Technical field
[0001]The present invention relates to a railway wheel.
BACKGROUND
[0002]Railway vehicle travels on rails which constitute the line. Railway vehicle includes a plurality of rail wheel. Railway wheel supports the vehicle, in contact with the rail, moves while rotating on the rail. Railway wheel is worn due to contact with the rail. Recently, for the purpose of high efficiency of railway transportation, an increase of the loaded weight of the rail vehicle, and, speed of railway vehicles has been developed. As a result, improvement of the wear resistance of the rail wheel has been demanded.
[0003]
　Technique for increasing the wear resistance of the rail wheel is, JP-A-9-202937 (Patent Document 1), JP 2012-107295 (Patent Document 2), JP 2013-231212 (Patent Document 3), JP 2004-315928 JP has been proposed (Patent Document 4).
[0004]
　Railway wheel disclosed in Patent Document 1, by mass%, C: 0.4 ~ 0.75%, Si: 0.4 ~ 0.95%, Mn: 0.6 ~ 1.2%, Cr: less than 0 ~ 0.2%, P: 0.03% or less, S: 0.03% or less, the balance being Fe and unavoidable impurities. In this railway wheel, region up to at least a depth of 50mm from the surface of the wheel tread portion is composed of pearlite. Method of manufacturing a rail wheel in Patent Document 1, the cooling curve of the wheel tread portion, through the pearlite generation region in the continuous cooling transformation curves, and the conditions in the long side of the martensitic transformation curve, the wheel tread portion including the cooling quench process.
[0005]
　Steel disclosed in Patent Document 2 wheels, by mass%, C: 0.65 ~ 0.84%, Si: 0.02 ~ 1.00%, Mn: 0.50 ~ 1.90%, Cr : 0.02 ~ 0.50%, V: 0.02 ~ 0.20%, include S ≦ 0.04%, the balance being Fe and impurities, P ≦ 0.05%, Cu ≦ 0.20 %, a Ni ≦ 0.20% of the chemical composition. The chemical composition further satisfies the following relation. [34 ≦ 2.7 + 29.5 × C + 2.9 × Si + 6.9 × Mn + 10.8 × Cr + 30.3 × Mo + 44.3 × V ≦ 43] and [0.76 × exp (0.05 × C) × exp ( 1.35 × Si) × exp (0.38 × Mn) × exp (0.77 × Cr) × exp (3.0 × Mo) × exp (4.6 × V) ≦ 25]. Steel for this vehicle, by satisfying the chemical composition and the formula, abrasion resistance, 耐転 dynamic fatigue properties, excellent resistance to spoke ring resistance, and are described in the patent document 2.
[0006]
　Steel disclosed in Patent Document 3 wheels, by mass%, C: 0.65 ~ 0.84%, Si: 0.4 ~ 1.0%, Mn: 0.50 ~ 1.40%, Cr : 0.02 ~ 0.13%, S: 0.04% or less, V: contains 0.02 ~ 0.12%, Fn1 of 32 to 43 as defined in formula (1), and the formula Fn2 represented by (2) is 25 or less, the balance being Fe and impurities. Here, equation (1) is a Fn1 = 2.7 + 29.5C + 2.9Si + 6.9Mn + 10.8Cr + 30.3Mo + 44.3V, equation (2) is, Fn2 = exp (0.76) × exp (0.05C) × is exp (1.35Si) × exp (0.38Mn) × exp (0.77Cr) × exp (3.0Mo) × exp (4.6V). The wheel steel has the chemical composition, by Fn1 and Fn2 satisfies the above range, wear resistance, 耐転 dynamic fatigue properties, excellent resistance to spoke ring resistance, and is described in Patent Document 3 there.
[0007]
　Wheels for railway vehicle disclosed in Patent Document 4, by mass%, C: 0.85 ~ 1.20%, Si: 0.10 ~ 2.00%, Mn: 0.05 ~ 2.00%, required further Cr, Mo, V, Nb, B, Co, Cu, Ni, Ti, Mg, Ca, Al, Zr, and N 1, two or more of containing a predetermined amount in accordance with the balance being Fe and other a rail vehicle wheel integral comprised of steel containing chemical components consisting of unavoidable impurities, at least a portion of the tread and / or flange surface of the wheel is pearlite structure. In Patent Document 4, the wheel of life for rail vehicles is dependent on the amount of wear of the tread and the flange surface (Patent Document 4, paragraph [0002]), and further, the increase in the amount of heat generated when braking in high-speed rail and has been described to depend on the cracks in the tread and flange surfaces occurs no. By wheels for railway vehicles having the above configuration, the wear resistance and thermal cracking of tread and flange face can be suppressed, it has been described as.
CITATION
Patent Document
[0008]
Patent Document 1: JP-A-9-202937
Patent Document 2: JP 2012-107295 Patent Publication
Patent Document 3: JP 2013-231212 Patent Publication
Patent Document 4: JP 2004-315928 JP
Non-patent literature
[0009]
非 特許 文献 1: F.Wever et al, the question of the heat treatment of steels, due to their time-temperature conversion diagrams, steel and iron, 74 (1954), P749 ~ 761st
Summary of the Invention
Problems that the Invention is to Solve
[0010]
　Wheels for railway vehicles proposed in Patent Document 1, at the same time as adequate hardenability, in order to have the property of pearlite structure is obtained, kept low Cr content, and containing an appropriate amount of Si. However, C content of the wheel for a railway vehicle described in Patent Document 1 is 0.4 to 0.75%, this wheel is made of so-called hypoeutectoid steel. Therefore, the improvement of the wear resistance is limited.
[0011]
　In Patent Documents 2 and steel wheels that are proposed in Patent Document 3, by C content contains V to from 0.65 to 0.84% ​​of the steel, to strengthen the pearlite structure, the wear resistance It is enhanced. However, only contain V, there is a limit to the improvement of wear resistance.
[0012]
　On the other hand, the railway vehicle wheel has been proposed in Patent Document 4, the use of over-eutectoid steel with an increased C content, to enhance the abrasion resistance.
[0013]
　Incidentally, an example of a method of manufacturing the rail wheel is as follows. The steel slab for forming the intermediate product of the rail wheel shape by hot working. Respect molded intermediate product, implementing a heat treatment (tread quenching). The tread quenching after heating the intermediate product, quenching the tread and the flange portion of the intermediate product. Thus, the matrix tissue of the surface layer portion of the tread, wear resistance is high fine pearlite is generated. However, the surface layer of the tread after the tread surface hardening, consisting of martensite in the upper layer of fine pearlite (or consists of martensite and bainite) hardening layer is formed. During use of the rail wheel hardened layer is subject to wear. Therefore, after the tread quenching, quenching layer formed on the outermost layer of the tread is removed by cutting, thereby exposing the fine pearlite tread. Through the above steps, the railway wheels are manufactured.
[0014]
　As described above, the wheel for rail vehicles consisting of over-eutectoid steel is excellent in abrasion resistance. However, when producing a rail wheel in the above-described manufacturing method using the over-eutectoid steel, unlike hypoeutectoid steel, the rail wheel, for example, the plate portion or boss portion of the rail wheel, the pro-eutectoid cementite generated it was found that tends to be. Pro-eutectoid cementite decreases the toughness of the steel. Accordingly, the railway wheels consisting of over-eutectoid steel, it is preferable that can suppress the generation of pro-eutectoid cementite.
[0015]
　An object of the present invention, even as high as C content 0.80% or more may suppress the formation of pro-eutectoid cementite, is to provide a railway wheel.
Means for Solving the Problems
[0016]
　Railway wheel according to the present embodiment, by mass%, C: 0.80 ~ 1.15%, Si: 0.45% or less, Mn: 0.10 ~ 0.85%, P: 0.050% or less, S: 0.030% or less, Al: 0.120 ~ 0.650%, N: 0.0030 ~ 0.0200%, Cr: 0 ~ 0.25%, and, V: a 0 to 0.12% containing, having a chemical composition the balance being Fe and impurities. In the microstructure, pro-eutectoid cementite amount defined by formula (1) is 1.50 present / 100 [mu] m or less.
[0017]
　Pro-eutectoid cementite amount (present / 100μm) = 200μm × 200μm /(5.66×100μm sum of the number of pro-eutectoid cementite which intersects the two diagonal lines of the square field of view) (1)
　rail wheel according to the present embodiment, in the microstructure, the average particle size of the aluminum nitride in the railway wheels may be 150nm or less.
The invention's effect
[0018]
　Railway wheel according to the present embodiment, even with a high C content, it is possible to reduce the pro-eutectoid cementite amount.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019]
[1] Figure 1 is a cross-sectional view parallel to the central axis of the rail wheel.
