Rail and method for manufacturing same

Abstract

A rail with excellent fatigue crack propagation resistance at the rail base underside is provided. The rail includes a chemical composition containing C: 0.70 mass% to 1.20 mass%, Si: 0.10 mass% to 1.20 mass%, Mn: 0.10 mass% to 1.50 mass%, P: 0.035 mass% or less, S: 0.020 mass% or less, and Cr: 0.05 mass% to 2.00 mass%, with a balance consisting of Fe and inevitable impurities. An area ratio of a pearlitic microstructure at a depth of 1 mm from a surface of a central portion of a rail base underside is 95 % or more, and a value of HC expressed by Expression (1) satisfies Expression (2), HC=12×%C+%Si / 10+%Mn / 20+%Cr / 182 HC≤120×GS / 1.1×BS

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a rail and a method of producing the same.BACKGROUND

[0002] In heavy haul railways mainly built to transport ore, the load applied to the axle of a freight car is much higher than that in passenger cars, and the operating environments for rails are harsh. Conventionally, steels having a pearlitic microstructure have therefore mainly been used in such rails from the viewpoint of the importance of wear resistance.

[0003] In recent years, the loading weight of freight cars has been further increased to improve the efficiency of transportation by rail. In addition, the number of wheels passing over the rails has increased due to the increased transportation capacity.

[0004] The passage of the wheels applies repetitive tensile and compressive stress to the rails. As the loading weight increases and the number of wheels passing over the rails increases, the frequency of rail replacement tends to increase each year due to breakage that starts from the rail base underside. Therefore, there is a growing demand for rail steels with improved fatigue crack propagation resistance at the rail base underside.

[0005] Against the above-described background, various studies have been conducted to further improve the breakage resistance of rail bottoms. For example, Patent Literature (PTL) 1 proposes a rail steel that exhibits a pearlitic microstructure containing C: 0.65 % to 1.40 %, in which 200 or more pearlite blocks with a grain size of 1 µm to 15 µm per 0.2 mm 2< examination area are present in at least part of a range starting at the bottom surface and extending to a depth of 10 mm.

[0006] PTL 2 proposes a rail containing, by mass%, C: 0.65 % to 1.20 %, Si: 0.05 % to 2.00 %, and Mn: 0.05 % to 2.00 %, the balance being Fe and inevitable impurities, in which 97 % or more of the head surface and bottom surface has a pearlitic microstructure, the part with a pearlitic microstructure has a surface hardness in a range of Hv320 to Hv500 and a maximum surface roughness of 180 µm or less, and the ratio of the surface hardness to the maximum surface roughness is 3.5 or more.

[0007] PTL 3 discloses a rail in which, while the rail head is subjected to accelerated cooling from 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 to 5 °C / s, so that the pearlite average hardness at the rail bottom becomes HB320 or more.CITATION LISTPatent Literature

[0008] PTL 1: JP 2006-57127 A PTL 2: WO 2011 / 021582 PTL 3: JP H01-139724 A SUMMARY(Technical Problem)

[0009] However, the above conventional technologies still have the following problems to be solved. When the C content is set to 0.65 % to 1.40 %, as in the technologies described in PTL 1 and 2, no improvement in rolling contact fatigue resistance can be expected, because a hard and brittle pro-eutectoid cementite microstructure is formed at the prior austenite grain boundary depending on heat treatment conditions. Furthermore, material property control cannot be considered sufficient with the technology described in PTL 3, since a pro-eutectoid cementite structure may also be formed depending on the combination of components and production conditions, resulting in an increase in the fatigue crack propagation rate.

[0010] To solve the above-described problems advantageously, it is an aim of the present disclosure to provide a rail with excellent fatigue crack propagation resistance at the rail base underside, together with a method of producing the same.(Solution to Problem)

[0011] In order to solve the above problem, we prepared rails having different C, Si, Mn, and Cr contents and intensely investigated the microstructure and fatigue crack propagation resistance of the rail base underside. As a result, we discovered a compositional parameter (the HC parameter in Expression (1) below) defined by the C content, Si content, Mn content, and Cr content and discovered that the value of the parameter is related to the amount of pro-eutectoid cementite. Furthermore, we discovered that by controlling the value of the HC parameter to be a value equal to or less than the value of a parameter configured by the prior austenite grain size and pearlite block size of the rail base underside, excellent fatigue crack propagation resistance of the rail base underside can be obtained even if a large amount of pro-eutectoid cementite is present. More details are as follows.

[0012] We found that the reason why the presence of pro-eutectoid cementite increases the fatigue crack propagation rate is that, as illustrated in the schematic diagram of FIG. 1, in a pro-eutectoid cementite portion that exists in the plastic zone at the tip of a fatigue crack, the {100} plane of ferrite in pearlite that is adjacent to the pro-eutectoid cementite portion fractures first in a brittle manner. Furthermore, we discovered that by controlling the ratio of the prior austenite grain size, which is the formation site of the microstructure, to the pearlite block size, which corresponds to the microstructure unit of the above-described brittle fracture, in accordance with the formation amount of pro-eutectoid cementite, the frequency with which the plastic zone forming at the tip of a fatigue crack encounters the {100} plane of ferrite in pearlite can be reduced, thereby making it possible to suppress brittle crack growth. In other words, we found that even when a large amount of pro-eutectoid cementite is present, coarsening the prior austenite grain size or refining the pearlite block size can stably suppress the aforementioned fatigue crack propagation rate.

