JP2007177275A - Power transmission shaft for constant-velocity universal joint - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stably provide a power transmission shaft which has all of high static-torsional strength, high torsional-fatigue strength and excellent hardening-cracking resistance. <P>SOLUTION: The power transmission shaft for the constant-velocity universal joint, which has a groove for torque transmission in both ends thereof, has a composition comprising 0.40-0.50 mass% C, 0.35-0.8 mass% Si, 0.5-0.8 mass% Mn, 0.005-0.05 mass% Al, 0.005-0.05 mass% Ti, 0.3-0.5 mass% Mo, 0.0005-0.005 mass% B, 0.05-0.5 mass% Cu, 0.005-0.025 mass% S, 0.02 mass% or less P, 0.2 mass% or less Cr and the balance Fe with unavoidable impurities; has a metallurgical structure of the base metal including a bainitic structure of 50% or higher by structure fraction; has a quench-hardened layer after having been induction hardened, of which the surface layer includes a retained austenite with an average particle diameter of 10 μm or smaller; and has an effective depth of the quench-hardened layer, of which the ratio to a shaft radius, in other words, a quench-hardened layer ratio is 0.5 to 0.75 in the groove part and is 1.0 in the region except the groove part. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a power transmission shaft for a constant velocity universal joint having a hardened layer formed by induction hardening and excellent in static torsional strength, torsional fatigue strength and anti-fire cracking resistance.

Conventionally, power transmission shafts for constant velocity universal joints, which are mechanical structural parts, are obtained by subjecting hot rolled steel bars to cutting, cold rolling, etc. and processing them into a predetermined shape, followed by induction hardening and tempering. Generally, static torsional strength and torsional fatigue strength, which are important characteristics as a machine structural component, are ensured.
In recent years, there has been a strong demand for weight reduction of automobile parts due to environmental problems, and further improvement in torsional fatigue strength of automobile parts has been demanded from this viewpoint.

In addition, the shaft end of the power transmission shaft has a groove part called spline or serration that acts as a power transmission action part, but if induction hardening is applied to a part having such a notch shape, if cracking occurs Since this causes a decrease in static torsional strength and torsional fatigue strength, improvement in this respect is also desired.
In addition, since the elements that promote the above-described burning crack are mainly C and P, it is effective to reduce these elements in order to suppress the burning crack. However, since C is an element effective for improving static torsional strength and torsional fatigue strength, positive reduction of C is not preferable from the viewpoint of static torsional strength and torsional fatigue strength.

In order to improve the static torsional strength and the torsional fatigue strength, for example, it is conceivable to increase the quenching depth by induction hardening. However, although the static torsional strength increases with an increase in the quenching depth, the torsional fatigue strength saturates at a certain depth even if the quenching depth is increased.
Further, improvement of the grain boundary strength is also effective for improving the torsional fatigue strength. From this viewpoint, a technique for refining the prior austenite grain size by dispersing TiC has been proposed (see, for example, Patent Document 1). )

In the technique described in Patent Document 1 described above, since fine TiC is dispersed in a large amount during induction quenching heating, the prior austenite grain size is refined, so that TiC is solutionized before quenching. It is necessary to adopt a process of heating to 1100 ° C or higher in the hot rolling process. Therefore, there is a problem that it is necessary to increase the heating temperature during hot rolling, resulting in poor productivity.
Further, even with the technique disclosed in the above-mentioned Patent Document 1, there is still a problem in that it cannot sufficiently meet the recent demands for torsional fatigue strength and fire cracking resistance.

In Patent Document 2, the ratio (CD / R) between the hardened layer depth CD and the radius R of the induction-hardened shaft component is limited to 0.3 to 0.7, and the CD / R and the surface after induction hardening are used. The value A defined by the austenite grain size γf up to 1 mm, the average Vickers hardness Hf up to (CD / R) = 0.1 as induction-quenched, and the average Vickers hardness Hc at the center of the shaft after induction hardening is expressed as C There has been proposed a shaft object part for a machine structure in which torsional fatigue strength is improved by controlling it within a predetermined range according to the amount.
However, in this part, since consideration is not given to static torsion strength, it is not possible to meet the recent demands for achieving both static torsional strength and torsional fatigue strength.
In Patent Document 2, no consideration is given to the occurrence of quenching cracks during induction hardening.

