Manufacturing method for flash butt welded rails

Abstract

A manufacturing method for a flash-butt welded rail according to one embodiment of the present invention is a manufacturing method for a flash-butt welded rail having a plurality of rail portions and a welded joint portion joining the plurality of rail portions, and includes a process of flash-butt welding the rails and a process of force-cooling the welded joint portion immediately after completion of the flash-butt welding, and in the forced cooling from the austenite temperature range, the cooling stop temperature (TC) of the outer surface of the head corner side at the weld center of the welded joint portion is 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 650°C or higher and 950°C or lower.

Description

[Technical field]

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

[0002] Flash butt welding is a widely used method for welding rails. It is known that flash butt welding has the advantages of being able to be automated, having high quality stability, and a short welding time.

[0003] Flash butt welding is a technique in which rails are joined together by heating the end faces of the rails to melt them, then pressing the molten surfaces together. During flash butt welding, the rails are heated from room temperature up to near their melting point, and then cooled. As a result, flash butt welding changes the metal structure and hardness of the rails. The area where the metallurgical and mechanical properties have changed 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 occurs when the metal structure is heated to above the A1 point during welding, and partial austenitization of the metal structure and decomposition of the pearlite structure occurs when the metal structure is heated to near the A1 point. In the part that has been austenitized during flash butt welding, there has been a problem in that martensite structure, which is harmful to toughness, is formed due to forced cooling such as air cooling that is performed immediately after welding to ensure the hardness of the weld.

[0005] There was a problem that the martensite structure is likely to become the starting point of fracture, and when the martensite structure is formed, the rail is likely to break. This martensite structure has the characteristic that it is likely to form at the head of the welded joint of the rail, where the cooling rate is relatively fast. The head is a part that requires wear resistance and needs to be cooled faster than other parts to suppress softening, but it is also characterized by the fact that the martensite structure is likely to form due to supercooling.

[0006] Regarding cooling of welded joints obtained by flash butt welding of rails, etc., 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 column, base and head are cooled in 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 the residual stress in the rail welds and improve the fatigue resistance of the rail welded joints.

[0008] In addition, in the welded joints of flash-butt welded rails, the following techniques have been proposed to control the welding conditions and the cooling conditions of the head and to improve the life of the welded joints.

[0009] Patent Document 2 describes a welding method and a cooling method that are 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 cooling starts.

[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 welding a rail, one or both of the head and bottom parts of a rail are heated to a temperature range of 800 to 900°C in a two-phase state in which austenite and cementite phases are mixed, are accelerated cooled 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 parts of the steel rail reaches a temperature of 680 to 550°C, and then the joint is allowed to cool naturally or slowly so as not to exceed 680°C, thereby suppressing the formation of pro-eutectoid cementite structures and improving the toughness of the rail welded joint. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] International Publication No. 2010 / 116680 [Patent Document 2] JP 2010-100937 A [Patent Document 3] JP 2004-043862 A Summary of the Invention [Problem to be solved by the invention]

[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, the 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 of the jaws of the head, where martensite structures harmful to toughness are formed. In the technology, the cooling rate is controlled at a position 20 mm away from the weld center in the rail axial direction. This position is outside the general HAZ (within 20 mm) where martensite is formed. Therefore, the temperature control by the technology does not control the formation of martensite structures in the welded 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, the technology does not have the effect of 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. In addition, the technology described in Patent Document 3 mainly controls the cooling rate of the rail head and bottom, and does not control the formation of structures in the jaw part of the head where martensite structures that are harmful to toughness form.

[0016] The present invention has been devised in view of the above-mentioned problems, and aims to improve the breakage resistance of the welded joint of a flash-butt welded rail. Preferably, the object of the present invention is to provide a manufacturing method that can satisfy the extremely strict breakage resistance requirement of the welded joint of a flash-butt welded rail for a freight railway, which has a severe track environment. [Means for solving the problem]

[0017] The gist of the present invention is as follows. (1) A method for producing a flash butt welded rail according to one embodiment of the present invention is a method for producing a flash butt welded rail having a plurality of rail portions and a weld joint portion joining the plurality of rail portions, the method comprising the steps of: (a) forming a welded joint portion by joining the rail portions together; and (b) forming a welded joint portion by joining the rail portions together. The welded joint portion is formed by joining the rail portions together. The welded joint portion is formed by joining the rail portions together. The welded joint portion is formed by joining the rail portions together. The welded joint portion is formed by joining the rail portions together. The welded joint portion is formed by joining the rail portions together. The welded joint portion is formed by joining the rail portions together. The welded joint portion is formed by joining the rail portions together. The method includes the steps of: flash butt welding a rail containing Fe: 0 to 0.0200%, Ca: 0 to 0.0200%, REM: 0 to 0.0500%, N: 0 to 0.0200%, Zr: 0 to 0.0200%, and Al: 0 to 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 the austenite temperature range, the cooling stop temperature (TC) of the outer surface of the head corner side at the weld center of the welded joint is 560°C or higher and 850°C or lower, and the cooling stop temperature (TA) of the outer surface of the head jaw is 650°C or higher and 950°C or lower. (2) Preferably, in the manufacturing method of a flash butt welded rail described above in (1), 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 equations 1 and 2. TA+TC≧1340 1 set 0≦TA-TC≦140 2 sets (3) In the manufacturing method of a flash butt welded rail described in (1) or (2) above, preferably, the rail has, as the chemical components, in unit mass%, one or two of the following: 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 Group f: one or more of Mg: 0.0005% to 0.0200%, Ca: 0.0005% to 0.0200%, and REM: 0.0005% to 0.0500%; Group g: one or more of N: 0.0025% to 0.0200%, Group h: Zr: 0.0001% to 0.0200%, and Group i: Al: 0.0010% to 1.0000%. (4) Preferably, in the method for manufacturing a flash-butt welded rail according to any one of (1) to (3) above, the forced cooling is air cooling. Effect of the Invention

