Technical field
[0001]
　The present invention is a steel part that has been subjected to gas nitriding treatment, in particular the surface fatigue strength and bending gears excellent in fatigue strength, the nitriding treatment components such as CVT sheaves, and a method of manufacturing the same.
BACKGROUND
[0002]
The steel parts used in automobiles and various industrial machines, fatigue strength, wear resistance, and to improve the mechanical properties such as galling resistance, carburizing and quenching, induction hardening, nitriding, and soft nitriding such as reduction surface hardening heat treatment is performed.
[0003]
　Nitriding and nitrocarburizing processes, A 1 carried out at points below the ferrite region, since there is no phase transformation during processing, it is possible to reduce the heat treatment distortion. Therefore, nitriding and nitrocarburizing treatment is applied high often used for components and large components with dimensional accuracy, and gears used for example in motor vehicle transmission components, to a crankshaft used in an engine.
[0004]
　Nitriding process is a processing method for entering nitrogen into the steel material surface. The medium used in the nitriding treatment, gas, salt bath, plasma and the like. The automotive transmission parts, mainly, gas nitriding is applied is excellent in productivity. By gas nitriding treatment, the steel material surface has a thickness of 10μm or more compound layers (Fe 3 are N layers nitrides are precipitated such) is formed, further, nitrogen diffusion layers on the steel surface layer of the lower compound layer cured layer is formed is. Compound layer mainly Fe 2 ~ 3 N (epsilon) and Fe 4 consists of N (γ '), the hardness of the compound layer is very high compared to steel as a base material. Therefore, compound layer, in the initial stage of use, to improve the wear resistance and surface fatigue strength of the steel parts.
[0005]
　Patent Document 1, a gamma 'phase fraction of the compound layer by a 30 mol% or more, nitrided parts with improved resistance to bending fatigue strength is disclosed.
[0006]
　Patent Document 2, the iron nitride compound layer having a predetermined structure to generate the steel member, the steel member having a bending fatigue strength and low distortion and excellent surface fatigue strength is disclosed.
[0007]
　Patent Document 3, by suppressing the formation of the compound layer, nitriding processing method of the low alloy steel is disclosed having a sufficient surface hardness and hardening depth.
[0008]
　Patent Document 4, the iron nitride compound layer having a predetermined structure is formed on the surface, has a bending fatigue strength and high resistance to pitting resistance, the steel member is a low distortion as compared to the carburized and carbonitrided It has been disclosed.
CITATION
Patent Document
[0009]
Patent Document 1: JP 2015-117412 Patent Publication
Patent Document 2: JP 2013-221203 Patent Publication
Patent Document 3: WO 2015/136917 Patent
Patent Document 4: WO 2013/157579
Summary of the Invention
Problems that the Invention is to Solve
[0010]
　Nitrided component in Patent Document 1, CO atmosphere gas 2 from being a gas soft using, since the surface of the compound layer easily becomes ε phase, bending fatigue strength is not believed to be still insufficient. Further, nitriding part of Patent Document 2, regardless of the composition of steel, NH 3 gas is 0.08 ~ 0.34, H 2 gas is 0.54 ~ 0.82, N 2 gas is 0.09 to due to the controlled so as to be 0.18, there is a possibility that the structure and thickness of the compound layer with the components of the steel is not as intended.
[0011]
　Nitriding method disclosed in Patent Document 3, the high K N value processing, low K N is characterized by thinning the compound layer by two-stage nitriding the value processing. This method, by decomposing by diffusing Grant compound layer of a nitride of the first stage, a compound layer which is applied by nitriding of the second stage, the N to hardened layer, are deeply the effective hardened layer but it requires a complicated process of two-stage nitriding. The ratio of gamma 'phase is low, it tends to be a starting point of pitting and bending fatigue fracture.
[0012]
　Nitriding of Patent Document 4, since the control range of the various gas partial pressure during the treatment is wide, or a ratio becomes lower in gamma 'phase, there is a possibility that the porosity increases.
[0013]
　An object of the present invention is to provide a superior part and a manufacturing method thereof rotating bending fatigue strength in addition to the surface fatigue strength.
Means for Solving the Problems
[0014]
　The present inventors focused on the form of the compound layer formed on the surface of the steel material by nitriding treatment, and investigated the relationship between fatigue strength.
[0015]
　As a result, the steel having an adjusted composition, by nitriding under nitriding potential control considering the amount of C fabric, the vicinity of the surface as the phase structure of the gamma 'phase mainly suppressing the occurrence of porous, surface layer of compressive residual by the stress a constant value or more has been found that excellent surface fatigue strength, and a nitride component having a rotary bending fatigue strength can be produced.
