JPH03153842A - Rolling bearing - Google Patents

Abstract

PURPOSE:To manufacture the rolling bearing having prolonged service life even if used under the conditions of quasi-high temps. to high temps. by subjecting an alloy steel obtd. by incorporating specified ratios of C, Cr, Si, Mn and O into Fe to carbonitriding heat treatment and executing hardening and high temp. tempering. CONSTITUTION:An alloy steel contg., by weight, 0.3 to 0.6% C, 0.5 to 2.5% Cr, 0.3 to 1.5% Si, 0.3 to 1.7% Mn and <=9ppm O, furthermore contg., at need, <=3.0% Mo and the balance Fe with inevitable impurities is subjected to carbonitriding heat treatment and is thereafter subjected to hardening and high temp. tempering. As for the impurities, about 40ppm Ti, about <=20ppm P and about <=80ppm S are preferably regulated, and as the tempering temp., about 240 to 550 deg.C is preferably regulated. In this way, the rolling bearing constituted of a bearing ring and a rolling element having prolonged service life even if used at quasi-high temps. to high temps. (about 120 to 550 deg.C) can be obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to rolling bearings used in automobiles, agricultural machinery, construction machinery, steel machinery, etc., and particularly to long-life bearings required for transmissions and engines. Regarding rolling bearings.

[Conventional technology]

Rolling bearings, consisting of raceway rings and rolling elements, are subjected to severe use in which they are repeatedly subjected to shear stress under high surface pressure, so they must withstand the shear stress to ensure a long rolling fatigue life (hereinafter referred to as life). There is a need.

Therefore, in the past, rolling bearings were constructed using high carbon chromium steel bearing steel class II (SUJ-2) as the bearing material, which was then quenched and tempered to achieve a Rockwell hardness of HRC58 to HRC58.
64 in order to improve the lifespan.

There is also a conventional example in which the life of a rolling bearing is improved by forming the rolling bearing using case-hardened steel. In this conventional example, it is necessary to set the hardness curve in accordance with the internal shear core force distribution caused by the contact surface pressure, so we used low carbon case hardening w4scr420H, SCM420H, 5 with good hardenability.
AE8620H.

By using 5AE4320H etc. and applying carburizing heat treatment to this, the hardness of the raceway ring and rolling element surface becomes H, C58 ~
64, and the core depth was set to H, C30-48 to ensure the required life.

As such conventional bearing materials, for example, Japanese Patent Application Laid-open No. 4
There is one described in No. 9-114516. In this conventional example, C: 0.36 to 0.50% by weight, Cr: 0.5
A medium carbon carburized steel for transfer contact bodies containing ~1.5% by weight, etc., is disclosed, and this carburized steel improves toughness, hardness, and fatigue strength by shortening the carburizing time and reducing the amount of retained austenite on the carburized surface. It is stated that this will improve.

By the way, as the loads and speeds of machines that use rolling bearings increase, the conditions under which bearings are used become harsher, and when rolling bearings are used at semi-high to high temperatures, the following problems arise. I came.

First, as the operating temperature of the rolling bearing increases, the hardness of the bearing decreases, causing plastic deformation and shortening its life. The second reason is that even if retained austenite is reduced as in the conventional example, if even a small amount of retained austenite exists on the bearing surface, this retained austenite will transform into martensite under semi-high temperature to high hot water conditions. As a result, dimensional changes occur, impairing so-called dimensional stability. In particular, in recent years, dimensional stability of bearings has been strictly required, and for example, in rolling bearings used in jet engines and the like, even a slight dimensional deviation may result in a serious accident. Therefore, strict dimensional stability is required for rolling bearings used at semi-high to high temperatures.

Conventionally, high-carbon chromium steel bearings (SUJ-2), case-hardened steel, and carburized steel were subjected to high-temperature tempering in order to prioritize the dimensional stability of rolling bearings used at semi-high to high temperatures. High-temperature tempered products with excellent dimensional stability are provided by previously converting retained austenite into martensite.

[Problem to be solved by the invention]

However, although such high-temperature tempered products can improve dimensional stability, high-temperature tempering reduces hardness and reduces lifespan due to plastic deformation and the like.

By the way, it has been known that nonmetallic inclusions in steel, particularly oxide inclusions, deteriorate the mechanical properties of steel materials. However, the present inventors have discovered that conventional high Cr bearing steels, low carbon alloy steels, case hardened steels, etc. do not take into account the reduction of oxide inclusions, and therefore have the problem of shortened service life. confirmed.

