JP2011208201A - Method for manufacturing bearing parts, and bearing parts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing bearing parts, which can form a large amount of fine carbides in the surface region of the bearing parts by carburizing and quenching treatment, and thereby can obtain the bearing parts having the surface with high hardness and high strength.SOLUTION: A steel for the bearing parts has a composition including, by mass%, 0.15-0.25% C, 0.90-1.30% Si, 0.70-1.10% Mn, 0.030% or less P, 0.100% or less S, 0.01-0.50% Cu, 0.01-0.50% Ni, 0.20-0.50% Cr, 0.50% or less Mo, 0.30% or less Al, 0.05% or less N, so as to satisfy expression (1): [Si]+[Ni]+[Cu]-[Cr]>0.5, and the balance Fe with unavoidable impurities. The method for obtaining the bearing parts in which the fine carbides are formed so that carbides with the size of 1 μm or less occupy 95% or more includes: subjecting the steel to vacuum carburization treatment so that the carbon concentration in the surface enters into the range of more than 1.1 to 1.5%; then, air-cooling the steel to make the structure of the surface layer pearlite; and subsequently subjecting the resultant steel to induction hardening treatment to divide cementite.

Description

  The present invention relates to a bearing part manufacturing method and a bearing part.

Conventionally, gears as machine parts are required to have high surface hardness, while the core part is required to have a predetermined toughness. Therefore, such gears have a low C content of about 0.2%. An amount of material, typically a JIS steel grade such as SCR420, is processed into a part shape, and then carburized and hardened and surface-hardened.
Conventionally, gas carburizing is mainly used as the carburizing method. In the case of this gas carburization, the hardness shows the maximum value at a surface carbon concentration of about 0.8%, and it is difficult to carburize at a higher concentration.

  On the other hand, in bearing components such as bearing balls, a large load is applied until reaching the core, and the hardness needs to be harder than that of gear components. Rather than hardening the surface hardness, a material containing a large amount of C, typically SUJ2, such as SUJ2 with a C content of about 1%, is processed into a part shape and then used after quenching and tempering. It is a fact.

  By the way, when it is desired to make the hardness of bearing parts such as bearing balls and rollers harder than that using SUJ2, it is possible to construct the bearing parts using a material having a higher C content than SUJ2. Originally, SUJ2 itself is a material that is hard, and if the C content is set to a higher content, for example, about 1.3%, the material hardness becomes too hard and the processing becomes difficult.

  For example, in the case of a bearing ball, high sphericity is required for a smooth bearing function, and thus high precision and precise processing is required. However, if the material is too hard, such processing becomes difficult. The processing cost is also extremely high.

Therefore, conventionally, when it is desired to impart a further hardness to the bearing component, a surface hardening treatment by nitriding or carbonitriding is performed as a means for that purpose.
For example, the following Patent Document 1 and Patent Document 2 disclose that hardness is increased by performing such nitriding treatment.
That is, these Patent Documents 1 and 2 show that an induction hardening process is performed after the carbonitriding process as a manufacturing method of the raceway member.

However, in the case of bearing parts subjected to surface hardening treatment by nitriding as described above, when martensite transformation is performed by quenching, a large amount of austenite that cannot be fully martensitic, that is, retained austenite, is generated.
This retained austenite gradually becomes martensite as the bearing parts are used, and the bearing parts are deformed due to distortion in order to cause volume expansion, and in the case of bearing balls, the sphericity is impaired. Cause problems.

  By the way, as a carburizing technique, in addition to the above-mentioned gas carburizing, vacuum carburizing capable of higher concentration carburizing is known. For example, bearing parts using the above-mentioned SCR420 material are subjected to vacuum carburizing treatment to increase the surface. It is conceivable that C is introduced at a concentration and quenching is performed, and induction quenching is used as quenching.

When carburizing and quenching is performed by the usual method of quenching directly from quenching following carburizing treatment, there is a problem that the residual distortion becomes large because the inside of the part is martensitic, but induction hardening is performed after carburizing treatment. In this case, induction hardening has an advantage that distortion is small because only the surface can be hardened without changing the internal structure of the part.
In the case of this vacuum carburization, the carburization thickness (carburization depth) can be increased as compared with gas carburization, that is, the surface hardened layer can be increased, and it can be used as a bearing component.

