JP2012241862A - Rolling bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive rolling bearing which has excellent toughness and secures a long life even when used under an environment in which water or solid foreign matter is mixed in lubricant.SOLUTION: An outer ring 2 of the cylindrical roller bearing 10 is composed of high-carbon steel having a content of carbon of≥0.6 mass%. A hardened layer subjected to quenching by heat treatment including high frequency quenching treatment is formed on a surface of the outer ring 2 including a raceway surface 2a, and a non-quenched part not subjected to quenching is formed in a core part. The surface hardness of the raceway surface 2a is≥Hv 651, and the hardness of the core part is≤Hv 500. Further, the residual austenite amount in the raceway surface 2a is≥12 vol.% and ≤40 vol.%. The valid hardened layer depth Y0 of the raceway surface 2a satisfies the expression of 0.07×Dw<Y0<0.07×Dw+5. The maximum diameter of nonmetallic inclusions estimated by external statistics method is≤35 μm.

Description

  The present invention relates to a rolling bearing.

  When manufacturing large-sized rolling bearings used in steel rolling mills, etc., low-medium carbon steels that ensure hardenability by adding chromium, nickel, molybdenum, etc. 0.4% by mass) is used as a material for the bearing ring, and the hardness required for the rolling bearing is obtained by carburizing (or carbonitriding) the low and medium carbon steel. The reason why low-medium carbon steel is used as a material for the bearing ring is to ensure toughness. In hardened steel, the hardness depends on the carbon content, and the hardness and toughness are in a trade-off relationship. Therefore, a bearing having higher toughness can be obtained by using a low carbon base material.

  A rolling mill for steel is a facility mainly for rolling steel plates, and a rolling bearing for rotating and supporting the roll is used at the roll neck portion of the rolling roll. The roll neck rolling bearing is subjected to high loads, impacts, vibrations, and the like during rolling, so toughness is one of the important performance requirements. Therefore, even in consideration of the demerit that the carburization time becomes longer due to the low carbonization of the steel, medium carbon steel or low carbon steel is used as a material for the race.

In addition, since the roll neck rolling bearing is likely to be mixed with rolling water and solid foreign matters such as iron dust, surface fatigue resulting from these problems becomes a problem.
It is known that the rolling fatigue life is reduced when water is mixed in the lubricant for the following reasons. When a rolling element contacts the raceway surface and a load is applied, the raceway surface is elastically deformed in the depth direction, and tensile stress is generated in the vicinity of the contact surface. When oxide inclusions and water are present on the surface on which tensile stress acts, a gap is formed between the oxide inclusions and the metal matrix, and water enters the gap to cause a corrosion reaction. As a result, stress concentration around the non-metallic inclusions increases, causing cracks and reducing the rolling fatigue life. Therefore, in rolling bearings used in an environment where water is mixed in the lubricant, measures are taken to reduce stress concentration by reducing non-metallic inclusions that are the starting points of cracks.

  On the other hand, when the solid foreign matter is mixed in the lubricant, the rolling fatigue life is reduced due to the stress concentration on the indentation edge caused by the solid foreign matter (hard particles) in the lubricant biting into the rolling contact portion. As measures against this, a method of increasing the surface hardness and a method of relaxing the stress concentration on the indentation edge by controlling the amount of retained austenite on the surface have been proposed (see, for example, Patent Documents 1 and 2). .

JP-A-6-117438 JP-A-6-129436

  However, in order to secure the amount of retained austenite on the surface, the amount of carbon corresponding to that is required. Therefore, considering the toughness, rolling bearings are obtained by subjecting medium carbon steel or low carbon steel to long-term carburizing treatment. Is currently manufacturing. On the other hand, it is generally known that low carbon steel is more difficult to clean up than high carbon steel. Therefore, in order to improve the surface fatigue life when water is mixed in the lubricant even when medium carbon steel or low carbon steel is used, in addition to the vacuum degassing performed in the production of ordinary bearing steel In addition, it is necessary to improve the cleanliness of the material by performing a special melting method.

For this reason, conventionally, in order to improve the toughness, rolling fatigue life when water is mixed in the lubricant, and rolling fatigue life when solid foreign matter is mixed in the lubricant, High carbon steel cannot be used as a material for the bearing ring, and it is necessary to use specially melted medium carbon steel or low carbon steel as the material for the bearing ring and to perform carburizing treatment for a long time. There was a problem.
Therefore, the present invention solves the problems of the prior art as described above, has excellent toughness, and has a long life even when used in an environment where water or solid foreign matter is mixed in the lubricant. An object is to provide an inexpensive rolling bearing.

