WO2018181862A1 - 鉄道車輪の製造方法及び鉄道車輪 - Google Patents
鉄道車輪の製造方法及び鉄道車輪 Download PDFInfo
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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
- C21D11/00—Process control or regulation for heat treatments
- C21D11/005—Process control or regulation for heat treatments for cooling
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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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- B—PERFORMING OPERATIONS; TRANSPORTING
- B60—VEHICLES IN GENERAL
- B60B—VEHICLE WHEELS; CASTORS; AXLES FOR WHEELS OR CASTORS; INCREASING WHEEL ADHESION
- B60B17/00—Wheels characterised by rail-engaging elements
- B60B17/0006—Construction of wheel bodies, e.g. disc 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
- 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
- C21D1/62—Quenching devices
- C21D1/667—Quenching devices for spray quenching
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/002—Heat treatment of ferrous alloys containing Cr
-
- 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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
-
- 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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
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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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- 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
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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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
- 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
- C22C38/06—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
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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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- B—PERFORMING OPERATIONS; TRANSPORTING
- B60—VEHICLES IN GENERAL
- B60B—VEHICLE WHEELS; CASTORS; AXLES FOR WHEELS OR CASTORS; INCREASING WHEEL ADHESION
- B60B17/00—Wheels characterised by rail-engaging elements
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B60—VEHICLES IN GENERAL
- B60B—VEHICLE WHEELS; CASTORS; AXLES FOR WHEELS OR CASTORS; INCREASING WHEEL ADHESION
- B60B17/00—Wheels characterised by rail-engaging elements
- B60B17/0065—Flange details
- B60B17/0068—Flange details the flange being provided on a single side
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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
- C21D2221/00—Treating localised areas of an article
-
- 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
- the present invention relates to a method for manufacturing a railway wheel and a railway wheel.
- the railway vehicle travel on the rails that make up the tracks.
- the railway vehicle includes a plurality of railway wheels.
- the railway wheel supports the vehicle, contacts the rail, and moves while rotating on the rail.
- Railway wheels wear due to contact with the rails.
- an increase in the weight of the railway vehicle and an increase in the speed of the railway vehicle are being promoted.
- Patent Document 1 Japanese Patent Application Laid-Open No. 9-202937
- Patent Document 2 Japanese Patent Application Laid-Open No. 2012-107295
- Patent Document 3 Japanese Patent Application Laid-Open No. 2013-231212
- Patent Document 4 Unexamined-Japanese-Patent No. 2004-315928
- the railway wheel disclosed in Patent Document 1 is mass%, 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: 0.03% or less, S: 0.03% or less, and the balance consists of Fe and other inevitable impurities.
- a region from the surface of the wheel tread part to a depth of at least 50 mm is made of a pearlite structure.
- the wheel tread portion is provided 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 long time side from the martensitic transformation curve. Includes a quenching process to cool.
- the steel for wheels disclosed in Patent Document 2 is in mass%, C: 0.65 to 0.84%, Si: 0.02 to 1.00%, Mn: 0.50 to 1.90%, Cr : 0.02 to 0.50%, V: 0.02 to 0.20%, S ⁇ 0.04%, the balance is Fe and impurities, P ⁇ 0.05%, Cu ⁇ 0.20 %, Ni ⁇ 0.20%.
- This chemical composition further satisfies the following relationship: [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].
- Patent Document 2 describes that this vehicle 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 is in mass%, C: 0.65 to 0.84%, Si: 0.4 to 1.0%, Mn: 0.50 to 1.40%, Cr : 0.02 to 0.13%, S: 0.04% or less, V: 0.02 to 0.12%, Fn1 defined by the formula (1) is 32 to 43, and the formula Fn2 represented by (2) is 25 or less, and the balance consists of Fe and impurities.
- Patent Document 3 describes that this vehicle steel has the above-mentioned chemical composition, and Fn1 and Fn2 satisfy the above ranges, thereby being excellent in wear resistance, rolling fatigue resistance, and spoke ring resistance. Yes.
- the wheel for a railway vehicle disclosed in Patent Document 4 is mass%, C: 0.85 to 1.20%, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, If necessary, further contains a predetermined amount of one or more of Cr, Mo, V, Nb, B, Co, Cu, Ni, Ti, Mg, Ca, Al, Zr, and N, with the balance being Fe and
- it is an integrated railway vehicle wheel made of steel containing a chemical component consisting of inevitable impurities, and at least a part of the tread surface and / or the flange surface of the wheel is a pearlite structure.
- Patent Document 4 the life of a rail vehicle wheel depends on the amount of wear on the tread surface and flange surface (paragraph [0002] of Patent Document 4), and further, the amount of heat generated when braking is applied in a high-speed railway. It is described that it depends on the cracks in the tread surface and the flange surface that occur. And it is described that the abrasion resistance and the thermal crack of a tread surface and a flange surface can be suppressed because the wheel for railway vehicles has the said structure.
- the railway wheel disclosed in Patent Document 4 is made of hypereutectoid steel with an increased C content as compared with Patent Documents 1 to 3.
- this railway wheel is applied to a freight railway that requires increased loading weight and higher speed, there is a possibility that sufficient wear resistance will be obtained.
- railway wheels are manufactured by the following method.
- the steel piece is hot-worked to form a railway wheel-shaped intermediate product.
- Heat treatment stamp surface quenching
- tread quenching after heating an intermediate product, cooling water is injected onto the tread surface and flange of the intermediate product to quench it.
- boss portion and the plate portion are allowed to cool.
- a layer made of martensite and / or bainite is further formed on the upper layer of the fine pearlite on the surface layer immediately below the tread surface and the surface layer portion of the flange after quenching the tread surface.
- the martensite and / or bainite layer formed in the surface layer of the tread surface and the surface layer of the flange by rapid cooling of the tread surface and the flange after the heat treatment is referred to as a “quenched layer” in the present specification.
- the hardened layer formed on the surface layer of the tread and the surface layer of the flange is removed by cutting to the intermediate product of the railway wheel after the step surface quenching, and the fine pearlite is treaded. And exposed on the surface of the flange.
- Conventional rail wheels are manufactured by the above manufacturing process.
- An object of the present invention is to provide a railway wheel manufacturing method and a railway wheel that can stably produce a hypereutectoid steel railway wheel having excellent toughness.
- a method for manufacturing a railway wheel according to an embodiment of the present invention includes a heating step and a cooling step.
- C 0.80 to 1.15%
- Si 1.00% or less
- Mn 0.10 to 1.25%
- P 0.050% or less
- S 0.005% by mass.
- An intermediate product of a railway wheel having a chemical composition composed of Fe and impurities in the balance including a boss portion, a rim portion including a tread surface and a flange, and a plate portion disposed between the boss portion and the rim portion, Heat above the A cm transformation point (° C.).
- the cooling rate at 800 to 500 ° C. on the surface other than the tread surface and the flange surface is Fn 1 ° C./second or less as defined by the formula (1), and the cooling rate in the intermediate product is the slowest.
- the cooling rate at 800 to 500 ° C. is not less than Fn 2 ° C./second defined by the formula (2), and the cooling rate at 800 to 500 ° C. on the tread surface and flange surface is not less than Fn 2 ° C./second. Cool the product.
- Fn1 ⁇ 5.0 + exp (5.651-1.427 ⁇ C ⁇ 1.280 ⁇ Si ⁇ 0.7723 ⁇ Mn ⁇ 1.815 ⁇ Cr ⁇ 1.519 ⁇ Al ⁇ 7.798 ⁇ V) (1)
- Fn2 0.515 + exp ( ⁇ 24.816 + 24.121 ⁇ C + 1.210 ⁇ Si + 0.529 ⁇ Mn + 2.458 ⁇ Cr-15.116 ⁇ Al-5.116 ⁇ V) (2)
- the content (mass%) of the corresponding element is substituted for each element symbol in the above formulas (1) and (2).
- the railway wheel according to the present embodiment is, in mass%, C: 0.80 to 1.15%, Si: 1.00% or less, Mn: 0.10 to 1.25%, P: 0.050% or less, S: 0.030% or less, Al: 0.025 to 0.650%, N: 0.0030 to 0.0200%, Cr: 0 to 0.60%, and V: 0 to 0.12%,
- the remainder has a chemical composition composed of Fe and impurities, and includes a boss portion, a rim portion including a tread surface and a flange, and a plate portion disposed between the boss portion and the rim portion.
- the pearlite area ratio is 95% or more, and the amount of proeutectoid cementite defined by the formula (A) is 1.0 / 100 ⁇ m or less.
- the pearlite area ratio is 95% or more, and the amount of pro-eutectoid cementite defined by the formula (A) is 1.0 / 100 ⁇ m or less.
- the area ratio of pearlite is 95% or more, and the amount of pro-eutectoid cementite defined by the formula (A) is 1.0 / 100 ⁇ m or less.
- the rail wheel manufacturing method according to the present embodiment can stably manufacture a hypereutectoid steel rail wheel having excellent toughness.
- FIG. 1 is a cross-sectional view parallel to the central axis of a railway wheel.
- FIG. 2 is a diagram showing the relationship between the Vickers hardness of the railway wheel and the wear amount of the railway wheel based on the result of the Nishihara-type wear test.
