Rail

Abstract

Provided is a rail that has excellent wear resistance and damage resistance. In a rail (1) according to the present embodiment: the metal structure of a head surface portion (10A) from a head portion outer surface (10S) to a depth of 20 mm is composed of pearlite; the Vickers hardness is 400 HV or more; the hardness H1 at a depth of 1 mm, the hardness H10 at a depth of 10 mm, and the hardness H20 at a depth of 20 mm from the head portion outer surface (10S) satisfy formula (1); and a hardness gradient S1 of a first head surface portion region (10A1) is 2.00 to 10.00 HV / mm, a hardness gradient S2 of a second head surface portion region (10A2) is 0.25 to 2.50 HV / mm, and a hardness gradient S3 of a third head surface portion region from a depth of 20 mm to a depth of 40 mm is 1.00 to 4.00 HV / mm, wherein the hardness gradient S1 of the first head surface portion region and the hardness gradient S2 of the second head surface portion region satisfy formula (2).　(1): H1 > H10 > H20 (2): S2 ≤ 0.95S1

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. This results in increased wear and damage to rails. Therefore, there is a demand for improved wear resistance and damage resistance in rails. Against this background, the development of rails with improved wear resistance and damage resistance is underway.

[0004] With the aim of improving the wear resistance of rails, rails disclosed in Patent Documents 1 and 2 have been proposed.

[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 850 to 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 uses hypereutectoid steel (C: more than 0.85 to 1.20%) to increase the volume fraction of cementite in lamellae in pearlite structures. Patent Document 2 also discloses that this improves the wear resistance of the rail.

[0006] The techniques disclosed in Patent Document 1 and Patent Document 2 improve the wear resistance of rails by increasing the hardness of steel by minimizing the lamellar spacing in pearlite or by increasing the volume ratio of cementite in pearlite lamellae.

[0007] 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.

[0008] In Patent Document 3, eutectoid 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, Patent Document 4 uses hypereutectoid steel (C: 0.70 to 0.85%) and balances the amounts of Si, Mn, and Cr to control the amount of pearlite and bainite formed and increase the hardness of the steel. Patent Document 4 also discloses that this improves the wear resistance and damage resistance of the rail.

[0009] The techniques disclosed in Patent Documents 3 and 4 increase the wear resistance and damage resistance of rails by increasing the hardness of steel by miniaturizing the lamellar spacing in pearlite or by controlling the metal structure of steel.

[0010] Japanese Patent Publication No. 63-023244 Publication of Patent No. 3078461 Publication of Patent No. 4390004 Publication of Patent No. 6822757

[0011] However, the wear resistance and damage resistance of rails may be improved by other techniques different from those disclosed in Patent Documents 1 to 4.

[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, N: 0.0200% or less, Al: 1.0000% or less, 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 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 is Fe and impurities, and the metal structure of the head surface from the outer surface of the head to a depth of 20 mm is In the head outer shell, the pearlite area ratio is 95% or more, the Vickers hardness is 400 HV or more, the hardness H1 at a depth of 1 mm starting from the head outer shell surface, the hardness H10 at a depth of 10 mm, and the hardness H20 at a depth of 20 mm satisfy formula (1), the hardness gradient S1 of the first head surface region from the head outer shell surface to the depth of 10 mm is 2.00 to 10.00 HV / mm, the hardness gradient S2 of the second head surface region from the depth of 10 mm to the depth of 20 mm is 0.25 to 2.50 HV / mm, the hardness gradient S3 of the third head surface region from the depth of 20 mm to the depth of 40 mm is 1.00 to 4.00 HV / mm, and the hardness gradient S1 of the first head surface region and the hardness gradient S2 of the second head surface region satisfy formula (2). H1>H10>H20 (1) S2≦0.95S1 (2)

[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 rail head area and its vicinity, illustrating a method for defining the head surface area (first and second head surface areas) and the third head surface area in FIG. 1A. FIG. 1C is a cross-sectional view of the rail head area and its vicinity, continuing from FIG. 1B. FIG. 1D is a cross-sectional view of the rail head area and its vicinity, continuing from FIG. 1C. FIG. 1E is a cross-sectional view of the rail head area and its vicinity, continuing from FIG. 1D. FIG. 1F is a cross-sectional view of the rail head area and its vicinity, continuing from FIG. 1E. FIG. 2 is a schematic diagram illustrating the contact area between the rail head and a wheel in the initial contact state. FIG. 3 is a perspective view of a rolling fatigue testing machine that reproduces damage caused by the rolling of a rail and a wheel. FIG. 4 is an enlarged view of the area near the head outer surface of the measurement surface of a test piece taken from a rail for hardness measurement. FIG. 5 is a graph showing the relationship between the hardness gradient in the first head surface area and the maximum crack depth and the number of internal cracks generated in crack damage, as determined by a rolling fatigue test. Fig. 6 is a schematic diagram illustrating the contact portion between the rail head and the wheel under normal contact conditions. Fig. 7 is a diagram showing the relationship between the hardness gradient in the second head surface region and the number of cracks and the amount of wear on the head, as obtained by a rolling fatigue test. Fig. 8 is a schematic diagram of a drop weight tester that reproduces rail breakage. Fig. 9 is a diagram showing the relationship between the hardness gradient in the third head surface region and the maximum value of the drop weight energy at which no breakage occurred, as obtained by a drop weight test. Fig. 10 is a schematic diagram illustrating how to determine the degree of Si segregation. Fig. 11 is a diagram showing the relationship between the hardness gradient in the second head surface region and the number of cracks on the rolling surface, as obtained by a rolling fatigue test. Fig. 12 is a schematic diagram showing an example of a cooling device used in the heat treatment step in the manufacturing process of the rail of this embodiment.

[0016] The rail of this embodiment will be described in detail. Hereinafter, mass % in the chemical composition will also be simply referred to as %.

[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 is a portion above a narrowed portion in the center of the rail 1 in the height direction. The rail head portion 10 receives a load from the wheels of a railway vehicle. The bottom portion 15 is located below the rail 1 when the rail 1 is upright, and is fixed to a sleeper. The bottom portion 15 is a plate-like portion extending in a direction perpendicular to the height direction of the rail 1. The bottom portion 15 supports the rail head portion 10, which receives a load from the wheels, via the pillar portion 14. The pillar portion 14 is located between the rail head portion 10 and the bottom portion 15. The pillar portion 14 is a plate-like portion extending in the height direction of the rail 1. 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. 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.

[0019] 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.

[0020] The region of the head surface 10A extending from the head outer casing surface 10S to a depth of 10 mm is referred to as the first head surface region 10A1, and the region extending from the head outer casing surface 10S to a depth of 10 mm to a depth of 20 mm is referred to as the second head surface region 10A2.

[0021] Furthermore, the region extending from the head outer surface 10S to a depth of 20 to 40 mm is referred to as the third head surface region 10B. D2 in Fig. 1A indicates the range extending from the head outer surface 10S to a depth of 20 to 40 mm. In Fig. 1A, the third head surface region 10B is indicated by hatching different from that of the head surface region 10A.

[0022] The head surface region 10A (first head surface region 10A1 and second head surface region 10A2) and the third head surface region 10B are defined as follows. Referring to FIG. 1B, the center position 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 line segment L11, 20 mm (D1) in the depth direction from point P11. Line segment L100 is drawn horizontally from point X (parallel to the bottom surface of bottom portion 15), and points A and B are the intersections of line segment L100 and head outer surface 10S. Referring to Figure 1C, the area from point A to point B on the surface of rail head 10 of rail 1 is divided into thirds, and the boundary positions of these three equal divisions are points C and D, respectively. Referring to Figure 1D, the area between points C and D is defined as head top 11. Furthermore, point E is the central position between points A and C on the surface of 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 free of contact with railway vehicles, 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 contact with railway vehicles, 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 and the line segment L at a depth of 40 mm Ｄ４０ By the above method, the head surface portion 10A (the first head surface portion region 10A1 and the second head surface portion region 10A2) and the third head surface portion region 10B shown in FIG. 1A can be defined.

[0028] Generally, the service life of the rail 1 is about 20 mm from the head outer surface 10S. Therefore, wear resistance and damage resistance are not considered important in the third head surface region 10B. Instead, resistance to breakage is required in the third head surface region 10B, and it is preferable that the third head surface region 10B have excellent breakage resistance.

[0029] [Technical Concept of the Rail of the Present Embodiment] Next, the technical concept of the rail 1 of the present embodiment will be described.

[0030] The inventors studied the wear resistance and damage resistance of the rail 1 on the premise that the hardness H1 (HV) at a depth of 1 mm, the hardness H10 (HV) at a depth of 10 mm, and the hardness H20 (HV) at a depth of 20 mm, starting from the head outer surface 10S, satisfy the following formula (1): H1 > H10 > H20 (1) The method for measuring the hardnesses H1, H10, and H20 will be described later.

[0031] The inventors investigated damage to the head surface 10A that occurs in rails 1 that satisfy formula (1) when used in harsh environments such as freight railways. As a result, the inventors identified the following two forms of damage occurrence: (Mode 1) Crack damage on the rolling surface caused by plastic deformation that occurs in the initial contact state where the contact area between the rail 1 and the wheel is small (wear depth of the rail head 10 is 0 to 10 mm). (Mode 2) Crack damage on the rolling surface caused by fatigue that occurs in the normal contact state where the contact area between the rail 1 and the wheel is large (wear depth of the rail head 10 is 10 to 20 mm).

[0032] Therefore, the present inventors investigated the causes of these two types of crack damage.

[0033] [(Mode 1) Crack Damage Occurring in the Initial Contact State] The present inventors first investigated crack damage occurring in the initial contact state in Mode 1. Fig. 2 is a schematic diagram illustrating the contact portion between the rail 1 and the wheel 2 in the initial contact state. Referring to Fig. 2, in the rail 1 where crack damage occurred on the rolling surface in the initial contact state, the contact area between the rail 1 and the wheel 2 is small. It was found that this increases the contact surface pressure at the gauge contact portion G.C. (head corner portion 12) of the rail 1, causing crack damage due to plastic deformation.

[0034] Therefore, the inventors considered controlling the contact pressure between the rail 1 and the wheel 2 in order to suppress crack damage caused by plastic deformation in mode 1. Controlling the contact pressure requires controlling the contact area between the rail 1 and the wheel 2. The inventors considered that controlling the hardness gradient in the region (first head surface region 10A1) starting from the head outer surface 10S and extending to a depth of 10 mm would be effective in controlling the contact area.

[0035] Therefore, the inventors investigated the relationship between the hardness gradient S1 in the first head surface region 10A1 and crack damage caused by plastic deformation. Specifically, rails 1 (with a C content of 0.80 to 1.20%) were prototyped with different hardness gradients S1 in the first head surface region 10A1. Then, using a rolling fatigue testing machine shown in Figure 3, which reproduces damage caused by the rolling of a rail and a wheel, the relationship between the hardness gradient S1 and the behavior of crack damage on the rolling surface was evaluated.

[0036] [Outline of Rolling Contact Fatigue Testing Machine] Referring to Figure 3, in the rolling contact fatigue testing machine, a rail 1 was placed on a sleeper 3. A motor 4 rotated a wheel 2. A load stabilizing device 5 held down the wheel 2 rotated by the motor 4. In the rolling fatigue test, the wheel 2 was placed on the rail 1. Then, while a predetermined load was applied to the wheel 2 using the load stabilizing device 5, the wheel 2 was repeatedly rolled back and forth on the rail head 10 of the rail 1 along the longitudinal direction of the rail 1.

[0037] [Conditions for Rolling Contact Fatigue Test] The chemical composition of the rail 1, the hardness gradient S1, the heat treatment method for the rail 1, the hardness measurement method, the gradient calculation method, the conditions for the rolling fatigue test of the rail 1 and the wheel 2 to reproduce crack damage, the method for evaluating crack damage on the rolling surface, and the method for evaluating internal crack damage immediately below the rolling surface were as follows:

[0038] [Chemical Composition, Shape, and Characteristics of Rail 1] The chemical composition of rail 1 was, by mass, 0.80 to 1.20% C, Si, and Mn, with the balance consisting of Fe and impurities. The rail cross-sectional shape was 136 pounds (mass: 67 kg / m). The hardness gradient S1 in the first head surface region 10A1 was 0 to 14.00 HV / mm. Furthermore, the Vickers hardness in the first head surface region 10A1 was 400 to 540 HV. In the metal structure of the head surface region 10A, the pearlite area ratio was 95% or more.

[0039] [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 heat treated under the following conditions to manufacture a rail 1 with the above-mentioned characteristics. Specifically, the rail preform after hot rolling was subjected to accelerated cooling under the following conditions. The controlled position was 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 0.5 to 10.0°C / sec. The cooling end temperature was 620 to 680°C.

[0040] [Method for measuring hardness of manufactured rail 1 and method for calculating gradient] The hardness of the manufactured rail 1 was determined by a Vickers hardness test in accordance with JIS Z 2244-1 (2020). Specifically, a test piece was taken from the rail 1, with the cross section perpendicular to the longitudinal direction of the rail 1 as the measurement surface.

[0041] Figure 4 is an enlarged view of the portion of the measurement surface of the sampled test piece near the head outer surface 10S. Referring to Figure 4, on the measurement surface, a line segment L11 extending in the depth direction from point P11, which is the center of the width of the head vertex 11 on the head outer surface 10S, was designated as the "hardness measurement line" L11. The hardness measurement line L11 was coaxial with the normal to the head outer surface 10S at point P11 and corresponded to the center line of the rail 1 on the measurement surface of the rail 1. On the hardness measurement line L11, measurement points were determined in the depth direction from point P11, from a depth position of 1 mm to a depth position of 10 mm, at 1.0 mm intervals.

[0042] Furthermore, in the head corner portion 12L, the line segment connecting point E and point P13 is a hardness measurement line L12. Ｌ At point E, the hardness measurement line L12 Ｌ The direction in which the line extends was defined as the depth direction. Ｌ Measurement points were set at 1.0 mm intervals in the depth direction along the line L12 from a depth of 1 mm to a depth of 10 mm. Similarly, in the head corner portion 12R, the line segment connecting point F and point P13 was set as the hardness measurement line L12. Ｒ At point F, the hardness measurement line L12 Ｒ The direction in which the line extends was defined as the depth direction. Ｒ Measurement points were determined at 1.0 mm intervals in the depth direction along the line from a depth of 1 mm to a depth of 10 mm.

[0043] Three hardness measurement lines L11 and L12 Ｌ , L12 Ｒ The Vickers hardness was measured at each of the measurement points (30 points in total). The test force was 10 kgf. The arithmetic mean value of all the measurement points was taken as the Vickers hardness (HV) of the first head surface region 10A1.

[0044] Furthermore, the hardness gradient S1 in the first head surface region 10A1 was determined by the following method. Ｌ , L12 ＲThe arithmetic mean value of the three measurement points at the same depth was taken as the hardness at that depth. Using this method, the arithmetic mean value of the three measurement points was taken as the hardness at each depth from 1 mm to 10 mm at 1.0 mm intervals.

[0045] The hardness gradient m was calculated by the following linear approximation using the obtained hardness at each depth position. Specifically, when x is the depth (mm) and y is the hardness (HV), the linear regression equation is expressed as follows: y = mx + b where m is the gradient and b is the intercept.

[0046] In this case, the gradient m is expressed by the following equation. Here, N is the number of data, Σxy is the sum of the products of x and y, Σx is the sum of x, Σy is the sum of y, Σx ２ is the sum of the squares of x.

[0047] The gradient m obtained based on the above formula was defined as the hardness gradient S1 in the first head surface region 10A1.

[0048] [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 tester shown in FIG. 3 was used. A rail 1 with a rail cross section 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 allowed to dry 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.

[0049] [Method for Evaluating Crack Damage on Rolling Contact Surface] The method for evaluating crack damage on the rolling contact surface of the rail 1 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, and the depth of cracks visible on the polished observation surface was measured. The crack depth was measured as the depth from the head outer surface 10S of the head corner portion 12. The maximum value of the measured crack depth was designated as the maximum crack depth of crack damage occurring in the rail 1.

