JP6755106B2 - Nitriding steel member and manufacturing method of nitrided steel member - Google Patents

Description

The present invention relates to a nitrided steel member and a method for manufacturing a nitrided steel member. More specifically, the present invention has wear resistance obtained by nitriding the surface by gas nitriding treatment, which is useful for gears and crank shafts for automobiles and transmissions. The present invention relates to a nitrided steel member having a compound layer having a sufficient thickness and excellent fatigue resistance, and a method for producing the nitrided steel member.

Among the surface hardening treatments of steel, there are many needs for nitriding treatment, which is a low strain treatment, and recently, there is increasing interest in atmosphere control technology for gas nitriding treatment. The background to this is that the gas nitriding treatment improves the carburizing and carburizing nitriding treatment accompanied by quenching of mechanical parts, and the strain due to induction hardening. The basis of atmosphere control by gas nitriding is to control the nitriding potential (K _N ) in the furnace, which causes γ'phase (Fe ₄ N) and ε phase (F e ₄ N) in the compound layer formed on the surface of the steel material. It is possible to obtain a wide range of nitriding qualities, such as controlling the volume fraction of Fe _2-3 N) or processing without forming a compound layer, and various proposals have been made. For example, in Patent Document 1, bending fatigue strength and surface fatigue are improved by selecting the γ'phase, and carburizing is substituted.

On the other hand, it is desired to form a thick compound layer from the viewpoint of wear resistance and fatigue resistance, but the compound layer is easily peeled off by the conventional ε-phase-based nitriding, and the film cannot be thickened (Patent Document). See 2-4). Specifically, the technique of Patent Document 2 relates to a soft nitride gear for the purpose of reducing gear noise, and stipulates that the compound layer thickness and the porous layer thickness are 12 μm or less. Further, in the technique of Patent Document 3, stress concentration avoidance due to unevenness is carried out by reducing the compound layer thickness to 5 μm or less, and in the technique of Patent Document 4, bending correction is performed when the compound layer thickness is 5 μm or less. The cracks are suppressed by. Further, even in the technique of Patent Document 1 described above, since the γ'phase is treated with a low K _N , when the γ'phase is thickened, the nitriding treatment takes a long time, and as a result, the diffusion layer It was difficult to thicken the film because of softening and reduction of compressive residual stress.

In contrast, the present inventors have found that, as a method of thickening a compound layer of gamma 'phase mainly first nitride potential in the furnace (hereinafter sometimes abbreviated as K _N) nitriding at high ambient After that, a proposal is made for a two-stage nitriding method that lowers the K _N in the furnace (see Non-Patent Document 1). Using this method, if the atmosphere of the first stage is set to a high K _N and the K _N of the γ'phase region is selected in the second stage, it is possible to thicken the compound layer mainly composed of the γ'phase. (Non-Patent Document 1).

Japanese Unexamined Patent Publication No. 2013-221203 Japanese Unexamined Patent Publication No. 11-72159 JP-A-2009-30134 Japanese Unexamined Patent Publication No. 2014-129607

Yasushi Hiraoka, Yoichi Watanabe: Heat Treatment, Vol. 55, No. 1, pp. 7-11

Although there is a problem that it is difficult to thicken the film as described above, conventionally, when gas nitriding or gas nitrocarburizing treatment is applied to a steel member, wear resistance and fatigue strength are improved as compared with an untreated material. Processing has been used. In order to improve wear resistance and fatigue resistance, it is desired that the compound layer be thick, but since thickening the compound layer reduces fatigue strength, as described above, it is suitable for the usage environment of parts. These problems have been addressed by optimizing the thickness according to the requirements (see Patent Documents 2 to 4 above). On the other hand, recently, as can be seen from the above-mentioned Patent Document 1 and non-patent documents (Yasu Hiraoka, Yoichi Watanabe, Akitake Ishida: Heat Treatment, Vol. 55, No. 1, pp. 1-2), it has been conventionally used. It has been clarified that the fatigue strength is improved by using the γ'phase-based compound layer rather than the ε-phase-based compound layer.

On the other hand, as described above, as a method of thickening the compound layer mainly composed of the γ'phase, the present inventors first perform nitriding in an atmosphere having a high nitriding potential (K _N ) in the furnace, and then perform nitriding. We have proposed a two-stage nitriding method that lowers the K _N in the furnace, and by using this method, it is possible to thicken the compound layer mainly composed of the γ'phase.

However, when the compound layer is formed by the above-mentioned two-stage nitriding method in the process of detailed examination, the present inventors tend to form an ε phase at the interface between the matrix phase and the compound layer, and therefore, the ε phase is easily formed in the compound layer. It was found that the volume ratio of the γ'phase that occupies can only be around 50%. Accordingly, by using the two-stage nitriding method, for example, although it is possible to obtain in a short time compound layer thickened above 13 .mu.m, over a long time in the conventional process low K _N, gas in one stage The problem is that only a fatigue strength equivalent to that of a nitrided 10 μm γ'phase-based compound layer (Yasu Hiraoka, Yoichi Watanabe, Akitake Ishida: Heat Treatment, Vol. 55, No. 1, pp. 1-2) can be obtained. there were.

In other words, 'to improve the ratio of phase, whereby, in a conventional process low K _N gas nitriding the gamma' gamma the thickened compounds layer is formed by a gas nitriding treatment with a compound layer of a phase mainly There has never been a technology that further improves the fatigue strength that can be realized, and if it can be realized, its industrial value is extremely large.

Therefore, an object of the present invention is to obtain a compound layer mainly composed of the γ'phase having a higher abundance ratio of the γ'phase on the surface of the nitrided steel member by a conventional method in a normal gas nitriding treatment time. By realizing a thicker film than the γ'phase-based compound layer gas-nitrided with low K _N , a nitrided steel member exhibiting high fatigue strength, which could not be achieved by the prior art, is realized. To develop technology.