[0016]
　The present invention is based on the above findings, which has been made repeatedly further studies, its gist the following.
[0017]
　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: 0.5% greater, 2.0%, Al: 0.01% or more, 0.05% or less, N: 0. 003% or more, 0.025% or less, Nb: 0% to 0.1% or less, B: 0% to 0.01% or less, Mo: 0% to less than 0.50%, V: 0% or more and less than 0.50%, Cu: 0% to less than 0.50%, Ni: 0% to less than 0.50%, and Ti: 0% or more, and contains less than 0.05%, the balance being a Fe and part steel was used as a raw material as an impurity, has formed on the surface of the steel material, iron, nitrogen and a compound layer of a thickness less than 3μm or 15μm containing carbon, Table The phase structure is gamma 'phase in the compound layer ranging from the surface to a depth of 5μm contains an area ratio 50% or more, the void area ratio in the range from the surface to a depth of 3μm is less than 10%, the compound layer surface nitrided component compressive residual stress is equal to or not less than 500 MPa.
Effect of the invention
[0018]
　According to the present invention, it is possible to obtain an excellent nitrided parts rotating bending fatigue strength in addition to the surface fatigue strength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019]
FIG. 1 is a diagram for explaining a method of measuring the depth of the compound layer.
It is an example of a structure photograph of FIG. 2 compound layer and the diffusion layer.
3 is a diagram showing a state in which voids are formed on the compound layer.
Is an example of a structure photograph of voids is formed in FIG. 4 compound layer.
It is a graph showing a relationship [5] and nitriding potential phase structure of the compound layer, and a rotating bending fatigue strength.
In the form of small roller for roller pitting test used to evaluate the FIG. 6 surface fatigue strength.
In the form of large rollers for roller pitting test used to evaluate the FIG. 7 surface fatigue strength.
8 is a cylindrical test piece for evaluating the rotary bending flexural fatigue strength.
DESCRIPTION OF THE INVENTION
[0020]
　It will be described in detail below each requirement of the present invention. First, a description will be given chemical composition of the steel as a material. Hereinafter, "%" representing the element concentration in the content and the component surface of the component element means "mass%".
[0021]
　[C: 0.05% or more, 0.30% or less]
　is C, is an element necessary for ensuring the core hardness of the component. In less than 0.05% content and C, since the core strength is too low, the surface fatigue strength and bending fatigue strength is significantly decreased. When the content of C exceeds 0.30%, the greater the compound layer thickness, surface fatigue strength and bending resistance is significantly decreased. The preferred range of C content is 0.08 to 0.25%.
[0022]
　[Si: 0.05% or more, 1.5% or
　less] Si is by solid solution strengthening, enhancing the core hardness. Also, increasing the temper softening resistance, increasing the surface fatigue strength of the part surface having a high temperature in wear conditions. To exhibit these effects, it is contained more than 0.05%. On the other hand, when the content of Si exceeds 1.5%, steel bars, since the strength after wire or hot forging is too high, cutting resistance is greatly reduced. A preferred range of Si content is 0.08 to 1.3%.
[0023]
　[Mn: 0.2% or more, 2.5% or
　less] Mn is by solid solution strengthening, enhancing the core hardness. Furthermore, Mn, at the time of nitriding treatment, fine nitrides in the cured layer (Mn 3 N 2 to form a), To obtain these effects, Mn is required more than 0.2%. On the other hand, when the content of Mn exceeds 2.5%, not only the effect of increasing the surface fatigue strength is saturated, since the steel bar as a material, the wire material and the hardness after hot forging too high, cutting sex is greatly reduced. A preferred range of Mn content is 0.4 to 2.3%.
[0024]
　[P: 0.025% or less]
　P is an impurity, since the brittle parts by grain boundary segregation, the content is preferably small. When the content of P exceeds 0.025%, the bending straightening property and bending fatigue strength is lowered. The preferable upper limit of the P content to prevent the deterioration of the bending fatigue strength is 0.018%. The complete content 0 and it is difficult to realistic lower limit is 0.001%.
[0025]
　[S: 0.003% or more, 0.05% or less]
　S forms MnS, in combination with Mn, to improve the machinability. To obtain this effect, S is a required 0.003% or more. However, when the content of S exceeds 0.05%, it becomes easy to generate a coarse MnS, surface fatigue strength and bending fatigue strength is significantly decreased. A preferred range of S content is 0.005 to 0.03%.
