Heat treatment method for the welded joint of a flash butt welded rail, and method for manufacturing a flash butt welded rail.

Abstract

A heat treatment method for a welded joint part of a flash-butt-welded rail according to one aspect of the present invention involves: beginning first accelerated cooling when the temperature of a vertex part corner-side outer surface and a column part outer surface is within the range of at least 700°C, the first accelerated cooling being performed such that the average cooling speed for the vertex part corner-side outer surface within the temperature range of 750°C–600°C is 1.0°C–3.5°C / sec and the average cooling speed for the column part outer surface within the temperature range of 750°C–600°C is 1.0°C–4.0°C / sec; suspending the first accelerated cooling when the temperature of the vertex part corner-side outer surface and the temperature of the column part outer surface are within the range of 500°C–600°C; beginning heating within 300 sec from suspension of the first accelerated cooling, the heating being performed such that the average heating speed of the vertex part corner-side outer surface and the column part outer surface is 0.5°C–2.0°C / sec; and performing maintenance such that the temperature of the vertex part corner-side outer surface and the column part outer surface is maintained within the range of 620°C–670°C for 30–180 sec.

Description

Technical Field

[0001] The present invention relates to a heat treatment method for a welded joint of a flash butt welded rail and a method for manufacturing a flash butt welded rail.

Background Art

[0002] Flash butt welding is widely used as a rail welding method. It is known that flash butt welding has advantages such as being automatable, having high quality stability, and short welding time.

[0003] Flash butt welding is a technique in which after melting the rail end face by heating, the molten surfaces are pressurized and crimped to join the rails together. During flash butt welding, the rail is heated from room temperature to nearly the melting point at maximum and then cooled. Therefore, flash butt welding causes changes in the metal structure and hardness of the rail. Thus, the portion where metallurgical properties, mechanical properties, etc. change due to the heat of welding is called the heat affected zone (HAZ).

[0004] In the HAZ, due to reheating to above the A1 point during welding, the rail steel is austenitized and then pearlite-transformed region (hereinafter referred to as the "re-γ region"), and in the vicinity of the re-γ region, due to reheating to near the A1 point, the rail steel is partially austenitized and then a region where decomposition and spheroidization of the pearlite structure occur (hereinafter referred to as the "annealed region") is formed. The length in the longitudinal direction of the rail of the region composed of these re-γ regions and annealed regions is collectively referred to as the HAZ width (see FIG. 3B). There are problems of wear resistance due to hardness and fracture resistance due to martensite in the HAZ.

[0005] First, let's explain the hardness issue. In the re-γ section, pearlite transformation occurs during natural cooling after welding. However, the cooling rate of the top surface during natural cooling is lower than the cooling rate of the top surface during rail manufacturing. This is because the top surface is accelerated cooling during rail manufacturing to ensure hardness. As a result, the hardness of the re-γ section is lower compared to the rail base material. Furthermore, in the tempered section, the hardness is also lower compared to the rail base material due to the decomposition and spheroidization of the pearlite structure.

[0006] Thus, in the welded joints of welded rails, a decrease in the hardness of the top surface is likely to occur due to structural changes caused by reheating. When the hardness of the welded joint decreases, in the harsh operating environment of freight railways, the wear of the top surface of the welded joint is accelerated by the passage of wheels. As a result, irregularities due to wear occur in the welded joint, and excessive load is applied to the welded joint during train operation, which easily leads to a reduction in the rail's service life.

[0007] Next, let's discuss the issue of martensite. The cooling rate during natural cooling after welding in the head and column sections of the re-γ section of welded rails is higher than the cooling rate during rail manufacturing. Normally, during rail manufacturing, the accelerated cooling rate of the rail is appropriately controlled to avoid the formation of martensite. However, in welded rails, the temperature of the welded joint decreases rapidly after welding is completed due to heat transfer caused by the temperature difference between the welded joint and the base material, resulting in a cooling rate that is higher than the accelerated cooling rate during rail manufacturing. Therefore, during cooling after welding, pearlite transformation is not completed in the welded joint, and a martensite structure with low toughness is easily formed. When a martensite structure is formed in the pearlite structure, it can easily lead to rail breakage in the harsh operating environment of freight railways. For this reason, the amount of martensite structure formed in welded joints is regulated by rail standards, etc. (for example, CN SPECIFICATION FOR THE MANUFACTURE OF STEEL RAIL, 12-16D).

[0008] As described above, at the welded joint of the welded rail, (1) On the surface of the crown, there is a problem of accelerated wear on the crown due to a decrease in hardness. (2) In the head section and column section, there was a problem of rail breakage due to the formation of a martensitic structure with low toughness.

[0009] To solve these problems, there was a need to develop a heat treatment method for flash butt welding that would ensure the hardness of the rail head surface and suppress the martensitic structure inside the rail head and in the column.

[0010] For example, the following techniques have been proposed to ensure the hardness of the HAZ.

[0011] Patent Document 1 describes a heat treatment method for rail welded joints in which, in order to prevent a decrease in hardness and bending of the rail top surface of the re-γ section, the rail welded joint is reheated after welding, and then the rail top is accelerated cooling while the bottom is controlled cooling.

[0012] Patent Document 2 describes a flash butt welding method for rails that reduces the tempered area on the top surface by setting the late flash speed during welding to 2.1 mm / sec or more, the HAZ width to 27 mm or less, and the length of the tempered area in the longitudinal direction of the rail to 10 mm or less.

[0013] Furthermore, in flash butt welding of rails, for example, the following heat treatment techniques have been proposed to control the microstructure of the welded joint.

[0014] Patent Document 3 describes a method for flash butt welding of rails in which, in order to prevent the formation of protereminate cementite structure in the welded joint heated to the range of 800-900°C and to improve the toughness of the welded joint, the rail head and / or bottom are accelerated cooling from a temperature range of 750°C or higher at a cooling rate of 1-10°C / sec, the accelerated cooling is stopped when the temperature of the rail head and / or bottom reaches 680-550°C, and thereafter the rail head and / or bottom are allowed to cool or slowly cool so that the temperature does not exceed 680°C.

[0015] Furthermore, Patent Document 4 indicates that in order to suppress the occurrence of brittle fracture originating from the bottom of a rail welded joint, the surface of the bottom is heated by induction heating to a temperature between 600°C and 800°C, and then cooled.

[0016] Furthermore, in flash butt welding, for example, the following heat treatment techniques have been proposed to control the microstructure of the welded joint.

[0017] Patent Document 5 describes how, when manufacturing a steel component having a flash butt weld joint, the temperature of the weld joint is increased or maintained at a high temperature, and then cooled to a temperature higher than the martensite onset temperature, in order to improve the metal structure.

[0018] Patent Document 6 indicates that, in order to reduce temperature variations in the width direction of the flash butt weld of a thin plate (steel strip), the welded area is post-heat treated by applying electric heating after welding. [Prior art documents] [Patent Documents]

[0019] [Patent Document 1] Japanese Patent Application Publication No. 3-104824 [Patent Document 2] International Publication No. 2011 / 052562 [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-43862 [Patent Document 4] International Publication No. 2015 / 156243 [Patent Document 5] Japanese Patent Application Laid-Open No. 2015-510452 [Patent Document 6] Japanese Patent Application Laid-Open No. Hei 8-118034 [Summary of the Invention] [Problems to be Solved by the Invention]

[0020] In the technology described in Patent Document 1, it is possible to suppress a decrease in the hardness of the surface of the top of the re-γ part. However, in the interior of the head and the column part of the re-γ part, it is difficult to prevent the generation of a coarse martensite structure that greatly reduces toughness, and there is a problem that the fracture resistance of the rail is not improved.

[0021] In the technology described in Patent Document 2, by reducing the HAZ width of the welded joint of the welded rail, the length in the longitudinal direction of the rail of the tempered part on the surface of the top of the head decreases. However, for the region of the re-γ part, there is no such effect. Therefore, the hardness of the pearlite structure decreases. For this reason, unevenness occurs due to wear on the top of the re-γ part, and there is a problem that a radical improvement in the rail service life cannot be achieved. Further, when the HAZ width is reduced, the cooling rate of the welded joint increases, and the generation of a coarse martensite structure that greatly reduces toughness increases in the interior of the head and the column part of the welded joint, and there is a problem that the fracture resistance of the rail further decreases.

[0022] The heat treatment method of Patent Document 3 aims to prevent the generation of the primary cementite structure in the HAZ of the welded joint of the welded rail, and does not control the decrease in hardness on the surface of the top of the head of the welded joint and the generation of the martensite structure generated in the interior of the head and the column part.

[0023] The heat treatment method of Patent Document 4 aims to improve the toughness of the bottom of the welded joint of the welded rail, and does not control the reduction of hardness on the surface of the top of the welded joint or the generation of martensite structure generated inside the head and in the column part.

[0024] The heat treatment method of Patent Document 5 is to stably form a pearlite structure or a bainite structure in order to ensure the hardness and toughness of the welded joint, that is, to suppress the generation of martensite structure. However, specific heat treatment conditions are not disclosed in Patent Document 5. Also, regarding the method for suppressing internal martensite generation and HAZ reduction technology in large structures such as rails, Patent Document 5 does not disclose them.

[0025] The heat treatment method of Patent Document 6 is a method for reducing the temperature variation in the width direction and does not control the generation of martensite structure as described above. Also, Patent Document 6 does not disclose specific heat treatment conditions.

[0026] The present invention has been devised in view of the above problems, and in a flash butt welded rail, it controls the hardness of the surface of the top of the welded joint, and at the same time prevents the generation of martensite structure inside the head and in the column part, and aims to provide a heat treatment method capable of improving wear resistance and fracture resistance, and a method for manufacturing a flash butt welded rail. Preferably, it is an object to provide a heat treatment method and a method for manufacturing a flash butt welded rail that can satisfy extremely strict wear resistance and fracture resistance requirements in a flash butt welded rail of a rail for a freight railway with a severe track environment.

Means for Solving the Problems

[0027] The gist of the present invention is as follows.

[0028] (1) A heat treatment method for a welded joint of a flash butt welded rail according to one aspect of the present invention comprises the steps of: performing first accelerated cooling on the welded joint of a flash butt welded rail having a welded joint after the completion of flash butt welding; stopping the first accelerated cooling; heating the welded joint; and maintaining the temperature of the welded joint, wherein the first accelerated cooling is started when the temperature of the top corner side outer surface and the column outer surface at the welding center of the welded joint is within the range of 700°C or higher; in the first accelerated cooling, the average cooling rate of the top corner side outer surface at the welding center of the welded joint in the temperature range of 750°C to 600°C is set to 1.0 to 3.5°C / sec; and in the first accelerated cooling, the front The average cooling rate of the outer surface of the column at the welding center in the temperature range of 750°C to 600°C is set to 1.0 to 4.0°C / sec. The first accelerated cooling is stopped when the temperature of the outer surface on the top corner side at the welding center of the welded joint and the temperature of the outer surface of the column are within the range of 500 to 600°C. The heating is started within 300 seconds of the stop of the first accelerated cooling. During the heating, the average heating rate of the outer surface on the top corner side at the welding center of the welded joint and the average heating rate of the outer surface of the column are set to 0.5 to 2.0°C / sec. During the holding, the temperature of the outer surface on the top corner side at the welding center of the welded joint and the temperature of the outer surface of the column are held within the range of 620 to 670°C for 30 to 180 seconds. (2) Preferably, the heat treatment method for the welded joint of a welded rail described in (1) above further comprises a second accelerated cooling step of the welded joint, after the step of maintaining the temperature of the welded joint, such that the outer surface of the top corner side and the outer surface of the column at the weld center of the welded joint are cooled to 200°C or less at an average cooling rate of 0.5°C / sec or more.

