Control device for rolling machine, rolling machine, method for operating rolling machine, and control program for rolling machine

Abstract

A control device for a rolling mill apparatus is a device for controlling a rolling mill apparatus which includes a pair of mill rolls for rolling a metal strip, and an unwinder, including a mandrel having a gripping portion for gripping a tail end portion of the metal strip, for unwinding the metal strip toward the pair of mill rolls. The control device includes: a mill roll control part configured to control the rotation of the pair of mill rolls on the basis of a deviation between a target speed and an actual speed of the metal strip; and an unwinder control part configured to control the torque of the unwinder on the basis of a deviation between a target tension and an actual tension of the metal strip between the unwinder and the pair of mill rolls. The control device is configured to, in a first period immediately before the tail end of the metal strip exits the unwinder, reduce the target speed so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip at the unwinding side or reduce the target tension.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a control device for a rolling mill apparatus, a rolling mill apparatus, an operation method for a rolling mill apparatus, and a control program for a rolling mill apparatus.BACKGROUND

[0002] In a rolling mill apparatus which includes an unwinder and a winder, an end portion (head and tail end portions) of a rolled material, a metal strip, in the longitudinal direction is usually gripped while being inserted in a slot in the winder or unwinder.

[0003] Patent Document 1 discloses a winding facility in which a metal strip is wound by rotating the winder, with the head end of the metal strip inserted in a slot in a mandrel of the winder and held by a gripper in the slot.

[0004] Patent Document 2 discloses a reverse-type rolling mill apparatus that continues rolling even after the tail end of a metal strip leaves the unwinder.Citation ListPatent Literature

[0005] Patent Document 1: JP6986114B Patent Document 2: JP2000-202503A SUMMARYProblems to be Solved

[0006] During rolling of a metal strip, a tail end portion of the metal strip remains inserted in the slot of the unwinder, and therefore the tail end portion bends downward according to the shape of the slot. Typically, this downward bend (plastic deformation) remains after the tail end portion leaves the unwinder.

[0007] Here, in a reverse-type rolling mill apparatus, in which a metal strip is rolled back and forth (e.g., Patent Document 2), the tail end of the metal strip that leaves the slot in the mandrel functioning as the unwinder becomes the head end in the next pass and is wound around the same mandrel (which now functions as the winder). If the head end of the metal strip (the tail end in the previous pass) bends downward as described above, the head end will be caught in a conveyor path during conveying the metal strip, making it difficult to feed the metal strip. This may reduce the production efficiency of products.

[0008] In view of the above, an object of at least one embodiment of the present invention is to provide a control device for a rolling mill apparatus, a rolling mill apparatus, an operation method for a rolling mill apparatus, and a control program for a rolling mill apparatus that enable smooth threading at the start of each rolling pass.Solution to the Problems

[0009] A control device for a rolling mill apparatus according to at least one embodiment of the present invention is a control device for controlling a rolling mill apparatus which includes a pair of mill rolls for rolling a metal strip, and an unwinder for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel having a gripping portion for gripping a tail end portion of the metal strip. The control device includes: a mill roll control part configured to control the rotation of the pair of mill rolls on the basis of a deviation between a target speed and an actual speed of the metal strip; and an unwinder control part configured to control the torque of the unwinder on the basis of a deviation between a target tension and an actual tension of the metal strip between the unwinder and the pair of mill rolls. The control device is configured to, in a first period immediately before the tail end of the metal strip exits the unwinder, reduce the target speed so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip at the unwinding side or reduce the target tension.

[0010] Further, a rolling mill apparatus according to at least one embodiment of the present invention includes: a pair of mill rolls for rolling a metal strip; an unwinder for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel having a gripping portion for gripping a tail end portion of the metal strip; and the above-described control device configured to control the pair of mill rolls and the unwinder.

[0011] Further, a rolling mill apparatus according to at least one embodiment of the present invention includes: a pair of mill rolls for rolling a metal strip; and an unwinder for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel having a gripping portion for gripping a tail end portion of the metal strip. The metal strip includes a gripped portion gripped by the gripping portion, and an adjacent portion adjacent to the gripped portion in a longitudinal direction of the metal strip. In a state where the metal strip is wound around the unwinder, an angle formed between the gripped portion and the adjacent portion on an upper surface side of the metal strip (or folding angle from the straight line along the gripped portion to the straight line along the adjacent portion in the rotational direction of the mandrel) is not smaller than 180 degrees. The rolling mill apparatus is configured such that the tail end of the metal strip exits the unwinder after the angle becomes smaller than 180 degrees at the end of unwinding of the metal strip from the unwinder.

[0012] Further, an operation method for a rolling mill apparatus according to at least one embodiment of the present invention is an operation method for operating a rolling mill apparatus which includes a pair of mill rolls for rolling a metal strip, and an unwinder for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel having a gripping portion for gripping a tail end portion of the metal strip. The operation method includes: a mill roll control step of controlling the rotation of the pair of mill rolls on the basis of a deviation between a target speed and an actual speed of the metal strip; and an unwinder control step of controlling the torque of the unwinder on the basis of a deviation between a target tension and an actual tension of the metal strip between the unwinder and the pair of mill rolls. In a first period immediately before the tail end of the metal strip exits the unwinder, the target speed is reduced so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip at the unwinding side, or the target tension is reduced.

[0013] Further, an operation method for a rolling mill apparatus according to at least one embodiment of the present invention is an operation method for operating a rolling mill apparatus which includes a pair of mill rolls for rolling a metal strip, and an unwinder for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel having a gripping portion for gripping a tail end portion of the metal strip. The metal strip includes a gripped portion gripped by the gripping portion, and an adjacent portion adjacent to the gripped portion in a longitudinal direction of the metal strip. In a state where the metal strip is wound around the unwinder, an angle formed between the gripped portion and the adjacent portion on an upper surface side of the metal strip is not smaller than 180 degrees. The operation method includes a step of allowing the tail end of the metal strip to exit the unwinder after the angle becomes smaller than 180 degrees at the end of unwinding of the metal strip from the unwinder.

