WO2020054339A1 - レール、及びレールの製造方法 - Google Patents
レール、及びレールの製造方法 Download PDFInfo
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- WO2020054339A1 WO2020054339A1 PCT/JP2019/032627 JP2019032627W WO2020054339A1 WO 2020054339 A1 WO2020054339 A1 WO 2020054339A1 JP 2019032627 W JP2019032627 W JP 2019032627W WO 2020054339 A1 WO2020054339 A1 WO 2020054339A1
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- rail
- head
- temperature
- cooling
- pearlite structure
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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
- C21D6/008—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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/08—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling structural sections, i.e. work of special cross-section, e.g. angle steel
- B21B1/085—Rail sections
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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
- C21D1/78—Combined heat-treatments not provided for above
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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
- C21D6/02—Hardening by precipitation
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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
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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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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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
- C21D9/04—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rails
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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
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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
- C22C38/002—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
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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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
- C22C38/06—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
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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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
- C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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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
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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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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- 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
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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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
- C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
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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
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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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
- C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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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
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- E—FIXED CONSTRUCTIONS
- E01—CONSTRUCTION OF ROADS, RAILWAYS, OR BRIDGES
- E01B—PERMANENT WAY; PERMANENT-WAY TOOLS; MACHINES FOR MAKING RAILWAYS OF ALL KINDS
- E01B5/00—Rails; Guard rails; Distance-keeping means for them
- E01B5/02—Rails
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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
- C21D2211/004—Dispersions; Precipitations
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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
Definitions
- the present invention relates to a high-strength rail used in a freight railway, which is excellent in wear resistance and internal fatigue damage resistance, and a method of manufacturing the same.
- Priority is claimed on Japanese Patent Application No. 2018-168799, filed Sep. 10, 2018, the content of which is incorporated herein by reference.
- high-strength rails as disclosed in Patent Documents 1 and 2 have been developed.
- the main feature of these rails is to improve the wear resistance by heat treatment to reduce the lamella spacing in the pearlite structure and increase the hardness of the steel, or increase the carbon content of the steel and increase the pearlite structure The reason is that the volume ratio of the cementite phase in the lamella inside is increased.
- Patent Literature 1 discloses that abrasion resistance is improved by rolling and cooling a rail head after rolling or reheated at a rate of 1 to 4 ° C./sec between 850 to 500 ° C. from an austenite zone temperature. It is disclosed that an excellent rail can be provided.
- Patent Document 2 discloses that a hypereutectoid steel (C: more than 0.85 to 1.20%) is used to increase the volume ratio of cementite in the lamella in the pearlite structure, thereby providing excellent wear resistance. It is disclosed that a modified rail can be provided.
- Patent Literature 1 or 2 by increasing the volume ratio of the cementite phase in the lamella in the pearlite structure, or by increasing the volume ratio of the cementite phase in the lamella in the pearlite structure, Abrasion resistance can be improved.
- high-strength rails as disclosed in Patent Documents 3, 4, and 5 have been proposed.
- the main feature of these rails is that, in addition to improving wear resistance, in order to improve internal fatigue damage resistance, a small amount of alloy is added to control pearlite transformation, or to control alloy or control small amount of alloy. The reason is that the hardness inside the head is improved by forming a precipitate in the pearlite structure by the addition.
- Patent Document 3 discloses that by adding B to hypereutectoid steel (C: more than 0.85 to 1.20%), the transformation temperature of the pearlite structure inside the head is controlled. It is disclosed that the hardness inside the head is improved.
- Patent Document 4 discloses a method in which V and N are added to hypereutectoid steel (C: more than 0.85 to 1.20%) to precipitate carbonitrides of V in a pearlite structure. It is disclosed that the hardness inside the part is improved. Further, in Patent Document 5, the hardness inside the head is improved by controlling the contents of Mn and Cr based on eutectoid steel (C: 0.73 to 0.85%). It is disclosed.
- the hardness of the inside of the head is improved by controlling the pearlite transformation temperature inside the head and strengthening the precipitation of the pearlite structure, and the internal fatigue damage resistance is improved in a certain range. Improvement can be achieved.
- the high-strength rails disclosed in Patent Literatures 3, 4 and 5 when used in a severe track environment required in recent years, sufficient characteristics cannot be obtained, and the further improvement in internal fatigue damage resistance is not achieved. Had been an issue.
- Japanese Patent Publication No. 63-023244 Japanese Patent Application Laid-Open No. 8-144016 Japanese Patent No. 3445619 Japanese Patent No. 3513427 Japanese Patent Application Laid-Open No. 2009-108397
- the present invention has been devised in view of the above-described problems, and has as its object to provide a rail having excellent wear resistance and internal fatigue damage resistance.
- the rail according to one embodiment of the present invention has a unit mass of C: 0.75 to 1.20%, Si: 0.10 to 2.00%, and Mn: 0.10 to 2.00%. , Cr: 0.10 to 1.20%, V: 0.010 to 0.200%, N: 0.0030 to 0.0200%, P ⁇ 0.0250%, S ⁇ 0.0250%, Mo: 0 to 0.50%, Co: 0 to 1.00%, B: 0 to 0.0050%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Nb: 0 to 0.0500 %, Ti: 0 to 0.0500%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, REM: 0 to 0.0500%, Zr: 0 to 0.0200%, and Al: 0 to 1.00%, the balance being Fe and impurities, and a tissue ranging from the outer surface of the head to a depth of 25 mm as a starting point has an area ratio of 9%
- the average value of CA / VA may satisfy the following formula 1. 0.01 ⁇ CA / VA ⁇ 0.70 Formula 1 (3)
- Group e: Nb One or two kinds of 0.0010 to 0.0500% and Ti: 0.0030 to 0.0500%
- Ca: 0.0005 to 0.05% the average value of CA / VA may satisfy the following formula 1. 0.01 ⁇ CA / VA ⁇ 0.70 Formula 1 (3)
- the method for manufacturing a rail according to another aspect of the present invention is such that, in unit mass%, C: 0.75 to 1.20%, Si: 0.10 to 2.00%, and Mn: 0.10 to 0.1% 2.00%, Cr: 0.10 to 1.20%, V: 0.010 to 0.200%, N: 0.0030 to 0.0200%, P ⁇ 0.0250%, S ⁇ 0.0250 %, Mo: 0 to 0.50%, Co: 0 to 1.00%, B: 0 to 0.0050%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Nb: 0 0.00.0%, Ti: 0 to 0.0500%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, REM
- the wear resistance and the internal fatigue damage resistance of the rail can be improved. Further, such a rail can greatly improve the service life of the rail when used in a freight railway.
- a rail having excellent abrasion resistance and internal fatigue damage resistance according to one embodiment of the present invention (may be referred to as a rail according to the present embodiment) will be described in detail.
- mass% in the composition is simply described as%.
- the rail according to the present embodiment has the following features.
- It has a predetermined chemical composition.
- a tissue in a range from the outer surface of the head to a depth of up to 25 mm includes a pearlite structure having an area ratio of 95% or more, and the hardness of the tissue is in a range of Hv 360 to 500.
- the number density of Cr-containing V nitrides having a grain size of 0.5 to 4.0 nm in the ferrite phase in the pearlite structure at a depth of 25 mm starting from the outer surface of the head is 1. The range is 0 to 5.0 ⁇ 10 17 cm ⁇ 3 .
- the reason why the area ratio of the pearlite structure is 95% or more will be described. It is most important to ensure the wear resistance of the rail head that comes into contact with the wheels.
- the present inventors have investigated the relationship between the metal structure and the wear resistance, and as a result, it has been confirmed that the pearlite structure has the highest wear resistance. Further, the pearlite structure is easy to obtain hardness (strength) even if the content of the alloy element is small, and is excellent in internal fatigue damage resistance. Therefore, the area ratio of the pearlite structure is limited to 95% or more for the purpose of improving the wear resistance and the internal fatigue damage resistance. When the area ratio of the pearlite structure is less than 95%, the wear resistance and the internal fatigue damage resistance are not sufficiently improved.
- the metal structure of the rail head has a pearlite structure.
- the area ratio of the pearlite structure in the rail head may be 100%.
- the necessary range of the metal structure (structure including pearlite) in which the pearlite structure is included at a rate of 95% or more in terms of the area ratio is determined from the surface of the head outline (the surface of the head corner and the top of the head). The reason why the range is limited to a depth of at least 25 mm starting from the surface will be described.
- the range of the structure including the pearlite structure is less than 25 mm from the head outer surface as a starting point, considering wear during use, it is not sufficient as an area required for wear resistance and internal fatigue damage resistance of the rail head. In addition, the wear resistance and the internal fatigue damage resistance cannot be sufficiently improved, and as a result, it is difficult to sufficiently improve the service life of the rail. In order to further improve the wear resistance and the internal fatigue damage resistance, it is desirable that a structure including a pearlite structure be formed up to a depth of about 30 mm starting from the outer surface of the head.
- FIG. 1 shows the names of the rails according to the present embodiment in terms of the head cross-sectional surface position, and the area where a tissue including a pearlite structure is required.
- the rail head refers to a portion above a portion confined at the center in the height direction of the rail when the rail is viewed in cross section.
- the rail head 3 has a head 1 and head corners 2 located at both ends of the head 1.
- One of the head corner portions 2 is a gauge corner (GC) portion that mainly contacts the wheel.
- the head outer surface refers to a surface of the rail head 3 in which the surface of the top 1 facing upward when the rail is erected and the surface of the head corner 2 are combined.
- the positional relationship between the crown 1 and the corner 2 is such that the crown 1 is located substantially at the center of the rail head in the width direction, and the corner 2 is located on both sides of the crown 1.
- ⁇ ⁇ A range from the surface of the head corner 2 and the top of the head 1 (the outer surface of the head) to a depth of 25 mm is referred to as a front surface part (3a, hatched part).
- a tissue containing a pearlite structure having a predetermined hardness is formed on the head surface 3a up to a depth of 25 mm starting from the surface of the head corner 2 and the top of the crown 1 (the outer surface of the head). Is required to improve the wear resistance and the internal fatigue damage resistance of the rail.
- the structure including the pearlite structure is arranged in the head portion 3a where the wheel and the rail are mainly in contact and wear resistance and internal fatigue damage resistance are required, and these characteristics are not required.
- the area ratio of the pearlite structure in the portion other than the head portion may be 95% or more, but may not be 95% or more.
- the area ratio of the pearlite structure is 95% or more, a very small amount of proeutectoid ferrite structure having an area ratio of less than 5%, if the area ratio of the pearlite structure is 95% or more, A proeutectoid cementite structure, a bainite structure, a martensite structure, or the like may be mixed. Even if these tissues are mixed, if it is less than 5%, the wear resistance of the head surface and the internal fatigue damage resistance inside the head are not significantly affected.
- the metal structure of the rail head of the rail according to the present embodiment only needs to have an area ratio of 95% or more of the surface of the head in a pearlite structure, and the wear resistance and the internal fatigue damage resistance are sufficiently improved.
- the area ratio of the pearlite structure may be 100%.
- the area ratio of the pearlite structure in the range of a depth up to 25 mm starting from the outer surface of the head can be determined by the following method. That is, the metal structure is observed in a visual field of an optical microscope of 200 times, and the area ratio of the pearlite structure can be determined by determining the area of each metal structure. In addition, an average value of the area ratios can be used as the area ratio of the observation site, using 10 or more visual fields (10 places) as the visual field of the optical microscope.
- the average area ratio of both pearlite structures at a position at a depth of 2 mm from the head outer surface and a position at a depth of 25 mm from the head outer surface is 95% or more, It can be said that 95% or more of the area ratio of the metal structure having a depth of at least 25 mm starting from the outer surface of the head is a pearlite structure.
- the hardness of the structure including the pearlite structure needs to be limited to the range of Hv 360 to 500.
- the reason why the hardness of the structure including the pearlite structure in the rail according to the present embodiment is limited to the range of Hv 360 to 500 will be described.
- the present inventors have studied the hardness of a metal structure including a pearlite structure necessary for ensuring the wear resistance and the internal fatigue damage resistance of the rail.
- Rail rolling was performed on a steel material (hypereutectoid steel) having a component of 0.0200% N, and the relationship between the hardness of the rail head, wear resistance, and internal fatigue damage resistance was investigated. Rail rolling, heat treatment conditions, and rolling fatigue test conditions are as shown below.
- ⁇ Evaluation Wear resistance Cumulative passing tonnage when the wear amount reached 25 mm.
- Internal fatigue damage resistance Using an ultrasonic flaw detector, the presence or absence of a crack inside the head over the entire length of the rail is investigated, and a crack with a crack length of 2 mm or more is judged to be damaged, and the cumulative time until crack initiation The passing tonnage was used. In the test, the number of evaluations was 3, and the minimum value of the cumulative passing tonnage up to the occurrence of a crack was used as a representative value.
- a pearlite structure with a depth of up to 25 mm starting from the outer surface of the head is used. It was confirmed by the above-mentioned test that the hardness of the contained metal structure had to be controlled in the range of Hv 360 to 500. For this reason, the hardness of the structure including the pearlite structure is limited to the range of Hv 360 to 500.
- a pearlite structure having a depth of 25 mm from the outer surface of the head as a starting point is included.
- the hardness of the metal structure may be Hv480 or less, Hv470 or less, or Hv460 or less.
- the hardness of the tissue including the pearlite tissue is measured at a measurement location (for example, a position at a depth of 2 mm starting from the outer surface of the head), and at least 20 points are measured, and the average value is determined as the hardness value at that position. adopt.
- the pearlite structure occupies 95% or more in area ratio, but other structures (e.g., proeutectoid cementite, proeutectoid ferrite, martensite, bainite, etc.) are present in the range of 5% or less, so that one point is obtained. This is because the hardness of the structure including the pearlite structure may not be able to be represented by the measurement of.
- the head is It can be said that the hardness in the range of at least 25 mm depth starting from the outer surface is Hv 360 to 500.
- Cr-containing V nitride in the present embodiment means an inclusion composed of V nitride and containing one or more Cr atoms. According to a three-dimensional atom probe (3DAP) method described later, the presence or absence of Cr atoms can be confirmed.
- the present inventors investigated in detail the generation of fatigue damage inside the head after the rolling fatigue test.
- a crack having a length of less than 2 mm which is hardly detected in the inspection for the presence or absence of a crack using an ultrasonic flaw detector after the rolling fatigue test, remains inside the head of the rail that passed the evaluation test. It was confirmed.
- the remaining cracks have a significant effect on the basic performance of the rail, and it is necessary to prevent it to ensure safety.
- the present inventors have studied a method for eliminating this crack.
- the pearlite structure did not change at the crack initiation site, although the pearlite structure did not change. It was confirmed that a microscopically softened portion was present in the ferrite phase in the inside. As a result, the present inventors have found that the strain is concentrated on the microscopically softened portion of the ferrite phase inside the head due to the contact with the wheel, and the crack is easily generated.
- the present inventors have considered that precipitation strengthening is effective for improving the microscopic hardness inside the head. Then, the present inventors searched for an element which is finely formed in the ferrite phase in the pearlite structure to cause precipitation strengthening.
- nitrides are effective as a component for precipitation strengthening due to the stability of hardness increase and the resistance to fatigue cracks.
- carbides and carbonitrides contain carbon that is easily diffused and decomposed, and therefore have low stability against heat and stress, and are not effective for stable precipitation strengthening.
- V nitride containing Cr which is a composite of Cr in V nitride, has extremely high stability against heat and stress, and has a microscopic view of the ferrite phase having a pearlite structure inside the head. It was confirmed that the natural softening was suppressed and the hardness of the ferrite phase in the pearlite structure was stably improved.
