JP2016108616A - Rolling bearing - Google Patents

Abstract

A rolling bearing that suppresses occurrence of surface-initiated peeling and has excellent dimensional stability is provided at a low cost. A rolling bearing in which at least one of an inner ring, an outer ring and a rolling element is obtained by carbonitriding and tempering a steel material and satisfying the following (A) to (E) after the heat treatment. (A) Hardness of raceway surface Hv750-880 (B) Si-Mn nitride on raceway surface is 1-5% by area ratio (C) N amount on raceway surface is 0.2 mass% (D) The amount of retained austenite on the raceway surface is 25 to 40% by volume. (E) The amount of retained austenite in the core is 28% by volume or less. [Selection] FIG.

Description

  The present invention relates to a rolling bearing made of a steel material having a specific composition, extending a surface-origin type peeling life, and having excellent dimensional stability.

  In a rolling bearing, when a load is applied and used for a long time, metal fatigue may occur, and the raceway surface may peel off. As described above, the bearing with a larger load is likely to peel off. The types of peeling include "inclusion starting type peeling" that starts from inclusions in the inner ring, outer ring, and steel forming the rolling elements, and "surface starting type" that starts from indentations with foreign objects such as dust. “Peeling” is broadly divided into “white structure peeling” in which hydrogen penetrates into a steel material to cause hydrogen embrittlement and is caused by a structural change called a white structure.

  Measures differ depending on the type of exfoliation, and for `` surface-origin type exfoliation '', it is possible to extend the life by specifying the alloy composition of the steel material and carburizing or carbonitriding to increase the amount of retained austenite on the surface Known (see, for example, Patent Documents 1 and 2).

  On the other hand, if the amount of retained austenite is increased, the surface-initiated peel life will be extended, but depending on the use conditions, decomposition of the retained austenite will gradually occur, and the dimensional stability will be reduced, which will hinder the rotation of the bearing. . Regarding such dimensional stability, it has also been pointed out that it is effective to reduce the amount of retained austenite (see, for example, Patent Documents 3 and 4).

Japanese Patent No. 2541160 Japanese Patent No. 2590645 Japanese Patent Laid-Open No. 11-30224 JP 2000-55132 A

  Carburizing or carbonitriding low-alloy alloy steel can keep the amount of retained austenite on the surface low while increasing the amount of retained austenite on the surface, and it is possible to achieve both surface-origin peeling life and dimensional stability. is there. However, in order to satisfy the hardness required for the rolling bearing, it is necessary to perform carburizing or carbonitriding for a long time at a high temperature, and the productivity is remarkably lowered, so that it is difficult to widely use it industrially. Further, if the amount of retained austenite is reduced with emphasis on dimensional stability, the surface-initiated peeling life is shortened.

  Thus, it has been difficult in the past to satisfy all of the surface-origin type peeling life, productivity, and dimensional stability in a balanced manner, and the present invention suppresses the occurrence of surface-origin type peeling and also provides dimensional stability. The object is to provide an excellent rolling bearing at a low cost.

  In order to solve the above-mentioned problems, the present inventors have studied that carbon steel is carbonitrided and quenched and tempered to control the retained austenite within a suitable range, and the surface of the raceway is finely coated with hard Si-Mn nitride. The inventors have found that it is effective to suppress the decomposition of retained austenite with a specific alloy composition, and to complete the present invention.

