JPH1171639A - Rolling bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rolling bearing having a long service life for driving a hard disk. SOLUTION: In a ball bearing composed of an inner ring, an outer ring and a rolling element, among the structural parts, either the inner ring or the outer ring, or both are those ones contg., by weight, 0.45 to 0.8% C, 0.5 to 1.5% Si, 0.05 to 0.3% Mn, <=0.005% S, <=0.5% Mo, <=0.015% O, and the balance Fe with inevitable impurity elements, having a structure mainly composed of graphite, cementite and ferrite and produced by using such a base stock that the area ratio of graphite in the cross-sectional area of the structure is regulated to 0.5 to 3%. By quenching and tempering treatment, on the surface of the finished race product, the average grain size of graphite is regulated to 3 μm, the total content of graphite is regulated to <=1.5% by area ratio, the content of residual austenite is regulated to <=6 vol.%, and its hardness is regulated to >=58 HRC.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hard disk drive (hereinafter referred to as HDD), a video tape recorder (VTR), a digital audio tape recorder (DAT), etc., for supporting a spindle rotating at high speed. Alternatively, the present invention relates to a bearing having a very small diameter for supporting a swing arm incorporated in an HDD or the like or a bearing requiring quietness.

[0002]

2. Description of the Related Art In general, a rolling bearing is subjected to rolling motion between a raceway surface and a rolling surface and repeatedly receives contact stress. Therefore, these materials are hard and endure a load, have a long rolling fatigue life, and have a long sliding fatigue life. It is required that the abrasion resistance is good. Therefore, these materials are generally quenched after quenching or carburizing or carbonitriding a steel material equivalent to Japanese Industrial Standards SCR420 for bearing steel, and case hardening steel for case hardening steel. And a hardness value of 58 to 64 on the Rockwell C scale (HRC) to secure required life and wear resistance.

[0003] In recent years, the density of HDD devices has been increasing, and bearings incorporated therein have been required to have higher functions and higher quality than ever before. For the purpose of cost reduction, there is an increasing demand for cost reduction for bearings.

[0004] The bearings built into the HDD device include a spindle motor bearing for rotating the magnetic disk and a swing arm bearing for swing drive provided for positioning the access to the effective area of the magnetic disk. It can be roughly divided into two.

[0005] In general, a bearing material for these HDD devices is SUJ2 (JIS G) which is a high carbon chromium bearing steel.
4805) is often used. SUJ2 is 82
After quenching in the temperature range of 0 to 860 ° C., tempering is performed in the temperature range of 160 to 200 ° C., and as a result of these heat treatments, the hardness of the bearing ring obtained is in the range of HRC 58 to 64, and the amount of retained austenite is 8 to 14% by volume. In addition, in the case of a swing arm bearing or the like which is required to have corrosion resistance or is particularly driven to swing, the race is often made of martensitic stainless steel having good corrosion resistance and fretting resistance. As the stainless steel used in such a case, SUS440C (JIS G4303) or 13Cr-based martensitic stainless steel is 1050.
After quenching at a temperature of about 100 ° C., a deep cooling treatment (sub-zero treatment) is performed, and tempering is performed in a temperature range of 150 to 200 ° C. As a result of these heat treatments, the hardness of the bearing ring obtained is H
RC 57-62, the amount of retained austenite is 8-12% by volume.

[0006]

The very small diameter ball bearing used for the spindle of the HDD device is extremely strictly required to reduce the torque, the sound and the noise. The accuracy is JIS Class 5 or higher.

However, in the conventional bearing, the basic static load rating C ₀ (maximum contact surface pressure, 400) is determined so as not to cause permanent deformation (plastic deformation) of the raceway surface or the rolling surface.
A load or impact load much smaller than 0 MPa) causes an extremely small permanent distortion on the raceway surface and the rolling surface, and this permanent distortion may cause unacceptable noise during rotational driving. HDDs are required to have a very high level of quietness that does not generate noise or noise. Therefore, even a slight acoustic deterioration of the bearing is a problem.

