Analysis on the causes of surface peeling of work rolls in cold tandem rolling

The cold rolling roll is an important large-scale component of the cold tandem rolling unit. Its manufacturing process is relatively complex and the working environment is harsh. It is subject to friction, thermal stress, impact and other stresses. During use, it may break, peel, wear, etc., and then fail. , increase the cost of consumables and affect production, causing economic losses. During the normal rolling production process of a company’s stainless steel production line, the surface of the cold continuous rolling work roll suddenly peeled off, causing a breakdown and shutting down, seriously affecting normal production. This article takes the peeling sample of the cold tandem rolling work roll as the analysis object. Through physical and chemical testing methods such as macroscopic fracture, spectral composition measurement, hardness, metallographic structure, and scanning electron microscopy, combined with daily use conditions, the causes of roll surface peeling are analyzed and discussed.

Physical and chemical inspection results and observation of the fracture of the peeled block

Carry out macroscopic morphology analysis of the roll peeling blocks (see Figure 1). There are typical fatigue fracture characteristics on the peeled block, and obvious ridge lines can be observed in the central circle area, which is an important feature of the fatigue expansion zone. The center of the shell line is the fatigue source, which is the initiation area of fatigue cracks. It can be seen that the fatigue source is located inside the roll rather than on the surface; the outer ring area occupies the largest area, and the macroscopic morphology is radial, which is an instant fracture zone. In cases of failure of structural materials and mechanical parts, fatigue damage is different from static load damage. Most of them occur without warning and in unexpected circumstances. There are no obvious signs in appearance before failure, and the damage is serious.

Using a scanning electron microscope to observe the fatigue source area, it was found that there were granular inclusions with a diameter of about 50 μm. EDS results show that it contains Ca, K, O, Mg, Al, Si and other elements (see Figure 2), and is a large-grained oxide mixed inclusion.

Chemical composition: Cut a sample from the peeled block for spectral composition analysis. The roll material is 8Cr3NiMoV. The test results are shown in Table 1. Except for the Cr element which is slightly lower than the lower limit, other components are within the scope of the standard GB/T1299-2014. A Rockwell hardness tester was used to test the hardness of the exfoliated block. The hardness (HRC) value reached 64.5, and each position on the sample was relatively uniform, meeting the standard requirements.

Table 1 Chemical composition w/% of exfoliated blocks
 CSiMnP SCrMoNiV
test value0.84120.41010.3020.00940.00062.790.23570.670.0697
standard value0.82-0.900.30-0.500.20-0.45≤0.020≤0.0152.80-3.200.20-0.400.60-0.800.05-0.15

Metallographic observation

A wire cutting machine was used to cut metallographic samples at the fatigue source. After measurement, the fatigue source was about 8mm away from the roll surface. After sample preparation and polishing, ZEISSImager.A1m metallographic microscope, ZEISSEVO18 scanning electron microscope and other instruments were used for observation and analysis. It was found that there were multiple root-shaped cracks in the peeled block (see Figure 3). The cracks originated from the internal fatigue source and expanded to the roll surface.

Figure 3 Internal cracks in the peeled block

Comprehensive observation of the sample revealed that there were many massive inclusions in the peeled block (see Figure 4a), with a size of up to 90 μm, and the inclusions seriously exceeded the standard. Using scanning electron microscope energy spectrum analysis (see Figure 4b), the massive large particle inclusions are mainly O and Al, with a small amount of Mg, S, Ca, Mn, and Fe. It can be inferred that they are mixed inclusions with Al2O3 as the main component. It may be caused by lax control of the melting process when producing rolls.

