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ROLL SPEED SET-UP IN HOT STRIP FINISHING ROLLING MILL
ROLL SPEED SET-UP IN HOT STRIP FINISHING ROLLING MILL Seong-Cheol Hong Rolling Technology & Process Control Research Group Technical Research Laboratories, POSCO Tel: +82-54-220-6316 Email: [email protected]
Abstract: Hot strip finishing mill is initially set up with rolling conditions such as roll speed, roll gap, and cooling pattern obtained by a set-up model. To improve the accuracy of initial roll speed setting for a 7-stand finishing mill, an equation for predicting forward slip and a compensation method for initial roll speed setting were proposed and applied for No.1 Hot Strip Mill at Pohang Steel Works, Pohang, Korea. As the results, mass flow was improved and width error for strip head-end part caused by excessive strip tension was decreased largely. Copyright © 2007 IFAC Keywords: hot rolling mill, roll speed, forward slip.
1. INTRODUCTION The purpose of hot rolling is to turn the slabs of 250 mm thickness produced in a continuous casting process into the strips of thickness 1.0-20 mm. As described in Fig.1, A hot strip rolling mill is composed of a reheating furnace, a roughing mill, a finishing mill, a run-out table, and a coiler. Slabs are heated in the reheating furnace and then rolled on the roughing mill which reduces the thickness of sheet bars to 30-60 mm. after rough rolling; the strip is further rolled by the finishing mill. The rolled strip is cooled down by spraying water in the run-out table and coiled by the down coiler. In addition, the final product may be processed through a cold rolling mill in order to produce thinner steel sheet. This paper focuses exclusively on the finishing mill that consists of six or seven stands. Each stand has two work rolls and two large backup rolls inside a mill housing. The work rolls perform the reduction of the strip under the support of the backup rolls that are pushed down by electrical screws or hydraulic cylinders. The majority of the power to decrease the thickness of the strip comes from one main drive per stand which turns the work rolls, and whose speed is used as a manipulated variable. Loopers are installed between stands to store some strip in order that
disturbance does not cause the strip to become too loose or tight and to measure the variations of the mass flow of the strip. A perfect looper keeps the strip tension constant and hence decouples upstream stands from downstream ones. At each stand and interstand are sensors to measure roll gap, rolling force and looper angle, from which the strip gauge and tension are estimated. Also at the exit of the mill are various sensors to measure the process variables such as thickness, profile, temperature, etc. The process control for finishing rolling is divided into three phases, mill set-up, real-time control, and set-up model learning. Mill set-up presets the initial settings of the mill parameters such as roll gap and roll speed using a mathematical set-up model before a sheet bar (at roughly 1100°C in temperature for mild steel) enters the process. Real-time control takes over once milling starts and includes automatic gauge control, rolling speed control, and looper tension control. The previously set parameter values are dynamically adjusted as the strip thickness, roll gap, rolling force, looper angle are measured during milling. After milling finishes, the prediction errors of the set-up model are used to update the parameters of the model for the next strip in model learning.
