JP2019039049A - Spring excellent in fatigue resistance, and production method thereof - Google Patents

Abstract

To provide a spring excellent in fatigue resistance.SOLUTION: A spring has a steel base material with spring properties and a nitrided layer formed on the surface of the steel base material. The steel base material contains one or two or more additive elements having a higher activity to nitrogen than iron has. The nitrided layer has a nitrogen compound layer formed on the outermost surface, the nitrogen compound layer containing γ-FeN and a nitride of the additive elements. A hard and fragile ε-FeN layer formed on a nitrogen compound layer obtained in a conventional nitriding is not formed on the outermost surface in the invention. The nitrogen compound layer containing γ-FeN and a nitride of the additive elements is nearly free from fissures even under repeated stress and cracking and peeling caused by shot-peening. Therefore, the fatigue resistance of the spring is greatly improved.SELECTED DRAWING: None

Description

  The present invention relates to a spring excellent in fatigue resistance, and a method of manufacturing the same.

  In the past, in various industrial fields including automobiles, efforts have been made to improve performance such as weight reduction, high output and life extension. Along with this, springs used in mechanical devices also need to have high load stress and excellent fatigue strength. For example, such characteristics are also required for springs used in automobile engines and suspensions.

There are several ways to increase the fatigue strength of a spring. The present application is to improve fatigue resistance by nitriding treatment.
The applicant grasps the following patent documents 1-6 as a prior art document which improves the fatigue strength of a spring by nitriding treatment.
Moreover, the applicant grasps | ascertains the following patent documents 7-8 about the technique which performs nitriding processing with gear wheels and cranks other than a spring.

Patent No. 3142689 Unexamined-Japanese-Patent No. 7-179985 Patent No. 4615208 gazette JP, 2015-86890, A JP, 2016-148068, A Patent No. 4711403 JP, 2013-221203, A Unexamined-Japanese-Patent No. 2017-36509

The following patent document 1 has the following description.
[Claim 1]
C: 0.4 to 0.8% Si: 0.8 to 4.0% Mn: 0.2 to 2.0% Cr: 0.4 to 3.0% by weight ratio, and further V: 0.05 to 0.50% Nb: 0.05 to 0.50 Ni: 0.2 to 2% Mo: at least one element selected from the group consisting of 0.1 to 1.0%, The steel consisting of the balance Fe and unavoidable impurities is nitrided, and the amount of nitrogen within 10 μm from the surface is 0.5 to 2%, the nitrogen diffusion depth is 0.07 mm or more, and the internal hardness is 470 or more in Vickers hardness A spring excellent in fatigue strength characterized by being.

The following patent document 2 has the following description.
[Claim 1]
C: 0.3 to 0.7% (meaning weight percent, the same applies hereinafter) Si: 1 to 4% Mn: 0.2 to 1.5% contained, the balance being iron and unavoidable impurities, 50 μm on the surface While the above nitrogen diffusion layer is formed, the outermost layer portion is an ε iron nitride layer having a thickness of 3 μm or more, and the internal hardness is HRC 49.0 or more. Spring.

The following patent document 3 has the following description.
[Claim 2]
In weight ratio, C: 0.50 to 1.00%, Si: 1.20 to 2.50%, Mn: 1.0% or less, Ni: 0.5% or less, Cr: 0.40 to 1.50%, Mo: 0.5% or less, V: 0.60% or less The valve spring is characterized in that the coil spring made of the steel to be treated is nitrided at a temperature T (absolute temperature K) and time t (s) satisfying all of the following equations (1) to (3) Production method.
(1) When (-1. 60 Si + 0.20 Cr + 2. 14)> = 0, T> {700-(1205 Si + 2.2 Cr-867)} / (-1. 60 Si + 0.20 Cr + 2.14)
In the case of (-1. 60 Si + 0.20 Cr + 2. 14) <0, T <{700-(1205 Si + 2.2 Cr-867)} / (-1. 60 Si + 0.20 Cr + 2. 14)
(2) When (−0.23Si + 1.85Mo−0.42)> = 0, T> {550− (236Si-1054Mo + 735)} / (− 0.23Si + 1.85Mo−0.42)
If (−0.23 Si + 1.85 Mo−0.42) <0, then T <{550− (236 Si-1054 Mo + 735)} / (− 0.23 Si + 1.85 Mo−0.42)
(3) t> {0.015 / exp ((40.4C-2.8Mn-15.5V + 17.2Mo-7.9Ni-21.5)-(254C-105V + 127Mo-40.7Ni-96.4) * 1000 / R / T)} ²
[0043]
Summarizing the results of the above multiple regression analysis, in order to meet the condition that the nitriding depth of the valve spring is 30 μm or more, the surface hardness is Hv 700 or more, and the internal hardness is Hv 550 or more, The relationship between temperature K) and time t (seconds) must satisfy all the following three inequalities.
(−1.5985 Si + 0.202 Cr + 0.5238 Mo + 2.1414) T + (1204.9 Si + 2.2 Cr−867.4)> 700 (9)
(-0.2275Si + 1.8458Mo-0.4153) T + (236.4Si-1053.6Mo + 734.7)> 550 (10)
2√ (t) exp ((40.4C-2.8Mn-15.5V + 17.2Mo-7.9Ni-21.5)-(253.5C-105.3V + 127Mo-40.7Ni-96.4) * 1000 / R / T)> 0.03 (0.03) 11)
The second invention is derived by modifying these equations.

The following patent document 4 has the following description.
[Claim 1]
A steel layer and a compound layer containing nitride formed on the surface of the steel layer,
The compound layer contains an ε phase, and the compressive residual stress of the ε phase is set to 800 to 1,400 MPa.
[Claim 5]
A method of manufacturing a spring,
Removing surface flaws formed on the surface of the spring wire;
Nitriding the spring wire material from which the surface flaws have been removed;
Shot peening the surface of the spring wire after the nitriding step;
The shot-peening process process WHEREIN: The shot peening of multiple times is performed, The manufacturing method of the spring made into the hardness of 1100-1300 HV of the projection material used for shot-peening performed finally.

