JP2002339988A - Roller bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a roller bearing of a long service life hardly causing breakage in an early stage even when a large load is applied thereto. SOLUTION: In the cylindrical roller bearing comprising an outer ring 1, an inner ring 2 and a plurality of rollers 3 disposed between the outer ring 1 and the inner ring 2 in a rolling manner, the outer ring 1 and the inner ring 2 are formed of case hardening steel with ferrite content (area ratio) of 0%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a long-life rolling bearing which is not easily damaged even when a large load is applied.

[0002]

2. Description of the Related Art In a rolling bearing, a high contact surface pressure is generated at a contact portion between a bearing ring and a rolling element. Therefore, in order to prevent plastic deformation at the contacting section, a rolling bearing and a rolling element are manufactured. Is subjected to a quenching process. In particular, in rolling bearings that require impact resistance and cracking resistance (resistance to cracking even if peeling occurs), such as rolling bearings used in steel rolling mills, raceways or rolling elements Or both of them are made of case hardened steel, subjected to carburizing treatment or carbonitriding treatment, and then subjected to quenching treatment in many cases.

In this case, a case hardening steel having a necessary hardenability is selected as the case hardening steel to be used in accordance with the thickness of the rolling bearing parts (the bearing ring and the rolling elements). That is, the hardness of at least the raceway surface of the bearing ring and the rolling surface of the rolling element is HRC5
The case hardened steel is selected so as to be 8 or more. The hardness inside the races and the rolling elements is 6 times or more the shear stress (unit: kgf / mm ² ) at which the Vickers hardness Hv at an arbitrary depth is expected to act at that depth. It is set as follows. The unit of the shear stress is M
In the case of Pa, the Vickers hardness Hv is at least about 0.6 times the expected shear stress at that depth.

[0004] As described above, the condition for selecting case hardening steel used for a rolling bearing is only the surface and internal hardness, and it has been conventionally considered that the presence of ferrite is acceptable.

[0005]

However, even if the Vickers hardness Hv at an arbitrary depth is about 0.6 times or more of the shear stress (unit: MPa) expected to act at the depth, the rolling bearing is required. Has been found to be damaged early, and it has been found that satisfying the above conditions may not be enough.

[0006] This is a macro-measurement of hardness, and the effects of both martensite and ferrite are averaged. Therefore, the adverse effect of ferrite is judged only from hardness (shear stress). It is thought that it is not possible. On the other hand, in order for a rolling bearing to receive a large load within a limited size, it is necessary to use a roller bearing as the bearing type and further increase the roller diameter. This will inevitably reduce the thickness of the bearing ring, but if the bearing ring is manufactured by carburizing and the thickness of the bearing ring is less than half the roller diameter, the boundary between the carburized layer and the core There was a problem that the residual tensile stress increased in the portion. In addition, when rolling contact stress is applied to the bearing ring and the rollers, cracks occur at the boundary between the carburized layer and the core, and there is a problem that the bearing ring eventually cracks. . Moreover, standards for the thickness of the bearing ring and the roller diameter have not been set in the past.

[0007] Therefore, an object of the present invention is to solve the problems of the prior art and to provide a long-life rolling bearing which is less likely to be damaged early even when a large load is applied.

[0008]

In order to solve the above problems, the present invention has the following arrangement. That is, the rolling bearing of claim 1 according to the present invention is a rolling bearing including an outer ring, an inner ring, and a plurality of rolling elements disposed so as to freely roll between the outer ring and the inner ring. At least one of the outer ring and the inner ring is made of case hardened steel having a ferrite content (area ratio) of 0.1% or less.

When ferrite is present in the races (outer race, inner race) and a shear stress is applied thereto, the ferrite undergoes plastic deformation as if it were rolled, and is pulled in a direction perpendicular to the raceway surface. Residual stress occurs. Even if the Vickers hardness Hv of the case hardening steel containing ferrite is 360, since the shear yield stress of the ferrite is about 49 MPa, the ferrite is plastically deformed when a shear stress exceeding 49 MPa acts thereon.

