Railway wheel

Abstract

To provide a railway wheel that achieves superior rolling contact fatigue damage resistance.SOLUTION: A railway wheel of the present disclosure comprises: a rim portion; a boss portion; and a plate portion disposed between the rim portion and the boss portion and linked to the rim portion and the boss portion, wherein the railway wheel comprises in mass%: C: 0.65-0.84%, Si: 0.10-1.00%, Mn: 0.05-1.20%, P: 0.050% or less, S: 0.040% or less, N: 0.0200% or less, and Sn: more than 0.50% to 1.50%, with the remainder being Fe and impurities. In the microstructure of the rim portion, the pearlite area ratio is 95.0% or more. A formula (1A) is satisfied. 115.4×Sn+165.4×Mn≥100.0 (1A).SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to railway wheels.

Background Art

[0002] Railway vehicles run on rails that make up a track. Railway vehicles are equipped with a plurality of railway wheels. The railway wheels support the railway vehicle, come into contact with the rails, and move while rotating on the rails. Therefore, repeated stress due to rolling contact is applied to the railway wheels.

[0003] Rolling fatigue cracks may occur on the tread surface of railway wheels due to the application of repeated stress. If the rolling fatigue cracks progress, damage called shelling may occur, where a part of the tread peels off. If a railway vehicle runs on the rails with a railway wheel having shelling, an impact load is applied to the rails. Therefore, this impact load may cause damage to the rails. Railway wheels with shelling affect the riding comfort and noise of railway vehicles. Therefore, it is required to suppress rolling fatigue cracks in railway wheels. In the following description, the property of suppressing rolling fatigue cracks is referred to as "rolling fatigue damage resistance property".

[0004] Techniques for enhancing the rolling fatigue damage resistance property of railway wheels have been proposed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2005-350769) and Patent Document 2 (International Publication No. 2015 / 190088).

[0005] The railway wheel disclosed in Patent Document 1 has a tread surface composed of any one of a bainite structure having a Vickers hardness of 360 or more with C: 0.35 to 0.55% by mass, a tempered martensite structure, or a mixed structure of bainite and tempered martensite. In this railway wheel, the rolling fatigue damage resistance property is enhanced by making the microstructure of the tread surface a martensite structure or a mixed structure of martensite and bainite.

[0006] The railway wheel disclosed in Patent Document 2 contains, by mass%, C: 0.65 - 0.84%, Si: 0.1 - 1.5%, Mn: 0.05 - 1.5%, P: 0.025% or less, S: 0.015% or less, Al: 0.001 - 0.08%, and Cr: 0.05 - 1.5%, with the balance being Fe and inevitable impurities. In this railway wheel, the microstructure in the region at least 15 mm deep from the tread surface is a pearlite structure, and at least the lamellar spacing of the pearlite structure is 150 nm or less. In this railway wheel, by making the microstructure pearlite and further reducing the pearlite lamellar spacing, the resistance to rolling contact fatigue damage characteristics are enhanced.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0008] The railway wheels disclosed in Patent Document 1 and Patent Document 2 exhibit resistance to rolling contact fatigue damage characteristics. However, it may be possible to obtain resistance to rolling contact fatigue damage characteristics by means different from those of Patent Document 1 and Patent Document 2.

[0009] An object of the present disclosure is to provide a railway wheel that exhibits excellent resistance to rolling contact fatigue damage characteristics.

Means for Solving the Problems

[0010] The railway wheel according to the present disclosure includes a rim portion, a boss portion, and a plate portion disposed between the rim portion and the boss portion and connecting the rim portion and the boss portion. The chemical composition of the railway wheel is, by mass percentage, C: 0.65 to 0.84%, Si: 0.10 to 1.00%, Mn: 0.05 to 1.20%, P: 0.050% or less, S: 0.040% or less, N: 0.0200% or less, and Sn: more than 0.50% to 1.50%, and the balance consists of Fe and impurities, In the microstructure of the rim portion, the pearlite area ratio is 95.0% or more, satisfies formula (1A). 115.4 × Sn + 165.4 × Mn ≥ 100.0 (1A) Here, for each element symbol in formula (1A), the content in mass percentage of the corresponding element in the chemical composition is substituted.

[0011] The railway wheel according to the present disclosure has a rim portion, a boss portion, and a plate portion disposed between the rim portion and the boss portion and connecting the rim portion and the boss portion. The chemical composition of the railway wheel is, by mass percentage, C: 0.65 to 0.84%, Si: 0.10 to 1.00%, Mn: 0.05 to 1.20%, P: 0.050% or less, S: 0.040% or less, N: 0.0200% or less, and Sn: more than 0.50% to 1.50%, and further contains one or more selected from the group consisting of the first group to the fourth group, and the balance consists of Fe and impurities, In the microstructure of the rim portion, the pearlite area ratio is 95.0% or more, satisfies formula (1). [First Group] Cu: 0.20% or less, Ni: 0.20% or less, Mo: 0.20% or less, and One or more selected from the group consisting of 0.50% or less of Cr [Group 2] 0.050% or less of Al, and One or more selected from the group consisting of 0.030% or less of Nb [Group 3] 0.10% or less of V, and One or more selected from the group consisting of 0.030% or less of Ti [Group 4] 0.0050% or less of B 115.4×Sn + 886.9×V + 509.2×Mo + 165.4×Mn - 1328040×B ≥ 100.0 (1) Here, in formula (1), the content in mass% of the corresponding element in the chemical composition is substituted for each element symbol. When an element is not contained, "0" is substituted for the corresponding element symbol. [Advantages of the Invention]

[0012] The railway wheel of the present disclosure can obtain excellent rolling contact fatigue damage characteristics. [Brief Description of the Drawings]

[0013]

Figure 1

Figure 2

Figure 3

[0014] The present inventors have studied railway wheels capable of obtaining excellent rolling contact fatigue damage characteristics. As a result, the present inventors have obtained the following findings.

