EP1639147B1 - Method for hot dip coating a metal bar and method for hot dip coating - Google Patents

Description

The invention relates to a device and a method for hot dip coating a metal strand, in particular a steel strip with Zn, Al, Zn-Al alloys, in which the metal strand is vertically passable through a container receiving the molten coating metal and through an upstream guide channel, wherein on both Side of the guide channel electromagnetic inductors are arranged, which generate a magnetic field for the retention of the coating metal in the container, and wherein the container is supplied by a pre-melt container with molten coating metal.

Known metal immersion coating systems for metal strips have a maintenance-intensive part, namely the coating vessel with the equipment located therein. The surfaces of the metal strips to be coated must be cleaned prior to coating and activated for connection to the coating metal. For this reason, the strip is treated in a continuous furnace in a reducing atmosphere prior to coating. Since the oxide layers are previously chemically removed, the reducing heat process activates the surfaces so that they are metallically pure after the heat process. In doing so, the strip is heated to the temperature required for coating with zinc, aluminum or zinc-aluminum alloys.

With the activation of the strip surface but increases the affinity of these strip surfaces for the surrounding atmospheric oxygen. In order to prevent atmospheric oxygen from getting back to the belt surfaces before the coating process, the belts are placed in a plunger from above in the Dip coating bath introduced. However, since the coating metal is in liquid form, and one would like to use gravitation with blowers ("air knives") to adjust the coating thickness, but the subsequent processes prohibit belt contact until the coating metal solidifies completely, the belt in the coating vessel must be deflected in a vertical direction become. This happens with a roller that runs in liquid metal. Due to the liquid coating metal, this roller is subject to heavy wear and is the cause of downtimes and thus failures in production.

Due to the desired low coating thickness of the coating metal, which can move in the micrometer range, high demands are placed on the quality of the strip surface. This means that the surfaces of the tape-guiding rollers also have to be of high quality. Disturbances on these surfaces generally lead to damage to the strip surface. This is another reason for possible plant shutdowns.

In order to avoid the problems associated with the rollers running in the liquid coating metal, solutions are known which provide a downwardly open coating vessel having in its lower region a guide channel of defined height for vertical tape feed-through upwards and an electromagnetic seal for sealing Has closure. These are electromagnetic inducers that work with backward, pumping or constricting electromagnetic alternating or traveling fields that seal the coating vessel down.

Such a solution is for example from the EP 0 673 444 B1 known. An electromagnetic closure for sealing the coating vessel down is also the solution according to the WO 96/03533 or the one according to the JP 5086446 one.

For a precise control of the position of the metal strand in the guide channel see the DE 195 35 854 A1 and the DE 100 14 867 A1 special solutions. According to the concepts disclosed therein additional correction coils are provided in addition to the coils for generating the electromagnetic traveling field, which are in communication with a control system and ensure that the metal strip is retrieved when departing from the central position in this.

At one of the EP 0 630 421 B1 known apparatus for hot dip coating a metal strand below the coating vessel, an electromagnetic closure device is arranged. It is envisaged there that the container for the molten coating metal is associated with a pre-melt, wherein the container is smaller in volume by a multiple than the pre-melt. The container is connected for refilling or for emptying with the pre-melt through feed and discharge channels, wherein the molten coating material between the pre-melt and the coating container to complete the circulation of atmospheric oxygen is circulated.

According to this embodiment, the pre-melt container for the coating metal is arranged laterally of the actual coating container. This arrangement of the pre-melt container is particularly favorable for new hot-dip coating equipment which can be designed for optimum performance of the hot-dip coating process.

From the JP 63 317656 A , from the JP 60 245774 A and from the WO 93/18198 A Coating devices are also known in which the metal strand to be coated is guided vertically through a guide channel.

The molten material necessary for the coating is supplied to the coating container from a pre-melt, which is in fluid communication with the coating container via lines and promotes in this melt.

The EP 0 451 020 A1 shows a solution in which a seal is provided down by a pair of cooperating rollers, which can accumulate in the nip from above from a guide channel leaking, overflowing melt.

It has been found that the vertical coating process (also known by the name CVGL method = Continuous Vertical Galvanizing Line method) is process-technically more favorable than the conventional hot-dip coating process, which operates with a deflection roller and stabilizing rollers running in the molten coating metal. Therefore, there is a desire to retrofit existing hot dipping sprinklers to vertical coating equipment. In particular, the space relationships that often require conceptual compromises that do not lead to optimal process conditions should be considered.

