TRACK AND METHOD FOR PRODUCING THE SAME

Abstract

Provided is a rail in which the rail bottom has excellent fatigue crack propagation resistance. The rail has a composition containing 0.70-1.20 mass% C, 0.10-1.20 mass% Si, 0.10-1.50 mass% Mn, 0.035 mass% or less P, 0.020 mass% or less S, 0.05-2.00 mass% Cr, with the remainder being Fe and unavoidable impurities. The area ratio of a pearlite structure at a depth position of 1 mm from the surface of the rail bottom is 95% or more, and the HC value represented by formula (1) satisfies formula (2).　Formula (1): HC = {(12 × [%C]) + ([%Si] / 10) + ([%Mn] / 20) + ([%Cr] / 18)}2. Formula (2): HC ≤ (120 × GS) / (1.1 × BS).

Description

1 / 34 TRACK AND METHOD FOR PRODUCING THE SAME TECHNICAL FIELD

[001] This disclosure relates to a rail and a method for producing it. BACKGROUND

[002] In heavy transport railways, built primarily to transport ore, the load applied to the axle of a freight car is much greater than in passenger cars, and the operating environments for rails are severe. Conventionally, steels having a pearlitic microstructure have been used primarily in such rails from the point of view of the importance of wear resistance.

[003] In recent years, the loading weight of freight wagons has been further increased to improve rail transport efficiency. In addition, the number of wheels passing over the rails has increased due to the increased transport capacity.

[004] The passage of wheels applies repetitive traction and compressive stress to the rails. As the load weight increases and the number of wheels passing over the rails increases, the frequency of rail replacement tends to increase each year due to cracking that starts from the bottom of the rail base. Therefore, there is a growing demand for rail steels with improved resistance to fatigue crack propagation in the bottom of the rail base.

[005] Given the background described above, several studies have been conducted to further improve the resistance to fracture of rail bottoms. For example, Patent Literature (PTL) 1 proposes a rail steel exhibiting a pearlitic microstructure containing C: 0.65% to 1.40%, where 200 or more pearlite blocks with grain sizes of 1 μm to 15 μm per 0.2 mm2 are present. Petition 870250083365, dated 09 / 16 / 2025, pp. 79 / 121 2 / 34 of the examination area is present in at least part of a band that begins on the background surface and extends to a depth of 10 mm.

[006] PTL 2 proposes a rail containing, in mass %, C: 0.65% to 1.20%, Si: 0.05% to 2.00% and Mn: 0.05% to 2.00%, the equilibrium being Fe and unavoidable impurities, wherein 97% or more of the head surface and bottom surface have a pearlitic microstructure, the part with a pearlitic microstructure has a surface hardness in the range of Hv320 to Hv500 and a maximum surface roughness of 180 μm or less, and the ratio between the surface hardness and the maximum surface roughness is 3.5 or more.

[007] PTL 3 discloses a rail in which, while the rail head is subjected to accelerated cooling of the austenite region after rail rolling, the rail bottom surface is subjected to accelerated cooling between 800 °C and 450 °C at a cooling rate of 1 °C / s 5 °C / s, so that the average pearlite hardness in the rail bottom becomes HB320 or more. LIST OF QUOTES Patent Literature

[008] PTL 1: JP 2006-57127 A PTL 2: WO 2011 / 021582 PTL 3: JP H01-139724 A SUMMARY (Technical Problem)

[009] However, the conventional technologies above still have the following problems to be solved. When the C content is set at 0.65% to 1.40%, as in the technologies described in PTL 1 and 2, no improvement in resistance to rolling contact fatigue can be expected, because a hard and brittle proeutectoid cementite microstructure is formed at the grain boundary of the previous austenite, depending on conditions of Petition 870250083365, dated 09 / 16 / 2025, pp. 80 / 121 3 / 34 heat treatment. Furthermore, material property control cannot be considered sufficient with the technology described in PTL 3, since a proeutectoid cementite structure can also be formed depending on the combination of components and production conditions, resulting in an increased rate of fatigue crack propagation.

[010] To solve the problems described above in an advantageous way, the objective of this disclosure is to provide a rail with excellent resistance to fatigue crack propagation in the lower part of the rail base, together with a method for producing the same. (Solution to the Problem)

[011] In order to solve the above problem, rails with different C, Si, Mn, and Cr contents were prepared, and the microstructure and resistance to fatigue crack propagation of the rail base bottom was intensively investigated. As a result, a compositional parameter (the HC parameter in Expression (1) below) defined by the C content, Si content, Mn content, and Cr content was discovered, and it was found that the parameter value is related to the amount of proeutectoid cementite. Furthermore, it was discovered that by controlling the value of the HC parameter to be equal to or less than the value of a parameter set by the previous austenite grain size and pearlite block size of the rail base bottom, excellent resistance to fatigue crack propagation of the rail base bottom can be obtained, even if a large amount of proeutectoid cementite is present. More details follow.

[012] It has been discovered that the reason why the presence of proeutectoid cementite increases the rate of fatigue crack propagation is that, as illustrated in the schematic diagram of FIG. 1, in a portion of proeutectoid cementite that exists in the plastic zone at the tip of a crack Petition 870250083365, dated 09 / 16 / 2025, pp. 81 / 121 4 / 34 due to fatigue, the {100} plane of ferrite in pearlite, which is adjacent to the proeutectoid cementite portion, fractures first in a brittle manner. Furthermore, it was discovered that by controlling the ratio of the grain size of the preceding austenite, which is the site of microstructure formation, to the block size of pearlite, which corresponds to the microstructure unit of the brittle fracture described above, according to the amount of proeutectoid cementite formation, the frequency with which the plastic zone that forms at the tip of a fatigue crack encounters the {100} plane of ferrite in pearlite can be reduced, thus making it possible to suppress brittle crack growth.In other words, it was found that even when a large amount of proeutectoid cementite is present, thickening of the austenite grain size or refinement of the pearlite block size can stably suppress the aforementioned fatigue crack propagation rate.

[013] This disclosure is based on the above findings and their main attributes are as follows.

[014] [1] A track comprising: a chemical composition containing (consisting of) C: 0.70% by mass to 1.20% by mass, Si: 0.10% by mass to 1.20% by mass, Mn: 0.10% by mass to 1.50% by mass, P: 0.035% by mass or less, S: 0.020% by mass or less, and Cr: 0.05% by mass to 2.00% by mass, with an equilibrium consisting of Fe and unavoidable impurities, wherein an area ratio of a pearlitic microstructure to a depth of 1 mm from the surface of a central portion of a Petition 870250083365, dated 09 / 16 / 2025, p. 82 / 121 5 / 34 of the bottom of a track base is 95% or more, and a value of HC expressed by Expression (1) satisfies Expression (2), HC = {(12 x [%C]) + ([%Si] / 10) + ([%Mn] / 20) + ([%Cr] / 18)}2...(1) HC < (120 x GS) / (1.1 x BS)...(2) where [%C], [%Si], [%Mn] and [%Cr] are the respective mass percentages of C, Si, Mn and Cr. GS is a previous austenite grain size in pm in the central portion of the lower rail base and BS is the size of the pearlite block in pm in the central portion of the bottom of the rail base.

[015] [2] The rail according to [1], wherein the chemical composition additionally contains at least one selected from the group consisting of V: 0.30% by mass or less, Cu: 1.0% by mass or less, Ni: 1.0% by mass or less, Nb: 0.05% by mass or less, Mo: 2.0% by mass or less, Al: 0.07% by mass or less, W: 1.0% by mass or less, Co: 1.0% by mass or less, B: 0.005% by mass or less, Ti: 0.05% by mass or less, Sb: 0.05% by mass or less, Mg: 0.01% by mass or less, Ca: 0.02% by mass or less, and Petition 870250083365, dated 09 / 16 / 2025, page 83 / 121 6 / 34 Sn: 0.05% by mass or less.

