Railway wheel

Abstract

The present invention provides a railway wheel in which the hub portion has excellent toughness. The railway wheel of the present disclosure includes a rim portion, a hub portion having a through-hole, and a plate portion. The chemical composition of the railway wheel is: C: 0.60 to 0.80%, Si: 1.00% or less, Mn: 0.10 to 1.50%, P: 0.050% or less, S: 0.030% or less, N: 0.0200% or less, and the remainder consisting of Fe and impurities. In a cross section of the hub portion after the hub portion is cut with a plane containing the central axis in the direction of the central axis of the through-hole, in a 15 mm x 15 mm region defined as a rectangular region by a plurality of axial line segments arranged at 15 mm intervals in the radial direction of the railway wheel from the inner peripheral surface of the through-hole in parallel with the central axis and a plurality of radial line segments arranged at 15 mm intervals in the direction of the central axis from the surface of the hub portion on which the opening of the through-hole is formed perpendicularly to the central axis, the average C concentration of each rectangular region in the cross section of the hub portion is less than 0.90 mass%.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a railway wheel. BACKGROUND

[0002] A railway vehicle travels on a track that constitutes a railway. The railway vehicle is provided with a plurality of railway wheels. The railway wheels support the vehicle and move while rotating on the track in contact with the track. The railway wheels are worn due to contact with the track. Recently, in order to increase the efficiency of railway transportation, an increase in the load of the railway vehicle and a speedup of the railway vehicle are being promoted. As a result, an improvement in the wear resistance of the railway wheel is required.

[0003] In Japanese Patent Application Publication No. 09-202937 (Patent Literature 1), Japanese Patent Application Publication No. 2000-345295 (Patent Literature 2), Japanese Patent Application Publication No. 2012-107295 (Patent Literature 3), and International Publication No. 2015 / 190088 (Patent Literature 4), a technology for improving the wear resistance of a railway wheel is proposed.

[0004] The railway wheel disclosed in Patent Literature 1 is an integrated railway wheel for a railway vehicle composed of a steel having a chemical composition containing, by weight %, C: 0.4% to 0.75%, Si: 0.4% to 0.95%, Mn: 0.6% to 1.2%, Cr: 0% to less than 0.2%, P: 0.03% or less, S: 0.03% or less, and the remainder composed of Fe and other unavoidable impurities, characterized in that a region from the surface of the wheel ground surface to a depth of 50 mm is composed of a pearlite structure. Thus, in Patent Literature 1, it is described that an integrated railway wheel for a railway vehicle that simultaneously satisfies wear resistance and heat check resistance that can withstand the speedup of the railway and has a long service life is obtained.

[0005] The railway wheel disclosed in Patent Literature 2 is characterized in that a ground surface portion and a flange portion are composed of a pearlite structure, the carbon content (C [wt%]) is 0.5% to 0.8%, and the Vickers hardness Hvt of the ground surface portion and the Vickers hardness Hvf of the flange portion are each within the following range. Thus, in Patent Literature 2, it is described that a railway wheel that is long in comprehensive service life from the viewpoint of fatigue peeling resistance and wear resistance is obtained.

[0006] 200 x C + 210 ≤ Hvf ≤ 357 x C + 132

[0007] 357 x C + 72 ≤ Hvt < 200 x C + 210

[0008] The railway wheel disclosed in Patent Literature 3 is characterized by having a chemical composition containing, in mass%, C: 0.65% to 0.84%, Si: 0.02% to 1.00%, Mn: 0.50% to 1.90%, Cr: 0.02% to 0.50%, V: 0.02% to 0.20%, and S: 0.04% or less, and Fn1 represented by the following formula (1) is 34 to 43, and Fn2 represented by the following formula (2) is 25 or less, with the remainder consisting of Fe and impurities, and P, Cu, and Ni in the impurities are each P: 0.05% or less, Cu: 0.20% or less, and Ni: 0.20%. Thus, it is described in Patent Literature 3 that a railway wheel excellent in wear resistance, rolling fatigue resistance, and peeling resistance is obtained.

[0009] Fn1 = 2.7 + 29.5 x C + 2.9 x Si + 6.9 x Mn + 10.8 x Cr + 30.3 x Mo + 44.3 x V... (1)

[0010] Fn2 = 0.76 x exp(0.05 x C) x exp(1.35 x Si) x exp(0.38 x Mn) x exp(0.77 x Cr) x exp(3.0 x Mo) x exp(4.6 x V)... (2)

[0011] C, Si, Mn, Cr, Mo, and V in the above formula (1) and formula (2) refer to the content of the element in mass%.

[0012] In the railway wheel disclosed in Patent Literature 4, there are C: 0.65% to 0.84%, Si: 0.1% to 1.5%, Mn: 0.05% to 1.5%, P: 0.025% or less, S: 0.015% or less, Al: 0.001% to 0.08%, and Cr: 0.05% to 1.5% in mass%, with the remainder being Fe and inevitable impurities, and the microstructure in at least a region from the ground surface to a depth of 15 mm is pearlite, and the pearlite interlamellar spacing in at least the region is 150 nm or less. Thus, it is described in Patent Literature 4 that a wheel excellent in wear resistance is obtained.

[0013] Prior art documents

[0014] Patent documents

[0015] Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 09-202937

[0016] Patent Literature 2: Japanese Patent Application Laid-Open (JP-A) No. 2000-345295

[0017] Patent Literature 3: Japanese Patent Application Laid-Open No. 2012-107295

[0018] Patent Literature 4: International Publication No. 2015 / 190088 SUMMARY

[0019] PROBLEMS TO BE SOLVED BY THE INVENTION

[0020] In addition, a load corresponding to the load weight of the railway vehicle is first applied to the railway axle. Thereafter, the load is transmitted to the railway wheel via the hub portion that is a contact portion of the railway axle and the railway wheel. Therefore, a large load is applied to the hub portion of the railway wheel. Recently, an increase in the load weight of the freight vehicle is required for the purpose of the high efficiency of the railway transportation. Therefore, an excellent toughness is required for the hub portion of the railway wheel.

[0021] However, in the conventional studies, the toughness of the hub portion has not been studied. The reason for this is that it has been considered that the improvement in the wear resistance of the rim portion that contacts the rail in the railway wheel is important. The toughness of the hub portion has not been studied in Patent Literatures 1 to 4 described above.

[0022] In Patent Literature 2, it is described that, in order to improve the wear resistance, the hardness is increased to a degree that does not impair the toughness in the flange portion that is a part of the rim portion, whereby the propagation of cracks is suppressed. However, the toughness of the hub portion has not been studied.

[0023] In Patent Literature 4, it is described that, by setting the average block size of the pearlite from the ground surface that is the surface of the railway wheel that contacts the rail, i.e., the ground surface, to at least 15 mm inside to 30 μm or less, the ductility and the toughness are improved. However, the toughness of the hub portion has not been studied.

[0024] The toughness has not been studied in Patent Literature 1 and Patent Literature 3. Therefore, even if the technologies disclosed in Patent Literatures 1 to 4 are utilized, there is a possibility that an excellent toughness cannot be obtained in the hub portion.

[0025] An object of the present disclosure is to provide a railway wheel in which the toughness of the hub portion is excellent.

