direct current (I) in the coated metal strip from one side edge to the other side edge of the metal strip, and a stabilizer.

2. Hot dip coating apparatus according to claim 1, wherein the wiping device comprises at least one magnet providing a magnetic field (B) near each main face of the metal strip.

3. Hot dip coating apparatus according to claim 1 or 2, wherein the means for providing a direct current (I) comprise rolls for contacting each side edge of the metal strip, or wherein the means for providing a direct current (I) comprise sliding contacts for contacting each side edge of the metal strip.

4. Hot dip coating apparatus according to any one of the preceding claims 1 - 3, wherein the stabilizer is an air stabilizer, a zinc-pad or a magnetic stabilizer.

5. Hot dip coating apparatus according to claim 4, wherein the air stabilizer or the magnetic stabilizer is positioned above the at least one magnet.

6. Hot dip coating apparatus according to claim 4, wherein the zinc-pad is positioned below the at least one magnet.

7. Device for wiping excess coating metal from a coated metal strip in a hot dip coating apparatus, wherein the metal strip has two main faces and two side edges, the device comprising at least one magnet for providing a magnetic field (B) near a main face of the coated metal strip, means for providing a direct current (I) in the coated metal strip from one side edge to the other side edge of the metal strip, and a stabilizer.

8. Device for wiping a metal strip in a hot dip coating apparatus according to claim 7, comprising at least one magnet for providing a magnetic field (B) near each main face of the metal strip.

9. Device for wiping a metal strip according to claim 7 or 8, wherein the means for providing a direct current (I) comprise rolls for contacting each side edge of the metal strip, or wherein the means for providing a direct current (I) comprise sliding contacts for contacting each side edge of the metal strip.

10. Device for wiping strip material in a hot dip coating apparatus according to claim 7, 8 or 9, wherein the stabilizer is an air stabilizer, a zinc-pad or a magnetic stabilizer.

11. Method for wiping excess coating metal from a coated metal strip in a hot dip coating apparatus, wherein the metal strip has two main faces and two side edges, wherein a magnetic field (B) is provided above a metal bath in the hot dip coating apparatus near a main face of the coated metal strip leaving the metal bath, wherein a direct current (I) is provided from one side edge to the other side edge of the coated metal strip, and wherein the coated metal strip is stabilized above or below the magnetic field.

12. Method according to claim 11, wherein the magnetic field (B) and the direct current (I) provide a Lorenz force (FL) in the coated metal strip which is determined by the strength of the magnetic field (B) and the strength of the direct current (I).

13. Method according to claim 11 or 12, wherein the strength of the magnetic field (B) and / or the strength of the direct current (I) is based on the thickness of the metal strip, the thickness of the coating to be wiped, and the velocity of the metal strip.

14. Method according to any one of claims 11 - 13, wherein a magnetic field (B) is provided at each main face of the coated metal strip.

15. Method according to any one of claims 11 - 14, wherein the metal strip is stabilized using an air stabilizer, a zinc-pad or a magnetic stabilizer.

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