WO2021193808A1 - 鉄道車輪 - Google Patents
鉄道車輪 Download PDFInfo
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- WO2021193808A1 WO2021193808A1 PCT/JP2021/012494 JP2021012494W WO2021193808A1 WO 2021193808 A1 WO2021193808 A1 WO 2021193808A1 JP 2021012494 W JP2021012494 W JP 2021012494W WO 2021193808 A1 WO2021193808 A1 WO 2021193808A1
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/55—Hardenability tests, e.g. end-quench tests
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/34—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tyres; for rims
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M17/00—Testing of vehicles
- G01M17/08—Railway vehicles
- G01M17/10—Suspensions, axles or wheels
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2261/00—Machining or cutting being involved
Definitions
- This disclosure relates to railway wheels.
- Railroad vehicles run on the rails that make up the railroad tracks.
- Railroad vehicles include a plurality of railroad wheels.
- Rail wheels support the vehicle, come into contact with the rails, and move while rotating on the rails.
- Rail wheels wear due to contact with rails.
- Recently, for the purpose of improving the efficiency of railway transportation the load weight on railway vehicles has been increased and the speed of railway vehicles has been increased. As a result, improvement in wear resistance of railway wheels is required.
- Patent Document 1 JP-A-9-202937
- Patent Document 2 JP-A-2012-107295
- Patent Document 3 JP-A-2013-231212
- Patent Document 4 Japanese Patent Application Laid-Open No. 2004-315928
- the railroad wheels disclosed in Patent Document 1 have a mass% of C: 0.4 to 0.75%, Si: 0.4 to 0.95%, Mn: 0.6 to 1.2%, Cr: It contains 0 to less than 0.2%, P: less than 0.03%, S: 0.03% or less, and the balance consists of Fe and unavoidable impurities.
- a region from the surface of the wheel tread to a depth of at least 50 mm is composed of a pearlite structure.
- the wheel tread portion is formed under the condition that the cooling curve of the wheel tread portion passes through the pearlite generation region in the continuous cooling transformation curve diagram and is on the martensitic transformation curve side for a long time. Includes a quenching step to cool.
- the steel for wheels disclosed in Patent Document 2 has C: 0.65 to 0.84%, Si: 0.02 to 1.00%, Mn: 0.50 to 1.90%, Cr in mass%. : Contains 0.02 to 0.50%, V: 0.02 to 0.20%, S ⁇ 0.04%, P ⁇ 0.05%, Cu ⁇ 0.20%, Ni ⁇ 0.20% However, the balance has a chemical composition consisting of Fe and impurities. This chemical composition further satisfies the following relational expression.
- Patent Document 2 describes that this wheel steel is excellent in wear resistance, rolling fatigue resistance, and spoke resistance by satisfying the above chemical composition and the above formula.
- the steel for wheels disclosed in Patent Document 3 has C: 0.65 to 0.84%, Si: 0.4 to 1.0%, Mn: 0.50 to 1.40%, Cr in mass%. : 0.02 to 0.13%, S: 0.04% or less, V: 0.02 to 0.12%, Fn1 defined in the formula (1) is 32 to 43, and the formula Fn2 represented by (2) is 25 or less, and the balance is composed of Fe and impurities.
- Patent Document 3 describes that this wheel steel has the above chemical composition and is excellent in wear resistance, rolling fatigue resistance, and spoke resistance when Fn1 and Fn2 satisfy the above ranges. There is.
- the railway wheels disclosed in Patent Document 4 have C: 0.85 to 1.20%, Si: 0.10 to 2.00%, Mn: 0.05 to 2.00%, in mass%, and are required. Depending on the situation, it further contains one or more of Cr, Mo, V, Nb, B, Co, Cu, Ni, Ti, Mg, Ca, Al, Zr, and N in a predetermined amount, and the balance is Fe and other unavoidable. It is an integrated railway wheel made of steel containing a chemical component composed of target impurities, and at least a part of the tread and / or flange surface of the railway wheel has a pearlite structure.
- Patent Document 4 the life of railway wheels depends on the amount of wear on the tread and flange surfaces (paragraph [0002] of Patent Document 4), and is further generated as the amount of heat generated when braking is applied in a high-speed railway. It is stated that it depends on cracks on the tread and flange surfaces. It is described that the railroad wheels have the above configuration, so that the wear resistance of the tread surface and the flange surface and thermal cracking can be suppressed.
- Japanese Unexamined Patent Publication No. 9-202937 Japanese Unexamined Patent Publication No. 2012-107295 Japanese Unexamined Patent Publication No. 2013-231212 Japanese Unexamined Patent Publication No. 2004-315928
- the railway wheel proposed in Patent Document 1 has a low Cr content and further contains an appropriate amount of Si in order to obtain an appropriate hardenability and a property of obtaining a pearlite structure.
- the C content of the railway wheel described in Patent Document 1 is 0.4 to 0.75%, and the railway wheel is made of so-called subeutectic steel. Therefore, there is a limit to the improvement of wear resistance.
- the pearlite structure is strengthened by containing V in the steel having a C content of 0.65 to 0.84% to improve wear resistance. I'm raising it.
- an example of a railway wheel manufacturing method is as follows. Steel pieces are hot-worked to form railroad wheel-shaped intermediates. Heat treatment (tread quenching) is performed on the molded intermediate product. In tread quenching, after heating the intermediate product, the rim portion (particularly the tread surface and flange portion of the rim portion) of the intermediate product is rapidly cooled. As a result, fine pearlite with high wear resistance is generated in the matrix structure of the surface layer portion of the tread surface. However, on the surface layer portion of the tread surface after quenching the tread surface, a quenching layer is formed on the upper layer of fine pearlite.
- the hardened layer is a hard layer made of martensite or a hard layer made of martensite and bainite. The hardened layer is prone to wear during the use of railroad wheels. Therefore, after quenching the tread, the hardened layer formed on the outermost surface layer of the tread is removed by cutting to expose fine pearlite on the tread.
- Rail wheels are manufactured by the above steps.
- railway wheels made of hypereutectoid steel have excellent wear resistance.
- a hardened layer is likely to be formed deeply after tread quenching because the C content is high.
- the hardened layer is removed by cutting, and if the hardened layer is deeply formed, it takes a long time to cut. Therefore, it is preferable that the formation of the hardened layer can be reduced as much as possible in the manufacturing process.
- An object of the present disclosure is to provide a railway wheel made of hypereutectoid steel having a high C content of 0.80% or more and capable of suppressing the formation of a hardened layer in a manufacturing process.
- the chemical composition of the railway wheel is mass%.
- the rest consists of Fe and impurities In the microstructure of the rim portion of the railroad wheel
- the initial cementite area ratio is 0.1 to 1.5%,
- the pearlite area ratio is 95.0% or more.