This is due to the miniaturization of HDD devices and the like.
As the ball bearings incorporated therein, smaller ones are being used, and impacts are applied due to an increased chance of dropping due to the portability of such devices themselves. Due to increased opportunities. When an impact is applied, the raceway surface and rolling surface are permanently deformed even with a relatively small impact load, causing deterioration of the performance of equipment incorporating ball bearings, such as acoustic deterioration and unevenness in rotational torque. Is often generated due to low yield stress of retained austenite and MnS contained in the steel constituting the bearing ring and the rolling elements.

Therefore, in recent years, the residual austenite has been reduced as much as possible by quenching and then performing a deep cooling process (subzero) or further tempering in a high temperature range of about 240 ° C. We try to reduce it.

On the other hand, a small amount of S combines with Mn to form an A-based inclusion of MnS. This MnS is B such as Al ₂ O ₃
Rolling contact fatigue life is not affected as much as that of system inclusions and other hard nonmetallic inclusions.It also acts as a free-cutting component and improves the machinability of steel. Steel often contains S in an amount of about 0.005 to 0.020% by weight.

However, when the content of MnS is large due to a low yield stress, the acoustic load is apt to deteriorate due to an impact load, and good silence cannot be obtained. Therefore, if S is reduced in order to improve the problem of acoustic degradation, the machinability of steel is reduced and the cost is increased. When manufacturing a rolling bearing, the largest manufacturing cost is the processing cost, and manufacturing a bearing using a material having good workability is the most effective in reducing costs.

Materials having good machinability include, for example, Pb and Ca,
Free-cutting steel containing S or the like can be used. However, although these free-cutting steels have good machinability, sufficient functions as rolling bearings cannot be obtained due to the effect of free-cutting components.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has excellent acoustic characteristics (quietness), minimizes manufacturing costs as much as possible, and is suitably used for inexpensive and high-performance HDDs. It is an object of the present invention to provide a rolling bearing having a good quietness.

[0014]

Means for Solving the Problems JP-A-7-188844
Japanese Patent Application Laid-Open No. 7-1888851 discloses an attempt to improve the machinability of carbon steel for machine structures by graphitizing carbon as a raw material. Such graphite is generated in a process in which lamellar (lamellar) cementite in pearlite changes from a metastable phase to a stable phase, and as the graphitization proceeds, the cementite disappears and the graphite grows coarsely. . These prior arts are structural carbon steels mainly made of graphite and ferrite, and focus on machinability, cold workability, etc., and require rolling fatigue life, quietness, etc. Are not sufficient for use in rolling bearings.

Further, Japanese Patent Application Laid-Open No. Hei 8-20841 proposes that such a graphite steel be subjected to a surface hardening treatment and that a residual austenite be imparted to a surface layer to secure a rolling fatigue life. However, since retained austenite causes acoustic deterioration, it is harmful for extremely small diameter ball bearings that require quietness. In addition, heat treatment in production or cost requires soaking after heating in RX gas. Therefore, the present invention cannot be applied to a rolling bearing with good silence, which is the object of the present invention.

Japanese Patent Application Laid-Open No. 2-274837 discloses that
By similarly performing a surface hardening treatment by induction hardening and forming a surface layer substantially free of graphite,
A bearing material having good rolling fatigue life and quietness is disclosed.
However, these prior arts do not provide residual austenite or Mn.
No mention is made of the machinability, quietness, or the effect on acoustic degradation caused by S.

On the other hand, in recent years, with the progress of graphitization technology, it has become possible to shorten the graphitization time and to reduce the size of graphite grains (Yuki Ota et al. (Kawasaki Steel Corporation) Materials and Processes, Vol. 9, PAGE.406, 1996 and Takashi Iwamoto et al. (Kawasaki Steel) Materials and Processes, Vol.
E. FIG. 407, 1996).

The inventors of the present invention have studied the use of various graphite steels mainly made of cementite, graphite, and ferrite for rolling bearings in terms of cost and function. As a result, the following findings (1) to (3) were obtained.

