JP2008088482A - Roller or ball in bearing having excellent rolling fatigue property and crushing strength, and bearing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a roller or a ball having an improved rolling fatigue life, and further, having improved crushing strength, and to provide a bearing using the same. <P>SOLUTION: In the roller or ball of a bearing using a bearing steel as the stock, the retained austenite grain size of the steel structure from the surface at least to a depth position of 1.5 mm is controlled to ≤3.5 μm. For obtaining the steel structure in a transferring section, treatment where the stock is heated to a two phase region of austenite and spheroidized carbides, is thereafter rapidly cooled, and is quenched is performed for two or more times. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a roller or ball of a roller or ball bearing made of bearing steel, and further to a bearing using the roller, in particular, a roller or ball having improved rolling fatigue characteristics and crushing strength than before, And a bearing using the same.

Bearings are used in rotating parts such as automobiles and machines, and excellent rolling fatigue characteristics are required. In addition, the roller bearing or ball bearing requires the crushing strength of the roller or steel ball so as not to break even when a load is applied to the bearing. As a method for improving rolling fatigue characteristics, for example, as described in Patent Document 1, there is a method of defining a heating method of bearing steel, and the prior austenite grain size is refined to an average of 4.0 μm or less. With this, the rolling fatigue life is more than doubled. However, this patent document does not describe the crushing strength which is an important characteristic of the bearing. Good crushing strength cannot be obtained by quenching only the surface layer.
JP 2006-152407 A

The present invention is, 10% cumulative failure probability in the rolling fatigue test is an important characteristic of the bearing (hereinafter, B ₁₀ life) while providing rollers or steel ball bearings with greatly improved, another Important characteristics An object of the present invention is to provide a roller or a ball of a bearing in which the crushing strength of a certain bearing is greatly improved.

The inventors made bearing rollers and steel balls with a refined prior austenite grain size in the hardened part, tested 20 times under the same conditions, and the rolling fatigue life until fatigue peeling occurred at the rolling part. investigated, 10% cumulative failure probability (hereinafter, B ₁₀ life) were determined. 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. Furthermore, as a result of diligent investigations on the crushing strength, if the entire part requires a hardness of Hv700 or higher and the average austenite grain size from the surface to a depth of 1.5 mm is 3.5 μm or less, the crushing strength is improved 1.3 times or more. I found out that
This invention is completed based on this knowledge, The summary structure is as follows.

(1) In a roller or ball of a bearing made of bearing steel, the steel structure at least from the surface to a depth of 1.5 mm in the rolling part has a prior austenite grain size of 3.5 μm or less. Roller or sphere of bearings with excellent rolling fatigue characteristics and crushing strength.

(2) The steel structure is obtained as described in (1) above, wherein the steel structure is obtained by heating the material in a two-phase region of austenite and spheroidized carbide, followed by quenching and quenching twice or more. Roller or sphere of bearings with excellent rolling fatigue characteristics and crushing strength.

(3) 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 850 ° C./s or less, the maximum heating temperature of Ac ₃ or more and Ac ₃ + 130 ° C. or less, Ac ₃ The roller or sphere of a bearing excellent in rolling fatigue characteristics and crushing strength according to claim 2, characterized in that the heating condition is that the residence time at a temperature above the point is 500 s or less.

(4) A roller or sphere of a bearing excellent in rolling fatigue characteristics and crushing strength, characterized in that the hardness of the roller and the sphere is Hv700 or more in the entire depth direction in (1) to (3).

(5) A roller or sphere of a bearing having excellent rolling fatigue characteristics and crushing strength, wherein the material is SUJ2 in (1) to (4).

(6) A roller or sphere, wherein the roller or sphere of the bearing according to the above (1) to (5) is used.

  According to the roller or sphere of the present invention and the bearing using the roller or sphere, not only the rolling fatigue life but also the crushing strength can be improved, which is very useful industrially.

The roller or steel ball of the bearing of the present invention constitutes a roller bearing or ball bearing such as a bearing inner and outer ring, bearing ball, bearing roller and needle through a molding process (forging, cutting, etc.) using a steel material, a steel bar or a wire. After processing into the shape of the part, it is manufactured by quenching the rolling part or the entire part.
In order to obtain the present invention, it is necessary to optimize the composition of the steel material, the material structure, the prior austenite grain size and depth of the quenched surface layer, the quenching conditions, and the structure after quenching.
The present invention will be specifically described below.

