JP2002030344A - Method for modifying surface of alloy steel for machine structure, and surface modified material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for modifying the surface of alloy steel for machine structures excellent in fatigue strength under the load condition in which stress ratio is positive, and to provide a surface modified material. SOLUTION: In this method for modifying the surface of alloy steel for machine structures, the surface of alloy steel for machine structures is subjected to vacuum carburizing treatment and is thereafter subjected to two step shot peening treatment in which the grain size of shots in the first shot peening treatment is smaller than that in the second shot peening treatment to introduce high compressive residual stress into the region directly below the surface of the alloy steel for machine structures and further to reduce the surface roughness. Moreover, as the prestage of the two step shot peening treatment superrapid-short time heating and rapid cooling treatment may be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and a material for modifying the surface of alloy steel for machine structures, and more particularly to a method and a material for modifying the tooth surface of a gear.

[0002]

2. Description of the Related Art From the viewpoints of higher efficiency of distribution, environmental protection and resource protection, higher output of automobile engines and weight reduction of automobiles have been attempted. For this reason, further improvement in fatigue strength is required for gears for automobiles.

[0003] In order to improve the fatigue strength of automotive gears, various surface modification treatments have been applied to the gear surfaces (tooth surfaces). As the surface modification treatment, conventionally, a carburizing quenching / tempering treatment, or a combined treatment of a carburizing quenching / tempering treatment and a shot peening treatment is often used.

[0004]

However, the gear subjected to these treatments has a fatigue limit of at most 10 under a load condition in which the stress ratio (R) is positive (for example, 0.1).
It is about 00 MPa, and further improvement in fatigue strength is desired.

An object of the present invention, which has been devised in consideration of the above circumstances, is to provide a method and a surface modifying material for an alloy steel for machine structural use having excellent fatigue strength under a positive load condition with a stress ratio. To provide.

[0006]

Means for Solving the Problems In order to achieve the above object, a method for modifying the surface of alloy steel for machine structure according to the present invention comprises applying a vacuum carburizing treatment to the surface of the alloy steel for machine structure and then applying the surface to the surface. First, a double shot peening process is performed in which the shot diameter of the second shot is smaller than that of the first shot.

[0007] According to the above method, the hardness of the alloy steel for machine structural use near the surface is increased, the formation of an abnormal surface structure on the surface can be prevented, the amount of retained austenite in the structure can be reduced, and High compressive residual stress can be introduced. Further, the surface roughness of the alloy steel for machine structural use can be reduced.

In addition, an ultra-rapid and short-time heating and quenching treatment may be performed before the double shot peening treatment.

Further, after the first-stage shot peening with a shot grain size of 400 to 800 μm is performed as the double shot peening process, the shot grain size becomes 1
It is preferable to perform a second-stage shot peening process of not more than 00 μm.

Further, it is preferable to carry out a contour induction hardening treatment as the heating and quenching treatment.

On the other hand, the surface modifying material for alloy steel for machine structure according to the present invention is obtained by subjecting the surface of the alloy steel for machine structure to vacuum carburizing, and applying the first stage shot grain to the surface after the vacuum carburizing. This is obtained by performing a double shot peening process in which the shot diameter of the second stage is smaller than the diameter.

Further, the surface modifying material for alloy steel for machine structure according to the present invention is obtained by subjecting the surface of the alloy steel for machine structure to vacuum carburizing treatment, and applying the ultra-rapid and short-time treatment to the surface after the vacuum carburizing treatment. The heating and quenching treatment is performed, and 1
This is obtained by performing a double shot peening process in which the second step shot diameter is smaller than the second step shot particle diameter.

According to the above construction, the hardness near the surface is hard, there is almost no abnormal structure on the surface, the amount of retained austenite in the structure is low, and the compressive residual stress introduced to the surface is high for mechanical structures. An alloy steel surface modifier can be obtained.

The chemical composition of the alloy steel for machine structural use is as follows: C: 0.15 to 0.25 wt%, Mn: 0.40 to
1.00 wt%, Mo: 0.15 to 0.60 wt%, C
r: 0.05-1.35 wt%, Ni: 0.05-2.
00 wt%, Si: 0.03 to 0.35 wt%, P:
It is preferable that 0.030 wt% or less, S: 0.030 wt% or less, and the balance be Fe and unavoidable impurities.

The surface carbon concentration is 0.7 to 0.9 wt.
%.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of the present invention will be described.

As a result of extensive studies by the present inventors, it has been found that when the stress ratio (R) is a positive load condition, the following is important for improving the fatigue strength.

(1) The hardness (HV) near the surface is improved as much as possible.

(2) Along with (1), a large compressive residual stress is introduced just below the surface.

(3) In order to shorten the crack propagation distance in Stage I of crack propagation and increase the yield stress, the crystal grain size is made as small as possible.

