CN117957336A

CN117957336A - Mechanical component

Info

Publication number: CN117957336A
Application number: CN202280060347.8A
Authority: CN
Inventors: 水田浩平
Original assignee: NTN Corp
Current assignee: NTN Corp
Priority date: 2021-09-07
Filing date: 2022-08-19
Publication date: 2024-04-30
Also published as: WO2023037846A1; JP2023038773A; WO2023037846A9

Abstract

The machine component is a machine component made of quenched and tempered steel. The mechanical component has a surface, and the amount of retained austenite A (unit:%) on the surface and the average grain size B (unit:. Mu.m) of prior austenite grains on the surface satisfy the following relational expression (1). 1.61-0.51 xA-0.027 xB.gtoreq.0 … (1).

Description

Mechanical component

Technical Field

The present invention relates to mechanical parts.

Background

Machine tools, construction machines, robots, automobiles, and other mechanical devices include mechanical components having surfaces that contact or slide with other components. If the surface of the mechanical part is subjected to a large load when it is stationary, the surface may be subject to indentations, thereby making the mechanical device unable to operate properly and smoothly. Therefore, for the mechanical component, there is a load (hereinafter referred to as an allowable load) that is allowed to act on the surface when the mechanical device is stationary. The mechanical component is used in a mechanical device in such a way that a load above the allowable load does not act on the surface. In recent years, with the increase in performance of mechanical devices, it has been demanded to increase the allowable load of the mechanical devices.

For bearings, the allowable load is defined as the rated static load. It is known that the rated static load is positively correlated with the surface hardness (for example, refer to Japanese patent application laid-open No. 2004-301149).

Prior art literature

Patent literature

Patent document 1: japanese patent application laid-open No. 2004-301149

Disclosure of Invention

Technical problem to be solved by the invention

However, it is difficult to increase the allowable load of the mechanical component without suppressing the decrease in toughness of the mechanical component by increasing the surface hardness.

The main object of the present invention is to provide a mechanical component capable of improving the allowable load without improving the surface hardness.

Technical proposal adopted for solving the technical problems

The machine part according to one embodiment of the present invention is a machine part comprising quenched and tempered steel, and has a surface, wherein the amount of retained austenite A (unit:%) on the surface and the average grain size B (unit:. Mu.m) of prior austenite grains on the surface satisfy the following relational expression (1).

[ Mathematics 1]

1.61-0.051×A-0.027×B≥0…(1)

In the mechanical component of the above-described one embodiment, the residual austenite amount a (unit:%) of the surface and the undissolved carbide amount C (unit:%) of the surface satisfy the following relational expression (2).

[ Math figure 2]

0.93+0.43×A-0.044×A²-0.069×C≥0…(2)

The machine part according to another embodiment of the present invention is a machine part made of quenched and tempered steel, and has a surface, wherein the relation (2) is satisfied by the amount of retained austenite a (unit:%) on the surface and the amount of undissolved carbide C (unit:%) on the surface.

In the mechanical component of the other embodiment, the retained austenite amount A (unit:%) of the surface and the average grain size B (unit:. Mu.m) of the prior austenite grains of the surface can satisfy the above-mentioned relational expression (1).

The mechanical component may comprise a plurality of martensite grains. The plurality of martensite grains may be divided into a first group and a second group. The minimum value of the grain size of the martensite grains belonging to the first group is larger than the maximum value of the martensite grains belonging to the second group. The total area of the martensite grains belonging to the first group divided by the total area of the martensite grains is 0.3 or more. The value obtained by dividing the total area of the martensite grains belonging to the first group by the total area of the martensite grains after removing the martensite grains having the smallest grain size belonging to the first group is less than 0.3. The average grain size of the martensitic grains belonging to the first group is 1.8 μm or less. The plurality of martensite grains may be divided into a third group and a fourth group. The minimum value of the grain size of the martensite grains belonging to the third group is larger than the maximum value of the martensite grains belonging to the fourth group. The total area of the martensite grains belonging to the third group divided by the total area of the martensite grains is 0.5 or more. The value obtained by dividing the total area of the martensite grains belonging to the third group by the total area of the martensite grains after removing the martensite grains having the smallest grain size belonging to the third group is less than 0.5. The average grain size of the martensitic grains belonging to the third group is 1.5 μm or less.

In the above mechanical part, the average aspect ratio of the martensite grains belonging to the first group may be 3.5 or less. The average aspect ratio of the martensitic grains belonging to the third group may be below 3.1.

In the mechanical component, the hardness of the surface layer portion, which is a region at a distance of 20 μm from the surface, may be 650HV or more and 780HV or less.

In the above-described machine component, the content of carbon in the high-carbon steel may be 0.8 mass% or more, the content of chromium in the high-carbon steel may be 4 mass% or less, the content of silicon in the high-carbon steel may be 0.1 mass% or more and 0.7 mass% or less, and the content of molybdenum in the high-carbon steel may be 0.25 mass% or less.

In the mechanical component, the surface may be configured as a sliding surface that slides with another component.

Effects of the invention

According to the present invention, a mechanical component capable of increasing the allowable load without increasing the surface hardness can be provided.

Brief description of the drawings

Fig. 1 is a cross-sectional view showing an example of a mechanical component of the present embodiment.

Fig. 2 is a flowchart showing an example of a method for manufacturing a mechanical component according to the present embodiment.

FIG. 3 is a graph showing the correlation between the value obtained by dividing the value of 2 times the residual indentation depth (unit: mm) obtained from the results of the indentation formation test on the surfaces of test pieces A to J by the diameter (unit: mm) of the ceramic ball and the maximum contact surface pressure (unit: GPa) at the time of forming each residual indentation.

Fig. 4 is a graph showing a correlation between actual measurement values of the retained austenite amount and the average grain size of the prior austenite grains on the surfaces of the test pieces a to J and the allowable load surface pressure calculated from the graph of fig. 3.

Fig. 5 is a graph showing a correlation between the calculated value of the allowable load surface pressure calculated from the graph of fig. 3 and the estimated value of the allowable load surface pressure calculated from the estimated expression derived from the graph of fig. 4 in the test pieces a to J.