(1) Good machinability and excellent quietness can be obtained by using a graphite steel material having an area ratio of graphite of 0.5% or more and 3% or less. (2) In a steel containing graphite, since the graphite itself acts as a free-cutting component, even if MnS is reduced, the machinability is hardly affected.

(3) By reducing coarse graphite, residual austenite and MnS on the finished bearing surface after heat treatment, acoustic deterioration can be significantly prevented.
From the above findings, it has been found that a rolling bearing with less rolling fatigue characteristics, quietness, and less acoustic degradation due to an impact load can be provided, and the present invention has been completed.

The rolling bearing according to the present invention is a ball bearing composed of an inner ring, an outer ring and a rolling element, wherein one or both of the inner ring and the outer ring of the component parts are expressed by weight% and C: 0. 45-0.8, Si: 0.5-1.
5, Mn: 0.05 to 0.3, S: 0.005 or less, M
o: 0.5 or less, O: 0.015 or less, the balance contains Fe and unavoidable impurity elements, and has a main structure of graphite, cementite and ferrite, and the area ratio of graphite in the structure cross section is 0.5% to 3%. Manufactured using the following materials, and the surface of the finished raceway ring is subjected to quenching and tempering treatment, the average particle size of graphite is 3 μm, the total content of graphite is 1.5% or less in area ratio, and the amount of retained austenite is 6% by volume or less and hardness of 58 or more HRC.

[0022]

[Effect] Graphite has a remarkable effect of improving the machinability of steel, and in order to sufficiently exhibit its effect, the amount of graphite must be 0.5% or more in area ratio. On the other hand, if graphite is present in excess in an area ratio exceeding 3.0%, coarse growth of graphite adversely affects characteristics such as quietness and acoustic deterioration of the bearing. Further, the time required for the graphitization process is inevitably increased, resulting in an increase in cost and the difficulty in dissolving the coarsely grown graphite by the heat treatment. Therefore, the material in the present invention is mainly made of graphite, cementite, and ferrite. It is assumed that the graphitization treatment has been performed in the range of 0.5% or more and 3% or less.

