JP2636661B2 - High-strength steel part with excellent fatigue strength and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel part used for various shafts of automobiles, construction machines, industrial machines, and the like, and a method of manufacturing the same. TECHNICAL FIELD The present invention relates to a high-strength steel part having increased resistance to fatigue fracture and a useful method for manufacturing such a high-strength steel part.

[0002]

2. Description of the Related Art Surface strengthening such as carburizing, carburizing, nitriding or nitriding is applied to mechanical structural parts which are subjected to high repetitive stress or surface pressure, such as various shafts of automobiles, construction machines and industrial machines. Processing is said to be effective in extending its life and is widely applied.

[0003] By the way, although much has not been clarified as to what effect the retained austenite (hereinafter abbreviated as residual γ) phase in a steel part has on the fatigue life of a material, it is not clear yet. Has been reported to be able to be a resistance to crack propagation. However, it is said that the residual γ phase, which is a soft phase, promotes crack generation.

[0004] Based on the above findings, the inventors of the present invention have determined that steel parts whose fracture progresses from the surface toward the inside such as bending fatigue and surface fatigue are most likely to crack. It was considered that the life of the material can be improved by reducing the residual γ near the surface and increasing the residual γ inside the material that is closely related to crack propagation. However, according to the surface treatment such as ordinary carburizing, carburizing or nitriding, or nitriding, the concentration of carbon and nitrogen becomes larger at the surface. The opposite distribution will be taken.

On the other hand, if a shot peening treatment is performed after a surface treatment such as carburization, the residual γ near the outermost surface undergoes a work-induced transformation and changes to a martensite phase, so that the residual γ that exhibits a desired fatigue strength is obtained. It is expected that it will approach the distribution. However, as shown in the examples described below, the target residual γ distribution is not achieved at the extent that hard shot peening is applied to a surface treatment material that has been subjected to ordinary carburization, carburization, nitriding, or the like. It was found that the desired fatigue strength could not be obtained.

Japanese Patent Application Laid-Open No. Sho 62-185826 also discloses a technique in which the carbon potential during carburization is set higher than usual to increase the total amount of residual γ, and then hard machining such as hard shot peening is performed. Proposed. However, simply increasing the potential of carburization also increases the amount of residual γ on the outermost surface as the internal residual γ increases. Therefore, even if part of the surface is transformed by shot peening, the outermost surface still has a few tens of percent. Residual γ remains. In steel parts having such a residual γ distribution,
Although an increase in compressive residual stress is recognized, the effect of suppressing crack generation, which is the object of the present invention, is not achieved, and the effect of improving fatigue strength is not sufficient. This is because the purpose of the above technique is to increase the compressive residual stress, and the residual γ amount on the outermost surface is not considered at all. In addition, an increase in the carburizing potential may cause coarse carbides to precipitate during the carburizing treatment, which may lead to a reduction in fatigue strength. Conversely, when shot peening is performed after carburizing (carburizing and carburizing) treatment with a reduced potential, the residual γ near the outermost surface is naturally very small, but at the same time, the amount of residual γ inside is also small. Therefore, the effect of crack propagation resistance due to residual γ cannot be obtained.

[0007]

SUMMARY OF THE INVENTION The present invention has been made under such a circumstance, and an object of the present invention is to achieve an excellent γ distribution by suppressing both generation and propagation of cracks. It is an object of the present invention to provide a high-strength steel part exhibiting fatigue strength and an optimum method for obtaining such a high-strength steel part.

[0008]

The high-strength steel part of the present invention, which has achieved the above-mentioned object, includes a surface treatment involving the introduction of carbon atoms and / or nitrogen atoms into a steel part in one of its manufacturing steps. One or more of them, the residual γ amount from the outermost surface to a depth of 40 μm is 15% by volume or less, and the residual γ amount at a depth of 100 to 400 μm from the outermost surface is 20 to 40. The point is volume%.

