Welded rails

Abstract

A welded rail according to an embodiment of the present invention, which has excellent fatigue damage resistance and breakage resistance at a weld joint part, comprises multiple rail parts and a weld joint part where the rail parts are joined together. When the gap between a softest part and a weld center of a heat-affected zone (HAZ) having a width (W) of 60 mm or less as measured along a longitudinal direction is defined as WX, and a region located at a distance of 0.6 WX to 0.7 WX from the weld center and at a depth of 2 to 5 mm from a top outer shell surface is defined as a pro-eutectoid cementite structure evaluation region, the total crossing number (N) of pro-eutectoid cementite structures intersecting with crosslines constituted by two 100 µm-long line segments, which are respectively parallel to the longitudinal direction and to the vertical direction, is 26 or less in the pro-eutectoid cementite structure evaluation region.

Description

[Technical Field]

[0001] The present invention relates to a welded rail. This application claims priority based on Japanese Patent Application No. 2021-181221, filed on November 5, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] Flash butt welding is a widely used method for welding rails. It is known to have advantages such as the ability to automate the process, high quality stability, and short welding time.

[0003] Flash butt welding is a technique for joining rails by heating the rail end faces and then pressing the molten surfaces together. During flash butt welding, the rail is heated from room temperature to near its maximum melting point and then cooled. As a result, flash butt welding changes the rail's metal structure and hardness. The area where the metallurgical and mechanical properties change due to the heat from welding, cutting, etc. is called the heat-affected zone (HAZ).

[0004] In the HAZ, when the rail is heated to above the A1 point during welding, the metallurgical structure of the rail undergoes austenitization and pearlite transformation, and when the rail is heated to near the A1 point, the metallurgical structure of the rail undergoes partial austenitization and decomposition of the pearlite structure, which causes a decrease in hardness in the HAZ.

[0005] When hardness of a welded rail decreases, the passage of wheels accelerates wear of the HAZ at the rail head. The difference in wear rate between the HAZ and the base metal makes the welded joint more likely to become uneven. This results in excessive loads being applied to the welded joint when a train is running, increasing the risk of breakage of the welded rail.

[0006] For this reason, it is necessary to suppress softening of the HAZ of the welded joint during flash butt welding of rails. For example, the following techniques have been proposed to suppress HAZ softening.

[0007] Patent Document 1 discloses that in flash welding of rails, in order to reduce the HAZ width in the longitudinal direction of the rail, a backing plate having a length in the longitudinal direction of the rail at the top surface of 15 mm or more and a thickness of 10 mm or more at the portion in contact with the top surface is set 20 mm to 50 mm from the end face of the rail before welding, and then the rail is flash butt welded, thereby softening the HAZ of the welded joint where hardness is reduced, i.e., the width of the softened region of the HAZ in the longitudinal direction of the rail (HAZ width) can be reduced to 15 mm or less.

[0008] Patent Document 2 describes a flash butt welding method for achieving a rail welded joint in which the latter flashing velocity is set to 2.1 mm / sec or more in order to reduce the HAZ width in the longitudinal direction of the rail during flash welding of rails, and the HAZ width is set to 27 mm or less and the softened width is set to 10 mm or less.

[0009] Patent Document 3 describes a heat treatment method for a welded joint in which, in rail welding, one or both of the rail head and base portions are heated to a temperature range of 800 to 900°C, where the rail is in a two-phase state in which austenite and cementite phases are mixed, and then accelerated cooling is performed from a temperature range of 750°C or higher at a cooling rate of 1 to 10°C / sec. The accelerated cooling is stopped when the temperature of one or both of the head and base portions of the steel rail reaches a temperature of 680 to 550°C, and then the rail is allowed to cool naturally or slowly so as not to exceed 680°C, thereby suppressing the formation of pro-eutectoid cementite structures and improving the toughness of the rail welded joint. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2007-289970 [Patent Document 2] International Publication No. 2011 / 052562 [Patent Document 3] Japanese Patent Publication No. 2004-43862 Summary of the Invention [Problem to be solved by the invention]

[0011] However, the fatigue damage resistance and breakage resistance required of welded rails are becoming increasingly high. The techniques of Patent Documents 1 to 3 have the following problems.

[0012] In the method of attaching a backing plate as in Patent Document 1, a separately prepared backing plate needs to be attached in a specified area. However, the location where the backing plate is placed is very close to the butt end faces of the rails. As a result, molten metal that splashes during flash butt welding adheres to the backing plate. Therefore, in the method of Patent Document 1, it is not easy to attach or remove the backing plate, and furthermore, removing the metal that adheres to the backing plate requires time and effort. Therefore, the method of Patent Document 1 has room for further improvement in work efficiency.

[0013] The technology described in Patent Document 1 primarily aims to prevent softening of the HAZ of a welded joint, reduce uneven rail wear, suppress train noise and vibration, reduce impacts on the rail when passing vehicles, and prevent rail fatigue failure. However, the track environment has become harsher due to the recent increase in freight car loads. This has led to frequent damage caused by fatigue failure at the bottom of welded joints and breakage caused by brittle fracture at the head. The technology described in Patent Document 1 is believed to be effective in preventing fatigue failure by reducing uneven wear at the head of welded joints. However, the technology described in Patent Document 1 is not intended to prevent brittle fracture and breakage caused by fatigue fracture that occur under such harsh operating conditions. The technology described in Patent Document 1 leaves room for further improvement in the operating performance of welded rails.

[0014] The technology described in Patent Document 2 primarily aims to reduce the heat-affected zone of welds in high-carbon hypereutectoid rail steel, reduce wear-induced irregularities in welded joints, and reduce uneven wear and surface damage to rail heads. However, the recent trend toward heavier freight car loads has led to harsher track environments, resulting in frequent occurrences of fatigue fracture-induced damage at the bottom of welded joints and brittle fracture-induced breakage at the heads. The technology described in Patent Document 2 is believed to be effective in reducing uneven wear and surface damage to rail heads in such harsh operating environments. However, the technology described in Patent Document 2 is not intended to prevent brittle fracture and fatigue fracture-induced breakage of welded joints that occur in such harsh operating environments. The technology described in Patent Document 2 leaves room for further improvement in the operating performance of welded rails.

[0015] Furthermore, rails with hypereutectoid components (C: 0.80% or more) are prone to the formation of pro-eutectoid cementite structures with low toughness in the weld joint, increasing the possibility of rail breakage. The technologies described in Patent Documents 1 and 2 are primarily intended to reduce the irregularities in the weld joint due to wear and to suppress uneven wear of the head, surface damage, and fatigue fracture in the weld joint, but do not take into consideration rails with hypereutectoid components. Patent Documents 1 and 2 do not aim to suppress the formation of pro-eutectoid cementite structures that reduce the toughness of the weld joint, which is a problem in hypereutectoid rails, and to improve the fracture resistance of the weld joint. Furthermore, if the technologies described in Patent Documents 1 and 2 are applied to rails with hypereutectoid components, it is believed that the fracture resistance will be insufficient.

[0016] The technology described in Patent Document 3 aims to improve the fracture resistance of welded joints by suppressing the formation of pro-eutectoid cementite structures that reduce the toughness of welded joints. The technology described in Patent Document 3 is targeted at rails with hyper-eutectoid components.

[0017] However, with the recent increase in the loading capacity of freight cars, the track environment has become more severe, resulting in frequent breakages caused by brittle fracture at the head of welded joints. The technology described in Patent Document 3 is considered to be effective in suppressing damage caused by pro-eutectoid cementite structures. However, the technology described in Patent Document 3 is not intended to prevent breakage caused by brittle fracture at the head under such harsh usage environments. The technology described in Patent Document 3 leaves room for further improvement in the usage performance of rails.

[0018] The present invention was devised in view of the above-mentioned problems, and aims to improve the fatigue damage resistance and breakage resistance of welded rail weld joints. Preferably, the present invention aims to provide a rail that can satisfy the extremely strict fatigue damage resistance and breakage resistance requirements of welded rail joints for freight railways, which are subject to harsh track environments. [Means for solving the problem]

[0019] The gist of the present invention resides in the rail shown below.

[0020] (1) A welded rail according to one aspect of the present invention is a welded rail including a plurality of rail portions and a weld joint portion joining the rail portions, wherein the rail portions have, as chemical compositions in mass%, C: 0.85 to 1.20%, Si: 0.10 to 2.00%, Mn: 0.10 to 2.00%, Cr: 0.10 to 1.50%, P≦0.0250%, S≦0.0250%, Mo: 0 to 0.50%, Co: 0-1.00%, B: 0-0.0050%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0.20%, Nb: 0-0.0500%, Ti: 0-0.0500%, Mg: 0-0.0200%, Ca: 0-0.0200%, REM: 0-0.0500%, N: 0-0.0200%, Zr: 0-0.0200%, and Al: 0-1.000%, with the balance being Fe and impurities. a HAZ width (W) that is the distance between two most-softened portions formed on both sides of the weld center of the welded joint, measured along the longitudinal direction of the welded rail in a cross section that is parallel to the longitudinal direction and the up-down direction of the welded rail and that passes through the center of the welded rail in the width direction, is 60 mm or less; when the distance between the most-softened portion and the weld center, measured along the longitudinal direction in the cross section, is defined as WX, and a region that is 0.6WX to 0.7WX away from the weld center and has a depth of 2 to 5 mm from the outer surface of the top of the welded joint is defined as a pro-eutectoid cementite structure evaluation region, the total number (N) of intersections of pro-eutectoid cementite structures that intersect with a cross line consisting of two line segments that are 100 μm long and parallel to the longitudinal direction and the up-down direction in the pro-eutectoid cementite structure evaluation region is 26 or less. (2) In the welded rail described in (1) above, the HAZ width (W) of the welded joint and the total intersection number (N) of the pro-eutectoid cementite structure may further satisfy the following formula: N≦4.6×LN(W)1 set Here, "LN" in the above formula 1 stands for natural logarithm. [Effects of the Invention]

[0021] According to the above-described aspects of the present invention, it is possible to improve the fatigue damage resistance and breakage resistance of the welded joint, and to significantly improve the service life of the rail. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 2 is a side view of a welded joint of a welded rail. [Figure 2] FIG. 2 is a cross-sectional view of a rail portion of a welded rail, taken perpendicular to the longitudinal direction. [Figure 3] FIG. 1 is a schematic diagram of the cross-sectional hardness distribution at a depth of 5 mm from the outer surface of the head portion, obtained by measuring the hardness of the welded joint of a welded rail along the longitudinal direction of the welded rail. [Figure 4] This is a schematic diagram of a rolling fatigue testing machine that reproduces damage caused by rolling contact between rails and wheels. [Figure 5] FIG. 1 is a schematic diagram of a pro-eutectoid cementite structure evaluation region. [Figure 6] FIG. 1 is a schematic diagram of a method for evaluating the pro-eutectoid cementite structure in the pro-eutectoid cementite structure evaluation region. [Figure 7] FIG. 1 is a schematic diagram of drop weight test conditions. [Figure 8] 1 is a graph showing the relationship between pro-eutectoid cementite structure and breakage resistance. [Figure 9] 1 is a graph showing the effect of HAZ width on breakage resistance. [Figure 10A] 1 is a graph showing the effect of the pro-eutectoid cementite structure on fracture resistance in a welded joint having a HAZ width of 10 mm. [Figure 10B] 1 is a graph showing the effect of the pro-eutectoid cementite structure on fracture resistance in a welded joint having a HAZ width of 30 mm. [Figure 10C] 1 is a graph showing the effect of the pro-eutectoid cementite structure on fracture resistance in a welded joint having a HAZ width of 60 mm. [Figure 11] 1 is a graph showing the influence of the HAZ width and the critical pro-eutectoid cementite structure on breakage resistance. [Figure 12] FIG. 1 is a schematic diagram of the heat distribution near the center of the weld after flash butt welding. [Figure 13] FIG. 1 is a schematic diagram of the change over time in heat distribution near the center of the weld after flash butt welding. [Figure 14A] FIG. 2 is a cross-sectional view of an example of a cooling device for a welded joint. [Figure 14B] FIG. 1 is a perspective view of an example of a cooling device for a welded joint. [Figure 15A] 1 is a diagram showing an example of a cooling gas outlet provided in a cooling device for a welded joint portion. [Figure 15B] 1 is a diagram showing an example of a cooling gas outlet provided in a cooling device for a welded joint portion. [Figure 15C] 1 is a diagram showing an example of a cooling gas outlet provided in a cooling device for a welded joint portion. [Figure 15D] 1 is a diagram showing an example of a cooling gas outlet provided in a cooling device for a welded joint portion. DETAILED DESCRIPTION OF THE INVENTION

[0023] As shown in FIG. 1 , a flash-butt welded rail (hereinafter simply referred to as “welded rail 1”) includes a plurality of rail portions 11 and a weld joint portion 12 joining the rail portions 11. The inventors have conducted extensive research into methods for improving the fatigue damage resistance and breakage resistance of the weld joint portion 12. The inventors have found that the fatigue damage resistance of the weld joint portion 12 improves as the HAZ width of the weld joint portion 12 is reduced. On the other hand, the inventors have also found that the breakage resistance of the weld joint portion 12 is reduced as the HAZ width of the weld joint portion 12 is reduced. As a result of various studies by the inventors into this phenomenon, they have found that the narrower the HAZ width of the weld joint portion 12, the smaller the width of the softened portion in the weld joint portion 12, which reduces macroscopic ductility and thereby reduces the breakage resistance of the welded rail 1.

