JP2008174810A - Bearing inner and outer rings and bearings with excellent rolling fatigue characteristics - Google Patents

Abstract

The present invention provides an inner ring and an outer ring for use in a bearing, in which rolling fatigue characteristics, which are important characteristics in a bearing, specifically, a 10% cumulative failure probability in a rolling fatigue test is greatly improved.
The steel structure at least from the surface to a depth of 0.5 mm in the rolling part is made to have a prior austenite grain size of 3.5 μm or less.
[Selection] Figure 2

Description

  The present invention relates to a bearing manufactured from bearing steel, a bearing inner ring, and a bearing outer ring, and more particularly, to a bearing having improved rolling fatigue characteristics as compared with the related art.

Bearings are used in rotating parts such as automobiles and machines, and excellent rolling fatigue characteristics are required. For example, as described in Patent Document 1, there is a method for defining the heating method of bearing steel, and by rolling the prior austenite grain size to an average of 4.0 μm or less, the rolling fatigue life is doubled or more. is doing. However, this Patent Document 1 only describes a prior austenite grain size of 0.1 mm at a minimum, and does not describe the quenching depth of the bearing. However, the quenching depth is important for the rolling fatigue life, and in the case of only the extreme surface layer portion, a sufficiently good rolling fatigue life cannot be obtained.
JP 2006-152407 A

The present invention, the rolling fatigue characteristics is an important property in bearings, specifically, 10% cumulative failure probability in rolling contact fatigue test (hereinafter, referred to as B ₁₀ life) were significantly improved, the bearing An inner ring and an outer ring to be provided are provided.

The inventors prepared inner and outer rings of bearings with a refined prior austenite grain size in the hardened part, conducted 20 rolling fatigue tests under the same conditions, and performed rolling until fatigue separation occurred at the rolling part. the dynamic fatigue life was investigated to determine the B ₁₀ life. As a result, it was found that the rolling fatigue life can be significantly improved when the average austenite grain size is 3.5 μm or less. However, if the fine-grained area is less than 0.5 mm deep from the surface of the transfer part, the effect of improving the rolling fatigue life cannot be sufficiently obtained, and the depth of the area with a grain diameter of 3.5 μm or less is 0.5 mm or more. I found it necessary.
The present invention has been made based on this finding, and the gist of the present invention is as follows.

(1) Rolling fatigue characteristics characterized in that the bearing steel is made of bearing steel, and the steel structure at least from the surface to the depth of 0.5 mm in the rolling part has a prior austenite grain size of 3.5 μm or less. Excellent bearing inner ring.
Here, the transfer portion refers to a portion where a steel ball or a roller rolls in contact with the inner ring or outer ring of the bearing.

(2) The steel structure is obtained by heating the raw material in a two-phase region of austenite and spheroidized carbide and then quenching and quenching it twice or more (1) The bearing inner ring with excellent rolling fatigue characteristics described in 1.

(3) The heating to the two-phase region is performed at a heating rate from 800 ° C. to the maximum heating temperature of 0.5 ° C./s to 950 ° C./s, the maximum heating temperature of Ac _{3 to} Ac ₃ + 130 ° C., Ac ₃ points The inner ring of the bearing having excellent rolling fatigue characteristics as described in (2) above, wherein the residence time at the above temperature is in accordance with a condition of 500 s or less.

(4) The inner ring of the bearing excellent in rolling fatigue characteristics according to any one of claims 1 to 3, wherein the hardness from the surface to the 0.5 mm depth position at the rolling part is Hv700 or more.

(5) The bearing inner ring having excellent rolling fatigue characteristics according to any one of (1) to (4), wherein the material is made of SUJ2.

(6) A bearing excellent in rolling fatigue characteristics having the inner ring according to any one of (1) to (5).

(7) Rolling fatigue characteristics characterized in that the bearing is made of bearing steel, and the steel structure at least from the surface to the depth of 0.5 mm in the rolling part has a prior austenite grain size of 3.5 μm or less. Excellent bearing outer ring.

