Welded joint portion of welded rail, and method for manufacturing welded joint portion of welded rail

Abstract

In a welded joint portion of a welded rail according to the present invention, when, on a welded surface, a distance from a central axis of a column portion to a tip of a foot is defined as T, a size of a heat affected zone is defined as W, a position at a center of the welded portion in a width direction and at intermediate point between a head surface of a head portion and a foot rear surface of the foot portion and at a surface of a column portion is defined as a first position, a center of a foot rear surface is defined as a second position, a position separated by T / 10 from the tip of the foot on the foot rear surface is defined as a third position, and a position separated by T / 3 from the central axis of the column portion on the foot surface is defined as a fourth position: the residual stress in a height direction measured at the first position is 543 - 6.25 × W (MPa) or less; the residual stress in a longitudinal direction measured at the second position is - 200 to - 20 (MPa); the residual stress in a longitudinal direction measured at the third position is - 350 to - 100 (MPa); and the residual stress in a longitudinal direction measured at the fourth position is 0 to -200 (MPa).

Description

Welded rail weld joint and method for manufacturing welded rail weld joint

[0001] This disclosure relates to a welded rail weld joint suitable for freight railway applications, and a method for manufacturing a welded rail weld joint. This application claims priority to Japanese Patent Application No. 2024-021377, filed February 15, 2024, the contents of which are incorporated herein by reference.

[0002] Freight rail is used as a means of transporting natural resources, such as ores, when they are mined. Compared to truck transport, freight rail transport of natural resources is more efficient and emits less CO per load. ２ In order to improve transport efficiency and reduce the environmental impact, rails for freight railways are required to have a longer lifespan and be easier to maintain.

[0003] In recent years, efforts have been made to extend the life of rails by welding rail joints. Rail joints are the parts of a rail that are most susceptible to damage. However, even in welded rail joints, problems arise with the wear resistance of the head and, in particular, breakage from the center of the rail.

[0004] A heat-affected zone (HAZ) exists in a welded joint. The HAZ is softer than the base material of the rail. For this reason, the wear amount and damage resistance of the head of the HAZ of the welded joint of a welded rail are lower than those of the base material. This deteriorates the maintainability and lifespan of the welded joint of the welded rail. Therefore, it is preferable that the softened width of the HAZ is narrow.

[0005] There are four main rail welding methods: flash butt welding, thermite welding, enclosed arc welding, and gas pressure welding. Of these, flash butt welding is widely used because it requires a short welding time. Furthermore, the welds obtained by flash butt welding have a narrow HAZ width. Furthermore, the welding process for flash butt welding is automated.

[0006] On the other hand, rail welding involves applying heat to the ends of two rails, joining them, and then cooling. During the cooling process, residual stresses are generated in both the circumferential and longitudinal directions in the welded joint of the welded rail. In particular, flash butt welding is prone to generating tensile residual stress in the circumferential direction of the rail in the center of the column. This is because flash butt welding narrows the heat distribution width along the longitudinal direction in order to narrow the HAZ width. This has led to the issue of fatigue cracks initiating and growing in the longitudinal direction in the column of the welded joint, leading to brittle fracture from the center of the column in the longitudinal direction, which can easily result in breakage.

[0007] Regarding countermeasures against fracture accompanied by fatigue cracks originating from the central axis of the column portion of the welded joint of this welded rail, Non-Patent Document 1 discloses post-weld heat treatment (PWHT) in which the rail weld is heated by induction heating to the Ac1 point or below at two points each spaced apart in the longitudinal direction from the weld center. PWHT distributes the distribution of tensile residual stress in the circumferential direction of the column portion, which is concentrated near the weld, longitudinally, thereby suppressing fatigue cracks and the resulting brittle fracture.

[0008] Furthermore, Patent Document 1 discloses that by arranging a coil at a distance from the center of the weld that is 0.2 to 3 times the HAZ width, heating the entire circumference of the rail, and heating the center of the pillar portion to a temperature range of 400 to 750°C, the residual stress in the circumferential direction at the center of the pillar portion can be increased to improve the fatigue properties of the pillar portion.

[0009] International Publication No. 2012 / 161207

[0010] 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

[0011] These PWHT technologies suppress longitudinal fatigue cracks originating from the central axis of the column at the welded joint, but still present the problem of breakage when attempting to further extend the service life or under heavy load conditions.

[0012] The present disclosure has been devised in light of the above-mentioned points, and aims to provide a welded rail weld joint and a manufacturing method thereof that improve the fracture resistance of the column portion of the weld under heavy load conditions, which is particularly required for welded rail weld joints in overseas freight railways. Reduced fracture resistance is a particularly significant issue in high-strength welded rails. Therefore, rails used in the welded rails according to the present disclosure comply with, for example, JIS E1101 (2001), JIS E1120 (2007), AREMA Chapter 4 "Rail", EN 13674-1, etc.

