JP6769491B2 - Nitriding parts and their manufacturing methods - Google Patents

Description

The present invention relates to steel parts subjected to gas nitriding treatment, particularly gears having excellent surface fatigue strength and bending fatigue strength, nitriding parts such as CVT sheaves, and a method for manufacturing the same.

Steel parts used in automobiles and various industrial machines are subjected to carburizing quenching, induction hardening, nitriding, and soft nitriding in order to improve mechanical properties such as fatigue strength, wear resistance, and seizure resistance. Surface hardening heat treatment is applied.

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, the 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-sized parts, and are applied to, for example, gears used for transmission parts of automobiles and crankshafts used for engines.

Nitriding treatment is a treatment method in which nitrogen penetrates into the surface of a steel material. The medium used for the nitriding treatment includes gas, salt bath, plasma and the like. Gas nitriding treatment, which is excellent in productivity, is mainly applied to transmission parts of automobiles. By gas nitriding treatment, the steel material surface is thick 10μm or more compound layers (Fe 3 _N layers nitrides are precipitated such) is formed, further, nitrogen diffusion layers on the steel surface layer of the lower 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 high as compared with the base steel. Therefore, the compound layer improves the wear resistance and surface fatigue strength of steel parts at the initial stage of use.

Patent Document 1 discloses a nitriding-treated component having improved bending fatigue resistance by setting the γ'phase ratio in the compound layer to 30 mol% or more.

Patent Document 2 discloses a steel member having a low strain and excellent surface fatigue strength and bending fatigue strength, in which an iron nitride compound layer having a predetermined structure is formed on the steel member.

Patent Document 3 discloses a method for nitriding a low alloy steel having a sufficient surface hardness and a hardened layer depth by suppressing the formation of a compound layer.

In Patent Document 4, a steel member having an iron nitride compound layer having a predetermined structure formed on the surface, having high pitting resistance and bending fatigue strength, and having low strain as compared with carburizing or carburizing nitriding treatment is provided. It is disclosed.

JP-A-2015-117412 Japanese Unexamined Patent Publication No. 2013-221203 International Publication No. 2015/136917 International Publication No. 2013/157579

Since the nitriding component of Patent Document 1 is gas nitrocarburizing using CO ₂ as the atmospheric gas, the surface side of the compound layer tends to be in the ε phase, and it is considered that the bending fatigue strength is not yet sufficient. Further, the nitriding parts of Patent Document 2 are 0.08 to 0.34 for NH ₃ gas, 0.54 to 0.82 for H ₂ gas, and 0.09 to 0.09 for N ₂ gas, regardless of the steel composition. Since it is controlled to be 0.18, the structure and thickness of the compound layer may not be as intended depending on the composition of the steel.

Nitriding method disclosed in Patent Document 3, the high K _N value processing is characterized by thinning the compound layer by two-stage nitriding low K _N value processing. In this method, the compound layer is imparted by the first-stage nitriding, and the compound layer imparted by the second-stage nitriding is decomposed by diffusing N into the cured layer to deepen the effective cured layer. However, a complicated process called two-stage nitriding is required. In addition, the ratio of the γ'phase becomes low, and it tends to be the starting point of pitting and bending fatigue fracture.

In the nitriding treatment of Patent Document 4, since the control range of various gas partial pressures during the treatment is wide, the ratio of the γ'phase may be low or the porosity may be high.

An object of the present invention is to provide a part having excellent rotational bending fatigue strength in addition to surface fatigue strength and a method for manufacturing the same.

The present inventors focused on 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.

As a result, by nitriding the steel whose composition has been adjusted under the nitriding potential control in consideration of the C amount of the dough, the vicinity of the surface has a phase structure mainly composed of the γ'phase, the generation of porous is suppressed, and the compression residue of the surface layer is obtained. It has been found that a nitrided part having excellent surface fatigue strength and rotational bending fatigue strength can be produced by setting the stress to a certain value or more.

The present invention has been further studied based on the above findings, and the gist thereof is as follows.