[0026] SOLUTION TO PROBLEM

[0027] The railway wheel of the present disclosure includes:

[0028] a rim portion;

[0029] a hub portion having a through hole; and

[0030] a plate portion disposed between the rim portion and the hub portion, connected to the rim portion and the hub portion,

[0031] The chemical composition of the railway wheel is as follows: by mass%,

[0032] C: 0.60%~0.80%

[0033] Si: below 1.00%

[0034] Mn: 0.10%~1.50%

[0035] P: below 0.050%

[0036] S: below 0.030%

[0037] N: below 0.0200%

[0038] Al: 0%~0.500%

[0039] Cu: 0%–0.50%

[0040] Ni: 0%~0.50%

[0041] Cr: 0%–0.50%

[0042] V: 0%~0.12%

[0043] Ti: 0%~0.010%

[0044] Mo: 0%–0.20%

[0045] Nb: 0%–0.050%, and

[0046] The remaining portion consists of Fe and impurities.

[0047] In the cross-section of the hub portion after cutting along the central axis of the through hole with a plane including the central axis,

[0048] When a 15mm × 15mm area is defined as a rectangular region, it is divided by a plurality of axial line segments arranged at 15mm intervals along the radial direction of the railway wheel, which are parallel to the central axis and extend from the inner circumferential surface of the through hole, and a plurality of radial line segments arranged at 15mm intervals along the central axis direction from the surface of the hub where the through hole is formed.

[0049] The average C concentration in each rectangular region of the cross-section of the hub is less than 0.90% by mass.

[0050] The effects of the invention

[0051] The disclosed railway wheel hub exhibits excellent toughness. Attached Figure Description

[0052] Figure 1 is a cross-sectional view of a railway wheel of the present embodiment including a center axis.

[0053] Figure 2 is a schematic view showing a measurement site of EPMA in a hub portion in a cross section of a railway wheel shown in Figure 1

[0054] Figure 3 is a cross-sectional view of a hub portion for explaining a configuration method of a measurement field of view of EPMA.

[0055] Figure 4 is a schematic view showing a manufacturing process of a railway wheel.

[0056] Figure 5 is a schematic view showing a manufacturing process of a railway wheel in a case where a blanking portion volume rate is appropriate.

[0057] Figure 6 is a schematic view showing a manufacturing process of a railway wheel in a case where a blanking portion volume rate is too small. DETAILED DESCRIPTION

[0058] [Structure of Railway Wheel]

[0059] Figure 1 is a cross-sectional view of a railway wheel 1 of the present embodiment including a center axis. Referring to Figure 1 , the railway wheel 1 is disc-shaped, including a hub portion 2, a plate portion 3, and a rim portion 4. The hub portion 2 is cylindrical, disposed at a central portion in a radial direction (a direction perpendicular to the center axis) of the railway wheel 1. The hub portion 2 has a through-hole 21. A railway axle not shown is inserted into the through-hole 21. An inner peripheral surface 22 of the hub portion 2 forms the through-hole 21. A center axis of the through-hole 21 coincides with the center axis of the railway wheel 1. In the present specification, the center axis direction of the through-hole 21 and the center axis of the railway wheel 1 are also simply referred to as the center axis direction. A direction perpendicular to the center axis of the railway wheel 1 is referred to as a radial direction of the railway wheel 1. In the present specification, the radial direction of the railway wheel 1 is also simply referred to as the radial direction.

[0060] The hub portion 2 has the through-hole 21. The hub portion 2 also has the inner peripheral surface 22, a flange side surface 23, and a ground surface side surface 24, which form the through-hole 21. The flange side surface 23 is continuous with the inner peripheral surface 22, forming an opening of the through-hole 21. The ground surface side surface 24 is disposed on the side opposite to the flange side surface 23, is continuous with the inner peripheral surface 22, and forms an opening of the through-hole 21. A thickness T2 of the hub portion 2 is thicker than a thickness T3 of the plate portion 3.

[0061] ​The rim portion 4 is formed at the edge portion of the outer periphery of the railway wheel 1. The rim portion 4 includes a ground surface 41 and a flange portion 42. The ground surface 41 is continuous with the flange portion 42. The ground surface 41 and the flange portion 42 are in contact with the rail surface when the railway wheel 1 is used. The thickness T4 of the rim portion 4 is thicker than the thickness T3 of the plate portion 3.

[0062] The plate portion 3 is disposed between the hub portion 2 and the rim portion 4, and is continuous with the hub portion 2 and the rim portion 4. Specifically, the inner peripheral edge portion of the plate portion 3 is continuous with the hub portion 2, and the outer peripheral edge portion of the plate portion 3 is continuous with the rim portion 4. The thickness T3 of the plate portion 3 is thinner than the thickness T2 of the hub portion 2 and the thickness T4 of the rim portion 4.

[0063] [Technical idea of the railway wheel of the present disclosure]

[0064] The present inventors first studied an appropriate chemical composition for improving the toughness of the hub portion 2. The present inventors considered that if the C content is limited to 0.80% or less, the toughness of the entire railway wheel 1 including the hub portion 2 can be improved. Therefore, the present inventors manufactured a railway wheel 1 having a C content of 0.80% or less, and investigated the toughness of the hub portion 2.

[0065] The present inventors considered that inclusions in the hub portion 2 have an influence on the toughness of the hub portion 2. Therefore, the present inventors studied inclusions in the hub portion 2 in detail. However, it was not possible to identify inclusions that have an influence on the toughness of the hub portion 2.

[0066] Therefore, the present inventors changed the perspective and studied from a chemical viewpoint. The hub portion 2 was cut with a plane including the center axis of the railway wheel 1 and parallel to the center axis, and a cross section of the hub portion 2 was obtained. The concentration of main elements was measured using an electron probe micro-analyzer (EPMA) for the observation plane. The measurement based on the EPMA was performed at a plurality of sites in the cross section of the hub portion 2. Figure 2 is a schematic view of the measurement sites of the EPMA in the cross section of the hub portion 2 of the railway wheel 1 shown in Figure 1 Figure 2 Referring to FIG. 6, the cross section of the hub portion 2 is divided into a plurality of rectangular regions P of 15 mm x 15 mm using a plurality of axial line segments arranged at 15 mm intervals in the radial direction from the inner peripheral surface 22 of the through-hole 21 parallel to the center axis of the through-hole 21 and a plurality of radial line segments arranged at 15 mm intervals in the direction of the center axis of the through-hole 21 from the flange side surface 23 perpendicular to the center axis of the through-hole 21. Further, the concentration of main elements in each of the rectangular regions P was measured using the EPMA, and the average concentration in each of the rectangular regions P was calculated. As a result, it was clarified that there is a deviation in the C concentration in the hub portion 2.

[0067] ​The average C concentration within the rectangular region P is less than 0.90 mass% in a plurality of the rectangular regions P, in most of the rectangular regions P. However, the rectangular regions P including a rectangular region P in which the C concentration is high are included in the plurality of the rectangular regions P. In the case where the C concentration is high, the average C concentration within 1 rectangular region P is 0.90 mass% or more.

[0068] Through the measurement test by the EPMA of the present inventor, the following was first clarified. In the wheel hub portion 2 of the railway wheel 1, the C concentration is not necessarily constant, and there is a case where the C concentration is locally high. Also, even in the railway wheel 1 having a chemical composition in which the C concentration is 0.80% or less as a whole, there is a case where, in the wheel hub portion 2, the C concentration reaches 0.90 mass% or more in a local portion.