- the railway wheel according to this embodiment can suppress the formation of a hardened layer in the manufacturing process even if the C content is as high as 0.80% or more.
- FIG. 1 is a cross-sectional view including a central axis of a railway wheel.
- FIG. 2 is a diagram showing the relationship between the distance from the water-cooled end and the Rockwell hardness HRC, which was obtained by a Jomini type one-side quenching test using a test material assuming a railway wheel.
- FIG. 1 is a cross-sectional view including a central axis of a railway wheel according to the present embodiment.
- the railroad wheel 1 has a disk shape and includes a boss portion 2, a plate portion 3, and a rim portion 4.
- the boss portion 2 has a cylindrical shape and is arranged at the central portion of the railway wheel 1 in the radial direction (direction perpendicular to the central axis) of the railway wheel 1.
- the boss portion 2 has a through hole 21.
- the central axis of the through hole 21 coincides with the central axis of the railroad wheel 1.
- a railroad 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 on the outer peripheral edge portion of the railway wheel 1.
- the rim portion 4 includes a tread surface 41 and a flange portion 42.
- the tread 41 is connected to the flange portion 42.
- the thickness T4 of the rim portion 4 is thicker than the thickness T3 of the plate portion 3.
- the plate portion 3 is arranged 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.
- the present inventors first examined a chemical composition suitable for enhancing wear resistance in railway wheels. As a result, in railway wheels, even if the same hardness is obtained, it is better to increase the C content to 0.80% or more and increase the hardness than to increase the V content to increase the hardness. It was found that the wear resistance when used as a railroad wheel is improved. This mechanism is not clear, but the following can be considered.
- the treads of railway wheels in use receive external force (load) from the rails. This external force crushes cementite in the surface layer of pearlite just below the tread, and the hardness is increased by strengthening the dispersion. Further, the carbon in the crushed fine cementite is supersaturated in the ferrite in the pearlite, and the hardness of the surface layer just below the tread is increased by strengthening the solid solution.
- the present inventors considered that in order to enhance the wear resistance, it is preferable to use a hypereutectoid steel having a C content of 0.80 to 1.15% in the chemical composition of the railway wheel.
- the railroad wheels of hypereutectoid steel having a C content of 0.80% or more are compared with the subeutectoid steel due to tread quenching during the manufacturing process of the railroad wheels.
- the hardened layer is formed deeper.
- the hardened layer formed on the rim portion 4 (tread surface 41 and flange portion 42) is removed by cutting. Therefore, it is preferable that the quenching layer is as thin as possible. Therefore, the present inventors have investigated a means capable of reducing the quenching layer formed during the manufacturing process in a railway wheel made of hypereutectoid steel having a C content of 0.80% or more.
- FIG. 2 is a diagram showing the relationship between the distance from the water-cooled end and the Rockwell hardness HRC, which was obtained by a Jomini type one-side quenching test using a test material assuming a railway wheel.
- FIG. 2 was obtained by the following method.
- test material having the chemical composition shown in Table 1 (a round bar test piece having a diameter of 25 mm and a length of 100 mm) was prepared.
- Test No. 1 containing no Nb, Test No. 2 having an Nb content of 0.009%, and Test No. 3 having an Nb content of 0.020% were prepared.
- a Jomini type one-side quenching test conforming to JIS G0561 (2011) was carried out. Specifically, the Jomini test piece was held in an air atmosphere at 950 ° C., which is a temperature equal to or higher than the A cm transformation point, for 30 minutes, and the structure of the Jomini test piece was defined as an austenite single phase. After that, quenching (water cooling) was carried out once. Specifically, water was sprayed onto one end of the Jomini test piece to cool it.
- the side surface of the water-cooled Jomini test piece is mechanically polished, and at regular intervals in the axial direction from one end (water-cooled end), Rockwell hardness using a C scale conforming to JIS Z2245 (2011) ( HRC) test was carried out.
- the HRC measurement interval was 1.0 mm pitch from the water-cooled end to the 15 mm position, and 2.5 mm pitch at the position 15 mm or more from the water-cooled end.
- the obtained HRC was plotted to create FIG.
- the Rockwell hardness HRC rapidly decreases as the distance D from the water-cooled end increases. If the distance D is equal to or greater than a predetermined distance, the Rockwell hardness HRC does not decrease so much even if the distance from the water-cooled end is increased.
- the region A from the water-cooled end to the sharp decrease in Rockwell hardness HRC is defined as the "quenched layer”. Further, a region B deeper than the region A and in which the Rockwell hardness HRC is not significantly reduced is defined as a “base material”.
- the depth of the hardened layer decreased as the Nb content increased. Therefore, if Nb is contained in the railway wheel, the formation of the quenching layer formed by the tread quenching treatment in the manufacturing process can be suppressed.
- the reason why the quenching layer generated by the tread quenching treatment in the manufacturing process can be suppressed by containing Nb is as follows.
- fine Nb carbides are generated when heated by tread quenching.
- Fine Nb carbides refine the old austenite grains.
- VC has a lower solid solution temperature than NbC. Therefore, the VC dissolves in a solid solution during the heating of tread quenching, and cannot function as pinning particles.
- V is contained, as described above, in the cooling step of tread quenching, VC is precipitated in the ferrite to strengthen the ferrite, but it is considered that the austenite crystal grains cannot be refined as pinning particles.
- the formation of the hardened layer is suppressed by a different mechanism and the former austenite crystal grains are refined.
- the formation of the hardened layer can be suppressed by containing Nb in the railway wheel having the above chemical composition.
- the railway wheel of the present embodiment completed based on the above findings has the following configuration.
- Carbon (C) increases the hardness of steel and enhances the wear resistance of railway wheels. If the C content is less than 0.80%, this effect cannot be obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the C content exceeds 1.15%, a large amount of pro-eutectoid cementite may be precipitated at the former austenite grain boundaries even if the content of other elements is within the range of the present embodiment. In this case, the toughness of the railroad wheels is reduced. Therefore, the C content is 0.80 to 1.15%.
- the lower limit of the C content is preferably 0.85%, more preferably 0.86%, still more preferably 0.87%, still more preferably 0.90%, still more preferably 0.95. %.
- the preferred upper limit of the C content is 1.10%, more preferably 1.05%.
- Si 1.00% or less Silicon (Si) is inevitably contained. That is, the Si content is more than 0%. Si enhances the hardness of steel by solid solution strengthening ferrite. However, if the Si content exceeds 1.00%, pro-eutectoid cementite, which causes a decrease in steel toughness, is likely to be generated even if the content of other elements is within the range of the present embodiment. If the Si content exceeds 1.00%, the hardenability of the steel becomes too high, and martensite is likely to be formed. In this case, the thickness of the hardened layer formed on the tread during tread quenching increases. As a result, the cutting amount increases and the yield decreases.
- the Si content is 1.00% or less.
