Patent Description:
A rolling fatigue life of a bearing part is improved by carbonitriding a surface of the bearing part (a raceway surface of each of an inner ring and an outer ring as well as a rolling contact surface of a rolling element) as described in Patent Literature <NUM> (<CIT>). Moreover, the rolling fatigue life of the rolling bearing is improved by attaining fine prior austenite grains in the surface of the bearing part as described in Patent Literature <NUM> (<CIT>). Furthermore, from <CIT> bearing parts made of chromium-molybdenum steel having a diffusion layer on the surface are known. The diffusion layer contains a plurality of chemical compound particles and a plurality of martensite blocks. The chemical compound particles have an average particle size of <NUM> or less. The chemical compound particles have an area ratio of <NUM>% or more in the diffusion layer. The martensite blocks have a maximum particle size of <NUM> or less. <CIT> discloses a rolling bearing comprising a roller, a roller shaft provided inwardly of the roller, and rolling elements provided between the roller and the roller shaft, the roller shaft having a nitrogen-rich layer and being arranged at its race surface, on which the rolling elements run, not smaller than <NUM> in the JIS austenite grain size number (JIS G0551) and not smaller than HV653 in the Vickers hardness number, at both ends not greater than HV300, and at its core beneath a widthwise midpoint of the race surface not smaller than HV550. From <CIT> it is known that an aggregate of hyperfine austenitic nanocrystal alloy steel particles obtained by heating-up hyperfine ferritic nanocrystal alloy steel particles to an austenite temperature region is subjected to rapid cooling such as quenching or a cooling operation at a suitable speed or heat refining treatment such as strong working, so as to obtain martensitic nanocrystal alloy steel powder composed of an aggregate of hyperfine martensitic nanocrystal alloy steel particles, and having high hardness and toughness. Alloy steel powder composed of an aggregate of hyperfine ferritic nanocrystal alloy steel particles is subjected to consolidation treatment such as cold press forming, discharge plasma sintering or the like in the air or in an oxidation suppression atmosphere or in a vacuum. Next, the consolidated body is subjected to heat refining treatment such as annealing and solution treatment, so as to be a nanocrystal alloy steel bulk material composed of an aggregate of martensitic nanocrystal alloy steel particles.

A steel used for the bearing part is generally quenched. That is, a quench-hardened layer having a structure mainly composed of a martensite phase is formed in the surface of the bearing part. However, it has not been conventionally known how states of martensite crystal grains affect the rolling fatigue life of the bearing part.

A locally high surface pressure may be applied to a rolling element or shaft of a rocker arm bearing due to an influence of an attachment error, imbalance of load, or the like. Further, since the rocker arm bearing is used inside an engine, a foreign matter may be introduced into a lubricant or deterioration of the lubricant may be caused. Further, since the rocker arm bearing is a full complement roller bearing, the following matters may occur: interference between rolling elements (rollers); occurrence of skew with respect to a roller; an insufficient amount of supply of lubricant between the bearing ring and the roller; and the like. This may cause a shorter rolling fatigue life of the rolling element or shaft of the rocker arm bearing than expected, and therefore, it has been desired to improve the rolling fatigue life. Similarly, it has been desired to improve the rolling fatigue life of a shaft used for a planetary gear mechanism bearing.

The present invention has been made in view of the above-described problem of the conventional art. More specifically, the present invention is to provide a bearing part having an improved rolling fatigue life.

A bearing part according to a first implementation of the present invention is a rolling element used for a rocker arm bearing, a shaft used for the rocker arm bearing, or a shaft used for a planetary gear mechanism bearing. The bearing part includes a quench-hardened layer in a surface of the bearing part. The quench-hardened layer includes a plurality of martensite crystal grains. A ratio of a total area of the plurality of martensite crystal grains in the quench-hardened layer is more than or equal to <NUM>%. The plurality of martensite crystal grains are classified into a first group and a second group. A minimum value of crystal grain sizes of the martensite crystal grains belonging to the first group is larger than a maximum value of crystal grain sizes of the martensite crystal grains belonging to the second group. A value obtained by dividing a total area of the martensite crystal grains belonging to the first group by the total area of the plurality of martensite crystal grains is more than or equal to <NUM>. A value obtained by dividing, by the total area of the plurality of martensite crystal grains, a total area of the martensite crystal grains belonging to the first group except for a martensite crystal grain that has a minimum crystal grain size and that belongs to the first group is less than <NUM>. An average grain size of the martensite crystal grains belonging to the first group is less than or equal to <NUM>. The steel is high-carbon chromium bearing steel SUJ2 defined in JIS.

A bearing part according to a second implementation of the present invention is a rolling element used for a rocker arm bearing, a shaft used for the rocker arm bearing, or a shaft used for a planetary gear mechanism bearing. The bearing part includes a quench-hardened layer in a surface of the bearing part. The quench-hardened layer includes a plurality of martensite crystal grains. A ratio of a total area of the plurality of martensite crystal grains in the quench-hardened layer is more than or equal to <NUM>%. The plurality of martensite crystal grains are classified into a first group and a second group. A minimum value of crystal grain sizes of the martensite crystal grains belonging to the first group is larger than a maximum value of crystal grain sizes of the martensite crystal grains belonging to the second group. A value obtained by dividing a total area of the martensite crystal grains belonging to the first group by the total area of the plurality of martensite crystal grains is more than or equal to <NUM>. A value obtained by dividing, by the total area of the plurality of martensite crystal grains, a total area of the martensite crystal grains belonging to the first group except for a martensite crystal grain that has a minimum crystal grain size and that belongs to the first group is less than <NUM>. An average grain size of the martensite crystal grains belonging to the first group is less than or equal to <NUM>. An average aspect ratio of the martensite crystal grains belonging to the first group is less than or equal to <NUM>.