Fig. 6 is a graph showing a correlation between actual measurement values of the amounts of retained austenite and undissolved carbide on the surfaces of the test pieces a to J and the allowable load surface pressure calculated from the graph of fig. 3.

Fig. 7 is a graph showing a correlation between the calculated value of the allowable load surface pressure calculated from the graph of fig. 3 and the estimated value of the allowable load surface pressure calculated from the estimated expression derived from the graph of fig. 6 in the test pieces a to J.

Fig. 8 is an EBSD image of the vicinity of the surface of test piece B.

Fig. 9 is an EBSD image of the vicinity of the surface of the test piece J.

Fig. 10 is a graph showing a correlation between the measured values of the average grain size and the average aspect ratio of the first group of martensite grains in the test pieces a to J and the allowable load surface pressure calculated from the graph of fig. 3.

Fig. 11 is a graph showing a correlation between the measured values of the average grain size and the average aspect ratio of the third group of martensite grains in the test pieces a to J and the allowable load surface pressure calculated from the graph of fig. 3.

Detailed Description

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

The bearing member 10 of the present embodiment is, for example, an inner ring 10 of a rolling bearing. Hereinafter, the inner race 10 will be described as an example of a mechanical component of the embodiment.

(Constitution of inner race 10)

Fig. 1 is a sectional view of an inner race 10. As shown in fig. 1, the inner ring 10 is annular. The center axis of the inner race 10 is set as the center axis a. The inner ring 10 has a width surface 10a, a width surface 10b, an inner peripheral surface 10c, and an outer peripheral surface 10d. The width surfaces 10a, 10b, 10c, and 10d constitute the surface of the inner ring 10.

Hereinafter, the direction of the central axis a is referred to as the axial direction. Hereinafter, a direction along a circumference centered on the central axis a as viewed in the axial direction is referred to as a circumferential direction. In the following, the direction perpendicular to the axial direction is referred to as the radial direction.

The width surfaces 10a and 10b are end surfaces of the inner ring 10 in the axial direction. The width surface 10b is an opposite surface of the width surface 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. The inner peripheral surface 10c is connected to the width surface 10a at one end in the axial direction and connected to the width surface 10b at the other end in the axial direction. The inner ring 10 is fitted to a shaft (not shown) at the 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 radially opposed surface of the inner peripheral surface 10 c. The outer peripheral surface 10d has one end in the axial direction connected to the width surface 10a and the other end in the axial direction connected to the width surface 10 b.

The outer peripheral surface 10d has a raceway surface 10da. The raceway surface 10da extends in the circumferential direction. The outer peripheral surface 10d is recessed toward the inner peripheral surface 10c side at the raceway surface 10da. At the cross-sectional angle, the raceway surface 10da is partially circular. The raceway surface 10da is located at the center of the axially outer peripheral surface 10 d. The raceway surface 10da is a part of the outer peripheral surface 10 d. The raceway surface 10da contacts a rolling surface of a rolling element (not shown in fig. 1).

The inner ring 10 is made of, for example, quenched and tempered high-carbon steel. The carbon content of the steel constituting the inner ring 10 is preferably 0.80 mass% or more and 1.20 mass% or less.

If the carbon content in the steel is 0.80 mass% or more, the solid solution carbon content in the steel can be made 0.5 mass% or more without carburizing the member to be processed in the method of manufacturing the inner ring 10. If the amount of solid-solution carbon in the steel is 0.5 mass% or more, the strength of the steel increases by solid-solution strengthening, and therefore a high allowable load surface pressure can be obtained. Further, if the carbon content in the steel is 0.80 mass% or more, a sufficient amount of undissolved carbide can be easily ensured from the viewpoint of suppressing coarsening of prior austenite grains and increase in the amount of retained austenite.

If the carbon content in the steel is more than 1.20 mass%, there is a concern that workability may be deteriorated, and therefore the carbon content is preferably 1.20 mass% or less.

The carbon content in the steel constituting the inner ring 10 may be 0.01 mass% or more and less than 0.8 mass%. The steel constituting the inner ring 10 may be low carbon steel or carburized steel. In this case, the inner ring 10 may have a carburized surface layer portion. Preferably, the solid solution carbon content of the surface layer portion is 0.5 mass% or more. Thus, a sufficient allowable load surface pressure can be obtained.

The content of chromium in the steel constituting the inner ring 10 is preferably 0.00 mass% or more and 4.00 mass% or less. In other words, the steel constituting the inner ring 10 may not contain chromium, but may contain chromium at most 4.00 mass%. The higher the chromium content in the steel, the better the strength, wear resistance, rolling fatigue life, etc. On the other hand, if the chromium content exceeds 4.00 mass%, the heating time during quenching and the heating time during tempering need to be set long, and productivity is deteriorated. On the other hand, if the content of chromium is too high, the workability (plastic workability) is lowered, and the manufacturing cost is increased.

The silicon content of the steel constituting the inner race 10 is preferably 0.10 mass% or more and 0.75 mass% or less. The higher the silicon content in the steel, the better the temper softening resistance. If the silicon content is too high, the invasion of carbon during carburizing treatment of low-carbon steel or carburized steel is inhibited, and productivity is lowered.

The content of manganese in the steel constituting the inner ring 10 is preferably 0.0 mass% or more and 1.5 mass% or less. In other words, the steel constituting the inner ring 10 may not contain manganese, but may contain manganese at most 1.5 mass%. The higher the manganese content in the steel, the better the hardenability. On the other hand, if the manganese content is too high, the workability (machinability) is lowered.

The content of molybdenum in the steel constituting the inner ring 10 is preferably 0.00 mass% or more and 0.25 mass% or less. In other words, the steel constituting the inner ring 10 may not contain molybdenum, but may contain molybdenum at most 0.25 mass%. The higher the molybdenum content in the steel, the better the temper softening resistance. In addition, as the content of molybdenum in the steel is higher, the strength, wear resistance, rolling fatigue life, and the like are better as with chromium. On the other hand, if the content of molybdenum is too high, the workability (plastic workability) is lowered, and the manufacturing cost is increased.

The nickel content of the steel constituting the inner ring 10 is preferably 0.00 mass% or more and 5 mass% or less. In other words, the steel constituting the inner ring 10 may not contain nickel, but may contain nickel at most 5 mass%. The higher the nickel content in the steel, the better the toughness after quenching and tempering.

