KR20190022801A - Nitrided parts and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a component having excellent rolling bending fatigue strength in addition to bending correction and a method for producing the same, and a method for producing the same, which comprises a steel material having a predetermined chemical composition and having a thickness of 3 탆 or more and less than 15 탆, , The phase structure in the compound layer in the range from the surface to the depth of 5 mu m contains the gamma prime phase at an area ratio of 50% or more, and the void area rate in the range from the surface to 3 mu m is 1% And the compressive residual stress on the surface of the compound layer is 500 MPa or more.

Description

Nitrided parts and method for manufacturing the same

The present invention relates to a steel part subjected to a gas nitriding treatment, particularly a gear having excellent bending correctability and bending fatigue strength, a nitrided part such as a CVT sheave, and a manufacturing method thereof.

Steel parts used in automobiles and various industrial machines are subjected to surface hardening heat treatment such as carburizing, high frequency welding, nitriding, and softening in order to improve mechanical properties such as fatigue strength, abrasion resistance, and corrosion resistance .

The nitriding treatment and the softening treatment are carried out in a ferrite region of not more than A ₁ point, and since there is no phase change during the treatment, the heat treatment deformation can be reduced. For this reason, the nitriding treatment and the softening treatment are often used for parts having a high dimensional accuracy or for large-sized parts. For example, the nitriding treatment and the softening treatment are applied to gears used for transmission parts of automobiles and crankshafts used for engines have.

The nitriding treatment is a treatment method in which nitrogen is introduced into the surface of a steel material. Examples of the medium used for the nitriding treatment include gas, salt bath, and plasma. In automotive transmission parts, gas nitriding treatment, which is mainly excellent in productivity, is applied. A compound layer (a layer in which nitrides such as Fe ₃ N precipitated) having a thickness of 10 탆 or more is formed on the surface of the steel by the gas nitriding treatment and a cured layer which is a nitrogen diffusion layer is formed on the lower surface side of the steel layer. The compound layer is mainly composed of Fe _{2 to 3} N (?) And Fe ₄ N (? '), And the hardness of the compound layer is very high as compared with the steel as the base material. Therefore, the compound layer improves the wear resistance of the steel part at the beginning of use.

Patent Document 1 discloses a nitrided component in which the bending fatigue strength is improved by setting the ratio of? 'Phase in the compound layer to 30 mol% or more.

Patent Document 2 discloses a steel member having a steel member having a predetermined structure and made of a steel member and having a bending fatigue strength at a low fatigue strength and a low surface fatigue strength.

Patent Document 3 discloses a method of manufacturing a nitrided component in which the fatigue strength after the nitriding treatment is increased and the deformation after the nitriding treatment is suppressed by optimizing the content of the element.

Japanese Patent Application Laid-Open No. 2015-117412 Japanese Patent Application Laid-Open No. 2013-221203 International Publication No. 2016/098143

Since the nitriding treatment component of Patent Document 1 is a gas softening process using CO ₂ as the atmospheric gas, the surface side of the compound layer is likely to be in the ε-phase, and therefore, the bending fatigue strength is not sufficient yet. Since the nitriding treatment component of Patent Document 2 is controlled so that the NH ₃ gas is 0.08 to 0.34, the H ₂ gas is 0.54 to 0.82, and the N ₂ gas is 0.09 to 0.18 regardless of the steel components, There is a possibility that the structure and thickness of the compound layer may not be as desired.

The nitriding treatment in Patent Document 3 is not suitable for controlling the gas condition at the time of treatment, and the ratio of the γ 'phase in the compound layer is lowered or the porosity is increased, which tends to become a starting point of fitting or bending fatigue failure. Further, in the gas softening treatment disclosed in Patent Document 3, the porosity tends to be high.

It is an object of the present invention to provide a component having excellent rotational bending fatigue strength in addition to the bending correction property and a manufacturing method thereof.

The present inventors paid attention to the form 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, it was found that by nitriding the steel whose composition was adjusted under the nitriding potential control considering the C amount of the base, the surface structure of the γ 'phase main body was suppressed to suppress the occurrence of porosity and the compressive residual stress It is possible to manufacture a nitrided component having excellent bending correction and rotational bending fatigue strength.

