After changing the rolls, the first coil of steel frequently exceeds the tolerance limits? Practical operation plan for roll gap calibration and hot roll diameter compensation
Roll change is the most common operation in the hot rolling workshop, but many teams have encountered the same common problem: after the new working rolls are put into operation, the thickness of the first few rolls or even the first 3-5 rolls frequently exceeds the tolerance limits and the plate shape fluctuates greatly. Either the rolls need to be downgraded and rejudged, or they have to be manually adjusted repeatedly, which not only lowers the yield rate but also disrupts the production rhythm. Field statistics show that the thickness qualification rate of the first 50 rolls after the new rolls are put into operation is only about 82%, which is a high-risk range for thickness scrap.
Many teams respond to problems by first thinking that the grinding accuracy of the rolls is poor. In fact, in most cases, the root cause lies in the improper calibration of the roll gap, inaccurate thermal roll diameter compensation, and non-standard non-steady-state process for the first roll. This article starts from on-site practical operations, dissects the core causes of thickness deviation, provides standardized calibration procedures, parameter optimization plans, and rapid handling techniques, and hands you a step-by-step guide to improve the hit rate of the first roll after roll change.
I. First, understand: The 4 core causes of thickness deviation in the first roll after roll change
1. Zero position calibration deviation of the roll gap, mechanical clearance not eliminated
If the roll gap calibration process is just a formality after roll change, the return stroke difference of the hydraulic cylinder, the gap of the bearing seat, and the wear amount of the liner plate cannot be completely eliminated. The actual roll gap will have a deviation of several tens of micrometers from the set value. Especially when the wear of the bearing seat liner plate of the supporting roll exceeds 0.5mm, it will directly cause the height of the rolling line to shift, resulting in periodic thickness fluctuations and wedge-shaped differences. This is a common equipment hazard that many teams easily overlook. In addition, the synchronization of the pressing systems on both sides is poor, and the deviation of the rolling force exceeds the standard, which can also cause the roll gap to be unevenly distributed, with thickness deviation accompanied by belt deviation.
2. Inaccurate thermal roll diameter compensation, insufficient thermal expansion effect estimation
After completing the calibration in the cold state, the roll surface temperature rapidly increases during the formal rolling process, causing the roll to expand thermally and the roll gap to be passively narrowed. If the parameters are still set according to the cold roll parameters, a continuous drift of thinner thickness will occur. Most production lines only use a fixed compensation value and do not distinguish the thermal expansion rates in the initial stage, heating stage, and steady state stage of the cold roll, nor do they match the thermal load differences of different steel grades and specifications, resulting in either insufficient or excessive compensation for the first roll.
3. Model self-learning parameters not reset, old roll data interfering with new roll
The AGC thickness control model relies on the self-learning coefficients of rolling force and roll gap for iterative optimization. The self-learning data from the previous set of rolls, which have been worn, is completely incompatible with the initial roll shape and roll diameter of the new roll. If the historical short-term self-learning coefficients are not cleared and the parameters are not initialized according to the steel grade and specification classification after roll change, it will directly cause a deviation in the setting of the rolling force in the model, resulting in thickness deviation at the head.
4. Non-steady-state conditions superimposed, control accuracy decreases
The first roll rolling is a typical non-steady-state condition: the roll temperature rises rapidly from room temperature, the thickness of the bearing oil film has not reached a stable value, and the rolling speed increases rapidly from the belt speed. Under the combined effect of multiple factors, the AGC system responds with a delay, and the closed-loop control accuracy of thickness significantly decreases. If you directly roll at full speed and with a large reduction during this period, the probability of deviation will increase exponentially.
II. Implementation measures 1: Standardized roll gap calibration process after roll change
Roll gap calibration is the basis for thickness control. It must be achieved through a three-step closed-loop process of "zeroing the mechanical clearance, ensuring synchronization on both sides, and manual verification": it is prohibited to directly start rolling after only performing automatic calibration.
