Abstract: In view of the frequent burn-out of rolling bearings of 950mm hot rolling mill in Taisteel, which seriously affects the normal production and cost control of the workshop, the stress situation of work roll and support roll bearings was analyzed and tracked to find out the causes of mill bearing burn-out. Through the control measures such as the grinding quality control of the grinder, the assembly quality control of the assembly personnel, the control of the work roll and the support roll arch window, and the control of the load distribution of the rolling line operators, the running quality of the roll system is continuous and stable, the production cost is constantly reduced, the bearing consumption is reduced year by year, and the effect is remarkable.

Key words: rolling mill; Bearings; Burning loss; Grinding quality

1 Introduction

The 950mm hot rolling line of Taishang, due to the poor operating environment of the field equipment and the decline of the equipment accuracy function, causes the burning bearing failure frequently, which not only increases the cost, but also interrupts the normal production, affects the normal production rhythm, and greatly affects the improvement of output and quality. According to the existing production process and equipment characteristics of 950mm hot rolling line in Taisteel, focusing on this engineering and technical problem, the mill utilization rate was improved, various causes of mill bearing burning were analyzed and investigated, the stress situation of work roll and support roll bearings were analyzed and tracked, the causes of mill bearing burning were identified, and rectification measures were formulated and implemented.

2 Analysis of the status quo and causes

2.1 Burn-out analysis of support roll bearing

The work roller coupling shaft of the 950mm hot rolling production line of Tai Iron and Steel Group rotates to drive the upper and lower support rollers. The thrust bearing on the operation side of the support roll (model 3510/480) mainly bears the axial force generated in the rolling process, and the four-row bearing of the support roll (model 314441B) mainly bears the radial force generated in the rolling process. From the bearing damage analysis, the wear and tear of support roller bearings are mainly concentrated in the lower support roller bearings. Its manifestations are mainly: bearing damage, general cage deformation fracture, outer ring fracture, rolling block, raceway pitting. The main reasons for the burn-out of support roller bearings are as follows.

(1) There is a gap between the bearing seat of the support roll and the bearing, and the assembly personnel does not carry out measurement and inspection, or the inspection is not in place, and there is a hidden danger of damage to the bearing outer ring during the assembly process. The wear of the support roll bearing table exceeds the usage standard, and the bearing table is not repaired in time, resulting in the fracture of the bearing inner sleeve and the bearing damage and burning of the bearing. The four rows on the operation side of the support roll and the thrust bearing are damaged. The bearing seat is not opened for gland inspection during assembly, and there are hidden dangers. The bearing is burned after the bearing is put on line. The four rows of bearings on the transmission side of the lower support roller are damaged, the inner sleeve is damaged, and the bearing outer ring is worn and stressed unevenly.

(2) During the rolling process, the horizontal deviation on both sides of the support roll changes greatly, the pressure pad and the step pad exceed the use standard, and the replacement is not timely. The grinding accuracy of the support roller is not up to the standard. The measuring ellipticity of the lower line is 0.15mm, and the internal control standard is 0.05mm. The grinding accuracy is not up to the standard. The inner and outer retaining ring of the support is not locked in place, and there is a phenomenon of rolling channeling.

(3) The support roller assembly is not carried out the original disassembly, the fixed frame is used, the bearing, the inner sleeve, the bearing seat and other reasons, resulting in the support roller burning bearing. During the assembly of the support roll, the fixing bolt of the bearing seat is not fastened in place or missing, and the locking nut bolt is not fastened in place. The inner sleeve of the support roller is not replaced after the use period, and the inner sleeve is spalling and falling block is damaged. The supporting roller bearing is damaged, and the supporting roller bearing itself has quality defects.

(4) Support roller bearing seat seal wear, aging, replacement is not timely. Bearing seat water, lubricating oil quickly emulsified, further reduce the service life of the bearing, resulting in burning of the bearing. The wear of the lining plate of the upper and supporting rollers exceeds the use standard and the window of the supporting rollers is not replaced and adjusted in time. The standard of the supporting rollers is 1000mm (0.8 ~ 1.02mm) on the operating side and 990mm (0.8 ~ 1.02mm) on the transmission side, which is also one of the reasons for the burning of the bearing.

2.2 Work roll bearing burn analysis

The 950mm hot rolling line of Taigang is a four-high rolling mill. In the process of steel rolling, due to rolling specifications and steel types, the impact force is uneven, the working roll F1-F3 operating side thrust (model 32048X/DF), the working roll F4-F7 operating side thrust (model 32040X/DF), Work roll F1-F3 four-row bearings (Model 527104) Work roll F4-F7 four-row bearings (model FC5068230M/HC) bear the axial and radial forces generated during rolling. In the normal rolling process, the working side bearing seat is fixed by the baffle plate. The analysis of roll force in the rolling process shows that the work roll bearing mainly bears radial force in the rolling process.

(1) Work roll operation side stop clamp deformation, bolt fastening is not in place, bearing seat gland bolt loose, assembly personnel assembly is not standard, bearing seat gland bolt fastening is not in place. The lower roller is not checked during assembly, the bearing clearance beyond the service life is too large, and the bearing is damaged online.