In order to solve the above problems, an aspect of the present invention has the following configuration. That is, a rolling bearing according to an aspect of 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 raceway surface of the inner ring and a raceway surface of the outer ring. A plurality of rolling elements arranged so as to be capable of rolling, wherein the outer ring satisfies the following five conditions.
Condition A: A hardened layer that is quenched by heat treatment including induction hardening is formed on the surface including the raceway surface, and a non-quenched portion that is not quenched is formed in the core. .
Condition B: The surface hardness of the raceway surface is Hv651 or higher, and the hardness of the core portion is Hv500 or lower.
Condition C: The amount of retained austenite on the raceway surface is 12% by volume to 40% by volume.
Condition D: The effective hardened layer depth Y0 of the raceway surface satisfies the expression 0.07 × Dw <Y0 <0.07 × Dw + 5. However, Dw is the diameter of the said rolling element, and the unit of Y0 and Dw is mm.
Condition E: The maximum diameter of the nonmetallic inclusion estimated by the extreme value statistical method is 35 μm or less.
In such a rolling bearing, the residual stress on the raceway surface is preferably −200 MPa or less.

  The rolling bearing of the present invention has excellent toughness and has a long life and is inexpensive even when used in an environment where water or solid foreign matter is mixed in the lubricant.

It is a fragmentary longitudinal cross-section which shows the structure of the cylindrical roller bearing which is one Embodiment of the rolling bearing which concerns on this invention. It is a graph which shows the correlation with the estimated maximum diameter of a nonmetallic inclusion, and the lifetime of a rolling bearing. It is a graph which shows the correlation with the amount of retained austenite of a raceway surface, and the lifetime of a rolling bearing. It is a graph which shows the correlation with the effective hardened layer depth Y0 of a raceway surface, and crushing strength.

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 cylindrical roller bearing which is an embodiment of a rolling bearing according to the present invention.
A cylindrical roller bearing 10 in FIG. 1 includes an inner ring 1 having a raceway surface 1a on an outer peripheral surface, an outer ring 2 having a raceway surface 2a opposite to the raceway surface 1a of the inner ring 1 on an inner peripheral surface, and both raceway surfaces 1a and 2a. A plurality of rolling elements 3 arranged so as to be freely rollable, a cage 4 that holds the rolling elements 3 between both raceway surfaces 1a and 2a, and lubrication that lubricates both the raceway surfaces 1a and 2a and the rolling elements 3. And a lubricant (not shown) such as oil and grease. In addition, the holder | retainer 4 does not need to be provided. Further, a sealing device such as a rubber seal may be provided.

In such a cylindrical roller bearing 10, the outer ring 2 is made of high carbon steel having a carbon content of 0.6% by mass or more and satisfies the following five conditions (in addition, the inner ring 1 and One or both of the rolling elements 3 may have the same configuration as the outer ring 2).
Condition A: A hardened layer formed by heat treatment including induction hardening is formed on the surface of the outer ring 2 including the raceway surface 2a, and a non-quenched portion not subjected to hardening is formed in the core portion. Has been.
Condition B: The surface hardness of the raceway surface 2a is Hv651 or higher, and the hardness of the core is Hv500 or lower.
Condition C: The amount of retained austenite on the raceway surface 2a is 12% by volume to 40% by volume.
Condition D: The effective hardened layer depth Y0 of the raceway surface 2a satisfies the expression 0.07 × Dw <Y0 <0.07 × Dw + 5. However, Dw is the diameter of the rolling element 3, and the unit of Y0 and Dw is mm.
Condition E: The maximum diameter of nonmetallic inclusions (carbide, nitride, oxide, etc.) estimated by the extreme value statistical method is 35 μm or less.

  Due to the action of the non-quenched portion formed in the core portion, the outer ring 2 has excellent toughness, and the same toughness as when carburized is applied. That is, the hardness of the core is Hv500 or less and the toughness of the core is excellent, so that the outer ring 2 is hardly cracked. Since heat treatment can be performed by induction hardening without performing carburizing or carbonitriding for a long time, the manufacturing cost of the cylindrical roller bearing 10 is low. In addition, although the hardness of a core part needs to be Hv500 or less, it is more preferable that it is Hv497Hv or less. Moreover, it is preferable that it is 205 or more.