FIG. 2 is obtained by one end quenching test and formastor test Jomini formula illustrates the C content, and the cooling rate, the relationship between the microstructure of the steel.
FIG. 3 is a diagram showing obtained by end quenching test and formastor test Jomini formula, the Si content, and the cooling rate, the relationship between the microstructure of the steel.
FIG. 4 is obtained by one end quenching test and formastor test Jomini formula illustrates the Mn content, and the cooling rate, the relationship between the microstructure of the steel.
FIG. 5 is obtained by one end quenching test and formastor test Jomini formula illustrates the Cr content, and the cooling rate, the relationship between the microstructure of the steel.
FIG. 6 is obtained by one end quenching test and formastor test Jomini formula illustrates the Al content, and the cooling rate, the relationship between the microstructure of the steel.
[7] FIG. 7 is a diagram showing obtained by end quenching test and formastor test Jomini formula, the V content, and the cooling rate, the relationship between the microstructure of the steel.
[8] FIG. 8 is a schematic view for explaining a method of measuring pro-eutectoid cementite amount.
FIG. 9 is after the manufacturing process of the railway wheel of the present embodiment and shows the average particle diameter of AlN, the relationship between the prior austenite grain size.
FIG. 10 is simulating the manufacturing process of the railway wheel, the hot forging simulated heating step (STEP1), is a schematic diagram for explaining a heat pattern of a tread quenching simulation step (STEP2).
[11] FIG. 11 is different from FIG. 10, to simulate the manufacturing process of the railway wheel, the hot forging simulated heating step (STEP1), tread quenching simulation step (STEP3) and schematic for illustrating a heat pattern of it is a diagram.
[12] FIG 12 is a STEP1 later shown in FIG. 10, in front of the test piece STEP2, a hot forging simulating the heating temperature, the relationship between the particle diameter and average particle diameter of each AlN in the steel It illustrates.
[13] FIG 13 is a diagram showing in test piece after STEP2 shown in FIG. 10, and the hot forging simulating the heating temperature, the relationship between the particle diameter and average particle diameter of each AlN in the steel.
[14] FIG 14 is a diagram showing in test piece after STEP3 shown in FIG. 11, and hot forging simulating the heating temperature, the relationship between the prior austenite grain size (formerly γ grain size).
[15] FIG. 15 is different from FIG. 10, to simulate the manufacturing process of the railway wheel, the hot forging simulated heating step (STEP1), tread quenching simulation step (STEP4) and schematic for illustrating a heat pattern of it is a diagram.
FIG. 16 is in the test piece after STEP4 shown in FIG. 15, and hot forging simulated heating temperature, Charpy impact value (J / cm 2 is a diagram showing a relationship between).
FIG. 17 is obtained by Jomini formula end quenching test in Example 1, and the distance from the water-cooling end is a diagram showing the relationship between the Rockwell hardness HRC.
DESCRIPTION OF THE INVENTION
[0020]
　[Configuration rail wheel]
　FIG 1 is a cross-sectional view including the central axis of the railway wheel according to the present embodiment. Referring to FIG. 1, the railway wheel 1 is disc-shaped, provided with a boss portion 2, and a plate portion 3 and the rim portion 4. Boss 2 is cylindrical, in the radial direction of the railway wheel 1 (the direction perpendicular to the central axis), is arranged in the center portion. Boss 2 has a through-hole 21. The central axis of the through-hole 21 is coincident with the central axis of the railway wheel 1. The through hole 21, railway axle (not shown) is inserted. The thickness T2 of the boss portion 2 is thicker than the thickness T3 of the plate portion 3. Rim portion 4 is formed on the edge of the outer periphery of the rail wheel 1. Rim portion 4 includes a tread surface 41, and a flange portion 42. Tread 41 is connected to the flange portion 42. In use of the rail wheel 1, tread 41 and flange 42 in contact with the rail surface. The thickness T4 of the rim portion 4 is greater 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. Inner peripheral edge portion of the plate portion 3 is connected to the boss portion 2, the outer peripheral edge portion of the plate portion 3 is connected to the rim portion 4. The thickness of the plate portion 3 T3 is less than the thickness T4 of the thickness T2 and the rim portion 4 of the boss portion 2.
[0021]
　The present inventors have initially in railway wheels, it was examined for the appropriate chemical composition to enhance the abrasion resistance. As a result, in the railway wheel, even when achieving the same hardness, than increasing the hardness by increasing the V content, who increased hardness by increasing the C content over 0.80% is, it was found that the wear resistance when used as a rail wheel is increased. This mechanism is not clear, it is believed the following matters. Tread rail wheel in use, receives an external force (load) from the rail. The external force cementite in pearlite of the surface layer immediately below the tread surface by is crushed, the hardness is increased by dispersion strengthening. Further, the carbon of the crushed fine in cementite is dissolved in supersaturation in the ferrite in the pearlite, enhancing the hardness of the surface layer immediately below the tread surface by solid solution strengthening.
[0022]
　If Takamere the C content of the steel, the volume fraction of cementite in the pearlite is increased, further, pearlite is likely to form a finer lamellae. In this case, the wear resistance of the rail wheel is increased by the above mechanisms. In contrast, when the content of the V in the steel, increasing the hardness of the steel by precipitation strengthening of V carbonitride. At this time, V carbonitrides to produce in ferrite mainly increase the hardness of ferrite. In other words, the content of V does not affect so much the miniaturization of the pearlite. Therefore, although it is possible to increase a degree abrasion resistance V content, the more enhanced dispersion strengthening and C solid solution by crushing cementite can not be enhanced abrasion resistance.
[0023]
　Accordingly, the present inventors have found that in order to improve the abrasion resistance, the chemical composition of the rail wheel, were considered the C content is preferably set to 0.80 to 1.15% of the over-eutectoid steel.
[0024]
　However, results of studies made by the present inventors, the railway wheels of C content 0.80% or more of the over-eutectoid steel, it was found that pro-eutectoid cementite is likely to generate. Accordingly, the present inventors have in the railway wheels consisting of C content is high over-eutectoid steel, the element content in the chemical composition were investigated the relationship between the pro-eutectoid cementite amount. As a result, we obtained the following findings.
[0025]
　2 to 7 are based on the results of heat treatment test assuming a heat treatment in the manufacturing process of the railway wheel, the content of each element in steel (2: C content, Figure 3: Si content, 4 : Mn content, Figure 5: Cr content, Figure 6: Al content, Figure 7: the V content) is a graph showing the average cooling rate at 800 ~ 500 ° C., the relationship between the pro-eutectoid cementite . Was defined as the value of the average cooling rate of 800 ~ 500 ° C., the precipitation temperature of the pro-eutectoid cementite is for 800 ~ 500 ° C..
[0026]
　Specifically, FIG. 2, Si: 0.29 ~ 0.30%, Mn: 0.79 ~ 0.80%, P: 0.001%, S: 0.002%, Al: 0.032 ~ 0.036%, N: 0.0040 and - 0.0042% and substantially constant content of each element, varying the C content, the balance to a plurality of samples (described later having a chemical composition consisting of Fe and impurities with steel No. 1, 2, 3) of example 1, which was developed on the basis of the results obtained in Jomini formula end quenching test and formastor test described below. 3, C: 1.00 ~ 1.03%, Mn: 0.80 ~ 0.81%, P: 0.001 ~ 0.002%, S: 0.001 ~ 0.002%, Al: 0.031 ~ 0.034%, N: 0.0040 and - 0.0042% and substantially constant content of each element, varying the Si content, the remainder of the plurality having a chemical composition consisting of Fe and impurities sample using (steel number 2,4,5 of example 1 described below), which was developed on the basis of the results obtained in Jomini formula end quenching test and formastor test described below. 4, C: 1.00 ~ 1.04%, Si: 0.29 ~ 0.31%, P: 0.001 ~ 0.002%, S: 0.001 ~ 0.002%, Al: 0.030 ~ 0.034%, N: 0.0040 and - 0.0058% and substantially constant content of each element, varying the Mn content, the remainder of the plurality having a chemical composition consisting of Fe and impurities sample using (steel number 2,7,8 of example 1 described below), which was developed on the basis of the results obtained in Jomini formula end quenching test and formastor test described below. Figure 5 is, C: 1.00 ~ 1.05%, Si: 0.29 ~ 0.30%, Mn: 0.78 ~ 0.80%, P: 0.001%, S: 0.001 ~ 0.002%, Al: 0.033 ~ 0.034%, N: substantially constant the 0.0030 to 0.0040% and the content of each element, varying the Cr content, the balance being Fe and impurities a plurality of samples using (steel number 2, 9, 10 of the first embodiment to be described later), Jomini formula end quenching test and later to formastor having a chemical composition It is those made on the basis of the results obtained in the test. 6, C: 1.00 ~ 1.03%, Si: 0.29 ~ 0.30%, Mn: 0.79 ~ 0.81%, P: 0.001%, S: 0.001 ~ 0.002%, N: 0.0034 and - 0.0046% and substantially constant content of each element, varying the Al content, the balance to a plurality of samples (described later having a chemical composition consisting of Fe and impurities carried using steel No. 2,11,12,13,14,15) of example 1, which was developed on the basis of the results obtained in Jomini formula end quenching test and formastor test described below. 7, C: 1.00 ~ 1.03%, Si: 0.29 ~ 0.30%, Mn: 0.80%, P: 0.001 ~ 0.002%, S: 0.001 ~ 0.002%, N: 0.0040 ~ 0.0048%, Al: 0.026 and - 0.034%, varying the V content, the remainder of the plurality having a chemical composition consisting of Fe and impurities sample ( using steel No. 2,17,18) of example 1 to be described later, it is those made on the basis of the results obtained in Jomini formula end quenching test and formastor test described below.