1. Pre-check of equipment status
First, check the wear amount of the working roll and the bearing seat liner plate of the supporting roll. If it exceeds 0.5mm, replace it immediately to ensure the precise height of the rolling line; clean the roll surface and the window of the frame to avoid foreign objects causing calibration deviation.
Confirm that the oil temperature of the hydraulic system is stable within the range of 40-50℃. Oil temperature fluctuations exceeding ±5℃ will cause servo valve response lag, which directly affects the calibration accuracy.
2. Standard AGC calibration steps
Initial reset: fully open the roll gap, clear the rolling force and position sensors, and eliminate the zero-point drift of the system. Stepped pressure application: Apply pressure gradually, first to a minimum rolling force of 200t to ensure uniform contact on both sides; then gradually increase the force to the calibrated rolling force (for conventional production lines, 2000t level), maintaining a stable pressure.
Eccentric measurement: Start the main drive in low speed operation, allowing the supporting rolls to rotate for more than 2 full turns. Collect rolling force fluctuation data throughout the process, record the eccentricity of the rolls and write it into the compensation parameters.
Zero point locking: After reducing the pressure to the calibrated pressure value and stabilizing it, lock the zero point of the roll gap, simultaneously record the reference values of the lowering positions on both sides to ensure that the deviation of the rolling force on the operating side and the transmission side is less than 245kN.
3. Manual review and verification
After automatic calibration is completed, it is necessary to measure the roll gap values at three points in the middle of the roll body and on both sides using a roll gap instrument. If the deviation of the three points exceeds ±0.05mm, re-calibrate; at the same time, manually rotate the rolls to confirm there is no jamming or uneven loading, and upload the calibration data to the secondary model to update the benchmark.
III. Implementation Measures Two: Precise Optimization of Hot Roll Diameter Compensation
Roll diameter expansion is a dynamic process. Compensation must be set in stages and regions, rather than a single fixed value. The core is to make the roll gap setting value match the real-time changes in roll diameter.
1. Staged Gradient Compensation
During the initial stage of cold rolls (the first 1-3 rolls): Pre-set the thermal expansion compensation amount in advance. For conventional plain carbon steel hot rolled rolls, set the pre-compensation amount according to 0.05-0.1mm of the roll diameter to offset the thermal expansion caused by the initial rapid heating.
During the temperature rise transition stage (rolls 4-10): Dynamically adjust the compensation coefficient based on the real-time collected roll surface temperature. For every 10℃ increase in roll surface temperature, the roll diameter compensation amount increases by 20-30μm, gradually approaching the steady state value.
During the steady-state rolling stage: Switch to real-time closed-loop compensation of the hot roll diameter, combined with the rolling rhythm and cooling water volume to calculate the thermal expansion amount, continuously correcting the roll gap setting.
2. Horizontal Zone Compensation
Due to the higher temperature in the middle of the roll body and faster heat dissipation at the edges, set zone roll diameter compensation. The compensation amount in the middle is 15%-20% higher than that at the edges to avoid lateral thickness differences and simultaneous plate shape and wave form deviations.
3. Roll Temperature Stopping Compensation Correction
If the rolls are replaced and the material is waiting or the temperature is maintained for more than 10 minutes, the roll temperature drops significantly. It is necessary to re-measure the roll temperature and reduce the roll diameter compensation amount; when restarting the rolling, execute the staged compensation again according to the initial stage of cold rolls, and prohibit directly using the steady-state parameters.
IV. Implementation Measures Three: Optimization of the First Roll Rolling Process Specifications
The first roll belongs to non-steady-state rolling and cannot directly apply steady-state process parameters. It is necessary to adapt and optimize the speed, reduction rate, and model.
