Method for manufacturing flash butt welding rail

Abstract

A method for manufacturing a flash butt welding rail according to one embodiment of the present invention is a method for manufacturing a flash butt welding rail having a plurality of rail parts and a welded joint part for joining the plurality of rail parts, the method comprising a step for flash-butt-welding a rail, and a step for forcibly cooling the welded joint part immediately after completion of the flash butt welding. In the forced cooling from the austenite temperature range, the cooling stop temperature (TC) of the head-top-corner-side outer shell surface at the welding center of the welded joint part is set to 560-850°C, and the cooling stop temperature (TA) of the head jaw outer shell surface is set to 650-950°C.

Description

Manufacturing method for flash butt welded rails

[0001] The present invention relates to a method for manufacturing flash-butt welded rails. This application claims priority to Japanese Patent Application No. 2023-118424, filed on July 20, 2023, the contents of which are incorporated herein by reference.

[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 up to near its 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, austenitization and pearlite transformation of the metal structure occur as a result of heating to the A1 point or higher during welding, and partial austenitization of the metal structure and decomposition of the pearlite structure occur as a result of heating to the vicinity of the A1 point. In the part that has been austenitized during flash butt welding, there has been a problem in that forced cooling, such as air cooling, performed immediately after welding to ensure the hardness of the welded part generates a martensite structure that is harmful to toughness.

[0005] Martensite structures tend to be the starting point of fractures, and when they form, rails are prone to breakage. Martensite structures tend to form in the heads of welded joints of rails, where the cooling rate is relatively fast. The heads are areas that require wear resistance and need to be cooled faster than other areas to prevent softening. However, the heads are prone to martensite formation due to supercooling.

[0006] Regarding cooling of welded joints obtained by flash butt welding of rails, the following techniques have been proposed to improve the fatigue strength of the welded joints by controlling residual stress.

[0007] Patent Document 1 describes a cooling method for rail welds, characterized in that the rail column, base, and head are cooled within a temperature range until the transformation from austenite to pearlite is completed. The purpose of the technology described in Patent Document 1 is to reduce residual stress in rail welds and improve the fatigue resistance of rail welded joints.

[0008] Furthermore, in the welded joint portion of a flash-butt welded rail, the following techniques have been proposed to control the welding conditions and the cooling conditions of the head portion and to improve the life of the welded joint.

[0009] Patent Document 2 describes a welding method and a cooling method characterized by performing flash butt welding with an upset amount of 20 mm or more, starting cooling of the rail head within 70 seconds after welding is completed, and completing cooling 25 to 60 seconds after the start of cooling.

[0010] Furthermore, in rails with a high carbon content, increasing the carbon content promotes the formation of pro-eutectoid cementite structures with low toughness in the welded joints of welded rails, which increases the possibility of rail breakage, etc. Therefore, the following techniques have been proposed to improve the toughness of welded joints.

[0011] 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 bottom 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, and the accelerated cooling is stopped when the temperature of one or both of the head and bottom 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.

[0012] International Publication No. 2010 / 116680 Japanese Patent Application Laid-Open No. 2010-100937 Japanese Patent Application Laid-Open No. 2004-043862

[0013] However, the purpose of the technology of Patent Document 1 is not to control the formation of martensite structures in the jaws of the head, which promote rail breakage. Therefore, this technology does not have the effect of improving the breakage resistance of rails. Furthermore, the technology of Patent Document 1 does not control the cooling rate in the jaws of the head, where martensite structures harmful to toughness form. This technology controls the cooling rate at a position 20 mm away from the weld center in the rail axial direction. This position is outside the HAZ (within 20 mm) where martensite typically forms. Therefore, the temperature control by this technology does not control the formation of martensite structures in the weld joint.

[0014] The technology described in Patent Document 2 aims to reduce defects in rail welds, ensure the hardness of the welded joints, and improve the service life of rail welded joints. However, the technology described in Patent Document 2 does not aim to control the formation of martensite structures in the jaws of the heads, which promote rail breakage. Therefore, this technology was not effective in improving the breakage resistance of rails.

[0015] The technology described in Patent Document 3 aims to suppress the formation of pro-eutectoid cementite structures that reduce the toughness of rail welded joints and improve the breakage resistance of rail welded joints. However, the technology described in Patent Document 3 does not control the formation of martensite structures that promote rail breakage, and does not have the effect of fundamentally improving the breakage resistance of rails. Furthermore, the technology described in Patent Document 3 mainly controls the cooling rate of the rail head and rail bottom, and does not control the formation of structures in the head jaws where martensite structures that are harmful to toughness form.

[0016] The present invention was devised in view of the above-mentioned problems, and aims to improve the breakage resistance of the welded joints of flash-butt welded rails. Preferably, it is an object of the present invention to provide a manufacturing method that can satisfy the extremely strict breakage resistance requirements of the welded joints of flash-butt welded rails used in freight railways, which are subject to harsh track environments.

[0017] The gist of the present invention is as follows: (1) A method for manufacturing a flash-butt welded rail according to one aspect of the present invention is a method for manufacturing a flash-butt welded rail having a plurality of rail portions and a weld joint portion joining the plurality of rail portions, and the method comprises the following chemical components, in mass %, C: 0.70 to 1.20%, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, P≦0.0300%, S≦0.0300%, Cr: 0 to 2.00%, Mo: 0 to 0.50%, Co: 0 to 1.00%, B: 0 to 0.0050%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, V: 0 to 0.200%, Nb: 0 to 0.0500%, Ti: 0 to 0.0500%, Mg and a step of forcibly cooling the welded joint immediately after completion of the flash butt welding, wherein the cooling stop temperature (TC) of the outer surface of the head corner side at the weld center of the welded joint from the austenite temperature range is set to be 560°C or higher and 850°C or lower, and the cooling stop temperature (TA) of the outer surface of the head jaw portion is set to be 650°C or higher and 950°C or lower. (2) Preferably, in the manufacturing method of a flash-butt welded rail described in (1) above, the cooling stop temperature (TC) of the outer surface of the head corner side and the cooling stop temperature (TA) of the outer surface of the head jaw portion satisfy the following formulas 1 and 2.TA+TC≧1340 1 0≦TA−TC≦140 2 (3) Preferably, in the method for manufacturing a flash butt welded rail described in (1) or (2) above, the rail has the following chemical compositions, in mass %, as follows: Group a: Cr: 0.05% or more and 2.00% or less, Mo: 0.01% or more and 0.50% or less, Group b: Co: 0.01% or more and 1.00% or less, Group c: B: 0.0001% or more and 0.0050% or less, Group d: Cu: 0.01% or more and 1.00% or less, and Ni: 0.01% or more and 1.00% or less, Group e: V: 0.005% or more and 0.200% or less, (4) Preferably, in the method for manufacturing a flash-butt welded rail according to any one of (1) to (3), the forced cooling is air cooling.

[0018] According to the above-described aspects of the present invention, in a flash-butt welded rail, it is possible to improve the breakage resistance of the welded joint and significantly improve the service life.

[0019] Side view of a welded rail Cross section perpendicular to the longitudinal direction of a welded rail Perspective view illustrating the position of the longitudinal cross section of the welded joint of a welded rail Schematic diagram of the cross-sectional hardness distribution 5 mm below the outer surface of the head jaw in the longitudinal cross section of the welded joint of a welded rail Schematic diagram of the martensite structure evaluation area in the longitudinal cross section of the welded joint of a welded rail Schematic diagram of a drop weight test Relationship between the number of martensite structures formed in the head jaw and rail breakage (when the drop weight height is 5.0 m and the drop weight energy is 49.0 kN m) Relationship between the number of martensite structures formed in the head jaw and rail breakage (when the drop weight height is 7.0 m and the drop weight energy is 73.5 kN m) Relationship between the cooling stop temperature (TC) on the outer surface of the corner side of the head top and the number of martensite structures formed in the head jaw Relationship between the cooling stop temperature (TA) of the outer surface of the head jaw and the number of martensite structures formed in the head jaw; Relationship between the sum of the cooling stop temperatures of the head corner side and the head jaw (TA + TC) and the number of martensite structures formed in the head jaw; Relationship between the difference in cooling stop temperatures of the head corner side and the head jaw (TA - TC) and the number of martensite structures formed in the head jaw; Relationship between the sum of the cooling stop temperatures of the head corner side and the head jaw (TA + TC) and the number of martensite structures formed in the head jaw (when TA - TC = 145 to 250); Relationship between the difference in cooling stop temperatures of the head corner side and the head jaw (TA - TC) and the number of martensite structures formed in the head jaw (when TA + TC = 1250 to 1330); Fe-Fe for determining the austenite range temperature ３ An example of a C-system equilibrium phase diagram (quoted from "Steel Materials," edited by the Japan Institute of Metals). Photograph of the appearance of the welded joint of a welded rail after trimming (bead cutting). Photograph of the appearance of the welded joint of a welded rail after trimming. Schematic diagram of an example of a cooling device suitable for implementing the welded rail manufacturing method.

[0020] To solve this problem, the inventors investigated the causes of breakage in welded joints and studied ways to prevent breakage due to brittle fracture originating from the jaw portion of the head of a welded joint. As a result, they found that breakage often originates from brittle cracks originating from the jaw portion of the rail head, and further confirmed that martensite structures are formed at the brittle crack initiation points. They then analyzed the relationship between this martensite structure and the cooling conditions of welded joints, and found that there is a correlation between the cooling conditions and the formation of martensite structures. They also confirmed that controlling the cooling conditions of welded joints suppresses the formation of martensite structures. The inventors then discovered a rail manufacturing method that consistently improves the performance of welded rail welded joints used in harsh environments.