[016] [3] A method for producing the rail according to [1] or [2], the method comprising: heating a steel material having the chemical composition according to [1] or [2] to a heating temperature of 1350 °C or less, hot rolling under a set of conditions including a finishing rolling temperature of 850 °C or more in the central portion of the rail base, subsequently cooling at an average cooling rate CR1 in °C / s until a temperature of the central portion of the rail base drops from 850 °C to 750 °C, and then cooling at an average cooling rate CR2 in °C / s until a temperature of the central portion of the rail base drops from 750 °C to a cooling stop temperature T, wherein the cooling stop temperature T is in a range of 400 °C to 650 °C, and the average cooling rate CR1 (°C / s) and the average cooling rate CR2 (°C / s) satisfy respectively Expression (3) and Expression (4), HC / 300 < CR1 < 3.0...(3) HC / 120 < CR2 < 6.0...(4). (Beneficial Effect)

[017] According to the present disclosure, a rail with excellent resistance to fatigue crack propagation in the underside of the rail base, together with a method for producing the same, can be provided. The rail of the present disclosure contributes to extending the service life of rails for heavy transport railways and preventing railway accidents. The rail is thus industrially beneficial. Furthermore, the method for producing a rail of the present disclosure can stably improve the resistance to fatigue crack propagation in the underside of the rail base, optimizing the Petition 870250083365, dated 09 / 16 / 2025, page 84 / 121 7 / 34 Heat treatment conditions after hot rolling. Thus, the method is also industrially beneficial. BRIEF DESCRIPTION OF THE DRAWINGS

[018] In the attached figures: FIG. 1 is a schematic diagram illustrating the effects of proeutectoid cementite, upstream austenite grain size, and pearlite block size on the rate of fatigue crack propagation; FIG. 2 is a cross-sectional view of a rail illustrating the rail parts and the position where a test specimen is collected for observation of the anterior austenite grain size and pearlite block size; FIG. 3 is a diagram illustrating the cross-sectional area and thickness of the rail base; FIG. 4 is a diagram illustrating the position in which a test specimen is collected for a fatigue crack propagation test; and Figures 5A, 5B, and 5C are diagrams illustrating the shape of the test specimen for the fatigue crack propagation test, where Figure 5A is a front view, Figure 5B is a side view, and Figure 5C is an enlarged front view of the notch. DETAILED DESCRIPTION<Partes de Trilho>

[019] Firstly, the designations of the various parts of the rail of the present disclosure are described with reference to the cross-sectional view of the rail in FIG. 2. In rail 1 illustrated in FIG. 2, 11 indicates the rail head, 12 indicates the rail weave, 13 indicates the rail base, and the underside surface of the rail base 13 is referred to as the underside of the rail base 14. The central portion of the underside of the rail base is the portion near the center of the width of the underside of the rail base. For example, if the dimension of Petition 870250083365, dated 09 / 16 / 2025, page 85 / 121 8 / 34 width of the bottom of the rail base 14 for W, the central portion of the bottom of the rail base is the region in a width strip of ±0.075 x W from the center width of the bottom of the rail base 14.

[020] Hereafter, the rail head, rail frame, rail base, rail base bottom and rail base bottom center portion are also referred to as the head, frame, base, base bottom and base bottom center portion, respectively. <Composição Química de Trilho>

[021] The chemical composition of the rail steel according to this disclosure will be described below. In the following description, % denotes % by mass, unless otherwise specified.

[022] C: 0.70% to 1.20%, Carbon (C) is an essential element to ensure the strength of the pearlitic microstructure, that is, resistance to contact fatigue during rolling. If the C content is less than 0.70%, it will be difficult to obtain excellent resistance to fatigue crack propagation in the bottom of the base. If the C content exceeds 1.20%, a large amount of proeutectoid cementite is formed at the austenite grain boundaries during cooling after hot rolling, resulting in an increased rate of fatigue crack propagation. Although proeutectoid cementite may be present even when the C content is 1.20% or less, its effect can be avoided by controlling the upstream austenite grain size and block size in the central portion of the bottom of the base, so that Expression (2) above is satisfied. From these perspectives, the C content is established in a range of 0.70% to 1.20%. The carbon content is preferably in the range of 0.70% to 0.89%.The C content is most preferably in a range of 0.70% to 0.85%.

[023] Si: 0.10% to 1.20% Petition 870250083365, dated 09 / 16 / 2025, p. 86 / 121 9 / 34 In addition to its effect as a deoxidizer, Si is an element that contributes to reducing the rate of fatigue crack propagation by increasing the equilibrium transformation temperature of pearlite and reducing lamellar spacing. From this perspective, the Si content needs to be 0.10% or more, but if the Si content exceeds 1.20%, weldability deteriorates due to the high bonding strength of Si with oxygen. Furthermore, since Si has the effect of shifting the eutectic point to the low C side, excessive Si addition promotes the formation of proeutectoid cementite and increases the rate of fatigue crack propagation. From these perspectives, the Si content is established in a range of 0.10% to 1.20%. The Si content is preferably in a range of 0.15% to 1.10%. The Si content is more preferably in a range of 0.20% to 1.00%.

[024] Mn: 0.10% to 1.50% Manganese (Mn) is an element that contributes to reducing the rate of fatigue crack propagation by lowering the pearlite transformation temperature and reducing lamellar spacing. If the Mn content is less than 0.10%, the effect is insufficient. On the other hand, if the Mn content exceeds 1.50%, a martensitic microstructure is likely to develop, causing hardening and embrittlement during heat treatment and rail welding, and material properties are likely to deteriorate. Furthermore, since Mn has the effect of shifting the eutectic point to the lower C side, excessive Mn addition promotes the formation of proeutectoid cementite and increases the rate of fatigue crack propagation. From these perspectives, the Mn content is established in a range of 0.10% to 1.50%. The Mn content is preferably in a range of 0.20% to 1.40%. The Mn content is most preferably in a range of 0.30% to 1.30%.

[025] P: 0.035% or less Petition 870250083365, dated 09 / 16 / 2025, page 87 / 121 10 / 34 P in an amount exceeding 0.035% degrades ductility. Therefore, the P content is set at 0.035% or less. The P content is preferably 0.020% or less. No specific lower limit is placed on the P content. The P content can be 0%, but it is generally more than 0% in industrial terms, and an excessive decrease in P content will increase refining costs. From an economic efficiency perspective, the P content is preferably 0.001% or more.

[026] S: 0.020% or less Sulfur (S) is an element present mainly in steel in the form of type A inclusions. When the S content exceeds 0.020% by mass, the amount of inclusions increases significantly, and at the same time, coarse inclusions are formed. As a result, the cleanliness of the steel material deteriorates. Therefore, the S content is established as 0.020% or less. The S content is preferably 0.015% or less. The S content is more preferably 0.010% or less. No specific lower limit is placed on the S content. The S content can be 0%, but it is generally more than 0% in industrial terms, and an excessive decrease in the S content will increase refining costs. From an economic efficiency perspective, the S content is preferably 0.0005% or more.

[027] Cr: 0.05% to 2.00% Chromium (Cr) is an element that contributes to reducing the rate of fatigue crack propagation by increasing the equilibrium transformation temperature of pearlite and reducing lamellar spacing. If the Cr content is less than 0.05%, fatigue crack growth cannot be sufficiently suppressed. On the other hand, if the Cr content exceeds 2.00%, the hardenability of the steel increases and martensite is easily formed. In the case of production under conditions where no martensite is formed, cementite... Petition 870250083365, dated 09 / 16 / 2025, pp. 88 / 121 11 / 34 proeutectoid is formed at the grain boundary of the previous austenite, which increases the rate of fatigue crack propagation. From these perspectives, the Cr content is established in a range of 0.05% to 2.00%. The Cr content is preferably in a range of 0.10% to 1.60%. The Cr content is most preferably in a range of 0.15% to 1.40%.