(8) The above steel structure is obtained by heating the material in a two-phase region of austenite and spheroidized carbide, quenching and quenching twice or more (7) The outer ring of the bearing with excellent rolling fatigue characteristics described in 1.

(9) The heating to the two-phase region is performed at a heating rate from 800 ° C. to the maximum heating temperature of 0.5 ° C./s to 950 ° C./s, the maximum heating temperature of Ac _{3 to} Ac ₃ + 130 ° C., Ac ₃ points The outer ring of a bearing having excellent rolling fatigue characteristics as described in (8) above, wherein the residence time at the above temperature is in accordance with a condition of 500 s or less.

(10) The bearing having excellent rolling fatigue characteristics according to any one of (7) to (9) above, wherein the hardness from the surface to the 0.5 mm depth position at the rolling part is Hv700 or more Outer ring.

(11) The outer ring of the bearing having excellent rolling fatigue characteristics according to any one of (7) to (10), wherein the material is SUJ2.

(12) A bearing having an outer ring according to any one of (7) to (11) and having excellent rolling fatigue characteristics.

(13) A bearing having excellent rolling fatigue characteristics, including the inner ring according to any one of (1) to (5) and the outer ring according to any one of (7) to (11).

  By applying the inner ring and the outer ring of the present invention, a bearing having excellent rolling fatigue characteristics can be obtained. Therefore, the present invention is very useful industrially.

The inner ring and outer ring of the bearing of the present invention are processed into the inner ring and outer ring of the bearing by subjecting a steel material, steel bar or steel pipe to a molding process (forging, cutting, etc.), and then the rolling part or the entire part is quenched. Manufactured.
In order to obtain the present invention, it is necessary to optimize the composition of the steel material, the structure, the prior austenite grain size and depth of the hardened surface layer, the quenching conditions, and the structure after quenching.
The present invention will be specifically described below.

[Steel composition]
First, the component composition of the steel material suitable for obtaining a quenched surface layer portion to be described later will be described. As the steel material, SUJ2, which is widely used as a bearing steel, is most suitable, but steel that satisfies the following composition can also be used.

C: 0.6 mass% to 1.5 mass%
C is an element necessary for securing the hardness necessary for obtaining a good fatigue life of the part in the quenched portion. If it is less than 0.6 mass%, sufficient hardness and fatigue strength can be obtained in the quenched portion. Absent. On the other hand, when it exceeds 1.5 mass%, the workability (shearability and forgeability) before quenching is deteriorated. Therefore, the preferable range of C content is 0.6 to 1.0 mass%.

Si: 0.1-1.0mass%
Si is preferably contained at 0.1 mass% or more in order to improve the rolling fatigue life. However, if added over 1.0 mass%, the processability before quenching (shearability and forgeability) is deteriorated as in C. Therefore, the range of suitable content of Si is 0.1 to 1.0 mass%.

Mn: 0.1-1.5mass%
Mn is preferably contained in an amount of 0.1 mass% or more in order to improve hardenability. However, if added excessively, the workability (shearability, forgeability) before quenching is deteriorated. For this reason, it is preferable that the upper limit of the content be 1.5 mass% or less.

Cr: 0.05-2.0mass%
Since Cr has the effect of reducing the hardness before quenching and improving the workability by improving the hardenability and promoting the spheroidization of the carbide, it is preferably contained in an amount of 0.05 mass% or more. However, even if added in excess of 2.0 mass%, the effect is saturated, so that it is preferably contained in the range of 0.05 to 2.0 mass%.

A composition containing the above-described elements and the balance being Fe and inevitable impurities is a basic component. Inevitable impurities include P, S, N and O. P can be up to 0.05 mass% and O can be up to 0.0150 mass%. S and N are also mixed as inevitable impurities, but may be positively added as described later.
That is, in addition to the above basic component composition, the following elements may be included in the ranges shown below.

