JP7094540B2 - Nitride steel member and manufacturing method and manufacturing equipment for nitrided steel member - Google Patents

Description

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

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

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

In order to control the thickness (depth) of each of these two layers and / or the type of iron nitride on the surface, in addition to the temperature and time of the gas nitriding treatment, the atmosphere in the gas nitriding treatment 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 (type of iron nitride) of the γ'phase (Fe ₄ N) and the ε phase (Fe _2-3 N) in the compound layer generated on the surface of the steel material is controlled through the control. It is proposed to do. Specifically, it is known that fatigue resistance is improved by forming a γ'phase rather than an ε phase (Non-Patent Document 1), and bending fatigue strength and surface fatigue are improved by forming a γ'phase. A nitrided steel member having improved the above has been proposed (Patent Document 1). Further, it is also known that the bending fatigue strength is improved as the thickness of the γ'phase in the compound layer is increased (Non-Patent Document 2). However, even if gas nitriding is performed to form a large number of γ'phases, the compound layer contains not a little ε phase, and in reality, it becomes a two-phase state of γ'phase and ε phase. There is.

Here, when forming a relatively thick compound layer, it is easy to form an ε phase in the region of the compound layer on the inner side in contact with the diffusion layer, and it is difficult to form a γ'phase in the region. It was recognized (Patent Document 2). The ε phase is relatively brittle and the growth rate of fatigue cracks is fast. Therefore, if the film is thickened, the fatigue strength may deteriorate. The reason why the ε phase is likely to be formed in the region is that the carbon in the matrix moves to the surface side due to the surface decarburization reaction during the nitride treatment, but the rate of carbon diffusion in the compound layer is slower than that in the matrix. This is due to the concentration of carbon at the interface between the diffusion layer / compound layer (Non-Patent Document 3).

Further, it is known that when a relatively thick compound layer is formed, the nitrogen concentration in the surface layer region of the compound layer increases, so that the proportion of the ε phase increases in the surface layer region. Specifically, when the thickness of the compound layer exceeds 17 μm, the proportion of the ε phase increases (Patent Document 3).

Japanese Unexamined Patent Publication No. 2013-221203 Japanese Unexamined Patent Publication No. 2016-211069 Japanese Unexamined Patent Publication No. 2017-36509 Yasushi Hiraoka, Yoichi Watanabe, Akitake Ishida: Heat Treatment, Vol. 55, No. 1, pp. 1-2 Y.Hiraoka, A.Ishida: Materials Transactions, Vol. 58, 2017, pp. 993-999 Dataly Toke et al .: Nitriding and Soft Nitriding of Iron, Agne Technology Center, 2013, pp. 37-49

As described above, it is possible to improve the fatigue strength of the nitrided steel member by controlling so that the γ'phase increases in the compound layer, and further improve the fatigue strength by thickening the γ'phase. It is possible. However, in order to form a relatively thick compound layer in order to thicken the γ'phase, the formation of the ε phase in the region near the interface between the diffusion layer / the compound layer 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 studies and various experiments, and controlled the temperature and nitriding potential of the nitriding process with high accuracy while limiting the configuration of the processing furnace, thereby forming a relatively thick γ'phase-based compound layer. It was found that the desired amount of γ'phase can be maintained even in the region near the interface of the diffusion layer / compound layer, and the increase of ε phase can be suppressed in the surface layer region of the compound layer.

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

The present invention is a nitrided steel member having a carbon steel or a low alloy steel having a carbon content of 0.10% or more in mass% as a matrix and an iron nitride compound layer formed on the surface thereof. The thickness of the material compound layer is 13 μm or more, and 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 Vaε, respectively, Vaγ'/ (Vaε + Vaγ' ) Is 0.5 or more, and when the volume ratios of the γ'phase and the ε phase occupying the lower 1/4 region of the iron nitride compound layer are Vbγ'and Vbε, respectively, Vbγ'/ ( It is a nitrided steel member characterized in that the value of Vbε + Vbγ') is 0.2 or more and 0.4 or less .