As a result of diligent studies to achieve the above problems, the present inventors have found that the ratio of the γ'phase and the ε phase in the iron nitrogen compound layer is extremely high at 0.7 or more, and in addition. The thickness of this compound layer mainly composed of γ'is 13 μm or more, and the thickness of the compound layer after gas nitriding treatment CLT [μm] with respect to the practical curing depth DLT [μm] of the nitriding diffusion layer after gas nitriding treatment. We have found that the above-mentioned problems can be achieved when the ratio satisfies a specific relationship, and have completed the present invention.

That is, the present invention is a nitrided steel member in which an iron nitrogen compound layer is formed on the surface of a steel member composed of a steel component formed by a γ'phase main compound layer, and γ'occupies the iron nitrogen compound layer. When the volume ratio of the phase and the ε phase is Vγ'and Vε, and the abundance ratio of the γ'phase is expressed by the ratio defined by Vγ'/ (Vγ'+ Vε), the value is 0.7 or more. 'The thickness of the phase-based compound layer is 13 μm to 30 μm, and the iron nitrogen compound layer represents the value of the thickness [μm] of the compound layer after the gas nitriding treatment as CLT, and is after the gas nitriding treatment. Provided is a nitriding steel member characterized in that the relationship of the following formula (1) is satisfied when the value of the practical curing depth [μm] of the nitriding diffusion layer is expressed as DLT.
CLT ÷ DLT ≧ 0.04 ・ ・ ・ (1)

The nitrided steel member of the present invention preferably has a thickness of the γ'phase main compound layer of 15 μm to 30 μm, and can be suitably applied to gears or crankshafts.

The present invention provides, as another embodiment, while adjusting the nitriding potential in the gas nitriding furnace (K _N), the material to be treated by performing gas nitriding treatment of the furnace, to produce a nitrided steel member of the The nitriding potential K _N = p _NH3 / p _H2 ^1.5 in the atmosphere during gas nitriding treatment is first set to K _N1, and then the nitriding potentials are set to K _{N2 to} K _Nx-1 and K _Nx. Good, but the first K _N1 satisfies equations (2) and (3) at the same time, and the final K _Nx satisfies equations (2) and (4) at the same time, and satisfies these requirements. Provided is a method for manufacturing a nitriding steel member, characterized in that the nitriding time at a controlled final K _Nx is 5 to 60 minutes.
K _N1 <K _N2 ~ K _Nx-1 , K _N2 ~ K _Nx-1 > K _Nx ... (2)
126.7034-5.68797 × 10 ^-1 × T ＋8.64682 × 10 ^-4 × T ² −4.43596 × 10 ^-7 × T ³ ＞ K _N1
... (3)
126.7034-5.68797 × 10 ^-1 × T ＋8.64682 × 10 ^-4 × T ² −4.43596 × 10 ^-7 × T ³ ＞ K _Nx ＞ 22.2265-1.15 × 10 ^-1 × T ＋ 2.03 × 10 ^-4 × T ² − 1.21466 × 10 ^-7 × T ³・ ・ ・ (4)
(However, p _NH3 and p _H2 are the NH ₃ partial pressure and the H ₂ partial pressure in the nitriding furnace, and T in the formulas (3) and (4) is the temperature [° C.].)

Method for manufacturing a nitride steel member of the present invention, the K _N1 is a 0.05 to 0.5, and the K _Nx is preferably a 0.15 to 0.5.

According to the present invention, the compound layer mainly composed of the γ'phase can be thickened in the same manner as the compound layer mainly composed of the ε phase even in a normal gas nitriding treatment time, and the compound layer mainly composed of the γ'phase can be thickened. Even if the film is thickened, the volume ratio of the γ'phase in the compound layer can be increased to 70% or more, and bending fatigue is compared with the conventional compound layer mainly composed of the γ'phase formed at low K _N. It becomes possible to provide a new nitriding member which can improve the strength and which has never existed before. As defined in the present invention, the "compound layer mainly composed of the γ'phase" in the present invention refers to the abundance ratio of the γ'phase in the iron nitrogen compound layer formed on the surface of the steel member. , Vγ'/ (Vγ'+ Vε) means a portion whose value is 0.7 or more when expressed by a ratio.

It is a Leherer diagram of iron (E. Lehrer: Zeitschrift fur Elektrochemie, 36, p.383 (1930).). It is the schematic of the pit type gas nitriding processing furnace. It is a heat pattern (pit furnace). It is the schematic of the batch type gas nitriding processing furnace. It is a heat pattern (batch furnace). Ono type rotary bending fatigue test piece shape. It is a figure which shows the example (black part: unanalyzable) of the EBSD analysis result.

Hereinafter, preferred embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. First, the premise of the technique of the present invention will be described. The present invention is a nitrided steel member in which an iron nitrogen compound layer mainly composed of a γ'phase is formed on the surface of the steel member, and the volume ratio of the γ'phase and the ε phase in the iron nitrogen compound layer is specified. Therefore, the steel material on which the compound layer is formed is required to be a component in which a compound layer mainly composed of the γ'phase is formed. Further, since the present invention also aims to provide a nitrided steel member that can be effectively applied to gears and crankshafts for automobiles and transmissions, it is necessary for functions such as machinability and manufacturability of steel materials. There are essential elements, and some elements are always present as impurity elements. Including these points, the "steel composed of the steel component formed by the γ'phase-based compound layer" constituting the present invention is preferably a steel type satisfying the following component range. The% below is based on mass.

[Steel component that is the base material]
<Carbon (C)>
It can be said that C is an essential element for ensuring the strength of the nitrided parts, and the content of C is required to be 0.05% or more. On the other hand, if the C content increases and exceeds 0.5%, the hardness before nitriding increases and the machinability decreases, so the C content is 0.05 to 0.5%. Is preferable.

