rail

Abstract

Provided is a rail capable of obtaining excellent wear resistance and damage resistance. A rail (1) according to an embodiment of the present invention contains 0.80%-1.20% of C, 0.80%-2.50% of Si, 0.10%-2.00% of Mn, 0.0250% or less of P, and 0.0250% or less of S in terms of mass%, with the balance being Fe and impurities. The metal structure of a head surface part (10A) has a pearlite area ratio of 95% or more and a Vickers hardness of 400 HV or more. In the head surface part (10A), the Si positive segregation degree, which is the ratio of the maximum value of the Si concentration in a Si positive segregation band to the Si concentration in a bulk region, is more than 1.00 to 1.35, and the Si negative segregation degree, which is the ratio of the minimum value of the Si concentration in a Si negative segregation band to the Si concentration in the bulk region, is 0.90 to less than 1.00.

Description

rail

[0001] The present disclosure relates to rails.

[0002] Economic development is driving new development of natural resources such as coal and iron ore. Specifically, mining of natural resources is progressing in previously unexplored areas with harsh natural environments. As a result, the track environment of freight railways that transport resources is becoming more severe. For example, the curvature of curved sections is becoming smaller and the gradient of gradient sections is becoming steeper.

[0003] As a result of the harsher track environment, the stress and slippage that wheels impose on rails are increasing. As a result, rail wear and damage are increasing. Therefore, there is a demand for improved wear resistance and damage resistance in rails. Against this background, development of rails with improved wear resistance and damage resistance is underway.

[0004] With the aim of improving the wear resistance of rails, rails have been proposed as disclosed in Patent Documents 1 and 2. In these rails, in order to enhance wear resistance, the lamellar spacing of pearlite is refined to increase the hardness of the steel, or the C content of the steel is increased to increase the volume fraction of cementite in the pearlite lamellae.

[0005] Specifically, in Patent Document 1, the head of a rail after rolling or the head of a reheated rail is accelerated cooled from the austenite temperature range to between 850 and 500°C at a rate of 1 to 4°C / second. Patent Document 1 discloses that this improves the wear resistance of the rail. Furthermore, Patent Document 2 discloses that hypereutectoid steel (C: more than 0.85 to 1.20%) is used to increase the volume fraction of cementite in the lamellae of the pearlite structure. Patent Document 2 also discloses that this improves the wear resistance of the rail.

[0006] Furthermore, in order to improve the damage resistance as well as the wear resistance of rails, rails have been proposed, for example, as disclosed in Patent Documents 3 and 4.

[0007] In Patent Document 3, hypereutectoid steel (C: 0.73 to 0.85%) is used, and the amounts of Mn and Cr are balanced to refine the lamellar spacing in pearlite. This increases the hardness of the steel. Patent Document 3 discloses that this improves the wear resistance and damage resistance of the rail. Furthermore, in Patent Document 4, hypereutectoid steel (C: 0.70 to 0.85%) is used, and the amounts of Si, Mn, and Cr are balanced to control the amount of pearlite and bainite formed, thereby increasing the hardness of the steel. Patent Document 4 also discloses that this improves the wear resistance and damage resistance of the rail.

[0008] Japanese Patent Publication No. 63-023244 Publication of Patent No. 3078461 Publication of Patent No. 4390004 Publication of Patent No. 6822757

[0009] The techniques disclosed in Patent Documents 1 and 2 improve the wear resistance of rails by increasing the hardness of the steel by minimizing the lamellar spacing in pearlite or by increasing the volume ratio of cementite in the pearlite lamellae. However, the rails disclosed in Patent Documents 1 and 2 may not provide sufficient wear resistance when used in the severe track environments that have become increasingly required in recent years.

[0010] The technologies disclosed in Patent Documents 3 and 4 improve the wear resistance and damage resistance of rails by increasing the hardness of steel by refining the lamellar spacing in pearlite or by controlling the metallographic structure of the steel. However, even the rails disclosed in Patent Documents 3 and 4 may not provide sufficient wear resistance and damage resistance when used in the severe track environments that have become increasingly required in recent years.

[0011] As mentioned above, a rail that is excellent in wear resistance and damage resistance and can be used in freight railways, which have a severe track environment, has not yet been provided.

[0012] An object of the present disclosure is to provide a rail that provides excellent wear and damage resistance.

[0013] The rail of the present disclosure contains, in mass %, C: 0.80 to 1.20%, Si: 0.80 to 2.50%, Mn: 0.10 to 2.00%, P: 0.0250% or less, S: 0.0250% or less, Al: 0 to 1.0000%, N: 0 to 0.0200%, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Co: 0 to 1.00%, B: 0 The head has a metal structure consisting of 0.0050%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0.200%, Nb: 0-0.0500%, Ti: 0-0.0500%, Mg: 0-0.0200%, Ca: 0-0.0200%, rare earth elements: 0-0.0500%, Zr: 0-0.0200%, and the balance being Fe and impurities. In the metal structure of the head surface portion from the outer surface of the head to a depth of 20 mm, the pearlite area ratio is 95% or more and the Vickers hardness is 400 HV or more. In the head surface portion, the Si positive segregation ratio, which is the ratio of the maximum Si concentration in the Si positive segregation zone to the Si concentration in the bulk region, is greater than 1.00 and less than 1.35. The negative Si segregation ratio, which is the ratio of the minimum Si concentration in the negative Si segregation zone to the Si concentration in the bulk region, is 0.90 to less than 1.00.

[0014] The rails of the present disclosure provide excellent wear and damage resistance.

[0015] FIG. 1A is a cross-sectional view perpendicular to the longitudinal direction of a rail according to this embodiment. FIG. 1B is a cross-sectional view of the vicinity of the rail head, illustrating a method for defining the head surface portion in FIG. 1A. FIG. 1C is a cross-sectional view of the vicinity of the rail head, continuing from FIG. 1B. FIG. 1D is a cross-sectional view of the vicinity of the rail head, continuing from FIG. 1C. FIG. 1E is a cross-sectional view of the vicinity of the rail head, continuing from FIG. 1D. FIG. 1F is a cross-sectional view of the vicinity of the rail head, continuing from FIG. 1E. FIG. 2A is a schematic view of crack generation in the vicinity of the rail tread. FIG. 2B is a schematic view of crack generation in the vicinity of the rail tread, different from FIG. 2A. FIG. 2C is a schematic view of crack generation in the vicinity of the rail tread, different from FIGS. 2A and 2B. FIG. 3 is a perspective view of a rolling fatigue testing machine that reproduces damage caused by rolling of a rail and a wheel. FIG. 4A is a schematic view showing the relationship between crack generation in the vicinity of the rail tread and a Si segregation zone. Fig. 4B is a schematic diagram different from Fig. 4A , showing the relationship between the state of crack generation near the rail tread and the Si segregation band. Fig. 4C is a schematic diagram different from Figs. 4A and 4B , showing the relationship between the state of crack generation near the rail tread and the Si segregation band. Fig. 5 is an enlarged view of a portion near the outer surface of the measurement surface of a test piece taken from a rail for hardness measurement. Fig. 6 is a schematic diagram for explaining how to determine the degree of positive Si segregation. Fig. 7 is a schematic diagram for explaining how to determine the degree of negative Si segregation. Fig. 8 is a schematic diagram showing an example of a cooling device used in a heat treatment step in the manufacturing process of a rail of this embodiment.

[0016] The rail of this embodiment will be described in detail. Hereinafter, % in the chemical composition means mass % unless otherwise specified.

[0017] [Rail configuration] Fig. 1A is a cross-sectional view perpendicular to the longitudinal direction of a rail 1 of this embodiment. Referring to Fig. 1A, the rail 1 includes a rail head portion 10, a bottom portion 15, and a pillar portion 14. The rail head portion 10 refers to the portion of the rail 1 above the narrowed portion at the center in the height direction. The rail head portion 10 receives the load from the wheels of a railway vehicle. The bottom portion 15 is fixed to a sleeper. The bottom portion 15 supports the rail head portion 10, which receives the load from the wheels, via the pillar portion 14. The pillar portion 14 is disposed between the rail head portion 10 and the bottom portion 15. The upper end of the pillar portion 14 is connected to the rail head portion 10, and the lower end of the pillar portion 14 is connected to the bottom portion 15.

[0018] The rail head portion 10 includes a top portion 11, a pair of top corner portions 12 (12L and 12R), and a pair of under-jaw portions 13. The top portion 11 is located at the center of the rail head portion 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 portion 10. In other words, the top portion 11 is located between the pair of head corner portions 12 (12L and 12R). In a cross section 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 the "head corner portion 12L," and the head corner portion on the right side of the top portion 11 is referred to as the "head corner portion 12R." The head corner portions 12L and 12R are collectively referred to as the head corner portion 12. One of the pair of head corner portions 12L and 12R is a gauge corner (G.C.) portion that mainly comes into contact with the wheels.

[0019] The surface of the rail head portion 10, which is the surface of the head top portion 11 that faces upward when the rail 1 is upright, combined with the surfaces of the pair of head corner portions 12, is referred to as the head outer surface 10S. The pair of under-chin portions 13 are located on the opposite side of the pair of head corner portions 12 in the height direction of the rail 1. The pair of under-chin portions 13 are the outer surfaces of the portion of the rail 1 that is narrowed in the center in the height direction.

[0020] The region extending from the head outer surface 10S to a depth of 20 mm is referred to as the head surface portion 10A. D1 in FIG. 1A indicates the range from the head outer surface 10S to a depth of 20 mm. The microstructure of the head surface portion 10A is made of pearlite with a Vickers hardness of 400 HV or more. This improves the wear resistance and damage resistance of the rail 1.

[0021] Generally, the service life of a rail is about 20 mm from the head outer surface 10S. Therefore, by making the microstructure of the head surface 10A a pearlite structure with a Vickers hardness of 400 HV or more, the wear resistance and damage resistance of the rail 1 are ensured.

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

[0023] Point X is located on the normal line L11, 20 mm (D1) in the depth direction from point P11. A line segment L100 is drawn horizontally from point X, and the intersections of line segment L100 and the surface of the rail head 10 are designated as points A and B. Referring to Figure 1C, the area from point A to point B on the surface of the rail head 10 of the rail 1 is divided into thirds, and the boundary positions of these thirds are designated as points C and D. Referring to Figure 1D, the area between points C and D is defined as the head top 11. Furthermore, point E is the central position between points A and C on the surface of the rail head 10. Similarly, point F is the central position between points B and D.

[0024] Referring to Figure 1E, the central position between points A and E on the surface of the rail head 10 is defined as point G. Similarly, the central position between points B and F is defined as point H. The area between points C and G on the surface of the rail head 10 is defined as a head corner portion 12L. Note that the area between points A and G is an area that is substantially less likely to come into contact with a railway vehicle, and is therefore excluded from the head corner portion 12L. Similarly, the area between points D and H is defined as a head corner portion 12R. The area between points B and H is an area that is substantially less likely to come into contact with a railway vehicle, and is therefore excluded from the head corner portion 12R.

[0025] The top 11, head corners 12L, and head corners 12R are defined by the above method. Of the surfaces of the rail head 10, the surfaces of the top 11, head corners 12L, and head corners 12R, that is, the surface from point G to point H, are defined as the head outer surface 10S.