[0013] The present disclosure is based on the above discoveries and primary features thereof are as follows. [1] A rail comprising: a chemical composition containing (consisting of) C: 0.70 mass% to 1.20 mass%, Si: 0.10 mass% to 1.20 mass%, Mn: 0.10 mass% to 1.50 mass%, P: 0.035 mass% or less, S: 0.020 mass% or less, and Cr: 0.05 mass% to 2.00 mass%, with a balance consisting of Fe and inevitable impurities, wherein an area ratio of a pearlitic microstructure at a depth of 1 mm from a surface of a central portion of a rail base underside is 95 % or more, and a value of HC expressed by Expression (1) satisfies Expression (2), HC = 12 × % C + % Si / 10 + % Mn / 20 + % Cr / 18 2 HC ≤ 120 × GS / 1.1 × BS where [%C], [%Si], [%Mn], and [%Cr] are respective contents by mass% of C, Si, Mn, and Cr, GS is a prior austenite grain size in µm at the central portion of the rail base underside, and BS is the pearlite block size in µm at the central portion of the rail base underside. [2] The rail according to [1], wherein the chemical composition further contains at least one selected from the group consisting of V: 0.30 mass% or less, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Nb: 0.05 mass% or less, Mo: 2.0 mass% or less, Al: 0.07 mass% or less, W: 1.0 mass% or less, Co: 1.0 mass% or less, B: 0.005 mass% or less, Ti: 0.05 mass% or less, Sb: 0.05 mass% or less, Mg: 0.01 mass% or less, Ca: 0.02 mass% or less, and Sn: 0.05 mass% or less. [3] A method of producing the rail according to [1] or [2], the method comprising: heating a steel material having the chemical composition according to [1] or [2] at a heating temperature of 1350 °C or less, hot rolling under a set of conditions including a rolling finish temperature of 850 °C or more at the central portion of the rail base underside, subsequently cooling at an average cooling rate CR 1 in °C / s until a temperature of the central portion of the rail base underside falls from 850 °C to 750 °C, and then cooling at an average cooling rate CR 2 in °C / s until a temperature of the central portion of the rail base underside falls 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 CR 1 (°C / s) and the average cooling rate CR 2 (°C / s) respectively satisfy Expression (3) and Expression (4), HC / 300 ≤ CR 1 ≤ 3.0 HC / 120 ≤ CR 2 ≤ 6.0 (Advantageous Effect)

[0014] According to the present disclosure, a rail with excellent fatigue crack propagation resistance at the rail base underside, together with a method of producing the same, can be provided. The rail of the present disclosure contributes to extending the service life of rails for heavy haul railways and preventing railway accidents. The rail is thus industrially beneficial. Furthermore, the method of producing a rail of the present disclosure can stably improve the fatigue crack propagation resistance at the rail base underside by optimizing the heat treatment conditions after hot rolling. The method is thus also industrially beneficial.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In the accompanying drawings: FIG. 1 is a schematic diagram illustrating the effects of pro-eutectoid cementite, prior austenite grain size, and pearlite block size on fatigue crack propagation rate; FIG. 2 is a rail cross-sectional view illustrating the parts of the rail and the position at which a test piece is collected for observation of the prior 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 at which a test piece is collected for a fatigue crack propagation test; and FIGS. 5A, 5B, and 5C are diagrams illustrating the shape of the test piece for the fatigue crack propagation test, where FIG. 5A is a front view, FIG. 5B is a side view, and FIG. 5C is an enlarged front view of the notch. DETAILED DESCRIPTION<Rail Parts>

[0016] First, the designations of the various parts of the rail of the present disclosure are described with reference to the rail cross-sectional view in FIG. 2. In the rail 1 illustrated in FIG. 2, 11 indicates the rail head, 12 indicates the rail web, 13 indicates the rail base, and the underside surface of the rail base 13 is referred to as the rail base underside 14. The central portion of the rail base underside is the portion near the widthwise center of the rail base underside. For example, if the width dimension of the rail base underside 14 is W, the central portion of the rail base underside is the region in a width range of ±0.075 × W from the widthwise center of the rail base underside 14.

[0017] In the following, the rail head, rail web, rail base, rail base underside, and central portion of the rail base underside are also referred to as the head, web, base, base underside, and central portion of the base underside, respectively.<Chemical Composition of Rail>

[0018] Next, the chemical composition of the steel of the rail according to the present disclosure will be described. In the following description, "%" denotes "mass%" unless otherwise specified.C: 0.70 % to 1.20 %,

[0019] C is an essential element to ensure the strength of the pearlitic microstructure, i.e., the rolling contact fatigue resistance. If the C content is less than 0.70 %, it is difficult to obtain excellent fatigue crack propagation resistance at the base underside. If the C content exceeds 1.20 %, a large amount of pro-eutectoid cementite is formed at the austenite grain boundary during cooling after hot rolling, resulting in an increase in the fatigue crack propagation rate. Although pro-eutectoid cementite can be present even when the C content is 1.20 % or less, the effect thereof can be avoided by controlling the prior austenite grain size and the block size at the central portion of the base underside so that the above Expression (2) is satisfied. From these perspectives, the C content is set in a range of 0.70 % to 1.20 %. The C content is preferably in a range of 0.70 % to 0.89 %. The C content is more preferably in a range of 0.70 % to 0.85 %.Si: 0.10 % to 1.20 %

