JP3941782B2 - Rolling bearing - Google Patents

Description

  The present invention relates to a rolling bearing used for iron making, automobiles, agricultural machines, construction machines, and the like, and more particularly, to a rolling bearing having a long life even under foreign matter-mixed lubrication.

  Generally, as a material for rolling elements (balls and various types of rolling bearings; the same applies hereinafter) and inner and outer rings (inner and outer rings; the same applies hereinafter) of rolling bearings, SUJ2 is used for case-hardened steel. A steel material equivalent to SCR420 is used. Since rolling bearings are used under repeated shear stress under high surface pressure, bearing steel is quenched and tempered and case-hardened steel is tempered after carburizing or carbonitriding to withstand the shearing stress and ensure a rolling fatigue life. It is hardened and tempered to have a hardness of HRC 58-64.

  However, rolling bearings are used in a wide variety of environments, and in addition to fatigue life, the life of the bearing is greatly influenced by inadequate lubrication conditions, wear due to contamination by foreign substances from peripheral devices, and early peeling. I can't overlook that. In addition, for example, in a tapered roller bearing that receives a large axial load, trouble may occur due to wear due to sliding of the transfer surface and wear due to pure slip at the large brim, and thrust bearings, spherical roller bearings, etc. Even in the case of rolling bearings other than these tapered roller bearings, wear due to sliding may be a problem when the usage conditions are severe. Accordingly, there is a demand for a rolling bearing that has a good wear resistance and is inexpensive and that can ensure not only fatigue life but also a long life even under lubrication mixed with foreign matter and prevent wear and early peeling.

The inventors of the present application have made studies on a new material for a rolling bearing that can realize the demand. The advantages and disadvantages of both the bearing steel SUJ2 and the case-hardened steel SCR420, which are generally used for rolling bearings, will be described below.
SUJ2 contains a large amount of C and Cr in the alloy composition, and is likely to cause giant carbides and segregation during steelmaking. As a result of performing a soaking process or the like to eliminate this, the material cost is higher than that of the SCR 420.

  Regarding the forming process, in the case of rolling elements of small bearings, the material is mostly made of cold drawing material (coil material) and is formed into the shape of rolling elements by cold die forging (header processing). I will not. Comparing the cold workability, SUJ2 has a higher content ratio of C and Cr in the alloy component, whereas SCR420 has a lower C content ratio, so the cold workability is better for SCR420. is there. On the other hand, since the inner and outer rings are formed by hot (warm) forging, there is not much difference in workability depending on the material. However, although soft annealing is performed after forging in preparation for cutting, SUJ2 has a lower cutting ability than SCR420 due to the carbon content of the material, and tends to be expensive.

  With regard to heat treatment, general SUJ2 is hardened and tempered to obtain the necessary hardness for the bearing, while SCR420 is carburized or carbonitrided to obtain the required hardness for the surface layer. After the treatment, tempering is performed, or in some cases, the carburization or carbonitriding / quenching / quenching treatment is performed, followed by further secondary quenching and tempering. Therefore, the heat treatment cost of SCR 420 is significantly higher than that of SUJ2. In addition, the “surface layer” referred to in the present specification means a region from the surface to the depth at which the maximum shear stress is generated at the time of rolling contact (specifically, 2% of the rolling element diameter), Under light load and a large amount of foreign matter, it can be shallow within the above range.

Regarding the grindability, SCR420 is worse than SUJ2 depending on the pro-eutectoid cementite and residual austenite amount (hereinafter referred to as γ _R ) in the black skin surface layer after heat treatment (that is, the surface layer before grinding). But there is no big difference. However, when carbonitriding SCR420, carbonitride precipitates on the black skin surface layer, and the grindability is remarkably reduced.
From the above comparison, SUJ2 is lower than SCR420 as the total cost of the bearing in consideration of material cost and processing cost.