[0026] The depth direction of the top of the head 11, the head corners 12L, and the head corners 12R is defined as follows. In the top of the head 11, the normal direction of the head outer surface 10S is defined as the depth direction. On the other hand, the depth direction of the head corners 12L and 12R is defined as follows. Referring to FIG. 1F, the line segment L13, which is a tangent to the underchin 13, Ｌ and L13 Ｒ Then, the line segment L13 Ｌ and L13 Ｒ The intersection of these points is defined as point P13. Point P13 is located on the normal line L11. In the head corner portion 12L, the direction of the line segment connecting an arbitrary point on the head outer surface 10S (i.e., the surface between points C and G) and point P13 is defined as the depth direction at the arbitrary point. For example, if the arbitrary point is point E, then the line segment L12 connecting point E and point P13 is defined as the depth direction at the arbitrary point. Ｌ The direction along the line segment L12 connecting an arbitrary point on the head outer surface 10S (i.e., the surface between points D and H) and point P13 in the head corner portion 12R is defined as the depth direction at the arbitrary point. For example, if the arbitrary point is point F, the line segment L12 connecting point F and point P13 is defined as the depth direction at the arbitrary point. Ｒ The direction along the line is defined as the depth direction at point F.

[0027] Based on the above definition, the line segment L at a depth of 20 mm from the head outer surface 10S between points G and H is Ｄ２０ By the above method, the head surface portion 10A shown in FIG. 1A can be defined.

[0028] [Technical Concept of the Rail 1 of the Present Embodiment] Next, the technical concept of the rail 1 of the present embodiment will be described.

[0029] The inventors first studied the chemical composition of a rail that would provide excellent wear resistance and damage resistance, and as a result, they found that the rail should have the following composition by mass: C: 0.80 to 1.20%, Si: 0.80 to 2.50%, Mn: 0.10 to 2.00%, P: 0.0250% or less, S: 0.0250% or less, Al: 0 to 1.0000%, N: 0 to 0.0200%, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Co: 0 to 1.00%, B: 0 to 0.0050%, Cu: 0 to 1.00%, Ni: 0 It was considered that excellent wear resistance and damage resistance could be obtained if the rail had a chemical composition of: Co: 0.00% or less, V: 0-0.200%, Nb: 0-0.0500%, Ti: 0-0.0500%, Mg: 0-0.0200%, Ca: 0-0.0200%, rare earth elements: 0-0.0500%, Zr: 0-0.0200%, and the balance being Fe and impurities.

[0030] The present inventors further investigated means for improving wear resistance from the viewpoint of metal structure, and as a result of the investigation, the present inventors found that sufficient wear resistance can be obtained if the metal structure of the head surface portion 10A has a pearlite area ratio of 95% or more and a Vickers hardness of 400 HV or more.

[0031] The present inventors further investigated means for improving the damage resistance of rails having the above-mentioned chemical composition and metal structure.

[0032] The inventors investigated damage that occurs on the head surface portion 10A of a rail when used in a severe operating environment such as a freight railway. As a result, they found that cracks occur on the surface of the head surface portion 10A due to contact with the wheels, and these cracks propagate further into the rail, causing spalling damage on the rolling surface of the rail (the surface that comes into contact with the railway wheels). Therefore, the inventors conducted a detailed study of the behavior of crack initiation and propagation on the head surface portion 10A. As a result, the inventors discovered the following:

[0033] 2A to 2C are schematic diagrams showing how cracks occur near the tread of the rail 1. Referring to Fig. 2A to 2C, there are three possible states for cracks occurring in the head surface portion 10A of the rail 1: Case 1 (Figure 2A): A large crack (initial crack CRA1) that forms in a straight line at a certain angle relative to the contact surface 10S with the wheel 2 (outer surface of the head). Case 2 (Figure 2B): A large crack (initial crack CRA1 + propagating crack CRA2) that consists of a crack (initial crack CRA1) that forms in a straight line at a certain angle relative to the contact surface 10S with the wheel 2 and a wavy crack (propagating crack CRA2) that propagates from the tip of the initial crack CRA1 due to repeated contact with the wheel 2. Case 3 (Figure 2C): A crack (initial crack CRA1) that forms in a straight line at a certain angle relative to the contact surface 10S with the wheel 2, but with a depth D that is shallower than in Case 1. In Case 3, where the depth D of the initial crack CRA1 is shallow, no peeling damage occurs.

[0034] Based on the above findings, the inventors further investigated the causes of crack initiation (CRA1 and CRA2) using the following method. Rails with a C content of 0.80 to 1.20% were fabricated. Then, using the rolling fatigue testing machine shown in Figure 3, the relationship between the initiation behavior of large cracks immediately below the contact surface and the element distribution in the steel material constituting the rail was investigated.

[0035] [Overview of Rolling Contact Fatigue Testing Machine] Referring to Figure 3, in the rolling contact fatigue testing machine, a rail 1 is placed on a sleeper 3. A motor 4 rotates a wheel 2. A load stabilizing device 5 holds down the wheel 2 that is rotated by the motor 4. In the rolling fatigue test, the wheel 2 is placed on the rail 1. Then, while a predetermined load is applied to the wheel 2 using the load stabilizing device 5, the wheel 2 repeatedly rolls back and forth on the rail head 10 of the rail 1 along the longitudinal direction of the rail 1.

[0036] [Conditions for Rolling Fatigue Test] The chemical composition and hardness of the rail 1, the heat treatment method for the rail 1, the conditions for the rolling fatigue test of the rail 1 and the wheel 2 to reproduce crack damage, the method for evaluating large and coarse cracks immediately below the contact surface, and the method for elemental analysis of the crack initiation area were as follows:

[0037] [Chemical composition, shape, and characteristics of rail 1] The chemical composition of rail 1 was, by mass%, 0.80 to 1.20% C, Si, Mn, and Cr, with the balance being Fe and impurities. The rail shape was 136 pounds (mass: 67 kg / m). The Vickers hardness of the head surface portion 10A was 400 to 550 HV. The pearlite area ratio in the metal structure of the head surface portion 10A was 95% or more.

[0038] [Heat Treatment Method in the Manufacturing Process of Rail 1] In the manufacturing process of rail 1 described above, a rail-shaped preform (hereinafter also referred to as rail preform) was manufactured by hot rolling. The rail preform was then subjected to heat treatment (accelerated cooling) under the following conditions to manufacture a rail 1 with the above-mentioned characteristics. Specifically, accelerated cooling was performed on the hot-rolled rail preform under the following conditions. The accelerated cooling cooled a portion of the rail preform corresponding to the head surface portion 10A. The cooling start temperature was 750 to 900°C, and the cooling rate was 2.0 to 30.0°C / sec. The cooling end temperature was 500 to 660°C.

[0039] [Test Conditions for Rolling Contact Fatigue Test] The test conditions for the rolling contact fatigue test were as follows. For the rolling contact fatigue test, a rolling contact fatigue testing machine shown in FIG. 3 was used. A rail 1 with a rail shape of 136 pounds and a length of 2 m was prepared as a test specimen. A wheel 2 of the AAR type with a diameter of 920 mm was also prepared. The radial load during the test was 300 kN, and the thrust load was 100 kN. Repeated lubrication of water supply and drying was adopted as the lubrication method during the test. Specifically, during the test, water was supplied to the rail 1 for a certain period of time, and then the water supply was stopped and the rail was dried for a certain period of time (water supply and drying). During the test, the number of repeated load applications using the wheel 2 was up to 5 million, and the cumulative passing tonnage was up to 150 million tons.

[0040] [Method for Evaluating Coarse Cracks] The method for evaluating coarse cracks immediately below the contact surface of the rail 1 with the railway wheel after the rolling fatigue test was as follows. A 300 mm long area on the surface of the head corner portion 12 of the rail 1 in the longitudinal direction of the rail 1 was designated as the evaluation area. A cross section parallel to the longitudinal direction of the rail 1 after the test was designated as the observation surface, and a sample including the evaluation area on the observation surface was taken. The observation surface was polished. On the polished observation surface, coarse cracks immediately below the contact surface between the head corner portion 12 and the railway wheel were identified. Specifically, cracks that propagated from the surface of the head corner portion 12 on the observation surface and had a depth length D (see Figures 2A to 2C) of 500 μm or more were recognized as cracks and evaluated. Here, the depth direction of the crack was defined as the normal direction to the surface of the head corner portion 12.

[0041] [Elemental analysis around the crack of the evaluation target] Elemental analysis of the area around the crack identified as the evaluation target was performed using the following method. The analysis area was a rectangular area including the crack, measuring 1000 μm in the longitudinal direction of the rail 1 and 1000 μm in the depth direction of the surface (contact surface) 10S. Area analysis of the analysis area was performed using an electron probe microanalyzer (EPMA), and element distribution maps of various elements were created. In the EPMA, the acceleration voltage was 15 kV, the probe current was 0.1 μA, the time was 50 ms, and the beam diameter was 2 μm.

[0042] The relationship between the obtained element distribution map and the crack morphology was examined. As a result, as shown in Figures 4A to 4C, the initial crack CRA1, which is a linear crack, initiated in the Si-positive segregation zone SE+. On the other hand, as shown in Figure 4B, the wave-shaped propagating crack CRA2, which propagated further from the initial crack CRA1 in the depth direction from the surface 10S, initiated not in the Si-positive segregation zone SE+ but in the Si-negative segregation zone SE-.

[0043] Generally, a Si segregation band can be formed as a central segregation in the center of a cross section perpendicular to the longitudinal direction of the bloom (hereinafter referred to as the cross-sectional center) during the casting of the bloom that will become the raw material for a rail. Such a Si segregation band formed in the cross-sectional center of a steel material exists in the web portion 14 when the rail 1 is manufactured. This Si segregation band is referred to as a Si macrosegregation band. Therefore, it seems that a Si segregation band would not normally exist in the head surface portion 10A, which is a part of the railhead portion 10 that is not the web portion 14.

[0044] However, as described above, the present inventors have found for the first time that Si segregation bands (Si positive segregation band SE+ and Si negative segregation band SE−) also exist in the head surface portion 10A. The Si segregation band present in the head surface portion 10A means that a further Si segregation band may occur around the Si macrosegregation band present in the column portion 14. In this specification, the Si segregation band formed around such an Si macrosegregation band is referred to as a Si semi-macrosegregation band. As described above, the Si semi-macrosegregation band may include not only a Si semi-macro positive segregation band but also a Si semi-macro negative segregation band.

[0045] Based on the above investigation results, the inventors have made the following findings: Linear initial cracks CRA1 tend to occur in the Si semi-macro positive segregation zone SE+. On the other hand, wavy propagating cracks CRA2 tend to occur in the Si semi-macro negative segregation zone SE-. As mentioned above, the microstructure of the head surface portion 10A of the rail 1 is essentially composed of pearlite, and the formation of hard structures such as island martensite (MA) due to a high Si content was not confirmed. Therefore, it was found that the initial cracks CRA1 and the propagating cracks CRA2 occur due to Si segregation zones (Si semi-macro positive segregation zone and Si semi-macro negative segregation zone) whose existence was not previously known in the head surface portion 10A.

[0046] Based on the above findings, the present inventors further investigated the relationship between the type of crack (initial crack CRA1 and propagating crack CRA2) and the Si semi-macro positive segregation degree or Si semi-macro negative segregation degree in the region where each crack occurred. Here, the Si positive segregation degree is the ratio of the Si concentration (mass%) in the Si positive segregation zone (Si semi-macro positive segregation zone) present in the head surface portion 10A to the Si concentration (mass%) in the bulk region. Also, the Si negative segregation degree is the ratio of the Si concentration (mass%) in the Si negative segregation zone (Si semi-macro negative segregation zone) present in the head surface portion 10A to the Si concentration (mass%) in the bulk region. Methods for measuring the Si positive segregation degree and the Si negative segregation degree will be described later.

[0047] As a result of the investigation, the inventors have found for the first time that in the head surface portion 10A of the rail 1 having the above-mentioned chemical composition and metal structure, if the Si positive segregation degree is more than 1.00 but less than 1.35, the occurrence of the linear initial crack CRA1 is sufficiently suppressed, and further, if the Si negative segregation degree is 0.90 to less than 1.00, the occurrence of the wavy propagating crack CRA2 is sufficiently suppressed.

