Railway axles

Abstract

The present invention provides a railway axle which can obtain a sufficient fatigue limit in a non-fitting part and increase the fatigue limit of a fitting part. The railway axle (1) according to the present disclosure comprises a fitting part (2) which has diameter DW mm and a non-fitting part (3). The non-fitting part (3) includes a parallel part (31) which has diameter DA mm smaller than diameter DW mm, and a fillet R part (32). DW / DA is 1.10 – 1.30, and when in the surface of the fillet R part (32), the curvature radius of a portion adjacent to the surface of the parallel part (31) is defined as Rf, and the smaller overall cured layer depth among the overall cured layer depths at positions of 0.23 Rf and 0.57 Rf in an axial direction of the railway axle (1) in a parallel part (31) side from an R end (2E), which is an end in contact with the parallel part (31) in the fillet R part (32), is defined as a reference overall cured layer depth CAT mm, the curvature radius Rf is a threshold curvature radius of Rcr mm to 60.0 mm defined in Equation (1).　

Description

[Technical Field]

[0001] This invention relates to an axle, and more specifically, to a railway axle used in railway vehicles. [Background technology]

[0002] A railway axle comprises a press-fit portion that can be pressed into a railway wheel, and a non-press-fit portion connected to the press-fit portion. Specifically, the press-fit portion of the railway axle is designed such that the diameter of the hole in the boss portion of the railway wheel is slightly smaller than the diameter of the press-fit portion of the railway axle. In use, the railway axle supports the weight of the railway vehicle. Furthermore, each time the railway vehicle passes over a curved rail (curve passage), the railway axle is subjected to a horizontal force due to the contact between the railway wheel and the rail. In other words, when passing over a curve, the railway axle is repeatedly subjected to rotational bending stress with each rotation of the railway wheel. The amplitude of this bending stress increases when passing over a curve.

[0003] As described above, in railway axles, the mating portion into which the railway wheel is pressed has contact pressure against the hole in the boss portion of the railway wheel. Therefore, minute slippage can repeatedly occur in the mating portion of the railway wheel due to contact with the railway wheel. Hereinafter, this minute slippage caused by contact between the mating portion of the railway wheel and the railway wheel will also be called fretting. It is known that the mating portion of railway axles may be damaged by fretting (hereinafter also called "fretting fatigue").

[0004] To suppress such fretting fatigue, high-frequency induction hardening is sometimes performed on the mating surfaces of railway axles. In the region of the mating surface that has been high-frequency induction hardened, the hardness increases. This region of the mating surface that has become harder due to high-frequency induction hardening is called the "hardened layer." Compressive residual stress is generated in the hardened layer. The compressive residual stress generated by the hardened layer suppresses the opening of cracks due to fretting. In other words, the hardened layer of a railway axle formed by high-frequency induction hardening can suppress fretting fatigue of the railway axle.

[0005] Railway axles that suppress fretting fatigue in the mating portion by performing high-frequency induction hardening are proposed in Japanese Patent Publication No. 10-8202 (Patent Document 1), Japanese Patent Publication No. 11-279696 (Patent Document 2), and Japanese Patent Publication No. 2000-73140 (Patent Document 3).

[0006] The railway axle disclosed in Patent Document 1 contains, by mass%, C: 0.3-0.48%, Si: 0.05-1%, Mn: 0.5-2%, Cr: 0.5-1.5%, Mo: 0.15-0.3%, and Ni: 0-2.4%. The mating portion of this railway axle has a hardened layer with a Vickers hardness of 400 or higher. The mating portion further has a region of martensite or bainite inside. In this railway axle, the depth of the hardened layer is 1-4.5 mm. Patent Document 1 states that this railway axle has a high fatigue limit.

[0007] The railway axle disclosed in Patent Document 2 contains, by mass%, C: 0.3-0.48%, Si: 0.05-1%, Mn: 0.5-2%, Cr: 0.5-1.5%, Mo: 0.15-0.3%, and Ni: 0-2.4%. The mating portion of this railway axle has a hardened layer with a Vickers hardness of 400 or more. The mating portion further has a region of tempered martensite or bainite inside. In this railway axle, the depth of the hardened layer is 5.0 mm or more and 10% or less of the diameter of the mating portion. Patent Document 2 states that this railway axle has a high fretting fatigue limit.

[0008] The railway axle disclosed in Patent Document 3 contains, in mass%, C: 0.3 to 0.48%, Si: 0.05 to 1%, Mn: 0.5 to 2%, Cr: 0 to 1.5%, Mo: 0 to 0.3%, and Ni: 0 to 2.4%. The fitting end portion of this railway axle and its surrounding region have a hardened layer with a Vickers hardness of 400 or more. The ratio (K / D) of the thickness (K) of the hardened layer to the diameter (D) of the fitting portion is 0.005 to 0.05. The upper portion of the hardened layer contains 0.02 to 2% of B. This railway axle has an excellent fatigue limit, as described in Patent Document 3.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0010]

Non-Patent Document 1

Summary of the Invention

Problem to be Solved by the Invention

[0011] In the railway axles disclosed in Patent Documents 1 to 3, a technique is disclosed in which induction hardening is performed to suppress fretting fatigue at the fitting portion. When induction hardening is performed on the fitting portion of a railway axle to form a hardened layer, fretting fatigue at the fitting portion can be suppressed.

[0012] As described above, in order to increase the fatigue limit of a railway axle, it is required to increase the fatigue limit of the fitting portion represented by fretting fatigue. On the other hand, for railway axles, a sufficient fatigue limit is also required even at non-fitting portions. Therefore, when increasing the fatigue limit of the fitting portion, it is desirable that a sufficient fatigue limit at the non-fitting portion can be obtained.

[0013] An object of the present disclosure is to provide a railway axle that can obtain a sufficient fatigue limit at a non-fitting portion and can increase the fatigue limit at a fitting portion. [Means for Solving the Problems]

[0014] A railway axle according to the present disclosure comprises: a fitting portion that is cylindrical with a diameter of DW mm and is capable of being press-fitted into a railway wheel; and a non-fitting portion connected to the fitting portion; the non-fitting portion comprises: a cylindrical parallel portion having a diameter of DA mm smaller than the diameter DW mm; and a fillet R portion disposed between the fitting portion and the parallel portion, the fillet R portion having a concavely curved surface in a cross section including the central axis of the railway axle; the fitting portion and the non-fitting portion each include: a hardened layer formed in the surface layer of the fitting portion and the non-fitting portion; and a base material portion inside the hardened layer; the chemical composition of the base material portion, in mass%, is: C: 0.30 to 0.42%, Si: 0.10 to 0.50%, Mn: 0.40 to 1.20%, P: more than 0% and 0.020% or less, S: More than 0% and less than 0.0400%, N: More than 0% and less than 0.0200%, O: more than 0% and less than 0.0040%, Ca: 0~0.0010%, Cr: 0~0.30%, Mo: 0~0.10%, Cu: 0~0.30%, Ni: 0~0.30%, Al: 0~0.100%, V: 0~0.060%, Ti: 0~0.020%, Nb: 0~0.030%, B: 0~0.0050%, and, The remainder consists of Fe and impurities. The ratio of the diameter DW to the diameter DA, DW / DA, is between 1.10 and 1.30. In the cross-section including the central axis of the railway axle, the radius of curvature of the portion of the fillet R portion adjacent to the surface of the parallel portion is Rf, and the smaller of the total hardened layer depths at 0.23Rf and 0.57Rf positions in the axial direction of the railway axle, from the R end (the end of the fillet R portion that is in contact with the parallel portion) toward the parallel portion, is defined as the reference total hardened layer depth C. AT When it is set to mm, The radius of curvature Rf is between the critical radius of curvature Rcr mm and 60.0 mm, as defined by equation (1).

[0015]

number

[0016] The railway axle according to this disclosure provides a sufficient fatigue limit in the non-fitted portion and an increased fatigue limit in the fitted portion. [Brief explanation of the drawing]

[0017] [Figure 1] Figure 1 is a side view of a railway axle according to this embodiment. [Figure 2] Figure 2 is a cross-sectional view of the plane including the central axis C1 of the railway axle of this embodiment shown in Figure 1. [Figure 3] Figure 3 is an S / N diagram organized by representative stress. [Figure 4] Figure 4 shows the location of the crack initiation point in a cross-section including the central axis of the axle under test. [Figure 5] Figure 5 is an enlarged view of the vicinity of the fitting portion 2 of the railway axle 1 in Figure 1. [Figure 6] Figure 6 shows the relationship between the contact angle θ and the fatigue limit improvement rate φ when DW / DA is changed. [Figure 7] Figure 7 shows the relationship between Rf / k and the contact angle θ. [Figure 8] Figure 8 shows the relationship between the radius of curvature Rf and the fatigue limit ratio of the mating section, based on Figures 6 and 7. [Figure 9] Figure 9 shows the relationship between the stress concentration factor Kt at the R end, the radius of curvature ratio Rf / DA, and DW / DA, obtained by FEM analysis. [Figure 10] Figure 10 shows the relationship between the fatigue limit of the non-fitted section and the radius of curvature Rf due to the nominal stress of the parallel section. [Figure 11] Figure 11 shows the relationship between the radius of curvature ratio Rcr / DA and 2CAT / DA. [Modes for carrying out the invention]

[0018] The inventors investigated and examined means to obtain a sufficient fatigue limit in the non-fitted portion of a railway axle and to increase the fatigue limit in the fitted portion. As a result, the inventors obtained the following findings.

