Method for improving banded shape of carbide of GCr15 bearing steel

Abstract

The invention provides a method for improving the banded form of GCr15 bearing steel carbides, which comprises the following steps: controlling the specific water flow of secondary cooling water to be 1.0 L / Kg-1. 5L / Kg in the cooling process of a continuous casting billet with the section size of 180mm * 240mm, and controlling the water distribution ratio of all areas of a secondary cooling section to be 4: 3: 3; in the continuous casting process, a soft reduction technology is adopted, a casting blank liquid core is kept unchanged at the initial reduction position, the pulling speed is kept constant, and the electromagnetic stirring speed is kept constant. According to the method, in the continuous casting process, a large-specific-water-flow forced cooling mode is adopted, solidification is accelerated, and therefore selective crystallization between dendrites is reduced, alloy impurity elements between the dendrites are reduced, and carbide banding is reduced after steel rolling is solidified.

Description

Technical Field

[0001] The invention belongs to the technical field of iron and steel metallurgy, and in particular relates to a method for improving the banded morphology of carbides in GCr15 bearing steel. Background Art

[0002] GCr15 bearing steel is the most widely used type of bearing steel. About 90% of bearing materials use this type of steel and it is widely used in all walks of life. The purity and uniformity of GCr15 bearing steel have an important influence on the stability, low noise and long life of bearings. Among them, the carbide band is an important factor affecting its uniformity. The carbon content of the carbide band is relatively high, and some liquid carbides are also easy to form on the band. At the same time, during the cooling process of steel rolling, network carbides are also easy to form in this area. These large pieces or large particles of carbides have high hardness in macroscopic mechanical properties, which are different from the surrounding body. Stress concentration is easy to occur during the processing and use of bearings, which leads to early cracks, thus affecting the fatigue life of the bearings.

[0003] The Chinese patent application number 201710513127.7 discloses a deformation method for reducing the grade of banded carbides in GCr15 bearing steel, including electromagnetic stirring in the continuous casting stage of molten steel, high temperature annealing after continuous casting, the high temperature annealing temperature is 1220℃, the holding time is 120-180min, and the blank is rolled after high temperature annealing, and formed by deformation methods such as forging or rolling, the deformation temperature is not less than 780℃ and not higher than 820℃, and the blank deformation is not less than 30% and not higher than 50%. The unevenness of carbides is eliminated in the continuous casting stage, and the grade of banded carbide GCr15 steel is reduced in the deformation stage. However, since C and Cr elements are easy to form liquid carbides in the cast point segregation area, it is difficult to ensure the consistency of the effect of reducing the grade of carbides only through high temperature annealing and rolling deformation. Summary of the invention

[0004] Aiming at the technical problem that the carbide banding level of GCr15 bearing steel is relatively high and affects the processing performance, the present invention proposes a method for accelerating solidification, alleviating selective crystallization between dendrites and reducing the carbide banding level of GCr15 bearing steel by adopting a high specific water volume strong cooling method during the continuous casting process.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A method for improving the banded morphology of carbides in GCr15 bearing steel, wherein the water volume of secondary cooling water in a continuous casting billet with a cross-sectional size of 180 mm×240 mm is controlled to be 1.0 L / Kg-1.5 L / Kg during the cooling process, and the water distribution ratio between each zone in the secondary cooling section is 4:3:3; a light reduction technology is adopted during the continuous casting process, and the liquid core of the billet is kept unchanged at the starting reduction position, the pulling speed is kept constant, and the electromagnetic stirring speed is kept constant.

[0007] Preferably, the pulling speed of the straightening machine is 0.8m / min-1.2m / min, the electromagnetic stirring parameter of the crystallizer is 150A-300A, the frequency is 2Hz-4Hz, and the terminal electromagnetic stirring parameter is between 400A-600A, the frequency is 5Hz-8Hz.

