Overall thickness deviation at both ends? Thermal rolling AGC system commissioning + process parameter optimization ideas
In the production of hot-rolled strip steel, the thickness deviation at the head and tail is a common and long-standing problem that has plagued the frontline workers - during the stages of head threading and tail steel throwing, the thickness deviation far exceeds the internal control requirements. This can lead to increased head and tail cutting losses and a decrease in yield, and in severe cases, it can cause tail slippage and steel pile-up faults. Many teams will only repeatedly adjust the AGC gain parameters, but the result is either an intensified oscillation or a treatment that addresses the symptoms but not the root cause.
In fact, the thickness deviation at the head and tail is not a problem of a single equipment, but a comprehensive result of the mismatch between the response characteristics of the AGC system and the non-steady-state rolling process. This article takes a dual-dimensional approach of system debugging and process optimization, dissects the underlying logic of the deviation, and provides direct and implementable debugging solutions and parameter optimization ideas to help you precisely narrow the tolerance of the head and tail thickness.
1. First, understand why the thickness at the head and tail is most prone to deviation:
The head and tail section belongs to a typical non-steady-state rolling interval, with much greater fluctuation in the working conditions than the steady-state rolling section, making it a natural weak link in thickness control. The core causes are divided into two levels: system and process.
System dynamic fluctuations, unstable control foundation
During the threading and steel throwing stages, the rolling speed, tension, and steel temperature are all in a dynamic state of change. The fluctuation amplitude of the rolling force can reach more than 20% of the steady-state value. The AGC system is in a continuous adjustment transition state and is difficult to quickly converge to the target thickness.
There is inherent lag in control and compensation
The detection signal of the exit thickness gauge has a tracking lag. The thermal expansion of the rolls, oil film thickness, eccentricity, etc., are compensated based on the steady-state working conditions, and the adaptability at the head and tail stage is poor, making the deviation prone to continuous accumulation.
Specific triggering factors
System side: Inappropriate AGC parameter setting, imbalance in response speed and stability; large synchronous error in thickness tracking; inaccurate compensation model; lag in hydraulic system response.
Process side: Excessive temperature drop at the head and tail leads to fluctuation in deformation resistance; excessive acceleration during threading causes a sudden change in rolling force; poor adaptability of the lowering procedure; fluctuations in live roll tension interfere with the stability of the roll gap.
2. Implementation measures one: Precise AGC system debugging plan
1. Segmented matching control mode, adapted to different rolling stages
Head threading stage:
: Prioritize the use of
position closed-loop + feedforward AGC
as the main mode, with weak monitoring AGC input. Avoid overshoot caused by thickness feedback lag. After the strip passes the exit thickness gauge and the rolling force stabilizes, smoothly switch to the rolling force closed-loop + full monitoring AGC mode.
Tail steel throwing stage:
: Reduce the monitoring AGC adjustment intensity in advance and switch to the position hold mode to avoid sudden changes in rolling force after the tail loses tension, causing significant fluctuations in the roll gap and reducing tail thickness drift.
2. Segmented parameter setting, balancing response and stability
For the transitional section of the head, appropriately reduce the AGC proportional gain and extend the integral time to suppress the thickness oscillation caused by the threading impact; restore high gain parameters in the steady-state section to ensure thickness control accuracy.
Set graded AGC dead zone and adjustment limit: Appropriately relax the dead zone threshold at the head and tail stages to limit the maximum single adjustment amount of the roll gap to avoid repeated oscillations in thickness caused by a single excessive adjustment.
3. Special calibration of core compensation items
Oil film thickness compensation:
: Establish a compensation table for oil film thickness under different rotational speeds and rolling forces, and separately correct the compensation coefficient for the low-speed threading stage to solve the problem of excessive thickness at the head caused by low-speed oil film thickness.
Roller eccentricity compensation:
: Complete the eccentricity test and calibration after changing the roll, and force the eccentricity compensation to be input at the head and tail stages to counteract the periodic thickness fluctuations caused by the eccentricity of the supporting roll.
Hot roll diameter compensation:
: In the initial stage of head rolling, set the compensation value based on the cold roll diameter, and adjust the thermal expansion amount in real time according to the rolling passes to avoid gradual thinning of the head thickness due to cumulative thermal expansion.
4. Synchronization optimization of thickness tracking and thickness gauge
Note: The above translation is based on the provided text and may not capture all nuances of the original Chinese. Calibrate the tracking delay time of the thickness measuring instrument at the machine frame outlet, dynamically correct the lag quantity according to the rolling speed, ensure that the AGC adjustment timing precisely matches the actual position of the strip steel, and avoid lagging or leading in the adjustment.
Regularly calibrate the zero point of the thickness measuring instrument to eliminate detection drift and avoid error signals misleading the AGC adjustment.