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 invented by the inventor of the present invention. In such a nitrided steel member, since 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, the iron nitride compound layer is concerned. It can be considered that the entire layer is a compound layer mainly composed of the γ'phase, and the thickness thereof is 13 μm or more, so that the fatigue strength is remarkably improved. 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 0.2 or more and is maintained at 0.4 or less . , Deterioration of fatigue strength due to the presence of the ε phase in the region is remarkably suppressed.

The thickness of the iron nitride compound layer is more preferably 20 μm to 35 μm or more. When the thickness is 20 μm, the fatigue strength is further improved. Further, 35 μm is a suitable value in consideration of productivity. (The thickness of the iron nitride compound layer generally corresponds to the nitriding time. If the nitriding time is not limited, there is no upper limit to the thickness of the iron nitride compound layer.)

Further, the value of Vbγ'/ (Vbε + Vbγ') is more preferably 0.3 or more. In this case, the 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.

Further, in the present invention, a nitriding steel member having a carbon steel having a carbon content of 0.10% or more in mass% or a low alloy steel as a parent phase by using a circulation type processing furnace equipped with a guide cylinder and a stirring fan. In the first stage of the nitriding treatment, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C., and the nitriding treatment is performed in at least two stages. 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 treatment, the temperature in the circulation type processing furnace is controlled in the range of 490 ° C. to 510 ° C. Moreover, it is a method for manufacturing a nitrided steel member, 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 nitride steel member having a carbon steel or a low alloy steel having a carbon content of 0.10% or more in mass% as a matrix and an iron nitride compound layer formed on the surface of the iron nitride compound layer. The thickness is 13 μm or more, and 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, Vaγ'/ (Vaε + Vaγ') When the value is 0.5 or more and 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, Vbγ'/ (Vbε +). It is possible to manufacture a nitrided steel member characterized in that the value of Vbγ') is 0.2 or more.

Here, the second-stage treatment (treatment in the range of 490 ° C to 510 ° C) is continued using the same circulation type treatment furnace as the first-stage treatment (treatment in the range of 560 ° C to 600 ° C). It may be carried out using a circulation type treatment furnace different from that of the first stage treatment. Depending on the temperature condition setting (elevation) performance in the circulating processing furnace, the latter may have better production efficiency. The temperature of the material may be maintained at the temperature conditions in the first stage treatment while the material is transferred from the circulation type processing furnace for the first stage treatment to the circulation type processing furnace for the second stage treatment. However, it may be temporarily naturally cooled to about room temperature. In any case, the inventor of the present invention has confirmed (in the examples described later) that the method of the present invention is effective.

Alternatively, the present invention uses a circulating processing furnace equipped with a guide tube and a stirring fan, and a nitrided steel member whose parent phase is carbon steel or low alloy steel having a carbon content of 0.10% or more in mass%. In the first stage of the nitriding treatment, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C., and the nitriding treatment is performed in at least three stages. 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 treatment, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C. Moreover, 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 treatment, the temperature in the circulation type processing furnace is in the range of 490 ° C. to 510 ° C. It is a method for manufacturing a nitriding steel member, which is controlled in the above-mentioned manner and in which the nitriding potential in the circulation type processing furnace is controlled in the range of 0.5 to 2.0.

Depending on the manufacturing method of the nitrided steel member,
A nitride steel member having a carbon steel or a low alloy steel having a carbon content of 0.10% or more in mass% as a matrix and an iron nitride compound layer formed on the surface of the iron nitride compound layer. The thickness is 13 μm or more, and 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, Vaγ'/ (Vaε + Vaγ') When the value is 0.5 or more and 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, Vbγ'/ (Vbε +). It is possible to manufacture a nitrided steel member characterized in that the value of Vbγ') is 0.2 or more.

Here, the third-stage treatment (treatment in the range of 490 ° C to 510 ° C) is the same as the first-stage and second-stage treatment (treatment in the range of 560 ° C to 600 ° C). It may be carried out subsequently using the above, or it may be carried out using a circulation type treatment furnace different from the first and second stage treatments. Depending on the temperature condition setting (elevation) performance in the circulating processing furnace, the latter may have better production efficiency. During the transfer of the material from the circulation type processing furnace for the first and second stages of processing to the circulation type processing furnace for the third stage processing, the temperature of the material is kept in the first and second stages of processing. It may be maintained at a temperature condition, or it may be temporarily naturally cooled to about room temperature. In any case, the inventor of the present invention has confirmed (in the examples described later) that the method of the present invention is effective.

