JP2024166081A - Method for controlling thickness of rolling mill, thickness control device for rolling mill, and method for manufacturing steel plate using the thickness control method for rolling mill - Google Patents

Abstract

To provide a thickness control method of a rolling machine and a thickness control apparatus of a rolling machine which can suppress variations of thicknesses more than before when the rolling machine is accelerated and decelerated, as well as a manufacturing method for a steel plate using the thickness control method of the rolling machine.SOLUTION: In a thickness control method of a rolling machine 1, which is provided with a plurality of rolling stands 3, 4, 5, 6 and 7 arranged along a conveying direction of a material 2 to be rolled and a motor 10 that drives mill rolls 8 on the plurality of rolling stands 3, 4, 5, 6 and 7, a ratio R of a droop rate determined by dividing a droop rate of the motor 10 on the rolling stands 4, 5, 6 and 7 at a downstream side in a conveying direction by a droop rate of the motor 10 on the rolling stands 3, 4, 5 and 6 at an upstream side in the conveying direction, of the rolling stands 3, 4, 5, 6 and 7 arranged adjacent to each other in the conveying direction, is set to 3/5-5/3, at the time when the material 2 to be rolled is rolled.SELECTED DRAWING: Figure 3

Description

The present invention relates to a method for controlling the thickness of a rolling mill that uses multiple rolling stands to roll steel plate, a rolling mill thickness control device, and a method for manufacturing steel plate using the rolling mill thickness control method.

Conventionally, a technique for controlling the thickness by controlling the roll gap and speed of each rolling stand in a rolling mill using AGC (Automatic Gage Control) as an example of a device for controlling the thickness is known. An example of a thickness control method using this type of AGC is described in Patent Document 1. In the method described in Patent Document 1, when rolling a material to be rolled in a tandem rolling mill, the gain value of the integral component used in PI control is changed based on the thickness deviation at the outlet side of the final rolling stand. In this way, the rolling speed of the upstream rolling stand is controlled. In addition, Patent Document 2 describes an example of a speed control method for a tandem rolling mill that suppresses thickness fluctuations by setting the droop rate. In the method described in Patent Document 2, the speed droop rate of the front stand of the tandem rolling mill is set to 0 (zero), and the speed droop rate of the rear stand is set to approximately 0 (zero), and rolling is performed. This suppresses thickness fluctuations due to speed changes of the tandem rolling mill.

JP 2005-254322 A Japanese Unexamined Patent Publication No. 49-101254

The method using AGC described in Patent Document 1 has a problem that it cannot adequately deal with thickness fluctuations that occur when the rolling mill accelerates and decelerates the material being rolled. In addition, as described above, the method described in Patent Document 2 sets the speed droop rate of the front stand to 0 (zero) and the speed droop rate of the rear stand to approximately 0 (zero). Therefore, the rolling mill to which the method described in Patent Document 2 can be applied may be limited to a tandem rolling mill consisting of two rolling stands. Therefore, in the case of a tandem rolling mill equipped with two or more rolling stands, the method described in Patent Document 2 may not be able to suppress thickness fluctuations. Note that the method described in Patent Document 2 has a problem that there is a risk of breaking the plate during acceleration and deceleration. The speed droop rate of the rolling mill is set to mitigate abrupt speed changes when conditions change, such as acceleration and deceleration, and setting it to approximately 0 (zero) would prevent the function from being exerted.

The present invention has been made to solve the above problems, and aims to provide a method for controlling the thickness of a rolling mill, a device for controlling the thickness of a rolling mill, and a method for manufacturing steel plate using the method for controlling the thickness of a rolling mill, which can suppress fluctuations in thickness during acceleration and deceleration of the rolling mill more than ever before.

