Abstract: The coil shape of hot-rolled strip steel is an important indicator of product quality. Tension control directly affects the coil shape, and the control of the work roller is a fundamental element of coiling and shaping. This article presents a practical and simple coil tension control model based on actual conditions, and a precise automatic step control (AJC) design for the work roller. It has significant reference value for the design and application of coiling machines. Keywords : tension control, AJC 1 Overview Laiwu Steel's 1500mm hot-rolled strip steel production line has an annual output of 3 million tons. Its production process includes heating, rough rolling, hot coiling, finish rolling, layer cooling, and coiling. The coiling area consists of two underground coiling machines. When the strip head leaves the finishing mill, the coiler is already in preparation. As the strip enters the coiler, the tension rollers, aided by guide plates, create a closed path between the tension rollers and the coil drum, allowing the strip to be smoothly wound onto the drum. During the first few turns of the strip onto the drum, the auxiliary coiling rollers provide step control. After 3-5 turns, a stable tension is established between the strip and the mill. At this point, the upper tension roller relaxes, the drive motor uses "zero current" control, and all auxiliary coiling rollers open (when coiling thick strip, the first auxiliary coiling roller must always press down on the strip), entering normal coiling mode. As the strip tail is about to leave the mill, the coiler enters the winding mode, both the mill and coiler simultaneously reduce speed, and the auxiliary coiling rollers close, pressing down on the outer layer of the coil. When the strip leaves the last stand, the tension rollers tighten, the drive motor operates in generator mode, establishing tension between the tension rollers and the drum, preventing the strip tail from deviating or the outer layer of the coil from becoming loose. 2. Design of Coiling Tension Control Model The effectiveness of coiling tension control directly affects the finished product and output of the 1500mm hot-rolled strip steel hot continuous rolling production line. The purpose of tension control is to ensure that the strip tension on coiler 1 or 2 remains constant at the set value during normal coiling. To maintain constant strip tension on the coiler, the tension control mainly considers tension torque, bending torque, friction torque, rotational inertia compensation varying with coil diameter, and dynamic compensation torque during acceleration and deceleration. In actual control, the coiler has two operating states: speed control state and tension control state. During jogging and threading, it operates in speed control state; during threading, once the coiling tension is established, it operates in tension control state. The switch from speed control state to tension control state is automatic under switching logic control. After switching to tension control state, although the coiler speed setpoint is slightly higher than the finishing mill speed reference setpoint. Because the finishing mill and the coiler are rigidly connected by strip steel, the actual speed of the coiler cannot reach the set speed value. The output of the coiler's speed regulator saturates, and the transmission system switches to tension control. Since the speed regulator's output reaches its limit, the tension setpoint is determined by calculations of tension torque, friction torque, bending torque, and acceleration/deceleration torque. 2.1 Coil Diameter Calculation: The coil diameter on the coiler can be calculated using the count value of a pulse generator installed on the coiler and its corresponding speed measuring roller. The calculation method involves defining a rotation angle αh on the coiler drum and measuring the strip length corresponding to that angle to calculate the coil diameter. The diameter of the steel coil is calculated using the following formula: D = 2L / αh Where: αh represents the rotation angle on the coiler drum (in radians), D represents the diameter of the steel coil, and L represents the strip length corresponding to αh. The calculation result of each sample of the steel coil diameter on the coiler needs to be verified. For the coiler, this means that each calculation result must be greater than the previous calculation value. In addition, necessary filtering methods must be adopted to filter out erroneous calculation values ​​caused by incorrect sampling values. 2.2 Calculation of Tension Torque The formula for calculating the strip tension torque is as follows: Mz = F * D / 2i Where: Mz represents the tension torque; F represents the tension setpoint; i represents the transmission ratio. The strip tension setpoint is automatically preset by the operating station, and can also be modified by the operator. 2.3 Calculation of Bending Torque The strip bending torque is calculated using the following formula: Where: Mb represents the bending torque; h represents the strip thickness; δy represents the yield coefficient; L represents the strip width. 