JPH10195589A - High torsional fatigue strength induction hardened steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an induction hardened steel material having excellent torsional fatigue strength as a shaft part and having excellent manufacturability such as cold workability when manufactured. SOLUTION: By weight ratio, C: 0.35 to 0.6%, S
i: 0.01 to 0.15%, Mn: 0.2% to 1.6
%, S: 0.005 to 0.15%, Al: 0.01 to
0.06%, Ti: 0.005 to 0.05%, B: 0.
0005-0.005%, N: 0.0015-0.00
8%, and if necessary, specific amounts of Cr and M
o, Ni, Nb, V, and a composition containing one or more of them, and the ratio of the hardened layer depth to the component radius is 0.3 to 0.1.
6, and the projected core hardness is HV400 or more, or the ratio of the hardened layer depth to the part radius is 0.4 to 0.75 and the ratio of the projected core hardness to the hardened layer hardness is 0.56 or more Or a high torsional fatigue strength induction hardened steel material having an average hardness in the cross section of HV560 or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the invention of a high torsional fatigue strength induction hardened steel material.
(A) to (C), having excellent torsional fatigue strength as a shaft component constituting a power transmission system of an automobile, such as a shaft having a spline portion, a shaft with a flange, a shaft with an outer cylinder, and the like. The present invention relates to an invention of an induction hardened steel material having excellent manufacturability such as cold workability during manufacturing.

[0002]

2. Description of the Related Art Shaft components constituting a power transmission system of an automobile are usually manufactured by forming medium carbon steel into a predetermined component and performing induction quenching and tempering. The trend toward improving the torsional fatigue strength has been strong in response to environmental regulations and environmental regulations. On the other hand, in the production of automobile parts, there is a strong tendency to improve manufacturability such as cold workability in order to reduce manufacturing costs.

On the other hand, Japanese Patent Publication No. 63-62571 discloses C: 0.30 to 0.38%, Mn: 0.6 to 1.0.
5%, B: 0.0005-0.0030%, Ti: 0.
Manufacture of a drive shaft in which steel consisting of 01 to 0.04% and Al: 0.01 to 0.04% is formed into a drive shaft and the ratio of the induction hardening depth to the steel member radius is 0.4 or more by induction hardening. The method is shown. Although the invention material refers to static torsional strength,
No mention is made of torsional fatigue strength.

The controlling factor is different between static torsional strength, which is a material resistance to a static load, and torsional fatigue strength, which is a material resistance to a repeated load, and they are different characteristics. Further, in the present invention, no consideration is given to cold workability. Therefore, at present, the material is not necessarily applied to parts requiring cold workability and torsional fatigue properties.

In Japanese Patent Publication No. 38847/1989, C: more than 0.35 to 0.65%, Si: 0.15% or less,
Mn: 0.60% or less, B: 0.0005 to 0.005
0%, Ti: 0.05% or less, Al: 0.015-0.
There is disclosed a method for manufacturing a component for a machine structure, which comprises performing cold forging and then induction hardening to manufacture a component for a machine structure using 050% steel as a material. From Table 1 on pages 3 and 4 of the publication, the maximum amounts of Ti and N added are Ti: 0.04% and N: 0.014%. The cold workability of this steel is not always sufficient. In addition, in the invention, as is apparent from the same publication, page 4, right column, line 16, and table 3, the maximum value of the hardened layer depth is 3 mm for a material having a diameter of 25 mm, that is, the hardened layer depth t is Radius ratio t / r
Is at most 0.24, which is extremely shallow. Further, the publication does not describe the torsional strength and the torsional fatigue strength, and the level at which the strength is achieved is unknown. That is, the invention does not disclose any technique regarding induction hardened steel having excellent torsional fatigue strength.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide an induction hardened steel material having excellent torsional fatigue strength as a shaft part and having excellent manufacturability such as cold workability at the time of its manufacture. Is what you do.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in order to realize an induction hardened steel material having excellent cold workability and excellent torsional fatigue strength as a part at the time of its manufacture. Obtained knowledge.

(1) The following method is effective for ensuring cold workability. 1) Reduce Si and P which are solid solution hardening elements. 2) Hardenability is ensured mainly by adding B.

(2) Further, in order to ensure cold workability, it is essential to adjust the amount of N. In order to bring out the above-described effect of improving the hardenability of B, it is necessary to reduce solid solution N. As disclosed in Table 1 on pages 3 and 4 of JP-B 1-38847, the amount of N added is 0.01 at the maximum.
Addition of a large amount such as 4% causes the following adverse effects in addition to the above. 1) TiN precipitates in the cooling process of the steel bar rolling before the cold working or in the cooling process of the softening annealing, and in the steel with a large amount of N added, the hardness hardens due to precipitation hardening. 2) Precipitation of a large amount of TiN significantly deteriorates machinability and causes cracking during cold working such as rolling.
In the case of high N steel, cold workability is significantly deteriorated.

The reason that the cold workability of the technique disclosed in Japanese Patent Publication No. 1-38474 is not necessarily sufficient is considered to be due to the adverse effect of such a large addition of N on the cold workability.
In order to suppress the adverse effects of TiN on the cold workability and to obtain the effect of improving the hardenability of B, N: 0.0
It is necessary to control in the range of 015 to 0.008%.

(3) Next, torsional fatigue fracture of induction hardened steel occurs in the following process. A. Cracks occur at the boundary between the surface or the hardened layer and the core. B. The crack propagates initially in a plane parallel or perpendicular to the axial direction. This is hereinafter referred to as mode III destruction. C. After the mode III fracture, a brittle fracture occurs with a grain boundary crack at a plane of 45 degrees in the axial direction, and a final fracture occurs. This is hereinafter referred to as mode I destruction.

