Use and management of hot-rolled plate and strip backup rollers

Abstract: The requirements for the output and quality of hot-rolled strips are getting higher and higher, which makes the requirements for the use and management of backup rollers changing with each passing day. In this case, it is particularly important to study the use and management of backup rolls, extend the service life of the backup rollers as much as possible, and avoid accidents on the backup rollers that affect production and product quality.

Keywords: roll; wear; roll type; use; management

As the output of hot-rolled strips gradually increases, the requirements for the use of backup rolls are also becoming higher and higher. How to extend the service life of backup rolls has become one of the main topics studied by hot-rolled strip mills. Now we are analyzing the use and management of backup rolls in our factory with colleagues to find the maximum cost-effectiveness of the rollers.

Overview of hot rolled plate and strip backup rolls

The backup roll is mainly used to provide support for the work roll during production rolling. Since the backup roll has a long service life and is subjected to alternating bending stress for a long time, the backup roll must have good toughness and fracture resistance. Therefore, the perfect combination of high wear resistance, high peeling resistance and fracture resistance is a must-have condition for the backup roller.

In general, the development of hot strip rolling mill backup rolls has gone through four stages (see Table 1).

As can be seen from the above table, the increase in Cr element content in the back-up roll material is effective in improving the water corrosion and wear resistance of the back-up roll matrix. Based on the summary of tests and experience, it is believed that the Cr content in the backup roll composition should be 5% optimal. Therefore, most hot-rolled strip mills now use forged Cr5 backup rolls. Practice has proved that this material is more suitable for the needs of today’s hot-rolled strip production plants.

Table 1 Development stages of back-up roll materials for hot strip rolling mill

development stageMaterial
The first stageSolid cast steel
second stageForged Cr2
The third phaseForged Cr3; composite casting
The fourth stageForged Cr4, 5; composite casting

Usage of backup rolls

Flat shape is an important quality indicator of strip steel, and roll wear is one of the key factors affecting the final flat shape of strip steel. Roller wear includes backup roll wear and work roll wear. Since the roll change cycle of the back-up roll is much longer than that of the work roll, its severe wear will affect the bending deformation of the work roll, thereby affecting the strip shape quality. Roller wear is one of the factors that is difficult to quantitatively control in current production.

Roller wear

The mechanism of roll wear is the same as other wear. From the perspective of tribology, roll wear can be understood as changes in macro and micro dimensions. The generally discussed roll wear includes both macro wear and micro wear, which is objectively manifested in the reduction of roll diameter.

The wear of the backup roller is mainly caused by the relative sliding and rolling friction with the work roller. The surface of the work roll and carbide particles grind away the metal particles on the surface of the back-up roll, causing wear of the back-up roll. The amount of wear is related to factors such as the material of the roll, surface hardness, roughness, pressure between the rolls, relative sliding amount and rolling distance.

Statistics from a large amount of measurement data show that the wear amount of the lower support roller is greater than that of the upper support roller. The main reason for this phenomenon is that the cooling water carrying a large amount of iron oxide scale acts on the surface of the lower roller, resulting in poor working conditions of the lower support roller and accelerating the wear of the lower support roller. In response to the above problems, the on-site under-support adopts the following measures to compensate for the wear of the rollers:

(1) Control from the rolling variety. Newly installed rollers should first roll wide, thin, plate and strip products with strict tolerance requirements, and then change product specifications sequentially as the rollers wear. Make it from wide to narrow, from thin to thick, and finally roll the product with low tolerance requirements;

(2) Consider the wear of the roll from the rolling reduction. Methods should be adopted to reduce the amount of reduction to compensate for the wear of the rollers;

(3) Consider the wear of the roll from the perspective of roll cooling. That is, changing the temperature distribution of the roll body is used to compensate for the impact of roll wear. As the roll wears, the amount of cooling water at its edge is gradually increased to correspondingly increase the thermal crown of the roll.

Below we mainly introduce the problem of compensating backup roll wear through roll profile design.

Roller shape design of backup roll

Back-up roller wear is a problem that steel rolling process personnel must pay special attention to. Only by carrying out accurate and reasonable statistics on the wear of backup rollers can we determine the reasonable service period of each backup roller, the actual wear amount of the backup roller and the distribution of wear along the roller body. Provide essential and accurate information for shape control to ultimately ensure product quality.