[0019] To obtain a sufficient fatigue limit in non-fitted and fitted sections, the chemical composition of the base material inside the hardened layer should be, in mass%, C: 0.30-0.42%, Si: 0.10-0.50%, Mn: 0.40-1.20%, P: greater than 0% and 0.020% or less, S: greater than 0% and 0.0400% or less, N: greater than 0% and 0.0200% or less, O: greater than 0% and 0.0040% Preferably, the chemical composition consists of 0-0.0010%, 0-0.30%, 0-0.10%, 0-0.30%, 0-0.30%, 0-0.00%, 0-0.00%, 0-0.00%, 0-0.00%, 0-0.00%, 0-0.00%, 0-0.00%, 0-0.00%, 0-0.00%, 0-0.00%, and the remainder being Fe and impurities.

[0020] The inventors further investigated, focusing on the shape of the railway axle, a means of increasing the fatigue limit of the fitted portion while maintaining a sufficient fatigue limit in the non-fitted portion of the railway axle having the above chemical composition.

[0021] In railway axles, fatigue design is generally performed separately for the mating and non-matting sections. The non-matting section includes a parallel section and a fillet radius section. The fillet radius section has a concave curved surface in the cross-section containing the central axis of the railway axle. The surface of the fillet radius section is smoothly concave. The surface of the fillet radius section in the cross-section containing the central axis of the railway axle may have a single curvature or multiple curvatures. For example, the radius of curvature of the portion of the fillet radius section adjacent to the end surface of the parallel section may be different from the radius of curvature of the portion adjacent to the end surface of the mating section. However, even if there are multiple curvatures, the surface of the fillet radius section is smoothly concave.

[0022] Normally, when a bending moment is applied to a railway axle, stress concentration occurs at the R-end of the fillet R section, which is the end that is in contact with the parallel section. The stress concentration at the R-end is influenced by the radius of curvature Rf of the surface of the fillet R section adjacent to the surface of the parallel section. Specifically, as the radius of curvature Rf of the fillet R section decreases, the stress concentration at the R-end increases. The maximum stress applied to the R-end is given by the stress concentration factor Kt × nominal stress. Therefore, if the radius of curvature Rf of the fillet R section is too small, the stress concentration factor Kt becomes excessively large. As a result, the maximum stress at the R-end becomes excessively large. Consequently, the stress applied to the R-end exceeds the fatigue limit at the R-end, making it more susceptible to crack formation.

[0023] Taking the above factors into consideration, in conventional railway axles, the radius of curvature Rf is set so that the stress at the R end does not exceed the fatigue limit at the R end. As described in Non-Patent Document 1, in conventional railway axles, the radius of curvature Rf of the fillet R section is set to about 100 mm.

[0024] On the other hand, in the fitted section, as will be discussed later, reducing the radius of curvature Rf increases the fatigue limit. However, if the radius of curvature Rf is reduced, stress concentration will occur at the R end of the fillet R section in the non-fitted section, as mentioned above. In other words, reducing the radius of curvature Rf lowers the fatigue limit of the non-fitted section.

[0025] Therefore, the inventors re-investigated whether the weakest point of the non-fitting section is the R-end when a bending moment is applied to a railway axle. As a result, in fatigue tests under high stress and low life conditions, cracks occurred at the surface origin of the R-end. On the other hand, in fatigue tests under low stress and long life conditions near the fatigue limit, cracks occurred not at the surface origin of the R-end, but internally at the quenching boundary (the boundary between the hardened layer and the base material) inside the parallel section. In other words, the inventors' investigation revealed for the first time that cracks in non-fitting sections do not necessarily originate from the surface origin of the R-end, but can also originate from the internal origin of the parallel section.

[0026] Stress concentration is less likely to occur at the internal starting point. Therefore, the stress applied at the internal starting point is close to the nominal stress. Furthermore, the local fatigue limit at the internal starting point is lower than the local fatigue limit at the surface starting point at the R end. For this reason, when reducing the radius of curvature Rf to increase the fatigue limit of the fitted section, even if stress concentration occurs at the R end and the stress applied at the R end increases, if that stress is below the local fatigue limit at the R end, and the stress applied at the internal starting point at that time is also below the local fatigue limit at the internal starting point, it is possible to increase the fatigue limit of the fitted section while maintaining a sufficient fatigue limit in the non-fitted section.

[0027] The railway axle of this embodiment was completed based on the above technical concept, and the gist of the railway axle of this embodiment is as follows.

[0028] The first configuration of the railway axle is, A cylindrical part having a diameter DW mm and capable of being press-fitted onto a railway wheel, comprising a fitting portion, It comprises a non-fitting portion connected to the aforementioned fitting portion, The aforementioned non-fitting portion is, A cylindrical parallel section having a diameter DAmm that is smaller than the aforementioned diameter DWmm, The system includes a fillet R portion, which is positioned between the aforementioned fitting portion and the aforementioned parallel portion, and has a surface that is concavely curved in a cross-section including the central axis of the railway axle, The aforementioned fitted portion and the aforementioned non-fitted portion are, A hardened layer formed on the surface of the aforementioned fitted portion and the aforementioned non-fitted portion, Including the base material portion located inside the hardened layer, The chemical composition of the aforementioned base material is, in mass%, C: 0.30~0.42%, Si: 0.10~0.50%, Mn: 0.40~1.20%, P: more than 0% and less than 0.020%, S: More than 0% and less than 0.0400%, N: More than 0% and less than 0.0200%, O: more than 0% and less than 0.0040%, Ca: 0~0.0010%, Cr: 0~0.30%, Mo: 0~0.10%, Cu: 0~0.30%, Ni: 0~0.30%, Al: 0~0.100%, V: 0~0.060%, Ti: 0~0.020%, Nb: 0~0.030%, B: 0~0.0050%, and, The remainder consists of Fe and impurities. The ratio of the diameter DW to the diameter DA, DW / DA, is between 1.10 and 1.30. In the cross-section including the central axis of the railway axle, the radius of curvature of the portion of the fillet R portion adjacent to the surface of the parallel portion is Rf, and the smaller of the total hardened layer depths at 0.23Rf and 0.57Rf positions in the axial direction of the railway axle, from the R end (the end of the fillet R portion that is in contact with the parallel portion) toward the parallel portion, is defined as the reference total hardened layer depth C. AT When it is set to mm, The radius of curvature Rf is between the critical radius of curvature Rcr mm and 60.0 mm, as defined by equation (1).

[0029]

number

[0030] The second configuration of the railway axle is, A railway axle of the first configuration, The chemical composition of the aforementioned base material is, in mass%, Ca: 0.0001~0.0010%, Cr: 0.01~0.30%, Mo: 0.01~0.10%, Cu: 0.01~0.30%, Ni: 0.01~0.30%, Al: 0.001~0.100%, V: 0.001~0.060%, Ti: 0.001~0.020%, Nb: 0.001~0.030%, and, It contains one or more substances selected from the group consisting of B: 0.0001 to 0.0050%.

[0031] The railway axle according to this embodiment will be described in detail below.

[0032] [Railway axle configuration] Figure 1 is a side view of a railway axle according to this embodiment. Referring to Figure 1, the railway axle 1 of this embodiment comprises a fitting portion 2 and a non-fitting portion 3. The fitting portion 2 is cylindrical with a diameter DW (mm). The central axis of the fitting portion 2 coincides with the central axis C1 of the railway axle 1. The central axis C1 of the railway axle 1 extends in the axial direction of the railway axle 1. The wheel seat end 2E of the fitting portion 2 is connected to the end of the non-fitting portion 3. In Figure 1, the railway axle 1 comprises a pair of fitting portions 2.

[0033] The non-recessed portion 3 is connected to the recessed portion 2. In Figure 1, the non-recessed portion 3 is positioned between the pair of recessed portions 2. The central axis of the non-recessed portion 3 coincides with the central axis C1 of the railway axle 1. The non-recessed portion 3 includes a parallel portion 31 and a fillet R portion 32. The parallel portion 31 is cylindrical with a diameter DA (mm). The diameter DA is smaller than the diameter DW.

[0034] The fillet R portion 32 has a concave curved surface in a cross-section including the central axis C1 of the railway axle 1. The surface of the fillet R portion 32 is smoothly concave. The surface of the fillet R portion 32 in a cross-section including the central axis C1 of the railway axle 1 may have a single curvature or may have multiple curvatures. For example, the radius of curvature of the portion of the fillet R portion 32 adjacent to the surface of the parallel portion 31 may be different from the radius of curvature of the portion adjacent to the surface of the fitting portion 2. However, even if there are multiple curvatures, the surface of the fillet R portion 32 is smoothly concave.

[0035] Rf (mm) is defined as the radius of curvature of the portion of the fillet R portion 32 adjacent to the surface of the parallel portion 31 in the cross-section including the central axis C1 of the railway axle 1. Note that if the surface of the fillet R portion 32 in the cross-section including the central axis C1 of the railway axle 1 has a single curvature, then the radius of curvature of the entire surface of the fillet R portion 32 is Rf (mm).

[0036] As described above, the diameter DA of the parallel section 31 is smaller than the diameter DW of the fitting section 2. Therefore, a step is formed between the parallel section 31 and the fitting section 2. The railway axle 1 may be solid or hollow. The diameter DA of the parallel section 31 is not particularly limited, but is, for example, 100 to 200 mm. The diameter DW of the fitting section 2 is not particularly limited, but is, for example, 110 to 260 mm.