As described above, based on the above (1) to (3), a method for improving the fatigue strength of a carburized and quenched alloy steel for a machine structure under a load condition of R ≧ 0 will be examined in detail. The following five items, namely, reduction / prevention of the formation of abnormal surface structure such as grain boundary oxide layer, refinement of crystal grains, reduction of retained austenite (γ _R ), high hardness just below the surface, just below the surface Introducing a large compressive residual stress (preferably a compressive residual stress equal to the yield stress) is an important factor.

The alloy steel for machine structure used herein is defined as an alloy steel material and a member using the same.

First, in order to achieve the above, it is necessary to reduce the oxygen partial pressure of the atmosphere in the carburizing furnace. Vacuum carburizing (hereinafter referred to as V)
C))).

Next, in order to achieve the following, it is important to rapidly heat the steel to the austenitizing temperature and to repeatedly heat and cool rapidly during quenching. As a quenching method that can realize them, Contour Induction Hardening (hereinafter referred to as CI) is an ultra-rapid and short-time heating and quenching treatment.
H)) method is useful. Here, by subjecting the VC-processed alloy steel for machine structural use to CIH processing,
It is possible to simultaneously improve the hardness and refine the crystal grains. Further, by the CIH treatment, it is possible to reduce the heat treatment strain of the alloy steel for machine structural use and to introduce a large compressive residual stress immediately below the surface.

Next, as a means for simultaneously achieving the above, there is a shot peening method. In particular, Double Shot Peening (hereinafter D)
SP)), and by setting the shot grain size used for the second stage peening to 100 μm or less, which is smaller than the shot grain size used for the first stage peening, to improve the fatigue strength of the alloy steel for machine structural use. It is possible to greatly improve the residual stress distribution and surface roughness just below the surface, which are important in the above.

Based on the above, the method for modifying the surface of alloy steel for machine structures according to the present invention firstly sets the surface carbon alloy target steel (for example, gears for automobiles) to a surface carbon concentration target value of 0.7. A VC treatment of about 0.9 wt% is performed.

Next, a first-stage shot peening treatment with a shot grain size of 400 to 800 μm and a shot grain size of 100 μm
By successively performing the following second-stage shot peening treatment, alloy steel for machine structure (surface modified material of alloy steel for machine structure) subjected to surface modification treatment is obtained.

Here, the alloy steel for machine structure may be any alloy steel that requires a high fatigue strength when the stress ratio is a positive load condition, and is not particularly limited. Steel, general case hardened steel, and the like. As the automotive gear steel, for example, the chemical component is C: 0.15 to 0.25 wt%, preferably 0.1%.
8 to 0.22 wt%, more preferably around 0.20, M
n: 0.40-1.00 wt%, preferably 0.80-1.00
0.90 wt%, more preferably around 0.85, Mo:
0.15 to 0.60 wt%, preferably 0.30 to 0.
50 wt%, more preferably around 0.40, Cr: 0.
05 to 1.35 wt%, preferably 0.08 to 0.12
wt%, more preferably around 0.10, Ni: 0.05
~ 2.00wt%, preferably 0.08 ~ 0.12wt
%, More preferably around 0.10, Si: 0.03 to
0.35 wt%, preferably 0.05 to 0.07 wt
%, P: 0.030 wt% or less, S: 0.030 wt%
In the following, those whose balance is Fe and inevitable impurities are mentioned.

The treatment conditions of the VC treatment are appropriately selected according to the amount of the abnormal surface structure (or the depth of the abnormal surface structure layer) allowed in the surface modifier of the alloy steel for machine structural use. There is no particular limitation.

Further, the processing conditions of the DSP processing are determined by the amount of residual austenite required for the surface modifier of the alloy steel for machine structural use, the hardness just below the surface, and the magnitude of the compressive residual stress introduced just below the surface. It is appropriately selected according to the
There is no particular limitation. Here, the shot grain size in the second-stage shot peening treatment is limited to 100 μm or less. When the shot grain size exceeds 100 μm, the depth from the surface where the maximum compressive residual stress is obtained becomes deep, This is because the effect of improving the roughness (reducing the surface roughness) cannot be expected (or hardly expected).

According to the method and the surface modifying material of the alloy steel for machine structure according to the present invention, by subjecting the surface of the alloy steel for machine structure to VC treatment, the grain boundary oxidation which is harmful to the fatigue strength can be obtained. No (or almost no) surface abnormal tissue is formed.

Further, by performing the DSP treatment after the VC treatment, (a) the amount of retained austenite in the region from the surface to a depth of about 100 μm of the surface modifier of the alloy steel for machine structure is significantly reduced due to the peening effect. (B) The maximum hardness of the surface portion of the surface modifier of the alloy steel for machine structural use is 1000H.
(C) The maximum compressive residual stress introduced just below the surface of the surface modifier of alloy steel for machine structure is 1
The pressure becomes extremely high at 800 MPa or more, and the maximum compressive residual stress value is located on the surface. Where (a)
(C) is due to the effect of superposition of the effect of normal double shot peening and the effect of (a), because work-induced martensite transformation occurs during peening and retained austenite is transformed into martensite.