The content of vanadium in the steel constituting the inner ring 10 is preferably 0.00 mass% or more and 1.00 mass% or less. In other words, the steel constituting the inner ring 10 may not contain vanadium, but may contain vanadium at most 1.0 mass%. The higher the vanadium content in the steel, the better the temper softening resistance. In addition, as the content of vanadium in the steel is higher, the strength, wear resistance, rolling fatigue life, and the like are better as in chromium and molybdenum. On the other hand, if the vanadium content is too high, the workability (plastic workability) is lowered and the manufacturing cost is increased.

In addition, the remaining portion of the steel constituting the inner ring 10 is iron and unavoidable impurities.

The steel constituting the inner ring 10 may be selected from SUJ2, SUJ3, SUJ4, SUJ5, SK85, 50100, 51100, 52100, A485Grade1, and 100Cr6, 100C4MnSi4-4, which are defined in the ISO standard, for example, which are defined in the JIS standard.

The steel constituting the inner ring 10 may be selected from, for example, W1-8, 4161, 1050, 1045, 4145, 4140, 5140, 4135, 5135, 4130, 5130, 1025, 4320, 5120, 1015 specified in SNCM815、SUP13、S55C、S53C、S50C、S45C、SCM445、SCM440、SCM435、SCr435、SCM430、S25C、SCM420、SCr420、SNCM420、SCM418、SCM415、SCr415、S15C,ASTM standard specified in JIS standard, and 60CrMo32, C50, 42CrMo4, 37Cr4, C25, 34CrMo4, 34Cr4, 22CrMoS, 20Cr4, 18CrMo4, C15 specified in ISO standard.

The steel constituting the surface (outer peripheral surface 10 d) of the inner ring 10 includes a plurality of martensite grains and a plurality of prior austenite grains. From a different perspective, the steel in the surface layer portion 11, which is the region from the surface to a distance of 20 μm, includes a plurality of martensite grains and a plurality of prior austenite grains.

The amount of retained austenite A (unit:%) on the surface (outer peripheral surface 10 d) of the inner ring 10 and the average grain size B (unit:. Mu.m) of the prior austenite grains on the surface satisfy the following relational expression (1).

[ Math 3]

1.61-0.051×A-0.027×B≥0…(1)

Preferably, the amount of retained austenite a (unit:%) of the surface (outer peripheral surface 10 d) of the inner ring 10 and the amount of undissolved carbide C (unit:%) of the surface satisfy the following relational expression (2).

[ Mathematics 4]

0.93+0.43×A-0.044×A²-0.069×C≥0…(2)

The retained austenite amount a and the undissolved carbide amount C of the surface (outer peripheral surface 10 d) were measured by an X-ray diffraction method performed on the surface. Specifically, first, the surface of the inner ring 10 is electropolished so that retained austenite in steel constituting the surface of the inner ring 10 is not transformed into martensite by working. Second, an X-ray profile is obtained by performing X-ray diffraction measurement on the surface and measuring the intensity of diffracted X-rays in a range of diffraction angles 2θ of 35 ° to 130 °. Thirdly, the residual austenite amount a and the undissolved carbide amount C are calculated by performing a leber analysis on the obtained X-ray profile.

The average grain size B of the prior austenite grains on the surface (outer peripheral surface 10 d) was measured according to the method specified in JIS standard (JIS G0551: 2005). Specifically, first, after polishing the surface, the prior austenite grain boundaries are exposed on the surface by using an etchant. Second, the surface is subjected to optical microscopy (hereinafter, an image obtained by optical microscopy will be referred to as an "optical microscopy image"). Wherein the optical microscope image is taken in such a manner as to contain a sufficient number (20 or more) of prior austenite grains. Thirdly, the area of each prior austenite grain in the optical microscope image is calculated by performing image processing on the obtained optical microscope image. Fourth, the circle equivalent diameter of each prior austenite grain can be calculated by calculating the square root of the calculated value obtained by dividing the area of each prior austenite grain by 4/pi. The calculated sum of the equivalent diameters of the circles of the prior austenite grains divided by the number of the prior austenite grains in the optical microscope image is the average grain diameter of the prior austenite grains on the surface.

The plurality of martensite grains contained in the surface layer portion 11 can be divided into a first group and a second group. The minimum value of the grain size of the martensite grains belonging to the first group is larger than the maximum value of the martensite grains belonging to the second group.

The total area of the martensite grains belonging to the first group divided by the total area of the martensite grains (sum of the total area of the martensite grains belonging to the first group and the total area of the martensite grains belonging to the second group) is 0.3 or more.

The value obtained by dividing the total area of the martensite grains belonging to the first group by the total area of the martensite grains after removing the martensite grains having the smallest grain size belonging to the first group is less than 0.3.

In other words, the martensite grains are allocated to the first group in order of the crystal grain size from large to small. The allocation to the first group is ended until the total area of the martensite grains allocated to the first group reaches 0.3 times or more of the total area of the martensite grains. The remaining martensite grains are then assigned to the second group.

The average grain size of the martensitic grains belonging to the first group is 1.8 μm or less. Preferably, the average grain size of the martensite grains belonging to the first group is 1.5 μm or less. More preferably, the average grain size of the martensite grains belonging to the first group is 1.3 μm or less.

Preferably, the average aspect ratio of the martensitic grains belonging to the first group is below 3.5. More preferably, the average aspect ratio of the martensitic grains belonging to the first group is 3.3 or less. Further preferably, the average aspect ratio of the martensitic grains belonging to the first group is 3.2 or less.

The plurality of martensite grains contained in the surface layer portion 11 can be divided into a third group and a fourth group. The minimum value of the grain size of the martensite grains belonging to the third group is larger than the maximum value of the martensite grains belonging to the fourth group.

The total area of the martensite grains belonging to the third group divided by the total area of the martensite grains (sum of the total area of the martensite grains belonging to the third group and the total area of the martensite grains belonging to the fourth group) is 0.5 or more.

The value obtained by dividing the total area of the martensite grains belonging to the third group by the total area of the martensite grains after removing the martensite grains having the smallest grain size belonging to the third group is less than 0.5.

