KR20190028520A - Nitrided parts and method of manufacturing the same - Google Patents

Abstract

And a compound layer having a thickness of not less than 3 占 퐉 and less than 20 占 퐉 and containing iron, nitrogen, and carbon and formed on the surface of the steel material and having a predetermined chemical composition and having excellent rolling bending fatigue strength in addition to the surface fatigue strength , 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 ratio in the range from the surface to 3 mu m is less than 1% , And the compressive residual stress on the surface of the compound layer is 500 MPa or more.

Description

Nitrided parts and method of manufacturing the same

The present invention relates to a steel component subjected to gas nitriding treatment, particularly a gear having excellent fatigue strength and bending fatigue strength, a nitrided component 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 machining, nitriding and softening in order to improve mechanical properties such as fatigue strength, abrasion resistance and endurance.

The nitriding treatment and the softening treatment are carried out in the ferrite region of A ₁ point or less, and there is no phase transformation during the treatment, so that 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 nitrification treatment is a treatment method in which nitrogen is introduced into the surface of the steel material. Examples of the medium used for the nitriding treatment include gas, salt bath, and plasma. In the transmission parts of automobiles, gas nitriding process with excellent productivity is mainly applied. By the gas nitrification treatment, 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, and a cured layer which is a nitrogen diffusion layer is further formed on the lower surface side of the compound 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 extremely high as compared with the steel as the base material. Therefore, the compound layer improves the wear resistance and the surface fatigue strength of the steel member at the beginning of use.

Patent Document 1 discloses a nitrided processed 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 with a steel structure having a predetermined structure and having a low strain and excellent surface fatigue strength and bending fatigue strength.

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

Patent Document 4 discloses a steel member having a high fatigue resistance and bending fatigue strength and having a low deformation compared with carburizing or carbo-nitriding treatment, wherein a steel compound layer having a predetermined structure is formed on the surface.

Japanese Patent Application Laid-Open No. 2015-117412 Japanese Patent Application Laid-Open No. 2013-221203 International Publication No. 2015/136917 International Publication No. 2013/157579

It is considered that the bending fatigue strength of the nitride-treated part of Patent Document 1 is not sufficient because the surface side of the compound layer tends to be epsilon in terms of gas softening using CO ₂ as the atmospheric gas. Further, 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, Therefore, there is a possibility that the structure and thickness of the compound layer may not be achieved.

The nitriding treatment method of Patent Document 3 is characterized in that the compound layer is thinned by nitriding in two stages of high K _N value treatment and low K _N value treatment. In this method, a nitrided compound layer is provided at the first stage, and a compound layer imparted by the second stage nitriding is diffused by diffusing N into the hardened layer to deepen the effective hardened layer, but a complicated process such as two-stage nitriding is required. Further, the ratio of? 'Phase is lowered, and it tends to be a starting point of fitting or bending fatigue failure.

Since the nitriding treatment of Patent Document 4 has a wide control range of various gas partial pressures during the treatment, there is a possibility that the ratio of? 'Phase is lowered or the porosity is increased.

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

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 amount of C on the base, the vicinity of the surface was made to have the phase structure of? 'Phase main body to suppress the occurrence of porosity, It has been found that a nitriding part having excellent surface fatigue strength and rotational bending fatigue strength can be produced by setting the value to be a predetermined value or more.

The present invention has been further studied on the basis of the above-described findings, and the gist of the present invention is as follows.

0.05 to 0.30%, Si: 0.05 to 1.5%, Mn: 0.2 to 2.5%, P: 0.025% or less S: 0.003 to 0.05%, Cr: more than 0.5% N: not less than 0% and not more than 0.1%, B: not less than 0% and not more than 0.01%, Mo: not less than 0% and not more than 0.50% V: a steel material containing 0% or more and less than 0.50%, Cu: 0% or more and less than 0.50%, Ni: 0% or more and less than 0.50%, and Ti: 0% or more and less than 0.05% And has a compound layer having a thickness of 3 占 퐉 or more and less than 15 占 퐉 and containing iron, nitrogen and carbon formed on the surface of the steel and having a phase structure in a compound layer in a range from the surface to a depth of 5 占 퐉, And the void area ratio was less than 10% in the range from the surface to the depth of 3 탆, and the compressive residual stress on the surface of the compound layer was 500 MPa Nitrided part, characterized in that merchant.

