Desicription:
This paper analyzes the reasons for the spalling of the backup rolls in our factory, and concludes that the spalling of the backup rolls is caused by the stress concentration of the contact high points, which leads to fatigue cracks in the secondary surface. The fatigue cracks extend to the surface and finally spall.
In the cold-rolled strip production process, although the back-up roll does not directly contact the strip steel, most of the huge rolling force exerted by the strip steel on the cold-rolled work roll is transmitted to the back-up roll, and the back-up roll has to bear a lot of Bending stress, long replacement cycle, fatigue hardening, stress concentration, rolling accidents, etc. are all likely to cause accidents such as roll surface peeling and roll shoulder falling off. The production cost of back-up rolls is high, the procurement cycle is long, and the occurrence of spalling accidents will not only increase the production cost, but also greatly affect the normal production of the rolling mill. Therefore, it is of great significance to study the reasons for the spalling of the backup rolls, to find cracks in time and prevent spalling.
Title: Analysis and Countermeasures of Spalling of Cold Rolling Back-up Rolls
Keywords: Cold Rolling rolls, Back-up Roll, Spalling of Back-up Rolls
1 . Analysis of the causes of spalling
- The main form of back-up roll spalling
There are two main ways for the backup roll to spall: one is spalling caused by surface cracks; the other is spalling caused by subsurface cracks.
1.1.1 Analysis of the stress state of the backup roll during the rolling process, the work roll of the rolling mill and the backup roll will be locally flattened at the contact point. At this time, the backup roll is mainly subjected to three stresses, as shown in Figure 1: caused by the rolling force The contact compressive stress P, the principal shear stress τ45 (Hertz stress) at an angle of 45° to the XY axis, and the orthogonal shear stress τ
Figure 1: The partial state of the stress of the backup roll
Calculated as follows:
pmax=0.83{[PE1E2 (d1+d2 )]/[(E1+E2 )d1 d2 ]}1/2;
b=1.52{[Pd1 d2 (E1+E2 )]/[(d1+d2 )E1E2 ]}1/2;
τ45 max =0.304 pmax, located at 0.39b on the surface of the roll;
τyx max=0.256 pmax, located at 0.25 b on the surface of the roll;
Among them: P is the rolling force per unit length;
d1 and d2 are the roll diameters of the work roll and backup roll;
E1 and E2 are the elastic modulus of work roll and backup roll;
1.1.2 Spalling of the backup roll caused by surface cracks
Due to the influence of various factors such as roll shape and wear, the pressure stress (p) on the surface of the backup roll is not uniformly distributed along the direction of the roll body, especially the increase in the local stress of the backup roll when accidents such as belt breakage and foreign matter intrusion occur. When the local compressive stress exceeds the strength of the backup roll, surface cracks will form (in this case, the surface cracks are generally accompanied by obvious plastic deformation marks). After the surface cracks are generated, the surface cracks first extend vertically to the inside of the roll body under the action of rolling cyclic stress, and then expand in the radial and circumferential directions after reaching the critical size, and finally lead to the occurrence of roll surface spalling. After this spalling occurs, there is generally a clear fatigue expansion track (C-type “beach mark”).
1.1.3 Spalling of backup roll caused by secondary surface cracks
During the rolling process, the maximum shear stress (τ45) borne by the backup roll is located in the subsurface layer. Cracks can also be initiated under certain conditions, mainly through the following two methods:
1) Instantaneous cracks: When there are accidents such as broken belt, slippage, and foreign body intrusion, the surface positive pressure stress increases and the maximum shear stress rises sharply. When the maximum shear stress exceeds the strength of the roll, cracks or even the surface layer of the roll Instant peeling. This kind of instantaneous cracking occurs less frequently on the back-up roll.