And, regarding the function of the rolling bearing manufactured using the above steel material as the material, in the rolling life, SCR420 carburized (or carbonitrided) has a lot of surface layer γ _{R and} also generates compressive residual stress. In particular, it tends to have a long life in an environment containing foreign matter. Further, SCR420 is superior to SUJ2 in terms of dimensional stability, which is an important function as a bearing.
Japanese Patent Laid-Open No. 2-125841 JP 63-303221 A Japanese Patent Laid-Open No. 2-27764

  However, in the case-hardened steel SCR420, which is a material of the above-mentioned conventional rolling bearing, (1) When trying to deepen the carburized hardened layer, it is necessary to perform carburizing treatment at a high temperature for a long time because the carbon content of the base is low. Therefore, the heat treatment productivity is lowered. (2) Further, when the surface carbon concentration is increased, pro-eutectoid carbides are likely to be formed due to a large Cr content, so that the rolling fatigue life is reduced. (3) Therefore, if the Cr content is reduced and Ni and Mo are added to improve the hardenability, there is a problem that the material cost increases. (4) Furthermore, when wear resistance is particularly required, a method of adding a large amount of carbide forming elements such as Cr, Mo, V and the like to precipitate a large amount of carbide on the bearing surface layer is known. This method not only increases the material cost, but also increases the processing cost due to a decrease in forgeability, machinability, grindability, and the like, and increases the heat treatment cost due to a decrease in carburization, resulting in a very expensive rolling bearing.

On the other hand, in the case of the bearing steel SUJ2, the cost can be kept low, but there is a problem that the bearing life is short especially in a foreign matter mixed environment.
As a countermeasure against such a problem, the present applicant previously proposed a rolling bearing having a low material cost, good heat treatment productivity, and a long life in Patent Document 1 filed earlier. In order to avoid the deterioration of the hardenability due to the Cr content being less than 0.35%, the Cr content that tends to generate proeutectoid carbide harmful to the rolling fatigue life of the bearing is reduced to 1.2 to 1.7% by weight. By adding Mn and using medium carbon manganese steel with a large amount of base carbon (C content: 0.4 to 0.7 wt%), generation of proeutectoid carbide is suppressed, and γ _R in the bearing surface layer is 25 to 25%. By obtaining the necessary hardened layer depth in the range of 45% by volume, it is intended to achieve a long life of the rolling bearing under the contamination with foreign matter.

Specifically, as disclosed in Patent Document 1 and FIG. 1 thereof, a long life can be obtained by allowing γ _R to exist in the surface layer in an amount of 25 to 45% by volume under the contamination with foreign matter. In order to carry out this, it has been shown that the above range of γ _R can be maintained if at least the amount of solute carbon in the surface layer is 0.8% by weight or more.
However, in this case, there is a problem such as a decrease in workability due to the amount of Mn, a decrease in grinding workability when carbonitriding, and the originally required functions and processing conditions differ between the rolling elements and the inner and outer rings. There is still room for improvement.

Further, as described above, the rolling elements of the bearing, the outer ring, and the inner ring have different processing methods. Rolling elements are processed through a grinding process (including lapping) after cold forging, and inner and outer rings are processed through hot (warm) forging, cutting and grinding (including super finishing). Is done.
In view of such a difference in processing method, it is necessary to eliminate the giant carbonitride of the material SUJ2 in the rolling elements, eliminate the need for soaking, and improve cold forgeability (die life). Each wheel had its own problems, such as the need to improve cutting workability (tool life) and grinding workability (dressing interval). In other words, there is still room for improvement in order to obtain a rolling bearing with as low a cost as possible while achieving a long life under lubrication mixed with foreign matter.

  By the way, in general, the same material is often used for the inner ring, the outer ring and the rolling element in the rolling bearing. Considering this, an expensive material may be used only for the inner and outer rings, the fixed ring, or the rolling elements. For example, in Patent Document 2, steel containing 0.3 wt% or more of C and 3 wt% or more of Cr is used for at least one of the inner and outer rings or the rolling elements of the inner and outer rings and rolling elements, and the rest A method for obtaining a long-life bearing by using high carbon Cr bearing steel or carburized steel is disclosed. However, no consideration is given to wear and cost.

On the other hand, in recent years, the use conditions of bearings have become very severe due to the speeding up, weight reduction, fuel consumption reduction and maintenance-free of steel facilities. For this reason, in addition to the improvement of the rolling life in the presence of foreign matter, there are cases where peeling due to surface damage due to foreign matter mixed in the lubricating oil, wear due to poor lubrication, or the like becomes a problem.
In contrast, as disclosed in Patent Document 3, carburization or carbonitriding is performed on high-chromium steel, fine carbides are precipitated on the bearing surface layer, and the amount of retained austenite γ _R is optimized, thereby introducing foreign matter. Conventional examples of extending the life under lubrication are known, but the consideration for wear due to poor lubrication has been insufficient.

