Roller is the key contact technology equipment for strip production. In view of the problems in the process of cold roll processing, the surface quality is improved in the production process from the aspects of roll grinding, non-destructive testing of rolls, and the use of rolls, and the production cost is reduced to ensure continuous and efficient operation. Non-destructive testing of rolls is of great significance for the rolling of cold rolled sheets, especially ultrasonic testing. Rolling accidents are reduced during the on-line operation of the rolls. According to the attenuation of the roll roughness, the schedule is reasonable, the chrome-plating quality of the chrome-plated rolls and the edge chamfering of the support rolls are optimized to reduce the roll consumption.
Title：Research on Operation and Maintenance of Cold Roll
Keyword：Rolling roll, Operation and Maintenance, Cold Roll
With the continuous upgrading of cold-rolled products, cold-rolled products are not only for ordinary building materials, but are also moving towards high-end automotive and home appliance plates. The variety of cold-rolled products has achieved full coverage from low-carbon and ultra-low-carbon mild steel series to high-strength and ultra-high-strength hard steel series. The surface quality of the strip needs to further meet the O5 automotive outer panel surface requirements. The rolls are in direct contact with the strip steel, and the surface appearance of the work roll is equivalent to a “mold”, which is the key to determining the surface quality of the strip steel. Higher requirements are put forward for cold-rolling work rolls or intermediate rolls, with higher hardness and higher wear resistance. At the same time, new requirements are put forward for the metallurgical quality and microstructure of the working layer of the support rolls, with stronger toughness and Anti-stripping performance. Grinding, inspection and improper use of rolls can cause a sharp increase in roll consumption. This paper studies the rolls from three aspects of roll processing, inspection and use, in order to improve the service life of the rolls, improve the quality of the strip, and provide a reference for enterprises to reduce the consumption of the rolls and improve the quality of the strip.
1 Roll processing
The rolls will bear high rolling loads during the working process, and at the same time, there will be frequent bending, torsion and friction between the rolls. After a certain period of use, the surface needs to be ground to remove surface pits, roll marks, cracks and other defects , The fatigue layer is gradually removed while repairing the roller curve. The roll material is a polymer composed of iron-based solid solution and alloy carbides. Currently, Cr5 rolls are generally used, and the single grinding amount is 0.2-0.3mm. If the roll processing cannot be qualified once, the grinding two will reach 0.1 mm or more.
In addition to the mechanical equipment factors of the grinder, the most critical media and consumables for roll grinding are metal cutting fluid and grinding wheel, and the improvement of surface quality also puts forward higher requirements for metal cutting fluid and grinding wheel. Metal cutting fluid needs
It has good chip settling properties, excellent cooling and cleaning properties, and reduces scratches on the roller surface. Select the appropriate grinding wheel model and grinding process parameters to ensure that roll shape problems, surface chatter marks, transverse thread lines, surface roughness, etc. cannot occur during the grinding process. For hardened steel rolls, corundum grinding wheels are generally used, and only the corundum grinding wheels that match them can be selected according to the material of the rolls to achieve high surface quality and grinding accuracy. Problems with roll shape are usually mainly caused by too soft grinding wheels. Vibration defects on the roll surface will occur if the grinding wheel is too hard or the feed rate and lateral movement speed are too high. It is necessary to reduce the linear speed of the grinding wheel, reduce the feed rate, reduce the lateral movement speed, and increase the roll speed. The horizontal thread line is also called the cutting pattern, which will appear when the grinding wheel is not corrected well or the feed rate or lateral movement speed is too high. When the grinding wheel is too soft, the filtering of the coolant fails, and the abrasive particles float in the coolant, it will cause the grinding point defects on the surface of the roll. It is necessary to check the filtration of the coolant, increase the rotation speed of the grinding wheel, reduce the rotation speed of the roll, and reduce the amount of knife feed. . The grinding wheel is too hard, the self-sharpening property is poor, the passivated sand particles are not easy to fall off, and the grinding roller surface is prone to fuzzing. It is necessary to increase the roller speed and reduce the linear speed of the grinding wheel.
