JP2007063626A - Steel component for bearing having excellent fatigue property, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a component for a bearing whose fatigue properties are improved by fining the old austenite grain size in the surface layer part and which is subjected to quenching treatment suitable for bearing inner and outer races of the bearing, a bearing ball or the like, and to provide a method for producing the same. <P>SOLUTION: Regarding the steel component for a bearing, in the surface layer, the average grain size of old austenite grains after quenching is controlled to ≤3.5 μm, and the steel component preferably has a componential composition comprising one or more selected from, by mass, 0.6 to 1.5 C, 0.1 to 1.0% Si, 0.1 to 1.5% Mn, 0.15 to 0.8% Mo and 0.15 to 1.0% W, ≤0.1% Al and 0.05 to 2.0% Cr, and, when needed, one or more selected from S, Cu, Ni, Ti, Nb, B, Sb and N are added thereto. The steel in which the aspect ratio of spheroidized carbide in the former structure is ≤3 on the average is heated at an Ac<SB>3</SB>point or above to an Ac<SB>3</SB>point+130°C or below in such a manner that the average heating rate in the temperature range from an Ac<SB>3</SB>point to 10°C to an Ac<SB>3</SB>point is controlled to ≥0.5°C/s, and holding time at an Ac<SB>3</SB>point or above is controlled to ≤500 s so as to be subjected to quenching treatment. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a steel part for a bearing that has been subjected to quenching treatment and a method for manufacturing the same, and in particular, refines the prior austenite grain size of the quenched surface layer to improve fatigue characteristics, and is suitable for bearing inner and outer rings, bearing balls, and the like. About things.

  Steel components for bearings such as bearings used in automobiles and machines are required to have excellent rolling fatigue characteristics. Steel parts for bearings are usually used after being quenched and tempered at sites where fatigue characteristics are required.

  As a method for improving the rolling fatigue life, for example, Patent Document 1 discloses that the S53C level hypoeutectoid steel (ferrite, pearlite structure) is subjected to high-frequency heat treatment to an austenite single-phase region at least twice, and old austenite. A method to refine the grain size and improve the fatigue life is described. The achieved prior austenite grain size is 6.2 μm even with the smallest grain size, and the rolling fatigue life is 1.2 to 1.5 times that of conventional materials. Degree.

On the other hand, although there is no description in this document about hypereutectoid steel, in general, in the case of hypereutectoid steel, the conventional austenite grains are 6 to 10 μm even in the case of normal quenching material. If the prior austenite grains are not refined, the rolling fatigue life cannot be expected to improve.
JP 2002-256336 A

  The present invention has been developed in view of the above-described situation, and an object of the present invention is to provide a bearing component that has improved rolling fatigue life and has excellent fatigue characteristics, and a method for manufacturing the same.

The present inventors have intensively studied to solve the above-mentioned problems. Steels with Mo and W added alone or in combination in an optimum amount, and steels in which the carbides in the previous structure are spheroidized are Ac ₃ or more. The effect of suppressing the grain growth by pinning the austenite grains by the remaining spheroidized cementite when quenching after heating at a temperature of ~ Ac ₃ point + 130 ° C or less and holding time of Ac ₃ point or more is 500 seconds or less In addition, due to the effect of suppressing the austenite grain growth of Mo and W, the average grain size became finer and the fatigue strength improved compared to the steel not containing Mo or W.

  Furthermore, we have found that the rolling fatigue life is significantly increased due to the fatigue improvement effect of addition of Mo and W compared to the steel without the optimum amount.

  The present invention has been completed by further studying the obtained knowledge, and the gist of the present invention is as follows.