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

[0019] [Figure 1] Side view of welded rail [Diagram 2] Sectional view perpendicular to the longitudinal direction of a welded rail [Diagram 3] FIG. 1 is a perspective view illustrating the position of a longitudinal cross section of a welded joint of a welded rail; [Figure 4] Schematic diagram of cross-sectional hardness distribution 5 mm below the outer surface of the head and jaw of the longitudinal cross section of the welded joint of a welded rail [Diagram 5] Schematic diagram of martensite structure evaluation area in a longitudinal cross section of a welded joint of a welded rail [Figure 6] Schematic diagram of drop weight test [Figure 7] Relationship between number of martensite structures formed in the head jaw and rail breakage (when the falling weight height is 5.0 m and the falling weight energy is 49.0 kN m) [Figure 8] Relationship between number of martensite structures formed in the head jaw and rail breakage (when the falling weight height is 7.0 m and the falling weight energy is 73.5 kN m) [Figure 9] Relationship between the cooling end temperature (TC) of the outer surface of the head corner and the number of martensite structures formed in the head jaw [Figure 10] Relationship between the cooling end temperature (TA) of the outer surface of the head jaw and the number of martensite structures formed in the head jaw [Figure 11] Relationship between the sum of the cooling end temperatures of the top corner side and the head jaw (TA+TC) and the number of martensite structures formed in the head jaw [Figure 12] Relationship between the difference in cooling end temperature between the top corner side and the head jaw (TA-TC) and the number of martensite structures formed in the head jaw [Figure 13] Relationship between the sum of the cooling end temperatures of the top corner side and the head jaw (TA+TC) and the number of martensite structures formed in the head jaw (when TA-TC=145~250) [Figure 14] Relationship between the difference in cooling end temperature between the top corner side and the head jaw (TA-TC) and the number of martensite structures formed in the head jaw (when TA+TC=1250~1330) [Figure 15] An example of an Fe-Fe3C equilibrium phase diagram for determining the austenite region temperature (quoted from "Iron and Steel Materials," edited by the Japan Institute of Metals). [Figure 16A] Appearance of the welded joint of a welded rail after trimming (bead cutting) [Figure 16B] Appearance of the welded joint of the welded rail after trimming [Figure 17] FIG. 1 is a schematic diagram of an example of a cooling device suitable for implementing a method for manufacturing a welded rail. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] In order to solve the problem, the inventors investigated the cause of breakage of the welded joint and studied how to prevent breakage caused by brittle fracture originating from the jaw of the head of the welded joint. As a result, it was confirmed that breakage often originates from a brittle crack originating from the jaw of the rail head, and further that a martensite structure is formed at the origin of the brittle crack. Then, as a result of analyzing the relationship between this martensite structure and the cooling conditions of the welded joint, the inventors clarified that there is a correlation between the cooling conditions and the formation of the martensite structure. And, it was confirmed that the formation of the martensite structure is suppressed by controlling the cooling conditions of the welded joint. And, the inventors found a manufacturing method of a rail that stably improves the usage performance of the welded joint of a welded rail used under a severe usage environment.

[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 a plurality of rail portions 11 and a welded joint portion 12 that joins the rail portions 11, and includes a step of flash-butt welding a rail containing, as chemical components, in mass %, C: 0.70-1.20%, Si: 0.05-2.00%, Mn: 0.05-2.00%, P≦0.0300%, S≦0.0300%, with the balance being Fe and impurities, and a step of forcibly cooling the welded joint portion 12 immediately after the completion of the flash-butt welding, in which the cooling stop temperature (TC) of the outer surface 1214 of the head corner side at the weld center A of the welded joint portion 12 is 560°C or higher, and the cooling stop temperature (TA) of the outer surface 1212 of the head jaw portion is 650°C or higher during the forcible cooling from the austenite temperature range. The method for manufacturing a flash-butt welded rail according to this embodiment will be 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 welded joint portion 12 that joins these rail portions 11. The present inventors have conducted extensive research into methods for improving the breakage resistance of the welded joint portion 12. The present inventors have found that the breakage resistance of the welded joint portion 12 can be improved by controlling the cooling stop temperature of the outer surface 1214 of the top corner side at the weld center A of the welded joint portion 12 and the cooling stop temperature of the outer surface 1212 of the head jaw portion of the welded joint portion 12 to a certain temperature or higher.

[0023] The inventors optimized the heat treatment conditions after completion of welding to reduce the amount of martensite structure formed in the martensite structure evaluation region C of the head 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 greatly increase its service life.

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

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

[0027] The rail head portion 111 of the rail portion 11 refers to the portion above the narrowed portion in the vertical center of the rail portion 11 in the cross section perpendicular to the longitudinal direction of the rail portion 11 shown in Fig. 2. The rail column portion 112 refers to the narrowed portion in the vertical center of the rail portion 11 in the cross section of the rail portion 11 shown in Fig. 2. The rail bottom portion 113 refers to the portion below the narrowed portion in the vertical center of the rail portion 11 in the cross section of the rail portion 11 shown in Fig. 2.

[0028] In addition, in the rail head 111, the upper outer surface is referred to as the rail top surface or rail top outer surface 1111, the outer surface on the upper corner side is referred to as the rail top corner side outer surface 1114, and the outer surface on the upper side 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] 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 as shown in Fig. 2. 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 as shown in Fig. 2. Note that the up-down direction of the welded rail 1 naturally means the up-down 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 width of the rail head 111 and running in the up-down direction of the rail part 11, and a dashed line 0.35W away from the center of the width of the rail head 111 and running in the up-down direction of the rail part 11. The area of ​​the rail top surface located between these dashed lines is the rail top corner side outer surface 1114. Moreover, the area of ​​the surface of the rail head jaw part 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 a rail that is the material of 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. Thus, like the rail portion 11, the welded joint 12 also has a head 121, a column portion 122, and a bottom portion 123. The head 121 of the welded joint 12 has a top outer surface 1211, a top corner side outer surface 1214, a head side outer surface 1213, and a head jaw outer surface 1212. Hereinafter, the name of the head in the rail portion 11 will be referred to as the "rail head portion 111", and the name of the head in the welded joint 12 will be simply referred to as the "head portion 121". Regarding other parts, when they are included in the rail portion 11, the term "rail" will be used, and when they are included in the welded joint 12, the term "rail" will not be used.

[0033] Heat-affected zone (HAZ) 12H is JIS Z As defined in 3001-1:2018, the term refers to a non-melted base material portion 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] As shown in Fig. 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, that is, 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." In addition, Fig. 3 shows a broken line 0.25W away from the center of the width of the rail head 111 and extending in the vertical direction of the rail portion 11, and a broken line 0.35W away from the center of the width of the rail head 111 and extending in the vertical direction of the rail portion 11. The longitudinal cross section is a cut surface along the vertical direction of the rail, formed at any point between these broken lines. Below, an overview of the hardness distribution of the welded joint 12 will be explained, and then the definition of the HAZ width will be explained.