A bearing part according to a third implementation of the present invention is a rolling element used for a rocker arm bearing, a shaft used for the rocker arm bearing, or a shaft used for a planetary gear mechanism bearing. The bearing part includes a quench-hardened layer in a surface of the bearing part. The quench-hardened layer includes a plurality of martensite crystal grains. A ratio of a total area of the plurality of martensite crystal grains in the quench-hardened layer is more than or equal to <NUM>%. The plurality of martensite crystal grains are classified into a third group and a fourth group. A minimum value of crystal grain sizes of the martensite crystal grains belonging to the third group is larger than a maximum value of crystal grain sizes of the martensite crystal grains belonging to the fourth group. A value obtained by dividing a total area of the martensite crystal grains belonging to the third group by the total area of the plurality of martensite crystal grains is more than or equal to <NUM>. A value obtained by dividing, by the total area of the plurality of martensite crystal grains, a total area of the martensite crystal grains belonging to the third group except for a martensite crystal grain that has a minimum crystal grain size and that belongs to the third group is less than <NUM>. An average grain size of the martensite crystal grains belonging to the third group is less than or equal to <NUM>. The steel is high-carbon chromium bearing steel SUJ2 defined in JIS.

The bearing part according to a fourth implementation of the present invention is a rolling element used for a rocker arm bearing, a shaft used for the rocker arm bearing, or a shaft used for a planetary gear mechanism bearing. The bearing part includes a quench-hardened layer in a surface of the bearing part. The quench-hardened layer includes a plurality of martensite crystal grains. A ratio of a total area of the plurality of martensite crystal grains in the quench-hardened layer is more than or equal to <NUM>%. The plurality of martensite crystal grains are classified into a third group and a fourth group. A minimum value of crystal grain sizes of the martensite crystal grains belonging to the third group is larger than a maximum value of crystal grain sizes of the martensite crystal grains belonging to the fourth group. A value obtained by dividing a total area of the martensite crystal grains belonging to the third group by the total area of the plurality of martensite crystal grains is more than or equal to <NUM>. A value obtained by dividing, by the total area of the plurality of martensite crystal grains, a total area of the martensite crystal grains belonging to the third group except for a martensite crystal grain that has a minimum crystal grain size and that belongs to the third group is less than <NUM>. An average grain size of the martensite crystal grains belonging to the third group is less than or equal to <NUM>. An average aspect ratio of the martensite crystal grains belonging to the third group is less than or equal to <NUM>.

In the bearing part, a hardness of the quench-hardened layer in the surface may be more than or equal to <NUM> Hv.

In the bearing part, the quench-hardened layer may contain nitrogen. An average nitrogen concentration of the quench-hardened layer may be more than or equal to <NUM> mass% between the surface and a position at a distance of <NUM> from the surface. In the bearing part, the quench-hardened layer may include a plurality of austenite crystal grains. A volume ratio of the plurality of austenite crystal grains in the quench-hardened layer may be less than or equal to <NUM>%.

According to the bearing part according to each of the first to fourth implementations of the present invention, a rolling fatigue life can be improved.

Details of embodiments will be described with reference to figures. In the below-described figures, the same or corresponding portions are denoted by the same reference characters, and will not be described repeatedly.

Hereinafter, a configuration of a rocker arm bearing (hereinafter, referred to as "bearing <NUM>") according to a first embodiment will be described.

<FIG> is a cross sectional view of bearing <NUM>. As shown in <FIG>, bearing <NUM> includes a shaft <NUM>, an outer ring <NUM>, and a rolling element <NUM>. Each of shaft <NUM>, outer ring <NUM> and rolling element <NUM> is composed of a steel. More specifically, each of shaft <NUM>, outer ring <NUM>, and rolling element <NUM> is composed of a bearing steel. Each of shaft <NUM>, outer ring <NUM> and rolling element <NUM> is preferably composed of high-carbon chromium bearing steel SUJ2 defined in JIS (JIS G <NUM>: <NUM>). Each of shaft <NUM>, outer ring <NUM> and rolling element <NUM> may be composed of high-carbon chromium bearing steel SUJ3 defined in JIS, <NUM> defined in ASTM, 100Cr6 defined in DIN, or GCr5 (GCr15) defined in GB.

Shaft <NUM> has an outer peripheral surface 11a. Outer peripheral surface 11a serves as a raceway surface (surface to be in contact with rolling element <NUM>). Shaft <NUM> has a cylindrical shape, for example. Shaft <NUM> may be solid or hollow. Shaft <NUM> has a central axis A1. Shaft <NUM> has a first end 11b and a second end 11c in an axial direction (direction along central axis A1). Second end 11c is an end opposite to first end 11b. Shaft <NUM> is fixed to a rocker arm <NUM> at first end 11b and second end 11c. Rocker arm <NUM> is pushed and moved by a cam (not shown) and is accordingly rocked. By rocking rocker arm <NUM>, shaft <NUM> is rotated about central axis A1.

Outer ring <NUM> has an annular shape (ring shape). Outer ring <NUM> has an upper surface 12a, a bottom surface 12b, an inner peripheral surface 12c, and an outer peripheral surface 12d. Upper surface 12a and bottom surface 12b constitute end surfaces of outer ring <NUM> in an axial direction. Bottom surface 12b is a surface opposite to upper surface 12a in the axial direction.