The grinding feed rate of the roll and the cooling performance of the cutting fluid not only affect the surface condition of the roll, but too large feed rate or insufficient cooling will also cause micro-cracks on the surface of the roll. The micro-cracks are small and dense, and they are not damaged. Testing such as eddy current and ultrasonic testing requires the sensitivity to be adjusted to a high level to detect. Ultrasonic and magnetic particle inspection results of micro cracks on the surface of the burnt roller. If this kind of grinding micro-cracks is not found, the use of the machine will cause serious accidents of peeling and roll bursting.
In order to achieve good deep drawability and high coating adhesion, the surface of cold-rolled strip is usually pitted, and the work roll needs to be textured. At present, the roll texturing method used in the production of high-quality strips mainly adopts EDM texturing. The topography parameters of EDM texturing are surface roughness Ra, peak density Pc value, etc., which can be adjusted by pulse voltage, discharge point spacing, current Intensity, intermittent time, etc. are controlled. Reducing the current or pulse width can reduce the Ra value and increase the PC value. The uniformity of the texturing and the point defects of the texturing are related to the quality of the carbon powder and the electrode. Uneven discharge and carbon powder adhesion will cause texturing quality problems, resulting in repeated grinding and texturing, resulting in increased costs. At the same time, watermarks often appear in the texturing process. The watermarks are wavy and curved, and there is a certain color difference on both sides, which is more obvious after chrome plating, which has a serious impact on rolled high-end surface products.
2 Roll inspection
Non-destructive testing is required before rolls are put on the machine. Commonly used in daily production are ultrasonic testing, eddy current testing, magnetic particle testing and penetrant testing. Eddy current testing has the advantages of fast detection speed and can detect soft spot defects, but it can only target surface opening defects, and there are many factors affected by interference, and excessive sensitivity can easily cause messy signals. Penetration testing is only effective when dealing with locally large opening defects. The use of ultrasonic and magnetic particle detection is more effective in ensuring the normal use of the roll.
Ultrasonic surface wave is more convenient to use the bottom wave method, that is, the probe is placed at a position of 1000mm in the middle of the roll body, and the water chestnut reflection wave at the end of the roll body is adjusted to 20% high, and then the gain is 32dB. When the roll is in good condition, the surface of the roll body will not cause abnormal wave reflection regardless of the circumferential and axial detection of the roll body. However, due to the restriction of the smelting process, the outer ring of the roll body sometimes has tiny pores and loose defects. After forging, The elongated strips are called hairline, which are randomly distributed along the length of the roller body, and the length is mostly 0.05-3mm, which causes abnormal reflections in surface wave detection. These small hairline defects are generally within 1mm in length, and will not affect the quality of the product during use. However, it is difficult to distinguish the small cracks produced by the roll on-line rolling from the small cracks caused by the on-line rolling during ultrasonic testing. In this case, magnetic powder inspection and microscopic observation are needed. The edge of the defect is generally a defect caused by the roll manufacturing, and the edge with sharp corners is generally a crack defect caused by rolling. Surface crack defects can cause strip-like peeling on the roll surface, effectively detecting the residual cracks on the roll surface that are not cleaned, and avoiding the roll strip from being damaged and running on the line. Magnetic particle and ultrasonic inspection results of hairline and magnified image of residual cracks.
The direct detection sensitivity of ultrasonic flaw detection shall be determined according to the AVG method to determine the equivalent diameter of the flaw, using 20% of the first bottom wave reflection less than the full screen height as the reference height, and then calculating the sensitivity increment of the flaw detection part to adjust the gain according to the following formula.