That is, the present invention provides the following rolling bearing.
(1) In a rolling bearing comprising a rolling element disposed via a cage between an inner ring and an outer ring,
A rolling bearing characterized in that at least one of an inner ring, an outer ring and a rolling element is obtained by carbonitriding and tempering a steel material, and satisfying the following (A) to (E) after the above treatment.
(A) The hardness of the raceway surface is HV750-880.
(B) Si-Mn nitride on the raceway surface is 1 to 5% in area ratio
(C) The amount of N on the raceway surface is 0.2% by mass or more. (D) The amount of retained austenite on the raceway surface is 25 to 40% by volume.
(E) The amount of retained austenite in the core is 28% by volume or less. (2) At least one of the inner ring, the outer ring, and the rolling element is
C: 0.85 to 1.15% by mass
Si: 0.40-0.90 mass%
Mn: 0.55 to 1.51% by mass
Cr: 1.06-1.90 mass%
As an essential component, Mo as an optional component: 0.30 mass% or less (including 0 mass%)
Ni: 0.30 mass% or less (including 0 mass%)
Cu: 0.20 mass% or less (including 0 mass%)
S: 0.025% by mass or less (including 0% by mass)
P: 0.020% by mass or less (including 0% by mass)
O: 15 mass ppm or less (including 0 mass ppm)
And the balance is made of a steel material made of iron and inevitable impurities, and satisfies the following relational expression.
2.7 ≦ 2 [Si] + [Mn] + ([Cr] −7 [MC] / 100) / (1− [MC]
/100)+[Mo]≦4.4
(In the formula, [Si], [Mn] and [Cr] are the amount of Si, Mn or Cr (mass%) in the steel material, and [MC] remains in the steel after carbonitriding and quenching and tempering. (The ratio (area%) of spherical carbides.)

  In the rolling bearing of the present invention, carbon steel is subjected to carbonitriding and tempering and tempering to a steel material constituting at least one of an inner ring, an outer ring, and a rolling element, and hard Si-Mn nitride is precipitated on the surface of the raceway surface to a specific hardness. At the same time, by controlling the amount of retained austenite on the raceway surface and the core part within a specific range, both suppression of surface-origin separation and improvement in dimensional stability are achieved.

It is a graph which shows the relationship between the amount of N of a surface, and a lifetime ratio. It is a graph which shows the relationship between the value of Formula (1), and dimensional stability.

  Hereinafter, the present invention will be described in detail.

In the present invention, the type and configuration of the rolling bearing is not limited, but carbonitriding and quenching and tempering are performed on the steel material constituting at least one of the inner ring, the outer ring and the rolling element, and the following (A) to (E) are satisfied. To do. (A) to (D) are qualities obtained by measuring the outermost surface of the raceway surface. (E) is a quality obtained by measuring a portion having a constant carbon / nitrogen concentration that is not affected by carbonitriding.
(A) The hardness of the raceway surface is HV750-880.
(B) Si-Mn nitride on the raceway surface is 1 to 5% in area ratio
(C) The amount of N on the raceway surface is 0.2% by mass or more. (D) The amount of retained austenite on the raceway surface is 25 to 40% by volume.
(E) The amount of retained austenite in the core is 28% by volume or less

[Race surface hardness is HV750-880]
When foreign matter is mixed in the lubricant, the rolling elements and the race ring bite the foreign matter, resulting in indentations on the raceway surface. And when a rolling element rolls and contacts on an indentation, stress concentration will arise in the periphery of an indentation and a fatigue crack will generate | occur | produce and will become the starting point of peeling. At this time, the higher the hardness, the smaller the size of the indentation generated when the foreign object is bitten. Or it becomes difficult to produce indentation. As a result, subsequent fatigue cracks are less likely to occur, and the rolling bearing has a long life. However, if the hardness after quenching and tempering is less than HV750, indentation tends to occur. Moreover, when it exceeds HV880, grindability will fall and productivity will fall. Preferably, the hardness of the raceway surface is set to Hv 800 to 850.

[Si-Mn nitride on the raceway surface is 1 to 5% in area ratio]
By performing carbonitriding on steel with a large amount of Si and Mn, nitrides containing Si and Mn (Si-Mn nitrides) precipitate. This Si-Mn nitride is thermally stable, the composition ratio of Si to Mn in the nitride (Si: Mn) is about 5: 1, and becomes a fine product of 0.01 to 1 μm. To precipitate. The Si—Mn nitride has a function of hardening the surface, strengthens the raceway surface, improves the surface-origin-type peeling life, and improves wear resistance and seizure resistance. Therefore, if the area ratio of the Si—Mn nitride is less than 1%, these effects cannot be obtained sufficiently. However, if it exceeds 5%, the surface hardness becomes excessive and the grindability is lowered. Preferably, the area ratio of the Si—Mn nitride is 2 to 5%.