<Silicon (Si)>
Si has a deoxidizing action. To obtain this effect, a Si content of 0.10% or more is required. However, if the Si content increases and exceeds 0.90%, the hardness before nitriding increases and the machinability decreases, which is not preferable. Therefore, the Si content is preferably 0.10 to 0.90%.

<Manganese (Mn)>
Mn has a deoxidizing action. To obtain this effect, a Mn content of 0.3% or more is required. However, if the Mn content increases to exceed 1.65%, the hardness before nitriding becomes too high and the machinability deteriorates, which is not preferable. Therefore, the Mn content is preferably 0.3 to 1.65%.

<Nickel (Ni)>
Ni does not necessarily have to be contained. However, since Ni is a component that contributes to the improvement of hardenability and toughness, it may be contained from this viewpoint. However, even if it is contained in an excessive amount, the above effect not only reaches saturation and causes an increase in cost, but also causes a decrease in machinability, which is not preferable. Therefore, its content is preferably limited to 2.10% or less.

<Chromium (Cr)>
Cr does not necessarily have to be contained. However, if the Cr content increases and exceeds 2.7%, the thickness of the nitrided compound layer decreases, and the effect of the γ'phase-based compound layer constituting the present invention cannot be sufficiently obtained. Therefore, the Cr content is preferably 0 to 2.7%.

<Molybdenum (Mo)>
Mo does not necessarily have to be contained. However, Mo is an element that is required for some mechanical parts because it combines with C in steel to form carbides at the nitriding temperature and brings about an action of improving the hardness of the core after nitriding. However, if the Mo content increases and exceeds 0.50%, not only the raw material cost increases, but also the hardness before nitriding increases and the machinability decreases, which is not preferable. Therefore, the Mo content is preferably 0.50% or less.

<Vanadium (V)>
V does not necessarily have to be contained. However, when V is contained, like Mo, it combines with C in steel at the nitriding temperature to form carbides, which has the effect of improving the hardness of the core after nitriding, and also penetrates from the surface during nitriding. -It is an element that is necessary in some cases because it also has the effect of improving the surface hardness by combining with diffused N and C to form nitrides and carbides. However, when the V content increases and exceeds 0.40%, not only the hardness before nitriding becomes too high and the machinability decreases, but also V in the matrix during hot forging and subsequent normalizing. Does not dissolve in solid solution, so that the above effect is saturated. Therefore, the V content is preferably 0 to 0.40%.

<Aluminum (Al)>
Al does not necessarily have to be contained. However, if the Al content increases and exceeds 1.1%, the amount of the nitride compound layer formed decreases, and the effect of the γ'phase-based compound layer of the present invention cannot be sufficiently obtained. Therefore, the Al content is preferably 0 to 1.1%.

<Sulfur (S)>
S has an action of combining with Mn to form MnS and improving machinability. However, when the S content exceeds 0.030%, coarse MnS is formed, and the hot forging property and bending fatigue strength are lowered. Therefore, the S content is preferably 0.002 to 0.030%. In order to ensure more stable machinability, the S content is preferably 0.010% or more. Further, in the case of a member applied to an application in which hot forging property and bending fatigue strength are more important, the S content is preferably 0.025% or less.

<Phosphorus (P)>
P is an impurity contained in the steel and segregates at the grain boundaries to embrittle the steel. In particular, when the content exceeds 0.030%, the degree of embrittlement may become significant. Therefore, in the present invention, it is required that the content of P in the impurities is 0.030% or less. The content of P in the impurities is preferably 0.020% or less.

<Other elements>
The steel member used in the present invention has a chemical composition in which the balance is composed of Fe and other impurities in addition to the above-mentioned elements. The "Fe and impurities" as the balance are unavoidably mixed from the ore as a raw material when the steel material is industrially manufactured, for example, copper (Cu) or titanium (Ti), or the manufacturing environment. It means that it contains components other than Fe, such as O (oxygen), which are inevitably mixed with the above.

[Nitride steel member]
The feature of the nitrided steel member of the present invention is that an iron nitrogen compound layer having the following peculiar composition is formed on the surface of the steel member composed of the steel material components as described above, in which the γ'phase main compound layer is formed. There is. That is, in the iron nitride compound layer that characterizes the steel nitride member of the present invention, the relationship between the volume ratios Vγ'and Vε of the γ'phase and the ε phase in the layer is defined by Vγ'/ (Vγ'+ Vε). When expressed as a ratio, the value is 0.7 or more (the abundance ratio of the γ'phase is 0.7 or more), and the thickness of the γ'phase main compound layer is 13 μm to 30 μm. Further, the iron nitrogen compound layer is characterized by satisfying the relationship of the following formula (1).
CLT ÷ DLT ≧ 0.04 ・ ・ ・ (1)
(However, CLT in the formula (1) represents the value of the thickness [μm] of the compound layer after the gas nitriding treatment, and DLT is the practical curing depth [μm] of the nitriding diffusion layer after the gas nitriding treatment. Represents a value.)
These will be described below.

<Nitrogen compound layer>
In the present invention, the "iron nitrogen compound layer" (sometimes also referred to as a nitrogen compound layer or simply a compound layer) is a γ'phase (Fe ₄ N) or ε phase on the surface of a steel member formed by gas nitrocarburizing treatment. A layer made of an iron nitrogen compound typified by (Fe _2-3 N). However, the steel material contains carbon in the base material, and a part of this carbon content is also contained in the compound layer, so that it is strictly a carbonitride. As shown in FIG. 7, the nitrogen compound layer is precipitated as a layer on the surface. In the present invention, a nitride compound layer composed of these γ'phase and ε phase is formed on the surface of the steel member (base material) in a thickness range of 13 to 30 μm. The thickness referred to here means the thickness of the compound layer mainly composed of the γ'phase in the present invention. In the nitrided steel member of the present invention in which the iron-nitrogen compound layer is formed on the surface of the steel member, first, the relationship between Vγ'and Vε, which is the volume ratio of the γ'phase and the ε phase in the nitrogen compound layer, is Vγ'/. The ratio specified by (Vγ'+ Vε) must be 0.7 or more. That is, in the nitrided steel member of the present invention, the thick iron-nitrogen compound layer can be made mainly of the γ'phase having a structure containing the γ'phase at a high level in this way, so that the fatigue strength and resistance can be increased. Abrasion is further improved.