[0021] A method for manufacturing a flash-butt welded rail according to one embodiment of the present invention is a method for manufacturing a flash-butt welded rail (1) having multiple rail portions (11) and a weld joint (12) joining the multiple rail portions (11). The method includes the steps of flash-butt welding a rail containing, in mass %, 0.70-1.20% C, 0.05-2.00% Si, 0.05-2.00% Mn, P≦0.0300%, S≦0.0300%, with the balance being Fe and impurities, and immediately forcibly cooling the weld joint (12) after the flash-butt welding is completed. During the forced cooling from the austenite temperature range, the cooling stop temperature (TC) of the outer surface (1214) of the top corner at the weld center (A) of the weld joint (12) is 560°C or higher, and the cooling stop temperature (TA) of the outer surface (1212) of the head and jaw is 650°C or higher. The method for manufacturing a flash-butt welded rail according to this embodiment is described in detail below.

[0022] 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 that joins these rail portions 11. The inventors have conducted extensive research into methods for improving the breakage resistance of the weld joint portion 12. The inventors have discovered that the breakage resistance of the weld joint portion 12 can be improved by controlling the cooling stop temperature of the outer surface 1214 of the top corner portion at the weld center A of the weld joint portion 12 and the cooling stop temperature of the outer surface 1212 of the head and jaw portions of the weld joint portion 12 to a certain temperature or higher.

[0023] The inventors then optimized the heat treatment conditions after completion of welding to reduce the amount of martensite structure formed in the martensite structure evaluation region C in the head and jaw portion of the head 121 of the welded joint 12, as shown in Figure 5. As a result, the inventors were able to improve the breakage resistance of the welded joint 12 and significantly extend its service life.

[0024] A method for manufacturing a flash-butt welded rail according to one embodiment of the present invention, which has been obtained based on the above findings, will now be described in detail. First, the terms used in this embodiment will be explained.

[0025] 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 the "welded rail 1."

[0026] As shown in Figures 1 and 2, a welded rail 1 comprises a plurality of 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, the term "rail" simply refers to the rail before welding, and the term "rail portion" refers to the base material portion of the welded rail 1.

[0027] The rail head portion 111 of the rail portion 11 refers to the portion above the narrowed portion at 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 at 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 at the vertical center of the rail portion 11 in the cross section of the rail portion 11 shown in Fig. 2.

[0028] In addition, the outer surface of the upper part of the rail head 111 is referred to as the rail top surface or rail top outer surface 1111, the outer surface on the corner side of the upper part is referred to as the rail top corner outer surface 1114, and the outer surface on the side of the upper part is referred to as the rail head side outer surface 1113. In addition, the narrowed portion at the lower part of the rail head 111 is referred to as the rail head jaw outer surface 1112.

[0029] As shown in Fig. 2, the rail head corner side outer surface 1114 is an outer surface located 0.25W to 0.35W away from the center (B) of the width (W) of the rail head 111 toward the rail head side outer surface 1113. Also, the rail head jaw outer surface 1112 is an outer surface located 0.25W to 0.35W away from the center (B) of the width (W) of the rail head toward the rail head side outer surface 1113. Naturally, the vertical direction of the welded rail 1 refers to the vertical direction when the welded rail 1 is used as a track.

[0030] 2 shows a dashed line 0.25W away from the center of the rail head 111 in the width direction and extending in the vertical direction of the rail portion 11, and another dashed line 0.35W away from the center of the rail head 111 in the width direction and extending in the vertical direction of the rail portion 11. The area of ​​the rail top surface located between these dashed lines is the rail top corner-side outer surface 1114. Furthermore, the area of ​​the surface of the rail head jaw portion located between these dashed lines is the rail head jaw outer surface 1112.

[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.

[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 portion outer surface 1211, a top portion corner side outer surface 1214, a head side portion outer surface 1213, and a head jaw portion outer surface 1212. 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 portion 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 portion of the base material that is not melted and whose metallurgical properties, mechanical properties, etc. have been changed by heat from welding, cutting, etc. In this embodiment, the base material refers to the rail portion 11.

[0034] 3 , the welded rail 1 according to this embodiment has a width of the heat-affected zone 12H along the longitudinal direction of the welded rail 1, i.e., a HAZ width. 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 direction of the welded rail 1 and passes through a cross section at a position 0.25W to 0.35W away from the center (B) of the width of the head of the welded rail 1 toward the head side outer surface 1213. In this embodiment, the cross section that is parallel to the longitudinal direction of the welded rail 1 and passes through a cross section at a position 0.25W to 0.35W away from the center (B) of the width of the head of the welded rail 1 toward the head side outer surface 1213 is referred to as a "longitudinal cross section." 3 shows a dashed line that is spaced 0.25W from the widthwise center of the rail head portion 111 and that runs along the vertical direction of the rail portion 11, and another dashed line that is spaced 0.35W from the widthwise center of the rail head portion 111 and that runs along the vertical direction of the rail portion 11. The longitudinal cross section is a cut surface that is formed at any point between these dashed lines and that runs along the vertical direction of the rail. Below, we will explain an overview of the hardness distribution of the welded joint portion 12, and then we will explain the definition of the HAZ width.

[0035] Figure 4 shows a schematic diagram of the hardness distribution in the longitudinal cross section of the welded joint 12. This graph was obtained by continuously measuring Vickers hardness along the head and jaw outer surface 1212 at a depth of 5 mm from the head and jaw outer surface 1212 toward the top corner outer surface 1214 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 that passes through the center of the heat-affected zone 12H in the longitudinal cross section of the welded joint 12 and runs along the vertical direction of the welded rail. Typically, the weld center A roughly coincides with the rail seam.

[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 that are heated to near the A1 point by the welding heat, resulting in partial austenitization, and then the subsequent cooling after welding results in decomposition of the pearlite structure. These regions exhibit significantly lower hardness. Therefore, as shown in Figure 4, a graph of the hardness distribution 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 portions of the welded rail 1 according to this embodiment. The distance between the two softest-softened portions is defined as the HAZ width. The weld center A approximately coincides with the center of this HAZ width.

[0037] 5, the martensitic structure evaluation region C refers to a region (C) in the longitudinal cross section that is within a range of ±5 mm (width 10 mm) in the longitudinal direction of the welded rail 1 from the weld center A as the center, and has a depth of 1 to 5 mm from the head jaw outer surface 1212 toward the top corner side outer surface 1214. The technical significance of the martensitic structure evaluation region C will be described later.

[0038] The martensitic structure evaluation region (C) is a region included in a longitudinal cross section located 0.25W to 0.35W away from the center (B) of the width of the head of the welded rail toward the head side outer surface 1213. This longitudinal cross section has two cross sections, one on the left and one on the right of the center of the width of the head. Either of these two cross sections may be set. This longitudinal cross section is also a cross section passing through the head jaw outer surface 1212. Therefore, the lower end of this cross section coincides with the head jaw outer surface 1212. When specifying the martensitic structure evaluation region C according to the above definition, the lower end of the cross section can be regarded as the head jaw outer surface 1212.

[0039] Forced cooling of the weld joint 12 refers to spraying a coolant such as air, water, or mist onto the weld joint 12. In this embodiment, the temperature drop that occurs when the weld joint 12 is left in the atmosphere after welding is referred to as natural cooling of the weld joint 12 and is considered a different concept from forced cooling of the weld joint 12. The cooling stop temperature of the top corner side outer surface 1214 at the weld center A refers to the temperature of the top corner side outer surface 1214 measured at the time when forced cooling is stopped, i.e., when the coolant spraying is stopped. Similarly, the cooling stop temperature of the head jaw outer surface 1212 at the weld center A refers to the temperature of the head jaw outer surface 1212 at the weld center A measured at the time when the coolant spraying is stopped.

[0040] Next, the technical concept of the present invention will be described. The inventors investigated damage that occurs in the welded joint 12 of the welded rail 1. As a result of investigating damaged rails that occurred in actual track, it was confirmed that the damage occurred most often as breakage that originated from a brittle crack that occurred in the head of the welded rail.

[0041] First, the inventors identified the origin of breakage in welded rails, and as a result, they confirmed that in many cases, breakage occurred in the heat-affected zone (HAZ).

[0042] Next, the location of breakage was identified in detail, and it was confirmed that in many cases, breakage occurred in region C shown in Figure 5, and that martensite structure was formed in this region C.

[0043] Therefore, we first conducted a detailed investigation into the relationship between the martensite structure formation in this region and rail weld joint fracture. A flash butt welding test was conducted using a hypereutectoid steel rail (0.80-1.20% C), and the rail drop weight test shown in Figure 6 was conducted to evaluate the relationship between the amount of martensite structure formed and the presence or absence of rail fracture. Control of the amount of martensite structure formed was achieved primarily by controlling the cooling stop temperature of the outer surface of the head jaw and the outer surface of the top corner in a region ±5 mm (10 mm wide) from the weld center (A) in the weld joint where martensite structure formed.

[0044] The rail, flash butt welding conditions, cooling conditions for the welded joint 12 after welding, properties of the welded joint 12, evaluation method for the martensitic structure, and drop weight test conditions are as follows:

[0045] Rail used as welding base material: Composition: 0.70-1.20% C, Si, Mn, balance iron and impurities. Rail shape: 136 lbs (weight: 67 kg / m). Hardness: 420 HV (top surface).

