EP1565590B1 - Method and device for hot-dip coating a metal strand - Google Patents

Abstract

A method for hot dip coating a metal strand includes passing the metal strand vertically through a coating tank that contains molten coating metal and through a guide channel upstream of the coating tank. A electromagnetic field is generated by inductors on both sides of the metal strand and an electromagnetic field superposed on the electromagnetic field of the inductors is generated by supplementary coils on both sides of the metal strand. A center position of the metal strand in the guide channel is stabilized by: (a) measuring the position of the metal strand in the guide channel; (b) measuring the induced current in the inductors; (c) measuring the induced current in the supplementary coils; and (d) influencing the induced current in the supplementary coils as a function of all of the parameters measured in steps (a) to (c).

Description

The invention relates to a process for hot dip coating a metal strand, in particular a steel strip, in which the metal strand is passed vertically through a container receiving the molten coating metal and through an upstream guide channel, wherein for retaining the coating metal in the container in the region of the guide channel an electromagnetic field by means of at least two inductors arranged on either side of the metal strand are produced, and wherein for stabilizing the metal strand in a central position in the guide channel, an electromagnetic field superimposed on the electromagnetic field of the inductors is generated by means of at least two additional coils arranged on both sides of the metal strand. Furthermore, the invention relates to a device for hot dip coating of a metal strand.

Conventional 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 of oxide residues prior to coating and activated for connection to the coating metal. For this reason, the belt surfaces are treated in a reducing atmosphere prior to coating in heating processes. Since the oxide layers are previously removed chemically or abrasive, the reducing heat process activates the surfaces in such a way that they are metallically pure after the heat process.

With the activation of the strip surface but increases the affinity of these strip surfaces for the surrounding atmospheric oxygen. To prevent that Air oxygen can get back to the belt surfaces before the coating process, the tapes are introduced into a dip proboscis from above into the dip coating bath. Since the coating metal is in liquid form and one would like to use the gravitation together with blowers for adjusting the coating thickness, but the subsequent processes prohibit band contact until complete solidification of the coating metal, the band must be deflected in the coating vessel in the vertical direction. 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 frequent shutdowns of the plant.

In order to avoid the problems associated with the rollers running in the liquid coating metal, there have been attempts to use a downwardly open coating vessel which has a guide channel for vertical tape feed-through in its lower portion and an electromagnetic shutter for sealing use. 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 known, for example, from EP 0 673 444 B1. An electromagnetic shutter for sealing the coating vessel The solution according to WO 96/03533 or the one according to JP 5086446 also begins at the bottom.

The coating of non-ferromagnetic metal bands is thus possible, but problems occur in the substantially ferromagnetic Stahlbändem that they are drawn in the electromagnetic seals by the ferromagnetism to the channel walls and the tape surface is damaged. Furthermore, it is problematic that the coating metal and the metal strip itself are inadmissibly heated by the inductive fields.

The position of the passing ferromagnetic steel strip through the guide channel between two traveling field inductors is an unstable equilibrium. Only in the middle of the guide channel is the sum of the magnetic forces of attraction acting on the tape zero. As soon as the steel strip is deflected from its center position, it gets closer to one of the two inductors, while it moves away from the other inductor. Causes of such a deflection may be simple flatness errors of the tape. Worth mentioning are any kind of tape waves in running direction, seen across the width of the tape (Centerbuckles, Quarterbuckles, Randwellen, Flattem, twisting, Crossbow, S-shape, etc.). The magnetic induction, which is responsible for the magnetic attraction decreases according to an exponential function with the distance from the inductor in their field strength. Similarly, the attraction force decreases with the square of the induction field strength with increasing distance from the inductor. For the deflected band, this means that with the deflection in one direction, the attractive force to one inductor expands exponentially, while the restoring force from the other inductor exponentially decreases. Both effects are self-reinforcing, so the balance is unstable.

To solve this problem, ie the exact position control of the metal strand in the guide channel, DE 195 35 854 A1 and DE 100 14 867 A1 notes. According to the concepts disclosed therein, it is provided that in addition to the coils for generating the electromagnetic traveling field additional additional coils are provided which are in communication with a control system and ensure that the metal strip is retrieved when departing from the central position in this.

A similar concept is also disclosed in JP 05078802 A. The additional coils are positioned in the guide channel below the inductors.

Other solutions for guiding the metal strip as accurately as possible are known from EP 0 855 450 A1, JP 10046310 A, WO 02/14572 A1 and JP 2000 053295 A.

In these previously known approaches, it has been found to be disadvantageous that the efficiency of the control is not sufficient to ensure a stable guidance of the metal strand in the middle of the guide channel. The problem in this context may be the large guy length between the lower deflection roller below the guide channel and the upper deflection roller above the coating bath, which in a production plant is well over 20 m. can lie. This reinforces the need for efficient position control of the metal strip in the guide channel.