[0010] On the other hand, the allowable shear stress of case hardening steel containing a small amount of ferrite and a carbon content of 0.2% is about 588 MPa based on the above-mentioned standard, but greatly exceeds the shear yield stress of ferrite of 49 MPa. However, when such a stress is actually applied to the bearing ring, the ferrite is plastically deformed and a residual stress is generated around the ferrite. The residual stress generated here becomes a tensile force in the direction perpendicular to the raceway surface, and is in the same direction as this tensile force and overlaps with the tensile stress due to the heat treatment originally present in case hardening steel, significantly increasing the fatigue resistance. Lower. Further, if non-metallic inclusions are present, fatigue cracks easily occur, causing breakage or peeling.

In order to prevent such a decrease in fatigue resistance and breakage, it is necessary to form the raceway ring from a case hardened steel having a completely hardened structure with a ferrite content of 0.1% or less. In the rolling elements, the presence of ferrite is allowed. Because the length of the rolling element is generally smaller than the width of the raceway, the carburized layer on the outer surface of the rolling element acts like a stick or a column for radial loads, even if ferrite is present. This is because plastic deformation is small and the probability of breakage is small.

According to a second aspect of the present invention, there is provided a roller bearing comprising an outer ring, an inner ring, and a plurality of rollers rotatably disposed between the outer ring and the inner ring. At least one of the outer ring and the inner ring is made of case hardened steel and satisfies the following expression. D _I ≧ 4.5 × T _max where D _I is the ideal critical diameter of the case hardened steel,
_max is the maximum value of the thickness of a portion of the outer ring and the inner ring having a raceway surface of the raceway ring made of the case hardened steel.

With a roller bearing having such a configuration, even if a large load is applied, premature breakage hardly occurs and a long life is obtained. In order to obtain a case hardened steel having a completely hardened structure with a ferrite content of 0.1% or less, it is necessary that the case hardened steel has sufficient hardenability. The present inventors have studied hardenability sufficient to prevent the above-mentioned breakage, and found that the ideal critical diameter D _I (unit: mm) needs to satisfy the above expression. That is, D
_{If I} is increased or _Tmax is decreased and the above expression is satisfied, even if a large load is applied, early damage is less likely to occur, and a long-life roller bearing can be obtained.

_Tmax is the maximum value of the thickness of the bearing ring in the portion having the raceway surface, and does not take into account the thickness of the portion having no raceway surface, that is, the portion having the flange. _DI is calculated by the following equation. D _I = 25.4 × (D _IC × f _Si × f _Mn × f _Ni × f _Cr × f
_Mo ) D _IC = 0.3112 × (% C) ^0.4978 f _Si = 1 + (% Si) × 0.7 f _Mn = 1 + ｛0.5173 × (% Mn) ³ −0.704
1 × (% Mn) ² + 3.533 × (% Mn)｝ f _Ni = 1 + ｛0.0565 (% Ni) ³ -0.1408
^{× (% Ni) 2 + 0.4396} × (% Ni)} f Cr = 1 + (% Cr) × 2.16 where _{f Mo = 1 + (% Mo} ) × 3, (% M) is hardening steel in Of the element M in (% by weight)
Represents

Such a roller bearing is effective for a roller bearing which receives a large load, particularly when the roller diameter is larger than the thickness of the race (the thickness of the race at the portion having the raceway surface). . More specifically, when the average diameter of the rollers is equal to or more than the sum of the average thickness of the inner ring and the average thickness of the outer ring (in each case, the average thickness of the races in the portion having the raceway surface), This is particularly effective when the diameter is at least 25% of the difference between the bearing outer diameter and the bearing inner diameter.

That is, in the roller bearing according to claim 3 of the present invention, in the roller bearing according to claim 2, the arithmetic mean value of the maximum value and the minimum value of the diameter of the roller is the bearing outer diameter and the bearing inner diameter. Is at least 25% of the difference between the two. If the arithmetic mean value of the maximum value and the minimum value of the diameter of the roller is less than 25% of the difference between the bearing outer diameter and the bearing inner diameter, early damage is likely to occur when a large load is applied.

Further, according to a fourth aspect of the present invention, there is provided a roller bearing including an outer ring, an inner ring, and a plurality of rollers rotatably disposed between the outer ring and the inner ring. The outer ring and the inner ring are made of case-hardened steel, and the minimum value of the thickness of the outer ring and the inner ring at the portion having the raceway surface is 50 times the arithmetic average of the maximum value and the minimum value of the roller diameter. % Or more.