[0027]
　Jomini formula end quench test was carried out in the following manner. Jomini test piece having a chemical composition described above using (diameter 25 mm, round bar specimens of length 100 mm), it was carried out Jomini Formula end quench test according to JIS G0561 (2011). Specifically, in the air atmosphere Jomini specimen, A cm and held for 30 minutes in an oven at 950 ° C. the temperature of the transformation point or higher, the tissue Jomini specimen was austenite single phase. After that, it was carried out one end quenching (water-cooled). Specifically, it was cooled by injecting water at one end of Jomini specimen.
[0028]
　After water cooling, and mechanically polishing the side surface of Jomini test piece was carried out water-cooling, at regular intervals in the axial direction from one end (water cooling end) was performed microstructure observation. Observation position of the microstructure observation, the water cooled end to 15mm position and 1.0mm pitch and a 2.5mm pitch in 15mm or more positions from the water-cooled end.
[0029]
　Samples were prepared according to the observation plane surface including the microstructure observation position described above. The observation surface of each sample was mirror finished by mechanical polishing. Then corrode the observation surface with a mixture of picric acid with sodium hydroxide. For any one field of view in the observation plane after the corrosion (200μm × 200μm), to produce a photographic image using a 500 × optical microscope. In the observation plane, pro-eutectoid cementite generated in prior austenite grain boundaries exhibiting a black color. As a result, it was able to confirm the presence or absence of the pro-eutectoid cementite. If the pro-eutectoid cementite is confirmed, pro-eutectoid cementite amount by the following method (in this / 100 [mu] m. Hereinafter, referred to as pro-eutectoid θ weight) was determined. As shown in FIG. 8, the field of view of square 200 [mu] m × 200 [mu] m, by subtracting the diagonal of two. Then, the total sum of the number of pro-eutectoid cementite which cross the two diagonal lines. The total number of pro-eutectoid cementite obtained, by dividing by two the total length of the diagonal (5.66 × 100 [mu] m), was determined proeutectoid θ per 100 [mu] m (the present / 100 [mu] m).
[0030]
　Then, the same observation plane again, mirror finished by mechanical polishing, and corroded by nital solution (mixed solution of nitric acid and ethanol). For any one field of view in the observation plane after the corrosion (200μm × 200μm), to produce a photographic image using a 500 × optical microscope. Hardened layer (martensite and / or bainite), and pearlite, the contrast is different. Therefore, based on the contrast, hardened layers in the observation plane, and were identified pearlite. Area ratio of pearlite was determined on the basis of the area of ​​the observation plane and the total area of ​​the identified perlite. If the hardened layer has been confirmed even a little, it is determined that the hardening layer was formed.
[0031]
　Incidentally, the relationship between the cooling time of the distance to the 800 ~ 500 ° C. from water cooled end at Jomini testing, non-patent document 1 shown experimentally (F.Wever et al., Zur Frage der Warmebehandlung der Stahle auf Grund ihrer Zeit-Temperatur-Umwandlungs-Schaubilder, Stahl u Eisen, 74 (1954), p749 ~ 761) is present. Based on this literature, by converting the distance from the water cooled end and the average cooling rate in the 800 ~ 500 ° C. at each position (° C. / sec).
[0032]
　Case the cooling rate is less than 1 ° C. / sec, can not be reproduced by Jomini formula end quenching test. Therefore, for the case the cooling rate is less than 1 ° C. / sec, it was continuously cooled test at low cooling rate (formastor test). The heat treatment was used Fuji radio Koki made of Four master testing machine. More specifically, each specimen soaked for 5 minutes at 950 ° C.. Then cooled at a constant cooling rate 1.0 ° C. / sec (or 0.1 ° C. / sec). On specimens after cooling was performed microstructure observation in the manner described above. When the pro-eutectoid cementite is confirmed, by the method described above, it was determined pro-eutectoid θ amount. Based on the results obtained in the above way, it was created FIGS. 2-7.
[0033]
　"○" mark in FIGS. 2-7 is made of microstructure substantially pearlite, pro-eutectoid θ amount in the microstructure indicates that 1.50 present / 100 [mu] m or less. "×" mark, hardened layer is not formed during the microstructure, although microstructure consists essentially of perlite, means that amount of pro-eutectoid θ exceeds 1.50 present / 100 [mu] m. Incidentally, "microstructure consists essentially of perlite" refers to the area ratio of pearlite in the microstructure means that 95% or more. Further, "●" mark, martensite and / or bainite (hereinafter, also referred to as quenched layer martensite and / or bainite) means that was produced.
[0034]
　The maximum cooling rate which the pro-eutectoid θ amount to produce more than 1.50 present / 100 [mu] m (boundary cooling rate of the "○" mark and "×" mark in the figure), defined as the pro-eutectoid cementite critical cooling rate to. The pro-eutectoid cementite critical cooling rate, indicated by the solid line in FIGS. 2-7.
[0035]
　2, as the C content increases, pro-eutectoid cementite critical cooling rate is increased. Further, FIGS. 3, 4, 5, and in FIG. 7, Si, Mn, Cr and V, effect on pro-eutectoid cementite critical cooling rate than the C and Al are small.
[0036]
　On the other hand, referring to FIG. 6, if the increase of Al content, pro-eutectoid cementite critical cooling rate is significantly reduced, the amount of pro-eutectoid θ is significantly decreased. Thus, for the pro-eutectoid theta content in steel, C is whereas has the effect of increasing the amount of pro-eutectoid theta, Al has an effect of significantly reduced the amount of pro-eutectoid theta.
[0037]
　Based on the above study results, the present inventors found that, in railway wheels over eutectoid steel of the C content of about 0.80% to 1.15% by increasing the Al content, the railway wheel manufacturing process in, in the cooling rate is slow plate portion and the boss portion at the time of heat treatment, it is possible to suppress the pro-eutectoid θ amount, even in railway wheels over eutectoid steel, it was considered excellent toughness can be obtained. Then, and based on the examination results, the result of examining the chemical composition of railway wheels over eutectoid steel, the chemical composition of the rail wheel, in mass%, C: 0.80 ~ 1.15%, Si: 0 .45% or less, Mn: 0.10 ~ 0.85%, P: 0.050% or less, S: 0.030% or less, Al: 0.120 ~ 0.650%, N: 0.0030 ~ 0 .0200%, Cr: 0 ~ 0.25%, and, V: 0 contains ~ 0.12%, if the chemical composition the balance being Fe and impurities, the slower the plate portion and the boss portion of the cooling rate It was also found for the first time that it is possible to suppress the pro-eutectoid θ amount 1.50 present / 100 [mu] m or less.
[0038]
　Incidentally, with reference to FIG. 6, if the increase of Al content at the time of tread quenching, quenching layer (in the figure "●" mark) is easily generated. In this case, the yield is likely to decrease in the manufacturing process. Therefore, from the viewpoint of suppressing the generation of the hardened layers, in the chemical composition described above, a preferred upper limit of Al content is 0.350%. However, in order to increase the toughness by suppressing the precipitation of pro-eutectoid cementite, Al content may be content exceeds 0.350%, it may be contained up to 0.650%.
[0039]
　[For grain refining prior austenite]
　As described above, in the railway wheel C content is from over eutectoid steel of 0.80 to 1.15%, as a technique for suppressing the amount of pro-eutectoid cementite, Al content increasing the the 0.120 to 0.650% is effective. However, as a result of further investigation by the present inventors, when having an increased Al content, in railway wheels, that coarse AlN is sometimes generated was found. Coarse AlN is upon heating in the tread surface hardening, it does not contribute to the grain growth inhibition of the prior austenite grains. Prior austenite grains finer particles, toughness is further enhanced. Therefore, when considering the further improvement of the toughness, the prior austenite grains it is preferable fine.