[0050] [Method for Evaluating Internal Crack Damage Immediately Below the Rolling Contact Surface] The method for evaluating internal crack damage immediately below the rolling contact surface of the rail 1 after the rolling fatigue test was as follows. Internal cracks were confirmed by ultrasonic testing (UST). The evaluation area was an area of ​​300 mm in the longitudinal direction of the rail 1 on the surface of the head corner portion 12 of the rail 1. The number of internal cracks with a length of 1 mm or more in the evaluation area was counted by phased array ultrasonic testing. For the internal crack measurement, ultrasonic testing based on the vanishing method was performed using a phased array ultrasonic testing device. The length of the internal cracks was confirmed by shear wave sector scanning and longitudinal wave sector scanning. Internal cracks with a maximum length of 1 mm or more were counted in the shear wave sector scanning or longitudinal wave sector scanning. The total number of internal cracks in the evaluation area was taken as the number of internal cracks occurring in the rail 1.

[0051] [Evaluation Results] Using the above conditions and evaluation method, a rolling fatigue test was performed to investigate the relationship between the hardness gradient S1 in the first head surface region 10A1, the maximum crack depth of crack damage, and the number of internal cracks that occurred. Fig. 5 is a diagram showing the relationship between the hardness gradient S1 in the first head surface region 10A1, the maximum crack depth of crack damage, and the number of internal cracks that occurred, obtained by the rolling fatigue test.

[0052] Referring to Figure 5, when the hardness gradient S1 is 2.00 HV / mm or more, the maximum crack depth of crack damage on the rolling surface is significantly reduced, and damage resistance is significantly improved, compared to when the hardness gradient is less than 2.00 HV / mm. A large hardness gradient S1 accelerates wear of the rail 1. Therefore, as shown in Figure 6, the head corner portion 12 of the rail 1 wears, and the contact area between the rail 1 and the wheel 2 increases. As a result, the contact surface pressure decreases. Due to the above mechanism, it is thought that when the hardness gradient S1 is 2.00 HV / mm or more, the maximum crack depth of crack damage is significantly reduced.

[0053] On the other hand, when the hardness gradient S1 exceeded 10.00 HV / mm, the maximum crack depth of the crack damage did not change significantly, but a large amount of internal crack damage occurred immediately below the rolling surface. This is thought to be due to the internal concentration of strain caused by the increase in the hardness gradient S1.

[0054] Through the above tests, the inventors have found that by setting the hardness gradient S1 of the first head surface region 10A1 of the rail 1 to 2.00 to 10.00 HV / mm, crack damage on the rolling surface of the head surface region 10A and internal crack damage directly below the rolling surface can be suppressed.

[0055] [(Mode 2) Crack damage occurring under normal contact conditions] Next, the inventors investigated the cause of crack damage on the rolling surface that occurs under normal contact conditions (wear depth of 10 to 20 mm), in which the contact area between the rail 1 and wheel 2 is large, which is mode 2. As a result of the investigation, it was found that in the rail 1 where crack damage occurred under normal contact conditions, the contact area between the rail 1 and wheel 2 was large and the crack damage occurred under conditions where the contact surface pressure was low.

[0056] The inventors further investigated the cause of crack damage under normal contact conditions. As a result, it was confirmed that crack damage occurs due to the accumulation of metal fatigue on the rolling surface. Therefore, the inventors considered controlling the wear rate of the rail 1 in order to suppress the occurrence of crack damage under normal contact conditions. Controlling the wear rate requires controlling the hardness of the rail 1. Therefore, the inventors considered that controlling the hardness gradient S2 of the second head surface region 10A2 from a depth of 10 mm to 20 mm starting from the head outer surface 10S would be effective in controlling the wear rate.

[0057] Therefore, the inventors investigated the relationship between the hardness gradient S2 in the second head surface region 10A2 and fatigue-induced crack damage and wear volume. Specifically, rails 1 (with a C content of 0.80 to 1.20%) were prototyped with different hardness gradients S2 in the second head surface region 10A2. Then, using the rolling fatigue testing machine shown in Figure 3, the relationship between the hardness gradient S2 and the occurrence of crack damage on the rolling surface was evaluated.

[0058] [Conditions for Rolling Fatigue Test] The chemical composition of the rail 1, the hardness gradient S2, the rail heat treatment method, the hardness measurement method, the hardness gradient S2 calculation method, the conditions for the rolling fatigue test of the rail 1 and the wheel 2 to reproduce crack damage, the method for evaluating crack damage on the rolling surface, and the method for evaluating the amount of wear on the rail head 10 in the rolling fatigue test were as follows:

[0059] [Chemical Composition, Shape, and Characteristics of Rail 1] The chemical composition of the rail 1 was, by mass, 0.80 to 1.20% C, Si, and Mn, with the balance consisting of Fe and impurities. The rail cross-sectional shape was 136 pounds (mass: 67 kg / m). The hardness gradient S1 in the first head surface region 10A1 was 5.00 HV / mm. The hardness gradient S2 in the second head surface region 10A2 was 0.00 to 3.50 HV / mm. The Vickers hardness in the first head surface region 10A1 was 435 to 500 HV, and the Vickers hardness in the second head surface region 10A2 was 400 to 450 HV. The pearlite area ratio in the metal structure of the head surface region 10A was 95% or more.

[0060] [Heat treatment method in the manufacturing process of rail 1] In the manufacturing process of rail 1, a rail-shaped preform after hot rolling was subjected to heat treatment under the following conditions to manufacture a rail 1 with the above-mentioned characteristics. Specifically, the preform after hot rolling was subjected to accelerated cooling under the following conditions. The controlled positions were the portion of the preform corresponding to the head portion 11 of rail 1 and the portion corresponding to the under-jaw portion 13. As will be described later, in the cooling device for accelerated cooling of rail 1, the nozzle for injecting compressed air onto the head portion 11 was a mechanism independent of the nozzle for injecting compressed air onto the under-jaw portion 13.

[0061] Specifically, the following two-stage accelerated cooling was performed on the top portion 11. In the first cooling stage, the cooling start temperature was 800°C, the cooling rate was 5.0°C / sec, and the cooling stop temperature was 650°C. In the second cooling stage, the cooling start temperature was 650°C, the cooling rate was 1.0 to 7.0°C / sec, and the cooling stop temperature was 580°C.

[0062] The lower jaw portion 13 was subjected to the following one-stage accelerated cooling: the cooling start temperature was 750°C, the cooling rate was 0.5 to 10.0°C / sec, and the cooling end temperature was 550°C.

[0063] [Method for measuring hardness of manufactured rail 1 and method for calculating gradient] The hardness of the manufactured rail 1 was determined by a Vickers hardness test in accordance with JIS Z 2244-1 (2020). Specifically, a test piece was taken from the rail 1, with the cross section perpendicular to the longitudinal direction of the rail 1 as the measurement surface.

[0064] On the measurement surface, a line segment extending in the depth direction from point P11 at the width center of the vertex 11 on the head outer surface 10S was defined as the hardness measurement line L11. Measurement points were determined at 1.0 mm intervals on the hardness measurement line L11 in the depth direction from point P11 to a depth of 11 mm to 20 mm.

[0065] Furthermore, in the head corner portion 12L, the line segment connecting point E and point P13 is a hardness measurement line L12. Ｌ The hardness measurement line L12 from point E ＬMeasurement points were set at 1.0 mm intervals in the depth direction along the line L12 from a depth of 11 mm to a depth of 20 mm. Similarly, in the head corner portion 12R, the line segment connecting point F and point P13 was set as the hardness measurement line L12. Ｒ The hardness measurement line L12 from point F Ｒ Measurement points were determined at 1.0 mm intervals in the depth direction along the line from a depth of 11 mm to a depth of 20 mm.

[0066] Three hardness measurement lines L11 and L12 Ｌ , L12 Ｒ The Vickers hardness was measured at each of the measurement points (30 points in total). The test force was 10 kgf. The arithmetic mean value of all the measurement points was taken as the Vickers hardness (HV) of the second head surface region 10A2.

[0067] Furthermore, the hardness gradient S2 in the second head surface region 10A2 was determined by the following method. Ｌ , L12 Ｒ The arithmetic mean value of the three measurement points at the same depth was taken as the hardness at that depth. Using this method, the arithmetic mean value of the three measurement points was taken as the hardness at each depth from 11 mm to 20 mm at 1.0 mm intervals.

[0068] The hardness at each depth position was used to perform the linear approximation described above to determine the gradient m defined by the above equation, which was defined as the hardness gradient S2 in the second head surface region 10A2.

[0069] [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 tester shown in FIG. 3 was used. A rail 1 with a rail cross section 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 10 million, and the cumulative passing tonnage was up to 300 million tons. In order to eliminate the influence of initial crack damage remaining on the rolling surface, when the area where the rail and wheel come into contact had worn down 10 mm from the head outer surface 10S, the rolling surface was redressed by about 1 mm in the depth direction, and the test was then continued.

[0070] [Method for evaluating the number of cracks on the rolling surface] The method for evaluating the number of cracks on the rolling surface of the rail 1 after the rolling fatigue test was as follows. An area of ​​300 mm in the longitudinal direction of the rail 1 on the surface of the head portion 11 of the rail 1 was designated as the evaluation area. Magnetic particle testing was performed on the evaluation area, and the number of cracks with a length of 0.5 mm or more in the longitudinal direction of the rail 1 was counted. The obtained number was designated as the number of cracks.

[0071] [Method for evaluating wear volume of rail head portion 10] A 300 mm long area in the longitudinal direction of the rail 1 was designated as the evaluation area. The cross-sectional shape of the test piece of the rail 1 perpendicular to the longitudinal direction before the rolling fatigue test was recorded. Furthermore, the cross-sectional shape of the test piece of the rail 1 perpendicular to the longitudinal direction after the rolling fatigue test was recorded. The two were compared to measure the wear depth (mm) at the center of the width of the rail 1 in the rail head portion 10. The maximum wear depth obtained was designated as the wear volume (mm).

[0072] [Evaluation Results] Using the above conditions and evaluation method, a rolling fatigue test was conducted to investigate the relationship between the hardness gradient S2 in the second head surface region 10A2, the number of cracks that occurred, and the amount of wear at the head. Figure 7 shows the relationship between the hardness gradient S2 in the second head surface region 10A2, the number of cracks that occurred, and the amount of wear at the rail head 10, obtained by the rolling fatigue test.

[0073] Referring to Figure 7, when the hardness gradient S2 was 0.25 HV / mm or more, the number of cracks on the rolling surface decreased significantly, and damage resistance improved significantly. This is thought to be because wear of the rail 1 was accelerated and metal fatigue was eliminated. On the other hand, when the hardness gradient S2 exceeded 2.50 HV / mm, there was no significant change in the number of cracks on the rolling surface, but rail wear accelerated and wear resistance decreased.

[0074] Through the above tests, the inventors have found that the damage resistance and wear resistance of the rail 1 can be improved by setting the hardness gradient S1 in the first head surface region 10A1 of the rail 1 to 2.00 to 10.00 HV / mm and, further, setting the hardness gradient S2 in the second head surface region 10A2 to 0.25 to 2.50 HV / mm.

[0075] [Regarding breakage resistance of rail 1] As described above, if the hardness gradient S1 in the first head surface region 10A1 of the rail 1 is set to 2.00 to 10.00 HV / mm and the hardness gradient S2 in the second head surface region 10A2 is set to 0.25 to 2.50 HV / mm, the damage resistance and wear resistance of the rail 1 are improved.

[0076] However, when wear of the rail 1 progresses and the cross section of the rail head 10 becomes smaller, the rail head 10 may break. Therefore, it is preferable that the breakage resistance of the rail 1 can be improved.

[0077] Breakage is likely to occur in the third head surface region 10B (the region from a depth of 20 mm to a depth of 40 mm from the head outer surface 10S) when the wear of the rail 1 progresses and the cross section of the railhead 10 becomes smaller. Therefore, the inventors further investigated the causes of breakage occurring in the third head surface region 10B. As a result, the following two types of brittle fracture were confirmed in rails 1 in which breakage occurred in the third head surface region 10B. (Type 1) Brittle fracture caused by localized concentration of strain due to an excessive increase in the hardness gradient S3 in the third head surface region 10B and a rapid change in ductility. (Type 2) Brittle fracture caused by insufficient distribution of strain in the deep region of the third head surface region 10B (the region from a depth of 30 mm to a depth of 40 mm from the head outer surface 10S).

[0078] Therefore, the inventors considered controlling the hardness gradient S3 of the third head surface region 10B in order to improve the breakage resistance of the rail 1. Therefore, the inventors carried out the following drop weight test to verify the relationship between the hardness gradient S3 in the third head surface region 10B and breakage.

[0079] [Outline of Drop Weight Test] In the drop weight test, rails 1 (C content: 0.80 to 1.20%) were prototyped with different hardness gradients S3 in the third head surface region 10B. Then, using the drop weight tester shown in Figure 8, the relationship between the hardness gradient of the third head surface region 10B and the maximum value of the drop weight energy at which breakage did not occur was evaluated.

[0080] [Conditions for Drop Weight Test] The rail 1 (composition, hardness gradient, hardness), heat treatment method for rail 1, hardness measurement method, gradient calculation method, and drop weight test conditions for reproducing rail breakage were as follows:

[0081] [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 cross-sectional shape was 136 pounds (mass: 67 kg / m). To clarify the correlation between chemical composition and breakage resistance, rails were used in which the Cr content varied from 0 to 1.00%. The hardness gradient S1 in the first head surface region 10A1 was 5.00 HV / mm. The hardness gradient S2 in the second head surface region 10A2 was 1.00 HV / mm. The hardness gradient S3 in the third head surface region 10B was 0.25 to 5.50 HV / mm. The Vickers hardness in the first head surface region 10A1 was 430 to 500 HV. The Vickers hardness of the second head surface region 10A2 was 420 to 440 HV. The Vickers hardness of the third head surface region 10B was 310 to 435 HV. The metal structure of the head surface region 10A had a pearlite area ratio of 95% or more. The metal structure of the third head surface region 10B was a structure consisting of ferrite and pearlite.

[0082] [Heat Treatment Method in the Process for Manufacturing Rail 1] In the process for manufacturing rail 1, a rail preform after hot rolling was subjected to heat treatment under the following conditions to manufacture a rail 1 with the above-described characteristics. Specifically, the rail preform after hot rolling was subjected to accelerated cooling under the following conditions.

[0083] The control positions were the portions of the rail preform corresponding to the top 11 of the rail 1 and the portions corresponding to the under-jaw 13. As will be described later, in the cooling device for accelerated cooling of the rail 1, the nozzle for injecting compressed air onto the top 11 and the nozzle for injecting compressed air onto the under-jaw 13 were independent mechanisms.

[0084] Specifically, the following three-stage accelerated cooling was performed on the top portion 11. In the first cooling stage, the cooling start temperature was 800°C, the cooling rate was 5.0°C / sec, and the cooling stop temperature was 650°C. In the second cooling stage, the cooling start temperature was 650°C, the cooling rate was 4.0°C / sec, and the cooling stop temperature was 580°C. In the third cooling stage, the cooling start temperature was 580°C, the cooling rate was 1.0 to 5.0°C / sec, and the cooling stop temperature was 500°C.

[0085] The lower jaw portion 13 was subjected to the following two-stage accelerated cooling. In the first cooling stage, the cooling start temperature was 750°C, the cooling rate was 6.0°C / sec, and the cooling stop temperature was 550°C. In the second cooling stage, the cooling start temperature was 550°C, the cooling rate was 0.5 to 8.0°C / sec, and the cooling stop temperature was 480°C.

[0086] [Method for measuring hardness of manufactured rail 1 and method for calculating gradient] The hardness of the manufactured rail 1 was determined by a Vickers hardness test in accordance with JIS Z 2244-1 (2020). Specifically, a test piece was taken from the rail 1, with the cross section perpendicular to the longitudinal direction of the rail 1 as the measurement surface.

[0087] On the measurement surface, a line segment extending in the depth direction from point P11 at the width center of the vertex 11 on the head outer surface 10S was defined as the hardness measurement line L11. Measurement points were determined at 1.0 mm intervals on the hardness measurement line L11 in the depth direction from point P11 to a depth of 21 mm to 40 mm.

[0088] Furthermore, in the head corner portion 12L, the line segment connecting point E and point P13 is a hardness measurement line L12. Ｌ The hardness measurement line L12 from point E Ｌ Measurement points were set at 1.0 mm intervals in the depth direction along the line L12 from a depth of 21 mm to a depth of 40 mm. Similarly, in the head corner portion 12R, the line segment connecting point F and point P13 was set as the hardness measurement line L12. Ｒ The hardness measurement line L12 from point F Ｒ Measurement points were determined at 1.0 mm intervals in the depth direction along the line from a depth of 21 mm to a depth of 40 mm.

