Abstract: This paper makes an in-depth analysis of the spalled pieces and “shoulder” on the side of the roll side of the 1422 hot continuous rolling back-up roll in a steel plant. The results show that work hardening, uneven wear, contact fatigue, unreasonable design of roll side chamfer and defects in the working layer are the main causes of roll spalling. Combined with the actual production, reasonable maintenance methods and preventive measures are put forward.
During the rolling process of the steel plate, although the back-up rolls are not in direct contact with the rolled piece, they bear most of the bending moment and play an important role in the normal operation of the rolling line. On the one hand, the replacement time of backup rolls is long, especially when an accident occurs, which will have a greater impact on the rolling line. On the other hand, the cost of the backup roll is high, and the spalling accident of the backup roll will reduce the effective utilization rate of the backup roll.” Meishan Hot Rolling Plant has used two types of backup rolls before and after: cast steel backup rolls and forged steel backup rolls , the back-up rolls of these two different materials have peeled off to varying degrees during use, which not only affects the normal operation of the rolling line, causes huge economic losses, but also increases the cost of plate production.
2 Peeling form of backup roller
During the use of the backup roll, there are mainly two types of failures: the peeling of the roll body and the peeling of the side of the roll, that is, “shoulder drop”. These two failure modes often cause a huge loss in the effective use diameter of the backup roll. As shown in Figure 1. This roll was used as the lower support roll of the F2 frame at that time, and appeared in the middle of the roll body in a “cat’s tongue shape”, and the cracks around the spalled edge continued to extend along the roll surface to the end of the roll, and extended radially to the entire working layer. The scope of the entire defect covers 2/3 of the circumferential roll surface, causing spalled cracks to extend to the bonding layer, thereby destroying the knot and layer. This situation is a serious defect in production, that is, the roll can no longer be used, and can only be scrapped in the end, which seriously reduces the effective use layer of the roll and increases the cost.
Another form of failure is the peeling of the roll side edge, see Figure 2. This kind of peeling is much smaller than that shown in Figure 1, but the incidence of this kind of peeling accident is also higher. It often appears at the end of the roll (usually on the working side), and the roll in Figure 2 was used as the upper support roll of F6 at that time.
3 Research and analysis of the failure mode of backup rollers
3.1 Basic parameters
The material of the support roller used in our factory is 75Cr2M n2N M a, its chemical composition, mechanical properties and basic dimensions are shown in Table 1~3
|Roll parameters||Maximum roll diameter||Minimum roll diameter||Roll length||weight||Roll length|
|rough rolling BUR||1385||1260||1429||2649||4486|
|Finish rolling F1-3||1380||1260||1429||4306||3925|
3.2 Effect of work hardening on spalling of backup rolls
The back-up roll is always in rolling contact with the high-hardness work roll or intermediate roll, and the roll surface is subjected to periodic contact pressure stress. Periodic rolling contact stress often produces a work-hardened layer on the surface of the back-up roll. When the stress value of the increased hardening and rolling stress exceeds the yield limit of the material, micro-cracks will appear, and then expand to cause spalling. Depending on the required hardness of the support roll, the critical value of the hardness increase that causes spalling is also different. It is generally believed that for a support roll with a hardness exceeding HSD65, there is a risk of spalling if the work hardening hardness increase value exceeds HSD3.
3.3 The effect of wear on the spalling of support rollers
The surface wear of the support roller is uneven, and the work-hardened layer is distributed in strips, and there is a hardness difference between different strips, and the stress state is also different. If there is overload in the rolling process, such as rolling accidents such as work roll peeling, roll wrapping, steel jamming, and sliding. Scale embedding, acid dipping and corrosion of the roll surface, etc., cause the local shear stress of the back-up roll to increase and yield, and cause surface subcutaneous micro-cracks at the bottom of the work-hardened layer, and the micro-cracks develop from the inside to the outside to the roll surface, resulting in peeling off blocks.
Before spalling occurs, such microcracks are difficult to be found in the use of rolls, and the expansion of cracks cannot be seen. If these subcutaneous microcracks are not found and ground during the grinding of the roll, the cracks will expand rapidly when the roll is used again, reaching the roll surface and causing peeling.
3.4 Effect of metallurgical defects on spalling of backup rolls
Metallurgical defects such as non-metallic inclusions exist in the material of the working layer of the back-up roll body, especially brittle oxide and silicate inclusions with edges and corners, and stress concentration exists at the sharp corners. After a certain cycle, microcracks are generated, and the microcracks expand along the direction of inclusions and stress, and finally cause the surface layer to peel off.
3.5 The influence of the chamfer design on the side of the roll side on the shoulder drop of the support roll
During the rolling process, due to the uneven wear of backup rolls and work rolls. The positive and negative roll bending forces of the work rolls form the roll shape of the back-up roll or work roll with the concave ends of the middle of the roll body and the contact stress at the end of the roll body increases rapidly. When the yield limit of the material is exceeded, plastic deformation occurs, and micro-cracks are generated by multiple alternating deformations, and the crack expansion will cause shoulder drop. In order to avoid or delay this kind of failure, a section of soft belt with low hardness is often made at the end of the back-up roll body, and it is designed as a conical surface or arc transition with a certain axial length. The influence of different chamfer designs on stress concentration is shown in Fig. 3″. Different types of chamfers have great differences in stress concentration factors at chamfered parts. When the taper is less than 0.50 radians and exceeds 500 mm, the stress concentration factor can be ignored The two recommended chamfers are shown in Figure 4 and Figure 5.
4 Process to prevent failure of backup rollers
4.1 Monitoring of Work Hardening
After the support roll is off the machine, the hardness of the roll surface is tested, the hardness value is recorded, and the hardness increase value and hardness unevenness are confirmed. If the hardness exceeds HSD3, the rolling cycle should be appropriately reduced, and the best roll changing cycle should be determined through practical adjustment. After grinding, test the hardness of the roller surface again to confirm whether the work-hardened layer has been ground cleanly. It is required to reach the hardness of the previous machine, and record the hardness test value.
4.2 Fatigue cracks
Use magnetic particle, penetrant or ultrasonic flaw detection to conduct non-destructive testing of microcracks on and below the surface of the roll surface to confirm that fatigue cracks and subcutaneous cracks are removed, and appropriately increase the amount of grinding to remove the fatigue layer. Due to the long period of use of the support roller in the machine, the fatigue layer must be completely removed when grinding. In actual production, through continuous efforts and exploration, the fatigue layer grinding after the machine was finally increased from the original 20mm to 25mm, and the peeling of the support roll was significantly reduced.
4.3 Chamfering at both ends of the roll body
Appropriate chamfering or large arc compound chamfering at both ends of the roller body can effectively reduce the contact stress at the end of the roller body, avoid shoulder drop, and ensure strict grinding when the machine is newly installed and every time it is ground. , Effectively prevent the support roller from falling off the shoulder.
The back-up roll is in long-term rolling contact with the work roll, and the long-term rolling contact causes contact fatigue cracks on the roll surface, and the micro-cracks expand and finally cause the roll body to peel off. The back-up roll is required to have high fatigue yield strength, good stress state and crack growth resistance; excellent wear resistance, delay and reduce the formation of concave roll shape; roll neck has good yield strength, toughness and fracture resistance.