WO2025017960A1 - Method for manufacturing flash butt welding rail - Google Patents

Abstract

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

Description

Manufacturing method for flash butt welded rails

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

Flash butt welding is a widely used method for welding rails. Flash butt welding is known to have the advantages of being able to be automated, having high quality stability, and a short welding time.

Flash butt welding is a technique in which the rail ends are melted by heating, and then the molten surfaces are pressed together to join the rails. During flash butt welding, the rail is heated from room temperature up to nearly its melting point, and then cooled. As a result, flash butt welding causes changes in the metal structure and hardness of the rail. The area where the metallurgical and mechanical properties have changed due to the heat from welding, cutting, etc. is called the heat affected zone (HAZ).

In the HAZ, the metal structure becomes austenitic and transforms into pearlite when it is heated to above the A1 point during welding, and partial austenitization of the metal structure and decomposition of the pearlite structure occurs when it is heated to near the A1 point. In the areas that become austenitic during flash butt welding, there is a problem in that forced cooling, such as air cooling, performed immediately after welding to ensure the hardness of the welded part can produce martensite structures that are harmful to toughness.

The martensite structure is prone to becoming the starting point of fracture, and when it forms, rails are prone to breakage. This martensite structure tends to form easily in the head of the welded joint of the rail, where the cooling rate is relatively fast. The head is an area that requires wear resistance, and needs to be cooled faster than other areas to prevent softening. However, it also has the characteristic that martensite is prone to form due to supercooling.

The following technologies have been proposed for cooling welded joints obtained by flash butt welding of rails, etc., to improve the fatigue strength of the welded joints by controlling residual stress.

Patent Document 1 describes a cooling method for rail welds, characterized by cooling the column, base, and head in a temperature range until the transformation from austenite to pearlite is complete. The technology described in Patent Document 1 aims to reduce residual stress in rail welds and improve the fatigue resistance of rail welded joints.

In addition, for the welded joints of flash butt welded rails, the following technologies have been proposed to control the welding conditions and head cooling conditions and improve the life of the welded joints.

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

Furthermore, in rails with high carbon content, increasing the carbon content promotes the formation of pro-eutectoid cementite structures with low toughness in the welded joints of welded rails, increasing the possibility of rail breakage. Therefore, the following technologies have been proposed to improve the toughness of welded joints.

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

WO 2010/116680 JP 2010-100937 A JP 2004-043862 A

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

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

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

The present invention was devised in consideration of the above-mentioned problems, and aims to improve the breakage resistance of the welded joints of flash-butt welded rails. Preferably, the objective is to provide a manufacturing method that can satisfy the extremely strict requirements for breakage resistance of the welded joints of flash-butt welded rails for freight railways, which have a harsh track environment.

The gist of the present invention is as follows.
(1) A method for manufacturing a flash butt welded rail according to one aspect of the present invention is a method for manufacturing a flash butt welded rail having a plurality of rail portions and a weld joint portion joining the plurality of rail portions, the method comprising the steps of: C: 0.70-1.20%, Si: 0.05-2.00%, Mn: 0.05-2.00%, P≦0.0300%, S≦0.0300%, Cr: 0-2.00%, Mo: 0-0.50%, Co: 0-1.00%, B: 0-0.0050%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0.200%, Nb: 0-0.0500%, Ti: 0-0.0500%, Mg ... The method includes the steps of flash butt welding a rail containing Fe: 0-0.0200%, Ca: 0-0.0200%, REM: 0-0.0500%, N: 0-0.0200%, Zr: 0-0.0200%, and Al: 0-1.0000%, with the balance being Fe and impurities, and immediately after completion of the flash butt welding, forcibly cooling the welded joint, wherein in the forcible cooling from the austenite temperature range, the cooling stop temperature (TC) of the outer surface of the head corner side at the weld center of the welded joint is 560°C or higher and 850°C or lower, and the cooling stop temperature (TA) of the outer surface of the head and jaw is 650°C or higher and 950°C or lower.
(2) Preferably, in the manufacturing method of a flash butt welded rail described above in (1), the cooling stop temperature (TC) of the outer surface of the head corner side and the cooling stop temperature (TA) of the outer surface of the head jaw portion satisfy the following formulas 1 and 2.
TA+TC≧1340 1 0≦TA−TC≦140 2 (3) Preferably, in the manufacturing method of a flash butt welded rail described in (1) or (2) above, the rail has, as the chemical components, in mass%, one or two of the following: group a: Cr: 0.05% or more and 2.00% or less, Mo: 0.01% or more and 0.50% or less, group b: Co: 0.01% or more and 1.00% or less, group c: B: 0.0001% or more and 0.0050% or less, group d: Cu: 0.01% or more and 1.00% or less, and Ni: 0.01% or more and 1.00% or less, group e: V: 0.005% or more and 0.200% or less, b: one or more of 0.0010% or more and 0.0500% or less, and Ti: 0.0010% or more and 0.0500% or less; f group: one or more of Mg: 0.0005% or more and 0.0200% or less, Ca: 0.0005% or more and 0.0200% or less, and REM: 0.0005% or more and 0.0500% or less; g group: N: 0.0025% or more and 0.0200% or less; h group: Zr: 0.0001% or more and 0.0200% or less; i group: Al: 0.0010% or more and 1.0000% or less.
(4) Preferably, in the method for manufacturing a flash-butt welded rail described in any one of (1) to (3) above, the forced cooling is air cooling.

The above aspect of the present invention makes it possible to improve the breakage resistance of the welded joint in flash butt welded rails and significantly improve their service life.