[0089] Three hardness measurement lines L11 and L12 Ｌ , L12 Ｒ The Vickers hardness was measured at each of the measurement points (60 points in total). The test force was 10 kgf. The arithmetic mean value of all the measurement points was taken as the Vickers hardness (HV) of the third head surface region 10B.

[0090] Furthermore, the hardness gradient S3 in the third head surface region 10B was determined by the following method.Ｌ , L12 Ｒ The arithmetic mean value of the three measurement points at the same depth was taken as the hardness at that depth. Using this method, the arithmetic mean value of the three measurement points was taken as the hardness at each depth from 21 mm to 40 mm at 1.0 mm intervals.

[0091] The hardness at each depth position was used to perform the linear approximation described above to determine the gradient m defined by the above equation, which was designated as the hardness gradient S3 in the third head surface region 10B.

[0092] [Drop Weight Test Conditions] The drop weight test was performed under the following two conditions. [Test Condition 1] Test Condition 1 for the drop weight test was as follows. A drop weight tester shown in FIG. 8 was used. In the test, the head surface portion 10A of the rail 1 was ground to eliminate the influence of the material of the head surface portion 10A and accurately evaluate the breakage susceptibility of the third head surface region 10B. In the drop weight test, the rail 1 was supported at two points with the rail head portion 10 facing downward and the bottom portion 15 facing upward. The distance between the two support points was 1000 mm. Then, a drop weight was dropped on the bottom portion 15 of the rail 1. The mass W of the drop weight was 500 kgf (4.9 kN). The height X of the drop weight was 5.0 m. In this case, the drop weight energy was 24.5 kN m. After the drop weight, the rail 1 was visually inspected for breakage.

[0093] [Test Condition 2] To further evaluate breakage susceptibility, a test was conducted under Test Condition 2 with an increased drop weight energy. Specifically, Test Condition 2 for the drop weight test was as follows. As with Test Condition 1, a drop weight tester shown in Figure 8 was used. In the test, the head surface portion 10A of the rail 1 was ground to eliminate the influence of the material and accurately evaluate the breakage susceptibility of the third head surface region 10B. In the drop weight test, the rail 1 was supported at two points with the rail head portion 10 facing downward and the bottom portion 15 facing upward. The distance between the two support points was 1000 mm. A drop weight was then dropped on the bottom portion 15 of the rail 1. The weight W of the drop weight was 1000 kgf (9.8 kN). The height X of the drop weight was 4.0 to 14.0 m. In this case, the drop weight energy was 39.2 to 137.2 kN m. After the weight was dropped, the rail 1 was visually inspected for breakage.

[0094] [Drop Weight Test Results] [Results Under Test Condition 1] In the drop weight test under test condition 1, no breakage occurred in any of the rails, regardless of the hardness gradient S3 of the third head surface region 10B.

[0095] [Results under test condition 2] Fig. 9 is a diagram showing the relationship between the hardness gradient S3 of the third head surface region 10B and the maximum value of the drop weight energy at which breakage did not occur, obtained by the drop weight test. The "●" marks in Fig. 9 represent the results when Cr was contained and when S3 / Cr was less than 1.25 or more than 20.00.

[0096] In the third head surface region 10B, when an external force from the wheel is applied, strain is distributed to a deep region (a region from a depth of 30 mm to 40 mm starting from the head outer surface 10S) that has a relatively low hardness and high ductility, thereby suppressing brittle fracture of the entire third head surface region 10B.

[0097] Referring to FIG. 9 , when the hardness gradient S3 of the third head surface region 10B exceeds 4.00 HV / mm, breakage occurs at a falling weight energy of less than 98.0 kN·m. When the hardness gradient S3 exceeds 4.00 HV / mm, the hardness gradient of the third head surface region 10B increases excessively. Therefore, when subjected to external force from the wheel, the change in ductility due to the change in hardness becomes abrupt. As a result, strain is concentrated partially in the third head surface region 10B. This is thought to have resulted in brittle fracture (Mode 1). On the other hand, when the hardness gradient S3 is less than 1.00 HV / mm, breakage also occurs at a falling weight energy of less than 98.0 kN·m. When the hardness gradient S3 is less than 1.00 HV / mm, ductility in the deep layer of the third head surface region 10B is not sufficiently ensured. Therefore, when an external force is applied from the wheel, the strain cannot be sufficiently distributed within the third head surface region 10B, which is thought to be the reason why brittle fracture occurs (mode 2).

[0098] Therefore, if the hardness gradient S3 of the third head surface region 10B is set in the range of 1.00 to 4.00 HV / mm, breakage will not occur up to a falling weight energy of 98.0 kN·m, and breakage resistance will be improved.

[0099] Furthermore, to clarify the correlation between chemical components and breakage resistance, the relationship between the Cr content and the hardness gradient S3 of the third head surface region 10B was investigated. As a result, it was found that when the hardness gradient S3 of the third head surface region 10B is in the range of 1.00 to 4.00 HV / mm and the ratio of the hardness gradient S3 of the third head surface region 10B to the Cr content (mass%) satisfies formula (3) (marked "▲" in Figure 9), breakage does not occur even when the falling weight energy exceeds 98.0 kN m, and breakage resistance is further improved. 1.25≦S3 / Cr≦20.00 (3)

[0100] Through the above drop weight tests, the inventors have found that by setting the hardness gradient S3 of the third head surface region 10B to 1.00 to 4.00 HV / mm, the difference in hardness between the third head surface region 10B and the head surface region 10A is suppressed (Mode 1), and furthermore, an increase in hardness in the deep layer of the third head surface region 10B is suppressed (Mode 2). As a result, the inventors have found that the breakage resistance of the rail 1 is improved. Furthermore, the inventors have found that when the hardness gradient S3 is 1.00 to 4.00 HV / mm, the breakage resistance of the rail 1 is further improved if the Cr content satisfies formula (3).

[0101] [(Preferred embodiment) Regarding Si segregation] As described above, if the hardness gradient S1 in the first head surface region 10A1 of the rail 1 is set to 2.00 to 10.00 HV / mm and the hardness gradient S2 in the second head surface region 10A2 is set to 0.25 to 2.50 HV / mm, the damage resistance and wear resistance of the rail 1 are improved.

[0102] Therefore, the inventors have investigated preferable means for further improving the damage resistance of a rail 1 in which the hardness gradient S1 in the first head surface region 10A1 is 2.00 to 10.00 HV / mm, the hardness gradient S2 in the second head surface region 10A2 is 0.25 to 2.50 HV / mm, and the hardness gradient S3 in the third head surface region is 1.00 to 4.00 HV / mm.

[0103] Specifically, the inventors conducted a detailed investigation into the location of damage in order to further suppress crack damage that occurs in the second head surface region 10A2, which is the region extending from a depth of 10 mm to a depth of 20 mm starting from the head outer surface 10S. As a result, the following points were discovered.

[0104] It was found that a Si segregation zone was present around the damage site in the second head surface region 10A2. Generally, a Si segregation zone 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 the rail. Such a Si segregation zone formed in the cross-sectional center of the steel material exists in the web portion 14 when the rail 1 is manufactured. This Si segregation zone is referred to as a Si macrosegregation zone. Therefore, it seems unlikely that Si segregation would normally exist in the second head surface region 10A2, which is a part of the railhead portion 10 that is not the web portion 14.

[0105] However, the present inventors have newly discovered that additional Si segregation bands may occur around the Si macrosegregation bands. Such Si segregation bands formed around the Si macrosegregation bands are referred to as Si semi-macrosegregation bands. Such Si semi-macrosegregation bands may exist in the second head surface region 10A2 in the rail head 10, away from the web portion 14. It has been found that there is a large difference in hardness at the location where damage occurred in the second head surface region 10A2 where the Si semi-macrosegregation bands were confirmed. It is believed that such a difference in hardness is due to the Si segregation bands.

[0106] Based on the above findings, the inventors prepared three types of rails 1, each manufactured under different conditions for the blooms that are the raw material for the rails 1, and analyzed in detail the state of crack damage generation in each rail 1 and the state of Si segregation in the areas where crack damage occurred.

[0107] [Conditions for Si Segregation Investigation] In the Si segregation investigation, the bloom manufacturing conditions, rail (components, hardness gradient, hardness), rail heat treatment method, hardness measurement method, gradient calculation method, Si analysis method, Si concentration state quantification method, and conditions for rolling fatigue tests of rails and wheels to reproduce internal crack damage were as shown below.

[0108] [Bloom Manufacturing Conditions] Blooms serving as the raw material for rail 1 were manufactured by continuous casting. The cross section (transverse section) of the bloom perpendicular to the longitudinal direction was a rectangle measuring 320 mm x 380 mm. In the continuous casting process, soft reduction was performed in the thickness direction of the bloom under the following conditions. (Soft Reduction Condition 1) Soft reduction was started when the central solid fraction was 15% and ended when the central solid fraction was 70%. The amount of reduction was achieved by controlling the load on each roll of the continuous casting machine. The cumulative reduction was 1 to 2%. (Soft Reduction Condition 2) Soft reduction was started when the central solid fraction was 15% and ended when the central solid fraction was 80 to 100%. The amount of reduction was achieved by controlling the load on each roll of the continuous casting machine. Note that the reduction rate in the region where the central solid fraction exceeded 70% was set to 40 to 70% of the reduction rate when the central solid fraction was 70% or less. This was to prevent cracking. The cumulative reduction ratio is 2 to 5%. Here, the central solid fraction refers to the solidification ratio of a 20 mm × 20 mm square area in the center of a bloom having the above-mentioned cross-sectional size. Each side of the square area is parallel or perpendicular to each side of the cross section of the bloom. The central solid fraction can be determined by computer simulation. An example of the computer simulation is Abaqus, a finite element analysis application made by Dassault Systèmes. The reduction ratio is the ratio of the amount of reduction during soft reduction to the height (thickness) of the bloom before reduction.

[0109] [Chemical Composition, Shape, and Characteristics of Rail 1] The chemical composition of rail 1 was, by mass, 0.80 to 1.20% C, Si, and Mn, with the balance consisting of Fe and impurities. The rail cross-sectional shape was 136 pounds (mass: 67 kg / m). The hardness gradient S1 in the first head surface region 10A1 was 5.00 HV / mm. The hardness gradient S2 in the second head surface region 10A2 was 0 to 3.50 HV / mm. The hardness gradient S3 in the third head surface region 10B was 1.00 to 4.00 HV / mm.

[0110] The Vickers hardness of the first head surface region 10A1 was set to 430 to 500 HV. The Vickers hardness of the second head surface region 10A2 was set to 405 to 448 HV. In the metal structure of the head surface region 10A, the pearlite area ratio was set to 95% or more.

[0111] [Heat treatment method in the manufacturing process of rail 1] In the manufacturing process of rail 1, a rail preform after hot rolling was subjected to heat treatment under the following conditions to manufacture a rail 1 with the above-mentioned characteristics. Specifically, the rail preform after hot rolling was subjected to accelerated cooling under the following conditions. The controlled positions were the portion of the rail preform corresponding to the head portion 11 of rail 1 and the portion corresponding to the under-jaw portion 13. As will be described later, in the cooling device for accelerated cooling of rail 1, the nozzle for injecting compressed air onto the head portion 11 was a mechanism independent of the nozzle for injecting compressed air onto the under-jaw portion 13.

[0112] Specifically, the following two-stage accelerated cooling was performed on the top portion 11. In the first cooling stage, the cooling start temperature was 800°C, the cooling rate was 5.0°C / sec, and the cooling stop temperature was 650°C. In the second cooling stage, the cooling start temperature was 650°C, the cooling rate was 6.0°C / sec, and the cooling stop temperature was 580°C. In addition, the following one-stage accelerated cooling was performed on the underchin portion 13. The cooling start temperature was 750°C, the cooling rate was 6.0°C / sec, and the cooling stop temperature was 550°C.

[0113] [Method for measuring hardness of rail 1 after manufacture, method for calculating gradient, and average hardness difference ΔHV ａｖｅ Calculation method] The hardness of the rail 1 after manufacture was determined by a Vickers hardness test in accordance with JIS Z 2244-1 (2020). Specifically, a test piece was taken from the rail 1, with the cross section perpendicular to the longitudinal direction of the rail 1 as the measurement surface.

[0114] On the measurement surface, a hardness measurement line L11 was assumed to extend in the depth direction from point P11 at the width center of the head vertex 11 on the head outer surface 10S. Measurement points were determined at 1.0 mm intervals on the hardness measurement line L11 in the depth direction from point P11 to a depth of 11 mm to 20 mm.

[0115] Furthermore, in the head corner portion 12L, the line segment connecting point E and point P13 is a hardness measurement line L12. Ｌ The hardness measurement line L12 from point E Ｌ Measurement points were set at 1.0 mm intervals in the depth direction along the line L12 from a depth of 11 mm to a depth of 20 mm. Similarly, in the head corner portion 12R, the line segment connecting point F and point P13 was set as the hardness measurement line L12. Ｒ The hardness measurement line L12 from point F Ｒ Measurement points were determined at 1.0 mm intervals in the depth direction along the line from a depth of 11 mm to a depth of 20 mm.

[0116] Three hardness measurement lines L11 and L12 Ｌ , L12 Ｒ The Vickers hardness was measured at each of the measurement points (30 points in total). The test force was 10 kgf. The arithmetic mean value of all the measurement points was taken as the Vickers hardness (HV) of the second head surface region 10A2.

[0117] Furthermore, the hardness gradient S2 in the second head surface region 10A2 was determined by the following method. Ｌ , L12 Ｒ The arithmetic mean value of the three measurement points at the same depth was taken as the hardness at that depth. Using this method, the arithmetic mean value of the three measurement points was taken as the hardness at each depth from 11 mm to 20 mm at 1.0 mm intervals.

[0118] The hardness at each depth position was used to perform the following linear approximation to determine the gradient m defined by the above equation: The gradient m thus obtained was designated as the hardness gradient S2 in the second head surface region 10A2.

[0119] Furthermore, when the hardness gradient S2 of the second head surface region 10A2 is 1.00 HV / mm, the absolute value of the difference between the hardness at each depth position and the hardness at the corresponding depth position in the above-mentioned linear approximation formula was defined as the hardness difference ΔHV. The arithmetic mean value of the hardness difference ΔHV at each depth position (total of 10 points) was defined as the average hardness difference ΔHV. ａｖｅ It was decided.

[0120] [Method for analyzing Si in rail 1] A test piece was taken from a manufactured rail 1, with a cross section perpendicular to the longitudinal direction as the measurement surface. Five analysis regions, each measuring 800 μm in the longitudinal direction of the rail 1 and 800 μm in the depth direction, were selected from the measurement surface of the test piece within the range from a depth of 10 mm to a depth of 20 mm starting from the center of the width of the head outer surface 10S (i.e., the second head surface region 10A2). The five analysis regions were arranged consecutively in the longitudinal direction of the rail 1, with the ends of adjacent analysis regions in contact with each other.

[0121] The analysis area was analyzed using an electron probe microanalyzer (EPMA) to generate an elemental distribution map of Si concentration. The EPMA was performed at an acceleration voltage of 15 kV, a probe current of 0.1 μA, a time of 50 ms, and a beam diameter of 2 μm.

[0122] Based on the element distribution map, regions where Si was enriched were identified as Si semi-macrosegregation zones. Specifically, for each analysis region, the average Si concentration in the analysis region was determined based on the element distribution map of the Si concentration obtained in the analysis region. The element distribution map was binarized using the obtained average Si concentration as a threshold value. At this time, in the element distribution map, regions where the Si concentration was higher than the threshold value and regions where the Si concentration was lower than the threshold value were colored differently. In the binarized element distribution map, regions where the Si concentration was higher than the threshold value were identified as Si semi-macrosegregation zones. Fifty Si semi-macrosegregation zones were selected from the five analysis regions. The degree of Si segregation in the selected macro-Si segregation zones was determined by the following method.

[0123] Fig. 10 is a schematic diagram for explaining how to determine the degree of Si segregation. Referring to Fig. 10, the element distribution map of the 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 the Si concentration, a line analysis that crosses the identified Si semi-macrosegregation zone was selected to obtain the Si concentration distribution along that line. Specifically, when the Si semi-macrosegregation 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 was selected.

[0124] 10, in the Si concentration distribution of the selected line analysis, the arithmetic mean value (mass%) of the Si concentration was determined for each of the bulk regions L1 (1000 μm) and R1 (1000 μm) located on either side of the Si semi-macrosegregation zone S1. The lower of the average Si concentration values ​​in the bulk region L1 and the bulk region R1 was determined as the Si concentration (mass%) in the bulk region.