[0040]
　The present inventors have found that in a railway wheel comprising a high over-eutectoid steel of Al content was investigated further refinement of the prior austenite grains. Fine result, as shown in FIG. 9, the railway wheels consisting of over-eutectoid steel having the chemical composition described above, the average particle diameter of the AlN in the microstructure if 150nm or less, the former austenite grain size 50μm or less It was found to be able of. This point will be described below.
[0041]
　Test piece having a chemical composition shown in Table 1 of the steel 14 to be described later were more prepared (diameter 40 mm, disc thickness 20 mm). Referring to FIG. 10, with respect to the test piece, to simulate the manufacturing process of the railway wheel, it was carried out hot forging simulated heating step (STEP1). At this time, in each specimen, each holding temperature in the furnace, 1150 ℃ (HP1), 1200 ℃ (HP2), 1250 ℃ (HP3), and a 1300 ℃ (HP4). Furnace atmosphere is an argon gas atmosphere, the retention time was 60 minutes none of the specimens. After the holding time, and allowed to cool.
[0042]
　After hot forging simulated heating step (STEP1), it was performed tread quenching simulation step (STEP2). The tread quenching simulated process, for any of the test piece, a furnace temperature of 950 ° C., and held 20 min. After holding time, it was carried out water quenching (WQ).
[0043]
　STEP1 or after the, STEP2 previous specimen, and, for each post STEP2 specimens, to obtain an average particle diameter of AlN (nm) by the method described below.
[0044]
　To measure the prior austenite grain size was carried out the following test. In the chemical composition, austenite grain boundary after tread quenching is difficult to determine. Therefore, in the cooling of the tread quenching simulation step, to precipitate a pro-eutectoid cementite in the old austenite grain boundaries, it devised so as to measure the austenite grain size. Specifically, with respect to the above-mentioned test piece, and hot forging simulating the heating step shown in FIG. 11 (STEP1), it was carried out and tread quenching simulation step (STEP3). Hot forging simulating the heating step of FIG. 11 (STEP1) was the same as hot forging simulating the heating step of FIG. 10 (STEP1). Tread quenching simulation process of FIG. 11 (STEP3), compared with the tread quenching simulation process of FIG. 10 (STEP2), differed only cooling method. Specifically, in the tread quenching simulation step (STEP3) of FIG. 11, the test piece after 20 minutes holding at 950 ° C., and immersed in a salt bath of 675 ° C., for 30 minutes isothermally held at 675 ° C.. Thus, to precipitate a pro-eutectoid cementite at grain boundaries of prior austenite, the austenite grains was identifiable state microstructure observation. The test piece after the isothermal holding was allowed to cool to room temperature (25 ° C.). Prior austenite grain size of more STEP1 and heat treated went specimens STEP3 was determined by a measurement method described later.
[0045]
　Figure 12 is a hot forging simulated heating step (STEP1) there later in tread quenching simulation step (STEP2) prior to testing specimens, the average particle diameter of AlN for each hot forging simulated heating temperature (HP1 ~ HP4) It illustrates. 13, in the test piece after tread quenching simulation step (STEP2), is a diagram showing an average particle diameter of AlN for each hot forging simulated heating temperature (HP1 ~ HP4). 14, in the test piece after tread quenching simulation step (STEP3), is a diagram illustrating a prior austenite grain size for each hot forging simulated heating temperature (HP1 ~ HP4).
[0046]
　"○" mark in FIG. 12 and FIG. 13 in shows the measured values ​​of the particle diameter of AlN has been identified in the observation field on the HP1 ~ HP4, bar graph of each HP is AlN measured in each HP It means the average particle diameter. Referring to FIGS. 12 and 13, when the hot forging simulating the heating temperature is 1200 ° C. or less, even after hot forging step, the tread quenching process before (Fig. 12), AlN is not completely dissolved, many of AlN remained. Therefore, after heating in the tread quenching process (FIG. 13), coarse AlN has a large number remaining, the average particle size also remained 150nm greater and coarse.
[0047]
　In contrast, hot forging simulated heating temperature exceeds the 1200 ° C. (HP3 and HP4), AlN residual amount after hot forging step (FIG. 12) is lowered, if it exceeds 1250 ° C., all AlN solid It was dissolved. Then, (FIG. 13) after heating in tread quenching process, fine AlN is in many precipitates, the average particle diameter became 150nm or less. Then, with reference to FIGS. 13 and 14, in accordance with an average particle size of AlN becomes finer, the former austenite grain size was also reduced.
[0048]
　Summarizes the 13 and 14, i.e., shows the average particle diameter of AlN after tread quenching simulation process, the relationship between the prior austenite grain size of 9. Referring to FIG. 9, in accordance with an average particle diameter of the AlN in the steel is reduced, prior austenite grain size was significantly decreased. When the average particle diameter of AlN becomes 150nm or less, the degree of decrease prior austenite grain size due to the lower average particle diameter of AlN is reduced. That is, prior austenite grain size, the average particle diameter of the AlN had an inflection point at 150nm vicinity. The average particle diameter of AlN is equal to or 150nm or less, the former austenite grain size is less sufficiently miniaturized 50 [mu] m.
[0049]
　The present inventors based on the above findings, the improvement of the toughness of the rail wheel due to miniaturization of miniaturization and prior austenite grains AlN, was confirmed by the following tests.
[0050]
　More specifically, as in the test of FIG. 12 and FIG. 13, a plurality prepared Charpy test pieces having a chemical composition of the steel 14 of the Examples below (10mm × 10mm × 55mm). For this specimen, the hot forging simulating the heating step shown in FIG. 15 (STEP1), were carried out and tread quenching simulation step (STEP4). Hot forging simulating the heating step of FIG. 15 (STEP1) was the same as hot forging simulating the heating step of FIG. 10 and FIG. 11 (STEP1). Tread quenching simulation process of FIG. 15 (STEP4), assuming that the rim portion in the tread surface hardening of actual rail wheel cools the specimen at a cooling rate at which the cooled and set. Specifically, in the tread quenching simulation step (STEP4) of FIG. 15, the test piece after 20 minutes holding at 950 ° C., and immersed in a salt bath of 400 ° C., and 10 minutes kept isothermally at 400 ° C.. It was then allowed to cool test piece after isothermal holding to room temperature (25 ° C.). Using the above STEP1 and heat treatment the Charpy test piece was conducted in STEP4, and the Charpy impact test according to JIS Z 2242 (2005) was performed at room temperature (25 ° C.).
[0051]
　Figure 16 is a diagram showing the Charpy impact test results. The horizontal axis represents the respective hot forged simulated heating temperature (HP1 ~ HP4), the vertical axis represents the Charpy impact value (J / cm 2 shows a). Bar graph in the figure, the Charpy impact value obtained in the Charpy test piece of a plurality (1-4) in each ~ HP4 HP1 (J / cm 2 shows the average).
[0052]
　Referring to FIG. 16, with the rise of the hot forging simulating the heating temperature, the Charpy impact value increases rapidly. When the hot forging simulated heating temperature exceeds 1200 ° C., as compared to the case of 1200 ° C. or less, increase cost of Charpy impact values ​​with increasing hot forging simulated heating temperature is reduced. In other words, if AlN average particle size and 150nm or less, it is possible to further enhance the toughness of the rail wheel with the chemical composition.
[0053]
　As described above, when the average particle diameter of AlN is 150nm or less, the former austenite grains is sufficiently miniaturized, as a result, is considered to further increase the toughness of the rail wheel. However, even beyond the average particle diameter of 150nm of AlN, as described above, if the pro-eutectoid cementite amount 1.50 present / 100 [mu] m or less, effective toughness is obtained in the railway wheel.
[0054]
　To the average particle diameter of AlN to 150nm or less, for example, the heating temperature in the hot forging to 1220 ° C. or higher. In this case, at the time of hot forging, much of the AlN in the steel forms a solid solution. Then, at the time of heating in the tread quenching process, the average particle diameter is less fine AlN 150 nm deposited. The pinning effect of fine AlN, the prior austenite grain coarsening is suppressed, the old austenite grains become fine. As a result, it is considered that further increases the toughness of the railway wheel.
[0055]
　Railway wheel according to the present embodiment has been completed based on the above findings, by mass%, C: 0.80 ~ 1.15%, Si: 0.45% or less, Mn: 0.10 ~ 0.85%, P: 0.050% or less, S: 0.030% or less, Al: 0.120 ~ 0.650%, N: 0.0030 ~ 0.0200%, Cr: 0 ~ 0.25%, and, V : 0 contains ~ 0.12%, with a chemical composition the balance being Fe and impurities. In the microstructure of the rail wheel, pro-eutectoid cementite amount defined by formula (1) is 1.50 present / 100 [mu] m or less.
