JP2011105998A - Treatment method for steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a treatment method for steel which improves a fatigue strength of the steel. <P>SOLUTION: The treatment method for the steel comprises: a process (S1) of applying a quenching treatment to the steel having 0.15-0.70% carbon content; a heat treatment process (S2) of applying a tempering treatment to the steel by heating it at 250-300°C for ≥2 hr thereby transforming an austenite in the structure into a martensite; a first shot peening process (S3) of applying shot peening to the steel by using shot particles with a large diameter; and after the first shot peening process (S3), a second shot peening process (S4) of applying shot peening to the steel by using shot particles with a small diameter. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a heat treatment of a steel material. More specifically, the present invention relates to a heat treatment of a steel material that is a material of equipment that is exposed to large bending, twisting and rolling loads, such as a gear of an automobile transmission.

Steel products that are constantly subjected to large bending, twisting, and rolling (pitching) loads, such as gears for automobile transmissions, are subjected to carburizing quenching and carbonitriding quenching for the purpose of improving fatigue strength. After that, a normalizing process is performed at 150 to 250 ° C.
Here, after carburizing and quenching or carbonitriding, the metal product may be shot peened, but the compressive residual stress applied near the surface of the metal product by shot peening is caused by heat treatment at about 350 ° C. It will be almost completely open.
Therefore, it has been found that when tempering at about 350 ° C. is performed after shot peening, the effect of improving the fatigue strength by such shot peening is small.

In recent years, with the increase in engine torque, steel products such as gears subjected to carbonitriding used in automobile transmissions have been required to further improve fatigue strength.
Here, it is conceivable to perform carburizing and quenching heat treatment in a high-concentration carbon environment (so-called “high-concentration carburizing and quenching heat treatment”), but fatigue strength sufficient to meet the above-described requirements has not been obtained.

There is also a so-called “high concentration carbonitriding and quenching treatment”, which is a combination of the nitriding method for infiltrating nitrogen into the metal surface to improve fatigue strength and the above-described high concentration carburizing and quenching heat treatment.
As shown in FIG. 8 (room temperature fatigue diagram), the fatigue strength at room temperature of the steel material subjected to the high concentration carbonitriding and quenching treatment is the fatigue strength of the steel material (for example, SCM420H carburized steel material) subjected to the known carburizing and quenching treatment. It is better than strength.
Here, the fatigue strength characteristic at normal temperature of the steel material subjected to the high concentration carbonitriding and quenching treatment is displayed as “high concentration carbonitriding material” or “normal temperature high concentration carbonitriding product” in FIG. Is indicated by a plot of “◆”.
In addition, the fatigue strength characteristics of steel materials that have been subjected to a known carburizing and quenching treatment are displayed as “current carburizing and quenching material” or “room temperature: current SCM420H carburizing” in FIG. ing.

However, as shown in FIG. 8, in a region where the number of repetitions is large at normal temperature, the fatigue strength at normal temperature of the steel material subjected to the high concentration carbonitriding and quenching treatment is the normal temperature of the steel material subjected to the high concentration carburizing and quenching treatment. Exceeds fatigue strength, but less in regions with fewer repeats.
Note that the fatigue strength characteristics at room temperature of steel materials that have been subjected to high-concentration carburizing and quenching treatment are displayed as “high-concentration carburized material” or “normal-temperature high-concentration carburized product” in FIG. It is shown.

Similarly, as shown in FIG. 9 (high temperature fatigue diagram), even in the results of high temperature fatigue tests, the fatigue strength (at high temperature) of the steel subjected to high concentration carbonitriding and quenching treatment in the region where the number of repetitions is large. Is higher than the fatigue strength (at high temperature) of steel that has been subjected to high-concentration carburizing and quenching treatment, but lower in regions where the number of repetitions is small.
That is, the high-concentration carbonitriding and quenching treatment may have a lower fatigue strength than the case where the high-concentration carburizing and quenching treatment is performed depending on the number of repetitions at room temperature and high temperature.
Here, the fatigue strength characteristics at high temperatures of steels that have been subjected to high-concentration carbonitriding and quenching are indicated as “high-concentration carbonitriding materials” or “high-temperature: high-concentration carbonitriding products”. 'Is shown in the plot.
The fatigue strength characteristics at high temperatures of steel materials that have been subjected to high-concentration carburizing and quenching treatment are displayed as “high-concentration carburized materials” or “high-temperature: high-concentration carburized products”, and are indicated by a plot of “◆”. Yes.

  The fatigue strength characteristics shown in FIGS. 8 and 9 are obtained by conducting fatigue fracture tests according to JIS Z 2273 “General Rules for Fatigue Test Methods for Metallic Materials” and JIS Z2274 “Test Methods for Rotating Bending Fatigue Metallic Materials”. It was.

