METHOD AND DEVICE FOR LEVELING METAL STRIP TO BE FED INTO CONTINUOUS ANNEALING FURNACE

Abstract: A method for leveling a metal strip performs leveling operation by opening leveling rolls at a predetermined position relative to the welded joint and close at a predetermined position relative to the welded joint. A tension to be exerted on the metal strip while the leveling rolls are held open is set at a given value in relation to the metal strip length where the leveling rolls are held open. The tension to be exerted on the metal strip while the leveling rolls is selected so that the tension may cause a given rate of elongation on the metal strip and whereby levels the metal strip. A leveling device is applicable for implementing the leveling process including quck open and quick close at the welded joint of the strips. The leveling device may features in that the leveling rolls as held open and thus released from the metal strip is self-propelled to rotate.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to a method device for leveling a series of metal strip to subject heat treatment, such as annealing. More specifically, the invention relates to a technic for effectively performing metal strip leveling operation with avoidance of breakage at welded joint, defect in the series of metal strip. Description of the Background Art In general, a plurality of hearth rolls are arranged in a continuous annealing furnace for defining a zig-zag path through known heat treatment stages, such as a heating stage, a soaking stage maintaining the metal strip at constant heated temperature, a cooling stage and so forth. The metal strip is wound around the hearth rolls and runs through the zig-zag path in the annealing furnace to be subject heat treatment. In the practical operation of the annealing furnace. The metal strip at the zone cross to the inlet has relatively low temperature. At this zone in the annealing furnace, the temperature at the transverse edge positions of the hearth roll tends to become much higher than that at the transverse central portion. This temperature difference between the edge portions and the central portion increases as increasing the line speed. This causes substantial difference of the heat expansion at the edge portion and the central portion to cause reduction of the effective crown magnitude. This reduction of the crown tends to lead unbalance of tension to be exerted on the metal strip in transverse direction in the case that the metal strip is in the configuration of edge wave or center wave. Unbalance tension tends to cause meander of the metal strip in the path. In order to prevent this, the line speed to feed the metal strip through the annealing furnace has to be lowered in order to minimize temperature difference between the edge portions and the central portions of the hearth roll. This clearly cause lowering efficiency of the annealing furnace. On the other hand, possibility of the meander of the metal strip can be reduced by providing greater magnitude of the initial crown on the periphery of the hearth roll. This may allow to maintain satisfactory line speed. On the other hand, when sufficient initial crown for the initial heating stage is provided for the hearth roll, the crown magnitude tends to be excessive at the high temperature zone to cause heat buckle when line speed is lowered in some reason, such as for welding the strips. In this view, there are various proposals for obtaining stable operation of the annealing furnace by controlling magnitude of crown on the peripheries of hearth rolls. For example such proposals have been disclosed in the Japanese Utility Model First (unexamined) Publication (Tokkai) Showa 57-177930 and the Japanese Utility Model First Publication (Jikkai) Showa 58-10546. In these prior proposal, crown magnitude have been controlled by heating and cooling the hearth rolls. On the other hand, the Japanese Patent First Publication (Jikkai) Showa 55-172359 proposed to control the crown magnitude by means of bending device. On the other hand, the meander of the metal strip in the annealing furnace is also caused due to configuration of the metal strip, such as bending, transverse curve and so forth. If the configuration of the metal strip is not suitable for passing through the annealing furnace to cause uneven contact with the hearth roll surface, meander of the metal strip is caused even when the crown magnitude is sufficient. This tendency increases as increasing the line speed. In order to regulate or adjust the configuration of the metal strip, tension leveler devices are provided. In general, such tension leveler devices exert substantial tension, e.g. 20 kg/mm². Such high tension to be exerted on the metal strip tends to cause breakage at the welded joint between leading and trailing metal coils. defects on the metal strip. On the other hand, because of the uneveness at the welded Joint, leveling rolls in the tension leveler tends to be damaged due to high tension. In order to prevent breakage of the metal strip and/or damaging of the leveling rolls, it has been taken ways to slow-down the line speed at the welded joint or to release the roll at the welded joint. slow-down of the line speed necessarily causes the lowering of the efficiency of the annealing furnace. On the other hand, releasing of the leveling rolls necessarily feed the metal strip with unleveled portions to cause meander in the annealing furnace. From Japanese Patent Abstracts, 1981, Vol. 5, No. 12 "C-40" there is known a tension setting and controlling device of continuous long material comprising a tension setting and controlling mechanism to control the operation of at least two tension applying bridle roll means. Before the junction of preceding and succeeding joining members of a continuous strip member enters one of the tension applying bridle rolls, the respective tension reference values of the preceding and succeeding strip members are set in setting devices and finally, the bridle roll is moved away from the strip to lower the tension to a predetermined value. The afore-mentioned prior art does not comprise a leveling roll assembly for leveling a metal strip, but merely comprises tension applying bridle rolls for exerting a predetermined magnitude of tension force to a metal Strip. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a method and device for leveling the metal strip which improves the aforementioned drawback in the prior art. Another object of the present invention to provide a method and device to perform tension leveling operation for the metal strip which successfully eliminates the metal strip to cause meander in the annealing furnace by controlling a tension to be exerted on the metal strip so that the expansion of the metal strip due to the exerted tension can be set in relation to open and close positions of the leveling rolls to release the rolls from the metal strip at the welded joint portion in order to establish optimal relationship. A further object of the invention is to provide a level device which is suitable for implementing quick open and quick close operation in the leveling operation for avoiding welded joint of the metal strip without requiring lowering of line speed. In order to accomplish the aforementioned and other objects, a method for leveling a metal strip according to claims 1 to 4 of the present invention, performs leveling operation by opening leveling rolls at a predetermined position relative to the welded joint and close at a predetermined position relative to the welded joint. A tension to be exerted on the metal strip while the leveling rolls are held open is set at a given value in relation to the metal strip length where the leveling rolls are held open. The tension to be exerted on the metal strip while the leveling rolls is selected so that the tension may cause a given elongation on the metal strip and whereby the metal strip is leveled. On the other hand, according to claims 5 and 6 of the invention. It is also provided a leveling device which is applicable for implementing the leveling process including quick open and quick close at the welded joint of the strips. The leveling device according to the present invention features in that the leveling rolls as held open and thus released from the metal strip is self-propelled to rotate. According to one aspect of the invention, a method for leveling a metal strip to be fed into an annealing furnace, which metal strip is formed into a series of strip by connecting coils at a joint, comprises the steps of: providing a leveling roll assemblies which is constituted of a first and second roll components disposed between opposite sides of a path through which the metal strip is fed; providing means for exerting a predetermined magnitude of tension force to the metal strip during leveling operation; depressing the first and second components of the leveling roll assembly with a predetermined pressure for leveling the metal strip; releasing the first component of the leveling roller assembly from the mating surface of the metal strip for releasing the metal strip from depressing pressure, at a joint of the series of metal strip; and rotatingly driving at least the released one of the first and second component while it is released from the metal strip. In the method as set forth above, the rotation speed of the first component is variable depending upon a line speed of the metal strip. In practice, the rotation speed of the first component is so adjusted as to have a given difference relative to the line speed so that the speed difference is reduced as increasing the line speed. In the practical operation, the rotation speed of the first component is adjusted as to satisfy the following formula: ΔS ≦αµρ¨ LS where ▵S is speed difference; and LS is line speed of the metal strip, when the line speed is lower then or equal to 150 m/min. and to satisfy the following formula: ΔS ≦αµρ¨ -a . LS+b where ΔS is speed difference; LS is line speed of the metal strip; a is 0.124; and b is 168.6 when the line speed is higher than 150 m/min. On the other hand, the first component is released from the metal strip at a point a given distance ahead the joint the contacted to the metal strip at a point a given distance after the joint. According to another aspect of the invention, a method for leveling a metal strip to be fed into an annealing furnace, which metal strip is formed into a series of strip by connecting coils at a join, comprises the steps of; providing a leveling roll assemblies which is constituted of a first and second roll components disposed between opposite sides of a path through which the metal strip is fed; providing means for exerting a predetermined magnitude of tension force to the metal strip during leveling operation; depressing the first and second components of the leveling roll assembly with a predetermined pressure for leveling the metal strip; releasing the first component of the leveling roller assembly from the mating surface of the metal strip for releasing the metal strip from depressing pressure, at a joint of the series of metal strip; and adjusting the tension force to be exerted metal strip during the leveling operation to adjust an elongation of the metal strip in a predetermined relationship with the distance to satisfy the following equation: Ø > 0.0082(x/8L) 1.45 where  is elongation (%); x is the distance; and L is a span between hearth rolls in the annealing furnace. The leveling operation is to be performed for smoothing the surface of the metal strip to have a steepness less than about 1%, where steepness means a ratio of the height of the peak of projecting portion on the surface of the metal strip versus a distance from