  However, since the SCR420 material contains a large amount of Cr that easily forms carbides, when high concentration of C is introduced into the surface by vacuum carburizing, coarse Cr carbides are easily generated at the carburizing stage, and the subsequent high frequency In the quenching process, the Cr carbide does not dissolve and remains after quenching, and the Cr carbide tends to cause breakage, and the strength is reduced.

In addition, there exist some which were disclosed by the following patent document 3, patent document 4, and patent document 5 as another prior art with respect to this invention.
However, those disclosed in these patent documents are different from the present invention in the composition of steels having different contents such as Si, Cu, Ni, etc., and the method of generating carbides is also different from the present invention. Are different.

  Patent Document 6 below discloses a method for producing a surface-hardened component and an invention about the surface-hardened component, where it is disclosed that annealing is performed after vacuum carburization and then induction hardening is performed. The disclosure is different from the present invention in that the content of Si, which is a characteristic component of the steel of the present invention, is substantially different from the present invention (less than the present invention).

  Furthermore, the following Patent Document 7 discloses an invention about a carburizing and quenching method and a carburized and quenched power transmission member, in which slow cooling is performed after vacuum carburizing, and then induction hardening is disclosed. Even in the disclosed one, the Si content of the steel is small, and the steel composition is different from the present invention.

  Patent Document 8 below discloses an invention for carburized parts, but the one disclosed in Patent Document 8 is completely different from the present invention in the quenching method following carburizing treatment, and generates carbide on the surface of the parts. This method is also different from the present invention, unlike the present invention.

  Further, the following patent document 9 discloses an invention relating to a machine part. However, what is disclosed in this patent document 9 is mainly intended for gears, and there is no mention regarding application to bearing parts. And something different.

JP 2008-19482 A JP 2008-63603 A WO 2006/118242 WO 2006/118243 WO2007 / 034911 JP 2005-48270 A JP 7-316640 A JP 2007-291486 A JP 2008-76850 A

  In view of the above circumstances, the present invention provides a bearing component manufacturing method and bearing component capable of producing a large amount of fine carbides on the surface by carburizing and quenching treatment and obtaining a bearing component having high hardness and high strength. It was made for the purpose of providing.

Thus, claim 1 relates to a method of manufacturing a bearing component, and in mass%, C: 0.15 to 0.25%, Si: 0.90 to 1.30%, Mn: 0.70 to 1.10%, P: 0.030% or less, S: 0.100% Hereinafter, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Cr: 0.20 to 0.50%, Mo: 0.50% or less, Al: 0.30% or less, N: 0.05% or less, and the condition of the following formula (1) Meet,
[Si] + [Ni] + [Cu]-[Cr]> 0.5 (1)
(However, each element symbol in the formula (1) represents mass%)
Vacuum carburization of steel having the composition of the balance Fe and inevitable impurities so that the surface carbon concentration after slow cooling after carburizing is within the range of more than 1.1 to 1.5% under a reduced pressure condition of 2 kPa or less. After the treatment, the above-mentioned slow cooling by air cooling is performed at a cooling rate that causes pearlite transformation to make the surface structure pearlite, and then the cementite in the pearlite structure is finely divided to the range from the surface to 0.1 mm. It is characterized in that induction hardening is performed under heating and cooling conditions in which fine carbides in which the number of carbides of 1 μm or less in the carbides accounts for 95% or more are generated.

  According to a second aspect of the present invention, in the first aspect, the slow cooling by the air cooling is performed at a cooling rate of 5 ° C./s to 0.2 ° C./s, and the induction hardening is performed at a heating temperature of 750 to 850 ° C. Features.

  A third aspect of the present invention is the method according to any one of the first and second aspects, wherein the steel is one or more of Nb: 0.02 to 0.20%, Ti: 0.02 to 0.20%, B: 0.0005 to 0.0100% in mass%. Is further contained.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the vacuum carburization is performed such that a surface carbon concentration after the slow cooling is 1.2% or more.