- FIG. 3 is a schematic diagram of the Nishihara type abrasion test.
- FIG. 4 is a diagram showing the relationship between the C content, the cooling rate, the hardened layer, and proeutectoid cementite based on the results of a heat treatment test assuming heat treatment during the manufacturing process of the railway wheel.
- FIG. 1 is a cross-sectional view parallel to the central axis of a railway wheel.
- FIG. 2 is a diagram showing the relationship between the Vickers hardness of the railway wheel and the wear amount of the railway wheel based on the result of the Nishihara-type wear test.
- FIG. 3 is a schematic diagram of the Nishihara type abrasion test.
- FIG. 4 is a diagram
- FIG. 5 is a diagram showing the relationship between the Si content, the cooling rate, the hardened layer, and proeutectoid cementite based on the results of a heat treatment test assuming heat treatment during the manufacturing process of the railway wheel.
- FIG. 6 is a diagram showing the relationship between the Mn content, the cooling rate, the hardened layer, and proeutectoid cementite based on the results of heat treatment tests assuming heat treatment during the manufacturing process of railway wheels.
- FIG. 7 is a diagram showing the relationship between the Cr content, the cooling rate, the hardened layer, and proeutectoid cementite based on the results of heat treatment tests assuming heat treatment during the manufacturing process of railway wheels.
- FIG. 8 is a diagram showing the relationship between the Al content, the cooling rate, the hardened layer, and proeutectoid cementite based on the results of a heat treatment test assuming heat treatment during the manufacturing process of the railway wheel.
- FIG. 9 is a diagram showing the relationship between the V content, the cooling rate, the hardened layer, and proeutectoid cementite based on the results of a heat treatment test assuming heat treatment during the manufacturing process of the railway wheel.
- FIG. 10 is a schematic diagram illustrating an example of a cooling device used in the method for manufacturing a railway wheel according to the present embodiment.
- FIG. 11 is a schematic diagram for explaining a method for measuring the amount of proeutectoid cementite.
- FIG. 12 is a diagram showing the distribution (Jomini curve) of the Rockwell hardness HRC with respect to the distance from the water-cooled end of the Jomini test piece obtained by the Jomini type one-end quenching test in the example.
- FIG. 1 is a cross-sectional view including a central axis of a railway wheel.
- railway 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 center portion of the railway wheel 1.
- the boss 2 has a through hole 21.
- the central axis of the through hole 21 coincides with the central axis of the railway wheel 1.
- An axle shaft (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 of the railway wheel 1.
- the rim portion 4 includes a tread surface 41 and a flange 42.
- the tread surface 41 is connected to the flange 42.
- a thickness T4 of the rim portion 4 is thicker than a thickness T3 of the plate portion 3.
- the plate portion 3 is disposed between the boss portion 2 and the rim portion 4.
- 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 examined a method for increasing the wear resistance of railway wheels. As a result, the present inventors obtained the following knowledge.
- FIG. 2 is a diagram showing the relationship between the Vickers hardness of the railway wheel and the wear amount of the railway wheel based on the result of the Nishihara-type wear test.
- FIG. 2 was obtained by the following experiment. A round bar having a diameter of 40 mm was produced from an ingot having the chemical composition shown in Table 1.
- An annular coarse test piece (corresponding to an intermediate product of a railway wheel) having a diameter of 32 mm and a width of 10 mm was prepared from a round bar.
- the outer peripheral surface of the coarse test piece after being allowed to cool was cut to produce a cylindrical wheel test piece 100 (corresponding to a railway wheel) shown in FIG.
- the wheel specimen 100 had a diameter D100 of 29.39 mm and a width W100 of 8 mm.
- steel number 29 shown in Table 2 was prepared as a rail material.
- An annular rail test piece 200 shown in FIG. 3 was prepared from the rail material of steel number 29.
- the diameter D200 of the rail test piece 200 was 30.0 mm, and the width W200 was 5 mm.
- the metal structure at a depth of 2 to 3 mm from the outer peripheral surface of the wheel specimen 100 toward the central axis was observed at 500 times using an optical microscope.
- the metal structure at a depth of 2 to 3 mm from the outer peripheral surface of the rail test piece 200 toward the central axis was observed with an optical microscope at a magnification of 500 times.
- the structures of the wheel test pieces 100 of steel numbers 1 to 4 and 21 to 28 were all pearlite single phase, and the structure of the rail test piece 200 was also pearlite single phase.
- a Vickers hardness test in accordance with JIS Z2244 (2009) was performed on the wheel test piece 100 at the same position as the structure observation, that is, a position having a depth of 2 to 3 mm from the outer peripheral surface toward the central axis.
- the test force was 2.9421N in all cases.
- a Vickers hardness test according to JIS Z2244 (2009) was performed on the rail test piece 200 at the same position as in the structure observation, that is, at a position having a depth of 2 to 3 mm from the outer peripheral surface toward the central axis.
- the test force was 2.9421N.
- the Vickers hardness (HV) of the rail test piece 200 was 430.
- the wheel test piece 100 and the rail test piece 200 are rotated against each other while being brought into contact with the center of the outer peripheral surface of the wheel test piece 100 and the width center of the outer peripheral face of the rail test piece 200 and pressed against each other with a force of 900 MPa.
- the test was conducted.
- the rotation speed of the wheel test piece 100 was 800 rpm, and the rotation speed of the rail test piece 200 was 775 rpm. Therefore, the slip ratio between the wheel test piece 100 and the rail test piece 200 was 1.1%.
- the mass (g) of the wheel test piece 100 after the test was obtained.
- the difference between the mass (g) of the pre-test wheel test piece 100 measured in advance before the test and the mass (g) of the post-test wheel test piece 100 is obtained, and this mass difference is divided by 50.
- the value obtained was defined as the amount of wheel wear (g / 10,000 rev.).
- Four wheel test pieces 100 were prepared for each steel number, and the same test was performed four times for each steel number using these.
- the average value of the amount of wear of the wheel specimen 100 obtained by the four tests was calculated as the amount of wear of the railway wheel of each steel number.
- FIG. 2 was created using the Vickers hardness and wear amount of the wheel specimen 100 obtained for each steel number.
- “ ⁇ ” indicates a steel group containing no V (hereinafter referred to as “V” -free steel group in which the Si content is approximately constant at about 0.3% and the C content is changed from 0.8 to 1.1%. It is a test result using “V-free hypereutectoid steel group”. “ ⁇ ” indicates that the C content is within the range of 0.75 to 0.79%, the Si content is approximately constant at approximately 0.3%, and the V content is approximately 0 to 0.1%. It is the test result using the steel group (henceforth "V content change low Si eutectoid steel group") changed to 1%.
- the Vickers hardness of the wheel increased as the V content increased. Specifically, as V content does not increase (steel 21) and V content increases to 0.028% (steel 22), 0.058% (steel 23), and 0.097% (steel 24), Vickers Hardness increased. However, the Vickers hardness is only about 350 HV, and the wear amount is also 0.015 g / 10000 rev. It decreased only to the extent.
- the C content is 0.84% (steel 1), 0.93% (steel 2), 1.00% (steel 3).
- Vickers hardness increased as it increased to 1.09% (steel 4).
- the wear amount is 0.010 g / 10000 rev. Decreased to a degree.
- the wear resistance of the railway wheel is increased by the above mechanism.
- V the hardness of the steel is increased by precipitation strengthening of V carbonitride.
- V carbonitride is produced in the ferrite, the hardness of the ferrite is mainly increased. That is, the inclusion of V does not significantly affect the pearlite refinement. Therefore, although the wear resistance can be increased to some extent by containing V, the wear resistance cannot be increased as much as the dispersion strengthening by crushed cementite and the solid solution strengthening of C.
- the present inventors set the chemical composition of the railway wheel in mass%, C: 0.80 to 1.15%, Si: 1.00. %: Mn: 0.10 to 1.25%, P: 0.050% or less, S: 0.030% or less, Al: 0.025 to 0.650%, N: 0.0030 to 0.0200 %, Cr: 0 to 0.60%, and V: 0 to 0.12%, with the balance being preferably a hypereutectoid steel composed of Fe and impurities.
- the railway wheel is manufactured by performing heat treatment (stepping quenching) on an intermediate product of the railway wheel.
- Abrasion resistance is required for treads and flanges that can come into contact with rails in railway wheels. Therefore, in the conventional heat treatment of intermediate products during the manufacturing process of railway wheels, in order to form a fine pearlite structure on the surface layer and the flange layer immediately below the tread surface, the tread surface and flange of the rim part of the intermediate product of the railway wheel are formed.
- a cooling medium water or a mixed fluid of water and air
- cooling surfaces are not sprayed on surfaces other than the treads and flange surfaces of railroad wheels (the surface of the boss, the surface of the plate, and the side of the rim) without spraying the cooling medium. It was. As described above, wear resistance is required for the tread surface and the flange surface of the rim portion, and on the surfaces other than the tread surface and the flange surface of the railway wheel (boss surface, plate surface, and rim surface). This is because wear resistance is not required.
- proeutectoid cementite is difficult to form.