[0069] Therefore, the present inventor investigated how the toughness changes in the case where the C concentration reaches 0.90 mass% or more in a local portion. Specifically, ingots were manufactured by the ingot method using molten steel having the chemical composition shown in Table 1. Here, in order to reproduce the deviation of the C concentration, the C concentration of each steel number was changed. In addition, the blank in Table 1 means that the content of the element is less than the detection limit value. After the ingots were heated to 1250°C, hot forging was performed, and cylindrical steels having a diameter of 40 mm and a length of 75 mm were manufactured. The obtained cylindrical steels were subjected to heat treatment simulating the manufacturing conditions of the wheel hub portion 2 of the railway wheel 1. Specifically, after heating treatment at 950°C for 30 minutes, cooling was performed at a cooling rate of 0.01°C / s or less.

[0070] [Table 1]

[0071] Table 1

[0072]

[0073] In order to evaluate the toughness of the steels of the steel numbers 1 to 8, Charpy impact values were found. Specifically, a U-notch test piece in accordance with JIS Z2242 (2005) was extracted from the center position in the cross section perpendicular to the length direction of the steel of each steel number. The cross section perpendicular to the length direction of the U-notch test piece was set to a square of 10 mm x 10 mm, and the length in the length direction of the U-notch test piece was set to 55 mm. The length direction of the U-notch test piece was parallel to the length direction of the steel. A U-notch was formed at the length center position of the U-notch test piece, that is, the center position of the length of 55 mm. The notch depth was set to 2 mm, and the notch root radius was set to 1 mm. Charpy impact tests in room temperature atmosphere were performed in accordance with JIS Z 2242 (2005). The Charpy impact values (J / cm 2 ) were found for four U-notch test pieces for each steel number, and the average of these values was taken as the Charpy impact value (J / cm 2). The results are shown in Table 2.

[0074] [Table 2]

[0075] Table 2

[0076]

[0077] As is apparent from Table 2, the Charpy impact value (J / cm 2 ) of the steel material having a C concentration of 0.90 mass% or more is lower than the Charpy impact value (J / cm 2 ) of the steel material having a C concentration of less than 0.90 mass%. That is, in the case where the C concentration is deviated and the C concentration locally reaches 0.90 mass% or more, the toughness of the portion having a C concentration of 0.90 mass% or more is lower than the toughness of the portion having a C concentration of less than 0.90 mass%. Therefore, it can be said that the rectangular region P having a higher C concentration among the plurality of rectangular regions P has lower toughness than the periphery thereof.

[0078] Therefore, the present inventors have found that, if the deviation of the C concentration in the hub portion 2 is suppressed and the average C concentration in each rectangular region P is controlled to be less than 0.90 mass%, the toughness of the hub portion 2 can be further improved.

[0079] The railway wheel 1 of the present embodiment, which is completed based on the above insight, has the following structure. [1]

[0081] A railway wheel, wherein

[0082] The railway wheel includes:

[0083] a rim portion;

[0084] a hub portion having a through hole; and

[0085] a plate portion disposed between the rim portion and the hub portion, connected to the rim portion and the hub portion,

[0086] the chemical composition of the railway wheel is:

[0087] C: 0.60% to 0.80%,

[0088] Si: 1.00% or less,

[0089] Mn: 0.10% to 1.50%,

[0090] P: 0.050% or less,

[0091] S: 0.030% or less,

[0092] N: 0.0200% or less,

[0093] Al: 0% to 0.500%,

[0094] Cu: 0% to 0.50%,

[0095] Ni: 0% to 0.50%,

[0096] Cr: 0% to 0.50%,

[0097] V: 0% to 0.12%,

[0098] Ti: 0% to 0.010%,

[0099] Mo: 0% to 0.20%,

[0100] Nb: 0% to 0.050%, and

[0101] the remainder consisting of Fe and impurities,

[0102] in a cross section of the hub portion after the hub portion is cut with a plane containing the central axis in a direction along the central axis of the through-hole,

[0103] when a 15 mm x 15 mm region divided with a plurality of axial line segments arranged at 15 mm intervals in a radial direction of the railway wheel parallel to the central axis and from an inner peripheral surface of the through-hole and a plurality of radial line segments arranged at 15 mm intervals in a direction of the central axis perpendicular to the central axis and from a surface of the hub portion on which an opening of the through-hole is formed is defined as a rectangular region,

[0104] an average C concentration of each rectangular region in the cross section of the hub portion is less than 0.90 mass%. [2]

[0106] The railway wheel according to [1], wherein

[0107] the railway wheel contains, from among Al: 0.001% to 0.500%,

[0108] Cu: 0.01% to 0.50%,

[0109] Ni: 0.01% to 0.50%,

[0110] Cr: 0.01% to 0.50%,

[0111] V: 0.01% to 0.12%,

[0112] Ti: 0.001% to 0.010%,

[0113] Mo: 0.01% to 0.20%, and

[0114] one or more elements selected from the group consisting of 0.010 to 0.050% of Nb.

[0115] Hereinafter, the railway wheel 1 of the present embodiment will be described in detail. In the present specification, " % " with respect to an element means mass % unless otherwise specifically noted.

[0116] Figure 4 is a schematic view showing a manufacturing process of the railway wheel 1. An example of the manufacturing process of the railway wheel 1 is as follows. A disc-shaped railway wheel steel material 5 is manufactured. The disc-shaped railway wheel steel material 5 is subjected to heat forging in the thickness direction once or plural times to manufacture an intermediate product 7 having an outer shape of the railway wheel 1. If necessary, hot rolling (wheel rolling) can be performed after the heat forging. The center axis portion 71 of the intermediate product 7 is removed by blanking processing so that an axle can pass therethrough. Thus, an intermediate product 8 shaped like a railway wheel is formed. The intermediate product 8 is subjected to heat treatment (wheel surface quenching). In the wheel surface quenching, after the intermediate product 8 is heated, a cooling liquid is sprayed to the wheel surface 41 and the flange portion 42 of the rim portion 4, and the wheel surface 41 and the flange portion 42 of the intermediate product 8 are quenched. Thus, in the surface layer portion of the wheel surface 41, a matrix structure of a fine pearlite having high wear resistance is generated. However, in the surface layer portion of the wheel surface 41 after the wheel surface quenching, a hardening layer composed of martensite (or composed of martensite and bainite) is formed in the upper layer of the fine pearlite. In the process of using the railway wheel 1, the hardening layer is easily worn. Therefore, the hardening layer formed in the outermost layer of the wheel surface 41 is removed by cutting processing after the wheel surface quenching. The railway wheel 1 is manufactured by the above process.

[0117] [Chemical composition of railway wheel]

[0118] As shown in Figure 1 , the railway wheel 1 of the present embodiment includes the rim portion 4, the hub portion 2 having the through hole 21, and the plate portion 3 disposed between the rim portion 4 and the hub portion 2 and connected to the rim portion 4 and the hub portion 2. The chemical composition of the railway wheel 1 of the present embodiment contains the following elements.

