EP1814678B2 - Method and device for descaling a metal strip - Google Patents

Description

The invention relates to a method for descaling a metal strip, in particular a hot rolled strip of normal steel or a hot or cold rolled strip of austenitic or ferritic stainless steel, in which the metal strip is guided in a conveying direction through at least one plasma descaling device, in which he Plasma descaling is subjected. Furthermore, the invention relates to a device for descaling a metal strip.

The EP 0 879 897 A1 discloses a plasma descaling; after carrying out the same, the belt to be removed is passed through a cooling device. The cooling device comprises two cooling rollers.

For further processing - eg. For example, by cold rolling, for a metallic coating or the direct processing into a final product - steel strip must have a scale-free surface. Therefore, for example, the scale formed during hot rolling and during the subsequent cooling must be removed completely. This takes place in prior art processes by a pickling process, wherein the from the various iron oxides (FeO, Fe ₃ O ₄ , Fe ₂ O ₃ ) or in stainless steels also chromium-rich iron oxides existing scale depending on the steel quality by means of various acids (eg hydrochloric acid, sulfuric acid, Nitric acid or mixed acid) at elevated temperatures by chemical reaction with the acid. Prior to pickling, additional mild mechanical treatment by stretch bend straightening is required in normal steel to break up the scale and thus allow faster penetration of the acid into the scale layer. In the case of the stainless, austenitic and ferritic steels, which are much more difficult to pickle, annealing and mechanical descaling of the strip are preceded by the pickling process in order to achieve the best bondable surface of the strip. After pickling, the steel strip must be rinsed, dried and oiled as needed to prevent oxidation.

The pickling of steel strip is carried out in continuous lines, the process part of which can have a very long length depending on the strip speed. Such systems therefore require very high investments. The pickling process also requires a great deal of energy and wastewater disposal and regeneration of hydrochloric acid, which is commonly used in mild steel.

There are therefore various approaches in the prior art to accomplish the descaling of metallic strands without the use of acids. Previously known developments are based here mostly on a mechanical removal of the scale (eg Ishiclean method, APO method). However, such methods are not suitable in terms of their economics and quality of the descaled surface for the industrial descaling of wide steel strip. Therefore, the descaling of such tape still relies on the use of acids.

The disadvantages in terms of cost-effectiveness and environmental impact must therefore be taken into account so far.

More recent approaches to descaling metallic strands rely on plasma technology. Such methods and devices of the type mentioned for the descaling of metal strands with different geometry, such as metal bands or metal wire, are already known in the prior art in various embodiments. It is exemplified on the WO 2004/044257 A1 , on the WO 2000/056949 A1 and on the RU 2 145 912 C1 pointed. In the plasma descaling technology disclosed therein, the material to be descaled passes between special electrodes located in a vacuum chamber. The descaling is carried out by the plasma generated between the steel strip and electrodes, wherein a metallic bare surface is produced without residues. The plasma technology thus represents an economic, qualitatively perfect and environmentally friendly way of descaling and cleaning steel surfaces. It can be used for normal steel and for stainless, austenitic and ferritic steel. A special pretreatment is not required.

In plasma descaling, therefore, the strip passes between electrodes arranged above and below the strip through a vacuum chamber. The plasma is located between the electrodes and the tape surface on both sides of the tape. The effect of the plasma acting on the scale is the removal of the oxides on the strip surface, which is associated with an increase in the temperature of the strip; This can be very disadvantageous. The increase in temperature may result in the formation of an oxide film on the belt surface as the descaled belt exits the vacuum in air, which is not permitted for further processing such as cold rolling or direct hot strip processing.

The fact that the plasma descaling subsequent cooling of the metal strip can be done to improve this situation has become known from various solutions, for example from the JP 07132316 A , of the JP 06279842 A , of the JP 06248355 A , of the JP 03120346 A , of the JP 2001140051 A and the JP 05105941 A , The concepts emerging from this literature, however, focus on measures for cooling, some of which are associated with considerable disadvantages or are relatively inefficient. Thus, for example, a medium sprayed on for cooling is used, which makes it necessary to carry out a subsequent drying of the metal strip. In the treatment of the metal strip with cooling gas, the cooling rate is very low, also this solution is not possible in a vacuum. The otherwise proposed solutions hardly offer opportunities to achieve a specific temperature control of the metal strip.

