SK1442001A3 - Steel for making a ball bearing part - Google Patents

Abstract

The invention concerns steel for making a ball bearing part whereof the chemical composition comprises, by weight: 0.6 % </= C </= 1.5 %; 0.4 % </= Mn </= 1.5 %; 0.75 % </= Si </= 2.5 %; 0.2 % </= Cr </= 2 %; 0 % </= Ni </= 0.5 %; 0 % </= Mo </= 0.2 %; 0 % < Al </= 0.05 % ; S </= 0.04 %; the rest being iron and the impurities resulting from the preparation. The composition further satisfies the relationships: Mn </= 0.75 + 0.55 x Si and Mn </= 2.5-0.8 x Si. The enclosure purity of the steel measured by method A of the ASTM E 45 standard further satisfying the following conditions: type A enclosures, fines: index </= 2.5; coarse particles: index 1.5; type B enclosures, fines: index </= 1.5 coarse particles: index </= 0.5; type C enclosures, fines: index = 0; coarse particles: index = 0; type D enclosures, fines: index </= 0.5; coarse particles </= 0.5. The invention also concerns a method for making a ball bearing part and the resulting ball bearing part.

Description

Technical field

The invention relates to bearing steel and bearings which can operate at a high load.

BACKGROUND OF THE INVENTION

Ball and roller bearings are well known. Generally, they consist of components made of 100Cr6 type bearing steel containing 0.6 to 1.5% carbon, 1.3 to 1.6% chromium, 0.3 to 1% manganese and less than 0.4% silicon, the remainder they form contaminants from melting. U.S. Patent 3,666,314 discloses a bearing steel containing 0.6 to 0.78% carbon, 0.5 to 2 chromium, 0.4 to 1.15 manganese, and 1 to 2% silicon. Japanese Patent Specification No. JP 09 302443 discloses bearing steel containing 0.7 to 0.88% carbon, 0.75 to 1% silicon, 0.4 to 2% manganese, and 0.4 to 0.95% chromium. Japanese Patent Publication No. P 09 302 444 discloses bearing steel containing 0.8 to 0.95% carbon, 0.75 to 1% silicon, 0.4 to 2% manganese, and 0.2 to 0.4% chromium. At the points of contact of various components of rolling bearings, the steel is subjected to strong Hertz pressures, which, due to the rotation of the bearing, create fatigue effects of the bearing. These fatigue phenomena lead to the destruction of the bearing after a certain time, which is dependent on the bearing loading conditions. To ensure sufficient service life, the bearings are designed so that the maximum stress due to Hertz pressure remains less than the fatigue limit of the steel used and at the minimum required service life. In addition, bearing deformations induced by the forces they must withstand activate the fatigue-induced destructive effects. To reduce this unfavorable phenomenon, rolling bearings are designed such that deformations are limited to low values. Both of these conditions lead to the possibility of limited bearing lightening. This is because, for a given application, the lighter the bearings, the higher the specific forces they must withstand. These restrictions on ······

99

9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

9999 99 ·· ···· ·· · bearing design has drawbacks, especially when it is desired to reduce the dimensions or weight of the mechanism into which the bearings are mounted. This is particularly the case with vehicle wheel bearings, where it is desired to improve the adhesion on the road by reducing the weight of unsprung masses.

SUMMARY OF THE INVENTION It is an object of the present invention to overcome this drawback by designing a bearing steel whose fatigue properties are improved over known bearing steels and thereby allow the use of bearings that are lighter than the bearings used today. In addition, · l · · · · · · · _ · _ · _ · _ · _ · _ · _ · l · _ · ··· ·· ··· A steel has been developed which has good castability and ability to overcome both heat and cold transformations and which requires only relatively standard heat treatment during or at the end of production.