[0014] Further, a control program for a rolling mill apparatus according to at least one embodiment of the present invention is a control program for controlling a rolling mill apparatus which includes a pair of mill rolls for rolling a metal strip, and an unwinder for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel having a gripping portion for gripping a tail end portion of the metal strip. The control program causes a computer to execute: a process for controlling the rotation of the pair of mill rolls on the basis of a deviation between a target speed and an actual speed of the metal strip; and a process for controlling the torque of the unwinder on the basis of a deviation between a target tension and an actual tension of the metal strip between the unwinder and the pair of mill rolls. The control program is configured to, in a first period immediately before the tail end of the metal strip exits the unwinder, reduce the target speed so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip at the unwinding side or reduce the target tension.Advantageous Effects

[0015] At least one embodiment of the present invention provides a control device for a rolling mill apparatus, a rolling mill apparatus, an operation method for a rolling mill apparatus, and a control program for a rolling mill apparatus that enable smooth threading at the start of each rolling pass.BRIEF DESCRIPTION OF DRAWINGS

[0016] FIG. 1 is a schematic configuration diagram of a rolling mill apparatus to which a control device is applied according to an embodiment. FIG. 2 is a partial cross-sectional view of an unwinder (winder) according to an embodiment. FIG. 3 is a schematic configuration diagram of a control device 50 according to an embodiment. FIG. 4 is a flowchart of the operation method for a rolling mill apparatus according to an embodiment. FIG. 5 is a graph showing examples of temporal changes in target speed and target tension of a metal strip, and peripheral speed of a mandrel in the operation method according to an embodiment. FIG. 6 is a schematic diagram for describing the change in shape of a tail end portion of a metal strip according to an embodiment. FIG. 7 is a schematic diagram for describing the change in shape of a tail end portion of a metal strip according to an embodiment. FIG. 8 is a schematic diagram for describing the change in shape of a tail end portion of a metal strip according to an embodiment. FIG. 9 is a schematic diagram for describing the change in shape of a tail end portion of a metal strip according to an embodiment. FIG. 10 is a schematic diagram for describing the change in shape of a tail end portion of a metal strip according to a typical operation method. FIG. 11 is a flowchart of the operation method for a rolling mill apparatus according to an embodiment. DETAILED DESCRIPTION

[0017] Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly identified, dimensions, materials, shapes, relative positions, and the like of components described in the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention.(Configuration of rolling mill apparatus)

[0018] FIG. 1 is a schematic configuration diagram of a rolling mill apparatus to which a control device is applied according to an embodiment. As shown in FIG. 1, the rolling mill apparatus 1 includes a rolling mill 10 including a pair of mill rolls 15, 16 for rolling a metal strip S (e.g., steel strip), an unwinder 2 disposed at the entry side of the rolling mill 10 (that is, at the upstream side of the rolling mill 10 in a traveling direction of the metal strip S being rolled), and a winder 3 disposed at the delivery side of the rolling mill 10 (that is, at the downstream side of the rolling mill 10 in the traveling direction of the metal strip S being rolled). Further, the rolling mill apparatus 1 includes a control device 50 for controlling the pair of mill rolls 15, 16 and the unwinder 2. The rolling mill apparatus 1 may include, for example, one rolling mill 10 as shown in FIG. 1, or may include two or more rolling mills 10.

[0019] The rolling mill 10 includes a pair of mill rolls (work rolls) 15, 16 disposed on opposite sides of the metal strip S with the metal strip S in between. As shown in FIG. 1, the rolling mill 10 may include a pair of intermediate rolls 17, 18 and a pair of backup rolls 19, 20 disposed on the opposite sides of the metal strip S to sandwich the pair of mill rolls 15, 16. The intermediate rolls 17, 18 and the backup rolls 19, 20 are configured to support the mill rolls 15, 16. Moreover, the rolling mill 10 includes a rolling-reduction device (such as a hydraulic cylinder; not shown) for reducing the thickness of the metal strip S between the pair of mill rolls 15, 16 by applying a load to the pair of mill rolls 15, 16.

[0020] The mill rolls 15, 16 are connected to a motor 11 via a spindle (not shown), etc., and the mill rolls 15, 16 are rotationally driven by the motor. In rolling of the metal strip S, the mill rolls 15, 16 are rotated by the motor 11 while the metal strip S is pressed by the rolling-reduction device, which create a friction force between the metal strip S and the mill rolls 15, 16. With this friction force, the metal strip S is sent to the delivery side of the mill rolls 15, 16.

[0021] The unwinder 2 is configured to unwind a coil of the metal strip S toward the rolling mill 10. The unwinder 2 includes a mandrel 4 and is configured to unwind the metal strip S toward the rolling mill 10 by rotationally driving the mandrel 4 with a motor 26. The mandrel 4 of the unwinder 2 is driven by the motor 26 to apply an entry-side tension to the metal strip S in rolling of the metal strip S.

[0022] The winder 3 is configured to wind the metal strip S from the rolling mill 10. The winder 3 includes a mandrel 5 and is configured to wind the metal strip S by rotationally driving the mandrel 5 with a motor (not shown). When the metal strip S is rolled while being wound around the mandrel 5 of the winder 3, a delivery-side tension is applied to the metal strip S by the mandrel 5 of the winder 3.

[0023] As shown in FIG. 1, guide rolls 6, 7 for guiding the metal strip S may be disposed between the rolling mill 10 and the mandrel 4 of the unwinder 2 and between the rolling mill 10 and the mandrel 5 of the winder 3, respectively.

[0024] In some embodiments, the rolling mill apparatus 1 is a reverse-type rolling mill apparatus (reverse mill) which is configured to reciprocate the metal strip S inserted between the pair of mill rolls 15, 16 to roll the metal strip S. In the reverse-type rolling mill apparatus 1, rolling is stopped immediately before the tail end St of the metal strip S unwound from the mandrel 4 of the unwinder 2, and rolling in an odd-number pass (first pass, etc.) is completed in a state where the metal strip S is pressed by the mill rolls 15, 16. Then, the metal strip S is unwound from the mandrel 5 of the winder 3 toward the rolling mill 10, and rolling in an even-number pass (second pass, etc.) is performed by moving the metal strip S in a traveling direction reverse to the preceding direction while winding the metal strip S around the mandrel 4 of the unwinder 2. That is, the roles of the unwinder 2 and the winder 3 switch in accordance with the traveling direction of the metal strip S.