[0119] C: 0.60 to 0.80%

[0120] Carbon (C) increases the hardness of the steel, and increases the wear resistance of the railway wheel 1. If the C content is less than 0.60%, the effect is not obtained even if the contents of the other elements are within the ranges of the present embodiment. On the other hand, if the C content is more than 0.80%, the toughness of the railway wheel 1 decreases even if the contents of the other elements are within the ranges of the present embodiment. Thus, the C content is 0.60% to 0.80%. The lower limit of the C content is preferably 0.63%, more preferably 0.65%, further preferably 0.67%, and still further preferably 0.70%. The upper limit of the C content is preferably 0.79%, more preferably 0.78%, further preferably 0.75%, and still further preferably 0.73%.

[0121] Si: 1.00% or less

[0122] Silicon (Si) is inevitably contained. That is, the Si content is more than 0%. Si increases the hardness of the steel by solid solution strengthening of ferrite. However, if the Si content is more than 1.00%, the toughness of the railway wheel 1 decreases even if the contents of the other elements are within the ranges of the present embodiment. Moreover, if the Si content is more than 1.00%, the hardenability of the steel becomes too high, and martensite is easily generated. In this case, the thickness of the hardened layer formed on the tread surface during tread surface quenching increases. As a result, the cutting amount increases, and the yield rate decreases. Moreover, if the Si content is more than 1.00%, the rim portion 4 is burned (Japanese: yaki) due to the frictional heat generated between the railway wheel 1 and the brake during use of the railway wheel 1. In this case, there is a case where the crack resistance of the steel decreases. Thus, the Si content is 1.00% or less. The upper limit of the Si content is preferably 0.90%, more preferably 0.80%, further preferably 0.70%, still further preferably 0.60%, and yet further preferably 0.50%. The lower limit of the Si content is not particularly limited. However, excessive reduction of the Si content increases the manufacturing cost. Thus, the lower limit of the Si content is preferably 0.01%, and more preferably 0.05%. From the viewpoint of increasing the hardness of the steel, the lower limit of the Si content is more preferably 0.10%, and further preferably 0.15%.

[0123] Mn: 0.10% to 1.50%

[0124] Manganese (Mn) solid-solution strengthens ferrite to increase the hardness of the steel. Mn further forms MnS to increase the machinability of the steel. If the content of Mn is less than 0.10%, these effects are not obtained even if the contents of other elements are within the ranges of the present embodiment. On the other hand, if the content of Mn is more than 1.50%, the hardenability of the steel becomes too high even if the contents of other elements are within the ranges of the present embodiment. In this case, the thickness of the hardened layer increases, and the yield in the manufacturing process decreases. Also, in use of the railway wheel 1, a burn is generated to the rim portion 4 due to the frictional heat generated between the wheel 1 and the brake. In this case, there is a case where the crack resistance of the steel decreases. Thus, the content of Mn is 0.10% to 1.50%. The lower limit of the content of Mn is preferably 0.50%, more preferably 0.60%, and further preferably 0.70%. The upper limit of the content of Mn is preferably 1.40%, more preferably 1.30%, further preferably 1.20%, more further preferably 1.10%, still more further preferably 1.00%, still more further preferably 0.95%, and still more further preferably 0.90%.

[0125] P: 0.050% or less

[0126] Phosphorus (P) is an impurity. P segregates to the grain boundaries to decrease the toughness of the steel. Thus, the content of P is 0.050% or less. The upper limit of the content of P is preferably 0.030%, and more preferably 0.020%. The content of P is preferably as low as possible. The content of P can also be 0%. However, an excessive decrease in the content of P increases the manufacturing cost. Thus, the lower limit of the content of P is preferably 0.001%, and more preferably 0.002% in consideration of the usual industrial production.

[0127] S: 0.030% or less

[0128] Sulfur (S) is inevitably contained. That is, the content of S is more than 0%. S forms MnS to increase the machinability of the steel. On the other hand, if the content of S is too high, the toughness of the steel decreases. Thus, the content of S is 0.030% or less. The upper limit of the content of S is preferably 0.020%. The content of S is preferably as low as possible. However, an excessive decrease in the content of S increases the manufacturing cost. Thus, the lower limit of the content of S is preferably 0.001%, more preferably 0.002%, and further preferably 0.005% in consideration of the usual industrial production.

[0129] N: 0.0200% or less

[0130] Nitrogen (N) is an unavoidable impurity. That is, the N content is greater than 0%. If the N content is greater than 0.0200%, even if the contents of other elements are within the range of the present embodiment, the AlN is coarsened, and the toughness of the steel is reduced. Thus, the N content is 0.0200% or less. The upper limit of the N content is preferably 0.0180%, more preferably 0.0150%, further preferably 0.0130%, still further preferably 0.0100%, and yet further preferably 0.0080%. The N content is preferably as low as possible. However, excessive reduction of the N content increases the manufacturing cost. Thus, in consideration of general industrial production, the lower limit of the N content is preferably 0.0010%, more preferably 0.0030%, and further preferably 0.0050%.

[0131] The remainder of the chemical composition of the railway wheel 1 of the present embodiment is composed of Fe and impurities. Here, the impurities refer to substances mixed from ores, scrap, or manufacturing environments, etc. as raw materials when the above railway wheel 1 is industrially manufactured, and are substances that are permitted within a range that does not adversely affect the railway wheel 1 of the present embodiment. Impurities other than the above impurities are, for example, O. The O content is, for example, 0.0070% or less.

[0132] [Regarding Arbitrary Elements]

[0133] The chemical composition of the railway wheel 1 of the present embodiment can also contain one or more elements selected from the group consisting of Al, Cu, Ni, Cr, V, Ti, Mo, and Nb instead of a part of Fe.

[0134] Al: 0% to 0.500%

[0135] Aluminum (Al) is an arbitrary element, and can not be contained. That is, the Al content can also be 0%. In the case of being contained, Al combines with N to form AlN, and the crystal grains are refined. By refining the crystal grains, the toughness of the steel is improved. However, if the Al content is greater than 0.500%, even if the contents of other elements are within the range of the present embodiment, the non-metallic inclusions increase, and the toughness of the steel is reduced. Thus, the Al content is 0% to 0.500%. The lower limit of the Al content is preferably greater than 0%, more preferably 0.001%, further preferably 0.005%, still further preferably 0.010%, yet further preferably 0.020%, and yet further preferably 0.030%. The upper limit of the Al content is preferably 0.450%, more preferably 0.400%, further preferably 0.350%, still further preferably 0.300%, yet further preferably 0.250%, and yet further preferably 0.200%. The Al content referred to in the present specification refers to the content of acid-soluble Al (sol. Al).

[0136] Cu: 0 to 0.50%

[0137] Copper (Cu) is an optional element and can not be contained. That is, the Cu content can also be 0%. In the case of being contained, Cu improves the hardness of the steel through solid solution strengthening. As a result, the wear resistance of the railway wheel 1 is improved. However, if the Cu content is greater than 0.50%, the hot workability of the steel is decreased even if the contents of the other elements are within the range of the present embodiment. Thus, the Cu content is 0% to 0.50%. The lower limit of the Cu content is preferably greater than 0%, more preferably 0.01%, further preferably 0.02%, still further preferably 0.05%, and yet further preferably 0.10%. The upper limit of the Cu content is preferably 0.45%, more preferably 0.40%, further preferably 0.35%, still further preferably 0.30%, and yet further preferably 0.25%.

