Title of invention: Nitrided parts
Technical field
[0001]
　The present invention relates to a steel part that has been subjected to a gas nitriding treatment.
Background technology
[0002]
　Steel parts used for automobiles and various industrial machines are manufactured by carburizing, induction hardening, nitriding, and nitrocarburizing to improve mechanical properties such as fatigue strength, wear resistance, and seizure resistance. A surface hardening heat treatment is performed.
[0003]
　The nitriding treatment and the soft nitriding treatment are performed in the ferrite region of A 1 point or less, and since there is no phase transformation during the treatment, heat treatment strain can be reduced. Therefore, the nitriding treatment and the soft nitriding treatment are often used for parts having high dimensional accuracy and large parts, and are applied to, for example, gears used for automobile transmission parts and crankshafts used for engines.
[0004]
　The nitriding treatment is a treatment method in which nitrogen penetrates into the surface of the steel material. The medium used for the nitriding treatment includes gas, salt bath, plasma and the like. Gas nitriding treatment, which excels in productivity, is mainly applied to automobile transmission parts. By the gas nitriding treatment, a compound layer ( a layer in which a nitride such as Fe 3 N is deposited) having a thickness of 10 μm or more is formed on the surface of the steel material, and a nitrogen diffusion layer is formed on the steel material surface layer below the compound layer. A hardened layer is formed. The compound layer is mainly composed of Fe 2 to 3 N(ε) and Fe 4 N(γ′), and the hardness of the compound layer is extremely higher than that of the steel core portion which is a non-nitrided layer. Therefore, the compound layer improves the wear resistance and surface fatigue strength of the steel part in the initial stage of use.
[0005]
　Patent Document 1 discloses a nitriding component in which the bending fatigue strength is improved by setting the γ'phase ratio in the compound layer to 30 mol% or more.
[0006]
　In Patent Document 2, the ratio of the γ′ phase in the compound layer is 0.5 or more, the thickness of the compound layer is 13 to 30 μm, and the compound layer thickness/cured layer depth≧0.04, A steel member having excellent wear resistance is disclosed.
[0007]
　In Patent Document 3, the compound layer has a thickness of 3 to 15 μm, a phase structure up to a depth of 5 μm from the surface has a γ′ phase with an area ratio of 50% or more, and a void area ratio up to a depth of 3 μm from the surface is 10. %, and the compressive residual stress on the surface of the compound layer is 500 MPa or more, a nitriding component excellent in surface bending strength and rotating bending fatigue strength is disclosed.
Prior art documents
Patent literature
[0008]
Patent Document 1: Japanese Patent Laid-Open No. 2015-117412
Patent Document 2: Japanese Patent Laid-Open No. 2016-211069
Patent Document 3: International Publication No. 2018/66666
Summary of the invention
Problems to be Solved by the Invention
[0009]
　Since the nitriding component of Patent Document 1 is gas soft nitriding using CO 2 as an atmospheric gas, it is considered that the bending fatigue strength is not yet sufficient because the surface side of the compound layer is likely to be in the ε phase.
[0010]
　In the nitriding component of Patent Document 2, the composition ranges of C, Cr, Mo, and V that affect the hardness and structure of the compound layer are not optimized, and the structure of the compound layer does not reach the target depending on the nitriding conditions. there is a possibility.
[0011]
　The nitriding component of Patent Document 3 focuses on controlling the γ'phase ratio in the surface layer portion of the compound layer, and is a finding on the phase ratio and various fatigue strengths in the entire depth direction of the compound layer. Is insufficient, so there is room for improvement.
[0012]
　An object of the present invention is to provide a part having excellent surface fatigue strength or wear resistance in addition to rotational bending fatigue strength.
Means for solving the problems
[0013]
　The present inventors paid attention to the morphology of the compound layer formed on the surface of the steel material by the nitriding treatment and investigated the relationship with the fatigue strength.
[0014]
　As a result, by nitriding the steel with adjusted components under the control of the nitriding potential, the structure of the compound layer generated in the surface layer of the steel after nitriding mainly consists of the γ'phase, and the void layer of the surface layer (hereinafter referred to as "porous layer"). It has been found that a nitrided component having excellent rotary bending fatigue strength and surface fatigue strength or wear resistance can be produced by suppressing the occurrence of the above) and making the hardness of the compound layer a certain value or more.
[0015]
　The present invention has been made through further studies based on the above findings, and its gist is as follows.
[0016]
　(1) In mass%, C: 0.05 to 0.35%, Si: 0.05 to 1.50%, Mn: 0.20 to 2.50%, P: 0.025% or less, S: 0.050% or less, Cr: 0.50 to 2.50%, V: 0.05 to 1.30%, Al: 0.050% or less, N: 0.0250% or less, Mo: 0-1. 50%, Cu:0 to 0.50%, Ni:0 to 0.50%, Nb:0 to 0.100%, Ti:0 to 0.050%, B:0 to 0.0100%, Ca: 0 to 0.0100%, Pb: 0 to 0.50%, Bi: 0 to 0.50%, In: 0 to 0.20%, and Sn: 0 to 0.100%, with the balance being Fe And a steel core portion which is an impurity, a nitrogen diffusion layer formed on the steel core portion, and a compound layer mainly containing iron nitride and having a thickness of 5 to 15 μm formed on the nitrogen diffusion layer In the cross section perpendicular to the surface of the compound layer, the void area ratio in the depth range from the surface to 3 μm is 10% or less, and C, Mn, Cr, V, Mo in the steel core portion When X defined based on the content is defined as X=−2.1×C+0.04×Mn+0.5×Cr+1.8×V-1.5×Mo, (i) 0≦X≦0.25 And the area ratio of the γ′ phase of the iron nitride in the compound layer is 50% or more and 80% or less, or (ii) 0.25≦X≦0.50, and in the compound layer A nitriding component, wherein the area ratio of the γ'phase of the iron nitride is 80% or more.
[0017]
　(2) 0≦X≦0.25, and the area ratio of the γ′ phase of the iron nitride in the compound layer is 50% or more and 80% or less, and (1) the nitriding treatment is performed. parts.
[0018]
　(3) The nitriding component according to (1) above, wherein 0.25≦X≦0.50, and the area ratio of the γ′ phase of the iron nitride in the compound layer is 80% or more.
Effect of the invention
[0019]
　According to the present invention, it is possible to obtain a nitriding component having excellent surface fatigue strength or wear resistance in addition to rotational bending fatigue strength. Nitrided parts having excellent surface fatigue strength in addition to rotary bending fatigue strength are suitable for gear parts, and nitrided parts having excellent wear resistance in addition to rotary bending fatigue strength are suitable for CVT and camshaft parts.
Brief description of the drawings
[0020]
FIG. 1 is a diagram illustrating a method for measuring the depth of a compound layer.