- the preferred upper limit of the Si content is 0.90%, more preferably 0.80%, still more preferably 0.70%, still more preferably 0.60%, still more preferably 0.45. %, More preferably 0.40%, still more preferably 0.35%.
- the lower limit of the Si content is not particularly limited. However, excessive reduction of Si content increases manufacturing costs. Therefore, the preferred lower limit of the Si content is 0.01%, more preferably 0.05%, still more preferably 0.10%, still more preferably 0.15%.
- Mn 0.10 to 1.20%
- Manganese (Mn) solid-solves and strengthens ferrite to increase the hardness of steel. Mn further forms MnS and enhances the machinability of steel. If the Mn content is less than 0.10%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 1.20%, the hardenability of the steel becomes too high even if the content of other elements is within the range of the present embodiment. In this case, the thickness of the hardened layer increases, and the yield during the manufacturing process decreases.
- the Mn content is 0.10 to 1.20%.
- the preferable lower limit of the Mn content is 0.30%, more preferably 0.50%, still more preferably 0.70%.
- the preferred upper limit of the Mn content is 1.00%, more preferably 0.90%, still more preferably 0.85%.
- P 0.050% or less Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is more than 0%. P segregates at the grain boundaries and reduces the toughness of the steel. Therefore, the P content is 0.050% or less.
- the preferred upper limit of the P content is 0.030%, more preferably 0.020%.
- the P content is preferably as low as possible. However, excessive reduction of P content increases manufacturing costs. Therefore, when considering normal industrial production, the preferable lower limit of the P content is 0.001%, and more preferably 0.002%.
- S 0.030% or less Sulfur (S) is inevitably contained. That is, the S content is more than 0%. S forms MnS and enhances the machinability of steel. On the other hand, if the S content is too high, the toughness of the steel will decrease. Therefore, the S content is 0.030% or less.
- the preferred upper limit of the S content is 0.020%, more preferably 0.015%, still more preferably 0.012%, still more preferably 0.010%. Excessive reduction of S content increases manufacturing costs. Therefore, the preferable lower limit of the S content is 0.001%, more preferably 0.002%, still more preferably 0.003%, still more preferably 0.005%.
- Al 0.005 to 0.190%
- Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, the above effect cannot be sufficiently obtained even if the other element content is within the range of the present embodiment. On the other hand, if the Al content exceeds 0.190%, the above effect is saturated. Therefore, the Al content is 0.005 to 0.190%.
- the lower limit of the Al content is preferably 0.008%, more preferably 0.010%.
- the preferable upper limit of the Al content is 0.180%, more preferably 0.170%, further preferably 0.150%, further preferably 0.120%, still more preferably 0.100. %, More preferably 0.080%, even more preferably 0.060%, still more preferably 0.050%.
- N 0.0200% or less Nitrogen (N) is an impurity that is inevitably contained. That is, the N content is more than 0%. If the N content exceeds 0.0200%, AlN becomes coarse and the toughness of the steel decreases. Therefore, the N content is 0.0200% or less.
- the preferred upper limit of the N content is 0.0100%, more preferably 0.0080%, still more preferably 0.0070%, still more preferably 0.0060%.
- the N content is preferably as low as possible. However, excessive reduction of N content raises manufacturing costs. Therefore, considering normal industrial production, the preferable lower limit of the N content is 0.0001%, more preferably 0.0010%, still more preferably 0.0025%.
- Niobium (Nb) combines with C to produce fine NbC during heating for tread quenching during the manufacturing process of railway wheels.
- the fine NbC functions as pinning particles and suppresses the coarsening of austenite during heating. Therefore, the old austenite crystal grains are maintained as fine, and the hardenability of the steel is suppressed. As a result, the formation of a hardened layer during the manufacturing process of railway wheels is suppressed.
- Nb also enhances the toughness of the steel material by suppressing the coarsening of the old austenite crystal grains. If the Nb content is less than 0.005%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
- the Nb content is 0.005 to 0.050%.
- the preferred lower limit of the Nb content is 0.007%, more preferably 0.009%.
- the preferred upper limit of the Nb content is 0.040%, more preferably 0.035%, even more preferably 0.030%, even more preferably 0.025%, still more preferably 0.023. %.
- the rest of the chemical composition of the railroad wheels according to this embodiment consists of Fe and impurities.
- the impurities are those mixed from ore, scrap, manufacturing environment, etc. as raw materials when the railway wheels of the present embodiment are industrially manufactured, and have an adverse effect on the railway wheels of the present embodiment. Means what is allowed within the range that does not give.
- the chemical composition of the railway wheel of the present embodiment may further contain Cr instead of a part of Fe.
- Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When included, Cr narrows the lamella spacing of perlite. This significantly increases the hardness of pearlite. However, if the Cr content exceeds 0.25%, the hardenability becomes excessively high and the thickness of the hardened layer after tread quenching becomes excessive even if the content of other elements is within the range of the present embodiment. Increases to. Therefore, the Cr content is 0 to 0.25%.
- the lower limit of the Cr content is preferably more than 0%, more preferably 0.01%, still more preferably 0.02%, still more preferably 0.03%.
- the preferred upper limit of the Cr content is 0.24%, more preferably 0.23%, still more preferably 0.22%.
- the chemical composition of the railway wheel of the present embodiment may further contain V instead of a part of Fe.
- V 0 to 0.12% Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, V forms any of carbides, nitrides, and carbonitrides to precipitate and reinforce steel (specifically, ferrite in steel). As a result, the hardness of the railroad wheels is increased, further enhancing the wear resistance. However, if the V content exceeds 0.12%, the hardenability becomes high, and the thickness of the hardened layer after tread quenching becomes excessively increased. Therefore, the V content is 0 to 0.12%.
- the lower limit of the V content is preferably more than 0%, more preferably 0.01%, still more preferably 0.02%, still more preferably 0.03%.
- the preferred upper limit of the V content is 0.11%, more preferably 0.10%.
- the area ratio of pearlite is 95.0% or more, and the area ratio of proeutectoid cementite is 0.1 to 1.5%.
- the phases other than pearlite and proeutectoid cementite are, for example, martensite and / or bainite.
- the lower limit of the area ratio of the pro-eutectoid cementite may be 0.2% or 0.3%.
- the preferred upper limit of the area ratio of the pro-eutectoid cementite is 1.4%, more preferably 1.3%, still more preferably 1.2%, still more preferably 1.1%, still more preferably. It is 1.0%, more preferably 0.9%.
- the pearlite area ratio and the proeutectoid cementite area ratio in the microstructure of the rim portion 4 are obtained by the following method.
- a sample is taken from the center position in the thickness direction of the rim portion 4 (the center position of the thickness T4 in FIG. 1).
- the observation surface of each sample is mirror-finished by mechanical polishing.
- the observation surface is etched with a sodium picric acid solution (100 ml of water + 2 g of picric acid + 25 g of sodium hydroxide). In etching, the sample is immersed in a boiled sodium picric acid solution.
