rail

Abstract

A rail according to the present disclosure has a chemical composition containing, by mass%, 0.55 to 1.20% of C, 0.10 to 2.00% of Si, 0.10 to 2.00% of Mn, 0.0020% to less than 0.0200% of Sn, 0.0040% or less of O, 0.025% or less of P, 0.025% or less of S, 1.0000% or less of Al, and 0.0020% to 0.0200% of N, with the balance being Fe and impurities, the chemical composition satisfying formula (1). In a metal structure of a head surface portion from an outline surface of the rail head portion to a depth position of 20 mm, the area ratio of a pearlite structure is 95% or greater, and the Vickers hardness is 230 HV or greater.　(1): 0.05 x 10-5 ≤ Sn x (O / 4) ≤ 1.50 x 10-5 In formula (1), Sn and O are substituted for the contents, in mass%, of the corresponding elements in the chemical composition of a rail 1.

Description

rail

[0001] This disclosure relates to rails.

[0002] Flash butt welding is a widely used method for welding rails. Flash butt welding allows for the automation of the welding process. Furthermore, it offers high stability in weld quality and short welding times. Therefore, flash butt welding is suitable for welding rails.

[0003] Flash butt welding typically involves an initial flashing step, a preheating step, a late flashing step, and an upsetting step.

[0004] In the initial flash process, the end faces of a pair of rails are placed facing each other with a gap between them. A flash (arc discharge) is then generated between the end faces of the pair of rails to heat them. In the preheating process, the end faces of the pair of rails heated in the initial flash process are forcibly brought into contact under pressure, and a large current is supplied for a certain period of time. This further heats the end faces of the pair of rails. In the preheating process, the process of bringing the end faces of the pair of rails into contact under pressure and supplying a large current, and the process of separating the end faces of the pair of rails are repeated multiple times. Through this repetition, the temperature of the end faces of the pair of rails is heated to the temperature required for the subsequent late flash process.

[0005] In the later flash process, similar to the initial flash process, a flash is generated between the end faces of a pair of rails to heat them. At this time, the pair of rails are moved so that they move closer to each other. This moving speed is called the flash speed. In the first half of the later flash process, the flash speed is relatively slow, and the melting length (melted length in the longitudinal direction of the rail) is small. On the other hand, in the second half of the later flash process, the flash speed is increased, and the melting length increases exponentially.

[0006] In the upset process, after the entire surface of the rail end face becomes molten, high pressure is applied to rapidly bring the rail end faces together, forming the HAZ (Heat Affected Zone), which is the welded area. Through these processes, a rail welded joint is manufactured by flash butt welding.

[0007] A technology relating to rail welded joints manufactured by flash butt welding is disclosed in International Publication No. 2011 / 052562 (Patent Document 1).

[0008] Patent Document 1 specifies that the late flash speed in the late flash process should be 2.1 to 2.8 mm / second or less. Patent Document 1 states that by adjusting the late flash speed, the heat distribution on the weld surface is made steeper, narrowing the HAZ width. This reduces the softening width of the rail welded joint.

[0009] International Publication No. 2011 / 052562

[0010] Incidentally, in the later flashing stage of flash butt welding, the rail end face becomes molten at a high temperature. The molten metal reacts with oxygen in the atmosphere to form oxides. Most of these oxides are expelled from the weld by the pressurization in the upsetting stage. However, oxides may remain within the rail weld joint. In this case, these oxides become the starting point for fracture. As a result, the bending strength of the rail weld joint decreases.

[0011] The purpose of this disclosure is to provide a rail that can increase the bending strength of a rail welded joint when manufacturing the rail welded joint by flash butt welding.

[0012] The rails of this disclosure have a chemical composition in mass percent of: C: 0.55 to 1.20%, Si: 0.10 to 2.00%, Mn: 0.10 to 2.00%, Sn: less than 0.0020 to 0.0200%, O: 0.0040% or less, P: 0.025% or less, S: 0.025% or less, Al: 1.0000% or less, N: 0.0020 to 0.0200%, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Co: 0 to 1. It contains 00%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0.200%, Nb: 0-0.0500%, Ti: 0-0.0500%, B: 0-0.0050%, Mg: 0-0.0200%, Ca: 0-0.0200%, rare earth elements: 0-0.0500%, Zr: 0-0.0200%, and Pb: 0-0.0040%, with the remainder being Fe and impurities, satisfying formula (1). In the metallographic structure of the rail head surface from the outer surface to a depth of 20 mm, the area ratio of pearlite is 95% or more, and the Vickers hardness is 230 HV or more. 0.05 × 10 －５ ≦Sn×(O / 4)≦1.50×10 －５ (1) Here, Sn and O in equation (1) are substituted with the mass percentage content of the corresponding elements in the chemical composition.

[0013] The rails of this disclosure can increase the bending strength of rail welded joints when rail welded joints are manufactured by flash butt welding.

[0014] Figure 1A is a cross-sectional view of the rail of this embodiment perpendicular to the longitudinal direction. Figure 1B is a cross-sectional view near the rail head to illustrate the method for defining the top surface portion in Figure 1A. Figure 1C is a cross-sectional view near the rail head, following Figure 1B. Figure 1D is a cross-sectional view near the rail head, following Figure 1C. Figure 1E is a cross-sectional view near the rail head, following Figure 1D. Figure 1F is a cross-sectional view near the rail head, following Figure 1E. Figure 2 is a perspective view of the rail welded joint. Figure 3 is a cross-sectional view of the rail welded joint when cut longitudinally at the center of the rail width. Figure 4 is a distribution diagram of Vickers hardness in the longitudinal direction of the rail obtained by a Vickers hardness test. Figure 5 is a schematic diagram of a four-point bending test. Figure 6 is a photographic image of the fracture surface after the rail welded joint fractured following the four-point bending test. Figure 7 is a diagram showing the relationship between Sn content and the maximum load at bending fracture. Figure 8 is a diagram showing the relationship between Sn content and the total area of ​​oxides on the fracture surface. Figure 9 shows the relationship between Sn content, maximum load, and Sn positive segregation degree when the Sn content is between 0.0020% and less than 0.0200%, and F1 satisfies formula (1). Figure 10 shows the relationship between Sn content, total oxide area, and Sn positive segregation degree when the Sn content is between 0.0020% and less than 0.0200%, and F1 satisfies formula (1). Figure 11A is a cross-sectional view perpendicular to the longitudinal direction of the rail bottom. Figure 11B is a schematic diagram of a plane extending in the longitudinal and height directions of the rail, including each analysis point in Figure 11A. Figure 11C is a schematic diagram for explaining surface analysis using an electron beam microanalyzer (EPMA) in each analysis area in Figure 11B. Figure 11D is a schematic diagram for explaining the measurement method of the Sn concentration distribution in each analysis area, following Figure 11C. Figure 12 is a schematic diagram showing the Sn concentration distribution of the merge line analysis. Figure 13 is a front view of the rail cooling system.

[0015] The rails of this embodiment will be described in detail below. Unless otherwise specified, percentages in the chemical composition refer to mass percentages.

[0016] [Rail Structure] Figure 1A is a cross-sectional view of the rail 1 of this embodiment, perpendicular to the longitudinal direction. In Figure 1A, direction L is the longitudinal direction of the rail 1, direction H is the height direction of the rail 1, and direction W is the width direction of the rail 1. Referring to Figure 1A, the rail 1 includes a rail head 10, a rail bottom 15, and a rail column 14.

[0017] The rail head 10 refers to the portion of the rail 1 above the constricted section in the height direction center. The rail head 10 receives the load from the wheels of the railway vehicle. The rail bottom 15 is the portion of the rail 1 from the bottom surface 15S to a height of 0.25h in the height direction. The rail bottom 15 is fixed to the sleepers. The rail bottom 15 supports the rail head 10, which receives the load from the wheels, via the rail column 14. The rail column 14 is positioned between the rail head 10 and the rail bottom 15. The upper end of the rail column 14 is connected to the rail head 10, and the lower end of the rail column 14 is connected to the rail bottom 15.

[0018] The rail head 10 includes a top portion 11, a pair of head corner portions 12 (12L and 12R), and a pair of jaw portions 13. The top portion 11 is located in the center of the rail head 10 in the width direction. The pair of head corner portions 12 (12L and 12R) are located at both ends of the top portion 11 in the width direction of the rail head 10. In other words, the top portion 11 is located between the pair of head corner portions 12 (12L and 12R). In a cross-sectional view perpendicular to the longitudinal direction of the rail 1, the head corner portion on the left side of the top portion 11 is referred to as "head corner portion 12L," and the head corner portion on the right side of the top portion 11 is referred to as "head corner portion 12R." The head corner portions 12L and 12R are collectively referred to as head corner portion 12. One of the pair of head corner portions 12L and 12R is the gauge corner (G.C.) portion that mainly contacts the wheel.

[0019] Of the rail head 10, the surface of the top portion 11 that faces upward when the rail 1 is upright, and the surface of the pair of head corner portions 12 combined are referred to as the head outer surface 10S. The pair of chin lower portions 13 are positioned on the opposite side of the pair of head corner portions 12 in the height direction of the rail 1. The pair of chin lower portions 13 are the outer surface of the portion that is constricted in the center in the height direction (H direction) of the rail 1.

[0020] The region from the outer surface 10S of the head to a depth of 20 mm is referred to as the head surface 10A. In Figure 1A, D1 indicates the range from the outer surface 10S of the head to a depth of 20 mm. As will be described later, the microstructure of the head surface 10A consists substantially of pearlite, and its Vickers hardness is 230 HV or higher.

[0021] Generally, the service life of a rail is approximately 20 mm in depth, starting from the outer surface 10S of the head. Therefore, by making the microstructure of the head surface 10A a pearlite structure with a Vickers hardness of 230 HV or higher, the wear resistance of the rail 1 is increased.

[0022] [Method for defining the area of ​​the top surface portion 10A] The top surface portion 10A is defined in the following way. Referring to Figure 1B, point P11 is defined as the center of the width of the surface of the rail head 10 in a cross section perpendicular to the longitudinal direction of the rail 1. A line segment L11 is assumed to extend in the depth direction starting from point P11. Line segment L11 is coaxial with the normal to the outer surface 10S of the top at point P11 and corresponds to the center line of the rail 1 in the cross section of the rail 1.

[0023] Let point X be located on the normal vector L11, 20 mm (D1) in the depth direction from point P11. Draw a line segment L100 horizontally from point X, and let points A and B be the intersections of line segment L100 and the head outer surface 10S.

[0024] Referring to Figure 1C, the area from point A to point B on the surface of the rail head 10 is divided into three equal parts, and the boundary positions of these three divisions are designated as points C and D, respectively. Then, referring to Figure 1D, the area between point C and point D is defined as the top portion 11. Furthermore, the midpoint between point A and point C on the surface of the rail head 10 is designated as point E. Similarly, the midpoint between point B and point D is designated as point F.

[0025] Referring to Figure 1E, point G is defined as the midpoint between points A and E on the surface of the rail head 10. Similarly, point H is defined as the midpoint between points B and F. The area between points C and G on the surface of the rail head 10 is defined as the head corner section 12L. Note that the area between points A and G is excluded from the head corner section 12L because it is an area with substantially little contact with railway vehicles. Similarly, the area between points D and H is defined as the head corner section 12R. The area between points B and H is excluded from the head corner section 12R because it is an area with substantially little contact with railway vehicles.

[0026] The top portion 11, the head corner portion 12L, and the head corner portion 12R are defined by the above method. Then, the surface of the rail head portion 10, specifically the surface of the top portion 11, the head corner portion 12L, and the head corner portion 12R, that is, the surface from point G to point H, is defined as the head outer surface 10S.