(4) The torsional fatigue fracture process The mode III fracture described in the column of "B." is a ductile fracture with a dimple pattern. When a large number of precipitates such as TiN are present, this becomes a core of the ductile fracture. Mode III destruction is likely to occur.

As disclosed in Japanese Patent Publication No. 38847/1989, the maximum amount of Ti and N added is 0.04% for Ti and 0.
Boron steel containing 014% is liable to cause ductile fracture with TiN as a nucleus. One of the reasons why the technique disclosed in Japanese Patent Publication No. 1-38474 is not widely used is considered to be the cause. Therefore, from the viewpoint of improving the mode III fracture strength, it is necessary to regulate the N amount to a range of 0.0015 to less than 0.008%.

(5) Tear fatigue fracture process In order to suppress the brittle fracture mode I accompanied by grain boundary cracking at a plane of 45 degrees in the axial direction, as described in the column of "C.", the following method is used. Grain boundary strengthening is effective. 1) Addition of B. B is due to the effect of driving out grain boundary segregation P from the grain boundaries. 2) Reduction in the amount of P, Cu and O, which are grain boundary segregation elements. 3) Reducing the amount of TiN grain boundary precipitation by optimizing the amounts of Ti and N.

(6) In order to suppress the brittle fracture mode I accompanied by grain boundary cracking, the size is further increased by adding the following method in addition to the above. 1) Strengthening of grain boundaries by adding Cr, Mo, Ni, Nb, and V. 2) Fine graining of prior austenite grain size.

(7) If the material hardness is reduced with emphasis on the cold workability, the hardness of the core usually becomes the hardness of the core, so that the hardness of the core decreases. When the core hardness is low and when the depth of the hardened layer is shallow, it becomes an internal starting point. In the case of the internal starting point, the torsional fatigue strength increases as the depth of the hardened layer increases and as the core hardness increases.

FIG. 2 is a schematic diagram showing the relationship between the depth of the hardened layer and the hardness of the core which affect the tear fatigue strength. In FIG.
When the core hardness is increased from (a) to (b), the starting point shifts from A to B, and the strength is improved.
This is equivalent to the case where the depth of the hardened layer is increased from (a) to (c) and the starting point shifts from A to C. Therefore, the core hardness Hco
As a new index capable of simultaneously describing the effects of both re and the hardened layer depth t / r (effective hardened layer depth t, component radius r), the projected core hardness was defined by the following equation. FIG. 3 shows the 1 × 10 ⁵ torsional fatigue strength of the internal starting material as measured by the projected core hardness Hp−.
Although they are arranged by core, there is a good correlation between the two. In order to make the 1 × 10 ⁵ torsional fatigue strength 600 MPa or more, the projected core hardness Hp-core can be achieved at 400 or more.

Definition of projection core hardness: effective hardened layer depth t,
When the component radius is r and the core hardness is Hcore, the projected core hardness is Hp-core = Hcore / (1-t)
/ R) (8) In order to achieve even better torsional fatigue strength,
The point is to move the fracture origin from the inside to the surface.
From FIG. 2, it is considered that Hp-core / Hcase is 1 or more, which is the surface starting point, but it is actually different. FIG. 4 shows the fracture origin, Hp-core / Hcase, and the number of repetitions N.
This shows the relationship. Hp-core / Hcas
When e is approximately 0.56 or more, it becomes the surface starting point.

(9) In the case of the starting point of the surface, the surface is softened during the fatigue process, and the soft core portion is originally work-hardened. In other words, micro plastic deformation progresses from the surface to the inside during the fatigue process, and the tear fatigue strength of the surface starting material is affected by the entire hardness distribution in the cross section. The average hardness Hav in the cross section was defined by the following equation as the average of the hardness in the cross section.

FIG. 5 shows the 1 × 10 ⁵ torsional fatigue strength of the surface starting material arranged in terms of the projected core hardness Hav. There is a good correlation between the two. In order to increase the torsional fatigue strength of 1 × 10 ⁵ times to 650 MPa or more, the average hardness in the cross section H
av can be achieved with 560 or more.

Definition of average hardness in cross section: The cross section of radius a is divided concentrically into N rings in the radial direction, the hardness of the n-th ring portion is H _n , the radius is r _n , and the interval is △ when the r _n,

[0022]

(Equation 1) The present invention has been made based on the above novel findings, and the gist of the present invention is as follows.

(1) According to the first and fourth aspects of the present invention, C: 0.35 to 0.60%, S
i: 0.01 to 0.15%, Mn: 0.2 to 1.60
%, S: 0.005 to 0.15%, Al: 0.010 to
0.06%, Ti: 0.005 to 0.050% B: 0.
0005-0.005%, N: 0.0015-0.00
8%, and optionally Cr: more than 0.1
1.2%, Mo: 0.02 to 0.8%, Ni: 0.1 to
3.5% Nb: 0.01-0.3% V: 0.03-0.
It contains 6% of one or more and P: 0.
020% or less, Cu: 0.05% or less, O: 0.002
5% or less, the balance being iron and unavoidable impurities, and the ratio of the effective hardened layer depth t to the part radius r, t /
A high torsional fatigue strength induction hardened steel material, wherein r is 0.3 to 0.6 and a projected core hardness Hp-core defined below is HV400 or more.