[0125] [Quantification of Si Concentration in Si Semimacrosegregation Zone] Furthermore, in the Si concentration analysis obtained by the selected line analysis, the maximum Si concentration in the Si semimacrosegregation zone S1 was determined. The arithmetic mean value of the maximum Si concentration and the Si concentration in the bulk region was defined as the Si concentration (mass%) in the Si semimacrosegregation zone S1.

[0126] [Determination of Si Segregation Degree] Based on the Si concentration in the bulk region and the Si concentration in the Si semi-macrosegregation band obtained by the above-mentioned method, the Si segregation degree in the Si semi-macrosegregation band was calculated by the following formula: Si segregation degree = Si concentration in Si semi-macrosegregation band / Si concentration in bulk region For each of the multiple macro Si segregation bands confirmed in the five analysis regions, the Si segregation degree was calculated by the above-mentioned method. The arithmetic mean value of the obtained Si segregation degrees of the top 20 was defined as the Si segregation degree in the Si semi-macrosegregation band. The arithmetic mean value of the Si segregation degrees of the 50 Si semi-macrosegregation bands was defined as the Si segregation degree in the second head surface region 10A2.

[0127] [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 tester shown in FIG. 3 was used. A rail 1 with a rail cross section 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 10 million, and the cumulative passing tonnage was up to 300 million tons. In order to eliminate the influence of initial crack damage remaining on the rolling surface, when the area where the rail and wheel come into contact had worn down 10 mm from the head outer surface 10S, the rolling surface was redressed by about 1 mm in the depth direction, and the test was then continued.

[0128] [Method for evaluating the number of cracks on the rolling surface] The method for evaluating the number of cracks on the rolling surface of the rail 1 after the rolling fatigue test was as follows. An area of ​​300 mm in the longitudinal direction of the rail 1 on the surface of the head portion 11 of the rail 1 was designated as the evaluation area. Magnetic particle testing was performed on the evaluation area, and the number of cracks with a length of 0.5 mm or more in the longitudinal direction of the rail 1 was counted. The obtained number was designated as the number of cracks.

[0129] [Evaluation Results] Table 1 shows the average hardness difference ΔHV in the second head surface region 10A2 when soft reduction is performed during continuous casting and the hardness gradient of the second head surface region 10A2 is 1.00 HV / mm. ａｖｅ This shows the relationship between

[0130]

[0131] Test No. 1 is a rail that was not soft reduced during continuous casting. Test No. 2 is a rail that was soft reduced under soft reduction condition 1. Test No. 3 is a rail that was soft reduced under soft reduction condition 2. Referring to Table 1, the degree of Si segregation in the second head surface region 10A2 correlated with the soft reduction conditions. Specifically, it was confirmed that continuation of soft reduction until the central solid fraction reached a high level suppresses the enrichment of Si in the rail. Specifically, the degree of Si segregation was highest in Test No. 1, where no soft reduction was performed, and lowest in Test No. 3, where soft reduction was performed until the central solid fraction reached its highest level.

[0132] In addition, the average hardness difference ΔHV ａｖｅ Specifically, in the rail of test No. 1, in which the Si segregation degree was in the range of more than 1.35 and not more than 1.50, the average hardness difference ΔHV ａｖｅ In contrast, in the rail of test No. 2 in which the degree of Si segregation was 1.15 or more and 1.35 or less, the average hardness difference ΔHV ａｖｅ Furthermore, in the rail of test number 3 in which the degree of Si segregation was less than 1.15, the average hardness difference ΔHV ａｖｅ From the above results, it can be seen that when the degree of Si segregation decreases, the average hardness difference ΔHV ａｖｅ was also found to decrease.

[0133] Furthermore, the results of the rolling fatigue test are shown in Figure 11. Figure 11 is a diagram showing the relationship between the hardness gradient S2 in the second head surface region 10A2 and the number of cracks occurring on the rolling surface, obtained by the rolling fatigue test. Referring to Figure 11, in the rails of test number 2 ("□" in the figure) and test number 3 ("△" in the figure), which were subjected to soft reduction, the degree of Si segregation decreased, and the average hardness difference ΔHV ａｖｅ As a result, the number of cracks was reduced compared to Test No. 1 (marked with "○" in the figure), in which soft reduction was not performed. Therefore, it was confirmed that the reduction in the degree of Si segregation in the second head surface region 10A2 reduced the number of cracks and further improved the damage resistance of the rail 1.

[0134] The rail of this embodiment has been completed based on the above technical concept, and its gist is as follows.

[0135] The rail of the first configuration has, 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, N: 0.0200% or less, Al: 1.0000% or less, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Co: 0 to 1.00%, B: 0 to 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 is Fe and impurities, and the metal structure of the head surface from the outer surface of the head to a depth of 20 mm is In the head outer shell, the pearlite area ratio is 95% or more, the Vickers hardness is 400 HV or more, the hardness H1 at a depth of 1 mm starting from the head outer shell surface, the hardness H10 at a depth of 10 mm, and the hardness H20 at a depth of 20 mm satisfy formula (1), the hardness gradient S1 of the first head surface region from the head outer shell surface to the depth of 10 mm is 2.00 to 10.00 HV / mm, the hardness gradient S2 of the second head surface region from the depth of 10 mm to the depth of 20 mm is 0.25 to 2.50 HV / mm, the hardness gradient S3 of the third head surface region from the depth of 20 mm to the depth of 40 mm is 1.00 to 4.00 HV / mm, and the hardness gradient S1 of the first head surface region and the hardness gradient S2 of the second head surface region satisfy formula (2). H1>H10>H20 (1) S2≦0.95S1 (2)

[0136] The rail of the second configuration is the rail of the first configuration, and contains one or more elements selected from the group consisting of 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: 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%.

[0137] The rail of the third configuration is a rail of the first or second configuration, in which the Si segregation degree in the second head surface region is 1.35 or less.

[0138] A rail of a fourth configuration is a rail of any one of the first to third configurations, containing 0.01 to 1.00% Cr, and the hardness gradient S3 of the third head surface region and the Cr content in mass % satisfy the formula (3): 1.25≦S3 / Cr≦20.00 (3).

[0139] 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 "%".

[0140] [Features of the Rail 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, N: 0.0200% or less, Al: 1.0000% or less, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Co: 0 to 1.00%, 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) The hardness H1 at a depth of 1 mm from the head outer casing surface 10S, the hardness H10 at a depth of 10 mm from the head outer casing surface 10S, and the hardness H20 at a depth of 20 mm from the head outer casing surface 10S satisfy formula (1). H1>H10>H20 (1) (Feature 4) The hardness gradient S1 of the first head surface region 10A1 from the head outer casing surface 10S to a depth of 10 mm is 2.00 to 10.00 HV / mm, the hardness gradient S2 of the second head surface region 10A2 from a depth of 10 mm to a depth of 20 mm from the head outer casing surface 10S is 0.25 to 2.50 HV / mm, and the hardness gradient S3 of the third head surface region 10B from a depth of 20 mm to a depth of 40 mm from the head outer casing surface 10S is 1.00 to 4.00 HV / mm, and the hardness gradient S1 of the first head surface region 10A1 and the hardness gradient S2 of the second head surface region 10A2 satisfy the formula (2). S2≦0.95S1 (2) Each feature will be described below.

[0141] [(Feature 1) Chemical Composition] The chemical composition of the rail according to this embodiment contains the following elements.

[0142] C: 0.80 to 1.20% Carbon (C) promotes pearlite transformation and improves the wear resistance of rails. If the C content is less than 0.80%, pro-eutectoid ferrite is formed. Therefore, rails mainly composed of pearlite structure do not achieve sufficient strength and wear resistance. On the other hand, if the C content exceeds 1.20%, excessive pro-eutectoid cementite is formed inside the head, reducing the breakage resistance of the rail. 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%.

[0143] Si: 0.80 to 2.50% Silicon (Si) dissolves in ferrite in the pearlite structure, increasing the hardness of the rail and improving its wear resistance. If the Si content is less than 0.80%, the above effect cannot be fully achieved. On the other hand, if the Si content exceeds 2.50%, the hardenability of the rail steel increases excessively. In this case, excessive martensite is formed in the rail head, reducing the breakage resistance and wear resistance of the head. Therefore, the Si content is 0.80 to 2.50%. The preferred lower limit of the Si content is 0.85%, more preferably 0.90%, even more preferably 0.95%, even more preferably 1.00%, even more preferably 1.02%, even more preferably 1.05%, and even more preferably 1.10%. The preferred upper limit of the Si content is 2.30%, even more preferably 2.20%, and even more preferably 2.00%.

[0144] Mn: 0.10 to 2.00% Manganese (Mn) improves the hardenability of rails and stabilizes pearlite transformation. Mn also refines the lamellar spacing of pearlite, thereby increasing the hardness and wear resistance of rails. If the Mn content is less than 0.10%, this effect is not fully achieved. In this case, soft pro-eutectoid ferrite is formed, reducing the wear resistance of the rail head. On the other hand, if the Mn content exceeds 2.00%, the hardenability of the rail steel increases excessively. As a result, bainite and martensite structures, etc., are formed in the rail head. As a result, the wear resistance and breakage resistance of the rail head are reduced. Furthermore, an excessive amount of Mn promotes Mn concentration in segregated regions. In this case, the formation of martensite and pro-eutectoid cementite is promoted in the rail head. As a result, the amount of martensite in the rail head increases, and the formation of pro-eutectoid cementite increases. As a result, the breakage resistance of the rail head 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%.

[0145] 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, reducing the rail's breakage resistance. Therefore, the P content is 0.0250% or less. The lower the P content, the better. 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%.

[0146] 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 MnS-based sulfide inclusions are formed. Stress concentration is likely to occur around the coarse MnS-based sulfides. This reduces the rail's breakage resistance. 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 0.0020%, even more preferably 0.0025%, 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%.

[0147] N: 0.0200% or less Nitrogen (N) is unavoidably contained. In other words, the N content is greater than 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 railhead is improved. Furthermore, when V is contained, N promotes the precipitation of V carbonitrides during the cooling process after welding of the rail. As a result, the hardness of pearlite increases, improving the damage resistance of the railhead. Even if even a small amount of N is contained, 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, which increases the likelihood of the formation of bubbles that serve as the starting point for fatigue damage. Therefore, the N content is 0.0200% or less. 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%.

[0148] Al: 1.0000% or less Aluminum (Al) is unavoidably contained. In other words, the Al content is greater than 0%. Al deoxidizes steel. Furthermore, Al increases the eutectoid transformation temperature. Therefore, Al suppresses the formation of pro-eutectoid cementite, which is harmful to toughness, and improves the breakage resistance of the rail head. 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 dissolve Al in the steel. As a result, coarse alumina-based inclusions are formed. Fatigue cracks are likely to initiate from these coarse inclusions. As a result, the damage resistance of the rail head is reduced. Furthermore, oxides are formed during rail welding, significantly reducing rail weldability. Therefore, the Al content is 1.0000% or less. The preferred lower limit of the Al content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the Al content is preferably 0.5000%, more preferably 0.1000%, even more preferably 0.0800%, and still more preferably 0.0500%.

[0149] The balance of the chemical composition of the rail according to this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of rails, and are acceptable within a range that does not adversely affect the rail according to this embodiment. For example, the impurity content is 0.0040% or less. The preferred lower limit of the O content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the O content is 0.0035%, and even more preferably 0.0030%.

[0150] [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 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. These elements will be described below.

[0151] [Group 1: Cr and Mo] Cr and Mo increase the equilibrium transformation temperature, thereby refining the lamellar spacing of pearlite structures. This increases the hardness of the rail head. Cr and Mo will be described below.

[0152] 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 Cr 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 rail head is improved. Even if even a small amount of Cr is contained, the above effects can be achieved to some extent. However, if the Cr content exceeds 1.00%, the hardenability of the rail steel material increases excessively. In this case, bainite, martensite, etc. are formed in the rail head. As a result, the wear resistance and breakage resistance of the rail are reduced. Furthermore, Cr concentration in segregated areas is promoted. This promotes the formation of martensite and pro-eutectoid cementite in the rail head. This increases the amount of martensite in the rail head and the amount of pro-eutectoid cementite formed. As a result, the breakage resistance of the rail head is reduced. Cr is also incorporated into the cementite phase in the pearlite structure, reducing the toughness of the cementite phase. 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%, more preferably 0.90%, and even more preferably 0.80%.

[0153] 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 rail head is improved. Even if even a small amount of Mo is contained, the above effects can be achieved to some extent. However, if the Mo content exceeds 0.50%, the transformation rate decreases significantly. As a result, martensite forms in the rail head, and the breakage resistance of the head decreases. 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%.

[0154] [Group 2: Co] Co: 0 to 1.00% Cobalt (Co) is an optional element and does not necessarily need to 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 pearlite immediately below the rolling surface, where deformation occurs due to contact with the wheel. This increases the hardness of the rolling surface and improves the wear resistance of the rail head. 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 above effect saturates. In this case, the lamellar structure is not sufficiently refined according to the Co content. Furthermore, the increase in alloy cost reduces economic efficiency. 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%.

[0155] [Third Group: B] 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 ) and promotes pearlite transformation. This reduces the cooling rate dependence of the pearlite transformation temperature. Therefore, the hardness gradient inside the rail head decreases. As a result, the generation of crack damage due to localized strain concentration in the rail head caused by contact with the wheel is suppressed, thereby extending the service life of the rail. The above effects can be achieved to some extent even with even a small amount of B. However, if the B content exceeds 0.0050%, coarse iron boride particles are formed. In this case, brittle fracture is promoted, and the breakage resistance of the rail head 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%.

[0156] [Fourth group: Cu and Ni] Cu and Ni dissolve in ferrite in pearlite to increase the hardness of the rail head, thereby improving the wear resistance of the rail head. Cu and Ni will be described below.

[0157] Cu: 0 to 1.00% Copper (Cu) is an optional element and does not necessarily need to be contained. In other words, the Cu content may be 0%. When copper is contained, that is, when the Cu content exceeds 0%, Cu dissolves in the ferrite of pearlite, increasing the hardness of the rail head through solid solution strengthening. As a result, the wear resistance of the rail head is improved. Even if even a small amount of Cu is contained, the above effects can be achieved to some extent. However, if the Cu content exceeds 1.00%, the hardenability of the rail steel material increases excessively, resulting in the formation of martensite in the rail head. As a result, the breakage resistance of the rail head decreases. 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 preferred upper limit of the Cu content is 0.90%, even more preferably 0.80%, and even more preferably 0.70%. In addition, in order to sufficiently increase the hardness of the rail head portion and suppress the formation of martensite, a more preferable upper limit of the Cu content is 0.40%.

[0158] 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 rail head through solid solution strengthening. As a result, the wear resistance of the rail head is improved. Even if even a small amount of Ni is contained, the above effects can be achieved to some extent. However, if the Ni content exceeds 1.00%, the hardenability of the rail steel material increases excessively, and martensite forms in the rail head. As a result, the wear resistance and breakage resistance of the rail head 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 preferred upper limit of the Ni content is 0.90%, even more preferably 0.80%, and even more preferably 0.70%.

[0159] [Group 5: V, Nb, and Ti] V, Nb, and Ti form precipitates such as carbides and nitrides during the cooling process of a rail after hot rolling in the rail manufacturing process. Precipitation hardening of these precipitates increases the fatigue strength of the rail head. V, Nb, and Ti also stably form carbides, nitrides, and the like during reheating during welding, suppressing softening of the heat-affected zone of the welded joint. V, Nb, and Ti will be described below.

[0160] V: 0 to 0.200% Vanadium (V) is an optional element and does not necessarily need to be present. In other words, the V content may be 0%. When vanadium (V) is present, that is, 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 rail manufacturing process. Precipitation hardening by these V precipitates increases the hardness of pearlite. As a result, the damage resistance of the rail head 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 rail head 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%.

[0161] 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 rail manufacturing process. Precipitation hardening by these Nb precipitates increases the hardness of pearlite. As a result, the damage resistance of the rail head is improved. Nb also contributes to the reduction of A ｃ１In a weld heat-affected zone reheated to a temperature range below the 1000°C point, Nb precipitates are stably formed over a wide temperature range, from low to high. This suppresses softening of the heat-affected zone of the welded joint. Even if even a small amount of Nb is contained, the above effect can be achieved to some extent. However, if the Nb content exceeds 0.0500%, excessive Nb precipitates are formed, resulting in excessive precipitation hardening. In this case, pearlite becomes embrittled, and the damage resistance of the rail head 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 upper limit of the Nb content is preferably 0.0400%, more preferably 0.0300%, even more preferably 0.0200%, still more preferably less than 0.0200%, still more preferably 0.0195%, and still more preferably 0.0190%.