Down Coiler
Finishing Mill
Run-out Table
Roughing Mill
Reheating Furnace
Fig. 1. A typical finishing rolling mill The initial rolling conditions are not perfect due to various reasons. If the errors are small, real time controllers can usually compensate, but large errors lead to mass flow unbalance, operation fault, quality decline, and cost increase. Of a particular interest is the precise setting of roll speed, since the high accuracy of the roll speed setting is essential for mass flow continuity and product quality. A roll speed is calculated from the target strip speed at the exit of a rolling stand and a forward slip computed by the setup model to consider the slippage between a strip and a roll. Therefore, the prediction accuracy of the forward slip is very important. In most steel works around the world, the forward slip is usually predicted using mathematical formulas based on the metallurgical and mechanical knowledge accumulated through decades of research and development. The forward slip is influenced by incoming thickness, reduction ratio, friction coefficient, etc (Hum, et al., 1996; Kwak, et al., 2002; Piispanen, 1978). Friction coefficient is known to be affected by working roll parameters such as roll speed, roll surface, lubricant oil (Jarl, 1988; Yamashita, et al., 1987) and the friction coefficient in the roll bite is not uniform (Liu, et al., 2001). Therefore, getting the precise forward slip is not easy. To improve the accuracy of the roll speed setting for a 7-stand finishing mill, a regression equation for predicting forward slip and a compensation method for initial roll speed setting were proposed in this paper and applied for No.1 Hot Strip Mill at Pohang Steel Works, Pohang, Korea. Standi+1
Standi
Vsi+1
Vsi Looper
Looper Controller Manual Operation
Roll Speed Controller
Roll Speed Reference Calculation
Initial Roll Speed Setting
Fig. 2. Roll speed setting & control in a finishing rolling mill
2. ROLL SPEED SET-UP AND CONTROL A roll speed is calculated from target strip thicknesses, the target strip speed at the exit of last stand, and a forward slip computed by the set-up model to consider the slippage between a strip and a roll as follows.
Vri =
Vsl * htl (1 + f i ) * ht i
(1)
Where, Vr = roll speed, Vs = strip speed, ht = target thickness, f = forward slip, the suffix ‘i’ indicates stand number, and the suffix ‘l’ means last stand. As depicted in Fig. 2, the initial roll speed is adjusted by a looper control and an operator during milling. Loopers are installed between two stands to store some strip in order that unexpected rolling condition change does not cause the strip to become too loose or tight, so it can contribute to the quality of products. It can also enable stable operation of the process by absorbing excessive loop of the strip arising from a mass flow unbalance. After the sheet bar has been going through the finishing mill, the looper control operates dynamically to keep a strip tension using the actual angle and motor current of the looper and hence to decouple upstream stands from downstream ones. For example, in the case of low tension, the looper angle increases to get proper tension, resulting in stable threading of a strip, while, in the case of high tension, the looper angle decreases to reduce strip tension. Fig. 2 shows the relation between looper angle and strip tension. Ideally, the looper angle needs to keep a desired value during operation to reduce the tension variation and to have the flexibility to absorb large changes in loop length during an abnormal rolling condition. However, if the initial roll speed setting has big error it is very difficult to control the tension of strip head-end part by looper. If strip length between stands increases largely and excessive strip tension generates, operation fault and strip dimension error can occur. The high strip tension between stands induces width shrinkage, thickness reduction and moreover can produce an edge wave on a strip. Fig. 4 and Fig. 5 show thickness and width variations for strip headend part caused by initial roll speed setting error.
Strip U nit Tension(0.001§ ¸ /§ ± )
700 600 500 400 300 200 100 0 100
120
140
160
180
200
Looperangle(0.1¡Æ )
Fig. 3. Looper angle and strip tension
Fig. 4. Strip thickness variation caused by roll speed setting error coolant and scale on a strip. The purge hood box has the following functions. To give the sensor a clean atmosphere in which to work (Air Purging). To keep the sensor at a range of temperature (Water Cooling). To protect the sensor from impact and handling using internal shock mounts. To measure a strip speed, a laser beam is split two beams and projected normally on the strip as described Fig. 7.
Fig. 5. Strip width variation caused by roll speed setting error
Standi+1
Standi
Speed & Looper System F6&F7
Looper
F6&F7
Metal-In
Angle
Mill Speed
3. FORWARD SLIP MEASUREMENT AND PREDICTION Data Processing
Speed Sensor
3.1 Hot Strip Speed Measurement System A hot strip speed measurement system was designed and installed to measure a strip speed and a forward slip. The system as shown in Fig. 6 consists of a speed sensor, a control panel, an operation panel, and a valve stand. This system was designed considering the following installation environment.
