Method for Manufacturing Welded Rail

Abstract

A method for manufacturing a welding rail according to an aspect of the present disclosure comprises: an installation step for installing electrodes on head parts and foot parts of a pair of base material rails arranged in the longitudinal direction; a preheating step for passing a preheating current through the pair of base material rails via the electrodes in a state of having end surfaces thereof in contact with each other to preheat the end surfaces; a flash step for passing a flash current through the pair of base material rails via the electrodes to generate, between the end surfaces, flash for melting the end surfaces; and an upset step for applying an upset force to the end surfaces of the pair of base material rails butted to each other to upset the pair of base material rails so as to join the base material rails. The sum SH of contact areas between the electrodes and the head parts of the base material rails and the sum SB of contact areas between the electrodes and the foot parts of the base material rails satisfy 0.20≤SH / SB≤0.70.

Description

Welded rail manufacturing method

[0001] This disclosure relates to a method for manufacturing a welded rail. This application claims priority to Japanese Patent Application No. 2024-061668, filed on April 5, 2024, the contents of which are incorporated herein by reference.

[0002] Rail joints are the most susceptible to damage and are the most expensive to maintain. Furthermore, joints are the primary source of noise and vibration when trains pass through. Meanwhile, freight rails are increasingly carrying heavier loads. For this reason, the technology of welding rails together to manufacture long rails and making the rail joints, which have the aforementioned problems, continuous has become common.

[0003] JIS E 1001:2002, "Railway - Track Terminology," lists gas pressure welding, flash butt welding, enclosed arc welding, and thermite welding as examples of rake welding methods for manufacturing long rails. Of these welding methods, flash butt welding is often used to weld rails for freight railways. The cross-sectional area of ​​freight rails is significantly larger than that of general structural members. Furthermore, unlike round or square bars, rails do not have a uniform cross-section. Rails have heads, columns, and bases, and their cross-sections have complex shapes. Therefore, a large heat input is required to properly melt the entire end face of the rail. Flash butt welding is suitable for welding base materials that require a large heat input.

[0004] Flash butt welding includes a flash process and an upset process. Flash butt welding may also include a preheating process before the flash process. Schematic diagrams of flash butt welding are shown in Figures 1A to 1D. In the flash process, shown schematically in Figure 1C, a pair of steel materials are placed face-to-face with their end faces in local contact, and a flash current is passed through the pair of steel materials. This causes resistance heating to rapidly heat and melt the contact area, and the electromagnetic pinch force caused by the flash current breaks the molten area, generating a flash between the pair of steel materials. The flash melts the end faces of the steel materials. In the upset process, shown schematically in Figure 1D, the end faces melted in the flash process are butted together and pressed together. The pair of steel materials are joined by the upset. The molten metal is forced out of the weld and forms a flash. In the preheating process, shown schematically in Figure 1B, a preheating current is passed through the pair of steel materials with their end faces in contact. The pair of end faces are heated using the resistance heating of the preheating current. The preheating step has the effect of stably generating an arc in the flash step.

[0005] Flash butt welding is prone to the formation of oxides on the rail joint surfaces. This is because the rail end surfaces are prone to oxidation when they reach high temperatures or melt during the preheating and flashing processes. Most of the oxides that form on the rail end surfaces during the preheating and flashing processes are pushed out of the weld zone along with the molten metal during the upsetting process. However, oxides may remain on the joint surfaces after welding is complete. Oxides on the joint surfaces may reduce the bending deflection of the welded rail.

[0006] Therefore, standards for bending performance are established for welded rails manufactured by flash butt welding. For example, the AREMA (American Railroad Engineering and Track Maintenance Association) standard requires that the deflection at break be 0.75 inches (approximately 19 mm) or more, and that the bottom stress at break be 125,000 lb / in. ２ (In the case of a four-point bending test (Chapter 3.10.3.6) using a 136 pound rail, the load must be 1700 kN or more.)

[0007] The stronger the rail, the more susceptible it is to cracking due to oxides, and the more likely it is to reduce bending deflection. One way to prevent this is to reduce the integrated current during the preheating process. The integrated current is the sum of the product of the preheating current and the current flow time when the rail end faces are brought into contact with each other. By reducing the integrated current during the preheating process, it is possible to prevent the arc from coarsening during the flashing process and reduce unevenness on the end faces.

[0008] However, reducing the integrated current makes it difficult for an arc to occur, which can lead to a phenomenon known as freezing, where the ends of the wire are welded together. This freezing can make flash butt welding impossible.

[0009] Thus, reducing the integrated current required to ensure the amount of bending deflection of the welded rail makes it difficult to suppress freezing. A technology that can achieve both the required amount of bending deflection and the suppression of freezing is desired. To solve this problem, the following rail welding technology has been proposed, for example.

[0010] As a technology capable of reducing the amount of bending deflection caused by oxide defects and suppressing the occurrence of freezing, Patent Document 1 discloses a flash welding method and an apparatus equipped with these control means, in which three-phase AC from a three-phase AC power source is converted into a square-wave AC voltage by an inverter, and the square-wave AC voltage is then stepped down and converted into a target voltage by a welding transformer, and then applied between a pair of workpieces to be welded, and when the output current reaches a certain limit value, the output frequency of the square-wave AC is switched to a higher value to control the current value, and when the short circuit disappears and the output drops, the original output frequency is restored.

[0011] Patent Document 2 discloses a flash butt welding method that can suppress softening of the heat-affected zone (HAZ) of a rail weld and reduce uneven wear and surface damage to the rail, and that can reduce the heat-affected width and softened width of the welded joint by setting the flashing velocity and flashing length in specific ranges in the later flashing process. Furthermore, the technology of Patent Document 2 not only suppresses HAZ softening, but also improves the amount of bending deflection by increasing the flashing velocity, and suppresses the occurrence of freezing by controlling the integrated current as heat input in the preheating process within an appropriate range.

[0012] Japanese Patent Publication No. 2002-160063

[0013] The technology of Patent Document 1 can suppress freezing by lowering the inverter output frequency and increasing the welding current. However, the technology of Patent Document 1 does not take into consideration the cross-sectional shape of the rail. When the technology of Patent Document 1 is applied to welding high-strength rails, there is a problem that the suppression of the decrease in bending deflection is insufficient and the standard is not satisfied.

[0014] The technology disclosed in Patent Document 2 is a welding method that takes into account the cross-sectional shape of the rail. This technology can suppress freezing and eliminate coarse oxides. However, when using high-strength rails that are highly susceptible to cracking as the base material, even the technology disclosed in Patent Document 2 sometimes fails to consistently achieve sufficient bending deflection.

[0015] The present disclosure has been devised in view of the above-mentioned points, and aims to provide a method for manufacturing a welded rail that can stably ensure the amount of bending deflection of the welded rail while avoiding the occurrence of freezing.

[0016] The gist of the present disclosure is as follows.