Further, due to the above-mentioned effects (a) to (c),
The fatigue limit of the surface modifier for a mechanical structural alloy steel according to the present invention is 1800 MPa or more when the stress ratio is a positive load condition (for example, R = 0.1). This value is more than twice as large as that of a conventional surface modifier in which only VC treatment is applied to alloy steel for machine structural use. At this time, it has reached the fatigue limit (e.g., the number of cycles to failure is 10 ⁷ times) when measuring the residual stress of the surface modifier of the machine structural alloy steel, although the maximum compressive residual stress of the surface is reduced slightly , But still 1
An extremely high compressive residual stress of 500 MPa or more exists, and the internal compressive residual stress distribution hardly changes before and after the fatigue test. That is, the surface modifier for alloy steel for machine structural use according to the present invention has excellent fatigue strength.

Next, a description will be given of a method of modifying the surface of alloy steel for machine structural use and a surface modifier according to another embodiment.

In the method for modifying the surface of alloy steel for machine structure according to the present embodiment, first, the target value of the surface carbon concentration on the surface of the alloy steel for machine structure (for example, gears for automobiles) is set to 0.7 to 0.7.
A 0.9 wt% VC process is performed.

Next, the surface of the alloy steel for machine structural use after the VC treatment is subjected to an ultra-rapid and short-time CIH treatment (heating and quenching treatment).

Thereafter, a first-stage shot peening treatment with a shot grain size of 400 to 800 μm and a shot grain size of 100 mm were applied to the surface of the alloy steel for machine structural use after the CIH treatment.
By successively performing the second-stage shot peening treatment of μm or less, alloy steel for machine structure (surface modified material of alloy steel for machine structure) which has been subjected to surface modification treatment can be obtained.

Here, the treatment conditions of the CIH treatment are appropriately selected according to the hardness and the grain size required for the surface modifier of the alloy steel for machine structural use, and are not particularly limited.

The surface modifying method and the surface modifying material for the alloy steel for machine structure according to the present embodiment also have the same functions as the surface modifying method and the surface modifying material for the alloy steel for machine structure according to the present invention. It goes without saying that it works.

According to the present embodiment, CIH processing is performed in the step between the VC processing and the DSP processing,
Due to the secondary quenching effect, a new effect of making the crystal grain size finer is exhibited. Thereby, the fatigue limit of the surface modified material of the alloy steel for machine structure is further increased (for example, as compared with the surface modified material of the alloy steel for machine structure according to the present invention, the fatigue limit is 1%).
0% or more).

[0041]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS High-strength gears as alloy steels for machine structural use aimed at increasing the contents of Ni and Mo to strengthen the carburized layer and reducing the contents of Si and Cr to reduce the surface abnormal layer. A steel for use (manufactured by Daido Steel Co., Ltd. (hereinafter, referred to as DSG1 steel)) was used. Here, the chemical composition of this DSG1 steel is as follows: C is 0.19 wt%, Mn is 0.84 wt%,
Mo: 0.4 wt%, Cr: 0.107 wt%, Ni: 0.09 wt%, Si: 0.06 wt%, P: 0.01
0 wt%, S is 0.019 wt%, and the balance is Fe and inevitable impurities.

In a steel bar of φ80 mm made of DSG1 steel,
After performing hot forging to form φ130 mm, annealing was performed to produce a steel bar having a Vickers hardness of about 200 HV. This bar is cut and the module is 3,
36 teeth, right twist angle 17 °, pressure angle 14 ° 3
Four gears having 0 'and an overbar diameter of 123.584 mm were produced. Each gear was subjected to a different surface modification treatment to obtain test gears 1-4.

Here, the test gear 1 was subjected to only the VC (vacuum carburizing) treatment, and the test gear 2 was subjected to the DSP after the VC treatment.
(Double shot peening) treatment, the test gear 3 was subjected to CIH (contour induction hardening) treatment after the VC treatment, and the test gear 4 was subjected to the CIH treatment after the VC treatment, and then subjected to the DSP treatment. It was done.

The heating in the VC treatment is performed by induction heating, propane gas is used as the carburizing gas, the furnace pressure is 6.67 × 10 ⁻² kPa, and the target value of the surface carbon concentration is 0.8.
wt%. As shown in FIG. 1, the carburizing conditions are as follows: first, carburizing is performed at 1223K for 2880 seconds (48 minutes), then primary carburizing is performed at 1173K for 60 seconds, and cooling with nitrogen gas is performed. . Next, 7 at 433K
A secondary carburizing treatment was performed for 200 seconds (2 hours), and natural cooling was performed.

In the CIH process, as shown in FIG.
Using high frequency of 3kHz, 925K in 1.5 seconds
After heating for ultra-rapid and short time until then, leave for 0.9 seconds, then use a high frequency of 150 kHz,
Ultra-rapid, short-time heating to 1137K in 0.2 seconds, spray hardening. Subsequently, using a high frequency of 3 kHz, 48 seconds in 0.5 seconds.
After heating up to 3K for a very rapid and short time, leave it for 2.0 seconds, and then perform water cooling.