(About improvement of mechanical characteristics)
The reason why the nitrided steel member of the present invention having the above structure is excellent in fatigue strength and wear resistance is considered as follows. The crystal structure of the γ'phase is FCC (face-centered cubic crystal) and has 12 slip systems, so that the crystal structure itself is rich in toughness. Further, since the crystal structure of the γ'phase forms a fine equiaxed structure, it is considered that the fatigue strength is improved. On the other hand, the crystal structure of the ε phase is HCP (hexagonal close-packed), and since bottom slip is prioritized, it is considered that the crystal structure itself has the property of being “hard to deform and brittle”. Therefore, in the nitrided steel member of the present invention, it is considered that the fatigue strength of the nitrided steel member can be improved by making the compound layer mainly composed of the γ'phase specified in the present invention.

(Volume ratio of γ'phase and ε phase)
As described above, in the iron nitrogen compound layer formed on the surface of the nitrided steel member of the present invention, the relationship between Vγ'and Vε, which is the volume ratio of the γ'phase and the ε phase in the iron nitrogen compound layer, is Vγ'/. The ratio specified by (Vγ'+ Vε) must be 0.7 or more, but this is specified for the following reasons. As described above, the "nitrogen compound layer" is a layer composed of the ε phase (Fe _2-3 N), the γ'phase (Fe ₄ N), etc. having the above-mentioned characteristics, and these phases in the compound layer. The distribution state of the compound layer is determined from the result (volume ratio) of the phase distribution analysis of the γ'phase and the ε phase in the depth direction cross section of the compound layer by EBSD (Electron Backscatter Diffraction) in a width of 100 μm × 3 visual field. According to the study by the present inventors, when this volume ratio is 0.7 or more, the fatigue strength of the nitrided steel member is excellent. The volume ratio is preferably 0.8 or more, and more preferably 0.9 or more. The technical feature of the present invention is that an iron nitrogen compound layer mainly composed of the γ'phase having a composition containing the γ'phase at a high level of 0.7 or more can be achieved in the prior art as described below. It is because it was made to have a thickness that was not available. As a result, the nitrided steel member having such an iron-nitrogen compound layer formed on the surface exhibits unprecedented characteristics excellent in fatigue strength and wear resistance.

(Thickness of compound layer)
In the present invention, it is defined that the thickness of the iron nitrogen compound layer containing the above-mentioned γ'phase formed on the surface of the nitrided steel member at a high level is 13 to 30 μm. This point will be described below. Since the rotational bending fatigue strength tends to increase as the compound layer thickness increases, it is advantageous to have a thick compound layer. On the other hand, in the conventional two-stage gas nitriding method, as described above, the thickness of the compound layer can be increased in a short time as compared with the conventional one-stage gas nitriding method, but the volume ratio of the γ'phase can be increased. It could not be improved, and the rotational bending fatigue strength could only be about the same as that of the 10 μm-thick compound layer mainly composed of the γ'phase formed by the conventional treatment method in one step. On the other hand, in the conventional one-stage gas nitriding treatment with low K _N , usually only a compound layer thickness of about several μm to 10 μm can be obtained. Since the thickness of this compound layer depends on the steel type and nitriding conditions, it can be increased by devising these points, but even in that case, the maximum thickness that can be practically realized by gas nitriding treatment in the low K _N region. Is about 13 μm at the carbon steel level. As described above, in the prior art, a thick compound layer having a thickness of 13 μm or more and a γ'phase ratio of 0.7 or more and an increased volume ratio of the γ'phase has not been obtained. Therefore, the lower limit of the thickness of the compound layer formed on the surface of the nitrided steel member provided in the present invention is defined as 13 μm. On the other hand, the upper limit of the thickness is the upper limit of the thickness of the compound layer in consideration of the fact that the nitriding time is practical in the two-stage nitriding method and can be reached by a steel type such as carbon steel in which the compound layer tends to be the thickest. Was defined as 30 μm.

(Nitriding time at K _Nx 5 to 60 minutes)
The nitrided steel member of the present invention having a compound layer having a composition that has not been achieved by the above-mentioned prior art can be stably obtained by the method for producing a nitrided steel member of the present invention. In the method for manufacturing a nitriding steel member of the present invention, the nitriding time at the nitriding potential K _Nx in the final furnace is defined as 5 to 60 minutes. This point will be described below. K _{N2 to} K _Nx-1 defined in the method for producing a nitrided steel member of the present invention is a region containing the ε phase, and the ε phase formed at this time is _γ'by changing the atmosphere to K _Nx . Transform into a phase. The transformation rate at this time is regulated by the rate at which nitrogen in the compound layer denitrifies into the atmosphere. According to the study by the present inventors, the holding time for nitriding under the control of _KNx, which is required to obtain a sufficient γ'phase inside the compound layer, varies depending on the temperature. For example, at 580 ° C, the holding time is as short as 5 to 20 minutes, but at a temperature of 500 ° C, it takes 20 to 60 minutes. Further, if the holding time is short, a sufficient γ'phase tends not to be obtained, and conversely, if it is long, the crystal grains of the γ'phase may become large, and in that case, the mechanical properties are deteriorated. Therefore, in the method for producing a nitrided steel member of the present invention, the lower limit is defined as 5 minutes as the holding time for obtaining a sufficient γ'phase even in a short time at 580 ° C. or higher. Further, the upper limit of the holding time for nitriding by controlling with K _Nx is defined as 60 minutes at which a sufficient γ'phase can be obtained at 500 ° C., which is the lower limit of the practical nitriding temperature. More preferably, it takes about 5 to 30 minutes. Within the above range, even at a nitriding temperature of 580 ° C. or higher, which can shorten the holding time, the crystal grains in the γ'phase do not become large, and there is no risk of deteriorating mechanical properties such as wear resistance.