[0015] First, the inventors of the present invention examined the chemical composition suitable for railway wheels. As a result, the inventors considered that a chemical composition consisting of, by mass%, C: 0.65 to 0.84%, Si: 0.10 to 1.00%, Mn: 0.05 to 1.20%, P: 0.050% or less, S: 0.040% or less, N: 0.0200% or less, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Mo: 0 to 0.20%, Cr: 0 to 0.50%, Al: 0 to 0.050%, Nb: 0 to 0.030%, V: 0 to 0.10%, Ti: 0 to 0.030%, B: 0 to 0.0050%, and the balance being Fe and impurities, is appropriate as a railway wheel.

[0016] The inventors further examined means for enhancing the rolling contact fatigue damage resistance characteristics in railway wheels satisfying the above chemical composition. Here, as disclosed in Patent Document 3 (Japanese Patent No. 7239082), there is a positive correlation between the rolling contact fatigue damage resistance characteristics and the 0.7% proof stress obtained from the repeated stress-strain curve (hereinafter also referred to as the repeated S-S curve). The following describes this point.

[0017] As described above, repeated stress due to rolling contact is applied to railway wheels. At this time, if the tread surface of the railway wheel is work-hardened, the strength of the tread surface increases. Thereby, the progress of cracks is suppressed, and the rolling contact fatigue damage resistance characteristics of the railway wheel are enhanced.

[0018] As a model for expressing the stress-strain response in repeated plastic deformation, the equation of the non-linear kinematic hardening rule shown in Equation (I) is known. σ eq =σ 0 +C / γ{1-exp(-γε peq )} (I) Here, in Equation (I), σ eq is the equivalent stress (MPa), σ 0 is the elastic limit (MPa), C is the initial kinematic hardening coefficient (GPa), γ is the kinematic hardening coefficient reduction rate, and ε peq is the equivalent plastic strain.

[0019] The equivalent stress σ eqcorresponds to the strength after work hardening. If the strength after work hardening is high, the crack propagation can be suppressed. That is, if the equivalent stress σ eq can be increased, the crack propagation can be suppressed.

[0020] Here, the fatigue crack propagation rate da / dN (mm / cycle) serving as an index of crack propagation is expressed by Equation (II) based on the Paris law. da / dN = C(ΔK) m = C(ΔK θ、max ) m (II) Here, ΔK θ、max in Equation (II) is the fluctuation range (MPa√m) of the stress intensity factor. Also, C and m are constants obtained from experiments (C = 3.93×10 -13 , m = 3.68).

[0021] When examining the correlation between the equivalent stress σ eq defined by Equation (I) and the fatigue crack propagation rate da / dN defined by Equation (II), when ε peq = 0.007 is substituted into Equation (I), the equivalent stress σ eq (that is, 0.7% proof stress) and the fatigue crack propagation rate da / dN defined by Equation (II) show a strong negative correlation as shown in FIG. 1. Specifically, referring to FIG. 1, as the 0.7% proof stress in the repeated S - S curve increases, the crack propagation rate da / dN exponentially decreases. If the crack propagation rate da / dN is low, it means that the resistance to rolling fatigue damage characteristics is high. That is, the higher the 0.7% proof stress in the repeated S - S curve, the higher the resistance to rolling fatigue damage characteristics.

[0022] Therefore, the present inventors focused on and studied the 0.7% proof stress in the repeated S-S curve. As a result, it was found that adding Sn in addition to the above chemical composition increases the 0.7% proof stress in the repeated S-S curve of railway wheels. Furthermore, the present inventors found that increasing the Sn content to more than 0.50% to 1.50% significantly increases the 0.7% proof stress in the repeated S-S curve of railway wheels. The reason for this is not clear, but increasing the Sn content causes it to dissolve in the ferrite structure in the pearlite structure of railway wheels and strengthen by solid solution. Furthermore, it narrows the interlamellar spacing of the pearlite structure. For the above reasons, it was considered that the 0.7% proof stress in the repeated S-S curve of railway wheels increases.

[0023] Furthermore, the present inventors also studied the influence of elements other than Sn on the 0.7% proof stress in the repeated S-S curve. As a result, the contents of V, Mo, and Mn had a positive correlation with the 0.7% proof stress in the repeated S-S curve, similar to Sn. On the other hand, the B content had a negative correlation with the 0.7% proof stress in the repeated S-S curve. Based on the above findings, the present inventors examined an appropriate relationship between the content of Sn, V, Mo, Mn, and B in the above chemical composition of railway wheels. As a result, in mass%, when C: 0.65 to 0.84%, Si: 0.10 to 1.00%, Mn: 0.05 to 1.20%, P: 0.050% or less, S: 0.040% or less, N: 0.0200% or less, Sn: more than 0.50% to 1.50%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Mo: 0 to 0.20%, Cr: 0 to 0.50%, Al: 0 to 0.050%, Nb: 0 to 0.030%, V: 0 to 0.10%, Ti: 0 to 0.030%, B: 0 to 0.0050%, and the balance consists of Fe and impurities, if the formula (1) is satisfied, the 0.7% proof stress in the repeated S-S curve increases and excellent anti-rolling fatigue damage characteristics can be obtained, the present inventors found. 115.4×Sn + 886.9×V + 509.2×Mo + 165.4×Mn - 1328040×B ≧ 100.0 (1) Here, for each element symbol in formula (1), the content in mass% of the corresponding element in the chemical composition is substituted. In addition, when the chemical composition of the railway wheel does not contain V, Mo, and B which are optional elements (that is, when V, Mo, and B are 0%), Equation (1) becomes the following Equation (1A). 115.4×Sn + 165.4×Mn ≥ 100.0 (1A)

[0024] Based on the above technical idea, the railway wheel of the present embodiment completed has the following configuration.

