The rolling process of a cold rolling mill depends on the mill configuration and the material being processed. This article focuses on a single-stand 20-high reversing cold rolling mill, particularly the Sendzimir-type mill. Its production process consists of three main stages: coil loading and strip threading, reversing rolling, and coil unloading and recoiling.
Because a 20-high mill uses small-diameter work rolls and relies heavily on strip tension, the rolling process must coordinate roll gap, rolling force, front and back tension, and rolling speed. The pass schedule also needs to account for work hardening, strip flatness, and surface quality.
Coil loading and strip threading
Before rolling begins, the steel coil must be loaded onto the uncoiler and threaded through the mill to the take-up reel. This stage prepares the strip path and establishes the initial connection between the two coilers.
The main operations are as follows:
A coil transfer car moves the steel coil onto the uncoiler mandrel.
A floating uncoiler is commonly used to keep the strip centered relative to the mill. A photoelectric edge-position sensor detects lateral strip displacement and sends a signal to a hydraulic cylinder, which adjusts the uncoiler position.
After unwinding, the strip passes through a straightener, typically a three-roll head straightener or a five-roll straightener, to correct the coil set. Some mills are also equipped with a hydraulic shear for trimming the strip head.
A swing-type guide table carries the strip past the entry-side coiler and directs it toward the 20-high mill.
The uncoiler continues feeding the strip through the mill until the leading end reaches the jaws of the delivery-side coiler. The coiler grips the strip and winds it around the mandrel for 2 to 3 turns before the feed stops.
Once threading is complete, the work rolls are installed if they were removed during threading, and the rolling line is adjusted to its specified position. The mill enclosure door is closed, the entry-side pressure plate is lowered, and the exit-side strip wiper is brought into contact with the strip.
The process lubrication and cooling system is then started to supply rolling fluid. The rolls are brought into the required working position, and the coilers begin rotating to establish front and back tension.
The thickness gauges and speed measuring devices on both sides of the mill are moved into their operating positions. The equipment is then ready for the first rolling pass.
Reversing rolling
A single-stand 20-high reversing mill reduces strip thickness through several passes. After each pass, the rolling direction changes, and the functions of the two coilers are exchanged. The former take-up reel becomes the payoff reel, while the opposite reel takes up the strip.
First rolling pass
After threading and equipment preparation, the coilers establish the required front and back tension. The main drive starts, and the mill applies the specified roll gap and rolling force.
During the first pass, the strip travels from the entry-side uncoiler toward the delivery-side coiler. The thickness gauges and speed measuring devices monitor the strip during rolling.
If an operator identifies an edge defect that could affect high-speed rolling, its position can be marked through the control panel. The position signal is sent to the mill automation system so that the rolling speed can be reduced when the defective section approaches or passes through the roll gap.
As the strip tail approaches the designated stopping position on the entry side, the mill reduces speed and stops. The first pass is then complete.
After stopping, the thickness gauges and speed measuring devices are withdrawn from the rolling line. The roll gap is released, strip tension is removed, the supply of rolling fluid is stopped, and the entry-side pressure plate is raised.
Subsequent reversing passes
For the second pass, the strip travels in the opposite direction. The two coilers exchange their operating roles.
The coiler that wound the strip during the first pass now operates as the payoff reel. It reverses its rotation and feeds the strip leading end toward the opposite coiler. Once the opposite coiler grips the strip, it winds the strip around its mandrel for 2 to 3 turns.
The rolling fluid supply is restarted, the required roll gap is established, and the two coilers apply the specified front and back tension. The thickness gauges and speed measuring devices are moved back into the rolling line, and the second pass begins.
From the second pass onward, rolling continues alternately between the two coilers and the 20-high mill. The front and back sides exchange positions whenever the rolling direction changes.
When the automatic gauge control (AGC) system is in operation, strip thickness can be controlled automatically through feedback from the thickness measuring devices and the corresponding roll gap adjustment system. If a marked strip defect approaches the roll gap, the mill control system can reduce the rolling speed. At the end of a pass, the mill decelerates and stops according to the programmed control sequence.
The number of passes depends on the product requirements and the coiler arrangement. Some reversing schedules use an odd number of passes so that the finished coil remains on the designated delivery side. Mills equipped with expandable mandrels on both sides may also be configured for an even number of passes, allowing the finished coil to be unloaded from the entry side.
Final rolling pass
The work rolls are often replaced before the finishing pass when the product requires a high-quality or specially specified surface finish. Using freshly ground and polished work rolls helps maintain the required strip surface condition.