The minimum value of the thickness of the outer ring and the inner ring having the raceway surface is less than 50% of the arithmetic average of the maximum and minimum values of the diameter of the rollers, and the raceway is subjected to carburizing. When manufactured, the residual tensile stress increases at the boundary between the carburized layer and the core, and the repetitive stress accompanying the passage of the rolling elements is superimposed, thereby causing cracks at the boundary. Here, the carburizing depth is
The Vickers hardness Hv refers to the thickness of the surface layer having a thickness of 633 or more. In the roller bearing according to the fourth aspect of the present invention, the carburized depth of the raceway surface of the race is 2 to 5% of the average diameter of the roller. A certain roller bearing is targeted.

If the minimum value of the thickness of the outer race and the inner race in the portion having the raceway surface is 50% or more of the arithmetic average of the maximum and minimum values of the diameter of the rollers, the carburization for the thickness is performed. Since the thickness of the layer is reduced, the residual tensile stress in the race is reduced, and cracks are less likely to occur. In addition, since no tensile stress exists inside the soot-baked race, no cracking occurs.

Further, the roller bearing according to claim 5 of the present invention is characterized in that, in the roller bearing according to claim 4, the nitrogen concentration of the case hardened steel is 100 ppm or less. Since the starting point of the crack generated by the residual tensile stress in the bearing ring is iron nitride, the nitrogen concentration in the case hardening steel is set to 100%.
Cracking can be suppressed by reducing it to ppm or less. If the nitrogen concentration is more than 100 ppm, the above-mentioned suppression effect becomes insufficient.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a rolling bearing according to the present invention will be described in detail with reference to the drawings. (First Embodiment) FIG. 1 is a longitudinal sectional view showing the structure of a four-row cylindrical roller bearing which is an embodiment of the rolling bearing according to the present invention.

This four-row cylindrical roller bearing (bearing outer diameter φD: 8
50 mm, bearing inner diameter φd: 600 mm, bearing widths B ₂ and C ₂ : 640 mm, roller inscribed circle diameter φF _W : 664 mm,
Chamfer dimensions r ₁ and r ₂ : 6 mm, basic dynamic load rating: 1
2544 kN, rated fatigue life: 208 hours)
An inner ring 2, a plurality of rollers 3 rotatably disposed between the outer ring 1 and the inner ring 2, a retainer 4 for holding the rollers 3,
It is composed of

Further, the thickness T ₁ and T ₂ in the portion having a raceway surface of the outer ring 1 and inner ring 2 is 28mm and 32mm
And the length of the roller 3 is 114 mm. The outer ring 1 and the inner ring 2 are made of case hardened steel, and the ferrite content (area ratio) is 0%. Ferrite content is determined by JI
Based on SHNCM2 steel, the content was adjusted by the concentration of elements such as Ni, Mo, Cr, and Mn, which have the effect of improving the hardenability of case hardened steel. However, the carbon concentration of the case hardened steel was fixed at 0.2%. The total carburizing depth was 8 mm. The ferrite content (area ratio) was measured at a depth of 8 mm (8 m from the raceway surface), which is the boundary between the carburized layer and the core.
m depth position) using a microscopic method.

The rollers 3 are manufactured by carburizing and quenching case hardening steel having a carbon concentration of 0.2%. The total carburized depth of the roller 3 is 8 mm, the same as the outer ring 1 and the inner ring 2, and 8 mm.
The ferrite content (area ratio) at the depth (at a position 8 mm deep from the rolling surface) is 5%. FIG. 2 shows the distribution of shear stress applied to the outer ring 1 of such a cylindrical roller bearing. This graph shows a shear stress distribution calculated by applying the Herz contact theory to the outer ring 1.

FIG. 3 shows the distribution of the hardness of the outer ring 1 in the depth direction. The above-mentioned conventional standard (the value of 1 / 0.6 of the Vickers hardness Hv is the yield stress (unit: MPa)). Fig. 4 shows the allowable shear stress distribution of the outer ring 1 according to
Shown in In the graphs of FIGS. 3 and 4, the outer ring composed of case hardening steel having a ferrite content of 0.1%, 1%, 5%, and 20% is also shown. The ferrite content is 0%, 0.1%, 1%, 5%, and 20% in order from the line located. The hardness and allowable shear stress are measured on the surface (0 mm depth)
And a depth of 8.0 mm at which ferrite begins to appear and a depth of 14.0 mm which is １ ／ of the thickness of the outer ring (that is, the center).