  In addition to this, when high concentration carbonitriding and quenching is performed, a large amount of austenite (residual austenite) is precipitated on the surface of the steel material. Austenite is an incompletely hardened structure and reduces the fatigue strength, so the desired fatigue strength cannot be obtained.

As other conventional technologies, carburized bearing parts with excellent wear resistance, seizure resistance, and rolling fatigue life are proposed even in the presence of contact fatigue with slip or in an environment where the lubricating oil is dilute. (For example, refer to Patent Document 1).
However, in this prior art (Patent Document 1), in addition to strictly defining the conditions of the material, the application is limited to bearing parts, so the above-described problem cannot be solved.

In addition, carburized parts or carbonitrided parts excellent in bending fatigue resistance and pitting resistance have been provided (see, for example, Patent Document 2).
However, in the related art (Patent Document 2), the conditions of the material are limited, and it is difficult to improve the fatigue strength of carbonitrided parts in general.

  Further, conventional techniques related to vacuum carbonitriding methods (for example, Patent Document 3) and conventional techniques related to gas carburizing methods, gas carbonitriding methods, and surface treatment apparatuses (for example, see Patent Document 4) have been proposed. It does not solve the problem.

JP 2006-97096 A JP 2008-88536 A International Publication No. 2003/050321 Pamphlet JP 2005-120404 A

  The present invention has been proposed in view of the above-described problems of the prior art, and an object of the present invention is to provide a heat treatment method for steel that can improve the fatigue strength of the steel.

As a result of various studies, the inventor conducted a heat treatment (tempering) on a carbonitrided steel material at a predetermined temperature for a predetermined time or longer and at a constant temperature, the fatigue strength in the entire range from a low cycle to a high cycle. Found to improve.
At the same time, the inventor has paid attention to the fact that shot peening improves various strengths of steel materials.
The present invention has been proposed based on such knowledge.

  The steel material heat treatment method of the present invention includes a step (S1) of performing a treatment for infiltrating nitrogen and carbon into a steel material having a carbon content of 0.15% to 0.70%, and heating at 250 ° C. to 300 ° C. for 2 hours or more. A tempering treatment is performed to transform the austenite (99.5% or more, more specifically 99.8% or more) of the structure into martensite, and a large diameter (φ0.3 to 1.. After the first stage shot peening process (S3) for performing shot peening using 2 mm) shot grains, and the first stage shot peening process (S3), shot grains having a small diameter (φ0.01 to 0.2 mm) And a second stage shot peening (S4) step in which shot peening is performed using.

Here, the first-stage and / or second-stage shot peening process (S1, S2) is preferably performed by impeller shot in which shot grains are injected by a flywheel or air shot in which shot grains are injected by air. .
In the first stage and / or second stage shot peening process (S1, S2), the arc height (shot peening strength measurement method: SAEJ442a) representing the strength is 0.4 mm (A type) or more. Is preferred.

In carrying out the present invention, in the step (S1) of performing the treatment for intruding nitrogen and carbon, a high concentration carbonitriding and quenching treatment is performed, and the steel surface has a carbon content of 1.0% or more and nitrogen of less than 1.0%. Penetration is preferable.
Here, high concentration carbonitriding and quenching is quenching performed with carbon and nitrogen concentrations higher than carbonitriding and quenching. The carbonitriding method is a surface hardening method in which a steel alloy absorbs carbon and nitrogen simultaneously by placing the steel alloy and carbon in a hot gas atmosphere.
In other words, high-concentration carbonitriding and quenching cause carbide to precipitate in the carbon-infiltrated layer, and carbonitriding and quenching does not cause carbide to precipitate in the carbon-infiltrated layer.

  Alternatively, in the present invention, in the step (S1) of injecting nitrogen and carbon, carbonitriding and quenching is performed, and carbon is 0.7 to 1.2% and nitrogen is 0.2 to 1.0% on the steel surface. Penetration is preferable.

  According to the present invention having the above-described configuration, the step of applying nitrogen and carbon to the steel material (S1) and the tempering process (S2) by heating at 250 ° C. to 300 ° C. for 2 hours or more are performed. Thus, in the structure after heat treatment (tempering treatment), most of the austenite that is an incompletely quenched structure (99.5% or more, more specifically 99.8% or more) is transformed into martensite that is a quenched structure. As a result, the fatigue strength against bending, twisting and rolling (pitting) in the steel material can be improved.