Further, the present invention includes a circulation type processing furnace having a guide cylinder and a stirring fan, and in the first stage processing, the temperature in the circulation type processing furnace is controlled in the range of 560 ° C. to 600 ° C. 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 treatment, the temperature in the circulation type processing furnace is controlled in the range of 490 ° C. to 510 ° C. Moreover, it is a nitriding steel member manufacturing apparatus characterized in that the 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 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 in the second stage treatment, 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 treatment, the temperature in the circulation type processing furnace is 490 ° C. to 510 ° C. It is an apparatus for manufacturing a nitrided steel member, which is controlled to a range and the nitriding potential in the circulation type processing furnace is controlled to a range of 0.5 to 2.0.

According to the equipment for manufacturing these nitrided steel members,
A nitride steel member having a carbon steel or a low alloy steel having a carbon content of 0.10% or more in mass% as a matrix and an iron nitride compound layer formed on the surface of the iron nitride compound layer. The thickness is 13 μm or more, and 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, Vaγ'/ (Vaε + Vaγ') When the value is 0.5 or more and 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, Vbγ'/ (Vbε +). It is possible to manufacture a nitrided steel member characterized in that the value of Vbγ') is 0.2 or more.

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, in order to control the nitriding potential, the manufacturing apparatus has a first control of changing the introduction ratio of each other while keeping the total flow rate of the introduction amount of the ammonia gas and the introduction amount of the ammonia decomposition gas constant. It is preferable that the second control for changing the introduction amount of the ammonia gas can be selectively performed while the introduction of the ammonia decomposition gas is stopped.

According to the nitrided steel member according to the present invention, since 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, the iron nitride is concerned. The entire physical compound layer can be considered to be a compound layer mainly composed of the γ'phase, and the thickness thereof is 13 μm or more, so that the fatigue strength is remarkably improved. 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 micrograph of a nitrided steel member according to one embodiment of the present 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 carbon content which a γ'phase can contain, and the temperature. It is a cross-sectional micrograph 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 the Ono type rotary 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 the fatigue limit and the volume ratio ratio of the γ'phase in the region of the lower 1/4. It is a schematic diagram 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 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 2nd control. It is a schematic diagram which shows the example of the jig inserted in the furnace.

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

(Structure of Nitride Steel Member 100 According to One Embodiment of the Present Invention)
FIG. 1 is a cross-sectional micrograph of a nitrided steel member 100 according to an embodiment of the present invention manufactured by the present inventor. As shown in FIG. 1, the nitrided steel member 100 of the present embodiment is provided with an iron nitride compound layer 101 as a hardened layer on the surface, and nitrogen is diffused into the matrix under the iron nitride compound layer 101. The diffusion layer 102 is provided. The matrix (base material) of the present 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 in FIG. 1 extends to a depth of about 1000 μm from the surface of the nitrided steel member 100.

As described above, the iron nitride compound layer 101 is a layer containing an ε phase (Fe _2-3 N) and a γ'phase (Fe ₄ N). The distribution state of these phases can be analyzed by the 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. (It is considered that the area ratio corresponds to the volume ratio.) For example, three cross sections (for three fields of view) having a width of 100 μm in the depth direction can be taken and determined from their average values.

FIG. 2 is an analysis result of the EBSD method in 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, respectively. The value of Vaγ'/ (Vaε + Vaγ')) when Vaγ'and Vaε are used 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). The value of Vbγ'/ (Vbε + Vbγ')) when is set to Vbγ'and Vbε, respectively, is larger than 0.2.

(Manufacturing method of nitrided steel member 100)
The iron nitride compound layer 101 of FIG. 1 is manufactured by a three-step nitriding treatment using a circulation type processing furnace equipped with a guide cylinder and a stirring fan (details will be described later) (Table 1 (compound layer described later). See (example) with a thickness of 16 μm).

