WO2019208534A1

WO2019208534A1 - Nitrided steel member, and method and device for producing nitrided steel member

Info

Publication number: WO2019208534A1
Application number: PCT/JP2019/017127
Authority: WO
Inventors: 泰平岡; 陽一渡邊
Original assignee: パーカー熱処理工業株式会社
Priority date: 2018-04-26
Filing date: 2019-04-23
Publication date: 2019-10-31
Also published as: JP7094540B2; JP2021120471A

Abstract

The present invention is a nitrided steel member in which a carbon steel having a carbon content of 0.10% by mass or more or a low alloy steel makes up a parent phase and an iron nitride compound layer is formed on the surface of the parent phase, the nitrided steel member being characterized in that the thickness of the iron nitride compound layer is 13 μm or more, the Vaγ'/(Vaε+Vaγ') value is 0.5 or more wherein Vaγ' and Vaε respectively represent the volume ratio of a γ' phase and the volume ratio of an ε phase in the entire region of the iron nitride compound layer, and the Vbγ'/(Vbε+Vbγ') value is 0.2 or more wherein Vbγ' and Vbε respectively represent the volume ratio of the γ' phase and the volume ratio of the ε phase in a lowermost region of the iron nitride compound layer when the iron nitride compound layer is divided into four regions in the depth direction.

Description

Nitride steel member and method and apparatus for producing nitride steel member

The present invention relates to a nitrided steel member and a method and apparatus for producing a nitrided steel member. More specifically, the present invention relates to a nitrided steel member having excellent fatigue resistance useful for gears and crankshafts for automobile transmissions, and a method and apparatus for manufacturing the nitrided steel member.

Among the surface hardening treatments for steel materials, there is a great need for nitriding treatment, which is low heat treatment strain treatment, and in recent years, there has been an increasing interest in atmosphere control technology for gas nitriding treatment.

In the basic structure obtained by gas nitriding, a compound layer that is iron nitride is formed on the surface, and a hardened layer called a diffusion layer is formed inside. The hardened layer is usually made of an alloy nitride such as Si or Cr as a base material component.

In order to control the thickness (depth) of each of these two layers and / or the type of surface iron nitride, etc., in addition to the temperature and time of the gas nitriding process, the atmosphere in the gas nitriding furnace is also It is controlled appropriately. Specifically, the nitriding potential (K _N ) in the gas nitriding furnace is appropriately controlled.

For example, the volume fraction (iron nitride type) of the γ 'phase (Fe ₄ N) and ε phase (Fe _2-3 N) in the compound layer generated on the surface of the steel material is controlled through the control. It has been proposed to do. Specifically, it is known that fatigue resistance is improved by forming a γ 'phase rather than an ε phase (Yasuhira Hiraoka, Yoichi Watanabe, Yasushi Ishida: Heat treatment, Volume 1, No. 1, 1-2 (Non-Patent Document 1)), a nitrided steel member having improved bending fatigue strength and surface fatigue by forming a γ 'phase has been proposed (Japanese Patent Laid-Open No. 2013-221203 (Patent Document 1)). Furthermore, it is also known that the bending fatigue strength increases as the thickness of the γ ′ phase in the compound layer increases (Y. Hiraoka, A. Ishida: Materials Transactions, 58, 2017, 993-999). Page (Non-Patent Document 2)). However, even if the gas nitriding treatment is performed to form a large amount of γ ′ phase, the compound layer contains not a little ε phase, and in fact, it becomes a two-phase state of γ ′ phase and ε phase. Yes.

Here, when a relatively thick compound layer is formed, an ε phase is likely to be formed in the region of the inner compound layer in contact with the diffusion layer, and it is difficult to form a γ ′ phase in the region. (Japanese Patent Laid-Open No. 2016-211069 (Patent Document 2)). The ε phase is relatively brittle and the fatigue crack growth rate is fast. Therefore, when the thickness is increased, the fatigue strength may be deteriorated. The reason why the ε phase is easily formed in the region is that the carbon in the matrix phase moves to the surface side by the surface decarburization reaction during the nitriding treatment, but the rate of carbon diffusion in the compound layer is slower than the matrix phase. This is caused by the concentration of carbon at the interface between the diffusion layer and the compound layer (Dietary Toke et al .: Nitriding and soft nitriding of iron, Agne Technical Center, 2013, pages 37-49 (Non-patent Document 3)).

It is also known that when a relatively thick compound layer is formed, the nitrogen concentration in the surface layer region of the compound layer increases, and thus the ratio of the ε phase increases in the surface layer region. Specifically, when the thickness of the compound layer exceeds 17 μm, the ratio of the ε phase increases (Japanese Patent Laid-Open No. 2017-36509 (Patent Document 3)).

Patent Document 1 cited in this specification is Japanese Patent Application Laid-Open No. 2013-221203, Patent Document 2 cited in this specification is Japanese Patent Application Laid-Open No. 2016-211069, and Patent Documents cited in this Specification are cited. 3 is JP-A-2017-36509. Non-patent document 1 cited in the present specification is “Heat Treatment”, Vol. 55, No. 1, page 1-2 (Yasuhira Hiraoka, Yoichi Watanabe, Yasushi Ishida), and the non-patent document cited in this specification. Reference 2 is “Materials Transactions”, 58, 2017, pp. 993-999 (Y. Hiraoka, I A. Ishida). Non-patent literature 3 cited in this specification is “Nitride and soft nitriding of iron” "Agne Technical Center, 2013, pages 37-49 (Dietary Toke et al.).

As described above, it is possible to improve the fatigue strength of the nitrided steel member by controlling the γ ′ phase to increase in the compound layer, and further increase the fatigue strength by increasing the γ ′ phase. It is possible. However, in order to form a relatively thick compound layer in order to make the γ ′ phase thicker, the formation of the ε phase in the region near the diffusion layer / compound layer interface is suppressed, and also in the surface layer region of the compound layer It is necessary to suppress the formation of the ε phase.

The present inventor repeated diligent examinations and various experiments, limited the configuration of the processing furnace, and then controlled the temperature and nitriding potential of the nitriding process with high accuracy, thereby making the compound layer mainly composed of a relatively thick γ ′ phase. It was found that a desired amount of γ ′ phase can be maintained even in the region near the interface of the diffusion layer / compound layer, and the increase in ε phase can be suppressed also in the surface layer region of the compound layer.

The present invention has been developed based on the above knowledge. An object of the present invention is to provide a nitrided steel member having significantly improved fatigue resistance, and a production method and a production apparatus for producing such a nitrided steel member.

The present invention is a nitrided steel member having a carbon steel or low alloy steel having a carbon content of 0.10% by mass or more as a parent phase and having an iron nitride compound layer formed on a surface thereof, the iron nitride The thickness of the physical compound layer is 13 μm or more. When the volume ratios of the γ ′ phase and the ε phase in the whole region of the iron nitride compound layer are Vaγ ′ and Vaε, respectively, Vaγ ′ / (Vaε + Vbγ ′) when the value of Vaγ ′) is 0.5 or more and the volume ratios of the γ ′ phase and the ε phase in the lower quarter region of the iron nitride compound layer are Vbγ ′ and Vbε, respectively. / (Vbε + Vbγ ′) is a nitrided steel member having a value of 0.2 or more.