- the present inventors conducted rail rolling using a steel material (hypereutectoid steel) containing V, Cr, and nitrogen to verify the effect of V nitride containing Cr, and A heat treatment was performed to promote the formation of V nitrides, and the precipitation inside the head and the hardness of the head were investigated. Furthermore, the internal fatigue damage resistance of the rail was evaluated.
- the V content is 0.010-0.
- Heat treatment condition rolling ⁇ accelerated cooling + controlled cooling Accelerated cooling condition (head outer surface): cooling from 800 ° C to 660-580 ° C at a cooling rate of 5 ° C / sec
- Accelerated cooling condition head outer surface: after stopping accelerated cooling Hold in the temperature range of 580 to 660 ° C for 5 to 120 seconds, and then maintain the temperature during accelerated cooling control cooling: control the accelerated cooling rate, and repeatedly execute and stop accelerated cooling to regain heat from inside the rail The temperature was controlled by performing accelerated cooling according to.
- the method for investigating V nitride containing Cr is as follows. [Method of investigating V nitride containing Cr] ⁇ Sampling position: Inside the head (25 mm depth starting from the outer surface of the head) ⁇ Pretreatment: three needle samples with a radius of curvature of 30 to 80 nm are prepared by FIB (focused ion beam) method ⁇ Measuring machine: three-dimensional atom probe (3DAP) method ⁇ Measurement method DC voltage is applied to the needle sample, and pulse voltage is applied Is applied, or the pulse of the needle sample is irradiated to the needle sample, so that the ions of the constituent atoms are field-evaporated from the tip of the needle. This ion is detected by a coordinate detector.
- the type of element is specified by the ion flight time.
- the three-dimensional element position and the number of atoms are specified based on the detected coordinates and the measurement order.
- Voltage DC, voltage pulse (pulse ratio 15% or more) or laser pulse (40 pJ)
- Sample temperature 40K to 70K ⁇ Determination method and counting method of V nitride containing Cr The measurement data was analyzed using IVAS software (manufactured by CAMECA). In the mass-to-charge ratio spectrum, a peak at 25.5 Da was identified as V 2+, and peaks at 25, 26 and 26.5 were identified as Cr 2+ . N cannot be directly recognized in the chemical composition of the rail according to the present embodiment, since the peak of NN + overlaps with the main peak of Fe 2+ .
- the NV 2+ peak appearing at 32.5 Da was identified as N.
- the ion corresponding to this peak contains V equivalent to N.
- nitride precipitates are determined using the atomic position data of V and CrN.
- the Maximum Separation Method included in the IRAS is used. This is a method in which a group of V, Cr, and N atoms whose distance from each other is equal to or less than a specific value is separated from the matrix and recognized as a precipitate. In this experiment, 1 nm was used as the “specific value”.
- the number of precipitates determined as Cr-containing V precipitates in the ferrite phase in the pearlite structure in the measurement region is counted using the IRAS software.
- the pearlite structure includes a ferrite phase and a cementite phase.
- V nitride containing Cr is used for strengthening the ferrite phase in the pearlite structure, so in this experiment, only the one present at the center of the ferrite phase in the pearlite structure was evaluated.
- the separation between the cementite phase and the ferrite phase in the measurement region can be determined from the C distribution (in the cementite phase, the C concentration becomes 25% in atomic ratio).
- the number density of Cr-containing nitride is measured as follows.
- the volume of the analysis region is estimated from the number of atoms contained in the analysis region measured by 3DAP.
- alloy elements other than iron are very small, so even if the volume of the analysis region is calculated from the number of elements in the analysis region, assuming that all atoms constituting the analysis region are iron atoms, It is considered that there is no significant difference from the true value.
- the number of atoms of iron is corrected by the detection of the ion detector, considered a value obtained by dividing the value in the Fe atom density (85 number / nm 3) the volume of the measurement site (nm 3).
- the detection rate varies depending on the apparatus, the detection rate was 35% in the apparatus used in this experiment, and the value obtained by dividing the number of detected atoms by 0.35 was calculated as the number of atoms contained in the analysis region. It was estimated.
- the grain size of the ferrite phase in the pearlite structure is 0.5 to 4.0 nm.
- the average value of the number densities of the three needle samples was taken as the number density of the rail.
- ⁇ Measurement method of particle size of Cr-containing V nitride In this experiment, only the number density of Cr-containing V nitride having a particle size of 0.5 to 4.0 nm was measured. This is because V nitride containing Cr having a particle size of less than 0.5 nm or more than 4.0 nm was considered not to contribute to improvement of the characteristics of the rail. Therefore, in the evaluation of Cr-containing V nitrides, of the Cr-containing V nitrides, only those having a particle size of 0.5 to 4.0 nm were extracted and counted.
- the method for measuring the particle diameter of each of the Cr-containing V nitrides is as follows. First, the total number of atoms of V and Cr constituting the V-nitride containing Cr is determined, and it is assumed that the same number of N as N is present in the precipitate. Estimate the volume of an object. VN and CrN respective lattice constants using literature values of 0.413nm and 0.415nm, if 0.414nm lattice constant of V nitrides containing Cr, the number of atoms entering into 1 nm 3 of about 113 Individual. The volume of the precipitate can be estimated based on the number of atoms contained in the precipitate.
- the V nitride containing Cr is a sphere, and the diameter of the sphere is defined as the particle diameter of the V nitride containing Cr. That is, the sphere equivalent diameter of the V nitride containing Cr was determined.
- V, Cr and N were included in the chemical components of the rail, and furthermore, after rolling It has been found that by controlling the heat treatment conditions, a certain amount of V nitride containing Cr can be generated in the ferrite phase having a pearlite structure.
- the ferrite phase having a pearlite structure can be microscopically observed in the pearlite structure inside the rail head. It was confirmed that the typical softened portion was reduced and the hardness of the ferrite phase in the pearlite structure was stable.
- the number density of Cr-containing V nitrides having a particle size of 0.5 to 4.0 nm is set to 1.0 to 5.0. It was confirmed that by controlling to a range of ⁇ 10 17 cm -3 , the number of microscopically softened portions was reduced and the hardness was stably uniformed.
- the particle diameter of the Cr-containing V nitride for controlling the number density is limited to the range of 0.5 to 4.0 nm.
- the size is the most effective size for reducing the microscopic softened portion in which the pearlite structure is generated and making the hardness uniform. Since V nitride containing Cr having a particle diameter of less than 0.5 nm or more than 4.0 nm does not contribute to the improvement of the characteristics of the rail, it is considered that the content thereof is preferably small.
- the present inventors used a rolling fatigue tester shown in FIG. 2 to obtain a V nitride containing Cr having a grain size of 0.5 to 4.0 nm at a position 25 mm deep from the surface of the head shell.
- the rails whose number density was in the range of 1.0 to 5.0 ⁇ 10 17 cm ⁇ 3 were evaluated for internal fatigue damage resistance. Rail components, metal structure, hardness, and rolling fatigue test conditions used in the test are as shown below.
- the number density of Cr-containing V nitrides having a grain size of 0.5 to 4.0 nm in the inside of the head is 1.0 to 5 mm.
- a microscopic softening portion is suppressed in the ferrite phase having a pearlite structure inside the rail head, and the above-described crack is formed inside the rail head. There is no residual, and the internal fatigue damage resistance of the rail is greatly improved.
- the number density of Cr-containing V nitrides having a grain size of 0.5 to 4.0 nm is set to 1.0 to 5.0.
- the range is 0 ⁇ 10 17 cm ⁇ 3 .
- V nitride containing Cr having a particle size of 0.5 to 4.0 nm becomes less than 1.0 ⁇ 10 17 cm ⁇ 3 , the inside of the head (starting from the outer surface of the head and having a depth of 25 mm It is not sufficient to improve the microscopically softened portion of the ferrite phase in the pearlite structure (position), and no improvement in internal fatigue damage resistance is observed.
- the number density of Cr-containing V nitrides having a particle size of 0.5 to 4.0 nm and having a particle size of 0.5 to 4.0 nm, which is located at a position 25 mm deep from the outer surface of the head, is set to 1.0 to 5.0 ⁇ 10 The range was limited to 17 cm -3 .
- V nitride containing Cr having a grain size of 0.5 to 4.0 nm is used.
- the number density of the objects it is desirable to control the number density of the objects to 1.5 ⁇ 10 17 cm ⁇ 3 or more, 1.8 ⁇ 10 17 cm ⁇ 3 or more, or 2.0 ⁇ 10 17 cm ⁇ 3 or more.
- the number density of Cr-containing V nitrides having a particle size of 0.5 to 4.0 nm is set to 4.0 ⁇ 10 17 cm ⁇ 3 or less, 3.5 ⁇ 10 17 cm ⁇ 3 or less, or 3 It may be controlled to 0.0 ⁇ 10 17 cm ⁇ 3 or less.
- the reason for choosing the position of 2mm depth starting from the outer surface of the head as the surface of the head and the position of 25mm depth starting from the outer surface of the head as the inside of the head is that the product rail has wear resistance and internal fatigue damage resistance. This is because it is the position that shows the sex most remarkably.
- the method for measuring the hardness is as described above.
- the measurement position of the hardness may be arbitrarily selected as long as the condition is satisfied, so that a numerical value representing the entire range from the top of the rail to the corner of the head can be obtained.
- the particle size and number density of the Cr-containing V nitride can be controlled mainly by the cooling rate during accelerated cooling and the temperature holding conditions during controlled cooling after stopping accelerated cooling.
- the particle size of the Cr-containing V nitride is controlled mainly by the temperature and the holding time during controlled cooling.
- the temperature is increased and the holding time is increased, the V nitride containing Cr grows, and the particle size of the V nitride containing Cr increases.
- the temperature is lowered and the holding time is shortened, the growth of Cr-containing V nitride is suppressed, and the grain size of the V nitride is reduced.
- the number density is mainly controlled by the temperature during controlled cooling. If the temperature at the time of controlled cooling is high, the formation of Cr-containing V-nitride is promoted, and the number density of V-nitride increases. On the other hand, when the temperature at the time of controlled cooling is low, the generation of Cr-containing V nitrides is suppressed, and the number density thereof is reduced.
- the control of the particle size and the number density of the Cr-containing V nitride can be controlled mainly by the temperature holding conditions during the control cooling after the stop of the accelerated cooling, and the temperature and the holding during the control cooling are controlled.
- the mutual control of the time makes it possible to keep both the particle size and the number density of the Cr-containing V nitride within a predetermined range.
- the present inventors have studied ways to improve the characteristics in long-term use. As a result of detailed observation of the rail subjected to the fatigue test, it was confirmed that a small crack (less than 0.5 mm in length) might be formed around the V nitride containing Cr. did. The present inventors have studied a method for eliminating such minute cracks.
- the present inventors have investigated in detail the relationship between the composition of the Cr-containing V-nitride and the minute cracks formed around it.
- the survey method is as shown below.
- the measured precipitates are five or more randomly selected from Cr-containing V nitrides having a particle size of 0.5 to 4.0 nm, and the average value thereof is used as a representative value.
- VA number of V atoms
- the average value of the ratio of the number of Cr atoms (CA) is described as “CA / VA”.
- CA / VA The average value of CA / VA in the three needle samples was defined as CA / VA of the rail.
- CA / VA may be 0.65 or less, 0.60 or less, or 0.55 or less.
- CA / VA cannot be set to zero. According to the experiments of the present inventors, no rails with a CA / VA of less than 0.01 were not confirmed, so the lower limit of the CA / VA may be set to 0.01, 0.02, or 0.05. . Further, V nitrides containing Cr having a particle size of less than 0.5 nm or more than 4.0 nm are considered to have no substantial effect on the characteristics of the rail, and are therefore excluded from the measurement of CA / VA. 0.01 ⁇ CA / VA ⁇ 0.70 Formula 1
- CA The control of CA / VA can be controlled mainly by the temperature holding condition at the time of controlled cooling after stopping the accelerated cooling.
- the control of CA / VA is mainly controlled by the temperature at the time of controlled cooling. If the temperature at the time of controlled cooling is high, the number of V atoms in the V nitride containing Cr increases, and CA / VA decreases. On the other hand, when the temperature at the time of controlled cooling is low, the number of Cr atoms in the Cr-containing V nitride increases, and CA / VA increases.
- the control of CA / VA can be controlled mainly by controlling the temperature during the cooling after the stop of the accelerated cooling, and by controlling the temperature during the temperature holding, the CA / VA falls within a predetermined range. It becomes possible.
- C 0.75 to 1.20% C is an element effective for promoting the pearlite transformation and ensuring abrasion resistance. If the C content is less than 0.75%, the present component system cannot maintain the minimum strength and wear resistance required for the rail. Further, when the C content is less than 0.75%, a proeutectoid ferrite structure is generated, and the wear resistance of the rail is significantly reduced. Further, when the C content is less than 0.75%, a soft pro-eutectoid ferrite structure that easily generates a fatigue crack is formed inside the head, thereby easily causing internal fatigue damage.
- the C content is set to 0.75 to 1.20%.
- the C content is desirably 0.80% or more, 0.83% or more, or 0.85% or more.
- the C content is preferably set to 1.10% or less, 1.05% or less, or 1.00% or less.
- Si 0.10-2.00%
- Si is an element that forms a solid solution with the ferrite phase in the pearlite structure, increases the hardness (strength) of the rail head, and improves wear resistance.
- the Si content is less than 0.10%, these effects cannot be sufficiently obtained.
- the Si content exceeds 2.00%, many surface flaws are generated during hot rolling of the rail. Further, when the Si content exceeds 2.00%, the hardenability is remarkably increased, a martensite structure is formed on the rail head, and the wear resistance is reduced. Therefore, the Si content is set to 0.10 to 2.00%.
- the Si content In order to stabilize the generation of the pearlite structure and improve the wear resistance and the internal fatigue damage resistance, the Si content should be 0.20% or more, 0.4% or more, or 0.50% or more. desirable. For the same reason, the Si content is preferably set to 1.80% or less, 1.50% or less, or 1.30% or less.
- Mn 0.10-2.00%
- Mn is an element that enhances hardenability, stabilizes pearlite transformation, refines lamella spacing of pearlite structure, secures pearlite structure hardness, and further improves wear resistance and internal fatigue damage resistance. .
- the Mn content is less than 0.10%, no improvement in wear resistance is observed.
- the Mn content is less than 0.10%, a soft pro-eutectoid ferrite structure that easily generates a fatigue crack is formed inside the head, and it is difficult to secure the internal fatigue damage resistance.
- the Mn content is set to 0.10 to 2.00%.
- the Mn content should be 0.40% or more, 0.50% or more, or 0.60% or more. Is desirable.
- the Mn content is preferably set to 1.80% or less, 1.50% or less, or 1.30% or less.
- Cr 0.10 to 1.20%
- Cr is an element that raises the equilibrium transformation temperature of steel, increases the degree of undercooling, refines the lamella spacing of the pearlite structure, increases the hardness of the pearlite structure, and improves the wear resistance of the rail. Further, Cr suppresses microscopic softening of the ferrite phase in the pearlite structure inside the rail head by precipitation strengthening due to the formation of V nitride containing fine Cr in the pearlite structure ferrite phase. It is an element that improves the resistance to internal fatigue damage inside.
- the Cr content is less than 0.10%, the effect is small, the number of fine Cr-containing V nitrides precipitated in the pearlite structure ferrite phase is reduced, and the ferrite phase in the pearlite structure is microscopically observed. Improvement of the softened portion is insufficient, and no improvement in the internal fatigue damage resistance is observed.