In addition, according to the present invention, the first stage shot peening process (S3) is performed using shot grains having a large diameter (φ0.3 to 1.2 mm), and a relatively deep region from the steel surface is obtained. Compressive residual stress integration peak is formed. Then, by performing the second-stage shot peening step (S4) using small diameter (φ0.01 to 0.2 mm) shot grains, a peak of the compressive residual stress integral value is formed in the vicinity of the steel material surface. As a result, the integrated compressive residual stress in the region from the steel surface to a certain depth (for example, 150 μm) can be increased.
Here, there is a correlation between the compressive residual stress integrated value and the fatigue strength (see FIG. 4). In the present invention, the compressive residual stress integrated value in the region from the steel surface to a certain depth (for example, 150 μm) is increased. Therefore, the fatigue strength of the steel material in the region is improved.

3 is a flowchart showing a processing method according to the first embodiment of the present invention. It is a normal temperature fatigue diagram of the steel materials according to Experimental Example 1. It is a characteristic view which shows the relationship between the magnitude | size of the compressive residual stress value in the area | region to the depth of 150 micrometers from the material surface, and 10 ⁶ times fatigue strength. It is a characteristic view which shows the relationship between the distance from the steel material surface after the shot peening process of the steel materials which concern on Experimental example 1, and compression residual stress distribution. It is a flowchart which shows the processing method which concerns on 2nd Embodiment of this invention. It is a fatigue diagram in 300 degreeC of the steel materials which concern on Experimental example 9. It is a characteristic view which shows the relationship between the distance from the steel material surface after the shot peening process of the steel materials which concern on Experimental example 9, and compression residual stress distribution. It is a fatigue diagram in the normal temperature of the steel materials which concern on a prior art. It is a fatigue diagram in the high temperature of the steel material which concerns on a prior art.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
First, a first embodiment will be described based on FIGS.
FIG. 1 shows the procedure of the steel material processing method according to the first embodiment.
1, the steel material processing method according to the first embodiment includes a high concentration carbonitriding process S1, a tempering process S2, a first (first stage) shot peening process S3, and a second (second stage). ) Of shot peening process S4.
Here, the high-concentration carbonitriding process S1 is a process for performing a process of intruding nitrogen and carbon.

In the first embodiment, the high concentration carbonitriding process S1 is performed on a steel material having a carbon content of 0.15% to 0.70%. And in the tempering treatment step S2, the tempering treatment is performed by heating at 250 ° C. to 300 ° C. for 2 hours or more, and the austenite in the structure (99.5% or more, more specifically 99.8% or more) is obtained. Transform to martensite.
In the high-concentration carbonitriding process S1 in the first embodiment, the carbon intrusion amount to the steel material surface is 1.0% or more, and the nitrogen infiltration amount is slightly less than 1.0%.
Here, high-concentration carbonitriding and quenching cause carbide to precipitate in the carbon-infiltrated layer, and carbonitriding and quenching does not cause carbide to precipitate in the carbon-infiltrated layer.
In both high concentration carbonitriding and quenching (high concentration carbonitriding treatment) and carbonitriding and quenching (carbonitriding treatment), the amount of nitrogen entering the steel surface is less than 1.0%.
Hereinafter, the carbonitriding process in which the carbon intrusion amount to the steel surface is 1.0% or more and the nitrogen intrusion amount is less than 1.0% is referred to as “high concentration carbonitriding”, “high concentration carbonitriding quenching” or “high “Concentration carbonitriding” is displayed.

In the first shot peening treatment step S3, shot peening is performed using shot grains having a relatively large diameter (for example, φ0.3 to 1.2 mm). Then, after the first stage shot peening process, in the second shot peening process S4, shot peening is performed using shot grains having a relatively small diameter (for example, φ0.01 to 0.2 mm).
Here, in the first-stage shot peening step S3, shot grains having a relatively large diameter (for example, φ0.3 to 1.2 mm) are collided with the steel surface, and a relatively deep region (for example, the steel surface) To about 50 μm), a compression residual stress integrated value peak is formed.
On the other hand, in the second-stage shot peening step S4, shot grains having a relatively small diameter (for example, φ0.01 to 0.2 mm) are collided with the steel material surface to form a peak of the compression residual stress integral value in the vicinity of the steel material surface. .

The first-stage shot peening step S3 and / or the second-stage shot peening step S4 are preferably performed by impeller shots in which shot particles are injected by a flywheel or air shots in which shot particles are injected by air.
In the first-stage shot peening step S3 and / or the second-stage shot peening step S4, the arc height (shot peening intensity measurement method: SAEJ442a) is preferably 0.4 mm (A type) or more.