Definition of projected core hardness: effective hardened layer depth t,
When the component radius is r and the core hardness is Hcore, the projected core hardness is Hp-core = Hcore / (1-t)
/ R) (2) In the invention of claims 2 and 5 of the present invention, C: 0.35 to 0.60%, Si: 0.01 to
0.15%, Mn: 0.2 to 1.60%, S: 0.00
5 to 0.15%, Al: 0.010 to 0.06%, T
i: 0.005 to 0.050% B: 0.0005 to 0.5%
005%, N: 0.0015 to 0.008%, and if necessary, Cr: more than 0.1 to l. 2%,
Mo: 0.02 to 0.8%, Ni: 0.1 to 3.5% N
b: 0.01 to 0.3% V: One or more of 0.03 to 0.6%, P: 0.020% or less, C
u: 0.05% or less, O: 0.0025% or less, the balance being iron and unavoidable impurities, the ratio t / r of the effective hardened layer depth t to the part radius being 0.4 to 0. .7
5, and a projection core hardness Hp-c defined below.
The ratio Hp-core / H between ore and the hardness of the hardened layer Hcase
A high-frequency fatigue strength induction hardened steel material having a case of 0.56 or more.

Definition of projected core hardness: effective hardened layer depth t,
When the component radius is r and the core hardness is Hcore, the projected core hardness is Hp-core = Hcore / (1-t)
/ R) (3) According to the third and sixth aspects of the present invention, the weight ratio of C: 0.35 to 0.60%, Si: 0.01
0.15%, Mn: 0.2-1.60%, S: 0.0
05-0.15%, Al: 0.010-0.06%, T
i: 0.005 to 0.050% B: 0.0005 to 0.5%
005%, N: 0.0015 to 0.008%, and if necessary, Cr: more than 0.1 to l. 2%,
Mo: 0.02 to 0.8%, Ni: 0.1 to 3.5% N
b: 0.01 to 0.3% V: One or more of 0.03 to 0.6%, P: 0.020% or less, C
u: 0.05% or less, O: 0.0025% or less, the balance being iron and unavoidable impurities, the ratio t / r of the effective hardened layer depth t to the part radius being 0.4 to 0. .7
5, and a projection core hardness Hp-c defined below.
The ratio Hp-core / H between ore and the hardness of the hardened layer Hcase
A high torsional fatigue strength induction hardened steel material having a case of 0.56 or more and an average in-section hardness Hav defined below as HV 560 or more.

Definition of projection core hardness: effective hardened layer depth t,
When the component radius is r and the core hardness is Hcore, the projected core hardness is Hp-core = Hcore / (1-t)
/ R) Definition of average hardness in cross section: A cross section of radius a is divided concentrically into N rings in the radial direction, the hardness of the n-th ring-shaped portion is H _n , the radius is r _n , and the interval is △ when the r _n,

[0027]

(Equation 1) (4) The invention according to claims 7 and 8 of the present invention provides the high-torsional fatigue strength induction-hardened steel material according to any one of claims 1 to 3, wherein the prior-austenite grain size of the induction-hardened layer is 9 or more. A high torsional fatigue strength induction hardened steel material according to any one of claims 4 to 6.

[0028]

Embodiments of the present invention will be described below.

First, the reason why the content range of the component of the present invention is limited as described above will be described.

C: 0.35 to 0.60%, C is an element effective for increasing the hardness of the induction hardened layer, but if it is less than 0.35%, the hardness is insufficient. If it exceeds 0.60%, the hardness before induction hardening becomes too hard and the cold workability deteriorates, and carbide precipitation at the austenite grain boundaries becomes remarkable, deteriorating the grain boundary strength. It was set to 0.35 to 0.60%.

Si: 0.01 to 0.15%, Si is added as a deoxidizing element and for the purpose of strengthening grain boundaries. However, if the content is less than 0.01%, the effect is insufficient. On the other hand, since Si increases the material hardness by solid solution hardening, addition of more than 0.15% deteriorates cold workability such as machinability at a stage before induction hardening. For the above reasons, the content is set to 0.01 to 0.15%.

Mn: 0.20 to 1.60%, Mn is (1) improved hardenability and (2) refinement of austenite grains during induction hardening heating by forming MnS in steel and (3) ) Added for the purpose of improving machinability. However, if less than 0.20%, this effect is insufficient. On the other hand, when Mn is excessively added, the pearlite fraction of the raw material before induction hardening is increased to increase the raw material strength and deteriorate the cold workability. In particular, this tendency becomes remarkable when the addition exceeds 1.60%. For the above reasons, M
The content of n was set to 0.20 to 1.60%. In the case of a steel material in which the cold workability is more important, Mn: 0.
It is desirable to limit to the range of 20 to 1.00%.

S: 0.005 to 0.15% S forms MnS in steel, and is added for the purpose of miniaturizing austenite grains and improving machinability during induction hardening heating. If less than 0.005%, the effect is insufficient. On the other hand, if the content exceeds 0.15%, the effect is saturated, and rather, grain boundary segregation is caused to cause grain boundary embrittlement. For the above reasons, the content of S is set to 0.005 to 0.15%.

Al: 0.010% to 0.06%, Al is added as a deoxidizing element and a crystal grain refining element. If less than 0.010%, the effect is insufficient. On the other hand, 0.06% Is exceeded, the effect is saturated, and rather the mode III breaking strength in the final part is deteriorated. Therefore, the content is set to 0.010 to 0.06%.

Ti: 0.005 to 0.050%, Ti combines with N in steel to form TiN. The purpose is to prevent BN precipitation by completely fixing solid solution N, that is, to secure solid solution B. Add as Furthermore, the addition of Ti also contributes to the refinement of the surface hardened layer. However, 0.005
%, The effect is insufficient, while 0.05%
If the content exceeds 0.005%, cracking during cold working due to a large amount of TiN or TiC and deterioration of the mode III fracture strength in the final part are caused.
And In order to further improve the cold workability and the high torsional fatigue strength characteristics, desirably, Ti: 0.00
It is desirable to limit the range to 5 to 0.030%.