The iron nitride compound layer formed on the surface of the nitrided steel member of the present invention satisfies the following formula (1) in addition to satisfying the above-mentioned specific relationship between the volume ratios of the γ'phase and the ε-phase described above. It is characterized by that.
CLT ÷ DLT ≧ 0.04 ・ ・ ・ (1)
(However, CLT in the formula (1) represents the value of the thickness [μm] of the compound layer after the gas nitriding treatment, and DLT is the practical curing depth [μm] of the nitriding diffusion layer after the gas nitriding treatment. Represents a value.)

The above formula (1) is an index showing the ratio of the thickness of the compound layer after the nitriding treatment to the depth of the nitriding diffusion layer (hereinafter, may be simply referred to as a diffusion layer). The method for forming a γ'phase-based compound layer described in Non-Patent Document 1 described above as a prior art is gas nitriding treatment in a two-stage atmosphere, and the nitriding potential K _N1 set first is ε. It is set in the region where the phase is formed. At this time, since the gas nitriding treatment atmosphere is a decarburized atmosphere, the carbon in the core of the steel material diffuses toward the surface side, and if the compound layer is present on the surface side, the decarburized amount is reduced and the compound layer is formed. And carbon is concentrated at the interface of the diffusion layer (see Dita Rietke et al., "Nitriding and Soft Nitriding of Iron", Agne Technology Center 2013, p. 48). Since this carbon is an element that stabilizes the ε phase, the ε phase is inevitably formed at the interface between the compound layer and the diffusion layer. On the other hand, in the production method of the present invention, in the first-stage gas nitriding treatment, the γ'phase is formed on the surface or the compound layer is not formed to increase the decarburization rate of the surface, and at the same time, the core portion. It is configured to slow down the amount of carbon from to the surface side. That is, in the method for producing a nitrided steel member of the present invention, the absolute value of the amount of carbon that may be concentrated on the surface in this first-stage treatment is made smaller than that in the conventional gas nitriding treatment in a two-stage atmosphere. This effect makes it possible to reduce the amount of carbon concentrated at the interface between the compound layer and the diffusion layer even in the ε phase region, so that the volume ratio of the γ'phase in the compound layer can be increased. It becomes possible to increase. As a result, for the first time by the present invention, it has been possible to provide a novel nitriding member in which a compound layer having a constitution which has not been obtained by the prior art is formed on the surface thereof. In the production method of the present invention, in the first-stage treatment, the same effect can be obtained not by the nitriding treatment but by the decarburization treatment, but it is desirable to carry out the treatment in a nitriding atmosphere from the viewpoint of treatment efficiency.

Here, the practical curing depth defined by the above formula (1) indicates the curing depth at the position of core hardness + 50 HV in the hardness distribution in the nitrided layer (JIS-0563). The practical curing depth indicates the sum of the compound layer thickness and the diffusion layer depth. Generally, the compound layer thickness is 1/10 or less of the diffusion layer thickness, and can be regarded as a practical curing depth ≈ diffusion layer depth. The growth of the nitriding diffusion layer proceeds by diffusing the N atoms supplied from the compound layer into the steel material. Since the growth rate of this layer is regulated by temperature and time regardless of the growth rate of the compound layer, the temperature is determined by the value on the right side of the equation (1) obtained by dividing the growth rate of this diffusion layer by the thickness of the compound layer. It is possible to indirectly know the growth rate of the compound layer with respect to time. As a result of diligent studies to achieve the above-mentioned object of the present invention, the present inventors are characterized in that the compound layer formed is a compound layer mainly composed of the γ'phase, and the compound layer grows rapidly. , Fatigue resistance can be improved by thickening the compound layer, and the effect of shortening the nitriding time can be achieved, and the compound layer mainly composed of the ε phase does not reduce the compressive residual stress and hardness in the diffusion layer. The present invention has been completed by discovering that the value on the right side of the above equation (1) is 0.04 or more for the nitrided steel member having a higher fatigue strength than the above. The total nitriding time is preferably 6 hours or less. Further, in the gas nitriding treatment with low K _N in the prior art, even if a compound layer mainly composed of γ'phase can be formed, the growth rate thereof is less than 0.040 defined by the equation (1). It is not a nitrided steel member that can achieve the effect of.

[Manufacturing method of nitrided steel member]
The method for producing a nitrided steel member of the present invention is for obtaining a nitrided steel member having the above-described configuration, and heat-treats the material to be treated in the processing furnace while flowing a nitride gas into the furnace. It is characterized in that the processing conditions for the gas nitriding treatment are defined as follows. That is, in the method for producing a nitrided steel member of the present invention, the nitriding potential K _N = p _NH3 / p _H2 ^1.5 in the atmosphere during gas nitriding treatment is first set to K _N1, and then the nitriding potential is set to K _{N2 to} K _Nx-1. , K _Nx , but the first K _{N 1} needs to satisfy the equations (2) and (3) at the same time. Further, the final K _Nx needs to satisfy the equations (2) and (4) at the same time, and the nitriding time at the final K _Nx controlled to satisfy these requirements needs to be 5 to 60 minutes. ..
K _N1 <K _N2 ~ K _Nx-1 , K _N2 ~ K _Nx-1 > K _Nx ... (2)
126.7034-5.68797 × 10 ^-1 × T ＋8.64682 × 10 ^-4 × T ² −4.43596 × 10 ^-7 × T ³ ＞ K _N1
... (3)
126.7034-5.68797 × 10 ^-1 × T ＋8.64682 × 10 ^-4 × T ² −4.43596 × 10 ^-7 × T ³ ＞ K _Nx ＞ 22.2265-1.15 × 10 ^-1 × T ＋ 2.03 × 10 ^-4 × T ² − 1.21466 × 10 ^-7 × T ³・ ・ ・ (4)
(However, p _NH3 and p _H2 are the NH ₃ partial pressure and the H ₂ partial pressure in the nitriding furnace, and T in the formulas (3) and (4) is the temperature [° C.].)