(1)Roller type design

Our factory conducted a large amount of data statistics based on the actual situation before the back-up roller grinding and found that the roll curves of the back-up rollers using flat rollers and sinusoidal rollers were not maintained well after they were removed from the machine. Uneven wear often occurs on the roll surface, and this phenomenon is particularly serious on the downstream stands. This problem has always been the crux of the difficulty in controlling rolling steel plate shape and must be overcome. Through experimentation and exploration, the roll shape of the backup roller was changed from the original sinusoidal curve to a “sixth power” curve. The experimental results show that the roll shape after disembarking from the machine can be basically consistent with the preset roll shape before going on the machine, which solves the original problem of uneven wear of the backup roller 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, the middle part of the backup roll and the work roll can achieve full contact during rolling. The self-retention of the backup roll after wear is significantly improved, and the original designed roll shape is basically maintained (see figure 2). Through the design and use of new roller types, uneven wear is improved.

(2) Edge chamfer design

Although a reasonable backup roller roll type can solve most of the problems caused by uneven wear of the backup roller. However, during the rolling process, if there is no chamfer on the shoulder of the backup roll or the chamfer is too small, or even the chamfer design is unreasonable, harmful contact will occur on the shoulder of the backup roll. Due to the alternating contact pressure stress acting on the shoulder of the backup roller, the edge of the backup roller will peel off, causing the backup roller to be scrapped in advance and even cause a production accident. Therefore, in order to solve the problem of stress concentration on the shoulder of the backup roller and prevent the edge of the backup roller from peeling off, it is very necessary to have a reasonable transition curve on the edge of the backup roller. The edge of the backup roller adopts arc compound chamfering, which has a better effect on reducing stress concentration on the shoulder (see Figure 3). If the chamfer is designed to be too small, the chamfer will not work in the later stages of rolling, and if the chamfer is designed to be too large, stress concentration will occur at the heel of the chamfer. According to the experience of using the back-up roller, a compound arc chamfer of 1~2 mm should be made in the radial direction within the 200mm single side length of the back-up roller. This can effectively reduce stress concentration points, reduce harmful contact surfaces, and avoid roll accidents during the service life of the backup roll.

Figure 1 Sixth power curve roller curve

Figure 2 Roll profile curve of backup roller after wear

Figure 3 Back-up roller shoulder chamfer

Management of backup rolls

In addition, the key to the quality of the roll usage lies in the management of the backup roll; the quality of management is directly related to the service life and cost indicators of the backup roll. The management of backup rolls must be discussed on a factory-by-factory basis and cannot be copied indiscriminately. Relevant measures must be formulated based on the rolling varieties and equipment conditions of the factory where the backup rolls are used, so as to gradually improve the management level of backup rolls.

Management of backup rolls entering the factory

Ultrasonic testing of backup rollers after entering the factory is particularly important. Since it is inevitable for backup rollers to have defects during the manufacturing process, they must be carefully inspected and confirmed after they enter the factory. It is important to know that any tiny defect on the roll may lead to a major accident or even a vicious accident. Therefore, the work of ultrasonic inspection personnel is very critical. Only when internal defects of the support roller are discovered before use can necessary measures be taken according to the defect situation. Develop a specific tracking plan for the use of problematic backup rollers to try to avoid production accidents caused by the spread of backup roller defects. If a roll defect is found, the location, depth direction, etc. of the defect should be recorded to facilitate tracking of the backup roll during use. Once the defect is found to have expanded, use must be stopped immediately to prevent accidents.

Normal and abnormal shutdown management

The support roller after normal dismounting should be processed according to the predetermined grinding amount. The support roller must be completely cooled before processing to prevent the processed roller shape from changing due to the influence of temperature, resulting in unstable production operations. It is also necessary to regularly measure the surface hardness of the back-up roller, and adjust the grinding amount of the back-up roller according to the change in hardness, thereby eliminating the hardened layer on the surface of the back-up roller and preventing peeling.