B: 0.0005 to 0.005% B is added for the purpose of increasing the hardenability by segregating at the austenite grain boundaries in the solid solution state and increasing the hardenability. at the same time,
There is also an effect of increasing grain boundary strength by driving out grain boundary impurities such as P and Cu from the grain boundaries. Grain boundary strengthening increases torsional strength and torsional fatigue strength. However,
If the content is less than 0.0005%, the effect is insufficient. On the other hand, excessive addition exceeding 0.005% rather causes grain boundary embrittlement, so that the content is 0.0005 to 0.005%.
And

N: 0.0015 to 0.008% N is added for the purpose of refining austenite grains during high frequency heating by precipitation of carbonitride such as AlN.
If it is less than 015%, the effect is insufficient.
If the content exceeds 008%, BN precipitates to cause a reduction in solid solution B, and also causes a large amount of TiN precipitation to cause cold work cracking and deterioration of the mode III fracture strength in the final part. 0015-0.008
%. In order to further improve the cold workability and high torsional fatigue strength characteristics, it is desirable that N: 0.00
It is desirable to limit to the range of 15 to 0.005%.

P: 0.020% or less (including 0%), P increases the hardness of the material by solid solution hardening, and deteriorates cold forgeability at a stage before induction hardening. Further, segregation occurs at the austenite grain boundaries, which lowers the strength of the grain boundaries to cause brittle fracture under torsional stress, thereby lowering the strength. In particular, when P exceeds 0.020%, the strength is significantly reduced, so 0.020% was made the upper limit. In addition,
In order to further strengthen the grain boundary, the content is preferably 0.015% or less.

Cu: 0.05% or less (including 0%), Cu also causes grain boundary segregation at austenite grain boundaries like P, causing a decrease in strength. In particular, when Cu exceeds 0.05%, the strength is significantly reduced, so 0.05% was made the upper limit.

O: 0.0025% or less (including 0%), O causes grain boundary segregation to cause grain boundary embrittlement, and forms hard oxide-based inclusions in the steel, and under torsional stress. It causes brittle fracture and lowers the cost, causing a reduction in strength. In particular, when O exceeds 0.0025%, the strength decreases remarkably,
The upper limit is 0.0025%.

Next, the invention steel according to claims 4, 5, 6, and 8 is as follows:
It is a steel in which the addition of Cr, Mo, Ni, Nb, and V has increased grain boundary strength and improved hardenability. Cr:
More than 0.1 to 1.2%, Mo: 0.02 to 0.80%, N
i: 0.1 to 3.50%, Nb: 0.01 to 0.3%,
V: 0.03 to 0.6%, all of these elements are added for the purpose of increasing grain boundary strength and improving hardenability by refining grain boundary carbides precipitated at austenite grain boundaries. Ni also has the effect of improving the toughness near the grain boundaries and suppressing brittle fracture. Also, N
b and V form carbonitrides in steel and also have the effect of making austenite grains fine during high-frequency heating. These effects are as follows: Cr: 01% or less; Mo: less than 0.02%;
i: less than 0.1%, Nb: less than 0.01%, V: 0.0
Less than 3% is insufficient. On the other hand, Cr: more than 1.2%,
Mo: more than 0.80%, Ni: more than 3.50%, Nb: 0.
Above 3%, V: above 0.6%, these effects are saturated,
Rather, excessive addition of these elements causes deterioration of cold workability. For the above reasons, the content was limited to the above ranges.

Next, in the first and fourth aspects, the induction hardened steel material is composed of the above components, the ratio t / r of the effective hardened layer depth t to the component radius r is set to 0.3 to 0.6, and The hardness Hp-core defined by the equation (1) is set to HV400 or more, and the reason for the limitation is described below.

The effective hardened layer depth t referred to in the present invention is JI
This is the effective hardened layer depth based on the induction hardened hardened layer depth measurement method specified in SG0559. Claims 1 and 4
Is an invention which aims to improve the torsional strength in the case of an internal starting point. When the effective hardened layer depth t / r exceeds 0.6, the starting point becomes the surface starting point, and the factor controlling tear fatigue strength is different. On the other hand, when t / r is less than 0.3, the effect of improving torsional fatigue strength is small. For the above reasons, the effective hardened layer depth t / r is 0.3
Limited to the range of 0.6. Next, the tear fatigue strength of the internal starting material increases in proportion to the projection core hardness Hp-core as shown above and in FIG. In order to set the time strength at 1 × 10 ⁵ times to 600 or more, it is necessary to set the projection core hardness to HV400 or more, and if it is less than that, the torsional fatigue strength is insufficient. For the above reasons, the projection core hardness Hp-core is set to HV400 or more. In order to make the time intensity at 1 × 10 ⁵ times, which is a higher intensity level at the internal starting point, 650 or more, the hardness of the projection core is preferably HV440 or more.

In the second and fifth aspects, the induction hardened steel material is composed of the above-mentioned components, the ratio t / r of the effective hardened layer depth t to the component radius r is 0.4 to 0.75, and Projection core hardness Hp-core and hardened layer hardness Hc defined above
The ratio Hp-core / Hcase of case is 0.56 or more, and the reason for such limitation is described below.

Claims 2 and 5 are steel materials aiming at a higher torsional fatigue strength level than claims 1 and 4. The ratio t / r of the effective hardened layer depth t to the component radius r is 0.4 to 0.5.
The reason for 75 is that the tear fatigue strength of the induction hardened material is improved as the induction hardened depth is increased. However, when the effective hardened layer depth is less than 0.4 at t / r, the effect of improving the torsional fatigue strength is reduced. If it is small and exceeds 0.75, the compressive residual stress of the surface layer is decreased, and the risk of occurrence of burning cracks in the shaft part manufacturing process is increased. Next, as is apparent from FIG.
The torsional fatigue strength is improved when the fatigue fracture starting point is on the surface than inside. Whether it becomes the surface starting point or the internal starting point is determined by the ratio H between the projected core hardness Hp-core and the hardened layer hardness Hcase.
Depends on p-core / Hcase. As shown in FIG. 4, when Hp-core / Hcase is 0.56 or more, the surface becomes the starting point. In the present invention, the projection core hardness Hp-core
Of the hardness of the hardened layer Hcase and the ratio Hp-core / Hcas
The reason why e is limited to the range of 0.56 or more is as described above.