Side view of welded rail Sectional view perpendicular to the longitudinal direction of a welded rail FIG. 1 is a perspective view illustrating the position of a longitudinal cross section of a welded joint of a welded rail; Schematic diagram of the cross-sectional hardness distribution 5 mm below the outer surface of the head jaw part of the longitudinal cross section of the welded joint part of the welded rail Schematic diagram of martensite structure evaluation area in a longitudinal cross section of a welded joint of a welded rail Schematic diagram of drop weight test Relationship between number of martensite structures formed in the head jaw and rail breakage (when the falling weight height is 5.0 m and the falling weight energy is 49.0 kN m) Relationship between number of martensite structures formed in the head jaw and rail breakage (when the falling weight height is 7.0 m and the falling weight energy is 73.5 kN m) Relationship between the cooling end temperature (TC) of the outer surface of the head corner and the number of martensite structures formed in the head jaw Relationship between the cooling end temperature (TA) of the outer surface of the head jaw and the number of martensite structures formed in the head jaw Relationship between the sum of the cooling end temperatures of the top corner side and the head jaw (TA + TC) and the number of martensite structures formed in the head jaw Relationship between the difference in cooling end temperature between the top corner side and the head jaw (TA-TC) and the number of martensite structures formed in the head jaw Relationship between the sum of the cooling end temperatures of the top corner side and the head jaw (TA + TC) and the number of martensite structures formed in the head jaw (when TA - TC = 145 to 250) Relationship between the difference in cooling end temperature between the top corner side and the head jaw (TA-TC) and the number of martensite structures formed in the head jaw (when TA+TC=1250-1330) An example of an Fe-Fe ₃ C equilibrium phase diagram for determining the austenite region temperature (quoted from "Steel Materials", edited by the Japan Institute of Metals). Appearance of the welded joint of a welded rail after trimming (bead cutting) Appearance of the welded joint of the welded rail after trimming FIG. 1 is a schematic diagram of an example of a cooling device suitable for implementing a method for manufacturing a welded rail.

In order to solve this problem, the inventors investigated the causes of breakage in welded joints and considered ways to prevent breakage caused by brittle fracture that occurs in the jaw of the head of a welded joint. As a result, they confirmed that breakage often begins with a brittle crack that occurs in the jaw of the rail head, and that martensite structures are formed at the origin of the brittle crack. As a result of analyzing the relationship between this martensite structure and the cooling conditions of the welded joint, the inventors clarified that there is a correlation between the cooling conditions and the formation of martensite structures. They also confirmed that the formation of martensite structures can be suppressed by controlling the cooling conditions of the welded joint. The inventors have discovered a manufacturing method for rails that stably improves the performance of welded joints of welded rails used in harsh operating environments.

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

As shown in FIG. 1, a flash butt welded rail (hereinafter simply referred to as "welded rail 1") comprises a plurality of rail portions 11 and a welded joint portion 12 that joins these rail portions 11. The inventors have conducted extensive research into methods for improving the breakage resistance of the welded joint portion 12. The inventors have discovered that the breakage resistance of the welded joint portion 12 can be improved by controlling the cooling stop temperature of the outer surface 1214 of the top corner side at the weld center A of the welded joint portion 12 and the cooling stop temperature of the outer surface 1212 of the head jaw portion of the welded joint portion 12 to a certain temperature or higher.

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

A method for manufacturing a flash butt welded rail according to one embodiment of the present invention, which was developed based on the above findings, will now be described in detail. First, the terms used in this embodiment will be explained.

A flash butt welded rail 1 is a rail obtained by joining rails together using flash butt welding. Hereinafter, the flash butt welded rail 1 will be referred to simply as "welded rail 1."

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

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

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

The rail head corner side outer surface 1114 is an outer surface located 0.25W to 0.35W away from the center (B) of the width (W) of the rail head 111 toward the rail head side outer surface 1113 as shown in Figure 2. The rail head jaw outer surface 1112 is an outer surface located 0.25W to 0.35W away from the center (B) of the width (W) of the rail head toward the rail head side outer surface 1113 as shown in Figure 2. Note that, of course, the up-down direction of the welded rail 1 means the up-down direction when the welded rail 1 is used as a track.

In FIG. 2, a dashed line is shown that is 0.25W away from the center of the width of the rail head 111 and runs along the vertical direction of the rail portion 11, and a dashed line is shown that is 0.35W away from the center of the width of the rail head 111 and runs along the vertical direction of the rail portion 11. The area of the rail head top surface that is located between these dashed lines is the rail head corner side outer surface 1114. Furthermore, the area of the surface of the rail head jaw that is located between these dashed lines is the rail head jaw outer surface 1112.

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

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

Heat-affected zone (HAZ) 12H is JIS Z
As defined in JIS C 3001-1:2018, the term refers to a non-melted base material portion whose metallurgical properties, mechanical properties, etc. have been changed by heat from welding, cutting, etc. In this embodiment, the base material refers to the rail portion 11.

As shown in Figure 3, the welded rail 1 according to this embodiment has a width of the heat-affected zone 12H along the longitudinal direction of the welded rail 1, i.e., the HAZ width. In the welded rail 1 according to this embodiment, the HAZ width is defined based on the hardness distribution of the welded joint 12 measured on a cross section that is parallel to the longitudinal direction of the welded rail 1 and passes through a cross section located 0.25W to 0.35W away from the center (B) of the width of the head of the welded rail 1 toward the head side outer surface 1213. In this embodiment, the cross section that is parallel to the longitudinal direction of the welded rail 1 and passes through a cross section located 0.25W to 0.35W away from the center (B) of the width of the head of the welded rail 1 toward the head side outer surface 1213 is referred to as the "longitudinal cross section." In addition, in FIG. 3, a dashed line is drawn that is 0.25W away from the center of the width direction of the rail head 111 and that runs along the vertical direction of the rail portion 11, and a dashed line is drawn that is 0.35W away from the center of the width direction of the rail head 111 and that runs along the vertical direction of the rail portion 11. The longitudinal cross section is a cut surface that runs along the vertical direction of the rail and is formed at any point between these dashed lines. Below, we will explain the outline of the hardness distribution of the welded joint portion 12, and then we will explain the definition of the HAZ width.

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

In the welded joint 12, a region is formed in which the welded joint is heated to above the A1 point by the welding heat, becomes austenitic overall, and then transforms into pearlite as the temperature is lowered after welding is completed. In addition, on both sides of this region, there are regions in which the welded joint is heated to near the A1 point by the welding heat, becomes partially austenitic, and then the pearlite structure decomposes as the temperature is lowered after welding is completed. In these regions, the hardness is significantly lower. Therefore, in a graph of the hardness distribution of a welded rail 1 obtained by flash butt welding, there are usually two Vickers hardness valleys, as shown in Figure 4. The locations where these Vickers hardness valleys occur are defined as the softest parts of the welded rail 1 according to this embodiment. The distance between the two softest parts is defined as the HAZ width. The weld center A approximately coincides with the center of this HAZ width.

As shown in Figure 5, the martensite structure evaluation area C refers to the area (C) in the longitudinal cross section that is within a range of ±5 mm (width 10 mm) in the longitudinal direction of the welded rail 1 centered on the weld center A, and has a depth of 1 to 5 mm from the head jaw outer surface 1212 toward the head corner side outer surface 1214. The technical significance of the martensite structure evaluation area C will be described later.