[0037] Figure 2 is a cross-sectional view of the railway axle 1 shown in Figure 1, including the central axis C1. Referring to Figure 2, the mating portion 2 and the non-matting portion 3 include a hardened layer HL and a base material portion BM. The hardened layer HL is formed on the surface of the mating portion 2 and the non-matting portion 3. Specifically, the hardened layer HL is formed on the surface of the mating portion 2 and the non-matting portion 3 in a range from the surface to a predetermined depth. In Figure 2, the hardened layer HL is formed on a part of the surface of the mating portion 2 and not on another part. Thus, the hardened layer HL does not have to be formed on the entire surface of the mating portion 2, and may be formed in at least a portion of the surface of the mating portion 2 in the axial direction of the railway axle 1. The hardened layer HL may also be formed on the entire surface of the mating portion 2. The hardened layer HL is further formed on the entire surface of the non-matting portion 3.

[0038] [Features of the railway axle 1 of the embodiment] The railway axle 1 of this embodiment includes the following features: (Feature 1) The chemical composition of the base material BM is as follows, in mass%, C: 0.30-0.42%, Si: 0.10-0.50%, Mn: 0.40-1.20%, P: greater than 0% and less than or equal to 0.020%, S: greater than 0% and less than or equal to 0.0400%, N: greater than 0% and less than or equal to 0.0200%, O: greater than 0% and less than or equal to 0.0040%, Ca: 0-0.0010%, Cr: 0-0.30%, Mo: 0-0.10%, Cu: 0-0.30%, Ni: 0-0.30%, Al: 0-0.100%, V: 0-0.060%, Ti: 0-0.020%, Nb: 0-0.030%, B: 0-0.0050%, with the remainder being Fe and impurities. (Feature 2) The ratio of diameter DW to diameter DA, DW / DA, is between 1.10 and 1.30. (Feature 3) The smaller of the two total hardened layer depths, C, is determined by moving from the R end 32E, which is the end of the fillet R portion 32 that contacts the parallel portion 31, toward the parallel portion 31, toward the axial direction of the railway axle 1, at positions 0.23Rf and 0.57Rf. AT When given in mm, the radius of curvature Rf is between the critical radius of curvature Rcr mm and 60.0 mm, as defined by equation (1).

[0039]

number

[0040] [Regarding the chemical composition of the base material BM] In this embodiment, the chemical composition of the base material BM contains the following elements. Unless otherwise specified, percentages for elements refer to mass percentages.

[0041] C: 0.30~0.42% Carbon (C) increases the hardness of steel. Furthermore, C increases the hardness of the hardened layer (HL) formed by high-frequency induction hardening. The above effects are sufficiently obtained when the C content is 0.30% or higher. On the other hand, if the C content is 0.42% or lower, quench cracking during high-frequency induction hardening is sufficiently suppressed. Therefore, the C content is between 0.30% and 0.42%. The preferred lower limit for the C content is 0.32%, and more preferably 0.35%. The preferred upper limit for the C content is 0.40%, and more preferably 0.38%.

[0042] Si: 0.10~0.50% Silicon (Si) deoxidizes steel. Furthermore, Si increases the tempering softening resistance of steel, thereby increasing the fatigue limit of the railway axle 1. The above effects are sufficiently obtained when the Si content is 0.10% or more. On the other hand, if the Si content is 0.50% or less, quench cracking during high-frequency induction hardening is sufficiently suppressed. Therefore, the Si content is 0.10 to 0.50%. The preferred lower limit for the Si content is 0.12%, more preferably 0.15%, and even more preferably 0.18%. The preferred upper limit for the Si content is 0.48%, more preferably 0.45%, and even more preferably 0.40%.

[0043] Mn: 0.40~1.20% Manganese (Mn) enhances the hardenability of steel, allowing the hardened layer HL to become sufficiently thick, thereby increasing the fatigue limit of the railway axle 1. A Mn content of 0.40% or more provides sufficient benefits. On the other hand, a Mn content of 1.20% or less suppresses excessive thickness of the hardened layer HL in the mating portion 2. Therefore, a sufficient fatigue limit is achieved in the mating portion 2. Consequently, the Mn content is between 0.40% and 1.20%. The preferred lower limit of the Mn content is 0.42%, more preferably 0.45%, even more preferably 0.55%, and even more preferably 0.65%. The preferred upper limit for the Mn content is 1.15%, more preferably 1.10%, even more preferably 1.07%, and even more preferably 0.95%.

[0044] P: More than 0% and less than 0.020% Phosphorus (P) is an unavoidable impurity. Therefore, the P content is greater than 0%. P segregates at grain boundaries, reducing the fatigue limit of steel. A sufficient fatigue limit can be obtained in a railway axle 1 if the P content is 0.020% or less. Therefore, the P content is greater than 0% and less than or equal to 0.020%. The preferred upper limit for the P content is 0.018%, more preferably 0.016%, even more preferably 0.015%, and even more preferably 0.014%. A low phosphorus (P) content is preferable. However, excessive reduction of the P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit for the P content is 0.001%, and more preferably 0.002%.

[0045] S: More than 0% and less than 0.0400% Sulfur (S) is an unavoidable impurity. Therefore, the S content is greater than 0%. S combines with Mn to form MnS. MnS reduces the fatigue limit of steel. A sufficient fatigue limit can be obtained in railway axle 1 if the S content is 0.0400% or less. Therefore, the S content is greater than 0% and less than or equal to 0.0400%. The preferred upper limit for the S content is 0.0300%, more preferably 0.0200%, even more preferably 0.0150%, and even more preferably 0.0100%. A low sulfur (S) content is preferable. However, excessive reduction of the S content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0030%.

[0046] N: More than 0% and less than 0.0200% Nitrogen (N) is an unavoidable impurity. Therefore, the N content is greater than 0%. N combines with Al and other elements to form fine nitrides, refining the crystal grains. However, if the N content is too high, coarse nitrides are formed, reducing the fatigue limit of the steel. If the N content is 0.0200% or less, the formation of coarse nitrides is suppressed. Therefore, a sufficient fatigue limit can be obtained in the railway axle 1. Consequently, the N content is greater than 0% and less than or equal to 0.0200%. The preferred upper limit for the N content is 0.0150%, more preferably 0.0120%, more preferably 0.0100%, more preferably 0.0090%, more preferably 0.0080%, and more preferably 0.0070%. The preferred lower limit for the N content is 0.0010%, more preferably 0.0020%, and even more preferably 0.0030%.

[0047] O: More than 0% and less than 0.0040% Oxygen (O) is an unavoidable impurity. Therefore, the O content is greater than 0%. O can form coarse oxides, which can be the starting point for fatigue failure. If the O content is 0.0040% or less, the formation of coarse oxides is suppressed. Therefore, a sufficient fatigue limit can be obtained in the railway axle 1. Thus, the O content is 0.0040% or less. The preferred upper limit for the O content is 0.0030%, more preferably 0.0025%, even more preferably 0.0020%, and even more preferably 0.0015%. A low oxygen content is preferable. However, excessive reduction of the oxygen content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the oxygen content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%.

[0048] The remainder of the chemical composition of the base material BM of the railway axle 1 according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are unintentionally included when the steel material of the railway axle 1 is industrially manufactured, from the raw materials such as ore, scrap, or the manufacturing environment, and are acceptable within a range that does not adversely affect the railway axle 1 according to this embodiment.

[0049] [About Optional Elements] The chemical composition of the base material BM of the railway axle 1 in this embodiment is further modified by replacing a portion of the Fe with: Ca: 0~0.0010%, Cr: 0~0.30%, Mo: 0~0.10%, Cu: 0~0.30%, Ni: 0~0.30%, Al: 0~0.100%, V: 0~0.060%, Ti: 0~0.020%, Nb: 0~0.030%, and, B: May contain one or more elements selected from the group consisting of 0 to 0.0050%. All of these elements are arbitrary elements. These arbitrary elements are described below.

[0050] [Group 1: Ca] The chemical composition of the base material BM of the railway axle 1 according to this embodiment may further include Ca in place of some of the Fe.

[0051] Ca: 0~0.0010% Calcium (Ca) is an impurity and may be present at 0%. Ca causes silicate inclusions (Group C as defined in JIS G 0555:2003) to aggregate, reducing the fatigue limit. If the Ca content is 0.0010% or less, the aggregation of silicate inclusions can be sufficiently suppressed. As a result, a sufficient fatigue limit can be obtained in the railway axle 1. Therefore, the Ca content is 0 to 0.0010%. A low Ca content is preferable. However, excessive reduction of the Ca content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit for the Ca content is 0.0009%, more preferably 0.0008%, and even more preferably 0.0006%.

[0052] [Group 2: Cr, Mo, Cu, and Ni] The chemical composition of the base material BM of the railway axle 1 according to this embodiment may further include one or more elements selected from the group consisting of Cr, Mo, Cu, and Ni in place of a portion of Fe. These elements are arbitrary and all increase the strength of the steel through solid solution.