  Claim 5 relates to a bearing component, and has a surface carbon concentration of more than 1.1 to 1.5%, and carbides of 1 μm or less generated by dividing cementite of pearlite structure within a range from the surface to 0.1 mm. It is characterized in that fine carbides occupying 95% or more of the carbides by number are formed.

A sixth aspect of the present invention is the same as in the fifth aspect, wherein C: 0.15 to 0.25%, Si: 0.90 to 1.30%, Mn: 0.70 to 1.10%, P: 0.030% or less, S: 0.100% or less, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Cr: 0.20 to 0.50%, Mo: 0.50% or less, Al: 0.30% or less, N: 0.05% or less and satisfying the condition of the following formula (1),
[Si] + [Ni] + [Cu]-[Cr]> 0.5 (1)
(However, each element symbol in the formula (1) represents mass%)
It has a composition of the remaining Fe and inevitable impurities, and the fine carbide is generated in a range from the surface to 0.1 mm by vacuum carburization, subsequent cooling, and induction hardening.

  A seventh aspect of the present invention is characterized in that, in any one of the fifth and sixth aspects, the carbide is formed in a structure in which a martensite structure occupies 50% or more.

  An eighth aspect of the present invention is the method according to any one of the fifth to seventh aspects, wherein the steel is in mass%, Nb: 0.02 to 0.20%, Ti: 0.02 to 0.20%, B: 0.0005 to 0.0100%. It further contains more than seeds.

  According to a ninth aspect of the present invention, in any one of the fifth to eighth aspects, the surface carbon concentration is 1.2% or more.

Effects and effects of the invention

  As described above, the method for manufacturing a bearing component according to the present invention includes a low-C steel material processed into a bearing component shape in advance, then vacuum carburized and then gradually cooled, and then induction-hardened to harden the surface. To obtain bearing parts.

  In the present invention, C is introduced to the surface of the bearing component so that the surface carbon concentration after slow cooling becomes more than 1.1 to 1.5% by vacuum carburizing treatment, and then the surface texture is changed to pearlite by slow cooling. Assume phase organization.

Subsequent to induction hardening, the thin and long cementite in the pearlite structure is finely divided and then cooled to cause martensite transformation and quenching. As a result, innumerable fine carbides are generated in a dispersed state on the surface (surface layer).
As a result, the carbide on the surface of the bearing part is made as fine as possible and a large amount of carbide is generated, so that the surface hardness of the bearing part is higher than before and the surface strength is further increased.

That is, the manufacturing method of the present invention is characterized in that carbide on the surface of the bearing component is finely and finely precipitated by dividing cementite in the pearlite structure.
In the present invention, a vacuum carburizing technique is used as a carburizing process.
In the case of normal gas carburization, the surface carbon concentration is limited to about 0.8%, but in the present invention, the surface carbon concentration can be made higher by using vacuum carburization.
Here, vacuum carburizing is generally performed by reducing the atmosphere in the furnace and inserting a hydrocarbon-based gas (such as methane, propane, ethylene, acetylene, etc.) directly into the furnace as the carburizing gas. Active carbon that decomposes in contact with the surface of the steel generates carbon during the carburizing period when carbon is supplied to the surface of the steel, stores the carbon, and decomposes and stores the carbon during the subsequent diffusion period. This is a method in which carbon dissolves in the matrix and carbon diffuses inward and carburizes. The supply route of carbon is not limited to the route via the carbide, and there is also a route that passes through the route of direct dissolution. In this invention, it carries out in the pressure reduction state of 2 kPa or less.
In such a case, a large amount of fine carbides can be generated on the surface of the bearing component, which greatly contributes to increasing the hardness and strength of the surface of the bearing component.

However, when steel grades such as the conventional SCR420 are used as steel materials, since a large amount of Cr that can easily form carbides is contained, if a large amount of C is introduced into the surface by vacuum carburization, Cr will be converted into carbides at the carburizing stage. Will form.
In this case, the carbide becomes coarse carbide. Such coarse carbides become foreign matter on the surface of the bearing component, and become the starting point of destruction, and on the contrary, lower the surface strength.