[0024] The inventors have optimized the welding conditions and the heat treatment conditions after welding completion, (1) Reduce the HAZ width in the welded joint 12 shown in FIG. 3, and (2) The amount of pro-eutectoid cementite precipitated in the pro-eutectoid cementite structure evaluation region C of the welded joint 12 shown in FIG. 5 was reduced. As a result, the inventors were able to improve the fatigue damage resistance and breakage resistance of the welded joint 12, and significantly increase its service life. Furthermore, the inventors were able to further improve the service life of the welded joint 12 by limiting the relationship between the HAZ width and the amount of pro-eutectoid cementite precipitation.

[0025] A welded rail 1 having excellent fatigue damage resistance and breakage resistance according to one embodiment of the present invention, which was obtained based on the above findings, will now be described in detail. Hereinafter, mass % in the composition will simply be abbreviated as %.

[0026] First, the terms used in this embodiment will be explained.

[0027] The flash-butt welded rail 1 is a rail obtained by joining rails together by flash-butt welding. Hereinafter, the flash-butt welded rail 1 will be simply referred to as a "welded rail 1."

[0028] As shown in Figures 1 and 2, a welded rail 1 comprises multiple rail portions 11 each having a rail head portion 111, a rail column portion 112, and a rail bottom portion 113, and a weld joint portion 12 that joins these rail portions 11. Note that the symbol "A" in Figure 1 indicates the weld center, which will be described later. Hereinafter, when simply referred to as a "rail," it means the rail before welding, and when referred to as a "rail portion," it means the base material portion of the welded rail.

[0029] The rail head portion 111 of the rail portion 11 refers to the portion above the narrowed portion in the vertical center of the rail portion 11 in the cross section perpendicular to the longitudinal direction of the rail portion 11 shown in Fig. 2. The rail post portion 112 refers to the narrowed portion in the vertical center of the rail portion 11 in the cross section of the rail portion 11 shown in Fig. 2. The rail bottom portion 113 refers to the portion below the narrowed portion in the vertical center of the rail portion 11 in the cross section of the rail portion 11 shown in Fig. 2.

[0030] Furthermore, the outer surface of the upper part of the rail head portion 111 is referred to as the rail top surface or rail top outer surface 1111. The narrowed portion at the bottom of the rail head portion 111 is referred to as the rail jaw subsection 1112. The head side surface of the rail head portion 111 is referred to as the rail head side outer surface 1113. On the rail top surface, the outer surface near the corner of the rail portion 11 is referred to as the rail top corner side outer surface 1114. Naturally, the up-down direction of the welded rail 1 refers to the up-down direction when the welded rail 1 is used as a track.

[0031] The welded joint 12 is a "welded joint" defined in JIS Z 3001-1:2018, and refers to a joint where members are joined together by welding. In this embodiment, the member refers to the rail that is the material for the rail portion 11. The welded joint 12 includes a heat-affected zone (HAZ) 12H.

[0032] In the welded rail 1, the shape of the welded joint 12 is substantially the same as that of the rail portion 11. Therefore, like the rail portion 11, the welded joint 12 also has a head portion 121, a column portion 122, and a bottom portion 123. The head portion 121 of the welded joint 12 has a top outer surface 1211, a subjaw portion 1212, a head side outer surface 1213, and a top corner side outer surface 1214. Hereinafter, the name of the head portion in the rail portion 11 will be referred to as the "rail head portion 111," and the name of the head portion in the welded joint 12 will be simply referred to as the "head portion 121." Regarding other portions, when they are included in the rail portion 11, the term "rail" will be used, and when they are included in the welded joint portion 12, the term "rail" will not be used.

[0033] The heat-affected zone (HAZ) 12H, as defined in JIS Z 3001-1:2018, refers to a non-melted portion of the base material where metallurgical properties, mechanical properties, etc. have changed due to heat from welding, cutting, etc. In this embodiment, the base material refers to the rail portion 11.

[0034] In the welded rail 1 according to this embodiment, the width of the heat-affected zone 12H along the longitudinal direction of the welded rail 1, i.e., the HAZ width, must be within a predetermined range. In the welded rail 1 according to this embodiment, the HAZ width is defined based on the hardness distribution of the welded joint 12 measured on a cross section that is parallel to the longitudinal and vertical directions of the welded rail 1 and passes through the center of the welded rail 1 in the width direction. In this embodiment, the cross section that is parallel to the longitudinal and vertical directions of the welded rail 1 and passes through the center of the welded rail 1 in the width direction is referred to as the "longitudinal cross section." Below, we will provide an overview of the hardness distribution of the welded joint 12, and then we will explain the definition of the HAZ width.

[0035] Figure 3 shows a schematic diagram of the hardness distribution in a longitudinal cross section 5 mm below the apex outer surface 1211 of the welded joint 12. This graph was obtained by measuring Vickers hardness continuously along the apex outer surface 1211 at a position 5 mm deep from the apex outer surface 1211 of the welded joint 12 in the longitudinal cross section of the welded joint 12. Note that the weld center A shown in this graph refers to a straight line along the vertical direction of the welded rail that passes through the center of the heat-affected zone 12H in the longitudinal cross section of the welded joint 12. Typically, the weld center A roughly coincides with the rail joint.

[0036] The welded joint 12 is heated to above the A1 point by the welding heat, resulting in austenitization throughout the weld joint. The subsequent cooling after welding results in a pearlite transformation region. On either side of this region, there are regions where the welded joint is heated to near the A1 point by the welding heat, resulting in partial austenitization, and then the pearlite structure decomposes as the temperature drops after welding. These regions exhibit significantly lower hardness. Therefore, as shown in Figure 3, the hardness distribution graph of a welded rail 1 obtained by flash butt welding typically exhibits two Vickers hardness valleys. The locations where these Vickers hardness valleys occur are defined as the softest-softened regions of the welded rail 1 according to this embodiment. The hardness of the softest-softened regions is generally 230 HV or higher, or 250 HV or higher. The distance between the two softest parts, which is determined by continuously measuring the Vickers hardness at a position 5 mm deep from the top outer surface 1211 of the welded joint 12 in a longitudinal cross section of the welded joint 12, along the top outer surface 1211, is defined as the HAZ width W.

[0037] As shown in Fig. 5, the pro-eutectoid cementite structure evaluation region C refers to a region in the longitudinal cross section that is 0.6WX to 0.7WX away from the weld center A and that is 2 to 5 mm deep from the outer surface of the head top. Here, WX is the distance between the softest part and the weld center A measured along the longitudinal direction of the welded rail 1 in the longitudinal cross section. The technical significance of the pro-eutectoid cementite structure evaluation region C will be described later. The pro-eutectoid cementite structure evaluation region C may be set on either the right or left side of the weld center A.

[0038] Next, the technical concept of the present invention will be explained. The inventors investigated damage that occurs in the welded joints of welded rails. As a result of investigating damaged rails that occurred in actual track, it was confirmed that the damage occurs in two ways: (1) breakage initiated by fatigue cracks that initiate from the bottom of the welded joint, and (2) breakage initiated by brittle cracks that initiate from the surface of the head of the welded joint.

[0039] Therefore, the causes of these occurrences were investigated. First, (1) breakage originating from fatigue cracks that initiate from the bottom of the welded joint was investigated. In welded joints where fatigue cracks initiate from the bottom of the welded joint, there was a large drop in the head of the welded joint due to wear. It was also found that in such welded rails, the HAZ width of the welded joint is significantly large. When a wheel passes over a welded joint with a worn head, the load of the vehicle causes bending deformation in the welded rail, which in turn generates a tensile load at the base of the welded joint. This tensile load causes fatigue cracks at the bottom of the welded joint.

[0040] (Relationship between HAZ width and breakage (Table 1)) Furthermore, to prevent fractures caused by fatigue cracks originating from the bottom of the welded joint, we investigated the relationship between the HAZ width and fracture in the welded joint. We performed flash butt welding tests using hypereutectoid steel rails (0.80–1.20% C) to create various welded joints with different HAZ widths. HAZ width was controlled primarily by controlling the late flash velocity just before upsetting during flash butt welding. The relationship between HAZ width and stress at the bottom of the welded joint was evaluated using a testing machine that reproduces damage caused by rail / wheel rolling, as shown in Figure 4. In Figure 4, reference numeral 1 denotes the welded rail described above, reference numeral 2 denotes the sleeper on which the welded rail 1 rests, and reference numeral 5 denotes a load stabilizer that holds down the wheel 3, which is rotated by a motor 4. In the rolling fatigue test, the load stabilizer 5 applies a predetermined load to the wheel 3, while the wheel 3 repeatedly rolls back and forth along the head of the welded rail 1 in the longitudinal direction.

[0041] The rail, flash butt welding conditions, cooling conditions for the welded joint after welding, characteristics of the welded joint, and conditions for the rail / wheel rolling fatigue test are as follows. Cooling of the welded joint after welding was performed on the top surface of the weld center (A), where wear-induced depression mainly occurs.

[0042] Rails as welding base material Composition: 0.80-1.20% C, 0.30% Si, 0.60% Mn, 0.0120% P, 0.0100% S, 0.35% Cr, 0.0035% N, 0.0020% Al, the balance being iron and impurities Rail profile: 136 lbs (weight: 67 kg / m). Hardness: 420 HV (top surface)

[0043] ●Flash butt welding conditions (preheating flash method) Initial flash time: 15 seconds Preheating times: 2 to 16 Late flash time: 15-30 seconds Average late flash velocity: 0.2-1.0mm / sec Late flash velocity just before upsetting (3 seconds): 0.3 to 3.0 mm / sec Upsetting load: 65~85KN

[0044] ● Cooling conditions for welded joints after welding Average cooling rate of the top surface of the weld center (A): 1.5 to 3.0°C / sec (temperature range: 800 to 550°C) + subsequent cooling (50°C) Cooling means: cooling device shown in Figures 14A and 14B As shown in Figures 14A and 14B, a cylindrical cooling device 6 was placed around the welded joint 12. The longitudinal direction of the cylindrical cooling device 6 was aligned with the longitudinal direction of the welded rail 1. As shown in Figures 15A to 15D, the cooling device 6 was provided with a plurality of cooling gas outlet ports 61 along the longitudinal direction of the cooling device 6. Using these cooling devices 6, cooling gas g was sprayed onto the top outer surface 1211, the underchin 1212, and the side outer surfaces 1213 of the head. Furthermore, as shown in Figures 15A to 15D, by changing the spacing between multiple cooling gas outlets 61, the cooling rate at the welding center A and the cooling rate at a location estimated to be 0.6WX to 0.7WX away from the welding center A were independently controlled. For example, the cooling device 6 of FIG. 15C has multiple cooling gas outlets 61 uniformly arranged. Therefore, the cooling device 6 of FIG. 15C can spray cooling gas uniformly along the longitudinal direction of the welded joint 12. On the other hand, the cooling device 6 of FIG. 15A has multiple cooling gas outlets 61 arranged at wide intervals in the longitudinal center and at narrow intervals near the longitudinal ends. During cooling, the longitudinal center of the cooling device 6 is arranged to face the weld center A, and the longitudinal ends of the cooling device 6 are arranged to face the location estimated to be the most softened part. Therefore, with the cooling device 6 of FIG. 15A, the amount of cooling gas sprayed on the most softened part is greater than the amount of cooling gas sprayed on the weld center A. The cooling devices 6 of Figures 15B and 15D, like those of Figure 15A, have multiple cooling gas outlets 61 arranged at wide intervals in the longitudinal center and at narrow intervals near the longitudinal ends. However, compared to Figure 15A, the spacing between the cooling gas outlets 61 in Figure 15B is wider in the longitudinal center. Therefore, compared to the cooling device of Figure 15A, the cooling device of Figure 15B has a smaller cooling gas blowing capacity to the weld center A. Compared to Figure 15A, the spacing between the cooling gas outlets 61 in Figure 15D is narrower near the longitudinal ends. Therefore, compared to the cooling device of Figure 15A, the cooling device of Figure 15D has a larger cooling gas blowing capacity to the most softened part.