III. Grounding Measures II: Process Parameter Collaborative Optimization Thought
1. Smooth Control of Spreading Speed and Acceleration
Reduce the initial speed of spreading, set a stepped acceleration curve to avoid excessive acceleration causing a sudden increase in rolling force and passive narrowing of the roll gap; after the head thickness is stable, gradually increase to the target rolling speed.
The tail steel throwing should be smoothly decelerated in advance to avoid sudden speed drop causing tension and rolling force fluctuations.
2. Adaptation Optimization of Head and Tail Reduction Procedures
Follow the principle of "light pressing at the head, gradual loading", reduce the head reduction rate before the final rolling by 5%-8% compared to the steady state, reduce the impact of head deformation resistance, and then restore the target reduction rate after the rolling force stabilizes.
For difficult-to-roll varieties such as thick specifications and high-strength steels, formulate separate head and tail reduction procedures, further reduce the single-pass reduction amount, and alleviate the pressure of AGC adjustment.
3. Temperature Compensation Control of Head and Tail
Due to the large temperature drop and high deformation resistance at the head and tail, preset the temperature compensation coefficient in the secondary model, correct the roll gap setting value in advance, and offset the thickness deviation caused by temperature drop.
Optimize the descaling rhythm at the entrance of the fine rolling, avoid excessive descaling at the head causing excessive temperature drop, and stabilize the head rolling temperature.
4. Stable Control of Live Roll Tension
Optimize the control curves for live roll take-up and release, reduce the fluctuation amplitude of live roll tension in the head and tail stages, avoid tension changes causing thickness fluctuations and deviation of the strip steel.
During the head spreading stage, use low tension spreading, and gradually increase to the target tension after stabilization, reducing the interference of tension changes on the roll gap.
IV. 90% of the teams have fallen into the following AGC debugging misunderstandings:
Misunderstanding: The greater the AGC gain, the higher the thickness accuracy
Correct solution: An excessively high gain will trigger system oscillation, and the fluctuations in the head and tail stages will be even greater, resulting in the "the more you adjust, the worse it gets" situation; the gain parameters must be matched according to the rolling stage to balance response speed and stability.
Misunderstanding: Head and tail out-of-tolerance is a problem of the AGC system, only adjusting the equipment without considering the process
Correct solution: More than half of the head and tail out-of-tolerance problems are caused by process condition fluctuations. Simply adjusting the equipment does not address the root cause; the process parameters and system control must be coordinated for optimization to converge deviations from the root.
Misunderstanding: Use the same compensation parameters for all steel types and specifications
Correct solution: Different steel types, specifications, and speeds have greatly different oil film thickness, thermal expansion, and deformation resistance. General parameters will inevitably lead to poor head and tail compatibility; parameters must be established separately for each steel type and specification.
Misunderstanding: Ignore the hydraulic system status and only adjust the electrical parameters
Correct solution: Problems such as hydraulic oil temperature fluctuations, servo valve sticking, and oil cylinder internal leakage will directly reduce the response speed and accuracy of AGC; no matter how good the parameters are, they will be ineffective. It is necessary to first ensure the stability of the hydraulic system status.
V. Emergency and Rapid Disposal for Head and Tail Thickness Out-of-Tolerance
Minor deviation (out-of-tolerance ≤ 0.05mm, stable fluctuation)
: Appropriately adjust the AGC monitoring gain, cooperate with the correction of the roll gap compensation amount, observe the convergence of the deviation after continuous rolling for 3-5 rolls, and no major changes are required.
Moderate deviation (out-of-tolerance 0.05-0.1mm, periodic fluctuation)
: Immediately check the eccentric compensation of the rolls, oil film compensation, whether they are normal, simultaneously reduce the spreading acceleration, appropriately lower the head reduction rate, stabilize the rolling condition.
Severe deviation (out-of-tolerance > 0.1mm, violent oscillation)
: Suspend acceleration, switch to manual roll gap control mode, check whether the hydraulic system pressure and thickness measuring instrument signal are abnormal, eliminate equipment faults, re-calibrate the AGC zero position, and gradually resume production. Summary
The control of the head and tail thickness of hot-rolled strip steel essentially involves "dynamic balance control under non-steady-state conditions". Simply relying on the AGC system to repeatedly adjust parameters is not sufficient to solve the problem completely. It is necessary to achieve the following:
- Fine-tuning of system debugging
- Adaptation of process parameters
- Hierarchical design of compensation models 。 From control mode matching, parameter segmented tuning, to process coordination of pressing procedures, speed and tension, multi-dimensional synchronous optimization is necessary to effectively narrow the tolerance of head and tail thickness, reduce cutting losses, and significantly improve the yield rate and product qualification rate.
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