CN117980521A

CN117980521A - Mechanical component and rolling bearing

Info

Publication number: CN117980521A
Application number: CN202280064409.2A
Authority: CN
Inventors: 水田浩平; 藤村直辉
Original assignee: NTN Corp
Current assignee: NTN Corp
Priority date: 2021-09-27
Filing date: 2022-09-21
Publication date: 2024-05-03

Abstract

The machine part (10) is made of steel and has surfaces (10 a,10b,10c,10 d). The steel has been quenched and tempered. The mechanical component is provided with a nitriding layer (11) on the surface and a core portion farther from the surface than the nitriding layer. The nitrogen concentration in the steel on the surface is 0.01 mass% or more. The hardness of the steel on the surface is 820Hv or more. The amount of retained austenite in the steel of the core is 0.1% by volume or more and 9% by volume or less. The dislocation density of the retained austenite in the steel of the core portion is 4.0X10 ¹⁴m^‑2 or more. The steel is high carbon steel or bearing steel.

Description

Mechanical component and rolling bearing

Technical Field

The present invention relates to a mechanical component and a rolling bearing. More specifically, the present invention relates to a machine component made of quenched and tempered steel, and a rolling bearing provided with the machine component.

Background art

The development of electric vehicles has been advanced, and electric vehicles (BEV: battery ELECTRIC VEHICLE), plug-in Hybrid electric vehicles (PHEV: plug-in Hybrid ELECTRIC VEHICLE), and Hybrid electric vehicles (HEV: hybrid ELECTRIC VEHICLE) have been mainly focused. For example, electric axles (e-axles), electric brakes, electric VTCs (variable valve trains), electric compressors, and the like are increasingly being used instead of engines.

For electric vehicles, in order to extend the travel distance with less electric power (increase electric power rate efficiency), it is desirable to make the unit smaller and lighter, and the motor rotates faster and outputs higher. Therefore, various components are required to be miniaturized, have a higher rotational speed, and have a higher strength. For example, as the rotational speed of the motor increases or is miniaturized, the heat generation amount of the rolling bearing increases, resulting in an increase in temperature of the rolling bearing at the time of use. Further, in order to reduce the torque of the rolling bearing or the like, there is a possibility that a decrease in viscosity of the lubricating oil, a decrease in oil amount, or the grease amount will occur, and if this occurs, the heat generation amount of the rolling bearing will further increase. Therefore, a rolling bearing used in an electric vehicle is required to have dimensional stability at high temperature and maintain high hardness.

The structure of the constituent material of the quenched and tempered rolling bearing includes precipitates such as martensite, retained austenite, undissolved carbides, and nitrides. An appropriate amount of retained austenite is considered to be effective for improving the cleaning oil rolling fatigue life and the indentation initiation point rolling fatigue life.

In the quenched and tempered rolling bearing, when the use temperature increases, the retained austenite is decomposed. As a result, the size of the constituent members of the rolling bearing changes due to the volume expansion caused by the decomposition of the retained austenite. Further, when the dimensional change rate of the raceway ring of the rolling bearing increases, creep may occur, leading to an increase in contact surface pressure and early damage caused thereby as the gap between the raceway surface and the rolling elements decreases, and problems such as abnormal noise, vibration increasing as the dimensional accuracy decreases.

Japanese patent application laid-open No. 2001-99163 (patent document 1) describes a raceway ring of a rolling bearing. The track ring described in patent document 1 is made of quenched and tempered steel. The amount of retained austenite in the steel of the track ring described in patent document 1 is substantially 0. According to the track ring described in patent document 1, the change in size with time due to use can be suppressed.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 2001-99163

Disclosure of Invention

Technical problem to be solved by the invention

However, the hardness of steel on the surface of the rail ring described in patent document 1 is lower than 752Hv. When the hardness of steel on the surface of the raceway ring becomes low in a state where the amount of retained austenite is small, the rolling fatigue life at the indentation start point is reduced. Therefore, the rolling fatigue life of the track ring described in patent document 1 at the indentation start point has room for improvement.

The present invention has been made in view of the above-described problems of the prior art. More specifically, the present invention provides a mechanical component and a rolling bearing that can suppress the change with time of the size and can improve the indentation starting point type rolling fatigue life of the surface.

Technical proposal adopted for solving the technical problems

The mechanical component of the first aspect of the invention is made of steel and has a surface. The steel has been quenched and tempered. The mechanical component is provided with a nitrided layer at the surface and a core portion farther from the surface than the nitrided layer. The nitrogen concentration in the steel on the surface is 0.01 mass% or more. The hardness of the steel on the surface is 820Hv or more. The amount of retained austenite in the steel of the core is 0.1% by volume or more and 9% by volume or less. The dislocation density of the retained austenite in the steel of the core portion is 4.0X10 ¹⁴m^-2 or more. The steel is high carbon steel or bearing steel.

In the mechanical part of the first aspect of the present invention, the dislocation density of martensite in the steel of the core may be 6.0×10 ¹⁴m^-2 or more.

The mechanical component of the second aspect of the invention is made of steel and has a surface. The steel has been quenched and tempered. The mechanical component is provided with a nitrided layer at the surface and a core portion farther from the surface than the nitrided layer. The nitrogen concentration in the steel on the surface is 0.01 mass% or more. The hardness of the steel on the surface is 820Hv or more. The amount of retained austenite in the steel of the core is 0.1% by volume or more and 5% by volume or less. The dislocation density of the retained austenite in the steel of the core portion is 1.0X10 ¹⁵m^-2 or more. The steel is low carbon steel or carburizing steel.

In the mechanical component according to the first aspect of the present invention or the second aspect of the present invention, when the nitrogen concentration in the steel on the surface is represented by X (unit: mass%), and the dislocation density of martensite in the steel on the surface is represented by Y (unit: m ^-2), the relationship of 934923.48+379.96×x— 330.96 ×y ²-5.41×10⁴×lοgY+783.83×lοgX² +.gtoreq.0 can be satisfied.