[028] Furthermore, it is not sufficient that each of the elements in the chemical composition of the present disclosure merely satisfies the aforementioned ranges. Instead, it is important to control the value of the component parameter HC, which corresponds to the amount of proeutectoid cementite, to be equal to or less than the value of a predetermined parameter set by the austenite grain size GS and the pearlite block size BS in the central portion of the lower rail base.

[029] The value of the HC component parameter corresponding to the amount of proeutectoid cementite is obtained by the following Expression (1), HC = {(12 x [%C]) + ([%Si] / 10) + ([%Mn] / 20) + ([%Cr] / 18)}2...(1), where [%C], [%Si], [%Mn] and [%Cr] are the respective contents (% by mass) of C, Si, Mn and Cr.

[030] From the perspective of achieving high strength of the pearlitic microstructure and suppression of proeutectoid cementite, that is, ensuring resistance to rolling contact fatigue, the HC value is preferably from 75 to 200. The HC value is more preferably from 90 to 150.

[031] In this disclosure, the predetermined parameter configured by the previous austenite grain size GS and the pearlite block size BS is expressed as (120 x GS) / (1.1 x BS). The value of this parameter and the value of HC satisfy the relationship in Expression (2) below: HC < (120 x GS) / (1.1 x BS)...(2) where Petition 870250083365, dated 09 / 16 / 2025, pp. 89 / 121 12 / 34 GS is the austenite grain size in sq m in the central portion of the lower rail base and BS is the size of the pearlite block in sq m in the central portion of the bottom of the rail base.

[032] In addition to the essential components mentioned above, the chemical composition of a rail used in this disclosure may optionally contain at least one selected from the group consisting of V: 0.30% by mass or less, Cu: 1.0% by mass or less, Ni: 1.0% by mass or less, Nb: 0.05% by mass or less, Mo: 2.0% by mass or less, Al: 0.07% by mass or less, W: 1.0% by mass or less, Co: 1.0% by mass or less, B: 0.005% by mass or less, Ti: 0.05% by mass or less, Sb: 0.05% by mass or less, Mg: 0.01% by mass or less, Ca: 0.02% by mass or less, and Sn: 0.05% by mass or less.

[033] The optional elements above are described below.

[034] V: 0.30% or less Vitamin V is an element that forms carbonitrides in steel and disperses and precipitates in the matrix, thereby improving the wear resistance of the steel. If the V content exceeds 0.30%, workability deteriorates and the alloy cost, i.e., the cost of rail production, also increases. From this perspective, the upper limit of the V content is preferably 0.30% when the chemical composition contains V. The V content is preferably 0.001% or more from the perspective of expressing the wear resistance improvement effect. The range of V content is most preferably from 0.001% to 0.15%.

[035] Cu: 1.0% or less Copper (Cu) is an element capable of further strengthening steel by strengthening it in solid solution, just like chromium (Cr). If the Cu content exceeds 1.0%, it is likely that... Petition 870250083365, dated 09 / 16 / 2025, pp. 90 / 121 13 / 34 Cu cracking occurs. Therefore, in cases where the chemical composition contains Cu, the Cu content is preferably 1.0% or less. From a high-strength perspective, the Cu content is preferably 0.001% or more. The range of Cu content is most preferably from 0.001% to 0.5%.

[036] Ni: 1.0% or less Ni is an element that can increase the strength of steel without deteriorating its ductility. Furthermore, if the chemical composition contains Cu, it is preferable to add Ni because Cu cracking can be suppressed by adding Ni in combination with Cu. However, if the Ni content exceeds 1.0% by mass, the hardenability of the steel increases further, the amount of martensite and bainite formed is increased, and the wear resistance and fatigue resistance from contact rolling of the rail head tend to decrease. From these perspectives, the Ni content is preferably 1.0% or less if the chemical composition contains Ni. From a high-strength perspective, the Ni content is preferably 0.001% or more. The range of Ni content is most preferably from 0.001% to 0.5%.

[037] Nb: 0.05% or less Nitrogen (Nb) is an element that combines with carbon (C) in steel to precipitate as carbides during and after hot rolling to form the rail and effectively refines the size of pearlite colonies. As a result, Nb greatly improves wear resistance, rolling contact fatigue resistance, and ductility, as well as contributing significantly to extending rail life. However, when the Nb content exceeds 0.05%, the effect of improving wear resistance and rolling contact fatigue resistance is saturated, and the effect does not increase as the content increases. From these perspectives, the upper limit of the Nb content is preferably 0.05% in cases where the chemical composition contains Nb. The Nb content is preferably... Petition 870250083365, dated 09 / 16 / 2025, pp. 91 / 121 14 / 34 0.001% or more in order to obtain a sufficient effect with respect to extending the service life of the rail. The Nb content range is more preferably from 0.001% to 0.03%.

[038] Mo: 2.0% or less Molybdenum (Mo) is an element capable of further strengthening steel by strengthening it in a solid solution. Molybdenum also has the effect of shifting the eutectic point towards the high-C side and thus inhibits the formation of proeutectoid cementite. However, if the Molybdenum content exceeds 2.0% by mass, the amount of bainite formed in the steel increases and the wear resistance of the railhead decreases. From this perspective, the Molybdenum content is preferably 2.0% or less when the chemical composition contains Molybdenum. From a high-strength perspective, the Molybdenum content is preferably 0.001% or more. The range of Molybdenum content is most preferably from 0.001% to 1.0%.

[039] Al: 0.07% or less Aluminum (Al) is an element that can be added as a deoxidizer. If the Al content exceeds 0.07% by mass, a large number of oxide-based inclusions are formed in the steel due to the strong bonding between Al and oxygen. As a result, the ductility of the steel decreases. Therefore, the Al content is preferably 0.07% or less when the chemical composition contains Al. There is no lower limit placed on the Al content, but the Al content is preferably 0.001% or more for deoxidation. The range of Al content is most preferably from 0.001% to 0.03%.

[040] W: 1.0% or less Iron (W) is an element that precipitates as carbides during and after hot rolling to shape the steel into a rail form, improving the rail's strength and ductility through precipitation strengthening. If the W content exceeds 1.0%, martensite is formed in the steel. As a result, the Petition 870250083365, dated 09 / 16 / 2025, pp. 92 / 121 15 / 34 ductility decreases. From these perspectives, the W content is preferably 1.0% or less in the case where the chemical composition contains W. No lower limit is placed on the W content, but the W content is preferably 0.001% or more in order to exert the effect of improving strength and ductility. The range of W content is most preferably from 0.001% to 0.5%.

[041] Co: 1.0% or less Cobalt (Co) is an element that can increase the equilibrium transformation temperature of pearlite and reduce lamellar spacing, further enhancing the strength of steel. Cobalt also has the effect of suppressing the precipitation of proeutectoid cementite. If the Cobalt content exceeds 1.0%, martensite is formed in the steel. As a result, ductility decreases. From these perspectives, the Cobalt content is preferably 1.0% or less in cases where the chemical composition contains Cobalt. No lower limit is placed on the Cobalt content, but the Cobalt content is preferably 0.001% or more to enhance strength. The range of Cobalt content is most preferably from 0.001% to 0.5%.