In other words, the martensite grains are allocated to the third group in order of the crystal grain size from large to small. The allocation to the third group is ended until the total area of the martensite grains allocated to the third group reaches 0.5 times or more the total area of the martensite grains. Then, the remaining martensite grains are allocated to the fourth group.

The average grain size of the martensitic grains belonging to the third group is 1.5 μm or less. Preferably, the average grain size of the martensite grains belonging to the third group is 1.2 μm or less. More preferably, the average grain size of the martensite grains belonging to the third group is 1.0 μm or less.

Preferably, the average aspect ratio of the martensitic grains belonging to the third group is below 3.1. More preferably, the average aspect ratio of the martensitic grains belonging to the third group is 3.0 or less. Further preferably, the average aspect ratio of the martensitic grains belonging to the third group is 2.9 or less.

The crystal grain size, average grain size and average aspect ratio of the martensitic grains belonging to the first group were measured as follows.

First, according to an Electron Back Scattering Diffraction (EBSD) method using a field emission scanning electron microscope (FE-SEM), a diffraction pattern of an electron beam diffracted in the vicinity of the surface of the inner ring 10 (hereinafter referred to as "EBSD image") is photographed. The EBSD image is taken in a manner that includes a sufficient number (more than 20) of martensite grains.

Second, boundaries of adjacent martensite grains are determined based on the crystal orientations of the respective grains shown in the EBSD image.

Third, the area and shape of each martensitic grain shown in the EBSD image are calculated from the boundaries of the martensitic grains determined. Further, the equivalent diameter of the circle, which is the value obtained by dividing the area of the martensite grains by pi/4, was calculated as the crystal grain diameter of the martensite grains.

Fourth, from the crystal grain sizes (circular equivalent diameters) of the respective martensite grains calculated as described above, the martensite grains belonging to the first group among the martensite grains shown in the EBSD image were determined. The value obtained by dividing the total area of the martensite grains belonging to the first group by the total area of the martensite grains shown in the EBSD image is the value obtained by dividing the total area of the martensite grains belonging to the first group by the total area of the martensite grains.

Fifth, the average grain size of the martensite grains belonging to the first group is calculated from the crystal grain sizes (circular equivalent diameters) of the martensite grains of the first group classified as above. The value obtained by dividing the sum of the circular equivalent diameters of the martensite grains shown in the EBSD image classified as the first group by the number of the martensite grains shown in the EBSD image classified as the first group is the average grain diameter of the martensite grains belonging to the first group.

Sixth, the average aspect ratio of the martensitic grains belonging to the first group is calculated from the shapes of the martensitic grains of the first group classified as above. Specifically, the shape of each martensitic grain shown in the EBSD image is approximated to an ellipse by the least square method, based on the shape of each martensitic grain shown in the EBSD image. The least squares-based elliptic approximation is performed according to the method described in S.Biggin and D.J.Dingley, J.App.Crystal (Journal of Applied Crystallography), 1977, 10, 376-378. In this elliptical shape, the aspect ratio of each martensitic grain shown in the EBSD image can be calculated by dividing the major axis dimension by the minor axis dimension. The value obtained by dividing the sum of the aspect ratios of the martensite grains shown in the EBSD image classified as the first group by the number of the martensite grains shown in the EBSD image classified as the first group is the average aspect ratio of the martensite grains belonging to the first group.

The crystal grain size, average grain size and aspect ratio of the martensite grains belonging to the third group were measured in the same manner as those of the martensite grains belonging to the first group.

The hardness of the surface layer 11 is 650Hv or more and less than 810HV. The hardness may be 650Hv or more and 780HV or less. The hardness may be 650Hv or more and 760HV or less. The hardness of the surface layer 11 can be measured as the cross-sectional hardness of the surface layer 11 using a vickers hardness tester. Specifically, the average value of the section hardness measured with the load of 300g and the number of n being 3 or more is the hardness of the surface layer 11.

(Method for manufacturing inner race 10)

Fig. 2 is a flowchart showing a method of manufacturing the inner ring 10. As shown in fig. 2, the method for manufacturing the inner ring 10 includes a preparation step S1, a quenching step S2, a tempering step S3, and a post-treatment step S4. The quenching step S2 is performed after the preparation step S1. The tempering step S3 is performed after the quenching step S2. The post-treatment step S4 is performed after the tempering step S3.

In the preparation step S1, the member to be processed is prepared. The object member is an annular member formed of the same steel as the inner ring 10.

In the quenching step S2, the member to be processed is quenched. Quenching is performed by maintaining the member to be processed in a non-oxidizing (for example, in argon, nitrogen, or the like or in vacuum) atmosphere at a temperature not lower than the a ₁ transformation point of the steel constituting the member to be processed, and then quenching to a temperature not higher than the M _S transformation point of the steel constituting the member to be processed. The quenching may be performed in an air atmosphere. In this case, in order to prevent decarburization, the surface of the member to be processed is subjected to a treatment of coating with a decarburization preventing agent or a copper plating treatment before the quenching step S2. The quenching step S2 may be performed twice. The heating maintenance temperature in the second quenching step S2 is preferably lower than that in the first quenching step S2.

In the tempering step S3, the member to be processed is tempered. Tempering is performed by maintaining the object member at a temperature less than the a ₁ transformation point of the steel constituting the object member. In the post-treatment step S4, the surface of the member to be processed is subjected to machining (grinding, polishing), cleaning, and the like. Through the above steps, the inner ring 10 shown in fig. 1 is formed.

The heat treatment conditions in the steps S2 to S3 are set so that at least one of the relational expression (1) and the relational expression (2) is established. Specifically, first, a plurality of test pieces having only different heat treatment conditions are prepared. Each test piece has the same structure as the above-described processing target member. Second, whether or not at least one of the relationship (1) and the relationship (2) is established is evaluated for each test piece. Third, the heat treatment conditions of the test piece that at least one of the relational expression (1) and the relational expression (2) holds are set as the heat treatment conditions in the manufacturing method of the inner ring 10.

(Effect of inner race 10)

In the inner ring 10, the retained austenite amount A (unit:%) of the surface (outer peripheral surface 10 d) of the inner ring 10 and the average grain size B (unit:. Mu.m) of the prior austenite grains of the surface satisfy the above-described relational expression (1).