In the mechanical component according to the first aspect of the present invention, the steel may contain 0.77 mass% or more of carbon, 4.0 mass% or less of chromium, 0.10 mass% or more and 0.70 mass% or less of silicon, and 0.25 mass% or less of molybdenum.

In the mechanical component according to the second aspect of the present invention, the steel may contain 0.01 mass% or more and less than 0.77 mass% of carbon, 4.0 mass% or less of chromium, 0.10 mass% or more and 0.70 mass% or less of silicon, and 0.25 mass% or less of molybdenum.

In the mechanical part according to the first or second aspect of the present invention, the dimensional change rate after holding at 160 ℃ for 2500 hours may be 40×10 ^-5 or less. In the mechanical part according to the first or second aspect of the present invention, the dimensional change rate after holding at 160 ℃ for 2500 hours may be 15×10 ^-5 or less. The dimensional change rate is a value obtained by dividing a value obtained by subtracting the size of the mechanical component before the holding (the difference in size between the mechanical components before and after the holding) by the size of the mechanical component before the holding.

The rolling bearing of the present invention includes an inner ring, an outer ring, and rolling elements. At least one of the inner ring, the outer ring and the rolling elements is the mechanical component described above.

Effects of the invention

According to the mechanical component of the first aspect or the second aspect of the present invention and the rolling bearing of the present invention, it is possible to suppress the time-dependent change in the dimension and to improve the indentation starting point type rolling fatigue life of the surface.

Brief description of the drawings

Fig. 1 is a sectional view of an inner race 10.

Fig. 2A is a cross-sectional view of outer race 30.

Fig. 2B is a sectional view of the rolling element 40.

Fig. 2C is a cross-sectional view of the rolling bearing of the embodiment.

Fig. 3 is a process diagram illustrating a method of manufacturing the inner ring 10.

Fig. 4 is a cross-sectional view of the processing target member 20.

Fig. 5A is a cross-sectional view of the processing target member 50.

Fig. 5B is a cross-sectional view of the processing target member 60.

Fig. 6 is a schematic view showing the surface shape of a track ring formed with indentations.

Fig. 7 is a graph showing the relationship between the hardness of steel of the surface of the rail ring and the bulge around the indentation.

Detailed Description

Embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and the repeated description is omitted.

(Constitution of mechanical parts of the embodiment)

The following describes the structure of the mechanical member according to the embodiment. The mechanical component of the embodiment is, for example, a raceway ring of a rolling bearing. The mechanical component of an embodiment may be a rolling element of a rolling bearing. The mechanical component of the embodiment may be a sliding member such as a gear, a shaft (axel), a shaft (shaft), or a ball screw, or may be a structural member such as a housing. Here, the inner ring 10 of the rolling bearing will be described as an example of the mechanical component of the embodiment.

The mechanical component of the embodiment may be a component (raceway ring (inner ring, outer ring), rolling element (ball, roller), or the like) of a rolling bearing used in an electric axle. The mechanical component of the embodiment may be a gear, an axle, a shaft, or the like used for the electric axle. A typical electric axle is a three-axis structure unit composed of a drive motor, a speed reducer, an inverter, and the like. Some electric axles are coaxial structural units including a drive motor, a planetary reducer, an inverter, and the like, or units including a drive motor, a CVT (continuously variable transmission), an inverter, and the like. Foreign matter may be mixed in the rolling bearing used in the decelerator of the electric vehicle axle due to frictional wear of the gear and the housing. Therefore, not only dimensional stability at high temperatures and high load capacity but also prevention of premature failure even if foreign matter is mixed in are required for the constituent members of the rolling bearing used in the speed reducer. In addition, in order to save space inside the speed reducer and miniaturize the speed reducer, a rolling bearing used in the speed reducer is required to have a high load capacity. For the same reasons as those of the rolling bearing members used in the decelerator of the electric axle, gears, axles, shafts, and the like used in the electric axle are also required to have dimensional stability at high temperatures, resistance to foreign matters, and high load capacity.

The mechanical component of the embodiment may be a component of a ball screw or a rolling bearing used in an electric brake. The electric brake is constituted by, for example, a motor, a reduction gear, a ball screw, a cylinder, a control device, and the like. The ball screw is a member composed of a screw shaft having a raceway surface, a nut (outer ring) having a raceway surface, rolling elements (balls) arranged between the raceway surface of the shaft and the raceway surface of the nut, a pipe, a ball return groove (japanese: koku side), an end cap, and the like. Rolling bearings and ball screws used in electric brakes are also required to have dimensional stability at high temperatures, resistance to foreign matters, and high load capacity.

The electric compressor is used for cooling a battery and an in-vehicle electronic device which are cooled indoors and easily reach high temperatures. The mechanical component of the embodiment may also be a rolling bearing for an electric compressor. Rolling bearings used in motor compressors are also required to have dimensional stability at high temperatures, resistance to foreign substances, and high load capacity.

The use of the mechanical component of the embodiment is not limited to automotive use. In other applications, sliding members such as rolling bearings, ball screws, shafts, and pins are required to have high dimensional stability and stability against geometric tolerances such as roundness, cylindricity, and coaxiality due to severe use environments. For example, high precision and dimensional stability are also required for constituent members of rolling bearings for spindles of machine tools.

Ball screws used in electric actuators, positioning devices, electric jacks, servo cylinders, electric servo presses, mechanical presses, transmissions, electric push rod diverters, electric injection molding machines, and the like are also required to have high dimensional stability, resistance to foreign matters, and high load capacity at high temperatures. The sliding members such as shafts and pins are more likely to become hot as the rotational speed increases, and therefore dimensional stability at high temperatures is required.

The following describes the structure of the inner ring 10.

Fig. 1 is a sectional view of an inner race 10. As shown in fig. 1, the inner ring 10 has a first end surface 10a, a second end surface 10b, an inner peripheral surface 10c, and an outer peripheral surface 10d. The first end surface 10a, the second end surface 10b, the inner peripheral surface 10c, and the outer peripheral surface 10d constitute the surface of the inner ring 10. The inner ring 10 is annular.