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

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

Next, the technical concept of the present invention will be explained. The inventors investigated damage that occurs in the welded joint 12 of the welded rail 1. As a result of investigating damaged rails that occurred on an actual track, it was confirmed that the most common form of damage was breakage that originated from a brittle crack that occurred at the head of the welded rail.

First, the inventors identified the starting point of the breakage in the welded rail. As a result, they confirmed that in many cases, the breakage occurred in the heat-affected zone (HAZ).

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

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

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

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

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

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

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

Evaluation of martensite structure Evaluation area (see Figure 5)
In a longitudinal cross section of the welded joint, this means a region within a range of ±5 mm (width 10 mm) from the weld center (A) in the longitudinal direction of the weld rail, and a region within a range of a depth of 1 to 5 mm from the head jaw outer surface 1212 to the outer surface of the top corner side (martensite structure evaluation region: C).
Reason for selecting the evaluation area: This is the location where the starting point of rail breakage occurs.
Method for Revealing Martensite Structure After the martensite structure evaluation region (C) is polished, it is etched with nital and observed under an optical microscope.
Polishing conditions: buff polishing with 1 μm diamond paste Martensite etching conditions Etching solution: alcohol + 5% nitric acid (nital)
Etching time: 5 to 10 seconds Method for examining structure Equipment: Optical microscope Magnification: 400x Method for evaluating structure Martensite structures that could be confirmed under an optical microscope at 400x were evaluated.
The target martensite structure had a major axis of 25 to 100 μm, and when martensite structures were generated, the number of martensite structures was investigated.

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

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

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

Furthermore, in order to control the amount of martensite structure generated in martensite structure evaluation region C, the inventors investigated the relationship between the heat treatment conditions after flash butt welding and the generation of martensite structure in martensite structure evaluation region C.

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

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

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

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

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

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

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

The inventors considered that in order to reduce the amount of martensite structure produced, it would be preferable to mutually control the cooling stop temperature of the head jaw outer surface 1212 and the cooling stop temperature of the top corner outer surface 1214. Experiments were then conducted focusing on the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner outer surface 1214, as well as the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner outer surface 1214.

A flash butt welding test was performed using hypereutectoid steel rails (0.70-1.20% C), and then the cooling stop temperature (TA) of the head jaw outer surface 1212 of the welded joint 12 and the cooling stop temperature (TC) of the head corner outer surface 1214 were changed to evaluate the relationship between the cooling stop temperature conditions and the amount of martensite structure generated.

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

Cooling conditions for welded joints after welding In the case of controlling the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 at the weld center (A) and the cooling stop temperature (TC) of the head corner outer surface 1214 (FIG. 11)
TA-TC=50 (fixed)
TA: 650-800℃, TC: 600-750℃
T A ≧ T C
In the case of controlling the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 at the weld center (A) and the cooling stop temperature (TC) of the head corner side outer surface 1214 (FIG. 12)
TA+TC=1400 (fixed)
TA: 700-800℃, TC: 600-700℃
T A ≧ T C
Subsequent cooling (outer surface of head jaw, outer surface of top corner): Cool naturally (until 50°C)

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

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

From these findings, it was confirmed that by controlling the sum or difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner outer surface 1214, it is possible to control the amount of martensite structure generated to 5 or less, thereby further improving the breakage resistance of the rail welded joint.

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

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

Therefore, in order to control the number of martensite structures formed to 5 or less, it is preferable to control both the sum and the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner outer surface 1214.

From these results, it is possible to control the cooling stop temperature (TC) of the head corner side outer surface 1214 at the weld center A of the weld joint 12 in a flash butt welded rail 1, and the cooling stop temperature (TA) of the head jaw outer surface 1212, and to suppress the amount of martensite structure generated in the martensite structure evaluation region C of the longitudinal cross section, thereby further suppressing breakage caused by brittle cracks that originate from the head 121 of the welded rail 1, and greatly improving the service life of the welded rail 1.

The manufacturing method for flash butt welded rail 1, which was obtained based on the above findings, is described in detail below. Hereinafter, the unit of "mass %" for the content of alloy components will be simply written as "%".

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

(C: 0.70-1.20%)
C is an element that promotes pearlite transformation and is effective in ensuring the wear resistance of the welded joint 12. If the C content is less than 0.70%, the minimum wear resistance required for the welded joint 12 cannot be obtained. On the other hand, if the C content exceeds 1.20%, a large amount of pro-eutectoid cementite structure is formed in the welded joint 12, and the breakage resistance of the welded joint 12 decreases. For this reason, the C content is limited to 0.70 to 1.20%. The C content is preferably 0.72% or more, 0.75% or more, or 0.80% or more. Preferably, the C content is 1.18% or less, 1.15% or less, or 1.10% or less. In order to stabilize the formation of pearlite structures, the C content is set to 0.80 to 1.10%. It is desirable.

(Si: 0.05-2.00%)
Silicon is an element that dissolves in the ferrite phase of the pearlite structure, increases the hardness of the welded joint 12, and improves the wear resistance. However, if the amount of silicon is less than 0.05%, these effects are not achieved. On the other hand, if the Si content exceeds 2.00%, the toughness of the pearlite structure decreases, and the fracture resistance of the welded joint 12 decreases. For this reason, the Si content is set to 0.05 to 2.00%. The Si content is preferably 0.10% or more, 0.20% or more, 0.30% or more, or 0.40% or more. The Si content is preferably 1.80%. In order to stabilize the formation of pearlite structure and improve the fracture resistance of the welded joint 12, the Si content is set to 0.40 to 1.60%, or 1.50%. It is preferable that the content be 2.00%.

(Mn: 0.05-2.00%)
Mn is an element that increases the hardenability of the welded rail 1 and stabilizes the pearlite transformation, while at the same time, refines the lamellar spacing of the pearlite structure, ensures the hardness of the welded joint 12, and further improves the wear resistance. However, if the Mn content is less than 0.05%, the effect is small and the wear resistance of the welded joint 12 decreases. On the other hand, if the Mn content exceeds 2.00%, the excessive amount of Mn This promotes Mn concentration in the segregated areas, promotes the formation of martensite structures in the welded joint 12, and reduces the breakage resistance. For this reason, the Mn content is limited to 0.05 to 2.00%. The Mn content is preferably 0.10% or more, 0.20% or more, 0.30% or more, or 0.40% or more. The Mn content is preferably 1.80% or less, 1.60% or less. or less, or 1.50% or less. In order to stabilize the formation of pearlite structures and improve the wear resistance and breakage resistance of the welded joint 12, the Mn content is preferably set to 0.40 to 1.50%.