[0020] In addition to its effect as a deoxidizer, Si is an element that contributes to the reduction of the fatigue crack propagation rate by increasing the pearlite equilibrium transformation temperature and by reducing the lamellar spacing. From this perspective, the Si content needs to be 0.10 % or more, but if the Si content exceeds 1.20 %, the weldability deteriorates due to the high bonding strength of Si with oxygen. Furthermore, since Si has the effect of moving the eutectic point to the low C side, excessive addition of Si promotes the formation of pro-eutectoid cementite and increases the fatigue crack propagation rate. From these perspectives, the Si content is set 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 %.Mn: 0.10 % to 1.50 %

[0021] Mn is an element that contributes to the reduction of the fatigue crack propagation rate by decreasing the pearlite transformation temperature and reducing the 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, which causes hardening and embrittlement during heat treatment and welding of rails, and the material properties are likely to deteriorate. Furthermore, since Mn has the effect of moving the eutectic point to the low C side, excessive addition of Mn promotes the formation of pro-eutectoid cementite and increases the fatigue crack propagation rate. From these perspectives, the Mn content is set 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 more preferably in a range of 0.30 % to 1.30 %.P: 0.035 % or less

[0022] P in an amount exceeding 0.035 % degrades ductility. Therefore, the P content is set to 0.035 % or less. The P content is preferably 0.020 % or less. No particular lower limit is placed on the P content. The P content may be 0 % but is usually more than 0 % in industrial terms, and an excessive decrease in P content will increase refining costs. From the perspective of economic efficiency, the P content is preferably 0.001 % or more.S: 0.020 % or less

[0023] S is an element mainly present in the steel in the form of A type inclusions. When the S content exceeds 0.020 mass%, the amount of the inclusions is significantly increased, and at the same time coarse inclusions are formed. As a result, the cleanliness of the steel material deteriorates. Therefore, the S content is set to 0.020 % or less. The S content is preferably 0.015 % or less. The S content is more preferably 0.010 % or less. No particular lower limit is placed on the S content. The S content may be 0 % but is usually more than 0 % in industrial terms, and an excessive decrease in S content will increase refining costs. From the perspective of economic efficiency, the S content is preferably 0.0005 % or more.Cr: 0.05 % to 2.00 %

[0024] Cr is an element that contributes to the reduction of the fatigue crack propagation rate by increasing the pearlite equilibrium transformation temperature and reducing the 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, pro-eutectoid cementite is formed at the prior austenite grain boundary, which increases the fatigue crack propagation rate. From these perspectives, the Cr content is set 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 more preferably in a range of 0.15 % to 1.40 %.

[0025] Furthermore, it is not sufficient merely for each of the elements in the chemical composition of the present disclosure to satisfy the aforementioned ranges. Rather, it is important to control the value of the component parameter HC, which corresponds to the amount of pro-eutectoid cementite, to be a value equal to or less than the value of a predetermined parameter configured by the prior austenite grain size GS and the pearlite block size BS at the central portion of the rail base underside.

[0026] The value of the component parameter HC corresponding to the amount of pro-eutectoid cementite is obtained by the following Expression (1), HC = 12 × % C + % Si / 10 + % Mn / 20 + % Cr / 18 2 where [%C], [%Si], [%Mn], and [%Cr] are the respective contents (mass%) of C, Si, Mn, and Cr.

[0027] From the perspective of attaining both high strength of the pearlitic microstructure and suppression of pro-eutectoid cementite, i.e., securing rolling contact fatigue resistance, the value of HC is preferably from 75 to 200. The value of HC is more preferably from 90 to 150.

[0028] In the present disclosure, the predetermined parameter configured by the prior austenite grain size GS and the pearlite block size BS is expressed as (120 × GS) / (1.1 × BS). The value of this parameter and the value of HC satisfy the relationship in Expression (2) below. HC ≤ 120 × GS / 1.1 × BS where GS is the prior austenite grain size in µm at the central portion of the rail base underside, and BS is the pearlite block size in µm at the central portion of the rail base underside.

[0029] In addition to the aforementioned essential components, the chemical composition of a rail used in the present disclosure may optionally contain at least one selected from the group consisting of V: 0.30 mass% or less, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Nb: 0.05 mass% or less, Mo: 2.0 mass% or less, Al: 0.07 mass% or less, W: 1.0 mass% or less, Co: 1.0 mass% or less, B: 0.005 mass% or less, Ti: 0.05 mass% or less, Sb: 0.05 mass% or less, Mg: 0.01 mass% or less, Ca: 0.02 mass% or less, and Sn: 0.05 mass% or less.

[0030] The above optional elements are described below.V: 0.30 % or less

[0031] V is an element that forms carbonitrides in the steel and disperses and precipitates in the matrix, thereby improving the wear resistance of the steel. If the V content exceeds 0.30 %, the workability deteriorates and the alloy cost, i.e., the rail production cost, also increases. From these perspectives, the upper limit of the V content is preferably 0.30 % in the case in which the chemical composition contains V. The V content is preferably 0.001 % or more from the perspective of expressing the effect of improving wear resistance. The range of the V content is more preferably 0.001 % to 0.15 %.Cu: 1.0 % or less

[0032] Cu is an element capable of further strengthening the steel by solid solution strengthening, as with Cr. If the Cu content exceeds 1.0 %, Cu cracking is liable to occur. Therefore, in the case in which the chemical composition contains Cu, the Cu content is preferably 1.0 % or less. From the perspective of high strength, the Cu content is preferably 0.001 % or more. The range of the Cu content is more preferably 0.001 % to 0.5 %.Ni: 1.0 % or less