The following patent document 5 has the following description.
[Claim 1]
A method of manufacturing a metal spring having a nitrided layer formed on its surface, comprising:
A temperature raising step of raising the temperature in the furnace to a predetermined nitriding treatment temperature in a state in which a metal spring main body formed in a predetermined spring shape is disposed in an airtight atmosphere furnace;
A first nitriding step of supplying ammonia gas so that the nitriding potential in the furnace becomes a predetermined high level value while maintaining the furnace temperature at the predetermined nitriding treatment temperature;
In the furnace, the supply of ammonia gas into the furnace is stopped and the hydrogen gas and the nitrogen gas are supplied into the furnace at a ratio of 3: 1 while maintaining the temperature in the furnace at the predetermined nitriding treatment temperature. A nitriding potential lowering step of lowering the nitriding potential of the metal spring main body to a low level value at which no compound layer is formed in the metal spring body;
Maintaining a furnace temperature at the predetermined nitriding temperature and holding a nitriding potential in the furnace at the low level value;
The high level value of the nitriding potential in the first nitriding step is a concentration that causes the formation of a compound layer in the metal wire spring body when the first nitriding step is continued for a predetermined time or more. The process time of the nitriding process is made into time shorter than the said predetermined time, The manufacturing method of the metal spring characterized by the above-mentioned.
[0022]
According to the method and apparatus for manufacturing a metal spring according to the present invention, since a nitriding process is performed with a high concentration of nitriding potential, a deep nitrided layer is formed as uniformly as possible on the entire surface of a metal spring body. Further, since the nitriding treatment at a high concentration of nitriding potential is limited by time, the formation of a compound layer on the metal spring body can be surely prevented.

The following Patent Document 6 has the following description.
[Claim 1]
At least 0.4% to 1.0% C, 0.9 to 3.0% Si, 0.1% to 2.0% Mn, 2.5% or less Cr, 0.7% by weight A steel spring member in which at least a nitrogen diffusion layer is provided on a base made of a steel material containing the following V, 0.25% or less of Mo, and 1.0% or less of W,
The Vickers hardness of the portion from the outermost surface to the depth of 20 μm is 750 or more, and the Vickers hardness of the core portion deeper than the portion is 600 or more,
When a compound layer is present above the nitrogen diffusion layer, the thickness of the compound layer is at most 0.5 μm,
Furthermore, a steel spring member having a fatigue strength of 639 MPa or more.

The following Patent Document 7 has the following description.
[0001]
The present invention relates to a nitrided steel member whose surface is nitrided by nitriding treatment in a gas atmosphere and a method of manufacturing the same. Further, the present invention relates to a high-strength nitrided steel member which is used for a gear of an automobile or the like and which has improved pitting resistance and bending fatigue strength.
[0008]
As a result of intensive studies to solve the above problems, the inventors of the present invention have produced an iron nitride compound layer having a predetermined structure on a steel member of a predetermined composition on the surface of the steel member, thereby reducing distortion and It has been found that a high strength, low strain nitrided steel member having excellent pitting resistance and bending fatigue strength can be obtained, and the present invention has been completed.
[0030]
In the present invention, “iron nitride compound layer” refers to the nitriding of iron represented by γ ′ phase −Fe ₄ N and ε phase −Fe _2-3 N on the surface of steel members formed by gas soft nitriding treatment. Refers to a compound. The iron nitride compound layer is a γ 'phase and an ε phase, and is precipitated as a layer state. In the present invention, an iron nitride compound layer composed of the γ ′ phase and the ε phase is formed on the surface of the steel member (base material) in a thickness range of 2 to 17 μm. On the other hand, the steel member (base material) is made of steel having the predetermined component range described above, but directly below the iron nitride compound layer (in contact with the interface between the iron nitride compound layer and the steel member (base material) The steel member (base material) at the position is in a state in which alloy carbo-nitrides are finely aligned and precipitated in the steel. Immediately below the iron nitride compound layer, since the alloy carbonitrides are thus precipitated, the hardness is higher than that of the core portion of the base material.
[0033]
The γ 'phase -Fe ₄ N appearing around 2θ: 41.2 degrees according to an X-ray diffraction (XRD) profile using an X-ray tube Cu of the iron nitride compound layer formed on the surface of the nitrided steel member of the present invention X-ray diffraction peak intensity IFe ₄ N (111) of (111) crystal face of E and X-ray diffraction peak intensity IFe of (111) crystal face of ε phase -Fe _2-3 N appearing near 2θ: 43.7 degrees _{In 3} N (111), the intensity ratio represented by IFe ₄ N (111) / {IFe ₄ N (111) + IFe ₃ N (111)} is 0.5 or more. As described above, the “iron nitride compound layer” is a layer composed of ε phase −Fe _2-3 N and / or γ ′ phase −Fe ₄ N, etc., and when X-ray diffraction analysis is performed on the surface of a steel member, Whether or not the γ 'phase is the main component is determined by measuring the ratio of the X-ray peak intensities. In the present invention, if the strength ratio is 0.5 or more, it can be determined that the iron nitride compound layer formed on the surface of the nitrided steel member is mainly composed of the γ 'phase, and the resistance of the nitrided steel member The pitching property and the bending fatigue strength become excellent. The strength ratio is preferably 0.8 or more, more preferably 0.9 or more.

The following Patent Document 8 has the following description.
[0007]
An object of the present invention is to provide a method of manufacturing a high strength and low strain nitrided steel member which has high pitting resistance and bending strength and which is low in strain as compared with carburizing or carbonitriding treatment.
[0008]
As a result of intensive studies to solve the above problems, the present inventors have carried out predetermined nitriding treatments on steel members made of carbon steel and alloy steel for machine structure, and controlled the iron nitride compound having a controlled structure (structure). By forming a layer on the surface of a steel member, it has been found that a high strength, low strain nitrided steel member having low strain and sufficient pitting resistance and flexural strength can be obtained, and the present invention has been completed.
[0010]
In the present specification, “iron nitride compound layer” refers to the nitriding of iron represented by γ ′ phase −Fe ₄ N and ε phase −Fe ₃ N on the surface of steel members formed by gas nitriding treatment. Refers to a compound.

Γ 'phase appearing around 2θ: 41.2 degrees according to an X-ray diffraction (XRD) profile when using a copper tube as an X-ray tube, of the iron nitride compound layer formed on the surface of the nitrided steel member of the present invention X-ray diffraction peak intensity IFe ₄ N (111) of (111) crystal face of -Fe ₄ N and X phase diffraction intensity IFe of (111) crystal face of ε-Fe ₃ N appearing near 2θ: 43.7 degrees _{In 3} N (111), the intensity ratio represented by IFe ₄ N (111) / {IFe ₄ N (111) + IFe ₃ N (111)} is 0.5 or more. As described above, the “iron nitride compound layer” is a layer composed of ε phase −Fe ₃ N and / or γ ′ phase −Fe ₄ N or the like, and when X-ray diffraction analysis is performed on the surface of a steel member, the above X It is determined whether the γ 'phase is the main component by measuring the ratio of the line peak intensities. In the present invention, if the strength ratio is 0.5 or more, it can be determined that the iron nitride compound layer formed on the surface of the nitrided steel member is mainly composed of the γ 'phase, and the resistance of the nitrided steel member Pitching property and bending strength become excellent. The strength ratio is preferably 0.8 or more, more preferably 0.9 or more.