In terms of mass%, C: 0.05% or more, 0.30% or less, Si: 0.05% or more, 1.5% or less, Mn: 0.2% or more, 2.5% or less, P: 0. 025% or less, S: 0.003% or more, 0.05% or less, Cr: more than 0.5%, 2.0% or less, Al: 0.01% or more, 0.05% or less, N: 0. 003% or more, 0.025% or less, Nb: 0% or more, 0.1% or less, B: 0% or more, 0.01% or less, Mo: 0% or more, less than 0.50%, V: 0% More than 0.50%, Cu: 0% or more, less than 0.50%, Ni: 0% or more, less than 0.50%, and Ti: 0% or more, less than 0.05%, and the balance A component made of steel, which is Fe and impurities, and has a compound layer having a thickness of 3 μm or more and less than 15 μm formed on the surface of the steel and containing iron, nitrogen, and carbon, up to a depth of 5 μm from the surface. The phase structure of the compound layer in the range of is 50% or more in area ratio of γ'phase, the void area ratio is less than 10% in the range from the surface to a depth of 3 μm, and the compressive residual stress on the surface of the compound layer is 500 MPa. Nitriding-treated parts characterized by the above.

According to the present invention, it is possible to obtain a nitrided component having excellent rotational bending fatigue strength in addition to surface fatigue strength.

It is a figure explaining the method of measuring the depth of a compound layer. This is an example of a microstructure photograph of a compound layer and a diffusion layer. It is a figure which shows the appearance that the void is formed in a compound layer. This is an example of a microstructure photograph in which voids are formed in the compound layer. It is a figure which shows the relationship between the nitriding potential, the phase structure of a compound layer, and the rotational bending fatigue strength. It is the shape of a small roller for a roller pitting test used to evaluate the surface fatigue strength. This is the shape of a large roller for the roller pitting test used to evaluate the surface fatigue strength. This is a cylindrical test piece for evaluating the rotational bending bending fatigue strength.

Hereinafter, each requirement of the present invention will be described in detail. First, the chemical composition of the steel material used as the material will be described. Hereinafter, "%" representing the content of each component element and the element concentration on the surface of the component shall mean "mass%".

[C: 0.05% or more, 0.30% or less]
C is an element necessary for ensuring the core hardness of the part. If the C content is less than 0.05%, the core strength becomes too low, so that the surface fatigue strength and the bending fatigue strength are greatly reduced. Further, when the C content exceeds 0.30%, the compound layer thickness becomes large, and the surface fatigue strength and bending resistance are greatly lowered. The preferred range of C content is 0.08 to 0.25%.

[Si: 0.05% or more, 1.5% or less]
Si increases the hardness of the core by strengthening the solid solution. In addition, the temper softening resistance is increased, and the surface fatigue strength of the component surface, which becomes hot under wear conditions, is increased. In order to exert these effects, 0.05% or more is contained. On the other hand, if the Si content exceeds 1.5%, the strength of steel bars, wire rods and after hot forging becomes too high, so that the machinability is greatly reduced. The preferred range of Si content is 0.08 to 1.3%.

[Mn: 0.2% or more, 2.5% or less]
Mn increases the hardness of the core portion by strengthening the solid solution. Further, Mn forms fine nitrides (Mn ₃ N ₂ ) in the hardened layer during the nitriding treatment, and improves wear resistance and surface fatigue strength by strengthening precipitation. In order to obtain these effects, Mn needs to be 0.2% or more. On the other hand, if the Mn content exceeds 2.5%, not only the effect of increasing the surface fatigue strength is saturated, but also the hardness of the steel bar, wire rod and hot forged material becomes too high, so that the cutting process is performed. The sex is greatly reduced. The preferred range of Mn content is 0.4 to 2.3%.

[P: 0.025% or less]
Since P is an impurity and segregates at grain boundaries to embrittle parts, it is preferable that the content is small. If the P content exceeds 0.025%, the bending straightness and bending fatigue strength may decrease. The preferable upper limit of the P content for preventing a decrease in bending fatigue strength is 0.018%. It is difficult to make the content completely 0, and the practical lower limit is 0.001%.

[S: 0.003% or more, 0.05% or less]
S combines with Mn to form MnS and improves machinability. In order to obtain this effect, S needs to be 0.003% or more. However, when the S content exceeds 0.05%, coarse MnS is likely to be generated, and the surface fatigue strength and bending fatigue strength are greatly reduced. The preferred range of S content is 0.005 to 0.03%.

[Cr: more than 0.5%, less than 2.0%]
Cr forms fine nitrides (CrN) in the hardened layer during the nitriding treatment, and improves surface fatigue strength and bending fatigue strength by precipitation strengthening. In order to obtain these effects, Cr needs to be more than 0.5%. On the other hand, if the Cr content exceeds 2.0%, not only the effect of improving the surface fatigue strength is saturated, but also the hardness of the steel bar, wire rod and hot forged material becomes too high, so that cutting is performed. Workability is significantly reduced. The preferred range of Cr content is 0.7 to 1.8%.