[042] B: 0.005% or less Iron (B) is an element that precipitates as nitrides in steel during and after hot rolling to shape the steel into a rail shape and improves the strength and ductility of the steel through precipitation strengthening. If the B content exceeds 0.005%, martensite is formed and, as a result, the ductility of the steel decreases. From this perspective, the B content is preferably 0.005% or less in the case where the chemical composition contains B. No lower limit is placed on the B content, but the B content is preferably 0.001% or more in order to exert the effect of improving strength and ductility. The range of B content is most preferably from 0.001% to 0.003%. Petition 870250083365, dated 09 / 16 / 2025, pp. 93-121 16 / 34

[043] Ti: 0.05% or less Titanium (Ti) is an element that precipitates as carbides, nitrides, or carbonitrides in steel during and after hot rolling to shape the steel into a rail shape, improving the strength and ductility of the steel through precipitation strengthening. If the Ti content exceeds 0.05% by mass, coarse carbides, nitrides, or carbonitrides are formed. As a result, the ductility of the steel decreases. From this perspective, the Ti content is preferably 0.05% or less in cases where the chemical composition contains Ti. No lower limit is placed on the Ti content, but the Ti content is preferably 0.001% or more in order to exert the effect of improving strength and ductility. The range of Ti content is most preferably from 0.001% to 0.03%.

[044] Sb: 0.05% or less Sb is an element that has a notable effect in preventing decarburization of steel during reheating of rail steel material in a heating furnace before hot rolling. If the Sb content exceeds 0.05%, the ductility and toughness of the steel will be negatively affected. Therefore, in cases where the chemical composition contains Sb, the Sb content is preferably 0.05% or less. No lower limit is placed on the Sb content, but the Sb content is preferably 0.001% or more in order to exert the effect of reducing the decarburized layer. The range of Sb content is most preferably from 0.001% to 0.03%.

[045] Mg: 0.01% or less Magnesium (Mg) is an element that combines with oxygen to precipitate as MgO, thus further enhancing strength. If the Mg content exceeds 0.01%, the increased MgO negatively affects the ductility and toughness of the steel. Therefore, when the chemical composition contains Mg, the Mg content is preferably 0.01% or less. No lower limit is placed on this. Petition 870250083365, dated 09 / 16 / 2025, pp. 94 / 121 17 / 34 Mg content, but the Mg content is preferably 0.001% or more in order to exert the effect of improving strength. The Mg content range is more preferably from 0.001% to 0.005%.

[046] Ca: 0.02% or less Calcium (Ca) is an element that combines with oxygen to precipitate as CaO, thus further enhancing strength. If the Ca content exceeds 0.02%, the increased CaO negatively affects the ductility and toughness of the steel. Therefore, in cases where the chemical composition contains Ca, the Ca content is preferably 0.02% or less. No lower limit is placed on the Ca content, but the Ca content is preferably 0.001% or more in order to exert the effect of improving strength. The range of Ca content is most preferably from 0.001% to 0.01%.

[047] Sn: 0.05% or less Tin (Sn) is an element that has a notable effect in preventing decarburization of steel during reheating of rail steel material in a heating furnace before hot rolling. If the Sn content exceeds 0.05%, the ductility and toughness of the steel will be negatively affected. Therefore, in cases where the chemical composition contains Sn, the Sn content is preferably 0.05% or less. No lower limit is placed on the Sn content, but the Sn content is preferably 0.001% or more in order to exert the effect of reducing the decarburized layer. The range of Sn content is most preferably from 0.001% to 0.01%.

[048] In the chemical composition of the rail steel of the present disclosure, the equilibrium, other than the essential and optional components above, consists of Fe and unavoidable impurities. As used in the present invention, examples of unavoidable impurities include N, O and the like. N contents of up to 0.008% and O contents of up to 0.004% are permitted. Impurities other than N and O may Petition 870250083365, dated 09 / 16 / 2025, pp. 95 / 121 18 / 34 Impurities will inevitably be mixed into the steel depending on the raw materials, materials, production facilities and other conditions. Raw materials include iron ore, reduced iron, scrap and the like. The above impurities are acceptable provided they do not interfere with the purpose of this disclosure. Impurities other than N and O include Pb, Zr, Bi, Zn, Se, As, Te, Tl, Cd, Hf, Ag, Hg, Ga, Ge and REM. <Microestrutura de Trilho>

[049] The rail of the present disclosure is a pearlitic rail. From the perspective of resistance to fatigue crack propagation, the microstructure at a depth of 1 mm from the surface of the central portion of the rail base bottom has a pearlite area ratio of 95.0% or more. The pearlite area ratio may be 100%. Residual microstructure other than pearlite at a depth of 1 mm from the surface of the central portion of the base bottom is acceptable at an area ratio of 5.0% or less and may be 0%. Among the residual microstructures, proeutectoid cementite is acceptable if the area ratio is 3.0% or less, since the resistance to fatigue crack propagation is not significantly affected. Examples of residual microstructure other than proeutectoid cementite include ferrite, bainite, and martensite.

[050] The microstructure and area ratio of the central portion of the lower part of the base can be measured by the measurement method in the Examples described below. (Anterior Austenite Grain Size GS in Central Portion of Lower Base)

[051] The rail of the present disclosure preferably has an anterior austenite grain size GS in the central portion of the lower base of 30 μm to 140 pm. The anterior austenite grain size GS in the portion Petition 870250083365, dated 09 / 16 / 2025, pp. 96 / 121 The 19 / 34 central diameter of the lower base is more preferably 30 μm to 80 μm from the perspective of preventing an excessive reduction in ductility and toughness.

[052] The austenite grain size GS in the central portion of the lower base can be measured by the measurement method in the Examples described below. (BS Perlite Block Size)

[053] The rail of the present disclosure preferably has a BS pearlite block size in the central portion of the lower base of 15 μm to 45 μm. Since a smaller pearlite block size, which corresponds to the brittle fracture microstructure unit, enables a reduction in brittle crack growth length, the BS pearlite block size in the central portion of the lower base is more preferably 15 μm to 30 μm.

[054] The size of the BS perlite block in the central portion of the base bottom can be measured by the measurement method in the Examples described below. <Formato de Trilho>

[055] The rail format of this disclosure is not limited and may be the rail format described by JIS E 1101:2001, BS EN13674-1:2011, the American Railway Engineering and Maintenance-of-Way Association (AREMA) or similar. <Método para Produzir Trilho>

[056] A method for producing the rail of the present disclosure is now described. The rail of the present disclosure can be produced by sequentially applying the following treatments (1) to (3) to a steel material having the chemical composition described above. Petition 870250083365, dated 09 / 16 / 2025, pp. 97 / 121 20 / 34

[057] (1) Hot Rolling (2) Primary Cooling (3) Secondary Cooling [05 8] The steel material used as rail material has the chemical composition of the rail described above and can be produced by any method. In general, a steel material is preferably produced by ingot casting, particularly continuous casting. (1) Hot Rolling

[059] Heating Temperature: 1350 °C or less The steel material is heated to a heating temperature of 1350 °C or less prior to hot rolling. If the heating temperature exceeds 1350 °C, the steel material may partially melt due to excessive temperature increase, resulting in defects within the track. No lower limit is placed on the heating temperature, but the heating temperature is preferably 1150 °C or higher to reduce resistance to deformation during rolling.

[060] Lamination finishing temperature: 850 °C or higher The heated steel material is hot-rolled into a rail shape. Here, finish rolling refers to a final rolling pass, performed using the end rolling gauge of a finish rolling mill.

[061] The finishing temperature for hot rolling is set at 850 °C or higher. If the finishing temperature is lower than 850 °C, then the rolling is carried out in a low austenite temperature range. As a result, not only is processing expansion introduced into the austenite crystal grains, but also austenite crystal grain elongation becomes noticeable. The increase in austenite grain boundary area increases the number of nucleation sites. Petition 870250083365, dated 09 / 16 / 2025, pp. 98 / 121 21 / 34 for proeutectoid cementite, resulting in lower fatigue crack propagation resistance. Therefore, the finishing temperature is set at 850 °C or higher. The finishing temperature is preferably 900 °C or higher. There is no upper limit placed on the finishing temperature, but an extreme thickening of the austenite grain size will reduce ductility and toughness. The finishing temperature is therefore preferably 1050 °C or lower. Here, the finishing temperature is the surface temperature of the central portion of the bottom rail base on the entry side of the rolling mill in the final rolling pass and can be measured with a radiation thermometer.