[0029] (3) A method for manufacturing a flash butt welded rail according to another aspect of the present invention comprises the steps of: obtaining a flash butt welded rail by flash butt welding rails; deburring the welded joint portion of the flash butt welded rail; and heat-treating the flash butt welded rail by the heat treatment method for the welded joint portion of the flash butt welded rail described in (1) or (2) above. [Effects of the Invention]

[0030] According to the above embodiment of the present invention, it is possible to improve the wear resistance and fracture resistance of the welded joint. [Brief explanation of the drawing]

[0031] [Figure 1A] This is a flowchart of the heat treatment method for the welded joint portion of the flash butt welded rail according to this embodiment. [Figure 1B] This is a schematic diagram of the heat treatment conditions. [Figure 2A] These are cross-sectional views perpendicular to the longitudinal direction of the welded rail, and side views of the welded rail. [Figure 2B] This is a magnified cross-sectional view of the head of a welded rail. [Figure 3A] This is a perspective view of a sample cut from a welded rail, allowing the longitudinal cross-section to be visible. [Figure 3B] This is a micrograph of the heat-affected zone in a longitudinal cross-section. [Figure 3C] This is a schematic diagram of the hardness distribution at 5 mm below the outer surface in the longitudinal cross-section directly below the outer surface 1111 and 1211 of the rail top. [Figure 4] This graph shows the relationship between the accelerated cooling rate of the welded joint in the first accelerated cooling method and the difference in hardness (Vickers hardness measured with a load of 10 kg) between the welded joint and the base material. [Figure 5] This graph shows the relationship between the accelerated cooling stop temperature during the first accelerated cooling of the welded joint and the difference in hardness (Vickers hardness measured with a load of 10 kg) between the welded joint and the base material. [Figure 6]This graph shows the relationship between the accelerated cooling stop temperature during the first accelerated cooling of a welded joint and the number of martensitic structures formed. [Figure 7] This graph shows the relationship between the timing of heating initiation at the welded joint and the number of martensitic structures formed. [Figure 8] This graph shows the relationship between the heating rate of the welded joint and the number of martensitic structures formed, and the relationship between the heating rate of the welded joint and the hardness of the outer surface of the top of the joint. [Figure 9] This graph shows the relationship between the maximum holding temperature of the welded joint and the number of martensitic structures formed, and the relationship between the maximum holding temperature of the welded joint and the hardness of the outer surface of the top of the joint. [Figure 10] This graph shows the relationship between the holding time of the welded joint and the number of martensitic structures formed, as well as the relationship between the holding time of the welded joint and the hardness of the outer surface of the top of the joint. [Figure 11] This graph shows the relationship between the cooling rate after heat treatment of the welded joint and the hardness of the outer surface of the top (Vickers hardness measured under a 10 kg load). [Figure 12] This graph shows the relationship between the cooling stop temperature after heat treatment of the welded joint and the hardness of the outer surface of the top (Vickers hardness measured under a load of 10 kg). [Figure 13] This is a flowchart of the manufacturing method for flash butt welded rails according to this embodiment. [Figure 14] This is a perspective view of the rail and electrode immediately before the start of flash butt welding in an example of a flash butt welding rail manufacturing method according to this embodiment. [Figure 15A] This is a schematic side view of the rail and electrode immediately before the start of flash butt welding in an example of a flash butt welding rail manufacturing method according to this embodiment. [Figure 15B] This is a schematic side view of the initial flash process of flash butt welding in an example of a flash butt welding rail manufacturing method according to this embodiment. [Figure 15C]This is a schematic side view of the preheating process for flash butt welding in an example of a flash butt welding rail manufacturing method according to this embodiment. [Figure 15D] This is a schematic side view of the late flash process of flash butt welding in an example of a flash butt welding rail manufacturing method according to this embodiment. [Figure 15E] This is a schematic side view of the flash butt welding rail immediately after the completion of the upset step in an example of a flash butt welding rail manufacturing method according to this embodiment. [Figure 15F] This is a schematic side view of trimming in an example of a flash butt weld rail manufacturing method according to this embodiment. [Figure 15G] This is a schematic side view of the first accelerated cooling of the welded joint in an example of a flash butt weld rail manufacturing method according to this embodiment. [Figure 15H] This is a schematic side view of the heating of the welded joint in an example of a flash butt weld rail manufacturing method according to this embodiment. [Modes for carrying out the invention]

[0032] (1. Heat treatment method for welded joints of flashbutt welded rails) The heat treatment method for the welded joint of the flash butt welded rail according to this embodiment will be described below. The heat treatment method for the welded joint of the flash butt welded rail according to this embodiment is as shown in Figure 1. (S1) In a flash butt welding rail 1 having a welded joint 12, after the completion of flash butt welding, a step is made to perform first accelerated cooling on the welded joint 12, (S2) The process of stopping the first accelerated cooling, (S3) A step of heating the welded joint 12, (S4) A step of maintaining the temperature of the welded joint 12, It is equipped with, (a) The first accelerated cooling is initiated when the temperature of the top corner side outer surface 1214 and the column outer surface 1221 at the welding center A of the welded joint 12 is within the range of 700°C or higher. (b) In the first accelerated cooling, the average cooling rate of the top corner side outer surface 1214 at the welding center A of the welded joint 12 in the temperature range of 750°C to 600°C is set to 1.0 to 3.5°C / sec. (c) In the first accelerated cooling, the average cooling rate of the column outer surface 1221 at the welding center A of the welded joint 12 in the temperature range of 750°C to 600°C is set to 1.0 to 4.0°C / sec. (d) The first accelerated cooling is stopped when the temperature of the top corner side outer surface 1214 and the column outer surface 1221 at the welding center A of the welded joint 12 are within the range of 500 to 600°C. (f) Start heating within 300 seconds of stopping the first accelerated cooling. (g) During heating, the average heating rate of the top corner side outer surface 1214 and the column outer surface 1221 at the welding center A of the welded joint 12 shall be 0.5 to 2.0 °C / sec. During the holding process, the temperature of the top corner side outer surface 1214 and the column outer surface 1221 at the welding center A of the welded joint 12 are maintained within the range of (h) 620 to 670°C for (i) 30 to 180 seconds.

[0033] The main reason for the decrease in wear resistance of the welded joint 12 was considered to be the decrease in hardness of the welded joint 12 after flash butt welding. Furthermore, the main reason for the deterioration of the fracture resistance of the welded joint 12 was considered to be the generation of martensite in the welded joint 12 after flash butt welding. The inventors found that softening of the welded joint 12 can be suppressed by performing a first accelerated cooling S1 on the welded joint 12 immediately after the completion of flash butt welding under predetermined conditions. Furthermore, the inventors found that the amount of martensite in the welded joint 12 can be suppressed by performing heating S3 and temperature holding S4 on the welded joint 12 immediately after stopping S2 of the first accelerated cooling S1 under predetermined conditions. Through these heat treatments S1 to S4, the inventors were able to significantly improve the wear resistance and fracture resistance of the welded joint 12.

[0034] Based on the above findings, a heat treatment method for the welded joint portion of a flash butt welded rail (welding rail) according to one embodiment of the present invention will be described in detail. First, the terms used in this embodiment will be explained.

[0035] (Definition of terms related to rails) A flash butt welded rail 1 is a rail obtained by joining rails together using flash butt welding. Hereafter, the flash butt welded rail 1 may be simply referred to as "welded rail 1".

[0036] As shown in Figure 2, the welded rail 1 comprises a plurality of rail sections 11 having a rail head 111, a rail column 112, and a rail bottom 113, and a welded joint 12 that joins these rail sections 11. In Figure 2, the symbol "A" indicates the welding center, which will be described later. Hereafter, when simply referred to as "rail," it means the rail before welding.

[0037] The rail head 111 of the rail section 11 refers to the portion above the constricted portion in the vertical center of the rail section 11, in a cross-section perpendicular to the longitudinal direction of the rail section 11 shown on the left side of Figure 2. The rail column 112 of the rail section 11 refers to the constricted portion in the vertical center of the rail section 11, in a cross-section of the rail section 11 shown on the left side of Figure 2. Furthermore, the rail bottom 113 of the rail section 11 refers to the portion below the constricted portion in the vertical center of the rail section 11, in a cross-section of the rail section 11 shown on the left side of Figure 2.

[0038] Furthermore, the rail head 111 of the rail section 11 has a top surface which becomes the top surface of the rail head 111 when the welded rail 1 is in use, a top surface which becomes the side surface of the rail head 111, and a corner section which is a rounded corner between the top surface and the top surface. The top surface and the corner section are areas that receive repeated loads from the wheels, so it is preferable to appropriately control their characteristics. In the welded rail 1 according to this embodiment, as shown in Figures 2A and 2B, the top surface of the rail head 111 is referred to as the rail top outer surface 1111. On this top surface, the outer surface close to the corner section of the rail section 11 is referred to as the rail top corner side outer surface 1114. The top surface of the rail head 111 is referred to as the rail side outer surface 1113. Also, the constricted lower part of the rail head 111 is referred to as the lower part of the rail jaw 1112. Naturally, the vertical direction of the welded rail 1 refers to the vertical direction when the welded rail 1 is used as a track. Furthermore, in the rail column portion 112 of the rail portion 11, the surface of the portion that is intermediate between the center of the rail column portion 112 and the lower end of the rail head portion 111, and is close to the lower part of the rail jaw 1112, is referred to as the outer surface of the rail column portion 1121.

[0039] The welded joint 12 refers to a "welded joint" as defined in JIS Z 3001-1:2018, meaning a joint where members are joined together by welding. In this embodiment, the member is the rail that forms the material of the rail section 11.

[0040] In the welded rail 1, the shape of the welded joint portion 12 is substantially the same as that of the rail portion 11. Therefore, the welded joint portion 12, like the rail portion 11, has a head portion 121, a column portion 122, and a bottom portion 123. The head portion 121 of the welded joint portion 12 has a top outer surface 1211, a jaw lower portion 1212, a top corner side outer surface 1214, and a head side outer surface 1213. The column portion 122 of the welded joint portion 12 has a column outer surface 1221. Hereinafter, the head portion in the rail portion 11 will be referred to as "rail head portion 111," and the head portion in the welded joint portion 12 will be simply referred to as "head portion 121." For other parts, the term "rail" will be used if they are included in the rail portion 11, and the term "rail" will not be used if they are included in the welded joint portion 12.

[0041] The heat-affected zone (HAZ) 12H, as defined in JIS Z 3001-1:2018, refers to the unmelted portion of the base material where metallurgical and mechanical properties have changed due to heat from welding, cutting, etc. In this embodiment, the base material is the rail portion 11.

[0042] The longitudinal cross-section is a cross-section that is parallel to the longitudinal and vertical directions of the welding rail 1 and passes through the center of the welding rail 1 in the width direction. Figures 3A and 3B show explanatory diagrams of the longitudinal cross-section of the welded joint 12.

[0043] Figure 3A is a perspective view of the welding rail 1 cut to allow visualization of its longitudinal cross-section. The welding center A shown in Figure 3A refers to a straight line along the vertical direction of the welding rail 1 that passes through the center of the heat-affected zone 12H in the longitudinal cross-section of the welding joint 12.

[0044] Figure 3B is a micrograph of the area near the top outer surface 1211 of the weld rail 1, taken in the longitudinal cross-section. Specifically, Figure 3B is a macroscopic image of the rectangular area indicated by the dashed line near the upper end of the longitudinal cross-section shown in Figure 3A. In Figure 3B, the microstructural changes of the heat-affected zone 12H in the longitudinal cross-section of the head 121 of the welded joint 12 can be visually observed. The heat-affected zone 12H includes a region 12Hγ in which the rail steel austenitizes and then undergoes pearlite transformation when the rail is reheated to above point A1 during welding, and a region 12HT surrounding this region in which the rail steel partially austenitizes when reheated to near point A1, and then undergoes decomposition and spheroidization of the pearlite structure. In this embodiment, the portion of the rail steel that is austenitized by reheating to point A1 or higher during welding is defined as the "re-γ region 12Hγ," and the region surrounding it, where the rail steel is partially austenitized by reheating to near point A1, and subsequently undergoes decomposition and spheroidization of the pearlite structure, is defined as the "tempered region 12HT." The tempered region 12HT is the region where the so-called two-phase tempering phenomenon occurs. For reference, the microstructure photograph in Figure 3B includes lines indicating the boundary between the tempered region 12HT and the re-γ region 12Hγ.

[0045] Furthermore, the HAZ width refers to the width of the heat-affected zone (HAZ) 12H measured along the longitudinal direction of the welding rail 1. In other words, the HAZ width is the sum of the lengths of the re-γ section 12Hγ and the tempering section 12HT in the longitudinal direction of the rail.