[Al: 0.01% or more, 0.05% or less]
Al is a deoxidizing element, and 0.01% or more is required for sufficient deoxidation. On the other hand, Al tends to form hard oxide-based inclusions, and when the Al content exceeds 0.05%, the bending fatigue strength is significantly reduced, and even if other requirements are satisfied, desired bending is achieved. Fatigue strength cannot be obtained. The preferred range of Al content is 0.02 to 0.04%.

[N: 0.003% or more, 0.025% or less]
N combines with Al and V to form AlN and VN. AlN and VN have the effect of refining the structure of the steel material before the nitriding treatment by the pinning action of the austenite grains and reducing the variation in the mechanical properties of the nitriding treated parts. If the N content is less than 0.003%, this effect is difficult to obtain. On the other hand, if the N content exceeds 0.025%, coarse AlN is likely to be formed, so that the above effect is difficult to obtain. The preferred range of N content is 0.005 to 0.020%.

The chemical composition of steel used as a material for the nitriding parts of the present invention contains the above elements, and the balance is Fe and unavoidable impurities. Inevitable impurities are components contained in raw materials or mixed in during the manufacturing process, and are components not intentionally contained in steel.

However, the steel used as the material for the nitrided parts of the present invention may contain the following elements instead of a part of Fe.

[Nb: 0% or more, 0.1% or less]
Nb combines with C and N to form NbC and NbN. By the pinning action of NbC and NbN, coarsening of austenite grains is suppressed, the structure of the steel material before the nitriding treatment is made finer, and there is an effect of reducing variations in the mechanical properties of the nitriding treatment parts. This effect can be obtained by adding a small amount of Nb, but in order to obtain the effect more reliably, it is preferable that Nb is 0.01% or more. When the content of Nb exceeds 0.1%, coarse NbC and NbN are likely to be formed, so that the above effect is difficult to obtain.

[B: 0 or more, 0.01% or less]
B has the effect of suppressing the grain boundary segregation of P and improving the toughness. In addition, it combines with N to form BN and improves machinability. These effects can be obtained by adding a small amount of Nb, but in order to obtain the effects more reliably, B is preferably 0.0005% or more. If the B content exceeds 0.01%, not only the above effect is saturated, but also a large amount of BN segregates, which may cause cracks in the steel material.

[Mo: 0% or more, less than 0.50%]
Mo forms fine nitrides (Mo ₂ N) in the hardened layer at the time of nitriding, and improves surface fatigue strength and bending fatigue strength by precipitation strengthening. In addition, Mo exerts an aging hardening action at the time of nitriding to improve the hardness of the core portion. The Mo content for obtaining these effects is preferably 0.01% or more. On the other hand, when the Mo content is 0.50% or more, the hardness of the steel bars and wires used as raw materials and after hot forging becomes too high, so that the machinability is remarkably lowered and the alloy cost is increased. The preferable upper limit of the Mo content for ensuring machinability is less than 0.40%.

[V: 0% or more, less than 0.50%]
V forms fine nitrides (VN) during nitriding and soft nitriding, and improves surface fatigue strength and bending fatigue strength by precipitation strengthening, and increases the core hardness of parts. It also has the effect of microstructure miniaturization. In order to obtain these effects, V is preferably 0.01% or more. On the other hand, when the V content is 0.50% or more, the hardness of the steel bars and wires used as raw materials and after hot forging becomes too high, so that the machinability is remarkably lowered and the alloy cost is increased. The preferable range of the V content for ensuring machinability is less than 0.40%.

[Cu: 0% or more, less than 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 effect of strengthening the solid solution of Cu, the content is preferably 0.01% or more. On the other hand, when the Cu content is 0.50% or more, the hardness of the steel bars and wires used as raw materials and after hot forging becomes too high, so that the machinability is remarkably lowered and the hot ductility is lowered. , It causes surface scratches during hot rolling and hot forging. The preferred range of Cu content for maintaining hot ductility is less than 0.40%.

