JP7620202B2 - CONTROL DEVICE, PLATING SYSTEM AND CONTROL METHOD - Google Patents

Description

This disclosure relates to a control device, a plating system, and a control method.

Patent Document 1 discloses a coating weight adhesion control device having a pass line position estimation means for estimating the pass line position of a steel sheet that has passed through a molten metal bath, a coating weight estimation means for estimating the coating weight at the gas wiping nozzle position based on the pass line position estimated by the pass line position estimation means, and a nozzle position correction amount calculation means for calculating a nozzle position correction amount based on the deviation between the coating weight estimated by the coating weight estimation means and a target value.

JP 2008-274425 A

In a plating system, it is necessary to deposit plating with a sufficient thickness relative to the target value, but it is also necessary to prevent excessive plating deposition and conserve plating materials. Therefore, this disclosure provides a plating system that is effective in both preventing insufficient plating deposition and conserving plating materials.

The control device according to one embodiment of the present disclosure includes a plating pot that contains a plating bath containing molten zinc, and a gas wiping device that adjusts the plating adhesion amount by spraying gas onto the steel strip that has passed through the plating pot. When the minimum value of the plating adhesion amount (adhesion amount of plating material) at multiple measurement points of the steel strip that has passed through the plating pot falls below a predetermined lower limit, the control device increases the plating adhesion amount using the gas wiping device. When the maximum value of the plating adhesion amount at the multiple measurement points exceeds a predetermined upper limit, the control device decreases the plating adhesion amount using the gas wiping device. When the difference between the maximum value and the minimum value exceeds a difference threshold, the control device prohibits the gas wiping device from decreasing the plating adhesion amount.

This control device can adjust the maximum plating adhesion amount to an upper limit or less while adjusting the minimum plating adhesion amount to an upper limit or less. In addition, if the difference between the maximum and minimum values is greater than the difference threshold, the gas wiping device is prohibited from reducing the plating adhesion amount, so that the minimum value is prevented from falling below the lower limit due to the control that adjusts the maximum value to an upper limit or less. Therefore, it is possible to achieve both prevention of insufficient plating adhesion amount and saving plating material.

The control device may further include a difference control unit that reduces the difference between the maximum and minimum values by the gas wiping device when the difference between the maximum and minimum values exceeds the difference threshold. In this case, after the difference between the maximum and minimum values of the plating adhesion amount is reduced, the maximum value is adjusted to be equal to or less than the upper limit value. Therefore, it is possible to more reliably achieve both prevention of insufficient plating adhesion amount and saving of plating material.

The multiple measurement points may include three or more measurement points aligned in the width direction of the steel strip, and the control device may further include a warp evaluation unit that evaluates the warp of the steel strip based on the coating weight at the three or more measurement points, and a warp control unit that reduces the warp of the steel strip in the plating pot using a warp correction device when the warp evaluation result exceeds a predetermined level. In this case, the variation in coating weight caused by the warp of the steel strip is also suppressed. Therefore, it is possible to more reliably achieve both suppression of insufficient coating weight and saving of plating material.

The multiple measurement points may include three or more one-side measurement points aligned in the width direction on one side of the steel strip, and three or more back-side measurement points on the back side of the one side that correspond to the three or more measurement points, and the warpage evaluation unit may evaluate the warpage of the steel strip based on the coating weight at the three or more one-side measurement points and the coating weight at the three or more back-side measurement points. In this case, the warpage of the steel strip can be evaluated with higher reliability, and the variation in coating weight caused by the warpage of the steel strip can be more reliably suppressed.

A plating system according to another embodiment of the present disclosure includes a plating pot containing a plating bath containing molten zinc, a gas wiping device that adjusts the coating weight by spraying gas onto the steel strip that has passed through the plating pot, a lower limit control unit that causes the gas wiping device to increase the coating weight when the minimum coating weight at multiple measurement points on the steel strip that has passed through the plating pot and the gas wiping device falls below a predetermined lower limit, an upper limit control unit that causes the gas wiping device to decrease the coating weight when the maximum coating weight at multiple measurement points exceeds a predetermined upper limit, and a difference control unit that prohibits the gas wiping device from decreasing the coating weight when the difference between the maximum and minimum values exceeds a difference threshold.

A control method according to yet another embodiment of the present disclosure includes increasing the coating weight by the gas wiping device when the minimum coating weight at multiple measurement points of a steel strip that has passed through a coating pot containing a coating bath containing molten zinc and a gas wiping device falls below a predetermined lower limit, decreasing the coating weight by the gas wiping device when the maximum coating weight at multiple measurement points exceeds a predetermined upper limit, and prohibiting the gas wiping device from decreasing the coating weight when the difference between the maximum and minimum values exceeds a difference threshold.

This disclosure provides a plating system that is effective in both preventing plating adhesion deficiencies and saving plating material.

FIG. 1 is a schematic diagram of a plating system. FIG. 2 is a block diagram illustrating a functional configuration of a control device. 1A and 1B are schematic diagrams illustrating displacement of a gas wiping nozzle to increase the amount of deposition. 1A and 1B are schematic diagrams illustrating the displacement of a gas wiping nozzle to reduce the amount of adhesion. 13 is a schematic diagram illustrating displacement of a gas wiping nozzle to reduce variability. FIG. 10A and 10B are schematic diagrams illustrating the relationship between the detection result of the adhesion amount and warpage. FIG. 1 is a schematic diagram illustrating the displacement of rolls to reduce warpage. FIG. 1 is a schematic diagram illustrating the displacement of rolls to reduce warpage. FIG. 2 is a block diagram illustrating a hardware configuration of a control device. 4 is a flowchart illustrating a procedure for controlling the amount of adhesion. 11 is a flowchart illustrating a procedure for reducing a difference. 1 is a flowchart illustrating a procedure for evaluating warpage. 11 is a flowchart illustrating a control procedure for reducing warpage. 10 is a flowchart showing a modified example of the adhesion amount control procedure.

The following describes the embodiments in detail with reference to the drawings. In the description, the same elements or elements having the same functions are given the same reference numerals, and duplicate descriptions are omitted.

〔Device〕
The plating system 1 shown in Fig. 1 is a system that performs a plating process on a steel strip 9 fed along a predetermined pass line PL. For example, the plating system 1 has a plating pot 2, a plurality of rolls 3, gas wiping devices 10, 20, adhesion weight sensors 30, 40, and a control device 100. The plating pot 2 contains a liquid plating material containing molten zinc. Hereinafter, the plating material contained in the plating pot 2 is also referred to as a "plating bath."