- the Cr content exceeds 1.20%, the hardenability increases remarkably, a bainite structure or a martensite structure is formed at the head of the rail, and the wear resistance and surface damage resistance of the rail decrease.
- the Cr content is set to 0.10 to 1.20%.
- the Cr content should be 0.30% or more and 0% or less. It is desirable to set it to 0.35% or more, or 0.40% or more.
- the Cr content is preferably set to 1.10% or less, 1.00% or less, or 0.90% or less.
- V 0.010 to 0.200%
- V forms fine nitrides containing Cr in the pearlite structure ferrite phase during the cooling process after the hot rolling of the rail, and the fineness of the ferrite phase in the pearlite structure inside the rail head by precipitation strengthening. It is an element that suppresses visual softening and improves the internal fatigue damage resistance of rails.
- the V content is less than 0.010%, the number of fine Cr-containing V nitrides precipitated in the ferrite phase of the pearlite structure is small, and the ferrite phase in the pearlite structure inside the rail head is microscopically observed. The improvement of the softened part is insufficient, and the improvement of the internal fatigue damage resistance of the rail is not recognized.
- the V content is set to 0.010 to 0.200%.
- the V content should be 0.030% or more, 0.035% or more, or 0.040% or more. It is desirable to do.
- the V content is preferably set to 0.180% or less, 0.150% or less, or 0.100% or less.
- N 0.0030 to 0.0200%
- N is an element that is contained at the same time as Cr and V to promote the formation of V nitride containing Cr in the ferrite phase in the pearlite structure in the cooling process after hot rolling of the rail.
- V nitride containing fine Cr When V nitride containing fine Cr is generated, microscopic softening of the ferrite phase in the pearlite structure inside the rail head is suppressed, and the internal fatigue damage resistance of the rail is improved.
- the N content is less than 0.0030%, the number of fine Cr-containing V nitrides generated in the ferrite phase in the pearlite structure is small, and the ferrite phase in the pearlite structure inside the rail head is microscopically observed.
- the N content exceeds 0.0200%, the number of V-nitrides containing fine Cr becomes excessive, and the pearlite structure inside the rail head (at a depth of 25 mm starting from the outer surface of the head) becomes brittle. And accelerated crack generation lowers the rail's resistance to internal fatigue damage. Further, when the N content exceeds 0.0200%, it is difficult to form a solid solution of N in steel, and bubbles serving as a starting point of fatigue damage are generated, so that internal fatigue damage is likely to occur. Therefore, the N content is set to 0.0030 to 0.0200%.
- the N content In order to stably generate Cr-containing V nitrides and improve internal fatigue damage resistance, the N content should be 0.0080% or more, 0.0090% or more, or 0.0100% or more. Is desirable. For the same reason, the N content is preferably set to 0.0180% or less, 0.0150% or less, or 0.0120% or less.
- P 0.0250% or less
- P is an impurity element contained in steel, and its content can be controlled by refining in a converter.
- the P content is preferably as low as possible, but if the P content exceeds 0.0250%, the pearlite structure is embrittled, a brittle crack is generated inside the head, and the internal fatigue damage resistance of the rail is reduced. . For this reason, the P content is limited to 0.0250% or less.
- the P content may be 0.220% or less, 0.200% or less, or 0.180% or less.
- the lower limit of the P content is not limited and may be 0%. However, in consideration of the dephosphorization ability in the refining process and economic efficiency, the lower limit of the P content may be set to 0.0020%, 0.0030%, or 0.0050%.
- S 0.0250% or less
- S is an impurity element contained in steel, and its content can be controlled by performing desulfurization in a hot metal ladle.
- the S content is preferably as small as possible, if the S content exceeds 0.0250%, coarse MnS-based sulfide inclusions are easily formed, and stress concentration around the inclusions inside the head due to concentration of Fatigue cracks are formed and the internal fatigue damage resistance of the rail is reduced. For this reason, the S content is limited to 0.0250% or less.
- the S content may be 0.220% or less, 0.200% or less, or 0.180% or less.
- the lower limit of the S content is not limited and may be 0%. However, in consideration of the desulfurization ability in the refining process and economic efficiency, the lower limit of the S content may be set to 0.0020%, 0.0030%, or 0.0050%.
- the rail according to the present embodiment contains the above-mentioned chemical components, and the balance is basically made of Fe and impurities. Impurities are components that are mixed in due to various factors in the production process, such as ore or scrap, when producing steel products industrially, and do not adversely affect the rail according to the present embodiment. Means acceptable. However, instead of a part of the remaining Fe, if necessary, the hardness (strength) of the pearlite structure is further increased to improve the wear resistance and internal fatigue damage resistance, the toughness, and the weld heat-affected zone.
- each optional element is as follows.
- Mo raises the equilibrium transformation point, reduces the lamella spacing of the pearlite structure, and improves the hardness of the rail.
- Co refines the lamellar structure of the worn surface and increases the hardness of the worn surface.
- C group B reduces the dependence of the pearlite transformation temperature on the cooling rate and makes the hardness distribution of the rail head uniform.
- (Group d) Cu forms a solid solution with the ferrite phase in the pearlite structure and increases the hardness of the rail. Ni improves the toughness and hardness of the pearlite structure, and at the same time, prevents the heat-affected zone of the weld joint from softening.
- (Group e) Nb and Ti improve the fatigue strength of the pearlite structure by precipitation hardening of carbides and nitrides generated during hot rolling and subsequent cooling processes. Further, Nb and Ti stably generate carbides and nitrides at the time of reheating, and prevent softening of the heat-affected zone of the weld joint.
- G group Zr suppresses the formation of a segregation zone in the center of the slab and suppresses the formation of a proeutectoid cementite structure or a martensite structure by increasing the equiaxed crystallization ratio of a solidified structure.
- H group Al acts as a deoxidizing material. Further, Al shifts the eutectoid transformation temperature to a higher temperature side, and contributes to increase the hardness (strength) of the pearlite structure. Therefore, these elements may be contained in order to obtain the above effects. Even if these elements are contained below the range described below, the characteristics of the rail according to the present embodiment are not impaired. Since these elements do not always need to be contained, the lower limit is 0%.
- Mo preferably 0.01 to 0.50% Mo raises the equilibrium transformation temperature, increases the degree of undercooling, refines the lamella spacing of the pearlite structure, and improves the hardness (strength) of the pearlite structure. As a result, the abrasion resistance and internal resistance of the rail are improved. It is an element that improves fatigue damage. However, if the Mo content is less than 0.01%, the effect is small, and the effect of improving the hardness of the rail steel cannot be obtained. On the other hand, if the Mo content exceeds 0.50%, the transformation speed is remarkably reduced, a martensite structure is formed on the rail head, and the wear resistance may be reduced. Therefore, when Mo is contained, the Mo content is preferably set to 0.01 to 0.50%.
- Co forms a solid solution in the ferrite phase of the pearlite structure, refines the lamellar structure of the pearlite structure, and improves the hardness (strength) of the pearlite structure. As a result, the wear resistance and the internal fatigue damage resistance of the rail are improved. It is an element to improve. However, if the Co content is less than 0.01%, the refinement of the lamellar structure is not promoted, and the effect of improving wear resistance and internal fatigue damage resistance cannot be obtained. On the other hand, if the Co content exceeds 1.00%, the above effect is saturated, and it may not be possible to achieve a finer lamellar structure in accordance with the content. On the other hand, if the Co content exceeds 1.00%, the economic efficiency may decrease due to an increase in alloy addition cost. Therefore, when it is contained, the Co content is preferably set to 0.01 to 1.00%.
- B preferably 0.0001 to 0.0050%
- B is an element that forms iron carbide boride (Fe 23 (CB) 6 ) at the austenite grain boundary and reduces the cooling rate dependence of the pearlite transformation temperature by the effect of promoting pearlite transformation.
- B is an element that imparts a more uniform hardness distribution to the rail from the outer surface of the head to the inside by the above-described effect, and prolongs the life of the rail.
- the B content is less than 0.0001%, the effect is not sufficient, and no improvement is observed in the hardness distribution of the rail head.
- the B content exceeds 0.0050%, a coarse iron carbide boride is generated, which promotes brittle fracture and lowers the toughness of the rail in some cases. Therefore, when it is contained, the B content is preferably made 0.0001 to 0.0050%.
- Cu preferably 0.01 to 1.00%
- Cu is an element that forms a solid solution in the ferrite phase having a pearlite structure, improves hardness (strength) by solid solution strengthening, and improves wear resistance and internal fatigue damage resistance of the rail.
- the Cu content is less than 0.01%, the effect cannot be obtained.
- the Cu content exceeds 1.00%, a martensite structure may be formed on the rail head due to remarkable improvement in hardenability, and the wear resistance may be reduced. Therefore, in the case where Cu is contained, the Cu content is preferably set to 0.01 to 1.00%.
- Ni preferably 0.01 to 1.00%
- Ni is an element that improves the toughness of the pearlite structure, and at the same time, improves the hardness (strength) by solid solution strengthening, thereby improving the wear resistance and the internal fatigue damage resistance of the rail.
- Ni is an element that finely precipitates an intermetallic compound of Ni 3 Ti in a composite with Ti in a heat affected zone of welding, and suppresses softening by precipitation strengthening.
- Ni is an element that suppresses embrittlement of grain boundaries in Cu-containing steel. However, when the Ni content is less than 0.01%, these effects are remarkably small.
- the Ni content exceeds 1.00%, a martensite structure is formed on the rail head due to a remarkable improvement in hardenability, and the wear resistance of the rail may decrease. Therefore, when Ni is contained, the Ni content is preferably set to 0.01 to 1.00%.
- Nb preferably 0.0010 to 0.0500% Nb precipitates as Nb carbide and / or Nb nitride in the cooling process after hot rolling, and increases the hardness (strength) of the pearlite structure by precipitation hardening, thereby improving the wear resistance and the internal fatigue damage resistance of the rail. It is an element to improve. Nb also stably generates Nb carbides and Nb nitrides from a low temperature range to a high temperature range in the heat-affected zone reheated to a temperature range of less than or equal to one point of Ac. It is an effective element for preventing softening.
- the Nb content is preferably set to 0.0010 to 0.0500%.
- Ti preferably 0.0030 to 0.0500%
- Ti precipitates as Ti carbides and / or Ti nitrides in the cooling process after hot rolling, and increases the hardness (strength) of the pearlite structure by precipitation hardening, thereby improving the wear resistance and internal fatigue damage resistance of the rail. It is an element to improve.
- Ti refines the structure of the heat-affected zone heated to the austenite region temperature and embrittles the weld joint. It is an effective ingredient for preventing
- the Ti content is less than 0.0030%, these effects are small.
- the Ti content is preferably set to 0.0030 to 0.0500%.
- Mg preferably 0.0005 to 0.0200%
- Mg is an element that combines with S to form fine sulfides. This Mg sulfide finely disperses MnS, reduces stress concentration, and improves the internal fatigue damage resistance of the rail. However, if the Mg content is less than 0.0005%, the effect is weak. On the other hand, when the Mg content exceeds 0.0200%, a coarse oxide of Mg is generated, a fatigue crack is generated due to stress concentration, and the internal fatigue damage resistance of the rail may be reduced. Therefore, when it is contained, the Mg content is preferably set to 0.0005 to 0.0200%.
- Ca preferably 0.0005 to 0.0200%
- Ca is an element that has a strong bonding force with S and forms CaS (sulfide). This CaS finely disperses MnS, relieves stress concentration, and improves the internal fatigue damage resistance of the rail. However, if the Ca content is less than 0.0005%, the effect is weak. On the other hand, when the Ca content exceeds 0.0200%, a coarse oxide of Ca is generated, and due to stress concentration, a fatigue crack is generated and the internal fatigue damage resistance may be reduced. Therefore, when Ca is contained, the Ca content is preferably set to 0.0005 to 0.0200%.
- REM preferably 0.0005 to 0.0500% REM is a deoxidizing / desulfurizing element, and when contained, generates REM oxysulfide (REM 2 O 2 S) which becomes a nucleus for generating Mn sulfide-based inclusions. Since this oxysulfide (REM 2 O 2 S) has a high melting point, the stretching of the Mn sulfide-based inclusion after rolling is suppressed. As a result, due to the inclusion of REM, MnS is finely dispersed, stress concentration is reduced, and the internal fatigue damage resistance of the rail is improved.
- the REM content is less than 0.0005%, the nuclei for forming MnS-based sulfides are insufficient, and the effect is small.
- the REM content exceeds 0.0500%, hard REM oxysulfide (REM 2 O 2 S) is excessively generated, fatigue cracks are generated due to stress concentration, and internal fatigue damage resistance is reduced. May decrease.
- the REM content is preferably made 0.0005 to 0.0500%.
- RREM is a rare earth metal such as Ce, La, Pr or Nd.
- the REM content is the sum of the contents of all these REMs. As long as the total sum of the contents is within the above range, the same effect can be obtained regardless of whether it is a single form or a composite form (two or more types).
- Zr preferably 0.0001 to 0.0200% Zr combines with O to form ZrO 2 inclusions. Since the ZrO 2 inclusions and ⁇ -Fe have good lattice matching, the ZrO 2 inclusions become solidification nuclei of high carbon rail steel in which ⁇ -Fe is the primary solidification crystal, and the equiaxed crystallization ratio of the solidification structure , The formation of a segregation zone at the center of the slab is suppressed.
- Zr is an element that suppresses the formation of a segregation zone at the center of the slab, thereby suppressing the formation of a martensite structure generated in the rail segregation portion.
- the Zr content is less than 0.0001%, the number of generated ZrO 2 -based inclusions is small, and does not exhibit a sufficient effect as a solidification nucleus.
- the Zr content exceeds 0.0200%, a large amount of coarse Zr-based inclusions are generated, fatigue cracks are generated due to stress concentration, and the internal fatigue damage resistance of the rail may be reduced. .
- the Zr content is preferably made 0.0001 to 0.0200%.
- Al preferably 0.0100 to 1.00%
- Al is an element that acts as a deoxidizing material. Further, Al is an element that shifts the eutectoid transformation temperature to a higher temperature side and contributes to increase the hardness (strength) of the pearlite structure, and as a result, improves the wear resistance and internal fatigue damage resistance of the pearlite structure. Element that causes However, if the Al content is less than 0.0100%, the effect is weak. On the other hand, when the Al content exceeds 1.00%, it is difficult to form a solid solution of Al in steel, and coarse alumina-based inclusions are generated. Since this coarse Al-based inclusion becomes a starting point of a fatigue crack, the internal fatigue damage resistance of the rail may be reduced. Further, when the Al content exceeds 1.00%, an oxide is generated at the time of welding, and the weldability may be significantly reduced. For this reason, when it is contained, the Al content is preferably set to 0.0100 to 1.00%.
- the rail according to the present embodiment controls the alloy composition and the structure of the rail steel, the hardness of the head surface and the inside of the head, the number density of V nitride containing fine Cr, and the V content containing Cr.
- the composition of the nitride By controlling the composition of the nitride, the wear resistance and the internal fatigue damage resistance of the rail when used in a freight railway can be improved, and the service life can be greatly improved.
- the rail according to the present embodiment can obtain the effects irrespective of the manufacturing method by being provided with the above components, structures, and the like. However, according to the manufacturing method including the following steps, the rail according to the present embodiment can be stably obtained, which is preferable.
- the method for manufacturing a rail according to the present embodiment includes heating a steel slab having the chemical composition of the rail according to the present embodiment, hot rolling the heated steel slab to form a rail, and accelerating and cooling the rail. Obtained by controlled cooling. Preferred production conditions are as shown in the table below, and the specific reasons will be described below. In addition, the final rolling reduction is the cross-sectional reduction rate of the rail head cross-section. Further, the temperature (excluding the billet temperature) indicated as the heat treatment condition means the temperature of the outer surface of the head of the rail.