[N content on raceway surface is 0.2 mass% or more]
In order to precipitate the Si—Mn nitride, it is necessary to increase the amount of N that penetrates by carbonitriding, and in order to obtain 1% of the precipitation amount of the Si—Mn nitride described above, N on the raceway surface is required. The amount needs to be 0.2% by mass or more.

[The amount of retained austenite on the raceway surface is 25 to 40% by volume]
Since retained austenite is softer than the martensite of the base structure, it has excellent deformability. Therefore, when the indentation is generated and the rolling elements are in rolling contact with the indentation, if the retained austenite is large, the indentation shape is deformed, and the stress concentration can be reduced. However, if the amount of retained austenite is less than 25% by volume, this effect cannot be obtained. Further, the more retained austenite, the better the surface-initiated peeling life, but when it exceeds 40% by volume, the surface hardness becomes too low and the indentation becomes large. Preferably, the amount of retained austenite on the raceway surface is 30 to 40% by volume.

[The amount of retained austenite in the core is 28% by volume or less]
Residual austenite decomposes gradually during use, and causes dimensional changes. Therefore, if the amount of retained austenite in the core is greater than 28% by volume, decomposition proceeds and dimensional stability decreases. The amount of retained austenite in the core is preferably 24% by volume or less, and more preferably 20% by volume or less. In order to obtain such a retained austenite amount in the core, it is preferable to use steel as an alloy component as described later.

  That is, the alloy composition of the steel material is preferably as follows. The steel material contains C, Si, Mn, and Cr as essential components, and each contains a specific amount.

[C: 0.85 to 1.15% by mass]
C is an element that dissolves in the matrix by quenching and improves the hardness, but if the content in the steel material is less than 0.85% by mass, the hardness after quenching decreases, wear resistance and rolling Fatigue life is reduced. In order to stably obtain wear resistance and rolling fatigue life, the C content is preferably set to 0.95% by mass or more. However, if the amount of C exceeds 1.15% by mass, the grindability and the fracture toughness value are lowered. In order to stably obtain grindability, the C content is preferably 1.10% by mass or less.

[Si: 0.40-0.90 mass%]
Si has the effect of improving the hardenability and temper softening resistance by dissolving in the base. In addition, there is also an effect of delaying the decomposition of retained austenite, which is important in the present invention. Therefore, these effects cannot be obtained when the Si amount is less than 0.40 mass%. However, if it exceeds 0.90% by mass, the hardness after spheroidizing annealing is increased, so that the turning property and the cold workability are deteriorated. In order to stably obtain turning properties and cold workability, the Si content is preferably 0.70% by mass or less.

[Mn: 0.55 to 1.51% by mass]
Mn has the effect of solid-dissolving in the base and improving the hardenability, and in the present invention, it has the effect of stabilizing the important amount of retained austenite. Residual austenite has the effect of extending the life against surface-origin peeling. It also has the effect of promoting the precipitation of Si—Mn nitrides together with Si. However, when the amount of Mn is less than 0.55% by mass, these effects cannot be obtained sufficiently. However, if it exceeds 1.51% by mass, the deformation resistance during hot forging will increase and hot forgeability will decrease, the amount of retained austenite will be excessive, and even if it has a decomposition suppression effect, dimensional stability will decrease. To do. In order to stably obtain hot forgeability and dimensional stability, the amount of Mn is preferably 0.55 to 1.4% by mass, and more preferably 0.8 to 1.2% by mass. .

[Cr: 1.06-1.90% by mass]
Cr is distributed between the solid solution in the base martensite and the solid solution in the spheroidized carbide. Cr dissolved in the base has the effect of improving the hardenability and further has the effect of stabilizing the martensite of the base structure, thereby extending the rolling fatigue life. However, if the Cr content is less than 1.06% by mass, these effects cannot be obtained. Preferably, the Cr amount is 1.30% by mass or more. Moreover, since the hardness after spheroidizing annealing will raise when it exceeds 1.90 mass%, turning property and cold workability will fall. In order to stably obtain turning and cold workability, the Cr content is preferably 1.70% by mass or less.

  Moreover, steel materials contain Mo, Ni, Cu, S, P, and O as arbitrary components.