The multiple rolls 3 are lined up along a pass line PL that passes through the plating bath in the plating pot 2, and guide the steel strip 9 along the pass line PL. Each of the multiple rolls 3 extends perpendicular to the pass line PL. The steel strip 9 bends around each roll 3 and follows the pass line PL. Hereinafter, one side of the width of the steel strip 9 along the pass line PL will be referred to as the "drive side", and the other side will be referred to as the "work side". On the drive side, each of the multiple rolls 3 is rotated in a direction that sends the steel strip 9 downstream of the pass line PL.

The multiple rolls 3 include a sink roll 4 and two support rolls 5, 6. The sink roll 4 is located in the plating bath contained in the plating pot 2. At the sink roll 4, the steel strip 9 that has entered the plating bath is folded back from bottom to top. Hereinafter, the surface of the steel strip 9 that contacts the sink roll 4 is referred to as the "back surface 9b," and the opposite surface is referred to as the "front surface 9a."

The support roll 5 is located above the sink roll 4 in the plating bath contained in the plating pot 2. The support roll 5 contacts the back surface 9b of the steel strip 9 that passes through the sink roll 4 and heads upward. On the support roll 5, the steel strip 9 is gently folded back from the front direction (the direction in which the front surface 9a faces) to the back direction (the direction in which the back surface 9b faces) as it heads upward.

The support roll 6 is located above the support roll 5 in the plating bath contained in the plating pot 2. The support roll 6 contacts the front surface 9a of the steel strip 9 that passes through the support roll 5 and moves upward. At the support roll 6, the steel strip 9 is gently folded back from the back direction to the front direction as it moves upward.

The arrangement of the support rolls 5 and 6 is an example and can be changed as appropriate. For example, the support roll 5 in contact with the back surface 9b may be arranged above the support roll 6 in contact with the front surface 9a.

The gas wiping device 10 removes excess plating material on the front surface 9a of the steel strip 9 by spraying gas onto the front surface 9a. The gas may be air or an inert gas such as nitrogen gas. For example, the gas wiping device 10 has a gas wiping nozzle 11, a gas supply unit 12, and a nozzle displacement device 13. The gas wiping nozzle 11 discharges gas toward the front surface 9a from a slit-shaped discharge port extending in the width direction of the steel strip 9. The gas supply unit 12 supplies gas to the gas wiping nozzle 11.

The nozzle displacement device 13 is configured to displace the gas wiping nozzle 11 along a direction perpendicular to the front surface 9a. In the following description of the gas wiping device 10, displacement toward the front surface 9a is referred to as "advancement," and displacement away from the front surface 9a is referred to as "retraction." When the nozzle displacement device 13 advances the gas wiping nozzle 11, the effect of the gas discharged by the gas wiping nozzle 11 is strengthened, and the amount of plating attached to the front surface 9a decreases. Conversely, when the nozzle displacement device 13 retreats the gas wiping nozzle 11, the effect of the gas discharged by the gas wiping nozzle 11 is weakened, and the amount of plating attached increases.

The nozzle displacement device 13 is further configured to change the inclination of the gas wiping nozzle 11 around an axis parallel to the pass line PL. By tilting the gas wiping nozzle 11 around an axis parallel to the pass line PL, a difference is created between the action of the gas on the drive side and the action of the gas on the working side, and the balance of the plating deposition amount on the drive side and the working side can be adjusted.

For example, as shown in FIG. 2, the nozzle displacement device 13 has a drive side actuator 14 and a work side actuator 15. The drive side actuator 14 advances or retreats the drive side end of the gas wiping nozzle 11 using a drive source such as an electric motor or a hydraulic motor. The work side actuator 15 advances or retreats the work side end of the gas wiping nozzle 11 using a drive source such as an electric motor or a hydraulic motor.

When the direction of displacement of the drive side end of the gas wiping nozzle 11 by the drive side actuator 14 is the same as the direction of displacement of the work side end of the gas wiping nozzle 11 by the work side actuator 15, the entire nozzle displacement device 13 moves forward or backward. When there is a difference between the displacement of the drive side end of the gas wiping nozzle 11 by the drive side actuator 14 and the displacement of the work side end of the gas wiping nozzle 11 by the work side actuator 15, the nozzle displacement device 13 tilts around an axis parallel to the pass line PL.

Returning to FIG. 1, the gas wiping device 20 removes excess plating material on the back surface 9b of the steel strip 9 by spraying gas onto the back surface 9b. The gas may be air or an inert gas such as nitrogen gas. The gas wiping device 20 is configured in the same manner as the gas wiping device 10. For example, the gas wiping device 20 has a gas wiping nozzle 21, a gas supply unit 22, and a nozzle displacement device 23. The gas wiping nozzle 21 discharges gas toward the back surface 9b from a slit-shaped discharge port extending in the width direction of the steel strip 9. The gas supply unit 22 supplies gas to the gas wiping nozzle 21.

The nozzle displacement device 23 is configured to displace the gas wiping nozzle 21 along a direction perpendicular to the back surface 9b. In the following description of the gas wiping device 20, displacement toward the back surface 9b is referred to as "advancement," and displacement away from the back surface 9b is referred to as "retraction."

The nozzle displacement device 23 is further configured to change the inclination of the gas wiping nozzle 21 around an axis parallel to the path line PL. For example, as shown in FIG. 2, the nozzle displacement device 23 has a drive side actuator 24 and a work side actuator 25, similar to the nozzle displacement device 13. The drive side actuator 24 advances or retreats the drive side end of the gas wiping nozzle 21 by a drive source such as an electric motor or a hydraulic motor. The work side actuator 25 advances or retreats the work side end of the gas wiping nozzle 21 by a drive source such as an electric motor or a hydraulic motor.

In the above, as an example of the configuration of the gas wiping device 10, 20, a configuration in which the plating adhesion amount is adjusted by changing the arrangement of the gas wiping nozzle 11, 21 has been exemplified, but the method of adjusting the plating adhesion amount is not limited to this. For example, the gas wiping device 10, 20 may be configured to adjust the plating adhesion amount by changing the gas discharge pressure from the gas wiping nozzle 11, 21. In this case, the gas wiping device 10, 20 may be configured to be able to change the gas discharge pressure from the gas wiping nozzle 11, 21 for each of multiple blocks arranged in the width direction of the steel strip 9. As an example, the inside of the gas wiping nozzle 11, 21 may be divided into multiple blocks arranged in the width direction of the steel strip 9, and the gas supply unit 12, 22 may be configured to be able to adjust the gas supply pressure for each of the multiple blocks.

