JP2000204432A - Power transmission shaft - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power transmission shaft excellent in workability such as forgeability and machinability and having high strength. SOLUTION: In a power transmission shaft using a constant velocity universal joint, graphite steel is subjected to induction hardening to harden the surface layer, and moreover, in the core part, a two phase structure of ferrite and martensite is formed. The graphite steel has a compsn. contg., as fundamental components, 0.35 to 0.70% C, 0.4 to 2.0% Si, 0.3 to 1.5% Mn, <=0.025% S, <=0.02% P, 0.01 to 0.1% Al, 0.001 to 0.004% B and 0.002 to 0.008% N, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power transmission shaft for transmitting torque through a constant velocity universal joint in an automobile, an industrial machine, or the like.

[0002]

2. Description of the Related Art In a power transmission shaft, for example, a drive shaft of an automobile, carbon steel or carburized steel is usually used as a material,
Further, a predetermined strength is secured by appropriately selecting the surface hardening by the heat treatment and the effective hardened layer depth.

In recent years, it has been desired to further increase the strength of a drive shaft in response to an increase in the output of an automobile or an increase in the weight of a vehicle due to safety. In addition, a demand for a lighter drive shaft has become apparent from the viewpoint of improving fuel efficiency, and in order to achieve this, it is urgently necessary to increase the strength of the drive shaft.

[0004]

In order to improve the load capacity of the shaft, it is common to increase the carbon strength of the material to improve the material strength or to increase the effective hardened layer depth. However, in the former case, the strength of a part having a notch or the like is reduced, and the machinability such as forgeability and machinability is reduced due to an increase in hardness of the material. On the other hand, in the latter case, the selection range of the depth of the hardened layer is extremely narrow in the case of the carburized material. In carbon steel, deep quenching becomes more difficult as the shaft diameter increases, and the effective hardened layer depth / shaft radius ratio (hereinafter, referred to as
Deep quenching exceeding 0.4) is considerably difficult due to concerns such as the occurrence of quenching cracks. In recent years, carbon steel added with boron B has been used to enable deep baking, but even if the effective hardened layer depth is increased, the static strength is γ.
> 0.65, and the torsional fatigue strength tends to saturate when γ> 0.5 (JP-A-5-320825), so that only about 15% strength improvement is achieved. In addition, in the case of the B additive, a hard nitrogen compound such as TiN is formed, which may cause a decrease in machinability.

[0005] Therefore, the present invention provides a method for improving forgeability and machinability.
To provide a power transmission shaft with excellent workability and high strength
I do.

[0006]

In order to achieve the above object, according to the present invention, in a power transmission shaft using a constant velocity universal joint, graphite steel is induction hardened to harden a surface layer, and ferrite is added to a core portion. A two phase structure of martensite was formed. Graphite steel is graphitized from cementite in carbon steel by graphitizing annealing.It contains graphite, a free-cutting element, and has excellent machinability, and because it is soft, it has excellent cold and warm forgeability. Has features. Therefore, good machinability can be ensured even when carbon is increased for higher strength.

[0007] Many conventional power transmission shafts use carbon steel
It is manufactured by wave quenching.
From the point of view, there are many things which do not give a heat influence to a core part. Also wick
Almost all of the core, even those that have affected the heat
Rutensite reduces residual compressive stress on the surface
Have lost. In contrast, in the present invention, induction hardening is used.
Not only hardens the surface layer, but also
Part of the core, and dissolve graphite in ferrite to form a core.
It has a two phase structure of ferrite and martensite
The residual compressive stress remains on the surface, thus increasing the strength and
Fatigue strength is achieved. To apply heat effects to the core
Is to perform induction hardening multiple times (for example, twice)
Good.

[0008] As the above graphite steel, C: 0.
35 to 0.70%, Si: 0.4 to 2.0%, Mn:
0.3-1.5%, S: 0.025% or less, P: 0.0
2% or less, Al: 0.01 to 0.1%, B: 0.001
~ 0.004%, N: 0.002-0.008%
Component, the balance being Fe and unavoidable impurities
Use of

[0009] Of the above elements, C produces graphite.
Element indispensable for This is less than 0.35%
And the surface hardness after induction hardening is too low to obtain sufficient strength
If not greater than 0.70%, cementite is present in the core
Precipitates, the core becomes hard (brittle) and the strength decreases.