[0027] In the crown portion 11, the head corner portion 12L, and the head corner portion 12R, the depth direction is defined as follows: In the crown portion 11, the direction normal to the head outer surface 10S is defined as the depth direction. On the other hand, the depth direction of the head corner portion 12L and the head corner portion 12R is defined as follows: Referring to Figure 1F, the line segment L13 is tangent to the lower jaw portion 13. Ｌ and L13 Ｒ This is assumed. And line segment L13 Ｌ and L13 Ｒ Let the intersection of the two lines be point P13. Point P13 is located on the normal vector L11. In the head corner portion 12L, the direction of the line segment connecting any point on the head outer surface 10S (i.e., the surface between point C and point G) and point P13 is defined as the depth direction at that arbitrary point. For example, if the arbitrary point is point E, then the line segment L12 between point E and point P13 is defined as the depth direction at that arbitrary point. Ｌ The direction along the line is defined as the depth direction at point E. Similarly, at the head corner portion 12R, the direction of the line segment connecting any point on the head outer surface 10S (i.e., the surface between point D and point H) and point P13 is defined as the depth direction at that arbitrary point. For example, if the arbitrary point is point F, then the line segment L12 between point F and point P13 is defined as the depth direction at that arbitrary point. ＲThe direction along [it] is defined as the depth direction at point F.

[0028] Based on the above definitions, a line segment L at a position 20 mm deep from the head outer surface 10S between point G and point H Ｄ２０ is defined. By the above method, the head surface part 10A shown in Fig. 1A can be defined.

[0029] [Regarding the technical idea of the rail 1 of this embodiment] The technical idea of the rail 1 of this embodiment is as follows. First, the inventors examined the chemical composition and microstructure of the rail in order to obtain wear resistance equivalent to that required for a normal rail. As a result, the inventors found that, in mass%, C: 0.55 to 1.20%, Si: 0.10 to 2.00%, Mn: 0.10 to 2.00%, O: 0.0040% or less, P: 0.025% or less, S: 0.025% or less, Al: 1.0000% or less, N: 0.0020 to 0.0200%, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Co: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, V: 0 to 0.200%, Nb: 0 to 0.0500%, Ti: 0 to 0.0500%, B: 0 to 0.0050%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, rare earth elements: 0 to 0.0500%, Zr: 0 to 0.0200%, and Pb: 0 to 0.0040%, having a chemical composition consisting of the balance Fe and impurities, and if the pearlite area ratio in the microstructure of the head surface part is 95% or more and the Vickers hardness is 230 HV or more, it is considered applicable as a rail.

[0030] Therefore, the inventors further examined means for increasing the bending strength of the rail welded joint when manufacturing the rail welded joint by flash butt welding in the rail 1 having the above chemical composition and microstructure. As a result, the inventors obtained the following findings.

[0031] First, the inventors found the mechanism of the generation of flash (arc discharge) in the late flash process of flash butt welding as follows. In the late flash process, a voltage is generated between the rail end faces according to the voltage applied from the electrodes attached to both ends of the rail. According to this voltage, flash (arc discharge) occurs.

[0032] Here, the voltage between the rail ends is affected not only by the voltage applied from the electrodes but also by the electrical resistance of the rail itself. If the electrical resistance of the rail itself is low, the voltage between the rail ends during flash butt welding increases. As a result, the generation of flash (arc discharge) is promoted. Based on the above findings, the inventors investigated ways to reduce the electrical resistance of the rail itself. As a result, the inventors found that if the above chemical composition contains 0.0020% or more of Sn by mass, the electrical resistance of the rail is sufficiently reduced, and the generation of Sn oxide can be suppressed during flash butt welding.

[0033] On the other hand, if the Sn content is 0.0200% or higher, excessive Sn oxides will be generated during flash butt welding. In this case, the bending strength of the welded joint produced by flash butt welding will decrease. Therefore, the Sn content should be between 0.0020% and less than 0.0200% by mass.

[0034] However, even when the Sn content was set to less than 0.0020% by mass, the bending strength of rail welded joints manufactured by flash butt welding was unstable and varied. Therefore, the inventors further investigated means to not only increase the bending strength of rail welded joints but also to stabilize the bending strength of rail welded joints. As a result, the inventors further found that the bending strength of rail welded joints manufactured by flash butt welding can be stabilized by satisfying formula (1) in the chemical composition of the rail. 0.05 × 10 －５ ≦Sn×(O / 4)≦1.50×10 －５ (1) Here, Sn and O in equation (1) are substituted with the mass percentage content of the corresponding elements in the chemical composition of the rail.

[0035] The following describes the range of Sn content, formula (1), and the bending strength of rail welded joints manufactured by flash butt welding.

[0036] [Regarding the structure of rail welded joints] First, a rail welded joint manufactured by flash butt welding two rails 1 will be described. Figure 2 is a perspective view of a rail welded joint. Referring to Figure 2, the rail welded joint comprises the base material portion 1A of a pair of rails 1 and a HAZ2. In flash butt welding, the HAZ2 is A １ This is a region that has been heated above the point (eutectoid transformation temperature) and has been affected by heat. In the base material 1A of the rail 1, in flash butt welding, １ These are regions that have been heated to below a certain temperature and are affected by heat, and regions that are not affected by heat.

[0037] The HAZ2 range of a rail welded joint can be determined by determining the hardness distribution. The method for determining the HAZ2 range is described below. As shown in Figure 3, a test specimen is prepared with a cross-section obtained by cutting the rail welded joint longitudinally at the center of the rail width. At the cross-section, the Vickers hardness is measured at a depth of 5 mm from the outer surface of the head along the longitudinal direction of the rail welded joint. A Vickers hardness test is performed for each set point in accordance with JIS Z 2244-1:2024. The test force is 10 kgf.

[0038] Figure 4 is a schematic diagram summarizing the Vickers hardness test results. As shown in Figure 4, in the Vickers hardness distribution, within the region between the two softest points 200, the range of ±10 mm on the horizontal axis of Figure 4 is A during welding. １ This is a region heated to the austenite region above a certain point. In Figure 4, the Vickers hardness of this region is approximately 380-390 HV. On the other hand, in the region between the two softest parts 200, the ranges of -10 to -15 mm and 10 to 15 mm on the horizontal axis of Figure 4 are A during welding. １ This region is heated to a two-phase state consisting of an austenite region above a certain point and a ferrite or cementite region. The minimum Vickers hardness of this region is approximately 270 HV, which is lower than the Vickers hardness within ±10 mm on the horizontal axis of Figure 4.

[0039] As described above, the Vickers hardness distribution in rail welded joints is as shown in Figure 4. In Figure 4, the distance between the pair of softest points 200 is referred to as the HAZ width 2. In Figure 4, the HAZ width 2 is approximately 30 mm.

[0040] [Regarding the Bending Strength Evaluation Test of Rail Welded Joints] To evaluate the bending strength of rail welded joints having the above configuration, the following test was conducted. Two rails were prepared with various chemical compositions, each containing, by mass%, C: 0.80-0.85%, Si: 0.70-0.80%, Mn: 0.70-0.80%, P: 0.008-0.013%, S: 0.008-0.013%, Al: 0.0020-0.0025%, and N: 0.0035-0.0040%, with the O content varied in the range of 0.0015-0.0020%, and the Sn content varied in the range of 0-0.0400%, with the remainder being Fe and impurities. In each rail, F1 (= Sn × (O / 4)) was 0.01 × 10 －５ ~2.00 x 10 －５ The rail shape was 136 pounds, and the mass was 67 kg / m. The perlite area ratio of the head surface was 95% or more, and the Vickers hardness of the head surface was 400 HV.

[0041] Rail welded joints were manufactured by performing the following flash butt welding on two rail test pieces. In the initial flash process, the initial flash time (time to generate the flash) was set to 15 seconds. In the preheating process, the number of preheating cycles (number of repetitions of the process of applying pressure and contact and supplying a large current, and the process of separating the pair of rail end faces) was set to 10. In the later flash process, the later flash time (time to generate the flash) was set to 20 seconds. The average flash velocity for the entire later flash process was set to 0.8 mm / second, and the average flash velocity for the 3 seconds before the upset process was set to 1.8 mm / second. The upset load in the upset process was set to 75 kN. After flash butt welding, the average cooling rate in the temperature range of 800 to 550°C of the welded joint was set to more than 1.5 to 3.0°C / second. After that, the welded joint was allowed to cool until the surface temperature reached 50°C. Rail welded joints were manufactured using the above manufacturing process.

[0042] Referring to Figure 5, the manufactured rail welded joint 50 was positioned with the rail head facing upwards and the rail bottom facing downwards, and the rail bottom was supported at two points. Furthermore, a load was applied to two points in the region of the rail head including HAZ2. The distance between the support points at the rail bottom was set to 1220 mm. The distance between the support points at the rail head of the weld was set to 305 mm. The HAZ2 was adjusted to be positioned at the center of the distance between the support points at the rail head. The applied load was increased in increments of 50 kN from 1500 kN. The maximum load at which fracture did not occur was then determined. If fracture did not occur when the load reached 3200 kN, the maximum load was set to 3200 kN. In the event that the rail welded joint 50 fractured, the fracture surface was the weld surface formed by flash butt welding.

[0043] Furthermore, the fracture surface of the rail after the rail weld joint fractured was observed, and the total area of ​​oxides (oxide area) at the bottom 15 of the rail fracture was measured using the following method. Figure 6 shows a macro photograph of the fracture surface after the rail weld joint fractured. First, in the macro photograph of the fracture surface of the entire bottom 15 of the rail, an area with a different color tone from the surrounding area was identified as the specific area OX (see Figure 6). The specific area OX could be identified visually because its color tone was different from other areas. The observation field including the identified oxide group area was observed using a SEM. When the specific area OX was observed with an SEM at 200 to 400x magnification, multiple gray particles were found to be present in clusters within the specific area OX. The diameter of each particle was approximately 10 to 30 μm, and the distance between adjacent particles was within 100 μm. Therefore, quantitative analysis of each particle within the specific OX region was performed using energy-dispersive X-ray spectroscopy (EDS) attached to the SEM. In the EDS analysis, the acceleration voltage was set to 15 kV, the irradiation current to 0.05 μA, the irradiation time to 60 ms, and the beam diameter to 1 μm. Particles with an O content exceeding 10% by mass were identified as oxides.

[0044] EDS analysis of each particle revealed that multiple particles within the specific region OX were all oxides. Therefore, in macro photographic images, the region with the same color tone as the specific region OX was identified as the oxide group region OX. Total area of ​​the observation field of view at the bottom of the rail (2000-2500 mm²) ２ ) Total area of ​​oxide region OX (mm²) ２ ) and the oxide area (mm²) at the fracture surface. ２ )

[0045] Figure 7 shows the relationship between Sn content and the maximum load at bending fracture. Referring to Figure 7, when the Sn content was 0.0020% or higher, the maximum load increased significantly. On the other hand, when the Sn content was 0.0200% or higher, the maximum load decreased significantly. Figure 8 shows the relationship between Sn content and oxide area (mm²) at the fracture surface. ２ This figure shows the relationship with ( ). Referring to Figure 8, when the Sn content was between 0.0020% and less than 0.0200%, the total area of ​​oxides decreased significantly. Based on these results, it was found that in the above chemical composition, when the Sn content is between 0.0020% and less than 0.0200%, the amount of Sn oxide generated during flash butt welding decreases significantly, and as a result, the bending strength increases significantly.