The said patent document 1 is related with the spring excellent in fatigue strength.
However, the document 1 nitrides the steel limited to a predetermined component range, specifies the nitrogen amount in the surface layer, the nitrogen diffusion depth, and the internal hardness, and only discloses it.
That is, in the document 1, the nitrogen amount is described only for the nitrided layer (compound layer), and the structure is not mentioned at all. Note that the nitrided layer obtained by ordinary nitriding is a layer in which an ε-Fe _{2 to 3} N layer is formed on the surface, and a layer mainly composed of γ'-Fe ₄ N is formed therebelow.

The patent document 2 relates to a high strength suspension spring.
However, Document 2 only specifies and discloses the nitrogen diffusion layer on the surface, the ε-iron nitride layer on the outermost layer, and the internal hardness in the steel limited to the predetermined component range.
That is, in the reference 2, as for the nitrided layer (compound layer), only the nitrogen diffusion layer and the ε-iron nitride layer of the outermost layer portion are specified.

The said patent document 3 is related with the manufacturing method of a valve spring.
However, Document 3 only discloses a method of setting the temperature and time at the time of nitriding treatment according to the concentrations of component elements for steel containing a predetermined alloying element. This method aims at satisfying the condition that the nitriding depth is 30 μm or more, the surface hardness is Hv 700 or more, and the internal hardness is Hv 550 or more as the characteristics of the valve spring which receives repeated load.
That is, in the case of Reference 3, with regard to the nitrided layer (compound layer), only the nitriding depth and the hardness of the surface and the inside are specified.

The patent document 4 relates to a spring and a method of manufacturing the spring.
However, reference 4 only discloses that the compound layer contains an ε phase, that the compressive residual stress of the ε phase is 800 to 1,400 MPa, and that the surface of the spring wire is shot peened.
That is, in the reference 4, with respect to the nitrided layer (compound layer), the compound layer includes only the ε phase, and the compressive residual stress thereof is specified.

The said patent document 5 is related with the manufacturing method of a metal spring.
However, Document 5 does not form a compound layer on the surface and forms a deep nitrided layer (diffusion layer) by processing the nitriding potential of the nitriding atmosphere in two steps.
That is, Document 5 discloses only a nitride layer (compound layer) having no compound layer and in which a deep diffusion layer is formed.

The said patent document 6 is related with a steel spring member.
However, Document 6 only specifies and discloses the thickness and fatigue strength of the surface and deep portions, and the compound layer, if any, provided with a nitrogen diffusion layer on the lower surface of the steel of a predetermined composition. Absent.
That is, in Document 6, the thickness and the fatigue strength are only specified for the nitrided layer (compound layer).

Patent Document 7 mentioned above relates to a nitrided steel member.
However, Document 7 is a layer in which the “iron nitride compound layer” is formed of ε phase −Fe _2-3 N and / or γ ′ phase −Fe ₄ N, etc., and when X-ray diffraction analysis is performed, X-rays are obtained. It remains to disclose to determine whether the γ 'phase is the main component by measuring the ratio of peak intensities.
That is, Document 7 only discloses a nitride layer (compound layer) in which ε phase −Fe _2-3 N and γ ′ phase −Fe ₄ N are simply mixed.

Patent Document 8 mentioned above relates to a method of manufacturing a nitrided steel member.
However, Document 8 is a layer in which the “iron nitride compound layer” is formed of ε phase −Fe _2-3 N and / or γ ′ phase −Fe ₄ N etc., and when X-ray diffraction analysis is performed, X-rays are obtained. It remains to disclose to determine whether the γ 'phase is the main component by measuring the ratio of peak intensities.
That is, Document 8 only discloses a nitride layer (compound layer) in which ε phase −Fe _2-3 N and γ ′ phase −Fe ₄ N are simply mixed.

The conventional nitriding treatment as described above has the following problems.
That is, in the nitrogen compound layer obtained by general nitriding treatment, an ε-Fe _{2 to 3} N layer is formed on the outermost surface, and a layer mainly composed of γ′-Fe ₄ N is formed on the inner side thereof. A compound with a higher nitrogen concentration forms on the surface side a compound with a lower nitrogen concentration on the inside. The outermost surface of ε-Fe _2-3 N has hard and brittle physical properties. Therefore, in a member subjected to repeated loading such as a spring, the outermost surface is cracked early. In addition, when shot peening is performed, it may be broken or peeled off. Such cracks, cracks and peeling cause fatigue failure.

The present invention has been made with the following object in order to solve the above problems.
The fatigue strength of the spring is remarkably improved by performing nitriding treatment with a predetermined nitrogen potential on the steel base material to which the spring characteristics are imparted. Provided is such a spring with excellent fatigue resistance, and a method of manufacturing the same.

The spring with excellent fatigue resistance according to claim 1 adopts the following constitution in order to achieve the above object.
A steel base material to which a spring characteristic is given,
And a nitride layer formed on the surface of the steel base material;
The steel base material contains one or more additional elements having higher activity to nitrogen than iron,
The nitrided layer is formed by forming a nitrogen compound layer containing γ′-Fe ₄ N and a nitride of the additional element on the outermost surface.

The spring having excellent fatigue resistance according to claim 2 adopts the following constitution in addition to the constitution according to claim 1.
The nitrogen compound layer has a thickness of 5 μm or less.

The spring with excellent fatigue resistance according to claim 3 adopts the following configuration in addition to the configuration according to claim 1 or 2.
In the nitrogen compound layer, the crystal grain diameter of at least the outermost layer portion is 1 μm or less.

The spring with excellent fatigue resistance according to claim 4 adopts the following constitution in addition to the constitution according to any one of claims 1 to 3.
The above-mentioned nitrogen compound layer has a compressive residual stress in the 45 ° direction of 1000 MPa or more.

The spring having excellent fatigue resistance according to claim 5 adopts the following constitution in addition to the constitution according to any one of claims 1 to 4.
In the nitrided layer, a nitrogen diffusion layer is formed under the nitrogen compound layer on the outermost surface.

The spring with excellent fatigue resistance according to claim 6 adopts the following constitution in addition to the constitution according to claim 5.
The nitrogen diffusion layer has a thickness of 20 to 150 μm.

In the spring with excellent fatigue resistance according to claim 7, in addition to the constitution according to claim 5 or 6, the following constitution is adopted.
Between the nitrogen compound layer and the nitrogen diffusion layer, there is no other compound layer layered.

The method for manufacturing a spring excellent in fatigue resistance according to claim 8 adopts the following configuration in order to achieve the above object.
For steel base material to which spring characteristics are given,
A gas nitriding process is performed in which the nitriding potential K _N and the absolute temperature T are controlled in the range according to the following equations (1) (2) (3).
(1) 133.52 exp (-0.008 T) K K _N 544 5449.1 exp (-0.01 T)
(2) 623 K ≦ T ≦ 923 K
(3) K _N = (partial pressure of NH ₃ ) / [(partial pressure of H ₂ ) ^3/2 ]

The method for producing a spring excellent in fatigue resistance according to claim 9 adopts the following constitution in addition to the constitution according to claim 8.
Prior to the gas nitriding treatment, the steel base material is subjected to a refining treatment for refining the crystal of the surface layer portion of the steel base material.

