Welding joint of welded rail and method for manufacturing welding joint of welded rail

Abstract

For the welded joint part of this welded rail, on the welding surface, the distance from the central axis of the column part to the toe is defined as T, the size of the heat-affected zone is defined as W, the middle between the head surface of the head part and the bottom surface of the foot part, and the center in the width direction of the welded part and the position on the surface of the column part is defined as the first position, the center on the bottom surface of the foot is defined as the second position, the position T / 10 away from the toe on the bottom surface of the foot is defined as the third position, and the position T / 3 away from the central axis of the column part on the surface of the foot is defined as the fourth position. When this is the case, the residual stress in the height direction measured at the first position is 543 - 6.25×W (MPa) or less, the residual stress in the longitudinal direction measured at the second position is -200 to -20 (MPa), the residual stress in the longitudinal direction measured at the third position is -350 to -100 (MPa), and the residual stress in the longitudinal direction measured at the fourth position is 0 to 200 (MPa).

Description

Technical Field

[0001] The present disclosure relates to a welded joint of a welded rail suitable for freight railway applications and a method for manufacturing the welded joint of the welded rail. This application claims priority based on Japanese Patent Application No. 2024-021377 filed in Japan on February 15, 2024, and incorporates its content herein by reference.

Background Art

[0002] In the mining of natural resources represented by ores, freight railways are used as a means of transporting natural resources. The transportation of natural resources by freight railways is superior in terms of transportation efficiency and CO emissions per unit of loaded weight compared to truck transportation. From the perspectives of improving these transportation efficiencies and reducing environmental impacts, there is a demand for further extending the lifespan and improving the maintainability of rails for freight railway applications. 2 In recent years, the lifespan of rails has been extended by welding the joints of rails. The joints of rails are the parts where damage is most likely to occur. However, problems such as wear resistance of the head and particularly breakage from the center of the web also occur in the welded joints of welded rails.

[0003] There is a heat-affected zone (HAZ) in the welded part. The heat-affected zone is softer than the rail base metal. Therefore, in the HAZ of the welded joint of the welded rail, the wear amount and damage resistance of the head are lower than those of the base metal. This deteriorates the maintainability and lifespan of the welded joint of the welded rail. Therefore, it is preferable that the softening width of the HAZ is narrow.

[0004]

[0005] ​As welding methods for rails, four methods are mainly used: flash butt welding, thermit welding, enclosed arc welding, and gas pressure welding. Among these, flash butt welding is widely popular. This is because flash butt welding has a short welding time. Also, in the welded part obtained by flash butt welding, the HAZ width is narrow. Moreover, in flash butt welding, the welding process is automated.

[0006] On the other hand, for rail welding, after applying heat to the ends of two rails, they are joined and cooled. Therefore, in the cooling process, residual stresses in the circumferential and longitudinal directions are generated at the welded joint of the welded rail. In particular, in flash butt welding, tensile residual stress in the circumferential direction of the rail is likely to occur at the center of the column part. This is because flash butt welding narrows the distribution width of heat along the longitudinal direction in order to narrow the HAZ width. Therefore, there are problems such as fatigue cracks occurring and growing in the longitudinal direction at the column part of the welded joint, and brittle fracture occurring longitudinally from the center of the column part, making breakage likely to occur.

[0007] Regarding countermeasures against fracture accompanied by fatigue cracks starting from the central axis of the column part of the welded joint of this welded rail, Non-Patent Document 1 discloses performing post-weld heat treatment (PWHT) in which two points separated from the welding center in the longitudinal direction are heated to below the Ac1 point by induction heating on the rail welded part. PWHT can disperse the distribution of tensile residual stress in the circumferential direction of the column part concentrated near the welded part in the longitudinal direction, thereby suppressing fatigue cracks and the accompanying brittle fracture.

[0008] Also, Patent Document 1 discloses that by arranging a coil at a position where the distance from the welding center is 0.2 to 3 times the HAZ width and heating the entire circumference of the rail to heat the central part of the column part in the temperature range of 400 to 750 °C, the circumferential residual stress at the central part of the column part can improve the fatigue characteristics of the column part.

Prior Art Documents

Patent Documents

[0009] [Patent Document 1] International Publication No. 2012 / 161207 [Non-Patent Document]

[0010] [Non-Patent Document 1] K. Saita, "Developing technologies to improve the reliability of flash-butt welds", 2017, 13th International Heavy Haul Conference, (IHHA 2017), Advancing heavy haul technologies and operations in a changing world, 2 - 6 September, 2017, Cape Town, South Africa [Summary of the Invention] [Problems to be Solved by the Invention]

[0011] By these PWHT techniques, longitudinal fatigue cracks starting from the central axis of the column part of the welded joint are suppressed. However, there still remained problems of breakage under further extended service life and heavy load conditions.

[0012] The present disclosure has been devised in view of the above-described viewpoints, and particularly aims to provide a welded joint of a welded rail and a method for manufacturing the same that improve the breakage resistance of the column part of the welded joint under heavy load conditions, which is required for welded joints of welded rails of overseas freight railways. In particular, the reduction of breakage is a significant problem in high-strength welded rails. Therefore, the rails used in the welded rails according to the present disclosure are targeted at, for example, JIS E1101(2001), JIS E1120(2007), AREMA Chapter4 "Rail", EN 13674-1, etc. [Means for Solving the Problems]

[0013] The gist of the present disclosure is as follows.

[0014] (1) The welded joint of the welded rail according to one aspect of the present disclosure is a welded joint of a welded rail including a base material part and a welded part, wherein the chemical composition of the base material part is, in mass% per unit mass, C: 0.60 to 1.10%, Si: 0.10 to 2.00%, Mn: 0.20 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.01 to 1.00%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Nb: 0 to 0.050%, Al: 0 to 0.100%, Ti: 0 to 0.080%, Mo: 0 to 0.10%, V: 0 to 0.200%, B: 0 to 0.0030%, Zr: 0 to 0.0200%, Ca: 0 to 0.0200%, Mg: 0 to 0.0100% Sb: 0 to 0.050%, Rare earth elements: 0 to 0.0500%, Co: 0 to 0.50%, N: 0 to 0.0200%, W: 0 to 0.10%, Pb: 0 to 0.09%, Bi: 0 to 0.10%, and Te: 0 to 0.05% and the balance consists of Fe and impurities, the welded part has a head, a column part, and a foot part, On the welding surface of the welded portion, the distance from the central axis of the column portion to the tip of the foot portion is defined as T, the size of the heat-affected zone along the longitudinal direction is defined as W in units of mm, the middle between the head surface of the head portion and the bottom surface of the foot portion, and the center in the width direction of the welded portion and the position on the surface of the column portion are defined as the first position, the center along the width direction on the bottom surface of the foot portion is defined as the second position, and the position on the bottom surface of the foot portion that is T / 10 away from the tip along the width direction is defined as the third position. When the position that is T / 3 away from the central axis of the column portion along the width direction on the foot surface of the foot portion is defined as the fourth position, the residual stress in the height direction measured at the first position is 543 - 6.25×W (MPa) or less, the residual stress in the longitudinal direction measured at the second position is -200 to -20 (MPa), the residual stress in the longitudinal direction measured at the third position is -350 to -100 (MPa), the welded joint of the rail is characterized in that the residual stress in the longitudinal direction measured at the fourth position is 0 to 200 (MPa). (2) Preferably, in the welded joint portion of the welded rail described in the above (1), the chemical composition of the base material portion is, in units of mass%, Cu: 0.50% or less, Ni: 0.50% or less, Nb: 0.050% or less, Al: 0.001 to 0.100%, Ti: 0.080% or less, Mo: 0.10% or less, V: 0.200% or less, B: 0.0030% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Mg: 0.0100% or less, Sb: 0.050% or less, rare earth elements: 0.0500% or less, Co: 0.50% or less, N: 0.0015 to 0.0200%, W: 0.10% or less, Pb: 0.09% or less, Bi: less than 0.10%, and Te: less than 0.05%, contains one or more elements selected from the group consisting of. (3) A method for manufacturing a welded joint of a welded rail according to another aspect of the present disclosure includes a step of flash butt welding a rail having the chemical composition described in (1) or (2) above, and a step of heating the welded portion formed by the flash butt welding, and is a method for manufacturing a welded joint of a welded rail including: the welded portion has a head, a column portion, and a foot portion, On the weld surface of the welded portion, the distance from the central axis of the column portion to the tip of the foot portion is defined as T, the size per unit mm of the heat-affected zone along the longitudinal direction is defined as W, the middle between the head surface of the head portion and the sole surface of the foot portion, and the center in the width direction of the welded portion and the position on the surface of the column portion are defined as the first position, the center along the width direction on the sole surface of the foot portion is defined as the second position, the position on the sole surface of the foot portion that is T / 10 away from the tip along the width direction is defined as the third position, and the position on the foot surface of the foot portion that is T / 3 away from the central axis of the column portion along the width direction is defined as the fourth position. When In the flash butt welding, the total preheating heat input is 9 to 60 (kA·s / cm 2 ), the final flash speed is 0.6 to 3.5 (mm / s), and the upset load is 45 to 120 MPa, the heating of the welded portion is started after the surface temperature at the first position drops below 500°C, in the heating of the welded portion, the surface temperature at the first position is raised to 500 to 680°C, in the heating of the welded portion, the average heating rate V of the surface at the first position is 2.0 to 6.0 (°C / s), in the heating of the welded portion, the average heating rate of the surface at the second position is 0.3×V to 1.0×V (°C / s), in the heating of the welded portion, the average heating rate of the surface at the third position is 0.25×V to 1.0×V (°C / s), In the heating of the welded part, the average temperature rise rate of the surface at the fourth position is set to 0.25×V~1.0×V (°C / s).