[0046] Referring to Figures 7 and 8, when F1 satisfies formula (1) (indicated by "◆" in Figures 7 and 8), the variation in the total oxide area was smaller compared to when F1 did not satisfy formula (1) (indicated by "◇" in Figures 7 and 8), and as a result, the variation in bending strength was also smaller. In other words, it was found that when F1 satisfies formula (1), the bending strength is stabilized.

[0047] Furthermore, the degree of positive Sn segregation at the bottom of the rail was determined by the method described later in [Method for measuring the degree of positive Sn segregation at the bottom of the rail]. Figure 9 shows the relationship between the Sn content, maximum load, and degree of positive Sn segregation when the Sn content is between 0.0020% and less than 0.0200%, and F1 satisfies formula (1). Figure 10 shows the relationship between the Sn content, total oxide area, and degree of positive Sn segregation when the Sn content is between 0.0020% and less than 0.0200%, and F1 satisfies formula (1).

[0048] Referring to Figures 9 and 10, it was found that if the degree of Sn positive segregation at the bottom 15 of the rail is 10.0 or less, the oxide area is further reduced, the maximum load is further increased, and the bending strength is further increased.

[0049] The rail of this embodiment was completed based on the above technical concept and has the following configuration.

[0050] The rail of the first form has a chemical composition in mass percent of: C: 0.55-1.20%, Si: 0.10-2.00%, Mn: 0.10-2.00%, Sn: less than 0.0020-0.0200%, O: 0.0040% or less, P: 0.025% or less, S: 0.025% or less, Al: 1.0000% or less, N: 0.0020-0.0200%, Cr: 0-1.00%, Mo: 0-0.50%, Co: 0-1 It contains 0.00%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0.200%, Nb: 0-0.0500%, Ti: 0-0.0500%, B: 0-0.0050%, Mg: 0-0.0200%, Ca: 0-0.0200%, rare earth elements: 0-0.0500%, Zr: 0-0.0200%, and Pb: 0-0.0040%, with the remainder being Fe and impurities, satisfying formula (1). In the metallographic structure of the rail head surface from the outer surface to a depth of 20 mm, the area ratio of pearlite is 95% or more, and the Vickers hardness is 230 HV or more. 0.05 × 10 －５ ≦Sn×(O / 4)≦1.50×10 －５ (1) Here, Sn and O in equation (1) are substituted with the mass percentage content of the corresponding elements in the chemical composition of rail 1.

[0051] The second form of the rail is the same as the first form of the rail, and its chemical composition contains, by mass%, Sn: 0.0030 to less than 0.0100%.

[0052] The third form of rail is the first or second form of rail, and its chemical composition is, in mass%, Cr: 0.01-1.00%, Mo: 0.01-0.50%, Co: 0.01-1.00%, Cu: 0.01-1.00%, Ni: 0.01-1.00%, V: 0.001-0.200%, Nb: 0.0001-0.0500%, Ti: 0. It contains one or more elements selected from the group consisting of: 0.0001 to 0.0500%, B: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0200%, Ca: 0.0001 to 0.0200%, rare earth elements: 0.0001 to 0.0500%, Zr: 0.0001 to 0.0200%, and Pb: 0.0001 to 0.0040%.

[0053] The fourth form of rail is a rail of any one of the first to third forms, and furthermore, the degree of positive segregation of Sn at the bottom of the rail is 10.0 or less.

[0054] The rails of this embodiment will now be described. In the following description, percentages related to elements refer to mass percentages unless otherwise specified.

[0055] [Features of Rail 1 of this Embodiment] The rail of this embodiment has the following features: (Feature 1) The chemical composition is, in mass%, C: 0.55 to 1.20%, Si: 0.10 to 2.00%, Mn: 0.10 to 2.00%, Sn: less than 0.0020 to 0.0200%, O: 0.0040% or less, P: 0.025% or less, S: 0.025% or less, Al: 1.0000% or less, N: 0.0020 to 0.0200%, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Co: 0 to 1. It contains 00%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0.200%, Nb: 0-0.0500%, Ti: 0-0.0500%, B: 0-0.0050%, Mg: 0-0.0200%, Ca: 0-0.0200%, rare earth elements: 0-0.0500%, Zr: 0-0.0200%, and Pb: 0-0.0040%, with the remainder being Fe and impurities. (Feature 2) The chemical composition satisfies formula (1). 0.05 × 10 －５ ≦Sn×(O / 4)≦1.50×10 －５(1) Here, the mass percentage content of the corresponding elements in the chemical composition of rail 1 is substituted for Sn and O in formula (1). (Feature 3) In the metallographic structure of the rail head surface up to a depth of 20 mm from the outer surface of the rail head, the area ratio of pearlite is 95% or more. (Feature 4) In the metallographic structure of the rail head surface up to a depth of 20 mm from the outer surface of the rail head, the Vickers hardness is 230 HV or more. Each feature will be explained below.

[0056] [(Feature 1) Chemical Composition] The chemical composition of rail 1 according to this embodiment contains the following elements.

[0057] C: 0.55-1.20% Carbon (C) promotes pearlite transformation, thereby increasing the wear resistance of the rail. If the C content is less than 0.55%, protereminate ferrite will be formed. As a result, sufficient strength and wear resistance cannot be obtained. On the other hand, if the C content exceeds 1.20%, protereminate cementite will be excessively formed on the top surface portion 10A, reducing the fracture resistance of the rail 1. Therefore, the C content is 0.55-1.20%. The preferred lower limit of the C content is 0.60%, more preferably 0.65%, still more preferably 0.70%, still more preferably 0.75%, and still more preferably 0.80%. The preferred upper limit of the C content is 1.15%, more preferably 1.10%, and still more preferably 1.05%.

[0058] Si: 0.10-2.00% Silicon (Si) solid-solves in the ferrite in the pearlite structure, increasing the hardness of the rail. As a result, the wear resistance of the top surface portion 10A of the rail is increased. If the Si content is less than 0.10%, the above effect cannot be sufficiently obtained. On the other hand, if the Si content exceeds 2.00%, the hardenability of the steel constituting the rail becomes excessively high. In this case, excessive martensite is formed on the top surface portion 10A. As a result, the wear resistance and fracture resistance of the top surface portion 10A decrease. Therefore, the Si content is 0.10-2.00%. The preferred lower limit of the Si content is 0.15%, more preferably 0.20%, more preferably 0.25%, more preferably 0.30%, more preferably 0.35%, and more preferably 0.40%. The preferred upper limit for the Si content is 1.95%, more preferably 1.90%, more preferably 1.80%, more preferably 1.70%, and still more preferably 1.60%.

[0059] Mn: 0.10-2.00% Manganese (Mn) enhances the hardenability of the steel constituting the rail and stabilizes the pearlite transformation. Mn further refines the lamellar spacing of the pearlite. This increases the hardness and wear resistance of the rail. If the Mn content is less than 0.10%, the effect is not sufficiently obtained. On the other hand, if the Mn content exceeds 2.00%, the hardenability of the steel constituting the rail becomes excessively high. As a result, bainite and / or martensite are formed on the top surface 10A of the rail. Consequently, the wear resistance and fracture resistance of the top surface 10A of the rail decrease. An excess amount of Mn further promotes the formation of protereminate cementite. As a result, the fracture resistance of the top surface 10A of the rail decreases. Therefore, the Mn content is 0.10-2.00%. The preferred lower limit of the Mn content is 0.20%, more preferably 0.30%, and even more preferably 0.40%. The preferred upper limit for the Mn content is 1.80%, more preferably 1.60%, and even more preferably 1.50%.

[0060] Sn: 0.0020% to less than 0.0200% Tin (Sn) dissolves in the steel that makes up the rail, lowering the electrical resistance of the rail. Therefore, during flash butt welding, the voltage between the end faces of the pair of rails being joined increases, promoting arc generation. As a result, the formation of oxides in the HAZ of the pair of rails being joined is suppressed. Consequently, the bending strength of the rail welded joint produced by flash butt welding increases. If the Sn content is less than 0.0020%, the above effect cannot be sufficiently obtained. On the other hand, if the Sn content is 0.0200% or more, the amount of Sn oxides produced during flash butt welding increases. In this case, the bending strength of the rail welded joint produced by flash butt welding decreases. Therefore, the Sn content is between 0.0020% and less than 0.0200%. The preferred lower limit of the Sn content is 0.0024%, more preferably 0.0026%, and even more preferably 0.0030%. The preferred upper limit for the Sn content is 0.0180%, more preferably 0.0160%, more preferably 0.0140%, and still more preferably 0.0100%.

[0061] O: 0.0040% or less. Oxygen (O) is an impurity. The O content may be 0%. If the O content exceeds 0.0040%, the formation of Sn oxide is promoted during flash butt welding. As a result, the bending strength of rail welded joints produced by flash butt welding decreases. Therefore, the O content is 0.0040% or less. It is preferable that the O content be as low as possible. However, excessive reduction of the O content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is greater than 0%, more preferably 0.0001%, more preferably 0.0003%, and more preferably 0.0005%. The preferred upper limit of the O content is 0.0035%, more preferably 0.0030%, and more preferably 0.0025%.

[0062] P: 0.025% or less. Phosphorus (P) is an impurity. The P content may be 0%. If the P content exceeds 0.025%, the perlite becomes excessively brittle, and the fracture resistance of the rail decreases. Therefore, the P content is 0.025% or less. It is preferable to have as low a P content as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, and more preferably 0.005%. The preferred upper limit of the P content is 0.020%, more preferably 0.015%, and more preferably 0.012%.

[0063] S: 0.025% or less. Sulfur (S) is an impurity. The S content may be 0%. If the S content exceeds 0.025%, coarse Mn sulfides will be formed. Stress concentration is likely to occur around coarse Mn sulfides. Therefore, the fracture resistance of the rail decreases. For this reason, the S content should be 0.025% or less. It is preferable to have as low an S content as possible. However, an extreme reduction in the S content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, and more preferably 0.005%. The preferred upper limit of the S content is 0.020%, more preferably 0.015%, and more preferably 0.012%.

[0064] Al: 1.0000% or less. Aluminum (Al) is inevitably present. In other words, the Al content is greater than 0%. Al deoxidizes steel. Al also increases the amount of eutectoid carbon and the eutectoid transformation temperature. Therefore, Al suppresses the formation of proutetic cementite, which reduces toughness, and increases the hardness of pearlite. As a result, the fracture resistance of the top surface 10A of the rail is increased. Even if only a small amount of Al is present, the above effects can be obtained to some extent. However, if the Al content exceeds 1.0000%, it becomes difficult to solid-solve Al in the steel. As a result, coarse Al-based inclusions are formed. Coarse Al-based inclusions become the initiation points for fatigue cracks. As a result, the damage resistance of the top surface 10A of the rail decreases. If the Al content exceeds 1.0000%, oxides are also formed during rail welding. In this case, the weldability of the rail decreases. Therefore, the Al content is 1.0000% or less. The preferred lower limit of the Al content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The preferred upper limit of the Al content is 0.5000%, more preferably 0.1000%, more preferably 0.0800%, more preferably 0.0500%, more preferably 0.0200%, more preferably 0.0100%, more preferably 0.0080%, and even more preferably 0.0050%.

[0065] N: 0.0020 to 0.0200% Nitrogen (N) forms nitrides, suppressing the coarsening of austenite grains. This refines the pearlite block. As a result, the toughness of the top surface of the rail is increased. If the N content is less than 0.0020%, the above effect cannot be sufficiently obtained. On the other hand, if the N content exceeds 0.0200%, it becomes difficult to solid-solve N in the steel. As a result, bubbles that become the starting point for fatigue damage are more likely to form. Therefore, the N content is 0.0020 to 0.0200%. The preferred lower limit of the N content is 0.0030%, more preferably 0.0040%, more preferably 0.0050%, and still more preferably 0.0060%. The preferred upper limit of the N content is 0.0180%, more preferably 0.0150%, and still more preferably 0.0120%.