[0026]
　[Cr: 0.5% greater,
　2.0%] Cr is during nitriding, fine nitride (CrN) is formed in the hardened layer, to improve the surface fatigue strength and bending fatigue strength by precipitation strengthening . To obtain these effects, Cr is required than 0.5%. Meanwhile, since the content of Cr exceeds 2.0%, not only the effect of improving the surface fatigue strength is saturated, steel bar as the material, hardness after wire or hot forging is too high, cutting processability is remarkably lowered. The preferred range of Cr content is 0.7 to 1.8%.
[0027]
　: [Al 0.01% or more, 0.05% or
　less] are Al, a deoxidizing element, it is required 0.01% or more for sufficient deoxidation. On the other hand, Al is liable to form a hard oxide inclusions, the content of Al exceeds 0.05%, bending decrease in fatigue strength is significantly desired bending even meet the other requirements fatigue strength can not be obtained. A preferred range of the Al content is 0.02 to 0.04%.
[0028]
　[N: 0.003% or more, 0.025% or less]
　N is, Al, combined with V to form AlN, the VN. AlN, VN by pinning action of the austenite grains, the steel of tissue prior to nitriding treatment finer and have the effect of reducing the variation in mechanical properties of the nitrided components. This effect is in a content less than 0.003% of N is inaccessible. On the other hand, when the content of N exceeds 0.025%, the order likely to be coarse AlN is formed, the above effect becomes difficult to obtain. A preferred range of N content is 0.005 to 0.020%.
[0029]
　Chemical composition of steel serving as a material for nitriding components of the present invention contains the above elements, the balance being Fe and unavoidable impurities. The inevitable impurities included in the raw material, or a component mixed in the manufacturing process, intentionally refer to components not intended to be contained in the steel.
[0030]
　However, the steel as the material for nitriding components of the present invention, instead of a part of Fe, and may contain elements below.
[0031]
　: [Nb 0% or more, 0.1% or
　less] is Nb, forms a NbC and NbN combines with C and N. NbC, the pinning effect of NbN, is suppressed coarsening of austenite grains, the steel of tissue prior to nitriding treatment finer and have the effect of reducing the variation in mechanical properties of the nitrided components. This effect can be obtained if added in a small amount of Nb, in order to obtain a more reliable effect, Nb is preferably 0.01% or more. When the content of Nb exceeds 0.1%, coarse NbC, because NbN made is easily formed, the above effect is difficult to obtain.　
[0032]
　[B: 0 or more, less 0.01%]
　B suppresses grain boundary segregation of P, it has the effect of improving the toughness. Moreover, combined with N to form a BN improving machinability. These effects can be obtained if trace amount of Nb, but to obtain a more reliable effect, B is preferably set to 0.0005% or more. When the content of B exceeds 0.01%, not only the effect is saturated, it may crack the steel material caused by a large amount of BN is segregated.
[0033]
　[Mo: 0% to less than
　0.50%] is Mo, fine nitrides during nitride (Mo 2 to N) is formed in the hardened layer, to improve the surface fatigue strength and bending fatigue strength by precipitation strengthening. Further, Mo enhances the core hardness and exhibit age hardening effect during nitriding. Mo content in order to obtain these effects is preferably 0.01% or more. On the other hand, the content of Mo is 0.50% or more, the steel bar as a material, the wire material and the hardness after hot forging too high, in addition to cutting workability is remarkably reduced, the alloy cost increases. The preferable upper limit of the Mo content for cutting ensuring less than 0.40%.
[0034]
　[V: 0% to less than 0.50%]
　of V, forming a nitride and nitrocarburizing during fine nitride (VN), to improve the surface fatigue strength and bending fatigue strength by precipitation strengthening, the core portion of the component to increase the hardness. Also it has the effect of refinement of the structure. To obtain these effects, V is preferably 0.01% or more. On the other hand, the content of V is 0.50% or more, the steel bar as a material, the wire material and the hardness after hot forging too high, in addition to cutting workability is remarkably reduced, the alloy cost increases. V content of the preferred range for cutting ensuring less than 0.40%.
[0035]
　[Cu: 0% to less than
　0.50%] Cu improves the hardness of the component core hardness and nitrogen diffusion layer as a solid solution strengthening element. Preferably it contains at least 0.01% in order to exhibit the effect of Cu solid solution strengthening. On the other hand, the content of Cu is 0.50% or more, the steel bar as a material, the wire material and the hardness after hot forging too high, in addition to cutting resistance is significantly reduced, since the hot ductility is lowered , during hot rolling, which causes surface flaws generated during hot forging. A preferred range of the Cu content for the hot ductility maintained is less than 0.40%.