[062] The reduction in lamination (thickness reduction) in the thickness direction of the rail base during the finishing lamination, which is the final lamination pass, is preferably greater than the reduction in area during this pass. For example, the area reduction can be set at 11%, while the thickness reduction in the central portion of the rail base is set at 15% or more. This can introduce a large expansion in the central portion of the rail base, which can further promote microstructure refinement after pearlite transformation and effectively increase the fatigue strength of the base bottom. By designing the gauge so that the width dimension of the base is enlarged during this finishing lamination, lamination with a relatively large thickness reduction compared to the area reduction can be stably achieved.

[063] The reduction in rail base area is a value calculated as area reduction (%) = {(So - Si) / So} x 100, where So is the cross-sectional area of ​​the base before finishing lamination and S1 is the cross-sectional area. Petition 870250083365, dated 09 / 16 / 2025, pp. 99 / 121 22 / 34 of the bottom after finishing lamination, based on the shaded area marked S in the cross-sectional view of the rail in FIG. 3.

[064] The reduction in rail base thickness is a value calculated as thickness reduction (%) = {(H0- H1) / H0} x 100, where H0 is the thickness of the base before finishing lamination and H1 is the cross-sectional area of ​​the base after finishing lamination, based on the height marked H in the same drawing.

[065] The other conditions for hot rolling are not limited. (2) Primary Cooling

[066] Average cooling rate from 850 °C to 750 °C (CR1 [°C / s]): HC / 300 < CR1 < 3.0 Next, accelerated cooling is performed. In this case, cooling from 850 °C to 750 °C is considered primary cooling. The temperature range of 850 °C to 750 °C corresponds to the temperature range for the formation of proeutectoid cementite, and if the average cooling rate CR1 in this temperature range is less than HC / 300 [°C / s], the amount of proeutectoid cementite increases. As a result, cracking is more likely to occur at the interface of the proeutectoid cementite microstructure, which can reduce the resistance to contact fatigue of the rolling track. Therefore, the average cooling rate CR1 of primary cooling is set at HC / 300 [°C / s] or more. The average cooling rate CR1 of primary cooling is preferably HC / 200 [°C / s] or more.

[067] On the other hand, if the average cooling rate CR1 of the primary cooling exceeds 3.0 °C / s, martensitic microstructures may be formed, resulting in reduced ductility and resistance to rolling contact fatigue. Therefore, the average cooling rate CR1 of the primary cooling is set at 3.0 °C / s or less. The average cooling rate CR1 of Petition 870250083365, dated 09 / 16 / 2025, pages 100 / 121 23 / 34 primary cooling is preferably 2.0 °C / s or less. (3) Secondary Cooling

[068] Average cooling rate (CR2 [°C / s]) from 750 °C to cooling stop temperature T: HC / 120 < CR2 < 6.0 Secondary cooling is performed following the primary cooling mentioned earlier. Secondary cooling is from 750 °C to the cooling stop temperature T. The cooling stop temperature T is in the range of 400 °C to 650 °C. If the average cooling rate from the secondary cooling start temperature of 750 °C to the secondary cooling stop temperature T in the range of 400 °C to 650 °C is less than HC / 120 [°C / s], the pearlite block size will become thicker, as will the lamellar spacing. In other words, the larger pearlite block size, which corresponds to the brittle fracture microstructure unit, can result in longer brittle crack growth, which can reduce the contact fatigue resistance of the rail. Furthermore, the increased cooling time at low temperatures can reduce productivity and increase rail production costs.Therefore, the average cooling rate CR2 of secondary cooling is set to HC / 120 [°C / s] or more. The average cooling rate CR2 of secondary cooling is preferably HC / 100 [°C / s] or more.

[069] On the other hand, if the average cooling rate CR2 of secondary cooling exceeds 6.0 °C / s, martensitic microstructures may be formed, resulting in reduced ductility and resistance to rolling contact fatigue. Therefore, the average cooling rate CR2 of secondary cooling is set at 6.0 °C / s or less. The average cooling rate CR2 of secondary cooling is preferably 4.0 Petition 870250083365, dated 09 / 16 / 2025, pp. 101 / 121 24 / 34 °C / s or less.

[070] The value of HC in relation to the average cooling rate CR1 and the average cooling rate CR2 can be obtained from Expression (1) above and is preferably 75 to 200. The value of HC is more preferably 90 to 150.

[071] In both primary and secondary cooling, the temperature used to determine the average cooling rate is the surface temperature of the central portion of the rail base bottom and can be measured with a radiation thermometer. The cooling interruption temperature T during secondary cooling is the temperature generated by measuring, with a radiation thermometer, the surface temperature of the central portion of the rail base bottom after accelerated cooling interruptions (before heat recovery).

[072] The accelerated cooling method is not limited and can be carried out, for example, by cooling using an online heat treatment plant. The coolant is not limited and can be one or more selected from air, sprayed water, mist and the like, but air is preferred. For example, the average cooling rate of the underside of a rail base can be controlled by installing a plurality of air injection devices at the underside of the base and adjusting the injection time and air pressure emitted by each device.

[073] In the production method of the present disclosure, it is important to cool the rail after hot rolling in order to satisfy a predetermined average cooling rate with respect to the surface temperature of the central portion of the rail base bottom and, if this condition is satisfied, the cooling method of other parts of the rail (such as the rail head) is not limited. For example, the rail base may be subjected to accelerated cooling, while other parts of the rail (such as Petition 870250083365, dated 09 / 16 / 2025, pages 102 / 121 25 / 34 (rail head) can be allowed to cool naturally or can be subjected to accelerated cooling like the base. (4) Other Treatments

[074] After cooling, the rail material can be subjected to known treatments, such as cold roller straightening. EXAMPLES

[075] The present disclosure is described below in greater detail by way of example, but the present disclosure is in no way limited to those examples and may be appropriately modified within a range consistent with the purpose of the present disclosure, all such modifications being included within the technical scope of the present disclosure.

[076] Steel materials having the chemical compositions illustrated in Table 1 were heated, subjected to hot rolling, and subjected to accelerated cooling after hot rolling under the set of conditions illustrated in Table 2, to produce 60 kg of rail materials conforming to JIS E1101. After cooling was stopped, the rail materials were allowed to cool naturally. The finishing temperature in Table 2 is the surface temperature of the central portion of the rail base bottom on the entry side of the end mill and was measured with a radiation thermometer. The cooling stop temperature T in Table 2 is the value generated by measuring, with a radiation thermometer, the surface temperature of the central portion of the rail base bottom at the moment secondary cooling was stopped.The average cooling rate was calculated as the cooling rate (°C / s) generated by converting the temperature change from the start of cooling to the interruption of cooling into a change per unit of time (second) for each of the primary cooling (from 850 °C to 750 °C) and cooling phases. Petition 870250083365, dated 09 / 16 / 2025, pp. 103 / 121 26 / 34 secondary (from 750 °C up to the cooling stop temperature T).

[077] Finishing lamination during hot rolling was performed under a thickness reduction condition of 15% in the central portion of the base. Under this condition, the thickness reduction is greater than the 11% area reduction during finishing lamination.