Each of inner peripheral surface 12c and outer peripheral surface 12d extends along a peripheral direction (direction along a perimeter having central axis A1 as its center). Inner peripheral surface 12c faces the central axis A1 side, and outer peripheral surface 12d faces the side opposite to central axis A1. That is, outer peripheral surface 12d is a surface opposite to inner peripheral surface 12c in a radial direction (direction passing through central axis A1 and orthogonal to central axis A1). Outer ring <NUM> is disposed such that inner peripheral surface 12c faces outer peripheral surface 11a. Inner peripheral surface 12c serves as a raceway surface.

Rolling element <NUM> has a cylindrical shape extending along the axial direction. Specifically, rolling element <NUM> is a needle roller. Rolling element <NUM> has an outer peripheral surface 13a. Outer peripheral surface 13a serves as a rolling contact surface. Rolling element <NUM> is disposed between shaft <NUM> and outer ring <NUM> such that outer peripheral surface 13a is in contact with outer peripheral surface 11a and inner peripheral surface 12c. Thus, shaft <NUM> is supported rotatably about central axis A1. Bearing <NUM> does not have a cage. That is, bearing <NUM> is a full complement roller bearing.

<FIG> is an enlarged cross sectional view of rolling element <NUM> in the vicinity of outer peripheral surface 13a. As shown in <FIG>, rolling element <NUM> includes a quench-hardened layer <NUM> in outer peripheral surface 13a. Quench-hardened layer <NUM> is a layer hardened by performing quenching. Quench-hardened layer <NUM> includes a plurality of martensite crystal grains.

When a deviation is more than or equal to <NUM>° between the crystal orientation of a first martensite crystal grain and the crystal orientation of a second martensite crystal grain adjacent to the first martensite crystal grain, the first and second martensite crystal grains are different martensite crystal grains. On the other hand, when the deviation is less than <NUM>° between the crystal orientation of the first martensite crystal grain and the crystal orientation of the second martensite crystal grain adjacent to the first martensite crystal grain, the first and second martensite crystal grains constitute one martensite crystal grain.

Quench-hardened layer <NUM> has a structure mainly composed of a martensite phase. More specifically, a ratio of a total area of the plurality of martensite crystal grains in quench-hardened layer <NUM> is more than or equal to <NUM>%. The ratio of the total area of the plurality of martensite crystal grains in quench-hardened layer <NUM> may be more than or equal to <NUM>%.

In addition to the martensite crystal grains, quench-hardened layer <NUM> includes austenite crystal grains, ferrite crystal grains, and cementite (Fe<NUM>C) crystal grains. A volume ratio of the austenite crystal grains in quench-hardened layer <NUM> is preferably less than or equal to <NUM>%. The volume ratio of the austenite crystal grains in quench-hardened layer <NUM> is more preferably more than or equal to <NUM>%. It should be noted that the volume ratio of the austenite crystal grains in quench-hardened layer <NUM> is measured by an X-ray diffraction method. More specifically, the volume ratio of the austenite crystal grains in quench-hardened layer <NUM> is calculated based on a ratio of the X-ray diffraction intensity of the austenite phase and the X-ray diffraction intensity of the other phases included in quench-hardened layer <NUM>.

The plurality of martensite crystal grains are classified into a first group and a second group. A minimum value of crystal grain sizes of the martensite crystal grains belonging to the first group is larger than a maximum value of crystal grain sizes of the martensite crystal grains belonging to the second group.

A value obtained by dividing a total area of the martensite crystal grains belonging to the first group by the total area of the plurality of martensite crystal grains (the sum of the total area of the martensite crystal grains belonging to the first group and the total area of the martensite crystal grains belonging to the second group) is more than or equal to <NUM>.

A value obtained by dividing, by the total area of the plurality of martensite crystal grains, the total area of the martensite crystal grains belonging to the first group except for a martensite crystal grain that has a minimum crystal grain size and that belongs to the first group is less than <NUM>.

From another viewpoint, it can be said that the plurality of martensite crystal grains are assigned to the first group in the order from one having the largest crystal grain size. The assignment to the first group is ended when the total area of the martensite crystal grains assigned to the first group until then becomes <NUM> or more time as large as the total area of the plurality of martensite crystal grains. A remainder of the plurality of martensite crystal grains are assigned to the second group.

An average grain size of the martensite crystal grains belonging to the first group is less than or equal to <NUM>. The average grain size of the martensite crystal grains belonging to the first group is preferably less than or equal to <NUM>. The average grain size of the martensite crystal grains belonging to the first group is more preferably less than or equal to <NUM>.

An aspect ratio of each of the martensite crystal grains belonging to the first group is less than or equal to <NUM>. The aspect ratio of each of the martensite crystal grains belonging to the first group is preferably less than or equal to <NUM>. The aspect ratio of each of the martensite crystal grains belonging to the first group is more preferably less than or equal to <NUM>.

The plurality of martensite crystal grains may be classified into a third group and a fourth group. A minimum value of crystal grain sizes of the martensite crystal grains belonging to the third group is larger than a maximum value of crystal grain sizes of the martensite crystal grains belonging to the fourth group.

A value obtained by dividing a total area of the martensite crystal grains belonging to the third group by the total area of the plurality of martensite crystal grains (the sum of the total area of the martensite crystal grains belonging to the third group and the total area of the martensite crystal grains belonging to the fourth group) is more than or equal to <NUM>.