- a hypereutectoid steel having a C content of 0.80% or more as in the chemical composition described above if a railway wheel is produced by a conventional production method, proeutectoid cementite may be generated inside the railway wheel.
- pro-eutectoid cementite is likely to be generated in the boss and plate portions, which were conventionally allowed to cool in tread quenching.
- Proeutectoid cementite reduces toughness.
- the toughness of the railway wheel is reduced. Therefore, on the other surfaces (boss surface, plate surface and rim side surface) except the tread surface and flange surface from which the hardened layer is removed by cutting, the generation of proeutectoid cementite is suppressed, It is preferable that generation can be suppressed.
- the present inventors investigated and examined a method for suppressing proeutectoid cementite not only in the rim part including the tread and the flange but also in the plate part and the boss part in the manufacturing process of the railway wheel. As a result, the present inventors obtained the following knowledge.
- FIG. 4 to 9 show the content of each element in steel based on the results of heat treatment tests assuming heat treatment during the manufacturing process of railway wheels (FIG. 4: C content, FIG. 5: Si content, FIG. 6). : Mn content, Fig. 7: Cr content, Fig. 8: Al content, Fig. 9: V content), average cooling rate (° C / sec) at 800-500 ° C, quenching layer and proeutectoid cementite It is a figure which shows the relationship.
- FIG. 4 is based on the results obtained in the Jomini-type one-end quenching test described later using a plurality of samples (steel numbers 1, 2, 3, 4 in Table 3 described later) with varying C content. It was created.
- FIG. 5 is created based on the results obtained in the Jomini type one-end quenching test using a plurality of samples (steel numbers 5, 3, and 6 in Table 3 to be described later) with varying Si content. is there.
- FIG. 6 is created based on the results obtained in the Jomini-type one-end quenching test using a plurality of samples (steel numbers 7, 3, and 8 in Table 3 to be described later) with varying Mn contents. is there.
- FIG. 7 was created based on the results obtained in the Jomini-type one-end quenching test using a plurality of samples with varying Cr contents (steel numbers 3, 9, 10, and 11 in Table 3 to be described later).
- FIG. 8 shows the results obtained in the Jomini-type one-time quenching test using a plurality of samples (steel numbers 3, 12, 13, 14, 15, 16 in Table 3 described later) with varying Al content. Based on this.
- FIG. 9 is created based on the results obtained in the Jomini-type one-end quenching test using a plurality of samples with varying V contents (steel numbers 3, 17, and 18 in Table 3 to be described later). is there.
- the “ ⁇ ” mark means that a hardened layer (martensite and / or bainite) was formed. “ ⁇ ” indicates that no hardened layer is formed, the microstructure is substantially pearlite, the amount of pro-eutectoid cementite in the micro-structure is 1.0 / 100 ⁇ m or less, and the pro-eutectoid cementite is substantially Means it didn't exist. “X” mark indicates that no quenching layer is formed in the microstructure and the microstructure is substantially made of pearlite, but the amount of pro-eutectoid cementite exceeds 1.0 / 100 ⁇ m, and the eutectoid in the microstructure This means that cementite was generated.
- the microstructure is substantially composed of pearlite means that the area ratio of pearlite in the microstructure is 95% or more.
- the measuring method of proeutectoid cementite amount (this / 100 micrometer) is mentioned later.
- the maximum cooling rate (cooling rate at the boundary between “ ⁇ ” mark and “ ⁇ ” mark in FIG. 4) in which pearlite is generated in the structure and the hardened layer is not generated is expressed as pearlite.
- critical cooling rate. 4 to 9 the pearlite critical cooling rate is indicated by a broken line. Referring to FIG. 4, the pearlite critical cooling rate decreases as the C content increases. Referring to FIG. 5, the pearlite critical cooling rate decreases as the Si content increases. Referring to FIG. 6, the pearlite critical cooling rate decreases as the Mn content increases. Referring to FIG.
- the pearlite critical cooling rate decreases as the Cr content increases.
- the pearlite critical cooling rate decreases as the Al content increases.
- the pearlite critical cooling rate decreases as the V content increases. That is, with reference to FIGS. 4 to 9, C, Si, Mn, Cr, Al, and V all have an action of lowering the pearlite critical cooling rate.
- proeutectoid cementite may be generated in the structure. Referring to FIG. 4, if the C content is increased, pro-eutectoid cementite is generated even if the cooling rate is high.
- the maximum cooling rate (the cooling rate at the boundary between “ ⁇ ” and “ ⁇ ” in the figure) generated when the amount of pro-eutectoid cementite exceeds 1.0 / 100 ⁇ m is determined as pro-eutectoid cementite critical cooling. Defined as speed.
- the pro-eutectoid cementite critical cooling rate is shown by solid lines in FIGS.
- C has an action of increasing the pro-eutectoid cementite critical cooling rate
- Al has an action of lowering the pro-eutectoid cementite critical cooling rate
- the present inventors have obtained a pearlite critical cooling rate, a pro-eutectoid cementite critical cooling rate, a C content, a Si content, a Mn content, a Cr content, an Al content, and a V content.
- the relationship with quantity was further examined.
- the intermediate product of the railway wheel after the heat treatment at the A cm transformation point or higher in the manufacturing process is cooled at 800 to 500 ° C.
- the cooling rate (° C./second) is set to Fn1 or less as an index of the pearlite critical cooling rate and defined by the formula (1), it has been found that the formation of a hardened layer can be suppressed. Further, it has been found that the generation of pro-eutectoid cementite can be suppressed by setting it to Fn2 or more defined by the formula (2) as an index of the pro-eutectoid cementite critical cooling rate.
- Fn1 ⁇ 5.0 + exp (5.651-1.427 ⁇ C ⁇ 1.280 ⁇ Si ⁇ 0.7723 ⁇ Mn ⁇ 1.815 ⁇ Cr ⁇ 1.519 ⁇ Al ⁇ 7.798 ⁇ V) (1)
- Fn2 0.515 + exp ( ⁇ 24.816 + 24.121 ⁇ C + 1.210 ⁇ Si + 0.529 ⁇ Mn + 2.458 ⁇ Cr-15.116 ⁇ Al-5.116 ⁇ V) (2)
- the content (mass%) of a corresponding element is substituted for each element symbol in the formulas (1) and (2).
- 800 to 500 ° C. is a temperature range where pearlite and pro-eutectoid cementite are generated.
- the method for manufacturing a railway wheel according to the present embodiment completed based on the above knowledge includes a heating step and a cooling step.
- C 0.80 to 1.15%
- Si 1.00% or less
- Mn 0.10 to 1.25%
- P 0.050% or less
- S 0.005% by mass.
- An intermediate product of a railway wheel having a chemical composition composed of Fe and impurities in the balance including a boss portion, a rim portion including a tread surface and a flange, and a plate portion disposed between the boss portion and the rim portion, Heat above the A cm transformation point.
- the intermediate product is cooled.
- the cooling rate at 800 to 500 ° C. of the surface other than the tread and the flange surface of the intermediate product of the railway wheel is Fn 1 ° C./sec or less as defined by the equation (1).
- the cooling rate at 800 to 500 ° C. in the slowest region is Fn 2 ° C./second or more as defined by the equation (2), and among the intermediate products of the railway wheel, at 800 to 500 ° C. on the tread surface and the flange surface.
- the intermediate product is cooled so that the cooling rate becomes Fn2 ° C./second or more.
- Fn1 ⁇ 5.0 + exp (5.651-1.427 ⁇ C ⁇ 1.280 ⁇ Si ⁇ 0.7723 ⁇ Mn ⁇ 1.815 ⁇ Cr ⁇ 1.519 ⁇ Al ⁇ 7.798 ⁇ V) (1)
- Fn2 0.515 + exp ( ⁇ 24.816 + 24.121 ⁇ C + 1.210 ⁇ Si + 0.529 ⁇ Mn + 2.458 ⁇ Cr-15.116 ⁇ Al-5.116 ⁇ V) (2)
- the content (mass%) of the corresponding element is substituted for each element symbol in the above formulas (1) and (2).
- the intermediate product is further cooled so that the cooling rate at 800 to 500 ° C. on the tread surface and the flange surface is Fn 2 ° C./second or more and 5 ° C./second or more and 200 ° C./second or less. May be.
- the chemical composition of the intermediate product of the railway wheel may contain one or more selected from the group consisting of Cr: 0.02 to 0.60% and V: 0.02 to 0.12%. Good.
- the railway wheel according to the present embodiment is, in mass%, C: 0.80 to 1.15%, Si: 1.00% or less, Mn: 0.10 to 1.25%, P: 0.050% or less, S: 0.030% or less, Al: 0.025 to 0.650%, N: 0.0030 to 0.0200%, Cr: 0 to 0.60%, and V: 0 to 0.12%,
- the remainder has a chemical composition composed of Fe and impurities, and includes a boss portion, a rim portion including a tread surface and a flange, and a plate portion disposed between the boss portion and the rim portion.
- the area ratio of pearlite is 95% or more, and the amount of pro-eutectoid cementite defined by the formula (A) is 1.0 / 100 ⁇ m or less.