Advantages of the Invention

[0015] According to the welded joint part of the welded rail and the manufacturing method of the welded joint part of the welded rail according to the present disclosure, sufficient anti-fracture properties of the column part and the foot part can be obtained.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0017] Hereinafter, the welded joint part of the welded rail according to an embodiment of the present disclosure will be described.

[0018] (Shape of the welded joint part of the welded rail) First, the shapes of the welded rail (hereinafter, also simply referred to as the rail) and its welded joint part (hereinafter, also simply referred to as the welded joint) will be described. The railway rail is installed and used on bases such as the ground and bridges. In the present embodiment, the vertical direction when the rail is installed on the base and used is referred to as the height direction Z. Further, the extending direction of the rail is referred to as the longitudinal direction X. Further, the direction perpendicular to the height direction and the longitudinal direction of the rail is referred to as the width direction Y.

[0019] The rail has a head, a web, and a foot. The head of the rail refers to the part above the constricted part at the center in the height direction of the rail. The head of the rail contacts the wheels of the railway vehicle. The web of the rail refers to the constricted part at the center in the height direction of the rail. The foot of the rail refers to the part below the constricted part at the center in the height direction of the rail, and is also referred to as the bottom. The foot of the rail is installed on the base. Figures 2 and 3 show the head 123, the web 124, and the foot 125 at the welded part 12 of the rail welded joint 1.

[0020] The foot has a bottom surface, a top surface, and a tip. The bottom surface of the foot refers to the surface that contacts the base when the rail is installed on the base. The top surface of the foot refers to the surface that is located on the upper side when the rail is installed on the base. The tip of the foot refers to the end in the width direction of the foot. Figures 2 and 3 show the bottom surface 1251, the top surface 1252, and the tip 1253 at the welded part 12 of the rail welded joint 1.

[0021] The rail welded joint 1 is obtained by welding the end faces of two or more railway rails. A rail with a length of 200 m or more after welding is called a continuous welded rail. The welding for manufacturing a continuous welded rail is called rail welding (see JIS E 1001:2001 "Railway - Track Terminology").

[0022] As shown in FIG. 1, the rail welded joint 1 has a base metal part 11 and a welded part 12. The welded part 12 is a general term for the part including the welding surface 121 and the heat affected zone 122 (HAZ). The welding surface 121 means the surface formed by the melting and solidification of the rail. The heat affected zone 122 means the non-molten part where the structure, metallurgical properties, and mechanical properties have changed due to the welding heat. The heat affected zone 122 is the region that is heated near the A1 point by the welding heat and partially austenitized, and then the decomposition of the pearlite structure occurs due to the temperature drop after the welding is completed, and it is also called HAZ. In the heat affected zone 122, the hardness is significantly reduced. Therefore, usually, in the graph of the hardness distribution of the rail obtained by flash butt welding, there are two valleys of Vickers hardness. The locations where these Vickers hardness valleys occur are defined as the softest parts of the rail welded joint according to this embodiment. The interval between the two softest parts is defined as the HAZ width. The welding center approximately coincides with the center of this HAZ width. The base metal part 11 means the region other than the welded part 12 in the rail welded joint. The region outside the heat affected zone 122 corresponds to the base metal part 11.

[0023] The structure of the base metal part 11 is substantially the same as the structure of the rail before welding. Also, the rail welded joint 1 has a uniform shape along the longitudinal direction X. Therefore, the base metal part 11 and the welded part 12 also have a head, a web, and a foot. In the rail welded joint 1 according to this embodiment, at least the web 124 and the foot 125 of the welded part 12 have a specific structure. On the other hand, the structure of the base metal part 11 is not particularly limited except for its chemical composition. Hereinafter, unless otherwise specified, the terms "head", "web", and "foot" mean the head 123 of the welded part 12, the web 124 of the welded part 12, and the foot 125 of the welded part 12, respectively.

[0024] (Findings of the present inventors) Next, the findings of the inventors on which the present disclosure is based will be described. The inventors newly found that under heavy load conditions, fracture due to fatigue cracks occurs from the foot surface 1252 of the foot portion 125 of the welded joint 12 (particularly, in the vicinity of the fourth position D shown in FIGS. 2 and 3). According to the conventional PWHT technology, it is possible to suppress brittle fracture due to a crack in the center of the column portion 124 of the welded joint 12. However, regarding the fracture at the fourth position D of the welded joint 12, it was not considered at all in the conventional PWHT technology.

[0025] Therefore, the inventors intensively studied why fracture occurs from the foot surface 1252 side of the foot portion 125 in order to further improve the fracture resistance of the rail welded joint 1. As a result, it was found that at the fourth position D of the foot surface 1252 near the column portion 124, a tensile residual stress of 250 MPa or more acts in the longitudinal direction X. It was found that in the usage environment under heavy load conditions, this tensile residual stress makes it easy to generate fatigue cracks.

[0026] Through further studies, it was found that in order to suppress fracture due to fatigue cracks, it is effective to suppress the tensile residual stress in the longitudinal direction X at the fourth position D to 200 MPa or less.

[0027] In order to reduce this residual stress, it is necessary to balance the residual stress throughout the welded joint 12 and the base material portion 11.

[0028] Therefore, the inventors considered a method of performing post-weld heat treatment so as to balance the compressive residual stress in the longitudinal direction X of the foot back surface 1251 and the foot tip 1253 with the tensile residual stress at the fourth position D of the foot surface, and to reduce the circumferential tensile residual stress at the center of the column portion 124. And by optimizing the temperature rising pattern at each of these parts, the inventors came to know a manufacturing method of the rail welded joint 1 that effectively reduces the residual stress at the fourth position D.

[0029] In the post-weld heat treatment of the rail, often only the heating conditions of the column part 124 are defined. This is because in the prior art, it was considered that the breakage of the welded part 12 of the rail was caused by the residual stress in the column part 124. Further, in the prior art, heating the foot part 125 of the welded part 12 was considered unfavorable because it caused softening of the foot part 125. Due to such circumstances, in the conventional post-weld heat treatment of the rail, the foot part 125 was often not the object of heating. Also, when the foot part 125 was heated, the temperature rising rate of the surface of the foot part 125 was not controlled.

[0030] However, the present inventors have found that heating the foot part 125 and further controlling the temperature rising rate at each of a plurality of specific parts in the foot part 125 is extremely important for suppressing breakage on the foot surface 1252 of the foot part 125.

[0031] In the welded surface 121 of the rail welded joint 1 according to the present embodiment obtained based on the above findings, the residual stress measured at each of the following four positions is within a predetermined range. (First position A) The position at the middle between the head surface of the head part 123 and the back surface 1251 of the foot part 125, at the center in the width direction of the welded part, and on the surface of the column part 124 (Second position B) The center along the width direction Y on the back surface 1251 of the foot part 125 (Third position C) The position on the back surface 1251 of the foot part 125 that is T / 10 away from the foot tip 1253 along the width direction Y (Fourth position D) The position on the foot surface 1252 of the foot part 125 that is T / 3 away from the central axis of the column part 124 along the width direction Y

[0032] As shown in FIG. 3, "T" is the distance from the central axis of the column part 124 to the foot tip 1253 of the foot part 125. In the four-point bending test described later, the rail welded joint 1 is bent with the rotation axis coinciding with the width direction Y. FIG. 1 shows the first position A. FIG. 3 shows the second position B, the third position C, and the fourth position D. Note that the first position A is the midpoint between the head surface of the head part 123 and the back surface 1251 of the foot part 125.