A value obtained by dividing, by the total area of the plurality of martensite crystal grains, the total area of the martensite crystal grains belonging to the third group except for a martensite crystal grain that has a minimum crystal grain size and that belongs to the third group is less than <NUM>.

From another viewpoint, it can be said that the plurality of martensite crystal grains are assigned to the third group in the order from one having the largest crystal grain size. The assignment to the third group is ended when the total area of the martensite crystal grains assigned to the third group until then becomes <NUM> or more time as large as the total area of the plurality of martensite crystal grains. A remainder of the plurality of martensite crystal grains are assigned to the fourth group.

An average grain size of the martensite crystal grains belonging to the third group is less than or equal to <NUM>. The average grain size of the martensite crystal grains belonging to the third group is preferably less than or equal to <NUM>. The average grain size of the martensite crystal grains belonging to the third group is more preferably less than or equal to <NUM>.

An aspect ratio of each of the martensite crystal grains belonging to the third group is less than or equal to <NUM>. The aspect ratio of each of the martensite crystal grains belonging to the third group is preferably less than or equal to <NUM>. The aspect ratio of each of the martensite crystal grains belonging to the third group is more preferably less than or equal to <NUM>.

The average crystal grain size of the martensite crystal grains belonging to the first group (third group) and the aspect ratio of each of the martensite crystal grains belonging to the first group (third group) are measured using an EBSD (Electron Backscattered Diffraction) method.

This will be described more in detail as follows. First, a cross section image (hereinafter, referred to as "EBSD image") in quench-hardened layer <NUM> is captured based on the EBSD method. The EBSD image is captured to include a sufficient number (more than or equal to <NUM>) of martensite crystal grains. A boundary between adjacent martensite crystal grains is specified based on the EBSD image. Second, based on the specified boundary between the martensite crystal grains, the area and shape of each martensite crystal grain in the EBSD image are calculated.

More specifically, by calculating the square root of a value obtained by dividing the area of each martensite crystal grain in the EBSD image by π/<NUM>, the equivalent circle diameter of each martensite crystal grain in the EBSD image is calculated.

Based on the equivalent circle diameter of each martensite crystal grain calculated as described above, the martensite crystal grains belonging to the first group (third group) among the martensite crystal grains in the EBSD image are determined. The value obtained by dividing, by the total area of the martensite crystal grains in the EBSD image, the total area of the martensite crystal grains belonging to the first group (third group) among the martensite crystal grains in the EBSD image is regarded as the value obtained by dividing the total area of the martensite crystal grains belonging to the first group (third group) by the total area of the plurality of martensite crystal grains.

Based on the equivalent circle diameter of each martensite crystal grain calculated as described above, the martensite crystal grains in the EBSD image are classified into the first group and the second group (or classified into the third group and the fourth group). The value obtained by dividing, by the number of the martensite crystal grains classified into the first group (third group) in the EBSD image, the total of the equivalent circle diameters of the martensite crystal grains classified into the first group (third group) in the EBSD image is regarded as the average grain size of the martensite crystal grains belonging to the first group (third group).

From the shape of each martensite crystal grain in the EBSD image, the shape of each martensite crystal grain in the EBSD image is approximated to an ellipse by the least squares method. This approximation to an ellipse by the least squares method is performed in accordance with a method described in <NPL>. By dividing the size in the major axis by the size in the minor axis in this elliptical shape, the aspect ratio of each martensite crystal grain in the EBSD image is calculated. A value obtained by dividing the total of the aspect ratios of the martensite crystal grains classified into the first group (third group) in the EBSD image by the number of the martensite crystal grains classified into the first group (third group) in the EBSD image is regarded as the average aspect ratio of the martensite crystal grains belonging to the first group (third group).

Quench-hardened layer <NUM> contains nitrogen. An average nitrogen concentration of quench-hardened layer <NUM> is preferably more than or equal to <NUM> mass% between outer peripheral surface 13a and a position at a distance of <NUM> from outer peripheral surface 13a. This average nitrogen concentration may be more than or equal to <NUM> mass%. This average nitrogen concentration is less than or equal to <NUM> mass%, for example. It should be noted that this average nitrogen concentration is measured using an EPMA (Electron Probe Micro Analyzer). The measurement of the average nitrogen concentration is preferably performed at the central position of the rolling contact surface in the axial direction (position at which an imaginary straight line that passes through the center of rolling element <NUM> in the direction along the central axis and that is orthogonal to the central axis intersects outer peripheral surface 13a). A penetration depth of the nitrogen in outer peripheral surface 13a at the central position of the rolling contact surface in the axial direction is preferably more than or equal to <NUM>. The penetration depth of the nitrogen is a depth until the concentration of the nitrogen measured using the EPMA becomes <NUM> mass%.

A hardness of quench-hardened layer <NUM> in outer peripheral surface 13a is preferably more than or equal to <NUM> Hv. It should be noted that the hardness of quench-hardened layer <NUM> in outer peripheral surface 13a is measured in accordance with JIS (JIS Z <NUM>: <NUM>).

<FIG> is an enlarged cross sectional view of shaft <NUM> in the vicinity of outer peripheral surface 11a. As shown in <FIG>, shaft <NUM> includes a quench-hardened layer <NUM> in outer peripheral surface 11a. The configuration of quench-hardened layer <NUM> is the same as the configuration of quench-hardened layer <NUM>.

More specifically, quench-hardened layer <NUM> includes a plurality of martensite crystal grains. A ratio of a total area of the plurality of martensite crystal grains in quench-hardened layer <NUM> is more than or equal to <NUM>% (preferably, more than or equal to <NUM>%).

