Method for controlling thickness of rolling mill, method for controlling thickness of rolling mill, and method for manufacturing steel plate using the method for controlling thickness of rolling mill

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

[Technical Field]

[0001] The present invention relates to a plate thickness control method and plate thickness control device for a rolling mill that rolls a steel plate using a plurality of rolling stands, and a steel plate manufacturing method that uses the plate thickness control method for a rolling mill. [Background technology]

[0002] Conventionally, a technique for controlling plate 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 plate thickness control device has been known. Patent Document 1 describes an example of a plate thickness control method using this type of AGC. In the method described in Patent Document 1, when rolling a material to be rolled in a tandem rolling mill, the gain value of an integral component used in PI control is changed based on the plate thickness deviation at the delivery side of the final rolling stand. This controls the rolling speed of the upstream rolling stand. Patent Document 2 also describes an example of a speed control method for a tandem rolling mill that suppresses plate thickness fluctuations by setting a droop ratio. In the method described in Patent Document 2, rolling is performed by setting the speed droop ratio of the front stand of the tandem rolling mill to 0 (zero) and the speed droop ratio of the rear stand to approximately 0 (zero). This suppresses plate thickness fluctuations associated with changes in the speed of the tandem rolling mill. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-254322 [Patent Document 2] Japanese Unexamined Patent Publication No. 49-101254 Summary of the Invention [Problem to be solved by the invention]

[0004] The method using AGC described in Patent Document 1 has the problem of being unable to adequately address thickness fluctuations that occur when the rolling mill accelerates or decelerates the material being rolled. Furthermore, the method described in Patent Document 2, as described above, 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 tandem rolling mills consisting of two rolling stands. Therefore, in the case of a tandem rolling mill having two or more rolling stands, the method described in Patent Document 2 may not be able to suppress thickness fluctuations. The method described in Patent Document 2 also has the problem of incurring a risk of sheet breakage during acceleration or deceleration. The speed droop rate of a rolling mill is set to mitigate sudden speed changes that occur when conditions change, such as acceleration or deceleration, and setting it to approximately 0 (zero) would prevent this function from being fully utilized.

[0005] The present invention has been made to solve the above-mentioned problems, and aims to provide a rolling mill plate thickness control method and rolling mill plate thickness control device that can suppress plate thickness fluctuations during acceleration and deceleration of the rolling mill more than conventional methods, and a steel plate manufacturing method using the rolling mill plate thickness control method. [Means for solving the problem]

[0006] 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 the conveying direction of the material to be rolled and motors provided in each of the rolling stands for driving the rolling rolls of the plurality of rolling stands, wherein, when rolling the material to be rolled, the ratio of the droop ratios obtained by dividing the droop ratio of the motor of the rolling stand downstream in the conveying direction by the droop ratio of the motor of the rolling stand upstream in the conveying direction of adjacent rolling stands 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 the conveying direction of the material to be rolled and motors provided in each of the rolling stands to drive the rolling rolls of the plurality of rolling stands, the plate thickness control device for a rolling mill including a control device for adjusting the droop rate of the motors, the control device setting a ratio of 3 / 5 to 5 / 3 of the droop rate obtained 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, among the rolling stands adjacent to each other in the conveying direction, 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 the droop rate of the motor to 0.1% or more. [Effects of the Invention]

[0007] According to the present invention, even if the conveying speed of the material to be rolled changes while the material is being rolled, large fluctuations in the thickness of the material to be rolled can be suppressed. [Brief explanation of the drawings]

[0008] [Figure 1] 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. [Figure 2] FIG. 10 is a diagram illustrating the behavior of a rolling mill of a comparative example. [Figure 3] FIG. 2 is a diagram illustrating the behavior of the rolling mill shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present invention will be specifically described below through embodiments 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 the present invention is not limited to this example.

[0010] FIG. 1 is a diagram showing an example of a rolling mill to which the rolling mill thickness control method, rolling mill thickness control device, and steel plate manufacturing method using the rolling mill thickness control method according to this embodiment can be applied. The rolling mill 1 shown in FIG. 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. The rolling stands 3, 4, 5, 6, and 7 are arranged at intervals determined by design along the conveying direction of the steel strip 2 (sometimes referred to as the rolling direction). In the following description, the rolling stands 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 from the upstream side in the conveying direction of the steel strip 2.

[0011] In the example shown in Figure 1, the first rolling stand 3 is equipped with 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 thereto. Therefore, each work roll 8 rotates under the torque generated by the motor 10, and continuously cold rolls the steel strip 2 while transporting it from the first rolling stand 3 to the fifth rolling stand 7.