[0017] (1) A method of manufacturing a welded rail according to one aspect of the present disclosure includes an installation step of installing electrodes on heads and foots of a pair of base rails aligned along the longitudinal direction, a preheating step of passing a preheating current through the pair of base rails via the electrodes while the end faces of the pair of base rails are in contact with each other to preheat the end faces, a flashing step of passing a flash current through the pair of base rails via the electrodes to generate a flash between the end faces that melts the end faces, and an upsetting step of butting the end faces of the pair of base rails together and applying an upsetting force to upset the pair of base rails so as to join the base rails, wherein the sum of the contact areas S between the electrodes and the heads of the base rails is Ｈ and the sum of the contact areas S between the electrode and the foot of the base rail Ｂ is within the range of formula 1. 0.20≦S Ｈ / S Ｂ ≦0.70 1 formula (2) Preferably, in the method for producing a welded rail described in (1) above, the duration of the flashing process is defined as a flashing time t, the average relative movement speed of the pair of base rails in a first period that is the period from the start of the flashing process until 0.90t has elapsed is defined as a first flashing velocity, and the average relative movement speed of the pair of base rails in a second period of the flashing process that is a period other than the first period is defined as a second flashing velocity, the first flashing velocity is slower than the second flashing velocity, the second flashing velocity is 1.8 to 3.0 mm / sec, and the average flashing voltage in the second period is 5.0 to 13.0 V. (3) Preferably, in the method for producing a welded rail described in (1) or (2) above, the flash-off distance in the flashing process is 5.0 to 60.0 mm. (4) In the method for manufacturing a welded rail according to any one of (1) to (3) above, it is preferable that the integrated current per unit area in the preheating step is set to 0.1745 to 0.4070 kA·sec / mm ２(5) Preferably, in the method for manufacturing a welded rail according to any one of (1) to (4) above, the chemical composition of the base rail 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%, V: 0 to 0.200%, Ti: 0 to 0.080%, Nb: 0 to 0.050%, Al: 0 to 0.1 (6) In the method for producing a welded rail according to any one of (1) to (5), the base rail preferably has a tensile strength of 1250 to 1600 MPa.

[0018] According to the present disclosure, it is possible to provide a method for manufacturing a welded rail that can stably ensure the amount of bending deflection of the welded rail while avoiding the occurrence of freezing.

[0019] 1 is a schematic diagram of an installation process; 2 is a schematic diagram of a preheating process; 3 is a schematic diagram of a flashing process; 4 is a schematic diagram of an upsetting process; 5 is a schematic diagram of an end face of a base rail; 6 is a cross-sectional view and a side view illustrating an example of an electrode arrangement; 7 is a cross-sectional view and a side view illustrating an example of an electrode arrangement; 8 is a cross-sectional view and a side view illustrating an example of an electrode arrangement; 9 is a cross-sectional view and a side view illustrating an example of an electrode arrangement; 10 is a graph showing the sum of the contact areas S between the electrodes and the foot portions; Ｂ , and the sum of the contact areas between the electrodes and the head S Ｈ is S Ｂ >S Ｈ 1 is a schematic diagram showing the relationship between the contact area between the electrode and the rail and the welding current when S Ｂ , and S Ｈ is S Ｂ <S Ｈ7 is a schematic diagram showing the relationship between the contact area between the electrode and the rail and the welding current when the condition (1) is satisfied. FIG. 8 is a diagram showing a method for measuring the contact area between the rail and the electrode. FIG. 9 is a diagram showing a four-point bending test procedure. FIG. 10 is a diagram showing the design dimensions of an example of a base rail. FIG. 11 is a photograph of pressure-sensitive paper inserted between the base rail and the electrode in FIG. 7.

[0020] The inventors focused on the possibility that the unique cross-sectional shape of railway rails, as exemplified in Figure 2, and the size of oxides contained in the welded portion of a welded rail may affect the amount of bending deflection.The inventors then conducted a detailed analysis of the fracture surface of welded rails after bending tests.As a result, they found that the location and size of oxide formation affect the amount of bending deflection, as shown in formula (2).

[0021] In equation (2), K is the stress intensity factor, σ is the stress in the vicinity of the oxide defect, area is the area of ​​the oxide defect, and F is the shape factor of the defect. When the end of the crack is open, i.e., when the crack reaches the outside of the material, F is 0.65. When the end of the crack is closed, i.e., when the crack remains inside the material, F is 0.50. The stress intensity factor K is a physical quantity that represents the strength of the stress distribution near the tip of the crack. The larger the stress intensity factor K, the greater the stress near the defect, making bending fracture more likely to occur and reducing the amount of bending deflection.

[0022] The location of the oxide affects the tensile stress during bending tests, and in turn affects the stress σ near the oxide defect. As shown in equation (2), even if the area of ​​the oxide defect is small, if the stress σ near the oxide defect is large, the stress intensity factor K becomes large and fracture is likely to occur.

[0023] The inventors have found that tensile stress during bending tests is large at the toe portions 132 of the welded rail. The toe portions 132 are the ends of the foot portions 13 of the welded rail, as shown in Figure 2. The inventors have found that if oxides are formed at the toe portions 132, even if the size of the oxides is small, the bending deflection of the welded rail may deteriorate, and the bending deflection may not be able to be increased stably.

[0024] Furthermore, when flash butt welding current is controlled by a single welding power supply controller, the welding current and welding voltage are uniquely specified. As a result, the welding current at the head 11 and the welding current at the foot 13 are different. In particular, when flash butt welding high-strength rails, the welding current at the toe 132 is likely to be too low compared to the head 11. The inventors have found that this results in low arc energy at the toe 132, making freezing more likely to occur.

[0025] One of the reasons why the welding current at the toe portion 132 is likely to be too small compared to the head portion 11 is the difference in cross-sectional area between the head portion 11 and the foot portion 13. As shown in the cross-sectional view of Figure 2, the cross-sectional area of ​​the toe portion 132 is smaller than the cross-sectional area of ​​the head portion 11. Therefore, the electrical resistance of the toe portion 132 is larger than that of the head portion 11. As a result, the current value at the toe portion 132 is smaller than that of the head portion 11.

[0026] Another reason why the welding current at the toe portion 132 is likely to be too small compared to the head portion 11 is the difference in distance between the head portion 11 and the toe portion 132 and the electrode 2. When the electrode 2 is positioned as illustrated in Fig. 3A, the distance between the toe portion 132 and the electrode 2 is longer than the distance between the head portion 11 and the electrode 2. This causes the current value at the toe portion 132 to be smaller than that at the head portion 11.

[0027] As explained above, oxides formed in the toe portion 132 tend to increase the stress intensity factor K. Furthermore, freezing is likely to occur in the toe portion 132. Based on this knowledge, the inventors attempted to optimize the welding conditions for the toe portion 132. As a result of extensive research, the inventors confirmed that both freezing and oxide suppression can be achieved when the following formula is satisfied: 0.20≦S Ｈ / S Ｂ ≦0.70 (1) Symbol S in formula (1) Ｈ is the sum of the contact areas between the electrodes 2 installed on the heads 11 of the pair of base rails 1 and the base rails 1 in flash butt welding. Ｂ is the sum of the contact areas between the electrodes placed on the foot portions 13 of the pair of base rails 1 and the base rails 1 in flash butt welding. Ｈand S Ｂ is within the range of the above formula (1), the value of the current flowing through the toe 132 can be stably increased. The inventors have confirmed that by increasing the current through the toe 132 to an appropriate value, the arc of the toe 13 can be generated stably and continuously, thereby achieving both the suppression of freezing and the suppression of oxides.

[0028] 4A and 4B are schematic diagrams showing the relationship between the contact area between the electrode 2 and the base rail 1 and the current. Ｂ , and the sum of the contact areas S between the electrode 2 and the head 11 Ｈ is S Ｂ >S Ｈ 4B is a schematic diagram of the current when the sum of the contact areas S between the electrode 2 and the foot 13 is satisfied. Ｂ , and the sum of the contact areas S between the electrode 2 and the head 11 Ｈ is S Ｂ <S Ｈ 1 is a schematic diagram of a current when Ｂ >S Ｈ At this time, the current flowing through the foot 13 is greater than the current flowing through the head 11. This makes it possible to suppress freezing in the foot 13 and reduce the amount of oxides produced in the toe 132. Ｂ <S Ｈ In this case, the current flowing through the head 11 becomes larger than the current flowing through the foot 13. In this case, it is difficult to prevent freezing from occurring in the foot 13, and it is not possible to reduce the amount of oxide produced in the toe 132. Ｂ , and the sum of the contact areas S between the electrode 2 and the head 11 Ｈ can be controlled by changing the size of the electrode 2 and the shape of the part of the electrode 2 that comes into contact with the base rail 1.

