JP2008267402A - Roller bearing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a long life roller bearing not easily producing indentation start point type peeling. <P>SOLUTION: An inner ring 1 and an outer ring 2 of a deep groove ball bearing are composed of high-carbon chromium bearing steel. Rolling elements 3 are composed of steel containing carbon of not less than 0.3 wt.% and not greater than 1.2 wt.%, silicon of not less than 0.3 wt.% and not greater than 2.2 wt.%, and manganese of not less than 0.2 wt.% and not greater than 2 wt.%. Heat treatment including carbonitriding treatment or nitriding treatment is applied on the rolling element and surface layer part 3a hardened by the heat treatment is formed on the rolling surface 3a. Nitrogen concentration of the surface layer part is not less than 0.2 wt.% and not greater than 2 wt.%. Nitride containing silicon and manganese deposit on the surface layer part and quantity of the Si-Mn retried is not less than 1% and not greater than 20% in area rate. Heat treatment including carbonitriding or nitriding is applied in the inner ring 1 and the outer ring 2, and surface layer parts hardened by the heat treatment are formed on raceway surfaces 1a, 2a. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a rolling bearing.

Foreign matter such as metal chips, shavings, burrs, and wear powder mixed in the lubricating oil of the bearing damages the bearing rings and rolling elements of the rolling bearing, resulting in a significant decrease in the life of the rolling bearing. That is well known. Therefore, even when a rolling bearing is used in an environment where foreign matter is mixed in the lubricant (hereinafter, sometimes referred to as a foreign matter-containing lubricating environment), the carbon content and residual austenite amount in the rolling surface layer of the bearing , And by making the content of carbonitride an appropriate value, a technology has been proposed to relieve stress concentration at the edge of the indentation caused by the foreign matter, suppress the generation of cracks, and improve the life of the rolling bearing. (See Patent Document 1).
Japanese Patent Laid-Open No. 64-55423

As described in Patent Document 1, the early separation that occurs when using a rolling bearing in a foreign matter-mixed lubrication environment is an indentation formed by foreign matter being caught between the rolling elements and the raceway. It is said that this is caused by the stress concentration occurring at the indentation edge.
In addition, it has been clarified that the above-described indentation origin type separation is not only caused by the stress concentration at the indentation edge but also by the tangential force acting between the rolling elements and the raceway. Factors affecting the tangential force include surface roughness and surface shape in addition to sliding speed and surface pressure. The smaller the surface roughness and the better the surface shape, the smaller the tangential force acting between the rolling elements and the races, and the longer the bearing life in a foreign matter-containing lubrication environment.

  However, when the amount of retained austenite on the rolling surface of the rolling element and the raceway surface of the raceway is increased as in the technique of Patent Document 1, not only the surface hardness decreases and the wear resistance becomes insufficient, but also the pressure resistance. There arises a problem that the traceability is lowered. That is, when the amount of retained austenite on the rolling surface or the raceway surface is large, the indentation is easily formed by the foreign matter. The surface on which the indentation is formed causes shape collapse and surface roughness deterioration, and the shape indentation and surface roughness deterioration become more noticeable as the size of the indentation increases and the number increases. That is, in a foreign matter-mixed lubrication environment, an indentation is more easily formed as the amount of retained austenite on the rolling surface or raceway surface increases, so that the tangential force acting between the rolling element and the raceway increases.

However, as described in Patent Document 1, due to the stress concentration relaxation effect due to the effect of retained austenite, the life of the member itself with a large amount of retained austenite is not significantly reduced, and the life of the counterpart member in contact with it is decreased. To do. This is because tangential forces of the same magnitude act on two objects that come into contact. For example, when the amount of retained austenite on the raceway surface of the raceway is large, the raceway has a long life due to the stress concentration relaxation effect, but the life of the rolling element that is the counterpart member decreases due to an increase in tangential force. .
The life of the rolling bearing is the same even if the rolling contact surface of the rolling element and the raceway surface of the raceway are separated. To extend the life of the rolling bearing, it is necessary to extend the life of both the rolling element and the raceway. There is. That is, a sufficient life extension effect cannot be obtained simply by increasing the amount of retained austenite on the rolling surface of the rolling element or the raceway surface of the raceway.

On the other hand, in recent years, as environmental problems such as global warming become more serious, machines and transport machines for improving fuel efficiency have been reduced in size and weight. Therefore, the demand for downsizing of a rolling bearing, which is one of machine parts, has become strict. In addition to the above-described rolling fatigue life (dynamic load rating), there are plastic deformation resistance (static load rating) against static overload as an obstacle to downsizing of rolling bearings. Therefore, it may not be sufficient for the performance of the rolling bearing to have a long life, and it is preferable that the rolling bearing is excellent in plastic deformation resistance.
SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a rolling bearing that solves the above-described problems of the prior art and that does not easily cause indentation-type peeling and has a long life.

In order to solve the above problems, the present invention has the following configuration. That is, the rolling bearing of claim 1 according to the present invention includes an inner ring having a raceway surface, an outer ring having a raceway surface facing the raceway surface of the inner ring, and a plurality of rolling bearings arranged between the raceway surfaces. In the rolling bearing provided with the rolling element, the following four conditions are satisfied.
Condition A: At least one of the inner ring and the outer ring is made of high carbon chrome bearing steel.
Condition B: The rolling element is made of steel, and a surface layer part formed by heat treatment including carbonitriding or nitriding is formed on the rolling surface.
Condition C: The nitrogen concentration in the surface layer portion is 0.2% by mass or more and 2% by mass or less.
Condition D: A nitride containing silicon and manganese is deposited on the surface layer portion, and the amount of the deposited nitride is 1% or more and 20% or less in terms of area ratio.