Such a nitrided steel member can be manufactured for the first time (provided to the world for the first time) by the method described later, which was invented by the present inventors. In such a nitrided steel member, since the volume ratio ratio Vaγ ′ / (Vaε + Vaγ ′) of the γ ′ phase in the entire region of the iron nitride compound layer is 0.5 or more, the iron nitride It can be considered that the entire compound layer is a compound layer mainly composed of γ ′ phase, and the fatigue strength is remarkably improved by the thickness being 13 μm or more. Since the value of the volume ratio Vbγ ′ / (Vbε + Vbγ ′) of the γ ′ phase in the lower 1/4 region of the iron nitride compound layer is maintained at 0.2 or more, ε in the region Deterioration of fatigue strength due to the presence of the phase is remarkably suppressed.

The thickness of the iron nitride compound layer is more preferably 20 μm to 35 μm or more.
If the thickness is 20 μm, the fatigue strength is further improved. Further, 35 μm is a preferable value in consideration of productivity. (The thickness of the iron nitride compound layer generally corresponds to the nitriding time. If there is no limitation on the nitriding time, there is no upper limit to the thickness of the iron nitride compound layer.)

The value of Vbγ ′ / (Vbε + Vbγ ′) is more preferably 0.3 or more. In this case, deterioration of fatigue strength due to the presence of the ε phase in the lower quarter region of the iron nitride compound layer is further suppressed.

The present invention also provides a nitrided steel member having a parent phase of carbon steel or low alloy steel having a carbon content of 0.10% or more by mass, using a circulation type processing furnace having a guide cylinder and a stirring fan. In which the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C. in the first stage of processing, and The nitriding potential in the circulation type processing furnace is controlled in the range of 0.15 to 0.4, and in the second stage processing, the temperature in the circulation type processing furnace is controlled in the range of 490 ° C. to 510 ° C. The method for producing a nitrided steel member is characterized in that the nitriding potential in the circulation type processing furnace is controlled in the range of 0.5 to 2.0.

According to the method for manufacturing the nitrided steel member,
A nitrided steel member having a carbon steel or low alloy steel having a carbon content of 0.10% by mass or more as a parent phase and having an iron nitride compound layer formed on a surface thereof, the iron nitride compound layer comprising: The thickness is 13 μm or more, and Vaγ ′ / (Vaε + Vaγ ′) when the volume ratios of the γ ′ phase and the ε phase in the entire region of the iron nitride compound layer are Vaγ ′ and Vε, respectively. Vbγ ′ / (Vbε + when the volume ratio of γ ′ phase and ε phase in the lower 1/4 region of the iron nitride compound layer is Vbγ ′ and Vbε respectively. A nitrided steel member having a value of Vbγ ′) of 0.2 or more can be produced.

Here, the second stage treatment (treatment in the range of 490 ° C. to 510 ° C.) is continued using the same circulation type processing furnace as the first stage treatment (treatment in the range of 560 ° C. to 600 ° C.). It may be performed using a circulation type processing furnace different from the first stage processing. Depending on the temperature condition setting (elevating / lowering) performance in the circulation type processing furnace, the latter may have better production efficiency. While the material is moved from the first-stage processing circulation processing furnace to the second-stage processing circulation processing furnace, the temperature of the material may be maintained at the temperature condition in the first-stage processing. However, it may be naturally cooled to about room temperature temporarily. In any case, it has been confirmed (in the examples described later) by the present inventors that the method of the present invention is effective.

Alternatively, according to the present invention, a nitrided steel member having a parent phase of carbon steel or low alloy steel having a carbon content of 0.10% by mass or more using a circulation type processing furnace including a guide cylinder and a stirring fan. In which the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C. in the first stage of processing, and The nitriding potential in the circulation type processing furnace is controlled in the range of 0.7 to 3.0, and in the second stage processing, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C. In addition, the nitriding potential in the circulation type processing furnace is controlled in the range of 0.15 to 0.4, and in the third stage processing, the temperature in the circulation type processing furnace is in the range of 490 ° C. to 510 ° C. And nitriding potency in the circulation type processing furnace. Le is a method for producing a nitride steel member characterized by being controlled in the range of 0.5-2.0.

Also by the method of manufacturing the nitrided steel member,
A nitrided steel member having a carbon steel or low alloy steel having a carbon content of 0.10% by mass or more as a parent phase and having an iron nitride compound layer formed on a surface thereof, the iron nitride compound layer comprising: The thickness is 13 μm or more, and Vaγ ′ / (Vaε + Vaγ ′) when the volume ratios of the γ ′ phase and the ε phase in the entire region of the iron nitride compound layer are Vaγ ′ and Vε, respectively. Vbγ ′ / (Vbε + when the volume ratio of γ ′ phase and ε phase in the lower 1/4 region of the iron nitride compound layer is Vbγ ′ and Vbε respectively. A nitrided steel member having a value of Vbγ ′) of 0.2 or more can be produced.

Here, the third stage treatment (treatment in the range of 490 ° C. to 510 ° C.) is the same circulation type processing furnace as the first and second stage treatment (treatment in the range of 560 ° C. to 600 ° C.). May be performed subsequently, or may be performed using a circulation type processing furnace different from the first stage and second stage processes. Depending on the temperature condition setting (elevating / lowering) performance in the circulation type processing furnace, the latter may have better production efficiency. While the material is transferred from the first and second stage processing circulation processing furnaces to the third stage processing circulation processing furnace, the temperature of the material in the first and second stage processing is It may be maintained at a temperature condition or may be naturally cooled to about room temperature temporarily. In any case, it has been confirmed (in the examples described later) by the present inventors that the method of the present invention is effective.

The present invention further includes a circulation type processing furnace having a guide cylinder and a stirring fan, and in the first stage of processing, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C., and The nitriding potential in the circulation type processing furnace is controlled in the range of 0.15 to 0.4, and in the second stage processing, the temperature in the circulation type processing furnace is controlled in the range of 490 ° C. to 510 ° C. And a nitriding potential in the circulation type processing furnace is controlled in the range of 0.5 to 2.0.

Alternatively, the present invention includes a circulation type processing furnace having a guide cylinder and a stirring fan, and in the first stage of processing, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C., and The nitriding potential in the circulation type processing furnace is controlled in the range of 0.7 to 3.0, and the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C. in the second stage processing. In addition, the nitriding potential in the circulation type processing furnace is controlled in the range of 0.15 to 0.4, and in the third stage processing, the temperature in the circulation type processing furnace is 490 ° C. to 510 ° C. The nitrided steel member manufacturing apparatus is characterized in that the nitriding potential in the circulation type processing furnace is controlled within a range of 0.5 to 2.0.