2.4 Calculation of Acceleration Torque The acceleration torque of the coiler motor is calculated from parameters such as the moment of inertia referred to the motor shaft and the strip line acceleration. The moment of inertia includes a fixed part and a variable part related to the coil specifications. The fixed part consists of the moment of inertia of the coiler transmission mechanism, drum, etc. The variable part is determined by parameters such as the coil diameter, width, and specific gravity of the steel. The formula for calculating the acceleration torque, determined by changes in linear velocity and coil diameter, is as follows: Where: K1 represents the moment of inertia coefficient of the fixed part; K2 and K3 represent the moment of inertia coefficients of the varying part depending on the coil diameter; L represents the strip width; D represents the coil diameter; dv/dt represents the acceleration coefficient. K1, K2, and K3 can be adjusted during unit commissioning and finalized. The moment of inertia of the coil is calculated using the following formula: Where: B represents the strip width; i represents the reduction ratio; ρ represents the specific gravity of the steel; D0 represents the expansion diameter of the drum; K represents the correction factor for moment of inertia calculation. 2.5 Calculation of Friction Torque: The friction torque compensation is a nonlinear function of velocity. This function will be determined based on actual tests during the commissioning phase, according to design experience and convention. 3. Auxiliary Roller Step Control The simplified structure of the coiler is shown in Figure 1: [align=center] Figure 1 Simplified Structure of the Coiler[/align] The coiler has three auxiliary rolls and an arc-shaped skirt guide plate. Their function is to guide the strip head to bend and shape it, and to coil it tightly onto the drum. It is a solid body with a wear-resistant surface. The radial closing and opening of each auxiliary roll is achieved by a motor, a gear machine (speed ratio i=1), and a directional connecting shaft. Each auxiliary roll is required to press the steel coil with the same pressure (the clamping force is generated by a hydraulic cylinder). The auxiliary rolls and skirt guide plates are replaceable wear-resistant parts. The roll gap between the auxiliary rolls and the drum is adjustable, and the size of the roll gap δ directly affects the coiling quality. If the δ value is too large, the steel coil is not easy to coil tightly, and the first few turns are prone to slippage; if the δ value is too small, the strip will impact when biting in, causing the auxiliary rolls to jump. Similarly, the steel coil is not easy to coil tightly, and may even slip. Typically, the coiler determines the roll gap δ value based on the strip thickness, material, and pressure of the auxiliary coiling rolls, set by a computer. The auxiliary coiling rolls have functions for controlling roll rotation speed, adjusting the position of the hydraulic cylinders, and controlling pressure. 3.1 The coiler's step control function: The automatic step-skipping control function relies on a highly responsive hydraulic servo control system for the radial motion control of the auxiliary coiling rolls, as well as a precise strip head detection device and an accurate mathematical model. The basic principle of automatic step-skipping control is: automatically tracking and calculating the strip position and sending signals to the electrical control system. Each auxiliary coiling roll is equipped with a position and pressure controller. After the strip coiling process begins, whenever the strip head rotates to a position very close to any auxiliary coiling roll, that auxiliary coiling roll quickly lifts up, disengaging from the strip; and when the strip head passes the auxiliary coiling roll, the auxiliary coiling roll quickly returns to press the strip on the drum and operates according to pressure control. This process continues until all auxiliary coiling rolls are opened after several coiling turns. A good step control system should minimize the time the strip head separates from the coiling rollers while ensuring the strip head does not collide with the coiling rollers, thus preventing damage to the coil shape. When the three coiling rollers jump, two are always under pressure control to prevent the coil from loosening. For safety, the jump amount of the coiling rollers is slightly greater than the strip thickness. 3.2 AJC Function Main Control Contents Both No. 1 and No. 2 coilers use a three-coiler fully hydraulic underground horizontal coiler, resulting in good coil quality. The three coiling rollers have a hydraulic step control function (AJC). Automatic step control is a new control function in modern hot strip mills, aiming to minimize indentations caused by the strip head colliding with the coiling rollers during coiling. This damage to the strip surface quality is more pronounced when the strip is thicker. Figure 2 shows the AJC control system: [align=center] Figure 2: Schematic diagram of AJC control system[/align] 4 Conclusions This system has a low failure rate and stable control due to the adoption of excellent tension control and AJC control model. The steel coil shape is good and the product quality is high, which meets market demand and creates huge economic benefits. It is a mature coil control scheme that can be promoted and applied in similar systems. References: [1] Peng Jian. Research on asymmetric cross rolling [D]. Beijing: Tsinghua University, 1990. [2] Lu Binglin. Technology of asymmetric cross control of roll shape [J]. Steel Rolling, 1994 Special Issue: 356-365. [3] Lu Binglin. Cross angle control model of asymmetric cross rolling [J]. Iron and Steel, 1996, 31(2): 30-33.