Abstract: We conducted a deeper analysis in response to multiple large-area peeling failures of cold rolling work rolls in a certain production line. The analysis covered peeling surface fracture observation, chemical composition, working layer structure, hardened layer depth, and crack propagation. The results show that fatigue crack propagation caused the large-area peeling failure. Abnormal indentation during the steel rolling process produced a roll surface crack, which was the crack source. Finally, to prevent severe roll surface peeling accidents, we proposed corresponding suggestions.These suggestions cover roll usage, maintenance, and design and manufacturing. The goal is to ensure that similar incidents do not occur in the near future.
The cold rolling process involves pressing hot rolled thick materials through the rolls under a certain deformation amount to form thinner products. In the cold rolling production process, the rolls are crucial.
The rolls are important tools in steel rolling production that directly contact the steel strip and participate in the rolling deformation. The surface quality of the rolls and their stability during use have a direct impact on efficient and high-quality production. During the rolling process, the rolls are in a complex stress state. They must withstand extremely high rolling forces. They also face shear stress between the roll sets. In addition, they experience friction forces generated during the steel rolling deformation process. Roll damage and failure are common problems during rolling. Wear, cracks, and peeling are common failure forms of cold rolling rolls [1-2]. Among them, peeling is a very serious form. It directly leads to the premature scrapping of the rolls. It also directly results in the steel strip becoming waste. In severe cases, it can even lead to breakage and steel stacking accidents during the cold rolling process. The causes of roll peeling are related to many factors. These factors include the manufacturing quality of the rolls. They also include rolling load. Roll grinding maintenance is another factor. Sudden rolling accidents are also involved. According to the manifestation of peeling, the peeling failure of cold rolling rolls can be divided into two types. One is peeling caused by surface damage. The other is peeling caused by internal defects.
In recent years, advanced metallurgical processes have been adopted for cold rolling roll billets. These include furnace external refining and electroslag remelting. Ultrasonic non-destructive testing is also used in the manufacturing process. As a result, internal defects in the working layer of cold rolling rolls have been strictly controlled. Therefore, the proportion of peeling caused by internal defects is constantly decreasing. In contrast, peeling caused by surface damage is increasing. This is driven by the improvement of product performance and the deterioration of rolling conditions [3-4]. Currently, the strength of cold rolling products is increasing. At the same time, the stability of the rolling process is relatively poor. As a result, the rolls inevitably suffer surface damage during machine installation. If these defects are not promptly removed by dismounting and grinding, cracks, peeling, and even breakage will occur. This directly affects production. Therefore, a comprehensive and in-depth analysis of roll peeling failure is helpful. It helps summarize experience in roll usage and maintenance. It also helps reduce the accidental wear of rolls. At the same time, it is conducive to optimizing the selection of rolls and manufacturing processes. This makes the rolls more in line with the development needs of products and processes.
Basic Information
After the working roll of the G1 machine frame of a certain cold rolling production line was dismounted, the assembly roll frame suddenly experienced roll surface cracking and large area peeling. Figure shows the macroscopic morphology of the peeling. Before the failure, this roll completed one machine installation use. The rolling time was approximately 14 hours. The cumulative rolled steel length was 104 km. The current roll diameter is 396.917 mm. The normal scrap roll diameter is 385 mm. There is still nearly 6 mm of effective working layer. The main peeling area is about 700 mm away from the transmission side roll end. The peeling surface has an axial length of 460 mm. The circumferential length is nearly 200 mm. The depth is 12-15 mm from the roll surface. Based on experience, it is located at the transition area between the end of the working roll and the core. To clarify the causes of the extensive peeling on the roller surface, samples were taken from the failed area. Peeling blocks were also collected. They were analyzed using a metallographic microscope. A scanning electron microscope was also used. The goal was to observe the microstructure and crack morphology of the peeling blocks. The chemical composition of the working layer was tested using a direct reading spectrometer. The hardness of the effective working layer was tested and analyzed using a microhardness tester. Finally, a comprehensive analysis was conducted from multiple perspectives. These included fracture surface observation of the peeling area. They also included working layer structure analysis. Hardened layer depth analysis was also performed. Crack propagation analysis was included as well. The goal was to clearly identify the failure cause.
Failure Cause Analysis 2.1 Fracture Surface Observation
The appearance morphology of the peeling blocks resulting from the failure of the roller surface is irregular, as shown in Figure. Since the roller is made of high-carbon high-alloy cold-formed die steel, most of them have the characteristics of brittle cracking. The main fracture surface shows fatigue crack extension zones and instantaneous fracture zones, etc. The surface of the fatigue crack extension zone is smooth, with fatigue crack extension lines, and the instantaneous fracture zone shows clear crack extension radiations (blue lines). Based on the characteristics of the fatigue extension arc, the crack source can be found to be located in the center area of the arc pointing in the opposite direction, as indicated by the red arrow in Figure.