2) Contact fatigue: Due to the long service life of the backup roll, under the action of cyclic stress, the fatigue life of the place with higher stress is short, and fatigue cracks are formed. This kind of situation is more common on the backing roll, and the position is usually in the protruding part of the roll body, the contact point of the backing roll and the edge of the work roll, and other places where stress is easy to concentrate. After the formation of fatigue cracks in the subsurface layer, there are two forms of expansion: one is that the cracks spread to the surface, forming pit-like small pieces and peeling off, and the bottom of the pit is the source of the crack. This kind of pit-like spalling is generally shallow, sometimes scattered on the roll body, and some are connected to each other into pit groups without obvious fatigue track; second, after the crack is formed, it expands in the radial and circumferential directions until the strength of the surrounding material It is reduced to the point where large flaking occurs, and even the entire roll is scrapped. This kind of spalling generally has obvious fatigue growth trajectory, but the crack source sometimes falls along with the spalling block and it is not easy to find.
- Analysis of the reasons for the spalling of the back up rolls
The material of the back-up roll in our factory is 3Cr forged steel, the size of the roll body is Φ1250～1150×1750mm, the roll shape is flat roll, and the work roll adopts cvc roll shape. The peeling position of the support roller body is mostly on the operating side (upper support roller) or transmission side (lower support roller) about 600-800mm from the edge, which corresponds to the highest point in the middle of the work roll cvc roll shape, as shown in Figure 2. , And there is no obvious regularity in the distribution of a small part of spalling position.
The pits on the peeling surface are dense, the size is generally within Φ20-100mm, and the depth is about 5-8mm. See Figure 3. According to the spalling morphology and the position of the spalling surface, it can be determined that the spalling of the backup roll in our factory belongs to contact fatigue spalling. There are two reasons for the analysis: one is, due to the stress concentration area at the height of the backup roll and the work roll, during the rolling process, When the number of stress cycles exceeds the fatigue life of the backup roll, fatigue cracks are formed on the secondary surface. After the fatigue cracks occur, they spread to the surface and finally cause peeling. Second, the rolling mill has a broken belt and roll accident, and the surface of the backup roll produces plastic deformation even in the secondary surface. The surface layer has cracks, the grinding amount is low after the machine is off, the surface hardened layer or the subsurface layer cracks are not cleanly removed, and the support roller develops fatigue peeling during the continued use.
2 Solution
The fatigue life of the backup roll is mainly related to the hardness of the roll body and the state of stress. The higher the hardness of the roll body, the longer the fatigue life; the greater the stress, the shorter the fatigue life.
1) The hardness of the work rolls purchased by our factory is generally 93-95HS, and the backup rolls are 64-65HS. The backup rolls have used 30-50mm when they peel off. The hardness of the backup rolls at this time is about 61-62HS. The best hardness difference between the backup roll and the work roll is generally controlled between 20～30HS. The hardness of the backup roll in our factory is obviously low, and the hardness of the backup roll decreases too fast as the diameter decreases, so we will support when we purchase the backup roll again The roll hardness is increased to 68～72HS.
2) Shorten the roll change cycle, shorten the backup roll change cycle appropriately, reduce the number of cycles that the backup roll bears, and avoid the use of the backup roll to exceed the fatigue limit.
3) Appropriately increase the amount of grinding of the accidental backup roll according to the surface condition of the backup roll after it is off the machine.
4) While performing surface wave inspection on the support roller, use dual crystal probes for subsurface inspection to find cracks as soon as possible to prevent the support roller from being damaged and causing peeling.
5) For the back-up rollers that have been peeled off or have subcutaneous cracks, combined with magnetic powder or coloring flaw detection and manually perform dish-shaped grinding treatment to reduce the amount of grinding, the specific grinding treatment method is shown in Figure 4.
3 Conclusion
Through analysis, the main reason for the back-up roll spalling in our factory is the secondary surface fatigue spalling at the stress concentration point of the roller body. For this reason, measures have been taken to improve the hardness of the roll, shorten the service cycle, strengthen the ultrasonic flaw detection of the back-up roll, and optimize the back-up roll grinding process. The back-up roll peeling phenomenon has been effectively controlled.
