1.What are the typical characteristics and main causes of longitudinal thickness difference (same plate difference)?
The thickness of the strip steel varies along its length, exhibiting periodic or random fluctuations.
Roll eccentricity, rolling speed/tension variations, and uneven material hardness and thickness.
2.What are the typical characteristics and main causes of deviations at the beginning and end of a test?
The thickness of the steel coil exceeds the allowable tolerance within a range of several tens of meters at the head and tail, while the thickness in the middle section is normal.
Process instability during the rolling acceleration and deceleration stages, fluctuations during tension build-up and release, and uneven properties of hot-rolled incoming materials at the beginning and end.
3.How to optimize upstream processes and model compensation?
Hot-rolled incoming material performance control: Research has found that uneven cooling during the laminar cooling stage of hot-rolled coils leads to fluctuations in their length-direction properties, which in turn causes thickness deviations in cold-rolled coils. By controlling the hot-rolled sheet shape, improving laminar cooling purging capacity, and adopting U-shaped cooling, the performance uniformity of hot-rolled coils can be significantly improved. One factory, for example, reduced the length of cold-rolled coils with excessive thickness from 41 meters to less than 10 meters.
Performance feedforward control: Modern AGC systems can pre-calculate and adjust the roll gap value based on detected fluctuations in incoming material hardness. This is called feedforward AGC (FF-AGC), which effectively counteracts the impact of changes in incoming material properties on the exit thickness.
4.How to precisely adjust the core parameters?
Dynamic roll gap adjustment: This is the core of thickness control.
Feedback AGC: Thickness deviation is detected by an exit thickness gauge and fed back to the hydraulic pressing system to adjust the roll gap. However, due to the distance of the thickness gauge from the roll gap, there is a certain lag.
Second-flow AGC: This is a more advanced control method. It utilizes an inlet thickness gauge, laser velocimeter, etc., based on the principle of "equal second-flow" (inlet thickness × inlet speed = outlet thickness × outlet speed) to calculate and control the current outlet thickness in real time without lag, resulting in higher accuracy. After adopting full-stand second-flow control, a steel plant reduced the thickness fluctuation length at the strip head from 30 meters to within 10 meters.
Tension optimization control: Tension fluctuations directly affect thickness. When rolling difficult-to-roll materials such as ultra-high-strength steel, developing a tension optimization model, especially optimizing tension settings during the beginning and end rolling processes, can effectively reduce the thickness deviation length.
5.How to optimize the control of the beginning and end of the acceleration and deceleration phases?
Acceleration/Deceleration Compensation: During the speed-up and deceleration phases of the rolling mill, thickness fluctuations are likely due to changes in lubrication conditions and friction coefficients. The acceleration/deceleration compensation function in the AGC system can pre-adjust the roll gap to compensate for these changes.
Head and Tail Control Strategy: During strip rolling, thickness control is difficult due to the loss of tension at both ends. By establishing a roll gap adjustment prediction model and optimizing the rolling speed and tension at both ends, the out-of-tolerance length at both ends can be effectively reduced.