[Mo: 0.30 mass% or less (including 0 mass%)]
Mo has the effect of improving the hardenability and temper softening resistance by dissolving in the base. In addition, it has the effect of stabilizing the martensite of the base structure, thus extending the rolling fatigue life. However, if the amount of Mo exceeds 0.30% by mass, a part of Mo forms a hard carbide and reduces machinability. Moreover, since Mo is an expensive element, it raises the cost of a raw material, Therefore In this invention, it adds selectively.

[Ni: 0.30% by mass or less (including 0% by mass)]
Ni has the effect of improving hardenability and the effect of stabilizing austenite, and improves toughness when added in a large amount. However, since Ni is a very expensive element, the steel material cost is increased. Therefore, in the present invention, Ni is not positively added, and is 0.30 mass% or less.

[Cu: 0.20% by mass or less (including 0% by mass)]
Cu has the effect of improving the hardenability and the grain boundary strength, but if it is excessive, the hot forgeability decreases. Therefore, in this invention, it does not add positively and Cu content shall be 0.20 mass% or less.

[S: 0.025% by mass or less (including 0% by mass)]
Since S forms MnS and acts as an inclusion, it is preferable that the amount of S in the steel material is small. However, if the amount of S is reduced, the productivity of the steel material is lowered and the steel material cost is increased, so that it is difficult to use it industrially. Therefore, in the present invention, the S amount is 0.025% by mass or less.

[P: 0.020% by mass or less (including 0% by mass)]
Since P segregates at the grain boundaries to reduce the grain boundary strength and fracture toughness value, it is preferable that the amount of P in the steel material is small. Therefore, in the present invention, the amount of P is set to 0.020% by mass or less.

[O: 15 mass ppm or less (including 0 mass ppm)]
O forms oxide inclusions such as Al ₂ O ₃ in the steel. Oxide inclusions serve as a starting point for delamination and affect the rolling fatigue life. However, if the amount of O is reduced, the productivity of the steel material is lowered and the cost of the steel material is increased, so that it is difficult to use it industrially. Therefore, in the present invention, the amount of O is set to 15 mass ppm or less.

And the remainder of steel materials is iron and an unavoidable impurity, and also the value of the following (1) formula satisfies 2.7 or more and 4.4 or less. [Si], [Mn] and [Cr] are the amount of Si, Mn or Cr (% by mass) in the steel material, and [MC] is the spherical carbide remaining in the steel after quenching and tempering. It is a ratio (area%).
(1) Formula: 2 [Si] + [Mn] + ([Cr] -7 [MC] / 100) /
(1- [MC] / 100) + [Mo]

  The present inventors conducted a rolling fatigue life test using steel materials having different alloy compositions, and quantified the inhibitory effect of each alloy element on the dimensional change due to decomposition of retained austenite. Each of Si, Mn, Cr and Mo Was found to be 2: 1: 1: 1. It was also found that the larger the value of equation (1), the greater the amount of alloying elements employed in the base martensite structure, the more stable retained austenite and the smaller the dimensional change.

  However, Cr is distributed into a part that dissolves in the base martensite structure and a part that dissolves in the spheroidized carbide, and is concentrated to about 7% by mass in the spheroidized carbide. Therefore, when the ratio of spheroidized carbide is large, Cr is distributed more in the spheroidized carbide, and the amount of Cr in the base martensite structure is reduced. After spheroidizing annealing, about 15% by mass of carbide in the steel material is present, but a part of the spheroidizing carbide is dissolved in the matrix martensite structure by the quenching treatment. That is, the amount of Cr dissolved in the base structure is determined by the amount of Cr in the steel material and the amount of spheroidized carbide remaining after quenching and tempering, and ((Cr) -7 [MC] / 100) / (1- [MC] / 100). In the present invention, the area ratio [MC] of the remaining spheroidized carbide is preferably 5 to 9%.

  And if the value of (1) Formula is less than 2.7, the decomposition suppression effect of retained austenite cannot be obtained stably. On the other hand, if it exceeds 4.4, the amount of retained austenite becomes excessive, and the effect of inhibiting decomposition of retained austenite cannot be substantially obtained. In order to improve the effect of suppressing the decomposition of retained austenite stably and to obtain stable dimensional stability, the value of formula (1) is preferably set to 2.8 or more and 4.4 or less, and 3.3 or more and 3 It is more preferable to make it .9 or less.