[0033] Ni is an element that can increase the strength of the steel without deteriorating the ductility. In addition, in the case in which the chemical composition contains Cu, it is preferable to add Ni because Cu cracking can be suppressed by the addition of Ni in combination with Cu. However, if the Ni content exceeds 1.0 mass%, the hardenability of the steel is further increased, the amount of martensite and bainite formed is increased, and the wear resistance and the rolling contact fatigue resistance of the rail head tend to be decreased. From these perspectives, the Ni content is preferably 1.0 % or less in the case in which the chemical composition contains Ni. From the perspective of high strength, the Ni content is preferably 0.001 % or more. The range of the Ni content is more preferably 0.001 % to 0.5 %.Nb: 0.05 % or less

[0034] Nb is an element that combines with C in the steel to precipitate as carbides during and after the hot rolling for forming the rail and effectively acts to refine the size of pearlite colonies. As a result, Nb greatly improves wear resistance, rolling contact fatigue resistance, and ductility, and contributes greatly to extending the service life of the rail. However, when the Nb content exceeds 0.05 %, the effect of improving the wear resistance and the 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 the case in which the chemical composition contains Nb. The Nb content is preferably 0.001 % or more in order to obtain a sufficient effect with respect to extending the service life of the rail. The range of the Nb content is more preferably 0.001 % to 0.03 %.Mo: 2.0 % or less

[0035] Mo is an element capable of further strengthening the steel by solid solution strengthening. Mo also has the effect of moving the eutectic point to the high C side and thus has the effect of inhibiting the formation of pro-eutectoid cementite. However, if the Mo content exceeds 2.0 mass%, the amount of bainite formed in the steel is increased, and the wear resistance of the rail head decreases. From these perspectives, the Mo content is preferably 2.0 % or less in the case in which the chemical composition contains Mo. From the perspective of high strength, the Mo content is preferably 0.001 % or more. The range of the Mo content is more preferably 0.001 % to 1.0 %.Al: 0.07 % or less

[0036] Al is an element that can be added as a deoxidizer. If the Al content exceeds 0.07 mass%, a large number of oxide-based inclusions are formed in the steel due to the high bonding strength between Al and oxygen. As a result, the ductility of the steel decreases. Therefore, the Al content is preferably 0.07 % or less in the case in which the chemical composition contains Al. No lower limit is placed on the Al content, but the Al content is preferably 0.001 % or more for deoxidation. The range of the Al content is more preferably 0.001 % to 0.03 %.W: 1.0 % or less

[0037] W is an element that precipitates as carbides during and after the hot rolling for shaping the steel into a rail shape and that improves the strength and the ductility of the rail through strengthening by precipitation. If the W content exceeds 1.0 %, martensite is formed in the steel. As a result, the ductility decreases. From these perspectives, the W content is preferably 1.0 % or less in the case in which 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 the strength and the ductility. The range of the W content is more preferably 0.001 % to 0.5 %.Co: 1.0 % or less

[0038] Co is an element that can increase the pearlite equilibrium transformation temperature and reduce the lamellar spacing, thereby further enhancing the strength of steel. Co also has the effect of suppressing the precipitation of pro-eutectoid cementite. If the Co content exceeds 1.0 %, martensite is formed in the steel. As a result, the ductility decreases. From these perspectives, the Co content is preferably 1.0 % or less in the case in which the chemical composition contains Co. No lower limit is placed on the Co content, but the Co content is preferably 0.001 % or more for enhancing strength. The range of the Co content is more preferably 0.001 % to 0.5 %.B: 0.005 % or less

[0039] B is an element that precipitates as nitrides in the steel during and after the hot rolling for shaping the steel into a rail shape and improves the strength and the ductility of the steel through strengthening by precipitation. If the B content exceeds 0.005 %, martensite is formed, and as a result, the ductility of the steel decreases. From these perspectives, the B content is preferably 0.005 % or less in the case in which 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 the strength and the ductility. The range of the B content is more preferably 0.001 % to 0.003 %.Ti: 0.05 % or less

[0040] Ti is an element that precipitates as carbides, nitrides, or carbonitrides in the steel during and after the hot rolling for shaping the steel into a rail shape and that improves the strength and the ductility of the steel through strengthening by precipitation. If the Ti content exceeds 0.05 mass%, coarse carbides, nitrides or carbonitrides are formed. As a result, the ductility of the steel decreases. From these perspectives, the Ti content is preferably 0.05 % or less in the case in which 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 the strength and the ductility. The range of the Ti content is more preferably 0.001 % to 0.03 %.Sb: 0.05 % or less

[0041] Sb is an element that has a remarkable effect of preventing the decarburization of the steel during reheating of the rail steel material in a heating furnace before the hot rolling. If the Sb content exceeds 0.05 %, the ductility and the toughness of the steel are adversely affected. Therefore, in the case in which 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 the Sb content is more preferably 0.001 % to 0.03 %.Mg: 0.01 % or less

[0042] Mg is an element that combines with oxygen to precipitate as MgO, thereby further enhancing strength. If the Mg content exceeds 0.01 %, the increase in MgO adversely affects the ductility and the toughness of the steel. Therefore, in the case in which the chemical composition contains Mg, the Mg content is preferably 0.01 % or less. No lower limit is placed on the Mg content, but the Mg content is preferably 0.001 % or more in order to exert the effect of improving the strength. The range of the Mg content is more preferably 0.001 % to 0.005 %.Ca: 0.02 % or less

[0043] Ca is an element that combines with oxygen to precipitate as CaO, thereby further enhancing strength. If the Ca content exceeds 0.02 %, the increase in CaO adversely affects the ductility and the toughness of the steel. Therefore, in the case in which 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 the strength. The range of the Ca content is more preferably 0.001 % to 0.01 %.Sn: 0.05 % or less

[0044] Sn is an element that has a remarkable effect of preventing the decarburization of the steel during reheating of the rail steel material in a heating furnace before the hot rolling. If the Sn content exceeds 0.05 %, the ductility and the toughness of the steel are adversely affected. Therefore, in the case in which 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 the Sn content is more preferably 0.001 % to 0.01 %.