The conditions of the first stage shot peening in the DSP processing are as follows: air pressure is 490 kPa, nozzle diameter is 1
The shot hardness is 0 mm, the shot particle diameter is 0.6 mm, the shot hardness is about 700 HV, and the arc height is 0.35 mmC. The condition of the second shot peening is that the air pressure is 3
92 kPa, nozzle diameter 4 mm, shot particle diameter 0.0
The shot hardness is 8 mm, the shot hardness is about 700 HV, and the arc height is 0.26 mmN.

(Test 1) The root portions R of the test gears 1 to 4 were masked by a window method, and then electrolytically polished to a predetermined depth to determine the amount of residual austenite (γ _R ) and residual stress (σ _r ). A measurement was made. gamma _R content was determined from the respective diffraction profile area ratio of martensite and gamma _R.

Here, as a device for measuring the amount of residual austenite (γ _R ) and the residual stress (σ _r ), a minute portion X-ray measuring device was used. Also, gamma _R amount and the measurement site of sigma _r is bent particularly important tooth bottom fillet R in fatigue strength
Of the tooth profile. Furthermore, the measurement of gamma _R amount and sigma _r, using Cr-K _alpha ray as an X-ray beam diameter of the incident X-ray was set to 0.3 mm.

(Test 2) A fatigue test was performed on each of the test gears 1 to 4. As a fatigue test method, a two-tooth simultaneous fatigue test in which a load was applied between the teeth on the pressing shaft side and the teeth on the fixing shaft side was adopted. Two teeth simultaneous fatigue test
Since the number of test pieces reaching the fatigue limit is two, this is a highly reliable fatigue test method. In advance, the root R of the loaded tooth
Strain gauge attached to the obtuse end of the section
2 mm) The relationship between the output and the load was determined, and the root stress was evaluated from this relationship. The load condition is the stress ratio (R; σ
_min / σ _max ) was fixed at 0.1, the frequency was 10 Hz, and the stress waveform was a sine wave. In addition, the fatigue limit of the gear, and the number of cycles to failure and repeated stress at the time of the 10 ^seven times.

First, the characteristics of the surface modified portion are evaluated.

(1) Surface Roughness The surface roughness (R _max ) of the test gear 1 was 6.0 μm. R _{max of the} test gear 2 was 5.6 μm, which was slightly smaller than that of the test gear 1. R _max of the test試歯vehicle 3 was 7.0 .mu.m. R _max of the test gear 4 is 4.6 μm
Which was smaller than the test gear 3. From the above results, the test試歯vehicles 2 and 4 subjected試歯vehicle 1,3 formed by subjecting the DSP process, it was found that R _max is improved.

(2) Carbon Concentration Distribution A steel bar (φ25 mm, length 100 mm) made of alloy steel for machine structure (SCM420H (JIS standard)) having a carbon concentration substantially equal to that of DSG1 steel constituting each of the test gears 1 to 4. Machine processing was performed to produce a test piece for measuring carbon concentration distribution. In addition, an infrared absorption method based on JIS standard was used as a carbon concentration measuring method. The carbon concentration at a position 50 μm deep from the surface was 0.77 wt%, which was almost the target carbon concentration. In addition, the calculation results of the carbon concentration and the measured values were substantially the same.

(3) Hardened Hardened Layer FIG. 3 is a schematic cross-sectional view of the test gear 4 after the surface modification treatment.

As shown in FIG. 3, the hardened layer of the test gear 4 (31 in FIG. 3) includes a VC layer 32, which is substantially uniformly carburized from the tooth tip portion 34 to the tooth bottom portion 35, and a secondary layer. It is composed of a quenched CIH layer 33. Here, the CIH layer 33
Is thicker at the tooth tip portion 34 than at the tooth bottom portion 35.

(4) Structure FIG. 4 shows a structure observation diagram of the tooth bottom R portions of the test gears 1, 3, and 4. Here, FIGS. 4A to 4C are observation views of the test gears 1, 3, and 4, respectively.

[0056] The test試歯vehicle 1 organization, as shown in FIG. 4 (a), was acicular martensite + gamma _R. Also, VC
Because of the treatment, no abnormal surface structure was observed. Subjected試歯vehicle 3 of tissue, as shown in FIG. 4 (b), was a very fine lath martensite + gamma _R. The surface immediately below, gamma for _R large amount, had become whitish. Further, since the composite treatment of the VC treatment and the CIH treatment was performed, no abnormal surface structure was observed as in the case of the test gear 1. The structure of the test gear 4 is shown in FIG.
(C), the it was a very fine lath martensite + gamma _R. Immediately below the surface, gamma _R as provided試歯vehicle 3 is not observed, also, the plastic flow was trace was slightly observed.