JP 2000-154819 A (Claims, paragraph [0008]) JP-A-8-53714 (Claims)

  The present invention has been developed in view of the above-mentioned present situation, and an object thereof is to provide a power transmission shaft that is superior in torsional fatigue strength and static torsional strength, and also excellent in resistance to cracking during induction hardening. And

The inventors have intensively studied to effectively improve the torsional fatigue strength, static torsional strength, and fire cracking resistance.
As a result, as described below, excellent torsional fatigue strength and static torsion are achieved by optimizing the chemical composition, structure, and prior austenite grain size and effective hardening depth of the hardened layer after quenching. It was found that a power transmission shaft having strength and fire cracking resistance could be obtained.

(1) By applying quenching to the power transmission shaft adjusted to an appropriate chemical composition and setting the prior austenite grain size of the hardened hardened layer to 10 μm or less, the torsional fatigue strength and the anti-cracking resistance are remarkably improved. Specifically, regarding the chemical composition, the addition of Mo and Si in an appropriate range increases the number of austenite nucleation sites during induction hardening and suppresses austenite grain growth. The grain size of the hardened hardened layer is effectively refined, and as a result, the torsional fatigue strength and the quench cracking resistance are remarkably improved.

(2) When the structure of the base material of the power transmission shaft, that is, the structure before quenching, is a structure in which the bainite structure is contained at a specific fraction, the bainite structure is finely dispersed compared to the ferrite-pearlite structure. Since it is a structure, the area of the ferrite / carbide interface, which is an austenite nucleation site, is increased during quenching heating, and the generated austenite is refined. As a result, the grain size of the quenched and hardened layer becomes fine, thereby improving the grain boundary strength and improving torsional fatigue strength and quench cracking resistance.

(3) By regulating the effective hardened layer depth separately for the groove part of the shaft and other parts, especially by increasing the hardening depth of the part other than the groove part and slightly reducing the hardening depth of the groove part. In addition to suppressing the cracking of the groove part, the strength of the part other than the groove part was reduced due to the grain boundary fracture when hardening to the conventional axis, but the prior austenite grains shown in the above (1) As a result of the grain refinement being suppressed by the effect of refinement of the diameter, the static torsional strength and the torsional fatigue strength are improved.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. A power transmission shaft for a constant velocity universal joint having a torque transmission groove at both ends of the shaft,
C: 0.40 to 0.50 mass%,
Si: 0.35-0.8 mass%
Mn: 0.5-0.8 mass%,
Al: 0.005 to 0.05 mass%,
Ti: 0.005 to 0.05 mass%,
Mo: 0.3-0.5 mass%,
B: 0.0005 to 0.005 mass%,
Cu: 0.05 to 0.5 mass%,
S: 0.005 to 0.025 mass%,
P: 0.02 mass% or less and
Cr: Contains 0.2 mass% or less, the balance is the composition of Fe and inevitable impurities, the base metal structure has a bainite structure, the bainite structure has a structure fraction of 50% or more, and induction hardening The former austenite average particle size of the subsequent hardened layer surface layer part is 10 μm or less, and the hardened layer ratio, which is the ratio of the effective hardened layer depth to the axial radius, is 0.5 to 0.75 for the groove part and 1.0 for the other part than the groove part. A power transmission shaft for a constant velocity universal joint.

2. 2. The power transmission shaft for a constant velocity universal joint according to 1 above, wherein the average hardness of the hardened layer in the groove is lower than the average hardness of the hardened layer in the portion other than the groove.

3. 3. The power transmission shaft for a constant velocity universal joint according to 1 or 2, wherein the groove portion has a fitting portion with another part at a rounded-up portion that expands from the minimum diameter portion of the groove portion separated from the shaft end surface. .

4). 4. The power transmission shaft for a constant velocity universal joint according to any one of the above 1 to 3, wherein a residual stress after high-frequency heat treatment in a portion other than the groove is −700 MPa or less.

5. 5. The power transmission shaft for a constant velocity universal joint according to any one of 1 to 4, wherein the hardened layer ratio of the rounded-up portion of the groove is 0.5 to 0.75.