(P≦0.0300%)
P is an impurity element contained in steel. If the P content exceeds 0.0300%, the pearlite structure becomes embrittled, and the fracture resistance of the welded joint 12 decreases. For this reason, the P content The lower limit of the P content does not need to be limited, and may be, for example, 0%, but taking into consideration the dephosphorization capacity of the refining process, the lower limit of the P content is set to 0. The P content is preferably 0.0025% or more, 0.0030% or more, or 0.0050% or more. The P content is preferably 0.0250% or less, 0.0200% or less. In order to stably maintain the toughness of the pearlite structure, the P content is preferably 0.0050 to 0.0150%.

(S≦0.0300%)
S is an impurity element contained in steel. If the S content exceeds 0.0300%, stress concentration occurs around the inclusions of coarse MnS-based sulfides, and the durability of the welded joint 12 is reduced. For this reason, the S content is limited to 0.0300% or less. It is not necessary to set a lower limit for the S content, and it may be, for example, 0%, but it is necessary to set the lower limit in consideration of the desulfurization capacity of the refining process. In this case, the lower limit of the S content may be set to about 0.0020%. The S content is preferably 0.0025% or more, 0.0030% or more, or 0.0050% or more. In order to stably maintain the fracture resistance of the pearlite structure, the S content is set to 0.0050 to 0.0150%. % is preferable.

The remainder of the chemical components of the rail, which is the material for the welded rail, includes iron and impurities. Impurities are, for example, components that are mixed in with raw materials such as ore or scrap during the industrial production of steel, or due to various factors in the manufacturing process, and are acceptable within the range that does not adversely affect the welded rail 1 according to this embodiment.

Furthermore, the rail, which is the material of the welded rail, may contain one or more of the following elements as necessary: Cr and Mo in group a, Co in group b, B in group c, Cu and one or two of Ni in group d, V, Nb, and Ti in group e, Mg, Ca, and one or two of REM in group f, N in group g, Zr in group h, and Al in group i. However, since the manufacturing method of the welded rail 1 according to this embodiment can exert its effect even if these elements are not contained in the rail, the lower limit of the content of these elements is 0%.

<Group A>

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

(Mo: preferably 0.50% or less)
Mo is an element that increases the equilibrium transformation temperature, refines the lamellar spacing of the pearlite structure by increasing the degree of supercooling, improves the hardness of the pearlite structure, and improves the wear resistance of the welded joint 12. In order to obtain the above-mentioned effects, it is preferable to set the Mo content to 0.01% or more. On the other hand, if the Mo content exceeds 0.50%, an excessive amount of Mo promotes Mo concentration in the segregation portion, promotes the formation of the martensite structure of the welded joint 12, and may reduce the breakage resistance. For this reason, it is desirable to set the Mo content to 0.01 to 0.50%. The Mo content is preferably 0.02% or more, 0.05% or more, or 0.10% or more. The Mo content is preferably 0.45% or less, 0.40% or less, or 0.30% or less. Therefore, in order to stably improve the hardness of the pearlite structure and improve the wear resistance and damage resistance of the welded joint 12, it is desirable to set the Mo content to 0.10 to 0.30%.

<Group B>
(Co: preferably 1.00% or less)
Co is an element that dissolves in the ferrite phase of the pearlite structure, refines the lamellar structure of the pearlite structure immediately below the rolling surface where deformation occurs due to contact with the wheel, improves the hardness of the rolling surface, and improves the wear resistance of the welded joint portion 12. In order to obtain the above-mentioned effect, it is preferable to set the Co content to 0.01% or more. On the other hand, if the Co content exceeds 1.00%, the above-mentioned effect is saturated and it is not possible to refine the lamellar structure according to the Co content. In addition, if the Co content exceeds 1.00%, the economic efficiency may decrease due to an increase in alloy cost. For this reason, it is desirable to set the Co content to 0.01 to 1.00%. The Co content is preferably 0.02% or more, 0.05% or more, or 0.10% or more. The Co content is preferably 0.90% or less, 0.80% or less, or 0.60% or less. Therefore, in order to stably improve the wear resistance of the pearlite structure and improve the wear resistance of the welded joint 12, the Co content is preferably set to 0.10 to 0.60%.

<Group C>
(B: preferably 0.0050% or less)
B is an element that forms iron boride (Fe ₂₃ (CB) ₆ ) at the austenite grain boundary, reduces the cooling rate dependency of the pearlite transformation temperature by promoting the pearlite transformation, uniforms the hardness distribution from the head surface to the inside of the welded joint 12, and extends the life of the welded joint 12 by improving the wear resistance. In order to obtain the above-mentioned effect, it is preferable that the B content is 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, coarse iron boride is generated, which promotes brittle fracture and may reduce the breakage resistance of the welded joint 12. For this reason, it is desirable to set the B content to 0.0001 to 0.0050%. The B content is preferably 0.0002% or more, 0.0003% or more, or 0.0005% or more. The B content is preferably 0.0040% or less, 0.0030% or less, or 0.0025% or less. Therefore, in order to stably maintain the toughness of the pearlite structure and improve the wear resistance of the welded joint 12, the B content is preferably set to 0.0005 to 0.0025%.

<Group d>
(Cu: preferably 1.00% or less)
Cu is an element that dissolves in the ferrite phase of the pearlite structure, improves the hardness of the welded joint 12 by solid solution strengthening, and improves the wear resistance of the welded joint 12. In order to obtain the above-mentioned effect, it is preferable to set the Cu content to 0.01% or more. On the other hand, if the Cu content exceeds 1.00%, an excessive amount of Cu promotes Cu concentration in the segregation portion, promotes the formation of the martensite structure of the welded joint 12, and may reduce the breakage resistance. For this reason, it is preferable to set the Cu content to 0.01 to 1.00%. The Cu content is preferably 0.02% or more, 0.05% or more, or 0.10% or more. The Cu content is preferably 0.90% or less, 0.80% or less, or 0.70% or less. Therefore, in order to stably maintain the toughness of the pearlite structure and improve the wear resistance of the welded joint 12, it is desirable to set the Cu content to 0.10 to 0.70%.