Returning to FIG. 1, the adhesion weight sensors 30, 40 measure the coating weight at multiple measurement points. The multiple measurement points may include three or more measurement points aligned in the width direction of the steel strip 9. The multiple measurement points may include three or more front side measurement points (one-side measurement points) aligned in the width direction on the front surface 9a (one side) of the steel strip 9, and three or more back side measurement points (rear side measurement points) corresponding to the three or more front measurement points on the back surface 9b (rear side) of the steel strip 9. For example, the adhesion weight sensor 30 measures the coating weight at three or more front side measurement points, and the adhesion weight sensor 40 measures the coating weight at three or more back side measurement points.

As shown in FIG. 2, the adhesion sensor 30 includes adhesion sensors 31, 32, and 33. The adhesion sensors 31, 32, and 33 are provided downstream of the gas wiping device 10 on the path line PL and measure the plating adhesion amount on the front surface 9a. For example, the adhesion sensor 31 faces the measurement point P31 on the front surface 9a and measures the plating adhesion amount at the measurement point P31. The adhesion sensor 32 faces the measurement point P32 on the front surface 9a and measures the plating adhesion amount at the measurement point P32. The adhesion sensor 33 faces the measurement point P33 on the front surface 9a and measures the plating adhesion amount at the measurement point P33. The measurement point P31 is located on the most driving side among the measurement points P31, P32, and P33, the measurement point P33 is located on the most working side among the measurement points P31, P32, and P33, and the measurement point P32 is located halfway between the measurement points P31 and P33. As an example, measurement point P32 is located at the center in the width direction of steel strip 9. Hereinafter, the coating adhesion amount detected by adhesion sensor 31 will be referred to as the "drive side adhesion amount," the coating adhesion amount detected by adhesion sensor 33 will be referred to as the "work side adhesion amount," and the coating adhesion amount detected by adhesion sensor 32 will be referred to as the "center adhesion amount."

The adhesion sensor 40 includes adhesion sensors 41, 42, and 43. The adhesion sensors 41, 42, and 43 are provided downstream of the gas wiping device 20 on the pass line PL and measure the plating adhesion amount on the back surface 9b. For example, the adhesion sensors 41, 42, and 43 face the adhesion sensors 31, 32, and 33, respectively, across the steel strip 9. For example, the adhesion sensor 41 faces the measurement point P41 on the back surface 9b and measures the plating adhesion amount at the measurement point P41. The adhesion sensor 42 faces the measurement point P42 on the back surface 9b and measures the plating adhesion amount at the measurement point P42. The adhesion sensor 43 faces the measurement point P43 on the back surface 9b and measures the plating adhesion amount at the measurement point P43. Measurement point P41 is located closest to the drive side among measurement points P41, P42, and P43, measurement point P43 is located closest to the work side among measurement points P41, P42, and P43, and measurement point P42 is located midway between measurement points P41 and P43. As an example, measurement point P42 is located at the center in the width direction of steel strip 9. Hereinafter, the coating adhesion amount detected by adhesion sensor 41 will be referred to as the "drive side adhesion amount," the coating adhesion amount detected by adhesion sensor 43 will be referred to as the "work side adhesion amount," and the coating adhesion amount detected by adhesion sensor 42 will be referred to as the "center adhesion amount."

The deposition amount sensors 31, 32, 33, 41, 42, and 43 measure the thickness of the thin layer of plating material formed on the front faces 9a and 9b as a value representing the plating deposition amount. Specific examples of the deposition amount sensors 31, 32, 33, 41, 42, and 43 include X-ray film thickness sensors.

The roll displacement device 50 (warping correction device) corrects the warping of the steel strip 9 (warping around an axis perpendicular to the pass line PL) by displacing the support roll 5 in a direction perpendicular to the back surface 9b. In the following description of the roll displacement device 50, the displacement of the support roll 5 in a direction approaching the back surface 9b is referred to as "forward movement," and the displacement of the support roll 5 in a direction away from the back surface 9b is referred to as "backward movement."

When the steel strip 9 is folded back on the roll 3, it tends to warp around an axis perpendicular to the pass line PL. Specifically, the center of the steel strip 9 tends to warp away from the roll 3 compared to the drive side and working side portions of the steel strip 9. For this reason, the sink roll 4 and the support roll 5 act to cause the steel strip 9 to warp convexly in the front direction, and the support roll 6 acts to cause the steel strip 9 to warp convexly in the back direction.

When the roll displacement device 50 moves the support roll 5 forward or backward, the relationship between the action of the sink roll 4 and the support roll 5 and the action of the support roll 6 changes, so the degree of warping of the steel strip 9 changes. The roll displacement device 50 may be configured to displace the support roll 6 instead of the support roll 5, or may be configured to displace both the support rolls 5 and 6.

The control device 100 repeatedly controls the gas wiping devices 10, 20 and the roll displacement device 50 based on the measurement results of the coating weight. In the plating process of the steel strip 9, it is first required that the coating weight is equal to or greater than a predetermined lower limit (hereinafter, referred to as the "quality lower limit"). At the same time, it is also required to prevent unnecessary adhesion of plating material and conserve plating material. Therefore, the control device 100 repeatedly executes a control cycle to achieve both prevention of insufficient coating weight and conservation of plating material.

For example, the control device 100 is configured to increase the coating weight by the gas wiping device 10, 20 when the minimum coating weight at multiple measurement points on the steel strip 9 that has passed through the coating pot 2 and the gas wiping device 10, 20 falls below a predetermined lower limit, to decrease the coating weight by the gas wiping device 10, 20 when the maximum coating weight at multiple measurement points exceeds a predetermined upper limit, and to prohibit the gas wiping device 10, 20 from decreasing the coating weight when the difference between the maximum and minimum values exceeds a difference threshold.

This allows the maximum plating adhesion amount to be adjusted to be equal to or less than the upper limit while adjusting the minimum plating adhesion amount to be equal to or more than the lower limit. Furthermore, if the difference between the maximum and minimum values is greater than the difference threshold, the gas wiping devices 10, 20 are prohibited from reducing the plating adhesion amount, so it is possible to prevent the minimum value from falling below the lower limit due to control that adjusts the maximum value to be equal to or less than the upper limit. Therefore, it is possible to achieve both prevention of insufficient plating adhesion amount and saving plating material.