[Experimental Example 1]
In Experimental Example 1, high concentration carbonitriding and quenching was performed on an alloy having a carbon content of 0.2%, in which 1.5% carbon and 0.6% nitrogen were infiltrated into the steel material surface.
And the alloy steel which performed the high concentration carbonitriding quenching was heated at 300 degreeC for 2 hours, and the tempering process was performed.
As described above, high concentration carbonitriding and quenching cause carbide to precipitate in the carbon intrusion layer, and carbonitriding and quenching does not cause carbide to precipitate in the carbon infiltration layer. In both high concentration carbonitriding and quenching (high concentration carbonitriding treatment) and carbonitriding and quenching (carbonitriding treatment), the amount of nitrogen entering the steel surface is less than 1.0%.
After the tempering treatment, first shot peening is performed by impeller shot using shot grains having a particle diameter of 0.8 mm, and then second shot peening is performed by air blow including shot grains having a particle diameter of 0.1 mm. went.
The steel material thus manufactured was subjected to a fatigue fracture test at room temperature according to JIS Z 2273 “General Rules for Fatigue Testing Methods for Metallic Materials” and JIS Z2274 “Testing Methods for Rotating Bending Fatigue Metallic Materials” at room temperature. The fatigue fracture characteristics obtained as a result of the fatigue fracture test are shown in FIG.

In FIG. 2, the fatigue fracture characteristics of the steel materials according to Experimental Example 1 are indicated by “◯” plots, and are labeled “present invention” or “present invention product”.
In FIG. 2, for comparison, the fatigue failure characteristics of SCM420H carburized steel at room temperature, the fatigue fracture characteristics of steel materials subjected to a known high-concentration carburizing and quenching treatment, and the known high-concentration carbonitriding and quenching treatment are performed. After that, the fatigue fracture characteristics at room temperature of the steel material subjected to two-stage shot peening are shown.
Here, the term “two-stage shot peening” means that shot peening is performed twice (first and second stages) using (two types of) shot grains having different particle diameters. is doing. In this case, shot peening with a relatively large particle size is performed, and then shot peening with a relatively small particle size is performed.
In the accompanying drawings, there is a phrase “two-stage peening”, which is an abbreviation of “two-stage shot peening”. In the drawings, the abbreviation is used because the area for explaining the plot is small and the number of characters that can be used is limited.

In FIG. 2, the fatigue strength characteristics of SCM420H carburized steel (steel material that has not been subjected to tempering treatment or two-stage shot peening) are displayed as “current carburizing and quenching material” or “normal temperature: current SCM420H carburizing”. It is shown by the plot of “■”.
Fatigue fracture characteristics at room temperature of steel materials that have been subjected to known high-concentration carburizing and quenching treatment (steel materials that have not been tempered or two-step shot peened) are "high-concentration carburized materials" or "normal-temperature high-concentration carburized products" Is indicated by a “●” plot.
The fatigue fracture characteristics at room temperature of a steel material (steel material not subjected to tempering treatment) that has been subjected to a known high-concentration carbonitriding and quenching treatment and subjected to two-stage shot peening is “high-concentration carbonitriding + two-stage peening” or “High-concentration carbonitriding two-stage peening product” is displayed, which is indicated by a plot of “♦”. As described above, the term “two-stage peening” is an abbreviation for “two-stage peening”.

As is clear from FIG. 2, the steel material according to Experimental Example 1 is a comparative example of SCM420H carburized steel, high-concentration carburized material, and steel material subjected to two-stage shot peening after performing high-concentration carbonitriding and quenching treatment. The fatigue strength is improved from the region where the number of repetitions N is small to the region where the number of repetitions N is large.
Compared to steel material that has been subjected to high-concentration carbonitriding and quenching treatment and subjected to two-stage shot peening (steel material that has not been tempered: “♦” plot), the steel material according to Experimental Example 1 (“○”) The fatigue strength of the plot (2) is improved because the steel according to Experimental Example 1 is subjected to tempering by heating at 300 ° C. for 2 hours after high concentration carbonitriding, and incomplete quenching. It is presumed that one of the causes is that the structure (retained austenite) is changed to a martensite structure (transformation).
Further, it is presumed that by performing two-stage shot peening after heating at 300 ° C. for 2 hours and performing tempering treatment, the integrated compressive residual stress increases and fatigue strength is improved.

Here, it is clear from FIG. 3 that the fatigue strength improves if the compression residual stress integral value increases.
FIG. 3 is a characteristic diagram showing the relationship between the integrated value of compressive residual stress (MPa · mm) from the surface of the steel material to 150 μm and the fatigue strength (MPa) of 10 ⁶ times.
In FIG. 3, a proportional relationship is substantially established between the integrated value of compressive residual stress (MPa · mm) from the surface of the steel material to 150 μm and the fatigue strength (MPa) of 10 ⁶ times. That is, if the integrated compressive residual stress value (MPa · mm) up to 150 μm is high, the fatigue strength (MPa) of 10 ⁶ times is improved.
From this, it is clear that the compressive residual stress integral value is increased and the fatigue strength is improved.
In FIG. 3, both fatigue test data at normal temperature (“●”) and fatigue test data at 300 ° C. (high temperature) (“■”) are used. That is, even at room temperature or high temperature (300 ° C.), if the compressive residual stress integral value is increased, the fatigue strength is improved.

