CN1561404A - Process method of hot-dip finishing - Google Patents

Abstract

The invention relates to a method and a device for coating the surface of especially strips, for example a (NE) nonferrous metal strip or a steel strip, which are coated with at least one metal coating during the passage through at least one vessel containing a molten coating material. The metal strip to be coated passes through the bath of coating material in the vessel from top to bottom. Suitable guide means are provided for this purpose. The easy downward sealing is achieved by means of a cyclically operating permanent magnet.

Description

Process method of hot-dip finishing

The invention relates to a method for coating the surface of, in particular, strips, such as a non-ferrous metal strip or a steel strip, which are coated with at least A metallic coating. The invention also relates to a device for carrying out the process.

The traditional thermal coating (called method 1) of the strip with Zn alloy, Zn-Al alloy, Al alloy or Al-Si alloy in the coating part is as follows: the strip is taken out of a heating furnace, the air is cut off, It is fed into the melt and reversed to vertical and stable by means of different arrangements of non-driven rollers, see Figure 1. This applies to all stated coating metals/alloys for hot dip finishing.

The disadvantage of process method 1 is that the rollers and roller bearings are located in the melt and all materials are subject to chemical attack by the melt. The service life of all components inserted in the melt is shortened. In addition, a large amount of melt with a correspondingly large melt container is required in order to accommodate the rollers or the entire melt bath system. Usually 200-400t of liquid zinc is required for hot-dip galvanizing. Due to the large capacity, it is impossible to quickly adjust the temperature and alloy composition of the melt. Larger variations of the above-mentioned parameters have to be accommodated and in some cases lead to a loss of quality, because the measures in terms of alloy technology and the quality of the strip influence each other in the same container.

A further disadvantage is that, especially with thin strips (<0.5 mm), it is not possible to increase the production speed in order to achieve an economical plant capacity (approximately 180 m/min). The reason for this is that there is a relative movement between the rollers and the belt in the tank. If the tension is increased in order to avoid this problem, there is a risk of tearing the tape, with the consequence being a lot of scrap and a longer standstill of the equipment.

Another limitation of the maximum strip speed of hot galvanizing plants is the nozzle dressing system located above the molten zinc, see Figure 1. The layer thickness is adjusted by means of air or nitrogen, wherein the minimum achievable coating thickness increases with increasing belt speed. That is, it is not possible to produce thin coatings at high belt speeds. But it is the thin coatings (<25g/m ² , one side in hot galvanized sheets) that are welcome for certain high-end applications.

As a further development, a process for hot-dip finishing of ferrite strip made of soft unalloyed steel is known as so-called vertical hot-dip galvanizing, which has been described in various patents such as EP0630421B1 and EP 0630420B1 and EP Described in 0673444B1.

In this process (referred to as method 2), the strip passes from top to bottom through a working container filled with molten metal composed of zinc alloy and/or aluminum alloy, wherein the strip has previously undergone a heat treatment and made Bringing into the melt is carried out with the air cut off. Compared with the first method, the melt volume is about 2-5t less liquid zinc. And there is no qualitative problem mentioned above, this is because the measures of alloy technology are all carried out in an advanced container located next to this assembly line, and the quality of the belt in the working container has nothing to do with it.

The connection between the working container and the furnace chamber below it is realized via a gas-tight ceramic channel, which is about 800 mm high and only has a maximum width of 20 mm for the passage of the belt. The downward sealing of the working vessel and the prevention of the melt flowing downwards into the furnace chamber are achieved below this channel by means of two inductors arranged laterally in the channel or belt. This inductor generates a moving electromagnetic field, which creates an upwardly directed force that prevents the melt from moving downward. This induction system works like a pump and thus also ensures the exchange of the melt in the channels.

Method 2 is characterized in that, at least in the coating area up to the melt pool, much higher strip speeds of up to 300 m/min can be easily produced even for thin steel strips, since the rollers are no longer in the coating vessel up.

When the belt passes through the coating unit from bottom to top, for example, after reaching a temperature of about 460°C during hot-dip galvanizing, the desired thickness of the metal finishing layer can be adjusted on the molten pool in accordance with the method of nozzle cleaning and trimming similar to method 1. Certainly. This is done analogously to method 1 by blowing compressed air or nitrogen.

Similar to method 1, even in method 2 the nozzle cleaning trimming method limits the maximum possible strip speed if the coating is thin. Of course, method 2 provides greater freedom for galvanizing parameters such as temperature, melt viscosity and alloy composition which also have an influence on coating thickness. The reason for this is that a higher belt speed can be selected for method 2 than for method 1 at the same coating thickness. Compared with Method 1, Method 2 has not been tested in large batches. So far only prototypes have been tested with narrow belts. These experiments were very successful.