[0066] The remainder of the chemical composition of the rail in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of the rail, and are acceptable as long as they do not adversely affect the rail in this embodiment.

[0067] [Optional Elements] The chemical composition of the rail in this embodiment may further include, in place of a portion of Fe, one or more elements selected from the group consisting of Cr: 0-1.00%, Mo: 0-0.50%, Co: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0.200%, Nb: 0-0.0500%, Ti: 0-0.0500%, B: 0-0.0050%, Mg: 0-0.0200%, Ca: 0-0.0200%, rare earth elements: 0-0.0500%, Zr: 0-0.0200%, and Pb: 0-0.0040%. All of these elements are optional and may not be included. Each element will be described below.

[0068] [Group 1: Cr, Mo, Co, Cu, Ni, V, Nb, and Ti] The chemical composition of rail 1 in this embodiment may further include one or more elements selected from the group consisting of Cr, Mo, Co, Cu, Ni, V, Nb, and Ti, in place of a portion of Fe. All of these elements enhance the wear resistance of the top surface of the rail. Each element will be described below.

[0069] Cr: 0-1.00% Chromium (Cr) is an optional element and may not be present. In other words, the Cr content may be 0%. If Cr is present, that is, if the Cr content is greater than 0%, Cr increases the equilibrium transformation temperature of the steel constituting the rail, increasing the degree of supercooling. This refines the lamellar spacing of the pearlite and increases the hardness of the pearlite. As a result, the wear resistance of the top surface of the rail is increased. Even if only a small amount of Cr is present, the above effect can be obtained to some extent. However, if the Cr content exceeds 1.00%, the hardenability of the steel constituting the rail becomes excessively high. In this case, bainite and / or martensite will form on the top surface of the rail. As a result, the wear resistance and fracture resistance of the rail will decrease. Therefore, the Cr content is 0-1.00%. The preferred lower limit of the Cr content is 0.01%, more preferably 0.02%, more preferably 0.03%, more preferably 0.04%, more preferably 0.06%, more preferably 0.08%, and more preferably 0.10%. The preferred upper limit of the Cr content is 0.95%, more preferably 0.90%, and more preferably 0.80%.

[0070] Mo: 0-0.50% Molybdenum (Mo) is an optional element and may not be present. In other words, the Mo content may be 0%. If present, i.e., if the Mo content is greater than 0%, Mo increases the equilibrium transformation temperature of the steel constituting the rail and increases the degree of supercooling. This refines the lamellar spacing of the pearlite and increases the hardness of the pearlite. As a result, the wear resistance of the top surface of the rail is increased. Even if only a small amount of Mo is present, the above effect can be obtained to some extent. However, if the Mo content exceeds 0.50%, the transformation rate decreases significantly. As a result, martensite is formed on the top surface of the rail. As a result, the fracture resistance of the top surface of the rail decreases. Therefore, the Mo content is 0-0.50%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.02%, more preferably 0.05%, and still more preferably 0.10%. The preferred upper limit for the Mo content is 0.45%, more preferably 0.40%, and even more preferably 0.30%.

[0071] Co: 0-1.00% Cobalt (Co) is an optional element and may not be included. In other words, the Co content may be 0%. If it is included, that is, if the Co content is greater than 0%, the Co will dissolve in the ferrite in the pearlite. This will refine the lamellar structure of the pearlite on the surface of the rail, which deforms due to contact with the wheels. This increases the hardness of the rail surface and improves the wear resistance of the top surface of the rail. Even if only a small amount of Co is included, the above effect can be obtained to some extent. However, if the Co content exceeds 1.00%, sufficient refinement of the lamellar structure corresponding to the Co content cannot be obtained. Furthermore, the manufacturing cost will increase. Therefore, the Co content is 0-1.00%. The preferred lower limit of the Co content is 0.01%, more preferably 0.02%, more preferably 0.05%, and still more preferably 0.10%. The preferred upper limit for the Co content is 0.90%, more preferably 0.80%, and even more preferably 0.60%.

[0072] Cu: 0-1.00% Copper (Cu) is an optional element and may not be included. In other words, the Cu content may be 0%. If it is included, that is, if the Cu content is greater than 0%, the Cu will dissolve in the ferrite in the pearlite, and solid solution strengthening will increase the hardness of the top surface of the rail. As a result, the wear resistance of the top surface of the rail will increase. Even if only a small amount of Cu is included, the above effect can be obtained to some extent. However, if the Cu content exceeds 1.00%, the hardenability of the steel constituting the rail will increase excessively. As a result, martensite will form on the top surface of the rail. As a result, the fracture resistance of the top surface of rail 1 will decrease. Therefore, the Cu content is 0-1.00%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.02%, more preferably 0.05%, and still more preferably 0.10%. The preferred upper limit for the Cu content is 0.90%, more preferably 0.80%, and even more preferably 0.70%. Furthermore, if the hardness of the rail head is sufficiently increased and the formation of martensite is suppressed, the even more preferred upper limit for the Cu content is 0.40%.

[0073] Ni: 0-1.00% Nickel (Ni) is an optional element and may not be included. In other words, the Ni content may be 0%. If Ni is included, that is, if the Ni content is greater than 0%, Ni increases the toughness of pearlite. Ni further increases the hardness of the top surface of rail 1 through solid solution strengthening. As a result, the wear resistance of the top surface of the rail is increased. Even if only a small amount of Ni is included, the above effects can be obtained to some extent. However, if the Ni content exceeds 1.00%, the hardenability of the steel constituting the rail becomes excessively high. As a result, martensite is formed on the top surface of the rail. As a result, the wear resistance and fracture resistance of the top surface of the rail are reduced. Therefore, the Ni content is 0-1.00%. The preferred lower limit of the Ni content is 0.01%, more preferably 0.02%, more preferably 0.05%, and still more preferably 0.10%. The preferred upper limit for the Ni content is 0.90%, more preferably 0.80%, and even more preferably 0.70%.

[0074] V: 0-0.200% Vanadium (V) is an optional element and may not be present. In other words, the V content may be 0%. If V is present, that is, if the V content is greater than 0%, V forms V precipitates such as V carbides, V nitrides, and V carbonitrides during the cooling process after hot rolling in the rail manufacturing process. The hardness of the pearlite increases due to the precipitation hardening caused by these V precipitates. As a result, the damage resistance of the top surface of the rail is increased. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.200%, fine V precipitates are excessively generated. In this case, the pearlite becomes brittle. As a result, the damage resistance of the top surface of the rail decreases. Therefore, the V content is 0-0.200%. The preferred lower limit of the V content is 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.020%, more preferably 0.030%, and more preferably 0.050%. The preferred upper limit of the V content is 0.180%, more preferably 0.150%, more preferably 0.120%, more preferably 0.100%, and more preferably 0.080%.

[0075] Nb: 0-0.0500% Niobium (Nb) is an optional element and may not be present. In other words, the Nb content may be 0%. If it is present, that is, if the Nb content is greater than 0%, Nb forms Nb precipitates such as Nb carbides and Nb nitrides during the cooling process after hot rolling in the rail manufacturing process. The hardness of the perlite increases due to the precipitation hardening caused by these Nb precipitates. As a result, the damage resistance of the top surface of the rail is increased. Nb is further A ｃ１Nb precipitates form in the heat-affected zone (HAZ) of the weld, which is reheated to a temperature range below 0.5°C. This suppresses the softening of the heat-affected zone (HAZ) of the welded joint. The above effect can be obtained to some extent even if only a small amount of Nb is present. However, if the Nb content exceeds 0.0500%, excessive Nb precipitates will be formed. In this case, the pearlite becomes brittle, and the damage resistance of the top surface of the rail decreases. Therefore, the Nb content is 0 to 0.0500%. The preferred lower limit of the Nb content is 0.0001%, more preferably 0.0010%, more preferably 0.0020%, more preferably 0.0025%, and more preferably 0.0030%. The preferred upper limit of the Nb content is 0.0400%, more preferably 0.0300%, and more preferably 0.0200%.

[0076] Ti: 0-0.0500% Titanium (Ti) is an optional element and may not be present. In other words, the Ti content may be 0%. If Ti is present, that is, if the Ti content is greater than 0%, Ti forms Ti precipitates such as Ti carbides and Ti nitrides during the cooling process after hot rolling in the rail manufacturing process. The hardness of the pearlite increases due to precipitation hardening by these Ti precipitates. As a result, the damage resistance of the top surface of the rail is increased. Furthermore, the precipitated Ti precipitates are less likely to dissolve in the matrix during reheating during welding. Therefore, the Ti precipitates refine the structure of the heat-affected zone (HAZ) of the weld, which is heated to the austenite region. As a result, embrittlement of the welded joint is suppressed. The above effects can be obtained to some extent even if only a small amount of Ti is present. However, if the Ti content exceeds 0.0500%, coarse Ti precipitates will be generated. In this case, stress concentration around coarse Ti precipitates makes fatigue cracks more likely to form. As a result, the damage resistance of the top surface of the rail decreases. Therefore, the Ti content is 0 to 0.0500%. The preferred lower limit of the Ti content is 0.0001%, more preferably 0.0010%, more preferably 0.0020%, more preferably 0.0030%, more preferably 0.0035%, and more preferably 0.0040%. The preferred upper limit of the Ti content is 0.0450%, more preferably 0.0400%, more preferably 0.0300%, and more preferably 0.0200%.

[0077] [Group 2: B] The chemical composition of the rail in this embodiment may further contain B instead of some of the Fe.

[0078] B: 0-0.0050% Boron (B) is an optional element and may not be present. In other words, the B content may be 0%. If it is present, that is, if the B content is greater than 0%, B is present in the austenite grain boundaries as iron carbon boride (Fe ２３ (CB) ６) forms, promoting pearlite transformation. This reduces the cooling rate dependence of the pearlite transformation temperature. As a result, the hardness gradient inside the top surface of the rail decreases. Consequently, the generation of crack damage on the top surface of the rail caused by contact with the wheel is suppressed. The above effect can be obtained to some extent even if only a small amount of B is included. However, if the B content exceeds 0.0050%, coarse iron carbon borides are formed. In this case, brittle fracture is promoted, and the fracture resistance of the top surface of the rail decreases. Therefore, the B content is 0 to 0.0050%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0002%, more preferably 0.0005%, and still more preferably 0.0010%. The preferred upper limit of the B content is 0.0040%, more preferably 0.0030%, and still more preferably 0.0020%.

[0079] [Group 3: Mg, Ca, and Rare Earth Elements (REM)] The chemical composition of the rail in this embodiment may further contain one or more elements selected from the group consisting of Mg, Ca, and rare earth elements (REM) in place of a portion of Fe. All of these elements enhance the damage resistance of the top surface of the rail. Each element will be described below.

[0080] Mg: 0-0.0200% Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content may be 0%. If it is included, that is, if the Mg content is greater than 0%, Mg combines with S to form fine sulfides (MgS). This MgS finely disperses MnS. Therefore, it alleviates stress concentration around MnS. As a result, the damage resistance of the rail head surface is increased. Even if only a small amount of Mg is included, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0200%, coarse Mg oxides are formed. In this case, stress concentration around the coarse Mg oxides makes fatigue cracks more likely to form. As a result, the damage resistance of the rail head surface decreases. Therefore, the Mg content is 0-0.0200%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0005%, more preferably 0.0010%, more preferably 0.0015%, and more preferably 0.0030%. The preferred upper limit of the Mg content is 0.0180%, more preferably 0.0150%, and more preferably 0.0120%.