According to the production equipment for these nitrided steel members,
A nitrided steel member having a carbon steel or low alloy steel having a carbon content of 0.10% by mass or more as a parent phase and having an iron nitride compound layer formed on a surface thereof, the iron nitride compound layer comprising: The thickness is 13 μm or more, and Vaγ ′ / (Vaε + Vaγ ′) when the volume ratios of the γ ′ phase and the ε phase in the entire region of the iron nitride compound layer are Vaγ ′ and Vε, respectively. Vbγ ′ / (Vbε + when the volume ratio of γ ′ phase and ε phase in the lower 1/4 region of the iron nitride compound layer is Vbγ ′ and Vbε respectively. A nitrided steel member having a value of Vbγ ′) of 0.2 or more can be produced.

In the apparatus for manufacturing a nitrided steel member of the present invention, for example, ammonia gas and ammonia decomposition gas are introduced into the circulation type processing furnace. In this case, the manufacturing apparatus, in order to control the nitriding potential, a first control for changing the introduction ratio of the ammonia gas and the introduction amount of the ammonia decomposition gas while keeping the total flow rate constant. It is preferable that the second control for changing the introduction amount of the ammonia gas can be selectively performed in a state where the introduction of the ammonia decomposition gas is stopped.

According to the nitrided steel member of the present invention, the value of the volume ratio ratio Vaγ ′ / (Vaε + Vaγ ′) of the γ ′ phase in the entire region of the iron nitride compound layer is 0.5 or more. It can be considered that the entire physical compound layer is a compound layer mainly composed of a γ ′ phase, and the fatigue strength is remarkably improved by having a thickness of 13 μm or more. Since the value of the volume ratio Vbγ ′ / (Vbε + Vbγ ′) of the γ ′ phase in the lower 1/4 region of the iron nitride compound layer is maintained at 0.2 or more, ε in the region Deterioration of fatigue strength due to the presence of the phase is remarkably suppressed.

It is a cross-sectional microscope picture of the nitrided steel member by one Embodiment of this invention. It is a cross-sectional phase distribution of the nitrided steel member of FIG. 1 analyzed by the EBSD method. It is a figure which shows the relationship between the amount of carbon which a γ 'phase can contain, and temperature. It is a cross-sectional microscope picture of a comparative example. It is a cross-sectional phase distribution of the nitrided steel member of FIG. 3 analyzed by the EBSD method. It is a figure which shows the form of an Ono type | formula rotation bending fatigue test piece. It is a figure which shows the relationship between a fatigue limit and a compound layer thickness. It is a figure which shows the relationship between a fatigue limit and the volume ratio ratio of (gamma) 'phase in the area | region of lower 1/4. It is the schematic of the manufacturing apparatus of the nitrided steel member by one Embodiment of this invention. It is a schematic sectional drawing of a circulation type processing furnace (horizontal type gas nitriding furnace). It is a graph which shows the example of the 1st control. It is a graph which shows the example of the 1st control. It is a graph which shows the example of the 2nd control. It is a graph which shows the example of the 2nd control. It is the schematic which shows the example of the jig inserted in a furnace.

Hereinafter, preferred embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

(Configuration of nitrided steel member 100 of one embodiment of the present invention)
FIG. 1 is a cross-sectional photomicrograph of a nitrided steel member 100 according to an embodiment of the present invention manufactured by the present inventors. As shown in FIG. 1, the steel nitride member 100 of the present embodiment includes an iron nitride compound layer 101 as a hardened layer on the surface, and nitrogen is diffused into the parent phase below the iron nitride compound layer 101. The diffusion layer 102 is provided. The parent phase (base material) of this embodiment is S45C having a carbon content of about 0.45% by mass.

The iron nitride compound layer 101 of the nitrided steel member 100 in FIG. 1 has a thickness of about 16 μm from the surface of the nitrided steel member 100. The diffusion layer 102 of the nitrided steel member 100 of FIG. 1 extends from the surface of the nitrided steel member 100 to a depth of about 1000 μm.

As described above, the iron nitride compound layer 101 is a layer including an ε phase (Fe _2-3 N) and a γ ′ phase (Fe ₄ N). The distribution state of these phases can be analyzed by an EBSD (Electron Back Scatter Diffraction) method. Specifically, it can be determined from the area ratio of the γ ′ phase and the ε phase in the cross section in the depth direction of the iron nitride compound layer 101. (The area ratio is considered to correspond to the volume ratio.) For example, it is possible to determine three cross sections (for three fields of view) in the depth direction having a width of 100 μm from the average value thereof.

FIG. 2 is an analysis result of the EBSD method of the cross section of FIG. In the embodiment of FIG. 2, in the iron nitride compound layer 101, the volume ratio of the γ ′ phase in the entire region (the volume ratio of the γ ′ phase and the ε phase in the entire region of the iron nitride compound layer 101 is The value of Vaγ ′ / (Vaε + Vaγ ′)) when Vaγ ′ and Vaε is about 0.70. Further, the volume ratio of the γ ′ phase in the lower 1/4 region of the iron nitride compound layer 101 (the volume ratio of the γ ′ phase and the ε phase in the lower 1/4 region of the iron nitride compound layer 101) Vbγ ′ / (Vbε + Vbγ ′)) where Vbγ ′ and Vbε are respectively greater than 0.2.

(Manufacturing method of the nitrided steel member 100)
The iron nitride compound layer 101 of FIG. 1 is manufactured by a three-stage nitriding process using a circulation type processing furnace (described in detail later) provided with a guide cylinder and a stirring fan (Table 1 (compound layer described later)). Refer to the example of a thickness of 16 μm).

In the first stage process (for example, 2 hours), the temperature in the circulation type processing furnace is controlled within a range of 580 ° C., and the nitriding potential in the circulation type processing furnace is controlled to 0.7. By this treatment, the entire thickness of the iron nitride compound layer 101 is adjusted.

In the second stage treatment (for example, 0.5 hour), the nitriding potential in the circulation type processing furnace is controlled to 0.3 while the temperature in the circulation type processing furnace is maintained at 580 ° C. By this treatment, the volume ratio of the γ ′ phase in the entire region of the iron nitride compound layer 101 (the volume ratios of the γ ′ phase and the ε phase in the entire region of the iron nitride compound layer 101 are Vaγ ′ and Vaε, respectively. The value of Vaγ ′ / (Vaε + Vaγ ′)) is adjusted.

In the third stage treatment (for example, 2 hours), the temperature in the circulation processing furnace is controlled to 500 ° C., and the nitriding potential in the circulation processing furnace is controlled to 0.7. By this treatment, the volume ratio of the γ ′ phase in the lower 1/4 region of the iron nitride compound layer 101 (the γ ′ phase and the ε phase occupying the lower 1/4 region of the iron nitride compound layer 101). The value of Vbγ ′ / (Vbε + Vbγ ′)) is adjusted when the volume ratios are Vbγ ′ and Vbε, respectively.