As can be seen by comparing FIGS. 2 and 4, at any depth, the applied shear stress is significantly lower than the allowable shear stress of the conventional standard. For such a cylindrical roller bearing, a case where the ferrite content of the case hardening steel constituting the outer ring 1 is set to 0%, 0.1%, 1%, 5%, and 20% is prepared. Petroleum Corporation VG
460) while using a radial load of 5880 kN,
A rotation test was performed under the conditions of a rotation speed of 1000 rpm and a temperature of 120 ° C. In addition, three rotation tests were performed for each type of bearing. The ferrite content is based on JIS HNC
Based on M2 steel, it was adjusted by the concentration of elements such as Ni, Mo, Cr, and Mn. However, the carbon concentration is 0.2
%, And the total carburized depth was 8 mm.

The results of the rotation test are shown in Table 1. None of the cylindrical roller bearings was damaged in a time shorter than the rated fatigue life (208 hours), but the higher the ferrite content, the shorter the cracks tended to occur. This cracking occurred at the boundary between the carburized layer and the core in the outer and inner rings 1 and 2. And the ferrite content is 0.1%
When the content was less than the above, the life was excellent. When the content of ferrite was 0%, cracking and peeling did not occur even for 10 times the rated fatigue life.

Such a long-life cylindrical roller bearing is unlikely to cause premature breakage even when a large load is applied, and thus can be suitably used for a steel rolling mill or the like.

[0029]

[Table 1]

Thus, the shear stress (unit is MPa)
Even though it has a Vickers hardness of 0.6 times or more, the crack life of the bearing differs depending on the presence or absence of ferrite,
It was found that the ferrite content at the boundary between the carburized layer and the core was preferably 0.1% or less, more preferably 0%. That is, it was found that it was difficult to predict the life of the rolling bearing only from the hardness and the allowable shear stress, and it was necessary to control the ferrite content.

In the present embodiment, the correlation between the ferrite content in the outer ring and the life of the bearing has been examined. However, the same tendency applies to the inner ring. Also,
The present embodiment is an example of the present invention, and the present invention is not limited to the present embodiment. For example, in the first embodiment, a four-row cylindrical roller bearing has been described as an example of the rolling bearing, but the rolling bearing of the present invention is applicable to other types of cylindrical roller bearings and other types of various rolling bearings. Can be applied. More specifically, for example, radial rolling bearings such as deep groove ball bearings, angular ball bearings, tapered roller bearings, needle roller bearings, and self-aligning roller bearings, and thrust types such as thrust ball bearings and thrust roller bearings. Rolling bearings.

(Second Embodiment) Rolling shaft of second embodiment
The structure of the bearing is the same as the four-row cylindrical roller bearing of the first embodiment.
Therefore, the description is omitted. However, outer ring and inner ring
Of case hardening steel that constitutes _IIs 200 mm. like this
Of the hardness of the outer ring of the cylindrical roller bearing in the depth direction, and
The allowable shear stress distribution of the outer ring based on the conventional standard described
It is the same as the case of the embodiment (ie, FIG. 3, FIG. 4)
It is the same as the graph in FIG.

With respect to such a cylindrical roller bearing, those having various cases of D _I (hardenability) of the case hardened steel constituting the outer ring were prepared and subjected to the same rotation test as in the first embodiment.
The results of the rotation test (life), the value of D _I / T _max of the bearing (the ratio of D _{I of the case} hardened steel constituting the outer ring to the maximum value T _max of the thickness of the outer ring in the portion having the raceway surface), and Table 2 shows the ferrite content (area ratio). The rotation test was 1
Three tests were performed for each type of bearing, and the bearing life was determined by averaging the three results.

[0034]

[Table 2]

FIG. 5 shows the relationship between the ferrite content and the life, and FIG. 6 shows the relationship between the value of D _I / T _max and the life.
Shown in As can be seen from FIG. 5, there is a tendency that the lower the ferrite content, the better the life. However, it can be seen that there is variation in the life when the ferrite content is 0%.