[0029] In this disclosure, the term "contact area" refers to the actual contact area between the electrode 2 and the base rail 1, as determined by a measurement method using pressure-sensitive paper 6, which will be described later. It should be noted that the theoretical contact area derived from the design dimensions of the base rail 1 and the electrode 2, as well as the catalog values ​​for the base rail 1, does not match the actual contact area measured using pressure-sensitive paper 6.

[0030] As shown in Fig. 5, the inventors measured the contact area between the top 111 of the base rail 1 and the electrode 2 by sandwiching pressure-sensitive paper 6 between the top 111 and the electrode 2. The top 111 is the flat part at the top of the head 11. In addition, by sandwiching pressure-sensitive paper 6 between the sole 131 of the base rail 1 and the electrode 2, the contact area S Ｂ The sole 131 is a flat portion on the underside of the foot 13 that contacts the base.

[0031] Figure 7 shows the design dimensions of the base rail 1. The design width of the top portion 111, which is the flat portion of the head portion 11 of the base rail 1, was 28 mm. The design width of the sole portion 131 of the base rail 1 was 152.4 mm. The design width of the electrode 2 was 70 mm.

[0032] Figure 8 shows a photograph of the pressure-sensitive paper 6. The pressure-sensitive paper 6 on the upper side of the photograph in Figure 8 is the pressure-sensitive paper 6 arranged between the top portion 111 of the base rail 1 and the electrode 2. The pressure-sensitive paper 6 on the lower side of the photograph in Figure 8 is the pressure-sensitive paper 6 arranged between the sole portion 131 of the base rail 1 and the electrode 2. The ruler used to measure the dimensions of the discolored area of ​​the pressure-sensitive paper is shown at the bottom of the photograph in Figure 8. To improve visibility, a scale was written between the lower pressure-sensitive paper 6 and the ruler. Furthermore, an arrow WD indicating the width direction of the rail and an arrow LD indicating the longitudinal direction of the rail were written in the upper left corner of each of the two pressure-sensitive papers 6.

[0033] Based on the design width of electrode 2 and the design width of top portion 111 shown in Fig. 7, the width of the contact area between top portion 111 and electrode 2 was predicted to be 28 mm. However, the actual width of the contact area between top portion 111 and electrode 2, as determined using pressure-sensitive paper 6, was approximately 15 mm. Top portion 111 was not in contact with either end of electrode 2. The actual contact area between top portion 111 and electrode 2 was much smaller than the contact area estimated from the design dimensions.

[0034] Furthermore, based on the design width of the electrode 2 and the design width of the sole 131 shown in Fig. 7, the width of the contact area between the sole 131 and the electrode 2 was predicted to be 70 mm. The width of the contact area between the sole 131 and the electrode 2, as determined using pressure-sensitive paper 6, was approximately 64 mm. However, as shown in the pressure-sensitive paper 6 at the bottom of Fig. 8, the sole 131 was not in contact with the center of the electrode 2. The actual contact area between the sole 131 and the electrode 2 was also much smaller than the contact area estimated from the design dimensions.

[0035] It is assumed that the current flows only in the area where the pressure-sensitive paper has changed color, so it is necessary to control the area where the pressure-sensitive paper changes color.

[0036] A method for manufacturing a welded rail according to an embodiment of the present disclosure, which has been obtained based on the above findings, includes, as illustrated in FIGS. 1A to 1D , an installation step S1 in which electrodes 2 are installed on the head portions 11 and foot portions 13 of a pair of base rails 1 aligned along the longitudinal direction; a preheating step S2 in which, with the end faces 14 of the pair of base rails 1 in contact with each other, a preheating current is passed through the pair of base rails 1 via the electrodes 2 to preheat the end faces 14; a flashing step S3 in which a flash current is passed through the pair of base rails 1 via the electrodes 2 to generate a flash between the end faces 14; and an upsetting step S4 in which the end faces 14 of the pair of base rails 1 are butted together and an upsetting force is applied to upset the pair of base rails 1 so as to join the base rails 1, and the sum S of the contact areas between the electrodes and the head portions 11 of the base rails 1 is 0.05 mm. Ｈ , and the sum of the contact areas S between the electrode and the foot portion 13 of the base rail 1 Ｂ is set to be within the range of formula (1). A method for manufacturing a welded rail according to this embodiment will now be described.

[0037] (Base rail 1) In the method for manufacturing a welded rail according to the present disclosure, a base rail 1 is flash butt welded to obtain a welded rail. The base rail 1 is a rail for a railway, and its type is not particularly limited. A suitable example of the base rail 1 is a high-strength rail for heavy-duty railways.

[0038] Railway rails are used by being installed on the ground, bridges, and other foundations. In this embodiment, the vertical direction when the rail is installed on a foundation and used is referred to as the height direction. The extension direction of the rail is referred to as the longitudinal direction. The direction perpendicular to the height direction and longitudinal direction of the rail is referred to as the width direction. In this embodiment, the "upper direction" and "lower direction" refer to the upper and lower directions when the rail is installed on a foundation and is ready for railway vehicles to pass through.

[0039] As shown in Figure 2, the base rail 1 has a head portion 11, a pillar portion 12, and a foot portion 13. The head portion 11 of the base rail 1 is the portion above the narrowed portion at the height center of the base rail 1. The head portion 11 of the base rail 1 comes into contact with the wheels of a railway vehicle. The pillar portion 12 of the base rail 1 is the narrowed portion at the height center of the base rail 1. The foot portion 13 of the base rail 1 is the portion below the narrowed portion at the height center of the base rail 1, and is also called the bottom. The foot portion 13 of the base rail 1 is installed on a foundation.

[0040] In the head 11, the flat portion at the top is referred to as the parietal portion 111, the flat portions at the sides are referred to as the head side portions 112, and the narrow portion at the bottom is referred to as the chin portion 113. In the foot 13, the flat portion that contacts the lower base is referred to as the sole portion 131, and the upper flat portions extending on both sides of the pillar portion 12 are referred to as the foot top portion 133. The ends of the foot 13 along the width direction are referred to as the toe portions 132.

[0041] The longitudinal ends of the base rail 1 are referred to as end faces 14. The end faces 14 become the welding surfaces in flash butt welding. A welded rail is obtained by welding the end faces 14 of a pair of base rails 1. A welded rail comprises two or more base rails 1 and a welded portion that joins the base rails 1. Naturally, a welded rail can also be used as the base rail 1. The welded rail can be extended by joining the end faces 14 of the welded rail to the end faces 14 of the base rails 1.

[0042] (Installation step S1) In the method for manufacturing a welded rail according to this embodiment, as shown schematically in Fig. 1A, first, electrodes 2 are installed on the head 11 and foot 13 of the base rail 1. The electrodes 2 are connected to a power source 3. A pressure is applied to the electrodes 2 installed on the base rail 1. This ensures a sufficient contact area between the electrodes 2 and the base rail 1, facilitating current flow. The pair of base rails 1 are aligned longitudinally so that their end faces 14 face each other.

[0043] The electrodes 2 can be installed at any location on the head 11 and foot 13. The most suitable locations are the top 111 and sole 131. FIG. 3A shows electrodes 2 installed on the top 111 and sole 131. Installing the electrodes 2 on the top 111 and sole 131 makes it extremely easy to apply pressure to the electrodes 2. Alternatively, a pair of electrodes 2 may be installed on both sides of the head 11. Alternatively, a pair of electrodes 2 may be installed on the sole 131 and sole 133 of the foot 13. FIG. 3B shows electrodes 2 installed on both sides of the head 11 and on the sole 131 and sole 133 of the foot 13. In this case, too, the electrodes 2 can be pressed against the head 11 and foot 13. The electrodes 2 may be installed in corners between the top and side of the base rail. In this case, the shape of the contact surface of the electrode 2 must match the shape of the corner. Electrodes 2 may also be installed on the toe portion 132. In this case, too, the shape of the installation surface of the electrodes 2 must match the shape of the toe portion 132. FIG. 3C shows an arrangement in which electrodes 2 are installed on both sides of the head portion 11 and on the toe portion 132 of the foot portion 13. By processing the electrodes 2 so that the shape of the installation surface of the electrodes matches the shape of the rail, the electrodes 2 can be installed at any location on the rail. Furthermore, in addition to the head portion 11 and the foot portion 13, electrodes 2 may also be installed on the column portion 12.