[0036]
　[Ni: 0% to less than
　0.50%] is Ni, improves the core hardness and surface hardness through solid solution strengthening. Preferably it contains at least 0.01% in order to exhibit the effects of Ni solid solution strengthening. On the other hand, at a content of Ni is 0.50% or more, bars, because the hardness after the wire or hot forging is too high, in addition to cutting workability is remarkably reduced, the alloy cost increases. Sufficient preferable range of the Ni content in order to obtain the machinability is less than 0.40%.
[0037]
　[Ti: 0% to less than
　0.05%] is Ti, TiN was formed by combining the N, to improve the core hardness and surface hardness. To obtain this effect, Ti is preferably 0.005% or more. On the other hand, the Ti content in 0.05% or more, in addition to the effect of improving the core hardness and surface hardness is saturated, the alloy cost increases. A preferred range of the Ti content is less than 0.007 to 0.04 percent.
[0038]
　It will now be described compound layer of nitrided component of the present invention.
[0039]
　[Compound layer thickness: 3 [mu] m or more but less than 15 [mu] m]
　from the compound layer is a layer of iron nitride formed by nitriding, the thickness will affect the surface fatigue strength and bending strength of the nitrided components. When the compound layer is too thick, easily become a starting point of destruction by pitting or bending. When the compound layer is too thin, the residual stress of the surface is not sufficiently obtained, the surface fatigue strength and bending strength is lowered. In the nitriding treatment component of the present invention, from the viewpoint of surface fatigue strength and bending strength, the thickness of the compound layer is less than 15μm more than 3 [mu] m.
[0040]
　The thickness of the compound layer after gas nitriding, polished vertical section of test material is measured by observing with an optical microscope by etching. Etching is performed with a 3% nital solution for 20 to 30 seconds. Compound layer is present on the surface layer of the low alloy steel, is observed as a layer of white non-corrosive. Structure photograph 5 field taken at 500 times by an optical microscope (field area: 2.2 × 10 4 [mu] m 2 ) to observe. In each field, it measures the 4 points 30μm each horizontally. The average value of the measured 20 points of value, is defined as a compound thickness ([mu] m). The outline of the measurement method in Fig. 1 shows an example of a structure photograph of a compound layer and the diffusion layer in FIG.
[0041]
　Surface ~ 5 [mu] m of the compound layer of gamma 'phase ratio: 50%]
　in the compound layer of the surface ~ 5 [mu] m gamma' phase ratio is low, the ε-phase ratio is high, it tends to become a starting point of pitting or bending fatigue fracture . This fracture toughness value of ε phase is due to the low compared to the gamma 'phase. Moreover, compared with the case the phase near the surface is ε-phase and a gamma 'phase, tends to introduce compressive residual stresses below the surface, it is possible to improve the fatigue strength.
[0042]
　Gamma 'phase fraction of the compound layer is back-scattered electron diffraction (Electron BackScatter Diffraction: EBSD) obtained by. Specifically, from the outermost surface of the compound layer to 5μm depth, area 150 [mu] m 2 performs EBSD measured, gamma 'phase, to create an analysis diagram to determine the ε phase. Then, the obtained EBSD analysis image, 'determined the area ratio of the phase, which gamma' gamma using an image processing application is defined as a phase fraction (%). The EBSD measurement, it is appropriate to measure about 10 fields at 4000 × before and after magnification.
[0043]
　The above gamma 'phase ratio surface-of 5μm depth "of the compound layer" gamma' means the ratio of the phase. That is, the thickness of the compound layer if less than the surface to 5 [mu] m, to calculate the gamma 'phase fraction in the region of the compound layer thickness min. As an example, if the 3 [mu] m thickness from the surface of the compound, gamma compound layer depth of the surface ~ 3 [mu] m 'ratio of phase gamma' becomes a phase ratio.
[0044]
　gamma 'phase fraction is preferably 60% or more, more preferably 65% ​​or more, more preferably 70% or more.
[0045]
　gamma 'phase ratio, is also conceivable determined using X-ray diffraction. However, measurement by X-ray diffraction, measurement region becomes ambiguous, it is impossible to obtain an accurate gamma 'phase fraction. Thus, gamma 'phase fraction of the compound layer in the present invention shall be determined by EBSD.
[0046]
　Surface ~ 3 [mu] m compound layer void area ratio of: less than 10%
　voids in the compound layer on the surface ~ 3 [mu] m is present the resulting stress concentration becomes a starting point of pitting and bending fatigue fracture. Therefore, the void area ratio is required to be less than 10%.