[0138] Ni: 0 to 0.50%

[0139] Nickel (Ni) is an optional element and can not be contained. That is, the Ni content can also be 0%. In the case of being contained, Ni improves the toughness of the steel. However, if the Ni content is greater than 0.50%, the hardenability is excessively increased and the thickness of the hardened layer after the ground face quenching is excessively increased even if the contents of the other elements are within the range of the present embodiment. Thus, the Ni content is 0% to 0.50%. The lower limit of the Ni content is preferably greater than 0%, more preferably 0.01%, further preferably 0.05%. The upper limit of the Ni content is preferably 0.45%, more preferably 0.40%, further preferably 0.35%, more further preferably 0.30%, still further preferably 0.25%, and yet further preferably 0.20%.

[0140] Cr: 0 to 0.50%

[0141] Chromium (Cr) is an optional element and can not be contained. That is, the Cr content can also be 0%. In the case of being contained, Cr reduces the interlamellar spacing of the pearlite. Thereby, the hardness of the pearlite is significantly increased. However, if the Cr content is greater than 0.50%, the hardenability is excessively increased and the thickness of the hardened layer after the ground face quenching is excessively increased even if the contents of the other elements are within the range of the present embodiment. Thus, the Cr content is 0% to 0.50%. The lower limit of the Cr content is preferably greater than 0%, more preferably 0.01%, further preferably 0.02%, still further preferably 0.03%, and yet further preferably 0.05%. The upper limit of the Cr content is preferably 0.45%, more preferably 0.40%, further preferably 0.35%, still further preferably 0.30%, yet further preferably 0.25%, and yet further preferably 0.20%.

[0142] V: 0% to 0.12%

[0143] Vanadium (V) is an optional element and can not be contained. That is, the V content can also be 0%. In the case of being contained, V forms any one of carbide, nitride, and carbonitride, and precipitates strengthen the steel (particularly ferrite in the steel). As a result, the hardness of the railway wheel 1 increases, and the wear resistance is improved. However, if the V content is greater than 0.12%, even if the contents of other elements are within the range of the present embodiment, the hardenability increases, and the thickness of the hardened layer after the ground surface quenching excessively increases. Thus, the V content is 0% to 0.12%. The lower limit of the V content is preferably greater than 0%, more preferably 0.01%, further preferably 0.02%, and still further preferably 0.03%. The upper limit of the V content is preferably 0.11%, more preferably 0.10%, further preferably 0.08%.

[0144] Ti: 0% to 0.010%

[0145] Titanium (Ti) is an optional element and can not be contained. That is, the Ti content can also be 0%. In the case of being contained, Ti forms any one of carbide, nitride, and carbonitride, and precipitates strengthen the steel (particularly ferrite in the steel). As a result, the hardness of the railway wheel 1 increases, and the wear resistance is improved. However, if the Ti content is greater than 0.010%, even if the contents of other elements are within the range of the present embodiment, the hardenability increases, and the thickness of the hardened layer after the ground surface quenching excessively increases. Thus, the Ti content is 0% to 0.010%. The lower limit of the Ti content is preferably greater than 0%, more preferably 0.001%, further preferably 0.002%, and still further preferably 0.003%. The upper limit of the Ti content is preferably 0.008%, more preferably 0.007%, and further preferably 0.005%.

[0146] Mo: 0% to 0.20%

[0147] Molybdenum (Mo) is an optional element and can not be contained. That is, the Mo content can also be 0%. In the case of being contained, Mo increases the hardness of the steel. As a result, the wear resistance of the railway wheel 1 is improved. However, if the Mo content is greater than 0.20%, even if the contents of other elements are within the range of the present embodiment, the hardenability excessively increases, and the thickness of the hardened layer after the ground surface quenching excessively increases. Thus, the Mo content is 0% to 0.20%. The lower limit of the Mo content is preferably greater than 0%, more preferably 0.01%, and further preferably 0.05%. The upper limit of the Mo content is preferably 0.18%, more preferably 0.15%, further preferably 0.12%, still further preferably 0.10%, and yet further preferably 0.05%.

[0148] Nb: 0% to 0.050%

[0149] Niobium (Nb) is an optional element and can not be contained. That is, the content of Nb can also be 0%. In the case of being contained, Nb combines with C at the time of heating for quenching of the tread surface in the manufacturing process of the railway wheel 1 to generate fine NbC. The fine NbC functions as pinning particles to suppress the coarsening of austenite at the time of heating. Therefore, the old austenite grains are maintained in a fine state, and the hardenability of the steel is suppressed. As a result, the generation of the hardened layer in the manufacturing process of the railway wheel 1 is suppressed. Nb also suppresses the coarsening of the old austenite grains to improve the toughness of the steel material. On the other hand, if the content of Nb is greater than 0.050%, even if the contents of the other elements are within the ranges of the present embodiment, the toughness of the steel material is rather decreased due to the coarsening of NbC. Thus, the content of Nb is 0% to 0.050%. The lower limit of the content of Nb is preferably greater than 0%, more preferably 0.010%, and further preferably 0.020%. The upper limit of the content of Nb is preferably 0.030%, and more preferably 0.020%.

[0150] [Deviation of C concentration in hub portion]

[0151] The deviation of the C concentration in the hub portion 2 of the railway wheel 1 of the present embodiment is small. Specifically, in the cross section of the hub portion 2 after the hub portion 2 is cut by a plane including the central axis in the direction of the central axis of the through-hole 21, when a 15 mm x 15 mm region divided by a plurality of axial line segments parallel to the central axis and arranged at an interval of 15 mm in the radial direction from the inner peripheral surface 22 of the through-hole 21 and a plurality of radial line segments perpendicular to the central axis and arranged at an interval of 15 mm in the direction of the central axis from the surface of the hub portion 2 on which the opening of the through-hole 21 is formed is defined as a rectangular region P, the average C concentration of each rectangular region P in the cross section of the hub portion 2 is less than 0.90 mass%. In the railway wheel 1 of the present embodiment, there is no 15 mm x 15 mm rectangular region in the hub portion 2 in which the average C concentration is 0.90 mass% or more. Therefore, the toughness is high in the entire region in the hub portion 2. As a result, the toughness of the hub portion 2 of the railway wheel 1 of the present embodiment is excellent.