The present invention has been made on the basis of the above-described findings and has been repeatedly reviewed.

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, in mass%, 0.20 to 0.60% of C, 0.05 to 1.5% of Si, 0.2 to 2.5% of Mn, 0.025% or less of P, 0.003 to 0.05% of S, 0.05 to 0.50% Mo: 0 to less than 0.50%, V: 0 to less than 0.50%, Cu: 0 to less than 0.05%, N: 0.003 to 0.025%, Nb: 0 to 0.1% The steel material is a steel material containing less than 0.50%, Ni: not less than 0%, less than 0.50%, and Ti: not less than 0% and less than 0.05% , And the phase structure in the compound layer at a depth of 5 탆 or less has a γ 'phase in an area ratio of 50% or more, and at a depth of 3 μm or less Wherein the void area ratio is less than 10% and the compressive residual stress on the surface of the compound layer is 500 MPa or more.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to obtain a nitrided component excellent in flexural fatigue strength in addition to bending correction.

1 is a view for explaining a method of measuring the depth of a compound layer.
2 is an example of a tissue photograph of a compound layer and a diffusion layer.
3 is a view showing a state in which a gap is formed in the compound layer.
4 is an example of a tissue photograph in which a cavity is formed in a compound layer.
5 is a graph showing the relationship between the nitriding potential and the phase structure of the compound layer and the rotational bending fatigue strength.
Fig. 6 shows the shape of the four-point bending test piece used for evaluating the bending proofness.
Fig. 7 shows the shape of a circumferential test piece for evaluating the rotational bending fatigue strength.

Hereinafter, each of the requirements of the present invention will be described in detail. First, the chemical composition of the steel to be the material will be described. Hereinafter, "%" representing the content of each component element and the element concentration on the surface of the component means "% by mass".

[C: 0.20% or more, 0.60% or less]

C is an element necessary for ensuring the hardness of the core portion of the component. If the content of C is less than 0.20%, the strength of the core portion becomes too low, so that the bending proofness and the bending fatigue strength are greatly lowered. On the other hand, if the content of C exceeds 0.60%, the thickness of the compound layer becomes large, and the bending proofness and bending resistance are largely reduced. The preferable range of the C content is 0.30 to 0.50%.

[Si: 0.05% or more, 1.5% or less]

Si increases the hardness of the core portion by solid solution strengthening. In order to exhibit this effect, 0.05% or more is contained. On the other hand, if the content of Si exceeds 1.5%, the strength after the steel bars, the wire and the hot forging becomes excessively high, and the cutting workability is greatly reduced. A preferable range of the Si content is 0.08 to 1.3%.

[Mn: 0.2% or more, 2.5% or less]

Mn increases hardness of the core portion by solid solution strengthening. Mn also forms fine nitrides (Mn ₃ N ₂ ) in the hardened layer during the nitriding treatment, and improves wear resistance and bending fatigue strength by precipitation strengthening. In order to obtain these effects, Mn is required to be not less than 0.2%. On the other hand, when the content of Mn exceeds 2.5%, not only the effect of increasing the flexural fatigue strength is saturated but also the hardness after the steel bars and the wire material or hot forging becomes excessively high. The preferable range of the Mn content is 0.4 to 2.3%.

[P: 0.025% or less]

P is an impurity and segregates by grain boundary, so it is preferable that the content is small. If the content of P exceeds 0.025%, the bending correction and the bending fatigue strength may be lowered. The preferable upper limit of the P content for preventing the lowering of the bending fatigue strength is 0.018%. It is difficult to completely reduce the content to 0, and the lower limit of practicality is 0.001%.

[S: 0.003% or more, 0.05% or less]

S combines with Mn to form MnS, and improves cutting workability. In order to obtain this effect, S must be 0.003% or more. However, when the content of S exceeds 0.05%, coarse MnS tends to easily form, and bending correction and bending fatigue strength are greatly reduced. The preferable range of the S content is 0.005 to 0.03%.