(Ni: preferably 1.00% or less)
Ni is an element that improves the toughness of the pearlite structure, and at the same time, improves the hardness of the welded joint 12 by solid solution strengthening, thereby improving the wear resistance of the welded joint 12. Furthermore, in the heat-affected zone, Ni combines with Ti to precipitate as a fine Ni ₃ Ti intermetallic compound, and is an element that suppresses the softening of the welded joint 12 by precipitation strengthening. Furthermore, when Cu is contained in the rail, Ni suppresses the embrittlement of the grain boundary. In order to obtain the above-mentioned effect, it is preferable to set the Ni content to 0.01% or more. If the Ni content exceeds 1.00%, an excessive amount of Ni may promote Ni concentration in the segregation portion, promote the formation of the martensite structure of the welded joint 12, and reduce the breakage resistance. For this reason, it is desirable to set the Ni content to 0.01 to 1.00%. The Ni content is preferably 0.02% or more, 0.05% or more, or 0.10% or more. The Ni content is preferably 0.90% or less, 0.80% or less, or 0.70% or less. Therefore, in order to stably maintain the toughness of the pearlite structure and improve the wear resistance of the welded joint 12, it is desirable to set the Ni content to 0.10 to 0.70%.

<Group e>
(V: preferably 0.200% or less)
V is an element that increases the hardness (strength) of the pearlite structure by precipitation hardening due to V carbo-nitrides formed during the cooling process after hot rolling, and improves the fatigue damage resistance of the welded joint 12. In order to obtain the above-mentioned effect, it is preferable to set the V content to 0.005% or more. On the other hand, if the V content exceeds 0.200%, the number of fine V carbo-nitrides becomes excessive, the pearlite structure becomes embrittled, and the fracture resistance of the welded joint 12 may decrease. For this reason, it is desirable to set the V content to 0.005 to 0.200%. The V content is preferably 0.010% or more, 0.015% or more, or 0.020% or more. The V content is preferably 0.180% or less, 0.150% or less, or 0.100% or less. Therefore, in order to stably maintain the breakage resistance of the welded joint 12 and improve the fatigue damage resistance of the welded joint 12, it is desirable to set the V content to 0.020 to 0.100%.

(Nb: preferably 0.0500% or less)
Nb is an element that increases the hardness of the pearlite structure and improves the fatigue damage resistance of the welded joint 12 by precipitation hardening due to Nb carbides and Nb nitrides generated during the cooling process after hot rolling in the manufacture of rails. In addition, in the heat-affected zone 12H reheated to a temperature range below the Ac1 point, Nb is an element that stably generates Nb carbides and Nb nitrides in a wide temperature range from low to high temperatures, and is effective in preventing softening of the heat-affected zone 12H of the welded joint 12. In order to obtain the above-mentioned effect, it is preferable to set the Nb content to 0.0010% or more. On the other hand, if the Nb content exceeds 0.0500%, the precipitation hardening of Nb carbides and nitrides becomes excessive, the pearlite structure itself becomes embrittled, and the fracture resistance of the welded joint 12 may decrease. For this reason, it is preferable to set the Nb content to 0.0010 to 0.0500%. The Nb content is preferably 0.0020% or more, 0.0025% or more, or 0.0030% or more. The Nb content is preferably 0.0400% or less, 0.0300% or less, or 0.0200% or less. Therefore, in order to stably maintain the breakage resistance of the welded joint 12 and improve the fatigue damage resistance of the welded joint 12, it is desirable to set the V content to 0.0030 to 0.0200%.

(Ti: preferably 0.0500% or less)
Ti is an element that increases the hardness of the pearlite structure by precipitation hardening due to Ti carbides and Ti nitrides generated during the cooling process after hot rolling in the manufacture of rails, thereby improving the fatigue damage resistance of the welded joint 12. Ti is also an element that refines the structure of the heat-affected zone 12H that is reheated to the austenite region by utilizing the fact that Ti carbides and Ti nitrides precipitated during reheating after welding do not dissolve in the matrix, thereby improving the breakage resistance of the welded joint 12. In order to obtain the above-mentioned effect, it is preferable that the Ti content is 0.0010% or more, or 0.0060% or more. On the other hand, if the Ti content exceeds 0.0500%, coarse Ti carbides and Ti nitrides are generated, and fatigue cracks are easily generated due to stress concentration around these, and the fatigue damage resistance of the welded joint 12 may be reduced. For this reason, it is preferable that the Ti content is 0.0040 to 0.0500%. The Ti content is preferably 0.0040% or more, 0.0050% or more, or 0.0060% or more. The Ti content is preferably 0.0400% or less, 0.0300% or less, or 0.0200% or less. Therefore, in order to improve the fatigue damage resistance and breakage resistance of the welded joint 12, it is desirable to set the Ti content to 0.0060 to 0.0200%.

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

(Ca: preferably 0.0200% or less)
Ca has a strong bond with S and forms sulfides (CaS), which finely disperse MnS, reduce stress concentration around MnS, and improve the fatigue damage resistance of the welded joint 12. In order to obtain the above-mentioned effect, it is preferable to set the Ca content to 0.0005% or more. On the other hand, if the Ca content exceeds 0.0200%, coarse oxides of Ca are generated, and fatigue cracks are likely to be generated due to stress concentration around the coarse oxides, and the fatigue damage resistance of the welded joint 12 may decrease. For this reason, it is desirable to set the Ca content to 0.0005 to 0.0200%. The Ca content is preferably 0.0010% or more, 0.0020% or more, or 0.0030% or more. The Ca content is preferably 0.0180% or less, 0.0150% or less, or 0.0120% or less. Therefore, in order to improve the fatigue damage resistance of the welded joint 12, the Ca content is preferably set to 0.0030 to 0.0120%.

(REM: preferably 0.0500% or less)
REM is a deoxidizing and desulfurizing element, and generates REM oxysulfide (REM ₂ O ₂ S), which becomes the nucleus for the formation of Mn sulfide-based inclusions. Oxysulfide (REM ₂ O ₂ S) has a high melting point, and suppresses the elongation of Mn sulfide-based inclusions after rolling. As a result, REM finely disperses MnS, relieves stress concentration around MnS, and improves fatigue damage resistance of the welded joint 12. In order to obtain the above-mentioned effect, it is preferable that the REM content is 0.0005% or more. On the other hand, if the REM content exceeds 0.0500%, coarse and hard REM oxysulfide (REM ₂ O ₂ S) is generated, and fatigue cracks are easily generated due to stress concentration around this oxysulfide, and the fatigue damage resistance of the welded joint 12 may be reduced. For this reason, it is desirable to set the REM content to 0.0005 to 0.0500%. The REM content is preferably 0.0010% or more, 0.0020% or more, or 0.0030% or more. The REM content is preferably 0.0400% or less, 0.0300% or less, or 0.0250% or less. Therefore, in order to improve the fatigue damage resistance of the welded joint 12, it is desirable to set the REM content to 0.0030 to 0.0250%.