& MMI Unit
Operation Panel
Signal Processor
Valve Stand
I/F Unit
Air & Water Supply
Control Panel
Fig. 6. Hot strip speed measurement system
Strip temperature: Max. 1300°C. Vibration: Max. 10G. The space for installation is limited. There are coolant and scale on a strip. There is a considerable amount of steam, spray, and dust. The up and down dynamic range of a strip: ±100 mm Speed sensor; The sensor, where Doppler Effect principle is applied, is placed at the inside of a purge hood box. The purge hood box with the sensor was installed below the side guide between stands to cope easily with some measurement problems caused by
Fig. 7. Principle of speed sensor used for measuring strip speed
Control Panel; The control panel has several units for sensor signal processing, data processing & monitoring, sensor on/off control, and data communication with a finishing mill set-up system, a speed and looper control system, and an AGC (Automatic Gauge Control) system. A signal processor transforms the beat frequency, which is proportional to the strip speed, into a voltage or a current or a digital signal. A data processing unit calculates the strip speed(V1) at the exit of Standi and the strip speed(V2) at the entry of Standi+1 considering the strip speed(Vm) measured by the sensor and a looper angle as follows.
V 2 =Vm / cos(θ ) V 1 = V 2 + dLp / dt
(2)
Where, θ is an angle between strip and the perpendicular line to the sensor and is calculated from the looper angle, LP is the length of strip between stands. A data monitoring system shows a strip speed, stand rolling speeds, and a looper angle in graphical form and sensor status, optical bed temperature, the internal temperature of purge hood box, water flow sensor status, and the reflectance of strip being measured. Operation Panel; The operation panel has the selection buttons of automatic and manual mode, several lamps to represent whether the speed measurement system is abnormal, and the same function as the data monitoring system of the control panel to show particularly an operator a strip speed and stand rolling speeds. A PLC (Programmable Logic Controller) in the control panel controls the on/off operation of the sensor in automatic or manual mode. The sensor is controlled automatically according to finishing mill state when an operator selects automatic mode in the operation panel. Valve Stand; The valve stand is a unit to supply the purge hood box dry air and clean water at a range of temperature. Dry air and clean water are used to give the sensor a clean atmosphere in which to work and to cool the sensor. Measured Strip Speed; To verify the accuracy of the measured strip speed, the following items are considered. Accuracy when the speed of an object rotating at a known speed is measured.
700
600
Roll & Strip Speed(mpm)
A beat frequency is measured, since the scattered light frequencies are too high (about 1014 Hz) to be measured by current electronic capabilities. The beat frequency occurs when two frequencies are superimposed into one receiver. It is equal to the difference between the frequencies. This frequency can be made to occur at a lower frequency (about 103~106 Hz) and is directly proportional to the strip speed.
500
400
300
F6Speed F7Speed Strip Speed
200
100
0 1
501
1001
1501
2001
2501
3001
3501
4001
4501
5001
5501
Time(ms)
Fig. 8. Measured strip speed Physical characteristics that the measured speed must be between the roll speeds of Standi and Standi+1. The speed of the head-end part of the strip calculated manually. Operator’s experience. The strip speed between No.6 and No.7 stands shown in Fig. 8 was measured when looper angle hunting occurred largely and there was a considerable amount of spray. The measured speed is between the roll speeds of No.6 and No.7 stand and is nearly same to the speed calculated for the head-end part of the strip. As a result, it is shown that the accuracy of the measured speed is enough. 3.2 Forward slip Prediction The forward slip (fr) is calculated by equation (3) that defines forward slip.
Vs − Vri fri = i Vri
(3)
Forward slip should accurately be predicted for initial roll speed setting. To do so, an equation for computing forward slip, which has reduction ratio (r) and rolling force per strip width (rfw) as input variables and forward slip as output, was derived as follows.
fr = a0 + a1 * r + a 2 * rfw
(4)
The terms a0 to a3 are parameters. The data set of 3000 coils is analyzed to find out the input variables and used to decide the parameters and the new data set of about 1000 coils for the verification of the equation. Fig. 9 shows the predicted values by the equation (4) against the measured values for selected coils processed at Pohang Works. The dots located on diagonal line correspond to the perfect predictions. The further a dot is from the line, the larger the error is. It is shown that the equation estimates accurately forward slip.