[Steel composition]
First, a steel material suitable for obtaining a quenched surface layer portion required in the present invention as described later will be described. SUJ2, which is widely used as a bearing steel, is most preferably used as the steel material, but steel that satisfies the following composition can be used.

C: 0.6mass% ~ 1.5mass%
C is an element necessary for securing the hardness necessary for obtaining the fatigue life of the part in the quenched portion. If it is less than 0.6 mass%, sufficient hardness and fatigue strength cannot be obtained in the quenched portion. On the other hand, when added in excess of 1.5 mass%, the workability before quenching (shearability, forgeability) is deteriorated. Therefore, a suitable C content range is 0.6 mass% to 1.0 mass%.

Si: 0.1-1.0mass%
Si is preferably contained in an amount of 0.1 mass% or more in order to improve the rolling fatigue life. However, if added over 1.0 mass%, like C, the workability before quenching (shearability, forgeability) is degraded. Therefore, the preferable content range of Si is 0.1 to 1.0 mass% or less.

Mn: 0.1 ~ 1.5mass%
In order to improve hardenability, Mn is preferably contained in an amount of 0.1 mass% or more. 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%
Cr is preferably contained in an amount of 0.05 or more because it has the effects of improving the hardenability and reducing the hardness before quenching by promoting the spheroidization of carbide and improving the workability. 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%.

  The balance other than the elements described above is Fe and inevitable impurities are basic components. 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. In addition to the basic component composition described above, the following elements may be contained within the ranges described below.

S: 0.03 mass% or less S may be added to improve the machinability by combining with Mn to form MnS, but if added over 0.03 mass%, MnS is in the rolling fatigue test. The upper limit of the content is preferably 0.03 mass%, since it becomes a crack starting point and the rolling fatigue characteristics are remarkably lowered.

Al: 0.1 mass% or less
Al is a component that has a strong deoxidizing effect and has an effect of improving the cleaning of steel, but may be added, but when added over 0.10 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 the effect of improving the hardness of the hardened part 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 increase the hardenability and improve the toughness of the hardened part. Further, when Cu is added, it is preferable to add 1/2 of the amount of Cu added to suppress hot brittleness.

Mo: 1.0 mass% or less
Mo may be added because it has an effect of improving hardenability and resistance to temper softening, but is preferably 1.0 mass% or less because workability deteriorates.

W: 1.0 mass% or less
W may be added because it has an effect of improving hardenability, but is preferably 1.0 mass% or less because workability is deteriorated.

Ti: 0.01 mass% or less
Ti may be added because it has the effect of suppressing the growth of austenite grains due to the formation of nitrides. However, if it exceeds 0.01 mass%, the rolling fatigue characteristics deteriorate, so 0.01 mass% or less is preferable.

Nb: 0.5 mass% or less
Nb may be added because it has an austenite grain growth suppression effect due to nitride (or carbonitride) formation, but if its content exceeds 0.5 mass%, the effect is saturated, so 0.5 mass% or less It is preferable to do.

B: 0.01 mass% or less Since B has an effect of improving hardenability, 0.01 mass% may be added to the upper limit, but if its content exceeds 0.01 mass%, the effect is saturated, so 0.01 mass% The following is preferable.

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 deterioration of the rolling fatigue characteristics, so may be added. However, if its 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, but excessive addition is preferably 0.01 mass% or less because it deteriorates the workability of steel.
The balance other than the elements described above is Fe and inevitable impurities.

[Structure of steel material]
Steel parts for bearings are formed by cutting, grinding, forging, etc. from the steel material before quenching, so the structure of the steel material before quenching is spheroidized, the parent phase is ferrite, and the carbide is spherical. Need to be. 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 the growth of austenite grains during the quenching process.

[Old austenite grain size of hardened surface layer]
Steel parts for bearings are quenched and tempered because they require rolling fatigue characteristics. In the present invention, the prior austenite grain size needs to be an average of 3.5 μm or less in the quenched surface layer portion which is particularly important for rolling fatigue characteristics. This is because the rolling fatigue life is remarkably improved as compared with that having a rolling fatigue life exceeding 3.5 μm.

On the other hand, in order to improve the crushing strength, which is another important characteristic of the bearing, the region in which the prior austenite grain size is 3.5 μm or less needs to be at least 1.5 mm deep from the surface. Thereby, a high crushing strength can be obtained.
In addition, the measurement of the area | region whose particle size is 3.5 micrometers or less was performed as follows. First, after corroding old austenite grain boundaries with our developed gamma R solution, after taking four SEM images at a magnification of 5000x at positions of 0.1mm, 0.2mm, 0.3mm ... and 0.1mm pitch from the surface Then, the area of each austenite grain was measured with an image analyzer, the equivalent circle diameter (2 × (area / π) ^1/2 ) was determined from the area, and the average austenite grain size at that position was determined. The position where the average prior austenite grain size was 3.5 μm or less was defined as “region where the prior austenite grain size was 3.5 μm or less”.