[0046] ●Flash butt welding conditions (preheating flash method) Initial flashing time: 15 seconds Number of preheating times: 10 Late flashing time: 25 seconds Average late flashing speed: 1.0 mm / sec Late flashing speed just before upsetting (for 3 seconds): 2.0 mm / sec Upsetting load: 65 KN

[0047] Cooling Conditions for the Welded Joint 12 After Welding Location: Weld Center (A) Cooling Start Time: 15 seconds after welding completion Cooling Stop Temperature (TA) of Head Jaw Outer Surface 1212: 500-800°C Cooling Stop Temperature (TC) of Crown Corner Outer Surface 1214: 550-900°C Subsequent Cooling (Head Jaw Outer Surface, Crown Corner Outer Surface): Natural Cooling (to 50°C) Note that in this embodiment, the phrase "immediately after completion of flash butt welding, the welded joint is forced to cool" means that forced cooling begins 5 to 30 seconds after completion of flash butt welding. The end of flash butt welding refers to the end of trimming. Trimming removes burrs formed on the welded joint during the upset process of flash butt welding. Forced cooling initiated under the above conditions is considered to be forced cooling initiated immediately after completion of flash butt welding.

[0048] Characteristics of welded joint 12: HAZ width: 32 mm; Hardness of weld center A: 390-440 HV; Hardness of softest part: 280 HV

[0049] Evaluation of Martensite Structure Evaluation Area (See Figure 5) This refers to the area (martensite structure evaluation area: C) in the longitudinal cross section of the welded joint, within a range of ±5 mm (10 mm width) from the weld center (A) in the longitudinal direction of the welded rail, and within a range of 1 to 5 mm depth from the outer surface 1212 of the head jaw toward the outer surface of the corner of the head. Reason for Selection of Evaluation Area This is because this is the location where the initiation point of rail breakage occurs. Method for Revealing Martensite Structure The martensite structure evaluation area (C) was polished, then etched with nital and observed under an optical microscope. Polishing Conditions: Buffing with 1 μm diamond paste Martensite Etching Conditions Etching Solution: Alcohol + 5% nitric acid (nital) Etching Time: 5 to 10 seconds Microstructure Investigation Method Equipment: Optical microscope Magnification: 400x Microstructure Evaluation Method The martensite structure visible under an optical microscope at 400x magnification was evaluated. The target martensite structure had a major axis of 25 to 100 μm, and when martensite structure was generated, the number of martensite structures was investigated.

[0050] Drop weight test conditions (see Figure 6) Posture: Welded rail supported at two points with the head on the bottom and the bottom on the top, and a drop weight was dropped onto the rail bottom. Span length (distance between two support points): 1000 mm Drop weight: 1000 kgf (9.8 kN) Drop height (X): 5.0, 7.5 m Drop energy: 49.0, 73.5 kN m

[0051] As a result, in the case of a falling weight height of 5.0 m (falling weight energy of 49.0 kN m), it was confirmed that breakage of the rail welded joint can be prevented when the number of martensite structures formed in the martensite structure evaluation region C near the outer surface 1212 of the head jaw is 10 or less, as shown in Figure 7.

[0052] Furthermore, in the case of a falling weight height of 7.5 m (falling weight energy of 73.5 kN m), it was confirmed that breakage of the rail welded joint can be prevented when the number of martensitic structures formed in the martensitic structure evaluation region C near the outer surface 1212 of the head and jaw is 5 or less, as shown in Figure 8. In this case, it is considered that breakage of the welded rail can be effectively suppressed even in an even harsher operating environment.

[0053] Furthermore, in order to control the amount of martensite structure formed in the martensite structure evaluation region C, the inventors investigated the relationship between the heat treatment conditions after flash butt welding and the formation of martensite structure in the martensite structure evaluation region C.

[0054] A flash butt welding test was performed using a hypereutectoid steel rail (0.70 to 1.20% C), and the rail drop weight test shown in Figure 6 was performed to evaluate the relationship between the amount of martensite structure formed in the martensite structure evaluation region C and the presence or absence of rail breakage. The amount of martensite structure formed was controlled mainly by controlling the cooling stop temperature of the head jaw outer surface 1212 and the top corner outer surface 1214 in a region (10 mm wide) ±5 mm from the weld center (A) in the weld joint 12 where the martensite structure formed. The rail, flash butt welding conditions, characteristics of the weld joint 12, and the martensite structure evaluation method are as described above.

[0055] Cooling conditions for welded joint 12 after welding Location: Weld center (A) Cooling start time: 15 seconds after welding is completed In the case of stop temperature control of top corner side outer surface 1214 (FIG. 9) Cooling stop temperature (TA) of head jaw outer surface 1212: 740°C Cooling stop temperature (TC) of head corner side outer surface 1214: 510 to 700°C Cooling stop temperature (TA) of head jaw outer surface ≧ cooling stop temperature (TC) of head corner side outer surface In the case of cooling stop temperature control of head jaw outer surface 1212 (FIG. 10) Cooling stop temperature of head jaw outer surface 1212: 620 to 800°C Cooling stop temperature of head corner side outer surface 1214: 620°C Cooling stop temperature (TA) of head jaw outer surface ≧ cooling stop temperature (TC) of head corner side outer surface Subsequent cooling (outer surface of head jaw, outer surface of top corner): Cool naturally (to 50°C)

[0056] Drop weight test conditions (see Figure 6) Posture: The welded rail was supported at two points with the head on the bottom and the bottom on the top, and the drop weight was attached to the rail bottom. Span length (distance between the two support points): 1000 mm Drop weight: 1000 kgf (9.8 kN) Drop height (X): 5.0 m Drop energy: 49.0 kN m

[0057] As a result, as shown in Figure 9, it was confirmed that when the cooling stop temperature of the outer surface 1214 on the corner side of the top portion is 560°C or higher, the number of martensite structures formed in the martensite structure evaluation region C is 10 or less.

[0058] Furthermore, as shown in Figure 10, it was confirmed that when the cooling stop temperature of the head jaw outer surface 1212 was 650°C or higher, the number of martensitic structures formed in the martensitic structure evaluation region C was 10 or less.

[0059] From these findings, it was confirmed that by controlling the cooling stop temperature of the head jaw outer surface 1212 and the cooling stop temperature of the head corner side outer surface 1214, it is possible to control the amount of martensite structure generated in the martensite structure evaluation region C to 10 or less, thereby preventing breakage of the rail weld joint.

[0060] Furthermore, the present inventors have investigated a method for further reducing the amount of martensite structure produced and further improving the fracture resistance of the welded joint 12 .

[0061] The inventors considered that, in order to reduce the amount of martensite structure produced, it is preferable to mutually control the stop cooling temperature of the head jaw outer surface 1212 and the stop cooling temperature of the top corner side outer surface 1214. Then, experiments were conducted focusing on the sum of the stop cooling temperature (TA) of the head jaw outer surface 1212 and the stop cooling temperature (TC) of the top corner side outer surface 1214, and the difference between the stop cooling temperature (TA) of the head jaw outer surface 1212 and the stop cooling temperature (TC) of the top corner side outer surface 1214.

[0062] A flash butt welding test was conducted using a hypereutectoid steel rail (0.70 to 1.20% C), and then the cooling stop temperature (TA) of the head jaw outer surface 1212 of the welded joint 12 and the cooling stop temperature (TC) of the top corner outer surface 1214 were changed to evaluate the relationship between the cooling stop temperature conditions and the amount of martensite structure formed.

[0063] The rail, the flash butt welding conditions, the properties of the welded joint 12, and the method for evaluating the martensitic structure are as described above.

[0064] Cooling conditions for welded joints after welding When controlling the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 at the weld center (A) and the cooling stop temperature (TC) of the top corner side outer surface 1214 (Fig. 11) TA - TC = 50 (fixed) TA: 650 to 800°C, TC: 600 to 750°C TA ≥ TC When controlling the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 at the weld center (A) and the cooling stop temperature (TC) of the top corner side outer surface 1214 (Fig. 12) TA + TC = 1400 (fixed) TA: 700 to 800°C, TC: 600 to 700°C TA ≥ TC Subsequent cooling (head jaw outer surface, top corner side outer surface): Natural cooling (down to 50°C)

[0065] As a result, as shown in Figure 11, it was confirmed that when the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner outer surface 1214 becomes 1340°C or higher, the number of martensitic structures formed in the martensitic structure evaluation region C becomes 5 or less.

[0066] Furthermore, as shown in Figure 12, it was confirmed that when the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner outer surface 1214 is 140°C or less, the number of martensitic structures formed in the martensitic structure evaluation region C is 5 or less.

[0067] From these findings, it was confirmed that by controlling the sum or difference of the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the head corner side outer surface 1214, it is possible to control the amount of martensite structure produced to 5 or less, and to further improve the breakage resistance of the rail weld joint.

[0068] In addition, when controlling the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214, and the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214 is in the range of 160 to 240°C, as shown in Figure 13, even if the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214 becomes 1340°C or higher, the number of martensitic structures generated in the martensitic structure evaluation region C will not be 5 or less.

[0069] Similarly, in the case of controlling the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214, if the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214 is in the range of 1250 to 1330°C, as shown in Figure 14, even if the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214 is 140°C or less, the number of martensitic structures formed in the martensitic structure evaluation region C will not be 5 or less.

[0070] Therefore, in order to control the number of martensite structures generated to five or less, it is preferable to control both the sum and the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214.