As shown in FIG. 2, the control device 100 has, as its functional configuration (hereinafter referred to as "functional blocks"), a nozzle position control unit 111, a roll position control unit 112, an adhesion amount acquisition unit 113, a lower limit control unit 114, an upper limit control unit 115, a difference monitoring unit 116, a difference control unit 117, a warpage evaluation unit 118, and a warpage control unit 119.

The nozzle position control unit 111 controls the gas wiping device 10 to displace the gas wiping nozzle 11 in response to a front side position command that specifies the target positions of the drive side end and working side end of the gas wiping nozzle 11. For example, the nozzle position control unit 111 displaces the drive side end of the gas wiping nozzle 11 using the drive side actuator 14 and displaces the working side end of the gas wiping nozzle 11 using the working side actuator 15 in response to the front side position command.

The nozzle position control unit 111 also controls the gas wiping device 20 to displace the gas wiping nozzle 21 in response to a back side position command that specifies the target positions of the drive side end and the working side end of the gas wiping nozzle 21. For example, the nozzle position control unit 111 displaces the drive side end of the gas wiping nozzle 21 by the drive side actuator 24 and displaces the working side end of the gas wiping nozzle 21 by the working side actuator 25 in response to the back side position command.

The roll position control unit 112 controls the roll displacement device 50 to displace the support roll 5 in response to a position command that specifies the target position of the support roll 5. The adhesion amount acquisition unit 113 acquires the detection results of the plating adhesion amount by the adhesion amount sensors 30 and 40.

The lower limit control unit 114 increases the coating weight by the gas wiping devices 10 and 20 when the minimum coating weight at multiple measurement points on the steel strip 9 that has passed through the coating pot 2 and the gas wiping devices 10 and 20 falls below a predetermined lower limit (hereinafter referred to as the "control lower limit"). The control lower limit is a value greater than the above-mentioned quality lower limit. For example, the control lower limit is a value obtained by adding a predetermined margin to the quality lower limit.

For example, when the minimum value of the plating adhesion amount detected by the adhesion amount sensors 31, 32, 33 (hereinafter referred to as the "front side minimum value") is below the control lower limit, the lower limit control unit 114 causes the gas wiping device 10 to increase the plating adhesion amount on the front surface 9a. For example, the lower limit control unit 114 changes the front side position command so as to retract the gas wiping nozzle 11. In response, the nozzle position control unit 111 causes the gas wiping device 10 to retract the gas wiping nozzle 11 according to the changed front side position command. The lower limit control unit 114 may retract the gas wiping nozzle 11 at a predetermined pitch for each control cycle.

The lower limit control unit 114 may retract the gas wiping nozzle 11 by an amount of retraction according to the deviation between the control lower limit value and the front-side minimum value (hereinafter referred to as the "front-side lower limit deviation"). In this case, the lower limit control unit 114 may calculate the amount of retraction by performing a proportional (P) calculation, a proportional-integral (PI) calculation, or a proportional-integral-derivative (PID) calculation on the front-side lower limit deviation.

In addition, the lower limit control unit 114 increases the plating adhesion amount on the back surface 9b by the gas wiping device 20 when the minimum value of the plating adhesion amount detected by the adhesion amount sensors 41, 42, 43 (hereinafter referred to as the "back side minimum value") is below the control lower limit. For example, the lower limit control unit 114 changes the back side position command so as to retract the gas wiping nozzle 21. In response, the nozzle position control unit 111 retracts the gas wiping nozzle 21 by the gas wiping device 20 according to the changed back side position command (see FIG. 3). The lower limit control unit 114 may retract the gas wiping nozzle 21 at a predetermined pitch for each control cycle.

The lower limit control unit 114 may retract the gas wiping nozzle 21 by an amount of retraction according to the deviation between the control lower limit value and the back side minimum value (hereinafter referred to as the "back side lower limit deviation") for each control cycle. In this case, the lower limit control unit 114 may calculate the amount of retraction by performing a proportional (P) calculation, a proportional-integral (PI) calculation, or a proportional-integral-derivative (PID) calculation on the back side lower limit deviation.

The upper limit control unit 115 reduces the plating adhesion amount using the gas wiping device 10, 20 when the maximum value of the plating adhesion amount at multiple measurement points exceeds a predetermined upper limit value (hereinafter referred to as the "control upper limit value"). The control upper limit value is set in advance based on the actual values of the plating adhesion amount at multiple measurement points, etc.

For example, when the maximum plating adhesion amount (hereinafter referred to as the "front side maximum value") detected by the adhesion amount sensors 31, 32, 33 exceeds the upper limit value for control, the upper limit control unit 115 causes the gas wiping device 10 to reduce the plating adhesion amount on the front surface 9a. For example, the upper limit control unit 115 changes the front side position command so as to advance the gas wiping nozzle 11. In response, the nozzle position control unit 111 causes the gas wiping device 10 to advance the gas wiping nozzle 11 according to the changed front side position command. The upper limit control unit 115 may advance the gas wiping nozzle 11 at a predetermined pitch for each control cycle.

The upper limit control unit 115 may advance the gas wiping nozzle 11 by an advance amount according to the deviation between the front-side maximum value and the upper limit value under control (hereinafter referred to as the "front-side upper limit deviation"). In this case, the upper limit control unit 115 may calculate the advance amount by performing a proportional (P) calculation, a proportional-integral (PI) calculation, or a proportional-integral-derivative (PID) calculation, etc., on the front-side upper limit deviation.

In addition, when the maximum plating adhesion amount (hereinafter referred to as the "back side maximum value") detected by the adhesion amount sensors 41, 42, 43 exceeds the upper limit value for control, the upper limit control unit 115 causes the gas wiping device 20 to reduce the plating adhesion amount on the back surface 9b. For example, the upper limit control unit 115 changes the back side position command so as to advance the gas wiping nozzle 21. In response, the nozzle position control unit 111 causes the gas wiping device 20 to advance the gas wiping nozzle 21 according to the changed back side position command (see FIG. 4). The upper limit control unit 115 may advance the gas wiping nozzle 21 at a predetermined pitch for each control cycle.

The upper limit control unit 115 may advance the gas wiping nozzle 11 by an advance amount according to the deviation between the back side maximum value and the controlled upper limit value (hereinafter referred to as the "back side upper limit deviation") for each control cycle. In this case, the upper limit control unit 115 may calculate the advance amount by performing a proportional (P) calculation, a proportional-integral (PI) calculation, or a proportional-integral-derivative (PID) calculation on the back side upper limit deviation.