[0010] Si is a deoxidizer at the steel making stage and promotes graphitization
It is added as an agent and for strengthening the grain boundaries. this
Is less than 0.4%, it is difficult for the carbide to be graphitized.
At the same time, the effect of strengthening the grain boundaries is reduced, and is greater than 2.0%.
In this case, the cold workability (forgeability, turning property) is significantly reduced.

[0011] Mn fixes and separates sulfur in steel as MnS.
Required to disperse, this is less than 0.3%
If this is the case, the hardenability will decrease (hardening depth cannot be obtained),
If it is more than 1.5%, it significantly inhibits graphitization and cold work
Reduce the quality.

[0012] S combines with Mn to form MnS inclusions
Although it is present, it becomes the starting point of crack generation during cold working
Make it smaller than 0.025%. In addition, P
Precipitates at the grain boundaries, embrittles the grain boundaries and reduces the strength
0.02% or less, because it increases the susceptibility to burning cracking
And

[0013] Al is a deoxidizing agent that oxidizes oxygen in steel during steelmaking.
In order to reduce the particle size by removing them as oxide inclusions.
Greater than 01%, but too much oxide inclusions
Since the toughness decreases and becomes the starting point of cracking during cold working,
0.10% or less.

[0014] B and N are formed during graphitizing annealing due to the formation of BN.
It is added to shorten the interval. Get enough shortening effect
To do this, B must be added in an amount greater than 0.001%.
Must be added, but if added over 0.004%, annealing
The time reduction effect saturates. N is 0.001% to 0.00
In order to convert 4% of B into BN, 0.002% or more of 0.1%.
008% or less.

[0015] Ni: 0.3 to 1.0 weight of the graphite steel
%, Mo: 0.2% by weight or more of one or two
Or both). When Ni is added,
Increased cold workability and strength due to increased ductility of light
You. If this is less than 0.3%, the cold workability and strength tend to increase.
On the other hand, if it is larger than 1.0%, the turning property is remarkable.
Lowers. The addition of Mo can improve toughness.
However, adding more than 0.2% reduces graphitization.
Let it down.

[0016] Maximum surface hardness (Vickers hardness)
If the difference between the minimum values is 200 Hv or less, maintain the intensity balance.
That is, a decrease in strength can be prevented. Strength in this range
Variations use graphite steel with a graphite particle size of 15 μm or less
This can be achieved by doing Graphite particle size is 1
If it is larger than 5 μm, graphite melts after quenching.
Voids (voids) due to the
Occurs, and the surface hardness varies greatly, leading to a decrease in strength.

[0017] In the above power transmission shaft, the hardness of the core is
(Rockwell hardness) is 25 to 45 HRC. 25
If it is smaller than the above, strength is reduced due to less martensite.
The effect is insufficient.
The shaft notch (for example, serration)
Portion) tends to be cracked.

The compressive residual stress on the surface is 60 kgf / mm ²
With the above, improvement in fatigue strength is achieved. In general, when induction hardening is performed on graphite steel, the graphite portion hardly forms a solid solution at the time of gamma conversion, so that the hardenability is low. Therefore, hardening cracks and the like are unlikely to occur even when water quenching is performed similarly to high carbon steel. With water firing, a high surface compressive residual stress of about 60 kgf / mm ² can be realized.

If the compressive residual stress on the surface is made to be 90 kgf / mm ² or more by shot peening after induction hardening, the fatigue strength can be further improved. In order to realize this, it is desirable to perform shot peening twice.

[0020]

FIG. 1 shows a pressure welding stub 1 for a constant velocity universal joint used as a propeller shaft or a drive shaft of an automobile. This stub 1 is formed of graphite steel,
One end has a tooth portion 2 (for example, serration) for transmitting torque. A constant velocity universal joint is fixed to the tooth portion 2. The other end is provided with a flange portion 3 for pressing and fixing the steel pipe.

As the graphite steel, one having a graphite particle size of 15 μm or less is used. This type of graphite steel can be produced, for example, by the method disclosed in Japanese Patent Application Laid-Open No. 8-283847. That is, after hot rolling, the cooling start temperature is set to A _r1.
Temperature, the cooling end temperature is below the Ms point, and the average cooling rate is 3
After water cooling at 0 to 100 ° C./s, further natural cooling is performed,
Next, after a graphitization treatment at a heating temperature of 600 to 720 ° C,
A steel bar is formed by performing wire drawing, drawing, or extrusion with a surface reduction rate of 30% or more.