[0081] Ca: 0-0.0200% Calcium (Ca) is an optional element and does not need to be included. In other words, the Ca content may be 0%. If it is included, that is, if the Ca content is greater than 0%, Ca combines with S to form a sulfide (CaS). This CaS finely disperses MnS. Therefore, it alleviates stress concentration around MnS. As a result, the damage resistance of the rail head surface is increased. Even if only a small amount of Ca is included, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0200%, coarse Ca oxides are formed. In this case, stress concentration around the coarse Ca oxides makes fatigue cracks more likely to form. As a result, the damage resistance of the rail head surface decreases. Therefore, the Ca content is 0-0.0200%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0005%, more preferably 0.0010%, more preferably 0.0020%, and more preferably 0.0030%. The preferred upper limit of the Ca content is 0.0180%, more preferably 0.0150%, and more preferably 0.0120%.

[0082] Rare Earth Elements (REM): 0-0.0500% Rare earth elements (REM) are optional elements and do not need to be included. In other words, the REM content may be 0%. If they are included, that is, if the REM content is greater than 0%, REM is oxysulfide (REM ２ O ２REM generates MnS, which acts as a nucleus for the formation of Mn sulfide inclusions. Oxysulfide has a high melting point. Therefore, it suppresses the elongation of Mn sulfide inclusions after hot rolling. As a result, REM finely disperses MnS, easing stress concentration around MnS. Consequently, the damage resistance of the rail head surface is increased. The above effect can be obtained to some extent even with a small amount of REM. However, if the REM content exceeds 0.0500%, coarse REM oxysulfide is generated. In this case, stress concentration around the coarse REM oxysulfide makes fatigue cracks more likely to form. Consequently, the damage resistance of the rail head surface decreases. Therefore, the REM content is between 0 and 0.0500%. The preferred lower limit of the REM content is 0.0001%, more preferably 0.0005%, more preferably 0.0010%, more preferably 0.0020%, and more preferably 0.0030%. The preferred upper limit of the REM content is 0.0400%, more preferably 0.0300%, and more preferably 0.0250%.

[0083] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and lanthanides, lanthanum (La), atomic number 57 to lutetium (Lu), atomic number 71. In this specification, REM content refers to the total content of these elements.

[0084] [Group 4: Zr] The chemical composition of the rail in this embodiment may further include Zr in place of a portion of Fe.

[0085] Zr: 0-0.0200% Zirconium (Zr) is an optional element and may not be present. In other words, the Zr content may be 0%. If it is present, that is, if the Zr content is greater than 0%, Zr is ZrO ２ It generates inclusions. ZrO ２Zr exhibits high lattice compatibility with austenite (γ-Fe). Therefore, γ-Fe acts as a solidification nucleus for the steel constituting the rail, which is the primary crystal of solidification, increasing the equiaxed crystallization rate of the solidification structure. This suppresses the formation of segregation zones in the center of the cast slab. Consequently, the formation of martensite on the top surface of the rail is suppressed. As a result, the fracture resistance of the top surface of the rail is increased. Even a small amount of Zr can provide some of the above effects. However, if the Zr content exceeds 0.0200%, excessive amounts of coarse Zr-based inclusions are formed. In this case, stress concentration around the coarse Zr-based inclusions makes fatigue cracks more likely to form. As a result, the damage resistance of the top surface of the rail decreases. Therefore, the Zr content should be between 0 and 0.0200%, and if present, it should be 0.0200% or less. The preferred lower limit of the Zr content is 0.0001%, more preferably 0.0010%, more preferably 0.0020%, and still more preferably 0.0030%. The preferred upper limit of the Zr content is 0.0180%, more preferably 0.0150%, and still more preferably 0.0120%.

[0086] [Group 5: Pb] The chemical composition of the rail in this embodiment may further contain Pb in place of a portion of Fe.

[0087] Pb: 0 to 0.0040% Lead (Pb) is an optional element and may not be present. In other words, the Pb content may be 0%. If Pb is present, that is, if the Pb content is greater than 0%, Pb improves the machinability of the steel material used for the rails. Even if only a small amount of Pb is present, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.0040%, coarse Pb inclusions will be formed. In this case, stress concentration around the coarse Pb inclusions makes it easier for fatigue cracks to form. As a result, the damage resistance of the top surface of the rail decreases. Therefore, the Pb content is 0 to 0.0040%. The preferred lower limit of the Pb content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0005%. The preferred upper limit of the Pb content is 0.0038%, more preferably 0.0036%, and still more preferably 0.0034%.

[0088] [Regarding (Feature 2) Equation (1)] The chemical composition of rail 1 in this embodiment further satisfies equation (1): 0.05 × 10 －５ ≦Sn×(O / 4)≦1.50×10 －５ (1) Here, Sn and O in equation (1) are substituted with the mass percentage content of the corresponding elements in the chemical composition of rail 1.

[0089] F1 is defined as follows: F1 = Sn × (0 / 4) F1 is an index of the bending strength of rail welded joints manufactured by flash butt welding. F1 = 0.05 × 10 －５ If it becomes less than 1.50 x 10 －５ If the Sn content in the rail exceeds 0, the Sn content is not appropriate in relation to the O content. As a result, excess Sn oxide remains in the weld during flash butt welding. Consequently, stable bending strength cannot be obtained in the rail welded joint after flash butt welding. When F1 satisfies equation (1), that is, when F1 is 0.05 × 10 －５ ~1.50 x 10 －５ Therefore, the amount of Sn oxide remaining in the HAZ during flash butt welding can be sufficiently reduced. As a result, the bending strength of rail welded joints produced by flash butt welding is stabilized. The preferred lower limit of F1 is 0.06 × 10 －５ And more preferably 0.07 × 10 －５ And more preferably 0.08 × 10 －５ And more preferably 0.10 × 10 －５ Therefore, the preferred upper limit for F1 is 1.48 × 10⁻⁶. －５ And more preferably 1.45 × 10 －５ And more preferably 1.40 × 10 －５ And more preferably 1.35 × 10 －５ Therefore, 1.30 × 10 －５ That is the case.

[0090] [(Feature 3) Regarding the metallic structure of the top surface] In the rail of this embodiment, the area ratio of perlite in the metallic structure of the top surface up to a depth of 20 mm from the outer surface of the rail head is 95% or more.

[0091] [Regarding the Metallurgical Structure of the Rail Head] In the metallurgical structure of steel materials of the same hardness, pearlite has the highest wear resistance. In other words, pearlite has superior wear resistance to ferrite, cementite, bainite, and martensite. Therefore, the metallurgical structure of the rail head of this embodiment is substantially pearlite. Specifically, the area ratio of pearlite in the metallurgical structure of the rail head is 95% or more. The remaining structure of the metallurgical structure of the rail head, other than pearlite, consists of one or more selected from the group consisting of ferrite, cementite, bainite, and martensite. A preferred pearlite area ratio is 98% or more, and most preferably 100%.

[0092] Furthermore, the microstructure of parts of the rail other than the top surface is not particularly limited. The microstructure of parts other than the top surface may consist of ferrite and pearlite, or it may be a ferrite structure or a bainite structure. Considering continuity with the microstructure of the top surface, the preferred microstructure of parts other than the top surface is one consisting of ferrite and pearlite.

[0093] [Method for Observing the Metallographic Structure of the Top Surface of the Rail] The pearlite area ratio of the top surface of the rail is determined by the following method. A sample is taken from the top surface of the rail 1, including a section perpendicular to the longitudinal direction of the rail 1, and including a depth of 10 mm from the surface of the top surface. The section of the sample taken perpendicular to the longitudinal direction of the rail 1 is defined as the observation surface. The observation surface of the sample is mirror polished. The mirror-polished observation surface is etched using 3% nitric acid alcohol (Nital etching solution). The etched observation surface is observed at a depth of 10 mm from the surface of the top surface using an optical microscope at a magnification of 200x. A photograph is taken, and the observation field of view (400 μm × 300 μm) is used as the evaluation area.

[0094] In the observation field, perlite and other tissues (ferrite, cementite, bainite, martensite, etc.) can be easily distinguished by contrast. At 200x magnification, perlite is observed as a gray color. When the gray tissue (i.e., perlite) is observed with a scanning electron microscope at 1000x magnification, lamellar structure is observed. Perlite is identified based on the contrast described above. Then, the area percentage (%) of perlite is calculated based on the total area of ​​the identified perlite and the total area of ​​the observation field. Alternatively, the area percentage (%) of perlite may be calculated using image analysis software.

[0095] Samples are taken from any 10 locations on the top surface of the rail, and the perlite area ratio is determined for each sample. The arithmetic mean of the perlite area ratios from the 10 locations is defined as the perlite area ratio (%) on the top surface of the rail.

[0096] [(Feature 4) Vickers hardness of the top surface of the rail] The Vickers hardness of the top surface of the rail is 230 HV or higher. If the Vickers hardness of the top surface of the rail is less than 230 HV, sufficient wear resistance and damage resistance cannot be obtained on the top surface of the rail. Therefore, the Vickers hardness of the top surface of the rail is 230 HV or higher.

[0097] The preferred lower limit of the Vickers hardness of the crown surface is 240 HV, more preferably 250 HV, and even more preferably 260 HV. The upper limit of the Vickers hardness of the crown surface is not particularly limited, but for example it is 550 HV, preferably 520 HV, and even more preferably 500 HV.

[0098] [Method for measuring Vickers hardness of the head surface] The Vickers hardness of the head surface is determined by the following method. Specifically, a test piece is taken from the rail with a cross section perpendicular to the longitudinal direction of the rail as the measurement surface. Referring to Figure 1F, on the measurement surface, the hardness measurement line L11 is defined as a line segment extending in the depth direction from the center of the width of the crown 11 of the head outer surface 10S. The hardness measurement line L11 is coaxial with the normal to the head outer surface 10S at position P11. Along the hardness measurement line L11, measurement points are set at a depth of 1 mm, a depth of 10 mm, and a depth of 20 mm from position P11 in the depth direction.

[0099] Furthermore, the line segment L13 is tangent to the lower jaw 13 on the measuring surface of the rail 1. Ｌ and L13 Ｒ Set it. Then, line segment L13 Ｌ and L13 Ｒ Let the intersection of these lines be position P13. Position P13 is located on the hardness measurement line L11.

[0100] At the head corner portion 12L of the measuring surface, the line segment connecting point E and point P13 is defined as the hardness measurement line L12 Ｌ Let's assume that point E is the hardness measurement line L12. Ｌ Along the depth direction, measurement points are set at a depth of 1 mm, a depth of 10 mm, and a depth of 20 mm. Similarly, at the head corner portion 12R, the line segment connecting point F and point P13 is set as the hardness measurement line L12. Ｒ Let's assume that point F is measured from the hardness measurement line L12. Ｒ Measurement points are set at depths of 1 mm, 10 mm, and 20 mm along the depth direction. At each measurement point, a Vickers hardness test is performed in accordance with JIS Z 2244-1:2024 to obtain the Vickers hardness (HV). The test force is set to 10 kgf.