If the roll comes off the machine abnormally (the roll has an accident on the machine such as steel jamming, steel sticking, etc.), the processing amount of the backup roll will change. If there are accidents such as steel jamming or steel sticking when the roll is in the machine, the amount of processing and grinding must be re-determined based on the surface defects after the support roll is removed from the machine. A combination of ultrasonic testing and eddy current testing should be used to accurately locate and determine the depth of cracks in defective parts. Adopt the principle of flaw detection while processing, constantly determine the depth of cracks, and completely process the cracks while reducing the processing amount as much as possible. Although this approach may cause the processing time of the roll to be extended, this method can save the abnormal processing consumption of the roll. Only by using the method of grinding and probing at the same time can the abnormal consumption of the roll be truly reduced. Otherwise, the consumption cannot be truly reduced only by the limited amount of grinding after normal shutdown. Although the amount of processing and grinding each time is small, and an accident may not occur immediately, the fatigue cracks accumulated over time have never been processed, which may eventually lead to an accident on the roll during the rolling process, and then the gain outweighs the loss.

Therefore, hot rolling mills should be equipped with corresponding detection equipment. Ultrasonic flaw detection and eddy current flaw detection can be carried out on the support rollers before and after grinding. If necessary, magnetic particle inspection, coloring flaw detection and other means can be used to check whether there are fatigue cracks inside the support roller and whether there are residual cracks on the surface. Determine whether there are obvious work-hardened areas through hardness inspection to ensure uniform surface hardness of the back-up roller and prevent defects in the back-up roller from entering the machine. The backup roller replacement cycle should be adjusted according to the work hardening condition of the shoulder to ensure that the hardness does not increase by more than 4HS during use. The work-hardened layer should be completely removed during grinding, and the roller surface hardness should be tested after grinding to restore the hardness value to the original hardness of the support roller.

Roll file management

Complete roll usage records should be established. In particular, it is necessary to establish roll eddy current and ultrasonic flaw detection files. Statistics show that most of the rolls that have accidents on the machine are rolls that were removed from the machine abnormally and repaired before being used again.

Therefore, the support roller must be regularly inspected by eddy current, surface ultrasonic wave and micro-crack ultrasonic flaw detection at the joint. Back-up rolls that have experienced rolling accidents must be 100% cracked and inspected. At the same time, the operating level of flaw detectors should be improved to reduce the probability of equipment leakage.

By taking these measures, accidents on the support roller can be reduced or avoided, the service life of the support roller can be extended, and the consumption of the support roller can be reduced.

Back-up roll failure modes and prevention

The main failure modes of backup rolls include peeling off of the roll body and peeling off of the roll side. In addition, there are also failure modes such as fatigue fracture of the roll neck and stress fracture of the roll body. Roll failure is often encountered during use. This problem is the direct cause of roll damage, shortened service life, and even premature scrapping. Therefore, roll failure is the last thing that roll users want to happen. However, due to the existence of this problem, it has always troubled the management of on-site rolls. Now we will analyze the failure modes of the backup rollers from several aspects, strengthen the use and management of the rollers, and try to avoid the occurrence of this problem.

Back-up roller peeling

(1) During the production process of the backup roll, due to the uneven wear of the backup roll and the work roll, the positive and negative bending forces of the work roll cause a depression in the middle of the backup roll body and bulges at both ends. The contact stress at the end of the roller body increases. When the yield limit of the material is exceeded, plastic deformation occurs. Multiple alternating deformations will produce micro cracks, and crack expansion will cause large pieces to peel off;

(2) The backup roller always maintains rolling contact with the high-hardness work roller, and the roller surface bears periodic contact pressure stress. Periodic rolling contact stress often produces a work-hardened layer on the surface of the backup roller, but not all work-hardening will lead to spalling. Only when the stress value increased by the hardening degree and the rolling stress exceed the yield limit of the material will microcracks propagate and cause spalling. If the work-hardened hardness increase of 4HS exceeds the original hardness, there is a risk of peeling. The fundamental solution is to process away the hardened layer to prevent micro-cracks in the skin from causing peeling.