SUMMARY OF THE INVENTION

The subject of the invention is a steel for producing bearing components having the following composition (the numbers represent percentages by weight):

0.60 <C <1.50 0.40 <Mn <1.50 0.75 <Si <2.50 0.20 <Cr <2.00 0 <Ni <0.50 0 <Mo <0.20 0 <Al <0.05 S < 0.40 O <0.0009,

the remainder being iron and melting impurities, the composition meeting the condition:

Mn < 0.75 + 0.55 x Si

Mn <2.50 - 0.80 x Si, the purity in terms of the inclusions measured by Method A according to ASTM E 45 further meets the following conditions:

- thin type A inclusions:

- Type A thick inclusions:

- thin type B inclusions:

- coarse inclusions of type B:

- thin type C inclusions:

- Type C thick inclusions:

index <2.5 index <1.5 index <1.5 index <0.5 index = 0 index = 0 «I · β · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · E e e e e · e e e e

- Thin D-type inclusions:

- Type D thick inclusions:

index <0.5 index <0.5

Preferably, the chemical composition is such that, separately or better at the same time:

0.8 <MN <1.2 0.8 <Si <1.7

0.9 <C <1.1

1.3 <Cr <1.3

An advantage is also that the silicon content must be higher than 1.2%.

The invention also relates to a method for producing bearing components, wherein:

- the semi-finished product made of the steel according to the invention is obtained by hot forming into a preform or a preform of a seamless tube,

- the molded semi-finished product is subjected to a spheroidization heat treatment consisting of heating to 750 to 850 ° C followed by cooling at 10 ° C per hour to 650 ° C to obtain a structure with a hardness of less than 270 HV formed by a homogeneous dispersion of globular carbides, and optionally undergo a cold forming operation, such as rolling or drawing or cold drawing the wire to obtain the product,

- the part is separated from the product and formed by hot or cold forming or by machining into a blank of bearing parts,

- the workpiece is subjected to a heat treatment by isothermal tempering or quenching, for example in oil, after austenitization at a temperature of 800 to 950 ° C and impregnation at a temperature of 100 to 400 ° C, preferably below 250 ° C, to obtain components for bearings with with a hardness of 58 to

Ιι · «· · · · ee ee e ·« e ee · · · · · _Λ e eeee fr * ee ······· e 'e · ee ee ee ee e ···· «e · ·· ·

HRc and consisting of fine carbides, martensite and 5-30% residual austenite. The product is, for example, a seamless oven.

The invention also relates to bearing components made of steel according to the invention having a structure consisting of martensite, residual carbides and 5 to 30% residual austenite.

The invention also relates to a bearing made of components according to the invention having for operation conditions corresponding to maximum Hertz pressures with a possibility of reaching 4.5 GPa and deformations with a possibility of reaching 150 µm and a service life of L10 at least four times that of that of 100Cr6 steel used under the same loading conditions.

The invention is illustrated, but not limited, by the following examples. Percentages are by weight unless otherwise specified.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

For the manufacture of bearings consisting of raceways and rolling elements having good fatigue strength, steels of the following chemical composition are used:

- more than 0,6% and preferably more than 0,9% carbon to obtain sufficient hardness and sufficient residual austenite content, but less than 1,5% and preferably less than 1,1% to prevent excessive segregation and to limit primary formation carbides

- more than 0.75%, preferably 0.8% and even more preferably more than 1.2% of silicon to increase the heat stability and residual austenite (at about 170 to 450 ° C and preferably above 300 ° C) and hardness, but less than 2.5%, and preferably less than 1.7%, since if the silicon content is very high, The steel becomes very brittle in particular to be able to be deformed by plastic deformation,

- more than 0,4% and preferably more than 0,8% of manganese to obtain the ability to obtain a tempered structure with a residual austenite content of more than 5% and preferably more than 15%, the manganese content of which must satisfy the following conditions: MN <0,75 + 0.55 x Si to obtain good castability, without which it would be difficult to obtain steel clean enough to have good bearing fatigue strength and satisfy the condition: MN <2.5 0.8 x Si to allow final operations and cold plastic deformation ; these relationships indicate that the manganese content must be less than 1.5% and preferably less than 1.2%,

