Description: The peeling off of the oxide film on the surface of the rolling mill rolls is an important factor affecting the surface quality of the strip. Effective control of the peeling off of the oxide film on the roll surface requires a systematic and comprehensive solution
Keyword: Rolling mill rolls
In a 1780 mm hot tandem rolling mill, the peeling of the surface oxide film on the work roll of the finishing mill seriously affected the surface quality of the strip, resulting in about 1500 t of steel coils being judged as waste products every month.
The reasons for the peeling of the oxide film on the roll surface were analyzed from the aspects of roll material, roll cooling water, anti-stripping water, rolling lubrication and water cutting plate, etc., and by optimizing the roll cooling water, adjusting the spraying position of the anti-stripping water, and eliminating the interaction between headers Interference, water cutting plate configuration and other technical improvement programs have solved the problem of peeling of oxide film on the surface of rolls in hot rolling production, and the waste and defective products of hot-rolled coils caused by peeling of oxide film have been reduced by more than 90%, and the company has achieved remarkable results in waste reduction and efficiency increase.
Fundamental analysis of rolling mill rolls surface oxide film peeling
During the hot rolling process, the roll forms an oxide film to protect the roll surface under high temperature conditions. The quality of the oxide film directly affects the service life of the roll, so the control of the thickness of the oxide film is very important. If the oxide film is too thin, it cannot protect the roll during the rolling process and is easily crushed, resulting in rough surface of the roll. In addition, the fine powder particles of the shattered oxide film are pressed into the surface of the strip steel to produce surface defects such as pinhole pits. If the oxide film is too thick, it is prone to massive spalling, which seriously affects the surface quality of the strip.
The peeling off of the oxide film on the surface of the roll is mainly affected by the following factors
(1) The influence of roll material:
At present, the main roll materials used in industrial production are infinite hard cast iron, high chromium cast iron and high-speed steel. The material of F1~F4 working rolls in the finishing rolling unit of the 1780 mm hot rolling mill has been changed from high-chromium cast iron to high-speed steel, F1~F4, and the material of working rolls is infinitely chilled cast iron.
The friction factor of high-speed steel rolls is higher than that of high-chromium cast iron rolls, so the rolling force of high-speed steel rolls is 10% to 20% greater than that of high-chromium cast iron rolls. When the rolling force increases, the heat transfer coefficient between the strip and the roll increases, which eventually leads to an increase in the heat transferred to the roll. In addition, high-speed steel has a large coefficient of thermal expansion and poor heat dissipation, and the average temperature of the rolling mill is much higher than that of high-chromium cast iron rolls. Therefore, during use, high-speed steel rolls need better cooling conditions. In general, the testing temperature of high-speed steel rolls should be controlled below 60 ℃ to ensure the surface condition of the rolls. The roll temperature of the F1-F4 work rolls in the 1780 mm hot rolling mill was above 70 ℃, which was obviously high, which was mainly caused by the insufficient cooling efficiency of the roll cooling water.
(2) The influence of roll cooling water.
During the hot rolling process, the rolling rod is in contact with the high-temperature rolled piece, and the temperature of the rolling roll rises rapidly. Roll cooling water can take away the heat of the roll, keep the temperature of the roll at a low level, control the thermal expansion and thermal crown of the roll, and effectively reduce the wear of the roll. In addition, the cooling water can lubricate the roll, reduce the friction factor, and reduce the rolling force to a certain extent.
The selection of roll cooling water pressure and water volume plays a vital role in the cooling effect of the roll. If the pressure of the cooling water is too high, the cooling water will splash heavily after spraying on the surface of the roll, and the roll cannot be fully cooled, and the cooling efficiency is low. The amount of cooling water mainly affects the formation of chlorinated film on the surface of the roll. If the amount of water is too large, it will be difficult to form an oxide film on the surface of the roll, and the wear of the roll will increase, which will affect the surface quality of the strip. If the amount of water is too small, the oxide film will grow too thick and fall off. Obvious peeling bands appear. Therefore, a cooling water system with lower pressure and higher water volume should be used. A large amount of cooling water is attached to the surface of the roll to take away more heat from the roll to ensure a higher cooling efficiency of the roll.
When high-chromium cast iron rolls are used for the work rolls of the finishing mill, the German Rolling Rod Manufacturing Company believes that the water pressure should be 0.5~1.0 Mpa. According to calculations, the minimum cooling water volume of the high-chromium cast iron rolls of the 1 780 mm finishing mill is 660 m3/h. However, according to the production practice of a hot rolling mill, the actual use value of cooling water for work rolls should be 20% larger than the theoretical calculation value. High chromium cast iron rolls can be calculated based on the minimum cooling water volume requirement of 4.5 m3/(cm h), and the minimum rolling cooling water volume of the 1780 mm finishing mill is 801 m3/h; , The amount of cooling water should be increased by 20% on this basis, and F1～F4 can be calculated. The minimum water volume of the work rolls should be 961.2 m3/h.
The 1780 mm hot rolling mill finishing mill F1~F4. The hydraulic pressure of the roll cooling water is 1.0 MPa, and the water volume is between 1 000 and 1250 m3/h, which meets the cooling water volume requirements for theoretical calculation. And through comparative analysis with similar hot-rolling lines, it was found that the existing cooling water of the 1780 mm hot-rolling mill was sufficient, indicating that the reason for the peeling of the oxide film on the surface of the hot-rolling mill was not the lack of cooling water for the rolls, but the high-speed steel rolls. The temperature measurement of the roll after the machine is obviously high and the fact exists. It is obvious that the cooling efficiency of the roll cooling water needs to be further investigated and analyzed.