[Ni: 0% or more, less than 0.50%]
Ni improves the core hardness and surface hardness by solid solution strengthening. In order to exert the effect of strengthening the solid solution of Ni, the content is preferably 0.01% or more. On the other hand, when the Ni content is 0.50% or more, the hardness of steel bars, wire rods and after hot forging becomes too high, so that the machinability is remarkably lowered and the alloy cost is increased. The preferred range of Ni content for obtaining sufficient machinability is less than 0.40%.

[Ti: 0% or more, less than 0.05%]
Ti combines with N to form TiN, which improves core hardness and surface hardness. In order to obtain this effect, Ti is preferably 0.005% or more. On the other hand, when the Ti content is 0.05% or more, the effect of improving the core hardness and the surface hardness is saturated, and the alloy cost increases. The preferred range of Ti content is 0.007 to less than 0.04%.

Next, the compound layer of the nitrided component of the present invention will be described.

[Thickness of compound layer: 3 μm or more and less than 15 μm]
The compound layer is a layer of iron nitride formed by nitriding treatment, and its thickness affects the surface fatigue strength and bending strength of the nitriding treated part. If the compound layer is too thick, it tends to be a starting point of fracture due to pitting or bending. If the compound layer is too thin, the residual stress on the surface cannot be sufficiently obtained, and the surface fatigue strength and the bending strength are lowered. In the nitrided component of the present invention, the thickness of the compound layer is 3 μm or more and less than 15 μm from the viewpoint of surface fatigue strength and bending strength.

The thickness of the compound layer is measured by polishing the vertical cross section of the test material after gas nitriding treatment, etching it, and observing it with an optical microscope. Etching is performed with a 3% nital solution for 20-30 seconds. The compound layer is present on the surface of the low alloy steel and is observed as a white uncorroded layer. Observe 5 visual fields (field of view area: 2.2 × 10 ⁴ μm ² ) of the tissue photograph taken at 500 times with an optical microscope. In each field of view, 4 points are measured every 30 μm in the horizontal direction. The average value of the measured 20 points is defined as the compound thickness (μm). FIG. 1 shows an outline of the measurement method, and FIG. 2 shows an example of a microstructure photograph of the compound layer and the diffusion layer.

[Γ'phase ratio of compound layer of surface ~ 5 μm: 50% or more]
When the ratio of the γ'phase is low and the ratio of the ε phase is high in the compound layer having a surface of about 5 μm, it is likely to be a starting point of pitting and bending fatigue fracture. This is because the fracture toughness value of the ε phase is lower than that of the γ'phase. Further, when the phase near the surface is the γ'phase, it becomes easier to introduce the compressive residual stress described later into the surface as compared with the case where it is the ε phase, and the fatigue strength can be improved.

The γ'phase ratio in the compound layer is determined by backscattered electron diffraction (Electron Backscatter Diffraction: EBSD). Specifically, EBSD measurement is performed on an area of 150 μm ² from the outermost surface of the compound layer to a depth of 5 μm, and an analysis diagram for discriminating between the γ'phase and the ε phase is created. Then, for the obtained EBSD analysis image, the area ratio of the γ'phase is obtained by using an image processing application, and this is defined as the γ'phase ratio (%). In the EBSD measurement, it is appropriate to measure about 10 fields of view at a magnification of about 4000 times.

The γ'phase ratio described above means the ratio of the "γ'phase" of the "compound layer" at a depth of ~ 5 μm on the surface. That is, when the thickness of the compound layer is less than 5 μm from the surface, the γ'phase ratio in the region corresponding to the thickness of the compound layer is calculated. As an example, if the thickness of the compound is 3 μm from the surface, the ratio of the γ'phase of the compound layer having a depth of the surface to 3 μm is the γ'phase ratio.

The γ'phase ratio is preferably 60% or more, more preferably 65% or more, still more preferably 70% or more.

A method of obtaining the γ'phase ratio by using X-ray diffraction can also be considered. However, in the measurement by X-ray diffraction, the measurement region becomes ambiguous, and an accurate γ'phase ratio cannot be obtained. Therefore, the γ'phase ratio in the compound layer in the present invention is determined by EBSD.

[Void area ratio of compound layer with surface to 3 μm: less than 10%]
If voids are present in the compound layer having a surface of 3 μm, stress concentration occurs, which is a starting point for pitting and bending fatigue fracture. Therefore, the void area ratio needs to be less than 10%.