In the above process, the cooling start temperature measured on the surface of the steel bar is set to A _r1 because the martensitic transformation strain and the rolling strain are simultaneously generated to increase the number of graphite forming sites.
Must be above the point. The cooling end temperature must be below the Ms point in order to obtain a martensitic transformation structure and facilitate graphite formation. Set the lower limit of the average cooling rate to 30
° C / s is for obtaining a martensitic transformation structure and for facilitating graphitization by retaining working strain. This is because site transformation does not increase. Annealing temperature 6
The reason for setting the temperature to 00 to 720 ° C. is that the graphitization time in this temperature range is the shortest. After graphitization, wire drawing is performed to ensure the roundness and the specified strength of the steel bar, to decompose the graphite, and to reduce the pores generated during quenching and tempering after cold forging. , In order to improve toughness. Particularly in cold forging, an undeformed portion called dead metal occurs. In this undeformed portion, graphite is not decomposed, the pores generated by quenching and tempering after cold forging are large, and the toughness is low. Therefore, it is necessary to decompose the graphite by wire drawing or the like before cold forging. In this case, if the area reduction ratio is smaller than 30%, the graphite is not sufficiently decomposed, so the graphite after cold forging is required. Voids generated by quenching and tempering are large, and the toughness value does not improve.

The graphite steel bar thus obtained is
After being formed into the stub shape by cold forging or the like, induction hardening is performed. The induction hardening is performed on the region A including the serration portion 2 and reaching the flange portion 3.
Due to induction hardening, the surface layer in the area A of the stub 1 is 50H
Hardened to more than RC. At this time, the thermal influence of the induction hardening is exerted on the core, and a two-phase structure of ferrite and martensite is generated in the core. It is preferable that the quenching be performed in two steps because the core has a thermal effect. However, even with one quenching, heating is performed at a low frequency, for example, in the case of a high frequency, the heating time is increased. By means such as increasing the time until cooling (delay time),
A phase structure can be formed.

After the induction hardening, the stub 1 is commercialized by performing tempering and, if necessary, finishing such as grinding.

The present invention is not limited to the pressing stub 1 described above.
The present invention can be widely applied to a power transmission shaft using a constant velocity universal joint, such as a welding stub or a shaft (whether hollow or solid) connected to the constant velocity universal joint.

[0026]

EXAMPLES The following tests were performed to confirm the effects of the present invention.

First, graphite steel equivalent to S53C having a material diameter of φ30 (C: 0.53%, Si: 1.2%, Mn: 0.4
%, P: 0.010%, S: 0.015%, Al: 0.
No. 03%, B: 0.0018%, N: 0.0055%) at both ends of a 170 mm long shaft for torque transmission. P. = 32/64, number of teeth N = 30, a spline for fitting is provided, and the shaft is machined into a shape with a stepped notch of φ20 and shape coefficient α = 1.33 at the center, and then quenched by high frequency heating. did. As a comparative example, S53C (C:
0.53%, Si: 0.25%, Mn: 0.75%,
P: 0.015%, S: 0.017%, Al: 0.02
Shafts of the same shape were formed using carbon steel (5%, Cr: 0.10%) and then quenched by the same quenching method. All samples were heat-treated so that the surface hardness was 58 to 62 HRC and the quenching depth to the effective hardened layer depth was 2.5 mm.
At this time, the hardness of the shaft core was about 25 HRC for graphite steel and about 18 HRC for carbon steel. However, the metal structure of the core was a structure containing ferrite and martensite for S53C equivalent graphite steel, and a structure containing ferrite and pearlite for S53C carbon steel.

This sample was subjected to a torsional strength test. In the static torsion test, both the graphite steel and the carbon steel had the same strength, but in the double torsion test by repetition, the graphite steel increased the strength by 5% or more than the carbon steel.