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

In the second stage treatment (for example, 0.5 hours), 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 ratio of the γ'phase and the ε phase in the entire region of the iron nitride compound layer 101 is 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 type processing furnace is controlled to 500 ° C., and the nitriding potential in the circulation type 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')) when the volume ratio is Vbγ'and Vbε, respectively, is adjusted.

This third-stage treatment is a novel feature of the manufacturing method of the present invention. The present inventor refers to the temperature range in which the γ'phase can contain (coexist) the most carbon at 490 to 510 ° C. (see FIG. 3), and performs the renitriding treatment in the temperature range. As a result of carrying out the invention, it was found that the volume ratio of the γ'phase can be maintained at 0.2 or more even in the lower 1/4 region of the iron nitride compound layer, which conventionally had many ε phases. I did it. The nitriding potential in this process is 0.5 to 2.0 (described later).

(Structure of Nitride Steel Member 120 in Comparative Example)
On the other hand, FIG. 4 is a cross-sectional micrograph of the nitrided steel member 120 as a comparative example manufactured by a conventional manufacturing method. As shown in FIG. 4, the nitrided steel member 120 of the comparative example also has an iron nitride compound layer 121 as a hardened layer on the surface, and nitrogen is diffused in the matrix under the hardened layer 201. It comprises layer 122. The matrix (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 to a depth of about 1000 μm from the surface of the nitrided steel member 120.

FIG. 5 is an analysis result of the EBSD method in 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, respectively). The value of Vaγ'/ (Vaε + Vaγ')) when Vaγ'and Vε are used is 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). The value of Vbγ'/ (Vbε + Vbγ')) when is set to Vbγ'and Vbε, respectively, is smaller than 0.2.

(Manufacturing method of nitrided steel member 120)
The iron nitride compound layer 121 of FIG. 4 is manufactured by a two-step nitriding treatment using a circulation type treatment furnace (details will be described later) equipped with a guide tube and a stirring fan.

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

In the second stage treatment (for example, 0.5 hours), 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 ratio of the γ'phase and the ε phase in the entire region of the iron nitride compound layer 101 is Vaγ'and Vε, respectively. The value of Vaγ'/ (Vaε + Vaγ')) is adjusted to be 0.5 or more. (Patent Document 2 discloses that the volume ratio of the γ'phase (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. Therefore, 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 ratio is Vbγ'and Vbε, respectively, is not adjusted (it remains a small value).

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

The result of the test (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 according to the embodiment of FIG. 1 is remarkably improved.

(Verification of effect by test piece (2): Thickness of iron nitride compound layer)
If the value of the volume 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. Can be thought of as a compound layer of. Therefore, it is considered that the thicker the iron nitride compound layer, the higher the fatigue strength.

Among the methods for producing the iron nitride compound layer 101 described above, a modified example was created in which the treatment content of the first stage was changed to further increase the thickness of the iron nitride compound layer. Specifically, as shown in Table 1 below, the nitriding potential in the circulating 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.

Even 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.

Then, a test piece corresponding to each modification was prepared and the fatigue limit was measured. The result of the test (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 method for producing the iron nitride compound layer 101, the treatment content of the second stage was changed without performing the treatment of the first stage to reduce the thickness of the iron nitride compound layer. A modified example and a comparative example were created. Specifically, as shown in Table 1 below, the treatment in the first stage was stopped, the treatment time was extended to 3 hours in the treatment in the second stage, and the thickness of the iron nitride compound layer was set to 13 μm. (Variation example). Further, the first-stage treatment was stopped, the treatment time was set to 2 hours in the second-stage treatment, and the thickness of the iron nitride compound layer was set to 10 μm (comparative example). Further, the first-stage treatment was stopped, the nitriding potential was set to 0.25, the treatment time was set to 3 hours, and the thickness of the iron nitride compound layer was set to 6 μm in the second-stage treatment (comparative example). Further, the first-stage treatment was stopped, the nitriding potential was set to 0.2 in the second-stage treatment, the treatment time was set to 3 hours, and the thickness of the iron nitride compound layer was set to 2 μm (comparative example).

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 even in the modified example in which the thickness of the iron nitride compound layer is 13 μm, and iron. 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 prepared, and the fatigue limit was measured. The result of the test (fatigue limit) was 43.9 kgf. This result is also plotted in FIG.