- a photographic image is generated using a 200x optical microscope for any one field of view (500 ⁇ m ⁇ 500 ⁇ m) in the observation plane of the sample after etching.
- each phase of the microstructure can be discriminated based on the contrast. Identify pearlite and pro-eutectoid cementite based on contrast.
- the area ratio (%) of pearlite is obtained based on the total area of the specified pearlite and the area of the observation field of view.
- the area ratio (%) of the proeutectoid cementite is determined based on the total area of the identified proeutectoid cementite and the area of the observation field of view.
- the content of each element in the chemical composition is within the range of the present embodiment, and the pearlite area ratio is 95.0% or more in the microstructure of the rim portion 4.
- the area ratio of the proeutectoid cementite is 0.1 to 1.5%. Therefore, the railway wheel 1 of the present embodiment can obtain appropriate toughness even if the prominent cementite is present in the rim portion 4.
- the railway wheel 1 of the present embodiment contains 0.005 to 0.050% of Nb in the above-mentioned chemical composition. Therefore, the depth of the quenching layer formed by the tread quenching treatment during the manufacturing process can be suppressed.
- the chemical composition of the railway wheel of the present embodiment preferably satisfies the formula (1).
- the content of the corresponding element is substituted in% by mass for the element symbol in the formula (1).
- F1 100Nb / (C + 0.5Si + 0.8Mn + 15Al + 40Cr + 10V).
- F1 is an index of the depth of the hardened layer that can be generated during the manufacturing process of railway wheels. Specifically, Nb suppresses the formation of a hardened layer as described above. On the other hand, C, Si, Mn, Al, Cr and V are considered to promote the formation of the hardened layer.
- F1 has a quenching layer forming element (Nb) as a numerator and a quenching layer promoting element (C, Si, Mn, Al, Cr, V) as a denominator.
- the content of the quenching layer suppressing element (Nb) is sufficiently higher than the content of the quenching layer promoting element.
- the depth of the quenching layer formed by the tread quenching treatment during the manufacturing process of the railway wheel can be sufficiently suppressed.
- the preferred lower limit of F1 is 0.080, more preferably 0.090, even more preferably 0.100, even more preferably 0.150, even more preferably 0.200, even more preferably. It is 0.250, more preferably 0.300, and even more preferably 0.340.
- F1 is a value obtained by rounding off the fourth decimal place of the obtained value.
- a more preferable lower limit of F1 is 0.380. In this case, the depth of the quenching layer formed by the tread quenching treatment during the manufacturing process of the railway wheel can be further sufficiently suppressed.
- a more preferable lower limit of F1 is 0.390, and even more preferably 0.395.
- This manufacturing method includes a process of manufacturing steel for railway wheels (material manufacturing process), a process of forming a wheel-shaped intermediate product from steel for railway wheels by hot processing (molding process), and a molded intermediate product. It includes a step of performing heat treatment (quenching of the tread) (heat treatment step) and a step of removing the hardened layer from the tread or the like of the intermediate product after the heat treatment by cutting to obtain a railroad wheel (cutting step).
- heat treatment step quenching of the tread
- cutting step cutting to obtain a railroad wheel
- molten steel having the above-mentioned chemical composition is manufactured using an electric furnace, a converter, or the like.
- the manufactured molten steel is cast into a casting material (slab or ingot).
- a slab may be produced by continuous casting, or an ingot may be produced by casting with a mold.
- the slab or ingot is hot-processed to produce a steel material for railway wheels (hereinafter, also referred to as a steel material) of a desired size.
- Hot working is, for example, hot forging, hot rolling, and the like.
- a steel material is produced by hot rolling, for example, the steel material is produced by the following method.
- a slabbing mill is used.
- a steel material is manufactured by performing ingot rolling on a material with a ingot rolling mill.
- a continuous rolling mill is installed downstream of the ingot rolling mill, the steel material after ingot rolling is further hot-rolled using the continuous rolling mill to produce a smaller steel material. You may.
- the heating temperature of the heating furnace in hot rolling is not particularly limited, but is, for example, 1100 to 1350 ° C.
- a steel material for railway wheels is manufactured by the above manufacturing process.
- the steel material for railway wheels may be a casting material (slab or ingot). That is, the above-mentioned hot working may be omitted.
- a steel material for railway wheels which is a material for railway wheels, is manufactured.
- the steel material for railway wheels is, for example, a columnar material.
- the prepared steel material for railway wheels is used to form an intermediate wheel-shaped product by hot working. 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. Hot working is, for example, hot forging, hot rolling, and the like.
- the preferable heating temperature of the steel material for railway wheels during hot working is 1220 ° C. or higher.
- NbC in the steel material for railway wheels is sufficiently solid-solved in the heating process during hot working.
- the preferred lower limit of the heating temperature during hot working is 1230 ° C, more preferably 1250 ° C, and even more preferably 1300 ° C.
- the preferred upper limit of the heating temperature during hot working is 1350 ° C.
- the cooling method of the intermediate product after hot working is not particularly limited. It may be left to cool or water cooled.
- tread quenching is performed on the molded wheel-shaped intermediate product.
- the intermediate product after the molding step hot forging or hot rolling
- reheat treatment the tread and flange of the intermediate product
- the cooling medium is, for example, air, mist, or spray, and is not particularly limited as long as a cooling rate suitable for the desired structure can be obtained.
- the plate portion and the boss portion are allowed to cool without being water-cooled. As a result, in the rim portion, the area ratio of the proeutectoid cementite can be suppressed to 1.5% or less, but the area ratio of the proeutectoid cementite becomes 0.1% or more.
- the diameter of the railway wheel of this embodiment is, for example, 700 mm to 1000 mm.
- the preferable cooling rate of the tread during tread quenching is 3.0 to 200.0 ° C./sec.
- the preferable cooling rate in the region where the cooling rate is the slowest is 1.5 ° C./sec or more.
- the pro-eutectoid cementite area ratio is 0.1% or more, but can be suppressed to 1.5% or less.
- the region where the cooling rate is the slowest among the intermediate products can be determined, for example, by measuring the temperature distribution change of the intermediate products during tread cooling using a plurality of thermography. More preferably, in the rim portion of the intermediate product during tread quenching, the preferable cooling rate in the region where the cooling rate is the slowest is 2.0 ° C./sec or more.
- the intermediate product is reheated, but the intermediate product after hot working may be directly (without reheating) tread quenching.
- Temper the intermediate product after quenching the tread as necessary It suffices to temper at a well-known temperature and time.
- the tempering temperature is, for example, 400 to 600 ° C.
- the railway wheel of this embodiment is manufactured by the above process.
- the railroad wheel of the present embodiment has a pearlite area ratio of 95.0% or more in the microstructure of the rim portion, and the content of each element in the chemical composition is within the range of the present embodiment.