[0008] Preferably, the starting position for pressing is the second group of rollers, and the specific pressing amount is 0 mm for the first group of rollers, 1 mm for the second group of rollers, 2 mm for the third group of rollers, 4 mm for the fourth group of rollers, 5 mm for the fifth group of rollers, 1 mm for the sixth group of rollers, 0 mm for the seventh group of rollers, 0 mm for the eighth group of rollers, and 0 mm for the ninth group of rollers.

[0009] Preferably, the first set of rollers adopts a fixed pressure mode, and the specific pressure is 2MPa-6MPa.

[0010] Compared with the prior art, the advantages and positive effects of the present invention are:

[0011] The carbide bands of GCr15 bearing steel are formed in the process of solidification of continuous casting billet, and there is selective crystallization between the dendrites, forming high concentrations of carbon, chromium, manganese, sulfur, phosphorus and other substances. After solidification, these alloy impurity elements form a large number of carbides. In the steel rolling process, these carbides cannot be completely homogenized, and finally appear as carbide bands. In order to reduce the degree of GCr15 carbide bands, the present invention adopts a large specific water volume strong cooling method in the process of continuous casting to accelerate solidification, thereby reducing the selective crystallization between the dendrites, reducing the alloy impurity elements between the dendrites, and reducing the carbide bands after solidification and steel rolling.

[0012] Specifically: 1. The water ratio of the secondary cooling water of 1.1L / Kg and 1.3L / Kg is used to accelerate the solidification speed of the continuous casting billet, shorten the length of the solidified liquid core by about 1m, and reduce the dendrite segregation of the continuous casting billet.

[0013] 2. In the steel rolling process, when the heating time of the heating furnace remains unchanged, the carbide banding is determined according to the SEP1520 standard, and the proportion of carbide banding of grade 7.5 and above is about 19.4%; when strong cooling is used, the proportion of carbide banding of grade 7.5 and above is about 6%, and the carbide banding structure is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The metallographic structure diagram of the GCr15 bearing steel prepared in Example 1 of the present invention;

[0015] Figure 2 The metallographic structure diagram of the GCr15 bearing steel prepared in Example 2 of the present invention;

[0016] Figure 3 This is the metallographic structure diagram of the GCr15 bearing steel prepared in the comparative example of the present invention. DETAILED DESCRIPTION

[0017] In order to better understand the present invention, a detailed description is given below in conjunction with the accompanying drawings and embodiments.

[0018] Embodiment 1:

[0019] The converter was used for smelting, with a tapping temperature of 1612°C and a tapping carbon of 0.12%; thereafter, the LF furnace was used for refining, with a refining time of 58 minutes and a tapping temperature of 1550°C. The RH refining time was 25 minutes, and the refined molten steel was cast by a continuous casting machine, with a pulling speed of 1.0m / min, and the terminal electromagnetic stirring parameters were set to 500A, a frequency of 6Hz, and a superheat of 23°C. The secondary cooling water volume during the continuous casting process was 1.1L / Kg, and the starting pressing position of the light pressing device was pressed from the second set of rollers. The specific continuous casting parameters are shown in Table 1.

[0020] Table 1 Continuous casting process parameters of Example 1

[0021]

[0022] Embodiment 2:

[0023] The converter smelting was adopted, the tapping temperature was 1610℃, and the tapping carbon was 0.11%. The LF furnace refining time was 62 minutes, the LF furnace tapping temperature was 1556℃, and the RH refining time was 27 minutes. During the continuous casting process of the continuous casting machine, the pulling speed was 1.0m / min, the end electromagnetic stirring parameters were 500A, the frequency was 6Hz, and the superheat was 24℃. During the continuous casting process, the secondary cooling water volume was 1.3L / Kg, and the starting pressing position of the light pressing device was pressed from the second set of rollers. The specific continuous casting parameters are shown in Table 2.

