Abstract: From the aspects of chemical composition, microstructure, hardness, etc., the raw materials and heat treatment quality of bearing rollers with similar specifications at home and abroad were compared. The results show that the raw material composition of foreign rollers is similar to that of domestic rollers, and the foreign rollers have similar banded structure and non-metallic inclusions. In terms of heat treatment quality, the martensitic structure level is close to that of foreign countries, but the content of residual austenite is higher than that of foreign countries.

Key words: cold-rolled sheet mill; Cylindrical bearing roller; Rolling mill bearing; Raw materials; Heat treatment

The early fatigue failure mode of rolling mill bearing rollers is that fine cracks are generated at the maximum alternating shear stress at a certain depth under the surface layer. With continuous load operation, cracks gradually spread to the surface, causing material spalling to produce pitting and pits. With the deterioration of surface contact, early bearing failure is finally caused by non-metallic inclusions and carbide liquid precipitation. It is also one of the reasons for reducing the contact fatigue strength and impact resistance of rollers [1]. The cylindrical bearings of two domestic and foreign companies have been used in the cold-rolled sheet mill of a domestic steel rolling mill, of which the foreign bearing life can reach more than two months and fail, while the domestic [2] bearing ring is made of carburized steel and the rolling body is made of high-carbon chromium bearing steel, but the service life can only reach 20 days. In order to improve the service life of domestic rolling mill bearings, the raw materials and heat treatment quality of foreign rolling mill bearings are analyzed, and a scheme is provided to improve the service life of domestic rolling mill bearings.

1 Roller raw material quality

1.1 Roller structure

The bearing roller of foreign rolling mill is cylindrical structure, the roller size is φ29.5mm, the length is 50mm; The size of the LYC roller is φ30mm and the length is 50mm.

1.2 Chemical composition comparison

Using the Swiss ARL4460 direct reading spectrograph and referring to the standard TB/T 2235-2010 "Technical Conditions for Rolling Bearings of Railway Vehicles", the components of foreign bearing rollers and LYC bearing rollers of similar specifications were analyzed. The results are shown in Table 1. From the result, the material of the roller used by the foreign bearing roller and the LYC is equivalent to the domestic GCr15 steel, and the composition is similar.

1.3 Non-metallic inclusions and carbide distribution

The performance of bearing steel directly affects the performance of bearing. There are four main factors affecting the quality of bearing steel, one is the content, shape, distribution and size of inclusions in steel; Secondly, the content, form, distribution and size of carbides in steel; Third, the center loose shrinkage hole and center segregation in steel; Fourth, the consistency of bearing steel product performance [3]. According to the corresponding grade chart of JB/T 18254-2002 "High carbon Chromium Bearing Steel", the raw materials used for foreign rolling mill bearing rollers and LYC rollers are rated in terms of non-metallic inclusion and carbide non-uniformity. The test results are shown in Table 2. According to the test results, all kinds of inclusion defects, banded carbides and carbide liquidations meet the TB/T 2235-2010 standard. The results show that the quality of GCr15 electroslag steel used for LYC roller reaches the quality of foreign rolling mill bearing roller, and is slightly superior in strip structure control.

2 Heat treatment quality inspection

2.1 Microstructure and hardness inspection

The microstructure of rolling mill bearing rollers at home and abroad was tested by OLYMPUS GX51 optical microscope. The etching agent was 4% nitrate alcohol solution. In order to grasp the hardness and distribution of roller bearings in foreign rolling mills, the roller was cut along 1/2 of the length, the cross section was ground, and the hardness was measured from the surface to the inside. The surface structure, core structure, carbide network, hardness of rolling surface and hardness of core of roller were tested. The results are shown in Table 3.

Table 3 reflects the domestic and foreign technical indicators of roller heat treatment quality, and the distribution of Figure 1 and Figure 2 represents the microstructure of foreign and domestic bearing rollers.