[0152] The deviation of the C concentration in the hub portion 2 is measured by the following method. The cross section of the hub portion 2 is obtained by cutting the railway wheel 1 with a plane including the central axis of the railway wheel 1 and parallel to the central axis. The cross section of the hub portion 2 resulting from the cutting is polished by mechanical polishing or ion milling or the like to obtain a smooth observation surface. The C concentration is measured for the observation surface using EPMA. The measurement field of view of the EPMA is set to a rectangle of 15 mm x 15 mm. Figure 3 is a cross-sectional view of the hub portion 2 for explaining the measurement method of EPMA. Referring to Figure 3In the cross-section of the hub portion 2, the hub portion 2 is divided by multiple axial line segments L1 and multiple radial line segments L2. The axial line segments L1 are parallel to the central axis of the through hole 21 and are arranged radially at 15mm intervals from the inner circumferential surface 22 of the through hole 21. The radial line segments L2 are perpendicular to the central axis of the through hole 21 and are arranged at 15mm intervals from the surface of the hub portion 2 where the through hole 21 is formed, along the central axis direction. Figure 3 In the diagram, radial segment L2 is arranged from the ground surface 24. However, radial segment L2 can also be arranged from the flange surface 23. A 15mm × 15mm region divided by axial segment L1 and radial segment L2 is defined as a rectangular region P. The average C concentration of each rectangular region P is calculated using EPMA, and the deviation of the C concentration is determined. (Refer to...) Figure 3 In cases where the entire rectangular region P containing the outer periphery of the hub portion 2 does not include the hub portion 2, the hub portion 2 is considered as the measurement object if its area within the rectangular region P is 50% or more. Furthermore, in this specification, the hub portion 2 refers to the region in a cross-section formed by cutting the railway wheel 1 along its central axis with a plane including the central axis, extending from the inner circumferential surface 22 to two points on the outer periphery of the railway wheel 1 where a straight line intersects a line parallel to the central axis of the railway wheel 1, with a straight line intersecting the line along the central axis of the railway wheel 1, where the straight-line distance between these two points is half the thickness T2 of the hub portion 2. Figure 3 In this diagram, the two points on the outer circumference of the railway wheel 1 that intersect with the straight line parallel to the central axis of the railway wheel 1 are designated as intersection point A and intersection point B, respectively. (Refer to...) Figure 3 The hub portion 2 refers to the region in the cross-section of the railway wheel 1 after cutting along the central axis direction with a plane including the central axis, up to half the thickness T2 of the hub portion 2, where the straight-line distance from the inner circumferential surface 22 to the intersection point AB is equal to half the straight-line distance from the inner circumferential surface 22 to the intersection point AB. Within the entire rectangular region P that is the object of measurement, if the average C concentration within the rectangular region P is less than 0.90% by mass, it is judged that the deviation in C concentration within the hub portion 2 is small.

[0153] [Microstructure of railway wheels]

[0154] The microstructure of the rim portion 4, plate portion 3, and hub portion 2 of the railway wheel 1 in this embodiment can also be a microstructure substantially composed of pearlite. Here, "substantially composed of pearlite" means that the area fraction of pearlite in the microstructure is 95% or more. However, even if proeutectoid ferrite precipitates, it does not affect the toughness. Therefore, the microstructure of the rim portion 4, plate portion 3, and hub portion 2 of the railway wheel 1 in this embodiment can also be a microstructure with a proeutectoid ferrite area fraction of 0% to 25% and the remainder composed of pearlite.

[0155] The area ratio of the pearlite is found using the following method. Samples are extracted from the center of the thickness direction of the rim portion 4 of the railway wheel 1, the center of the thickness direction of the plate portion 3, and the center of the thickness direction of the hub portion 2. The observation surface of each sample is mirror finished by mechanical polishing. Thereafter, the observation surface is etched using a nitric acid alcohol solution (a mixture of nitric acid and alcohol). For an arbitrary 1 field of view (200 μm x 200 μm) in the etched observation surface, a photograph image is generated using a 500x optical microscope. The contrast of the hardenability layer (martensite and / or bainite) and the pearlite is different. Thus, the hardenability layer and the pearlite in the observation surface are determined based on the contrast. The area ratio of the pearlite is found based on the total area of the determined pearlite and the area of the observation surface.

[0156] As above, in the railway wheel 1 of the present embodiment, the content of each element in the chemical composition is within the range of the present embodiment, and in the cross section of the hub portion 2 after the hub portion 2 is cut with a plane containing the center axis in the direction of the center axis of the through-hole 21, when a 15 mm x 15 mm region defined as a rectangular region P by a plurality of axial line segments L1 arranged at 15 mm intervals in the radial direction of the railway wheel 1 from the inner peripheral surface 22 of the through-hole 21 in parallel with the center axis and a plurality of radial line segments L2 arranged at 15 mm intervals in the direction of the center axis from the surface of the hub portion 2 on which the opening of the through-hole 21 is formed in perpendicular to the center axis, the average C concentration of each rectangular region P in the cross section of the hub portion 2 is less than 0.90 mass%. Thus, the toughness of the hub portion 2 is excellent.

[0157] [Manufacturing method of railway wheel]

[0158] An example of a method of manufacturing the above railway wheel 1 will be described. The manufacturing method includes a step (raw material manufacturing step) of manufacturing the railway wheel steel material 5, a step (forming step) of forming an intermediate product 8 in a wheel shape from the railway wheel steel material 5 by hot working, a step (heat treatment step) of subjecting the formed intermediate product 8 to heat treatment (ground face quenching), and a step (machining step) of removing the hardenability layer from the ground face 41 and the like of the intermediate product 8 after the heat treatment by machining to produce the railway wheel 1. Each step will be described below.

[0159] [Raw material manufacturing step]

[0160] In the raw material manufacturing step, after molten steel having the above chemical composition is smelted using an electric furnace or a converter or the like, casting is performed to produce a cast product (a cast slab or a cast block). The cast slab can be manufactured by continuous casting, and the cast block can be manufactured by mold casting.

[0161] The cast piece or the ingot is subjected to hot working to manufacture the steel material 5 for railway wheel of a desired size. The hot working is, for example, hot forging, hot rolling, or the like. In the case where the steel material 5 for railway wheel is manufactured by hot rolling, the steel material 5 for railway wheel is manufactured, for example, by the following method. In the hot rolling, for example, an initial rolling mill is used. The initial rolling mill is used to perform initial rolling on a raw material to manufacture the steel material 5 for railway wheel. In the case where a continuous rolling mill is provided downstream of the initial rolling mill, the steel material after the initial rolling can be further subjected to hot rolling using the continuous rolling mill to manufacture a steel material 5 for railway wheel of a smaller size. In the continuous rolling mill, a horizontal stand having a pair of horizontal rollers and a vertical stand having a pair of vertical rollers are alternately arranged in a line. The heating temperature of the heating furnace in the hot rolling is not particularly limited and is, for example, 1100°C to 1350°C. The steel material 5 for railway wheel is manufactured by the above manufacturing process.

[0162] In addition, the steel material 5 for railway wheel can also be a cast product (cast piece or ingot). The steel material 5 for railway wheel can also be obtained by cutting a cylindrical cast product perpendicularly to the axial direction into a wheel shape. That is, the above hot working can also be omitted. The steel material 5 for railway wheel, which is a raw material of the railway wheel 1, is manufactured by the above process. The steel material 5 for railway wheel is, for example, a cylindrical raw material.

[0163] [Forming Process]

[0164] In the forming process, the prepared steel material 5 for railway wheel is used and subjected to hot working to form an intermediate product 8 of a wheel shape. The intermediate product 8 has a wheel shape and thus includes the hub portion 2, the plate portion 3, and the rim portion 4 including the ground surface 41 and the flange portion 42. The hot working is, for example, hot forging, hot rolling (wheel rolling), or the like. As an example, if the following forming process is used, the deviation of the C concentration in the hub portion 2 can be reduced.

[0165] Figure 4 is a schematic view showing a manufacturing process of the railway wheel 1. Referring to Figure 4 The intermediate product 6 of a disc shape is manufactured by subjecting the steel material 5 for railway wheel to hot forging. The intermediate product 7 having an outer shape of a wheel is manufactured by further subjecting the intermediate product 6 of a disc shape to hot forging. If necessary, hot rolling (wheel rolling) can also be performed after the hot forging. Thereafter, the center axis portion 71 of the intermediate product 7 having an outer shape of a wheel is removed by blanking. Thus, the intermediate product 8 having a shape of a wheel is manufactured.

