Method for regulating and controlling recrystallization of bearing steel through electric pulse-assisted cold rolling

Abstract

The invention discloses a method for regulating and controlling recrystallization of bearing steel through electric pulse-assisted cold rolling, which comprises the following steps: prefabricating a plurality of groups of sample ring blanks, applying pulse current to samples in the cold rolling process, and setting the duty ratio, peak current and frequency of the pulse current, the pressing amount of rolling, the rolling time and the rotating speed of an upper pressing roller; the temperature change of the sample ring blank in the rolling process is recorded, if the temperature is higher than 350 DEG C after rolling is finished, the duty ratio or the peak current is reduced or the rolling time is shortened during next rolling, and the group with the temperature lower than 350 DEG C is reserved; the method comprises the following steps: performing EBSD test on a sample without electric pulse auxiliary rolling and a sample subjected to electric pulse auxiliary rolling under different duty ratios and peak currents, comparing a recrystallization grain ratio with an average grain size, establishing a corresponding relation between the recrystallization grain ratio and the duty ratio and between the recrystallization grain ratio and the peak current, and further evaluating and optimizing the electric pulse auxiliary cold ring rolling treatment process. The production of recrystallized grains is efficiently promoted in the cold ring rolling process, work hardening is avoided, and the secondary machining performance of the cold ring rolling is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of regulating the microstructure and properties of metal materials, and particularly to a method for regulating the recrystallization of bearing steel by electric pulse-assisted cold rolling. Background Art

[0002] Cold rolling is a way for metal materials to undergo plastic deformation and is widely used in the pretreatment of metals such as light alloys, stainless steels, and bearing steels. According to the law of conservation of energy, during the plastic deformation process of materials, the mechanical energy generated by the external force doing work will be converted into internal energy and heat energy, and the internal energy is stored in the metal lattice in the form of microscopic defects. Compared with traditional production processes, cold rolling has the advantages of high processing accuracy, excellent surface finish, high material utilization rate, and high production efficiency. It has been proven that cold rolling can refine the microstructure of metals such as bearing steel, such as refining primary carbides, grains, martensite, bainite, etc. It strengthens the metal by refining the metal microstructure (grains), significantly improving its strength and hardness. However, while its strength and hardness are improved, a large number of dislocations are introduced, resulting in work hardening, and the toughness and plasticity of the metal matrix are significantly reduced, which is not conducive to subsequent secondary processing.

[0003] The occurrence of metal grain recrystallization can significantly reduce the dislocation density inside the metal, release the deformation stored energy accumulated during cold rolling, eliminate the high residual stress inside the metal, and at the same time convert the long-axis grains formed by cold rolling into equiaxed grains, significantly improving its toughness and plasticity. In the industrial field, the method for promoting the recrystallization of cold-rolled metals is the recrystallization annealing method. Generally, the heavily deformed metal is heated above its lowest recrystallization temperature and held for a certain time. The proportion of recrystallized grains is closely related to the degree of metal deformation, heating temperature, holding time, cooling method, etc. Obviously, this method requires a relatively high heating temperature and a long holding time, resulting in high production costs and a long production process.

[0004] Therefore, how to efficiently promote the recrystallization of cold-rolled metals, so that they have a refined microstructure (grains) and can ensure high toughness and plasticity for subsequent secondary processing, is an urgent problem to be solved in the current field of cold ring rolling processing of bearing steel. Summary of the Invention

[0005] The main purpose of the present invention is to address the deficiencies of the above-mentioned recrystallization annealing process, such as the need for a relatively high heating temperature and a long holding time, high requirements for the degree of cold rolling deformation, time-consuming and energy-consuming, and the extension of the overall processing flow of bearing steel. A method for regulating the recrystallization of bearing steel by electric pulse-assisted cold ring rolling is proposed. Through the thermo-electric-mechanical composite field, the recrystallization ratio of bearing steel during cold ring rolling is promoted to improve its secondary processing performance and avoid long-time annealing treatment; by changing the peak current and duty cycle of the electric pulse, the recrystallization ratio during the cold rolling process of bearing steel is comprehensively regulated.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A method for regulating the recrystallization of bearing steel by electric pulse-assisted cold rolling, comprising the following steps:

[0008] S1. First, prefabricate multiple groups of sample ring blanks through rolling equipment, and then apply pulsed current to the samples during cold rolling, ensuring that the current flows from the upper pressure roller of the rolling equipment through the samples to the lower pressure roller, so as to ensure that electricity and force always act on the same local area of the samples; the parameters of the pulsed current include: duty cycle, peak current, and frequency; the parameters of rolling include: reduction, rolling time, and rotational speed of the upper pressure roller, and the pulse time is equal to the rolling time; the peak current of one group of samples is set to 0A, and the rolling parameters are the same as those of other samples, serving as a control group;