  Further, by controlling the surface hardness of the raceway surface 2a, the amount of retained austenite, and the effective hardened layer depth Y0 (distance from the surface to the depth position of the hardness Hv550), solid lubricant (for example, iron) Even when used in an environment where hard particles such as dust are mixed, surface fatigue and surface peeling hardly occur, and the cylindrical roller bearing 10 of this embodiment has a long life. Since the surface hardness of the raceway surface 2a is Hv651 or more and the amount of retained austenite is 12% by volume or more (preferably 14% by volume or more), it is difficult to cause fracture starting from the surface, and the cylindrical roller bearing 10 Has a long life. In order to prevent burning cracks during heat treatment due to excessive retained austenite, the amount of retained austenite is preferably 40% by volume or less, and more preferably 25% by volume or less.

Furthermore, if the effective hardened layer depth Y0 is less than 0.07 × Dw, there is a risk of premature failure without being able to withstand the shear stress of the core. On the other hand, since the effective hardened layer depth Y0 is smaller than 0.07 × Dw + 5, the core portion having high toughness remains sufficiently, and the outer ring 2 is hardly cracked.
Furthermore, since the maximum diameter of non-metallic inclusions is small and is made of high carbon steel with higher cleanliness than carburized steel, surface fatigue and surface even when used in an environment where water is mixed into the lubricant Separation hardly occurs, and the cylindrical roller bearing 10 of this embodiment has a long life. The maximum diameter of non-metallic inclusions estimated by the extreme value statistical method correlates with the life in a clean lubrication environment.

Furthermore, if a residual stress (that is, a compressive residual stress) of −200 MPa or less (preferably −205 MPa or less) is applied to the raceway surface 2a by induction hardening, cracking is suppressed, so water is added to the lubricant. When used in a mixed environment, the cylindrical roller bearing 10 of this embodiment has a longer life.
Since the cylindrical roller bearing 10 of this embodiment is excellent in toughness, it is suitable for a large bearing that requires toughness. Also, the roll neck rolling bearing incorporated in the roll neck of the rolling roll of a steel rolling mill and rotatingly supporting the roll is used in an environment where water and solid foreign matter are mixed in the lubricant. The cylindrical roller bearing 10 of the embodiment is suitable for a roll neck rolling bearing.

  Here, the reason why the cylindrical roller bearing 10 has a long life when the outer ring 2 satisfies the above conditions will be described. Mainly, large radial roller bearings have a tendency that fatigue of the outer ring progresses rapidly and is easily damaged. This is considered to be caused by the following. That is, since the raceway surface of the inner ring is convex, high surface pressure is generated in the contact area with the rolling elements, whereas the raceway surface of the outer ring is concave, In comparison, the contact surface pressure with the rolling element is low. For this reason, in general radial roller bearings, the rolling fatigue of the inner ring is often faster than that of the outer ring.

  However, in the case of a large-sized radial roller bearing such as the above-mentioned roll neck rolling bearing, the raceway surface of both the inner ring and the outer ring has a substantially flat curvature, so the difference in contact surface pressure with the rolling element is small. . On the other hand, since the roll neck rolling bearing is used for inner ring rotation, the outer ring, which is a fixed ring, has a load zone of about 1/5 of the entire circumference. The number of stress repetitions in this load zone is larger than the number of stress repetitions in the inner ring.

For this reason, the larger the size of the rolling bearing, the greater the influence of the number of stress repetitions than the surface pressure, and the faster the rolling fatigue of the outer ring than the inner ring. Therefore, as the size of the rolling bearing is larger, the effect of extending the life is obtained because the outer ring satisfies the above conditions.
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 cylindrical roller bearing has been described as an example of a rolling bearing, but the present invention can be applied to various types of rolling bearings other than the cylindrical roller bearing. For example, radial rolling bearings such as deep groove ball bearings, angular contact ball bearings, self-aligning 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 This is a rolling bearing. The present invention can be applied not only to large cylindrical roller bearings but also to large tapered roller bearings, large self-aligning roller bearings, and the like.

〔Example〕
Hereinafter, the present invention will be described more specifically with reference to examples. In order to confirm the effect of the present invention elementally, a race ring made of various steels having different alloy components was prepared, and a thrust ball bearing (nominal number 51305) using this as a fixed ring was produced. The steel types used are M1 to M9, SUJ2 to SUJ5, and SCr420, and have alloy components as shown in Tables 1 and 2, respectively. In addition, components (remainder) other than carbon (C), silicon (Si), manganese (Mn), chromium (Cr), and molybdenum (Mo) shown in Tables 1 and 2 are iron and inevitable impurities.