FIG. 2 is an example of a structure photograph of a compound layer and a diffusion layer.
FIG. 3 is a diagram showing the relationship between the γ′ phase ratio and rotary bending fatigue strength.
FIG. 4 is a diagram showing a relationship between a γ′ phase ratio and surface fatigue strength.
FIG. 5 is a diagram showing how voids are formed in a compound layer.
FIG. 6 is an example of a structure photograph in which voids are formed in the compound layer.
FIG. 7 is a shape of a small roller for a roller pitting test used to evaluate surface fatigue strength and wear resistance.
FIG. 8 is a shape of a large roller for a roller pitting test used for evaluating surface fatigue strength and wear resistance.
FIG. 9 is a shape of a cylindrical test piece for evaluating the rotational bending fatigue strength.
MODE FOR CARRYING OUT THE INVENTION
[0021]
　In the present invention, the steel component adjusted to the desired characteristics, by nitriding under nitriding potential control, depending on the component of the steel, nitriding component excellent in surface fatigue strength in addition to rotating bending fatigue strength, It is possible to obtain a nitriding component having excellent wear resistance as well as rotational bending fatigue strength. Hereinafter, embodiments of the present invention will be described in detail.
[0022]
　(1) Nitriding component according to the present invention
[0023]
　First, the chemical composition of the raw material steel will be described. Hereinafter, “%” indicating the content of each component element and the element concentration on the surface of the component means “mass %”. Further, the steel core portion of the nitriding-treated component according to the present invention has the same chemical composition as the steel material used as the raw material.
[0024]
　[C: 0.05 to 0.35%]
　C is an element necessary for ensuring the hardness of the core of the component. Therefore, C needs to be 0.05% or more. On the other hand, if the content of C exceeds 0.35%, the strength after hot forging becomes too high, and the machinability is greatly reduced. The preferable lower limit of the C content is 0.08%. Moreover, the preferable upper limit of the C content is 0.30%.
[0025]
　[Si: 0.05 to 1.50%]
　Si is an element that enhances the hardness of the core by solid solution strengthening. Further, the temper softening resistance is increased, and the surface fatigue strength and wear resistance of the surface of the component which becomes high temperature under wear conditions are increased. In order to exert these effects, Si needs to be 0.05% or more. On the other hand, when the Si content exceeds 1.50%, the bar steel, the wire rod, and the strength after hot forging become too high, so that the machinability is greatly reduced. The preferable lower limit of the Si content is 0.08%. The preferable upper limit of the Si content is 1.30%.
[0026]
　[Mn: 0.20 to 2.50%]
　Mn forms a fine nitride (Mn 3 N 2 ) in the compound layer and the diffusion layer by the nitriding treatment, and increases hardness, so that surface fatigue strength and It is an element effective in improving wear resistance and rotating bending fatigue strength. In addition, the solid solution strengthens the core hardness. To obtain these effects, Mn needs to be 0.20% or more. On the other hand, when the content of Mn exceeds 2.50%, not only the effect is saturated, but also the bar steel or wire as a raw material and the hardness after hot forging become too high, so that the machinability is greatly reduced. .. The preferable lower limit of the Mn content is 0.40%. The preferable upper limit of the Mn content is 2.30%.
[0027]
　[P: 0.025% or less]
　P is an impurity and segregates at the grain boundaries to embrittle the component. Therefore, the content is preferably small. If the P content exceeds 0.025%, the surface fatigue strength, wear resistance, and rotary bending fatigue strength may decrease. The preferable upper limit of the P content for preventing the reduction of the rotational bending fatigue strength is 0.018%. The P content may be 0, but it is difficult to completely set it to 0, and the P content may be 0.001% or more.
[0028]
　[S: 0.050% or less]
　S is not an essential element, but is usually contained as an impurity even if not intentionally added. S in steel is an element that combines with Mn to form MnS and improves machinability. In order to obtain the effect of improving the machinability, S is preferably contained in 0.003% or more. However, if the S content exceeds 0.050%, coarse MnS is likely to be generated, and surface fatigue strength, wear resistance, and rotary bending fatigue strength are significantly reduced. The preferable lower limit of the S content is 0.005%. The preferable upper limit of the S content is 0.030%.
[0029]
　[Cr: 0.50 to 2.50%]
　Cr forms a fine nitride (CrN) in the compound layer or the diffusion layer by nitriding treatment to increase the hardness, so that surface fatigue strength and wear resistance are improved. , And an element effective in improving the rotating bending fatigue strength. To obtain these effects, Cr needs to be 0.50% or more. On the other hand, when the content of Cr exceeds 2.50%, not only the effect is saturated, but also the bar steel or wire as a raw material and the hardness after hot forging become too high, so that the machinability is significantly lowered. .. The preferable lower limit of the Cr content is 0.70%. The preferable upper limit of the Cr content is 2.00%.
[0030]
　[V: 0.05 to 1.30%]
　V forms a fine nitride (VN) in a compound layer or a diffusion layer by nitriding treatment to increase hardness, so that surface fatigue strength and wear resistance , And an element effective in improving the rotating bending fatigue strength. To obtain these effects, V needs to be 0.05% or more. On the other hand, when the content of V exceeds 1.30%, not only the effect is saturated, but also the bar steel or wire as a raw material and the hardness after hot forging become too high, so that the machinability is remarkably lowered. .. The preferable lower limit of the V content is 0.10%. The preferable upper limit of the V content is 1.10%.
[0031]
　[Al: 0.050% or less] Although
　Al is not an essential element, it is a deoxidizing element, and is often contained in the deoxidized steel to some extent. Further, it has the effect of forming AlN by combining with N and making the microstructure of the steel material before nitriding treatment fine by the pinning action of austenite grains, and reducing variations in mechanical properties of nitriding components. In order to obtain the effect of refining the structure of the steel material, it is preferable to contain 0.010% or more. On the other hand, Al easily forms a hard oxide-based inclusion, and when the Al content exceeds 0.050%, the rotational bending fatigue strength is remarkably reduced, and even if other requirements are satisfied, it is desirable. Rotating bending fatigue strength cannot be obtained. The preferable lower limit of the Al content is 0.020%. The preferable upper limit of the Al content is 0.040%.
[0032]
　[N: 0.0250% or less]
　N is not an essential element, but N is usually contained as an impurity even if not intentionally added. N in steel combines with Mn, Cr, Al, and V to form Mn 3 N 2 , CrN, AlN, and VN. Among them, Al and V, which have a high tendency to form a nitride, have the effect of refining the microstructure of the steel material before nitriding by the pinning action of the austenite grains and reducing the variation in the mechanical properties of the nitriding components. In order to obtain the effect of refining the structure of the steel material, it is preferable to contain 0.0030% or more. On the other hand, if the content of N exceeds 0.0250%, coarse AlN is likely to be formed, so that the above effect is difficult to obtain. The preferable lower limit of the N content is 0.0050%. The preferable upper limit of the N content is 0.0200%.