The inventors focused on the microstructure of steel constituting the surface of the machine component, and examined, through experiments, the allowable loads (specifically, allowable loads and allowable load surface pressures defined substantially similarly) of various machine components whose only microstructure of the surface is different from each other due to the heat treatment conditions to be applied. As a result, it was found that the mechanical component satisfying the relation (1) has high pressure mark formation property and can secure an allowable load. Furthermore, the present inventors have found that the hardness of the surface layer portion of the mechanical part that satisfies the relation (1) can be controlled within a range of 650Hv or more and less than 810 Hv. Details are set forth in the examples.

Preferably, the amount of retained austenite a (unit:%) of the surface (outer peripheral surface 10 d) of the inner ring 10 and the amount of undissolved carbide C (unit:%) of the surface satisfy the above-described relational expression (2).

The present inventors have found that, in the mechanical component in which the relational expression (1) and the relational expression (2) are established, the allowable load can be ensured, and the hardness of the surface layer portion can be controlled within a range of 650Hv or more and less than 810 Hv.

If the average grain size of the martensite grains of the first group is 1.8 μm or less and the average grain size of the martensite grains of the third group is 1.5 μm or less, the ratio of the fine martensite grain sizes among the plurality of martensite grains contained in the steel constituting the surface is relatively high. In this case, the allowable load of the inner ring 10 can be further improved.

If the average aspect ratio of the martensite grains belonging to the first group is 3.5 or less and the average aspect ratio of the martensite grains belonging to the third group is 3.1 or less, the martensite grains having a relatively large average aspect ratio are less likely to become stress concentration sources than those in the case where the average aspect ratios are higher than the above values. In this case, the allowable load of the inner ring 10 can be further improved.

If the hardness of the surface layer 11 is 650HV or more and less than 810HV, toughness is improved compared to the case where the hardness is 810HV or more, and therefore impact resistance is improved. If the hardness is 650HV or more and 780HV or less, toughness is further improved compared to the case where the hardness is 810HV or more, and therefore impact resistance is further improved.

(Modification)

In the inner ring 10, at least the relation (2) of the relation (1) and the relation (2) may be satisfied. The inventors found that the allowable load can be ensured and the hardness of the surface layer portion 11 can be controlled within a range of 650Hv or more and less than 810Hv in the mechanical component in which the relational expression (2) is established.

The mechanical component of the present embodiment is not limited to the inner ring, and may be any mechanical component having a surface that contacts or slides with other components. The mechanical component of the present embodiment may be, for example, an outer ring or rolling element of a rolling bearing, a ball screw, a shaft, a housing, or the like. In such a mechanical component, at least one of the above-described relational expression (1) and relational expression (2) may be satisfied for a surface that contacts or slides with other components.

(Other specific examples of the machine component of the present embodiment)

The mechanical component of the present embodiment may be at least one of an inner ring, an outer ring, and balls of a ball bearing for a spindle of a spindle device of a machine tool. As described above, the ball bearing of the present embodiment has high impact resistance, and thus can contribute to improvement in performance of the spindle device of the machine tool.

The mechanical component of the present embodiment may be at least one of an inner ring, an outer ring, and a roller of a self-aligning roller bearing for a continuous casting apparatus or a rolling apparatus of steel equipment. Self-aligning roller bearings for the above-mentioned applications are required to have a high allowable load and a high dimensional stability accuracy for suppressing creep. As a countermeasure for achieving high dimensional stability accuracy, it is effective to raise the tempering temperature, but if the tempering temperature is raised, the surface hardness is lowered. Therefore, in the above-described technique of increasing the rated static load (allowable load) by increasing the surface hardness, it is difficult to achieve both high rated static load and high dimensional stability accuracy. In contrast, in the self-aligning roller bearing of the present embodiment, even if the hardness of the surface layer portion is 780HV or less, the allowable load increases, and therefore, both high allowable load and high dimensional stability accuracy can be achieved. The self-aligning roller bearing according to the present embodiment can contribute to improvement of performance of a continuous casting device or a rolling device of steel equipment.

The mechanical component of the present embodiment is at least one of an inner ring, an outer ring, and a roller of a tapered roller bearing for a reduction gear of a construction machine, or at least one of an inner ring, an outer ring, and a ball of an angular contact ball bearing. Tapered roller bearings used for the above applications are required to have a high allowable load and a high impact resistance for preventing breakage. In view of preventing breakage, it is possible to construct the tapered roller bearing from carburized steel, but in this case, it is necessary to perform gas carburization and a large amount of carbon dioxide is discharged in the manufacturing process thereof. In contrast, the tapered roller bearing of the present embodiment has the high impact resistance as described above even if it is made of high carbon steel, and therefore, compared with a tapered roller bearing made of carburized steel, it is possible to reduce the environmental load of the manufacturing process, and at the same time, it has high allowable load and high impact resistance that can prevent breakage. The tapered roller bearing according to the present embodiment can contribute to improvement of performance of a continuous casting device or a rolling device of a steel plant.

The mechanical component of the present embodiment may be at least one of an inner ring, an outer ring, and balls of the deep groove ball bearing for a robot. Deep groove ball bearings for the above-mentioned applications are required to have a high allowable load and a high dimensionally stable accuracy for achieving a precise motion. Therefore, in the deep groove ball bearing of the present embodiment, similarly to the self-aligning roller bearing of the present embodiment described above, both high allowable load and high dimensional stability accuracy can be achieved. The deep groove ball bearing of the present embodiment can contribute to improvement of performance of the robot.

The mechanical component of the present embodiment may be a sliding member used in an Electric Vehicle (EV), a Fuel Cell Vehicle (FCV), a plug-in hybrid vehicle (PHV), a Hybrid Vehicle (HV), or the like. Examples of the sliding member include ball bearings, needle roller bearings, tapered roller bearings, cylindrical roller bearings, and pinion shafts used in transmissions. The sliding member used for the above-mentioned applications is required to have a high allowable load and quietness at the time of sliding. The sliding member according to the present embodiment has high pressure mark formation properties as described below, and thus can contribute to improvement in performance of the sliding member.