[0047]
　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. N 2 generation of the nitride potential K below N tends to occur 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 the easy to generate. The schematic gap is formed in the compound layer in Fig. 3 shows a structure photograph of voids is formed in FIG.
[0048]
　Void area ratio can be measured by light microscopy. Specifically, the depth of the surface ~ 3 [mu] m in the cross section of the test material, 5 perimetry at a magnification of 1000 times (field area: 5.6 × 10 3 [mu] m 2 ) to, 3 [mu] m from the outermost surface of each field the percentage of voids occupying the depth range of the void area ratio.
[0049]
　Void area ratio preferably less than 5%, more preferably less than 2%, more preferably less than 1%, and most preferably 0.
[0050]
　[Compound layer surface of the compressive residual stress: 500 MPa or more]
　nitrided component of the present invention, together with the surface of the steel is hardened by nitriding, compressive residual stress is introduced into the surface layer of the steel, the fatigue strength of the component, the wear sex can be improved. Nitriding component of the present invention, the compound layer is improved as described above, further by introducing compressive residual stress than 500MPa on the surface comes to have excellent surface fatigue strength, rotating bending fatigue strength. Manufacturing method for introducing such compressive residual stress on the surface of the component will be described later.
[0051]
　Next, an example of a manufacturing method of nitriding part of the present invention.
[0052]
　The nitriding part of the manufacturing method of the present invention, subjected to a gas nitriding process to the steel having the components described above. Treatment temperature of the gas nitriding treatment is 550 ~ 620 ° C., the processing time of the entire gas nitriding is 1.5 to 10 hours.
[0053]
　Process Temperature: 550 ~ 620 ℃]
　Gas nitriding temperature (nitriding temperature) mainly is correlated with the nitrogen diffusion rate, affects the surface hardness and hardening depth. If nitriding treatment temperature is too low, the diffusion rate of nitrogen is slow, the surface hardness becomes low, case depth becomes shallower. 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 of it becomes shallow. Therefore, in the present embodiment, the nitriding treatment temperature is 550 ~ 620 ° C. ambient ferrite temperature range. In this case, it is possible to suppress the surface hardness becomes low, and can prevent the case depth becomes shallower.
[0054]
　[Gas nitriding overall processing time: 1.5-10 hours]
　Gas nitriding treatment, NH 3 , H 2 , N 2 is carried out in an atmosphere containing. Nitriding the whole time, i.e., time from the start to the end of the nitriding treatment (treatment time) is correlated with the formation of the compound layer and the decomposition and nitrogen diffusion coating, influence on the surface hardness and hardening depth on. When the processing time is too short surface hardness becomes low, case depth becomes shallower. On the other hand, if the treatment time is too long, the surface hardness of the denitrification and decarburization occurs steel decreases. Processing time further if too long, the production cost is high. Therefore, the processing time of the entire nitriding process is 1.5 to 10 hours.
[0055]
　Incidentally, the atmosphere gas nitriding process of the present embodiment, NH 3 , H 2 and N 2 includes other inevitably oxygen, an impurity such as carbon dioxide. Preferred atmosphere is, NH 3 , H 2 and N 2 99.5% in total (% by volume) or more.
[0056]
　Carbon monoxide, when subjected to gas nitrocarburizing treatment in an atmosphere containing a few percent of carbon dioxide, high ε phase of the solid solubility limit of C is preferentially produced. Since gamma 'layers can hardly solid solution C, when subjected to soft nitriding treatment, the compound layer becomes ε single phase. Furthermore, since the growth rate of the ε-phase is faster than gamma 'phase, in gas soft that ε phase is stably generated is thicker than required compound layer. Thus, rather than a gas nitrocarburizing treatment in the present invention, nitride potential K as described below N it is necessary to apply the gas nitriding process to control.
[0057]
　[Gas conditions nitriding]
　In nitriding method of the present invention, nitriding treatment is performed under nitriding potential is controlled in consideration of the amount of C fabric. Thus, the phase structure in the compound layer of the depth of the surface ~ 5 [mu] m and gamma 'phase ratio of 50% or more, the void area ratio at a depth of a surface ~ 3 [mu] m and less than 10%, 500 MPa compressive residual stress in the compound layer surface it can be equal to or greater than.
[0058]
　Nitriding potential K gas nitriding N is defined by the following formula.