[0048] The rail of this embodiment has been completed based on the above technical concept, and its gist is as follows.

[0049] The rail of the first configuration contains, in mass %, C: 0.80 to 1.20%, Si: 0.80 to 2.50%, Mn: 0.10 to 2.00%, P: 0.0250% or less, S: 0.0250% or less, Al: 0 to 1.0000%, N: 0 to 0.0200%, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Co: 0 to 1.00%, B: The alloy comprises 0 to 0.0050%, 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%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, rare earth elements: 0 to 0.0500%, Zr: 0 to 0.0200%, and the balance being Fe and impurities. In the metal structure of the head surface portion from the outer surface of the head to a depth of 20 mm, the pearlite area ratio is 95% or more and the Vickers hardness is 400 HV or more. In the head surface portion, the Si positive segregation ratio, which is the ratio of the maximum Si concentration in the Si positive segregation zone to the Si concentration in the bulk region, is greater than 1.00 to 1.35. The negative Si segregation ratio, which is the ratio of the minimum Si concentration in the negative Si segregation zone to the Si concentration in the bulk region, is 0.90 to less than 1.00.

[0050] The rail of the second configuration is the rail of the first configuration, and further contains Al: 0.0001 to 1.0000%, N: 0.0001 to 0.0200%, Cr: 0.01 to 1.00%, Mo: 0.01 to 0.50%, Co: 0.01 to 1.00%, B: 0.0001 to 0.0050%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, V: The steel sheet contains one or more elements selected from the group consisting of: Cr: 0.001 to 0.200%, Nb: 0.0001 to 0.0500%, Ti: 0.0001 to 0.0500%, Mg: 0.0001 to 0.0200%, Ca: 0.0001 to 0.0200%, rare earth elements: 0.0001 to 0.0500%, and Zr: 0.0001 to 0.0200%.

[0051] The rail of this embodiment will be described in detail below. In the following description, the unit of element content "mass %" will be simply written as "%".

[0052] [Features of Rail 1 of the Present Embodiment] The rail of the present embodiment has the following features: (Feature 1) The chemical composition, in mass %, is: C: 0.80 to 1.20%, Si: 0.80 to 2.50%, Mn: 0.10 to 2.00%, P: 0.0250% or less, S: 0.0250% or less, Al: 0 to 1.0000%, N: 0 to 0.0200%, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Co: 0 to 1.00%, and B: 0 to 0. 0.0050%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0.200%, Nb: 0-0.0500%, Ti: 0-0.0500%, Mg: 0-0.0200%, Ca: 0-0.0200%, rare earth elements: 0-0.0500%, Zr: 0-0.0200%, and the balance being Fe and impurities. (Feature 2) In the metal structure of the head surface portion 10A from the head outer surface 10S to a depth of 20 mm, the pearlite area ratio is 95% or more and the Vickers hardness is 400 HV or more. (Feature 3) In the head surface portion 10A, the Si positive segregation ratio, which is the ratio of the maximum Si concentration in the Si positive segregation zone to the Si concentration in the bulk region, is greater than 1.00 to 1.35. (Feature 4) In the head surface portion 10A, the negative Si segregation ratio, which is the ratio of the minimum value of the Si concentration in the negative Si segregation zone to the Si concentration in the bulk region, is 0.90 to less than 1.00. Each feature will be described below.

[0053] (Feature 1) Chemical Composition The chemical composition of the rail 1 according to this embodiment contains the following elements.

[0054] C: 0.80 to 1.20% Carbon (C) promotes pearlite transformation and improves the wear resistance of the rail 1. If the C content is less than 0.80%, pro-eutectoid ferrite is formed. As a result, sufficient strength and wear resistance cannot be obtained. On the other hand, if the C content exceeds 1.20%, excessive pro-eutectoid cementite is formed in the head surface portion 10A, reducing the breakage resistance of the rail 1. Therefore, the C content is 0.80 to 1.20%. The preferred lower limit of the C content is 0.85%, more preferably 0.90%, and even more preferably 0.95%. The preferred upper limit of the C content is 1.18%, more preferably 1.15%, and even more preferably 1.10%. To stabilize the formation of pearlite structures, the preferred C content is 0.95 to 1.10%.

[0055] Si: 0.80 to 2.50% Silicon (Si) dissolves in ferrite in the pearlite structure, increasing the hardness of the rail 1. As a result, the wear resistance of the head surface portion 10A of the rail 1 is improved. If the Si content is less than 0.80%, the above effect is not sufficiently achieved. On the other hand, if the Si content exceeds 2.50%, the hardenability of the steel constituting the rail 1 is excessively increased. In this case, excessive martensite is formed in the head surface portion 10A. As a result, the wear resistance and breakage resistance of the head surface portion 10A are reduced. If the Si content exceeds 2.50%, the fluctuations in the Si concentration in the steel become even greater. As a result, the degree of Si segregation fluctuates. In this case, initial cracks and / or propagating cracks are more likely to occur. As a result, large and coarse cracks are more likely to occur in the head surface portion 10A. Large and coarse cracks reduce the damage resistance of the rail 1. Therefore, the Si content is set to 0.80 to 2.50%. The preferred lower limit of the Si content is 0.85%, more preferably 0.90%, more preferably 0.95%, more preferably 1.00%, more preferably 1.02%, more preferably 1.05%, and even more preferably 1.10%. The preferred upper limit of the Si content is 2.30%, more preferably 2.20%, and even more preferably 2.00%. In addition, when stabilizing the formation of pearlite structure and improving the wear resistance and breakage resistance of the head surface portion 10A, the preferred Si content is 1.05 to 2.20%.

[0056] Mn: 0.10 to 2.00% Manganese (Mn) improves the hardenability of the steel constituting the rail 1 and stabilizes pearlite transformation. Mn also refines the lamellar spacing of pearlite, thereby increasing the hardness and wear resistance of the rail 1. If the Mn content is less than 0.10%, this effect is not fully achieved. On the other hand, if the Mn content exceeds 2.00%, the hardenability of the steel constituting the rail 1 is excessively increased. As a result, bainite and / or martensite, etc., are formed in the head surface portion 10A of the rail 1. As a result, the wear resistance and breakage resistance of the head surface portion 10A of the rail 1 are reduced. An excessive amount of Mn also promotes the formation of pro-eutectoid cementite. As a result, the breakage resistance of the head surface portion 10A of the rail 1 is reduced. Therefore, the Mn content is 0.10 to 2.00%. The lower limit of the Mn content is preferably 0.20%, more preferably 0.30%, and even more preferably 0.40%. The upper limit of the Mn content is preferably 1.80%, more preferably 1.60%, and even more preferably 1.50%. In order to stabilize the formation of pearlite structures and improve the wear resistance and breakage resistance of the rail head portion, the Mn content is preferably 0.30 to 1.60%.

[0057] P: 0.0250% or less Phosphorus (P) is an impurity. In other words, the lower limit of the P content is greater than 0%. If the P content exceeds 0.0250%, pearlite becomes embrittled, and the breakage resistance of the rail 1 decreases. Therefore, the P content is 0.0250% or less. The lower the P content, the more preferable it is. 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 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, even more preferably 0.0025%, even more preferably 0.0030%, and even more preferably 0.0050%. The preferred upper limit of the P content is 0.0200%, even more preferably 0.0150%, and even more preferably 0.0120%.

[0058] S: 0.0250% or less Sulfur (S) is an impurity. In other words, the lower limit of the S content is greater than 0%. If the S content exceeds 0.0250%, coarse Mn sulfides are formed. Stress concentration is likely to occur around the coarse Mn sulfides. As a result, the breakage resistance of the rail 1 decreases. Therefore, the S content is 0.0250% or less. The lower the S content, the more preferable it is. 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 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, even more preferably 0.0030%, and even more preferably 0.0050%. The preferred upper limit of the S content is 0.0200%, even more preferably 0.0150%, and even more preferably 0.0120%.

[0059] [Optional Elements] The chemical composition of the rail of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Al: 0-1.0000%, N: 0-0.0200%, Cr: 0-1.00%, Mo: 0-0.50%, Co: 0-1.00%, B: 0-0.0050%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0.200%, Nb: 0-0.0500%, Ti: 0-0.0500%, Mg: 0-0.0200%, Ca: 0-0.0200%, rare earth elements: 0-0.0500%, and Zr: 0-0.0200%. All of these elements are optional elements. Each optional element will be described below.

[0060] Al: 0 to 1.0000% Aluminum (Al) is an optional element and does not necessarily need to be contained. In other words, the Al content may be 0%. When contained, that is, when the Al content exceeds 0%, Al deoxidizes the steel. Al also increases the eutectoid carbon content and the eutectoid transformation temperature. Therefore, Al suppresses the formation of pro-eutectoid cementite, which reduces toughness, and increases the hardness of pearlite structures. As a result, the breakage resistance of the head surface portion 10A of the rail 1 is improved. Even if even a small amount of Al is contained, the above effects can be achieved 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 alumina-based inclusions are formed. These coarse Al-based inclusions become the initiation points for fatigue cracks. This reduces the damage resistance of the head surface portion 10A of the rail 1. If the Al content exceeds 1.0000%, oxides are also formed during welding of the rail 1. In this case, the weldability of the rail 1 is reduced. Therefore, the Al content is 0 to 1.0000%. A preferred lower limit of the Al content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. A preferred upper limit of the Al content is 0.5000%, more preferably 0.1000%, even more preferably 0.0800%, and even more preferably 0.0500%.

[0061] N: 0 to 0.0200% Nitrogen (N) is an optional element and does not necessarily need to be present. In other words, the N content may be 0%. When N is present, that is, when the N content exceeds 0%, N segregates at austenite grain boundaries, promoting pearlite transformation from the austenite grain boundaries. This reduces the pearlite block size. As a result, the toughness of the head surface portion 10A of the rail 1 is improved. Furthermore, when V is present, N promotes the precipitation of V carbonitrides during the cooling process after welding of the rail 1. As a result, the hardness of pearlite increases, improving the damage resistance of the head surface portion 10A of the rail 1. Even if even a small amount of N is present, the above effects can be achieved to some extent. However, if the N content exceeds 0.0200%, it becomes difficult to dissolve N in the steel. As a result, bubbles, which are the initiation points of fatigue damage, are more likely to form. Therefore, the N content is 0 to 0.0200%. Excessive reduction of N increases the smelting costs of the steelmaking process. Therefore, in consideration of industrial production, the lower limit of the N content is preferably 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0030%. The upper limit of the N content is preferably 0.0180%, more preferably 0.0150%, and even more preferably 0.0120%.

[0062] Cr: 0 to 1.00% Chromium (Cr) is an optional element and does not necessarily need to be contained. That is, the Cr content may be 0%. When chromium is contained, that is, when the Cr content exceeds 0%, Cr raises the equilibrium transformation temperature and increases the degree of supercooling. This refines the lamellar spacing of pearlite and increases the hardness of pearlite. As a result, the wear resistance of the head surface portion 10A of the rail 1 is improved. Even if even a small amount of Cr is contained, the above effect can be achieved to some extent. However, if the Cr content exceeds 1.00%, the hardenability of the steel constituting the rail 1 increases excessively. In this case, bainite and / or martensite, etc., are formed in the head surface portion 10A of the rail 1. As a result, the wear resistance and breakage resistance of the rail 1 are reduced. If the Cr content exceeds 1.00%, Cr concentration in the segregated portions is further promoted. This promotes the formation of martensite and pro-eutectoid cementite in the head surface portion 10A of the rail 1. This increases martensite and / or pro-eutectoid cementite in the head surface portion 10A of the rail 1. As a result, the breakage resistance of the head surface portion 10A of the rail 1 decreases. Therefore, the Cr content is 0 to 1.00%. A preferred lower limit of the Cr content is 0.01%, more preferably 0.02%, even more preferably 0.03%, even more preferably 0.04%, even more preferably 0.06%, even more preferably 0.08%, and even more preferably 0.10%. A preferred upper limit of the Cr content is 0.95%, even more preferably 0.90%, and even more preferably 0.80%.