The center axis of the inner race 10 is set as the center axis a. With the direction along the central axis a as the axial direction. Radial to the direction orthogonal to the central axis a and passing through the central axis a. The circumferential direction is a direction along a circumference centered on the central axis a.

The first end face 10a and the second end face 10b are end faces of the inner ring 10 in the axial direction. The second end face 10b is a face opposite to the first end face 10a in the axial direction.

The inner peripheral surface 10c extends in the circumferential direction. The inner peripheral surface 10c faces the central axis a side. One end and the other end of the inner peripheral surface 10c in the axial direction are connected to the first end surface 10a and the second end surface 10b, respectively. Although not shown, the inner ring 10 is fitted to the shaft at an inner peripheral surface 10 c.

The outer peripheral surface 10d extends in the circumferential direction. The outer peripheral surface 10d faces the opposite side of the central axis a. That is, the outer peripheral surface 10d is a surface opposite to the inner peripheral surface 10c in the radial direction. One end and the other end of the outer peripheral surface 10d in the axial direction are connected to the first end surface 10a and the second end surface 10b, respectively. The outer peripheral surface 10d has a raceway surface 10da. The raceway surface 10da is a portion of the outer peripheral surface 10d that contacts rolling elements (not shown). The raceway surface 10da is located, for example, at the axial center of the outer peripheral surface 10 d. The raceway surface 10da extends in the circumferential direction. In cross section, the raceway surface 10da is partially arcuate.

The inner ring 10 is made of steel. The steel constituting the inner ring 10 has been quenched and tempered. The steel constituting the inner ring 10 is, for example, high carbon steel. The high carbon steel is hypereutectoid steel having a carbon concentration of 0.77 mass% or more. The steel constituting the inner ring 10 may be bearing steel. The bearing steel is high-carbon chromium steel having a carbon concentration of 0.9 mass% or more and 1.05 mass% or less and a chromium concentration of 0.9 mass% or more and 1.7 mass% or less. The steel constituting the inner ring 10 may also be low carbon steel or carburized steel. Low carbon steel means less than 0.77 mass% hypoeutectoid steel. The carburizing steel is a steel containing any one of chromium, molybdenum, and nickel, the carbon concentration of which is 0.1 mass% or more and 0.5 mass% or less.

Specific examples of the high carbon steel include SK85 and the like defined in JIS standards. Specific examples of the bearing steel include SUJ2, SUJ3, SUJ4 and SUJ5 specified in JIS standard, 50100, 51100, 52100, A485 Grade 1 specified in ASTM standard, 100Cr6, 100CrMnSi4-4, 100CrMnSi6-4, 100CrMo7-3 and 100CrMnMoSi8-4-6 specified in DIN, 105Cr4 specified in GB/T, GCr15SiMn, GCrSiMo, GCr Mo specified in ISO standard. Specific examples of the low carbon steel include S55C, S53C, S50C, S45C, S C and S15C specified in JIS standard, 1045, 1046, 1050, 1053 and 1055 specified in AISI standard, C45E, C45R, C55, C55E and C55R specified in iso standard, and 45, 50Mn and 55 specified in GB/T. As concrete examples of the carburizing steel, SCr420, SCr435, SCM420, SCM435, SNCM420 and SNCM815 specified in JIS standard, 5120, 4118, 4135, 4320, 8620, 5135 and 9315 specified in AISI standard, 20Cr4, 20CrMo4, 20NiCrMo7, 18NiCrMo14-6, 17NiCrMo6-4, 37Cr4 and 25CrMo4, 34CrMo4 specified in ISO standard, G20CrMo, G20CrNi2Mo specified in GB/T, and the like can be cited.

The carbon concentration in the steel constituting the inner ring 10 is, for example, 0.77 mass% or more. The carbon concentration in the steel constituting the inner ring 10 may be 0.01 mass% or more and less than 0.77 mass%. The steel constituting the inner ring 10 may contain 4.0 mass% or less of chromium, 0.10 mass% or more and 0.70 mass% or less of silicon, and 0.25 mass% or less of molybdenum. In this case, the steel constituting the inner ring 10 may be free of chromium and molybdenum.

The surface of the inner ring 10 has been subjected to nitriding or carburizing nitriding treatment. That is, the inner ring 10 has a nitrided layer 11 on the surface (the surface of the inner ring 10 is the nitrided layer 11). In the nitrided layer 11, nitrogen is solid-dissolved in the steel. The portion of the inner ring 10 farther from the surface than the nitrided layer 11 is taken as a core 12. In other words, the core 12 is an inner portion of the inner ring 10 other than the nitrided layer 11. In the core 12, nitrogen is not solid-dissolved in the steel. The nitrogen concentration in the steel on the surface of the inner ring 10 is 0.01 mass% or more. The nitrogen concentration in the steel on the surface of the inner ring 10 may be 0.1 mass% or more. The nitrogen concentration on the surface of the inner ring 10 can be measured by using EPMA (Electron Probe Micro Analyzer: electron probe microanalyzer), for example. In the measurement with the EMPA, a calibration curve is drawn using a standard sample with a known nitrogen concentration.

The hardness of steel on the surface of the inner ring 10 is 820Hv or higher. The hardness of the steel on the surface of the inner ring 10 was measured by the vickers hardness test method defined in JIS standard (JIS Z2244:2009). The load when the steel hardness of the surface of the inner ring 10 was measured was 300g. The hardness of the steel of the surface of the inner ring 10 is obtained by measuring at least 3 or more positions and averaging the obtained measured values.

When the steel constituting the inner ring 10 is high carbon steel or bearing steel, the amount of retained austenite in the steel of the core 12 is 9 vol% or less. In this case, the amount of retained austenite in the steel of the core 12 is 0.1% by volume or more. When the steel constituting the inner ring 10 is low carbon steel or carburized steel, the amount of retained austenite in the steel of the core 12 is 5 vol% or less. In this case, the amount of retained austenite in the steel of the core 12 is 0.1% by volume or more.

