GB2066786A

GB2066786A - Method and apparatus for reducing oscillation of running strip

Info

Publication number: GB2066786A
Application number: GB8041034A
Authority: GB
Original assignee: Hitachi Ltd; Nisshin Steel Co Ltd
Current assignee: Hitachi Ltd; Nippon Steel Nisshin Co Ltd
Priority date: 1979-12-26
Filing date: 1980-12-22
Publication date: 1981-07-15
Also published as: BR8008487A; FR2473025A1; FR2473025B1; AU6585980A; GB2066786B; AU542225B2; DE3048672A1; DE3048672C2

Abstract

In a continuous galvanizing line, oscillation of a steel strip 10 withdrawn from a molten zinc metal bath (12) is reduced by liquid-cooled anti-vibration magnets (34, 36, 38, 40) disposed in the vicinity of the respective side edges of the steel strip (10). The magnets cause tension in the strip in the direction of the strip width and amplitude of the oscillation of the strip perpendicular to the strip surfaces is reduced. Also, by magnetic force of the magnet, bending of the running strip is cured and a uniform coating is obtained. The gap between the magnets and the strip side edge is automatically controlled using a motor 48, actuated by detecting signals derived from sensors scanning the gap. Such anti-vibration magnets can also reduce oscillation in other strip processing lines, and an application to a strip inspection line is disclosed. <IMAGE>

Description

SPECIFICATION Method and apparatus for reducing oscillation of running strip The present invention relates to a method and apparatus for reducing oscillation of magnetic strip material which is being continuously treated while the strip is running in various processing lines such as continuous galvanizing lines, annealing lines, pickling lines and inspection lines.

In these processing lines, various treatments and inspections of the strip usually have to be conducted while the strip is running at a high line speed. For example, thickness control of the material to be coated on the strip in a continuous galvanizing line is conducted while the strip is running at a line speed of 200 m/min or more. Also, the flow and/or thickness of the strip material have to be detected while the strip is running.

Usually, these running strips are supported at spaced locations by a pair of rollers or pinch rollers with a tension imparted on the strip in the longitudinal direction. In the strip between the adjacent pairs of rollers, various types of problems arise such as oscillation in the strip thickness and strip width directions, and also deformation of the strip surfaces into a convex or concave shape. These problems become serious as the line speed of the strip increases and cause deteriorations of the strip quality due to lack of accuracy in the treatment and detection errors in the flaw inspections or in the detection of thickness of the strip.

According to one aspect of the present invention there is provided a method of reducing oscillation of a strip formed of magnetic material, which is continuously running along a predetermined passage, comprising a step of applying a magnetic field to the strip in the direction of the strip width, thereby imparting a tension to the strip in the direction of strip width in a non-contacting manner.

According to a second aspect of the present invention there is provided an apparatus for reducing oscillation of a strip formed of magnetic material, which is running continuously along a predetermined passage comprising magnetic means for reducing oscillation of the strip, which is disposed in the vicinity of at least one side edge of the strip in a non-contacting manner with the strip, said magnetic means being adapted to impart tension to the strip in a direction of the strip width by applying a magnetic field to the strip.

The magnetic field acts to attract the side edge of the strip in the direction of the magnetic field and causes a tension in the strip in the direction of the strip width, so that amplitude of strip oscillation is reduced and also bending of strip in the strip width is cured. Strip tension in the direction of strip width is not uniform and small even if tension is imparted in the longitudinal direction, especially where the strip edge portions have a wavy configuration, which results in oscillation of the strip. A magnetic field applied to the strip advantageously effects those side edge portions and helps to prevent oscillation and bending.

As a magnetic means to cause magnetic field, any type of means to cause magnetic field, such as a permanent magnet, linear motor, solenoid, are applicable. Also any kind of strip or sheet which is made of magnetic material can be applicable to the present invention.

According to a preferred feature of the present invention, the location of the magnetic means is preferably adjusted so that a desired gap between the magnetic means and the side edge is maintained.

According to another aspect of the present invention, the magnetic means is adapted for the reduction of strip oscillation in a continuous metal coating apparatus. This can help to give uniform thickness of the coating metal due to suppression of the strip oscillation. Also, by the use of the magnetic means, bending of the strip may be advantageousiy eliminated which results in an improved coating on the surfaces of the strip. The magnetic means is further applicable for the prevention of strip oscillation or bending other than in the continuous metal coating apparatus such as continuous strip inspection lines.