A rolling bearing according to a second aspect of the present invention is the rolling bearing according to the first aspect, wherein the amount of retained austenite on the raceway surface of the raceway ring made of high carbon chromium bearing steel among the inner ring and the outer ring is γ. _{R (AB)} and the amount of retained austenite γ _{R (C) on} the rolling surface of the rolling element satisfy the following three expressions.
γ _{R (AB)} −15 ≦ γ _{R (C)} ≦ γ _{R (AB)} +15
γ _{R (AB)} ≧ 0
γ _{R (C)} ≦ 50
However, the unit of numerical values in the above three formulas is volume%.

Furthermore, the rolling bearing according to claim 3 according to the present invention is the rolling bearing according to claim 2, wherein the raceway surface of the bearing ring made of high carbon chromium bearing steel among the inner ring and the outer ring is carburized. Or the surface layer part hardened | cured by the heat processing including a carbonitriding process is formed, and while the hardness of this surface layer part is HRC58 or more and 66 or less, the hardness of the inner core part of this surface layer part is HRC56 or more and 64 It is characterized by the following.
Furthermore, the rolling bearing according to claim 4 according to the present invention is the rolling bearing according to any one of claims 1 to 3, wherein the rolling element has a carbon content of 0.3 mass% or more and 1.2. Characterized in that it is made of steel with a silicon content of 0.3% by mass or more and 2.2% by mass or less, and a manganese content of 0.2% by mass or more and 2% by mass or less. To do.

  The present inventors examined a method based on the above-described knowledge as a method of extending the life by indentation-origin type peeling. That is, while ensuring a sufficient life by indentation origin type peeling of a certain member (for example, a rolling element), the pressure resistance and wear resistance of the member are improved, thereby suppressing deterioration of surface roughness and surface shape. This is a method in which the tangential force acting between two objects is suppressed to extend the life of the mating member (eg, raceway). As a result, it was found that the factors related to the material that improves the scratch resistance and wear resistance include the amount of retained austenite, the surface nitrogen concentration, and the amount of nitride deposited on the surface in addition to the surface hardness.

  Further, the present inventors examined the influence of the surface roughness of the rolling surface of the rolling element and the raceway surface of the raceway and the effect of the deterioration of the surface shape. The surface roughness of only the raceway surface of the raceway was reduced. Compared to the case where the surface shape is deteriorated or the deterioration of the surface shape is suppressed, the surface-origin type peeling is more effective when the surface roughness is reduced or the deterioration of the surface shape is suppressed only for the rolling surface of the rolling element. It was found that it can be suppressed. That is, the life of the rolling bearing can be effectively extended if the surface roughness of the rolling surface of the rolling element is made smaller than that of the raceway surface of the bearing ring or the deterioration of the surface shape is suppressed.

  On the other hand, in order to improve the plastic deformation resistance of the rolling bearing, it is most effective to improve the hardness of the material constituting the bearing part of the rolling bearing, and not only the surface of the bearing part but also the core part. The higher the hardness, the more effective. As a method for improving the surface hardness, there are carburizing treatment and carbonitriding treatment. Hardness can be improved by solid solution of carbon and nitrogen to precipitate hard carbides and nitrides. In addition, the hardness of the core after quenching is greatly affected by the carbon content of the material, and high-carbon bearing steel has a higher core hardness than low- and medium-carbon case-hardened steel. Become. That is, in order to improve the plastic deformation resistance, high-carbon steel is quenched (more preferably, the high-carbon steel is subjected to carburizing or carbonitriding) to improve the hardness from the surface to the core. This is very important.

  Therefore, by specifying the amount of retained austenite in the surface layer portion of the rolling element, the nitrogen concentration, and the amount of nitride containing silicon and manganese, the pressure resistance and wear resistance are improved, and the rolling element of the rolling element is rotated when the rolling bearing rotates. Increase in tangential force generated between the rolling surface and the raceway surface of the bearing ring is suppressed, so that indentation-based delamination, which tends to occur when used in a contaminated environment with foreign matter, is suppressed, extending the life of the rolling bearing. Is achieved. Furthermore, the plastic ring resistance is improved by configuring the raceway ring with high carbon chromium bearing steel and further subjecting the raceway surface to carburizing or carbonitriding to harden the surface to the core.

  The rolling bearing according to the present invention has a long life because indentation-origin-type peeling hardly occurs even when used under severe conditions such as a foreign matter-mixed lubrication environment.

Embodiments of a rolling bearing according to the present invention will be described in detail with reference to the drawings.
FIG. 1 is a partial longitudinal sectional view showing a structure of a deep groove ball bearing which is an embodiment of a rolling bearing according to the present invention. This deep groove ball bearing has an inner ring 1 having a raceway surface 1a on its outer peripheral surface, an outer ring 2 having a raceway surface 2a opposite to the raceway surface 1a of the inner ring 1 on its inner peripheral surface, and rolling between both raceway surfaces 1a and 2a. A plurality of rolling elements 3 arranged freely, a retainer 4 for holding the rolling elements 3 between the inner ring 1 and the outer ring 2, seals 5 and 5 covering the openings of the gaps between the inner ring 1 and the outer ring 2, The lubrication between the raceway surfaces 1a and 2a and the rolling surface 3a of the rolling element 3 is performed by a lubricant 6 such as grease or lubricating oil. The cage 4 and the seal 5 may not be provided.