The amount of retained austenite in the steel of the core 12 can be measured using an X-ray diffraction method. When the amount of retained austenite in the steel of the core 12 is measured by an X-ray diffraction method, a Cr tube bulb type X-ray diffraction apparatus is used. In the Cr tube spherical X-ray diffraction device, the wavelength of Cr-K alpha rays is 2.29093 X10 ^-10 m, the tube voltage is 30kV, the tube current is 10mA, and the collimator size is 2mm multiplied by 2mm. In determining the amount of retained austenite in the steel of the core 12, the inner ring 10 is preferably electropolished so that no work-induced transformation of the retained austenite occurs.

When the steel constituting the inner ring 10 is high carbon steel or bearing steel, the dislocation density of retained austenite in the steel of the core 12 is 4.0×10 ¹⁴m^-2 or more. When the steel constituting the inner ring 10 is high carbon steel or bearing steel, it is preferable that the dislocation density of martensite in the steel of the core 12 is 6×10 ¹⁴m^-2 or more. When the steel constituting the inner ring 10 is low carbon steel or carburized steel, the dislocation density of retained austenite in the steel of the core 12 is 1.0×10 ¹⁵m^-2 or more.

The nitrogen concentration in the steel on the surface of the inner ring 10 is denoted as X (unit: mass%), the dislocation density of martensite in the steel on the surface of the inner ring 10 is denoted as Y (unit: m ^-2), and the hardness of the steel on the surface of the inner ring 10 is denoted as Z (unit: hv). In this case, Z can be calculated from 935743.48+379.96×X-330.96 ×Y ²-5.41×10⁴×lοgY+783.83×lοgX² (formula 1). The formula 1 is obtained by obtaining the Z value by the test when X and Y are changed, and then performing multiple regression analysis. According to equation 1, in order to set the hardness of the steel on the surface of the inner ring 10 to 820Hv or more, the relation 934923.48+379.96×x-330.96 ×y ²-5.41×10⁴×lοgY+783.83×lοgX² ∈ 0 (equation 2) is satisfied.

The dislocation density of retained austenite in the steel of the core 12 and the dislocation density of martensite in the steel of the surface of the inner ring 10 were measured by using a cobalt (Co) tube-type X-ray diffraction apparatus. More specifically, first, the X-ray profiles of austenite and martensite of the core 12 (the surface of the inner ring 10) were measured using a cobalt (Co) tube-ball type X-ray diffraction apparatus. In the Co tube spherical X-ray diffraction device, the wavelength of Co-K alpha rays is 1.7889 X10 ^-10 m, the tube voltage is 40kV, the tube current is 50mA, and the size of a collimator is 1mm in diameter. The X-ray profile of austenite and martensite of the core 12 (the surface of the inner ring 10) is measured in a range of 30 ° or more and 135 ° or less in 2θ. Second, the half-value width of the peak of the X-ray profile of martensite and austenite obtained by X-ray diffraction was separated into crystallite size and strain on the basis of Rietveld analysis. Third, the dislocation density of martensite and the dislocation density of austenite are obtained by applying the separated crystallite size and strain to the following Williamson-Hall (Williamson-Hall) formula. In this formula, ρ is the dislocation density (unit: m- ²), ε is the strain, and b is the length of the bergs vector (b=0.25X10 ^-9 m).

[ Number 1]

Number 1

In addition, in the X-ray profile of martensite obtained by X-ray diffraction of the core 12 (inner ring 10 surface), peaks of {110} plane, {200} plane, {211} plane and {220} plane are objects to be measured. In contrast, in the austenitic X-ray profile obtained by X-ray diffraction of the core 12 (inner ring 10 surface), peaks of {111} plane, {200} plane, {220} plane, {311} plane and {222} plane are objects to be measured. The reason why the Rietveld analysis is performed is to reduce the influence of {200} planes of martensite and {200} planes of austenite having different elastic moduli.

The dimensional change rate of the inner ring 10 after being kept at 160 ℃ for 2500 hours is 40×10 ^-5 or less. Preferably, the dimensional change rate of the inner ring 10 after being kept at 160 ℃ for 2500 hours is 15×10 ^-5 or less. The dimensional change rate of the inner ring 10 is calculated by dividing the size of the inner ring 10 before holding by the value obtained by subtracting the size of the inner ring 10 before holding from the size of the inner ring 10 after holding.

< Modification >

Fig. 2A is a cross-sectional view of outer race 30. Fig. 2B is a sectional view of the rolling element 40. As shown in fig. 2A, the mechanical component of the embodiment may be the outer ring 30 of the rolling bearing. As shown in fig. 2B, the mechanical component of the embodiment may also be a rolling element 40. The outer ring 30 and the rolling elements 40 are formed in the same manner as the inner ring 10 except for the shape, and fig. 2C is a sectional view of the rolling bearing according to the embodiment. The rolling bearing (rolling bearing 100) of the embodiment has an inner ring 10, an outer ring 30, rolling elements 40, and a cage 70. In the rolling bearing 100, at least one of the inner ring 10, the outer ring 30, and the rolling elements 40 may be a mechanical component of the embodiment.

(Method for manufacturing machine component of embodiment)

The following describes a method for manufacturing the inner ring 10.

Fig. 3 is a process diagram illustrating a method of manufacturing the inner ring 10. As shown in fig. 3, the method for manufacturing the inner ring 10 includes a preparation step S1, a nitriding step S2, a quenching step S3, a cooling step S4, a tempering step S5, and a post-treatment step S6.

In the preparation step S1, the processing target member 20 is prepared. Fig. 4 is a cross-sectional view of the processing target member 20. As shown in fig. 4, the processing target member 20 is annular, and has a first end surface 20a, a second end surface 20b, an inner peripheral surface 20c, and an outer peripheral surface 20d. The first end face 20a, the second end face 20b, the inner peripheral face 20c, and the outer peripheral face 20d are faces that become the first end face 10a, the second end face 10b, the inner peripheral face 10c, and the outer peripheral face 10d, respectively, after the post-treatment step S6 is completed. The processing target member 20 is formed of the same steel as the inner ring 10.