[0063] Mo: 0 to 0.50% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. That is, the Mo content may be 0%. When contained, that is, when the Mo content exceeds 0%, Mo raises the equilibrium transformation temperature and increases the degree of supercooling. This refines the lamellar spacing of pearlite and increases the hardness of pearlite. As a result, the wear resistance of the head surface portion 10A of the rail 1 is improved. Even if even a small amount of Mo is contained, the above effect can be achieved to some extent. However, if the Mo content exceeds 0.50%, the transformation rate significantly decreases. As a result, martensite forms in the head surface portion 10A of the rail 1. As a result, the breakage resistance of the head surface portion 10A of the rail 1 is reduced. Therefore, the Mo content is 0 to 0.50%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.02%, even more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Mo content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.30%.

[0064] Cobalt (Co) is an optional element and need not be contained. In other words, the Co content may be 0%. When Co is contained, that is, when the Co content exceeds 0%, Co dissolves in ferrite in pearlite and refines the lamellar structure of the surface layer of pearlite, including the head outer surface (rolling surface) 10S of the rail 1. This increases the hardness of the head outer surface 10S and improves the wear resistance of the head surface portion 10A of the rail 1. Even if even a small amount of Co is contained, the above effect can be achieved to some extent. However, if the Co content exceeds 1.00%, the lamellar structure cannot be sufficiently refined in accordance with the Co content. Furthermore, production costs increase. Therefore, the Co content is 0 to 1.00%. The preferred lower limit of the Co content is 0.01%, more preferably 0.02%, even more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Co content is preferably 0.90%, more preferably 0.80%, and even more preferably 0.60%.

[0065] B: 0 to 0.0050% Boron (B) is an optional element and may not be contained. In other words, the B content may be 0%. When it is contained, that is, when the B content is more than 0%, B forms iron boride (Fe ２３ (CB) ６ ) to promote pearlite transformation. This reduces the cooling rate dependency of the pearlite transformation temperature. Therefore, the hardness gradient inside the head surface portion 10A of the rail 1 decreases. As a result, the generation of crack damage in the head surface portion 10A of the rail 1 due to contact with the wheel is suppressed. Even if even a small amount of B is contained, the above effect can be obtained to some extent. However, if the B content exceeds 0.0050%, coarse iron carbides are generated. In this case, brittle fracture is promoted and the breakage resistance of the head surface portion 10A of the rail 1 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%, even more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the B content is 0.0040%, even more preferably 0.0030%, and even more preferably 0.0020%.

[0066] Cu: 0 to 1.00% Copper (Cu) is an optional element and does not necessarily need to be contained. That is, the Cu content may be 0%. When copper is contained, that is, when the Cu content exceeds 0%, Cu dissolves in ferrite in pearlite, increasing the hardness of the head surface portion 10A of the rail 1 through solid solution strengthening. As a result, the wear resistance of the head surface portion 10A of the rail 1 is improved. Even if even a small amount of Cu is contained, 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 1 is excessively increased. As a result, martensite is formed in the head surface portion 10A of the rail 1. As a result, the breakage resistance of the head surface portion 10A of the rail 1 is reduced. Therefore, the Cu content is 0 to 1.00%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.02%, even more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Cu content is preferably 0.90%, more preferably 0.80%, and even more preferably 0.70%. In order to sufficiently increase the hardness of the rail head portion and suppress the formation of martensite, the upper limit of the Cu content is more preferably 0.40%.

[0067] Ni: 0 to 1.00% Nickel (Ni) is an optional element and does not necessarily need to be contained. That is, the Ni content may be 0%. When contained, that is, when the Ni content exceeds 0%, Ni increases the toughness of pearlite. Ni also increases the hardness of the head surface portion 10A of the rail 1 through solid solution strengthening. As a result, the wear resistance of the head surface portion 10A of the rail 1 is improved. Even if even a small amount of Ni is contained, the above effect can be obtained to some extent. However, if the Ni content exceeds 1.00%, the hardenability of the steel constituting the rail 1 is excessively increased. As a result, martensite is formed in the head surface portion 10A of the rail 1. As a result, the wear resistance and breakage resistance of the head surface portion 10A of the rail 1 are reduced. Therefore, the Ni content is 0 to 1.00%. The preferred lower limit of the Ni content is 0.01%, more preferably 0.02%, even more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Ni content is preferably 0.90%, more preferably 0.80%, and still more preferably 0.70%.

[0068] V: 0 to 0.200% Vanadium (V) is an optional element and need not be present. In other words, the V content may be 0%. When vanadium (V) is present, i.e., when the V content exceeds 0%, V forms V precipitates such as V carbides, V nitrides, and V carbonitrides during the cooling process after hot rolling in the manufacturing process of the rail 1. Precipitation hardening by these V precipitates increases the hardness of pearlite. As a result, the damage resistance of the head surface portion 10A of the rail 1 is improved. Even if even a small amount of V is present, the above effect can be achieved to some extent. However, if the V content exceeds 0.200%, excessive fine V precipitates are formed. In this case, pearlite becomes embrittled. As a result, the damage resistance of the head surface portion 10A of the rail 1 is reduced. Therefore, the V content is 0 to 0.200%. The lower limit of the V content is preferably 0.001%, more preferably 0.005%, even more preferably 0.010%, even more preferably 0.020%, even more preferably 0.030%, and even more preferably 0.050%. The upper limit of the V content is preferably 0.180%, more preferably 0.150%, even more preferably 0.120%, even more preferably 0.100%, and even more preferably 0.080%.

[0069] Nb: 0 to 0.0500% Niobium (Nb) is an optional element and does not necessarily need to be contained. In other words, the Nb content may be 0%. When it is contained, that is, when the Nb content exceeds 0%, Nb forms Nb precipitates such as Nb carbides and Nb nitrides during the cooling process after hot rolling in the manufacturing process of the rail 1. Precipitation hardening by these Nb precipitates increases the hardness of pearlite. As a result, the damage resistance of the head surface portion 10A of the rail 1 is improved. Nb also contributes to the formation of Nb precipitates. ｃ１Nb precipitates are formed in the weld heat-affected zone (HAZ) reheated to a temperature range below this point. This suppresses softening of the weld heat-affected zone (HAZ) of the welded joint. Even a small amount of Nb can achieve the above effect to some extent. However, if the Nb content exceeds 0.0500%, excessive Nb precipitates are formed. In this case, pearlite becomes embrittled, and the damage resistance of the head surface portion 10A of the rail 1 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%, even more preferably 0.0020%, even more preferably 0.0025%, and even more preferably 0.0030%. The preferred upper limit of the Nb content is 0.0400%, even more preferably 0.0300%, and even more preferably 0.0200%.

[0070] Ti: 0 to 0.0500% Titanium (Ti) is an optional element and does not necessarily need to be included. In other words, the Ti content may be 0%. When titanium is included, i.e., when the Ti content exceeds 0%, Ti forms Ti precipitates such as Ti carbides and Ti nitrides during the cooling process after hot rolling in the manufacturing process of the rail 1. Precipitation hardening by these Ti precipitates increases the hardness of pearlite. As a result, the damage resistance of the head surface portion 10A of the rail 1 is improved. Furthermore, the precipitated Ti precipitates are difficult to dissolve in the matrix during reheating during welding. Therefore, the Ti precipitates refine the structure of the weld heat-affected zone (HAZ) heated to the austenite temperature range. As a result, embrittlement of the welded joint is suppressed. Even if even a small amount of Ti is included, the above effects can be achieved to some extent. However, if the Ti content exceeds 0.0500%, coarse Ti precipitates are formed. In this case, stress concentration around the coarse Ti precipitates makes fatigue cracks more likely to occur. As a result, the damage resistance of the head surface portion 10A of the rail 1 decreases. Therefore, the Ti content is 0 to 0.0500%. A preferred lower limit of the Ti content is 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, even more preferably 0.0030%, even more preferably 0.0035%, and even more preferably 0.0040%. A preferred upper limit of the Ti content is 0.0450%, more preferably 0.0400%, even more preferably 0.0300%, and even more preferably 0.0200%.

[0071] Mg: 0 to 0.0200% Magnesium (Mg) is an optional element and does not necessarily need to be contained. In other words, the Mg content may be 0%. When magnesium (Mg) is contained, that is, when the Mg content exceeds 0%, Mg combines with S to form fine sulfides (MgS). This MgS finely disperses MnS. Therefore, stress concentration around the MnS is alleviated. As a result, the damage resistance of the head surface portion 10A of the rail 1 is improved. Even if even a small amount of Mg is contained, the above effect can be achieved 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 occur. As a result, the damage resistance of the head surface portion 10A of the rail 1 is reduced. Therefore, the Mg content is 0 to 0.0200%. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0015%, and even more preferably 0.0030%. The upper limit of the Mg content is preferably 0.0180%, more preferably 0.0150%, and even more preferably 0.0120%.

[0072] Ca: 0 to 0.0200% Calcium (Ca) is an optional element and does not necessarily need to be contained. In other words, the Ca content may be 0%. When contained, that is, when the Ca content exceeds 0%, Ca combines with S to form sulfides (CaS). This CaS finely disperses MnS. Therefore, stress concentration around the MnS is alleviated. As a result, the damage resistance of the head surface portion 10A of the rail 1 is improved. Even if even a small amount of Ca is contained, the above effect can be achieved 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 occur. As a result, the damage resistance of the head surface portion 10A of the rail 1 is reduced. Therefore, the Ca content is 0 to 0.0200%. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0030%. The upper limit of the Ca content is preferably 0.0180%, more preferably 0.0150%, and even more preferably 0.0120%.

[0073] Rare earth elements (REM): 0 to 0.0500% Rare earth elements (REM) are optional elements and may not be contained. In other words, the REM content may be 0%. When contained, that is, when the REM content is more than 0%, the REM is converted into oxysulfides (REM ２ O ２S), which serve as nuclei for the formation of Mn sulfide-based inclusions. Oxysulfides have a high melting point. Therefore, they suppress the elongation of Mn sulfide-based inclusions after hot rolling. As a result, REM finely disperses MnS, mitigating stress concentration around the MnS. As a result, the damage resistance of the head surface portion 10A of the rail 1 is improved. Even if even a small amount of REM is contained, the above effect can be achieved to some extent. However, if the REM content exceeds 0.0500%, coarse REM oxysulfides are formed. In this case, stress concentration around the coarse REM oxysulfides makes fatigue cracks more likely to form. As a result, the damage resistance of the head surface portion 10A of the rail 1 is reduced. Therefore, the REM content is 0 to 0.0500%. The lower limit of the REM content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0030%. The upper limit of the REM content is preferably 0.0400%, more preferably 0.0300%, and even more preferably 0.0250%.

[0074] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, in this specification, the REM content refers to the total content of these elements.