[0033] Note that in Fig. 2, the third position C and the fourth position D are only shown on one side of the welded joint 1 of the rail. However, in reality, as shown in Fig. 3, the third position C and the fourth position D exist on both sides of the welded joint 1 of the rail. On both sides of the welded joint 1 of the rail, the residual stresses at the third position C and the fourth position D need to be within a predetermined range. When measuring the residual stress, for the third position C and the fourth position D, it is necessary to measure both sides of the welded joint 1 of the rail. And it is necessary to confirm whether the residual stress is within the predetermined range on each of the two sides.

[0034] The first position A also exists on both sides of the welded joint 1 of the rail. On both sides, the residual stress at the first position A needs to be within a predetermined range. However, the two first positions A are close to each other. If the residual stress at one first position A is within the predetermined range, it is usually the case that the residual stress at the other first position A is also within the predetermined range. Therefore, for the first position A, it is sufficient to measure only the residual stress on one side.

[0035] Next, the residual stresses at the first position A to the fourth position D will be described. Hereinafter, the residual stress described by a negative value indicates the residual stress in the compression direction, and the residual stress described by a positive value indicates the residual stress in the tensile direction. An increase in the residual stress may be expressed as "the residual stress becomes larger on the tensile side" or "the residual stress swings to the tensile side", and a decrease in the residual stress may be expressed as "the residual stress becomes larger on the compression side" or "the residual stress swings to the compression side".

[0036] (Residual stress in the height direction Z measured at the first position A: 543 - 6.25×W (MPa) or less) The double arrows shown in Fig. 1 indicate the residual stress in the height direction Z at the first position A. When the residual stress in the height direction Z measured at the first position A increases on the tensile side, the generation of fatigue cracks in the column part 124 of the welded part 12 and the propagation of the cracks along the longitudinal direction X are promoted. As a result, the fracture resistance of the welded joint of the rail decreases. Therefore, the smaller the residual stress in the height direction Z measured at the first position A, the more preferable.

[0037] If the residual stress in the height direction Z measured at the first position A is 543 - 6.25×W (MPa) or less, the occurrence of fatigue cracks in the column portion 124 of the welded portion 12 can be sufficiently suppressed. Here, "W" is the size in mm of the heat-affected zone 122 along the longitudinal direction X as shown in FIG. 1. The size of the heat-affected zone 122 along the longitudinal direction X is generally referred to as the "HAZ width". Although the HAZ width is not particularly defined, the smaller the HAZ width, the greater the residual stress in the height direction Z at the first position A, which is the central portion of the column portion 124. The HAZ width may be, for example, in the range of 18 to 50 mm.

[0038] The residual stress in the height direction Z measured at the first position A is preferably 493 - 6.25×W (MPa) or less, and more preferably 473 - 6.25W (MPa) or less. The lower limit value of the residual stress in the height direction Z measured at the first position A is not sometimes limited. Considering the productivity in operation, it is preferable that the residual stress in the height direction Z measured at the first position A is 100 (MPa) or more.

[0039] (Residual stress in the longitudinal direction X measured at the second position B: -200 to -20 (MPa)) The two arrows marked with the symbol B in FIG. 2 indicate the residual stress in the longitudinal direction X at the second position B. The residual stress in the longitudinal direction X at the second position B is a compressive stress. The greater the residual stress in the longitudinal direction X measured at the second position B, which is the central portion of the sole surface 1251, becomes on the tensile side, the more the occurrence of fatigue cracks from the sole surface 1251 is promoted. This is due to the synergistic effect of the bending stress applied to the welded portion 12 when the vehicle passes through the welded portion 12 and the axial force generated by the thermal expansion of the rail. However, by balancing the residual stress in the longitudinal direction X at the second position B and the tensile residual stress in the longitudinal direction X at the fourth position D described later, the fracture resistance of the welded portion 12 can be improved.

[0040] In order to suppress the generation of fatigue cracks from the bottom surface 1251, the residual stress in the longitudinal direction X measured at the second position B needs to be -20 (MPa) or less. The residual stress in the longitudinal direction X measured at the second position B is preferably -50 (MPa) or less, and more preferably -100 (MPa) or less.

[0041] The lower the residual stress in the longitudinal direction X measured at the second position B, the more preferable. However, from the viewpoint of surely reducing the tensile residual stress in the longitudinal direction X at the fourth position D, the residual stress in the longitudinal direction X measured at the second position B is -200 (MPa) or more. The residual stress in the longitudinal direction X measured at the second position B is preferably -180 (MPa) or more, and more preferably -150 (MPa) or more.

[0042] (Residual stress in the longitudinal direction X measured at the third position C: -350 to -100 (MPa)) The two arrows described at the position marked with the symbol C in FIG. 2 indicate the residual stress in the longitudinal direction X measured at the third position C. The residual stress in the longitudinal direction X at the third position C is a compressive stress. The more the residual stress in the longitudinal direction X at the third position C fluctuates in the tensile direction, the more it promotes the generation of fatigue cracks from the periphery of the toe 1253. This is due to the synergistic effect of the bending stress applied to the welded portion 12 when the vehicle passes through the welded portion 12 and the axial force generated by the thermal expansion of the rail. However, by balancing the residual stress in the longitudinal direction X measured at the third position C and the tensile residual stress in the longitudinal direction X at the fourth position D, the fracture resistance can be improved.

[0043] In order to suppress the generation of fatigue cracks from the periphery of the toe 1253, the residual stress in the longitudinal direction X measured at the third position C needs to be -100 (MPa) or less. The residual stress in the longitudinal direction X measured at the third position C is preferably -150 (MPa) or less, and more preferably -200 (MPa) or less. On the other hand, if the residual stress in the longitudinal direction X measured at the third position C is too low, it becomes difficult to reduce the tensile residual stress in the longitudinal direction X at the fourth position D. Therefore, the lower limit value of the residual stress in the longitudinal direction X measured at the third position C is set to -350 (MPa).

[0044] (Residual stress in the longitudinal direction X measured at the fourth position D: 200 (MPa) or less) The double arrows shown at the position marked with the symbol D in Fig. 2 indicate the residual stress in the longitudinal direction X measured at the fourth position D. The residual stress in the longitudinal direction X at the fourth position D is a tensile stress. The more the residual stress in the longitudinal direction X at the fourth position D fluctuates in the tensile direction, the more it promotes the generation of fatigue cracks from the periphery of the fourth position D and reduces the fracture resistance. This is due to the synergistic effect of the bending stress applied to the welded joint 1 when the vehicle passes through and the axial force generated by the thermal expansion of the rail.

[0045] In order to suppress the generation of fatigue cracks, the residual stress in the longitudinal direction X measured at the fourth position D needs to be 200 (MPa) or less. The residual stress in the longitudinal direction X measured at the fourth position D is preferably 180 (MPa) or less, and more preferably 150 (MPa) or less. The lower limit value of the residual stress in the longitudinal direction X measured at the fourth position D is not always limited, but it is preferably set to 50 (MPa) in consideration of the productivity during operation.

[0046] As described above, in the welded joint of the welded rail in the present embodiment, in order to improve the fracture resistance, the residual stress is reduced throughout the welded joint. That is, the fracture resistance is improved by balancing the residual stress in the entire welded part and the base material part. Specifically, in order to improve the fracture resistance, it is important to balance the compressive residual stress in the longitudinal direction X at the second position B and the third position C and the tensile residual stress in the longitudinal direction X at the fourth position D, and to reduce the value of the tensile residual stress in the longitudinal direction X at the fourth position D.

[0047] Next, a method for measuring the residual stress of the rail welded joint 1 according to the present embodiment will be described. (1) First, the surface of the residual stress measurement location on the rail welded joint 1 is polished using paper or the like. The polishing is performed so that a strain gauge can be attached to the residual stress measurement location. (2) Next, a strain gauge with two axes and a gauge length of 2 mm is attached to the welded surface 121 using an adhesive or the like. At this time, the longitudinal direction X of the rail and the circumferential direction of the rail are aligned with the two axes of the strain gauge, respectively. (3) Then, a region with a length and width of 10 mm and a thickness of 5 mm is cut out centering on the strain gauge attachment part. The length and width of the cut-out region are made to coincide with the two axes of the strain gauge. In the cut-out region, the residual stress is released. And the shape of the cut-out region changes. This shape change is measured by the strain gauge. (4) Based on the obtained strain and the Young's modulus (210 GPa) of the rail, the residual stress is calculated. As described above, the plus side is the compressive stress and the minus side is the tensile stress.