S: 0.03 mass% or less S can be added because it combines with Mn to form MnS to improve machinability. However, if added over 0.03 mass%, MnS is a rolling fatigue test. The upper limit of the content is preferably set to 0.03 mass%, since it becomes a crack initiation point in the inside and significantly deteriorates rolling fatigue characteristics.

Al: 0.1 mass% or less
Al is a component that has a strong deoxidizing action and has an effect of improving the cleaning of steel, but may be added, but if added over 0.1 mass%, the cleaning of the steel is rather Since it deteriorates and the rolling fatigue characteristics are reduced, the content is preferably 0.1 mass% or less.

Cu: 1.0 mass% or less
Cu may be added because it has an effect of improving the hardness of the quenched portion by improving the hardenability, but 1.0 mass% or less is sufficient to obtain this effect.

Ni: 1.0 mass% or less
Ni may be added up to 1.0 mass% in order to improve the hardenability and the toughness of the hardened part. Moreover, in order to suppress hot brittleness when Cu is added, it is preferable to add Ni at a half of the Cu addition amount.

Mo: 1.0 mass% or less
Mo may be added because it is effective in improving hardenability and temper softening resistance. However, if excessively added, workability deteriorates, so 1.0 mass% or less is preferable.

W: 1.0 mass% or less W may be added because it has an effect of improving hardenability. However, if excessively added, workability deteriorates, and therefore, W is preferably 1.0 mass% or less.

Ti: 0.01 mass% or less
Ti may be added because it is effective in suppressing austenite grain growth due to nitride formation. However, if it exceeds 0.01 mass%, rolling fatigue characteristics deteriorate, so 0.01 mass% or less is preferable.

Nb: 0.5 mass% or less
Nb may be added because it is effective in suppressing the growth of austenite grains due to the formation of nitride (or carbonitride), but if its content exceeds 0.5 mass%, the effect is saturated, so 0.5 mass% The following is preferable.

B: 0.01 mass% or less Since B is effective in improving hardenability, 0.01 mass% may be added to the upper limit, but if its content exceeds 0.01 mass%, the effect is saturated. It is preferable to set it as 0.01 mass% or less.

Sb: 0.0050 mass% or less
Sb is effective for delaying the microstructure change (white layer formation) during the rolling fatigue test, and has the function of preventing the deterioration of rolling fatigue characteristics, so may be added. However, if the content exceeds 0.01 mass%, the toughness deteriorates, so it is preferable to set it to 0.01 mass% or less.

N: 0.01 mass% or less N forms nitrides and carbonitrides and is effective in refining austenite grains. However, excessive addition degrades the workability of steel, so 0.01 mass% or less. It is preferable to add.
The balance other than the above elements is naturally Fe and inevitable impurities.

[Structure of steel material]
Steel parts for bearings are formed from various processes such as cutting, grinding, forging, etc. before quenching, so that the structure of the steel material before quenching is spheroidized and the parent phase is converted to a ferrite structure. , And the carbide must be spheroidized. At this time, the carbide needs to have an average aspect ratio (ratio of the major axis / minor axis of the carbide) of 3 or less in consideration of workability and the effect of suppressing austenite grain growth during the quenching process.

[Old austenite grain size of hardened surface layer]
Steel parts for bearings are subjected to quenching and tempering because rolling fatigue characteristics are required. In the present invention, it is important that the prior austenite grain size is an average of 3.5 μm or less in the quenched surface layer, which is particularly important for rolling fatigue characteristics. This is because the rolling fatigue life is remarkably improved by setting the prior austenite grain size to an average of 3.5 μm or less in the quenched surface layer portion.

