GB2242484A

GB2242484A - Rolling bearing component

Info

Publication number: GB2242484A
Application number: GB9106195A
Authority: GB
Inventors: Yasuo Utsumi; Kousuke Nonaka
Original assignee: NSK Ltd
Current assignee: NSK Ltd
Priority date: 1990-03-22
Filing date: 1991-03-22
Publication date: 1991-10-02
Anticipated expiration: 2011-03-22
Also published as: JPH03271343A; JP3123055B2; GB2242484B; GB9106195D0

Abstract

A rolling member 1 used as an outer race, inner race, a ball or a roller of a bearing contains a metal flow in the structure formed by plastic working (cold forging), and the angle of metal flow theta 1, theta 2 determined by a tangent line 3, 4 of the metal flow and the axis of rotation 5 is 10 DEG or larger to improve the member against rolling fatigue. <IMAGE>

Description

ROLLING MEMBER BACKGROUND OF THE INVEI4TION FIELD OF THE INVENTION The present invention relates to a rolling member1 and in particular, to a rolling member of a rolling bearing used in a cylinder head, rocker arm, and the like of an engine of automobiles and autobicycles, which are operated in a high engine speed range.

DESCRIPTION OF THE PRIOR ART Recently, it has been an increasing trend to change the bearing system of a cylinder head and a rocker arm of an engine for automobiles and autobicycles from a conventional slipper system to a rolling system to reduce friction of the engine and as a countermeasure for wear of the cam.

In this case, for an outer race of a rolling bearing, a conventional bearing steel type 2 (SUJ - 2) is used, and the outer race was formed by turning a bar material and thereafter by hardening through quenching.

The rolling members which are used for such engines are required to provide high reliability and semipermanent service life from the standpoint of improving the safety and durability.

Accordingly, in order to meet such requirements, the applicants of the present application proposed a long life rolling member in Japanese Patent Laid Open Publication No. Hei 2125,841, and Japanese Patent Laid Mizen Publication No. Hei 2-209,452.

In such prior art rolling members, in order to reduce non-metallic inclusions which serve as a starting point of causing a crack at the time of plastic working, the cold forgeability is improved by reducing the content of S (sulfur) which is an element acting to produce the non-metallic inclusions, and at the same time, a long service life is attained.

Furthermore, a rolling bearing which provides longer service life has been proposed in Japanese Patent Laid Open Publication No. Hei 2125,838, in which the stress concentration is moderated to suppress the flaking due to fatigue by adding Nb (niobium) and V (Vanadium) and forming fine crystal particles.

In the engines for automobiles and autobicycles, since they are often operated at high engine speeds exceeding 10,000 revolutions per minute, the rolling bearing is also required to provide characteristics which enable to bear such severe operation.

However, in the prior art rolling bearing and rolling member, in a high engine speed region in which a phenomenon of jumping of the cam and the rocker arm is caused, the follow of the rolling bearing to the rotation of the cam becomes unsatisfactory and a fatigue break (crack) of an outer race sometimes occurs. This fatigue break is caused due to the presence of a non-metallic inclusion which acts as a starting point when a bending stress acting on the outer race becomes large. Accordingly, in order to achieve a long service life, it is necessary to reduce the fatigue break.

SUMMARY OF THE INVENTION In order to solve the problems in the prior art, it is an object of the present invention to provide a rolling member which enables to reduce the occurrence of a fatigue break as far as possible and which achieves a long service life.

In one aspect of the present invention, in a rolling member which bears a load and rolls relative to a counterpart member, an angle of a metal flow formed by plastic working with respect to an axis of rotation is equal to 10 degrees or larger.

In another aspect of the present invention, a rolling member is formed of an alloyed steel containing 15 ppm or less 0 (oxygen) and 150 ppm or less S (sulfur), and at least a rolling surface is hardened by a heat treatment.

In another aspect of the present invention, a rolling member is formed of an alloyed steel containing 80 ppm or less Ti (titanium), and at least a rolling surface is hardened by a heat treatment.

In still another aspect of the present invention, a rolling member is formed by subjecting a case hardened steel to a carburiziny treatment or a carbonitriding treatment, and a residual stress due to compression is formed in a region from a rolling surface to a depth of 0.2 mm.

