JP4631618B2 - Manufacturing method of steel parts for bearings with excellent fatigue characteristics - Google Patents

Description

  The present invention relates to a bearing component such as a bearing inner and outer ring, a bearing ball, and the like, and a method for manufacturing the same, and particularly relates to an improved austenite grain size in a surface layer portion and improved fatigue characteristics.

  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, in Patent Document 1, the S53C level hypoeutectoid steel (ferrite, pearlite structure) is subjected to high-frequency heat treatment to the austenite single phase region at least twice, and the old austenite A method to refine the grain size and improve the fatigue life is described. The previous 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 hypereutectoid steel is not described in this document, generally, in the case of hypereutectoid steel, the conventional austenite grains are usually 6 to 10 μm even in the case of quenching, but further optimized heat treatment 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 is based on the fatigue life improvement effect by optimizing the residual carbide density in the pre-quenching structure and the refinement of the γ grain size utilizing the hydrating effect of the residual carbide. It is an object of the present invention to provide a bearing component and a method for manufacturing the same that have improved rolling fatigue life by utilizing the fatigue life improvement effect.

As a result of diligent studies to solve the above problems, the inventors have spheroidized steel in which the carbides in the previous structure are spheroidized, and the carbide density is 1.15 μm ² or more per unit area. the reduction carbide non-vanishing Ac ₃ or more ~Acm temperature below the temperature, after heating at the conditions Ac ₃ point or more residence time is less than 500 seconds and the quenching Ru, than steel conventional spheroidized carbides density of carbides pinning The knowledge that the grain growth suppression effect due to the steel works favorably, the austenite grain size before quenching is refined to an average grain size of 3.5 μm or less, the rolling fatigue life is increased, and the carbide density is optimized to improve the fatigue life Got.

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

1. Component composition is C: 0.6-1.5 mass%, Si: 0.1-1.0 mass%, Mn: 0.1-1.5 mass%, Al: 0.1 mass% or less, Cr: 0.05 -2.0 mass%, having a composition composed of the balance Fe and inevitable impurities, the number of spheroidized carbide per unit area is 1.15 / μm ² To the above steel, Ac ₃ Point-10 ℃ ~ Ac ₃ The average heating rate between the point temperatures is 0.5 ° C./s or more, and Ac ₃ More than point Ac ₃ Ac + ₃ A method for producing a steel component for a bearing having excellent fatigue characteristics, wherein the quenching treatment is performed by performing heating with a retention time not less than a point being 500 seconds or less.

2. The component composition is further S: 0.03 mass% or less, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Mo: 1.0 mass% or less, W: 1.0 mass% or less, Ti: 0.01 mass % Or less, Nb: 0.5 mass% or less, B: 0.01 mass% or less, Sb: 0.0050 mass% or less, and N: 0.01 mass% or less. 2. A method for producing a steel part for a bearing excellent in fatigue characteristics as described in 1.

   According to the present invention, the surface layer portion has a fine structure in which the average grain size of the prior austenite grains is 3.5 μm or less, and a steel component for bearings having excellent rolling fatigue life characteristics can be easily obtained. Useful.

   The present invention will be specifically described below.

[Microscopic organization]
In the steel part for bearing according to the present invention, the number per unit area of spheroidized carbide in the pre-structure before quenching is 1.15 / μm ² or more, and the average grain size of prior austenite grains in the surface-hardened structure is 3.5 μm or less. To do.

FIG. 1 shows the effect of spherical carbide density on the prior austenite grain size of 1.0 mass% C steel. When the spherical carbide density increases, the prior austenite grain size after quenching rapidly becomes finer, the spherical carbide density is 1.15 μm ² or more, and the average grain size is 3.5 μm or less, which is substantially constant.

Figure 2 shows the effect of globular carbides density on the relationship of the old γ grain size and B ₁₀ fatigue life. Globular carbides density 1.20μm ^two materials, the average particle size of prior austenite grains is 3.5μm or less, the life ratio becomes 2 or more, thereby improving the B ₁₀ fatigue life.

On the other hand, the material with a spherical carbide density of 1.01 μm ² has a life ratio of 1.5 or less even when the prior austenite grains change, the B ₁₀ fatigue life hardly changes, and the fatigue life is inferior to that of the material with a spherical carbide density of 1.20 μm ^2. . In the present invention, the spheroidized carbide refers to a carbide having an aspect ratio of 3.0 or less.