Further, the steel of the present invention satisfies the function as a rolling bearing, and further has a residual austenite and M
In order to avoid the problem of acoustic deterioration of the rolling bearing caused by nS, the composition and structure were limited to the following. Less than,
Unless otherwise stated,% means% by weight. 1) C: 0.45 to 0.8% C is the most important element for converting the base material into martensite and obtaining the required hardness as a bearing. Further, in the raw material stage, most of C exists as graphite and cementite, and particularly graphite acts as a free-cutting component to enhance the machinability of steel. At least 0.45 to achieve these effects
% Is necessary, and if added over 0.8%,
Both the function and the workability of the bearing cannot be satisfied, for example, the machinability is reduced due to the coarsening of graphite or the increase in the amount of cementite. In addition, residual austenite acting on acoustic deterioration is easily generated. For the above reasons, the C content is in the range of 0.45% or more and 0.8% or less. 2) Si: 0.5 to 1.5% Si is an element necessary as a deoxidizing agent in steel making, and is also an element that improves hardenability and destabilizes cementite in steel to promote graphitization. Useful. Further, it enhances temper softening resistance and improves strength and rolling fatigue life. If the addition amount is less than 0.5%, the effect is difficult to obtain. If the addition amount exceeds 1.5%, the effect is saturated and the cost is further increased. Therefore, the Si content of the material is 0.5%. The range is at least 1.5%. 3) Mn: 0.05 to 0.3% Mn is an element necessary for deoxidation at the time of steel making like Si, and further has a low melting point by fixing S in steel as sulfide MnS. It is an element effective for preventing generation of FeS. Further, Mn is an element that enhances the hardenability of steel, contributes to the improvement in strength after heat treatment and the rolling fatigue life, and promotes the generation of retained austenite effective for the rolling life. However, in HDD devices, VTR devices, and other bearings that require particularly low noise, retained austenite and MnS are deemed harmful, and large amounts of them are not preferred. And On the other hand, if the amount of Mn is too small, a small amount of S in the steel is formed as FeS and the hot workability is reduced. Therefore, it is preferable that Mn contains about 10 times the S content. For this reason, Mn
The lower limit of the content was set to 0.05%. 4) S: 0.005% or less S combines with Mn to form MnS and has an effect of improving the machinability of steel. However, in a ball bearing having a relatively small diameter,
Since the yield stress of MnS is low, an extremely small permanent deformation occurs on the raceway surface, which causes acoustic deterioration. When the S is reduced, the machinability of the steel decreases, but if the graphite steel is finely dispersed graphite, since graphite acts as a free-cutting component,
Since reducing the S has almost no effect on the machinability, in the present invention it is preferable to reduce the amount as much as possible. Further, if the S content exceeds 0.005%, acoustic deterioration is apt to occur gradually, so the upper limit value is set to 0.005%. 5) Mo: 0.5% or less Mo is selectively added because it enhances the hardenability and contributes to the improvement of the strength after heat treatment, the rolling fatigue life, and the tempering softening resistance. Preferably, 0.15% or more is added. However, if the content is too large, it adversely affects graphitization and processability, and further increases the cost. Therefore, the upper limit was set to 0.5%. 6) Oxygen: 0.015% or less Oxygen is a factor for forming hard B-based inclusions. When the B-based inclusions coarsely aggregate, not only the rolling fatigue life,
Since it also affects the quietness of the bearing, it is preferable to reduce the oxygen in steel as much as possible,
5%. 7) Average particle size of graphite on the surface of the finished bearing product: 3 μm or less If the size and amount of graphite distributed in the steel are too large, respectively, it is good due to the influence of pit edges and the like that occur when graphite is dropped when the bearing rotates. The quietness cannot be obtained. By providing no graphite or having an average particle size of graphite of 3 μm or less and a total content of preferably 1% or less in area ratio, a bearing with good quietness can be obtained. It should be noted that extremely fine and small amount of graphite exhibits vibration damping ability (vibration damping property) and absorbs vibration, thus improving quietness. 8) The total content of graphite on the surface of the finished bearing product: 1.5% or less in area ratio On the surface of the finished bearing product, the self-lubricating effect of graphite and the microelastic fluid lubricating effect (M- Since the EHL effect is obtained and the rotation performance tends to be improved, graphite is preferably contained in an area ratio of 0.1% or more, more preferably about 0.5% or more. 9) Amount of retained austenite on the finished bearing surface;
% By volume or less Retained austenite (γR) causes an extremely small permanent distortion on the raceway surface and rolling surface when an impact load is applied, and this causes acoustic deterioration and uneven rotation torque. . If the amount of retained austenite is less than 6% by volume, there is no practical effect, so the maximum allowable value is set to 6% by volume. 10) Rockwell C scale hardness on the finished bearing surface: HRC 58 or more If the hardness on the finished bearing surface is not HRC 58 or more, not only the rolling fatigue life is reduced but also the internal friction is increased and the rotational performance is reduced. In addition, there is a problem that the built-in scratches are easily formed on the raceway surface and the rolling surface, and the acoustic defect rate is increased. For this reason, the lower limit value of the hardness on the surface of the finished bearing product was set to HRC58.

For the above reasons, in the rolling bearing according to the present invention, the average particle size of graphite is 3 on the surface of the finished bearing product.
μm or less, the total content of graphite is 1.5% or less in area ratio, the amount of retained austenite is 6% by volume or less, and the hardness is HRC58.
All of the above requirements were satisfied. 11) Inevitable impurity elements; 0.001% ≦ (N, B)
≦ 0.015%, (Al, Ti, Nb, Zr) ≦ 0.0
The greater the number of precipitation nuclei, the finer the 5% graphite is dispersed and deposited. B, Al, Ti, Nb, Z
and nitrides such as r, other REMs (rare earth elements) and oxides thereof, which contribute to crystallization of graphite and promote graphitization. These elements are added in very small amounts or inevitably contained in steel and exist as nitrides and oxides to promote graphitization.However, excessive addition of graphite causes coarsening of graphite and rolling life. And undesired effects such as reduced silence. For the above reasons,
Nitrogen and B are 0.015% or less, other elements are 0.0
It is adjusted to be 5% or less.

