The demand for output and quality of hot-rolled strip is getting higher and higher, which makes the use and management of backup rolls more and more stringent. In this case, it is particularly important to study the use and management of backup rolls, prolong the service life of backup rolls as much as possible, and avoid accidents of backup rolls on the machine that will affect production and product quality.
The back-up roll is mainly used to provide support for the work roll during production and rolling. Because the back-up roll has a long service life and bears alternating bending stress for a long time, the back-up roll must have good toughness and fracture resistance. Therefore, the perfect combination of high wear resistance, high peeling resistance and crack resistance is a must-have condition for backup rolls.
According to the summary of experiments and experience, it is considered that the content of cr in the backup roll composition should be 5% should be the best. Therefore, most hot-rolled strip mills have used forged Cr5 back-up rolls.
Use of backup rolls
Shape is an important quality index of strip steel, and roll wear is one of the key factors affecting the final shape of strip steel. The roll wear includes back-up roll wear and work roll wear. Since the roll changing cycle of the backup roll is much longer than that of the working roll, its serious wear will affect the bending deformation of the working roll, which will also affect the strip shape quality. Roll wear is one of the factors that are difficult to quantitatively control in current production.
Backup Roll wear
The mechanism of roll wear and other wear is the same. From the perspective of tribology, roll wear can be understood as the change of macroscopic and microscopic dimensions. The general discussion of roll wear includes both macroscopic wear and microscopic wear, which is objectively manifested as the reduction of roll diameter.
The wear of the backup roll is mainly due to the relative sliding and rolling friction with the working roll. The surface of the work roll and the carbide particles will grind the metal particles on the surface of the support roll, causing the wear of the support roll. The amount of wear is related to the material of the roll, surface hardness, roughness, pressure between the rolls, relative slippage and rolling distance.
The statistical results of a large number of measurement data show that the wear of the lower back-up roller is greater than that of the upper back-up roller. The reason for this phenomenon is that the cooling water entrained with a large amount of oxide scale acts on the surface of the lower roller, resulting in poor working conditions of the lower backup roller, which accelerates the wear of the lower backup roller.
For the above problems, the following measures are adopted to compensate for the wear of the roll under the on-site support:
(1) Control from the type of rolling. The newly installed rolls should be rolled wide, thin, and strip products with strict tolerance requirements. Then, with the wear of the rolls, the product specifications are changed sequentially, so that the width becomes narrower, the thinner becomes thicker, and finally the products with low tolerance requirements are rolled;
(2) Consider the wear of the roll from the rolling reduction. The method of reducing the amount of reduction should be adopted to compensate for the wear of the roll;
(3) Consider the wear of the roll from the cooling of the roll. That is, the influence of roll wear is compensated by changing the temperature distribution of the roll body. With the wear of the roll, the amount of cooling water at the edge is gradually increased to increase the thermal crown of the roll accordingly.
In the following, we mainly introduce the problem of compensating the wear of the back-up roll through the design of the roll profile.
Roll profile design of backup rolls
The wear of backup rolls is a problem that steel rolling technicians must focus on. Only by making accurate and reasonable statistics on the wear of backup rolls can the reasonable service period of each backup roll, the actual amount of wear of the backup rolls and the distribution of wear along the roll body be determined. Provide necessary and accurate information for shape control, so as to finally achieve the goal of ensuring product quality.
(1) Roll shape design
According to the actual situation of the back-up roll before grinding, our factory conducted a large amount of statistical data and found that the roll profile curve of the back-up roll adopting flat roll and sinusoidal roll type is not well maintained after off-machine, and uneven wear often occurs on the surface of the roll. And this phenomenon is particularly serious in the downstream rack. This problem has always been the crux of the increase in the difficulty of rolling steel shape control. It must be overcome. Through experimentation and exploration, the roll shape of the backup roll was changed from the original sinusoidal curve to the “sixth power” curve. The experimental results show that the roll shape after getting off the machine is basically consistent with the preset roll shape before starting the machine, which solves the original problem of uneven wear of the back-up rolls to a large extent, and greatly reduces the difficulty of plate shape control (see Figure 1)
Due to the adoption of the “sixth power” curve, during rolling, the middle part of the back-up roll can fully contact with the work roll, and the self-retention of the back-up roll after wear is significantly improved, and the roll shape of the original design is basically maintained. (See Figure 2). Through the design and use of a new roll shape, the uneven wear has been improved.