The plurality of martensite crystal grains in quench-hardened layer <NUM> are classified into a first group and a second group. A value obtained by dividing a total area of the martensite crystal grains belonging to the first group by the total area of the plurality of martensite crystal grains is more than or equal to <NUM>. A value obtained by dividing, by the total area of the plurality of martensite crystal grains, the total area of the martensite crystal grains belonging to the first group except for a martensite crystal grain that has a minimum crystal grain size and that belongs to the first group is less than <NUM>.

The average grain size of the martensite crystal grains belonging to the first group is less than or equal to <NUM> (preferably less than or equal to <NUM>, and more preferably less than or equal to <NUM>). The aspect ratio of each of the martensite crystal grains belonging to the first group is less than or equal to <NUM> (preferably less than or equal to <NUM> and more preferably less than or equal to <NUM>).

The plurality of martensite crystal grains in quench-hardened layer <NUM> may be classified into a third group and a fourth group. A value obtained by dividing a total area of the martensite crystal grains belonging to the third group by the total area of the plurality of martensite crystal grains is more than or equal to <NUM>. A value obtained by dividing, by the total area of the plurality of martensite crystal grains, the total area of the martensite crystal grains belonging to the third group except for a martensite crystal grain that has a minimum crystal grain size and that belongs to the third group is less than <NUM>.

The average grain size of the martensite crystal grains belonging to the third group is less than or equal to <NUM> (preferably less than or equal to <NUM>, and more preferably less than or equal to <NUM>). The aspect ratio of each of the martensite crystal grains belonging to the third group is less than or equal to <NUM> (preferably less than or equal to <NUM> and more preferably less than or equal to <NUM>).

Quench-hardened layer <NUM> contains nitrogen. An average nitrogen concentration of quench-hardened layer <NUM> is preferably more than or equal to <NUM> mass% between outer peripheral surface 11a and a position at a distance of <NUM> from outer peripheral surface 11a. This average nitrogen concentration may be more than or equal to <NUM> mass%. This average nitrogen concentration is less than or equal to <NUM> mass%, for example. The average nitrogen concentration is preferably measured at the central position of the rolling contact surface in the axial direction (position at which an imaginary straight line that passes through the center of rolling element <NUM> in the direction along the central axis and that is orthogonal to the central axis intersects outer peripheral surface 11a). A penetration depth of the nitrogen in outer peripheral surface 11a at the central position of the rolling contact surface in the axial direction is preferably more than or equal to <NUM>. The hardness of quench-hardened layer <NUM> in outer peripheral surface 11a is preferably more than or equal to <NUM> Hv. Moreover, the volume ratio of the austenite crystal grains in quench-hardened layer <NUM> is preferably less than or equal to <NUM>% (preferably more than or equal to <NUM>% and less than or equal to <NUM>%).

The following describes a method for manufacturing rolling element <NUM>.

<FIG> is a process chart showing the method for manufacturing rolling element <NUM>. As shown in <FIG>, the method for manufacturing rolling element <NUM> includes a preparing step S1, a carbonitriding step S2, a first tempering step S3, a quenching step S4, a second tempering step S5, and a post-process step S6.

In preparing step S1, a processing target member having a cylindrical shape is prepared. The processing target member is formed into rolling element <NUM> by performing carbonitriding step S2, first tempering step S3, quenching step S4, second tempering step S5 and post-process step S6 thereto. In preparing step S1, first, the material of the processing target member is cut. In preparing step S1, second, the processing target member is subjected to cold forging or cold heading. In preparing step S1, third, cutting is performed to provide the processing target member with a shape close to the shape of rolling element <NUM> as required.

In carbonitriding step S2, first, by heating the processing target member to a temperature of more than or equal to a first temperature, the processing target member is carbonitrided. The first temperature is a temperature of more than or equal to an A<NUM> transformation point of the steel of the processing target member. In carbonitriding step S2, second, the processing target member is cooled. This cooling is performed such that the temperature of the processing target member becomes less than or equal to an Ms transformation point.

In first tempering step S3, the processing target member is tempered. First tempering step S3 is performed by holding the processing target member at a second temperature for a first period of time. The second temperature is a temperature of less than the A<NUM> transformation point. The second temperature is more than or equal to <NUM> and less than or equal to <NUM>, for example. The first period of time is more than or equal to <NUM> hour and less than or equal to <NUM> hours, for example.

In quenching step S4, the processing target member is quenched. In quenching step S4, first, the processing target member is heated to a third temperature. The third temperature is a temperature of more than or equal to the A<NUM> transformation point of the steel of the processing target member. The third temperature is preferably lower than the first temperature. In quenching step S4, second, the processing target member is cooled. This cooling is performed such that the temperature of the processing target member becomes less than or equal to an Ms transformation point.

In second tempering step S5, the processing target member is tempered. Second tempering step S5 is performed by holding the processing target member at a fourth temperature for a second period of time. The fourth temperature is a temperature of less than the A<NUM> transformation point. The fourth temperature is more than or equal to <NUM> and less than or equal to <NUM>, for example. The second period of time is more than or equal to <NUM> hour and less than or equal to <NUM> hours, for example. It should be noted that each of quenching step S4 and second tempering step S5 may be repeated multiple times.

In post-process step S6, the processing target member is post-processed. In post-process step S6, cleaning of the processing target member, machining of a surface of the processing target member, such as grinding or polishing, and the like are performed, for example. In this way, rolling element <NUM> is manufactured.

