JPH02125841A - Rolling bearing - Google Patents

Abstract

PURPOSE:To prevent the cracks of the title bearing at the time of working and to prolong its service life by forming at least one of the inner ring, outer ring and rolling element in a rolling bearing with a medium carbon Mn steel having specific compsn., subjecting the steel to carburizing treatment and specifying the amt. of retained austenite on the surface layer. CONSTITUTION:At least one of the inner ring, outer ring and rolling element in a bearing is formed with the compsn. of a medium carbon Mn steel constituted of, by weight, 0.4 to 0.7% C, 0.15 to 1.2% Si, 1.2 to 1.7% Mn, 200 to 300ppm Al, <=40ppm Ti, 100 to 200ppm N, <=80ppm S, <=9ppm O and the balance Fe. The steel is worked into a rolling bearing, which is subjected to carburizing heat treatment or carbonitriding heat treatment to regulate the amt. of retained austenite in the surface layer part to 25 to 45vol.%. In this way, the coarsening of the crystal grains is prevented to prolong the service life of the bearing. At the time of furthermore incorporating at least one kind of 0.03 to 0.08% Nb and 0.1 to 0.15% V into the above steel, the crystal grains are converted into fine ones having >=8 of grain size number even after the carburizing heat treatment, by which the service life can moreover be prolonged.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to rolling bearings used in automobiles, agricultural machinery, construction machinery, steel machinery, etc., and in particular, rolling bearings that have a long life required for transmissions and engines. Regarding rolling bearings.

[Conventional technology]

Conventionally, rolling bearings with a long rolling fatigue life (hereinafter also referred to as "life") that are used under conditions of high surface pressure required for automatic applications, for example, have been designed to Since it is necessary to set a hardness curve, low carbon case hardening steel 5CR42011, SC with good hardenability is used.
By using M420H, 5AE8620H, 5AE4320H, etc., and performing carburizing heat treatment or carbonitriding treatment, the surface hardness of the inner and outer rings and rolling elements becomes HRC58 to 6.
4, and the hardness of the entire bearing was increased so that the core hardness was HRC 30 to 48.

In addition, in U.S. Patent No. 4,191,599, high carbon alloy steel is heat treated in a carburizing atmosphere, the MS point of the surface is lowered than the core, and thermal stress type transformation is caused by quenching, resulting in compressive residual stress on the surface. Disclosed is a rolling bearing with a long life.

Further, in U.S. Pat. No. 4,023,988, C: 0.6
-1.5% by weight, Cr, Mn, Ni, Cu, M
A long-life rolling bearing is disclosed that uses a hot-forming low alloy steel selected from the following and has fine carbides.

[Problem to be solved by the invention]

However, when trying to deepen the carburized layer in the conventional case hardening steel 5CR42011, etc., the carburizing process must be carried out at high temperature and for a long time because the carbon content of the matrix (base carbon content) is low. Heat treatment productivity decreases. On the other hand, when the surface carbon concentration is increased, pro-eutectoid formation tends to occur due to the high Cr content of the case-hardened steel, resulting in a decrease in rolling fatigue life. Sokote, S A E 86
201-1 and S A E 4320 II, Cr
Hardenability is ensured by reducing the content and adding other elements (Ni, Mo), but this increases material cost. Further, during carburizing heat treatment or carbonitriding heat treatment of this case hardening steel, crystal grains sometimes become coarse, which becomes a stress concentration source, resulting in a reduction in rolling fatigue life.

On the other hand, in other conventional examples, expensive Mo, Ni, and Cr
etc., it is necessary to increase the cost in order to obtain a long-life rolling bearing. In the conventional example of US Pat. No. 4,023,988, in order to form fine carbides, for example, spt+eroidzin
g anneal, rough forming, na
Complex heat treatments such as rdning, austenitizing, etc. are required, and a decrease in heat treatment productivity cannot be avoided.

In US Pat. No. 4,191,599, Mo, an expensive element,
, W, and Cr, which leads to high costs, and it has not been possible to achieve a long life under lubrication contaminated with foreign matter only by a mechanism that applies residual compressive stress to the surface.