The above high-strength steel part has a desired high strength by defining its residual γ distribution, and its production method is particularly limited as long as the above residual γ distribution can be achieved. Although not intended, an optimum manufacturing method includes the following configuration. That is, the method for producing a high-strength steel part according to the present invention means that a part made of carburizing steel has a surface carbon concentration of 0.7% by weight or more,
Surface nitrogen concentration: 0.2% by weight or more and (surface carbon concentration + surface nitrogen concentration): 1.3% by weight or less, after performing carburizing and nitriding treatment for T hours, and surface carbonizing at 900 ° C. or more. Concentration: Carburizing treatment to be 0.4-0.9% by weight
０．６0.6T from the outermost surface
The gist is that the residual γ amount up to the depth is set to 40% by volume or less, and shot peening at an arc height of 0.6 mmA or more is performed to reduce the residual austenite amount to 15% by volume or less.

[0010]

The present inventors have studied from various angles the residual γ distribution of a steel part that can suppress both the initiation and propagation of cracks. As a result, the amount of residual γ from the outermost surface to a depth of 40 μm was suppressed to 15% by volume or less, and
When the amount of residual γ at a depth of 0 to 400 μm is set to 20 to 40% by volume, it has been found that both effects are most synergistically exhibited and good fatigue strength is exhibited, and the present invention has been completed. Here, “the residual γ amount from the outermost surface to a depth of 40 μm is 15 vol% or less” means that the residual γ amount is always 15 vol% or less in any part from the outermost surface to a depth of 40 μm. Means In any of the regions of the above steel parts, a martensite structure or a bainite structure is mainly used except for the residual γ structure.

[0011] In the high-strength steel part of the present invention, the details defining the residual γ distribution as described above are as follows. That is, the range of initial stage crack generation due to fatigue is from the outermost surface to a depth of 50 μm, particularly to a depth of 40 μm. If the amount of residual γ in this part exceeds 15% by volume, the residual γ softer than the base structure is distorted. This is because they concentrate and promote the generation of cracks. For fatigue crack propagation of steel parts, the amount of residual γ at a depth of 100 to 400 μm from the outermost surface is important,
This is because, when the residual γ amount in this portion is set to 20 to 40% by volume, the effect of suppressing fatigue crack propagation is most effectively exhibited.

The high-strength steel part of the present invention exerts the effects of the present invention as long as the above-mentioned residual γ distribution is achieved. The production method is not particularly limited. It can be manufactured by each method of 1) to (3). As is apparent from the constitutions of the following production methods, in order to produce the high-strength steel part of the present invention, a surface treatment involving the introduction of carbon atoms and / or nitrogen atoms into the steel part is performed. It is necessary to increase the concentration of carbon and nitrogen in the surface layer of steel parts by including it as one or more of the steps. Is impaired, shortening the service life, and also impairing workability.

(1) After subjecting a steel part to nitriding treatment (a treatment mainly for the formation of nitrides of Fe and other alloy elements using a method such as a salt bath), the nitride is decomposed to such an extent that the nitrides are decomposed. A method in which carburizing treatment or the like is performed at a high temperature to diffuse solid-solution nitrogen into the interior, adjust the concentration of nitrogen and carbon near the outermost surface, and then perform strong processing such as shot peening. (2) In the optimum manufacturing method according to the present invention, which will be described in detail later, a method in which two-stage processing is performed continuously without dividing the steps, that is, a method in which the temperature is raised or the potential is gradually reduced. (3) A steel part that has been carburized and quenched so that the amount of residual γ becomes relatively large is rapidly subjected to sub-zero treatment and heating to room temperature to transform only the residual γ near the outermost surface.

As another preferable condition, it is preferable that the material defect of the oxide layer generated by inclusions or carburization in the material or the decarburized layer at the time of heat treatment is as small as possible. This is because, in the present invention, the structure is adjusted to suppress the occurrence of cracks, so that the crack initiation site is likely to shift to such a material defect. Further, a treatment for increasing the compressive residual stress such as shot peening is effective from the viewpoint of further increasing the effect of the present invention. this is,
The present inventors have studied from various angles also an optimal manufacturing method for achieving the above-mentioned residual γ distribution because the residual compressive stress has an effect of controlling both crack generation and propagation. As a result, it has been found that a desired residual γ distribution can be easily obtained by appropriately defining each processing condition. Each manufacturing condition in the manufacturing method according to the present invention will be described.