Attempts to extremely reduce the contents of all of these elements lead to an increase in cost and to a reduction in the number of graphite nuclei resulting in the inhibition of graphitization. Is not performed. Note that B and N
Is preferably left at 0.001% or more. The reason for this is that B and N form compound BN, which acts as a precipitation nucleus of graphite in particular.

[0026]

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings and tables. Table 1
In addition, the chemical composition (% by weight), the area ratio of graphite (%), and the tool life (minute) as a result of the machinability test are also described for various Examples and Comparative Examples. Although only the main elements are shown in the column of the chemical components in Table 1, Al, Ti, Nb, B, N, and the like are contained as inevitable impurity elements or trace addition elements at about several tens ppm. .

The size and amount of graphite were adjusted by changing the treatment time in the range of 680 ° C. to 710 ° C. for graphitization of steel. The average area ratio of graphite was determined by image analysis after observing a reflected electron image (composition image) in 30 visual fields under a scanning electron microscope (1000-fold magnification).

Also, the machinability test was conducted according to JIS B 4011.
The cutting conditions were determined according to the following conditions. The time (minutes) required for the flank wear of the cutting tool to reach 0.2 mm was defined as the tool life.

[Machinability test] Cutting machine: High-speed lathe Tool: P10 (JIS B 4053) Cutting speed: 200-250 m / min Feeding amount: 0.2-0.3 mm / rev Cutting depth: 1.0- In steel containing 1.5 mm graphite, graphite acts as a free-cutting component, and therefore, no effect on machinability was observed even when the amount of S was reduced. Furthermore, as is clear from Table 1, the machinability of the steel types A1 to A18 containing graphite was remarkably superior to the steel types A19 and SUJ2 not containing graphite.

Next, these steel types A1 to A19, SUJ
2 was used as a material to actually manufacture a ball bearing 1810, and various performances required for the bearing were evaluated. Examples 1 to 15 (steel type A1)
In A15), after quenching from a temperature range of 840 to 870 ° C., tempering was performed in a temperature range of 160 to 220 ° C. so that the amount of retained austenite was 6% by volume or less.
On the other hand, Comparative Examples 16 to 20 (A16 to A19, SUJ2)
After quenching from a temperature range of 840 to 870 ° C,
Tempering was performed in a temperature range of 0 to 180 ° C.

Table 2 shows the hardness of the surface of the finished bearing after the heat treatment, the average particle size and content of graphite, the amount of retained austenite, and the results of the evaluation of various functions of the bearing. The measurement of the particle size and area ratio of graphite was performed by the same method as the measurement of the graphite area ratio of the raw material. Twelve SUJ2 balls were used for the rolling elements incorporated in the bearings (deep groove balls), plastic ones were sealed in the retainers, and mineral oil-based ones were used for the grease.

[Measurement Evaluation of Acoustic Deterioration] The sound degradation level was measured by using a bearing sound pressure measuring device for two bearings 1810 (deep-small small-diameter ball bearings) to be incorporated.

Next, an inner ring rotary type rolling bearing W was incorporated into the shaft 8 of the HDD spindle motor shown in FIG. 1, and a 25 kgf impact load (drop test) was applied thereto. Reference numeral 1 is an outer ring, 2 is an inner ring, 3 is a rolling element, 4 is a stator, 5 is a rotor, 6 is a seal, 7 is a housing, and 7a is a flange. The flange 7a was fixed with bolts to apply an impact to the bearing W. Thereafter, the bearing W was removed from the shaft 8, and the sound pressure value of the bearing W was measured in a state of the bearing alone.