[0012] Note that a reducer (not shown) for increasing 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 work rolls 8 described above correspond to the rolling rolls according to 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 of the rolling stands 4, 5, 6, and 7, the same components as those in the first rolling stand 3 are designated by the same reference numerals as those in the first rolling stand 3, and their description will be omitted.

[0013] In the conveying direction of the steel strip 2, thickness gauges 11 for detecting the thickness of the steel strip 2 are provided 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 on the delivery side of the fifth rolling stand 7. The thickness gauges 11 may be conventionally known. Furthermore, between adjacent rolling stands of the rolling stands 3, 4, 5, 6, and 7 in the conveying direction of the steel strip 2, tension rolls (not shown) for adjusting the tension of the steel strip 2 and tension gauges (not shown) for detecting the tension of the steel strip 2 may be provided. Furthermore, the rolling stands adjacent to each other in the conveying direction as described above refer to the first rolling stand 3 and the second rolling stand 4. Furthermore, the rolling stands adjacent to each other in the conveying direction refer to the second rolling stand 4 and the third rolling stand 5. Furthermore, the rolling stands adjacent to each other in the conveying direction refer to the third rolling stand 5 and the fourth rolling stand 6. Moreover, the rolling stands adjacent to each other in the conveying direction refer to the fourth rolling stand 6 and the fifth rolling stand 7.

[0014] A tension reel 12 is provided downstream of a thickness gauge 11 installed downstream of the fifth rolling stand 7 in the conveying direction of the steel strip 2, for winding up the steel strip 2 after rolling by the rolling mill 1.

[0015] A screw down 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 screw down device 13 adjusts the screw down position of the upper work roll 8 in the height direction of a pair of work rolls 8. That is, the screw down device 13 presses the upper work roll 8 toward the lower work roll 8 in the height direction of the rolling mill 1. A screw down position detector 14 is provided near the screw down device 13 to detect the screw down position of the upper work roll 8, i.e., the position of the upper work roll 8 in the height direction, i.e., the roll gap between 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 screw down device 13, the screw down position detector 14, and the rolling load detector 15 may be conventionally known.

[0016] 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 to 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 position detector 14. The output data can include a command signal for operating the motor 10, a droop ratio, and a command signal for operating the roll down device 13. The droop ratio will be described later.

[0017] In this embodiment, the control device 16 is configured to set the ratio R of the droop rates 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 fluctuations in the thickness of the steel strip 2 that accompany changes in the mass flow when the mass flow changes in each of the rolling stands 3, 4, 5, 6, and 7 of the rolling mill 1. The mass flow is a value obtained by multiplying the thickness of the steel strip 2 at the delivery side of each of the rolling stands 3, 4, 5, 6, and 7 by the conveying speed (sometimes referred to as the feed rate) of the steel strip 2. During steady-state operation of the rolling mill 1, that is, when rolling is performed at a substantially constant speed, the mass flow of each of the rolling stands 3, 4, 5, 6, and 7 is substantially constant.

[0018] An example of a case where the mass flow changes is when, in order to continuously roll multiple steel strips, 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 two steel strips 2 are connected together, for example, by welding. Therefore, while the two steel strips 2 are being welded together, 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 welding is completed, the conveying speed at the trailing end of the steel strip 2 is increased to its original conveying speed. When the leading end of a steel strip to be rolled is connected to the trailing end of the 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.

[0019] The droop ratio is one of a plurality of control parameters used to stably control the motor 10, and functions to reduce the rotational 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, and 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, and 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, and 7 due to the droop ratio can be expressed by the following formula. Δv=(I / I max)×D×V Δv is the decrease in the conveying speed of the steel strip 2 at the exit of each rolling stand 3, 4, 5, 6, and 7 due to the droop ratio. D is the droop ratio (%). I is the current load current (A) of the motor 10. 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 outlet of each rolling stand 3, 4, 5, 6, and 7. As shown in the above formula, the droop ratio means the ratio of the speed reduction to the speed when the rated current is flowing through the motor 10. In other words, the droop ratio means the ratio of the difference between the speed of the motor 10 when the rated current is flowing through the motor 10 and the speed when the current load current is flowing through the motor 10 to the speed of the motor 10 when the rated current is flowing through the motor 10. The larger this value (droop ratio), the more the control device 16 outputs a command signal to the motor 10 to reduce the speed of the motor 10. Normally, the droop ratio can be freely changed by the operator by changing the constants on the control panel.