A method for manufacturing shaft <NUM> is the same as the method for manufacturing rolling element <NUM> and is therefore not described here in detail.

The following describes effects of rolling element <NUM> and shaft <NUM>.

When material failure is considered in accordance with the weakest link model, portions each having a relatively low strength, i.e., martensite crystal grains each having a relatively large crystal grain size have a great influence on the material failure. In each of quench-hardened layer <NUM> and quench-hardened layer <NUM>, the average grain size of the martensite crystal grains belonging to the first group (third group) is less than or equal to <NUM> (less than or equal to <NUM>). Accordingly, in each of rolling element <NUM> and shaft <NUM>, even such relatively large martensite crystal grains belonging to the first group (third group) are fine crystal grains, with the result that rolling fatigue strength and static load capacity are improved.

As the average aspect ratio of the martensite crystal grains becomes smaller, the shape of each of the martensite crystal grains becomes closer to a spherical shape, with the result that stress concentration is less likely to take place. Accordingly, when the average aspect ratio of the martensite crystal grains belonging to the first group (third group) is less than or equal to <NUM> (less than or equal to <NUM>), the rolling fatigue strength and static load capacity can be further improved.

Since the volume ratio of the austenite crystal grains in each of quench-hardened layer <NUM> and quench-hardened layer <NUM> is less than or equal to <NUM>%, the hardness in each of outer peripheral surface 13a and outer peripheral surface 11a can be suppressed from being decreased while maintaining dimensional stability of each of rolling element <NUM> and shaft <NUM>.

The following describes a configuration of a planetary gear mechanism bearing according to a second embodiment (hereinafter, also referred to as "bearing <NUM>").

<FIG> is a schematic diagram of a planetary gear mechanism <NUM>. As shown in <FIG>, planetary gear mechanism <NUM> includes a ring gear <NUM> (internal gear), a sun gear <NUM> (sun gear), and a plurality of pinion gears <NUM> (planetary gears).

Ring gear <NUM> has a ring shape (annular shape). Ring gear <NUM> has an inner peripheral surface 31a. Internal teeth 31b are formed in inner peripheral surface 31a. Sun gear <NUM> has a disc shape. Sun gear <NUM> has an outer peripheral surface 32a. External teeth 32b are formed in outer peripheral surface 32a. Sun gear <NUM> is disposed at the center of ring gear <NUM>. Each of pinion gears <NUM> has a disc shape. Pinion gear <NUM> has an outer peripheral surface 33a. External teeth 33b are formed in outer peripheral surface 33a. Pinion gear <NUM> is disposed between ring gear <NUM> and sun gear <NUM>, and external teeth 33b are engaged with internal teeth 31b and external teeth 32b.

<FIG> is a cross sectional view of bearing <NUM>. As shown in <FIG>, bearing <NUM> includes a shaft <NUM>, a rolling element <NUM>, and a cage <NUM>. Each of shaft <NUM> and rolling element <NUM> is composed of a steel. Each of shaft <NUM> and rolling element <NUM> is composed of high-carbon chromium bearing steel SUJ2 defined in JIS, for example. Each of shaft <NUM> and rolling element <NUM> may be composed of high-carbon chromium bearing steel SUJ3 defined in JIS, <NUM> defined in ASTM, 100Cr6 defined in DIN, or GCr5 (GCr15) defined in GB.

Shaft <NUM> has a cylindrical shape extending along a central axis A2. Shaft <NUM> has an outer peripheral surface 21a. Outer peripheral surface 21a serves as a raceway surface. Shaft <NUM> is inserted into a through hole 33c formed in pinion gear <NUM>. Outer peripheral surface 21a faces an inner wall surface of through hole 33c.

Shaft <NUM> has a first end 21b and a second end 21c in an axial direction (direction along central axis A2). Second end 21c is an end opposite to first end 21b in the axial direction. Shaft <NUM> is fixed to a carrier <NUM> at first end 21b and second end 21c. A revolution motion of pinion gear <NUM> is input and output from carrier <NUM>.

An oil supply flow path 21d is formed inside shaft <NUM>. Oil supply flow path 21d has a supply opening 21da opened at second end 21c and a discharge opening 21db opened at outer peripheral surface 21a. Lubricant supplied from supply opening 21da passes through oil supply flow path 21d and is discharged from discharge opening 21db. Thus, the lubricant is supplied to the surroundings of rolling element <NUM>.

Rolling element <NUM> extends along the axial direction. Rolling element <NUM> has a cylindrical shape. Rolling element <NUM> is a needle roller, for example. Rolling element <NUM> has an outer peripheral surface 22a. Outer peripheral surface 22a serves as a rolling contact surface. Rolling element <NUM> is disposed between shaft <NUM> and pinion gear <NUM> such that outer peripheral surface 22a is in contact with outer peripheral surface 21a and the inner wall surface of through hole 33c. Thus, pinion gear <NUM> is supported rotatably about central axis A2 by bearing <NUM>.

Cage <NUM> holds rolling element <NUM> so as to maintain an interval between rolling elements <NUM> in the peripheral direction (direction along the perimeter passing through central axis A2) to fall within a predetermined range.

<FIG> is an enlarged cross sectional view of shaft <NUM> in the vicinity of outer peripheral surface 21a. As shown in <FIG>, shaft <NUM> includes a quench-hardened layer <NUM> in outer peripheral surface 21a. The configuration of quench-hardened layer <NUM> is the same as the configuration of quench-hardened layer <NUM>.