[0071] From these results, it has become possible to further suppress breakage caused by brittle cracks originating from the head 121 of the welded rail 1 and significantly improve the service life of the welded rail 1 by controlling the cooling stop temperature (TC) of the outer surface 1214 of the head corner side at the weld center A of the weld joint 12 and the cooling stop temperature (TA) of the outer surface 1212 of the head jaw, and suppressing the amount of martensitic structure produced in the martensitic structure evaluation region C of the longitudinal cross section.

[0072] A method for manufacturing a flash-butt welded rail 1, which was obtained based on the above findings, will be described in detail below. Hereinafter, the unit of "mass %" for the content of alloy components will be simply referred to as "%".

[0073] (1) Reasons for Limiting the Chemical Composition of the Rail The manufacturing method for a welded rail 1 according to this embodiment includes a step of flash butt welding the rails that will become the raw material for the welded rail 1. The reasons for limiting the chemical composition of the rail before welding are explained in detail below. However, the chemical composition of the rail before welding is the same as the chemical composition of the rail portion 11 of the welded rail 1. Therefore, the upper and lower limit values ​​of alloying elements explained regarding the chemical composition of the rail also apply to the chemical composition of the rail portion 11.

[0074] (C: 0.70 to 1.20%) C is an element effective in promoting pearlite transformation and ensuring the wear resistance of the welded joint 12. If the C content is less than 0.70%, the minimum strength and wear resistance required of the welded joint 12 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 12, reducing the breakage resistance of the welded joint 12. For this reason, the C content is limited to 0.70 to 1.20%. The C content is preferably 0.72% or more, 0.75% or more, or 0.80% 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, it is desirable to set the C content to 0.80 to 1.10%.

[0075] (Si: 0.05 to 2.00%) Si is an element that dissolves in the ferrite phase of the pearlite structure, increasing the hardness of the welded joint 12 and improving its wear resistance. However, if the Si content is less than 0.05%, 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 breakage resistance of the welded joint 12 decreases. For this reason, the Si content is limited to 0.05 to 2.00%. The Si content is preferably 0.10% or more, 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 the pearlite structure and improve the breakage resistance of the welded joint 12, it is desirable to set the Si content to 0.40 to 2.00%.

[0076] (Mn: 0.05 to 2.00%) Mn is an element that improves the hardenability of the welded rail 1, stabilizes pearlite transformation, and refines the lamellar spacing of the pearlite structure, ensuring the hardness of the welded joint 12 and further improving its wear resistance. However, if the Mn content is less than 0.05%, this effect is small, and the wear resistance of the welded joint 12 decreases. On the other hand, if the Mn content exceeds 2.00%, the excessive Mn promotes Mn concentration in the segregated areas, promoting the formation of a martensitic structure in the welded joint 12 and reducing breakage resistance. For this reason, the Mn content is limited to 0.05 to 2.00%. The Mn content is preferably 0.10% or more, 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. In order to stabilize the formation of pearlite structures and improve the wear resistance and breakage resistance of the welded joint 12, the Mn content is preferably set to 0.40 to 1.50%.

[0077] (P≦0.0300%) P is an impurity element contained in steel. If the P content exceeds 0.0300%, the pearlite structure becomes embrittled, resulting in a decrease in the fracture resistance of the welded joint 12. For this reason, the P content is limited to 0.0300% 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 0.0250% or less, 0.0200% or less, or 0.0150% or less. To stably maintain the toughness of the pearlite structure, the P content is preferably 0.0050 to 0.0150%.

[0078] (S≦0.0300%) S is an impurity element contained in steel. If the S content exceeds 0.0300%, stress concentration occurs around coarse MnS-based sulfide inclusions, reducing the breakage resistance of the welded joint 12. For this reason, the S content is limited to 0.0300% or less. The lower limit of the S content does not need to be limited 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 0.0250% or less, 0.0200% or less, or 0.0150% or less. In order to stably maintain the breakage resistance of the pearlite structure, it is desirable to set the S content to 0.0050 to 0.0150%.

[0079] The remainder of the chemical composition of the rail, which is the raw material for the welded rail, includes iron and impurities. The impurities are components that are mixed in, for example, with 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 1 according to this embodiment.

[0080] Furthermore, the rail, which is the material for the welded rail, may contain one or more of the following elements as necessary: ​​Cr and Mo in group a; Co in group b; B in group c; Cu and one or two of Ni in group d; V, Nb, and Ti in group e; Mg, Ca, and one or more of REM in group f; N in group g; Zr in group h; and Al in group i, for the purposes of increasing the hardness of the welded joint 12 to improve wear resistance, improve toughness, prevent softening of the heat-affected zone 12H, and control the cross-sectional hardness distribution inside the head. However, since the manufacturing method for the welded rail 1 according to this embodiment can exhibit its effects even if these elements are not contained in the rail, the lower limit of the content of these elements is 0%.

[0081] <Group a>

[0082] (Cr: preferably 2.00% or less) Cr is an element that increases the equilibrium transformation temperature, refines the lamellar spacing of the pearlite structure by increasing the degree of supercooling, improves the hardness of the pearlite structure, and improves the wear resistance of the welded joint 12. To fully obtain these effects, the Cr content is preferably 0.03% or more, or 0.05% or more. On the other hand, if the Cr content exceeds 2.00%, the excessive amount of Cr may promote Cr concentration in the segregation zone, promote the formation of a martensite structure in the welded joint, and reduce breakage resistance. For this reason, it is desirable to set the Cr content to 0.05 to 2.00%. The Cr content is preferably 0.06% or more, 0.08% or more, or 0.10% or more. The Cr content is preferably 1.80% or less, 1.50% or less, or 1.20% or less. Therefore, in order to stabilize the formation of pearlite structures and improve the wear resistance and damage resistance of the welded joint 12, it is desirable that the Cr content be 0.10 to 1.20%.

[0083] (Mo: preferably 0.50% or less) Mo is an element that increases the equilibrium transformation temperature, refines the lamellar spacing of the pearlite structure by increasing the degree of supercooling, improves the hardness of the pearlite structure, and improves the wear resistance of the welded joint 12. To achieve the above-mentioned effects, the Mo content is preferably 0.01% or more. On the other hand, if the Mo content exceeds 0.50%, the excessive amount of Mo promotes Mo concentration in the segregation region, promotes the formation of a martensitic structure in the welded joint 12, and may reduce breakage resistance. For this reason, the Mo content is desirably 0.01 to 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. Therefore, in order to stably improve the hardness of the pearlite structure and improve the wear resistance and damage resistance of the welded joint 12, it is desirable that the Mo content be 0.10 to 0.30%.

[0084] <Group b> (Co: preferably 1.00% or less) Co is an element that dissolves in the ferrite phase of the pearlite structure, refines the lamellar structure of the pearlite structure immediately below the rolling surface where deformation occurs due to contact with the wheel, improves the hardness of the rolling surface, and improves the wear resistance of the welded joint 12. To achieve the above-mentioned effects, the Co content is preferably 0.01% or more. On the other hand, if the Co content exceeds 1.00%, the above-mentioned effects saturate, and the lamellar structure cannot be refined according to the Co content. Furthermore, if the Co content exceeds 1.00%, the alloy cost increases, which may reduce economic efficiency. For this reason, the Co content is desirably 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. Therefore, in order to stably improve the wear resistance of the pearlite structure and improve the wear resistance of the welded joint 12, it is desirable to set the Co content to 0.10 to 0.60%.

[0085] <Group c> (B: preferably 0.0050% or less) B is an element that forms iron boride (Fe ２３ (CB) ６ ), and by accelerating pearlite transformation, B reduces the cooling rate dependency of the pearlite transformation temperature, uniforms the hardness distribution from the head surface to the interior of the welded joint 12, and improves the wear resistance, thereby extending the life of the welded joint 12. To achieve the above effects, the B content is preferably 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, coarse iron boride particles are formed, which may promote brittle fracture and reduce the breakage resistance of the welded joint 12. For this reason, the B content is preferably 0.0001 to 0.0050%. The B content is preferably 0.0002% or more, 0.0003% or more, or 0.0005% or more. The B content is preferably 0.0040% or less, 0.0030% or less, or 0.0025% or less. Therefore, in order to stably maintain the toughness of the pearlite structure and improve the wear resistance of the welded joint 12, the B content is preferably set to 0.0005 to 0.0025%.

[0086] <Group d> (Cu: preferably 1.00% or less) Cu is an element that dissolves in the ferrite phase of a pearlite structure, improving the hardness of the welded joint 12 through solid solution strengthening, and improving the wear resistance of the welded joint 12. To achieve the above-mentioned effects, the Cu content is preferably 0.01% or more. On the other hand, if the Cu content exceeds 1.00%, the excessive amount of Cu promotes Cu concentration in the segregation region, promoting the formation of a martensitic structure in the welded joint 12 and possibly reducing breakage resistance. For this reason, the Cu content is preferably 0.01 to 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. Therefore, in order to stably maintain the toughness of the pearlite structure and improve the wear resistance of the welded joint 12, it is desirable that the Cu content be 0.10 to 0.70%.