- the area ratio of pearlite is 95% or more, and the amount of pro-eutectoid cementite defined by the formula (A) is 1.0 / 100 ⁇ m or less.
- the area ratio of pearlite is 95% or more, and the amount of proeutectoid cementite defined by the formula (A) is 1.0 / 100 ⁇ m or less.
- the chemical composition of the intermediate product of the railway wheel may contain one or more selected from the group consisting of Cr: 0.02 to 0.60% and V: 0.02 to 0.12%. Good.
- the railway wheel of the present embodiment has a shape including a boss portion 2, a plate portion 3, a rim portion 4 including a tread surface 41 and a flange 42.
- the chemical composition of the railway wheel of this embodiment contains the following elements.
- C 0.80 to 1.15% Carbon (C) increases the hardness of the steel and increases the wear resistance. If the C content is too low, these effects cannot be obtained. On the other hand, if the C content is too high, pro-eutectoid cementite precipitates at the prior austenite grain boundaries, and the ductility, toughness and fatigue life of the steel decrease. Therefore, the C content is 0.80 to 1.15%.
- the minimum with preferable C content is 0.85%, More preferably, it is 0.86%, More preferably, it is 0.87%, More preferably, it is 0.90%.
- the upper limit with preferable C content is 1.05%, More preferably, it is 1.00%.
- Si Silicon (Si) is inevitably contained. That is, the Si content is more than 0%. Si enhances the hardness of steel by solid-solution strengthening of ferrite. However, if the Si content is too high, proeutectoid cementite is likely to be generated. If the Si content is too high, the hardenability of the steel becomes too high and martensite is likely to be generated. Further, during use as a railway wheel, there is a case where the steel is cracked by frictional heat generated between the brake and the steel and the crack resistance of the steel is lowered. Therefore, the Si content is 1.00% or less.
- the upper limit with preferable Si content is 0.80%, More preferably, it is 0.65%, More preferably, it is 0.45%, More preferably, it is 0.35%.
- the minimum with preferable Si content is 0.01%, More preferably, it is 0.05%, More preferably, it is 0.20%.
- Mn 0.10 to 1.25%
- Manganese (Mn) enhances the hardness of steel by solid solution strengthening of ferrite. Mn further forms MnS and improves the machinability of the steel. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, the hardenability of the steel becomes too high and martensite is likely to be generated. Further, during use as a railway wheel, there is a case where the steel is cracked by frictional heat generated between the brake and the steel and the crack resistance of the steel is lowered. Therefore, the Mn content is 0.10 to 1.25%.
- the minimum with preferable Mn content is 0.50%, More preferably, it is 0.60%, More preferably, it is 0.70%.
- the upper limit with preferable Mn content is 1.00%, More preferably, it is 0.82%.
- P 0.050% or less Phosphorus (P) is an unavoidable impurity. That is, the P content is more than 0%. P segregates at the grain boundaries and lowers the toughness of the steel. Therefore, the P content is 0.050% or less.
- the upper limit with preferable P content is 0.030%, More preferably, it is 0.020%.
- the P content is preferably as low as possible. However, if the P content is excessively reduced, the refining cost becomes excessively high. Therefore, when considering normal industrial production, the preferable lower limit of the P content is 0.0001%, more preferably 0.0005%.
- S 0.030% or less Sulfur (S) is unavoidably contained. That is, the S content is more than 0%. When S is contained actively, S forms MnS and improves the machinability of steel. However, S decreases the toughness of the steel. Therefore, the S content is 0.030% or less. The upper limit with preferable S content is 0.020%. When the effect of improving machinability is obtained, the preferable lower limit of the S content is 0.001%, more preferably 0.005%.
- Al 0.025 to 0.650%
- Aluminum (Al) suppresses the formation of pro-eutectoid cementite and increases the toughness of steel in the chemical composition of the railway wheel of this embodiment having a C content of 0.80% or more.
- Al is further combined with N to form AlN, and the crystal grains are refined. As the crystal grains become finer, the toughness of the steel increases. If the Al content is too low, these effects cannot be obtained. On the other hand, if the Al content is too high, coarse nonmetallic inclusions increase and the toughness of the steel decreases. Therefore, the Al content is 0.025 to 0.650%.
- the minimum with preferable Al content is 0.030%, More preferably, it is 0.040%, More preferably, it is 0.050%.
- the upper limit with preferable Al content is 0.450%, More preferably, it is 0.350%, More preferably, it is 0.250%, More preferably, it is 0.115%.
- the Al content referred to in the present specification means the content of acid-soluble Al (sol. Al).
- N 0.0030 to 0.0200% Nitrogen (N) combines with Al to form AlN, and crystal grains are refined. The refinement of crystal grains increases the toughness of steel. If the N content is too low, this effect cannot be obtained. On the other hand, if the N content is too high, the effect is saturated. Therefore, the N content is 0.0030 to 0.0200%.
- the minimum with preferable N content is 0.0035%, More preferably, it is 0.0040%.
- the upper limit with preferable N content is 0.0100%, More preferably, it is 0.0080%.
- the balance of the chemical composition of the railway wheel according to the present embodiment is composed of Fe and impurities.
- the impurities are industrially produced from the ore as a raw material, scrap, or production environment, and do not adversely affect the railway wheel of this embodiment. It means what is allowed in the range.
- the chemical composition of the railway wheel according to the present embodiment may further include one or more selected from the group consisting of Cr and V 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 significantly increases the hardness of pearlite by reducing the lamella spacing of pearlite. However, if the Cr content is too high, proeutectoid cementite is likely to be generated. If the Cr content is too high, the hardenability is further increased and martensite is easily generated. Therefore, the Cr content is 0 to 0.60%.
- the upper limit with preferable Cr content is 0.30%, Preferably it is 0.25%, More preferably, it is 0.10%.
- the preferable lower limit of the Cr content when obtaining the effect of reducing the lamella spacing of pearlite is 0.02%.
- 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 strengthen the steel. As a result, the hardness of the railway wheel is remarkably increased and the wear resistance is further enhanced. However, if the V content is too high, the hardenability increases and the thickness of the hardened layer after the tread quenching increases excessively. Therefore, the V content is 0 to 0.12%.
- the upper limit with preferable V content is 0.09%.
- the minimum with preferable V content is 0.02%, More preferably, it is 0.03%.
- the method for manufacturing a railway wheel according to the present embodiment includes a heat treatment step.
- the heat treatment step includes a heating step and a cooling step.
- heating process In the heating step, first, an intermediate product having the above-described chemical composition and having a rough shape of a railway wheel including a boss portion, a plate portion, and a rim portion is prepared.
- the intermediate product is manufactured by the following method.
- the molten steel having the above-described chemical composition is manufactured using an electric furnace or a converter.
- the material is manufactured using molten steel.
- a slab is manufactured by a continuous casting method.
- an ingot is manufactured by the ingot-making method.
- a billet as a raw material is manufactured by carrying out ingot rolling or hot forging on a slab or ingot.
- the raw material may be a slab manufactured by a continuous casting method.
- the shape of the material is preferably a cylindrical shape.
- the above intermediate product is molded.
- the material is cut in a direction perpendicular to the longitudinal direction.
- Hot working in a direction perpendicular to the cut surface to form a disk shape.
- an intermediate product of a railway wheel is formed by hot working so that the wheel has a rough shape. In hot working, for example, hot forging is performed, and then hot rolling (wheel rolling) is performed as necessary.
- An intermediate product is manufactured by the above process.
- the manufactured intermediate product is heated. Specifically, the intermediate product is heated to an A cm transformation point (° C.) or higher.
- the intermediate product is charged into a heating furnace and heated at a temperature (quenching temperature) equal to or higher than the A cm transformation point.
- a temperature quenching temperature
- Well-known conditions are sufficient for the heating rate and the holding time at the quenching temperature.
- the quenching temperature is, for example, 850 to 1000 ° C.
- a cooling process is implemented with respect to the heated intermediate goods.
- the microstructure of the surface layer just below the tread and the surface layer of the flange of the intermediate product of the railway wheel is made into a fine pearlite structure with high wear resistance.
- a hardened layer (a layer made of martensite and / or bainite) may be formed somewhat on the fine pearlite layer.
- the hardened layer is removed by cutting in a subsequent process.
- the surface of the intermediate product other than the tread surface and the flange surface suppresses the formation of a hardened layer in the microstructure.
- the microstructure is substantially pearlite (perlite is 95% or more in area ratio).
- the surface other than the tread surface and the flange surface means the surface of the plate portion, the surface of the boss portion, and the surface other than the tread surface and the flange surface of the rim portion. Suppressing the formation of a hardened layer on the surface other than the tread surface and the flange surface of the intermediate product is because cutting of the generated hardened layer is performed on the surface of the intermediate product other than the tread surface and the flange surface. This is because it is difficult.
- the intermediate product of the railway wheel having the chemical composition which is the hypereutectoid steel described above suppresses the formation of proeutectoid cementite in any region of the intermediate product. That is, in the intermediate product of the railway wheel having the chemical composition which is the hypereutectoid steel described above, the generation of proeutectoid cementite is suppressed not only in the rim portion but also in the plate portion and the boss portion. It is the above hypereutectoid steel by suppressing the formation of a quenching layer in all the microstructures of the rim, plate and boss other than the tread and flange, and suppressing the formation of proeutectoid cementite. Even a railway wheel having a chemical composition can suppress a decrease in toughness.