Fast Specifically, for a rolling bearing, such as for passenger cars which are used under conditions of low load transmission, and cost reduction it is strongly demanded with abrasion resistance, to be of material having a large effect on the bearing manufacturing costs Consideration must be given to the quality of machinability and grindability.

Accordingly, the present invention provides a rolling bearing that is excellent in wear resistance, has a long life even under lubrication mixed with foreign matter, has excellent workability, is advantageous in terms of cost, and is suitable for use at high speed and low load. It has been with the purpose to provide to.

"In particular high speed, to satisfy both wear resistance and low cost rolling bearings for transmissions such as a passenger car to be used under conditions of low load" in order to achieve the above Symbol purpose of, in a first aspect of the present invention In such a rolling bearing, at least one of the inner ring, the outer ring, and the rolling element is
C: 0.3 to 0.9% by weight
Si: 0.1 to 0.7% by weight
Mn: 0.5 to 1.5% by weight
Cr: 0.1 to 0.8% by weight
And the balance of carbon and nitrogen in the surface layer of the finished product.
C: 0.6 to 1.2% by weight
N: 0.2 to 0.9% by weight
Furthermore, the total content of Cr and nitrogen (total amount of Cr content in material and N content in surface layer: “Cr + N”) is 0.4 to 1.0% by weight.
It is characterized by being.

Next, the action of the alloy components used in this rolling bearing and the critical significance of numerical limitation will be described.
In general, when bearing steel such as SUJ2 used as a rolling bearing material or case-hardened steel equivalent to SCR420 is carbonitrided, the wear resistance is greatly improved when the nitrogen concentration is increased. Tends to decrease remarkably, resulting in a significant increase in processing cost.
Therefore, as a result of repeated research on the correlation between the material components and nitrogen concentration, grindability, wear resistance, etc., the present inventors have optimized the Cr content and the amount of nitrogen added to increase the It has been found that a bearing having excellent life and wear resistance and good workability including grindability can be provided at low cost.

[C content]
C is an element necessary for obtaining the core strength necessary for the bearing. However, if the carbon content of the material exceeds 0.9% by weight, soaking is necessary to eliminate giant carbides and segregation during steelmaking, which increases the material cost. Further, since the deformation resistance tends to increase as the amount of carbon increases and the cold workability and the machinability deteriorate, the upper limit is set to 0.9% by weight as a preferable value.
On the other hand, when the carbon content of the raw material is less than 0.3% by weight, the carburizing (or carbonitriding) treatment becomes longer and the heat treatment productivity is lowered. However, when used for inner and outer rings, it is desirable that the carbon content of the material be 0.6% by weight or less when dimensional stability and core toughness are problems.

[Si content]
Si is an element necessary as a deoxidizer during steelmaking, and is an element effective for improving hardenability, strengthening base martensite, and further improving resistance to temper softening. Addition is necessary. However, if the content is too large, the cold workability and machinability are reduced, and the carbon and nitrogen penetration depth during carbonitriding is reduced, leading to an increase in the cost of heat treatment. The content was 0.7% by weight.

[Mn content]
Mn is an element effective for improving hardenability. Furthermore, in the present invention, by performing carbonitriding, fine nitrides are formed on the surface of the finished bearing product and wear resistance is improved. However, if Mn is added, grindability is less likely to deteriorate even when carbonitriding. I found In order to exert this effect, 0.5% by weight or more is necessary. However, if added in a large amount, the cold workability and machinability of the material deteriorate, so the upper limit was made 1.5% by weight.

[Cr content]
Since Cr is an element effective for improving the hardenability and temper softening resistance, 0.1% by weight or more is necessary. By performing carbonitriding, nitride is formed on the surface of the finished bearing product, resulting in resistance to resistance. Abrasion is improved, but if the relationship between the Cr content and the nitrogen content exceeds a certain amount, the grindability deteriorates. In addition, excessive addition not only increases the cost of the material, but also reduces the carbon and nitrogen penetration depth during carbonitriding, leading to an increase in the cost of heat treatment, so the upper limit is preferably 0.8% by weight. did.