Thus, when the carbide density is 1.15 μm ² or more in the previous structure before quenching, the prior γ grains become finer than the steel having a carbide density of less than 1.15 μm ² , and the average grain diameter of the prior austenite grains is 3.5. At μm or less, the fatigue life is significantly improved compared to steels with a carbide density of less than 1.15 μm ² .

Further, the increase in the carbide surface area of the spheroidized carbide promotes the penetration of the carbide into the matrix during quenching, and the rolling fatigue life of the steel component for bearing is greatly improved.
Adjustment of the spheroidizing carbide density is preferably any of spheroidizing annealing, low temperature rolling + sphering annealing, and low temperature processing, but is not particularly defined in the present invention.

1 Spheroidizing annealing Spheroidizing annealing involves melting layered carbides in pearlite by heating directly above the Ae ₃ point to form seeds of spheroidizing carbides, and then gradually cooling to form spheroidizing carbides. Ae Spherical carbide density is set to 1.15 / μm ² or more by setting the heating conditions just above ₃ points to low temperature and soaking for a short time. Specifically, the soaking is performed at 720 ° C. ± 10 ° C. for 1 to 4 hours.

2 Low temperature rolling + spheroidizing annealing In the material rolling process, at least in the finishing process, rolling is performed at a temperature below the pearlite transformation finish temperature and the area reduction rate is 10% or more, and carbides in the pearlite structure are divided. Thereafter, spheroidizing annealing is performed, and the spheroidizing carbide density is set to 1.15 / μm ² or more.

3 Low-temperature processing In the material rolling process, rolling with a reduction in area of 30% or more is performed in a warm region below the pearlite transformation finish temperature to break up carbides in the pearlite structure to a carbide density of 1.15 pieces / μm ² or more. There is no problem even if the rolling is changed to forging of the product.

When manufacturing a product from a material with a spheroidized carbide density of 1.15 pieces / μm ² or more, softening annealing is performed during the forging process or heating for warm forging is performed before quenching. However, if the carbide form before quenching is spheroidized carbide and the density is 1.15 / μm ² or more, there is no problem because the pinning effect during quenching does not disappear.

The matrix structure on which the carbide precipitates may be any of ferrite, bainite, and martensite.
In the steel part for bearing according to the present invention, the structure other than the surface layer is not particularly specified after quenching. Even if the average grain size of the prior austenite grains is 3.5 μm or less in the whole thickness direction, not limited to the surface layer, the effect is not impaired.

  In order to improve the fatigue life, it is advantageous that the hardness of the surface layer portion is harder after quenching, and the hardness of the surface layer portion is preferably Hv 700 or more.

The component composition suitable for the steel component for bearing according to the present invention will be described.
[Ingredient composition]
C: 0.6% ~ 1.5mass%
C is an element necessary for securing the hardness necessary for obtaining the fatigue life of the part 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, when it exceeds 1.5 mass%, the workability (shearability and forgeability) before quenching is deteriorated. Therefore, the preferred C content range is preferably 0.65% to 1.5 mass%.

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

Mn: 0.1 ~ 1.5mass%
In order to improve hardenability, Mn is contained more than 0.1 mass%. However, adding excessively degrades the workability (shearability and forgeability) before quenching. For this reason, the upper limit of the content is preferably 1.5 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 promoting the spheroidization of carbide. 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%.

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 preferred basic component composition of the present invention, with the balance being 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 unavoidable impurities, S is allowed up to 0.01% and N is allowed up to 0.0080%, but may be positively added. In order to improve desired characteristics, one or more of S, Cu, Ni, Mo, W, 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.

Mo: 1.0 mass% or less
Mo may be added because it has an effect of improving hardenability and resistance to temper softening. However, since workability deteriorates, it is preferable to add 1.0 mass% or less.

W: 1.0 mass% or less
W may be added because it has an effect of improving hardenability. However, since workability is deteriorated, W is preferably set to 1.0 mass% or less.

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.

[Quenching condition]
The steel part for bearings according to the present invention is a steel material having a pre-quenching structure, preferably a bar steel or a wire rod, for which the carbide form is spheroidized carbide and the number per unit area is 1.15 / μm ² or more. After processing into the shape of bearing steel parts such as bearing inner and outer rings and bearing balls, surface layer quenching is performed. The conditions for obtaining the surface layer part in which the average grain size of the prior austenite grains is 3.5 μm or less will be described.