[0044] One or both of the pair of base rails 1 on which the electrode 2 is installed can be moved along the longitudinal direction. This makes it possible to control the distance between the end faces 14 of the pair of base rails 1. A typical flash butt welding machine is configured so that one base rail 1 is fixed and the other base rail 1 is moved along the longitudinal direction. Alternatively, both base rails 1 may be moved in the longitudinal direction. The means for moving the base rails 1 is not particularly limited. A moving means provided in a known flash butt welding machine can be appropriately used as a means for moving the base rails 1.

[0045] (Preheating step S2) After the electrodes 2 are installed on the base rail 1, the end faces 14 of the base rail 1 are preheated as shown in FIG. 1B. The pair of base rails 1 are pressed together, bringing most of their end faces 14 into contact, and current is passed through the pair of base rails 1 via the electrodes 2, causing resistance heating at the end faces 14. The end faces 14 are preheated by the resistance heating. The current passed to preheat the end faces 14 is called a preheating current. Preheating the end faces 14 stabilizes the flash 4 in the flashing step S3.

[0046] As shown schematically in FIG. 1B , the end face 14 may be repeatedly brought into contact with and separated from the substrate during preheating. By repeatedly bringing the end face 14 into contact with and separating the substrate, the entire end face 14 can be preheated. For example, it is preferable to bring the end face 14 into contact with the substrate 2 to 18 times. The number of times the end face 14 is brought into contact with the substrate 2 is referred to as the number of preheating times. It is also preferable to bring the end face 14 into contact with the substrate 2 for 1 to 5 seconds during each preheating. The sum of the preheating current during multiple preheatings, i.e., the time integral of the preheating current during the period from the start to the end of preheating, is referred to as the integrated current.

[0047] (Flashing Step S3) After preheating the end faces 14, a flash 4 is generated between the end faces 14, as shown schematically in Fig. 1C. With the end faces 14 of the base rails 1 in localized contact, a current is passed through the pair of base rails 1 via electrodes 2, causing resistance heating to rapidly heat and melt the contact area, and the electromagnetic pinching force caused by the current ruptures the molten area, generating a flash 4 between the end faces 14. The current during the flashing step S3 is called a flash current.

[0048] The flash 4 melts the end faces 14. The molten metal generated on the end faces 14 splashes. Because the flash 4 wears down the base rail 1, the pair of base rails 1 are moved closer together during the flashing step S3. The length of the base rail 1 worn down during the flashing step S3 is called the flashing loss.

[0049] (Upsetting process S4) After melting the end faces 14 using the flash 4, the pair of base rails 1 are upset. Upsetting involves butting the end faces 14 of the pair of base rails 1 together and applying pressure. The pressure applied to the end faces 14 during upsetting is called the upset force. The end faces 14 of the pair of base rails 1 are joined by upsetting. The molten metal on the end faces 14 is forced out of the weld and forms burrs 5.

[0050] (Contact area ratio S Ｈ / S Ｂ In the method for manufacturing a welded rail according to this embodiment, the following formula 1 is satisfied: 0.20≦S Ｈ / S Ｂ ≦0.70 (1) Formula In formula 1, S Ｈ is the contact area between the electrode 2 and the head 11 of the base rail 1, and S Ｂ is the contact area between the electrode 2 and the foot 13 of the base rail 1. When the electrode 2 is installed at multiple locations on the head 11, the total contact area between the electrode 2 and the head 11 is S Ｈ When the electrodes 2 are placed at multiple locations on the foot 13, the total contact area between the electrodes 2 and the foot 13 is S Ｂ Hereafter, S Ｈ / S Ｂ is called the contact area ratio. Ｈ / S Ｂ is in the range of 0.20 to 0.70. Preferably, the contact area ratio S Ｈ / S Ｂ may be 0.22 or more, 0.25 or more, 0.30 or more, or 0.35 or more. Ｈ / S Ｂ may be 0.60 or less, 0.50 or less, or 0.45 or less.

[0051] The method for measuring the contact area is as follows. First, as shown in FIG. 5, pressure-sensitive paper 6 is sandwiched between all the electrodes 2 and the base rail 1. The size of the pressure-sensitive paper 6 is larger than the size of the electrodes 2. The inventors used pressure-sensitive paper for extremely low pressure (model number LLLW) manufactured by Fujifilm Corporation to measure the contact area. However, commercially available equivalent products can also be used to measure the contact area. By using pressure-sensitive paper of the same model number for all the electrodes, the contact area ratio S Ｈ / S Ｂ When calculating the above, the influence of the type of pressure-sensitive paper on the measurement results of the contact area can be offset.

[0052] Then, a pressure is applied to all of the electrodes 2. The pressure may be any value within the range of 15 to 25 tons. The pressure application time may be 2 minutes or more. However, the pressure and applied force must be the same at all measurement points.

[0053] After the pressure application is completed, the area of ​​the discolored part of the pressure-sensitive paper 6 is measured using known image analysis software. An example of the pressure-sensitive paper 6 is shown in FIG. 8. The contact area of ​​all the electrodes 2 installed on the head 11 of the base rail 1 is measured according to the above-mentioned procedure, and the total value of these is taken as S. Ｈ Similarly, the contact areas of all the electrodes 2 installed on the foot portions 13 of the base rail 1 are measured according to the above procedure, and the total value of these is taken as S Ｂ It is considered to be.

[0054] The cross-sectional shape and material of a pair of base rails 1 are usually approximately the same. Therefore, in flash butt welding to produce a welded rail, the shape, arrangement, and pressure applied to the electrode 2 are usually approximately the same for the pair of base rails 1. In this case, it is sufficient to measure the contact area for only one of the base rails 1. If the size, arrangement, and pressure applied to the electrode 2 for one base rail 1 differ from those for the other base rail 1, the contact area is measured for both base rails 1. Then, it is determined whether or not Equation 1 is satisfied for each base rail 1. If Equation 1 is satisfied for both base rails 1, the method is considered to be a method for producing a welded rail according to this embodiment.

[0055] It is not necessary to always measure the contact area before the start of flash butt welding. If the shape, arrangement, and pressure applied to the electrode 2 are the same, the contact area between the electrode 2 and the base rail 1 will also essentially be the same. Therefore, if the shape, arrangement, and pressure applied to the electrode 2 that can satisfy Equation 1 are specified before flash butt welding, and the electrode 2 is arranged so as to reproduce these conditions in the flash butt welding installation step S1, it is estimated that the flash butt welding will satisfy Equation 1.

[0056] However, when welding base rails multiple times, the electrode 2 gradually wears and deforms. Therefore, when manufacturing multiple welded rails using the same electrode 2, the contact area also gradually changes. Therefore, when performing multiple welding operations using the same electrode, it is preferable to measure the contact area between the base rail 1 and the electrode 2 every few times to confirm whether it is within a predetermined range. If the electrode is significantly worn or deformed, it can be polished and reshaped to adjust the contact area. Furthermore, if polishing the electrode is not possible, it is preferable to replace the electrode.

[0057] (Effects) The contact area ratio S included in the formula (1) Ｈ / S Ｂ As the current flow to the base 13 decreases, the amount of current flowing to the base 13 increases. This increases the frequency of flashes 4 at the base 13. The flashes 4 scatter the molten metal and oxides contained in the molten metal. Therefore, by increasing the frequency of flashes 4 at the base 13, the amount of oxides at the welded surface of the base 13 is reduced, improving the amount of bending deflection of the welded rail.