The process in the method of the present invention is roughly divided into three stages. First, the first step is a step of efficiently introducing a large amount of carbon and nitrogen atoms into a material by carburizing and nitriding. From such a viewpoint, it is necessary to perform the treatment under the condition that the surface carbon concentration is at least 0.7% by weight and the surface nitrogen concentration is at least 0.2% by weight. However, if carbon and nitrogen atoms are excessively introduced, the action of reducing the carbon and nitrogen concentrations near the outermost surface in the next step (carburizing treatment) becomes insufficient, and as a result, the residual γ amount near the outermost surface becomes the target amount. And the object of the present invention cannot be achieved. Also, excessive introduction of carbon and nitrogen atoms often leads to precipitation of coarse carbides, which hinder the diffusion of interstitial atoms in the next step, and if these survive after completion of the entire process, they themselves will be removed. It becomes a source of cracks and reduces fatigue strength. From such a viewpoint, it is necessary to perform the carburizing and nitriding treatment under the condition that (surface carbon concentration + surface nitrogen concentration) is 1.3% by weight or less. The treatment temperature is not particularly limited, but is desirably performed at a temperature of 800 ° C. to 900 ° C. for the purpose of efficiently taking in carbon and nitrogen atoms.

Next, in the second step, while adjusting the conditions for increasing the internal residual γ by diffusing each atom taken in in the previous step into the inside, the carbon and nitrogen concentrations on the outermost surface are adjusted to be relatively low, thereby making the most possible. This is a treatment for suppressing an increase in surface residual γ and precipitation of coarse carbides. The treatment temperature needs to be 900 ° C. or higher in consideration of the carburization efficiency and the diffusion of atoms taken in at the previous stage. Regarding the upper limit of the processing temperature, if the temperature is too high, a problem such as coarsening of crystal grains occurs. Therefore, an appropriate temperature may be selected according to the material and the processing history. The processing time is preferably about 0.20 to 0.60 times the time required in the previous stage. If the time is shorter than this, the amount of diffusion of each atom into the inside becomes insufficient, and if the time is longer than this, the diffusion proceeds too much and the concentration of carbon and nitrogen at each position becomes thin, so that the intended effect cannot be achieved. As for the processing time, only the first and second time ratios are specified, but the absolute value of each time may be appropriately set according to the size of the target component. In the second stage carburizing treatment, at least the surface carbon concentration is set to 0.1.
It is necessary to carry out the reaction under the condition of 9% by weight or less. That is, when the surface carbon concentration exceeds 0.9% by weight, the amount of residual γ after heat treatment becomes larger than 40% by volume, and the amount of residual γ near the outermost surface is reduced to 15% by volume or less even after shot peening. It is not possible. On the other hand, the lower limit of the surface carbon concentration is preferably 0.4% by weight or more from the viewpoint of obtaining a surface strengthening effect. However, for parts requiring pitting resistance and wear resistance, the surface hardness must be kept high by carburizing at a relatively high concentration of about 0.7% by weight or more. Needless to say, it is necessary to determine the optimum surface carbon concentration within the above range according to the characteristics.

If the residual γ amount immediately below the outermost surface of the material (ie, from the outermost surface to a depth of 40 μm) immediately after the first and second stage treatments is 40 vol% or less, then the final stage ( By performing shot peening with an arc height of 0.60 mmA or more as after (quenching), the residual γ near the outermost surface causes work-induced transformation and does not promote the generation of fatigue cracks (up to 15% by volume or less). Reduced and 20-40 at 100-400 μm depth
% Of the residual γ phase is obtained.