[0162] 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 rail manufacturing process. Precipitation hardening by these Ti precipitates increases the hardness of pearlite. As a result, the damage resistance of the rail head is improved. Furthermore, Ti precipitates precipitated during reheating during welding are less likely to dissolve in the matrix. Therefore, Ti precipitates refine the structure of the weld heat-affected zone heated to the austenite region, suppressing the embrittlement of the weld joint. Even if only a small amount of Ti is included, the above effects can be achieved to some extent. However, when 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 rail head is reduced. 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%.

[0163] [Group 6: Mg, Ca, and Rare Earth Elements (REM)] Mg, Ca, and rare earth elements (REM) finely disperse MnS-based sulfides and reduce fatigue damage originating from inclusions in the rail head. Mg, Ca, and REM will be described below.

[0164] 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. This alleviates stress concentration around the MnS. As a result, the damage resistance of the railhead 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 railhead 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%.

[0165] 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 bonds with S to form sulfides (CaS). This CaS finely disperses MnS. Therefore, stress concentration around the MnS is alleviated. As a result, damage resistance of the railhead 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 form. As a result, damage resistance of the railhead 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%.

[0166] 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 of REM (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 railhead 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 occur. As a result, the damage resistance of the railhead is reduced. Therefore, the REM content is 0 to 0.0500%. The preferred lower limit of the REM content is 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%.

[0167] 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.

[0168] [Group 7: Zr] 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 it is contained, that is, when the Zr content is more than 0%, ZrO has high lattice matching with austenite (γ-Fe). ２Zr generates inclusions. As a result, it becomes the solidification nucleus of high-carbon rail steel, in which γ-Fe is the solidification primary crystal, and increases the equiaxed crystallization rate of the solidification structure. This suppresses the formation of segregation zones in the center of the cast slab. Therefore, martensite formation in the railhead is suppressed. As a result, the breakage resistance of the railhead is improved. 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 railhead is reduced. Therefore, the Zr content is 0 to 0.0200%. The preferred lower limit of the Zr content is 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0030%. The upper limit of the Zr content is preferably 0.0180%, more preferably 0.0150%, and even more preferably 0.0120%.

[0169] [(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 is 95% or more of a pearlite structure and has a Vickers hardness of 400 HV or more.

[0170] [Regarding the Metallic Structure of the Head Surface Portion 10A] The reason for limiting the structure to pearlite is that, as a result of investigating the correlation between the metallic structure and wear resistance, pearlite has the highest wear resistance at the same hardness. Therefore, the metallic structure of the head surface portion 10A of this embodiment is essentially pearlite. Specifically, the area ratio of pearlite in the metallic structure of the head surface portion 10A is set to 95% or more.

[0171] The structure other than pearlite in the metal structure of the head surface portion 10A is composed of one or more types 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 structures is 5% or less, the 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%.

[0172] The metal structure of the third head surface region 10B is not particularly limited. The metal structure of the third head surface region 10B 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 region 10A, a preferred metal structure of the third head surface region 10B is a structure consisting of ferrite and pearlite.

[0173] [Vickers hardness of pearlite in head surface portion 10A] The Vickers hardness of pearlite in the head surface portion 10A is 400 HV or more. If the Vickers hardness of pearlite 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 pearlite in the head surface portion 10A is 400 HV or more.

[0174] The preferred lower limit of the Vickers hardness of the pearlite in the head surface portion 10A is 420 HV, more preferably 450 HV, even more preferably 470 HV, and even more preferably 500 HV. The upper limit of the Vickers hardness of the pearlite in the head surface portion 10A is not particularly limited, but is, for example, 650 HV.

[0175] [Vickers hardness of the third head surface region 10B] The lower limit of the Vickers hardness of the third head surface region 10B is preferably 300 HV. If the Vickers hardness is 300 HV or more, the rail 1 can have a higher strength.

[0176] [Method for Observing Metallic Structure of Head Surface Portion 10A] The pearlite area ratio of the head surface portion 10A is determined by the following method. A test piece is taken from the rail 1, with the observation surface being a cross section perpendicular to the longitudinal direction of the rail 1. Referring to FIG. 4, points P11, E, F, P13, line segment L11, and line segment L12 are marked on the observation surface by the method described with reference to FIGS. 1B to 1F. Ｌ , line segment L12 ＲOn the line segment L11, rectangular regions are selected as the observation field of view, with centers at positions 0.5 mm, 5.0 mm, 10.0 mm, 15.0 mm, and 20.0 mm deep from point P11, and with a width of 450 μm in the direction parallel to the line segment L100 (a line segment extending horizontally) and 350 μm in the direction perpendicular to the line segment L100. Similarly, the line segment L12 Ｌ In this example, rectangular regions are selected as observation fields, each having a center at a depth of 0.5 mm, 5.0 mm, 10.0 mm, 15.0 mm, and 20.0 mm from point E, and extending 450 μm in a direction parallel to line segment L100 (a line segment extending horizontally) and 350 μm in a direction perpendicular to line segment L100. Ｒ In this example, rectangular regions are selected as the observation field of view, with their centers at positions 0.5 mm, 5.0 mm, 10.0 mm, 15.0 mm, and 20.0 mm deep from point F, and each rectangular region is 450 μm in the direction parallel to line segment L100 (a line segment extending horizontally) and 350 μm in the direction perpendicular to line segment L100.

[0177] The observation surface including each observation field (15 in total) is mirror-polished. The mirror-polished observation surface is etched using 3% nitric acid alcohol (Nital etching solution) to reveal the metal structure. Each observation field in the etched observation surface is observed using an optical microscope at a magnification of 200x. Photographs are taken, and each observation field (450 μm × 350 μm) is designated as the evaluation area.

[0178] In each 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 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. Based on the above contrast, the total area of ​​bainite, martensite, and pro-eutectoid cementite is calculated. The area of ​​pearlite is then determined by subtracting the total area of ​​bainite, martensite, and pro-eutectoid cementite from the total area of ​​the observation field. The area ratio of pearlite is calculated based on the pearlite area and the total area of ​​the observation field. The area ratio of pearlite may also be calculated using image analysis software, etc.

[0179] The arithmetic mean value of the area ratio of pearlite in each observation field is defined as the pearlite area ratio (%) of the head surface portion 10A of the rail 1.

[0180] [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 4, on the measurement surface, a line segment extending in the depth direction from point P11 at the width center 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 point 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 point P11.

[0181] 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.

[0182] 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.

[0183] 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 Ｒ The 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 10 mm. Ｌ , L12 Ｒ The arithmetic mean value of the three hardnesses at a depth of 1 mm is defined as the Vickers hardness H20 (HV) at a depth of 20 mm.

[0184] If 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 are all 400 HV or more, the Vickers hardness of the head surface is determined to be 400 HV or more.

[0185] The Vickers hardness of the third head surface region 10B is determined by the following method. On the hardness measurement line L11, measurement points are set at a depth of 30 mm and a depth of 40 mm in the depth direction from point P11. Ｌ At this point, the hardness measurement line L12 Ｌ The measurement points are 30 mm and 40 mm in the depth direction along the hardness measurement line L12. Ｒ At this point, the hardness measurement line L12 Ｒ Measurement points are set at a depth of 30 mm and a depth of 40 mm in the depth direction along the line.

[0186] At each measurement point, a Vickers hardness test in accordance with JIS Z 2241-1 (2020) is performed to obtain the Vickers hardness (HV). At this time, the test force is 10 kgf. The arithmetic mean value of the hardness at the three measurement points at a depth of 30 mm is taken as the Vickers hardness (HV) at the depth of 30 mm. The arithmetic mean value of the Vickers hardness at the three measurement points at a depth of 40 mm is taken as the Vickers hardness (HV) at the depth of 40 mm. The lower of the obtained Vickers hardness values ​​at the depth of 30 mm and the depth of 40 mm is taken as the Vickers hardness (HV) of the third head surface region 10B.

[0187] [(Feature 3) Hardness Distribution in the Head Surface Portion 10A] In the rail 1 of this embodiment, the hardness H1 (HV) at a position 1 mm deep from the head outer casing surface 10S, the hardness H10 (HV) at a position 10 mm deep from the head outer casing surface 10S, and the hardness H20 (HV) at a position 20 mm deep from the head outer casing surface 10S satisfy the formula (1). H1>H10>H20 (1)

[0188] In order to ensure that the hardness gradient of the first head surface region 10A1 and the hardness gradient of the second head surface region 10A2 satisfy Feature 4, it is effective to provide a distribution in which the hardness decreases in the depth direction of the head surface region 10A. If the hardness decreases in the order of hardness H1 at a depth of 1 mm in the head surface region 10A, hardness H10 at a depth of 10 mm, and hardness H20 at a depth of 20 mm (i.e., if formula (1) is satisfied), the hardness gradient (S1) of the first head surface region 10A1 and the hardness gradient (S2) of the second head surface region 10A2 are more likely to be satisfied.

[0189] Preferably, in the rail 1 of this embodiment, the hardness difference (H1-H20) obtained by subtracting the hardness H20 (HV) at a depth of 20 mm from the head outer casing surface 10S from the hardness H1 (HV) at a depth of 1 mm from the head outer casing surface 10S is 31 HV or more. In this case, even better wear resistance and damage resistance can be obtained, provided that Features 3 and 4 are satisfied. The preferred lower limit of the hardness difference (H1-H20) is 32 HV, more preferably 33 HV, even more preferably 34 HV, even more preferably 35 HV, and even more preferably 40 HV.

[0190] [(Feature 4) Hardness gradient S1 of the first head surface region 10A1, hardness gradient S2 of the second head surface region 10A2, and hardness gradient S3 of the third head surface region 10B] In the rail 1 of this embodiment, the hardness gradient S1 of the first head surface region 10A1 from the head outer casing surface 10S to a depth of 10 mm is 2.00 to 10.00 HV / mm, the hardness gradient S2 of the second head surface region 10A2 from a depth of 10 mm to a depth of 20 mm starting from the head outer casing surface 10S is 0.25 to 2.50 HV / mm, and the hardness gradient S3 of the third head surface region 10B from a depth of 20 mm to a depth of 40 mm starting from the head outer casing surface 10S is 1.00 to 4.00 HV / mm. This point will be explained below.

[0191] [Hardness Gradient of First Head Surface Region 10A1] The reason why the hardness gradient S1 of the first head surface region 10A1 is set to 2.00 to 10.00 HV is as follows.

[0192] As shown in Figure 2, in the initial contact state (wear depth 0 to 10 mm) where the contact area between the rail 1 and the wheel 2 is small, the contact pressure between the rail 1 and the wheel 2 increases, making it easy for crack damage caused by plastic deformation to occur. In order to suppress crack damage caused by plastic deformation, it is effective to control the contact pressure between the rail 1 and the wheel 2. In order to control the contact pressure, it is effective to increase the contact area between the rail 1 and the wheel 2. If the hardness gradient in the first head surface region 10A1 is large, the contact area is likely to increase. Therefore, the contact pressure is easily reduced, and damage resistance is improved.

[0193] Referring to FIG. 5, if the hardness gradient S1 of the first head surface region 10A1 is 2.00 HV / mm or more, wear of the rail 1 is accelerated and the contact area between the rail 1 and the wheel 2 increases. This reduces the contact pressure and significantly reduces the maximum crack depth of crack damage on the rolling surface. As a result, damage resistance is significantly improved. On the other hand, if the hardness gradient S1 of the first head surface region 10A1 exceeds 10.00 HV / mm, the maximum crack depth of crack damage does not change significantly. However, a large amount of internal crack damage occurs due to the concentration of strain caused by the increased hardness gradient in the interior immediately below the rolling surface. Therefore, the hardness gradient S1 of the first head surface region is 2.00 to 10.00 HV / mm.

[0194] The hardness gradient S1 of the first head surface region is preferably 2.10 HV / mm, more preferably 2.20 HV / mm, and even more preferably 2.30 HV / mm. The hardness gradient S1 of the first head surface region is preferably 9.50 HV / mm, more preferably 9.00 HV / mm, and even more preferably 8.50 HV / mm.

[0195] [Hardness gradient S2 of second head surface region 10A2] The reason why the hardness gradient S2 of the second head surface region 10A2 is set to 0.25 to 2.50 HV / mm is as follows.

[0196] As shown in Figure 7, under normal contact conditions (wear depth of 10 to 20 mm) where the contact area between the rail 1 and the wheel 2 is large, metal fatigue accumulates on the rolling surface, making it prone to crack damage. When the hardness gradient S2 of the second head surface region 10A2 is 0.25 HV / mm or higher, wear of the rail 1 is accelerated and metal fatigue is eliminated. This significantly reduces the number of cracks occurring on the rolling surface. As a result, the damage resistance of the rail 1 is significantly improved. On the other hand, when the hardness gradient S2 exceeds 2.50 HV / mm, there is no significant change in the number of cracks occurring on the rolling surface, but wear of the rail 1 is excessively accelerated, reducing the wear resistance of the rail 1. Therefore, the hardness gradient S2 of the second head surface region 10A2 is 0.25 to 2.50 HV / mm.

[0197] The hardness gradient S2 of the second head surface region is preferably 0.30 HV / mm, more preferably 0.35 HV / mm, and even more preferably 0.40 HV / mm. The hardness gradient S2 of the second head surface region is preferably 2.40 HV / mm, more preferably 2.30 HV / mm, and even more preferably 2.20 HV / mm.

[0198] [Hardness gradient S3 of third head surface region 10B] The reason why the hardness gradient S3 of the third head surface region 10B is set to 1.00 to 4.00 HV / mm is as follows.

[0199] As described above, there are two types of brittle fracture that cause breakage in the third head surface region 10B: (Type 1) Brittle fracture caused by localized concentration of strain due to an excessive increase in the hardness gradient S3 of the third head surface region 10B and a sudden change in ductility; (Type 2) Brittle fracture caused by insufficient distribution of strain in the deep region of the third head surface region 10B (the region from a depth of 30 mm to a depth of 40 mm starting from the head outer surface 10S).

[0200] Referring to FIG. 9 , if the hardness gradient S3 of the third head surface region 10B exceeds 4.00 HV / mm, the hardness gradient of the third head surface region 10B increases excessively. Therefore, when an external force from the wheel is applied, the change in ductility accompanying the change in hardness becomes abrupt. As a result, strain is concentrated partially in the third head surface region 10B. As a result, the maximum value of the falling weight energy at which breakage does not occur decreases, making breakage more likely (Mode 1). On the other hand, if the hardness gradient S3 is less than 1.00 HV / mm, the ductility of the deep region of the third head surface region 10B is not ensured. In this case, when an external force from the wheel is applied, strain distribution within the third head surface region 10B is not sufficient. As a result, the maximum value of the falling weight energy at which breakage does not occur decreases, making breakage more likely (Mode 2). If the hardness gradient S3 of the third head surface region 10B is 1.00 to 4.00 HV / mm, the concentration of strain due to the difference in hardness is suppressed, and further, the distribution of strain within the third head surface region 10B is controlled, resulting in excellent breakage resistance.

[0201] The hardness gradient S3 of the third head surface region 10B has a lower limit of 1.05 HV / mm, more preferably 1.10 HV / mm, even more preferably 1.15 HV / mm, and even more preferably 1.20 HV / mm. The hardness gradient S3 of the third head surface region 10B has an upper limit of 3.80 HV / mm, more preferably 3.60 HV / mm, and even more preferably 3.40 HV / mm.

[0202] Preferably, the hardness gradient S3 of the third head surface region 10B is greater than the hardness gradient S2 of the second head surface region 10A2, so that excessive hardness in the deep layer of the third head surface region 10B can be prevented.

[0203] [Method of measuring the hardness gradient S1 of the first head surface region 10A1, the hardness gradient S2 of the second head surface region 10A2, and the hardness gradient S3 of the third head surface region 10B] The hardness gradient S1 of the first head surface region 10A1, the hardness gradient S2 of the second head surface region 10A2, and the hardness gradient S3 of the third head surface region 10B are measured using the following method.

[0204] 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.

[0205] 4, on the measurement surface, a line segment extending in the depth direction from point P11 at the width center of the vertex 11 on the head outer surface 10S as the origin point is defined as a hardness measurement line L11. On the hardness measurement line L11, measurement points are determined in the depth direction from point P11 at a depth of 1 mm to a depth of 40 mm at 1.0 mm intervals.