[0045] ●Characteristics of welded joints HAZ width: 10~80mm Hardness of weld center: 390~440 HV Hardness of softest part: 280 HV

[0046] Conditions for rolling fatigue testing of rails and wheels Testing machine: Rolling fatigue testing machine (see Figure 4) Shape of the welded rail used as the test specimen: 2m long (with a welded joint in the center of the length) Wheels: AAR type (diameter 920mm) Radial load: 300KN Thrust load: 50KN Bottom stress: 400 MPa (measured using a strain gauge at the beginning of the test) Lubrication: Repeated water-dry lubrication (i.e., spraying water on the welded rail for a certain period of time, then stopping the water supply and allowing the water to dry, repeating the cycle) Number of repeated load applications using wheels: Up to 4 million times Cumulative transit tonnage: up to 120 million tons Pass criteria: No breakage after 2 million load applications *Cumulative passing tonnage: The total weight of freight cars that have traveled on the welded rails. In this test, it is evaluated as twice the passing weight applied by the wheels. In other words, the cumulative passing tonnage is calculated by multiplying the above-mentioned radial load (300kN) by the number of wheel passes x 2.

[0047] ●Evaluation Inspection method for bottom cracks in welded joints: visual inspection and magnetic particle inspection

[0048] [Table 1]

[0049] As a result, as shown in Table 1, the smaller the HAZ width, the more repeated cycles until fracture occurred, and the longer the service life of the welded joint. In addition, the smaller the HAZ width, the less unevenness formed in the welded joint.

[0050] Specifically, when the HAZ width exceeded 60 mm, the irregularities formed in the welded joint increased, and the number of wheel passes required to fracture was less than 2 million, failing to meet the pass criteria. Furthermore, when the HAZ width was between 40 mm and 60 mm, the irregularities formed in the welded joint decreased, and the number of wheel passes required to fracture exceeded 2 million, falling within the range of 2 million to 3 million, thereby meeting the pass criteria. Furthermore, when the HAZ width was between 20 mm and 40 mm, the irregularities formed in the welded joint further decreased, and the number of wheel passes required to fracture fell within the range of 3 million to 4 million. Furthermore, when the HAZ width was between 10 mm and 20 mm, the irregularities formed in the welded joint further decreased, and fracture no longer occurred even after 4 million wheel passes.

[0051] This test revealed that the service life of the welded joint is further improved as the HAZ width decreases.

[0052] (Relationship between the total number of intersections in the pro-eutectoid cementite structure and breakage (Fig. 8)) Next, (2) we investigated the cause of breakage, which originates from a brittle crack on the surface of the head of the welded joint. As a result of investigating the relationship between the fracture origin and the metal structure of the welded rail where the breakage occurred, it was confirmed that a pro-eutectoid cementite structure was formed at the fracture origin.

[0053] Therefore, we identified the origin of the breakage and found that the breakage occurred in the heat-affected zone 12H (HAZ).

[0054] Furthermore, the fracture location was identified in detail. As a result, in the longitudinal cross-sectional hardness distribution of the welded joint shown in Figure 3, where WX is the distance between the weld center (A) and the softest part, it was confirmed that the fracture occurred in a location within a range of 0.6WX to 0.7WX from the weld center A and within a depth range of 2 to 5 mm from the outer surface of the top. This location corresponds to the pro-eutectoid cementite structure evaluation region C described above.

[0055] Therefore, we investigated the relationship between the pro-eutectoid cementite structure in this region and fracture in welded joints. First, we investigated the relationship between the amount of pro-eutectoid cementite structure and fracture in welded joints. A flash butt welding test was performed using a hyper-eutectoid steel rail (1.00% C). The drop weight test shown in Figure 4 was then performed on the welded rail to evaluate the relationship between the amount of pro-eutectoid cementite structure and the presence or absence of fracture in welded joints. The amount of pro-eutectoid cementite structure was controlled by controlling the cooling rate of the outer surface of the crown at a distance of 0.6WX to 0.7WX from the weld center in the welded joint where pro-eutectoid cementite structure was formed. The HAZ width was controlled primarily by controlling the number of preheats, the average late flashing velocity, and the late flashing velocity immediately before upsetting in flash butt welding. The rail, flash butt welding conditions, cooling conditions for the welded joint after welding, welded joint properties, the pro-eutectoid cementite structure evaluation method, and drop weight test conditions are as follows:

[0056] Rails as welding base material Composition: 1.00% C, 0.30% Si, 0.60% Mn, 0.0120% P, 0.0100% S, 0.35% Cr, 0.0035% N, 0.0020% Al, balance iron and impurities Rail profile: 136 lbs (weight: 67 kg / m). Hardness: 420 HV (top surface) ●Flash butt welding conditions (preheating flash method) Initial flash time: 15 seconds Preheating times: 2 to 14 times Late flash time: 15-30 seconds Average late flash velocity: 0.3-1.0mm / sec Late flash velocity just before upsetting (3 seconds): 0.5 to 3.0 mm / sec Upsetting load: 65~85KN

[0057] ● Cooling conditions for welded joints after welding Average cooling rate of the top surface of the weld center (A): 1.5 to 3.5 / sec (temperature range: 800 to 550°C) + subsequent cooling (50°C) Average cooling rate of the outer surface of the top of the welded joint 0.6WX-0.7WX: 0.8-4.0°C / sec (temperature range: 800-550°C) + 0.1-1.5°C / sec (temperature range: 550-450°C) + subsequent cooling (50°C) Cooling means: cooling device shown in Figures 14A and 14B

[0058] ●Characteristics of welded joints HAZ width: 10~60mm Hardness of weld center: 380~440 HV Hardness of softest part: 280 HV Total number of intersections of proeutectoid cementite structure in proeutectoid cementite structure evaluation region C (N): 20 to 34

[0059] ●Method for evaluating pro-eutectoid cementite structure Evaluation area (see Figure 5): In the longitudinal cross section of the welded joint, if the distance between the weld center (A) and the softest part is WX, the area is 0.6WX to 0.7WX from the weld center and 2 to 5 mm deep from the outer surface of the top of the weld. Reason for selecting the evaluation area: This is the location where breakage originating from the pro-eutectoid cementite structure occurs. Evaluation method of pro-eutectoid cementite structure: After polishing the area for evaluation of the pro-eutectoid cementite structure, cementite etching was performed, and the pro-eutectoid cementite structure was observed with an optical microscope and photographed. Polishing conditions: Buffing with 1 μm diamond paste Proeutectoid cementite etching conditions Etching solution: Picric acid caustic soda solution Etching conditions: 80°C x 120 minutes Survey methodology Equipment: Optical microscope Magnification: 500x Evaluation method (see Figure 6): The number of pro-eutectoid cementite structures intersecting two perpendicular line segments, each 100 μm long, was counted. One of the two perpendicular line segments was parallel to the longitudinal direction of the welded rail, and the other was perpendicular to the up-down direction of the welded rail. The two perpendicular line segments were arranged so that they intersected each other at their midpoints to form a cross. The total number of intersections (N) of the pro-eutectoid cementite structure was defined as the sum (Xn+Yn) of the numbers of cementite strands (Xn, Yn) that intersected with each of the orthogonal 100 μm line segments. In the chemical composition of the rail portion of the welded rail according to this embodiment, pro-eutectoid cementite usually precipitates in a network form as shown in Fig. 6. Since granular cementite can be difficult to distinguish from inclusions such as MnS, it is preferable to measure only the network-form cementite when measuring the total number of intersections of the pro-eutectoid cementite structure. Quantification: Two perpendicular lines, each 100 μm long, were drawn at 20 locations in the pro-eutectoid cementite evaluation area, and the total number of intersections of the pro-eutectoid cementite structures was measured. The average value of the total number of intersections in each photograph was considered to be the total number (N) of intersections of the pro-eutectoid cementite structures in the weld joint.

[0060] Drop weight test conditions (see Figure 7) Position: The welded rail is supported at two points with the head on the bottom and the bottom on the top, and a drop weight is dropped onto the bottom of the welded joint. Span (distance between two support points): 1000mm Falling weight: 1000kgf (9.8kN) Drop height (X): 3.0 m Falling weight energy: 29.4 kN m

[0061] As a result, as shown in FIG. 8, it was found that when the total number of intersections (N) of the pro-eutectoid cementite structure in the evaluation region of the pro-eutectoid cementite structure exceeds 26, fracture of the welded joint occurs in the drop weight test.

[0062] (Relationship between HAZ width and falling weight energy (Fig. 9)) Furthermore, in order to drastically improve the performance of welded joints used in increasingly severe track environments due to the recent trend toward heavier loadings on freight cars, the inventors conducted a detailed investigation into the relationship between breakage due to brittle fracture occurring at the head and the HAZ width of the welded joint. In the pro-eutectoid cementite structure evaluation region C shown in Figure 5, the number of pro-eutectoid cementite structures formed in the longitudinal cross section of the welded joint was further controlled (total number of pro-eutectoid cementite structures: N = 18), and the correlation between the HAZ width of the welded joint and breakage resistance was investigated under drop weight test conditions that reproduced even more severe track conditions. The number of pro-eutectoid cementite structures formed was mainly controlled by controlling the cooling rate of the outer surface of the head top at a distance of 0.6WX to 0.7WX from the weld center in the welded joint where the pro-eutectoid cementite structures formed. The range between the upper and lower limits of the cooling rate was narrowed to maintain a constant total number of pro-eutectoid cementite structures. In addition, the HAZ width was controlled mainly by controlling the number of preheatings, the average late flash velocity, and the lower limit of the late flash velocity just before upsetting in flash butt welding.

[0063] A flash butt welding test was conducted using a hypereutectoid steel rail (1.00% C), and the drop weight test of the welded rail shown in Figure 7 was conducted to evaluate the relationship between the amount of pro-eutectoid cementite structure formed and the presence or absence of fracture in the welded joint. The rail, flash butt welding conditions, and evaluation method for the pro-eutectoid cementite structure were the same as those used in the welding test for the graph in Figure 8. The cooling conditions for the welded joint after welding, the properties of the welded joint, and the drop weight test conditions are as follows:

[0064] ● Cooling conditions for welded joints after welding Average cooling rate of the top surface of the weld center (A): 1.5 to 3.5°C / sec (temperature range: 800 to 550°C) + subsequent cooling (50°C) Average cooling rate of the outer surface of the top of the welded joint 0.6WX-0.7WX: 1.7-2.8°C / sec (temperature range: 800-550°C) + 0.8-1.5°C / sec (temperature range: 550-450°C) + subsequent cooling (50°C) Cooling means: cooling device shown in Figures 14A and 14B

[0065] ●Characteristics of welded joints HAZ width: 10~60mm Hardness of weld center: 380~440 HV Hardness of softest part: 280 HV Total number of intersections of proeutectoid cementite structure (N): 18 Drop weight test conditions (see Figure 7) Position: The welded rail is supported at two points with the head on the bottom and the bottom on the top, and a drop weight is dropped onto the bottom of the welded joint. Span (distance between two support points): 1000mm Falling weight: 1000kgf (9.8kN) Drop height (X): 4.0 to 11.0 m Falling weight energy: 39.2 to 107.8 kN m

[0066] As a result, as shown in Figure 9, when the total number of intersections (N) of pro-eutectoid cementite structures is the same, there is a correlation between the HAZ width of a welded joint and the fracture resistance of the welded joint. As the HAZ width decreases, the falling weight energy that causes fracture decreases. In other words, the inventors have found that as the HAZ width decreases, the fracture resistance of the welded joint decreases. The inventors have determined that this decrease in fracture resistance is caused by a decrease in the softened area of the welded joint as the HAZ width decreases, i.e., a decrease in macroscopic ductility.

[0067] (Favorable relationship between HAZ width and total number of intersections of pro-eutectoid cementite structure (FIGS. 10A to 10C and FIG. 11)) Furthermore, the inventors conducted a detailed investigation into the fracture resistance of welded joints, which varies depending on the HAZ width. The correlation between the formation of pro-eutectoid cementite structures in longitudinal cross sections of welded joints in the pro-eutectoid cementite structure evaluation region C shown in Figure 5 and the fracture resistance of the welded joint was investigated under drop weight test conditions. A flash butt welding test was conducted using a hyper-eutectoid steel rail (1.00% C). Next, a drop weight test was conducted on the welded joint shown in Figure 7 to evaluate the relationship between the amount of pro-eutectoid cementite structures formed and the presence or absence of fracture in the welded joint. This allowed for the investigation of the formation of pro-eutectoid cementite structures that can prevent fracture. The number of pro-eutectoid cementite structures formed was mainly controlled by controlling the cooling rate of the outer surface of the crown at a distance of 0.6WX to 0.7WX from the weld center in the welded joint where the pro-eutectoid cementite structures formed. The cooling rate was limited to a certain range, and the total number of intersections of the pro-eutectoid cementite structures was controlled to be within a certain range. In addition, the HAZ width was controlled mainly by controlling the number of preheatings, the average late flash velocity, and the lower limit of the late flash velocity just before upsetting in flash butt welding.