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

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 cavity 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 diagram 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 small roller for testing the roller fitting used for evaluating the surface fatigue strength.
Fig. 7 shows the shape of the large roller for testing the roller fitting used for evaluating the surface fatigue strength.
8 is a cylindrical 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 a steel material to be a 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: 0.05% or more and 0.30% or less]

C is an element necessary for securing the hardness of the core portion of the component. If the content of C is less than 0.05%, the strength of the core portion becomes excessively low, so that the surface fatigue strength and the bending fatigue strength are significantly lowered. On the other hand, if the content of C exceeds 0.30%, the thickness of the compound layer becomes large, and the surface fatigue strength and bending resistance are greatly reduced. The preferable range of the C content is 0.08 to 0.25%.

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

Si enhances hardness of the core portion by solid solution strengthening. Further, the tempering softening resistance is increased, and the surface fatigue strength of the surface of the component which becomes hot under the wear condition is increased. In order to exhibit these effects, 0.05% or more is contained. On the other hand, if the content of Si exceeds 1.5%, the strength after the steel bar, the wire or the hot forging becomes too high, and the cutting workability is greatly reduced. A preferable range of the Si content is 0.08 to 1.3%.

[Mn: not less than 0.2% and not more than 2.5%]

Mn improves the hardness of the core portion by solid solution strengthening. Further, Mn forms fine nitrides (Mn ₃ N ₂ ) in the hardened layer during the nitriding treatment and improves wear resistance and surface fatigue strength by 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 surface fatigue strength is saturated but also the hardness after the steel bars and the wire material or hot forging becomes too high, resulting in a drastic decrease in cutting workability. 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 zero, and the practical lower limit is 0.001%.

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

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

[Cr: more than 0.5% to 2.0% or less]

Cr improves the surface fatigue strength and the bending fatigue strength by precipitation strengthening by forming fine nitride (CrN) in the hardened layer at the time of nitriding treatment. In order to obtain these effects, Cr needs to be more than 0.5%. On the other hand, when the content of Cr exceeds 2.0%, not only the effect of improving the surface fatigue strength is saturated but also the hardness after the steel strip or the wire to be worked or the hot forging becomes excessively high. The preferable range of the Cr content is 0.7 to 1.8%.

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

Al is an element of deoxidation 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: 0.003% or more and 0.025% or less]

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 composition 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 and 0.1% or less]

Nb combines with C and N to form NbC or NbN. The coarsening of the austenite grains is suppressed by the pinning action of NbC and NbN, and the structure of the steel material prior to the nitriding treatment is refined to reduce the fluctuation of the mechanical characteristics of the nitrided parts. 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: not less than 0 and not more than 0.01%]

B has an effect of suppressing grain boundary segregation of P and improving toughness. Also, it bonds with N to form BN to improve cutting performance. These effects can be obtained by adding a small amount of Nb, but in order to obtain a more reliable effect, B is preferably 0.0005% or more. If 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.

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

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

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

V forms fine nitride (VN) at the time of nitriding and softening, and improves surface fatigue strength and bending fatigue strength by precipitation strengthening, thereby increasing 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 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 range of the V content for ensuring cutting workability is less than 0.40%.

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

Cu improves the core hardness of the component and the hardness of the nitrogen diffusion layer as solid solution strengthening elements. 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, if the content of Cu is 0.50% or more, the hardness after the steel strip or the wire material or hot forging becomes too high, so that the cutting workability is significantly lowered and the hot ductility is lowered. This can cause surface scratches. The preferable range of the Cu content for maintaining hot ductility is less than 0.40%.

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

Ni improves core 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, if the content of Ni is 0.50% or more, the hardness after the bar or wire or hot forging becomes excessively high, so that the cutting workability is significantly 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 and less than 0.05%]

Ti combines with N to form TiN, which improves the core portion hardness and surface hardness. In order to obtain this action, Ti is preferably 0.005% or more. On the other hand, when the content of Ti is 0.05% or more, the effect of improving the core portion hardness and surface hardness 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 the thickness thereof affects the surface fatigue strength and bending strength of the nitrided component. If the compound layer is too thick, it tends to be a starting point of fracture due to fitting or bending. If the compound layer is too thin, residual stress on the surface can not be sufficiently obtained, and surface fatigue strength and bending 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 surface fatigue strength 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 noncorrosive. Observation of a 5-field (field of view: 2.2 x 10 ⁴ 탆 ² ) tissue photograph taken at 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.