On the other hand, even in the comparative example in which the thickness of the iron nitride compound layer is 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 is 0.5. The volume ratio of the γ'phase in the lower quarter region of the iron nitride compound layer was larger 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 in lower 1/4)
The volume of the γ'phase in the lower quarter region of the iron nitride compound layer, while maintaining the condition that the volume ratio of the γ'phase in the entire region of the iron nitride compound layer is greater than 0.5. In order to make the ratios different, the processing contents of the three stages 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.

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

As shown in FIG. 8, when the value of the volume ratio (Vbγ'/ (Vbε + Vbγ') of the γ'phase in the region of the lower 1/4 of the iron nitride compound layer is 0.2 or more, It can be seen that the fatigue strength is significantly improved.

(Supplement to the manufacturing method according to the present invention)
As described above, the first-stage treatment described as the method for manufacturing the nitride steel member 100 is a treatment for adjusting the overall thickness of the iron nitride compound layer, and generally, in order to increase productivity. This is a process that can increase the overall thickness of the iron nitride compound layer in the shortest possible time.

When the total thickness of the desired iron nitride compound layer is not so thick (in the case of 13 to 16 μm), the first-stage treatment can be omitted as described with reference to Table 1. On the other hand, when the overall thickness of the desired iron nitride compound layer is thick, it is preferable to carry out the nitriding treatment 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. Further, it is preferable that the range of the nitriding temperature is also controlled to a high temperature range in which the nitriding layer grows faster, specifically, 560 ° C to 600 ° C.

Next, in the second-stage treatment described as the method for manufacturing the nitrided steel member 100, 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 carry out the nitriding process 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 range in which the nitriding layer grows faster, specifically, 560 ° C. to 600 ° C., as in the first stage treatment.

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

Then, as described above, the third-stage treatment described as the method for manufacturing the nitrided steel member 100 is the volume ratio of the γ'phase in the region of the lower 1/4 of the iron nitride compound layer (Vbγ'/ (Vbε). This is a process for setting the value of + Vbγ')) to 0.2 or more. This treatment needs to be carried out in the temperature range of 490 ° C to 510 ° C (see FIG. 3) where the γ'phase can contain (coexist) the most carbon. Further, even in the temperature range, the volume ratio of the γ'phase decreases at the nitriding potential where the ε phase is likely to be formed. Therefore, the range of the nitriding potential also needs to be controlled to 0.5 to 2.0.

(Structure of manufacturing equipment for nitrided steel members)
Here, if the basic matters of the gas nitriding treatment are chemically explained, in the gas nitriding treatment, it is represented by the following formula (1) in the processing furnace (gas nitriding furnace) in which the object to be treated is arranged. A 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 _{NH 3} is the partial pressure of ammonia in the furnace, and P _{H 2} is the partial pressure of hydrogen in the furnace. The nitriding potential K _N is well known as an index showing 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) can be almost ignored quantitatively. Therefore, if the concentration of ammonia in the furnace consumed in the reaction of the formula (3) or the concentration of the hydrogen gas generated in the reaction of the formula (3) is known, the nitriding potential can be calculated. That is, since the generated hydrogen and nitrogen are 1.5 mol and 0.5 mol, respectively, from 1 mol of ammonia, the hydrogen concentration in the furnace can be known by measuring the ammonia concentration in the furnace, and the nitriding potential can be calculated. Can be done. Alternatively, if the hydrogen concentration in the furnace is measured, the ammonia concentration in the furnace can be known, and the nitriding potential can also be calculated.

The ammonia gas flowing in the gas nitriding furnace is circulated in the furnace and then discharged to the outside of the furnace. That is, in the gas nitriding process, fresh (new) ammonia gas is continuously inflowed into the furnace with respect to the existing gas in the furnace, so that the existing gas is continuously discharged to the outside of the furnace (extruded by the supply pressure). ..