[0025] FIG. 2 is a partial cross-sectional view of the unwinder 2 (winder 3) according to an embodiment and a diagram for describing the operation of the unwinder 2 (winder 3). As shown in FIGs. 1 and 2, the unwinder 2 includes a gripper 22 (gripping portion) for gripping a tail end portion including the tail end St of the metal strip S. Further, the winder 3 includes a gripper 23 for gripping a head end portion including the head end of the metal strip S.

[0026] In the exemplary embodiment shown in FIG. 2, the gripper 22, 23 is disposed radially inward of the outer peripheral surface of the mandrel 4, 5 so as to be movable along the radial direction. The mandrel 4, 5 has a slot 24 which opens to the surface of the mandrel 4, 5 and can receive the tail end or head end of the metal strip S. The tail end portion or head end portion of the metal strip S is gripped by moving the gripper 22, 23 radially outward by an actuator (not shown), etc., and applying a force to the metal strip S in a direction from the gripper 22, 23 toward the mandrel 4, 5.

[0027] When the metal strip S is rolled with the rolling mill apparatus 1, basically, the mill rolls 15, 16, and the mandrels 4, 5 of the unwinder 2 and the winder 3 are rotated while applying tension to the metal strip S in a state where the tail end portion and head end portion of the metal strip S are held by the grippers 22, 23.

[0028] As shown in FIG. 1, the rolling mill apparatus 1 includes a speed measurement part 28 for measuring the speed of the metal strip S (speed in the traveling direction; material speed). The speed measurement part 28 may include a motor speed detector for detecting the motor speed for driving the pair of mill rolls 15, 16, and the speed of the metal strip S may be calculated based on the detected motor speed. Alternatively, the speed measurement part 28 may include a speed sensor for detecting the speed of the metal strip S.

[0029] As shown in FIG. 1, the rolling mill apparatus 1 includes a tension measurement part 30 for measuring the tension (hereinafter also referred to as unwinding-side tension) acting on the metal strip S between the unwinder 2 and the pair of mill rolls 15, 16 (i.e., at the upstream or entry side of the rolling mill 10). The tension measurement part 30 may include a tension meter for detecting the tension acting on the metal strip S at the unwinder side. Alternatively, the tension measurement part 30 may include a torque detector for detecting the load torque of the unwinder 2, and the tension may be calculated based on the detected load torque.

[0030] A signal indicating the detection result of the speed of the metal strip S by the speed measurement part 28 and a signal indicating the detection result of the tension by the tension measurement part 30 are transmitted to the control device 50.

[0031] FIG. 3 is a schematic configuration diagram of the control device 50 according to an embodiment. As shown in FIG. 3, the control device 50 includes a mill roll control part 56 and an unwinder control part 58. The control device 50 may further include a target speed acquisition part 52 and / or a target tension acquisition part 54.

[0032] The control device 50 includes a calculator with a processor (such as CPU), a main storage device (memory device; such as RAM), an auxiliary storage device, an interface, and the like. The control device 50 receives signals indicating detected values from the speed measurement part 28 and / or the tension measurement part 30 via the interface. The processor is configured to process the signals thus received. Also, the processor is configured to process programs loaded into the main storage device. Thereby, the function of each functional unit (target speed acquisition part 52, target tension acquisition part 54, mill roll control part 56, and / or unwinder control part 58) is implemented.

[0033] The processing contents in the control device 50 may be implemented as programs executed by the processor. The programs may be stored in, for example, an auxiliary storage device or may be stored in a computer-readable storage medium. When the programs are executed, these programs are loaded into the main storage device. The processor is configured to read out the programs from the main storage device to execute instructions included in the programs.

[0034] The target speed acquisition part 52 is configured to acquire a target speed of the metal strip S. The target speed may be set in advance according to the steel type and target thickness of the metal strip S to be rolled. The preset target speed may be stored in a storage unit (e.g., auxiliary storage device; not shown) of the control device 50. The target speed acquisition part 52 may acquire the target speed of the metal strip S by reading the target speed stored in advance from the storage unit.

[0035] The target tension acquisition part 54 is configured to acquire a target value of the tension (target tension) of the metal strip S between the unwinder 2 and the pair of mill rolls 15, 16. The target tension may be set in advance according to the steel type and target thickness of the metal strip S to be rolled. The preset target tension may be stored in a storage unit (e.g., auxiliary storage device; not shown) of the control device 50. The target tension acquisition part 54 may acquire the target speed of the metal strip S by reading the target tension stored in advance from the storage unit.

[0036] The mill roll control part 56 is configured to control the rotation of the pair of mill rolls 15, 16 on the basis of a deviation between the target speed of the metal strip S acquired by the target speed acquisition part 52 and the speed measurement (actual speed) of the metal strip S acquired by the speed measurement part 28. The mill roll control part 56 may be configured to calculate a command value for the rotation speed of the motor 11 (the motor that drives the pair of mill rolls 15, 16) such that the speed measurement approaches the target speed, and adjust the rotation speed of the motor 11 according to the command value.

[0037] The unwinder control part 58 is configured to control the torque of the unwinder 2 on the basis of a deviation between the target tension of the metal strip S acquired by the target tension acquisition part 54 and the tension measurement (actual tension) of the metal strip S acquired by the tension measurement part 30. The unwinder control part 58 may be configured to calculate a torque command value such that the tension measurement approaches the target tension, and control the motor 26 that drives the unwinder 2 according to the command value.(Operation method for rolling mill apparatus)

[0038] Next, the method for operating the rolling mill apparatus 1 according to some embodiments will be described.