The method of manufacturing a spring excellent in fatigue resistance according to claim 10 adopts the following configuration in addition to the configuration according to claim 9.
The above-mentioned miniaturization process is a halogenation process.

The method for manufacturing a spring excellent in fatigue resistance according to claim 11 adopts the following configuration in addition to the configuration according to claim 9.
The above-described miniaturization process is shot peening.

In the spring excellent in fatigue resistance according to claim 1, a nitrided layer is formed on the surface of a steel base material to which spring characteristics are imparted. The steel base material contains one or more additional elements having higher activity to nitrogen than iron. In the nitrided layer, a nitrogen compound layer containing γ′-Fe ₄ N and a nitride of the above-described additive element is formed on the outermost surface.
That is, the present invention does not form a hard and brittle ε-Fe _2-3 N layer on the outermost surface like the nitrogen compound layer obtained by the conventional nitriding treatment. On the outermost surface, a nitrogen compound layer containing γ'-Fe ₄ N and a nitride of the above additive element is formed. Such a nitrogen compound layer is less likely to crack even if stress is repeatedly applied, and is less likely to be cracked or peeled off by shot peening. Thereby, the fatigue strength of the spring can be significantly improved.

In the spring excellent in fatigue resistance according to claim 2, the nitrogen compound layer has a thickness of 5 μm or less.
By limiting the thickness of the nitrogen compound layer to 5 μm or less, cracking, cracking and peeling due to repeated stress and shot peening are less likely to occur. Thereby, the fatigue strength of the spring can be further improved.

In the spring excellent in fatigue resistance according to claim 3, in the nitrogen compound layer, the crystal grain diameter of at least the outermost layer portion is 1 μm or less.
By limiting the crystal grain size of the nitrogen compound layer to 1 μm or less, cracking, cracking and peeling due to repeated stress and shot peening are less likely to occur. Thereby, the fatigue strength of the spring can be further improved.

In the spring excellent in fatigue resistance according to claim 4, the nitrogen compound layer has a compressive residual stress in a 45 degree direction of 1000 MPa or more.
By setting the compressive residual stress in the 45 ° direction of the nitrogen compound layer to 1000 MPa or more, cracking, cracking and peeling due to repeated stress and shot peening are less likely to occur. Thereby, the fatigue strength of the spring can be further improved.

In the spring excellent in fatigue resistance according to claim 5, the nitrided layer has a nitrogen diffusion layer formed under the nitrogen compound layer on the outermost surface.
The nitrogen diffusion layer is strengthened by the diffusion and penetration of nitrogen atoms. The outermost nitrogen compound layer is supported by the reinforced nitrogen diffusion layer. Therefore, cracking, cracking and peeling due to repeated stress and shot peening are less likely to occur. Thereby, the fatigue strength of the spring can be further improved.

In the spring excellent in fatigue resistance according to claim 6, the nitrogen diffusion layer has a thickness of 20 to 150 μm.
By setting the thickness of the nitrogen diffusion layer to 20 to 150 μm, the nitrogen compound layer on the outermost surface is supported by the nitrogen diffusion layer of the thickness. Therefore, cracking, cracking and peeling due to repeated stress and shot peening are less likely to occur. Thereby, the fatigue strength of the spring can be further improved.

The spring with excellent fatigue resistance according to claim 7 has no other compound layer layered between the nitrogen compound layer and the nitrogen diffusion layer.
The absence of another compound layer layered between the nitrogen compound layer and the nitrogen diffusion layer makes it difficult to cause cracking, cracking or peeling of the other compound layer due to repeated stress or shot peening. Thereby, the fatigue strength of the spring can be further improved.

The method for producing a spring excellent in fatigue resistance according to claim 8 is characterized in that the nitriding potential K _N and the absolute temperature T are expressed by the following formulas (1), (2), (3) for a steel base material to which spring characteristics are given. Control gas nitriding to a range according to
(1) 133.52 exp (-0.008 T) K K _N 544 5449.1 exp (-0.01 T)
(2) 623 K ≦ T ≦ 923 K
(3) K _N = (partial pressure of NH ₃ ) / [(partial pressure of H ₂ ) ^3/2 ]
By this, a nitrided layer is formed on the surface of the steel base material to which the spring characteristics are imparted. As the nitrided layer, a nitrogen compound layer containing γ′-Fe ₄ N and a nitride of the additive element is formed on the outermost surface.
That is, the resulting spring does not form a hard and brittle ε-Fe _2-3 N layer on the outermost surface like the nitrogen compound layer obtained by the conventional nitriding treatment. On the outermost surface, a nitrogen compound layer containing γ'-Fe ₄ N and a nitride of the above additive element is formed. Such a nitrogen compound layer is less likely to crack even if stress is repeatedly applied, and is less likely to be cracked or peeled off by shot peening. Thereby, the fatigue strength of the spring can be significantly improved.

In the method for producing a spring excellent in fatigue resistance according to claim 9, prior to the gas nitriding treatment, the steel base material is subjected to a refining treatment for refining the crystal of the surface layer portion of the steel base material.
By performing the above-described gas nitriding process on the above-described steel base material in which the crystal of the surface layer portion is refined, the nitrided layer to be formed has γ′-Fe ₄ N and a nitride of the above additive element on the outermost surface. A nitrogen compound layer is formed, and the crystal grain size is refined to about 1 μm or less. The formation of such a nitrided layer makes it more difficult to cause cracking, cracking and peeling due to repeated stress and shot peening. Thereby, the fatigue strength of the spring can be further improved.

In the method for producing a spring excellent in fatigue resistance according to claim 10, the above-mentioned refining treatment is a halogenation treatment.
By the halogenation treatment, fine iron halide is formed in the surface layer portion of the steel base material, and the crystals in the surface layer portion of the steel base material are refined. During the subsequent gas nitriding treatment, the halogen is desorbed from the steel base material from the fine iron halide, and nitrogen is bonded by using fine crystals of the steel as nuclei to form a nitrogen compound having a fine crystal grain diameter.

In the method for producing a spring excellent in fatigue resistance according to claim 11, the above-mentioned refining process is shot peening.
By the above-mentioned shot peening, the crystal of the surface layer portion of the above-mentioned steel base material receives modification by processing, and causes processing distortion. During the subsequent gas nitriding treatment, the heated and deformed crystal is divided into polygonal fine grains and refined. Nitrogen bonds with such fine crystals of steel as nuclei to form nitrogen compounds having a fine crystal grain size.