In the third step of the shot peening treatment, in addition to the original effects of surface strengthening and compressive residual stress, reduction of surface residual γ is an important object in the present invention. The condition was that the treatment be performed with a high arc height. If necessary, tempering may be performed at a low temperature (about 200 ° C.) at which residual γ does not transform after the second stage of carburizing and quenching, as is often done in conventional carburizing.

Further, the present invention is intended to satisfy both the characteristics of crack generation suppression and crack propagation suppression. Therefore, as a means for further strengthening the surface, after the above-mentioned shot peening treatment is performed, this treatment is performed. It was confirmed that performing shot peening treatment using particles smaller than the used shot particles exhibited a greater effect.

In the method of the present invention, as described above, the carburizing and nitriding treatment and the carburizing treatment are performed in two steps, and then the shot peening treatment is performed. For example, Japanese Patent Application Laid-Open No. 3-24258 discloses a multistage treatment. A technique has been proposed to increase the penetration depth. However, this technique is aimed at shortening the time of the conventional carburizing and nitriding treatment and eliminating the gap between the depth of carbon and nitrogen penetration, and the preferred surface carbon concentration is 1.2 to 1.8% by weight. As can be seen from the fact that no mention is made of shot peening, the concept of reducing residual γ on the surface is not included. Therefore, such a method not only increases the amount of residual surface γ excessively, but also tends to precipitate coarse carbides, which is an adverse effect on improving the resistance to fatigue fracture caused by repeated bending stress and tensile compression stress, which is the technical subject of the present invention. In addition, it is a problem that the method is characterized by performing nitriding alone instead of carburizing and nitriding, resulting in poor efficiency.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention, and any design change in the spirit of the foregoing or the following is not limited to the present invention. It is included in the technical scope.

[0022]

EXAMPLES Example 1 SCM420 steel (A) and SCR42 were used as test materials.
No. 0 steel (B), these test materials were forged, subjected to normalizing treatment, processed into test pieces, and (a) subjected to long-term carburization at a low potential, followed by shot peening (S
P) and (b) ordinary carburizing after nitrocarburizing, followed by shot peening.

That is, as shown in Table 1 below and FIG.
The M420 steel was subjected to carburizing and nitriding under various carbon potential and nitrogen potential (NP) conditions for 10 hours while gradually reducing the NH ₃ flow rate, and then subjected to shot peening at an arc height of 0.84 mmA (No. 1-3). The carbon potentials of No. 1 to No. 3 were No. 1 = 0.6%, No. 2 = 0.8% and N
o. 3 = 0.5%. On the other hand, for SCR420 steel, after nitrocarburizing treatment at 570 ° C. for various times shown in Table 2 below, gas carburizing treatment at 950 ° C. for the time shown in Table 2 and then shot peening at an arc height of 0.84 mmA. (Nos. 4 to 7).

The steel parts after the treatment were subjected to a fatigue test using an Ono-type rotary bending fatigue tester. Since the present invention is particularly effective when a crack occurs at or just below the outermost surface, the fatigue test was performed using a notched fatigue test piece, and the stress concentration factor was 2.0.
The results are also shown in Table 1. Further, the residual γ distribution (that is, the relationship between the depth from the outermost surface and the residual γ amount) of the steel member after the treatment was examined by an X-ray diffractometer.
The result is shown in FIG.

[0025]

[Table 1]

[0026]

[Table 2]

From these results, the following can be considered. That is, N has a residual γ distribution as defined in the present invention.
o. The steel parts of Nos. 1 and 4 indicate that only these parts have excellent fatigue properties. That is, it was confirmed that appropriately adjusting the residual γ distribution is effective for improving the fatigue strength of the steel part.

On the other hand, Nos. 2, 3 and 5 to 7 do not satisfy any of the requirements specified in the present invention, and all of them are inferior in fatigue limit to Nos. 1 and 4. These can be considered as follows.