[0020] Furthermore, the ratio R (hereinafter sometimes referred to simply as ratio R) of the droop rates of the motors 10 of adjacent rolling stands in the conveying direction of the steel strip 2 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 adjacent rolling stands in the conveying direction of the steel strip 2, and can be expressed as follows: Ratio R = Droop ratio at downstream rolling stand / Droop ratio at upstream rolling stand

[0021] In this embodiment, the droop ratio R is set within a range of 3 / 5 to 5 / 3 (0.6 to 1.7). As described above, this is to suppress thickness fluctuations of the steel strip 2 due to changes in mass flow in each rolling stand 3, 4, 5, 6, and 7 of the rolling mill 1. Note that, if the droop ratio R 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 changes in mass flow, as described below, will fall below the set or target value of the tension. Therefore, in order to increase the tension and return it to the original value, a command to widen the roll gap of the downstream rolling stand is output from the above-mentioned control device 16 to the screw down device 13. This widens the roll gap, increasing the risk of thickness fluctuations toward the over-side. Note that thickness fluctuations toward the over-side mean that the thickness of the steel strip 2 exceeds the set or target value of the thickness of the steel strip 2.

[0022] On the other hand, when the above-mentioned ratio is less than 3 / 5, the tension increases above the set value or target value of the tension as the mass flow changes, in contrast to when the droop ratio exceeds 5 / 3. Therefore, a command to narrow the roll gap of the downstream rolling stand is output from the above-mentioned control device 16 to the screw down device 13 in order to reduce the tension and return it to its original value. 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 value or target value of the thickness of the steel strip 2.

[0023] Furthermore, the droop ratio is preferably 0.1% or more, and more preferably 0.2% or more. This more reliably reduces the risk of sheet breakage due to changes in the conveying speed of the steel strip 2 when the steel strip 2 is rolled by the rolling mill 1. That is, a droop ratio of 0.1% or more allows for a margin of mass flow in each rolling stand 3, 4, 5, 6, and 7. This facilitates mitigating sudden tension fluctuations due to changes in the conveying speed of the steel strip 2, further reducing the risk of sheet breakage. Furthermore, the droop ratio is preferably set to 10.0% or less at most. If the droop ratio is 10.0% or less, the ratio R of the droop ratio of the upstream rolling stand to the downstream rolling stand adjacent to each other in the conveying direction can be maintained within the range of 3 / 5 to 5 / 3 in this embodiment, thereby more effectively suppressing the speed reduction caused by the droop ratio. Furthermore, large thickness fluctuations resulting from imbalance in the mass flow throughout the entire apparatus can be prevented, and an increase in the off-gauge length can be avoided. Therefore, the drooping rate is preferably 10.0% or less, and more preferably 1.4% or less.

[0024] (Actions and Effects) The functions and effects of this embodiment will be explained in comparison with a comparative example. Table 1 shows the droop ratios of the motors 10 of the rolling stands 3, 4, 5, 6, and 7 of the rolling mill 1 shown in FIG. 1. Additionally, as a comparative example, Table 1 shows the droop ratios of the motors of the rolling stands of the rolling mill before applying the rolling mill thickness control method and rolling mill thickness control device and the steel plate manufacturing method using the rolling mill thickness control method according to this embodiment. The rolling mill of the comparative example is configured almost identically to the rolling mill 1 of this embodiment, except that the droop ratio is not adjusted as in this embodiment. Furthermore, FIG. 2 is a diagram illustrating the behavior of the rolling mill of the comparative example. FIG. 3 is a diagram illustrating the behavior of the rolling mill 1 shown in FIG. 1.

[0025] [Table 1]

[0026] First, the behavior of the rolling mill of the comparative example will be described. When rolling a plurality of 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. 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 description, 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."

[0027] The behavior of each rolling stand during deceleration will be explained using the fourth and fifth rolling stands of a comparative rolling mill as an example. Since the conveying speed of the steel strip in the fifth rolling stand is inherently 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. Furthermore, 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 trapped 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.

[0028] Therefore, the decrease in the forward slip ratio of the steel strip at the fifth rolling stand during deceleration is greater than the decrease in the forward slip ratio of the steel strip at the fourth rolling stand. Furthermore, 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 smaller than the mass flow at the fifth rolling stand. As a result, the steel strip is left in a surplus state between the fourth and fifth rolling stands, and the tension of the steel strip between the fourth and fifth rolling stands decreases. Figure 2(b) shows this state. Furthermore, as shown in Figure 2(c), a command signal is output to increase the roll gap of the work rolls 8 of the fifth rolling stand 7. The forward slip ratio mentioned above is the ratio of the work roll rotation speed to the steel strip conveying speed (sometimes referred to as strip feed) at the delivery side of each rolling stand minus the work roll rotation speed.

[0029] As a result, in the comparative example, as shown in Figure 2(d), the thickness of the plate at the delivery side of the rolling mill increases to the over side, resulting in large thickness fluctuations. This is because the droop rate in the fifth rolling stand is greater than that in the fourth rolling stand. In other words, the deceleration in the fifth rolling stand due to the droop rate is greater than that in the fourth rolling stand. This causes the tension between the rolling stands to decrease, resulting in an imbalance in the mass flow between the two rolling stands.