This third stage process is a novel feature in the production method of the present invention. The inventor of the present invention refers to the temperature range in which the γ ′ phase can contain (coexist with) the most carbon (490 to 510 ° C.) (see FIG. 3), and performs renitriding treatment in the temperature range. In practice, the present inventors found that the volume ratio of the γ ′ phase can be maintained at 0.2 or more even in the region of the lower quarter of the iron nitride compound layer in which the ε phase was large in the past, It was done. Note that the nitriding potential in this process is 0.5 to 2.0 (described later).

(Configuration of the nitrided steel member 120 of the comparative example)
On the other hand, FIG. 4 is a cross-sectional micrograph of a nitrided steel member 120 as a comparative example manufactured by a conventional manufacturing method. As shown in FIG. 4, the steel nitride member 120 of the comparative example also includes an iron nitride compound layer 121 as a hardened layer on the surface, and diffusion in which nitrogen is diffused in the parent phase below the hardened layer 201. A layer 122 is provided. The parent phase (base material) of the comparative example is also S45C having a carbon content of about 0.45% by mass.

The iron nitride compound layer 121 of the nitrided steel member 120 in FIG. 4 also has a thickness of about 16 μm from the surface of the nitrided steel member 120. The diffusion layer 122 of the nitrided steel member 120 in FIG. 4 also extends from the surface of the nitrided steel member 120 to a depth of about 1000 μm.

FIG. 5 is an analysis result of the EBSD method of the cross section of FIG. In the comparative example of FIG. 5, in the iron nitride compound layer 121, the volume ratio of the γ ′ phase in the entire region (the volume ratio of the γ ′ phase and the ε phase in the entire region of the iron nitride compound layer 121 is The value of Vaγ ′ / (Vaε + Vaγ ′)) when Vaγ ′ and Vε are about 0.55. Further, the volume ratio of the γ ′ phase in the lower 1/4 region of the iron nitride compound layer 121 (the volume ratio of the γ ′ phase and the ε phase in the lower 1/4 region of the iron nitride compound layer 121) Vbγ ′ / (Vbε + Vbγ ′)) where Vbγ ′ and Vbε are respectively smaller than 0.2.

(Manufacturing method of the nitrided steel member 120)
The iron nitride compound layer 121 in FIG. 4 is manufactured by a two-stage nitriding process using a circulation type processing furnace (details will be described later) including a guide cylinder and a stirring fan.

In the first stage process (for example, 2 hours), the temperature in the circulation type processing furnace is controlled within a range of 580 ° C., and the nitriding potential in the circulation type processing furnace is controlled to 0.7. By this treatment, the entire thickness of the iron nitride compound layer 121 is adjusted.

In the second stage treatment (for example, 0.5 hour), the nitriding potential in the circulation type processing furnace is controlled to 0.3 while the temperature in the circulation type processing furnace is maintained at 580 ° C. By this treatment, the volume ratio of the γ ′ phase in the entire region of the iron nitride compound layer 101 (the volume ratios of the γ ′ phase and the ε phase in the entire region of the iron nitride compound layer 101 are Vaγ ′ and Vε, respectively. The value of Vaγ ′ / (Vaε + Vaγ ′)) is adjusted to be 0.5 or more. It is disclosed in Japanese Patent Application Laid-Open No. 2016-211069 (Patent Document 2) that the (γ ′ phase volume ratio ratio (Vaγ ′ / (Vaε + Vaγ ′)) is preferably 0.5 or more. )

Unlike the method for manufacturing the nitrided steel member 100, the third stage treatment is not performed. For this reason, the volume ratio of the γ ′ phase in the lower 1/4 region of the iron nitride compound layer 101 (the volume of the γ ′ phase and the ε phase in the lower 1/4 region of the iron nitride compound layer 101). The value of Vbγ ′ / (Vbε + Vbγ ′)) when the ratios are Vbγ ′ and Vbε, respectively, is not adjusted (it remains small).

(Verification of effect by test piece (1): embodiment and comparative example)
Using a test piece for a bending fatigue test, the fatigue strength was verified. Specifically, a test piece having the configuration (cross section) shown in FIG. 1 was prepared, and the fatigue limit was measured. As shown in FIG. 6, the shape of the test piece corresponds to an Ono type rotating bending fatigue tester (Shimadzu Corporation, H7 type).

The test result (fatigue limit) was 45.4 kgf. On the other hand, when a test piece having the configuration (cross section) of FIG. 4 was prepared and the fatigue limit was measured, it was 40.8 kgf. From this result, it can be seen that the fatigue resistance of the nitrided steel member 100 of the embodiment of FIG. 1 is remarkably improved.

(Verification of effect by test piece (2): thickness of iron nitride compound layer)
If the volume ratio ratio Vaγ ′ / (Vaε + Vaγ ′) of the γ ′ phase in the entire region of the iron nitride compound layer is 0.5 or more, the entire iron nitride compound layer is mainly composed of the γ ′ phase. It can be considered that this is a compound layer. Therefore, it is considered that the fatigue strength increases as the thickness of the iron nitride compound layer increases.

In the manufacturing method of the iron nitride compound layer 101 described above, a modification was made by changing the processing content of the first stage and further increasing the thickness of the iron nitride compound layer. Specifically, as shown in Table 1 below, the nitriding potential in the circulation type processing furnace was set to 1.3, and the thickness of the iron nitride compound layer was set to 20 μm. Further, the nitriding potential in the circulation type processing furnace was set to 1.3, the processing time was extended to 3 hours, and the thickness of the iron nitride compound layer was set to 25 μm.

Also in such a modification, as shown in Table 1, the volume ratio of the γ ′ phase in the entire region of the iron nitride compound layer is larger than 0.5, and the lower 1/4 of the iron nitride compound layer The volume ratio of the γ ′ phase in the region was greater than 0.2.

And the test piece corresponding to each modification was created, and the fatigue limit was measured. The test result (fatigue limit) was 47.4 kgf when the thickness of the iron nitride compound layer was 20 μm, and 49.0 kgf when the thickness of the iron nitride compound layer was 25 μm. These results are plotted in FIG.

For further comparison, in the manufacturing method of the iron nitride compound layer 101, the first stage process is not performed, and the second stage process content is changed to reduce the thickness of the iron nitride compound layer. A modification and a comparative example were created. Specifically, as shown in Table 1 below, the first-stage treatment was stopped, the treatment time was extended to 3 hours in the second-stage treatment, and the thickness of the iron nitride compound layer was 13 μm. (Modification). In addition, the treatment of the first step was stopped, the treatment time was 2 hours in the treatment of the second step, and the thickness of the iron nitride compound layer was 10 μm (comparative example). Further, the first-stage treatment was stopped, the nitriding potential was 0.25 in the second-stage treatment, the treatment time was 3 hours, and the thickness of the iron nitride compound layer was 6 μm (comparative example). Further, the first stage treatment was stopped, the nitriding potential was 0.2 in the second stage treatment, the treatment time was 3 hours, and the thickness of the iron nitride compound layer was 2 μm (comparative example).