[0059]
　　K N (ATM -½ ) = (NH2 . 3 sub-pressure (ATM)) / [(H 2 partial-pressure (ATM)) ] 3/2 ]
[0060]
　NH atmosphere gas nitriding 3 and H 2 partial pressure of can be controlled by adjusting the flow rate of the gas. To form the compound layer by nitridation treatment, K during gas nitriding N it is necessary is not less than a predetermined value, as described above, K N is too high, N 2 tends to generate gas ε the proportion of the phases is increased, so that many voids. Thus, K N provided the condition, it is important to suppress the generation of voids.
[0061]
　Results of study of the present inventors, the phase structure of the nitriding potential compound layer of the gas nitriding process, and affects the rotating bending fatigue strength of nitrided components, that optimal nitriding potential is determined by the C content of the steel heading was.
[0062]
　Specifically, when the C content of the steel (mass%) and (wt% C), nitriding potential during the gas nitriding treatment, always 0.15 ≦ K gas nitriding N ≦ -0.17 satisfies the × ln (wt% C) +0.20, the phase structure of the compound layer becomes gamma 'phase ratio of 50% or more was further found that nitrided component has a higher rotational bending fatigue strength and surface fatigue strength .
[0063]
　Even meet the above equation mean nitriding potential of the gas nitriding process, if you take the nitriding potential values ​​that do not satisfy the above equation, even one o'clock, gamma 'phase fraction in the compound layer does not become 50% or more.
[0064]
　Figure 5 shows the nitriding potential, the result of examining the relationship gamma 'ratio and rotary bending fatigue strength of the compound layer. Figure 5 is for steel j embodiments described later (Table 1).
[0065]
　Thus, in this nitriding treatment, nitriding potential K corresponding to the C content of the steel as a dough N implementing gas nitriding treatment under. Thus, stable it is possible to impart gamma 'phase on the surface of the steel, excellent surface fatigue strength, rotating bending fatigue strength, preferably, the surface fatigue strength than 2000 MPa, rotating bending fatigue strength of more than 600MPa nitriding treatment component can be obtained.
Example
[0066]
　Steel a ~ aa having the chemical components shown in Table 1, to produce a molten steel was melted in 50kg vacuum melting furnace, to produce an ingot by casting the molten steel. Incidentally, a ~ s in Table 1 are steels having chemical compositions defined in the present invention. On the other hand, the steel t ~ aa is at least one element or a steel of the comparative examples deviating from the chemical components defined in the present invention.
[0067]
[Table 1]

[0068]
　And a round bar having a diameter of 40mm The ingot was hot-forged. Subsequently, after the annealing of each round bar, roller pitting small roller for study to evaluate the surface fatigue strength shown in subjecting Figure 6 the cutting, to produce a large roller shown in FIG. Further, to prepare a cylindrical test piece for evaluating flexural fatigue strength shown in FIG.
[0069]
　Relative collected specimens were carried out gas nitriding treatment under the following conditions. Test pieces were charged into the gas nitriding furnace, NH in the furnace 3 , H 2 , N 2 by introducing gases the However, the test number 34, CO in the atmosphere 2 and 3% added gas nitrocarburizing treatment gas by volume. In addition, test number 35, was changed in the first half and the second half of the nitride conditions, was a two-stage nitriding treatment. Test No. 36 corresponds to Example 16 in Patent Document 3. On specimens after gas nitriding treatment was carried out oil cooling using 80 ° C. oil.
[0070]
　H in the atmosphere 2 partial pressure, the heat conduction H mounted directly to the gas nitriding furnace body 2 was measured using a sensor. The difference in thermal conductivity of the standard gas and measurement gas was measured in terms of gas concentration. H 2 partial pressure during the gas nitriding treatment was continuously measured.
[0071]
　Further, NH 3 partial pressure, NH manual glass tube type out of the furnace 3 was measured by attaching a spectrometer.
[0072]
　Residual NH every 10 minutes 3 simultaneously measuring the partial pressure of the nitriding potential K N is calculated, so as to converge to the target value, NH 3 flow rate and N 2 were adjusted flow rate. NH 3 nitride potential K every 10 minutes to measure the partial pressure N is calculated, so as to converge to the target value, NH 3 flow rate and N 2 were adjusted flow rate.
[Table 2]

[0073]
　[Compound layer thickness and void area ratio Measurement
　of small roller after gas nitriding, a cross section in a direction perpendicular to the length direction was mirror-polished and etched. Scanning electron microscope (Scanning Electron Microscope: SEM) was used to observe the etched section was performed to confirm the presence or absence of voids in the measurement and the surface layer portion of the compound layer thickness. The etching was carried out for 20 to 30 seconds with 3% nital solution.