[0025] [1] A railway wheel, a rim portion, a boss portion, and a plate portion disposed between the rim portion and the boss portion and connecting the rim portion and the boss portion, The chemical composition of the railway wheel is, by mass%, C: 0.65 to 0.84%, Si: 0.10 to 1.00%, Mn: 0.05 to 1.20%, P: 0.050% or less, S: 0.040% or less, N: 0.0200% or less, and Sn: more than 0.50% to 1.50%, and the balance consists of Fe and impurities, In the microstructure of the rim portion, the pearlite area ratio is 95.0% or more, satisfying Equation (1A), A railway wheel. 115.4×Sn + 165.4×Mn ≥ 100.0 (1A) Here, in each element symbol in Equation (1A), the content in mass% of the corresponding element in the chemical composition is substituted.

[0026] [2] A railway wheel, a rim portion, a boss portion, and a plate portion disposed between the rim portion and the boss portion and connecting the rim portion and the boss portion, The chemical composition of the railway wheel is, by mass%, C: 0.65 to 0.84%, Si: 0.10 to 1.00%, Mn: 0.05 to 1.20%, P: 0.050% or less, S: 0.040% or less, N: 0.0200% or less, and Sn: more than 0.50% to 1.50%, containing furthermore, containing one or more selected from the group consisting of the first group to the fourth group, the balance being composed of Fe and impurities, in the microstructure of the rim portion, the pearlite area ratio is 95.0% or more, satisfying formula (1), Railway wheel. [First group] Cu: 0.20% or less, Ni: 0.20% or less, Mo: 0.20% or less, and Cr: 0.50% or less, one or more selected from the group consisting of [Second group] Al: 0.050% or less, and Nb: 0.030% or less, one or more selected from the group consisting of [Third group] V: 0.10% or less, and Ti: 0.030% or less, one or more selected from the group consisting of [Fourth group] B: 0.0050% or less 115.4 × Sn + 886.9 × V + 509.2 × Mo + 165.4 × Mn - 1328040 × B ≥ 100.0 (1) Here, in formula (1), the content in mass% of the corresponding element in the chemical composition is substituted for each element symbol. When an element is not contained, "0" is substituted for the corresponding element symbol.

[0027] [3] The railway wheel according to [2], wherein the chemical composition contains the first group, Railway wheel.

[0028] [4] The railway wheel according to [2] or [3], wherein the chemical composition contains the second group, railway wheel.

[0029] [5] The railway wheel according to [2] to [4], wherein the chemical composition contains the third group, railway wheel.

[0030] [6] The railway wheel according to [2] to [5], wherein the chemical composition contains the fourth group, railway wheel.

[0031] Hereinafter, the railway wheel of the present embodiment will be described in detail. Note that "%" regarding elements means mass % unless otherwise specified.

[0032] [Configuration of Railway Wheel] FIG. 2 is a cross-sectional view including the central axis of the railway wheel according to the present embodiment. Referring to FIG. 2, the railway wheel 1 is disk-shaped and includes a boss portion 2, a plate portion 3, and a rim portion 4. The boss portion 2 is cylindrical and is disposed at the central portion of the railway wheel 1 in the radial direction of the railway wheel 1 (the direction perpendicular to the central axis). The boss portion 2 has a through hole 21. The central axis of the through hole 21 coincides with the central axis of the railway wheel 1. A railway axle (not shown) is inserted into the through hole 21. The thickness T2 of the boss portion 2 is thicker than the thickness T3 of the plate portion 3. The rim portion 4 is formed at the edge of the outer periphery of the railway wheel 1. The rim portion 4 includes a tread surface 41 and a flange portion 42. The tread surface 41 is connected to the flange portion 42. When the railway wheel 1 is in use, the tread surface 41 and the flange portion 42 are in contact with the rail surface. The thickness T4 of the rim portion 4 is thicker than the thickness T3 of the plate portion 3. The plate portion 3 is disposed between the boss portion 2 and the rim portion 4 and is connected to the boss portion 2 and the rim portion 4. Specifically, the inner peripheral edge portion of the plate portion 3 is connected to the boss portion 2, and the outer peripheral edge portion of the plate portion 3 is connected to the rim portion 4. The thickness T3 of the plate portion 3 is thinner than the thickness T2 of the boss portion 2 and the thickness T4 of the rim portion 4.

[0033] [Features of the Railway Wheel of the Present Embodiment] The railway wheel of the present embodiment satisfies the following Features 1 to 3.

[0034] (Feature 1) The chemical composition is, by mass%, C: 0.65 to 0.84%, Si: 0.10 to 1.00%, Mn: 0.05 to 1.20%, P: 0.050% or less, S: 0.040% or less, N: 0.0200% or less, Sn: over 0.50% to 1.50%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Mo: 0 to 0.20%, Cr: 0 to 0.50%, Al: 0 to 0.050%, Nb: 0 to 0.030%, V: 0 to 0.10%, Ti: 0 to 0.030%, B: 0 to 0.0050%, and the balance consists of Fe and impurities. (Feature 2) In the microstructure of the rim portion, the pearlite area ratio is 95.0% or more. (Feature 3) The above chemical composition satisfies Formula (1). 115.4 × Sn + 886.9 × V + 509.2 × Mo + 165.4 × Mn - 1328040 × B ≥ 100.0 (1) Here, for each element symbol in Formula (1), the content in mass% of the corresponding element in the chemical composition is substituted. When the chemical composition of the railway wheel does not contain the optional elements V, Mo, and B (that is, when V, Mo, and B are 0%), Formula (1) becomes the following Formula (1A). 115.4 × Sn + 165.4 × Mn ≥ 100.0 (1A)

[0035] [Regarding (Feature 1) Chemical Composition] The chemical composition of the railway wheel of the present embodiment contains the following elements.