The difference monitoring unit 116 prohibits the upper limit control unit 115 from reducing the plating adhesion amount by the gas wiping device 10, 20 when the difference between the maximum and minimum plating adhesion amounts at multiple measurement points exceeds the difference threshold. The difference threshold is set in advance based on the control upper limit and control lower limit. The difference threshold is less than or equal to the difference between the control upper limit and control lower limit.

For example, when the difference between the front side maximum value and the front side minimum value exceeds the difference threshold, the difference monitoring unit 116 prohibits the upper limit control unit 115 from reducing the plating adhesion amount by the gas wiping device 10. Also, when the difference between the back side maximum value and the back side minimum value exceeds the difference threshold, the difference monitoring unit 116 prohibits the upper limit control unit 115 from reducing the plating adhesion amount by the gas wiping device 20.

When the difference between the maximum and minimum plating adhesion amounts at multiple measurement points exceeds the difference threshold, the difference control unit 117 causes the gas wiping device 10, 20 to reduce the difference between the maximum and minimum values. For example, when the difference between the front-side maximum and front-side minimum values exceeds the difference threshold, the difference control unit 117 causes the gas wiping device 10 to reduce the difference between the front-side maximum and front-side minimum values.

For example, when the difference between the front-side maximum value and the front-side minimum value exceeds the difference threshold value, and the absolute value of the difference between the drive-side adhesion amount and the work-side adhesion amount exceeds the second difference threshold value, the difference control unit 117 reduces the difference between the drive-side adhesion amount and the work-side adhesion amount. The second difference threshold value is set in advance together with the difference threshold value, and is, for example, equal to or less than the above difference threshold value.

For example, when the plating adhesion amount detected by the adhesion amount sensor 31 is greater than the plating adhesion amount detected by the adhesion amount sensor 33, the differential control unit 117 changes the front side position command to cause at least one of the drive side end of the gas wiping nozzle 11 to advance and the work side end of the gas wiping nozzle 11 to retreat, in accordance with the changed front side position command. In response, the nozzle position control unit 111 executes at least one of the following by the gas wiping device 10: advancing the drive side end of the gas wiping nozzle 11 and retracting the work side end of the gas wiping nozzle 11.

When the plating adhesion amount detected by the adhesion amount sensor 31 is smaller than the plating adhesion amount detected by the adhesion amount sensor 33, the differential control unit 117 changes the front side position command to cause at least one of the drive side end of the gas wiping nozzle 11 to retract and the work side end of the gas wiping nozzle 11 to advance. In response to this, the nozzle position control unit 111 executes at least one of the following by the gas wiping device 10: retracting the drive side end of the gas wiping nozzle 11 and advancing the work side end of the gas wiping nozzle 11 according to the changed front side position command.

The differential control unit 117 may displace at least one of the driving side end and the working side end of the gas wiping nozzle 11 at a predetermined pitch for each control cycle. The differential control unit 117 may displace at least one of the driving side end and the working side end of the gas wiping nozzle 11 by an amount of displacement corresponding to the difference between the front side maximum value and the front side minimum value for each control cycle. In this case, the differential control unit 117 may calculate the amount of displacement by performing a proportional (P) operation, a proportional-integral (PI) operation, or a proportional-integral-derivative (PID) operation on the difference between the front side maximum value and the front side minimum value.

In addition, when the difference between the back side maximum value and the back side minimum value exceeds the difference threshold, the difference control unit 117 causes the gas wiping device 20 to reduce the difference between the back side maximum value and the back side minimum value.

For example, the difference control unit 117 reduces the difference between the drive side adhesion amount and the work side adhesion amount when the difference between the rear side maximum value and the rear side minimum value exceeds the difference threshold value and the absolute value of the difference between the drive side adhesion amount and the work side adhesion amount exceeds a second difference threshold value.

For example, when the plating adhesion amount detected by the adhesion amount sensor 41 is greater than the plating adhesion amount detected by the adhesion amount sensor 43, the differential control unit 117 changes the back side position command to cause at least one of the drive side end of the gas wiping nozzle 21 to advance and the work side end of the gas wiping nozzle 21 to retreat, in accordance with the changed back side position command. In response, the nozzle position control unit 111 executes at least one of the following by the gas wiping device 20: advance the drive side end of the gas wiping nozzle 21 and retreat the work side end of the gas wiping nozzle 21 (see FIG. 5).

When the plating adhesion amount detected by the adhesion amount sensor 41 is smaller than the plating adhesion amount detected by the adhesion amount sensor 43, the differential control unit 117 changes the back side position command so as to cause at least one of the drive side end of the gas wiping nozzle 21 to retract and the work side end of the gas wiping nozzle 21 to advance. In response, the nozzle position control unit 111 causes the gas wiping device 20 to retract the drive side end of the gas wiping nozzle 21 and advance the work side end of the gas wiping nozzle 21 according to the changed back side position command.

The differential control unit 117 may displace at least one of the driving side end and the working side end of the gas wiping nozzle 21 at a predetermined pitch for each control cycle. The differential control unit 117 may displace at least one of the driving side end and the working side end of the gas wiping nozzle 21 by an amount of displacement corresponding to the difference between the back side maximum value and the back side minimum value for each control cycle. In this case, the differential control unit 117 may calculate the amount of displacement by performing a proportional (P) operation, a proportional-integral (PI) operation, or a proportional-integral-derivative (PID) operation on the difference between the back side maximum value and the back side minimum value.

The warpage evaluation unit 118 evaluates the warpage of the steel strip 9 based on the amount of plating adhesion at three or more measurement points. The warpage evaluation unit 118 may evaluate the warpage of the steel strip 9 based on the amount of plating adhesion at three or more front side measurement points and the amount of plating adhesion at three or more back side measurement points.

As an example, the warpage evaluation unit 118 evaluates the warpage of the steel strip 9 based on the coating weight detected by the coating weight sensors 31, 32, and 33 and the coating weight detected by the coating weight sensors 41, 42, and 43. For example, the warpage evaluation unit 118 evaluates the warpage of the front surface 9a based on the coating weight detected by the coating weight sensors 31, 32, and 33, and evaluates the warpage of the back surface 9b based on the coating weight detected by the coating weight sensors 41, 42, and 43.