Furthermore, it is also clear from the following experiment that the depth of the region where the prior austenite grain size is 3.5 μm or less is 0.5 mm or more.
Incidentally, the prior austenite grain size is measured by first applying 0.1mm, 0.2mm, 0.3mm ... and 0.1mm pitch from the surface after corroding the former austenite grain boundary with the gamma R liquid developed by the applicant. At the position, four SEM images were taken at a magnification of 5000, and then the area of each austenite grain was measured with an image analyzer, and the equivalent circle diameter (2 × (area / π) ^1/2 was calculated from the area. ) And the average austenite grain size at that position. In this way, the average austenite grain size at each position in the depth direction is obtained, and the average austenite grain size may be 3.5 μm or less at all positions from 0.1 to 0.5 mm.

  That is, induction hardening was performed on the inner and outer rings of the ball bearing shown in FIG. 1 to produce bearings having a prior austenite grain size of 3.5 μm or less in depths of 0.2 mm, 0.5 mm, and 1.5 mm, respectively. In such a bearing, the inner ring or the outer ring was rotated until fatigue failure occurred. In addition, when the steel ball was damaged, the test was continued by replacing the steel ball. Fifteen fatigue tests were performed on bearings of each quenching depth, and the above-mentioned B10 life was calculated. As shown in FIG. 2, the rolling fatigue life can be increased to a high level by setting the region in which the prior austenite grain size is 3.5 μm or less to 0.5 mm or more from the transfer portion surface.

  As for the depth of the quenching part having a prior austenite grain size of 3.5 μm or less, it may be 0.5 mm or more from the transfer part surface, and the prior austenite grain size of the steel structure at a depth of more than 0.5 mm is more than 3.5 μm. Even if it has become, or it may be 3.5 micrometers or less about the area | region exceeding 0.5 mm in depth, either may be sufficient.

[Hardness of hardened part]
In the present invention, the hardness of the hardened portion in the inner ring and the outer ring of the bearing is 700 or more in terms of Vickers hardness (hereinafter referred to as Hv) at a position 0.5 mm from the surface layer portion of the transfer portion. This is because sufficient rolling fatigue life cannot be obtained with a member having a hardness of less than Hv700. Note that the depth of the quenched portion may be Hv 700 or more over the entire region.

[Hardening conditions]
In the present invention, since the prior austenite grain size of the quenched surface layer portion needs to be an average of 3.5 μm or less, the optimization of the quenching condition is very important. As for the number of times of quenching, N times of quenching (N: 1 or more) may be carried out, but the heating conditions in the Nth (last) quenching process are:
(I) Heating temperature: Ac ₃ points or higher and Ac ₃ + 130 ° C or lower
(ii) Heating rate: (Ac ₃ points -20 ° C) or more and Ac ₃ points or less in an average temperature range of 0.5 ° C / s to 950 ° C / s
(iii) Ac ₃ or more dwell time: It is necessary to be 500 seconds or less.

Here, Ac ₃ point is the temperature at which the transformation from ferrite, bainite or martensite to austenite is completed during heating. If the heating temperature is less than Ac ₃ points, the reverse transformation to austenite will not be completed, so that a complete martensitic quenched structure cannot be obtained and sufficient hardness cannot be obtained. Conversely, when the heating temperature exceeds Ac ₃ + 130 ° C., the spheroidizing carbides dissolve and the effect of suppressing the austenite grain growth is lost, and the grain growth is accelerated rapidly. Therefore, the prior austenite grain size after quenching is over 3.5 μm. turn into.

The heating rate of the quenching treatment needs to be an average of 0.5 ° C./s or more and 1000 ° C./s or less in a temperature range of (Ac ₃ points−20 ° C.) to Ac ₃ points. If the heating rate is slower than 0.5 ° C / s in this temperature range, the austenite grain size will not be refined due to the reduction of nucleation driving force to austenite, etc., and the prior austenite grain size of the quenched structure will be 3.5 μm. Become super. On the other hand, when the temperature exceeds 1000 ° C./s, the region having a prior austenite grain size of 3.5 μm or less becomes a depth region of less than 0.5 mm, and a sufficient fatigue life cannot be obtained.