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

[0014] (1) A welded joint of a welded rail according to an aspect of the present disclosure is a welded joint of a welded rail including a base metal portion and a weld portion, wherein the chemical composition of the base metal portion is, in 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%, the weld zone comprises 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%, with the balance being Fe and impurities; the weld zone 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 position midway between the head surface of the head and the sole of the foot, which is also the center in the width direction of the weld, and on the surface of the column is 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 is defined as a fourth position, the height direction residual stress measured at the first position is 543-6.25×W (MPa) or less, the longitudinal direction residual stress measured at the second position is -200 to -20 (MPa), and the longitudinal direction residual stress measured at the third position is -350 to -100 (MPa), A rail welded joint, characterized in that the longitudinal residual stress measured at the fourth position is 0 to 200 (MPa).(2) Preferably, in the welded joint portion of the welded rail described in (1) above, the chemical composition of the base metal portion is, in 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, The steel sheet contains one or more elements selected from the group consisting of Bi: 0.10% or less, and Te: 0.05% or less. (3) A manufacturing method of a welded joint of a welded rail according to another aspect of the present disclosure is a manufacturing method of a welded joint of a welded rail, comprising the steps of flash butt welding a rail having the chemical composition described in (1) or (2) above, and heating the weld formed by the flash butt welding, wherein the weld has a head, a column, and a foot, and at the weld 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, 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 on the surface of the column is defined as a first position, the center in the width direction of the sole of the foot is defined as a second position, a position on the sole of the foot along the width direction away from the toe at T / 10 is defined as a third position, and a position on the foot surface of the foot along the width direction away from the central axis of the column is defined as a fourth position. In the flash butt welding, the total preheating heat input is 9 to 60 (kA·s / cm ２), a final flashing velocity of 0.6 to 3.5 (mm / s), and an upset load of 45 to 120 MPa; the heating of the welded portion is started after the surface temperature at the first position has dropped to less than 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, an 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, an average temperature rise rate V of the surface at the second position is set to 0.3 x V to 1.0 x V (°C / s); in the heating of the welded portion, an average temperature rise rate V of the surface at the third position is set to 0.25 x V to 1.0 x V (°C / s); and in the heating of the welded portion, an average temperature rise rate V of the surface at the fourth position is set to 0.25 x V to 1.0 x V (°C / s).

[0015] According to the welded joint of a welded rail and the manufacturing method of a welded joint of a welded rail disclosed herein, sufficient breakage resistance of the post and foot portions can be obtained.

[0016] Fig. 1 is a side view of a welded joint of a welded rail. Fig. 2 is a perspective view of a welded joint of a welded rail. Fig. 3 is a cross-sectional view of a welded joint of a welded rail. Fig. 4 is a schematic diagram illustrating an example of the arrangement of a coil and a magnetic body when performing PWHT by induction heating. Fig. 5 is a schematic diagram illustrating a four-point bending test.

[0017] Hereinafter, a welded joint portion of a welded rail according to one embodiment of the present disclosure will be described.

[0018] (Shape of welded joint of welded rail) First, the shape of a welded rail (hereinafter also simply referred to as a rail) and its welded joint (hereinafter also simply referred to as a weld joint) will be described. Railway rails are used by being installed on the ground or on the foundation of a bridge or the like. In this embodiment, the vertical direction when the rail is installed on the foundation and used is referred to as the height direction Z. The extension direction of the rail is referred to as the longitudinal direction X. The direction perpendicular to the height direction and longitudinal direction of the rail is referred to as the width direction Y.

[0019] A rail has a head, a post, and a foot. The head of a rail is the part above the narrowed part at the center of the rail's height. The head of a rail comes into contact with the wheels of a railway vehicle. The post of a rail is the narrowed part at the center of the rail's height. The foot of a rail is the part below the narrowed part at the center of the rail's height, and is also called the bottom. The foot of a rail is installed on a foundation. Figures 2 and 3 show the head 123, post 124, and foot 125 of the weld 12 of a rail weld joint 1.

[0020] The foot has a sole, a foot surface, and a toe. The sole of the foot is the surface that comes into contact with the base when the rail is installed on the base. The foot surface of the foot is the surface that is located on the upper side when the rail is installed on the base. The toe of the foot is the end of the foot in the width direction. Figures 2 and 3 show the sole 1251, foot surface 1252, and toe 1253 of the welded portion 12 of the rail weld joint 1.

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

[0022] As shown in FIG. 1 , a rail weld joint 1 has a base metal portion 11 and a weld portion 12. The weld portion 12 is a collective term that includes a weld surface 121 and a heat-affected zone 122 (HAZ). The weld surface 121 refers to the surface formed by melting and solidifying the rail. The heat-affected zone 122 refers to the unmelted portion where the structure, metallurgical properties, and mechanical properties have changed due to the welding heat. The heat-affected zone 122 is a region that is heated to near point A1 by the welding heat and partially austenitized, and then the pearlite structure decomposes as the temperature drops after welding is completed. This region is also called the HAZ. The hardness of the heat-affected zone 122 is significantly reduced. Therefore, two Vickers hardness valleys are typically present in a graph of the hardness distribution of a rail obtained by flash butt welding. The locations where these Vickers hardness valleys occur are defined as the softest parts of the rail weld joint according to this embodiment. The distance between the two softest parts is defined as the HAZ width. The weld center is approximately the same as the center of the HAZ width. The base material 11 refers to the area of ​​the rail weld joint other than the weld 12. The area outside the heat-affected zone 122 corresponds to the base material 11.

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

[0024] (Discovery of the Inventors) Next, the discovery of the inventors, which forms the basis of the present disclosure, will be described. The inventors have newly discovered that under heavy load conditions, breakage due to fatigue cracking occurs from the foot surface 1252 of the foot portion 125 of the weld 12 (particularly near the fourth position D shown in FIGS. 2 and 3 ). Conventional PWHT technology makes it possible to suppress brittle fracture due to a crack in the center of the column portion 124 of the weld 12. However, conventional PWHT technology has not taken into consideration breakage at the fourth position D of the weld 12.

[0025] Therefore, in order to further improve the breakage resistance of the rail welded joint 1, the inventors conducted extensive research into why fracture occurs on the foot surface 1252 side of the foot portion 125. As a result, they found that a tensile residual stress of 250 MPa or more acts in the longitudinal direction X at a fourth position D on the foot surface 1252, which is close to the column portion 124. They found that this tensile residual stress makes fatigue cracks more likely to occur in an operating environment under heavy load conditions.

[0026] After further investigation, it was found that in order to suppress breakage 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 portion 12 and the base material portion 11 .