- a V-nitride containing Cr at a position at a depth of 25 mm from the head outer surface as a tissue, hardness, and a structure up to a depth of 25 mm starting from the head outer surface is used.
- the configuration of the other parts is not particularly limited, so that the heat treatment conditions are also determined with respect to the head shell surface.
- the rail of the present embodiment is a steel slab obtained by casting molten steel whose components have been adjusted by smelting in a commonly used melting furnace such as a converter or an electric furnace by an ingot-bulking method or a continuous casting method. Bloom or slab), and remanufacture the steel slab by hot rolling to form a rail shape, followed by heat treatment after hot rolling.
- the chemical composition of the billet may be within the same range as the chemical composition of the rail according to the present embodiment described above.
- the heating of the steel slab is the most important step in stably generating fine V nitride containing Cr by rail heat treatment. Since no controlled cooling is performed during the production of the billet, the V nitride containing Cr is coarsened at the stage of the billet. Therefore, in order to stably generate fine Cr-containing V nitride after the rail heat treatment, it is necessary to re-dissolve Cr-containing Cr-nitride coarsened in the steel slab before rolling. Therefore, it is necessary to control the heating condition of the steel slab in the temperature range (1000 to 1200 ° C.) in which the V nitride containing Cr is re-dissolved.
- the heating conditions of the billet are preferably as follows. Heating rate: 1-8 ° C / min Speed control temperature range: 1000-1200 ° C
- the above temperature is a temperature condition of the steel slab, and the temperature control of the heating furnace is desirably controlled to meet the above heating condition. It should be noted that the heating rate of the slab before hot rolling is not the average heating rate. That is, this heating rate indicates a sequential heating rate during heating. In the method of manufacturing a rail according to the present embodiment, it is necessary to always increase the temperature at a rate of 1 to 8 ° C./min while raising the temperature of the slab from 1000 ° C. to 1200 ° C.
- T (t) when the relationship between the temperature T [° C.] of the slab and the time t [min] is defined as T (t), in the method of manufacturing a rail according to the present embodiment, the temperature of the slab is changed from 1000 ° C. to 1200 ° C. It is necessary that dT (t) / dt [° C./min] is always 1 or more and 8 or less while the temperature is raised to ° C.
- the reason why the heating rate of the steel slab is preferably in the range of 1 to 8 ° C./min will be described. If the heating rate is less than 1 ° C / min, the Cr-containing V nitride coarsened during casting is redissolved, but re-precipitated during heating, and the Cr-containing V nitride is coarsened and dissolved. In some cases, it may be difficult to stably generate fine V nitride containing Cr in the rail heat treatment. Further, when the heating rate is less than 1 ° C./min, the heating of the slab becomes excessive, and the decarburization of the slab surface proceeds, and at the same time, the slab cracks and the like are generated. Quality may not be guaranteed. On the other hand, if the heating rate is less than 1 ° C./min, a large amount of the heating fuel is used, so that the economy may decrease.
- the heating rate is preferably in the range of 1 to 8 ° C./min.
- the heating rate may be 2 ° C / min or more, or 3 ° C / min or more.
- the heating rate may be 7 ° C / min or less, 6 ° C / min or less, or 5 ° C / min or less.
- this heating rate indicates a sequential heating rate during heating of the billet.
- the heating rate after the billet temperature exceeds 1200 ° C. is not particularly limited.
- the temperature at which heating of the billet is stopped can be an arbitrary value of 1200 ° C. or more.
- the heating end temperature of the billet may be 1220 ° C. or higher, 1250 ° C. or higher, or 1300 ° C. or higher.
- the accelerated cooling is cooling performed by injecting a coolant such as water onto the rail surface.
- the start time and the end time of the accelerated cooling are the start time and the end time of the injection of the refrigerant.
- the cooling rate during accelerated cooling means the average cooling rate.
- the difference between the rail surface temperature between the start time and the end time of the accelerated cooling is defined as the start time and the end time of the accelerated cooling. Is the value obtained by dividing by the elapsed time between.
- Hot rolling conditions Final rolling temperature of the outer surface of the head: 850 to 1000 ° C
- Controlled cooling head outer surface
- the temperature of the outer surface of the head is maintained in a range of 580 to 660 ° C. for 5 to 150 seconds, and then the cooling and the accelerated cooling are performed.
- Temperature is controlled by repeatedly stopping and performing accelerated cooling in response to reheating from inside the rail
- Accelerated cooling head outer surface
- Average cooling rate 2-30 ° C / sec
- Accelerated cooling start temperature 750 ° C or higher
- Accelerated cooling stop temperature 600 to 650 ° C
- Controlled cooling head outer surface
- the temperature of the head surface is maintained in the range of 600 to 650 ° C. for 20 to 150 seconds, and then the substrate is cooled down and accelerated.
- Temperature maintenance during controlled cooling The accelerated cooling rate is controlled, and the accelerated cooling is repeatedly executed and stopped in accordance with the reheating from inside the rail, so that the temperature is controlled to be within a predetermined temperature range.
- the final rolling temperature (the outer surface of the head) of the hot rolling be in the range of 850 to 1000 ° C.
- the final rolling temperature (outer surface of the head) is lower than 850 ° C.
- the fineness of the austenite grains after rolling becomes remarkable.
- the hardenability is significantly reduced, and it may be difficult to secure the hardness of the rail head.
- the final rolling temperature (the outer surface of the head) exceeds 1000 ° C., austenite grains after rolling are coarsened, the hardenability is excessively increased, and a bainite structure harmful to wear resistance is formed on the rail head. It will be easier.
- the final rolling temperature (outer surface of the head) be in the range of 850 to 1000 ° C.
- the final rolling temperature may be 860 ° C or higher, 880 ° C or higher, or 900 ° C or higher.
- the final rolling temperature may be 980 ° C or lower, 960 ° C or lower, or 940 ° C or lower.
- the reason why the final rolling reduction (area reduction rate) of the hot rolling is preferably in the range of 2 to 20% will be described. If the final reduction (cross-sectional area reduction rate of the rail head) is less than 2%, the austenite grains after rolling become coarse, the hardenability increases excessively, and a bainite structure harmful to wear resistance is formed on the rail head. In some cases, the grain size of the pearlite structure itself becomes coarse, and the ductility and toughness required for the rail may not be secured. On the other hand, when the final reduction (rail cross-sectional area reduction ratio) exceeds 20%, the austenite grains after rolling are remarkably refined, hardenability is significantly reduced, and the hardness of the rail head is secured. Becomes difficult.
- the final reduction amount (cross-sectional area reduction rate of the rail head) be in the range of 2 to 20%.
- the final rolling reduction (area reduction ratio) may be 4% or more, 6% or more, or 8% or more.
- the final rolling reduction (area reduction ratio) may be 18% or less, 16% or less, or 14% or less.
- ⁇ ⁇ Other rolling conditions of the rail head are not particularly limited as long as the above conditions are satisfied.
- a pearlite structure may be mainly obtained by referring to a method described in JP-A-2002-226915. That is, after roughly rolling the slab, intermediate rolling by a reverse rolling mill is performed over a plurality of passes, and then finish rolling by a continuous rolling mill is performed by two or more passes. At the time of final rolling of finish rolling, the temperature may be controlled within the above range.
- the reason why the average cooling rate of accelerated cooling (the outer surface of the head) is preferably set to 2 to 30 ° C./sec will be described.
- the average cooling rate is less than 2 ° C./sec, pearlite transformation starts in a high temperature range during accelerated cooling.
- a portion having a hardness of less than Hv360 occurs on the surface of the rail head, and the required wear resistance and internal fatigue damage resistance of the rail can be secured. It can be difficult.
- the average cooling rate in the accelerated cooling be 2 to 30 ° C./sec.
- the average cooling rate in accelerated cooling may be 3 ° C./sec or more, 4 ° C./sec or more, or 5 ° C./sec or more.
- the average cooling rate in accelerated cooling may be 25 ° C / sec or less, 20 ° C / sec or less, or 15 ° C / sec or less.
- the start temperature of the accelerated cooling ie, the rail temperature at the start of the spraying of the refrigerant
- the stop temperature ie, the rail temperature at the end of the spraying of the refrigerant
- the start temperature of the accelerated cooling of the outer surface of the head is lower than 750 ° C.
- a pearlite structure may be generated in a high temperature region before the accelerated cooling. In this case, a predetermined hardness cannot be obtained, and it becomes difficult to secure the wear resistance and surface damage resistance required for the rail.
- the temperature of the outer surface of the head of the rail at the start of accelerated cooling be 750 ° C. or higher.
- accelerated cooling needs to be started within 180 seconds after completion of hot rolling in order to set the start temperature of accelerated cooling to 750 ° C. or higher.
- the stop temperature of accelerated cooling exceeds 660 ° C.
- pearlite transformation starts in a high temperature range immediately after cooling, and many pearlite structures with low hardness are generated.
- the hardness cannot be secured on the surface of the rail head, and it may be difficult to secure the wear resistance and surface damage resistance required for the rail.
- the stop temperature of the accelerated cooling is set to less than 580 ° C., immediately after cooling, many bainite structures harmful to wear resistance are generated on the surface of the rail head, and it is difficult to secure the required wear resistance as a rail. May be. For this reason, it is preferable that the stop temperature of the accelerated cooling be in the range of 580 to 660 ° C.
- the heat treatment refrigerant on the rail head during accelerated cooling is not particularly limited.
- heat treatment is performed by air jet cooling, mist cooling, mixed jet cooling of water and air, or a combination thereof. It is desirable to control the cooling rate of the rail head at the time.
- the reasons for limiting the preferable conditions of the controlled cooling performed after the accelerated cooling will be described.
- This step has a significant effect on the number density and grain size of the Cr-containing V nitride.
- the temperature of the rail in the controlled cooling, is kept within a certain range for a certain period of time by injecting a refrigerant in accordance with the degree of reheating, and then the temperature of the rail is lowered. That is, it can be said that the controlled cooling step is a combination of the temperature holding step and the subsequent cooling step.
- the end point of the accelerated cooling is defined as the start point of the temperature holding in the control cooling.
- the end of the accelerated cooling causes reheating of the rail, and the temperature of the surface of the rail usually rises.
- the temperature of the surface of the rail rises to some extent due to the reheating, the temperature of the surface of the rail is lowered again by injecting the refrigerant to the rail.
- the injection of the refrigerant to the rail is stopped to increase the temperature of the surface of the rail again.
- the maintenance of the temperature in the controlled cooling of the rail is usually achieved by repeating the temperature increase by the recuperation and the temperature processing by the cooling. In this way, accelerated cooling is stopped on the low temperature side of the temperature range where the temperature is to be maintained, cooling is started in anticipation of recuperation occurring from inside the rail head, and cooling is performed before reaching the lower limit of the predetermined temperature range. It is desirable to stop. In order to control the holding time, it is desirable to repeatedly execute this temperature control. If the amount of recuperation is small, it is also effective to heat with an IH coil or the like. However, the degree of recuperation is small, and the temperature fluctuation on the rail surface may be kept within a certain range without injecting the refrigerant. In this case, the temperature can be maintained simply by leaving the rail alone.
- the temperature of the rail surface be in the range of 580 to 660 ° C.
- the fluctuation range of the temperature of the rail surface be within 60 ° C.
- the temperature holding time be 5 to 60 ° C. It is preferable to set the range to 150 sec.
- the reason why the holding temperature after accelerated cooling is preferably in the range of 580 to 660 ° C. and the fluctuation width of the temperature on the rail surface is preferably within 60 ° C. will be described.
- the holding temperature exceeds 660 ° C.
- pearlite transformation starts in a high temperature range immediately after cooling, and a large pearlite structure with low hardness is generated on the surface of the rail head.
- the hardness cannot be secured, and it becomes difficult to secure the wear resistance and surface damage resistance required for the rail.
- the formation of V nitride containing Cr is promoted inside the rail head, and the number density is excessively increased.
- the pearlite structure inside the rail head is embrittled, crack generation is promoted, and internal fatigue damage resistance is reduced.
- the holding temperature after accelerated cooling is preferably in the range of 580 to 660 ° C.
- the fluctuation range of the temperature of the rail surface exceeds 60 ° C., the macro hardness of the pearlite structure becomes uneven on the rail head surface, and as a result, the abrasion resistance and internal fatigue damage resistance required for the rail are reduced. There is concern that it will be difficult to secure them. For this reason, it is preferable that the fluctuation range of the temperature of the rail surface be 60 ° C. or less.
- the holding time is preferably in the range of 5 to 150 sec.
- the holding time is from the end of the above-described accelerated cooling to the end of the last recuperation (when the rail temperature starts to drop naturally, or In the case where the temperature is maintained only by the recuperation or the transformation heat generation, the temperature is maintained until the end of the recuperation or the transformation heat generation from the end of the above-mentioned accelerated cooling (the rail temperature starts to decrease naturally). (The time point or the time point at which refrigerant spraying starts). If the holding time exceeds 150 sec, tempering of the pearlite structure proceeds during the holding, and the pearlite structure softens.
- V nitride containing Cr grows inside the rail head, and the grain size increases.
- the number density of V nitride containing fine Cr decreases, and improvement of microscopic softening of the ferrite phase in the pearlite structure cannot be expected.
- the holding time is less than 5 seconds, the pearlite transformation is not completed during the holding, and a martensite structure is generated. As a result, it becomes difficult to secure wear resistance and internal fatigue damage resistance on the rail head surface and inside the head.
- the time for maintaining the temperature after the accelerated cooling be 5 to 150 sec.
- the method of maintaining the temperature during controlled cooling is not particularly limited. Controls recuperation from inside the rail head by repeatedly cooling and stopping the outer surface of the rail head using air-jet cooling, mist cooling, mixed injection cooling of water and air, or a combination of these. It is desirable to perform cooling.
- the holding temperature is set in the range of 600 to 650 ° C. in the above-mentioned controlled cooling.
- the reason why the holding time is preferably in the range of 20 to 120 sec will be described.
- the holding temperature is lower than 600 ° C.
- the number of Cr atoms in the Cr-containing V nitride increases, so that CA / VA increases, making it difficult to satisfy a predetermined CA / VA value.
- the holding temperature exceeds 650 ° C.
- the number of V atoms in the Cr-containing V nitride increases, making it difficult to stably maintain the CA / VA value.
- the holding temperature is in the range of 600 to 650 ° C.
- the holding temperature is preferably set in the range of 20 to 120 seconds.
- the rail is cooled down and accelerated. If the cooling rate of the rail after the isothermal holding is too low, as in the case where the isothermal holding is continued for a long time, the tempering of the pearlite structure proceeds during the holding, and the rail head surface, the hardness inside the head may not be secured, Further, the number density of V nitride containing fine Cr may be reduced. Therefore, it is considered that a cooling rate of 0.5 ° C./sec or more needs to be maintained at least up to about 200 ° C. to prevent this. Such cooling conditions can be achieved by leaving the rail in the normal temperature atmosphere or performing accelerated cooling after the above-mentioned temperature holding.
- the following rails had the following manufacturing conditions as described in the remarks column of the table.
- No. In No. 49 the end temperature of the accelerated cooling was 560 ° C., but other conditions were as described above.
- No. In No. 50 the average cooling rate during accelerated cooling was 35.0 ° C./sec, but other conditions were as described above.
- No. In No. 53 the average cooling rate during accelerated cooling was 1.0 ° C./sec, but other conditions were as described above.