[0087] (Ni: preferably 1.00% or less) Ni is an element that improves the toughness of the pearlite structure and, at the same time, improves the hardness of the welded joint 12 by solid solution strengthening, thereby improving the wear resistance of the welded joint 12. Furthermore, in the heat-affected zone, Ni combines with Ti to form fine Ni ３ Ni precipitates as a Ti intermetallic compound and suppresses softening of the welded joint 12 through precipitation strengthening. Furthermore, when Cu is contained in the rail, Ni suppresses embrittlement of the grain boundary. To achieve the above-mentioned effect, the Ni content is preferably 0.01% or more. If the Ni content exceeds 1.00%, the excessive amount of Ni promotes Ni enrichment in the segregation zone, promoting the formation of a martensitic structure in the welded joint 12 and possibly reducing breakage resistance. For this reason, the Ni content is preferably 0.01 to 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. Therefore, to stably maintain the toughness of the pearlite structure and improve the wear resistance of the welded joint 12, the Ni content is preferably 0.10 to 0.70%.

[0088] <Group e> (V: preferably 0.200% or less) V is an element that increases the hardness (strength) of the pearlite structure and improves the fatigue damage resistance of the welded joint 12 through precipitation hardening by V carbonitrides formed during the cooling process after hot rolling. To achieve the above-mentioned effect, the V content is preferably 0.005% or more. On the other hand, if the V content exceeds 0.200%, the number of fine V carbonitrides becomes excessive, embrittling the pearlite structure and reducing the fracture resistance of the welded joint 12. Therefore, the V content is desirably 0.005 to 0.200%. The V content is preferably 0.010% or more, 0.015% or more, or 0.020% or more. The V content is preferably 0.180% or less, 0.150% or less, or 0.100% or less. Therefore, in order to stably maintain the breakage resistance of the welded joint 12 and improve the fatigue damage resistance of the welded joint 12, it is desirable that the V content be 0.020 to 0.100%.

[0089] (Nb: preferably 0.0500% or less) Nb is an element that increases the hardness of the pearlite structure and improves the fatigue damage resistance of the welded joint 12 through precipitation hardening by Nb carbides and Nb nitrides formed during the cooling process after hot rolling in the manufacture of a rail. Furthermore, in the heat-affected zone 12H reheated to a temperature range below the Ac1 point, Nb stably forms Nb carbides, Nb nitrides, and the like over a wide temperature range from low to high, and is an effective element for preventing softening of the heat-affected zone 12H of the welded joint 12. To achieve the above-mentioned effect, the Nb content is preferably 0.0010% or more. On the other hand, if the Nb content exceeds 0.0500%, precipitation hardening of Nb carbides and nitrides, etc., becomes excessive, the pearlite structure itself becomes embrittled, and the fracture resistance of the welded joint 12 may decrease. For this reason, 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. Therefore, in order to stably maintain the breakage resistance of the welded joint 12 and improve the fatigue damage resistance of the welded joint 12, it is desirable to set the V content to 0.0030 to 0.0200%.

[0090] (Ti: preferably 0.0500% or less) 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 in the manufacture of a rail, thereby improving the fatigue damage resistance of the welded joint 12. Furthermore, Ti is an element that refines the structure of the heat-affected zone 12H that is reheated to the austenite region, taking advantage of the fact that Ti carbides and Ti nitrides precipitated during reheating after welding do not dissolve in the matrix, thereby improving the fracture resistance of the welded joint 12. To achieve the above-mentioned effect, the Ti content is preferably 0.0010% or more, or 0.0060% or more. On the other hand, if the Ti content exceeds 0.0500%, coarse Ti carbides and Ti nitrides are formed, and stress concentration around them makes fatigue cracks more likely to occur, which may reduce the fatigue damage resistance of the welded joint 12. For this reason, it is desirable that the Ti content be 0.0040 to 0.0500%. The Ti content is preferably 0.0040% or more, 0.0050% or more, or 0.0060% or more. The Ti content is preferably 0.0400% or less, 0.0300% or less, or 0.0200% or less. Therefore, in order to improve the fatigue damage resistance and breakage resistance of the welded joint 12, it is desirable that the Ti content be 0.0060 to 0.0200%.

[0091] <Group f> (Mg: preferably 0.0200% or less) Mg is an element that combines with S to form fine sulfides (MgS), which finely disperse MnS, alleviating stress concentration around the MnS and improving the fatigue damage resistance of the welded joint 12. To achieve the above-mentioned effect, the Mg content is preferably 0.0005% or more. On the other hand, if the Mg content exceeds 0.0200%, coarse Mg oxides are formed, and stress concentration around these coarse oxides may facilitate fatigue crack formation and reduce the fatigue damage resistance of the welded joint 12. For this reason, the Mg content is desirably 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. Therefore, in order to improve the fatigue damage resistance of the welded joint 12, the Mg content is preferably 0.0030 to 0.0120%.

[0092] (Ca: preferably 0.0200% or less) Ca has a strong bonding force with S and forms sulfides (CaS). This CaS finely disperses MnS, alleviating stress concentration around the MnS and improving the fatigue damage resistance of the welded joint 12. To achieve the above-mentioned effect, the Ca content is preferably 0.0005% or more. On the other hand, if the Ca content exceeds 0.0200%, coarse Ca oxides are generated, and stress concentration around these coarse oxides may make fatigue cracks more likely to occur, thereby reducing the fatigue damage resistance of the welded joint 12. 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. Therefore, in order to improve the fatigue damage resistance of the welded joint 12, the Ca content is preferably 0.0030 to 0.0120%.

[0093] (REM: preferably 0.0500% or less) REM is a deoxidizing and desulfurizing element, and REM oxysulfide (REM ２ O２ S), which act as nuclei for the formation of Mn sulfide-based inclusions. ２ O ２ Since REM has a high melting point, it suppresses the elongation of Mn sulfide-based inclusions after rolling. As a result, REM finely disperses MnS, alleviating stress concentration around the MnS and improving the fatigue damage resistance of the welded joint 12. To obtain the above-mentioned effect, it is preferable that the REM content be 0.0005% or more. On the other hand, if the REM content exceeds 0.0500%, coarse and hard REM oxysulfides (REM ２ O ２ S) is generated, and stress concentration around this oxysulfide may make fatigue cracks more likely to occur, resulting in a decrease in the fatigue damage resistance of the welded joint 12. For this reason, it is desirable to set the REM content to 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. Therefore, in order to improve the fatigue damage resistance of the welded joint 12, it is desirable to set the REM content to 0.0030 to 0.0250%.

[0094] REM refers to a total of 17 elements consisting of Sc, Y, and La (lanthanoid). The "REM content" refers to the total content of all these REM elements. As long as the total content is within the above range, the same effect can be obtained whether the REM element is one type or two or more types.

[0095] <Group g> (N: preferably 0.0200% or less) N is an element that can be mixed into rails as an impurity during the steelmaking process. Even when degassing is actively performed, approximately 0.0025% N can remain in the steel. In normal rail refining, the N content is approximately 0.0030 to 0.0050%. While it is possible to reduce the N content to less than 0.0025%, to avoid rising refining costs, 0.0025% or more N may be contained in the rail. Furthermore, N segregates at austenite grain boundaries, thereby promoting pearlite transformation from the austenite grain boundaries and effectively improving the toughness of the welded joint 12 by primarily reducing the pearlite block size. Furthermore, the simultaneous inclusion of N and V promotes the precipitation of V carbonitrides during the cooling process of the welded joint 12 after rail welding, increasing the hardness of the pearlite structure and improving the fatigue damage resistance of the welded joint 12. To achieve the above-mentioned effects, the N content is preferably 0.0060% or more. On the other hand, if the N content exceeds 0.0200%, it becomes difficult to dissolve N in the steel, which may lead to the formation of bubbles that serve as the starting point for fatigue damage. For this reason, the N content is preferably 0.0060 to 0.0200%. The N content is preferably 0.0060% or more, 0.0070% or more, or 0.0080% or more. The N content is preferably 0.0180% or less, 0.0160% or less, or 0.0150% or less. Therefore, to stably improve toughness and fatigue damage resistance, the N content is preferably 0.0080 to 0.0150%.

[0096] <H group> (Zr: preferably 0.0200% or less) Zr is ZrO, which has good lattice matching with γ-Fe. ２Zr forms inclusions, which act as solidification nuclei in high-carbon rail steel, where γ-Fe is the primary crystal. This increases the equiaxed crystallization rate of the solidification structure, thereby suppressing the formation of segregation zones in the center of the slab and suppressing alloy enrichment in the segregated zones. As a result, Zr suppresses the formation of martensite in the welded joint 12, improving breakage resistance and fatigue damage resistance. To achieve the above-mentioned effect, the Zr content is preferably 0.0001% or more. On the other hand, if the Zr content exceeds 0.0200%, a large amount of coarse Zr-based inclusions is formed. Stress concentration around these coarse inclusions may increase fatigue crack initiation and reduce the fatigue damage resistance of the welded joint 12. For this reason, the Zr content is preferably 0.0001 to 0.0200%. The Zr content is preferably 0.0005% or more, 0.0010% or more, or 0.0015% or more. The Zr content is preferably 0.0180% or less, 0.0150% or less, or 0.0120% or less. Therefore, in order to stably improve breakage resistance and fatigue damage resistance, the Zr content is desirably 0.0015 to 0.0120%.

[0097] <Group i> (Al: preferably 1.0000% or less) Al is a component that functions as a deoxidizer. To achieve the above-mentioned effect, the Al content is preferably 0.0005% or more, or 0.0010% or more. On the other hand, if the Al content exceeds 1.0000%, coarse alumina-based inclusions are formed, which may facilitate fatigue crack initiation and reduce the fatigue damage resistance of the welded joint 12. Furthermore, if the Al content exceeds 1.0000%, oxides may be formed during rail welding, which may significantly reduce the weldability of the rail. For this reason, the Al content is preferably 0.0005 to 1.0000%. The Al content is preferably 0.5000% or less, 0.4000% or less, or 0.3000% or less. Therefore, to stably perform deoxidation, the Al content is preferably 0.0005 to 0.3000%.