First, since No. 2 has a high nitrogen potential in the first half of the carburizing and nitriding treatment, the amount of residual γ at a depth of 100 to 400 μm from the outermost surface can be made 20 to 40% by volume as in No. 1. However, since the nitrogen potential in the latter half of the carburizing and nitriding treatment is too high, the amount of residual γ from the outermost surface to a depth of 40 μm cannot be set to 15% by volume or less. In No. 3, the nitrogen potential in the first half of the carburizing and nitriding treatment is too low, so that the residual γ amount at a depth of 100 to 400 μm from the outermost surface cannot be 20 to 40% by volume. In No. 5, since the pre-nitrocarburizing time was too long and the amount of invading nitrogen was excessive, the amount of residual γ from the outermost surface to a depth of 40 μm could not be reduced to 15% by volume or less. In Nos. 6 and 7, since the pre-nitrocarburizing time was too short and the amount of invading nitrogen was too small, the residual γ amount at a depth of 100 to 400 μm from the outermost surface was 20 to
It cannot be 40% by volume.

Example 2 As test materials, SCR420 steel (B), a typical carburizing steel, and SNCM, a material having a relatively high residual γ, were used.
420 steel (C) was used. These test materials were forged, subjected to normalizing treatment, processed into test pieces, and then subjected to carburizing and carburizing-nitriding treatments and oil-quenched from the γ region. Subsequently, tempering was performed at 180 ° C. for 2 hours, and thereafter, shot peening treatment was performed. In this embodiment, the first
The carburizing and nitriding treatment of the second stage and the carburizing treatment of the second stage were continuously performed in the same furnace, but two furnaces for carburizing and nitriding were used, or the temperature was temporarily reduced after the carburizing and nitriding treatment. Any procedure, such as lowering the temperature to room temperature and then raising the temperature again to perform carburization, may be employed.

Since the fatigue strength of the shot peening treatment largely depends on the strength, consideration was given to making the comparison between the treatments as uniform as possible. As a result, regarding the first-stage shot peening, all shot materials having a diameter of 0.6 mm were used, and the arc height was changed by changing the initial projection speed as necessary. When performing two-stage shot peening,
The shot material of the second stage uses the thing of diameter 0.15mm,
All arc heights were unified to 0.2 mmA. Table 3 shows the processing conditions.

[0032]

[Table 3]

For the carburized steel part after the treatment,
A fatigue test was performed in the same manner as in Example 1. Table 4 shows the measured values of the residual γ amount by an X-ray diffractometer immediately after the carburizing (nitriding) treatment and after the shot peening treatment, and also shows the fatigue limit of each treated material. Note that the residual γ content is as follows: the residual γ volume ratio at a depth of 40 μm after carburizing (nitriding) treatment and before shot peening, the residual γ volume ratio at the same position after shot peening, and a depth of 300 after shot peening.
Three types of measured values of the residual γ volume ratio inside μm are shown.

[0034]

[Table 4]

From the results shown in Table 4, the following can be considered. First
Nos. 8 to 11 all satisfy all the requirements specified by the method of the present invention, and show excellent fatigue limit values. In particular, No. 9 was obtained by applying two-stage shot peening, and its fatigue strength was remarkably excellent. No.
In Nos. 10 and 11, the processing time was adjusted so that the value of the processing time ratio (u / t) became lower and higher within the specified ranges, respectively. Is shown.

On the other hand, No. 12 to No. 18 do not satisfy any of the requirements specified in the method of the present invention, and all of them are inferior in fatigue limit to No. 8 to No. 11.
These can be considered as follows.