Therefore, in order to solve these problems, the invention of this application does not reduce hardness even when tempered at high temperatures, and the generation of oxide inclusions is extremely small. The purpose is to provide a rolling bearing that has a long life even when used under high temperature conditions.

[Means to solve the problem]

In order to achieve such an object, the invention according to claim (1) provides a rolling bearing comprising a raceway ring and a rolling element, in which at least one of the raceway ring and the rolling element has a C;0.
3-0.6% by weight, Cr; 0.5-2°5% by weight, S
i; 0.3-1.5% by weight, Mn; 0.3-1.5% by weight
It is characterized by being made of an alloy steel containing 1.7% by weight, O: 9 ppm or less, the balance being Fe and unavoidable impurities, which is subjected to carbonitriding heat treatment, followed by quenching and high-temperature tempering.

The invention according to claim (2) relates to a rolling bearing characterized in that the alloy steel according to claim (1) contains Mo in an amount of 3.0% by weight or less.

[Function] The invention related to this application uses carbonitriding instead of conventional carburizing as surface heat treatment, so there is no risk of hardness decreasing even when high temperature tempering is performed, and the amount of oxygen in the steel is minimized. By reducing the amount of oxide inclusions and avoiding the generation of oxide inclusions, a long-life rolling bearing can be provided.

In the present invention, the reason why carbonitriding heat treatment is adopted is that N
This takes advantage of the property that improves resistance to temper softening. In the case of conventional bearing steel, case hardened steel, etc., when tempered at a high temperature, e.g. 250°C, the surface hardness becomes H,
Below C60, it becomes impossible to maintain the surface hardness necessary for a rolling bearing. On the other hand, if carbonitriding is performed instead of carburizing as the surface hardening heat treatment, sufficient hardness can be maintained even during high-temperature tempering due to the effect of N on improving the temper softening resistance. Therefore, even if high-temperature tempering is performed to improve dimensional stability, a sufficient life can be ensured.

Next, the critical significance of the action and content of each contained element in the present invention will be explained.

C: 0.3 to 0.6% by weight C is a necessary element to improve hardness after quenching and tempering. Note that carbonitriding increases the carbon concentration on the bearing surface, so this value corresponds to the C content in the core.

If the C content exceeds 0.6% by weight, the amount of retained austenite increases from the surface layer to the core, and the dimensional stability in a semi-high temperature to high temperature environment is inhibited. Furthermore, since huge carbides are generated at the material stage, machinability and toughness are reduced, and fracture strength is also reduced.

On the other hand, if the content is less than 0.3% by weight, the carbonitriding treatment time becomes long and the heat treatment productivity decreases.

Furthermore, if you try to increase the carbonitriding temperature and shorten the treatment time, the decomposition rate of NH3 gas will increase, making it difficult for nitrogen (N) to enter, making it impossible to raise the temperature too much (82
0-880°C). Therefore, when the bearing is loaded, it takes a long time to obtain 0% of the required hardness to the depth where shear stress is applied, which is disadvantageous in terms of cost.

In addition, the amount of oxygen that produces harmful oxide inclusions when improving the life of a bearing increases as the carbon content decreases, which is disadvantageous in improving the life of the bearing. Therefore, in the present invention, medium carbon steel (C weight %; 0.
3 to 0.6), it was possible to obtain low oxygen, high cleanliness steel. Preferably, the carbon content is 0.35-0.45% by weight.

Cr; 0.5 to 2.5% by weight Cr is an element effective in improving hardenability and temper softening resistance. In addition, precipitation hardening that uniformly forms fine carbides provides sufficient surface hardness even if high-temperature tempering is performed, and the toughness of the matrix can be improved.

In addition, the hard and fine Cr carbide also serves to improve wear resistance. Moreover, since Cr is also a carbide-forming element, as a result of increasing the C concentration in the carbonitrided layer, the carbonitriding properties of the material can be improved even if a large amount of Si, which inhibits carburization, is contained.

By exerting these effects 1 and achieving the required surface hardness H, C
In order to ensure 60 or more (particularly 61 to 70), the lower limit of the Cr content was set at 0.5% by weight.

On the other hand, if the content exceeds 2.5% by weight, it becomes difficult to form uniform fine carbides, and huge carbides are formed at the material stage. There is a risk of shortening the lifespan. Further, an increase in the Cr content more than necessary is disadvantageous in terms of cost, and when attempting to refine giant carbides, quenching at a high temperature is required, resulting in a decrease in heat treatment productivity. Therefore, C
The upper limit of the content of r was set to 2.5% by weight.

