Cold Rolling mill Roll Failure Guide

In the cold rolling of stainless steel, the work rolls serve as the critical components that determine both product quality and production efficiency. A company’s 950mm six-high, six-stand continuous cold rolling mill for stainless steel—the fourth such narrow-strip production line in China—has been plagued by severe roll failure issues since it commenced operations in July 2018. Frequent incidents involving roll spalling (flaking), breakage, and cracking have not only disrupted production but also significantly inflated operating costs. To resolve this challenge, the technical team conducted an in-depth analysis of the root causes of these failures and formulated targeted corrective measures; ultimately, they achieved a drastic reduction in the incidence of roll failures, thereby providing valuable practical experience to the industry.

Ⅰ、The “Three Major Killers” of Cold Rolling Roll Failure

To address the issue of roll failure, it is first necessary to identify the specific modes of failure and their distribution patterns. Through a statistical analysis of roll scrap data collected during the initial stages of operation, the technical team discovered that roll failures at a certain company’s 950mm cold continuous rolling mill were primarily concentrated in three categories of issues, and that the failure characteristics differed significantly among the various types of rolls:

(1) Spalling (Flaking): This constitutes the primary mode of failure. The work rolls are the most severely affected; a total of 54 work rolls were scrapped due to spalling—accounting for over 70% of all work roll failures—with the majority of these incidents concentrated in stands F1 through F4. Additionally, five intermediate rolls were scrapped due to spalling, which represented the sole cause of failure for this roll type.

(2) Roll Fracture: Sixteen work rolls were scrapped due to fracture, while seven backup rolls failed for the same reason. This accounts for 50% of all backup roll failures and represents a critical issue that threatens the continuity of production operations.

(3) Cracking: Eight work rolls were scrapped due to cracking, and seven backup rolls similarly failed due to cracks. If not detected in a timely manner, cracks can easily propagate into spalling or complete roll fracture, thereby acting as a “hidden killer.”

From a macroscopic perspective, spalling (flaking) manifests as the localized detachment of material from the roll surface, resulting in the formation of irregular pits. Roll fractures, conversely, present distinct fracture cross-sections—sometimes accompanied by signs of plastic deformation. Cracks, meanwhile, exist as minute fissures located either on the surface or within the interior of the roll, requiring specialized inspection techniques for identification. These various modes of failure not only lead directly to the scrapping of the rolls but can also scratch the steel strip or trigger strip-breakage accidents, thereby initiating a “chain reaction of failures.”

II. Digging Deeper into the Roots: Three Major Factors Causing “Short Lifespans” for Rolling Mills

Through methods such as metallographic analysis, traceability of process parameters, and a review of management workflows, the technical team ultimately identified the three core causes of the roll failure: inherent quality defects in the roll itself, issues with the rolling process and raw materials, and improper operational management.

(I) Roll Material and Design: “Inherent Flaws” Harbor Hidden Risks

The “inherent quality” of a rolling roll directly determines its service life. A technical team conducted a sampling analysis on rolls that had failed due to spalling; within the test specimens and in the vicinity of cracks, they discovered granular Al₂O₃ inclusions and TiN precipitates—specifically, the Al₂O₃ inclusions reached a maximum size of 74.5 μm, while the TiN precipitates reached a maximum size of 18.3 μm. The presence of these impurities stemmed from the failure to employ the electroslag remelting (ESR) process during the roll’s manufacture, resulting in the incomplete removal of impurities during the smelting stage.

During the rolling process, the rolls are continuously subjected to alternating tensile and compressive stresses. Given the weak bonding force between the inclusions and the matrix structure, separation occurs readily, leading to the formation of “microcracks.” As the number of rolling cycles increases, these microcracks progressively propagate and extend, ultimately triggering roll failure—manifesting as spalling—or even catastrophic roll fracture. Furthermore, certain rolls suffered from design flaws regarding their geometry; for instance, the transition fillets at points where the cross-sectional dimensions changed were inadequately designed, leading to stress concentration. In one such instance, when a support roll was subjected to high rolling pressures, the stress at the fillet location far exceeded the material’s ultimate tensile strength, directly precipitating a roll fracture accident.