It is clear from FIG. 4 that the compressive residual stress increases by performing two-stage shot peening.
FIG. 4 shows the distribution of compressive residual stress after the two-stage shot peening treatment from the surface of the steel material according to Experimental Example 1 to a depth of 150 μm.
In Fig. 4, compared with the residual stress value of the steel material not subjected to shot peening (“□” plot), which is only the high concentration carbonitriding and quenching, which is the object of comparison, the two-stage shot peening treatment is performed after the high concentration carbonitriding and quenching. Steel materials (plots of “♦”) and steel materials according to Experimental Example 1 (steel materials tempered at 300 ° C. for 2 hours after quenching with high concentration carbonitriding and then subjected to two-stage shot peening treatment: plots of “◯”) In both cases, the absolute value of the residual stress value is increased.

Here, in FIG. 4, the residual stress value is a compressive stress, and the sign is “minus”. Therefore, the large residual stress (compressive stress) value in FIG. 4 means that the absolute value is large, and the characteristic curve is located below in FIG.
In FIG. 4, the characteristic curve of the steel material not subjected to the two-stage shot peening treatment ("□" plot) and the characteristic curve of the steel material subjected to the two-stage shot peening treatment ("♦" plot) and the experimental example The plot of “◯” in the steel material according to 1) is located below, and the absolute value of residual stress (compressive stress) is large.
In the result of Experimental Example 1 shown in FIG. 2, the cause of the improvement in the fatigue strength (the plot of “◯” in FIG. 2) of the steel material according to Experimental Example 1 (the product of the present invention) is the compression residual stress due to two-stage shot peening. It is reasonable from the result of FIG. 2 that the increase is estimated.

[Experiment 2]
In Experimental Example 2, the fatigue fracture characteristics were determined under the same conditions as in Experimental Example 1 by changing the heating temperature in the tempering process by 10 ° C. from 230 ° C. to 320 ° C.
The results are shown in Table 1 below.
In Table 1, the case where the heating temperature is 300 ° C. is not shown. This is because the fatigue strength in this case is the same as in Experimental Example 1.
Table 1

In Table 1, the mark “◯” means that the steel had a fatigue strength equivalent to or higher than that of the steel material according to Experimental Example 1 (heating temperature 300 ° C.).
On the other hand, the “x” mark means that the fatigue strength was significantly inferior to the steel material according to Experimental Example 1 (heating temperature 300 ° C.).
From Table 1, it is understood that the heating temperature in the heat treatment (tempering treatment) in the present invention is preferably 250 ° C to 300 ° C.

When the treatment of Experimental Example 1 was performed, the surface of the steel material with improved fatigue strength transformed from austenite, which was an incompletely quenched structure with reduced fatigue strength, to martensite, in which 99.8% or more was a quenched structure.
On the other hand, if the temperature during the tempering process is low (less than 250 ° C.), the precipitation of hard micronitrides does not proceed, and the transformation from austenite to martensite does not proceed. Is estimated to have not improved.
On the other hand, if the tempering temperature is high (higher than 300 ° C.), hard micronitrides are precipitated, but the martensite itself is tempered because of the high temperature, so the hardness decreases and the fatigue strength also decreases. Resulting in. Therefore, it is estimated that the fatigue strength was not improved.

[Experiment 3]
In Experimental Example 3, the fatigue fracture characteristics were determined under the same conditions as in Experimental Example 1 by changing the heating time in the tempering process from 1 hour to 5 hours in 0.5 hour increments.
The results are shown in Table 2 below.
In Table 2, the case where the heating time is 2 hours is not shown because the conditions are the same as in Experimental Example 1.
Table 2

In Table 2, the mark “◯” means that the steel material according to Experimental Example 1 (heating time: 2 hours) had the same or higher fatigue strength.
On the other hand, the “x” mark means that the fatigue strength was significantly inferior to the steel material according to Experimental Example 1 (heating time: 2 hours).
From Table 2, it is understood that the heating time in the heat treatment (tempering treatment) in the present invention may be 2 hours or more.
If the tempering time is too short (less than 2 hours), the precipitation of hard micronitrides will not proceed, and the heat treatment (tempering) will not be sufficiently performed, and the transformation from austenite to martensite will not proceed. It is estimated that the fatigue strength was not improved.

Here, if the heating time is too long, there is no problem in terms of fatigue strength, but it is disadvantageous in terms of the efficiency of heat treatment, energy consumption for heating, and other costs.
According to the inventor's calculations, it has been clarified that the heat treatment time (heating time, tempering time) exceeds 5 hours is not appropriate from the viewpoint of the efficiency and cost described above.
Accordingly, it can be said that the heating time in the heat treatment (tempering treatment) in the present invention is appropriate from 2 hours to 5 hours.