For most applications, controlled cooling of the metal strip during descaling is required before the strip comes in contact with the air. Such targeted cooling is not possible with the solutions known from the prior art.

The invention is therefore based on the object to provide a method and an associated device for descaling a metal strip, with which it is possible to achieve a quality increase in the production of the metal strip, in particular by preventing oxidation processes, without the microstructure of the metal strip negative influence.

The solution of this problem by the invention is characterized according to the method by the features of claim 1.

For the purpose of achieving a complete descaling, it is preferably provided that the metal strip is subjected to plasma descaling at least twice, each time with subsequent controlled cooling.

Oxidation of the descaled metal strip in the ambient atmosphere is prevented by the last controlled cooling in the conveying direction being such that the metal strip leaves the last cooling device in the conveying direction at a temperature of less than or equal to 100 ° C.

On the other hand, the microstructure of the metal strip is not adversely affected by the plasma descaling in each of the plasma descaling devices being such that the metal strip behind the plasma descaling device has a temperature of at most 200 ° C.

As a particularly advantageous embodiment of the cooling of the metal strip, it has been found that the cooling of the metal strip in the at least one cooling device takes place in that the metal strip is brought into contact with a cooling roller via a predeterminable wrap angle. The cooled roll dissipates heat on contact with the metal strip therefrom. In order to optimize the heat transfer, it has been proven that the metal strip is held under tension at least in the area of contact with the cooling roll.

Advantageously, the metal strip is cooled at least substantially to the same temperature in each of the cooling subsequent to the plasma descaling. It is also advantageous if, alternatively or in addition thereto, the metal strip is cooled at least essentially by the same temperature difference in each of the cooling subsequent to the plasma descaling.

The cooling of the metal strip in the one or more cooling devices is preferably carried out under reduced pressure relative to the ambient pressure, in particular under vacuum. However, it can be provided that the cooling of the metal strip takes place in the last cooling device in the conveying direction under a protective gas, in particular under nitrogen.

The device for descaling the metal strip has the features of claim 10.

Preferably, in the conveying direction of the metal strip at the end or behind the or each cooling device, a temperature sensor is arranged, which is in communication with a control device which is suitable for influencing the cooling device with respect to the cooling power generated by it and / or the conveying speed of the metal strip.

Preferably, at least two plasma descaling devices are provided, to each of which a cooling device is connected.

By changing the wrap angle, the cooling capacity applied by the cooling device to the metal strip can be influenced, i. H. how much the cooling device cools the metal strip. Movement means are therefore preferably provided with which at least one cooling roller can be moved relative to another cooling roller perpendicular to the axes of rotation of the cooling rollers.

The cooling rolls are preferably liquid-cooled, in particular water-cooled.

Furthermore, means for generating a tensile force in the metal strip may be provided, at least in the region of the cooling devices. This ensures a good contact of the metal strip on the cooling rolls.

According to an installation concept, at least two plasma descaling devices and at least two downstream cooling devices are arranged in a straight line. An alternative to this, which is space-saving, provides that a plasma descaling device is arranged so that the metal strip is guided vertically upwards (or downwards) in it, and another plasma descaling device is arranged so that the metal strip in her vertically down (or up) is performed with a cooling device is disposed between the two plasma descaling.

A good cooling effect of the cooling rollers can be achieved if they have on their lateral surface a coating with a wear-resistant and highly thermally conductive material, in particular with hard chrome or ceramic.

The technology described offers great advantages in terms of environmental protection, energy consumption and quality compared to pickling.

Furthermore, the investment costs for corresponding systems are much lower than in known descaling and / or cleaning systems.

It is particularly advantageous that the metal strip to be descaled has a very good and unoxidized surface following descaling, so that the subsequent operations can be carried out with high quality.

The invention thus ensures that the metal strip is cooled during and after the descaling controlled to a temperature which is below the temperature at which an oxidation or tarnishing on the strip surface can occur in air.

In a method for descaling a metal strip, in particular a hot rolled strip of normal steel, in which the metal strip is guided in a conveying direction through at least one plasma descaling device in which it is subjected to plasma descaling, it can be provided that the plasma descaling directly or indirectly Coating the metal strip is followed by a coating metal, in particular a hot dip galvanizing of the metal strip.