The invention is therefore based on the object to provide a method and an associated apparatus for hot dip coating of a metal strand, with which or with which it is possible to overcome the disadvantages mentioned. The efficiency of the control should therefore be improved, which should make it possible in a simpler manner, to keep the metal strand in the middle of the guide channel.

• a) Messen der Lage des Metallstranges im Führungskanal;
• b) Messen des Induktionsstroms in den Induktoren;
• c) Messen des Induktionsstroms in den Zusatzspulen;
• d) Einwirken auf den Induktionsstrom in den Zusatzspulen in Abhängigkeit aller in den Schritten a) bis c) gemessenen Parameter, um den Metallstrang in einer mittigen Lage im Führungskanal zu halten,

wobei die Zusatzspulen in Förderrichtung des Metallstranges gesehen innerhalb der Erstreckung der Induktoren angeordnet sind.The solution of this object by the invention according to the method is characterized in that the stabilization of the central position of the metal strand in the guide channel is carried out by the sequence of the following steps in a closed loop:

• a) measuring the position of the metal strand in the guide channel;
• b) measuring the induction current in the inductors;
• c) measuring the induction current in the auxiliary coils;
• d) acting on the induction current in the auxiliary coils as a function of all parameters measured in steps a) to c) in order to keep the metal strand in a central position in the guide channel,

wherein the additional coils are arranged in the conveying direction of the metal strand within the extension of the inductors.

Thus, the concept of the invention is based on the fact that the three sizes of position of the metal strand in the guide channel, induction current in the inductors and induction current in the auxiliary coils are detected and taken into account in the regulation of the position of the metal strand; the manipulated variable of the control loop is then in turn the induction current in the additional coils.

With this procedure, it is possible to take into account both the magnetic field generated by the inductors (main coils) itself and the superimposed magnetic field caused by the additional coils in the control, resulting in an overall improvement in the efficiency of the control.

A first development is based on the fact that the electromagnetic field generated for sealing is a multi-phase traveling field, which is generated by applying an alternating current with a frequency between 2 Hz and 2 kHz. Alternatively, a single-phase alternating field can be provided, which is generated by applying an alternating current with a frequency between 2 kHz and 10 kHz.

Particularly preferably, the determination of the position of the Metatlstranges in the guide channel is made inductively.

In order to ensure the most accurate possible recording of the tape position, provides a development that the determination of the position in a region of the guide channel takes place in which there is no or only a weakened effect of the magnetic field of the inductors and / or the magnetic field of the additional coils. Alternatively, however, it is also possible for this determination to take place in a region of the guide channel in which an effect of these magnetic fields is present.

The measuring means (the measuring coils) for determining the position of the metal strand thus lies within or outside the range of the electromagnetic elements, by which both the inductor and the additional coils are to be understood.

It is possible, in particular, for the measuring device to be arranged in front of the additional coil in the region of the extension of the inductor, for the measuring device to be arranged in the region of the extension of the inductor next to the additional coil, or for the measuring device to be arranged outside the region of the extent of the inductor. Combinations of these arrangements are possible.

The device according to the invention for hot dip coating a metal strand with at least two on both sides of the metal strand in the region of the guide channel arranged inductors for generating an electromagnetic field for retaining the coating metal in the container and with at least two arranged on both sides of the metal strand auxiliary coils for generating a the electromagnetic field of the inductors superimposed electromagnetic Field for stabilizing the metal strand in a central position in the guide channel is characterized by measuring means for measuring the position of the metal strand in the guide channel, the induction current in the inductors and the induction current in the auxiliary coils and by control means for controlling the induction current in the auxiliary coils in dependence measured parameters are suitable to hold the metal strand in a central position in the guide channel, the auxiliary coils in Förderrichtun g of the metal strand seen within the extension of the inductors are arranged.

Advantageously, the measuring means for detecting the position of the metal strand in the guide channel is an inductive sensor.

Furthermore, it can be provided that the measuring means for detecting the position of the metal strand in the guide channel in the conveying direction of the metal strand is arranged within the extension of the inductors. But it is also possible that the measuring means is arranged outside the extension of the inductors. In both cases, it is possible that the measuring means for detecting the position of the metal strand in the guide channel in the conveying direction of the metal strand is arranged outside the extension of the additional coils. This ensures a precise position detection of the metal strand.

Finally, a development provides that several measuring means are arranged for detecting the position of the metal strand in the guide channel at different points in the conveying direction of the metal strand. The individual measuring means can be arranged both inside and outside the magnetic fields of inductor or additional coil.

In the drawing, an embodiment of the invention is shown. The single figure shows schematically a hot-dip coating device with a guided through this metal strand.