In this deep groove ball bearing, the inner ring 1 and the outer ring 2 are made of high carbon chromium bearing steel, and the rolling element 3 has a carbon content of 0.3 mass% or more and 1.2 mass% or less, and a silicon content. Is 0.3 mass% or more and 2.2 mass% or less, and the manganese content is 0.2 mass% or more and 2 mass% or less.
The rolling element 3 is subjected to a heat treatment including a carbonitriding process or a nitriding process, and a surface layer portion (not shown) cured by the heat treatment is formed on the rolling surface 3a. The nitrogen concentration in the surface layer portion is 0.2% by mass or more and 2% by mass or less. In addition, a nitride containing silicon and manganese (hereinafter sometimes referred to as Si—Mn nitride) is deposited on the surface layer portion, and the amount of the deposited Si—Mn nitride is deposited. Is an area ratio of 1% to 20%.

On the other hand, the inner ring 1 and the outer ring 2 are subjected to a heat treatment including a carburizing process or a carbonitriding process, and surface layer portions (not shown) hardened by the heat treatment are formed on the raceway surfaces 1a and 2a. The hardness of the surface layer portion formed on the raceway surfaces 1a and 2a is HRC 58 or more and 66 or less, and the hardness of the inner core portion of the surface layer portion is HRC 56 or more and 64 or less.
Further, the retained austenite amount γ _{R (AB) of} the raceway surfaces 1a and 2a and the retained austenite amount γ _{R (C) of the} rolling surface 3a satisfy the following three expressions.
γ _{R (AB)} −15 ≦ γ _{R (C)} ≦ γ _{R (AB)} +15
γ _{R (AB)} ≧ 0
γ _{R (C)} ≦ 50
However, the unit of numerical values in the above three formulas is volume%.
The deep groove ball bearing of this embodiment has a long life even when used under harsh conditions such as a foreign matter-mixed lubrication environment because it has excellent resistance to plastic deformation in addition to being hard to cause indentation-type peeling. It is.

Below, the structure of this invention and its effect are demonstrated in detail.
[About the point that at least one of the inner ring and outer ring is made of high carbon chromium bearing steel]
In the present invention, at least one of the inner ring and the outer ring needs to be made of high carbon chrome bearing steel (for example, SUJ2 and SUJ3 defined in Japanese Industrial Standard JIS G4805). High carbon chrome bearing steel is extremely stable in quality, including cleanliness. Therefore, race rings made of high carbon chrome bearing steel are less likely to cause internal origin flaking starting from inclusions. Sufficient rolling life can be secured. Moreover, since it is high carbon steel, it can be made high hardness from the surface to a core part by performing appropriate quenching and tempering. In the present invention, the quality of the high carbon chromium bearing steel is preferably higher than the level (bearing quality) that satisfies the cleanliness regulations defined in Japanese Industrial Standard JIS G4805.

  Furthermore, for example, when carburizing or carbonitriding the case-hardened steel such as SCR420, SCM420, etc., a very long heat treatment is required to form a sufficient hardened layer that can withstand an excessive static load. Therefore, the heat treatment cost is remarkably increased. On the other hand, high carbon chromium bearing steel ensures sufficient hardness only by quenching regardless of the presence or absence of carburizing and carbonitriding (for example, a short time equivalent to the soaking time required for quenching and tempering). But it is possible). Therefore, the heat treatment can be performed at low cost.

  In addition, although SUJ2 and SUJ3 were illustrated as high carbon chromium bearing steel, the high carbon chromium bearing steel which can be used in this invention is not limited to these. Since high carbon steel having a hypereutectoid composition can have the same strength as SUJ2 and SUJ3, if the quality such as cleanliness satisfies the bearing quality, the hypereutectoid composition It is possible to use a high carbon steel, for example, a carbon steel or alloy steel having a carbon content of 0.8% by mass or more and 1.2% by mass or less. However, in addition to being inexpensive, SUJ2 has good workability at the time of raw material and workability after heat treatment, and therefore SUJ2 is used in consideration of the balance of bearing life, workability, cost, etc. preferable.

[About the hardness of the surface layer and the core formed on the raceway surface]
As described above, two major functions required for a rolling bearing are life (dynamic load rating) and plastic deformation resistance (static load rating). The factor relating to the material that improves the life and plastic deformation resistance is the hardness. The higher the hardness, the longer the life and the plastic deformation resistance is improved. In particular, a surface layer portion to which a tangential force or a large shear stress acts is required to have high hardness in order to extend the life and improve the plastic deformation resistance.
Furthermore, if the surface layer has a high hardness, the indentation is difficult to form when the rolling bearing is used in a foreign matter-mixed lubrication environment. It is also effective for improving acoustic performance and vibration.

  It is preferable to form a hardened surface layer portion on the raceway surface of the race by performing a heat treatment including a carburizing treatment or a carbonitriding treatment, and the surface layer portion formed on the raceway surface has a hardness of HRC 58 or more, The hardness of the inner core portion of the surface layer portion is preferably HRC56 or higher. And it is more preferable that the hardness of these surface layer parts and core parts is HRC60 or more. However, if the hardness is too high, the toughness may decrease and cracking may occur. Therefore, the hardness of the surface layer portion is preferably HRC 66 or less, and more preferably HRC 64 or less. Moreover, it is preferable that the hardness of a core part is HRC64 or less. The surface layer portion in the present invention is a portion from the surface to a depth of 200 μm.