Preferred embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, wherein: Fig. 1 is a perspective view of a continuous galvanizing apparatus having magnetic means in accordance with the present invention, Fig. 2 is a circuit diagram of one embodiment of the present invention, Fig. 3 is an enlarged cross-section view of the magnetic means in Fig. 1, Fig. 4 is a cross-section view taken along the line IV--IV of Fig. 3, Fig. 5 is a graph showing a test result of amplitudes of strip oscillation with and without the magnetic means of the embodiment, Fig. 6 (a) and (b) are schematic graphs respectively showing test results of amplitudes of oscillation without and with the magnetic means of the embodiment, Fig. 7 is a graph showing the relation between the magnet force and the gap between the strip and the magnetic means, Fig. 8 is a schematic side view of another embodiment of the present invention, and Fig. 9 is a schematic section view of another embodiment of a magnetic means of the present invention.

Fig. 1 shows one embodiment of the invention in which an anti-vibration magnet is adopted in a continuous galvanizing apparatus having gas wiping nozzles. The galvanizing apparatus comprises a bottom roller 14, a pair of deflector rollers 16 and 18, and a top roller 32 which continuously feed a steel strip 10 into a molten zinc bath 12, and a pair of gas wiping nozzles 20 and 22 disposed above the surface of the molten zinc bath 12 and adapted for effecting a gas wiping on the coated steel strip 10. In this galvanizing apparatus, the coating thickness on the surface of the steel strip is controlled by spraying a jet of gas from the wiping nozzles 20 and 22 installed above the surface of the bath 1 2 on both sides of the strip as it is withdrawn from the molten zinc bath 12.The thickness is usually adjusted by varying the distance between the front nozzle 20 and rear nozzle 22, pressure of the gas jet and the velocity at which the strip runs.

The wiping nozzles 20 and 22 are slidably supported by a column 28. The distance between the strip 10 and the wiping nozzles 20 and 22 is adjusted by a control motor 30. By operating the control motor 30, the wiping nozzles 20 and 22 can be moved in parallel relationship close to or away from the front and rear surfaces of the strip 10. The wiping gas in instroduced to the wiping nozzles 20 and 22 respectively from pipes 24 and 26.

The steel strip after withdrawal from the zinc bath 12 cannot be supported in any direct contacting manner until the coating metal is cooled and solidified, so that the position of the top roller 32 must be higher in proportion to the strip running speed. In practice, a free span of about 40 m height is necessary when the running speed is 200 m/min. The increase of the length of the free span and the increase of the strip running speed act to promote the oscillation and deformation of the strip 10, so that the distance between the wiping nozzles 20 and 22 and the strip surfaces must be continuously controlled or maintained larger. For instance, the strip 10 makes a lateral oscillation of amplitude of 3 to 10 mm at a frequency of 1 to 7 Hz. The dynamic pressure of the gas jet from the nozzles 20 and 22 varies by the lateral oscillation of the strip.The thickness of the coating metal on the strip 10 is reduced when the distance is reduced, and increased when the distance is increased, thereby causing non-uniform thickness of the coating layers.

For the above reasons, there has been an increased demand for a galvanizing apparatus capable of making a uniform coating layer at high line speed. Although it has been proposed to reduced the thickness of the coating layer by increasing the pressure of the wiping gas, the pressure can be increased only to 0.6 kg/cm2 or so, because the level of noise increases due to the gas pressure.

Accordinq to the present embodiment of the invention, there are provided four anti-vibration magnet units 34, 36, 38 and 40 in close proximity to the side edges of the strip. By the magnetic force of the magnet unit, the side edges of the oscillating strip are attracted to a region where the magnetic flux density is higher, so that the oscillation of the strip is prevented. The magnet units 38 and 40 are secured to a side plate 42 and the magnet units 34 and 36 are secured to another side plate. The side plates are located near the respective side edge of the strip.

Furthermore, in order to maintain the steel strip in a region where the steel strip is stably held, it is necessary to keep a constant small gap between the magnet units and the side edges of the strip. In practice, winding of the strip or changes in the strip width are inevitable. For the above reason, the side plate 42 is slidably supported by a column 50 with a screw rod 44 which is coupled to a control motor 48 through a belt 46. The side plate 42 can be moved parallel away from or close to the side edge of the strip by driving the control motor 48 so that a desired gap between the magnet units 38, 40 and the corresponding side edge of the strip is produced. The magnet units 34 and 36 are also capabie of moving in the direction of strip width so as to keep a desired gap between the magnet units 34, 36 and the corresponding side edge of the strip.