[0053] Cr: 0~0.30% Chromium (Cr) is an optional element and does not need to be included. In other words, the Cr content may be 0%. When Cr is included, that is, when the Cr content is greater than 0%, Cr increases the strength of the steel. Even if only a small amount of Cr is included, this effect can be obtained to some extent. On the other hand, if the Cr content is 0.30% or less, it is possible to suppress the hardening layer HL from becoming excessively thick. As a result, a sufficient fatigue limit can be obtained in the railway axle 1. Therefore, the Cr content is 0 to 0.30%. The preferred lower limit for the Cr content is 0.01%, more preferably 0.03%, more preferably 0.06%, and still more preferably 0.10%. The preferred upper limit for the Cr content is 0.28%, more preferably 0.25%, and even more preferably 0.20%.

[0054] Mo: 0~0.10% Molybdenum (Mo) is an optional element and does not need to be present. In other words, the Mo content may be 0%. When Mo is present, that is, when the Mo content is greater than 0%, Mo increases the strength of the steel. Even if only a small amount of Mo is present, the above effect can be obtained to some extent. However, when the Mo content exceeds 0.10%, the above effect saturates. Therefore, the Mo content is between 0% and 0.10%. The preferred lower limit for the Mo content is 0.01%, more preferably 0.02%, more preferably 0.03%, and still more preferably 0.04%. The preferred upper limit for the Mo content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0055] Cu: 0~0.30% Copper (Cu) is an optional element and does not need to be present. In other words, the Cu content may be 0%. When Cu is present, that is, when the Cu content is greater than 0%, Cu increases the strength of the steel. Even a small amount of Cu will provide this effect to some extent. On the other hand, if the Cu content is 0.30% or less, sufficient hot workability can be obtained in the steel. Therefore, the Cu content is between 0% and 0.30%. The preferred lower limit for the Cu content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Cu content is 0.25%, more preferably 0.20%, more preferably 0.15%, more preferably 0.10%, and still more preferably 0.05%.

[0056] Ni: 0~0.30% Nickel (Ni) is an optional element and does not need to be present. That is, the Ni content may be 0%. If Ni is present, i.e., if the Ni content is greater than 0%, Ni increases the strength of the steel. Even a small amount of Ni will provide some degree of this effect. However, if the Ni content exceeds 0.30%, the above effect saturates. Therefore, the Ni content is between 0% and 0.30%. The preferred lower limit for the Ni content is 0.01%, more preferably 0.02%, and even more preferably 0.04%. The preferred upper limit for the Ni content is 0.25%, more preferably less than 0.20%, even more preferably 0.15%, and even more preferably 0.10%.

[0057] [Group 3: Al] The chemical composition of the base material BM of the railway axle 1 according to this embodiment may further include Al instead of some of the Fe.

[0058] Al: 0~0.100% Aluminum (Al) is an optional element and does not need to be present. In other words, the Al content may be 0%. When Al is present, that is, when the Al content is greater than 0%, Al deoxidizes the steel. Al further combines with N to form AlN, refining the crystal grains. As a result, the toughness of the steel increases. Even if only a small amount of Al is present, these effects can be obtained to some extent. On the other hand, if the Al content is 0.100% or less, the formation of coarse oxide inclusions is suppressed. Therefore, a sufficient fatigue limit can be obtained in the railway axle 1. Thus, the Al content is 0 to 0.100%. The preferred lower limit of the Al content is 0.001%, more preferably 0.005%, more preferably 0.007%, more preferably 0.010%, more preferably 0.015%, and more preferably 0.020%. The preferred upper limit for the Al content is 0.080%, more preferably 0.060%, more preferably 0.050%, more preferably 0.045%, and more preferably 0.040%. In this specification, the Al content refers to the content of acid-soluble Al (sol.Al).

[0059] [Group 4: V, Ti, and Nb] The chemical composition of the base material BM of the railway axle 1 according to this embodiment may further include one or more elements selected from the group consisting of V, Ti, and Nb in place of a portion of Fe. These elements are arbitrary and all form precipitates to increase the strength of the steel.

[0060] V: 0~0.060% Vanadium (V) is an optional element and may not be present at all. That is, the V content may be 0%. If V is present, i.e., if the V content is greater than 0%, V combines with N and C to form V(C,N). V(C,N) increases the strength of the steel. Furthermore, V(C,N) refines the crystal grains, increasing the fatigue strength of the steel. Even a small amount of V present provides some degree of these effects. On the other hand, if the V content is 0.060% or less, sufficient toughness can be obtained in the steel. Therefore, the V content is between 0 and 0.060%. The preferred lower limit of the V content is 0.001%, more preferably 0.005%, more preferably 0.008%, and still more preferably 0.010%. The preferred upper limit for the V content is 0.055%, more preferably 0.050%, even more preferably 0.045%, and even more preferably 0.040%.

[0061] Ti: 0~0.020% Titanium (Ti) is an optional element and does not need to be present. That is, the Ti content may be 0%. If Ti is present, i.e., if the Ti content is greater than 0%, Ti combines with N to form fine TiN. TiN increases the strength of the steel. Furthermore, TiN refines the crystal grains and increases the fatigue limit of the steel. Even a small amount of Ti present provides some degree of these effects. On the other hand, if the Ti content is 0.020% or less, sufficient toughness can be obtained in the steel. Therefore, the Ti content is between 0 and 0.020%. The preferred lower limit for the Ti content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the Ti content is 0.018%, more preferably 0.015%, more preferably 0.013%, more preferably 0.010%, and more preferably 0.007%.

[0062] Nb: 0~0.030% Niobium (Nb) is an optional element and may not be present at all. That is, the Nb content may be 0%. If Nb is present, i.e., if the Nb content is greater than 0%, it combines with N and C to form Nb(C,N). Nb(C,N) refines the crystal grains, increasing the strength and toughness of the steel. Even a small amount of Nb can provide this effect to some extent. On the other hand, if the Nb content is 0.030% or less, sufficient toughness can be obtained in the steel. Therefore, the Nb content is typically between 0% and 0.030%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.002%, more preferably 0.003%, and still more preferably 0.005%. The preferred upper limit for the Nb content is 0.029%, more preferably 0.027%, more preferably 0.025%, and still more preferably 0.020%.

[0063] [Group 5:B] The chemical composition of the base material BM of the railway axle 1 according to this embodiment may further include B in place of some of the Fe.

[0064] B: 0~0.0050% Boron (B) is an optional element and does not need to be present. That is, the B content may be 0%. If B is present, i.e., if the B content is greater than 0%, B enhances the hardenability of the steel. Even a small amount of B will provide some degree of this effect. On the other hand, if the B content is 0.0050% or less, sufficient toughness can be obtained in the steel. Therefore, the B content is between 0 and 0.0050%. The preferred lower limit for the B content is 0.0001%, more preferably 0.0003%, even more preferably 0.0005%, and even more preferably 0.0007%. The preferred upper limit for the B content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0065] [About the microstructure of the base material BM] Preferably, the microstructure of the base material BM of the railway axle 1 according to this embodiment is mainly composed of ferrite and pearlite. In this specification, "mainly composed of ferrite and pearlite" means that the total area ratio of ferrite and pearlite in the microstructure is 70% or more. The remainder of the microstructure of the base material BM of the railway axle 1 according to this embodiment is, for example, tempered martensite, bainite, and retained austenite.

[0066] The total area ratio of ferrite and pearlite in the microstructure can be determined by the following method: Take five samples for microstructure observation from any position within the base material BM in a cross section perpendicular to the central axis C1 direction of the mating portion 2 or non-matting portion 3. The cross section perpendicular to the central axis C1 will be used as the observation surface. After polishing the observation surface of each sample to a mirror finish, immerse it in Nital etching solution for about 10 seconds to reveal the microstructure by etching. Observe the etched observation surface with an optical microscope. 40,000 μm per field of view. 2 (Magnification 500x) is used, and one field of view is observed for each sample (i.e., a total of 5 fields of view using 5 samples). In each field of view, ferrite and pearlite are identified based on contrast. The total area of ​​identified ferrite and pearlite and the area of ​​each field of view (40,000 μm²) are calculated. 2 Based on the above, the total area ratio of ferrite and pearlite in each field of view is determined. The arithmetic mean of the total area ratios of ferrite and pearlite obtained in each field of view is defined as the total area ratio of ferrite and pearlite (%).

[0067] [About the hardened layer HL] In this specification, the hardened layer HL refers to the region formed by high-frequency induction hardening, which has a higher Vickers hardness compared to the base material BM. In this specification, the hardened layer HL refers to the region from the surface defined as the total hardened layer depth according to JIS G 0559:2019.

[0068] [Method for measuring the total hardened layer depth] The total hardened layer depth can be determined in accordance with JIS G 0559:2019 by the following method. A macrostructural test is performed according to the same standard.

[0069] Specifically, a test piece is taken at a position 0.23Rf in the axial direction of the railway axle 1, on the parallel section 31 side from the R end 32E, which is the end of the fillet R section 32 that contacts the parallel section 31, and the cross section of the test piece having a cross section perpendicular to the axial direction of the railway axle 1 is taken. This cross section of the test piece is used as the measuring surface. The position of this measuring surface on the railway axle 1 is 0.23Rf in the axial direction of the railway axle 1, on the parallel section 31 side from the R end 32E, which is the end of the fillet R section 32 that contacts the parallel section 31. Four test pieces are taken at 90° intervals around the central axis C1 of the railway axle 1. Each test piece has a measuring surface at the 0.23Rf position. The size of the measuring surface is such that the radial length of the railway axle 1 is 20 mm and the length perpendicular to the radial length (approximately circumferential length) is 15 mm. One side of the measuring surface corresponds to the outer circumferential surface of the parallel section 31 of the railway axle 1. Therefore, that side is slightly curved. This curved side will be referred to as the "curved side." These four test specimens will also be referred to as "0.23 Rf position test specimens."