Quench-hardened layer <NUM> includes a plurality of martensite crystal grains. A ratio of a total area of the plurality of martensite crystal grains in quench-hardened layer <NUM> is more than or equal to <NUM>% (preferably, more than or equal to <NUM>%).

The average grain size of the martensite crystal grains belonging to the first group is less than or equal to <NUM> (preferably less than or equal to <NUM>, and more preferably less than or equal to <NUM>). The aspect ratio of the martensite crystal grains belonging to the first group is less than or equal to <NUM> (preferably less than or equal to <NUM> and more preferably less than or equal to <NUM>).

Quench-hardened layer <NUM> contains nitrogen. An average nitrogen concentration of quench-hardened layer <NUM> is preferably more than or equal to <NUM> mass% between outer peripheral surface 21a and a position at a distance of <NUM> from outer peripheral surface 21a. This average nitrogen concentration may be more than or equal to <NUM> mass%. This average nitrogen concentration is less than or equal to <NUM> mass%, for example. The average nitrogen concentration is preferably measured at the central position of the rolling contact surface in the axial direction (position at which an imaginary straight line that passes through the center of rolling element <NUM> in the direction along the central axis and that is orthogonal to the central axis intersects outer peripheral surface 21a). A penetration depth of the nitrogen in outer peripheral surface 21a at the central position of the rolling contact surface in the axial direction is preferably more than or equal to <NUM>. A hardness of quench-hardened layer <NUM> in outer peripheral surface 21a is preferably more than or equal to <NUM> Hv. Moreover, the volume ratio of the austenite crystal grains in quench-hardened layer <NUM> is preferably less than or equal to <NUM>% (preferably more than or equal to <NUM>% and less than or equal to <NUM>%).

The effect of the method for manufacturing shaft <NUM> and the effect of shaft <NUM> are the same as the effect of the method for manufacturing rolling element <NUM> and the effect of rolling element <NUM> and therefore are not described here in detail.

The following describes a rolling fatigue test and a static load capacity test, each of which was performed to confirm the effects of rolling element <NUM>, shaft <NUM>, and shaft <NUM>.

In each of the rolling fatigue test and the static load capacity test, samples <NUM>, <NUM>, and <NUM> were used. Each of samples <NUM> and <NUM> was composed of SUJ2. Sample <NUM> was composed of SCM435, which is a chromium-molybdenum steel defined in JIS (JIS G <NUM>: <NUM>).

Sample <NUM> was prepared by performing the same heat treatment as that for rolling element <NUM> (shaft <NUM> or shaft <NUM>). More specifically, in the preparation of sample <NUM>, the first temperature was set to <NUM>, the second temperature was set to <NUM>, the third temperature was set to <NUM>, and the fourth temperature was set to <NUM>. For each of samples <NUM> and <NUM>, quenching step S4 and second tempering step S5 were not performed. In the preparation of sample <NUM>, the first temperature was set to <NUM> and the second temperature was set to <NUM>. In the preparation of sample <NUM>, the first temperature was set to <NUM> and the second temperature was set to <NUM>. The heat treatment conditions for samples <NUM> to <NUM> are shown in Table <NUM>.

It should be noted that in each of samples <NUM> to <NUM>, at a position at a distance of <NUM> from the surface, the ratio of the total area of the austenite crystal grains was more than or equal to <NUM>% and less than or equal to <NUM>%, the nitrogen concentration in the surface was more than or equal to <NUM> mass% and less than or equal to <NUM> mass%, and the hardness in the surface was <NUM> Hv.

In sample <NUM>, the average grain size of the martensite crystal grains belonging to the first group was <NUM>, and the average aspect ratio of the martensite crystal grains belonging to the first group was <NUM>. Moreover, in sample <NUM>, the average grain size of the martensite crystal grains belonging to the third group was <NUM>, and the average aspect ratio of the martensite crystal grains belonging to the third group was <NUM>.

Table <NUM> shows results of measurements of the average grain size and average aspect ratio of the martensite crystal grains in each of samples <NUM> to <NUM>.

<FIG> shows an EBSD image at a cross section of sample <NUM>. <FIG> shows an EBSD image at a cross section of sample <NUM>. <FIG> shows an EBSD image at a cross section of sample <NUM>. As shown in <FIG>, it is understood that the martensite crystal grains in sample <NUM> are finer than those in each of samples <NUM> and <NUM>.

In the rolling fatigue test, an inner ring, an outer ring, and a tapered roller were prepared using each of samples <NUM> and <NUM>, and were used to produce a tapered roller bearing. The rolling fatigue test was performed under conditions that the rotating speed of the inner ring was <NUM> rotations/min and the maximum contact pressure was <NUM> GPa. In the rolling fatigue test, bath lubrication was performed using VG56, which is a turbine oil. In this turbine oil, hard gas-atomized powder was mixed at a ratio of <NUM>/l. The test conditions for the rolling fatigue test are shown in Table <NUM>. It should be noted that the rolling fatigue test was performed onto six tapered roller bearings each produced using sample <NUM> and six tapered roller bearings each produced using sample <NUM>.

In the static load capacity test, flat plate-like members were produced using samples <NUM> to <NUM>. The static load capacity test was performed by finding a relation between the maximum contact pressure and the indentation depth by pressing a ceramic ball composed of silicon nitride against a surface of each of the flat plate-like members having been mirror-finished. It should be noted that the static load capacity was evaluated in accordance with the maximum contact pressure when a value obtained by dividing the indentation depth by the diameter of the ceramic ball reached <NUM>/<NUM> (when a value obtained by dividing the indentation depth by the diameter of the ceramic ball and multiplying by <NUM> reached <NUM>).