Examples

As described above, the present inventors have focused on the microstructure of steel constituting the surface of the machine component, and have examined, through experiments, the allowable loads (specifically, allowable loads and allowable load surface pressures defined substantially similarly) of various machine components in which only the microstructure of the surface is different from each other due to the heat treatment conditions to be applied. The results of the experiment are shown below.

(Test piece)

Test pieces A to J were used. Test pieces A to J are inner rings of the jade bearings. Test pieces a to J were prepared by subjecting a member to be processed composed of SUJ2 to heat treatment under mutually different conditions. That is, the prepared test pieces a to J differ from each other only in the heat treatment conditions to be applied.

(Evaluation of microstructure)

The evaluation parameters of the microstructure of each surface (raceway surface) of the test pieces a to J are the surface retained austenite amount, the undissolved carbide amount, the average grain size of prior austenite grains, the average grain size and average aspect ratio of the martensite grains of the first group, the average grain size and average aspect ratio of the martensite grains of the third group, and the cross-sectional hardness of the surface layer portion. The evaluation parameters were measured by the above-described measurement methods. The measurement conditions are as follows.

1. Retained austenite amount and undissolved carbide amount

The measurement was performed using an X-ray diffraction apparatus. The measurement conditions were that the voltage applied to the X-ray tube was 40kV, the current was 500mA, the spot diameter of the X-ray was phi 1mm, and the diffraction angle 2 theta was 35 DEG to 130 deg.

2. Average grain size of prior austenite grains

Bitter alcohol (picral) is used as the etching solution. The observation field of the optical microscope image was 1000 μm wide by 750 μm long.

3. Average grain size and average aspect ratio of martensite grains

The EBSD image has an observation field of view of 115 μm wide by 87 μm long.

4. Section hardness of the surface layer portion

The value of n is 3.

(Measurement of allowable load surface pressure)

The allowable load surface pressures of the test pieces a to J are defined as parameters corresponding to the allowable loads of the machine components as follows.

The allowable load surface pressures of the test pieces A to J are maximum contact surface pressures at which the value obtained by dividing the value of 2 times the residual indentation depth (unit: mm) formed by pressing the ceramic ball against the surface of each test piece in the indentation formation test by the diameter (unit: mm) of the ceramic ball takes 1/10000.

The allowable load surface pressures of the test pieces a to J were calculated as follows. First, the surfaces of the test pieces a to J corresponding to the outer peripheral surface 10d were mirror polished. Second, the ceramic balls were pressed against the surfaces of the test pieces a to J after grinding and then the load was removed, whereby indentations (residual indentations) were formed only on the surfaces. The maximum contact surface pressure between the test piece and the ceramic ball is in the range of 3.0GPa to 6.0 GPa. In addition, when an uncured portion is present in the interior, the load condition is adjusted so that the internal stress at the time of the test does not reach the uncured portion. Third, the depth of each residual indentation formed on each surface of test pieces a to J was measured. Further, the present indentation formation test was repeated by varying the maximum contact surface pressure within the above range. Fourth, the maximum contact surface pressure at 1/10000 of the value is calculated from the correlation (see FIG. 3) between the value obtained by dividing the 2-fold value of the depth (unit: mm) of the plurality of residual indentations formed on each test piece A to J by the diameter (unit: mm) of the ceramic ball and the maximum contact surface pressure (unit: GPa) at the time of forming each residual indentation. The maximum contact surface pressure thus calculated was defined as the allowable load surface pressure of each test piece a to J.

In order to convert the residual indentation depth formed in the indentation test into a residual indentation of a bearing in which both the inner ring and the rolling element are made of the same steel, the allowable load surface pressure was calculated from a value 2 times the residual indentation depth in the indentation test, focusing on the fact that the plastic deformation amounts of the raceway surface and the rolling surface in the bearing are equal. The allowable load surface pressure thus calculated is considered to be defined substantially the same as the rated static load of the bearing.

(Evaluation results)

Table 1 shows the evaluation results of the surface microstructure and allowable load surface pressure of each of the test pieces a to J.

TABLE 1

<1. Retained austenite amount and average grain size of prior austenite grains >

FIG. 4 is a graph showing the relationship among the amount of retained austenite (unit:%), the average grain size of the prior austenite grains (unit: μm), and the allowable load surface pressure (unit: GPa). As shown in table 1 and fig. 4, the larger the amount of retained austenite, the lower the allowable load surface pressure. Further, the larger the average grain size of the prior austenite grains is, the lower the allowable load face pressure becomes.

Further, from the correlation between the amount of retained austenite, the average grain size of the prior austenite grains and the allowable load surface pressure shown in FIG. 4, the following estimation formula (3) for estimating the allowable load surface pressure D (unit: GPa) from the measured values of the amount of retained austenite A (unit:%) and the average grain size B (unit: μm) of the prior austenite grains is derived by multiple regression analysis.

[ Math 5]

D＝5.81-0.051×A-0.027×B…(3)

Further, according to the above-mentioned estimation formula (3), an estimated value D (unit: GPa) of the allowable load surface pressure is calculated from the measured values of the retained austenite amount A (unit:%) and the average grain size B (unit: μm) of the prior austenite grains. Fig. 5 is a graph showing a relationship between the calculated value of the allowable load surface pressure shown in table 1 and the estimated value D (unit: GPa) of the allowable load surface pressure calculated by the above-described estimated expression (3). As shown in fig. 5, the plots showing the calculated and estimated values of the allowable load surface pressures of the test pieces a to J are distributed in the vicinity of the broken line indicating the state where the calculated and estimated values are equal to each other. From this, it was confirmed that by using the estimation formula (3), the allowable load surface pressures of the mechanical components having the same structure as the test pieces a to J can be predicted with high accuracy. In fig. 5, "R ²" represents a contribution ratio. The contribution ratio (R ²) was 0.942.