[0058] Contact area ratio S Ｈ / S Ｂ If the contact area ratio S is less than 0.20, the current in the head portion 11 becomes too small, and freezing tends to occur in the head portion 11. Ｈ / S Ｂ If the contact area ratio S exceeds 0.70, the current in the head 11 becomes excessive, while the current in the foot 13 becomes insufficient. As a result, the amount of oxides generated in the foot 13 during welding increases, and the amount of bending deflection of the welded rail deteriorates. Ｈ / S ＢThe contact area ratio S may be 0.22 or more, 0.25 or more, 0.30 or more, or 0.35 or more. Ｈ / S Ｂ may be 0.60 or less, 0.50 or less, or 0.45 or less.

[0059] The contact area ratio can be controlled by changing the size of the electrode 2 and the shape of the part of the electrode 2 that contacts the base rail 1. For example, if the cross-sectional shape of the base rail 1 is 136RE of the AREMA standard (Chapter 4), S Ｈ The range is 5 to 60 cm ２ and the preferred range is 16 to 24 cm ２ In this case, S Ｂ The range is 14 to 100 cm ２ and the preferred range is 42 to 62 cm ２ is.

[0060] The most basic aspect of the method for manufacturing a welded rail according to this embodiment has been described above. Next, a more preferred aspect of the method for manufacturing a welded rail according to this embodiment will be described.

[0061] (First flashing velocity: slower than the second flashing velocity) (Second flashing velocity: 1.8 to 3.0 mm / sec) In the method for manufacturing a welded rail according to this embodiment, the duration of the flashing step S3 is defined as the flashing time t. Furthermore, the period from the start of the flashing step S3 until 0.90t has elapsed is defined as the first period, and the period in the flashing step S3 other than the first period is defined as the second period. In other words, if the start of the flashing step S3 is defined as 0 second and the time from the start to the end of the flashing step S3 is defined as t seconds, then the period from 0 seconds to 0.90t seconds is the first period, and the period from 0.90t seconds to t seconds is the second period.

[0062] In the manufacturing method of a welded rail according to this embodiment, the average relative movement speed of the pair of base rails 1 during a first period is defined as a first flashing speed. The first flashing speed is the value obtained by dividing the relative movement distance of the pair of base rails 1 during the first period by the length of the first period. For example, when each of the pair of base rails 1 is moved using a jig, the relative movement distance of the pair of base rails 1 during the first period is the difference between the distance between the jigs at the start of the first period and the distance between the jigs at the end of the first period. Furthermore, the average relative movement speed of the pair of base rails 1 during a second period is defined as a second flashing speed. The second flashing speed is the value obtained by dividing the relative movement distance of the pair of base rails 1 during the second period by the length of the second period.

[0063] In the method for manufacturing a welded rail according to this embodiment, the first flashing velocity is preferably slower than the second flashing velocity, and the second flashing velocity is preferably within the range of 1.8 to 3.0 mm / sec.

[0064] The flashing velocity is an index that affects the amount of molten metal splashed onto the end face 14 of the base rail 1. The flashing velocity is also related to the effect of removing oxides formed on the end face 14 of the base rail 1. Setting the second flashing velocity to 1.8 mm / sec or higher increases the amount of oxide removal, reducing the amount of oxide on the welded rail's weld surface and increasing the amount of bending deflection of the welded rail. Setting the second flashing velocity to 3.0 mm / sec or lower maintains the distance between the end faces 14 of the base rail 1, making it easier to suppress freezing. The second flashing velocity may be 1.9 mm / sec or higher, 2.0 mm / sec or higher, or 2.2 mm / sec or higher. The second flashing velocity may be 2.9 mm / sec or lower, 2.8 mm / sec or lower, or 2.6 mm / sec or higher. A particularly preferred range for the second flashing velocity is 2.0 to 2.8 mm / sec.

[0065] During the first period of the flashing step S3, the flashing is not stable. Therefore, it is preferable to stabilize the flashing by making the first flashing velocity slower than the second flashing velocity. Furthermore, reducing the first flashing velocity also has the effect of suppressing freezing. The first flashing velocity may be set to 98% or less, 95% or less, or 90% or less of the second flashing velocity. On the other hand, setting the first flashing velocity to 0.3 mm / sec or more is preferable because it can increase the frequency of arc generation. The first flashing velocity may be set to 0.4 mm / sec or more, 0.8 mm / sec or more, or 1.0 mm / sec or more.

[0066] The length of the flashing time t is not particularly limited. The longer the flashing time t, the higher the temperature of the end face and the more stable the flashing. On the other hand, the shorter the flashing time t, the less time and power are required for flash butt welding. The flashing time t can be, for example, 10 seconds or more, or 20 seconds or more. The flashing time t can be, for example, 2 minutes or less, or 1 minute or less.

[0067] (Average flash voltage in the second period: 5.0 to 13.0 V) The average flash voltage in the second period is the average value of the inter-electrode voltage from the start to the end of the second period. The average flash voltage in the second period is determined by dividing the time integral of the voltage from the start to the end of the second period by the length of the second period. The average flash voltage in the second period is preferably within the range of 5.0 to 13.0 V.

[0068] The flash voltage is an index that affects the arc length, i.e., the distance between the end faces 14 of the base rail 1. Increasing the flash voltage and increasing the arc length increases the arc energy. Increasing the arc energy increases the depth of the irregularities that occur on the end faces 14 of the base rail 1 and increases the amount of oxide formation. Setting the average flash voltage during the second period to 13.0 V or less reduces the amount of oxide formation and further increases the amount of bending deflection of the welded rail. On the other hand, setting the average flash voltage during the second period to 5.0 V or more further suppresses contact and freezing of the end faces 14 of the pair of base rails 1. The average flash voltage during the second period may be 6.0 V or more, 7.0 V or more, 8.0 V or more, or 10.0 V or more. The average flash voltage during the second period may be 12.0 V or less, 11.5 V or less, 11.0 V or less, or 10.5 V or less. A particularly preferred range for the average flash voltage during the second period is 9.5 to 11.0 V.

[0069] (Flash-off distance: 5.0 to 60.0 mm) The flash-off distance refers to the length of one base rail 1 that has been melted and damaged by flashing during the flashing process. Suppressing the flash-off distance can reduce the amount of molten metal that splashes, thereby improving work efficiency. Furthermore, it can improve the yield of welded rails. Therefore, from the perspective of further improving work efficiency and cost, it is preferable to set the upper limit of the flash-off distance to 60 mm. On the other hand, increasing the flash-off distance can heat the entire end face uniformly and suppress the occurrence of freezing. Therefore, it is preferable to set the flash-off distance to 5.0 mm or more. The flash-off distance may be 8.0 mm or more, 10.0 mm or more, or 15.0 mm or more. The flash-off distance may be 50.0 mm or less, 40.0 mm or less, or 30.0 mm or less. A particularly preferred range of the flash-off distance is 8.0 to 30.0 mm.

[0070] (Integrated current per unit area during preheating: 0.1745 to 0.4070 kA·sec / mm ２The integrated current is the sum of the product of the current value when the end faces 14 of the pair of base rails 1 are brought into contact with each other during the preheating process and the time during which the current flows. The integrated current is determined by calculating the time integral of the preheating current from the start to the end of preheating.

[0071] The integrated current is an index that represents the amount of heat generated by resistance. Therefore, the integrated current means the amount of heat input in the preheating step. The integrated current affects the frequency of arc generation during the flashing step S3. The integrated current per unit area is set to 0.1745 kA·sec / mm ２ By setting the value to the above, the frequency of arc generation can be increased during the flashing step S3, thereby further suppressing freezing. Note that the "integrated current per unit area" is the value obtained by dividing the integrated current by the cross-sectional area of ​​the base rail 1.

