Technical field
[0001]
The present invention is a steel part that has been subjected to gas nitriding treatment, particularly bending straightening property 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, improve the wear resistance of the steel part.
[0005]
　Patent Document 1, a gamma 'phase fraction of the compound layer by a 30 mol% or more, nitrided component fatigue strength with improved bending 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 optimizing the content of the element to increase the fatigue strength after nitriding method of manufacturing a nitride component that suppresses deformation after nitriding is disclosed.
CITATION
Patent Document
[0008]
Patent Document 1: JP 2015-117412 Patent Publication
Patent Document 2: JP 2013-221203 Patent Publication
Patent Document 3: WO 2016/098143
Summary of the Invention
Problems that the Invention is to Solve
[0009]
　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.
[0010]
　Nitriding of Patent Document 3 is not an appropriate control of the gas condition at the time of processing, or lower the ratio of the gamma 'phase compound layer, the porosity is high, it tends to be a starting point of pitting or bending fatigue fracture . Further, a gas nitrocarburizing process is Patent Document 3 discloses the porosity tends to increase.
[0011]
　An object of the present invention is to provide a bending excellent rotary bending fatigue strength in addition to the straightening property component and a manufacturing method thereof.
Means for Solving the Problems
[0012]
　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.
[0013]
　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 bending straightening property, and a nitride component having a rotary bending fatigue strength can be produced.
[0014]
　The present invention is based on the above findings, which has been made repeatedly further studies, its gist the following.
[0015]
　By mass%, C: 0.20 ~ 0.60%, Si: 0.05 ~ 1.5%, Mn: 0.2 ~ 2.5%, P: 0.025% or less, S: 0.003 ~ 0.05%, Cr: 0.05 ~ 0.50%, Al: 0.01 ~ 0.05%, N: 0.003 ~ 0.025%, Nb: 0 ~ 0.1%, B: 0 ~ 0.01%, Mo: 0% to less than 0.50%, V: 0% to less than 0.50%, Cu: 0% to less than 0.50%, Ni: 0% or more, 0 less than .50%, and Ti: 0% or more, the thickness and contains less than 0.05%, the balance being the material steel is Fe and impurities, containing formed on the steel surface, iron, nitrogen and carbon of has a compound layer of less than 15μm more than 3 [mu] m, the phase structure is gamma 'phase in the depth of the compound layer on the surface ~ 5 [mu] m contained in an area ratio 50% or more, air-in depth of the surface ~ 3 [mu] m The area ratio is less than 10%, nitrided component compressive residual stress in the compound layer surface and wherein the at least 500 MPa.
Effect of the invention
[0016]
　According to the present invention, it is possible to obtain a bending excellent nitrided parts rotating bending fatigue strength in addition to the straightening property.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017]
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 a four-point bending test specimen used to evaluate the [6] straightenability.
7 is a cylindrical shape test pieces for evaluating the rotating bending fatigue strength.
DESCRIPTION OF THE INVENTION
[0018]
　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%".
[0019]
　[C: 0.20% or more 0.60% or less]
　is C, it is an element necessary for ensuring the core hardness of the component. In less than 0.20% content and C, since the core strength is too low, bending straightening property and bending fatigue strength is significantly decreased. When the content of C exceeds 0.60%, the large compound layer thickness, bending straightening property and bending resistance is significantly decreased. The preferred range of C content is 0.30 to 0.50 percent.
[0020]
　[Si: 0.05% or more, 1.5% or
　less] Si is by solid solution strengthening, enhancing the core hardness. Order to exert its effect, 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%.
[0021]
　[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%. Meanwhile, since the Mn content exceeds 2.5%, not only the effect is saturated to increase the bending fatigue strength, steel bar as a material, the hardness after the wire or hot forging is too high, cutting sex is greatly reduced. A preferred range of Mn content is 0.4 to 2.3%.
[0022]
　[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%.
[0023]
　[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, bending straightening property and bending fatigue strength is significantly decreased. A preferred range of S content is 0.005 to 0.03%.
[0024]
　[Cr: 0.05% or more, 0.50% or
　less] Cr is during nitriding, fine nitride (CrN) is formed in the hardened layer, to improve the bending fatigue strength by precipitation strengthening. To obtain this effect, Cr is necessary more than 0.05%. On the other hand, when the content of Cr exceeds 0.5%, the steel bar as a material, the wire material and the hardness after hot forging too high, the bending straightening property is lowered. The preferred range of Cr content is from 0.10 to 0.30 percent.