[0009] S2. Record the temperature change of the sample ring blanks during rolling. If the surface temperature of the sample at the end of rolling is higher than 350°C, reduce the duty cycle or peak current, or reduce the rolling time during the next rolling, and repeat step S1 for electric pulse-assisted rolling, retaining the group where the surface temperature of the sample at the end of rolling is still lower than 350°C;

[0010] S3. Conduct EBSD testing on the sample ring blanks without electric pulse-assisted rolling, and obtain the proportion of recrystallized grains and the average grain size thereof;

[0011] S4. Conduct EBSD testing on the sample ring blanks of electric pulse-assisted rolling under different duty cycles and peak currents by the method of controlling variables, and obtain the proportion of recrystallized grains and the average grain size thereof; by comparing with the proportion of recrystallized grains and the average grain size of the sample ring blanks without electric pulse-assisted rolling in S3, establish the corresponding relationships between the duty cycle, peak current, the proportion of recrystallized grains, and the average grain size respectively, and then evaluate and optimize the process of electric pulse-assisted cold ring rolling.

[0012] In the above solution, in step S1, the selection range of the duty cycle of the pulsed current is 0.01 - 0.5, the selection range of the peak current is 0 - 1000A, and the selection range of the frequency is 0 - 1000Hz.

[0013] In the above solution, in step S1, the range of controlling the reduction speed of rolling is 0.1 mm / s - 100 mm / s, reduction speed = reduction / rolling time; the range of controlling the rolling ratio is 0.1 - 0.9, rolling ratio = reduction / initial thickness of the sample ring blank in the rolling direction; the selection range of the rotational speed of the upper pressure roller is 0 - 6.28 r / s; the rolling time is controlled within 1 - 15 s.

[0014] In the above solution, in step S4, if the proportion of recrystallized grains increases and the average grain size decreases compared with non-electric pulse assisted rolling, a larger duty cycle or peak current is selected, and steps S1 - S2 are repeated. If the surface temperature of the specimen is still lower than 350 °C at the end of rolling, the proportion of recrystallized grains and the average grain size are continuously compared until the average grain size becomes larger than that under the previous parameter, indicating that the previous parameter is the optimal choice.

[0015] In the above solution, in step S4, if the proportion of recrystallized grains increases and the average grain size becomes larger compared with non-electric pulse assisted rolling, a lower duty cycle or peak current is selected, and steps S1 - S2 are repeated. If the surface temperature of the specimen is still lower than 350 °C at the end of rolling, the proportion of recrystallized grains and the average grain size are continuously compared until the average grain size is smaller than that of the control group, indicating that this parameter is the optimal choice.

[0016] In the above solution, in step S4, if the proportion of recrystallized grains decreases compared with non-electric pulse assisted rolling and the grain size necessarily becomes larger, a larger duty cycle or peak current is selected, and steps S1 - S2 are repeated. If the surface temperature of the specimen is still lower than 350 °C at the end of rolling, the proportion of recrystallized grains and the average grain size are continuously compared until the proportion of recrystallized grains is larger than that of the control group, indicating that this parameter is the optimal choice.

[0017] In the above solution, the equipment for regulating the recrystallization of bearing steel by electric pulse assisted cold rolling includes an upper pressure roll, a lower pressure roll and a pulse power supply. An insulating ring is arranged inside the upper pressure roll, and an insulating core roll is arranged inside the lower pressure roll. The specimen ring blank is installed on the lower pressure roll, and insulating guide rolls are arranged on both sides of the specimen ring blank; the brush A of the pulse power supply is in contact with the upper pressure roll, and the brush B is in contact with the lower pressure roll to ensure that the current flows from the upper pressure roll through the specimen to the lower pressure roll, so as to ensure that electricity and force always act on the same local area of the specimen.

[0018] In the above solution, an infrared thermometer is used to record the temperature change of the specimen ring blank during rolling.