[0101] Three hardness measurement lines L11, L12 Ｌ , L12 ＲThe arithmetic mean of the three measurements taken at the same depth is taken as the hardness at that depth. Specifically, the arithmetic mean of the three hardnesses at a depth of 1 mm on hardness measurement lines L11, L12L, and L12R is taken as the Vickers hardness H1 (HV) at a depth of 1 mm. Hardness measurement lines L11, L12 Ｌ , L12 Ｒ The arithmetic mean of the three hardnesses at a depth of 10 mm is defined as the Vickers hardness H10 (HV) at a depth of 1 mm. Hardness measurement lines L11, L12 Ｌ , L12 Ｒ The arithmetic mean of the three hardnesses at a depth of 20 mm is defined as the Vickers hardness H20 (HV) at a depth of 1 mm.

[0102] The minimum value among the obtained Vickers hardness values ​​H1 at a depth of 1 mm, H10 at a depth of 10 mm, and H20 at a depth of 20 mm is defined as the Vickers hardness of the top surface portion 10A.

[0103] [Effects of Rail 1 in this Embodiment] Rail 1 in this embodiment satisfies features 1 to 4. Therefore, in Rail 1 in this embodiment, the bending strength is stabilized in the welded portion of the welded joint after flash butt welding.

[0104] [Preferred form of rail 1 in this embodiment 1] Preferably, rail 1 in this embodiment further satisfies the following feature 5. (Feature 5) The degree of positive segregation of Sn at the bottom of the rail is 10.0 or less. Feature 5 will be described below.

[0105] [(Feature 5) Regarding the degree of positive Sn segregation at the bottom of the rail] As described below, rails are manufactured using bloom as the material. Bloom is manufactured by continuous casting. Due to solidification shrinkage and bulging in continuous casting, central segregation is generated in the center of the manufactured bloom, and V-segregation is generated in the area surrounding the central segregation. In order to suppress central segregation, light reduction may be performed on the cast slab during continuous casting. When light reduction is performed, axial cracks may occur in the bloom. Since Sn is an element that easily segregates, Sn is concentrated in the unsolidified molten steel during continuous casting. Therefore, Sn is easily concentrated in central segregation and V-segregation. Furthermore, Sn-enriched molten steel is easily filled into fine cracks caused by axial cracks. Therefore, Sn segregates in the center of the bloom, and V-segregation and segregation originating from axial cracks can also occur in the surrounding area other than the center of the bloom. In this specification, positive segregation of Sn formed in the peripheral region other than the center of the bloom is referred to as semi-macro Sn positive segregation.

[0106] When manufacturing rails by hot-rolling a bloom containing semi-macro Sn positive segregation, semi-macro Sn positive segregation may remain at the bottom of the rail. Reducing semi-macro Sn positive segregation at the bottom of the rail can significantly suppress the formation of Sn oxides during flash butt welding. As a result, the bending strength of rail welded joints manufactured by flash butt welding is further stabilized.

[0107] If the degree of positive Sn segregation at the bottom of the rail is 10.0 or less, semi-macro positive Sn segregation at the bottom of the rail is sufficiently reduced. Therefore, the amount of residual Sn oxide in the HAZ of rail welded joints manufactured by flash butt welding is sufficiently suppressed. As a result, the bending strength of rail welded joints manufactured by flash butt welding is further stabilized.

[0108] The preferred upper limit for the Sn positive segregation at the bottom of the rail is 9.5, more preferably 9.0, even more preferably 8.5, and still more preferably 8.0. The lower limit for the Sn positive segregation at the bottom of the rail is not particularly limited. However, excessive reduction of the Sn positive segregation increases manufacturing costs. Therefore, considering manufacturing costs, the preferred lower limit for the Sn positive segregation is 2.0, more preferably 2.2, even more preferably 2.4, and still more preferably 2.6.

[0109] [Method for measuring the degree of positive Sn segregation at the bottom of the rail] The degree of positive Sn segregation at the bottom of the rail is determined by the following method. As shown in Figure 11A, a test piece is taken that includes a cross section perpendicular to the longitudinal direction (L direction) of the bottom of the rail 15 and has a length of 10 mm in the longitudinal direction of the rail 1. Of the test piece, the cross section perpendicular to the longitudinal direction of the bottom of the rail 15 is used as the observation surface.

[0110] On the observation surface, let W (mm) be the width of the bottom surface 15S of the rail bottom 15. From one endpoint of the bottom surface 15S (the left end face of the bottom surface 15S in Figure 11A), position P is W / 8 in the width direction of the bottom surface 15S. Ｗ／８ , 2W / 8 position P ２Ｗ／８ , 6W / 8 position P ６Ｗ／８ , and 7W / 8 position P ７Ｗ／８ This will be the point of analysis.

[0111] As shown in Figure 11B, analysis regions AR1 to AR5 are selected in a plane that includes each analysis point and extends in the longitudinal direction (L direction) and height direction (H direction) of the rail, each being a square with dimensions of 800 μm in the longitudinal direction of the rail and 800 μm in the height direction of the rail.

[0112] Specifically, analysis point P Ｗ／８ A rectangular area is selected that includes the rail 1, with a length of 800 μm and a height of 4000 μm. The selected area is divided into five equal parts in the height direction to define the analysis areas AR1 to AR5. Similarly, analysis point P ２Ｗ／８ A rectangular area is selected that includes the rail 1, with a length of 800 μm and a height of 4000 μm. The selected area is divided into five equal parts in the height direction to partition the analysis areas AR1 to AR5. Analysis point P ６Ｗ／８A rectangular area is selected that includes the rail 1, with a length of 800 μm and a height of 4000 μm. The selected area is divided into five equal parts in the height direction to partition the analysis areas AR1 to AR5. Analysis point P ７Ｗ／８ A rectangular area is selected that includes the rail 1, with a length of 800 μm and a height of 4000 μm. The selected area is divided into five equal parts in the height direction to partition the analysis areas AR1 to AR5.

[0113] In the plane containing each analysis point, the analysis regions AR1 to AR5 are arranged continuously in the height direction of rail 1, and the opposite sides of adjacent analysis regions ARn (where n is a natural number from 1 to 5) are touching each other.

[0114] For each analysis area of ​​ARn, surface analysis is performed using a Wavelength-Dispersive X-ray Electron Probe Micro Analyzer (WDS-EPMA) to create an elemental distribution map of Sn concentration (mass%). In the EPMA, the acceleration voltage is set to 15 kV, the irradiation current to 0.05 μA, the irradiation time to 50 msec, and the beam diameter to 2 μm.

[0115] As shown in Figure 11C, in each analysis region ARn, line analysis is performed using raster scanning with scan lines L1 to Li (where i is an integer) parallel to the L direction. The interval between each scan line L1 to Li is 2 μm. For each scan line Li, the Sn concentration at each position in the L direction (Sn concentration distribution in the L direction) is measured. Here, as shown in Figure 11D, the average value of the Sn concentration at each position in the L direction of the five consecutively arranged scan lines is calculated. The Sn concentration distribution consisting of the average Sn concentrations at each position in the L direction is taken as the Sn concentration distribution in the merged line analysis LLj (where j is an integer).

[0116] Figure 12 is a schematic diagram showing the Sn concentration distribution in the L direction for each merge line analysis LLj. Referring to Figure 12, in the Sn concentration distribution DSn of the merge line analysis LLj, the region SE with high Sn concentration corresponds to the semi-macro Sn positive segregation zone. The peak value PSn of Sn concentration is determined from the Sn concentration distribution DSn.

[0117] In each analysis region ARn, the maximum value of the peak Sn concentration PSn from all merge line analyses LLj is selected. The selected maximum value of the peak PSn is taken as the maximum Sn concentration (mass%) in that analysis region ARn.

[0118] The arithmetic mean of the top 10 maximum Sn concentrations from the 20 analysis regions ARn is calculated and taken as the average maximum Sn concentration (mass%). The average maximum Sn concentration is rounded to four decimal places by rounding the fifth decimal place of the obtained value. Furthermore, the arithmetic mean of all Sn concentrations in the Sn concentration distribution obtained from all line analyses in the 20 analysis regions ARn is taken as the average Sn concentration (mass%). The average Sn concentration is rounded to four decimal places by rounding the fifth decimal place of the obtained value. Using the obtained average maximum Sn concentration (mass%) and average Sn concentration (mass%), the degree of positive Sn segregation is calculated using the following formula. The degree of positive Sn segregation is rounded to one decimal place by rounding the second decimal place of the obtained value. Degree of positive Sn segregation = Average maximum Sn concentration / Average Sn concentration

[0119] [Method for Manufacturing Rails of This Embodiment] An example of a method for manufacturing rails of this embodiment will be described. The rail manufacturing method described below is just one example for manufacturing rails of this embodiment. Therefore, rails having the above-described configuration may be manufactured by other manufacturing methods other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a rail manufacturing method of this embodiment.

[0120] An example of the rail manufacturing method of this embodiment includes the following steps: (Step 1) Bloom manufacturing step (Step 2) Hot rolling step (Step 3) Accelerated cooling step In the bloom manufacturing step, a bloom is manufactured. In the hot rolling step, the bloom is hot-rolled to manufacture rail material. In the accelerated cooling step, the rail material after the hot rolling step is accelerated-cooled to manufacture the rail. Each step will be described below.

[0121] [(Process 1) Bloom Manufacturing Process] In the bloom manufacturing process, bloom, which will be the material for the rails of this embodiment, is manufactured. Specifically, molten steel having a chemical composition that satisfies features 1 and 2 is produced by melting in a commonly used melting furnace such as a converter or electric furnace. Bloom is manufactured by continuous casting using the manufactured molten steel. Preferred conditions for continuous casting will be described later.

[0122] [(Process 2) Hot Rolling Process] In the hot rolling process, the bloom is reheated and hot rolling is performed to manufacture the rail material. First, the bloom is placed in a heating furnace and reheated. The reheating temperature HT (°C) is set to 1000 to 1350°C. If the reheating temperature HT is less than 1000°C, the hot workability of the bloom will decrease. As a result, it may not be possible to secure the desired shape in the rail material. On the other hand, if the reheating temperature HT exceeds 1350°C, the bloom will be excessively heated and partially melted. In this case, it will be difficult to make the rail material into the desired shape. Therefore, the reheating temperature HT is 1000 to 1350°C. The preferred temperature for the reheating temperature HT will be described later. The holding time at the reheating temperature HT is not particularly limited, but for example, it is 60 to 200 minutes.

[0123] The heated bloom is extracted from the heating furnace. Hot rolling is performed on the extracted bloom to manufacture rail profiles. In hot rolling, the temperature at which the bloom passes through the final pass is defined as the final rolling temperature FT (°C). The temperature at which it passes through the final pass refers to the surface temperature (°C) of the rail profile when it passes through the rolling stand where the final reduction is performed on the bloom. The final rolling temperature FT (°C) can be measured, for example, by a thermometer installed on the exit side of the rolling stand where the final reduction is performed.

[0124] In this embodiment, the final rolling temperature FT is set to 750 to 1100°C. If the final rolling temperature FT is less than 750°C, pearlite transformation begins immediately after the completion of hot rolling. In this case, the hardness of the top surface of the rail cannot be sufficiently increased in the heat treatment process after rolling. On the other hand, if the final rolling temperature FT exceeds 1100°C, the austenite grains in the rail material after hot rolling become coarser. In this case, the hardenability becomes excessively high. As a result, bainite is excessively generated on the top surface of the rail, and the pearlite area ratio becomes excessively low. Therefore, the final rolling temperature FT is set to 750 to 1100°C. Preferred conditions for the final rolling temperature FT will be described later.