Here, the said hardened layer surface layer part refers to the part from the hardened layer surface to the depth of 500 micrometers.
Further, the portion other than the groove portion refers to an intermediate shaft portion excluding the groove portion, but excludes a shoulder portion formed in the intermediate shaft portion. Here, when the shoulder part is fitted with another part in the groove part, the other part is brought into contact with the large diameter part formed between the groove part and the shaft intermediate part, so that the positional relationship in the axial direction is reached. Refers to the part to be determined. Hereinafter, the portion other than the groove is referred to as a shaft intermediate portion.
Furthermore, the effective hardened layer depth is an “effective hardened layer depth” defined in JIS G0559.

  Thus, according to the present invention, it is possible to stably obtain a power transmission shaft that is excellent in resistance to cracking and has high static torsional strength and torsional fatigue strength. Play great achievement.

The present invention will be specifically described below.
First, the reason why the component composition of the power transmission shaft is limited to the above range in the present invention will be described.
C: 0.40 ~ 0.50mass%
C is an element having the greatest influence on hardenability, and increases the hardness and depth of the hardened hardened layer and effectively contributes to the improvement of torsional strength. However, if the content is less than 0.40 mass%, in order to ensure the required torsional strength, the quench hardening depth must be dramatically increased. Add 0.40mass% or more because it is difficult to generate the structure. On the other hand, if the content exceeds 0.50 mass%, the grain boundary strength decreases, and accordingly, the torsional fatigue strength decreases, and the machinability, cold forgeability, and fire cracking resistance also decrease. For this reason, C was limited to the range of 0.40 to 0.50 mass%. Preferably, it is in the range of 0.42 to 0.46 mass%.

Si: 0.35-0.8 mass%
Si is an element useful for the formation of a bainite structure. Moreover, it has the effect | action which refines | miniaturizes the particle size of a hardening hardening layer. Furthermore, it is an element that improves the temper softening resistance, and increases the hardness of the induction hardening layer. Furthermore, the carbide | carbonized_material production | generation is suppressed and the fall of the grain boundary strength by a carbide | carbonized_material is suppressed. These improve the static torsional strength, the torsional fatigue strength and the anti-fire cracking resistance.

  Thus, Si is a very important element in the present invention, and it is essential to contain 0.35 mass% or more. This is because when the Si content is less than 0.35 mass%, the bainite structure fraction is reduced, and the old austenite grain size of the hardened hardened layer surface layer cannot be made fine particles of 10 μm or less, and the hardened layer This is because the hardness decreases and the static torsional strength and torsional fatigue strength decrease. However, if the amount of Si exceeds 0.8 mass%, the hardness increases due to the solid solution hardening of ferrite, leading to a decrease in machinability, cold forgeability, and fire cracking resistance. Therefore, Si was limited to the range of 0.35 to 0.8 mass%. Preferably it is the range of 0.40-0.8 mass%.

Mn: 0.5 to 0.8 mass%
Mn is an indispensable component for improving the hardenability and securing the hardening depth during quenching, so it is actively added, but if the content is less than 0.5 mass%, the effect of addition is poor, so 0.5% More than mass%. On the other hand, if the amount of Mn exceeds 0.8 mass%, the hardness of the rolled material is increased, the rolling property and machinability are lowered, and the quenching resistance is improved. Therefore, Mn is set to 0.8 mass% or less. Furthermore, it is suitable to set it as 0.7 mass% or less.

Al: 0.005 to 0.05 mass%
Al is an element effective for deoxidation. Moreover, it is an element useful also in refine | miniaturizing the particle size of a hardening hardening layer by suppressing the austenite grain growth at the time of quenching heating. However, if the content is less than 0.005 mass%, the effect of addition is poor. On the other hand, even if the content exceeds 0.05 mass%, the effect is saturated, and a disadvantage that causes an increase in the component cost occurs. Limited to a range of ~ 0.05 mass%. Preferably it is the range of 0.02-0.04 mass%.