[0019] The beneficial effects of the present invention are:

[0020] The method for regulating the recrystallization of bearing steel by electric pulse assisted cold rolling proposed by the present invention efficiently promotes the generation of recrystallized grains during cold ring rolling, avoids work hardening, and improves its secondary processing performance. Specifically, by utilizing the thermal effect and non-thermal effect of the pulse current, as well as the deformation stored energy generated by the cold rolling of bearing steel, the recrystallization ratio of bearing steel during cold ring rolling is promoted, and fine grains are maintained; in addition, through the feedback of the GOS diagram and grain size, the parameter range of the optimal pulse current that can both promote the recrystallization of bearing steel during cold ring rolling and avoid the growth of recrystallized grains is selected, establishing a connection between the macroscopic process parameters and the microscopic tissue state of bearing steel. Description of the Drawings

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0022] Figure 1 It is a schematic structural diagram of the equipment used in the method of implementing the present invention;

[0023] Figure 2 It is a temperature change diagram of the cold rolling ring process of M50 bearing steel assisted by a 125A peak current in the embodiment of the present invention;

[0024] Figure 3 It is the microstructure of the cold rolling ring specimen of M50 bearing steel without current in the embodiment of the present invention: (a) Grain orientation spread diagram; (b) Grain size statistical diagram;

[0025] Figure 4 It is the microstructure of the cold rolling ring specimen of M50 bearing steel assisted by a 125A peak current in the embodiment of the present invention: (a) Grain orientation spread diagram; (b) Grain size statistical diagram;

[0026] Figure 5 It is a temperature change diagram of the cold rolling ring process of M50 bearing steel assisted by a 200A peak current in the embodiment of the present invention;

[0027] Figure 6 It is the microstructure of the cold rolling ring specimen of M50 bearing steel assisted by a 200A peak current in the embodiment of the present invention: (a) Grain orientation spread diagram; (b) Grain size statistical diagram.

[0028] Figure 1 Among them: 1. Upper pressure roller; 2. Lower pressure roller; 3. Specimen ring blank; 4. Insulating collar; 5. Insulating core roller; 6. Insulating guide roller; 7. Pulse power supply; 8. Brush A; 9. Brush B; 10. Infrared thermometer; 11. Electric power coupling area acting on the specimen. Specific embodiments

[0029] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the following further details the present invention in conjunction with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0030] It should be noted that the illustrations provided in the embodiments of the present invention only schematically illustrate the basic concept of the present invention. Therefore, only the components related to the present invention are shown in the drawings, rather than being drawn according to the number, shape, and size of the components in actual implementation. The types, quantities, and ratios of the components in actual implementation can be arbitrarily changed, and the component layout type may also be more complex.

[0031] In this embodiment, taking the selection of the peak current parameter as an example, the electro-pulse assisted cold rolling treatment of M50 bearing steel is carried out to further elaborate in detail on a method for electro-pulse assisted cold rolling to regulate the recrystallization of bearing steel according to the present invention.

[0032] 1) First, prefabricate multiple groups of M50 bearing steel ring blanks through rolling equipment. The size is an inner diameter of 44 mm, an outer diameter of 60 mm, an axial thickness of 8 mm, and a radial height of 8 mm. The equipment for electro-pulse assisted cold rolling to regulate the recrystallization of bearing steel is as Figure 1 shown, including an upper pressure roller 1 and a lower pressure roller 2. An insulating sleeve 4 is provided inside the upper pressure roller 1, and an insulating core roller 5 is provided inside the lower pressure roller 2. The specimen ring blank 3 is installed on the lower pressure roller 2, and insulating guide rollers 6 are provided on both sides of the specimen ring blank 3. The brush A8 of the pulse power supply 7 contacts the upper pressure roller 1, and the brush B9 contacts the lower pressure roller 2 to ensure that the current flows from the upper pressure roller 1 through the specimen to the lower pressure roller 2, so as to ensure that the electricity and force always act on the same local area of the specimen (that is, the electric-force coupling area 11 in the figure). Set the duty cycle of the pulse current to 0.2, the peak current to 125 A, and the frequency to 100 Hz; the rolling reduction is 2 mm, the rolling time is 10 s, and the rotational speed of the upper pressure roller 1 is 3.14 r / s. After setting each parameter, the specimen ring blank 3 is sleeved on the lower pressure roller 2 for electro-pulse assisted rolling.

[0033] The peak current of the control group is 0 A, and the rolling parameters are the same as those of other specimens, which is used to determine whether recrystallization has occurred and grain growth in the 125 A group.

[0034] After testing, the initial contact area between the rolling roller and the specimen ring blank 3 is 37 mm 2 , and the final contact area at the end of rolling is 186 mm 2 , and its average contact area is 111.5 mm 2 , then the current density at 125 A is 1.12 A / mm 2 .