1 C: 0.6-1.5 mass%,
Si: 0.1-1.0mass%,
Mn: 0.1 ~ 1.5mass%
Al: 0.1 mass% or less,
Cr: 0.05 to 2.0 mass%
N: 0.01 mass% or less,
Mo: 0.15 to 0.8 mass%, W: 0.15 to 0.8 mass%, one or more remaining Fe and unavoidable impurities,
A steel part for bearings with excellent fatigue characteristics, wherein the average grain size of the prior austenite grains in the surface layer is 3.5 μm or less.
2 Furthermore,
S: 0.03 mass% or less,
Cu: 1.0 mass% or less,
Ni: 1.0 mass% or less,
Ti: 0.01 mass% or less,
Nb: 0.5 mass% or less,
B: 0.01 mass% or less,
The steel part for bearings with excellent fatigue characteristics according to claim 1, comprising one or more of Sb: 0.0050 mass% or less.

  3. The steel part for bearings having excellent fatigue characteristics according to 1 or 2, wherein the surface layer has a hardness of Hv700 or more.

4 1 or 2-component composition according to the steel aspect ratio carbide forms in the tissue and 3 below spheroidized carbides on average, an average heating rate of at Ac ₃ point -10 ° C. to Ac ₃ point 0.5 ℃ / s or more, Ac ₃ point or more, Ac ₃ point + 130 ° C or less, and the holding time of Ac ₃ point or more is heated for 500 seconds or less, and then it is quenched, and it has excellent fatigue characteristics. Manufacturing method.

  According to the present invention, the average old austenite grain size in the quenched surface layer portion of the bearing steel part becomes a fine structure of 3.5 μm or less, and a bearing steel part having excellent rolling fatigue life characteristics can be easily obtained, Very useful to.

Hereinafter, the present invention will be specifically described.
[Ingredient composition]
C: 0.6mass% ~ 1.5mass%
C is an element necessary to ensure the hardness required to obtain the fatigue life of the parts in the quenched portion. If it is less than 0.6 mass%, sufficient hardness and fatigue strength cannot be obtained in the quenched portion.

  On the other hand, if added over 1.5 mass%, the workability (shearability and forgeability) before quenching is degraded. Therefore, a suitable C content range is 0.65 mass% to 1.50 mass%.

Si: 0.1-1.0mass%
Si is preferably contained in an amount of 0.1 or more in order to improve the rolling fatigue life. However, if added over 1.0 mass%, as with C, the workability before quenching (shearability, forgeability) deteriorates. Therefore, the preferable content range of Si is 0.1 to 1.0 mass% or less.

Mn: 0.1-1.0mass%
In order to improve hardenability, Mn is preferably contained in an amount of 0.1 mass% or more. However, if added excessively, the workability (shearability and forgeability) before quenching is degraded. For this reason, it is preferable that the upper limit of the content be 1.0 mass% or less.

Cr: 0.05-2.0mass%
Cr is preferably contained in an amount of 0.05 or more because it has the effect of improving the hardenability and reducing the hardness and improving the workability before quenching by accelerating carbide spheroidization. However, even if added in excess of 2.0 mass%, the effect is saturated, so that it is preferably contained in the range of 0.05 to 2.0 mass%.

Mo: 0.15-0.8 mass%, W: 0.15-1.0 mass% or less
Mo and W are very important components for the present invention, and one or more of them are added. Mo and W have the effect of suppressing the growth of austenite grains by the drag effect in the austenite region and improving the fatigue life by strengthening the martensite matrix, so when adding one or more, add 0.15-0.8 Mass%.

  Figure 1 shows the effect of Mo and W addition on the B10 life. The figure shows the results of investigating the effects of addition of Mo and W on the same heat-treated material, and adding 0.15 mass% or more of Mo and W significantly improves the fatigue life. On the other hand, if it exceeds 0.8 mass%, the fatigue life is greatly deteriorated.

In steel parts for bearings, to which an appropriate amount of one or more of Mo and W is added, the steel that has not been added to the austenite grain size in the hardened surface layer is refined even if quenching is performed under the same conditions, and Mo and W are added. In addition to the effect of strengthening martensite, the fatigue life is improved by more than 4 times compared to conventional steel.
Al: 0.1 mass% or less
Al is preferably contained because it is a component having a strong deoxidizing action and an effect of improving the cleaning of steel. If added in excess of 0.10 mass%, the steel cleaning is rather deteriorated and the fatigue life is reduced, so the content is preferably 0.1 mass% or less. More preferably, it is 0.005-0.1 mass%.