the peak to bottom of adjacent depression. According to a further aspect of the invention, a leveling device for leveling a metal strip to be fed into an annealing furnace, which metal strip is formed into a series of strip by connecting coils at a join, comprises a leveling roll assembly which is constituted of a first and second roll components disposed between opposite sides of a path through which the metal strip is fed, means for exerting a predetermined magnitude of tension force to the metal strip during leveling operation, means for depressing the first and second components of the leveling roll assembly with a predetermined pressure for leveling the metal strip, means for releasing the first component of the leveling roller assembly from the mating surface of the metal strip for releasing the metal strip from depressing pressure, at a joint of the series of metal strip, and means for rotatingly driving the first component while it is released from the metal strip. According to a still further aspect, a leveling device for leveling a metal strip to be fed into an annealing furnace, which metal strip is formed into a series of strip by connecting coils at a join, comprises a leveling roll assembly which is constituted of a first and second roll components disposed between opposite sides of a path through which the metal strip is fed, means for exerting a predetermined magnitude of tension force to the metal strip during leveling operation, means for depressing the first and second components of the leveling roll assembly with a predetermined pressure for leveling the metal strip, means for releasing the first component of the leveling roller assembly from the mating surface of the metal strip for releasing the metal strip from depressing pressure, at a joint of the series of metal strip, and means of adjusting the tension force to be exerted metal strip during the leveling operation to adjust an elongation of the metal strip in a predetermined relationship with the distance to satisfy the following equation:  ≧ 0.0082(x/8L) 1.45 where  is elongation (%); x is the distance; and L is a span between hearth rolls in the annealing furnace. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention but are for explanation and understanding only. In the drawings: Fig. 1 is a fragmentary illustration of a general construction of an inlet portion of an annealing furnace, to which the preferred embodiment of a method for leveling a metal strip and a leveling device therefor are applicable; Fig. 2 is an illustration of the annealing furnace for performing continuous annealing for a series of metal strip; Fig. 3 is an illustration of an arrangement of leveling rollers of the preferred embodiment of the leveling device according to the invention; Fig.4 is a graph showing steepness of the metal strip, which steepness is defined by a ratio of the height of a peak of the uneveness (H) versus pitch between peak and adjacent bottom (P); Fig. 5 is a graph showing elongation % of the metal strip relative to a distance from leveling rollers open point and/or close point to a welded joint on the metal strip; Fig. 6 is a chart showing axial direction temperature distribution of the hearth roll at the position adjacent the inlet of the annealing furnace; Fig. 7 is a perspective view of the preferred embodiment of a leveling roller assembly to be employed in the preferred embodiment of the leveling device according to the invention; and Fig. 8 is a graph showing relationship between a speed difference between the metal strip and the leveling roll and a line speed. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Figs. 1 to 3, a annealing furnace 10 to which the preferred embodiment of a method for leveling a metal strip 12 and a leveling device 14 for implementing the preferred embodiment of the leveling method according to the invention, has an inlet side looper 16 at the position upstream of a furnace chamber 18. The preferred embodiment of the leveling device 14 is provided at a position upstream of the inlet side looper 16, the leveling device 14 to be employed is a tension leveling device. The furnace chamber 18 is divided into a plurality of zones, i.e. heating zone 18a, a soaking zone 18b, a moderate cooling zone 18c and quick cooling zone 18d. A plurality of hearth rolls 18e are arranged in the furnace chamber 18 to define a zig zag path for the metal strip 12. As is well known, the hearth rolls are separated into two groups, in which one group is located adjacent the ceiling of the annealing furnace and the other group of rolls are located adjacent the floor of the furnace. Therefore, the rolls in the one group is distanced from preceding the trailing rolls in other group at a predetermined span L. As shown in Fig. 3, the preferred embodiment of the leveling device 14 comprises a plurality of leveling roll assemblies 20. Each leveling roller assembly comprises a work roll 22, an intermediate rolls 24, back-up rolls 26 and deflector rolls 28. The deflector rolls 28 forming a deflector roll assembly 30 are oriented opposite side to the work roll 22, the intermediate roll 24 and the back-up rolls 26 which forms a work roll assembly 32 with respect to the metal strip path. The leveling roll assemblies 20 are arranged along a metal strip path in the leveling device 14 with a given interval. The leveling roll assemblies 20 are respectively associated with known mechanism for shifting the work roll assembly 32 toward and away from the deflector roll assembly 30 so as to release the work rolls from the metal strip while the welding joint 34 on the metal strip 12 passes through the leveling device 14. Bridle rolls 36 and 37 are provided at upstream end and downstream end of the leveling device 14 for controlling tension to be exerted on the metal strip 12 passing through the leveling device. In the shown