FIG. It is a diagram for calculating the nitriding potential. It is a microscope picture which shows the cross-sectional structure of the spring of a present Example. It is the figure which plotted the cross-sectional hardness distribution of the spring of a present Example. It is a X-ray-diffraction result from the surface of the spring of a present Example. It is the electron micrograph which observed the crystal grain of the nitrogen compound layer of the spring of a present Example.

  Below, the form for implementing this invention is demonstrated.

Description of Spring The spring having excellent fatigue resistance according to this embodiment has a steel base material to which spring characteristics are imparted, and a nitrided layer formed on the surface of the steel base material.

[Steel base material]
The steel base material contains one or more additional elements having higher activity to nitrogen than iron. The steel which comprises the said steel base material is based on iron, and contains carbon as an alloying element. The above steel can contain unavoidable impurities.
The additive element is an alloy element having higher activity to nitrogen than iron, and examples thereof include chromium, manganese, silicon, titanium, niobium, vanadium, tungsten and the like. The steel base material can contain one or more of the above-described additive elements.

As said steel base material, the steel type which can provide spring characteristics can be used. Preferably, spring steel can be used as the steel base material.
As the above-mentioned spring steel, for example, SUP3 which is C system, SUP6, SUP7 (Si-Mn system) which is Si system, SUP12 (SAE 9245, Si-Cr system), SUP 9 which is Cr system, SAE 5160 (Mn- Cr system), SUP11, SAE51 B60 (Mn-Cr-B system), SUP10 (Cr-V system), SUP13 (SAE 4160, Cr-Mo system), etc. can be mentioned.

Spring characteristics are imparted to the steel base material.
As a method of imparting spring characteristics, for example, a method of hot-forming or cold-forming a material and strengthening it by quenching and tempering, pre-patenting and cold working (hard steel wire, piano wire) or quenching and tempering ( Oil-tempered wire) can be mentioned as a method of cold-forming the wire.

  In the present embodiment, for example, an oil temper wire for spring (JIS G 3560), an oil temper wire for valve spring (JIS G 3561) or the like can be applied as a steel base material to which the above-mentioned spring characteristics are imparted.

[Nitrided layer]
The nitrided layer is formed by forming a nitrogen compound layer containing γ′-Fe ₄ N and a nitride of the additional element on the outermost surface.

  The nitrided layer is formed on the surface of the steel base material by performing gas nitriding treatment described later on the steel base material.

The nitride of the above-mentioned additive element is, for example, chromium nitride, manganese nitride, silicon nitride, titanium nitride, niobium nitride, vanadium nitride, tungsten nitride or the like.
The nitrogen compound layer is a layer containing γ′-Fe ₄ N and one or more of the chromium nitride, manganese nitride, silicon nitride, titanium nitride, niobium nitride, vanadium nitride, tungsten nitride, etc. described above.

The nitrogen compound layer preferably has a thickness of 5 μm or less.
If the thickness of the nitrogen compound layer exceeds 5 μm, the risk of cracking when repeated stress is applied or cracking or peeling due to shot peening may be increased. The thickness of the nitrogen compound layer is more preferably 3 μm or less, and even more preferably 2 μm or less.
The lower limit of the thickness of the nitrogen compound layer is preferably at least zero μm. As a preferable range, the lower limit of the thickness of the nitrogen compound layer can be 0.1 μm or more. If the thickness of the nitrogen compound layer is less than 0.1 μm, the effect of improving the fatigue strength and the compressive residual stress may not be sufficiently obtained.

It is preferable that the crystal grain diameter of at least the outermost layer portion of the nitrogen compound layer be 1 μm or less.
If the crystal grain size of the nitrogen compound layer exceeds 1 μm, the risk of cracking when repeated stress is applied or cracking or peeling due to shot peening is increased.
Of the nitrogen compound layer, the outermost layer portion having a crystal grain diameter of 1 μm or less is about 1 to 3 μm of the surface layer portion, and the thicker portion is a dense layer having a crystal grain diameter exceeding 1 μm.

The above-mentioned nitrogen compound layer preferably has a compressive residual stress in the 45 ° direction of 1000 MPa or more.
If the compressive residual stress is less than 1000 MPa, there is a risk that a sufficient fatigue resistance can not be obtained.
The compressive residual stress can be measured, for example, by the sin ² ψ method which is an X-ray stress measurement method. This measurement method is measured based on the parallel inclination method (ψ0 constant method; PSPC method) which is a scanning method, using a CrKα ray as a specially made X-ray, and using an α-Fe (211) plane as a measurement diffraction plane. At this time, K = -318 MPa / degree (which is a standard value of the Japan Society for Materials Science) is used as a stress constant. Measure the direction of 45 degrees with respect to the wire direction of the spring wire.

The nitrided layer preferably has a nitrogen diffusion layer formed under the nitrogen compound layer on the outermost surface.
That is, the nitrided layer includes two layers of the nitrogen compound layer formed on the outermost surface of the steel base material and the nitrogen diffusion layer formed therebelow.
The nitrogen diffusion layer is formed by diffusion and penetration of nitrogen atoms into the steel base material by gas nitriding treatment described later.

The nitrogen diffusion layer preferably has a thickness of 20 to 150 μm.
If the thickness of the nitrogen diffusion layer is less than 20 μm, the reinforcement of the steel base material by the diffusion and penetration of nitrogen atoms is not sufficient and the effect of supporting the surface nitrogen compound layer to make it difficult to cause cracking, cracking and peeling is sufficient. The risk of becoming unobtainable increases.
If the thickness of the nitrogen diffusion layer exceeds 150 μm, the strengthening of the steel base material saturates the effect of supporting the nitrogen compound layer on the surface, and making it thicker than that will unnecessarily extend the nitriding treatment time.

Between the nitrogen compound layer and the nitrogen diffusion layer, it is preferable that no other compound layer which is layered be present.
That is, in the most preferable form, a layer containing γ'-Fe ₄ N and a nitride of the above additive element is formed as the nitrogen compound layer on the outermost surface of the steel base material, and the nitrogen diffusion layer is formed therebelow It is a thing. In this case, no other compound layer exists between the nitrogen compound layer and the nitrogen diffusion layer.

Moreover, what the other compound present in the granular form between the nitrogen compound layer and the nitrogen diffusion layer is also included as an aspect of the present invention. In this case, the other compounds are granular and not layered. Examples of the other compound existing in the form of particles include compounds containing ε-Fe _{2 to 3} N, and ε-Fe _{2 to 3} N and a nitride of the above-described additive element.
Such particulate other compounds are formed on the nitrogen compound layer side at the interface between the nitrogen compound layer and the nitrogen diffusion layer.

Description of Manufacturing Method In the method of manufacturing a spring according to this embodiment, gas nitriding is performed on a steel base material to which spring characteristics are imparted.

  The application of the above-described steel base material and spring characteristics is as described above.