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

<Group g>
(N: preferably 0.0200% or less)
N is an element that may be mixed into rails as an impurity in the steelmaking process. Even if degassing is actively performed, about 0.0025% N may remain in the steel. In normal rail refining, the N content is about 0.0030 to 0.0050%. It is possible to make the N content less than 0.0025%, but in order to avoid an increase in refining costs, 0.0025% or more of N may be contained in the rail. In addition, N is an element that is effective in improving the toughness of the welded joint 12 by promoting pearlite transformation from the austenite grain boundary by segregating at the austenite grain boundary, mainly by refining the pearlite block size. In addition, when N and V are simultaneously contained, the precipitation of V carbonitrides is promoted during the cooling process of the welded joint 12 after welding of the rail, the hardness of the pearlite structure is increased, and the fatigue damage resistance of the welded joint 12 is improved. In order to obtain the above-mentioned effect, it is preferable that the N content is 0.0060% or more. On the other hand, if the N content exceeds 0.0200%, it becomes difficult to dissolve N in the steel, and bubbles that are the starting point of fatigue damage may be easily generated. For this reason, it is desirable to set the N content to 0.0060 to 0.0200%. The N content is preferably 0.0060% or more, 0.0070% or more, or 0.0080% or more. The N content is preferably 0.0180% or less, 0.0160% or less, or 0.0150% or less. Therefore, in order to stably improve toughness and fatigue damage resistance, it is desirable to set the N content to 0.0080 to 0.0150%.

<H group>
(Zr: preferably 0.0200% or less)
Zr forms _ZrO2 inclusions that have good lattice matching with γ-Fe, and thus serves as the solidification nucleus of high-carbon rail steel in which γ-Fe is the solidification primary crystal, and by increasing the equiaxed crystallization rate of the solidification structure, it suppresses the formation of a segregation zone in the center of the slab and suppresses alloy concentration in the segregation portion. As a result, Zr suppresses the formation of a martensite structure in the welded joint 12, improving the breakage resistance and fatigue damage resistance. In order to obtain the above-mentioned effect, it is preferable that the Zr content is 0.0001% or more. On the other hand, if the Zr content exceeds 0.0200%, a large amount of coarse Zr-based inclusions are formed, and fatigue cracks are easily generated due to stress concentration around these coarse inclusions, and the fatigue damage resistance of the welded joint 12 may decrease. For this reason, it is preferable that the Zr content is 0.0001 to 0.0200%. The Zr content is preferably 0.0005% or more, 0.0010% or more, or 0.0015% or more. The Zr content is preferably 0.0180% or less, 0.0150% or less, or 0.0120% or less. Therefore, in order to stably improve the breakage resistance and fatigue damage resistance, it is desirable to set the Zr content to 0.0015 to 0.0120%.

<i group>
(Al: preferably 1.0000% or less)
Al is a component that functions as a deoxidizer. In order to obtain the above-mentioned effect, it is preferable that the Al content is 0.0005% or more, or 0.0010% or more. On the other hand, if the Al content exceeds 1.0000%, coarse alumina-based inclusions are generated, and fatigue cracks are likely to occur from these coarse inclusions, and the fatigue damage resistance of the welded joint 12 may decrease. Furthermore, if the Al content exceeds 1.0000%, oxides may be generated during welding of the rail, and the weldability of the rail may decrease significantly. For this reason, it is preferable that the Al content is 0.0005 to 1.0000%. The Al content is preferably 0.5000% or less, 0.4000% or less, or 0.3000% or less. Therefore, in order to stably perform deoxidation, it is preferable that the Al content is 0.0005 to 0.3000%.

(2) Reason for limiting the cooling stop temperature (TC) of the outer surface 1214 on the top corner side at the weld center A of the welded joint 12 The manufacturing method of the welded rail 1 according to this embodiment includes a process of immediately forcibly cooling the welded joint 12 obtained by flash butt welding the rails after the flash butt welding is completed. The forced cooling increases the hardness of the welded joint 12 and improves the wear resistance of the welded joint 12. However, excessive forced cooling will cause martensite to form in the welded joint 12. Therefore, the forced cooling conditions need to be appropriately managed.
As described above, forced cooling means spraying a coolant such as air, water, or mist onto the welded joint 12. Examples of forced cooling include air cooling, water cooling, and air-water cooling. Natural cooling is considered to be a different concept from forced cooling.
In both forced cooling and natural cooling, the cooling rate after flash butt welding is influenced by the heat input in flash butt welding and the width of the HAZ formed by flash butt welding. The larger the HAZ width, the slower the cooling rate tends to be. When a rail is flash butt welded to produce a welded joint and then the welded joint is natural cooling, the average cooling rate of the outer surface of the top corner side in the temperature range from 900°C to 600°C is usually within the range of about 1.0 to 2.0°C/sec. On the other hand, when a rail is flash butt welded to produce a welded joint and then the welded joint is air-cooled, the average cooling rate of the outer surface of the top corner side in the temperature range from 900°C to 600°C is usually within the range of about 1.5 to 5.0°C/sec. When the cross-sectional shape of the rail to be welded and the flash butt welding conditions are the same, the cooling rate by natural cooling is always slower than the cooling rate by forced cooling. The "average cooling rate V in the temperature range from X°C to Y°C" is a value calculated by the following formula when the time required for the temperature of the object to be cooled to decrease from X°C to Y°C is defined as t.
V = (X - Y) / t
The average cooling rate can be measured using a radiation thermometer, which is capable of measuring the temperature of the surface of an object in a non-contact manner.
In addition, in this embodiment, the description "immediately after the completion of flash butt welding, the welded joint is forcedly cooled" means that forced cooling is started within 5 to 30 seconds after the completion of flash butt welding. The time of the end of flash butt welding is the time of the end of trimming, as described above. After the completion of flash butt welding, it usually takes 5 seconds or more to install a cooling device on the welded joint to start cooling. In addition, if forced cooling is started more than 30 seconds after the completion of flash butt welding, pearlite transformation may start in a high temperature range before forced cooling, and the hardness of the welded joint may be impaired.