[Experimental Example 4]
In Experimental Example 4, the carbon content of the alloy steel before performing the treatment according to the embodiment is changed in 0.05% increments in the range of 0.10% to 0.80%, and the same conditions as in Experimental Example 1 The fatigue fracture characteristics were obtained.
The results are shown in Table 3 below.
In Table 3, the case where the carbon content of the alloy steel is 0.20% is not shown because the conditions are the same as in Experimental Example 1.
Table 3

In Table 3, the mark “◯” means that the steel had a fatigue strength equivalent to or higher than that of the steel material according to Experimental Example 1 (the carbon content of the alloy steel was 0.2%).
On the other hand, the “x” mark means that the fatigue strength is significantly inferior to the steel material according to Experimental Example 1 (the carbon content of the alloy steel is 0.2%).
In Table 3, when the carbon content of the alloy steel is 0.35% to 0.60%, all are equal to or higher than the steel material according to Experimental Example 1 (the carbon content of the alloy steel is 0.2%). Since the fatigue strength was as follows (“◯” mark in Table 3), the display was omitted.
From Table 3, it is understood that 0.15% to 0.70% is suitable for the carbon content of the alloy steel before the heat treatment (tempering treatment) in the present invention.

[Experimental Example 5]
In Experimental Example 5, when performing high-concentration carbonitriding on the alloy steel, the carbon amount to be infiltrated into the steel material surface is changed in 0.5% increments in the range of 0.5% to 2.0%. In the range of 0% to 2.5%, the fatigue fracture characteristics were obtained under the same conditions as in Experimental Example 1 by changing in increments of 0.1%.
The results are shown in Table 4 below.
In Table 4, the case where the carbon intrusion amount is 1.5% is not shown because the conditions are the same as those in Experimental Example 1.
Table 4

In Table 4, “◯” marks mean that the steel had a fatigue strength equivalent to or higher than that of the steel material according to Experimental Example 1 (carbon intrusion amount was 1.5%).
On the other hand, the “x” mark means that the fatigue strength is significantly inferior to the steel material according to Experimental Example 1 (carbon intrusion amount is 1.5%).
From Table 4 and Experimental Example 1 (carbon intrusion amount is 1.5%), the carbon intrusion amount in the high-concentration carbonitriding process before performing the heat treatment (tempering treatment) in the present invention is suitably 1.0% or more. It is understood that there is.

[Experimental Example 6]
In Experimental Example 6, when performing high-concentration carbonitriding on the alloy steel, the amount of nitrogen to be infiltrated into the steel surface is changed in a range of 0.0% to 1.2% in increments of 0.1%, Fatigue fracture characteristics were determined under the same conditions as in Experimental Example 1.
The results are shown in Table 5 below.
Table 5

Also in Table 5, the mark “◯” means that the steel had a fatigue strength equivalent to or higher than that of the steel material according to Experimental Example 1 (the nitrogen penetration amount was 0.6%).
From the results shown in Table 5, it was found that 0.2% to 0.9% of the nitrogen intrusion amount in the high concentration carbonitriding treatment before the heat treatment (tempering treatment) in the present invention is appropriate. In other words, it was confirmed that “0.2% to 0.9%” is appropriate for “a little less than 1%”.
When the nitrogen intrusion amount is 1.0% or more, it is presumed that a large amount of residual austenite is precipitated, resulting in a decrease in fatigue strength.

In Experimental Example 5 and Experimental Example 6, the inventor suggested that, in a high concentration carbonitriding process, the amount of carbon and the amount of nitrogen that can be contained in steel (the amount of carbon intrusion and the amount of nitrogen intrusion) are actually total. And found that the limit is about 3.0%.
Carbon and nitrogen are solidified on the surface of the steel material, and when the total amount exceeds about 3.0%, the steel is saturated, and excess nitrogen is present as a void (blow hole etc.) in the vicinity of the steel material surface. This is a kind of material defect (defective), and therefore causes a reduction in the strength of the steel material.

[Experimental Example 7]
In Experimental Example 7, when two-stage shot peening is performed on the alloy steel (steel material) of Experimental Example 1, the shot grains used in the second-stage shot peening treatment are fixed to 0.1 mm as in Experimental Example 1. However, the grain size of the shot grains used in the first stage shot peening treatment was increased by 0.2 mm in the range of 0.1 mm to 2.1 mm, and then changed in increments of 0.3 mm. And the fatigue fracture test similar to Experimental example 1 was done, and the fatigue fracture characteristic was calculated | required.
The results are shown in Table 6 below.
Table 6

In Table 6, “◯” indicates a fatigue strength equivalent to or higher than that of the steel material according to Experimental Example 1 (first stage shot particle size is 1.0 mm, second stage shot particle size is 0.1 mm). It means that it had.
On the other hand, the “x” mark indicates that the fatigue strength is significantly inferior to the steel material according to Experimental Example 1 (the first stage shot particle size is 1.0 mm and the second stage shot particle size is 0.1 mm). It means that it was.
From the results shown in Table 6, it was found that 0.3 to 1.2 mm is appropriate as the shot particle size of the first stage shot peening in the present invention.