[0046] (Definition of terms related to heat treatment conditions) In step S1, which involves first accelerated cooling of the welded joint, the accelerated cooling start temperature refers to the temperature of the outer surface 1214 on the top corner side of the weld center A at the time the injection of cooling gas is started. In step S1, where the first accelerated cooling is performed on the welded joint, the average accelerated cooling rate in the temperature range of 750°C to 600°C on the outer surface of the top corner is the value obtained by dividing 150°C (=750°C - 600°C) by the time required for the outer surface of the top corner to decrease from 750°C to 600°C. Similarly, in step S1, where the first accelerated cooling is performed on the welded joint, the average accelerated cooling rate in the temperature range of 750°C to 600°C on the outer surface of the column is the value obtained by dividing 150°C (=750°C - 600°C) by the time required for the outer surface of the column to decrease from 750°C to 600°C. In step S2, which involves stopping the first accelerated cooling of the welded joint, the accelerated cooling stop temperature of the top corner side outer surface refers to the temperature of the top corner side outer surface 1214 of the weld center A at the time the injection of cooling gas is stopped. Similarly, the accelerated cooling stop temperature of the column outer surface 1221 refers to the temperature of the column outer surface 1221 of the weld center A at the time the injection of cooling gas is stopped. In step S3, which involves heating the welded joint, the average heating rate of the top corner side outer surface is the difference between the heating start temperature of the top corner side outer surface and 620°C, which is the lower limit of the holding temperature range, divided by the time required to raise the temperature from the heating start temperature to 620°C. The heating start temperature of the top corner side outer surface refers to the temperature of the top corner side outer surface of the weld center A at the time when heating of the welded joint using the heating means is started. Similarly, the average heating rate of the column outer surface is the difference between the heating start temperature of the column outer surface (the temperature of the column outer surface at the start of heating) and 620°C, divided by the time required to raise the temperature from the heating start temperature to 620°C. It is permissible to set the lower limit of the holding temperature range to a value greater than 620°C, but even if the lower limit of the holding temperature range is set to a value other than 620°C, the definition of the average heating rate does not change. In step S4, which involves maintaining the temperature of the welded joint, the temperature holding time for the top corner side outer surface is the time during which the temperature of the top corner side outer surface 1214 of the weld center A was within the range of 620 to 670°C. That is, the temperature holding time is the length of time from when the temperature of the top corner side outer surface 1214 of the weld center A rises to 620°C or higher due to heating, until the temperature of that area subsequently decreases to below 620°C. Similarly, the temperature holding time for the column outer surface is the time during which the temperature of the column outer surface was within the range of 620 to 670°C. Even if the holding temperature range is set to a range other than 620 to 670°C, the definition of the holding time does not change. In step S5, where a second accelerated cooling is performed on the heated welded joint, the cooling stop temperature of the top corner side outer surface refers to the temperature of the top corner side outer surface 1214 of the weld center A at the time when the injection of cooling gas for the second accelerated cooling is stopped. Similarly, the cooling start temperature of the top corner side outer surface refers to the temperature of the top corner side outer surface 1214 of the weld center A at the time when the injection of cooling gas for the second accelerated cooling is started. Likewise, the cooling stop temperature of the column outer surface refers to the temperature of the column outer surface of the weld center A at the time when the injection of cooling gas for the second accelerated cooling is stopped, and the cooling start temperature of the column outer surface refers to the temperature of the column outer surface of the weld center A at the time when the injection of cooling gas for the second accelerated cooling is started. In step S5, where a second accelerated cooling is performed on the heated welded joint, the average cooling rate of the top corner side outer surface is the difference between the cooling start temperature and the cooling stop temperature of the top corner side outer surface divided by the cooling gas injection time. Similarly, the average cooling rate of the column outer surface is the difference between the cooling start temperature and the cooling stop temperature of the column outer surface divided by the cooling gas injection time.

[0047] Next, we will explain the experimental results that led to the technical concept of the heat treatment method according to this embodiment. First, we will explain the experimental results regarding the relationship between the average accelerated cooling rate of the top corner side outer surface and the absolute value of the difference in hardness between the welded joint and the base material in the first accelerated cooling step (S1) of the welded joint 12 (see Figure 4), the relationship between the temperature of the top corner side outer surface at the time of stopping the first accelerated cooling (S2) and the absolute value of the difference in hardness between the welded joint and the base material (see Figure 5), and the relationship between the temperature of the top corner side outer surface at the time of stopping the cooling and the number of martensitic structures formed (see Figure 6). The inventors investigated a method for controlling the hardness of the welded joint in flash butt welding of rails. As shown in Figure 3B, the welded joint has a re-γ section 12Hγ and a tempered section 12HT. After the completion of flash butt welding, softening occurs in both the re-γ section 12Hγ and the tempered section 12HT. However, the width of the re-γ section 12Hγ is greater than the width of the tempered section 12HT. Therefore, the inventors first considered how to ensure the hardness of the re-γ section 12Hγ. The inventors considered that in order to control the hardness of the re-γ section 12Hγ, it is important to perform a first accelerated cooling after flash butt welding and to suitably control the average cooling rate of the outer surface on the top corner side during this first accelerated cooling, as well as the stopping temperature of the first accelerated cooling. The inventors then confirmed suitable cooling conditions through experiments.

[0048] Flash butt welding tests were conducted using eutectoid steel rails and hypereutectoid steel rails (both 0.75-1.20%C). After welding, the welded joint was subjected to a first accelerated cooling process. Various accelerated cooling rates and accelerated cooling stop temperatures were applied to the first accelerated cooling process. The relationship between these conditions and the hardness and martensitic structure of the welded joint was then evaluated. The welding rails, flash butt welding conditions, cooling conditions of the welded joint, characteristics of the welded joint, evaluation method for hardness and martensitic structure of the welded joint, and evaluation of the effect of hardness of the welded joint on the wear resistance of the welded joint are shown below.

[0049] ● Welded rails Composition: Contains 0.75-1.20% C, 0.40% Si, 0.80% Mn, and 0.20% Cr, with the remainder being iron and impurities. Rail shape: 136 lbs (weight: 67 kg / m). Hardness of the base material: 350HV (0.75%C), 400HV (0.90%C), 450HV (1.10%C) The hardness of the base material was measured using the method described later.

[0050] ● Flash butt welding conditions (preheating flash method) Initial flash time: 15 sec Preheating cycles: 10 Late flash duration: 20 seconds Average late flash speed: 0.6mm / sec Late flash speed immediately before upset (3 seconds): 1.8mm / sec Loss of melted material from the later flash: 10 mm Upset load: 65kN

[0051] ●Conditions for the first accelerated cooling applied to the welded joint immediately after welding. Cooling method: Nozzles are placed at equal intervals around the head of the welded joint, and air is sprayed in. Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column section (see Figure 2) Temperature measurement method: Infrared thermometer (the same as in other experiments) Temperature of the outer surface of the top corner and column section at the start of the first accelerated cooling: 700°C or higher Average accelerated cooling rate of the outer surface on the corner side of the top of the head: 0.4~4.5℃ / sec Average accelerated cooling rate of the outer surface of the column: 0.4~5.0℃ / sec Accelerated cooling stop temperature: 400~700℃ *In the experiment to change the average accelerated cooling rate of the outer surface on the corner side of the top of the head (see Figure 4), the temperature of the outer surface on the corner side of the top of the head was fixed at 570°C when the first accelerated cooling stopped, and the temperature of the outer surface of the column was set in the range of 550 to 570°C when the first accelerated cooling stopped. *In the experiment to change the temperature of the outer surface on the top corner side when the first accelerated cooling is stopped (see Figures 5 and 6), the average accelerated cooling rate of the outer surface on the top corner side was fixed at 2.0°C / sec, and the average accelerated cooling rate of the outer surface of the column was set in the range of 1.8 to 2.2°C / sec.

[0052] ● Characteristics of welded joints HAZ width: 10~50mm

[0053] ● Heating conditions for welded joints none.

[0054] ● Evaluation method for hardness of base material and welded joint Evaluation site for hardness of the base material: Longitudinal cross section directly below the outer surface 1111 of the top of the rail. Evaluation site for hardness of welded joint 12: Longitudinal cross-section directly below the outer surface 1211 of the top of the weld center A (see Figures 2 and 3A) Hardness evaluation: Longitudinal cross-sections were cut from the outer surface 1111 of the rail top and directly below the outer surface 1211 of the rail top, polished, and then evaluated using a Vickers hardness tester. Polishing conditions: The outer surface 1111 of the rail top and the longitudinal cross-section directly below the outer surface 1211 of the rail top were buffed with 1 μm diamond paste. Hardness testing machine: Vickers hardness tester (load 10 kgf) Under the conditions described above, the hardness of the base material and welded joint sections of the rail was measured at a depth of 5 mm from the head surface in the longitudinal cross-section, at 80 points on both the left and right sides along the longitudinal direction of the welded rail from the weld center A, for a total of 161 points. The measurement interval was 1 mm. The average of 20 hardness points at 71-80 mm to the left and 71-80 mm to the right of the weld center A was considered the hardness of the base material, i.e., the base material hardness. The average of 10 hardness points at 2-6 mm to the left and 2-6 mm to the right of the weld center A was considered the hardness of the welded joint. For reference, Figure 3C shows a schematic diagram of the hardness distribution obtained by the measurement method described above. Typically, hardness dips appear on both sides of the weld center A. These hardness dips are called the softest areas. The "20 hardness points 71-80 mm to the left and 71-80 mm to the right of the weld center A" mentioned above are measurement points in an area outside the softest areas, unaffected by welding heat. The "10 hardness points 2-6 mm to the left and 2-6 mm to the right of the weld center A" are areas inside the softest areas.

[0055] ● Evaluation method for martensitic structure of flashbutt weld joints Evaluation area B (see Figure 3A): Region B of the longitudinal cross-section of the welded joint 12, extending from 0 to (2 / 3) × h from the top outer surface 1211 and from ±5 mm in the longitudinal direction from the weld center A (total width 10 mm). Here, "h" represents the height of welding rail 1. This area will be referred to as martensite evaluation region B. Reason for selecting the evaluation area: Martensite evaluation area B is the area that is heated to point A1 or higher in flash butt welding, and it has been confirmed in previous flash butt welding tests to be the area where martensite structure is most easily formed. Observation of martensite structure: After polishing martensite evaluation area B, nital etch was performed, and the presence and number of martensite structures were investigated using a light microscope. Polishing conditions: Buff polishing with 1μm diamond paste Nital etching conditions: Alcohol + 5% nitric acid Optical microscope observation conditions: 200x magnification Field of view: Entire martensite evaluation area B Evaluation of martensitic structure: Martensitic structure visible under a 200x optical microscope was used as the evaluation target. The presence or absence of martensitic structure, and if present, the number of martensitic cells, were investigated. The specific procedure for evaluating the martensitic structure was as follows: An optical microscope image of the martensitic evaluation region B was taken at a magnification of 200x. Next, this optical microscope image was binarized using image analysis software. Since martensite usually appears white, the white area within the martensitic evaluation region B in the binarized optical microscope image can be considered as the martensitic structure. In this martensitic structure, those with a major axis of 20 μm or more were selected for evaluation, and their number was calculated. In the welded joint of the welded rail according to this embodiment, the metal structure other than the martensitic structure is pearlite. In an optical microscope image of such a metal structure, martensite without carbides appears as a white area and can be clearly distinguished from pearlite. In addition, the welded joint of the welded rail may contain trace amounts of bainite structure, but in the welded rail according to this embodiment, the bainite structure in the welded joint is considered as martensitic structure. This is because it is difficult to distinguish between the two using an optical microscope image, and furthermore, the effect of both on the fracture resistance of the welded rail is almost the same.

[0056] ● Evaluation of hardness affecting the wear resistance of flashbutt welded joints. The smaller the difference in hardness between the welded joint and the outer surface of the rail's top, the less wear-induced irregularities the welded joint will have, thus improving the rail's service life. To ensure wear resistance at the welded joint, a hardness difference of Δ30HV or less is desirable. This hardness difference is regulated by rail standards, etc. (e.g., AREMA: American Railway Engineering and Maintenance-of-Way Association).

[0057] As a result, as shown in Figure 4, when the average accelerated cooling rate of the outer surface on the top corner side exceeded 3.5°C / sec, the hardness of the re-γ portion on the top outer surface 1211 of the welded joint increased excessively, and the difference in hardness between the welded joint and the base material (i.e., the value obtained by subtracting the hardness of the base material from the hardness of the welded joint) exceeded Δ30HV. In the welded joint where the average accelerated cooling rate of the outer surface on the top corner side exceeded 3.5°C / sec, the unevenness due to wear increased, and the wear resistance of the welded joint decreased.

[0058] Furthermore, when the average accelerated cooling rate of the outer surface on the corner side of the top of the welded joint fell below 1.0°C / sec, the pearlite transformation temperature increased, and the hardness of the pearlite produced by the transformation decreased. As a result, the hardness of the re-γ portion of the outer surface 1211 of the top of the welded joint decreased, and the hardness difference between the welded joint and the base material exceeded Δ30HV. It was found that the unevenness of the welded joint increased due to wear, and the wear resistance of the welded joint decreased.