[0035] FIG. 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 the Vickers hardness along the head jaw outer surface 1212 at a depth of 5 mm from the head jaw outer surface 1212 of the welded joint 12 toward the top corner outer surface 1214 in the longitudinal cross section of the welded joint 12. The weld center A in this graph means a straight line along the vertical direction of the welded rail that passes through the center of the heat-affected zone 12H in the longitudinal cross section of the welded joint 12. Usually, the weld center A roughly coincides with the rail joint.

[0036] In the welded joint 12, a region is formed in which the welded joint is heated to the A1 point or higher by the welding heat, and is generally austenitized, and then transformed into pearlite by the cooling after the completion of welding. In addition, on both sides of this region, there are regions in which the welded joint is heated to the vicinity of the A1 point by the welding heat, and is partially austenitized, and then the pearlite structure is decomposed by the cooling after the completion of welding. In these regions, the hardness is significantly reduced. Therefore, in a graph of the hardness distribution of a welded rail 1 obtained by flash butt welding, there are usually two Vickers hardness valleys as shown in FIG. 4. The locations where these Vickers hardness valleys occur are defined as the most softened parts of the welded rail 1 according to this embodiment. The distance between the two most softened parts is defined as the HAZ width. The weld center A almost coincides with the center of this HAZ width.

[0037] 5, the martensite structure evaluation region C means a region (C) in the longitudinal cross section, which is within a range of ±5 mm (width 10 mm) in the longitudinal direction of the welded rail 1 centered on the weld center A, 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 martensite structure evaluation region C will be described later.

[0038] The martensite structure evaluation region (C) is a region included in a longitudinal 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 toward the head side outer surface 1213. There are two longitudinal cross sections on the left and right of the center of the width of the head. Either of these two cross sections may be set. In addition, this longitudinal cross section is a cross section that passes 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 martensite structure evaluation region C according to the above definition, the lower end of the cross section may be regarded as the head jaw outer surface 1212.

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

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

[0041] First, the inventors identified the initiation point of breakage in welded rails, and confirmed that in many cases, breakage occurred in the heat-affected zone (HAZ).

[0042] Next, the location of breakage was identified in detail. As a result, 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, first, the relationship between the state of martensite structure formation in this area and breakage of rail welded joints was investigated in detail. A flash butt welding test was performed using hypereutectoid steel rails (0.80-1.20%C), and the rail drop weight test shown in Figure 6 was performed to evaluate the relationship between the amount of martensite structure formed and the presence or absence of rail breakage. The amount of martensite structure formed was mainly achieved by controlling the cooling stop temperature of the head jaw outer surface and the outer surface of the top corner side in an area (width 10 mm) ±5 mm from the weld center (A) in the welded joint where martensite structure has formed.

[0044] The rail, flash butt welding conditions, cooling conditions for the welded joint 12 after welding, characteristics of the welded joint 12, evaluation method for the martensite structure, and drop weight test conditions are as shown below.

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

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

[0047] Cooling conditions for the welded joint 12 after welding Part: Weld center (A) Cooling start time: 15 seconds after welding is completed Cooling stop temperature (TA) of head jaw outer surface 1212: 500 to 800°C Cooling stop temperature (TC) of the outer surface 1214 of the top corner: 550 to 900°C Subsequent cooling (outer surface of head jaw, outer surface of top corner): Cool naturally (until 50°C) In this embodiment, the statement "immediately after completion of flash butt welding, the welded joint is forcedly cooled" means that forced cooling is started within 5 seconds to 30 seconds after completion of flash butt welding. The time point at which flash butt welding is completed refers to the time point at which trimming is completed. In trimming, burrs formed on the welded joint in the upset process of flash butt welding are removed. Forced cooling that is started under the above conditions is considered to be forced cooling that is started immediately after completion of flash butt welding.

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

[0049] ●Evaluation of martensite structure Evaluation area (see Figure 5) In a longitudinal cross section of the welded joint, this refers to a region within a range of ±5 mm (width 10 mm) from the weld center (A) in the longitudinal direction of the welded rail, and a region within a range of a depth of 1 to 5 mm from the head jaw outer surface 1212 to the outer surface of the top corner side (martensite structure evaluation region: C). Reasons for selecting evaluation areas This is because this is the location where the starting point of rail breakage occurs. Method for revealing martensite structure The martensite structure evaluation area (C) is polished, etched with nital, and observed under an optical microscope. Polishing conditions: Buff polishing with 1 μm diamond paste Martensite Etch Conditions Etching solution: Alcohol + 5% nitric acid (Nital) Etching time: 5 to 10 seconds How to research an organization Equipment: Optical microscope Magnification: 400x How to evaluate your organization The martensite structure that could be confirmed under an optical microscope at 400x magnification was used for evaluation. 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) Position: The welded rail is supported at two points with the head on the bottom and the bottom on the top, and a drop weight is dropped onto the bottom of the rail. Span length (distance between two support points): 1000mm Falling weight: 1000kgf (9.8kN) Drop height (X): 5.0, 7.5m Falling weight energy: 49.0, 73.5kN·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 head jaw outer surface 1212 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 martensite structures formed in the martensite structure evaluation region C near the head jaw outer surface 1212 is 5 or less, as shown in Fig. 8. In this case, it is considered that breakage of the welded rail can be effectively suppressed even in a 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 a rail drop weight test as shown in Figure 6 was performed to evaluate the relationship between the amount of martensite structure generated in martensite structure evaluation region C and the presence or absence of rail breakage. The amount of martensite structure generated was controlled mainly by controlling the cooling stop temperature of the head jaw outer surface 1212 and the head corner side outer surface 1214 in a region (width 10 mm) ±5 mm from the weld center (A) in the weld joint 12 where the martensite structure was generated. The rail, flash butt welding conditions, characteristics of the weld joint 12, and the method of evaluating the martensite structure are as described above.

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

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

[0057] As a result, as shown in FIG. 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 FIG. 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 martensite structures formed in the martensite 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 top corner side outer surface 1214, the amount of martensite structure generated in the martensite structure evaluation region C can be controlled to 10 or less, thereby preventing breakage of the rail welded joint.

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

[0061] The inventors considered that, in order to reduce the amount of martensite structure generated, 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, an experiment was carried out 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 hypereutectoid steel rails (0.70 to 1.20% C). 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 condition 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 of evaluating the martensite structure are as described above.