In other words, the cause of reduced rolling fatigue life is microscopic damage that originates from damage (indentation) caused on the bearing surface layer by foreign substances such as metal chips, shavings, firmness, and wear powder mixed in the bearing lubricating oil. There is flaking caused by cranks. Furthermore, there are non-metallic inclusions that exist in the base of the bearing and become sources of stress concentration due to their high hardness and low plastic deformability. However, the rolling fatigue life described above will be reduced.

Furthermore, in any of the above-mentioned conventional rolling bearings, cracking during pre-processing of the bearing, such as forging, cannot be sufficiently suppressed depending on the processing rate.

In order to solve these various problems, the present invention aims to achieve good heat treatment productivity without increasing material costs, and to use bearings under clean lubrication as well as under lubrication contaminated with foreign matter. The purpose of the present invention is to provide a rolling bearing that has a longer lifespan than conventional bearings and does not crack during pre-processing such as forging, which requires a high processing rate.

[Means to solve the problem]

The invention according to claim (1) provides a rolling bearing comprising an inner ring, an outer ring, and a rolling element, in which at least one of the inner ring, outer ring, and rolling element contains C: 0.4 to 0.7% by weight, and Si: 0.
．． 15-1.2% by weight, 4121.2-1.7% by weight,
8j2: 200-300 ppm, Ti: 40 ppm
Below, N: 10 (1-200ppmX5:80pp
m or less, 0:9 ppm or less, the balance is iron, medium carbon manganese copper, and has been subjected to carburizing heat treatment or carbonitriding heat treatment, and the amount of retained austenite in the surface layer is 25 to 45.
It is characterized by being vol%.

Moreover, the invention described in claim (2) further provides the medium carbon manganese steel described in claim (1) with an additional content of Nb0.03 to 0.0.
It is characterized by containing at least one of 8% by weight and V: 0.1 to 0.15% by weight.

Further, the invention according to claim (3) provides a rolling bearing comprising an inner ring, an outer ring, and a rolling element, in which at least one of the inner ring, outer ring, and rolling element contains C: 0.4 to 0.7% by weight, S
i: 0.15 to 1.2% by weight, Mn: 1.2 to 1.7% by weight, Ti: 40 ppm or less, S: 80 ppm or less, ○: 9 ppm or less, Nb: 0.03 to 0.08
Weight% and V: 0.1 to 0.15% by weight of at least one medium carbon manganese steel with the balance being iron, subjected to carburizing heat treatment or carbonitriding heat treatment, and the amount of retained austenite in the surface layer is 25 to 45vo1%. It is a sign of a ship.

Furthermore, the invention according to claim (4) is characterized in that the medium carbon manganese steel Vat is microcrystalline with an average grain size number of 8 or more even after carburizing heat treatment or carbonitriding heat treatment.

[Function] As a result of various studies on extending the life of rolling bearing steel and cracking during pre-processing, the inventors of the present application have determined the relationship between the amount of retained austenite in the bearing surface layer and life, and the relationship between grain size and life. We have obtained various new knowledge regarding the relationship between the S content and the cracking incidence during pre-processing, and based on this knowledge, we have arrived at the present invention as described in the claims.

First, in the present invention, the reason why medium carbon manganese steel with C: 0.4 to 0.7 wt% is used will be explained.

The present inventors have discovered that by setting the amount of retained austenite in the bearing surface layer to 25 to 45 vol%, it is possible to extend the life of a rolling bearing under foreign matter-containing lubrication. However, in order to keep the amount of retained austenite in the bearing surface layer within the above range, it is necessary to increase the surface carbon concentration, but 5CR42011SCM4
Since 2011 has a high Cr content, it tends to cause pro-eutectoid, which is harmful to the rolling fatigue life of the bearing. On the other hand, if the Cr content is reduced, the hardenability decreases, making it impossible to obtain the hardened layer depth necessary for a rolling bearing. Therefore, in the present invention, the Cr content is less than 0.35% by weight, and the Cr content is less than 0.35% by weight.
In order to avoid a decrease in hardenability due to a decrease in content, Mn is added, and by using a medium carbon manganese steel with a large amount of heath carbon, the occurrence of pro-eutectoid is suppressed, and the residual austenite in the bearing surface layer is reduced to 25 to 45 vol. % range to obtain the required hardened layer depth.

Here, the effect of retained austenite, which is a feature of the present invention, will be explained with reference to FIGS. 1 to 4.