For example, the warpage evaluation unit 118 calculates the difference between the average plating adhesion amount detected by the adhesion amount sensors 31 and 33 and the plating adhesion amount detected by the adhesion amount sensor 32 as an evaluation value of the warpage of the front surface 9a (hereinafter referred to as the "front side warpage evaluation value"). When the front side warpage evaluation value is a positive value, the front surface 9a is evaluated as being aligned so as to be convex in the front direction (see (a) in FIG. 6), and when the front side warpage evaluation value is a negative value, the front surface 9a is evaluated as being aligned so as to be convex in the back direction.

The warpage evaluation unit 118 also calculates the difference between the average plating adhesion amount detected by the adhesion amount sensors 41 and 43 and the plating adhesion amount detected by the adhesion amount sensor 42 as an evaluation value of the warpage of the back surface 9b (hereinafter referred to as the "back-side warpage evaluation value"). When the back-side warpage evaluation value is a positive value, the back surface 9b is evaluated as being aligned so as to be convex in the back direction, and when the back-side warpage evaluation value is a negative value, the back surface 9b is evaluated as being aligned so as to be convex in the front direction.

The warpage evaluation unit 118 evaluates the warpage of the steel strip 9 based on the front side warpage evaluation value and the back side warpage evaluation value. For example, the warpage evaluation unit 118 calculates the difference between the front side warpage evaluation value and the back side warpage evaluation value as the overall warpage evaluation value. A positive overall warpage evaluation value indicates that the steel strip 9 is aligned so as to be convex in the front direction, and a negative overall warpage evaluation value indicates that the steel strip 9 is aligned so as to be convex in the back direction.

When warping occurs in the steel strip 9, either the front surface 9a or the back surface 9b will be convex, and the other will be concave. For this reason, it is predicted that the signs of the front side warp evaluation value and the back side warp evaluation value will be reversed. For example, when the steel strip 9 is aligned so as to be convex in the front direction (see (a) of FIG. 6), the front side warp evaluation value will be a positive value, and the back side warp evaluation value will be a negative value. For this reason, the absolute value of the overall evaluation value of the warp will be greater than the absolute values of both the front side warp evaluation value and the back side warp evaluation value. When the signs of the front side warp evaluation value and the back side warp evaluation value match each other, there is a possibility that the steel strip 9 is not warped, and the coating weight is simply variable (see (b) of FIG. 6). In such a case, the absolute value of the overall evaluation value of the warp will be smaller than the absolute values of at least one of the front side warp evaluation value and the back side warp evaluation value.

When the warpage evaluation result exceeds a predetermined level, the warpage control unit 119 reduces the warpage of the steel strip 9 in the plating pot 2 using the roll displacement device 50. For example, when the warpage direction of the steel strip 9 is convex in the front direction, the warpage control unit 119 advances the support roll 5 using the roll displacement device 50 (see Figure 7). Also, when the warpage direction of the steel strip 9 is convex in the back direction, the warpage control unit 119 retreats the support roll 5 using the roll displacement device 50 (see Figure 8).

The warpage control unit 119 may displace each of the support rolls 5 at a predetermined pitch for each control cycle. The warpage control unit 119 may displace the support rolls 5 by an amount of displacement corresponding to the warpage evaluation value of the steel strip 9 for each control cycle. In this case, the warpage control unit 119 may calculate the amount of displacement by performing a proportional (P) calculation, a proportional-integral (PI) calculation, or a proportional-integral-derivative (PID) calculation, etc., on the warpage evaluation value of the steel strip 9.

Figure 9 is a block diagram illustrating an example of the hardware configuration of the control device 100. As shown in Figure 9, the control device 100 is composed of one or more control computers. For example, the control device 100 has a circuit 190. The circuit 190 has one or more processors 191, a memory 192, a storage 193, and an input/output port 194. The storage 193 has a storage medium readable by a computer, such as a hard disk. The storage medium may be a removable medium such as a non-volatile semiconductor memory, a magnetic disk, or an optical disk. The storage 193 stores a program that causes the gas wiping devices 10 and 20 to increase the coating weight when the minimum coating weight at multiple measurement points of the steel strip 9 that has passed through the coating pot 2 and the gas wiping devices 10 and 20 falls below a predetermined lower limit, causes the gas wiping devices 10 and 20 to decrease the coating weight when the maximum coating weight at multiple measurement points exceeds a predetermined upper limit, and prohibits the gas wiping devices 10 and 20 from decreasing the coating weight when the difference between the maximum and minimum values exceeds a difference threshold. For example, the storage 193 stores a program for configuring each of the above-mentioned functional blocks in the control device 100.

The memory 192 temporarily stores the program loaded from the storage 193 and the results of calculations by the processor 191. The processor 191 configures each functional block of the control device 100 by executing the above program in cooperation with the memory 192. The input/output port 194 inputs and outputs electrical signals between the nozzle displacement devices 13, 23, the adhesion amount sensors 30, 31, 32, 33, 40, 41, 42, 43, and the roll displacement device 50 in accordance with instructions from the processor 191. Note that the control device 100 is not necessarily limited to one that configures each functional block by a program. For example, each functional block of the control device 100 may be configured by a dedicated logic circuit or an ASIC (Application Specific Integrated Circuit) that integrates the logic circuit.

[Control Procedure]
Next, as an example of a control method, a coating weight control procedure executed by the control device 100 will be illustrated. This procedure includes increasing the coating weight by the gas wiping device 10, 20 when the minimum value of the coating weight at a plurality of measurement points of the steel strip 9 that has passed through the coating pot 2 and the gas wiping device 10, 20 falls below a predetermined lower limit, decreasing the coating weight by the gas wiping device 10, 20 when the maximum value of the coating weight at a plurality of measurement points exceeds a predetermined upper limit, and prohibiting the gas wiping device 10, 20 from decreasing the coating weight when the difference between the maximum value and the minimum value exceeds a difference threshold.

In addition, this control procedure includes a control procedure for gas wiping device 10 and a control procedure for gas wiping device 20. Since the control procedure for gas wiping device 10 and the control procedure for gas wiping device 20 are the same, the control procedure for gas wiping device 10 will be described in detail below, and the explanation of the control procedure for gas wiping device 20 will be omitted.

As shown in FIG. 10, the control device 100 first executes steps S01 and S02. In step S01, the adhesion amount acquisition unit 113 acquires the detection results of the plating adhesion amount by the adhesion amount sensors 30 and 40. In step S02, the lower limit control unit 114 checks whether the minimum value of the plating adhesion amount detected by the adhesion amount sensors 31, 32, and 33 (the above-mentioned front side minimum value) is below the above-mentioned lower limit value for control.