[0064] ● Cooling conditions for welded joints after welding In the case of 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 head corner outer surface 1214 (Figure 11) TA-TC=50 (fixed) TA: 650~800℃, TC: 600~750℃ TA≧TC In the case of 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 head corner outer surface 1214 (FIG. 12) TA+TC=1400 (fixed) TA: 700~800℃, TC: 600~700℃ TA≧TC Subsequent cooling (outer surface of head jaw, outer surface of top corner): Cool naturally (until 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 martensite structures formed in the martensite 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 martensite structures formed in the martensite 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 top corner side outer surface 1214, it is possible to control the amount of martensite structure generated to 5 or less, and further improve the breakage resistance of the rail welded 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 martensite structures formed in the martensite structure evaluation area 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 becomes 140°C or less, the number of martensite structures formed in the martensite structure evaluation area C will not be 5 or less.

[0070] Therefore, in order to control the number of martensite structures formed 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 is possible 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 in a flash butt welded rail 1 and the cooling stop temperature (TA) of the outer surface 1212 of the head jaw, and to suppress the amount of martensite structure generated in the martensite structure evaluation region C of the longitudinal cross section, thereby further suppressing breakage caused by brittle cracks originating from the head 121 of the welded rail 1 and significantly improving the service life of the welded rail 1.

[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 abbreviated as "%".

[0073] (1) Reasons for limiting the chemical composition of rails The manufacturing method of the welded rail 1 according to this embodiment includes a step of flash butt welding the rails that are 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 the alloying elements explained for the chemical composition of the rail also apply to the chemical composition of the rail portion 11.

[0074] (C:0.70~1.20%) C is an element effective for 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 for 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 generated in the welded joint 12, and the breakage resistance of the welded joint 12 decreases. 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. In order to stabilize the generation of pearlite structure, it is preferable that the C content is 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, increases the hardness of the welded joint 12, and improves the 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 preferable that the Si content is 0.40 to 2.00%.

[0076] (Mn: 0.05-2.00%) Mn is an element that improves the hardenability of the welded rail 1, stabilizes the pearlite transformation, refines the lamellar spacing of the pearlite structure, ensures the hardness of the welded joint 12, and further improves the wear resistance. However, if the Mn content is less than 0.05%, the 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 amount of Mn promotes Mn concentration in the segregation portion, promotes the formation of the martensite structure of the welded joint 12, and reduces the 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 fracture resistance of the welded joint 12 decreases due to embrittlement of the pearlite structure. 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 limited, and may be, for example, 0%, but considering the dephosphorization ability of the refining process, the lower limit of the P content may be about 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. In order to stably maintain the toughness of the pearlite structure, it is desirable to set the P content to 0.0050 to 0.0150%.

[0078] (S≦0.0300%) S is an impurity element contained in steel. When the S content exceeds 0.0300%, stress concentration is generated around the inclusions of coarse MnS-based sulfides, and the fracture resistance of the welded joint portion 12 decreases. For this reason, the S content is limited to 0.0300% or less. Note that the lower limit of the S content does not need to be limited and may be, for example, 0%, but considering the desulfurization ability of the refining process, the lower limit value of the S content may be set to about 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 addition, in order to stably maintain the fracture resistance of the pearlite structure, it is desirable that the S content be 0.0050 to 0.0150%.

[0079] The balance of the chemical components of the rail, which is the material of the welded rail, contains iron and impurities. Impurities are components that are mixed in, for example, by raw materials such as ore or scrap when industrially manufacturing steel materials, or by various factors in the manufacturing process, and are those that are allowed within a range that does not adversely affect the welded rail 1 according to the present embodiment.

[0080] Furthermore, in the rail, which is the material of the welded rail, for the purpose of improving wear resistance due to an increase in the hardness of the welded joint portion 12, improving toughness, preventing softening of the heat-affected zone 12H, and controlling the cross-sectional hardness distribution inside the head, one or two of the elements of group a of Cr and Mo, group b of Co, group c of B, group d of Cu and Ni, one or more of the elements of group e of V, Nb, and Ti, one or more of the elements of group f of Mg, Ca, and REM, group g of N, group h of Zr, and group i of Al may be contained as necessary in one or more groups. However, even if these elements are not contained in the rail, the manufacturing method of the welded rail 1 according to the present embodiment can exhibit its effects, so the lower limit value of the content of these elements is 0%.

[0081] <Group a>

[0082] (Cr: Preferably 2.00% or less) Cr is an element that raises the equilibrium transformation temperature, refines the interlamellar spacing of the pearlite structure with an increase in supercooling degree, improves the hardness of the pearlite structure, and improves the wear resistance of the welded joint 12. In order to fully obtain these effects, it is preferable that the Cr content is 0.03% or more, or 0.05% or more. On the other hand, when the Cr content exceeds 2.00%, an excessive amount of Cr promotes the Cr enrichment in the segregation part, promotes the formation of the martensite structure in the welded joint, and may reduce the fracture resistance. For this reason, it is desirable that the Cr content is 0.05 - 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 the pearlite structure and improve the wear resistance and damage resistance of the welded joint 12, it is desirable that the Cr content is 0.10 - 1.20%.

[0083] (Mo: Preferably 0.50% or less) Mo is an element that raises the equilibrium transformation temperature, refines the interlamellar spacing of the pearlite structure with an increase in supercooling degree, improves the hardness of the pearlite structure, and improves the wear resistance of the welded joint 12. In order to obtain the above effects, it is preferable that the Mo content is 0.01% or more. On the other hand, when the Mo content exceeds 0.50%, an excessive amount of Mo promotes the Mo enrichment in the segregation part, promotes the formation of the martensite structure in the welded joint 12, and may reduce the fracture resistance. For this reason, it is desirable that the Mo content is 0.01 - 0.50%. The Mo content is preferably 0.02% or more, 0.05% or more, or 0.10% or more. The Mo content is preferably 0.45% or less, 0.40% or less, or 0.30% or less. 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 is 0.10 - 0.30%.

[0084] <Group b> (Co: Preferably 1.00% or less) Co dissolves in the ferrite phase of the pearlite structure, refines the lamellar structure of the pearlite structure directly under the rolling surface where deformation occurs due to contact with the wheel, improves the hardness of the rolling surface, and is an element that improves the wear resistance of the welded joint 12. In order to obtain the above effects, it is preferable that the Co content is 0.01% or more. On the other hand, when the Co content exceeds 1.00%, the above effects are saturated, and the refinement of the lamellar structure according to the Co content cannot be achieved. Also, when the Co content exceeds 1.00%, the economic efficiency may decrease due to an increase in alloy cost. For this reason, it is desirable that the Co content is 0.01 to 1.00%. The Co content is preferably 0.02% or more, 0.05% or more, or 0.10% or more. The Co content is preferably 0.90% or less, 0.80% or less, or 0.60% or less. 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 that the Co content is 0.10 to 0.60%.