[0125] In the hot rolling process, other conditions may be those that are well known. In the hot rolling process, hole rolling, which is a well known form of rail hot rolling, is performed. Specifically, the bloom is roughly rolled using a first reverse rolling mill to produce intermediate steel material. Subsequently, intermediate rolling is performed on the intermediate steel material using a second reverse rolling mill. After intermediate rolling, finish rolling is performed on the intermediate steel material using a tandem continuous rolling mill for two or more passes. Here, a pass means that the intermediate steel material moves from the inlet side to the outlet side of one rolling stand and is reduced by a pair of work rolls of that rolling stand during the movement.

[0126] [(Step 3) Accelerated Cooling Step] In the accelerated cooling step, the following accelerated cooling is performed on the rail material immediately after the completion of hot rolling. For example, the cooling device 20 shown in Figure 13 is used for accelerated cooling. Referring to Figure 13, the cooling device 20 comprises a plurality of air injection nozzles 21, a supply passage 22, and an air supply device 23. The air injection nozzles 21 inject air (compressed air) onto the top surface portion 10A of the rail material 100. The supply passage 22 supplies air to the plurality of air injection nozzles 21. The air supply device 23 supplies air to the supply passage 22.

[0127] Multiple air injection nozzles 21 are arranged along the entire length of the rail profile 100, in the longitudinal direction of the rail profile 100. Alternatively, the cooling device 20 may not have multiple air injection nozzles 21 arranged along the entire length of the rail profile 100, but rather only on a portion of the rail profile 100. In this case, the cooling device 20 further includes a drive device that moves the rail profile 100 in the longitudinal direction during accelerated cooling. Even in this case, the entire length of the rail profile 100 can be continuously cooled. Multiple air injection nozzles 21 are arranged to surround the rail head 10. The air injection nozzles 21 inject air toward the top surface 10A.

[0128] In accelerated cooling, the cooling device 20 is used to perform accelerated cooling on the top surface portion 10A of the rail material 100. Specifically, accelerated cooling is performed under the following conditions.

[0129] The cooling start temperature should be set between 900 and 750°C. If the cooling start temperature exceeds 900°C, the hardenability of the rail material will increase excessively. In this case, excessive bainite will form on the top surface, and the pearlite area ratio will decrease excessively. On the other hand, if the cooling start temperature is below 750°C, pearlite will form in the high-temperature range before accelerated cooling. As a result, the Vickers hardness on the top surface of the rail will be excessively low. Therefore, the cooling start temperature should be set between 900 and 750°C.

[0130] Furthermore, the accelerated cooling rate of the outer surface of the rail head should be set to 1.0 to 10.0 °C / second. If the accelerated cooling rate is less than 1.0 °C / second, the pearlite transformation temperature will rise. In this case, the Vickers hardness of the rail head surface will decrease excessively. On the other hand, if the accelerated cooling rate exceeds 10.0 °C / second, the accelerated cooling will be excessive. In this case, excessive bainite will be formed on the rail head surface, and the pearlite area ratio will decrease excessively. Therefore, the accelerated cooling rate should be set to 1.0 to 10.0 °C / second.

[0131] The accelerated cooling rate can be calculated using the following formula, based on the time from the start to the end of accelerated cooling by the cooling device, the temperature (°C) of the top surface of the rail material at the start of accelerated cooling, and the temperature (°C) of the top surface of the rail material at the end of accelerated cooling: Temperature difference (temperature of the top surface of the rail material at the start of accelerated cooling - temperature of the top surface of the rail material at the end of accelerated cooling) / time from the start to the end of accelerated cooling

[0132] The end temperature of accelerated cooling should be 550 to 450°C. If the end temperature of accelerated cooling is 550 to 450°C, the pearlite area ratio in the microstructure of the rail head can be 95% or more, provided that other conditions are met.

[0133] Through the above manufacturing process, a rail 1 that satisfies features 1 to 4 is produced.

[0134] [Preferred Conditions] Preferably, the above manufacturing process satisfies the following preferred conditions 1 to 4. (Preferred Condition 1) In continuous casting, light reduction is started when the central solid fraction of the bloom is 10 to 25%. (Preferred Condition 2) In continuous casting, light reduction is ended when the central solid fraction of the bloom exceeds 70%. (Preferred Condition 3) In the hot rolling process, the reheating temperature HT is set to 1250 to 1350°C. (Preferred Condition 4) In the hot rolling process, the final rolling temperature FT is set to 900 to 1100°C. Preferred conditions 1 to 4 will be explained below.

[0135] [(Preferred Condition 1) Regarding the central solid phase ratio at the start of light reduction] The degree of Sn positive segregation correlates with the central solid phase ratio of the bloom during light reduction in continuous casting during the bloom manufacturing process. Specifically, during continuous casting, the bloom immediately after coming out of the mold is solidified only on the surface, with the interior still molten. As the bloom moves from the upstream to the downstream of the continuous casting apparatus, the internal solidification of the bloom progresses, and the central solid phase ratio increases. Here, the central solid phase ratio refers to the solidification ratio in the area of ​​the center of the bloom with a height of 20 mm and a width of 20 mm in a cross-section perpendicular to the longitudinal direction of the bloom (hereinafter also referred to as the bloom center). When the entire center of the bloom is solidified, the central solid phase ratio of the bloom is 100%. The cross-section perpendicular to the longitudinal direction of the bloom should be in the range of a height of 300 to 450 mm and a width of 350 to 500 mm.

[0136] If the bloom is lightly reduced when the central solid fraction is low, that is, when internal solidification of the bloom has not progressed sufficiently, semi-macro Sn positive segregation in the bloom will be suppressed. If the central solid fraction at the start of light reduction exceeds 25%, the reduction will not sufficiently penetrate into the interior of the bloom even if light reduction is performed. Therefore, the degree of Sn positive segregation at the bottom 15 of the rail when the rail 1 was manufactured will not be sufficiently reduced.

[0137] On the other hand, if the central solid fraction at the start of light reduction is less than 10%, the bloom has not solidified sufficiently. Even if light reduction is applied to a bloom in this state, the reduction does not penetrate into the interior of the bloom. Therefore, the degree of Sn positive segregation at the bottom of the rail of the manufactured rail is not sufficiently reduced.

[0138] Therefore, preferably, the central solid phase ratio at the start of light reduction is 10 to 25%. A more preferable upper limit for the central solid phase ratio at the start of light reduction is 20%.

[0139] [(Preferred condition 2) Regarding the central solid phase ratio at the end of light reduction] Preferably, light reduction is further terminated when the central solid phase ratio of the bloom exceeds 70%. If light reduction is terminated when the central solid phase ratio is 70% or less, the reduction will not penetrate sufficiently to the center of the bloom. As a result, V segregation may not be sufficiently suppressed, and the degree of Sn positive segregation at the bottom of the rail may not be sufficiently reduced.

[0140] If the central solid fraction at the end of light reduction exceeds 70%, the reduction penetrates sufficiently to the center of the bloom. As a result, V segregation is sufficiently suppressed, and the degree of Sn positive segregation is sufficiently reduced. The preferred lower limit for the central solid fraction at the end of light reduction is over 80%. The preferred upper limit for the central solid fraction at the end of light reduction is 99%. In this case, the formation of fine cracks (axial cracks) in the bloom associated with light reduction is suppressed, and the degree of Sn positive segregation is reduced.

[0141] Note that the reduction ratio under light reduction refers to the ratio (%) of the amount of reduction under light reduction to the height (thickness) of the bloom before the start of light reduction. Reduction ratio under light reduction (%) = Amount of reduction under light reduction / Bloom thickness before the start of light reduction × 100

[0142] For light reduction, the reduction ratio shall be 1-3% if the central solid fraction is 70% or less. If the central solid fraction exceeds 70%, the reduction ratio for light reduction shall be 2-5%.

[0143] The central solid fraction of the bloom is determined by the following method: The temperature at the center of the bloom in the thickness direction during continuous casting is determined by a one-dimensional heat transfer and solidification calculation. The enthalpy method is used in the heat transfer and solidification calculation.

[0144] Specifically, the bloom is divided into N elements in the thickness direction. Let N = 1000. The initial temperature of all elements is the molten steel temperature (°C) at the time it is poured from the tundish into the mold. At each time step, the solid fraction and enthalpy of each element are calculated. Here, the time step Δt is 1 second.

[0145] The solid fraction FSni of element i (i=1 to N) at the nth time step is given by the following equation (A). Here, TC ｎ ｉ is the temperature (°C) of element i at the nth time step.

[0146] Note that TS is the solidus temperature, TL is the liquidus temperature, and TC ｎ ｉ The following equation is satisfied: TS ≤ TC ｎ ｉ ≦TL

[0147] Also, the enthalpy H of element i at the n-th time step ｎ ｉ is obtained based on the following formula (B). H ｎ ｉ = ρ × c × (TC ｎ ｉ - Tref) + ρ × L × FS ｎ ｉ (B) In formula (B), ρ is density, c is specific heat, Tref is reference temperature, and L is latent heat. Here, let the density ρ be 7800 kg / m ３ , the specific heat c be 700 J / (kg·K), the reference temperature Tref be 0 °C, and the latent heat L be 250 kJ / kg.

[0148] Using the following formula (C) obtained by discretizing the heat conduction equation, the enthalpy H at the next time step (the (n + 1)-th time step) ｎ＋１ ｉ is obtained. Here, H ｎ＋１ ｉ and H ｎ ｉ are the enthalpies of element i at the (n + 1)-th time step and the n-th time step, respectively. TC ｎ ｉ＋１ , TC ｎ ｉ and TC ｎ ｉ－１ are the temperatures of the adjacent elements of element i. k is the thermal conductivity. Δt is the time step, and Δx is the size of element i (that is, the value obtained by dividing the bloom thickness by N). Note that the thermal conductivity k at temperature T uses the following formula. k = 39.0 - 0.0138 × (T - 500)

[0149] Using the obtained enthalpy H ｎ＋１ ｉ and formulas (A) and (B), the temperature TC at the next time step (the (n + 1)-th time step) ｎ ｉ－１ is obtained. By tracking the time change of the solid fraction at the center position of the bloom (i = N / 2), the central solid fraction, which is the solid fraction at the center position of the bloom, is obtained.

[0150] [Regarding (Preferred Condition 3) Reheating Temperature HT] The preferred reheating temperature HT is 1250 to 1350°C. In this case, combined with the other preferred conditions 1, 2 and 4, the degree of Sn positive segregation at the bottom of the rail becomes 10.0 or less. A more preferred lower limit for the reheating temperature HT is 1280°C.

[0151] [Regarding (Preferred Condition 4) Final Rolling Temperature FT] The preferred final rolling temperature FT is 900 to 1100°C. In this case, combined with the other preferred conditions 1 to 3, the degree of Sn positive segregation at the bottom 15 of the rail 1 becomes 10.0 or less. The preferred lower limit of the final rolling temperature FT is 950°C or higher.

[0152] Furthermore, if the central solid fraction at the start of light reduction is set to 20% or less, the preferred lower limit of the central solid fraction at the end of light reduction is set to over 80%, the reheating temperature HT is set to 1280°C or higher, and the final rolling temperature FT is set to 950°C or higher, the Sn positive segregation degree of the top surface of the manufactured rail can be reduced to 8.0 or less.

[0153] [Regarding the manufacturing method of rail welded joints] The rail welded joints of this embodiment are manufactured by flash butt welding the ends of a pair of rails of this embodiment. Flash butt welding will be described below.

[0154] Flash butt welding is a well-known welding method. Flash butt welding includes preheated flash welding and continuous flash welding. The rail welding joint in this embodiment can be constructed using either flash butt welding method.

[0155] Preheated flash butt welding includes an initial flash process, a preheating process, a late flash process, and an upset process.