Next, in the third and sixth aspects, the induction hardened steel material is composed of the above components, the ratio t / r of the effective hardened layer depth t to the component radius r is 0.4 to 0.75, and Projection core hardness Hp-core and hardened layer hardness Hc defined above
ratio Hp-core / Hcase is 0.56 or more, and the average hardness Hav in the cross section as defined above.
Is HV560 or more, but the reason for such limitation is described below.

Claims 3 and 6 are inventions for improving tear strength in the case of a surface starting point, and are steel materials aiming at a higher torsional fatigue strength level than claims 2 and 5. The ratio t / r of the effective hardened layer depth t to the component radius r is 0.4 to 0.5.
75 and the Hp-core / Hcase is set to 0.
The reason for limiting to the range of 56 or more is the same as in claims 2 and 5 above.

Next, as shown in FIG. 5 and FIG. 5, the torsional fatigue strength of the surface starting material is calculated from the projected core hardness Hp-core.
It increases in proportion to Time intensity at 1 × 10 ⁵ times is 65
In order to set the average hardness Hav in the cross section to HV5
It is necessary to be 60 or more, and if it is less than 60, the torsional fatigue strength is insufficient.

For the above reasons, the average hardness HaV in the cross section was HV560 or more. Note that the time intensity at a higher intensity level of 1 × 10 ⁵ times at the surface starting point was 700 times.
In order to achieve the above, the average hardness Hav in the cross section is set to HV56.
It is desirable to be 0 or more.

Next, claims 7 and 8 are induction hardened steel materials in which austenite grains at the time of high frequency heating are refined in a single layer to increase the strength by preventing grain boundary destruction. In the present invention, the reason why the former austenite crystal grain size of the induction hardened layer of the induction hardened steel material is 9 or more is that brittle fracture due to grain boundary fracture is suppressed by refining the former austenite grain boundary of the induction hardened layer. If the particle size is less than 9, this effect is small.

Next, a method for producing the steel material of the present invention will be described.

In the induction hardened steel material of the present invention, the induction hardening conditions and tempering conditions for production are not particularly limited, and any conditions may be used as long as the requirements of the present invention are satisfied.
For example, if the requirements of the present invention are satisfied, tempering may not be performed. In the present invention, if the requirements of the present invention are satisfied, heat treatments such as normalizing, annealing, spheroidizing annealing, and quenching-tempering can be performed as necessary before induction hardening. When normalizing, annealing, and spheroidizing annealing are not performed before induction hardening, the steel material is manufactured by hot rolling at a finishing temperature of 700 to 900 ° C.
Average cooling rate in the temperature range of 500 ° C: 0.1 to 1.7 ° C
/ Second is desirable. However, the present invention is not particularly limited.

In the steel material of the present invention, one or two of Ca and Pb can be contained as required for the purpose of improving machinability. In addition, Ca not only improves machinability, but also has the effect of forming phosphide by combining with P in steel, reducing the amount of segregation of P at the grain boundary and increasing the grain boundary strength. Ca, P
The appropriate addition range of b is as follows. Ca: 0.000
5 to 0.010%, Pb: 0.05 to 0.5% In the present invention, a large compressive residual stress is applied to the surface of the induction hardened shaft part, thereby suppressing brittle fracture and further increasing the strength. Can also be planned. By setting the residual stress on the poor surface of the induction hardened steel material to −80 kgf / mm ² or less, brittle fracture is suppressed and torsional fatigue strength is significantly improved. The application of the compressive residual stress to the induction hardened steel material is performed after induction hardening and tempering, and after the arc height l. 0mm
Hard shot peening at an intensity of A or more is effective. Here, the arc height is, for example, “Automotive technology, Vol. 41, No. 7, 1987, 726-72.
Page 7 "is an index of shot peening strength. However, in the present invention, the conditions for applying the compressive residual stress are not particularly limited, and any conditions may be used as long as the requirements of the present invention are satisfied.

One of the causes of the occurrence of cracks in the process of torsional fatigue is uneven hardness of the hardened layer. The target parts of the present invention are:
Cold working while hot rolling - besides When induction hardening, after a heat treatment of simple annealing or the like in the hot rolling after A ₃ following transformation point temperature, it may be cold worked over induction hardening . However, the structure that has undergone heat treatment such as simple annealing after hot rolling is greatly affected by the structure of the rolled material. Therefore, even when such a heat treatment is performed after hot rolling, it is important to optimize the texture of the rolled material in order to suppress the unevenness in hardness of the hardened layer during induction hardening. If the structure of the rolled material has a ferrite fraction of more than 35% and a ferrite crystal grain size of more than 30 μm, the hardened layer will have remarkable unevenness in hardness, which will easily cause torsional fatigue failure. Therefore, the structure fraction of ferrite in the structure of the rolled material is 35% or less, and the ferrite crystal grain size is 30% or less.
It is desirable that the thickness be not more than μm. However, in the present invention, the present tissue factor is not particularly limited.

[0055]

EXAMPLES The effects of the present invention will be more specifically described below with reference to examples. (Example-1) Tables 1 and 2 show examples of the first invention and the seventh invention of the present application.

[0056]

[Table 1] A steel material having the composition shown in Table 1 was rolled into a bar of 40 mmφ. From this steel bar, a test piece for drilling for evaluation of machinability, a torsion test piece, and a test piece for evaluating susceptibility to quench cracking were collected.

