In cold rolling production, rolls directly affect three key indicators: strip output, surface quality, and consumption cost per ton. Various defects occurring on rolls during cold rolling can not only cause mass product downgrading or scrapping, reducing the yield, but also lead to equipment damage in severe cases. Therefore, domestic cold rolling enterprises attach great importance to research on roll usage technology, striving to effectively reduce roll consumption. Based on the actual conditions at a stainless steel cold rolling site, this paper sorts out common roll defect types in production, analyzes the causes of each defect, and proposes corresponding treatment measures.
Keywords: cold rolling rolls; roll defects; treatment measures
During the initial stage of operation at a cold rolling mill of a certain steel plant, roll surface defects were prominent, significantly impacting product quality and production rhythm. Common roll surface defects observed on-site mainly include chatter marks, roll indentations, grinding chatter (knife marks), cracks, and spalling, all of which are directly transferred to the strip surface, affecting the final product quality. An in-depth analysis of the formation mechanisms and solutions for various defects holds significant practical importance for stable production.
1 Chatter Marks
During strip rolling, bright and dark alternating stripes perpendicular to the rolling direction often appear on the strip surface; these defects are known as chatter marks. Practical experience shows that most of these chatter marks originate from the rolls and are formed by the transfer of roll surface chatter marks to the strip surface.
1.1 Causes of Roll Chatter Marks
The causes and manifestations of roll chatter marks mainly include the following:
- Chatter marks caused by grinding wheel spindle vibration, usually distributed spirally on the roll surface.
- Chatter marks caused by poor grinding wheel dressing or inadequate grit shedding, also spirally distributed. The chatter mark spacing can usually be calculated from roll speed and grinding wheel speed, and compared with measured values to accurately determine that the marks are grinding-wheel-related.
- Chatter marks caused by poor contact of the roll rest (bearing pads), distributed parallel to the roll busbar.
- Chatter marks caused by poor tailstock center shape or abnormal headstock rotation, also distributed parallel to the roll busbar.
- Chatter marks caused by resonance between the grinding wheel headstock and the headstock, resulting in marks parallel to the roll busbar.
1.2 Methods for Eliminating Roll Chatter Marks
To eliminate roll chatter marks, the primary task is to identify and remove the abnormal vibration source, and then select the corresponding grinding process based on the severity of the chatter marks. For pronounced chatter marks, elimination should start from the rough grinding stage; for slight chatter marks, treatment can begin from the semi-rough grinding stage.
2 Roll Indentations
2.1 Manifestations and Causes of Roll Indentations
Roll indentations mainly manifest as pinholes, dents, indentations, and pits on the roll surface. Irregularly distributed circular indentations on the roll surface, often retaining the original surface texture inside, are mostly caused by metal chips, foreign objects entering the bite zone during rolling, or abnormal roll-to-roll contact friction.
Preventive measures mainly include: identifying and cutting off sources of foreign objects, cleaning the interior of the mill stand, optimizing process lubrication and cooling conditions, improving the surface quality of pickled coils and edge trimming quality, enhancing the surface hardness and hardened layer depth of work rolls, and reasonably matching the hardness difference between work rolls and intermediate rolls.
2.2 Methods for Eliminating Roll Indentations
Depending on the depth and severity of the indentations, grinding or turning processes are used to remove the defects.
3 Grinding Chatter (Knife Marks)
3.1 Causes of Grinding Chatter
Grinding chatter on rolls is mainly caused by mismatched grinding parameters: improper coordination among roll speed, grinding wheel speed, Z-axis longitudinal feed rate, and grinding wheel infeed; excessively sharp edges or localized defects on the grinding wheel; insufficient semi-finish grinding passes failing to remove marks left by rough grinding; excessive load during the finish grinding stage; and excessive roll crown. All these factors can induce grinding chatter defects.
3.2 Prevention and Elimination Methods for Grinding Chatter
Preventive measures: Reasonably match roll speed, Z-axis longitudinal feed rate, grinding wheel infeed, and grinding wheel speed to optimize grinding parameters; chamfer sharp grinding wheel edges; dress defective grinding wheels until the defects are eliminated; select standard grinding programs for different roll types and strictly follow procedures, ensuring that defects from the previous grinding stage are completely removed before proceeding to the next; ensure semi-finish grinding defects are properly removed, control the number and load of finish grinding passes, and consider using a softer grinding wheel or appropriately reducing the grinding wheel crown.
Elimination method: Grinding chatter is typically eliminated by repeated grinding.
4 Cracks
4.1 Causes of Cracks
During the rolling process, or when grinding rolls at high temperatures, localized stress concentration can easily induce cracks. Accidents such as strip breakage or cobbles in the mill can directly cause roll cracking. Inhomogeneous incoming material thickness can cause impact on the rolls during rolling, leading to a sudden significant increase in rolling force and generating cracks. Additionally, long-term use can lead to roll fatigue cracks.
4.2 Crack Treatment Methods
To prevent crack propagation leading to more severe defects such as chunking or spalling, cracks must be addressed promptly and completely removed. During grinding, the roll must be carefully inspected, and any cracks found should be marked at the roll neck end for subsequent tracking and inspection. For small cracks, if visual inspection after some grinding cannot confirm complete removal, penetrant testing or magnetic particle inspection must be conducted to confirm that the cracks are entirely eliminated before the roll is put into use. For deeper cracks that cannot be removed by prolonged conventional grinding, spot plunge grinding can be employed to determine the crack depth and then address them accordingly.
5 Spalling (Chunking)
Roll spalling refers to the detachment and loss of localized material from the roll body surface.
5.1 Roll Surface Spalling
During rolling, occurrences of strip breakage, tail pinching, or overlapped rolling can cause cobbles or skidding, subjecting the roll surface to localized thermal shock, resulting in roll burns and stress concentration. When rolling stresses exceed the tensile strength of the roll material, surface cracks continuously propagate during the rolling campaign, ultimately leading to roll spalling.
High-cycle fatigue spalling primarily occurs on the second intermediate rolls and backup rolls, often developing spontaneously without apparent equipment failures. This type of spalling is often of a “brittle” nature, with cracks originating beneath the roll surface layer. Even when stress levels are below the material’s strength, repeated cyclic contact stress can induce fatigue cracks. These cracks propagate parallel to the tangential direction of the roll surface, extending towards the surface under cyclic stress, eventually resulting in spalling.
5.2 Roll End Chunking
Abnormal thrust bearings in the mill, inaccurate positioning or abnormal operation of the roll change cart, can cause end damage during roll changes. Lifting, handling, and other accidental impacts can also result in chunking at the roll ends.
5.3 Spalling Treatment Methods
Roll surface spalling requires rough grinding combined with non-destructive inspection until the defects are completely removed. Slight spalling at the roll ends can be addressed by grinding and dressing, while severe spalling requires scrapping the roll.
6 Bearing Pad Marks
6.1 Causes of Bearing Pad Marks
Bearing pad marks commonly occur on rolls with smaller diameters that require support from rests during grinding, such as work rolls, first intermediate rolls, and second intermediate rolls in a 20-high mill at this plant. Burrs on uneven rest pad surfaces, impurities in the pad material, or grinding wheel abrasive grains and iron chips trapped between the pad and the roll can scratch the roll surface during grinding, forming bearing pad marks.
6.2 Methods for Eliminating Bearing Pad Marks
Bearing pad marks are typically eliminated by using pad paper, or by employing semi-pass grinding or repeated grinding passes.
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