InitialRollSpeed C orrection Ratio(% )
8 Actual
Predicted
6 4 2 0 -2 -4 -6 -8 1
51
101
151
201
251
CoilNumber
Fig. 9. Predicted & measured forward slip
Fig. 11. Initial roll speed correction ratio prediction
4. COMPENSATION FOR INITIAL ROLL SPEED SETTING The initial roll speed is adjusted by a looper control and an operator during milling so that a constant loop angle is maintained and a roll speed in steady state is achieved. Several factors make the deviation of the initial roll speed from a roll speed in steady state. These include forward slip prediction error and the variations of rolling conditions such as the temperature, thickness, and width of a sheet bar, roll eccentricity, wear, thermal expansion, surface roughness, bearing oil film thickness, and mill chatter. Estimating or measuring these factors under continuous rolling is impossible or not easy. Therefore, to cope with the initial speed setting error occurred by the unknown factors, a method to compensate for the initial roll speed setting for next strip using the deviation of the actual looper angle from a reference value for strip head-end part is normally employed. But the performance of the method is not so good because deciding the looper angle error and computing a speed correction from the deviation are not simple. Fig. 10 shows roll speed correction ratio and looper angle for strip head-end part.
Consequently, another compensation method for initial roll speed was proposed to increase setting accuracy. After milling finishes, this method predicts a correction ratio for initial roll speed setting using the actual rolling data of last coil such as looper angle and the output of looper height controller including manual speed correction ratio. The correction ratio (Cr) is defined as:
Cr =
(5)
Vr
Where, Vr = initial roll speed correction value. A model for predicting the correction was developed using the actual data set of about 5000 coils and certified using new data set. Fig.11 shows correction ratio prediction results by the compensation method. 5. CONCLUSIONS In this paper, an equation for predicting forward slip and a compensation method for initial roll speed are proposed to increase initial roll speed setting accuracy for hot strip finishing rolling.
70
Looper Angle for Strip Top Part(¡Æ )
∆Vr
Standi+1
Standi
60 Vsi+1
50
Vsi Looper
40 30
Looper Controller
20
Manual Operation
10
Initial Roll Speed Correction Ratio Learning
0 -8
-6
-4
-2
0
2
4
6
RollSpeed Correction Ratio(% )
Fig. 10. Roll speed correction ratio and looper angle for strip head-end part
Roll Speed Controller
Roll Speed Reference Calculation Initial Roll Speed Setting Compensation Initial Roll Speed Setting
Fig. 12. New roll speed setting & control in a finishing rolling mill
T he In cid en ce o f m in us w id th in strip head en d p art(% )
90
end part before and after the application of the new roll speed setting method.
81.8
80 70 60
REFERENCES
50 40
33.4
30 20 10 0 Before
After
Fig. 13. The incident of width shrinkage in strip head-end part before and after the application of the new roll speed setting method A new speed set-up system, as depicted in Fig. 12, was developed to produce the initial speed settings based on the suggested speed compensation method alongside with the speed settings for all finishing mill stands using the forward slip prediction equation. The system was applied for No.1 hot strip mill at Pohang Steel Works, Pohang, Korea. As the application results, mass flow unbalance between stands was improved and the number of coils having big width error for strip head-end part caused by excessive strip tension was decreased largely. Fig. 13 shows the incident of width shrinkage in strip head-
Hum, B., H.W. Colquhoun and J.G. Lenard (1996). Measurements of friction during hot rolling of aluminum strips. Journal of Materials processing Technology, 60, 331-338. Jarl, M. (1988), Friction and forward slip in hot rolling, Scandinavian Journal of Metallurgy, 17, 2-7. Kwak, W.J., Y.H. Kim, J.H. Lee and S.M. Hwang (2002). A Precision On-line Model for the Prediction of Roll Force and Roll Power in HotStrip Rolling. Metallurgical and Materials Transactions, 33A, 3255-3272. Liu, Y.J., A.K. Tieu, D.D. Wang and W.Y.D. Yuen (2001). Friction measurement in cold rolling, Journal of Materials processing Technology, 111, 142-145. Piispanen, V. (1978), Forward Slip in the Hot Rolling of Sheet. Scandinavian Journal of Metallurgy, 7, 88-90. Yamashita, M., I. Yarita, H. Abe, T. Mikuriya and F. Yanagishima (1987). Technologies of flying gauge change in fully continuous cold rolling mill for thin gauge steel strips. IRSID Rolling Conf, vol. 2, E.36.1-E.36.11.