[0068] The rail, flash butt welding conditions, and evaluation method for the pro-eutectoid cementite structure were the same as those for the welding test shown in Figure 8. The cooling conditions for the welded joint after welding, the properties of the welded joint, and the drop weight test conditions are as follows:

[0069] ● Cooling conditions for welded joints after welding Average cooling rate of the top surface of the weld center (A): 1.5 to 3.5 / sec (temperature range: 800 to 550°C) + subsequent cooling (50°C) Average cooling rate of the outer surface of the top of the welded joint 0.6WX to 0.7WX: 1.5 to 3.5°C / sec (temperature range: 800 to 550°C) + 0.2 to 1.5°C / sec (temperature range: 550 to 450°C) + subsequent cooling (50°C) Cooling means: cooling device shown in Figures 14A and 14B

[0070] ●Characteristics of welded joints HAZ width: 10, 20, 30, 40, 50, 60mm (6 levels) Hardness of weld center: 380~440 HV Hardness of softest part: 280 HV Total intersection number of proeutectoid cementite structure (N) = 6 to 26

[0071] Drop weight test conditions (see Figure 7) Position: The welded rail is supported at two points with the head on the bottom and the bottom on the top, and a drop weight is dropped onto the bottom of the welded joint. Span (distance between two support points): 1000mm Falling weight: 1000kgf (9.8kN) Drop height (X): 6 levels within the range of 7.0 to 12.0 m Falling weight energy: 6 levels within the range of 68.6 to 117.6 kN·m Drop weight test conditions to prevent breakage under severe track conditions Drop height (X): 9.0 m Standard energy to prevent breakage: 88.2 kN m

[0072] The test results are plotted in Figures 10A to 10C, with the horizontal axis representing the total number of intersections N of pro-eutectoid cementite structures and the vertical axis representing the drop weight energy. Figure 10A shows the evaluation results for various welded joints with a HAZ width of 10 mm, Figure 10B shows the evaluation results for various welded joints with a HAZ width of 30 mm, and Figure 10C shows the evaluation results for various welded joints with a HAZ width of 60 mm. In Figures 10A to 10C, the type of data point is changed depending on whether or not breakage occurred. The "breakage prevention reference energy" shown in Figures 10A to 10C is an evaluation standard for the breakage resistance of welded joints under severe track conditions. In this test, the breakage prevention reference energy was set to 88.2 kN. Welded rails that did not experience breakage at the welded joints in a drop weight test with a drop weight energy of 88.2 kN were determined to have excellent welded joint breakage resistance even under severe track conditions. In addition, the maximum total number of intersections of pro-eutectoid cementite structures in various welded joints that can withstand a falling weight energy of 88.2 kN was considered to be the critical total number of intersections of cementite in the pro-eutectoid cementite structure.

[0073] 10A to 10C, it was confirmed that the total number of intersections (N) of pro-eutectoid cementite structures capable of preventing breakage under severe track conditions, i.e., the critical total number of intersections of pro-eutectoid cementite structures, significantly decreased as the HAZ width decreased. Specifically, as shown in Fig. 10C, the critical total number of intersections of pro-eutectoid cementite structures in a welded joint with a HAZ width of 60 mm was 20, while as shown in Fig. 10B and Fig. 10A, the critical total number of intersections of pro-eutectoid cementite structures in a welded joint with a HAZ width of 30 mm was 18, and the critical total number of intersections of pro-eutectoid cementite structures in a welded joint with a HAZ width of 10 mm was 12.

[0074] Figure 11 shows the relationship between HAZ width and the total number of intersections of the critical pro-eutectoid cementite structure for HAZ widths of 10 to 60 mm. It can be seen that as the HAZ width decreases, the total number of intersections of the critical pro-eutectoid cementite structure, which is necessary to prevent breakage under severe track conditions, decreases significantly. These experimental results demonstrate that the more the HAZ width is reduced to improve the service life of the welded joint, the more the total number of intersections of the critical pro-eutectoid cementite structure increases, making it more difficult to ensure breakage resistance under severe track conditions.

[0075] Furthermore, in order to reliably prevent breakage under severe track conditions, the inventors estimated the critical total number of intersections of pro-eutectoid cementite structures for preventing breakage of welded joints for each HAZ width. As a result, they found that breakage of welded joints can be reliably prevented by reliably controlling the total number of intersections (N) of pro-eutectoid cementite structures to a value calculated using the following formula (1) based on the HAZ width (W). Here, "LN" in formula (1) refers to the natural logarithm, i.e., the logarithm with Napier's constant e as the base. N≦4.6×LN(W) 1 set

[0076] From these results, the present inventors have found that in order to further suppress breakage due to brittle cracks initiating from the head of a welded joint, it is necessary to control the amount of pro-eutectoid cementite structures produced, i.e., the total number of intersections of pro-eutectoid cementite structures. Furthermore, the present inventors have found that in order to prevent breakage of a welded joint under severe track conditions, it is desirable to control the total number of intersections of pro-eutectoid cementite structures within a predetermined range defined according to the HAZ width.

[0077] The welded rail according to this embodiment, which has excellent fatigue damage resistance and breakage resistance at the welded joint, was obtained based on the above findings and will be described in detail below. Hereinafter, the unit of "mass%" for the content of alloy components will be simply referred to as "%".

[0078] (1) Reasons for limiting the chemical composition of steel The reasons for limiting the chemical composition of the rail portion of the welded rail of this embodiment will be described in detail below.

[0079] C is an element that promotes pearlite transformation and is effective in ensuring the wear resistance of welded joints. If the C content is less than 0.85%, the minimum strength and wear resistance required of welded joints cannot be maintained. On the other hand, if the C content exceeds 1.20%, a large amount of pro-eutectoid cementite structure is formed in the welded joint, reducing the fracture resistance of the welded joint. For this reason, the C content is limited to 0.85 to 1.20%. The C content is preferably 0.90% or more, 0.95% or more, or 1.00% or more. The C content is preferably 1.18% or less, 1.15% or less, or 1.10% or less. Note that, to stabilize the formation of pearlite structure, the C content is preferably 0.95 to 1.10%.

[0080] Silicon dissolves in the ferrite phase of a pearlite structure, increasing the hardness of a welded joint and improving its wear resistance. However, if the Si content is less than 0.10%, these effects cannot be fully expected. On the other hand, if the Si content exceeds 2.00%, the toughness of the pearlite structure decreases, and the fracture resistance of the welded joint decreases. For this reason, the Si content is limited to 0.10 to 2.00%. The Si content is preferably 0.20% or more, 0.30% or more, or 0.40% or more. The Si content is preferably 1.80% or less, 1.60% or less, or 1.50% or less. In order to stabilize the formation of a pearlite structure and improve the fracture resistance and wear resistance of a welded joint, the Si content is preferably 0.30 to 1.50%.

[0081] Mn is an element that improves the hardenability of welded rails and stabilizes pearlite transformation, while also refining the lamellar spacing of pearlite structures, ensuring the hardness of welded joints, and further improving wear resistance. However, if the Mn content is less than 0.10%, this effect is small, and the wear resistance of welded joints decreases. On the other hand, if the Mn content exceeds 2.00%, the excessive Mn promotes Mn concentration in segregated areas, promoting the formation of pro-eutectoid cementite structures in welded joints and reducing breakage resistance. For this reason, the Mn content is limited to 0.10 to 2.00%. The Mn content is preferably 0.20% or more, 0.30% or more, or 0.40% or more. The Mn content is preferably 1.80% or less, 1.60% or less, or 1.50% or less. Note that, in order to stabilize the formation of pearlite structures and improve the wear resistance and breakage resistance of welded joints, the Mn content is preferably 0.30 to 1.50%.

[0082] Cr is an element that increases the equilibrium transformation temperature, refines the lamellar spacing of pearlite structures by increasing the degree of supercooling, improves the hardness of pearlite structures, and improves the wear resistance of welded joints. However, if the Cr content is less than 0.10%, these effects cannot be fully expected. On the other hand, if the Cr content exceeds 1.50%, the excessive Cr promotes Cr concentration in segregated areas, promotes the formation of pro-eutectoid cementite structures in welded joints, and reduces breakage resistance. For this reason, the Cr content is limited to 0.10 to 1.50%. The Cr content is preferably 0.15% or more, 0.20% or more, or 0.25% or more. The Cr content is preferably 1.40% or less, 1.30% or less, or 1.00% or less. Note that, to stabilize the formation of pearlite structures and improve the wear resistance and damage resistance of welded joints, the Cr content is preferably 0.20 to 1.00%.

[0083] P is an impurity element contained in steel. If the P content exceeds 0.0250%, the pearlite structure becomes embrittled, reducing the fracture resistance of the welded joint. For this reason, the P content is limited to 0.0250% or less. The lower limit of the P content does not need to be set and may be, for example, 0%. However, taking into account the dephosphorization capacity of the refining process, the lower limit of the P content may be set to approximately 0.0020%. The P content is preferably 0.0025% or more, 0.0030% or more, or 0.0050% or more. The P content is preferably 0.0200% or less, 0.0150% or less, or 0.0120% or less.

[0084] S is an impurity element contained in steel. If the S content exceeds 0.0250%, stress concentration occurs around coarse MnS-based sulfide inclusions, reducing the fracture resistance of the welded joint. For this reason, the S content is limited to 0.0250% or less. The lower limit of the S content does not need to be set and may be, for example, 0%. However, taking into account the desulfurization capacity of the refining process, the lower limit of the S content may be set to approximately 0.0020%. The S content is preferably 0.0025% or more, 0.0030% or more, or 0.0050% or more. The S content is preferably 0.0200% or less, 0.0150% or less, or 0.0120% or less.

[0085] The remainder of the chemical composition of the rail portion of the welded rail includes iron and impurities, which are components that are mixed in, for example, from raw materials such as ore or scrap during the industrial production of steel, or due to various factors in the manufacturing process, and are acceptable as long as they do not adversely affect the welded rail according to this embodiment.

[0086] Furthermore, the rail portion of the welded rail may contain one or more of the following elements as needed: Mo (group a), Co (group b), B (group c), Cu and Ni (group d), V, Nb, and Ti (group e), Mg, Ca, and REM (group f), N (group g), Zr (group h), and Al (group i), for the purposes of increasing the hardness of the welded joint to improve wear resistance, improve toughness, prevent softening of the heat-affected zone, and control the cross-sectional hardness distribution inside the head. However, since the welded rail according to this embodiment can exhibit its effects even if these elements are not contained in the rail portion, the lower limit of the content of these elements is 0%.

[0087] Mo in group a raises the equilibrium transformation point, refines the interlamellar spacing of the pearlite structure, and improves the hardness of the welded joint. Co in group b dissolves in the ferrite phase of the pearlite structure, refines the lamellar structure directly below the rolling surface of the welded joint, and increases the hardness of the wear surface. B in group c reduces the cooling rate dependence of the pearlite transformation temperature and makes the hardness distribution inside the head of the welded joint uniform. Cu and Ni in group d dissolve in the ferrite in the pearlite structure, increase the hardness of the welded joint, and simultaneously improve the toughness. V, Nb, and Ti in group e improve the fatigue strength of the welded joint by precipitation hardening of carbides and nitrides generated during the cooling process of the welded joint after rail welding. Also, V, Nb, and Ti in group e stably generate carbides and nitrides during reheating of the welded joint and prevent softening of the heat-affected zone. Mg, Ca, and REM in group f finely disperse MnS-based sulfides and reduce fatigue damage generated from inclusions in the welded joint. N in group g promotes the precipitation of carbides and nitrides of V during the cooling process of the welded joint after rail welding and improves the fatigue damage resistance of the welded joint. Zr in group h increases the equiaxed crystal ratio of the solidification structure, suppresses the formation of the segregation band in the center of the slab, and suppresses the enrichment of Mn and Cr in the segregation part. Furthermore, Al in group i improves the fracture resistance of the welded joint by deoxidation.

[0088] <Group a> Mo is an element that raises the equilibrium transformation temperature, refines the interlamellar spacing of the pearlite structure due to an increase in supercooling, improves the hardness of the pearlite structure, and improves the wear resistance of the welded joint. To obtain the above effects, it is preferable that the Mo content is 0.01% or more. On the other hand, when the Mo content exceeds 0.50%, the pearlite structure may become brittle and the fracture resistance of the welded joint may decrease. Therefore, it is desirable that the Mo content is 0.01 - 0.50%. The Mo content is preferably 0.02% or more, 0.05% or more, or 0.10% or more. The Mo content is preferably 0.45% or less, 0.40% or less, or 0.30% or less.

[0089] <Group b> Co dissolves in the ferrite phase of the pearlite structure, refines the lamellar structure of the pearlite structure directly under the rolling surface where deformation occurs due to contact with the wheel, improves the hardness of the rolling surface, and is an element that improves the wear resistance of the welded joint. In order to obtain the above effects, it is preferable that the Co content is 0.01% or more. On the other hand, when the Co content exceeds 1.00%, the above effects are saturated, and the refinement of the lamellar structure according to the Co content cannot be achieved. Further, when the Co content exceeds 1.00%, the economic efficiency decreases due to an increase in alloy cost. Therefore, it is desirable that the Co content is 0.01 to 1.00%. The Co content is preferably 0.02% or more, 0.05% or more, or 0.10% or more. The Co content is preferably 0.90% or less, 0.80% or less, or 0.60% or less.