[0166] The present inventors have reached the insight that by adjusting the volume of the center axis portion 71 (hereinafter referred to as the center axis portion 71) of the intermediate product 7 having an outer shape of a wheel with respect to the volume of the steel material 5 for railway wheel, the deviation of the C concentration in the hub portion 2 can be suppressed. The reason is not certain, but for example, the following matters are considered.

[0167] In the present specification, the volume of the center axis portion 71 with respect to the volume of the steel material for railway wheel 5 is referred to as a punch portion volume rate. The punch portion volume rate is defined by the following equation (i).

[0168] (D71 2 × T71) / (D5 2 × T5) (i)

[0169] In the equation (i), the diameter (cm) of the center axis portion 71 is substituted for D71, the thickness (cm) of the center axis portion 71 is substituted for T71, the diameter (cm) of the steel material for railway wheel 5 is substituted for D5, and the thickness (cm) of the steel material for railway wheel 5 is substituted for T5.

[0170] Figure 5 is a schematic view of a manufacturing process of the railway wheel 1 in a case where the punch portion volume rate is appropriate. Referring to Figure 5 , the center axis portion of the steel material for railway wheel 5 is flattened by hot working and extends along the radial direction of the steel material for railway wheel 5. In a case where the punch portion volume rate is appropriate, the volume of the center axis portion 71 is large enough with respect to the volume of the steel material for railway wheel 5. In this case, the hub portion 2 is less strained by the hot working.

[0171] Figure 6 is a schematic view of a manufacturing process of the railway wheel 1 in a case where the punch portion volume rate is too small. Referring to Figure 6 In a case where the punch portion volume rate is too small, the volume of the center axis portion 71 is too small with respect to the volume of the steel material for railway wheel 5. In this case, the reduction is stronger than in the case where the punch portion volume rate is appropriate, and thus the hub portion 2 is more strained by the hot working. It is considered that the C concentration is locally deviated in a region where the strain is generated.

[0172] The present inventors have studied the punch portion volume rate and the deviation of the C concentration of the hub portion 2. Specifically, the railway wheels 1 having different punch portion volume rates were manufactured, and it was investigated whether the C concentration was deviated. As a result, it was found that, as described in the examples described later, for example, if the punch portion volume rate is 0.07% or more, the deviation of the C concentration of the hub portion 2 can be suppressed. That is, by setting the punch portion volume rate to 0.07% or more, the strain of the hub portion 2 can be suppressed, and the deviation of the C concentration can be suppressed.

[0173] As described above, in the manufacturing method of the railway wheel 1 of the present disclosure, the punch portion volume rate at the time of hot working is adjusted to 0.07% or more. Thereby, the strain of the hub portion 2 can be suppressed, and the deviation of the C concentration of the hub portion 2 can be suppressed. Thereby, the toughness of the railway wheel 1 is improved.

[0174] The preferable heating temperature of the steel material 5 for a railway wheel at the time of hot working is 1220°C or higher. The cooling method of the intermediate product after hot working is not particularly limited. It can be either natural cooling or water cooling.

[0175] [Heat treatment step]

[0176] In the heat treatment step, the intermediate product 8 in the shape of a formed wheel is subjected to land quenching. Specifically, the intermediate product 8 after the forming step (hot forging or hot rolling) is heated again to A cm or higher (reheating treatment). After heating, the land 41 and the flange portion 42 of the intermediate product 8 are rapidly cooled (land quenching). The land 41 and the flange portion 42 are rapidly cooled, for example, by spraying a cooling medium. The cooling medium is not particularly limited as long as a cooling speed that corresponds to the desired structure is obtained, and is, for example, air, mist, or spray. In addition, at the time of land quenching, the plate portion 3 and the hub portion 2 are not water-cooled but are naturally cooled.

[0177] In the above description, the intermediate product 8 is reheated, but the land quenching can also be directly performed on the intermediate product 8 after hot working (without reheating).

[0178] The intermediate product 8 after land quenching is tempered as needed. The tempering is performed at a temperature and for a time that are well known. The tempering temperature is, for example, 400°C to 600°C.

[0179] [Cutting step]

[0180] As described above, the surface layer of the land 41 of the intermediate product 8 after heat treatment is formed with fine pearlite, and a quenched layer is formed on the upper layer. In use of the railway wheel 1, since the quenched layer has low wear resistance, the quenched layer is removed by cutting. The cutting is performed by a well-known method.

[0181] The railway wheel 1 of the present embodiment is manufactured by the above-described steps. In the railway wheel 1 of the present embodiment, the content of each element in the chemical composition is within the range of the present embodiment, and in a cross section of the hub portion 2 after the hub portion 2 is cut with a plane including the central axis in the direction of the central axis of the through-hole 21, when a 15 mm x 15 mm region defined by a plurality of axial line segments L1 arranged at 15 mm intervals in the radial direction from the inner peripheral surface 22 of the through-hole 21 in parallel with the central axis and a plurality of radial line segments L2 arranged at 15 mm intervals in the direction of the central axis from the surface of the hub portion 2 on which the opening of the through-hole 21 is formed in perpendicular to the central axis is defined as a rectangular region P, the average C concentration in each rectangular region P in the cross section of the hub portion 2 is less than 0.90 mass%. Therefore, the toughness of the hub portion 2 is excellent.

[0182] [Example]

[0183] The deviation of the C concentration of the hub portion with respect to the punch portion volume fraction was investigated. Specifically, a molten steel having C: 0.71%, Si: 0.30%, Mn: 0.80%, P: 0.010%, S: 0.010%, N: 0.0040%, Al: 0.030%, Cu: 0.02%, Ni: 0.02%, Cr: 0.10%, Mo: 0.02%, and the remainder consisting of Fe and impurities was manufactured. A material manufacturing process simulating a manufacturing process of a railway wheel was performed using the above molten steel and a bloom (cylindrical steel material having a diameter of 46 cm and a thickness of 50 cm) was manufactured by an ingot casting method. After the bloom was heated to 1250°C, 1 time of hot forging was performed, and a disc-shaped intermediate product having a diameter of 87 cm and a thickness of 14 cm was manufactured. Further, 1 time of forging was performed, and an intermediate product having an outer shape of a railway wheel having a diameter of 97 cm, a thickness of the rim portion (T4) of 15 cm, a width of the rim portion of 9 cm, a thickness of the plate portion (T3) of 5 cm, a width of the plate portion of 26 cm, a thickness of the hub portion (T2) of 7 cm, and a width of the hub portion of 20 cm was manufactured. An intermediate product having a shape of a railway wheel with a punch portion volume fraction of 0.20% or 0.05% was manufactured, and whether or not the C concentration of the hub portion was deviated was investigated.