The means for solving the above problems are as follows.
(1) A method for controlling the thickness of a rolling mill having a plurality of rolling stands arranged along a conveying direction of a material to be rolled and a motor provided in each of the rolling stands for driving the rolling rolls of the plurality of rolling stands, wherein, when the material to be rolled is being rolled, a ratio of droop rates calculated by dividing the droop rate of the motor of the rolling stand downstream in the conveying direction by the droop rate of the motor of the rolling stand upstream in the conveying direction among the rolling stands adjacent to each other in the conveying direction is set to 3/5 to 5/3.
(2) The method for controlling a thickness of a rolling mill according to (1), wherein the droop rate of the motor is set to 0.1% or more.
(3) A method for manufacturing a steel plate using the method for controlling the plate thickness of a rolling mill according to (1) or (2).
(4) A plate thickness control device for a rolling mill including a plurality of rolling stands arranged along a conveying direction of a material to be rolled and a motor provided for each of the rolling stands to drive the rolling rolls of the plurality of rolling stands, the plate thickness control device further comprising a control device for adjusting the droop rate of the motor, the control device setting a ratio of droop rates obtained by dividing the droop rate of the motor of the rolling stand downstream in the conveying direction by the droop rate of the motor of the rolling stand upstream in the conveying direction, between adjacent rolling stands in the conveying direction, to 3/5 to 5/3 when the material to be rolled is being rolled.
(5) The plate thickness control device for a rolling mill according to (4), wherein the control device sets a droop rate of the motor to 0.1% or more.

According to the present invention, even if the conveying speed of the rolled material changes while rolling the rolled material, it is possible to suppress large fluctuations in the thickness of the rolled material.

1 is a diagram showing an example of a rolling mill to which a rolling mill plate thickness control method, a rolling mill plate thickness control device, and a steel plate manufacturing method using the rolling mill plate thickness control method according to the present embodiment can be applied. FIG. 4 is a diagram illustrating the behavior of a rolling mill of a comparative example. FIG. 2 is a diagram for explaining the behavior of the rolling mill shown in FIG. 1 .

The present invention will be specifically described below through an embodiment of the present invention (hereinafter, referred to as the present embodiment). The present embodiment described below shows a preferred example of the present invention, and is not intended to be limiting in any way.

Figure 1 is a diagram showing an example of a rolling mill to which the rolling mill thickness control method and rolling mill thickness control device according to the present embodiment and the manufacturing method of steel plate using the rolling mill thickness control method can be applied. The rolling mill 1 shown in Figure 1 cold rolls a steel strip 2, which is a metal strip corresponding to the rolled material in this embodiment, and is equipped with five rolling stands 3, 4, 5, 6, and 7. Each rolling stand 3, 4, 5, 6, and 7 is arranged at a design-defined interval along the conveying direction of the steel strip 2 (sometimes referred to as the rolling direction). In the following description, from the upstream side in the conveying direction of the steel strip 2, they are referred to as the first rolling stand 3, the second rolling stand 4, the third rolling stand 5, the fourth rolling stand 6, and the fifth rolling stand 7.

In the example shown in FIG. 1, the first rolling stand 3 includes a pair of work rolls 8 that sandwich and roll the steel strip 2, a pair of backup rolls 9 that support each work roll 8, and a motor 10 that drives each work roll 8. The backup rolls 9 are arranged on the opposite side of the work rolls 8 from the steel strip 2. One motor 10 is installed for each rolling stand 3, 4, 5, 6, and 7, and is connected to each of the work rolls 8 of each rolling stand 3, 4, 5, 6, and 7 so that torque can be transmitted. Therefore, each work roll 8 rotates by receiving the torque generated by the motor 10, and the steel strip 2 is continuously cold rolled while being transported from the first rolling stand 3 to the fifth rolling stand 7.

In addition, a reduction gear (not shown) that increases the torque generated by the motor 10 may be provided between the motor 10 and each work roll 8 in the torque transmission direction. The above-mentioned work roll 8 corresponds to the rolling roll in this embodiment. The second rolling stand 4, the third rolling stand 5, the fourth rolling stand 6, and the fifth rolling stand 7 are configured in the same manner as the first rolling stand 3. Therefore, in each rolling stand 4, 5, 6, and 7, the same configuration as the first rolling stand 3 is assigned the same reference numerals as the configuration of the first rolling stand 3, and the description thereof is omitted.

In the conveying direction of the steel strip 2, between the first rolling stand 3 and the second rolling stand 4, between the second rolling stand 4 and the third rolling stand 5, and at the outlet side of the fifth rolling stand 7, a thickness gauge 11 for detecting the thickness of the steel strip 2 is provided. The thickness gauge 11 may be a conventionally known one. In addition, between adjacent rolling stands in the conveying direction of the steel strip 2 among the rolling stands 3, 4, 5, 6, and 7, a tension roll (not shown) for adjusting the tension of the steel strip 2 and a tension gauge (not shown) for detecting the tension of the steel strip 2 may be provided. In addition, the rolling stands adjacent to each other in the conveying direction described above refer to the first rolling stand 3 and the second rolling stand 4. In addition, the rolling stands adjacent to each other in the conveying direction refer to the second rolling stand 4 and the third rolling stand 5. In addition, the rolling stands adjacent to each other in the conveying direction refer to the third rolling stand 5 and the fourth rolling stand 6. Additionally, the rolling stands adjacent to each other in the conveying direction refer to the fourth rolling stand 6 and the fifth rolling stand 7.