[0033]
　The chemical composition of the steel used as the material for the nitriding component according to the present invention contains the above elements, and the balance is Fe and impurities. Impurities are components that are contained in raw materials or are mixed during the manufacturing process, and are components that are not intentionally contained in steel. The impurities are, for example, Te of 0.05% or less, W, Co, As, Mg, Zr, and REM of 0.01% or less. Te is for the purpose of improving machinability, and adding 0.30% or less has no great influence.
[0034]
　However, the steel that is the material of the nitriding component of the present invention may contain the following elements instead of a part of Fe.
[0035]
　[Mo: 0 to 1.50%]
　Mo forms a fine nitride (Mo 2 N) in the compound layer or diffusion layer formed by the nitriding treatment, and increases the hardness. It is an element effective in improving wear resistance and rotating bending fatigue strength. In order to obtain these effects, Mo is preferably 0.01% or more. On the other hand, when the content of Mo exceeds 1.50%, not only the effect is saturated, but also the bar steel used as a raw material, the wire rod and the hardness after hot forging become too high, so that the machinability is remarkably lowered. .. The more preferable lower limit of the Mo content is 0.10%. The preferable upper limit of the Mo content is 1.10%.
[0036]
　[Cu: 0 to 0.50%]
　Cu improves the hardness of the core of the component and the hardness of the nitrogen diffusion layer as a solid solution strengthening element. In order to exert the action of solid solution strengthening of Cu, the content of 0.01% or more is preferable. On the other hand, when the content of Cu exceeds 0.50%, the bar steel or wire as a raw material and the hardness after hot forging become too high, so that the machinability is remarkably lowered and the hot ductility is lowered. Therefore, it causes surface scratches during hot rolling and hot forging. A preferable lower limit of the Cu content for maintaining hot ductility is 0.05%. The preferable upper limit of the Cu content is 0.40%.
[0037]
　[Ni:0 to 0.50%]
　Ni improves core hardness and surface hardness by solid solution strengthening. In order to exert the effect of solid solution strengthening of Ni, the content of 0.01% or more is preferable. On the other hand, if the Ni content exceeds 0.50%, the bar steel, the wire rod, and the hardness after hot forging become too high, so that the machinability is remarkably lowered and the alloy cost is increased. The preferable lower limit of the Ni content for obtaining sufficient machinability is 0.05%. The preferable upper limit of the Ni content is 0.40%.
[0038]
　[Nb:0-0.100%]
　Nb combines with C and N to form NbC and NbN, and by the pinning action of austenite grains, the structure of the steel material before nitriding is refined, and the machine of nitriding parts It has the effect of reducing the variation in dynamic characteristics. To obtain this effect, Nb is preferably 0.010% or more. On the other hand, if the content of Nb exceeds 0.100%, coarse NbC and NbN are formed, so that the above effect is difficult to obtain. The preferable lower limit of the Nb content is 0.015%. The preferable upper limit of the Nb content is 0.090%.
[0039]
　[Ti: 0 to 0.050%]
　Ti combines with N to form TiN, and improves core hardness and surface hardness. To obtain this effect, Ti is preferably 0.005% or more. On the other hand, when the Ti content exceeds 0.050%, the effect of improving the core hardness and the surface hardness is saturated, and the alloy cost increases. The preferable lower limit of the Ti content is 0.007%. The preferable upper limit of the Ti content is 0.040%.
[0040]
　[B:0-0.0100%]
　Solid solution B has the effect of suppressing the grain boundary segregation of P and improving the toughness. Further, BN that is combined with N and precipitates improves the machinability. In order to obtain these effects, B is preferably 0.0005% (5 ppm) or more. On the other hand, when the content of B exceeds 0.0100%, not only the above effects are saturated, but also a large amount of BN segregates, which may cause cracks in the steel material. The preferable lower limit of the B content is 0.0008%. The preferable upper limit of the B content is 0.0080%.
[0041]
　[Ca: 0 to 0.0100%, Pb: 0 to 0.50%, Bi: 0 to 0.50%, In: 0 to 0.20%, and Sn: 0 to 0.100%]
　Other required Accordingly, a free-cutting element for improving machinability can be contained. Examples of the free-cutting element include Ca, Pb, Bi, In, and Sn. In order to improve machinability, it is preferable to contain one or more elements of Ca, Pb, Bi, In and Sn in an amount of 0.005% or more. Even if a large amount of free-cutting element is added, the effect is saturated, and the hot ductility decreases, so the Ca content is 0.0100% or less, the Pb content is 0.50% or less, and the Bi content is The content is 0.50% or less, the In content is 0.20% or less, and the Sn content is 0.100% or less.
[0042]
　As for the components of the nitriding component of the present invention, the content (% by mass) of C, Mn, Cr, V, and Mo is 0≦−2.1×C+0.04×Mn+0.5×Cr+1.8×V−. It is necessary to satisfy 1.5×Mo≦0.50. The elements not contained are calculated as 0. Here, the value of X is defined by the following mathematical expression, and X will be used in the following description.
[0043]
　　X = -2.1 x C + 0.04 x Mn + 0.5 x Cr + 1.8 x V-1.5 x Mo
[0044]
　C, Mn, Cr, V and Mo are elements that affect the phase structure and thickness of the compound layer. C and Mo have the effect of stabilizing the ε phase and increasing the thickness. On the other hand, Mn, Cr and V have the effect of thinning the compound layer. Therefore, by designing these elements within a certain range, the ratio of the γ'phase in the compound layer and the compound layer thickness can be stably controlled, and the surface fatigue strength, wear resistance and rotational bending fatigue strength can be improved. Improve.
[0045]
　To obtain these effects, X needs to be 0 or more. If it is less than 0, the γ'phase cannot be obtained in an effective ratio for the rotational bending fatigue strength. On the other hand, when X exceeds 0.50, the compound layer becomes thin and desired characteristics cannot be obtained. The area ratio of the γ'phase will be described later.
[0046]
　Next, the nitriding component of the present invention will be described.
[0047]
　The nitriding component according to the present invention is manufactured by processing a steel material into a base material and then nitriding the material under predetermined conditions. The nitriding component according to the present invention includes a steel core portion, a nitrogen diffusion layer formed on the steel core portion, and a compound layer formed on the nitrogen diffusion layer. That is, the nitriding component according to the present invention has a structure in which the compound layer is on the surface, the nitrogen diffusion layer is inside the compound layer, and the steel core is inside the nitrogen diffusion layer.
[0048]
　The steel core part is a part to which nitrogen that has penetrated from the surface did not reach during the nitriding treatment. The steel core portion has the same chemical composition as the steel material used as the material of the nitriding component.