Abstract: Hot strip finishing mill is initially set up with rolling conditions such as roll speed, roll gap, and cooling pattern obtained by a set-up model. To improve the accuracy of initial roll speed setting for a 7-stand finishing mill, an equation for predicting forward slip and a compensation method for initial roll speed setting were proposed and applied for No.1 Hot Strip Mill at Pohang Steel Works, Pohang, Korea. As the results, mass flow was improved and width error for strip head-end part caused by excessive strip tension was decreased largely. Copyright © 2007 IFAC Keywords: hot rolling mill, roll speed, forward slip.
1. INTRODUCTION The purpose of hot rolling is to turn the slabs of 250 mm thickness produced in a continuous casting process into the strips of thickness 1.0-20 mm. As described in Fig.1, A hot strip rolling mill is composed of a reheating furnace, a roughing mill, a finishing mill, a run-out table, and a coiler. Slabs are heated in the reheating furnace and then rolled on the roughing mill which reduces the thickness of sheet bars to 30-60 mm. after rough rolling; the strip is further rolled by the finishing mill. The rolled strip is cooled down by spraying water in the run-out table and coiled by the down coiler. In addition, the final product may be processed through a cold rolling mill in order to produce thinner steel sheet. This paper focuses exclusively on the finishing mill that consists of six or seven stands. Each stand has two work rolls and two large backup rolls inside a mill housing. The work rolls perform the reduction of the strip under the support of the backup rolls that are pushed down by electrical screws or hydraulic cylinders. The majority of the power to decrease the thickness of the strip comes from one main drive per stand which turns the work rolls, and whose speed is used as a manipulated variable. Loopers are installed between stands to store some strip in order that
disturbance does not cause the strip to become too loose or tight and to measure the variations of the mass flow of the strip. A perfect looper keeps the strip tension constant and hence decouples upstream stands from downstream ones. At each stand and interstand are sensors to measure roll gap, rolling force and looper angle, from which the strip gauge and tension are estimated. Also at the exit of the mill are various sensors to measure the process variables such as thickness, profile, temperature, etc. The process control for finishing rolling is divided into three phases, mill set-up, real-time control, and set-up model learning. Mill set-up presets the initial settings of the mill parameters such as roll gap and roll speed using a mathematical set-up model before a sheet bar (at roughly 1100°C in temperature for mild steel) enters the process. Real-time control takes over once milling starts and includes automatic gauge control, rolling speed control, and looper tension control. The previously set parameter values are dynamically adjusted as the strip thickness, roll gap, rolling force, looper angle are measured during milling. After milling finishes, the prediction errors of the set-up model are used to update the parameters of the model for the next strip in model learning.