[0075] Zr: 0 to 0.0200% Zirconium (Zr) is an optional element and may not be contained. In other words, the Zr content may be 0%. When contained, that is, when the Zr content is more than 0%, Zr is used as ZrO ２ Generates inclusions. ZrO ２Zr has a high lattice matching with austenite (γ-Fe). Therefore, γ-Fe serves as the solidification nucleus of the steel constituting the rail 1, where the solidification primary crystal is γ-Fe, and increases the equiaxed crystallization rate of the solidification structure. This suppresses the formation of a segregation zone in the center of the cast slab. Therefore, the formation of martensite in the head surface portion 10A of the rail 1 is suppressed. As a result, the breakage resistance of the head surface portion 10A of the rail 1 is improved. Even if Zr is contained even slightly, the above effect can be achieved to some extent. However, if the Zr content exceeds 0.0200%, excessive 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 head surface portion 10A of the rail 1 is reduced. Therefore, the Zr content is 0 to 0.0200%, and if contained, it is 0.0200% or less. The lower limit of the Zr content is preferably 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, and still more preferably 0.0030%. The upper limit of the Zr content is preferably 0.0180%, more preferably 0.0150%, and still more preferably 0.0120%.

[0076] The balance of the chemical composition of the rail 1 of this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition refer to impurities that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of the rail 1, and are acceptable to the extent that they do not adversely affect the rail 1 of this embodiment. The chemical composition of the rail 1 of this embodiment may contain impurities other than the above-mentioned impurities, such as O: 0.0060% or less. The O content may be 0.0001% or more, 0.0010% or more, or 0.0020% or more.

[0077] [(Feature 2) Metallic Structure and Hardness of Head Surface Portion 10A] In the rail 1 of this embodiment, the metallic structure of the head surface portion 10A has an area ratio of pearlite of 95% or more and a Vickers hardness of 400 HV or more.

[0078] [Regarding the Metal Structure of the Head Surface Portion 10A] In the metal structures of steel materials with the same hardness, pearlite has the highest wear resistance. In other words, pearlite has better wear resistance than ferrite, cementite, bainite, and martensite. Therefore, the metal structure of the head surface portion 10A of this embodiment is substantially pearlite. Specifically, the area ratio of pearlite in the metal structure of the head surface portion 10A is 95% or more.

[0079] The remaining structure of the metal structure of the head surface portion 10A other than pearlite is one or more selected from the group consisting of ferrite, cementite, bainite, and martensite. If the pearlite area ratio is 95% or more and the area ratio of the remaining structure is 5% or less, the remaining structure other than pearlite will have almost no effect on the wear resistance and damage resistance of the head surface portion 10A. The pearlite area ratio is preferably 98% or more, and most preferably 100%.

[0080] The metal structure of the portion other than the head surface portion 10A of the rail 1 is not particularly limited. The metal structure of the portion other than the head surface portion 10A may be a structure consisting of ferrite and pearlite, or may be a ferrite structure or a bainite structure. Considering the continuity from the metal structure of the head surface portion 10A, a preferred metal structure of the portion other than the head surface portion 10A is a structure consisting of ferrite and pearlite.

[0081] [Vickers hardness of head surface portion 10A] The Vickers hardness of the head surface portion 10A is 400 HV or more. If the Vickers hardness of the head surface portion 10A is less than 400 HV, sufficient wear resistance and damage resistance cannot be obtained in the head surface portion 10A of the rail 1. Therefore, the Vickers hardness of the head surface portion 10A is 400 HV or more.

[0082] The lower limit of the Vickers hardness of the head surface portion 10A is preferably 420 HV, more preferably 450 HV. The upper limit of the Vickers hardness of the head surface portion 10A is not particularly limited, but is, for example, 650 HV, preferably 600 HV, more preferably 550 HV.

[0083] [Method for Observing Metal Structure of Head Surface Portion 10A] The pearlite area ratio of the head surface portion 10A is determined by the following method. A sample is taken from the head surface portion 10A, including a position 10 mm deep from the surface of the head surface portion 10A, in a cross section perpendicular to the longitudinal direction of the rail 1. Of the surface of the taken sample, a cross section 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 etchant). Of the etched observation surface, a position 10 mm deep from the surface of the head surface portion 10A is observed using an optical microscope at a magnification of 200x. A photograph is taken, and the observation field (400 μm × 300 μm) is defined as the evaluation area.

[0084] In the observation field, pearlite can be easily distinguished from other structures (pro-eutectoid ferrite, pro-eutectoid cementite, bainite, martensite, etc.) by contrast. Ferrite is observed as a white area. Pearlite is observed as a black and white layer or a dark gray color. Bainite and martensite are observed as areas brighter than the gray color of pearlite. Pro-eutectoid cementite, like ferrite, is observed as a white area. Pearlite is identified based on the above-mentioned contrast. Then, the area ratio of pearlite is calculated based on the total area of ​​the identified pearlite and the total area of ​​the observation field. The area ratio of pearlite may also be calculated using image analysis software, etc.

[0085] The above-mentioned samples are taken from any 10 locations on the head surface portion 10A of the rail 1, and the pearlite area ratio is determined for each sample. The arithmetic mean value of the pearlite area ratios at the 10 locations is defined as the pearlite area ratio (%) of the head surface portion 10A of the rail 1.

[0086] [Method for Measuring Vickers Hardness of Head Surface Portion 10A] The Vickers hardness of the head surface portion 10A is determined by the following method. Specifically, a test piece is taken from the rail 1, with a cross section perpendicular to the longitudinal direction of the rail 1 as the measurement surface. Referring to Figure 5, on the measurement surface, a line segment extending in the depth direction from the width center position P11 of the head top portion 11 on the head outer casing surface 10S as the origin is taken as the hardness measurement line L11. The hardness measurement line L11 is coaxial with the normal to the head outer casing surface 10S at position P11. On the hardness measurement line L11, measurement points are taken at a depth of 1 mm, a depth of 10 mm, and a depth of 20 mm in the depth direction from position P11.

[0087] Also, the line segment L13, which is a tangent to the lower jaw 13 on the measurement surface of the rail 1, Ｌ and L13 Ｒ Then, the line segment L13 Ｌ and L13 Ｒ The intersection of these lines is defined as position P13. Position P13 is located on the hardness measurement line L11.

[0088] At the head corner portion 12L of the measurement surface, the line segment connecting point E and point P13 is the hardness measurement line L12. Ｌ From point E to hardness measurement line L12 Ｌ The measurement points are located at a depth of 1 mm, a depth of 10 mm, and a depth of 20 mm in the depth direction along the line 12R. Similarly, the line segment connecting point F and point P13 in the head corner portion 12R is defined as the hardness measurement line L12. Ｒ From point F to hardness measurement line L12 Ｒ Measurement points are set at 1 mm, 10 mm, and 20 mm depth positions in the depth direction along the line. Vickers hardness tests are performed at each measurement point in accordance with JIS Z 2241-1 (2020) to obtain the Vickers hardness (HV). The test force is 10 kgf.

[0089] Three hardness measurement lines L11 and L12 Ｌ , L12 Ｒ The arithmetic mean value of the same depth measurement points (three points) is taken as the hardness at that depth position. Ｌ , L12 ＲThe arithmetic mean value of the three hardnesses at a depth of 1 mm is defined as the Vickers hardness H1 (HV) at a depth of 1 mm. Ｌ , L12 Ｒ The arithmetic mean value of the three hardnesses at a depth of 10 mm is defined as the Vickers hardness H10 (HV) at a depth of 1 mm. Ｌ , L12 Ｒ The arithmetic mean value of the three hardnesses at a depth of 20 mm is defined as the Vickers hardness H20 (HV) at a depth of 1 mm.

[0090] If the obtained Vickers hardness H1 at a depth of 1 mm, the Vickers hardness H10 at a depth of 10 mm, and the Vickers hardness H20 at a depth of 20 mm are all 400 HV or more, the Vickers hardness of the head surface portion 10A is determined to be 400 HV or more.

[0091] [(Feature 3) Degree of Positive Si Segregation in the Head Surface Portion 10A] In the rail 1 of this embodiment, the degree of positive Si segregation in the head surface portion 10A is more than 1.00 and 1.35. If the degree of positive Si segregation exceeds 1.35, early cracks are more likely to occur. If early cracks occur, large cracks are more likely to form. As a result, the damage resistance of the rail 1 decreases. If the degree of positive Si segregation is 1.35 or less, the occurrence of early cracks is sufficiently suppressed. Therefore, the rail 1 can achieve excellent damage resistance. The upper limit of the degree of positive Si segregation is preferably 1.34, more preferably 1.32, even more preferably 1.30, even more preferably 1.28, even more preferably 1.25, even more preferably 1.20, and even more preferably 1.15. The lower limit of the degree of positive Si segregation is higher than 1.00. The lower limit of the degree of positive Si segregation is preferably as close to 1.00 as possible. However, excessively lowering the lower limit of the positive Si segregation degree increases the production cost. Therefore, in consideration of the production cost, the lower limit of the positive Si segregation degree is preferably 1.01, more preferably 1.03, and even more preferably 1.05.

[0092] [(Feature 4) Degree of Negative Si Segregation in the Head Surface Region 10A] In the rail 1 of this embodiment, the degree of negative Si segregation in the head surface region 10A is 0.90 to less than 1.00. If the degree of negative Si segregation is less than 0.90, a propagation crack originating from an initial crack is likely to grow. The reason for this is unclear, but the following is thought to be the reason. As mentioned above, Si dissolves in the pearlite to increase the strength of ferrite. However, if the degree of negative Si segregation is less than 0.90, the strength of ferrite in pearlite decreases in the semi-macrosegregation region of negative Si segregation. As a result, a propagation crack is likely to grow. If a propagation crack grows, coarse and large cracks are generated. As a result, the damage resistance of the rail 1 is reduced. If the degree of negative Si segregation is 0.90 or more, the growth of a propagation crack is sufficiently suppressed. Therefore, the rail 1 can achieve excellent damage resistance.

[0093] The lower limit of the negative Si segregation is preferably 0.91, more preferably 0.92, even more preferably 0.95, even more preferably 0.97, and even more preferably 0.99. The upper limit of the negative Si segregation is less than 1.00. The upper limit of the negative Si segregation is preferably as close to 1.00 as possible. However, an excessive increase in the lower limit of the negative Si segregation increases production costs. Therefore, when production costs are taken into consideration, the upper limit of the positive Si segregation is preferably 0.99, and even more preferably 0.98.

[0094] [Method for Measuring the Degrees of Positive and Negative Si Segregation in the Head Surface Portion 10A] The degrees of positive and negative Si segregation in the head surface portion 10A are determined by the following method. A test piece is taken, with the measurement surface extending in the depth direction (the direction normal to point P11 on the surface of the rail head 10) and the longitudinal direction of the rail 1, starting from point P11 (see FIG. 1B ) at the width center of the surface of the rail head 10 of the rail 1. Five analysis regions, each 800 μm in length and 800 μm in depth, are selected from the measurement surface of the test piece, ranging from a depth of 10 mm to a depth of 20 mm starting from the surface of the rail head 10. The five analysis regions are arranged consecutively in the longitudinal direction of the rail 1, with the ends of adjacent analysis regions in contact with each other.

[0095] The analysis area is subjected to area analysis using an electron probe microanalyzer (EPMA) to create an element distribution map of Si concentration. In the EPMA, the acceleration voltage is 15 kV, the irradiation current is 0.1 μA, the time is 50 ms, and the beam diameter is 2 μm.

[0096] Based on the element distribution map, the Si semi-macro positive segregation zone and the Si semi-macro negative segregation zone are identified. Specifically, for each analysis region, the average Si concentration [Si] in the analysis region is determined based on the element distribution map of the Si concentration obtained. The above-described element distribution map is divided into the following four Si concentration regions using the average Si concentration [Si] as the median. That is, the element map is quaternized using the average Si concentration [Si] as the median. Area 1: Region higher than 1.15 × [Si] Area 2: Region equal to or greater than [Si] and equal to or less than 1.15 × [Si] Area 3: Region equal to or greater than 0.85 × [Si] and less than [Si] Area 4: Region less than 0.85 × [Si] Of the quaternized analysis regions, the region corresponding to Area 1 is identified as the Si semi-macro positive segregation zone. Of the quaternized analysis regions, the region corresponding to Area 4 is identified as the Si semi-macro negative segregation zone. Fifty Si semi-macro positive segregation zones are selected from the five analysis regions. Similarly, 50 Si semi-macro negative segregation zones are selected from the five analysis regions. In the selected Si semi-macro positive segregation zones and semi-macro negative segregation zones, the Si positive segregation degree and the Si negative segregation degree are determined by the following method.