After the final pass, the mill stops and the roll gap is released. The thickness gauges and speed measuring devices are withdrawn, and the supply of rolling fluid is stopped.
The coiler pinch roll is lowered to hold the coil, or the coil car is raised so that its support roll holds the wound coil in position. The coiler then rotates to wind the remaining strip tail completely onto the mandrel and prevent the coil from loosening.
The reversing rolling process is complete once the strip is fully wound and the finished coil is secured.
Coil unloading and recoiling
The unloading method depends mainly on the coiler mandrel design. The two common arrangements are expandable mandrels and solid mandrels.
Unloading from an expandable mandrel
When an expandable mandrel is used, the finished coil can be removed directly from the coiler.
The operator first secures the coil with a radial strapping band. The coil car is then raised to support the coil from below. The coiler mandrel contracts and its gripping jaws open, allowing the coil to rest on the coil car.
The coil car and the coiler's auxiliary pusher plate move in coordination to slide the coil off the mandrel. The coil car then transports the finished coil to the designated storage station.
Recoiling from a solid mandrel
When a solid mandrel is used, the finished coil cannot be removed directly by contracting the mandrel. An additional recoiling operation is therefore required.
The finished coil and solid mandrel are transferred from the coiling position to a separate recoiling station. The strip is then unwound from the solid mandrel and rewound onto an expandable mandrel, allowing the finished coil to be removed.
Sendzimir and Sundwig 20-high mills equipped with solid-mandrel coilers may use this type of recoiling arrangement.
Because recoiling takes place outside the main rolling area, the recoiling operation can run simultaneously with rolling when the equipment layout provides an independent recoiling station and the two operations do not interfere with each other.
Rolling process schedules
The quality and productivity of a 20-high reversing mill depend heavily on its rolling process schedules. The main schedules include the reduction schedule, tension schedule, speed schedule, and roll profile arrangement.
Reduction schedule
The reduction schedule determines the amount of thickness reduction in each pass. It must be developed according to the mill's technical specifications, the mechanical properties of the material, and the product quality requirements.
The design must also account for production capacity, energy consumption, rolling force, motor torque, and available drive power.
A 20-high mill is suitable for rolling materials that require high reductions and close thickness tolerances. For carbon steel, particularly soft low-carbon steel, the mill can achieve a high total reduction over one rolling campaign.
The original process data gives a total reduction exceeding 95% and a maximum reduction of up to 66% in an individual pass. These figures represent demanding process conditions and should not be treated as standard limits for every material or mill. The actual schedule must be checked against rolling force, torque, motor power, strip tension, and work roll capacity.
For a reversing mill, all passes take place on the same stand. One common approach is to distribute the reductions so that rolling forces remain reasonably balanced across the passes.
The first and second passes generally use higher reductions. As rolling progresses, the material undergoes work hardening, increasing its deformation resistance. The reduction per pass is therefore gradually decreased to keep the rolling force within the mill's operating limits.
The rolling schedule should also make effective use of the available drive power of both the mill and the front and back coilers. Increasing the reduction per pass can reduce the total number of passes and improve productivity, but excessive reduction may exceed the equipment's force, torque, or power limits.
In some cases, additional passes are used to improve strip flatness, surface quality, and longitudinal thickness consistency. Pass distribution also affects the material's work-hardening history and mechanical properties. The resulting strength must be evaluated according to the material and rolling conditions rather than assuming that more passes always produce higher strength at the same total reduction.
The finishing pass has a significant influence on final strip flatness and surface quality. A reduction of approximately 10% is used as a reference in the original process description, but the actual finishing reduction depends on the target thickness, material condition, and product quality requirements.
Tension schedule
Strip tension is an important part of cold rolling, particularly in 20-high and other multi-roll mills with small-diameter work rolls.
Front and back tension help reduce rolling force, improve rolling stability, and influence strip flatness. The tension schedule must account for the material's mechanical properties, work-hardening condition, strip thickness, and edge quality.
The specific tension applied during rolling depends on the material's physical and mechanical properties, as well as the condition of the strip edges.
The original process data gives a unit tension range of 20% to 70%. However, it does not specify the reference basis for this percentage. The actual process specification must define whether this represents a proportion of yield strength or another reference value before it is used for tension setting.
Because 20-high mills rely on relatively high strip tension, their coilers require substantial drive power. The original design data indicates that the coiler motor power is generally 70% to 80% of the main mill drive power, with some configurations reaching 100%. These values are equipment-specific references rather than universal sizing requirements.
First-pass tension
During the first pass, the back tension is generally kept relatively low. This is particularly important when the incoming strip comes directly from a pickling line. Excessive back tension may cause relative movement between coil layers and lead to surface scratches.