When a bearing is used under lubrication contaminated with foreign matter, impressions as shown in FIG. 2 are generated on the rolling surfaces of the inner and outer races and the rolling elements due to repeated contact with the foreign matter. As can be seen from the cross-sectional view of the indentation not shown in FIG. 2, an edge portion is formed in the indentation, and the maximum stress p ma is applied to this edge portion. The curvature r of this edge portion and the radius C of the indentation are closely related to retained austenite as will be explained below. Normally, retained austenite is soft, for example, about ■v300 (however, it varies depending on the carbon content of the material). therefore,
By allowing this retained austenite 1- to exist in the surface layer in a desired proportion, it is possible to alleviate stress concentration at the edge portion of the indentation, thereby delaying the propagation of microcracks that occur in the indentation area after the indentation is formed. Can be done. The retained austenite in the surface layer transforms into martensite due to the deformation energy applied to the surface after a predetermined number of relative passes of the partner part (for example, the rolling element to the raceway ring) passing through the indentation during transfer. , harden.

FIG. 3 shows the relationship between the r/c value and retained austenite T8.

In order to reduce P, , /P (that is, to alleviate stress concentration), it is necessary to increase r, assuming C is constant. In other words, since the value of r/c is a factor indicating the degree of relaxation of stress concentration, the larger this value is, the longer the life will be. However, as can be seen from Figure 3, even if the proportion of retained austenite TR is increased, the value of r/C is saturated at a predetermined level,
Constant plate 1 - does not get bigger. Especially retained austenite γ
, becomes more than 45 vol%, this becomes noticeable.
r/c i,l is almost saturated. Therefore, γ8
If it is made larger than that, it will only lower the surface hardness and reduce the rolling fatigue life.

Next, the critical significance of each numerical limitation indicated in the claims of the present invention will be explained.

First, the service life of inner and outer rings and rolling elements when used for foreign matter lubrication is determined by the time required until flaking occurs, as is clear from the relationship between bearing life and retained austenite γR (vol%) shown in the graph of Figure 1. The rolling fatigue life Lto indicated by the elapsed time changes in accordance with the change in the amount of residual austenite LrR.

In other words, when the retained austenite TR becomes 25vo1% or more, the rolling fatigue life LIO improves;
When it exceeds 1%, the life span decreases rapidly. Therefore, the retained austenite in the surface layer of the inner and outer rings and rolling elements is
It must be in the range of at least 20VO1% to 45VO1%.

In particular, if the retained austenite γ8 exceeds 45 vol%, the surface hardness after carburizing heat treatment or carbonitriding heat treatment decreases, which is not preferable.

In order to obtain a life equivalent to or longer than that of conventional carburized steel bearings under clean lubrication, it is desirable that the II RC of the rolling elements is 63 or more, and the HRC of the inner and outer rings is 58.
It is preferable that it is above. For this purpose, it is necessary that the retained austenite TR is 45 vol% or less.

Note that the experimental conditions of Part 1II are as follows. The bearing life test was conducted using a ball bearing life tester made by NSK Ltd. Copper powder (hardness, Hv300-500, turbidity 80-160
μm] at a mixing ratio of 1100pp,
The test was conducted at a bearing load (radial load) of 600 kgf and a bearing rotation speed of 200 rpm.

Next, the effects of the elements contained in the medium carbon manganese steel used in the present invention and the critical significance of their contents will be explained.

AN forms oxide-based nonmetallic inclusions such as A E 203. Since A E 203 has high hardness and low plastic deformability, it becomes a stress concentration source and causes a reduction in rolling fatigue life. Therefore, it is necessary to reduce the AN content in order to improve bearing life. On the other hand, however, in order to prevent crystal grain coarsening during carburizing heat treatment or carbonitriding heat treatment, it is necessary for 82 to precipitate at grain boundaries in the form of iN.

In the invention described in claims (]) and (2), the i content is set to 200 to 300 ppm. AN is 200pp
If it is less than 300 ppm, the crystal grains will become coarse and the rolling fatigue life of the bearing will be reduced.
The amount of 203 increases, which has a negative effect on the lifespan.

Ti appears as a nonmetallic inclusion in the form of TiN.