[0074]
　Compound layer can be confirmed as a layer of white non-corrosion present in the surface layer. Photographed structure photograph 10 fields with 4000-fold (field area: 6.6 × 10 2 [mu] m 2 ) from observing the compound layer, to measure the thickness of the compound layer of the three points 10μm for each. Then, the average value of the measured 30 points was defined as a compound thickness ([mu] m).
[0075]
　Similarly, the 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%) was calculated by binarizing by an image processing application. Then, the average value of the measured 10 field was defined as the void area ratio (%). In the case where the compound layer is less than 3μm also it was measured from similarly surface to 3μm depth.
[0076]
　[Gamma 'Measurement of the phase fraction]
　phase fraction gamma compound layer', electron backscatter diffraction method: obtained in (Electron BackScatter Diffraction EBSD). From the outermost surface of the compound layer to 5μm depth performs EBSD measured area 150μm2, γ 'phase, to create an analysis diagram to determine the ε-phase, the obtained EBSD analysis image, using the image processing application gamma' It was determined phase proportions (%). In the EBSD measurement, it was 10 fields measured at 4000-fold magnification.
[0077]
　Then, 'the mean value of phase ratio, gamma' measured 10 fields of gamma is defined as the phase fraction (%). When a compound layer is less than 5μm was calculated gamma 'phase fraction in the region of the compound layer thickness min.
[0078]
　[Compound layer residual stress]
　relative small roller contact portion after the nitriding, using a small portion X-ray residual stress measuring device, gamma 'phase under the conditions shown in Table 3, the residual stress of the ε-phase and the base layer (matrix) sigma gamma ' , sigma epsilon , sigma m was measured. Moreover, obtained in EBSD, the area 90μm of 3μm depth range from the outermost surface 2 occupied in gamma 'phase, epsilon phase and the area ratio V of the base layer gamma' , V epsilon , V m with the following formula residual stress σ obtained by c was used as a residual stress on the surface.
[0079]
　　p c = V c p c + V e p e + V m p m
[0080]
[table 3]

[0081]
　[Surface fatigue strength evaluation test]
　The small roller for roller pitting tests, after finishing gripping portion for the purpose of removing the heat treatment distortion, and subjected to the roller pitting test pieces, respectively. The shape after finishing is shown in FIG.
[0082]
　Roller pitting test, the combination of the roller pitch for computing test small roller and shape of the roller pitting large roller test shown in FIG. 3, was carried out under the conditions shown in Table 4.
[0083]
　The unit of size in FIG. 2 and 3 is "mm". The roller pitting large roller test, using a steel that satisfies the SCM420 standard of JIS, general manufacturing process, i.e. "normalizing → specimen processing → co 析浸 coal → cold tempering by the gas carburizing furnace → polishing are those manufactured by a process of "the position of 0.05mm from the surface, i.e., in Vickers hardness Hv is 740 to 760 at the position of depth 0.05mm, also Vickers hardness Hv of more than 550 depth was in the range of 0.8 ~ 1.0mm.
[0084]
　Table 4 shows the test conditions were evaluated in the surface fatigue strength. Test truncation number, 2 × 10 showing a general steel fatigue source 7 and times, 2 × 10 without pitting causes generated in the small roller test piece 7 the maximum surface pressure has been reached once the small roller test piece It was a fatigue limit.
[0085]
[Table 4]

[0086]
　Detection of pitting generation is carried out by the vibration meter is installed in the test machine, after the vibration generating, the rotation of both the small roller test piece and large roller test piece was stopped and check the number and rotation pitting occurred.
[0087]
　In the present invention the component, the maximum surface pressure in the fatigue limit was targeted to be at least 2000 MPa.
[0088]
　[Anti-rotation bending fatigue strength]
　to cylindrical test piece was subjected to gas nitriding treatment was carried out rotating bending fatigue test Ono-type. 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 It was fatigue limit of the bending fatigue test piece.
[0089]
　In the present invention component, the maximum stress in the fatigue limit was the goal of at least 600 MPa.
[0090]
　[Test Results]
　Table 2 shows the results. Components of the test numbers 1 to 25 are steel, and the conditions of the gas nitriding is in the range of the present invention, compounds thickness of 3 ~ 15 [mu] m, the compound layer gamma 'layer ratio of 50% or more, the compound layer void area ratio less than 10%, the compressive residual stress of the compound layer is not less than 500 MPa. As a result, the surface fatigue strength than 2000 MPa, rotating bending fatigue strength 600MPa or more, a good result was obtained.