  FIG. 1 and FIG. 2 show the relationship between the crushing strength of SUJ2 (component composition of steel symbol 1 in Table 2 to be described later), the particle size, and the region where the average prior austenite particle size is 3.5 μm or less. In a cylindrical test piece of 6.0mmφ × 9.0mm length, steel with depth of 1.0mm and 2.0mm in the area where the prior austenite grain size is 3.5μm or less (Note that the quenching hardness is Hv750 or more at all depths) Yes, all the prior austenite grain size in the center is 4.5μm), and for comparison, the surface layer (old austenite grain size at 0.1mm from the surface) and the central part have an average austenite grain size of 4.3μm and hardness A specimen having a Hv of 750 or more in the entire region (a region with a prior austenite grain size of 3.5 μm or less is assumed to be 0 mm) was also produced. In such a test piece, compression was performed until the test piece was broken as shown in FIG. 3 (crush test), and the breaking load was measured. This crushing test was performed 10 times, and the average crushing strength was calculated.

  As shown in FIG. 1, the calculation result shows that the crushing strength can be increased to a high level by setting the area where the prior austenite grain size is 3.5 μm or less to a depth of at least 1.5 mm from the surface. As described above, the crushing strength is improved by 1.3 times or more in the steel in which the depth of the region where the prior austenite grain size is 3.5 μm or less is at least 1.5 mm from the surface.

[Hardness of hardened part]
In the present invention, the roller of the roller bearing or the ball of the ball bearing has a Vickers hardness (hereinafter referred to as Hv) of 700 or more even in a region other than the former austenite grain size of 3.5 μm or less. It is assumed that the Vickers hardness (hereinafter referred to as Hv) is 700 or more over the entire depth direction of the sphere. This is because sufficient crushing strength cannot be obtained with a member having a hardness of less than Hv700.

[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. Regarding the number of times of quenching, it is only necessary to perform N quenching (N = 1 or more), but the heating conditions in the Nth (last) quenching process are:
(1) Heating temperature: Ac ₃ points or higher and Ac ₃ + 130 ° C or lower (2) Heating rate: 800 ° C to Ac ₃ points average temperature 0.5 ° C / s or higher and 850 ° C / s or lower (3) Ac ₃ points or higher Dwell time of: need to be 500 seconds or less.

Here, the Ac ₃ point is a 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 point, 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 spheroidized 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 exceeds 3.5 µm. End up.

The heating rate of the quenching treatment needs to be an average of 0.5 ° C./s to 850 ° C./s between “800 ° C.” and “Ac ₃ points” temperatures. 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 a decrease in nucleation driving force to austenite, and the prior austenite grain size of the quenched structure exceeds 3.5 μm. End up. If it exceeds 850 ° C./s, the area of the prior austenite grain size of 3.5 μm or less becomes less than 1.5 mm, and the crushing strength becomes less than 1.3 times.

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. When the residence time of ₃ points or more of Ac exceeds 500 seconds, the time is sufficient for grain growth, and the prior austenite grain size of the structure after quenching exceeds 3.5 μm.

  The quenching treatment under the above conditions should be performed twice or more. The reason for limiting to two or more times is to make both the prior austenite grain size 3.5μm and the former austenite grain size 3.5μm area 1.5mm or more compatible. With only one heating, the prior austenite grain size can be 3.5μm or less. However, the depth of the region was not obtained. This heating rate condition may be applied only at the time of the final quenching process (only the Nth time when performing the N times quenching process). In the quenching process that is performed prior to the final quenching process (in the case of N quenching process, the first to N-1 quenching process), the structure after quenching is 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, if heating is performed at such a high temperature that the residual spherical carbide dissolves, the effect of suppressing the austenite grain growth by the spherical carbide during the final quenching disappears, and the austenite grains become coarse and non-uniform, Since the amount of carbon penetration into the parent phase becomes higher than the optimum value and the rolling fatigue characteristics are deteriorated, the temperature at which the spheroidized carbide dissolves in austenite and becomes a single austenite phase during the quenching process before the final quenching. It is necessary to do the following. In addition, regarding the number of times of quenching, quenching may be performed for a plurality of times, but from the viewpoint of industrial and cost, it is preferable that the number of quenching is up to two.