Voids, the smaller the steel surface binding by the base material, from the energy stable location such as grain boundaries, N ₂ gas is formed by elimination from the surface of the steel material along the grain boundaries. The generation of N ₂ is more likely to occur as the nitriding potential K _N, which will be described later, is higher. This is because the phase transformation of bcc → γ'→ ε occurs as K _N increases, and the solid solution amount of N ₂ is larger in the ε phase than in the γ'phase, so the ε phase is the N ₂ gas. This is because it is easy to generate. FIG. 3 shows an outline in which voids are formed in the compound layer, and FIG. 4 shows a microstructure photograph in which voids are formed.

The void area ratio can be measured by observation with an optical microscope. Specifically, the depth of the surface to 3 μm in the cross section of the test material is measured in 5 visual fields (visual field area: 5.6 × 10 ³ μm ² ) at a magnification of 1000 times, and ³ μm from the outermost surface for each visual field. The ratio of voids in the depth range is defined as the void area ratio.

The void area ratio is preferably less than 5%, more preferably less than 2%, still more preferably less than 1%, and most preferably 0.

[Compressive residual stress on the surface of the compound layer: 500 MPa or more]
In the nitriding-treated component of the present invention, the surface of the steel is hardened by the nitriding treatment, and compressive residual stress is introduced into the surface layer portion of the steel, so that the fatigue strength and wear resistance of the component are improved. The nitriding component of the present invention has excellent surface fatigue strength and rotational bending fatigue strength by improving the compound layer as described above and further introducing a compressive residual stress of 500 MPa or more on the surface. A manufacturing method for introducing such compressive residual stress on the surface of a component will be described later.

Next, an example of the method for manufacturing the nitrided component of the present invention will be described.

In the method for manufacturing a nitrided part of the present invention, a steel material having the above-mentioned components is subjected to gas nitriding treatment. 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.

[Treatment temperature: 550 to 620 ° C]
The temperature of the gas nitriding treatment (nitriding treatment temperature) mainly correlates with the diffusion rate of nitrogen and affects the surface hardness and the depth of the hardened layer. If the nitriding treatment temperature is too low, the diffusion rate of nitrogen is slow, the surface hardness is low, and the cured layer depth is shallow. On the other hand, if it exceeds nitriding temperature the _C1 point A, 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 The shallowness becomes shallow. Therefore, in the present embodiment, the nitriding treatment temperature is 550 to 620 ° C. around the ferrite temperature range. In this case, it is possible to suppress the decrease in surface hardness and the shallow depth of the hardened layer.

[Processing time for the entire gas nitriding process: 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), correlates 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. To exert. If the treatment time is too short, the surface hardness becomes low and the cured layer depth becomes shallow. On the other hand, if the treatment time is too long, denitrification and decarburization will occur and the surface hardness of the steel will decrease. If the processing time is too long, the manufacturing cost will be higher. Therefore, the processing time of the entire nitriding treatment is 1.5 to 10 hours.

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 . The preferred atmosphere is NH ₃ , H ₂ and N ₂ totaling 99.5% (volume%) or more.

When gas nitrocarburizing treatment is performed in an atmosphere containing about several% of carbon monoxide and carbon dioxide, the ε phase having a high solid solution limit of C is preferentially generated. Since C can hardly be dissolved in the γ'layer, the compound layer becomes an ε single phase when soft nitriding treatment is performed. Furthermore, since the growth rate of the ε phase is faster than that of the γ'phase, the compound layer is formed thicker than necessary in gas nitrocarburizing in which the ε phase is stably formed. Therefore, in the present invention, it is necessary to perform the gas nitriding treatment in which the nitriding potential _KN is controlled as described later, instead of the gas nitrocarburizing treatment.

[Gas conditions for nitriding]
In the nitriding treatment method of the present invention, the nitriding treatment is performed under a nitriding potential controlled in consideration of the C amount of the dough. As a result, the phase structure of the compound layer at a depth of ~ 5 μm is set to γ'phase ratio of 50% or more, the void area ratio at a depth of ~ 3 μm is set to less than 10%, and the compressive residual stress on the surface of the compound layer is set to 500 MPa. It can be the above.

The nitriding potential K _N of the gas nitriding process is defined by the following equation.

K _N (atm- ^{1 / 2} ) = (NH ₃ partial pressure (atm)) / [(H ₂ partial pressure (atm)) ^3/2 ]

The partial pressure of NH ₃ and H _{2 in} the atmosphere of gas nitriding can be controlled by adjusting the flow rate of the gas. In order to form a compound layer by nitriding treatment, K _N at the time of gas nitriding treatment needs to be a certain value or more, but as described above, if K _N becomes too high, N ₂ gas is likely to be generated ε. The proportion of phases increases and the voids increase. Therefore, it is important to set the K _N condition to suppress the generation of voids.