[0048] Note that, as described above, the residual stresses at the third position C and the fourth position D are measured on both sides of the welded joint 1 of the rail. This is because it is necessary that the residual stresses at the third position C and the fourth position D be within a predetermined range on both sides of the welded joint 1 of the rail. For the residual stress at the first position A, it is sufficient to measure it on one side of the welded joint 1 of the rail. Although the residual stress at the first position A also needs to be satisfied on both sides of the welded joint 1 of the rail, it is estimated that the residual stress at the first position A will be approximately the same value on both sides of the welded joint 1 of the rail.

[0049] Next, the chemical composition (steel components) of the base material part 11 included in the welded joint 1 of the rail according to the present embodiment will be described. The unit “%” of the content of each element means “mass %”. The chemical composition of the base material part 11 is the same as the chemical composition of the rail before welding.

[0050] (C: 0.60 to 1.10%) C increases the cementite fraction and refines the interlamellar spacing of the pearlite structure, thereby increasing the strength of the rail and improving its wear resistance and resistance to surface damage. The appropriate C content for the rail ranges from 0.60% to 1.10%. Preferably, the C content is from 0.78% to 1.07%. The C content may be 0.65% or more, 0.70% or more, or 0.80% or more. The C content may be 1.00% or less, 0.90% or less, or 0.85% or less.

[0051] (Si: 0.10 - 2.00%) Si is a solid solution strengthening element of the ferrite phase in the pearlite structure. It is also an element that improves hardenability, refines the interlamellar spacing, increases strength, and improves wear resistance. The appropriate Si content for the rail ranges from 0.10% to 2.00%. Preferably, the Si content is from 0.20% to 1.30%. The Si content may be 0.25% or more, 0.30% or more, or 0.40% or more. The Si content may be 1.20% or less, 1.00% or less, or 0.80% or less.

[0052] (Mn: 0.20 - 1.50%) Mn improves hardenability and refines the interlamellar spacing. By solid - solving in the lamellar cementite, it suppresses the granulation of the lamellar structure after the formation of the pearlite structure, thereby improving strength and wear resistance. The appropriate Mn content for the rail ranges from 0.20% to 1.50%. Preferably, the Mn content is from 0.30% to 1.15%. The Mn content may be 0.25% or more, 0.30% or more, or 0.40% or more. The Mn content may be 1.00% or less, 0.90% or less, or 0.80% or less.

[0053] (P: 0.030% or less) P is an element contained in steel, which reduces the toughness of the welded joint 1 of the rail according to the present embodiment and impairs the fracture resistance. The appropriate P content as a rail is 0.030% or less. Preferably, the P content is 0.025% or less, 0.020% or less, or 0.015% or less. The P content may be 0%, but considering the refining cost, the P content may be more than 0%, 0.001% or more, or 0.005% or more.

[0054] (S: 0.030% or less) S is an element contained in steel, which is an element that reduces the surface damage resistance by precipitating sulfides. The appropriate S content as a rail is 0.030% or less. Preferably, the S content is 0.025% or less, 0.020% or less, or 0.015% or less. The S content may be 0%, but considering the refining cost, the S content may be more than 0%, 0.001% or more, or 0.005% or more.

[0055] (Cr: 0.01 - 1.00%) Cr is an element that improves the strength and wear resistance of the welded joint 1 of the rail by refining the interlamellar spacing of pearlite. Also, Cr dissolves in V carbonitride and promotes the precipitation of (V, Cr)(C, N), which also improves the surface damage resistance. The appropriate Cr content as a rail is in the range of 0.01 - 1.00%. Preferably, the Cr content is 0.03 - 0.60%. The Cr content may be 0.10% or more, 0.20% or more, or 0.25% or more. The Cr content may be 0.80% or less, 0.60% or less, or 0.40% or less.

[0056] Furthermore, in the base metal part 11 of the welded joint 1 of the rail manufactured with the above chemical composition, one or more elements of V, Cu, Ni, Nb, Al, Ti, Mo, B, Zr, Ca, Mg, Sb, rare earth elements (REM), Co, N, W, Pb, Bi, Te may be contained as necessary. However, even if these elements are not contained in the base metal part 11, the welded joint 1 of the rail according to the present embodiment can exhibit its effects, so the lower limit value of the content of these elements is 0%.

[0057] (Cu: below 0.50%) Cu is an element that improves hardenability, refines the lamellar spacing of pearlite to improve strength, and improves wear resistance. For example, when the Cu content is 0.01% or more, the effect can be preferably obtained. Also, by setting the Cu content to 0.50% or less, it is possible to prevent the formation of a martensite structure, which is harmful to the toughness of the rail weld joint, due to the lengthening of the time until the end of pearlite transformation. Therefore, the Cu content is preferably 0.50% or less. The Cu content is more preferably 0.05% or more, 0.08% or more, or 0.10% or more. The Cu content is more preferably 0.40% or less, 0.30% or less, or 0.03% or less.

[0058] (Ni: below 0.50%) Ni is an element that improves hardenability, refines the lamellar spacing of pearlite to improve strength, and improves wear resistance. For example, when the Ni content is 0.01% or more, the effect can be preferably obtained. Also, by setting the Ni content to 0.50% or less, it is possible to prevent the formation of a martensite structure, which is harmful to the toughness of the rail weld joint, due to the lengthening of the time until the end of pearlite transformation. Therefore, the Ni content is preferably 0.50% or less. The Ni content is more preferably 0.03% or more, 0.05% or more, or 0.10% or more. The Ni content is more preferably 0.40% or less, 0.30% or less, or 0.10% or less.

[0059] (Nb: below 0.050%) Nb is an element effective in refining austenite grains by the pinning effect of Nb carbides and Nb nitrides during welding and suppressing the formation of martensite harmful to toughness. For example, when the Nb content is 0.005% or more, the effect can be preferably obtained. Further, when the Nb content is made 0.050% or less, an increase in hardenability can be suppressed and the formation of martensite can be suppressed. Therefore, the Nb content is preferably 0.050%. The Nb content is more preferably 0.008% or more, 0.010% or more, or 0.015% or more. The Nb content is more preferably 0.040% or less, 0.030% or less, or 0.025% or less.

[0060] (Al: 0.001~0.1000%) Al is an element effective in preventing the coarsening of austenite during welding and suppressing the formation of martensite harmful to toughness. For example, when the Al content exceeds 0.001%, the effect can be preferably obtained. Further, when the Al content is 0.100% or less, the formation of coarse Al-based oxide inclusions can be suppressed and rail damage can be further suppressed. Therefore, the Al content is preferably 0.001 to 0.100%. The Al content is more preferably 0.010 to 0.070%. The Al content may be more than 0.003%, 0.005% or more, or 0.010% or more. The Al content may be 0.090% or less, 0.080% or less, or 0.060% or less.

[0061] (Ti: 0.080% or less) Ti is an element effective in forming Ti carbides, preventing the coarsening of austenite during welding, and suppressing the formation of martensite harmful to toughness. For example, when the Ti content is 0.002% or more, the effect can be preferably obtained. Further, when the Ti content is 0.080% or less, the formation of coarse Ti nitrides can be prevented and toughness can be further improved. Therefore, the Ti content is preferably 0.080% or less. The Ti content is more preferably 0.004% or more, 0.006% or more, or 0.008% or more. The Ti content is more preferably 0.070% or less, 0.050% or less, or 0.010% or less.

[0062] (Mo: less than 0.10%) Mo is an element that improves hardenability, refines the interlamellar spacing of pearlite to improve strength, and improves wear resistance. For example, when the Mo content is 0.005% or more, the effect can be preferably obtained. Also, when the Mo content is 0.10% or less, it is possible to prevent the time until the end of pearlite transformation from becoming long and the generation of martensite, which is harmful to toughness. Therefore, the Mo content is preferably 0.10% or less. The Mo content is more preferably 0.010% or more, 0.015% or more, or 0.020% or more. The Mo content is more preferably 0.090% or less, 0.070% or less, or 0.025% or less.