Downstream of the thickness gauge 11 installed downstream of the fifth rolling stand 7 in the conveying direction of the steel strip 2, a tension reel 12 is provided to wind up the steel strip 2 after rolling by the rolling mill 1.

A reduction device 13 is provided above the upper backup roll 9 in each rolling stand 3, 4, 5, 6, and 7 in the height direction of the rolling mill 1. The reduction device 13 adjusts the reduction position of the upper work roll 8 in the height direction of the pair of work rolls 8. In other words, the reduction device 13 presses the upper work roll 8 toward the lower work roll 8 in the height direction of the rolling mill 1. A reduction position detector 14 is provided near the reduction device 13 to detect the reduction position of the upper work roll 8, that is, the position of the upper work roll 8 in the height direction, that is, the roll gap of the pair of work rolls 8. The roll gap means the distance between the pair of work rolls 8 in the height direction. A rolling load detector 15 is provided below the lower backup roll 9 in each rolling stand 3, 4, 5, 6, and 7 in the height direction of the rolling mill 1 to detect the rolling load acting on the steel strip 2. The reduction devices 13, the reduction position detector 14, and the rolling load detector 15 may be conventionally known.

The rolling mill 1 is equipped with a control device 16 that controls the rolling of the steel strip 2. The control device 16 is mainly composed of a microcomputer, and is configured to perform calculations based on input data, pre-stored data, arithmetic expressions, etc., and output the results. The input data can include, for example, the thickness of the steel strip 2 detected by the thickness gauge 11, the tension of the steel strip 2 detected by the tension gauge, and the roll gap detected by the roll down position detector 14. The output data can include a command signal for operating the motor 10, the droop rate, and a command signal for operating the roll down device 13. The droop rate will be described later.

In this embodiment, the control device 16 sets the ratio R of the droop rate of the motors 10 of the rolling stands adjacent to each other in the conveying direction of the steel strip 2 within a preset range. This is to suppress the thickness fluctuation of the steel strip 2 that accompanies the change in mass flow when the mass flow changes in each rolling stand 3, 4, 5, 6, 7 of the rolling mill 1. The mass flow is the product of the thickness of the steel strip 2 at the exit side of each rolling stand 3, 4, 5, 6, 7 and the conveying speed (sometimes called the feed speed) of the steel strip 2. When the rolling mill 1 is in steady operation, that is, when rolling is performed at a substantially constant speed, the mass flow of each rolling stand 3, 4, 5, 6, 7 is substantially constant.

An example of a case where the mass flow changes is when the leading end of a steel strip to be rolled is connected to the trailing end of a steel strip 2 currently being rolled in order to continuously roll multiple steel strips. The two steel strips 2 are connected to each other by, for example, welding. Therefore, while the two steel strips 2 are being welded to each other, the conveying speed at the trailing end of the steel strip 2 is reduced below the conveying speed at the trailing end of the steel strip 2 before the welding. After the above-mentioned welding is completed, the conveying speed at the trailing end of the steel strip 2 is increased to the original conveying speed. In this way, when the leading end of a steel strip to be rolled is connected to the trailing end of a steel strip 2 currently being rolled, the mass flow changes and the conveying speed at the trailing end of the steel strip 2 is decelerated and then accelerated.