[0090] <Group c> B forms iron carbide boride (Fe 23 (CB)6) at the austenite grain boundary, and due to the promoting effect of pearlite transformation, reduces the cooling rate dependence of the pearlite transformation temperature, equalizes the hardness distribution from the head surface to the inside of the welded joint, and is an element that extends the life of the welded joint. In order to obtain the above effects, it is preferable that the B content is 0.0001%. On the other hand, when the B content exceeds 0.0050%, coarse iron carbide boride is generated, promoting brittle fracture, and the fracture resistance of the welded joint may decrease. Therefore, it is desirable that the B content is 0.0001 to 0.0050%. The B content is preferably 0.0002% or more, 0.0005% or more, or 0.0010% or more. The B content is preferably 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[0091] <Group d> Cu dissolves in the ferrite phase of the pearlite structure, and as an element, it improves the hardness of the welded joint through solid solution strengthening and also improves the wear resistance of the welded joint. To obtain the above effects, it is preferable that the Cu content is 0.01% or more. On the other hand, when the Cu content exceeds 1.00%, the pearlite structure may become brittle and the fracture resistance may decrease. Therefore, it is preferable that the Cu content is 0.01 - 1.00%. The Cu content is preferably 0.02% or more, 0.05% or more, or 0.10% or more. The Cu content is preferably 0.90% or less, 0.80% or less, or 0.70% or less. Note that it is desirable to control the Cu content to 0.40% or less.

[0092] Ni is an element that improves the toughness of the pearlite structure, and at the same time, through solid solution strengthening, it improves the hardness of the welded joint and the wear resistance of the welded joint. Furthermore, in the heat-affected zone, Ni combines with Ti and precipitates as fine intermetallic compounds of Ni3Ti, and as an element, it suppresses the softening of the welded joint through precipitation strengthening. Also, when Cu is contained in the rail part, Ni suppresses grain boundary embrittlement. To obtain the above effects, it is preferable that the Ni content is 0.01% or more. When the Ni content exceeds 1.00%, the pearlite structure may become brittle and the fracture resistance may decrease. Therefore, it is desirable that the Ni content is 0.01 - 1.00%. The Ni content is preferably 0.02% or more, 0.05% or more, or 0.10% or more. The Ni content is preferably 0.90% or less, 0.80% or less, or 0.70% or less.

[0093] <e group> V is an element that increases the hardness (strength) of pearlite structures and improves the fatigue damage resistance of welded joints through precipitation hardening due to V carbonitrides formed during the cooling process after hot rolling. To achieve the above-mentioned effects, the V content is preferably 0.01% or more. On the other hand, if the V content exceeds 0.20%, the number of fine V carbonitrides becomes excessive, embrittling the pearlite structure and reducing the fatigue damage resistance of the welded joints. Therefore, the V content is preferably 0.01 to 0.20%. The V content is preferably 0.02% or more, 0.03% or more, or 0.05% or more. The V content is preferably 0.18% or less, 0.15% or less, or 0.10% or less.

[0094] Nb is an element that increases the hardness of pearlite structures and improves the fatigue damage resistance of welded joints through precipitation hardening by Nb carbides and Nb nitrides formed during the cooling process after hot rolling. Furthermore, in heat-affected zones reheated to temperatures below the Ac1 point, Nb stably forms Nb carbides and Nb nitrides over a wide temperature range, from low to high, and is an effective element for preventing softening of the heat-affected zone of welded joints. To achieve the above-mentioned effects, the Nb content is preferably 0.0010% or more. On the other hand, if the Nb content exceeds 0.0500%, excessive precipitation hardening of Nb carbides and nitrides may occur, embrittle the pearlite structure itself, and reduce the fatigue damage resistance of the welded joints. Therefore, the Nb content is preferably 0.0010 to 0.0500%. The Nb content is preferably 0.0020% or more, 0.0025% or more, or 0.0030% or more. The Nb content is preferably 0.0400% or less, 0.0300% or less, or 0.0200% or less.

[0095] Ti is an element that increases the hardness of the pearlite structure through precipitation hardening by Ti carbides and Ti nitrides formed during the cooling process after hot rolling, and improves the fatigue damage resistance of the welded joint. Also, Ti is an effective component for preventing embrittlement of the welded joint by refining the structure of the heat-affected zone reheated to the austenite region, taking advantage of the fact that Ti carbides and Ti nitrides precipitated during post-weld reheating do not dissolve in the matrix. To obtain the above effects, it is preferable that the Ti content is 0.0030% or more. On the other hand, when the Ti content exceeds 0.0500%, coarse Ti carbides and Ti nitrides are formed, and due to stress concentration around these, fatigue cracks are likely to be generated, and the fatigue damage resistance of the welded joint may decrease. Therefore, it is desirable that the Ti content is 0.0020 to 0.0500%. The Ti content is preferably 0.0030% or more, 0.0035% or more, or 0.0040% or more. The Ti content is preferably 0.0400% or less, 0.0300% or less, or 0.0200% or less.

[0096] <f group> Mg is an element that combines with S to form fine sulfides (MgS), and this MgS finely disperses MnS, relieves stress concentration around MnS, and improves the fatigue damage resistance of the welded joint. To obtain the above effects, it is preferable that the Mg content is 0.0005% or more. On the other hand, when the Mg content exceeds 0.0200%, coarse oxides of Mg are formed, and due to stress concentration around these coarse oxides, fatigue cracks are likely to be generated, and the fatigue damage resistance of the welded joint may decrease. Therefore, it is desirable that the Mg content is 0.0005 to 0.0200%. The Mg content is preferably 0.0010% or more, 0.0015% or more, or 0.0030% or more. The Mg content is preferably 0.0180% or less, 0.0150% or less, or 0.0120% or less.

[0097] Ca has a strong bond with S, forming sulfides (CaS). This CaS finely disperses MnS, alleviating stress concentration around the MnS and improving the fatigue damage resistance of the welded joint. To achieve the above-mentioned effects, the Ca content is preferably 0.0005% or more. On the other hand, if the Ca content exceeds 0.0200%, coarse Ca oxides are formed, and stress concentration around these coarse oxides may increase the likelihood of fatigue crack initiation and reduce the fatigue damage resistance of the welded joint. For this reason, the Ca content is desirably 0.0005 to 0.0200%. The Ca content is preferably 0.0010% or more, 0.0020% or more, or 0.0030% or more. The Ca content is preferably 0.0180% or less, 0.0150% or less, or 0.0120% or less.

[0098] REM is a deoxidizing and desulfurizing element that generates REM oxysulfides (REM2O2S), which act as nuclei for the formation of Mn sulfide inclusions. Oxysulfides (REM2O2S) have a high melting point, which inhibits the elongation of Mn sulfide inclusions after rolling. As a result, REM finely disperses MnS, alleviating stress concentration around the MnS and improving the fatigue damage resistance of welded joints. To achieve the above effects, the REM content is preferably 0.0005% or more. On the other hand, if the REM content exceeds 0.0500%, coarse and hard REM oxysulfides (REM2O2S) are generated. The stress concentration around these oxysulfides increases the likelihood of fatigue crack initiation, potentially reducing the fatigue damage resistance of welded joints. Therefore, the REM content is preferably 0.0005 to 0.0500%. The REM content is preferably 0.0010% or more, 0.0020% or more, or 0.0030% or more.The REM content is preferably 0.0400% or less, 0.0300% or less, or 0.0250% or less.

[0099] REM is a total of 17 elements consisting of Sc, Y, and La (lanthanoids). The "content of REM" means the total value of the contents of all these REM elements. If the total content is within the above range, the same effect can be obtained whether the type of REM element is one or two or more.

[0100] <g group> N is an impurity element mixed in the steelmaking process. Even if degassing is actively carried out, about 0.0020% of N remains in the steel. In normal rail refining, the N content is about 0.0030 to 0.0060%. Also, N segregates at the austenite grain boundaries, promoting the pearlite transformation from the austenite grain boundaries, and mainly by refining the pearlite block size, it is an element effective in improving the toughness of the welded joint part. Also, when N and V are contained simultaneously, during the cooling process of the welded joint part after welding of the rail, it promotes the precipitation of V carbonitrides, increases the hardness of the pearlite structure, and improves the fatigue damage resistance of the welded joint part. To obtain the above effects, it is preferable that the amount of N is 0.0050% or more. On the other hand, when the amount of N exceeds 0.0200%, it becomes difficult to dissolve N in the steel, and bubbles that are the starting points of fatigue damage may be easily generated. Therefore, it is desirable that the N content is 0.0020 to 0.0200%. The N content is preferably 0.0030% or more, 0.0040% or more, or 0.0080% or more. The N content is preferably 0.0180% or less, 0.0150% or less, or 0.0120% or less.

[0101] <h group> Zr forms ZrO₂ inclusions with good lattice compatibility with γ-Fe, so it serves as the solidification nucleus of high-carbon rail steel where γ-Fe is the primary solidifying crystal, and by increasing the equiaxed crystal ratio of the solidification structure, it suppresses the formation of segregation bands in the center of the slab. To obtain the above effects, it is preferable that the Zr content is 0.0001% or more. On the other hand, when the Zr content exceeds 0.0200%, a large amount of coarse Zr-based inclusions are generated, and due to stress concentration around these coarse inclusions, fatigue cracks are likely to occur, and the fatigue damage resistance of the welded joint may decrease. Therefore, it is desirable that the Zr content is 0.0001 - 0.0200%. The Zr content is preferably 0.0010% or more, 0.0020% or more, or 0.0030% or more. The Zr content is preferably 0.0180% or less, 0.0150% or less, or 0.0120% or less.

[0102] Al is a component that functions as a deoxidizer. To obtain the above effects, it is preferable that the Al content is 0.0100% or more, and more preferably 0.500% or more. On the other hand, when the Al content exceeds 1.00% or 1.000%, it becomes difficult to dissolve Al in the steel, coarse alumina-based inclusions are generated, and fatigue cracks are likely to occur from these coarse inclusions, and the fatigue damage resistance of the welded joint may decrease. Furthermore, when the Al content exceeds 1.000%, oxides are generated during the welding of the rail, and the weldability of the rail may be significantly reduced. Therefore, it is desirable that the Al content is 0.0100 - 1.000%. The Al content is preferably 0.0200% or more, 0.0500% or more, or 0.1000% or more. The Al content is preferably 0.900% or less, 0.800% or less, or 0.700% or less. The chemical composition of the rail part is measured in accordance with JIS G 0321:2017 "Product Analysis Methods for Steel Materials and Their Allowable Variation Values".

[0103] (2) Reasons for Limiting the HAZ Width (W) of the Welded Joint Next, in this embodiment, the reason for limiting the HAZ width (W) of the welded joint to 60 mm or less will be explained.

[0104] As shown in Table 1, the results of the rail / wheel rolling test showed that as the HAZ width decreased, the amount of unevenness generated in the welded joint decreased, the number of rolling cycles until fracture increased, and the service life of the welded joint improved. Specifically, in the above-mentioned experiment, when the HAZ width exceeded 60 mm, the amount of unevenness generated in the welded joint increased, and the number of rolling cycles until fracture was less than 2 million, which did not meet the pass criteria. Furthermore, when the HAZ width was between 40 mm and 60 mm, the amount of unevenness generated in the welded joint decreased, and the number of rolling cycles until fracture exceeded 2 million, which met the pass criteria. Furthermore, when the HAZ width was between 20 mm and 40 mm, the amount of unevenness generated in the welded joint further decreased, and the number of rolling cycles until fracture was in the range of 3 to 4 million. Furthermore, when the HAZ width is between 10 mm and 20 mm, the unevenness generated in the welded joint is further reduced, and the welded joint does not fracture even after 4 million cycles.It was found that as the HAZ width decreases, the service life of the welded joint is further improved.

[0105] For this reason, the HAZ width of the welded joint is limited to 60 mm or less. The HAZ width of the welded joint may also be 55 mm or less, 50 mm or less, 40 mm or less, or 30 mm or less. The lower limit of the HAZ width is not particularly limited, but may be, for example, 5 mm or more, 10 mm or more, or 15 mm or more. To stably increase the number of rolling cycles until fracture, it is desirable to control the HAZ width within the range of 10 to 30 mm. The HAZ width is measured as follows. The hardness measurement target is a longitudinal cross section, i.e., a cross section parallel to the longitudinal and vertical directions of the welded rail 1 and passing through the center of the welded rail 1 in the width direction. In the longitudinal cross section, Vickers hardness measurements are performed continuously along the outer surface 1211 of the top of the welded joint 12 at a depth of 5 mm from the outer surface 1211 of the top of the welded joint 12. The Vickers hardness measurements are performed in accordance with JIS Z 2244:2009, "Vickers Hardness Test - Test Method." The test force, i.e., the force with which the indenter is pressed into the sample surface, is 10 kgf. The measurement interval is 1 mm. This results in a hardness distribution graph such as that shown in Figure 3. The hardness distribution graph of the welded rail 1 shows two Vickers hardness valleys. The locations where these Vickers hardness valleys occur are the softest areas. The distance between the two softest areas is considered to be the HAZ width W.

