Significant Differences in Roll Wear Between Leveling and Conventional Cold Rolling
Although the structural design of leveling mills is similar to that of cold rolling mills, differences in lubrication methods, rolling tension, roll service life, and drive modes result in wear patterns for work rolls and support rolls that differ significantly from those in cold rolling mills.
The flatness of the finished strip is primarily determined by the roll gap profile. Within the industry, the roll gap is divided into two major components: the secondary and the fourth-order components, which correspond to two typical wave-shaped defects:
1. Secondary component: Controls bilateral waves and central waves, forming the basis for daily flatness adjustments.
2. Fourth-order component: Directly induces 1/4-wave and edge-center composite waves, and is also the cause of the vast majority of frequent quality issues in single-stand leveling machines.
To achieve stable control of sheet flatness, it is necessary not only to understand the patterns of roll gap changes caused by roller wear but also to adopt a comprehensive set of equipment and process coordination measures to reduce wave defects at their source.
Field Test Conclusions: The Actual Impact of Two Types of Roll Wear on Strip Shape
(I) With Minor Work Roll Wear, Initial Roll Profile Deviation Is the Primary Cause of 1/4 Waves
Field measurements of the roll surface profiles before and after the rolls were put into service, combined with comparative data from multiple strip specifications, led to two key conclusions:
1. Rolling wear is uniform and minimal
After the entire set of work rolls completed a full production cycle, the overall wear rate was very low, with only the drive side showing slightly more wear than the operating side. Fluctuations in the secondary and fourth-order convexity of the roll gap before and after service were minimal. Wear caused by rolling had virtually no effect on the basic roll profile and had limited impact on the final plate shape.
2. Poor Match Between Original and Target Roll Profiles Causes Widespread 1/4-Wave Defects
Work rolls are shipped from the factory with standard design crown, but when adapted to different strip widths, the measured roll profile deviates significantly from the target profile required for production; the narrower the strip width, the lower the actual crown—sometimes falling below half the standard value.
The proportion of the fourth-order component in the roll profile is disproportionately high, with the fourth-order convexity exceeding 50% of the second-order convexity. This inherent roll profile deviation directly causes the 1/4-wave defect and is the root cause of the recurring issues observed on-site.
In summary: Work rolls are not affected by long-term rolling wear; their greatest vulnerability lies in substandard roll profile accuracy during the grinding stage.
(2) Severe localized wear on the support rolls has almost no effect on sheet flatness
Comparing the profiles of new support rolls with those of rolls at the end of their service life, the degree of localized wear on the support rolls is far greater than that on the work rolls. The difference in maximum diameter loss can reach up to 120 micrometers, and the wear distribution is extremely uneven.
Calculations based on a simulation model of the roll train’s elastic deformation, using actual measurement data, yielded the following counterintuitive conclusions:
1. Uneven wear on the support rolls only slightly alters the secondary curvature of the roll gap, with an increase of just 1–6 micrometers throughout the entire process. The fluctuations in these values are minimal, and their impact on overall sheet flatness is essentially negligible;
2. Throughout the entire service life of the support rolls—from new to scrapped—the trend in roll gap changes caused by wear remains stable, and the degree of interference with sheet flatness remains essentially constant throughout the entire process.
Principle: Support roll wear indirectly affects the roll gap by altering the bending profile of the work rolls. Localized pitting-type wear is unlikely to significantly alter the overall roll gap profile; even severe wear visible to the naked eye will not cause batch-wide sheet flatness defects.
Comprehensive Sheet Shape Control Measures for the Leveling Machine
Deformation during the leveling process is extremely low (typically ≤3%). The corrective capability of any single adjustment method is limited; therefore, a coordinated approach involving three elements—fundamental roll profile optimization, dynamic equipment adjustment, and process parameter matching—is required. This is categorized into four major control methods:
(1) Roll Profile Optimization: Eliminating 1/4-wave distortion and ensuring long-term stable sheet shape (fundamental solution)
Based on the empirical findings discussed earlier, the roll profile is the core factor determining the baseline sheet shape. Priority should be given to roll profile design and grinding control:
1. Precisely customize the crown of work rolls by specification
Abandon the practice of grinding to a uniform, fixed crown; instead, set target crown values in segments based on the mainstream strip widths of the production line; For narrow-gauge strip steel, increase the reference crown separately to reduce the discrepancy between measured and theoretical roll profiles, lower the proportion of fourth-order crown, and suppress the 1/4-wave at its source.
For production lines with frequent 1/4-wave occurrences, composite curve roll profiles can be adopted to fine-tune the roll gap height at the 1/4-width position of the strip steel, thereby offsetting the fourth-order wave extension difference.
2. Simplified management of support rolls eliminates the need for frequent replacement
Localized wear on support rolls does not affect strip flatness, so there is no need to take them offline prematurely due to surface pitting; grinding is scheduled only at the end of the service cycle or when extensive spalling or scratches appear on the roll surface, significantly reducing grinding and roll replacement costs. Support rolls feature smooth transition surfaces to minimize additional bending caused by harmful contact between rolls.