- 0.2% to 2%, and preferably 1.3% to 1.6% of chromium, in order to achieve sufficient hardenability, on the one hand, and to produce globular carbides with dimensions less than 2 µm, uniformly distributed and in sufficient quantity,

- less than 0,5% nickel, the residual content of which is unnecessary but has a favorable effect on hardenability,

less than 0.2% molybdenum; this element reduces the softening rate during annealing,

- 0% to 0,05% aluminum and less than 0,04% sulfur,

- less than 0,0009 oxygen to achieve sufficient purity in terms of inclusions, the remainder being iron and smelt from melting.

In addition, in order to achieve good durability, the purity of the steel, expressed in terms of inclusions, is such that, when measured by method A according to ASTM E 45, it meets the following conditions:

- thin inclusions: type A: index <2.5

- thick inclusions: type A: index <1,5

- thin inclusions of type B: index <1,5

- coarse inclusions of type B: index <0,5 e · ····

- thin type C inclusions;

- Type C thick inclusions:

- Thin D-type inclusions:

- Type D thick inclusions:

·· ee ·· ee · ······· · · · · · · ·

Index * index · index index index index index index index index index index index index = = index index index index = = = index index index = index = Type A interfaces are essentially sulfides, Type B interfaces are essentially aluminates, Type C interfaces are essentially silicates, Type D interfaces are globular interfaces.

This steel is cast and optionally rolled into a blank, which may be a billet (hut) or a pipe.

The blank is then deformed by hot plastic deformation to obtain a round billet such as round bars or seamless pipes.

The product billet is then subjected to a spheroidization annealing consisting of one or more thermal operations at a temperature of 750 to 850 ° C followed by cooling at a maximum cooling rate of 10 ° C per hour up to a temperature below 650 ° C to obtain a structure with a hardness less than 270 HV with fine dispersion of carbides. This heat treatment is needed to impart good ductility of the steel to plastic cold deformations and good machinability, in the method used to make the product, for example by cold rolling, cold drawing or wire drawing. This product is used to produce workpiece blanks, for example, ball bearing blanks, rollers, or bearing raceways.

The production of semi-finished products, which is carried out by hot or cold forming or by drawing of wire or by machining parts cut off from the product, results in a heat treatment consisting of tempering and annealing. This provides a bearing component. The pre-tempering austenitization temperature, above 800 ° C, is selected so as to obtain, after tempering, a structure consisting of martensite, 5 to 30% residual austenite, and en · · a ♦ · · · · ··· · · · · · · · · · ····

II of homogeneous distribution of residual carbides. The residual austenite content, the presence of which is absolutely necessary in order to obtain good penetration resistance, depends on the value of Ms (martensite start, which is the temperature of the start of the martensitic transformation), which depends both on the composition of the steel and the austenitization conditions. It is known to those skilled in the art how these parameters can be determined, for example by dilatometric tests. The annealing (impregnation) is carried out at temperatures above 100 ° C to stabilize the structure, but below 400 ° C and preferably below 250 ° C, in order not to destabilize the residual austenite.

The steel thus obtained has a fatigue limit at a hardness of 62 HRc substantially higher than 100Cr6 steel at the same purity expressed by the inclusions as the steel of the invention. This fatigue behavior is visibly evidenced by a compression fatigue test on 7 mm diameter test specimens having 4 facets 3 mm wide and 12 mm high. These test specimens shall be subjected to a periodic load of compressive forces exerting a compressive stress of 2 450 MPa to 50 MPa and the number of cycles after which the test specimen is broken shall be determined. Because fatigue tests are statistical in nature, they are repeated several times.