<2 > Retained austenite amount and undissolved carbide amount >

Fig. 6 is a graph showing the relationship between the amount of retained austenite (unit:%), the amount of undissolved carbide (unit:%) and the allowable load surface pressure (unit: GPa). As shown in table 1 and fig. 6, it was confirmed that the allowable load surface pressure tends to be lower as the amount of undissolved carbide is lower. The reason for this is considered to be that the less the amount of undissolved carbide, the more the amount of solid-solution carbon tends to be, so that the formation of martensite is promoted, the formation of microcracks due to lenticular martensite is promoted, and as a result, the depth of the residual indentation is increased. On the other hand, as shown in fig. 6, when the amount of undissolved carbide exceeds 10%, the amount of solid-solution carbon becomes insufficient, and the depth of the residual indentation becomes deep, thus allowing the load face pressure to decrease.

Further, from the correlation between the amount of retained austenite, the amount of undissolved carbide, and the allowable load surface pressure shown in FIG. 6, the following estimation formula (4) for estimating the allowable load surface pressure D (unit: GPa) from the measured values of the amount of retained austenite A (unit:%) and the amount of undissolved carbide C (unit:%) was derived by multiple regression analysis.

[ Math figure 6]

D＝5.13+0.93+0.43×A-0.044×A²-0.069×C…(4)

Further, according to the above-mentioned estimation formula (4), an estimated value D (unit: GPa) of the allowable load surface pressure is calculated from measured values of the retained austenite amount A (unit:%) and the undissolved carbide amount C (unit:%). Fig. 7 is a graph showing a relationship between the calculated value of the allowable load surface pressure shown in table 1 and the estimated value D (unit: GPa) of the allowable load surface pressure calculated by the above-described estimated expression (4). As shown in fig. 7, the plots showing the calculated and estimated values of the allowable load surface pressures of the test pieces a to J are distributed in the vicinity of the broken line indicating the state where the calculated and estimated values are equal to each other. From this, it was confirmed that by using the estimation formula (4), the allowable load surface pressures of the mechanical components having the same structure as the test pieces a to J can be predicted with high accuracy. In fig. 7, "R ²" represents a contribution ratio. The contribution ratio (R ²) was 0.938.

Using the estimated expression (3), the amount of retained austenite and the average grain size of prior austenite grains on the surface of the machine component for realizing the allowable load required for the machine component can be calculated. Using the estimated expression (4), the amount of retained austenite and the amount of undissolved carbide on the surface of the machine component for realizing the allowable load required for the machine component can be calculated. The above-mentioned relational expression (1) is a relational expression which is to be satisfied by the retained austenite amount a and the average grain size B of the prior austenite grains so that the allowable load surface pressure D of the estimated expression (3) is 4.2GPa or more. The above-mentioned relational expression (2) is a relational expression to be satisfied by the retained austenite amount a and the undissolved carbide amount C so that the allowable load surface pressure D of the estimated expression (4) is 4.2GPa or more. The test pieces A, B, D, F to H satisfy the relational expression (1) and the relational expression (2). The test piece C, E, I, J does not satisfy the relation (1) and the relation (2).

The estimation formulae (3) and (4) are derived from the relationship between the allowable load and the microstructure of the surface (raceway surface) of the bearing inner ring confirmed in the present embodiment, but are applicable to mechanical components other than the bearing inner ring. That is, the allowable load surface pressure of the surface of the mechanical component other than the bearing inner race can be estimated by the estimation formulae (3) and (4).

On the other hand, the relationship between the surface microstructure of the mechanical component other than the bearing inner race and the allowable load can be confirmed by performing the same test as the above test. In this case, an estimated expression for estimating the allowable load surface pressure from the actual measurement value of the residual austenite amount and the average grain size of the prior austenite grains may be derived by multiple regression analysis from the correlation between the actual measurement value of the residual austenite amount and the average grain size of the prior austenite grains and the allowable load surface pressure calculated from the result of the indentation test. Further, from this estimated expression, a relational expression can be derived in which the amount of retained austenite on the surface of the machine component and the average grain size of the prior austenite grains should satisfy in order to achieve the allowable load required for the machine component. The estimation formula and the relational expression thus derived are considered to be substantially equivalent to the above-described estimation formula (3), estimation formula (4), relational expression (1), and relational expression (2).

When the required allowable load is less than 4.2GPa, the mechanical component may be set so as to satisfy a relational expression derived by substituting the required value of the allowable load into the allowable load surface pressures D of the estimated expressions (3) and (4).

The estimated expression (3) and the estimated expression (4) are derived from the relationship between the microstructure of the inner ring surface (raceway surface) made of high carbon steel and the allowable load confirmed in the present example, but are applicable to machine parts made of low carbon steel or carburized steel having a carbon content of 0.01 mass% or more and less than 0.8 mass%. That is, the allowable load surface pressure of the surface of the machine member composed of the low carbon steel or the carburizing steel having the carbon content of 0.01 mass% or more and less than 0.8 mass% can be estimated by the estimation formula (3) and the estimation formula (4).

The relationship between the microstructure of the surface of the mechanical part composed of low carbon steel or carburized steel having a carbon content of 0.01 mass% or more and less than 0.8 mass% and the allowable load can also be confirmed by performing the same test as the above test. In this case, an estimated expression for estimating the allowable load surface pressure from the actual measurement value of the residual austenite amount and the average grain size of the prior austenite grains may be derived by multiple regression analysis from the correlation between the actual measurement value of the residual austenite amount and the average grain size of the prior austenite grains and the allowable load surface pressure calculated from the result of the indentation test. Further, from this estimated expression, a relational expression can be derived in which the amount of retained austenite on the surface of the machine component and the average grain size of the prior austenite grains should satisfy in order to achieve the allowable load required for the machine component. The estimation formula and the relational expression thus derived are considered to be substantially equivalent to the above-described estimation formula (3), estimation formula (4), relational expression (1), and relational expression (2).

<3. Average grain size and average aspect ratio of the martensite grains of the first group, and average grain size and average aspect ratio of the martensite grains of the third group >

As shown in Table 1, the allowable load surface pressure of the test pieces A to H in which the average grain size of the martensite grains of the first group was smaller than 1.8 μm and the average grain size of the martensite grains of the third group was smaller than 1.5 μm was higher than that of the test piece I, J in which the average grain size of the martensite grains of the first group was larger than 1.8 μm and the average grain size of the martensite grains of the third group was larger than 1.5 μm.