[Cr: 0.05% or more, 0.50% or less]

Cr forms fine nitride (CrN) in the hardened layer at the time of nitriding treatment and improves bending fatigue strength by precipitation strengthening. In order to obtain this effect, Cr must be 0.05% or more. On the other hand, if the content of Cr exceeds 0.5%, the hardness after the steel strip or the wire to be worked or the hot forging becomes excessively high, and the bend proofing property is lowered. The preferable range of the Cr content is 0.10 to 0.30%.

[Al: 0.01% or more, 0.05% or less]

Al is a deoxidizing element and requires 0.01% or more for sufficient deoxidation. On the other hand, Al tends to form hard oxide inclusions. When the content of Al exceeds 0.05%, the lowering of the bending fatigue strength becomes remarkable, and the desired bending fatigue strength can not be obtained even if other requirements are satisfied. The preferable range of the Al content is 0.02 to 0.04%.

[N: not less than 0.003%, not more than 0.025%]

N combines with Al and V to form AlN and VN. AlN and VN have the effect of making the structure of the steel material before nitriding finer by the pinning action of the austenite particles and reducing the fluctuation of the mechanical properties of the nitrided parts. If the content of N is less than 0.003%, this effect is difficult to obtain. On the other hand, if the content of N exceeds 0.025%, coarse AlN tends to be easily formed, so that it is difficult to obtain the above effect. The preferable range of the N content is 0.005 to 0.020%.

The chemical component of the steel to be the material of the nitrided part of the present invention contains the above-mentioned elements, and the balance is Fe and inevitable impurities. Inevitable impurities are components that are included in the raw material or are incorporated in the manufacturing process and are not intentionally contained in the steel.

However, the steel to be the material of the nitrided part 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 or N to form NbC or NbN. The coarsening of the austenite grains is suppressed by the pinning effect of NbC and NbN so that the structure of the steel material prior to the nitriding treatment is miniaturized and the fluctuation of the mechanical properties of the nitrided parts is reduced. This effect can be obtained by adding a trace amount of Nb, but in order to obtain a more reliable effect, Nb is preferably 0.01% or more. When the content of Nb is more than 0.1%, coarse NbC and NbN are easily formed, so that it is difficult to obtain the above effect.

[B: 0% or more, 0.01% or less]

B has an effect of suppressing grain boundary segregation of P and improving toughness. It also combines with N to form BN, which improves machinability. Such an effect can be obtained by adding a trace amount of B, but B is preferably 0.0005% or more in order to obtain a more reliable effect. When the content of B exceeds 0.01%, not only the above effect is saturated but also a large amount of BN segregates and cracks may occur in the steel material.

[Mo: 0% or more, less than 0.50%]

Mo forms fine nitride (Mo ₂ N) in the hardened layer at the time of nitriding and improves bending fatigue strength by precipitation strengthening. Mo also exhibits an age hardening effect at the time of nitriding to improve the hardness of the core portion. The Mo content for obtaining such an effect is preferably 0.01% or more. On the other hand, if the content of Mo is 0.50% or more, the hardness after the steel strip or the wire material or hot forging becomes excessively high, so that the cutting workability is remarkably reduced and the alloy cost is increased. A preferable upper limit of the Mo content for ensuring cutting workability is less than 0.40%.

[V: 0% or more, less than 0.50%]

V forms fine nitride (VN) at the time of nitriding and softening, improves the bending fatigue strength by precipitation strengthening, and increases the hardness of the core portion of the component. It also has the effect of tissue refinement. In order to obtain these effects, V is preferably 0.01% or more. On the other hand, if the content of V is 0.50% or more, the hardness after the bar or the wire to be a material or the hot forging becomes excessively high, so that the cutting workability is remarkably decreased and the alloy cost is increased. A preferable range of the V content for ensuring cutting workability is less than 0.40%.

[Cu: 0% or more, less than 0.50%]

Cu improves the core hardness of the component and the hardness of the nitrogen diffusion layer as the solid solution strengthening element. In order to exhibit the action of strengthening the solubility of Cu, the content of 0.01% or more is preferable. On the other hand, from the viewpoint that the content of Cu is 0.50% or more, the hardness after the steel strip or the wire material or the hot forging becomes excessively high, so that the cutting workability is remarkably decreased. In addition, It causes surface scratches during hot forging. The preferable range of the Cu content for maintaining hot ductility is less than 0.40%.