[C content of finished product surface; 0.6 to 1.2% by weight]
Normally, the carbon concentration on the surface of the finished product after carbonitriding is required to be 0.8 or more in order to obtain the required hardness for the bearing, but in the present invention, it is possible to obtain both long life and wear resistance at the same time. Since the upper limit of the nitrogen content is increased, the minimum required surface carbon amount is 0.6% by weight. However, if its content exceeds 1.2% by weight, the amount of solid solution becomes excessive together with the nitrogen content, and depending on the processing conditions, more than necessary γ _R is generated and the surface hardness is reduced, In some cases, precipitation may occur and the rolling life may be reduced. Therefore, the upper limit is set to a preferable value of 1.2% by weight.

[N content of finished product surface; 0.2-0.9 wt%]
When the amount of nitrogen on the surface of the finished product is less than 0.2% by weight, it becomes difficult to simultaneously obtain life and wear resistance due to insufficient solid solution of nitrogen. Therefore, the lower limit is preferably 0.2% by weight. On the other hand, when the amount of nitrogen is increased, nitrides precipitate and wear resistance is improved. However, depending on the amount of Cr added, the wear resistance tends to be improved and the grindability tends to deteriorate. If the amount exceeds 0.9% by weight, the grindability is not improved even if the amount of Cr is reduced.

[Total content of Cr and nitrogen (Cr + N); 0.4 to 1.0% by weight]
Cr has the effect of improving the wear resistance by forming a nitride or carbonitride by adding nitrogen, but the grindability is lowered. The inventors have experimentally found that if the sum of the Cr content in the material and the N content in the surface layer is within an appropriate range, both the grindability and wear resistance will be good ( The details of the experiment will be described in detail in Examples below).
In conclusion, if the Cr content in the surface layer is 0.2% by weight or more and the Cr + N amount exceeds 1.0% by weight, the grindability deteriorates rapidly, while the content becomes less than 0.4% by weight. As a result, it has been found that the wear resistance is significantly reduced. Therefore, an appropriate range of Cr + N amount that can satisfy the grindability and the wear resistance at the same time is defined as 0.4 to 1.0% by weight. Even if the amount of Cr + N is within this range, sufficient wear resistance cannot be obtained if the content of the surface layer N is not 0.2% by weight or more.

According to the invention according to claim 1 of the present application, high-speed In particular, excellent wear resistance under conditions of low load, and the effect is obtained that with a low cost can be provided a rolling bearing of long lifetime It is done.

Hereinafter, an embodiment of the present invention (that is, in the case where both wear resistance and low cost in addition to the rolling life under lubrication mixed with foreign matters under high speed and low load conditions) will be described.
(1) Relationship between material composition of bearing alloy steel and tool life in cutting and die life in cold die forging (upsetting) Comparison of tool life and die life for each steel type in Examples and Comparative Examples .

Tool life test conditions:
Cutting machine: High-speed lathe Tool: P10 (JIS B 4053)
Cutting speed: 180-220 m / sec
Feed amount: 0.2-0.3mm / rev
Cutting depth: 0.6 to 1.0 mm
Each sample was cut under the above conditions according to the tool cutting test method of “JIS B 4011”, and the tool life was defined as the tool flank wear amount reaching 0.2 mm.

Mold life test conditions:
Mold: V30 (JIS B 4053)
Upsetting rate: 15-20%
Workability: 300 to 400 pieces per minute Lubrication: Zinc phosphate coating + lubricating oil Each steel type is processed under the above conditions until cracks or breakage occurs in the mold and the workpiece after processing becomes scratched or deformed The mold life is indicated by the number of workpieces processed so far.
The results are shown in Table 1 .

  In the alloy steel of the rolling bearing of the present invention, good results were obtained in both tool life and die life for all steel types. On the other hand, in the comparative example, N8 and N10 had a large Mn content or C content, both the tool life and the mold life were reduced, and the cost was increased. Therefore, the following heat treatment experiment is not performed for N8 and N10.