However, the reason for preventing the speed increase is that the belt must subsequently be cooled to below 300° C. before the upward section makes the first deflection. If the temperature is higher than this, the danger is that in the cooling tower, metal particles will be generated at the first contact roller or the reversing roller and irreparable surface defects will be produced on the material.

The cooling is usually carried out by means of a plurality of air cooling segments arranged one behind the other. However, the cooling effect and more precisely the cooling rate are limited by the medium and cannot be increased arbitrarily over a defined distance (eg 2×15 m) when the cooling medium air is used. As the belt speed increases or the mass flow rate increases, the distance of this cooling section must be lengthened. But this has led to an increase in the reversing rolls in the cooling tower of the hot-dip finishing plant.

In the equipment described in method 1, the height of the upper reversing roller is usually between 30-60m. For method 2 at high belt speeds it is necessary to correspondingly continue to lengthen the cooling section and thus increase the height of the cooling tower as much as possible in the direction of 80-90 m. This makes the investment costs of buildings and foundations even higher.

As a result, the idling, unstable belt section lengthens in the tower and the belt run deteriorates, so that vibrations can occur and have a negative effect on the product quality. There are still problems in the application of other cooling media in the upper section, and they have not been used on a large scale at present.

Another problem with the electromagnetic seal according to method 2 is that the forces acting on the liquid melt also act on the ferrite strip, in particular. An undesired contact between the strip and the channel by the magnetic force of the sealed inductor is only possible through additional cost-intensive measures. For this purpose, additional stabilizing coils and complex and expensive adjustment techniques are necessary.

The task of the present invention is to avoid the disadvantages of the above-mentioned methods 1 and 2 and to propose a high-speed hot-dip finishing plant without cooling towers, which combines the lowest possible manufacturing costs, optimized investment costs and the best production quality. When reaching high equipment capacity.

This object is solved in a method of the type stated in the preamble of claim 1 in that the container is sealed by means of a cyclically operating permanent magnet. The sealing of containers by means of cyclically operating permanent magnets is much more reliable and cost-effective than an electromagnetic solution, and requires much less energy for rotation than electromagnetic sealing, which is especially advantageous in the event of a power outage .

Some technical solutions of the method are described in other dependent claims. The device for carrying out the method and its design are described in the other claims.

The invention is described in terms of some schematically illustrated embodiments. Shown as:

Figure 1: A conventional coating method for tape;

Figure 2: A further developed coating method according to the technical background;

Fig. 3: according to the coating method of the present invention and a kind of high-speed hot-dip finishing equipment of corresponding design in operation;

Figure 4: Equipment in the starting state shown in Figure 3;

Figure 5: The device as shown in Figure 3, when stopped after running.

According to FIG. 3, after a change of direction in the furnace, with the air cut off, it enters vertically downwards into a container in which the melt bath is located. The solution pool is sealed downwards. Forces are required for this, but they are not electromagnetic, but are generated by means of permanent magnets running around them. The use of permanent magnets to secure the seal of the melt is known per se. But there already work with rectangular channels. The spacing and shape of this channel form cannot be changed.

Instead, the present invention recommends the use of two rotors 5, 5' arranged side by side. These rotors are tubes 6, 6' made of heat-resistant and melt-resistant material, preferably ceramic. Rollers rotate in these tubes 6 , 6 ′, the diameter of which can be freely selected, and permanent magnets 4 are arranged on the outer surfaces of these rollers. The rotors 5, 5' can be adjusted towards the melt or towards the belt. The gap 7 can also be closed when the plant is stopped or when the plant is started.

Permanent magnets are much cheaper than electromagnetic seals by means of coils or inductors, and require much less energy for rotation than electromagnetic seals, which is especially advantageous in the event of a power outage.

In addition, greater magnetic field strengths (factor 3) can be generated with permanent magnets compared to electromagnetic modes of operation. Such high magnetic field strengths and the resulting high forces are required for the cleaning and conditioning process to set the desired coating thickness on the steel strip. In the previously known methods this had to be done by means of additional cleaning and trimming nozzles.

Additional measures within the magnetic sealing and cleaning and reconditioning are no longer necessary in the method according to the invention, since the narrowest passage of the strip 1 through the sealing unit is only a few millimeters long. In addition, the belt 1 can be tensioned shorter than in previously known methods, since the belt 1 can be immediately cooled and reversed in a water bath 9 directly under the sealing unit. In the present invention, the stretched length is preferably only about 5000 mm, which is about 8-10 times larger in method 1 and larger in method 2.

