JP2883460B2 - Bearing steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolling bearing for use in an engine of an automobile or the like and an auxiliary machine such as an alternator driven by the engine, particularly used under vibration or impact load, or a rolling of an automobile or the like. The present invention relates to bearing steel used for sliding parts.

[0002]

2. Description of the Related Art High carbon chromium bearing steel (especially JIS SUJ2) is most often used as a material for a race and a rolling element of a rolling bearing. Various materials are used. For example, in the case of a bearing to which an impact load acts, a case-hardened steel (such as SAE5120) subjected to a carburizing quenching / tempering treatment to enhance toughness is used.

In recent years, the size, weight, and performance of engines such as engines of motor vehicles and alternators driven by the engines have been rapidly reduced, and the rolling bearings used for them have also been reduced in size. The operating speed has also been increased. For this reason, the vibration / impact load applied to the rolling bearing increases remarkably, and the phenomenon that the bearing temperature rises occurs. In the above-mentioned conventional steel, there is a problem that peeling occurs in a short time and the steel cannot be used. . For example, in an alternator that is driven by an engine to generate power, in addition to the impact load caused by pushing up from the road surface when driving a car,
Vibration caused by the drive belt from the engine is constantly applied to the bearing. The miniaturization, weight reduction and high performance of engines and accessories are
It is clear that the demand for improvement in fuel efficiency of automobiles and the like will increase further in the future, and there is a strong demand for the development of bearing steel that exhibits a long life under high-speed rotation, vibration and impact loads. In conducting the research for solving the above-described problem, the present inventors first investigated the cause of the reduction in the life of the bearing due to high-speed rotation. As a result, they found that although the mode of exhaustion of life is surface peeling, a phenomenon different from that conventionally interpreted occurred in the process of generation of the peeling.

First, when the microstructure of the section under the rolling surface of the bearing which was peeled off in a short time at high speed rotation was observed,
In all the bearings examined, a structure different from the matrix (matrix), which is hardly corroded and observed white (hereinafter, referred to as a white layer), near the position where the maximum shear stress is generated below the rolling surface, along with the separation. Had occurred. FIG. 1 shows a micrograph of a microstructure of a cross section including a white layer. Next, when the hardness of the white layer portion and the surrounding matrix was measured by a micro Vickers hardness tester, the hardness of the matrix was about HV750, whereas the hardness of the white layer was HV1100 to 1300.
It was found that the white layer portion was much harder than the matrix portion. From this result, the life is shortened with high speed rotation because the white layer is generated near the position where the maximum shear stress is generated, and the white layer is hard and brittle, and the crack is early generated by the repeated loading of the shear stress. However, it was presumed that this was due to the fact that the matrix easily spread to the peeling.
However, in the normal bearing life test, there was no case where such a white layer was formed and the life was shortened.Therefore, it is considered that the short life caused by the generation of the white layer is largely affected by vibration and impact loads. As a result, a test was conducted to confirm this.

[0005] The inner and outer races are made of the same material using the two materials shown in Table 1 to form ball bearings 6303 (inner diameter 17 mm, outer diameter 47 m).
m) was prepared and heat-treated in the middle column to give the hardness shown in the right column. For this sample bearing, as shown in Table 2,
A rotating life test was performed by applying a load to the battery by two methods, static and dynamic. In the static load test, a sample bearing is incorporated into a life tester and continuously rotated under a static load. On the other hand, in the dynamic load test, a life tester is installed on a vibrating table, and a continuous rotation test is performed while applying a static load to the sample bearing and simultaneously vibrating the entire tester. Rotational life tests were carried out twice for each material and each test condition, and the results are shown in Table 3.

[Table 1]

[Table 2]

[Table 3]