[0085] <c group> (B: Preferably 0.0050% or less) B forms iron carbide boride (Fe 23 (CB)6) at the austenite grain boundary, reduces the cooling rate dependence of the pearlite transformation temperature due to the promoting effect of pearlite transformation, equalizes the hardness distribution from the head surface to the inside of the welded joint 12, and is an element that extends the life of the welded joint 12 by improving wear resistance. In order to obtain the above effects, it is preferable that the B content is 0.0001% or more. On the other hand, when the B content exceeds 0.0050%, coarse iron carbide boride is generated, promoting brittle fracture, and the fracture resistance of the welded joint 12 may decrease. For this reason, it is desirable that the B content is 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, it is desirable that the B content is 0.0005 to 0.0025%.

[0086] <d group> (Cu: preferably 1.00% or less) Cu is an element that dissolves in the ferrite phase of the pearlite structure, and improves the hardness of the welded joint 12 by solid solution strengthening, thereby improving the wear resistance of the welded joint 12. In order to obtain the above-mentioned effects, it is preferable that the Cu content is 0.01% or more. On the other hand, if the Cu content exceeds 1.00%, an excessive amount of Cu promotes Cu concentration in the segregation portion, promotes the formation of the martensite structure of the welded joint 12, and may reduce the breakage resistance. For this reason, it is preferable that the Cu content is 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 preferable that the Cu content is 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 precipitate as a fine Ni3Ti intermetallic compound, and is an element that suppresses the softening of the welded joint 12 by precipitation strengthening. Furthermore, when Cu is contained in the rail, Ni suppresses the embrittlement of the grain boundary. In order to obtain the above-mentioned effect, it is preferable that the Ni content is 0.01% or more. If the Ni content exceeds 1.00%, an excessive amount of Ni promotes Ni concentration in the segregation part, promotes the formation of the martensite structure of the welded joint 12, and may decrease the breakage resistance. For this reason, it is preferable that the Ni content is 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, in order to stably maintain the toughness of the pearlite structure and improve the wear resistance of the welded joint 12, the Ni content is preferably set to 0.10 to 0.70%.

[0088] <e group> (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 by precipitation hardening due to the carbide and nitride of V generated during the cooling process after hot rolling. To obtain the above effects, it is preferable to set the V content to 0.005% or more. On the other hand, when the V content exceeds 0.200%, the number of fine V carbides and nitrides becomes excessive, the pearlite structure becomes brittle, and the fracture resistance of the welded joint 12 may decrease. For this reason, it is desirable to set the V content to 0.005 - 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 fracture 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 0.020 - 0.100%.

[0089] (Nb: Preferably 0.0500% or less) Nb is an element that increases the hardness of the pearlite structure by precipitation hardening due to Nb carbides and Nb nitrides generated during the cooling process after hot rolling in the manufacture of rails, and improves the fatigue damage resistance of the welded joint 12. In addition, in the heat-affected zone 12H reheated to a temperature range below the Ac1 point, Nb stably generates Nb carbides and Nb nitrides in 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. In order to obtain the above-mentioned effect, it is preferable that the Nb content is 0.0010% or more. On the other hand, if the Nb content exceeds 0.0500%, the precipitation hardening of Nb carbides and nitrides becomes excessive, the pearlite structure itself becomes embrittled, and the fracture resistance of the welded joint 12 may decrease. For this reason, it is preferable that the Nb content is 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 and improves the fatigue damage resistance of the welded joint 12 through precipitation hardening by Ti carbides and Ti nitrides generated during the cooling process after hot rolling in the production of rails. Also, Ti is an element that utilizes the fact that Ti carbides and Ti nitrides precipitated during post-welding reheating do not dissolve in the matrix, refines the structure of the heat-affected zone 12H reheated to the austenite region, and improves the fracture resistance of the welded joint 12. To obtain the above effects, it is preferable to set the Ti content to 0.0010% or more, or 0.0060% or more. On the other hand, when the Ti content exceeds 0.0500%, coarse Ti carbides and Ti nitrides are generated, and fatigue cracks are likely to be generated due to stress concentration around them, and the fatigue damage resistance of the welded joint 12 may decrease. For this reason, it is desirable to set the Ti content to 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, to improve the fatigue damage resistance and fracture resistance of the welded joint 12, it is desirable to set the Ti content to 0.0060 to 0.0200%.

[0091] <f group> (Mg: preferably 0.0200% or less) Mg is an element that combines with S to form fine sulfides (MgS), which finely disperse MnS, reduce stress concentration around MnS, and improve the fatigue damage resistance of the welded joint 12. In order to obtain the above-mentioned effects, it is preferable that the Mg content is 0.0005% or more. On the other hand, if the Mg content exceeds 0.0200%, coarse oxides of Mg are generated, and fatigue cracks are likely to be generated due to stress concentration around the coarse oxides, and the fatigue damage resistance of the welded joint 12 may decrease. For this reason, it is preferable that the Mg content is 0.0005 to 0.0200%. The Mg content is preferably 0.0010% or more, 0.0015% or more, or 0.0030% or more. The Mg content is preferably 0.0180% or less, 0.0150% or less, or 0.0120% or less. Therefore, in order to improve the fatigue damage resistance of the welded joint 12, the Mg content is preferably set to 0.0030 to 0.0120%.

[0092] (Ca: preferably 0.0200% or less) Ca is an element that forms sulfides (CaS) because of its strong bonding force with S, and this CaS finely disperses MnS, relieves stress concentration around MnS, and improves fatigue damage resistance of the welded joint 12. In order to obtain the above-mentioned effect, it is preferable that the Ca content is 0.0005% or more. On the other hand, if the Ca content exceeds 0.0200%, coarse oxides of Ca are generated, and fatigue cracks are likely to be generated due to stress concentration around the coarse oxides, and the fatigue damage resistance of the welded joint 12 may decrease. For this reason, it is preferable that the Ca content is 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 set to 0.0030 to 0.0120%.

[0093] (REM: preferably 0.0500% or less) REM is a deoxidizing and desulfurizing element, which generates REM oxysulfide (REM2O2S) and serves as the nucleation site for the formation of Mn sulfide inclusions. Since the oxysulfide (REM2O2S) has a high melting point, it suppresses the elongation of Mn sulfide inclusions after rolling. As a result, REM finely disperses MnS, relieves the stress concentration around MnS, and improves the fatigue damage resistance of the welded joint 12. To obtain the above effects, it is preferable that the REM content is 0.0005% or more. On the other hand, when the REM content exceeds 0.0500%, coarse and hard REM oxysulfide (REM2O2S) is generated, and due to the stress concentration around this oxysulfide, fatigue cracks are likely to be generated, and the fatigue damage resistance of the welded joint 12 may decrease. Therefore, it is desirable that the REM content is 0.0005 - 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, to improve the fatigue damage resistance of the welded joint 12, it is desirable that the REM content is 0.0030 - 0.0250%.