[0039] FIG. 4 is a flowchart of the operation method for a rolling mill apparatus according to an embodiment. The operation method shown in FIG. 4 can be performed using the above-described control device 50. Further, a part or the whole of the procedure shown in FIG. 4 may be performed manually. FIG. 5 is a graph showing examples of temporal changes in target speed and target tension of the metal strip S, and peripheral speed (circumferential speed) of the mandrel 4 of the unwinder 2 in the operation method according to an embodiment. FIGs. 6 to 9 are each a schematic diagram for describing the change in shape of the tail end portion of the metal strip S according to an embodiment.

[0040] As shown in FIG. 4, in the operation method according to an embodiment, the metal strip S is rolled with the rolling mill 10 while unwinding the metal strip S from the unwinder 2 (S2). In step S2, for example, as shown in FIG. 5, rolling is performed with the target speed Vt* of the metal strip S as Vt1 and the unwinding-side target tension T* as T1 (see FIG. 5). During the rolling of the metal strip S, since the unwinding-side tension is somewhat large, as shown in FIG. 5, the speed of the metal strip S (≈ Vt* = Vt1) is almost equal to the peripheral speed Vm of the mandrel 4 of the unwinder 2.

[0041] During the rolling of the metal strip S, the control device 50 determines whether it is within the first period that is immediately before the tail end St of the metal strip S leaves the unwinder 2 (S4). The control device 50 may be configured, for example, to determine that it is within the first period when the number of remaining windings of the metal strip S in the unwinder 2 falls below a predetermined value. The first period may be a period when the number of remaining windings of the metal strip S in the unwinder 2 is not more than 2, not more than 1, or not more than 0.5. The number of windings of the metal strip S in the unwinder 2 may be calculated based on the number of windings or the length of the metal strip S at the start of the rolling pass and / or the angular position of the gripper 22 around the rotational axis of the mandrel 4.

[0042] As a result of the determination in step S4, if the first period has not yet started (No in S4; until time t1 in FIG. 5), the rolling of the metal strip S is continued while keeping the target speed Vt* of the metal strip S at Vt1 and the unwinding-side target tension T* at T1 (S2).

[0043] As a result of the determination in step S4, if the first period has started (Yes in S4, time t1 in FIG. 5), the target speed Vt* is reduced so that the peripheral speed Vm of the mandrel 4 becomes greater than the speed (actual speed) of the metal strip S at the unwinding side, and / or the target tension T* is reduced (S6).

[0044] In the graph shown in FIG. 5, the first period is the period from time t1, when the number of remaining windings of the metal strip S around the mandrel 4 becomes not more than a predetermined value (e.g., not more than 0.5 windings), to time t6, when the tail end St of the metal strip S leaves the slot 24 of the unwinder 2. In the example shown in FIG. 5, the target speed Vt* of the metal strip S is reduced from Vt1 to Vt2 during the period from time t1 to t3 in the first period, and the unwinding-side target tension T* of the metal strip S is reduced from T1 to T2 during the period from time t2 (provided that t1 < t2 < t3) to time t4 (provided that t3 < t4) in the first period.

[0045] In steps S2 and S6, the rotation of the pair of mill rolls 15, 16 may be controlled based on the deviation between the target speed Vt* and the actual speed of the metal strip S. Further, in steps S2 and S6, the torque of the unwinder 2 may be controlled based on the deviation between the target tension T* and the actual tension of the metal strip between the unwinder 2 and the pair of mill rolls 15, 16.

[0046] A phenomenon that occurs in step S6 will now be described specifically.

[0047] When the number of remaining windings of the metal strip S around the mandrel 4 is zero (see FIG. 6; the time between t4 and t5 in FIG. 5) or when the number of remaining windings is more than zero (e.g., the period before t4 in FIG. 5), as shown in FIG. 6, the angle θ formed on the upper surface Sa side of the metal strip S between a gripped portion 60 (tail end portion) and an adjacent portion 62 adjacent to the gripped portion 60 is larger than 180 degrees, and the gripped portion 60 bends downward (i.e., toward the lower surface Sb of the metal strip S). Also, when the number of remaining windings is zero (see FIG. 6), the adjacent portion 62 adjacent to the gripped portion 60 (tail end portion) of the metal strip S is roughly tangent to the mandrel 4.

[0048] Generally, when the target speed Vt* of the metal strip S decreases, the peripheral speed Vm of the mandrel 4 also decreases. However, if the rate of change of the target speed Vt* of the metal strip S is increased to some extent in the first period (that is, if the target speed Vt* is reduced steeply; step S6), the rate of decrease of the mandrel peripheral speed becomes smaller than that of the speed of the metal strip S due to the inertia of the mandrel 4. As a result, a period during which the peripheral speed Vm of the mandrel 4 is greater than the speed (actual speed) of the metal strip S occurs (from time t1 to t3 in FIG. 5). This allows the unwinding-side tension (actual tension) of the metal strip S to be relieved.

[0049] Alternatively, in the first period, the unwinding-side target tension T* of the metal strip S may be reduced (step S6) to relieve the unwinding-side tension (actual tension) of the metal strip S.

[0050] As a result of relieving the unwinding-side tension (actual tension) of the metal strip S, even if the mandrel 4 exceeds the angular position where the number of remaining windings of the metal strip S around the mandrel 4 is zero (see FIG. 6; time between t4 and t5 in FIG. 5) and the angular position where the opening of the slot 24 in the mandrel 4 and the gripper 22 are at the top of the mandrel (see FIG. 7; time t5 in FIG. 5), the gripped portion 60 (tail end portion) of the metal strip S remains caught in the slot 24 because the tension acting on the metal strip S is smaller than the gripping force by the gripper 22 (gripping portion).

[0051] Then, the mandrel 4 rotates further, and after the angle θ formed between the gripped portion 60 and the adjacent portion 62 becomes smaller than 180 degrees and the gripped portion 60 bends upward (i.e., toward the upper surface Sa of the metal strip S) (see FIG. 8), the gripped portion 60 (tail end portion) of the metal strip S exits the slot 24 (S8 in FIG. 4; time t6 in FIG. 5; FIG. 9).