[0036] Therefore, referring to FIG. 6, there is a tendency of excellent life as the value of D _I / T _max is large, D _I
When the value of / T _max is 4.5 or more, it can be seen that the life is extremely excellent. As described above, it may be difficult to determine that ferrite is completely 0% with a metallurgical microscope, and in such a case, the life can be predicted from the value of D _I / T _max . In addition, the form of damage of the bearing is that the outer ring has cracked in all bearings,
The starting point was ferrite with a depth of about 8 mm.

Next, in the above four-row cylindrical roller bearing,
The bearing outer diameter, inner ring shape, and basic dynamic load rating remain unchanged.
By adjusting the roller diameter, the roller length, the number of rollers, the raceway diameter of the inner and outer rings, and the width of the raceway surface, bearings having various roller diameters were manufactured and subjected to the same rotation test as described above. As a result, when the roller diameter was less than 62.5 mm, that is, when the roller diameter was less than 25% of the difference between the bearing outer diameter and the bearing inner diameter, no crack occurred in the outer ring.

[0038] In the present embodiment has described the correlation between D _I and the bearing life of hardening steel constituting the outer ring, there is a similar tendency for the inner ring. (Third Embodiment) The configuration of the rolling bearing of the third embodiment is as follows.
Since it is almost the same as the four-row cylindrical roller bearing of the first embodiment, it will be described with reference to FIG.

Four-row cylindrical roller bearing of the third embodiment (bearing outer diameter φD: 850 mm, bearing inner diameter φd: 600 mm, bearing widths B ₂ and C ₂ : 640 mm, roller inscribed circle diameter φF _W : 6)
64 mm, chamfer dimensions r ₁ and r ₂ : 6 mm, basic dynamic load rating: 9800 kN, rated fatigue life: 208 hours)
Is composed of an outer ring 1, an inner ring 2, a plurality of rollers 3 rotatably disposed between the outer ring 1 and the inner ring 2, and a retainer 4 for holding the rollers 3.

The thicknesses T ₁ and T ₂ of the outer race 1 and the inner race 2 having the raceway surfaces are both 30 mm, and the diameter of the rollers 3 is 65 mm. Outer ring 1 and inner ring 2
Is made of case hardened steel SAE9315 and the carburizing depth is 2% of the roller diameter. In such a cylindrical roller bearing, the bearing outer diameter, bearing inner diameter, bearing width, and basic dynamic load rating are not changed, and the roller diameter, roller length, number of rollers, inner and outer raceway diameter, and raceway width are changed. Adjusted to produce bearings with various roller diameters and subjected to the same rotational tests as above. FIG. 7 shows the result.

The ratio between the thickness of the bearing ring (the minimum value of the thickness of the outer ring and the inner ring having the raceway surface) and the diameter (hereinafter, referred to as [thickness of the bearing ring] / [roller average diameter]) Is 0.5 or more, the life is excellent, and the damaged form is peeled. On the other hand, when [thickness of raceway ring] / [roller average diameter] is less than 0.5, cracks occur at the boundary between the carburized layer and the core in all bearings, and the life is shortened. Was not enough.

When the race was manufactured from soaked steel, cracking did not occur. However, regardless of the value of [thickness of race] / [roller average diameter], the time for peeling (peeling life) was 400. ~ 500 hours. In the present embodiment, the outer ring and the inner ring have the same thickness, but when the thicknesses are different, the thinner of the outer ring and the inner ring may be considered. When the thickness of the bearing ring is not constant as in this embodiment, the minimum value may be considered. Further, when the rollers are tapered rollers, convex rollers, or the like, the roller diameter may be an arithmetic average of the maximum value and the minimum value of the roller diameter. That is, the present invention is not limited to the cylindrical roller bearing as in the present embodiment, but can be applied to various roller bearings such as tapered roller bearings and convex roller bearings.

(Fourth Embodiment) The configuration of the rolling bearing of the fourth embodiment is the same as that of the four-row cylindrical roller bearing of the third embodiment, and a description thereof will be omitted. However, the basic dynamic load rating is 12544 kN. In such a cylindrical roller bearing, the bearing outer diameter, bearing inner diameter, bearing width, and basic dynamic load rating are not changed, and the roller diameter, roller length, number of rollers, inner and outer raceway diameter, and raceway width are changed. Adjusted to produce bearings with various roller diameters and subjected to the same rotational tests as above. Since the result is the same as that of FIG. 7, the description is omitted.