[Experimental Example 8]
In Experimental Example 8, when performing two-stage shot peening of the same alloy steel (steel material) as in Experimental Example 1, the grain size of shot grains used in the first stage shot peening is fixed at 1.0 mm, which is the same as in Experimental Example 1. However, the particle size of the shot grains used in the second stage shot peening was changed in the range of 0.005 mm to 0.30 mm. And the fatigue test similar to Experimental example 1 was done, and the fatigue fracture characteristic was calculated | required.
The results are shown in Table 7 below.
In Table 7, the case where the shot particle diameter is 0.10 mm is not shown because the conditions are the same as in Experimental Example 1.
Table 7

In Table 7, “◯” indicates the fatigue strength equivalent to or higher than that of the steel material according to Experimental Example 1 (first stage shot grain size is 1.0 mm, second stage shot grain size is 0.1 mm). It means that it had.
On the other hand, the “x” mark means that the fatigue strength is significantly inferior to the steel material according to Experimental Example 1.
From the results shown in Table 7, it was found that 0.01 mm to 0.20 mm was appropriate for the shot particle size of the first stage shot peening in the present invention.

Next, a second embodiment of the present invention will be described with reference to FIGS.
In FIG. 5, the carbonitriding process S1A is performed on a steel material having a carbon content of 0.15% to 0.70%. In the tempering step S2A, tempering is performed by heating at 250 ° C. to 300 ° C. for 2 hours or more, and the austenite in the structure (99.5% or more, more specifically 99.8% or more) is martensified. Transform to the site. Here, the carbonitriding treatment step S1A is a step of performing a treatment for intruding nitrogen and carbon.
In the first shot peening treatment step S3A, shot peening is performed using shot grains having a relatively large diameter (for example, φ0.3 to 1.2 mm). Then, after the first stage shot peening step S3A, in the second stage shot peening step S4A, shot peening is performed using shot grains having a relatively small diameter (for example, φ0.01 to 0.2 mm).

Here, in the carbonitriding process S1A in the second embodiment, 0.7 to 1.2% of carbon and 0.2 to 1.0% of nitrogen are infiltrated into the steel material surface.
In other words, the first embodiment and the second embodiment differ in the amount of carbon and nitrogen that enter the steel material in the step of performing the process of intruding nitrogen and carbon.

[Experimental Example 9]
In Experimental Example 9, a carbonitriding process in which 0.8% carbon and 0.5% nitrogen were infiltrated into the steel surface was performed as a process for infiltrating nitrogen and carbon into an alloy having a carbon content of 0.2%. .
And the alloy steel which performed the carbonitriding process concerned was heated at 300 degreeC for 2 hours, and the tempering process was performed.
After the tempering treatment, first shot peening is performed by impeller shot using shot particles having a particle size of 1.0 mm, and then second shot peening is performed by air blow including shot particles having a particle size of 0.1 mm. went.
The steel material thus manufactured was subjected to a fatigue fracture test at room temperature according to JIS Z 2273 “General Rules for Fatigue Testing Methods for Metallic Materials” and JIS Z2274 “Testing Methods for Rotating Bending Fatigue Metallic Materials” at room temperature. The fatigue fracture characteristics obtained as a result of the fatigue fracture test are shown in FIG.

In FIG. 6, the fatigue fracture characteristics of rotating bending at a high temperature (300 ° C.) of the steel material according to Experimental Example 9 are indicated by “◯” plots, which are labeled “present invention” or “present invention product”. Has been.
Here, in FIG. 6, for comparison, a carbonitriding material tempered at 300 ° C. for 2 hours but not subjected to two-step shot peening (a plot of “●”: “carburizing” Nitride ”or“ 300 ° C. carbonitrided product ”) and carbonitriding material that has been subjected to two-step shot peening but not tempered (“ ◆ ”) : "Carbonitriding material + 2-stage shot peening") is also shown.

As is clear from FIG. 6, the steel material according to Experimental Example 9 (“◯” plot) is tempered with a carbonitrided material that has not been subjected to two-stage shot peening (“●” plot). The fatigue strength is improved over a region having a small number of repetitions N to a region having a large number of repetitions N with respect to a non-carbonitriding material (plot “♦”).
Compared with carbonitriding material that has not been tempered (“♦” plot: “carbonitriding material + two-stage shot peening”), the fatigue strength of the steel material according to Experimental Example 9 (“○” plot) is It is estimated that the fatigue strength was improved because the steel material according to Experimental Example 9 was tempered at 300 ° C. for 2 hours, and the retained austenite structure was transformed into a martensite structure.
In addition, compared with a carbonitrided material that has not been subjected to two-stage shot peening (“●” plot), the fatigue strength of the steel material according to Experimental Example 9 (“○” plot) is improved. Since the steel material according to Experimental Example 9 is subjected to two-stage shot peening, it is presumed that one factor is an increase in the integrated compressive residual stress.