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

By the way, 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 gases, ammonia and ammonia decomposition gas, as the gas to be introduced into the circulation type processing furnace 2. ing. The ammonia decomposition gas is a gas also called AX gas, which 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 the 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 the furnace wall (also called a bell) 201, and a cylinder 204 called an internal retort is further arranged inside the cylinder 202. As shown by the arrow in the figure, the introduced gas supplied from the gas introduction pipe 205 passes around the object to be treated and then passes through the space between the two cylinders 202 and 204 by the action of the stirring fan 203. And circulate. 206 is a gas hood with 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 product S to be treated is carbon steel or low alloy steel, and is, for example, a crankshaft, a gear, or the like which is an automobile part.

Further, 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 atmosphere gas concentration detecting device 3. , A nitride potential adjuster 4, a programmable logic controller 30, and an in-core introduction gas supply unit 20 are provided.

The stirring fan 8 is arranged 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 so as to rotate the stirring fan 8 at an arbitrary rotation speed.

The atmosphere gas concentration detecting device 3 is composed of a sensor that can detect the hydrogen concentration or the ammonia concentration in the processing furnace 2 as the atmosphere 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 atmosphere gas pipe 12 is formed by a path that directly connects the sensor main body of the atmosphere gas concentration detection device 3 and the processing furnace 2, and is connected to the exhaust gas combustion decomposition device 41 on the way. 40 is connected. As a result, the atmospheric gas is distributed into the discarded gas and the gas supplied to the atmospheric gas concentration detecting device 3.

Further, the atmosphere gas concentration detection device 3 is adapted to output an information signal including the detected concentration to the nitride potential regulator 4 after detecting the atmosphere gas concentration in the furnace.

The nitriding potential regulator 4 includes an in-core nitriding potential arithmetic unit 13 and a gas flow rate output adjusting device 30. Further, the programmable logic controller 31 has a gas introduction amount control device 14 and a parameter setting device 15.

The in-core nitriding potential calculation device 13 calculates the nitriding potential in the processing furnace 2 based on the hydrogen concentration or the ammonia concentration detected by the in-core atmosphere gas concentration detecting device 3. Specifically, a formula for calculating the 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 setting parameter value input for setting is transmitted to the gas flow rate output adjusting means 30.

Then, the gas flow rate output adjusting means 30 sets the nitriding potential calculated by the in-core nitriding potential calculation device 13 as the output value, sets the target nitriding potential (set nitriding potential) as the target value, and sets the ammonia gas and the ammonia decomposition gas. Control is carried out with each introduction amount as an input value. More specifically, the first control of changing the introduction ratio of each other while keeping the total flow rate of the introduction amount of ammonia gas and the introduction amount of ammonia decomposition gas constant, and the ammonia gas with the introduction of ammonia decomposition gas stopped. The second control for changing the introduction amount of the gas can be selectively implemented. The output value of the gas flow rate output adjusting means 30 is transmitted to the gas introduction amount controlling means 14.

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

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

In the present 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 furnace introduction gas supply unit 21 is formed of, for example, a tank filled with the first 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 the control signal output from the gas introduction amount control means 14. Further, 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 a solenoid valve that switches an open / closed state according to a control signal output by the gas introduction amount control means 14, and is formed between the first supply amount control device 22 and the first flow meter 24. Being intervened.

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

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. Further, the third supply amount control device 26 detects the supply amount from the second 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 a solenoid valve that switches an open / closed state according to a control signal output by the gas introduction amount control means 14, and is formed between the second supply amount control device 26 and the second flow meter 28. Being intervened.

(Operation of nitriding steel member manufacturing equipment (manufacturing method))
Next, the operation of the manufacturing apparatus 1 of the present embodiment will be described. First, the product 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. After that, only the mixed gas of ammonia gas and the ammonia decomposition gas or only the ammonia gas is introduced into the processing furnace 2 from the furnace introduction gas supply unit 20 at the set initial flow rate. This set 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 to rotate the stirring fan 8 to stir the atmosphere in the processing furnace 2.

The in-core nitriding potential calculation device 13 of the nitriding potential regulator 4 calculates the nitriding potential in the furnace (at first, the value is extremely high (because there is no hydrogen in the furnace), but the decomposition of ammonia gas (hydrogen generation)). Decreases as it progresses), and it is determined whether or not the sum of the target nitriding potential and the reference deviation value has been exceeded. 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 nitriding potential in the furnace is less than the sum of the target nitriding potential and the reference deviation value, the nitriding potential regulator 4 determines the amount of gas introduced into the furnace via the gas introduction amount control means 14. Starts control of.