[0097] [Method for Measuring the Degree of Positive Si Segregation] Fig. 6 is a schematic diagram for explaining how to determine the degree of positive Si segregation. Referring to Fig. 6, the element distribution map of the Si concentration obtained by EPMA described above is composed of a plurality of line analyses (line scans). Therefore, from the plurality of line analyses constituting the element distribution map of the Si concentration, a line analysis that crosses the identified Si semi-macro positive segregation band is selected to obtain the Si concentration distribution along that line. Specifically, when the Si semi-macro positive segregation band has a shape with a large aspect ratio, such as an ellipse, a line analysis that passes through approximately the center of the Si segregation band in the longitudinal direction is selected.

[0098] 6, in the Si concentration distribution obtained by the selected line analysis, the arithmetic mean value (mass %) of the Si concentration is determined for each of the bulk regions L1 (1000 μm) and R1 (1000 μm) located on either side of the Si semi-macro positive segregation band. The lower of the average Si concentration values ​​in the bulk region L1 and the bulk region R1 is taken as the Si concentration (mass %) in the bulk region.

[0099] [Quantification of Si Concentration in Si Semi-macro Positive Segregation Zone] Furthermore, in the Si concentration analysis obtained by the selected line analysis, the maximum value of the Si concentration in the Si semi-macro positive segregation zone is determined, and the arithmetic mean value of the maximum value of the Si concentration and the Si concentration in the bulk region is defined as the Si concentration (mass%) in the Si semi-macro positive segregation zone.

[0100] [Determination of the degree of positive Si segregation] Based on the Si concentration in the bulk region and the Si concentration in the Si semi-macro positive segregation band obtained by the above-mentioned method, the degree of Si segregation in the Si semi-macro positive segregation band is calculated using the following formula: Positive Si segregation = Si concentration in Si semi-macro positive segregation band / Si concentration in bulk region The degree of positive Si segregation is calculated for each of the 50 Si semi-macro positive segregation bands by the above-mentioned method. The arithmetic mean value of the top 20 obtained positive Si segregations is taken as the positive Si segregation degree in the Si semi-macro positive segregation band.

[0101] [Method for Measuring the Degree of Negative Si Segregation] Fig. 7 is a schematic diagram for explaining how to determine the degree of negative Si segregation. Referring to Fig. 7, the element distribution map of Si concentration obtained by EPMA described above is composed of a plurality of line analyses (line scans). Therefore, among the plurality of line analyses constituting the element distribution map of Si concentration, a line analysis that crosses the identified Si semi-macro negative segregation zone is selected to obtain the Si concentration distribution along that line. Specifically, when the Si semi-macro negative segregation zone has a shape with a large aspect ratio, such as an ellipse, a line analysis that passes through approximately the center of the Si segregation zone in the longitudinal direction is selected.

[0102] 7, in the Si concentration distribution obtained by the selected line analysis, the arithmetic mean value (mass %) of the Si concentration is determined for each of the bulk regions L1 (1000 μm) and R1 (1000 μm) located on either side of the Si semi-macro negative segregation zone. The lower of the average Si concentration values ​​in the bulk region L1 and the bulk region R1 is taken as the Si concentration (mass %) in the bulk region.

[0103] [Quantification of Si concentration in the Si semi-macro negative segregation zone] Furthermore, in the Si concentration analysis obtained by the selected line analysis, the maximum Si concentration in the Si semi-macro negative segregation zone is determined, and the arithmetic mean value of the minimum Si concentration and the Si concentration in the bulk region is determined as the Si concentration (mass%) in the Si semi-macro negative segregation zone.

[0104] [Determination of the degree of negative Si segregation] Based on the Si concentration in the bulk region and the Si concentration in the Si semi-macro negative segregation band obtained by the above-mentioned method, the degree of Si segregation in the Si semi-macro negative segregation band is calculated using the following formula: Negative Si segregation = Si concentration in Si semi-macro negative segregation band / Si concentration in bulk region The degree of negative Si segregation is calculated for each of the 50 Si semi-macro negative segregation bands by the above-mentioned method. The arithmetic mean value of the top 20 obtained negative Si segregations is taken as the negative Si segregation degree in the Si semi-macro negative segregation band.

[0105] [Advantages of the Rail 1 of the Present Embodiment] The rail 1 of the present embodiment satisfies Features 1 to 4. Therefore, the rail 1 of the present embodiment has excellent wear resistance and damage resistance.

[0106] [Manufacturing Method of Rail 1 of the Present Embodiment] An example of a manufacturing method of the rail 1 of the present embodiment will be described. The manufacturing method of the rail 1 described below is one example for manufacturing the rail 1 of the present embodiment. Therefore, the rail 1 having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a manufacturing method of the rail 1 of the present embodiment.

[0107] An example of a method for manufacturing a rail 1 according to this embodiment includes the following steps: (Step 1) Material Manufacturing Step, (Step 2) Hot Rolling Step, and (Step 3) Heat Treatment Step. The rail 1 according to this embodiment is obtained by preparing a bloom having a chemical composition that satisfies Feature 1 (Step 1), reheating the bloom, and then hot rolling it to form a preform having a rail shape (Step 2). The preform is then subjected to controlled cooling (Step 3). Each step will be described below.

[0108] [(Step 1) Material Production Step] In the material production step, a bloom is produced as the material for the rail 1 of this embodiment. Specifically, molten steel having a chemical composition that satisfies Feature 1 is produced by melting in a commonly used melting furnace such as a converter or an electric furnace. The produced molten steel is then used to produce a bloom by continuous casting.

[0109] [Continuous Casting Conditions for Satisfying Feature 3] In continuous casting, the bloom is lightly reduced. Lightly reducing the bloom in continuous casting reduces the degree of positive Si segregation in the bloom. If the degree of positive Si segregation in the bloom is reduced, the degree of positive Si segregation in the rail 1 to be produced also decreases.

[0110] The soft reduction in continuous casting satisfies the following conditions: (Condition 1) The soft reduction is started when the central solid fraction of the bloom is 10 to 25%. (Condition 2) The soft reduction is ended when the central solid fraction of the bloom is more than 70%. These conditions will be explained below.

[0111] [Regarding Condition 1] The degree of Si segregation correlates with the central solid fraction of the bloom during soft reduction of the bloom. Specifically, immediately after the bloom leaves the mold, only the surface layer is solidified, and the interior is molten. As the bloom leaves the mold and travels downstream, internal solidification of the bloom progresses, and the central solid fraction increases. Here, the central solid fraction of the bloom refers to the solidification ratio in a 20 mm x 20 mm region (hereinafter also referred to as the bloom center) in a cross section (height 320 mm x width 380 mm) perpendicular to the longitudinal direction of the bloom. When the entire center of the bloom is solidified, the central solid fraction of the bloom is 100%.

[0112] If the bloom is soft reduced when its central solid fraction is low, that is, when the internal solidification of the bloom has not progressed, the Si positive segregation in the bloom is suppressed. In this embodiment, the soft reduction is started when the central solid fraction is 10 to 25%. If the central solid fraction of the bloom at the start of the soft reduction exceeds 25%, the soft reduction does not sufficiently penetrate into the interior of the bloom. Therefore, the Si positive segregation degree is not sufficiently reduced. On the other hand, if the central solid fraction of the bloom at the start of the soft reduction is less than 10%, the bloom is not sufficiently solidified. Even if the bloom in this state is soft reduced, the Si positive segregation degree is not sufficiently reduced. Therefore, the soft reduction is started when the central solid fraction is 10 to 25%. Note that the Si positive segregation degree can be further suppressed by starting the soft reduction when the central solid fraction is 20% or less.

[0113] [Regarding Condition 2] Furthermore, when the central solid fraction of the bloom exceeds 70%, the soft reduction is terminated. If the soft reduction is terminated when the central solid fraction is 70% or less, the reduction does not sufficiently penetrate to the center of the bloom. As a result, the degree of positive Si segregation may not be reduced sufficiently. If the soft reduction is terminated when the central solid fraction is more than 70%, the reduction will sufficiently penetrate to the center of the bloom. As a result, the degree of positive Si segregation is reduced sufficiently. When the central solid fraction of the bloom exceeds 70%, the reduction rate of the soft reduction is set to 40 to 70% of the reduction rate when the central solid fraction is 70% or less.

[0114] The reduction rate during soft reduction refers to the ratio of the amount of reduction during soft reduction to the height (thickness) of the bloom before the start of soft reduction. Specifically, it can be defined by the following formula: Reduction rate during soft reduction (%) = (thickness of bloom before the start of soft reduction - reduction amount during soft reduction) / thickness of bloom before the start of soft reduction x 100

[0115] When the central solid fraction exceeds 70%, the total reduction rate of the soft reduction (that is, the reduction rate of the soft reduction in this embodiment) is 2 to 5%.

[0116] By carrying out soft reduction in continuous casting under the above conditions, the degree of positive Si segregation in the manufactured rail 1 is reduced to 1.35 or less.

[0117] The central solid fraction of the bloom is calculated as follows: The temperature at the center of the bloom in the thickness direction during continuous casting is calculated by one-dimensional heat transfer solidification calculation. The heat transfer solidification calculation uses the enthalpy method.

[0118] Specifically, the bloom is divided into N elements in the thickness direction. N=1000. The initial temperature of all elements is set to the molten steel temperature (°C) at the time of pouring 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 set to 1 second. The solid fraction FS of element i (i=1 to N) at the nth time step is ｎ ｉ is expressed by the following formula (A).

[0119]

[0120] Here, T.C. ｎ ｉ is the temperature (°C) of element i at the nth time step. TS is the solidus temperature, TL is the liquidus temperature, and TC ｎ ｉ satisfies the following formula: TS≦TC ｎ ｉ ≦TL

[0121] Also, the enthalpy H of element i at the nth time step ｎ ｉ is calculated based on the following formula (B): 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, density ρ is 7800 kg / m ３ , the specific heat C is 700 J / (kg·K), the reference temperature Tref = 0°C, and the latent heat L is 250 kJ / kg. Using the following equation (C) which is a discretized version of the heat conduction equation, the enthalpy H of the next time step (the n+1th time step) is calculated. ｎ＋１ ｉ Ask for.

[0122]

[0123] Here, H ｎ＋１ ｉ and H ｎ ｉ is the enthalpy of element i at the n+1th time step and the nth time step. ｎ ｉ＋１ and T.C. ｎ ｉ－１ is the temperature of the adjacent element of element i. k is the thermal conductivity. Δt is the time step, and Δx is the size of element i (i.e., the value obtained by dividing the thickness of the bloom by N). The thermal conductivity k at temperature T is calculated using the following formula: k = 39.0 - 0.0138 x (T - 500)

[0124] The resulting enthalpy H ｎ＋１ ｉ Using equations (A) and (B), the temperature TC of the next time step (n+1 time step) is calculated. ｎ ｉ－１ The change in the solid fraction at the center position of the bloom (i=N / 2) over time is tracked to determine the central solid fraction, which is the solid fraction at the center position of the bloom.

[0125] [(Step 2) Hot Rolling Step] The produced bloom is reheated and then hot rolled to form a rail blank.