[0206] 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.

[0207] At the head corner portion 12L, 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 Measurement points are determined at 1.0 mm intervals from a depth of 1 mm to a depth of 40 mm in the depth direction along the line L12. Similarly, in the head corner portion 12R, the line segment connecting point F and point P13 is defined as the hardness measurement line L12. Ｒ From point F to hardness measurement line L12 Ｒ Measurement points are set at 1.0 mm intervals in the depth direction along the line from a depth of 1 mm to a depth of 40 mm.

[0208] Three hardness measurement lines L11 and L12 Ｌ , L12 Ｒ At each measurement point, the Vickers hardness is measured in accordance with JIS Z 2244-1 (2020). The test force is 10 kgf.

[0209] Of the obtained Vickers hardness, the above-mentioned hardness measurement lines L11 and L12 Ｌ , L12 ＲThe arithmetic mean value of the hardness at the same depth measurement points (3 points) is calculated by this method. The calculated mean value of the three measurement points obtained at each depth position at 1.0 mm intervals from the depth position 1 mm to the depth position 40 mm is taken as the Vickers hardness at that depth position.

[0210] Using the obtained hardness at each depth position from 1 mm to 10 mm, a linear approximation is performed to determine the hardness gradient m. Specifically, when x is the depth (mm) and y is the hardness (HV), the linear regression equation is expressed as follows: y = mx + b, where m is the gradient and b is the intercept.

[0211] In this case, the gradient m is expressed by the following equation.

[0212] The gradient m obtained based on the above formula is defined as the hardness gradient S1 in the first head surface region 10A1.

[0213] Similarly, a linear approximation is performed using the hardness values ​​at each depth position from 11 mm to 20 mm, and a hardness gradient m is calculated based on the above formula. The obtained gradient m is defined as the hardness gradient S2 in the second head surface region 10A2.

[0214] Similarly, a linear approximation is performed using the hardness values ​​obtained at each depth position from the 21 mm depth position to the 40 mm depth position, and a hardness gradient m is calculated based on the above formula. The obtained gradient m is defined as the hardness gradient S3 in the third head surface region 10B.

[0215] [(Feature 4) Relationship between the hardness gradient (S1) of the first head surface region 10A1 and the hardness gradient (S2) of the second head surface region 10A2] The hardness gradient S1 of the first head surface region 10A1 and the hardness gradient S2 of the second head surface region 10A2 satisfy the formula (2): S2≦0.95S1 (2)

[0216] If the hardness gradient S2 of the second head surface region 10A2 does not satisfy formula (2), the hardness gradient S2 will be equal to or greater than the hardness gradient S1. In this case, the second head surface region 10A2 will be excessively soft in the depth direction. As a result, crack damage that occurs internally just below the rolling surface due to contact between the rail 1 and the wheel will occur in large quantities in the excessively soft second head surface region 10A2. As a result, the first head surface region 10A1 and the second head surface region 10A2 are likely to peel off at the same time, significantly reducing damage resistance.

[0217] If the hardness gradient S2 satisfies the formula (2), crack damage occurring in the second head surface region 10A2 immediately below the rolling surface due to contact between the rail 1 and the wheel can be sufficiently suppressed, thereby significantly improving damage resistance.

[0218] [Effects of the rail 1 of this embodiment] The rail 1 of this embodiment satisfies Features 1 to 4. Therefore, the rail 1 of this embodiment has excellent wear resistance and excellent damage resistance. Therefore, the rail 1 of this embodiment is suitable as a rail for freight railways that are used in harsh environments.

[0219] [Preferred embodiment (part 1) of the rail 1 of this embodiment] In the chemical composition of the rail 1 of this embodiment, Cr dissolves in cementite in pearlite and hardens the cementite. This increases the hardness of the rail head portion 10. However, the hardening of cementite by Cr reduces the toughness of the rail 1. Therefore, if the hardness gradient S3 of the third head surface region 10B of the rail 1 is 1.00 to 4.00 HV / mm and the rail 1 does not contain Cr, the rail 1 will have sufficiently excellent breakage resistance.

[0220] On the other hand, when the rail 1 contains Cr, the rail 1 preferably satisfies Features 1 to 4, and further satisfies Feature 5. (Feature 5) The rail 1 contains 0.01 to 1.00% Cr, and the hardness gradient S3 of the third head surface region and the Cr content in mass% satisfy the formula (3): 1.25≦S3 / Cr≦20.00 (3) Feature 5 will now be described.

[0221] As described above, by setting the hardness gradient S3 of the third head surface region 10B to 1.00 to 4.00 HV / mm, the concentration of strain due to differences in hardness is suppressed and the distribution of strain within the third head surface region 10B is controlled, thereby providing the rail 1 with excellent breakage resistance.

[0222] Cr dissolves in cementite in pearlite and hardens the cementite. Therefore, increasing the Cr content in the rail 1 can improve the wear resistance of the rail 1. On the other hand, hard cementite has low toughness. Therefore, it is preferable to have less Cr-containing cementite, especially in the third head surface region 10B. In other words, although Cr can improve the wear resistance of the rail 1, in the third head surface region 10B, Cr affects the damage resistance together with the hardness gradient S3.

[0223] F3 is defined as follows: F3 = S3 / Cr When the rail 1 contains Cr and F3 satisfies formula (3), the inclusion of Cr enhances the wear resistance of the rail 1 while suppressing the excessive formation of Cr-containing cementite. This increases the maximum value of the falling weight energy at which breakage does not occur, significantly improving breakage resistance.

[0224] The lower limit of F3 is preferably 1.30, more preferably 1.50, even more preferably 2.00, and even more preferably 2.50. The upper limit of F3 is preferably 19.00, more preferably 18.00, and even more preferably 15.00.

[0225] [Preferred embodiment (part 2) of the rail 1 of this embodiment] [(Feature 6) Regarding the degree of Si segregation in the second head surface region 10A2] Preferably, the rail 1 of this embodiment further satisfies the following Feature 6: (Feature 6) The degree of Si segregation in the second head surface region 10A2 is 1.35 or less.

[0226] The rail 1 of this embodiment satisfies Features 1 to 4, thereby achieving excellent damage resistance and excellent damage tolerance in the head surface portion 10A of the rail 1. If such a rail 1 also satisfies Feature 6, the damage tolerance can be further improved.

[0227] 11, when the hardness gradient S2 of the second head surface region 10A2 is 0.25 to 2.50 HV / mm, the number of crack damage occurrences is further reduced when the Si segregation degree in the second head surface region 10A2 is 1.35 or less (Test Nos. 2 and 3) compared to when the Si segregation degree in the second head surface region 10A2 is more than 1.35 (Test No. 1). Therefore, the preferred Si segregation degree in the second head surface region 10A2 is 1.35 or less.

[0228] The upper limit of the degree of Si segregation in the second head surface region 10A2 is preferably 1.30, and more preferably 1.25.

[0229] [Method for Measuring the Degree of Si Segregation in the Second Head Surface Region 10A2] The degree of Si segregation in the second head surface region 10A2 of the rail 1 of this embodiment is determined by the following method. A test piece is taken, with the measurement surface being a cross section perpendicular to the longitudinal direction of the rail 1. Five analysis regions 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 center of the width of the head outer surface 10S (i.e., the second head surface region 10A2), each measuring 800 μm in the longitudinal direction of the rail 1 and 800 μm in the depth direction. 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.

[0230] 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.

[0231] Based on the element distribution map, regions where Si is enriched are identified as Si semi-macrosegregation zones. 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 in the analysis region. The element distribution map is binarized using the obtained average Si concentration [Si] as a threshold value. At this time, regions in the element distribution map where the Si concentration is higher than the threshold value and regions where the Si concentration is lower than the threshold value are colored differently. In the binarized element distribution map, regions where the Si concentration is higher than the threshold value are identified as Si semi-macrosegregation zones. Fifty Si semi-macrosegregation zones are selected from the five analysis regions. The degree of Si segregation in the selected Si semi-macrosegregation zones is determined by the following method.

[0232] Fig. 10 is a schematic diagram for explaining how to determine the degree of Si segregation. Referring to Fig. 10, 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-macrosegregation zone is selected to obtain the Si concentration distribution along that line. Specifically, when the Si semi-macrosegregation 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.

[0233] 10, 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-macrosegregation 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.

[0234] [Quantification of Si Concentration in Si Semimacrosegregation Zone] Furthermore, in the Si concentration analysis obtained by the selected line analysis, the maximum Si concentration in the Si semimacrosegregation zone is determined, and the arithmetic mean value of the maximum Si concentration and the Si concentration in the bulk region is defined as the Si concentration (mass%) in the Si semimacrosegregation zone.

[0235] [Determination of Si Segregation] Based on the Si concentration in the bulk region and the Si concentration in the Si semi-macrosegregation band obtained by the above-mentioned method, the Si segregation degree in the Si semi-macrosegregation band is calculated using the following formula: Si segregation degree = Si concentration in Si semi-macrosegregation band / Si concentration in bulk region The Si segregation degree is calculated for each of the multiple Si semi-macrosegregation bands confirmed in the five analysis regions by the above-mentioned method. The arithmetic mean value of the obtained Si segregation degrees for the top 20 is defined as the Si segregation degree in the Si semi-macrosegregation band. The arithmetic mean value of the Si segregation degrees of the 50 Si semi-macrosegregation bands is defined as the Si segregation degree in the second head surface region 10A2.

[0236] [Method for manufacturing rail 1] An example of a method for manufacturing the rail 1 of this embodiment will be described. The method for manufacturing the rail 1 described below is one example for manufacturing the rail 1 of this 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 method for manufacturing the rail 1 of this embodiment.

[0237] 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.

[0238] [(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 satisfying 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. Preferred conditions for continuous casting will be described later.

[0239] [(Step 2) Hot Rolling Step] The produced bloom is reheated and then hot rolled to form a rail blank.

[0240] First, the bloom is charged into a heating furnace and reheated. If the reheating temperature is less than 1000°C, sufficient formability during hot rolling cannot be ensured. Furthermore, rolling defects are more likely to occur during hot rolling. On the other hand, if the reheating temperature exceeds 1350°C, the steel may melt, making rail manufacturing difficult. Therefore, the reheating temperature is set to 1000 to 1350°C. After the bloom has been heated to within this temperature range, it is removed from the heating furnace. The bloom is then hot rolled to form a rail preform.

[0241] In hot rolling, the temperature of a rail preform when it passes through the final pass is referred to as the final rolling temperature. The temperature when it passes through the final pass refers to the temperature of the rail preform 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 exit side of the rolling stand that performs the final reduction.

[0242] 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 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, forms in the rail head 10 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.

[0243] Other hot rolling conditions are not particularly limited. In order to ensure the hardness of the rail head portion 10 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 two or more 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.

[0244] [(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. 12. Referring to FIG. 12, the cooling device 20 includes a plurality of air injection nozzles 21 that inject air (compressed air) toward the head surface portion 10A, a supply path 210 that supplies air to the plurality of air injection nozzles 21, a plurality of air injection nozzles 22 that inject air toward the pair of lower jaw portions 13, an air supply path 220 that supplies air to the plurality of air injection nozzles 22, and an air supply device 23 that supplies air to the air supply path 210 and / or 220.

[0245] The plurality of air injection nozzles 21 and 22 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 plurality of air injection nozzles 21 and 22 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.

[0246] The plurality of air injection nozzles 21 and 22 are arranged to surround the rail head portion 10. The air injection nozzle 21 injects air toward the head surface portion 10A. The air injection nozzle 22 injects air toward the underchin portion 13.

[0247] Air supply device 23 supplies air to air supply path 210 and / or air supply path 220. When air supply device 23 supplies air to air supply path 210 but not to air supply path 220, air is sprayed from air injection nozzle 21 to accelerate cooling of head surface 10A. At this time, air is not supplied to air supply path 220, so air is not sprayed from air injection nozzle 22. On the other hand, when air supply device 23 supplies air to air supply path 220 but not to air supply path 210, air is sprayed from air injection nozzle 22 to accelerate cooling of underchin 13. At this time, air is not supplied to air supply path 210, so air is not sprayed from air injection nozzle 22. When the air supply device 23 supplies air to the air supply paths 210 and 220, air is sprayed from the air injection nozzles 21 and 22 to accelerate cooling of the head surface 10A and the underchin 13. In this way, the air supply device 23 can independently control the accelerated cooling by the air injection nozzle 21 and the accelerated cooling by the air injection nozzle 22.

[0248] [Cooling Conditions] In order for the rail 1 to satisfy Features 2 to 4, it is desirable to perform stepwise accelerated cooling of the head surface portion 10A and the under-chin portion 13. Specifically, it is desirable to perform cooling of the head surface portion 10A in two stages (first cooling stage and second cooling stage) and cooling of the under-chin portion 13 in one stage (first cooling stage).

[0249] Furthermore, when the rail 1 satisfies Feature 5, it is desirable to further accelerate the cooling of the head surface portion 10A and the underchin portion 13 in stages. Specifically, it is desirable to perform the cooling of the head surface portion 10A in three stages (the first and second cooling stages described above, plus a third cooling stage), and to perform the cooling of the underchin portion 13 in two stages (the first cooling stage described above, plus a second cooling stage).

[0250] The cooling start temperature, cooling rate, and cooling end temperature for each cooling stage for the head surface portion 10A, and the cooling start temperature, cooling rate, and cooling end temperature for each cooling stage for the under-chin portion 13 are as follows. Note that the cooling start temperature for the first to third stages of cooling the head surface portion refers to the temperature at the surface of the head surface portion 10A. The cooling start temperature for the first and second stages of cooling the under-chin portion refers to the temperature at the surface of the under-chin portion 13. [Accelerated cooling for head surface] First stage of cooling for head surface: Starting temperature: 750-900°C, Cooling rate: 2.0-8.0°C / sec, End temperature: 620-680°C Second stage of cooling for head surface: Starting temperature: 620-680°C, Cooling rate: 2.0-6.0°C / sec, End temperature: 550-600°C Third stage of cooling for head surface: Starting temperature: 550-600°C, Cooling rate: 2.0-4.0°C / sec, End temperature: 450-520°C [Accelerated cooling for under-chin] First stage of cooling for under-chin: Starting temperature: 700-850°C, Cooling rate: 2.0-8.0°C / sec, End temperature: 530-580°C Second stage of cooling for under-chin: Starting temperature: 530-580°C, Cooling rate: 2.0-6.0°C / sec Cooling end temperature: 400 to 520°C

[0251] The cooling start temperature refers to the temperature of the rail preform when accelerated cooling of the rail preform begins. In other words, the cooling start temperature refers to the surface temperature of the rail preform when air (compressed air) as a refrigerant begins to be sprayed onto the rail preform. The cooling rate refers to the value (°C / second) obtained by dividing the difference between the cooling start temperature and the cooling end temperature by the cooling time. The cooling end temperature refers to the temperature of the rail preform when cooling using air (compressed air) at the above-mentioned cooling rate is completed. The conditions for the above-mentioned accelerated cooling are explained below.

[0252] [Accelerated cooling conditions for satisfying features 2 to 4] In order to satisfy features 2 to 4 of the rail 1 of this embodiment, the first and second head surface cooling stages of the head surface accelerated cooling are performed, and the first under-chin cooling stage of the under-chin accelerated cooling is performed. The conditions for the first head surface cooling stage, the second head surface cooling stage, and the first under-chin cooling stage are described below.

[0253] [First Stage of Cooling of Head Surface Portion] (Cooling Start Temperature: 750 to 900°C) If the cooling start temperature in the first stage of cooling of the head surface portion is less than 750°C, pearlite will form in the high temperature range before cooling. As a result, the Vickers hardness (400 HV) at the head surface portion 10A, which is necessary for improving the wear resistance and damage resistance of the rail 1, will not be sufficiently obtained. If the cooling start temperature in the first stage of cooling of the head surface portion is less than 750°C, pro-eutectoid cementite may also form in the rail 1. In this case, the rail 1 will not have sufficient wear resistance and damage resistance. On the other hand, if the cooling start temperature of the rail 1 exceeds 900°C, the hardenability of the rail preform will be excessively increased. In this case, bainite will form in excess, and the rail 1 will not have sufficient wear resistance. Therefore, the preferred cooling start temperature in the first stage of cooling of the head surface portion is 750 to 900°C.