Here, one of the features of the present invention is that the cold workability at the stage before induction hardening is excellent. Cold workability means machinability (cutability), rollability,
Although there are cold forgeability and the like, generally there is a correlation between them, and if the machinability is excellent, the rollability and the cold forgeability are also excellent. Therefore, in the present application, evaluation of cold workability is represented by evaluation of machinability by a drill. The evaluation of the machinability by the drill was performed by changing the peripheral speed of the drill (material: SKH51-φ10 mm) at a feed rate of 0.33 mm / s and calculating the total hole depth at which the drill could not be cut at each speed. create a circumferential speed Doriru life curve, the maximum speed at which the drill life is l000mm defined as V _L1000, was criteria machinability. Table 1 also shows the evaluation results of _VL1000 . The steel of the present invention has relatively excellent machinability as compared with the comparative steel having the same carbon content.

On the other hand, the comparative steel material 11 is a case where the S content is lower than the range of the present invention, and the comparative steel materials 7, 8, 1
0, 13, and 15 represent C, Si, Mn, Al, and T, respectively.
This is the case where the content of i exceeds the range of the present invention, and all of these steel materials have relatively poor machinability as compared with other steel materials having the same carbon content.

Next, from the steel materials shown in Table 1, the parallel portion was 20 mm.
φ torsion test pieces were prepared. Induction hardening was performed under the condition of fixed hardening at a frequency of 10 KHz, and then tempering was performed at 170 ° C. × 1 hour. These samples were subjected to a torsional test and a torsional fatigue test. The torsional fatigue was evaluated by 1 × 10 ⁶ times strength. The hardness distribution was measured at the center of the parallel portion. Table 2 shows the evaluation results of the torsional strength and torsional fatigue strength of each steel material, together with the evaluation results of hardness and the like. The starting point of torsional fatigue fracture is an internal starting point. The effective hardened layer depth is the effective hardened layer depth based on the induction hardened hardened layer depth measuring method specified in JIS G 0559.

[0060]

[Table 2] As is clear from Table 2, in each of the examples of the present invention, the static torsional strength is 1580 MPa or more, and the torsional fatigue strength is 600 MPa.
a. Inventive Example 2, which has a γ-grain size of No. 9 or more, which is the seventh inventive example, shows particularly excellent strength characteristics.

On the other hand, Comparative Examples 6, 9, 14, and 16 are cases in which the contents of C, Mn, Ti, and B are respectively below the range of the present invention, and all of them have the same carbon content as other steel materials. In comparison, static torsional strength and torsional fatigue strength are relatively inferior. Comparative Examples 12 and 17 are the cases where the contents of S and B exceeded the range of the present invention, respectively, and in both cases, the static torsional strength and the torsional fatigue strength were relative to other steel materials having the same carbon content. Inferior. In Comparative Example 18, the components were within the scope of the present invention, but the depth of the hardened layer was less than the range of the present invention, and the static torsional strength and torsional fatigue strength were higher than those of other steel materials having the same carbon content. Is relatively inferior. In Comparative Example 19, the components were within the range of the present invention, but the projected core hardness was lower than the range of the present invention, and the static torsional strength and torsion were lower than those of other steel materials having the same carbon content. Fatigue strength is relatively poor. (Example-2) Tables 3 and 4 show examples of the second invention and the seventh invention of the present application.

[0062]

[Table 3] A steel material having the composition shown in Table 3 was prepared in the same procedure as in Example 1, and the static tear strength and torsional fatigue strength were evaluated under the same conditions. In addition, the machinability by a drill was evaluated as an index of the cold workability. The evaluation results are shown in Table 3, and the steel material of the present invention has relatively excellent machinability as compared with the comparative steel material having the same carbon content.

Next, Table 4 shows the evaluation results of the strength characteristics.
The starting point of the torsional fatigue fracture is the internal starting point in Comparative Example 7, and the other starting points are the surface starting points.

[0064]

[Table 4] As is clear from Table 4, in each of the examples of the present invention, the static tear strength was 1650 MPa or more, and the torsional fatigue strength was 680 MPa.
a. Inventive Example 3, which is a seventh invention example and has a γ grain size of No. 9 or more and is a high carbon steel, exhibits particularly excellent strength characteristics.

On the other hand, in Comparative Example 6, the components were within the range of the present invention, but the depth of the hardened layer was lower than the range of the present invention, and the static torsional strength was higher than other steel materials having the same carbon content. ,
The torsional fatigue strength is relatively poor. In Comparative Example 7, the components were within the range of the present invention, but the ratio of the projected core hardness to the surface hardness was lower than the range of the present invention, the starting point of torsional fatigue fracture was inside, and the same carbon content. As compared with other steel materials, the torsional fatigue strength is relatively inferior. (Example 3) Tables 5 and 6 show examples of the third invention and the seventh invention of the present application.

[0066]

[Table 5] A steel material having the composition shown in Table 5 was prepared in the same procedure as in Example 1, and the static torsional strength and the torsional fatigue strength were evaluated under the same conditions. In addition, the machinability by a drill was evaluated as an index of the cold workability. The evaluation results are shown in Table 5, and the steel material of the present invention is relatively excellent in machinability as compared with the comparative steel material having the same carbon content.

Next, Table 6 shows the evaluation results of the strength characteristics.
The starting point of the torsional fatigue fracture is the internal starting point in Comparative Example 7, and the other starting points are the surface starting points.

[0068]

[Table 6] As is clear from Table 6, in each of the examples of the present invention, the static torsional strength was 1650 MPa or more, and the torsional fatigue strength was 710 MPa.
a. The seventh invention example, invention example 3 having a γ grain size of No. 9 or more and a high carbon steel, shows particularly excellent strength characteristics.