The calculation formula n=20lg (λD/2π),
n———Sensitivity increment (dB) value;
D———The thickness of the workpiece (unit: mm);
λ———When f=2.5MHz, the wavelength of ultrasonic wave in steel (usually λ=2.36mm).
After finding the defect signal, use the gain of the instrument to divide the thickness of the detected workpiece into 12, 7, 5, 4, 3, 3, 2, 2, 2 as the principle, and decrease the corresponding gain value according to the defect position. As a benchmark, raise or lower the height of this defect wave to 20% of the full scale of the screen. Generally, the outer surface layer of the work roll and the intermediate roll with a depth of 90mm from the roll surface is not allowed to have defect echoes, and the middle area is not allowed to have defects exceeding Φ2+15dB. Although the abnormal wave defect near the center of the work roll caused a certain drop in the bottom wave, the evaluation result did not exceed the standard. The abnormal wave comes from the superimposed equivalent of the loose center, and the roll has no abnormalities when used on-line.
The use of support rollers, the stress concentration position during use is 5-10mm depth. It is difficult to find defects in this depth area through ultrasonic surface wave inspection after offline. The defect formation position is relatively shallow when detected by a straight probe. If a certain scale is not formed, the inspection will be more difficult. In this case, improve the sensitivity of surface waves, screen grass-like waves, and focus on direct ultrasonic detection of beam positions that appear in clusters. Ultrasonic detection and internal and external conditions of damage under the roll skin. In the follow-up use, it is necessary to increase the intensity of detection and tracking of the accident roll to prevent the continuous expansion of cracks and the entire roll to be scrapped.
3 Roll use
In the case of large reduction in acid tandem rolling, the roll body hardness of the work roll of the rolling mill must be absolutely guaranteed, otherwise it will cause product thickness fluctuations and poor plate shape. The pursuit of roll height and hardness will lead to poor roll anti-accident ability, which puts forward higher requirements for a low production line operation accident rate. A more serious broken belt accident will directly cause the work roll worth 100,000 yuan to be scrapped. Downstream users such as automotive appliances put forward higher requirements on the shape of cold-rolled products, and at the same time, the requirements for the surface roughness of automotive steel are very narrow. To obtain the surface roughness and surface quality of the strip steel that meets the requirements of users, it is necessary to arrange the production plan reasonably according to the attenuation of the roll roughness, and extend the life cycle of the roll as long as possible to reduce the frequency of roll change. Reduce roll consumption.
The relationship between the roller change cycle of the support roller and the structure and specifications of the product is relatively small. In addition to the internal damage caused by the accident, the abnormal consumption of the roll is at the position where the shoulder chamfer of the support roll is the most stressed, and the chamfer at the end of the roll body is in a straight line, it is easy to form small micro cracks, and the crack depth can reach 2 -3mm. Grinding and removal will cause a lot of roll consumption. If it is not handled in time in subsequent use, it will cause the side of the roll to peel off. Changing to arc chamfer can effectively reduce the stress concentration at this position. The quality problems such as the inclusion of the roll itself will also cause the fatigue resistance of the chamfered part to decrease. Even if the chamfering curve is optimized, the inclusion defects in this area will cause the initiation and expansion of cracks and affect the life of the roll.
During the rolling process, the chrome-plated roll can reduce the coefficient of friction, and the amount of iron powder generated during the roll gap friction process is reduced, which greatly improves the cleanliness of the strip after rolling. Improve the average reflectivity of the surface of the strip. The pressure in the subsequent continuous annealing and galvanizing cleaning section is reduced, which is beneficial to improve the quality of the board surface. Table 1 shows the comparison of the effects of using chromium-plated rolls in the cold rolling mill of Tangshan Iron and Steel Co., Ltd. However, the quality of the coating is very high, and it is necessary to ensure that it cannot fall off under a large rolling load. Once the coating falls off, it will directly cause surface defects of the product. The thickness of the normal chromium plating layer is about 6μm, and the microstructure is in the form of a net.