Also in the modified example in which the thickness of the iron nitride compound layer is 13 μm, as shown in Table 1, the volume ratio of the γ ′ phase in the entire region of the iron nitride compound layer is larger than 0.5, The volume ratio of the γ ′ phase in the lower quarter region of the nitride compound layer was greater than 0.2.

Then, a test piece corresponding to the modified example was created and the fatigue limit was measured. The test result (fatigue limit) was 43.9 kgf. This result is also plotted in FIG.

On the other hand, in the comparative example in which the thickness of the iron nitride compound layer was 2 to 10 μm, as shown in Table 1, the volume ratio of the γ ′ phase in the entire region of the iron nitride compound layer was 0.5 The volume ratio of the γ ′ phase in the lower quarter region of the iron nitride compound layer was greater than 0.2.

However, when a test piece corresponding to the comparative example was prepared and the fatigue limit was measured, the test result (fatigue limit) was less than 40.8 kgf. That is, it can be seen that when the thickness of the iron nitride compound layer is less than 13 μm, the effect of the present invention of improving fatigue strength cannot be obtained. These results are also plotted in FIG.

(Verification of effect by test piece (3): Volume ratio of γ ′ phase of lower quarter)
While maintaining the condition that the volume ratio of the γ 'phase in the entire region of the iron nitride compound layer is larger than 0.5, the volume of the γ' phase in the lower 1/4 region of the iron nitride compound layer In order to make the ratios different, the contents of the three steps were changed as shown in Table 2 below. Under any of the conditions shown in Table 2, the thickness of the produced iron nitride compound layer was 20 μm.

And the test piece corresponding to each condition of Table 2 was created, and the fatigue limit was measured. The results of the test (fatigue limit) are plotted in FIG. 8 with the volume ratio (Vbγ ′ / (Vbε + Vbγ ′)) of the γ ′ phase in the lower quarter region of the iron nitride compound layer as the horizontal axis. Has been.

As shown in FIG. 8, when the volume ratio ratio (Vbγ ′ / (Vbε + Vbγ ′)) of the γ ′ phase in the lower quarter region of the iron nitride compound layer is 0.2 or more, It can be seen that the fatigue strength is significantly improved.

(Supplementary manufacturing method according to the present invention)
As described above, the first-stage process described as the method for manufacturing the nitrided steel member 100 is a process for adjusting the overall thickness of the iron nitride compound layer, and in general, to increase productivity. In this process, the total thickness of the iron nitride compound layer can be increased in as short a time as possible.

When the total thickness of the desired iron nitride compound layer is not so thick (in the case of 13 to 16 μm), as described with reference to Table 1, the first-stage process can be omitted. On the other hand, when the total thickness of the desired iron nitride compound layer is thick, it is preferable to perform nitriding with a high nitriding potential as the first stage treatment. The specific range is preferably 0.5 to 3.0, particularly 0.8 to 3.0. Also, the range of the nitriding temperature is preferably controlled to a high temperature range where the nitrided layer grows faster, specifically, 560 ° C. to 600 ° C.

Next, as described above, the second-stage treatment described as the method for manufacturing the nitrided steel member 100 is performed as described above. The volume ratio of the γ ′ phase in the entire region of the iron nitride compound layer (Vaγ ′ / (Vaε + Vaγ ′) )) Is a process for setting the value to 0.5 or more. In this process, it is necessary to perform nitriding with a low nitriding potential. The specific range is preferably 0.15 to 0.4. Further, the range of the nitriding temperature is preferably controlled to a high temperature region where the nitrided layer grows faster, specifically, 560 ° C. to 600 ° C., as in the first stage treatment.

If details of particularly preferable conditions are disclosed for the second stage treatment, the range of nitriding potential suitable for a nitriding temperature of 580 ° C. is 0.25 to 0.3, and the nitriding temperature is 560 ° C. The preferred nitriding potential range is 0.3 to 0.4.

Then, as described above, the third-stage process described as the method for manufacturing the nitrided steel member 100 is the volume ratio ratio (Vbγ ′ / (Vbε) of the γ ′ phase in the lower quarter region of the iron nitride compound layer. This is a process for setting the value of + Vbγ ′)) to 0.2 or more. This treatment needs to be carried out in a temperature range of 490 ° C. to 510 ° C. (see FIG. 3) in which the γ ′ phase can contain the most carbon (coexist). Further, even in the temperature range, the volume ratio of the γ ′ phase is lowered at the nitriding potential at which the ε phase is easily formed. For this reason, the range of the nitriding potential needs to be controlled to 0.5 to 2.0.

(Configuration of manufacturing equipment for nitrided steel members)
Here, the basic matters of the gas nitriding treatment will be described chemically. In the gas nitriding treatment, the following expression (1) is expressed in the processing furnace (gas nitriding furnace) in which the article to be processed is arranged. Nitriding reaction occurs.
NH ₃ → [N] + 3 / 2H ₂ (1)

At this time, the nitriding potential K _N is defined by the following equation (2).
K _N = P _NH3 / P _H2 ^3/2 (2)
Here, P _NH3 is the in-furnace ammonia partial pressure, and P _H2 is the in-furnace hydrogen partial pressure. The nitriding potential K _N is well known as an index representing the nitriding ability of the atmosphere in the gas nitriding furnace.

On the other hand, in the furnace during the gas nitriding treatment, a part of the ammonia gas introduced into the furnace is thermally decomposed into hydrogen gas and nitrogen gas according to the reaction of the formula (3).
NH ₃ → 1 / 2N ₂ + 3 / 2H ₂ (3)

In the furnace, the reaction of the formula (3) mainly occurs, and the nitriding reaction of the formula (1) is almost negligible in quantity. Therefore, if the in-furnace ammonia concentration consumed in the reaction of equation (3) or the hydrogen gas concentration generated in the reaction of equation (3) is known, the nitriding potential can be calculated. That is, since hydrogen and nitrogen generated are 1.5 mol and 0.5 mol, respectively, from 1 mol of ammonia, if the in-furnace ammonia concentration is measured, the in-furnace hydrogen concentration can also be known and the nitriding potential can be calculated. Can do. Alternatively, if the in-furnace hydrogen concentration is measured, the in-furnace ammonia concentration can be found, and the nitriding potential can also be calculated.

The ammonia gas that has flowed into the gas nitriding furnace is circulated through the furnace and then discharged outside the furnace. That is, in the gas nitriding treatment, fresh (new) ammonia gas is continuously flowed into the furnace with respect to the existing gas in the furnace, so that the existing gas is continuously discharged out of the furnace (extruded at the supply pressure). .

Here, if the flow rate of ammonia gas introduced into the furnace is small, the gas residence time in the furnace becomes long, so the amount of ammonia gas to be decomposed increases, and the nitrogen gas generated by the decomposition reaction + The amount of hydrogen gas increases. On the other hand, if the flow rate of ammonia gas introduced into the furnace is large, the amount of ammonia gas discharged outside the furnace without being decomposed increases, and the amount of nitrogen gas + hydrogen gas generated in the furnace decreases. To do.