[0184] A railway wheel was manufactured by performing a grounding surface quenching and a cutting process of a hardening layer on the intermediate product having a shape of a railway wheel with a changed punch portion volume fraction. The railway wheel was cut along the center axis direction of the through hole with a plane including the center axis. Then, the cross section of the hub portion was polished by mechanical polishing to obtain a smooth observation surface. The cross section of the hub portion was divided by a plurality of axial line segments parallel to the center axis of the through hole and arranged at an interval of 15 mm in the radial direction from the inner peripheral surface of the through hole, and a plurality of radial line segments perpendicular to the center axis of the through hole and arranged at an interval of 15 mm in the center axis direction from the surface of the hub portion on which the opening of the through hole is formed. A region of 15 mm x 15 mm divided was defined as a rectangular region P. The average C concentration in each rectangular region P was measured by EPMA. The highest C concentration among the obtained average C concentrations was indicated in the column of "Maximum C concentration (mass%)" of Table 3.

[0185] [Table 3]

[0186] Table 3

[0187] Test number Punching portion volume fraction (%) Maximum C concentration (mass %) 1 0.20 0.84 2 0.05 0.93

[0188] [Charpy impact test]

[0189] To investigate the relationship between the C concentration and the toughness, a Charpy impact test was performed. Steel materials having the chemical compositions shown in Table 4 were produced. In Table 4, a blank means that the content of the element is less than the detection limit value. The C content was adjusted in combination with the maximum C concentration of Table 3. The shape of the steel material was set to a round bar having a diameter of 20 mm and a length of 125 mm.

[0190] [Table 4]

[0191] Table 4

[0192]

[0193] To evaluate the toughness of the steel materials of Test No. 1 and Test No. 2, the Charpy impact values were found. Specifically, a U-notch test piece in accordance with JIS Z 2242 (2005) was extracted from the center position in the section perpendicular to the length direction of each steel material No. The section perpendicular to the length direction of the U-notch test piece was set to a square of 10 mm x 10 mm, and the length in the length direction of the U-notch test piece was set to 55 mm. The length direction of the U-notch test piece was parallel to the length direction of the steel material. A U-notch was formed at the length center position of the U-notch test piece, that is, the center position of the length of 55 mm. The notch depth was set to 2 mm, and the notch bottom radius was set to 1 mm. The Charpy impact test in the atmosphere at room temperature was performed in accordance with JIS Z 2242 (2005). The Charpy impact values (J / cm 2 ) were found for four U-notch test pieces for each test No., and the average of these values was taken as the Charpy impact value (J / cm 2 ) of the test No. The results are shown in Table 5.

[0194] [Table 5]

[0195] Table 5

[0196]

[0197] [Results of Evaluation]

[0198] Referring to Table 5, in Test No. 1, the punch portion volume fraction is 0.07% or more. Therefore, the variation in the C concentration of the hub portion is suppressed. Specifically, in a cross section of the hub portion after the hub portion is cut by a plane including the central axis in the direction of the central axis of the through-hole, when a 15 mm x 15 mm region divided by a plurality of axial line segments parallel to the central axis and arranged at 15 mm intervals in the radial direction from the inner peripheral surface of the through-hole and a plurality of radial line segments perpendicular to the central axis and arranged at 15 mm intervals in the direction of the central axis from the surface of the hub portion on which the opening of the through-hole is formed is defined as a rectangular region P, the average C concentration of each rectangular region P in the cross section of the hub portion is less than 0.90 mass%. Also, the Charpy impact value in the case where the C concentration is less than 0.90 mass% is 6.0 (J / cm 2 ) or more. That is, in the railway wheel of Test No. 1, since the average C concentration in each rectangular region P within the hub portion is less than 0.90 mass%, the Charpy impact value in the entire region within the hub portion is 6.0 (J / cm 2 ) or more. Thus, the hub portion of the railway wheel manufactured under the conditions of Test No. 1 has excellent toughness.

[0199] On the other hand, in Test No. 2, the punch portion volume fraction is less than 0.07%. Therefore, the variation in the C concentration of the hub portion is not suppressed. Specifically, in a cross section of the hub portion after the hub portion is cut by a plane including the central axis in the direction of the central axis of the through-hole, when a 15 mm x 15 mm region divided by a plurality of axial line segments parallel to the central axis and arranged at 15 mm intervals in the radial direction from the inner peripheral surface of the through-hole and a plurality of radial line segments perpendicular to the central axis and arranged at 15 mm intervals in the direction of the central axis from the surface of the hub portion on which the opening of the through-hole is formed is defined as a rectangular region P, the rectangular region P including the average C concentration of 0.90 mass% or more is included in the rectangular regions P in the cross section of the hub portion. Also, the Charpy impact value in the case where the C concentration is 0.90 mass% or more is 6.0 (J / cm 2 ) or less. That is, in the rectangular region P in which the average C concentration is 0.90 mass% or more, the Charpy impact value is 6.0 (J / cm 2 ) or less. Thus, the hub portion of the railway wheel manufactured under the conditions of Test No. 2 does not have excellent toughness.

[0200] The above describes the embodiments of the present disclosure. However, the above-described embodiments are nothing more than examples for implementing the present disclosure. Thus, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately changed and implemented without departing from the scope of the gist thereof.

[0201] Explanation of Reference Signs

[0202] 1: railway wheel; 2: hub portion; 3: plate portion; 4: rim portion; 21: through hole; 22: inner peripheral surface; 23: flange side surface; 24: ground surface side surface; 41: ground surface; 42: flange portion; L1: axial line segment; L2: radial line segment.

Claims

1. A railway wheel, wherein the railway wheel comprises: a rim portion; a hub portion having a through-hole; and a plate portion disposed between the rim portion and the hub portion, connected to the rim portion and the hub portion, a chemical composition of the railway wheel is: C：0.60％～0.80％、 in terms of mass%, Si: 1.00% or less, Mn: 0.10% to 1.50%, P: 0.050% or less, S: 0.030% or less, Al：0％～0.500％、 N: 0.0200% or less, Cu: 0% to 0.50%, Cr：0％～0.50％、 V：0％～0.12％、 Ni: 0% to 0.50%, Ti: 0% to 0.010%, Mo: 0% to 0.20%, Nb: 0% to 0.050%, and the remainder consists of Fe and impurities, the hub portion is a region in which, in a cross section of the railway wheel after the railway wheel is cut with a plane containing a central axis in a direction along the central axis, a straight line distance between two points on an outer periphery of the railway wheel intersected by a straight line parallel to the central axis from an inner periphery surface of the through-hole becomes half of a thickness of the hub portion, in a cross section of the hub portion after the hub portion is cut with a plane containing a central axis in a direction along the central axis of the through-hole, when a 15 mm x 15 mm region divided by a plurality of axial line segments arranged at 15 mm intervals in a radial direction of the railway wheel from an inner periphery surface of the through-hole parallel to the central axis and a plurality of radial line segments arranged at 15 mm intervals in a direction along the central axis from a surface of the hub portion on which an opening of the through-hole is formed perpendicular to the central axis is defined as a rectangular region, an average C concentration of each rectangular region in the cross section of the hub portion is less than 0.90 mass%.

2. The railway wheel according to claim 1, wherein Al：0.001％～0.500％、 the railway wheel contains one or more elements selected from the group consisting of Cu: 0.01% to 0.50%, Cr：0.01％～0.50％、 V：0.01％～0.12％、 Ni: 0.01% to 0.50%, Ti: 0.001% to 0.010%, Mo: 0.01% to 0.20%, and Nb: 0.010% to 0.050%.

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