The processing target member 20 is subjected to nitriding treatment in the nitriding step S2. Nitriding treatment of the processing target member 20 is performed by heating and holding the processing target member 20 in an atmosphere gas containing a nitrogen source. The heating temperature in the nitriding process S2 and the nitrogen concentration in the atmosphere gas are set so that the compound layer is not formed on the surface of the processing object member 20. By performing the nitriding step S2, nitrogen permeates into the interior from the surface of the member to be processed 20, and nitrogen is solid-dissolved in the member to be processed 20. The nitriding step S2 proceeds to the post-treatment step S6, and the post-treatment nitrogen reaches a position further inside than the position of the specific surface of the inner ring 10.

Instead of the nitriding step S2, the processing target member 20 may be subjected to carburizing and nitriding. The carburizing and nitriding treatment of the processing target member 20 is performed by heating and holding the processing target member in an atmosphere gas containing a nitrogen source and a carbon source. The heating temperature in the carburizing and nitriding process and the carbon concentration and nitrogen concentration in the atmosphere gas are set so that a compound layer is not formed on the surface of the processing object member 20. By performing the carburizing and nitriding process, carbon and nitrogen permeate into the interior from the surface of the processing target member 20, and carbon and nitrogen are solid-dissolved in the processing target member 20. The carburizing and nitriding step is performed until the post-treatment step S6 is performed such that carbon and nitrogen reach a position further inside than the position of the specific surface of the inner ring 10.

In the quenching step S3, the member 20 to be processed is quenched. The quenching of the processing target member 20 is performed by heating and holding the processing target member 20 at a temperature equal to or higher than the a ₁ transformation point of the steel constituting the processing target member 20, and then cooling to a temperature equal to or lower than the Ms transformation point of the steel constituting the processing target member 20. By performing the quenching step S3, martensite and retained austenite are generated in the steel constituting the processing target member 20. In addition, after the quenching process S3 is performed, the quenching process S3 may be repeated by heating the processing object member 20 again to a point of a ₁ phase transition or more. By performing the quenching step S3 a plurality of times, the crystal grains become finer, and the effect of the cooling step S4 is improved.

In the cooling step S4, the member 20 to be processed is subjected to a sub-zero treatment. In the cooling step S4, the member to be processed 20 may be subjected to a deep cooling process (a super-zero sub-process). In the subzero treatment, the object member 20 is cooled to a temperature higher than-100 ℃ and lower than room temperature. In the cryogenic treatment, the object 20 is cooled to a temperature of-100 ℃ or lower. By performing the cooling step S4, a part of retained austenite in the steel constituting the processing target member 20 is transformed into martensite. In addition, before the cooling step S4, a low-temperature tempering step or a cleaning step may be performed to prevent cracking.

In the tempering step S5, the member to be processed 20 is tempered. Tempering of the processing target member 20 is performed by heating the processing target member 20 to a temperature lower than the a ₁ transformation point of steel constituting the processing target member 20. More specifically, tempering of the processing target member 20 is performed by heating the processing target member 20 to a temperature of about 180 ℃. In the post-treatment step S6, the surface of the member 20 is subjected to mechanical processing such as grinding and polishing. Through the above steps, the inner ring 10 having the structure shown in fig. 1 is manufactured. In addition, in the tempering step S5, when the member 20 to be processed is heated at a temperature of 180 ℃ or higher, the higher the heating temperature is, the lower the dislocation density of martensite is, and the lower the hardness is. On the other hand, if the cooling step S4 is performed, the dislocation density of martensite is less likely to be reduced by heating in the tempering step S5, and therefore, although the hardness is reduced with an increase in the heating temperature, it is possible to obtain a higher hardness than usual.

< Modification >

Fig. 5A is a cross-sectional view of the processing target member 50. Fig. 5B is a cross-sectional view of the processing target member 60. In the case where the mechanical component of the embodiment shown in fig. 5A is the outer ring 30, a processing target member 50 that is an annular member is used as the processing target member. In contrast, when the mechanical component of the embodiment shown in fig. 5B is the rolling element 40, the processing target member 60, which is a spherical member, is used as the processing target member. The processing target member 50 and the processing target member 60 have the same configuration as the processing target member 20 except for their shapes.

(Effects of mechanical parts of the embodiment)

As a countermeasure for suppressing the dimensional change of the quenched and tempered steel rail ring or rolling element, it is considered to reduce the amount of retained austenite by tempering at high temperature. However, when tempering is performed at a high temperature, although the change in size with time due to the reduction in the amount of retained austenite can be suppressed, the hardness of the steel on the surface of the raceway ring or the rolling element is reduced.

When foreign matter is sandwiched between the surface of the raceway ring and the rolling elements, an indentation is formed in the surface of the raceway ring. Fig. 6 is a schematic view showing the shape of the surface of the track ring on which the indentations are formed. As shown in fig. 6, the surface of the rail ring is convex around the indentation. Fig. 7 is a graph showing the relationship between the hardness of steel of the surface of the rail ring and the bulge around the indentation. In FIG. 7, the horizontal axis represents hardness (unit: hv), and the horizontal axis represents the protrusion (unit: μm) around the indentation. As shown in fig. 7, as the hardness of the steel of the surface of the track ring decreases, the amount of protrusion around the indentation increases.

When the protrusion amount around the indentation increases, stress concentration occurs at the protrusion around the indentation, and fatigue fracture starting from the indentation easily occurs. Therefore, when the tempering is performed at high temperature to suppress the change in size with time, there is a possibility that the rolling fatigue life of the rail ring is insufficient.