In the present embodiment of the invention, four magnet units 34, 36, 38 and 40 are employed to prevent the oscillation of the strip, however, the number of the magnets is not limited to the embodiment. For example, only one upper magnet units 34 and 38 can be employed. Also, if the case allows, only one magnet unit, for example, the magnet unit 34 can be employed. Further, the location of the magnet units are not limited to the embodiment. In the case of the gas wiping type galvanizing apparatus, prevention of strip oscillation between the wiping nozzles 20 and 22 is most effective for obtaining uniform thickness of the coating metal. For the above reason, it is preferable to locate the magnet unit near the side edge of the strip between the two wiping nozzles 20 and 22 so that the magnetic force of the magnet unit is greatest there.

In the present embodiment, the deflector rollers 1 6 and 1 8 are employed to secure stable galvanizing, however, these deflector rollers may promote strip oscillation due to fitting error in bearing portions or eccentricity of the rollers by heat deformation. Therefore, it is preferable to omit these deflector rollers to ensure the effect of the magnet unit. Referring now to Fig. 2, in which the magnet units 34 and 38 are located in the vicinity of respective side edges of the strip 1 0. The magnet units 34 and 38 are respectively provided with detectors 68 and 66 adapted to detect the gap between the side edges of the strip 10 and the magnet units. As the detector, a photodetector or other type of position detector can be used. Signals from the detectors 66 and 68 are delivered to comparators 62 and 64 where a preset signal produced in a preset circuit 60 is introduced. The comparator 62 and 64 produce signals corresponding to the actual detected signals and the preset signal. The output signals from the comparators 62 and 64 are respectively transmitted through amplifier 70 and 72 to the control motors 48 and 49. The screw rods 44 and 45 connected to the magnetic units 38 and 34 are coupled to the output shafts of the control motors 48 and 49, and therefore the magnetic units can be shifted as the screw rods are driven by the motors.

According to this arrangement, if the actuai gap between the magnetic unit and the strip side edge is not equal to the predetermined desired value, the control motor is actuated in accordance with the signal representing the deviation, so that the magnetic unit is located at the predetermined desired positiOn. In this way, the gap between the magnetic unit and the strip side edge is adjusted such that the magnetic force applied to the strip side edge is maintained substantially constant.

Hereafter the details of the magnetic units will be explained with respect to Figs. 3 and 4. In the case of the galvanizing apparatus, it is preferable to locate the magnet unit near the wiping nozzle for the above reasons, however, the magnet unit has to be disposed under strong thermal radiation from the melting metal and the hot wiping gas. Especially, permanent magnets are thermolabile and their magnetic force is easily decreased by the heat. In order to solve the problem, the magnet unit 34 is formed of a cylindrical sealed vessel 1 12 enclosing a plurality of permanent magnets 102, 104, 106 and 108, through which cooling liquid circulates.The permanent magnets 102, 104, 106 and 108 are secured to a T-shaped partition wall 110 which defines a passage for the cooling liquid in the sealing vessel 112. The magnets are secured in recesses tormed on the surface of the wall 110 and projecting toward an inner wall of the vessel 112. At the upper edge of the vessel, there are secured seal caps 114 and 1 16 having an entrance pipe 52 and an exit pipe 54 for the cooling liquid. This sealing vessel 112 is located near the side edge of the strip 10 such that the magnets 102, 104, 106 and 108 face the side edge of the strip 1 0.In order to enhance the effect of the magnet unit 34, it is preferable to use nonmagnetic materials for the sealing vessel 112, such as stainless steel. Also, it is preferable to use a thin material for the sealing vessel 112. According to this embodiment, reduction of magnet force by the heat can be eliminated. Also, adhesion of splashed molten metal to the magnets or the sealing vessel 11 2 can be prevented.

In the present embodiment, it is preferable to locate the magnets 102, 104, 106 and 108 on partition wall 110 such that the direction of magnet field of the respective magnets may be in the order of SNS or NSN so that the most effective magnetic field can be obtained. Also, it is preferable to use a non-magnetic material to enhance the effect of the anti-vibration magnets.