The chemical composition of the steel in ppm tested as the first sample was ("inv." = Steel of the invention, "comp." = 100Cr6 for comparison):

C Are you Mn Ni Cr Mo Al WITH ABOUT inv. 1,011 1.53 1.00 0.16 1.44 0,030 0,039 0.03 207 comp. 1,012 0.23 0.33 0.33 1.43 0,027 0,036 0,011 3.0

In-purity values

type A type B thin coarse type C thin coarse type D thin coarse thin thick inv. 1.5 0.5 1 0 0 0 0.5 0 comp. 2.0 0.5 1 0 0 0 0.5 0

I · · · e e e e i i i i i i i i i i i i i i i i i i i

The results are as follows: Steel according to the invention:

Test Sample 1: No Failure to Test Sample 2: Failure to Test Sample 3: No Failure to Test Sample 4: Failure to Test Sample

111 111 000 cycles

111 000 cycles

111 111 000 cycles

111 000 cycles

100Cr6 steel according to the state of the art:

Sample 1: Failure in Test Sample 2: Failure in Test Sample 3: Failure in Test Sample 4: Failure in Test Sample

630 000 cycles 31 111 000 cycles

788 000 cycles

111 000 cycles

These results indicate that the fatigue strength of the steel of the invention is 17% greater than 100Cr6 steels, i.e. an increase of approximately 400 MPa.

Example 2

In a second example, a wheel bearing consisting of two rows of tapered bodies was made of steel according to the invention and tested at a radial force Fr = 590 daN, corresponding to an acceleration of 0.6 g and at an axial force Fa = 354 daN. These loads resulted in a radial deformation of 0.14 mm (while normal design rules for this application do not allow deformation greater than 0.04 mm) and the maximum Hertz (Hz) stress calculated without respecting the deformations was Po = 3.4 GPa. The bearing components were subjected to a tempering stage after austenitization at 890 ° C and were subsequently annealed at 230 ° C to obtain a content of approximately 17% residual austenite. The bearings were subsequently tested on a test stand to measure parameter L10 corresponding to the test duration leading to 10% of damaged bearings. The parameter L10 is calculated for at least 8 test bearings using Weibull statistics theory.

······················

Q · · «· <<<<<<<<<<<<<<<

For comparison, bearings were made of 100Cr6 steel tempered after austenitization at 835 ° C and annealed at 170 ° C according to the prior art. These bearings were tested under the same conditions as above.

The purity expressed in the inclusions was in both cases in accordance with the invention.

The results were as follows:

Steel according to the invention 100Cr6% residual austenite 17 19 16 6

L10 (in hours) 85 109 220 43

The results demonstrate a very substantial increase in the service life resulting from the improved fatigue behavior of the steel of the invention.

In addition, the measured lifetimes can be compared with the results used in the prior art to design bearings for each respective application. For conventional 100Cr6 steel, these results only apply to the load level, resulting in maximum Hertz stresses of less than 3.5 GPa and deformations of less than 50 gm. In this example, a prior life evaluated in the prior art would be greater than 80 hours, which could be considered sufficient. However, the above tests show that these assumptions could be false for the prior art 100Cr6 steel while they are valid for the steel of the invention.

Example 3

In the third example, the roller bearings of the gearbox were tested on a test stand, the rollers having a relatively low stiffness due to the small thickness (3.2 mm) and made both of the steel according to the invention and of steel »» · ···· ·· · · · · • · · · · · · · • · f · · · * ^ί φ ί ί / * ί ί · < »· · · · · · · · ····

100Cr6 at a radial load of 3000 daN, which is directed to a maximum Hertz stress calculated without respecting the deformations (i.e. underestimated) of 3.8 kPa and at a deformation of 55 µm. The steel of the invention had a hardness of 61.5 HRc and a residual austenite content of 12-14%. The 100 Cr6 steel had a hardness of 62 HRc and a residual austenite content of 5-8%. In both cases the purity was expressed in the inclusions of the invention. The bearings were tested at 100 ° C at 900 rpm. The bearings of the invention had an L10 service life of greater than 150 hours, while the L10 service life of 100Cr6 steel bearings was only 25 and 60 hours.

Industrial Applicability

Steel composition for the production of longer-life rolling bearings compared to prior art steel bearings.