[Ni: 0% or more, less than 0.50%]

Ni improves core part hardness and surface hardness by solid solution strengthening. In order to exert the action of strengthening the solubility of Ni, it is preferable to contain 0.01% or more. On the other hand, when the content of Ni is 0.50% or more, the hardness after the steel bars and the wire or hot forging becomes excessively high, so that the cutting workability is remarkably lowered and the alloy cost is increased. A preferable range of the 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 to improve the core portion hardness and surface hardness. In order to obtain this action, it is preferable that Ti is 0.005% or more. On the other hand, if the content of Ti is 0.05% or more, the effect of improving the hardness of the core portion and the hardness of the surface layer is saturated, and the alloy cost is increased. The preferable range of the Ti content is less than 0.007 to 0.04%.

Next, the compound layer of the nitride-treated part of the present invention will be described.

[Thickness of compound layer: 3 m or more and less than 15 m]

The compound layer is a layer of iron nitride formed by nitriding treatment, and its thickness affects the bending corrective property and the bending strength of the nitrided component. If the compound layer is excessively thick, it tends to be a bending fatigue fracture origin. If the compound layer is excessively thin, the residual stress on the surface can not be sufficiently obtained, and the bending proofness and the bending fatigue strength are lowered. In the nitriding treated part of the present invention, the thickness of the compound layer is set to be not less than 3 탆 and less than 15 탆 from the viewpoint of bending correction and bending strength.

After the gas nitriding treatment, the thickness of the compound layer is measured by polishing with a vertical cross section of the specimen, etching and observing with an optical microscope. The etching is carried out for 3 to 20% for 20 to 30 seconds. The compound layer is present in the surface layer of the low alloy steel and is observed as a layer of white non-corrosive. Observation of a 5-field (field of view: 2.2 x 10 ⁴ 탆 ² ) tissue photograph taken 500 times by an optical microscope. In each field of view, four points are measured every 30 탆 in the horizontal direction. The average value of the measured 20 points is defined as the compound thickness (占 퐉). Fig. 1 schematically shows a measurement method, and Fig. 2 shows an example of a tissue photograph of a compound layer and a diffusion layer.

[Gamma] 'phase ratio of the surface layer to the 5 [micro] m compound layer: 50% or more]

When the ratio of the? 'Phase is low and the? Phase ratio is high in the compound layer of the surface to 5 占 퐉, the compound layer tends to become a fracture origin upon bending correction or bending fatigue. This is because the fracture toughness value of the? Phase is lower than that of the? 'Phase. Further, when the phase in the vicinity of the surface is? 'Phase, the compressive residual stress, which will be described later, can be easily introduced into the surface, and the fatigue strength can be improved.

The ratio of the? 'Phase in the compound layer is determined by Electron Back Scattering Diffraction (EBSD). More specifically, an EBSD measurement is performed for an area of 150 mu m ² from the outermost surface of the compound layer to a depth of 5 mu m, and an analysis chart for discriminating the? 'Phase and the? Phase is prepared. Then, with respect to the obtained EBSD analysis image, the area ratio of? 'Phase is determined using an image processing application, and this is defined as a ratio of?' Phase (%). It is appropriate to measure the EBSD at 10 magnifications at a magnification of about 4000 times.

The? 'Phase ratio means the ratio of?' Of the "compound layer" to the surface to a depth of 5 μm. That is, when the thickness of the compound layer is not satisfied by 5 mu m from the surface, the ratio of the? 'Phase in the region corresponding to the thickness of the compound layer is calculated. As an example, if the thickness of the compound is 3 mu m from the surface, the ratio of the gamma prime phase of the compound layer at the surface to the depth of 3 mu m becomes the gamma prime phase ratio.

The ratio of? 'phase is preferably at least 60%, more preferably at least 65%, even more preferably at least 70%.

The γ 'phase ratio may be determined by X-ray diffraction. However, in the measurement by X-ray diffraction, the measurement region becomes ambiguous and an accurate? 'Phase ratio can not be obtained. Therefore, the ratio of the? 'Phase in the compound layer in the present invention is determined by EBSD.