Fig. 5 is a graph showing the strip oscillation preventing effect performed by the anti-vibration magnet, in which the magnet is removed apart from the strip side edge between times 2 and 6 seconds so as not to effect magnetic force to the strip as shown. As is apparent from the graph, if the magnet force is effected to the strip, the amplitude of strip oscillation is kept at most 2 to 3 mm, while the amplitude is about 10 mm if the magnet is removed apart from the strip side edge. The oscillation preventing effect will be more apparent from the experiments in which continuous galvanizing is effected by the apparatus shown in Fig. 1 with a steel strip having a thickness of 1.20 mm, the strip width of 1000 mm, at a line speed of 123 m/min, and amount of zinc 300 g/m2. Fig. 6(a) and 6(b) show the results of experiments, respectively without and with the anti-vibration magnets.The results show that, if the magnet is not employed, the maximum amplitude of strip oscillation about 7 mm appears at the side edge of the strip and the minimum amplitude of strip oscillation of 4 mm appears at the middle of the strip width. Also, the strip is deformed or bent backward so that the cross-section of the strip becomes concave or convex if the magnet if not employed. The amount of the bending or deformation, usually called "canoeing", is about 8 mm. On the other hand, if the anti-vibration magnet is located near the side edges of the strip as shown in Fig. 6(b), not only the amplitude of oscillation at the edge portion becomes smaller (2.5 mm) but also the canoeing disappears.

.The effect of the anti-vibration magnet will be more apparent from the following Table 1, in which thickness of coating metal on the surface of the resultant strips are measured in accordance with the present invention with the anti-vibration magnet and the conventional galvanizing method without the anti-vibration magnet. The strip to be measured was made by gas wiping method such that a steel strip having a thickness of 1.20 mm and a width of 1000 mm was coated at a line speed of 1 23 m/min with an expected amount of zinc to be coated 300 g/m2.The measurement was made by the so-called "3 point method" in accordance with the Japanese Industrial Standard H0401 "Test method for galvanized strip". 10 test pieces were sampled from the resultant strips at an equal distance with respect to the direction of the strip width, and the weight of zinc coated on the test pieces were measured by the so-called "antimony chloride method".

As is shown from Table 1, the thickness of coating is not uniform by the conventional method.

Especially, due to the deformation or bending of the strip (canoeing), the thickness of the coating is thinner at the side portion and thicker at the middle portion on the front side of the strip, and thicker at the side portion and thinner at the middle portion on the rear side of the strip. On the other hand, according to the present embodiment with the anti-vibration magnet, the thickness of coating is uniform at the entire portion on both sides of the strip, which proves that the anti-vibration magnet is effective not only to prevent lateral oscillation but also deformation or bending such as canoeing of the strip.

TABLE 1 Weight of zinc (g/m2)

Test points in the direction of strip width Side of strip 1 2 3 4 5 6 7 8 9 10 Average Present Front 150 152 151 150 146 148 150 154 150 148 149.9 embodiment (with Rear 150 154 152 156 155 152 154 148 151 152 152.4 magnet) Total 300 306 303 306 301 300 304 302 301 300 302.3 Conven- Front 142 147 150 157 163 153 150 158 140 145 150.5 tional Method Rear 154 152 150 143 145 143 150 150 155 152 149.4 (without magnet) Total 296 299 300 300 308 296 300 303 295 297 299.4 Fig.7 shows the relation between magnet force applied to a steel strip and the gap between the strip side edge and the magnet surface. The magnet used in this test result is formed of a plurality of rectangular magnet pieces made of cobalt magnet stacked to form a magnet unit as shown in Fig. 3.

The effective length of the magnet unit facing the strip side edge is about 500 mm. As is shown from Fig. 7, the magnet force decreases with the increase of the gap. Also the magnet force decreases if the strip becomes thinner. Considering the reliability of the gap controlling mechanism, it is preferable to keep a gap of about 4 to 6 mm so that magnetic force of about 5 to 25 kg may be obtained.

Fig. 8 shows another embodiment of the present invention in which the anti-vibration magnet is applied to a continuous strip inspection line. A strip 120 is running over a pair of rollers 122 and 124. A strip detector 1 26 for detecting the flaw or thickness of the strip 120 by laser beam or x-ray is disposed along the passage of the strip between the rollers 122 and 124. Anti-vibrating magnets 128 are disposed in the vicinity of respective side edges of the strip 120 below the detector 126. By this arrangement of the magnets 128, detection error due to oscillation, deformation or bending of the strip is prevented for the oscillation or deformation of the strip is effectively suppressed by the magnet.

It is effective and preferable to make the magnet have a narrowed tip 132 and 1 34 as shown in Fig. 9, because, by so doing, the magnetic flux density acting on the side edge of the strip 130 is increased.

Claims

1. A method for reducing oscillation of a strip formed of magnetic material, which is continuously running along a predetermined passage, comprising a step of applying a magnetic field to the strip in the direction of the strip width, thereby imparting a tension to the strip in the direction of strip width in a non-contacting manner.

2. A method for reducing oscillation of a strip according to claim 1, wherein the magnetic field is applied to the strip from both side edges of the strip.

