A strip break in an 18-high rolling mill can expose the work rolls to abnormal impact loads, localized overload, and pressure from trapped strip fragments. Depending on the severity of the incident, the rolls may develop dents, material pickup, surface spalling, or cracks.
After a strip break, the mill must be stopped and isolated before any maintenance work begins. The broken strip must be removed safely, and the work rolls and associated roll assemblies must be inspected. Based on the inspection results, the affected rolls can then be ground, repaired where technically permissible, or replaced.
The main stages include emergency shutdown, energy isolation, strip removal, work roll removal, damage inspection, repair or replacement, and commissioning before production resumes.
Emergency shutdown and energy isolation
When a strip break occurs, the mill and associated equipment must be stopped according to the site's incident response procedure. This prevents broken strip from continuing to wrap around or damage the roll stack.
The main actions include:
Stop the equipment: Activate the specified emergency stop or fault shutdown procedure. Stop the main drive, coilers, uncoiler, and related feeding equipment to prevent further strip movement.
Notify the control room: Report the break location, time of occurrence, and equipment operating condition. Record the rolling speed, rolling force, and strip tension immediately before the break.
Isolate energy sources: Isolate the relevant electrical supplies to the main drive motor, coilers, hydraulic system, and pneumatic equipment. Authorized personnel must apply lockout/tagout (LOTO) to prevent unexpected startup.
Release or restrain stored energy: Check hydraulic accumulators, pneumatic circuits, coiler mandrels, and residual strip tension. Release stored pressure or restrain mechanical movement according to the equipment procedure.
Verify the safe condition: Confirm that the mill has stopped, the relevant energy sources are isolated, and the roll chocks and other movable components are secured with the designated safety devices.
For mills equipped with hydraulic safety pins or mechanical locking devices, installation and verification must follow the manufacturer's specified procedure. Pressing the emergency stop button or disconnecting the main motor power alone does not make the equipment safe for maintenance.
Strip removal and initial damage assessment
Broken strip can only be removed after the equipment has been isolated, strip tension has been released, and the required mechanical restraints are in place.
Removing broken strip fragments
After a strip break, fragments may become trapped around the work rolls, intermediate rolls, or backup rolls. They may also be lodged near the chocks, side support assemblies, or guiding equipment.
The following precautions apply:
Use suitable clamps, lifting devices, or approved cutting tools to remove strip fragments. Do not pull them out by hand.
Confirm that any residual strip tension or spring-back hazard has been released or securely restrained.
Account for sharp strip edges, pointed fragments, and pinch hazards between rolls.
Never place hands or other body parts inside roll gaps or other areas that have not been properly isolated.
After removing the visible strip fragments, inspect the mill stand for remaining metal pieces, weld remnants, and other foreign objects that could cause further damage during restart.
Initial work roll inspection
Once the strip has been removed, inspect the work roll surfaces and record the roll identification number and damage locations.
The inspection should cover:
Roll dents: Check for indentations or pressure marks caused by trapped strip or foreign objects.
Metal pickup: Look for material transferred from the strip to the roll surface, such as copper pickup when rolling copper strip.
Surface scoring: Check for scratches, abrasion marks, and localized grooves along the rolling direction.
Surface spalling: Look for local material loss, small spalls, or larger areas of surface damage.
Cracks and abnormal deformation: Inspect the barrel and the transitions near the roll necks for cracks, deformation, or other structural damage.
If visible cracks, extensive spalling, or suspected roll body damage are found, the roll must be removed from service and sent for a dedicated safety assessment and inspection. Even when external damage is not obvious, a roll exposed to a severe impact event should not automatically be returned to service.
Work roll removal and roll stack inspection
After the initial assessment, determine whether the work roll needs to be removed for further inspection.
Removal using dedicated roll changing equipment
The roll arrangement of an 18-high mill is relatively complex. The work rolls operate together with intermediate rolls, backup or backing assemblies, and side support systems.
Depending on the mill design, work rolls should be removed using a dedicated roll changing trolley, hydraulic extraction device, or lifting equipment specifically approved for that roll assembly.
A C-hook, as mentioned in the original procedure, is suitable only for equipment designed to use that lifting method. It should not be treated as a universal roll changing tool for all 18-high mills.
The roll changing sequence must follow the manufacturer's instructions. Before removal, confirm that the chocks, locating components, drive connections, and associated support assemblies are in a safe condition.
Inspection of rolls and associated components
After removing the work roll, inspect the roll surface and other related components according to the load conditions during the strip break.
The inspection should include:
Work roll barrels, necks, and drive connections.
Intermediate rolls and their bearing support assemblies.
Backup or backing bearings and support components.
Side support rolls, side support roller boxes, and locking mechanisms.
Thrust bearings and axial locating components.
Roll chocks, seals, and lubrication connections.
Different 18-high mill designs use different roll support and changing arrangements. The inspection scope must therefore match the actual roll stack configuration.
Work roll damage inspection and assessment
Visual inspection alone is generally insufficient to determine whether a work roll exposed to a strip break can return to service. Suitable inspection methods must be selected according to the damage characteristics, roll material, and manufacturer's inspection requirements.
Visual and dimensional inspection
After cleaning the roll surface, carry out a detailed visual and dimensional inspection.