The phase structure (γ'phase or ε-phase) of the surface nitrogen compound layer formed during the gas nitriding treatment is determined by the temperature and the nitriding potential K _N from the Iron Leherer diagram shown in FIG. And, in general, K _N during the gas nitriding treatment is performed constantly. On the other hand, in the present invention, it is the last K _Nx during the gas nitriding treatment that controls the phase structure of the final compound layer in the present invention, regardless of the atmosphere control process. In that case, the last K _Nx satisfies the above equations (2) and (4) at the same time, and the nitriding time at the final K _Nx controlled to satisfy these requirements is 5 to 60. It was discovered that the nitriding steel member having a phase structure capable of achieving the object of the present invention can be obtained by dividing the amount.

In the method for producing a nitride steel member of the present invention, the relationship between K _{N1 to} K _{Nx-1 in the} first and intermediate stages and the final K _Nx is determined by the formula (2), K _N1 <K _{N2 to} K _Nx-1. In addition to satisfying K _{N2 to} K _Nx-1 > K _Nx , the range of the first K _N1 is limited to "in the α phase or γ'phase region" defined in equation (3). The final range of K _{Nx needs} to be limited to "in the γ'phase region" defined by the formula (4). In other words, K _{N2 to} K _Nx-1 in the middle stage are regions outside the "γ'phase region". On the other hand, in the prior art, since the first K _{N 1} is set in the ε phase region, the volume ratio of the γ'phase in the compound layer could not be 0.7 or more. By utilizing the above-mentioned effects, in the production method of the present invention, the carbon content on the surface is controlled by carrying out the first K _{N 1} in the α phase or γ'phase region until it reaches K _{N x.} The setting of K _{N in} the middle can be freely set, and it is effective to increase the K _N in the middle in order to thicken the compound layer. Further, the nitriding treatment time during the final K _Nx retention is controlled by the diffusion rate at which nitrogen in the compound layer flows into the atmosphere, and according to the study by the present inventors, the required compound layer thickness, steel type, and atmosphere. It takes 10 to 60 minutes including the change time of.

(About nitriding potential)
Nitriding potential K _N is a parameter defined by K _N = p _NH3 / p _H2 ^1.5 (p _NH3 and p _H2 are the partial pressures of ammonia and hydrogen in the furnace). For example, when ammonia or ammonia decomposition gas is used. The formula for this is described in a non-patent document (Dita Rietke et al., "Iron nitriding and soft nitriding", Agne Technology Center, 2013, p. 131). In addition, even when a plurality of types of gases other than ammonia and ammonia decomposition gas such as N ₂ gas are used, non-patent documents (H. Du, MAJ Somers, J. Agren: METALLUGICAL AND MATERIALS TRANSACTIONS A, 31A, 200,195-211). The present invention is defined by a _KEN value calculated based on these conventional calculation methods.

The processing furnace for performing the gas nitriding treatment for carrying out the production method of the present invention may be a pit type or a batch type, and regardless of the shape of the furnace, as described above, the compound of the nitrided steel member which is the treatment result. properties of the layers, the temperature and time of the processing furnace, also determined in an atmosphere of K _N. This is commonly referred to as Lehrer diagram, can be known from the state diagram of K _N and the temperature was axis shown in FIG. 1 (Source: Dita Ritoke al, soft nitriding and nitriding the "Iron , Agne Technology Center p.131). The nitriding treatment method when using the pit type and batch type processing furnaces performed by the production method of the present invention is described below.

(Pit type example)
A schematic diagram of the pit furnace is shown in FIG. 2, and an example of processing conditions in the pit furnace is shown in FIG. As shown in FIG. 2, in order to control the K _N in the furnace, the H ₂ sensor 21, the PLC + K _N regulator 22, and the mass flow meter (MFC) 23 are set for each process gas. is there. Then, the processed product 24 to be subjected to the gas nitriding treatment is installed in the center of the furnace in advance, sealed in the furnace, evacuated in the furnace, recompressed in the furnace with N ₂ gas, and then constant. Start heating while flowing N ₂ gas of the flow rate into the furnace. For heating, the retort 25 is heated from the outside with a heater (not shown) set on the outer circumference, and for temperature adjustment, the temperature is adjusted to a desired temperature based on the temperature measured by the thermocouple 26 in the furnace. Will be done. Reference numeral 27 in FIG. 2 is a stirrer.