The device has a container 3 which is filled with molten coating metal 2. This may be, for example, zinc or aluminum. To be coated metal strand 1 in the form of a steel strip passes through the container 3 in the conveying direction R vertically upwards. It should be noted at this point that it is also possible in principle that the metal strand 1, the container 3 passes from top to bottom. For the passage of the metal strand 1 through the container 3, this is open in the bottom area; Here is an exaggeratedly large or broad guide channel. 4

So that the molten coating metal 2 can not flow down through the guide channel 4 down, are located on both sides of the metal strand 1, two electromagnetic inductors 5, which generate a magnetic field, which causes buoyancy forces in the liquid coating metal 2, which counteract the gravitational force of the coating metal 2 and thus seal the guide channel 4 downwards.

The inductors 5 are two oppositely disposed alternating field or traveling field inductors, which are operated in the frequency range from 2 Hz to 10 kHz and establish an electromagnetic transverse field perpendicular to the conveying direction R. The preferred frequency range for single-phase systems (AC field inductors) is between 2 kHz and 10 kHz, that for multi-phase systems (eg traveling field inductors) between 2 Hz and 2 kHz.

The aim is to keep the metal strand 1 located in the guide channel 4 so that it is defined as possible in a position, preferably in the center plane 11 of the guide channel 4, is located.

The metal strand 1 located between the two opposing inductors 5 is generally attracted to the closer inductor upon application of an electromagnetic field between the inductors 5, the attraction increasing as it approaches an inductor, resulting in a highly unstable midband position. This results in the operation of the device, the problem that the metal strand 1 due to the attraction of the inductors 5 can not run freely and centrally through the guide channel 4 between the activated inductors.

To stabilize the metal strand 1 in the center plane 11 of the guide channel 4 therefore additional coils 6 are arranged on both sides of the guide channel 4 and der Metallstranges 1. These are controlled by a control means 10 so that the superposition of the magnetic fields of the inductors 5 and the additional coils 6 keeps the metal strand 1 always in the middle in the guide channel 4.

By means of the additional coils 6, therefore, the magnetic field of the inductors 5 can be amplified or attenuated depending on the control (superposition principle), without violating the sealing condition (minimum required field strength for the seal). In this way, the position of the metal strand 1 in the guide channel 4 can be influenced.

For this purpose, the control means 10 are initially supplied with a signal s, s' or s "representing the position of the metal strand 1 in the guide channel 4. The position s, s' or s" is determined by position measuring means 7, 7 'and 7, respectively The determination of the position of the metal strand 1 between the inductors 5 in the electromagnetic field thus takes place inductively, the feedback effect of the metal strand 1 in the magnetic field being utilized.

The control means 10 are further supplied with the determined by current measuring means 8 and 9 induction currents in the inductors 5 - current I _Ind - or in the auxiliary coils 6 - current I _Korr -versorgt.

Algorithms are stored in the control means 10, which, starting from the three parameters position s, s' or s "of the metal strand 1 in the guide channel, induction current I _Ind in the inductors 5 and induction _current I _Korr in the auxiliary coils 6, a new control signal in the form of an induction current I _Korr to the auxiliary coils 6. In this way, the position of the metal strand 1 in the closed loop is maintained so that the positional deviations of the metal strand 1 from the center plane 11 are minimal, ie that the value s, s' or s "zero possible becomes.

As can be seen, the position s, s 'or s "of the metal strand 1 in the guide channel 4 by means of the position measuring means 7, 7' and 7" determined, the position measuring means 7 - viewed in-conveying direction R - above the inductors , the position measuring means 7 'are positioned below the inductors 5 and the position measuring means 7 "in the area of the inductors 5. In the present case, all three position measuring means 7, 7' and 7" are arranged outside the range of the additional coils 6. From the means the position measuring means 7, 7 ', 7 "measured values, an average value can be formed in the control means 10.

Since the position measuring means 7, 7 'and 7 "are inductive displacement transducers, the influence of the magnetic fields which are caused by the inductors 5 and the additional coils 6 should remain as small as possible. This is achieved by the arrangement of the position measuring means 7 7 'outside the extension of the inductors 5. However, as can be seen in the figure, a position measuring means (in the present case 7 ") can be positioned in the region of the inductors 5.

Although a positioning of the position measuring means 7 or 7 'outside of the effect of the additional coils 6 has proven successful, they can therefore be arranged in principle in the area of action of the inductors 5 and the additional coils 6.