In a state where the amount of retained austenite in steel is low and the periphery of the retained austenite is surrounded by martensite having a high dislocation density (i.e., poor deformability), the retained austenite is subjected to constraint or stress from the martensite, and the lattice plane spacing (lattice constant) becomes small, with the result that the dislocation density of the retained austenite in steel becomes high. Since such retained austenite is constrained by martensite having a high dislocation density around the retained austenite even when the volume thereof expands due to the decomposition, even if the retained austenite is decomposed with use, the dimensional change due to the decomposition is small.

The inner ring 10 is subjected to a subzero treatment or a cryogenic treatment, whereby the amount of retained austenite in the steel of the core 12 is reduced. More specifically, in the inner ring 10, when the steel constituting the inner ring 10 is high carbon steel or bearing steel, the amount of retained austenite in the steel of the core 12 is 9% by volume or less, and when the steel constituting the inner ring 10 is low carbon steel or carburizing steel, the amount of retained austenite in the steel of the core 12 is 5% by volume or less.

Further, since the inner ring 10 is subjected to the subzero treatment or the cryogenic treatment, the dislocation density of martensite in the steel of the core 12 is high, and as a result, the dislocation density of retained austenite in the steel of the core 12 is also high. More specifically, in the inner ring 10, when the steel constituting the inner ring 10 is high carbon steel or bearing steel, the dislocation density of the retained austenite in the steel of the core 12 is 4.0×10 ¹⁴m^-2 or more, and when the steel constituting the inner ring 10 is low carbon steel or carburizing steel, the dislocation density of the retained austenite in the steel of the core 12 is 1.0×10 ¹⁵m^-2 or more.

As described above, since the steel of the core 12 is in a state in which the retained austenite is surrounded by martensite having a high dislocation density, even if the retained austenite in the steel of the core 12 is decomposed by a temperature increase due to the use of the inner ring 10, dimensional change is less likely to occur because volume expansion due to the decomposition is restrained by the martensite having a high dislocation density around. Thus, the inner ring 10 is suppressed in the dimensional change with time due to use.

Further, since tempering at high temperature is not performed on the inner ring 10, the progress of decomposition of martensite in the surface of the inner ring 10 is slight. The nitrogen concentration in the steel on the surface of the inner ring 10 is 0.01 mass% or more, and the steel on the surface of the inner ring 10 is solid solution-strengthened. As a result, the hardness of the steel on the surface of the inner ring 10 is 820Hv or more. As shown in fig. 7, when the hardness of the steel on the surface of the inner ring 10 is 820Hv or more, the protrusion amount around the indentation drastically decreases. Therefore, according to the inner ring 10, the indentation starting point type rolling fatigue life is also improved. By increasing the hardness of the steel on the surface of the inner ring 10, an indentation is not easily formed on the surface of the inner ring 10, thereby increasing the static load capacity of the rolling bearing using the inner ring 10.

In addition, a high residual compressive stress may be applied to the surface of the rolling element in a pressurizing step, and the surface of the raceway ring (inner ring, outer ring) is less likely to form an indentation than the surface of the raceway ring. Therefore, even if only the raceway ring is formed as a mechanical part of the embodiment, the indentation-starting-point rolling fatigue life of the rolling bearing can be improved.

(Evaluation test of hardness)

Samples 1 to 19 were prepared for evaluating the relationship among the hardness of the surface of a machine part made of quenched and tempered steel, the dislocation density of martensite, and the nitrogen concentration. Samples 1 to 19 were annular with an inner diameter of 54mm, an outer diameter of 60mm and a width of 15 mm. For samples 1 to 19, as shown in table 1, the steel grade, the nitrogen concentration in the steel at the sample surface, and the dislocation density of martensite in the steel at the sample surface were changed. The "OK" and "NG" described in the column "satisfy formula 2" in table 1 indicate that the above formula 2 is satisfied and the above formula 2 is not satisfied, respectively.

TABLE 1

In addition, the nitrogen concentration in the steel of each sample surface is adjusted by changing the heating temperature and the holding time in the nitriding treatment or the carburizing-nitriding treatment. The dislocation density of martensite in the steel at the surface of each sample was adjusted by changing the cooling temperature and the holding time in the subzero treatment or the cryogenic treatment.

Samples 1 to 7 and samples 9 to 14 satisfy the above formula 2. Whereas sample 8 and samples 15 to 19 do not satisfy formula 2 above.

The hardness of the steel on the sample surfaces of samples 1 to 7 and samples 9 to 14 was 820Hv or higher. While the hardness of the steel at the sample surface of sample 8 and samples 15 to 19 was lower than 820Hv. From this comparison, it is evident that the rolling fatigue life is improved by satisfying the above equation 2 and making the hardness of the steel on the machine member surface 820Hv or more. From another point of view, it is known that by increasing the nitrogen concentration in the steel of the machine part surface and the dislocation density of martensite in the steel of the machine part surface, the hardness of the steel of the machine part surface can be increased, and further the rolling fatigue life of the machine part can be increased.

(Evaluation test of the dimensional Change with time)

For evaluation of the change in size with time, the above-mentioned samples 3 to 7 and samples 9 to 14 were used. The amounts of retained austenite in the steels of the core portions of samples 3 to 7 and samples 9 to 14, and the dislocation densities in the steels of the core portions of samples 3 to 7 and samples 9 to 14 are shown in table 2.

TABLE 2

The dislocation density of martensite in the steel of the core 12 of each sample and the amount of retained austenite in the martensite in the steel of the core 12 of each sample were adjusted by changing the cooling temperature and the holding time in the subzero treatment or the cryogenic treatment.

When the steel is high carbon steel or bearing steel, the condition a is satisfied if the dislocation density of martensite in the steel of the core 12 is 4.0×10 ¹⁴m^-2 or more. Whereas when the steel is low carbon steel or carburized steel, the condition a is satisfied if the dislocation density of martensite in the steel of the core 12 is 1.0×10 ¹⁵m^-2 or more.

When the steel is high carbon steel or bearing steel, the condition B is satisfied if the amount of retained austenite in the steel of the core 12 of the sample is 9 vol% or less. Whereas when the steel is low carbon steel or carburized steel, condition B is satisfied if the amount of retained austenite in the steel of the core 12 of the sample is 5 vol% or less.