[Pore area ratio of the surface layer to 3 탆 compound layer: less than 10%] [

The voids of the compound layer of the surface to 3 mu m form stress concentration and become a starting point of bending fatigue fracture. Therefore, the void area ratio should be less than 10%.

The voids are formed by removing N ₂ gas from the surface of the steel material along the grain boundaries from an energy-stabilized place such as a grain boundary on a steel material surface having a small binding force by the base metal. The generation of N ₂ is more likely to occur as the nitridation potential K _N described later is higher. This is because the phase transition from bcc to gamma 'to epsilon takes place as K _N increases and the epsilon phase is more likely to generate N ₂ gas because the epsilon phase is larger in N ₂ than in the gamma phase. Fig. 3 schematically shows the formation of voids in the compound layer, and Fig. 4 shows a tissue photograph in which voids are formed.

The void area ratio can be measured by observation under an optical microscope. In the, range of 3㎛ depth from the outermost surface for each field: Specifically, the surface to a depth of 3㎛, magnification 1000 times 5 visual field measurement (5.6 × 10 ³ ㎛ ² visual field area) of the Disclosure The section The ratio of the voids occupied is defined as the void area ratio.

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

[Compressive residual stress on the surface of the compound layer: 500 MPa or more]

In the nitriding treated part of the present invention, the surface of the steel is hardened by the nitriding treatment, and the compressive residual stress is introduced into the surface layer part of the steel, thereby improving the fatigue strength and wear resistance of the part. The nitriding treated part of the present invention has excellent bending fatigue strength by making the above-mentioned improvement in the compound layer and further introducing a compression residual stress of 500 MPa or more on the surface. A manufacturing method for introducing such compressive residual stress to the surface of the component will be described later.

Next, an example of a method for producing a nitrided processed part of the present invention will be described.

In the method for producing a nitrided part of the present invention, the steel having the above-mentioned components is subjected to a gas nitriding treatment. The treating temperature of the gas nitriding treatment is 550 to 620 占 폚, and the treating time of the entire gas nitriding treatment is 1.5 to 10 hours.

[Processing temperature: 550 to 620 DEG C]

The temperature (nitriding temperature) of the gas nitriding process is mainly correlated with the diffusion rate of nitrogen, and affects the surface hardness and the depth of the hardened layer. When the nitriding treatment temperature is excessively low, the diffusion rate of nitrogen is slow, the surface hardness is low, and the depth of the hardened layer becomes shallow. On the other hand, when the nitriding treatment temperature exceeds the A _C1 point, austenite phase (? Phase) having a nitrogen diffusion rate lower than that of the ferrite phase (? Phase) is generated in the steel to lower the surface hardness, Loses. Therefore, in this embodiment, the nitriding treatment temperature is 550 to 620 DEG C around the ferrite temperature region. In this case, lowering of the surface hardness can be suppressed, and the depth of the hardened layer can be suppressed to be shallow.

[Processing time of the entire gas nitriding treatment: 1.5 to 10 hours]

The gas nitriding treatment is performed in an atmosphere containing NH ₃ , H ₂ , and N ₂ . The time of the entire nitriding treatment, that is, the time from the start to the end of the nitriding treatment (treatment time) is correlated with the formation and decomposition of the compound layer and the diffusion penetration of nitrogen, and affects the surface hardness and the depth of the hardened layer. If the treatment time is too short, the surface hardness becomes low and the depth of the hardened layer becomes shallow. On the other hand, if the treatment time is too long, denitrification or decarburization occurs and the surface hardness of the steel is lowered. If the treatment time is too long, the manufacturing cost also increases. Therefore, the entire nitriding treatment time is 1.5 to 10 hours.

Further, the atmosphere of the gas nitriding treatment of the present embodiment includes impurities such as oxygen and carbon dioxide inevitably in addition to NH ₃ , H ₂ and N ₂ . A preferable atmosphere is not less than 99.5% (vol%) in total of NH ₃ , H ₂ and N ₂ .