[0070] Similarly, a test piece is taken at a position 0.57Rf in the axial direction of the railway axle 1, on the parallel section 31 side from the R end 32E, which is the end of the fillet R portion 32 that contacts the parallel section 31, and has a cross section perpendicular to the axial direction of the railway axle 1. This cross section of the test piece is designated as the measuring surface. The position of this measuring surface on the railway axle 1 is 0.57Rf in the axial direction of the railway axle 1, on the parallel section 31 side from the R end 32E, which is the end of the fillet R portion 32 that contacts the parallel section 31. Four test pieces are taken at 90° intervals around the central axis C1 of the railway axle 1. Each test piece has a measuring surface at the 0.57Rf position. The size of the measuring surface is such that the radial length of the railway axle 1 is 20 mm and the length perpendicular to the radial length (approximately circumferential length) is 15 mm. One side of the measuring surface corresponds to the outer circumference of the parallel section 31 of the railway axle 1. Therefore, that side is slightly curved. This curved side will be referred to as the "curved side." These four test specimens will also be referred to as the "0.57 Rf position test specimens."

[0071] The measurement surface of each test specimen is immersed in Nital etching solution for approximately 10 seconds to reveal the microstructure by etching. After etching, the interface between the hardened layer HL and the base material BM can be clearly observed on the measurement surface. The distance from the surface of the hardened layer HL (the center of the curved edge) to this interface (the radial distance of the railway axle 1) is defined as the total hardened layer depth (mm). The arithmetic mean of the four total hardened layer depths measured on the measurement surface of four 0.23Rf position test specimens is defined as the total hardened layer depth (mm) at the 0.23Rf position. The arithmetic mean of the four total hardened layer depths measured on the measurement surface of four 0.57Rf position test specimens is defined as the total hardened layer depth (mm) at the 0.57Rf position.

[0072] [About the microstructure of the hardened layer HL] Preferably, the microstructure of the hardened layer HL consists mainly of martensite and bainite. The microstructure of the hardened layer is well known. In the hardened layer HL, in the region from the surface to half the depth of the total hardened layer, the total area ratio of martensite and bainite is 80% or more.

[0073] The microstructure in the region from the surface of the hardened layer HL to a depth of half the total hardened layer depth can be observed by the following method: Take five specimens from the hardened layer HL that include at least the region from the surface to the total hardened layer depth. The cross section of each specimen perpendicular to the central axis C1 is used as the observation surface. After polishing the observation surface of each sample to a mirror finish, immerse it in Nital etching solution for about 10 seconds to reveal the microstructure by etching. After etching, the interface between the hardened layer HL and the base material BM can be clearly observed on the observation surface. On the observation surface of each specimen, the viewing area is 40,000 μm², centered at a depth of 1 / 2 the total hardened layer depth from the surface of the hardened layer HL. 2 The rectangular observation field (200 μm × 200 μm) is observed using an optical microscope with a magnification of 500x.

[0074] In each observation field, martensite and bainite are identified from other phases (ferrite, etc.) based on contrast. Distinguishing between martensite and bainite by contrast alone is difficult. However, martensite and bainite are easily distinguishable from other structures (ferrite, pearlite, austenite) by contrast alone. The total area of ​​identified martensite and bainite is compared with the area of ​​the observation field (40,000 μm²). 2 Based on the above, the total area percentage of martensite and bainite in each observation field is determined. The arithmetic mean of the total area percentages of martensite and bainite obtained in the five observation fields is defined as the total area percentage of martensite and bainite (%).

[0075] Although the microstructure of the hardened layer HL differs from that of the base material BM, the chemical composition of the hardened layer HL is the same as that of the base material BM. In other words, the hardened layer HL has the chemical composition described in Feature 1.

[0076] [(Feature 2) About DW / DA] In the railway axle 1 according to this embodiment, the ratio of the diameter DW of the fitting portion 2 to the diameter DA of the parallel portion 31, DW / DA, is 1.10 to 1.30.

[0077] As mentioned above, fretting fatigue is prone to occur in the mating portion 2. Fretting fatigue is thought to be caused by repeated minute slippage due to contact between the mating portion 2 and the railway wheel 1 as a result of a bending moment being applied to the railway axle 1. When a bending moment is applied, the stress on the surface of the mating portion 2 changes depending on DW / DA. Specifically, when DW / DA is large, the bending stress on the surface of the mating portion 2 is more easily relieved. Therefore, increasing DW / DA can suppress fretting fatigue in the mating portion 2. In other words, increasing DW / DA raises the fatigue limit in the mating portion 2.

[0078] Therefore, in this embodiment, DW / DA is set to 1.10 or higher. In this case, the fatigue limit in the mating portion 2 is sufficiently increased and fretting fatigue is suppressed. On the other hand, if the diameter ratio DW / DA is too large, the heating and cooling from the non-matting portion 3 to the mating portion 2 during the high-frequency induction hardening process described later will be uneven. As a result, a part of the mating portion 2 may become coarse-grained, or the hardened layer HL of the mating portion 2 may become extremely thin. Therefore, in the railway axle 1 according to this embodiment, DW / DA is 1.10 to 1.30.

[0079] A preferred lower limit for DW / DA is 1.11, more preferably 1.12, even more preferably 1.13, and even more preferably 1.14. A preferred upper limit for DW / DA is 1.25, more preferably 1.22, even more preferably 1.20, even more preferably 1.19, and even more preferably 1.18.

[0080] [(Feature 3) Regarding the radius of curvature Rf of the fillet R section 32] In the railway axle 1 according to this embodiment, the smaller of the total hardened layer depths at positions 0.23Rf and 0.57Rf in the axial direction of the railway axle 1, from the R end 32E, which is the end of the fillet R portion 32 that contacts the surface of the parallel portion 31, toward the parallel portion 31, is set as the reference total hardened layer depth C. AT When the radius of curvature is given as mm, the radius of curvature Rf is between the critical radius of curvature Rcr mm and 60.0 mm, as defined by equation (1).

[0081]

number

[0082] Here, if the fillet R portion 32 has multiple radii of curvature, the radius of curvature Rf of the portion adjacent to the surface of the parallel portion 31 refers to the radius of curvature (mm) in the range of 1 / 4 of the length of the fillet R portion, from the point of contact at the end of the parallel portion 31 in the axial direction of the fillet R portion.

[0083] In the railway axle 1 of this embodiment, the radius of curvature Rf of the portion of the fillet R portion 32 in the cross-section including the central axis C1 of the railway axle 1 that is adjacent to the surface of the parallel portion 31 satisfies the above range, thereby maintaining a sufficient fatigue limit in the non-fit portion 3 while increasing the fatigue limit of the fitted portion 2. This point will be explained below.

[0084] [Examination of fatigue limit at fitting section 2] As described above, fretting fatigue occurs in the interlocking portion 2. Therefore, in the interlocking portion 2, there are fatigue limits related to fracture and fatigue limits related to crack initiation due to fretting. Even if a stress below the fatigue limit related to fracture is applied to the interlocking portion 2, if the applied stress exceeds the fatigue limit related to crack initiation, fretting fatigue will occur. However, if the fatigue limit related to fracture is increased, the fatigue limit related to crack initiation also increases. Therefore, by increasing the fatigue limit related to fracture of the interlocking portion 2, the initiation of cracks caused by fretting can also be suppressed. For these reasons, it is preferable to increase the fatigue limit related to fracture in the interlocking portion 2 as much as possible.

[0085] [Study on Fatigue Limit at Non-Fit Portion 3] In order to increase the fatigue limit of the fit portion 2 while ensuring a sufficient fatigue limit at the non-fit portion 3, the present inventors first performed the following fatigue test on the railway axle 1 having a chemical composition satisfying Feature 1, and investigated the fatigue limit at the non-fit portion 3.

[0086] Specifically, a test axle that satisfies the chemical composition of Feature 1 and has been subjected to induction hardening was prepared. In order to cause fracture at the non-fit portion 3 of the test axle, the diameter DW of the fit portion 2 was set to 169 mm, the diameter DA of the parallel portion 31 of the non-fit portion 3 was set to 130 mm, and DW / DA was set to 1.30. That is, the step difference between the fit portion 2 and the non-fit portion 3 was increased. The total hardened layer depth at the R end 32E of the test axle was 7.7 mm, and the hardened layer depth at the parallel portion 31 was 10.7 mm.

[0087] For the fatigue tests, a fatigue test that implements a rotational bending load mode applying a repeated load with a 90° phase difference using an electro-hydraulic servo fatigue tester equipped with two orthogonal axes of actuators, and a bending fatigue test using a conventional single-axis electro-hydraulic servo fatigue tester were performed. Strain gauges were attached from the fillet R portion 32 to the parallel portion 31 of the test axle, and the stress generated by applying a static bending load before the fatigue test (the axial stress distribution on the surface of the test axle) was measured. Based on the obtained relationship between the stress distribution and the test force, the stress on the surface directly above the position where the crack originated in the fatigue test was determined, and this was taken as the representative stress. In the fatigue test, the stress ratio applied to the test axle was set to -1, and the test frequency was set to 1 Hz. The test environment was room temperature in the atmosphere, and the number of repetitions was 10 7 When no macroscopic crack was confirmed after the cycles, the fatigue test was terminated. Here, the macroscopic crack means a crack having a total length of 10 mm or more as a result of visual confirmation.