[0052] In a typical conventional operation method, the unwinding-side tension of the metal strip S is not relieved during the rolling of the metal strip S until the tail end exits the unwinder. Therefore, in the vicinity of the angular position where the opening of the slot 24 in the mandrel 4 and the gripper 22 are at the top of the mandrel (see FIG. 7), the tension acting on the metal strip S is greater than the gripping force by the gripper 22 (gripping portion), and the gripped portion 60 (tail end portion) of the metal strip S comes off the slot 24.

[0053] At this time, the angle θ formed between the gripped portion 60 and the adjacent portion 62 is around 180 degrees, which is smaller than the angle θ (over 180 degrees) when the number of remaining windings of the metal strip S is more than zero. However, even if the downward bending deformation (plastic deformation) formed in the gripped portion 60 when the number of remaining windings of the metal strip S is more than zero is momentarily straightened, it is an elastic deformation, and as shown in FIG. 10, most of the downward bend of the gripped portion 60 eventually remains. FIG. 10 is a schematic diagram for describing the change in shape of the tail end portion of the metal strip according to a typical operation method.

[0054] In this regard, according to the above-described embodiments, in the first period immediately before the tail end St of the metal strip S exits the unwinder, the target speed Vt* of the metal strip is reduced so that the peripheral speed Vm of the mandrel 4 becomes greater than the actual speed of the metal strip S, or the target tension T* is reduced, allowing the actual tension of the metal strip S to be relieved. This makes it easier for the tail end of the metal strip S (gripped portion 60) to exit the mandrel 4 (unwinder) 2 at the position (e.g., the position shown in FIG. 8) where the tail end portion bends in the direction (i.e., upward) opposite to that during unwinding. Thus, the downward bend (plastic deformation) formed in the tail end portion at the gripping portion during unwinding is folded back to the opposite side upon exiting the mandrel 4, thereby mitigating the downward bending deformation (plastic deformation) in the tail end portion. This prevents the head end portion of the metal strip S from being caught in the conveyor path at the start of the next pass of rolling, enabling smooth threading of the strip. As a result, it is possible to produce products more efficiently.

[0055] In step S6, in the first period, the target speed Vt* may be reduced so that the peripheral speed Vm of the mandrel 4 becomes greater than the speed (actual speed) of the metal strip S at the unwinding side, and the target tension T* may also be reduced.

[0056] In the example shown in FIG. 5, in the first period (t1 to t6), the target speed Vt* is reduced so that the peripheral speed Vm of the mandrel 4 becomes greater than the speed (actual speed) of the metal strip S at the unwinding side during the period from time t1 to t3, and the target tension T* is also reduced during the period from time t2 to t4.

[0057] Thus, by reducing the target speed Vt* of the metal strip S so that the peripheral speed Vm of the mandrel 4 becomes greater than the speed of the metal strip S, and also reducing the target tension T* in the first period immediately before the tail end of the metal strip S exits the unwinder 2, the tension of the metal strip S can be relieved more reliably. This facilitates the folding back of the downward bend (plastic deformation), formed in the tail end portion at the gripping portion during unwinding, to the opposite side upon exiting the mandrel 4, thereby mitigating the downward bending deformation (plastic deformation) in the tail end portion more reliably. This prevents the head end portion of the metal strip S from being caught in the conveyor path at the start of the next pass of rolling, enabling smooth threading of the strip.

[0058] Alternatively, in the above-described step S6, the target speed Vt* may be reduced so that the peripheral speed of mandrel 4 becomes greater than the speed (actual speed) of the metal strip S at the unwinding side while reducing the target tension T* in the first period.

[0059] In the example shown in FIG. 5, in the first period (t1 to t6), during the period from time t2 to t3, the target speed Vt* is reduced so that the peripheral speed Vm of the mandrel 4 becomes greater than the speed (actual speed) of the metal strip S at the unwinding side, and simultaneously, the target tension T* is reduced.

[0060] Thus, by reducing the target speed Vt* of the metal strip S so that the peripheral speed Vm of the mandrel 4 becomes greater than the speed of the metal strip S while reducing the target tension T* in the first period immediately before the tail end of the metal strip S exits the unwinder 2, the tension of the metal strip S can be relieved more reliably. This facilitates the folding back of the downward bend (plastic deformation), formed in the tail end portion at the gripping portion during unwinding, to the opposite side upon exiting the mandrel 4, thereby mitigating the downward bending deformation (plastic deformation) in the tail end portion more reliably. This prevents the head end portion of the metal strip S from being caught in the conveyor path at the start of the next pass of rolling, enabling smooth threading of the strip.

[0061] FIG. 11 is a flowchart of the operation method for a rolling mill apparatus according to an embodiment. A part or the whole of the procedure shown in FIG. 11 may be performed using the control device 50 or manually.

[0062] As shown in FIG. 4, in the operation method according to an embodiment, when the angle formed between the gripped portion 60 (tail end portion) inserted in the slot 24 and the adjacent portion 62 adjacent to the gripped portion 60 on the upper surface Sa side of the metal strip S (or folding angle from the straight line along the gripped portion to the straight line along the adjacent portion in the rotational direction of the mandrel) θ (see FIG. 6) is not smaller than 180 degrees (No in S14; that is, until the angle θ becomes 180 degrees, as shown in FIG. 7, for example), the metal strip S is rolled with the rolling mill 10 while unwinding the metal strip S from the unwinder 2 (S12).

[0063] Then, after the angle θ becomes smaller than 180 degrees at the end of unwinding of the metal strip S from the unwinder 2 (Yes in S14; see FIG. 8, for example), the tail end St of the metal strip S is released from the unwinder 2 (S16).

[0064] According to the above-described embodiments, in a state where the metal strip S is wound around the unwinder 2, the angle θ formed between the gripped portion 60 (tail end portion) and the adjacent portion 62 on the upper surface Sa side of the metal strip S is not smaller than 180 degrees (that is, the tail end portion of the metal strip bends downward), and after the angle θ becomes smaller than 180 degrees (that is, the tail end portion of the metal strip S bends upward) at the end of unwinding of the metal strip S from the unwinder 2, the tail end of the metal strip S is released from the unwinder 2. Thus, the downward bend (plastic deformation) formed in the tail end portion at the gripping portion during unwinding is folded back to the opposite side upon exiting the mandrel 4, thereby mitigating the downward bending deformation (plastic deformation) in the tail end portion. This prevents the head end portion of the metal strip S from being caught in the conveyor path at the start of the next pass of rolling, enabling smooth threading of the strip. As a result, it is possible to produce products more efficiently.