[0028] Therefore, the inventors have investigated a method of performing post-weld heat treatment that balances the compressive residual stress in the longitudinal direction X of the sole 1251 and toe 1253 with the tensile residual stress at the fourth position D on the foot surface, and that can reduce the circumferential tensile residual stress at the center of the column portion 124. They have discovered a manufacturing method for a rail welded joint 1 that effectively reduces the residual stress at the fourth position D by optimizing the temperature rise pattern at each of these locations.

[0029] In rail post-weld heat treatment, only the heating conditions for the base portion 124 are often specified. This is because, in the prior art, it was believed that breakage of the rail weld 12 was caused by residual stress in the base portion 124. Furthermore, in the prior art, heating the base portion 125 of the weld 12 was considered undesirable because it would soften the base portion 125. For these reasons, the base portion 125 was often not targeted for heating in prior rail post-weld heat treatment. Furthermore, when the base portion 125 was heated, the temperature rise rate of the surface of the base portion 125 was not controlled.

[0030] However, the inventors have discovered that it is extremely important to also heat the foot portion 125 and further to control the rate of temperature rise at each of several specific locations on the foot portion 125 in order to prevent breakage on the foot surface 1252 of the foot portion 125.

[0031] Based on the above findings, the residual stresses measured at each of the following four positions on the weld surface 121 of the rail welded joint 1 according to this embodiment are within a predetermined range: (First position A) Midway between the head surface of the head portion 123 and the sole 1251 of the foot portion 125, at the center of the weld in the width direction, and at the surface of the column portion 124; (Second position B) Center along the width direction Y on the sole 1251 of the foot portion 125; (Third position C) Position T / 10 away from the toe 1253 along the width direction Y on the sole 1251 of the foot portion 125; (Fourth position D) Position T / 3 away from the central axis of the column portion 124 along the width direction Y on the foot surface 1252 of the foot portion 125.

[0032] As shown in Figure 3, "T" refers to the distance from the central axis of the column portion 124 to the toe 1253 of the foot portion 125. In the four-point bending test described below, the rotation axis is aligned with the width direction Y to bend the welded joint 1 of the rail. Figure 1 shows a first position A. Figure 3 shows a second position B, a third position C, and a fourth position D. The first position A is the midpoint between the head surface of the head portion 123 and the sole surface 1251 of the foot portion 125.

[0033] In Figure 2, the third position C and the fourth position D are shown on only one side of the rail weld joint 1. However, in reality, as shown in Figure 3, the third position C and the fourth position D exist on both sides of the rail weld joint 1. On both sides of the rail weld joint 1, the residual stress at the third position C and the fourth position D must be within a predetermined range. When measuring the residual stress, it is necessary to measure both sides of the rail weld joint 1 for the third position C and the fourth position D. Then, it is necessary to confirm that the residual stress on each side is within the predetermined range.

[0034] First positions A also exist on both sides of the welded joint 1 of the rail. The residual stress at the first positions A on both sides must be within the specified range. However, the two first positions A are close to each other. If the residual stress at one first position A is within the specified range, the residual stress at the other first position A will usually also be within the specified range. Therefore, it is sufficient to measure the residual stress at only one of the first positions A.

[0035] Next, the residual stresses at the first position A to the fourth position D will be described. Hereinafter, residual stresses described as negative values ​​indicate residual stresses in the compressive direction, and residual stresses described as positive values ​​indicate residual stresses in the tensile direction. An increase in residual stress is sometimes expressed as "residual stress increases toward the tensile side" or "residual stress deflects toward the tensile side," and a decrease in residual stress is sometimes expressed as "residual stress increases toward the compressive side" or "residual stress deflects toward the compressive side."

[0036] (Residual stress in the height direction Z measured at first position A: 543-6.25×W (MPa) or less) The double-headed arrow in Figure 1 indicates the residual stress in the height direction Z at first position A. When the residual stress in the height direction Z measured at first position A increases toward the tensile side, fatigue crack initiation in the column portion 124 of the welded portion 12 and the propagation of the crack along the longitudinal direction X are promoted. This reduces the breakage resistance of the welded joint of the rail. Therefore, the smaller the residual stress in the height direction Z measured at first position A, the better.

[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 weld 12 can be sufficiently suppressed. Note that "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." While the HAZ width is not specifically defined, the narrower the HAZ width, the greater the residual stress in the height direction Z at the first position A, which is the center 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 of the residual stress in the height direction Z measured at the first position A is not particularly limited. In consideration of operational productivity, it is preferable that the residual stress in the height direction Z measured at the first position A be 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 at the position marked with the symbol B in Figure 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 compressive stress. The greater the tensile residual stress in the longitudinal direction X measured at the second position B, which is the center of the sole 1251, the more likely fatigue crack initiation from the sole 1251 is promoted. This is due to the synergistic effect of the bending stress applied to the weld 12 when a vehicle passes over it and the axial force generated by thermal expansion of the rail. However, the breakage resistance of the weld 12 can be improved by balancing the residual stress in the longitudinal direction X at the second position B with the tensile residual stress in the longitudinal direction X at the fourth position D described below.

[0040] In order to suppress the occurrence of fatigue cracks from the sole 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 reliably 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 at the position marked with the symbol C in Figure 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 compressive stress. The more the residual stress in the longitudinal direction X at the third position C deflects toward the tensile direction, the more it promotes the initiation 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 a vehicle passes over it and the axial force generated by thermal expansion of the rail. However, by balancing the residual stress in the longitudinal direction X measured at the third position C with the tensile residual stress in the longitudinal direction X at the fourth position D, breakage resistance can be improved.

[0043] In order to suppress the initiation 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. For this reason, the lower limit 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-headed arrow at the position marked with the symbol D in FIG. 2 indicates 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 deflects in the tensile direction, the more it promotes fatigue crack initiation from the periphery of the fourth position D, reducing breakage resistance. This is due to the synergistic effect of the bending stress applied to the welded portion 12 when a vehicle passes over it and the axial force generated by thermal expansion of the rail.