[0072] On the other hand, by suppressing the integrated current, the hardness of the head of the weld can be increased. Therefore, the integrated current per unit area is set to 0.4070 kA·sec / mm ２ It is preferable that the integrated current per unit area is 0.1861 kA·sec / mm or less. ２ Above, 0.2094kA・sec / mm ２ or more, or 0.2326 kA·sec / mm ２ The integrated current per unit area may be 0.3838 kA·sec / mm ２ Below, 0.3722kA・sec / mm ２ or less, or 0.2326 kA·sec / mm ２ The preferred range of the integrated current per unit area is 0.2094 to 0.3722 kA·sec / mm ２ is.

[0073] For example, the cross-sectional area of ​​the base rail is 8598 mm ２ In the case of a 136RE rail, the integrated current in the preheating step is preferably 1500 to 3500 kA·sec. ２ In the case of a 136RE rail, the integrated current may be 1600 kA·sec or more, 1800 kA·sec or more, or 2000 kA·sec or more.２ In the case of a 136RE rail, the integrated current may be 3300 kA·sec or less, 3200 kA·sec or less, or 2000 kA·sec or less. ２ For a 136 RE rail, the preferred range of integrated current is 1800 to 3200 kA·sec.

[0074] (Chemical Composition of Base Rail 1) While the base rail 1 is not particularly limited, a preferred example of the chemical composition (steel composition) of the base rail 1 according to this embodiment will be described below. The unit "%" for the content of each element means "mass %."

[0075] C: 0.60 to 1.10% C increases the cementite fraction. C also refines the lamellar spacing of the pearlite structure, increasing the strength of the rail. As a result, C improves the wear resistance and surface damage resistance of welded rails. A preferred C content for rails used for freight railways is in the range of 0.60 to 1.10%. When the C content is 0.60% or more, sufficient hardness is obtained at the rail head, improving wear resistance. On the other hand, when the C content is 1.10% or less, bending deflection can be improved. A more preferred range of C content is 0.78 to 1.07%.

[0076] Si: 0.10 to 2.00% Si is a solid solution strengthening element for the ferrite phase in the pearlite structure. Si also dissolves in the ferrite phase, increasing the strength of the rail. This improves the wear resistance of welded rails. On the other hand, Si is an element that easily generates oxides. By reducing the Si content, the amount of bending deflection can be further improved. The preferred Si content is in the range of 0.10 to 2.00%. A more preferred range of Si content is 0.20 to 1.30%.

[0077] Mn: 0.20 to 1.50% Mn improves hardenability and refines lamellar spacing. Furthermore, Mn dissolves in lamellar cementite, suppressing granularity of the lamellar structure after the formation of pearlite, improving the strength of the welded rail. As a result, Mn improves the wear resistance of the welded rail. However, Mn is an element that easily forms oxides, and if the Mn content is excessive, the amount of oxides increases, resulting in a deterioration in bending deflection. The appropriate Mn content is in the range of 0.20 to 1.50%. The preferred range of Mn content is 0.30 to 1.15%.

[0078] P: 0.030% or less P is an impurity element contained in steel. By reducing the P content, the toughness of the welded rail can be improved and the amount of bending deflection can be increased. The P content is preferably 0.030% or less. A more preferable range of the P content is 0.025% or less. The P content may be 0%, but in consideration of refining costs, the P content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[0079] S: 0.030% or less S is an impurity element contained in steel. It is an element that generates sulfides and reduces the amount of bending deflection. A preferable S content is 0.030% or less. A more preferable range of the S content is 0.025% or less. The S content may be 0%, but in consideration of refining costs, the S content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[0080] Cr: 0.01 to 1.00% Cr refines the lamellar spacing of pearlite. As a result, Cr improves the strength of the welded rail and improves its wear resistance. On the other hand, reducing the Cr content reduces the hardenability of the welded rail, suppresses martensite formation in the weld, and increases the amount of bending deflection. The preferred Cr content is in the range of 0.01 to 1.00%. A more preferred range for the Cr content is 0.03 to 0.60%.

[0081] Furthermore, the base rail may contain elements such as V, Ti, Nb, Al, Cu, Ni, Mo, B, Ca, Zr, Mg, REM, Sn, Sb, Co, N, and O, as needed. However, the welded rail according to this embodiment can still exhibit its effects even if the base rail does not contain these elements. Therefore, the lower limit of the content of these elements is 0%.

[0082] V: 0 to 0.200% V forms precipitates during the cooling process of the base rail after hot rolling. V precipitates increase the hardness (strength) of the pearlite structure through precipitation hardening, improving the wear resistance and surface damage resistance of the welded rail. Therefore, the V content may be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, setting the V content to 0.200% prevents excessive precipitates in some areas and further increases the bending deflection of the welded rail. For this reason, the V content is preferably 0.200% or less. A more preferable range of the V content is 0.050% or less.

[0083] Ti: 0 to 0.080% Ti forms Ti carbide in the base rail. Ti carbide prevents coarsening of austenite grains during welding of the base rail and suppresses the formation of martensite, which is harmful to toughness. Therefore, the Ti content may be 0.001% or more, 0.005% or more, or 0.008% or more. On the other hand, if the Ti content is 0.080% or less, the formation of coarse Ti nitrides and Ti oxides, which are both important, is suppressed, and the bending deflection of the welded rail can be further improved. The Ti content is preferably 0.080% or less. More preferably, the Ti content is 0.010% or less.

[0084] Nb: 0 to 0.050% Nb forms Nb carbides and Nb nitrides. When welding base rails, these fine inclusions refine the austenite grains through a pinning effect, suppressing the formation of martensite, which is detrimental to toughness. Therefore, the Nb content may be 0.001% or more, 0.005% or more, or 0.008% or more. On the other hand, setting the Nb content to 0.050% or less suppresses the formation of coarse Nb carbides and Nb nitrides, further improving the bending deflection of welded rails. The Nb content is preferably 0.050% or less. More preferably, the Nb content is 0.025% or less.

[0085] Al: 0 to 0.100% Al has a deoxidizing effect and improves the toughness of the weld and the bending deflection of the welded rail. Al also improves hardenability. Furthermore, Al refines the lamellar spacing of pearlite, improving the strength and wear resistance of the welded rail. Therefore, the Al content may be 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, setting the Al content to 0.100% or less suppresses the formation of coarse Al-based oxide inclusions and further increases the bending deflection of the welded rail. For this reason, the Al content is preferably 0.100% or less. More preferably, the Al content is 0.070% or less.

[0086] Cu: 0 to 0.50% Cu improves hardenability. Furthermore, Cu refines the lamellar spacing of pearlite, improving the strength and wear resistance of the welded rail. Therefore, the Cu content may be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, setting the Cu content to 0.50% or less avoids excessive increases in hardenability, suppresses martensite formation in the welded portion, and further improves the bending deflection of the welded rail. Therefore, the Cu content is preferably 0.50% or less, 0.30% or less, or 0.15% or less. More preferably, the Cu content is 0.05% or less.

[0087] Ni: 0 to 0.50% Ni improves hardenability. Ni also refines the lamellar spacing of pearlite, improving the strength and wear resistance of welded rails. Therefore, the Ni content may be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, setting the Ni content to 0.50% or less avoids excessive increases in hardenability, suppresses martensite formation in the welded portion, and further improves the bending deflection of the welded rail. Therefore, the Ni content is preferably 0.50% or less, 0.30% or less, or 0.15% or less. More preferably, the Ni content is 0.10% or less.