Advantageously, the energy introduced by the plasma descaling into the metal strip can be used to preheat the metal strip prior to coating.

The metal strip is preferably first plasma-demineralized in a coupled system and then coated, in particular hot-dip galvanized. The metal strip preheated by the plasma descaling is preferably conducted without air access from the plasma descaling into the protective gas atmosphere of a continuous furnace required for the coating, where the strip is further heated to the temperature required for the coating. The band heating can be done inductively after the plasma descaling according to the "heat-to-coat" method. The strip, in particular the hot strip to be galvanized, can be heated very rapidly under reduced atmosphere to 440 ° C. to 520 ° C., in particular to approximately 460 ° C., before it enters the coating bath.

The plasma descaling downstream coating can be carried out according to the conventional method with deflection roller in the coating container or by the vertical method (Continuous Vertical Galvanizing Line - CVGL method), in which the coating metal is retained in the coating container by an electromagnetic closure. The metal strip dives only very briefly into the coating metal.

The plasma descaling system can be coupled to a continuous furnace for hot dip galvanizing of hot-rolled steel strip, wherein on the outlet side of Plasmaentzunderungsanlage a vacuum lock and on the inlet side of the continuous furnace furnace sluice of conventional design can be located, which are connected to each other gas-tight.

The latter coupling of plasma descaling and coating has particular advantages because hot rolled steel strip must be completely free of oxides prior to hot dip galvanizing to obtain a well adherent zinc layer.

In addition, the strip must be heated to a temperature which is about 460 ° C to 650 ° C, depending on the heating rate. In this case, the strip heating arising during plasma descaling can be used as preheating of the strip before it enters the continuous furnace, thereby achieving an energy saving and a shortening of the furnace.

Fig. 1

schematisch eine Vorrichtung zur Entzunderung eines Metallbandes in der Seitenansicht gemäß einer ersten Ausführungsform,

Fig. 2

eine zu Fig. 1 analoge Darstellung einer zweiten Ausführungsform der Vorrichtung,

Fig. 3

schematisch drei Kühlwalzen einer Kühlvorrichtung bei geringer Kühlleistung,

Fig. 4

die zu Fig. 3 analoge Darstellung bei hoher Kühlleistung der Kühlvorrichtung und

Fig. 5

schematisch eine Vorrichtung zur Entzunderung und nachfolgenden Feuerverzinkung des Metallbandes in der Seitenansicht.

In the drawings, embodiments of the invention are shown.
Show it:

Fig. 1

schematically a device for descaling a metal strip in the side view according to a first embodiment,

Fig. 2

one too Fig. 1 analogous representation of a second embodiment of the device,

Fig. 3

schematically three cooling rollers of a cooling device with low cooling power,

Fig. 4

the too Fig. 3 Analog representation at high cooling capacity of the cooling device and

Fig. 5

schematically a device for descaling and subsequent hot dip galvanizing of the metal strip in the side view.

In Fig. 1 is a device for descaling a steel strip 1 to see, this system is designed in a horizontal design. The steel strip 1 coming from a uncoiler 19 is directed in a stretch-bending machine 20 with the associated S-roll stands 21 and 22 so that the metal strip 1 has the greatest possible flatness before the strip enters the process part of the system under high tension.

Through several vacuum locks 23, the belt 1 enters a first plasma descaling device 2 in which the vacuum required for plasma descaling is generated and maintained by means of known vacuum pumps. In the plasma descaling device 2 are located on both sides of the belt 1 arranged electrodes 24, which generate the plasma required for descaling.

The plasma heats the strip surface on both sides, resulting in a heating of the entire strip cross-section to a temperature of max. 200 ° C at the end of the plasma descaling device 2 can lead. The amount of band heating over the total cross-section depends mainly on the conveying speed v of the metal strip 1 and the strip thickness with the same energy of the plasma, with increasing band speed v and strip thickness, the band heating is lower.