  Therefore, in the present invention, the composition of the steel material is reduced in Cr, which is easy to form carbide, contains a large amount of Si which is difficult to form carbide, does not form carbide during vacuum carburization, and C is obtained by vacuum carburization. Even when it is introduced into the surface at a high concentration, the composition of the steel material is adjusted so that a pearlite single phase structure is obtained by subsequent slow cooling.

In the present invention, a pearlite single phase structure is formed by slow cooling after carburizing C at a high concentration on the surface by vacuum carburization, and the cooling rate after carburizing is 5 ° C./s to 0.2 ° C./s. It is desirable to be within the range.
If the cooling rate is slower than 0.2 ° C./s, coarse carbides are generated at the grain boundaries, and the surface hardness, strength increase, and strength enhancement, which are the objects of the present invention, cannot be realized sufficiently.
On the other hand, if the cooling rate is higher than 5 ° C./s, the cooling after carburizing causes baking, and the structure becomes martensite.

After performing the carburizing treatment as described above, in the present invention, the surface of the bearing component is induction-hardened.
By heating for a short time with this high frequency (heating time is about 20 to 30 seconds), cementite in the pearlite structure is finely divided in the longitudinal direction to refine the carbide.

In this induction hardening, it is desirable that the surface of the bearing component is within a heating temperature range of 750 to 850 ° C.
When the temperature is higher than 850 ° C., not only the cementite is divided, but also carbides generated by the division are dissolved in the matrix, and the object of the present invention cannot be sufficiently achieved.
On the other hand, if the temperature is lower than 750 ° C., the structure does not become austenite by high-frequency heating, so that sufficient cooling does not occur by subsequent cooling. That is, the structure does not become martensite well.

That is, in the present invention, it is preferable to perform heating at a low temperature of 750 to 850 ° C. while heating at a high temperature of 1150 ° C. or higher in normal induction hardening.
In addition, it is desirable that the cooling during induction hardening is 10 ° C./s or more, preferably water cooling.

  In the production method of the present invention as described above, as indicated by the height of the surface carbon concentration in the table of the later examples (the height of the surface carbon concentration in the examples is the same as the amount of carbide as it is. It is possible to generate a large amount of carbide on the surface of the bearing part, and the hardness (surface hardness) and strength of the bearing part can be increased to high hardness and strength, and nitriding is applied to SUJ2 containing about 1% of C It is possible to obtain a bearing component having the same high hardness and high strength as those subjected to.

Moreover, the bearing part obtained by the manufacturing method of the present invention does not cause a problem that a large amount of retained austenite remains after quenching, unlike the bearing part whose hardness and strength are increased by the conventional nitriding treatment. As a result, the problem that the retained austenite becomes martensite and the volume expansion at that time causes distortion and deformation due to the bearing component can be solved, and the amount of change over time can be reduced.
According to the present invention, a bearing component having a thick hardened surface layer can be obtained, and the thick carburized layer, that is, the hardened surface layer, can increase the overall strength required for the bearing component.

  In the present invention, since the steel material is soft with low C, it is not necessary to perform spheroidizing annealing prior to processing the steel material into a bearing part. Therefore, round and large carbides are not generated by the spheroidizing annealing.

In the present invention, the steel may contain one or more of Nb, Ti, and B in the above range.
In the vacuum carburization, carburization can be performed so that the surface carbon concentration after slow cooling is 1.2% or more.

Claims 5 to 9 relate to a bearing component, and this bearing component can make the surface hardness and strength high hardness and high strength by generating a large amount of fine carbides on the surface, A bearing component having the same high hardness and strength as those obtained by nitriding SUJ2 containing about 1% of C can be obtained.
In addition, due to residual austenite, the problem of distortion and deformation caused by volume expansion due to transformation from residual austenite to martensite can be eliminated, and the bearing component can be reduced in size over time. .