Si; 0.3 to 1.5% by weight Si is an element that acts as a deoxidizing agent during molten steel, and is effective in improving solid solution strengthening and temper softening resistance to extend the life of bearings. be.

However, if the Si content increases, it leads to a decrease in mechanical strength, machinability, and carbonitriding property, so the Si content is set in the range of 0.3 to 1.5% by weight.

Mn: 0.3 to 1.7% by weight Mn is included because it acts as a deoxidizing and desulfurizing agent during molten steel, plays a major role in improving hardenability, and is inexpensive.

However, if the content increases, the life of the bearing will not be improved; on the contrary, the life will be reduced due to the formation of many nonmetallic inclusions, and other machinability such as forgeability and machinability will be reduced.

Therefore, the Mn content is limited within the above range.

0: 9 ppm or less 0 is an element that generates oxide nonmetallic inclusions (especially All! was set at 9 ppm.

Note that Al generates oxide-based nonmetallic inclusions such as Al2O, which is harmful to the service life. but,
Since Al itself has the effect of preventing coarsening of crystal grains, it is effective to contain it in an amount of 200 to 300 ppm or less.

Mo; 3.0% by weight or less Mo is an element effective in improving temper softening resistance like the above-mentioned Cr, and is also an element necessary for forming carbides in the surface layer. It is also effective in improving hardenability and hardness after hardening.

Therefore, in the rolling bearing described in claim (2), claim (1)
By further containing MO in the rolling bearing described in ), the bearing life is further improved.

However, when the MO content exceeds 3.0% by weight, the improvement of the action and effect of Mo is not so great, and on the contrary, huge carbides are formed at the material stage, which may reduce the lifespan and increase the cost. Since this is also disadvantageous in terms of performance, the upper limit of the MO content was set at 3.0% by weight. In particular, 0.1 to 3.0
Preferably, it is % by weight.

In the present invention, unavoidable impurities may be contained in addition to the various elements described above. Examples of such impurity elements include Ti, S, and P.

Ti appears as a nonmetallic inclusion in the form of TiN. Since this TiN is hard and has low plastic deformability, it becomes a stress concentration source and shortens the life. Therefore, it is necessary to reduce the Ti content as much as possible, and preferably,
It is desirable that the content be 40 ppm or less.

P is an element that reduces impact resistance. Therefore, it is better to reduce the content, and it is desirable to reduce the content to 20 ppm or less.

S causes the formation of sulfide-based nonmetallic inclusions such as MnS. Since MnS has low hardness and high plastic deformability,
Acts as a starting point for cracking during pre-processing such as rolling and forging. Therefore, in order to prevent cracking during pre-processing such as forging and to enable stronger processing, it is preferable to reduce the S content as much as possible. It is desirable that the content be 80 ppm or less.

In the present invention, fine carbides are produced in the surface layer of at least one of the raceway rings (inner ring, outer ring, etc.) and the rolling elements by carbonitriding, quenching, and tempering treatments.

This carbide is hard and has excellent wear resistance, and as a result, it is possible to ensure the necessary hardness of the bearing when the operating temperature is between semi-high and high temperatures, thereby improving the life of the bearing. Moreover, since the size thereof is minute (0.5 to 1.0 μm), stress concentration due to the applied load does not occur, and the life of the rolling bearing can be improved.

Due to the presence of fine carbides in the surface layer of the bearing and improved resistance to temper softening, a highly hard rolling bearing with a surface height of H, C60 or higher can be obtained.

By carbonitriding the alloy steel of the present invention, quenching it, and subjecting it to high-temperature tempering, the fine carbides can be precipitated in the surface layer, and retained austenite can be reduced as much as possible. Retained austenite transforms into martensite at semi-high to high temperatures, and dimensional changes occur at this time, so it is preferable to reduce the average amount of retained austenite from the core to the surface layer to 3% by volume or less by high-temperature tempering.

The tempering temperature at this time is preferably about 240 to 550'C from the viewpoint that if the tempering temperature is low, it is difficult to convert all the residual austenite into martensite.

According to the present invention, semi-high temperature to high temperature (approximately 120 to 50
It is possible to provide a rolling bearing with a long life even when used at temperatures below 50°C.

〔Example〕

Next, examples of the present invention will be described.

After melting the test materials A-H having the compositions shown in Table 1 below, carburizing, carbonitriding heat treatment, quenching, and
Tempering was performed.

Below is the margin Table 1 Table 2 The numerical values in Table 1 indicate the content (% by weight) of each element.

Further, the oxygen content is expressed in ρpm (*l). The C content is a value depending on the stage of the material, and for carburized materials, it is 0.8 to 1.0% by weight after carburizing, and for carbonitrided materials, it is 0.7 to 0.9% by weight after carbonitriding.