[0035] 2) Use an infrared thermometer 10 to record the temperature change of the specimen ring blank 3 during the rolling process. The surface temperature of the specimen at the end of rolling is as Figure 2 shown. The surface temperature at the end of cold ring rolling of M50 assisted by a peak current of 125 A is 247.2 °C, which is lower than 350 °C, indicating that the duty cycle, peak current, and pulse time (rolling time) parameters are appropriate.

[0036] 3) EBSD tests were carried out on the M50 cold ring-rolled specimens without electric pulse assistance (i.e., the control group), and their grain orientation spread (GOS) maps and average grain sizes are as Figure 3 shown. Its recrystallization ratio is 24.4%, and the average grain size is 4.05 μm.

[0037] 4) EBSD tests were respectively carried out on the M50 cold ring-rolled specimens assisted by a peak current of 125 A. Their GOS maps and average grain sizes are as Figure 4 shown. Its recrystallization ratio is 30.5% > 24.4%, and the average grain size is 1.91 μm < 4.05 μm. It can be seen from this that the M50 cold ring rolling assisted by a peak current of 125 A can effectively promote its recrystallization ratio during the rolling process, and no growth of recrystallized grains occurs, indicating that a larger peak current can be continued to be selected. Select a peak current of 200 A and repeat steps 1) and 2). If its temperature is still lower than 350 °C, then continue to repeat step 4).

[0038] 5) It can be seen from Figure 5 that the surface temperature at the end of the M50 cold ring rolling assisted by a peak current of 200 A is 276.8 °C, which is still lower than 350 °C. EBSD tests were respectively carried out on the M50 cold ring-rolled specimens assisted by a peak current of 200 A. Their GOS maps and average grain sizes are as Figure 6 shown. Its recrystallization ratio is 34.4% > 30.5%, and the average grain size is 1.41 μm < 1.91 μm. It can be seen from this that the M50 cold ring rolling assisted by a peak current of 200 A can further promote its recrystallization ratio during the rolling process, and no growth of recrystallized grains still occurs, indicating that a larger peak current can be continued to be selected until the average grain size becomes larger compared with the average grain size of the previous parameter, indicating that the proportion of the thermal effect of the pulsed current is larger and the recrystallized grains grow. Then, the previous parameter is the optimal peak current.

[0039] Regarding the duty cycle, the experiment can also be set up by the method of controlling variables according to the above steps. By the recrystallization ratio in the GOS map and the average grain size in the grain size map, it is determined whether the recrystallization is promoted and the growth of recrystallized grains occurs, so as to select the best electric pulse-assisted cold ring rolling parameters for regulating the recrystallization of bearing steel, so that the rolled bearing rings have better performance.

[0040] The method for regulating the recrystallization of bearing steel by electric pulse-assisted cold ring rolling proposed by the present invention efficiently promotes the generation of recrystallized grains during the cold ring rolling process, avoids work hardening, and improves its secondary processing performance. By utilizing the thermal effect and non-thermal effect of pulsed current, as well as the deformation stored energy generated by the cold rolling of bearing steel, the recrystallization ratio of bearing steel during the cold ring rolling process is promoted, and fine grains are maintained. In addition, through the GOS diagram and the feedback of grain size, the optimal parameter range that can not only promote the recrystallization of bearing steel during the cold ring rolling process but also avoid the growth of recrystallized grains is selected, establishing a connection between the macroscopic process parameters and the microscopic tissue state of bearing steel.

[0041] It should be noted that according to the needs of implementation, each step / component described in the present application can be split into more steps / components, or two or more steps / components or partial operations of steps / components can be combined into new steps / components to achieve the purpose of the present invention.

[0042] The magnitudes of the sequence numbers of the steps in the above embodiments do not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

[0043] It should be understood that those of ordinary skill in the art can make improvements or transformations according to the above description, and all such improvements and transformations should fall within the protection scope of the appended claims of the present invention.