(5) Grain Size FIG. 5 shows an observation diagram of the prior austenite grain size at the roots R of the test gears 1, 3, and 4. Here, FIG. 5A is a 200-times observation view of the test gear 1, and FIGS. 5B and 5C are 600-times observation views of the test gears 3 and 4. The austenite grain size number (N) was determined according to JIS standards.

The test gears 1 and 2 shown in FIGS.
N of Nos. 3 and 4 was No. 8.6, No. 13.4 and No. 13.5, respectively, and the test gears 3 and 4 had particularly fine crystal grains.

Here, the average crystal grain size (d _γ ) was obtained from N, assuming that the crystal grain size of each of the test gears 1, 3, and 4 was spherical. As a result, the d _gamma of the test試歯vehicle 1 20.3Myuemu, the d _gamma of the test試歯vehicle 3 3.8 .mu.m, the d _gamma of the test試歯wheel 4 was 3.7 .mu.m. From now on, the d of the test gears 3 and 4
_It can be seen that _γ is extremely fine. Thus, d
_{The reason} why _γ becomes fine is due to the secondary quenching effect of the CIH process, which is an ultra-rapid and short-time heating and quenching process.

(6) Distribution of Retained Austenite (γ _R ) FIG. 6 shows the relationship between the depth (μm) from the surface of each of the test gears 1 to 4 and the amount of γ _R (vol%). Here, a white diamond indicates the test gear 1, a black square indicates the test gear 2, a white triangle indicates the test gear 3, and a black circle indicates the test gear 4.

The γ _R amount on the surface of the test gear 1 is 11.5 vo.
1%. The maximum value of gamma _R content, the depth 100μ
m, and the value was 26.8 vol%.

The amount of γ _{R on} the surface of the test gear 2 was 1.8 vol.
% Was very low. The maximum value of gamma _R content is the position in the depth 165 .mu.m, significantly less compared to the test試歯vehicle 1, the value was 16.5vol%.

The γ _R amount on the surface of the test gear 3 is 24.5 vo.
1%, which is larger than the maximum value of the test gear 2. The maximum value of gamma _R content is the position of the depth 15 [mu] m, the value was 31.3vol%. Furthermore, γ _R
The amount was a high value of 21.2 vol% even at a position at a depth of 210 μm. These values are very large compared to the test gear 1 subjected to only the VC processing. Thus, the γ _{R of the} test gear 3 subjected to the CIH treatment after the VC treatment is obtained.
It is considered that the large amount is due to the large amount of carbon and the slightly higher quenching heating temperature shown in FIG.

The γ _R amount on the surface of the test gear 4 is 3.6 vol.
% Was very low. The maximum value of the amount of γ _R was at a position at a depth of 100 μm, and the value was 23.8 vol%. Thus, after VC processing and CIH processing, D
Gamma _R content of the test試歯wheel 4 which has been subjected to SP treatment was very small as compared to gamma _R of test試歯vehicle 3.

Since γ _R is harmful in terms of fatigue strength, it is important to reduce the amount of γ _R near the surface in order to improve the fatigue limit of the gear. As is evident from the above results, the test gears 2 and 4 subjected to the DSP treatment
Γ _R near the surface compared to the untreated test gears 1 and 3
The amount was significantly lower. This is due to the DSP processing
This is because work-induced martensitic transformation has occurred and γ _R has been converted to martensite. From this, it can be seen that by performing the DSP treatment, it is possible to convert a considerable amount of harmful γ _R into martensite in terms of fatigue strength.

(7) Hardness Distribution The hardness distribution on the surface and near the surface of each of the test gears 1 to 4 was examined. The hardness was evaluated using a microhardness tester. The test load was 2.942N according to JIS. However, the hardness at a position of 10 μm from the surface is 2.9.
Since it is impossible to measure with a load of 42N, 0.9807
It was measured at a load of N.

The depth (μ) from the surface of each test gear 1-4
FIG. 7 shows the relationship between m) and Vickers hardness (HV).
Here, the white diamond indicates the test gear 1 and the black square indicates the test gear 2.
, The white triangle indicates the test gear 3, and the black circle indicates the test gear 4.

The Vickers hardness of the test gear 1 at a depth of 10 μm was 735 HV, and the maximum hardness was 799 HV.

The Vickers hardness of the test gear 2 at a depth of 10 μm was 1040 HV, which was an extremely high value. Also, even at a position of 200 μm depth, 80
It showed a high value of 4 HV. Vickers hardness is 104
0 HV, which was an extremely high value. That is, the maximum hardness of the test gear 2 is compared with the maximum hardness of the test gear 1,
About 240 HV was also high. This is, as mentioned above,
This is because, by the SP treatment, a work-induced martensitic transformation occurs, and γ _R is transformed into martensite.

The Vickers hardness of the test gear 3 at a depth of 10 μm was 784 HV, which was about 50 HV higher than that of the test gear 1. The position showing the maximum hardness was a position at a depth of 200 μm, and the value was 893 HV.