Down Coiler
Finishing Mill
Run-out Table
Roughing Mill
Reheating Furnace
Fig. 1. A typical finishing rolling mill The initial rolling conditions are not perfect due to various reasons. If the errors are small, real time controllers can usually compensate, but large errors lead to mass flow unbalance, operation fault, quality decline, and cost increase. Of a particular interest is the precise setting of roll speed, since the high accuracy of the roll speed setting is essential for mass flow continuity and product quality. A roll speed is calculated from the target strip speed at the exit of a rolling stand and a forward slip computed by the setup model to consider the slippage between a strip and a roll. Therefore, the prediction accuracy of the forward slip is very important. In most steel works around the world, the forward slip is usually predicted using mathematical formulas based on the metallurgical and mechanical knowledge accumulated through decades of research and development. The forward slip is influenced by incoming thickness, reduction ratio, friction coefficient, etc (Hum, et al., 1996; Kwak, et al., 2002; Piispanen, 1978). Friction coefficient is known to be affected by working roll parameters such as roll speed, roll surface, lubricant oil (Jarl, 1988; Yamashita, et al., 1987) and the friction coefficient in the roll bite is not uniform (Liu, et al., 2001). Therefore, getting the precise forward slip is not easy. To improve the accuracy of the roll speed setting for a 7-stand finishing mill, a regression equation for predicting forward slip and a compensation method for initial roll speed setting were proposed in this paper and applied for No.1 Hot Strip Mill at Pohang Steel Works, Pohang, Korea. Standi+1
Standi
Vsi+1
Vsi Looper
Looper Controller Manual Operation
Roll Speed Controller
Roll Speed Reference Calculation
Initial Roll Speed Setting
Fig. 2. Roll speed setting & control in a finishing rolling mill
2. ROLL SPEED SET-UP AND CONTROL A roll speed is calculated from target strip thicknesses, the target strip speed at the exit of last stand, and a forward slip computed by the set-up model to consider the slippage between a strip and a roll as follows.
Vri =
Vsl * htl (1 + f i ) * ht i
(1)
Where, Vr = roll speed, Vs = strip speed, ht = target thickness, f = forward slip, the suffix ‘i’ indicates stand number, and the suffix ‘l’ means last stand. As depicted in Fig. 2, the initial roll speed is adjusted by a looper control and an operator during milling. Loopers are installed between two stands to store some strip in order that unexpected rolling condition change does not cause the strip to become too loose or tight, so it can contribute to the quality of products. It can also enable stable operation of the process by absorbing excessive loop of the strip arising from a mass flow unbalance. After the sheet bar has been going through the finishing mill, the looper control operates dynamically to keep a strip tension using the actual angle and motor current of the looper and hence to decouple upstream stands from downstream ones. For example, in the case of low tension, the looper angle increases to get proper tension, resulting in stable threading of a strip, while, in the case of high tension, the looper angle decreases to reduce strip tension. Fig. 2 shows the relation between looper angle and strip tension. Ideally, the looper angle needs to keep a desired value during operation to reduce the tension variation and to have the flexibility to absorb large changes in loop length during an abnormal rolling condition. However, if the initial roll speed setting has big error it is very difficult to control the tension of strip head-end part by looper. If strip length between stands increases largely and excessive strip tension generates, operation fault and strip dimension error can occur. The high strip tension between stands induces width shrinkage, thickness reduction and moreover can produce an edge wave on a strip. Fig. 4 and Fig. 5 show thickness and width variations for strip headend part caused by initial roll speed setting error.
Strip U nit Tension(0.001§ ¸ /§ ± )
700 600 500 400 300 200 100 0 100
120
140
160
180
200
Looperangle(0.1¡Æ )
Fig. 3. Looper angle and strip tension
Fig. 4. Strip thickness variation caused by roll speed setting error coolant and scale on a strip. The purge hood box has the following functions. To give the sensor a clean atmosphere in which to work (Air Purging). To keep the sensor at a range of temperature (Water Cooling). To protect the sensor from impact and handling using internal shock mounts. To measure a strip speed, a laser beam is split two beams and projected normally on the strip as described Fig. 7.
Fig. 5. Strip width variation caused by roll speed setting error
Standi+1
Standi
Speed & Looper System F6&F7
Looper
F6&F7
Metal-In
Angle
Mill Speed
3. FORWARD SLIP MEASUREMENT AND PREDICTION Data Processing
Speed Sensor
3.1 Hot Strip Speed Measurement System A hot strip speed measurement system was designed and installed to measure a strip speed and a forward slip. The system as shown in Fig. 6 consists of a speed sensor, a control panel, an operation panel, and a valve stand. This system was designed considering the following installation environment.