The heat output of the work roll is mainly concentrated on the position slightly above the exit side, so the cooling efficiency of this part is very important. The cooling water of the work rolls has ⒉ discharge pipes, which are respectively arranged on the inlet side and the outlet side. The rotation is close to 1 circle, and the water cutting plate at the entrance is cut off. It adheres to the surface of the roll for a long time, and the cooling efficiency is high. It can be seen that the distribution of roll cooling water has a great influence on the cooling effect of the roll, and the outlet cooling water volume should be much larger than the inlet cooling water volume to achieve a better cooling effect. The ratio of the outlet cooling water volume to the inlet cooling water volume is 70% and 30% in the original design, and the cooling efficiency is low. The ratio of the outlet cooling water volume should be increased to reduce the roll temperature to prevent the oxide film from growing too thick and peeling off.
In addition, the cooling water quality of the work rolls of the 1780 mm finishing mill is poor, and it is easy to block the cooling water nozzles of the rolling rolls, causing the local oxide film to grow too fast and peel off.
(3) Anti-stripping water.
The peeling of the oxide film on the surface of the roll and the intrusion of the oxide scale can be prevented by the control of the anti-stripping water. The anti-stripping water can reduce the surface temperature of the strip steel, reduce the temperature difference when the strip steel is in contact with the roll, reduce the thermal cracks on the surface of the roll and cause spalling, and improve the state of the stick surface. The spraying position of the anti-stripping water at the entrance of the finishing mill originally designed is far from the biting point of the rolling mill, and the surface of the strip turns red when biting, and the anti-stripping water does not play a good role in reducing the temperature difference between the roll and the strip surface. In addition, the anti-stripping water is blocked by the upper edge of the guide plate on the inlet side, which also leads to poor cooling effect.
(4) The influence of rolling lubrication.
The rolling lubricant can form a protective film covering the surface of the roll, reduce the friction factor, reduce the rolling force and roll temperature, reduce the online wear of the roll, reduce the accident of steel sticking of the work roll, and increase the service life of the roll. At the same time, the lubrication of the rolling process can also improve the state of the roll surface, prevent the scale from being pressed in, and improve the surface quality of the strip. The original design of the 1780 mm hot rolling mill and the finishing unit have been lubricated by the rolling process, but there is a problem of mutual interference and influence between the header arrangement and other functional headers in the rolling mill, and the process lubrication cannot effectively play its due role. effect.
(5) The influence of the water cutting board.
The upper work rolls of the finishing mill are all equipped with water cutting plates. In order to increase the strength and rigidity of the upper guide frame, the original design provided reinforcement ribs in the guide frame, but no drainage holes were designed on the reinforcement ribs, resulting in rolling During the process, the water accumulation in the guide is serious, the counterweight of the upper water cutting plate is insufficient, and the water cutting is not thorough, which hinders the formation of lubricating oil film on the surface of the roll and affects the effect of rolling lubrication. In addition, no water cutting plate is designed between the anti-stripping water of the lower working wheel at the entrance and the rolling lubricating oil. The anti-stripping water and the rolling lubricating oil interfere with each other, resulting in the falling anti-stripping water covering the surface of the lower working roll, and the lubricating oil cannot cover the high-speed Steel rolls seriously affect the effect of rolling lubrication. In addition, in the original design, the water cutting plate is made of cast iron, and it is easy to scratch the stable oxide film established on the surface of the roll when it is in direct contact with the roll, causing the oxide film to peel off.
Technical transformation implementation plan
(1) Calculate and confirm that the original roll cooling water is sufficient, and there is no need to change the cooling water supply system. In order to improve the cooling effect, it is necessary to optimize the configuration, change the ratio of the cooling water inlet and outlet of the work roll, and adjust the original design of 30% and 70% to 15% and 85%.
(2) Optimize the filter device and nozzle selection on the pipeline, improve the cooling water quality of the work rolls, and prevent nozzle clogging.
(3) Adjust the distance between the injection position of the anti-stripping water and the biting point. After the transformation, the distance between the position where the anti-stripping water is sprayed on the surface of the strip and the biting point is greatly reduced.
(4) Increase the counterweight of the water cutting board, set drainage holes on the rib plate, replace the material of the water cutting board, and enhance the water retaining effect and drainage effect of the water cutting board.
(5) A water cutting plate is added between the anti-stripping water of the work roll at the entrance and the rolling lubricating oil to eliminate the interference of the anti-stripping water to the rolling lubrication, so that the rolling lubricating oil can evenly cover the rolling-mixing surface, improving The effect of rolling lubricants.
(6) Adjust the relative position and injection angle of each header in the rolling mill to solve the mutual interference and influence between headers.
(1) Through the optimization of the cooling water of the roll, the cooling efficiency of the roll is improved, the temperature of the roll after the roll is lowered, and the temperature of the roll is reduced from above 70 °C before the transformation to below 60 °C, which effectively improves the use of high-speed steel rolls. life.
(2) Effectively shorten the time from when the anti-stripping water is sprayed onto the strip surface to when the strip bites in, and reduce the temperature difference when the strip steel surface contacts the rolls, and give full play to the effect of the anti-stripping water.
(3) It solves the mutual interference and influence among various headers in the rolling mill such as anti-stripping water, lubricating oil, cooling water and descaling water, and ensures the normal function of each header in the rolling mill.
(4) Through the transformation, the surface quality of the roll has been greatly improved, and the peeling of the oxide film on the roll surface of the work roll has been reduced.
(5) The consumption of high-speed steel rolls is 15% lower than that before the transformation.
(6) After the transformation, the strip quality problem caused by the peeling of the oxide film on the roll surface has been effectively solved. The monthly waste and defective products have been reduced from 1 500 t to less than 100 t, and the waste and defective products have been reduced by more than 90%. The increased income can reach more than 40 million yuan, and the economic benefits are remarkable.