Claims

1. A method for regulating the recrystallization of bearing steel by electro-pulse assisted cold rolling, characterized in that, It includes the following steps: S1. First, prefabricate multiple groups of sample ring blanks through rolling equipment, and then apply pulsed current to the samples during cold rolling, ensuring that the current flows from the upper pressing roll of the rolling equipment through the samples to the lower pressing roll to ensure that electricity and force always act on the same local area of the samples; The parameters set for the pulsed current include: duty cycle, peak current, and frequency; the parameters set for rolling include: reduction, rolling time, and rotational speed of the upper pressing roll, and the pulse time is equal to the rolling time; the peak current of one group of samples is set to 0A, and the rolling parameters are the same as those of other samples, serving as a control group; S2. Record the temperature change of the sample ring blanks during rolling. If the surface temperature of the sample at the end of rolling is higher than 350°C, it is necessary to lower the duty cycle or peak current, or reduce the rolling time during the next rolling, and repeat step S1 for electro-pulse assisted rolling, retaining the group where the surface temperature of the sample at the end of rolling is still lower than 350°C; S3. Conduct EBSD tests on the sample ring blanks without electro-pulse assisted rolling, and obtain the proportion of recrystallized grains and the average grain size thereof; S4. Conduct EBSD tests on the sample ring blanks of electro-pulse assisted rolling under different duty cycles and peak currents through the method of controlling variables, and obtain the proportion of recrystallized grains and the average grain size thereof; by comparing with the proportion of recrystallized grains and the average grain size of the sample ring blanks without electro-pulse assisted rolling in S3, establish the corresponding relationships between the duty cycle, peak current, proportion of recrystallized grains, and average grain size respectively, and then evaluate and optimize the electro-pulse assisted cold ring rolling process.

2. The method for regulating the recrystallization of bearing steel by electro-pulse assisted cold rolling according to claim 1, characterized in that, In step S1, the selection range of the duty cycle of the pulsed current is 0.01 - 0.5, the selection range of the peak current is 0 - 1000A, and the selection range of the frequency is 0 - 1000Hz.

3. The method for regulating the recrystallization of bearing steel by electro-pulse assisted cold rolling according to claim 1, characterized in that, In step S1, the range of controlling the reduction speed of rolling is 0.1mm / s - 100mm / s, reduction speed = reduction / rolling time; the range of controlling the rolling ratio is 0.1 - 0.9, rolling ratio = reduction / initial thickness of the sample ring blank in the rolling direction; the selection range of the rotational speed of the upper pressing roll is 0 - 6.28r / s; the rolling time is controlled within 1 - 15s.

4. The method for regulating the recrystallization of bearing steel by electro-pulse assisted cold rolling according to claim 1, characterized in that, In step S4, if, compared with the rolling without electro-pulse assistance, the proportion of recrystallized grains increases and the average grain size becomes smaller, then select a larger duty cycle or peak current, and repeat steps S1 - S2. If the surface temperature of the sample at the end of rolling is still lower than 350°C, then continue to compare the proportion of recrystallized grains and the average grain size until the average grain size becomes larger compared with the average grain size under the previous parameter, indicating that the previous parameter is the optimal choice.

5. The method for regulating the recrystallization of bearing steel by electro-pulse assisted cold rolling according to claim 1, wherein In step S4, if, compared with the rolling without electro-pulse assistance, the proportion of recrystallized grains increases and the average grain size becomes larger, then select a lower duty cycle or peak current, and repeat steps S1 - S2. If the surface temperature of the sample at the end of rolling is still lower than 350°C, then continue to compare the proportion of recrystallized grains and the average grain size until the average grain size is smaller than that of the control group, indicating that this parameter is the optimal choice.

6. The method for regulating the recrystallization of bearing steel by electro-pulse assisted cold rolling according to claim 1, characterized in that In step S4, if the proportion of recrystallized grains decreases compared with non-electric pulse assisted rolling and the grain size necessarily increases, a larger duty cycle or peak current is selected, and steps S1 - S2 are repeated. If the surface temperature of the specimen is still lower than 350 °C at the end of rolling, the proportion of recrystallized grains and the average grain size are continuously compared until the proportion of recrystallized grains is greater than that of the control group, indicating that this parameter is the optimal choice.

7. The method for regulating the recrystallization of bearing steel by electro-pulse assisted cold rolling according to claim 1, wherein The equipment for regulating the recrystallization of bearing steel by electric pulse assisted cold rolling includes an upper pressure roll, a lower pressure roll and a pulse power supply. An insulating ring is provided inside the upper pressure roll, and an insulating core roll is provided inside the lower pressure roll. The specimen ring blank is installed on the lower pressure roll, and insulating guide rolls are provided on both sides of the specimen ring blank; the brush A of the pulse power supply is in contact with the upper pressure roll, and the brush B is in contact with the lower pressure roll to ensure that the current flows from the upper pressure roll through the specimen to the lower pressure roll, so as to ensure that electricity and force always act on the same local area of the specimen.

8. The method for regulating the recrystallization of bearing steel by electro-pulse assisted cold rolling according to claim 1, characterized in that An infrared thermometer is used to record the temperature change of the specimen ring blank during rolling.

Images