[0059] Furthermore, as shown in Figure 5, when the temperature of the outer surface on the top corner side exceeded 600°C at the cessation of the first accelerated cooling, the pearlite transformation temperature increased, and the hardness of the pearlite produced by the transformation decreased. As a result, the hardness of the re-γ portion of the top outer surface 1211 of the welded joint decreased, and the difference in hardness between the welded joint and the base material exceeded Δ30HV. Consequently, the unevenness due to wear of the welded joint increased, and the wear resistance of the welded joint decreased.

[0060] Furthermore, as shown in Figure 6, it was found that when the temperature of the outer surface on the top corner side falls below 500°C at the cessation of the first accelerated cooling, the number of coarse martensitic structures that significantly reduce toughness increases in the interior of the head and column of the welded joint. The greater the number of coarse martensitic structures, the lower the fracture resistance of the welded joint.

[0061] Therefore, it was found that in order to control the hardness of the welded joint and ensure wear resistance and fracture resistance, it is necessary to control the average accelerated cooling rate of the outer surface on the top corner side and the accelerated cooling stop temperature of the outer surface on the top corner side within a certain range. Furthermore, the inventors have newly discovered that in order to ensure fracture resistance of the welded joint, it is necessary to suppress the martensitic structure, which is detrimental to toughness, in the interior of the head and column of the welded joint.

[0062] Next, we will explain the experimental results regarding the relationship between the start time of heating in the step of heating the welded joint 12 (S3) and the number of martensitic structures formed (see Figure 7). The inventors investigated a method to further suppress the formation of martensitic structure in the head and column portions of the welded joint during flash butt welding of rails. First, they investigated in detail the causes of martensitic structure formation. As a result, the following was revealed.

[0063] The head and column sections of the welded joint are locally heated during flash butt welding. Therefore, when the welded rail is allowed to cool naturally after welding, the cooling rate of the head and column sections of the welded joint is greater than the cooling rate of the rail after rolling during rail manufacturing. For this reason, martensite is more likely to form in the head and column sections of the welded joint than in the rail section, which is not affected by welding heat. In addition, alloy segregation exists in the head and column sections of the welded joint. The inventors have newly discovered that martensite structures are easily formed in these segregated areas. Therefore, they investigated the cause of segregation formation. As a result, they found that alloy segregation inevitably occurs during the casting stage. Furthermore, the inventors have newly discovered that in the segregated areas, during the first accelerated cooling immediately after welding, which has a faster cooling rate compared to cooling after rolling, the pearlite transformation is not completed, the austenite structure remains, and as a result, a martensite structure is formed. For the reasons stated above, the amount of martensite generated differs between the welded joint, which is affected by welding heat, and the rail section, which is the base material and is not affected by welding heat.

[0064] For the reasons stated above, if the welded joint is subjected to first-stage accelerated cooling after flash butt welding, martensite, which causes fracture, is generated in the welded joint. However, as explained with reference to Figure 4, accelerated cooling is essential to increase the hardness of the welded joint and ensure wear resistance. Therefore, the inventors investigated a heat treatment method in which the pearlite transformation is completed after the first-stage accelerated cooling, which is started immediately after welding, has stopped. As a result, the inventors discovered a method in which the pearlite transformation is completed and the formation of a martensite structure is prevented by heating the welded joint within a certain period of time after the first-stage accelerated cooling and holding it in the pearlite transformation temperature range.

[0065] This heat treatment method was investigated in detail. First, the effect of the timing of heating initiation at the weld joint was examined. Flash butt welding tests were performed using eutectoid steel rails and hypereutectoid steel rails (both 0.75-1.20%C). After welding, a first accelerated cooling was performed, and the relationship between the time from the cessation of the first accelerated cooling to the start of heating and the amount of martensitic structure formed was evaluated. The welding rails, the conditions for the first accelerated cooling of the weld joint immediately after welding, the heating conditions of the weld joint after the first accelerated cooling, and the characteristics of the flash butt welded joint are shown below. Other experimental conditions were the same as those for the welding tests described above.

[0066] ● Welded rails Composition: Contains 0.90% C, 0.40% Si, 0.80% Mn, 0.20% Cr, with the remainder being iron and impurities. Rail shape: 136 lbs (weight: 67 kg / m). Hardness of the base material: 400 HV (outer surface of rail top 1111, see Figure 2)

[0067] ● First accelerated cooling conditions for welded joints immediately after welding Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Average accelerated cooling rate of the outer surface on the corner side of the top of the head: 2.0°C / sec Average accelerated cooling rate of the outer surface of the column: 2.3°C / sec Temperature of the outer surface on the top corner side at the cessation of the first accelerated cooling: 570°C Temperature of the outer surface of the column at the cessation of the first accelerated cooling: 560°C

[0068] ● Heating conditions for welded joints Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Heating start time: 50-500 seconds after cooling of the welded joint has stopped. Average heating rate of the outer surface on the top corner and the outer surface of the column: 0.5~2.0℃ / sec Temperature range for the outer surface of the top corner and the outer surface of the column: 620~670℃ Holding time for the outer surface of the top corner and the outer surface of the column: 30-180 seconds Cooling after holding: Allow the outer surface of the top corner and the outer surface of the column to cool to below 200°C (average cooling rate of 0.1~0.4°C / sec on the outer surface of the top corner and the outer surface of the column). Heating method: Electrical heating or high-frequency heating

[0069] ● Characteristics of flashbutt welded joints Evaluation site: Outer surface of the top of the weld center A 1211 (see Figure 2) HAZ width: 10~50mm Hardness (re-γ region): 380~420HV

[0070] As a result, as shown in Figure 7, it was found that when the time from the cessation of the first accelerated cooling to the start of heating exceeds 300 seconds, the number of martensitic structures in the welded joint increases. This is presumed to be because, before the start of heating, the temperature inside the head and column of the welded joint decreases, causing martensitic transformation. Even after the cessation of the first accelerated cooling, i.e., the cessation of the injection of the refrigerant for the first accelerated cooling, there is a temperature difference between the rail and the atmosphere. Therefore, even after the cessation of the first accelerated cooling, the temperature of the welded joint continues to decrease. This is why the temperature inside the head and column of the welded joint decreases. Furthermore, the martensitic structure does not disappear with subsequent heating. If heating is carried out at a higher temperature than the above conditions in order to eliminate the martensitic structure, the effect of the first accelerated cooling, which is performed to ensure the hardness of the welded joint, is lost. Therefore, it was found that in order to prevent the formation of martensitic structures in the welded joint, it is necessary to control the time from the cessation of the first accelerated cooling to the start of heating within a predetermined range.

[0071] Furthermore, we will explain the experimental results evaluating the relationship between the average heating rate of the outer surface on the top corner side of the welded joint and the amount of martensitic structure formed inside the top and column of the welded joint (see Figure 8), and the relationship between the average heating rate of the outer surface on the top corner side and the hardness of the outer surface on the top (see Figure 8), during the heating process (S3) of the welded joint 12. The cooling conditions for the welding rail, the welded joint, the heating conditions for the welded joint, and the characteristics of the flash butt welded joint are as shown below. Other test conditions were the same as those for the welding test described above.

[0072] ● Welded rails The welding rail used to obtain the experimental results shown in Figure 7 was the same as the one used in the experiment. ● First accelerated cooling conditions for welded joints Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Average accelerated cooling rate of the outer surface on the corner side of the top of the head: 2.0°C / sec Average accelerated cooling rate of the outer surface of the column: 2.3°C / sec Temperature of the outer surface on the top corner side at the cessation of the first accelerated cooling: 570°C Temperature of the outer surface of the column at the cessation of the first accelerated cooling: 560°C ● Heating conditions for welded joints Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Heating start time: Within 200 seconds after the cessation of the first accelerated cooling of the welded joint. Average heating rate of the outer surface on the top corner and the outer surface of the column: 0.1~4.0℃ / sec Temperature range for the outer surface of the top corner and the outer surface of the column: 620~670℃ Temperature retention time for the outer surface of the top corner and the outer surface of the column: 30-180 seconds Cooling after holding: The outer surface of the top corner and the outer surface of the column are allowed to cool to 200°C (average cooling rate of 0.1~0.4°C / sec on the outer surface of the top corner and the outer surface of the column). Heating method: Electrical heating or high-frequency heating

[0073] ● Characteristics of flashbutt welded joints Evaluation site: Outer surface of the top of the weld center A 1211 (see Figure 2) HAZ width: 10~50mm

[0074] As a result, as shown in Figure 8, when the average heating rate of the outer surface on the corner side of the top of the joint exceeded 2.0°C / sec, the formation of a martensitic structure was observed in the martensitic evaluation region B inside the head and column. This is presumed to be because the temperature difference between the outer surface of the head side of the welded joint and the inside of the head and column of the welded joint increased, preventing the inside of the head of the welded joint from heating sufficiently and thus not promoting pearlite transformation. The effect of the above phenomenon was particularly pronounced when the rail was heated by high-frequency heating.

[0075] Furthermore, it was found that when the average heating rate of the outer surface on the corner side of the crown falls below 0.5°C / sec, the hardness of the outer surface on the crown decreases. This is presumed to be because the welded joint is tempered. Therefore, it was found that in order to prevent the formation of martensitic structure in the welded joint and suppress the decrease in hardness, it is necessary to control the average heating rate of the outer surface on the corner side of the crown within a certain range. Furthermore, the average heating rate was approximately the same on the outer surface of the top corner and the outer surface of the column.

[0076] Next, the relationship between the heating and holding temperature of the welded joint 12 during the process of maintaining the temperature of the welded joint 12 (S4) and the amount of martensitic structure formed inside the head and column of the welded joint (see Figure 9), and the relationship between the heating and holding temperature and the hardness of the outer surface of the top (see Figure 9) were evaluated. The cooling conditions for the welding rail and welded joint, and the heating conditions for the welded joint are as shown below. Other test conditions were the same as those for the welding test described above.

[0077] ● Welded rails The welding rail used to obtain the experimental results shown in Figure 7 was the same as the one used in the experiment. ● First accelerated cooling conditions for welded joints Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Average accelerated cooling rate of the outer surface on the corner side of the top of the head: 2.0°C / sec Average accelerated cooling rate of the outer surface of the column: 2.3°C / sec Temperature of the outer surface on the top corner side at the cessation of the first accelerated cooling: 570°C Temperature of the outer surface of the column at the cessation of the first accelerated cooling: 560°C ● Heating conditions for welded joints Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Heating start time: Within 200 seconds after the cooling of the welded joint has stopped immediately after welding. Average heating rate of the outer surface on the top corner and the outer surface of the column: 0.5~2.0℃ / sec Temperature range for the outer surface of the top corner and the outer surface of the column: 500~700℃ Holding time for the outer surface of the top corner and the outer surface of the column: 0-210 sec Cooling after holding: The outer surface of the top corner and the outer surface of the column are allowed to cool to 200°C (average cooling rate of 0.1~0.4°C / sec on the outer surface of the top corner and the outer surface of the column). *As stated above, the term "holding time of the outer surface on the top corner side" refers to the time during which the temperature of the outer surface 1214 on the top corner side of the welding center A was within the range of 620 to 670°C. However, in this experiment, there were cases where the temperature was held in a range exceeding 670°C. In this case, the holding time refers to the isothermal holding time at the maximum temperature.

[0078] As a result, as shown in Figure 9, it was found that when the maximum holding temperature of the outer surface on the corner side of the crown exceeded 650°C, the welded joint was tempered, and the hardness of the outer surface on the crown decreased. Furthermore, when the maximum holding temperature of the outer surface on the corner side of the crown fell below 580°C, the pearlite transformation rate decreased significantly, the pearlite transformation was not completed, and the formation of a martensitic structure was observed in the martensitic evaluation region B inside the crown and column. Therefore, it was found that in order to prevent the formation of a martensitic structure in the welded joint and suppress the decrease in hardness, it is necessary to control the holding temperature of the outer surface on the corner side of the crown within a certain range. Furthermore, the maximum temperature maintained was approximately the same on the outer surface of the top corner and the outer surface of the column.

[0079] Furthermore, the relationship between the heating and holding time in the process of maintaining the temperature of the welded joint 12 (S4) and the amount of martensitic structure formed (Figure 10), and the relationship between the heating and holding time and the hardness of the top outer surface (Figure 10) were evaluated. The cooling conditions for the welding rail and welded joint, and the heating conditions for the welded joint are as shown below. Other test conditions were the same as those for the welding test described above.