[0036] C: 0.65 to 0.84% Carbon (C) increases the hardness of the steel and enhances the rolling fatigue damage resistance characteristics of the railway wheel 1. When the C content is less than 0.65%, even if the contents of other elements are within the range of the present embodiment, the above effects cannot be sufficiently obtained. On the one hand, if the C content exceeds 0.84%, even if the contents of other elements are within the range of this embodiment, the toughness of the railway wheel 1 will decrease. Therefore, the C content is 0.65 to 0.84%. The preferable lower limit of the C content is 0.67%, more preferably 0.69%, and even more preferably 0.70%. The preferable upper limit of the C content is 0.82%, more preferably 0.80%, even more preferably 0.77%, and even more preferably 0.73%.

[0037] Si: 0.10 to 1.00% Silicon (Si) solid-solution strengthens ferrite to increase the hardness of steel. If the Si content is less than 0.10%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be fully obtained. On the one hand, if the Si content exceeds 1.00%, even if the contents of other elements are within the range of this embodiment, the toughness of the railway wheel 1 will decrease. If the Si content exceeds 1.00%, furthermore, the hardenability of the steel becomes too high and martensite is likely to form. In this case, the thickness of the hardened layer formed on the tread surface during tread hardening increases. As a result, the cutting amount increases and the yield decreases. If the Si content exceeds 1.00%, furthermore, during the use of the railway wheel 1, the rim portion 4 is burned by the frictional heat generated between the brake. In this case, the crack resistance of the steel may decrease. Therefore, the Si content is 0.10 to 1.00%. The preferable lower limit of the Si content is 0.15%, more preferably 0.20%, and even more preferably 0.25%. The preferable upper limit of the Si content is 0.90%, more preferably 0.80%, even more preferably 0.70%, even more preferably 0.60%, and even more preferably 0.50%.

[0038] Mn: 0.05 to 1.20% Manganese (Mn) solid-solution strengthens ferrite to increase the hardness of steel. Mn further forms MnS, improving the machinability of steel. If the Mn content is less than 0.05%, these effects cannot be obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Mn content exceeds 1.20%, even if the contents of other elements are within the range of this embodiment, the hardenability of the steel becomes too high. In this case, the thickness of the hardened layer increases, and the yield during the manufacturing process decreases. Furthermore, when the railway wheel 1 is in use, the rim portion 4 is hardened by the frictional heat generated between the brake. In this case, the crack resistance of the steel may decrease. Therefore, the Mn content is 0.05 to 1.20%. The preferable lower limit of the Mn content is 0.06%, more preferably 0.08%, and even more preferably 0.10%. The preferable upper limit of the Mn content is 1.10%, more preferably 1.00%, and even more preferably 0.95%.

[0039] P: 0.050% or less Phosphorus (P) is an impurity inevitably contained. P segregates at grain boundaries, reducing the toughness of steel. Therefore, the P content is 0.050% or less. It is preferable that the P content is as low as possible. However, excessive reduction of the P content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the P content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferable upper limit of the P content is 0.040%, more preferably 0.025%, even more preferably 0.020%, and even more preferably 0.015%.

[0040] S: 0.040% or less Sulfur (S) is inevitably contained. S forms MnS and improves the machinability of steel. If even a small amount of S is contained, the above effects can be obtained to a certain extent. However, if the S content exceeds 0.040%, the MnS in the steel coarsens and the toughness of the steel decreases. Therefore, the S content is 0.040% or less. It is preferable that the S content is as low as possible. However, excessive reduction of the S content increases the manufacturing cost. Therefore, considering normal industrial production, the lower limit of the S content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.005%. The preferable upper limit of the S content is 0.030%, more preferably 0.025%, even more preferably 0.020%, and even more preferably 0.015%.

[0041] N: 0.0200% or less Nitrogen (N) is an impurity inevitably contained. If the N content exceeds 0.0200%, even if the contents of other elements are within the range of this embodiment, AlN coarsens and the toughness of the steel decreases. Therefore, the N content is 0.0200% or less. The preferable lower limit of the N content is more than 0%, more preferably 0.0001%, even more preferably 0.0002%, and even more preferably 0.0003%. The preferable upper limit of the N content is 0.0180%, more preferably 0.0150%, even more preferably 0.0130%, even more preferably 0.0100%, and even more preferably 0.0080%.

[0042] Sn: More than 0.50% to 1.50% Tin (Sn) solid-solution strengthens the ferrite in pearlite and increases the hardness of the steel. Sn further narrows the interlamellar spacing of pearlite. Thereby, the hardness of pearlite increases. If the Sn content is 0.50% or less, these effects cannot be obtained sufficiently. On the one hand, if the Sn content exceeds 1.50%, even if the contents of other elements are within the scope of this embodiment, the toughness of the railway wheel 1 will decrease. Further, if the Sn content exceeds 1.50%, the hot workability of the steel will decrease. Therefore, the Sn content is more than 0.50% and up to 1.50%. The preferable lower limit of the Sn content is 0.55%, more preferably 0.60%, still more preferably 0.65%, and even more preferably 0.70%. The preferable upper limit of the Sn content is 1.40%, more preferably 1.30%, still more preferably 1.25%, and even more preferably 1.20%.

[0043] The balance of the chemical composition of the railway wheel according to this embodiment consists of Fe and impurities. Here, the impurities refer to those mixed from ores, scraps, or manufacturing environments as raw materials when industrially manufacturing the railway wheel of this embodiment, and are those allowed within a range that does not adversely affect the railway wheel of this embodiment.