Furthermore, it is necessary to adjust the heating conditions so that the residence time of Ac ₃ points or more is 500 seconds or less. If the residence time of Ac ₃ points or more exceeds 500 seconds, sufficient time is given for grain growth, and the prior austenite grain size of the structure after quenching exceeds 3.5 μm.

The quenching process under the above conditions is performed twice or more. This is because both the prior austenite grain size of 3.5 μm or less and the prior austenite grain size of 3.5 μm or less in the region depth of 0.5 mm or more are compatible. This is because the depth of the region could not be obtained even if it could be reduced to μm or less. This heating rate condition may be applied only at the time of the final quenching process (only the Nth time when performing the N-time quenching process). In the quenching process performed prior to the final quenching process (in the case of N quenching process, the quenching process from 1 to N-1), the structure after quenching is a bainite or martensite structure (single phase or complex) However, the residual spherical carbide may be used, and heat treatment limited to the final quenching step is not particularly required.
However, when heating at a high temperature where residual spherical carbide is dissolved, the effect of suppressing the austenite grain growth by the spherical carbide disappears at the time of final quenching, and the problem that the austenite grains become coarse and non-uniform, Since the amount of carbon penetration into the parent phase is higher than the optimum value and the rolling fatigue characteristics are reduced, during the quenching process before final quenching, the spheroidized carbides dissolve in austenite and become an austenite single phase. Must be below the temperature. Note that the number of times of quenching may be three or more, but is preferably two in terms of industrial productivity and cost.

  By performing quenching treatment under the above-described conditions, the area where the average prior austenite particle size is 3.5 μm or less and the average prior austenite particle size is 3.5 μm or less exists to a depth of 0.5 mm or more from the surface, An outer ring can be manufactured.

[Tempering]
In the present invention, a tempering process may be performed after the quenching process. However, when performing tempering treatment, if the tempering temperature is too high, the quenched surface layer portion is softened, the fatigue strength is reduced, and the effect of refining the prior austenite grain size of the quenched surface layer portion is reduced. When tempering, the temperature should be 200 ° C or less. The time is preferably 2 hours or less.

[Others]
After the quenching treatment and the tempering treatment are performed under the above-described conditions, the bearing can be finished by performing a surface treatment such as shot peening or a finishing surface polishing treatment as necessary.
Further, the present invention can be applied to both the inner ring and the outer ring constituting the bearing, and may be appropriately used as a member on the side that becomes the rate limiting factor of rolling fatigue failure when viewed as a bearing. It goes without saying that the present invention can be applied to both the inner ring and the outer ring.

  A 100 kg steel ingot having various compositions shown in Table 1 was soaked at 1250 ° C. for 20 hours, and then hot forged to 20 mmφ at a finishing temperature of 1000 ° C. to obtain a round bar. The round bar was subjected to spheroidizing annealing at 780 ° C. for 5 hours. This round bar was roughly processed into an inner ring and an outer ring of a ball bearing, subjected to induction hardening under the heat treatment conditions shown in Table 2, and then tempered at 170 ° C. for 2 hours. After that, the rolling part was finished to ▽▽▽▽ by surface polishing. Next, a steel ball was assembled between the inner ring and the outer ring, and the steel ball was fixed with a retaining ring to produce a # 6206 ball bearing shown in FIG. The steel balls used were 3 / 8-inch steel balls, heated at 850 ° C for 15 minutes and then quenched to austenite grain size of 6 µm, and salted at a heating rate of 50 ° C / s and a heating temperature of 850 ° C. Steel balls with an austenite grain size of 3.0 μm were prepared by quenching twice with a bath, and assembled between the inner ring and the outer ring, respectively.