Ti: 0.005 to 0.05 mass%
Ti combines with N mixed as an unavoidable impurity to prevent B from becoming BN and the effect of improving the hardenability of B to disappear, and has the effect of sufficiently exerting the effect of improving the hardenability of B. . In order to obtain this effect, it is necessary to contain at least 0.005 mass%, but if it exceeds 0.05 mass%, a large amount of TiN is formed, which becomes the starting point of fatigue fracture and torsional fatigue strength Therefore, Ti is limited to the range of 0.005 to 0.05 mass%. Preferably
The range is 0.015 to 0.03 mass%.
Furthermore, it is preferable to satisfy Ti (mass%) / N (mass%) ≧ 3.42 from the viewpoint of securing N securely and improving the hardenability by B to obtain a bainite and martensite structure.

Mo: 0.3-0.5 mass%
Mo promotes the formation of a bainite structure, thereby minimizing the austenite grain size during quenching and heating and reducing the grain size of the quenched hardened layer. Moreover, it has the effect | action which refines | miniaturizes the particle size of a hardening hardening layer by suppressing the grain growth of austenite at the time of quenching heating. Furthermore, since it is an element useful for improving hardenability, it is used for adjusting hardenability.
Thus, Mo is a very important element in the present invention, and if the content is less than 0.3 mass%, the prior austenite grain size of the hardened layer surface layer is adjusted no matter how the manufacturing conditions and quenching conditions are adjusted. Cannot be made into fine grains of 10 μm or less. However, if the content exceeds 0.5 mass%, the hardness of the rolled material is remarkably increased, and the workability is reduced. Moreover, when Mo exceeds 0.5 mass%, the fire cracking resistance also decreases. Therefore, Mo is limited to the range of 0.3 to 0.5 mass%. Preferably it is the range of 0.35-0.45 mass%.

B: 0.0005-0.005 mass%
B has an effect of promoting the formation of a bainite structure or a martensite structure. B also has the effect of improving the torsional strength by improving the hardenability by adding a small amount and increasing the quenching depth during quenching. Further, B preferentially segregates at the grain boundary, reduces the concentration of P segregating at the grain boundary, improves the grain boundary strength, and thus has an effect of improving torsional fatigue strength. Also, the crack resistance is improved by strengthening the grain boundaries.
For this reason, in the present invention, B is positively added. However, if the content is less than 0.0005 mass%, the effect of addition is poor. On the other hand, if the content exceeds 0.005 mass%, the effect is saturated, rather the component In order to raise the cost, B is limited to the range of 0.0005 to 0.005 mass%. Preferably it is the range of 0.0010-0.0030 mass%.

Cu: 0.05 to 0.5 mass%,
Cu is effective for improving the hardenability, and also dissolves in ferrite, and this solid solution strengthening improves torsional fatigue strength. Further, by suppressing the formation of carbides, the decrease in grain boundary strength due to carbides is suppressed, and the torsional fatigue strength is improved. For that purpose, 0.05 mass% or more is added. However, if the content exceeds 0.5 mass%, cracks occur during hot working, and the resistance to fire cracking also deteriorates. In addition, Preferably it is 0.3 mass% or less.

S: 0.005 to 0.025 mass%,
S is a useful element that improves the machinability by forming MnS in steel and is contained at 0.005 mass% or more, but if it exceeds 0.025 mass%, the amount of MnS increases and the torsional strength decreases. Therefore, S is limited to 0.025 mass% or less.

P: 0.02 mass% or less P segregates at the grain boundaries of austenite and lowers the grain boundary strength, thereby lowering the torsional fatigue strength. In addition, there is a harmful effect that promotes burning cracks. Therefore, it is desirable to reduce the P content as much as possible, but it is allowed up to 0.02 mass%.

Cr: 0.2 mass% or less
Cr stabilizes the carbide and promotes the formation of residual carbide, lowers the grain boundary strength, and degrades the torsional fatigue strength. In addition, Cr promotes fire cracking. Therefore, it is desirable to reduce the Cr content as much as possible, but up to 0.2 mass% is acceptable. Preferably it is 0.05 mass% or less.