- the intermediate product at the quenching temperature is cooled so as to satisfy all of the following (A) to (C).
- (A) The cooling rate of the intermediate product other than the tread surface and the flange surface, that is, the boss surface, the plate surface, and the rim side surface (the rim surface other than the tread surface and the flange surface) at 800 to 500 ° C is Fn1 ° C / Cool the intermediate product so that it is less than a second.
- the region where the cooling rate is the slowest at 800 to 500 ° C. that is, the region where the cooling rate is the slowest inside the boss part, the plate part and the rim part (hereinafter referred to as the slowest area)
- the intermediate product is cooled so that the cooling rate at is at least Fn2 ° C./sec.
- the intermediate product is cooled so that the cooling rate at 800 to 500 ° C. of the tread surface and flange surface of the intermediate product is Fn 2 ° C./second or more.
- the cooling rate is set to Fn2 ° C./second or more on the tread surface and the flange surface.
- the upper limit of the cooling rate of the tread surface and the flange surface is not particularly limited. However, if the cooling rate on the tread surface and the flange surface is too fast, the thickness of the hardened layer to be generated increases, and the range that must be removed in the cutting process increases. Therefore, the preferable upper limit of the cooling rate of the tread surface and the flange surface is 200 ° C./second.
- the cooling rate of the tread surface and the flange surface is preferably Fn 2 ° C./second or more and 5 ° C./second or more. In this case, the pearlite structure of the surface layer immediately below the tread surface and the surface layer of the flange becomes finer, and further excellent wear resistance can be obtained.
- the reason why the “cooling rate at 800 to 500 ° C.” is defined is that this temperature range is a temperature range where pearlite transformation occurs and a temperature range where proeutectoid cementite is generated.
- the “cooling rate at 800 to 500 ° C.” means an average cooling rate (° C./second) at 800 to 500 ° C. in each region of the intermediate product of the railway wheel.
- the cooling speed on the surface and inside of the intermediate product varies depending on the shape of the intermediate product (that is, the railway wheel) and the cooling method.
- the temperature change of the surface of the intermediate product during cooling (that is, the cooling rate at each part) can be specified by using a heat distribution measuring device represented by thermography. Therefore, the cooling rate in the slowest region can also be specified by the heat distribution measuring device.
- FIG. 10 is a side view of the cooling device 10 used in the cooling process.
- the cooling device 10 includes a rotating device 11 having a rotating shaft and a plurality of cooling nozzles 12-14.
- the plurality of cooling nozzles 12 to 14 includes one or more tread surface cooling nozzles 14, one or more plate part cooling nozzles 13, and one or more boss part cooling nozzles 12.
- One or a plurality of tread cooling nozzles 14 are arranged around the rotation axis as in the prior art.
- the nozzle opening of the tread cooling nozzle 14 is arranged to face the tread 41 of the intermediate product.
- the nozzle opening of the tread surface cooling nozzle 14 may be disposed to face the surface of the intermediate flange 42.
- One or a plurality of plate portion cooling nozzles 13 are arranged such that the nozzle openings face the surface of the plate portion 3.
- One or a plurality of boss portion cooling nozzles 12 are arranged such that the nozzle opening faces the surface of the boss portion 2.
- the tread surface cooling nozzle 14 injects a cooling medium from the nozzle opening, and mainly cools the surfaces of the tread surface 41 and the flange 42 of the rim portion 4.
- the plate portion cooling nozzle 13 mainly cools the plate portion 3 by ejecting a cooling medium from the nozzle opening.
- the boss portion cooling nozzle 12 mainly cools the boss portion 2 by jetting a cooling medium from the nozzle opening.
- the tread surface cooling nozzle 14 may cool not only the surface of the tread surface 41 and the flange 42 of the rim portion 4 but also at least a part of the plate portion 3.
- the plate portion cooling nozzle 13 may cool not only the plate portion 3 but also at least a portion of the rim portion 4 and / or at least a portion of the boss portion 2.
- the boss portion cooling nozzle 12 may cool not only the boss portion 2 but also at least a part of the plate portion 3.
- the arrangement and number of the tread surface cooling nozzle 14, the plate portion cooling nozzle 13, and the boss portion cooling nozzle 12 in FIG. 10 are merely examples, and are not limited thereto.
- the configuration of the plurality of cooling nozzles of the cooling device is not particularly limited as long as cooling satisfying the above (A) to (C) is possible in the cooling step.
- the cooling medium is not particularly limited as long as a cooling rate suitable for a desired tissue can be obtained.
- the cooling medium is, for example, water, air (air), mist, brackish water (spray) or the like.
- the cooling device 10 further includes one or more thermography (infrared heat distribution measuring device) 20.
- the thermography 20 is arranged so as to be able to measure the upper surface temperature, the lower surface temperature, the side surface temperature, and the internal temperature of the intermediate product when the intermediate product of the railway wheel is mounted on the cooling device 10.
- the arrangement and the number of the thermography 20 in FIG. 10 are examples, and are not limited thereto.
- the plurality of thermography 20 includes a tread surface 41, a surface of the flange 42, a surface other than the surfaces of the tread surface 41 and the flange 42 (for example, side surfaces of the rim portion 4), and the surface of the plate portion 3.
- the temperature distribution of the surface of the boss part 2 are arranged so as to be measurable.
- a sample intermediate product heated above the A cm transformation point (a sample product having the same shape and composition as the intermediate product of the railway wheel that is actually the product and intended for temperature measurement) is placed in the cooling device 10. To do. While rotating the sample intermediate product by the rotating device 11, the cooling medium is ejected from each of the cooling nozzles 12 to 14, and cooling is started. During cooling, the temperature distribution of the sample intermediate product is measured by a plurality of thermographs 20.
- the plurality of thermographs 20 are connected to a temperature distribution analyzer (not shown).
- the temperature distribution analyzer includes, for example, a computer and a temperature distribution analysis program stored in a memory in the computer. When the temperature distribution analysis program is executed by the CPU, the temperature distribution analysis apparatus analyzes the temperature change per unit time in each region of the sample intermediate product (including the internal region of the sample intermediate product) three-dimensionally.
- the temperature distribution analyzer can be analyzed by a well-known method using, for example, a well-known heat conduction analysis program using a three-dimensional FEM (finite element method).
- the sample intermediate product ⁇ Cool the sample intermediate product to room temperature (rapid cooling) to identify the temperature change in each region of the sample intermediate product. Then, based on the result of the temperature change, the region (the slowest region) where the cooling rate at 800 to 500 ° C. is the slowest is specified among the sample intermediate products.
- the surface other than the surface of the tread surface 41 and the flange 42 that is, the surface of the boss portion 2, the surface of the plate portion 3, and the surface of the rim portion 4 other than the tread surface 41 and the flange 42.
- the cooling rate at 800 to 500 ° C on the surface is less than Fn1 ° C / second, and the cooling rate at 800 to 500 ° C in the slowest region specified in the sample intermediate product by three-dimensional analysis is Fn2 ° C / second or more.
- the cooling rate of the sample intermediate product is adjusted by the cooling device 10 so that the cooling rate at 800 to 500 ° C.
- the flow rate of each cooling medium of the tread surface cooling nozzle 14, the plate portion cooling nozzle 13, and the boss portion cooling nozzle 12 is adjusted, or a plurality of tread surface cooling nozzles 14 disposed in the cooling device 10, A cooling nozzle to be used is selected from the plate portion cooling nozzle 13 and the boss portion cooling nozzle 12 to adjust the cooling rate.
- cooling is performed using the cooling device 10 for the intermediate product for products heated to the A cm transformation point or higher instead of the sample intermediate product.
- the cooling rate at 800 to 500 ° C.
- the cooling rate at 800 to 500 ° C. of the surface of the tread 41 and the flange 42 among the sample intermediate products measured by the thermography 20 is Fn 2 ° C./s and 5 ° C./s or more.
- the cooling rate of the sample intermediate product is adjusted by the cooling device 10 so as to be 200 ° C./second or less.
- the cooling rate is Fn1 ° C. / Cool the intermediate product so that it is less than a second. Thereby, generation
- cooling is promoted not only in the tread surface 41 and the flange 42 but also in portions other than the tread surface 41 and the flange 42 (side surfaces of the boss portion 2, the plate portion 3, and the rim portion 4).
- a cooling process is implemented by the above process.
- the temperature of the intermediate product after the cooling step is, for example, room temperature (25 ° C.). However, the temperature of the intermediate product after the cooling step is not particularly limited as long as it is 500 ° C. or lower.
- Tempering is performed on the intermediate product after the cooling process as necessary. Tempering may be performed at a known temperature and time.
- the tempering temperature is not higher than the Ac1 transformation point.
- the tempering temperature is, for example, 400 to 600 ° C.
- the holding time at the tempering temperature is, for example, 60 to 180 minutes.
- the tempering temperature and the holding time are not limited to this. Tempering may not be performed.