[0126] First, the bloom is loaded into a heating furnace and reheated. If the reheating temperature of the bloom is less than 1000°C, sufficient formability during hot rolling cannot be ensured. Furthermore, rolling defects are likely to occur during hot rolling. On the other hand, if the reheating temperature of the bloom exceeds 1350°C, the steel may melt, making rail manufacturing difficult. Therefore, the reheating temperature of the bloom is set to 1000 to 1350°C. After heating the bloom to within this temperature range, it is removed from the heating furnace. The bloom is then hot rolled to form a rail blank. The bloom heating temperature refers to the temperature at the center of the cross section perpendicular to the longitudinal direction of the bloom. The bloom heating temperature can be measured by embedding a thermocouple in the center of the bloom. The bloom heating temperature can also be calculated by one-dimensional heat transfer and solidification calculation based on actual measurements using a thermocouple and the heating pattern of the heating furnace. The heat transfer and solidification calculation uses the enthalpy method or the equivalent specific heat method.

[0127] [Conditions for reheating in a heating furnace] The following conditions must be further satisfied when reheating in a heating furnace: (Condition 3) The average heating rate HR1 in the temperature range from 50°C to 600°C is 2.0 to 5.0°C / min. (Condition 4) The average heating rate HR2 in the temperature range from 700 to 900°C is more than 5.0 to 8.0°C / min. These conditions are explained below.

[0128] [(Condition 3) Average Heating Rate HR1] In steel, Si actively diffuses at temperatures higher than 900°C. However, the distribution of Si in the bloom at temperatures below 900°C affects the diffusion of Si above 900°C. Therefore, it is preferable to reduce the fluctuation in the distribution of Si in the bloom at temperatures below 900°C to some extent.

[0129] In the bloom temperature range of 600°C or less, carbides are generated in the bloom. The interface between the carbide and the matrix becomes a diffusion path for Si. Therefore, if an appropriate amount of carbides of an appropriate size are generated in the temperature range from room temperature to 600°C, a sufficient amount of interfaces between the carbide and the matrix are formed, and a sufficient number of Si diffusion paths are formed. In this case, fluctuations in the Si distribution in the bloom below 900°C can be suppressed to some extent.

[0130] If the average heating rate HR1 from room temperature to 600°C is less than 2.0°C / min, the generated carbides become excessively coarse. In this case, the interface between the carbide and the base material is insufficient. As a result, a sufficient number of Si diffusion paths cannot be secured. Furthermore, if the average heating rate HR1 exceeds 5.0°C / min, the bloom temperature exceeds the carbide generation temperature range (600°C or less) before a sufficient amount of carbide is generated. Therefore, the amount of carbide generated is insufficient, and the interface between the carbide and the base material is insufficient. As a result, a sufficient number of Si diffusion paths cannot be secured.

[0131] If the average heating rate HR1 is 2.0 to 5.0°C / min, a sufficient amount of carbides of appropriate size are generated. In this case, a sufficient amount of interfaces between the carbides and the matrix are formed. Therefore, sufficient diffusion paths for Si are formed. As a result, fluctuations in the Si distribution in the bloom below 900°C can be suppressed to some extent.

[0132] [(Condition 4) Average Heating Rate HR2] In the temperature range of 700 to 900°C, carbides in the bloom dissolve. As described above, Si actively diffuses above 900°C. However, the distribution of Si in the bloom at temperatures below 900°C affects the diffusion of Si above 900°C. Therefore, it is preferable to reduce the fluctuation of the Si distribution in the bloom at temperatures below 900°C to some extent.

[0133] When the average heating rate HR2 in the temperature range of 700 to 900°C is 5.0°C / min or less, the carbides in the bloom dissolve. However, because the heating rate is slow, C in the dissolved carbides may regenerate as carbides. In this case, the regenerated carbides inhibit the diffusion of Si. Therefore, the fluctuations in the Si distribution are not resolved and tend to remain as they are. As a result, the temperature exceeds 900°C with localized fluctuations in the Si distribution remaining within the bloom. In this case, even if the temperature exceeds 900°C and Si diffusion is promoted, the fluctuations in the Si distribution cannot be fully resolved.

[0134] Furthermore, if the average heating rate HR2 in the temperature range of 700 to 900°C exceeds 8.0°C / min, some of the carbides in the bloom will not completely dissolve and will remain. In this case, some of the remaining carbides will hinder the diffusion of Si. Therefore, even in this case, the fluctuations in the Si distribution are not resolved and tend to remain as they are. As a result, the temperature exceeds 900°C with localized fluctuations in the Si distribution remaining within the bloom. In this case, even if the temperature exceeds 900°C and Si diffusion is promoted, the fluctuations in the Si distribution cannot be fully resolved.

[0135] If the average heating rate HR2 is greater than 5.0 to 8.0°C / min, the bloom temperature exceeds 900°C at a relatively early stage after the carbide dissolves. Therefore, a C-enriched region does not exist for a long time, and C diffuses and disperses early after the carbide dissolves. As a result, fluctuations in the Si distribution are reduced to a certain extent at temperatures below 900°C. As a result, Si diffusion is promoted after the temperature exceeds 900°C, making it easier for Si to be uniformly dispersed.

[0136] The bloom temperature, like the heating temperature described above, refers to the temperature at the center of the bloom in a cross section perpendicular to the longitudinal direction. The bloom temperature can be measured by embedding a thermocouple in the center of the bloom. The bloom heating temperature can also be determined by FEM analysis based on the actual measurement value using the thermocouple and the heat pattern of the heating furnace.

[0137] The average heating rate HR1 (°C / min) can be calculated by determining the time it takes for the bloom temperature to rise from room temperature to 600°C. Also, the average heating rate HR2 (°C / min) can be calculated by determining the time it takes for the bloom temperature to rise from 700°C to 900°C.

[0138] The bloom heated under the above-described reheating conditions is subjected to hot rolling to produce a rail-shaped preform (rail preform). In hot rolling, the temperature of the rail preform when it passes through the final pass is referred to as the final rolling temperature. The temperature when it passes through the rolling stand that performs the final reduction on the rail preform. The final rolling temperature can be measured, for example, by a thermometer installed on the outlet side of the rolling stand that performs the final reduction.

[0139] If the final rolling temperature is less than 750°C, pearlite transformation begins immediately after the completion of hot rolling. In this case, the rail 1 cannot be hardened by heat treatment after rolling, and sufficient wear resistance cannot be obtained. On the other hand, if the final rolling temperature exceeds 1100°C, austenite grains coarsen in the rail preform after hot rolling. In this case, hardenability increases excessively. As a result, bainite, which reduces wear resistance, is formed in the head surface portion 10A of the rail 1. As a result, sufficient wear resistance cannot be obtained in the rail 1. Therefore, the final rolling temperature is set to 750 to 1100°C.

[0140] Other hot rolling conditions are not particularly limited. To ensure the hardness of the head surface portion 10A of the rail 1, groove rolling, which is a normal rail manufacturing process, may be carried out while controlling the bloom reheating temperature and final rolling temperature as described above. For example, after rough rolling the bloom, intermediate rolling is carried out over multiple passes using a reversing rolling mill, followed by multiple passes of finish rolling using a continuous rolling mill. During the final rolling of this finish rolling, the final rolling temperature may be controlled within the above temperature range.

[0141] [(Step 3) Heat Treatment Step] The rail preform immediately after hot rolling is subjected to the next heat treatment (accelerated cooling). The equipment for performing the accelerated cooling is not particularly limited. For example, accelerated cooling may be performed using a cooling device 20 shown in FIG. 8. Referring to FIG. 8, the cooling device 20 includes a plurality of injection nozzles 21 that inject a refrigerant onto the head surface portion 10A, a supply path 210 that supplies the refrigerant to the plurality of injection nozzles 21, and a supply device 22 that supplies the refrigerant to the supply path 210.

[0142] The multiple jet nozzles 21 are arranged in the longitudinal direction of the rail preform 100 over the entire length of the rail preform 100. Note that the cooling device 20 may not arrange the multiple jet nozzles 21 in the longitudinal direction of the rail preform 100 over the entire length of the rail preform 100, but may arrange them only on a portion of the rail preform 100. In this case, the cooling device 20 further includes a drive device that moves the rail preform 100 in the longitudinal direction during accelerated cooling. Even in this case, the entire length of the rail preform can be continuously cooled.

[0143] The multiple injection nozzles 21 are arranged to surround the rail head 10. The injection nozzles 21 inject refrigerant toward the head surface portion 10A. The supply device 22 supplies the refrigerant to the supply path 210. The supply device 22 supplies the refrigerant to the supply path 210, and the refrigerant is injected from the injection nozzles 21 to accelerate and cool the head surface portion 10A. The refrigerant is not particularly limited. The refrigerant may be compressed air (air), a mixed refrigerant (mist) of water and air, or water. A combination of these refrigerants may also be used.

[0144] [Cooling Conditions] The following conditions are satisfied in accelerated cooling. (Condition 5) The cooling start temperature TS at the head outer surface 10S is 750°C or higher. (Condition 6) The average cooling rate CR at the head outer surface 10S is 2.0 to 20.0°C / sec. (Condition 7) The cooling end temperature TE at the head outer surface 10S is 500 to 660°C.

[0145] Here, the cooling start temperature TS refers to the temperature (°C) of the head outer surface 10S when accelerated cooling of the rail preform begins. In other words, the cooling start temperature TS refers to the temperature (°C) of the rail preform when injection of refrigerant onto the rail preform begins. The average cooling rate CR refers to the value (°C / sec) obtained by dividing the difference between the cooling start temperature TS and the cooling stop temperature TE by the cooling time. The cooling stop temperature TE refers to the temperature (°C) of the rail preform when cooling of the rail preform at the average cooling rate CR is completed. These conditions will be explained below.

[0146] [Condition 5: Cooling Start Temperature TS] If the cooling start temperature TS is less than 750°C, pearlite will form before the accelerated cooling starts. In this case, sufficient hardness will not be obtained in the head surface portion 10A. Furthermore, if the C content of the rail 1 is high, pro-eutectoid cementite will form if the cooling start temperature TS is less than 750°C. In this case, sufficient toughness will not be obtained in the rail 1. If the cooling start temperature TS is 750°C or higher, accelerated cooling can be started at a temperature higher than the pearlite transformation start temperature and the pro-eutectoid cementite transformation start temperature. Therefore, by performing accelerated cooling to the cooling stop temperature TE at the above-mentioned average cooling rate CR, an appropriate metal structure and hardness can be obtained in the rail 1.

[0147] [(Condition 6) Average Cooling Rate CR] If the average cooling rate CR is less than 2.0°C / sec, the pearlite lamellae formed in the head surface portion 10A become coarse. In this case, sufficient hardness cannot be obtained in the head surface portion 10A. On the other hand, if the average cooling rate CR exceeds 20.0°C / sec, excessive bainite and martensite are formed in the head surface portion 10A, resulting in an excessively low pearlite area ratio. If the average cooling rate CR is 2.0 to 20.0°C / sec, an appropriate metal structure and hardness can be obtained in the head surface portion 10A.

[0148] [Condition 7: Cooling Stop Temperature TE] If the cooling stop temperature TE exceeds 660°C, the pearlite lamellae formed in the head surface portion 10A become coarse. In this case, sufficient hardness cannot be obtained in the head surface portion 10A. On the other hand, if the cooling stop temperature TE is less than 500°C, excessive bainite and martensite are formed in the head surface portion 10A, resulting in an excessively low pearlite area ratio. If the cooling stop temperature TE is 500 to 660°C, an appropriate metal structure and hardness can be obtained in the head surface portion 10A.

[0149] By the above manufacturing method, the rail 1 satisfying features 1 to 4 is manufactured.

[0150] The effects of the rail of this embodiment will be described more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the rail of this embodiment. Therefore, the rail of this embodiment is not limited to this one example of conditions.

[0151] Rails were manufactured having the chemical compositions shown in Table 1 (Tables 1A to 1D).