The in-core nitriding potential calculation device 13 of the nitriding potential regulator 4 calculates the in-core nitriding potential based on the input hydrogen concentration signal or ammonia concentration signal. Then, the gas flow rate output adjusting means 30 sets the nitriding potential calculated by the in-core nitriding potential calculation device 13 as the output value, sets the target nitriding potential (set nitriding potential) as the target value, and introduces the gas into the furnace. Is performed as an input value for PID control. Specifically, in the PID control, the first control of changing the introduction ratio of each other while keeping the total flow rate of the introduction amount of ammonia gas and the introduction amount of ammonia decomposition gas constant, and the introduction of ammonia decomposition gas were stopped. A second control that changes the amount of ammonia gas introduced in the state is selectively implemented. In the PID control, each setting parameter value set and input by the parameter setting device 15 is used. As the setting parameter value, for example, different values are prepared depending on the value of the target nitriding potential.

Then, 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 means 30 determines the flow rate of each gas, and the output value is transmitted to the gas introduction amount control means 14.

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

With the above control, the nitriding potential in the furnace 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 of the product S to be treated after the nitriding treatment.

(About the selection between the first control and the second control)
An example in which the first control is adopted is shown in FIGS. 11 (a) and 11 (b). In the examples of FIGS. 11A and 11B, the total flow rate of the amount of ammonia gas introduced and the amount of ammonia decomposition gas introduced is constant at 166 (l / min), and the nitriding potential is 0. It is controlled with high precision to .16.

An example in which the second control is adopted is shown in FIGS. 12 (a) and 12 (b). In the examples of FIGS. 12 (a) and 12 (b), the introduction of the ammonia decomposition gas is stopped, and only the amount of the introduced ammonia gas is feedback-controlled in small steps in the vicinity of 220 (l / min), whereby nitriding is performed. The potential is controlled to 0.16 with high accuracy.

From the viewpoint of control stability and processing safety, it is preferable that the first control is carried out. 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 ), the decomposition reaction of the formula (3) occurs frequently, so that the nitriding potential in the first control Is difficult to control with high precision. In such a case, it is preferable to shift to the second control and perform the nitriding potential control.

(About the importance of the guide tube (internal retort))
According to the experiment of the present inventor, when the guide tube 5 (internal retort) is removed from the manufacturing apparatus 1 and the nitriding treatment is performed (comparative example), the ε phase and the α phase are formed on the surface of the processed product S. It was confirmed that it would be done. (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 the case of the comparative example, (1) the treatment was carried out at (1) treatment temperature: 580 ° C., nitriding potential: 0.2, and treatment time: 1.5 hours. After that, (2) nitriding treatment was carried out at (2) treatment temperature: 580 ° C., nitriding potential: 1.5, treatment time: 1.5 hours, and (2) treatment temperature: 580 ° C., nitriding potential: 0.3, treatment time. : 20 minutes of nitriding treatment was carried out, and finally (4) treatment temperature: 500 ° C., nitriding potential: 0.7, treatment time: 2 hours of nitriding treatment was carried out (4 steps in total). As the product S to be treated, the jig shown in FIG. 13 is used as a steel material at the center of the A side (furnace lid side), the B side (center of the furnace), and the C side (depth side of the furnace), respectively. A coin-shaped test piece of S45C steel having a diameter of 20 × 5 mm was used.

An X-ray structure analysis of the surface of each test piece after the nitriding treatment revealed that, as shown in Table 3 below, in the case of the examples, the compound layer thickness was uniform on all surfaces and the γ'phase. was gotten.

On the other hand, in the case of the comparative example, the A surface installed on the furnace lid side had an ε phase, while the C surface installed in the depth direction had two phases, an α phase and a γ'phase. In addition, the thickness of the compound layer tended to become thinner toward the depth. It is considered that this is because the uniformity in the furnace of the nitriding potential 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 thickness of the compound layer was 23 μm in common), and the rotational bending fatigue strength was tested. Specifically, using an Ono-type rotary bending fatigue tester (Shimadzu Corporation, H7 type), it was determined whether or not it was possible to reach ¹⁰⁷ rotations with a test load of 47 kgf and a rotation speed of 3600 rpm.