[0045] In order to suppress the occurrence 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 of the residual stress in the longitudinal direction X measured at the fourth position D is not particularly limited, but is preferably 50 MPa in consideration of operational productivity.

[0046] As described above, in the welded joint of the welded rail in this embodiment, residual stress is reduced throughout the welded joint to improve breakage resistance. That is, breakage resistance is improved by balancing residual stress throughout the weld and base material. Specifically, to improve breakage 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 with 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 residual stress in a rail welded joint 1 according to this embodiment will be described. (1) First, the surface of the rail welded joint 1 at the location where residual stress is to be measured is polished using paper or the like. Polishing is performed so that a strain gauge can be attached to the location where residual stress is to be measured. (2) Next, a biaxial strain gauge with 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. (3) Then, a region measuring 10 mm in length and width and 5 mm in thickness is cut out from the strain gauge attachment location. The length and width of the cut-out region are aligned with the two axes of the strain gauge. Residual stress is released in the cut-out region, and the shape of the cut-out region changes. This shape change is measured using a strain gauge. (4) The residual stress is calculated based on the obtained strain and the rail's Young's modulus (210 GPa). As described above, the positive side is compressive stress and the negative side is tensile stress.

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

[0049] Next, the chemical composition (steel components) of the base material portion 11 of the rail welded joint 1 according to this embodiment will be described. The unit "%" for the content of each element means "mass %." The chemical components of the base material portion 11 are the same as the chemical components of the rail before welding.

[0050] (C: 0.60 to 1.10%) C increases the cementite fraction and refines the lamellar spacing of the pearlite structure, thereby increasing the strength of the rail and improving its wear resistance and surface damage resistance. The C content appropriate for a rail is in the range of 0.60 to 1.10%. Preferably, the C content is 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 to 2.00%) Si is a solid solution strengthening element for the ferrite phase in the pearlite structure, and is also an element that improves hardenability and refines lamellar spacing to increase strength and improve wear resistance. The Si content appropriate for a rail is in the range of 0.10 to 2.00%. Preferably, the Si content is 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 to 1.50%) Mn is an element that improves hardenability and refines lamellar spacing, and by dissolving in lamellar cementite, suppresses granularization of the lamellar structure after the formation of pearlite structure, thereby improving strength and wear resistance. The appropriate Mn content for a rail is in the range of 0.20 to 1.50%. Preferably, the Mn content is 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, and reduces the toughness of the welded joint 1 of the rail according to this embodiment, impairing breakage resistance. The P content appropriate for 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 in consideration of refining costs, 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, and reduces surface damage resistance by precipitating sulfides. The S content appropriate for rails 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 in consideration of refining costs, the S content may be more than 0%, 0.001% or more, or 0.005% or more.

[0055] (Cr: 0.01 to 1.00%) Cr is an element that refines the lamellar spacing of pearlite, thereby improving the strength and wear resistance of the rail weld joint 1. Cr also dissolves in V carbonitrides, promoting the precipitation of (V, Cr)(C, N), and improving surface damage resistance. The Cr content appropriate for a rail is in the range of 0.01 to 1.00%. Preferably, the Cr content is 0.03 to 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, the base metal portion 11 of the rail welded joint 1 manufactured with the above chemical composition may contain one or more of the following elements as needed: V, Cu, Ni, Nb, Al, Ti, Mo, B, Zr, Ca, Mg, Sb, rare earth elements (REM), Co, N, W, Pb, Bi, and Te. However, since the rail welded joint 1 according to this embodiment can exhibit its effects even if these elements are not contained in the base metal portion 11, the lower limit of the content of these elements is 0%.

[0057] (Cu: 0.50% or less) Cu is an element that improves hardenability, refines the lamellar spacing of pearlite, improves strength, and improves wear resistance. For example, when the Cu content is 0.01% or more, this effect can be preferably obtained. Furthermore, by setting the Cu content to 0.50% or less, it is possible to prevent the time until the completion of pearlite transformation from increasing and the formation of a martensite structure that is harmful to the toughness of rail welded joints. 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: 0.50% or less) Ni is an element that improves hardenability and refines the lamellar spacing of pearlite to improve strength and wear resistance. For example, when the Ni content is 0.01% or more, this effect can be preferably obtained. Furthermore, when the Ni content is 0.50% or less, the time until the completion of pearlite transformation is extended, making it possible to prevent the formation of martensite structures that are harmful to the toughness of rail welded joints. Therefore, the Ni content is preferably 0.50%. 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%.