2. Analysis and discussion

The process of crack generation

From the macroscopic morphology analysis, it can be inferred that the roll spalling failure originated from the internal fatigue source. Subsequently, under the continuous stress cyclical stress during operation, the cracks gradually expanded to form ridge lines. The fatigue cracks expand to a certain extent, resulting in insufficient strength and inability to withstand the external force during the rolling process. Eventually, a termination zone is formed and the surface of the roll peels off. During the operation of the roll, the maximum combined shear stress caused by the load of the rolling mill and the local compression of the roll at the contact point is located in a small area below the surface of the roll. The preparation process before the roll is manufactured and used will produce residual stress. At the same time, although the cold rolling processing temperature is low, the temperature of the roll and the strip will also increase under the action of friction, resulting in thermal stress. If strip breakage, flicking, overlap, slippage, etc. occur during the rolling process, the roll surface will be subject to local overload heat and impact stress. Since non-metallic inclusions exist in the steel in the form of mechanical mixtures, and their properties are very different from those of steel, they destroy the uniformity and continuity of the steel matrix and cause stress concentration there, becoming a source of fatigue. In addition, during the heating process, the linear expansion coefficients of the non-metallic inclusions and the matrix are different, creating an additional stress field in the matrix near the inclusions. Under such complex stress conditions, if there are non-metallic inclusions in the surface layer, especially brittle inclusions, the spherical inclusions will first peel off from the matrix at the two extremes of the maximum stress, forming primary microcracks. Before the crack propagates rapidly, the interface between the steel matrix and the inclusions gradually separates from the matrix and forms a crack channel. As the number of stress cycles increases, the microcracks gradually expand outward along the shell tear matrix of the ball. When the overall size of the crack exceeds the critical size that the roll can withstand, the fatigue crack enters the unstable expansion stage, and the roll surface eventually peels off due to instantaneous fracture. Roll surface peeling has gone through several processes: crack initiation caused by inclusions → crack propagation → peeling.

Effect of inclusions on fatigue properties

The influence of inclusions on the fatigue life of the workpiece is related to the nature, size, quantity and distribution of the inclusions. Generally speaking, hard and brittle massive or spherical inclusions that have poor connection with the matrix and do not deform, such as TiN, Al2O3, etc., are more harmful than ductile and elongated inclusions. When there are a large number of inclusions, they are concentrated and distributed, or they are on the surface of the part or in a high-stress area, they will have the most serious impact on the fatigue life. At the same time, the influence of inclusions on fatigue properties also depends on the structure and properties of the matrix. Experiments have pointed out that the fatigue strength of mild steel has little relationship with inclusions. As the strength of steel increases, the harmful effects of inclusions become more and more serious. serious. In metal materials with high hardness and high strength, the influence of inclusions on fatigue strength has become a prominent issue. According to relevant research data, fatigue life is very sensitive to the size of inclusions. Reducing the size of inclusions can significantly improve fatigue life. For high-hardness, high-strength workpieces, the critical size of surface inclusions is 8 to 10 μm, which decreases as the hardness increases and increases as the depth increases. If the inclusions are smaller than the critical size, fatigue fracture caused by the inclusions can be avoided and the fatigue performance will be better; when the inclusions are larger than the critical size, as the size of the inclusions increases, the fatigue strength and fatigue life of the steel decrease sharply. According to the literature, for high-strength steel, if the size of the inclusions is reduced by 1/3, the fatigue life will be extended by 10 times, and if the size of the inclusions is reduced by approximately half, the fatigue life will be extended by 100 times. At the same time, if the size of the inclusions is reduced by half, the fatigue strength can be increased by 1.12 to 1.15 times. To sum up, the roll is under the action of complex stress during use, causing fatigue crack sources at large inclusions. As the alternating stress continues to change, the fatigue cracks expand and generate secondary cracks, forming on the fracture surface. Fatigue bands and radial bands. When the overall size of the crack exceeds the critical size, the fatigue crack enters the unstable expansion stage, and the roll eventually fails due to instantaneous fracture and spalling.

Conclusion

(1) The surface spalling of the cold tandem rolling work roll is caused by fatigue fracture caused by large-sized Al2O3-like brittle and hard inclusions near the surface.

(2) Strengthen the detection of rolls. Ultrasonic testing can detect internal defects, and magnetic particle and eddy current can detect surface defects. Different methods or a combination are used according to the situation to ensure the quality of the rolls.

(3) Develop a scientific and reasonable roll use and maintenance system to ensure cooling and lubrication during use and prevent overheating.

 

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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