(2) The thrust bearing on the operating side of the work roll is damaged, the two sides of the mill are not level, and the bottom plate wear on both sides of the step pad exceeds the use standard. The outer ring of the four-row bearing of the work roller is seriously worn, which exceeds the use standard and is not shut down. Work roll channeling roll slide weld open, bolt loose gap. The lining plate of the support roller bearing seat is worn 1mm online, and the lining plate is not replaced in time, resulting in uneven force on the roll surface, which leads to the work roller shaft moving.

(3) The bearing table on the lower support operating side and transmission side falls off, and the uneven force on the roll surface leads to the work roller shaft moving. The bearing seat of the work roll has been used for a long time, and the inner diameter is worn and deformed, which can not reach the use standard.

(4) The grinding accuracy of the grinder is poor, the roller grinding can not reach the internal control index, and the ellipticity and conicity are 0.05mm as the process standard, which leads to the burning of the bearing after the line is put on. The window of the work roll is not up to the process standard, the operating side is 560mm (0.8 ~ 1.02mm), the transmission side is 550mm (0.8 ~ 1.02mm), and the rolling roller shaft is shifting during the rolling process. The inlet temperature of finishing rolling can not reach the opening temperature of 950℃, the rolling force of each frame is increased, and the low temperature steel is rolled. In the rolling process of the mill, the load distribution of rolling force is uneven, and the load of some stands is too large.

3 Burning bearing control measures

(1) The grinding quality of the roll is mainly checked to ensure that the internal control index of the work roll ellipticity and cone is ≤0.02mm, and the internal control index of the support roll ellipticity and cone is ≤0.05mm, and the stability and profile quality of the rolling mill are significantly improved. The whole process of roll assembly is controlled from grinding quality, assembly accuracy, on-line time and frame and off-line inspection. The assembly personnel will conduct a special measurement of the arch window, and adjust the shims that exceed the technical requirements of the drawing in time to ensure that the precision of the arch window is controlled within 0.8 ~ 1.02mm. The gap of the arch of the lower support roll will be measured once a month by fixed repair, and adjusted within the standard range. The work roll and the bearing seat liner of the support roll will be measured once a month. Replace the liner plate for the bearing seat that is not up to standard. New bearings will be disassembled and inspected once every half month after being used for 1 month, and a tracking record will be made for the use of bearings. If the bearing outer ring is found to have pitting points or serious wear, the bearing will be replaced before being put on the line. The original disassembly and installation of the fixed frame will be carried out according to assembly standards. Through the operation analysis, the precision of the gate window of the rolling mill and the original disassembly of the rolling roll are mainly controlled, and the running quality of the rolling roll is improved month by month.

(2) In view of the problem of high consumption of roll bearings, the eight working methods of "fixed disassembly method, fixed plate measurement, fixed frame use, fixed cooling method, fixed grinding accuracy, fixed refueling method, fixed disassembly cycle, and fixed replacement cycle" are carried out. The roll assembly quality has been greatly improved by the acceptance of the roll assembly station and roll assembler before rolling up. The advantages and disadvantages of equipment lubrication are analyzed to provide reference and improvement direction for lubrication work. Roll running quality is stable and bearing consumption is continuously reduced. Strictly implement the roll change cycle of the finishing rolling support roll, increase the management of the finishing mill yard, and fully release the bearing stress on the lower line, reduce the fatigue degree of the bearing, and extend the service life. During the finishing rolling cycle, the backing plate of the support roll was replaced, and the window accuracy was controlled within 0.8 ~ 1.02mm. The window size of the work roll was adjusted by replacing the support roll every day to ensure that the window accuracy was within the range. At the same time, the roll bending seat is replaced according to the 12-month cycle to ensure the normal operation of the roll bending function. The bolt of the roll bending seat and the slide board are fastened, the wear amount of the slide board is ≤0.4mm, the offset of the work roll slide gap to the center line of the window is ≤0.1/1000mm, and the total length offset is ≤0.15mm. Roll base thickness: operating side inlet 320 (-0.1 ~ -0.4) mm, operating side outlet 310 (-0.1 ~ -0.4) mm, transmission side inlet 325 (-0.1 ~ -0.4) mm, transmission side outlet 315 (-0.1 ~ -0.4) mm, slide mounting aperture M16, depth 35mm.

(3) Oil grease in strict accordance with the uniform regulations of the rolling line. Work roller shaft regularly disassembly, regular replacement, to avoid due to dynamic balance does not meet the technical requirements, shake caused by rolling burn bearing. In the process of bearing assembly, the disc bearing has no jamming resistance, no foreign matter in the bearing seat, and no pitting in the bearing raceway. The bolt of the liner plate of the work roll bearing seat is tightened, and the measurement size meets the requirements. Grease the lining plate, wipe the roll surface, install the locking device, so that the bearing seat and the rolling roller shaft movement ≤2mm. Tighten in place, the clearance between the bearing outer ring and the inner hole of the bearing seat is less than 0.03mm, and the interference fit between the inner ring and the roll is 0.02 ~ 0.05mm. Work roller bearing seat liner size requirements: drive side 550 (-0.4 ~ -0.58) mm; 560 (-0.4 to -0.58) mm on the operating side; Cooling time ≥24h. Support roller bearing seat liner size requirements: drive side 990 (-0.3 ~ -0.5) mm; 1000 mm (-0.3 to -0.5) on the operating side.