According to JB/T 1255-2014 "Technical Conditions for Heat treatment of high-carbon chromium bearing steel parts of rolling bearings", the microstructure and hardness of the roller heat treatment quality at home and abroad meet the standard requirements, and T stands for troostite structure. As can be seen from Table 3, the heat treatment quality of LYC rollers is comparable to that of foreign bearing rollers. As domestic rollers are salt bath quenched and tempered in the roll bottom furnace production line with protective atmosphere, both the control level of martensitic structure and the uniformity of martensitic structure (see Figure 2(a)) and the control of troostitic structure (see Figure 2(b)). Neither is lower than the microstructure level of foreign bearing rollers (see Figure 1(a) and (b)). The surface martensitic structure of foreign bearing rollers is level 3, while the surface martensitic structure of LYC rollers can reach level 2 ~ 3 compared with the actual control. There is only a small amount of troostite in LYC roller core superficial corrosion, but troostite grade 2 in foreign roller core superficial corrosion. In contrast to the ribbon structure, foreign raw materials exist, but the carbide distribution is uniform and small in the belt (see Figure 1(c)); The LYC rollers have no zonal distribution (see Figure 2(c)). In contrast to the network carbide, foreign rollers exist, and the size of a single particle carbide is larger (see Figure 1 (d)), while the LYC tissue network carbide is absent, and the distribution of carbide particles is fine and uniform (see Figure 2(d)). Compared with the hardness of the roller core, the hardness gradient of the LYC roller has little change, and is slightly higher than the hardness of the foreign roller core. From the mesh carbide and ribbon carbide point of view, the quality of domestic bearing roller raw materials is equivalent to foreign raw materials.

2.2 Surface residual austenite

A portable Xstrees3000X X-ray stress meter was used to test the residual austenite content of the longitudinal section samples of two kinds of rollers. The results of the domestic and foreign rolling mill bearing rollers are shown in Table 4.

From the test value of residual austenite on the surface, the content of residual austenite in China is greater than that in foreign rollers. It can be seen that the bearing rollers of this kind of rolling mill belong to the martensitic quenching process, and the salt bath quenching and tempering heat treatment method, and the bearing industry JB/T 1255-2014 has specified the residual austenite. For the residual austenite of salt quenching, the residual austenite changes little after re-tempering. Although the residual austenite content is high, tempering at 200℃ has little effect on it. In terms of residual austenite, if the LYC roller is used in the rolling mill, the original martensitic quenching of the roller can be replaced by the heat treatment method of Martensitic compound quenching [4]. The quenched structure obtained after quenching is not a single bainite structure, but a composite structure dominated by bainite and containing a small amount of martensitic. This "Martensitic composite structure" not only has high strength and hardness, but also has a small amount of martensitic. Moreover, it has good impact performance and can meet the service conditions of rolling mill bearings. In order to obtain such composite structures, the process scheme adopts martensite transformation after quenching [5]. The martensite transformation occurs first during quenching, and the parts with low carbon content in austenite are transformed into martensite, while the parts with high carbon content in austenite are transformed into lower bainite during the subsequent isothermal process of lower bainite transition temperature above Ms point. After the final quenching, the residual austenite volume is small and the dimensional stability is high. The martensite transformed during the first quenching process is tempered in time when the lower bainite transition temperature is isothermal, so the quenching process is not easy to crack. The quenching heating temperature is 860℃, the heating time is 84min, the first cooling temperature is 180℃, the isothermal temperature is 230℃, and the isothermal time is 5h.

2.3 Grain size comparison

According to the crystal size displayed in GB/T 6394-2002 "Method for the Determination of Average Grain Size of Metals", the etching agent was used 2g bitric acid +1g sodium decyl sulfonic acid +100mL H2O at room temperature. The austenite grain size of high carbon chromium bearing steel after quenching and tempering can be displayed. According to GB/T 6394-2002 standard, metallographic microscope was used for grading by manual cut-off method. The grain size comparison of the two rollers is shown in Figure 3, the foreign roller is 10.2, the LYC roller is 10.4. There are many factors affecting the grain size, the most important are the chemical composition of the material and the heating temperature and holding time during heat treatment; The higher the heating temperature during heat treatment, the longer the holding time, the coarser the grain, and vice versa. This further shows that the two heat treatment quenching heating temperature is the same, belong to a reasonable temperature distribution.

3 Conclusion

1) Domestic electroslag bearing steel roller materials with similar specifications are close to the chemical composition of foreign rolling mill bearing roller materials, and the quality of raw materials is intact, so GCr15 electroslag bearing steel can be used to replace foreign rolling mill bearing roller materials.

2) The heat treatment quality of domestic bearing rollers, such as microstructure, grain size and hardness, is equivalent to that of foreign countries, but the control of residual austenite is slightly higher than that of foreign countries, and good mechanical properties can be obtained by carrying out the heat treatment process of first horse and then shell.

3) With the popularization and application of Mabai production line, the heat treatment process of "Mabai composite structure" can be realized: it is recommended to use the quenching heating temperature of 860℃, the heating time of 84min, the first cooling temperature of 180℃, the isothermal temperature of 230℃, and the isothermal time of 5h for small batch trial production.

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