[0065] The contents described in the above embodiments would be understood as follows, for instance. [1] A control device (50) for a rolling mill apparatus according to at least one embodiment of the present invention is a control device for controlling a rolling mill apparatus (1) which includes a pair of mill rolls (15, 16) for rolling a metal strip (S), and an unwinder (2) for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel (4) having a gripping portion (e.g., the above-described gripper 22) for gripping a tail end portion of the metal strip. The control device includes: a mill roll control part (56) configured to control rotation of the pair of mill rolls on the basis of a deviation between a target speed and an actual speed of the metal strip; and an unwinder control part (58) configured to control a torque of the unwinder on the basis of a deviation between a target tension and an actual tension of the metal strip between the unwinder and the pair of mill rolls. The control device is configured to, in a first period immediately before a tail end of the metal strip exits the unwinder, reduce the target speed so that a peripheral speed of the mandrel becomes greater than a speed of the metal strip at an unwinding side or reduce the target tension. According to the above configuration [1], in the first period immediately before the tail end of the metal strip exits the unwinder, the target value of the speed of the metal strip is reduced so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip, or the target value of the tension is reduced, allowing the tension of the metal strip to be relieved. This makes it easier for the tail end of the metal strip to exit the mandrel (unwinder) at the position where the tail end portion (gripped portion) bends in the direction opposite to that during unwinding. Thus, the downward bend (plastic deformation) formed in the tail end portion at the gripping portion during unwinding is folded back to the opposite side upon exiting the mandrel, thereby mitigating the downward bending deformation (plastic deformation) in the tail end portion. This prevents the head end portion of the metal strip from being caught in the conveyor path at the start of the next pass of rolling, enabling smooth threading of the strip. As a result, it is possible to produce products more efficiently. [2] In some embodiments, in the above configuration [1], the control device for a rolling mill apparatus is configured to reduce the target speed so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip at the unwinding side and also reduce the target tension in the first period. According to the above configuration [2], in the first period immediately before the tail end of the metal strip exits the unwinder, the target value of the speed of the metal strip is reduced so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip, and the target value of the tension is also reduced, allowing the tension of the metal strip to be relieved more reliably. This facilitates the folding back of the downward bend (plastic deformation), formed in the tail end portion at the gripping portion during unwinding, to the opposite side upon exiting the mandrel, thereby mitigating the downward bending deformation (plastic deformation) in the tail end portion more reliably. This prevents the head end portion of the metal strip from being caught in the conveyor path at the start of the next pass of rolling, enabling smooth threading of the strip. [3] In some embodiments, in the above configuration [2], the control device for a rolling mill apparatus is configured to reduce the target speed so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip at the unwinding side while reducing the target tension in the first period. According to the above configuration [3], in the first period immediately before the tail end of the metal strip exits the unwinder, the target value of the speed of the metal strip is reduced so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip while reducing the target value of the tension, allowing the tension of the metal strip to be relieved more reliably. This facilitates the folding back of the downward bend (plastic deformation), formed in the tail end portion at the gripping portion during unwinding, to the opposite side upon exiting the mandrel, thereby mitigating the downward bending deformation (plastic deformation) in the tail end portion more reliably. This prevents the head end portion of the metal strip from being caught in the conveyor path at the start of the next pass of rolling, enabling smooth threading of the strip. [4] In some embodiments, in any of the above configurations [1] to [3], the first period is a period during which the number of remaining windings of the metal strip in the unwinder is not more than two. According to the above configuration [4], the tension of the metal strip is relieved by reducing the target value of the speed or the target value of the tension of the metal strip in the first period, when the number of remaining windings of the metal strip in the unwinder is not more than two (i.e., the first period immediately before the tail end of the metal strip exits the unwinder), so the tension acting on the metal strip is maintained until immediately before the tail end exits the unwinder (i.e., before the first period). Therefore, the yield is improved. [5] A rolling mill apparatus (1) according to at least one embodiment of the present invention includes: a pair of mill rolls (15, 16) for rolling a metal strip (S); an unwinder (2) for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel (4) having a gripping portion (e.g., the above-described gripper 22) for gripping a tail end portion of the metal strip; and a control device (50) in any of the above configurations [1] to [4] configured to control the pair of mill rolls and the unwinder. According to the above configuration [5], in the first period immediately before the tail end of the metal strip exits the unwinder, the target value of the speed of the metal strip is reduced so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip, or the target value of the tension is reduced, allowing the tension of the metal strip to be relieved. This makes it easier for the tail end of the metal strip to exit the mandrel (unwinder) at the position where the tail end portion (gripped portion) bends in the direction opposite to that during unwinding. Thus, the downward bend (plastic deformation) formed in the tail end portion at the gripping portion during unwinding is folded back to the opposite side upon exiting the mandrel, thereby mitigating the downward bending deformation (plastic deformation) in the tail end portion. This prevents the head end portion of the metal strip from being caught in the conveyor path at the start of the next pass of rolling, enabling smooth threading of the strip. As a result, it is possible to produce products more efficiently. [6] A rolling mill apparatus (1) according to at least one embodiment of the present invention includes: a pair of mill rolls (15, 16) for rolling a metal strip (S); and an unwinder (2) for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel (4) having a gripping portion (e.g., the above-described gripper 22) for gripping a tail end portion of the metal strip. The metal strip includes a gripped portion (60) gripped by the gripping portion, and an adjacent portion (62) adjacent to the gripped portion in a longitudinal direction of the metal strip. In a state where the metal strip is wound around