(4) The roll is installed on the machine tool to check that the bearing inner sleeve surface has no pitting and no cracks. The ovality of the inner sleeve is measured by the dial indicator, and the working roller and support roller can meet the standard before assembly. According to the grinding of roll bearing table, the roller is replaced, and the inner sleeve and the roller diameter are interference fit. At the same time, the inner sleeve is heated 90 ~ 100℃ before installation, not exceeding 120℃, no overheating phenomenon, to ensure the stable use of the support roller on the line. Replace vulnerable spare parts (stop clamp, inner and outer ring, end cover, liner) strictly according to the cycle, ensure that the bolts of spare parts are fastened in place and not loose. Check the bearings after rolling off the line and before rolling on the line, and deal with any hidden problems in advance. Do not bring any hidden problems into the line. Replace the spacer ring, stop clamp, outer stop ring, inner stop ring and seal that do not meet the standard during assembly. Bearing seat gland bolt complete to ensure assembly accuracy.

(5) The equalizing plate under the lower support roll should be replaced at the same time, and it is strictly prohibited to wear inconsistent on both sides of the line. Every time the burning bearing occurs in the rolling process, it is analyzed in detail, and specific preventive measures are worked out until the hidden danger is eliminated. To ensure the level of the two sides of the finished rolling step pad, both sides of the fixed repair are trimmed at the same time. The roll grinding quality before each line is recorded, so that it can be traced. The wear of the leveling plate is ≤ 0.1mm, the wear of the step pad is ≤ 1mm, the elevation is (810±5) mm, the centering accuracy of the step pad on both sides is ≤ 0.1mm, and the horizontal dislocation tolerance is ≤ 0.2mm.

(6) Analyze the bearing seat, measure the bearing inner diameter, and scrap the bearing seat immediately if the bearing seat wear exceeds the use standard. Comprehensively inspect the work roller bearings on the offline line, focus on tracking and checking the bearings that exceed the period, find hidden dangers and replace them in time, prevent the bearings with hidden dangers from being used online, and ensure the online use requirements of bearings. Open the bearing seat gland once every half month to check the four rows of bearings and thrust bearings, and the gland bolts are complete. Inspect damaged seals each time they are assembled, and replace them in time if they do not meet the standard. The fixed bolts of the work roller slide seat are tightened by fixed repair, and other frame checks are tightened according to the condition. The support roller ensures the original disassembly and assembly at the same time, the support roller can be used up and down the original fixed frame to ensure the assembly quality. At the same time, continue to adhere to the measurement and adjustment of the frame arch window according to the offline disassembly of hidden bearings. The bearing seat of the support roll down the line is measured strictly according to the period, and the liner plate that exceeds the standard is replaced in time during use to ensure the assembly quality.

4 Effect evaluation

(1) The rolling system operation quality is continuous and stable, and the production cost is constantly reduced. Through the continuous optimization and improvement of the assembly standard, the eight fixed working method is summarized. The closed-loop management is formed through the roll offline, disassembly, cooling, grinding, assembly and on-line, which ensures the continuous stability of the rolling operation quality. Through the internal special inspection, the continuous consolidation and improvement of the management standard and the field environment are constantly improved.

(2) From 2021 to 2023, bearing consumption shows a downward trend. Through the control of grinding quality, assembly quality control, work roll, support roll window control, rolling line operator load distribution control, temperature control, production plan preparation, process optimization, improve slab hot loading rate, a series of measures, All are conducive to the reduction of bearing consumption. In 2023, compared with 2021, the cost of tons of steel decreased by 1.433 yuan, and the annual savings of 2.58817 million yuan.

(3) Before 2020, the ball form of the four-row bearing of the support roll was in the form of a pin. Due to rolling specifications, steel grades and other reasons, it was easy to break the ball after the force was large, and now it is changed to solid ball, from the original unmodified steel volume of 200,000 t to 400,000 t after the modification. A set of bearings saves nearly 40,000 yuan, further reducing cost consumption.

(4) Through tracking analysis, proper control measures have formed a virtuous cycle, and bearing consumption has a downward trend year by year. The reduction of burned-bearing failure improves the production utilization. According to the process standard, it takes 40 minutes to replace 1 set of support roller and 12 minutes to replace 1 set of work roller each time the bearing is burned. It takes 2 min to produce 1 coil of hot rolled strip steel, and according to the calculation of less burning 1 set of support roll and 52 min cumulative time of work roll, 26 more coils of strip steel can be produced, which lays the foundation for a stable production rhythm.