The Vickers hardness of the test gear 4 at a depth of 10 to 75 μm was 1057 to 1067 HV, which was an extremely high value. In addition, it had a higher hardness than the test gear 2 up to a depth of about 200 μm.

Here, 100 near the surface of the test gear 4
The reason why the ultra-high hardness exceeding 0 HV was obtained was that the surface treatment such as grain boundary oxidation was not generated due to the VC treatment.

The ultra-rapid and short-time heating and quenching treatment by the CIH treatment increased the hardness near the surface.

The hardness immediately below the surface was significantly increased due to the superposition effect of the plastic deformation effect by the DSP treatment and the resulting work-induced martensitic transformation.

This is considered to be due to the combined effect of the three factors.

[0076] (8) Residual stress (sigma _r) shows distribution relationship between the depth ([mu] m) and the residual stress (sigma _r) from the surface of each test試歯vehicle 1-4 in FIG. Here, a white diamond indicates the test gear 1, a black square indicates the test gear 2, a white triangle indicates the test gear 3, and a black circle indicates the test gear 4.

The compressive residual stress is also introduced near the surface of the test gear 1, but its value is as small as about 390 MPa.

In the test gear 2, an extremely large compressive residual stress was introduced, and the maximum value (1838 MPa) was at the surface. Further, even at a position at a depth of 90 μm from the surface, a high compressive residual stress of 1173 MPa was introduced. However, the depth from the surface is about 200 μm
In the above region, the compressive residual stress distribution is almost the same as that of the test gear 1.

A compressive residual stress of 801 MPa was introduced on the surface of the test gear 3. The maximum residual stress is 1
Although the value is not so high as 054 MPa, the area from the surface to a depth of about 350 μm is about 900 MPa.
The above compressive residual stress was introduced. This is CIH
This is due to the effect of ultra-rapid and short-time heating and quenching of only the surface by the treatment.

In the test gear 4, an extremely large compressive residual stress was introduced, and its maximum value (1862 MPa) was on the surface. Further, similarly to the test gear 3, a compressive residual stress of about 900 MPa or more was introduced into a region from the surface to a depth of about 250 μm.

Here, it is considered that the extremely high compressive residual stress was obtained in the vicinity of the surfaces of the test gears 2 and 4 due to the same effect as in the above (7) hardness distribution.

Next, the fatigue strength and the residual stress distribution after the fatigue test are evaluated.

(A) SN Diagram The fatigue test was performed on the test gears 1, 2, and 4. FIG. 9 shows an SN diagram of the test gears 1, 2, and 4. Here, the horizontal axis indicates the number of cycles (times) until fracture, and the vertical axis indicates the fatigue strength (σ _R (MPa)). Also, white diamonds indicate the test gear 1, black squares indicate the test gear 2, and black circles indicate the test gear 4.

As shown in FIG. 9, the fatigue limit (σ _up ) of the test gear 1 subjected to only the VC treatment was 883 MPa.
The σ _{up of the} test gear 2 was 1931 MPa, which was 2.18 times the σ _up of the test gear 1. Further, the σ _{up of the} test gear 4 was 2207 MPa, which was 2.50 times the σ _up of the test gear 1.

Here, the reason why extremely high σ _up was obtained in the test gears 2 and 4 was that the surface treatment such as grain boundary oxidation was not generated due to the VC treatment.

The amount of γ _R near the surface was significantly reduced by the DSP treatment.

Due to the effect of the DSP processing, 1000H
An ultra-high hardness of V or more was obtained.

An extremely high compressive residual stress of 1800 MPa or more could be introduced to the surface.

It is considered that this is due to the combined effect of the four factors.

Further, from the σ _{up of the} test gear 2, the test gear 4
Σ _up was higher than 270MPa by CIH
It is considered that d _γ became extremely fine due to the effect of the treatment.

(A) Observation of fracture surface Observation of the fracture surface of each of the test gears 1 to 4 revealed that the fatigue fracture starting point was on the obtuse angle side of the end in the direction of the tooth trace. Also,
Fatigue cracks originated from the surface.

[0092] (c) to confirm the effectiveness of the high compressive residual stress on the fatigue strength of the residual stress distribution subjected試歯car after fatigue test were measured residual stress distribution of the gear has reached the fatigue limit (10 ⁷ times) . Here, the test gear 2 was used for the measurement.

FIG. 10 shows the relationship between the depth (μm) from the surface of the test gear 2 and the residual stress (σ _r ). Here, the black squares indicate the values before the fatigue test, the white squares indicate the values after the fatigue test was performed 10 ⁷ times under a load of 1710 MPa, and the black circles indicate the values of 193.
The figure shows a state after a fatigue test was performed 10 ⁷ times under a load of 1 MPa.