The invention is therefore based on the object to provide a way to convert existing conventional hot dip coating equipment so that the vertical coating process can be optimally performed, with maximum benefits to be drawn from the existing system.

This object is achieved in that the pre-melt is arranged vertically below the guide channel, wherein one of an oven trunk is provided from which the metal strand runs in the feed direction, wherein the metal strand with at least one deflection roller, preferably with two pulleys, deflected in the vertical direction and fed to the guide channel, and wherein the line of intersection of the extension of the metal strand in the feed direction the extension of the metal strand in the vertical direction through the guide channel is below the level of the molten coating metal in the pre-melt, so that the pass line of the metal strand is not changed compared to the conventional method.

In this case, it is preferably provided that the premelting container is suitable for receiving a deflecting roller arranged in the molten coating metal, that is to say it is such a container which is suitable for the classical implementation of the hot-dip coating method.

With this embodiment, a simple and cost-effective concept for the conversion or modernization of existing hot dip coating equipment on the vertical coating process is created in which, nevertheless, optimal process conditions can be realized. The container for the hot-dip coating together with the upstream guide channel is located directly above the conventional plant and there via its coating container, which acts as a pre-melt container; the coating container, which in the classical method has the deflection roller immersed in the coating metal, is therefore used as a premelting container for the vertical coating installation.

Preferably, the end of the oven trunk and the lower end of the guide channel are connected to a gas-tight and heated roller chamber. It can also be provided that between the end of the Ofenrüssels and the roller chamber a lock, in particular a roller lock, is arranged.

The apparatus preferably further includes a controllable or controllable pump for pumping molten coating metal from the premelting container into the container. Further, a controllable or controllable drain may be provided for passing molten coating metal from the vessel into the premelting vessel. Lines between the container, the Vorschmelzbehälter, the pump or the drain can be designed to be heated.

Above the container, a deflection roller can be arranged, which deflects the metal strand from the vertical direction. This deflection roller is advantageously water-cooled in order to be able to manage with the cooling section above the coating vessel of the coating system to be converted or modernized.

At least one of the existing pulleys and the guide rollers which are in contact with the metal strand may be provided with a ceramic coating which is not wettable by molten coating metal.

The method for hot dip coating of the metal strand in the vertical coating process, in which the container is supplied by a pre-melt with molten coating metal, according to the invention is characterized in that to start the coating process molten coating metal is conveyed in the conveying direction moving metal strand from Vorschmelzbehälter in the initially empty preheated receptacle , wherein between Vorschmelzbehälter and receptacle by means of a pump and a drain, a transfer of molten coating metal whose volume flow is at least five times as large as the discharge of coating metal from the container through the metal strand.

It is advantageously provided that before starting the coating process in the roller chamber by acting on the roller chamber with a protective gas and setting a desired temperature in the roller chamber an atmosphere with very low dew point is produced, which favors the adhesion of the coating metal to the surface of the metal strand ,

Another development provides that the metal strand is fed to the guide channel with a temperature between 450 ° C and 530 ° C.

Furthermore, it can be provided that the level height of the coating metal in the container is controlled or regulated according to a predetermined value.

Between the pre-melt container and the container can advantageously be carried out by means of the pump and the drain, a transfer of molten coating metal, the volume flow is substantially greater, preferably at least five times greater than the discharge of coating metal from the container through the metal strand.

The pre-melt can be supplied in solid form new coating metal. From the pre-melt, impurities can be removed, preferably periodically.

In the drawing, an embodiment of the invention is shown. The single figure shows schematically the side view of a hot dip coating plant for coating a metal strand with coating metal.

The illustrated hot dip coating apparatus operates according to the vertical coating method, i. H. the metal strand 1 runs vertically upwards in the conveying direction R through a guide channel 4 and comes into contact with the molten coating metal 2, which is located in a container 3 and in the upper part of the guide channel 4.

It is notable that this vertical coater is based on a converted hot dip coater in which the classic hot dip coating process (with deflection roll in the molten coating metal) is carried out. In this case, the metal strand 1 enters in a feed direction Z in a container 6, in which molten coating metal 2 is located. A deflection roller 7 deflects the metal strand 1 in the vertical direction V. Above the container 6, a blower 22 is arranged, which represents an "air knife", via which the layer thickness of the coating metal 2 is set on the metal strand 1. Further above, a cooling section 23 is arranged, which cools the metal strand 1 together with the coating metal 2.

It is shown that the section line 12 of the extension of the metal strand 1 in the feed direction Z with the extension of the metal strand 1 in the vertical direction V through the guide channel 4 below the level 13 of the coating metal 2 in the container 6.