[0156] The initial flashing process is a process in which the rails begin at room temperature. In the initial flashing process, a flash is generated between the opposing end faces (i.e., weld surfaces) of a pair of rails, adjusting the end faces perpendicular to the longitudinal direction of the raw rail material. The initial flashing process further heats the end faces through the resistance heating of the flash and arc heating. The time required for the initial flashing process (initial flashing time) is, for example, 10 to 40 seconds.

[0157] In the preheating process, the opposing end faces of a pair of rails are forced into contact, and a large current is passed through the pair of rails for a certain period of time to heat the rail base material near the welded surface (end face) by resistive heating. After that, the pair of rails are separated. This contact and separation of the welded surfaces is repeated one or more times. It is preferable that the number of preheating cycles (pressurized contact and separation of the rail end faces) be two or more times. A more preferable lower limit for the number of preheating cycles is four, more preferably eight, and still more preferably twelve.

[0158] In the late flash process, a partial flash is generated between the opposing weld surfaces, and the weld surface (end face) is heated by the resistance heating and arc heating of this flash. Subsequently, the flash that was generated in a part of the weld surface (end face) is generated across the entire weld surface (end face) by increasing the flash velocity. The entire weld surface (end face) is heated uniformly by the resistance heating and arc heating of this flash. In the late flash process, oxides that were generated on the end face (weld surface) during the preheating process are further scattered and reduced by the flash.

[0159] In the late flash process, the late flash time, which is the duration for which the late flash process is carried out, is preferably 10 to 30 seconds. If the late flash time is less than 10 seconds, the removal of oxides by flashing will be insufficient. On the other hand, if the late flash time exceeds 30 seconds, the unevenness of the welded surface (end face) due to flashing will become significant, and the formation of oxides will actually be promoted. Therefore, the late flash time should be set to 10 to 30 seconds.

[0160] Furthermore, the average flash speed for the entire late flash process is preferably 0.3 mm / second or higher. An average flash speed of 0.3 mm / second or higher improves the amount of erosion on the rail end face, enabling even better welding. In addition, the average flash speed for the 3 seconds before the start of the upset process is preferably 0.5 mm / second or higher. Here, flash speed refers to the speed at which the jigs that grip the pair of rails are brought closer together.

[0161] Furthermore, in order to reduce the HAZ width of the welded joint, the late flash allowance, which is the amount of material rail erosion in the late flash process, is preferably 10 mm or more.

[0162] In the upsetting process, after the entire surface of the rail end face is molten, the rail end faces are rapidly pressed together under high pressure to form the Hazardous Zone (HAZ). At this time, the upsetting load is set to 50 to 90 kN. The average cooling rate of the top surface of the HAZ at 800 to 550°C is set to more than 1.5 to 4.0°C / second. In the temperature range below 550°C, the rails are allowed to cool to 50°C.

[0163] Through the above process, a rail welded joint is manufactured, comprising a pair of rails and a HAZ formed between the pair of rails.

[0164] The effects of the rail of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the rail of this embodiment. Therefore, the rail of this embodiment is not limited to this one example of conditions.

[0165] Rails having the chemical compositions shown in Table 1 (Tables 1A to 1D) were manufactured.

[0166]

[0167]

[0168]

[0169]

[0170] In Table 1, a "-" indicates that the corresponding element was intentionally omitted.

[0171] The rails for each test number were manufactured under the manufacturing conditions shown in Table 2 (Tables 2A and 2B).

[0172]

[0173]

[0174] Specifically, blooms were produced by continuous casting using molten steel. The central solid fraction (%) of the bloom at the start of light reduction and at the end of light reduction during continuous casting are shown in Table 2. The reduction rate for light reduction was set to 1-3% when the central solid fraction was 70% or less, and to 2-5% when the central solid fraction exceeded 70%.

[0175] The manufactured bloom was subjected to hot rolling. The reheating temperature HT (°C) and the final rolling temperature FT (°C) are shown in Table 2.

[0176] Accelerated cooling was performed on the rail profiles after hot rolling. The cooling device shown in Figure 13 was used. The starting temperature for cooling was 900 to 750°C. The accelerated cooling rate was 1.0 to 10.0°C. Rails for each test number were manufactured using the above manufacturing process. The cross-sectional shape of the rails for each test number was set to 136 pounds (mass: 67 kg / m).

[0177] [Evaluation Tests] The following evaluation tests were conducted using the manufactured rails: (Test 1) Measurement of the perlite area ratio on the top surface of the rail (Test 2) Measurement of the Vickers hardness on the top surface of the rail (Test 3) Measurement of the degree of Sn segregation on the bottom of the rail (Test 4) Evaluation of the bending strength of the rail welded joints. Tests 1 to 4 are described below.

[0178] [(Test 1) Measurement Test of Perlite Area Ratio on the Top Surface] Based on the method described in [Method for Observing Metal Structure on the Top Surface] above, the perlite area ratio on the top surface of each test number rail was determined. The obtained area ratios are shown in Table 3 (Table 3A and Table 3B).

[0179]

[0180]

[0181] [(Test 2) Vickers hardness measurement test of the top surface] The Vickers hardness (HV) of the top surface of the rail for each test number was determined in accordance with the method described in [Method for measuring Vickers hardness of the top surface] above. The results obtained are shown in Table 3.

[0182] [(Test 3) Measurement Test of Sn Positive Segregation Degree at the Bottom of the Rail] The Sn positive segregation degree at the bottom of the rail for each test number was determined in accordance with the method described in [Method for Measuring Sn Positive Segregation Degree at the Bottom of the Rail] above. The results obtained are shown in Table 3.

[0183] [(Test 4) Bending Strength Evaluation Test of Rail Welded Joints] The rail for each test number was cut at the center of its longitudinal direction to obtain two rail test pieces. A rail welded joint was manufactured by performing the following flash butt welding on the two rail test pieces. Two rail welded joints were manufactured for each test number. In other words, four rail test pieces were prepared for each test number.

[0184] In the initial flash process, the initial flash time was set to 15 seconds. In the preheating process, the number of preheating cycles was set to 10. In the later flash process, the later flash time was set to 20 seconds. The average flash speed for the entire later flash process was set to 0.8 mm / second, and the average flash speed for the 3 seconds before the upset process was set to 1.8 mm / second. The upset load was set to 75 kN. After flash butt welding, the average cooling rate for the surface temperature of the weld in the temperature range of 800 to 550°C was set to more than 1.5 to 3.0°C / second. After that, the weld was allowed to cool until the surface temperature reached 50°C.

[0185] Referring to Figure 5, the manufactured rail welded joint was positioned with the rail head facing upwards and the rail bottom facing downwards, and the rail bottom was supported at two points. Furthermore, a load was applied to the two points on the rail head of the weld. The distance between the support points on the rail bottom was set to 1220 mm. The distance between the support points on the rail head of the weld was set to 305 mm. A load was applied to the support points on the rail head, and the load was increased until the rail fractured. The load at which the rail fractured (fracture load) was determined, and this fracture load was set as the maximum load. If the rail did not fracture when the load was 3200 kN, the maximum load was set to 3200 kN. The lower limit of the acceptable load was defined by the following formula: Lower limit of acceptable load = 3.5 × (HV - 260) + 1800 Here, HV is the Vickers hardness (HV) of the top surface. Based on the obtained maximum load and lower limit of acceptable load, the bending strength of the weld was evaluated as follows. Evaluation AA: Maximum load is 120% or more of the lower limit of the passing load, or the rail welded joint does not break. Evaluation A: Maximum load is 110% to less than 120% of the lower limit of the passing load. Evaluation B: Maximum load is between the lower limit of the passing load and less than 110% of the lower limit of the passing load. Evaluation X: Maximum load is less than the lower limit of the passing load. The evaluation results are shown in the "Bending Strength Evaluation" column of Table 3.

[0186] Furthermore, the absolute difference in the maximum loads of each of the two rail welded joints for each test number was determined. If the absolute difference was 50 kN or less, it was determined that stable bending strength was obtained (indicated as "A" in the "Fluctance of Fracturing Load" column in Table 3). On the other hand, if the absolute difference was 180 kN or more, it was determined that stable bending strength was not obtained (indicated as "X" in the "Fluctance of Fracturing Load" column in Table 3).

[0187] [Evaluation Results] Referring to Tables 1 to 3, rails numbered 1 to 68 satisfied features 1 to 4. Therefore, excellent bending strength was obtained in the rail welded joints after flash butt welding, and the bending strength was consistently obtained.

[0188] Furthermore, test numbers 1 to 58 met the preferred conditions 1 to 4. Therefore, the rails in these test numbers met not only features 1 to 4 but also feature 5. As a result, even better bending strength was obtained in the welded joints after flash butt welding.

[0189] On the other hand, in test number 69, the Sn content was too high. As a result, sufficient bending strength was not obtained in the welded joint after flash butt welding compared to test numbers 1 to 68.

[0190] On the other hand, in test number 70, the Sn content was too low. As a result, sufficient bending strength was not obtained in the welded joint after flash butt welding compared to test numbers 1 to 68.

[0191] On the other hand, in test number 71, the oxygen content was too high. As a result, sufficient bending strength was not obtained in the welded joint after flash butt welding compared to test numbers 1 to 68.

[0192] On the other hand, in test numbers 72 and 73, F1 was below the lower limit of formula (1). Therefore, although excellent bending strength was obtained in the welded joint after flash butt welding, stable bending strength was not obtained compared to test numbers 1 to 68.

[0193] On the other hand, in test numbers 74 and 75, F1 exceeded the upper limit of formula (1). Therefore, although excellent bending strength was obtained in the welded joint after flash butt welding, stable bending strength was not obtained compared to test numbers 1 to 68.

[0194] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

[0195] 1 Rail 10A Top surface 15 Rail bottom

Claims

1. The chemical composition, in mass%, is as follows: C: 0.55-1.20%, Si: 0.10-2.00%, Mn: 0.10-2.00%, Sn: less than 0.0020-0.0200%, O: 0.0040% or less, P: 0.025% or less, S: 0.025% or less, Al: 1.0000% or less, N: 0.0020-0.0200%, Cr: 0-1.00%, Mo: 0-0.50%, Co: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0.200%, Nb: 0-0.0500%, Ti: 0-0.0500%. A rail containing B: 0-0.0050%, Mg: 0-0.0200%, Ca: 0-0.0200%, rare earth elements: 0-0.0500%, Zr: 0-0.0200%, and Pb: 0-0.0040%, with the remainder being Fe and impurities, satisfying formula (1), and having a pearlite area ratio of 95% or more in the metallographic structure of the rail head surface up to a depth of 20 mm from the outer surface of the rail head, and a Vickers hardness of 230 HV or more. 0.05 × 10 －５ ≦Sn×(O / 4)≦1.50×10 －５ (1) Here, Sn and O in formula (1) are substituted with the mass percentage content of the corresponding elements in the chemical composition.

2. A rail according to claim 1, wherein the chemical composition contains, by mass%, Sn: 0.0030 to less than 0.0100%.

3. The rail according to claim 1, wherein the chemical composition is, in mass%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, Co: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, V: 0.001 to 0.200%, Nb: 0.0001 to 0.0500%, Ti: 0.0001 to 0.0500%, B: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0200%, Ca: 0.0001 to 0.0200%, rare earth elements: 0.0001 to 0.0500%, Zr: 0.0001 to 0.0200%, and A rail containing one or more Pb selected from the group consisting of Pb: 0.0001 to 0.0040%.

4. A rail according to any one of claims 1 to 3, wherein the degree of positive segregation of Sn at the bottom of the rail is 10.0 or less.

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