[0059] (Nb: 0.050% or less) Nb is an element that is effective in refining austenite grains during welding due to the pinning effect of Nb carbides and Nb nitrides, thereby suppressing the formation of martensite, which is detrimental to toughness. For example, when the Nb content is 0.005% or more, this effect can be suitably obtained. Furthermore, when the Nb content is 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 to 0.1000%) Al is an element effective in preventing coarsening of austenite grains during welding and suppressing the formation of martensite, which is harmful to toughness. For example, when the Al content exceeds 0.001%, this effect can be suitably obtained. Furthermore, when the Al content is 0.100% or less, the formation of coarse Al-based oxide inclusions is 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 that forms Ti carbides, prevents austenite from becoming coarse during welding, and is effective in suppressing the formation of martensite, which is harmful to toughness. For example, when the Ti content is 0.002% or more, this effect can be suitably obtained. Furthermore, 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: 0.10% or less) Mo is an element that improves hardenability and refines the lamellar spacing of pearlite to improve strength and wear resistance. For example, when the Mo content is 0.005% or more, this effect can be preferably obtained. Furthermore, when the Mo content is 0.10% or less, the time until the completion of pearlite transformation is extended, and the formation of martensite, which is harmful to toughness, can be prevented. 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: 0.200% or less) V is an element that increases the hardness (strength) of pearlite structures through precipitation hardening by (V, Cr)(C, N) formed during the cooling process after hot rolling, thereby improving the wear resistance and surface damage resistance of rail joints. For example, when the V content is 0.010% or more, this effect can be suitably obtained. Furthermore, when the V content is 0.200% or less, it is possible to prevent partial excessive precipitation of (V, Cr)(C, N), which would reduce the toughness of the rail. For this reason, 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: 0.0030% or less) B is an element that segregates to austenite grain boundaries during hot rolling to improve hardenability and refine lamellar spacing, thereby improving strength and simultaneously improving wear resistance. For example, when the B content is 0.0004% or more, this effect can be suitably obtained. Furthermore, when the B content is 0.0030% or less, it is possible to prevent the formation of B carbonitrides during heat treatment, which would impair 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 also as a deoxidizer of ZrO ２Zr is an element that increases the equiaxed crystallization rate of the solidification structure through inclusions, suppresses segregation in the center of the slab, and suppresses the formation of martensite and pro-eutectoid cementite structures in rail segregation regions. For example, when the Zr content is 0.0050% or more, this effect can be favorably achieved. Furthermore, when the Zr content is 0.0200% or less, the formation of large amounts of coarse Zr-based inclusions can be prevented, and rail breakage due to stress concentration can be further suppressed. 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 used as a deoxidizer and also forms Ca sulfides, which inhibit the coarsening of MnS and improve surface damage resistance. For example, when the Ca content is 0.0050% or more, this effect can be suitably obtained. Furthermore, when the Ca content is 0.0200% or less, the generation of large amounts 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 used as a deoxidizer. For example, when the Mg content is 0.0050% or more, the effect can be suitably obtained. Furthermore, when the Mg content is 0.0100% or less, the generation of large amounts 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 a solid solution strengthening element that strengthens pearlite and improves wear resistance. For example, when the Sn content is 0.005% or more, this effect can be preferably obtained. Furthermore, when the Sn content is 0.050% or less, the time until the pearlite transformation is completed is extended, and the formation of martensite, which is harmful to toughness, can be prevented. 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, this effect can be suitably obtained. Furthermore, when the Sb content is 0.050% or less, the time until the pearlite transformation is completed is extended, and the formation of martensite, which is harmful to toughness, can be prevented. 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 elements (REM): 0.0500% or less) Rare earth elements (REM) are a collective term for 17 elements, including two elements, scandium and yttrium, and 15 elements (lanthanoids) from lanthanum to lutetium. REM are used as deoxidizers and also as inclusion control agents to reduce coarse Al ２ O ３REM is an element that suppresses the formation of oxide-based inclusions and improves the breakage resistance of rails. For example, when the REM content is 0.0010% or more, this effect can be suitably obtained. Furthermore, when the REM content is 0.0500% or less, the formation of coarse oxide-based inclusions is prevented, and rail breakage due to stress concentration can be further suppressed. 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 pro-eutectoid cementite, improves ductility, and improves surface damage resistance. For example, when the Co content is 0.01% or more, this effect can be suitably obtained. Furthermore, when the Co content is 0.50% or less, it is possible to avoid a situation in which pearlite transformation is promoted, which impairs hardenability and makes it impossible to obtain sufficient hardness. 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 to 0.0200%) N, when present in a solid solution state in ferrite, reduces the ductility of steel. On the other hand, when N and V are simultaneously contained in a rail, they form (V, Cr)(C, N) 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%, this effect is preferably achieved. Furthermore, when the N content is 0.0200% or less, excessive precipitation of (V, Cr)N in parts, which reduces toughness, can be avoided. Therefore, the N content is preferably 0.0015 to 0.0200% or less. The N content is more preferably 0.0020 to 0.0080%.

[0073] (W: 0.10% or less) W is an element that improves hardenability and refines the lamellar spacing of pearlite to improve strength and wear resistance. For example, when the W content is 0.03% or more, this effect can be preferably obtained. Furthermore, when the W content is 0.10% or less, the time until the pearlite transformation is completed is extended, and the formation 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, or 0.05% or more. The W content is more preferably 0.09% or less, or 0.07% or less.

[0074] The chemical composition of the base metal portion 11 of the rail weld joint 1 according to this embodiment is composed of the balance iron and impurities. Impurities refer to components that are mixed in during the industrial production of steel due to raw materials such as ore or scrap, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the function and effects of this embodiment. Pb, Bi, and Te may also be contained within a range that does not impair the function and effects of the rail weld joint according to this embodiment. Regarding the contents of these elements, Pb is 0.09% or less, Bi is 0.10% or less, and Te is 0.05% or less, so that the function and effects of this embodiment are not impaired. Similarly, O (oxygen) may also be contained within a range that does not impair the effect of the rail weld joint according to this embodiment. For example, an O content of approximately 0.0010 to 0.0040% does not impair the function and effects of this embodiment.

[0075] Next, an example of a manufacturing method for the rail weld joint 1 according to this embodiment will be described. The manufacturing method described below makes it possible to suitably obtain the rail weld joint 1 according to this embodiment. However, even if a rail weld joint is obtained by a method other than the manufacturing method described below, it also falls under the category of the rail weld joint 1 according to this embodiment as long as it satisfies the above-mentioned requirements.

[0076] The method for manufacturing a rail welded joint according to this embodiment includes the steps of flash butt welding the rail and heating the weld 12 formed by the flash butt welding. The rail used as the weld base material is manufactured by hot rolling a continuously cast steel billet into a rail shape, and then heat treating the head and other parts as necessary. Note that flash butt welding and heating do not change the chemical composition of the rail outside the weld 12. Therefore, the chemical composition of the rail used as the weld base material is the same as the chemical composition of the base material 11 of the rail welded joint 1.

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

[0078] In the flashing process, the end faces of two rails are first butted together with a gap in place while a voltage is applied. Here, "butting" means bringing two objects close together but facing each other with a gap between them. The term "butting" is distinct from the term "butting together," which means bringing two objects into contact. Next, the rails are moved so that their end faces approach each other. This causes the unevenness of the end faces of the two rails to come into local contact with each other, resulting in the flow of a short-circuit current.