Now, FIG. 9 is a schematic view showing a manufacturing apparatus for manufacturing a nitrided steel member according to an embodiment of the present invention. As shown in FIG. 9, the manufacturing apparatus 1 of the present embodiment includes a circulation type processing furnace 2, and uses only two types of ammonia and ammonia decomposition gas as gases introduced into the circulation type processing furnace 2. ing. The ammonia decomposition gas is a gas called AX gas, and is a mixed gas composed of nitrogen and hydrogen in a ratio of 1: 3. However, as the introduced gas, (1) only ammonia gas, (2) only two types of ammonia and ammonia decomposition gas, (3) only two types of ammonia and nitrogen gas, or (4) ammonia and ammonia decomposition gas Only three types of nitrogen gas can be selected.

An example of a cross-sectional structure of the circulation type processing furnace 2 is shown in FIG. In FIG. 10, a cylinder 202 called a retort is arranged in a furnace wall (also called a bell) 201, and a cylinder 204 called an internal retort is arranged inside thereof. As shown by the arrows in the figure, the introduced gas supplied from the gas introduction pipe 205 passes through the space between the two

cylinders

202 and 204 by the action of the stirring fan 203 after passing around the object to be processed. Circulate. 206 is a gas hood with a flare, 207 is a thermocouple, 208 is a lid for cooling work, and 209 is a fan for cooling work. The circulation type processing furnace 2 is also called a horizontal gas nitriding furnace, and its structure itself is known.

The article S to be processed is carbon steel or low alloy steel, for example, a crankshaft or gear that is an automobile part.

As shown in FIG. 9, the processing furnace 2 of the surface hardening processing apparatus 1 of the present embodiment includes a furnace opening / closing lid 7, a stirring fan 8, a stirring fan drive motor 9, and an atmospheric gas concentration detection device 3. A nitriding potential controller 4, a programmable logic controller 30, and an in-furnace gas supply unit 20 are provided.

The stirring fan 8 is disposed in the processing furnace 2 and rotates in the processing furnace 2 to stir the atmosphere in the processing furnace 2. The stirring fan drive motor 9 is connected to the stirring fan 8 and rotates the stirring fan 8 at an arbitrary rotation speed.

The atmospheric gas concentration detection device 3 includes a sensor that can detect the hydrogen concentration or ammonia concentration in the processing furnace 2 as the atmospheric gas concentration in the furnace. The detection main body of the sensor communicates with the inside of the processing furnace 2 via the atmospheric gas pipe 12. In the present embodiment, the atmospheric gas pipe 12 is formed by a path that directly communicates the sensor main body of the atmospheric gas concentration detection device 3 and the processing furnace 2, and the in-furnace gas disposal pipe that is connected to the exhaust gas combustion decomposition apparatus 41 on the way. 40 is connected. Thereby, the atmospheric gas is distributed into the gas to be discarded and the gas supplied to the atmospheric gas concentration detection device 3.

The atmospheric gas concentration detection device 3 outputs an information signal including the detected concentration to the nitriding potential controller 4 after detecting the atmospheric gas concentration in the furnace.

The nitriding potential controller 4 has an in-furnace nitriding potential calculation device 13 and a gas flow rate output adjusting device 30. The programmable logic controller 31 includes a gas introduction amount control device 14 and a parameter setting device 15.

The in-furnace nitriding potential calculation device 13 calculates the nitriding potential in the processing furnace 2 based on the hydrogen concentration or ammonia concentration detected by the in-furnace atmospheric gas concentration detection device 3. Specifically, a calculation formula for nitriding potential programmed according to the actual gas introduced into the furnace is incorporated, and the nitriding potential is calculated from the value of the atmospheric gas concentration in the furnace.

The parameter setting device 15 is composed of, for example, a touch panel and can set and input the total flow rate of the gas introduced into the furnace, the gas type, the processing temperature, the target nitriding potential, and the like. Each set parameter value that has been set and input is transmitted to the gas flow rate output adjusting means 30.

Then, the gas flow rate output adjusting means 30 uses the nitriding potential calculated by the in-furnace nitriding potential calculation device 13 as an output value, a target nitriding potential (set nitriding potential) as a target value, and the ammonia gas and the ammonia decomposition gas. Control is performed with each introduction amount as an input value. More specifically, the first control for changing the introduction ratio of ammonia gas with the total flow rate of the introduction amount of ammonia gas and the introduction amount of ammonia decomposition gas being constant, and ammonia gas in a state where introduction of ammonia decomposition gas is stopped The second control for changing the introduction amount of can be selectively performed. The output value of the gas flow rate output adjusting means 30 is transmitted to the gas introduction amount control means 14.

The gas introduction amount control means 14 sends control signals to the first supply amount control device 22 for ammonia gas and the second supply amount control device 26 for ammonia decomposition gas in order to realize the introduction amount of each gas. It has become.

The in-furnace introduction gas supply unit 20 of the present embodiment includes a first in-furnace introduction gas supply unit 21 for ammonia gas, a first supply amount control device 22, a first supply valve 23, a first flow meter 24, ,have. The in-furnace introduced gas supply unit 20 of the present embodiment includes a second in-furnace introduced gas supply unit 25 for ammonia decomposition gas (AX gas), a second supply amount control device 26, a second supply valve 27, And a second flow meter 28.

In this embodiment, the ammonia gas and the ammonia decomposition gas are mixed in the furnace introduction gas introduction pipe 29 before entering the processing furnace 2.

The first in-furnace introduction gas supply unit 21 is formed by, for example, a tank filled with a first in-furnace introduction gas (ammonia gas in this example).

The first supply amount control device 22 is formed by a mass flow controller, and is interposed between the first furnace introduction gas supply unit 21 and the first supply valve 23. The opening degree of the first supply amount control device 22 changes according to a control signal output from the gas introduction amount control means 14. The first supply amount control device 22 detects the supply amount from the first furnace introduction gas supply unit 21 to the first supply valve 23, and sends an information signal including the detected supply amount to the gas introduction control means 14. It is designed to output. The control signal can be used for correction of control by the gas introduction amount control means 14.

The first supply valve 23 is formed by an electromagnetic valve that switches between open and closed states according to a control signal output from the gas introduction amount control means 14, and is interposed between the first supply amount control device 22 and the first flow meter 24. It is intervened.

The second in-furnace introduction gas supply unit 25 is formed by, for example, a tank filled with the second in-furnace introduction gas (in this example, ammonia decomposition gas).

The second supply amount control device 26 is formed by a mass flow controller, and is interposed between the second furnace introduction gas supply unit 25 and the first supply valve 27. The opening degree of the first supply amount control device 26 changes according to the control signal output from the gas introduction amount control means 14. The third supply amount control device 26 detects the supply amount from the second in-furnace introduction gas supply unit 25 to the second supply valve 27 and sends an information signal including the detected supply amount to the gas introduction control means 14. It is designed to output. The control signal can be used for correction of control by the gas introduction amount control means 14.