[0056]
　Pro-eutectoid cementite amount (present / 100μm) = /(5.66×100μm sum of 200 [mu] m × 200 [mu] m the number of pro-eutectoid cementite which intersects the two diagonal lines of the square field of view) (1)
　rail wheel of this embodiment, in the microstructure, the average particle diameter of AlN may also be 150nm or less.
[0057]
　In this case, the fine AlN, austenite grains are refined. As a result, further increases the toughness of the railway wheel.
[0058]
　Chemical composition of the train wheels, Al: 0.120 may contain ~ 0.350%. The chemical composition of the train wheels, Cr: 0.02 ~ 0.25%, and, V: 0.02 ~ 0.12%, may contain one or more selected from the group consisting of .
[0059]
　It described in detail below rail wheel of this embodiment. In this specification, "%" related to elements, unless otherwise specified, means mass%.
[0060]
　[Chemical composition of the railway wheel]
　chemical composition of railroad wheel of the present embodiment contains the following elements.
[0061]
　C: 0.80 ~ 1.15%
　carbon (C) increases the hardness of the steel, improve the wear resistance of the rail wheel. If the C content is too low, the effect can not be obtained. On the other hand, if the C content is too high, pro-eutectoid cementite many precipitated in prior austenite grain boundaries. In this case, the toughness of the railway wheel is reduced. Therefore, C content is from 0.80 to 1.15%. The preferable lower limit of C content is 0.90%, more preferably 0.95%. The preferable upper limit of C content is 1.10%, more preferably 1.05%.
[0062]
　Si: 0.45% or less
　silicon (Si) is inevitably contained. That, Si content is over 0%. Si is a ferrite solid solution strengthening to increase the hardness of steel. However, if Si content is too high, pro-eutectoid cementite which causes a decrease toughness of the steel is likely to generate. Further if the Si content is too high, too high hardenability of the steel, martensite is easily generated. In this case, the thickness of the hardened layer formed on the tread surface when the tread surface hardening is increased. As a result, hardened layer is removed by cutting. Thus, quenching layer is thick, the amount of cutting is yield decreases increases. Further if the Si content is too high, during use of the railway wheel, is baked enters the frictional heat generated between the brake. In this case, there is a case where the resistance crack resistance of railway wheels decreases. Therefore, Si content is 0.45% or less. The preferable upper limit of the Si content is 0.35%, more preferably 0.25%. The lower limit of Si content is not particularly limited, for example, 0.05%.
[0063]
　Mn: 0.10 ~ 0.85%
　manganese (Mn) of increasing the hardness of the steel by solid solution strengthening of the ferrite. Mn is further configured to form a MnS, to improve the machinability of the steel. If Mn content is too low, these effects can not be obtained. On the other hand, if the Mn content is too high, hardenability of steel becomes too high. In this case, the thickness of the hardened layer is increased, the yield during the manufacturing process is reduced. Furthermore, when using the rail wheel, it contains the baked by frictional heat generated between the brake and may withstand crack resistance of the steel is lowered. Therefore, Mn content is 0.10 to 0.85%. The preferable lower limit of the Mn content is 0.50%, more preferably 0.70%. The preferable upper limit of the Mn content is 0.82%.
[0064]
　P: 0.050% or less
　phosphorus (P) is an impurity contained in the unavoidable. That, P content is over 0%. P lowers the toughness of the steel is segregated at the grain boundaries. Accordingly, P content is 0.050% or less. The preferable upper limit of the P content is 0.030%, more preferably 0.020%. P content is preferably as small as possible. However, if an attempt excessively reduced P content, refining cost becomes excessively high. Therefore, when considering the usual industrial production, preferable lower limit of the P content is 0.0001%, more preferably from 0.0005%.
[0065]
　S: 0.030% or less
　Sulfur (S) is contained inevitably. That, S content is over 0%. S forms MnS, enhances the machinability of the steel. On the other hand, if the S content is too high, toughness of the steel is lowered. Therefore the S content is 0.030% or less. The preferable upper limit of the S content is 0.020%. The preferable lower limit of the S content for improving the machinability is 0.005%.
[0066]
　Al: 0.120 ~ 0.650%
　of aluminum (Al) inhibits generation of pro-eutectoid cementite which causes a decrease toughness of the steel. Al further, AlN was formed by combining the N, refining the old austenite grains. Prior austenite grains further be miniaturized, it increases the toughness of the steel. If the Al content is too low not these effects can not be obtained. However, if the Al content is too high, AlN is coarsened, not a solid solution at the heating temperature during hot working. Therefore, the prior austenite grains of grain growth effects upon heating during tread quenching is not exhibited, toughness rail wheel is lowered. Therefore, Al content is from 0.120 to 0.650%. A preferable lower limit of Al content is 0.150%, more preferably 0.250%. The preferable upper limit of Al content is 0.630%, more preferably 0.600%, still more preferably 0.550%, still more preferably 0.500%. In consideration of reduction in cutting of hardened layer after tread quenching, preferable upper limit of Al content is 0.350%, more preferably 0.320%, still more preferably from 0.300% . Al content referred herein means the content of acid-soluble Al (sol. Al).
[0067]
　N: 0.0030 ~ 0.0200%
　nitrogen (N) combines with Al to form AlN, refining the old austenite grains. By prior austenite grains finer, it increases the toughness of the steel. This effect can not be obtained if the N content is too low. On the other hand, if the N content is too high, its effect is saturated. Therefore, N content is 0.0030 to 0.0200%. The preferable lower limit of the N content is 0.0035%, more preferably 0.0040%. The preferable upper limit of the N content is 0.0100%, more preferably 0.0080%.
[0068]
　The remainder of the chemical composition of the rail wheel according to the present embodiment is composed of Fe and impurities. Here, the impurity, when the industrial production of the rail wheel, the ore as a raw material, there is to be mixed etc. Scrap or manufacturing environment, does not adversely affect the railway wheels of the embodiment means what is allowed in the range.
[0069]
　The chemical composition of the rail wheel of this embodiment further, in place of part of Fe, and may contain Cr.
[0070]
　Cr: 0 ~ 0.25%
　chromium (Cr) is an optional element and may not be contained. That, Cr content may be 0%. If contained, Cr decreases the lamellar spacing of pearlite. Thus, the hardness of the pearlite is increased remarkably. However, if the Cr content is too high, the hardenability is increased, the thickness of the hardened layer after tread quenching increases excessively. Therefore, Cr content is 0 to 0.25%. The preferable upper limit of the Cr content is 0.22%. A preferable lower limit of the Cr content is over 0%. A preferable lower limit of the Cr content for obtaining the effect of the lamellar spacing reduction of pearlite more effectively is 0.02%.
[0071]
　The chemical composition of the rail wheel of this embodiment further, in place of part of Fe, and may contain V.
[0072]
　V: 0 ~ 0.12%
　vanadium (V) are optional elements may not be contained. That, V content may be 0%. If contained, V is a carbide, to form one of the nitrides, and carbonitrides, to precipitation strengthening steel. As a result, the hardness of the rail wheel is significantly increased, further increase the abrasion resistance. However, if the V content is too high, the hardenability is increased, the thickness of the hardened layer after tread quenching increases excessively. Therefore, V content is from 0 to 0.12%. The preferable upper limit of the V content is 0.10%. The preferable lower limit of V content is 0 percent, more preferably 0.02%, more preferably from 0.03%.
[0073]
　[For pro-eutectoid cementite amount]
　Railway wheels according to the present embodiment, in the microstructure, pro-eutectoid cementite amount defined by formula (1) (the pro-eutectoid θ weight) 1.50 present / 100 [mu] m or less.
[0074]
　Eutectoid θ amount (present / 100μm) = /(5.66×100μm sum of 200 [mu] m × 200 [mu] m the number of pro-eutectoid cementite which intersects the two diagonal lines of the square field of view) (1)
　More specifically, the present embodiment in the rim portion of the rail wheel form, and 1.50 present / 100 [mu] m or less pro-eutectoid θ quantity defined in equation (1), the plate portion, pro-eutectoid cementite amount defined by formula (1) is 1 and at .50 present / 100 [mu] m or less, in the boss portion, pro-eutectoid cementite amount defined by formula (1) is 1.50 present / 100 [mu] m or less.