The two deflection rollers 10 and 11 are thus arranged so that the pass line of the metal strand 1 in both the furnace trunk 9 and in the vertical part of the hot dip coating plant - in comparison with the original classical coating plant - is not changed.

In the illustrated hot dip coating system, however, the metal strand 1 does not enter the coating metal located in the container 6, but the metal strand 1 is deflected from the feed direction Z via the deflection rollers 10 and 11 in the vertical direction V, so that the metal strand 1 is above the guide roller 10 and the guide rollers 24 may enter the guide channel 4. Electromagnetic inductors 5 hold back the coating metal 2 located in the container 3, so that it can not run down through the guide channel 4.

The deflection roller 7 running in the original system in the molten coating metal 2 is shown in dashed lines, which indicates that it is no longer needed in the hot-dip coating system shown and consequently can be dismantled.

The metal strand 1 is first heated in an oven 8 and transported in the conveying direction R. It passes via a furnace trunk 9, which has the original hot-dip coating, via a roller lock 15 in a roller chamber 14 (preferably electrically heated), which connects the end of the oven trunk 9 and the lower end of the guide channel 4 gas-tight. In the roller chamber 14, the metal strand 14 is maintained at the temperature T set in the furnace.

The double roller lock 15 has the task to separate different inert gas atmospheres in the oven 8 on the one hand and in the roller chamber 14 on the other hand from each other and to prevent that in the event of a fault, air from the roller chamber 14 can get into the oven 8. In addition, it fulfills an important procedural function when starting the hot-dip coating installation: the sealing of the protective gas atmosphere in the roller chamber 14 makes it possible to achieve the low dew point required for the coating within a short time. This causes within a very short time after filling the coating metal 2 in the container 3 a flawless Adhesion of the coating metal 2 can be achieved on the metal strand 1, which is an important advantage over the conventional hot dip coating process.

The lock 15 can be acted upon with nitrogen or another protective gas, so that the required sealing of the atmosphere of the roller chamber 14 takes place in relation to that in the furnace 8. The roller chamber 14 is likewise filled with inert gas, preferably using nitrogen, forming gas (nitrogen with a maximum of 5% hydrogen) or a protective gas with low heat conductivity (eg argon).

The container 6 of the original hot dip coater serves as a pre-melt container, i. H. from it molten coating metal 2 is conveyed by a submersible in the melt controllable or controllable pump 16 and a heated line 19 into the container 3. In the bottom region of the container 3, a controllable or controllable drain 17 is arranged, which consists of an actuatable plug (movable in the direction of the double arrow). Via the outflow 17, coating metal 2 can pass from the container 3 back into the premelting container 6 via a further heatable line 20.

By appropriate control of the pump 16 and the drain 17, a desired level height h of coating metal 2 can be maintained in the container 3. In the lines 19 and 20, the conveying movement of the coating metal 2 is indicated schematically by arrows.

Above the hot-dip coating installation and the air cooling section 23, a liquid-cooled deflection roller 21 is provided which deflects the metal strand from the vertical direction V and conveys it away from the hot-dip coating installation in the conveying direction R.

The pump 16 is arranged laterally below the roller chamber 14; the pump 16 dips into the molten coating metal 2 in the premelting tank 6.

The volume of the Vorschmelzbehälters 6 is a multiple of that of the container. 3

The return line 20 for molten coating metal 2 from the container 3 into the pre-melt container 6 ends below the level 13 in the pre-melt container 6.

The quantity of molten coating metal 2 conveyed by the pump 16 from the premelting container 6 into the container 3 preferably remains substantially constant. This results in a constant circulation of coating metal, in which constantly fresh and impurity-free coating metal from the pre-melt 6 in the container 3 is promoted. The temperature control of the coating metal 2 takes place in the premelting tank 6, the level of which 13 is continuously regulated or kept constant by the melting of blocks of solid coating metal. The level 13 in the pre-melt 6 is set so that in the event of a malfunction of the hot-dip coating system, the entire coating metal 2 can be taken from the container 3 from the pre-melt 6.

The "air knife" 22 and the cooling section 23 are arranged above the container 3, similar to the conventional hot-dip coating. The air cooling section 23 is adjusted accordingly in terms of their performance because of the shorter available cooling length. As an additional measure for cooling the metal strand 1, a water-cooled deflection pulley 21 can be used.

The pre-melt container 6 is provided with a charging device, not shown, by means of which solid blocks coating metal can be used in the pre-melt container 6 for melting.