As shown in FIG. 10, a very high compressive residual stress of 1540 MPa was maintained at the root R of the test gear 2 after the fatigue test was performed 10 ⁷ times under a load of 1710 MPa even after the fatigue test. You can see that it exists. In addition, a high compressive residual stress of 1627 MPa exists at the root R of the test gear 2 after the fatigue test is performed 10 ⁷ times under a load of 1931 MPa even after the fatigue test.

Although not shown, also in the other test gears 1, 3, and 4, the compressive stress on the surface was reduced by 211 to 298 MPa before and after the fatigue test, and the depth from the surface was about 50 μm or more. In the region of No.1, there was almost no difference in the distribution of compressive residual stress before and after the fatigue test.

That is, when R ≧ 0, it is considered that the compressive residual stress does not disappear much by the fatigue test.

Next, the effect of the surface modification characteristics on the fatigue strength will be evaluated.

The gear 1 was made of SCr42 by subjecting a base material made of S50C to a soft nitriding treatment mainly containing nitrogen gas.
The gear 2 is formed by subjecting a base material made of 0H to a carburizing process mainly containing an endothermic gas, and the gear 3 is formed by performing a carburizing process mainly made of an endothermic gas to a base material made of SCM420H. The base material made of SCM420H was subjected to carburizing treatment mainly containing nitrogen gas, and the gear 5 was made of a base material made of DSG1 steel subjected to VC treatment.
The gear 6 is made by subjecting a base material made of C to a CIH treatment.
The gear 7 is made by subjecting the base material made of S50C to CIH processing and DSP processing, and the VC7 is made to the base material made of DSG1 steel.
Gear 8 with DSG processing and DSP processing
VC treatment, CIH treatment, and DS
The gear 9 is manufactured using the P treatment.

Table 1 shows the surface modification characteristics and fatigue strength of each of the gears 1 to 9.

[0100]

[Table 1]

(I) Surface hardness and maximum compressive residual stress (σ
_Rmax ) and fatigue limit (σ _up ) For gears used under the load condition of R = 0.1, which property among the surface modification properties of hardness, maximum compressive residual stress and grain size is most effective I examined whether it is. So, first,
The sum of the yield stress (σ _Y ) and the maximum compressive residual stress (σ _rmax ) was used as the first factor, and the relationship between this first factor and the gear fatigue limit (σ _up ) was determined.

The yield stress (σ _Y ) and the maximum compressive residual stress (σ
_rmax ) and the fatigue limit (σ _up ) are shown in FIG.
Here, σ _Y is HV · N / 3 (MPa) (in the unit system of FIG. 7, the unit of HV is Kg / cm ² , and σ _Y =
3.27 HV (MPa)).

As shown in FIG. 11, σ _up is σ _Y + σ _rmax
Σ _up increases with an increase in σ _Y + σ _rmax . When this relationship is obtained by the least square method, the following equation (straight line L _{1 in} FIG. 11) is obtained.

Σ _up = 0.478 (σ _Y + σ _rmax ) -454... As a result, the gears 8 and 9 having a high σ _up have an ultra-high hardness of 1000 HV or more and a high compression residual of 1800 MPa or more on the surface. Stress has been introduced. Also, σ _up is 171
The 0 MPa gear 7 is also introduced with a high compressive residual stress of 1514 MPa immediately below the surface. Furthermore, by introducing a high compressive residual stress just below the surface, the stress intensity factor (K value) becomes smaller not only near the surface but also toward the inside. Therefore, by combining the CIH treatment and the DSP treatment, it is possible to increase the hardness immediately below the surface and introduce a high compressive residual stress, and it is considered to be extremely effective in improving the fatigue strength of the gear.

(Ii) Effect of Grain Size on Fatigue Limit (σ _up ) As shown in FIG. 11, the gear 9 having a fine grain size (d _γ ) (d _γ = 3.7 μm), the gear 7 (D _γ = 7.4 μ
m) and the gear 6 (d _γ = 7.9 μm), σ _up is located above the straight line L ₁ . On the other hand, the gear 8 (d _γ = 20.3 μ) having a large crystal grain size (d _γ )
m), gear 5 (d _γ = 20.3 μm), gear 3 (d _γ
= 21.0 μm) and the gear 2 (d _γ = 29.7 μm), σ _up is located below the straight line L ₁ .

As a result, under the load condition of R = 0.1, the first factor that determines the fatigue limit of the gear is (σ _Y +
σ _rmax ), indicating that the second factor is the crystal grain size.

Thus, the equation relating to the fatigue limit is expressed as the following equation.

Σ _up = 0.478 (σ _Y + σ _rmax ) + a + bd _γ ^-1/2 (where a and b are constants and the unit of d _γ is m) From all the experimental results in FIG. When a and b are obtained, the equation is expressed as the following equation.

Σ _up = 0.478 (σ _Y + σ _rmax ) +1.
363d _γ ^-1/2 -894 ... parameter X (0.47
FIG. 12 shows the relationship between 8 (σ _Y + σ _rmax ) + 1.363d _γ ^-1/2 ) and the fatigue limit (σ _up ).