Although the preferred component composition range has been described above, in the present invention, it is not sufficient to limit the component composition to the above range, and adjustment of the steel structure is also important.
That is, in the present invention, the base material structure of the power transmission shaft, that is, the structure before quenching has a bainite structure, and the structure fraction of the bainite structure needs to be 50% or more. The reason for this is that the bainite structure is a structure in which carbides are finely dispersed as compared with the ferrite-pearlite structure. Therefore, the area of the ferrite / carbide interface, which is an austenite nucleation site during quenching heating, increases, and austenite formed. This is because it contributes to making the grain size of the hardened and hardened layer fine. And grain boundary intensity | strength rises by refinement | miniaturizing the particle size of a hardening hardening layer, and a torsional fatigue strength and a quench cracking resistance improve.
Here, the structure fraction of the bainite structure is more preferably 60 vol% or more.

The remaining structure other than the bainite structure may be a martensite structure, ferrite, pearlite, or the like, and is not particularly defined.
Also, regarding the refinement of the grain size of the hardened layer after quenching, the martensite structure has the same effect as the bainite structure, but from an industrial point of view, the bainite structure is better than the martensite structure. Since the addition amount of the alloy element is small and it can be produced at a low cooling rate, it is advantageous in production. Further, when the martensite structure fraction increases in the base material structure, the spline rollability deteriorates. That is, problems such as rolling die breakage occur. From this point of view, it is preferable that the base material structure is 50% or more of bainite, preferably 60% or more of bainite, and the balance is ferrite and / or pearlite structure.

  In the power transmission shaft of the present invention, it is also important to adjust the prior austenite grain size of the hardened layer after induction hardening. That is, regarding the hardened layer after induction hardening, the prior austenite grain size needs to be 10 μm or less in the surface layer in the above range. This is because if the prior austenite grain size of the hardened hardened layer surface layer exceeds 10 μm, sufficient grain boundary strength cannot be obtained, and satisfactory improvement in torsional fatigue strength cannot be expected. In addition, Preferably it is 8 micrometers or less.

Here, the measurement of the prior austenite particle size of the hardened hardened layer surface is performed as follows.
Using an optical microscope, observe the field of view from the surface layer to 500 μm from 400 times (area of 1 field of view: 0.25 mm x 0.225 mm) to 1000 times (area of 1 field of view: 0.10 mm x 0.09 mm), and use an image analyzer This is done by measuring the average particle size.

  Furthermore, in the hardened layer after induction hardening, it is important that the ratio of the effective hardened layer depth to the axial radius (hereinafter referred to as hardened layer ratio) is 0.5 to 0.75 for the groove and 1.0 for the middle shaft portion. . In other words, if the entire curing can be performed up to the axis, a significant increase in strength can be obtained. However, conventionally, the strength has decreased due to grain boundary fracture. Here, in the present invention, the grain boundary strength is improved because the prior austenite grain size of the hardened hardened layer surface layer has been refined to 10 μm as described above. Strength is improved.

  However, when the groove is fully cured, burn cracks are generated and the strength is lowered. Therefore, the groove has a cured layer ratio in the range of 0.5 to 0.75. That is, when the hardened layer ratio is less than 0.5, desired static torsion and torsional fatigue strength cannot be obtained. On the other hand, when the hardened layer ratio exceeds 0.75, the occurrence of burning cracks becomes significant, and the static torsional fatigue strength and torsional fatigue strength are increased. Fatigue strength decreases. The lower limit of the cured layer ratio is preferably 0.6.

  Furthermore, by setting the hardened layer ratio of the rounded-up portion of the groove portion, which is often the starting point of fracture in torsional fatigue, to 0.5 to 0.75, a torsional fatigue strength with a more stable quality can be obtained.

Next, conditions for manufacturing the power transmission shaft of the present invention will be described.
A steel material adjusted to a predetermined component composition is rolled into a predetermined length after steel bar rolling or hot forging, and then subjected to cutting, followed by rolling a groove (spline portion) 2 as shown in FIG. Processing is performed, and then the entire power transmission shaft 1 is subjected to induction hardening and tempering to obtain a product.