As a result, only the test piece (the condition at the top of Table 4) satisfying both Vaγ'/ (Vaε + Vaγ')> 0.5 and Vbγ'/ (Vbε + Vbγ')> 0.2 is 10 We were able to reach the ^7th turn.

In Comparative Example 1 and Comparative Example 2, since the treatment temperature of the third-stage nitriding treatment did not satisfy the conditions of the production method according to the present invention, Vbγ'/ (Vbε + Vbγ')> 0.2 could be satisfied. As a result, it was not possible to achieve the same fatigue strength as in the examples. 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, resulting in that. It was not possible to achieve the same fatigue strength as in the examples.

(Further Examples and Comparative Examples Using SCM435 as a Base Material)
The present invention is applicable not only to carbon steel but also to low alloy steel having a carbon content of 0.1% or more in mass%. For example, SCr440, SCM435 and the like can also be used as the parent phase.

Test pieces having the form shown in FIG. 6 were prepared from SCM435 steel based on the conditions shown in Table 5 (the thickness of the compound layer was 18 μm in common), and the rotational bending fatigue strength was tested. Specifically, using an Ono-type rotary bending fatigue tester (Shimadzu Corporation, H7 type), it was determined whether or not it was possible to reach ¹⁰⁷ rotations with a test load of 55 kgf and a rotation speed of 3600 rpm.

As a result, Vaγ'/ (Vaε + Vaγ')> 0.5 and Vbγ'/ (Vbε + Vbγ')> 0. Both of 2 and were satisfied, and it was possible to reach ¹⁰⁷ rotations.

1 Manufacturing equipment for steel nitride members 2 Circulation type processing furnace 3 Atmospheric gas concentration detector 4 Nitrating potential regulator 5 Internal retort 6 Retort 7 Furnace opening / closing lid 8 Stirring fan 9 Stirring fan drive motor 12 Atmospheric gas piping 13 In-furnace nitride potential calculation Device 14 Gas introduction amount control device 15 Parameter setting device (touch panel)
20 In-core gas supply unit 21 1st in-core gas supply unit 22 1st in-core gas supply control device 23 1st supply valve 25 2nd in-core introduction gas supply unit 26 2nd in-core gas supply control device 27 2nd Supply valve 29 In-furnace introduction gas introduction pipe 30 Gas flow rate output regulator 31 Programmable logic controller 40 In-furnace gas waste pipe 41 Exhaust gas combustion decomposition device 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 inlet pipe 206 Flared gas exhaust or gas hood 207 Thermocouple 208 Cooling work lid 209 Cooling work blower

Claims (5)

A nitride steel member having a carbon steel or a low alloy steel having a carbon content of 0.10% or more in mass% as a matrix and an iron nitride compound layer formed on the surface thereof.
The thickness of the iron nitride compound layer is 13 μm or more, and the thickness is 13 μm or more.
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 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 or more. There is less than 0.4
A nitrided steel member characterized by this. The nitrided steel member according to claim 1, wherein the iron nitride compound layer has a thickness of 20 μm to 35 μm or more. The nitrided steel member according to claim 1 or 2, wherein the value of Vbγ'/ (Vbε + Vbγ') is 0.3 or more. It is a method of manufacturing a nitrided steel member whose parent phase is carbon steel or low alloy steel having a carbon content of 0.10% or more in mass% by using a circulation type processing furnace equipped with a guide cylinder and a stirring fan. hand,
It has at least two stages of nitriding and
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 manufacturing a nitrided steel member, which is characterized by being controlled. It is a method of manufacturing a nitrided steel member whose parent phase is carbon steel or low alloy steel having a carbon content of 0.10% or more in mass% by using a circulation type processing furnace equipped with a guide cylinder and a stirring fan. hand,
It has at least three stages of nitriding and
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 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 third 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 manufacturing a nitrided steel member, which is characterized by being controlled.

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