[0075]
　As described above, the more the amount of pro-eutectoid theta, toughness rail wheel is lowered. Railway wheel according to the present embodiment, contains from 0.120 to 0.650% of Al. If the chemical composition of the over-eutectoid steel, by containing 0.120 to 0.650% of Al, the rail wheel after tread quenching process in the manufacturing process, reduces the amount of pro-eutectoid theta, 1. it can be suppressed to 50 lines / 100 [mu] m or less. The preferable upper limit of the pro-eutectoid θ weight was 1.20 present / 100 [mu] m, more preferably from 1.00 present / 100 [mu] m.
[0076]
　Eutectoid θ amount is measured by the following method. Central position in the thickness direction of the rim portion of the rail wheel, the thickness direction of the center of the plate portion, and respectively take samples from the thickness direction of the center position of the boss. The observation surface of the sample is mirror finished by mechanical polishing. Thereafter, corrode observation surface with a mixture of picric acid with sodium hydroxide. For any one field of view in the observation plane after the corrosion (200μm × 200μm), to produce a photographic image using a 500 × optical microscope. In the observation plane, pro-eutectoid cementite generated in prior austenite grain boundaries exhibiting a black color. Therefore, it is possible to confirm the presence or absence of pro-eutectoid cementite. As shown in FIG. 8, a square field of view 100 of the 200 [mu] m × 200 [mu] m, catching diagonal 101 of two. Then, a sum of the number of pro-eutectoid cementite which intersects the diagonal 101 of these two. The total number of pro-eutectoid cementite obtained, by dividing by two the total length of the diagonal line 101 (5.66 × 100 [mu] m) (i.e., based on the equation (1)), the pro-eutectoid θ amount (present / 100 [mu] m) the seek. The rim center position, the plate portion center position, and the pro-eutectoid θ amount determined by the boss center, both suppressed to 1.50 present / 100 [mu] m or less.
[0077]
　Average particle size of the AlN]
　Preferably, the microstructure of the rail wheel according to the present embodiment, the average particle diameter of AlN is 150nm or less. As shown in FIG. 9, the average particle diameter of AlN is equal to or 150nm or less, the former austenite grains are finer. Therefore, further increases the toughness of the railway wheel. Preferred upper limit of the average particle diameter of AlN is 120 nm, more preferably from 100 nm. Although the lower limit of the average particle diameter of AlN is not particularly limited, for example, 10 nm.
[0078]
　The average particle diameter of AlN is obtained by observing the carbon extraction replica sample with TEM-EDS. Specifically, it determined by the following method. Central position in the thickness direction of the rim portion of the rail wheel, the thickness direction of the center of the plate portion, and respectively take samples from the thickness direction of the center position of the boss. Polishing the observation surface of the sample taken. The observation surface after polishing corroded by nital solution. Depositing carbon on the observation surface after corrosion. The sample after the carbon deposition, the precipitate only the base material was immersed in a stripping solution in eluting without melting, separating the replica film from the base material. Of replica film, the precipitate was confirmed field (6.5 [mu] m × 9.0 .mu.m), and generates an image (field the same size) in TEM. Further, to identify the precipitates in photographic images by EDS and electron beam diffraction image analysis. Of precipitates exhibit characteristic rectangular or hexagonal form, in the spectrum of the X-ray intensity obtained by EDS measurements, Al element characteristic X-ray energy (Al K [alpha line: 1.49keV) peak around the the appeared precipitate, identified as AlN. Using photographic images, determine the area of ​​each AlN identified, obtaining the circle-equivalent diameter (nm) from the area determined. The average value of equivalent circle diameter determined, defined as the average particle diameter of AlN (nm).
[0079]
　The railway wheel according to the present embodiment, the average particle diameter of 150nm or less of AlN, the average particle diameter of the AlN in the rim portion of the rail wheel is not less 150nm or less, an average particle diameter of the AlN in the plate portion is not more 150nm or less, the average particle diameter of the AlN in the boss means that at 150nm or less.
[0080]
　[Microstructure rail wheel]
　rim portion of the rail wheel in this embodiment, the plate portion, and the boss portion of the microstructure consists essentially pearlite. Here, "substantially made of perlite" is the area ratio of pearlite in the microstructure means that 95% or more.
[0081]
　Area ratio of pearlite is obtained by the following method. Central position in the thickness direction of the rim portion of the rail wheel, the thickness direction of the center of the plate portion, and respectively take samples from the thickness direction of the center position of the boss. The observation surface of the sample is mirror finished by mechanical polishing. Thereafter, corrode observation surface by nital solution (mixed solution of nitric acid and ethanol). For any one field of view in the observation plane after the corrosion (200μm × 200μm), to produce a photographic image using a 500 × optical microscope. Hardened layer (martensite and / or bainite), and pearlite, the contrast is different. Therefore, based on the contrast, hardened layers in the observation plane, and, identifies the perlite. Area ratio of pearlite is determined based on the area of ​​the observation plane and the total area of ​​the identified perlite.
[0082]
　[Method rail wheel]
　An example of a method of manufacturing a rail wheel described above. This manufacturing method includes a process for manufacturing a steel rail wheel (Material manufacturing process), by hot working, a step of forming the intermediate product of the wheel shape from the train wheel steel (molding step), molded intermediate product including heat treatment step of carrying out (tread quenching) (heat treatment step), a step of a rail wheel removed by cutting the hardened layer from the tread surface of the intermediate product or the like after the heat treatment (cutting step) to the. Hereinafter, the respective steps will be described.
[0083]
　[Material manufacturing process]
　The material production process, after the smelted molten steel having the above chemical composition in an electric furnace or a converter, etc., cast to be a steel ingot. Incidentally, the steel ingot may be any of the cast slab by continuous casting, by mold ingot.
[0084]
　The slab or ingot by hot working to produce a steel for railway wheels of the desired size. Hot working example, hot forging, a hot rolling and the like. With the above-described manufacturing steps, the steel for railway wheels is produced.
[0085]
　Incidentally, steel for railway wheels may be cast material (cast slab or ingot). In other words, hot working steps described above may be omitted. Through the above process, steel for railway wheels which is a material of the rail wheel is manufactured.
[0086]
　Molding Step]
　In the molding process, using a prepared railroad wheel steel material, molding the intermediate product of the wheel shape by hot working. For intermediate products having wheels shape, it comprises a boss portion, and a plate portion and a rim portion including a tread and a flange portion. Hot working example, hot forging, a hot rolling and the like.
[0087]
　Preferred heating temperature of the steel for railway wheels during hot working is 1220 ° C. or higher. In this case, in the heating step during hot working, AlN in the steel for railway wheels is sufficiently dissolved. Therefore, when heated in the heat treatment step follows step (tread quenching), the average particle diameter is less fine AlN 150 nm deposited. Thus, as shown in FIG. 9, prior austenite grain size is refined to 50μm or less, further enhanced toughness rail wheel.
[0088]
　A preferred lower limit of the heating temperature during the hot working is 1230 ° C., more preferably from 1250 ° C., more preferably from 1300 ° C.. Preferred upper limit of the heating temperature of the hot working is 1350 ° C..
[0089]
　Incidentally, the intermediate product cooling method after hot working is not particularly limited. Also it may in cool, may be water-cooled.
[0090]
　[Heat treatment step]
　In the heat treatment step, implementing the tread quenching the intermediate products of the molded wheel shape. Specifically, hot working (hot forging or hot rolling) after the intermediate product of A cm reheating above transformation point (reheating). After heating, quenching the tread and the flange portion of the intermediate product (tread quenching). For example, to cool the tread and the flange portion by the cooling medium. Cooling medium, for example, air, a mist, brackish (spray) is not particularly limited as long as the cooling rate that matches the desired tissue is obtained. At the time of tread quenching, the plate portion and the boss portion is cooled without water cooling. In the present embodiment, by setting the Al content in the chemical composition of the rail wheel and from 0.120 to 0.650%, even if the plate portion and the boss portion was allowed to cool as in the previous manufacturing method at the time of tread quenching, can sufficiently suppress generation of pro-eutectoid cementite, specifically, the rim portion, the plate portion, the boss portion, it is possible to suppress the pro-eutectoid θ amount 1.50 present / 100 [mu] m or less.
[0091]
　The diameter of the rail wheel in this embodiment for example, a 700 mm ~ 1000 mm. Also preferred cooling rate of tread at the tread surface hardening is 5 ~ 200 ° C. / sec. Further, the rim portion of the intermediate product, in the region of the plate portion and the boss portion, preferably the cooling rate of the most slow cooling rate region at the time of tread quenching is about 0.1 ° C. / sec. In this case, the railway wheels having the above chemical composition prepared, the amount of pro-eutectoid θ is 1.50 present / 100 [mu] m or less. Slowest area cooling rate of the intermediate products, for example, by measuring with a plurality of thermographic temperature distribution change of the intermediate product in the tread cooling, it can be obtained.