[Gas nitriding treatment]
In the gas nitriding process, the steel base material is heated and held in an atmosphere to which a nitrogen source is added, nitrogen is permeated from the surface of the steel base material, and the above-described nitrogen compound layer and nitrogen diffusion layer are formed.

NH ₃ gas is used as the nitrogen source. NH ₃ gas supplies nitrogen atoms [N] to the steel base material according to the following reaction formula (A).
_{(A) 2NH 3 = 2 [} N] + 3H 2
The above reaction formula (A) can be rewritten as the following formula (B).
(B) NH ₃ = [N] + 3 / 2H ₂

As the atmosphere described above, an atmosphere in which NH ₃ is used as a nitrogen source and N ₂ , CO, CO ₂ , H _{2 and the} like are mixed as needed can be used.

In the present embodiment, the gas nitriding process is performed by controlling the nitriding potential K _N and the absolute temperature T in the ranges according to the following formulas (1), (2), and (3).
(1) 133.52 exp (-0.008 T) K K _N 544 5449.1 exp (-0.01 T)
(2) 623 K ≦ T ≦ 923 K
(3) K _N = (partial pressure of NH ₃ ) / [(partial pressure of H ₂ ) ^3/2 ]

[Nitriding potential]
The nitriding potential K _N is a nitrogen potential of a reactive species that contributes to the nitriding reaction of the steel base material. The partial pressure of NH ₃ and H _{2 in} the atmosphere is given by the above equation (3) based on the above equation (B).

FIG. 1 is a Railer state diagram.
The Rayleigh phase diagram shows the formation of iron nitride (ε phase, γ ′ phase) in iron nitriding. This figure shows what kind of iron nitride is generated as a phase by the correlation between the nitriding potential K _{N taken} on the vertical axis and the temperature taken on the horizontal axis. In FIG. 1, an isoconcentration line is drawn in which the nitrogen concentrations of ε nitrides become equal.

From FIG. 1, the temperature range in which γ′-Fe ₄ N is generated is 350 ° C. to 650 ° C. The absolute temperature T is 623 K to 923 K. Thereby, the above equation (2) is given.

FIG. 2 is a diagram for calculating equation (1) in the manufacturing method of the present invention.
This figure is a plot of estimated values of the nitrogen concentration near the outer surface of the compound layer under the assumption that thermodynamic equilibrium is established based on the above-mentioned Lehrer phase diagram. The vertical axis is the nitriding potential K _N , and the horizontal axis is the absolute temperature.
First straight line is drawn by connecting the plots arranged in high nitriding potential K _N side. Second straight line is drawn by connecting the plots arranged in a lower side of the nitride potential K _N. The equation (1) is given within the range of the condition between the first straight line and the second straight line.

  The heating and holding time can be appropriately determined according to the heating temperature (the above absolute temperature T) to be employed and the characteristics of the target nitrided layer. For example, if it is the temperature within the range of 350-650 ° C, about 0.5 to 24 hours can be adopted.

  By the nitriding treatment, the above-described nitrided layer is formed on the surface layer portion of the steel base material.

After the nitriding treatment, a treatment for improving the compressive residual stress of the surface layer can be performed as needed. Shot peening etc. are employable as processing which improves compressive residual stress, for example.

[Micronization treatment]
In the spring manufacturing method of the present embodiment, prior to the above-described gas nitriding treatment, the above-described steel base material can be subjected to a refining process in which crystals of the surface layer portion of the above-described steel base material are refined.
By performing the above-described gas nitriding process on the steel base material in which the crystal of the surface layer portion is refined by the above-described refining process, the nitrided layer to be formed has γ′-Fe ₄ N and the above addition on the outermost surface. A nitrogen compound layer containing an element nitride is formed, and the crystal grain size is refined to about 1 μm or less. The formation of such a nitrided layer makes it more difficult to cause cracking, cracking and peeling due to repeated stress and shot peening. Thereby, the fatigue strength of the spring can be further improved.

  A halogenation process or a shot peening process can be performed as the above-mentioned miniaturization process.

[Halogenation treatment]
The halogenation treatment as the above-mentioned refinement processing is performed by heating and holding the above-mentioned steel base material in the atmosphere gas containing halogen using a heating furnace which can control the atmosphere.

The halogen used in the atmospheric gas, for _example, can be used F _2, Cl 2, HCl, halogen gas or halide gas such as NF _3.

  As the above-mentioned atmosphere gas, a mixed gas containing 0.5 to 20% by volume of halogen and the remainder being nitrogen gas, hydrogen gas or inert gas can be used.

  The halogenation treatment activates the surface of the steel base material by heating and holding the steel base material at 200 to 550 ° C. for about 10 minutes to 3 hours in the atmosphere gas.

By subjecting the surface of the steel base material activated by the halogenation treatment to the gas nitriding process at the above-described nitriding potential and absolute temperature, the above-described nitrided layer is formed on the surface of the steel base material.
When spring steel is used as the above-mentioned steel base material, alloy elements more active than Fe, such as Cr, Al, Si, etc., are often added. By subjecting such a steel base material to a halogenation treatment, a dense and thick nitrided layer can be formed by a subsequent gas nitriding treatment. In addition, halogenation produces iron halide in the surface layer portion of the steel base material. In the subsequent gas nitriding treatment, the halogen in the surface layer part is released to form iron of fine crystal grains, and the nitrogen compound layer of extremely fine crystal grains is formed by nitriding.

[Shot peening processing]
The shot peening treatment as the above-mentioned refinement treatment makes small spheres of innumerable steels or non-ferrous metals collide with the surface of the steel base material at high speed, and aims at work hardening and compressive residual stress by plastic deformation. In the case of a shot material of steel balls, one having a diameter of about 0.2 to 4 mm can be used. The speed of the steel ball to be projected is about 40 to several hundreds m / s.

  By performing shot peening as the above-mentioned refinement processing, the crystal of the surface layer portion of the above-mentioned steel base material receives deformation by processing and causes processing distortion. During the subsequent gas nitriding treatment, the heated and deformed crystal is divided into polygonal fine grains and refined. Nitrogen bonds with such fine crystals of steel as nuclei to form nitrogen compounds having a fine crystal grain size.

[Residual stress improvement treatment]
After the gas nitriding treatment, for example, a shot peening treatment can be performed to improve the compressive residual stress in the surface layer portion.

Shot peening treatment as the above-mentioned residual stress improvement treatment causes innumerable small balls of steel or non-ferrous metal to collide with the surface of a steel base material at high speed, and aims at work hardening and compressive residual stress by plastic deformation. In the case of a shot material of steel balls, one having a diameter of about 0.2 to 4 mm can be used. The speed of the steel ball to be projected is about 40 to several hundreds m / s.

[Effect of the embodiment]
The above embodiment has the following effects.