[0024] Table 2 Continuous casting process parameters of Example 1

[0025]

[0026] Comparative Example:

[0027] The converter smelting was adopted, the tapping temperature was 1605℃, and the tapping carbon was 0.13%; the LF furnace refining time was 57 minutes, the LF furnace tapping temperature was 1548℃, the RH refining time was 24 minutes, the casting speed of the continuous casting machine was 1.0m / min, the terminal electromagnetic stirring parameters were 500A, the frequency was 6Hz, and the superheat was 21℃. The continuous casting secondary cooling water volume was 0.3L / Kg, and the specific continuous casting parameters are shown in Table 3.

[0028] Table 3 Parameters of comparative continuous casting process

[0029]

[0030] The ingots prepared in Example 1, Example 2 and the comparative example were heated in a steel rolling furnace and rolled into bars with a diameter of 40 mm. Samples were taken longitudinally, and the sides of the sample tubes covered the entire radius from the edge to the center. The samples were then subjected to spheroidizing heat treatment and then quenching treatment. After the heat treatment, the samples were ground, polished, and corroded with nitric acid and alcohol. Finally, they were observed under a microscope. Figure 1-3 The metallographic organization diagrams of Example 1, Example 2 and Comparative Example are shown in Table 4. The samples were further compared with the SEP1520 standard spectrum, and the specific rating results are shown in Table 4.

[0031]

[0032]

[0033] Combination Figure 1-3 As can be seen from Table 4, the proportion of carbide banding of the bearing steel prepared in Example 1 with a grade greater than or equal to 7.5 is 8.3%, and its banding is 7.25 on average; the proportion of carbide banding of the bearing steel prepared in Example 2 with a grade greater than or equal to 7.5 is 5.5%, and its banding is 7.24 on average; the proportion of carbide banding of the bearing steel prepared in the comparative example with a grade greater than or equal to 7.5 is 19.4%, and its banding is 7.34 on average. In the steel rolling process, when the heating time of the heating furnace remains unchanged, the proportion of carbide banding of 7.5 and above is significantly reduced when the continuous casting machine adopts strong cooling, and the carbide banding structure is reduced. According to the SEP1520-2002 version standard, the carbide banding is reduced from the maximum grade 7.5 to grade 7.2.

[0034] The above description is only a preferred embodiment of the present invention and does not limit the present invention in other forms. Any technician familiar with the profession may use the technical content disclosed above to change or modify it into an equivalent embodiment with equivalent changes and apply it to other fields. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for improving the banded morphology of carbides in GCr15 bearing steel, characterized in that: During the cooling process of the continuous casting billet with a cross-sectional size of 180mm×240mm, the water volume of the secondary cooling water is controlled to be 1.0L / Kg-1.5L / Kg, and the water distribution ratio between each zone of the secondary cooling section is 4:3:3; the light pressure reduction technology is adopted during the continuous casting process and the starting pressure reduction position keeps the liquid core of the billet unchanged, the pulling speed is kept constant, and the electromagnetic stirring speed is kept constant.

2. The method for improving the banded morphology of carbides in GCr15 bearing steel according to claim 1, characterized in that: The pulling speed of the straightening machine is 0.8m / min-1.2m / min, the electromagnetic stirring parameters of the crystallizer are 150A-300A, the frequency is 2Hz-4Hz, and the end electromagnetic stirring parameters are between 400A-600A, and the frequency is 5Hz-8Hz.

3. The method for improving the banded morphology of carbides in GCr15 bearing steel according to claim 1, characterized in that: The starting position for pressing is the second set of rollers, and the specific pressing amount is 0 mm for the first set of rollers, 1 mm for the second set of rollers, 2 mm for the third set of rollers, 4 mm for the fourth set of rollers, 5 mm for the fifth set of rollers, 1 mm for the sixth set of rollers, 0 mm for the seventh set of rollers, 0 mm for the eighth set of rollers, and 0 mm for the ninth set of rollers.

4. The method for improving the banded morphology of carbides in GCr15 bearing steel according to claim 3, characterized in that: The first set of rollers adopts a fixed pressure mode, and the specific pressure is 2MPa-6MPa.

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