  The above is the basic component composition, and the balance is Fe and inevitable impurities. Inevitable impurities include P, S, N, and O. P allows 0.05 mass% and O allows 0.0150 mass%.

  When S and N are also mixed as inevitable impurities, S is allowed up to 0.01 mass% and N is allowed up to 0.008 mass%, but may be positively added. When improving the desired characteristics, one or more of S, Cu, Ni, Ti, Nb, B, Sb, and N are added.

S: 0.03 mass% or less
S may be added to combine with Mn to form MnS and improve machinability. If added over 0.03 mass%, MnS becomes the starting point of cracking and the fatigue life is remarkably reduced. Therefore, when added, the upper limit of the content is preferably 0.03 mass%.

Cu: 1.0 mass% or less
Cu may be added because it has the effect of improving the hardness of the hardened part by improving the hardenability. In order to acquire this effect, when adding, it is 1.0 mass% or less.

Ni: 1.0 mass% or less
Ni is added to the upper limit of 1.0 mass% in order to increase hardenability and improve the toughness of the hardened portion. In addition, when Cu is added, it is preferable to add Ni to the amount of Cu added to prevent hot brittleness.

Ti: 0.01 mass% or less
Ti may be added because it has the effect of suppressing the growth of austenite grains due to nitride formation. However, if it exceeds 0.01 mass%, the fatigue characteristics deteriorate, so when added, it is preferably 0.01 mass% or less.

Nb: 0.5 mass% or less
Nb may be added because it has an austenite grain growth suppressing effect due to the formation of nitride (or carbonitride), but if its content exceeds 0.5 mass%, the effect is saturated, so when adding, 0.5% It is preferable to make it mass% or less.

B: 0.01 mass% or less
Since B has an effect of improving hardenability, it may be added to the upper limit of 0.01 mass%, but if its content exceeds 0.01 mass%, the effect is saturated, so if added, 0.01 mass% or less It is preferable to do.

Sb: 0.0050 mass% or less
Sb is effective for delaying the microstructure change, and has an effect of preventing deterioration of rolling fatigue characteristics, so may be added. However, if the content exceeds 0.0050 mass%, the toughness deteriorates. Therefore, when added, the content is preferably 0.0050 mass% or less.

N: 0.01 mass% or less
N exists as an unavoidable impurity, but forms a nitride (or carbonitride) and is effective in refining γ grains. Since excessive addition deteriorates the workability of steel, when adding, it is preferable that it is 0.01 mass% or less.
[Microscopic organization]
In the steel part for bearing according to the present invention, the average prior austenite grain size in the surface layer after quenching is regulated to 3.5 μm or less. When the average prior austenite grain size is specified to be 3.5 μm or less, fatigue characteristics are improved.

  In order to improve the fatigue life, the hardness of the surface layer portion is more advantageous after quenching, and the hardness of the surface layer portion is preferably Hv700 or more.

[Production conditions]
The steel part for bearing of the present invention is manufactured by processing a steel material, preferably a steel bar or a wire, into a shape of a part such as a bearing inner / outer ring or a bearing ball through a forging process and then quenching. In order to obtain fine prior austenite grains in the surface layer portion after quenching, the microstructure before quenching and quenching conditions are defined.

1 Pre-structure The carbide before quenching shall be spheroidized carbide. When a steel having a spheroidized carbide structure is quenched, the austenite grain growth during heating is suppressed by the pinning effect of the spheroidized carbide. In order to make this effect effective, the spheroidization state of the carbide should have an average aspect ratio of the carbide of 3 or less.

  Although the method for setting the average aspect ratio of the carbide to 3 or less is not particularly specified, it is preferable to perform spheroidizing annealing near the Ac1 transformation point in order to make the aspect ratio and the carbide particle size uniform.