[0092]
　The tread quenching, fine pearlite generates in the surface layer of the tread. C content of the railway wheel of this embodiment from 0.80 to 1.15% and higher. Therefore, it increases the wear resistance of the fine pearlite. Further, Al content of the railway wheel of this embodiment from 0.120 to 0.650% and high. Therefore, when the tread surface hardening, the generation of eutectoid cementite which causes a decrease toughness of the steel is suppressed.
[0093]
　Although reheating the intermediate product in the above description, (without reheating) directly to the intermediate product after hot working may be carried out tread quenching.
[0094]
　In the above description, at the time of tread quenching, but was allowed to cool the plate portion and the boss portion, if allowed to cool, the surface of the plate portion and the boss portion hardly generated hardened layer. On the other hand, when the tread surface quenching, may be cooled plate portion and the boss portion in cooling or cooling rate. In this case, it is preferable to cool at a degree of cooling rate is not hardened layer is formed on the surface of the plate portion and the boss portion.
[0095]
　Preferably, after heating and hot working an intermediate article above 1220 ° C., once cooling the intermediate product. Then, the cooled intermediate product A cm reheated to above the transformation point, carrying out the tread quenching. In this case, coarse AlN in the steel is dissolved by heating at the time of hot working, then precipitated as fine AlN during reheating. Therefore, as described above, the prior austenite grains can refinement.
[0096]
　The intermediate product after tread quenching, performing the tempering as necessary. Tempering suffices be performed at a known temperature and time.
[0097]
　[Cutting Step]
　As described above, although the surface layer of the tread of the intermediate product after the heat treatment fine pearlite is formed, hardened layer is formed thereon. In use of the rail wheel, since the low abrasion resistance of the hardened layer is removed hardened layer by cutting. Cutting the reporting may be made in a known manner.
[0098]
　Railway wheel according to the present embodiment is manufactured by the above steps. The railway wheels manufactured by the above manufacturing process, the pro-eutectoid θ weight is 1.50 present / 100 [mu] m or less. Therefore, it is believed that the toughness of the railway wheel is increased. Furthermore, when the heating temperature in the hot working step was 1220 ° C. or more, an average particle diameter of the AlN in the steel becomes 150nm or less, the former austenite grain size is 50μm or less. In this case, further increases the toughness of the railway wheel.
Example 1
[0099]
　Chemical compositions shown in Table 1 were produced molten steel of the steel Nos. 1 to 20 having a.
[0100]
[Table 1]

　
[0101]
　It was produced round ingots (top diameter 107mm, base diameter 97 mm, the truncated cone-shaped height 230 mm) by ingot using the above-molten steel. To simulate the hot working process of the railway wheel manufacturing process, after heating the ingot to 1250 ° C., and hot forging to produce a round bar with a diameter of 40 mm.
[0102]
　[Simulated tread quenching Test
　was performed a simulated tread quenching test simulating the tread quenching in the manufacturing process of the railway wheel, were investigated pro-eutectoid θ amount after simulated tread quenching test.
[0103]
　[Eutectoid θ amount measurement test]
　D / 4 position of the round bar of steel No. 1 and steel No. 15 ( "D", the diameter of the round bar) from to produce a heat-treated test piece of diameter 3 mm, length 10 mm. Longitudinal heat treatment specimen was consistent with the direction of the central axis of the round bar.
[0104]
　It was continuously cooling test using the produced heat-treated specimens. The heat treatment was used Fuji radio Koki made of Four master testing machine. Specifically, to prepare a test piece of each steel number by two, soaking for 5 minutes at 950 ° C.. Then, one of the specimens were cooled at a constant cooling rate 1.0 ° C. / sec. Another one of the specimens were cooled at a constant cooling rate 0.1 ° C. / sec. For each test piece after cooling, it was determined eutectoid θ amount in the following manner.
[0105]
　Samples were prepared for the observation plane cross section perpendicular to the longitudinal direction of the heat treatment the test piece. In observation surface was measured pro-eutectoid θ amount by the following method. After the observation surface was mechanically polished, corroded with observation surface with a mixture of picric acid with sodium hydroxide. For any one field of view in the observation plane after the corrosion (200μm × 200μm), to produce a photographic image using a 500 × optical microscope. Based on the contrast, it was confirmed eutectoid cementite in the observation field. If the pro-eutectoid cementite is observed, by the method described above, it was calculated pro-eutectoid θ amount.
[0106]
　[Hardened layer depth measurement test]
　Further, the depth of the hardened layer, was carried out Jomini Formula end quenching test. Jomini formula end quench test was carried out in the following manner. A round bar having a diameter of 40mm each steel numbers, were prepared Jomini specimen diameter 25 mm, length 100 mm. The central axis of Jomini specimen was consistent with the central axis of the round bar. Using Jomini specimens was performed Jomini Formula end quench test according to JIS G0561 (2011). Specifically, in the air atmosphere Jomini specimen, A cm and held for 30 minutes in an oven at 950 ° C. the temperature of the transformation point or higher, the tissue Jomini specimen was austenite single phase. After that, it was carried out one end quenching (water-cooled). Specifically, it was cooled by injecting water at one end of Jomini specimen.
[0107]
　After water cooling, Rockwell hardness and mechanically polishing the side surface of Jomini test piece was carried out water-cooling, at regular intervals in the axial direction from one end (water cooling end), using the C scale conforming to JIS Z 2245 (2011) ( HRC) test was performed to obtain a HRC distribution. Measurement interval of HRC, the water cooled end to 15mm position and 1.0mm pitch and a 2.5mm pitch in 15mm or more positions from the water-cooled end. From the resulting HRC distribution was determined hardened layer thickness by the following method.
[0108]
　Figure 17 is a diagram illustrating HRC distribution of steel Nos. 1 to 3 (Jomini curve). 17, in Jomini curve, as the distance D from the water-cooled end leaves, Rockwell hardness HRC rapidly decrease. Then, if D is a predetermined distance or more, even apart distance from the water-cooling end, Rockwell hardness HRC is not significantly reduced. The area A Rockwell hardness HRC is rapidly lowered is defined as "hardened layers", the region B Rockwell hardness HRC is not significantly decrease is defined as "base material". The regions A and B can be classified through the inflection point. HRC distribution of each steel number (Jomini curve) to identify the area A from was obtained hardened layer thickness a (mm).
[0109]
　In the method described in FIGS. 2-7 above, relative Jomini specimen after Jomini formula end quenching test of each steel numbers, to implement a microstructure observation test, in the region where the hardened layers does not produce to determine the pearlite area ratio in the microstructure. Specifically, of the Jomini specimens of each steel numbers, the portion corresponding to the region B in FIG. 17, samples were taken. The observation surface of each sample was mirror finished by mechanical polishing. Then it corrodes the observation surface with nital solution (mixed solution of nitric acid and ethanol). For any one field of view in the observation plane after the corrosion (200μm × 200μm), to produce a photographic image using a 500 × optical microscope. Based on the contrast were identified pearlite in the observation plane. Area ratio of pearlite was determined on the basis of the area of ​​the observation plane and the total area of ​​the identified perlite.
[0110]
　[Test Results]
　Table 1 shows the test results. Referring to Table 1, in any of the steel numbers, microstructure in the region other than the hardened layer was tissue consisting essentially pearlite. In other words, pearlite area ratio was 95% or more.
[0111]
　Furthermore, the chemical composition of the steel Nos. 13 to 16 and 19 were suitable. Therefore, the cooling rate is 0.1 ° C. / sec, in any of 1.0 ° C. / sec, the amount of pro-eutectoid θ was 1.50 present / 100 [mu] m or less. Therefore, it was expected that superior toughness can be obtained. In steel No. 13, 14, 16, and 19, Al content was less than 0.350%. Therefore, while the Al content is hardened layer thickness of steel No. 15 was 0.610% were 17.5 mm, the hardened layer thickness of steel No. 13, 14, 16 and 19 11.0mm was following and thinner.
[0112]
　On the other hand, the steels Nos. 1 to 12, 17 and 18, Al content was too low. As a result, the amount of pro-eutectoid θ exceeds 1.50 present / 100 [mu] m.
[0113]
　In steel No. 10, Si content was too high. As a result, the amount of pro-eutectoid θ exceeds 1.50 present / 100 [mu] m.
Example 2
[0114]
　Using a molten steel having the chemical composition of the steel No. 14 in Table 1, were produced round ingots (top diameter 107mm, base diameter 97 mm, the truncated cone-shaped height 230 mm) by an ingot-making method. After heating the ingot to 1250 ° C., and hot forging to produce a plurality of round bars with a diameter of 40 mm. And round bars produced with the test piece.