- the end temperature of the accelerated cooling was 680 ° C., but other conditions were as described above.
- No. 57 the heating rate of the steel slab in the range of 1000 to 1100 ° C.
- the heating rate of the steel slab in the range of 1100 to 1200 ° C. was set to 5 ° C./min.
- the conditions were as described above.
- No. 58 the heating rate of the steel slab in the range of 1100 to 1200 ° C. was 12 ° C./min
- the heating rate of the steel slab in the range of 1000 to 1100 ° C. was 6 ° C./min
- the conditions were as described above.
- No. In No. 59 the heating rate of the steel slab in the range of 1000 to 1100 ° C. was 0.5 ° C./min, but the heating rate of the steel slab in the range of 1100 to 1200 ° C. was 4 ° C./min.
- the heating rate of the steel slab in the range of 1000 to 1200 ° C. was 6.0 ° C./min, but other conditions were as described above.
- the heating rate of the steel slab in the range of 1000 to 1200 ° C. was 5.0 ° C./min, but other conditions were as described above.
- the heating rate of the steel slab in the range of 1000 to 1200 ° C. was 3.0 ° C./min, but other conditions were as described above.
- the heating rate of the steel slab in the range of 1000 to 1200 ° C. was 2.0 ° C./min, but other conditions were as described above.
- the area ratio of the pearlite structure is obtained by cutting a sample from the cross section of each rail head, polishing each sample with 3% nital etching after polishing the diamond, and then observing the structure using an optical microscope ( ⁇ 200).
- the measurement visual fields were any 10 visual fields at a depth of 2 mm from the outer surface of the head and any 10 visual fields at a depth of 25 mm from the outer surface of the head.
- the average value of the area ratio of the pearlite tissue in any 10 visual fields at a depth of 2 mm from the surface of the head is referred to as “surface pearlite ratio”, and the area ratio of the pearlite tissue in any 10 visual fields at a depth of 25 mm from the surface of the head is calculated.
- the average value was defined as “25% position pearlite ratio”. In the case of both rails having a surface area of 95% or more, it was determined that a tissue extending from the outer surface of the head to a depth of 25 mm contained a pearlite structure having an area ratio of 95% or more.
- the hardness was determined by cutting out a sample from the cross section of each rail head and polishing a portion corresponding to the rail cross section of each sample with diamond abrasive grains having an average particle diameter of 1 ⁇ m.
- the hardness was measured according to JIS Z 2244 using Twenty points were measured at an arbitrary position at a depth of 2 mm from the outer surface of the head, and the average value was defined as the surface hardness. Twenty points were measured at an arbitrary position at a depth of 25 mm from the outer surface of the head, and the average value was taken as the hardness at the position of 25 mm.
- Both of the rails having a range of Hv 360 to 500 were determined to be the rails having a tissue hardness in the range of Hv 360 to 500 in a range up to a depth of 25 mm starting from the outer surface of the head.
- the state of the inclusions was determined by collecting several needle samples having a radius of curvature of 30 to 80 nm from the ferrite phase in the pearlite structure at a depth of 25 mm starting from the outer surface of the head by FIB (focused ion beam) method. These were determined by evaluating them using a three-dimensional atom probe (3DAP) method. The details of the evaluation conditions are as described above.
- the V-nitride containing Cr having a grain size of 0.5 to 4.0 nm in the ferrite phase in the pearlite structure at a depth of 25 mm from the outer surface of the head as a starting point was obtained in each needle sample.
- the average value of the number density is referred to as “Cr-containing V-nitride number density”.
- the average value of the ratio of CA to VA of the contained V nitride was defined as “CA / VA”.
- the characteristics of the rail were evaluated by a rolling fatigue tester using the rolling fatigue tester shown in FIG.
- the shape of the test piece was a 141-lb rail of 2 m
- the AAR type wheel (920 mm in diameter) was brought into contact with the rail
- the load applied to the wheel was radial: 275 to 325 KN
- the thrust was 50 to 80 KN.
- No lubricant was used in the evaluation of wear resistance, and oil lubrication was performed in the evaluation of internal fatigue damage resistance.
- abrasion resistance In the evaluation of abrasion resistance, the above test was conducted five times until the abrasion amount of the rail head surface layer exceeded 25 mm, and the average value of the cumulative passing tonnage when the abrasion amount reached 25 mm was used as the abrasion resistance of the rail. Sex index.
- the evaluation criteria were as follows. Rails determined to be ranked A to C in the following evaluation criteria were determined to be rails having excellent wear resistance.
- the rail having the chemical composition, the area ratio of the pearlite structure, the hardness, and the number density of the V nitride containing Cr within the range of the present invention has the wear resistance and the internal fatigue damage resistance. Excellent.
- rails having a CA / VA within the range of the present invention were more excellent in wear resistance and internal fatigue damage resistance.
- the comparative example rail in which one or more of the chemical composition, the area ratio of the pearlite structure, the hardness, and the number density of the V nitride containing Cr are out of the range of the present invention has abrasion resistance and internal fatigue damage resistance.
- One or both genders failed.
- No. In No. 2 the internal fatigue damage resistance was impaired. This is presumably because the amount of C was excessive and the amount of pearlite structure was insufficient due to the generation of a large amount of proeutectoid cementite.
- No. In No. 7 the internal fatigue damage resistance was impaired.
- the wear resistance and the internal fatigue damage resistance of the rail can be improved. Therefore, according to the present invention, for example, the service life of a rail used in a freight railway can be greatly improved.
Abstract
Description
本願は、2018年9月10日に、日本に出願された特願2018-168799号に基づき優先権を主張し、その内容をここに援用する。
(2)上記(1)に記載のレールでは、さらに、前記頭部外郭表面から深さ25mmの位置の、前記パーライト組織中の前記フェライト相における粒径が0.5~4.0nmの前記Crを含有するV窒化物において、Vの原子数をVA、Crの原子数をCAとしたとき、CA/VAの平均値が下記式1を満足してもよい。
0.01≦CA/VA≦0.70… 式1
(3)上記(1)または(2)に記載のレールでは、単位質量%で、a群:Mo:0.01~0.50%、b群:Co:0.01~1.00%、c群:B:0.0001~0.0050%、d群:Cu:0.01~1.00%、及びNi:0.01~1.00%の1種または2種、e群:Nb:0.0010~0.0500%、及びTi:0.0030~0.0500%の1種または2種、f群:Mg:0.0005~0.0200%、Ca:0.0005~0.0200%、及びREM:0.0005~0.0500%の1種または2種、g群:Zr:0.0001~0.0200%、h群:Al:0.0100~1.00%の群から選択される1群または2群以上を含有してもよい。
(4)本発明の別の態様に係るレールの製造方法は、単位質量%で、C:0.75~1.20%、Si:0.10~2.00%、Mn:0.10~2.00%、Cr:0.10~1.20%、V:0.010~0.200%、N:0.0030~0.0200%、P≦0.0250%、S≦0.0250%、Mo:0~0.50%、Co:0~1.00%、B:0~0.0050%、Cu:0~1.00%、Ni:0~1.00%、Nb:0~0.0500%、Ti:0~0.0500%、Mg:0~0.0200%、Ca:0~0.0200%、REM:0~0.0500%、Zr:0~0.0200%、及びAl:0~1.00%を含有し、残部がFeおよび不純物からなる鋼片を、加熱終了温度を1200℃以上とし、1000~1200℃の範囲内での加熱速度を1~8℃/minとして加熱する工程と、加熱された前記鋼片を、最終圧延温度を850~1000℃の範囲内とし、且つ最終圧下量を2~20%として熱間圧延し、これによりレールを形成する工程と、前記レールを、加速冷却の開始温度を750℃以上とし、前記加速冷却の際の平均冷却速度を2~30℃/secとし、前記加速冷却の終了温度を580~660℃として加速冷却する工程と、前記レールを、保持温度を580~660℃の範囲内とし、温度保持時間を5~150secとして、レール表面温度の変動幅を60℃以下とするように制御冷却する工程と、前記レールを常温まで放冷又は加速冷却する工程とを備える。
(i)所定の化学組成を有している。
(ii)頭部外郭表面を起点として深さ25mmまでの範囲の組織が、面積率で、95%以上のパーライト組織を含み、かつ、前記組織の硬さがHv360~500の範囲である。
(iii)前記頭部外郭表面を起点として深さ25mmの位置の、パーライト組織中のフェライト相において、粒径が0.5~4.0nmのCrを含有するV窒化物の個数密度が1.0~5.0×1017cm-3の範囲である。