[0045] In the chemical composition of the steel for the rail of the present disclosure, the balance other than the above essential and optional components consists of Fe and inevitable impurities. As used herein, examples of the inevitable impurities include N, O, and the like. N content up to 0.008 % and O content up to 0.004 % are allowable. Impurities other than N and O may 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 as long as they do not interfere with the aim of the present disclosure. Impurities other than N and O include Pb, Zr, Bi, Zn, Se, As, Te, Tl, Cd, Hf, Ag, Hg, Ga, Ge, and REM.<Rail Microstructure>

[0046] The rail of the present disclosure is a pearlitic rail. From the perspective of fatigue crack propagation resistance, the microstructure at a depth of 1 mm from the surface of the central portion of the rail base underside has an area ratio of pearlite of 95.0 % more. The area ratio of pearlite may be 100 %. The residual microstructure other than pearlite at a depth of 1 mm from the surface of the central portion of the base underside is acceptable at an area ratio of 5.0 % or less and may be 0 %. Among residual microstructures, pro-eutectoid cementite is acceptable if the area ratio is 3.0 % or less, since the fatigue crack propagation resistance is not significantly affected. Examples of the residual microstructure other than pro-eutectoid cementite include ferrite, bainite, and martensite.

[0047] The microstructure and area ratio of the central portion of the base underside can be measured by the method of measurement in the Examples described below.(Prior Austenite Grain Size GS at Central Portion of Base Underside)

[0048] The rail of the present disclosure preferably has a prior austenite grain size GS at the central portion of the base underside of 30 µm to 140 µm. The prior austenite grain size GS at the central portion of the base underside is more preferably 30 µm to 80 µm from the perspective of preventing an excessive reduction in ductility and toughness.

[0049] The prior austenite grain size GS at the central portion of the base underside can be measured by the method of measurement in the Examples described below.(Pearlite Block Size BS)

[0050] The rail of the present disclosure preferably has a pearlite block size BS at the central portion of the base underside of 15 µm to 45 µm. Since a smaller pearlite block size, which corresponds to the microstructure unit of brittle fracture, enables a reduction in length of brittle crack growth, the pearlite block size BS at the central portion of the base underside is more preferably 15 µm to 30 µm.

[0051] The pearlite block size BS at the central portion of the base underside can be measured by the method of measurement in the Examples described below.<Shape of Rail>

[0052] The shape of the rail of the present disclosure is not limited and can be the shape of the rail described by JIS E 1101:2001, BS EN13674-1:2011, the American Railway Engineering and Maintenance-of-Way Association (AREMA), or the like.<Method of Producing Rail>

[0053] A method of 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. (1) Hot Rolling (2) Primary Cooling (3) Secondary Cooling

[0054] 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 casting, particularly continuous casting.(1) Hot RollingHeating temperature: 1350 °C or less

[0055] The steel material is heated at 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 inside the rail. No lower limit is placed on the heating temperature, but the heating temperature is preferably 1150 °C or more to reduce deformation resistance during rolling.Rolling finish temperature: 850 °C or more

[0056] The heated steel material is hot rolled into the shape of a rail. Here, finish rolling refers to one pass of final rolling, performed using the final rolling caliber of one final rolling mill.

[0057] The rolling finish temperature in hot rolling is set to 850 °C or more. If the rolling finish temperature is lower than 850 °C, then the rolling is performed in an austenite low temperature range. As a result, not only is processing strain introduced into austenite crystal grains, but also the elongation of austenite crystal grains becomes remarkable. The increase in austenite grain boundary area increases the number of nucleation sites for pro-eutectoid cementite, resulting in lower fatigue crack propagation resistance. Therefore, the rolling finish temperature is set to 850 °C or more. The rolling finish temperature is preferably 900 °C or more. No upper limit is placed on the rolling finish temperature, but an extreme coarsening of the prior austenite grain size will reduce ductility and toughness. The rolling finish temperature is therefore preferably 1050 °C or less. Here, the rolling finish temperature is the surface temperature of the central portion of the rail base underside at the entry side of the rolling mill in the final rolling pass and can be measured with a radiation thermometer.

[0058] The rolling reduction (thickness reduction) in the thickness direction of the rail base during the finish rolling, which is the final rolling pass, is preferably greater than the reduction in area during this pass. For example, the reduction in area may be set to 11 %, while the thickness reduction at the central portion of the rail base is set to 15 % or more. This can introduce a large strain in the central portion of the rail base, which can further promote the refinement of the microstructure after pearlite transformation and effectively increase the fatigue strength of the base underside. By designing the caliber so that the width dimension of the base is widened during this finish rolling, rolling with relatively large thickness reduction compared to the reduction in area can be performed stably.

[0059] The reduction in area of the rail base is a value calculated as reduction in area (%) = {(S 0 - S 1 ) / S 0 } × 100, where S 0 is the cross-sectional area of the bottom before finish rolling and S 1 is the cross-sectional area of the bottom after finish rolling, based on the area of the shaded area marked S in the cross-sectional view of the rail in FIG. 3.

[0060] The thickness reduction of the rail base is a value calculated as thickness reduction (%) = {(H 0 - H 1 ) / H 0 } × 100, where H 0 is the thickness of the bottom before finish rolling and H 1 is the cross-sectional area of the bottom after finish rolling, based on the height marked H in the same drawing.