[0049]
　The nitrogen diffusion layer is a portion in which nitrogen invading from the surface in the nitriding treatment forms a solid solution in the matrix or is deposited as iron nitride and alloy nitride. Since the solid solution strengthening of nitrogen and the particle dispersion strengthening of iron nitrides and alloy nitrides act on the nitrogen diffusion layer, the hardness is higher than that of the steel core.
[0050]
　The compound layer is a layer mainly containing an iron nitride formed by combining nitrogen atoms that have penetrated into the steel by the nitriding treatment and iron atoms contained in the material. The compound layer is mainly composed of iron nitride, but in addition to iron and nitrogen, oxygen mixed from the outside air and each element contained in the steel material (that is, each element contained in the steel core). 1) or two or more thereof are also included in the compound layer. Generally, 90% or more (mass %) of the elements contained in the compound layer are nitrogen and iron. The iron nitride contained in the compound layer is Fe 2 to 3 N (ε phase) or Fe 4 N (γ′ phase).
[0051]
　[Thickness of compound layer: 5 to 15 μm]
　The thickness of the compound layer affects the surface fatigue strength, wear resistance, and rotational bending fatigue strength of the nitrided component. The compound layer is harder than the inner nitrogen diffusion layer and the steel core, but has a property of being easily cracked. If the compound layer is excessively thick, cracking is likely to occur due to pitting or bending, which easily becomes a fracture starting point, leading to deterioration in surface fatigue strength and rotary bending fatigue strength. On the other hand, when the compound layer is too thin, the contribution of the hard compound layer becomes small, and thus the surface fatigue strength and the rotational bending fatigue strength also decrease. In the nitriding component according to the present invention, from the above viewpoint, the thickness of the compound layer is 5 to 15 μm.
[0052]
　After the gas nitriding treatment, the thickness of the compound layer is measured by polishing the vertical cross section of the test material, etching it, and observing it with a scanning electron microscope (SEM). The etching is performed with a 3% Nital solution for 20 to 30 seconds. The compound layer is present on the surface layer of the low alloy steel and is observed as an uncorroded layer. The compound layer is observed from 10 visual fields (visual field area: 6.6×10 2 μm 2 ) photographed at 4000×, and the thickness of the compound layer is measured at 3 points every 10 μm in the horizontal direction. Then, the average value of the measured 30 points is defined as the compound layer thickness (μm). FIG. 1 shows an outline of the measuring method, and FIG. 2 shows an example of a structure photograph of the compound layer and the nitrogen diffusion layer. As shown in FIG. 2, the compound layer that is not corroded by etching and the corroded nitrogen diffusion layer have distinct contrasts and can be distinguished.
[0053]
　No clear contrast difference such as the interface between the compound layer and the nitrogen diffusion layer is generated between the nitrogen diffusion layer in which the nitrogen has penetrated by the nitriding treatment and the steel core portion in which the nitrogen has not penetrated. It is difficult to identify the boundary with the steel core. When measuring the hardness profile in the depth direction, the area where the hardness continuously decreases with the depth is the nitrogen diffusion layer, and the area where the hardness is constant regardless of the depth is the steel core. is there. In the nitriding component, if the difference between the value of Vickers hardness at a certain point A and the value of Vickers hardness at a point B deeper than the point A by 50 μm is within 1%, the points A and B are It may be determined that both of them are in the steel core. Alternatively, under normal nitriding conditions, since nitrogen does not penetrate 5.0 mm or more from the surface, the point 5.0 mm deep from the surface may be the steel core.
[0054]
　[Area ratio of γ'phase of compound layer: 50% or more] The
　γ'phase has an fcc structure and is more tough than the ε phase having an hcp structure. On the other hand, the ε phase has a wider solid solution range of N and C and a higher hardness than the γ′ phase. Therefore, the inventors of the present invention have conducted investigations and researches with the aim of clarifying the structure of a compound layer effective for surface fatigue strength and rotational bending fatigue strength. As a result, as shown in FIG. 3, it was found that the rotational bending fatigue strength increases as the proportion of the γ'phase in the compound layer increases. In particular, it was found that the ratio of the γ'phase effective for the rotational bending fatigue strength is 50% or more in the area ratio in the cross section perpendicular to the surface.
[0055]
　On the other hand, as shown in FIG. 4, in the surface fatigue strength, the ratio of the γ′ phase forms a peak at around 70% in the above area ratio, and at least the surface fatigue strength decreases even if the γ′ phase is more than that. I found out that. That is, in parts (gear parts, etc.) where surface fatigue strength is particularly important, the area ratio of the γ′ phase of the compound layer is preferably 80% or less. On the other hand, in parts where rotational bending fatigue strength is more important than surface fatigue strength (CVTs in automobiles, camshaft parts, etc.), it is desirable that the area ratio of the γ'-phase of the compound layer is high, especially 80% or more. Is desirable.
[0056]
　The area ratio of the γ'phase is determined by image-processing the structure photograph. Specifically, for 10 micrographs of the cross section of the surface layer of the nitriding-processed component taken at 4000 times by the electron backscatter diffraction method (EBSD), the γ in the compound layer was compared. The'phase and ε phase are discriminated, and the area ratio of the γ'phase in the compound layer is binarized and obtained by image processing. Then, the average value of the measured area ratios of the γ′ phase of the 10 visual fields is defined as the area ratio (%) of the γ′ phase.
[0057]
　[Void area ratio
　of the compound layer within the depth of 3 μm from the surface : 10% or less] Stress concentration occurs in the voids present in the compound layer within the depth of 3 μm from the surface, resulting in pitting and bending fatigue fracture. It is easy to start from. Therefore, the void area ratio needs to be 10% or less.
[0058]
　The voids are formed by desorbing N 2 gas along the grain boundaries from the steel material surface from an energy-stable place such as a grain boundary on the surface of the steel material having a small binding force by the base material . The generation of N 2 is more likely to occur as the nitriding potential K N described later is higher. This, K N accordance becomes high, bcc → γ '→ occur phase transformation epsilon, gamma' towards the epsilon phase than phase N 2 is large solid solution amount of Trip epsilon phase N 2 gas This is because it is easy to generate. Fig. 5 shows the outline of voids formed in the compound layer (Dieter Rietke et al.: "Nitriding and soft nitriding of iron", Agne Technical Center, Tokyo, (2011), P.21), and Fig. 6 shows the voids formed. The organization photograph is shown.
[0059]
　The void area ratio can be measured by a scanning electron microscope (SEM). The ratio of the total area of ​​voids in the area 90 μm 2 within the range of 3 μm depth from the outermost surface (void area ratio, unit: %) is determined by analysis using an image processing application. Then, the average value of the measured 10 fields of view is defined as the void area ratio (%). Even when the compound layer has a thickness of less than 3 μm, the measurement target is from the surface to a depth of 3 μm.
[0060]
　The void area ratio is preferably 5% or less, more preferably 2% or less, still more preferably 1% or less, and most preferably 0.