It is clear from FIG. 7 that the compressive residual stress increases by performing two-stage shot peening.
In FIG. 7, compared with the residual stress value of the steel material not subjected to shot peening only by carbonitriding and quenching (“□” plot: comparison target), the steel material subjected to two-stage shot peening treatment after carbonitriding (“◆”) ) And the steel materials according to Experimental Example 9 (steel materials tempered at 300 ° C. for 2 hours after carbonitriding and quenching, and then subjected to two-stage shot peening treatment: “◯” plots) The absolute value is increasing.

Also in FIG. 7, the residual stress value is a compressive stress, and the sign is “minus”. Also in FIG. 7, a large residual stress (compressive stress) value means a large absolute value, and the characteristic curve is located below in FIG.
In FIG. 7, the characteristic curve (“♦” plot) of the steel material subjected to the two-stage shot peening treatment with respect to the characteristic curve (“□” plot) of the steel material not subjected to the two-stage shot peening treatment, and an experimental example 9) is located below, and the absolute value of the residual stress (compressive stress) is large.
In the result of Experimental Example 9 shown in FIG. 6, an increase in compressive residual stress due to two-stage shot peening is estimated as the cause of the improved fatigue strength (“◯” plot in FIG. 6) of the steel according to Experimental Example 9. This is reasonable from the results of FIG.

[Experimental Example 10]
In Experimental Example 10, when carbonitriding the alloy steel, the carbon amount to be infiltrated into the steel material surface was changed in the range of 0.5% to 1.4% in increments of 0.1%. The fatigue fracture characteristics were determined under the same conditions as in No. 9.
The results are shown in Table 8 below.
In Table 8, the case where the carbon intrusion amount is 0.8% is not shown because the conditions are the same as those in Experimental Example 9.
Table 8

In Table 8, the mark “◯” means that the steel had a fatigue strength equivalent to or higher than that of the steel material according to Experimental Example 9 (carbon intrusion amount 0.8%).
On the other hand, the “x” mark in Table 8 means that the fatigue strength was significantly inferior to the steel material according to Experimental Example 9.
From the results of Experimental Example 10 shown in Table 8, it is understood that 0.70% to 1.2% is appropriate for the carbon intrusion amount in the carbonitriding process.

[Experimental Example 11]
In Experimental Example 11, when carbonitriding the alloy steel, the amount of nitrogen to be infiltrated into the surface of the steel material is changed in the range of 0.1% to 1.2% in increments of 0.1%. The fatigue fracture characteristics were determined under the same conditions as in No. 9.
The results are shown in Table 9 below.
In Table 9, the case where the nitrogen intrusion amount is 0.5% is not shown because the conditions are the same as those in Experimental Example 9.
Table 9

Also in Table 9, “◯” marks mean that the steel had a fatigue strength equivalent to or higher than that of the steel material according to Experimental Example 9 (the nitrogen penetration amount was 0.5%).
On the other hand, the “x” mark in Table 9 means that the fatigue strength was significantly inferior to the steel material according to Experimental Example 9.
From the results of Experimental Example 11 shown in Table 9, it is understood that 0.2% to 1.0% is appropriate as the nitrogen penetration amount in the carbonitriding process.

  The illustrated embodiment is merely an example, and is not intended to limit the technical scope of the present invention.

Claims (3)

  A step of infiltrating nitrogen and carbon into a steel material having a carbon content of 0.15% to 0.70%, and a tempering treatment by heating at 250 ° C. to 300 ° C. for 2 hours or more, and austenite in the structure is martensified. After the heat treatment process for transforming into the site, the first stage shot peening process in which shot peening is performed using large diameter shot grains, and the first stage shot peening process, shot peening is performed using small diameter shot grains. A method for heat treating a steel material, comprising a second stage shot peening process.   The heat treatment of the steel material according to claim 1, wherein in the step of performing the treatment to infiltrate nitrogen and carbon, a high concentration carbonitriding and quenching treatment is performed to infiltrate the steel material surface by 1.0% or more of carbon and less than 1.0% of nitrogen Method.   In the step of performing the treatment for infiltrating nitrogen and carbon, carbonitriding and quenching is performed, and 0.7 to 1.2% of carbon and 0.2 to 1.0% of nitrogen are infiltrated into the steel surface. A heat treatment method for steel.

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