- the area ratio of is 0.1 to 1.5%. Therefore, the railway wheel of the present embodiment can obtain appropriate toughness even if pro-eutectoid cementite is formed. Further, the railway wheel of the present embodiment contains 0.005 to 0.050% of Nb in the above-mentioned chemical composition. Therefore, the depth of the quenching layer formed by the tread quenching treatment during the manufacturing process can be suppressed.
- the blank in the "Chemical composition” column means that the corresponding element content was below the detection limit.
- the Nb content of test number 7 in Table 2 means that the fourth decimal place was rounded to 0%.
- the Cr content of Test No. 1 means that the third decimal place was rounded to 0%.
- the V content of test number 1 means that the third decimal place was rounded to 0%.
- a round ingot (top diameter 107 mm, bottom diameter 97 mm, height 230 mm truncated cone type) was manufactured by the ingot method using the above molten steel.
- the ingot was heated to 1250 ° C. and then hot forged to produce a round bar having a diameter of 40 mm.
- a heat treatment test piece having a diameter of 3 mm and a length of 10 mm was prepared from a D / 4 depth position (“D” is the diameter of the round bar) in the radial direction from the surface of the round bar of each test number.
- the longitudinal direction of the heat treatment test piece was parallel to the central axis direction of the round bar.
- a continuous cooling test simulating tread quenching was carried out.
- a four-master testing machine manufactured by Fuji Denpa Koki was used for the heat treatment. Specifically, test pieces of each test number were prepared and heated at 950 ° C. for 5 minutes. Then, it cooled at the cooling rate (° C./sec) shown in Table 2. For each test piece after cooling, the area ratio (%) of pearlite area ratio (%) and the area ratio (%) of proeutectoid cementite (primigenesis ⁇ ) were determined by the following methods.
- a sample was prepared in which the cross section perpendicular to the longitudinal direction of the above-mentioned heat treatment test piece was used as the observation surface. After the observation surface was mechanically polished, the observation surface was etched with a sodium picric acid solution (100 ml of water + 2 g of picric acid + 25 g of sodium hydroxide). For etching, the sample was immersed in a boiled sodium picric acid solution. A photographic image was generated using a 200x optical microscope for any one field of view (500 ⁇ m ⁇ 500 ⁇ m) in the observation plane after etching.
- the pearlite area ratio was 95.0% or more in all the test numbers.
- the area ratio of the initial cementite (initial analysis ⁇ ) of each test number is as shown in Table 2.
- a Jomini type one-sided quenching test was carried out for the depth of the quenching layer.
- the Jomini type one-sided quenching test was carried out by the following method.
- a Jomini test piece having a diameter of 25 mm and a length of 100 mm was prepared from a round bar having a diameter of 40 mm for each test number.
- the central axis of the Jomini test piece coincided with the central axis of the round bar.
- a Jomini type one-side quenching test conforming to JIS G0561 (2011) was carried out.
- the Jomini test piece was held in an air atmosphere at 950 ° C., which is a temperature equal to or higher than the A cm transformation point, for 30 minutes, and the structure of the Jomini test piece was defined as an austenite single phase. After that, quenching (water cooling) was carried out once. Specifically, water was sprayed onto one end of the Jomini test piece to cool it.
- the side surface of the water-cooled Jomini test piece is mechanically polished, and at regular intervals in the axial direction from one end (water-cooled end), Rockwell hardness using a C scale conforming to JIS Z2245 (2011) ( HRC) test was carried out.
- the HRC measurement interval was 1.0 mm pitch from the water-cooled end to the 15 mm position, and 2.5 mm pitch at the position 15 mm or more from the water-cooled end. From the obtained HRC distribution, the quenching layer depth was determined by the following method.
- the Jomini curve shown in Fig. 2 was created for the steel materials of each test number. As described above, in the Jomini curve, the region A in which the Rockwell hardness HRC rapidly decreases is defined as the “quenched layer”, and the region B in which the Rockwell hardness HRC does not decrease so much is defined as the “base material”. Area A and area B could be separated via an inflection point. Region A was specified from the HRC distribution (Jomini curve) of each steel number, and the quenching layer depth (mm) was determined. For test numbers 15 to 18, the quenching layer depth measurement test was not carried out (“ ⁇ ” in the “quenching layer depth” column in Table 2).
- a continuous cooling test was carried out using the prepared heat treatment test pieces.
- a four-master testing machine manufactured by Fuji Denpa Koki was used for the heat treatment. Specifically, the test pieces of each test number were heated at 950 ° C. for 5 minutes. Then, it was cooled at a cooling rate of 0.01 to less than 0.1 ° C./sec.
- a sample was prepared in which the cross section perpendicular to the longitudinal direction of each test piece after cooling was used as the observation surface. After the observation surface was mechanically polished, the observation surface was etched with a sodium picric acid solution (100 ml of water + 2 g of picric acid + 25 g of sodium hydroxide). For etching, the sample was immersed in a boiled sodium picric acid solution.
- a photographic image was generated using a 200x optical microscope for any one field of view in the observation plane after etching.
- the observation field of view was a square field of view of 500 ⁇ m ⁇ 500 ⁇ m.
- the portion where the pro-eutectoid cementite was precipitated was judged to be the grain boundary of the former austenite crystal grains, and the former austenite crystal grains were identified.
- the particle size of the identified old austenite crystal grains was determined by a cutting method. Specifically, two diagonal lines were drawn in the square field of view. Then, the total number of proeutectoid cementites (former austenite grain boundaries) intersecting these two diagonal lines was calculated.
- the particle size ( ⁇ m) of the old austenite crystal grains was determined by the following formula.
- the particle size ( ⁇ m) of the obtained former austenite crystal grains is shown in Table 2.
- Grain size of former austenite grains total length of two diagonals / total number of proeutectoid cementites intersecting the diagonals
- the toughness of the round bar of each test number was evaluated by the following method. Specifically, four square bar-shaped heat-treated materials having a width of 12 mm, a height of 12 mm, and a length of 70 mm were collected from the round bars of each test number. The square bar-shaped heat-treated material was collected while avoiding a radius of 4 mm from the central axis of the round bar. The longitudinal direction of the square bar heat-treated material was parallel to the longitudinal direction of the round bar.
- a continuous cooling test was conducted on the square rod-shaped heat-treated material, simulating tread quenching.
- a thermodynamic cycle tester manufactured by Fuji Denpa Koki was used for the heat treatment.
- the square bar heat treatment material was soaked at 950 ° C. for 5 minutes. Then, the square rod-shaped heat-treated material was cooled at the cooling rates shown in Table 2.
- a heat treatment simulating the manufacturing process of railway wheels was performed.
- the square rod-shaped heat-treated material was machined to prepare a U-notch test piece having a width of 10 mm, a height of 10 mm, and a length of 55 mm.