The second supply valve 27 is formed by an electromagnetic valve that switches between open and closed states according to a control signal output from the gas introduction amount control means 14, and is interposed between the second supply amount control device 26 and the second flow meter 28. It is intervened.

(Operation of Manufacturing Equipment for Nitride Steel Members (Manufacturing Method))
Next, the operation of the manufacturing apparatus 1 according to this embodiment will be described. First, the article S to be processed is put into the circulation type processing furnace 2, and the circulation type processing furnace 2 is heated to a desired processing temperature. Thereafter, a mixed gas of ammonia gas and ammonia decomposition gas or only ammonia gas is introduced into the processing furnace 2 at a set initial flow rate from the in-furnace introduced gas supply unit 20. This setting initial flow rate can also be set and input in the parameter setting device 15, and is controlled by the first supply amount control device 22 and the second supply amount control device 26 (both are mass flow controllers). Further, the stirring fan drive motor 9 is driven and the stirring fan 8 rotates to stir the atmosphere in the processing furnace 2.

The in-furnace nitriding potential calculation device 13 of the nitriding potential controller 4 calculates the in-furnace nitriding potential (because initially there is a very high value (since there is no hydrogen in the furnace), the decomposition of ammonia gas (hydrogen generation). It is determined whether or not the sum of the target nitriding potential and the reference deviation value is below. This reference deviation value can also be set and input in the parameter setting device 15.

When it is determined that the calculated value of the in-furnace nitriding potential is less than the sum of the target nitriding potential and the reference deviation value, the nitriding potential controller 4 introduces the introduction amount of the in-furnace introduction gas via the gas introduction amount control means 14. Start controlling.

The in-furnace nitriding potential calculation device 13 of the nitriding potential controller 4 calculates the in-furnace nitriding potential based on the input hydrogen concentration signal or ammonia concentration signal. The gas flow rate output adjusting means 30 uses the nitriding potential calculated by the in-furnace nitriding potential calculation device 13 as an output value, sets the target nitriding potential (set nitriding potential) as a target value, and introduces the amount of introduced gas into the furnace. PID control is carried out using as an input value. Specifically, in the PID control, the first control for changing the introduction ratio with the total flow rate of the introduction amount of ammonia gas and the introduction amount of ammonia decomposition gas being constant and the introduction of ammonia decomposition gas were stopped. And second control for changing the introduction amount of ammonia gas in the state is selectively performed. In the PID control, each setting parameter value set and input by the parameter setting device 15 is used. For this setting parameter value, for example, different values are prepared according to the value of the target nitriding potential.

And the gas flow rate output adjusting means 30 controls the amount of each introduced gas in the furnace as a result of the PID control. Specifically, the gas flow rate output adjusting unit 30 determines the flow rate of each gas, and the output value is transmitted to the gas introduction amount control unit 14.

The gas introduction amount control means 14 sends control signals to the first supply amount control device 22 for ammonia gas and the second supply amount control device 26 for ammonia decomposition gas in order to realize the introduction amount of each gas.

With the above control, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. As a result, the nitriding treatment can be performed with extremely high quality without forming an ε phase or an oxide film that inhibits decarburization on the surface after the nitriding treatment of the workpiece S.

(Selection between first control and second control)
An example in which the first control is employed is shown in FIGS. 11A and 11B. In the example of FIGS. 11A and 11B, the total flow rate of the introduction amount of ammonia gas and the introduction amount of ammonia decomposition gas is constant at 166 (l / min), and the nitriding potential is highly accurate to 0.16. It is controlled.

An example in which the second control is employed is shown in FIGS. 12A and 12B. In the example of FIGS. 12A and 12B, the introduction of ammonia decomposition gas is stopped, and only the introduction amount of ammonia gas is feedback-controlled in the vicinity of 220 (l / min), so that the nitriding potential becomes 0.16. It is controlled with high accuracy.

From the viewpoint of control stability and process safety, the first control is preferably performed. However, when the amount of the processed product S inserted into the furnace is large (for example, when the surface area of the processed product S exceeds 7 m ² ), many decomposition reactions occur in the formula (3). Is difficult to control with high precision. In such a case, it is preferable to shift to the second control and perform nitriding potential control.

(Importance of guide tube (internal retort))
According to the experiments of the present inventors, when the guide tube 5 (internal retort) is removed from the manufacturing apparatus 1 and nitriding is performed (comparative example), an ε phase or an α phase is formed on the surface of the workpiece S. It was confirmed that it would be. (In the comparative example, in addition to removing the guide cylinder 4, the positions of the stirring fan 9 and the gas introduction pipe 29 were also moved to the center of the ceiling in the furnace.)

Specifically, in the case of using the manufacturing apparatus 1 and in the case of the comparative example, (1) the treatment temperature: 580 ° C., the nitriding potential: 0.2, the treatment time: 1.5 hours, Thereafter, (2) nitriding treatment was performed at 580 ° C., nitriding potential: 1.5, processing time: 1.5 hours, and (2) processing temperature: 580 ° C., nitriding potential: 0.3, processing time. Nitriding treatment was performed for 20 minutes, and finally (4) nitriding treatment was performed at a treatment temperature of 500 ° C., a nitriding potential of 0.7, and a treatment time of 2 hours (total of 4 stages). As an article to be processed S, using the jig shown in FIG. 13, as a steel material, respectively, in the center of A side (furnace lid side), B side (center in the furnace), C side (depth side in the furnace), A coin-shaped specimen of S45C steel and φ20 × 5 mm was used.

When the X-ray structure analysis of the surface of each test piece after nitriding was performed, as shown in Table 3 below, in the case of the example, the uniform compound layer thickness and the γ ′ phase in any surface was gotten.

On the other hand, in the case of the comparative example, the ε phase was observed on the A surface installed on the furnace lid side, whereas the C surface installed in the depth direction had two phases of α phase and γ ′ phase. Moreover, the tendency for the compound layer thickness to become thin was recognized as it went to the depth direction. This is considered to be because the uniformity of the nitriding potential in the furnace is not good.

(Further examples and comparative examples using S45C as a base material)
Test pieces having the form shown in FIG. 6 were prepared from S45C steel based on the conditions shown in Table 4 (the compound layer thickness is 23 μm in common), and the rotary bending fatigue strength test was performed. Specifically, using an Ono type rotating bending fatigue tester (Shimadzu Corporation, H7 type), it was determined whether or not 10 ⁷ rotations could be reached with a test load of 47 kgf and a rotation speed of 3600 rpm.

As a result, only the test piece that satisfies both Vaγ ′ / (Vaε + Vγγ)> 0.5 and Vbγ ′ / (Vbε + Vbγ ′)> 0.2 (the uppermost condition in Table 4) is 10 We were able to reach ⁷ turns.