The maximum diameter of non-metallic inclusions contained in each steel type was estimated by the extreme value statistical method, and the cleanliness of each steel type was evaluated. Specifically, to observe the microstructure of the test reference area S0 = 100 mm ^2, the dimensions of the oxide inclusions and titanium nitride (TiN) inclusions in an area S = 30,000 mm ² to forecast (length and breadth Square root of the product), and the largest dimension among these was taken as the predicted maximum diameter. Tables 1 and 2 show the predicted maximum diameters of the nonmetallic inclusions of each steel type.

  A specific method for manufacturing the thrust ball bearing will be described below. A steel material was roughly processed by turning to form a raceway shape, and then subjected to induction hardening as a heat treatment to form a hardened layer on the raceway surface. The hardened layer is composed of martensite, and a non-hardened portion (ferrite structure) in which pearlite structure and spheroidized carbide are dispersed is formed in the core portion inside the hardened layer by induction hardening. And after performing the tempering process, the grinding process was finally performed.

  A fixed ring, a rotating wheel made of SUJ2, a rolling element (ball), and a cage are assembled to obtain a thrust ball bearing (inner diameter: 25 mm, outer diameter: 52 mm, width: 18 mm). Obtained. For Comparative Example 1, carburization, quenching, and tempering were performed as the heat treatment, and for Comparative Example 2, through-quenching and tempering were performed as the heat treatment. Moreover, the raw material of a rolling element is SUJ2, and the diameter Dw is 9.3 mm.

  Although the raceway surface is hardened by the heat treatment as described above, the surface hardness of the raceway surface of the fixed ring of each thrust ball bearing has values as shown in Tables 3 and 4. Moreover, the hardness of the core is as shown in Tables 3 and 4. Further, the amount of retained austenite on the raceway surface and the effective hardened layer depth (distance from the surface to the depth position of the hardness Hv550) Y0 are also as shown in Tables 3 and 4. The surface hardness, core hardness, and effective hardened layer depth Y0 were measured with a Vickers hardness meter. The amount of retained austenite was measured by the X-ray diffraction method.

These thrust ball bearings were subjected to a rotation test in an environment in which water or solid foreign matters were mixed in the lubricant, and the life under each environment was evaluated. The test conditions (surface pressure and lubrication conditions) in a water-mixed environment were as follows, and the life (L ₁₀ life) at which the cumulative failure probability of the test bearing was 10% was determined.
Surface pressure: 3.2 GPa
Lubricant: Lubricating oil (oil bath) whose ISO viscosity grade is ISO VG10
Amount of water mixed: 30 ml / day
Test surface: Polished surface

Moreover, test conditions under solid contaminated environment (surface pressure and lubrication condition) and as follows, the cumulative failure probability of the test bearing was determined lifetime (L ₁₀ life) of 10%. In addition, the hardness of a solid foreign material is Hv870, and a particle size is 74-147 micrometers.
Surface pressure: 4.2 GPa
Lubricant: Turbine oil (oil bath) whose ISO viscosity grade is ISO VG68
Solid foreign matter content: 200ppm
Test surface: Polished surface

Tables 3 and 4 show the results of the rotation test in a water-mixed environment. The correlation between the predicted maximum diameter of nonmetallic inclusions and the L ₁₀ life of the test bearing is shown in the graph of FIG. The numerical values of the lifetime of Tables 3 and 4 and Figure 2 are indicated by the relative value when the 1.0 L ₁₀ life of Comparative Example 1.
From the graph of FIG. 2, it can be seen that by using a material having a small predicted maximum diameter of non-metallic inclusions and excellent cleanliness, the life is superior compared to Comparative Example 1 using carburized steel. Further, it can be seen that the life expectancy is excellent when the predicted maximum diameter of the nonmetallic inclusion is 35 μm or less (preferably 34 μm or less), and the life is further improved when it is 20 μm or less.

Next, Tables 3 and 4 show the results of the rotation test in a solid foreign matter mixed environment. The correlation between the amount of retained austenite on the raceway surface and the L ₁₀ life of the test bearing is shown in the graph of FIG. The numerical values of the lifetime of Tables 3 and 4 and FIG. 3 is shown as a relative value when the 1.0 L ₁₀ life of Comparative Example 2.
From the graph of FIG. 3, each example using induction hardening as the heat treatment has a longer life than Comparative Example 2 using through-quenching as the heat treatment, and the longer the amount of retained austenite on the raceway surface, the better the life. I understand that

  Next, in order to evaluate toughness, the following crushing tests were performed. A ring-shaped member obtained by turning a steel material made of SUJ2 was subjected to heat treatment (quenching and tempering) and then ground to produce an inner ring for a large cylindrical roller bearing (nominal number NU2326). In Examples 18 to 26 and Comparative Examples 9, 11, and 12, quenching was performed by induction hardening to form a hardened layer on the raceway surface. In Comparative Example 10, the ring-shaped member was subjected to quenching (through quenching) by furnace heating and oil cooling. Table 5 shows the types of quenching in Examples 18 to 26 and Comparative Examples 9 to 12, the surface hardness of the raceway surface, the hardness of the core, the amount of retained austenite on the raceway surface, and the effective hardened layer depth Y0. In addition, the raw material of a rolling element is SUJ3, The diameter Dw is 38 mm.