- the manufactured U-notch test piece was subjected to a Charpy impact test conforming to JIS Z 2242 (2005) at room temperature and in the air to obtain a Charpy impact value (J / cm 2).
- the average value of the four values was defined as the Charpy impact value (J / cm 2 ) of the test number. If the obtained Charpy impact value (J / cm 2) is 12.5 J / cm 2 or more was evaluated as excellent in toughness ( " ⁇ " in "Toughness Evaluation” column in Table 2). On the other hand, if the obtained Charpy impact value (J / cm 2) is less than 12.5 J / cm 2, it was evaluated to be low toughness ( " ⁇ " in "Toughness Evaluation” column in Table 2). No toughness test was performed on test numbers 12 to 14, 17 and 18 (“ ⁇ ” in the “Toughness evaluation” column in Table 2).
- Test results The test results are shown in Table 2. With reference to Table 2, the microstructure was substantially pearlite tissue in all test numbers. That is, the pearlite area ratio was 95.0% or more.
- Test Nos. 1 to 6 the chemical composition was appropriate and the cooling conditions in the heat treatment step were appropriate. Therefore, the initial cementite area ratio was 0.1 to 1.5%. Therefore, the Charpy impact value (J / cm 2 ) was 12.5 J / cm 2 or more, and the toughness was high. Furthermore, the depth of the hardened layer was 8.0 mm or less, and it was expected that the hardened layer generated during tread hardening could be suppressed.
- the test numbers 1 to 6 satisfied the formula (1).
- test numbers 7 to 10 the Nb content was less than 0.005%. Therefore, the depth of the quenching layer was as deep as 9.0 mm or more. The old austenite crystal grains of test numbers 7 to 10 were larger than those of test numbers 1 to 6.
- test numbers 11 to 14 the Al content was too high and the Nb content was less than 0.005%. Therefore, the depth of the quenching layer was as deep as 9.0 mm or more. Test numbers 7 to 14 did not satisfy the formula (1).
- test numbers 15 and 16 the cooling conditions in the heat treatment step were too slow. Therefore, the initial cementite area ratio exceeded 1.5%. Therefore, Charpy impact value (J / cm 2) is less than 12.5 J / cm 2, toughness was low. In test numbers 17 and 18, the cooling conditions in the heat treatment step were too slow, so that the pro-eutectoid cementite area ratio exceeded 1.5%.
Abstract
Description
リム部と、
ボス部と、
前記リム部と前記ボス部との間に配置され、前記リム部と前記ボス部とにつながる板部とを備え、
前記鉄道車輪の化学組成は、質量%で、
C:0.80~1.15%、
Si:1.00%以下、
Mn:0.10~1.20%、
P:0.050%以下、
S:0.030%以下、
Al:0.005~0.190%、
N:0.0200%以下、
Nb:0.005~0.050%、
Cr:0~0.25%、
V:0~0.12%、及び、
残部がFe及び不純物からなり、
前記鉄道車輪の前記リム部のミクロ組織において、
初析セメンタイト面積率が0.1~1.5%であり、
パーライト面積率が95.0%以上である。
図1は本実施形態による鉄道車輪の中心軸を含む断面図である。図1を参照して、鉄道車輪1は円盤状であり、ボス部2と、板部3と、リム部4とを備える。ボス部2は円筒状であり、鉄道車輪1の径方向(中心軸に対して垂直な方向)において、鉄道車輪1の中央部に配置される。ボス部2は貫通孔21を有する。貫通孔21の中心軸は、鉄道車輪1の中心軸と一致する。貫通孔21には、図示しない鉄道用車軸が挿入される。ボス部2の厚さT2は、板部3の厚さT3よりも厚い。リム部4は、鉄道車輪1の外周の縁部に形成されている。リム部4は、踏面41と、フランジ部42とを含む。踏面41は、フランジ部42と繋がっている。鉄道車輪1の使用時において、踏面41及びフランジ部42はレール表面と接触する。リム部4の厚さT4は、板部3の厚さT3よりも厚い。板部3は、ボス部2とリム部4との間に配置され、ボス部2及びリム部4とつながっている。具体的には、板部3の内周縁部はボス部2とつながっており、板部3の外周縁部はリム部4とつながっている。板部3の厚さT3は、ボス部2の厚さT2及びリム部4の厚さT4よりも薄い。
図2は、鉄道車輪を想定した試験材を用いたジョミニ式一端焼入れ試験により得られた、水冷端からの距離と、ロックウェル硬さHRCとの関係を示す図である。図2は次の方法により求めた。
鉄道車輪であって、
リム部と、
ボス部と、
前記リム部と前記ボス部との間に配置され、前記リム部と前記ボス部とにつながる板部とを備え、
前記鉄道車輪の化学組成は、質量%で、
C:0.80~1.15%、
Si:1.00%以下、
Mn:0.10~1.20%、
P:0.050%以下、
S:0.030%以下、
Al:0.005~0.190%、
N:0.0200%以下、
Nb:0.005~0.050%、
Cr:0~0.25%、