[Nitrogen concentration in surface layer portion formed on rolling surface and amount of Si-Mn nitride]
In the present invention, carbonitriding or nitriding is performed to enrich the surface layer of the rolling element with nitrogen. Similar to carbon, nitrogen not only acts on solid solution strengthening of martensite and stabilizes retained austenite, but also has the effect of improving the pressure resistance and wear resistance by forming nitrides or carbonitrides.
FIGS. 2 and 3 show the influence of nitrogen on the scratch resistance and wear resistance. FIG. 2 is a graph showing the relationship between the nitrogen concentration in the surface layer portion and the depth of the indentation generated in the surface layer portion, and FIG. 3 is a graph showing the relationship between the nitrogen concentration in the surface layer portion and the wear amount.
The indentation depth in the graph of FIG. 2 was obtained by a pressure-resistant indentation test as shown in FIG. 4, and the wear amount in the graph of FIG. 3 was obtained by a two-cylinder abrasion test as shown in FIG. In the indentation test, a steel ball having a diameter of 2 mm is placed on the sample on which the surface layer portion is formed, and the depth of the indentation generated in the sample is measured when a 5 GPa load is applied to the steel ball from above. (See FIG. 4).

  Next, a method of a two-cylinder wear test will be described. As shown in FIG. 5, cylindrical test pieces 50 and 50 are mounted on two cylindrical shafts 51 and 51 arranged vertically, and the two test pieces 50 and 50 are attached to each other while applying a load from above. Rotated at low speed in the opposite direction in contact. And after rotating for 20 hours with a predetermined slip ratio, the amount of wear of both the test pieces 50 and 50 was calculated | required, and the average value of both was made into the amount of wear.

The lower test piece 50 is a drive-side test piece that is rotationally driven by a motor, and is rotated at a rotational speed of 10 min ⁻¹ . The upper test piece 50 is a driven side test piece that is rotationally driven by a motor via a gear, and is decelerated by the gear and rotated at a rotational speed of 7 min ⁻¹ . Since the rotational speeds of the two test pieces 50 and 50 are different, the slip is forcedly generated. In addition, the surface pressure which acts between the two test pieces 50 and 50 is 0.8 GPa. Further, the outer peripheral surface of the upper test piece 50 is a cylindrical surface, whereas the outer peripheral surface of the lower test piece 50 is substantially spherical as shown in FIG. 5, and both test pieces are in substantially elliptical contact. .
The nitrogen concentration in the surface layer was measured with an electron beam microanalyzer (EPMA). In order to investigate only the influence of the nitrogen concentration, the elements other than the nitrogen concentration in the surface layer portion, that is, the hardness and the amount of retained austenite are made constant for each sample.

2 and 3, it can be seen that the higher the nitrogen concentration in the surface layer portion, the better the wear resistance and pressure scar resistance. And if a nitrogen concentration is 0.2 mass% or more, an effect will be remarkable, but it turns out that 0.45 mass% or more is more preferable.
On the other hand, if the nitrogen concentration is too high, the toughness and static strength may decrease. Since the toughness and static strength are necessary performances for rolling elements of a rolling bearing, it is not preferable that the nitrogen concentration is too high. As can be seen from the result of the Charpy impact test shown in FIG. 6, when the nitrogen concentration exceeds 2% by mass, the toughness rapidly decreases. Therefore, the upper limit of the nitrogen concentration in the surface layer portion is set to 2% by mass.

  As described above, the higher the nitrogen concentration in the surface layer portion, the higher the pressure scar resistance and wear resistance of the material. However, the present inventors, even when the nitrogen concentration is the same, depending on the presence state of nitrogen inside the material (nitrogen may be present in the material as a solid solution and may be deposited as a nitride), We obtained the knowledge that scratch resistance and wear resistance change. Although detailed numerical values will be described later, when carbonitriding is performed on a material containing a large amount of silicon or manganese, even if the nitrogen concentration is the same, the surface layer is more than the amount of nitrogen present in solid solution in the material. The amount of nitrogen present as Si—Mn nitride deposited on the part increases.

FIGS. 7 and 8 show the influence of the amount of Si—Mn nitride on the indentation resistance and wear resistance. FIG. 7 is a graph showing the relationship between the amount of Si—Mn nitride in the surface layer portion and the depth of the indentation generated in the surface layer portion, and FIG. 8 shows the amount and wear of the Si—Mn nitride in the surface layer portion. It is a graph which shows the relationship with quantity.
The depth of the indentation in the graph of FIG. 7 is obtained in the same manner as described above by a pressure-resistant indentation test as shown in FIG. 4, and the wear amount in the graph of FIG. It was obtained in the same manner. In order to investigate only the effect of the amount of Si-Mn nitride, all samples other than the amount of Si-Mn nitride in the surface layer, i.e., the hardness, the amount of retained austenite, and the nitrogen concentration were kept constant. is there.

The amount of the Si—Mn nitride in the surface layer portion was measured using a field emission scanning electron microscope (FE-SEM). That is, the surface was observed at a pressurization voltage of 10 kV, a photograph magnified 5000 times was taken, and at least three fields of view were photographed. Calculated.
From the graphs of FIGS. 7 and 8, it can be seen that the greater the amount of the Si—Mn nitride in the surface layer portion, the better the wear resistance and pressure scar resistance. The effect is remarkable if the amount of Si—Mn nitride is 1% or more in terms of area ratio, but 2% or more is more preferable.