The value obtained by subtracting the sound pressure value before the test from the sound pressure value after the impact test was determined as an acoustic deterioration value (dB). Table 2 shows the results. From Table 2, it can be seen that Mn affects the impact force.
The relationship between the size of S, retained austenite (γR) and graphite and the area ratio became clear. That is, Examples 1 to 15
In this case, the acoustic deterioration value was extremely small at 1.5 dB or less, and was less than 1.1 dB in most examples. On the other hand, in Comparative Examples 16 to 20, the sound degradation values were 2.6 dB, respectively.
It becomes large at 2.9 dB, 3.5 dB, 3.2 dB, and 4.5 dB, and the degree of deterioration is large.

[Measurement Evaluation of Anderon Value] With respect to the anderon value, the anderon value (Medium, High Band) of the bearing 1810 alone (without applying an impact load) is also measured by the Anderon meter of the bearing 1810 alone incorporated in FIG.
Was measured. Table 2 shows the results. Symbol M. in Table 2. B is an abbreviation for Medium Band, and the symbol H.B. B is an abbreviation for High Band. From Table 2, it was found that Examples 1 to 15 had low Anderon values and were excellent in quietness. On the other hand, in Comparative Examples 16 to 20, it was found that the Anderon value was high and the quietness was inferior.

[Measurement and Evaluation of Torque] As for the torque, the torque of the shaft 8 was measured in a state of being incorporated in the spindle (without applying an impact load). From Table 2, in Examples 1 to 15, the torque was 0.30 to 0.39 g. cm range. On the other hand, in the comparative examples 19 and 20, the torque was 0.41, 0.40.
And high. Since these comparative examples 19 and 20 are steel types containing no graphite, they are inferior in quietness. The torque was measured by applying an axial load of 1 kgf to each of the two single bearings and measuring the torque of the inner ring in the relative rotation between the inner ring and the outer ring while rotating the inner ring at 2 rpm, as in the measurement and evaluation of the sound deterioration. did.

As can be seen from Table 2, in Examples 1 to 15, both the amount of S and the amount of retained austenite were reduced, so that there was little acoustic degradation and good quietness. In contrast, Comparative Example 16,
No. 17 has a large S content and a large amount of retained austenite, and is relatively good in silence, but tends to easily deteriorate in sound.
In Comparative Example 18, although the S content was small, the graphite was coarse, so that the quietness and the acoustic deterioration tended to be slightly inferior to those of the steel of the present invention. Further, Comparative Examples 19 and 20 do not contain graphite in the raw material, so that the machinability is poor and the cost cannot be avoided. In addition, the amount of retained austenite is large, so that the acoustic deterioration is large.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

As described above, the rolling bearing according to the present invention has a larger size and amount of graphite distributed in the steel, and a residual austenite and M, which are present, than the conventional graphite steel.
Since the amount of nS is reduced as much as possible, it can be manufactured at low cost with excellent acoustic characteristics (quietness), good quietness, little acoustic degradation, and low noise.

[Brief description of the drawings]

FIG. 1 is an enlarged sectional view showing a rolling bearing according to an embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Outer ring, 2 ... Inner ring, 3 ... Rolling element, 4 ... Stator, 5 ...
Rotor, 6: seal, 7: housing, 7a: flange, 8: shaft, W: rolling bearing.

Claims (1)

[Claims] In a ball bearing composed of an inner ring, an outer ring and a rolling element, one or both of an inner ring and an outer ring of the component parts are expressed as C: 0.45-0.
8, Si: 0.5 to 1.5, Mn: 0.05 to 0.3,
S: 0.005 or less, Mo: 0.5 or less, O: 0.01
5 or less, containing the balance Fe and unavoidable impurity elements, graphite, cementite and ferrite as main structures,
Manufactured using a material having an area ratio of graphite of 0.5% or more and 3% or less in the cross section of the structure, and the surface of the finished raceway ring is subjected to a quenching and tempering process to have an average graphite particle size of 3 μm and a total content of graphite. A rolling bearing having an area ratio of 1.5% or less, a retained austenite amount of 6% by volume or less, and a hardness of 58 or more HRC.