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Analysis of Causes and Improvement Measures for Cold Rolling Roll Bursting

Abstract: This paper analyzes phenomena such as roll cracking and roll bursting occurring during the cold rolling production process, and—based on actual production conditions—proposes measures to prevent such roll defects.

Company’s cold rolling plant employs a rolling process utilizing a single-stand, reversible, six-high rolling mill; however, the production line frequently experiences abnormal shutdowns caused by surface spalling on the work rolls. Such abnormal shutdowns not only increase manufacturing costs but also disrupt and destabilize the normal production rhythm, leading to imbalances in process capacity and delays in customer deliveries. To address this issue of roll surface spalling, Longwei Company conducted a comprehensive analysis involving stress calculations on the rolls, combined with an examination of the plant’s on-site process parameters and actual operating environment. Through this investigation, the company successfully identified the root causes of the roll failures, formulated effective preventive measures, and achieved highly positive results.

Ⅰ. Roll Bursting Phenomenon

During production operations, roll changes, or post-change storage, the rolls exhibited localized deep spalling, extensive surface detachment, and cracks of varying severity, as shown in Figures 1 through 4.

(1) Roll Bursting During Production. During normal operations in the steel rolling workshop, incidents involving strip breakage—accompanied by loud, violent noises—have occurred. These incidents exhibit no discernible pattern; sudden, irregular shutdowns have taken place while processing different materials, varying thickness specifications, and during different rolling passes and stages. On-site inspections following these incidents reveal severe cracking of the roll bodies and localized spalling of the roll surfaces. The phenomena of bursting and surface spalling are particularly pronounced in the intermediate rolls; extensive peeling of the intermediate roll bodies not only renders these rolls immediately unserviceable but also causes varying degrees of damage to the work rolls. Consequently, the products being rolled at the time of the incident must be cut and re-coiled, resulting in significant material loss.

(2) Roll Bursting After Removal: In accordance with the rolling process and roll-change protocols of a cold rolling mill, rolls must be removed from the mill stand for maintenance and grinding—after reaching a prescribed usage limit (defined by the number of coils, rolling distance, rolling passes, or tonnage)—so that they may be paired, reassembled, and readied for future use. Occasionally, either during the roll-changing procedure itself or immediately after removal, the body of a roll may burst. This event is typically accompanied by an audible cracking or loud explosive sound, followed by visible cracking and spalling of the roll surface. In severe cases, fragments of the roll body can be ejected with explosive force, flying distances of several meters; this not only results in the immediate scrapping of the roll but also poses a significant safety hazard, as flying debris presents a risk of injury to personnel.

Ⅱ. Analysis of the Causes of Roll Bursting

（1）Analysis of Force Conditions

During the rolling process, the rolls of a cold rolling mill primarily experience stresses such as bending stress, fatigue stress, and thermal shock fatigue stress.

① Bending Stress. During rolling operations on a reversible six-high HC mill, the maximum positive roll bending force is 300 kN, and the maximum negative roll bending force is 200 kN; the maximum rolling force is 10 MN, the rolling speed ranges from 0 m/s to 10 m/s, and the intermediate roll shifting distance is 200 mm. Since the work rolls, intermediate rolls, and backup rolls are pressed tightly against one another, the rolling pressure is distributed uniformly across the rolled strip, and the rolling force acting on the rolls is likewise distributed evenly. Under safe rolling load conditions, this factor is not the primary cause of roll spalling. However, if the intermediate roll shifting distance is excessive—or if roll shifting is performed while under an excessive load—localized bending stress concentrations will occur within the contact zones between the intermediate roll ends, the work rolls, and the backup rolls. Simultaneously, the “edge effect” of the rolling emulsion can lead to an accumulation of metal debris and fine particulate matter in this region, further exacerbating the stress concentration and triggering the formation of localized cracks. As the cumulative rolling volume increases, this process ultimately results in roll cracking and surface spalling; specifically, crack propagation is most frequently observed in the form of spalling at the shoulders of the backup rolls, or bursting and spalling at the ends of the intermediate rolls.

② Fatigue Stress. During the rolling process, with every full rotation of the roll, the central section of the roll body—a region of high stress concentration—is subjected to alternating tensile and compressive stresses. This continuous, cyclic fluctuation generates significant fatigue stress within these concentrated areas. After a certain number of cycles, cracks begin to initiate at the sharp corners of non-metallic inclusions; these cracks then propagate along the orientation of the inclusions and the direction of the applied stress. This process can ultimately lead to the spalling of the roll’s surface layer and constitutes a primary contributing factor to roll bursting during cold rolling operations.