Samples 3 to 7 and samples 9 to 11 satisfy the condition a and the condition B. While samples 12-14 did not meet condition a and condition B.

The dimensional change rate of samples 3 to 7 and samples 9 to 11 after being kept at 160℃for 2500 hours was 40X 10 ^-5 or less. And samples 12-14 have a dimensional change rate of greater than 50X 10 ^-5 after 2500 hours at 160 ℃. From this comparison, it is clear that by satisfying the condition a and the condition B, the change with time of the size of the mechanical component can be suppressed.

When the steel is high carbon steel or bearing steel, the condition C is satisfied if the dislocation density of martensite in the steel of the core 12 of the sample is 6.0×10 ¹⁴m^-2 or more. Samples 3 to 5 satisfy the condition C, and the dimensional change rate after holding at 160℃for 2500 hours is 15X 10 ^-5 or less. While samples 9 to 11 did not satisfy the condition C, the dimensional change rate after holding at 160℃for 2500 hours was greater than 15X 10 ^-5. From this comparison, it is clear that by satisfying the condition C, the change with time in the size of the mechanical component can be further suppressed.

(Constitution of inner race 10 of modification)

The nitrogen concentration in the steel on the surface of the inner ring 10 according to the modification example is, for example, 0.3 mass% or more. The nitrogen concentration in the steel on the surface of the inner ring 10 according to the modification is preferably 0.4 mass% or more. The hardness of the steel on the surface of the inner ring 10 according to the modification example is, for example, 850Hv or more. The hardness of the steel on the surface of the inner ring 10 according to the modification example after being held at 160 ℃ for 2500 hours is preferably 850Hv.

The steel constituting the inner ring 10 of the modification is high carbon steel or bearing steel. The steel constituting the inner ring 10 of the modification preferably contains less than 0.77 mass% of carbon, 4.0 mass% or less of chromium, 0.10 mass% or more and 0.70 mass% or less of silicon, and 0.25 mass% or less of molybdenum.

In the inner ring 10 according to the modification, the retained austenite amount in the steel of the core 12 is 9 vol% or less. In the inner ring 10 according to the modification, the amount of retained austenite in the steel of the core 12 is, for example, 0.1% by volume or more. In the inner ring 10 according to the modification, the dislocation density of retained austenite in the steel of the core 12 is 4.0×10 ¹⁴m^-2 or more. In the inner ring 10 according to the modification, the dislocation density of retained austenite in the steel of the core 12 is preferably 6×10 ¹⁴m^-2 or more.

According to the above formula 1, in order to set the hardness of steel on the surface of the inner ring 10 in the modification to 850Hv or more, it is preferable that the inner ring 10 in the modification satisfies the relationship 934893.48+379.96×x-330.96 ×y ²-5.41×10⁴×lοgY+783.83×lοgX² +.gtoreq.0 (formula 3).

The method for manufacturing the inner ring 10 according to the modification is the same as the method for manufacturing the inner ring 10, and therefore, the description thereof will be omitted here. The effect of the inner ring 10 according to the modification is the same as that of the inner ring 10, and therefore, the description thereof is omitted here. Fig. 6 shows a relationship between the hardness of steel on the surface of the inner ring 10 and the convex portion around the indentation in the modification.

(Evaluation test of hardness)

Samples 20 to 38 were prepared for evaluating the relationship among the hardness of the surface of a machine part made of quenched and tempered steel, the dislocation density of martensite, and the nitrogen concentration. Samples 20 to 38 were ring-shaped with an inner diameter of 54mm, an outer diameter of 60mm, and a width of 15 mm. For samples 20 to 38, as shown in table 3, the steel grade, the nitrogen concentration in the steel at the sample surface, and the dislocation density of martensite in the steel at the sample surface were changed. The "OK" and "NG" described in the column "satisfy formula 3" in table 3 indicate that the above formula 3 is satisfied and the above formula 3 is not satisfied, respectively.

TABLE 3

In addition, the nitrogen concentration in the steel of each sample surface is adjusted by changing the heating temperature and the holding time in the nitriding treatment or the carburizing-nitriding treatment. The dislocation density of martensite in the steel at the surface of each sample was adjusted by changing the cooling temperature and the holding time in the subzero treatment or the cryogenic treatment. Samples 20 to 24, sample 28 and samples 30 to 33 satisfy the above formula 3. Whereas samples 25 to 27, sample 29 and samples 34 to 38 do not satisfy the above formula 3.

The hardness of the steel on the sample surfaces of samples 20 to 24, 28 and 30 to 33 was 850Hv or higher. While the hardness of the steel at the sample surfaces of samples 25 to 27, sample 29 and samples 34 to 38 was lower than 850Hv. From this comparison, it is evident that the rolling fatigue life is improved by satisfying the above equation 3 and the hardness of the steel on the machine member surface is 850Hv or more. From another point of view, it is known that by increasing the nitrogen concentration in the steel of the machine part surface and the dislocation density of martensite in the steel of the machine part surface, the hardness of the steel of the machine part surface can be increased, and further the rolling fatigue life of the machine part can be increased.

(Evaluation test of the dimensional Change with time)

For evaluation of the change in size with time, the above-described samples 22, 28 and 31 were used. The amounts of retained austenite in the steels of the cores 12 of samples 22, 28 and 31, and the dislocation densities in the steels of the cores 12 of samples 22, 28 and 31 are shown in table 4.

TABLE 4

The dislocation density of martensite in the steel of the core 12 of each sample and the amount of retained austenite in the steel of the core 12 of each sample were adjusted by changing the cooling temperature and the holding time in the subzero treatment or the cryogenic treatment.

The dislocation density of retained austenite in the steel of the core 12 of the sample was set to 4.0X10 ¹⁴m^-2 or more as condition D. The condition E was defined that the amount of retained austenite in the steel of the core 12 of the sample was 9 vol% or less.