- the surface of the tread 41 and the flange 42 other than the surfaces of the tread 41 and the flange 42 (the surface of the boss 2, the surface of the plate 3, and the surface of the rim 4). It is difficult to form a hardened layer on the surface other than the surface. Therefore, in the manufacturing method of the railway wheel of the present embodiment, not only the intermediate rim part 4 of the railway wheel but also the plate part 3 and the boss part 2 are cooled, except for the surfaces of the tread 41 and the flange 42. It is not necessary to cut the surfaces (the surface of the boss portion 2, the surface of the plate portion 3, and the side surface of the rim portion 4).
- the railway wheel of this embodiment is manufactured by the above process.
- a railway wheel is produced by the production method of the present embodiment, in the region of the plate portion 3 and the boss portion 2, the pro-eutectoid cementite which is a factor of toughness reduction, despite being a railway wheel using hypereutectoid steel. Generation is suppressed.
- the formation of a hardened layer that causes a reduction in toughness in the region of the plate portion 3 and the boss portion 2 can be suppressed.
- generation of proeutectoid cementite is suppressed.
- the microstructure of the railway wheel manufactured by the above manufacturing method is as follows.
- the structure of the surface layer immediately below the tread and the surface layer portion of the flange is a pearlite structure.
- the amount of proeutectoid cementite is 1.0 / 100 ⁇ m or less.
- the microstructure of portions other than the tread surface and the flange of the boss portion, the plate portion, and the rim portion is substantially made of pearlite. That is, 95% or more of the area ratio is pearlite. Further, the amount of proeutectoid cementite is 1.0 / 100 ⁇ m or less.
- the area ratio of pearlite is 95% or more in the microstructure of the boss part, and the amount of proeutectoid cementite is 1.0 / 100 ⁇ m. It is as follows. In the microstructure of the plate portion, the area ratio of pearlite is 95% or more, and the amount of proeutectoid cementite is 1/100 ⁇ m or less. In the microstructure of the rim portion, the area ratio of pearlite is 95% or more, and the amount of proeutectoid cementite is 1.0 / 100 ⁇ m or less.
- the amount of proeutectoid cementite is defined by the formula (A).
- Amount of pro-eutectoid cementite (lines / 100 ⁇ m) total number of pro-eutectoid cementites intersecting two diagonal lines of a square field of 200 ⁇ m ⁇ 200 ⁇ m / (5.66 ⁇ 100 ⁇ m) (A)
- the microstructure can be observed by the following method.
- a sample for microstructural observation is collected at a position deeper than 5 mm from the surface of each part (boss part, plate part, rim part) of the railway wheel.
- the sample observation surface is mirror-finished by mechanical polishing, the observation surface is corroded with a mixed solution of picric acid and sodium hydroxide.
- a photographic image is generated using an optical microscope with a magnification of 500 times for an arbitrary field of view (200 ⁇ m ⁇ 200 ⁇ m) in the observation surface after corrosion.
- the pro-eutectoid cementite produced at the prior austenite grain boundaries exhibits a black color, so that the presence or absence of pro-eutectoid cementite is specified.
- two diagonal lines 101 are drawn on a square field of view 100 ⁇ m ⁇ 200 ⁇ m. Then, the sum total of the number of pro-eutectoid cementites intersecting these two diagonals 101 is obtained. As defined by equation (1), the total number of pro-eutectoid cementite obtained is divided by the total length of two diagonal lines 101 (5.66 ⁇ 100 ⁇ m), and the amount of pro-eutectoid cementite per 100 ⁇ m 100 ⁇ m).
- pro-eutectoid cementite is 1.0 / 100 ⁇ m or less, generation of pro-eutectoid cementite can be sufficiently suppressed.
- the same observation surface is mirror-finished again by mechanical polishing, and corroded with a nital solution (mixed solution of nitric acid and ethanol).
- a photographic image is generated using an optical microscope with a magnification of 500 times for an arbitrary field of view (200 ⁇ m ⁇ 200 ⁇ m) in the observation surface after corrosion. Ferrite, bainite, martensite, and pearlite have different contrasts. Therefore, the hardened layer and the pearlite in the observation surface are specified based on the contrast. The area ratio of pearlite is obtained based on the total area of the specified pearlite and the area of the observation surface.
- the microstructure of the rim portion including the tread and the flange has a pearlite area ratio of 95% or more, and is substantially made of pearlite.
- the amount of proeutectoid cementite is 1.0 piece / 100 micrometers or less. Therefore, the railway wheel is excellent in wear resistance.
- the microstructures of portions other than the tread and the flange of the boss portion, the plate portion, and the rim portion of the railway wheel are substantially made of pearlite.
- the railway wheel according to the present embodiment is excellent in toughness even if it has a chemical composition that becomes hypereutectoid steel.
- the railway wheel may include a hardened layer on the surface layer immediately below the tread surface of the rim portion and the surface layer of the flange.
- the hardened layer is removed by the above-described cutting process.
- the microstructure of the tread surface and the flange surface of the rim portion is substantially made of pearlite.
- a round ingot (conical frustum type having a top surface diameter of 107 mm, a bottom surface diameter of 97 mm, and a height of 230 mm) was manufactured by the ingot method using the molten steel.
- the ingot was heated to 1250 ° C. and then hot forged within a temperature range of 850 to 1100 ° C. to produce a round bar for a railway wheel having a diameter of 40 mm.
- Jomini type one-side quenching test 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 and steel numbers 1 to 18. Specifically, a round bar having a diameter of 40 mm was converted into a bar steel having a diameter of 25 mm by lathe processing. Then, the round bar was cut
- a heat treatment process (heating process and cooling process) during the manufacturing process of the railway wheel was simulated, and a Jomini-type one-time quenching test according to JIS G0561 (2011) was performed using a Jomini test piece.
- the Jomini test piece was held in a furnace at 950 ° C., which is a temperature equal to or higher than the A cm transformation point, in an air atmosphere for 30 minutes to make the Jomini test piece have an austenite single phase.
- one-end quenching water cooling
- water was sprayed onto one end of the Jomini test piece to cool it.
- the side surface of the Jomini test piece subjected to water cooling is mechanically polished, and Rockwell hardness (HRC) using a C scale conforming to JIS Z2245 (2011) at regular intervals in the axial direction from one end (water cooling end) thereof.
- HRC Rockwell hardness
- FIG. 12 shows the results for steel numbers 1 to 4.
- the Jomini curve is based on the hardness at the water-cooled end position of the test piece. Region A in which the hardness sharply decreases as the distance from the water-cooled end increases, and water-cooled end from region A
- the region B is separated from the region A, and the region B has a lower hardness than the region A with respect to the distance from the water-cooled end.
- the region A corresponds to a quenched layer made of martensite and / or bainite.
- Region B was a structure consisting essentially of pearlite.
- the quenching layer depth was obtained based on the HRC distribution as shown in FIG.
- Microstructure observation at each distance from the water-cooled end was carried out by the following method.
- the measurement surface of the sample side where HRC measurement was performed was used as the observation surface, and after mirror finishing by mechanical polishing, the observation surface was mixed with a mixture of picric acid and sodium hydroxide. Corroded.
- a photographic image was generated using an optical microscope with a magnification of 500 times for an arbitrary field of view (200 ⁇ m ⁇ 200 ⁇ m) in the observation surface after corrosion.
- the pro-eutectoid cementite produced at the prior austenite grain boundaries was black, so the presence or absence of pro-eutectoid cementite could be identified.
- the same observation surface was again mirror-finished by mechanical polishing, and was corroded with a nital solution (mixed solution of nitric acid and ethanol).
- a photographic image was generated using an optical microscope with a magnification of 500 times for an arbitrary field of view (200 ⁇ m ⁇ 200 ⁇ m) in the observation surface after corrosion. Ferrite, bainite, martensite, and pearlite have different contrasts. Therefore, the quenching layer and pearlite in the observation surface were specified based on the contrast.
- the area ratio of pearlite was determined based on the total area of the identified pearlite and the area of the observation surface.
- the “numerical value” in the column corresponding to the distance from the water-cooled end indicates the number of pro-eutectoid cementite per 100 ⁇ m at the distance where the structure is substantially composed of pearlite (the area ratio is 95% or more is pearlite).
- the cooling rate (° C./second) is Fn1 or less defined by the formula (1) and the range of Fn2 or more defined by the formula (2) is colored in gray. did. Referring to Table 4, in the range of the cooling rate colored in gray, a hardened layer was not formed, and the amount of proeutectoid cementite was 1.0 / 100 ⁇ m or less.
- the region where the cooling rate is the slowest at 800 to 500 ° C. that is, the region where the cooling rate is the slowest in the boss part, the plate part and the rim part (hereinafter, the slowest).
- the intermediate product is cooled so that the cooling rate in the region) is at least Fn2 ° C / second, and the cooling rate at 800 to 500 ° C of the tread surface and flange surface of the intermediate product is at least Fn2 ° C / second.
- the pearlite area ratio is 95% or more in any of the boss portion, the plate portion, and the rim portion, and the amount of proeutectoid cementite is 1.0. / 100 [mu] m or less and becomes, on the surface of the boss portion and the plate portion was found to be inhibiting the formation of hardened layer.