  By performing quenching treatment under the conditions described above, the roller or steel of the bearing has an average prior austenite grain size of 3.5 μm or less and an area having an average prior austenite grain size of 3.5 μm or less at least 1.5 mm deep from the surface. A sphere 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 and tempering treatments are performed under the above conditions, surface treatment such as shot peening and finishing surface polishing treatments are performed as necessary, and there is no problem even if the steel parts for bearings are finished.

  By making it explained for each of the above items, a roller or sphere having both rolling fatigue characteristics and crushing strength can be obtained, and by using such a roller or sphere as a roller or sphere of a bearing, The rolling fatigue life of the bearing can be improved, and the bearing can be used with a high load. In addition, it is preferable to satisfy the above-described conditions for the rolling surfaces of parts constituting the bearings other than the rollers and balls, such as bearing inner and outer rings.

A 100 kg steel ingot of 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. A cylindrical test piece having a diameter of 12.2 mmφ was roughly machined from the center portion of the round bar, subjected to quenching treatment under the heat treatment conditions shown in Table 2, and then tempered at 170 ° C. for 2 hours.
In the rolling fatigue test, a cylindrical specimen having a length of 12 mmφ × 22 mm was finished and subjected to the rolling fatigue test. In the rolling fatigue characteristic evaluation, the rolling fatigue life until the test piece was peeled was investigated using a radial rolling fatigue tester manufactured by NTN. In the fatigue test, 20 tests were conducted at a Hertzian stress of 5880 MPa (600 kgf / mm ² ) stress load number of 46400 cpm, and the life when 10% cumulative failure was established (hereinafter referred to as B ₁₀ life) was obtained.

  In addition, the crushing test produced a cylindrical test piece of 6.2 mmφ x 9.2 mm length and was subjected to induction quenching. After reaching a predetermined temperature in a salt bath heated to 1100 ° C, the quenching operation was repeated twice. Thus, samples with various grain sizes and various average prior austenite grain sizes of 3.5 μm or less were prepared. Then, finish polishing to 6.0 mmφ × 9.0 mm was performed to prepare a test piece simulating a roller bearing. This sample was subjected to a crush test 10 times by the test method of FIG. 3, and the average value of the crush strength was calculated.

The quenching hardness of the test specimens was determined by rounding at a half of the length of the cylindrical specimens and polishing to a mirror surface for both rolling fatigue specimens and crush specimens, and Vickers hardness at 0.1 mm pitch from the surface. The prior austenite grain size was investigated. As described above, the previous austenite grain size was obtained by taking four 5000-fold SEM images at each position, measuring the area of each austenite grain with an image analyzer, and calculating the equivalent circle diameter (2 × (area / π) ^1/2 ) and the average austenite grain size at that position. The position where the average prior austenite grain size was 3.5 μm or less was defined as “region where the prior austenite grain size was 3.5 μm or less”.
The hardness was all Hv700 or higher in all cross sections.

It is a figure which shows the relationship between the depth and the crushing strength whose prior γ grain size is 3.5 μm or less. It is a figure which shows the relationship between the depth and the improvement rate of the crushing strength where the former γ grain size is 3.5 μm or less. It is a figure which shows a crush test method.

Claims (6)

In bearing rollers or balls made of bearing steel,
A roller or sphere of a bearing excellent in rolling fatigue characteristics and crushing strength, characterized in that the steel structure at least 1.5 mm deep from the surface in the rolling part has a prior austenite grain size of 3.5 μm or less.   2. The rolling fatigue property according to claim 1, wherein the steel structure is obtained by heating the material to a two-phase region of austenite and spheroidized carbide and then quenching and quenching twice or more. Bearing roller or sphere with excellent crushing strength. 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 850 ° C./s or less, the maximum heating temperature of Ac ₃ or more, Ac ₃ + 130 ° C. or less, and Ac ₃ points or more. The roller or sphere of a bearing excellent in rolling fatigue characteristics and crushing strength according to claim 2, wherein the heating condition is such that the residence time at temperature is 500 s or less.   4. The roller or sphere of a bearing excellent in rolling fatigue characteristics and crushing strength according to claim 1, wherein the hardness of the roller and the sphere is Hv 700 or more throughout the depth direction.   5. A roller or sphere of a bearing according to claim 1, wherein said material is made of SUJ2 and has excellent rolling fatigue characteristics and crushing strength.   A roller or sphere, wherein the roller or sphere according to claim 1 is used.

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