Cold Rolling Mill Roll Cracking and Bursting: Causes, Analysis, and Prevention

Abstract: This paper analyzes the phenomena of roll cracking and roll bursting in the cold rolling production process, and proposes measures to avoid roll cracking and bursting based on actual production conditions. Longwei Company’s cold rolling mill employs a single-stand, reversible, six-high rolling process. During production, abnormal shutdowns frequently occur due to roll surface spalling. These shutdowns not only increase manufacturing costs but also disrupt normal production rhythms, causing imbalances in process capacity and affecting customer deliveries. To address the roll surface spalling problem, Longwei Company conducted stress analysis on the rolls, combined with analysis of the factory’s on-site process conditions and actual operating environment. This identified the causes of roll breakage and led to effective preventative measures, achieving positive results.

mill Roll bursting phenomenon

During production, during roll changing, and during post-roll placement, rolls may experience localized deep bursts, large-area surface peeling, and cracks of varying degrees, as shown in Figures 1 to 4. (1) Roll bursting during production. During normal production in the steel rolling workshop, a strip breakage accident accompanied by a violent noise occurred. There was no obvious pattern to the occurrence of this accident. When producing different materials, different thicknesses, different passes, and different rolling stages, the accident occurred without any pattern and was a sudden stop. At the accident site, the roll body was severely cracked and the surface was peeling off in some areas. Among them, the bursting and surface peeling of the intermediate roll were particularly obvious. Because the intermediate roll body peeled off over a large area, it not only caused the intermediate roll to be scrapped directly, but also caused varying degrees of damage to the work rolls. As a result, the products rolled online were cut and coiled, resulting in a large amount of material loss. (2) Roll bursting after being replaced. According to the rolling process and roll replacement system of the cold rolling mill, after a certain number of rolls (including the number of coils, kilometers, passes, and tonnage), the rolls need to be taken off the machine for maintenance and grinding so that they can be matched, disassembled and put back into use. Sometimes, during the roll replacement process, or when the roll has just been replaced, the roll body will burst. A loud or soft popping sound can be heard, and the roll will crack and peel off. In severe cases, the roll body fragments can fly up to several meters away, which not only makes the roll scrapped, but also poses a safety hazard of flying fragments injuring people.

Analysis of the causes of roll bursting

Analysis of the stress situation

The rolls of a cold rolling mill mainly bear bending stress, fatigue stress, and thermal shock fatigue stress during the rolling process. ① Bending stress. During reversible six-roll HC mill rolling, the maximum positive bending roll force is 300KN, the maximum negative bending roll force is 200KN, the maximum rolling force is 10MN, the rolling speed is 0m/s~10m/s, and the roll pulling distance is 200mm. Because the work rolls, intermediate rolls, and support rolls are tightly pressed together, the rolling pressure is evenly distributed along the workpiece, and the rolling force on the rolls is evenly distributed. Within the safe rolling load, this is not the main cause of roll spalling. However, if the intermediate roll pulling distance is too large, or if the load is too high during roll pulling, local bending stress concentration will occur in the contact area between the intermediate roll head and the work rolls and support rolls. Simultaneously, the edge effect of the emulsion will cause the accumulation of metal slag and impurities in this area, exacerbating the stress concentration and leading to localized cracks. As the amount of steel rolled increases, this causes roll cracks and surface bursting. Crack propagation often occurs during support roll shoulder detachment, intermediate roll head bursting, and detachment. ② Fatigue stress. During the rolling process, the roll is subjected to tensile and compressive stresses in the middle of the roll body, where stress is highly concentrated, with each revolution. These stresses continuously cycle and change, resulting in significant fatigue stress at the stress concentration points. After a certain number of cycles, cracks form at the sharp corners of inclusions and propagate along the direction of the inclusions and stresses, potentially leading to surface peeling of the roll. This is a contributing factor to cold rolling cracking. ③ Thermal shock fatigue stress. During the rolling process, the deformation zone and cooling zone of the roll change continuously. That is, the roll is subjected to continuous heating and cooling. Under this working environment, the roll is subjected to alternating thermal stress in the cooling zone and the deformation heating zone for a long time, which easily leads to the appearance of initial microcracks on the surface of the roll. These microcracks will continue to intensify and are a factor causing the surface of the roll to peel off.

Actual process conditions

① Insufficient cooling. The temperature of the affected roll was high and uneven, reaching 300℃ in some areas. Inspection revealed uneven spray flow and pressure from the mill’s emulsion nozzles, with many nozzles showing varying degrees of blockage. Because the mill’s emulsion applied the emulsion unevenly and insufficiently to the roll, the heat generated during rolling deformation could not be fully released, causing the roll temperature to rise and resulting in axial opening cracks. Additionally, excessively low emulsion temperature, excessively low concentration, and substandard emulsion cleanliness also contributed to the roll cracking. ② Process slippage. Occasionally, process slippage occurs in the rolling mill during a roll burst accident. Due to improper rolling processes and imbalances in tension, slippage occurs between the work roll surface and the rolled material. The severe vibration and frictional heat caused by process slippage are transferred to the work roll and intermediate roll, causing a rapid increase in roll surface temperature and resulting in roll surface cracks.