As a result of the study by the present inventors, it was found that the nitriding potential of the gas nitriding treatment affects the phase structure of the compound layer and the rotational bending fatigue strength of the nitriding treated part, and the optimum nitriding potential is determined by the C content of the steel. I found it.

Specifically, when the C content (mass%) of the steel is (mass% C), the nitriding potential during the gas nitriding process is always 0.15 ≤ K _N ≤ -0.17 during the gas nitriding process. It was found that when × ln (mass% C) +0.20 is satisfied, the phase structure of the compound layer becomes a γ'phase ratio of 50% or more, and the nitriding part has high rotational bending fatigue strength and surface fatigue strength. ..

Even if the average nitriding potential of the gas nitriding treatment satisfies the above equation, the γ'phase ratio in the compound layer does not become 50% or more when the nitriding potential value that does not satisfy the above equation is taken even for a while.

FIG. 5 shows the results of investigating the relationship between the nitriding potential, the γ'ratio of the compound layer, and the rotational bending fatigue strength. FIG. 5 shows the steel j (Table 1) of the embodiment described later.

As described above, in this nitriding treatment method, the gas nitriding treatment is carried out under the nitriding potential K _N according to the amount of C of the steel as the dough. This makes it possible to stably impart the γ'phase to the surface of the steel, and has excellent surface fatigue strength, rotational bending fatigue strength, preferably surface fatigue strength of 2000 MPa or more, and rotational bending fatigue strength of 600 MPa or more. Nitrided parts can be obtained.

Steels a to aa having the chemical components shown in Table 1 were melted in a 50 kg vacuum melting furnace to produce molten steel, and molten steel was cast to produce an ingot. Reference numerals a to s in Table 1 are steels having the chemical components specified in the present invention. On the other hand, steels t to aa are comparative steels having at least one element or more that deviates from the chemical composition specified in the present invention.

This ingot was hot forged to form a round bar with a diameter of 40 mm. Subsequently, after each round bar was annealed, a small roller for a roller pitting test for evaluating the surface fatigue strength shown in FIG. 6 and a large roller shown in FIG. 7 were produced by cutting. Further, a cylindrical test piece for evaluating the bending fatigue resistance shown in FIG. 8 was prepared.

The collected test pieces were subjected to gas nitriding treatment under the following conditions. The test piece was charged into a gas nitriding furnace, NH ₃ , H ₂ , and N ₂ gases were introduced into the furnace, and the nitriding treatment was carried out under the conditions shown in Table 2. However, Test No. 34 was a gas nitrocarburizing treatment in which 3% of CO ₂ gas was added in a volume fraction to the atmosphere. Further, the test number 35 was a two-stage nitriding treatment in which the nitriding conditions were changed between the first half and the second half. Test number 36 corresponds to Example 16 in Patent Document 3. The test piece after the gas nitriding treatment was oil-cooled using oil at 80 ° C.

The H ₂ partial pressure in the atmosphere was measured using a heat conduction type H ₂ sensor directly mounted on the gas nitriding furnace body. The difference in thermal conductivity between the standard gas and the measured gas was converted into gas concentration and measured. H ₂ partial pressure during the gas nitriding treatment was continuously measured.

The NH ₃ partial pressure was measured by attaching a manual glass tube type NH ₃ analyzer outside the furnace.

The partial pressure of residual NH ₃ was measured every 10 minutes, and at the same time, the nitriding potential K _N was calculated, and the NH ₃ flow rate and N ₂ flow rate were adjusted so as to converge to the target value. The nitriding potential K _N was calculated every 10 minutes when the NH ₃ partial pressure was measured, and the NH ₃ flow rate and the N ₂ flow rate were adjusted so as to converge to the target value.

[Measurement of compound layer thickness and void area ratio]
The cross section of the small roller after the gas nitriding treatment in the direction perpendicular to the length direction was mirror-polished and etched. The etched cross section was observed using a scanning electron microscope (SEM), the thickness of the compound layer was measured, and the presence or absence of voids in the surface layer was confirmed. Etching was performed with a 3% nital solution for 20-30 seconds.

The compound layer can be confirmed as a white uncorroded layer existing on the surface layer. The compound layer was observed from 10 visual fields (visual field area: 6.6 × 10 ² μm ² ) of the tissue photograph taken at 4000 times, 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 thickness (μm).