[0006] As a result of the test, in the static load test, peeling did not occur in any of the sample bearings even when the rotation time was 1000 hours, and there was no problem in the life time. But,
In the dynamic load test with vibration superimposed, JIS SUJ2
Only 43 and 61 hours in the case of
E 189 and 2 even with case hardening steel
It was found that peeling occurred in 02 hours, and the life was greatly reduced. Observation of the microstructure under the rolling surface of the sample having a short life in the dynamic load test revealed that a white layer similar to that shown in FIG. 1 was formed. When the C concentration distribution of the tissue containing the white layer was analyzed by EPMA (electron probe microanalyzer), it was found that the carbon (C) concentration was higher in the white layer portion than in the matrix portion. FIG. 2 is a simulated result of an investigation of the carbon concentration distribution in the white layer and its surroundings by EPMA. It is clearly shown that the white layer 2 has a higher carbon concentration than the matrix 1. Further, it was confirmed that a portion (high carbon portion) 3 having a higher carbon concentration than the white layer 2 was present in the white layer 2. The high carbon concentration in the white layer portion is due to the diffusion and aggregation of carbon atoms therein, but the diffusion of carbon atoms is induced by stress. It is considered that the repeated stress load due to the vibration promotes the stress-induced diffusion of the carbon atoms and promotes the aggregation of the carbon in the white layer portion. In addition, microscopic strain is accumulated below the rolling surface due to the repeated stress load, and carbon atoms are fixed thereto due to the accumulation of this strain. That is, due to the repetition of the impact stress, the carbon atoms diffuse and stick to the position of the maximum shear stress below the rolling surface, thereby producing a white layer having extremely high hardness and being hardly corroded. Then, due to the repeated impact load, cracks are generated from this very hard and brittle white layer,
It spreads and spreads to the matrix, leading to early exfoliation.

Based on the results of such research, the present inventors have considered that it is necessary to prevent the formation of this white layer in order to prolong the life of the bearing, and therefore, it is necessary to suppress the diffusion of carbon. In addition, a steel for bearings in which the chemical composition is optimized in order to improve the matrix strength has already been proposed (Japanese Patent Application No. Hei 2-1).
33489). The contents are as follows. First, as a result of investigating the relationship between the concentration of carbon and other alloying elements and the diffusion rate of carbon, it was found that lowering the carbon concentration and increasing the chromium concentration are effective means to reduce the diffusion rate of carbon. Was found. In addition, for other alloy components, component design was conducted to give sufficient mechanical properties to be used as bearings and to minimize carbides and non-metallic inclusions that are harmful to rolling bearing steel. Was. As a result, in terms of weight ratio, C: 0.65 to
0.90%, Si: 0.15 to 0.50%, Mn: 0.
15-1.00%, Cr: 2.00-5.00%, Ni: 0.20-0.50%, Mo:
A rolling bearing steel containing one or more of 0.1 to 2.0% and V: 0.05 to 1.00%, the balance being Fe and unavoidable impurities. is there.

[0008]

Since the above bearing steel has a relatively high Cr content, the quenching temperature must be adjusted to the value of a normal bearing steel (JIS S) in order to obtain the best mechanical properties.
UJ2, etc.). Conventionally, most bearing steels have been used as materials for rolling bearing units.
It is also being used for rolling and sliding automobile parts such as VJs (constant velocity joints) and face cams. Some of these automotive parts are larger in size than rolling bearings and may require a longer heating time for quenching. However, if the quenching temperature is increased or lengthened, austenite crystal grains may become coarse, and the toughness after heat treatment may be reduced. this is,
The rolling life is reduced, and the effect of the invention steel, which incorporates the above research results, is diminished.

[0009]

Accordingly, the present inventors have made the most of the characteristics of the bearing steel according to the above-mentioned prior application without causing the crystal grains to become coarse even when the quenching temperature becomes high. Bearing steel that can be used. The rolling bearing steel according to the present invention has a weight ratio of C: 0.
65 to 0.90%, Si: 0.15 to 0.50%, M
n: 0.15 to 1.00%, Cr: 2.0 to 5.0%,
N: 0.0090 to 0.0200%,
A1: 0.010 to 0.050%, Nb: 0.005 to
0.50%, one or two of which contain Fe
And unavoidable impurities. Also,
In addition to them, if necessary, Ni: 0.20-0.
50%, Mo: 0.10 to 2.00%, V: 0.05 to
One or more of 1.00% may be contained.

[0010]

The reasons for limiting each chemical component are as follows.

C: 0.65 to 0.90% In the rolling bearing, since the contact portion between the rolling element and the race (race) is a line (roller bearing) or a point (ball bearing), the contact pressure is extremely high. . Therefore, in order to prevent the plastic deformation at the contact portion and to guarantee the smooth movement of the bearing, it is most important that the hardness of the rolling bearing steel is high. In addition, it is necessary to have high hardness also from the point of abrasion resistance. For these reasons, the content of carbon is required to be 0.65% or more. However, when the content exceeds 0.90%, the diffusion rate of carbon increases, and the carbides become large and cause stress concentration, so that the structure changes during use. This causes a reduction in the rolling life as described above. Therefore, the upper limit is set to 0.90%.