[0106] (3) Reason for limiting the total number of intersections (N) of pro-eutectoid cementite structures in the pro-eutectoid cementite structure evaluation region Next, the reason why the total number (N) of intersections of pro-eutectoid cementite structures intersecting with each of the orthogonal 100 μm line segments in the pro-eutectoid cementite structure evaluation region C set in the welded joint of the welded rail according to this embodiment is limited to 26 or less will be explained. Hereinafter, the total number of intersections of pro-eutectoid cementite structures in the pro-eutectoid cementite structure evaluation region C set in the welded joint may be simply referred to as "the total number of intersections of pro-eutectoid cementite structures in the welded joint."

[0107] As described above with reference to Figure 6, the total intersection number of the pro-eutectoid cementite structure is the total number of intersection points between the pro-eutectoid cementite structure and the cross lines arranged in the pro-eutectoid cementite evaluation region in a cross section that is parallel to the longitudinal and vertical directions of the welded rail and passes through the center of the welded rail in the width direction. The cross lines arranged in the pro-eutectoid cementite evaluation region are cross lines consisting of two 100 µm long line segments that are parallel to the longitudinal and vertical directions of the rail, as shown in Figure 6. In consideration of variations, two 100 µm long line segments that intersect at right angles were drawn at 20 locations in the pro-eutectoid cementite evaluation region, the total intersection number of the pro-eutectoid cementite structure was measured, and the average value of the total intersection number in each photograph was regarded as the total intersection number (N) of the pro-eutectoid cementite structure in the welded joint.

[0108] As shown in Fig. 8, if the total number of intersections of the pro-eutectoid cementite structure in the welded joint exceeds 26, fracture of the welded joint occurs in a drop weight test. For this reason, the total number of intersections of the pro-eutectoid cementite structure in the welded joint is limited to 26 or less. The reasons for selecting the pro-eutectoid cementite structure evaluation region C and the method for calculating the total number of intersections of the pro-eutectoid cementite structure are as described above. To stably suppress fracture of the welded joint, the total number of intersections of the pro-eutectoid cementite structure in the welded joint is desirably 24 or less, 23 or less, or 22 or less. The fewer the total number of intersections of the pro-eutectoid cementite structure in the welded joint, the better, so the lower limit is not particularly limited. The method for measuring the total number of intersections of the pro-eutectoid cementite structure in the welded joint is the same as the "method for evaluating the pro-eutectoid cementite structure" explained with reference to the graph shown in FIG.

[0109] (4) Reasons for limiting the preferable relationship between the HAZ width (W) of the welded joint and the total number of intersections of the pro-eutectoid cementite structure of the welded joint Next, the reason why it is preferable that the HAZ width (W) of the welded joint and the total number of intersections (N) of the pro-eutectoid cementite structure in the welded rail according to this embodiment satisfy formula 1 will be explained. N≦4.6×LN(W) 1 set

[0110] The present inventors further investigated the fracture resistance of welded joints. Under conditions where the total number of intersections (N) of pro-eutectoid cementite structures was kept constant, the correlation between the HAZ width of welded joints and fracture resistance was investigated under drop weight test conditions that replicated even more severe track conditions. As shown in Figure 9, when the total number of intersections (N) of pro-eutectoid cementite structures was the same, there was a correlation between the HAZ width of welded joints and their fracture resistance. As the HAZ width decreased, the drop weight energy required to cause fracture decreased. In other words, the present inventors found that the fracture resistance of welded joints decreased as the HAZ width decreased. The present inventors determined that this decrease in fracture resistance was due to a decrease in the softened portion of the welded joint as the HAZ width decreased, i.e., a decrease in macroscopic ductility.

[0111] Furthermore, the inventors investigated the fracture resistance of welded joints, which varies depending on the HAZ width. The correlation between the total number of intersections (N) of pro-eutectoid cementite structures and the fracture resistance of welded joints was investigated under drop weight test conditions. As a result, as shown in Figures 10A to 10C, it was confirmed that as the HAZ width decreases, the total number of intersections (N) of pro-eutectoid cementite structures that can prevent fracture under severe track conditions, i.e., the critical total number of intersections of pro-eutectoid cementite structures, decreases significantly.

[0112] The relationship between HAZ width and the total number of cementite intersections in the critical pro-eutectoid cementite structure for HAZ widths of 10 to 60 mm is summarized in Figure 11. It was confirmed that as the HAZ width decreases, the total number of intersections in the critical pro-eutectoid cementite structure, which can prevent breakage under severe track conditions, decreases significantly.

[0113] Furthermore, in order to reliably prevent breakage under severe track conditions, the inventors estimated the critical total number of intersections (N) of pro-eutectoid cementite structures that prevents breakage of welded joints for each HAZ width. As a result, it was confirmed that breakage of welded joints can be more reliably prevented by reliably controlling the total number of intersections (N) of pro-eutectoid cementite structures to a value calculated using the following formula (1) based on the HAZ width (W). Here, "LN" in formula (1) refers to the natural logarithm, i.e., the logarithm with Napier's constant e as the base. N≦4.6×LN(W) 1 set From these results, the inventors confirmed that in order to prevent breakage under severe track conditions, it is preferable to control the upper limit of the total number of intersections of the pro-eutectoid cementite structure in accordance with the HAZ width.

[0114] Next, a method for manufacturing a welded rail according to another aspect of the present invention will be described. The method for manufacturing a welded rail according to this embodiment makes it possible to preferably obtain a welded rail having excellent fatigue damage resistance and breakage resistance at the welded joint as described above. However, the welded joint of a welded rail that satisfies the above requirements will have excellent fatigue damage resistance and breakage resistance regardless of the manufacturing method. Therefore, the method for manufacturing a welded rail according to this embodiment is not particularly limited. The manufacturing method described below does not limit the scope of the welded rail according to this embodiment, but should be understood as a desirable example of the manufacturing method.

[0115] In order to obtain a welded rail with excellent fatigue damage resistance and breakage resistance at the welded joint, it is preferable to suppress both (1) breakage initiated by a fatigue crack that initiates at the bottom of the welded joint and (2) breakage initiated by a brittle crack that initiates at the head of the welded joint. Narrowing the HAZ width in the welded joint is effective in suppressing breakage initiated by a fatigue crack that initiates at the bottom of the welded joint. Furthermore, reducing the total number of cementite intersections in the pro-eutectoid cementite structure in the welded joint is effective in suppressing both breakage initiated by a fatigue crack that initiates at the bottom of the welded joint and breakage initiated by a brittle crack that initiates at the head of the welded joint.

[0116] As a result of further investigations, the inventors have found that it is possible to achieve both a reduction in the HAZ width and a reduction in the total number of cementite intersections in the pro-eutectoid cementite structure by strictly controlling the flash butt welding conditions and the cooling rate of the welded joint after welding is completed. They have also found that it is possible to more effectively reduce the total number of cementite intersections in the pro-eutectoid cementite structure by even more strictly controlling the cooling rate of the welded joint.

[0117] The method for manufacturing a welded rail according to this embodiment, which was obtained based on the above findings, includes a step of flash butt welding the rails to form a welded joint, and a step of heat treating the welded joint.

[0118] There are no particular limitations on the manufacturing method of the base rail that serves as the material for the rail to be subjected to flash butt welding, i.e., the welded rail. The HAZ width is controlled by the flash butt welding conditions described below. The state of cementite in the weld joint is controlled by the heat treatment conditions after flash butt welding. The metal structure of the base rail before welding is transformed into a different structure in the weld joint by the welding heat. Therefore, the metal structure of the base rail before flash butt welding does not affect the HAZ width or the state of cementite in the weld joint.

[0119] A preferred example of a method for manufacturing a base rail is as follows: Casting a bloom having the above-mentioned chemical composition; hot rolling the bloom at a rolling start temperature of 1000 to 1350°C and a rolling end temperature of 750 to 1100°C; cooling the rail at a cooling start temperature of 700 to 900°C, a cooling stop temperature of 500 to 650°C, and an average cooling rate between the cooling start temperature and the cooling stop temperature of 1 to 20°C / sec; When a welded rail is manufactured using the rail thus obtained as a base material, the wear resistance and breakage resistance of the rail portion are significantly improved.

[0120] When flash butt welding of rails is performed using the preheat flash method, which includes an initial flash process, a preheat process, a later flash process, and an upset process, Preheating times are 2 to 14 times. - Late flash time is 10 to 30 seconds, The average late flash velocity is 0.3 mm / sec or more. The late flash velocity just before upsetting (for 3 seconds) is 0.5 mm / sec or more, -Upset load must be 50kN or more. When flash butt welding rails is performed using the continuous flash method, which includes the flash process and the upset process, Flash time is 150-250 seconds. · Flash speed must be 0.10mm / sec or more. Under these conditions, the ends of a plurality of rails are flash butt welded to obtain a welded rail having a rail portion and a weld joint portion.

[0121] Heat treatment after flash butt welding involves: The average cooling rate of the outer surface of the top of the welded joint at the weld center A in the temperature range of 800 to 550 ° C is more than 1.5 to 3.5 ° C / sec, The average cooling rate CR1 in the temperature range of 800 to 550 ° C on the outer surface of the top of the welded joint at a location 0.6WX to 0.7WX away from the weld center A is 1.5 to 3.5 ° C / sec, The average cooling rate CR2 in the temperature range of 550 to 450 ° C on the outer surface of the top of the welded joint at a location 0.6WX to 0.7WX away from the weld center A is 0.2 to 1.5 ° C / sec, and ·CR2≧2.0-0.5×CR1 In this manner, the cooling is controlled to cool the welded joint of the welded rail. These manufacturing conditions will be described in detail below.

[0122] (5) Desirable flash butt welding conditions First, the desirable flash butt welding conditions for the manufacturing method of a welded rail according to this embodiment will be described. Rail flash butt welding can be performed using either the preheat flash method or the continuous flash method. Either method can be used in the manufacturing method of a welded rail according to this embodiment.

[0123] In the case of the preheat flash method, flash butt welding includes an initial flash step, a preheat step, a later flash step, and an upset step.

[0124] The initial flashing process begins when the material rails are at room temperature. To facilitate contact between the welding surfaces in the subsequent preheating process, a flash is generated between the end faces (i.e., the welding surfaces) of a pair of material rails in the initial flashing process, and the welding surfaces are adjusted perpendicular to the longitudinal direction of the rails. Furthermore, in the initial flashing process, the welding surfaces are heated by resistance heating and arc heating of the flashing. The time required for the initial flashing process, i.e., the initial flashing time, is preferably 10 seconds or more and 40 seconds or less.

[0125] In the preheating process, a large current is passed through the pair of material rails for a certain period of time while the opposing welding surfaces of the pair of material rails are forcibly brought into contact with each other, and the base material near the welding surfaces is heated by resistance heating. The pair of material rails are then separated. The contact and separation of the welding surfaces is repeated one or more times. The number of preheating cycles (contact and separation of the welding surfaces) is preferably two or more. The number of preheating cycles is more preferably four or more, and even more preferably ten or more. On the other hand, from the viewpoint of reducing the HAZ width, the number of preheating cycles is preferably 14 or less, 13 or less, or 12 or less.

[0126] In the latter flashing process, first, a flash is partially generated between the opposing welding surfaces, and the welding surfaces are heated by the resistance heating and arc heating of this flashing. Next, in the latter flashing process, the flash that was generated in a portion of the welding surfaces is generated over the entire welding surfaces by increasing the flashing velocity, and the entire welding surfaces are uniformly heated by the resistance heating and arc heating of this flashing. Furthermore, in the latter flashing process, oxides generated during the preheating process are scattered and reduced by the flashing. The flashing velocity is the speed at which the jigs holding the pair of material rails are brought closer to each other.

[0127] If the time required for the latter flashing process, i.e., the latter flashing time, is long, the HAZ width of the welded joint increases. Furthermore, if the flashing velocity in the latter flashing process, i.e., the latter flashing velocity, is increased, the heat distribution near the weld surface becomes steeper, resulting in a reduction in the HAZ width of the welded joint. Therefore, the latter flashing time is set to 10 seconds or more and 30 seconds or less. Furthermore, it is desirable to set the average latter flashing velocity to 0.3 mm / sec or more or 0.4 mm / sec or more, and the latter flashing velocity immediately before upsetting (for 3 seconds) to 0.5 mm / sec or more. Here, the average latter flashing velocity refers to the average value of the flashing velocity throughout the entire latter flashing process, and the latter flashing velocity immediately before upsetting refers to the average value of the flashing velocity for 3 seconds before the start of upsetting. To reliably reduce the HAZ width of the welded joint, it is desirable to set the latter flash-off distance, i.e., the amount of erosion of the base rail in the latter flashing process, to 10 mm or more.