[0152]

[0153]

[0154]

[0155]

[0156] In Table 1, "-" indicates that the content of the corresponding element is at or below the impurity level. Note that in all test numbers, the O content was 0.0060% or less.

[0157] The rails of each test number were manufactured under the manufacturing conditions shown in Table 2 (Table 2A and Table 2B).

[0158]

[0159]

[0160] Specifically, blooms were produced by continuous casting using molten steel. The central solid fractions (%) of the blooms at the start and end of soft reduction during continuous casting are shown in Table 2. The reduction ratios during soft reduction were 2 to 5%. The reduction ratios when the central solid fraction exceeded 70% were all 40 to 70% of the reduction ratios when the central solid fraction was 70% or less.

[0161] The produced blooms were subjected to hot rolling. First, the blooms were reheated to 1000 to 1350°C. The average heating rates HR1 (°C / min) and HR2 (°C / min) were as shown in Table 2. The reheated blooms were hot rolled to produce rail preforms having a rail shape. The final rolling temperature was 750 to 1100°C for all test numbers.

[0162] The rail preform after hot rolling was subjected to heat treatment (accelerated cooling). The cooling device shown in Figure 8 was used. The cooling start temperature TS (°C), average cooling rate CR (°C / sec), and cooling end temperature TE (°C) were as shown in Table 2.

[0163] Rails with each test number were manufactured according to the above manufacturing process.

[0164] [Evaluation Tests] The following evaluation tests were carried out using the manufactured rails: (Test 1) Pearlite area ratio measurement test of head surface portion (Test 2) Vickers hardness measurement test of head surface portion (Test 3) Si positive segregation measurement test (Test 4) Si negative segregation measurement test (Test 5) Wear resistance and damage resistance evaluation test Each test will be explained below.

[0165] [(Test 1) Test for measuring pearlite area ratio in head surface portion] Based on the method described in the above-mentioned [Method for observing metallographic structure of head surface portion 10A], the pearlite area ratio in the head surface portion of the rail of each test number was measured. The measurement results are shown in Table 3 (Table 3A and Table 3B). Note that for each test number, the pearlite area ratio was 95% or more, and the remainder of the metallographic structure other than pearlite was one or more types selected from the group consisting of ferrite, cementite, bainite, and martensite.

[0166]

[0167]

[0168] [(Test 2) Vickers hardness measurement test of head surface portion] Based on the method described in the above [Method for measuring Vickers hardness of head surface portion 10A], the Vickers hardness H1 at a depth of 1 mm, the Vickers hardness H10 at a depth of 10 mm, and the Vickers hardness H20 at a depth of 20 mm on the head surface portion of the rail for each test number were obtained. The obtained Vickers hardnesses H1, H10, and H20 are shown in Table 3.

[0169] [(Test 3) Si positive segregation measurement test] The Si positive segregation in the head surface portion 10A of each test number was determined based on the method described in the above [Method for measuring Si positive segregation and Si negative segregation in the head surface portion 10A]. The obtained Si positive segregation is shown in Table 3.

[0170] [(Test 4) Si Negative Segregation Measurement Test] The Si negative segregation in the head surface portion 10A of each test number was determined based on the method described in the above [Method for measuring Si positive segregation and Si negative segregation in the head surface portion 10A]. The obtained Si negative segregation is shown in Table 3.

[0171] [(Test 5) Wear Resistance and Damage Resistance Evaluation Test] The wear resistance and damage resistance evaluation test was conducted as follows. Two rails were prepared for each test number. The rail shape for each test number was 136 pounds (mass: 67 kg / m) and 2 m in length.

[0172] The following rolling fatigue tests were conducted using rails with each test number. [Rolling Fatigue Test] The rolling fatigue test was conducted as follows. For the rolling fatigue test, a rolling fatigue testing machine shown in Figure 3 was used. An AAR type wheel 2 with a diameter of 920 mm was prepared. The radial load during the test was 300 kN, and the thrust load was 100 kN. Repeated lubrication of water supply and drying was adopted as the lubrication method during the test. Specifically, during the test, water was supplied to the rail 1 for a certain period of time, and then the water supply was stopped and the rail 1 was dried for a certain period of time (water supply and drying). During the test, the number of repeated load applications using the wheel 2 was set to a maximum of 5 million times, and the cumulative passing tonnage was set to a maximum of 150 million tons.

[0173] [Method for Evaluating Damage Resistance of Rolling Contact Surface] The number of cracks on the rolling contact surface of a rail after a rolling fatigue test was evaluated as follows. A 300 mm area in the longitudinal direction of the head corner 12 of the rail 1 was designated as the evaluation area. A test specimen was taken from the evaluation area of ​​the rail 1 after the test, with the cross section parallel to the longitudinal direction of the evaluation area and including a normal to the head outer surface 10S of the head corner 12. The length of the observation surface in the longitudinal direction of the rail 1 was 300 mm. The observation surface was mirror-polished. The mirror-polished observation surface was visually inspected to confirm the presence or absence of cracks propagating from the head outer surface 10S in the depth direction of the rail 1. If a crack was detected, the depth D (see Figures 2A to 2C) of the crack from the contact surface (head outer surface 10S) was measured. The number of cracks with a length D of 500 μm or more was counted. The resulting number was designated the number of cracks. The number of cracks detected is shown in Table 3. If the number of cracks that occurred was 5 or less, it was determined that excellent damage resistance was obtained.

[0174] [Method for Evaluating Wear Amount of Rail Head] A 1000 mm long area in the longitudinal direction of the rail 1 after the rolling fatigue test was designated as the evaluation area. The evaluation area was cut into cross sections perpendicular to the longitudinal direction of the rail 1 at 100 mm intervals, and the shapes of the cross sections (a total of 11 cross sections) were recorded. The cross-sectional shapes perpendicular to the longitudinal direction of the test pieces of the rail 1 before the rolling fatigue test were also recorded in advance. The cross-sectional shapes of the rail 1 before the rolling fatigue test and each cross-sectional shape after the rolling fatigue test were compared, and the wear depth (mm) at the center of the width of the rail 1 at the rail head 10 was measured for each of the 11 cross-sectional shapes after the rolling fatigue test. The maximum wear depth obtained was designated the wear amount (mm). The wear amount is shown in Table 3. A wear amount of less than 5.00 mm was determined to indicate excellent wear resistance.

[0175] [Evaluation Results] Referring to Tables 1 to 3, the rails of test numbers 1 to 54 satisfied features 1 to 4. Therefore, the number of cracks that occurred was less than 5, and excellent damage resistance was obtained. Furthermore, the wear amount was less than 5.00 mm, and excellent wear resistance was obtained.

[0176] On the other hand, in test numbers 55 to 58, the central solid fraction of the bloom at the start of soft reduction during continuous casting was too high. Therefore, the degree of positive Si segregation was too high. As a result, the number of cracks that occurred was five or more, and the damage resistance was low.

[0177] In test numbers 59 and 60, the central solid fraction of the bloom at the end of soft reduction in continuous casting was too low. Therefore, the degree of positive Si segregation was too high. As a result, the number of cracks that occurred was five or more, and the damage resistance was low.

[0178] In the test samples 61 and 62, the average heating rate HR1 was too slow in the hot rolling process. Therefore, the degree of negative Si segregation was too low. As a result, the number of cracks that occurred was five or more, and the damage resistance was low.

[0179] In test numbers 63 to 65, the average heating rate HR1 was too fast in the hot rolling process. Therefore, the degree of negative Si segregation was too low. As a result, the number of cracks that occurred was five or more, and the damage resistance was low.

[0180] In the test numbers 66 and 67, the average heating rate HR2 was too slow in the hot rolling process. Therefore, the degree of negative Si segregation was too low. As a result, the number of cracks that occurred was five or more, and the damage resistance was low.

[0181] In the test numbers 68 and 69, the average heating rate HR2 was too fast in the hot rolling process. Therefore, the degree of negative Si segregation was too low. As a result, the number of cracks that occurred was five or more, and the damage resistance was low.

[0182] In test numbers 70 and 71, the accelerated cooling start temperature TS in the heat treatment process was too low. Therefore, sufficient hardness was not obtained in the head surface portion 10A. As a result, the number of cracks that occurred was 5 or more, and sufficient damage resistance was not obtained. Furthermore, the wear amount was 5.00 mm or more, and sufficient wear resistance was not obtained.

[0183] In test numbers 72 and 73, the average cooling rate CR of the accelerated cooling in the heat treatment process was too slow. Therefore, sufficient hardness was not obtained in the head surface portion 10A. As a result, the number of cracks that occurred was five or more, and sufficient damage resistance was not obtained. Furthermore, the wear amount was 5.00 mm or more, and sufficient wear resistance was not obtained.

[0184] In test numbers 74 and 75, the average cooling rate CR of the accelerated cooling in the heat treatment process was too fast. As a result, the area ratio of pearlite in the metal structure of the head surface portion 10A was less than 95%, with the remainder being bainite. As a result, the number of cracks that occurred was five or more, and sufficient damage resistance was not obtained. Furthermore, the wear amount was 5.00 mm or more, and sufficient wear resistance was not obtained.

[0185] In test numbers 76 and 77, the cooling stop temperature TE was too low. As a result, the pearlite area ratio in the metal structure of the head surface portion 10A was less than 95%, with the remainder being bainite and martensite. As a result, the number of cracks that occurred was five or more, and sufficient damage resistance was not obtained. Furthermore, the wear amount was 5.00 mm or more, and sufficient wear resistance was not obtained.

[0186] In test numbers 78 and 79, the cooling stop temperature TE was too high. Therefore, sufficient hardness was not obtained in the head surface portion 10A. As a result, the number of cracks that occurred was five or more, and sufficient damage resistance was not obtained. Furthermore, the wear amount was 5.00 mm or more, and sufficient wear resistance was not obtained.

[0187] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

[0188] 1 Rail 10A Head surface 10S Head outer surface

Claims

1. By mass: C: 0.80-1.20%, Si: 0.80-2.50%, Mn: 0.10-2.00%, P: 0.0250% or less, S: 0.0250% or less, Al: 0-1.0000%, N: 0-0.0200%, Cr: 0-1.00%, Mo: 0-0.50%, Co: 0-1.00%, B: 0-0.0050%, Cu: 0-1.00%, Ni: 0-1.00%, V: 0-0.200%, Nb: 0-0.0500%, Ti: 0-0.0500%, Mg: 0-0.0200%, Ca: 0-0.0200%, A rail comprising: rare earth elements: 0-0.0500%, Zr: 0-0.0200%, and the balance being Fe and impurities; in a metal structure of a head surface portion from the head outer surface to a depth of 20 mm, a pearlite area ratio is 95% or more and a Vickers hardness is 400 HV or more; and in the head surface portion, a positive Si segregation degree, which is a ratio of a maximum Si concentration in a Si-positive segregation band to a Si concentration in a bulk region, is greater than 1.00 to 1.35, and a negative Si segregation degree, which is a ratio of a minimum Si concentration in a Si-negative segregation band to a Si concentration in the bulk region, is 0.90 to less than 1.

00.

2. A rail as claimed in claim 1, comprising: Al: 0.0001-1.0000%, N: 0.0001-0.0200%, Cr: 0.01-1.00%, Mo: 0.01-0.50%, Co: 0.01-1.00%, B: 0.0001-0.0050%, Cu: 0.01-1.00%, Ni: 0.01-1.00%, V: 0.001-0.200%, Nb: 0.0001-0.0500%, Ti: 0.0001-0.0500%, Mg: 0.0001-0.0200%, Ca: 0.0001-0.0200%, A rail comprising one or more elements selected from the group consisting of: rare earth elements: 0.0001 to 0.0500%; and Zr: 0.0001 to 0.0200%.

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