[0080] ● Welded rails The welding rail used to obtain the experimental results shown in Figure 7 was the same as the one used in the experiment. ● First accelerated cooling conditions for welded joints Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Average accelerated cooling rate of the outer surface on the corner side of the top of the head: 2.0°C / sec Average accelerated cooling rate of the outer surface of the column: 2.3°C / sec Temperature of the outer surface on the top corner side at the cessation of the first accelerated cooling: 570°C Temperature of the outer surface of the column at the cessation of the first accelerated cooling: 560°C ● Heating conditions for welded joints Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Timing of heating start for the outer surface of the top corner and the outer surface of the column: Within 200 seconds after cooling of the welded joint stops. Average heating rate of the outer surface on the top corner and the outer surface of the column: 0.5~2.0℃ / sec Temperature range for the outer surface of the top corner and the outer surface of the column: 620~670℃ Temperature retention time for the outer surface of the top corner and the outer surface of the column: 5 to 210 seconds Cooling after holding: Allow the outer surface of the top corner and the outer surface of the column to cool to below 200°C (average cooling rate of 0.1~0.4°C / sec on the outer surface of the top corner and the outer surface of the column).

[0081] As a result, as shown in Figure 10, it was found that if the temperature holding time of the outer surface on the corner side of the top of the welded joint exceeds 180 seconds, the welded joint is tempered, and the hardness of the outer surface on the top of the welded joint decreases. Furthermore, if the temperature holding time of the outer surface on the corner side of the top of the welded joint is less than 30 seconds, the pearlite transformation does not complete within the holding time, and the formation of a martensitic structure was observed in the martensitic evaluation region B inside the head and column. Therefore, it was found that in order to prevent the formation of a martensitic structure in the welded joint and suppress the decrease in hardness, it is necessary to control the temperature holding time of the outer surface on the corner side of the top of the welded joint within a certain range. Furthermore, the temperature retention time was approximately the same on the outer surface of the top corner and the outer surface of the column.

[0082] Next, we will explain the experimental results regarding the relationship between the cooling rate and the hardness of the top surface during the second accelerated cooling process (S5) of the welded joint (see Figure 11), and the relationship between the cooling stop temperature and the hardness of the top surface (see Figure 12). After holding S4, the welded joint may be allowed to cool. This significantly improves the wear resistance and fracture resistance of the welded joint. However, to further suppress the decrease in hardness of the outer surface of the top of the welded joint, it was considered preferable to perform accelerated cooling again after holding S4. Therefore, the relationship between the cooling conditions after heating and temperature holding and the hardness of the outer surface of the top was evaluated. Hereinafter, the accelerated cooling performed after holding S4 will be referred to as the second accelerated cooling S5.

[0083] The heating conditions, temperature holding conditions, and cooling conditions in the second accelerated cooling S5 of the welded joint are as follows. The cooling conditions and heating conditions for the weld rail and welded joint were set to conditions that would easily cause tempering. Other test conditions were the same as those for the welding test described above.

[0084] ● Welded rails The welding rail used to obtain the experimental results shown in Figure 7 was the same as the one used in the experiment. ● First accelerated cooling conditions for welded joints Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Average accelerated cooling rate of the outer surface on the corner side of the top of the head: 2.0°C / sec Average accelerated cooling rate of the outer surface of the column: 2.3°C / sec Temperature of the outer surface on the top corner side at the cessation of the first accelerated cooling: 570°C Temperature of the outer surface of the column at the cessation of the first accelerated cooling: 560°C ● Heating conditions for welded joints Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Heating start time: Within 200 seconds after the cooling of the welded joint has stopped immediately after welding. Average heating rate of the outer surface on the top corner and the outer surface of the column: 0.6°C / sec Temperature range for the outer surface of the top corner and the outer surface of the column: 630°C Temperature retention time for the outer surface of the top corner and the outer surface of the column: 80 seconds

[0085] ●Cooling conditions in the second type of accelerated cooling Average cooling rate of the outer surface on the top corner and the outer surface of the column: 0.1~2.0℃ / sec Cooling start temperature: When the temperature of the outer surface of the head side and the outer surface of the column side of the welding center A is within the range of 600°C or higher. Cooling stop temperature for the outer surface of the top corner and the outer surface of the column: 100~340℃ Cooling method: Nozzles are placed at equal intervals around the head of the welded joint, and air is sprayed in. *In experiments where the cooling rate is varied, the cooling stop temperature for the outer surface of the top corner and the outer surface of the column is fixed at 140°C. *In experiments where the cooling stop temperature is varied, the average cooling rate of the outer surface on the top corner and the outer surface of the column is fixed at 0.7°C / sec.

[0086] As a result, as shown in Figure 11, it was found that the hardness of the top corner outer surface of the welded joint was further improved by setting the average cooling rate of the top corner outer surface to 0.5°C / sec or higher during the second accelerated cooling. Furthermore, as shown in Figure 12, it was found that the hardness of the top corner outer surface of the welded joint was further improved by setting the temperature of the top corner outer surface to 200°C or lower when the cooling of the second accelerated cooling stopped. Therefore, it was found that in order to further improve the hardness of the welded joint, it is preferable to control the average cooling rate and the cooling stop temperature of the second accelerated cooling within a certain range. Furthermore, the average cooling rate and cooling stop temperature were approximately the same on the outer surface of the top corner and the outer surface of the column.

[0087] The above describes the experimental results that led to the discovery of the heat treatment method according to this embodiment. Next, the heat treatment method according to this embodiment will be described in detail.

[0088] (S1) Reasons for limiting the conditions of the first accelerated cooling process for the welded joint 12

[0089] The reasons for limiting the first accelerated cooling conditions will now be explained. The temperature measurement sites are the outer surface 1214 on the top corner side of the weld center A and the outer surface 1221 of the column section, as shown in Figure 2. The first accelerated cooling should be performed on the welded joint so that the temperature at these measurement sites satisfies the following requirements. Preferred examples of specific means for the first accelerated cooling will be described later.

[0090] (a) Conditions for initiating the first accelerated cooling of the welded joint The conditions for initiating the first accelerated cooling S1 of the welded joint will now be described. The first accelerated cooling S1 is initiated when the temperature of the outer surface on the top corner side and the outer surface of the column at the weld center of the welded joint is within the range of 700°C or higher. For example, this condition can be achieved by initiating the use of a cooling means for the first accelerated cooling S1 within 60 seconds from the end of the flash butt welding. Here, the end of the flash butt welding refers to the time when the upsetting process is completed and pressurization of the rail is finished. After the upsetting is complete, there is usually a process called trimming, which involves removing excess weld material from the weld joint. This process is performed within the above time (e.g., 60 seconds). Typically, at the end of flash butt welding, the temperature of the weld joint is close to the melting point of the steel. After welding is complete, the temperature of the weld joint decreases due to the temperature difference between the weld joint and the atmosphere, and heat transfer from the weld joint to the base metal. It is desirable to start using cooling means for the first accelerated cooling S1 before the temperature of the outer surface of the top corner side of the weld center A and the outer surface of the column falls below 700°C due to this temperature decrease. If the first accelerated cooling S1 is started before the temperature of the outer surface on the top corner side of the weld center A and the outer surface of the column falls below 700°C, the hardness of the top outer surface of the welded joint can be further increased. More preferably, the first accelerated cooling S1 is started within 55 seconds, 50 seconds, or 45 seconds after the completion of flash butt welding. Even more preferably, the first accelerated cooling S1 is started when the temperature of the outer surface on the top corner side of the weld center A is 720°C or higher, 750°C or higher, or 800°C or higher.

[0091] (b) Average accelerated cooling rate of the outer surface 1214 on the top corner side In the first accelerated cooling S1, if the average accelerated cooling rate of the top corner side outer surface 1214 exceeds 3.5°C / sec, the hardness of the re-γ portion of the welded joint increases. As a result, the hardness difference between the welded joint and the base material exceeds Δ30HV. In this case, as the number of years of use of the welding rail increases, the unevenness due to wear of the welded joint increases. That is, the wear resistance of the welded joint decreases.

[0092] Furthermore, when the average accelerated cooling rate of the outer surface 1214 on the top corner side falls below 1.0°C / sec, the pearlite transformation temperature rises, and the hardness of the pearlite produced by the transformation decreases. As a result, the hardness of the re-γ portion of the welded joint decreases, and the hardness difference between the welded joint and the base material exceeds Δ30HV. In this case as well, as the number of years of use of the welding rail increases, the unevenness due to wear of the welded joint increases. In other words, the wear resistance of the welded joint decreases.

[0093] Therefore, the average accelerated cooling rate of the top corner side outer surface 1214 in the first accelerated cooling S1 was limited to the range of 1.0 to 3.5°C / sec. Furthermore, the lower limit of the average accelerated cooling rate of the top corner side outer surface 1214 is preferably 1.1°C / sec or higher, or 1.2°C / sec or higher. Furthermore, the upper limit of the average accelerated cooling rate of the top corner side outer surface 1214 is preferably 2.9°C / sec or lower, or 2.8°C / sec or lower. In order to stably ensure the hardness of the re-γ portion of the welded joint and improve the wear resistance of the welded joint, it is desirable to set the average accelerated cooling rate to 1.1 to 2.9°C / sec.

[0094] (c) Average accelerated cooling rate of the outer surface 1221 of the column section In the first accelerated cooling S1, if the average accelerated cooling rate of the outer surface 1221 of the column exceeds 4.0°C / sec, the hardness of the re-γ portion of the welded joint increases. As a result, the toughness of the column decreases, and the fracture resistance of the welded joint decreases. Furthermore, in the first accelerated cooling S1, if the average accelerated cooling rate of the outer surface 1221 of the column falls below 1.0°C / sec, the pearlite transformation temperature increases, and the hardness of the pearlite produced by the transformation decreases. As a result, the strength of the column decreases, and the fatigue damage resistance of the welded joint decreases. Therefore, the average accelerated cooling rate of the column outer surface 1221 in the first accelerated cooling S1 was limited to the range of 1.0 to 4.0°C / sec. Furthermore, the lower limit of the average accelerated cooling rate of the column outer surface 1221 is preferably 1.1°C / sec or higher, or 1.2°C / sec or higher. Furthermore, the upper limit of the average accelerated cooling rate of the column outer surface 1221 is preferably 2.9°C / sec or lower, or 2.8°C / sec or lower. In order to stably ensure the hardness of the re-γ portion of the welded joint and improve the fracture resistance and fatigue damage resistance of the welded joint, it is desirable to set the average accelerated cooling rate to 1.1 to 2.9°C / sec. As mentioned above, the outer surface 1221 of the column refers to the surface of the portion midway between the center of the column 122 and the lower end of the head 121, and close to the lower jaw portion 1212. The cooling rate of the column is controlled in the region slightly towards the head from the center of the column.

[0095] (S2) Reasons for limiting the conditions for stopping the first accelerated cooling process. (d) Stop temperature of the first accelerated cooling When the first accelerated cooling stop temperature of the outer surface on the top corner and the outer surface of the column exceeds 600°C, the pearlite transformation temperature rises, and the hardness of the pearlite produced by the transformation decreases. As a result, the hardness of the re-γ portion of the welded joint decreases, and at the top, the hardness difference between the welded joint and the base material exceeds Δ30HV. In this case, as the number of years of service of the welding rail increases, the unevenness due to wear of the welded joint increases. That is, the wear resistance of the welded joint decreases. In addition, the strength of the column decreases, and the fatigue damage resistance of the welded joint decreases.

[0096] Furthermore, if the first accelerated cooling stop temperature for the outer surface of the top corner and the outer surface of the column falls below 500°C, a bainite structure detrimental to wear resistance may form on the outer surface of the top of the welded joint. Moreover, depending on the selection of the accelerated cooling temperature, a martensitic structure detrimental to toughness may form in the interior of the head and the column of the welded joint immediately after the accelerated cooling stops. Even after subsequent heating S3, this martensitic structure remains in the welded joint.