[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 flashing. These phenomena are repeated continuously during the pre-flashing process. The end surface of the rail is then flattened by the pre-flashing process.

[0080] Because the rails are worn away by the splashing of molten metal, the rails must be brought closer together during 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 amount of relative movement 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 flash-off length and the flash-off distance are essentially the same value.

[0081] In the preheating process, the entire end faces of the two rails are brought into contact, a current is applied for 2 to 5 seconds, and then the end faces of the rails are separated for 1 to 2 seconds. This contact and separation is repeated 2 to 18 times. Since the entire end faces of the two rails are in contact at this time, a large current flows through the rails. This preheats the end faces of the rails prior to the implementation of the latter flashing process.

[0082] In the latter flashing process, similar to the former flashing process, the end faces of two rails are first placed opposite each other with a gap therebetween while a voltage is applied. The rails are then moved so that their end faces approach each other. This causes flashing between the two end faces. The principle of the latter flashing process is similar to that of the former flashing process. However, unlike the former flashing process, the end faces of the rails are flattened and preheated at the start of the latter flashing process. The flashing velocity in the latter flashing process, i.e., the latter flashing velocity, is generally faster than the flashing velocity in the former flashing process, i.e., the former flashing velocity. Furthermore, the flashing length in the latter flashing process is longer than the flashing length in the former flashing process.

[0083] The entire end faces are melted in the late flash process. In the subsequent upsetting process, the end faces of the rails are butted together and a large rolling force is applied. The large rolling force joins the end faces of the rails to create a joint. During the upsetting process, molten material and oxides on the end faces are pushed out of the weld and remain as burrs. Burrs are usually removed immediately after welding is completed. Specifically, burrs are removed by trimming (or bead cutting) immediately after welding is completed, before the weld joint is reheated.

[0084] (Total preheating heat input: 9 to 60 kA·s / cm ２ As mentioned above, in the preheating process, current is applied multiple times. The total value of the heat input during the multiple current applications in the preheating process is called the total preheating heat input. The total preheating heat input in flash butt welding affects the adhesion of the joint during welding. When the total preheating heat input is 9 kA·s / cm ２ If the total preheating heat input is less than 9 kA·s / cm, unbonded portions will occur after welding, and the desired bending performance will not be obtained. ２ On the other hand, the total preheating heat input is 60 kA·s / cm ２ If the total preheating heat input exceeds 9 to 60 kA·s / cm ２ The range is as follows.

[0085] (Final flashing velocity: 0.6 to 3.5 mm / s) The final flashing velocity refers to the flashing velocity immediately before the upset process. The final flashing velocity affects the adhesion of the joint during welding. If it is less than 0.6 mm / s, an unbonded portion occurs after welding, making it impossible to obtain the desired bending performance. If it exceeds 3.5 mm / s, the entire rail will short-circuit, making welding impossible. For this reason, the flashing velocity is set in the range of 0.6 to 3.5 mm / s. Note that the faster the final flashing velocity, the smaller the HAZ width, and the slower the final flashing velocity, the larger the HAZ width tends to be, but the effect of the final flashing velocity is small relative to the total heat input in the preheating process.

[0086] (Upset load: 45 to 120 MPa) The upset load is the load applied to both sides of the rail during the upset process of flash butt welding. The upset load affects the adhesion of the joint during welding and the HAZ width. If it is less than 45 MPa, unbonded areas will occur after welding, making it impossible to achieve the specified bending performance. There is no particular impact on the upper limit, but due to equipment constraints, the general limit of 120 MPa was set. For these reasons, the upset load range was set to 45 to 120 MPa. The higher the upset load, the smaller the HAZ width, and the lower the upset load, the larger the HAZ width tends to be, but the effect of the upset load is small compared to the total heat input in the preheating process.

[0087] The HAZ width in flash butt welding is controlled via the total heat input in the preheating step, the final flashing velocity, and the upset load.

[0088] Next, the process of heating the weld 12 formed by flash butt welding will be described. Heating the weld 12 corresponds to what is known as PWHT (Post Weld Heat Treatment). PWHT is initiated after the surface temperature at the first position A has dropped to less than 500°C following flash butt welding. That is, the PWHT heating start temperature T1 is set to less than 500°C. There is no particular lower limit set for the PWHT heating start temperature T1. For example, it is also acceptable to start PWHT after the temperature of the weld has dropped to room temperature. On the other hand, if the PWHT heating start temperature T1 is too high, the effect of reducing residual stress cannot be obtained.

[0089] PWHT is performed, for example, by induction heating using a coil. Figure 4 shows an example of the arrangement of the coil L and magnetic material M when performing PWHT by induction heating. As shown in Figure 4, induction heating may be performed by placing the coil L at a distance from the center of the weld that is, for example, 0.2 to 3 times the HAZ width. Furthermore, the length of the heated region along the longitudinal direction X may be, for example, 0.5 to 5 times the HAZ width. The coils L are placed on both sides of the longitudinal direction X with the weld at the center.

[0090] In PWHT, the heating conditions at the first position A to the fourth position D are individually controlled. The heating conditions can be controlled via the distance between the coil and the rail or the magnetic material coverage. The magnetic material coverage is the length of the magnetic material disposed on the coil along the circumferential direction of the rail divided by the circumferential length of the coil. The heating conditions can be controlled to achieve a predetermined heating rate for each of the first position A (the center of the surface of the column portion 124 of the welded portion 12 along the height direction Z), the second position B (the center of the sole surface 1251 of the welded portion 12 along the width direction Y), the third position C (a position T / 10 away from the toe 1253 of the sole 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).