[0061] The other conditions for hot rolling are not limited.(2) Primary Cooling

[0062] Average cooling rate from 850 °C to 750 °C (CR 1 [°C / s]): HC / 300 ≤ CR 1 ≤ 3.0

[0063] 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 of pro-eutectoid cementite formation, and if the average cooling rate CR 1 in this temperature range is less than HC / 300 [°C / s], the amount of pro-eutectoid cementite increases. As a result, cracks are more likely to occur at the interface of the pro-eutectoid cementite microstructure, which may reduce the rolling contact fatigue resistance of the rail. Therefore, the average cooling rate CR 1 of primary cooling is set to HC / 300 [°C / s] or more. The average cooling rate CR 1 of primary cooling is preferably HC / 200 [°C / s] or more.

[0064] On the other hand, if the average cooling rate CR 1 of primary cooling exceeds 3.0 °C / s, martensitic microstructures may be formed, resulting in reduced ductility and rolling contact fatigue resistance. Therefore, the average cooling rate CR 1 of primary cooling is set to 3.0 °C / s or less. The average cooling rate CR 1 of primary cooling is preferably 2.0 °C / s or less.(3) Secondary Cooling

[0065] Average cooling rate (CR 2 [°C / s]) from 750 °C to cooling stop temperature T: HC / 120 ≤ CR 2 ≤ 6.0

[0066] Secondary cooling is performed following the aforementioned primary cooling. The secondary cooling is from 750 °C to the cooling stop temperature T. The cooling stop temperature T is in a range of 400 °C to 650 °C. If the average cooling rate from the secondary cooling start temperature of 750 °C to the cooling stop temperature T of the secondary cooling in the range of 400 °C to 650 °C is less than HC / 120 [°C / s], the pearlite block size becomes coarse, as does the lamellar spacing. In other words, the larger pearlite block size, which corresponds to the microstructure unit of brittle fracture, may result in longer brittle crack growth, which may reduce the rolling contact fatigue resistance of the rail. In addition, the increased cooling time at low temperatures may reduce productivity and increase rail production costs. Therefore, the average cooling rate CR 2 of secondary cooling is set to HC / 120 [°C / s] or more. The average cooling rate CR 2 of secondary cooling is preferably HC / 100 [°C / s] or more.

[0067] On the other hand, if the average cooling rate CR 2 of secondary cooling exceeds 6.0 °C / s, martensitic microstructures may be formed, resulting in reduced ductility and rolling contact fatigue resistance. Therefore, the average cooling rate CR 2 of secondary cooling is set to 6.0 °C / s or less. The average cooling rate CR 2 of secondary cooling is preferably 4.0 °C / s or less.

[0068] The value of HC in relation to the average cooling rate CR 1 and the average cooling rate CR 2 can be obtained by the above Expression (1) and is preferably 75 to 200. The value of HC is more preferably 90 to 150.

[0069] In both primary cooling and secondary cooling, the temperature in determining the average cooling rate is the surface temperature of the central portion of the rail base underside and can be measured with a radiation thermometer. The cooling stop temperature T during secondary cooling is the temperature yielded by measuring, with a radiation thermometer, the surface temperature of the central portion of the rail base underside after accelerated cooling stops (before heat recuperation).

[0070] The method of accelerated cooling is not limited and can be performed by, for example, cooling using an on-line heat treatment facility. The coolant is not limited and can be one or more selected from air, spray water, mist, and the like, but air is preferred. For example, the average cooling rate of the rail base underside can be controlled by installing a plurality of air injection apparatuses at the base underside and adjusting the injection time and pressure of air emitted from each apparatus.

[0071] In the production method of the present disclosure, it is important to cool the rail after hot rolling so as to satisfy a predetermined average cooling rate with respect to the surface temperature of the central portion of the rail base underside, and if this condition is satisfied, the method of cooling 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 the rail head) may be allowed to cool naturally or may be subjected to accelerated cooling like the base.(4) Other Treatments

[0072] After cooling, the rail material may be subjected to known treatments, such as cold roller straightening.EXAMPLES

[0073] The present disclosure is described below in greater detail through examples, but the present disclosure is not restricted by any means to these examples and may be changed appropriately within a range conforming to the purpose of the present disclosure, all such changes being included within the technical scope of the present disclosure.

[0074] 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 rail materials conforming to JIS E1101. After cooling was stopped, the rail materials were allowed to cool naturally. The rolling finish temperature in Table 2 is the surface temperature of the central portion of the rail base underside at the entry side of the final rolling mill and was measured with a radiation thermometer. The cooling stop temperature T in Table 2 is the value yielded by measuring, with a radiation thermometer, the surface temperature of the central portion of the rail base underside at the time that secondary cooling was stopped. The average cooling rate was calculated as the cooling rate (°C / s) yielded by converting the temperature change from the start of cooling to the stop of cooling into the change per unit time (second) for each of the primary cooling (from 850 °C to 750 °C) and secondary cooling (from 750 °C to the cooling stop temperature T).

[0075] The finish rolling during hot rolling was performed under a condition of 15 % thickness reduction in the central portion of the base. Under this condition, the thickness reduction is greater than the reduction in area of 11 % during finish rolling.