  Next, in order to investigate the influence of the amount of the Si—Mn nitride on the surface layer on the lifetime due to the indentation-origin type peeling, a thrust-type lifetime test was performed in a lubricating environment containing foreign matter. A steel material having a composition as shown in Table 1 (steel type 1 corresponds to SUJ3 and steel type 2 corresponds to SUJ2) was turned into a disk having a diameter of 65 mm and a thickness of 6 mm, and the following heat treatment was performed. . That is, carbonitriding was performed at 820 to 900 ° C. for 2 to 10 hours in a mixed gas of RX gas, propane gas, and ammonia gas, followed by oil quenching and further tempering at 160 to 270 ° C. for 2 hours. Test pieces having surface layer portions with various nitrogen concentrations were produced by changing the carbonitriding treatment temperature, treatment time, and the flow rate of ammonia gas. After such heat treatment, the surface of the test piece was polished and lapped to a mirror finish so that the surface layer portion was not scraped off.

A thrust type life test was performed using the disk obtained as described above. That is, the bearing ring of the thrust ball bearing of the reference number 51305 is a rotating ring, the disk is a fixed ring, and there are six rolling elements (steel balls having a diameter of 3/8 inch) and a steel cage between these rings. To obtain a thrust ball bearing, and the thrust ball bearing was rotated in a lubricating environment containing foreign matter. And when the indentation origin type peeling occurred in the disk, it was considered as the life. The test conditions are as follows.
Load: 5880N
Rotational speed: 1000min ^-1
Lubricant: Lubricating oil whose ISO viscosity grade is ISO VG68 In the lubricant, 200 ppm of fine powder having a hardness of Hv870 and a particle size of 74 to 147 μm is mixed.

Table 2 shows the nitrogen concentration in the surface layer portion of the test piece, the amount of Si—Mn nitride, and the lifetime. Further, the relationship between the nitrogen concentration in the surface layer portion and the amount of Si-Mn nitride is shown in the graph of FIG. 9, and the relationship between the amount of Si-Mn nitride and the lifetime is shown in the graph of FIG. The values of the lifetime of the graph of Table 2 and FIG. 10 shows a relative value when the 1 L ₁₀ life of Comparative Example 1.

  From the graph of FIG. 9, it can be seen that the amount of Si—Mn nitride deposited increases in proportion to the nitrogen concentration. Although details will be described later, it can be seen that, when compared with the same amount of nitrogen, steel having a higher Si and Mn content has a larger amount of Si-Mn nitride and a longer life. Similarly to the scratch resistance and wear resistance, when the amount of the Si—Mn nitride is 1% or more in terms of area ratio and the nitrogen concentration is 0.2% by mass, the life is remarkably improved.

  On the other hand, if the amount of Si—Mn nitride is too large as in the case of the nitrogen concentration, there is a problem that toughness and static strength are lowered. Since toughness and static strength are necessary performances for rolling elements of a rolling bearing, it is not preferable that the amount of Si-Mn nitride is too large. As can be seen from the result of the Charpy impact test shown in FIG. 11, when the amount of Si-Mn nitride exceeds 20%, the toughness rapidly decreases. Therefore, the upper limit of the amount of Si—Mn nitride is 20%, more preferably 10%.

  Note that the maximum diameter of the Si—Mn nitride is preferably 1 μm or less. Nitride having a maximum diameter exceeding 1 μm does not contribute much to the strengthening of steel, and it is effective for strengthening that fine nitrides are dispersed in the steel. The reason for this is that in the precipitation strengthening theory, the smaller the distance between the precipitated particles, the better the strengthening performance. Even if the amount of nitride (area ratio) is the same, the greater the number of precipitated particles, the smaller the distance between the precipitated particles, and the greater the degree of strengthening.

That is, the amount of Si—Mn nitride in the steel is increased (2% to 10% by area ratio) and the number of fine Si—Mn nitride having an average particle size of 0.05 μm to 1 μm. (100 or more in an area of 375 μm ² ) is preferable. In particular, when the number of Si—Mn nitrides having an average particle size of 0.05 μm or more and 0.5 μm or less is 20% or more of the entire Si—Mn nitride, the degree of strengthening can be further increased. FIG. 15 shows the relationship between the number of Si—Mn nitrides having an average particle size of 0.05 μm or more and 1 μm or less (the number in an area of 375 μm ² ) and the lifetime. From this graph, it can be seen that by dispersing 100 or more Si—Mn nitrides in an area of 375 μm ² , the base structure is strengthened and the life in a lubricating environment containing foreign matters is improved.

  In order to obtain such a nitride, the carbonitriding temperature in the heat treatment is preferably set to 800 ° C. or higher and 880 ° C. or lower. If the carbonitriding temperature exceeds 880 ° C., the nitride may become coarse and the number of fine Si—Mn nitrides may decrease. Further, since the solid solution source of nitrogen is increased, the amount of nitride is decreased, and a desired amount may not be obtained. It is preferable that a mixed gas of RX gas and enriched gas is used as an atmosphere from the initial stage of the carbonitriding process, and the carbon potential value is 1.2 or more.

