1780 Cause Analysis and Preventive Measures for Spalling of backup rolls in Hot Continuous Rolling

Abstract: An analysis of the peeling and chipping of the backup rolls at the edges of the backup rolls of Angang’s 1780 hot tandem roughing mill was conducted. It was believed that work hardening, uneven wear, fatigue micro-cracks, and unreasonable roll side chamfer design were the main reasons for the roll peelings. Based on actual usage conditions, reasonable usage and maintenance methods and preventive measures are proposed.

Keywords: hot rolling; backup roll; spalling; fatigue microcracks

Introduction

In the four-high rolling mill for plate and strip steel production, the work rolls are in direct contact with the rolled piece, causing plastic deformation of the rolled piece, which is closely related to the surface quality and shape of the plate and strip. The backup roll bears most of the bending moment, which has a great influence on the stable operation of production, especially the shape of the cross-section of the plate and strip steel. backup roll play an important role in influencing production. On the one hand, the replacement cycle of the backup roll is relatively long, especially when an accident occurs, which has a great impact on the rolling line; on the other hand, the manufacturing cost of the backup roll is high. Each one can cost over one million yuan. When a backup roll accident occurs, the diameter of the support roller is lost, causing great economic losses. Shortly after the 1780 hot continuous rolling operation of Anyang Iron and Steel Co., Ltd. was put into operation, a serious accident occurred in which the end of the support roll of the rough rolling mill fell off. This article analyzes the reasons why the support roller peels off and blocks, and takes corresponding improvement and preventive measures in a targeted manner.

Basic situation of 1780 hot continuous rolling

1780 hot continuous rolling is one of the important production lines in the strategic planning of Anyang Iron and Steel’s product structure adjustment. In the first phase, 2 walking beam heating furnaces, 1 four-high reversing roughing mill with vertical rollers, 1 flying shear, 7 continuous four-high finishing mills, and 2 coilers will be built, with an annual output of 175 Million tons. In the second phase, a walking beam heating furnace, a two-roll roughing mill and a coiler will be built, with the annual output reaching 3.5 million tons, and a leveling and coiling unit with an annual output of 800,000 tons will be added. The process layout is shown in Figure 1. The roll conditions of the rolling line are shown in Table 1.

No. 1-3 heating furnace (reserved); No. 2-2 heating furnace; No. 3-3 heating furnace; 4-rough descaler; 5-R1 vertical roller mill (reserved); 6-R1 two-roll rough rolling mill (reserved); 7-R2 vertical roller mill; 8-R2 four-high roughing mill; 9-intermediate billet insulation cover; 10-head flying shear; 11-F1E vertical roller mill; 12-seven-stand finishing rolling mill; 13—Measurement room; 14—Laminar flow cooling; 15—No. 1 coiler; 16—No. 2 coiler; 17—No. 3 coiler (reserved); 18—No. 1 baler;19—No. 2 baling machine; 20—No. 3 baling machine (reserved); 21—Uncoiling inspection line; 22—No. 4 baling machine; 23—No. spraying machine; 24—Warehouse walking beam

Figure 1 Schematic diagram of the hot continuous rolling process (the dotted box represents the reserved equipment)

Table 1 Overview of 1780 hot continuous rolling rolls

 Roll size/mm Maximum/minimum roll diameter × roll body length × total roll lengthRoll materialRoll weight/t
roughing mill R2 work rollΦ1200/1100× 1780×4 940High chromium steel25.9
R2 backup rollΦ1600/1450× 1780×5 760Forged steel 45Cr4NiMoV41.9
Finishing millF1-3 work rollΦ800/710×2080×5 270Improved high chromium iron12.5
F4 work rollΦ800/710×2080×5 270Improved high nickel chromium infinite hardness12.5
F5-7 work rollΦ700/630×2080×5 250Improved high nickel chromium infinitely chilled cast iron9.8
F1-4 backup rollΦ1 600/1 450×1780×5 760Forged steel 45Cr4NiMoV41.9
F5-6 backup rollΦ1 600/1 450×1780×5 760Forged steel 45Cr4NiMoV41.9
F7 backup rollΦ1 600/1 450×1780×5 760Forged steel 45Cr4NiMoV41.9

Peeling off the edges of the R2 backup roll of the roughing mill

The backup roll where the roll end peeled off is the lower backup roll of the R2 roughing mill. A total of 114,000 tons of steel have been rolled. The edge block occurred at one end of the transmission side of the roll body. The circumferential direction is nearly 1000mm, the axial width is 200mm, and the radial direction is 90mm. After getting off the machine, the hardness of the roller surface is measured, and the maximum is HS71.2.

Judging from the morphology of the broken edge of the backup roll, the origin of the crack occurred on the roller surface, and no visible inclusions or crack source expansion defects were found in other parts. Judging from this phenomenon, the meat loss in this part is mainly caused by fatigue micro-cracks caused by excessive edge stress. The fatigue micro-cracks are caused by the crack expansion induced by frequent alternating impact loads.

The factory inspection performance of the backup roll is shown in Table 2.