From the plasma descaling device 2, the not yet completely descaled belt 1 runs in a cooling device 4 provided with cooling rollers 6, 7, 8 which is connected in a gas-tight manner to the plasma descaling device 2 and in which the same vacuum prevails as in the plasma descaling device 2 ,

The belt 1 runs around the cooling rollers 6, 7, 8, the circumference of which is cooled from the inside with water, which dissipates the heat through a cooling circuit. The high strip tension causes the band 1 - the cooling rollers 6, 7, 8 wrapped around - good at these, in order to ensure the highest possible heat transfer.

The cooling rollers 6, 7, 8 wrap around the metal strip 1 alternately from above and from below. Preferably, three to seven cooling rolls are provided. The cooling water for cooling the cooling rolls is fed continuously via rotary feedthroughs and discharged again.

At the in Fig. 1 The arrangement shown are three cooling rollers 6, 7, 8 in the cooling device 4, which are driven individually. Depending on the performance and maximum belt speed v of the system, more cooling rollers are possible and useful. On the inlet side and the outlet side of the cooling device 4 are temperature sensors 12 for continuously measuring the temperature of the metal strip 1. By employment of one (or more) of the cooling rollers 6, 7, 8 (s. FIG. 3 and FIG. 4 ), for example, in the vertical direction of the wrap angle α (s. FIG. 3 and FIG. 4 ) and thus the cooling capacity of the cooling device 4 are regulated, which acts on the metal strip 1. At the end of the cooling device 4, the maximum strip temperature should be about 100 ° C.

From the cooling device 4, the cooled strip 1 passes into a second plasma descaling device 3, which is connected in a gastight manner to the cooling device 4 and in which the same vacuum as in the first plasma descaling device 2 is produced by means of vacuum pumps. In the second plasma descaling device 3, which is constructed similarly to the first one, the complete descaling of the strip 1 which has not yet completely descaled in the first plasma descaling device 2 takes place. The strip 1 heats up similarly as in the plasma descaling device 2 to a final temperature, which is dependent on the belt speed v and the belt cross-section about 100 ° C to 200 ° C above the inlet temperature in the plasma descaling device 3. From there, the belt 1 passes through a gas-tight lock 25 into the second cooling device 5 filled with protective gas (eg nitrogen), which is provided with cooling rollers 9, 10, 11 as the first cooling device 4.

Preferably, the individual plasma descaling devices 2 and 3 or more of these devices are all designed to be the same length.

The number of cooling rollers 6, 7, 8, 9, 10, 11 depends on the performance of the system. In the cooling device 5, the belt 1 is cooled by the cooling rollers 9, 10, 11 to a final temperature not exceeding 100 ° C. As in the case of the first cooling device 4, temperature sensors 13 for measuring the strip temperature are again located on the inlet side and outlet side of the cooling device 5. At the end of the cooling device 5 is another gas-tight lock 26, which prevents the entry of air into the cooling device 5. This measure ensures that the strip 1 exits the process part of the line at a maximum temperature of 100 ° C. and that the bare surface of the strip can not be oxidized by atmospheric oxygen.

Behind the process part of the system is a train roller stand 18 consisting of two or three rollers which applies the required strip tension or holds it together with the S-roller stand 22. The elements marked with the reference numerals 17 and 18 thus represent means for generating a tensile force in the belt 1. The tensile force generated in the belt 1 serves to ensure good contact of the belt 1 to the cooling rollers 6, 7, 8, 9, 10, 11 to ensure. Thereafter, the tape 1 passes through the necessary other facilities, such as tape storage and Besäumschere, to the reel 27 (as shown) or other coupled devices, eg. B. to a tandem mill.

Depending on the calculated required cooling capacity, the proposed plasma entrainment system can have one or more plasma descaling devices 2, 3 with adjoining cooling devices 4, 5. The embodiment according to Fig. 1 depends on two such units. If only one cooling device 4 is used, this is similar to the second cooling device 5 described here with the associated locks 25 and 26 are formed.

Fig. 2 shows an alternative embodiment of the plant for the descaling of steel strip 1, in which the plasma descaling devices 2 and 3 are arranged vertically (vertically). All functions in this system are identical to those in Fig. 1 explained attachment. A vertical arrangement may, under certain conditions, be more favorable than a horizontal arrangement because of its shorter length.