When processing in a pit furnace having the above configuration, for example, as shown in FIG. 3, after the temperature inside the furnace reaches about 400 ° C., the process gas is changed from N ₂ gas to NH ₃ + AX gas (NH ₃ decomposition gas). Switch to H ₂ + N ₂ = 3: 1) and start atmosphere control so that the desired K _N value (= K _{N 1} ) is obtained. Generally, in the heat equalizing region, K _N is kept constant (for example, K _N = 1.0), and the treatment is performed while maintaining this K _N until about 400 ° C. during cooling. On the other hand, in the present invention, the first K _N1 is held in the α phase region or the γ'phase region (for example, K _N1 = 0.25) for a certain period of time, and then the high K _N2 (for example, K _N2 = 1.3). Then, it is also a preferable form to change to K _Nx (for example, K _N = 0.3) from 20 minutes before cooling. The nitrided compound layer formed at high K _N is a compound layer containing an ε phase and has a faster growth rate than the γ'phase. In the present invention, the fast growth rate at high K _N is utilized to form a compound layer. The film is thickened, and then the atmosphere is changed to K _Nx to transform from the ε phase to the γ'phase. Then, after cooling to about 400 ° C. with K _Nx , the atmosphere is replaced with N ₂ gas again to cool the inside of the furnace to 100 ° C. or lower. When cooling, the heater is stopped and the inside of the furnace is cooled while cooling the outer circumference of the retort with a fan.

(In case of batch type)
A schematic view of the straight-through type batch furnace is shown in FIG. 4, and an example of processing conditions in the batch furnace is shown in FIG. As shown in FIG. 4, an H ₂ sensor 41, a PLC + K _N regulator 42, and a mass flow meter (MFC) 43 are set for each process gas in order to control the K _N in the furnace. Is similar to the pit furnace. In the case of a batch furnace, the processed product 44 is inserted into the furnace by opening the door 48 into the furnace previously heated to 580 ° C. in an NH ₃ gas atmosphere. When processing a batch furnace having the above-described structure, for example, as shown in FIG. 5, after the treatment product 44 into the furnace has been inserted, switching the process gas to NH ₃ + AX gas, desired K _N value ( = Start atmosphere control so that K _N1 ). Similar to the above-mentioned pit furnace, the gas nitriding treatment is first performed with K _N1 and then with K _N2 , and then the atmosphere is changed to K _Nx and held for 20 minutes, as in the above-mentioned treatment with the pit furnace. It is possible to form a nitride compound layer mainly composed of the γ'phase, which is thick and contains the γ'phase at a high level, as defined in the present invention. In FIG. 4, 46 is a thermocouple, 47 is a stirrer, and 49 is a cooling tank.

Hereinafter, the present invention will be specifically described based on Examples and Comparative Examples, but the present invention is not limited to these Examples.

Table 1 shows the steel components used in Examples and Comparative Examples. The balance is iron (Fe). As shown below, each steel material is composed of a steel material component capable of thickening the γ'phase-based compound layer, and is a steel type that is satisfactory from the viewpoint of the purpose of use such as machinability and manufacturability of the steel material. is there.

(Examples 1 to 15)
Table 2 shows the gas nitriding treatment conditions for each. Using the batch type or pit type furnace described above, in each case, the nitriding potential K _{N of the} atmosphere during the gas nitriding process is different for the first K _N1 , the second stage K _{N 2} , and the final K _{N x.} went. The first-stage nitriding potentials K _N1 of the examples are all on the Leherer diagram of each alloy steel [Fig. 1 is for pure iron and is different from that (Yasu Hiraoka, Yoichi Watanabe, heat treatment, volume 55). No. 1, p. 7, "Control of nitriding potential in gas nitriding and control of compound layer phase structure of low alloy steel utilizing thermodynamic calculation method")], carried out in the α region or γ'phase region. During this period, it promotes decarburization from the surface and promotes the growth of the diffusion layer. Subsequently, the second stage nitriding potential K _N2 is carried out in the ε phase or the two phase region of ε + γ', and during this period, the compound layer is thickened by utilizing the fast growth rate of the ε phase. The nitriding potential K _{Nx in the} third stage was carried out in the γ'phase region, and the nitriding time was set between 15 and 50 minutes depending on the thickness of the compound layer to be formed and the characteristics of the furnace.

For the nitrided steel member obtained after the gas nitriding treatment performed under each condition, the thickness CLT [μm] of the iron nitrogen compound layer formed on the surface and the practical curing depth DLT [μm] of the diffusion layer were measured. , CLT ÷ DLT values were calculated respectively, and the results are summarized in Table 3. Further, regarding the relationship between the γ'phase and the ε phase in the iron nitrogen compound layer, the abundance ratio of the γ'phase was determined by the method described later, and the results are summarized in Table 3. Further, each of the obtained nitrided steel members was subjected to a rotational bending fatigue test described below to examine and evaluate the mechanical properties. The results are summarized in Table 3.

<Evaluation>
(1) Ono-type rotary bending fatigue test Using the notch test piece of the shape shown in Fig. 6, gas nitriding treatment was performed under the conditions of each example, and then the Ono-type rotary bending fatigue test (JIS Z 2274) was carried out. did. The test load was selected from two levels of 40 kgf and 55 kgf according to the steel material composition, and the rotation speed was common to 3000 rpm. Evaluation of test results, and ○ a case where greeted 10 ⁷ rotating at pass, otherwise was evaluated as × in failure.

(2) Measurement method of nitrogen compound layer Using a coin-shaped test piece of φ20 × 5 mm, gas nitriding treatment is performed under the conditions of each example, and the flat surface portion of the test piece after gas nitriding treatment is cut perpendicular to the flat surface portion. Then, the thickness of the compound layer in the cross section was measured according to JIS-0562.

(3) Phase distribution analysis method using EBSD A backscattered electron diffraction (EBSD) device (EBSD) device (EBSD) mounted on a scanning electron microscope (FEI Crystal) after mechanically mirror-polishing the cross section of a nitrided steel material. Phase Map was measured using Inca Crystal) manufactured by Oxford Instruments. The Phase Map is a color-coded phase determined by matching the actually measured electron diffraction pattern with the electron diffraction figure of the candidate phase. FIG. 7 shows an example of the EBSD analysis result having a compound layer mainly composed of γ'phase and a compound layer mainly composed of ε phase. The upper part of FIG. 7 is a micrograph of the γ'phase main compound layer, and the lower part is a micrograph of the ε phase main compound layer. The right column of each is Phase Map, and the gray-colored portion is the γ'phase portion. In the present invention, using this Phase Map, the ratio defined by Vγ'/ (Vγ'+ Vε) was determined, and the abundance ratio of the γ'phase and the ε phase in the iron nitrogen compound layer was compared.