Another advantage of the method according to the invention is: the molten metal, preferably the surface of the molten zinc in the coating site, is in a protective gas atmosphere, this protective gas consists of a nitrogen/hydrogen mixture, and Interfering oxidation of liquid zinc does not occur. In the case of the previously known methods 1 and 2, a considerable amount of effort is added to this implementation. Also in these methods the surface of the zinc pool must be made accessible for some manual work. In the present invention it is not necessary to provide access spaces to the surface of the molten pool for removal of oxide particles.

In the embodiment shown in FIG. 3, the apparatus for hot-dip finishing of a non-ferrous metal strip, or a steel strip 1, is in a state of continuous operation. The strip 1 fed in and to be finished passes through a tensioning roller 17 in the furnace 2 and then through a gate 18 which closes the protective air inside the hot-dip finishing plant in an oxygen-tight manner with respect to the ambient air.

In the subsequent galvanizing chamber 14 the belt 1 is deflected by means of guide rollers 13 so that it points perpendicularly to the coating area. When entering the coating area 19, the strip 1 passes in the vertical direction from top to bottom through the molten bath 3 held in the gap 7 between the rotors 5, 5' and thus obtains the desired coating.

The melt pool 3 is held at the lower end in a gap formed between the spaced rotors 5, 5' by means of the magnetic force of the magnetic field or the force of the moving magnetic field of the rotating permanent magnet 4 when the strip runs through. down. The rotors 5, 5' are located within the tubes 6, 6' surrounding them in a coating area 19 which is surrounded outwards by a channel-shaped casing, and the rotors 5, 5' are accommodated in this area at a variable distance. The pipes 6, 6' surrounding them are made of high temperature and melt resistant, especially non-magnetic material, preferably ceramics.

The permanent magnet 4 rotates inside these tubes 6, 6'. The melt required for coating and currently to be replenished is transported in adjustable quantities from a collecting tank 8 to the gap 7 between the rotors 5, 5' by means of a metal pump, in the collecting tank 8 The solution can be adjusted. The strip 1 coated on the inside passes through the gap 7 at the lower end and then through a device 15 for air stabilization and then through a device 16 for water cooling.

After passing through the pool 9 and the tension wheel 20, it is pulled out from the equipment for further use or processing.

The additional FIGS. 4 and 5 both show the method according to the invention.

a) in a starting state, and

b) Shutdown after operation.

a) Starting state:

-band stop

- The rotor turns

-Close the gap between the rotors

- input melt

-Close the oven chamber.

b) Shutdown after operation

-melt return

- The rotor turns

- gap closure

- The cooking chamber is open.

Claims (9)

1. to ribbon especially, the method of coating is carried out on the surface of for example a kind of non-ferrous metal band or a kind of steel band, these ribbons have been coated the coating of at least a metal when the container of liquation shape coated material is housed by at least one, it is characterized in that container is realized sealing by means of the permanent magnet of circular flow.

2. by the described method of claim 1, it is characterized in that, just carried out the adjustment of desirable coat-thickness on metal strip simultaneously by means of the permanent magnet of circular flow.

3. by claim 1 or 2 described methods, it is characterized in that, the coated material of liquation shape is in the wedge gap between the rotor of two reverse rotations of a liquid-collecting box introducing, and wherein metal strip passes through liquid-bath from the top down and passes between spaced apart rotor.

4. by one in the aforesaid right requirement or multinomial described method, it is characterized in that metal strip is preferably under the shielding gas atmosphere under the situation of cut-out air in preheating oven back warp commutation back and vertically is passed down through liquid-bath.

5. by one in the aforesaid right requirement or multinomial described method, it is characterized in that the line space gas of bringing into that coating is crossed is stablized and/or water cooling.

6. especially for the device of implementing by one of aforesaid right requirement described method, it comprises that at least one container is used to place the liquation shape coated material that metal ribbon is used, it is characterized in that, for making container sealing be provided with two antiports, the rotor that can mutually adjust in case of necessity, wherein within rotor, be provided with rotating running roller, on the outside surface of running roller, fixed permanent magnet.

7. by the described device of claim 6, it is characterized in that the container that liquid-bath is housed is made of the top intermediate cavity between the rotor.

8. by claim 6 or 7 described devices, it is characterized in that these rotors are surrounded by a shell keeping under a kind of shielding gas atmosphere at least.

9. by one or multinomial described device in the claim 6 to 8, it is characterized in that, rotor case and one are used for metal strip from being delivered to the upper chamber of rotor case, and the liquid-collecting box of using with a liquation, and be arranged in for example being used for bringing the stable and water-cooled device of line space gas under the rotor case, and be connected with another pond in case of necessity.