[0088] Mo: 0 to 0.100% Mo improves hardenability. Furthermore, Mo refines the lamellar spacing of pearlite, improving the strength and wear resistance of welded rails. Therefore, the Mo content may be 0.001% or more, 0.010% or more, or 0.020% or more. On the other hand, setting the Mo content to 0.100% or less avoids excessive increases in hardenability, suppresses martensite formation in the welded portion, and further improves the bending deflection of the welded rail. Therefore, the Mo content is preferably 0.100% or less, 0.070% or less, or 0.050% or less. More preferably, the Mo content is 0.025% or less.

[0089] B: 0 to 0.0050%. B segregates at austenite grain boundaries during hot rolling of the slab, which is the material for the base rail, improving hardenability. B also refines the lamellar spacing, thereby improving the strength of the welded rail and simultaneously improving its wear resistance. Therefore, the B content may be 0.0001% or more, 0.0010% or more, or 0.0020% or more. On the other hand, setting the B content to 0.0050% or less avoids excessive increases in hardenability, suppresses martensite formation in the weld, and further improves the bending deflection of the welded rail. For this reason, the B content is preferably 0.0050% or less. More preferably, the B content is 0.0030% or less.

[0090] Ca: 0 to 0.0100% Ca is used as a deoxidizer. Ca also forms sulfides. Ca sulfides suppress the coarsening of MnS and improve the bending deflection of the welded rail. Therefore, the Ca content may be 0.0001% or more, 0.0010% or more, or 0.0020% or more. On the other hand, if the Ca content is 0.0100% or less, the generation of coarse Ca-based oxides is suppressed and the bending deflection of the welded rail can be further improved. For this reason, the Ca content is preferably 0.0100% or less. More preferably, the Ca content is 0.0050% or less.

[0091] Zr: 0 to 0.0200% Zr is used as a deoxidizer. Zr also acts as ZrO ２ Forms inclusions. ZrO ２Inclusions increase the equiaxed crystallization rate of the solidification structure, suppress segregation in the center of the slab, and suppress the formation of martensite and pro-eutectoid cementite structures in the segregated areas. Therefore, the Zr content may be 0.0001% or more, 0.0010% or more, or 0.0020% or more. On the other hand, if the Zr content is 0.0200% or less, the formation of coarse Zr-based oxides is suppressed, and the bending deflection of the welded rail can be further improved. For this reason, the Zr content is preferably 0.0200% or less. More preferably, the Zr content is 0.0100% or less.

[0092] Mg: 0 to 0.0100% Mg is used as a deoxidizer. It also forms Mg sulfides. Mg sulfides suppress the coarsening of MnS and improve the bending deflection of welded rails. Therefore, the Mg content may be 0.0001% or more, 0.0010% or more, or 0.0020% or more. On the other hand, setting the Mg content to 0.0100% or less suppresses the formation of coarse Mg-based oxides, further improving the bending deflection of welded rails. For this reason, the Mg content is preferably 0.0100% or less. More preferably, the Mg content is 0.0050% or less.

[0093] REM: 0 to 0.0500% REM is used as a deoxidizer. It also has the effect of controlling inclusions. REM reduces coarse Al ２ O ３ This suppresses the formation of coarse oxide-based inclusions and improves the breakage resistance of the rail. Therefore, the REM content may be set to 0.0001% or more, 0.0010% or more, or 0.0020% or more. On the other hand, if the REM content is set to 0.0500% or less, the formation of coarse oxide-based inclusions is suppressed and the bending deflection of the welded rail can be further improved. For this reason, the REM content is preferably 0.0500% or less. More preferably, the REM content is 0.0100% or less.

[0094] Sn: 0 to 0.050% Sn is a solid solution strengthening element. Sn strengthens pearlite and improves the wear resistance of welded rails. Therefore, the Sn content may be 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Sn content is 0.050% or less, the brittleness of the welded portion is reduced, and the bending deflection of the welded rail can be further improved. Therefore, the Sn content is preferably 0.050% or less. More preferably, the Sn content is 0.030% or less.

[0095] Sb: 0 to 0.050% Sb suppresses decarburization during heating. Therefore, the Sb content may be 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Sb content is 0.050% or less, the brittleness of the welded joint is reduced, and the amount of bending deflection of the welded rail can be further improved. Therefore, the Sb content is preferably 0.050% or less. More preferably, the Sb content is 0.030% or less.

[0096] Co: 0 to 0.500% Co suppresses the precipitation of pro-eutectoid cementite, improves the ductility of the welded rail, and improves surface damage resistance. Therefore, the Co content may be 0.001% or more, 0.010% or more, or 0.020% or more. On the other hand, if the Co content is 0.500% or less, the brittleness of the welded portion is reduced, and the bending deflection of the welded rail can be further improved. Therefore, the Co content is preferably 0.500% or less. More preferably, the Co content is 0.100% or less.

[0097] N: 0 to 0.0200%. When present in a solid solution state in ferrite, N reduces the ductility of the steel. On the other hand, when the base rail contains V, Ti, and Nb, N forms carbonitrides during the cooling process after hot rolling. Carbonitrides increase the strength of the pearlite structure and improve the wear resistance and surface damage resistance of the welded rail. Therefore, the N content may be 0.0001% or more, 0.0010% or more, or 0.0020% or more. On the other hand, setting the N content to 0.0200% or less suppresses excessive precipitation of carbonitrides and further improves the bending deflection of the welded rail. Therefore, the N content is preferably 0.0200% or less, 0.0150%, or 0.0100%. More preferably, the N content is 0.0080% or less.

[0098] O: 0 to 0.0100% O is an impurity element contained in steel. O exists as an oxide in steel. When O is set to 0.0100% or less, fracture originating from oxides is suppressed, and the bending deflection of the welded rail can be further improved. A preferred O content is 0.0100% or less. More preferably, O is 0.0030% or less. From the viewpoint of improving the mechanical properties of the welded rail, the smaller the O content, the better. On the other hand, taking into consideration refining costs, the O content may be set to 0.0001% or more, 0.0010% or more, or 0.0015% or more.

[0099] The balance of the chemical composition of the base rail is iron. The base rail may also contain elements other than those listed above as impurities. The term "impurities" refers to components that are mixed in with raw materials such as ore or scrap during the industrial production of steel, or due to various factors in the manufacturing process, and are acceptable as long as they do not adversely affect the welded rail according to this embodiment.

[0100] (Tensile strength of base rail 1: 1250 to 1600 MPa) The higher the tensile strength of the base rail 1, the better the wear resistance of the welded rail, in response to heavier vehicle loads. A preferred tensile strength of the base rail 1 is 1250 MPa or higher. On the other hand, if the tensile strength of the base rail 1 is set to 1600 MPa or less, the bending deflection of the welded rail can be further improved. A more preferred range for the tensile strength of the base rail 1 is 1350 to 1550 MPa.

[0101] (Post-heat treatment) The method for manufacturing a welded rail may further include a step of post-heat treating the welded portion formed in the upsetting step. The post-heat treatment reduces the hardness of the welded portion and further improves the amount of bending deflection of the welded rail. The conditions for the post-heat treatment are not particularly limited. Known post-heat treatment conditions can be adopted in the method for manufacturing a welded rail according to this embodiment. For example, the maximum heating temperature of the welded portion in the post-heat treatment may be 700°C or higher. The heating means for the post-heat treatment may be, for example, electrical heating, induction heating, or gas burner heating.

[0102] The effects of the welded rail manufacturing method of this embodiment will be explained more specifically using examples. The conditions in the examples below are one example of conditions adopted to confirm the feasibility and effects of the welded rail manufacturing method of this embodiment. Therefore, the welded rail manufacturing method of this embodiment is not limited to this one example of conditions.

[0103] Cast slabs were produced using a continuous casting facility in a steelmaking process. The chemical compositions of the cast slabs were as shown in Tables 1A to 2C. In these tables, the contents of elements that were not added to the cast slabs are indicated by the symbol "-".