[0097] Therefore, the stopping temperature of the first accelerated cooling of the outer surface on the corner side of the crown and the outer surface on the column was limited to the range of 500 to 600°C. Furthermore, the lower limit of the stopping temperature of the first accelerated cooling of the outer surface on the corner side of the crown and the outer surface on the column is preferably 510°C or higher, or 520°C or higher. Furthermore, the upper limit of the stopping temperature of the first accelerated cooling of the outer surface on the corner side of the crown and the outer surface on the column is preferably 590°C or lower, or 580°C or lower. In order to stably ensure the hardness of the re-γ portion of the crown in the welded joint, suppress the formation of bainite structure on the outer surface of the crown, ensure the strength of the column, prevent the formation of martensitic structure inside the crown and column, and improve the wear resistance, fatigue damage resistance, and fracture resistance of the welded joint, it is desirable to set the stopping temperature of the first accelerated cooling of the outer surface on the corner side of the crown and the outer surface on the column to 510 to 590°C.

[0098] (S3) Reasons for limiting the conditions for the process of heating the welded joint 12 The reasons for limiting the heating conditions in heating S3 will be explained below. The temperature control points are the outer surface 1214 on the top corner side of the welding center A and the outer surface 1221 of the column, as shown in Figure 2. Heating should be performed on the welded joint so that the temperature at these measurement points satisfies the following requirements.

[0099] (e) Heating start time If the start time of heating S3 exceeds 300 seconds after the stop of the first accelerated cooling S1, the temperature inside the head and column of the welded joint will decrease before the start of heating S3. This will cause martensitic transformation to occur inside the head and column of the welded joint. The martensitic structure will not disappear in subsequent heating S3. For this reason, in order to prevent the formation of a martensitic structure in the welded joint, the start time of heating S3 is limited to within 300 seconds after the stop of the first accelerated cooling S1. Furthermore, the start time of heating S3 is preferably within 200 seconds or 100 seconds after the stop of the first accelerated cooling S1.

[0100] (f) Average heating rate When the average heating rate of the outer surface on the top corner and the outer surface of the column exceeds 2.0°C / sec, the temperature difference between the outer surface of the top side of the welded joint and the inside of the top, and between the outer surface of the column and the inside of the column, increases, preventing the top and inside of the column from heating up sufficiently. As a result, pearlite transformation in the inside of the top and inside of the column of the welded joint is not promoted, and a martensitic structure is formed. This phenomenon is particularly pronounced when the welded joint is heated by high-frequency heating. Furthermore, when the average heating rate falls below 0.5°C / sec, tempering occurs in the welded joint, and the hardness of the top and column decreases.

[0101] Therefore, in order to prevent the formation of martensitic structure inside the head and column sections of the welded joint and to suppress the decrease in hardness of the top outer surface, the average heating rate of the top corner side outer surface and the column outer surface was limited to the range of 0.5 to 2.0°C / sec. Furthermore, the lower limit of the average heating rate of the top corner side outer surface and the column outer surface is preferably 0.6°C / sec or higher, or 0.7°C / sec or higher. Furthermore, the upper limit of the average heating rate of the top corner side outer surface and the column outer surface is preferably 1.9°C / sec or lower, or 1.8°C / sec or lower. In order to stably ensure the hardness of the top and column sections of the welded joint, prevent the formation of martensitic structure inside the head and column sections, and improve the wear resistance, fatigue damage resistance, and fracture resistance of the welded joint, it is desirable to set the average heating rate of the top corner side outer surface and the column outer surface to 0.7 to 1.8°C / sec.

[0102] (S4) Reasons for limiting the conditions for the process of maintaining the temperature of the welded joint 12 (h) Holding temperature range When the temperature of the outer surface on the top corner side of the welded joint and the outer surface of the column section falls within the temperature range, the heat input from the heating means to the welded joint is reduced. This maintains the temperature of the outer surface on the top corner side of the welded joint and the outer surface of the column section within the temperature range.

[0103] If the holding temperature of the outer surface on the corner side of the top and the outer surface of the column exceeds 670°C, tempering occurs in the welded joint, reducing the hardness of the top and column of the welded joint. Furthermore, if the holding temperature of the outer surface on the corner side of the top and the outer surface of the column falls below 620°C, the pearlite transformation rate decreases significantly, preventing the completion of pearlite transformation inside the top and column of the welded joint, resulting in the formation of a martensitic structure.

[0104] Therefore, in order to suppress the decrease in hardness of the top and column portions of the welded joint and to prevent the formation of a martensitic structure, the holding temperature of the outer surface on the corner side of the top portion and the outer surface of the column portion was limited to the range of 620 to 670°C. When the temperature of the outer surface on the corner side of the top portion is maintained in the range of 620 to 670°C, the holding temperature of the outer surface of the column portion is often also within the range of 620 to 670°C or close to it. Furthermore, the lower limit of the holding temperature range for the outer surface on the corner side of the top portion and the outer surface of the column portion is preferably 625°C or higher, or 630°C or higher. Furthermore, the upper limit of the holding temperature range for the outer surface on the corner side of the top portion and the outer surface of the column portion is preferably 660°C or lower, or 650°C or lower. To ensure stable hardness at the top and column portions of the welded joint, prevent the formation of martensitic structure within the top and column portions, and improve the wear resistance and fracture resistance of the welded joint, it is desirable to set the temperature range of the outer surface on the corner side of the top portion and the outer surface of the column portion to 625 to 660°C.

[0105] It is not necessary to maintain a constant holding temperature. The temperature of the outer surface of the top corner side of the welded joint and the outer surface of the column may fluctuate within the holding temperature range of 620 to 670°C. On the other hand, if the maximum holding temperature of the outer surface of the top corner side and the outer surface of the column, which is the highest temperature during reheating, falls below 620°C or exceeds 670°C, the above-mentioned temperature holding effect will not be obtained.

[0106] (i) Retention time If the temperature holding time of the outer surface on the corner side of the crown and the outer surface of the column exceeds 180 seconds, tempering occurs in the welded joint, and the hardness of the crown and column decreases. Also, if the temperature holding time of the outer surface on the corner side of the crown and the outer surface of the column is less than 30 seconds, the pearlite transformation rate decreases significantly, and the pearlite transformation inside the crown and column is not completed, resulting in the formation of a martensitic structure.

[0107] Therefore, in order to prevent the formation of martensitic structure inside the head and column sections of the welded joint and to suppress the decrease in hardness of the head and column sections, the temperature holding time of the outer surface on the corner side of the head and the outer surface of the column section was limited to the range of 30 to 180 seconds. Furthermore, the lower limit of the temperature holding time of the outer surface on the corner side of the head and the outer surface of the column section is preferably 34 seconds or more, or 40 seconds or more. Furthermore, the upper limit of the temperature holding time of the outer surface on the corner side of the head and the outer surface of the column section is preferably 116 seconds or less, or 110 seconds or less. In order to stably ensure the hardness of the head and column sections of the welded joint, prevent the formation of martensitic structure inside the head and column sections, and improve the wear resistance and fracture resistance of the welded joint, it is desirable to set the temperature holding time of the outer surface on the corner side of the head and the outer surface of the column section to 40 to 110 seconds.

[0108] (S5) Reasons for limiting the conditions for the second accelerated cooling of the welded joint, which may be performed after holding S4. The cooling conditions for the welded joint after holding S4 are not particularly limited. For example, after holding S4 is completed, the welded joint may be left in the atmosphere for slow cooling (so-called air cooling). Alternatively, the welded joint may be accelerated again after holding S4, similar to the first accelerated cooling S1 performed immediately after flash butt welding. Hereinafter, the accelerated cooling performed after holding S4 will be referred to as the second accelerated cooling S5.

[0109] (j) Average cooling rate In the second accelerated cooling S5, if the average cooling rate of the outer surface on the corner side of the crown and the outer surface of the column is set to 0.5°C / sec or higher, tempering of the welded joint is suppressed, and the hardness of the crown and column of the welded joint is further increased. Therefore, in order to further increase the hardness of the welded joint, it is preferable to control the average cooling rate of the outer surface on the corner side of the crown and the outer surface of the column in the second accelerated cooling S5 to 0.5°C / sec or higher. Furthermore, the upper limit of the average cooling rate of the outer surface on the corner side of the crown and the outer surface of the column is preferably 0.6°C / sec or higher, or 0.7°C / sec or higher. In this case, it is preferable to start the second accelerated cooling S5 as quickly as possible. For example, it is preferable to start the second accelerated cooling S5 when the temperature of the outer surface on the crown side of the weld center A and the outer surface of the column are within the range of 600°C or higher.

[0110] (k) Cooling stop temperature Furthermore, if the cooling stop temperature of the outer surface on the corner side of the top and the outer surface of the column is set to 200°C or lower, tempering of the welded joint is suppressed, and the hardness of the top and column of the welded joint is further increased. Therefore, in order to further increase the hardness of the welded joint, it is preferable to control the cooling stop temperature of the outer surface on the corner side of the top and the outer surface of the column in the second accelerated cooling S5 to 200°C or lower. In addition, the upper limit of the cooling stop temperature of the outer surface on the corner side of the top and the outer surface of the column is preferably 180°C or 140°C.

[0111] Furthermore, it is desirable that the above temperature control be based on values ​​obtained by measuring the temperature of the outer surface 1214 on the top corner side of the weld center A of the welded joint, and the temperature of the outer surface of the column using a radiation thermometer or a contact thermometer. In addition, the average heating rate and average cooling rate can be controlled by adjusting the surface temperature and elapsed time based on the above temperature measurements.

[0112] The various conditions specified in the heat treatment method for the welded joint of a flash butt welded rail according to this embodiment have been described above. In the heat treatment method according to this embodiment, the cooling means and heating means for the welded joint are not particularly limited as long as the above conditions are met. Cooling means and heating means can be appropriately adopted according to the size of the welded joint, etc. Preferred examples of cooling means and heating means for the welded joint are described below.

[0113] (1) Preferred accelerated cooling method, cooling range, and cooling location for flash butt weld joints First, a preferred method for accelerating the cooling of a flash butt welded joint will be described. In the heat treatment method according to this embodiment, the accelerating cooling method is not particularly limited in either the first accelerating cooling S1 performed immediately after flash butt welding or the second accelerating cooling S5 performed after holding S4. It is desirable to use a method in which air or mist, etc., can be used as a refrigerant to selectively accelerate the cooling of the heat-affected zone of the welded joint, and to spray this refrigerant onto the welded joint.

[0114] Furthermore, all accelerated cooling methods must be applied to the re-gamma portion of the heat-affected zone, where hardness is prone to decrease. In addition, since wear resistance, fatigue damage resistance, and fracture resistance are characteristics of the head and column portions of the weld rail, accelerated cooling is performed to improve the hardness and microstructure of the head and column portions. Therefore, it is desirable to apply the refrigerant described above over the entire outer surface of the head and column portions at the weld joint. Note that if the refrigerant is applied to the head, it may flow to the column portion, resulting in sufficient cooling capacity and allowing for control of the cooling rate. In such cases, it may not be necessary to actively perform accelerated cooling on the column portion. Figure 15E shows a schematic diagram of an example of a cooling device 6. The cooling device 6 has a cylindrical shape and extends along the longitudinal direction of the welding rail 1. The cooling device 6 has a cooling gas outlet facing the welded joint portion of the welding rail. The cooling device 6 accelerates the cooling of the welded joint portion by blowing cooling gas onto the region including at least the heat-affected zone 12H (tempered portion 12HT and re-γ portion 12Hγ). In the schematic diagram of Figure 15E, the cooling device 6 is arranged to inject cooling gas onto the top outer surface 1211, the top corner side outer surface 1214, and the lower jaw portion 1212 of the welded joint portion, and the cooling gas is not directly blown onto the column portion 122. Since the cooling gas flows from the top to the column portion, even if the cooling device 6 is arranged as illustrated in Figure 15E, the heat treatment conditions for the column portion outer surface described above can be achieved. Alternatively, a further cooling device 6 that blows cooling gas onto the column portion may be provided.

[0115] As described above, the temperature measurement location for controlling accelerated cooling is preferably the outer surface 1214 on the top corner side of the welding center A (see Figure 2), which is representative of the outer surface of the head.

[0116] (2) Preferred heating method, heating range and control position of flash butt weld joint Next, a preferred method for heating the flash butt weld joint will be described. In the heat treatment method according to this embodiment, the method for performing heating S3 is not particularly limited. High-frequency heating or electric current heating using electrodes is desirable because it allows for selective heating of the weld joint and temperature control.