In the heat treatment shown in Table 2 below, carbonitriding is R1I
Approximately 3 hours in an atmosphere of gas engineer enriched gas (1.5 vol. 1%) + ammonia gas (3 to 5 vol. 1%), 820
~B50''C Carbonitriding treatment was carried out, and then from this state oil quenching was carried out to 60°C, and further tempering was carried out once for 2 hours at the temperature shown in Table 2.

On the other hand, in the case of carburizing treatment, after carburizing at 930''C for 3 hours in an RX gas + enriched gas atmosphere, the temperature was lowered to 830°C, held at 830°C for 930 minutes, and then oil quenching was performed.

In addition, if only quenching is performed without carburizing or carbonitriding, R
Oil quenching was carried out at 830°C in a gas atmosphere.

In Table 1.2, test materials A and B are related to examples of the alloy steel of the present invention, test material C is a comparative example of medium carbon steel with a low tempering temperature, and test material C is a comparative example of medium carbon steel with a low tempering temperature. A comparative example of medium carbon steel that is carburized rather than carbonitrided. Sample E is a comparative example of high carbon bearing steel that is not carbonitrided. Sample F is a high carbon steel that is not tempered at a high temperature. Comparative example of one type of bearing steel, test material G is a low carbon steel that is carburized instead of carbonitriding, comparative example with an oxygen content of 12 ppm, which exceeds the upper limit of the present invention, test material H is carburized, In addition, it is a low carbon steel whose tempering temperature is not high, and whose oxygen content exceeds the upper limit of the present invention.
This is a comparative example of pm.

For each of the sample materials thus obtained, the surface hardness (HllC) was measured and the average amount of retained austenite was determined. In the carbonitrided and carburized test materials, the amount of retained austenite has a predetermined gradient from the surface layer to the core, so the average value from the surface layer to the core was used as the amount of retained austenite. Table 2 shows the measurement results of the surface hardness and amount of retained austenite for each sample material.

Sample material A has an average amount of retained austenite of 0VA1% due to high temperature tempering. Despite the high-temperature tempering, the carbonitriding treatment ensures a sufficient surface hardness (HllC of 60 or more).

Test material B also has similar characteristics to test material A.

In particular, since sample B further contains Mo,
Surface hardness increases.

Since the tempering temperature of sample C is low, retained austenite exists as it is from the core to the surface layer.

Since the tempering temperature of the test material is the same as that of test material A,
The retained austenite ffi is O, but because carburization is performed instead of carbonitriding, the tempering softening resistance is insufficient. Therefore, the surface hardness is lower than that of sample material A and the like.

Test material E is a conventional bearing steel type II (SUJ-2). When this sample material is subjected to high-temperature tempering to convert retained austenite to martensite, it becomes softened, making it impossible to secure a surface hardness HRC of 60 or higher, which is necessary to improve bearing life.

Test material F is also a conventional bearing steel type 1 (SUJ-2). If the softening caused by high-temperature tempering is avoided, the surface hardness will be H8060 or higher, but retained austenite will remain and impair dimensional stability.

Test material G is conventional case hardening steel 5Cr420.

Since this sample material G has been carburized, it is not possible to prevent softening during high-temperature tempering, and the required surface hardness cannot be ensured.

Sample material H is the same as sample material G (conventional dephosphorization m5cr420
It is. If you avoid softening during high temperature tempering, the surface hardness will be H or C.
60 or more, retained austenite remains as it is, impairing dimensional stability. Since the oxygen content of both specimens G and H exceeds the upper limit of the present invention, their lifespan is shortened compared to specimens A and B, which have an oxygen content of 9 ppm or less.

Figure 1 shows the carbonitriding time and 0 for medium carbon steel (base C content - 0.4% by weight) and low carbon steel (base C content = 0.2% by weight).
% depth gradient characteristic diagram is shown. In FIG. 1, during carbonitriding, the gas atmosphere was the same as that described in Table 2, and the temperature was 850°C.

As can be seen from Fig. 1, medium carbon steel as in the present invention is considerably more advantageous in terms of time to obtain the required 0% to the depth where shear stress acts, compared to low carbon steel. As a result, it has been demonstrated that it is advantageous in terms of cost. Therefore, in low carbon steels like test materials G and H, the heat treatment productivity decreases.

Figure 2 shows the test results for dimensional stability at high temperatures.

The test was conducted by leaving each sample material in a high-temperature bath at 170°C for 500 hours, and then measuring the expansion rate against room temperature (20'C). do).