If it is determined in step S02 that the front-side minimum value is below the lower limit value for control, the control device 100 executes step S03. In step S03, the lower limit control unit 114 increases the plating adhesion amount on the front surface 9a by the gas wiping device 10.

If it is determined in step S02 that the front-side minimum value is not below the lower limit value for control, the control device 100 executes step S04. In step S04, the upper limit control unit 115 checks whether the maximum value of the plating adhesion amount detected by the adhesion amount sensors 31, 32, and 33 (the above-mentioned front-side maximum value) is above the above-mentioned upper limit value for control.

If it is determined in step S04 that the front maximum value is greater than the upper limit value for control, the control device 100 executes step S05. In step S05, the difference monitoring unit 116 checks whether the difference between the front maximum value and the front minimum value is less than the difference threshold value.

If it is determined in step S05 that the difference between the front-side maximum value and the front-side minimum value is below the difference threshold, the control device 100 executes step S06. In step S06, the upper limit control unit 115 causes the gas wiping device 10 to reduce the amount of plating applied on the front surface 9a.

If it is determined in step S05 that the difference between the front maximum value and the front minimum value exceeds the difference threshold, the control device 100 executes step S08. If it is determined in step S04 that the front maximum value is below the upper limit value for control, the control device 100 executes step S08 without executing steps S05 and S06. In step S08, the difference monitoring unit 116 checks whether the absolute value of the difference between the drive side adhesion amount and the working side adhesion amount exceeds the second difference threshold.

If it is determined in step S08 that the difference between the drive side adhesion amount and the working side adhesion amount exceeds the second difference threshold, the control device 100 executes step S09. In step S09, the difference control unit 117 causes the gas wiping device 10 to reduce the difference between the front side maximum value and the front side minimum value. The specific processing procedure of step S09 will be described later.

In this way, if it is determined that the difference between the front-side maximum value and the front-side minimum value is not below the difference threshold, the upper limit control unit 115 prohibits the gas wiping device 10 from reducing the plating adhesion amount on the front surface 9a, and the difference control unit 117 causes the gas wiping device 10 to reduce the difference.

If it is determined in step S08 that the difference between the drive side adhesion amount and the work side adhesion amount does not exceed the second difference threshold, the control device 100 executes steps S11 and S12. In step S11, the warp evaluation unit 118 evaluates the warp of the steel strip 9 based on the coating adhesion amount detected by the coating amount sensors 31, 32, and 33 and the coating adhesion amount detected by the coating amount sensors 41, 42, and 43. For example, the warp evaluation unit 118 calculates a comprehensive evaluation value of the warp. The specific processing procedure of step S11 will be described later. In step S12, the warp control unit 119 checks whether the warp evaluation result by the warp evaluation unit 118 (for example, the absolute value of the comprehensive evaluation value of the warp) exceeds a predetermined level.

If it is determined in step S12 that the evaluation result of the warpage exceeds the predetermined level, the control device 100 executes step S13. In step S13, the warpage control unit 119 reduces the warpage of the steel strip 9 in the plating pot 2 by the roll displacement device 50. The specific processing procedure of step S13 will be described later.

After steps S06, S09, and S13, the control device 100 returns the process to step S01. If it is determined in step S12 that the warpage evaluation result does not exceed the predetermined level, the control device 100 returns the process to step S01 without executing any of steps S06, S09, or S13. The control device 100 repeats the above control cycle.

Figure 11 is a flow chart illustrating the procedure for reducing the difference in step S09. As shown in Figure 11, the control device 100 first executes step S21. In step S21, the difference control unit 117 checks whether the plating adhesion amount on the drive side (the plating adhesion amount detected by the adhesion amount sensor 31) is greater than the plating adhesion amount on the work side (the plating adhesion amount detected by the adhesion amount sensor 33).

If it is determined in step S21 that the plating adhesion amount on the driving side is greater than the plating adhesion amount on the working side, the control device 100 executes step S22. In step S22, the differential control unit 117 executes at least one of the following: moving the driving side end of the gas wiping nozzle 11 forward and moving the working side end of the gas wiping nozzle 11 backward by the gas wiping device 10.

If it is determined in step S21 that the plating adhesion amount on the driving side is not greater than the plating adhesion amount on the working side, the control device 100 executes step S23. In step S23, the difference control unit 117 executes at least one of the following: retracting the driving side end of the gas wiping nozzle 11 and advancing the working side end of the gas wiping nozzle 11 by the gas wiping device 10. This completes the procedure for reducing the difference.

Figure 12 is a flow chart illustrating the procedure for evaluating the warpage of the steel strip 9 in step S11. As shown in Figure 12, the control device 100 executes steps S31, S32, and S34. In step S31, the warpage evaluation unit 118 evaluates the warpage of the front surface 9a based on the coating weight detected by the coating weight sensors 31, 32, and 33. For example, the warpage evaluation unit 118 calculates the front side warpage evaluation value. In step S32, the warpage evaluation unit 118 evaluates the warpage of the back surface 9b based on the coating weight detected by the coating weight sensors 41, 42, and 43. For example, the warpage evaluation unit 118 calculates the back side warpage evaluation value. In step S34, the warpage evaluation unit 118 calculates the difference between the front side warpage evaluation value and the back side warpage evaluation value as the overall evaluation value of the warpage.

Figure 13 is a flow chart illustrating the procedure for reducing the warpage of the steel strip 9 in step S13. As shown in Figure 13, the control device 100 first executes step S41. In step S41, the warpage control unit 119 checks whether the warpage of the steel strip 9 is convex in the front direction (for example, whether the overall evaluation value of the warpage is a positive value).

If it is determined in step S41 that the warp of the steel strip 9 is convex in the front direction, the control device 100 executes step S42. In step S42, the warp control unit 119 advances the support roll 5 by the roll displacement device 50.

If it is determined in step S41 that the warp of the steel strip 9 is not convex in the front direction, the control device 100 executes step S43. In step S43, the warp control unit 119 causes the roll displacement device 50 to move the support roll 5 backward. This completes the procedure for reducing the warp of the steel strip 9.

The above-mentioned procedure for controlling the amount of adhesion can be modified as appropriate. For example, as shown in FIG. 14, the control device 100 may execute step S05 (checking whether the difference between the front maximum value and the front minimum value is below the difference threshold value) prior to step S04 (checking whether the front maximum value is above the upper limit value for control), and step S04 may be executed if the difference between the front maximum value and the front minimum value is below the difference threshold value.

