Common Problems and Solutions for Rollers in Cold Rolling

　　In the early stages of commissioning its cold‑rolling mill, a steel plant encountered significant challenges due to surface defects on the rolls, which severely impacted product quality and production throughput. These roll surface defects—primarily including vibration marks, roll prints, tool marks, cracks, and spalling—directly degrade the surface finish of the final products. Investigating the root causes of these defects and developing effective mitigation strategies is of great practical importance for optimizing on‑site operations.

　　1. Vibrational pattern

　　During the rolling process of strip steel, alternating light and dark stripes perpendicular to the strip’s direction of travel frequently appear on its surface; this defect is known as strip waviness. Practical experience has shown that, in the vast majority of cases, such waviness originates from the rolls—specifically, roll‑induced vibrations are reproduced on the strip surface.

　　1.1 Causes of Roll Vibration Marks

　　The causes and manifestations of roll‑induced vibration marks are as follows: (1) Vibration of the grinding wheel spindle generates roll vibration marks, which appear on the roll surface in a helical pattern. (2) Poor grinding wheel geometry or inadequate dressing can also produce roll vibration marks, likewise arranged in a helical fashion; by comparing the calculated pitch of these marks—derived from the rotational speeds of the roll and the grinding wheel—with the actual measured spacing, one can accurately identify whether the marks originate from the grinding wheel. (3) Improper contact between the roll support (support block) and the roll gives rise to vibration marks that run parallel to the roll’s generatrix. (4) Defective tailstock geometry results in vibration marks aligned parallel to the roll’s generatrix. (5) Malfunctioning headstock rotation produces vibration marks that are likewise oriented parallel to the roll’s generatrix. (6) Resonance between vibrations of the grinding wheel headstock and the bedhead housing leads to roll vibration marks, which are distributed parallel to the generatrix on the roll surface.

　　1.2 Methods for Eliminating Roll Vibration Marks

　　To eliminate roll vibration marks, first address any abnormal vibration sources, and then select the grinding method based on the severity of the marks. For pronounced marks, begin with rough grinding; for less noticeable marks, start with semi‑rough grinding.

　　2. Roller imprint

　　① Forms and causes of roll marks. Roll marks primarily manifest as pinholes, dents, indentations, and holes on the roll surface. Irregularly distributed depressions on the roll surface are typically circular, with the surface texture or roll pattern often preserved within the depressions; these are generally caused by debris (foreign matter) entering the bite zone or by friction resulting from contact between rolls. Preventive measures include: identifying the source of debris, cleaning the mill stands, optimizing process lubrication and cooling conditions, improving the surface quality of pickled coil stock and edge‑trimming quality, increasing the surface hardness and case depth of the work rolls, and appropriately balancing the hardness differential between work rolls and intermediate rolls.

　　② Methods for eliminating roll marks. Depending on the severity of the roll marks, they can be removed by grinding or turning.

　　3. Knife Flower

　　3.1 Causes of Roll Grooving

　　Surface defects caused by mismatched speeds among the roll, the grinding wheel, the Z-axis feed rate, and the wheel’s infeed; surface defects resulting from an excessively sharp wheel edge or edge defects; surface defects arising from insufficient passes during semi‑finish grinding, which fail to eliminate the imperfections left by the rough‑grinding passes; surface defects due to excessive load during the finish‑grinding passes; and surface defects also caused by excessive roll crown.

　　3.2 Preventive Measures and Elimination Methods for Roll Grooving

　　① Preventive measures. Adjust the roll rotational speed, Z-axis feed rate, wheel feed rate, and wheel peripheral speed according to the following formula: a = reasonable configuration of these parameters. Here, a represents the grinding depth (mm); Vw is the workpiece circumferential speed (m/min); Vs is the wheel circumferential speed (m/s); Ap is the back‑feed amount (mm); Ds is the wheel diameter (mm); Dw is the workpiece diameter (mm); and m is the number of effective abrasive points per unit length of the wheel circumference. Chamfer the sharp edges of the grinding wheel and perform tailstock dressing on wheels with defects until the defects are eliminated. Select standard grinding programs based on the type of roll, adhere strictly to these procedures, and ensure that any defects from the previous pass have been fully removed before advancing to the next program. While ensuring that defects from the semi‑finishing passes are eliminated, control the number of passes and the load during finishing, and consider using a softer grinding wheel or reducing the wheel’s crown.