& MMI Unit
Operation Panel
Signal Processor
Valve Stand
I/F Unit
Air & Water Supply
Control Panel
Fig. 6. Hot strip speed measurement system
Strip temperature: Max. 1300°C. Vibration: Max. 10G. The space for installation is limited. There are coolant and scale on a strip. There is a considerable amount of steam, spray, and dust. The up and down dynamic range of a strip: ±100 mm Speed sensor; The sensor, where Doppler Effect principle is applied, is placed at the inside of a purge hood box. The purge hood box with the sensor was installed below the side guide between stands to cope easily with some measurement problems caused by
Fig. 7. Principle of speed sensor used for measuring strip speed
Control Panel; The control panel has several units for sensor signal processing, data processing & monitoring, sensor on/off control, and data communication with a finishing mill set-up system, a speed and looper control system, and an AGC (Automatic Gauge Control) system. A signal processor transforms the beat frequency, which is proportional to the strip speed, into a voltage or a current or a digital signal. A data processing unit calculates the strip speed(V1) at the exit of Standi and the strip speed(V2) at the entry of Standi+1 considering the strip speed(Vm) measured by the sensor and a looper angle as follows.
V 2 =Vm / cos(θ ) V 1 = V 2 + dLp / dt
(2)
Where, θ is an angle between strip and the perpendicular line to the sensor and is calculated from the looper angle, LP is the length of strip between stands. A data monitoring system shows a strip speed, stand rolling speeds, and a looper angle in graphical form and sensor status, optical bed temperature, the internal temperature of purge hood box, water flow sensor status, and the reflectance of strip being measured. Operation Panel; The operation panel has the selection buttons of automatic and manual mode, several lamps to represent whether the speed measurement system is abnormal, and the same function as the data monitoring system of the control panel to show particularly an operator a strip speed and stand rolling speeds. A PLC (Programmable Logic Controller) in the control panel controls the on/off operation of the sensor in automatic or manual mode. The sensor is controlled automatically according to finishing mill state when an operator selects automatic mode in the operation panel. Valve Stand; The valve stand is a unit to supply the purge hood box dry air and clean water at a range of temperature. Dry air and clean water are used to give the sensor a clean atmosphere in which to work and to cool the sensor. Measured Strip Speed; To verify the accuracy of the measured strip speed, the following items are considered. Accuracy when the speed of an object rotating at a known speed is measured.
700
600
Roll & Strip Speed(mpm)
A beat frequency is measured, since the scattered light frequencies are too high (about 1014 Hz) to be measured by current electronic capabilities. The beat frequency occurs when two frequencies are superimposed into one receiver. It is equal to the difference between the frequencies. This frequency can be made to occur at a lower frequency (about 103~106 Hz) and is directly proportional to the strip speed.
500
400
300
F6Speed F7Speed Strip Speed
200
100
0 1
501
1001
1501
2001
2501
3001
3501
4001
4501
5001
5501
Time(ms)
Fig. 8. Measured strip speed Physical characteristics that the measured speed must be between the roll speeds of Standi and Standi+1. The speed of the head-end part of the strip calculated manually. Operator’s experience. The strip speed between No.6 and No.7 stands shown in Fig. 8 was measured when looper angle hunting occurred largely and there was a considerable amount of spray. The measured speed is between the roll speeds of No.6 and No.7 stand and is nearly same to the speed calculated for the head-end part of the strip. As a result, it is shown that the accuracy of the measured speed is enough. 3.2 Forward slip Prediction The forward slip (fr) is calculated by equation (3) that defines forward slip.