the unwinder, an angle (θ) formed between the gripped portion and the adjacent portion on an upper surface (Sa) side of the metal strip (or folding angle from the straight line along the gripped portion to the straight line along the adjacent portion in the rotational direction of the mandrel) is not smaller than 180 degrees. The rolling mill apparatus is configured such that a tail end of the metal strip exits the unwinder after the angle becomes smaller than 180 degrees at the end of unwinding of the metal strip from the unwinder. According to the above configuration [6], in a state where the metal strip is wound around the unwinder, the angle formed between the gripped portion and the adjacent portion on the upper surface side of the metal strip (or folding angle from the straight line along the gripped portion to the straight line along the adjacent portion in the rotational direction of the mandrel) is not smaller than 180 degrees (that is, the tail end portion of the metal strip bends downward), and after the angle becomes smaller than 180 degrees (that is, the tail end portion of the metal strip bends upward) at the end of unwinding of the metal strip from the unwinder, the tail end of the metal strip exits the unwinder. Thus, the downward bend (plastic deformation) formed in the tail end portion at the gripping portion during unwinding is folded back to the opposite side upon exiting the mandrel, thereby mitigating the downward bending deformation (plastic deformation) in the tail end portion. This prevents the head end portion of the metal strip from being caught in the conveyor path at the start of the next pass of rolling, enabling smooth threading of the strip. As a result, it is possible to produce products more efficiently. [7] An operation method for a rolling mill apparatus according to at least one embodiment of the present invention is an operation method for operating a rolling mill apparatus (1) which includes a pair of mill rolls (15, 16) for rolling a metal strip (S), and an unwinder (2) for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel (4) having a gripping portion (e.g., the above-described gripper 22) for gripping a tail end portion of the metal strip. The operation method includes: a mill roll control step (S2, S6) of controlling rotation of the pair of mill rolls on the basis of a deviation between a target speed and an actual speed of the metal strip; and an unwinder control step (S2, S6) of controlling a torque of the unwinder on the basis of a deviation between a target tension and an actual tension of the metal strip between the unwinder and the pair of mill rolls. In a first period immediately before a tail end of the metal strip exits the unwinder, the target speed is reduced so that a peripheral speed of the mandrel becomes greater than a speed of the metal strip at an unwinding side, or the target tension is reduced (S6). According to the above method [7], in the first period immediately before the tail end of the metal strip exits the unwinder, the target value of the speed of the metal strip is reduced so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip, or the target value of the tension is reduced, allowing the tension of the metal strip to be relieved. This makes it easier for the tail end of the metal strip to exit the mandrel (unwinder) at the position where the tail end portion (gripped portion) bends in the direction opposite to that during unwinding. Thus, the downward bend (plastic deformation) formed in the tail end portion at the gripping portion during unwinding is folded back to the opposite side upon exiting the mandrel, thereby mitigating the downward bending deformation (plastic deformation) in the tail end portion. This prevents the head end portion of the metal strip from being caught in the conveyor path at the start of the next pass of rolling, enabling smooth threading of the strip. As a result, it is possible to produce products more efficiently. [8] An operation method for a rolling mill apparatus according to at least one embodiment of the present invention is an operation method for operating a rolling mill apparatus (1) which includes a pair of mill rolls (15, 16) for rolling a metal strip (S), and an unwinder (2) for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel (4) having a gripping portion (e.g., the above-described gripper 22) for gripping a tail end portion of the metal strip. The metal strip includes a gripped portion (60) gripped by the gripping portion, and an adjacent portion (62) adjacent to the gripped portion in a longitudinal direction of the metal strip. In a state where the metal strip is wound around the unwinder, an angle (θ) formed between the gripped portion and the adjacent portion on an upper surface (Sa) side of the metal strip (or folding angle from the straight line along the gripped portion to the straight line along the adjacent portion in the rotational direction of the mandrel) is not smaller than 180 degrees. The operation method includes a step (S16) of allowing a tail end of the metal strip to exit the unwinder after the angle becomes smaller than 180 degrees at the end of unwinding of the metal strip from the unwinder. According to the above method [8], in a state where the metal strip is wound around the unwinder, the angle formed between the gripped portion and the adjacent portion on the upper surface side of the metal strip (or folding angle from the straight line along the gripped portion to the straight line along the adjacent portion in the rotational direction of the mandrel) is not smaller than 180 degrees (that is, the tail end portion of the metal strip bends downward), and after the angle becomes smaller than 180 degrees (that is, the tail end portion of the metal strip bends upward) at the end of unwinding of the metal strip from the unwinder, the tail end of the metal strip exits the unwinder. Thus, the downward bend (plastic deformation) formed in the tail end portion at the gripping portion during unwinding is folded back to the opposite side upon exiting the mandrel, thereby mitigating the downward bending deformation (plastic deformation) in the tail end portion. This prevents the head end portion of the metal strip from being caught in the conveyor path at the start of the next pass of rolling, enabling smooth threading of the strip. As a result, it is possible to produce products more efficiently. [9] A control program for a rolling mill apparatus according to at least one embodiment of the present invention is a control program for controlling a rolling mill apparatus (1) which includes a pair of mill rolls (15, 16) for rolling a metal strip (S), and an unwinder (2) for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel (4) having a gripping portion (e.g., the above-described gripper 22) for gripping a tail end portion of the metal strip. The control program causes a computer to execute: a process for controlling rotation of the pair of mill rolls on the basis of a deviation between a target speed and an actual speed of the metal strip; and a process for controlling a torque of the unwinder on the basis of a deviation between a target tension and an actual tension of the metal strip between the unwinder and the pair of mill rolls. The control program is configured to, in a first period immediately before a tail end of the metal strip exits the unwinder, reduce the target speed so that a peripheral speed of the mandrel becomes greater than a speed of the metal strip at an unwinding side or reduce the target tension.