Since TiN has high hardness and low plastic deformability, it becomes a stress concentration source and is detrimental to rolling fatigue life. Therefore, it is necessary to reduce the Ti content as much as possible, and the upper limit is 40p.
It was set as pm.

N is necessary to form AlN and suppress coarsening of crystal grains. However, when the N content is high, the amount of TiN, which is a nonmetallic inclusion, increases. Therefore, claim (1), (
2) In the invention described, the N content is 100 to 200 ppm.
And so. If the N content is less than 1100 ppm, the amount of IN precipitated will be insufficient and the crystal grains will become coarse and ζ.
If it exceeds 100%, the amount of TiN increases and the rolling fatigue life decreases.

S causes the formation of sulfide-based nonmetallic inclusions such as MnS. Since MnS has low hardness and high plastic deformability, it acts as a starting point for cracking during pre-processing of at least one of the inner ring, outer ring, and transfer body during forging, rolling, etc. Therefore, in order to prevent the occurrence of cracks during pre-processing such as forging and to enable stronger cutting, it is necessary to reduce the S content, and the upper limit is set at 80 ppm.

0 decreases the rolling fatigue life as an element that generates oxide-based nonmetallic inclusions, so it is necessary to reduce its content as much as possible, so the second limit was set at 9 ppm.

Since Si is necessary as a deoxidizing agent, its content is reduced to 0.
The content was 15 to 1.2% by weight. If it is less than 0.15% by weight, the deoxidizing effect will not be sufficient, and if it exceeds 1.2% by weight, there will be no change in the deoxidizing effect, so the content was set within the above range.

Since Mn is necessary to compensate for the decrease in hardenability due to the decrease in Cr content, the content is set at 1.2 to 1.7% by weight.
Closed. If it is less than 1.2% by weight, hardenability cannot be improved, and if it exceeds 1.7% by weight, hardness increases and machinability such as forgeability and machinability decreases. , the content was within the above range.

Nb and V are effective elements for precipitating at grain boundaries by themselves and suppressing their coarsening, making the crystal grains finer and extending the life of the bearing. It is effective because it further enhances the anti-inflammatory effect.

In other words, high-temperature heat treatment (
950° C. to 970° C.) or a long time heat treatment, ANN alone may not be able to sufficiently prevent crystal grain coarsening. Therefore, in the invention described in claim (2), Nb: 0.
03-0.08% by weight and V: O, ]-0.15
% by weight of at least one type.

In addition, in the invention of claim (3), in place of i and N, the above Nb: 0.03 to 0.08
Weight % and V: Contained at least one of 0.1 to 0.15 weight %.

Nb: less than Q, Q3 weight, V: less than 0.1% by weight, there is little effect in preventing coarsening of crystal grains, and Nb
: 0.08% by weight, V : 0.15% by weight, the effect of preventing crystal grain coarsening will not improve and the cost will increase, so the contents of Nb and (2) were selected within the above ranges.

The critical significance of the heath carbon value of the carbon steel used in the present invention is as follows.

If the proportion of heath carbon is lower than 0.4% by weight, the carburizing or carbonitriding heat treatment time becomes longer, and the heat treatment productivity decreases. Furthermore, the medium carbon manganese steel used in the present invention is a steel type that does not contain elements that improve hardenability such as Cr and Mo, and if the amount of heath carbon is less than 0.4% by weight, the hardenability will be insufficient. Unable to obtain hardening depth.

On the other hand, when the heath carbon content exceeds 0.7% by weight, the proportion of carbon that enters into solid solution in the rematrix, where the amount of carbon that enters through carburization is small, decreases, resulting in a non-uniform solid solution state, which shortens rolling fatigue life. It will drop.

Therefore, from the above, the amount of heath carbon is 0.4
The range was selected to be 0.7% by weight.

As shown in FIG. 4, carbon steel in such a range is subjected to carburizing heat treatment or carbonitriding heat treatment to adjust the amount of solid solute carbon or solid solute carbon nitrogen to a range of O18 to 1.1% by weight. As a result, the amount of retained austenite in the surface layer portion can be within the range of 25 to 45 vol%. Furthermore, if carburizing heat treatment or carbonitriding heat treatment is performed on carbon steel with the amount of heath carbon within the above range, carbon and nitrogen atoms will uniformly diffuse into Fe atoms, solid solution strengthening will occur, and micro- It is possible to delay the occurrence of scratches and improve rolling fatigue life even under clean lubrication.