Vs − Vri fri = i Vri
(3)
Forward slip should accurately be predicted for initial roll speed setting. To do so, an equation for computing forward slip, which has reduction ratio (r) and rolling force per strip width (rfw) as input variables and forward slip as output, was derived as follows.
fr = a0 + a1 * r + a 2 * rfw
(4)
The terms a0 to a3 are parameters. The data set of 3000 coils is analyzed to find out the input variables and used to decide the parameters and the new data set of about 1000 coils for the verification of the equation. Fig. 9 shows the predicted values by the equation (4) against the measured values for selected coils processed at Pohang Works. The dots located on diagonal line correspond to the perfect predictions. The further a dot is from the line, the larger the error is. It is shown that the equation estimates accurately forward slip.
InitialRollSpeed C orrection Ratio(% )
8 Actual
Predicted
6 4 2 0 -2 -4 -6 -8 1
51
101
151
201
251
CoilNumber
Fig. 9. Predicted & measured forward slip
Fig. 11. Initial roll speed correction ratio prediction
4. COMPENSATION FOR INITIAL ROLL SPEED SETTING The initial roll speed is adjusted by a looper control and an operator during milling so that a constant loop angle is maintained and a roll speed in steady state is achieved. Several factors make the deviation of the initial roll speed from a roll speed in steady state. These include forward slip prediction error and the variations of rolling conditions such as the temperature, thickness, and width of a sheet bar, roll eccentricity, wear, thermal expansion, surface roughness, bearing oil film thickness, and mill chatter. Estimating or measuring these factors under continuous rolling is impossible or not easy. Therefore, to cope with the initial speed setting error occurred by the unknown factors, a method to compensate for the initial roll speed setting for next strip using the deviation of the actual looper angle from a reference value for strip head-end part is normally employed. But the performance of the method is not so good because deciding the looper angle error and computing a speed correction from the deviation are not simple. Fig. 10 shows roll speed correction ratio and looper angle for strip head-end part.
Consequently, another compensation method for initial roll speed was proposed to increase setting accuracy. After milling finishes, this method predicts a correction ratio for initial roll speed setting using the actual rolling data of last coil such as looper angle and the output of looper height controller including manual speed correction ratio. The correction ratio (Cr) is defined as:
Cr =
(5)
Vr
Where, Vr = initial roll speed correction value. A model for predicting the correction was developed using the actual data set of about 5000 coils and certified using new data set. Fig.11 shows correction ratio prediction results by the compensation method. 5. CONCLUSIONS In this paper, an equation for predicting forward slip and a compensation method for initial roll speed are proposed to increase initial roll speed setting accuracy for hot strip finishing rolling.
70
Looper Angle for Strip Top Part(¡Æ )
∆Vr
Standi+1
Standi
60 Vsi+1
50
Vsi Looper
40 30
Looper Controller
20
Manual Operation
10
Initial Roll Speed Correction Ratio Learning
0 -8
-6
-4
-2
0
2
4
6
RollSpeed Correction Ratio(% )
Fig. 10. Roll speed correction ratio and looper angle for strip head-end part
Roll Speed Controller
Roll Speed Reference Calculation Initial Roll Speed Setting Compensation Initial Roll Speed Setting
Fig. 12. New roll speed setting & control in a finishing rolling mill
T he In cid en ce o f m in us w id th in strip head en d p art(% )
90
end part before and after the application of the new roll speed setting method.
81.8
80 70 60
REFERENCES
50 40
33.4
30 20 10 0 Before
After
Fig. 13. The incident of width shrinkage in strip head-end part before and after the application of the new roll speed setting method A new speed set-up system, as depicted in Fig. 12, was developed to produce the initial speed settings based on the suggested speed compensation method alongside with the speed settings for all finishing mill stands using the forward slip prediction equation. The system was applied for No.1 hot strip mill at Pohang Steel Works, Pohang, Korea. As the application results, mass flow unbalance between stands was improved and the number of coils having big width error for strip head-end part caused by excessive strip tension was decreased largely. Fig. 13 shows the incident of width shrinkage in strip head-
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