On the other hand, in Comparative Example 6, the components were within the range of the present invention, but the hardened layer depth was lower than the range of the present invention, and the static torsional strength was higher than other steel materials having the same carbon content. ,
The torsional fatigue strength is relatively poor. In Comparative Example 7, the components are also in the range of the present invention, but the ratio of the projected core hardness to the surface hardness is lower than the range of the present invention, the starting point of torsional fatigue fracture is inside, and the same carbon The torsional fatigue strength is relatively inferior to that of other steel materials. Comparative Example 8
Although the components are also within the scope of the present invention, the average hardness in the cross section is lower than the range of the present invention, and the static torsional strength and the torsional fatigue strength are relative to other steel materials having the same carbon content. Is inferior to (Embodiment-4) Tables 7 and 8 show embodiments of the fourth and eighth inventions of the present application.

[0070]

[Table 7] A steel material having the composition shown in Table 7 was prepared in the same procedure as in Example 1, and the static torsional strength and the torsional fatigue strength were evaluated under the same conditions. In addition, the machinability by a drill was evaluated as an index of the cold workability. The evaluation results are shown in Table 7, and the steel material of the present invention is relatively superior in machinability as compared with the comparative steel material having the same carbon content.

Next, Table 8 shows the evaluation results of the strength characteristics.

[0072]

[Table 8] The starting point of torsional fatigue fracture is an internal starting point.

As is clear from Table 8, all of the examples of the present invention have excellent characteristics of static torsional strength of 1600 MPa or more and torsional fatigue strength of 600 MPa or more. The eighth invention example, invention example 3 and the like, in which the γ grain size is 9 or more and high carbon steel, shows particularly excellent strength properties. (Embodiment 5) Tables 9 and 10 show embodiments of the fifth and eighth inventions of the present application.

[0074]

[Table 9] A steel material having the composition shown in Table 9 was prepared in the same procedure as in Example 1, and the static torsional strength and the torsional fatigue strength were evaluated under the same conditions. In addition, the machinability by a drill was evaluated as an index of the cold workability. The evaluation results are shown in Table 9, and the steel material of the present invention is relatively superior in machinability as compared with the comparative steel material having the same carbon content.

Next, Table 10 shows the evaluation results of the strength characteristics.

[0076]

[Table 10] The starting point of the torsional fatigue fracture is a surface starting point.

As is clear from Table 10, all of the examples of the present invention have excellent characteristics of static torsional strength of 1600 MPa or more and torsional fatigue strength of 680 MPa or more. Eighth invention example, invention example 9 in which the γ grain size is 9 or more and high carbon steel
Others show particularly good strength properties. (Embodiment-6) Tables 11 and 12 show embodiments of the sixth and eighth inventions of the present application.

[0078]

[Table 11] A steel material having the composition shown in Table 11 was prepared in the same procedure as in Example 1, and the static torsional strength and the torsional fatigue strength were evaluated under the same conditions. In addition, the machinability by a drill was evaluated as an index of the cold workability. The evaluation results are shown in Table 11, and the steel material of the present invention is relatively excellent in machinability as compared with the comparative steel material having the same carbon content.

Next, Table 12 shows the evaluation results of the strength characteristics.

[0080]

[Table 12] The starting point of the torsional fatigue fracture is a surface starting point.

As is clear from Table 12, the present invention has excellent characteristics in which the static torsional strength is 1690 MPa or more and the torsional strength ratio is 690 MPa or more.

The eighth invention example, invention example 10 which is a high-carbon steel having a γ grain size of No. 9 or more, shows particularly excellent strength characteristics.

[0083]

As described above, the induction hardened steel material of the present invention has excellent torsional fatigue strength as a shaft part, and has excellent cold workability, that is, excellent manufacturability when manufactured.
The industrial effects of the present invention are very remarkable.

[Brief description of the drawings]

FIG. 1 (A) is a shaft having a seresin portion,
(B) is a diagram showing a shaft with a flange, and (C) is a diagram showing a shaft with an outer cylinder.

FIG. 2 is a diagram schematically showing the relationship between the depth of a hardened layer and the hardness of a core on the torsional fatigue strength.

FIG. 3 is a diagram showing a relationship between a 1 × 10 ⁵ torsional fatigue strength of an internal starting material and a projected core hardness Hp-core.

FIG. 4 is a diagram showing a relationship between a fracture starting point, Hp-core / Hcase, and the number of repetitions N.

FIG. 5 is a diagram showing a relationship between a 1 × 10 ⁵ torsional fatigue strength of a surface starting material and a projected core hardness Hav.

[Explanation of symbols]

 10 shaft, 11, 12 serration, 20, 21 shaft, 22 flange 30, 31, 32 shaft, 33 outer cylinder

Claims (8)