First, No. 12 and No. 13 are those in which the surface carbon concentration and the surface nitrogen concentration in the first stage are respectively lower than the ranges specified by the method of the present invention, whereby the surface residual γ is suppressed low. However, the amount of internal residual γ is also 1
Since it is very low, that is, more than 0% by volume, the intended effect cannot be obtained, and particularly good fatigue strength has not been obtained. N
o.14 is because the surface carbon concentration in the second stage was too high,
The surface residual γ after the heat treatment is 54.8%, which greatly exceeds the specified range of the present invention. Even after the shot peening, the residual γ of 30% or more remains on the surface to reduce the fatigue strength. In No. 15, the value of u / t is smaller than the specified range of the present invention, and the diffusion of atoms taken in the first stage becomes insufficient.
The amount is small and the desired effect has not been obtained. No. 16
Contrary to No. 15, the value of u / t is large, and the diffusion of the atoms taken in in the first stage progresses too much, particularly the nitrogen atoms are totally diluted, and the intended effect is not obtained. . In No. 17, (surface carbon concentration + surface nitrogen concentration) in the first stage was higher than the range specified in the present invention, and as a result, the amount of surface residual γ after heat treatment was higher than the value specified in the present invention. Is also larger, this is also No. 14
The fatigue strength is low as well. No. 18 had a low shot peening strength, indicating a low strength because the residual γ was not sufficiently transformed.

Nos. 19 to 26 cover typical prior art. No. 19 is obtained by performing carburizing treatment alone at a normal surface carbon concentration. The residual γ content is extremely low on both the surface and the inside, and does not show sufficient strength. No. 20 is a single treatment of carburizing and nitriding. When the surface nitrogen concentration was set to 0.3% by weight, the amount of internal residual γ after the heat treatment showed a value similar to that specified in the present invention.
Correspondingly, the surface residual γ amount was as large as 53% by volume, and 31.6% by volume still remained after shot peening. As a result, although the fatigue limit was improved as compared with the single carburized material, the result was inferior to the example of the present invention. The result of applying two-stage shot peening to this is N
o. No. 21 shows a much higher fatigue limit than the single shot peened material, but it is a very low value as compared with the two-stage shot peened material of the present invention (No. 9). It should be noted that the difference in fatigue limit during single shot peening (No. 8 or No. 10)
And the difference between No. 20 and 62: 69 MPa), the difference in the fatigue limit value during two-step peening (the difference between No. 9 and No. 21: 9).
3MPa), which shows that the effect of the present invention is more remarkably exhibited when two-stage peening is applied.

No. 22 is an example of high-concentration carburization + hard shot peening disclosed in JP-A-62-185826. Here, as described in Table 2,
Carburization was performed in two stages (temperature was lowered in the second stage and excess carburization was performed). Shot peening was also performed so as to satisfy the conditions described in the gazette. As shown in Table 3, although the internal residual γ was successfully increased as shown in Table 3, the amount of residual γ on the outermost surface that remained in a very large amount could not be reduced even by shot peening, and good fatigue strength was obtained. Could not. Nos. 23 to 25 are disclosed in JP-A-3-2425.
No. 8 discloses a multi-stage treatment using a combination of carburizing, carbonitriding and carbonitriding.
Starting at ~ 800 ° C. The conditions described in the publication are very high potentials with a surface carbon concentration of 1.2 to 1.8% by weight and a surface nitrogen concentration of 0.3 to 1.0% by weight. After treatment, the amount of surface residual γ after heat treatment was 70% by volume.
As described above, furthermore, a large amount of coarse carbides precipitated,
Under the conditions of the fatigue test, the fatigue strength was significantly reduced.

By the way, No. 23 and No. 25 cannot be expected to reduce the amount of residual γ on the outermost surface even if the improvement is made since the final step is carburizing and nitriding.
24 is a single stage carburizing process, which is considered to be a process similar to the method of the present invention. Therefore, an attempt was made to adjust the conditions in order to examine whether the same effect as the method of the present invention can be obtained by such a method. No. 26 and No. 27 show the results.

