Selection of Rollers
Cold rolling work rolls must be defect-free and meet finish requirements. The surface is hard, the core is tough, and service life is long. Work rolls are forged from alloy steel with a body hardness of Shore 95-100° and a neck hardness of Shore 45-50°. Support roll hardness is lower, typically Shore 60-65°, with a neck hardness of Shore 42°. Avoid grinding work roll surfaces. Support rolls are replaced and reground every six months. Rolling generates high contact stress, causing fatigue and surface peeling. Minor peeling can be cleaned and reused, but severe peeling requires roll replacement and regrinding. To prevent peeling, support rolls should be tempered every six months.
Roll type
Cold rolling changes due to pressure and deformation heat. Rolls bend under rolling force (pressure roll type) and expand unevenly from heat (temperature roll type). To compensate, rolls are pre-ground into a specific arc, called roll type. Cold rolling mills mainly use pressure roll types, so rolls are usually ground with a convex arc. Convexity depends on rolling reduction, material properties, roll heating, tension, and lubricant performance. Rolls are ground on special grinders to remove defects like dents, bruises, and scratches. After grinding, the maximum convexity should be at the roll center, transitioning smoothly to the edges.
Roller configuration method
Roller configuration method. The roll configuration method refers to the problem of how to distribute the cold rolling roll shape or roll convexity required by a certain rolling mill to each roll, whether it is evenly distributed on the upper and lower working rolls or only ground on the upper or lower rolls. The configuration of the roll shape should take into account conditions such as roll grinding and whether the roll shape adjustment is simple. Under the premise of ensuring product quality, it should be determined according to the specific situation. The usual configuration method is:
⑴ For double cold rolling mills, most of them adopt the configuration method in which both the upper and lower rolls have convexity. Only a very small number of rolls have convexity and the lower roll is cylindrical.
⑵ For quadruple cold rolling mills, most of the working rolls have convexity and the support rolls are cylindrical. If the total convexity is not large, only the upper working roll has convexity and the lower working roll is cylindrical. If the convexity is large, the total convexity is evenly distributed on the upper and lower working rolls.
Roller shape control
During the strip rolling process, due to the fluctuation of rolling pressure, deformation heat of metal and other rolling factors, the roll shape changes greatly, causing uneven thickness or waves on the strip. These waves sometimes appear in the middle and sometimes appear on the edge. The appearance of waves indicates that the roll shape has changed, causing uneven pressure distribution in the width direction of the strip. The appearance of waves is not conducive to rolling. If the waves are too large, they may become final waste or cause strip breakage during the rolling process. In order to obtain a strip in a good rolling state and facilitate continued normal rolling, the plate shape needs to be controlled in time.
1. General method of roll shape control
When the rolling speed is low, the waves on the strip should be eliminated by adjusting factors such as spraying, bending rolls, tension, pressure reduction and speed according to the size and position of the waves. The general measures for plate shape control are as follows:
2. Use hydraulic bending roll device to control the plate type.
Waves appear during the rolling process, whether at the edge or the middle of the strip, are not conducive to continued rolling. Although the general measures to eliminate waves mentioned above have certain adjustment capabilities, the adjustment speed is too slow and the adjustment range is small. For example, tension, if the adjustment range is too large, it is easy to cause the strip to crack and break.
In recent years, hydraulic devices have been installed on the bearing boxes of the support rolls and working rolls, and the required roll shape is controlled by using hydraulic cylinders to force the support rolls or working rolls to bend. This adjustment method is called hydraulic bending roll method. Once waves appear during the rolling process, especially during high-speed rolling, they can be eliminated quickly and effectively, which can significantly improve the flatness of the product. The control method is: when waves appear on the edge of the strip, increase the P force or reduce the Q force to increase the roll convexity. This type of bending roll is called positive bending. When waves appear in the middle of the strip, increase the Q force or reduce the P force to reduce the roll convexity. This type of bending roll is called negative bending.
3.Automatic control of roll shape
Modern cold rolling mills are equipped with a plate shape meter, which connects hydraulic roll bending, roll tilting, rolling force, coolant and tension through a microcomputer to form a closed loop, thus realizing a plate shape automatic control system, that is, relying on the plate shape meter to send signals and realize automatic control of the plate shape through an electronic computer.
Tension control(one)
Modern rolling mills use strong front and rear tension systems. The front tension can exceed the local yield strength, allowing controlled extension. This prevents uneven elongation and wave formation, ensuring a straighter strip. Higher tension reduces rolling force, with rear tension having a greater effect. However, excessive rear tension causes back slip, reducing or eliminating front slip, making stable rolling difficult. Proper tension improves strip centering, reduces deviation, and enables high-speed rolling.
Tension control (two)
When rolling with tension, proper adjustment is crucial to prevent strip breakage or coil misalignment, which can cause surface scratches. The maximum cold rolling tension should not exceed the yield strength of the strip. For aluminum alloy strips, unit tension is typically 50-60% of the yield strength, depending on plasticity and edge condition. The tension adjustment range is usually 10:1 to 20:1, ensuring precise control. Maintaining stable tension prevents coil displacement and surface defects.
Conclusion
In severe cases, the strip can be broken. The tension fluctuation range is generally specified as±
1-3%. During the rolling process, as the number of rolling passes increases, the cold work hardening degree of the strip increases, that is, the yield strength of the rolled piece increases accordingly. Therefore, as the number of rolling passes increases, the unit tension value selected for each pass should also increase accordingly. The unwinding tension should be less than the tension during the previous coiling pass to avoid the displacement between the coils during unwinding and scratching the strip surface.
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