In the FIGS. 3 and 4 It can be seen how by vertical displacement of the cooling roller 7 (see double arrow), which is located between the two cooling rollers 6 and 7, the wrap angle α of the belt 1 about the rollers 6, 7, 8 can be changed (entered for the wrap angle around the roller 7), which also changes the heat transferred from the metal strip 1 to the cooling rollers 6, 7, 8 heat flow. The vertical displacement of the central cooling roller 7 is effected by means of movement 16, which is shown schematically and in the present case are designed as a hydraulic piston-cylinder system.

By measuring the strip temperature in or at the end of the cooling devices 4, 5 by the temperature sensors 12, 13 can by means of in Fig. 1 only schematically illustrated control devices 14 and 15 on the cooling capacity in the cooling devices 4, 5 are influenced, so that a desired outlet temperature of the belt 1 can be achieved. If the measured temperature is too high, a higher wrap angle α can be set by controlling the movement means 16, so that the band 1 is cooled better. In principle, the conveying speed v of the belt 1 can also be reduced or increased by the system in order to increase or reduce the cooling capacity. Here, of course, then a vote between the two control devices 14 and 15 is required.

In Fig. 5 A solution is sketched in which the heat introduced by the plasma descaling into the metal strip is used to coat the strip with a coating metal immediately after descaling. Fig. 5 shows the process part of a coupled plasma descaling and hot-dip galvanizing line for hot rolled steel strip. The strip 1, after being stretch-straightened in the stretch-bender 20 (stretch-straightening unit), passes through a vacuum lock 23 into the plasma descaling device 2 where it descalculates to about 200 ° C to 300 ° C, depending on the belt speed and belt thickness is heated.

Subsequently, the belt 1 passes through a vacuum outlet lock 25 and through the furnace inlet lock 29 connected thereto into a continuous furnace 28. On the inlet side of the furnace 28 there is a train roller pair 30 (hot letter), which has the required high strip tension in the plasma descaling device 2 generated.

Behind the tension roller pair 30, the belt temperature is measured with a temperature sensor 12, via which the required further belt heating in the continuous furnace 28 is controlled. From the point of the sensor 12, the belt 1 passes through the inductively heated continuous furnace 28, in which it is heated very quickly to about 460 ° C after the "heat-to-coat" process. Subsequently, the tape passes over a trunk 31 in the coating container 32, where it is hot-dip galvanized. With the wiping nozzles 34, the layer thickness is regulated. In the subsequent air cooling section 35, the metal strip 1 is cooled and then fed to the other required process steps, such as the skin-pass, the stretch-straightening and the chromating.

LIST OF REFERENCE NUMBERS

1

metal band

2

Plasma descaling

3

Plasma descaling

4

cooler

5

cooler

6

chill roll

7

chill roll

8th

chill roll

9

chill roll

10

chill roll

11

chill roll

12

temperature sensor

13

temperature sensor

14

control device

15

control device

16

means

17

Means for generating a tensile force

18

Means for generating a tensile force

19

uncoiler

20

Tension-leveling machine

21

S-roll stand

22

S-roll stand

23

vacuum lock

24

electrodes

25

lock

26

lock

27

coiler

28

Continuous furnace

29

Furnace inlet lock

30

Zugrollenpaar

31

trunk

32

coating tank

33

idler pulley

34

wiping

35

Air cooling section

R

conveying direction

α

Wrap angle

v

conveyor speed

Claims (18)