  In order to produce the rolling bearing of the present invention, preferably, a steel material having the above alloy composition is used, and the inner ring, the outer ring, and the rolling element are brought close to the completed shape by hot working and turning, and then carbonitriding and quenching and tempering are performed. Then, processing is performed so as to satisfy the above-mentioned (A) to (E). After that, grinding is performed to finish the finished shape.

  Carbonitriding and quenching and tempering are preferably not performed under extremely high temperature and long time in order to minimize the influence on productivity, and quenching is performed at 820 to 860 ° C. for 2 to 6 hours. After carbonitriding with a mixed gas of RX gas, propane gas and ammonia gas, oil cooling is performed. The oil temperature during oil cooling is preferably 40 to 120 ° C. Tempering is carried out at 160 to 200 ° C. for a predetermined time, and then air-cooled or gradually cooled.

  In the present invention, the type of rolling bearing is not limited, and includes ball bearings such as deep groove ball bearings, angular ball bearings, thrust ball bearings, roller bearings such as cylindrical roller bearings, tapered roller bearings, and self-aligning roller bearings, or needle bearings. It is applicable to.

  Examples The present invention will be further described below with reference to examples and comparative examples, but the present invention is not limited thereby.

(1) Rolling fatigue element test After steel material having the alloy composition shown in Table 1 (the balance is iron and inevitable impurities) is turned into a disk shape, and subjected to carbonitriding and quenching and tempering treatment shown in Table 2. Then, grinding and polishing and lapping were performed to prepare a disk specimen as a completed shape. Then, a disc test piece, an upper race of thrust ball bearing 51305, 6 steel balls having a diameter of 3/8 inch, and a cage made of brass are combined and set in a thrust type rolling fatigue tester. A rolling fatigue life test was conducted. This test was performed five times, and the average value of the life (L50) at which the cumulative failure probability was 50% was determined.
・ Maximum contact surface pressure: 5.0 GPa
・ Rotational speed: 1000 min ^-1
Lubricating oil: ISO-VG68 (including 0.009 g of iron powder having a size of about 100 μm and a hardness of HV870)

  The results are shown in Table 2 as relative values with respect to Comparative Example 5 (steel type J: SUJ2). FIG. 1 is a graph showing the relationship between the surface N amount and the life ratio. As a result of the test, it can be seen that if the surface N amount is 0.2% by mass or more, a long life exceeding twice is obtained. In particular, Examples 1 to 5 have a lifetime that is three times or more. However, in Example 10, the Cr amount of the steel material is the lower limit of the preferred amount of the present invention, and although it has a longer life than the comparative example, it is shorter than the other examples. Moreover, in the comparative example 2, the amount of C of steel materials is less than the preferable amount of this invention, the surface hardness falls and a lifetime is short.

  In Table 2, in addition to the surface N amount, carbon steel nitriding and quenching, surface hardness of each steel material after tempering, area ratio of Si-Mn nitride, surface retained austenite amount (γR), core The amount of retained austenite (γR) is also shown. In the examples, (A), (B), (D), and (E) are all satisfied at the same time. On the other hand, in Comparative Example 1, the area ratio and the N amount of the Si—Mn nitride on the surface are outside the range of the present invention, and the residual austenite amount on the surface is insufficient, so that a sufficient life is not obtained.

(2) Dimensional stability test A steel material having the alloy composition shown in Table 1 (the balance is iron and inevitable impurities) is turned into a predetermined outer ring shape, subjected to the heat treatment shown in Table 2, and then ground. An outer ring for the deep groove ball bearing 6206 was produced as a completed shape. And this outer ring | wheel was hold | maintained at 130 degreeC for 5000 hours, and the dimensional change rate before holding | maintenance was calculated | required.

  The results are shown in Table 2. In Table 2, the values of [MC] and (1) are also shown. FIG. 2 is a graph showing the relationship between the value of equation (1) and the dimensional change rate. As a result of the test, when the value of the formula (1) is in the range of 2.7 to 4.4, it can be seen that the dimensional stability is excellent. In Example 11, the amount of Mn in the steel material is the lower limit of the preferred amount of the present invention, and the amount of retained austenite in the core part is also the upper limit of regulation, so that the dimensional stability is worse than in other examples. ing. Moreover, in Example 12, the value of the formula (1) is the lower limit, the effect of suppressing the decomposition of retained austenite is not sufficient, and the dimensional stability is worse compared to other examples.