[0094] Note that REM is a total of 17 elements consisting of Sc, Y, and La (lanthanoids). The "REM content" means the total value of the contents of all these REM elements. If the total content is within the above range, the same effects can be obtained whether the type of REM element is one or two or more.

[0095] <g group> (N: preferably 0.0200% or less) N is an element that can be mixed into the rail as an impurity in the steelmaking process. Even if degassing is actively carried out, about 0.0025% of N can remain in the steel. In normal rail refining, the N content is about 0.0030 to 0.0050%. It is possible to make the N content less than 0.0025%, but in order to avoid the increase in refining cost, the rail may contain 0.0025% or more of N. Also, N segregates at the austenite grain boundaries, promoting the pearlite transformation from the austenite grain boundaries. Mainly, by refining the pearlite block size, it is an element effective in improving the toughness of the welded joint 12. Also, when N and V are contained simultaneously, during the cooling process of the welded joint 12 after welding of the rail, the precipitation of V carbonitrides is promoted, increasing the hardness of the pearlite structure and improving the fatigue damage resistance of the welded joint 12. To obtain the above effects, it is preferable to make the N amount 0.0060% or more. On the other hand, when the N amount exceeds 0.0200%, it becomes difficult to dissolve N in the steel, and bubbles that are the starting points of fatigue damage may be easily generated. For this reason, it is desirable to make the N content 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, in order to stably improve the toughness and fatigue damage resistance, it is desirable to make the N content 0.0080 to 0.0150%.

[0096] <h group> (Zr: preferably 0.0200% or less) Zr forms ZrO₂ inclusions with good lattice compatibility with γ-Fe, serving as the solidification nuclei of high-carbon rail steel where γ-Fe is the primary solidifying crystal. By increasing the equiaxed crystal ratio of the solidification structure, the formation of the segregation band in the center of the slab is suppressed, and the alloy enrichment in the segregation part is inhibited. As a result, Zr suppresses the formation of the martensite structure in the welded joint part 12, improving the fracture resistance and fatigue damage resistance. To obtain the above effects, it is preferable that the Zr content is 0.0001% or more. On the other hand, when the Zr content exceeds 0.0200%, a large amount of coarse Zr-based inclusions are generated, and due to the stress concentration around these coarse inclusions, fatigue cracks are likely to occur, and the fatigue damage resistance of the welded joint part 12 may decrease. For this reason, it is desirable that the Zr content is 0.0001 - 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, to stably improve the fracture resistance and fatigue damage resistance, it is desirable that the Zr content is 0.0015 - 0.0120%.

[0097] (Al: preferably 1.0000% or less) Al is a component that functions as a deoxidizer. To obtain the above effects, it is preferable that the Al content is 0.0005% or more, or 0.0010% or more. On the other hand, when the Al content exceeds 1.0000%, coarse alumina-based inclusions are generated, and fatigue cracks are likely to occur from these coarse inclusions, and the fatigue damage resistance of the welded joint part 12 may decrease. Furthermore, when the Al content exceeds 1.0000%, oxides are generated during the welding of the rail, and the weldability of the rail may be significantly reduced. For this reason, it is desirable that the Al content is 0.0005 - 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, it is desirable that the Al content is 0.0005 - 0.3000%.

[0098] (2) Reason for limiting the cooling stop temperature (TC) of the outer surface 1214 on the top corner side at the weld center A of the weld joint 12 The manufacturing method of the welded rail 1 according to this embodiment includes a step of forcefully cooling the welded joint 12 obtained by flash butt welding the rails immediately after the flash butt welding is completed. The forceful cooling increases the hardness of the welded joint 12, improving the wear resistance of the welded joint 12. However, excessive forceful cooling will cause martensite to form in the welded joint 12. Therefore, the forced cooling conditions must be appropriately controlled. As described above, forced cooling means 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 to be 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 in 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 produce a welded joint and then the welded joint is natural cooling, the average cooling rate of the outer surface of the top corner side in the temperature range from 900°C to 600°C is usually within the range of about 1.0 to 2.0°C / sec. On the other hand, when a rail is flash butt welded to produce a welded joint and then the welded joint is air-cooled, the average cooling rate of the outer surface of the top corner side in the temperature range from 900°C to 600°C is usually within the range of about 1.5 to 5.0°C / sec. When the cross-sectional shape of the rail to be welded and the flash butt welding conditions are the same, 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 by the following formula, where t is defined as the time required for the temperature of the object to be cooled to drop from X°C to Y°C. V=(XY) / t The average cooling rate can be measured using a radiation thermometer, which is capable of measuring the temperature of the surface of an object in a non-contact manner. In addition, in this embodiment, the description "immediately after the completion of flash butt welding, the welded joint is forcedly cooled" means that forced cooling is started within 5 seconds to 30 seconds after the completion of flash butt welding. The time of the end of flash butt welding is the time of the end of trimming, as described above. After the completion of flash butt welding, it usually takes 5 seconds or more to install a cooling device on the welded joint to start cooling. In addition, if forced cooling is started more than 30 seconds after the completion of flash butt welding, pearlite transformation may start in a high temperature range before forced cooling, and the hardness of the welded joint may be impaired.

[0099] In the forced cooling of the welded joint 12, the cooling stop temperature (TC) of the outer surface 1214 on the apex corner side at the weld center A and the cooling stop temperature (TA) of the outer surface 1212 of the head jaw part at the weld center A are each controlled independently. Hereinafter, the reason for limiting the cooling stop temperature (TC) of the outer surface 1214 on the apex corner side at the weld center A of the welded joint 12 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.

[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 martensite structures generated in the martensite structure evaluation region C near the head jaw exceeds 10, and as shown in FIG. 7, the 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 be 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, for example, 850°C or lower, 800°C or lower, 750°C or lower, or 720°C or lower. In order to stably control the number of martensite structures generated in the martensite 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] Although there is no particular limitation on the cooling start temperature in the forced cooling from the austenite temperature range, there is no problem as long as it is within the austenite temperature range. The temperature of the outer surface 1214 on the corner side of the top portion immediately after the completion of flash butt welding of the rail is about 1200°C, and therefore the upper limit of the austenite temperature range in this embodiment is substantially 1200°C.