[0061]
　Next, an example of a method for manufacturing a nitriding component according to the present invention will be described.
[0062]
　In the method for manufacturing a nitriding component according to the present invention, the gas nitriding treatment is performed on the steel material having the above components. The treatment temperature of the gas nitriding treatment is 550 to 620° C., and the treatment time of the entire gas nitriding treatment is 1.5 to 10 hours.
[0063]
　[Treatment temperature: 550 to 620° C.]
　The temperature of the gas nitriding treatment (nitriding treatment temperature) mainly has a correlation with the diffusion rate of nitrogen and affects the surface hardness and the depth of the hardened layer. If the nitriding temperature is too low, the diffusion rate of nitrogen becomes slow, the surface hardness becomes low, and the depth of the hardened layer becomes shallow. On the other hand, nitriding treatment temperature A C1 if it exceeds point, ferrite phase (alpha phase) the nitrogen diffusion rate is small austenite phase than (gamma phase) is generated in the steel, the surface hardness becomes low, hardening depth Becomes shallower. Therefore, in this embodiment, the nitriding temperature is 550 to 620° C. around the ferrite temperature range. In this case, it is possible to prevent the surface hardness from lowering and also to prevent the hardened layer depth from becoming shallow.
[0064]
　[Overall treatment time of gas nitriding treatment: 1.5 to 10 hours] The
　gas nitriding treatment is carried out in an atmosphere containing NH 3 , H 2 and N 2 . The time of the entire nitriding treatment, that is, the time from the start to the end of the nitriding treatment (treatment time) is correlated with the formation and decomposition of the compound layer and the diffusion and permeation of nitrogen, and affects the surface hardness and the depth of the hardened layer. Exert. If the treatment time is too short, the surface hardness will be low and the hardened layer will be shallow. On the other hand, if the treatment time is too long, the void area ratio on the surface of the compound layer increases, and the surface fatigue strength and rotary bending fatigue strength decrease. If the processing time is too long, the manufacturing cost will be further increased. Therefore, the total nitriding treatment time is 1.5 to 10 hours.
[0065]
　The atmosphere of the gas nitriding treatment of the present embodiment inevitably contains impurities such as oxygen and carbon dioxide in addition to NH 3 , H 2 and N 2 . A preferable atmosphere is a total of 99.5% (volume %) of NH 3 , H 2 and N 2 . If the content of impurities, especially carbon dioxide, becomes high in the atmosphere, the formation of the non-γ'phase (ε phase) is promoted by the presence of carbon, so that it is difficult to produce the nitriding component of the present invention.
[0066]
　[Gas condition for nitriding treatment] In
　the nitriding treatment method for the nitriding component according to the present invention, the nitriding potential is controlled. As a result, the area ratio of the γ'phase in the compound layer can be set within a predetermined range, and the void area ratio in the depth range of 3 μm from the surface can be set to 10% or less.
[0067]
　The nitriding potential K N of the gas nitriding treatment is defined by the following formula.
[0068]
　　K N (atm −1/2 )=(NH 3 partial pressure (atm))/[(H 2 partial pressure (atm)) 3/2 ]
[0069]
　The partial pressures of NH 3 and H 2 in the gas nitriding atmosphere can be controlled by adjusting the gas flow rate.
[0070]
　As a result of studies by the present inventors, the nitriding potential of the gas nitriding treatment affects the thickness, phase structure, and void area ratio of the compound layer, and the optimum nitriding potential has a lower limit of 0.15 and an upper limit of 0.40. It was found that the average is 0.18 or more and less than 0.30.
[0071]
　As described above, when nitriding the component steel in the present invention, the γ′ phase ratio in the compound layer can be stably increased without complicating the nitriding conditions, and the depth of 3 μm from the surface can be obtained. The void area ratio in the above range can be 10% or less. Therefore, it is possible to obtain a nitriding component having excellent rotary bending fatigue strength, preferably surface fatigue strength of 2400 MPa or more and rotary bending fatigue strength of 600 MPa or more.
[0072]
　(2) Nitrided component having excellent surface fatigue strength
　As described above, the rotational bending fatigue strength can be increased by increasing the ratio of the γ'phase in the compound layer. On the other hand, surface fatigue (contact fatigue accompanied by tangential force due to slip) has a peak in the area ratio of γ′ around 70%, and if there are more γ′ phases than that, at least the surface fatigue strength decreases. It turned out to do. It is considered that this is because it is desirable that the hardness of the compound layer is high in order to secure the surface fatigue strength. That is, when the γ'phase exceeds 70% and becomes excessively large, the ratio of the hard ε phase decreases as compared with the γ'phase, and particularly when it exceeds 80%, the hardness of the compound layer becomes insufficient, and as a result, The surface fatigue strength is thought to decrease. On the other hand, as described above, when the γ′ phase rich in toughness is reduced to less than 50%, the rotational bending fatigue strength becomes insufficient. In the nitriding component according to the present invention, which requires particularly surface fatigue strength, the ratio of the γ′ phase in the compound layer is 50% or more and 80% or less in the area ratio in the cross section perpendicular to the surface. Stipulate.
[0073]
　By precipitating a nitride such as CrN or VN in the compound layer or by making a substitutional element a solid solution in the compound layer, the inventors of the present invention have a hardness of the compound layer having a γ′ phase of 50-80%. We have found that Specifically, the value X concerning the content ratios of C, Mn, Cr, V and Mo is 0≦X≦0.25, whereby the hardness of the compound layer and the surface fatigue strength can be increased. it can. That is, also in the nitriding component according to the present invention, in particular, 0≦X≦0.25, and the area ratio of the γ′ phase of the iron nitride in the compound layer is 50% or more and 80% or less. It is possible to achieve both a high level of surface fatigue strength and a high level of rotary bending fatigue strength as compared with conventional ones. In this nitriding component, the hardness of the compound layer can be realized to be 730 HV or higher, but the hardness of the compound layer is preferably harder, specifically, it is preferably 750 Hv or higher.
[0074]
　(3) Nitrided component excellent in rotational bending fatigue strength
　As described above, the rotational bending fatigue strength can be increased by increasing the ratio of the γ'phase in the compound layer. Therefore, in products where surface fatigue strength is not required so much (products whose tangential force or contact surface pressure is below a certain level), in the nitrided component according to the present invention, the ratio of the γ′ phase in the compound layer is The area ratio in the cross section is preferably 80% or more. However, in a product in which the tangential force and the contact surface pressure are constant or less, when the γ'phase is 80% or more, the wear resistance becomes a problem instead of the surface fatigue strength. As described above, in addition to the hardness of the γ'phase being lower than that of the ε phase, when the γ'phase is 80% or more, the thickness of the compound layer becomes insufficient, resulting in insufficient wear resistance. Was sometimes.