[0117] Furthermore, it is preferable to apply heating to the interior of the head and column sections of the welded joint where the martensitic structure is formed. In other words, it is preferable to heat the outer surface of the head and column sections of the welded joint. Heating may or may not be applied to the bottom section. Depending on the heating method selected, the bottom section of the welded joint may also be heated, but as long as it is within the range of the above heating conditions, it will not significantly affect the characteristics of the welded joint, so the entire welded joint may be heated. Figure 15F shows a schematic diagram of an example of a heating means. In Figure 15F, electrodes 3, positioned at the top and bottom and sandwiching the rail section, are used to heat the welded joint. These electrodes 3 are electrodes for flash butt welding (see schematic diagrams of flash butt welding shown in Figures 15A to 15C). By using electrodes for flash butt welding for heat treatment of the welded joint, the manufacturing equipment for flash butt welded rails can be simplified.

[0118] As described above, the temperature measurement position for controlling heating is preferably the outer surface 1214 on the top corner side of the welding center A (see Figure 2), as it represents the outer surface of the head and column. Of course, the outer surfaces of the head side and column may be controlled separately.

[0119] (2. Method for manufacturing flash butt welded joints) Next, a method for manufacturing a flash butt welded joint according to another aspect of the present invention will be described. As shown in the flowchart of Figure 13, the method for manufacturing a flash butt welded joint according to this embodiment comprises the steps of: obtaining a flash butt welded rail 1 by flash butt welding rail 2; deburring the welded joint portion of the flash butt welded rail; and heat treating the flash butt welded rail 1 according to the heat treatment method for the welded joint portion of the flash butt welded rail according to the above embodiment.

[0120] A schematic overview of a preferred example of a method for manufacturing a flash butt welded joint is shown schematically in Figures 14 and 15A to 15H. First, as shown in the perspective view of Figure 14 and the side view of Figure 15A, electrodes 3 are attached to a pair of rails 2 before welding. Typically, electrodes 3 are positioned to sandwich the top and bottom of the rails. Next, rail 2 is flash butted. Specifically, as shown in Figure 15B, current is first passed through electrode 3 to generate flash F in the space between the end faces of rail 2 (initial flash process). Next, as shown in Figure 15C, with the contact surfaces forced into contact, a large current is passed through the pair of rails 2 (rails that will be used as welding material) for a certain period of time to preheat the base material near the welding surface through resistive heating. (Preheating process) Furthermore, after preheating, the rails 2 (rails to be welded) are separated, and as shown in Figure 15D, a space is created at the end faces of the rails 2. Current is then passed through the electrodes 3 to generate a flash F in the space between the end faces of the rails 2, and the flash F accelerates the melting of the end faces of the rails 2. (Later flash process) At the end of the flash butt welding, the end faces of the pair of rails 2 are butted together and pressed, as shown in Figure 15E. This joins the pair of rails 2 to form a welded rail 1. During pressing, molten metal is discharged to the outside of the welded joint. This molten metal solidifies to form a burr 4. (Upset process) After the flash butt welding is complete, the burrs 4 are removed using the trimmer 5, as shown in Figure 15F. The process of removing the burrs 4 is sometimes called trimming. In Figure 15F, the trimmer 5 is moved from right to left across the page to remove the burrs 4. During trimming, the electrode 3 may get in the way of the work, so the electrode 3 may be removed from the rail. Note that in the schematic diagram of Figure 15E, the burrs 4 are formed only on the top and bottom surfaces of the welding rail 1, but in reality, burrs 4 are also formed on the sides of the welding rail 1. In the schematic diagram of Figure 15F, the trimmer 5 is positioned only on the top and bottom surfaces of the welding rail 1, but in reality, the trimmer 5 is also positioned on the sides of the welding rail 1 to remove the burrs 4 formed on the sides of the welding rail 1. Next, the welding rail 1 is subjected to heat treatment. Specifically, as shown in Figure 15G, the welding joint is first cooled using a cooling device 6 having a cooling gas outlet facing the welding joint. The cooling device 6 performs a first accelerated cooling of the welding joint by blowing cooling gas onto the welding joint. Furthermore, as shown in Figure 15H, the electrodes 3 are reattached to the welding rail 1 and the welding joint is heated by applying an electric current. Preferably, after the electric current heating is completed, the cooling device 6 is reattached to the welding rail 1 to perform a second accelerated cooling. The period from the end of flash butt welding shown in Figure 15E to the start of cooling shown in Figure 15G is preferably within approximately 60 seconds. In the inventors' welding equipment, by quickly removing electrode 3, trimming, and attaching cooling device 6, it was possible to reduce the period from the end of flash butt welding to the start of heat treatment to approximately 45 seconds. Although trimming lowers the temperature of the welded joint, in the inventors' welding equipment, the temperature of the weld center A at the start of heat treatment was 1000°C or higher.

[0121] In the method for manufacturing a flash butt welded joint according to this embodiment, heat treatment is performed after flash butt welding to ensure the hardness of the welded joint and to suppress the formation of martensite in the welded joint. Therefore, according to the method for manufacturing a flash butt welded joint according to this embodiment, a flash butt welded rail with excellent wear resistance and fracture resistance can be obtained.

[0122] The rails used for flash butt welding and the flash butt welding conditions are not particularly limited, but a preferred example is described below.

[0123] (1) Preferred chemical composition of rail steel First, the preferred chemical composition of the rail steel constituting the rails to be welded will be described. In the heat treatment method according to this embodiment, the chemical composition of the rail steel is not particularly limited. In order to ensure the minimum hardness and strength required for rails, a composition containing C: 0.75~1.20 mass%, Si: 0.10~2.00 mass%, and Mn: 0.20~2.00 mass% is used as a base, and if necessary, N: 0.020% or less, P: 0.025% or less, S: 0.025% or less, Cr: 0.05~2.00%, Mo: 0~0.50%, V: 0~0.100%, Nb: 0~ A compositional system containing one or more elements such as 0.0500%, B:0~0.0050%, Co:0~1.00%, Cu:0~1.00%, Ni:0~1.00%, Ti:0~0.0500%, Mg:0~0.0200%, Ca:0~0.0200%, Al:0~1.00%, Zr:0~0.0200%, and REM:0~0.0500%, with the remainder being iron and impurities, is desirable. The shape of the rail is not particularly limited. For example, the rail shape may be 115~140 pounds (57~70 kg / m).

[0124] (2) Preferred HAZ width of welded joint, flash butt welding conditions Next, the preferred HAZ width of the welded joint and the flash butt welding conditions will be described. Note that the HAZ is formed by flash butt welding. Therefore, the HAZ width will be a value corresponding to the flash butt welding conditions. Heat treatment after flash butt welding changes the hardness and structure of the HAZ, but does not change the HAZ width.

[0125] In the heat treatment method according to this embodiment, the HAZ width is not particularly limited, but it is desirable to set the HAZ width to a range of 10 to 40 mm, for example. Setting the HAZ width to 10 mm or more reduces the amount of heat transfer from the welded joint to the base material after the completion of flash butt welding, and further suppresses the formation of martensitic structure in the head and column portions of the welded joint. Furthermore, setting the HAZ width to 40 mm or less can further improve the hardness of the welded joint.

[0126] Furthermore, the flash butt welding conditions in the rail manufacturing method according to this embodiment are not particularly limited. For example, a welding method for controlling the HAZ width to the preferred range of 10 to 40 mm as described above is as follows.

[0127] There are two types of flash butt welding for rails: preheating flash welding and continuous flash welding. Either welding method can be applied to the rail manufacturing method according to this embodiment.

[0128] In the case of the preheating flash method, flash butt welding is performed as follows: (s11) Initial flash process (see Figure 15B) (s12) Preheating process (see Figure 15C) (s13) Late flash process (see Figure 15D), and (s14) Upset process (see Figure 15E) Includes.

[0129] (s11) Initial flash process The initial flash process is a flash process that begins with the rails at room temperature. Specifically, as shown in Figure 15B, current is first passed through electrode 3 to generate a flash F in the space between the end faces of rail 2. In order to facilitate contact of the welding surfaces in the subsequent preheating process, the initial flash process generates a flash between the end faces (i.e., the welding surfaces) of a pair of rails. This adjusts the welding surfaces perpendicular to the longitudinal direction of the rails. Furthermore, in the initial flash process, the welding surfaces are heated by the resistance heating of the flash and the arc heating. The time for the initial flash process, i.e., the initial flash time, is preferably between 10 seconds and 40 seconds.

[0130] (s12) Preheating process In the preheating process, as shown in Figure 15C, a large current is passed through a pair of rails for a certain period of time while the opposing weld surfaces of the pair of rails are forced into contact with each other. This generates resistive heating, which heats the base material near the weld surfaces. After that, the pair of rails are separated. The contact and separation of the weld surfaces is repeated one or more times. It is preferable to perform preheating (contact and separation of the weld surfaces) two or more times. More preferably, the number of preheating cycles is four or more, and even more preferably twelve or more.

[0131] (s13) Late flash process After preheating, in the later flash process, the rails (rails that will be welded) are separated, and as shown in Figure 15D, a space is created at the end faces of rail 2. Current is then passed through electrode 3 to generate a flash F in the space between the end faces of rail 2, and the flash F accelerates the melting of the end faces of rail 2. Specifically, in the later flash process, a flash is first generated partially between the opposing weld surfaces, and the weld surfaces are heated by the resistance heating and arc heating of this flash. Next, in the later flash process, the flash speed is increased. This causes the flash that was occurring in a part of the weld surface to occur across the entire weld surface. The entire weld surface is heated uniformly by the resistance heating and arc heating of this flash. Furthermore, in the later flash process, oxides generated during the preheating process are scattered and reduced by the flash. The flash speed is the speed at which the jigs that grip the pair of rails are brought closer together.

[0132] If the duration of the late flash process, i.e., the late flash time, is long, the HAZ width of the weld joint increases. Also, if the flash speed during the late flash process, i.e., the late flash speed, increases, the heat distribution near the weld surface becomes steeper, and as a result, the HAZ width of the weld joint decreases. For this reason, it is desirable to set the late flash time to 10 seconds or more and 30 seconds or less, the average late flash speed to 0.3 mm / sec or more, and the late flash speed immediately before upsetting (for 3 seconds) to 0.5 mm / sec or more. Here, the average late flash speed is the average value of the flash speed throughout the entire late flash process, and the late flash speed immediately before upsetting is the average value of the flash speed during the 3 seconds before upsetting begins. Furthermore, in order to reliably reduce the HAZ width of the weld joint, it is desirable that the late flash allowance, i.e., the amount of rail erosion during the late flash process, be 10 mm or more.

[0133] (s14) Upset process In the later flash process, the entire weld surface is melted. In the subsequent upset process, as shown in Figure 15E, the end faces of the pair of rails 2 are butted together and pressurized. That is, the weld surfaces are rapidly brought into close contact with high pressure, and most of the molten metal on the weld surface is discharged to the outside. Furthermore, in the upset process, pressure and deformation are applied to the high-temperature heated portion behind the weld surface, thereby forming the joint. In other words, some of the oxides generated during welding are discharged in the upset process, and the oxides remaining in the weld joint are finely dispersed. This makes it possible to reduce the possibility of oxides remaining on the joint surface as defects that hinder bending performance. In addition, the discharge of most of the molten metal to the outside in the upset process contributes to a reduction in the HAZ width of the weld joint. To reliably reduce the HAZ width of the weld joint, it is desirable to set the upset load to 50kN or more. More preferably, the upset load should be 65kN or more.

[0134] In the case of a continuous flash system, flash butt welding is (s21) Flash process, (s22) Upset process and It is equipped with [features]. Unlike the preheated flash welding method, the continuous flash butt welding method does not include a preheating process. (s21) In the flash process, a longer flash time increases the HAZ width of the weld joint. Conversely, increasing the flash speed makes the heat distribution near the weld surface steeper, resulting in a reduction of the HAZ width of the weld joint. For this reason, the flash time should be between 150 sec and 250 sec, and the flash speed should be 0.10 mm / sec or higher. (s22) In the case of the continuous flash method, the upset process should be performed under the same conditions as the upset process in the case of the preheating flash method described above. In order to reliably reduce the HAZ width of the welded joint, it is desirable to preheat with pulse flash or the like before the flash process to reduce the flash time and increase the flash speed.

[0135] In the upset step of flash butt welding, molten metal is discharged from the weld joint. The molten metal discharged from the weld joint solidifies and forms burrs at the weld joint. To remove these burrs, the manufacturing method of the flash butt welded joint according to this embodiment may include a step of deburring the weld joint after the completion of flash butt welding and before the start of heat treatment of the weld joint. Deburring is preferably performed for a short time, for example, within 60 seconds, so as not to interfere with the heat treatment of the weld joint.