When gas softening treatment is carried out in an atmosphere containing about several percent of carbon monoxide and carbon dioxide, an 竜 phase with a high solid solubility limit of C is preferentially produced. Since the γ 'layer can hardly employ C, when the softening treatment is carried out, the compound layer becomes an ε single phase. Further, since the growth rate of the epsilon phase is faster than that of the gamma prime phase, the compound layer is formed thicker than necessary in the gas softening in which the epsilon phase is stably produced. Therefore, in the present invention, it is necessary to perform the gas nitriding treatment in which the nitriding potential K _N is controlled as described later, instead of the gas softening treatment.

[Gas Condition for Nitriding Treatment]

In the nitriding treatment method of the present invention, the nitriding treatment is carried out under the controlled nitriding potential in consideration of the C amount of the base. Thereby, the phase structure in the compound layer at the depth of 5 탆 or less from the surface is set to 50% or more of the γ 'phase ratio, the void area ratio at the depth of 3 탆 to the surface is made less than 1% The residual stress can be set to 500 MPa or more.

The nitridation potential K _N of the gas nitridation treatment 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 pressures of the atmosphere NH ₃ and H _{2 in} the gas nitriding process can be controlled by adjusting the flow rate of the gas. In order to form a compound layer by a nitriding treatment, a K _N at the time of gas nitriding process, but is a predetermined value or greater, as described above, K _N is ground excessively high, the proportion of the easy to generate N ₂ gas ε increases And there is a lot of gap. Therefore, it is important to provide a condition of K _N and to suppress the generation of voids.

As a result of the studies conducted by the present inventors, it has been 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 nitrided component, and that the optimum nitriding potential is determined by the C content of the steel.

Concretely, when the C content (% by mass) of the steel is (mass% C), the nitriding potential in the gas nitriding treatment is always 0.15? K _N ? -0.17 x ln (mass% 0.20, it was found that the phase structure of the compound layer was 50% or more of the? 'Phase ratio, and that the nitrided parts had high bending correction and rotational bending fatigue strength.

Even if the average nitridation potential of the gas nitriding process satisfies the above formula, if the nitridation potential value that does not satisfy the above formula is temporarily taken, the ratio of the? 'Phase in the compound layer does not become 50% or more.

Fig. 5 shows the results of investigation of the relationship between the nitriding potential, the γ 'ratio of the compound layer and the rotational bending fatigue strength. Fig. 5 relates to the strength a (Table 1) of the embodiment to be described later.

Thus, in the present nitriding treatment method, the gas nitriding treatment is carried out under the nitriding potential K _N in accordance with the amount of C of the base steel. As a result, it becomes possible to stably impart the? 'Phase to the surface of the steel, and to provide a nitrided component having excellent bending correction and rotational bending fatigue strength, preferably 1.2% or more, and rotational bending fatigue strength of 520 MPa or more Can be obtained.

Example

The steels a to a having the chemical compositions shown in Table 1 were dissolved in a 50 kg vacuum melting furnace to prepare molten steel and molten steel was cast to produce ingots. Further, a to s in Table 1 are steels having the chemical components specified in the present invention. On the other hand, the steels t to aa are rivers of comparative examples excluded from the chemical components prescribed in the present invention by at least one element.

The ingot was hot-forged to form a round bar having a diameter of 25 mm. Subsequently, each of the round rods was annealed, and then subjected to a cutting process to produce a rectangular test piece for evaluation of the bending correction shown in Fig. Further, a circumferential test piece for evaluating the bending fatigue strength shown in Fig. 3 was produced.

The obtained test specimens were subjected to gas nitriding under the following conditions. The test pieces were charged into a gas nitriding furnace, and respective gases of NH ₃ , H ₂ and N ₂ were introduced into the furnace, and nitriding treatment was carried out under the conditions shown in Table 2. Test No. 32 was a gas softening treatment in which CO ₂ gas was added in an amount of 3% by volume in the atmosphere. The test pieces after the gas nitriding treatment were subjected to air cooling using oil at 80 캜.

The H ₂ partial pressure in the atmosphere was measured using a thermally conductive H ₂ sensor mounted directly on the gas nitriding furnace. The difference between the thermal conductivity of the standard gas and that of the measurement gas was measured in terms of gas concentration. The H ₂ partial pressure was continuously measured during the gas nitriding process.