The coated to be coated metal strand 1 of hot-rolled or cold-rolled steel is supplied to the roller chamber 14 in the coating with zinc at a temperature between 450 ° C and 530 ° C through the end zone of the furnace 8 and the furnace trunk 9 and via the acted upon with inert gas lock 15 , wherein at the beginning of the coating process of the container 3 is initially empty, d. H. There is initially no coating metal 2 in it.

After start of the metal strand 1 in the conveying direction R, molten coating metal 2 is pumped from the premelting container 6 into the container 3 via the pump 16. Before that, the electromagnetic inducers 5 were activated so that the coating metal 2 filled in the container 3 is retained therein and can not leak down.

Subsequently, the desired level h in the container 3 is maintained by appropriate control or regulation of both the pump 16 and the drain 17.

The level height h in the container 3 is thereby controlled or regulated by the pump 16 as much as possible by supplying molten coating metal 2 and by appropriately controlled or regulated discharge of molten coating metal 2 via the outflow 17 as a function of the belt speed and the desired coating quality.

The quantity of molten coating metal 2 circulated by the pumping or the return flow between the premelting container 6 and the container 3 is in this case a multiple of the amount of coating metal discharged as a coating by the metal strand 1 per unit of time.

By pumping molten coating metal 2 from the pre-melt container 6 into the container 3, fresh and clean coating metal is constantly supplied to the container 3. Impurities, in particular hard zinc, can be precipitated in the premelting tank 6 and then removed from it at desired time intervals.

LIST OF REFERENCE NUMBERS

1

metal strand

2

molten coating metal

3

container

4

guide channel

5

Inductor (magnet)

6

Premelter

7

idler pulley

8th

oven

9

furnace snout

10

idler pulley

11

idler pulley

12

intersection

13

water level

14

roll chamber

15

Lock (roller lock)

16

pump

17

outflow

18

19

management

20

management

21

idler pulley

22

blow-off

23

cooling section

24

guide rollers

Z

feed

V

vertical direction

T

temperature

H

level height

R

Conveying direction of the metal strand

Claims (8)

1. Device for hot dip coating a metal strand (1), especially a steel strip, in which the metal strand (1) can be vertically guided through a tank (3) containing the molten coating metal (2) and through an upstream guide channel (4), wherein electromagnetic inductors (5) are arranged in the area of the guide channel (4) and induce a magnetic field for keeping the coating metal (2) in the coating tank (3), and wherein the receiving tank (3) is supplied with molten coating metal (2) from a premelting tank (6), characterised in that the premelting tank (6) is arranged vertically below the guide channel (4), wherein a furnace snout (9) which extends from a furnace (8) and from which the metal strand (1) runs out in a feed direction (Z) is provided, wherein the metal strand (1) is deflected into the vertical direction (V) by at least two deflecting rollers (10, 11) and fed to the guide channel (4), wherein the line of intersection (12) of the extension of the metal strand (1) in the feed direction (Z) with the extension of the metal strand (1) in vertical direction (V) through the guide channel (4) is located below the level (13) of the molten coating metal (2) in the premelting tank (6), so that the pass line of the metal strand (1) is not changed by comparison with the conventional process, and wherein the premelting tank (6) is constructed for reception of a deflecting roller (7) arranged in the molten coating metal (2).
2. Device according to claim 1, characterised in that the end of the furnace snout (9) and the lower end of the guide channel (4) are connected with a gastight roller chamber (14).
3. Device according to claim 2, characterised in that a lock (15), especially a roller lock, is arranged between the end of the furnace snout (9) and the roller chamber (14).
4. Device according to any one of claims 1 to 3, characterised by a controllable or regulable pump (16) for pumping molten coating metal (2) from the premelting tank (6) into the receiving tank (3).
5. Device according to claim 4, characterised by a controllable or regulable outlet (17) for transferring molten coating metal (2) from the receiving tank (3) to the premelting tank (6).
6. Device according to claim 4 or claim 5, characterised in that lines (19, 20) between receiving tank (3), premelting tank (6), pump (16) and/or outlet (17) are designed to be heatable.
7. Device according to any one of claims 1 to 6, characterised in that a deflecting roller (21) which deflects the metal strand (1) out of the vertical direction (V) is positioned above the receiving tank (3).
8. Device according to any one of claims 1 to 7, characterised in that at least one of the deflecting rollers (10, 11, 21) and the guide rollers (24) are provided with a ceramic coating which cannot be wetted by molten coating metal (2).

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