[0063] (V: less than 0.200%) V is an element that increases the hardness (strength) of the pearlite structure and improves the wear resistance and surface damage resistance of the rail joint by precipitation hardening due to (V, Cr)(C, N) generated during the cooling process after hot rolling. For example, when the V content is 0.010% or more, the effect can be preferably obtained. Also, when the V content is 0.200% or less, it is possible to prevent the precipitation amount of (V, Cr)(C, N) from becoming excessive in part and reducing the toughness of the rail. Therefore, the V content is preferably 0.200% or less. The V content is more preferably 0.015% or more, 0.020% or more, or 0.040% or more. The V content is more preferably 0.150% or less, 0.100% or less, or 0.050% or less.

[0064] (B: less than 0.0030%) B is an element that segregates at austenite grain boundaries during hot rolling to improve hardenability, improves strength by refining the interlamellar spacing, and simultaneously improves wear resistance. For example, when the B content is 0.0004% or more, the effect can be suitably obtained. Also, when the B content is 0.0030% or less, it is possible to prevent the formation of B carbonitrides during heat treatment from impairing toughness. Therefore, the B content is preferably 0.0030% or less. The B content is more preferably 0.0010% or more, 0.0015% or more, or 0.0018% or more. The B content is more preferably 0.0028% or less, 0.0025% or less, or 0.0020% or less.

[0065] (Zr: 0.0200% or less) Zr is used as a deoxidizer and, together with ZrO 2 is an element that increases the equiaxed crystal ratio of the solidification structure by inclusions, suppresses segregation in the center of the slab, and suppresses the formation of martensite structure and primary cementite structure generated in the rail segregation part. For example, when the Zr content is 0.0050% or more, the effect can be suitably obtained. Also, when the Zr content is 0.0200% or less, it is possible to prevent the formation of a large amount of coarse Zr-based inclusions and further suppress rail fracture due to stress concentration. Therefore, the Zr content is preferably 0.0200% or less. The Zr content is more preferably 0.0060% or more, 0.0080% or more, or 0.0090% or more. The Zr content is more preferably 0.0180% or less, 0.0150% or less, or 0.0100% or less.

[0066] (Ca: 0.0200% or less) Ca is an element that is used as a deoxidizer, forms Ca sulfide, suppresses coarsening of MnS, and improves surface damage resistance. For example, when the Ca content is 0.0050% or more, the effect can be preferably obtained. Also, when the Ca content is 0.0200% or less, generation of a large amount of coarse Ca-based inclusions can be prevented, and rail breakage due to stress concentration can be further suppressed. Therefore, the Ca content is preferably 0.0200% or less. The Ca content is more preferably 0.0060% or more, 0.0080% or more, or 0.0090% or more. The Ca content is more preferably 0.0180% or less, 0.0150% or less, or 0.0100% or less.

[0067] (Mg: 0.0100% or less) Mg is an element that is used as a deoxidizer. For example, when the Mg content is 0.0050% or more, the effect can be preferably obtained. Also, when the Mg content is 0.0100% or less, generation of a large amount of coarse Mg-based inclusions can be prevented, and rail breakage due to stress concentration can be further suppressed. Therefore, the Mg content is preferably 0.0100% or less. The Mg content is more preferably 0.0060% or more, 0.0080% or more, or 0.0090% or more. The Mg content is more preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less.

[0068] (Sn: 0.050% or less) Sn is an element that strengthens pearlite and improves wear resistance as a solid solution strengthening element. For example, when the Sn content is 0.005% or more, the effect can be preferably obtained. Also, when the Sn content is 0.050% or less, it is possible to prevent the time until the end of pearlite transformation from becoming long and the generation of martensite, which is harmful to toughness. Therefore, the Sn content is preferably 0.050% or less. The Sn content is more preferably 0.006% or more, 0.008% or more, or 0.010% or more. The Sn content is more preferably 0.040% or less, 0.035% or less, or 0.030% or less.

[0069] (Sb: 0.050% or less) Sb is an element that suppresses decarburization during heating. For example, when the Sb content is 0.005% or more, the effect can be preferably obtained. Also, when the Sb content is 0.050% or less, it is possible to prevent the time until the end of pearlite transformation from becoming long and the generation of martensite, which is harmful to toughness. Therefore, the Sb content is preferably 0.050% or less. The Sb content is more preferably 0.006% or more, 0.008% or more, or 0.010% or more. The Sb content is more preferably 0.040% or less, 0.035% or less, or 0.030% or less.

[0070] (Rare earth element (REM): 0.0500% or less) The rare earth element (REM) is a general term for a total of 17 elements, namely, two elements, scandium and yttrium, and 15 elements (lanthanoids) from lanthanum to lutetium. REM is used as a deoxidizer and suppresses the formation of coarse Al 2 O 3 and improves the anti-fracture property of the rail. For example, when the REM content is 0.0010% or more, the effect can be preferably obtained. Also, when the REM content is 0.0500% or less, it is possible to prevent the generation of coarse oxide inclusions and further suppress rail fracture due to stress concentration. Therefore, the REM content is preferably 0.0500% or less. The REM content is more preferably 0.0015% or more, 0.0020% or more, or 0.0030% or more. The REM content is more preferably 0.0400% or less, 0.0300% or less, or 0.0100% or less.

[0071] (Co: 0.50% or less) Co is an element that suppresses primary cementite, improves ductility, and enhances surface damage resistance. For example, when the Co content is 0.01% or more, the effect can be preferably obtained. Also, when the Co content is 0.50% or less, the situation where pearlite transformation is promoted, hardenability is impaired, and sufficient hardness cannot be obtained can be avoided. Therefore, the Co content is preferably 0.50% or less. The Co content is more preferably 0.02% or more, 0.03% or more, or 0.05% or more. The Co content is more preferably 0.40% or less, 0.25% or less, or 0.10% or less.

[0072] (N: 0.0015~0.0200%) When N exists in a solid solution state in ferrite, it reduces the ductility of steel. On the other hand, when N and V are simultaneously contained in the rail, (V, Cr)(C, N) is formed during the cooling process after hot rolling, increasing the hardness (strength) of the pearlite structure and improving wear resistance and surface damage resistance. When the N content exceeds 0.0015%, the effect can be preferably obtained. Also, when the N content is 0.0200% or less, the situation where excessive precipitation of (V, Cr)N occurs and toughness is reduced can be avoided. Therefore, the N content is preferably 0.0015~0.0200% or less. The N content is more preferably 0.0020~0.0080%.

[0073] (W: 0.10% or less) W is an element that improves hardenability, refines the lamellar spacing of pearlite to improve strength, and improves wear resistance. For example, when the W content is 0.03% or more, the effect can be preferably obtained. Also, when the W content is 0.10% or less, the time until the end of pearlite transformation becomes long, and the generation of martensite, which is harmful to toughness, can be prevented. Therefore, the W content is preferably 0.10% or less. The W content is more preferably 0.03% or more, 0.05% or more. The W content is more preferably 0.09% or less, 0.07% or less.

[0074] In the chemical composition of the base material portion 11 of the rail welding joint 1 according to the present embodiment, the balance consists of iron and impurities. The impurities are components that are mixed in due to raw materials such as ore or scrap, or various factors in the manufacturing process when the steel material is industrially manufactured, and are those that are allowed within a range that does not adversely affect the functions and effects of the present embodiment. Further, within a range that does not impair the effects of the rail welding joint according to the present embodiment, Pb, Bi, and Te may be contained. Regarding the content of these elements, if Pb is 0.09% or less, Bi is 0.10% or less, and Te is 0.05% or less, the functions and effects of the present embodiment are not impaired. Also, regarding O (oxygen), similarly, it may be contained within a range that does not impair the effects of the rail welding joint according to the present embodiment. For example, if the O content is about 0.0010 to 0.0040%, the functions and effects of the present embodiment are not impaired.

[0075] Next, an example of the manufacturing method of the rail welding joint 1 according to the present embodiment will be described. According to the manufacturing method described below, the rail welding joint 1 according to the present embodiment can be suitably obtained. However, even a rail welding joint obtained by a method other than the manufacturing method described below that satisfies the above requirements corresponds to the rail welding joint 1 according to the present embodiment.

[0076] The manufacturing method of the rail welding joint according to the present embodiment includes a step of flash butt welding the rail and a step of heating the welded portion 12 formed by the flash butt welding. The rail used as the welding base material is manufactured by hot rolling a continuously cast steel slab into a rail shape and then performing heat treatment on the head or the like as necessary. Note that the flash butt welding and heating do not change the chemical composition of the rail outside the welded portion 12. Therefore, the chemical composition of the rail used as the welding base material is the same as the chemical composition of the base material portion 11 of the rail welding joint 1.

