Foreword
The bending roll technology of medium and heavy plate rolling mills is a technique that uses mechanical force to bend the roll body to control the crown and straightness of the strip. It is usually powered by hydraulic pressure and is also called hydraulic bending roll. It uses a hydraulic cylinder to apply additional bending force to the work roll or support roll, causing the roll to deflect additionally to compensate for the original roll crown and ensure the strip shape.
1. Bending Roller Control Method
Bending roll control methods mainly include two basic forms: bending the work roll (positive bending roll, negative bending roll) and bending the support roll. When the ratio of the work roll body length L to the diameter D, L/D < 3.5, the work roll bending method is used; when L/D ≥ 3.5, the support roll bending method is usually used. Both bending roll methods have positive bending and negative bending.
Positive bending refers to the bending direction of the roll caused by the bending force being opposite to the bending direction caused by the rolling force; that is, the crown of the work roll increases during bending. Negative bending, on the other hand, refers to the bending direction of the roll caused by the bending force being the same as the bending direction caused by the rolling force; that is, the crown of the work roll decreases during bending. Hydraulic roll bending devices can quickly adjust the roll profile with a large adjustment range.
Hydraulic roll bending devices not only reduce the number of roll grinding cycles but also the number of roll changes, thus improving mill operating rates. Combined with a computer, it can effectively control sheet shape. The disadvantage of hydraulic roll bending is that it adds additional loads to the mill, bearings, and the rolls themselves, thereby affecting the full utilization of the mill's capacity. (As shown in Figure 1:)
(Figure 1: Positive and negative bending of the bending roller)
2. Functional design of bending roller control
The bending roll system is controlled by four bending roll controllers, forming an independent control unit within the automated control system. Each bending roll controller includes the work roll bending control loop, a signal processor, and output unit components. Its main functions include: controller calculation, controller mode selection, controller monitoring, digital filtering, and servo valve output. Auxiliary functions include: actual value adjustment and servo valve monitoring. The structural diagram of the bending roll control is shown in Figure 2.
(Figure 2: Structural diagram of bending roller control)
2.1 Equipment Composition:
Each stand's bending roll system comprises 16 hydraulic cylinders. Eight are located at the outlet and inlet on the drive side (DS, two for the upper inlet, two for the upper outlet, two for the lower inlet, and two for the lower outlet work roll bearing housings). Thus, the bending roll/balancing force for each roll bearing housing requires four hydraulic cylinders to generate. Furthermore, the offset of work roll movement caused by the asymmetry of the resultant bending roll force at the mill stand centerline must be considered; therefore, the bending roll hydraulic cylinders must be supplied with a variable pressure that corresponds to the amount of movement.
To achieve the adjustment of the bending roller force, the upper inlet/outlet on the outer drive side, the lower inlet/outlet on the inner drive side, the lower inlet/outlet on the inner operating side, and the upper inlet/outlet on the outer operating side of the piston side are controlled by individual servo valves.
The emergency balancing controller is installed parallel to the servo circuit. Its control includes a single solenoid valve that supplies power to the piston side of the hydraulic cylinder. The circuit is typically installed to operate during the bending operation of the work roll.
2.2 Control Algorithm
To ensure the bending of the work rolls throughout the entire operating range, a closed-loop rolling force controller is provided, connected to each servo loop. The controller's output, specified only by a PI controller, forms the drive signal for the servo valves. An input is established that differentiates the actual value from the rolling force reference value. The bending roll rolling force reference value is controlled by a ramp generator before being fed back to the control loop. The ramp is set according to the permissible hydraulic cylinder speed.
When the steel sheet is on the stand (negative bending), the bending roll force under the balancing force should be reduced first. Before the tail of the steel sheet leaves the previous stand, the rolling force must be increased to reach the balancing force. The minimum rolling force is limited to 4MN per roll neck.
Regarding the axial force displacement control of the four sets of hydraulic cylinders, the individual bending force of each roller is determined by decomposing the total bending force based on the displacement position. These individual bending forces are used to determine the pressure of the hydraulic cylinder.
2.3 Calculation of Optimal Bending Roller Force
Before rolling steel plates, the bending roll force value required to obtain good plate shape or transverse thickness accuracy is calculated in advance based on the incoming material, width, thickness, original crown of the billet, reduction, rolling pressure, and original parameters of the rolling mill. This value is called the optimal bending roll force.
The optimal bending roll force is calculated based on the strip shape prediction model, and the bending roll device is set in the corresponding position to ensure that the strip achieves a good strip shape and minimal camber after passing through the mill. This is the optimal bending roll force preset control on the strip mill. The preset calculation of the optimal bending roll force is complex and is generally executed by a computer central processing unit program. Because the system has a fast response speed, it can calculate the optimal bending roll force value at every instant by continuously measuring the slab thickness and camber, and correct the bending roll force value at any time. Therefore, it can be used for online real-time control.
2.4 Bending Roller Controller Mode Selection
The operating mode of the bending roll controller is determined by various control signals and the current frame status. The operating mode is specified by the frame controller and drive coordination control. The following modes are available:
(1) Bending rolls during frame calibration
(2) Bending rolls during rolling operation
(3) Repositioning of bending roller value
(4) Balance bending roller
(5) Positive bending (lifting roller) during roller changing and maintenance operations.
(6) Negative bending (lowering the roll) during roll changing and maintenance operations
(7) Stop exercising
2.5 Controller Monitoring
To ensure the safe operation of the controller, the following variables are checked according to the corresponding limits: bending roll force, change in bending roll force for flatness control (last pass), valve current, etc. If the limits are exceeded, the corresponding limit value will be displayed and a signal will be generated. This ensures the safe use of the work roll.
2.6 Setpoint Assignment Management
The bending roller setpoint is set by the process computer/human-machine interface (HMI). To avoid changes during operation mode transitions, a ramp setpoint can be used. Set the current actual value as the "stop movement" setpoint.
To ensure roll contact during rolling, the minimum roll bending limit is determined by rolling force, roll weight, acceleration mode, etc., and the controller's roll bending setting applies to the current steel plate condition. For open roll gaps, the roll bending system is limited by a calculated balance value. This limit can only be used remotely during roll changes or during maintenance operations to reduce the rolling force on the rolls.
The setpoint for positive or negative bends can be specified from the process computer or manually. The setpoints in the main program counter include the following:
(1) Fixed setting points, such as the bending roller force during frame calibration.
(2) Variable setpoints of the process computer during rolling
(3) Manually set points for human intervention
(4) Supplementary setting points for flatness control
3. Conclusion
The bending roll control function of medium and heavy plate rolling mills greatly improves the plate shape and reduces the transverse thickness difference, ensuring the flatness and shape quality of the steel plate. It has good application prospects in both hot rolling mills and cold rolling mills.
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