(2) Heat treatment quality of bearing alloy steel The following F, G, and H heat treatments were performed on the alloy steels of the examples and comparative examples in Table 1 (excluding N8 and N10), and the heat treatment quality was evaluated. .
[Heat treatment F]
Enriched gas and ammonia gas are added to the endothermic gas atmosphere at a temperature of 840 to 900 ° C. for 1 to 4 hours, carbonitriding is performed under the condition that residual ammonia is contained at least 0.1% by volume or more, and directly quenched as it is. Or directly quenching and holding at 830 to 860 ° C. for 30 minutes, followed by secondary quenching and subsequent tempering at 160 to 180 ° C. for 2 hours.

[Heat treatment G]
Enriched gas and ammonia gas are added to the endothermic gas atmosphere at a temperature of 870 to 930 ° C. for 1 to 4 hours, and carbonitriding is performed under the condition that the residual ammonia is less than 0.1% by volume, and then directly quenched. Alternatively, after direct quenching and holding at 830 to 860 ° C. for 30 minutes, secondary quenching is performed, followed by tempering at 160 to 180 ° C. for 2 hours.

[Heat treatment H]
After carrying out a normal carburizing treatment at a temperature of 930 to 960 ° C. for 5 to 7 hours, the mixture is allowed to cool to room temperature, and then kept at 830 to 860 ° C. for 30 minutes, followed by quenching, and subsequently at 160 to 180 ° C. Temper for 2 hours.
The heat treatment performed on the alloy steel of this example is [Heat Treatment F], and sufficient carburization and nitriding depth can be obtained in a short time treatment of 1 to 4 hours. In most cases, quenching is performed directly, so the cost is almost equivalent to that of normal quenching. However, since deformation at the time of quenching is a serious problem for thin-walled bearings, etc., secondary quenching or press quenching can suppress deformation, reduce the defective rate, and reduce the grinding cost. May also be advantageous.

  Further, unless the residual ammonia amount is controlled to be at least 0.1% by volume or more with an ammonia analyzer, sufficient nitrogen cannot be given to the finished product surface. When the processing temperature exceeds 900 ° C, the decomposition rate of ammonia gas increases, making it difficult to leave a sufficient amount of ammonia, not only reducing nitrousability, but also lowering toughness due to coarsening of the crystal grain size. As a result, the function as a bearing is reduced. Moreover, since the heat processing time for obtaining sufficient hardened layer depth will become long when process temperature will be 840 degrees C or less, and it leads to a cost increase, process temperature was 840 degreeC or more and 900 degrees C or less.

Table 2 shows the results of the heat treatment quality, the result of the thrust life test under the contamination with foreign matters (using the thrust type tester), the grindability and the wear resistance. FIG. 3 and FIG. 4 show the relationship between Cr + N and grindability and wear resistance. The conditions for the life test, grinding test, and wear test are as follows.
Life test conditions:
Surface pressure: 4900 MPa
Rotation speed: 1000rpm
Lubricating oil: # 68 Turbine oil Contaminated foreign matter:
Composition: Fe ₃ C powder Hardness: H _R C52
Particle size: 74-147 μm
Inclusion amount: 300ppm in lubricating oil

Grinding test conditions:
Whetstone: WA100
Grinding fluid: Soluble type Grinding peripheral speed: 2800-3000m / min
In the grinding test, an inner ring having a nominal number 6206 was used. The inner ring raceway surface was ground with a grindstone under the above conditions, and the number of the inner rings ground before dressing the grindstone was investigated.

Abrasion test conditions:
Tester: Two-cylinder wear tester (shown in Fig. 2)
Load: 50kgf
Rotation speed: 100rpm
Slip rate: 30%
Lubricating oil: S10
Oil temperature: 60 ° C
The wear test was carried out under the above conditions using two cylindrical test pieces of the same steel type, and the weight loss amount (wear amount) of each was measured, and the average value was used to indicate the wear rate.
The abrasion test is performed using a two-cylinder abrasion tester as shown in FIG. 2, and the specimens S are respectively attached to a pair of cylinders 10 which are vertically opposed to each other, and are in contact with each other while applying a load P from above. The average value of the wear rates (g / m) of both specimens S is obtained by rotating in the reverse direction at a low speed. In particular, in order to test the wear characteristics in a poorly lubricated state, a low-viscosity lubricating oil that easily breaks the oil film was poured during rotation.