[0098] (2) Reason for Limiting the Cooling Stop Temperature (TC) of the Corner-Side Outer Surface 1214 of the Welded Joint 12 at the Weld Center A: The manufacturing method for a welded rail 1 according to this embodiment includes a step of immediately forcibly cooling the welded joint 12 obtained by flash butt welding the rail after the flash butt welding is completed. Forced cooling increases the hardness of the welded joint 12 and improves its wear resistance. However, excessive forced cooling can cause martensite to form in the welded joint 12. Therefore, the forced cooling conditions must be properly managed. Note that, as mentioned above, forced cooling refers to spraying a coolant such as air, water, or mist onto the welded joint 12. Examples of forced cooling include air cooling, water cooling, and air-water cooling. Natural cooling is considered a different concept from forced cooling. In both forced cooling and natural cooling, the cooling rate after flash butt welding is affected by the heat input during flash butt welding and the width of the HAZ formed by flash butt welding. The larger the HAZ width, the slower the cooling rate tends to be. When a rail is flash-butt welded to form a welded joint and then allowed to cool, the average cooling rate of the outer surface of the top corner side in the temperature range from 900°C to 600°C is typically within the range of approximately 1.0 to 2.0°C / sec. On the other hand, when a rail is flash-butt welded to form a welded joint and then allowed to cool, the average cooling rate of the outer surface of the top corner side in the temperature range from 900°C to 600°C is typically within the range of approximately 1.5 to 5.0°C / sec. Given the same cross-sectional shape of the rail to be welded and the same flash-butt welding conditions, the cooling rate by natural cooling is always slower than the cooling rate by forced cooling. The "average cooling rate V in the temperature range from X°C to Y°C" is a value calculated using the following formula, where t is defined as the time required for the temperature of the target to be cooled to drop from X°C to Y°C. V = (X - Y) / t The average cooling rate can be measured using a radiation thermometer, which can measure the temperature of the surface of an object without contact.Furthermore, in this embodiment, the phrase "immediately after completion of flash butt welding, the welded joint is forcedly cooled" means that forced cooling is started within 5 to 30 seconds after completion of flash butt welding. The time at which flash butt welding is completed refers to the time at which trimming is completed, as described above. After completion of flash butt welding, it usually takes 5 seconds or more to install a cooling device at the welded joint to start cooling. Furthermore, if forced cooling is started more than 30 seconds after completion of flash butt welding, pearlite transformation may begin in a high temperature range before forced cooling, which may impair the hardness of the welded joint.

[0099] In the forced cooling of the welded joint 12, the stop cooling temperature (TC) of the outer surface 1214 on the corner side of the vertex at the weld center A and the stop cooling temperature (TA) of the outer surface 1212 of the head jaw at the weld center A are each controlled independently. The reason why the stop cooling temperature (TC) of the outer surface 1214 on the corner side of the vertex at the weld center A of the welded joint 12 is limited to 560°C or higher in the forced cooling from the austenite temperature range after completion of flash butt welding in the manufacturing method of the welded rail 1 according to this embodiment will be explained below.

[0100] As shown in FIG. 9 , when the cooling stop temperature (TC) of the vertex corner side outer surface 1214 is less than 560°C, the number of martensitic structures formed in the martensitic structure evaluation region C near the head jaw exceeds 10, and as shown in FIG. 7 , breakage of the welded joint 12 cannot be prevented. For this reason, the cooling stop temperature (TC) of the vertex corner side outer surface 1214 is limited to 560°C or higher. TC may also be set to 580°C or higher, 600°C or higher, 620°C or higher, or 650°C or higher. It is not necessary to limit the upper limit of TC, but TC may be set to, for example, 850°C or lower, 800°C or lower, 750°C or lower, or 720°C or lower. To stably control the number of martensitic structures formed in the martensitic structure evaluation region C to 10 or less, it is desirable to control the cooling stop temperature (TC) to 600°C or higher and 750°C or lower.

[0101] The cooling start temperature in the forced cooling from the austenite temperature range is not particularly limited, but there is no problem as long as it is within the austenite temperature range. Immediately after the completion of flash butt welding of the rail, the temperature of the outer surface 1214 of the top corner side is about 1200°C. Therefore, the upper limit of the austenite temperature range in this embodiment is substantially 1200°C.

[0102] The austenite temperature range varies depending on the carbon content and alloying elements of the rail. To accurately determine the austenite temperature range, it is best to directly measure the transformation point by reheating and cooling experiments. However, since actual measurement is not always easy, the Fe-Fe transformation point is calculated based only on the carbon content. ３ It may be easily determined by reading from the equilibrium phase diagram of the C system.

[0103] For example, in Fig. 15, the Fe-Fe alloy is quoted from Izumi Osamu et al., "Lectures on Current Metallurgy, Materials, Vol. 4, Steel Materials," 2nd Edition, Japan Institute of Metals, December 1985, p. 19. ３ The austenite temperature range in the rail's component system is the region higher than the A3 line and the Acm line in the parallel phase diagram. In the above-mentioned range of carbon content of the rail, Ar3 is approximately 715°C to 750°C, and Arcm is approximately 715°C to 900°C. The A3 line is the solubility line of the ferrite phase in the austenite phase, and the Acm line is the solubility line of the cementite phase in the austenite phase.

[0104] (3) Reason for limiting the cooling stop temperature (TA) of the head jaw outer surface 1212 at the weld center A of the welded joint 12 Next, we will explain the reason why, in this embodiment, the cooling stop temperature (TA) of the head jaw outer surface 1212 at the weld center A of the welded joint 12 is limited to 650°C or higher during forced cooling from the austenite temperature range after completion of flash butt welding.

[0105] As shown in FIG. 10 , when the cooling stop temperature (TA) of the head jaw outer surface 1212 is less than 650°C, the number of martensite structures formed in the martensite structure evaluation region C near the head jaw outer surface 1212 exceeds 10, and as shown in FIG. 7 , breakage of the welded joint 12 cannot be prevented. For this reason, the cooling stop temperature (TA) of the head jaw outer surface 1212 is limited to 650°C or higher. TA may also be set to 680°C or higher, 700°C or higher, 720°C or higher, or 750°C or higher. It is not necessary to limit the upper limit of TA, but TA may be set to, for example, 950°C or lower, 900°C or lower, 850°C or lower, or 820°C or lower. To stably control the number of martensite structures formed in the martensite structure evaluation region C to 10 or less, it is desirable to control the cooling stop temperature (TA) to 670°C or higher and 850°C or lower. After the forced cooling is stopped, the welded joint 12 is allowed to cool naturally. If the welded joint 12 is forced to cool again after the forced cooling has been stopped, martensite may form in the welded joint 12. The welded joint 12 is preferably left in the air until its temperature reaches room temperature. If the forced cooling is stopped at a temperature of 600°C or higher and the welded joint 12 is then allowed to cool naturally, the average cooling rate of the outer surface of the corner side of the top portion is typically about 0.5°C / second in the temperature range from 600°C to 200°C.

[0106] The cooling start temperature in the forced cooling from the austenite temperature range is not particularly limited, but there is no problem as long as it is within the austenite temperature range. The austenite temperature range is specified as described above.

[0107] (4) Desirable temperature control method for the head outer surface at the weld center A of the welded joint 12 Next, in this embodiment, a method for controlling the cooling stop temperature (TC) of the head corner side outer surface 1214 at the weld center A of the welded joint 12 to 560°C or higher and the cooling stop temperature (TA) of the head jaw outer surface 1212 to 650°C or higher during forced cooling from the austenite temperature range after completion of flash butt welding will be described.

[0108] When forcibly cooling the head of a rail, a method of injecting a coolant such as air onto the top outer surface 1211, top corner outer surfaces 1214, and head side outer surfaces 1213 shown in Figure 2 is generally used. The cooling stop temperature (TC) of the top corner outer surface 1214 is adjusted mainly by controlling the amount of coolant sprayed onto the top outer surface 1211 and the top corner outer surfaces 1214. The cooling stop temperature (TA) of the head jaw outer surface 1212 is adjusted mainly by controlling the amount of coolant sprayed onto the head side outer surfaces 1213.

[0109] In addition, when the amount of refrigerant sprayed cannot be controlled, the cooling stop temperature (TC) of the top corner outer surface 1214 and the cooling stop temperature (TA) of the head chin outer surface 1212 can be controlled by adjusting the distance between the refrigerant spray hole and the outer surface at the top outer surface 1211, the top corner outer surface 1214, and the head side outer surface 1213.

[0110] The device for performing the cooling according to the above procedure is not particularly limited. For example, a cooling device 2 as shown in Fig. 17 may be used as the cooling means for the welded rail 1 according to this embodiment. This cooling device 2 includes a rail installation section, a plurality of coolant injection means 21 having coolant injection holes, a coolant supply means 23, and a control means 24.

[0111] The rail installation section is configured so that the welded joints 12 of the welded rail 1 can be placed thereon. Figure 17 shows the state in which the welded joints 12 of the welded rail 1 are placed on the rail installation section. The cooling device 2 may also include a drive device for moving the welded rail 1 placed on the rail installation section in its longitudinal direction. This allows the multiple welded joints 12 on the welded rail 1 to be continuously cooled.