V:0~0.12%、及び、
残部がFe及び不純物からなり、
前記鉄道車輪の前記リム部のミクロ組織において、
初析セメンタイト面積率が0.1~1.5%であり、
パーライト面積率が95.0%以上である、
鉄道車輪。
[1]に記載の鉄道車輪であって、
前記化学組成は式(1)を満たす、
鉄道車輪。
100Nb/(C+0.5Si+0.8Mn+15Al+40Cr+10V)≧0.070 (1)
ここで、式(1)中の元素記号には、対応する元素の含有量が質量%で代入される。
[1]又は[2]に記載の鉄道車輪であって、
前記化学組成は、
Cr:0.02~0.25%、及び、
V:0.02~0.12%、
からなる群から選択される1元素以上を含有する、
鉄道車輪。
本実施形態の鉄道車輪の化学組成は、次の元素を含有する。
炭素(C)は、鋼の硬度を高め、鉄道車輪の耐摩耗性を高める。C含有量が0.80%未満であれば、他の元素含有量が本実施形態の範囲内であっても、この効果が得られない。一方、C含有量が1.15%を超えれば、他の元素含有量が本実施形態の範囲内であっても、旧オーステナイト結晶粒界に初析セメンタイトが多く析出する場合がある。この場合、鉄道車輪の靱性が低下する。したがって、C含有量は0.80~1.15%である。C含有量の好ましい下限は0.85%であり、さらに好ましくは0.86%であり、さらに好ましくは0.87%であり、さらに好ましくは0.90%であり、さらに好ましくは0.95%である。C含有量の好ましい上限は1.10%であり、さらに好ましくは1.05%である。
シリコン(Si)は不可避に含有される。つまり、Si含有量は0%超である。Siは、フェライトを固溶強化して鋼の硬さを高める。しかしながら、Si含有量が1.00%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼の靭性低下の要因となる初析セメンタイトが生成しやすくなる。Si含有量が1.00%を超えればさらに、鋼の焼入れ性が高くなりすぎ、マルテンサイトが生成しやすくなる。この場合、踏面焼入れ時に踏面上に形成される焼入れ層の厚みが増大する。その結果、切削量が増大して歩留まりが低下する。Si含有量が1.00%を超えればさらに、鉄道車輪の使用中に、ブレーキとの間に発生する摩擦熱により焼きが入る。この場合、鉄道車輪の耐き裂性が低下する場合がある。したがって、Si含有量は1.00%以下である。Si含有量の好ましい上限は0.90%であり、さらに好ましくは0.80%であり、さらに好ましくは0.70%であり、さらに好ましくは0.60%であり、さらに好ましくは0.45%であり、さらに好ましくは0.40%であり、さらに好ましくは0.35%である。Si含有量の下限は特に制限されない。しかしながら、Si含有量の過剰な低減は製造コストを高める。したがって、Si含有量の好ましい下限は0.01%であり、さらに好ましくは0.05%であり、さらに好ましくは0.10%であり、さらに好ましくは0.15%である。
マンガン(Mn)はフェライトを固溶強化して鋼の硬さを高める。Mnはさらに、MnSを形成し、鋼の被削性を高める。Mn含有量が0.10%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。一方、Mn含有量が1.20%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼の焼入れ性が高くなりすぎる。この場合、焼入れ層の厚みが増大し、製造工程時における歩留まりが低下する。さらに、鉄道車輪の使用時に、ブレーキとの間に発生する摩擦熱により焼きが入り、鋼の耐き裂性が低下する場合がある。したがって、Mn含有量は0.10~1.20%である。Mn含有量の好ましい下限は0.30%であり、さらに好ましくは0.50%であり、さらに好ましくは0.70%である。Mn含有量の好ましい上限は1.00%であり、さらに好ましくは0.90%であり、さらに好ましくは0.85%である。
りん(P)は、不可避に含有される不純物である。つまり、P含有量は0%超である。Pは粒界に偏析して鋼の靭性を低下する。したがって、P含有量は0.050%以下である。P含有量の好ましい上限は0.030%であり、さらに好ましくは0.020%である。P含有量はなるべく低い方が好ましい。しかしながら、P含有量の過剰な低減は製造コストを高める。したがって、通常の工業生産を考慮した場合、P含有量の好ましい下限は0.001%であり、さらに好ましくは0.002%である。
硫黄(S)は、不可避に含有される。つまり、S含有量は0%超である。SはMnSを形成し、鋼の被削性を高める。一方、S含有量が高すぎれば、鋼の靭性が低下する。したがってS含有量は0.030%以下である。S含有量の好ましい上限は0.020%であり、さらに好ましくは0.015%であり、さらに好ましくは0.012%であり、さらに好ましくは0.010%である。S含有量の過剰な低減は製造コストを高める。したがって、S含有量の好ましい下限は0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%であり、さらに好ましくは0.005%である。
アルミニウム(Al)は、鋼を脱酸する。Al含有量が0.005%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。一方、Al含有量が0.190%を超えれば、上記効果が飽和する。したがって、Al含有量は0.005~0.190%である。Al含有量の好ましい下限は0.008%であり、さらに好ましくは0.010%である。Al含有量の好ましい上限は0.180%であり、さらに好ましくは0.170%であり、さらに好ましくは0.150%であり、さらに好ましくは0.120%であり、さらに好ましくは0.100%であり、さらに好ましくは0.080%であり、さらに好ましくは0.060%であり、さらに好ましくは0.050%である。
窒素(N)は、不可避に含有される不純物である。つまり、N含有量は0%超である。N含有量が0.0200%を超えれば、AlNが粗大化して、鋼の靭性を低下する。したがって、N含有量は0.0200%以下である。N含有量の好ましい上限は、0.0100%であり、さらに好ましくは0.0080%であり、さらに好ましくは0.0070%であり、さらに好ましくは0.0060%である。N含有量はなるべく低い方が好ましい。しかしながら、N含有量の過剰な低減は製造コストを引き上げる。したがって、通常の工業生産を考慮すれば、N含有量の好ましい下限は0.0001%であり、さらに好ましくは0.0010%であり、さらに好ましくは0.0025%である。
ニオブ(Nb)は、鉄道車輪の製造工程中の踏面焼入れのための加熱時において、Cと結合して微細なNbCを生成する。微細なNbCはピンニング粒子として機能して、加熱時におけるオーステナイトの粗大化を抑制する。そのため、旧オーステナイト結晶粒が微細なまま維持され、鋼の焼入れ性が抑制される。その結果、鉄道車輪の製造工程中における焼入れ層の生成を抑制する。Nbはさらに、旧オーステナイト結晶粒の粗大化を抑制することにより、鋼材の靭性も高める。Nb含有量が0.005%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。一方、Nb含有量が0.050%を超えれば、他の元素含有量が本実施形態の範囲内であっても、NbCが粗大化して鋼材の靭性がかえって低下する。したがって、Nb含有量は0.005~0.050%である。Nb含有量の好ましい下限は0.007%であり、さらに好ましくは0.009%である。Nb含有量の好ましい上限は0.040%であり、さらに好ましくは0.035%であり、さらに好ましくは0.030%であり、さらに好ましくは0.025%であり、さらに好ましくは0.023%である。
本実施形態の鉄道車輪の化学組成はさらに、Feの一部に代えて、Crを含有してもよい。