　　② Methods for eliminating roll surface defects. Typically, these defects are removed by repeated grinding.

　　4. Cracks

　　① Causes of roll cracking. During rolling, or when relatively hot rolls are ground, localized stress concentrations can readily lead to crack formation; accidents such as strip breakage or steel entanglement in the mill can also induce cracks; uneven incoming material thickness may subject the rolls to impact during rolling, resulting in excessive rolling forces over a short period and causing cracks; additionally, fatigue cracks may develop in the rolls.

　　② Methods for eliminating roll cracks. To prevent the roll from losing metal or developing more serious defects due to cracking, timely treatment is essential, and it must ensure that all cracks are completely removed. During grinding, carefully inspect the roll; if a crack is detected, mark its end on the roll neck to facilitate subsequent checks. If the crack is minor and cannot be visually confirmed as fully ground after a period of grinding, perform an ultrasonic or magnetic particle inspection to verify whether the crack has been eliminated; if it persists, continue grinding. If the crack is large and deep and remains prominent even after prolonged grinding, employ localized plunge‑cut grinding to assess its depth.

　　5. Spalling (material loss)

　　Roll spalling refers to the phenomenon in which a specific area of the roll body separates from the roll.

　　5.1 Spalling of the Roll Surface

　　① During rolling, incidents such as strip breakage, tail‑slip, and strip overlap can cause steel to jam or slip, subjecting the roll‑face contact zone to thermal shock. This results in varying degrees of burn damage on the roll surface, creating conditions that promote stress concentration. If the rolling stresses acting on the roll exceed its tensile strength, surface cracks may propagate during the rolling cycle, ultimately leading to roll spalling.

　　② High‑cycle fatigue spalling, initiated by subsurface cracks, is commonly observed in intermediate and backup rolls and typically occurs without any accompanying mill incidents. In general, this type of fatigue fracture is characterized as “brittle” spalling, indicating that the failure process originates beneath the roll surface. The number of stress cycles is critical: even stresses below the material’s yield strength can, through repeated loading, trigger crack initiation, leading to fatigue‑induced cracking and subsequent spalling. Because very small cracks propagate parallel to the roll surface along the tangential direction, high‑cycle contact‑stress fatigue can develop at numerous locations within the stressed zone. Subsequently, cyclic loading further propagates these cracks toward the surface until spalling ultimately occurs.

　　5.2 Roll-end metal loss

　　Abnormalities in the mill’s thrust bearings cause spalling at the roll ends; improper positioning or malfunctions of the roll‑changing carriage result in abnormal conditions during roll changes, also leading to spalling at the roll ends; and lifting operations or other unforeseen incidents can cause impact damage to the roll ends.

　　5.3 Methods for Addressing Roll Spalling

　　For surface spalling on the roll, perform rough grinding followed by nondestructive testing until the defects are eliminated. As for spalling at the roll ends, mild spalling is addressed by grinding, while severe spalling results in scrapping the roll.

　　6. Towa Print

　　① Causes of roll‑bearing‑pad marks. Roll‑bearing‑pad marks typically occur on rolls with smaller diameters that require a roll‑body support during grinding. In cold‑rolling mills, such marks are commonly found on the work rolls of 20‑high rolling mills, as well as on the first and second intermediate rolls. These marks arise when the bearing pad is uneven, resulting in burrs; when impurities are present in the pad material; or when abrasive grains, metal chips, or other foreign particles become lodged between the pad and the roll, thereby scratching the roll surface during grinding.

　　② Methods for eliminating roll‑shoe marks. Typically, these marks are removed by inserting shim paper, performing semi‑pass grinding, or repeating the grinding process.