[0112] The multiple coolant injection means 21 are arranged to surround the rail installation portion. The coolant injection direction of the coolant injection means 21 is directed toward the rail installation portion. The coolant injection means 21 may be configured to have an adjustable installation location. The coolant injection means 21 may also be configured to have an adjustable coolant injection direction. This allows the cooling device 2 to be used for welded rails 1 of various shapes. It is preferable that the cooling device 2 not inject the coolant toward the column portion 122 of the welded joint portion 12. A portion of the coolant injected toward the column portion 122 flows toward the head jaw outer surface 1212 of the welded joint portion 12, lowering the temperature of the head jaw outer surface 1212. As a result, the cooling stop temperature (TA) of the head jaw outer surface 1212 may be excessively reduced, or the relationship TA≧TC may not be satisfied. Therefore, it is preferable that the cooling device 2 does not include the coolant injection means 21 positioned to inject the coolant toward the column portion 122. If the cooling device 2 has a refrigerant injection means 21 positioned to inject refrigerant toward the column portion 122, it is preferable to use the control means 24 described below to suppress the amount of refrigerant injected toward the column portion 122.

[0113] The refrigerant supply means 23 supplies the refrigerant to the refrigerant injection means 21. The control means 24 is configured to start and end the injection of the refrigerant from the refrigerant injection means 21. The control means 24 can also independently control the start and end of the injection of the refrigerant from each of the plurality of refrigerant injection means 21.

[0114] Optimizing the amount of coolant injected using the control means 24 and / or the coolant injection means 21 is extremely important for controlling the cooling conditions of the welded joint 12 as described above. It is necessary to keep the distance between the coolant injection means 21 and the welded joint 12 within a predetermined range and to independently set the amount of coolant injected by each of the multiple coolant injection means 21. Therefore, it is preferable that the control means 24 be able to independently control the amount of coolant injected by each of the multiple coolant injection means 21. This allows the cooling stop temperature to be preferably controlled as described above. Furthermore, if the control means cannot control the amount of coolant injected, it is necessary to change the position of the coolant injection means 21 and adjust the distance between the coolant injection holes and the outer surface to preferably control the cooling stop temperature as described above.

[0115] When the welded joint 12 is cooled using a cooling device 2 not equipped with a temperature measuring device, the cooling stop temperature and average cooling rate are measured as follows: (A) After flash butt welding is completed and immediately before the cooling device is placed, a radiation thermometer is used to measure the temperatures of the outer surface of the top corner and the outer surface of the head jaw of the welded joint 12. These temperatures are the cooling start temperatures of the outer surface of the top corner and the outer surface of the head jaw. (B) The welded joint 12 is placed on the rail mounting portion of the cooling device. The placement may be performed by moving the welded joint 12 while the cooling device is stationary. Alternatively, the placement may be performed by moving the cooling device while the welded joint 12 is stationary. In either case, the time required to place the welded joint 12 in the cooling device and start cooling is approximately 5 to 10 seconds. (C) The welded joint 12 is cooled. (D) After cooling is completed, the cooling device is removed from the welded joint 12. When removing the cooling device, the welded joint 12 may be moved, or the cooling device may be moved. In either case, the time required to remove the cooling device from the welded joint 12 is approximately 5 to 10 seconds. (E) Immediately after removing the cooling device from the welded joint 12, a radiation thermometer is used to measure the temperatures of the outer surface of the top corner side and the outer surface of the head jaw of the welded joint 12. These temperatures are the cooling stop temperature (TC) of the outer surface of the top corner side and the cooling stop temperature (TA) of the outer surface of the head jaw. (F) The average cooling rate is calculated by dividing the difference between the cooling start temperature and the cooling end temperature by the time required from the first temperature measurement (A) to the second temperature measurement (E).

[0116] The above-described temperature measurement procedure applies to a cooling device 2 that is not equipped with a temperature measuring device. Alternatively, the cooling device 2 may further include a temperature measuring device connected to the control device 24 and disposed facing the rail installation area. The temperature measuring device is a radiation thermometer. The radiation thermometer can measure the surface temperature of an object. By using the temperature measuring device provided in the cooling device 2, the cooling conditions of the welded joint 12 can be controlled with high precision according to the temperature of the welded joint 12. If the cooling device 2 is equipped with a temperature measuring device, cooling is preferably initiated 5 to 10 seconds after the completion of measurement of the cooling start temperature. Furthermore, the cooling stop temperature is preferably measured 5 to 10 seconds after the completion of cooling. However, if the shape of the welded joint 12 and the flash butt welding conditions are constant, the temperature distribution of the welded joint 12 disposed in the rail installation area will also be approximately constant, and therefore, cooling of the welded joint 12 can be performed under the same refrigerant injection conditions. Therefore, if the refrigerant injection conditions that can keep the cooling conditions within the range of the manufacturing method according to this embodiment are determined in advance, there is no need to measure the temperature using a temperature measuring means.

[0117] (5) Why TA≧TC (i.e., 0≦TA−TC) is preferable Next, we will explain why, in the manufacturing method according to this embodiment, it is preferable to control the cooling stop temperature (TC) of the outer surface 1214 of the head corner side at the weld center A of the weld joint 12 and the cooling stop temperature (TA) of the head jaw outer surface 1212 so as to satisfy the relationship TA≧TC during forced cooling from the austenite temperature range after completion of flash butt welding.

[0118] When forcibly cooling the rail head, a method of injecting a coolant such as air onto the head top outer surface 1211, the head top corner outer surface 1214, and the head side outer surface 1213 shown in Figure 2 is used. However, excessive cooling of the head jaw outer surface 1212 may cause martensite to form in the welded joint 12. In particular, when TA < TC, martensite tends to form in the welded joint 12. For the above reasons, it was determined that it is preferable to make TA ≥ TC. For example, by suppressing the spraying of coolant toward the column portion 122 of the welded joint 12, it becomes easier to satisfy the relationship TA ≥ TC.

[0119] (6) Reason why TA + TC ≧ 1340 is preferable Next, we will explain why, in the manufacturing method according to this embodiment, it is preferable to control the cooling stop temperature (TC) of the outer surface 1214 of the head corner side at the weld center A of the weld joint 12 and the cooling stop temperature (TA) of the head jaw outer surface 1212 so as to satisfy the relationship TA + TC ≧ 1340 during forced cooling from the austenite temperature range after completion of flash butt welding.

[0120] As shown in Figure 11, when the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214 is 1340°C or higher, the temperatures of the top corner side outer surface 1214 and the head jaw outer surface 1212 are ensured, and the cooling rate of the head jaw outer surface 1212 during the natural cooling process after cooling is stopped is reduced.As shown in Figure 8, the number of martensite structures formed in the martensite structure evaluation region C near the head jaw is 5 or less, further improving the breakage resistance of the welded joint 12.For this reason, it is preferable to satisfy TA + TC ≥ 1340.To stably control the number of martensite structures formed in the martensite structure evaluation region C to 5 or less, it is more desirable to control TA + TC to 1360°C or higher and 1500°C or lower.

[0121] (7) Reason why 0≦TA−TC≦140 is preferable Next, we will explain why, in the manufacturing method according to this embodiment, it is preferable to control the cooling stop temperature (TC) of the outer surface 1214 of the head corner side at the weld center A of the weld joint 12 and the cooling stop temperature (TA) of the head jaw outer surface 1212 so as to satisfy the relationship 0≦TA−TC≦140 during forced cooling from the austenite temperature range after completion of flash butt welding.

[0122] As shown in FIG. 12 , when the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214 is controlled to 140°C or less, the temperature difference between the top corner side outer surface 1214 and the head jaw outer surface 1212 is reduced, and the cooling rate of the head jaw outer surface 1212 during the cooling process in the head jaw where martensite structures are formed is slowed. As shown in FIG. 9 , the number of martensite structures formed in the martensite structure evaluation region C near the head jaw is 5 or less, further improving the breakage resistance of the welded joint 12. For this reason, it is preferable to set TA-TC≦140. To stably control the number of martensite structures formed in the martensite structure evaluation region C to 5 or less, it is even more desirable to control TA-TC to 20°C or more and 120°C or less. As will be described later, it is expected that there will be few cases where TA-TC is less than 0°C, so the lower limit of TA-TC is set to 0°C.

[0123] In addition, when the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214 is in the range of 160 to 240°C, as shown in Figure 13, even if the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214 becomes 1340°C or higher, the number of martensite structures formed will not be five or less.

[0124] Furthermore, when the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214 is in the range of 1250 to 1330°C, as shown in Figure 14, even if the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214 is 140°C or less, the number of martensite structures formed will not be five or less.

[0125] Therefore, in order to control the number of martensite structures generated to 5 or less, it is preferable to control both the sum and the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner side outer surface 1214. It is also preferable that both TA+TC≧1340 and 0≦TA−TC≦140 are satisfied.

[0126] (8) Reasons for limiting the temperature measurement positions on the outer surface 1214 of the head corner and the outer surface 1212 of the head jaw at the weld center A of the welded joint 12

[0127] The inventors considered that temperature control of the head jaw outer surface 1212, which is located in the vicinity thereof, is effective for controlling the generation of martensitic structure in the martensitic structure evaluation region C near the head jaw shown in Figure 5. Furthermore, as a result of examining the temperature control position that affects the generation of martensitic structure on the head jaw outer surface 1212, it became clear that there is a strong correlation between the temperature of the head corner side outer surface 1214 and the amount of martensitic structure generated, as shown in Figures 9, 11, and 12. Therefore, the inventors considered that temperature control of the head jaw outer surface 1212 and the head corner side outer surface 1214 is effective for controlling the generation of martensitic structure in the martensitic structure evaluation region C.