Similarly, the ratio of the total area of voids (void area ratio, unit is%) to the area 90 μm ² in the range of 3 μm depth from the outermost surface was quantified by an image processing application. Then, the average value of the measured 10 visual fields was defined as the void area ratio (%). Even when the compound layer was less than 3 μm, the measurement target was similarly up to a depth of 3 μm from the surface.

[Measurement of γ'phase ratio]
The γ'phase ratio in the compound layer was determined by backscattered electron diffraction (Electron Backscatter Diffraction: EBSD). EBSD measurement was performed on an area of 150 μm2 from the outermost surface of the compound layer to a depth of 5 μm, an analysis diagram for discriminating between the γ'phase and the ε phase was created, and the obtained EBSD analysis image was γ'using an image processing application. The phase ratio (%) was determined. In the EBSD measurement, 10 visual fields were measured at a magnification of 4000 times.

Then, the average value of the measured γ'phase ratios of the 10 visual fields was defined as the γ'phase ratio (%). When the compound layer was less than 5 μm, the γ'phase ratio in the region corresponding to the thickness of the compound layer was calculated.

[Compound layer residual stress]
Residual stress of γ'phase, ε phase and matrix (matrix) σ _γ' , σ _ε , σ under the conditions of Table 3 using a micro part X-ray residual stress measuring device for the small roller contact part after nitriding. _m was measured. Further, using the area ratios V _γ' , V _ε , and V _m of the _γ'phase , the ε phase, and the mother layer in the 90 μm ² area in the depth range of 3 μm from the outermost surface, which are determined by EBSD, the following equation is used. The residual stress σ _c obtained in the above was taken as the residual stress on the surface.

_{σ c = V γ 'σ γ} ' + V ε σ ε + V m σ m

[Surface fatigue strength evaluation test]
The small rollers for the roller pitting test were subjected to finishing of the grip portion for the purpose of removing heat treatment strain, and then subjected to each roller pitting test piece. The shape after finishing is shown in FIG.

The roller pitting test was carried out under the conditions shown in Table 4 with 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 unit of dimensions in FIGS. 2 and 3 is "mm". The large roller for the roller pitting test uses steel that meets the JIS SCM420 standard, and is used in the general manufacturing process, that is, "normalizing-> test piece processing-> eutectoid carburizing by gas carburizing furnace-> low-temperature tempering-> polishing. The Vickers hardness Hv is 740 to 760 at a position 0.05 mm from the surface, that is, a position at a depth of 0.05 mm, and the Vickers hardness Hv is a depth of 550 or more. Was in the range of 0.8 to 1.0 mm.

Table 4 shows the test conditions for which the surface fatigue strength was evaluated. The number of test censoring was 2 × 10 ⁷ times, which indicates the source of fatigue of general steel, and the maximum surface pressure reached 2 × 10 ⁷ times without pitting in the small roller test piece was set as the small roller test piece. It was set as the fatigue limit.

The occurrence of pitting was detected by a vibrometer provided in the testing machine, and after the vibration was generated, the rotation of both the small roller test piece and the large roller test piece was stopped, and the pitting occurrence and the rotation speed were confirmed.

In the parts of the present invention, the target is that the maximum surface pressure at the fatigue limit is 2000 MPa or more.

[Rotating bending fatigue strength]
The Ono-type rotary bending fatigue test was performed on the cylindrical test piece subjected to the gas nitriding treatment. Rpm 3000 rpm, test abort count is set to 1 × 10 ⁷ times showing the fatigue limit of general steel, the rotary bending fatigue test piece, the rotation of the maximum stress fracture reaches 1 × 10 ⁷ times without causing The fatigue limit of the bending fatigue test piece was set.

In the parts of the present invention, it was aimed that the maximum stress in the fatigue limit is 600 MPa or more.

[Test results]
The results are shown in Table 2. In test numbers 1 to 25, the steel components and the conditions for gas nitriding treatment are within the scope of the present invention, the compound thickness is 3 to 15 μm, the γ'layer ratio of the compound layer is 50% or more, and the compound layer void area ratio. It was less than 10%, and the compressive residual stress of the compound layer was 500 MPa or more. As a result, good results were obtained with a surface fatigue strength of 2000 MPa or more and a rotary bending fatigue strength of 600 MPa or more.