[078] Accelerated cooling was carried out by air injection using an air injection apparatus. Petition 870250083365, dated 09 / 16 / 2025, pp. 104 / 121 [Table 1] Steel No. Chemical composition (% by mass)*1 HC*2 Notes C Si Mn PS Cr V Cu Ni Nb Mo Al W Co B Ti Sb Mg Ca Sn 1 0.83 0.48 0.64 0.010 0.005 0.85 - - - - - - - - - - - - - - 102 Steel conforming to standard 2 0.74 0.70 0.25 0.016 0.004 1.98 - - - - - - - - - - - - - - 82 3 0.72 1.19 0.52 0.033 0.008 0.68 - - - - - - - - - - - - - - 78 4 0.91 0.20 0.12 0.014 0.018 0.41 - - - - - - - - - - - - - - 120 5 1.18 0.11 0.33 0.015 0.007 0.06 - - - - - - - - - - - - - - 201 6 0.82 0.37 1.22 0.013 0.004 0.24 - - - - - - - - - - - - - - 99 7 0.79 0.18 1.12 0.011 0.003 1.02 - - - - - - - - - - - - - - 92 8 0.73 0.52 1.49 0.012 0.005 0.13 - - - - - - - - - - - - - - 79 9 0.85 0.91 0.46 0.010 0.003 0.70 - - - - - - - - - - - - - - 107 10 0.89 0.24 0.37 0.014 0.009 1.14 - - - - - - - - - - - - - - 116 11 0.80 0.86 0.73 0.016 0.010 0.35 - - - - - - - - - - - - - - 95 12 0.83 0.15 1.31 0.015 0.008 0.16 - - - - - - - - - - - - - - 101 13 0.79 0.48 0.44 0.009 0.004 0.92 0.06 - - 0.015 - - - - - - - - - - 92 14 0.80 0.37 0,41 0,010 0,003 0,76 0,14 - - - - - - - - - - - - - 94 15 0,75 0,29 0,38 0,011 0,005 0,59 0,30 - - - - - - - - - - - - - 82 16 0,93 0,39 0,20 0,012 0,003 0,84 - 0,37 0,16 - - - - - - - - - - - 127 17 0,84 0,45 0,15 0,013 0,006 0,38 - 0,52 0,25 - - - - - - - - - - - 103 18 0,79 0,22 0,24 0,010 0,004 0,19 - 0,98 0,45 - - - - - - - - - - - 91 19 0,86 0,16 0,30 0,009 0,005 0,20 - - 0,93 - - - - - - - - - - - 107 20 0,82 0,37 0,56 0,012 0,004 0,67 - - - 0,045 - - - - - - - - - - 99 21 0,81 0,75 0,68 0,010 0,005 0,22 - - - - 0,31 - - - - - - - - - 97 22 0,79 0,29 0,45 0,013 0,003 0,31 - - - - 1,02 - - - - - - - - - 91 23 0,90 0,12 0,27 0,011 0,006 0,25 - - - - 1,93 - - - - - - - - - 117 24 0,83 0,50 0,98 0,015 0,006 0,57 - - - - - 0,029 0,13 - - - - - - - 102 25 0,86 0,72 0,41 0,014 0,005 0,63 - - - - - 0,065 - - - - - - - - 109 26 0,84 0,49 0,22 0,016 0,003 0,82 - - - - - - 0,48 - - - - - - - 104 27 0,80 0,26 0,55 0,015 0,004 0,30 - - - - - - 0,95 - - - - - - - 94 28 0,82 0,42 0,76 0,011 0,004 0.68 - - - - - - - 0.28 - - - - - - 99 29 0.88 0.19 0.28 0.012 0.006 0.57 - - - - - - - 0.45 - - - - - - 113 30 0.90 0.11 0.46 0.014 0.005 0.34 - - - - - - - 0.91 - - - - - - 118 31 0.77 1.04 0.29 0.013 0.009 0.25 - - - - - - - - 0.003 0.02 - - - - 88 32 0.82 0.81 0.33 0.015 0.007 0.50 - - - - - - - - 0.005 0.04 - - - - 99 33 0.80 0.52 0.19 0.008 0.003 1.53 - - - - - - - - - - 0.03 - - - 95 34 0.81 0.49 0.25 0.010 0.005 1.41 - - - - - - - - - - 0.05 - - - 97, 27 / 34 Petition 870250083365, dated 09 / 16 / 2025, pp. 105 / 121 Steel No. Chemical composition (% by mass)*1 HC*2 Notes C Si Mn PS Cr V Cu Ni Nb Mo Al W Co B Ti Sb Mg Ca Sn 35 0.85 0.27 1.06 0.017 0.004 0.39 - - - - - - - - - - - 0.002 - - 106 Steel conforming to 36 0.83 0.38 0.91 0.012 0.003 0.75 - - - - - - - - - - - 0.010 - - 102 37 0.84 0.45 0.14 0.020 0.003 1.36 - - - - - - - - - - - - 0.007 - 104 38 0.81 0.59 0.46 0.011 0.006 0.93 - - - - - - - - - - - - 0.019 - 97 39 0.80 0.52 0.21 0.010 0.004 1.52 - - - - - - - - - - - - - 0.03 95 40 0.85 0.81 0.39 0.015 0.003 1.01 - - - - - - - - - - - - - 0.05 107 41 0.65 0.28 0.41 0.015 0.008 0.65 - - - - - - - - - - - - - - 62 Comparative steel 42 1.22 0.46 0.79 0.019 0.010 0.84 - - - - - - - - - - - - - - 218 43 0.72 0.08 0.56 0.011 0.006 0.09 - - - - - - - - - - - - - - 75 44 1.19 1.26 0.68 0.016 0.005 0.70 - - - - - - - - - - - - - - 210 45 0.81 0.30 0.05 0.013 0.011 0.26 - - - - - - - - - - - - - - 95 46 1.15 0.62 1.53 0.012 0.003 1.12 - - - - - - - - - - - - - - 196 47 0.85 0.17 0.80 0.010 0.005 0,03 - - - - - - - - - - - - - - 105 48 1.12 0.91 1.00 0.016 0.004 2.07 - - - - - - - - - - - - - - 188 * The underlined area indicates a value outside the range of this disclosure. *1 The balance consists of Fe and incidental impurities. *2 HC = {(12 x [%C]) + ([%Si] / 10) + ([%Mn] / 20) + ([%Cr] / 18)}2 28 / 34 Petition 870250083365, dated 09 / 16 / 2025, pp. 106 / 121 [Table 2] Test ns. Steel ns. HC*1 Production Conditions Measurement Results Notes Heating Temperature [°C] Lamination Finishing Temperature [°C] Primary Cooling Secondary Cooling Microstructure*3 Pearlite Area Ratio [%] Proeutectoid Area Ratio θ [%] Previous Austenite Grain Size GS [pm] Pearlite Block Size BS [pm] (120 x GS) / (1.1 x BS) Crack Growth Rate da / dN @ΔK = 20 MPaVm [x 10'8 m / cycle] HC / 300 Average Cooling Rate CR1 [°C / s] Cooling Interruption Temperature T [°C] HC / 120 Average Cooling Rate CR2 [°C / s] 1 1 102 1250 900 0.5 1.0 568 1.0 2.0 P + θ 99.6 0.4 48 23 229 4.9 Example 2 2 82 1200 910 0.3 0.8 545 0.7 2.7 P + B 99.5 0.5 50 20 273 3.7 3 3 78 1275 880 0.3 0.5 648 0.6 0.8 P + θ 99.0 1.0 43 27 174 5.8 4 4 120 1300 950 0.4 1.2 526 1.0 3.1 P + θ 98.1 1.9 63 25 273 6.7 5 5 201 1275 900 0.7 2.9 476 1.7 2.6 P + θ + B 95.4 2.8 45 24 202 6.9 6 6 99 1250 920 0.3 1.4 551 0.8 2.0 P + θ 99.8 0.2 43 22 218 4,2 7 7 92 1300 1000 0.3 1.9 409 0.8 5.8 P + B 97.8 0.0 131 44 327 3.1 8 8 79 1275 880 0.3 0.6 530 0.7 2.2 P 100.0 0.0 38. 