[0254] (Cooling rate: 2.0 to 8.0°C / sec) If the cooling rate in the first stage of head surface cooling is less than 2.0°C / sec, the pearlite transformation temperature rises. In this case, the Vickers hardness (400 HV) at the head surface 10A, which is necessary to improve the wear resistance and damage resistance of the rail 1, cannot be sufficiently obtained. Furthermore, the hardness gradient S1 of the first head surface region 10A1 may be less than 2.00 HV / mm. In this case, crack damage in the rail 1 becomes excessively deep, and sufficient damage resistance cannot be obtained. On the other hand, if the cooling rate in the first stage of head surface cooling exceeds 8.0°C / sec, the difference in pearlite transformation temperature in the depth direction from the surface becomes excessively large. Therefore, the hardness gradient S1 of the first head surface region 10A1 may exceed 10.00 HV / mm. In this case, numerous internal cracks occur in the rail 1, and sufficient damage resistance cannot be obtained. Therefore, the preferred cooling rate in the first stage of cooling the head surface is 2.0 to 8.0° C. / sec.

[0255] (Cooling end temperature: 620 to 680°C) If the cooling end temperature of the first stage of cooling the head surface exceeds 680°C, pearlite transformation begins in the high-temperature range immediately after the end of cooling. In this case, a large amount of low-hardness pearlite is produced. As a result, the Vickers hardness (400 HV) required to improve the wear resistance and damage resistance of the rail 1 cannot be sufficiently obtained in the head surface region 10A. In this case, the difference in pearlite transformation temperature from the surface to the depth direction becomes excessively small. As a result, the hardness gradient S1 of the first head surface region 10A1 may be less than 2.00 HV / mm. In this case, crack damage in the rail 1 becomes excessively deep, and sufficient damage resistance cannot be obtained. On the other hand, if the cooling end temperature of the first stage of cooling the head surface is less than 620°C, excessive cooling occurs. As a result, the difference in pearlite transformation temperature from the surface to the depth direction becomes excessively large. As a result, the hardness gradient S1 of the first head surface region 10A1 may exceed 10.00 HV / mm. In this case, a large number of internal cracks occur in the rail 1, and sufficient damage resistance cannot be obtained. Therefore, the preferable cooling end temperature of the first stage of cooling of the head surface portion is 620 to 680°C.

[0256] [Head surface cooling second stage] (Cooling start temperature: 620 to 680°C) The head surface cooling second stage is carried out continuously after the head surface cooling first stage is completed. Therefore, the cooling start temperature of the head surface cooling second stage is the same as the cooling end temperature of the head surface cooling first stage. In other words, the cooling start temperature of the head surface cooling second stage is 620 to 680°C.

[0257] (Cooling rate: 2.0 to 6.0°C / sec) If the cooling rate in the second head surface cooling stage is less than 2.0°C / sec, the pearlite transformation temperature will rise even if the first under-chin cooling stage described below is performed. In this case, the difference in pearlite transformation temperature from the surface of the railhead 10 to the depth direction will be excessively small. As a result, the hardness gradient S2 of the second head surface region 10A2 will be less than 0.25 HV / mm. In this case, the rail 1 will not have sufficient damage resistance. On the other hand, if the cooling rate in the second head surface cooling stage exceeds 6.0°C / sec, the difference in pearlite transformation temperature from the surface of the railhead 10 to the depth direction will be excessively large. As a result, the hardness gradient S2 of the second head surface region 10A2 may exceed 2.50 HV / mm. In this case, the rail 1 will not have sufficient wear resistance. Therefore, the preferred cooling rate in the second head surface cooling stage is 2.0 to 6.0°C / sec.

[0258] (Cooling end temperature: 550 to 600°C) If the cooling end temperature of the second stage of head surface cooling exceeds 600°C, pearlite transformation begins in the high-temperature region immediately after the end of cooling. In this case, the difference in pearlite transformation temperature from the surface of the railhead 10 to the depth direction becomes excessively small. As a result, the hardness gradient S2 of the second head surface region 10A2 may become less than 0.25 HV / mm. In this case, the rail 1 does not have sufficient damage resistance. On the other hand, if the cooling end temperature of the second stage of head surface cooling is less than 550°C, the railhead outer surface S10 is excessively cooled. As a result, the difference in pearlite transformation temperature from the surface of the railhead 10 to the depth direction becomes excessively large. As a result, the hardness gradient S2 of the second head surface region 10A2 may exceed 2.50 HV / mm. In this case, the rail 1 does not have sufficient wear resistance. Therefore, the preferred cooling end temperature of the second stage of head surface cooling is 550 to 600°C.

[0259] [First Under-Jaw Cooling Stage] (Cooling Start Temperature: 700-850°C) The first under-jaw cooling stage, which is accelerated cooling of the under-jaw 13, is performed around the end of the first head surface cooling stage. Note that, in the first head surface cooling stage, the under-jaw 13 is not directly cooled by the refrigerant (compressed air). Therefore, at the end of the first head surface cooling stage, the surface temperature of the under-jaw 13 is higher than the surface temperature of the head surface 10A. If the cooling start temperature in the first under-jaw cooling stage is less than 700°C, pearlite forms in the high-temperature region before the start of cooling. As a result, the Vickers hardness (400 HV) of the head surface 10A, which is necessary for improving the wear resistance and damage resistance of the rail 1, is not sufficiently obtained. Furthermore, pro-eutectoid cementite may form in the rail 1. In this case, the rail 1 does not achieve sufficient wear resistance and damage resistance. On the other hand, if the cooling start temperature in the first stage of under-chin cooling exceeds 850°C, the hardenability of the rail preform will be excessively increased. In this case, martensite will be excessively formed in the second head surface region 10A2. As a result, the hardness gradient S2 of the second head surface region 10A2 may exceed 2.50 HV / mm. In this case, sufficient wear resistance will not be obtained in the rail 1. Therefore, the preferred cooling start temperature in the first stage of under-chin cooling is 700 to 850°C.

[0260] (Cooling rate: 2.0 to 8.0°C / sec) If the cooling rate in the first stage of under-chin cooling is less than 2.0°C / sec, the difference in pearlite transformation temperature from the surface of the railhead 10 to the depth direction will be excessively small. As a result, the hardness gradient S2 of the second head surface region 10A2 may become less than 0.25 HV / mm. In this case, the rail 1 will not have sufficient damage resistance. On the other hand, if the cooling rate in the first stage of under-chin cooling exceeds 8.0°C / sec, the difference in pearlite transformation temperature from the surface of the railhead 10 to the depth direction will become excessively large. As a result, the hardness gradient S2 of the second head surface region 10A2 may exceed 2.50 HV / mm. In this case, the rail 1 will not have sufficient wear resistance. Therefore, the preferred cooling rate in the first stage of under-chin cooling is 2.0 to 8.0°C / sec.

[0261] (Cooling end temperature: 530 to 580°C) If the cooling end temperature in the first stage of under-chin cooling exceeds 580°C, pearlite transformation begins in the high-temperature region immediately after the end of cooling. In this case, the difference in pearlite transformation temperature from the surface of the railhead 10 to the depth direction becomes excessively small. As a result, the hardness gradient S2 of the second head surface region 10A2 may become less than 0.25 HV / mm. In this case, the rail 1 does not have sufficient damage resistance. On the other hand, if the cooling end temperature in the first stage of under-chin cooling is less than 530°C, the surface of the under-chin region 13 is excessively cooled. As a result, the difference in pearlite transformation temperature from the surface of the railhead 10 to the depth direction becomes excessively large. As a result, the hardness gradient S2 of the second head surface region 10A2 may exceed 2.50 HV / mm. In this case, the rail 1 does not have sufficient wear resistance. Therefore, the preferred cooling end temperature for the first stage of under-chin cooling is 530 to 580°C.

[0262] [Head surface cooling third stage] The head surface cooling third stage is carried out continuously after the head surface cooling second stage is completed. The cooling start temperature, cooling rate and cooling end temperature of the head surface cooling third stage are as follows.

[0263] (Cooling start temperature: 550 to 600°C) As described above, the third stage of head surface cooling is carried out immediately after the second stage of head surface cooling is completed. Therefore, the cooling start temperature of the third stage of head surface cooling is the same as the cooling end temperature of the second stage of head surface cooling. In other words, the cooling start temperature of the third stage of head surface cooling is 550 to 600°C.

[0264] (Cooling rate: 2.0 to 4.0°C / sec) If the cooling rate in the third stage of head surface cooling is less than 2.0°C / sec, heat removal from the rail head will be insufficient even if under-chin cooling is applied. As a result, the difference in pearlite transformation temperature from the surface of the rail head 10 to the depth direction will be excessively large. As a result, the hardness gradient S3 of the third head surface region 10B will exceed 4.00 HV / mm. In this case, the rail 1 will not have excellent breakage resistance. On the other hand, if the cooling rate exceeds 4.0°C / sec, heat removal from the rail head will be excessive. In this case, the difference in pearlite transformation temperature from the surface of the rail head 10 to the depth direction will be excessively small. As a result, the hardness gradient S3 of the third head surface region 10B will be less than 1.00 HV / mm. In this case, the rail 1 may not have excellent breakage resistance. Therefore, the cooling rate in the third stage of head surface cooling is 2.0 to 4.0°C / sec.

[0265] (Cooling end temperature: 450 to 520°C) If the cooling end temperature in the third stage of head surface cooling exceeds 520°C, heat removal in the third head surface region 10B will be insufficient. As a result, the difference in pearlite transformation temperature from the surface of the railhead 10 in the depth direction will be excessively large. As a result, the hardness gradient S3 in the third head surface region 10B will exceed 4.00 HV / mm. In this case, the rail 1 will not have excellent breakage resistance. On the other hand, if the cooling end temperature in the third stage of head surface cooling is less than 450°C, heat removal in the third head surface region 10B will be excessive. In this case, the difference in pearlite transformation temperature from the surface of the railhead 10 in the depth direction will be excessively small. As a result, the hardness gradient S3 in the third head surface region 10B will be less than 1.00 HV / mm. In this case, the rail 1 may not have excellent damage resistance. Therefore, the cooling end temperature in the third stage of head surface cooling is 450 to 520°C.

[0266] [Second stage of under-chin cooling] The second stage of under-chin cooling is carried out immediately after the first stage of under-chin cooling. The cooling start temperature, cooling rate, and cooling end temperature of the second stage of under-chin cooling are as follows:

[0267] (Cooling start temperature: 530 to 580°C) As described above, the second stage of under-chin cooling is carried out immediately after the first stage of under-chin cooling is completed. Therefore, the cooling start temperature of the second stage of under-chin cooling is the same as the cooling end temperature of the first stage of under-chin cooling. In other words, the cooling start temperature of the second stage of under-chin cooling is 530 to 580°C.

[0268] (Cooling rate: 2.0 to 6.0°C / sec) If the cooling rate in the second stage of under-chin cooling is less than 2.0°C / sec, heat removal in the third head surface region 10B will be insufficient. As a result, the difference in pearlite transformation temperature from the surface of the railhead 10 to the depth direction will be excessively large. As a result, the hardness gradient S3 in the third head surface region 10B will exceed 4.00 HV / mm. In this case, the rail 1 may not have excellent breakage resistance. On the other hand, if the cooling rate in the second stage of under-chin cooling exceeds 6.0°C / sec, heat removal in the third head surface region 10B will be excessive. In this case, the difference in pearlite transformation temperature from the surface of the railhead 10 to the depth direction will be excessively small. As a result, the hardness gradient S3 in the third head surface region 10B will be less than 1.00 HV / mm. In this case, the rail 1 may not have excellent breakage resistance. Therefore, the cooling rate in the second stage of cooling the lower jaw is 2.0 to 6.0° C. / sec.

[0269] (Cooling end temperature: 400 to 520°C) If the cooling end temperature in the second stage of under-chin cooling exceeds 520°C, heat removal in the third head surface region 10B will be insufficient. As a result, the difference in pearlite transformation temperature from the surface of the railhead 10 in the depth direction will be excessively large. As a result, the hardness gradient S3 in the third head surface region 10B will exceed 4.00 HV / mm. In this case, the rail 1 may not achieve excellent breakage resistance. On the other hand, if the cooling end temperature in the second stage of under-chin cooling is less than 400°C, heat removal in the third head surface region 10B will be excessive. In this case, the difference in pearlite transformation temperature from the surface of the railhead 10 in the depth direction will be excessively small. As a result, the hardness gradient S3 in the third head surface region 10B will be less than 1.00 HV / mm. As a result, the rail 1 may not achieve excellent breakage resistance. Therefore, the preferred cooling end temperature in the second stage of cooling the lower jaw is 400 to 520°C.

[0270] The above manufacturing process produces the rail 1 of this embodiment that satisfies Features 1 to 4. Regarding Feature 4, formula (2) can be satisfied by adjusting the accelerated cooling conditions to fall within the above range.

[0271] [Preferable continuous casting conditions for satisfying feature 6] To further satisfy feature 6 in the rail 1, that is, to set the Si segregation degree in the second head surface region 10A2 to 1.35 or less, it is desirable to lightly reduce the bloom during continuous casting in the material production process. Lightly reducing the bloom during continuous casting reduces the Si segregation degree of the bloom. If the Si segregation degree of the bloom is reduced, the Si segregation degree of the produced rail 1 will also be reduced. This is thought to be because the segregated state of the rail 1 is maintained even after the above-mentioned heat treatment process.

[0272] 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.

[0273] [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%.

[0274] 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.

[0275] [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.

[0276] The reduction rate during soft reduction refers to the ratio of the reduction amount 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 (%) = Reduction amount during soft reduction / Thickness of bloom before the start of soft reduction × 100

[0277] The cumulative reduction ratio of soft reduction is 1 to 3% when the central solid fraction is 70% or less. When the central solid fraction exceeds 70%, the cumulative reduction ratio of soft reduction (i.e., the reduction ratio of soft reduction in this embodiment) is 2 to 5%.

[0278] 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.

[0279] 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.

[0280] 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).

[0281]

[0282] 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

[0283] 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 ｎ＋１ ｉ Ask for.

[0284]

[0285] 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)

[0286] 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.

[0287] 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.

[0288] Rails were manufactured having the chemical compositions shown in Table 2 (Tables 2A to 2D).

[0289]

[0290]

[0291]

[0292]

[0293] In Table 2, "-" means that the corresponding element was not intentionally added.

[0294] The rails of each test number were manufactured under the manufacturing conditions shown in Table 3 (Tables 3A to 3D).

[0295]

[0296]

[0297]

[0298]

[0299] Specifically, blooms were produced by continuous casting using molten steel. In the continuous casting, soft reduction was performed on the blooms of test numbers 75 to 78. The center solid fraction (%) at the start of soft reduction and the center solid fraction (%) at the end of soft reduction were as shown in Table 3D. For test number 75, soft reduction was terminated when the center solid fraction reached 70%. For test number 76, when the center solid fraction exceeded 70%, the soft reduction was set to 50% of the reduction rate when the center solid fraction was 70% or less, and the soft reduction was terminated when the center solid fraction reached 80%. For test number 77, when the center solid fraction exceeded 70%, the soft reduction was set to 60% of the reduction rate when the center solid fraction was 70% or less, and the soft reduction was terminated when the center solid fraction exceeded 90%. In Test No. 78, when the central solid fraction exceeded 70%, the soft reduction was set to 70% of the reduction when the central solid fraction was 70% or less, and the soft reduction was terminated when the central solid fraction was 100%. In Test No. 75, the cumulative reduction in soft reduction was 2%. In Test Nos. 76 to 78, the cumulative reduction in soft reduction was 4%.

[0300] The produced blooms were subjected to hot rolling. First, the blooms were reheated to 1000 to 1350°C. The reheated blooms were then hot rolled to produce rail preforms in the shape of a rail. The final rolling temperature was 750 to 1100°C for all test numbers.

[0301] The rail preform after hot rolling was subjected to heat treatment (accelerated cooling). The cooling device shown in Figure 12 was used. The cooling start temperatures (°C), cooling rates (°C / sec), and cooling end temperatures (°C) for the first to third head surface cooling stages, the first under-jaw cooling stage, and the second under-jaw cooling stage were as shown in Table 3 (Tables 3A to 3D).

[0302] Rails with each test number were manufactured according to the above manufacturing process.

[0303] [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) Hardness gradient measurement test in first head surface portion region, second head surface portion region, and third head surface portion region (Test 4) Si segregation measurement test (Test 5) Wear resistance and damage resistance evaluation test (Test 6) Breakage resistance evaluation test Each test will be explained below.

[0304] [(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 4 (Tables 4A to 4D). Note that for each test number, 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.

[0305]

[0306]

[0307]

[0308]

[0309] [(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 4.