Next, the reasons for limiting each chemical component of the steel material in the present invention will be described in detail below.
C: 0.15-0.25%
If the C content is less than the lower limit, ferrite is generated in the core and the strength is lowered. On the other hand, when the upper limit of 0.25% is exceeded, workability, particularly machinability, deteriorates.
Further, if the C content exceeds a certain amount, the steel becomes hard, and a spheroidizing annealing treatment is required in some cases when processing into bearing parts. In that case, round and large carbides are generated during the spheroidizing annealing treatment.

Si: 0.90 to 1.30%
In the present invention, Si is an important component for making the structure pearlite by cooling after vacuum carburization. If the Si content is less than 0.90%, carbides are likely to occur during air cooling after vacuum carburization. In addition, the hardenability is reduced and the strength is reduced.
On the other hand, if the Si content exceeds 1.30%, workability, particularly machinability, deteriorates.
Si is an element that promotes grain boundary oxidation in the case of ordinary gas carburization, and this grain boundary oxide layer causes a reduction in impact strength and fatigue strength. However, in the present invention, by using vacuum carburization (atmospheric pressure is, for example, 2 kPa or less), the problem of grain boundary oxidation is effectively suppressed even though Si is contained.
In the present invention, the more desirable range of Si is 1.0 to 1.3%.

Mn: 0.70 to 1.10%
Mn is added as a deoxidizing material when steel is melted, but if its content exceeds 1.10% and the amount increases, workability, particularly machinability deteriorates.
On the other hand, if the content is less than 0.70%, ferrite is generated in the core part, resulting in a decrease in strength.

P: 0.030% or less
S: 0.100% or less These are impurities and are undesirable components for the mechanical properties of the bearing parts, and their contents are regulated to the upper limit value or less.
Particularly when toughness and hot workability are required, a more desirable range of S is 0.03% or less.

Cu: 0.01 to 0.50%
Ni: 0.01-0.50%
Cu and Ni are components that suppress the formation of carbides, and are each contained in a lower limit of 0.01% or more.
On the other hand, a large amount of addition exceeding 0.50% reduces hot workability.
A more desirable range of Cu is 0.05 to 0.3%, and a more desirable range of Ni is 0.04 to 0.3%. Cu and Ni can improve the core strength by containing 0.05% and 0.04% or more, respectively.

Cr: 0.20 to 0.50%
Cr is a component that promotes the formation of carbides, and if it is added in a large amount exceeding 0.50%, Cr forms carbides during post-carburization treatment. Moreover, the pearlite single phase structure which is the object of the present invention cannot be obtained. Furthermore, a large amount of addition deteriorates workability, especially machinability.
On the other hand, if it is less than 0.20%, the hardenability is lowered and the strength is lowered.
A more desirable range of Cr is 0.2 to 0.4%. Furthermore, the core part intensity | strength can be improved by setting it as 0.25% or more.

Mo: 0.50% or less
Mo is a component that improves hardenability. However, if it is added in a large amount exceeding 0.50%, the workability of the steel, particularly the machinability, deteriorates.
If the addition amount of Mo is less than 0.01%, the martensite transformation is not sufficient at the time of quenching, and the strength is lowered due to incomplete quenching. Therefore, it is desirable to add 0.01% or more.
A more desirable range of Mo is 0.05 to 0.4%. Furthermore, the production | generation of a carbide | carbonized_material can be suppressed by setting it as 0.3% or less.

Al: 0.30% or less
Al is added as a deoxidizer. It also has the function of making the crystal grains finer and improving the strength.
However, if it is contained in a large amount exceeding 0.30%, alumina is formed in the steel and the strength is reduced.
In order to secure the function of refining the crystal grains and improving the strength, it is desirable to add 0.01% or more of Al.
A more desirable range of Al is 0.01 to 0.04%.

N: 0.05% or less
N has an effect of preventing coarsening of crystal grains. Since this effect is saturated at about 0.05%, it should be less than that.
The N content is preferably 0.002% or more. In order to further reduce the content, the cost becomes high.
A more desirable range of N is 0.01 to 0.03%.