Sample 22 and sample 28 met conditions D and E. Whereas sample 31 did not satisfy condition D and condition E. Sample 22 and sample 28 had dimensional change rates of 40X 10 ^-5 or less after 2500 hours at 160 ℃. Whereas sample 31 had a dimensional change of greater than 50X 10 ^-5 after 2500 hours at 160 ℃. From this comparison, it is clear that by satisfying the condition D and the condition E, the change with time of the size of the mechanical component can be suppressed.

The dislocation density of retained austenite in the steel of the core 12 of the sample was set to 6×10 ¹⁴m^-2 or more as condition F. Sample 22 satisfied condition F and had a dimensional change rate of 15X 10 ^-5 or less after being held at 160℃for 2500 hours. Whereas sample 28 did not meet condition F and had a dimensional change rate of greater than 15 x10 ^-5 after 2500 hours at 160 ℃. From this comparison, it is clear that by satisfying the condition F, the change with time in the size of the mechanical component can be further suppressed.

The nitrogen concentration on the sample surface was set to 0.4 mass% or more as condition G. Sample 22 satisfied condition G, and the hardness of the surface after 2500 hours at 160 ℃ was 850Hv or more. While sample 28 did not meet condition G and the hardness of the surface after 2500 hours at 160 ℃ was less than 850Hv. From this comparison, it is clear that the hardness of the surface of the machine component can be maintained even after use in a high-temperature environment by satisfying the condition G.

While the embodiments of the present invention have been described above, various modifications can be made to the above embodiments. The scope of the present invention is not limited to the above embodiment. The scope of the present invention is indicated by the claims, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Symbol description

The method comprises the steps of A central shaft, S1 preparation step, S2 nitriding step, S3 quenching step, S4 cooling step, S5 tempering step, S6 post-treatment step, 10 inner ring, 10a first end face, 10b second end face, 10c inner peripheral face, 10d outer peripheral face, 10da track face, 11 nitriding layer, 12 core part, 20 processing object component, 20a first end face, 20b second end face, 20c inner peripheral face, 20d outer peripheral face, 30 outer ring, 40 rolling element, 50,60 processing object component, 70 retainer and 100 rolling bearing.

Claims

1. A machine part having a surface made of quenched and tempered steel, comprising a nitriding layer and a core portion, wherein the nitriding layer is located on the surface and is solid-dissolved with nitrogen, the core portion is located farther from the surface than the nitriding layer,

The nitrogen concentration in the steel of the surface is 0.01 mass% or more,

The hardness of the steel of the surface is above 820Hv,

The amount of retained austenite in the steel of the core is 0.1% by volume or more and 9% by volume or less,

The dislocation density of retained austenite in the steel of the core is 4.0 x 10 ¹⁴m^-2 or more,

The steel is high carbon steel or bearing steel.

2. The mechanical component of claim 1, wherein the dislocation density of martensite in the steel of the core is above 6.0 x 10 ¹⁴m^-2.

3. A machine part having a surface made of quenched and tempered steel, comprising a nitriding layer and a core portion, wherein the nitriding layer is located on the surface and is solid-dissolved with nitrogen, the core portion is located farther from the surface than the nitriding layer,

The nitrogen concentration in the steel of the surface is 0.01 mass% or more,

The hardness of the steel of the surface is above 820Hv,

The amount of retained austenite in the steel of the core is 0.1% by volume or more and 5% by volume or less,

The dislocation density of retained austenite in the steel of the core is 4.0 x 10 ¹⁵m^-2 or more,

The steel is low carbon steel or carburizing steel.

4. The machine component of claim 1, wherein the relationship 934923.48+379.96×x-330.96 ×y ²-5.41×10⁴×lοgY+783.83×lοgX² ++0 is satisfied when the nitrogen concentration in the steel of the surface is expressed as X (unit: mass%), and the dislocation density of martensite in the steel of the surface is expressed as Y (unit: m ^-2).

5. The machine component of claim 1, wherein the steel contains 0.77 mass% or more of carbon, 4.0 mass% or less of chromium, 0.10 mass% or more and 0.70 mass% or less of silicon, and 0.25 mass% or less of molybdenum.

6. The machine component of claim 2, wherein the steel contains 0.01 mass% or more and less than 0.77 mass% carbon, 4.0 mass% or less chromium, 0.10 mass% or more and 0.70 mass% or less silicon, and 0.25 mass% or less molybdenum.

7. The mechanical part of claim 1, wherein the dimensional change rate after 2500 hours of holding at 160 ℃ is 40 x 10 ^-5 or less.

8. The mechanical part of claim 1, wherein the dimensional change rate after 2500 hours of holding at 160 ℃ is 15 x 10 ^-5 or less.

9. A machine part having a surface made of quenched and tempered steel, comprising a nitriding layer and a core portion, wherein the nitriding layer is located on the surface and is solid-dissolved with nitrogen, the core portion is located farther from the surface than the nitriding layer,

The nitrogen concentration in the steel of the surface is 0.3 mass% or more,

The hardness of the steel of the surface is above 850Hv,

The amount of retained austenite in the steel of the core is 9 vol% or less,

The steel is high carbon steel or bearing steel.

10. The mechanical part according to claim 9, wherein the relationship 934893.48+379.96×x-330.96 ×y ²-5.41×10⁴×lοgY+783.83×lοgX² ++0 is satisfied when the nitrogen concentration in the steel of the surface is expressed as X (unit: mass%), and the dislocation density of martensite in the steel of the surface is expressed as Y (unit: m ^-2).

11. The mechanical part of claim 9 or 10, wherein the hardness of the steel of the surface after 2500 hours at 160 ℃ is above 850 Hv.

12. The mechanical component of claim 10, wherein the steel contains less than 0.77 mass% carbon, 4.0 mass% or less chromium, 0.10 mass% or more and 0.70 mass% or less silicon, and 0.25 mass% or less molybdenum.

13. A rolling bearing comprising an inner ring, an outer ring, and rolling elements,

At least one of the inner ring, the outer ring, and the rolling elements is the bearing component of any one of claims 1 to 12.