(2) Edge chamfer design
Although a reasonable backup roll type can solve most of the problems caused by uneven wear of the backup roll. However, in the rolling process, if the shoulder of the back-up roll is not chamfered or the chamfer is too small, or even the design of the chamfer is unreasonable, it will cause harmful contact to the shoulder of the back-up roll. Since the alternating contact pressure stress acts on the shoulder of the backup roll, the edge of the backup roll will peel off, which will lead to the early scrapping of the backup roll and even cause production accidents. Therefore, it is very necessary to solve the problem of stress concentration on the shoulder of the back-up roll, prevent the edge of the back-up roll from peeling off, and set a reasonable transition curve on the side of the back-up roll. The edge of the back-up roll adopts circular arc compound chamfering, which has a good effect on reducing the stress concentration on the shoulder (see Figure 3). If the design chamfer is too small, the chamfer will not work in the later stage of rolling, and if the design of the chamfer is too large, stress concentration will occur at the heel of the chamfer. According to the use experience of the back-up roll, within the range of 200mm of the length of one side of the back-up roll, compound arc chamfering of l~2mm in the radial direction can effectively reduce the stress concentration point and reduce the harmful contact surface. Avoid roll accidents during the service life of backup rolls.
Backup Roll Management
In addition, the key to the quality of roll use lies in the management of the back-up rolls; the quality of management is directly related to the service life and cost indicators of the back-up rolls. The management of back-up rolls should be discussed by the factory, and it cannot be copied. It is necessary to formulate relevant measures in combination with the availability of rolling products in the factory where the back-up rolls are used, and gradually improve the management level of the back-up rolls.
Management of back-up rolls entering the factory
Ultrasonic testing of backup rolls after they enter the factory is particularly important. Since it is inevitable for backup rolls to have defects during the manufacturing process, it is necessary to carefully inspect and confirm the backup rolls after they enter the factory. It is necessary to know that any tiny defect on the rolls may lead to major accidents or even serious accidents. Therefore, the work of ultrasonic testing personnel is very critical. Only when the internal defects of the backup roll are found before use, can the necessary measures be taken according to the defect situation, and a specific tracking plan for the use of the problematic backup roll can be formulated to avoid production accidents caused by the spread of backup roll defects. If a roll defect is found, the position, depth direction, etc. of the defect should be recorded, so as to facilitate tracking of the backup roll during use. Once a defect is found to expand, it must be stopped immediately to prevent accidents.
Normal and abnormal disembarkation management
The back-up roller after being off the machine normally should be processed according to the pre-determined grinding amount. The back-up roll must be completely cooled before processing, so as to prevent the processed roll shape from changing due to temperature, resulting in unstable production operation. It is also necessary to measure the surface hardness of the back-up roll regularly, and adjust the grinding amount of the back-up roll according to the change in hardness, thereby eliminating the hardened layer on the surface of the back-up roll and preventing peeling. If the roll is off the machine abnormally (the roll has an accident on the machine such as steel jamming, steel sticking, etc.), the processing amount of the back-up roll must be changed.
If there are accidents such as steel jamming and steel sticking on the roll, the amount of processing and grinding should be re-determined according to the surface defects after the backup roll is off the machine. The combination of ultrasonic testing and eddy current testing should be used to accurately locate and determine whether there are cracks in the defect site and determine the depth value. The principle of flaw detection while processing is adopted to determine the depth of cracks from time to time, and the cracks are completely processed while reducing the amount of processing as much as possible. Although this method may cause the processing time of the roll to be prolonged, this method can save the abnormal processing consumption of the roll. Only by adopting the method of grinding while exploring can the abnormal consumption of the roll be really reduced, otherwise the consumption cannot be really reduced only by the limited amount of grinding after the machine is off the machine normally. Because although the amount of machining and grinding each time is small, and accidents may not occur immediately, the fatigue cracks accumulated over time have not been processed, which may eventually lead to accidents in the rolling process of the rolls, and the gains outweigh the losses.
Therefore, the hot rolling mill should be equipped with corresponding detection equipment. Ultrasonic flaw detection and eddy current flaw detection are carried out on the back-up roll before and after grinding, and if necessary, magnetic particle flaw detection and coloring flaw detection can be used to check whether there are fatigue cracks inside the back-up roll and whether there are residual cracks on the surface. Determine whether there is obvious work hardening zone through hardness inspection, so as to ensure that the surface hardness of the back-up roll is uniform, and prevent the back-up roll from being defective on the machine. The roll changing cycle of the back-up roll should be adjusted according to the work-hardening condition of the shoulder, so as to ensure that the increase in hardness does not exceed 4HS during use. The work-hardened layer should be completely removed during grinding, and the hardness of the roll surface should be tested after grinding to restore the hardness value to the original hardness of the backup roll.
Roll file management
A complete roll usage record should be established. In particular, it is necessary to establish roll eddy current and ultrasonic flaw detection files. The statistical results show that most of the rolls that have accidents on the machine are abnormal rolls that have been repaired and reused.