[0044] [Regarding Optional Elements] The chemical composition of the railway wheel 1 according to this embodiment may further contain one or more selected from the group consisting of the first group to the fourth group. [First Group] Cu: 0.20% or less, Ni: 0.20% or less, Mo: 0.20% or less, and Cr: 0.50% or less, one or more selected from the group consisting of [Second Group] Al: 0.050% or less, and Nb: 0.030% or less, one or more selected from the group consisting of [Third Group] V: 0.10% or less, and Ti: 0.030% or less, one or more selected from the group consisting of [Fourth Group] B: 0.0050% or less Hereinafter, the elements of the first group to the fourth group will be described.

[0045] [Regarding the first group (Cu, Ni, Mo, and Cr)] The chemical composition of the railway wheel 1 of the present embodiment may contain one or more selected from the group consisting of Cu, Ni, Mo, and Cr in place of a part of Fe. All of these elements increase the hardness of the steel.

[0046] Cu: 0.20% or less Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu increases the hardness of the steel by solid solution strengthening. As a result, the resistance to rolling fatigue damage of the railway wheel 1 increases. However, if the Cu content exceeds 0.20%, even if the contents of other elements are within the range of the present embodiment, the hot workability of the steel decreases. Therefore, the Cu content is 0 to 0.20%, and when contained, the Cu content is 0.20% or less. The lower limit of the Cu content is preferably more than 0%, more preferably 0.01%, still more preferably 0.02%, still more preferably 0.05%, and still more preferably 0.10%. The upper limit of the Cu content is preferably 0.17%, more preferably 0.15%, still more preferably 0.12%, and still more preferably 0.10%.

[0047] Ni: 0.20% or less Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, Ni increases the hardness and toughness of the steel. However, if the Ni content exceeds 0.20%, even if the contents of other elements are within the range of the present embodiment, the hardenability becomes excessively high and the thickness of the hardened layer after tread hardening increases excessively. Therefore, the Ni content is 0 to 0.20%, and when contained, the Ni content is 0.20% or less. The lower limit of the Ni content is preferably more than 0%, more preferably 0.01%, still more preferably 0.02%, and still more preferably 0.05%. The upper limit of the Ni content is preferably 0.17%, more preferably 0.15%, still more preferably 0.12%, and even more preferably 0.10%.

[0048] Mo: 0.20% or less Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When contained, Mo increases the hardness of the steel. As a result, the rolling contact fatigue resistance of the railway wheel 1 is enhanced. However, if the Mo content exceeds 0.20%, even if the contents of other elements are within the scope of this embodiment, the hardenability becomes excessively high, and the thickness of the hardened layer after tread hardening increases excessively. Therefore, the Mo content is 0 to 0.20%, and when contained, the Mo content is 0.20% or less. The lower limit of the Mo content is preferably more than 0%, more preferably 0.01%, still more preferably 0.02%, and even more preferably 0.05%. The upper limit of the Mo content is preferably 0.18%, more preferably 0.15%, still more preferably 0.12%, even more preferably 0.10%, and even more preferably 0.07%.

[0049] Cr: 0.50% or less Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When contained, Cr narrows the interlamellar spacing of pearlite. Thereby, the hardness of pearlite increases significantly. However, if the Cr content exceeds 0.50%, even if the contents of other elements are within the scope of this embodiment, the hardenability becomes excessively high, and the thickness of the hardened layer after tread hardening increases excessively. Therefore, the Cr content is 0 to 0.50%, and when contained, the Cr content is 0.50% or less. The lower limit of the Cr content is preferably more than 0%, more preferably 0.01%, still more preferably 0.02%, even more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Cr content is preferably 0.45%, more preferably 0.40%, still more preferably 0.35%, still more preferably 0.30%, still more preferably 0.25%, and still more preferably 0.20%.

[0050] [Regarding the second group (Al and Nb)] The chemical composition of the railway wheel 1 of the present embodiment may contain one or more selected from the group consisting of Al and Nb in place of a part of Fe. All of these elements form carbides or nitrides to suppress the coarsening of crystal grains due to the pinning effect.

[0051] Al: 0.050% or less Aluminum (Al) is an optional element and may not be contained. That is, the Al content may be 0%. When contained, Al combines with N to form AlN and refine the crystal grains. By refining the crystal grains, the toughness of the steel is increased. However, if the Al content exceeds 0.050%, even if the contents of other elements are within the range of the present embodiment, non-metallic inclusions increase and the toughness of the steel decreases. Therefore, the Al content is 0 to 0.050%, and when contained, the Al content is 0.050% or less. The lower limit of the Al content is preferably more than 0%, more preferably 0.001%, still more preferably 0.005%, still more preferably 0.010%, still more preferably 0.020%, and still more preferably 0.030%. The upper limit of the Al content is preferably 0.045%, more preferably 0.040%, and still more preferably 0.035%. Note that the Al content referred to in this specification means the acid-soluble Al (sol.Al) content.

[0052] Nb: 0.030% or less Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb combines with C during heating for surface hardening in the manufacturing process of railway wheels to form fine NbC. The fine NbC functions as pinning particles to suppress the coarsening of austenite during heating. Therefore, the prior austenite grain size is maintained fine, and the hardenability of the steel is suppressed. As a result, the formation of a hardened layer during the manufacturing process of the railway wheel 1 is suppressed. Nb also enhances the toughness of the steel material by suppressing the coarsening of the prior austenite grain size. On the other hand, if the Nb content exceeds 0.030%, even if the contents of other elements are within the range of this embodiment, the NbC coarsens and the toughness of the steel material decreases instead. Therefore, the Nb content is 0 to 0.030%, and when contained, the Nb content is 0.030% or less. The lower limit of the Nb content is preferably more than 0%, more preferably 0.010%, still more preferably 0.012%, and still more preferably 0.015%. The upper limit of the Nb content is preferably 0.025%, more preferably 0.020%, and still more preferably 0.017%.