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

[0092] In induction heating, the closer the coil and the rail are to each other, the faster the temperature rise rate. If the coil and the rail are too close, they may come into contact with each other. If the coil and the rail are too far apart, it is difficult to achieve the desired temperature rise effect. In the manufacturing method for a rail welded joint according to the present disclosure, the distance between the coil and the rail may be set to, for example, 3 to 20 mm.

[0093] In induction heating, for example, as shown in Figure 4, by placing a magnetic material M around the welded joint 1 of the rail, the heating rate can be increased or decreased. The effect can be changed by adjusting the coverage of the magnetic material M for each part that controls the heating rate. Specifically, as shown in Figure 4, by placing a magnetic material M on the induction heating coil L provided along the circumferential direction of the welded joint 1, the magnetic field lines can be converged, thereby increasing the heating effect of the part where it is placed. In other words, the "coverage of the magnetic material" can be controlled by the number of magnetic materials placed on the coil. The coverage can be selected from 0 to 100% depending on the heating rate to be controlled.

[0094] When heating the welded portion 12, it is necessary to consider the amount of heat transfer (heat removal) from the welded portion 12 to the base material portion 11. The amount of heat removal in the column portion 124 is much smaller than the amount of heat removal in the foot portion 125. Therefore, when the welded portion 12 is heated uniformly along the circumferential direction of the rail, the temperature rise rate in the foot portion 125 is significantly smaller than the temperature rise rate in the column portion 124. In order to achieve the temperature rise rate described below, it is necessary to manage the amount of heat input at the first position A to the fourth position D while taking into account the amount of heat removal.

[0095] (Heating rate (V) at 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)) While the welded portion 12 is being heated, heat transfer from the welded portion 12 to the base material portion 11 is constantly occurring. The slower the heating rate, the greater the heating width in the longitudinal direction X, and the greater the reduction in circumferential residual stress. If the heating rate (V) at first position A exceeds 6°C / s, the heating width in the longitudinal direction X becomes too narrow, and the desired effect of reducing residual stress cannot be achieved. There is no particular limit to the lower limit of the heating rate (V) at first position A. However, in order to avoid the PWHT time becoming too long, the lower limit of the heating rate (V) at first position A is set to 2°C / s.

[0096] (Maximum heating temperature at first position A: 500 to 680°C) The higher the maximum heating temperature, the greater the heating width in the longitudinal direction X, and the greater the reduction in circumferential residual stress. If the maximum heating temperature at first position A is 500°C or less, the heating width in the longitudinal direction X is narrow, and the desired effect of reducing residual stress cannot be achieved. If the maximum heating temperature at first position A exceeds 680°C, the column portion 124 softens. For this reason, 680°C is set as the upper limit of the maximum heating temperature at first position A.

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

[0098] The maximum heating temperature at the second position B is not particularly limited. This is because the maximum heating temperature at the second position B is automatically determined by performing heating in a state where the requirements for the temperature rise rate at the first position A, the maximum heating temperature at the first position A, and the temperature rise rate at the second position B are satisfied. 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 third position C (position T / 10 away from the toe 1253 along the width direction Y on the sole 1251 of the welded portion 12): 0.25 × V to 1.0 × V (°C / s)) The slower the heating rate at third position C, the greater the effect of reducing residual stress at fourth position D. If the heating temperature at third position C exceeds 1.0 × V (°C / s), the effect of reducing residual stress in the longitudinal direction X at fourth position D cannot be obtained. Preferably, it is 0.7 × V (°C / s) or less. If the heating temperature is below 0.25 × V (°C / s), the tensile residual stress at the center of sole 1251 and third position C will exceed the upper limit, impairing the breakage resistance of the center of sole 1251 and third position C. Therefore, the residual stress at the center of sole 1251 will shift to the tensile side, impairing the breakage resistance of the 1 / 10T portion from the toe portion toward the center portion. Therefore, 0.3 × V (°C / s) was set as the lower limit. As with the second position B, there is no particular limitation on the maximum heating temperature at the third position C. 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 fourth position D (position T / 3 away from the central axis of column portion 124 along the width direction Y on foot surface 1252 of foot portion 125 of welded portion 12): 0.25 × V to 1.0 × V (°C / s)) The slower the heating rate at fourth position D, the greater the effect of reducing residual stress at fourth position D. If the heating rate at fourth position D exceeds 1.0 × V (°C / s), the effect of reducing residual stress in the longitudinal direction X at fourth position D cannot be obtained. The heating rate at fourth position D is preferably 0.7 × V (°C / s) or less. If the heating rate at fourth position D is below 0.25 × V (°C / s), the residual stress at second position B and third position C will shift to the tensile side, impairing breakage resistance. Therefore, 0.25 × V (°C / s) is set as the lower limit of the heating rate at fourth position D. As with second position B, the maximum heating temperature at fourth position D is not particularly limited. On the other hand, the maximum heating temperature at the fourth position D may be set to 0.3×tmax (° C.) to 1.0×tmax (° C.).

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

[0102] As mentioned above, fracture of the rail welded joint 1 occurs due to the longitudinal tensile residual stress at the fourth position D, the axial force acting on the rail, and the bending stress caused by the wheels. For this reason, fracture resistance was evaluated by a simple four-point bending fatigue test with the rail head at the top and the welded joint at the center.

[0103] Figure 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 sole of the foot facing downwards. At this time, the center of the two supports is aligned with the welding surface. The distance between the two supports is 4 feet (1219 mm). Then, a load is applied to the top of the rail welded joint, divided into two points. The distance between the two load application points is 1 foot (305 mm). The center of the two load application points is also aligned with the welding surface.

[0104] For a 136 pound rail, five tests were conducted with a stress range of TS / 4 [MPa] and a stress ratio of 0.1, where TS [MPa] is the tensile strength of the base metal head, and three or more specimens were deemed to have passed the condition of 2 million cycles. The tensile test to determine TS was conducted by taking a No. 14 test specimen of JIS Z2241, with the center at the center of the width direction of the head of the base metal and at a depth of 6 mm from the top, and the longitudinal direction of the rail as the longitudinal direction of the test specimen, and measuring it.