[0136] (3) Preferred metallographic structure of welded joints

[0137] Next, a preferred metal structure of a welded joint in a welded rail obtained by the manufacturing method of a welded rail according to this embodiment will be described. However, the following description is not limited to the manufacturing method of a welded rail according to this embodiment. The metal structure described below is an example of a metal structure possessed by a welded joint in a welded rail that has excellent wear resistance, fatigue damage resistance, and fracture resistance.

[0138] Ensuring wear resistance is paramount at the rail head, where it comes into contact with the wheels. Investigations into the relationship between metal structure and wear resistance revealed that pearlite structure is the most suitable for the rail head at welded joints. Therefore, pearlite structure is desirable for the rail head (1 / 3h from the top surface) at welded joints. Note that "h" represents the height of the welded rail. For parts of the rail other than the welded joint, other metal structures are acceptable as long as they provide the necessary strength and ductility for the rail. [Examples]

[0139] The effects of one aspect of the present invention will be further explained in detail by the examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as it does not depart from the spirit of the invention and achieves the objectives of the present invention.

[0140] Rails having the components described in the table and below were manufactured, and welded rails were produced by flash butt welding under the welding conditions described below. Furthermore, the welded joints were heat-treated under the heat treatment conditions described below. In principle, "holding time" in Table 2 refers to the time during which the temperature of the outer surface 1214 on the top corner side of the weld center A was within the range of 620 to 670°C. However, in some experimental examples, the temperature was held in a temperature range of less than 620°C or more than 670°C. For these examples, the "holding time" column shows the isothermal holding time at the maximum temperature. The "average heating rate" column shows the average heating rate between the heating start temperature and the maximum holding temperature. In addition, "maximum holding temperature" in the table refers to the highest temperature of the outer surface on the top corner side when reheating was performed. In addition, "heating start time" in the table refers to the time from the cessation of the first accelerated cooling to the start of heating.

[0141] ● Rail Composition: 0.90% C, 0.50% Si, 0.80% Mn, 0.30% Cr, 0.0100% P, 0.0100% S, 0.0040% N and impurities Rail shape: 136 lbs (weight: 67 kg / m). Hardness of the base material: 400HV (measured on the outer surface 1111 of the top of the rail, see Figure 2)

[0142] ● Flash butt welding conditions (preheating flash method) Initial flash time: 20 seconds Preheating cycles: 10 Late flash duration: 30 seconds Average late flash speed: 0.6mm / sec Late flash speed immediately before upset (3 seconds): 1.4mm / sec Loss of melted material from the later flash: 10 mm Upset load: 75kN HAZ width: 30mm

[0143] ● Heat treatment conditions for welded joints • First accelerated cooling conditions for the welded joint immediately after welding Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Cooling method: Air is sprayed uniformly onto the head. Timing for starting cooling: When the temperature of the outer surface of the head side and the outer surface of the column side of the welding center A is within the range of 700°C or higher. Average cooling rate in the temperature range of 750°C to 600°C: As shown in the "Average Cooling Rate" column of Tables 1A and 1B. Tables 1A and 1B also show the average cooling rates for the outer surface of the top corner and the outer surface of the column, respectively. • First accelerated cooling shutdown conditions The first accelerated cooling shutdown temperature is as shown in the "Cooling Shutdown Temperature" column of Tables 1A and 1B. Tables 1A and 1B also show the cooling shutdown temperatures for the outer surface of the top corner and the outer surface of the column, respectively. • Heating conditions for welded joints Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Heating method: The head is uniformly heated using high-frequency heating. Average heating rate: As shown in the "Average heating rate" column of Table 2. Note that in all examples, the average heating rates on the top corner outer surface 1214 and the column outer surface 1221 were approximately the same. • Temperature holding conditions for welded joints Holding temperature and holding time: As described in the "Maximum Holding Temperature" and "Holding Time" columns of Table 2. In all examples, the holding temperature and holding time were approximately the same on the outer surface 1214 on the top corner side and the outer surface 1221 on the column side. • Second accelerated cooling conditions after heat treatment Control position: The outer surface 1214 on the top corner side of the welding center A, and the outer surface 1221 of the column (see Figure 2) Cooling method: Air is uniformly sprayed onto the head (see Figure 15E). Timing for starting cooling: When the temperature of the outer surface 1214 on the top corner side of the welding center A is within the range of 600°C or higher. Temperature measurement means: radiation thermometer Average cooling rate and cooling stop temperature: As shown in the "Average Cooling Rate" and "Cooling Stop Temperature" columns of Table 2. In all examples, the average cooling rate and cooling stop temperature were approximately the same on the top corner outer surface 1214 and the column outer surface 1221.

[0144] However, for some welded rails, the heat treatment conditions for the welded joints and the rail steel composition were modified as described above. Changes to the rail steel composition are noted in the "Remarks" column of the table. For example, in Example 29, "C: 1.20%" is written in the remarks column. In this Example 29, the carbon content was 1.20%, but the other chemical components were as described above. The same applies to other examples.

[0145] Furthermore, the wear resistance of the flash butt weld joint was estimated from the hardness characteristics of the weld joint, which have a high correlation with wear characteristics. It was evaluated that as the absolute value of the difference in hardness between the weld joint and the base material increases, the irregularities due to wear increase, and the wear resistance of the weld joint decreases. The evaluation index for wear resistance is as shown below. Absolute difference in hardness between the welded joint and the base material: 0-10 HV. Rating: A Absolute difference in hardness between welded joint and base material: Greater than 10-20 HV. Rating: B Absolute difference in hardness between welded joint and base material: Over 20-30 HV. Rating: C Absolute difference in hardness between welded joint and base material: Over 30 HV. Rating: X

[0146] Furthermore, the fracture resistance of the flash butt weld joint was evaluated based on the presence or absence of martensitic structure in the weld joint that could cause fracture. Martensite formation at welded joints: None. Rating: A Martensite formation at welded joints: Yes. Rating: X The methods for evaluating the hardness of the flash butt weld joint and the martensitic structure are as described above.

[0147] [Table 1A]

[0148] [Table 1B]

[0149] [Table 2]

[0150] In Example 2, heating was not performed after the first accelerated cooling S1. As a result, martensite formed in the welded joint in Example 2, resulting in insufficient fracture resistance.

[0151] In Example 3, the average cooling rate of the outer surface on the top corner side and the outer surface of the column section was excessive during the first accelerated cooling S1. As a result, the hardness of the welded joint in Example 3 became excessive, the difference in hardness between the rail section and the welded joint section became excessive, and the wear resistance was insufficient.

[0152] In Example 6, the average cooling rate of the outer surface on the top corner side and the outer surface of the column section was insufficient during the first accelerated cooling S1. As a result, the hardness of the welded joint in Example 6 was insufficient, the difference in hardness between the rail section and the welded joint section became excessive, and the wear resistance was insufficient.

[0153] In Example 7, the stopping temperature of the first accelerated cooling S1 was too high on the outer surface of the top corner and the outer surface of the column. As a result, in Example 7, the hardness of the welded joint was insufficient, the difference in hardness between the rail and the welded joint became excessive, and the wear resistance was insufficient.

[0154] In Example 10, the stopping temperature of the first accelerated cooling S1 was too low on the outer surface of the top corner and the outer surface of the column. As a result, martensite formed in the welded joint in Example 10, resulting in insufficient fracture resistance. This martensite is presumed to have formed during the first accelerated cooling S1.

[0155] In Example 11, the time between the cessation of the first accelerated cooling S1 and the start of heating S3 was too long. As a result, martensite formed in the welded joint in Example 11, resulting in insufficient fracture resistance. It is presumed that this martensite formed after the first accelerated cooling S1 but before the start of heating S3.

[0156] In Example 13, the average heating rate during heating S3 was excessive. As a result, martensite formed in the welded joint in Example 13, resulting in insufficient fracture resistance. This is presumed to be because the inside of the head of the welded joint did not heat up sufficiently, and the pearlite transformation was not promoted.

[0157] In Example 16, the average heating rate during heating S3 was insufficient. As a result, the hardness of the welded joint in Example 16 was insufficient, and the hardness difference between the rail and the welded joint became excessive, resulting in insufficient wear resistance. This is presumed to be because the welded joint was tempered.

[0158] In Example 17, the maximum holding temperature during holding S4 was too high. As a result, the hardness of the welded joint in Example 17 was insufficient, and the difference in hardness between the rail and the welded joint became excessive, resulting in insufficient wear resistance. This is presumed to be because the welded joint was tempered.

[0159] In Example 20, the heating termination temperature, i.e., the holding temperature during holding S4, was too low. As a result, martensite formed in the welded joint in Example 20, resulting in insufficient fracture resistance. This is presumed to be because the pearlite transformation rate decreased significantly, and the pearlite transformation was not completed.

[0160] In Example 21, the holding time in holding S4 was excessive. As a result, the hardness of the welded joint in Example 21 was insufficient, the hardness difference between the rail and the welded joint became excessive, and the wear resistance was insufficient. This is presumed to be because the welded joint was tempered.

[0161] In Example 24, the holding time during holding S4 was insufficient. As a result, martensite formed in the welded joint in Example 24, resulting in insufficient fracture resistance. This is presumed to be because the pearlite transformation was not completed within the holding time.

[0162] On the other hand, the welded rails obtained by the manufacturing method to which the heat treatment method according to this embodiment was applied exhibited excellent fracture resistance and wear resistance at the welded joints. [Explanation of Symbols]

[0163] S1 First Accelerated Cooling S2 Stop of the first accelerated cooling S3 heating S4 retention S5 Second Accelerated Cooling 1. Flash butt welding rail (welding rail) 11 Rail section 111 Rail Head 1111 Rail top outer surface 1112 Rail submandibular 1113 Rail head side outer surface 1114 Outer surface of the corner at the top of the rail 112 Rail column section 1121 Outer surface of rail column 113 Rail bottom 12 Welded joint 121 Head (of welded joint) 1211 Top surface of the welded joint 1212 (Sub-jaw portion of welded joint) 1213 Outer surface of the head side (of the welded joint) 1214 (Top corner side outer surface of welded joint) 122 Column section (of welded joint) 1221 Outer surface of column section (welded joint) 123 Bottom (of the welded joint) 12H Heat Affected Zone (HAZ) 12HT tempering section 12Hγ Re-γ part A Welding Center B. Martensite evaluation area 2. Rails before welding 3 electrodes 4 Bali 5 Trimmer 6 Cooling device F Flash

Claims

1. In a flash butt welding rail having a welded joint, the process includes a step of performing a first accelerated cooling on the welded joint after the completion of flash butt welding, The first step of stopping accelerated cooling, The process of heating the welded joint, The process includes a step of maintaining the temperature of the welded joint, The first accelerated cooling is initiated when the temperature of the outer surface of the top corner and the outer surface of the column at the welding center of the welded joint is within the range of 700°C or higher. In the first accelerated cooling described above, the average cooling rate of the outer surface on the top corner side of the welded joint at the weld center is set to 1.0 to 3.5°C / sec in the temperature range of 750°C to 600°C. In the first accelerated cooling described above, the average cooling rate of the outer surface of the column portion at the welding center of the welded joint portion in the temperature range of 750°C to 600°C is set to 1.0 to 4.0°C / sec. The first accelerated cooling is stopped when the temperature of the outer surface on the top corner side of the welded joint and the temperature of the outer surface of the column are within the range of 500 to 600°C. The heating is started within 300 seconds of the cessation of the first accelerated cooling. In the heating described above, the average heating rate of the outer surface on the top corner side of the welded joint and the average heating rate of the outer surface of the column at the welding center of the welded joint are set to 0.5 to 2.0°C / sec. During the holding process, the temperature of the outer surface of the top corner side at the welding center of the welded joint and the temperature of the outer surface of the column are maintained within the range of 620 to 670°C for 30 to 180 seconds. A heat treatment method for the welded joint portion of a flash butt welded rail, characterized by the features described herein.

2. Furthermore, after the step of maintaining the temperature of the welded joint, A second accelerated cooling process is performed on the welded joint so that the outer surface of the top corner side and the outer surface of the column at the weld center of the welded joint are cooled to 200°C or less at an average cooling rate of 0.5°C / sec or more. A heat treatment method for a welded joint of a welded rail according to claim 1, characterized by comprising the above.

3. The process of obtaining a flash-butt welded rail by flash-butting the rails, The process of deburring the welded joint portion of the flash butt weld rail, The step of heat-treating the flash butt weld rail according to the heat treatment method for the welded joint portion of the flash butt weld rail described in claim 1 or 2, A method for manufacturing a flash butt welded rail equipped with a

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