  In addition, even if soft annealing is performed during the forging process as product manufacture or heating for warm forging is performed, if the carbide form before quenching is spheroidized carbide, the pinning effect during quenching process disappears do not do.

  Further, the remaining structure other than carbide may be any of ferrite, bainite, and martensite.

For 2 quenching condition average prior austenite grain size to obtain a hardened surface layer portion is 3.5μm or less, quenching treatment, between Ac ₃ point -10 ° C. to Ac ₃ point and heated at 0.5 ° C. / s or higher, the heating temperature: Ac ₃ points or more, Ac ₃ + 130 ° C. or less, holding time 500 seconds or less.

The heating temperature is set to Ac ₃ points or more in order to obtain a uniform quenched structure after quenching. On the other hand, when the Ac ₃ point is higher than + 130 ° C, even if spheroidized carbide is present, the pinning effect is reduced, austenite grain growth occurs, and the average grain size of the prior austenite grains in the structure after quenching is 3.5 μm. Therefore, Ac ₃ point or higher and Ac ₃ point + 130 ° C or lower.

When holding for more than 500 seconds at Ac ₃ points or more, due to the pinning effect due to the spheroidizing carbide, it will be sufficient time for grain growth, and the prior austenite grain size of the structure after quenching will exceed 3.5 μm, Ac ₃ points or more holding time is 500 seconds or less.

Heating rate, and 0.5 ° C. / s or greater between Ac ₃ point -10 ° C. to Ac ₃ point. When the heating rate in the temperature range is less than 0.5 ° C./s, the austenite grain size becomes coarse due to a decrease in nucleation driving force to austenite, and the prior austenite grain size of the quenched structure is 3.5 μm. Since it becomes super, it shall be 0.5 ℃ / s or more. Heating rate is an average value of between Ac ₃ point -10 ° C. to Ac ₃ point.

  In addition, it is preferable that the quenching process is performed a plurality of times because a finer surface layer portion is obtained. When performing the quenching process a plurality of times, the above-described conditions are applied at least only during the final quenching process (only when the N quenching process is performed).

  The quenching process performed prior to the final quenching process (in the case of performing the N-time quenching process, the quenching process from 1 to N-1 times) is performed so that a structure in which spherical carbides remain after quenching is obtained. If the heating temperature is Acm point or less (the temperature at which the spheroidized carbide dissolves in austenite and becomes an austenite single phase), other conditions may be selected as appropriate, and the conditions are not limited to those in the final quenching process.

  However, it is preferable to perform the quenching process twice in consideration of productivity and cost. As the quenching apparatus, it is preferable to use a high-frequency heating apparatus because the surface layer portion is most heated, but it is only necessary to select an apparatus suitable for the shape of the part, and it is not particularly defined.

In the present invention, a tempering process may be performed after the quenching process. However, when tempering is performed, when the tempering temperature becomes high, the surface layer portion softens, the fatigue strength decreases, and the effect of refining the prior austenite grain size of the quenched surface layer portion is impaired. Is 200 ° C. or lower, and a surface layer of Hv 700 or higher can be obtained.
After the quenching and tempering treatments are performed under the above conditions, a finishing polishing treatment is performed as necessary to obtain a bearing steel part.

  100kg steel ingots with various compositions shown in Table 1 were soaked at 1250 ° C for 15h, then hot forged at 850 ° C or higher to obtain a φ30mm steel bar. The obtained steel bar was spheroidized and annealed (SA) to produce spheroidized carbide with an average aspect ratio of 1.3 to 1.6.

  In Table 1, steel no. 1 to 4, 6 to 8, and 10 to 16 are component compositions within the scope of the present invention. No. 9 has an added amount of W outside the scope of the present invention. No. 17 is outside the scope of the present invention because neither Mo nor W is added.

  In Table 1, the value <in the left of the numerical value means that it is not actively added and is a contamination level as an inevitable impurity.

  Radial rolling fatigue test specimens with a length of φ12mm × 22mm were roughly machined from a quarter of the diameter of these steel bars, and quenched under various heat treatment conditions. Tempering was performed at 170 ° C, and after finishing, it was subjected to microstructure observation, hardness test and fatigue test.