[0077] (Flash Butt Welding) Flash butt welding is a type of resistance butt welding, and includes, for example, a pre-flash step, a preheat step, a post-flash step, and an upset step.

[0078] In the flash process, the end faces of two rails are first butted together with a gap in a state where a voltage is applied. Here, "butting together" means bringing two objects close together but facing each other with a gap between them. There is a distinction between the term "butting against" which means bringing two objects that are facing each other into contact, and the term "butting together". Next, the rails are moved so that the end faces of the rails are brought closer together. As a result, the protruding parts of the end faces of the two rails, which have projections and recesses, come into local contact with each other, causing a short-circuit current to flow.

[0079] The area through which the short-circuit current flows is rapidly heated by resistance heating and melts. This melted area is broken by the pinch force of the short-circuit current, generating an arc. This causes some of the molten metal to splash, and the end surface is heated by radiant heat. The splashing of molten metal is called the occurrence of a flash. These phenomena are continuously repeated in the pre-flashing process. The end surface of the rail is flattened by the pre-flashing process.

[0080] Because the rails are worn away by the splashing of molten metal, it is necessary to bring the rails closer to each other in the early flashing process and the later flashing process described below. The relative movement speed of the rails at this time is called the flashing speed. The relative movement amount of the rails is called the flashing length. The length of the rail worn away during the flashing process is called the flash-off distance. Usually, the flashing length and the flash-off distance are substantially the same value.

[0081] In the preheating process, the entire end faces of the two rails are brought into contact, electricity is passed through them for about 2 to 5 seconds, and then the end faces of the rails are separated for about 1 to 2 seconds. This contact and separation is repeated about 2 to 18 times. At this time, since the entire end faces of the two rails are in contact, a large current flows through the rails. Thereby, prior to the implementation of the post-stage flash process, the end face of the rail is preheated.

[0082] In the post-stage flash process, similar to the pre-stage flash process, first, the end faces of two rails are opposed to each other with a gap while applying a voltage. Next, the rails are moved so as to bring the end faces of the rails closer. Thereby, a flash is generated between the two end faces. The principle of the post-stage flash process is the same as that of the pre-stage flash process. However, different from the pre-stage flash process, at the start of the post-stage flash process, the end face of the rail is flattened and preheated. The flash speed in the post-stage flash process, that is, the post-stage flash speed, is generally faster than the flash speed in the pre-stage flash process, that is, the pre-stage flash speed. Also, the flash length in the post-stage flash process is longer than the flash length in the pre-stage flash process.

[0083] By the post-stage flash process, the entire end face is brought into a molten state. In the subsequent upsetting process, the end faces of the rails are abutted against each other and a large upsetting force is applied. Due to the large upsetting force, the end faces of the rails are joined and a joint is produced. Also, during the upsetting process, the melt and oxide of the end face are extruded from the welded part and remain as burrs. The burrs are usually removed immediately after welding is completed. Specifically, the burrs are removed by trimming (or bead cutting) between immediately after welding is completed and reheating of the welded joint part.

[0084] (Total preheating heat input: 9 - 60 kA·s / cm 2 ) As described above, in the preheating process, energization is carried out in a plurality of times. The total value of the heat input in the multiple energizations of the preheating process is referred to as the total preheating heat input. The total preheating heat input in flash butt welding affects the adhesion of the joint during welding. If the total preheating heat input is less than 9 kA·s / cm 2 , an uncrimped part occurs after welding and a predetermined bending performance cannot be obtained. Therefore, the total preheating heat input is set to 9 kA·s / cm 2 or more. On the other hand, if the total preheating heat input is 60 kA·s / cm 2When it exceeds this value, the welding cost becomes excessive. Therefore, the total preheating heat input should be in the range of 9 to 60 kA·s / cm 2 within this range.

[0085] (Final flash speed: 0.6 to 3.5 mm / s) The final flash speed refers to the flash speed immediately before the upsetting process. The final flash speed affects the adhesion of the joint during welding. If it is less than 0.6 mm / s, uncrimped parts will occur after welding and the specified bending performance cannot be obtained. Also, if it exceeds 3.5 mm / s, the entire rail will short-circuit and welding will be impossible. Therefore, the flash speed is set in the range of 0.6 to 3.5 mm / s. Note that the faster the final flash speed, the smaller the HAZ width, and the slower the final flash speed, the larger the HAZ width tends to be. However, the influence of the final flash speed is small compared to the total heat input in the preheating process.

[0086] (Upsetting load: 45 to 120 MPa) The upsetting load refers to the load applied to both sides of the rail during the upsetting process of flash butt welding. The upsetting load affects the adhesion of the joint and the HAZ width during welding. If it is less than 45 MPa, uncrimped parts will occur after welding and the specified bending performance cannot be obtained. Also, although there is no particular influence on the upper limit, it is generally set to 120 MPa due to equipment constraints. For the above reasons, the range of the upsetting load is set to 45 to 120 MPa. The higher the upsetting load, the smaller the HAZ width, and the lower the upsetting load, the larger the HAZ width tends to be. However, the influence of the upsetting load is small compared to the total heat input in the preheating process.

[0087] The HAZ width in flash butt welding is controlled through the total heat input in the above preheating process, the final flash speed, and the upsetting load.

[0088] Next, a process of heating the welded portion 12 formed by flash butt welding will be described. The heating of the welded portion 12 corresponds to so-called PWHT (Post Weld Heat Treatment). PWHT starts after the surface temperature at the first position A drops below 500°C after flash butt welding. That is, the temperature rise start temperature T1 of PWHT is less than 500°C. The lower limit of the temperature rise start temperature T1 of PWHT is not particularly set. For example, it is also allowed to start PWHT after the temperature of the welded portion has dropped to room temperature. On the other hand, if the temperature rise start temperature T1 of PWHT is too high, the effect of reducing residual stress cannot be obtained.

[0089] PWHT is performed, for example, by induction heating using a coil. Fig. 4 shows an example of the arrangement of the coil L and the magnetic body M when performing PWHT by induction heating. As shown in Fig. 4, induction heating may be performed by arranging the coil L at a position that is, for example, 0.2 to 3 times the HAZ width from the welding center. Also, the length of the heated region along the longitudinal direction X may be, for example, 0.5 to 5 times the HAZ width. Note that the coil L is arranged on both sides of the longitudinal direction X with the welded portion as the center.

[0090] In PWHT, the heating conditions for the first position A to the fourth position D are controlled individually. The heating conditions can be controlled through the distance between the coil and the rail or the coverage rate of the magnetic body. The coverage rate of the magnetic body is the value obtained by dividing the length of the magnetic body arranged on the coil along the circumferential direction of the rail by the circumferential length of the coil. For each of the first position A (the center along the height direction Z on the surface of the column portion 124 of the welded portion 12), the second position B (the center along the width direction Y on the bottom surface 1251 of the welded portion 12), the third position C (a position T / 10 away from the tip 1253 of the bottom surface 1251 of the welded portion 12 along the width direction Y), and the fourth position D (a position T / 3T away from the central axis of the column portion 124 along the width direction Y on the foot surface 1252 of the welded portion 12), control may be performed so as to achieve a predetermined temperature rise rate.

[0091] Note that the heating rate of the welded portion 12 described below is an average value. The value obtained by dividing the difference between the surface temperature at the start of heating and the surface temperature at the end of heating by the heating time is the average value of the heating rate, that is, the average heating rate. The surface temperatures at the first position A to the fourth position D can be measured by a radiation thermometer.

[0092] In induction heating, the closer the distance between the coil and the rail, the higher the heating rate. If the coil and the rail are too close, there is a possibility that they will come into contact. If the coil and the rail are too far apart, it is difficult to obtain the heating effect. In the method for manufacturing a welded joint of a rail according to the present disclosure, for example, the distance between the coil and the rail may be set to 3 to 20 mm.

[0093] In induction heating, for example, as shown in FIG. 4, by disposing the magnetic body M around the welded joint 1 of the rail, the heating rate can be increased or decreased. The effect can be changed by the coverage rate of the magnetic body M with respect to each part for controlling the heating rate. Specifically, as shown in FIG. 4, by disposing the magnetic body M in the induction heating coil L provided along the circumferential direction of the welded joint 1, the magnetic field lines can be converged, and the heating effect of the disposed part can be enhanced. That is, the "coverage rate of the magnetic body" can be controlled by the number of magnetic bodies disposed in the coil. The coverage rate can be selected from 0 to 100% according to the heating rate to be controlled.