This kind of micro-crack is difficult to find when the roll is in use, and it is difficult to detect the expansion of the crack. If the roll is not found when it is removed from the machine for grinding and a large amount of grinding is performed, the crack will expand rapidly when it is used again, which may cause peeling. Due to the long service life of the backup roller, the wear of the middle of the backup roller is large and the wear of both ends is small, resulting in a U-shaped roller shape. This causes local contact pressure peaks at both ends of the roller body and an increase in alternating shear stress at both ends, accelerating fatigue damage. Moreover, due to less wear on the edge of the roll, the maximum alternating shear stress point basically does not move. Under its repeated action, local cracks may appear. As the cracks propagate, they eventually cause spalling. The back-up roller spalling is mainly small pieces of spalling, which are distributed within the range of the roll body in contact with the rolling stock.

Prevention of failure modes of backup rollers

(1) Monitoring of work hardening After the support roller is machined, the hardness of the roller surface is tested. The first is to confirm whether the hardness has an increasing trend, and the second is to determine the uniformity of the hardness. If the hardness increases by more than 4HS compared with before running the machine, the amount of grinding should be appropriately increased to effectively reduce the rolling cycle. Process the roller surface multiple times and check the hardness of the roller surface repeatedly to confirm whether the work-hardened layer is clean and must reach the hardness when the machine was put on the machine last time;

(2) Fatigue crack monitoring can use magnetic particle, penetrant or ultrasonic flaw detection to confirm whether the fatigue cracks have been removed, and appropriately increase the amount of grinding to remove the fatigue layer;

(3) Adjust the roll replacement cycle. The fatigue cracks of the back-up roll are closely related to the roll service cycle, grinding system and the roll surface condition before the roll is put on the machine. Therefore, the roll replacement cycle of the back-up roll should be gradually adjusted according to the change in hardness of the back-up roll surface, and various non-destructive testing methods should be used to detect cracks on the surface of the roll surface to confirm the removal of fatigue cracks. After long-term experience accumulation, a reasonable roller replacement system and grinding system have been summarized and formulated. This is a very effective maintenance method for preventing contact fatigue cracks on the support rollers.

Shortcomings in the manufacturing of backup rolls

(1) Hardness drop control problem According to the requirements of the technical agreement, the hardness of new backup rollers is basically around HSD68. When the backup rollers are in the middle and late stages of use, the hardness drop drops significantly, and is basically HSD60 or below. When the hardness of the support roller decreases, it is no longer wear-resistant, and it is extremely easy to produce plush and flocculent iron hair on the roller body, causing the support roller to be unable to serve according to the theoretical design cycle. This results in frequent replacement of backup rollers, which not only increases costs but also increases the labor intensity of employees;

(2) Hardness uniformity problem Due to the poor hardness uniformity of the back-up roller, the roller body wear is inconsistent, which directly affects the plate shape control and product quality;

(3) The problem of surface manufacturing defects. Because some backup rollers did not meet the process requirements during forging, many “pores” inside the roller body were exposed on the surface at a certain diameter. Due to the existence of “pores”, the grinding amount of the backup roller increases, which reduces the utilization rate of the backup roller and significantly shortens the service cycle. At the same time, the back-up roller is prone to cracks in the “pores” after long-term use, which may cause peeling accidents on the roller surface.

Conclusion

The final use quality of the backup roller is evaluated by the number of accidents of the backup roll on the machine and various rolling indicators. Nowadays, energy saving and consumption reduction are the leading trends in major steel plants, which requires that the use and management level of on-site support rollers must be improved. Reducing the consumption of support rollers has become the primary task of technical personnel.

To sum up, if you hope that the backup roller can be used to the expected scrap diameter, you must work hard on the flaw detection, use and management of the backup roller to minimize accidents. Improve the use of support rollers to a new level to ensure the smooth progress of the production process. At the same time, it is also hoped that the manufacturing industry can provide better and more sophisticated backup rollers for hot-rolled plate and strip mills, so that both parties can achieve a win-win situation and achieve common success.

 

MM GROUP is one of the professional roll manufacturing base in China, which supply all kinds of large-size rolls for iron and steel enterprises with production capacity of 100,000 tons of all kinds of hot strip mill rolls, section mil rolls, rod mil rolls, cold rolling m rolls, casting and forging backup rolls.

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