In manufacturing a rolling member, when an element which produces a non-metallic inclusion is mixed, the non-metallic inclusion is produced and this becomes the cause of cracking. Accordingly, it is practiced to change the non-metallic inclusion into fine particles by plastic working (cold forging), and at the same time, to make the material structure dense and uniform. By the plastic working, the non-metallic inclusion is changed to the fine particles, and also extended uniformly. As a result, a flow (metal flow) of fiber of the non-metallic inclusion is formed.

The inventors of the present application studied and discovered that in the rolling member which has been subjected to plastic working, when an angle of the metal flow with respect to the axis of rotation becomes 10 degrees or larger, a life against rolling fatigue is significantly improved.

The present invention was made based on such a novel discovery, and in a rolling member which bears a load and rolls relative to a counterpart member, since the metal flow formed by plastic working forms an angle of 10 degrees or larger with the axis of rotation, the non-metallic inclusion is made fine, and the material structure is made dense and uniform. Furthermore, since the stress concentration acting on the non-metallic inclusion is moderated, a rolling member having an excellent wear resistance and an impact resistance can be achieved, and moreover, this rolling member provides a long life even when it is used in a high engine speed region in which the jumping phenomenon occurs.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a half sectional view showing a condition of metal flows in a ring-shaped test piece of a rolling member in an embodiment in the present invention.

Fig. 2 is a schematic view for explaining swaging working.

Fig. 3 is a sectional view showing a condition of metal flows in a steel ball which is a rolling member in an embodiment of the present invention.

Fig. 4 is a characteristic chart showing a relationship between an angle of the metal flow and a bearing life.

Fig. 5 is a characteristic chart showing a relationship between a residual stress and a distance from the surface of a rolling member.

Fig. 6 is a schematic diagram showing an arrangement of a ring crack test machine of a bearing.

Fig. 7 is a schematic diagram for explaining 5-stage header working.

Figs. 8A and 8B are half sectional views showing a condition of metal flows in a ring-shaped test piece of a rolling member in another embodiment of the present invention.

Fig. 9 is a characteristic chart showing a relationship between a test load and a bearing life in the another embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a half sectional view of a ringshaped test piece 1 obtained by grinding a disk 21 in a ring shape, which disk 21 is obtained by cold forging a bar material 20 in Fig. 2. The reference numerals 3 and 4 denote tangent lines of respective metal flows, numeral 5 denotes an axis of rotation, and numeral 6 denotes a metal flow.

The angle of the metal flow 6 is determined by an angle between the metal flow 6 and the axis of rotation 5. Accordingly, angles 61 and 62 respectively between the tangent line 4 and a reference line 10 parallel to the axis of rotation 5 and between the tangent line 3 and a reference line 2 parallel to the axis of rotation 5 are angles of metal flow.

In the case where the rolling member is a steel ball (rolling body) not of the ring shape, metal flows formed due to plastic working are shown in Fig. 3 which is a sectional view of the steel ball. In Fig. 3, the reference numeral 31 denotes a flow specific portion (end flow) at which the metal flow is exposed substantially perpendicular to a rolling surface, that is, a pole portion. Numeral 32 denotes an equator portion at which burrs are removed. The angle of metal flow in such a steel ball is defined similar to the case of the ringshaped test piece mentioned above, that is, by an angle 63 between a tangent line 4 and an axis of rotation 5.

As will be seen from Figs. 1 and 3, since the metal flows curve towards a surface of the member, the angles between the metal flows and the axis of rotation assume various values. Hence, among these angles, an angle which assumes a maximum value is defined as the angle between the metal flow and the axis of rotation.

When the angle of metal flow mentioned above is less than 10 degrees, the non-metallic inclusion is not sufficiently changed to fine particles and the material structure does not become sufficiently dense and is not homogenized.

Fig. 4 shows a relationship between the angle of metal flow of test pieces and the life (L10) ratio against a crack of a rolling member, in which the test pieces are cold forged by changing the upset ratio (the upset ratio will be described later) in various rates in producing the ring-shaped test pieces by using a conventional bearing steel Type 2 (SUJ - 2), and in which the life ratio of a standard which is not subjected to plastic working is made "1". In this case, as the angle of metal flow increases, the upset ratio becomes also high. As will be seen from Fig. 4, the life (L10) is significantly improved when the angle of metal flow is equal to 10 degrees or larger. The upper limit of the angle of metal flow is a working limit, that is, until a crack due to the plastic working is caused.