(II) Rolling Process and Raw Materials: “Subsequent Use” Accelerates Failure

Even if the rolls are “inherently sound,” improper processing and substandard raw materials will accelerate their failure.

(1) Cold Work Hardening and Excessive Rolling Forces: Stainless steel exhibits a high degree of cold work hardening; when the total reduction reaches 67.5%, the tensile strength of the rolled material soars to as high as 2000 MPa, resulting in a drastic increase in rolling forces. If the superposition of localized stresses on the rolls exceeds the material’s ultimate strength, it will directly induce plastic deformation, subsequently leading to the formation of cracks.

(2) Tension Fluctuations and Slippage: Unstable rolling tension control or the occurrence of slippage can easily trigger strip breakage. At the instant of breakage, the steel strip undergoes intense friction against the roll surface, generating localized high-temperature thermal shock that causes roll surface burns and accelerates spalling.

(3) Poor Raw Material Quality: Upstream raw materials exhibit defects such as transverse wedging, non-uniform hardness, inclusions, edge cracks, and poor weld quality. Specifically, wedged raw materials result in a disparity in rolling forces between the operating and drive sides of the rolling mill, triggering unilateral stress concentration; conversely, raw materials containing inclusions or edge cracks are prone to fracture during the rolling process, leading—via thermal shock—to steel adhesion and cracking on the roll surface.

(III) Usage and Management: “Extensive Operations” Exacerbate the Situation

During the initial stages of production, the lack of standardized procedures for the management and utilization of mill rolls further amplified the risk of failure.

(1) Omission of Flaw Detection: New rolls entering the facility—as well as accident-damaged rolls following grinding—are installed directly into the machinery without undergoing non-destructive testing; this results in rolls containing internal cracks or inclusions being put into service while in a defective state.

(2) Unreasonable Roll Replacement Cycle: When the service life of the rolls is excessively prolonged, the work-hardened layer progressively deepens under the influence of continuous cyclic stress; this increases material brittleness and significantly elevates the risk of cracking and spalling.

(3) Disorganized Disassembly, Assembly, and Spare Parts Management: The lack of specialized equipment for the disassembly and assembly of mill rolls—relying entirely on manual labor—leads to wear on the rolls, bearing housings, and seals, as well as a decline in assembly precision. This can even result in bearing burnout and seizure, subsequently triggering roll torsion fractures. Furthermore, there is a shortage of spare rolls; rolls removed from the mill are often sent directly for grinding without allowing sufficient time for residual stresses to dissipate (as the surface temperatures of some rolls remain elevated). This practice not only increases the risk of grinding burns on the roll surface but also exacerbates the internal stress state, potentially leading to the formation of cracks during the grinding process itself.

(4) Insufficient Personnel Training: Employees are unfamiliar with the grinding, documentation, and inspection procedures for defective rolls; furthermore, the allocation of responsibilities is unclear. Consequently, defective rolls are reinstalled into the machinery without being identified.

III. Tailored Solutions: Three Major Improvement Measures to Achieve “Cost Reduction and Efficiency Gains”

In response to the aforementioned issues, the technical team formulated an improvement plan spanning three dimensions—inspection, process, and management—and significantly reduced the incidence of roll failure through “end-to-end process control.”

(I) Strengthen Non-Destructive Testing: Strictly Guard the “First Checkpoint Before Machining”

Given that existing rolls are predominantly manufactured by small domestic vendors—resulting in significant quality variability—the technical team has established a “comprehensive coverage, dual-inspection” flaw detection mechanism:

(1) Comprehensive Inspection Coverage: Every newly rolled work roll and every roll involved in an incident must undergo flaw detection prior to installation; rolls scheduled for routine replacement are subject to random spot checks. This ensures the complete elimination of “defective rolls” from being put into service.

(2) Dual-Method Inspection Approach: A combined strategy utilizing “ultrasonic testing + magnetic particle testing” is employed. Ultrasonic testing leverages its excellent penetration and directionality to locate, quantify, and characterize internal defects within the roll; meanwhile, magnetic particle testing serves to assist in the detection of surface defects. Where conditions permit, eddy current testing is additionally incorporated during the grinding process to further enhance the accuracy of surface defect identification.