[0075]
　The present inventors not only optimize the hardness of the compound layer by appropriately controlling the value of X, specifically 0.25≦X≦0.50, but It was found that the layer thickness can be secured. That is, also in the nitriding component according to the present invention, in particular, by setting 0.25≦X≦0.50 and setting the area ratio of the γ′ phase of the iron nitride in the compound layer to 80% or more, It is possible to achieve both a high level of rotational bending fatigue strength and high wear resistance compared to. In this nitriding component, the hardness of the compound layer can be 710 HV or more, but the hardness of the compound layer is preferably harder, specifically, 730 HV or more.
Example
[0076]
　[Embodiment 1] In
　Embodiment 1, a nitriding component which is particularly excellent in rotary bending fatigue strength and surface fatigue strength will be described. Among the nitriding parts according to the present invention, it is particularly characterized in that 0≦X≦0.25 and the area ratio of the γ′ phase of the iron nitride in the compound layer is 50% or more and 80% or less. ..
[0077]
　Ingots of steels a to ag having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured using a 50 kg vacuum melting furnace. Note that a to y in Table 1-1 are steels having the chemical composition defined in this example. On the other hand, the steels z to ag shown in Table 1-2 are steels of comparative examples in which at least one element is out of the chemical composition specified in this example.
[0078]
[Table 1-1]

[0079]
[Table 1-2]

[0080]
　This ingot was hot forged into a round bar having a diameter of 40 mm. The hot forging was performed at a temperature between 1000°C and 1100°C, and after the forging, it was left to cool in the atmosphere. Subsequently, each round bar was annealed and then subjected to a cutting process to produce a small roller for a roller pitting test for evaluating the surface fatigue strength shown in FIG. 7. Multiple small rollers are made from one ingot for the roller pitting test, but at that time, cross-section observation (measurement of compound layer thickness and void area ratio, measurement of γ'phase ratio, and compound layer hardness) The number of small rollers was made larger than the number required for the roller pitting test. Further, using the same round bar as a raw material, a cylindrical test piece for evaluating the rotary bending fatigue strength shown in FIG. 9 was produced. A plurality of cylindrical test pieces were also prepared from one ingot for the rotating bending fatigue test.
[0081]
　As shown in FIG. 7, the small roller, which is a roller pitting test piece, includes a central test surface portion having a diameter of 26 and a width of 28 mm, and grip portions having a diameter of 22 provided on both sides thereof. In the roller pitting test, the test surface portion was brought into contact with a large roller, a predetermined surface pressure was applied, and then the roller was rotated.
[0082]
　Gas nitriding treatment was performed on the collected test pieces under the following conditions. The test piece was placed in a gas nitriding furnace, and NH 3 , H 2 , and N 2 gases were introduced into the furnace, and nitriding treatment was performed under the conditions shown in Tables 2-1 and 2-2. However, test number 42 was a gas soft nitriding treatment in which 3% by volume of CO 2 gas was added to the atmosphere . The test piece after the gas nitriding treatment was oil-cooled using oil at 80°C.
[0083]
　The H 2 partial pressure in the atmosphere was measured using a heat conduction type H 2 sensor directly attached to the gas nitriding furnace body . The difference in thermal conductivity between the standard gas and the measurement gas was converted into the gas concentration and measured. The H 2 partial pressure was continuously measured during the gas nitriding process.
[0084]
　The NH 3 partial pressure was measured using an infrared absorption NH 3 analyzer installed outside the furnace . The NH 3 partial pressure was continuously measured during the gas nitriding treatment. As for the test number 42 under the atmosphere of the CO 2 gas mixture , (NH 4 ) 2 CO 3 was deposited in the infrared absorption NH 3 analyzer, and there was a risk that the device might break down. The NH 3 partial pressure was measured every 10 minutes using an analyzer .
[0085]
　The NH 3 flow rate and the N 2 flow rate were adjusted so that the nitriding potential K N calculated in the apparatus converges to the target value . The nitriding potential K N was recorded every 10 minutes , and the lower limit value, upper limit value and average value were derived.
[0086]
[Table 2-1]

[0087]
[Table 2-2]

[0088]
　[Measurement of compound layer thickness and void area ratio] With
　a small roller after the gas nitriding treatment, the test surface portion (the position of φ26 in FIG. 7) was cut by a surface perpendicular to the longitudinal direction, and the obtained cross section was mirror-polished. And then etched. The etched cross section was observed using a scanning electron microscope (SEM, JSM-7100F manufactured by JEOL Ltd.), and the thickness of the compound layer was measured and the presence or absence of voids in the surface layer was confirmed. The etching was performed with a 3% Nital solution for 20 to 30 seconds.
[0089]
　The compound layer can be confirmed as an uncorroded layer existing on the surface layer. The compound layer was observed from 10 fields of view (field area: 6.6×10 2 μm 2 ) of a structure photograph taken with a scanning electron microscope at 4000×, and the thickness of the compound layer at 3 points was measured every 10 μm. .. Then, the average value of the measured 30 points was defined as the compound layer thickness (μm).
[0090]
　Area 90μm ranging from the outermost surface of 3μm depth 2 ratio of the total area of the voids occupying the (void area ratio, unit is%) and the above-described structure photograph (10 fields) image processing application (Nippon Denshi; It was determined by analysis by Analysis Station). Specifically, a region of 3 μm in the depth direction and 30 μm in the direction parallel to the surface in the vicinity of the sample surface in the micrograph was extracted, and the area of ​​the void portion in the extracted region was calculated. The calculated area was divided by the area (90 μm 2 ) of the extracted region to measure the void area ratio in the micrograph. This calculation was performed in the measured 10 fields of view, and the average value was defined as the void area ratio (%). Even when the compound layer has a thickness of less than 3 μm, the measurement target is from the surface to a depth of 3 μm.
[0091]
　[Measurement of
　γ'Phase Ratio] The γ'phase ratio was determined by image-processing the structure photograph. Specifically, a backscattered electron diffraction method (Electron Back Scatter Diffraction: EBSD, manufactured by EDAX) was used to analyze a cross-sectional field of view perpendicular to the surface of the nitrided component obtained at a magnification of 4000, and a phase map was drawn. The γ'phase and the ε phase in the compound layer were discriminated from the 10 phase maps, and the area ratio of the γ'phase in the compound layer was binarized by image processing to be obtained. Then, the average value of the measured area ratios of the γ′ phase in the 10 visual fields was defined as the γ′ phase ratio (%).
[0092]
　[Hardness of
　Compound Layer ] The hardness of the compound layer was measured by the following method using a nanoindentation device (manufactured by Hysitron; TI950). At a position near the center in the thickness direction of the compound layer, indentation was randomly performed at 50 points with a pushing load of 10 mN. The indenter has a triangular pyramid (Birkovich) shape, and the hardness is derived in accordance with ISO14577-1, and the nanoindentation hardness H IT is converted to the Vickers hardness HV by the following formula.