In this respect, the occurrence of the crack varies depending on the conditions for producing the material. For example, when the material is subjected to spheroidizing annealing, the limit point of occurrence of the crack is improved to a high degree of working. And the limit is lowered as the concentration of carbon increases.

The rolling member in the present invention is obtained by plastic working of a material with a upset ratio in a range from a predetermined rate to a working limit (until the crack occurrence limit).

Here, the upset ratio is expressed by (Hg - H/Ho) x 100%, in which as shown in Fig. 2, lio represents a size of a material (bar material) before the plastic working (cold forging), and H represents a size of the material after the cold forging. As the upset ratio is increased, the angle of metal flow increases.

In obtaining the rolling member of the present invention, it is preferable that the upset ratio is in a range from about 10 to 60%. When the upset ratio is 10% or lower, there is a fear that a fiber angle of metal flow becomes less than 10 degrees. On the other hand, when the upset ratio exceeds 60%, there is a fear that a crack occurs due to the working limit.

The fatigue break (crack) of a rolling member is caused by a crack which in turn is caused by a non-metallic inclusion located at a position of a maximum shearing stress. Accordingly, it is necessary or preferable to perform plastic working so that the angle of metal flow at the position of the maximum shearing stress becomes equal to 10 degrees or larger. In a rolling bearing, the position of the maximum shearing stress is located within several mm from a surface layer.

The angle of metal flow becomes a small value at a position near the core of the member, and a large value at a position near the surface layer portion. Accordingly, in order to obtain a rolling member having a long life, it is preferable that the angle of metal flow at each position from the surface layer portion to at least the maximum shearing stress position is 10 degrees or larger.

Since the maximum shearing stress position is different depending on a load which is applied, it is preferable to determine the maximum shearing stress position from expected use conditions of the rolling member, and to perform plastic working with a upset ratio which enables to obtain a maximum value of the angle of metal flow which is equal to 10 degrees or larger among the angles of metal flows formed between the surface of the rolling member and at least the maximum shearing stress position. By virtue of this, it is possible to obtain a rolling member in which the non-metallic inclusion at the maximum shearing stress position is made fine, the structure is made dense and uniform, and the service life is long.

The rolling members in the present invention include various kinds of rolling members such as an inner race and an outer race which constitute raceway rings, and a roller and a ball which constitute rolling bodies. As materials for forming the rolling member, a high carbon chrome bearing steel (SUJ 2 - 4), a carburized bearing steel (SCr 420H, SCM 420H, SNCM 220H, SNCIT 420H, SNCM 815, SAE 4320), and a high temperature bearing high-speed steel (M50), which are known as bearing steels, and a rolling bearing stainless steel (SUS 440C, 51440C), and the like.

In another aspect of the present invention, furthermore, as a rolling member forming material, a high cleanness steel is used, in which the contents of oxygen and sulfur which act as elements for producing the non-metallic inclusion are respectively selected, that is, O : 15 ppm or less, S : 150 ppm, and Ti : 80 ppm or less. By the use of the high cleanness steel, the non-metallic inclusion which acts as a starting source for causing a fatigue break is limited as far as possible, and the life of the rolling member is further improved.

When the content of S (sulfur) is limited to 150 ppm or less, the production of a non-metallic inclusion (MnS etc.) which is produced due to the action of S, is suppressed and the occurrence of a crack can be avoided at the time of plastic working.

As a result, it becomes possible to perform the plastic working with a high upset ratio. Thus, the limitation of the content of S as mentioned above is preferable to achieve a fine-particled non-metallic inclusion, and a dense and fine material structure.

In particular, the content of S is preferably 80 ppm or less.

Furthermore, when the content of O (oxygen) is made equal to 15 ppm or less, this is preferable in suppressing the production of oxygen family nonmetallic inclusion (A1203) which has a coarse particle diameter ed which acts as a starting point of the occurrence of a crack at the time of rolling fatigue. In particular, it is preferable that the content of O (oxygen) is equal to 9 ppm or less.