In the present invention, the base material structure of the power transmission shaft has the above-described bainite structure, and the bainite structure has a structure fraction of 50 vol% or more, after hot working such as rolling and forging. It is necessary to cool at a rate of 0.2 ° C / s or higher. This is because when the cooling rate is less than 0.2 ° C./s, it becomes difficult to obtain a bainite or martensite structure, and the structure fraction may not reach 50 vol%. The preferable range of the cooling rate after hot working is 0.3 to 30 ° C./s.
In addition, it is preferable to perform hot working in a temperature range of over 900 ° C. to 1150 ° C. If the temperature is below 900 ° C., the necessary bainite structure cannot be obtained. On the other hand, if it exceeds 1150 ° C., the heating cost increases, which is economically disadvantageous.

  In the present invention, the cutting process and the rolling process of the spline part are not particularly limited, and a conventionally known method may be used.

  Next, in the present invention, after the spline portion is rolled, induction hardening is performed in order to obtain the above-described hardened layer, and the heating temperature range during this induction hardening needs to be 800 to 1000 ° C. This is because when the heating temperature is less than 800 ° C., the austenite structure is not sufficiently generated, and as a result of insufficient generation of the hardened layer structure described above, sufficient torsional fatigue strength and fire cracking resistance can be ensured. On the other hand, when the heating temperature exceeds 1000 ° C., the growth of austenite grains is promoted to become coarse, and the hardened layer has a coarse particle size, which also causes a decrease in torsional fatigue strength and fire cracking resistance. is there. A more preferable heating temperature range is 800 to 950 ° C.

  In particular, in order to adjust the hardened layer ratio to the above range, the heating temperature is set to a low temperature range of 1000 ° C. or lower, and the high frequency induction current penetration range is deepened to refine the prior austenite grain size and the hardened layer depth. Ensuring both.

Furthermore, it is preferable that the hardened layer average hardness of a groove part is lower than the hardened layer average hardness of parts other than this groove part. Specifically, it is recommended to reduce the average hardness of the hardened layer in the groove by about 50 to 120 Hv. That is, the stability of the static torsional strength is improved as a result of a decrease in notch sensitivity in the groove (notched portion) without reducing the hardness of the portion other than the groove.
This hardness adjustment can be achieved, for example, by subjecting only the groove to a tempering process by high frequency induction heating.

  Moreover, as shown in FIG. 2, it is advantageous that the groove portion has a fitting portion with another component at a rounded-up portion that expands from the groove portion minimum diameter portion separated from the shaft end surface. In other words, the weakest part in the groove is the end of the fitting part with the other part, and when fitting with other parts using only the groove, the diameter is the smallest at the groove bottom. The groove bottom is the weakest. On the other hand, by having a fitting part with other parts in the rounded-up part, the weakest part becomes the rounded-up part and becomes a part whose diameter is larger than the groove bottom, and fitting with other parts is possible only with the groove part. It is more advantageous than doing so.

Furthermore, the residual stress after high-frequency heat treatment of the shaft middle portion is set to −700 MPa or less. Because if the fracture occurs from the smallest diameter part of the shaft middle part during the torsional fatigue test, especially during the middle to high cycle fatigue (10 ⁴ times or more) fatigue test, the fatigue strength will be low, This is because by setting the residual stress to −700 MPa or less, the fracture from the shaft middle portion minimum diameter portion is suppressed, and high torsional fatigue strength can be obtained.
This residual stress can be adjusted, for example, by setting the cooling rate during induction hardening to 300 ° C./s or more at the shaft surface portion with respect to the shaft intermediate portion.

Steel materials having the composition shown in Table 1 were melted by a converter and cast into continuous slabs. The slab size was 300 × 400mm. The slab was rolled into a 150 mm square billet through a breakdown process, and then rolled into a 23 mmφ steel bar. The finishing temperature of rolling was over 900 ° C. as a suitable temperature from the viewpoint of bainite structure formation. Cooling after rolling was performed under the conditions shown in Table 2.
Next, after cutting this steel bar to a predetermined length, cutting and rolling of the spline part were performed to produce a power transmission shaft 1 having a groove part (spline part) 2 having the dimensions and shape shown in FIG. . This power transmission shaft was quenched using an induction hardening apparatus having a frequency of 3 kHz under the conditions shown in Table 2, and then the maximum heating temperature was set to 240 ° C. and the heating time was set to 12 using the same induction hardening apparatus. Tempering was performed in seconds, followed by cooling after heating or water cooling after cooling, and then the static torsional strength and torsional fatigue strength were investigated.