In No. 26, the nitrogen potential in the first stage was increased to 0.8% by weight, and the carbon potential in the second stage was further increased, considering that the total processing time was as close as possible to the method of the present invention. Was adjusted to 0.8% by weight. As a result, the amount of residual γ on the outermost surface after carburization was 57.2% by volume, which greatly exceeded the range specified in the present invention, and thus the fatigue strength was not improved. This is presumably because when this method was used, the concentration gradient of nitrogen introduced in the first half was very large, and diffusion in the second half was insufficient. Therefore, we extended the carburizing time in the second half,
I decided to spread enough. As a result, as shown in No. 27, it was found that the residual γ distribution specified in the present invention can be obtained by setting the latter half carburizing time to 4.5 hours (Example of the present invention). However, due to the lack of the requirements specified by the method of the present invention, the fatigue strength is slightly inferior to those of Nos. 8 to 11. In addition, the total time must be longer than that of the method of the present invention, and in particular, the use time at a high temperature of the furnace is overwhelmingly long, so that the durability of the furnace is slightly inferior. Further, when the first half and the second half are continuously performed in the same furnace, the temperature difference between the first stage and the second stage is large, so that it takes time to raise the temperature, and the total time further increases. . In view of the above, it can be seen that the method of the present invention can achieve the object more efficiently than the single nitriding → single carburizing treatment.

Example 3 The above S was used as a test material so that the amount of residual γ was relatively large.
Using NCM420 steel (C), surface carbon concentration = 0.9
870 ° C so that 5% and surface nitrogen concentration = 0.33%
For 3 hours (treatment No. 1 in Table 2 above).
0 to the first stage treatment). At this time, the residual γ amount from the outermost surface to a depth of 40 μm after tempering is 50% by volume.
Met. Thereafter, the test material was immersed in liquid nitrogen for only 1 second, pulled up, and then heated with hot air to quickly return to room temperature. As a result, the amount of residual γ from the outermost surface to a depth of 40 μm was reduced to 13% by volume. Also, 30 from the outermost surface
The residual γ amount at a depth of 0 μm was 28.7% by volume, and the amount of decrease in the residual γ amount was smaller than that of the surface. The fatigue limit of the steel part thus obtained was as good as 921 MPa.

[0043]

The present invention has been configured as described above, and a high-strength steel part exhibiting excellent fatigue strength and an optimum method for obtaining such a high-strength steel part have been realized.

[Brief description of the drawings]

FIG. 1 is a graph showing a residual γ distribution of a steel member subjected to a treatment in Example 1.

FIG. 2 is a graph for explaining conditions of carburizing and nitriding in Example 1.

Claims (5)

(57) [Claims] Claims: 1. A method for manufacturing a semiconductor device comprising a surface treatment involving the introduction of carbon atoms and / or nitrogen atoms into a steel part as one or more of its manufacturing steps. A high-strength steel part excellent in fatigue strength, characterized in that the amount of retained austenite is 15% by volume or less and the amount of retained austenite at a depth of 100 to 400 µm from the outermost surface is 20 to 40% by volume. 2. A part made of carburizing steel has a surface carbon concentration of 0.7% by weight or more, a surface nitrogen concentration of 0.2% by weight or more, and (surface carbon concentration + surface nitrogen concentration): 1.
After performing the carburizing and nitriding treatment for 3 hours by weight so as to be 3% by weight or less, the carburizing treatment is performed at a temperature of 900 ° C. or more so that the surface carbon concentration becomes 0.4 to 0.9% by weight. By performing the treatment for 6 T hours, the amount of retained austenite from the outermost surface to a depth of 40 μm is reduced to 40% by volume or less, and the arc height is reduced to 0.1%.
The residue provide Reinforced above shot peening 6mmA
A method for producing a high-strength steel part having excellent fatigue strength, characterized in that the amount of retained austenite is 15% by volume or less . 3. A high-strength steel with excellent fatigue strength, characterized in that after the treatment according to claim 2, shot peening is performed again using particles smaller than the shot particles used in the shot peening. Manufacturing method for high strength steel parts. 4. A high-strength steel part excellent in fatigue strength according to claim 1, which is obtained by the method according to claim 2 or 3. 5. A method for producing a high-strength steel part having excellent fatigue strength, comprising producing the high-strength steel part according to claim 1 by the method according to claim 2 or 3.