Further, as described in claim (4), even after carburizing heat treatment or carbonitriding heat treatment, the grain size of the medium carbon manganese steel constituting the rolling bearing is made fine with a grain size number of 8 or more. , it is possible to provide a rolling bearing with a longer life.

〔Example] Next, examples of the present invention will be described.

In 5MN443, which is a conventional carbon steel, 8! .

A sample material was prepared by melting the material with adjusted S and N contents. The composition of each sample material is shown in Table 1 below.

(Hereafter, margin) No. ■ table <S 8j2. Ti.

0 is ppm.

Others are wt%> Next, a plurality of each sample material in Table 1 above was subjected to heat treatment at 930'CX for 8 hours, and the size of crystal grains was examined.

The results are shown in Table 2 below.

(Rolling life test) Each of the test materials in Table 1 above was subjected to carburizing heat treatment or carbonitriding heat treatment, and the amount of retained austenite in the surface layer was reduced to 25 to 25%.
A test piece adjusted to 45vo1% was created.

The heat treatment conditions in this example will be explained next.

As shown in the graph shown in Figure 5, direct quenching in carburizing heat treatment involves heat treatment at 930±5°C for approximately 8 hours in an atmosphere of P x gas enriched gas, followed by oil quenching and further heat treatment at 160°C2. Time tempered. Furthermore, as for carbonitriding heat treatment, as shown in the graph of Fig. 6, carbonitriding heat treatment was performed at 830 to 870°C for about 3 to 4 hours in an atmosphere of 5% Rx gas - enriched ammonia gas. , then oil quenched.

Disk-shaped test pieces that can be applied to both the inner and outer rings of rolling bearings were created using each of the test pieces that had been subjected to the carburizing heat treatment or carbonitriding heat treatment. 1st edition) Electrical Steel Research Institute,
Rikogakusha, May 25, 1965, pp. 10-21j
A rolling fatigue life test was conducted using the testing machine described above. The test conditions are as follows.

P, a, -=560 kB-f/mm2N =30
00 c, p, m lubricating oil #68 Turbine oil The rolling fatigue life test results are shown in Table 2 and Figure 7. FIG. 7 shows the relationship between the average grain size number of each sample material and the bearing life LIG indicated by the number of cycles of stress due to transfer. As can be seen from FIG. 7, the larger the average grain size number, that is, the smaller the grains, the larger the value of 1-10, and the longer the rolling fatigue life of the bearing.

Sample material 2 has a low i and N content, and sample material 4 has a low content of 8! Since the content of N is small, and the content of N is small in sample material 5, the value of L+o is small.

On the other hand, sample material 1.3 has a good LIO value because both the i and N contents are within the range of the present invention.

For sample material 6.7, IF! Prevention of N crystal grain coarsening 1
Since Nb or V, which enhances the F action, is contained, the crystal grains are further refined and the LIO value becomes even larger.

Sample material 8 is AI2. Although the content of H is insufficient compared to the above sample material 1, since it contains Nb which itself prevents the coarsening of crystal grains, the crystal grains become smaller and the value of L+o also decreases. In good condition.

Sample material 9 has small crystal grains, but the Affi content exceeds the range of the present invention, so it is classified as I! , 20J increases, the value of L+o becomes smaller, and the life becomes shorter.

In addition to i and N, sample material 10.11 has a high content of niNb and V(7). The value of becomes large, and the degree of improvement in grain refinement effect is small compared to the addition of Nb and V, resulting in high cost.

Rolling fatigue life in the present invention. In order to improve,
It is desirable to perform carburizing heat treatment and carbonitriding heat treatment by controlling the temperature for 9 hours, etc. so that the crystal grain size becomes fine with a grain size number of 8 or more even after carburizing heat treatment or carbonitriding heat treatment.

(Crack occurrence test) Next, cylindrical samples of φ20×30 man were prepared using the test materials shown in Table 2 above, cold worked (forged) at an upsetting rate of 80%, and the crack occurrence rate was investigated. Ten cylindrical samples were prepared for each sample material. The results are shown in Table 2 and FIG. 8 above. FIG. 8 is a graph showing the relationship between the S content and cracking incidence of each sample material.