- Charpy impact test Charpy test pieces (10 mm ⁇ 10 mm ⁇ 55 mm) were prepared from the round bars of the respective test numbers (9-1 to 9-4). The central axis of the Charpy specimen coincided with the central axis of the round bar. A Charpy impact test according to JIS Z 2242 (2005) was performed at room temperature (25 ° C.) using Charpy test pieces.
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Abstract
Description
Fn1=-5.0+exp(5.651-1.427×C-1.280×Si-0.7723×Mn-1.815×Cr-1.519×Al-7.798×V)・・・(1)
Fn2=0.515+exp(-24.816+24.121×C+1.210×Si+0.529×Mn+2.458×Cr-15.116×Al-5.116×V)・・・(2)
ここで、上記の式(1)及び式(2)における各元素記号には、対応する元素の含有量(質量%)が代入される。
初析セメンタイト量(本/100μm)=200μm×200μmの正方形視野の2本の対角線と交差する初析セメンタイトの本数の総和/(5.66×100μm) (A)
図1は、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42とを含む。踏面41は、フランジ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42の表面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との間に配置される。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、西原式摩耗試験の結果に基づく、鉄道車輪のビッカース硬さと鉄道車輪の摩耗量との関係を示す図である。図2は次の実験により得られた。表1に示す化学組成を有するインゴットから、直径40mmの丸棒を製造した。
上述のとおり、鉄道車輪は、鉄道車輪の中間品に対して熱処理(踏面焼入れ)を実施して製造される。耐摩耗性は、鉄道車輪においてレールと接触し得る、踏面及びフランジに要求される。したがって、従来の鉄道車輪の製造工程中の中間品に対する熱処理においては、踏面直下の表層及びフランジの表層に微細なパーライト組織を形成するために、鉄道車輪の中間品のリム部の踏面及びフランジに対して冷却媒体(水、又は水と空気の混合流体)を噴き付けて、踏面及びフランジを急冷していた。一方、従来の熱処理では、鉄道車輪の踏面及びフランジ表面以外の表面(ボス部の表面、板部の表面及びリム部の側面)に対しては、冷却媒体を吹き付けず、放冷を実施していた。上述のとおり、耐摩耗性が要求されるのはリム部の踏面及びフランジ表面であり、鉄道車輪のうちの踏面及びフランジ表面以外の表面(ボス部表面、板部表面及びリム部の側面)には耐摩耗性が要求されないからである。
Fn1=-5.0+exp(5.651-1.427×C-1.280×Si-0.7723×Mn-1.815×Cr-1.519×Al-7.798×V)・・・(1)
Fn2=0.515+exp(-24.816+24.121×C+1.210×Si+0.529×Mn+2.458×Cr-15.116×Al-5.116×V)・・・(2)
ここで、式(1)及び式(2)中の各元素記号には、対応する元素の含有量(質量%)が代入される。なお、800~500℃は、パーライト及び初析セメンタイトが生成する温度域である。
Fn1=-5.0+exp(5.651-1.427×C-1.280×Si-0.7723×Mn-1.815×Cr-1.519×Al-7.798×V)・・・(1)
Fn2=0.515+exp(-24.816+24.121×C+1.210×Si+0.529×Mn+2.458×Cr-15.116×Al-5.116×V)・・・(2)
ここで、上記の式(1)及び式(2)における各元素記号には、対応する元素の含有量(質量%)が代入される。
初析セメンタイト量(本/100μm)=200μm×200μmの正方形視野の2本の対角線と交差する初析セメンタイトの本数の総和/(5.66×100μm) (A)
本実施形態の鉄道車輪はたとえば、図1に示すとおり、ボス部2と、板部3と、踏面41及びフランジ42を含むリム部4を備えた形状を有する。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、耐摩耗性を高める。C含有量が低すぎれば、これらの効果が得られない。一方、C含有量が高すぎれば、旧オーステナイト粒界に初析セメンタイトが析出し、鋼の延性、靭性及び疲労寿命が低下する。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%である。C含有量の好ましい上限は1.05%であり、さらに好ましくは1.00%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度を高める。しかしながら、Si含有量が高すぎれば、初析セメンタイトが生成しやすくなる。Si含有量が高すぎればさらに、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。さらに、鉄道車輪として使用中に、ブレーキとの間に発生する摩擦熱により焼きが入り、鋼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0.80%であり、さらに好ましくは0.65%であり、さらに好ましくは0.45%であり、さらに好ましくは0.35%である。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0.20%である。
マンガン(Mn)はフェライトを固溶強化して鋼の硬度を高める。Mnはさらに、MnSを形成し、鋼の被削性を高める。Mn含有量が低すぎれば、これらの効果は得られない。一方、Mn含有量が高すぎれば、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。さらに、鉄道車輪として使用中に、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0.10~1.25%である。Mn含有量の好ましい下限は0.50%であり、さらに好ましくは0.60%であり、さらに好ましくは0.70%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0.82%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含有量を過剰に低減しようとすれば、精錬コストが過剰に高くなる。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0.0001%であり、さらに好ましくは0.0005%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を積極的に含有させる場合、SはMnSを形成して鋼の被削性を高める。しかしながら、Sは鋼の靱性を低下する。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%である。被削性向上の効果を得る場合の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0.005%である。
アルミニウム(Al)は、C含有量が0.80%以上の本実施形態の鉄道車輪の化学組成において、初析セメンタイトの生成を抑制し、鋼の靱性を高める。Alはさらに、Nと結合してAlNを形成し、結晶粒を微細化する。結晶粒が微細化することにより、鋼の靭性が高まる。Al含有量が低すぎればこれらの効果が得られない。一方、Al含有量が高すぎれば、粗大な非金属介在物が増加して、鋼の靭性が低下する。したがって、Al含有量は0.025~0.650%である。Al含有量の好ましい下限は0.030%であり、さらに好ましくは0.040%であり、さらに好ましくは0.050%である。Al含有量の好ましい上限は0.450%であり、さらに好ましくは0.350%であり、さらに好ましくは0.250%であり、さらに好ましくは0.115%である。本明細書でいうAl含有量は、酸可溶Al(sol.Al)の含有量を意味する。
窒素(N)は、Alと結合してAlNを形成し、結晶粒を微細化する。結晶粒が微細化することにより、鋼の靭性を高める。N含有量が低すぎればこの効果が得られない。一方、N含有量が高すぎれば、その効果が飽和する。したがって、N含有量は0.0030~0.0200%である。N含有量の好ましい下限は、0.0035%であり、さらに好ましくは0.0040%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%である。
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減少させることにより、パーライトの硬度を顕著に増大させる。しかしながら、Cr含有量が高すぎれば、初析セメンタイトが生成しやすくなる。Cr含有量が高すぎればさらに、焼入れ性が高くなり、マルテンサイトが生成しやすくなる。したがって、Cr含有量は0~0.60%である。Cr含有量の好ましい上限は0.30%であり、好ましくは0.25%であり、さらに好ましくは0.10%である。パーライトのラメラ間隔減少の効果を得る場合のCr含有量の好ましい下限は0.02%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、鋼を析出強化する。その結果、鉄道車輪の硬さが顕著に増大して、耐摩耗性をさらに高める。しかしながら、V含有量が高すぎれば、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含有量の好ましい上限は0.09%である。V含有量の好ましい下限は0.02%であり、さらに好ましくは0.03%である。
上述の鉄道車輪の製造方法を説明する。本実施形態による鉄道車輪の製造方法は、熱処理工程を含む。熱処理工程は、加熱工程と、冷却工程とを備える。
加熱工程では、初めに上述の化学組成を有し、ボス部、板部及びリム部を備える鉄道車輪の粗形状を有する中間品を準備する。中間品はたとえば、次の方法で製造される。