[0104] These cast pieces were heated and then hot-rolled by the universal rolling method to produce base rails having the final shape of the 136RE rail specified by the AREMA standard. The design cross-sectional area of ​​the 136RE rail was 8598 mm ２ If necessary, the base rail was heat treated to apply a predetermined tensile strength to the base rail. The tensile strengths of the base rail are shown in Tables 2A to 2C.

[0105] The above base rails were 25m and 150m long. These base rails were cut into 6m lengths. Then, the end faces to be welded were ground using a #60 grinding wheel until the rust was evenly removed. After that, the end faces of the pair of base rails were butted together and flash butt welding was performed. The flash butt welding machine was configured as follows: Primary power source of flash butt welding machine: AC Transformer capacity of flash butt welding machine: 700kVA Upset load of flash butt welding machine: Maximum 1000kN

[0106] The electrodes were placed on the top and bottom of the base rail. The shape of the contact surface between the electrode and the base rail was a rectangle with a length of 11 cm along the longitudinal direction of the base rail and a length of 7 cm along the width direction of the base rail. The distance between the end of the electrode on the end face of the base rail and the end face of the base rail was 30 cm. The sum of the contact areas S between the electrode and the head of the base rail Ｈ , and the sum of the contact areas S between the electrode and the base rail foot Ｂ , and S Ｈ / S ＢThe contact area was measured using the pressure-sensitive paper method described above before the start of flash butt welding. However, in some flash butt welding, an electrode of the same shape as in welding condition No. 1 was used, and the same pressure was applied to the electrode. Before these flash butt weldings were performed, S Ｈ and S Ｂ It was assumed that the contact area was the same as that of welding condition No. 1, and measurement of the contact area was omitted.

[0107] The integrated current in the preheating process, the first flashing speed and second flashing speed in the flashing process, the average flashing voltage, and the flash-off distance are shown in Tables 4A to 4C. The number of preheating cycles was varied from 5 to 15. The current and current application time were varied in the preheating process. The upset load was 75 tons. Welded rails were manufactured using the above manufacturing process. No post-heat treatment was performed on the welded rails.

[0108] The amount of bending deflection of the welded rail was then evaluated. It was also determined whether freezing occurred during flash butt welding. The evaluation results of freezing and bending deflection are shown in Tables 5A to 5C.

[0109] A method for evaluating the bending deflection of a joint according to this embodiment will be described with reference to FIG. 6 . The bending deflection was evaluated using a four-point bending test in accordance with the AREMA standard (Chapter 3.10.3.6). In the four-point bending test, the foot of the welded rail was first placed on two supports. The distance between the two supports was 48 inches. Next, pressure was applied to the head of the welded rail at two points. The distance between the two pressure points was 12 inches. As shown in FIG. 6 , the center of the two supports and the center of the two pressure points were aligned with the weld 15. The loading rate was 45 kN / min or less. In this example, the test was performed at 10 kN / min. A four-point bending test was performed under the above conditions, and a pressure was applied to the welded rail until fracture occurred. The amount of deflection at fracture, i.e., the amount of depression of the pressure points, was considered to be the bending deflection.

[0110] Six welded rails were prepared for each welding condition and subjected to a four-point bending test. The evaluation standard for the amount of bending deflection was 19 mm or more. Welding conditions under which four or more welded rails had an amount of bending deflection equal to or greater than the evaluation standard were deemed to be acceptable.

[0111] A method for determining freezing during the manufacture of welded rails according to this embodiment will be described. The welding current during the flashing process in the welding chart typically fluctuates between approximately 10 and 30 kA, with the upper limit repeated at intervals of several milliseconds to several hundred milliseconds. However, when freezing occurs, a current of 50 kA or more flows for a long period of time, approximately 0.5 seconds or longer. When this current flow occurs, it is determined that freezing has occurred. Six welded rails were produced for each welding condition. The number of welded rails that experienced freezing was evaluated as the amount of freezing. Welding conditions under which freezing occurred in one or fewer welded rails were deemed acceptable.

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[0119]

[0120]

[0121]

[0122]

[0123]

[0124]

[0125]

[0126]

[0127] S Ｈ / S Ｂ According to the welding conditions in which the electrodes were positioned so that the value of the welded joint was within the preferred range, freezing was suppressed and a welded rail with a large amount of bending deflection was obtained.

[0128] On the other hand, under the welding conditions of Examples 37 to 40 and Examples 71 to 74, S Ｈ / S Ｂ The welded rails obtained under these welding conditions had unacceptable bending deflections. It is believed that the flash butt welding performed under these welding conditions had insufficient welding current at the base, making it impossible to sufficiently remove the oxides at the toe.

[0129] In addition, under the welding conditions of Examples 57 to 60 and Examples 75 to 78, S Ｈ / S Ｂ In flash butt welding under these welding conditions, freezing occurred. It is believed that in flash butt welding under these welding conditions, the welding current at the head was insufficient, causing the end faces of the pair of base rails to come into contact.

[0130] S1 Installation process S2 Preheating process S3 Flashing process S4 Upsetting process 1 Base rail 11 Head 111 Head top 112 Head side 113 Head jaw 12 Column 13 Foot 131 Foot sole 132 Foot tip 133 Foot top 14 End face 15 Welding part 2 Electrode 3 Power source 4 Flashing 5 Burr 6 Pressure-sensitive paper S Ｈ Contact area between the electrode and the head of the base rail S Ｂ Contact area between the electrode and the base rail foot

Claims

1. A method for manufacturing welded rails, comprising: an installation step of installing electrodes on the head and foot portions of a pair of base rails aligned along the longitudinal direction; a preheating step of preheating the end faces of the pair of base rails by passing a preheating current through the electrodes while the end faces of the pair of base rails are in contact with each other; a flashing step of passing a flash current through the pair of base rails through the electrodes to generate a flash between the end faces that melts the end faces; and an upsetting step of butting the end faces of the pair of base rails together and applying an upsetting force to upset the pair of base rails so as to join them, wherein the sum of the contact areas S between the electrodes and the heads of the base rails is 1 / 2 mm. Ｈ and the sum of the contact areas S between the electrode and the foot of the base rail Ｂ A method for manufacturing a welded rail, in which the value of S is within the range of formula (1). Ｈ / S Ｂ ≦0.70 Formula (1) 2. The method for manufacturing a welded rail according to claim 1, wherein the duration of the flashing process is defined as a flashing time t, the average relative movement speed of the pair of base rails during a first period that is the period from the start of the flashing process until 0.90t has elapsed is defined as a first flashing velocity, the average relative movement speed of the pair of base rails during a second period of the flashing process that is a period other than the first period is defined as a second flashing velocity, the first flashing velocity is slower than the second flashing velocity, the second flashing velocity is 1.8 to 3.0 mm / sec, and the average flashing voltage during the second period is 5.0 to 13.0 V.

3. A method for manufacturing a welded rail according to claim 1 or 2, characterized in that the flashing distance in the flashing step is 5.0 to 60.0 mm.

4. The integrated current per unit area in the preheating process is 0.1745 to 0.4070 kA·sec / mm ２ 3. The method for manufacturing a welded rail according to claim 1 or 2, characterized in that:

5. The chemical composition of the base rail 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%, V: 0 to 0.200%, Ti: 0 to 0.080%, Nb: 0 to 0.050%, Al: 0 to 0.100%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Mo: 0 to 0.100%, B: 0 to 0.0050%, Ca: 0 to 0.0100%, Zr: 0 to 0.0200%, Mg: 0 to 0.0100%, 3. The method for manufacturing a welded rail according to claim 1, characterized in that the steel sheet contains REM: 0 to 0.0500%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Co: 0 to 0.500%, N: 0 to 0.0200%, and O: 0 to 0.0100%, with the balance including Fe and impurities.

6. A method for manufacturing a welded rail according to claim 1 or 2, characterized in that the tensile strength of the base rail is 1250 to 1600 MPa.

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