[0066] According to the above program [9], in the first period immediately before the tail end of the metal strip exits the unwinder, the target value of the speed of the metal strip is reduced so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip, or the target value of the tension is reduced, allowing the tension of the metal strip to be relieved. This makes it easier for the tail end of the metal strip to exit the mandrel (unwinder) at the position where the tail end portion (gripped portion) bends in the direction opposite to that during unwinding. Thus, the downward bend (plastic deformation) formed in the tail end portion at the gripping portion during unwinding is folded back to the opposite side upon exiting the mandrel, thereby mitigating the downward bending deformation (plastic deformation) in the tail end portion. This prevents the head end portion of the metal strip from being caught in the conveyor path at the start of the next pass of rolling, enabling smooth threading of the strip. As a result, it is possible to produce products more efficiently.

[0067] Embodiments of the present invention were described in detail above, but the present invention is not limited thereto, and various amendments and modifications may be implemented.

[0068] In the present specification, an expression of relative or absolute arrangement such as "in a direction", "along a direction", "parallel", "orthogonal", "centered", "concentric" and "coaxial" shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function.

[0069] For instance, an expression of an equal state such as "same", "equal", and "uniform" shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a difference that can still achieve the same function.

[0070] Further, an expression of a shape such as a rectangular shape or a cylindrical shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved.

[0071] On the other hand, an expression such as "comprise", "include", and "have" are not intended to be exclusive of other components.Reference Signs List

[0072] 1Rolling mill apparatus 2Unwinder 3Winder 4Mandrel 5Mandrel 6Guide roll 7Guide roll 10Rolling mill 11Motor 15Mill roll 16Mill roll 17Intermediate roll 18Intermediate roll 19Backup roll 20Backup roll 22Gripper 23Gripper 24Slot 26Motor 28Speed measurement part 30Tension measurement part 50Control device 52Target speed acquisition part 54Target tension acquisition part 56Mill roll control part 58Unwinder control part 60Gripped portion 62Adjacent portion SMetal strip SaUpper surface SbLower surface StTail end θAngle

Claims

1. A control device for controlling a rolling mill apparatus which includes a pair of mill rolls for rolling a metal strip, and an unwinder for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel having a gripping portion for gripping a tail end portion of the metal strip, the control device comprising: a mill roll control part configured to control rotation of the pair of mill rolls on the basis of a deviation between a target speed and an actual speed of the metal strip; and an unwinder control part configured to control a torque of the unwinder on the basis of a deviation between a target tension and an actual tension of the metal strip between the unwinder and the pair of mill rolls, wherein the control device is configured to, in a first period immediately before a tail end of the metal strip exits the unwinder, reduce the target speed so that a peripheral speed of the mandrel becomes greater than a speed of the metal strip at an unwinding side or reduce the target tension.

2. The control device for a rolling mill apparatus according to claim 1, wherein the control device is configured to reduce the target speed so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip at the unwinding side and also reduce the target tension in the first period.

3. The control device for a rolling mill apparatus according to claim 2, wherein the control device is configured to reduce the target speed so that the peripheral speed of the mandrel becomes greater than the speed of the metal strip at the unwinding side while reducing the target tension in the first period.

4. The control device for a rolling mill apparatus according to any one of claims 1 to 3, wherein the first period is a period during which the number of remaining windings of the metal strip in the unwinder is not more than two.

5. A rolling mill apparatus, comprising: a pair of mill rolls for rolling a metal strip; an unwinder for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel having a gripping portion for gripping a tail end portion of the metal strip; and a control device according to any one of claims 1 to 3 configured to control the pair of mill rolls and the unwinder.

6. A rolling mill apparatus, comprising: a pair of mill rolls for rolling a metal strip; and an unwinder for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel having a gripping portion for gripping a tail end portion of the metal strip, wherein the metal strip includes a gripped portion gripped by the gripping portion, and an adjacent portion adjacent to the gripped portion in a longitudinal direction of the metal strip, wherein, in a state where the metal strip is wound around the unwinder, an angle formed between the gripped portion and the adjacent portion on an upper surface side of the metal strip is not smaller than 180 degrees, and wherein the rolling mill apparatus is configured such that a tail end of the metal strip exits the unwinder after the angle becomes smaller than 180 degrees at the end of unwinding of the metal strip from the unwinder.

7. An operation method for operating a rolling mill apparatus which includes a pair of mill rolls for rolling a metal strip, and an unwinder for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel having a gripping portion for gripping a tail end portion of the metal strip, the operation method comprising: a mill roll control step of controlling rotation of the pair of mill rolls on the basis of a deviation between a target speed and an actual speed of the metal strip; and an unwinder control step of controlling a torque of the unwinder on the basis of a deviation between a target tension and an actual tension of the metal strip between the unwinder and the pair of mill rolls, wherein, in a first period immediately before a tail end of the metal strip exits the unwinder, the target speed is reduced so that a peripheral speed of the mandrel becomes greater than a speed of the metal strip at an unwinding side, or the target tension is reduced.

8. An operation method for operating a rolling mill apparatus which includes a pair of mill rolls for rolling a metal strip, and an unwinder for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel having a gripping portion for gripping a tail end portion of the metal strip, wherein the metal strip includes a gripped portion gripped by the gripping portion, and an adjacent portion adjacent to the gripped portion in a longitudinal direction of the metal strip, wherein, in a state where the metal strip is wound around the unwinder, an angle formed between the gripped portion and the adjacent portion on an upper surface side of the metal strip is not smaller than 180 degrees, and wherein the operation method comprises a step of allowing a tail end of the metal strip to exit the unwinder after the angle becomes smaller than 180 degrees at the end of unwinding of the metal strip from the unwinder.

9. A control program for a rolling mill apparatus which includes a pair of mill rolls for rolling a metal strip, and an unwinder for unwinding the metal strip toward the pair of mill rolls, the unwinder including a mandrel having a gripping portion for gripping a tail end portion of the metal strip, the control program causing a computer to execute: a process for controlling rotation of the pair of mill rolls on the basis of a deviation between a target speed and an actual speed of the metal strip; and a process for controlling a torque of the unwinder on the basis of a deviation between a target tension and an actual tension of the metal strip between the unwinder and the pair of mill rolls, wherein the control program is configured to, in a first period immediately before a tail end of the metal strip exits the unwinder, reduce the target speed so that a peripheral speed of the mandrel becomes greater than a speed of the metal strip at an unwinding side or reduce the target tension.

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