(Comparative Examples 1 to 3)
In Comparative Examples 1 to 3, as shown in Table 2, the same steel materials used in Examples 1, 5 and 9, respectively, were used, and low K _N (= 0) in the γ'phase region was obtained as in the conventional technique. Only one step of gas nitriding treatment in .25) was performed, and the obtained nitriding steel material was subjected to the same test as in Examples. As a result, as shown in Table 3, in all of Comparative Examples 1 to 3, the compound layer thickness of 13 μm or more specified in the present invention was not satisfied, and therefore the result of the rotational bending fatigue strength was unacceptable. ..

(Comparative Examples 4 to 6)
In Comparative Examples 4 to 6, the same steel materials as those used in Examples 3, 5 and 7, respectively, were used to perform the two-stage nitriding treatment according to the prior art. As a result, as shown in Table 3, although a thick compound layer could be formed, the volume ratio of the γ'phase in the compound layer was about 50%, which did not reach the level specified in the present invention. The result of rotational bending fatigue strength was unacceptable.

(Comparative Examples 7-9)
In Comparative Examples 7 to 9, the same steel materials as those used in Examples 3, 5 and 7, respectively, were used. Then, K _N was taken to 0.25 forming the γ'phase by the conventional technique, and all of them were treated in one step. Then, in order to increase the thickness of the compound layer to 13 μm or more specified in the present invention, the nitriding time is set to the longer side in Comparative Examples 7 and 8, and the nitriding temperature is increased in Comparative Example 9 to shorten the time. Nitriding treatment was carried out. As a result, as shown in Table 3, the thickness of the iron nitrogen compound layer of the obtained nitrided steel material satisfied 13 μm or more specified in the present invention, but satisfied the formula (1) specified in the present invention. It didn't become something to do. As a result of conducting the same test as in Examples for the obtained nitrided steel material, as shown in Table 3, the fatigue strength was unacceptable (55 kgf).

According to the present invention, the compound layer mainly composed of the γ'phase containing the γ'phase at a high level of 0.7 or more in volume ratio is the same as the compound layer mainly composed of the ε phase even in a normal gas nitriding treatment time. The film can be thickened, and even if the compound layer mainly composed of the γ'phase is thickened, the bending fatigue strength is higher than that of the compound layer mainly composed of the γ'phase formed at low K _N. It becomes possible to provide a nitrided steel member that can be improved. As an example of utilization of the present invention, since the nitrided steel member provided by the present invention realizes high fatigue strength, it is expected to be used for, for example, gears and crankshafts for automobiles and transmissions. ..

Claims (4)

While adjusting the gas nitriding furnace nitriding potential (K _N), a method for manufacturing a steel nitriding member performs gas nitriding process on a target material in the furnace,
The nitriding potential K _N = p _NH3 / p _H2 ^1.5 of the atmosphere during the gas nitriding process is first set to K _N1 , then the nitriding potential is adjusted to K _N2 , the final nitriding potential is K _Nx , and the first K _N1 simultaneously satisfies the following equation (2) and the following equation (3), the final K _Nx satisfies the following equations (2) and (4) at the same time, and the K _N2 satisfies the following equation (2). _Moreover , a method for manufacturing a nitrided steel member, characterized in that the nitriding time at the final K _Nx whose nitriding potential is controlled so as to satisfy these requirements is 5 to 60 minutes.
K _N1 <K _N2 > K _Nx ... (2)
126.7034-5.68797 × 10 ^-1 × T ＋8.64682 × 10 ^-4 × T ² −4.43596 × 10 ^-7 × T ³ ＞ K _N1・ ・ ・ (3)
126.7034-5.68797 × 10 ^-1 × T ＋8.64682 × 10 ^-4 × T ² −4.43596 × 10 ^-7 × T ³ ＞ K _Nx ＞ 22.2265-1.15 × 10 ^-1 × T + 2.03 × 10 ^-4 × T ² − 1.21466 × 10 ^-7 × T ³・ ・ ・ (4)
(However, p _NH3 and p _H2 are the NH ₃ partial pressure and the H ₂ partial pressure in the nitriding furnace, and T in the formulas (3) and (4) is the temperature [° C.].) The method for producing a nitrided steel member according to claim 1, wherein the K _N1 is 0.05 to 0.5 and the K _Nx is 0.15 to 0.5. A nitrided steel member in which an iron-nitrogen compound layer is formed on the surface of a steel member composed of a steel component formed by a compound layer mainly composed of the γ'phase.
When the volume ratio of the γ'phase and the ε phase in the iron nitrogen compound layer is Vγ'and Vε, and the abundance ratio of the γ'phase is expressed by the ratio defined by Vγ'/ (Vγ'+ Vε), The thickness of the γ'phase-based compound layer having a value of 0.7 or more is 12.1 μm to 30 μm, and the iron nitride compound layer is the thickness of the compound layer after gas nitriding treatment [μm]. When the value of CLT is expressed as CLT and the value of the practical curing depth [μm] of the nitrided diffusion layer after gas nitriding treatment is expressed as DLT, the nitrided steel member is characterized by satisfying the relationship of the following formula (1). ..
CLT ÷ DLT ≧ 0.04 ・ ・ ・ (1) The nitrided steel member according to claim 3, wherein the thickness of the compound layer mainly composed of the γ'phase having a ratio value defined by Vγ'/ (Vγ'+ Vε) of 0.7 or more is 15 μm to 30 μm.