[0310] [(Test 3) Hardness Gradient Measurement Test in the First Head Surface Region, the Second Head Surface Region, and the Third Head Surface Region] Based on the above-described [Method for Measuring the Hardness Gradient S1 in the First Head Surface Region 10A1, the Hardness Gradient S2 in the Second Head Surface Region 10A2, and the Hardness Gradient S3 in the Third Head Surface Region 10B], the hardness gradient S1 (HV / mm) in the first head surface region, the hardness gradient S2 (HV / mm) in the second head surface region, and the hardness gradient S3 (HV / mm) in the third head surface region of the rail for each test number were determined. The obtained hardness gradients S1 to S3 are shown in Table 4. Furthermore, S2 / S1 is shown in Table 4. Furthermore, F3 (= S3 / Cr) is shown in Table 4.

[0311] [(Test 4) Si Segregation Measurement Test] The Si segregation in the second head surface region 10A2 was determined for each test number based on the method described in the above-mentioned [Method for measuring Si segregation in the second head surface region 10A2]. The obtained Si segregation values ​​are shown in Table 4 (Tables 4A to 4D).

[0312] (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 cross section for each test number was 136 pounds (mass: 67 kg / m) and 2 m in length.

[0313] The following two types of rolling fatigue tests were conducted using rails with each test number. [Rolling Fatigue Test (First Condition)] The rolling fatigue test (first condition) was conducted as follows. For the rolling fatigue test, a rolling fatigue testing machine shown in FIG. 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.

[0314] [Method for evaluating crack damage on rolling contact surfaces] The method for evaluating crack damage on the rolling contact surfaces of rails after rolling fatigue tests was as follows. A region of 300 mm in the longitudinal direction of the rail 1 on the surface of the head corners of the rail was designated as the evaluation area. A cross section parallel to the longitudinal direction of the rail 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, and the depth of cracks visible on the polished observation surface was measured. The crack depth was measured as the depth from the head outer casing surface at the head corners. The crack depth was defined as the distance from the head outer casing surface to the crack tip in the normal direction of the head outer casing surface. The maximum crack depth measured was designated as the maximum crack depth of crack damage occurring in the rail.

[0315] [Method for evaluating internal crack damage immediately below the rolling surface] The method for evaluating internal crack damage immediately below the rolling surface of the rail after the rolling fatigue test was as follows. Internal cracks were confirmed by ultrasonic flaw detection (UST). An area of ​​300 mm in the longitudinal direction of the rail on the surface of the corner of the head of the rail was selected as the evaluation area. The number of internal cracks in the evaluation area was measured by ultrasonic flaw detection. The total number of internal cracks in the evaluation area was taken as the number of internal cracks that occurred in the rail.

[0316] [Rolling fatigue test (second condition)] The rolling fatigue test (second condition) was conducted as follows. For the rolling fatigue test, a rolling fatigue testing machine shown in Figure 3 was used. The type and diameter of the wheel 2, the radial load, thrust load during the test, and the lubrication method during the test were the same as those for the rolling fatigue test (first condition). During the test, the number of repeated load applications using the wheel 2 was set to a maximum of 10 million times, and the cumulative passing tonnage was set to a maximum of 300 million tons. In order to eliminate the influence of initial crack damage remaining on the rolling surface, when the contact area between the rail and the wheel had worn 10 mm from the head outer surface, the rolling surface was redressed by about 1 mm in the depth direction, and the test was then continued.

[0317] [Method for evaluating the number of cracks on the rolling contact surface] The method for evaluating the number of cracks on the rolling contact surface of the rail after the rolling fatigue test was as follows. An area of ​​300 mm in the longitudinal direction of the rail 1 on the surface of the top of the rail 1 was designated as the evaluation area. Magnetic particle testing was performed on the evaluation area, and the number of cracks with a length of 0.5 mm or more in the longitudinal direction of the rail 1 was counted. The obtained number was designated as the number of cracks.

[0318] [Method for evaluating wear volume of rail head] A 300 mm long area in the longitudinal direction of the rail 1 was designated as the evaluation area. The cross-sectional shape of the test piece of the rail 1 perpendicular to the longitudinal direction before the rolling fatigue test was recorded. Furthermore, the cross-sectional shape of the test piece of the rail 1 perpendicular to the longitudinal direction after the rolling fatigue test was recorded. The two were compared to measure the wear depth (mm) at the center of the width of the rail 1 in the rail head 10. The maximum wear depth obtained was designated as the wear volume (mm).

[0319] [Method of evaluating the results of the rolling fatigue test (first and second conditions)] Based on the results of the rolling fatigue test (first and second conditions), the damage resistance and wear resistance of the rail were evaluated as follows.

[0320] (Damage resistance = maximum crack depth: 1st condition) A: Maximum crack depth 215 μm or less B: Maximum crack depth over 215 to 240 μm or less C: Maximum crack depth over 240 to 350 μm or less F: Maximum crack depth over 350 μm (Damage resistance = number of internal cracks: 1st condition) A: Number of internal cracks 0 to 1 B: Number of internal cracks 2 to 3 C: Number of internal cracks 4 to 6 F: Number of internal cracks over 6

[0321] (Damage resistance = Number of cracks: Second condition) AAA: Number of cracks: 0 AA: Number of cracks: 1-2 A: Number of cracks: 3 B: Number of cracks: 4 C: Number of cracks: 5-6 F: Number of cracks: 6 or more (Wear resistance: Second condition) A: Maximum wear depth: 12.9 mm or less B: Maximum wear depth: 12.9 mm or more but less than 13.4 mm C: Maximum wear depth: 13.4 mm or more but less than 13.5 mm F: Maximum wear depth: over 13.5 mm

[0322] (Test 6) Breakage Resistance Evaluation Test A drop weight test was carried out under the following two conditions to evaluate the breakage resistance of the rails of each test number.

[0323] [Test Condition 1] The test condition 1 for the drop weight test was as follows. A drop weight tester shown in Figure 8 was used. In the test, the head surface portion 10A of the rail 1 was ground to eliminate the influence of the material of the head surface portion 10A and accurately evaluate the breakage susceptibility of the third head surface region 10B. In the drop weight test, the rail 1 was supported at two points with the rail head portion 10 facing downward and the bottom portion 15 facing upward. The distance between the two support points was 1000 mm. Then, a drop weight was dropped on the bottom portion 15 of the rail 1. The mass W of the drop weight was 500 kgf (4.9 kN). The height X of the drop weight was 5.0 m. In this case, the drop weight energy was 24.5 kN m. After the drop weight, the rail 1 was visually inspected for breakage.

[0324] [Test Condition 2] In order to evaluate breakage susceptibility more strictly, a test was conducted under test condition 2 in which the drop weight energy was increased. Specifically, test condition 2 for the drop weight test was as follows. As with test condition 1, a drop weight tester shown in Figure 8 was used. In the test, the head surface portion 10A of the rail 1 was ground to eliminate the influence of the material of the head surface portion 10A and to accurately evaluate the breakage susceptibility of the third head surface portion region 10B.

[0325] In the drop weight test, the rail 1 was supported at two points with the rail head 10 facing downward and the bottom 15 facing upward. The distance between the two support points was 1000 mm. A weight was then dropped on the bottom 15 of the rail 1. The weight W of the weight was 1000 kgf (9.8 kN). The height X of the weight was 4.0 to 14.0 m. In this case, the drop weight energy was 39.2 to 137.2 kN m. After the weight was dropped, the rail 1 was visually inspected for breakage.

[0326] [Method for evaluating drop weight test results] Based on the results of the drop weight test, breakage resistance was evaluated based on the maximum value of the drop weight energy at which the rail did not break. The evaluation results were as follows: A: Maximum drop weight energy without breakage: 117.6 to 137.2 kN m (test condition 2) B: Maximum drop weight energy without breakage: 98.0 kN m (test condition 2) F: Maximum drop weight energy without breakage: 24.5 kN m (test condition 1)

[0327] [Evaluation Results] Referring to Tables 2 to 4, the rails of test numbers 1 to 78 satisfied characteristics 1 to 4. Therefore, excellent wear resistance and damage resistance were obtained.

[0328] Furthermore, in the rails of test numbers 1 to 16, 18 to 27, and 32 to 78, either no Cr was contained or F3 satisfied formula (3). Therefore, these test numbers exhibited even better breakage resistance than test numbers 17 and 28 to 31, in which F3 did not satisfy formula (3).

[0329] Furthermore, for test numbers 75 to 78, soft reduction was carried out under appropriate conditions during continuous casting. As a result, the Si segregation degree of these rails was 1.35 or less. Therefore, in the rolling fatigue test under the second condition, the number of cracks generated in the second head surface region was significantly small, and damage resistance was significantly excellent.

[0330] On the other hand, in test numbers 79 and 80, the cooling start temperature in the first stage of cooling the head surface of the heat treatment was too high, which resulted in a low pearlite area ratio and low wear resistance.

[0331] In test numbers 81 and 82, the cooling start temperature of the first stage of cooling the head surface of the heat treatment was too low. As a result, the pearlite area ratio was low, and the Vickers hardness of the head surface was less than 400 HV. Furthermore, the hardness gradient S1 of the first head surface region was less than 2.00 HV / mm. As a result, the damage resistance and wear resistance were low.

[0332] On the other hand, in test numbers 83 and 84, the cooling rate in the first stage of cooling the head surface of the heat treatment was too slow. As a result, the Vickers hardness of the head surface was less than 400 HV, and the hardness gradient S1 of the first head surface region was less than 2.00 HV / mm. As a result, the damage resistance and wear resistance were low.

[0333] In test numbers 85 and 86, the cooling rate in the first stage of cooling the head surface portion of the heat treatment was too fast. As a result, the hardness gradient S1 in the first head surface portion region exceeded 10.00 HV / mm. As a result, many internal cracks occurred in the first head surface portion region, and the damage resistance was low.

[0334] In test numbers 87 and 88, the cooling end temperature of the first stage of head surface cooling and the cooling start temperature of the second stage of head surface cooling in the heat treatment were too low. As a result, the hardness gradient S1 of the first head surface region exceeded 10.00 HV / mm. As a result, many internal cracks occurred in the first head surface region, and the damage resistance was low.

[0335] In test numbers 89 and 90, the cooling end temperature of the first stage of head surface cooling and the cooling start temperature of the second stage of head surface cooling in the heat treatment were too high. As a result, the Vickers hardness of the head surface was less than 400 HV, and the hardness gradient S1 of the first head surface region was less than 2.00 HV / mm. As a result, the damage resistance and wear resistance were low.

[0336] In test numbers 91 and 92, the cooling rate in the second stage of cooling the head surface portion of the heat treatment was too slow. Therefore, the hardness gradient S2 of the second head surface portion region was less than 0.25 HV / mm. As a result, the damage resistance was low.

[0337] In test numbers 93 and 94, the cooling rate in the second stage of cooling the head surface portion of the heat treatment was too fast. As a result, the hardness gradient S2 of the second head surface portion region exceeded 2.50 HV / mm. As a result, the wear resistance was low.

[0338] In test numbers 95 and 96, the cooling end temperature of the second stage of cooling the head surface portion of the heat treatment was too low. As a result, the hardness gradient S2 of the second head surface portion region exceeded 2.50 HV / mm. As a result, the wear resistance was low.

[0339] In test numbers 97 and 98, the cooling end temperature of the second stage of cooling the head surface portion of the heat treatment was too high. Therefore, the hardness gradient S2 of the second head surface portion region was less than 0.25 HV / mm. As a result, the damage resistance was low.

[0340] In test numbers 99 and 100, the cooling rate in the third stage of cooling the head surface portion of the heat treatment was too slow. Therefore, the hardness gradient S3 of the third head surface portion region was less than 1.00 HV / mm. As a result, the breakage resistance was low.

[0341] In test numbers 101 and 102, the cooling rate in the third stage of cooling the head surface portion of the heat treatment was too fast. As a result, the hardness gradient S3 of the third head surface portion region exceeded 4.00 HV / mm. As a result, the breakage resistance was low.

[0342] In test numbers 103 and 104, the cooling end temperature of the third stage of cooling the head surface portion of the heat treatment was too low. Therefore, the hardness gradient S3 of the third head surface portion region exceeded 4.00 HV / mm. As a result, the breakage resistance was low.

[0343] In test numbers 105 and 106, the cooling end temperature of the third stage of cooling the head surface portion of the heat treatment was too high. Therefore, the hardness gradient S3 of the third head surface portion region was less than 1.00 HV / mm. As a result, the breakage resistance was low.

[0344] In test numbers 107 and 108, the cooling start temperature in the first stage of cooling the lower jaw portion of the heat treatment was too low. As a result, the Vickers hardness of the head surface was less than 400 HV. As a result, the wear resistance was low.

[0345] In test numbers 109 and 110, the cooling start temperature in the first stage of cooling the lower jaw portion of the heat treatment was too high. As a result, the hardness gradient S2 of the second head surface region exceeded 2.50 HV / mm. As a result, the wear resistance was low.

[0346] In test numbers 111 and 112, the cooling rate in the first stage of cooling the lower jaw portion of the heat treatment was too slow. As a result, the hardness gradient S2 of the second head surface region was less than 0.25 HV / mm. As a result, the damage resistance was low.

[0347] In test numbers 113 and 114, the cooling rate in the first stage of cooling the lower jaw portion of the heat treatment was too fast. As a result, the hardness gradient S2 in the second head surface region exceeded 2.50 HV / mm. As a result, the wear resistance was low.

[0348] In test numbers 115 and 116, the cooling end temperature of the first stage of cooling the lower jaw portion of the heat treatment was too low. As a result, the hardness gradient S2 of the second head surface region exceeded 2.50 HV / mm. As a result, the wear resistance was low.

[0349] In test numbers 117 and 118, the cooling end temperature of the first stage of cooling the lower jaw portion of the heat treatment was too high. As a result, the hardness gradient S2 of the second head surface region was less than 0.25 HV / mm. As a result, the damage resistance was low.

[0350] In test numbers 119 and 120, the cooling rate in the second stage of cooling the lower jaw portion of the heat treatment was too slow. As a result, the hardness gradient S3 in the third head surface region exceeded 4.00 HV / mm. As a result, the breakage resistance was low.

[0351] In test numbers 121 and 122, the cooling rate in the second stage of cooling the lower jaw portion of the heat treatment was too fast. As a result, the hardness gradient S3 of the third head surface region was less than 1.00 HV / mm. As a result, the breakage resistance was low.

[0352] In test numbers 123 and 124, the cooling end temperature of the second stage of cooling the lower jaw portion of the heat treatment was too low. As a result, the hardness gradient S3 of the third head surface region was less than 1.00 HV / mm. As a result, the breakage resistance was low.

[0353] In test numbers 125 and 126, the cooling end temperature of the second stage of cooling the lower jaw portion of the heat treatment was too high. As a result, the hardness gradient S3 of the third head surface region exceeded 4.00 HV / mm. As a result, the breakage resistance was low.

[0354] 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.

[0355] 1 Rail 10A Head surface 10S Head outer surface 10A1 First head surface region 10A2 Second head surface region 10B Third head surface region

Claims

1. 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, N: 0.0200% or less, Al: 1.0000% or less, 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 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 head outer shell surface to a depth of 20 mm, the pearlite area ratio is 95% or more and the Vickers hardness is 400 HV or more; a hardness H1 at a depth of 1 mm starting from the head outer shell surface, a hardness H10 at a depth of 10 mm, and a hardness H20 at a depth of 20 mm satisfy formula (1); a hardness gradient S1 of a first head surface portion region from the head outer shell surface to the depth of 10 mm is 2.00 to 10.00 HV / mm; a hardness gradient S2 of a second head surface portion region from the depth of 10 mm to the depth of 20 mm is 0.25 to 2.50 HV / mm; A rail, wherein a hardness gradient S3 of the third head surface region from the 20 mm depth position to the 40 mm depth position is 1.00 to 4.00 HV / mm, and a hardness gradient S1 of the first head surface region and a hardness gradient S2 of the second head surface region satisfy the following formula (2): H1>H10>H20 (1) S2≦0.95S1 (2) 2. A rail according to claim 1, containing one or more elements selected from the group consisting of 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: 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%. Rail.

3. A rail according to claim 1 or 2, wherein the degree of Si segregation in the second head surface region is 1.35 or less.

4. A rail according to claim 1 or 2, containing 0.01 to 1.00% Cr, wherein the hardness gradient S3 of the third head surface region and the Cr content in mass % satisfy the formula (3): 1.25≦S3 / Cr≦20.00 (3)

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