1. Method for descaling a metal strip (1), particularly a hot-rolled strip of normal steel or a hot-rolled or cold-rolled strip of austenitic or ferritic stainless steel, in which the metal strip (1) is led in a conveying direction (R) through at least one plasma descaling device (2, 3), in which it is subjected to plasma descaling, wherein the metal strip (1) subsequently to the plasma descaling in the at least one plasma descaling device (2, 3) is subjected to a regulated cooling in a cooling device (4, 5) in such a manner that it has a defined temperature behind the cooling device (4, 5), wherein the cooling of the metal strip (1) is carried out in the at least one cooling device (4, 5) in such a manner that the metal strip (1) is brought into contact with a cooling roller (6, 7, 8, 9, 10, 11) over a predeterminable angle (α) of looping, wherein at least three cooling rollers (6, 7, 8, 9, 10, 11), which are arranged to be movable relative to one another so that the angle (α) of looping between the metal strip (1) and the roller surface is variable and thus the cooling performance can be influenced, are arranged in the cooling device (4, 5).
2. Method according to claim 1, characterised in that the metal strip (1) is subjected to at least one double plasma descaling each with a respective subsequent regulated cooling.
3. Method according to claim 1 or 2, characterised in that the regulated cooling which is last in conveying direction (R) is carried out in such a manner that the metal strip (1) leaves the last cooling device (5) at a temperature of less than or equal to 100° C.
4. Method according to one of claims 1 to 3, characterised in that the plasma descaling in each of the plasma descaling devices (2, 3) is so carried out that the metal strip (1) behind the plasma descaling device (2, 3) has a temperature of at most 200° C.
5. Method according to one of claims 1 to 4, characterised in that the metal strip (1) is held under tension at least in the region of contact with the cooling roller (6, 7, 8, 9, 10, 11).
6. Method according to one of claims 2 to 5, characterised in that the metal strip (1) is cooled to at least substantially the same temperature in each cooling subsequent to the plasma descaling.
7. Method according to one of claims 2 to 5, characterised in that the metal strip (1) in each cooling subsequent to the plasma descaling is cooled substantially by the same temperature difference.
8. Method according to one of claims 1 to 7, characterised in that the cooling of the metal strip (1) in the cooling device or cooling devices (4, 5) is carried out under a pressure reduced relative to ambient pressure, particularly under vacuum.
9. Method according to one of claims 1 to 8, characterised in that the cooling of the metal strip (1) in the cooling device (5) which is last in conveying direction (R) is carried out in a protective gas, particularly in nitrogen.
10. Device for descaling a metal strip (1), particularly a hot-rolled strip of normal steel or a hot-rolled or cold-rolled strip of austenitic or ferritic stainless steel, which comprises at least one plasma descaling device (2, 3), through which the metal strip (1) is led in a conveying direction (R), wherein at least one cooling device (4, 5) arranged behind the plasma descaling device (2, 3) in conveying direction (R) and suitable for regulated cooling of the metal strip (1) to a defined temperature is provided, particularly for carrying out the method according to one of claims 1 to 9, wherein the cooling device or at least one of the cooling devices (4, 5) comprises at least three cooling rollers (6, 7, 8, 9, 10, 11) which are so arranged and movable relative to one another that the angle (α) of looping between the metal strip (1) and the roller surface is variable.
11. Device according to claim 10, characterised in that at least one temperature sensor (12, 13) is arranged in or in the conveying direction (R) of the metal strip (1) at the end or behind the or each cooling device (4, 5) and is connected with a regulating device (14, 15) suitable for influencing the cooling device (4, 5) with respect to the cooling performance it produces and/or the transport speed (v) of the metal strip (1).
12. Device according to claim 10 or 11, characterised by at least two plasma descaling devices (2, 3) with each of which a respective cooling device (4, 5) is connected.
13. Device according to one of claims 10 to 12, characterised by movement means (16) by which at least one cooling roller (6, 7, 8, 9, 10, 11) can be moved relative to another cooling roller (6, 7, 8, 9, 10, 11) perpendicularly to the axes of rotation of the cooling rollers (6, 7, 8, 9, 10, 11).
14. Device according to one of claims 10 to 13, characterised in that the cooling rollers (6, 7, 8, 9, 10, 11) are liquid-cooled, in particular water-cooled.
15. Device according to one of claims 10 to 14, characterised by means (17, 18) for generating a tension force in the metal strip (1) at least in the region of the cooling devices (4, 5).
16. Device according to one of claims 10 to 15, characterised in that at least two plasma descaling devices (2, 3) as well as at least two downstream cooling devices (4, 5) are arranged in a straight line.
17. Device according to one of claims 10 to 15, characterised in that a plasma descaling device (3) is so arranged that the metal strip (1) is guided vertically upwardly or downwardly therein and a plasma descaling device (3) is so arranged that the metal strip (1) is guided vertically downwardly or upwardly therein, wherein a cooling device (4) is arranged between the two plasma descaling devices (2, 3).
18. Device according to one of claims 10 to 17, characterised in that the cooling rollers (6, 7, 8, 9, 10, 11) of the at least one cooling device (4, 5) have on the circumferential surface thereof a coating with a wear-resistant and good thermally conductive material, particularly with hard chrome or ceramic.

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