The droop ratio is one of a number of control parameters used to stably control the motor 10, and functions to reduce the rotation speed (hereinafter sometimes simply referred to as speed) of the motor 10. Therefore, in this embodiment, when the droop ratio of the motor 10 of each rolling stand 3, 4, 5, 6, 7 is changed, the amount of reduction in the conveying speed of the steel strip 2 on the delivery side of each rolling stand 3, 4, 5, 6, 7 changes according to the droop ratio. In other words, the droop ratio causes a speed change. The amount of reduction in the conveying speed of the steel strip 2 on the delivery side of each rolling stand 3, 4, 5, 6, 7 due to the droop ratio can be expressed by the following formula.
Δv=(I/I _max )×D×V
Δv is the amount of decrease in the conveying speed of the steel strip 2 at the exit of each rolling stand 3, 4, 5, 6, 7 due to the drooping rate. D is the drooping rate (%). I is the load current (A) of the motor 10 at the present time. I _max is the maximum value (A) of the rated current of the motor 10. V is the set speed (mpm) of the steel strip 2 at the exit of each rolling stand 3, 4, 5, 6, 7. As shown in the above formula, the drooping rate means the ratio of the speed decrease to the speed when the rated current is passed through the motor 10. In other words, the drooping rate means the ratio of the difference between the speed of the motor 10 when the rated current is passed through the motor 10 and the speed when the current load current is passed through the motor 10 to the speed of the motor 10 when the rated current is passed through the motor 10. The larger this value (drooping rate) is, the more the control device 16 outputs a command signal to the motor 10 to decrease the speed of the motor 10. Normally, the droop rate can be changed by the operator by changing the constants on the control panel.

In addition, the ratio R of the droop rates of the motors 10 of adjacent rolling stands in the conveying direction of the steel strip 2 (hereinafter, sometimes simply referred to as ratio R) is a value obtained by dividing the droop rate of the downstream rolling stand by the droop rate of the upstream rolling stand among the rolling stands adjacent to each other in the conveying direction of the steel strip 2, and can be expressed as shown in the following formula.
Ratio R = sag ratio at downstream rolling stand / sag ratio at upstream rolling stand

In this embodiment, the ratio R of the droop rate is set within the range of 3/5 to 5/3 (0.6 to 1.7). As described above, this is to suppress the thickness fluctuation of the steel strip 2 due to the change in mass flow at each rolling stand 3, 4, 5, 6, and 7 of the rolling mill 1. If the ratio R of the droop rate exceeds 5/3, the tension of the steel strip 2 between adjacent rolling stands in the conveying direction of the steel strip 2 due to the change in mass flow will be lower than the set value or target value of the tension, as described below. Therefore, in order to increase the tension and return it to the original value, a command to expand the roll gap of the downstream rolling stand is output from the above-mentioned control device 16 to the reduction device 13. This will expand the roll gap, increasing the risk of thickness fluctuation to the over side. The thickness fluctuation to the over side means that the thickness of the steel strip 2 will exceed the set value or target value of the thickness of the steel strip 2.

On the other hand, when the above-mentioned ratio is less than 3/5, the tension increases above the set or target value of the tension as the mass flow changes, in contrast to when the sag ratio exceeds 5/3. Therefore, a command to narrow the roll gap of the downstream rolling stand in order to reduce the tension and return it to its original value is output from the above-mentioned control device 16 to the reduction device 13. This increases the risk of the thickness of the steel strip 2 fluctuating to the underside. Note that a thickness fluctuation to the underside means that the thickness of the steel strip 2 becomes less than the set or target value of the thickness of the steel strip 2.

In addition, the droop rate is preferably 0.1% or more, and more preferably 0.2% or more. By doing so, the risk of plate breakage caused by changes in the conveying speed of the steel strip 2 when the steel strip 2 is rolled by the rolling mill 1 can be more reliably reduced. In other words, if the droop rate is 0.1% or more, the mass flow at each rolling stand 3, 4, 5, 6, and 7 can be given a margin. Therefore, it becomes easier to alleviate the steep tension fluctuation caused by changes in the conveying speed of the steel strip 2, and the risk of plate breakage can be further reduced. Furthermore, it is preferable to set the droop rate to 10.0% or less at most. If the droop rate is 10.0% or less, the ratio R of the droop rate of the upstream rolling stand adjacent to each other in the conveying direction to the droop rate of the downstream rolling stand can be maintained within the range of 3/5 to 5/3 in this embodiment, so that the amount of speed reduction imparted by the droop rate can be more effectively suppressed. And, it is possible to prevent large plate thickness fluctuations caused by the imbalance of the mass flow in the entire device, and to avoid the off-gauge length from becoming long. Therefore, the droop rate is preferably 10.0% or less, and more preferably 1.4% or less.