(3) Deep groove ball bearing life test Based on Examples 1, 3, and 5 that showed excellent results in the above (1) rolling fatigue element test and (2) dimensional stability test, An inner ring and an outer ring were produced. That is, a steel material having the alloy composition shown in Table 1 (the balance is iron and inevitable impurities) is turned into a predetermined inner ring shape or outer ring shape, subjected to carbonitriding and quenching and tempering as shown in Table 2, and then subjected to grinding. Then, a finished shape was formed, and a test bearing was manufactured in combination with a SUJ2 3/8 inch steel ball and a resin cage. For comparison, test bearings were produced in the same manner for Comparative Examples 4 and 5. Then, the test bearing was mounted on a radial type bearing life tester, and a rolling fatigue life test was performed under the following conditions. This test was performed five times, and the average value of the life (L50) at which the cumulative failure probability was 50% was determined.
・ Maximum contact surface pressure: 3.0 GPa
・ Rotation speed: 3000 min ^-1
-Lubricating oil: ISO-VG68 (including 0.05 g of iron powder having a size of about 100 μm and a hardness of HV870)

  The results are shown in Table 2 as relative values with respect to Comparative Example 5, and Examples 1, 3, and 5 show good lifetimes. On the other hand, in Comparative Example 4, the amount of N on the surface and the amount of retained austenite are less than the values specified in the present invention, and since the amount of Si and Mn in the steel material are small, carbonitriding and quenching and tempering are also performed. The area ratio of the Si—Mn-based nitride specified in the above is not satisfied, and the amount of retained austenite in the core and the value of the formula (1) are not satisfied, so the effect of suppressing the decomposition of retained austenite is insufficient and the dimensional stability is poor. It has become. In Comparative Example 3, an attempt was made to produce a test bearing. However, the area ratio of Si-Mn carbide on the surface was large, and the hardness was excessive, so that the bearing could not be processed.

  From the above, it can be seen that a rolling bearing having a long life and excellent dimensional stability can be provided by satisfying (A) to (E), and preferably satisfying formula (1) with a specific alloy composition.

Claims (2)

In a rolling bearing comprising a rolling element disposed via a cage between an inner ring and an outer ring,
A rolling bearing characterized in that at least one of an inner ring, an outer ring and a rolling element is obtained by carbonitriding and tempering a steel material, and satisfying the following (A) to (E) after the above treatment.
(A) The hardness of the raceway surface is HV750-880.
(B) Si-Mn nitride on the raceway surface is 1 to 5% in area ratio
(C) The amount of N on the raceway surface is 0.2% by mass or more. (D) The amount of retained austenite on the raceway surface is 25 to 40% by volume.
(E) The amount of retained austenite in the core is 28% by volume or less At least one of the inner ring, the outer ring, and the rolling element is
C: 0.85 to 1.15% by mass
Si: 0.40-0.90 mass%
Mn: 0.55 to 1.51% by mass
Cr: 1.06-1.90 mass%
As an essential component, Mo as an optional component: 0.30 mass% or less (including 0 mass%)
Ni: 0.30 mass% or less (including 0 mass%)
Cu: 0.20 mass% or less (including 0 mass%)
S: 0.025% by mass or less (including 0% by mass)
P: 0.020% by mass or less (including 0% by mass)
O: 15 mass ppm or less (including 0 mass ppm)
The rolling bearing according to claim 1, wherein the balance is made of a steel material made of iron and inevitable impurities, and satisfies the following relational expression.
2.7 ≦ 2 [Si] + [Mn] + ([Cr] −7 [MC] / 100) / (1− [MC]
/100)+[Mo]≦4.4
(In the formula, [Si], [Mn] and [Cr] are the amount of Si, Mn or Cr (mass%) in the steel material, and [MC] remains in the steel after carbonitriding and quenching and tempering. (The ratio (area%) of spherical carbides.)

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