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Research on the influence of work roll size of cold rolling mill on rolling performance

Description:

Cold-rolled steel strips and sheets generally have a thickness of 0.1-3mm and a width of 100-2000mm. They have the advantages of smooth surface, flatness, dimensional accuracy and good mechanical properties. They are widely used in automobile manufacturing, electrical appliances, building materials, packaging materials, and household appliances. Various industries such as electrical appliances. With the progress of the times, various industries have higher and higher requirements for the quality of cold-rolled strip and thin plates. Especially in the automobile manufacturing industry, the quality requirements for plates have reached unprecedented heights. Facing the challenge, various advanced rolling mills were designed and manufactured. These rolling mills use different rolling methods and different roll system configurations. From the 4-high rolling type, the 6-high rolling type, the 8-high rolling type, the 12-high rolling type to the 20-high rolling type, the overall development trend is multi-roll and smaller roll diameter. There must be an inevitability for such a development trend. Based on the theoretical research point of view, this paper analyzes and studies the influence of the change of work roll diameter on the rolling performance, and strives to find a rule to provide reference for related researchers.

Title: Research on the influence of work roll size of cold rolling mill on rolling performance

Keyword: Rolling roll, work roll, cold rolling mill,rolling performance

Related product: work roll

 

  • Introduction to the force status of the cold rolling process
  1. Introduction to the rolling process

The rolling process of the steel strip is a process in which when the steel strip passes through the roll gap defined by the upper and lower work rolls, it is subjected to the rolling pressure of the upper and lower work rolls to produce plastic elastic deformation and make the steel strip thinner, thereby obtaining a steel strip with a certain thickness. Because the rolling process produces a great extrusion force on the steel strip, the crystal grains of the steel strip are flattened and elongated, the crystal lattice is distorted, and the crystal grains are broken, resulting in a decrease in the plasticity of the steel strip and an increase in hardness and strength. The phenomenon of work hardening occurs. At the same time, the rolling process of the steel strip generates great deformation heat, which increases the temperature of the strip steel and the rolls. Excessive temperature of the roll surface will cause the hardness of the quenched layer of the work roll to decrease, which will affect the surface quality of the strip and the life of the roll. The increase of roll temperature and uneven roll temperature distribution will destroy the normal roll shape and directly affect the strip shape and dimensional accuracy. Therefore, in order to ensure the normal production of cold rolling, effective cooling measures, such as emulsions, should be taken for the rolls and strip steel.

During the rolling process, due to the uneven chemical composition and structure of the strip, the original thickness of the strip is uneven; the thermal expansion of the roll, the wear of the roll, the eccentric operation of the roll, etc.; there are gaps between the contact surfaces of the rolling mill and the elasticity of the rolling mill itself Deformation will cause fluctuations in strip thickness. In order to ensure that the longitudinal thickness of the strip is constant, constant roll gap control is used in the production to realize the control of its thickness. That is, the thickness value fed back by the thickness sensor controls the hydraulic equipment to dynamically adjust the rolling pressure, so as to ensure the control method of the constant thickness of the rolling gap. This method requires the rolling mill to have high rigidity.

When the constant roll gap control is adopted, the rolling pressure is dynamically and drastically changed, which causes the bending degree of the work roll during the rolling process to also dynamically change, resulting in the strip steel in the width direction, the edge part and the middle part Uneven thickness phenomenon. The constant roll gap method, although the thickness of the middle part is guaranteed to be uniform, but the accuracy of the edge part is lost. For this reason, another control method has been produced: constant pressure control.

When using constant pressure control, the rolling pressure remains unchanged, and the strip deviation due to the thickness deviation of the blank is not adjusted. At this time, the obtained strip shape is the best. Constant pressure control is generally used in the operation of the leveler to improve the shape of the rolled sheet.

In the rolling process, you can also use HC, CVC, UPC roll shape control method and other dynamic control methods to control the shape of the shape.

  1. Force analysis of work roll during rolling

work roll

N0-rolling pressure

N0’——Reaction force of strip steel to work roll

f Friction-the reaction static friction force exerted by the strip steel on the work roll

Fx——X-direction bearing reaction force

Fy——Y-direction bearing reaction force

M drive-drive torque

With constant roll gap control, the rolling force changes rapidly dynamically, so N0 and N0′ also change rapidly dynamically, which belong to alternating forces. This also leads to a sharp change in Fx Fy at all times, which is also an alternating force. When the strip rolling linear speed is constant, that is, when V0 is constant, there are:

M drive = f friction·r-torque balance, r is the radius of the work roll

From the contact form, the work roll and the steel belt, the work roll and the support roll (or the intermediate roll) are all in line contact, and there is a sharply changing alternating stress at the contact point, and there is a large stress concentration phenomenon.