(3) Institutional Safeguards: Formulate the *Management System for Non-Destructive Testing of Rolling Rolls* to clearly define inspection standards, responsible personnel, and procedures for the disposition of non-conforming rolls, thereby ensuring that the inspection process is standardized and traceable.

(II) Optimize Processes and Raw Materials: Reduce “In-Use Harm”

1. Raw Material Quality Control: Establish standards for incoming raw material inspection; classify and appropriately handle materials exhibiting defects such as edge cracks, inclusions, or surface peeling; and strictly prohibit high-risk materials from entering the rolling process. Furthermore, collaborate with upstream suppliers to enhance raw material stability, thereby mitigating the root causes of strip breakage at the source.

2. Rolling Process Optimization:

(1) Rationally distribute the reduction amounts across stands F1–F6, maintaining the total reduction ratio between 50% and 70% to prevent rolling forces from exceeding limits due to excessive reduction.

(2) Reduce the hardness of the work rolls from the original 90–93 HSD to 86–90 HSD; while maintaining wear resistance, this enhances material toughness and minimizes crack formation.

(3) Strengthen the monitoring of rolling process parameters, and strictly control the rolling force differential between the operating and drive sides of the mill, as well as the range of inter-stand tension fluctuations, to prevent unilateral edge waves in the strip profile and reduce the risk of strip breakage.

(III) Standardized Usage and Management: Extending Roll Service Life

1. Scientifically establish roll-change cycles: Based on the stress conditions and wear patterns specific to different stands and roll types, formulate differentiated roll-change schedules that both fully leverage roll performance and prevent fatigue failure caused by overuse.

2. Optimize Roll Spare Parts and Grinding Management: Increase the inventory of spare rolls to ensure that removed rolls have sufficient time to relieve residual stresses—specifically, work rolls, intermediate rolls, and backup rolls must undergo grinding only after 12 hours, 24 hours, and 48 hours of removal, respectively, to prevent high-temperature grinding from compromising the stress state of the roll surface.

3. Enhance Disassembly and Assembly Precision: Install a specialized roll disassembly and assembly platform equipped with dedicated tools to minimize component wear caused by manual handling, improve assembly precision, and prevent roll torsion fractures resulting from bearing-related issues.

4. Establish Comprehensive Roll Lifecycle Records: Assign a unique identifier to each roll and document information such as incoming inspection results, number of mounting cycles, rolling volume, defect status, and grinding records, thereby enabling full lifecycle tracking. Concurrently, establish a roll evaluation mechanism to periodically analyze failure data and continuously optimize management strategies.

IV. Significant Improvements: Monthly strip breakage rates dropped by 67%, and roll failure rates plummeted.

Following the implementation of a series of improvement measures, the issue of roll failure at a certain company’s 950mm cold continuous rolling mill has been fundamentally resolved.

(1) Significant Reduction in Strip Breakage Rate: The monthly strip breakage rate dropped from a peak of 1.59% to 0.53%—a reduction of 67%—thereby minimizing roll damage caused by strip breakage.

(2) Drastic Reduction in Roll Failures: The monthly number of failed rolls plummeted from a peak of 19 to a stable range of 2–5. Based on a cost of tens of thousands of yuan per roll, this translates to monthly production cost savings amounting to hundreds of thousands of yuan.

(3) Enhanced Production Efficiency: Downtime caused by roll failure has been significantly reduced, the utilization rate of the continuous rolling mill unit has increased, and product quality stability has improved, generating substantial economic and social benefits for the enterprise.

V. Conclusion

While the failure of cold-rolling rolls may appear to be merely a matter of “equipment malfunction,” it is, in reality, a systemic issue encompassing manufacturing, process engineering, and management. Practical experience at a certain company demonstrates that the complex challenge of roll failure can be effectively resolved through a comprehensive, end-to-end control strategy characterized by “strengthening inspection to control sources, optimizing processes to mitigate damage, and standardizing management to extend service life.” This experience not only offers a valuable blueprint for the production of cold-rolled stainless steel narrow strips but also provides key insights for equipment management and cost control within other steel rolling enterprises—underscoring that only by shifting from “reactive maintenance” to “proactive prevention” can stable and efficient production be truly achieved.

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