(iv)好ましくは、さらに、頭部外郭表面から深さ25mmの位置の、パーライト組織中のフェライト相における粒径が0.5~4.0nmのCrを含有するV窒化物において、Vの原子数をVA、Crの原子数をCAとしたとき、CA/VAの平均値が下記式1を満足する(なお、粒径が0.5~4.0nmのCrを含有するV窒化物のCA/VAの平均値を単に「CA/VA」と記載する場合がある)。
0.01≦CA/VAの平均値≦0.70… 式1
本実施形態に係るレールでは、頭部外郭表面を起点として少なくとも25mm深さの範囲において、95%(面積率)以上をパーライト組織とする必要がある。
車輪と接触するレール頭部では耐摩耗性の確保が最も重要である。本発明者らが金属組織と耐摩耗性との関係を調査した結果、パーライト組織が最も耐摩耗性に優れることが確認された。また、パーライト組織は合金元素の含有量が少なくても硬さ(強度)が得られ易く、耐内部疲労損傷性にも優れる。そこで、耐摩耗性および耐内部疲労損傷性を向上させる目的からパーライト組織の面積率を95%以上に限定した。パーライト組織の面積率が95%未満では、耐摩耗性および耐内部疲労損傷性が十分に向上しない。なお、耐摩耗性を十分に確保するには、レール頭部の金属組織の96%以上、97%以上、98%以上、又は99%以上をパーライト組織とすることが望ましい。レール頭部におけるパーライト組織の面積率を100%としてもよい。
[金属組織の評価手順および方法]
●評価手順
測定用試験片採取:レール頭部の横断面からサンプルを切り出し
事前処理:サンプルをダイヤモンド研磨後に3%ナイタールエッチング処理
組織観察:光学顕微鏡(200倍)
視野:頭部外郭表面から深さ2mmの任意の10視野以上、及び頭部外郭表面から深さ25mmの任意の10視野以上
●評価方法
組織判断:金属組織学の教科書(例えば、「入門・金属材料の組織と性質 材料を生かす熱処理と組織制御」:日本熱処理技術協会)等で判断、不明な場合はSEM観察
比率判断:各組織の面積測定、視野内の面積率を算定、全視野の平均値をその部位の代表値とする
本実施形態に係るレールでは、パーライト組織を含む組織の硬さをHv360~500の範囲に限定とする必要がある。次に、本実施形態に係るレールにおいて、パーライト組織を含む組織の硬さをHv360~500の範囲に限定した理由について説明する。
●鋼成分
0.90%C-0.50%Si-0.70%Mn-0.50%Cr-0.010~0.200%V-0.0150%P-0.0120%S-0.0030~0.0200%N(残部Fe及び不純物)
●レール形状
141ポンド(重さ:70kg/m)。
●圧延・熱処理条件
最終圧延温度(頭部外郭表面):950℃。
熱処理条件:圧延→加速冷却
加速冷却条件(頭部外郭表面):冷却速度で2~15℃/secで800℃から580~680℃まで冷却
なお、加速冷却は空気、冷却水などの冷媒をレール表面に噴射することにより実施した。本実施形態において、加速冷却の開始時点及び終了時点とは、冷却水の噴射の開始時点及び終了時点である。
●試験条件
試験機:転動疲労試験機(図2参照)
試験片形状 レール:141ポンドレール×2m
車輪:AARタイプ(直径920mm)
荷重 ラジアル:275~325KN
スラスト:50~80KN
潤滑:無潤滑(耐摩耗性)、油潤滑(耐内部疲労損傷性)
累積通過トン数
無潤滑(耐摩耗性):レール頭表層部の摩耗量が25mm超まで
油潤滑(耐摩耗性):き裂発生まで(最大200MGT)(Million Gloss Tonnage)※レールの上を走行した貨車の総重量、本試験の場合は車輪から作用した通貨重量の2倍で評価。
●評価
耐摩耗性:摩耗量が25mmに達した際の累積通過トン数とした。
耐内部疲労損傷性:超音波探傷装置を用いて、レール全長での頭部内部のき裂の有無を調査し、き裂長さ2mm以上のき裂を損傷と判断し、き裂発生までの累積通過トン数とした。なお、試験は評価数3とし、き裂発生までの累積通過トン数はその最小値を代表値とした。
[レール頭部の硬さの測定方法および測定条件]
●測定方法
装置:ビッカース硬度計(荷重98N)
測定用試験片採取:レール頭部の横断面からサンプルを切り出し。
事前処理:横断面を平均粒径1μmのダイヤモンド砥粒で研磨。
測定方法:JIS Z 2244に準じて測定。
●算定方法
頭部表面:頭部外郭表面から深さ2mmの任意位置において20点の測定を行い、平均値を頭部表面の硬さとした。
頭部内部:頭部外郭表面から深さ25mmの任意位置において20点の測定を行い、平均値を頭部内部の硬さとした。
次に、頭部外郭表面を起点として深さ25mmの位置における横断面において、粒径が0.5~4.0nmのCrを含有するV窒化物の個数密度を1.0~5.0×1017cm-3の範囲に限定した理由を説明する。なお、本実施形態における「Crを含有するV窒化物」とは、V窒化物から構成される介在物であって1個以上のCr原子を含有するものを意味する。後述する3次元アトムプローブ(3DAP)法によれば、Cr原子の有無を確認することが出来る。
●鋼成分
0.90%C-0.50%Si-0.70%Mn-0.50%Cr-0.0150%P-0.0120%S-0.010~0.200%V-0.0030~0.0200%N(残部Fe及び不純物)
●レール形状
141ポンド(重さ:70kg/m)。
●圧延・熱処理条件
最終圧延温度(頭部外郭表面):950℃。
熱処理条件:圧延→加速冷却+制御冷却
加速冷却条件(頭部外郭表面):冷却速度5℃/secで800℃から660~580℃まで冷却
制御冷却条件(頭部外郭表面):加速冷却停止後に580~660℃の温度域で5~120sec保持し、その後に加速冷却
制御冷却時の温度保持:加速冷却速度の制御、さらには、加速冷却の実行、停止を繰返し行い、レール内部からの復熱に応じて加速冷却を行うことによって温度を制御した。
[Crを含有するV窒化物の調査方法]
●試料採取位置:頭部内部(頭部外郭表面を起点として深さ25mmの位置)
●事前処理:FIB(集束イオンビーム)法によって曲率半径30~80nmの針試料を3個作成
●測定機:3次元アトムプローブ(3DAP)法
●測定方法
針試料にDC電圧印加し、さらにパルス電圧を印可するか、または針試料にパルスレーザーを照射することによって、針先端から構成原子のイオンを電界蒸発させる。このイオンを座標検出機により検出する。イオン飛行時間によって元素の種類を特定する。検出した座標及び測定順番によって3次元での元素位置や原子数を特定する。
電圧:DC、電圧パルス(パルス比15%以上)またはレーザーパルス(40pJ) 試料温度:40Kから70K
●Crを含有するV窒化物の判定方法およびカウント方法
IVASソフトウエア(CAMECA製)を用いて、測定データの解析を行った。質量電荷比スペクトルにおいて、25.5DaのピークをV2+と同定し、25、26、26.5のピークをCr2+と同定した。Nは、NN+のピークがFe2+の主ピークと重なるので、本実施形態に係るレールの化学組成においては直接的に認識できない。そこで、32.5Daに現れるNV2+のピークを、Nと同定した。このピークに対応するイオンはNと等量のVを含んでいることになる。
イオンを検出した座標及び測定順番に基づいて3D元素マップを得た後に、V、CrNの原子位置データを用いて、窒化析出物を判定する。これには、例えば、IVASに含まれているMaximum Separation Methodを用いる。これは、互いの距離が特定の値以下であるV、Cr、N原子の群をマトリックスと切り離し、析出物と認識する方法である。本実験では、「特定の値」として1nmを用いた。
上記の方法で析出物の認識を行った後、IVASソフトウエアを用いて、測定領域内の、パーライト組織中のフェライト相における、Crを含有するV析出物と判定された析出物の数をカウントする。
なお、パーライト組織中にはフェライト相とセメンタイト相とが存在する。本実施形態に係るレールにおいては、Crを含有するV窒化物はパーライト組織中のフェライト相の強化に用いられるので、本実験では、パーライト組織中のフェライト相の中央部に存在するもののみを評価対象とした。測定領域におけるセメンタイト相とフェライト相との分離は、C分布から判断可能である(セメンタイト相ではC濃度が原子数比率で25%になる)。
●Crを含有するV窒化物の個数密度の測定方法
上記の方法で判定された、Crを含有する窒化物の個数密度の測定は以下のように行う。
分析領域の体積については、3DAPによって測定される分析領域に含まれる原子の個数から推定する。一般の鋼の場合、鉄以外の合金元素は非常に少ない、そこで、分析領域を構成する原子を全て鉄原子と仮定して分析領域の体積を分析領域中の元素の個数から算出したとしても、真値と大きな差異はないと考えられる。そこで、鉄の原子数をイオン検出器の検出率で補正し、その値をFeの原子密度(85個数/nm3)で割った値を測定部位の体積(nm3)とみなせる。検出率は装置によって様々であるが、本実験で用いた装置では検出率が35%であったので、検出された原子数を0.35で割った値を、分析領域に含まれる原子の個数と推定した。
析出物が分布したフェライト相の中央部の領域に含まれている析出物個数を、その切り出した領域の体積で割ることで、パーライト組織中のフェライト相における粒径が0.5~4.0nmのCrを含有するV窒化物の個数密度を求めることができる。例えば、フェライト相中の鉄3000万原子の相当する体積の測定で、1個の析出物が観察された場合は、分析領域の体積は3×107/0.35(イオン検出器の検出率)/85個数(Feの原子密度)=1.0×106nm3となり、個数密度は1.0×10-6nm-3となる。単位をcm-3に変換する場合には、この値に1021を掛ければよく、上述の場合1.0×1017(cm-3)が個数密度となる。3個の針試料における個数密度の平均値を、そのレールの個数密度とした。
●Crを含有するV窒化物の粒径の測定方法
本実験においては、粒径が0.5~4.0nmのCrを含有するV窒化物の個数密度のみを測定対象とした。粒径が0.5nm未満、又は4.0nm超のCrを含有するV窒化物は、レールの特性の向上に寄与しないと考えられたためである。従って、Crを含有するV窒化物の評価にあたっては、Crを含有するV窒化物のうち、その粒径が0.5~4.0nmのものだけを抽出し、その個数を数えた。
Crを含有するV窒化物それぞれの粒径の測定方法は以下の通りである。まず、Crを含有するV窒化物を構成するV及びCrの合計原子数を求め、この合計原子数と同数のNが析出物中にあるものと仮定し、結晶構造はNaCl型として、各析出物の体積を推定する。VN及びCrNそれぞれの格子定数は0.413nm及び0.415nmとの文献値を用い、Crを含有するV窒化物の格子定数を0.414nmとすれば、1nm3中に入る原子数は約113個である。析出物に含まれる原子の個数と、をベースに、析出物の体積を見積もることができる。ここでは、Crを含有するV窒化物を球と仮定し、この球の直径を、Crを含有するV窒化物の粒径とした。即ち、Crを含有するV窒化物の球相当径を求めた。
[レール]
●鋼成分
0.90%C-0.50%Si-0.70%Mn-0.50%Cr-0.0150%P-0.0120%S-0.010~0.200%V-0.0030~0.0200%N(残部Fe及び不純物)
●レール形状
141ポンド(重さ:70kg/m)。
●金属組織
パーライト
●硬さ
Hv360~500(頭部外郭表面を起点として深さ25mmまでの範囲)
[転動疲労試験条件]
●試験条件
試験機:転動疲労試験機(図2参照)
試験片形状 レール:141ポンドレール×2m
車輪:AARタイプ(直径920mm)
荷重 ラジアル:275~325KN スラスト:50~80KN
潤滑:油潤滑
累積通過トン数:き裂発生まで(最大200MGT)
(Million Gloss Tonnage)
※レールの上を走行した貨車の総重量、本試験の場合は車輪から作用した通貨重量の2倍で評価。
●評価
超音波探傷装置を用いて、レール全長での頭部内部のき裂の有無を調査し、き裂長さ0.5mm以上のき裂を損傷と判断し、き裂発生までの累積通過トン数を耐内部疲労損傷性の評価指標とした。なお、試験は評価数3とし、き裂発生までの累積通過トン数はその最小値を代表値とした。
次に、本発明者らは、レールの耐内部疲労損傷性をより一層向上させるため、Crを含有するV窒化物のVとCrの原子数の比を限定した理由を説明する。
●サンプル作製
レールを切断し、頭部内部の頭部外郭表面を起点として深さ25mmの位置からサンプル作製。
●事前処理:断面をダイヤモンド研磨。
●観察方法
装置:走査型電子顕微鏡
倍率:1万~10万
観察位置:観察面において粒径が1~3nmのCrを含有するV窒化物の周囲を詳細観察、粒径は走査型電子顕微鏡で観察される窒化物を円と仮定し、その直径を粒径とした。
試料採取位置、事前処理、測定機、測定方法、Crを含有するV窒化物の判定方法については、前述の「Crを含有するV窒化物の調査方法」同様。
●VとCrの原子数と組成の比率の算定
上記の方法でCrを含有するV窒化物と判定されたものについて詳細な分析を行う。個々の窒化物について、VとCrとの原子数をカウントし、Vの原子数(VA)に対するCrの原子数(CA)の比を算定する。測定析出物は粒径が0.5~4.0nmのCrを含有するV窒化物の中からランダムに選んだ5個以上とし、それらの平均値を代表値とする。以下、頭部外郭表面から深さ25mmの位置の、パーライト組織中のフェライト相における、粒径が0.5~4.0nmのCrを含有するV窒化物の、Vの原子数(VA)に対するCrの原子数(CA)の比の平均値を「CA/VA」と記載する。なお、3つの針試料におけるCA/VAの平均値を、レールのCA/VAとした。
なお、微小き裂防止の観点からは、CA/VAの下限値を定める必要はないが、Crを含有するV窒化物が必ずCrを含有するので、CA/VAを0にすることはできない。本発明者らの実験によれば、CA/VAが0.01未満になるレールは確認されなかったので、CA/VAの下限値を0.01、0.02、又は0.05としてもよい。また、粒径が0.5nm未満又は4.0nm超のCrを含有するV窒化物は、レールの特性に実質的影響を及ぼさないと考えられるので、CA/VAの測定にあたっては除外される。
0.01≦CA/VA≦0.70… 式1
本実施形態に係るレールにおいて、レール鋼(レールの素材となる鋼材)の化学成分の限定理由について詳細に説明する。以下、各元素の含有量を示す単位「%」は、「質量%」を意味する。
Cは、パーライト変態を促進させて、かつ、耐摩耗性を確保するために有効な元素である。C含有量が0.75%未満になると、本成分系では、レールに要求される最低限の強度や耐摩耗性が維持できない。また、C含有量が0.75%未満になると、初析フェライト組織が生成して、レールの耐摩耗性が大幅に低下する。さらに、C含有量が0.75%未満になると、頭部内部に疲労き裂を生成し易い軟質な初析フェライト組織が生成し、内部疲労損傷を発生し易くする。一方、C含有量が1.20%を超えると、頭部内部に初析セメンタイト組織が生成し易くなり、パーライト組織と初析セメンタイト組織との界面から疲労き裂が発生し、内部疲労損傷が発生し易くなる。このため、C含有量を0.75~1.20%とする。パーライト組織の生成を安定化し、耐内部疲労損傷性を向上させるには、C含有量を0.80%以上、0.83%以上、又は0.85%以上とすることが望ましい。同じ理由で、C含有量を1.10%以下、1.05%以下、又は1.00%以下とすることが好ましい。
Siは、パーライト組織中のフェライト相に固溶し、レール頭部の硬度(強度)を上昇させ、耐摩耗性を向上させる元素である。しかしながら、Si含有量が0.10%未満では、これらの効果が十分に得られない。一方、Si含有量が2.00%を超えると、レールの熱間圧延時に表面疵が多く生成する。さらに、Si含有量が2.00%を超えると、焼入れ性が著しく増加し、レール頭部にマルテンサイト組織が生成し、耐摩耗性が低下する。このため、Si含有量を0.10~2.00%とする。パーライト組織の生成を安定化し、耐摩耗性と耐内部疲労損傷性とを向上させるには、Si含有量を0.20%以上、0.4%以上、又は0.50%以上とすることが望ましい。同じ理由で、Si含有量を1.80%以下、1.50%以下、又は1.30%以下とすることが好ましい。
Mnは、焼入れ性を高め、パーライト変態を安定化すると同時に、パーライト組織のラメラ間隔を微細化し、パーライト組織の硬度を確保し、耐摩耗性や耐内部疲労損傷性をより一層向上させる元素である。しかしながら、Mn含有量が0.10%未満では、耐摩耗性の改善が認めらない。また、Mn含有量が0.10%未満では、頭部内部に疲労き裂を生成し易い軟質な初析フェライト組織が生成し、耐内部疲労損傷性の確保が困難となる。一方、Mn含有量が2.00%を超えると、焼入れ性が著しく増加し、レール頭部にマルテンサイト組織が生成し、レールの耐摩耗性や耐表面損傷性が低下する。このため、Mn含有量を0.10~2.00%とする。パーライト組織の生成を安定化し、レールの耐摩耗性や耐内部疲労損傷性を向上させるには、Mn含有量を0.40%以上、0.50%以上、又は0.60%以上とすることが望ましい。同じ理由で、Mn含有量を1.80%以下、1.50%以下、又は1.30%以下とすることが好ましい。