19 218 3.3 9 9 107 1225 890 0.4 1.8 563 0.9 1.6 P + θ 99.3 0.7 40 22 196 6.0 10 10 116 1200 920 0.4 2.0 538 1.0 2.5 P + θ 98.7 1.3 52 26 218 5.2 11 11 95 1150 870 0.3 1.1 581 0.8 0.8 P + θ 99.6 0.4 30 16 205 4.6 12 12 101 1250 900 0.3 0.9 510 0.8 3.6 P + θ 99.1 0.9 43 18 262 4.0 13 13 92 1300 950 0.3 1.6 554 0.8 2.0 P 100.0 0.0 72 40 196 4.8 14 14 94 1250 900 0.3 0.8 566 0.8 1.7 P 100.0 0.0 48 27 194 4.6 15 15 82 1275 930 0.3 0.7 584 0.7 1.4 P + B 99.1 0.0 39 26 164 5.5 16 16 127 1275 910 0.4 1.5 540 1.1 1.8 P + θ 97.8 2.2 43 25 185 6.8 17 17 103 1250 890 0.3 0.9 563 0.9 2.1 P + θ + B 98.5 0.4 50 29 188 5.3 18 18 91 1300 900 0.3 1.3 575 0.8 2.8 P + B 98.0 0.0 57 23 270 4.0 19 19 107 1275 920 0.4 2.2 588 0.9 1.5 P + θ + B 97.2 0.6 52 32 177 6.1 20 20 99 1225 880 0.3 1.2 549 0.8 1.9 P + θ + B 99.4 0.1 36 18 218 3.7 21 21 97 1250 890 0.3 1.0 506 0.8 2.6 P + θ 99.6 0.0.4 35 17 229 3.9 22 22 91 1275 940 0.3 1.6 570 0.8 2.3 P + B 96.9 0.0 60 30 218 3.5 23 23 117 1300 950 0.4 2.5 601 1.0 1.6 P + θ + B 95.6 0.2 66 34 212 4.3 24 24 102 1250 950 0.3 1.3 487 0.8 3.0 P + θ 99.5 0.5 83 36 251 4.1 25 25 109 1300 910 0.4 1.9 529 0.9 3.2 P + θ 99.3 0.7 78 32 266 4.8 26 26 104 1250 890 0.3 2.0 552 0.9 2.4 P + θ + B 97.5 0.5 52 24 236 4.2, 29 / 34 Petition 870250083365, dated 09 / 16 / 2025, pp. 107 / 121 Test ns. Steel ns. HC*1 Production Conditions Measurement Results Notes Heating Temperature [°C] Lamination Finishing Temperature [°C] Primary Cooling Secondary Cooling Microstructure*3 Pearlite Area Ratio [%] Proeutectoid Area Ratio θ [%] Previous Austenite Grain Size GS [pm] Pearlite Block Size BS [pm] (120 x GS) / (1.1 x BS) Crack Growth Rate da / dN @ΔK = 20 MPaVm [x 10-8 m / cycle] HC / 300 Average Cooling Rate CR1 [°C / s] Cooling Interruption Temperature T [°C] HC / 120 Average Cooling Rate CR2 [°C / s] 27 27 94 1275 920 0.3 1.4 568 0.8 1.8 P + θ + B 96.9 0.2 50 26 210 5.0 Example 28 28 99 1275 910 0.3 1.7 525 0.8 2.9 P + θ + B 98.2 0.1 40 23 190 5.4 29 29 113 1300 900 0.4 2.3 537 0.9 2.0 P + θ + B 97.1 0.5 68 34 218 4.9 30 30 118 1300 960 0.4 1.5 571 1.0 1.2 P + θ + B 95.6 0.2 61 39 171 4.3 31 31 88 1175 900 0.3 0.7 552 0.7 1.4 P 100.0 0.0 45 26 185 3.7 32 32 99 1250 890 0.3 1.0 546 0.8 2.6 P + θ 99,4 0.6 54 24 245 3.9 33 33 95 1300 930 0.3 1.7 539 0.8 1.8 P + θ 99.7 0.3 60 32 207 4.3 34 34 97 1250 900 0.3 1.5 561 0.8 2.4 P + θ 99.8 0.2 61 29 229 4.1 35 35 106 1200 910 0.4 1.2 506 0.9 2.8 P + θ 99.2 0.8 53 27 218 5.0 36 36 102 1250 930 0.3 0.9 542 0.8 3.3 P + θ 99.5 0.5 49 22 243 4.4 37 37 104 1250 920 0.3 1.6 528 0.9 2.4 P + θ 99.4 0.6 50 26 207 4.6 38 38 97 1275 900 0.3 0.8 536 0.8 1.3 P + θ 99.7 0.3 56 34 180 4.0 39 39 95 1300 930 0.3 1.7 540 0.8 1.8 P + θ 99.9 0.1 59 31 208 4.3 40 40 107 1275 950 0.4 2.0 551 0.9 2.0 P + θ 99.2 0.8 51 28 199 5.2 41 41 70 1250 900 0.2 0.4 579 0.6 0.8 P + F 98.9 0.0 43 25 188 7.2 Comparative Examples 42 42 218 1175 870 0.7 0.7 521 1.8 1.9 P + θ 94.9 5.1 33 23 157 12.6 43 43 75 1300 930 0.3 1.5 594 0.6 0.6 P 100.0 0.0 57 36 175 8.2 44 44 209 1150 850 0.7 0.8 516 1.7 1.7 P + B + M + θ 92.5 3.8 30 19 172 11.7 45 45 95 1250 920 0.3 1.9 568 0.8 1.0 P 100.0 0.0 52 33 172 7.8 46 46 196 1200 880 0.7 0.7 522 1.6 1.8 P + B + M + θ 92.4 3.2 36 24 164 10.10.2 47 47 105 1300 950 0.4 2.0 551 0.9 1.2 P 100.0 0.0 66 41 175 7.4 48 48 187 1200 900 0.6 0.6 539 1.6 1.6 P + B + M + θ 91.9 3.7 39 25 170 9.9 49*2 10 116 1360 950 0.4 - - 1.0 - - - - - - - - 50 5 201 1250 840 0.7 1.1 496 1.7 1.7 P + θ + B 94.8 3.1 29 16 198 8.1 51 5 201 1200 900 0.7 0.6 482 1.7 3.4 P + θ 96.5 3.5 38 21 197 13.3 52 5 201 1300 980 0.7 3.1 396 1.7 5.9 P + B + M + θ 94.0 0.6 93 30 338 7.2 53 5 201 1250 920 0.7 0.7 652 1.7 0.9 P + θ 96.9 3.1 64 46 152 7.8 54 4 120 1300 960 0.4 0.4 ​​383 1.0 6.1 P + B + M + θ 92.5 2.3 77 27 316 7.1 * The underlined area indicates a value outside the range of this disclosure. *1 HC = {(12 x [%C]) + ([%Si] / 10) + ([%Mn] / 20) + ([%Cr] / 18)}2*2Part of the steel material melted during heating and the properties could not be evaluated. *3 P: pearlite, F: ferrite, B: bainite, M: martensite, θ: proeutectoid cementite 30 / 34 Petition 870250083365, dated 09 / 16 / 2025, pp. 108 / 121 31 / 34