[0128] The manufacturing method for the welded rail 1 according to this embodiment is not particularly limited in any respects other than those described above. For example, the flash butt welding conditions are not particularly limited. Welding conditions suitable for rails having the above-mentioned chemical compositions can be appropriately adopted. The cooling means for the welded joint 12 is also not limited. Any cooling means can be appropriately adopted that can set the cooling stop temperature (TC) of the top corner-side outer surface 1214 at the weld center A and the cooling stop temperature (TA) of the head-jaw outer surface 1212 within the above-mentioned ranges.

[0129] Immediately after flash butt welding, the welded joint 12 retains the steel expelled from the weld during the upset process as a reinforcement. This reinforcement is removed immediately after welding is completed. However, as shown in Figures 16A and 16B, a reinforcement of several millimeters in thickness may remain on the surface of the head jaw after welding is completed, even after trimming (or bead cutting), which is the process of removing the reinforcement. The evaluation region (C) of the martensitic structure must be identified based on the head jaw surface of the base rail, regardless of this remaining reinforcement.

[0130] Furthermore, it is preferable that the metal structure in the martensite structure evaluation region C be pearlite. It has been confirmed that a pearlite structure is best for ensuring the wear resistance of not only this region but also the rail head. Therefore, for the head of the rail welded joint (the region from the head top surface to a depth of 1 / 3h), a pearlite structure is desirable in the portions other than the martensite structure specified above. For other portions, however, the portions other than the martensite structure specified above may have a metal structure other than pearlite, as long as the strength, ductility, and toughness required for the rail can be ensured. In the manufacturing method for a welded rail according to this embodiment, the rail's chemical composition and cooling stop temperature are within the above-mentioned ranges, so that the metal structure of the welded joint is mainly composed of pearlite.

[0131] 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 achieve the object of the present invention.

[0132] The inventors manufactured welded rails under various conditions and evaluated their breakage resistance. The test conditions were as follows:

[0133] ●Flash butt welding conditions (preheating flash method) Initial flashing time: 15 seconds Number of preheating times: 10 Late flashing time: 25 seconds Average late flashing speed: 1.0 mm / sec Late flashing speed just before upsetting (for 3 seconds): 2.0 mm / sec Upsetting load: 65 KN

[0134] ●Cooling conditions for welded joints after welding Location: Weld center (A) Cooling start time: 15 seconds after welding is completed Cooling method: Air Cooled locations: Top outer surface 1211, top corner outer surface 1214, head side outer surface 1213 Cooling after air cooling: Natural cooling (up to 50°C) ●Characteristics of flash butt welded joints HAZ width: 32 mm Hardness of weld center: 390 to 440 HV Hardness of softest part: 280 HV

[0135] Evaluation of Martensite Structure Evaluation Area (See Figure 5) This refers to the area in the longitudinal cross section of the welded joint, within a range of ±5 mm (10 mm wide) from the weld center (A), and extending 1 to 5 mm deep from the outer surface 1212 of the head jaw toward the outer surface of the corner of the head (martensite structure evaluation area: C). Reason for Selecting Evaluation Area This is because this is the location where rail breakage occurs starting from the martensite structure. Method for Revealing Martensite Structure After polishing the martensite structure evaluation area (C), it was etched with nital and observed under an optical microscope. Polishing Conditions: Buffing with 1 μm diamond paste Martensite Etching Conditions: Etching Solution: Alcohol + 5% Nitric Acid (Nital) Etching Time: 5 to 10 seconds Microstructure Investigation Method: Equipment: Optical microscope Magnification: 400x Microstructure Evaluation Method: The martensite structure visible under an optical microscope at 400x magnification was evaluated. The target martensite structure had a major axis of 25 to 100 μm, and when martensite structure was generated, the number of martensite structures was investigated.

[0136] Drop weight test conditions (see Figure 6) Posture: Welded rail supported at two points with the head on the bottom and the bottom on the top, and a drop weight was dropped onto the rail bottom. Span length (distance between two support points): 1000 mm Drop weight: 1000 kgf (9.8 kN) Drop height (X): 5.0, 7.5 m Drop energy: 49.0, 73.5 kN m

[0137] The chemical compositions of the rails subjected to flash butt welding are shown in Tables 1A, 1B, 2A, 2B, 3A, and 3B. The stop cooling temperatures (TC) of the outer surface of the top corners and the stop cooling temperatures (TA) of the outer surface of the head jaws are shown in Tables 4A and 4B.

[0138] For reference, Tables 4A and 4B also show the calculated results of TA+TC and TA-TC.

[0139] In addition, Tables 4A and 4B also show the number of martensite sites in the martensite structure evaluation region C of the welded rail, and the evaluation results of the breakage resistance of the welded rail. The evaluation criteria were as follows: Number of martensite sites in the martensite structure evaluation region C 5 or less: A More than 5 and 10 or less: B More than 10: C Evaluation of breakage resistance No breakage at a falling weight height of 7.5 m: A Breakage at a falling weight height of 7.5 m, but no breakage at a falling weight height of 5.0 m: B Breakage at a falling weight height of 5.0 m: C Welded rails that were evaluated for breakage resistance as "B" or "A" were determined to have excellent breakage resistance.

[0140]

[0141]

[0142]

[0143]

[0144]

[0145]

[0146]

[0147]

[0148] The welded rails obtained by the manufacturing methods of Examples 31 and 32 lacked breakage resistance. The number of martensite particles in these welded rails was excessive. This is thought to be because the cooling stop temperature (TA) of the outer surface of the head and jaw portion at the weld center of the welded joint was too low.

[0149] The welded rails obtained by the manufacturing method of Example 37 had insufficient breakage resistance. The number of martensite particles in these welded rails was excessive. This is thought to be because the cooling stop temperature (TC) of the outer surface of the top corner of the welded joint at the weld center was too low.

[0150] The welded rail obtained by the manufacturing method of Example 38 lacked breakage resistance. The number of martensite particles in this welded rail was excessive. This is thought to be because both TA and TC were too low.

[0151] On the other hand, all of the welded rails obtained by the manufacturing method in which the chemical composition, TA, and TC of the rail were all appropriate were excellent in breakage resistance.

[0152] 1 Flash butt welded rail (welded rail) 11 Rail part 111 Rail head 1111 Rail head top outer surface 1112 Rail head jaw outer surface 1113 Rail head side outer surface 1114 Rail head corner side outer surface 112 Rail column part 113 Rail bottom part 12 Welded joint part 121 (of welded joint part) head 1211 (of welded joint part) head top outer surface 1212 (of welded joint part) head jaw outer surface 1213 (of welded joint part) head side outer surface 1214 (of welded joint part) head corner side outer surface 122 (of welded joint part) column part 123 (of welded joint part) bottom part 12H Heat affected zone (HAZ) A Weld center B Width direction center C Martensite structure evaluation area 2 Cooling device 21 Coolant injection means 22 Rail installation section 23 Coolant supply means 24 Control means

Claims

1. A manufacturing method for a flash butt welded rail having a plurality of rail portions and a weld joint portion joining the plurality of rail portions, the chemical components being, in mass %, C: 0.70-1.20%, Si: 0.05-2.00%, Mn: 0.05-2.00%, P≦0.0300%, S≦0.0300%, Cr: 0-2.00%, Mo: 0-0.50%, Co: 0-1.00%, B: 0-0.0050%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0. 200%, 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: 0-1.0000%, with the balance being Fe and impurities; and immediately after completion of the flash butt welding, forcibly cooling the welded joint, wherein during the forced cooling from an austenite temperature range, a cooling stop temperature (TC) of an outer surface of a head corner side at a weld center of the welded joint is set to 560°C or higher and 850°C or lower, and a cooling stop temperature (TA) of an outer surface of a head jaw portion is set to 650°C or higher and 950°C or lower.

2. The manufacturing method of flash butt welded rails according to claim 1, characterized in that the cooling stop temperature (TC) of the outer surface of the corner side of the top portion and the cooling stop temperature (TA) of the outer surface of the head jaw portion satisfy the following formulas 1 and 2. TA + TC ≥ 1340 Formula 1 0 ≤ TA - TC ≤ 140 Formula 2 3. The rail has the following chemical components, in mass %, as follows: group a: Cr: 0.05% or more and 2.00% or less, Mo: 0.01% or more and 0.50% or less; group b: Co: 0.01% or more and 1.00% or less; group c: B: 0.0001% or more and 0.0050% or less; group d: Cu: 0.01% or more and 1.00% or less, and Ni: 0.01% or more and 1.00% or less; group e: V: 0.005% or more and 0.200% or less, Nb: 0.0010% or more and 0.0500% or less, and Ti: 0.0010% or more and 0.0500% or less; 3. A method for manufacturing a flash butt welded rail as claimed in claim 1 or 2, characterized in that it contains one or more of the following: group f: one or more of Mg: 0.0005% or more and 0.0200% or less, Ca: 0.0005% or more and 0.0200% or less, and REM: 0.0005% or more and 0.0500% or less; group g: N: 0.0025% or more and 0.0200% or less; group h: Zr: 0.0001% or more and 0.0200% or less; group i: Al: 0.0010% or more and 1.0000% or less.

4. A method for manufacturing flash-butt welded rails as described in claim 1 or 2, characterized in that the forced cooling is air cooling.

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