Crは、鋼の平衡変態温度を上昇させ、過冷度の増加により、パーライト組織のラメラ間隔を微細化し、パーライト組織の硬さを上昇させ、レールの耐摩耗性を改善する元素である。さらにCrは、パーライト組織のフェライト相中の微細なCrを含有するV窒化物の生成による析出強化によって、レール頭部内部のパーライト組織中のフェライト相の微視的な軟化を抑制し、頭部内部の耐内部疲労損傷性を向上させる元素である。しかしながら、Cr含有量が0.10%未満ではその効果は小さく、パーライト組織のフェライト相中に析出する微細なCrを含有するV窒化物の個数が少なくなり、パーライト組織中のフェライト相の微視的な軟化部の改善が不十分となり、耐内部疲労損傷性の向上が認められない。一方、Cr含有量が1.20%を超えると、焼入れ性が著しく増加し、レール頭部にベイナイト組織やマルテンサイト組織が生成し、レールの耐摩耗性や耐表面損傷性が低下する。さらに、Cr含有量が1.20%を超えると、微細なCrを含有するV窒化物の数が過剰となり、レール頭部内部(頭部外郭表面を起点として深さ25mmの位置)のパーライト組織が脆化し、き裂発生の促進によりレールの耐内部疲労損傷性が低下する。このため、Cr含有量を0.10~1.20%とする。パーライト組織の生成を安定化し、Crを含有するV窒化物を安定的に生成させ、レールの耐摩耗性や耐内部疲労損傷性を向上させるには、Cr含有量を0.30%以上、0.35%以上、又は0.40%以上とすることが望ましい。同じ理由で、Cr含有量を1.10%以下、1.00%以下、又は0.90%以下とすることが好ましい。
Vは、レールの熱間圧延後の冷却過程において、パーライト組織のフェライト相中に微細なCrを含有するV窒化物を生成させ、析出強化によってレール頭部内部のパーライト組織中のフェライト相の微視的な軟化を抑制し、レールの耐内部疲労損傷性を向上させる元素である。しかしながら、V含有量が0.010%未満では、パーライト組織のフェライト相中に析出する微細なCrを含有するV窒化物の個数が少なく、レール頭部内部のパーライト組織中のフェライト相の微視的な軟化部の改善が不十分で、レールの耐内部疲労損傷性の向上が認められない。一方、V含有量が0.200%を超えると、微細なCrを含有するV窒化物の数が過剰となり、レール頭部内部(頭部外郭表面を起点として深さ25mmの位置)のパーライト組織が脆化し、き裂発生の促進によりレールの耐内部疲労損傷性が低下する。このため、V含有量を0.010~0.200%とする。Crを含有するV窒化物を安定的に生成させ、レールの耐内部疲労損傷性を向上させるには、V含有量を0.030%以上、0.035%以上、又は0.040%以上とすることが望ましい。同じ理由で、V含有量を0.180%以下、0.150%以下、又は0.100%以下とすることが好ましい。
Nは、Cr、Vと同時に含有させることで、レールの熱間圧延後の冷却過程において、パーライト組織中のフェライト相中にCrを含有するV窒化物の生成を促進させる元素である。微細なCrを含有するV窒化物が生成すると、レール頭部内部のパーライト組織中のフェライト相の微視的な軟化が抑制され、レールの耐内部疲労損傷性が向上する。しかしながら、N含有量が0.0030%未満では、パーライト組織のフェライト相中に生成する微細なCrを含有するV窒化物の個数が少なく、レール頭部内部のパーライト組織中のフェライト相の微視的な軟化部の改善が不十分となり、レールの耐内部疲労損傷性の向上が認められない。一方、N含有量が0.0200%を超えると、微細なCrを含有するV窒化物の数が過剰となり、レール頭部内部(頭部外郭表面を起点として深さ25mmの位置)のパーライト組織が脆化し、き裂発生の促進によりレールの耐内部疲労損傷性が低下する。さらに、N含有量が0.0200%を超えると、Nを鋼中に固溶させることが困難となり、疲労損傷の起点となる気泡が生成し、内部疲労損傷が発生し易くなる。このため、N含有量を0.0030~0.0200%とする。Crを含有するV窒化物を安定的に生成させ、耐内部疲労損傷性を向上させるには、N含有量を0.0080%以上、0.0090%以上、又は0.0100%以上とすることが望ましい。同じ理由で、N含有量を0.0180%以下、0.0150%以下、又は0.0120%以下とすることが好ましい。
Pは、鋼中に含有される不純物元素であり、転炉での精錬を行うことによりその含有量を制御することが可能である。P含有量は低いほど好ましいが、P含有量が0.0250%を超えると、パーライト組織が脆化し、頭部内部において、脆性的なき裂が発生し、レールの耐内部疲労損傷性が低下する。このため、P含有量を0.0250%以下に制限する。P含有量を0.220%以下、0.200%以下、又は0.180%以下としてもよい。P含有量の下限は限定されておらず、0%としてもよい。しかし、精錬工程での脱燐能力、及び経済性を考慮すると、P含有量の下限値を0.0020%、0.0030%、又は0.0050%としてもよい。
Sは、鋼中に含有される不純物元素であり、溶銑鍋での脱硫を行うことによりその含有量を制御することが可能である。S含有量は少ないほど好ましいが、S含有量が0.0250%を超えると、粗大なMnS系硫化物の介在物が生成し易くなり、頭部内部において、介在物の周囲の応力集中により、疲労き裂が生成し、レールの耐内部疲労損傷性が低下する。このため、S含有量を0.0250%以下に制限する。S含有量を0.220%以下、0.200%以下、又は0.180%以下としてもよい。S含有量の下限は限定されておらず、0%としてもよい。しかし、精錬工程での脱硫能力、及び経済性を考慮すると、S含有量の下限値を0.0020%、0.0030%、又は0.0050%としてもよい。
(a群)Moは、平衡変態点を上昇させ、パーライト組織のラメラ間隔を微細化し、レールの硬度を向上させる。
(b群)Coは、摩耗面のラメラ組織を微細化し、摩耗面の硬度を高める。
(c群)Bは、パーライト変態温度の冷却速度依存性を低減させ、レール頭部の硬度分布を均一にする。
(d群)Cuは、パーライト組織中のフェライト相に固溶し、レールの硬度を高める。Niは、パーライト組織の靭性と硬度を向上させ、同時に、溶接継手熱影響部の軟化を防止する。
(e群)Nb、Tiは、熱間圧延やその後の冷却過程で生成した炭化物や窒化物の析出硬化により、パーライト組織の疲労強度を向上させる。また、Nb、Tiは、再加熱時に炭化物や窒化物を安定的に生成させ、溶接継手熱影響部の軟化を防止する。
(f群)Mg、Ca、REMは、MnS系硫化物を微細分散し、介在物から生成する内部疲労損傷を低減する。
(g群)Zrは、凝固組織の等軸晶化率を高めることにより、鋳片中心部の偏析帯の形成を抑制し、初析セメンタイト組織やマルテンサイト組織の生成を抑制する。
(h群)Alは、脱酸材として作用する。また、Alは、共析変態温度を高温側へ移動させ、パーライト組織の高硬度(強度)化に寄与する。
そのため、上記の効果を得るため、これらの元素を含有させてもよい。これらの元素は後述する範囲以下で含有されていても、本実施形態に係るレールの特性を損なうものではない。これらの元素は必ずしも含有させる必要がないので、その下限は0%である。
Moは、平衡変態温度を上昇させ、過冷度の増加により、パーライト組織のラメラ間隔を微細化し、パーライト組織の硬さ(強度)を向上させ、その結果として、レールの耐摩耗性と耐内部疲労損傷性を向上させる元素である。しかしながら、Mo含有量が0.01%未満ではその効果が小さく、レール鋼の硬度を向上させる効果が得られない。一方、Mo含有量が0.50%を超えると、変態速度が著しく低下し、レール頭部にマルテンサイト組織が生成し、耐摩耗性が低下する場合がある。このため、含有させる場合には、Mo含有量を0.01~0.50%とすることが好ましい。
Coは、パーライト組織のフェライト相に固溶し、パーライト組織のラメラ組織を微細化し、パーライト組織の硬さ(強度)を向上させ、その結果として、レールの耐摩耗性と耐内部疲労損傷性を向上させる元素である。しかしながら、Co含有量が0.01%未満では、ラメラ組織の微細化が促進せず、耐摩耗性や耐内部疲労損傷性の向上効果が得られない。一方、Co含有量が1.00%を超えると、上記の効果が飽和し、含有量に応じたラメラ組織の微細化が図れない場合がある。また、Co含有量が1.00%を超えると、合金添加コストの増大により経済性が低下する場合がある。このため、含有させる場合には、Co含有量を0.01~1.00%とすることが好ましい。
Bは、オーステナイト粒界に鉄炭ほう化物(Fe23(CB)6)を形成し、パーライト変態の促進効果により、パーライト変態温度の冷却速度依存性を低減させる元素である。またBは、上記の効果により、頭部外郭表面から内部までより均一な硬度分布をレールに付与し、レールを高寿命化する元素である。しかしながら、B含有量が0.0001%未満では、その効果が十分でなく、レール頭部の硬度分布には改善が認められない。一方、B含有量が0.0050%を超えると、粗大な鉄炭ほう化物が生成し、脆性破壊を助長し、レールの靭性が低下する場合がある。このため、含有させる場合には、B含有量を0.0001~0.0050%とすることが好ましい。
Cuは、パーライト組織のフェライト相に固溶し、固溶強化により硬さ(強度)を向上させ、レールの耐摩耗性と耐内部疲労損傷性を向上させる元素である。しかし、Cu含有量が0.01%未満ではその効果が得られない。一方、Cu含有量が1.00%を超えると、著しい焼入れ性向上により、レール頭部にマルテンサイト組織が生成し、耐摩耗性が低下する場合がある。このため、含有させる場合には、Cu含有量を0.01~1.00%とすることが好ましい。
Niは、パーライト組織の靭性を向上させ、同時に、固溶強化により硬さ(強度)を向上させ、レールの耐摩耗性と耐内部疲労損傷性を向上させる元素である。さらにNiは、溶接熱影響部においては、Tiと複合でNi3Tiの金属間化合物を微細に析出し、析出強化により軟化を抑制する元素である。また、Niは、Cu含有鋼において粒界の脆化を抑制する元素である。しかしながら、Ni含有量が0.01%未満では、これらの効果が著しく小さい。一方、Ni含有量が1.00%を超えると、著しい焼入れ性向上により、レール頭部にマルテンサイト組織が生成し、レールの耐摩耗性が低下する場合がある。このため、含有させる場合には、Ni含有量を0.01~1.00%とすることが好ましい。
Nbは、熱間圧延後の冷却過程でNb炭化物および/またはNb窒化物として析出し、析出硬化により、パーライト組織の硬さ(強度)を高め、レールの耐摩耗性や耐内部疲労損傷性を向上させる元素である。またNbは、Ac1点以下の温度域に再加熱された熱影響部において、低温度域から高温度域までNbの炭化物やNb窒化物を安定的に生成させ、溶接継手の熱影響部の軟化を防止するのに有効な元素である。しかしながら、Nb含有量が0.0010%未満では、これらの効果が得られず、パーライト組織の硬度(強度)の向上は認められない。一方、Nb含有量が0.0500%を超えると、Nbの炭化物や窒化物の析出硬化が過剰となり、パーライト組織自体が脆化し、レールの耐内部疲労損傷性が低下する場合がある。このため、含有させる場合には、Nb含有量を0.0010~0.0500%とすることが好ましい。
Tiは、熱間圧延後の冷却過程でTi炭化物および/またはTi窒化物として析出し、析出硬化により、パーライト組織の硬さ(強度)を高め、レールの耐摩耗性や耐内部疲労損傷性を向上させる元素である。またTiは、溶接時の再加熱において、析出したTi炭化物、Ti窒化物が溶解しないことを利用して、オーステナイト域温度まで加熱される熱影響部の組織を微細化し、溶接継手部の脆化を防止するのに有効な成分である。しかしながら、Ti含有量が0.0030%未満ではこれらの効果が少ない。一方、Ti含有量が0.0500%を超えると、粗大なTi炭化物やTi窒化物が生成し、応力集中により、疲労き裂が生成し、耐内部疲労損傷性が低下する場合がある。このため、含有させる場合には、Ti含有量を0.0030~0.0500%とすることが好ましい。
Mgは、Sと結合して微細な硫化物を形成する元素である。このMg硫化物は、MnSを微細に分散させ、応力集中を緩和し、レールの耐内部疲労損傷性を向上させる。しかし、Mg含有量が0.0005%未満ではその効果は弱い。一方、Mg含有量が0.0200%を超えると、Mgの粗大酸化物が生成し、応力集中により、疲労き裂が生成し、レールの耐内部疲労損傷性が低下する場合がある。このため、含有させる場合には、Mg量を0.0005~0.0200%とすることが好ましい。
Caは、Sとの結合力が強く、CaS(硫化物)を形成する元素である。このCaSはMnSを微細に分散させ、応力集中を緩和し、レールの耐内部疲労損傷性を向上させる。しかしながら、Ca含有量が0.0005%未満ではその効果は弱い。一方、Ca含有量が0.0200%を超えると、Caの粗大酸化物が生成し、応力集中により、疲労き裂が生成し、耐内部疲労損傷性が低下する場合がある。このため、含有させる場合には、Ca含有量を0.0005~0.0200%とすることが好ましい。
REMは、脱酸・脱硫元素であり、含有されるとMn硫化物系介在物の生成核となるREMのオキシサルファイド(REM2O2S)を生成する。このオキシサルファイド(REM2O2S)は融点が高いため、圧延後のMn硫化物系介在物の延伸を抑制する。この結果、REMの含有により、MnSが微細に分散し、応力集中が緩和され、レールの耐内部疲労損傷性が向上する。しかしながら、REM含有量が0.0005%未満では、MnS系硫化物の生成核としては不十分であり、その効果は小さい。一方、REM含有量が0.0500%を超えると、硬質なREMのオキシサルファイド(REM2O2S)が過剰に生成し、応力集中により、疲労き裂が生成し、耐内部疲労損傷性が低下する場合がある。このため、含有させる場合には、REM含有量を0.0005~0.0500%とすることが好ましい。
Zrは、Oと結合してZrO2介在物を形成する。このZrO2介在物とγ-Feとは格子整合性が良いので、ZrO2介在物が、γ-Feが凝固初晶である高炭素レール鋼の凝固核となり、凝固組織の等軸晶化率を高めることにより、鋳片中心部の偏析帯の形成を抑制する。またZrは、鋳片中心部の偏析帯の形成を抑制することにより、レール偏析部に生成するマルテンサイト組織の生成を抑制する元素である。しかしながら、Zr含有量が0.0001%未満では、生成するZrO2系介在物の数が少なく、凝固核として十分な作用を示さない。一方、Zr含有量が0.0200%を超えると、粗大なZr系介在物が多量に生成し、応力集中により、疲労き裂が生成し、レールの耐内部疲労損傷性が低下する場合がある。このため、含有させる場合には、Zr含有量を0.0001~0.0200%とすることが好ましい。
Alは、脱酸材として作用する元素である。また、Alは、共析変態温度を高温側へ移動させる元素であり、パーライト組織の高硬度(強度)化に寄与し、その結果として、パーライト組織の耐摩耗性や耐内部疲労損傷性を向上させる元素である。しかしながら、Al含有量が0.0100%未満では、その効果が弱い。一方、Al含有量が1.00%を超えると、Alを鋼中に固溶させることが困難となり、粗大なアルミナ系介在物が生成する。この粗大なAl系介在物は疲労き裂の起点となるので、レールの耐内部疲労損傷性が低下する場合がある。さらに、Al含有量が1.00%を超えると、溶接時に酸化物が生成し、溶接性が著しく低下する場合がある。このため、含有させる場合には、Al含有量を0.0100~1.00%とすることが好ましい。
本実施形態に係るレールは、上記の成分、組織等を備えることで、製造方法に関わらず、その効果を得ることができる。しかしながら、以下に示す工程を含む製造方法によれば、本実施形態に係るレールを安定的に得られるので好ましい。
加熱速度:1~8℃/min
速度制御温度範囲:1000~1200℃
なお、上記温度は鋼片の温度条件であり、加熱炉の温度制御は上記の加熱条件に合うように制御することが望ましい。また、熱間圧延前の鋼片の加熱速度は、平均加熱速度ではないことに留意する必要がある。即ち、この加熱速度は加熱中の逐次の加熱速度を示すものである。本実施形態に係るレールの製造方法では、鋼片の温度を1000℃から1200℃まで上昇させる間、昇温速度を常に1~8℃/minとする必要がある。換言すると、鋼片の温度T〔℃〕と時間t〔min〕との関係をT(t)と定義した場合、本実施形態に係るレールの製造方法では、鋼片の温度を1000℃から1200℃まで上昇させる間は常にdT(t)/dt〔℃/min〕が1以上8以下とされる必要がある。
加熱速度が1℃/min未満では、鋳造中に粗大化したCrを含有するV窒化物が再溶解するが、加熱中に再度析出し、Crを含有するV窒化物が粗大化し、溶解させることが困難となり、レール熱処理においてCrを含有する微細なV窒化物を安定的に生成させることが困難となる場合がある。さらに、加熱速度が1℃/min未満では、鋼片の加熱が過剰となり、鋼片表面の脱炭が進むと同時に、鋼片の割れ等が発生し、熱間圧延、熱処理後のレール製品としての品質が確保できない場合がある。また、加熱速度が1℃/min未満では、加熱燃料を多量に使用するため、経済性が低下する場合がある。
頭部外郭表面の最終圧延温度:850~1000℃
頭部断面の最終圧下量(レール頭部の断面減面率):2~20%
●熱間圧延後の熱処理条件(頭部外郭表面):圧延後、加速冷却及び制御冷却を行う
加速冷却(頭部外郭表面)
平均冷却速度:2~30℃/sec
加速冷却開始温度:750℃以上
加速冷却停止温度:580~660℃
制御冷却(頭部外郭表面)
加速冷却停止後に頭部外郭表面の温度を580~660℃の範囲に5~150sec間保持し、その後放冷および加速冷却を実施
温度保持:加速冷却速度の制御、さらには、加速冷却の実行、停止を繰返し行い、レール内部からの復熱に応じて加速冷却を行うことによって温度を制御
平均冷却速度:2~30℃/sec
加速冷却開始温度:750℃以上
加速冷却停止温度:600~650℃
制御冷却(頭部外郭表面)