[0025]
　: [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%.
[0026]
　[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%.
[0027]
　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.
[0028]
　However, the steel as the material for nitriding components of the present invention, instead of a part of Fe, and may contain elements below.
[0029]
　: [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 shows the effect of reducing the variation in the 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.
[0030]
　[B: 0% or more, less 0.01%]
　B suppresses grain boundary segregation of P, has the effect of improving the toughness. Moreover, combined with N to form a BN improving machinability. These effects can be obtained if the B dopants, 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.
[0031]
　[Mo: 0% to less than
　0.50%] is Mo, fine nitrides during nitride (Mo 2 N) was formed in the cured layer, to improve the 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%.
[0032]
　[V: 0% to less than 0.50%]
　of V, except that fine nitrides during nitriding and nitrocarburizing form a (VN), to improve the bending fatigue strength by precipitation strengthening, the core hardness of the component higher. 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%.
[0033]
　[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%.
[0034]
　[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%.
[0035]
　[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.
[0036]
　It will now be described compound layer of nitrided component of the present invention.
[0037]
　[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 treatment and has a thickness of, affects the bending straightening property and bending strength of the nitrided components. When the compound layer is too thick, easily become a starting point of bending fatigue fracture fracture. When the compound layer is too thin, the residual stress of the surface is not sufficiently obtained, bending straightening property and bending fatigue strength is lowered. In the nitriding treatment component of the present invention, the bending point of view of straightening property and bending strength, the thickness of the compound layer is less than 15μm more than 3 [mu] m.
[0038]
　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.
[0039]
　Surface ~ 5 [mu] m of the compound layer of gamma 'phase ratio: more than 50%]
　gamma in the compound layer on the surface ~ 5 [mu] m' low ratio of phase and ε-phase ratio is high, destroyed during compound layer straightenability or when bending fatigue It tends to become a starting point of. 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.
[0040]
　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.
[0041]
　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.
[0042]
　gamma 'phase fraction is preferably 60% or more, more preferably 65% ​​or more, more preferably 70% or more.
[0043]
　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.
[0044]
　[Compound layer void area ratio of the surface ~ 3 [mu] m: less than 10%
　void of the compound layer on the surface ~ 3 [mu] m, the resulting stress concentration becomes a starting point of bending fatigue fracture. Therefore, the void area ratio is required to be less than 10%.
[0045]
　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 'occurs phase transformation → ε, γ' bcc → γ accordance rises 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.
[0046]
　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.
[0047]
　Void area ratio preferably less than 5%, more preferably less than 2%, more preferably less than 1%, and most preferably 0.
[0048]
　[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 is directed to improved the above-mentioned compound layer, further by introducing compressive residual stress than 500MPa on the surface comes to have excellent bending fatigue strength. Manufacturing method for introducing such compressive residual stress on the surface of the component will be described later.
[0049]
　Next, an example of a manufacturing method of nitriding part of the present invention.
[0050]
　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.
[0051]
　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.
[0052]
　[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.
[0053]
　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.
[0054]
　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.
[0055]
　[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 depth of the compound layer on 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 1%, 500 MPa compressive residual stress in the compound layer surface it can be equal to or greater than.
[0056]
　Nitriding potential K gas nitriding N is defined by the following formula.
[0057]
　　K N (ATM -½ ) = ((NH2 . 3 sub-pressure (ATM)) / [(H 2 partial-pressure (ATM)) ] 3/2 ]
[0058]
　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.
[0059]
　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.
[0060]
　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 high bending straightening property and rotary bending fatigue strength .
[0061]
　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 temporarily, gamma 'phase fraction in the compound layer does not become 50% or more.
[0062]
　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 the steel a embodiment to be described later (Table 1).
[0063]
　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 flexural straightening property, rotary bending fatigue strength, preferably, the bending force of 1.2% or more, rotary bending fatigue strength There can be obtained the above nitriding processing part 520 MPa.
Example
[0064]
　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.
[0065]
[Table 1]

[0066]
　It was a round bar having a diameter of 25mm this ingot by hot forging. Subsequently, after the annealing of each round bar, to produce a rectangular test piece for evaluating straightening property bending shown in subjecting Figure 2 the cutting. Further, to prepare a cylindrical test piece for evaluating flexural fatigue strength shown in FIG.