  Moreover, the quenching after carbonitriding is preferably an oil quenching in which the oil temperature is 60 ° C. or higher and 120 ° C. or lower. If the oil temperature exceeds 120 ° C., sufficient hardness may not be obtained. The heating temperature during tempering after quenching is preferably 160 ° C. or higher and 270 ° C. or lower. If desired, subzero treatment may be performed between quenching and tempering after carbonitriding.

[Residual austenite content]
As described above, it has been clarified that when the amount of retained austenite is small, the indentation resistance and the wear resistance are improved, but as the amount of retained austenite on the surface is large, the peeling life is extended. That is, when considering the rolling element as a center, the smaller the amount of retained austenite on the rolling surface of the rolling element, the better the pressure resistance and wear resistance of the rolling element, and the life of the raceway is extended. Lifespan is reduced. Therefore, there is an optimum amount of retained austenite of the rolling elements in order to maximize the bearing life, but the optimum range varies depending on the amount of retained austenite of the race.

If the amount of retained austenite on the rolling surface of the raceway is large, the life of the raceway will be longer, the pressure resistance of the raceway will be reduced, and the tangential force acting between the raceway and the rolling element will also increase. It is necessary to extend the life of the rolling element rather than to improve the pressure resistance and wear resistance of the rolling element. For this reason, when the amount of retained austenite on the raceway surface of the raceway is large, the amount of retained austenite on the rolling surface of the rolling element must also be increased. That is, the range of the retained austenite amount (γ _{R (C)} ) on the rolling contact surface of the rolling element with the longest bearing life varies depending on the retained austenite amount (γ _{R (AB)} ) on the raceway surface. It is preferable to satisfy the following three expressions. However, the unit of numerical values in these three formulas is volume%.
γ _{R (AB)} −15 ≦ γ _{R (C)} ≦ γ _{R (AB)} +15
γ _{R (AB)} ≧ 0
γ _{R (C)} ≦ 50
In addition, if the amount of retained austenite is too large, not only the hardness decreases, but the pressure resistance and wear resistance decrease, and the dimensional stability when used at high temperatures also deteriorates. The amount of retained austenite is preferably 50% by volume or less.

[Surface hardness of the rolling surface of the rolling element]
The surface hardness Hv of the rolling surface of the rolling element is preferably 750 or more, more preferably 800 or more, and further preferably 820 or more. Surface hardness is the most important factor related to a material that improves the scratch resistance and wear resistance. In order to investigate the influence of surface hardness on the pressure scar resistance and wear resistance, the pressure scar resistance test and the two-cylinder wear test described above were performed. The results are shown in FIGS.

  FIG. 12 is a graph showing the relationship between surface hardness and pressure dent resistance, and FIG. 13 is a graph showing the relationship between surface hardness and wear resistance. From these graphs, it can be seen that the higher the surface hardness is, the better the pressure dent and wear resistance is. In particular, it can be seen that when the surface hardness is Hv 750 or higher, both the scratch resistance and the wear resistance are extremely excellent. Also, it is known that the higher the surface hardness, the higher the fatigue strength. By increasing the surface hardness of the rolling surface of the rolling element, not only the indentation resistance and wear resistance but also the indentation origin peel strength Can also be improved.

Next, alloy elements (carbon, silicon, manganese, etc.) contained in the steel constituting the rolling elements will be described.
[Carbon content]
Carbon is an important element for obtaining the strength and life required for steel. When the carbon content is too small, not only the sufficient strength cannot be obtained, but also the heat treatment time for obtaining the required hardened layer depth becomes longer during the carburizing treatment or carbonitriding treatment described later, and the heat treatment cost is reduced. It leads to increase. Therefore, the carbon content is preferably 0.3% by mass or more, more preferably 0.5% by mass or more, and further preferably 0.95% by mass or more. On the other hand, if the carbon content is too large, huge carbides are produced during steelmaking, which adversely affects the subsequent hardenability and rolling fatigue life, and the heading property is lowered, leading to an increase in cost. Therefore, the carbon content is preferably 1.2% by mass or less, and more preferably 1.1% by mass or less.

[Contents of silicon and manganese]
Silicon and manganese are elements necessary for precipitation of Si—Mn nitride, and in order to deposit a sufficient amount of Si—Mn nitride, it is necessary that a large amount of silicon and manganese be contained in the steel. is there. In the case of SUJ2 (Si content: 0.25 mass%, Mn content: 0.4 mass%), which is a general bearing material, Si-Mn-based nitridation is possible even when nitrogen is excessively added by carbonitriding or the like. The amount of things is insufficient. For this reason, it is preferable that content of silicon and manganese is the following values.

  In order to precipitate a sufficient amount of Si—Mn nitride, the silicon content is preferably 0.3% by mass or more, and more preferably 0.4% by mass or more. However, if the silicon content is too large, the toughness of the steel is reduced or the diffusion of nitrogen into the core is hindered. Therefore, the content is preferably 2.2% by mass or less, and 0.7% by mass or less. More preferably. Moreover, precipitation of Si-Mn type nitride is accelerated | stimulated as content of manganese is 0.2 mass% or more (preferably 0.9 mass% or more). However, since manganese has a function of stabilizing the retained austenite, if the manganese content is too large, the amount of retained austenite after the heat treatment may be more than necessary. Therefore, the manganese content is preferably 2% by mass or less, and more preferably 1.15% by mass or less.