(Action and Effects)
The operation and effect of this embodiment will be described in comparison with a comparative example. Table 1 shows the droop ratio of each motor 10 of each rolling stand 3, 4, 5, 6, and 7 of the rolling mill 1 shown in FIG. 1. In addition, as a comparative example, Table 1 shows the droop ratio of each motor of each rolling stand of the rolling mill before applying the rolling mill thickness control method and rolling mill thickness control device according to this embodiment and the manufacturing method of the steel plate using the rolling mill thickness control method. The rolling mill of the comparative example is configured almost the same as the rolling mill 1 of this embodiment, except that the droop ratio is not adjusted as in this embodiment. Also, FIG. 2 is a diagram for explaining the behavior of the rolling mill of the comparative example. FIG. 3 is a diagram for explaining the behavior of the rolling mill 1 shown in FIG. 1.

First, the behavior of the rolling mill in the comparative example will be explained. When rolling multiple steel strips continuously, the leading end of the steel strip to be rolled next is welded to the trailing end of the steel strip currently being rolled. In order to perform this welding, the conveying speed of the trailing end side of the steel strip currently being rolled is gradually reduced. Figure 2(a) shows this state. In the following explanation, this state in which the conveying speed of the trailing end side is gradually reduced will be referred to as "during deceleration" or "at deceleration."

The behavior of each rolling stand during deceleration will be explained using the fourth and fifth rolling stands of the comparative rolling mill as examples. Since the conveying speed of the steel strip in the fifth rolling stand is originally higher than that in the fourth rolling stand, the change in the conveying speed of the steel strip in the fifth rolling stand during deceleration is greater than that in the fourth rolling stand. Also, the increase in the coefficient of friction between the steel strip and the work roll in the fifth rolling stand is greater than that between the steel strip and the work roll in the fourth rolling stand. This is because the conveying speed of the steel strip in the fifth rolling stand is higher than that in the fourth rolling stand. Therefore, the amount of lubricating oil that is caught between the steel strip and the work roll is greater than that in the fourth rolling stand, and the amount of oil drawn in during deceleration is also greater.

Therefore, the forward slip ratio of the steel strip at the fifth rolling stand during deceleration is greater than that at the fourth rolling stand. Also, compared to the steady state before the conveying speed of the tail end of the steel strip is decelerated, the mass flow at the fourth rolling stand during deceleration is less than the mass flow at the fifth rolling stand. As a result, the steel strip is left in a surplus between the fourth and fifth rolling stands, and the tension of the steel strip between the fourth and fifth rolling stands is reduced. Figure 2(b) shows this state. Also, as shown in Figure 2(c), a command signal is output to increase the roll gap of the work roll 8 of the fifth rolling stand 7. The forward slip ratio mentioned above is the ratio of the value obtained by subtracting the rotational speed of the work roll from the conveying speed of the steel strip (sometimes called the plate feed) at the exit side of each rolling stand to the rotational speed of the work roll.

As a result, in the comparative example, as shown in FIG. 2(d), the thickness of the strip at the exit of the rolling mill rises to the over side, resulting in large thickness fluctuations. This is because the droop rate at the fifth rolling stand is greater than the droop rate at the fourth rolling stand. In other words, the deceleration at the fifth rolling stand due to the droop rate is greater than the deceleration at the fourth rolling stand. This causes the balance of the mass flows between the two rolling stands to be lost in the direction of decreasing the tension between them.

In contrast, in this embodiment, as shown in Table 1, the droop ratios at the fourth rolling stand and the fifth rolling stand are both 0.4%. The ratio R of the droop ratios is 1/1 (1.0), which is within the above-mentioned range. Therefore, as shown in FIG. 3(a), when the deceleration of the tail end side of the steel strip 2 is started, the balance of the mass flows of the rolling stands 6 and 7 is maintained. Therefore, the deceleration at the fourth rolling stand 6 due to the droop ratio and the deceleration at the fifth rolling stand 7 due to the droop ratio are almost the same. Even if the tension starts to decrease almost simultaneously with the deceleration of the tail end side of the steel strip 2, the amount of decrease in tension is smaller than that in the comparative example, as shown in FIG. 3(b) and FIG. 2(b). Also, almost simultaneously with the deceleration of the tail end side of the steel strip 2, a command signal to increase the roll gap of the work roll 8 of the fifth rolling stand 7 is output, but the increase in the roll gap due to the command signal is also smaller than that in the comparative example, as shown in FIG. 3(c) and FIG. 2(c). As a result, in this embodiment, as shown in Figures 3(d) and 2(d), thickness fluctuations can be suppressed compared to the comparative example.