[0088] In the fatigue test described above, a macroscopic crack several tens of millimeters in size occurred on the surface of the axle in the region from the fillet radius 32 to the parallel section 31 of the non-fitting section 3. Figure 3 is an S / N diagram organized by representative stress. The fatigue limit of the axle under representative stress was 338 MPa. The fatigue limit under representative stress (338 MPa) is calculated by comparing the representative stress under the conditions in which the longest lifespan and macroscopic crack occurred during the fatigue test with 10 7 The average was calculated using the representative stress under the condition where no macroscopic crack was observed after a certain number of cycles. Here, the representative stress under the latter condition where no macroscopic crack was observed is the stress at the surface position directly above the crack initiation point where the representative stress was determined under the above macroscopic crack initiation conditions, and in Figure 3, 10 7 Of the two points plotted with arrows at the number of repetitions, this corresponds to the higher stress.

[0089] For the axles in which macroscopic cracks were confirmed during fatigue testing, the fracture surface was prepared to determine the crack initiation location. Figure 4 shows the crack initiation location in a cross-section including the central axis of the axle. As a result of testing on multiple axles, a surface-initiated crack (initiation location) was confirmed near the R end 32E of the fillet R portion 32 in one axle. In addition, internally-initiated cracks (initiation location) were confirmed in the base material BM at a depth of 12-14 mm from the surface of the parallel portion 31 in four axles. Referring to Figures 3 and 4, the plots of internally-initiated cracks in the SN curve plots in Figure 3 are marked with a " / ". Referring to Figures 3 and 4, cracks originating on the surface occurred under high stress and short life, while cracks originating internally occurred under low stress and long life.

[0090] The above test results suggest that the fatigue limit at the non-fitting portion 3 of the railway axle 1 is determined by the relationship between the fatigue limit at the R end 32E or the fatigue limit inside the parallel portion 31 and the applied stress. Further investigation revealed that the depth position of the starting point of the internal starting point corresponds to the total hardened layer depth of the hardened layer HL, i.e., the internal firing boundary position. The axial position of the internal starting point on the test axle was 14 to 34 mm from the R end 32E. When this axial position of the internal starting point is expressed using the radius of curvature Rf of the fillet R portion 32, it is 0.23Rf to 0.57Rf mm.

[0091] Considering the above, when a crack occurs in the non-fitting portion 3 of the railway axle 1 that satisfies the chemical composition of Feature 1, the crack does not necessarily originate at a surface point near the R end 32E. Rather, the crack originates at either a surface point near the R end 32E, or at an internal point corresponding to the internal firing boundary position of the hardened layer HL in the axial direction from the R end 32E toward the parallel portion 31, in the range of 0.23Rf to 0.57Rf.

[0092] Here, the fatigue limit (338 MPa) obtained in the fatigue test described above is the surface stress directly above the corresponding internal origin. Specifically, the fatigue limit (338 MPa) of the axle under test is determined from the lowest surface stress among the five origin locations obtained (one surface origin and four internal origins).

[0093] Referring to Figure 3, the fatigue limit at the internal origin is inversely proportional to the ratio of the depth of the internal origin to the radius (DA / 2) of the parallel section 31, relative to the surface stress (fatigue limit). Therefore, in Figure 4, the fatigue limit of 338 MPa is shown for the internal origin at (74,14) in the xz coordinate system. Consequently, the fatigue limit at this internal origin where the surface stress is 338 MPa is 275 MPa. From the above, it can be considered that the fatigue limit when a crack occurs at the internal origin in the parallel section 31 is at least 275 MPa.

[0094] On the other hand, the representative stress (minimum fracture stress) when a crack occurs at the surface origin near the R end 32E is 501 MPa, and the number of fracture cycles is 2.32 × 10⁻⁶. 5 In general, in an SN diagram where internally originating fractures and surface-originating fractures coexist, the number of fracture cycles at the inflection point of the SN curve for surface-originating fractures is 1.00 × 10⁻⁶. 5It is around the number of cycles to fracture. Therefore, based on the number of cycles to fracture, the representative stress (minimum fracture stress) at the surface origin obtained in this fatigue test is considered to be close to the fatigue limit. Furthermore, if the lifespan is longer than the surface origin and fracture occurs at the internal origin, no crack occurs on the surface at least until the fracture life. On the surface, the highest stress occurs near the R end 32E of the fillet R portion 32. Therefore, considering that no crack occurred even at this stress, when determining the fatigue limit at the surface origin, the maximum stress on the surface of the fillet R portion 32 (i.e., the maximum stress near the R end 32E) is used. In this embodiment, the arithmetic mean of the representative stress (minimum fracture stress) of 501 MPa when a crack occurs at the surface origin and the lifespan is longer than the surface origin (494 MPa) is calculated to be 498 MPa. From the above, it can be considered that the fatigue limit when a crack occurs at the surface origin near the R end 32E is at least 498 MPa.

[0095] Based on the experimental results described above, it is assumed that for a railway axle 1 with a chemical composition satisfying characteristic 1, the fatigue limit at the surface starting point of the non-fitting portion 3 (near the R end 32E) is 498 MPa, and the fatigue limit at the internal starting point (internal quenching boundary position) is 275 MPa.

[0096] [Relationship between the shape near the mating portion 2 and the fatigue limit of the mating portion 2] Next, we examine the relationship between the shape near the mating portion 2 and the fatigue limit of the mating portion 2. Figure 5 is an enlarged view of the vicinity of the mating portion 2 of the railway axle 1 in Figure 1. Referring to Figure 5, the fatigue strength of the mating portion 2 can be increased by increasing the step difference k (mm) between the mating portion 2 and the parallel portion 31, or by increasing the mating contact angle θ (°) at the wheel seat end 2E, which is the boundary between the mating portion 2 and the fillet R portion 32. The mating contact angle θ, as shown in Figure 5, refers to the angle (°) made between the tangent to the surface of the fillet R portion 32 at the wheel seat end 2E and the axial direction of the railway axle 1 in a cross-section of the railway axle 1 including the central axis.

[0097] Figure 6 shows the relationship between the contact angle θ and the fatigue limit improvement rate φ of the fitted portion 2 when DW / DA is changed. Here, the fatigue limit improvement rate means the value obtained by subtracting 1.0 from the ratio of the fatigue limit to the fatigue limit of the fitted portion in a railway axle with DW / DA = 1.00 (i.e., a railway axle without a step). As shown in Figure 6, the fatigue limit of the fitted portion 2 increases as DW / DA increases (i.e., as the step k increases) and as the contact angle θ increases. Here, the relationship between the radius of curvature Rf of the fillet R portion 32 and the contact angle θ can be expressed geometrically by the following equation.

[0098]

number

[0099] In the above equation, k = (DW - DA) / 2. Figure 7 shows the relationship between Rf / k obtained from the above equation and the fit contact angle θ. Referring to Figure 7, when the step height k is constant, the larger Rf is, the smaller the fit contact angle θ becomes. In the above explanation, it was assumed that the surface of the fillet R portion 32 has a single radius of curvature Rf, but the above equation can also be applied when the surface of the fillet R portion 32 has multiple radii of curvature. Specifically, when the fillet R portion 32 has multiple radii of curvature, the radius of curvature of the portion of the fillet R portion 32 adjacent to the fit portion 2 is smaller than the radius of curvature Rf of the portion of the surface of the fillet R portion 32 adjacent to the surface of the parallel portion 31. Therefore, in this case, the fit contact angle θ will be larger than when obtained using the radius of curvature Rf in the above equation. A larger fit contact angle θ increases the fatigue limit of the fit portion 2. Based on the above, even when the surface of the fillet R portion 32 has multiple radii of curvature, examining the fit contact angle θ using the radius of curvature Rf results in a more safe and reasonable evaluation.

[0100] Combining the relationship between Figures 6 and 7, we get the result shown in Figure 8. In Figure 8, the fatigue limit of the mating section is normalized as a ratio (matting section fatigue limit ratio) to the fatigue limit of the mating section of a conventional railway axle with a radius of curvature Rf = 100 mm. Referring to Figure 8, if no crack occurs in the non-matting section 3 due to repeated load stress applied to the railway axle 1, the mating section fatigue limit ratio is 1.11 to 1.28 times when DW / DA is 1.10 to 1.30 and the radius of curvature Rf is 60.0 mm. Furthermore, as shown in Figure 8, if the radius of curvature Rf is 60.0 mm or less, the mating section fatigue limit ratio increases even further. Therefore, if the upper limit of the radius of curvature Rf is set to 60.0 mm, a superior fatigue limit can be obtained in the mating section 2 compared to a conventional railway axle with a radius of curvature of about 100 mm.

[0101] [Stress concentration at the R end 32E of the non-fitting portion 3] As described above, in this embodiment, in order to increase the fatigue limit of the mating portion 2 when no crack occurs in the non-matting portion 3, the radius of curvature Rf of the fillet R portion 32 is set to 60.0 mm or less. However, as mentioned above, the smaller the radius of curvature Rf, the greater the stress concentration at the R end 32E. The load stress applied at the surface origin of the R end 32E is given by the stress concentration factor × nominal stress. Therefore, if the stress concentration increases, the stress applied to the R end 32E increases and exceeds the fracture stress (498 MPa). In this case, a sufficient fatigue limit cannot be obtained in the non-matting portion 3.