[079] The resulting rails are pearlitic rails. The anterior austenite grain size GS, the pearlite block size BS, and the resistance to fatigue crack propagation were evaluated for each rail. Table 2 lists the test results. The details of each evaluation are described below. <Microestrutura de Trilho>

[080] The method for measuring the microstructure and area ratio of the central portion of the lower base was as follows.

[081] Specimens for microstructure observation were collected from the central portion of the lower rail base illustrated in FIG. 2 at a depth of 1 mm from the surface, embedded in resin and mirror polished. Subsequently, a plane perpendicular to the longitudinal lamination direction at a depth of 1 mm from the surface was observed by optical microscopy at 200x magnification in 10 fields of view (one field of view being 300 sq m x 400 sq m). The area ratio of the constituent phase was determined for each field of view, and the average value was taken as the area ratio of the microstructure. <Tamanho de Grão de Austenita Anterior GS>

[082] The method for measuring the austenite grain size prior to GS in the central portion of the base bottom was as follows.

[083] Specimens for microstructure observation were collected from the central portion of the lower rail base illustrated in FIG. 2 at a depth of 1 mm from the surface, embedded in resin and mirror-polished. Subsequently, the specimens were etched with nitro to reveal the proeutectoid cementite that had precipitated at the grain boundary of the previous austenite. A plane perpendicular to the longitudinal lamination direction at a depth of 1 mm from the surface was observed using a Petition 870250083365, dated 09 / 16 / 2025, pp. 109 / 121 32 / 34 scanning electron microscope with 200x magnification. Grain size was measured in a region surrounded by proeutectoid cementite by a trace operation using image interpretation software. At least 400 regions were measured and the average value was taken as the previous austenite grain size GS. <Tamanho de Bloco de Perlita BS>

[084] The method for measuring the BS perlite block size in the central portion of the base bottom was as follows.

[085] Specimens for microstructure observation were collected from the central portion of the lower rail base illustrated in FIG. 2 at a depth of 1 mm from the surface. After the specimens were embedded in resin and mirror-polished, an orientation analysis was performed using EBSD (Electron Backscatter Diffraction Standard). Grain boundaries where the orientation difference between adjacent crystal orientations was 15° or more were defined as pearlite block boundaries, and the average of the grain diameters, measured as a circular equivalent, was taken as the pearlite block size BS. The measurement region was 300 µm square, the measurement steps were 0.3 µm apart, and measurement points with a confidence index, indicating the reliability of the measurement orientation, of 0.1 or less were excluded from the measurement. Furthermore, crystal grains extending beyond the edge of the measurement region were also excluded from the measurement. <Resistência à Propagação de Trincamento por Fadiga>

[086] Fatigue crack propagation test specimens were collected from the central portion of the lower rail base illustrated in FIG. 4 at a depth of 1 mm from the surface, and a fatigue crack propagation test was performed. Petition 870250083365, dated 09 / 16 / 2025, pages 110 / 121 33 / 34

[087] FIGS. 5A, 5B and 5C are schematic diagrams illustrating an example of a test specimen, where FIG. 5A is a front view, FIG. 5B is a side view and FIG. 5C is an enlarged front view of the notch. The test specimen was a plate with a width W of 20 mm, a height H of 100 mm and a thickness B of 5 mm in FIGS. 5A, 5B and 5C, with a notch formed at one end of the central portion H / 2 of the height H. The notch was formed on the side of the central portion of the lower rail base. The notch had a length L of 2 mm and a width C of 0.2 mm, and the edge of the notch had a curvature R of 0.1 mm. The stress ratio (ratio R = minimum stress / maximum stress) was set at 0.2, and the fatigue crack propagation rate da / dN (m / cycle) was measured in the stress intensity factor range ΔK = 20 MPa-m1 / 2 to evaluate the resistance to fatigue crack propagation.The test specimen was evaluated as exhibiting fatigue crack propagation inhibition performance if the da / dN value was 7.0 x 10-8 or less.

[088] As illustrated in Table 2, the fatigue crack propagation rates in the test results for the rail materials of the Examples (Tests No. 1 to 40 in Table 2) met 7.0 x 10-8 or less, confirming that these Examples exhibit fatigue crack propagation inhibition performance. On the other hand, for the Comparative Examples (Tests No. 41 to 48 and 50 to 54 in Table 2), the chemical composition of the rail material did not meet the conditions of this disclosure, the pearlite area ratio did not meet the conditions of this disclosure, or the conditions of Expression (2) above were not met, and these Comparative Examples had a fatigue crack propagation rate da / dN (m / cycle) exceeding 7.0 x 10-8. In Test No. 49, the heating temperature was too high, causing some of the steel material to crack. Petition 870250083365, dated 09 / 16 / 2025, pages 111 / 121 34 / 34 melted during heating. For this reason, it was not possible to subject it to lamination for fear of fracture during lamination, and the properties could not be evaluated. INDUSTRIAL APPLICABILITY

[089] According to the present disclosure, a rail with excellent resistance to fatigue crack propagation in the underside of the rail base, together with a method for producing the same, can be provided. The rail of the present disclosure contributes to extending the service life of rails for heavy transport railways and preventing railway accidents. The rail is thus industrially beneficial. Furthermore, the method for producing a rail of the present disclosure can stably improve the resistance to fatigue crack propagation in the underside of the rail base by optimizing the heat treatment conditions after hot rolling. Thus, the method is also industrially beneficial. LIST OF REFERENCE SIGNS Rail Rail head (head) Rail pattern (weave) Rail base (base) Bottom of rail base (bottom of base) Petition 870250083365, dated 09 / 16 / 2025, pages 112 / 121

Claims

1 / 3 CLAIMS 1. Rail, characterized in that it comprises: a chemical composition containing C: 0.70% by mass to 1.20% by mass, Si: 0.10% by mass to 1.20% by mass, Mn: 0.10% by mass to 1.50% by mass, P: 0.035% by mass or less, S: 0.020% by mass or less, and Cr: 0.05% by mass to 2.00% by mass, with an equilibrium consisting of Fe and unavoidable impurities, wherein an area ratio of a pearlitic microstructure at a depth of 1 mm from a surface of a central portion of a lower part of a rail base is 95.0% or more, and a value of HC expressed by Expression (1) satisfies Expression (2), HC = {(12 x [%C]) + ([%Si] / 10) + ([%Mn] / 20) + ([%Cr] / 18)}2...(1) HC < (120 x GS) / (1.1 x BS)...(2) where [%C], [%Si], [%Mn] and [%Cr] are respective contents in % by mass of C, Si, Mn and Cr, GS is the austenite grain size in qm in the central portion of the lower rail base and BS is the pearlite block size in pm in the central portion of the lower rail base.

2. Rail, according to claim 1, characterized in that the chemical composition further comprises at least one selected from the group consisting of V: ​​0.30% by mass or less, Petition 870250083365, dated 09 / 16 / 2025, p. 113 / 121 2 / 3 Cu: 1.0% by mass or less, Ni: 1.0% by mass or less, Nb: 0.05% by mass or less, Mo: 2.0% by mass or less, Al: 0.07% by mass or less, W: 1.0% by mass or less, Co: 1.0% by mass or less, B: 0.005% by mass or less, Ti: 0.05% by mass or less, Sb: 0.05% by mass or less, Mg: 0.01% by mass or less, Ca: 0.02% by mass or less, and Sn: 0.05% by mass or less.

3. Method for producing the rail defined in claim 1 or 2, characterized in that it comprises: heating a steel material having the chemical composition defined in claim 1 or 2 to a heating temperature of 1350 °C or less, hot rolling under a set of conditions, including a finishing rolling temperature of 850 °C or more in the central portion of the rail base bottom, subsequently cooling at an average cooling rate CR1 in °C / s until a temperature of the central portion of the rail base bottom drops from 850 °C to 750 °C, and then cooling at an average cooling rate CR2 in °C / s until a temperature of the central portion of the rail base bottom drops from 750 °C to a cooling stop temperature T, wherein the cooling stop temperature T is in a range of 400 °C to 650 °C, and Petition 870250083365, of 16 / 09 / 2025, p.114 / 121 3 / 3 the average cooling rate CR1 in °C / s and the average cooling rate CR2 in °C / s satisfy respectively Expression (3) and Expression (4), HC / 300 < CR1 < 3.0...(3) HC / 120 < CR2 < 6.0...(4). Petition 870250083365, dated 16 / 09 / 2025, p. 115 / 121.

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