In Table 2 , No. 1A to No. 8A are alloy steels of the examples, but all have a long life, and wear resistance and grindability are good, so a low-cost long-life wear-resistant bearing is provided. Can be provided. On the other hand, No. 9A to No. 15A in the comparative examples are examples in the case where Cr + N exceeds 1.0% by weight, and the grindability is greatly lowered, resulting in a large cost increase.
In addition, No. 16A is not improved in wear resistance and life because the content of Mn and Cr is less than the minimum required amount. No. 17A sufficient carbon is not given in the short-term treatment due to low C wt% of the material became short life for gamma _R becomes insufficient. No. 18A and No. 19A are examples of normal carburizing, but since N is not contained, wear resistance and life are not improved.

No. 20A~No. 22A is a comparative example in which Cr + N is less than 0.4 wt%, sufficient wear resistance is obtained, of gamma _R than necessary in the vicinity of the surface, especially in No. 22A As a result, the service life was shortened. No. 23A and No. 24 are examples of low N carbonitriding, but since the Cr content is high and the solid solution amount of N is not sufficient, the relationship between grindability and wear resistance is not improved.
With No. 25A, although comparatively good results were obtained in the comparative examples with respect to the number of grindings and wear resistance, the rolling life was increased due to the occurrence of some proeutectoids (carbides) due to the excessive amount of carbon in the surface layer. Declined.
In No. 26A, since the amount of nitrogen in the surface layer was too large, the grindability decreased.
In No. 27A, although Cr and the amount of nitrogen in the surface layer individually satisfy the conditions, the wear resistance is not improved because Cr + N is less than the lower limit.

(3) Cr + N and Grindability and Abrasion Resistance Regarding the relationship between Cr + N and grindability and wear resistance, as shown in FIG. When the content of Cr + N exceeds 1.0% by weight, the grindability deteriorates rapidly. On the other hand, when Cr + N is less than 0.4% by weight, the wear resistance is remarkably lowered as shown in FIG. Further, even if Cr + N is in the range of 0.4 to 1.0% by weight, sufficient wear resistance cannot be obtained if N is not contained in an amount of 0.2% by weight or more.

FIG. 5 shows the relationship between wear resistance and grindability.
In a comparative example in which at least one of the Cr content, the N content in the surface layer, or the sum of these Cr + N is outside the scope of the present invention, the wear resistance is improved and the grindability tends to be lowered. However, in the example of the present invention, good results were obtained in both grindability and wear resistance. Comparative Examples No. 17A and 25A show wear resistance and grindability relatively similar to those of the present invention, but the rolling life is reduced as described above because the carbon content of the surface layer is outside the scope of the present invention. is doing.

Therefore, by including N in an amount of 0.2% by weight or more and further making Cr + N in the range of 0.4 to 1.0% by weight, a low-cost rolling bearing having both excellent grindability and wear resistance is obtained. It becomes possible to provide.
All the above-mentioned present inventions can be applied to various rolling bearings (ball bearings, cylindrical roller bearings, tapered roller bearings, spherical roller bearings, etc., regardless of radial type or thrust type).

It is a graph showing the relationship between the surface nitrogen concentration of a rolling bearing member and the wear rate. It is a conceptual diagram of a two-cylinder abrasion tester. It is a graph showing the relationship between the total content of Cr and N and grindability in Examples and Comparative Examples. It is a graph showing the relationship between the total content of Cr and N and the wear resistance in Examples and Comparative Examples. It is a graph showing the relationship between abrasion resistance and grindability in the examples and comparative examples.

Explanation of symbols

  10 cylinder

Claims (1)

At least one of the inner ring, the outer ring, and the rolling element,
C: 0.3 to 0.9 % by weight
Si: 0.1 to 0.7 % by weight
Mn: 0.5 to 1.5% by weight
Cr: 0.1 to 0.8 % by weight
And the balance of carbon and nitrogen in the surface layer of the finished product.
C: 0.6 to 1.2 % by weight
N: 0.2 to 0.9 % by weight
Der is, furthermore, a rolling bearing total content of Cr and nitrogen (Cr + N) is characterized by 0.4 to 1.0 wt% der Rukoto.

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