[0030] 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 droop ratio ratio R is 1 / 1 (1.0), which is within the above-mentioned range. Therefore, as shown in FIG. 3(a), when deceleration of the tail end side of the steel strip 2 is initiated, the balance of the mass flows of the rolling stands 6 and 7 is maintained. Therefore, the deceleration of the fourth rolling stand 6 due to the droop ratio and the deceleration of the fifth rolling stand 7 due to the droop ratio are approximately the same. Even if the tension begins 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 in the comparative example, as shown in FIG. 3(b) and FIG. 2(b). Furthermore, almost simultaneously with the deceleration of the tail end side of the steel strip 2, a command signal to increase the roll gap between the work rolls 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 in the comparative example, as shown in FIG. 3(c) and FIG. 2(c). As a result, in this embodiment, as shown in FIG. 3(d) and FIG. 2(d), it is possible to suppress the thickness fluctuation compared to the comparative example.

[0031] In addition, in this embodiment, the droop ratio of each rolling stand 3, 4, 5, 6, and 7 is 0.1% or more, so the risk of sheet breakage due to changes in the speed of the steel strip 2 when the steel strip 2 is rolled by the rolling mill 1 can be reduced.

[0032] The present invention is not limited to the above-described embodiment. In the above-described embodiment, when rolling a steel strip 2 in the rolling mill 1 shown in FIG. 1, the ratio R of the droop ratios of adjacent rolling stands in the conveying direction of the steel strip 2 is always set within the above-described 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 when deceleration of the tail end of the steel strip 2 currently being rolled begins. That is, the ratio R of the droop ratio is set within the above-described range when the balance of mass flows of adjacent rolling stands changes depending on the conveying speed of the steel strip 2. Specifically, the deceleration of the tail end of the steel strip 2 can be determined based on, for example, the rotational 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 deceleration has begun, the ratio R of the droop ratios of adjacent rolling stands in the conveying direction of the steel strip 2 may be set within the range of 3 / 5 to 5 / 3. Even with this configuration, the same functions and effects as those of the above-described embodiment can be obtained. The determination of the start of deceleration at the tail end of the steel strip 2 and the change or setting of the droop rate R may be performed by the control device 16 or by the operator. [Example]

[0033] An example conducted to verify the effectiveness of the rolling mill thickness control method and rolling mill thickness control device according to an embodiment of the present invention will now be described. A rolling mill configured similarly to the rolling mill shown in FIG. 1 was prepared, and a steel strip was rolled using this rolling mill. As a result, as in the above-described embodiment, it was possible to suppress a decrease in tension during deceleration. Furthermore, it was possible to suppress fluctuations in the roll gap in the fifth rolling stand. In other words, it was possible to suppress thickness fluctuations of the steel strip at the delivery side of the fifth rolling stand to an unprecedented extent. Furthermore, as shown in Table 2, the off-gauge length in the rolling mill of this example was approximately 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" refers to the length of the steel strip that deviates from the product thickness within the total length of the steel strip.

[0034] [Table 2] [Explanation of symbols]

[0035] 1. Rolling mill 2 Steel strips 3. No. 1 Rolling Stand 4. No. 2 Rolling Stand 5. No. 3 Rolling Stand 6. No. 4 Rolling Stand 7. No. 5 Rolling Stand 8 Work Rolls 9 Backup Role 10 Motor 11 Thickness gauge 12 Tension reel 13 Screw down device 14. Rolling position detector 15 Rolling load detector 16 Control device

Claims

1. A method for controlling 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, In order to weld the leading end of another rolled material to be rolled next to the trailing end of the material currently being rolled, the conveying speed of the trailing end side of the material to be rolled is reduced and the material to be rolled is rolled, and the ratio of the droop ratios obtained by dividing the droop ratio of the motor of the rolling stand downstream in the conveying direction by the droop ratio of the motor of the rolling stand upstream in the conveying direction between the rolling stands adjacent to each other in the conveying direction is set to 1.0 to 5 / 3 between each of the rolling stands adjacent to each other in the conveying direction. A method for controlling thickness in a rolling mill.

2. 2. The method for controlling thickness of a rolling mill according to claim 1, wherein the droop ratios of the motors are all set to 0.1% or more.

3. A method for manufacturing a steel plate using the method for controlling plate thickness of a rolling mill according to claim 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, a control device for adjusting a droop rate of the motor; The control device, when rolling the material to be rolled by reducing the conveying speed on the tail end side of the material to be rolled in order to weld the front end of another material to be rolled to the tail end of the material currently being rolled, sets 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 the rolling stands adjacent to each other in the conveying direction to 1.0 to 5 / 3. Plate thickness control device for rolling mills.

5. 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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