[0128] In the upset process, after the entire weld surface is melted in the latter flash process, the weld surfaces are rapidly brought into contact with each other using a large applied pressure. This expels most of the molten metal from the weld surface, while applying pressure and deformation to the highly heated area behind the weld surface, thereby forming a joint. In other words, oxides generated during welding are expelled and refined during the upset process, reducing the likelihood of them remaining on the weld surface as defects that impair bending performance. Furthermore, expelling most of the molten metal contributes to reducing the HAZ width of the welded joint. To ensure a reduction in the HAZ width of the welded joint, an upset load of 50 kN or more is desirable. An upset load of 65 kN or more is even more desirable.

[0129] In the case of the continuous flashing method, flash butt welding does not include a preheating process and consists of a flashing process and an upset process. In the flashing process, a long flashing time increases the HAZ width of the welded joint. Furthermore, increasing the flashing velocity sharpens the heat distribution near the weld surface, resulting in a reduction in the HAZ width of the welded joint. Therefore, the flashing time should be between 150 seconds and 250 seconds. Furthermore, the flashing velocity should be 0.10 mm / sec or greater. The upset process in the case of the continuous flashing method can be carried out under the same conditions as the upset process in the preheating flashing method described above. To reliably reduce the HAZ width of the welded joint, it is desirable to preheat using pulse flashing or the like before the flashing process, reduce the flashing time, and increase the flashing velocity.

[0130] (6) Desirable cooling conditions after flash butt welding Next, desirable cooling conditions after flash butt welding will be described. Whether flash butt welding is performed by the preheating flash method or the continuous flash method, the cooling conditions after flash butt welding can be controlled in the same way.

[0131] The welded joint is heated to the austenite region during flash butt welding. Therefore, without proper cooling, the head of the welded joint loses hardness. Furthermore, pro-eutectoid cementite, which can initiate fracture, forms at the head of the welded joint. This requires independent temperature control at locations near the weld center A and away from the weld center A. Figure 12 shows a schematic diagram of the temperature distribution in the welded joint after flash butt welding. The solid line in Figure 12 represents the heat distribution immediately after welding under welding conditions that result in a welded joint with a large HAZ width, while the dashed line represents the heat distribution immediately after welding under welding conditions that result in a welded joint with a small HAZ width. During flash butt welding, the weld center A is intensely heated, while resistance heating is minimal at locations away from the weld center A. The temperature rise at locations away from the weld center A is due to heat transfer from the weld center A. Therefore, after flash butt welding is completed, a steep temperature gradient occurs in the welded joint as shown in Figure 12. Furthermore, in order to narrow the HAZ width, welding must be performed under conditions (corresponding to the dashed line graph) that produce a steeper temperature gradient than normal welding conditions (corresponding to the solid line graph). For this reason, when heat treating the welded joint, it is necessary to take into consideration the distance between the location where temperature control is performed and the weld center A. In the manufacturing method of a welded rail according to this embodiment, the cooling rate at the weld center A, and Cooling rate at a point 0.6WX to 0.7WX away from the weld center A These are controlled independently. For example, as shown in Fig. 15A, this type of heat treatment is possible by optimizing the spacing between the cooling gas outlets of the cooling device. The temperature at each point where the cooling conditions are controlled may be measured, and the injection position of the coolant may be optimized. On the other hand, in normal cooling, it is estimated that the cooling rate at the welding center A differs from the cooling rate at points away from the welding center A due to the heat distribution as shown in Fig. 12.

[0132] First, it is desirable to cool the outer surface 1211 of the apex of the welded joint 12 at the weld center A at an average cooling rate in the temperature range of 800 to 550°C within a range of more than 1.5 to 3.5°C / sec. The average cooling rate in the temperature range of 800 to 550°C is calculated by dividing 250°C (i.e., the difference between 800°C and 550°C) by the time required to reduce the temperature of that location from 800°C to 550°C. By setting the average cooling rate for that location in this temperature range to more than 1.5°C / sec, the hardness of the welded joint can be ensured and the wear resistance of the apex of the welded joint can be improved. Furthermore, if the average cooling rate for that location in this temperature range exceeds 3.5°C / sec, the hardness of the welded joint becomes excessive, reducing the rolling contact fatigue damage resistance of the apex of the welded joint.

[0133] The above temperature is preferably controlled by measuring the outer surface of the top of the welded joint with a radiation thermometer after welding. The cooling rate can be controlled by adjusting the temperature and elapsed time based on the above temperature measurement.

[0134] Furthermore, the average cooling rate CR1 of the outer surface 1211 of the top of the welded joint at a position 0.6WX to 0.7WX away from the weld center A in the temperature range of 800 to 550°C is set to a range of more than 1.5 to 3.5°C / sec. The average cooling rate CR1 in the temperature range of 800 to 550°C is the value obtained by dividing 250°C (i.e., the difference between 800°C and 550°C) by the time required to reduce the temperature of that location from 800°C to 550°C. If the average cooling rate CR1 of that location in this temperature range is 1.5°C / sec or less, the pro-eutectoid cementite structure in the pro-eutectoid cementite structure evaluation region C will increase, the total number of cementite intersections (N) will exceed 26, and it will be difficult to ensure the minimum breakage resistance required for the welded joint of a welded rail. Furthermore, if the average cooling rate CR1 at that position in this temperature range exceeds 3.5°C / sec, excessive recuperation after cooling occurs, making it difficult to control the average cooling rate CR2 in a temperature range below 550°C. As a result, the pro-eutectoid cementite structure increases due to the temperature rise, and the total number of cementite intersections (N) in the pro-eutectoid cementite structure exceeds 26.

[0135] Furthermore, the average cooling rate CR2 at the outer surface of the head portion of the welded joint at a position 0.6WX to 0.7WX away from the weld center A in the temperature range of 550 to 450°C is set to 0.2 to 1.5°C / sec. The average cooling rate CR2 at that position in the temperature range of 550 to 450°C is the value obtained by dividing 100°C (i.e., the difference between 550°C and 450°C) by the time required to reduce the temperature from 550°C to 450°C. If the average cooling rate CR2 at that position in this temperature range is 0.2°C / sec or less, the pro-eutectoid cementite structure at the head portion in the position 0.6WX to 0.7WX increases, the total number of cementite intersections (N) of the pro-eutectoid cementite structure exceeds 26, and it becomes difficult to ensure the minimum breakage resistance required for the welded joint of a welded rail. On the other hand, even if the average cooling rate CR2 of the relevant portion in this temperature range exceeds 1.5°C / sec, there is no significant change in the total intersection number (N) of the pro-eutectoid cementite structure, and the effect saturates. For this reason, the preferable upper limit of the average cooling rate CR2 was set to 1.5°C / sec.

[0136] The cooling rate control for the outer surface of the apex at the weld center (A) is performed within a range of 800 to 550°C, while the cooling rate control for the outer surface of the apex at the 0.6WX to 0.7WX positions is performed within a range of 800 to 450°C. This difference in temperature range is due to the difference in the purpose of the cooling rate control. The purpose of the cooling rate control for the outer surface of the apex at the weld center (A) is to sufficiently induce pearlite transformation to maintain hardness. On the other hand, the purpose of the cooling rate control for the outer surface of the apex at the 0.6WX to 0.7WX positions is to suppress the formation of pro-eutectoid cementite structures.

[0137] Furthermore, in order to control the total cementite intersection number (N) and HAZ width (W) of the pro-eutectoid cementite structure to satisfy the relational expression N≦4.6×LN(W) and further improve the fracture resistance of the welded joint, it is desirable to control the average cooling rate CR1 (800 to 550°C) and the average cooling rate CR2 (550 to 450°C) of the outer surface 1211 of the top portion at the position 0.6WX to 0.7WX to satisfy the relational expression CR2 ≧ 2.0 − 0.5 × CR1. This is because, by controlling the average cooling rate (CR2) in the low temperature range, which is important for controlling the formation of the pro-eutectoid cementite structure after pearlite transformation, pearlite transformation is sufficiently promoted and the formation of the pro-eutectoid cementite structure is further suppressed. Therefore, in order to fundamentally prevent fracture of a welded joint, it is desirable to control the total number of cementite intersections (N) in the pro-eutectoid cementite structure and the HAZ width (W) so that they satisfy the relationship N≦4.6×LN(W). In addition to controlling the average cooling rate CR1 in the high-temperature region immediately after welding and the average cooling rate CR2 in the subsequent low-temperature region, it is also desirable to control the average cooling rate CR1 immediately after welding (800 to 550°C) and the average cooling rate CR2 thereafter (550 to 450°C) to be in the range of CR2≧2.0-0.5×CR1. When independently controlling the cooling rate at the weld center A and the cooling rate at a location 0.6WX to 0.7WX away from the weld center A, it is necessary to consider the heat distribution at the weld center and its surrounding area after welding is completed. Figure 13 shows a schematic diagram of the change over time in heat distribution at the weld center and its surrounding area when the weld joint is subjected to accelerated cooling. The meanings of the four heat distribution curves shown in Figure 13 are as follows. (Curve 1) Heat distribution in the welded joint immediately after welding is completed (Curve 2) Heat distribution in the welded joint at the start of accelerated cooling X seconds after welding completion (Curve 3) Heat distribution in the welded joint Y seconds after completion of welding when accelerated cooling is performed using the cooling device shown in Figure 15C X seconds after completion of welding. (Curve 4) Heat distribution in the welded joint Y seconds after completion of welding when accelerated cooling is performed using the cooling device in Figure 15A X seconds after completion of welding. According to the temperature distribution immediately after welding is completed, shown in curve 1, the temperature at the weld center is close to the melting point of the steel. However, because heat transfer from the weld joint to the base material occurs constantly during and after welding, the temperature decreases the further away from the weld center. As shown in curve 1 in Figure 13, immediately after welding, the temperature at a location 0.6WX to 0.7WX away from the weld center A is significantly lower than that at the weld center A. According to the temperature distribution at the start of accelerated cooling (X seconds after welding), shown by curve 2, the temperature of the welded joint is lower than the temperature immediately after welding is completed. However, the amount of temperature drop is not uniform in the welded joint. The amount of temperature drop at the weld center A is greater than the amount of temperature drop at a location 0.6WX to 0.7WX away from the weld center A. According to the temperature distribution after accelerated cooling using the cooling device of Figure 15C, shown by curve 3, the cooling rate at the weld center is greater than the cooling rate at a location 0.6WX to 0.7WX away from the weld center A. In the cooling device of Figure 15C, multiple cooling gas discharge ports 61 are uniformly arranged. Therefore, with the cooling device of Figure 15C, the cooling gas is sprayed uniformly along the weld joint, but the cooling rate of the weld joint is not uniform. According to the temperature distribution after accelerated cooling using the cooling device of FIG. 15A, shown by curve 4, the temperature at the weld center is the same as that of curve 3, but the temperature at the location 0.6WX to 0.7WX away from the weld center A is below curve 3. The cooling rate at the weld center is approximately the same as the cooling rate at the location 0.6WX to 0.7WX away from the weld center A. In the cooling device of FIG. 15A, the spacing between the multiple cooling gas outlet ports 61 is wide in the center and narrow at the ends. Therefore, with the cooling device of FIG. 15A, the amount of cooling gas injected is particularly large at the location 0.6WX to 0.7WX away from the weld center A. To mitigate the effect of the temperature difference caused by welding, it is necessary to increase the amount of cooling gas injected at the location 0.6WX to 0.7WX away from the weld center A. By comparing curves 2 to 4 in FIG. 13, it is possible to understand the effect that the temperature difference between the weld center A and its peripheral area immediately after the completion of welding has on the cooling rate.