Figure 2 Photo of spalling of backup roll in roughing mill

Table 2 Support roller performance inspection parameters

Roll body hardness HSDRoll neck hardness HSDHardened layer thickness/mmYield strength σs/MPaTensile strength σb/MPaElongation δ/%Section shrinkage ψ/%Impact energy Akv/J
65~6745~50≥80≥650≥900≤12≥40≥24

Analysis of causes of edge peeling

Effect of work hardening

The backup roll always maintains rolling contact with the high-hardness work roller, and the roller surface is subject to periodic contact stress. Periodic rolling contact stress produces a work-hardened layer on the surface of the backup roll. When the stress value increased by the hardening degree and the rolling stress exceed the yield limit of the material,microcracks will appear and then expand to cause spalling.

Effect of uneven wear

The surface wear of the backup roll is uneven, and the work-hardened layer is distributed in strips. There are differences in hardness and different stress states between different strips. If there is overload during the rolling process, such as work roll peeling, roll wrapping, steel jamming, slipping and other rolling accidents, iron oxide scale embedding, cooling water erosion of the roll surface, etc., the local shear stress of the backup roll will increase and yield. Subcutaneous micro-cracks are produced at the bottom of the work-hardened layer, and the micro-cracks develop from the inside to the outside to the roller surface, causing peeling and falling off.

Before spalling occurs, this kind of micro-crack is difficult to detect during use of the roll, and the expansion of the crack cannot be seen. If these subcutaneous micro-cracks are not discovered and removed during off-line grinding of the roll, the cracks will develop rapidly when used again, reaching the roll surface and causing peeling.

Impact of manufacturing defects

During the manufacturing process, there are defects such as non-metallic inclusions or folds in the working layer of the roll body. There are stress concentrations at the sharp corners. After a certain period of use, micro-cracks will occur and expand along the direction of defects and stress, eventually causing peeling.

Effect of roll side chamfering

In order to control the plate shape, hydraulic bending rolls have become one of the necessary devices in plate and strip steel rolling mills. During the rolling process, under the action of the roll bending force, the contact stress at the ends of the backup roll and the work roll increases rapidly. When the yield limit of the material is exceeded, plastic deformation occurs, and multiple alternating deformations will produce microcracks. Crack propagation will cause the ends to peel off.

Countermeasures

Strengthen the assessment of roller replacement cycle

The steel rolling workshop and the roller grinding workshop have established complete roll usage files, kept detailed records of the online usage and grinding conditions of each roll, strictly implemented the roll replacement system for various rolls, and resolutely put an end to the phenomenon of rolls being out of service. The technical department evaluates the implementation of the roll replacement system. The adjusted roll replacement cycle is shown in Table 3.

Table 3 Roll replacement cycle

Roll nameRoll changing cycleRoll changing cycle
rough rolling mill R2 work roll30,000 tons
R2 backup roll100,000 tons
finish rolling millF1-3 work rollper class
F4 work rollper class
F5-7 work rollper class
F1-4 backup roll200,000 tons
F5-6 backup roll150,000 tons
F7 backup roll100,000 tons

Strengthen the detection of surface work hardening of the backup roll body

After the backup roll comes off the production line, the hardness of the roller surface is tested and recorded to confirm the hardness increase and hardness unevenness. After grinding is completed, check the hardness of the roller surface again to ensure that the work-hardened layer has been completely removed. The hardness value must not exceed the hardness value when the machine was last used. In the later period of use of the support roller, use the gap between the replacement of the work roll to detect the surface hardness value of the backup roll. If any abnormality is found, please consult a technician in time to confirm whether the roller needs to be replaced.

Appropriately increase the grinding amount of the backup roll

Since the backup roll has a relatively long online service life, the fatigue layer must be completely removed during grinding. In actual production, through inspection and exploration, rough rolling and finishing rolling backup rolls are generally used for one cycle, and after the wear is removed, the fatigue layer of about 0.5 to 1 mm is processed to reduce fatigue cracks and extend the service life of the rolls. . After grinding, insist on eddy current testing.

Optimize the chamfering of the end of the backup roll body

The original chamfer of the backup roll was 150mm×1.0mm, which was too small. After adjustment, the chamfer of the backup roll was 200~250mm×1.5mm. The chamfering length is changed each time when grinding. The chamfering length is 200mm and 250mm, alternately; the chamfering form is a smooth transition shape.

Add ultrasonic flaw detection device

The grinding machine is only equipped with an eddy current flaw detection device, which cannot detect and locate deep defects. It is impossible to conduct effective and comprehensive inspections on new rolls entering the factory. It is impossible to find hidden defects in new rolls entering the factory, and it is also impossible to conduct in-depth inspections of rolls with defects coming off the production line. After applying for the purchase of ultrasonic flaw detectors, we can conduct comprehensive inspections on new rolls entering the factory and conduct focused inspections on rolls coming off the production line.

Conclusion

The manufacturing of heavy-weight backup roll requires the use of large-scale special equipment, which results in a long manufacturing cycle and high procurement costs. It also has the characteristics of long service life, high cost of use and large losses after abnormal failure. Strict attention must be paid to the manufacture and use of backup rolls. Long-term rolling contact between the backup roll and the work roll can easily cause fatigue cracks on the roll surface, and the expansion of micro-cracks will eventually lead to roll peeling. Detecting and monitoring the surface hardness of the backup roll, designing reasonable backup roll body chamfers, reasonably adjusting the roll replacement cycle, and adding ultrasonic non-destructive testing devices for detection can prevent the occurrence of backup roll peeling and extend the service life of the backup roll.

 

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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