[0094] Note that when heating the welded portion 12, it is necessary to consider the amount of heat transfer (heat extraction) from the welded portion 12 to the base material portion 11. The amount of heat extraction in the column portion 124 is extremely smaller than the amount of heat extraction in the foot portion 125. Therefore, when the welded portion 12 is uniformly heated along the circumferential direction of the rail, the heating rate of the foot portion 125 becomes significantly smaller than the heating rate of the column portion 124. In order to achieve the heating rate described later, it is necessary to manage the amount of heat input at the first position A to the fourth position D while considering the amount of heat extraction.

[0095] (Heating rate (V) at the first position A (center along the height direction Z on the surface of the column portion 124 of the welded portion 12): 2 to 6 (°C / s)) During the heating of the welded part 12, heat transfer always occurs from the welded part 12 to the base material part 11. The slower the heating rate, the greater the heating width in the longitudinal direction X and the lower the residual stress in the circumferential direction. If the heating rate (V) at the first position A exceeds 6 °C / s, the heating width in the longitudinal direction X becomes too narrow to obtain the effect of reducing the predetermined residual stress. The lower limit of the heating rate (V) at the first position A is not particularly limited. However, in order to avoid the PWHT time becoming too long, the lower limit of the heating rate (V) at the first position A is set to 2 °C / s.

[0096] (Maximum heating temperature at the first position A: 500 - 680 °C) The higher the maximum heating temperature, the greater the heating width in the longitudinal direction X and the lower the residual stress in the circumferential direction. If the maximum heating temperature at the first position A is 500 °C or lower, the heating width in the longitudinal direction X is narrow and the effect of reducing the predetermined residual stress cannot be obtained. When the maximum heating temperature at the first position A exceeds 680 °C, the column part 124 softens. Therefore, 680 °C is set as the upper limit of the maximum heating temperature at the first position A.

[0097] (Heating rate at the second position B (center along the width direction Y on the bottom surface 1251 of the welded part 12): 0.3×V - 1.0×V (°C / s)) The slower the heating rate at the second position B, the greater the effect of reducing the residual stress at the fourth position D. When the heating rate at the first position A is defined as V (°C / s), if the heating rate at the second position B exceeds 1.0×V (°C / s), the effect of reducing the longitudinal residual stress at the fourth position D cannot be obtained. Preferably, the heating rate at the second position B is 0.8×V (°C / s) or less. On the other hand, if the heating rate at the second position B is less than 0.3×V (°C / s), the tensile residual stress at the second position B exceeds the upper limit, impairing the fold resistance of the second position B. Therefore, 0.3×V (°C / s) is set as the lower limit of the heating rate at the second position B.

[0098] Note that the maximum heating temperature at the second position B is not particularly limited. By performing heating while satisfying the requirements for the heating rate at the first position A, the maximum heating temperature at the first position A, and the heating rate at the second position B described above, the maximum heating temperature at the second position B will naturally be determined. On the other hand, when the maximum heating temperature at the first position A is defined as tmax (°C), the maximum heating temperature at the second position B may be specified as 0.3×tmax (°C) to 1.0×tmax (°C).

[0099] (Heating rate at the third position C (a position 10T away from the tip 1253 along the width direction Y on the bottom surface 1251 of the leg portion 12 of the welded portion 12): 0.25×V to 1.0×V (°C / s)) The slower the heating rate at the third position C, the greater the effect of reducing the residual stress at the fourth position D. When the heating temperature at the third position C exceeds 1.0×V (°C / s), the effect of reducing the residual stress in the longitudinal direction X at the fourth position D cannot be obtained. Preferably, it is 0.7×V (°C / s) or less. If it is less than 0.25×V (°C / s), the tensile residual stress at the center of the bottom surface 1251 and the third position C portion exceeds the upper limit, impairing the fold resistance of the center of the bottom surface 1251 and the third position C portion. Therefore, since the residual stress at the center of the bottom surface 1251 swings to the tensile side and impairs the fold resistance of the 1 / 10T portion from the tip portion toward the center portion, 0.3×V (°C / s) is set as the lower limit. Note that, similar to the second position B, the maximum heating temperature at the third position C is not particularly limited. On the other hand, the maximum heating temperature at the third position C may be specified as 0.3×tmax (°C) to 1.0×tmax (°C).

[0100] (Heating rate at the fourth position D (a position 13T away from the central axis of the column portion 12 along the width direction Y on the foot surface 1252 of the leg portion 125 of the welded portion 12): 0.25×V to 1.0×V (°C / s)) The slower the temperature rise rate of the fourth position D, the greater the effect of reducing the residual stress at the fourth position D. When the temperature rise rate of the fourth position D exceeds 1.0×V (°C / s), the effect of reducing the residual stress in the longitudinal direction X of the fourth position D cannot be obtained. The temperature rise rate of the fourth position D is preferably 0.7×V (°C / s) or less. When the temperature rise rate of the fourth position D is less than 0.25×V (°C / s), the residual stresses at the second position B and the third position C shift to the tensile side and the folding resistance is impaired. Therefore, 0.25×V (°C / s) is set as the lower limit of the temperature rise rate of the fourth position D. Similar to the second position B, the maximum heating temperature of the fourth position D is not particularly limited. On the other hand, the maximum heating temperature of the fourth position D may be specified as 0.3×tmax (°C) to 1.0×tmax (°C).

[0101] Next, a method for evaluating the folding resistance of the foot surface 1252 of the foot portion 125 of the welded joint 12 in the rail welded joint 1 according to the present embodiment will be described.

[0102] As described above, the fracture of the rail welded joint 1 is caused by the tensile residual stress in the longitudinal direction of the fourth position D, the axial force applied to the rail, and the bending stress caused by the wheels. Therefore, the folding resistance was evaluated by a fatigue test under four-point bending with the welded joint portion as the center and the rail head upward in a simple manner.

[0103] FIG. 5 shows a schematic diagram for explaining the four-point bending test. In the four-point bending test, first, the rail welded joint is placed on two supports with the bottom surface of the foot facing downward. At this time, the center between the two supports is aligned with the welding surface. The distance between the two supports is set to 4 feet (1219 mm). Then, the load is applied to the top of the rail welded joint at two points. The distance between the two points where the load is applied is set to 1 foot (305 mm). The center of the two points where the load is applied is also aligned with the welding surface.

[0104] When the tensile strength at the base metal crown is TS [MPa] for a 136-pound rail, five tests were conducted at a stress range of TS / 4 [MPa] and a stress ratio of 0.1, and the condition that the number of cycles to withstand 2 million cycles was 3 or more was defined as passing. Note that for the tensile test to obtain TS, a No. 14 test piece of JIS Z2241 was collected and measured with the longitudinal direction of the rail as the longitudinal direction of the test piece, centered at a position 6 mm deep from the crown at the center in the width direction of the head of the base metal.

Example

[0105] The effect of the welded joint of the rail of the present embodiment will be further specifically described by way of examples. The conditions in the following examples are one example of the conditions adopted to confirm the feasibility and effect of the welded joint of the rail of the present embodiment. Therefore, the welded joint of the rail of the present embodiment is not limited to this one example of conditions.

[0106] The welded joint of the rail was manufactured by the following method. A slab produced by continuous casting was hot-rolled by the universal rolling method to manufacture a rail having the final shape of a 136RE rail (cross-sectional area: 8600 mm 2 ) defined by AREMA. The chemical components of the slabs in Examples 1 to 30 were as described in Tables 1 and 2. The chemical components of the slabs in Examples 31 to 58 were the same as those of the slab in Example 1. The content of the elements not added to the slab was indicated by the symbol "-" in Table 1. The remainder of the chemical components described in Table 1 was iron and impurities.

[0107] Welding consisting of a preheating process, a flash process, and an upsetting process was performed on the end faces of the obtained rails using a stationary flash butt welder. After upsetting, the head was accelerated and cooled with compressed air. The total heat input Q1 (kAs) in the preheating process, the final flash speed V0 (mm / s) in the flash process, the upsetting load L1 (t), and the obtained HAZ width W were as shown in Tables 3A and 3B. Note that in the preheating process, the total heat input Q1 was 800 kAs (9 kAs / cm 2 ) to 3600 kAs (42 kAs / cm 2Within the range of ( ), the upset load L1 was changed within the range of 40 t (47 MPa) to 90 t (107 MPa) respectively for implementation.