Si: 0.15 to 0.50% Silicon is used as a deoxidizing agent when refining steel.
If the steel is not sufficiently deoxidized, oxide-based inclusions increase in the steel, which also becomes a source of stress concentration and promotes a structural change during use. At least 0.15% for this deoxidation
Of silicon is required. However, when the content exceeds 0.50%, the amount of retained austenite after quenching increases, the quenching hardness decreases, and the rolling life decreases.
In addition, since spheroidization of carbides is prevented during spheroidizing annealing, adverse effects of stress concentration due to irregular shaped carbides also occur. Furthermore,
Since the decrease in hardness after annealing is not sufficient, the machinability decreases.

Mn: 0.15 to 1.00% Manganese is an element used as a deoxidizing agent in the refining, like silicon. Further, it has a large effect of improving hardenability and is a useful element for performing complete hardening in a relatively large part. In order to exert these effects, it is necessary to contain at least 0.15% of manganese. However, when the content exceeds 1.00%, the amount of retained austenite after quenching increases, the quenching hardness decreases, and the rolling life decreases.

Cr: 2.00 to 5.00% Chromium is an important element for securing hardenability, and also contributes to an improvement in life by suppressing a structural change due to diffusion of carbon.
In order to exhibit such an effect, chromium must be at least 2.0.
% Is required. However, if it exceeds 5.0%, the effect is saturated, and conversely, workability in steps such as rolling and forging decreases, and the material price also increases.

N: 0.0090 to 0.0200% As will be described later, by combining with niobium and aluminum to form a nitride, it plays a role in preventing austenite crystal grains from becoming coarse. The minimum nitrogen content corresponding to the following niobium and aluminum contents is 0.0090%. However, if the content exceeds 0.0200%, the effect is saturated and steelmaking becomes difficult.

A1: 0.010 to 0.050% Aluminum is finely distributed in steel as nitride AlN, and prevents coarsening of austenite crystal grains during quenching and heating. For this purpose, a content of at least 0.010% is required. However, if the content exceeds 0.050%, a large amount of alumina (Al ₂ O ₃ ), which is a nonmetallic inclusion, is generated, and the rolling life is reduced.

Nb: 0.005 to 0.50% Niobium, like aluminum, forms fine carbonitrides in the steel, which are finely dispersed in the steel to form austenite crystal grains during quenching and heating. Prevent growth.
In order to achieve such effects sufficiently, 0.005%
It is necessary to contain the above amount. However, 0.
If the content exceeds 50%, such an effect is saturated, and the workability of the steel is reduced and the material cost is increased unnecessarily.

Ni: 0.20 to 0.50% Nickel has the effect of strengthening the matrix and improving the toughness, thereby improving the rolling life. In order to ensure such effects, the content of 0.20% or more is necessary. However, when the content exceeds 0.50%, the amount of retained austenite increases and the quenching hardness decreases, and conversely, the rolling life decreases. Also, nickel is an expensive element, and the extra addition only increases the material cost unnecessarily.

Mo: 0.1 to 2.0% Molybdenum strengthens the matrix and suppresses the diffusion of carbon, thereby preventing a reduction in rolling life due to a structural change. In order to achieve such an effect, the amount of molybdenum must be contained at 0.1% or more. However, if the content exceeds 2.0%, such an effect is saturated, the workability of steel is reduced, and the material price is increased unnecessarily.

V: 0.05-1.00% Vanadium forms fine and stable carbides, thereby suppressing the diffusion of carbon and preventing a structural change during use.
That is, it is an element effective for improving the rolling life, and such an effect can be obtained by containing 0.05% or more. However, when the content exceeds 1.0%, such an effect is saturated, and the workability of the steel is reduced, and the material cost is increased unnecessarily.

[0021]

EXAMPLES Next, the features of the steel of the present invention will be described with reference to examples in comparison with comparative steels and conventional steels. Tables 4 and 5 show the chemical components of these test steels.

[Table 4]

[Table 5]

In Tables 4 and 5, A1 to A12 are steels of the present invention (of which A1 to A4 are first steels of the invention, A5 to A5
13 is a second invention steel), and B1 to B3 are comparative steels in which any component is out of the range specified in the present invention.
C1 and C2 are conventional steels, and C1 is JIS SUJ2
Steel, C2, is SAE 5120 steel. In order to perform a rolling life test, first, a disk-shaped test piece was prepared from these test steels and subjected to a predetermined heat treatment. Here, except for the C2 steel, all other test steels were quenched at the temperatures shown in the left column of Table 6. Since C2 steel is case hardened steel, the temperature of the secondary quenching after carburizing is described in Table 6. Since the steel of the present invention has a low carbon content and contains a relatively large amount of Cr, the quenching temperature is 50 to 70 ° C. higher than that of the conventional steels C1 and C2. The same is true for the comparative steel. The middle column of Table 6 shows the austenitic crystal grain roughness (grain number according to JIS G0551) of each test steel when heat treatment was performed at the quenching temperature shown in the left column of Table 6. Is the grain size of the surface carburized part). After that, each test steel including the case hardened steel C2 was tempered so that the surface hardness became substantially HRC 61 to 62, and then the ball was rolled under the load conditions shown in Table 2 to obtain a dynamic load. The test was performed. Table 6 shows the results.
In the right column. The “rolling life” here is
This is the time (hr) until the separation occurs on the bearing surface.