[0105] The effects of the rail weld joint of this embodiment will be explained more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the rail weld joint of this embodiment. Therefore, the rail weld joint of this embodiment is not limited to this one example of conditions.

[0106] The welded joints of the rails were produced by the following method. A cast piece produced by continuous casting was hot-rolled by the universal rolling method to form a 136RE rail (cross-sectional area: 8600 mm) specified by AREMA. ２ ) were manufactured. The chemical compositions of the casts of Examples 1 to 30 were as shown in Tables 1 and 2. The chemical compositions of the casts of Examples 31 to 58 were the same as the chemical composition of the cast of Example 1. The contents of elements not added to the casts are indicated by the symbol "-" in Table 1. The remainder of the chemical compositions shown in Table 1 was iron and impurities.

[0107] The end faces of the obtained rails were welded together using a fixed flash butt welding machine, in a process consisting of a preheating process, a flashing process, and an upset process. After upsetting, the head was subjected to accelerated cooling with compressed air. The total heat input Q1 (kAs) in the preheating process, the final flashing velocity V0 (mm / s) in the flashing process, the upset 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 preheating heat input Q1 was set to 800 kAs (9 kAs / cm ２ ) ~ 3600kAs (42kAs / cm ２ ), and the upset load L1 was changed within the range of 40t (47 MPa) to 90t (107 MPa).

[0108] Furthermore, heating coils were installed on both sides of a position that was twice the length of the HAZ width W from the weld center in the longitudinal direction. Induction heating was performed after the temperature at the center (first position A) along the height direction of the surface of the column portion at the weld center had dropped to a predetermined temperature. The temperature increase was stopped when the surface temperature of the center of the column portion at the weld center reached a predetermined temperature. The temperature increase start temperature T1 (°C), the temperature increase rate V1 (°C / s) at the first position A, the temperature increase stop time T2 (°C) 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 Tables 4A and 4B.

[0109]

[0110]

[0111]

[0112]

[0113]

[0114]

[0115] Rail welded joints were manufactured using the above manufacturing process. Residual stresses were then measured at the first position A, the second position B, the third position C, and the fourth position D on the welded surface of the rail welded joint. Furthermore, the breakage resistance of the rail welded joint was evaluated by a four-point bending fatigue test. The methods for measuring residual stress and evaluating breakage resistance were as described above. The results are shown in Tables 5A and 5B.

[0116]

[0117]

[0118] In Examples 33 and 37, the residual stress at the first position A was excessive, which resulted in poor breakage resistance in Examples 33 and 37. This is presumably because the temperature rise 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, which resulted in poor breakage resistance in Examples 34 and 38. This is presumably 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, which resulted in poor breakage resistance in Example 41. This is presumably because the temperature increase rate at the second position B in Example 41 was too slow.

[0121] In Example 42, the residual stress at the fourth position D was excessive. As a result, the breakage resistance of Example 42 was poor. This is presumably because the heating rate at the second position B in Example 42 was too fast. The reason the residual stress at the fourth position D was inappropriate is presumably because optimization of the residual stress at the fourth position D via 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, which resulted in poor breakage resistance in Example 45. This is presumably because the temperature increase 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 breakage resistance of Example 46 was poor. This is presumably because the heating rate at the third position C in Example 46 was too fast. The reason the residual stress at the fourth position D was inappropriate is presumably because 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, which resulted in poor breakage resistance in Example 49. This is presumably because the temperature increase rate at the fourth position D in Example 49 was too fast.

[0125] In Example 50, the residual stress was inappropriate at all positions related to the foot. Specifically, the residual stress was insufficient at the second position B and the third position C, and excessive at the fourth position D. As a result, the breakage resistance of Example 50 was poor. This is presumably because the temperature rise rate at all of the second position B, the third position C, and the fourth position D in Example 50 was too fast.

[0126] On the other hand, examples with appropriate residual stress had good breakage resistance. In these examples, the flash butt welding conditions and heating conditions were appropriate.

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

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

Claims

1. A welded joint of a welded rail having a base metal portion and a weld portion, wherein the chemical composition of the base metal portion is, in 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%, with the balance being Fe and impurities; the weld has a head, a column, and a foot; 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 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 is defined as a fourth position, the height direction residual stress measured at the first position is 543-6.25×W (MPa) or less, the longitudinal direction residual stress measured at the second position is -200 to -20 (MPa), and the longitudinal direction 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 metal portion is, in 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: 0.10% or less, and The welded joint of a welded rail according to claim 1, characterized in that it contains one or more elements selected from the group consisting of: Te: 0.05% or less.

3. A manufacturing method for a welded rail weld joint, comprising the steps of flash butt welding a rail having the chemical composition defined in claim 1 or 2, and heating the weld formed by the flash butt welding, wherein the weld has a head, a column, and a foot; at the weld 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, 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 is defined as a first position, the center in 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, ２ ), a final flashing velocity of 0.6 to 3.5 (mm / s), and an upset load of 45 to 120 MPa; the heating of the welded portion is started after the surface temperature at the first position has dropped to less than 500°C; during the heating of the welded portion, the surface temperature at the first position is raised to 500 to 680°C; during the heating of the welded portion, an average temperature rise rate V of the surface at the first position is set to 2.0 to 6.0 (°C / s); during the heating of the welded portion, an average temperature rise rate V of the surface at the second position is set to 0.3 x V to 1.0 x V (°C / s); during the heating of the welded portion, an average temperature rise rate V of the surface at the third position is set to 0.25 x V to 1.0 x V (°C / s); and during the heating of the welded portion, an average temperature rise rate V of the surface at the fourth position is set to 0.25 x V to 1.0 x V (°C / s).

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