In a similar shape, graphite steel equivalent to S45C (C: 0.45%, Si: 1.41%, Mn: 0.31%)
%, P: 0.015%, S: 0.010%, Al: 0.
027%, B: 0.0014%, N: 0.005%)
And S45C carbon steel (C: 0.45%, Si: 0.20
%, Mn: 0.9%, P: 0.016%, S: 0.01
5%, Al: 0.025%, Cr: 0.10%). Similarly, quenching was performed by high-frequency heating. In this case, heat treatment was performed so that the surface hardness was 56 to 61 HRC and the quenching depth to the effective hardened layer depth was 4.0 mm. At that time, the hardness of the shaft core was 28% for graphite steel.
About HRC and about 12 HRC for carbon steel. The metal structure of the core was a structure containing ferrite and martensite for S45C equivalent graphite steel, and a structure containing ferrite and pearlite for S45C carbon steel.

When a torsional strength test was carried out on this sample, the graphite steel and the carbon steel had the same strength in the static torsion test, but in the double torsional torsion fatigue test by repetition, the graphite steel was at least 12% stronger than the carbon steel. Up. In particular, in the low load region (high cycle fatigue), the strength increased by 15 to 20%.

Further, graphite steel equivalent to S45C was processed into a similar shape, and twice hardened so that the surface hardness was 56 to 61 HRC and the quenching depth to the effective hardened layer depth was 4.0 mm. The hardness and the metal structure of the core at this time were about 28 HRC, similar to the above-described S45C-equivalent graphite steel, and were a structure containing ferrite and martensite. In addition, the variation in the hardness of the surface layer is 2 in Vickers hardness.
It was within 00Hv.

When a torsional strength test was performed on this sample, the static torsional strength was increased by 10% with respect to the once quenched graphite steel. In the double torsional torsion fatigue test, the strength was increased by 12% or more than that of the carbon steel as in the case of the once quenched graphite steel. Especially in the low load region (high cycle fatigue)
Strength increased by 20%.

Measurement of surface compressive stress and double torsion of S45C-equivalent graphite steel, its water-quenched product (using a coolant of water + 15% water-soluble coolant) and shot-peened product (shot-peening treatment after induction hardening) When a torsional fatigue (high cycle fatigue) test was performed, the surface compressive stress was 50 kgf / mm ² for S45C equivalent graphite steel,
The water-quenched product was 65 kgf / mm ² , and the shot-peened product was 97 kgf / mm ² . The torsional torsional fatigue was increased by 9% for the water-quenched product and by 20% for the shot-peened product, based on S45C graphite steel.

[0034]

As described above, according to the present invention, machinability,
It is possible to provide a high-strength power transmission shaft having excellent machinability such as cold / warm forgeability and excellent static strength and fatigue strength.

[Brief description of the drawings]

FIG. 1 is a side view showing a stub of a constant velocity universal joint as an example of a power transmission shaft.

[Explanation of symbols]

 1 Stub 2 Tooth (serration) 3 Flange

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C21D 9/28 C21D 9/28 Z C22C 38/06 C22C 38/06 38/12 38/12 (72) Invention Person Hiroaki Makino 1578 Higashikaizuka, Iwata-shi, Shizuoka F-term in NTN Corporation (reference) 4K042 AA14 AA25 BA01 BA03 BA05 BA09 CA02 CA08 CA10 DA01 DB01 DC05

Claims (7)

[Claims] In a power transmission shaft using a constant velocity universal joint, graphite steel is induction hardened to harden a surface layer, and a two-phase structure of ferrite and martensite is formed in a core portion. Power transmission shaft. 2. The graphite steel in weight%, C: 0.35
0.70%, Si: 0.4 to 2.0%, Mn: 0.3 to
1.5%, S: 0.025% or less, P: 0.02% or less, Al: 0.01 to 0.1%, B: 0.001 to 0.
The power transmission shaft according to claim 1, wherein 004%, N: 0.002 to 0.008% are basic components, and the balance is Fe and unavoidable impurities. 3. The power transmission shaft according to claim 2, wherein one or two types of Ni: 0.3 to 1.0% by weight and Mo: 0.2% by weight or less are added to the graphite steel. 4. The difference between the maximum value and the minimum value of the surface hardness is 20
2. The power transmission shaft according to claim 1, wherein the power transmission shaft is 0 Hv or less. 5. The power transmission shaft according to claim 1, wherein the core has a hardness of 25 to 45 HRC. 6. A compressive residual stress on a surface of 60 kgf / mm.
The power transmission shaft according to claim 1, wherein the number is ^two or more. 7. The power transmission shaft according to claim 1, wherein the compressive residual stress on the surface is increased to 90 kgf / mm ² or more by shot peening.