LIST OF REFERENCE NUMBERS

1

Metal strand (steel strip)

2

coating metal

3

container

4

guide channel

5

inductor

6

additional coil

7

Position measuring means

7 '

Position measuring means

7 "

Position measuring means

8th

Current measuring means

9

Current measuring means

10

control means

11

midplane

s

Position of the metal strand in the guide channel

s'

Position of the metal strand in the guide channel

s "

Position of the metal strand in the guide channel

I _Ind

Induction current in the inductor

I _corr

Induction current in the additional coil

R

conveying direction

Claims (12)

1. Method of hot-dip coating a metal strip (1), particularly a steel strip, in which the metal strip (1) is guided vertically through a container (3), which receives the molten coating metal (2), and through an upstream guide channel (4), wherein for restraint of the coating metal (2) in the container (3) an electromagnetic field is generated in the region of the guide channel (4) by means of at least two inductors (5) arranged on either side of the metal strip (1) and wherein for stabilisation of the metal strip (1) in a central position in the guide channel (4) an electromagnetic field, which is superimposed on the electromagnetic field of the inductors (5), is generated by means of at least two additional coils (6) arranged on either side of the metal strip (1), characterised in that the stabilisation of the central position of the metal strip (1) in the guide channel (4) is carried out by the sequence of the following steps in a closed regulating loop:

   a) measuring the position (s, s', s") of the metal strip (1) in the guide channel (4);

   b) measuring the induction current (I_Ind) in the inductors (5);

   c) measuring the induction current (I_Korr) in the additional coils (6);

   d) acting on the induction current (I_Korr) in the additional coils (6) in dependence on all parameters (s, I_Ind, I_Korr), which are measured in the steps a) to c), in order to keep the metal strip (1) in a central position in the guide channel (4),

   wherein the additional coils (6) are arranged within the length of the inductors (5) as seen in conveying direction (R) of the metal strip (1).
2. Method according to claim 1, characterised in that the electromagnetic field is a multi-phase travelling field produced by application of an alternating current with a frequency between 2 Hz and 2 kHz.
3. Method according to claim 1, characterised in that the electromagnetic field is a single-phase alternating field which is produced by application of an alternating current with a frequency between 2 kHz and 10 kHz.
4. Method according to one of claims 1 to 3, characterised in that determination of the position (s, s', s") of the metal strip (1) in the guide channel (4) is carried out inductively.
5. Method according to one of claims 1 to 4, characterised in that determination of the position (s, s', s") in a region of the guide channel (4) is carried out in that no or only an attenuated effect of the magnetic field of the inductors (5) and/or of the magnetic field of the additional coils (6) is present.
6. Method according to one of claims 1 to 4, characterised in that determination of the position (s, s', s") in a region of the guide channel (4) is carried out in that an action of the magnetic field of the inductors (5) and/or of the magnetic field of the additional coils (6) is present.
7. Device for hot-dip coating a metal strip (1), particularly a steel strip, in which the metal strip (1) is guided vertically through a container (3), which receives the molten coating metal (2), and through an upstream guide channel (4), with at least two inductors (5), which are arranged on either side of the metal strip (1) in the region of the guide channel (4), for generating an electromagnetic field for restraint of the coating metal (2) in the container (3) and with at least two additional coils (6), which are arranged on either side of the metal strip (1), for generating an electromagnetic field - which is superimposed on the electromagnetic field of the inductors (5) - for stabilising the metal strip (1) in a central position in the guide channel (4), characterised by measuring means (7, 7', 7", 8, 9) for measuring the position (s; s', s") of the metal strip (1) in the guide channel (4), the induction current (I_Ind) in the inductors (5) and the induction current (I_Korr) in the additional coils (6) as well as through regulating means (10), which are suitable for controlling the induction current (I_Korr) in the additional coils (6) in dependence on the measured parameters (s, s', s", I_Ind, I_Korr) in order to keep the metal strip (1) in a central position in the guide channel (4), wherein the additional coils (6) are arranged within the length of the inductors (5) as seen in the conveying direction (R) of the metal strip (1).
8. Device according to claim 7, characterised in that the measuring means (7, 7', 7") for detection of the position (s, s', s") of the metal strip (1) in the guide channel (4) is an inductive measurement pick-up.
9. Device according to claim 7 or 8, characterised in that the measuring means (7, 7', 7") for detection of the position (s, s', s") of the metal strip (1) in the guide channel (4) is arranged within the length of the inductors (5) as seen in conveying direction (R) of the metal strip (1).
10. Device according to claim 7 or 8, characterised in that the measuring means (7, 7', 7") for detection of the position (s, s', s") of the metal strip (1) in the guide channel (4) is arranged outside the length of the inductors (5) as seen in conveying direction (R) of the metal strip (1).
11. Device according to one of claims 7 to 10, characterised in that the measuring means (7, 7', 7") for detection of the position (s, s', s") of the metal strip (1) in the guide channel (4) is arranged outside the length of the additional coils (6) as seen in conveying direction (R) of the metal strip (1).
12. Device according to one of claims 7 to 11, characterised in that several measuring means (7, 7', 7") for detection of the position (s, s', s") of the metal strip (1) in the guide channel (4) are arranged at different positions as seen in conveying direction (R) of the metal strip (1).

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