In the forced cooling of the welded joint 12, the cooling stop temperature (TC) of the outer surface 1214 of the top corner at the weld center A and the cooling stop temperature (TA) of the outer surface 1212 of the head jaw at the weld center A are each controlled independently. Below, the reason for limiting the cooling stop temperature (TC) of the outer surface 1214 of the top corner at the weld center A of the welded joint 12 to 560°C or higher in the forced cooling from the austenite temperature range after completion of flash butt welding in the manufacturing method of the welded rail 1 according to this embodiment will be explained.

As shown in FIG. 9, if the cooling stop temperature (TC) of the outer surface 1214 on the corner side of the top of the head is less than 560°C, the number of martensite structures formed in the martensite structure evaluation area C near the head jaw exceeds 10, and as shown in FIG. 7, the breakage of the welded joint 12 cannot be prevented. For this reason, the cooling stop temperature (TC) of the outer surface 1214 on the corner side of the top of the head is limited to 560°C or more. TC may be 580°C or more, 600°C or more, 620°C or more, or 650°C or more. It is not necessary to limit the upper limit of TC, but TC may be, for example, 850°C or less, 800°C or less, 750°C or less, or 720°C or less. In order to stably control the number of martensite structures formed in the martensite structure evaluation area C to 10 or less, it is desirable to control the cooling stop temperature (TC) to 600°C or more and 750°C or less.

The cooling start temperature for the forced cooling from the above austenite temperature range is not particularly limited, but there is no problem as long as it is within the austenite temperature range. The temperature of the outer surface 1214 on the corner side of the top portion immediately after the flash butt welding of the rail is completed is about 1200°C, so the upper limit of the austenite temperature range in this embodiment is essentially 1200°C.

The austenite temperature range differs depending on the carbon content and alloy components of the rail. In order to accurately determine the austenite temperature range, it is most preferable to directly measure the transformation point by a reheating and cooling experiment or the like. However, since actual measurement is not necessarily easy, it may be simply determined by reading from an equilibrium phase diagram of the Fe-Fe ₃ C system based only on the carbon content.

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

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

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

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

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

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

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

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

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

The plurality of coolant injection means 21 are arranged so as to surround the rail installation portion. The coolant injection direction of the coolant injection means 21 is directed toward the rail installation portion. The coolant injection means 21 may be configured so that the installation location thereof can be changed. The coolant injection means 21 may be configured so that the coolant injection direction thereof can be changed. This allows the cooling device 2 to be used for welded rails 1 of various shapes.
In addition, it is preferable that the cooling device 2 does not inject the refrigerant toward the column portion 122 of the welded joint portion 12. A part of the refrigerant injected into the column portion 122 flows to the head jaw outer surface 1212 of the welded joint portion 12, lowering the temperature of the head jaw outer surface 1212. As a result, there is a risk that the cooling stop temperature (TA) of the head jaw outer surface 1212 will be excessively lowered or the relationship TA≧TC will not be satisfied. Therefore, in the cooling device 2, it is preferable that the refrigerant injection means 21 positioned to inject the refrigerant toward the column portion 122 is omitted. When the cooling device 2 has the refrigerant injection means 21 positioned to inject the refrigerant toward the column portion 122, it is preferable to suppress the amount of refrigerant injected into the column portion 122 using the control means 24 described later.

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

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

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

The above-mentioned temperature measurement procedure is applied to a cooling device 2 that is not equipped with a temperature measuring means. On the other hand, the cooling device 2 may further include a temperature measuring means that is connected to the control means 24 and is arranged facing the rail installation portion. The temperature measuring means is a radiation thermometer. The radiation thermometer can measure the surface temperature of an object. By using the temperature measuring means equipped to the cooling device 2, the cooling conditions of the welded joint 12 can be controlled with high accuracy according to the temperature of the welded joint 12. Note that, when the cooling device 2 is equipped with a temperature measuring means, it is desirable to start cooling 5 to 10 seconds after the completion of the measurement of the cooling start temperature. It is also desirable to measure the cooling stop temperature 5 to 10 seconds after the completion of cooling.
However, if the shape of the welded joint 12 and the flash butt welding conditions are constant, the temperature distribution of the welded joint 12 disposed in the rail installation portion will also be approximately constant, and therefore it is possible to cool the welded joint 12 under the same coolant injection conditions. Therefore, if the coolant injection conditions that can bring the cooling conditions within the range of the manufacturing method according to this embodiment are determined in advance, there is no need to measure the temperature using a temperature measuring means.

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

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

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

As shown in FIG. 11, when the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the head corner outer surface 1214 is 1340°C or higher, the temperatures of the head corner outer surface 1214 and the head jaw outer surface 1212 are secured, and the cooling rate of the head jaw outer surface 1212 during the cooling process after cooling is stopped is relaxed, and as shown in FIG. 8, the number of martensite structures formed in the martensite structure evaluation area C near the head jaw is 5 or less, and the breakage resistance of the welded joint 12 can be further improved. For this reason, it is preferable to set TA+TC≧1340. To stably control the number of martensite structures formed in the martensite structure evaluation area C to 5 or less, it is even more preferable to control TA+TC to 1360°C or higher and 1500°C or less.

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

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

In addition, if the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner outer surface 1214 is in the range of 160 to 240°C, as shown in Figure 13, even if the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner outer surface 1214 is 1340°C or higher, the number of martensite structures formed will not be 5 or less.

Furthermore, when the sum of the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner outer surface 1214 is in the range of 1250 to 1330°C, as shown in Figure 14, even if the difference between the cooling stop temperature (TA) of the head jaw outer surface 1212 and the cooling stop temperature (TC) of the top corner outer surface 1214 is 140°C or less, the number of martensite structures formed will not be 5 or less.

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

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

The inventors considered that in order to control the generation of martensite structure in the martensite structure evaluation area C near the head jaw shown in Figure 5, it would be effective to control the temperature of the head jaw outer surface 1212, which is in the vicinity. Furthermore, after examining the temperature control position that affects the generation of martensite structure on the head jaw outer surface 1212, it became clear that there is a strong correlation between the temperature of the head corner side outer surface 1214 and the amount of martensite structure generated, as shown in Figures 9, 11, and 12. For this reason, the inventors considered that in order to control the generation of martensite structure in the martensite structure evaluation area C, it would be effective to control the temperature of the head jaw outer surface 1212, as well as the head corner side outer surface 1214.