Next, in the above four-row cylindrical roller bearing,
The outer ring and the inner ring were formed using case hardening steel SAE9315 having variously changed nitrogen concentrations, and various cylindrical roller bearings were manufactured and subjected to the same rotation test as described above. FIG. 8 shows the result. In the plots in FIG.
ppm indicates the case, and □ indicates the case of 100 ppm. The mark x is 110 ppm, and the mark o is 12 ppm.
In the case of 0 ppm, * mark shows the case of 150 ppm, respectively.

As can be seen from FIG. 8, when the nitrogen concentration of the case hardening steel is 100 ppm or less, the life is excellent irrespective of [thickness of the bearing ring] / [roller average diameter]. In other words, when the nitrogen concentration is 100 ppm or less, the amount of iron nitride, which is the starting point of cracking, becomes small, so that cracking is suppressed and the form of breakage is exfoliated, so that the roller bearing has a long life.
Further, when [thickness of raceway ring] / [roller average diameter] is 0.5 or more, the roller bearing can be stably provided with a long life.

In the case of the present embodiment, similarly to the third embodiment, when the thicknesses of the outer ring and the inner ring are different, it is only necessary to consider the smaller thickness of the outer ring and the inner ring. When the thickness of the bearing ring is not constant as in this embodiment, the minimum value may be considered. Further, when the rollers are tapered rollers, convex rollers, or the like, the roller diameter may be an arithmetic average of the maximum value and the minimum value of the roller diameter.

[0047]

As described above, the rolling bearing according to the first aspect of the present invention and the roller bearing according to the second to fifth aspects of the present invention hardly cause early damage even when a large load is applied, and have a long life.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing a structure of a four-row cylindrical roller bearing which is an embodiment of a rolling bearing of the present invention.

FIG. 2 is a graph showing a correlation between a depth from a raceway surface and a magnitude of applied shear stress.

FIG. 3 is a graph showing a correlation between a depth from a raceway surface and Vickers hardness.

FIG. 4 is a graph showing a correlation between a depth from a raceway surface and an allowable shear stress.

FIG. 5 is a graph showing a correlation between ferrite content and life.

FIG. 6 is a graph showing the correlation between the value of D _I / T _max and the lifetime.

FIG. 7 is a graph showing a correlation between [thickness of raceway ring] / [roller average diameter] and service life.

FIG. 8: [Ring ring thickness] / various nitrogen concentrations
It is a graph which shows the correlation between [roller average diameter] and life.

[Explanation of symbols]

 1 outer ring 2 inner ring 3 balls

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3J101 AA01 AA13 AA24 AA32 AA43 AA52 AA62 AA81 BA53 BA54 BA70 DA03 EA02 FA31

Claims (5)

[Claims] 1. A rolling bearing comprising an outer ring, an inner ring, and a plurality of rolling elements rotatably disposed between the outer ring and the inner ring, wherein at least one of the outer ring and the inner ring is A rolling bearing comprising a case hardened steel having a ferrite content (area ratio) of 0.1% or less. 2. A roller bearing comprising an outer ring, an inner ring, and a plurality of rollers rotatably disposed between the outer ring and the inner ring, wherein at least one of the outer ring and the inner ring is case-hardened. A roller bearing made of steel and satisfying the following expression. D _I ≧ 4.5 × T _max where D _I is the ideal critical diameter of the case hardened steel,
_max is the maximum value of the thickness of a portion of the outer ring and the inner ring having a raceway surface of the raceway ring made of the case hardened steel. 3. The roller bearing according to claim 2, wherein an arithmetic average of a maximum value and a minimum value of the diameter of the roller is at least 25% of a difference between a bearing outer diameter and a bearing inner diameter. 4. A roller bearing comprising an outer ring, an inner ring, and a plurality of rollers rotatably disposed between the outer ring and the inner ring, wherein the outer ring and the inner ring are made of case-hardened steel. A roller bearing, wherein a minimum value of a thickness of a portion having a raceway surface of the outer ring and the inner ring is 50% or more of an arithmetic average of a maximum value and a minimum value of the diameter of the roller. 5. The roller bearing according to claim 4, wherein said case hardened steel has a nitrogen concentration of 100 ppm or less.