Claims (12)

PATENT CLAIMS ················ · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · The patent claims 1. Steel for the manufacture of bearing components, characterized in that its chemical composition by weight is: 0.60 <C <1.50 0.40 <Mn <1.50 0.75 <Si <2.50 0.20 <Cr <2.00 0 <Ni <0.50 0 <Mo <0.20 0 <Al <0.05 S <0,04 O <0,0009, the remainder being iron and melting impurities, the composition meeting the condition: Mn < 0.75 + 0.55 x Si Mn <2.50 - 0.80 x Si and the purity in terms of the inclusions measured by Method A according to ASTM E 45 further satisfy the following conditions: - thin A-type inclusions: index £ 2.5 - Type A thick inclusions: index £ 1.5 - thin inclusions of type B: index <1,5 - thick inclusions of type B: index <0,5 - thin C-type inclusions: index = 0 ···················· ··· ·· ···· ·· ·· ···· ·· · - type C thick inclusions: index = O - thin D-type inclusions: index <0.5 - coarse inclusions of type D: index <0,5 excluding steel containing by weight 0,55 to 0,78% carbon, 0,5 to 2% chromium, 0,1 to 1,16% manganese and 1 to 2% silicon, and the remainder being iron and residual impurities, excluding steels containing by weight 0.7 to 0.8% carbon, 0.5 to 1% silicon, 0.1 to 2% manganese, 0.4 to 0.95% chromium, less as 0.05% aluminum and less than 0.003 oxygen and the remainder being iron and residual impurities and excluding steels containing by weight 0.8 to 0.95% carbon, 0.5 to 1% silicon, 0.1 to 2% manganese, 0.15 to 0.4% chromium, less than 0.05% aluminum and less than 0.030 oxygen and the remainder being iron and residual impurities. Steel according to claim 1, characterized in that its chemical composition is in percent by weight 0.8 <Mn <1.2 0.8 <Si <1.7. Steel according to claim 1 or 2, characterized in that its chemical composition in weight percent is Si> 1.2. Steel according to claim 1, characterized in that its chemical composition in percent by weight is: 0.9 <C <1.1 1.3 <Cr <1.6. Steel according to claim 4, characterized in that its chemical composition is in percent by weight 0.8 <Mn <1.2 0.8 <Si <1.7. t · ································································································································ · ·· ·· ······ Steel according to claim 4 or 5, characterized in that its chemical composition in percent by weight is Si> 1.2. 7. A method for producing bearing components, characterized in that: - a blank made of steel according to claims 1 to 6 is obtained by hot forming into a forging or seamless tube, - the formed semi-finished product is subjected to a spheroidization heat treatment in one or more thermal operations at a temperature of 750 to 850 ° C, followed by cooling at a rate of 10 ° C per hour to a temperature of 650 ° C to obtain a structure with a hardness less than 270 globular carbides and optionally undergo a cold forming operation, such as rolling or cold drawing or cold drawing the wire to obtain a product, - a part is separated from the product and formed hot or cold or machined into a component blank for bearings, - the workpiece is subjected to a heat treatment by isothermal tempering or quenching, for example in oil, after austenitization at a temperature of 800 to 950 ° C and a soaking at a temperature of 100 to 400 ° C preferably below 200 ° C, to obtain a bearing component with hardness 58 to 67 HRc and consisting of homogeneously distributed fine carbides, martensite and 5 to 30% residual austenite. Method according to claim 7, characterized in that the product is a seamless pipe. A seamless tube, characterized in that it is made of steel according to claim 1 to 6. A bearing component made of steel according to claims 1 to 6, characterized in that it has a structure consisting of uniformly distributed fine carbides, martensite and 5 to 30% residual austenite by weight. «> · ···> ·· «· 1-4 • t · e · · e s «s s s s s s s s s s s s s s s E Bearing component made of steel according to claim 11, characterized in that it is produced by the method according to claim 7. Bearing, characterized in that it consists of the components according to claims 10 or 11.