Therefore, it is necessary to carry out eddy current, surface ultrasonic and ultrasonic flaw detection of micro-cracks in the joint part on the back-up roll regularly. 100% crack detection must be carried out on backup rolls that have had rolling accidents. At the same time, the operating level of flaw detection personnel should be improved to reduce the probability of missed detection of equipment.
Taking these measures can reduce or avoid accidents of backup rollers in the machine, prolong the service life of backup rollers, and reduce the consumption of backup rollers.
Backup Roll Failure Forms and Prevention
The failure forms of the back-up roll mainly include the peeling of the roll body and the peeling off of the side of the roll, and there are also failure forms such as roll neck fatigue fracture and roll body stress fracture. Roll failure is often encountered during use, which is the direct cause of roll damage, shortened life, or even early scrapping. Therefore, roll failure is the last thing that roll users want to happen, but because of the existence of this problem, it has always troubled the management of on-site rolls. Now analyze the failure form of the backup roll from several aspects, strengthen the use and management of the roll, and try to avoid the occurrence of this problem.
Spalling of backup rolls
(1) During the production process of the back-up roll, due to the uneven wear of the back-up roll and the work roll, the positive and negative bending force of the work roll will cause the center of the back-up roll body to protrude at both ends, increasing the contact stress at the end of the roll body big. When the yield limit of the material is exceeded, plastic deformation occurs, and multiple alternating deformations will produce micro-cracks, and crack expansion will cause large flakes to peel off;
(2) The back-up roll always maintains rolling contact with the high-hardness work roll, and the roll surface is subjected to periodic contact pressure stress. Cyclic rolling contact stress often produces a work-hardened layer on the surface of the backup roll, but not all work-hardening results in spalling. Only when the stress value of the increased degree of hardening is superimposed with the rolling stress exceeds the yield limit of the material, will microcracks propagate and cause spalling. The 4HS whose work-hardening hardness increase value exceeds the original hardness has the risk of spalling. The fundamental solution is to process the hardened layer to prevent micro-cracks in the skin from peeling off.
This kind of micro-crack is difficult to be found during the use of the roll, and it is difficult to find the crack expansion. For example, if the roll is not found when the roll is ground down and a large amount of grinding is performed, the crack will expand rapidly when the roll is used again, which may cause peeling. Due to the long service period of the backup roll, the large amount of wear in the middle of the backup roll and the small amount of wear at both ends lead to a U-shaped roll shape, resulting in local contact pressure peaks at both ends of the roll body and an increase in the alternating shear stress at both ends. Added to the fatigue damage. And because the wear of the edge of the roll is less, the point of maximum alternating shear stress basically does not move. Under its repeated action, cracks may appear locally. As the crack propagates, eventually spalling occurs. The spalling of backup rolls is mainly small pieces of spalling, which are distributed in the range of the roll body in contact with the rolled piece.
Prevention of Backup Roll Failure Forms
(1) Monitoring of work hardening
After the back-up roll is machined, the hardness of the roll surface is tested. One is to confirm whether the hardness has an increasing trend. The second is to determine the uniformity of hardness. If the hardness increases by more than 4HS compared with before the machine, the amount of grinding should be appropriately increased to effectively reduce the rolling cycle. Process and test the hardness of the roller surface several times to confirm whether the work-hardened layer is ground clean, and it is required to reach the hardness of the previous machine.
(2) Fatigue crack monitoring
Magnetic particle, penetrant or ultrasonic flaw detection can be used to confirm whether the fatigue cracks are cleared, and the fatigue layer can be removed by appropriately increasing the amount of grinding.
(3) Adjustment of roll changing cycle
The fatigue cracks of the backup roll are closely related to the service period of the roll, the grinding system and the state of the roll surface before the roll is put on the machine. Therefore, the roll changing cycle of the backup roll should be gradually adjusted according to the changes in the hardness of the backup roll surface, and various non-destructive testing methods should be used to detect the cracks on the surface of the roll surface to confirm the removal of fatigue cracks. After long-term accumulation of experience, a reasonable roll change system and grinding system have been formulated, which is an effective maintenance method for preventing contact fatigue cracks of backup rolls.
The quality of the final use of the backup roll is evaluated by the number of accidents on the backup roll and various rolling indicators. Nowadays, energy saving and consumption reduction is the leading trend of major steel mills, which requires the use and management level of on-site backup rolls to be improved, and reducing the consumption of backup rolls has become the primary task of technicians.
To sum up, if you want the back-up roll to be able to use the expected scrapped diameter, you must work hard on the flaw detection, use and management of the back-up roll. Minimize accidents, improve the use of backup rolls to a new level, and ensure the smooth progress of the production process.