1. Staged Speed Increase
The first roll adopts low-speed threading and low-speed rolling, with the speed set at 60%-70% of the normal production speed; after the head of the strip passes through the final machine frame and the AGC closed-loop stabilizes, gradually increase the speed in 2-3 stages, with each roll increase not exceeding 20%, to avoid sudden speed changes causing fluctuations in oil film thickness and rolling force oscillations.
2. Reduction Rate Adaptation Adjustment
The total reduction rate of the first roll is appropriately reduced by 5%-8%, especially the reduction rate at the final machine frame is controlled within 15%, to reduce the fluctuation of rolling force and alleviate the AGC adjustment pressure; for high-strength steel and thick-section products, the reduction rate of the first roll is reduced by an additional 10%, prioritizing the stability of threading and thickness.
3. Classification Initialization of Model Parameters
After replacing the rolls, clear the short-term self-learning coefficients of the previous set of rolls, load the initial self-learning parameters by group according to the steel type and specification, and prohibit using default coefficients for all steel types. For steel types with significant differences such as plain carbon steel, silicon steel, and stainless steel, establish exclusive initial compensation tables to significantly reduce the deviation of the first roll setting.
4. Pre-matching of Bending Force
Based on the initial convexity of the new rolls, the bending force value is preset, and it is increased by 10% - 15% compared to the final bending force of the old rolls. This is to avoid excessive central reduction caused by the large convexity of the new rolls and poor plate shape, which may indirectly lead to thickness fluctuations.
V. Four Adjustment Mistakes That 90% of the Shifts Have Committed
Mistake: Relying solely on automatic calibration without manual verification: The correct approach is to conduct manual verification of the three-point roll gap to serve as the final defense line. Omitting this step can easily result in batch out-of-tolerance.
Mistake: Using the same set of self-learning parameters for both new and old rolls: When the old roll wears down, its diameter decreases and the roll shape becomes flatter. There is a significant difference in its state compared to the new roll. Using the same parameters for both will directly lead to inaccurate model setting. The rolls must be replaced and reset, or initialized for different steel grades.
Mistake: Adjusting the roll gap repeatedly and drastically for the first roll: The deviation of the first roll's thickness is mostly caused by the dynamic process of thermal expansion. It is necessary to observe the trend of 2-3 rolls before making a slight adjustment to the compensation coefficient. Frequent and drastic changes to the roll gap will exacerbate the oscillation and make the situation worse.
Mistake: Ignoring the influence of oil temperature on the roll gap: Fluctuations in the oil temperature will change the viscosity of the oil and the response speed of the servo valve. During cold machine operation, calibration is performed, and during hot machine operation, the roll gap will show hidden deviations. It is necessary to warm up the machine and stabilize the oil temperature before calibration.
VI. Emergency and Quick Disposal for First Roll Deviation
Minor deviation (within ±0.05mm): Adjust the AGC gain coefficient, reduce the response speed, and maintain the current speed for 2-3 rolls of production. After the roll temperature stabilizes, the deviation will automatically converge.
Moderate deviation (±0.05~0.1mm): Pause the speed increase, manually correct the thermal compensation value of the roll gap, and simultaneously adjust the corresponding machine frame's bending force. After the thickness of 2-3 rolls is stable, gradually resume the production rhythm.
Severe deviation (more than 0.1mm out of tolerance, with obvious wedge-shaped deviation): Immediately stop the machine, re-execute the roll gap calibration process, check if the pressing system, sensors, and liners are abnormal, eliminate equipment faults before restarting the rolling, and prohibit forced production under faulty conditions. Summary
After changing the rolls, the thickness of the first roll exceeded the standard. Although it seems like a minor issue, it actually reflects the management level of standardization in calibration and refinement of parameters in the workshop. By solidifying the roll gap calibration, accurately adjusting the thermal compensation parameters, and implementing the process specifications for the first roll, not only can the hit rate of the first roll thickness be increased to over 95%, but it can also reduce cutting losses and re-judgments, and decrease abnormal wear of the rolls. This is a cost reduction and quality improvement measure with minimal investment and quick results.