[0102] Therefore, the influence of the radius of curvature Rf and DW / DA on the stress concentration factor at the R end 32E was evaluated by FEM analysis. For the FEM analysis, the general-purpose code Abaqus from Dassault Systèmes was used. A finite element division using axisymmetric Fourier elements was created, and the element dimensions near the R end were set to Rf / 10 or less. The input material properties were set to Young's modulus of 205.8 GPa and Poisson's ratio of 0.3, and elastic analysis was performed. The obtained results are shown in Figure 9. Referring to Figure 9, the stress concentration factor Kt shows a negative correlation with Rf / DA, which is obtained by dividing the radius of curvature by the diameter of the parallel section. On the other hand, the stress concentration factor Kt is substantially independent of DW / DA.

[0103] [Regarding the lower limit of the radius of curvature Rf] As described above, reducing the radius of curvature Rf increases the stress concentration factor Kt at the R end 32E of the non-fitting portion 3. Therefore, the stress applied to the surface origin of the R end 32E increases, potentially exceeding the fatigue limit (498 MPa) at the surface origin. For this reason, the lower limit of the radius of curvature Rf is defined as follows.

[0104] As described above, in the non-fitted section 3, the fatigue limit at the internal starting point (internal quenching location) is 275 MPa, and the fatigue limit at the surface starting point is 498 MPa. Therefore, even if the stress at the R end 32E increases as a result of reducing the radius of curvature Rf to increase the fatigue limit of the fitted section 2, as long as the stress does not exceed 498 MPa, the internal starting point (internal quenching location) will be the crack initiation point in the non-fitted section 3. Consequently, the fatigue limit in the non-fitted section 3 remains constant. This is because the stress concentration occurring in the fillet R section 32 does not affect the internal starting point, and therefore no increase in stress associated with this stress concentration occurs.

[0105] Therefore, using the fatigue test results described above and the relationship between the stress concentration factor Kt and Rf / DA shown in Figure 9, the relationship between the fatigue limit of the non-fitting portion 3 under the nominal stress of the parallel portion 31 and Rf / DA was determined. An example of the determined results is shown in Figure 10. Figure 10 shows DW / DA = 1.10, DW = 200 mm, total hardened layer depth, i.e., the depth of the internal starting point (depth of the internal firing boundary) C. AT This is an example where =12.0 mm. In this case, if the radius of curvature Rf is greater than or equal to the critical radius of curvature Rcr = 26.0 mm in Figure 10, the crack initiation point will be internal, and the fatigue limit at the non-fitted section 3 will remain constant. On the other hand, if the radius of curvature Rf is less than the critical radius of curvature Rcr, the fatigue limit at the non-fitted section 3 will decrease. In this case, it means that the crack initiation point will be surface-initiated, and that it will be greatly affected by the stress concentration factor Kt.

[0106] In other words, the critical radius of curvature Rcr is equal to the total hardened layer depth C of the hardened layer in the parallel section 31. ATThe radius of curvature Rf is such that the stress at the position (i.e., the internal starting point) is 275 MPa, and the stress at the R end 32E (i.e., the surface starting point) is 498 MPa. If the lower limit of the radius of curvature Rf is the critical radius of curvature Rcr, the fatigue limit at the nominal stress of the non-fitting portion 3 can be maximized (i.e., the fatigue limit at the internal starting point).

[0107] Similar to Figure 10, DW / DA is 1.10 to 1.30, and the total hardened layer depth C of the hardened layer in the parallel section 31. AT The critical radius of curvature Rcr was determined by varying the conditions within the range of 4.0 to 12.0 mm. The calculated critical radius of curvature was divided by the parallel section diameter to obtain Rcr / DA and 2C. AT The relationship with / DA is shown in Figure 11. Here, 2C AT / DA represents the total hardened layer depth C relative to the radius (DA / 2) in the parallel section 31. AT This is the ratio. Referring to Figure 11, in the range of DW / DA from 1.10 to 1.30, the critical radius of curvature Rcr is hardly affected by DW / DA. Figure 11, part 2C AT The Rcr / DA curve for / DA approximates the curve as shown in equation (1).

[0108]

number

[0109] Therefore, in a railway axle 1 that satisfies features 1 and 2, if the radius of curvature Rf of the fillet R section 32 is Rcr mm or more and 60.0 mm or less, the fatigue limit at the nominal stress in the non-fitted section 3 will be maximized. Thus, it is possible to increase the fatigue limit in the fitted section 2 while maintaining the fatigue limit in the non-fitted section 3.

[0110] The preferred lower limit of the radius of curvature Rf is 1.05 Rcr mm, more preferably 1.10 Rcr mm, and even more preferably 1.15 Rcr mm. The preferred upper limit of the radius of curvature Rf is 58.0 mm, more preferably 55.0 mm, and even more preferably 52.0 mm. Note that Rcr is the value obtained by rounding the second decimal place of the value obtained in equation (1) to the first decimal place.

[0111] In this embodiment, the positions 0.23Rf and 0.57Rf in the axial direction of the railway axle 1 from the R end 32E toward the parallel section 31 both correspond to internal starting points, and the fatigue limit is approximately constant. Therefore, the total hardened layer depth at the 0.23Rf and 0.57Rf positions is approximately the same value, but in this embodiment, the smaller value is used as the reference total hardened layer depth C. AT This definition is used because, if the total hardened layer depth changes from 0.23Rf to 0.57Rf, the position with the smallest total hardened layer depth becomes the internal starting point, which affects the fatigue limit.

[0112] As described above, the railway axle 1 of this embodiment satisfies features 1 to 3. Therefore, a sufficient fatigue limit can be obtained in the non-fitted portion, and the fatigue limit of the fitted portion can be increased.

[0113] [Manufacturing method] An example of a method for manufacturing railway axles according to this embodiment will be described.

[0114] Molten steel having a chemical composition satisfying Feature 1 is manufactured. An ingot is manufactured using the molten steel. A rough product having an axle shape is manufactured by hot forging the ingot. The heating temperature of the ingot during hot forging can be within a known temperature range. For example, the heating temperature is 1000 to 1300°C. The manufactured rough product is subjected to quenching and tempering, or normalizing.

[0115] When performing quenching and tempering treatments, the upper limits for quenching and tempering treatments can be limited to well-known conditions. Specifically, in the quenching treatment, the quenching temperature should be set to A c3 The temperature shall be between the transformation point and 1000°C. The crude product shall be held at the quenching temperature, and then rapidly cooled by water cooling or oil cooling. In the tempering process, the tempering temperature shall be set to A c1The temperature should be below the transformation point. The tempering temperature is, for example, 500-700°C. The crude product is held at the tempering temperature and then allowed to cool. When normalizing is performed, the crude product is A c1 The material is held at a heat treatment temperature higher than the transformation point, and then allowed to cool. Alternatively, tempering may be performed after the normalizing process.

[0116] Machining is performed on the rough product, which has undergone hardening and tempering or normalizing treatment, as necessary. Subsequently, high-frequency induction hardening and tempering treatment are performed on the rough product.

[0117] [About high-frequency induction hardening treatment] In high-frequency induction hardening, the surface layer of the rough product is heated by high-frequency induction. c3 After heating to a temperature higher than the transformation point, it is cooled. For example, the hardening temperature in high-frequency induction hardening is A c3 The transformation point is ~1100°C. In this case, the surface layer of the crude product transforms from austenite to martensite or bainite. As a result, a hardened layer is formed on the surface of the crude product.

[0118] High-frequency induction hardening can be carried out using a well-known high-frequency heating device and a well-known cooling device. For example, an annular high-frequency heating device may be used as the high-frequency heating device, and an annular cooling device may be used as the cooling device. In this case, by arranging the central axis C1 of the railway axle 1, the annular high-frequency heating device, and the annular cooling device coaxially, high-frequency induction hardening can be efficiently carried out on the surfaces of the mating portion 2 and non-matting portion 3 of the railway axle 1.

[0119] In the high-frequency induction hardening process according to this embodiment, the surface layer of the crude product is heated by high-frequency induction heating. c3 The heating method is not particularly limited as long as it can be heated above the transformation point. In other words, in this embodiment, well-known high-frequency heating methods can be used as the high-frequency heating method.

[0120] After high-frequency induction hardening, tempering may be performed. In other words, tempering is an optional process. For example, in tempering, the tempering temperature is set to 150-250°C, and the holding time at the tempering temperature is set to 30-150 minutes. After holding, the railway axle is air-cooled.

[0121] If necessary, the rough product that has undergone high-frequency induction hardening is subjected to final machining. At this time, the railway axle 1 is machined (turning and grinding) to satisfy features 2 and 3. The railway axle 1 according to this embodiment is manufactured through the above process.

[0122] The effects of the railway axle 1 of this embodiment will be further described in detail below with reference to examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the railway axle 1 of this embodiment. Therefore, the railway axle 1 of this embodiment is not limited to this one example of conditions. [Examples]

[0123] Multiple railway axles with different radii of curvature in the fillet R section were manufactured, and the fatigue limits in the fitted and unfitted sections were determined using the following method. Railway axles having the chemical compositions shown in Table 1 (Tables 1A and 1B) were manufactured using the following manufacturing method.

[0124] [Table 1A]

[0125] [Table 1B]

[0126] In Table 1, a "-" indicates that the content of the corresponding element is at an impurity level. Specifically, a "-" for Cr, Mo, Cu, and Ni content means that the content of each element is less than 0.01%. A "-" for Al, V, Ti, and Nb content means that the content of the element is less than 0.001%. A "-" for B and Ca content means that the content of each element is less than 0.0001%.