In Comparative Example 1 and Comparative Example 2, the treatment temperature of the third stage nitriding treatment did not satisfy the conditions of the manufacturing method according to the present invention, and therefore Vbγ ′ / (Vbε + Vbγ ′)> 0.2 could be satisfied. As a result, the fatigue strength equivalent to that of the example could not be realized. In Comparative Example 3, since the second-stage nitriding treatment was skipped, the γ ′ phase ratio in the entire compound layer was low, and Vaγ ′ / (Vaε + Vaγ ′)> 0.5 could not be satisfied. The fatigue strength equivalent to that of the example could not be realized.

(Further examples and comparative examples using SCM435 as a base material)
In addition to carbon steel, the present invention is also applicable to low alloy steel having a carbon content of 0.1% or more by mass%. For example, SCr440, SCM435, etc. can also be used as a parent phase.

Test pieces having the form shown in FIG. 6 were prepared from the SCM435 steel based on the conditions shown in Table 5 (the compound layer had a common thickness of 18 μm), and the rotary bending fatigue strength test was performed. Specifically, Ono-type rotating bending fatigue tester (Shimadzu Corporation, H7 type) using a test load 55 kgf, the rotational speed as 3600 rpm, it is determined whether it is possible to welcome 10 ⁷ rotation.

As a result, Vaγ ′ / (Vaε + Vaγ ′)> 0.5 and Vbγ ′ / (Vbε + Vbγ ′)> 0 only when the third processing step is performed (the upper condition of Table 5). Satisfying both 2 and 10 ⁷ revolutions.

DESCRIPTION OF SYMBOLS 1 Manufacturing apparatus of nitrided steel member 2 Circulating type processing furnace 3 Atmospheric gas concentration detection apparatus 4 Nitriding potential controller 5 Internal retort 6 Retort 7 Furnace opening / closing lid 8 Stirring fan 9 Stirring fan drive motor 12 Atmospheric gas piping 13 Nitriding potential calculation in furnace Device 14 Gas introduction amount control device 15 Parameter setting device (touch panel)
20 In-furnace gas supply unit 21 First in-furnace gas supply unit 22 First in-furnace gas supply controller 23 First supply valve 25 Second in-furnace gas supply unit 26 Second in-furnace gas supply controller 27 Second Supply valve 29 Furnace introduction gas introduction pipe 30 Gas flow rate output adjustment device 31 Programmable logic controller 40 Furnace gas disposal pipe 41 Exhaust gas combustion decomposition apparatus 100 Steel nitride member 101 of one embodiment Iron nitride compound layer 102 Diffusion layer 120 Comparative example Nitride steel member 121 Iron nitride compound layer 122 Diffusion layer 201 Furnace wall or bell 202 Retort 203 Stirring fan 204 Guide tube (internal retort)
205 Gas introduction pipe 206 Gas exhaust or gas hood with flare 207 Thermocouple 208 Cooling lid 209 Cooling fan

Claims

A nitrided steel member having a carbon steel or low alloy steel having a carbon content of 0.10% by mass or more as a parent phase, and an iron nitride compound layer formed on the surface,
The iron nitride compound layer has a thickness of 13 μm or more,
When the volume proportions of the γ ′ phase and the ε phase in the entire region of the iron nitride compound layer are Vaγ ′ and Vaε, respectively, the value of Vaγ ′ / (Vaε + Vaγ ′) is 0.5 or more,
When the volume ratios of the γ ′ phase and the ε phase in the lower 1/4 region of the iron nitride compound layer are Vbγ ′ and Vbε, respectively, the value of Vbγ ′ / (Vbε + Vbγ ′) is 0.2. A nitrided steel member characterized by the above.
The nitrided steel member according to claim 1, wherein the iron nitride compound layer has a thickness of 20 袖 m to 35 袖 m or more.
3. The nitrided steel member according to claim 1, wherein the value of Vbγ ′ / (Vbε + Vbγ ′) is 0.3 or more.
A method of manufacturing a nitrided steel member having a carbon steel or low alloy steel having a carbon content of 0.10% by mass or more as a parent phase using a circulation type processing furnace having a guide cylinder and a stirring fan. And
Has at least two stages of nitriding treatment;
In the first stage treatment, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C., and the nitriding potential in the circulation type processing furnace is in the range of 0.15 to 0.4. Controlled,
In the second stage treatment, the temperature in the circulation type processing furnace is controlled in the range of 490 ° C. to 510 ° C., and the nitriding potential in the circulation type processing furnace is in the range of 0.5 to 2.0. A method for producing a nitrided steel member, characterized by being controlled.
A method of manufacturing a nitrided steel member having a carbon steel or low alloy steel having a carbon content of 0.10% by mass or more as a parent phase using a circulation type processing furnace having a guide cylinder and a stirring fan. And
Has at least three stages of nitriding,
In the first stage treatment, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C., and the nitriding potential in the circulation type processing furnace is in the range of 0.7 to 3.0. Controlled,
In the second stage process, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C., and the nitriding potential in the circulation type processing furnace is in the range of 0.15 to 0.4. Controlled,
In the third stage of processing, the temperature in the circulation type processing furnace is controlled in the range of 490 ° C. to 510 ° C., and the nitriding potential in the circulation type processing furnace is in the range of 0.5 to 2.0. A method for producing a nitrided steel member, characterized by being controlled.
A circulation type processing furnace having a guide cylinder and a stirring fan;
In the first stage treatment, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C., and the nitriding potential in the circulation type processing furnace is in the range of 0.15 to 0.4. Controlled,
In the second stage treatment, the temperature in the circulation type processing furnace is controlled in the range of 490 ° C. to 510 ° C., and the nitriding potential in the circulation type processing furnace is in the range of 0.5 to 2.0. An apparatus for producing a nitrided steel member, characterized by being controlled.
A circulation type processing furnace having a guide cylinder and a stirring fan;
In the first stage treatment, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C., and the nitriding potential in the circulation type processing furnace is in the range of 0.7 to 3.0. Controlled,
In the second stage process, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C., and the nitriding potential in the circulation type processing furnace is in the range of 0.15 to 0.4. Controlled,
In the third stage of processing, the temperature in the circulation type processing furnace is controlled in the range of 490 ° C. to 510 ° C., and the nitriding potential in the circulation type processing furnace is in the range of 0.5 to 2.0. An apparatus for producing a nitrided steel member, characterized by being controlled.
Ammonia gas and ammonia decomposition gas are introduced into the circulation processing furnace,
In order to control the nitriding potential, the manufacturing apparatus
A first control for changing a mutual introduction ratio while keeping a total flow rate of the introduction amount of the ammonia gas and the introduction amount of the ammonia decomposition gas constant;
A second control for changing the introduction amount of the ammonia gas in a state where the introduction of the ammonia decomposition gas is stopped;
The apparatus for producing a nitrided steel member according to any one of claims 6 and 7, characterized in that can be selectively implemented.