[0093]
　　HV=0.0924×H IT
[0094]
　The average value of the measured 50 points was defined as the hardness (HV) of the compound layer.
[0095]
　[Surface fatigue strength evaluation test] The
　surface fatigue strength was evaluated by the following method using a roller pitting tester (RP102, manufactured by Komatsu Equipment Co., Ltd.). A small roller for a roller pitting test was subjected to a finishing process of a grip portion for the purpose of removing heat treatment distortion, and then subjected to a roller pitting test. The shape after finishing is shown in FIG.
[0096]
　The roller pitting test was carried out under the conditions shown in Table 3 using a combination of the small roller for the roller pitting test and the large roller for the roller pitting test having the shape shown in FIG. The large roller was produced under conditions different from those of the present invention, and is not the product of the present invention.
[0097]
　The unit of dimensions in FIGS. 7 and 8 is “mm”. The large roller for the roller pitting test uses a steel satisfying the SCM420 standard of JIS G 4053 (2016), and a general manufacturing process, that is, "normalizing → test piece processing → eutectoid carburizing in a gas carburizing furnace → The Vickers hardness HV at a position of 0.05 mm from the surface, that is, a depth of 0.05 mm is 740 to 760, and the Vickers hardness Hv is 550. The above depth was in the range of 0.8 to 1.0 mm.
[0098]
　Table 3 shows the test conditions for evaluating the surface fatigue strength. The number of times the test was cut off was set to 2×10 7 times, which is the fatigue limit of general steel, and the maximum surface pressure that reached 2×10 7 times without causing pitting in the small roller test piece was applied to the small roller test piece. The fatigue limit was set. In the roller pitting test, especially in the vicinity of the fatigue limit, the surface pressure was tested at intervals of 50 MPa. That is, the values ​​of the pitting strengths shown in Tables 2-1 and 2-2 were the same as those of the target test number, although pitting did not occur in the small roller test piece tested under the same surface pressure. It shows that pitting occurred in the small roller test piece tested under a surface pressure higher by 50 MPa than that.
[0099]
[Table 3]

[0100]
　The occurrence of pitting was detected by a vibrometer attached to the tester. After the occurrence of vibration, the rotation of both the small roller test piece and the large roller test piece was stopped, and the occurrence of pitting and the number of rotations were confirmed. In this example, assuming application to a gear component, the target was that the surface pressure at the fatigue limit in the roller pitting test shown in Table 3 was 2400 MPa or more.
[0101]
　[Rotary Bending Fatigue Strength Evaluation Test]
　An Ono-type rotating bending fatigue test according to JIS Z 2274 (1978) was performed on a cylindrical test piece subjected to gas nitriding treatment. Rpm 3000 rpm, test truncation number, 1 × 10 showing the fatigue limit of general steel 7 as times, in rotating bending fatigue test piece, break of 1 × 10 without causing 7 rotates the maximum stress reaches times The fatigue limit of the bending fatigue test piece was used. In the rotating bending fatigue test, the stress was tested at intervals of 10 MPa, especially near the fatigue limit. That is, the values ​​of the rotary bending fatigue strengths shown in Tables 2-1 and 2-2 were lower than those of the same stress, although no fracture occurred in the cylindrical test piece tested under the same stress. It shows that fracture occurred in the cylindrical test piece tested under the stress of 10 MPa higher.
[0102]
　In this example, assuming application to gear parts, the stress at the fatigue limit in the Ono-type rotary bending fatigue test was set to 600 MPa or more.
[0103]
　[Test Results] The
　results are shown in Tables 2-1 and 2-2. In Test Nos. 1 to 31, the composition of the steel and the conditions of the gas nitriding treatment were within the ranges assumed in this example, the compound layer thickness was 5 to 15 μm, and the γ′ phase ratio of the compound layer was 50% to 80%. Hereafter, the compound layer void area ratio was 10% or less. As a result, the hardness of the compound layer was 730 Hv or more (measurement load 10 mN), the surface fatigue strength was 2400 MPa or more, and the rotary bending fatigue strength was 600 MPa or more, which were favorable results.
[0104]
　In Test Nos. 32 to 50, the components of steel and a part of the conditions of the gas nitriding treatment are out of the ranges assumed in this example, and any one of the thickness of the compound layer, the γ′ phase, and the void area ratio, Or multiple characteristics did not reach the target value. As a result, the surface fatigue strength or rotary bending fatigue strength did not meet the target. For example, in Test No. 42, since the atmosphere in the gas nitriding treatment contains carbon dioxide and was soft nitriding treatment, the formed compound layer was thick and the γ′ phase ratio was low (ε phase was formed. ), the void area ratio became high, and sufficient characteristics could not be obtained from the viewpoint of pitting strength and rotary bending fatigue strength.
[0105]
　Although the test number 46 is a comparative example in which the surface fatigue strength does not reach the target value, it is a component suitable as a nitriding component excellent in rotary bending fatigue strength and wear resistance of Example 2 described later. The steel ac used for the test number 46 is also the steel b of the present invention example of Example 2.
[0106]
　[Embodiment 2] In
　Embodiment 2, a nitriding component which is particularly excellent in rotary bending fatigue strength and wear resistance will be described. Among the nitriding components according to the present invention, it is particularly characterized in that 0.25≦X≦0.50 and the area ratio of the γ′ phase of the iron nitride in the compound layer is 80% or more. ..
[0107]
　Ingots of steels a to ag having the chemical compositions shown in Tables 4-1 to 4-2 were manufactured in a 50 kg vacuum melting furnace. Note that a to y in Table 4-1 are steels having the chemical composition specified in this example. On the other hand, the steels z to ag shown in Table 4-2 are steels of Comparative Examples in which at least one element is out of the chemical composition defined in this Example.
[0108]
[Table 4-1]

[0109]
[Table 4-2]

[0110]
　This ingot was hot forged into a round bar having a diameter of 40 mm. As in Example 1, hot forging was performed at a temperature between 1000° C. and 1100° C., and after the forging, it was left to cool in the atmosphere. Subsequently, each round bar was annealed and then subjected to a cutting process to produce a small roller for a roller pitting test shown in FIG. 7 for evaluating the wear resistance. In the same manner as in Example 1, in addition to the quantity used for the roller pitting test, the quantity used for cross-section observation was also made under the same conditions. Further, using the same round bar as a raw material, a cylindrical test piece for evaluating the rotary bending fatigue strength shown in FIG. 9 was produced.
[0111]
　Gas nitriding treatment was performed on the collected test pieces under the following conditions. The test piece was placed in a gas nitriding furnace, NH 3 , H 2 , and N 2 gases were introduced into the furnace, and nitriding treatment was performed under the conditions shown in Tables 5-1 and 5-2. However, test number 42 was a gas soft nitriding treatment in which 3% by volume of CO 2 gas was added to the atmosphere . The test piece after the gas nitriding treatment was oil-cooled using oil at 80°C.