[0138] The means for independently controlling the cooling rate at the welding center A and the cooling rate at a location 0.6WX to 0.7WX away from the welding center A is not particularly limited, but as described above, it is preferable to use multiple cylindrical cooling devices 6 as shown in Figures 14A and 14B. As illustrated in FIG. 15A and other figures, the cooling device 6 is provided with multiple cooling gas outlets 61. The cooling device 6 is connected to a compressor via a cooling gas supply pipe (not shown). The cooling device 6 is arranged around the welded joint so that the cooling gas outlets 61 face the top outer surface 1211 of the welded joint, the rail top corner outer surface 1114, and the head side outer surface 1213. The cooling device 6 is arranged so that its longitudinal direction coincides with the longitudinal direction of the welded rail. Additionally, the longitudinal centers of the multiple cylindrical cooling devices 6 are aligned with the weld center A. The cooling device 6 can cool the weld center A and HAZ by spraying cooling gas g onto the welded joint. The cooling gas g is, for example, air. The cooling rate can be controlled by the arrangement and number of cooling gas discharge ports 61. As shown in Fig. 15A, it is most preferable that the cooling gas discharge ports 61 are arranged at wide intervals in the longitudinal center and at narrow intervals near the longitudinal ends (cementite control positions). This makes it possible to increase the cooling capacity at locations 0.6WX to 0.7WX away from the weld center A compared to the cooling capacity at the weld center A. In the cooling device 6 shown in Fig. 15C, cooling gas discharge ports 61 are provided at equal intervals along the longitudinal direction. With this type of cooling device 6, the amount of cooling gas discharged can be made uniform. However, as described above with reference to Fig. 13, if the amount of cooling gas discharged is made uniform, the cooling rate of the welded joint will not be made uniform. On the other hand, it is also undesirable that the spacing between the cooling gas outlets 61 is too wide at the center in the longitudinal direction. For example, in the cooling device 6 shown in Fig. 15B, the spacing between the cooling gas outlets 61 at the center in the longitudinal direction is wider than that of the cooling device shown in Fig. 15A. With the cooling device 6 shown in Fig. 15B, the cooling rate at the center of the weld may be insufficient. It is also undesirable for the spacing between the cooling gas discharge ports 61 to be too narrow near the longitudinal ends. For example, in the cooling device 6 shown in Fig. 15D, the spacing between the cooling gas discharge ports 61 near the longitudinal ends is narrower than in the cooling device shown in Fig. 15A. With the cooling device 6 shown in Fig. 15D, the cooling rate at a location 0.6WX to 0.7WX away from the weld center A may be excessive. It is desirable to optimize the size and spacing of the cooling gas discharge ports 61 depending on various conditions such as the flow rate of the cooling gas g and the shape of the welded rail. In order to keep the relationship between CR1 and CR2 within the above range, it is preferable to appropriately control the flow rate of the cooling gas. The arrangement of the cooling gas discharge ports 61 in the cooling device 6 needs to be determined according to the HAZ width W of the welded joint for which the cooling device 6 is used. For example, it is preferable that the distance between the location where the cooling gas discharge ports 61 are sparsely arranged and the location where they are densely arranged is approximately 0.6WX to 0.7WX. When such a cooling device 6 is arranged in a welded joint, the location where the cooling gas discharge ports 61 are sparsely arranged faces the weld center A, and the location where the cooling gas discharge ports 61 are densely arranged faces a location 0.6WX to 0.7WX away from the weld center A. Immediately after welding is completed, when the welded joint is still hot, the softest part of the welded joint has not yet formed. However, in welded rails of the same shape and components that are welded using the same flash butt welding conditions, the distance between the weld center and the softest part is essentially the same. Furthermore, the cooling conditions after welding do not substantially affect the location of the softest part. Therefore, the location of the softest part can be easily estimated before cooling begins. The placement of the cooling gas outlet 61 of the cooling device 6 can be determined based on the estimated location of the softest part. Other specific configurations of the cooling device 6 are not particularly limited. For example, the size of the cooling device 6 along its longitudinal direction is not particularly limited, but is preferably within a range of 2.0 to 3.0 times the HAZ width. Such a cooling device 6 can ensure the cooling efficiency of the entire welded joint. There are also no particular limitations on the diameter of the cooling gas outlet 61 of the cooling device 6 and the flow rate of the cooling gas. These configurations can be changed as appropriate depending on the welding target, etc.

[0139] (7) Desirable metal structure of welded joints

[0140] Next, a description will be given of a desirable metal structure of the welded joint in this embodiment. The metal structure of the welded joint is not particularly limited as long as the above-mentioned requirements are met, but by having the configuration described below, the fatigue damage resistance and breakage resistance of the welded joint of the welded rail are further improved.

[0141] Ensuring wear resistance is paramount at the head of the welded joint, which comes into contact with the wheel. Research into the relationship between metal structure and wear resistance confirmed that a pearlite structure is the best for ensuring wear resistance at the head of the welded joint. Therefore, it is desirable for the head of the welded joint (the region from the top surface to a depth of 1 / 3h) to be primarily made of pearlite. However, for other areas, a metal structure other than pearlite may be used as long as it can ensure the strength, ductility, and toughness required for welded rails. [Example]

[0142] The effects of one embodiment of the present invention will be explained in more detail using examples. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0143] Various rails having the chemical compositions shown in Table 2 were used as the raw materials for welded rails. The remainder of the chemical compositions shown in Table 2 was iron and impurities. The contents of elements that were not intentionally added are indicated by "-" in Table 2.

[0144] These rails were flash butt welded and then heat treated to produce various welded rails. Rolling fatigue tests and drop weight tests were then conducted on the welded joints of the welded rails. The heat treatment conditions were as shown in Table 4. For reference, values outside the preferred ranges in Table 4 are underlined. The HAZ width (W) and the total number of pro-eutectoid cementite intersections (N) in the welded joints were as shown in Table 3. Values outside the ranges of the invention are underlined in Table 3. The results of the rolling fatigue tests and drop weight tests on the welded joints were as shown in Table 5. The methods for evaluating the pro-eutectoid cementite structure and measuring the HAZ width were as described above. As is clear from the above measurement method, the total number of pro-eutectoid cementite intersections is an integer greater than or equal to 0. However, 4.6 × LN(W) is not an integer. When comparing these values, the value of 4.6 × LN(W) should not be rounded to the nearest whole number. For example, if the total number of pro-eutectoid cementite intersections is 10 and 4.6 × LN(W) is 9.7, it is determined that the relationship N≦4.6 × LN(W) is not satisfied. Other experimental conditions were as follows:

[0145] Rails as welding base material Rail profile: 136 lbs (weight: 67 kg / m) Hardness: 420 HV (top surface)

[0146] ●Flash butt welding conditions (preheating flash method) Flash butt welding was carried out under the following welding conditions: Initial flash time: 15 seconds Preheating times: 2 to 14 times Late flash time: 15-30 seconds Average late flash velocity: 0.3-1.0mm / sec Late flash velocity just before upsetting (3 seconds): 0.5 to 3.0 mm / sec Upsetting load: 65~85KN However, in Comparative Example 28, flash butt welding was performed under the following welding conditions. Preheating times: 16 Average late flash velocity: 0.2mm / sec Late flash velocity just before upsetting (3 seconds): 0.3 mm / sec Other welding conditions: same as above In Comparative Example 35, flash butt welding was performed under the following welding conditions. Average late flash velocity: 0.1mm / sec Late flash velocity just before upsetting (3 seconds): 0.2 mm / sec Other welding conditions: same as above Cooling conditions The cooling rate at the weld center A and the cooling rate at a location 0.6WX to 0.7WX away from the weld center A were controlled independently. The cooling rates were as shown in Table 4. The cooling means was a cooling device 6 having a configuration as shown in Figs. 14A and 14B. 5, the cooling rate at a distance of 0.6WX to 0.7WX from the weld center A tends to be smaller than the cooling rate at the weld center. Taking this tendency into consideration, the arrangement and spacing of the cooling gas outlet ports in the cooling device were determined. For example, in Example 1, a cooling device was used in which the spacing between cooling gas outlets was wide in the longitudinal center and narrow at both longitudinal ends, as shown schematically in FIG. 15A. In Comparative Examples 28 and 35, as shown schematically in Fig. 15B, a cooling device was used in which the spacing between cooling gas outlet ports in the longitudinal center was wider than that in Fig. 15A. Therefore, in Comparative Examples 28 and 35, the cooling rate at the weld center A was insufficient. In Comparative Examples 31 and 38, a cooling device with uniform spacing between cooling gas discharge ports was used, as shown schematically in Fig. 15C. Therefore, in Comparative Examples 31 and 38, the cooling rate was insufficient at a location 0.6WX to 0.7WX away from the weld center A. In Comparative Example 32, as shown schematically in Fig. 15D, a cooling device was used in which the spacing between the cooling gas discharge ports at both longitudinal ends was narrower than that in Fig. 15A. Therefore, in Comparative Example 32, the cooling rate at the location 0.6WX to 0.7WX away from the weld center A was excessive.

[0147] ●Characteristics of welded joints Hardness of weld center: 390~440 HV Hardness of softest part: 280 HV

[0148] Conditions for rolling fatigue testing of rails and wheels Testing machine: Rolling fatigue testing machine (see Figure 4) Shape of the welded rail used as the test specimen: 2m long (with a welded joint in the center of the length) Wheels: AAR type (diameter 920mm) Radial load: 300KN Thrust load: 50KN Bottom stress: 400 MPa (measured using a strain gauge at the beginning of the test) Lubrication: Repeated water-dry lubrication (i.e., spraying water on the welded rail for a certain period of time, then stopping the water supply and allowing the water to dry, repeating the cycle) Number of repeated load applications using wheels: Up to 4 million times Cumulative transit tonnage: up to 120 million tons Evaluation criteria for rail / wheel rolling fatigue tests The number of repeated load applications until fracture is less than 2 million: X (Fail) The number of repeated load applications until fracture is 2 million to less than 3 million times: C (pass) The number of repeated load applications until breakage is between 3 million and 4 million times: B (pass) No breakage even after 4 million repeated load applications: A (pass)

[0149] Drop weight test conditions (see Figure 7) Position: The welded rail is supported at two points with the head on the bottom and the bottom on the top, and a drop weight is dropped onto the bottom of the welded joint. Span (distance between two support points): 1000mm Falling weight: 1000kgf (9.8kN) Drop height (X): 3.0m and 9.0m Falling weight energy: 29.4 kN m and 88.2 kN m (breakage prevention standard energy) Drop weight test evaluation criteria Breakage due to falling weight energy of 29.4 kN m: X Breakage with a falling weight energy of 88.2 kN m: B No breakage with a falling weight energy of 88.2 kN m: A

[0150] [Table 2]

[0151] [Table 3]

[0152] [Table 4]

[0153] [Table 5]

[0154] In Comparative Examples 28 and 35, the welded rails had excessive HAZ widths due to inappropriate flash butt welding conditions. In Comparative Examples 28 and 35, the fatigue damage resistance of the welded joints was insufficient, and the rolling fatigue test results were unacceptable.

[0155] In Comparative Examples 31 and 38, the CR1 was too small, resulting in an excessive total intersection number of the pro-eutectoid cementite structure. In Comparative Examples 31 and 38, the welded joints lacked breakage resistance, resulting in failing the drop weight test. In Comparative Example 32, the CR1 was too large, resulting in excessive recuperation after cooling, making it difficult to control the average cooling rate CR2 in the temperature range below 550°C. As a result, the pro-eutectoid cementite structure increased with the temperature rise, and the total number of cementite intersections (N) of the pro-eutectoid cementite structure exceeded 26. In Comparative Example 32, the welded joints lacked breakage resistance, resulting in failing the drop weight test.

[0156] On the other hand, welded rail weld joints whose chemical composition, HAZ width, and total number of intersections of pro-eutectoid cementite structures were within the ranges of the present invention were excellent in fatigue damage resistance and breakage resistance, and showed good results in both rolling fatigue tests and drop weight tests. Furthermore, welded rail weld joints whose welded rails satisfied the relationship N≦4.6×LN(W) showed even better test results. [Explanation of symbols]

[0157] 1. Flash butt welded rail (welded rail) 11 Rail section 111 Rail head 1111 Rail head outer surface 1112 Rail jaw bottom 1113 Rail head side outer surface 1114 Rail top corner outer surface 1114 112 Rail column 113 Rail bottom 12 Welded joint 121 (Welded joint) head 1211 (Welded joint) outer surface of the top 1212 (Welded joint) underside of jaw 1213 (Welded joint) Head side outer surface 1214 (Welded joint) Top corner outer surface 122 (Welded joint) column 123 (Bottom of welded joint) 12H Heat Affected Zone (HAZ) A Welding center 2 sleepers 3 wheels 4 motors 5 Load stabilization device 6 Cooling device 61 Cooling gas outlet g Cooling gas

Claims

1. a plurality of rail portions; a welded joint portion that joins the rail portion; A welded rail comprising: The rail portion has, as a chemical composition, in mass %, C: 0.85-1.20%, Si: 0.10-2.00%, Mn: 0.10-2.00%, Cr: 0.10-1.50%, P≦0.0250%, S≦0.0250%, Mo: 0 to 0.50%, Co: 0-1.00%, B: 0 to 0.0050%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, V: 0-0.20%, Nb: 0 to 0.0500%, Ti: 0 to 0.0500%, Mg: 0 to 0.0200%, Ca: 0-0.0200%, REM: 0-0.0500%, N: 0 to 0.0200%, Zr: 0 to 0.0200%, and Al: 0-1.000% and the balance being Fe and impurities, a HAZ width (W) that is the distance between two most-softened zones formed on both sides of a weld center of the welded joint, measured along the longitudinal direction of the welded rail in a cross section that is parallel to the longitudinal direction and up-down direction of the welded rail and passes through the center in the width direction of the welded rail, is 60 mm or less; In the cross section, the distance between the softest part and the weld center measured along the longitudinal direction is defined as WX, and a region having a distance of 0.6WX to 0.7WX from the weld center and a depth of 2 to 5 mm from the outer surface of the top portion is defined as a pro-eutectoid cementite structure evaluation region. In the pro-eutectoid cementite structure evaluation region, the total number of intersections (N) of network-like cementite intersecting with a cross line consisting of two line segments having a length of 100 μm parallel to the longitudinal direction and the up-down direction is 26 or less. A welded rail characterized by:

2. The welded rail according to claim 1, characterized in that the HAZ width (W) of the welded joint and the total number of intersections (N) of the network-like cementite satisfy the following formula: N≦4.6×LN(W) 1 set Here, "LN" in the above formula stands for natural logarithm.

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