  Table 2 (Nos. 1 and 2) shows the heat treatment conditions. When the induction hardening apparatus is used, the heating rate is as fast as 700 ° C./s or more.

In the evaluation, the surface layer Vickers hardness and the surface layer former austenite grain size were investigated, and the rolling fatigue life was evaluated based on the B ₁₀ life by the radial fatigue test.

  The Vickers hardness of the surface layer portion was measured by averaging the Vickers hardness of 0.1 mm from the surface layer in the longitudinal section (hereinafter referred to as L section) of the radial test piece at a load of 2.94 N (300 gf).

  For the prior austenite grain size of the surface layer part, prior austenite grain corrosion was performed, and a four-view photograph was taken at a magnification of 5000 using an SEM immediately below the L cross-sectional surface layer, and a cutting method was performed.

  In the cutting method, in each visual field, three line segments that are divided into four in the vertical and horizontal directions are drawn, and the number (X) at which these line segments (108 μm per visual field) intersect with the prior austenite grain boundaries is measured. Average old austenite grain size (μm) = 108 / (0.89 × X) ”and then averaged over 4 fields of view.

In the radial fatigue test, 20 tests were performed at Hertz stress of 5884 MPa (600 kgf / mm ² ) and a rotational speed of about 46400 cpm, and the B ₁₀ life was determined.

  Table 2 (Nos. 1 and 2) shows the test results obtained together with the heat treatment conditions. In Table 2, a conventional example that is a standard for comparing fatigue characteristics is Steel No. With the component composition of 17, the quenching conditions were outside the scope of the present invention.

In addition, Test No. About 2-1-16, steel No. without Mo and W addition. No. 17 using No. 17 The ratio 2-17 _{B 10} life, Test No. Nos. 3-1 to 16 are No. The ratio 3-17 _{B 10} life, Test No. Nos. 4-1 to 16 are no. 4-17 _{B 10} the ratio lifetime respectively.

  In the comparative example, the component composition and / or quenching conditions were out of the scope of the present invention. In all the examples, the average aspect ratio of the carbide was 3 or less.

  When the steel component composition and quenching conditions are within the range of the present invention (invention example), the austenite grain size is as fine as 3.5 μm or less, and the fatigue life is superior to that of the conventional example.

Mo on B ₁₀ life, shows the effect of W added.

Claims (4)

C: 0.6-1.5mass%,
Si: 0.1-1.0mass%,
Mn: 0.1 ~ 1.5mass%
Al: 0.1 mass% or less,
Cr: 0.05 to 2.0 mass%
N: 0.01 mass% or less,
Mo: 0.15 to 0.8 mass%, W: 0.15 to 0.8 mass%, one or more remaining Fe and unavoidable impurities,
A steel part for bearings with excellent fatigue characteristics, wherein the average grain size of the prior austenite grains in the surface layer is 3.5 μm or less. Furthermore,
S: 0.03 mass% or less,
Cu: 1.0 mass% or less,
Ni: 1.0 mass% or less,
Ti: 0.01 mass% or less,
Nb: 0.5 mass% or less,
B: 0.01 mass% or less,
The steel part for bearings with excellent fatigue characteristics according to claim 1, comprising one or more of Sb: 0.0050 mass% or less.   The steel part for bearings having excellent fatigue characteristics according to claim 1 or 2, wherein the surface layer has a hardness of Hv700 or more. In the component composition according to claim 1 or 2, the average heating rate of the carbides form in tissues steel aspect ratio is 3 or less of the spheroidized carbides on average, at Ac ₃ point -10 ° C. to Ac ₃ point Bearing steel with excellent fatigue characteristics, characterized by heat treatment at 0.5 ° C / s or higher, Ac ₃ point or higher, Ac ₃ point + 130 ° C or lower, and holding time of Ac ₃ point or higher for 500 seconds or less, followed by quenching A manufacturing method for parts.

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