embodiment, the leveling roll assemblies 20 is operated to be open at a point 40 of the metal strip x (m) ahead of the welded joint 34 and return to closed position after x (m) from the welded joint at a point 42. Namely, when the point 40 of the metal strip arrives the leveling device 14, the leveling roll assemblies 20 are operated at the open position, in which the work roll assembly 32 are shifted away fro the deflector roll assembly 30 to make the metal strip free from rolling pressure. The leveling roll assemblies 20 are operated to the closed position to contact the work roll 22 and the deflector rolls 28 onto the opposite sides of the metal strip when the point 42 pass the outlet of the leveling device. On the other hand, while the leveling roll assemblies 20 are in closed position to contact the work rolls 22 and the deflector rolls 28 onto the surfaces of the metal strip 12, the depression and so forth are set so that the steepness which is the ratio of the height of the peak H of the projecting uneveness versus the pitch P from the peak to the bottom of the depression, is suitable for satisfactorily high line speed in the annealing furnace. In order to check an optimum steepness, an experimentation has been taken place with respect to tin plate. The result is shown in Fig. 4. As seen from Fig. 4, when the steepness is less than or equal to 1.0%, the line speed in the annealing furnace was higher than or equal to 550 m/min. This is satisfactory for obtaining high efficiency of the annealing furnace. In this view, the preferred steepness is 1.0% or smaller. By repeating bending in the leveling device with a preset depression and a given tension force exerted on the metal strip, the steepness can be reduced. In this leveling operation, it is also observed elongation of the metal strip due to bending pressure and tension. The bridle rolls 36 and 37 are controlled to provide a tension for the metal strip passing the leveling device. The tension is so determined as to provide a given elongation expansion  on the metal strip. Throughout the disclosure, the metal strip elongation  (%) represents magnitude of tension force to be exerted on the metal strip. The tension force is determined to satisfy the following equation:  ≧ 0.0082(x/8L) 1.45 The relationship between the elongation  (%) and the aforementioned x(m) is shown in Fig. 5. When the elongation (%) relative to the distance x(m) is maintained in the hutched range, breakage of the metal strip in the leveling device and meander of the metal strip in the annealing furnace was not occurred. Therefore, by setting the operation condition of the leveling device to satisfy the foregoing equation, it becomes unnecessary to slow-down the line speed at the welded joint of the metal strip but can provide satisfactorily stable and high efficiency annealing furnace operation. Here, in the conventional construction, the work rolls 22 are idle roll rotatingly driven by the metal strip fed at a given line speed. Therefore, the rotation speed of the work roll was determined corresponding to the line speed of the metal strip. In the quick open and quick close operation for avoiding breakage of the metal strip at the welded joints set forth above, substantial thrusting force may be exerted onto the bearing section of the work roll when it is released from the metal strip in the quick open operation. On the other hand, while it is held at open position, the work roll stops. As a result, when the work roll is operated toward the metal strip in quick close operation, the substantial slip is created between the metal strip and the work roll to for scratches on the metal strip surface. In order to prevent this, the leveling roll assembly 20 to be employed in the preferred embodiment of the invention, has a driving mechanism 50 for driving the work roll 22 at least when the leveling roll assembly 20 is held open position and thus the work roll 22 is held away from the metal strip. As shown in Fig. 7, the driving mechanism 50 comprises an electric motor 52 which drives a driving sproket 54. The driving sproket 54 is connected to a driven sproket 56 associated with a power distribution mechanism 58, via a chain 60. The power distribution mechanism 58 is associated with the back-up rolls 26 to rotatingly drive the latter. Since rotation torque thus exerted on the back-up rolls 26 are transmitted to the work roll 22 via the intermediate rolls 24. A clutch 62 may be provided between the driven sproket 56 and the power distribution mechanism 58 for establishing and releasing the power train in synchronism with quick open and quick close operation of the leveling roll assembly. Fig. 8 shows a graph showing a relationship between the line speed LS and the speed difference ΔS (m/min) as a result of experimentation. As will be seen from Fig. 8, when the line speed is lower than or equal to 150 m/min, scratch was not formed even when the work roll 22 is not driven to rotate. On the other hand, when the line speed is higher than 150 m/min, scratches were made when the work roll is held stopped. As further be observed from Fig. 8, higher speed of rotation was required to reduce the difference ΔS. Namely, when the speed difference ΔS in relation to the line speed is held in a hutched range, scratch will never formed on the metal strip. Therfore, the speed of rotation of the work roll is set to satisfy the following formulae: When LS < 150 m/min, ΔS ≦αµρ¨ LS; and When LS > 150 (m/min) ΔS ≦αµρ¨ -a . LS + b (were a = 0.124. b = 168.6) Therefore, by driving the work roll while it is held released from the metal strip, formation of scratch of the metal strip can be successfully prevented during leveling operation. While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding of the invention, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention set out in the appended claims.