Furthermore, when the content of Ti (titanium) is limited to 80 ppm or less, it is possible to suppress the production of a non-metallic inclusion (TiN) which is produced due to the action of Ti (titanium). In particular, it is preferable that the content of Ti (titanium) is equal to 40 ppm or less.

As a material for a rolling member of the present invention, when a carburized bearing steel is used, the unbalance (the carbon concentration is higher at the surface layer portion) in the concentration of carbon is caused between a core portion and a surface layer portion as a result of carburization, or carbonitriding. Thus, a residual stress of compression is formed -from the surface through the core portion. Since the residual stress of compression acts to cancel out a stress due to rolling, the resistance for a rolling fatigue break is improved. The residual stress of a case hardened steel after carburization, or carbonitriding increases, as shown in Fig. 5, at a position near the surface layer, and it decreases gradually from the surface layer until a depth of 0.2 mm, and thereafter the residual stress is maintained substantially at a constant value.On the other hand, in the case of a normal hardened steel (SUJ 2), the residual stress of compression exists within a depth of 0.1 mm, however, a certain amount of tensile stress is present at positions deeper than 0.1 mm. This tensile stress acts in a direction opposite to the direction of cancelling the stress applied to the rolling member, and there is a possibility that the position of maximum shearing stress is changed within several mm from the surface layer face. Accordingly, in another aspect of the present invention, a case hardened steel is subjected to a carburizing treatment, or a carbonitriding treatment so as to form a residual stress of compression from a rolling surface to a depth of 0.2 mm. As a result, the resistance to a rolling fatigue break is improved and the longer life is achieved.As the residual stress of compression, from the standpoint of view of improving the impact resistance and the wear resistance of the rolling member thereby to improve the life, it is preferable that the magnitude of the residual stress of compression is 10 kg/mm2 or larger in a region from the surface layer portion to a depth of 0.2 mm or larger. In order to achieve this value of the residual stress of compression, it is necessary to adjust the concentration of a carbon source or a nitrogen source at the time of carburizing, or carbonitriding, and to adjust the carburizing, or carbonitriding temperature and time.

EXAMPLE 1 A steel ingot of SUJ - 2 is produced by a 100 kgf vacuum furnace, and after subjecting the steel ingot to a red heating treatment, the steel ingot is worked into a bar material 20 having a diameter of 22 mm (Fig. 2). Then, the bar material 20 is cold forged with various upset ratios as shown in Table 1 (described later), and a disk 21 is produces as shown in Fig. 2. Thereafter, the disk 21 is cut in a ring shape as shown in Fig. 1, and the disk 21 is subjected to a heat treatment and grinding to produce a plurality of test pieces as shown in Table 1. In Fig. 1, the metal flows 6 are shown, and a maximum value of the angles between the metal flows 6 and the axis of rotation 5 is defined as the angle (e) of metal flow. Furthermore, in Fig. 1, the solid line 11 represents a raceway groove of a rolling body.

The heat treatment includes spheroidizing tempering, quenching, and tempering, and the spheroidizing tempering is performed by heating the steel to a temperature just above a transformation point and after maintaining this temperature, the steel is cooled gradually at a suitable speed. The quenching is performed by oil cooling the steel after heating for suitable hours at 800 to 850"C, and the tempering is performed normally at a low temperature of 150 to 200'C.

For each of the test pieces a ring crack test was carried out in the manner as shown in Fig.

6. Specifically, a test piece 1 is fixed between a load roll 60 and a drive roll 61 by using a support roll 62. A fatigue breaking test is carried out by applying a load to the test piece 1 by the load roll 60, and by rotating the test piece 1 at a high speed.

In this case, a maximum test load is 500 kgf (a maximum acting stress of 118 kgf/mm2) and a rotation speed of test piece 1 is 9600 rpm. The life of the test piece 1 is determined by the number of rotations until a crack is caused from an inner peripheral surface. As the life, a 100% break life (L10) is employed. Further, the lubrication is performed by oil mist lubrication of 68 &num; turbine oil.

For each type of test piece, six pieces are prepared with the same upset ratio, and some of the six pieces are used in the fatigue breaking test and the rest of them are used as test pieces in obtaining an angle of metal flow. In the technique of detecting the angle of metal flow, a test piece is put into a treatment liquid containing water and concentrated hydrochloric acid in the ratio, that is, water : concentrated hydrochloric acid = 1 : 1, and it is boiled. This process is repeated a plurality of times, and the test pieces are observed by a scanning type atomic microscope, and an angle (o) of metal flow is obtained from a photograph of a microscope image.