  The static torsional strength is measured by using an electric torsion tester, and as shown in FIG. 3, the spline portions 2a and 2b are incorporated in the disc-shaped grippers 3a and 3b, respectively, and twisted between the grippers 3a and 3b. Torque was applied, the torque value at the time when the shaft broke, was measured, and this was converted into a stress value (Gpa) at the minimum part (diameter 19 mmφ) of the shaft middle part and evaluated.

The torsional fatigue strength is calculated by converting the torque value when the number of repetitions of fracture is 5 × 10 ⁴ times in the torsional fatigue test of the power transmission shaft to the stress value (Gpa) at the minimum part (diameter 19 mmφ) of the shaft middle part. evaluated.
In the torsional fatigue test, a hydraulic fatigue tester is used, and as shown in FIG. 3, the spline portions 2a and 2b are respectively incorporated in the disc-shaped grippers 3a and 3b, and the frequency is 1 between the grippers 3a and 3b. This was done by repeatedly applying torque at ~ 3 Hz.

  Moreover, about the same power transmission shaft, the base material structure | tissue of steel materials and the hardened layer particle size (old austenite particle size) were measured using the optical microscope.

  Also, for the same power transmission shaft, in the cross section parallel to the axial direction at the rounded-up portion of the groove and the shaft middle portion, the hardness is measured at a 0.2 mm pitch from the surface to the center with a Vickers hardness tester, and the hardened layer ratio and hardened layer The average hardness was investigated. For the average hardness of the hardened layer, the average value of the hardness up to a position where the ratio of the depth from the surface of the hardened layer to the axial radius becomes 0.2 was used as the average hardness of the hardened layer.

Furthermore, the same power transmission shaft was also investigated for fire cracking resistance.
The anti-fire cracking resistance was evaluated by conducting a magnetic particle flaw detection test on the spline part after induction hardening, and evaluating the case where the crack was generated as x and the case where the crack was not generated as being ◯.
In addition, after spline rolling of each power transmission shaft, the presence or absence of defects in the rolling dies is visually observed. It was evaluated as good formability (◯).
The obtained results are also shown in Table 2.

It is a front view of a typical power transmission shaft. It is a figure which shows the shape of a groove part. It is a figure which shows the test point in the torsional fatigue test of a power transmission shaft.

Explanation of symbols

1 Power transmission shaft 2 Groove 3 Grab

Claims (5)

A power transmission shaft for a constant velocity universal joint having a torque transmission groove at both ends of the shaft,
C: 0.40 to 0.50 mass%,
Si: 0.35-0.8 mass%
Mn: 0.5-0.8 mass%,
Al: 0.005 to 0.05 mass%,
Ti: 0.005 to 0.05 mass%,
Mo: 0.3-0.5 mass%,
B: 0.0005 to 0.005 mass%,
Cu: 0.05 to 0.5 mass%,
S: 0.005 to 0.025 mass%,
P: 0.02 mass% or less and
Cr: Contains 0.2 mass% or less, the balance is the composition of Fe and inevitable impurities, the base metal structure has a bainite structure, the bainite structure has a structure fraction of 50% or more, and induction hardening The former austenite average particle size of the subsequent hardened layer surface layer part is 10 μm or less, and the hardened layer ratio, which is the ratio of the effective hardened layer depth to the axial radius, is 0.5 to 0.75 for the groove part and 1.0 for the other part than the groove part. A power transmission shaft for a constant velocity universal joint.   2. The power transmission shaft for a constant velocity universal joint according to claim 1, wherein the average hardness of the hardened layer of the groove is lower than the average hardness of the hardened layer other than the groove.   3. The power transmission for a constant velocity universal joint according to claim 1, wherein the groove portion has a fitting portion with another part in a rounded-up portion that expands from the smallest diameter portion of the groove portion separated from the shaft end surface. shaft.   The power transmission shaft for a constant velocity universal joint according to any one of claims 1 to 3, wherein a residual stress after high-frequency heat treatment in a portion other than the groove is -700 MPa or less. 5. The power transmission shaft for a constant velocity universal joint according to claim 1, wherein the hardened layer ratio of the rounded-up portion of the groove is 0.5 to 0.75.

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