Hardening 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 point or more, Ac ₃ + 130 ° C. or less, performed as follows retention time 500 seconds.
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 exceeds 130 ° C, the pinning effect is reduced even when spheroidized carbide is present, austenite grain growth occurs, and the average grain size of the prior austenite grains in the structure after quenching is 3.5. Since it exceeds μm, set 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 by the spheroidized carbide, it becomes a sufficient time for grain growth, and the prior austenite grain size of the structure after quenching is over 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 the effect of reduction of nucleation driving force to austenite, and the prior austenite grain size of the structure after quenching 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. In the case of performing the quenching process a plurality of times, the above-described conditions are applied only at least 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), the other conditions may be appropriately selected and are not limited to the conditions of the final quenching step.

  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 part 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 is softened, the fatigue strength is reduced, 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-described conditions, a finishing polishing treatment is performed as necessary to obtain a bearing steel part.

   100 kg steel ingots with various compositions shown in Table 1 were soaked at 1250 ° C. for 15 hours, and then hot forged at 850 ° C. or higher to obtain a φ20 mm bar steel. After reheating the obtained steel bar to 1000 ° C for 30 minutes, it was hot-rolled to φ13mm at various temperatures from 800 ° C to 600 ° C, and then averaged with ferrite by spheroidizing annealing (SA) at 770 ° C x 1 hour. The aspect ratio was 1.3 to 1.5, and the spheroidized carbide density was varied.

  A radial type rolling fatigue test piece having a diameter of φ12 mm × 22 mm was roughly machined from the central part of the diameter of these steel bars and subjected to quenching treatment under various heat treatment conditions. Tempering was performed at 170 ° C., and after finishing, the samples were subjected to a structure observation, a hardness test, and a fatigue test. Table 2 shows the heat treatment conditions. When an induction hardening apparatus is used, the heating rate is as fast as 700 ° C / s or more.

In the evaluation, the surface Vickers hardness and the former austenite grain size of the surface layer were investigated, and the rolling fatigue characteristics were evaluated by the B ₁₀ life by the radial fatigue test. The Vickers hardness of the surface layer portion was measured by averaging the Vickers hardness within 0.1 mm from the surface layer in a 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 a Hertzian 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 to 3) shows the test results obtained together with the heat treatment conditions. In Table 2, the spherical carbide density in the previous structure before quenching is less than 1.15 / μm ² and the quenching condition is outside the scope of the present invention, and the conventional example is the spherical carbide density in the previous structure before quenching. A comparative example having a quenching condition outside the range of the present invention was used.

When the spherical carbide density and the quenching condition in the pre-structure before quenching are within the range of the present invention (invention example), the average prior austenite grain size is fine, and the fatigue life is excellent as compared with the conventional example. Even when the quenching conditions are within the range of the present invention, the fatigue characteristics were inferior to those of the examples of the present invention when the spherical carbide density in the previous structure before quenching was outside the range of the present invention. Incidentally, when compared with the same particle size, globular carbides density was superior to higher B ₁₀ life.

The figure which shows the influence of the spherical carbide density which acts on a prior austenite particle size. It shows the effect of globular carbides density on the relationship of the former austenite grain size and the B ₁₀ fatigue life.

Claims (2)

Component composition is C: 0.6-1.5 mass%, Si: 0.1-1.0 mass%, Mn: 0.1-1.5 mass%, Al: 0.1 mass% or less, Cr: 0.05 -2.0 mass%, having a composition composed of the balance Fe and inevitable impurities, the number of spheroidized carbide per unit area is 1.15 / μm ² To the above steel, Ac ₃ Point-10 ℃ ~ Ac ₃ The average heating rate between the point temperatures is 0.5 ° C./s or more, and Ac ₃ More than point Ac ₃ Ac + ₃ A method for producing a steel component for a bearing having excellent fatigue characteristics, wherein the quenching treatment is performed by performing heating with a retention time not less than a point being 500 seconds or less. The component composition is further S: 0.03 mass% or less, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Mo: 1.0 mass% or less, W: 1.0 mass% or less, Ti: 0.01 mass % Or less, Nb: 0.5 mass% or less, B: 0.01 mass% or less, Sb: 0.0050 mass% or less, and N: 0.01 mass% or less. The method for manufacturing a steel part for bearings having excellent fatigue characteristics according to claim 1.

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