Chemical Composition Analysis
Chemical composition analysis was conducted on the peeling blocks, and the results are shown in Table 1. They basically fall within the conventional MC5 cold rolling working roller chemical composition range. The harmful elements P and S are well controlled, while the contents of Ni and Mo are relatively low. However, no V element was detected (in conventional MC5 materials, wNi = 0.3% – 0.5%, wMo = 0.2% – 0.6%, wV = 0.1% – 0.2%). However, the relatively low content of these alloy elements may affect the wear resistance and other usage properties of the roller, but will not fundamentally affect the accident resistance of the roller.
Depth Analysis of the Hardened Layer
We tested the hardness distribution of the fracture surface of the peeling block along the radial direction of the roller. The test results are shown in Figure. From the test results, we can see that the hardness of the peeling block gradually decreases as the distance from the roller surface increases.At a distance of 6mm from the roller surface, the hardness (HV) is 700 – 710. It enters the transition zone of the hardened layer, and the depth of the working layer meets the technical requirements for roller use.
Crack Propagation Analysis
The magnified fracture surface morphology near the crack source is shown in Figure. Clear first and second fatigue extension steps can be seen at the depth positions of about 2 and 10mm from the roller surface. There are a large number of dense fatigue crack extension lines. The yellow lines in the figure show the fatigue crack propagation process. We inspected the condition of the roller surface in the green box area in the figure. We found a roller surface indentation less than 0.5 mm in length, as shown in Figure. We found a crack at the indentation root, as shown in Figure. We preliminarily determined that an external foreign object falling into the roller during the rolling process formed this indentation. We observed abnormal adhesion of the steel strip along the roller surface circumference at the edge of the indentation. This indicates that the accident occurred during the rolling process. Therefore, we judged that this crack should be the crack source causing the peeling of the roller.
Using wire cutting to cut the sample along the red dotted line in Figure 6 to observe the fracture surface crack propagation morphology, as shown in Figure. We can clearly see that the crack first extends at a small angle from the roller surface to the interior. It then splits into multiple cracks. These cracks extend along the grain to the first fatigue extension step of the peeling surface. The crack then widens and elongates further. This increases the fatigue extension area. It forms the second fatigue extension step shown in Figure. After the roller was unloaded normally, the internal stress of the roller redistributed. This caused instantaneous instability. The crack then expanded rapidly to the periphery and the roller surface. This formed a large area of instantaneous fracture zone and the cracking of the roller.The above process conforms to the typical process of surface cracks causing peeling, as shown in Figure, including four typical steps: formation of surface cracks, inward extension of cracks to the hardened layer, extension of fatigue cracks along the hardened layer, and formation of a tearing and brittle peeling.
Conclusion
This paper conducts a detailed analysis and research on the large-area peeling failure of the rolling mill rolls from multiple perspectives, including the fracture surface of the peeling area, the microstructure of the working layer, the depth of the hardened layer, and the crack propagation. In this failure of the peeling rolling mill roll, the microstructure of the working layer and the depth of the hardened layer are normal. Thus, it is clearly determined that the large-area peeling failure of the rolling mill roll is caused by fatigue crack propagation. The crack source is the foreign object falling into the machine frame during the rolling process and forming a indentation, which led to the crack on the roll surface. Currently, with the increasing strength of sheet and strip products and the increasingly complex rolling conditions, the process stability is getting worse. During the use of the rolling mill rolls, they inevitably suffer impact damage on the roll surface. How to better deal with such problems will be an important research direction in the field of rolling mill roll usage technology.
Suggestions can be made from the following three aspects:
(1) Monitor the stability of the rolling process, and trace, identify, and judge the minor accidents in the rolling process from multiple angles such as the surface quality information of the strip steel, the actual process data of the rolling process, and the usage status of the rolling mill rolls, to create a relatively safe environment for the use of the rolling mill rolls.
(2) Improve the rolling mill roll maintenance system. Conduct a full roll surface inspection on rolled mill rolls. These rolls are replaced at the end of the process. Confirm the damage condition of the rolls during their service in the machine. Once you find an abnormality, suspend their use. Analyze the cause. Or eliminate the defect through grinding technology. This allows the rolling mill rolls to return to a normal state.
(3) Develop and use rolling mill rolls with better accident resistance performance, which can resist rolling accidents to a certain extent, prevent the occurrence of roll surface cracks or slow down the expansion speed after the cracks occur, and minimize the occurrence of severe accidents such as roll surface peeling, such as the use of semi-high-speed steel rolling mill rolls, high-speed steel rolling mill rolls, etc.