[0053] [Regarding the third group (V and Ti)] The chemical composition of the railway wheel 1 of this embodiment may contain one or more selected from the group consisting of V and Ti instead of a part of Fe. All of these elements form either carbide, nitride, or carbonitride to precipitate-strengthen the steel.

[0054] V: 0.10% or less Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, V forms either carbide, nitride, or carbonitride to precipitate-strengthen the steel (specifically, ferrite in the steel). As a result, the hardness of the railway wheel 1 increases and the resistance to rolling fatigue damage improves. However, if the V content exceeds 0.10%, the hardenability increases and the thickness of the hardened layer after surface hardening increases excessively. Therefore, the V content is 0 to 0.10%, and when contained, the V content is 0.10% or less. The lower limit of the V content is preferably more than 0%, more preferably 0.01%, still more preferably 0.02%, and even more preferably 0.03%. The upper limit of the V content is preferably 0.09%, more preferably 0.08%, and still more preferably 0.07%.

[0055] Ti: 0.030% or less Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, Ti forms any one of carbides, nitrides, and carbonitrides to precipitate and strengthen the steel (specifically, ferrite in the steel). As a result, the hardness of the railway wheel 1 increases and the resistance to rolling fatigue damage is enhanced. However, if the Ti content exceeds 0.030%, the hardenability increases and the thickness of the hardened layer after tread hardening increases excessively. Therefore, the Ti content is 0 to 0.030%, and when contained, the Ti content is 0.030% or less. The lower limit of the Ti content is preferably more than 0%, more preferably 0.001%, still more preferably 0.002%, and even more preferably 0.003%. The upper limit of the Ti content is preferably 0.020%, more preferably 0.010%, and still more preferably 0.005%.

[0056] [Regarding Group 4 (B)] The chemical composition of the railway wheel 1 of the present embodiment may contain B in place of a part of Fe.

[0057] B: 0.0050% or less Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B reduces the cooling rate dependence of the pearlite transformation temperature and makes the hardness distribution of the wheel tread uniform. However, if the B content exceeds 0.0050%, the hardenability increases and the thickness of the hardened layer after tread hardening increases excessively. Therefore, the B content is 0 to 0.0050%, and when contained, the B content is 0.0050% or less. The lower limit of the B content is preferably more than 0%, more preferably 0.0001%, and even more preferably 0.0003%. The upper limit of the B content is preferably 0.0040%, more preferably 0.0030%, even more preferably 0.0025%, even more preferably 0.0010%, and even more preferably 0.0005%.

[0058] [(Feature 2) Regarding the microstructure of the rim portion of the railway wheel] The railway wheel of this embodiment further satisfies the following Feature 2. (Feature 2) In the microstructure of the rim portion, the pearlite area ratio is 95.0% or more.

[0059] In the microstructure of the rim portion 4 of the railway wheel 1 of this embodiment, the area ratio of pearlite is 95.0% or more. Among the microstructures of the rim portion 4, the phases other than pearlite are, for example, martensite, bainite, and proeutectoid ferrite.

[0060] [Measurement method of pearlite area ratio] The pearlite area ratio in the microstructure of the rim portion 4 is determined by the following method. A sample is taken from the central position in the thickness direction of the rim portion 4 (the central position of the thickness T4 in FIG. 2). The observation surface of each sample is polished to a mirror finish by mechanical polishing. Thereafter, etching is performed on the observation surface using 3% nitric acid alcohol (nital etching solution). For an arbitrary one field (500 μm × 500 μm) in the observation surface of the etched sample, a photographic image is generated using an optical microscope with a magnification of 200 times. In the observation surface, each phase (pearlite, primary ferrite, martensite, and bainite) has a different contrast. Therefore, based on the contrast, the pearlite in the microstructure can be discriminated. Based on the contrast, the pearlite is specified. The area ratio (%) of the pearlite is determined based on the total area of the specified pearlite and the area of the observation field.

[0061] [(Feature 3) Regarding formula (1) (or formula (1A))] The railway wheel of the present embodiment further satisfies formula (1). 115.4×Sn + 886.9×V + 509.2×Mo + 165.4×Mn - 1328040×B ≧ 100.0 (1) Here, for each element symbol in formula (1), the content in mass % of the corresponding element in the chemical composition is substituted. When an element is not contained, "0" is substituted for the corresponding element symbol. In addition, when the chemical composition of the railway wheel does not contain the optional elements V, Mo, and B (that is, when V, Mo, and B are 0%), formula (1) becomes the following formula (1A). 115.4×Sn + 165.4×Mn ≧ 100.0 (1A)

[0062] Define Fn1 as follows. Fn1 = 115.4×Sn + 886.9×V + 509.2×Mo + 165.4×Mn - 1328040×B In addition, when the chemical composition of the railway wheel does not contain the optional elements V, Mo, and B (that is, when V, Mo, and B are 0%), define Fn1 as follows. Fn1 = 115.4×Sn + 165.4×Mn

[0063] Fn1 corresponds to the left side of formula (1) or formula (1A). Fn1 is an index for enhancing the rolling contact fatigue damage characteristics of a railway wheel that satisfies Feature 1 and Feature 2. Among the elements in the above chemical composition, Sn, V, Mo, and Mn increase the 0.7% proof stress in the repeated S-S curve of the railway wheel. On the other hand, B decreases the 0.7% proof stress in the repeated S-S curve of the railway wheel. If Fn1 is less than 100.0, the contents of Sn, V, Mo, and Mn are too small relative to the B content. In this case, even if the railway wheel satisfies Feature 1 and Feature 2, sufficient rolling contact fatigue damage characteristics cannot be obtained. Therefore, Fn1 is 100.0 or more.

[0064] A preferable lower limit of Fn1 is 110.0, more preferably 120.0, and even more preferably 140.0. The upper limit of Fn1 is not particularly limited, and for example, it is 400.0.

