The surface of cold-rolled work rolls should be free of defects, and the smoothness should meet the requirements. The surface is hard, the center is soft and soft, and it has a long service life. The hardness of the backup roll should be lower than that of the work rolls. Avoid grinding the work roll surface. The support rollers are generally replaced and reground every six months. Due to the large contact stress and frequent alternating stress during rolling, the back-up roll fatigues after a period of use. The surface peels off, and when the peeling off is not large, some can continue to be used after cleaning.
Keyword：work rolls
Work Rolls selection
1. The surface state of the work roll
The surface of cold-rolled work rolls should be free of defects, and the smoothness should meet the requirements. The surface is hard, the center is soft and soft, and it has a long service life. The working rolls are generally made of forged alloy steel, and the hardness of the roll body is required to be above 95-100° Shore. The hardness of the roll neck is above 45-50° Shore.
The hardness of the backup roll should be lower than that of the work roll. Avoid grinding the work roll surface. The hardness of the backup roll is generally Shore 60-65°, and the roll neck is Shore 42°. The support rollers are generally replaced and reground every six months. Due to the large contact stress and frequent alternating stress during rolling, the back-up roll fatigues after a period of use. The surface peels off, and when the peeling off is not large, some can continue to be used after cleaning. When peeling is found in some rolling mills, regardless of the size of the area and the depth, the roll must be changed, and the peeled part can be ground before it can be reused. In order to prevent surface peeling, the backup roller should be tempered and recovered after being used for a certain period of time (generally half a year).
2. Selection of roll type and grinding roll
The cold-rolled roll shape changes due to the influence of pressure and deformation heat, and the roll shape is bent by the rolling force (referred to as the pressure roll shape). The roll expands unevenly due to uneven heating, that is, the middle part expands more than the edge part (referred to as temperature roll type). In order to compensate for the change of roll shape caused by pressure and temperature, the roll must be ground into a certain arc in advance, which is generally called the roll shape of the roll. For the cold rolling mill, the pressure roll type is the main one, so the curvature selected by the roll type of the cold rolling mill is usually the convexity. The size of the crown is related to the rolling reduction, the yield strength and width of the rolled piece, the heating condition of the roll, the material of the rolled piece and the roll, the tension during rolling, the performance of the lubricant and many other factors.
The roll should be ground on a special roll grinder. The purpose of grinding is to remove the defects such as depressions, bruises, scratches, and aluminum sticking on the surface of the roll to obtain the required roll shape and roughness. For the roll profile after grinding, the convexity point should be at the center point of the roll body length, and it should gradually and smoothly transition symmetrically to both sides.
3. Roll configuration method
The configuration method of roll shape refers to the problem of how to distribute the cold roll shape or roll crown required by a rolling mill to each roll, whether it is evenly distributed on the upper and lower work rolls or only ground on the upper roll or the lower roll. The configuration of the roll type should consider the conditions such as roll grinding and whether the adjustment of the roll type is simple, and on the premise of ensuring the product quality, it should be determined according to the specific situation. The usual configuration method is:
（1）Double cold rolling mills, most of them adopt the configuration method that both the upper and lower rolls have convexity, and only a few of them only have convexity on the upper roll and the lower roll is cylindrical.
（2）In the quadruple cold rolling mill, most of the work rolls have a crown, and the backup rolls are cylindrical. If the total crown is not large, only the upper work roll has a crown, and the lower work roll is cylindrical. If the crown is large, the total crown is evenly distributed on the upper and lower work rolls.
Control of work rolls shape
In the strip rolling method, during the rolling process, due to the fluctuation of some rolling factors such as rolling pressure and metal deformation heat, the roll shape changes greatly, resulting in uneven thickness or waves of the strip. These waves sometimes appear in the middle and sometimes on the sides. 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 good for rolling. If it is too large, it may become the final waste product or produce a broken strip during the rolling process. In order to obtain a strip in a good rolling state and to facilitate the continuation of normal rolling, it is necessary to control the strip shape in time.
1. General method of roll shape control
When the rolling speed is low, the waves in the strip should be eliminated by adjusting factors such as spraying, bending rolls, tension, reduction and speed according to the size and position of the waves.
2. Use the hydraulic bending device to control the plate shape
Waves appear during the rolling process, no matter in the edge or middle of the strip, it is not conducive to continuous rolling. Although the above-mentioned general measures to eliminate waves have certain adjustment capabilities, the adjustment speed is too slow and the adjustment range is small, such as tension, If the adjustment range is too large, it is easy to cause the belt plate to break and cause the belt to break.
In recent years, hydraulic devices have been installed on the support rolls and work roll bearing boxes, and hydraulic cylinders are used to force the support rolls or work rolls to bend to control the required roll shape. This adjustment method is called hydraulic bending method. Once waves appear in the rolling process, especially in the high-speed rolling process, they can be quickly and effectively eliminated, and the flatness of the product can be obviously improved. The control method is: when the edge of the strip appears waves, increase the P force or reduce the Q force to increase the crown of the roll. This type of bending roll is called positive bending. When the wave appears in the middle of the strip, the Q force is increased or the P force is reduced to reduce the crown of the roll. This type of bending roll is called negative bending.
3. Automatic control of roll shape
The modern cold rolling mill is equipped with a shape meter, which connects hydraulic bending, roll tilt, rolling force, coolant and tension through a microcomputer to form a closed loop to realize the automatic control system of the shape. That is to rely on the shape instrument to send out a signal, and realize the automatic control of the shape through the electronic computer.
Tension control
Modern strip rolling mills are generally equipped with a stronger front and rear tension system. The front tension value can exceed the yield strength of the part with a small extension of the rolled piece. In this way, it is possible to avoid waves caused by uneven partial metal extension of the strip, and it is easier to roll out the strip to be straight.
When the tension is increased, the rolling force can be reduced. When the back tension increases, the reduction of rolling force is greater than that of the front tension, but if the back tension is too large, it will cause excessive back slip, reduce the front slip value, or even make the front slip value equal to zero. In this way, the rolled piece “slips” on the roll, and it is difficult to establish a stable rolling process, so the increase in post tension should not exceed a certain limit.
The tension improves the centering of the strip during rolling, prevents or reduces the deviation phenomenon when there is no tension, and creates favorable conditions for high-speed rolling. However, in strip tension rolling, the tension should be properly adjusted according to the specific situation, otherwise it will cause strip breakage or mutual misalignment between coil layers, causing scratches on the strip surface.
The maximum tension applied in cold rolling. In theory, it should not exceed the yield strength of the rolled piece. When rolling aluminum alloy strip, according to the plasticity and edge state of the material, the unit tension is generally taken as 50-60% of the yield strength of the rolled piece in this pass.
The adjustment range of tension (ratio of maximum and minimum tension) is generally 10:1 to 20:1, within which a given tension value can be accurately achieved. During rolling, the tension on the strip should be kept balanced and constant. If the fluctuation of tension is light, it can cause the layers of the coil to shift each other, resulting in scratches on the surface of the strip. When the fluctuation is severe, 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 degree of cold hardening of the strip increases, that is, the yield strength of the rolled piece increases accordingly. Therefore, as the number of rolling passes increases, the selected unit tension value for each pass should also increase accordingly. The uncoiling tension should be lower than the tension during the previous coiling, so as to avoid the misalignment between the coil layers during the uncoiling and rolling of the strip and scratch the surface of the strip.