As can be seen from Figure 2, the tempering temperature is at the normal value (16
It can be seen that sample materials C, F, and H, which have a temperature of 0°C), have large expansion coefficients and problems with dimensional stability because the retained austenite I cannot be reduced to 0vo1%. especially,
Test material F, which is a high carbon steel and whose tempering temperature is not high, has the largest retained austenite l. Therefore, the dimensional stability of test materials A and B, which are examples of the alloy steel of the present invention whose tempering temperature is high (260°C), and of test materials E and G is lower than that of other test materials. Improve than.

FIG. 3 shows the relationship between tempering temperature and surface hardness of test materials C, D, E, F, and H. Generally, the value of surface hardness decreases as the tempering temperature increases, but when carbonitriding is performed as in sample C, the degree of decrease in surface hardness decreases due to the effect of N on improving tempering softening resistance. The sample material was carburized after changing to

It will be less than H. Since the conditions for the test material were the same as those for test material A except that carburization was performed instead of carbonitriding, it was demonstrated that carbonitriding improves the temper softening resistance. Test materials E and F are softened by high temperature tempering.

Next, using the sample materials A, B, D, E, and G, single-row deep groove ball bearings (6206) each having a bearing outer diameter of 62++ ua, a bearing inner diameter of 30 ann, and a width of 16 mm were manufactured. Then, the life (L) was determined using a ball bearing life tester manufactured by NSK Ltd.

) was measured.

The conditions for measurement are: lubricating oil = turbine oil (FBK oil RO6B manufactured by Nippon Oil Co., Ltd.), bearing load = 14
00kgf (radial load), bearing rotation speed = 200O
rpm, test temperature = 150°C. The lifespan was expressed as the number of rotations (cycles) until flaking occurred.

FIG. 4 is a characteristic diagram comparing the lifespan when the lifespan of the sample material is set to 1. The harder the surface hardness, the longer the life. Since the test materials A and B have excellent tempering softening resistance, it is possible to prevent a decrease in surface hardness after high-temperature tempering, and as a result, it is possible to ensure a good bearing life.

In addition, it can be seen that in sample material G, the oxygen content is 12 ppm (which exceeds the upper limit of the present invention, so the life is shortened. In contrast, the test material with an oxygen content of 9 ppm or less) It is verified that A and B have good lifespans.

In this example, when measuring the bearing life, the inner ring.

Although both the outer ring and the rolling elements were made using the above-mentioned sample materials, it is possible to improve the bearing life by forming at least one of them as in the present invention.

〔Effect of the invention〕

As explained above, according to the invention described in claim (1),
There is no decrease in hardness even after high-temperature tempering, and there is very little generation of oxide inclusions, making it possible to provide long-life rolling bearings even when used under semi-high to high-temperature conditions. can.

Further, the invention as claimed in claim (2) further includes MO in the alloy steel as claimed in claim (1), so that the tempering softening resistance is further improved, and as a result, the bearing life is further improved. It becomes possible to provide bearings.

[Brief explanation of the drawing]

Fig. 1 is a characteristic diagram showing the relationship between the distance from the surface of the sample material and the weight % of contained C, and Fig. 2 is a characteristic diagram showing the state of dimensional change consisting of the relationship between the average amount of retained austenite and the expansion coefficient.
Figure 3 is a characteristic diagram showing the relationship between tempering temperature and surface hardness.
FIG. 4 is a characteristic diagram showing the relationship between surface hardness and bearing life.

Claims (2)

[Claims] (1) In a rolling bearing consisting of a bearing ring and rolling elements,
At least one of the bearing ring and the rolling element has a C; 0.3.
~0.6% by weight, Cr; 0.5-2.5% by weight, Si;
0.3 to 1.5% by weight, Mn; 0.3 to 1.7% by weight,
A rolling bearing characterized in that it is made of an alloy steel containing 0.9 ppm or less, the balance being Fe and unavoidable impurities, which is subjected to carbonitriding heat treatment, followed by quenching and high temperature tempering. (2) In a rolling bearing consisting of a bearing ring and rolling elements,
At least one of the bearing ring and the rolling element has a C; 0.3.
~0.6% by weight, Cr; 0.5-2.5% by weight, Si;
0.3 to 1.5% by weight, Mn; 0.3 to 1.7% by weight,
Mo: 3.0% by weight or less, O: 9ppm or less, balance Fe
A rolling bearing characterized in that it is made of an alloy steel containing unavoidable impurities that has been subjected to carbonitriding heat treatment, followed by quenching and high-temperature tempering.