[0108] Furthermore, heating coils were installed on both sides at positions longitudinally separated from the welding center by a length twice the HAZ width W. After the temperature at the center (the first position A) along the height direction on the surface of the column part at the welding center dropped to a predetermined temperature, induction heating was carried out. When the surface temperature of the central part of the column at the welding center reached the predetermined temperature, the temperature increase was stopped. The temperature increase start temperature T1 (°C), the temperature increase rate V1 (°C / s) at the first position A, the T2 (°C) at which the temperature increase was stopped at the first position A, the temperature increase rate V2 (°C / s) at the second position B, the temperature increase rate V3 (°C / s) at the third position C, and the temperature increase rate V4 (°C / s) at the fourth position D were as shown in Table 4A and Table 4B.

[0109] [Table 1]

[0110] [Table 2]

[0111] [Table 3A]

[0112] [Table 3B]

[0113] [Table 4A]

[0114] [Table 4B]

[0115] The welded joint of the rail was manufactured by the above manufacturing process. Then, the residual stresses at the first position A, the second position B, the third position C, and the fourth position D on the welding surface of the welded joint of the rail were measured. Furthermore, the evaluation of the bending fatigue resistance of the welded joint of the rail was carried out by a four-point bending fatigue test. The measurement method of the residual stress and the evaluation method of the bending fatigue resistance were as described above. The results were as shown in Table 5A and Table 5B.

[0116]

Table 5A

[0117]

Table 5B

[0118] In Examples 33 and 37, the residual stress at the first position A was excessive. As a result, the bending fatigue resistance of Examples 33 and 37 was inferior. It is presumed that this is because the heating rate at the first position A in Examples 33 and 37 was excessive.

[0119] In Examples 34 and 38, the residual stress at the first position A was excessive. As a result, the bending fatigue resistance of Examples 34 and 38 was inferior. It is presumed that this is because the maximum heating temperature at the first position A in Examples 34 and 38 was too low.

[0120] In Example 41, the residual stress at the second position B was excessive. As a result, the bending fatigue resistance of Example 41 was inferior. It is presumed that this is because the heating rate at the second position B in Example 41 was too slow.

[0121] In Example 42, the residual stress at the fourth position D became excessive. As a result, the bending fatigue resistance of Example 42 was inferior. It is presumed that this is because the heating rate at the second position B in Example 42 was too fast. It is presumed that the reason why the residual stress at the fourth position D became inappropriate was that the optimization of the residual stress at the fourth position D through the residual stress at the second position B could not be achieved.

[0122] In Example 45, the residual stress at the third position C was excessive. As a result, the folding resistance of Example 45 was inferior. It is presumed that this was because the heating rate at the third position C in Example 45 was too slow.

[0123] In Example 46, the residual stress at the third position C was insufficient and the residual stress at the fourth position D was excessive. As a result, the folding resistance of Example 46 was inferior. It is presumed that this was because the heating rate at the third position C in Example 46 was too fast. The reason why the residual stress at the fourth position D became inappropriate is presumed to be that the optimization of the residual stress at the fourth position D via the residual stress at the third position C could not be achieved.

[0124] In Example 49, the residual stress at the fourth position D was excessive. As a result, the folding resistance of Example 49 was inferior. It is presumed that this was because the heating rate at the fourth position D in Example 49 was too fast.

[0125] In Example 50, the residual stress at all positions related to the foot portion was inappropriate. Specifically, the residual stresses at the second position B and the third position C were insufficient, and the residual stress at the fourth position D was excessive. As a result, the folding resistance of Example 50 was inferior. It is presumed that this was because the heating rates at all of the second position B, the third position C, and the fourth position D in Example 50 were too fast.

[0126] On the other hand, in examples where the residual stress was appropriate, the folding resistance was good. In these examples, the flash butt welding conditions and the heating conditions were appropriate.

Industrial Applicability

[0127] According to the above aspect of the present invention, a welded joint of a welded rail that improves the folding resistance of a welded portion under heavy load conditions can be obtained. Therefore, the welded joint of the obtained welded rail can be suitably applied to freight railways and the like, and thus has high industrial applicability.

Explanation of Signs

[0128] 1 Welded joint of rail 11 Base material part 12 Welded part 121 Weld surface 122 Heat affected zone 123 Head part 124 Column part 1241 Central axis 125 Foot part 1251 Bottom surface of foot 1252 Surface of foot 1253 Tip of foot X Longitudinal direction Y Width direction Z Height direction Size of heat affected zone (HAZ width) along the longitudinal direction W T Distance from the central axis of the column part to the tip of the foot A First position B Second position C Third position D Fourth position L Coil M Magnetic material

Claims

1. A welded joint portion of a welded rail including a base metal portion and a weld portion, The chemical composition of the base material portion is, in unit mass%, C: 0.60-1.10%, Si: 0.10-2.00%, Mn: 0.20-1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.01-1.00%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Nb: 0 to 0.050%, Al: 0-0.100%, Ti: 0 to 0.080%, Mo: 0 to 0.10%, V: 0 to 0.200%, B: 0 to 0.0030%, Zr: 0 to 0.0200%, Ca: 0-0.0200%, Mg: 0-0.0100% Sb: 0 to 0.050%, Rare earth elements: 0 to 0.0500%, Co: 0 to 0.50%, N: 0 to 0.0200%, W: 0 to 0.10%, Pb: 0 to 0.09%, Bi: 0 to 0.10%, and Te: 0-0.05% with the remainder being Fe and impurities, The weld portion has a head portion, a column portion, and a foot portion, On the welding surface of the weld, the distance from the central axis of the column to the toe of the foot is defined as T, the size of the heat-affected zone along the longitudinal direction is defined as W in mm, the midpoint between the head surface of the head and the sole surface of the foot, the center in the width direction of the weld, and the position of the surface of the column are defined as a first position, the center along the width direction of the sole surface of the foot is defined as a second position, a position T / 10 away from the toe along the width direction of the sole surface of the foot is defined as a third position, and a position T / 3 away from the central axis of the column along the width direction of the foot surface of the foot is defined as a fourth position. The residual stress in the height direction measured at the first position is 543-6.25×W (MPa) or less; The longitudinal residual stress measured at the second position is −200 to −20 (MPa); The longitudinal residual stress measured at the third position is −350 to −100 (MPa); A welded joint portion of a welded rail, wherein the longitudinal residual stress measured at the fourth position is 0 to 200 (MPa).

2. The chemical composition of the base material portion is, in unit mass%, Cu: 0.50% or less, Ni: 0.50% or less, Nb: 0.050% or less, Al: 0.001-0.100%, Ti: 0.080% or less, Mo: 0.10% or less, V: 0.200% or less, B: 0.0030% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Mg: 0.0100% or less, Sb: 0.050% or less, Rare earth elements: 0.0500% or less, Co: 0.50% or less, N: 0.0015-0.0200%, W: 0.10% or less, Pb: 0.09% or less, Bi: 0.10% or less, and Te: 0.05% or less, 2. The welded joint of a welded rail according to claim 1, further comprising one or more elements selected from the group consisting of:

3. flash butt welding a rail having the chemical composition according to claim 1 or 2; and heating the welded portion formed by the flash butt welding. The weld portion has a head portion, a column portion, and a foot portion, On the welding surface of the weld, the distance from the central axis of the column to the toe of the foot is defined as T, the size in mm of the heat-affected zone along the longitudinal direction is defined as W, the midpoint between the head surface of the head and the sole of the foot, the center in the width direction of the weld, and the position of the surface of the column are defined as a first position, the center along the width direction of the sole of the foot is defined as a second position, a position T / 10 away from the toe along the width direction of the sole of the foot is defined as a third position, and a position T / 3 away from the central axis of the column along the width direction of the foot surface of the foot is defined as a fourth position. In the flash butt welding, the total preheating heat input is 9 to 60 (kA·s / cm ２ ), the final flash velocity is 0.6 to 3.5 (mm / s), and the upset load is 45 to 120 MPa; The heating of the weld is initiated after the surface temperature of the first location has decreased to less than 500° C.; In the heating of the welded portion, the surface temperature of the first position is increased to 500 to 680° C.; In the heating of the welded portion, the average temperature rise rate V of the surface at the first position is set to 2.0 to 6.0 (° C. / s); In the heating of the welded portion, the average temperature rise rate of the surface at the second position is set to 0.3×V to 1.0×V (° C. / s); In the heating of the welded portion, an average temperature rise rate of the surface at the third position is set to 0.25×V to 1.0×V (° C. / s); In the heating of the welded portion, the average temperature rise rate of the surface at the fourth position is set to 0.25×V to 1.0×V (° C. / s). A manufacturing method for welded joints of welded rails.