In Test No. 26, the nitriding temperature was too high, and as a result, the γ'phase ratio of the compound layer was low, the void area ratio was large, and the residual stress was low, so that the surface fatigue strength and the rotational bending fatigue strength were low.

In Test No. 27, the nitriding temperature was too low, the compound layer was not formed, and the residual stress on the surface was also low, so that the surface fatigue strength and the rotational bending fatigue strength were low.

In Test No. 28, the nitriding time was too long, the void area ratio became large, and the residual stress on the surface was released and lowered accordingly, so that the rotational bending fatigue strength became low.

In Test No. 29, the nitriding time was too short, a sufficient compound layer thickness could not be obtained, and the residual stress on the surface was low, so that the surface fatigue strength and the rotational bending fatigue strength were low.

In Test No. 30, the lower limit of the nitriding potential was low, a sufficient compound layer thickness could not be obtained, and the residual stress on the surface was low, so that the surface fatigue strength and the rotational bending fatigue strength were low.

In Test No. 31, the lower limit of the nitriding potential was too low, the compound layer was not formed, and the residual stress on the surface was low, so that the surface fatigue strength and the rotational bending fatigue strength were low.

In test number 32, the upper limit of the nitriding potential was high, the void area ratio was increased, and the rotational bending fatigue strength was low.

In Test No. 33, the upper and lower limits of the nitriding potential were not appropriate, the compound layer thickness became thicker, and the void area ratio increased, so that the surface fatigue strength and the rotational bending fatigue strength became low.

Test number 34 was a soft nitriding treatment, in which a γ'phase was hardly generated on the surface and the residual stress was low, so that the surface fatigue strength and the rotational bending fatigue strength were low.

In test number 35, the average K _N was appropriate and the γ'phase ratio was high, but the upper limit of K _N during the nitriding treatment was high, and the void area ratio increased.

In test number 36, the amount of C in the steel was too high and the compound layer thickness became thick, so that the surface fatigue strength became low.

In test number 37, the amount of C in the steel was too low, and sufficient core strength could not be obtained. Therefore, the steel was broken at an early stage starting from the mother layer.

In Test No. 38, the amount of Si in the steel was too low, and sufficient core hardness could not be obtained. Therefore, the steel was broken at an early stage starting from the mother layer.

In Test No. 39, the amount of Mn of the steel was too low, and sufficient hardness of the hardened layer and hardness of the core could not be obtained. Therefore, the steel was broken at an early stage starting from the mother layer.

In test number 40, the amounts of P and S in the steel were too high, and the steel was broken at an early stage due to grain boundary segregation of P and formation of coarse MnS.

In Test No. 41, the amount of Cr in the steel was too low, and sufficient diffusion layer hardness and core hardness could not be obtained. Therefore, the steel was broken at an early stage starting from the mother layer.

In test number 42, the Al content of the steel was too high, oxide-based inclusions were formed, and the steel fractured at an early stage starting from the matrix layer.

In test number 43, the C amount, Mn amount, and Cr amount of the steel were low, the upper limit of the nitriding potential was high, and the void area ratio was increased, so that the surface fatigue strength and the rotational bending fatigue strength were low.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the spirit of the present invention.

Claims (1)

By mass%
C: 0.05% or more, 0.30% or less,
Si: 0.05% or more, 1.5% or less,
Mn: 0.2% or more, 2.5% or less,
P: 0.025% or less,
S: 0.003% or more, 0.05% or less,
Cr: Over 0.5%, 2.0% or less,
Al: 0.01% or more, 0.05% or less,
N: 0.003% or more, 0.025% or less,
Nb: 0% or more, 0.1% or less,
B: 0% or more, 0.01% or less,
Mo: 0% or more, less than 0.50%,
V: 0% or more, less than 0.50%,
Cu: 0% or more, less than 0.50%,
Parts made of steel containing Ni: 0% or more and less than 0.50%, and Ti: 0% or more and less than 0.05%, with the balance being Fe and impurities.
It has a compound layer having a thickness of 3 μm or more and less than 15 μm, which is formed on the surface of a steel material and contains iron, nitrogen and carbon.
The phase structure in the compound layer in the range from the surface to a depth of 5 μm contains γ'phase in an area ratio of 50% or more.
The void area ratio is less than 10% in the range from the surface to a depth of 3 μm.
A nitriding component characterized in that the compressive residual stress on the surface of the compound layer is 500 MPa or more.