[0102] The austenite temperature range also differs depending on the carbon content and alloying components of the rail. To accurately determine the austenite temperature range, it is best to directly measure the transformation point by a reheating and cooling experiment or the like. However, since actual measurement is not necessarily easy, it can also be determined simply by reading it from an equilibrium phase diagram of the Fe-Fe3C system based only on the carbon content.

[0103] For example, Fig. 15 shows a phase diagram of the Fe-Fe3C system, 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. The austenite temperature range in the rail component system is the higher temperature region than the A3 and Acm lines in the parallel phase diagram. In the above-mentioned range of carbon content of rails, Ar3 is about 715°C to 750°C, and Arcm is about 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 and jaw outer surface 1212 at the weld center A of the weld joint 12 Next, the reason why the cooling stop temperature (TA) of the head jaw outer surface 1212 at the weld center A of the welded joint 12 in the forced cooling from the austenite temperature range after completion of flash butt welding in this embodiment is limited to 650°C or higher will be explained.

[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 generated in the martensite structure evaluation region C near the head jaw outer surface 1212 exceeds 10, and as shown in FIG. 7, the 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 be 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, for example, 950°C or lower, 900°C or lower, 850°C or lower, or 820°C or lower. In order to stably control the number of martensite structures generated 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 is stopped, martensite may be generated in the welded joint 12. The welded joint 12 is preferably left in the air until its temperature reaches room temperature. When the forced cooling is stopped at a temperature of 600°C or higher and then the welded joint 12 is allowed to cool naturally, the average cooling rate of the outer surface on the corner side of the top portion is usually about 0.5°C / sec 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) A desirable method for controlling the temperature of the outer surface of the head at the weld center A of the weld 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 in 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 spraying a coolant such as air onto the top outer casing surface 1211, the top corner side outer casing surface 1214, and the head side outer casing surface 1213 shown in Fig. 2 is used. The cooling stop temperature (TC) of the top corner side outer casing surface 1214 is adjusted mainly by controlling the amount of coolant sprayed onto the top outer casing surface 1211 and the top corner side outer casing surface 1214. The cooling stop temperature (TA) of the head jaw outer casing surface 1212 is adjusted mainly by controlling the amount of coolant sprayed onto the head side outer casing surface 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 in 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 in the above-mentioned procedure is not particularly limited. For example, a cooling device 2 as illustrated 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. Fig. 17 shows a state in which the welded joints 12 of the welded rail 1 are placed on the rail installation section. The cooling device 2 may 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 provided 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 so that its installation location can be changed. The coolant injection means 21 may be configured so that its coolant injection direction can be changed. This allows the cooling device 2 to be used for welded rails 1 of various shapes. It is preferable that the cooling device 2 does not inject the refrigerant toward the column 122 of the welded joint 12. A part of the refrigerant injected into the column 122 flows to the head jaw outer surface 1212 of the welded joint 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 lowered, or the relationship TA≧TC may not be satisfied. Therefore, it is preferable that the cooling device 2 does not include the refrigerant injection means 21 positioned to inject the refrigerant toward the column 122. When the cooling device 2 has the refrigerant injection means 21 positioned to inject the refrigerant toward the column 122, it is preferable to suppress the amount of refrigerant injected into the column 122 by using the control means 24 described later.

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

[0114] Optimization of the coolant injection amount 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 set the distance between the coolant injection means 21 and the welded joint 12 within a predetermined range and to set the coolant injection amount of each of the multiple coolant injection means 21 independently. Therefore, it is preferable that the control means 24 can independently control the injection amount of the coolant for each of the multiple coolant injection means 21. This makes it possible to preferably control the cooling stop temperature as described above. In addition, when the control means cannot control the coolant injection amount, it is necessary to preferably control the cooling stop temperature as described above by changing the arrangement of the coolant injection means 21 and adjusting the distance between the coolant injection hole and the outer surface.

[0115] When cooling the welded joint 12 using a cooling device 2 not equipped with a temperature measuring means, the method for measuring the cooling stop temperature and the average cooling rate is as follows. (A) After the 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 side and the outer surface of the head jaw of the welded joint 12. These temperatures are the cooling start temperature of the outer surface of the top corner side and the cooling start temperature of the outer surface of the head jaw. (B) The welded joint 12 is installed on a rail installation portion of the cooling device. Installation may be performed by moving the welded joint 12 while the cooling device is stationary. Alternatively, installation may be performed by moving the cooling device while the welded joint 12 is stationary. In either case, the time required to install 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-mentioned temperature measurement procedure is applied to a cooling device 2 that is not equipped with a temperature measuring means. On the other hand, the cooling device 2 may further include a temperature measuring means that is connected to the control means 24 and arranged facing the rail installation portion. The temperature measuring means is a radiation thermometer. The radiation thermometer can measure the surface temperature of an object. By using the temperature measuring means equipped to the cooling device 2, the cooling conditions of the welded joint 12 can be controlled with high accuracy according to the temperature of the welded joint 12. Note that, when the cooling device 2 is equipped with a temperature measuring means, it is preferable to start cooling 5 to 10 seconds after the completion of the measurement of the cooling start temperature. Also, it is preferable to measure the cooling stop temperature 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 portion will also be approximately constant, and therefore it is possible to cool the welded joint 12 under the same coolant injection conditions. Therefore, if the coolant injection conditions that can bring 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 of this embodiment, it is preferable to control the cooling stop temperature (TC) of the head corner side outer surface 1214 at the weld center A of the welded joint 12 and the cooling stop temperature (TA) of the head jaw outer surface 1212 during forced cooling from the austenite temperature range after completion of flash butt welding, so that the relationship TA≧TC is satisfied.

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

[0119] (6) Reason why preferably TA + TC ≥ 1340 Next, in the manufacturing method according to the present embodiment, when forcibly cooling from the austenite temperature range after the completion of the flash butt welding, the cooling stop temperature (TC) of the top corner side outer surface 1214 and the cooling stop temperature (TA) of the head jaw outer surface 1212 at the welding center A of the welded joint portion 12 will be described. The reason for preferably controlling so as to satisfy the relationship of TA + TC ≥ 1340 will be described.

[0120] As shown in FIG. 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 becomes 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 in the heat dissipation process after the cooling stop is alleviated. As shown in FIG. 8, the number of martensite structures generated in the martensite structure evaluation region C near the head jaw becomes 5 or less, and the anti-fracture property of the welded joint portion 12 can be further improved. For this reason, it is preferable to set TA + TC ≥ 1340. In order to stably control the number of martensite structures generated 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 preferably 0 ≤ TA - TC ≤ 140 Next, we will explain why, in the manufacturing method of this embodiment, it is preferable to control the cooling stop temperature (TC) of the head corner side outer surface 1214 at the weld center A of the welded joint 12 and the cooling stop temperature (TA) of the head jaw outer surface 1212 during forced cooling from the austenite temperature range after completion of flash butt welding, so that they satisfy the relationship 0≦TA-TC≦140.