[Si] + [Ni] + [Cu]-[Cr]> 0.5 (1)
Si, Ni and Cu suppress the formation of carbides, while Cr increases.
In the present invention, by balancing the addition amounts of Si, Ni, Cu and Cr, even when high-concentration carburization is performed by vacuum carburization, a pearlite structure single phase can be generated by subsequent cooling.

Surface carbon concentration: more than 1.1 to 1.5% (mass%)
If the carbon concentration is 1.1% or less, the surface of the bearing part cannot be effectively strengthened. On the other hand, even if the surface carbon concentration exceeds 1.5%, the strength is not improved further. On the other hand, graphite adheres during carburizing and contaminates the carburizing furnace, so 1.5% was made the upper limit.

Cooling rate after vacuum carburization: 5 ° C / s to 0.2 ° C / s
When the cooling rate is faster than the upper limit value, it becomes organized martensite and does not become pearlite. On the other hand, if the cooling rate is further slower than the lower limit value, coarse carbides are precipitated at the grain boundaries.
A more desirable range of the cooling rate is 4 ° C./s to 0.4 ° C./s.

Induction hardening temperature: 750-850 ° C
When the quenching temperature is lower than the lower limit value, austenite is not formed, and quenching becomes poor and the strength is lowered.
On the other hand, when the temperature is higher than the upper limit value, the carbide is excessively dissolved and the carbide does not remain on the surface of the carburized component.

It is explanatory drawing of the method of the thrust test performed in the Example. 2 is a scanning electron micrograph of Example 1. FIG. It is a figure showing the relationship between the carburizing depth and the hardness in carburizing by vacuum carburizing and carburizing by gas carburizing.

Next, embodiments of the present invention will be described in detail below.
Steels having chemical compositions shown in Table 1 were melted, and each material was machined into a test piece shape for the following thrust test.
This part was vacuum carburized under the temperature conditions of 900 to 1050 ° C., specifically the temperature conditions shown in Table 2, and then cooled at a cooling rate of 5 ° C./s to 0.2 ° C./s, specifically, the cooling rate shown in Table 2. Furthermore, after that, it cooled on the heating conditions shown in Table 2, and the cooling conditions of 10 degree-C / s or more, and induction hardening was implemented.
And about the obtained test piece, the thrust test was done on condition of the following, and fatigue strength was evaluated.
In addition, surface carbon concentration, surface hardness, carburization depth, the ratio of carbides of 1 μm or less, the amount of retained austenite, and the amount of deformation were measured under the following conditions.
The results are also shown in Table 2.

<Vacuum carburizing>
Using acetylene gas, vacuum carburization was performed at 900 to 1050 ° C. under a reduced pressure of 1/100 atm or less.
After carburizing, air cooling was performed to make the structure a pearlite single phase.

<Induction hardening>
Heating was performed at a heating temperature of 750 to 850 ° C. for 20 to 30 seconds, and then cooling (specifically, water cooling) was performed at a cooling rate of 10 ° C./s or more.

<Measurement of surface carbon concentration>
The surface of the test piece for measuring the carbon concentration was directly measured by a spark discharge emission spectrometry (JIS G 1253).

<Carbide measurement>
The surface layer of the test piece for carbide measurement after induction hardening is mirror-polished, corroded with a picric acid alcohol solution, and observed with a SEM (scanning electron microscope). Observation was performed at 1 mm for ² minutes, and a ratio (number ratio) of carbides having a cross-sectional size of 1 μm or less to all carbides was determined.

<Surface hardness measurement>
According to JIS Z 2244, the surface layer part of the test piece after induction hardening was mirror-polished, and a value obtained by measuring a position 0.05 mm from the surface with a load of 2.94 N was used.

<Carburization depth measurement>
In accordance with JIS G 0557, the effective hardened layer depth was measured based on Vickers hardness of 550 HV.

<Measurement of residual γ amount>
The amount of residual γ was measured using an X-ray diffraction method.
Specifically, the sample was quantified by applying X-rays generated by a Cu target to a sample through a Zr filter.
As the diffraction plane, γ-phase (220) (311) and α-phase (211) were used.