[The ratio of? 'Phase of the compound layer of 5 占 퐉 from the surface: 50% or more]

When the ratio of the gamma prime phase to the 5 mu m compound layer from the surface is low and the epsilon phase ratio is high, it is likely to become a starting point of fitting or bending fatigue failure. This is because the fracture toughness value of? Phase is lower than that of? 'Phase. Further, as compared with the case where the phase in the vicinity of the surface is? 'Phase, it is easier to introduce the compressive residual stress, which will be described later, on the surface, and the fatigue strength can be improved.

The ratio of the γ 'phase in the compound layer is determined by Electron Back Scattering Diffraction (EBSD). Specifically, 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? 'Phase and? Phase is prepared. Then, with respect to the obtained EBSD analysis image, the area ratio of? 'Phase is determined by using an image processing application, and this is defined as a ratio of?' Phase (%). In the EBSD measurement, it is appropriate to measure about 10 fields at a magnification of about 4000 times.

The above-mentioned? 'Phase ratio means a ratio of?' Of the "compound layer" at a depth of 5 μm from the surface. That is, when the thickness of the compound layer is not satisfied with 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 a depth of 3 mu m from the surface 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 ratio of the? 'phase may be determined by using 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 3 占 퐉 compound layer from the surface: less than 10%]

When voids are present in the compound layer of 3 mu m from the surface, stress concentration occurs and becomes a starting point of fitting and bending fatigue failure. Therefore, the void area ratio should be less than 10%.

The voids are formed on the surface of a steel material having a small binding force by the base material, and N ₂ gas is desorbed from the surface of the steel material along the grain boundaries from an energy stabilized place such as grain boundary. The generation of N ₂ is more likely to occur as the nitridation potential K _N described later is higher. This is because due to K _N is the higher, bcc → γ 'is the transformation of a → ε occurs, γ' phase than the side on the ε a capacity of N ₂ in size, easy to this side on the ε generating a N ₂ gas . Fig. 3 schematically shows the formation of voids in the compound layer, and Fig. 4 shows a photograph of the structure formed with voids.

The void area ratio can be measured by optical microscope observation. Specifically, the depth of 3 mu m from the surface in the cross section of the specimen was measured at 5 times the field of view (visual field area: 5.6 x 10 ³ mu m ² ) at a magnification of 1000 times, and in the range of depth from the outermost surface to 3 mu m 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 nitride-treated part of the present invention has excellent surface fatigue strength and rotational bending fatigue strength by introducing the compound layer into the above-described structure and introducing a compressive residual stress of 500 MPa or more on the surface thereof. A manufacturing method for introducing such a compressive residual stress to the surface of the component will be described later.

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

In the method for producing a nitrided part of the present invention, a steel material having the above-described 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 of the gas nitriding treatment (nitriding treatment temperature) is mainly correlated with the diffusion rate of nitrogen, and affects the surface hardness and the depth of the hardened layer. If the nitriding treatment temperature is too low, the diffusion rate of nitrogen is slow, the surface hardness becomes 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 from being 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 total 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 excessively long, denitrification and decarburization are generated to lower the surface hardness of the steel. If the processing time is too long, the manufacturing cost becomes higher. Therefore, the entire nitriding treatment time is 1.5 to 10 hours.

In addition, NH ₃ , H ₂ and N _{2 as} well as impurities such as oxygen and carbon dioxide are inevitably contained in the atmosphere of the gas nitriding process of the present embodiment. A preferable atmosphere is not less than 99.5% (volume%) of NH ₃ , H ₂ and N ₂ in total.

When the gas softening treatment is carried out in an atmosphere containing about several percent of carbon monoxide and carbon dioxide, the high ε phase solidified by 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 not to perform the gas softening treatment but to perform the gas nitriding treatment in which the nitriding potential K _N is controlled as described later.