The results are shown in the following Table 1.

Table 1

Test -Upset Metal L10 life Life piece ratio flow number (%) angle (Cycle) ratio 1 O O 0.13 x i06 1 2 10 5 0.20 x 106 1.5 3 20 15 0.46 x 106 3.5 4 30 30 0.52 x 106 4.0 5 35 40 0.56 x 106 4.3 6 60 50 0.57 x 106 4.4 7 70 Crack occur - - * only turning without conducting cold forging As will be seen form Table 1, when the angle (e) of metal flow is increased to 10 degrees or larger, it is seen that the life is significantly improved. For example, when the angle (e) is 15 degrees, the life is about 3.5 times as long as the turned product (test piece number 1) which is not cold forged.

The upset ratio at the time of cold forging can be increased to a working limit at which a crack occurs. As the upset ratio is increased, the angle of metal flow is increased resulting in a long life.

EXAMPLE 2 In this EXAMPLE 2, in order to examine the influence of cleanness of a material and a residual stress of compression on the life of a rolling member, as shown in Table 2, generally, two classes of test pieces are prepared so that in one class (high cleanness steel), the contents of O (oxygen), S (sulfur), and Ti (titanium) are high, and in the other class (low cleanness steel), the contents of 0, S and Ti are low. Furthermore, each of the two classes are divided into two kinds, in one kind a residual stress by compression is present on the surface, and in the other kind the residual stress is not present. The fatigue breaking test was conducted on these test pieces.

As materials for test, a bearing steel type 2 (SUJ - 2) for normal quenching, arid case hardened steels (SCr 420, SAE 4320) for carburizing, and carburizing and nitriding are used.

A bar material 21 (a diameter of 22 mm) of each material is cold forged in a disk 21 as shown in Fig. 2. Further, as shown in Fig. 7, the disk 21 is cold forged by a 5-stage header to obtain a ringshaped test piece 70. In this cold forging, the swaging rate is adjusted so that for all the test pieces, the angle of fiber of metal flow after the forging equally becomes 15 degrees.

Figs. 8A and 8B are views of half cross sections of ring-shaped test pieces used in the example. Fig. 8A shows a ring-shaped test piece obtained by directly turning the bar material 20 in Fig. 2, and Fig. 8B shows a ring-shaped test piece formed by the 5-stage header. According to Fig. 8A, in the ring-shaped test piece formed by turning only, a metal flow is not clearly produced, and also, the angle of metal flow is seen as being parallel to the axis of rotation 5. In contrast, according to Fig.

8B, the metal flow is clearly produced, and the angle of metal flow is large with respect to the axis of rotation 5.

The forged product is heat treated after turning1 and is cut subsequently to form a ringshaped test piece. In the heat treatment, as to the SUJ - 2, the quenching and annealing are performed similar to that in the above-mentioned EXAMPLE 1, and as to the case hardened steel, the carburizing treatment is carried out. In this carburizing treatment, the case hardened steel is heat treated in an atmosphere of Rx plus enrich gas 5 % for about eight hours at 930 + 5 C, and thereafter, oil quenching is carried out, and furthermore, annealing is performed at 1600C for two hours.In this respect, in place of the caburizing, carbonitriding may be carried out, however, in the latter case, a carbonitriding heat treatment is carried out in an atmosphere of Rx gas plus enrich gas plus ammonia gas 5 % for about three to four hours at 830 to 870 e C. Thereafter, oil quenching is carried out.

The residual stress by compression is defined by a minimum value of residual stresses by compression at positions 0.08 to 0.12 mm below the surface of the test piece.

In the following Table 2, the comparison example steel means test pieces which are directly formed by turning the bar material 20 in Fig. 2 without performing the cold forging by the 5-stage header, and each of these test pieces corresponds to the one shown in Fig. 8A.