加速冷却停止後に頭部表面の温度を600~650℃の範囲に20~150sec秒間保持し、その後放冷および加速冷却する。
制御冷却時の温度保持:加速冷却速度の制御、レール内部からの復熱に応じて加速冷却の実行、停止を繰返し行い、所定の温度範囲となるように制御する。
最終圧延温度(頭部外郭表面)が850℃未満では、圧延後のオーステナイト粒の微細化が顕著となる。この場合、焼入れ性が大幅に低下し、レール頭部の硬さの確保が困難になる場合がある。また、最終圧延温度(頭部外郭表面)が1000℃を超えると、圧延後のオーステナイト粒が粗大化し、焼入れ性が過剰に増加し、レール頭部に耐摩耗性に有害なベイナイト組織が生成し易くなる。そのため、最終圧延温度(頭部外郭表面)を850~1000℃の範囲とすることが好ましい。最終圧延温度を860℃以上、880℃以上、又は900℃以上としてもよい。最終圧延温度を980℃以下、960℃以下、又は940℃以下としてもよい。
最終圧下量(レール頭部の断面減面率)が2%未満では、圧延後のオーステナイト粒が粗大化し、焼入れ性が過剰に増加し、レール頭部に耐摩耗性に有害なベイナイト組織が生成し易くなり、パーライト組織自体の粒径が粗大化し、レールとして必要な延性や靭性を確保できない場合がある。一方、最終圧下量(レール頭部の断面減面率)が20%を超えると、圧延後のオーステナイト粒の微細化が顕著となり、焼入れ性が大幅に低下し、レール頭部の硬さの確保が困難になる。そのため、最終圧下量(レール頭部の断面減面率)を2~20%の範囲とすることが好ましい。最終圧下量(断面減面率)を4%以上、6%以上、又は8%以上としてもよい。最終圧下量(断面減面率)を18%以下、16%以下、又は14%以下としてもよい。
平均冷却速度が2℃/sec未満になると、加速冷却の途中の高温度域でパーライト変態が開始する。その結果、本実施形態に係るレールの成分系では、レール頭部表面において、硬さがHv360未満となる部位が発生し、レールとして必要な耐摩耗性や耐内部疲労損傷性を確保することが困難となる場合がある。一方、平均冷却速度が30℃/secを超えると、本実施形態に係るレールの成分系では、パーライト組織の硬さが大幅に増加する、さらに、レール頭部表面において、ベイナイト組織やマルテンサイト組織が頭表部に生成し、レールの耐摩耗性や靭性が低下することが懸念される。このため、加速冷却における平均冷却速度を2~30℃/secとすることが好ましい。加速冷却における平均冷却速度を3℃/sec以上、4℃/sec以上、又は5℃/sec以上としてもよい。加速冷却における平均冷却速度を25℃/sec以下、20℃/sec以下、又は15℃/sec以下としてもよい。
頭部外郭表面の加速冷却の開始温度が750℃未満になると、加速冷却前の高温度域でパーライト組織が生成する場合がある。この場合、所定の硬度が得られず、レールとして必要な耐摩耗性や耐表面損傷性を確保することが困難となる。また、上述の場合、炭素量が比較的多い鋼では、初析セメンタイト組織が生成し、パーライト組織が脆化し、レールの靭性が低下することが懸念される。このため、加速冷却を開始する際のレールの頭部外郭表面の温度を750℃以上とすることが好ましい。なお、上述の最終圧延温度を考慮すると、加速冷却の開始温度を750℃以上とするためには、熱間圧延完了後180秒以内に加速冷却を開始する必要があると考えられる。
保持温度が660℃を超えると、本実施形態に係るレールの成分系では、冷却直後の高温度域でパーライト変態が開始し、レール頭部表面において硬さの低いパーライト組織が多く生成する。その結果、硬さが確保できず、レールとして必要な耐摩耗性や耐表面損傷性を確保することが困難となる。さらにこの場合、レール頭部内部においてCrを含有するV窒化物の生成が促進され、個数密度が過剰に増加する。その結果、レール頭部内部のパーライト組織が脆化し、き裂の発生が促進され、耐内部疲労損傷性が低下することが懸念される。
保持時間が150secを超えると、保持中にパーライト組織の焼戻しが進み、パーライト組織が軟化する。その結果、レール頭部表面、頭部内部の硬さが確保できず、レールとして必要な耐摩耗性や耐内部疲労損傷性を確保することが困難となる。さらにこの場合、レール頭部内部においてCrを含有するV窒化物が成長し、その粒径が増加する。その結果、微細なCrを含有するV窒化物の個数密度が低下し、パーライト組織中のフェライト相の微視的な軟化の改善が期待できない。
一方、保持時間が5sec未満では、保持中にパーライト変態が完了せず、マルテンサイト組織が生成する。その結果、レール頭部表面、頭部内部の耐摩耗性や耐内部疲労損傷性を確保することが困難となる。またこの場合、Crを含有するV窒化物の成長が抑制され、粒径は減少する。その結果、微細なCrを含有するV窒化物の個数密度が低下し、パーライト組織中のフェライト相の微視的な軟化が改善せず、耐内部疲労損傷性を向上が期待できない。このため、加速冷却後の温度保持を行う時間を5~150secとすることが好ましい。
表2-1~表2-4に記載の化学成分を有する鋼片を加熱し、加熱された鋼片を熱間圧延してレールを形成し、このレールを加速冷却及び制御冷却することにより、表3-1~表3-4に記載の金属組織、硬度、及びCr含有V窒化物を有するレールを得た。これら表において、発明範囲外の値には下線を付した。製造条件は、表の備考欄に特別の記載がない限り、以下の通りとした。
●鋼片の加熱速度:1000~1200℃の範囲内で4℃/min
●鋼片の加熱の終了温度:1250℃
●最終圧延温度:950℃
●最終圧下量(減面率):5~10%
●加速冷却の開始温度:800℃
●加速冷却時の平均冷却速度:6~8℃/sec
●加速冷却の終了温度:600℃
●制御冷却時の保持温度:600~660℃
●制御冷却時の温度保持時間:20~40秒
●温度保持終了後の冷却:常温の大気中に放置することにより室温まで冷却
No.49は、加速冷却の終了温度が560℃とされたが、その他条件は上述の通りであった。
No.50は、加速冷却時の平均冷却速度が35.0℃/secとされたが、その他条件は上述の通りであった。
No.53は、加速冷却時の平均冷却速度が1.0℃/secとされたが、その他条件は上述の通りであった。
No.54は、加速冷却の終了温度が680℃とされたが、その他条件は上述の通りであった。
No.57は、1000~1100℃の範囲内での鋼片の加熱速度が10℃/minとされたが、1100~1200℃の範囲内での鋼片の加熱速度は5℃/minとされ、その他条件は上述の通りであった。
No.58は、1100~1200℃の範囲内での鋼片の加熱速度が12℃/minとされたが、1000~1100℃の範囲内での鋼片の加熱速度は6℃/minとされ、その他条件は上述の通りであった。
No.59は、1000~1100℃の範囲内での鋼片の加熱速度が0.5℃/minとされたが、1100~1200℃の範囲内での鋼片の加熱速度は4℃/minとされ、その他条件は上述の通りであった。
No.60は、1100~1200℃の範囲内での鋼片の加熱速度が0.8℃/minとされたが、1000~1100℃の範囲内での鋼片の加熱速度は3℃/minとされ、その他条件は上述の通りであった。
No.61は、1000~1200℃の範囲内での鋼片の加熱速度が10.0℃/minとされたが、その他条件は上述の通りであった。
No.79は、1000~1200℃の範囲内での鋼片の加熱速度が8.0℃/minとされたが、その他条件は上述の通りであった。 No.80は、1000~1200℃の範囲内での鋼片の加熱速度が6.0℃/minとされたが、その他条件は上述の通りであった。
No.81は、1000~1200℃の範囲内での鋼片の加熱速度が5.0℃/minとされたが、その他条件は上述の通りであった。
No.82は、1000~1200℃の範囲内での鋼片の加熱速度が3.0℃/minとされたが、その他条件は上述の通りであった。
No.83は、1000~1200℃の範囲内での鋼片の加熱速度が2.0℃/minとされたが、その他条件は上述の通りであった。
A:摩耗量が25mmに達した際の累積通過トン数が175超~200MGT
B:摩耗量が25mmに達した際の累積通過トン数が150超~175MGT
C:摩耗量が25mmに達した際の累積通過トン数が100超~150MGT
X:摩耗量が25mmに達した際の累積通過トン数が100MGT未満
A:損傷発生した際の累積通過トン数が175超~200MGT
B:損傷発生した際の累積通過トン数が150超~175MGT
C:損傷発生した際の累積通過トン数が100超~150MGT
X:損傷発生した際の累積通過トン数が100MGT未満
No.2は、耐内部疲労損傷性が損なわれた。これは、Cが過剰であったので、初析セメンタイトが多く生成することによりパーライト組織の量が不足したからと考えられる。
No.7は、耐内部疲労損傷性が損なわれた。これは、Cが不足したので、初析フェライトが多く生成することによりパーライト組織の量及び硬さが不足したからと考えられる。
No.8は、耐摩耗性が損なわれた。これは、Siが過剰であったので、マルテンサイトが多く生成することによりパーライト組織の量が不足し、且つ硬さが過剰となったからと考えられる。マルテンサイトは高い硬度を有するが、耐摩耗性は低いので、No.8の耐摩耗性には寄与しなかった。
No.13は、耐摩耗性が損なわれた。これは、Siが不足したので硬さが不足したからと考えられる。
No.14は、耐内部疲労損傷性及び耐摩耗性が損なわれた。これは、Mnが過剰であったので、マルテンサイトが多く生成することによりパーライト組織の量が不足し、且つ硬さが過剰となったからと考えられる。
No.19は、耐内部疲労損傷性及び耐摩耗性が損なわれた。これは、Mnが不足したので、初析フェライトが多く生成することによりパーライト組織の量及び硬さが不足したからと考えられる。
No.20は、耐内部疲労損傷性及び耐摩耗性が損なわれた。これは、Crが過剰であったので、マルテンサイトが多く生成することによりパーライト組織の量が不足し、硬さが過剰となり、さらにCrを含むV窒化物の個数密度が過剰となったからと考えられる。
No.25は、耐内部疲労損傷性及び耐摩耗性が損なわれた。これは、Crが不足したので、パーライト組織が軟化し、Crを含むV窒化物の個数密度が不足したのでパーライト組織中のフェライト相の局部軟化が抑制されなかったからと考えられる。
No.26は、耐内部疲労損傷性が損なわれた。これは、Vが過剰であったので、Crを含むV窒化物の個数密度が過剰となり、パーライト組織が脆化したからであると考えられる。
No.33は、耐内部疲労損傷性が損なわれた。これは、Vが不足したので、Crを含むV窒化物の個数密度が不足し、パーライト組織中のフェライト相の局部軟化が抑制されなかったからと考えられる。
No.34は、耐内部疲労損傷性が損なわれた。これは、Nが過剰であったので、Crを含むV窒化物の個数密度が過剰となり、パーライト組織が脆化したからであると考えられる。
No.41は、耐内部疲労損傷性が損なわれた。これは、Nが不足したので、Crを含むV窒化物の個数密度が不足し、パーライト組織中のフェライト相の局部軟化が抑制されなかったからと考えられる。
No.42は、耐内部疲労損傷性が損なわれた。これは、Pが過剰であったので、パーライト組織が脆化したからであると考えられる。
No.45は、耐内部疲労損傷性が損なわれた。これは、Sが過剰であったので、粗大MnSが多数生成したからであると考えられる。
No.49は、耐内部疲労損傷性及び耐摩耗性が損なわれた。これは、加速冷却停止温度が低すぎたので、ベイナイトが生成してパーライト組織が不足したからであると考えられる。
No.50は、耐内部疲労損傷性が損なわれた。これは、加速冷却速度が高すぎたので、パーライト組織の硬さが過剰となったからであると考えられる。
No.53は、耐内部疲労損傷性及び耐摩耗性が損なわれた。これは、加速冷却速度が低すぎたので、パーライト組織の硬さが不足したからであると考えられる。
No.54は、耐内部疲労損傷性が損なわれた。これは、加速冷却停止温度が高すぎたので、Crを含むV窒化物が過剰に生成し、パーライト組織が脆化したからであると考えられる。
No.57は、耐内部疲労損傷性が損なわれた。これは、鋼片の加熱の際に加熱速度が高すぎる時期があったので、鋳造中に粗大化したCrを含むV窒化物が残存し、Crを含有するV窒化物の個数密度が不足し、パーライト組織中のフェライト相の局部軟化が抑制されなかったからと考えられる。
No.58は、耐内部疲労損傷性が損なわれた。これは、鋼片の加熱の際に加熱速度が高すぎる時期があったので、鋳造中に粗大化したCrを含むV窒化物が残存し、Crを含有するV窒化物の個数密度が不足し、パーライト組織中のフェライト相の局部軟化が抑制されなかったからと考えられる。
No.59は、耐内部疲労損傷性が損なわれた。これは、鋼片の加熱の際に加熱速度が低すぎる時期があったので、加熱中にCrを含むV窒化物が一旦溶解した後再析出し、粗大化したので、Crを含有するV窒化物の個数密度が不足し、パーライト組織中のフェライト相の局部軟化が抑制されなかったからと考えられる。
No.60は、耐内部疲労損傷性が損なわれた。これは、鋼片の加熱の際に加熱速度が低すぎる時期があったので、加熱中にCrを含むV窒化物が一旦溶解した後再析出し、粗大化したので、Crを含有するV窒化物の個数密度が不足し、パーライト組織中のフェライト相の局部軟化が抑制されなかったからと考えられる。
No.61は、耐内部疲労損傷性が損なわれた。これは、鋼片の加熱の際に加熱速度が高すぎる時期があったので、鋳造中に粗大化したCrを含むV窒化物が残存し、Crを含有するV窒化物の個数密度が不足し、パーライト組織中のフェライト相の局部軟化が抑制されなかったからと考えられる。
2 頭部コーナー部
3 レール頭部
3a 頭表部
4 レール移動用スライダー
5 レール
6 車輪
7 モーター
8 荷重負荷装置
Claims (4)
- 単位質量%で、
C:0.75~1.20%、
Si:0.10~2.00%、
Mn:0.10~2.00%、
Cr:0.10~1.20%、
V:0.010~0.200%、
N:0.0030~0.0200%、
P≦0.0250%、
S≦0.0250%、
Mo:0~0.50%、
Co:0~1.00%、
B:0~0.0050%、
Cu:0~1.00%、
Ni:0~1.00%、
Nb:0~0.0500%、
Ti:0~0.0500%、
Mg:0~0.0200%、
Ca:0~0.0200%、
REM:0~0.0500%、
Zr:0~0.0200%、及び
Al:0~1.00%
を含有し、
残部がFeおよび不純物からなり、
頭部外郭表面を起点として深さ25mmまでの範囲の組織が、面積率で95%以上のパーライト組織を含み、かつ、前記組織の硬さがHv360~500の範囲であり、
前記頭部外郭表面を起点として深さ25mmの位置の、前記パーライト組織中のフェライト相において、粒径が0.5~4.0nmのCrを含有するV窒化物の個数密度が1.0~5.0×1017cm-3の範囲であることを特徴とするレール。 - さらに、前記頭部外郭表面から深さ25mmの位置の、前記パーライト組織中の前記フェライト相における粒径が0.5~4.0nmの前記Crを含有するV窒化物において、Vの原子数をVA、Crの原子数をCAとしたとき、CA/VAの平均値が下記式1を満足することを特徴とする請求項1に記載のレール。
0.01≦CA/VAの平均値≦0.70… 式1 - 単位質量%で、下記a群からh群の成分の1群または2群以上を含有することを特徴とする請求項1または2に記載のレール。
a群:Mo:0.01~0.50%。
b群:Co:0.01~1.00%。
c群:B:0.0001~0.0050%。
d群:Cu:0.01~1.00%、及びNi:0.01~1.00%の1種または2種。
e群:Nb:0.0010~0.0500%、及びTi:0.0030~0.0500%の1種または2種以上。
f群:Mg:0.0005~0.0200%、Ca:0.0005~0.0200%、及びREM:0.0005~0.0500%の1種または2種以上。
g群:Zr:0.0001~0.0200%。
h群:Al:0.0100~1.00%。 - 単位質量%で、C:0.75~1.20%、Si:0.10~2.00%、Mn:0.10~2.00%、Cr:0.10~1.20%、V:0.010~0.200%、N:0.0030~0.0200%、P≦0.0250%、S≦0.0250%、Mo:0~0.50%、Co:0~1.00%、B:0~0.0050%、Cu:0~1.00%、Ni:0~1.00%、Nb:0~0.0500%、Ti:0~0.0500%、Mg:0~0.0200%、Ca:0~0.0200%、REM:0~0.0500%、Zr:0~0.0200%、及びAl:0~1.00%を含有し、残部がFeおよび不純物からなる鋼片を、加熱終了温度を1200℃以上とし、1000~1200℃の範囲内での加熱速度を1~8℃/minとして加熱する工程と、
加熱された前記鋼片を、最終圧延温度を850~1000℃の範囲内とし、且つ最終圧下量を2~20%として熱間圧延し、これによりレールを形成する工程と、
前記レールを、加速冷却の開始温度を750℃以上とし、前記加速冷却の際の平均冷却速度を2~30℃/secとし、前記加速冷却の終了温度を580~660℃として加速冷却する工程と、
前記レールを、保持温度を580~660℃の範囲内とし、温度保持時間を5~150secとして、レール表面温度の変動幅を60℃以下とするように制御冷却する工程と、
前記レールを常温まで放冷又は加速冷却する工程と
を備えるレールの製造方法。
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