As shown in FIG. 12, it can be seen that there is a very good correlation between the fatigue limit (σ _up ) of the gear obtained under the condition of R = 0.1 and the parameter X.

From these results, the yield stress (σ _Y ), the maximum compressive residual stress (σ _rmax ), and the average crystal grain size (d
It has been found that there is a proportional relationship between _γ ) and the gear fatigue limit (σ _up ). Further, in order to improve the fatigue limit of the gear under the load condition of R ≧ 0, it is important to increase the hardness near the surface and introduce a large compressive residual stress near the surface. It turned out to be important.

As described above, the embodiments of the present invention are not limited to the above-described embodiments, and it goes without saying that various other embodiments can be envisaged.

[0113]

In summary, according to the present invention, the surface of alloy steel for machine structural use is subjected to vacuum carburizing and double shot peening or vacuum carburizing, ultra-rapid and short-time heating and quenching, and double shot peening. By performing the above, an excellent effect that an alloy steel for machine structural use having excellent fatigue strength when the stress ratio is a positive load condition can be obtained.

[Brief description of the drawings]

FIG. 1 is a view showing processing conditions of a carburizing process in an example.

FIG. 2 is a diagram showing processing conditions of a contour induction hardening process in the embodiment.

FIG. 3 is a schematic cross-sectional view of a test gear 4 according to an example after a surface modification treatment.

FIG. 4 is a structural observation view of a root portion R after surface modification treatment of test gears 1, 3, and 4 in an example.

FIG. 5 is an observation diagram of the prior austenite grain size at the root portion R after the surface modification treatment of the test gears 1, 3, and 4 in Examples.

FIG. 6 is a diagram showing the relationship between the depth from the surface of each test gear 1 to 4 and the retained austenite content in the example.

FIG. 7 is a diagram showing the relationship between the depth from the surface of each test gear 1 to 4 and Vickers hardness in the example.

FIG. 8 is a diagram showing the relationship between the depth from the surface of each of the test gears 1 to 4 and the residual stress in the example.

FIG. 9 is an SN diagram of test gears 1, 2, and 4 in the embodiment.

FIG. 10 is a diagram showing the relationship between the depth from the surface and the residual stress before and after the fatigue test of the test gear 2 in the example.

FIG. 11 is a diagram showing the relationship between the sum of the yield stress and the maximum compressive residual stress and the fatigue limit in Examples.

FIG. 12 is a diagram showing a relationship between a parameter X and a fatigue limit.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C21D 1/10 C21D 1/10 A 7/06 7/06 A C22C 38/00 301 C22C 38/00 301N 38 / 44 38/44 C23C 8/22 C23C 8/22

Claims (8)

[Claims] After performing vacuum carburizing on the surface of the alloy steel for machine structural use, the surface is subjected to a double shot peening process in which the second step shot diameter is smaller than the first step shot diameter. A method for modifying the surface of alloy steel for machine structure, characterized by introducing a high compressive residual stress directly below the surface of the alloy steel for machine structure and reducing the surface roughness. 2. An ultra-rapid and short-time heating and quenching treatment is performed before the double-shot peening treatment.
The surface modification method of the alloy steel for a machine structure according to the above. 3. After performing a first-stage shot peening process with a shot grain size of 400 to 800 μm as the double shot peening process, a shot grain size of 100 to 800 μm is obtained.
The method for modifying the surface of alloy steel for machine structural use according to claim 1 or 2, wherein a second-stage shot peening treatment of not more than μm is performed. 4. The method for modifying the surface of alloy steel for machine structural use according to claim 2, wherein a contour induction hardening treatment is performed as the heating and quenching treatment. 5. A double-shot peening process in which the surface of the alloy steel for machine structure is subjected to vacuum carburizing, and the surface after the vacuum carburizing is subjected to a double-shot peening process in which the second-stage shot particle size is smaller than the first-stage shot particle size. A surface modifying material for alloy steel for machine structural use, characterized by being subjected to a heat treatment. 6. A vacuum carburizing treatment is performed on the surface of the alloy steel for machine structural use, an ultra-rapid and short-time heating and quenching treatment is performed on the surface after the vacuum carburizing treatment.
A surface modifying material for alloy steel for machine structural use, characterized by being subjected to a double shot peening treatment in which the second step shot diameter is smaller than the first step shot diameter. 7. The chemical composition of the alloy steel for machine structural use is as follows:
C: 0.15 to 0.25 wt%, Mn: 0.40 to 1.
00 wt%, Mo: 0.15 to 0.60 wt%, Cr:
0.05-1.35 wt%, Ni: 0.05-2.00
wt%, Si: 0.03 to 0.35 wt%, P: 0.0
30 wt% or less, S: 0.030 wt% or less, the balance is F
The surface-modified material for alloy steel for machine structural use according to claim 5 or 6, which is e and inevitable impurities. 8. The surface modifying material for alloy steel for machine structural use according to claim 5, wherein the surface carbon concentration is 0.7 to 0.9 wt%.