[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. As a result, the phase structure in the compound layer at a depth of 5 占 퐉 from the surface is set to 50% or more of the? 'Phase ratio, the void area ratio at a depth of 3 占 퐉 from the surface is made less than 10% The residual stress can be 500 MPa or more.

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

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

The partial pressures of NH ₃ and H ₂ in the atmosphere of the gas nitriding treatment can be controlled by adjusting the flow rate of the gas. In order to form the compound layer by the nitriding treatment, it is necessary that K _N in the gas nitriding treatment is a certain value or more. However, as K _N becomes excessively high as described above, the ratio of the ε phase, which easily generates N ₂ gas, , And more pores. 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.

More specifically, Steel C content (mass%) of (weight% C) that have been time, the potential at the time of nitriding gas nitriding treatment, always 0.15≤K _N ≤-0.17 × ln the gas nitriding treatment (weight% C) +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 component had a higher rotational bending fatigue strength and surface 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? 'Phase ratio in the compound layer does not become 50% or more.

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

Thus, in the present nitriding treatment method, the gas nitriding treatment is performed under the nitriding potential K _N according to the amount of C of the underlying steel. As a result, it is possible to stably impart the? 'Phase to the surface of the steel and to provide a nitriding treatment part having excellent surface fatigue strength and rotational bending fatigue strength, preferably surface fatigue strength of not less than 2000 MPa and rotational bending fatigue strength of not less than 600 MPa Can be obtained.

Example

Steels a to aa 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 at least one element or more, and are comparative steels deviating from the chemical components specified in the present invention.

The ingot was hot-forged to form a round bar having a diameter of 40 mm. Subsequently, each of the round rods was annealed, and then a cutting process was performed to produce a small roller for testing the roller fitting and a large roller shown in Fig. 7 for evaluating the surface fatigue strength shown in Fig. Further, a cylindrical test piece for evaluating the bending fatigue strength shown in Fig. 8 was produced.

The obtained test specimens were subjected to gas nitridation under the following conditions. The test piece was charged into a gas nitriding furnace, and each gas of NH ₃ , H ₂ and N ₂ was introduced into the furnace, and nitriding treatment was carried out under the conditions shown in Table 2. Test No. 34 was a gas softening treatment in which CO ₂ gas was added in an amount of 3% by volume in the atmosphere. Test No. 35 was a two-stage nitriding treatment in which the nitriding conditions were changed to the first half and the second half. Test No. 36 corresponds to Example 16 in Patent Document 3. 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. H ₂ partial pressure was continuously measured during the gas nitriding process.

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

The nitridation potential K _N was calculated at the same time as the partial pressure of the residual NH ₃ was measured every 10 minutes, 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 checked. The etching was carried out for 3 to 20% for 20 to 30 seconds in the leaving solution.

The compound layer can be identified as a white noncorrosive 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 the pores occupied in the area of the range of ² 90㎛ 3㎛ depth from the outermost surface was determined by binarized by the (pore area ratio, the unit is%), the 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 prepare an analysis chart for discriminating the phases of? 'And? Phases. The resulting EBSD analysis image was subjected to γ- 'Percent (%). EBSD measurements were made at 10 magnifications with a magnification of 4000 times.

The average value of the measured γ 'phase ratios at 10 fields was defined as a γ' phase ratio (%). When the compound layer is not satisfied by 5 mu m, the ratio of the? '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 occupying 90 μm ^{2 in the} depth of 3 μm from the outermost surface obtained by EBSD, The stress σ _c was regarded as the surface residual stress.

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

[Evaluation of surface fatigue strength evaluation]

The small rollers for testing the roller fittings were subjected to finishing of the gripping parts for the purpose of excluding the heat treatment deformation, and then provided to the roller fitting test pieces. The shape after finishing is shown in Fig.

The roller fitting test was carried out under the conditions shown in Table 4 with a combination of the above-described small roller test roller test piece and the roller fitting test rollers of the shape shown in FIG.

The unit of dimensions in Figs. 2 and 3 is " mm ". The above-described roller fitting test bench roller was manufactured by a general manufacturing process, that is, a process of "Bonding -> test piece processing - gas carburization by gas carburization -> low temperature tempering - polishing" using steel satisfying the standard of SCM420 of JIS And the Vickers hardness Hv at a position of 0.05 mm from the surface, that is, at a depth of 0.05 mm, was in a range of 740 to 760, and a depth of Vickers hardness Hv of 550 or more was in a range of 0.8 to 1.0 mm.