As shown in Table 2, sample materials 2, 3, and 4.6 cracked due to their high S content. In particular, although sample material 3.6 has small crystal grains and a large value of L+o, the occurrence of cracks cannot be avoided due to the large S content.

It can be seen from FIG. 8 that the cracking incidence decreases as the S content in the sample material decreases, and it can be seen that the cracking incidence is 0% when the S content is 80 ppm or less. therefore,
If the S content is 80 ppm or less, stronger processing becomes possible.

In addition, in the rolling life test in the above example, the life was shown for a disk-shaped test piece that can be applied to both the inner ring and the outer ring. You can get similar results by doing so.

21'I [Effect of the invention] As explained above, according to the invention described in claim (+1.
Further, since a long and complicated heat treatment is not required, a rolling bearing with good heat treatment productivity can be provided without increasing material cost.

A predetermined amount of retained austenite exists in the surface layer,
In addition, coarsening of crystal grains during carburizing heat treatment or carbonitriding heat treatment is prevented, and the amount of nonmetallic inclusions is also limited, so it is not only possible to use it under lubrication contaminated with foreign matter (even under clean lubrication, it is different from conventional rolling bearings). This results in a rolling bearing with a longer life in comparison.

Furthermore, since the amount of S is also limited, it is possible to provide a rolling bearing that does not generate cracks during pre-processing such as forging with a high rate of addition.

Further, according to the invention described in claim (2), in addition to the above-mentioned effects, the crystal grains can be further made finer, so that a rolling bearing with a longer life can be provided.

Furthermore, according to the invention described in claim (4), in addition to the above-mentioned effects, the medium carbon manganese steel constituting the rolling bearing has a grain size number of 8 even after carburizing heat treatment or carbonitriding heat treatment.
Because of the above fine crystal grains, it is possible to provide a rolling bearing with a longer life.

[Brief explanation of drawings]

Fig. 1 is a graph showing the relationship between the rolling fatigue life of a bearing and the amount of retained austenite under lubrication containing foreign matter, Fig. 2 is a cross-sectional view of an indentation shown along with stress, and Fig. 3 is a graph showing the relationship between the rolling fatigue life of a bearing and the amount of retained austenite under lubrication containing foreign matter. This is a graph showing the relationship between the value of c and the amount of TR, showing that r/c is saturated with respect to γ. )
Fig. 5 and Fig. 6 are graphs showing the relationship between temperature and time of direct carburizing heat treatment and carbonitriding heat treatment, respectively. Average particle size number and bearing life. The eighth circle is a graph showing the relationship between the S content and the crack occurrence rate.

Claims (4)

[Claims] (1) In a rolling bearing consisting of an inner ring, an outer ring, and a rolling element, at least one of the inner ring, outer ring, and rolling element is C
:0.4-0.7% by weight, Si: 0.15-1.2% by weight, Mn: 1.2-1.7% by weight, Al: 200-30
0ppm, Ti: 40ppm or less, N: 100-200
ppm, S: 80 ppm or less, O: 9 ppm or less, the balance is iron, the steel is made of medium carbon manganese steel, has been subjected to carburizing heat treatment or carbonitriding heat treatment, and has a residual austenite amount of 25 to 45 vol% in the surface layer. rolling bearings. (2) Nb: 0.03 to 0.03 to the medium carbon manganese steel.
08% by weight and at least one of V: 0.1 to 0.15% by weight (
1) The rolling bearing described. (3) In a rolling bearing consisting of an inner ring, an outer ring, and a rolling element, at least one of the inner ring, outer ring, and rolling element is C
: 0.4 to 0.7% by weight, Si: 0.15 to 1.2% by weight, Mn: 1.2 to 1.7% by weight, Ti: 40ppm or less, S: 80ppm or less, O: 9ppm or less, Nb: 0
．． 03 to 0.08% by weight and V: 0.1 to 0.15% by weight, the balance is made of medium carbon manganese steel, and is subjected to carburizing heat treatment or carbonitriding heat treatment to reduce the amount of retained austenite in the surface layer. 25-45vol%
A rolling bearing characterized by: (4) The medium carbon manganese steel is microcrystalline with an average grain size number of 8 or more even after carburizing heat treatment or carbonitriding heat treatment, any one of claims (1) and (3) above. The rolling bearing described in paragraph 1.