  A pre-crack is formed on the raceway surface (outer peripheral surface) of these inner rings to a depth of 1 mm by wire cutting, the direction in which the pre-cracks extend is horizontal, the inner ring is mounted on a crushing test apparatus, and a load is applied from above. The inner ring was compressed. And the load which the crack propagated from the pre-crack was made into crushing strength. The results are shown in Table 5 and the graph of FIG. In addition, the numerical value of crushing strength of Table 5 and FIG. 4 is shown by the relative value when the crushing strength of Comparative Example 10 is 1.0.

From the graph of FIG. 4, it can be seen that when the effective hardened layer depth Y0 is smaller than 0.07 × Dw + 5, the crushing strength is high and the toughness is excellent.
Further, a test was conducted on the correlation between the effective hardened layer depth Y0 of the raceway surface and the life of the rolling bearing. A ring-shaped member obtained by turning a steel material made of SUJ2 was subjected to heat treatment (quenching and tempering) and then ground to produce an outer ring for a large tapered roller bearing (reference number HR30326J).

  In Example 27 and Comparative Example 13, quenching was performed by induction hardening, and a hardened layer was formed on the raceway surface. In Comparative Example 14, the ring-shaped member was subjected to quenching (through quenching) by furnace heating and oil cooling. The type of quenching in Example 27 and Comparative Examples 13 and 14, the predicted maximum diameter of non-metallic inclusions, the surface hardness of the raceway surface, the hardness of the core, the amount of retained austenite on the raceway surface, and the effective hardened layer depth Y0 Is shown in Table 6. In addition, the raw material of a rolling element is SUJ2, and the diameter (effective outer diameter) Dw is 38 mm.

The outer ring, the inner ring and the rolling element thus obtained were assembled to produce a tapered roller bearing with a nominal number HR30326J. Then, by performing a rotation test under the following conditions, it was evaluated life (L ₁₀ life).
Surface pressure: 1.7 GPa
Lubricant: Turbine oil whose ISO viscosity grade is ISO VG68 (forced circulation) Rotational speed: 1500 min ^-1
Calculation life: 600 hours Table 6 shows the results of the rotation test. The numerical values of the lifetime of the table 6 is shown as a relative value when the 1.0 L ₁₀ life of Comparative Example 14. From Table 6, the comparative example 13 having a small effective hardened layer depth Y0 has a shorter life than the comparative example 14 in which the type of quenching is through-quenching, whereas the implementation having a sufficient effective hardened layer depth Y0. The lifetime of Example 27 was three times that of Comparative Example 14.

DESCRIPTION OF SYMBOLS 1 Inner ring 1a Raceway surface 2 Outer ring 2a Raceway surface 3 Rolling element 10 Cylindrical roller bearing

Claims (2)

An inner ring having a raceway surface, an outer ring having a raceway surface opposite to the raceway surface of the inner ring, and a plurality of rolling elements arranged in a freely rollable manner between the raceway surface of the inner ring and the raceway surface of the outer ring, And the outer ring satisfies the following five conditions.
Condition A: A hardened layer that is quenched by heat treatment including induction hardening is formed on the surface including the raceway surface, and a non-quenched portion that is not quenched is formed in the core. .
Condition B: The surface hardness of the raceway surface is Hv651 or higher, and the hardness of the core portion is Hv500 or lower.
Condition C: The amount of retained austenite on the raceway surface is 12% by volume to 40% by volume.
Condition D: The effective hardened layer depth Y0 of the raceway surface satisfies the expression 0.07 × Dw <Y0 <0.07 × Dw + 5. However, Dw is the diameter of the said rolling element, and the unit of Y0 and Dw is mm.
Condition E: The maximum diameter of the nonmetallic inclusion estimated by the extreme value statistical method is 35 μm or less.   The rolling bearing according to claim 1, wherein a residual stress of the raceway surface is −200 MPa or less.

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