Claims: A method for leveling a metal strip to be fed into an annealing furnace, which metal strip is formed of preceding and succeeding strips being connected at a joint, comprising the steps of: providing a leveling roll assembly which comprises a first (32) and a second (30) roll component disposed between opposite sides of a path through which the metal strip is fed: providing means for exerting a predetermined magnitude of tension force to said metal strip during leveling operation: urging said first and second components of said leveling roll assembly with a predetermined pressure for leveling said metal strip: releasing said first component (32) of said leveling roller assembly from the mating surface of said metal strip for releasing said metal strip from urging pressure at a point a given distance ahead the joint (34) and contacted again to said metal strip at a point (42) a given distance after said joint, and rotatingly driving at least said first component (32) while it is released from said metal strip, wherein the rotation speed of said first component (32) is so adjusted so as to have a given speed difference relative to the line speed of said metal strip So that said speed difference is reduced as the line speed increases, and wherein said rotation speed of the first component (32) is adjusted so as to satisfy the following formula: ▵S ≦αµρ¨ -ax . x LS+b where S is the speed difference, LS is the line speed of the metal Strip, a is 0.124; and b is 168.6 when the line speed is higher than 150 m/min. A method as set forth in Claim 1, which further comprises a step of adjusting said tension force to be exerted metal strip during said leveling operation to adjust an elongation of said metal strip in a predetermined relationship with said distance to satisfy the following equation: Ø ≧ 0.0082 (x/8L) 1.45 where ⊘ is elongation (%); x is said distance; and L is a span between hearth rolls in said annealing furnace. A method as set forth in Claim 1, wherein said leveling operation is to be performed for smoothing the surface of said metal strip to have steepness less than about 1%. A method as set forth in Claim 2, wherein said leveling operation is to be performed for smoothing the surface of said metal strip to have steepness less than about 1%. A leveling device for leveling a metal strip to be fed into an annealing furnace, which metal Strip is comprised of preceding and succeeding strips which are connected at a joint (34), comprising; a leveling roll assembly (20) which consists of first and second roll components (32, 30) disposed between opposite sides of a path through which the metal strip is fed: means (36, 37) for exerting a predetermined magnitude of tension force to said metal strip during leveling operation: means for urging said first and second components of said leveling roll assembly (20) with a predetermined pressure for leveling said metal Strip: means for releasing said first component (32) of said leveling roller assembly (20) from the mating surface of said metal strip for releasing said metal strip from said urging pressure at a joint (34) of said preceding and succeeding metal strips: means (52 to 58) for rotatingly driving at least said first component (32) while it is released from said metal strip, wherein said driving means is able to adjust the rotational speed of said first component (32) as to have a given speed difference relative to the line speed of said metal strip so that said speed difference is reduced as the line speed increases: and wherein said driving means (52 to 58) is controlled to adjust the rotational speed of said first component (32) as to satisfy the following formula: ▵S ≦αµρ¨ -a . LS+b where ΔS is speed difference; LS is line speed of the metal strip; a is 0.124; and b is 168.6 when the line speed is higher than 150 m/min. A leveling device as set forth in Claim 5, wherein said first component (32) is released from said metal strip at a point (4O) a given distance ahead the joint (34) and recontacted to said metal strip at a point (42) a given distance after said joint.

Inventors: NAKAZONO, YOJI C/O KAWASAKI STEEL CORPORATION, OHNISHI, TATEO C/O KAWASAKI STEEL CORPORATION, OHNO, HIRONOBU C/O KAWASAKI STEEL CORPORATION, SHIMOYAMA, YUJI C/O KAWASAKI STEEL CORPORATION, SHIODA, ISAMU C/O KAWASAKI STEEL CORPORATION, YAMAGUCHI, YASUHIRO C/O KAWASAKI STEEL CORPORATION
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1987
Main classification: C21D9
IPC codes: C21D9, B21D1, C21D7