[Ratio of silicon and manganese]
Unlike the nitride by tempering, the Si—Mn nitride in the present invention is formed by reacting nitrogen with silicon while incorporating manganese. Therefore, if the content of manganese is less than the content of silicon in the steel, precipitation of Si—Mn nitride is not promoted even if nitrogen is sufficiently diffused. In the case where nitrogen is intruded into a steel having a silicon content of 0.3% by mass to 2.2% by mass and a manganese content of 0.2% by mass to 2% by mass, the Si—Mn system In order to promote the precipitation of nitride, the ratio Si / Mn between the silicon content and the manganese content is preferably 5 or less.

[Chromium content]
Steel is often also added with chromium as an alloy component. Chromium improves the hardenability and is a carbide forming element, and has an effect of promoting precipitation and further refinement of carbides strengthening the steel. In order to sufficiently obtain such an action, the chromium content is preferably 0.5% by mass or more, and more preferably 0.9% by mass or more. If it is less than 0.5% by mass, the hardenability may be lowered and sufficient hardness may not be obtained, or the carbide may be coarsened during carbonitriding.

However, if the chromium content is too large, a chromium oxide film is formed on the surface during the carbonitriding process, and the diffusion of carbon and nitrogen may be inhibited. 1.2% by mass or less is more preferable.
In this invention, you may further add other alloy components, such as molybdenum (Mo), nickel (Ni), vanadium (V), to the steel which comprises a rolling element as needed.

  In addition, this embodiment shows an example of this invention and this invention is not limited to this embodiment. For example, in the present embodiment, a deep groove ball bearing has been described as an example of a rolling bearing, but the present invention can be applied to various types of rolling bearings. For example, radial rolling bearings such as angular contact ball bearings, self-aligning ball bearings, cylindrical roller bearings, tapered roller bearings, needle roller bearings, and self-aligning roller bearings, and thrust types such as thrust ball bearings and thrust roller bearings This is a rolling bearing.

Hereinafter, the present invention will be described more specifically with reference to examples. Tapered roller bearings with various configurations (nominal number L44649 / 610) were prepared, and a life test and an excessive static load test were performed.
First, the tapered roller bearing used for the test will be described (see Tables 3 to 6). The inner and outer rings of the tapered roller bearings of Examples 1 to 46 and Comparative Examples 2 and 3 are made of high carbon chromium bearing steel (SUJ2). And the heat processing which consists of a carbonitriding process, a carburizing process, or submerged quenching and tempering is performed. The condition of the carbonitriding process is to hold at 830 to 850 ° C. for 1 to 3 hours in an atmosphere composed of RX gas, enriched gas, and ammonia gas. The carburizing condition is to hold at 830 to 850 ° C. for 1 to 3 hours in an atmosphere composed of RX gas and enriched gas. The sub-quenching is an oil cooling after holding at 830 to 850 ° C. for 1 hour in an RX gas atmosphere. Moreover, the conditions of tempering are that it cools, after hold | maintaining at 180-240 degreeC.

By such heat treatment, the amount of retained austenite on the raceway surfaces of the inner and outer rings is set to 10% by volume, 20% by volume, or 30% by volume.
On the other hand, the inner and outer rings of the tapered roller bearing of Comparative Example 1 are made of case-hardened steel SCr420. And the heat processing which consists of a carburizing quenching process and tempering is given. The conditions for the carburizing and quenching treatment are oil cooling after holding at 920 to 950 ° C. for 3 to 8 hours in an atmosphere composed of RX gas and enriched gas. Moreover, the conditions of tempering are that it cools, after hold | maintaining at 180-240 degreeC.

  Next, the rolling elements of the tapered roller bearings of Examples 1 to 46 and Comparative Examples 1 to 3 are made of steel having compositions shown in Tables 3 to 6. A method for manufacturing these rolling elements will be described. First, a tapered roller-shaped member is manufactured from a steel wire by header processing and rough grinding, and a mixed gas atmosphere of RX gas, enriched gas, and ammonia gas is formed in this member. After carbonitriding and quenching at 830 ° C. for 5 to 20 hours, tempering was performed at 180 to 270 ° C. And post-processes, such as finishing, were performed and the rolling element was obtained.

  For the raceway and rolling element thus obtained, the surface hardness HRC (surface layer hardness) of the raceway surface, the hardness HRC of the raceway core, and the residual austenite of the raceway raceway surface Amount, surface hardness Hv (roller surface hardness) of rolling element, amount of retained austenite on rolling element surface, nitrogen concentration in rolling element surface layer, and precipitation on rolling element surface layer part The amount (area ratio) of the Si—Mn-based nitride being measured was measured.

  The nitrogen concentration was measured by quantitative analysis using an electron beam microanalyzer (EPMA). The amount of retained austenite was measured by the X-ray diffraction method. In both cases, the surface of the rolling element was directly analyzed and measured. Furthermore, the amount of Si-Mn nitride was measured using a field emission scanning electron microscope (FE-SEM). That is, the surface was observed with a pressurization voltage of 10 kV, a photograph magnified 5000 times was taken, and at least three fields of view were photographed. After binarizing the photograph, the amount of Si-Mn nitride was measured by area ratio using an image analyzer. Calculated. Furthermore, the hardness was measured with a hardness meter. The measurement results are summarized in Tables 3-6.