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

Incidentally, immediately after flash butt welding, the steel expelled from the weld in the upset process remains as a weld metal in the weld joint 12. The weld metal is removed immediately after welding is completed. However, as shown in Figures 16A and 16B, after trimming (or bead cutting), which is the process of removing the weld metal, a weld metal of several mm in thickness may still remain on the surface of the head jaw after welding is completed. The evaluation area (C) of the martensitic structure needs to be identified based on the head jaw surface of the base rail, regardless of this remaining weld metal.

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

The effect of one aspect of the present invention will be explained in more detail using an example. However, the conditions in the example are merely one example of conditions adopted to confirm the feasibility and effect of the present invention. The present invention is not limited to this one example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

The inventors manufactured welded rails under various conditions and evaluated their breakage resistance. The test conditions are as follows:

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

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

Evaluation of martensite structure Evaluation area (see Figure 5)
In the longitudinal cross section of the welded joint, this refers to a range of ±5 mm (width 10 mm) from the weld center (A) and a region 1 to 5 mm deep from the head jaw outer surface 1212 to the outer surface of the top corner side (martensite structure evaluation region: C).
Reason for selecting the evaluation site: This is because this is the location where rail breakage occurs starting from the martensite structure.
Method for Revealing Martensite Structure After the martensite structure evaluation region (C) is polished, it is etched with nital and observed under an optical microscope.
Polishing conditions: buff polishing with 1 μm diamond paste Martensite etching conditions Etching solution: alcohol + 5% nitric acid (nital)
Etching time: 5 to 10 seconds Method for examining structure Equipment: Optical microscope Magnification: 400x Method for evaluating structure Martensite structures that could be confirmed under an optical microscope at 400x were evaluated.
The target martensite structure had a major axis of 25 to 100 μm, and when martensite structures were generated, the number of martensite structures was investigated.

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

The chemical composition of the rails used in flash butt welding is shown in Tables 1A, 1B, 2A, 2B, 3A, and 3B. The cooling stop temperature (TC) of the outer surface of the top corner and the cooling stop temperature (TA) of the outer surface of the head jaw are shown in Tables 4A and 4B.

For reference, Tables 4A and 4B also show the calculation results of TA+TC and TA-TC.

In addition, Tables 4A and 4B also show the number of martensites in the martensite structure evaluation region C of the welded rail and the evaluation results of the breakage resistance of the welded rail. The evaluation criteria were as follows.
Number of martensites in martensite structure evaluation area C: 5 or less: A
More than 5 but less than 10: B
Over 10: C
●Breakage resistance evaluation No breakage when dropped from a weight height of 7.5 m: A
B: Breakage occurred at a drop height of 7.5 m, but no breakage occurred at a drop height of 5.0 m
Breakage occurred at a falling weight height of 5.0 m: C
Welded rails whose breakage resistance was rated as "B" or "A" were determined to have excellent breakage resistance.

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

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

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

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

1. Flash butt welded rail (welded rail)
11 Rail portion 111 Rail head portion 1111 Rail head top outer surface 1112 Rail head jaw outer surface 1113 Rail head side outer surface 1114 Rail head corner side outer surface 112 Rail column portion 113 Rail bottom portion 12 Welded joint portion 121 (of welded joint portion) Head portion 1211 (of welded joint portion) Head portion outer surface 1212 (of welded joint portion) Head jaw outer surface 1213 (of welded joint portion) Head side outer surface 1214 (of welded joint portion) Head corner side outer surface 122 (of welded joint portion) Column portion 123 (of welded joint portion) Bottom portion 12H Heat-affected zone (HAZ)
A: welding center B: width direction center C: martensite structure evaluation area 2: cooling device 21: coolant injection means 22: rail installation section 23: coolant supply means 24: control means

Claims (4)

A method for manufacturing a flash butt welded rail having a plurality of rail portions and a welded joint portion that joins the plurality of rail portions,
The chemical components, in mass%, are: C: 0.70-1.20%, Si: 0.05-2.00%, Mn: 0.05-2.00%, P≦0.0300%, S≦0.0300%, Cr: 0-2.00%, Mo: 0-0.50%, Co: 0-1.00%, B: 0-0.0050%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0. 200%, Nb: 0-0.0500%, Ti: 0-0.0500%, Mg: 0-0.0200%, Ca: 0-0.0200%, REM: 0-0.0500%, N: 0-0.0200%, Zr: 0-0.0200%, and Al: 0-1.0000%, with the balance being Fe and impurities;
and immediately after completion of the flash butt welding, forcibly cooling the welded joint.
In the forced cooling from the austenite temperature range, the cooling stop temperature (TC) of the outer surface of the top corner side at the weld center of the weld joint is set to 560° C. or more and 850° C. or less;
A method for manufacturing a flash butt welded rail, characterized in that the cooling stop temperature (TA) of the head and jaw outer surface is 650°C or higher and 950°C or lower. 2. A method for manufacturing a flash butt welded rail as claimed in claim 1, characterized in that the cooling stop temperature (TC) of the outer surface of the top corner side and the cooling stop temperature (TA) of the outer surface of the head jaw satisfy the following formulas 1 and 2.
TA+TC≧1340 1st formula 0≦TA-TC≦140 2nd formula The rail has, as the chemical composition, in mass %,
Group a: Cr: 0.05% or more and 2.00% or less, Mo: 0.01% or more and 0.50% or less,
Group b: Co: 0.01% or more and 1.00% or less,
Group c: B: 0.0001% or more and 0.0050% or less,
Group d: Cu: 0.01% or more and 1.00% or less, and Ni: 0.01% or more and 1.00% or less, or one or two of these;
Group e: one or more of V: 0.005% or more and 0.200% or less, Nb: 0.0010% or more and 0.0500% or less, and Ti: 0.0010% or more and 0.0500% or less;
Group f: one or more of Mg: 0.0005% or more and 0.0200% or less, Ca: 0.0005% or more and 0.0200% or less, and REM: 0.0005% or more and 0.0500% or less;
Group g: N: 0.0025% or more and 0.0200% or less,
Group h: Zr: 0.0001% or more and 0.0200% or less,
Group i: Al: 0.0010% or more and 1.0000% or less,
3. The method for manufacturing a flash butt welded rail according to claim 1, further comprising the step of: The method for manufacturing flash butt welded rails according to claim 1 or 2, characterized in that the forced cooling is air cooling.

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