③ Thermal Shock Fatigue Stress. During the rolling process, the deformation zone and the cooling zone of the roll alternate continuously; that is, the roll is subjected to a ceaseless cycle of heating and cooling. In this operational environment, the roll endures prolonged, alternating thermal stresses across its cooling and heated deformation zones. This readily leads to the formation of incipient micro-cracks on the roll’s surface; these micro-cracks progressively propagate and intensify, constituting a primary factor responsible for surface spalling on the roll.

(2) Actual Process Operating Conditions

① Insufficient Cooling. The damaged roll exhibited elevated and uneven temperatures, with localized spots reaching as high as 300°C. Inspection revealed that the emulsion nozzles on the rolling mill suffered from inconsistent flow rates and spray pressures, and a significant number of nozzles were clogged to varying degrees. Because the cooling effect of the mill’s emulsion on the roll was both uneven and inadequate, the heat generated by the deformation of the rolled material during the rolling process could not be fully dissipated; this resulted in a rise in the roll’s temperature, ultimately leading to the formation of axial open cracks. Furthermore, excessively low emulsion temperatures, insufficient emulsion concentrations, and substandard emulsion cleanliness are additional factors contributing to the development of cracks in the rolls.

② Process-Induced Slippage. In the event of a roll burst accident, the rolling mill occasionally experiences a phenomenon known as process-induced slippage. This occurs when an improper rolling process—specifically, an imbalance in front and back tensions—causes sliding to take place between the work roll surface and the rolled material. The intense vibration and frictional heat generated by this process-induced slippage are transmitted to the work rolls and intermediate rolls; this leads to a rapid rise in roll surface temperature, thereby resulting in the formation of cracks on the roll surfaces.

(3) Actual Production Situation

Roll damage resulting from production accidents. In 90% of cases, roll bursts are accompanied by strip breakage incidents during rolling operations. Although it is often impossible to precisely determine the chronological sequence between the strip breakage and the subsequent roll burst, a strip breakage event during rolling inevitably results in damage to the rolls. During a strip breakage, the roll temperature undergoes a sudden, drastic fluctuation; consequently, the formation of cracks is virtually unavoidable. These cracks are typically deep, localized in nature, and characterized by significant depth and wide apertures. Furthermore, a strip breakage often leads to steel pile-ups at the mill entry or instances of overlapping—phenomena where the inertia of the high-speed rotating rolls generates instantaneous impact forces and concentrated energy sufficient to inflict severe structural damage upon the rolls. When a strip breakage is accompanied by steel adhesion to the rolls, the consequences for the work rolls and intermediate rolls can be catastrophic. Steel adhesion creates indentations on the roll surfaces, which are then directly imprinted onto the surfaces of all other rolls in the stack. If these indentations are not detected promptly—and if the affected work rolls, intermediate rolls, and backup rolls are not comprehensively replaced—then, after a certain number of rolling cycles, these indentations will continuously transfer and replicate across the various rolls. Concurrently, any minor cracks that may have formed within these indented areas will propagate and intensify, ultimately leading to the spalling of the working layer on the roll body and the peeling away of residual surface material.

(4) Roll problems.

① Excessive Hardness. The hardness of a rolling mill roll’s working surface is a primary quality indicator for the roll. While this hardness metric generally determines the roll’s service life, a mismatch between the roll’s inherent hardness and the specifications of the work rolls, intermediate rolls, or backup rolls can still result in damage to the roll itself. Longwei Company employs the following hardness matching scheme for its rolling mill rolls: Work Roll Hardness: 90–95 HSD; Intermediate Roll Hardness: 75–80 HSD; and Backup Roll Hardness: 60–65 HSD. Even after new rolls have undergone proper pairing—in terms of both diameter and hardness—any localized fluctuations in hardness values that occur during operation can lead to localized cracking and spalling within the roll itself; furthermore, such anomalies can act as a catalyst for localized bursting in adjacent rolls.

② Maintenance Grinding of Mill Rolls. After a mill roll is taken out of service, any fatigue layers or surface crack layers present on its working surface may persist as residual defects if the subsequent grinding process is incomplete. When such a defective roll is subsequently reinstalled and put back into operation, the influence of cyclic fatigue stresses causes these micro-cracks to manifest rapidly and propagate continuously—potentially reaching a critical depth or penetrating into the transition layer—thereby creating a latent risk for surface spalling of the roll.