[0023]

[Table 6] First, looking at the crystal grain size in the middle column of Table 6,
The steels A1 to A12 of the present invention are all conventional steel C1 (JIS
SUJ2) and quenching from a higher temperature than C2 (SAE 5120), but the size of the crystal grains is equal to or smaller than that of conventional steel (the coarsest one is 10.0). It is superior to conventional steel, and the finest one is 11.7).

Looking at the results of the dynamic load test shown in the right column of Table 6, all of the steels A1 to A12 of the present invention have achieved a long service life of 1100 hours or more. In contrast, comparative steels B1 to B
3 has a long life of about 850 hours, and the conventional steel has an extremely short life of several tens of hours to about 200 hours. As described above, in the steel of the present invention, since coarsening of the crystal grains is suppressed to a minimum, quenching can be performed from a temperature at which the effect of the added element for improving the rolling life is maximized. As a result, as shown in Table 6, each of the steels of the present invention has a sufficient rolling life under a dynamic load. In the above example, the rolling life was measured using the test steel for the raceway. However, the same high life can be obtained by using the steel of the present invention for the rolling elements (balls and rollers). It is natural if you take into account.

[0025]

As is evident from the above results, the bearing steel according to the present invention exhibits a long life under severe conditions, in particular, where vibration and impact loads are applied. Therefore, it can be used as a material most suitable for a bearing used at a high speed rotation, an engine related to an automobile or an aircraft which inevitably generates vibrations and impacts, and a bearing for an accessory driven by the engine. Furthermore, in the heat treatment for manufacturing such bearings and the like, the temperature can be raised sufficiently and quenching can be performed over a long time without worrying about coarsening of crystal grains, thereby simplifying the manufacturing process. And contributes to maintaining stable product quality.

[Brief description of the drawings]

FIG. 1 is a photograph showing a metal structure of a cross section below a rolling surface of a bearing that has been separated in a short time by high-speed rotation.

FIG. 2 shows the carbon concentration distribution of the white layer and the matrix by EPMA.
Fig. 7 is a simulated view of the result analyzed in Fig. 4.

[Explanation of symbols]

 1 ... matrix (matrix), 2 ... white layer, 3 ... high carbon part

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masao Goto 3-5-8 Minamisenba, Chuo-ku, Osaka-shi Inside Koyo Seiko Co., Ltd. (72) Inventor Toshihiro Kawaguchi 3-5-2, Minamisenba, Chuo-ku, Osaka-shi Koyo Seiko Co., Ltd. (72) Inventor Teruo Hoshino 3-5-8 Minamisenba, Chuo-ku, Osaka-shi Koyo Seiko Co., Ltd. (72) Inventor Masayuki Kitamura 5-58 Minamisenba, Chuo-ku, Osaka-shi Within Seiko Co., Ltd. (72) Inventor Yoshitaka Natsume 1-1-1, Showa-cho, Kariya-shi, Aichi Japan Inside Denso Co., Ltd. (72) Inventor Akihiro Mitsuya 1-1-1, Showa-cho, Kariya-shi, Aichi Japan Nihon Denso Co., Ltd. (58 ) Surveyed field (Int.Cl. ⁶ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. A weight ratio of C: 0.65 to 0.90%, S
i: 0.15 to 0.50%, Mn: 0.15 to 1.00%, Cr: 2.0 to
5.0%, N: 0.0090-0.0200%
1: 0.010 to 0.050%, Nb: 1 of 0.005 to 0.50%
A bearing steel containing one or two kinds, the balance being Fe and unavoidable impurities. 2. C: 0.65 to 0.90 in weight ratio
%, Si: 0.15 to 0.50%, Mn: 0.15 to
1.00%, Cr: 2.0-5.0%, N: 0.009
0-0.0200%, and A1: 0.01
0 to 0.050%, Nb: one or two of 0.005 to 0.50%, Ni: 0.20 to 0.50%, M
o: 0.10 to 2.00%, V: 0.05 to 1.00%
A bearing steel comprising one or more of the above, and the balance being Fe and unavoidable impurities.