[0076] Accelerated cooling was performed by air injection using an air injection apparatus.[Table 2]

[0077] Table 2Test No.Steel No.IIC *1< Production conditionsMeasurement resultsNotesHeating temperature [°C]Rolling finish temperature [°C]Primary coolingSecondary coolingMicrostructure *3< Pearlite area ratio [%]Pro-eutectoid θ area ratio [%]Prior austenite grain size GS [µm]Pearlite block size BS [µm](120 × GS) / (1.1 × BS)Crack growth rate da / dN @ΔK - 20 MPa√m [× 10 -8< m / cycle]HC / 300Average cooling rate CR 1 [°C / s]Cooling stop temperature T [°C]HC / 120Average cooling rate CR 2 [°C / s]1110212509000.51.05681.02.0P + θ99.60.448232294.9Example228212009100.30.85450.72.7P + B99.50.550202733.7337812758800.30.56480.60.8P + θ99.01.043271745.84412013009500.41.25261.03.1P + θ98.11.963252736.75520112759000.72.94761.72.6P + θ + B95.42.845242026.9669912509200.31.45510.82.0P + 099.80.243222184.2792130010000.31.94090.85.8P + B97.80.0131443273.1887912758800.30.65300.72.2P100.00.038191183.39910712258900.41.85630.91.6P + θ99.30.740221966.0101011612009200.42.05381.02.5P + θ98.71.352262185.211119511508700.31.15810.80.8P + θ99.60.430162054.6121210112509000.30.95100.83.6P + θ99.10.943182624.013139213009500.31.65540.82.0P100.00.072401964.814149412509000.30.85660.81.7P100.00.048271944.615158212759300.30.75840.71.4P + B99.10.039261645.5161612712759100.41.55401.11.8P + 097.82.243251856.817171031250890030.95630.82.1P + 0 + B98.50.450291885.318189113009000.31.35750.82.8P + B98.00.057232704.0191910712759200.42.25880.91.5P + θ + B97.20.652321776.120209912258800.31.25490.81.9P + θ + B99.40.136182183.721219712508900.31.05060.82.6P + θ99.60.435172293.922229112759400.31.65700.82.3P + B96.90.060302183.5232311713009500.42.56011.01.6P + θ + B95.60.266342124.3242410212509500.31.34870.83.0P + 099.50.583362514.1252510913009100.41.95290.93.2P + 099.30.778322664.826261041250890032.05520.82.4P + 0 + B97.50.552242364.227279412759200.31.45680.81.8P + θ + B9690.250262105.028289912759100.31.75250.82.9P + θ + B98.20.1402319054292911313009000.42.35370.92.0P + θ + B97.10.568342184.9303011813009600.41.55711.01.2P + θ + B95.60.261391714.331318811759000.30.75520.71.4P100.00.045261853.732329912508900.31.05460.82.6P + 099.40.654242453.933339513009300.31.75390.81.8P + 09970.360322074.334349712509000.31.55610.82.4P + 099.80.261292294.1353510612009100.41.25060.92.8P + θ99.20.853272185.0363610212509300.30.95420.83.3P + θ99.50.549222434.4373710412509200.31.65280.92.4P + θ99.40.650262074.638389712759000.30.85360.81.3P + θ99.70.356341804.039399513009300.31.75400.81.8P + θ99.90.159312084.3404010712759500.42.05510.92.0P + θ99.20.851281995.241417012509000.20.45790.60.8P + F98.90.043251887.2Comparative Example424221811758700.70.75211.81.9P + θ9495.1332315712.643437513009300.31.55940.60.6P100.00.057361758.2444420911508500.70.85161.71.7P + B + M + θ92.53.8301917211.74545 9512509200.31.95680.81.0P100.00.052331727.8464619612008800.70.75221.61.8P + B + M + θ92.43.2362416410.2474710513009500.42.05510.91.2P100.00.066411757.4484818712009000.60.65391.61.6P + B + M + 091.93.739251709.949*21011613609500.4-1.0----50520112508400.71.14961.71.7P + θ + B94.83.129161988.151520112009000.70.64821.73.4P + θ96.53.5382119713.352520113009800.73.13961.75.9P + B + M + θ94.00.693303387.253520112509200.70.76521.70.9P + θ96.93.164461527.845429113109200.40.43831.40.1P + B + M + θ92.52.377273307.1* Underlining indicates a value outside the range of the present disclosure. *1 HC - {(12 × |%C|) + (|%Si| / 10) + (|%Mn| / 20) + (|%Cr| / 18)} 2< *2 Part of the steel material melted during heating, and the properties could not be evaluated. *3 P: pearlite, F: ferrite, B: bainite, M: martensite, θ: pro-eutectoid cementite

[0078] The resulting rails are pearlitic rail. The prior austenite grain size GS, pearlite block size BS, and fatigue crack propagation resistance were evaluated for each rail. Table 2 lists the test results. The following describes the details of each evaluation.<Rail Microstructure>

[0079] The method of measuring the microstructure and the area ratio of the central portion of the base underside were as follows.

[0080] Test pieces for microstructure observation were collected from the central portion of the rail base underside 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 rolling longitudinal direction at the depth of 1 mm from the surface was observed by optical microscopy at 200x magnification over 10 fields of view (one field of view being 300 µm × 400 µm). 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.<Prior Austenite Grain Size GS>

[0081] The method of measuring the prior austenite grain size GS at the central portion of the base underside was as follows.

[0082] Test pieces for microstructure observation were collected from the central portion of the rail base underside illustrated in FIG. 2 at a depth of 1 mm from the surface, embedded in resin, and mirror polished. Subsequently, the test pieces were niter-etched to reveal the pro-eutectoid cementite that had precipitated at the prior austenite grain boundary. A plane perpendicular to the rolling longitudinal direction at a depth of 1 mm from the surface was observed using a scanning electron microscope at 200x magnification. The grain size was measured in a region surrounded by pro-eutectoid cementite by a tracing operation using image interpretation software. At least 400 regions were measured, and the average value was taken as the prior austenite grain size GS.<Pearlite Block Size BS>

[0083] The method of measuring the pearlite block size BS at the central portion of the base underside was as follows.