An entry-side pressure plate can provide additional controlled resistance to increase back tension. Front tension can then be set according to the rolling process and strip specifications.
Tension distribution in subsequent passes
For later passes, front and back tension are adjusted according to the material grade, strip dimensions, reduction schedule, and flatness requirements.
The front tension from the previous pass may be used as the back tension reference for the next pass. In some operating schedules, unit front tension is set higher than unit back tension, but this relationship must be adjusted according to the rolling conditions.
Finishing-pass coiling tension
The coiling tension used during the finishing pass depends partly on the mandrel arrangement.
For mills using expandable mandrels, the finished coil may be removed directly and sent to a batch annealing furnace. In this case, relatively low coiling tension is used to reduce the risk of interlayer sticking during annealing.
The original process data gives a coiling tension below 50 MPa as a reference condition associated with a low probability of annealing adhesion. This value should be verified against the material, surface condition, residual rolling oil, and annealing process.
For mills using solid mandrels, the finished coil must be rewound onto an expandable mandrel before unloading. The recoiling operation can use a relatively low tension of 10 to 40 MPa. This arrangement allows higher tension to be used during the actual rolling process, subject to mill and strip limits.
Tension adjustment for strip flatness
Tension also affects the appearance and distribution of strip flatness defects. However, center buckle and edge wave cannot be corrected by simply increasing or decreasing the overall strip tension.
Front tension and back tension have different effects on flatness deviation. Increasing tension may reduce some center buckle conditions, while excessive tension may worsen edge buckling or increase the risk of strip damage.
Flatness correction should therefore be based on measured strip shape, tension distribution, roll profile, and available roll adjustment mechanisms. In a 20-high mill, intermediate roll shifting and other profile control functions may also be used to correct transverse thickness and elongation differences.
Speed schedule
Rolling speed should be selected according to the mill's mechanical and electrical capacity. Within the permitted speed range, higher rolling speeds can improve production output, but speed selection must also account for rolling force, temperature rise, lubrication, and strip quality.
The first pass generally uses a lower rolling speed. This is because the first pass often has a relatively high reduction, which increases heat generation at the roll-strip contact area. The cooling conditions of a multi-roll mill may limit how quickly the rolls can dissipate this heat.
The incoming strip may also have relatively large longitudinal thickness variations, and its flatness may not fully match the roll profile. A lower first-pass speed gives operators more time to monitor strip behavior and make necessary adjustments. The combination of high speed and high reduction may also exceed the available motor capacity.
For subsequent passes, the rolling speed is determined by the reduction schedule, tension settings, and available main drive and coiler power. The mill should operate at a speed that makes effective use of its drive capacity without exceeding the process limits.
Each pass includes acceleration at startup and deceleration at shutdown. During steady rolling, unnecessary speed changes should be avoided because they can disturb tension and affect longitudinal thickness consistency. Speed adjustments should be made according to process requirements, defect positions, and equipment operating conditions.
An increase in rolling speed can also change friction and lubrication conditions, sometimes reducing rolling force. This effect depends on the rolling regime and should not be assumed to occur under all operating conditions.
Roll profile
Roll profile control is important for maintaining strip flatness and transverse thickness accuracy.
A 20-high mill uses a rigid housing and a cluster roll arrangement with multiple roll adjustment mechanisms. Depending on the mill design, cylindrical work rolls without a preset crown can be used, with strip profile controlled through intermediate roll shifting and other adjustment mechanisms.
The main roll profile arrangements include:
Work rolls: Generally use cylindrical surfaces or the profile specified by the roll grinding requirements. A suitable roll profile is selected according to the strip specifications and surface quality requirements.
First intermediate rolls: Commonly have a tapered section at one end to allow axial adjustment. Their lateral movement changes the roll contact conditions and provides a means of controlling strip profile.
Second intermediate rolls: May use a specified crown or eccentric adjustment arrangement, depending on the mill configuration. These adjustments help compensate for roll deflection and control strip flatness.
Backing assemblies: The backing rolls and bearing components must maintain the specified contact geometry. Their profiles and support conditions should follow the mill manufacturer's design requirements rather than being modified independently.
Different Sendzimir 20-high mill configurations use different combinations of tapered intermediate rolls, axial shifting, and secondary crown adjustment. The appropriate roll profile and adjustment strategy must therefore be determined from the actual roll stack design.
A suitable roll profile, combined with tension control and accurate roll gap adjustment, helps maintain strip flatness and thickness consistency throughout the reversing rolling process.