Each of the tapered roller bearings prepared using sample <NUM> had an L<NUM> life (<NUM>% failure life) of <NUM> hours. On the other hand, each of the tapered roller bearings prepared using sample <NUM> had an L<NUM> life of <NUM> hours. Thus, each of the tapered roller bearings produced using sample <NUM> had a rolling fatigue life improved twice or more as compared with that in each of the tapered roller bearings produced using sample <NUM>. This test result is shown in Table <NUM>.

<FIG> is a graph showing a relation between the average grain size of the martensite crystal grains and the rolling fatigue life. <FIG> is a graph showing a relation between the average aspect ratio of the martensite crystal grains and the rolling fatigue life. In <FIG>, the horizontal axis represents the average grain size (unit: µm) of the martensite crystal grains, and the vertical axis represents rolling fatigue life L<NUM> (unit: hour). In <FIG>, the horizontal axis represents the average aspect ratio of the martensite crystal grains, and the vertical axis represents rolling fatigue life L<NUM> (unit: hour).

As shown in <FIG> and <FIG>, rolling fatigue life L<NUM> was more improved as the average grain size of the martensite crystal grains belonging to the first group (third group) was smaller, and rolling fatigue life L<NUM> was more improved as the average aspect ratio of the martensite crystal grains belonging to the first group (third group) was smaller.

<FIG> is a graph showing a relation between the maximum contact pressure and the indentation depth. In <FIG>, the horizontal axis represents the maximum contact pressure (unit: GPa), and the vertical axis represents a value obtained as follows: the indentation depth / the diameter of the ceramic ball × <NUM><NUM>. As shown in <FIG>, when the value of the vertical axis was <NUM>, the value of the maximum contact pressure in a curve corresponding to sample <NUM> was larger than those in curves corresponding to samples <NUM> and <NUM>. That is, the value of the static load capacity in sample <NUM> was larger than each of those in samples <NUM> and <NUM>.

<FIG> is a graph showing a relation between the average grain size of the martensite crystal grains and the static load capacity. <FIG> is a graph showing a relation between the average aspect ratio of the martensite crystal grains and the static load capacity. In <FIG>, the horizontal axis represents the average grain size (unit: µm) of the martensite crystal grains, and the vertical axis represents the static load capacity (unit: GPa). In <FIG>, the horizontal axis represents the average aspect ratio of the martensite crystal grains, and the vertical axis represents the static load capacity (unit: GPa).

As shown in <FIG> and <FIG>, the static load capacity was more improved as the average grain size of the martensite crystal grains belonging to the first group (third group) was smaller, and the static load capacity was more improved as the average aspect ratio of the martensite crystal grains belonging to the first group (third group) was smaller. In view of this as well as the results shown in <FIG> and <FIG>, when the average grain size of the martensite crystal grains belonging to the first group (third group) is less than or equal to <NUM> (less than or equal to <NUM>) and the average aspect ratio of the martensite crystal grains belonging to the first group (third group) is less than or equal to <NUM> (less than or equal to <NUM>), it is possible to achieve a rolling fatigue life L<NUM> that is <NUM> or more times as large as rolling fatigue life L<NUM> of the conventional one (i.e., rolling fatigue life L<NUM> of sample <NUM>) and it is possible to achieve a static load capacity of more than or equal to <NUM> GPa.

From such test results, it has been also experimentally indicated that the rolling fatigue strength and static load capacity of rolling element <NUM> (shaft <NUM>, or shaft <NUM>) are improved because quench-hardened layer <NUM> (quench-hardened layer <NUM>, or quench-hardened layer <NUM>) is included.

Although the embodiments of the present invention have been illustrated, the embodiments described above can be modified in various manners. Further, the scope of the present invention is not limited to the above-described embodiments. The scope of the present invention is defined by the terms of the claims, and is intended to include any modifications within the scope of the claims.

The above embodiments are particularly advantageously applied to a rolling element of a rocker arm bearing, a shaft of the rocker arm bearing, and a shaft of a planetary gear mechanism bearing.

Claim 1:
A bearing part composed of a steel, the bearing part comprising a quench-hardened layer (<NUM>, <NUM>, <NUM>) in a surface of the bearing part, wherein
the bearing part is a rolling element (<NUM>, <NUM>) used for a rocker arm bearing, a shaft (<NUM>) used for the rocker arm bearing, or a shaft (<NUM>) used for a planetary gear mechanism bearing,
the quench-hardened layer (<NUM>, <NUM>, <NUM>) includes a plurality of martensite crystal grains,
a ratio of a total area of the plurality of martensite crystal grains in the quench-hardened layer (<NUM>, <NUM>, <NUM>) is more than or equal to <NUM>%,
the plurality of martensite crystal grains are classified into a first group and a second group,
a minimum value of crystal grain sizes of the martensite crystal grains belonging to the first group is larger than a maximum value of crystal grain sizes of the martensite crystal grains belonging to the second group,
a value obtained by dividing a total area of the martensite crystal grains belonging to the first group by the total area of the plurality of martensite crystal grains is more than or equal to <NUM>,
a value obtained by dividing, by the total area of the plurality of martensite crystal grains, a total area of the martensite crystal grains belonging to the first group except for a martensite crystal grain that has a minimum crystal grain size and that belongs to the first group is less than <NUM>,
an average grain size of the martensite crystal grains belonging to the first group is less than or equal to <NUM>, and
the steel is high-carbon chromium bearing steel SUJ2 defined in JIS G <NUM>: <NUM>.