The main inspection items include:
Roll barrel diameter and remaining usable diameter.
Barrel roundness, cylindricity, and radial runout.
Roll crown and surface profile.
Roll surface roughness.
Roll neck dimensions and bearing fit surfaces.
Drive end dimensions and locating conditions.
A dial indicator can be used to measure roll runout under the specified support and measuring conditions. If roll bending is suspected, the results should be considered together with roll neck condition, shaft straightness, and other geometric measurements. A single runout measurement is not sufficient to confirm that a roll is bent.
Non-destructive testing
Different inspection methods are used according to defect location and detection requirements.
Magnetic particle testing (MT): Suitable for ferromagnetic roll materials. It can detect surface and near-surface cracks.
Eddy current testing (ET): Mainly used to detect surface and near-surface defects. Some automated systems can perform continuous inspection during roll grinding.
Ultrasonic testing (UT): Used to detect internal and subsurface defects, including internal cracks, inclusions, and other discontinuities.
Liquid penetrant testing (PT): Used to detect surface-breaking defects in non-porous materials. Its suitability depends on the material and surface condition.
If the roll has experienced a severe impact, abnormal overload, or suspected crack propagation, qualified personnel should assess whether additional monitoring, such as acoustic emission testing, is required.
These methods have different detection capabilities. They must be selected according to roll material, defect characteristics, and inspection requirements rather than treated as interchangeable techniques.
Work roll repair or replacement
After inspection, the work roll should be classified according to defect depth, defect location, remaining barrel diameter, roll profile, and mill operating requirements.
Repairing minor damage
If the roll has only minor material pickup, shallow scratches, or small dents, and inspection confirms that there are no cracks or internal defects, roll grinding may be used to restore the surface.
The following conditions must be checked:
Whether the defect can be completely removed.
Whether the remaining roll diameter is within the permitted operating range.
Whether the roll crown and profile meet the current rolling process requirements.
Whether the surface roughness and geometric accuracy meet the required specifications.
Whether new cracks or thermal damage develop during grinding.
A work roll with cracks must not be returned to service simply because the visible surface marks have been removed by grinding. The crack depth, propagation condition, and any internal defects must first be assessed by qualified personnel before a repair or rejection decision is made.
Replacing severely damaged rolls
If a work roll has deep cracks, extensive spalling, significant bending deformation, or a remaining diameter below the minimum permitted value, it must be scrapped or replaced according to the inspection results.
Before installing a replacement roll, confirm that its barrel diameter, crown, hardness, and surface roughness meet the current rolling process requirements. The roll neck dimensions, bearing fits, and drive end connections must also be checked.
Roll suitability cannot be determined by barrel diameter alone. Compatibility with the intermediate rolls and other supporting roll assemblies must also be considered.
Checks and commissioning before restarting the mill
After the work roll has been repaired or replaced and the assembly has been completed, the entire roll stack must be inspected and commissioned. Normal rolling speed should not be restored immediately.
The main tasks include:
Check the assembly: Confirm that the work rolls, intermediate rolls, and associated support components are correctly installed. Verify chock positioning and drive connections.
Check lubrication and cooling: Confirm that the bearing lubrication system, rolling fluid supply, and associated pipelines operate correctly without leakage or blockage.
Perform low-speed inching: Follow the equipment operating procedure to carry out an unloaded inching or low-speed test. Check for abnormal resistance, binding, or noise.
Calibrate the roll gap and measuring systems: Check the roll gap zero position, position feedback, and relevant AGC settings.
Check rolling force and roll adjustment systems: Verify the condition of the rolling force measurement system, screwdown mechanism, roll bending, and roll shifting functions within the scope of the maintenance work.
Conduct a low-load rolling trial: Use a reduced speed and load consistent with the process requirements. Monitor strip surface quality, flatness, thickness, and equipment operating conditions.
Resume production gradually: Increase rolling speed and load according to the process requirements only after all relevant checks have been completed successfully.
Before restoring power, authorized personnel must follow the site's energy control procedure. Tools and foreign objects must be removed, personnel must leave the hazardous area, protective devices must be restored, and all lockout/tagout devices must be removed through the approved procedure. Relevant operators must be notified before restarting.
Root cause analysis and preventive measures
Repairing or replacing the work roll does not eliminate the cause of the strip break. The incident must also be investigated to prevent recurrence.
The investigation should consider:
Incoming strip edge cracks, thickness variations, and surface defects.
Weld quality and strip joint condition.
Rolling speed, front and back tension, and pass reduction.
Work roll surface damage and abnormal roll gap conditions.
Rolling fluid lubrication and cooling conditions.
Side support systems, backing bearings, and thrust bearing condition.
Possible faults in sensors, hydraulic systems, and automatic control programs.
For recurring strip breaks, rolling records, equipment alarms, rolling force, and tension variations should be analyzed together. The rolling schedule or maintenance plan can then be adjusted according to the identified causes.
An 18-high rolling mill contains precision components in its work roll assemblies, bearing supports, side support systems, and drive connections. In addition to roll-related sleeves, connecting components, and locating structures, we provide custom precision machining for non-standard parts used in tobacco and packaging machinery, filling equipment, yacht equipment, and remote-controlled racing cars. Hansheng Automation manufactures precision components according to customer-supplied drawings and specified machining requirements.