[0091]
　Test No. 26 nitriding temperature is too high, as a result, low gamma 'phase fraction of the compound layer, larger void area ratio, the residual stress is lowered, the surface fatigue strength, rotating bending fatigue strength is lowered.
[0092]
　Test No. 27 nitriding temperature is too low, not compound layer is formed, the residual stress of the surface also becomes lower, the surface fatigue strength, rotating bending fatigue strength is lowered.
[0093]
　Test No. 28 is a nitridation time is too long, the void area ratio is increased, the residual stress of the surface along with it is lowered is open, rotating bending fatigue strength is lowered.
[0094]
　Test No. 29 is a nitridation time is too short, sufficient compound layer thickness is obtained, the residual stress of the surface is lowered, the surface fatigue strength, rotating bending fatigue strength is lowered.
[0095]
　Test No. 30 is the lower limit of the nitriding potential is low, can not be obtained sufficient compound layer thickness, the residual stress of the surface is lowered, the surface fatigue strength, rotating bending fatigue strength is lowered.
[0096]
　Test No. 31 is too lower limit of the nitriding potential low, not compound layer is produced, the residual stress of the surface is lowered, the surface fatigue strength, rotating bending fatigue strength is lowered.
[0097]
　Test No. 32 has a high upper limit of the nitriding potential, the void area ratio is increased, rotating bending fatigue strength is lowered.
[0098]
　Test No. 33 is not bounds nitride potential appropriate, thickened compound layer thickness, because the void area ratio is increased, the surface fatigue strength, rotating bending fatigue strength is lowered.
[0099]
　Test No. 34 is a nitrocarburizing treatment, gamma 'phase is hardly generated on the surface, the residual stress is lowered, the surface fatigue strength, rotating bending fatigue strength is lowered.
[0100]
　Test No. 35, K average N but is appropriate gamma 'phase fraction is higher, K in nitriding N limit is high, the void area ratio is increased.
[0101]
　Test No. 36 is too high C content of the steel, since the compound layer thickness is thicker, the surface fatigue strength is lowered.
[0102]
　Test No. 37 is too low C content of the steel, so sufficient core strength is not obtained, and destroyed early mother layer as a starting point.
[0103]
　Test No. 38 is too low Si content of the steel, so sufficient core hardness was not obtained, and destroyed early mother layer as a starting point.
[0104]
　Test No. 39 is too low Mn content of the steel, a sufficient hardened layer hardness, the core hardness was not obtained, and destroyed early mother layer as a starting point.
[0105]
　Test No. 40 P of the steel, S amount is too high, the grain boundary segregation of P, and the formation of coarse MnS, destroyed prematurely.
[0106]
　Test No. 41 is too low Cr content of the steel, a sufficient diffusion layer hardness, the core hardness was not obtained, and destroyed early mother layer as a starting point.
[0107]
　Test No. 42 is too high Al content of the steel, oxide inclusions are generated and destroyed early mother layer as a starting point.
[0108]
　C amount of test No. 43 steel, Mn amount, Cr amount is low, even higher upper limit of the nitriding potential, the void area ratio is increased, the surface fatigue strength, rotating bending fatigue strength is lowered.
[0109]
　It has been described an embodiment of the present invention. However, the above-described embodiments are merely examples for carrying out the present invention. Accordingly, the present invention is not limited to the embodiments described above, it can be implemented by changing the above-described embodiments without departing from the scope and spirit thereof as appropriate.

The scope of the claims
[Requested item 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: 0.5% greater,
　　2.0%, Al: 0.01% or more, 0.05% or
　　less, N: 0. 003% or more, 0.025% or
　　less, Nb: 0% to 0.1% or less,
　　B: 0% to 0.01% or
　　less, Mo: 0% to less than
　　0.50%, V: 0% or more and less than
　　0.50%, Cu: 0% to less than
　　0.50%, Ni: 0% to less than 0.50%, and
　　Ti: 0% to less than 0.05%
containing, the remainder a Fe and part steel was used as a material which is an impurity,
　it is formed on the surface of the steel material, iron, compounds of less than nitrogen and thickness 3μm or 15μm containing carbon A layer,
　the phase structure is gamma 'phase in the compound layer ranging from the surface to a depth of 5μm contains an area ratio 50% or more,
　in a range from the surface to a depth of 3μm in the void area ratio is less than 10% There,
　the compressive residual stress in the compound layer surface is not less than 500MPa
nitrided component, characterized in that.