From the perspective of elastic deformation, the work roll is subjected to two alternating pressures of N0 and N0′, which produce dynamic local elastic deformation on its upper and lower contact surfaces.

 3. Detailed explanation of rolling deformation zone

Assuming that the rigidity of the work roll is infinite, there is no elastic deformation of the work roll under the rolling force. Then, the rolling deformation zone is the part indicated by the theoretical deformation zone in the figure.

The actual situation is that the work roll has a certain thickness of hardened layer after surface quenching, and the hardness is relatively high. However, under the action of the rolling force, deformation will still occur, and the deformation amount is A, which is the amount of flattening deformation.

After the strip is rolled by the work rolls, when it exits the roll gap, there will be a certain amount of elastic recovery deformation, and the amount of deformation is B.

The influence of the change of roll diameter on the stiffness of the rolling mill

From the force analysis, it can be seen that the work roll is subjected to a sharply changing alternating stress, and there is a large local stress concentration phenomenon on the contact part of the upper and lower lines, and at the same time, the contact part will produce local elastic deformation.

In the production of actual work rolls, surface quenching treatment is generally used to produce a certain thickness of hardened layer when the roll diameter is larger, or surface carburization, nitriding and other treatments are carried out to improve the surface hardness. Due to the large roll diameter, the quenching depth cannot reach the roll core, which makes the hardness of the core material lower. In practical applications, when subjected to a sharply changing alternating stress, because the shell is hard and soft, internal stress gradients will be generated at various positions on the contact line, resulting in inconsistent internal elastic deformations at each point, which makes it easier to accumulate and accumulate internal defects. Expansion, resulting in fatigue cracks or partial peeling of the surface. Coupled with the effect of rolling heat, the failure rate of the roll surface will be faster, which directly affects the quality of the steel strip and the production schedule.

From the analysis of the deformation zone, we know that the actual deformation zone length is greater than the theoretical deformation zone length. When a larger diameter work roll is used, because the shell is hard and soft, the elastic deformation generated on the contact line is larger than the deformation when the whole is hardened, so that the actual deformation zone length becomes longer. In the case of constant pressure, the pressure at the unit contact point is actually reduced, but the yield limit of the steel strip is fixed. Therefore, only by providing greater rolling force can the compressive stress at the unit contact point be increased, and the ideal can be obtained. The rolling effect.

When the work roll diameter is reduced, it will bring the following benefits:

  1. The reduction of work roll diameter is beneficial to the overall quenching, so that the whole has a uniform hardness. Under the action of the rolling force, the elastic deformation will be reduced, thereby reducing the bouncing of the rolling mill, improving the rigidity of the rolling mill, and facilitating thickness control. At the same time, the length of the rolling deformation zone is also reduced, 5 increases the pressure at the unit contact point, and the ideal rolling effect can be obtained under a relatively small rolling force. When multiple rolling mills are used for rolling at the same time, it is beneficial to overcome the work hardening caused by the previous rolling and ensure the thickness of the plate.
  2. The reduction of the work roll diameter is beneficial to increase the fatigue strength of the work roll, increase the working time of the work roll, and reduce the consumption of the work roll. (Reference 2)
  3. The diameter of the work roll is reduced, which is beneficial to reduce the bounce of the rolling mill and improve the rigidity of the rolling mill. When the wall thickness of the rolling mill is the same, under the same rolling force, the elastic deformation of the rolling mill stand with a small cavity height is smaller than that of the rolling mill stand with a large cavity height, which is beneficial to reduce the bouncing of the rolling mill and improve the rolling mill. rigidity.
  4. The diameter of the work roll is reduced, the rigidity is improved, and thinner steel strip can be rolled, and the range of the plate suitable for processing becomes larger.

      4. Conclusion

The reduction of work roll diameter is conducive to energy saving and consumption reduction, is conducive to the control of the thickness of the steel strip, and is conducive to the rolling of thinner steel strip.

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