[0115]
　Referring to FIG. 10, with respect to the test piece, to simulate the manufacturing process of the railway wheel, it was carried out hot forging simulated heating step (STEP1). At this time, in each specimen, each holding temperature in the furnace, 1150 ℃ (HP1), 1200 ℃ (HP2), 1250 ℃ (HP3), and a 1300 ℃ (HP4). Furnace atmosphere is an argon gas atmosphere, the retention time was 60 minutes none of the specimens. After the holding time, and allowed to cool.
[0116]
　After hot forging simulating the heating process was carried out tread quenching simulation step (STEP2). The tread quenching simulated process, for any of the test piece, a furnace temperature of 950 ° C., and held 20 min. After holding time, it was carried out water quenching (WQ).
[0117]
　[AlN average particle size measurement
　test] There STEP1 later, STEP2 previous specimen, and, for each post STEP2 specimens, to obtain an average particle diameter of AlN. Specifically, samples were taken from the center of the cross section perpendicular to the longitudinal direction of the test piece. The observation surface of the samples taken was polished. The observation surface after polishing was corroded with nital solution. It was deposited carbon on the observation surface after corrosion. The sample after the carbon deposition, the precipitate only the base material was immersed in a stripping solution in eluting without melting, was peeled off the replica film from the base material. Of replica film, in any field (6.5 [mu] m × 9 .mu.m), to produce a photographic image (perspective and the same size) in TEM. Moreover, the elements within each precipitates in photographic images were identified by EDS. Measuring the area of each AlN identified, obtaining the circle-equivalent diameter (nm) from the area determined. The average value of equivalent circle diameter determined was defined as the average particle diameter of AlN (nm).
[0118]
　[Prior austenite grain size measurement test]
　on specimens having the chemical composition of the steel No. 14 in Table 1 of a hot forging simulating the heating step shown in FIG. 11 (STEP1), and a tread quenching simulation step (STEP3) Carried out. Hot forging simulating the heating step of FIG. 11 (STEP1) was the same as hot forging simulating the heating step of FIG. 10 (STEP1). In tread quenching simulation process of FIG. 11 (STEP3), a test piece of 950 ° C., and immersed in a salt bath of 675 ° C., for 30 minutes isothermally held at 675 ° C.. Thus, to precipitate a pro-eutectoid cementite at grain boundaries of prior austenite, the austenite grains was identifiable state microstructure observation. The test piece after the isothermal holding was allowed to cool to room temperature (25 ° C.).
[0119]
　Samples were taken from the center of the cross section perpendicular to the longitudinal direction of the test piece was left to cool. The observation surface of the samples taken was polished. The observation surface after polishing was corroded with a mixture of picric acid with sodium hydroxide. Of corroded observation surface, in any field (200 [mu] m × 200 [mu] m), to produce a photographic image (perspective and the same size) in TEM. In the preparation photographic images were identified prior austenite grains. Crystal grains surrounded by pro-eutectoid cementite were identified as prior austenite grains.
[0120]
　The particle size of the prior austenite grain identified in the field of view, determined by the cutting method. Specifically, as shown in FIG. 8, the field of view of the square visual field 100 of the 200 [mu] m × 200 [mu] m, by subtracting the diagonal 101 of the two. Then, the total sum of the number of pro-eutectoid cementite which intersects the diagonal 101 of these two. Then, the following equation to determine the prior austenite grain size.
[0121]
　Prior austenite grain size = 2 Total length of the diagonal line 101 (566μm) / total number of pro-eutectoid cementite crossing diagonally 101
　[Charpy impact test]
　The Charpy test pieces having a chemical composition of the steel No. 14 of Table in 1 (10 mm × 10mm × 55mm) was more prepared. For this specimen, the hot forging simulating the heating step shown in FIG. 15 (STEP1), were carried out and tread quenching simulation step (STEP4). Hot forging simulating the heating step of FIG. 15 (STEP1) was the same as hot forging simulating the heating step of FIG. 10 and FIG. 11 (STEP1). Tread quenching simulation process of FIG. 15 (STEP4), assuming that the rim portion in the tread surface hardening of actual rail wheel cools the specimen at a cooling rate at which the cooled and set. Specifically, in the tread quenching simulation step (STEP4) of FIG. 15, the test piece after 20 minutes holding at 950 ° C., and immersed in a salt bath of 400 ° C., and 10 minutes kept isothermally at 400 ° C.. It was then allowed to cool test piece after isothermal holding to room temperature (25 ° C.). Using the above STEP1 and heat treatment the Charpy test piece was conducted in STEP4, and the Charpy impact test according to JIS Z 2242 (2005) was performed at room temperature (25 ° C.).
[0122]
　[Test Results]
　Fig. 12 is a hot forging simulated heating step (STEP1) there later in tread quenching simulation step (STEP2) prior to the test piece, the AlN for each hot forging simulated heating temperature (HP1 ~ HP4) is a diagram showing an average particle diameter. 13, in the test piece after tread quenching simulation step (STEP2), is a diagram showing an average particle diameter of AlN for each hot forging simulated heating temperature (HP1 ~ HP4). 14, in the test piece after tread quenching simulation step (STEP3), is a diagram illustrating a prior austenite grain size for each hot forging simulated heating temperature (HP1 ~ HP4). 16, in the test piece after tread quenching simulation step (STEP4), the hot forging simulated heating temperature (HP1 ~ HP4) Charpy impact test values for (J / cm 2 is a diagram showing a). Figure 9 is based on the results of FIGS. 13 and 14, is a diagram showing the relationship between the prior austenite grain size and AlN average particle size.
[0123]
　Referring to FIGS. 12 and 13, when the hot forging simulating the heating temperature is 1200 ° C. or less, AlN is not completely dissolved even in hot forging simulated heating (STEP1) after (FIG. 12), a number of AlN remaining. Therefore, even after heating tread quenching simulation step (STEP2) (FIG. 13), coarse AlN has a large number remaining, the average particle size also remained 150nm greater and coarse.
[0124]
　In contrast, hot forging simulated heating temperature exceeds the 1200 ° C., AlN residual amount after hot forging simulated heating step (FIG. 12) is lowered, if it exceeds 1250 ° C., AlN was all dissolved. Then, when the hot forging simulated heating temperature exceeds the 1200 ° C., after heating of the tread quenching simulated process (FIG. 13), fine AlN is in many precipitates, the average particle diameter became 150nm or less.
[0125]
　Referring to FIG. 9, in accordance with an average particle size of AlN becomes finer, the former austenite grain size was also reduced. The average particle diameter of AlN is equal to or 150nm or less, the former austenite grain size is less sufficiently miniaturized 50 [mu] m. Then, referring to FIG. 16, in accordance with an average particle size of AlN becomes finer, Charpy impact value is increased, when the average particle diameter of AlN is 150nm or less, the Charpy impact value 10J / cm 2 next , it was found that further excellent toughness as railway wheels obtained.
[0126]
　It has been described an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the embodiments described above, it can be implemented by changing the embodiment described above without departing from the scope and spirit thereof as appropriate.
DESCRIPTION OF SYMBOLS
[0127]
　1 Railway wheels
　2 boss
　3 plate portion
　4 the rim portion
　41 tread
　42 flange

The scope of the claims

[Requested item 1]A railway wheel,
　by
　mass%, C: 0.80
　~ 1.15%, Si: 0.45% or
　less, Mn: 0.10
　~ 0.85%, P: 0.050% or
　less, S: 0.030% or
　less,
　Al: 0.120 - 0.650%, N: 0.0030
　~ 0.0200%, Cr: 0 - 0.25%, and,
　V: 0 contains ~ 0.12% ,
　has a chemical composition the balance being Fe and impurities,
　defined by equation (1), eutectoid cementite amount in the microstructure of the rail wheel is 1.50 present / 100 [mu] m or less, the railway wheels.
　Pro-eutectoid cementite amount (present / 100μm) = 200μm × 200μm /(5.66×100μm sum of the number of pro-eutectoid cementite which intersects the two diagonal lines of the square field of view) (1)
[Requested item 2]
　A rail wheel according to claim 1,
　the average particle diameter of the AlN in the microstructure is 150nm or less, the railway wheels.
[Requested item 3]
　A rail wheel according to claim 1 or claim 2,
　wherein the chemical
　composition, Al: 0.120 ~ 0.350%,
　containing, railway wheels.
[Requested item 4]
　A rail wheel according to any one of claims 1 to 3,
　wherein the chemical
　composition, Cr: 0.02 ~ 0.25%,
　and, V: 0.02 ~ 0.12%,
　It contains one or more selected from the group consisting of railway wheels.