The NH ₃ partial pressure was measured by addition of a passive glass tube NH ₃ analyzer in addition to the furnace.

The partial pressure of the residual NH ₃ was measured every 10 minutes, the nitridation potential K _N was calculated, and the NH ₃ flow rate and the N ₂ flow rate were adjusted so as to converge to the target value. The nitridation potential K _N was calculated every 10 minutes to measure the NH ₃ partial pressure, 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 pore area ratio]

The cross section of the small rollers after the gas nitriding treatment in the direction perpendicular to the longitudinal direction was mirror-polished and etched. The cross section was observed using a scanning electron microscope (SEM), and the thickness of the compound layer and the presence or absence of voids in the surface layer were examined. The etching was carried out for 3 to 20% for 20 to 30 seconds.

The compound layer can be identified as a white non-corrosive layer present in the surface layer. A compound layer was observed from a 10-field (field of view: 6.6 x 10 ² 탆 ² ) tissue photograph taken at 4000 times, and the thickness of three compound layers was measured every 10 탆. The average value of the measured 30 points was defined as the compound thickness (占 퐉).

Similarly, the ratio of the total area of voids (void area ratio, unit:%) in the area of 90 mu m ² from the outermost surface to a depth of 3 mu m was determined by binarization by an image processing application. The average value of the measured 10 fields of view was defined as a void area ratio (%). In the case where the compound layer is less than 3 mu m, the measurement object is similarly measured from the surface to a depth of 3 mu m.

[Measurement of? 'phase ratio]

The ratio of the? 'Phase in the compound layer was determined by Electron Back Scattering Diffraction (EBSD). An EBSD measurement was performed for an area of 150 mu m ² from the outermost surface of the compound layer to a depth of 5 mu m to obtain an analysis chart for discriminating the gamma prime and the epsilon phase. 'Percent (%). EBSD measurements were made at 10 magnifications with a magnification of 4000 times.

The average value of the measured ratios of the gamma prime ratios of the 10 fields of view was defined as the gamma prime ratio (%). When the compound layer is not satisfied at 5 mu m, the ratio of? 'Phase in the region corresponding to the thickness of the compound layer is calculated.

[Compound layer residual stress]

The residual stresses σ _{γ '} , σ _ε and σ _m of the γ' phase, ε phase and the matrix in the condition of Table 3 were measured using a micro-X-ray residual stress measuring device for the nitrile small roller contact portion did. Further, using the γ 'phase, the ε phase and the area ratios V _{γ '} , V _ε , and V _m of the parent layer occupied in an area of 90 μm ^{2 in a} depth of 3 μm from the outermost surface determined by EBSD, The stress σ _c was regarded as the surface residual stress.

σ _c = Vγ'σ _{γ '} + V _ε σ _ε + V _m σ _m

[Bending Correction Property]

A rectangular bending test was performed on the square test piece provided in the gas nitriding treatment. The static bending test was performed by four-point bending with the inner point distance of 30 mm and the outer point distance of 80 mm, and the deformation speed was 2 mm / min. The strain gage was mounted on the R portion in the longitudinal direction of the rectangular test piece, and the maximum strain amount (%) at the time when the R portion was cracked and the strain gauge could not be measured was obtained as bending correction.

In the component of the present invention, it was aimed that the bending correction property was 1.2% or more.

[Rotational bending fatigue strength]

On the circumferential test piece provided for the gas nitriding treatment, Ono type rotational bending fatigue test was carried out. The number of revolutions was 3000 rpm, and the number of times of stopping the test was 1 x 10 ⁷ cycles, which indicates the fatigue limit of ordinary steel. In the rotational bending fatigue test piece, the maximum stress reached at 1 x 10 ⁷ times without occurrence of rupture, Of the fatigue limit.

In the parts of the present invention, it was aimed that the maximum stress in the fatigue limit was 520 MPa or more.

[Test result]

The results are shown in Table 2. Test Nos. 1 to 23 show that the composition of the steel and the conditions of the gas nitriding treatment are within the range of the present invention, the thickness of the compound is 3 to 15 탆, the proportion of the γ 'layer of the compound layer is 50% The compressive residual stress of the compound layer became 500 MPa or more. As a result, good results were obtained with a bending correction of 1.2% or more and a rotational bending fatigue strength of 520 MPa or more.