3. Improvement Measures to Prevent Roll Bursting

3.1 Ensure the roll grinding process and maintenance system.

Establish robust institutional safeguards governing roll usage cycles, maintenance, and grinding procedures. Based on the material composition and performance characteristics of the rolls—and taking into account actual production conditions and process requirements—formulate a rational roll maintenance system to ensure that the rolls meet the necessary process specifications for operational deployment. Generally, work rolls, intermediate rolls, and backup rolls may be maintained in accordance with the guidelines presented in Table 1.

Name	Grinding Amount（mm)	Ellipticity(mm)	Taper (mm)	Roll type accuracy(mm)
Working Roll	0.15-0.25	0-0.01	0-0.01	0-0.01
Middle Roll	0.40-0.55	0-0.01	0-0.01	0-0.03
Support Roller	0.70-1.20	0.01	0.01	0-0.02

To prevent rolls from being installed with residual defects due to incomplete cleaning, the grinding allowance for damaged rolls may be moderately increased to 0.40–0.55 mm. Furthermore, post-grinding flaw detection inspections should be intensified to ensure that internal stresses are relieved and micro-cracks are completely eliminated.

3.2. Strictly pair the mill rolls for use.

The pairing of support rolls must be managed to ensure both operational service life and surface quality; this includes the proper matching of support roll bearings to guarantee the stable operation of the support rolls. The internal matching of work rolls and intermediate rolls—as well as the matching *between* work rolls and intermediate rolls—is of particular importance. Roll pairing must strictly adhere to specific parameters, such as roll diameter, roll surface hardness, and service life cycles—with special attention paid to the pairing of damaged rolls, rolls in their initial stages of use, and rolls nearing the end of their service life.

3.3 Ensure the Operation of the Emulsion Process

To effectively balance the cooling and lubricating functions of rolling mill emulsions—and driven by environmental regulations—closed-loop, zero-discharge emulsion systems are becoming increasingly widespread. As a result, the service life of these emulsions has been significantly extended; to ensure their continued efficacy, it is essential to conduct real-time monitoring of key physicochemical parameters, such as concentration and cleanliness. This ensures that the emulsion effectively fulfills its critical roles—including cleaning, lubrication, and cooling—within the rolling mill and its rolls, while simultaneously preventing contamination from the mill’s AGC hydraulic fluid, oil-air lubrication oils, and maintenance lubricants.

3.4 Implement the Roll Change System

Depending on the material and thickness of the rolled products, different rolling processes are employed; accordingly, a corresponding roll-changing schedule is established, which is generally implemented in accordance with Table 2.

Table 2 Work System for Rolling Products Implementation
Name	Rolling Passes (times)	Reference Tonnage (tons)	Remarks
Working Roll	30	100	1. When work rolls show vibration marks ortool marks, replace the work roll immediately. 2. If soft spots or roller marks appear on theboard surface, replace the working roll.
Middle Roller	120	400
Support Roller	670	2000

3.5. Adhere to the roll preheating protocol.

Following a roll change, ensure that the specified preheating duration, temperature, and pressure are maintained. Prior to rolling, allow for a sufficient roll-warming period—typically 30 to 40 minutes—and apply a specific warming pressure, generally ranging from 4 MN to 5 MN. Furthermore, ensure that the rolls reach the required temperature; the emulsified fluid temperature requirements are typically 45°C to 50°C in the summer and 50°C to 55°C in the winter.

3.6. Optimization of Rolling Process

Products rolled on single-stand mills are becoming increasingly thin, while production batches for specific specifications are shrinking—sometimes organized on a coil-by-coil basis. This results in significant variations in the intended application and quality between successive products, necessitating timely adjustments to the rolling process. Production is therefore organized to meet the specific quality requirements of downstream processes and end-users; by optimizing and adjusting rolling parameters—such as rolling speed, reduction ratio, and tension—the process is tailored to accommodate the varying priorities users place on factors such as strip shape, dimensions, mechanical properties, and chemical composition.

3.7. Establish a linkage mechanism that integrates both vertical and horizontal dimensions.

The steel rolling process must respond promptly to variations in upstream raw materials and timely convey technical requirements to downstream processes. Furthermore, information regarding roll usage and real-time operational metrics must be shared in real time with the roll maintenance workshop; similarly, details concerning operational maintenance and incident handling must be coordinated with the repair workshop. Through this mechanism of vertical and horizontal coordination, the efficient and smooth operation of the rolling mill is ensured.

One stop solution for rolling mill

What causes roll bursting in cold rolling mills?

Roll bursting is mainly caused by fatigue stress, thermal shock, improper cooling, and process instability.

Why is intermediate roll damage more severe?

Intermediate rolls experience high stress concentration and shifting forces, making them more prone to failure.

Why is roll hardness matching important?

Improper hardness leads to uneven stress distribution and accelerates roll damage.

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