クロム(Cr)は、任意元素であり、含有されなくてもよい。つまり、Cr含有量は0%であってもよい。含有される場合、Crは、パーライトのラメラ間隔を狭める。これにより、パーライトの硬度が顕著に増大する。しかしながら、Cr含有量が0.25%を超えれば、他の元素含有量が本実施形態の範囲内であっても、焼入れ性が過剰に高くなり、踏面焼入れ後の焼入れ層の厚さが過剰に増大する。したがって、Cr含有量は0~0.25%である。Cr含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.02%であり、さらに好ましくは0.03%である。Cr含有量の好ましい上限は0.24%であり、さらに好ましくは0.23%であり、さらに好ましくは0.22%である。
バナジウム(V)は任意元素であり、含有されなくてもよい。つまり、V含有量は0%であってもよい。含有される場合、Vは、炭化物、窒化物、及び炭窒化物のいずれかを形成して、鋼(具体的には鋼中のフェライト)を析出強化する。その結果、鉄道車輪の硬さが増大して、耐摩耗性をさらに高める。しかしながら、V含有量が0.12%を超えれば、焼入れ性が高くなり、踏面焼入れ後の焼入れ層の厚さが過剰に増大する。したがって、V含有量は0~0.12%である。V含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.02%であり、さらに好ましくは0.03%である。V含有量の好ましい上限は0.11%であり、さらに好ましくは0.10%である。
本実施形態の鉄道車輪1のリム部4のミクロ組織において、パーライトの面積率は95.0%以上であり、初析セメンタイトの面積率は0.1~1.5%である。リム部4のミクロ組織のうち、パーライト及び初析セメンタイト以外の相はたとえば、マルテンサイト及び/又はベイナイトである。初析セメンタイトの面積率の下限は0.2%であってもよく、0.3%であってもよい。初析セメンタイトの面積率の好ましい上限は1.4%であり、さらに好ましくは1.3%であり、さらに好ましくは1.2%であり、さらに好ましくは1.1%であり、さらに好ましくは1.0%であり、さらに好ましくは0.9%である。
本実施形態の鉄道車輪の化学組成は、好ましくは、式(1)を満たす。
100Nb/(C+0.5Si+0.8Mn+15Al+40Cr+10V)≧0.070 (1)
ここで、式(1)中の元素記号には、対応する元素の含有量が質量%で代入される。
上述の鉄道車輪を製造する方法の一例を説明する。本製造方法は、鉄道車輪用鋼を製造する工程(素材製造工程)と、熱間加工により、鉄道車輪用鋼から車輪形状の中間品を成形する工程(成形工程)と、成形された中間品に対して熱処理(踏面焼入れ)を実施する工程(熱処理工程)と、熱処理後の中間品の踏面等から焼入れ層を切削加工により除去して鉄道車輪とする工程(切削加工工程)とを含む。以下、各工程について説明する。
素材製造工程では、電気炉又は転炉等を用いて上述の化学組成を有する溶鋼を製造する。製造した溶鋼を鋳造して鋳造材(鋳片又はインゴット)にする。連続鋳造による鋳片を製造してもよいし、鋳型によって鋳込んでインゴットを製造してもよい。
成形工程では、準備された鉄道車輪用鋼材を用いて、熱間加工により車輪形状の中間品を成形する。中間品は車輪形状を有するため、ボス部と、板部と、踏面及びフランジ部を含むリム部とを備える。熱間加工はたとえば、熱間鍛造、熱間圧延等である。
熱処理工程では、成形された車輪形状の中間品に対して踏面焼入れを実施する。具体的には、成形工程(熱間鍛造又は熱間圧延)後の中間品をAcm変態点以上に再加熱する(再加熱処理)。加熱後、中間品の踏面及びフランジ部を急冷(踏面焼入れ)する。たとえば、冷却媒体により踏面及びフランジ部を冷却する。冷却媒体はたとえば、エアー、ミスト、スプレーであり、所望の組織に合った冷却速度が得られるものであれば特に限定されるものではない。なお、踏面焼入れ時において、板部及びボス部は水冷せずに放冷する。これにより、リム部において、初析セメンタイトの面積率を1.5%以下に抑えることができるものの、初析セメンタイトの面積率が0.1%以上となる。
上述のとおり、熱処理後の中間品の踏面の表層には微細パーライトが形成されるが、その上層には焼入れ層が形成されている。鉄道車輪の使用において、焼入れ層の耐摩耗性は低いため、切削加工により焼入れ層を除去する。切削加工は周知の方法で行えば足りる。
表2に示す化学組成を有する試験番号1~18の溶鋼を製造した。
鉄道車輪の製造工程中の踏面焼入れを模擬した模擬踏面焼入れ試験を実施して、模擬踏面焼入れ試験後のパーライト面積率、初析セメンタイト面積率、及び、焼入れ層深さを以下のとおり求めた。
各試験番号の丸棒の表面から径方向にD/4深さ位置(「D」は、丸棒の直径)から、直径3mm、長さ10mmの熱処理試験片を作製した。熱処理試験片の長手方向は、丸棒の中心軸方向と平行であった。
焼入れ層の深さについて、ジョミニ式一端焼入れ試験を実施した。ジョミニ式一端焼入れ試験は次の方法で実施した。各試験番号の直径40mmの丸棒から、直径25mm、長さ100mmのジョミニ試験片を作製した。ジョミニ試験片の中心軸は、丸棒の中心軸と一致した。ジョミニ試験片を用いて、JIS G0561(2011)に準拠したジョミニ式一端焼入れ試験を実施した。具体的には、ジョミニ試験片を大気雰囲気中、Acm変態点以上の温度である950℃の炉内で30分保持して、ジョミニ試験片の組織をオーステナイト単相とした。その後、一端焼入れ(水冷)を実施した。具体的には、ジョミニ試験片の一端に水を噴射して冷却した。
各試験番号の丸棒の表面から径方向にD/4深さ位置から、直径3mm、長さ10mmの熱処理試験片を作製した。熱処理試験片の長手方向は、丸棒の中心軸の方向と平行であった。
旧オーステナイト結晶粒の粒径=2本の対角線の総長さ/対角線に交差する初析セメンタイトの総本数
次の方法により、各試験番号の丸棒の靭性を評価した。具体的には、各試験番号の丸棒から、幅12mm、高さ12mm、長さ70mmの角棒状熱処理素材を4本ずつ採取した。角棒状熱処理素材は丸棒の中心軸から半径4mmの範囲を避けて採取した。角棒状熱処理素材の長手方向は、丸棒の長手方向に平行であった。
試験結果を表2に示す。表2を参照して、いずれの試験番号においても、ミクロ組織は実質的にパーライトからなる組織であった。つまり、パーライト面積率が95.0%以上であった。
2 ボス部
3 板部
4 リム部
41 踏面
42 フランジ部
Claims (3)
- 鉄道車輪であって、
リム部と、
ボス部と、
前記リム部と前記ボス部との間に配置され、前記リム部と前記ボス部とにつながる板部とを備え、
前記鉄道車輪の化学組成は、質量%で、
C:0.80~1.15%、
Si:1.00%以下、
Mn:0.10~1.20%、
P:0.050%以下、
S:0.030%以下、
Al:0.005~0.190%、
N:0.0200%以下、
Nb:0.005~0.050%、
Cr:0~0.25%、
V:0~0.12%、及び、
残部がFe及び不純物からなり、
前記鉄道車輪の前記リム部のミクロ組織において、
初析セメンタイト面積率が0.1~1.5%であり、
パーライト面積率が95.0%以上である、
鉄道車輪。 - 請求項1に記載の鉄道車輪であって、
前記化学組成は式(1)を満たす、
鉄道車輪。
100Nb/(C+0.5Si+0.8Mn+15Al+40Cr+10V)≧0.070 (1)
ここで、式(1)中の元素記号には、対応する元素の含有量が質量%で代入される。 - 請求項1又は請求項2に記載の鉄道車輪であって、
前記化学組成は、
Cr:0.02~0.25%、及び、
V:0.02~0.12%、
からなる群から選択される1元素以上を含有する、
鉄道車輪。
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