3. A method for reducing oscillation of a strip according to claim 1 or 2, further comprising a step of detecting the position of the strip side edge and a step of controlling the magnitude of the magnet force by the magnetic field applied to the strip in accordance with the result of the detection.

4. A method for reducing oscillation of a strip according to claim 3, wherein the magnitude of the magnetic force applied to the strip is controlled to be substantially constant at both side edges of the strip.

5. A method for reducing oscillation of a strip according to claim 4, wherein the magnitude of the magnetic force applied to the strip is controlled to be the same at both side edges of the strip.

6. An apparatus for reducing oscillation of a strip formed of magnetic material, which is running continuously along a predetermined passage, comprising magnetic means for reducing oscillation of the strip, which is disposed in the vicinity of at least one side edge of the strip in a non-contacting manner with the strip, said magnetic means being adapted to impart tension to the strip in a direction of the strip width by applying a magnetic field to the strip.

7. An apparatus for reducing oscillation of a strip according to claim 6, wherein a said magnetic means is disposed in the vicinity of respective side edges of the strip.

8. An apparatus for reducing oscillation of a strip according to claim 6 or 7 wherein said magnetic means is disposed along the side edge of the strip.

9. An apparatus for reducing oscillation of a strip according to claim 6, 7 or 8, wherein said magnetic means is formed of a permanent magnet.

1 0. An apparatus for reducing oscillation of a strip according to claim 9, wherein said magnetic means is formed of a plurality of permanent magnets arranged along the side edge of the strip.

1 An apparatus for reducing oscillation of a strip according to claim 8, wherein said magnetic means has magnetic poles arranged along the side edge of the strip in the order of S and N alternately.

12. An apparatus for reducing oscillation of a strip according to claim 8 or claim 1 further comprising means for detecting a position of the side edge of the strip, and control means for controlling a gap between the surface of said magnetic means and the corresponding side edge of the strip by shifting said magnetic means in response to the detected value so as to maintain substantially a constant gap therebetween.

13. An apparatus for reducing oscillation of a strip according to any one of claims 6 to 12, further comprising cooling mear.s for cooling said magnetic means.

14. An apparatus for reducing oscillation of a strip according to claim 1 3, wherein said cooling means comprises a non-magnetic casing for enclosing said magnetic means, having an inlet and outlet for passing cooling medium through said casing.

15. An apparatus for continuously coating molten metal on the surface of a magnetic strip comprising a bath for storing molten metal, guide means for guiding the strip into said bath to be immersed therein, means for withdrawing the strip from said bath, thickness control means for controlling a thickness of molten metal coated on the strip surfaces withdrawn from said bath, and magnetic means for reducing oscillation of the strip which is disposed in the vicinity of a side edge of the strip in a non-contacting manner for applying a magnetic field in the direction of the strip width.

1 6. An apparatus for continuously coating molten metal according to claim 1 5 wherein said magnetic means is disposed in the vicinity of said thickness control means.

1 7. An apparatus for continuously coating molten metal according to claim 1 5 or 1 6, wherein said magnetic means comprises permanent magnet.

1 8. An apparatus for continuously coating molten metal according to claim 1 5, 1 6 or 1 7, wherein said apparatus is a continuous galvanizing apparatus.

1 9. An apparatus for continuously coating molten metal according to any one of claims 1 5 to 1 8 wherein said thickness control means comprises a pair of wiping nozzles which direct wiping gas toward respective surfaces of the strip to remove excess coating metal.

20. An apparatus for continuously coating molten metal according to any one of claims 1 5 to 1 9, further comprising means for detecting the position of the side edge of the strip, and control means for controlling the gap between said magnetic means and the corresponding side edge of the strip by shifting said magnetic means in response to the detected value so as to maintain substantially a constant gap therebetween.

21. An apparatus for continuously coating molten metal according to any one of claims 1 5 to 20, further comprising cooling means for cooling said magnetic means.

22. An apparatus for continuously coating molten metal according to claim 21, wherein said cooling means comprises a non-magnetic casing for enclosing said magnetic means, having an inlet and outlet for passing cooling medium therethrough.

23. A method for reducing oscillation of a strip formed of magnetic material, substantially as described herein with reference to the accompanying drawings.

24. An apparatus for reducing oscillation of a strip formed of magnetic material, substantially as any described herein with reference to the accompanying drawings.

25. An apparatus for continuously coating molten metal on the surface of a strip of a magnetic material, substantially as described herein with reference to Figs. 1 to 4 of the accompanying drawings.