In the rolling fatigue test, the inner ring was rotated while applying a load of 11760 N (1200 kgf) to the bearing, and the rolling fatigue test was conducted until separation occurred in the inner ring or the outer ring. Fatigue test was performed fifteen test to determine the B ₁₀ life at 10% cumulative failure probability.

  For the hardness and grain size of the test piece, the inner ring and outer ring were cut perpendicular to the rolling surface, the cut surface was polished to a mirror surface, and the Vickers hardness and old austenite grain size were investigated at a pitch of 0.1 mm from the rolling surface. did. The Vickers hardness was measured at a load of 2.94 N (300 gf), and the old austenite particle size was 0.1 mm pitch as described above, and the average prior austenite particle size at each depth position was determined. The depth from the surface of the transfer surface, which is μm or less, was obtained.

  As shown in Table 2, if an inner ring and an outer ring having a fine surface and a hardness of Hv 700 or more reaching a depth of 0.5 mm or more from the surface are used, the life of the bearing is greatly improved.

It is a figure which shows the structure of a ball bearing. It is a figure which shows the relationship between a prior-austenite particle size and quenching depth, and a rolling fatigue life.

Claims (13)

  A bearing made of bearing steel, which has excellent rolling fatigue characteristics characterized by having a steel structure at least 0.5 mm deep from the surface of the rolling part with a prior austenite grain size of 3.5 μm or less. Bearing inner ring.   2. The rolling according to claim 1, wherein the steel structure is obtained by heating the raw material in a two-phase region of austenite and spheroidized carbide, followed by quenching and quenching twice or more. Bearing inner ring with excellent dynamic fatigue characteristics. The heating to the two-phase region is performed at a heating rate from 800 ° C. to the maximum heating temperature of 0.5 ° C./s or more and 950 ° C./s or less, a maximum heating temperature of Ac ₃ or more, Ac ₃ + 130 ° C. or less, and a temperature of Ac ₃ points or more. The inner ring of a bearing having excellent rolling fatigue characteristics according to claim 2, characterized in that the residence time at is in accordance with a condition of 500 s or less.   The bearing inner ring having excellent rolling fatigue characteristics according to any one of claims 1 to 3, wherein a hardness from a surface to a depth position of 0.5 mm at the rolling portion is Hv700 or more.   The bearing inner ring having excellent rolling fatigue characteristics according to any one of claims 1 to 4, wherein the material is made of SUJ2.   A bearing having an inner ring according to claim 1 and having excellent rolling fatigue characteristics.   A bearing made of bearing steel, which has excellent rolling fatigue characteristics characterized by having a steel structure at least 0.5 mm deep from the surface of the rolling part with a prior austenite grain size of 3.5 μm or less. Bearing outer ring.   8. The rolling structure according to claim 7, wherein the steel structure is obtained by subjecting the raw material to a two-phase region of austenite and spheroidized carbide, followed by quenching and quenching twice or more. Bearing outer ring with excellent dynamic fatigue characteristics. The heating to the two-phase region is performed at a heating rate from 800 ° C. to the maximum heating temperature of 0.5 ° C./s or more and 950 ° C./s or less, a maximum heating temperature of Ac ₃ or more, Ac ₃ + 130 ° C. or less, and a temperature of Ac ₃ points or more. The outer ring of a bearing having excellent rolling fatigue characteristics according to claim 8, wherein the residence time is in accordance with a condition of 500 s or less.   10. The outer ring of a bearing having excellent rolling fatigue characteristics according to claim 7, wherein a hardness from a surface to a depth of 0.5 mm at the rolling part is Hv 700 or more.   11. The outer ring of a bearing having excellent rolling fatigue characteristics according to claim 7, wherein the material is made of SUJ2.   A bearing having an outer ring according to any one of claims 7 to 11 and having excellent rolling fatigue characteristics.   A bearing excellent in rolling fatigue characteristics comprising the inner ring according to any one of claims 1 to 5 and the outer ring according to any one of claims 7 to 11.

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