<Size change>
A φ10 mm × 100 mm part was prepared, and after the induction hardening, tempering at 300 ° C. for 3 hours (corresponding to use for 1000 hours) was performed to measure the diameter change, and the amount of change was evaluated.

<Thrust test>
As shown in FIG. 1, three spherical rolling elements 14 are formed on a disk-shaped test piece 12 having a hole 10 having a diameter of 28.3 mm and a thickness of 8.8 mm and an outer diameter of 63 mm. In this state, the rolling element 14 is pressed downward in the drawing so that the contact stress between the test piece 12 and the rolling element 14 becomes the following stress, and the pressing jig 16 is rotated at a high speed in this state, test piece 12 to determine the lifetime until broken by wear and surface of curling or the like, and 10% of the total was evaluated for fatigue strength at a load number L ₁₀ to break.
Specific conditions are as follows.
-Specimen: Lapping process (mirror surface process) after vacuum carburizing induction hardening of a disk-shaped test specimen of φ63 (outer diameter)-28.3 (hole diameter) x 8.8 (thickness) mm
・ Rolling elements: 3/8 inch SUJ2 balls 3 pieces ・ Contact stress: Pmax = 5.5 GPa
・ Load rotation speed 1800rpm
・ Lubrication: Turbine oil # 68 oil tank lubrication ・ Temperature: normal temperature ・ Evaluation: L ₁₀ (10% destruction load)

As can be seen from the results in Table 2, in Comparative Example 1 where Si is as low as 0.71% and the value of formula (1) is 0.28, which is lower than the lower limit of the present invention, the proportion of carbides of 1 μm or less is 8%. Low, resulting in low fatigue strength.
In Comparative Example 2, the Cr content is 0.80%, which is higher than the upper limit of 0.50% of the present invention, and the value of formula (1) is 0.30, which is lower than the lower limit of 0.5 of the present invention. (1 μm or less) The proportion of carbide is as low as 61%, and the fatigue strength is low.

In Comparative Example 3 where the surface carbon concentration is as low as 0.70 and no carbide is formed on the surface, and in Comparative Example 4 where the surface carbon concentration is as low as 0.85 and the amount of fine carbide on the surface is as low as 28%, fatigue is observed. Low strength.
Further, in Comparative Example 5 where the heating temperature during induction hardening is low at 710 ° C. and the amount of fine carbide on the surface is as low as 42%, the fatigue strength is low.
Furthermore, the heating temperature of induction hardening is as high as 910 ° C., and the fatigue strength is low in Comparative Example 6 where no carbide is generated on the surface.

  In Comparative Example 7, a test piece was prepared using JIS steel type SUJ2, and this was subjected to quenching and tempering (tempering) treatment. In Comparative Example 7, the surface hardness was lower than that of the example of the present invention. Furthermore, both the amount of change and the fatigue strength are inferior to those of the examples of the present invention.

  In Comparative Example 8, instead of the tempering treatment of Comparative Example 7, gas carbonitriding was performed and induction hardening was performed, and N was 0.21% on the surface due to the nitriding treatment (filled in the surface carbon concentration column in parentheses) ) Is contained. As a result, the surface hardness of the comparative example 8 is sufficient, but the amount of retained austenite is large, and the amount of change is large due to this.

On the other hand, in each of the examples of the present invention, a large amount of fine carbides are formed on the surface, and the surface hardness and fatigue strength are high, and the amount of change is kept small.
In each example, the surface hardness was obtained at the same level as that obtained by subjecting SUJ2 of Comparative Example 8 to nitriding treatment.
Incidentally, FIG. 2 (a) shows an electron micrograph after vacuum carburization and slow cooling for Example 1, and (b) shows an electron micrograph after induction hardening (both magnifications are 10,000 times). .

  From Fig. 2 (a), the structure after vacuum carburization and slow cooling is a pearlite single-phase structure well, and after (b) induction hardening, fine carbides due to cementite fragmentation are fine. It can be seen that a large number of the particles are dispersed (the granular white dots in FIG. 2B are carbides).