[Claims] 1. A weight ratio of C: 0.35 to 0.60
%, Si: 0.01-0.15%, Mn: 0.2-1.
60%, S: 0.005 to 0.15%, Al: 0.01
0 to 0.06%, Ti: 0.005 to 0.050% B:
0.0005-0.005%, N: 0.0015-0.
008%, P: 0.020% or less, Cu: 0.
O: 0.0025% or less, with the balance being iron and unavoidable impurities, the ratio t / r of the effective hardened layer depth t to the part radius r being 0.3 to 0.6. And projection core hardness Hp-core as defined below
Is high-torsion fatigue strength induction hardened steel material having a HV of 400 or more. Definition of projected core hardness: effective hardened layer depth t, component radius r,
When the core hardness is Hcore, the projected core hardness Hp-core = Hcore / (1-t
/ R) 2. A weight ratio of C: 0.35 to 0.60
%, Si: 0.01-0.15%, Mn: 0.2-1.
60%, S: 0.005 to 0.15%, Al: 0.01
0 to 0.06%, Ti: 0.005 to 0.050% B:
0.0005-0.005%, N: 0.0015-0.
008%, P: 0.020% or less, Cu: 0.
O: 0.0025% or less, with the balance being iron and unavoidable impurities, the ratio t / r of the effective hardened layer depth t to the part radius r being 0.4 to 0.75. And projection core hardness Hp-cor as defined below
e and the ratio Hp-core / Hca of the hardness of the hardened layer Hcase
The high torsional fatigue strength induction hardened steel material, wherein se is 0.56 or more. Definition of projected core hardness: effective hardened layer depth t, component radius r,
When the core hardness is Hcore, the projected core hardness Hp-core = Hcore / (1-t
/ R) 3. The weight ratio of C: 0.35 to 0.60
%, Si: 0.01-0.15%, Mn: 0.2-1.
60%, S: 0.005 to 0.15%, Al: 0.01
0 to 0.06%, Ti: 0.005 to 0.050% B:
0.0005-0.005%, N: 0.0015-0.
008%, P: 0.020% or less, Cu: 0.
O: 0.0025% or less, with the balance being iron and unavoidable impurities, the ratio t / r of the effective hardened layer depth t to the part radius r being 0.4 to 0.75. And projection core hardness Hp-cor as defined below
e and the ratio Hp-core / Hca of the hardness of the hardened layer Hcase
The high torsional fatigue strength induction hardened steel material, wherein se is 0.56 or more, and the average in-section hardness Hav defined below is HV560 or more. Definition of projected core hardness: effective hardened layer depth t, component radius r,
When the core hardness is Hcore, the projected core hardness Hp-core = Hcore / (1-t
/ R) Definition of average hardness in cross section: A cross section of radius a is divided concentrically into N rings in the radial direction, the hardness of the n-th ring-shaped portion is H _n , the radius is r _n , and the interval is when the Δr _n, [number 1] 4. The weight ratio of C: 0.35 to 0.60
%, Si: 0.01-0.15%, Mn: 0.2-1.
60%, S: 0.005 to 0.15%, Al: 0.01
0 to 0.06%, Ti: 0.005 to 0.050% B:
0.0005-0.005%, N: 0.0015-0.
008%, Cr: more than 0.1 to 1.2%, Mo:
0.02 to 0.8%, Ni: 0.1 to 3.5% Nb:
0.01 to 0.3% V: Contains one or more of 0.03 to 0.6%, P: 0.020% or less, Cu:
The content is limited to 0.05% or less and O: 0.0025% or less, the balance being iron and unavoidable impurities, and the ratio t / r of the effective hardened layer depth t to the component radius r is 0.3 to 0.1%. 6, and a projection core hardness Hp-cor defined below.
The high torsional fatigue strength induction hardened steel, wherein e is HV400 or more. Definition of projected core hardness: effective hardened layer depth t, component radius r,
When the core hardness is Hcore, the projected core hardness is Hp-core = Hcore / (1-t
/ R) 5. A weight ratio of C: 0.35 to 0.60
%, Si: 0.01-0.15%, Mn: 0.2-1.
60%, S: 0.005 to 0.15%, Al: 0.01
0 to 0.06%, Ti: 0.005 to 0.050% B:
0.0005-0.005%, N: 0.0015-0.
008%, and Cr: more than 0.1 to l. 2
%, Mo: 0.02 to 0.8%, Ni: 0.1 to 3.5
% Nb: 0.01 to 0.3% V: One or more of 0.03 to 0.6%, P: 0.020% or less,
Cu: 0.05% or less, O: 0.0025% or less, the balance being iron and unavoidable impurities,
The ratio t / r of the effective hardened layer depth t to the component radius is 0.4 to 0.4.
75 and a projection core hardness Hp− defined below.
ratio of core and hardened layer hardness Hcase-Hp-core /
High swing fatigue strength induction hardened steel, characterized in that Hcase is 0.56 or more. Definition of projected core hardness: effective hardened layer depth t, component radius r,
When the core hardness is Hcore, the projected core hardness Hp-core = Hcore / (1−t
/ R) 6. A weight ratio of C: 0.35 to 0.60.
%, Si: 0.01-0.15%, Mn: 0.2-1.
60%, S: 0.005 to 0.15%, Al: 0.01
0 to 0.06%, Ti: 0.005 to 0.050% B:
0.0005-0.005%, N: 0.0015-0.
008%, and Cr: more than 0.1 to l. 2
%, Mo: 0.02 to 0.8%, Ni: 0.1 to 3.5
% Nb: 0.01 to 0.3% V: One or more of 0.03 to 0.6%, P: 0.020% or less,
Cu: 0.05% or less, O: 0.0025% or less, the balance being iron and unavoidable impurities,
The ratio t / r of the effective hardened layer depth t to the component radius is 0.4 to 0.4.
75 and a projection core hardness Hp− defined below.
ratio of core and hardened layer hardness Hcase-Hp-core /
A high torsional fatigue strength induction hardened steel material having an Hcase of 0.56 or more and an average in-section hardness Hav defined below of HV560 or more. Definition of projected core hardness: effective hardened layer depth t, component radius r,
When the core hardness is Hcore, the projected core hardness Hp-core = Hcore / (1-t
/ R) Definition of average hardness in cross section: A cross section of radius a is divided concentrically into N rings in the radial direction, the hardness of the n-th ring-shaped portion is H _n , the radius is r _n , and the interval is △ when the r _n, [number 1] 7. The high torsional fatigue strength induction hardened steel according to claim 1, wherein the prior-austenite grain size of the induction hardened layer is 9 or more. 8. The high torsional fatigue strength induction hardened steel material according to claim 4, wherein the prior austenite crystal grain size of the induction hardened layer is 9 or more.