In the spring excellent in fatigue resistance of this embodiment, a nitrided layer is formed on the surface of a steel base material to which spring characteristics are imparted. The steel base material contains one or more additional elements having higher activity to nitrogen than iron. In the nitrided layer, a nitrogen compound layer containing γ′-Fe ₄ N and a nitride of the above-described additive element is formed on the outermost surface.
That is, the present invention does not form a hard and brittle ε-Fe _2-3 N layer on the outermost surface like the nitrogen compound layer obtained by the conventional nitriding treatment. On the outermost surface, a nitrogen compound layer containing γ'-Fe ₄ N and a nitride of the above additive element is formed. Such a nitrogen compound layer is less likely to crack even if stress is repeatedly applied, and is less likely to be cracked or peeled off by shot peening. Thereby, the fatigue strength of the spring can be significantly improved.

In the spring excellent in fatigue resistance of the present embodiment, the nitrogen compound layer has a thickness of 5 μm or less.
By limiting the thickness of the nitrogen compound layer to 5 μm or less, cracking, cracking and peeling due to repeated stress and shot peening are less likely to occur. Thereby, the fatigue strength of the spring can be further improved.

In the spring excellent in fatigue resistance of this embodiment, the crystal grain diameter of at least the outermost layer portion of the nitrogen compound layer is 1 μm or less.
By limiting the crystal grain size of the nitrogen compound layer to 1 μm or less, cracking, cracking and peeling due to repeated stress and shot peening are less likely to occur. Thereby, the fatigue strength of the spring can be further improved.

In the spring excellent in fatigue resistance of the present embodiment, the compressive residual stress in the 45 ° direction of the nitrogen compound layer is 1000 MPa or more.
By setting the compressive residual stress in the 45 ° direction of the nitrogen compound layer to 1000 MPa or more, cracking, cracking and peeling due to repeated stress and shot peening are less likely to occur. Thereby, the fatigue strength of the spring can be further improved.

The spring excellent in fatigue resistance of the present embodiment is a spring in which the nitrogen diffusion layer is formed under the nitrogen compound layer on the outermost surface of the nitride layer.
The nitrogen diffusion layer is strengthened by the diffusion and penetration of nitrogen atoms. The outermost nitrogen compound layer is supported by the reinforced nitrogen diffusion layer. Therefore, cracking, cracking and peeling due to repeated stress and shot peening are less likely to occur. Thereby, the fatigue strength of the spring can be further improved.

In the spring excellent in fatigue resistance of the present embodiment, the nitrogen diffusion layer has a thickness of 20 to 150 μm.
By setting the thickness of the nitrogen diffusion layer to 20 to 150 μm, the nitrogen compound layer on the outermost surface is supported by the nitrogen diffusion layer of the thickness. Therefore, cracking, cracking and peeling due to repeated stress and shot peening are less likely to occur. Thereby, the fatigue strength of the spring can be further improved.

In the spring excellent in fatigue resistance of the present embodiment, no other compound layer in the form of a layer is present between the nitrogen compound layer and the nitrogen diffusion layer.
The absence of another compound layer layered between the nitrogen compound layer and the nitrogen diffusion layer makes it difficult to cause cracking, cracking or peeling of the other compound layer due to repeated stress or shot peening. Thereby, the fatigue strength of the spring can be further improved.

The method of manufacturing a spring excellent in fatigue resistance according to the present embodiment applies the nitriding potential K _N and the absolute temperature T to the steel base material to which spring characteristics are imparted, according to the following formulas (1) (2) (3) Perform gas nitriding process controlled to the range.
(1) 133.52 exp (-0.008 T) K K _N 544 5449.1 exp (-0.01 T)
(2) 623 K ≦ T ≦ 923 K
(3) K _N = (partial pressure of NH ₃ ) / [(partial pressure of H ₂ ) ^3/2 ]
By this, a nitrided layer is formed on the surface of the steel base material to which the spring characteristics are imparted. As the nitrided layer, a nitrogen compound layer containing γ′-Fe ₄ N and a nitride of the additive element is formed on the outermost surface.
That is, the resulting spring does not form a hard and brittle ε-Fe _2-3 N layer on the outermost surface like the nitrogen compound layer obtained by the conventional nitriding treatment. On the outermost surface, a nitrogen compound layer containing γ'-Fe ₄ N and a nitride of the above additive element is formed. Such a nitrogen compound layer is less likely to crack even if stress is repeatedly applied, and is less likely to be cracked or peeled off by shot peening. Thereby, the fatigue strength of the spring can be significantly improved.

In the method of manufacturing a spring having excellent fatigue resistance according to the present embodiment, prior to the gas nitriding treatment, the steel base material is subjected to a refining treatment for refining the crystal of the surface layer portion of the steel base material.
By performing the above-described gas nitriding process on the above-described steel base material in which the crystal in the surface layer portion is refined by doing this, the nitrided layer to be formed has γ′-Fe ₄ N and A nitrogen compound layer containing a nitride of an additive element is formed, and the crystal grain size is refined to about 1 μm or less. The formation of such a nitrided layer makes it more difficult to cause cracking, cracking and peeling due to repeated stress and shot peening. Thereby, the fatigue strength of the spring can be further improved.

In the method of manufacturing a spring having excellent fatigue resistance according to the present embodiment, the above-described microfabrication treatment can be a halogenation treatment.
By so doing, fine halides of iron halide are formed in the surface layer portion of the steel base material by the halogenation treatment, and the crystals of the surface layer portion of the steel base material are refined. During the subsequent gas nitriding treatment, the halogen is desorbed from the steel base material from the fine iron halide, and nitrogen is bonded by using fine crystals of the steel as nuclei to form a nitrogen compound having a fine crystal grain diameter.

In the method of manufacturing a spring with excellent fatigue resistance according to the present embodiment, the above-described refining process can be shot peening.
By doing this, the crystals of the surface layer portion of the steel base material are subjected to deformation due to processing by the above-described shot peening to cause processing distortion. During the subsequent gas nitriding treatment, the heated and deformed crystal is divided into polygonal fine grains and refined. Nitrogen bonds with such fine crystals of steel as nuclei to form nitrogen compounds having a fine crystal grain size.

  Below, an Example is described.

  As described below, the spring of the present invention was obtained by the production method of the present invention.

[Steel base material]
The following steel base material was used.
Material: Oil temper wire for spring (high strength spring steel included in SWOSC-B)
Wire diameter: 3.2 mm
Average coil diameter: 21 mm
Total volume: 6.0

[Micronization treatment]
A halogenation treatment was carried out as a miniaturization treatment.
The halogenation treatment heats and holds the steel base material at 200 to 550 ° C. for about 10 minutes to 3 hours in an atmosphere gas containing a halogen gas such as F ₂ , Cl ₂ , HCl, or NF ₃ or a halide gas. did.

