The main factors affecting rolling - 2, cold rolling technology (125)
Plate control
The modern cold rolling mill is equipped with a shape meter, which connects the hydraulic bending rolls, rolling force, coolant and tension through a microcomputer to form a closed loop, and realizes the automatic control system of the shape, that is, relying on the shape meter. Signal, through the microcomputer to realize the automatic control of the board type.
一、Selection of rolls
1. The surface state of the roll
The surface of cold-rolled work rolls should be free of defects, and the smoothness should meet the requirements. Hard surface, soft center and long service life. The work roll is generally made of alloy steel by forging, and the hardness of the roll body is required to be above 95-100° Shore. Roll neck hardness is above Shore 45-50°. The hardness of the backup roll should be lower than that of the work roll. Avoid scratching the work roll surface. The hardness of the backup roll is generally 60-65° Shore and the roll neck is 42° Shore. The support rollers are generally replaced and reground once every six months. Due to the high contact stress and frequent alternating stress during rolling, the back-up rolls become fatigued after a period of use. The surface is peeled off. When the peeling is not large, some can continue to be used after cleaning. When some rolling mills are found to be peeled off, no matter the size of the area or the depth, the rolls must be changed, and the peeled parts can be ground before reuse. In order to prevent the surface from peeling off, the back-up roller should be tempered and restored after being used for a certain period of time (usually half a year).
2. Selection of roller type and grinding roller
The roll shape of cold rolling changes due to the influence of pressure and deformation heat. The roll shape of the roll is bent due to the rolling force (referred to as the pressure roll shape). The part expands greatly (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 cold rolling mills, the pressure roll shape is the main, so usually the selected arc of the roll shape of the cold rolling mill is the crown. The size of the crown is related to the rolling reduction, the yield strength and width of the rolling piece, the heating conditions of the roll, the material of the rolling piece and the roll, and the tension during rolling, lubricant properties and many other factors. The roll should be ground on a special roll grinder. The purpose of grinding is to remove defects such as dents, bumps, scratches, and aluminum sticking on the surface of the roll to obtain the required roll shape and roughness. For the roll shape after grinding, the large crown should be at the center point of the length of the roll body, and gradually and smoothly transition to both sides symmetrically.
3. Configuration method of roll shape The configuration method of roll shape refers to the problem of how to distribute the cold rolling 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 on the upper or lower roll. The configuration of the roll shape should consider the conditions such as roll grinding and whether the adjustment of the roll shape is easy. On the premise of ensuring the quality of the product, it should be determined according to the specific situation. The usual configuration method is:
(1) Most of the double cold rolling mills adopt the configuration method that both the upper and lower rolls have crowns, and there are only a few rolls in which only the upper rolls have crowns and the lower rolls are cylindrical.
(1) For quadruple cold rolling mills, most of the work rolls have crowns, and the backup rolls are cylindrical. If the total crown is not large, only the upper work rolls have crowns, and the lower work rolls are cylindrical. If the crown is larger, the total crown is evenly distributed on the upper and lower work rolls.
二、Roll shape control
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 of the strip or waves. Sometimes in the middle, sometimes in the sides. The appearance of waves indicates that the roll shape has changed, causing uneven pressure distribution of the strip in the width direction. The appearance of waves is not good for rolling. If it is too large, it may become a waste product or break the strip during the rolling process. In order to obtain good rolling The strip in the out state is beneficial to continue normal rolling, and the plate shape needs to be controlled in time.
1. General method of roll shape control
When the rolling speed is low, the wave of the strip should be eliminated by adjusting the spray, bending roll, tension, reduction and speed according to the size and position of the wave. The general measures for plate shape control are as follows: 2. Use hydraulic bending roll device to control the plate shape
Waves appear in the rolling process, no matter at the edge or the middle of the strip, it is not conducive to continued rolling. Although the above general measures to eliminate waves have a certain adjustment ability, 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 strip to be broken and the strip to break.
In recent years, hydraulic devices have been installed on the support rolls and work roll bearing boxes, and the required roll shape is controlled by a hydraulic cylinder forcing the support rolls or work rolls to bend. This adjustment method is called the hydraulic bending method. In the rolling process, especially in the high-speed rolling process, once the wave appears, it can be quickly and effectively eliminated, and the flatness of the product can be obviously improved. The control method is: when the wave appears on the edge of the strip, increase the P force or Reducing the Q force increases the crown of the roll. This type of bending is called positive bending. When there is a wave in the middle of the strip, increasing the Q force or reducing the P force reduces the crown of the roll. This type of bending is called negative bending.
2. Automatic control of roll shape
The modern cold rolling mill is equipped with a shape meter. It connects the hydraulic bending roll, roll inclination, rolling force, coolant and tension through a microcomputer to form a closed loop to realize the automatic control system for the shape of the plate, that is, relying on the plate shape. The type instrument sends out a signal, and the electronic computer realizes the automatic control of the plate type.
三、Tension control
Modern strip rolling mills are generally equipped with relatively strong front and rear tension systems, and the front tension value can exceed the yield strength of the smaller part of the partial extension of the rolled piece. . In this way, the uneven metal extension of the strip can be avoided to cause waves, and the rolled strip can be easily straightened.
When the tension is increased, the rolling force can be reduced. The increase in the back tension reduces the rolling force more than 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. ”, it is difficult to establish a stable rolling process, so the post tension increase should not exceed a certain limit.
The tension improves the neutrality of the strip during rolling, prevents or reduces the deviation phenomenon without tension, and creates favorable conditions for high-speed rolling. However, in the strip tension rolling, the tension should be adjusted properly according to the specific situation, otherwise it will cause the strip to be broken or the coil layers to dislocate each other, causing the strip surface to be scratched.
High tension used 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 (the ratio of large and small tension) is generally 10:1 to 20:1, within this range, the given tension value can be accurately achieved. During rolling, the tension of the strip should be balanced and Constant, the fluctuation of the tension, the light one can cause the mutual dislocation of the coil layers, resulting in scratches on the surface of the strip, and the strip can be broken when the fluctuation is serious. The tension fluctuation range is generally specified as ±1-3%.
During the rolling process, with the increase of the rolling passes, the cold work hardening degree of the strip increases, that is, the yield strength of the rolled piece increases accordingly. Therefore, as the rolling passes increase, the unit of each pass should be selected. The tension value should also be increased accordingly. The uncoiling tension should be lower than the tension of the previous coiling, so as to avoid the dislocation of the coil layers during the uncoiling and rolling of the strip and scratch the surface of the strip.
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