Table 2

Test Method Clean- Material Content Content Content Residual piece of ness of O of S of Ti stress number working m) (wt%) (dim) (kgfXmm) 8 X 8 0.006 14 (a a) 9 Su-2 6 0.004 20 +5 0 o 10 c SUJ2 5 0.003 10 a) U 11 c u SCr420 7 0.008 14 12 ap, ---- SAE4320 8 0.008 20 -20 (a c 12 O X 13 11 0.011 50 (a a) a) 14 H0 suJ-2 10 0.011 42 +2 a) o 14 ri H > 15 U 14 0.012 40 H 16 EP, -15 17 SAE4320 13 0.014 50 18 ro 4 0.002 10 e 20 6 6 0.003 12 H (a UH SCr420 9 0.008 18 q) ~~~~ H ---- -16 8 22 '"Y SAE4320 8 0.007 26 to C: : x (a a) c c o 24 SUJ-2 13 0.01D 38 +3 a) H to u H (OH SCr420 12 0.020 48 U EP, a) 27 ou ZX SAE4320 14 0.017 46 For each of the test pieces shown in Table 2, the rolling fatigue test was conducted in the manner shown in Fig. 6. The test loads are divided into three grades of 350 kgf, 400 kgf and 500 kgf, and the number of test pieces for each grade is that 10 test pieces for each of 400 kgf and 500 kgf, and 3 test pieces for 350 kgf. When the number of repetition times reached 100 x 106, the test was interrupted. The test results are shown in Table 3 and Fig. 9.

As will be seen form the results, similar to EXAMPLE 1, it is confirmed that the products formed by cold forging provide a longer life than the product formed by turning. Further, the high cleanness steel containing small contents of O (oxygen), S (sulfur), and Ti (titanium) provides a longer life as compared with the normal cleanness steel containing relatively much contents of 0, S and Ti. For example, when comparing the cold forged product with the turned product, in the case of the high cleanness steel, the cold forged product is improved in the life to provide about six times as long as the turned product, and in the case of the normal cleanness steel, the cold forged product is improved in the life to provide about four times as long as the turned product.Furthermore, in the same cold forged product, for both the high cleanness steel and the normal cleanness steel, it is confirmed that the case hardened steel in which the stress by compression is present in the surface is longer in the life than the quenched steel (SUJ - 2) in which the tensile stress is present in the surface.

Although in the EXAMPLES, as a rolling member which retains a residual stress by compression in the surface, it is described as to a case hardened steel which is subjected to a carburizing heat treatment, the present invention is not limited to this, and a bearing steel other than the case hardened steel may be subjected to induction hardening.

While in the EXAMPLES, as the test pieces the ring-shaped test pieces are tested, the same test results will be obtained for such test pieces as a roller and a steel ball which are rolling bodies.

The rolling member of the present invention is applicable to a raceway ring (inner race, outer race) and a rolling body (ball, roller).

As described in the foregoing, in the present invention, the following advantages are provided.

In one aspect of the invention, the angle of a metal flow formed by plastic working with respect to the axis of rotation is equal to 10 degrees or larger. As a result, the non-metallic inclusion is made to become fine particles, and the concentration of a stress acting on the non-metallic inclusion can be moderated. Accordingly, the structure can be made dense and uniform, and a rolling member having a long life can be provided.

In another aspect of the invention, the contents of oxygen, sulfur, and titanium which are the elements acting as the cause of producing the non-metallic inclusion are reduced as far as possible. Accordingly, a rolling member having a further long life can be provided.

In another aspect of the invention, since the concentration of the stress is moderated by a residual stress by compression, a rolling member having a further l-ng life can be provided.

Claims

WHAT IS CLAIMED IS:

1. In a rolling member which bears a load and rolls'relative to a counterpart member, the improvement in which: an angle of a metal flow formed by plastic working of said rolling member is equal to 10 degrees or larger with respect to rotation axis of said rolling member.

2. A rolling member according to claim 1, wherein said rolling member is formed of an alloyed steel containing 15 ppm or less oxygen, and 150 ppm or less sulfur, and at least a rolling surface is hardened by a heat treatment.

3. A rolling member according to claim 1, wherein said rolling member is formed of an alloyed steel containing 80 ppm or less titanium, and at least a rolling surface is hardened by a heat treatment.

4. A rolling member according to claim 1 or 3, wherein said rolling member is formed by subjecting a case hardened steel to a carburizing treatment or a carbonitriding treatment and then hardening and a residual stress by compression is retained in said rolling member in a region from a rolling surface to a depth of 0.2 mm or larger.