Table 2 shows the carburization depth of the examples formed by vacuum carburization. As shown in the table, a deep carburized layer is formed in this vacuum carburization as compared with the case of gas carburization.
In the case of vacuum carburization, the carburization rate is faster than that of gas carburization, and the carburized layer becomes deeper when carburized for the same time.
Here, the carburization depth refers to a depth at which the Vickers hardness is 550 HV.

FIG. 3 shows a comparison between carburization by vacuum carburization and carburization by gas carburization. As shown in the figure, in the case of vacuum carburization, a deeper carburization depth can be obtained than by gas carburization.
The results of FIG. 3 show the results when vacuum carburizing, gas carburizing, and quenching are performed on SCR 420 under the condition of 950 ° C. for 2 hours.
From this result, it can be seen that deep carburization depth can be obtained by vacuum carburization as compared with gas carburization.

  Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be implemented in variously modified forms without departing from the spirit of the present invention.

Claims (9)

In mass%
C: 0.15-0.25%
Si: 0.90 to 1.30%
Mn: 0.70 to 1.10%
P: 0.030% or less
S: 0.100% or less
Cu: 0.01 to 0.50%
Ni: 0.01-0.50%
Cr: 0.20 to 0.50%
Mo: 0.50% or less
Al: 0.30% or less
N: 0.05% or less and satisfying the condition of the following formula (1),
[Si] + [Ni] + [Cu]-[Cr]> 0.5 (1)
(However, each element symbol in the formula (1) represents mass%)
Vacuum carburization of steel having the composition of the balance Fe and inevitable impurities so that the surface carbon concentration after slow cooling after carburizing is within the range of more than 1.1 to 1.5% under a reduced pressure condition of 2 kPa or less. After the treatment, the above-mentioned slow cooling by air cooling is performed at a cooling rate that causes pearlite transformation to make the surface structure pearlite, and then the cementite in the pearlite structure is finely divided to the range from the surface to 0.1 mm. A method for manufacturing a bearing component, comprising performing induction hardening under heating and cooling conditions in which fine carbides in which the number of carbides of 1 μm or less in the carbides accounts for 95% or more are generated.   2. A bearing part according to claim 1, wherein the slow cooling by the air cooling is performed at a cooling rate of 5 [deg.] C./s to 0.2 [deg.] C./s, and the induction hardening is performed at a heating temperature of 750 to 850 [deg.] C. Method. In any one of Claims 1 and 2, the said steel is mass%.
Nb: 0.02-0.20%
Ti: 0.02-0.20%
B: 0.0005-0.0100%
1 or 2 types or more of these are further contained, The manufacturing method of the bearing components characterized by the above-mentioned.   4. The method of manufacturing a bearing component according to claim 1, wherein the vacuum carburization is performed so that a surface carbon concentration after the slow cooling is 1.2% or more.   The surface carbon concentration is more than 1.1 to 1.5%, and fine particles of 1 μm or less generated by dividing the cementite of pearlite structure within the range from the surface to 0.1 mm account for 95% or more of the carbides. Bearing parts characterized by carbides. In claim 5, in mass%
C: 0.15-0.25%
Si: 0.90 to 1.30%
Mn: 0.70 to 1.10%
P: 0.030% or less
S: 0.100% or less
Cu: 0.01 to 0.50%
Ni: 0.01-0.50%
Cr: 0.20 to 0.50%
Mo: 0.50% or less
Al: 0.30% or less
N: 0.05% or less and satisfying the condition of the following formula (1),
[Si] + [Ni] + [Cu]-[Cr]> 0.5 (1)
(However, each element symbol in the formula (1) represents mass%)
A bearing component having a composition of the balance Fe and inevitable impurities, wherein the fine carbide is generated within a range of 0.1 mm from the surface by vacuum carburization, subsequent cooling, and induction hardening.   7. The bearing part according to claim 5, wherein the carbide is formed in a structure in which a martensite structure occupies 50% or more. In any one of Claims 5-7, the said steel is mass%.
Nb: 0.02-0.20%
Ti: 0.02-0.20%
B: 0.0005-0.0100%
A bearing component characterized by further containing one or more of the above.   The bearing component according to claim 5, wherein the surface carbon concentration is 1.2% or more.