Table 4 shows the test conditions under which the surface fatigue strength was evaluated. The number of times of stopping the test was 2 x 10 ⁷ circuits representing a general steel fatigue source, and the maximum surface pressure reached at 2 x 10 ⁷ times without occurrence of fitting in the small roller test specimen was defined as the fatigue limit of the small roller test piece.

The occurrence of the fitting was detected by a vibration system provided in the tester, and after the occurrence of the vibration, the rotation of both the small roller test piece and the large roller test piece was stopped to confirm the occurrence of fitting and the number of revolutions.

In the parts of the present invention, it is aimed that the maximum surface pressure in the fatigue limit is 2000 MPa or more.

[Internal bending fatigue strength]

On-cylinder rotary bending fatigue tests were performed on the cylindrical test specimens provided for the gas nitriding treatment. 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 600 MPa or more.

[Test result]

The results are shown in Table 2. Test Nos. 1 to 25 show that the steel composition and the conditions for the gas nitriding treatment were within the range of the present invention, the compound thickness was 3 to 15 탆, the γ 'layer ratio of the compound layer was 50% The residual compressive stress of 500 MPa or more was obtained. As a result, a satisfactory result was obtained that the surface fatigue strength was 2000 MPa or more and the rotational bending fatigue strength was 600 MPa or more.

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

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

Test No. 28 was too long in nitriding time to increase the porosity area ratio, resulting in lowering of the residual stress on the surface and lowering the rotational bending fatigue strength.

Test No. 29 had an excessively short nitriding time and a sufficient compound layer thickness was not obtained, and the residual stress on the surface was lowered, so that the surface fatigue strength and the rotational bending fatigue strength were lowered.

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

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

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

In test No. 33, the upper and lower limits of the nitriding potential were not suitable, and the thickness of the compound layer became thick, and the void area ratio increased, so that the surface fatigue strength and the rotational bending fatigue strength were lowered.

Test No. 34 was a softening treatment, and the surface fatigue strength and the rotational bending fatigue strength were lower because the residual stress was low and the? 'Phase was hardly generated on the surface.

In Test No. 35, the average K _N was appropriate and the γ 'phase ratio was high, but the upper limit of K _{N in} the nitriding treatment was high and the void area ratio was increased.

Test No. 36 had a too high C content of steel and a thicker compound layer, so that the surface fatigue strength was lowered.

Test No. 37 was prematurely destroyed with the parent layer as the starting point because the amount of C in the steel was too low and sufficient core strength was not obtained.

In Test No. 38, the amount of Si in the steel was too low, and sufficient core hardness was not obtained, so that it was destroyed prematurely from the parent layer.

Test No. 39 was prematurely destroyed starting from the parent layer because the amount of Mn in the steel was too low and sufficient hardened layer hardness and core hardness were not obtained.

Test No. 40 was excessively high in P and S contents of the steel and was destroyed prematurely due to grain boundary segregation of P and formation of coarse MnS.

In Test No. 41, the amount of Cr in the steel was too low, and sufficient hardness of the diffusion layer and hardness of the core portion were not obtained.

In Test No. 42, the amount of Al in the steel was excessively high, and oxide inclusions were generated, and were destroyed prematurely from the parent layer.

Test No. 43 had lower surface fatigue strength and rotational bending fatigue strength because the C amount, Mn amount and Cr amount of the steel were low, the upper limit of the nitriding potential was high and the void area ratio was increased.

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 0.05% and not more than 0.30%
Si: not less than 0.05% and not more than 1.5%
Mn: not less than 0.2% and not more than 2.5%
P: 0.025% or less,
S: not less than 0.003% and not more than 0.05%
Cr: more than 0.5% and not more than 2.0%
Al: 0.01% or more and 0.05% or less,
N: not less than 0.003% and not more than 0.025%
Nb: 0% or more and 0.1% or less,
B: not less than 0% and not more than 0.01%
Mo: 0% or more and less than 0.50%
V: not less than 0% and not more than 0.50%
Cu: not less than 0% and not more than 0.50%
Ni: not less than 0% and less than 0.50%
Ti: 0% to 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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