[0065] [Regarding the effects of the railway wheel of the present embodiment] The railway wheel of the present embodiment satisfies Features 1 to 3. Therefore, excellent rolling contact fatigue damage characteristics can be obtained with the railway wheel of the present embodiment.

[0066] [Regarding the uses of the railway wheel of the present embodiment] The railway wheel of the present embodiment is suitable for railway vehicle applications.

[0067] [Manufacturing method of the railway wheel of the present embodiment] Hereinafter, the manufacturing method of the railway wheel of the present embodiment will be described. The manufacturing method of the railway wheel described hereinafter is an example of the manufacturing method of the railway wheel of the present embodiment. Therefore, the railway wheel having the above-described configuration may be manufactured by other manufacturing methods other than the manufacturing method described hereinafter. However, the manufacturing method described hereinafter is a preferable example of the manufacturing method of the railway wheel of the present embodiment.

[0068] An example of the manufacturing method of the present embodiment includes the following steps. (Step 1) Stock preparation step (Step 2) Forming step (Process 3) Heat treatment process (Process 4) Machining process The following describes each process.

[0069] [(Process 1) Material preparation process] In the material preparation process, molten steel having the above-described chemical composition is produced using an electric furnace, a converter, or the like. The produced molten steel is cast into a casting (a slab or an ingot). A slab may be produced by continuous casting, or an ingot may be produced by casting into a mold.

[0070] The slab or ingot is hot-worked to produce a steel material for railway wheels (hereinafter also referred to as a steel material) having a desired size. The hot working is, for example, hot forging, hot rolling, or the like. When producing the steel material by hot rolling, for example, the steel material is produced by the following method. In hot rolling, for example, a slab rolling mill is used. The slab rolling mill performs slab rolling on the material to produce a steel material. When a continuous rolling mill is installed downstream of the slab rolling mill, hot rolling may be further performed on the steel material after slab rolling using the continuous rolling mill to produce a steel material having an even smaller size. In the continuous rolling mill, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. The heating temperature of the heating furnace in hot rolling is not particularly limited, but is, for example, 1100 to 1350°C. Through the above manufacturing process, a steel material for railway wheels is produced.

[0071] Note that the steel material for railway wheels may be a casting (a slab or an ingot). That is, the above-described hot working may be omitted. Through the above processes, a steel material for railway wheels, which is a material for railway wheels, is produced. The steel material for railway wheels is, for example, a cylindrical material.

[0072] [(Process 2) Forming process] In the forming process, an intermediate product with a wheel shape is formed by hot working using the prepared steel material for railway wheels. Since the intermediate product has a wheel shape, it includes a boss portion, a plate portion, and a rim portion including a tread surface and a flange portion. The hot working is, for example, hot forging, hot rolling, or the like. The cooling method of the intermediate product after hot working is not particularly limited. It may be air cooling or water cooling.

[0073] [(Process 3) Heat treatment process] In the heat treatment process, surface hardening of the tread is performed on the formed intermediate product with a wheel shape. Specifically, the intermediate product after the forming process (hot forging or hot rolling) is reheated to above the A c3 transformation point or the A cm transformation point and soaked for 30 to 150 minutes (reheating treatment). After heating, the tread surface and flange portion of the intermediate product are rapidly cooled (surface hardening of the tread). For example, the tread surface and flange portion are cooled by a cooling medium. The cooling medium is, for example, air, mist, spray, salt bath, or the like, and is not particularly limited as long as the cooling rate can obtain a desired structure. Furthermore, the temperature and holding time of the cooling medium are not particularly limited. Thereby, in the tread surface and flange portion of the rim portion, a hardened layer (martensite) on the outermost surface and a hard pearlite structure below it are formed.

[0074] Subsequently, tempering is performed on the intermediate product after surface hardening of the tread as necessary. The tempering temperature is, for example, 400 to 600 °C.

[0075] [(Process 4) Machining process] As described above, fine pearlite is formed on the surface layer of the tread of the intermediate product after the heat treatment process, and a hardened layer composed of martensite is formed on the upper layer. In the use of railway wheels, the wear resistance of martensite is low. Therefore, the hardened layer is removed by well-known machining.

[0076] Through the above processes, the railway wheel of the present embodiment is manufactured.

Example

[0077] The effects of the railway wheel of this embodiment will be further specifically described by way of examples. The conditions in the following examples are one example of the conditions adopted to confirm the feasibility and effects of the railway wheel of this embodiment. Therefore, the railway wheel of this embodiment is not limited to this one example of conditions.

[0078] Molten steel having the chemical compositions shown in Table 1A and Table 1B was produced. In the following description, Table 1A and Table 1B are also collectively referred to as "Table 1". "-" in Table 1 indicates that the corresponding element content is below the impurity level.

[0079]

Table 1A

[0080]

Table 1B

[0081] Simulating the material preparation process of the manufacturing process of the railway wheel, ingots of each test number were produced by the ingot-making method using the above molten steel. Next, simulating the forming process of the manufacturing process of the railway wheel, the ingot was hot forged to produce a round bar with a diameter of 35 mm. From this round bar, a round bar with a diameter of 17 mm and a length of 140 mm was processed by cutting.

[0082] For this round bar, quenching simulating the tread quenching of the manufacturing process of the railway wheel was carried out. Specifically, each round bar of each test number was soaked at a heat treatment temperature of 900 °C for 30 minutes. After soaking, in order to form a pearlite structure, the round bar was taken out of the furnace and immersed in a salt bath at 550 °C. The immersion time in the salt bath was 3 minutes. Then, the round bar was taken out of the salt bath and air-cooled to room temperature (25 °C).

[0083] [Evaluation Test] The following evaluation tests were carried out on the round bars of each test number. (Test 1) Measurement test of 0.7% proof stress in the repeated S-S curve The following describes Test 1.