(3) Actual production situation

Roll damage caused by production accidents. 90% of roll bursts are accompanied by strip breakage during rolling. Although the exact relationship between strip breakage and roll bursting cannot be determined, strip breakage during rolling will inevitably damage the roll. During strip breakage, the roll temperature changes drastically, making cracks inevitable. These cracks are deep, localized, and have a large depth and opening. Strip breakage can also cause steel buildup and overlapping. Due to the inertia of the high-speed rolling, the instantaneous impact and energy accumulation are sufficient to severely damage the roll. When strip breakage occurs along with steel adhesion, the impact on the work rolls and intermediate rolls is fatal. Steel adhesion creates indentations on the roll surface, which are directly transmitted to all roll surfaces. If the indentations are not detected and replaced promptly, after a certain number of rolling cycles, the indentations will continuously transfer and replicate between the rolls. Even minor cracks at the indentation sites will expand and intensify, eventually causing the working layer of the roll body to peel off and the remaining layer to flake off.

(4) Roll problems

① Excessive Hardness. The hardness of the working surface of the roll is a major quality indicator. Although the hardness index determines the service life of the roll, if the hardness of the roll itself is not matched with the index of the work roll, intermediate roll, and support roll, it will still cause damage to the roll. The hardness matching of the rolls of Longwei Company’s rolling mill is: working hardness 90~95HSD, intermediate roll hardness 75~80HSD, and support roll hardness 60~65HSD. After the roll diameter and hardness are matched, changes in the hardness value of the roll during use will cause local cracks and spalling of the roll itself. At the same time, the roll will also induce local bursting of adjacent rolls. ② Maintenance and grinding of rolls. After the rolls are taken off the production line, the fatigue layer and surface crack layer on the roll surface will remain due to incomplete grinding. When the defective rolls are put back into use, under the action of cyclic fatigue stress, the microcracks will appear quickly and continue to expand until they reach a certain critical depth or transition layer, which will create a hidden danger for the surface of the rolls to peel off.

Improvement measures to prevent roll bursting

3.1 Ensure proper grinding processes and maintenance procedures for the rolls. Establish strict systems for roll usage cycles, roll maintenance, and grinding. Based on the roll material and performance characteristics, and in conjunction with actual production conditions and processes, establish a reasonable roll maintenance system to ensure that the rolls meet the process requirements for online use. Generally, work rolls, intermediate rolls, and support rolls can be maintained according to (Table 1).

Name	Grinding Amount（mm）	Ovality（mm）	Taper（mm）	Roller Profile Accuracy（mm）
Work roll	0.15-0.25	0-0.01	0-0.01	0-0.01
Intermediate roll	0.40-0.55	0-0.01	0-0.01	0-0.03
Support roll	0.70-1.20	±0.01	±0.01	0-0.02

To prevent rolls from being used on the machine with defects due to incomplete cleaning of their own defects, the grinding amount for repairing damaged rolls can be appropriately increased to 0.40mm~0.55mm. Furthermore, the flaw detection inspection after grinding should be strengthened to ensure that the rolls eliminate internal stress and thoroughly remove micro-cracks. 3.2. Strictly match and use the rolls. The pairing of support rolls must ensure service life and surface quality, including the pairing of support roll bearings to ensure stable operation of the support rolls. The matching of the work roll and intermediate roll themselves, and the matching between the work roll and intermediate roll, are particularly important. Roll pairing must meet indicators such as roll diameter, roll surface hardness, and roll service life, especially for the pairing of damaged rolls, initial rolls, and final rolls. 3.3 Ensure the smooth operation of the emulsion process Balancing the cooling and lubrication functions of rolling mill emulsions, and due to environmental requirements, the full-cycle, emission-free emulsion system is gradually being promoted, significantly extending the service life of emulsions. To ensure the cleanliness of the emulsion, its concentration, cleanliness, and other physicochemical indicators must be monitored in real time. This ensures the emulsion effectively cleans, lubricates, and facilitates cold rolling of the rolling mill and rolls, while preventing contamination from rolling mill AGC hydraulic oil, oil-air lubricating oil, and maintenance oils. 3.4 Implement the roll changing system Depending on the material and thickness of the rolled product, different rolling processes are implemented, and corresponding roll changing procedures are established. Generally, these procedures are followed as shown in Table 2.

name	Rolling passes (times)	Reference tonnage (tons)	备注
Work roll	30	100	1. If vibration marks or knife marks appear on the work roll, the work roll should be replaced immediately. 2. If soft spots appear on the board surface, indicating roller marks, the work roller must be replaced.
intermediate roll	120	400
Backup rolls	670	2000

3.5. Adhere to the roll preheating system.

After changing rolls, ensure the preheating time, temperature, and pressure of the rolls. Provide sufficient time for the rolls to heat before rolling, generally 30-40 minutes, and ensure a certain pressure, generally 4-5 minutes, to meet a specific temperature requirement. The emulsion temperature requirements are generally: 45℃-50℃ in summer; 50℃-55℃ in winter.

3.6 Optimize rolling process

As products rolled by single-stand rolling mills become increasingly thinner, and production batches of the same specification become smaller, sometimes even organized by coil, significant differences in the uses and quality of the products are generated. Therefore, the rolling process needs timely adjustments. Production is organized according to the quality requirements of downstream processes and end users. Based on the different emphases of users regarding plate shape, dimensions, mechanical properties, and chemical composition, rolling processes such as high-speed rolling, high-reduction rolling, and high-tension rolling are optimized and adjusted.

3.7 Establish a coordinated mechanism that combines vertical and horizontal collaboration.

The rolling mill process must respond promptly to changes in upstream raw materials and transmit technical requirements to downstream processes in a timely manner. It must also share information on roll usage and online indicators with the roll maintenance workshop in real time, and communicate with the overhaul workshop regarding operation, maintenance, and accident handling. Through this vertical and horizontal linkage mechanism, the efficient and smooth operation of the rolling mill can be guaranteed. One stop solution for steel industry