[0084] Test pieces for microstructure observation were collected from the central portion of the rail base underside illustrated in FIG. 2 at a depth of 1 mm from the surface. After the test pieces were embedded in resin and mirror polished, orientation analysis was performed using EBSD (Electron Backscatter Diffraction Pattern). 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 circle 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, which indicates 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.<Fatigue Crack Propagation Resistance>

[0085] Fatigue crack propagation test pieces were collected from the central portion of the rail base underside illustrated in FIG. 4 at a depth of 1 mm from the surface, and a fatigue crack propagation test was performed.

[0086] FIGS. 5A, 5B, and 5C are schematic diagrams illustrating an example test piece, 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 piece was a plate with a width W of 20 mm, height H of 100 mm, and thickness B of 5 mm in FIGS. 5A, 5B, and 5C, with a notch formed at one width end of the central H / 2 portion of the height H. The notch was formed on the side of the central portion of the rail base underside. The notch had a length L of 2 mm and width C of 0.2 mm, and the edge of the notch had a curvature R of 0.1 mm. The stress ratio (R ratio = minimum stress / maximum stress) was set to 0.2, and the fatigue crack propagation rate da / dN (m / cycle) was measured in the stress intensity factor range ΔK = 20 MPa·m 1 / 2< to evaluate the fatigue crack propagation resistance. The test piece was evaluated as exhibiting fatigue crack propagation inhibition performance if the value of da / dN was 7.0 × 10 -8< or less.

[0087] As illustrated in Table 2, the fatigue crack propagation rates in the test results for the rail materials of the Examples (Test Nos. 1 to 40 in Table 2) all satisfied 7.0 × 10 -8< or less, confirming that these Examples exhibit fatigue crack propagation inhibition performance. On the other hand, for the Comparative Examples (Test Nos. 41 to 48 and 50 to 54 in Table 2), the chemical composition of the rail material did not satisfy the conditions of the present disclosure, the pearlite area ratio did not satisfy the conditions of the present disclosure, or the conditions of Expression (2) above were not satisfied, and these Comparative Examples had a fatigue crack propagation velocity da / dN (m / cycle) exceeding 7.0 × 10 -8< . In Test No. 49, the heating temperature was too high, causing part of the steel material to melt during the heating. For this reason, it could not be subjected to rolling because of fear of fracture during the rolling, and the properties could not be evaluated.INDUSTRIAL APPLICABILITY

[0088] According to the present disclosure, a rail with excellent fatigue crack propagation resistance at the rail base underside, together with a method of producing the same, can be provided. The rail of the present disclosure contributes to extending the service life of rails for heavy haul railways and preventing railway accidents. The rail is thus industrially beneficial. Furthermore, the method of producing a rail of the present disclosure can stably improve the fatigue crack propagation resistance at the rail base underside by optimizing the heat treatment conditions after hot rolling. The method is thus also industrially beneficial.REFERENCE SIGNS LIST

[0089] 1Rail 11Rail head (head) 12Rail web (web) 13Rail base (base) 14Rail base underside (base underside)

Claims

1. A rail comprising: a chemical composition containing C: 0.70 mass% to 1.20 mass%, Si: 0.10 mass% to 1.20 mass%, Mn: 0.10 mass% to 1.50 mass%, P: 0.035 mass% or less, S: 0.020 mass% or less, and Cr: 0.05 mass% to 2.00 mass%, with a balance consisting of Fe and inevitable impurities, wherein an area ratio of a pearlitic microstructure at a depth of 1 mm from a surface of a central portion of a rail base underside is 95.0 % or more, and a value of HC expressed by Expression (1) satisfies Expression (2), HC = 12 × % C + % Si / 10 + % Mn / 20 + % Cr / 18 2 HC ≤ 120 × GS / 1.1 × BS where [%C], [%Si], [%Mn], and [%Cr] are respective contents by mass% of C, Si, Mn, and Cr, GS is the prior austenite grain size in µm at the central portion of the rail base underside, and BS is the pearlite block size in µm at the central portion of the rail base underside.

2. The rail according to claim 1, wherein the chemical composition further comprises at least one selected from the group consisting of V: 0.30 mass% or less, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Nb: 0.05 mass% or less, Mo: 2.0 mass% or less, Al: 0.07 mass% or less, W: 1.0 mass% or less, Co: 1.0 mass% or less, B: 0.005 mass% or less, Ti: 0.05 mass% or less, Sb: 0.05 mass% or less, Mg: 0.01 mass% or less, Ca: 0.02 mass% or less, and Sn: 0.05 mass% or less.

3. A method of producing the rail according to claim 1 or 2, the method comprising: heating a steel material having the chemical composition according to claim 1 or 2 at a heating temperature of 1350 °C or less, hot rolling under a set of conditions including a rolling finish temperature of 850 °C or more at the central portion of the rail base underside, subsequently cooling at an average cooling rate CR1 in °C / s until a temperature of the central portion of the rail base underside falls 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 underside falls 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 in °C / s and the average cooling rate CR2 in °C / s respectively satisfy Expression (3) and Expression (4), HC / 300 ≤ CR 1 ≤ 3.0 HC / 120 ≤ CR 2 ≤ 6.0

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