[Gas nitriding treatment]
The steel base material after the halogenation treatment was subjected to gas nitriding treatment under the following conditions to form a nitrided layer, and the spring of this example was obtained.
Atmosphere: Gas composition satisfying the above equations (1) (2) (3) Heating temperature: 500 ° C.
Holding time: 4 hours

[Residual stress improvement treatment]
In order to improve the residual stress in the surface layer portion, shot peening was performed under the following conditions after the gas nitriding treatment.
Shot grain: Steel ball φ 0.3 (HV 500 to 620)
Projection speed: 60 m / s

[Cross-sectional organization]
FIG. 3 is a photomicrograph showing the cross-sectional structure of the spring of this example. The layer which looks white on the surface is a nitrogen compound layer.

[Section hardness]
FIG. 4 is a diagram in which the cross-sectional hardness distribution of the spring of this embodiment is plotted. The topmost nitrogen compound layer exceeds Hv 900. The portion where the hardness decreases as the depth from the surface increases is the nitrogen diffusion layer. In the present embodiment, the thickness of the nitrogen diffusion layer is about 100 μm and is in the range of 20 to 150 μm. Therefore, in the present embodiment, the depth of 100 μm or more is the core portion hardly affected by nitrogen, and is less than Hv600.

[Nitrogen compound layer]
FIG. 5 is the result of X-ray diffraction from the surface of the spring of this example. It has been identified that the surface nitrogen compound layer consists of a nitrogen compound layer containing γ'-Fe ₄ N. With regard to the nitrides of the above additive elements, if the additive amount of the additive elements is several% or less, detection is difficult with detection sensitivity by X-ray diffraction, and it is present in the form of precipitates less than detection sensitivity Conceivable.

Grains
FIG. 6 is an electron micrograph of the crystal grains of the nitrogen compound layer of the spring of this example.
It is understood that the crystal grain size of the nitrogen compound layer is 1 μm or less.

[Compressed residual stress]
The result of measuring the compressive residual stress in the 45 ° direction of the nitrogen compound layer of the spring of this example is −1,28 ± 99 MPa after gas nitriding treatment, and after shot peening, it improves to −1,520 ± 110 MPa, 1000 MPa It was over.

〔Fatigue strength〕
The fatigue strength was measured by the SN method under the following conditions.
Number of tests: n = 3
Average stress: 700MPa
Stress amplitude: 590MPa
The measurement result (the number of repetitions) was 1 × 10 ⁷ or more.
It is known that a conventional common nitriding spring has a fatigue strength of about 1 × 10 ⁶ times. On the other hand, it can be seen that the spring of this example has sufficiently high fatigue strength.

[Modification]
Although the particularly preferred embodiments of the present invention have been described above, the present invention is not intended to limit the present invention to the illustrated embodiments, and can be modified into various aspects and implemented, and the present invention includes various modifications. The purpose is to

In the method for producing a spring excellent in fatigue resistance according to claim 10, the above-mentioned refining treatment is a halogenation treatment.
By the halogenation treatment, fine iron halide is formed in the surface layer portion of the steel base material, and the crystals in the surface layer portion of the steel base material are refined. During the subsequent gas nitriding treatment, the halogen is desorbed from the steel base material from the fine iron halide, and nitrogen is bonded by using fine crystals of the steel as nuclei to form a nitrogen compound having a fine crystal grain diameter.

The above-mentioned nitrogen compound layer preferably has a compressive residual stress in the 45 ° direction of 1000 MPa or more.
If the compressive residual stress is less than 1000 MPa, there is a risk that a sufficient fatigue resistance can not be obtained.
The compressive residual stress can be measured, for example, by the sin ² ψ method which is an X-ray stress measurement method. This measurement method is a scanning method parallel傾法; based on (.phi.0 constant method PSPC method), using the CrKα lines as characteristic X-ray is measured using the alpha-Fe (211) plane to the measured diffraction plane . At this time, K = -318 MPa / degree (which is a standard value of the Japan Society for Materials Science) is used as a stress constant. Measure the direction of 45 degrees with respect to the wire direction of the spring wire.

In the method of manufacturing a spring having excellent fatigue resistance according to the present embodiment, the above-described microfabrication treatment can be a halogenation treatment.
By so doing, fine halides of iron halide are formed in the surface layer portion of the steel base material by the halogenation treatment, and the crystals of the surface layer portion of the steel base material are refined. During the subsequent gas nitriding treatment, the halogen is desorbed from the steel base material from the fine iron halide, and nitrogen is bonded by using fine crystals of the steel as nuclei to form a nitrogen compound having a fine crystal grain diameter.

Claims (11)

A steel base material to which a spring characteristic is given,
And a nitride layer formed on the surface of the steel base material;
The steel base material contains one or more additional elements having higher activity to nitrogen than iron,
A spring excellent in fatigue resistance characterized in that the nitrided layer is formed on the outermost surface of a nitrogen compound layer containing γ'-Fe ₄ N and a nitride of the additive element. The spring having excellent fatigue resistance according to claim 1, wherein the nitrogen compound layer has a thickness of 5 μm or less. The spring excellent in fatigue resistance according to claim 1 or 2, wherein the crystal grain diameter of at least the outermost layer portion of the nitrogen compound layer is 1 μm or less. The spring having excellent fatigue resistance according to any one of claims 1 to 3, wherein the nitrogen compound layer has a compressive residual stress in a 45 degree direction of 1000 MPa or more. The spring having excellent fatigue resistance according to any one of claims 1 to 4, wherein the nitrided layer has a nitrogen diffusion layer formed under the nitrogen compound layer on the outermost surface. The spring having excellent fatigue resistance according to claim 5, wherein the nitrogen diffusion layer has a thickness of 20 to 150 m. 7. The spring with excellent fatigue resistance according to claim 5, wherein another compound layer in a layered form is not present between the nitrogen compound layer and the nitrogen diffusion layer. For steel base material to which spring characteristics are given,
A method of manufacturing a spring excellent in fatigue resistance characterized by performing gas nitriding processing in which the nitriding potential K _N and the absolute temperature T are controlled in the range according to the following formulas (1), (2) and (3).
(1) 133.52 exp (-0.008 T) K K _N 544 5449.1 exp (-0.01 T)
(2) 623 K ≦ T ≦ 923 K
(3) K _N = (partial pressure of NH ₃ ) / [(partial pressure of H ₂ ) ^3/2 ] The method for manufacturing a spring excellent in fatigue resistance according to claim 8, wherein prior to the gas nitriding treatment, the steel base material is subjected to a refining treatment for refining the crystal of the surface layer portion of the steel base material. The method for producing a spring excellent in fatigue resistance as set forth in claim 9, wherein the miniaturization treatment is a halogenation treatment. The method for manufacturing a spring excellent in fatigue resistance as set forth in claim 9, wherein the miniaturization process is shot peening.