[0067]
　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 32, CO in the atmosphere 2 and 3% added gas nitrocarburizing treatment gas by volume. On specimens after gas nitriding treatment was carried out oil cooling using 80 ° C. oil.
[0068]
　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.
[0069]
　Further, NH 3 partial pressure, NH manual glass tube type out of the furnace 3 was measured by attaching a spectrometer.
[0070]
　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]

[0071]
　[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.
[0072]
　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).
[0073]
　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.
[0074]
　[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.
[0075]
　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.
[0076]
　[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.
[0077]
　　p c = V c p c + V e p e + V m p m
[0078]
[table 3]

[0079]
　[Straightenability]
　to square specimens were subjected to gas nitriding treatment was carried out static bending test. Static bending test performs inner support span 30 mm, in four-point bending of the outer support span 80 mm, strain rate was 2 mm / min. Attaching a strain gauge to the R portion of the square test piece longitudinally, cracks R portion occurs was determined as the maximum strain the strain amount when the measurement is no longer gauge (%) bending straightening property.
[0080]
　In the present invention component, bending straightening property was targeted to be 1.2% or more.
[0081]
　[Rotating 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.
[0082]
　In the present invention component, the maximum stress in the fatigue limit was the goal of at least 520 MPa.
[0083]
　[Test Results]
　Table 2 shows the results. Components of the test Nos. 1 to 23 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 bending straightening property is 1.2% or more, rotary bending fatigue strength 520MPa or more, a good result was obtained.
[0084]
　Test No. 26 nitriding temperature is too high, as a result, the compound layer of gamma 'phase fraction is low, large void area ratio, residual stress becomes tensile stress, rotational bending fatigue strength is lowered.
[0085]
　Test No. 27 nitriding temperature is too low, not compound layer is formed, the residual stress of the surface also becomes lower, rotary bending fatigue strength is lowered.
[0086]
　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.
[0087]
　Test No. 29 is a nitridation time is too short, can not be obtained sufficient compound layer thickness, the residual stress of the surface is lowered, it becomes shallower for case depth, and destroyed early mother layer as a starting point.
[0088]
　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, rotating bending fatigue strength is lowered.
[0089]
　Test No. 31 is the lower limit of the nitriding potential is too low, not compound layer is produced, the residual stress of the surface is lowered, rotating bending fatigue strength is lowered.
[0090]
　Test No. 32 has a high upper limit of the nitriding potential, the void area ratio is increased, the bending straightening property, rotary bending fatigue strength is lowered.
[0091]
　Test No. 33 is the upper limit of the nitriding potential is too high, becomes thick compound layer thickness, gamma 'phase fraction is low, because the void area ratio is increased, the bending straightening property, rotary bending fatigue strength is lowered.
[0092]
　Test No. 34 is a nitrocarburizing treatment, gamma 'phase is hardly generated on the surface, the residual stress is lowered, the bending straightening property, rotary bending fatigue strength is lowered.
[0093]
　Test No. 35 is too high C content of the steel, since the compound layer thickness is thicker, the bending straightening property, rotary bending fatigue strength is lowered.
[0094]
　Test No. 36 is too low C content of the steel, so sufficient core strength is not obtained, and destroyed early mother layer as a starting point.
[0095]
　Test No. 37 is too high Si content of the steel, since the hardness of the base material becomes too high, the bending straightening property was lower.
[0096]
　Test No. 38 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.
[0097]
　Test No. 39 P of the steel, S amount is too high, the grain boundary segregation of P, and the formation of coarse MnS, destroyed prematurely.
[0098]
　Test No. 40 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.
[0099]
　Test No. 41 is too high Al content of the steel, oxide inclusions are generated and destroyed early mother layer as a starting point.
[0100]
　C amount of test No. 42 steel, Mn amount is low, because the amount of Cr is higher, the higher the hardness of the base material, the bending straightening property, rotary bending fatigue strength is lowered.
[0101]
　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: 0.20% or more 0.60% 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: 0.05% or more, 0.50% or
　　less, 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 part steel was used as a material of Fe and impurities,
　formed on the surface of the steel material, iron, nitrogen and less than a thickness of 3μm or more 15μm containing carbon Has a compound 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,
　the void area ratio in the range from the surface to a depth of 3μm is less than 10% , and the
　compressive residual stress in the compound layer surface is not less than 500MPa
nitrided component, characterized in that.