[0112]
　The partial pressures of H 2 and NH 3 in the atmosphere were measured by the same method as in Example 1. Also, the control of the nitriding potential K N during the nitriding treatment was performed by the same method as in Example 1.
[0113]
[Table 5-1]

[0114]
[Table 5-2]

[0115]
　Using a small roller after the gas nitriding treatment, the thickness of the compound layer, the ratio of the γ'phase in the compound layer (area ratio), the void area ratio, and the hardness of the compound layer were measured by the same method as in Example 1. ..
[0116]
　[Abrasion resistance evaluation test]
　Abrasion resistance was evaluated by the following method using a roller pitting tester (RP102, manufactured by Komatsu Equipment Co., Ltd.). A small roller for a roller pitting test was subjected to finish processing of the grip portion for the purpose of removing heat treatment strain, and then provided as a roller pitting test piece. The shape after finishing is the same as that of the first embodiment shown in FIG. 7.
[0117]
　The roller pitting test was carried out under the conditions shown in Table 6 using a combination of the above-mentioned small roller for the roller pitting test and the large roller for the roller pitting test having the shape shown in FIG. The large roller was produced under conditions different from those of the present invention, and is not the product of the present invention.
[0118]
　The unit of dimensions in FIGS. 7 and 8 is “mm”. The large roller for the roller pitting test uses a steel satisfying the SCM420 standard of JIS G 4053 (2016), and a general manufacturing process, that is, "normalizing → test piece processing → eutectoid carburizing in a gas carburizing furnace → The Vickers hardness HV at a position of 0.05 mm from the surface, that is, a depth of 0.05 mm is 740 to 760, and the Vickers hardness Hv is 550. The above depth was in the range of 0.8 to 1.0 mm.
[0119]
　Table 6 shows the test conditions for evaluating the wear resistance. The test was stopped at a repetition number of 2×10 6 times, the roughness part was used to scan the worn part of the small roller along the main axis direction, and the maximum wear depth was measured. The average value of the In the present embodiment, assuming that it is applied to a CVT or a camshaft component, the wear depth by the roller pitting test shown in Table 6 is set to be 10 μm or less.
[0120]
[Table 6]

[0121]
　[Rotary Bending Fatigue Strength Evaluation Test]
　An Ono-type rotating bending fatigue test according to JIS Z 2274 (1978) was performed on a cylindrical test piece subjected to gas nitriding treatment. Rpm 3000 rpm, test truncation number, 1 × 10 showing the fatigue limit of general steel 7 as times, in rotating bending fatigue test piece, break of 1 × 10 without causing 7 rotates the maximum stress reaches times The fatigue limit of the bending fatigue test piece was used.
[0122]
　For nitriding parts with excellent rotational bending fatigue strength and wear resistance, assuming application to CVT and camshaft parts, aiming for a wear depth of 10 μm or less and a maximum stress at the fatigue limit of 640 MPa or more. did.
[0123]
　[Test Results] The
　results are shown in Tables 5-1 and 5-2. Test Nos. 1 to 31 have steel components and gas nitriding conditions within the ranges assumed in this example, the compound layer thickness is 5 to 15 μm, the γ′ phase ratio of the compound layer is 80% or more, and the compound is The layer void area ratio was 10% or less. As a result, the hardness of the compound layer was 710 Hv (measurement load: 10 mN), the wear depth was 10 μm or less, and the rotary bending fatigue strength was 640 MPa or more, which were favorable results.
[0124]
　In Test Nos. 32 to 50, the components of the steel and a part of the conditions of the gas nitriding treatment are outside the range assumed in this example, and any one of the thickness of the compound layer, the γ′ phase, and the void area ratio, Or, multiple characteristics did not reach the target value. As a result, the wear resistance or rotary bending fatigue strength did not meet the target. For example, in Test No. 42, the atmosphere in the gas nitriding treatment contains carbon dioxide and was the soft nitriding treatment, so the ratio of the γ′ phase in the formed compound layer is low (the ε phase is formed), In terms of rotational bending fatigue strength, sufficient characteristics could not be obtained.
[0125]
　In addition, although the test number 46 is a comparative example in which the rotating bending fatigue strength does not reach the target value, the target value of the rotating bending fatigue strength in the above-described Example 1 (an example in which gear parts are assumed) has been cleared. Therefore, it is a component suitable as a nitriding component excellent in rotary bending fatigue strength and surface fatigue strength. The steel ac used for the test number 46 is also the steel k of the present invention example of Example 1.
[0126]
　The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit thereof.
claims
[Claim 1]
　% By mass,
　　C: 0.05 to 0.35%,
　　Si: 0.05 to 1.50%,
　　Mn: 0.20 to 2.50%,
　　P: 0.025% or less,
　　S: 0.050 % 　　Or less,
　　Cr: 0.50 to 2.50%,
　　V: 0.05 to 1.30%,
　　Al: 0.050% or less,
N: 0.0250% or less,
　　Mo: 0 to 1.50%,
　　Cu: 0 to 0.50%,
　　Ni: 0 to 0.50%,
　　Nb: 0 to 0.100%,
　　Ti: 0 to 0.050%,
　　B: 0 to 0.0100%,
　　Ca: 0 to 0 0.0100%,
　　Pb:0 to 0.50%,
　　Bi:0 to 0.50%,
　　In:0 to 0.20%, and
　　Sn:0 to 0.100%
, with the balance being Fe and impurities. A steel core part, and
　a nitrogen diffusion layer formed on the steel core part,
　A compound layer mainly containing iron nitride and having a thickness of 5 to 15 μm, formed on the nitrogen diffusion layer, and having
　a depth ranging from the surface to 3 μm in a cross section perpendicular to the surface of the compound layer; Has a void area ratio of 10% or less, and
　X determined based on the contents of C, Mn, Cr, V, and Mo in the steel core portion is
　　X=−2.1×C+0.04×Mn+0.5 When
　defined as ×Cr+1.8×V−1.5×Mo ,
　(i) 0≦X≦0.25, and the area ratio of the γ′ phase of the iron nitride in the compound layer is 50% or more, 80 % or less, or,
　(ii) 0.25 ≦ X ≦ 0.50 and, the area ratio of the gamma 'phase of the iron nitride in the compound layer is 80% or more
nitriding treatment component, characterized in that ..
[Claim 2]
　The nitriding component according to claim 1, wherein 0≦X≦0.25 and an area ratio of the γ′ phase of the iron nitride in the compound layer is 50% or more and 80% or less.
[Claim 3]
　2. The nitriding component according to claim 1, wherein 0.25≦X≦0.50, and the area ratio of the γ′ phase of the iron nitride in the compound layer is 80% or more.