[0127] Molten steel was used to produce an ingot. After heating the ingot to 1250°C, a rough product having an axle shape was produced by hot forging. The rough product had two interlocking parts 2 and a non-interlocking part 3 between the two interlocking parts 2.

[0128] The crude products for each test number were subjected to quenching and tempering. The heat treatment temperature during quenching was the A temperature of the steel for each test number. c3 The temperature was set to 890°C, which is higher than the transformation point. After holding at the heat treatment temperature, oil quenching was performed. Tempering was then performed on the rough products of each test number after quenching. The tempering conditions were as follows: the temperature was held at 640°C for 120 minutes, and then air-cooled to room temperature. After air-cooling, rough machining was performed to dimensions several millimeters larger than the DW and DA of each test number shown in Table 2 below.

[0129] High-frequency induction hardening was performed on the rough products of each test number after rough machining. As a result of the high-frequency induction hardening, a hardened layer HL was formed on the rough products of each test number, as shown in Figures 1 and 2, in the area from near the ring seat end 2E of the mating portion 2 to near the ring seat end 2E of the opposite mating portion 2, including the non-matting portion 3 (parallel portion 31 and fillet R portions 32 on both sides).

[0130] The hardened layer depth was formed by adjusting the output of the high-frequency heating device so that it was approximately the same in the region where the hardened layer HL described above was formed. The hardening temperature during high-frequency induction hardening was the A temperature of the steel for each test number. c3The temperature was above the transformation point. Tempering treatment was performed on the crude products after high-frequency induction hardening. During the tempering treatment, the entire crude product for each test number was held at 200°C for 120 minutes. After holding, the crude products were air-cooled to room temperature.

[0131] After air cooling, the rough products were subjected to finish machining to adjust the radius DA (mm) of the parallel section 31, the radius DW (mm) of the fitting section 2, and the radius of curvature Rf of the fillet R section 32 as shown in Table 2. Through the above manufacturing process, railway axles for each test number (hereinafter referred to as "test axles") were manufactured.

[0132] [Evaluation Test] For each test axle with the manufactured test number, the following fatigue tests and the standard total hardened layer depth C were performed. AT A measurement test was conducted.

[0133] [Fatigue Test] A wheel fixture was pressed into the wheel seat of the mating section 2 on one side of each test axle. A fatigue test was performed on the test axle with the wheel fixture pressed in using a resonant fatigue testing machine. This involved rotating the eccentric mass by a motor attached to the end of the axle opposite to the side where the wheel fixture was pressed in, causing the axle to resonate and performing a rotational bending load mode. A bending fatigue test was also performed using a standard single-axis electro-hydraulic servo-type fatigue testing machine. A strain gauge was attached at a position 0.57Rf from the R end 32E of the fillet R section 32 of the test axle, and the stress generated by applying a static bending load before the fatigue test was measured. The test force was then determined so that this stress was 330 MPa, and the fatigue test described above was performed. In the fatigue test, the stress ratio applied to the test axle was set to -1, and the test frequency was set to 1-20 Hz. The test environment was room temperature, air, with a cycle count of 10 7 If no macroscopic cracks were detected in the first cycle, the fatigue test was terminated.

[0134] If no macroscopic cracks were observed after the fatigue test, the test axle was removed from the wheel jig, and the occurrence of fatigue cracks on the surface of the mating portion 2 was investigated. For the investigation, wear particles were wiped off the surface of the mating portion 2, and it was polished with a buff using alumina polishing powder. Then, the surface of the mating portion 2 was observed using a video microscope at a magnification of 100 to 400 times, and the presence or absence of cracks, and if cracks had occurred, the length of the crack on the observed surface was determined.

[0135] [Reference total hardening layer depth C AT [Measurement Test] Using the axle after testing, the total hardened layer depth at positions 0.23Rf and 0.57Rf in the axial direction of the axle, from the R end 32E toward the parallel section 31, was determined by macrostructural testing according to JIS G 0559:2019 as follows: A sample was taken with a surface (hereinafter referred to as the measurement surface) that included positions 0.23Rf and 0.57Rf and had a cross-section parallel to the central axis C1. The measurement surface of the sample was immersed in Nital etching solution for about 10 seconds to reveal the microstructure by etching. On the measurement surface after etching, the interface between the hardened layer HL and the base material BM was clearly confirmed. Based on the confirmed interface, the total hardened layer depth at positions 0.23Rf and 0.57Rf was determined. Of the two obtained total hardened layer depths, the smaller value was used as the reference total hardened layer depth C. AT (mm)

[0136] Furthermore, in all of the test axles with the specified test numbers, the total area ratio of ferrite and pearlite in the microstructure of the base material BM was 70% or more, and in the hardened layer HL, the total area ratio of martensite and bainite in the region from the surface to half the depth of the total hardened layer was 80% or more.

[0137] [Table 2]

[0138] [Evaluation Results] In Table 2, the "Rf≧Rcr?" column displays "T" when the radius of curvature Rf is greater than or equal to the critical radius of curvature Rcr, and "NT" when the radius of curvature Rf is less than the critical radius of curvature Rcr. Also, in Table 2, the "-" for test numbers 1, 6, 7, and 12 in the "Crack Length at Fit" column means that no crack was observed. Referring to Table 2, test numbers 1, 2, 7, 8, and 13-24 satisfied features 1-3. Therefore, in the fatigue test, 1 × 10 7 No macroscopic cracks were observed after a certain number of cycles, and sufficient fatigue strength was obtained in the non-fitted portion 3. Furthermore, the cracks on the surface of the fitted portion 2 were also very small compared to the comparative examples, test numbers 3-6 and 9-12, and a sufficient fatigue limit was obtained in the fitted portion 2 as well.

[0139] On the other hand, in test numbers 3 and 9, the DW / DA ratio was less than 1.10. Therefore, 1.0 × 10 7 Although no macroscopic cracks were observed in the number of cycles, a large crack occurred in the mating section 2, and a sufficient fatigue limit could not be obtained in the mating section 2.

[0140] In test numbers 4, 5, 10, and 11, the radius of curvature Rf was too large. Therefore, 1.0 × 10 7 Although no macroscopic cracks were observed in the number of cycles, relatively large cracks occurred in the mating section 2, and a sufficient fatigue limit could not be obtained in the mating section 2.

[0141] In tests 6 and 12, the radius of curvature Rf was less than the critical radius of curvature Rcr (mm). Therefore, 1.0 × 10 7 Macroscopic cracks of several tens of millimeters in size occurred on the surface of the fillet radius (i.e., the surface of the non-fitting portion 3) after fewer than a certain number of cycles, and a sufficient fatigue limit could not be obtained in the non-fitting portion 3.

[0142] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure. [Explanation of Symbols]

[0143] 1. Railway axle 2 Fitting section 3. Non-fitting parts 31 Parallel section 32 Fillet R section HL hardened layer BM base metal part

Claims

1. It is a railway axle, A cylindrical part having a diameter DW mm and capable of being press-fitted onto a railway wheel, comprising a fitting portion, It comprises a non-fitting portion connected to the aforementioned fitting portion, The aforementioned non-fitting portion is, A cylindrical parallel portion having a diameter DA mm smaller than the aforementioned diameter DW mm, The vehicle comprises a fillet R portion, which is positioned between the fitting portion and the parallel portion and has a surface that is concavely curved in a cross-section including the central axis of the railway axle, The aforementioned fitted portion and the aforementioned non-fitted portion are, A hardened layer formed on the surface of the aforementioned fitted portion and the aforementioned non-fitted portion, Including the base material portion located inside the hardened layer, The chemical composition of the aforementioned base material is, in mass%, C: 0.30-0.42%, Si: 0.10 to 0.50%, Mn: 0.40 to 1.20%, P: more than 0% and less than 0.020%, S: more than 0% and less than 0.0400%, N: more than 0% and less than 0.0200%, O: more than 0% but not more than 0.0040%, Ca: 0-0.0010%, Cr: 0 to 0.30%, Mo: 0 to 0.10%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, Al: 0-0.100%, V: 0 to 0.060%, Ti: 0 to 0.020%, Nb: 0 to 0.030%, B: 0-0.0050%, and, The remainder consists of Fe and impurities. The ratio of the diameter DW to the diameter DA, DW / DA, is between 1.10 and 1.

30. In the cross-section including the central axis of the railway axle, the radius of curvature of the portion of the fillet R portion adjacent to the surface of the parallel portion is Rf, and the smaller of the total hardened layer depths at 0.23Rf and 0.57Rf positions in the axial direction of the railway axle toward the parallel portion is defined as the reference total hardened layer depth C. ＡＴ When set to mm, The radius of curvature Rf is between 60.0 mm and the critical radius of curvature Rcr mm as defined by equation (1). Railway axle. [Math 1]

2. A railway axle according to claim 1, The chemical composition of the aforementioned base material is, in mass%, Ca: 0.0001 to 0.0010%, Cr: 0.01-0.30%, Mo: 0.01 to 0.10%, Cu: 0.01-0.30%, Ni: 0.01 to 0.30%, Al: 0.001-0.100%, V: 0.001-0.060%, Ti: 0.001 to 0.020%, Nb: 0.001–0.030%, and, B: Contains one or more selected from the group consisting of 0.0001 to 0.0050%, Railway axle.

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