[0084] [(Test 1) Measurement test of 0.7% proof stress in repeated S-S curve] The 0.7% proof stress in the repeated S-S curve of the round bar for each test number was evaluated by the following method. From the round bar for each test number, a cylindrical test piece 100 shown in Fig. 3 was taken so that the longitudinal direction of the test piece for the repeated stress-strain test (hereinafter also referred to as the repeated S-S test) was the longitudinal direction of the round bar. Referring to Fig. 3, the total length 106 of the test piece 100 was 130 mm, the length of the parallel portion 101 at the longitudinal center of the test piece 100 was 20 mm, and the length of the gripping portion 102 located at both ends of the parallel portion 101 was 41 mm. The diameter 103 of the parallel portion 101 was 6 mm, and the diameter 104 of the gripping portion 102 was 15 mm. The radius of curvature 105 between the parallel portion 101 and the gripping portion 102 was 20 mm. Further, the surface roughness of the parallel portion 101 was 0.4 μm, and the surface roughness of the gripping portion 102 was 1.6 μm.

[0085] A repeated S-S test was carried out using the test piece 100. The repeated S-S test was performed using a hydraulic servo fatigue testing machine (manufactured by Shimadzu Corporation). The repeated S-S test was conducted by the Incremental Step method with a maximum strain amplitude of 1.2%. The strain increment per waveform was 2.0×10 -1 %, the strain waveform was a triangular wave, and after applying a repeated load of 20 blocks, the test was terminated. Here, the strain was measured with a strain gauge attached to the parallel portion of the test piece. The gauge length of the strain gauge was 12.5 mm, the strain ratio was -1, and the strain rate was 4×10 -2 % / s. The strain, load, and stroke in the repeated S-S test were measured at a sampling frequency of 10 Hz.

[0086] Next, a nominal stress-nominal strain hysteresis loop obtained from the above repeated S-S test was obtained. The absolute value of the nominal stress at the absolute value of the nominal strain at the apex of the hysteresis loop obtained from the 20th block of repeated loading was averaged, and the averaged nominal stress and nominal strain were plotted to obtain a repeated S-S curve. The stress value at which the plastic strain in the repeated S-S curve becomes 0.7% was defined as the 0.7% proof stress (MPa). The obtained 0.7% proof stress is shown in the column of "0.7% proof stress (MPa)" in Table 2.

[0087]

Table 2

[0088] [Test Results] Referring to Table 1 and Table 2, the round bars manufactured by simulating the manufacturing processes of railway wheels with test numbers 1 to 19 satisfied Features 1 to 3. Therefore, for the round bars with these test numbers, the 0.7% proof stress in the repeated S-S curve was 650 MPa or more, and excellent rolling contact fatigue damage resistance characteristics were obtained.

[0089] On the other hand, in Test No. 20, the Sn content was too low. Therefore, the 0.7% proof stress in the repeated S-S test was less than 650 MPa, and sufficient rolling contact fatigue damage resistance characteristics were not obtained.

[0090] In Test Nos. 21 and 22, Fn1 was too low. Therefore, the 0.7% proof stress in the repeated S-S test was less than 650 MPa, and sufficient rolling contact fatigue damage resistance characteristics were not obtained.

[0091] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the gist thereof.

Claims

1. A railway wheel, comprising: a rim portion; a boss portion; a plate portion disposed between the rim portion and the boss portion and connecting the rim portion and the boss portion; The chemical composition of the railway wheel is, by mass%, C: 0.65 to 0.84%, Si: 0.10 to 1.00%, Mn: 0.05 to 1.20%, P: 0.050% or less, S: 0.040% or less, N: 0.0200% or less, and Sn: more than 0.50% to 1.50%, and the balance consists of Fe and impurities; In the microstructure of the rim portion, the pearlite area ratio is 95.0% or more, satisfying formula (1A); A railway wheel. 115.4 × Sn + 165.4 × Mn ≥ 100.0 (1A) Here, in each element symbol in formula (1A), the content in mass% of the corresponding element in the chemical composition is substituted.

2. A railway wheel, comprising: a rim portion; a boss portion; a plate portion disposed between the rim portion and the boss portion and connecting the rim portion and the boss portion; The chemical composition of the railway wheel is, by mass%, C: 0.65 to 0.84%, Si: 0.10 to 1.00%, Mn: 0.05 to 1.20%, P: 0.050% or less, S: 0.040% or less, N: 0.0200% or less, and Sn: more than 0.50% to 1.50%, and further contains one or more selected from the group consisting of Group 1 to Group 4, and the balance consists of Fe and impurities; In the microstructure of the rim portion, the pearlite area ratio is 95.0% or more, satisfying formula (1); A railway wheel. [Group 1] Cu: 0.20% or less, Ni: 0.20% or less, Mo: 0.20% or less, and Cr: 0.50% or less, one or more selected from the group consisting of [Group 2] Al: 0.050% or less, and Nb: 0.030% or less, one or more selected from the group consisting of [Group 3] V: 0.10% or less, and Ti: 0.030% or less, one or more selected from the group consisting of [Group 4] B: 0.0050% or less 115.4 × Sn + 886.9 × V + 509.2 × Mo + 165.4 × Mn - 1328040 × B ≥ 100.0 (1) Here, in each element symbol in formula (1), the content in mass% of the corresponding element in the chemical composition is substituted. When an element is not contained, "0" is substituted for the corresponding element symbol.

3. The railway wheel according to claim 2, wherein the chemical composition contains the Group 1. A railway wheel.

4. The railway wheel according to claim 2, wherein The chemical composition contains the second group, Railway wheel.

5. The railway wheel according to claim 2, The chemical composition contains the third group, Railway wheel.

6. The railway wheel according to claim 2, The chemical composition contains the fourth group, Railway wheel.

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