加熱された中間品に対して、冷却工程を実施する。この冷却工程により、鉄道車輪の中間品のうち踏面直下の表層及びフランジの表層のミクロ組織を、耐摩耗性が高い微細パーライト組織とする。踏面直下の表層及びフランジの表層においては、微細パーライトの上層に焼入れ層(マルテンサイト及び/又はベイナイトからなる層)が多少生成しても構わない。この場合には、その後の工程で切削加工を行い焼入れ層を除去する。一方、中間品のうち踏面及びフランジ表面以外の表面では、ミクロ組織中に焼入れ層が生成するのを抑制する。そして、ミクロ組織を実質的にパーライト組織(パーライトが面積率で95%以上)とする。ここで、中間品のうち、踏面及びフランジ表面以外の表面とは、板部の表面、ボス部の表面、及び、リム部の踏面及びフランジ表面以外の表面を意味する。中間品のうち踏面及びフランジ表面以外の表面において、焼入れ層が生成するのを抑制するのは、中間品のうち、踏面及びフランジ表面以外の表面においては、生成した焼入れ層を切削加工することが困難なためである。
(A)中間品のうち踏面及びフランジ表面以外の表面、すなわちボス部表面、板部表面及びリム部側面(踏面及びフランジ表面以外のリム部表面)の800~500℃における冷却速度がFn1℃/秒以下となるように、中間品を冷却する。
(B)中間品のうち、800~500℃における冷却速度が最も遅くなる領域、すなわち、ボス部内部、板部内部及びリム部内部において冷却速度が最も遅くなる領域(以下、最遅領域という)での冷却速度がFn2℃/秒以上となるように、中間品を冷却する。
(C)中間品のうち踏面及びフランジ表面の800~500℃における冷却速度がFn2℃/秒以上となるように、中間品を冷却する。
冷却工程後の中間品に対して、必要に応じて焼戻しを実施する。焼戻しは周知の温度及び時間で行えば足りる。焼戻し温度はAc1変態点以下である。焼戻し温度はたとえば、400~600℃であり、焼戻し温度での保持時間はたとえば、60~180分である。ただし、焼戻し温度及び保持時間はこれに限定されない。焼戻しは実施しなくてもよい。
熱処理工程(加熱工程及び冷却工程)後の中間品の踏面41の直下の表層及びフランジ42の表層には微細パーライトが形成されるものの、微細パーライトの上層には焼入れ層が形成される場合がある。鉄道車輪の使用において、焼入れ層の耐摩耗性は低い。そのため、本工程において、切削加工により踏面41の直下の表層及びフランジ42の表層の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。
上述の製造方法により製造された鉄道車輪のミクロ組織は、次のとおりである。踏面直下の表層及びフランジの表層部分の組織は、パーライト組織である。初析セメンタイト量は、1.0本/100μm以下である。ボス部、板部、リム部のうちの踏面及びフランジ以外の部分のミクロ組織は、実質的にパーライトからなる。つまり、面積率の95%以上がパーライトである。さらに、初析セメンタイト量は、1.0本/100μm以下である。
初析セメンタイト量(本/100μm)=200μm×200μmの正方形視野の2本の対角線と交差する初析セメンタイトの本数の総和/(5.66×100μm) (A)
鋼番号1~鋼番号18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。具体的には、旋盤加工により、直径40mmの丸棒を直径25mmの棒鋼とした。その後、100mmの長さで丸棒を切断して、ジョミニ試験片を作製した。
水冷端からの各距離におけるミクロ組織観察は、次の方法で実施した。ジョミニ試験片の水冷端からの各距離にて、HRC測定を行ったサンプル側面の測定面を観察面として、機械研磨により鏡面仕上げした後、観察面をピクリン酸と水酸化ナトリウムとの混合液で腐食した。腐食後の観察面内の任意の1視野(200μm×200μm)に対して、500倍の光学顕微鏡を用いて写真画像を生成した。観察面において、旧オーステナイト粒界に生成した初析セメンタイトは黒色を呈するため、初析セメンタイトの生成有無が特定できた。
上記のジョミニ試験片を用いて、ジョミニ式一端焼入れ試験では再現できない、低冷速での連続冷却試験を実施した。熱処理には富士電波工機製のフォーマスタ試験機を使用した。鋼番号1~鋼番号18の直径40mmの丸棒から、直径3mm、長さ10mmの試験片を各鋼番号につき1個ずつ用意した。試験片を950℃で5分間均熱した。その後、一定の冷却速度1.0℃/秒で冷却した。冷却後の試験片に対して、上述の方法により、初析セメンタイト量(本/100μm)を算出した。
結果を表4に示す。表4中において、水冷端からの距離に対応した欄の「●」印は、その距離における組織が焼入れ層(マルテンサイト及び/又はベイナイト)であったことを示す。また、水冷端からの距離に対応した欄の「○」印は、その距離における組織が実質的にパーライトからなり(面積率で95%以上がパーライトであり)、マルテンサイト又はベイナイトが確認されず、初析セメンタイトも確認されなかったことを示す。水冷端からの距離に対応した欄の「数値」は、組織が実質的にパーライトからなり(面積率で95%以上がパーライトであり)、その距離における初析セメンタイトの100μmあたりの本数を示す。また、表4中の各鋼番号においては、冷却速度(℃/秒)が式(1)で定義されるFn1以下であり、式(2)で定義されるFn2以上である範囲をグレーで色づけした。表4を参照して、グレーで色づけされた冷却速度の範囲では、焼入れ層が生成せず、かつ、初析セメンタイト量が1.0本/100μm以下であった。
冷却後の各試験番号(9-1~9-4)の丸棒の中央部から、ミクロ組織観察用のサンプルを採取した。サンプルの観察面は、丸棒の中心軸に対して垂直な面とした。機械研磨により観察面を鏡面仕上げした後、観察面をピクリン酸と水酸化ナトリウムとの混合液で腐食した。腐食後の観察面内の任意の1視野(200μm×200μm)に対して、500倍の光学顕微鏡を用いて写真画像を生成した。観察面において、旧オーステナイト粒界に生成した初析セメンタイトは黒色を呈するため、初析セメンタイトの生成有無が特定できた。また、実施例1と同じ方法により、パーライト面積率を求めた。その結果、いずれの試験番号においても、パーライト面積率が95%以上であった。
各試験番号(9-1~9-4)の丸棒から、シャルピー試験片(10mm×10mm×55mm)を作製した。シャルピー試験片の中心軸は、丸棒の中心軸と一致した。シャルピー試験片を用いて、JIS Z 2242(2005)に準拠したシャルピー衝撃試験を室温(25℃)にて実施した。
試験結果を表5に示す。表5を参照して、冷却速度がFn2以上(3.4)の場合(鋼番号9-1)、初析セメンタイト量は1.0本/100μm以下であった。そのため、シャルピー衝撃値が20.0J/cm2以上と高く、十分な靱性が得られた。一方、冷却速度がFn2未満の場合(鋼番号9-2~9-4)、シャルピー衝撃値は20.0J/cm2未満と低かった。
2 ボス部
3 板部
4 リム部
10 冷却装置
Claims (5)
- 質量%で、
C:0.80~1.15%、
Si:1.00%以下、
Mn:0.10~1.25%、
P:0.050%以下、
S:0.030%以下、
Al:0.025~0.650%、
N:0.0030~0.0200%、
Cr:0~0.60%、及び、
V:0~0.12%、
を含有し、残部がFe及び不純物からなる化学組成を有し、
ボス部と、
踏面及びフランジを含むリム部と、
前記ボス部と前記リム部との間に配置される板部とを備える、鉄道車輪の中間品をAcm変態点以上に加熱する加熱工程と、
加熱された前記中間品を冷却する冷却工程とを備え、
前記冷却工程では、前記中間品において前記踏面及びフランジ表面以外の表面の800~500℃における冷却速度が式(1)で定義されるFn1℃/秒以下であり、前記中間品において冷却速度が最も遅くなる領域での800~500℃における冷却速度が式(2)で定義されるFn2℃/秒以上であり、前記踏面及びフランジ表面での800~500℃における冷却速度がFn2℃/秒以上となるように、前記中間品を冷却する、
鉄道車輪の製造方法。
Fn1=-5.0+exp(5.651-1.427×C-1.280×Si-0.7723×Mn-1.815×Cr-1.519×Al-7.798×V)・・・(1)
Fn2=0.515+exp(-24.816+24.121×C+1.210×Si+0.529×Mn+2.458×Cr-15.116×Al-5.116×V)・・・(2)
ここで、上記の式(1)及び式(2)における各元素記号には、対応する元素の含有量(質量%)が代入される。 - 請求項1に記載の鉄道車輪の製造方法であって、
前記冷却工程ではさらに、前記踏面及びフランジ表面での800~500℃における冷却速度がFn2℃/秒以上かつ5℃/秒以上であり、200℃/秒以下となるように、前記中間品を冷却する、
鉄道車輪の製造方法。 - 請求項1又は請求項2に記載の鉄道車輪の製造方法であって、
前記化学組成は、
Cr:0.02~0.60%、及び、
V:0.02~0.12%、
からなる群から選択される1種以上を含有する、
鉄道車輪の製造方法。 - 質量%で、
C:0.80~1.15%、
Si:1.00%以下、
Mn:0.10~1.25%、
P:0.050%以下、
S:0.030%以下、
Al:0.025~0.650%、
N:0.0030~0.0200%、
Cr:0~0.60%、及び、
V:0~0.12%、
を含有し、残部がFe及び不純物からなる化学組成を有し、
ボス部と、
踏面及びフランジを含むリム部と、
前記ボス部と前記リム部との間に配置される板部とを備え、
前記ボス部のミクロ組織において、パーライトの面積率は95%以上であり、式(A)で定義される初析セメンタイト量は1.0本/100μm以下であり、
前記板部のミクロ組織において、パーライトの面積率は95%以上であり、式(A)で定義される初析セメンタイト量は1.0本/100μm以下であり、
前記リム部のミクロ組織において、パーライトの面積率は95%以上であり、式(A)で定義される初析セメンタイト量は1.0本/100μm以下である、
鉄道車輪。
初析セメンタイト量(本/100μm)=200μm×200μmの正方形視野の2本の対角線と交差する初析セメンタイトの本数の総和/(5.66×100μm) (A) - 請求項4に記載の鉄道車輪であって、
前記化学組成は、
Cr:0.02~0.60%、及び、
V:0.02~0.12%、
からなる群から選択される1種以上を含有する、
鉄道車輪。
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