[Effects of this embodiment]
As described above, the control device 100 comprises a plating pot 2 containing a plating bath containing molten zinc, and a gas wiping device 10, 20 that adjusts the plating adhesion weight by spraying gas onto the steel strip 9 that has passed through the plating pot 2, a lower limit control unit 114 that increases the plating adhesion weight using the gas wiping device 10, 20 when the minimum value of the plating adhesion weight at multiple measurement points of the steel strip 9 that has passed through the plating pot 2 falls below a predetermined lower limit value, an upper limit control unit 115 that reduces the plating adhesion weight using the gas wiping device 10, 20 when the maximum value of the plating adhesion weight at multiple measurement points exceeds a predetermined upper limit value, and a difference monitoring unit 116 that prohibits the gas wiping device 10, 20 from reducing the plating adhesion weight when the difference between the maximum value and the minimum value exceeds a difference threshold value.

This control device 100 can adjust the maximum plating adhesion amount to an upper limit or less while adjusting the minimum plating adhesion amount to a lower limit or more. Furthermore, if the difference between the maximum and minimum values is greater than the difference threshold, the gas wiping devices 10, 20 are prohibited from reducing the plating adhesion amount, thereby preventing the minimum value from falling below the lower limit due to the control that adjusts the maximum value to an upper limit or less. Therefore, it is possible to achieve both prevention of insufficient plating adhesion amount and saving plating material.

The control device 100 may further include a difference control unit 117 that causes the gas wiping device 10, 20 to reduce the difference between the maximum and minimum values when the difference between the maximum and minimum values exceeds a difference threshold. In this case, after the difference between the maximum and minimum values of the plating adhesion amount is reduced, the maximum value is adjusted to be equal to or less than the upper limit value. Therefore, it is possible to more reliably achieve both prevention of insufficient plating adhesion amount and saving of plating material.

The multiple measurement points include three or more measurement points aligned in the width direction of the steel strip 9, and the control device 100 may further include a warp evaluation unit 118 that evaluates the warp of the steel strip 9 based on the coating weight at the three or more measurement points, and a warp control unit 119 that reduces the warp of the steel strip 9 in the plating pot 2 by the roll displacement device 50 when the evaluation result of the warp exceeds a predetermined level. In this case, the variation in the coating weight caused by the warp of the steel strip 9 is also suppressed. Therefore, it is possible to more reliably achieve both suppression of insufficient coating weight and saving of plating material.

The multiple measurement points may include three or more one-side measurement points aligned in the width direction on the front surface 9a of the steel strip 9, and three or more back-side measurement points on the back surface 9b that correspond to the three or more measurement points, and the warpage evaluation unit 118 may evaluate the warpage of the steel strip 9 based on the coating weight at the three or more one-side measurement points and the coating weight at the three or more back-side measurement points. In this case, the warpage of the steel strip 9 can be evaluated with higher reliability, and the variation in coating weight caused by the warpage of the steel strip 9 can be more reliably suppressed.

Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

1...plating system, 2...plating pot, 9...steel strip, 9a...front surface (one side), 9b...back surface (reverse side), 10, 20...gas wiping device, 100...control device, 50...roll displacement device (warping correction device), 114...lower limit control device, 115...upper limit control device, 116...difference monitoring device, 117...difference control device, 118...warping evaluation device, 119...warping control device.

Claims (5)

a coating pot containing a coating bath containing molten zinc, and a gas wiping device that adjusts the coating weight by blowing gas onto the steel strip that has passed through the coating pot; and a lower limit control unit that increases the coating weight by the gas wiping device when the minimum value of the coating weight at multiple measurement points of the steel strip that has passed through the coating pot falls below a predetermined lower limit value;
an upper limit control unit that reduces the coating weight by the gas wiping device when a maximum value of the coating weights at the plurality of measurement points exceeds a predetermined upper limit;
a difference monitoring unit that prohibits the gas wiping device from reducing the coating weight when a difference between the maximum value and the minimum value exceeds a difference threshold;
A control device comprising: a difference control unit that reduces the difference between the maximum value and the minimum value by the gas wiping device when the difference between the maximum value and the minimum value exceeds the difference threshold . The plurality of measurement points includes three or more measurement points arranged in the width direction of the steel strip,
The control device includes:
a warpage evaluation unit that evaluates the warpage of the steel strip based on the coating weights at the three or more measurement points;
The control device according to claim 1 , further comprising a warpage control unit that reduces the warpage of the steel strip in the plating pot by a warpage correction device when the evaluation result of the warpage exceeds a predetermined level. The plurality of measurement points include three or more one-side measurement points arranged in the width direction on one side of the steel strip, and three or more back-side measurement points corresponding to the three or more measurement points on the back side of the one side,
The control device according to claim 2 , wherein the warpage evaluation unit evaluates the warpage of the steel strip based on the coating weights at the three or more one-side measurement locations and the coating weights at the three or more back-side measurement locations. a coating pot containing a coating bath containing molten zinc;
a gas wiping device for blowing gas onto the steel strip that has passed through the coating pot to adjust the coating weight;
a lower limit control unit that increases a coating weight by the gas wiping device when a minimum value of a coating weight at a plurality of measurement points of the steel strip that has passed through the coating pot and the gas wiping device falls below a predetermined lower limit value;
an upper limit control unit that reduces the coating weight by the gas wiping device when a maximum value of the coating weights at the plurality of measurement points exceeds a predetermined upper limit;
a difference monitoring unit that prohibits the gas wiping device from reducing the coating weight when a difference between the maximum value and the minimum value exceeds a difference threshold;
a difference control unit that reduces the difference between the maximum value and the minimum value by the gas wiping device when the difference between the maximum value and the minimum value exceeds the difference threshold . When a minimum value of coating weight at a plurality of measurement points of a steel strip that has passed through a coating pot containing a coating bath containing molten zinc and a gas wiping device falls below a predetermined lower limit value, increasing the coating weight by the gas wiping device;
When a maximum value of a coating weight at a plurality of measurement points exceeds a predetermined upper limit, reducing the coating weight by the gas wiping device;
prohibiting the gas wiping device from reducing the plating weight when a difference between the maximum value and the minimum value exceeds a difference threshold;
and when the difference between the maximum value and the minimum value exceeds the difference threshold, reducing the difference between the maximum value and the minimum value by the gas wiping device .

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