In Test No. 26, the nitriding temperature was too high. As a result, the γ 'phase ratio of the compound layer was low, the pore area ratio was large, the residual stress became tensile stress, and the rotational bending fatigue strength was low.

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

In Test No. 28, the nitriding time was excessively long, the porosity area ratio became large, and the residual stress on the surface due to this became too low to lower the rotational bending fatigue strength.

Test No. 29 was prematurely destroyed from the parent layer because the nitriding time was too short, a sufficient compound layer thickness could not be obtained, the residual stress on the surface was lowered, and the depth of the hardened layer became shallow.

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

In Test No. 31, the lower limit of the nitriding potential was excessively low, and no compound layer was formed, and the residual stress on the surface was lowered, so that the rotational bending fatigue strength was lowered.

In Test No. 32, the upper limit of the nitriding potential was high, and the porosity area ratio was increased, and the bending correction and the rotational bending fatigue strength were lowered.

In Test No. 33, the upper limit of the nitriding potential was too high, the thickness of the compound layer became thick, the ratio of? 'Phase was low, and the porosity area ratio was increased, so that the bending corrective property and the rotational bending fatigue strength were lowered.

Test No. 34 is a softening treatment, in which the? 'Phase is hardly generated on the surface and the residual stress is lowered, so that the bending corrective property and the rotational bending fatigue strength are lowered.

In Test No. 35, the amount of C in the steel was too high and the thickness of the compound layer became thick, so that the bending correction and the rotational bending fatigue strength were lowered.

Test No. 36 was prematurely destroyed from the parent layer because the amount of C in the steel was excessively low and sufficient core strength was not obtained.

In Test No. 37, the amount of Si in the steel was too high and the hardness of the base material was excessively high, so that the bending correction was low.

Test No. 38 was prematurely destroyed from the parent layer since the amount of Mn in the steel was excessively low and sufficient hardened layer hardness and core hardness were not obtained.

Test No. 39 was prematurely destroyed due to grain boundary segregation of P and formation of coarse MnS because the amounts of P and S in the steel were too high.

In Test No. 40, the amount of Cr in the steel was excessively low, sufficient hardness of the diffusion layer and hardness of the core portion were not obtained, and therefore, it was destroyed prematurely from the parent layer.

In Test No. 41, the amount of Al in the steel was too high, oxide inclusions were generated, and the alloy was destroyed prematurely from the parent layer.

Test No. 42 had a low amount of C and Mn in the steel and a high amount of Cr, so that the hardness of the base material was high and the bending correction and the rotational bending fatigue strength were low.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for practicing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be carried out by appropriately changing the above-described embodiment within the scope not departing from the gist of the present invention.

Claims (1)

In terms of% by mass,
C: not less than 0.20%, not more than 0.60%
Si: not less than 0.05%, not more than 1.5%
Mn: not less than 0.2%, not more than 2.5%
P: 0.025% or less,
S: not less than 0.003%, not more than 0.05%
Cr: not less than 0.05%, not more than 0.50%
Al: 0.01% or more, 0.05% or less,
N: not less than 0.003%, not more than 0.025%
Nb: 0% or more, 0.1% or less,
B: not less than 0%, not more than 0.01%
Mo: 0% or more, less than 0.50%
V: not less than 0%, not more than 0.50%
Cu: not less than 0%, not more than 0.50%
Ni: not less than 0%, less than 0.50%
Ti: 0% or more and less than 0.05%, and the balance being Fe and impurities.
A compound layer formed on the surface of the steel and containing iron, nitrogen and carbon and having a thickness of 3 탆 or more and less than 15 탆,
The phase structure in the compound layer ranging from the surface to the depth of 5 mu m contains the gamma prime phase at an area ratio of 50% or more,
The void area ratio is less than 10% in the range from the surface to the depth of 3 mu m,
When the compressive residual stress on the surface of the compound layer is 500 MPa or more
Wherein the nitriding treatment is performed at a temperature higher than the melting point.

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