Next, the method of a life test and an excessive static load test will be described. The life test was performed by rotating the tapered roller bearing under the following conditions in a foreign matter-mixed lubricating environment. And when peeling generate | occur | produced on the raceway surface of a bearing ring or the rolling surface of a rolling element, the rotation time until then was made into the lifetime.
In the life test, 12 tests were performed for one type of bearing. Then, a Weibull plot was created, the L ₁₀ life was obtained from the result of the Weibull distribution, and this was taken as the life. The results are shown in Tables 3 to 6 and the graph of FIG. In addition, the lifetime of Tables 3-6 and FIG. 14 is shown by the relative value when the lifetime of the comparative example 2 which was the shortest lifetime is set to 1.
Radial load: 12kN
Axial load: 3.5kN
Rotational speed: 3000min ^-1
Lubricant: Lubricating oil whose ISO viscosity grade is ISO VG68 200 ppm of fine powder having a hardness of Hv 870 and a particle size of 74 to 134 μm is mixed in the lubricant as foreign matter.

  On the other hand, the excessive static load test was performed by applying a radial load of 32 kN to a tapered roller bearing similar to that used in the life test for 30 seconds to cause permanent deformation of the race and the tapered roller. Then, the permanent deformation that occurred in the inner ring after unloading and the permanent deformation that occurred in the center of the tapered roller were measured, the sum of the permanent deformation amount of both was calculated, and this was used as the permanent deformation amount of the tapered roller bearing. . The amount of permanent deformation was measured using Foam Talysurf manufactured by Taylor Hobson. The results are shown in Tables 3-6. The values of the amount of permanent deformation in Tables 3 to 6 are shown as relative values when the value of Comparative Example 1 having the largest amount of permanent deformation is 1.

  The rolling bearing of the present invention can be suitably used for transmissions, engines, and the like of automobiles, agricultural machines, construction machines, and steel machines.

It is a fragmentary longitudinal cross-section which shows the structure of the deep groove ball bearing which is one Embodiment of the rolling bearing which concerns on this invention. It is a graph which shows the relationship between the nitrogen concentration of a surface layer part, and the depth of an indentation. It is a graph which shows the relationship between the nitrogen concentration of a surface layer part, and the amount of wear. It is the schematic explaining the test method of a pressure | voltage resistant test. It is the schematic explaining the test method of a two-cylinder abrasion test. It is a graph which shows the relationship between the nitrogen concentration of a surface layer part, and the impact absorption energy by a Charpy impact test. It is a graph which shows the relationship between the quantity of the Si-Mn type nitride of a surface layer part, and the depth of an indentation. It is a graph which shows the relationship between the quantity of the Si-Mn type nitride of a surface layer part, and the amount of wear. It is a graph which shows the relationship between the nitrogen concentration of a surface layer part, and the quantity of Si-Mn type nitride. It is a graph which shows the relationship between the quantity of Si-Mn type nitride, and lifetime. It is a graph which shows the relationship between the quantity of Si-Mn type nitride, and the impact absorption energy by a Charpy impact test. It is a graph which shows the relationship between the surface hardness of the rolling surface of a rolling element, and the depth of an indentation. It is a graph which shows the relationship between the surface hardness of the rolling surface of a rolling element, and wear amount. It is a graph which shows the relationship between the amount of retained austenite of the rolling surface of a rolling element, and the lifetime of a bearing. 3 is a graph showing the relationship between the number of Si—Mn nitrides (average number in an area of 375 μm 2) having an average particle diameter of 0.05 μm or more and 1 μm or less and the lifetime.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Inner ring 1a Raceway surface 2 Outer ring 2a Raceway surface 3 Rolling element 3a Rolling surface

Claims (4)

In a rolling bearing comprising: an inner ring having a raceway surface; an outer ring having a raceway surface facing the raceway surface of the inner ring; and a plurality of rolling elements arranged to be freely rollable between the both raceway surfaces. A rolling bearing characterized by satisfying two conditions.
Condition A: At least one of the inner ring and the outer ring is made of high carbon chrome bearing steel.
Condition B: The rolling element is made of steel, and a surface layer part formed by heat treatment including carbonitriding or nitriding is formed on the rolling surface.
Condition C: The nitrogen concentration in the surface layer portion is 0.2% by mass or more and 2% by mass or less.
Condition D: A nitride containing silicon and manganese is deposited on the surface layer portion, and the amount of the deposited nitride is 1% or more and 20% or less in terms of area ratio. Of the inner ring and the outer ring, the retained austenite amount γ _{R (AB)} of the raceway surface of the bearing ring made of high carbon chrome bearing steel and the retained austenite amount γ _{R (C)} of the rolling surface of the rolling element The rolling bearing according to claim 1, wherein the following three expressions are satisfied.
γ _{R (AB)} −15 ≦ γ _{R (C)} ≦ γ _{R (AB)} +15
γ _{R (AB)} ≧ 0
γ _{R (C)} ≦ 50
However, the unit of numerical values in the above three formulas is volume%.   Of the inner ring and the outer ring, the raceway surface of the raceway ring made of high carbon chromium bearing steel is formed with a surface layer portion that is hardened by heat treatment including carburizing treatment or carbonitriding treatment. The rolling bearing according to claim 2, wherein the hardness is HRC58 or more and 66 or less, and the hardness of the inner core portion of the surface layer portion is HRC56 or more and 64 or less.   The rolling element has a carbon content of 0.3% by mass to 1.2% by mass, a silicon content of 0.3% by mass to 2.2% by mass, and a manganese content of 0.2% by mass. The rolling bearing according to any one of claims 1 to 3, wherein the rolling bearing is made of steel having a mass% of 2% by mass or less.

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