As shown in table 1, the allowable load surface pressures of the test pieces a to H in which the average aspect ratio of the first group of martensite grains is 3.5 or less and the average aspect ratio of the third group of martensite grains is 3.1 or less were higher than the allowable load surface pressures of the test pieces I, J in which the average aspect ratio of the first group of martensite grains is more than 3.5 and the average aspect ratio of the third group of martensite grains is more than 3.1.

<4. Cross-sectional hardness >

As shown in table 1, in the test piece C, E, I, J which did not satisfy the above-described relational expression (1) and relational expression (2), the cross-sectional hardness of the test piece C, E was higher than 770GPa. That is, in the test piece C, E, the section hardness was increased, but the allowable load face pressure was not sufficiently increased. In contrast, the cross-sectional hardness of the test pieces A, B, D, F to H satisfying the above-mentioned relational expression (1) and relational expression (2) is 650GPa to 770GPa. It is considered that the test pieces A, B, D, F to H have higher toughness and higher impact resistance than the test piece C, E. Further, according to the test results, it was confirmed that the allowable load of the mechanical component of the present embodiment can be increased to be equal to or higher than the allowable load of the mechanical component having a higher surface hardness without increasing the surface hardness.

(Rolling fatigue life test)

Further, the test pieces a to J were each assembled into a 6206-type deep groove ball bearing defined in JIS standards, and rolling fatigue life test was performed on each bearing. An outer race and rolling elements (balls) of each bearing, which were composed of SUJ2, were prepared and heat-treated under the same conditions as the test pieces assembled together with them. In the rolling fatigue test, the inner ring was rotated at 3000rpm with the outer ring fixed. The maximum contact surface pressure between the rolling element and the inner ring is 3.3GPa. VG64 lubricant was used.

The time until the raceway surface of the inner ring of each bearing was peeled off was measured as the life, and whether the measured life was acceptable was determined. Table 1 also shows the results of whether the rolling fatigue life of each bearing assembled from the test pieces a to J is acceptable. In the judgment of whether or not the bearing whose life exceeded the calculated life was judged to be acceptable (denoted as "a" in table 1), the bearing whose life did not exceed the calculated life was judged to be unacceptable (denoted as "F" in table 1). As shown in table 1, the rolling fatigue life of the bearings assembled from the test pieces A, B, D, F to H satisfying the above-described relational expressions (1) and (2) is sufficiently longer than the rolling fatigue life of the bearings assembled from the test pieces C, E, I, J not satisfying the above-described relational expressions (1) and (2).

From the test results, it was confirmed that by using the mechanical parts satisfying the above-described relational expression (1) and relational expression (2), both the improvement of the allowable load and the improvement of the rolling contact fatigue life can be achieved compared with the mechanical parts not satisfying the above-described relational expression (1) and relational expression (2).

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 appended claims, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Symbol description

10 Inner ring, 10a,10b width surface, 10c inner peripheral surface, 10d outer peripheral surface, 10da raceway surface, 11 surface layer portion.

Claims

1. A mechanical part comprising quenched and tempered steel, wherein,

The mechanical component has a surface, and the residual austenite amount A (unit:%) of the surface and the average grain size B (unit:. Mu.m) of prior austenite grains of the surface satisfy the following relational expression (1):

1.61-0.051×A-0.027×B≥0…(1)。

2. The mechanical component of claim 1, wherein the amount of retained austenite a (unit:%) of the surface and the amount of undissolved carbide C (unit:%) of the surface satisfy the following relation (2):

0.93+0.43×A-0.044×A2-0.069×C≥0…(2)。

3. a mechanical part comprising quenched and tempered steel, wherein,

The mechanical component has a surface, and the residual austenite amount A (unit:%) of the surface and the undissolved carbide amount C (unit:%) of the surface satisfy the following relation (2):

0.93+0.43×A-0.044×A²-0.069×C≥0…(2)。

4. The mechanical part according to claim 3, wherein the residual austenite amount A (unit:%) of the surface and the average grain size B (unit: μm) of the prior austenite grains of the surface satisfy the following relational expression (1):

1.61-0.051×A-0.027×B≥0…(1)。

5. A mechanical part according to any one of claims 1 to 4, comprising a skin portion having said surface,

The surface layer portion includes a plurality of martensite grains,

The plurality of martensite grains is divided into a first group and a second group,

The minimum value of the grain size of the martensite grains belonging to the first group is larger than the maximum value of the martensite grains belonging to the second group,

The total area of the martensite grains belonging to the first group divided by the total area of the martensite grains has a value of 0.3 or more,

A value obtained by dividing a total area of the martensite grains belonging to the first group after removal of the martensite grains having the smallest grain size belonging to the first group by a total area of the martensite grains is less than 0.3,

The martensite grains belonging to the first group have an average grain size of 1.8 μm or less,

The plurality of martensite grains are divided into a third group and a fourth group,

The minimum value of the grain size of the martensite grains belonging to the third group is larger than the maximum value of the martensite grains belonging to the fourth group,

The total area of the martensite grains belonging to the third group divided by the total area of the martensite grains has a value of 0.5 or more,

The value obtained by dividing the total area of the martensite grains belonging to the third group after the martensite grains having the smallest grain size belonging to the third group by the total area of the martensite grains is less than 0.5,

The martensite grains belonging to the third group have an average grain size of 1.5 μm or less.

6. The mechanical part according to claim 5, wherein,

The martensitic grains belonging to the first group have an average aspect ratio of less than 3.5,

The average aspect ratio of the martensitic grains belonging to the third group is 3.1 or less.

7. The mechanical component according to any one of claims 1 to 6, comprising a surface layer portion which is a region having a distance of 20 μm from the surface, wherein the hardness of the surface layer portion is 650HV or more and 780HV or less.

8. The mechanical part according to any one of claims 1 to 7, wherein,

The steel is a high-carbon steel,

The content of carbon in the high-carbon steel is more than 0.8 mass percent,

The content of chromium in the high-carbon steel is below 4 mass%,

The silicon content in the high-carbon steel is 0.1 to 0.7 mass%,

The content of molybdenum in the high-carbon steel is 0.25 mass% or less.

9. A machine component as claimed in any one of claims 1 to 8, wherein the surface is configured as a sliding surface for sliding with other components.