In addition, in this embodiment, the droop rate of each rolling stand 3, 4, 5, 6, and 7 is 0.1% or more, which reduces the risk of plate breakage caused by changes in the speed of the steel strip 2 when the steel strip 2 is rolled by the rolling mill 1.

The present invention is not limited to the above-mentioned embodiment. In the above-mentioned embodiment, when rolling the steel strip 2 in the rolling mill 1 shown in FIG. 1, the ratio R of the droop ratios of the rolling stands adjacent to each other in the conveying direction of the steel strip 2 is always set within the above-mentioned range of 3/5 to 5/3. However, instead of this, the ratio R of the droop ratio may be set within the range of 3/5 to 5/3 at the time when the deceleration of the tail end side of the steel strip 2 currently being rolled is started. That is, when the balance of the mass flows of the adjacent rolling stands changes due to the conveying speed of the steel strip 2, the ratio R of the droop ratio is set within the above-mentioned range. Specifically, the deceleration of the tail end side of the steel strip 2 can be determined based on, for example, the rotation speed of the motor 10 or the work roll 8, or the current value applied to the motor 10. Then, when it is determined that the above-mentioned deceleration has started, the ratio R of the droop ratios of the rolling stands adjacent to each other in the conveying direction of the steel strip 2 may be set within the range of 3/5 to 5/3. Even with such a configuration, the same action and effect as the above-mentioned embodiment can be obtained. In addition, the determination that deceleration has begun at the tail end of the steel strip 2 as described above, and the change or setting of the droop rate ratio R may be performed by the control device 16 or by the operator.

An example conducted to confirm the effect of the rolling mill thickness control method and rolling mill thickness control device according to the embodiment of the present invention will be described. A rolling mill configured similarly to the rolling mill shown in FIG. 1 was prepared, and the rolling mill was used to roll a steel strip. As a result, as in the above-mentioned embodiment, it was possible to suppress the decrease in tension during deceleration. In addition, it was possible to suppress the fluctuation of the roll gap in the fifth rolling stand. In other words, it was possible to suppress the thickness fluctuation of the steel strip at the exit side of the fifth rolling stand more than ever before. And, as shown in Table 2, the off-gauge length in the rolling mill of this embodiment was about half the off-gauge length of the rolling mill before applying the rolling mill thickness control method and rolling mill thickness control device according to this embodiment. Note that off-gauge means the length of the total length of the steel strip that is outside the product thickness.

REFERENCE SIGNS LIST 1 Rolling mill 2 Steel strip 3 First rolling stand 4 Second rolling stand 5 Third rolling stand 6 Fourth rolling stand 7 Fifth rolling stand 8 Work roll 9 Backup roll 10 Motor 11 Plate thickness gauge 12 Tension reel 13 Rolling device 14 Rolling position detector 15 Rolling load detector 16 Control device

Claims (5)

A method for controlling a thickness of a rolling mill including a plurality of rolling stands arranged along a conveying direction of a material to be rolled, and a motor provided for each of the rolling stands for driving rolls of the plurality of rolling stands, comprising:
A method for controlling plate thickness in a rolling mill, in which, when the rolled material is being rolled, a ratio of droop rates calculated by dividing the droop rate of the motor of the rolling stand downstream in the conveying direction by the droop rate of the motor of the rolling stand upstream in the conveying direction, is set to 3/5 to 5/3. The method for controlling the thickness of a rolling mill according to claim 1, wherein the droop rates of the motors are all set to 0.1% or more. A method for manufacturing steel plates using the method for controlling plate thickness of a rolling mill according to claim 1 or 2. A plate thickness control device for a rolling mill including a plurality of rolling stands arranged along a conveying direction of a material to be rolled, and a motor provided for each of the rolling stands for driving the rolling rolls of the plurality of rolling stands,
A control device for adjusting a droop rate of the motor is provided,
The control device is a plate thickness control device for a rolling mill that sets a ratio of droop rates calculated by dividing the droop rate of the motor of the rolling stand that is downstream in the conveying direction by the droop rate of the motor of the rolling stand that is upstream in the conveying direction, between adjacent rolling stands in the conveying direction, to 3/5 to 5/3 when the rolled material is being rolled. The plate thickness control device for a rolling mill according to claim 4, wherein the control device sets the droop rate of the motor to 0.1% or more.

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