[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 is relaxed in the head jaw where the martensite structure is generated. As shown in FIG. 9, the number of martensite structures generated in the martensite structure evaluation region C near the head jaw is 5 or less, and the breakage resistance of the welded joint 12 is further improved. For this reason, it is preferable to set TA-TC≦140. In order to stably control the number of martensite structures generated in the martensite structure evaluation region C to 5 or less, it is more preferable to control TA-TC to 20°C or more and 120°C or less. Note that, as described later, it is expected that there are 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 5 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 preferable that both TA+TC≧1340 and 0≦TA−TC≦140 are satisfied.

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

[0127] The inventors considered that, in order to control the generation of martensite structure in the martensite structure evaluation region C in the vicinity of the head jaw shown in Fig. 5, it is effective to control the temperature of the head jaw outer surface 1212 in the vicinity. Furthermore, as a result of examining the temperature control position that affects the generation of martensite structure in 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 martensite structure generated, as shown in Figs. 9, 11, and 12. For this reason, the inventors considered that, in order to control the generation of martensite structure in the martensite structure evaluation region C, it is effective to control the temperature of the head jaw outer surface 1212 as well as the head corner side outer surface 1214.

[0128] In the manufacturing method of the welded rail 1 according to this embodiment, matters other than those described above are not particularly limited. For example, the flash butt welding conditions are not particularly limited. Welding conditions suitable for rails having the above-mentioned chemical components can be appropriately adopted. The cooling means for the welded joint 12 is also not limited. Cooling means capable of setting the cooling stop temperature (TC) of the head 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 can be appropriately adopted.

[0129] In addition, the steel expelled from the welded portion by the upset process remains as a reinforcement in the welded joint 12 immediately after flash butt welding. The reinforcement is removed immediately after the completion of welding. However, as shown in Figures 16A and 16B, the reinforcement may still remain in some places of several mm thickness on the surface of the head jaw after the completion of welding, even after trimming (or bead cutting), which is the work of removing the reinforcement. The evaluation region (C) of the martensite structure needs to be specified based on the head jaw surface of the base rail, regardless of this remaining reinforcement.

[0130] In addition, it is preferable that the metal structure in the martensite structure evaluation region C is pearlite. It has been confirmed that the pearlite structure is the 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 top surface to a depth of 1 / 3h), the portion other than the martensite structure limited above is preferably a pearlite structure. For other parts, the portion other than the martensite structure limited above may be a metal structure other than a pearlite structure as long as it can ensure the strength, ductility, and toughness required for the rail. In the manufacturing method of the welded rail according to this embodiment, the rail components and the cooling stop temperature are within the above-mentioned ranges, so that the metal structure of the welded joint is mainly composed of pearlite. EXAMPLES

[0131] The effects of one embodiment of the present invention will be explained more specifically with reference to an example. However, the conditions in the example are merely an example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0132] The inventors produced 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 flash time: 15sec Preheating times: 10 times Late flash time: 25sec Average late flash velocity: 1.0mm / sec Late flash velocity just before upsetting (3 seconds): 2.0mm / sec Upset load: 65KN

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

[0135] ●Evaluation of martensite structure Evaluation area (see Figure 5) In the longitudinal cross section of the welded joint, this refers to a range of ±5 mm (width 10 mm) from the weld center (A) and a region 1 to 5 mm deep from the head jaw outer surface 1212 to the outer surface of the top corner side (martensite structure evaluation region: C). Reasons for selecting evaluation areas This is because the martensite structure is the location where rail breakage occurs as a starting point. Method for revealing martensite structure The martensite structure evaluation area (C) is polished, etched with nital, and observed under an optical microscope. Polishing conditions: Buff polishing with 1 μm diamond paste Martensite Etch Conditions Etching solution: Alcohol + 5% nitric acid (Nital) Etching time: 5 to 10 seconds How to research an organization Equipment: Optical microscope Magnification: 400x How to evaluate your organization The martensite structure that could be confirmed under an optical microscope at 400x magnification was used for evaluation. The martensite structures targeted had major axes of 25 to 100 μm, and when martensite structures were generated, the number of martensite structures was investigated.

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

[0137] The chemical compositions of the rails used in flash butt welding are shown in Tables 1A, 1B, 2A, 2B, 3A, and 3B. The cooling stop temperatures (TC) of the outer surface of the top corner side and the cooling stop temperatures (TA) of the outer surface of the head jaw 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 martensites 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 martensites in martensite structure evaluation area C 5 or less:A More than 5 but less than 10: B More than 10 pieces:C ●Breakage resistance evaluation No breakage at a falling weight height of 7.5m: A B: Breakage occurred at a drop height of 7.5m, but no breakage occurred at a drop height of 5.0m. Breakage occurred at a falling weight height of 5.0m: C Welded rails rated as "B" or "A" for breakage resistance were judged to have excellent breakage resistance.

[0140] [Table 1A]

[0141] [Table 1B]

[0142] [Table 2A]

[0143] [Table 2B]

[0144] [Table 3A]

[0145] [Table 3B]

[0146] [Table 4A]

[0147] [Table 4B]

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

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

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

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

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

Claims

1. A method for manufacturing a flash butt welded rail having a plurality of rail portions and a welded joint portion that joins the plurality of rail portions, The chemical components, in mass%, are: 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; Immediately after the flash butt welding is completed, the welded joint is forcedly cooled. Equipped with In the forced cooling from the austenite temperature range, the cooling stop temperature (TC) of the outer surface of the top corner side at the weld center of the weld joint is set to 560° C. or more and 850° C. or less; The cooling end temperature (TA) of the outer surface of the head and jaw is set to 650°C or higher and 950°C or lower. A method for manufacturing a flash butt welded rail.

2. 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. The method for manufacturing a flash butt welded rail according to claim 1 . TA+TC≧1340 1 set 0≦TA-TC≦140 2 sets

3. The rail has, as the chemical composition, in mass %, 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, or one or two of these; Group e: one or more of 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; 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, 3. The method for manufacturing a flash butt welded rail according to claim 1, further comprising the step of:

4. 3. The method for manufacturing a flash-butt welded rail according to claim 1, wherein the forced cooling is air cooling.