Rail and method for manufacturing same

Abstract

A rail with excellent fracture resistance at the rail web 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 1.80 mass%, P: 0.035 mass% or less, S: 0.020 mass% or less, and Cr: 0.05 mass% to 1.80 mass%, with a balance consisting of Fe and inevitable impurities. When a Vickers hardness at a depth of 0.5 mm from a surface of a rail web is measured over a range of ±17.5 mm above and below a rail height center position, an average value of the Vickers hardness is Hv280 or more with a standard deviation of 5 or less.

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 bending stress to the outer track web of the rail laid in curved sections. As a result, the frequency of rail replacement due to web breakage has been increasing each year. Therefore, there is a growing demand for rail steels with improved fracture resistance at the rail web.

[0005] Against the aforementioned background, various studies have focused on the rail web. For example, Patent Literature (PTL) 1 discloses a method in which the web is rapidly cooled at a cooling rate of 15 °C / s or more, subsequently cooled to a temperature of 250 °C to 450 °C, and then cooled to the Ms point or lower when bainite transformation reaches 30 % or more to obtain a martensitic microstructure, thereby forming the web into a tempered martensite structure with high toughness.

[0006] PTL 2 discloses a method of producing rails in which compressive residual stress is imparted by cooling from the head to the upper neck or web with high-pressure gas or water-containing gas, thereby improving the fracture resistance of the rail fastening portion.

[0007] PTL 3 discloses a rail with excellent rolling contact fatigue resistance in the web, wherein the rail has a predetermined chemical composition, 90 % or more by area of the metallic structure of a cross-section of the web of the rail is a pearlitic microstructure, the minimum hardness of the cross-section of the rail column is Hv300 or more, and the difference between the maximum and minimum hardness of the cross-section of the rail column is Hv40 or less.CITATION LISTPatent Literature

[0008] PTL 1: JP S62-99438 A PTL 2: JP S59-47326 A PTL 3: WO 2020 / 189232 SUMMARY(Technical Problem)

[0009] However, the above conventional technologies still have the following problems to be solved. The technology disclosed in PTL 1 requires holding the temperature until the bainite transformation starts, which results in low production efficiency. The technology disclosed in PTL 2 places the highest priority on obtaining wear resistance / rolling contact fatigue resistance in the head and thus does not necessarily provide the desired crack growth inhibition capability in the web. Depending on the production conditions, a crack-sensitive martensitic microstructure may be generated. Furthermore, with regard to the technology described in PTL 3, the surface layer hardness may vary depending on the combination of ingredients and production conditions, and it is difficult to say that the fracture resistance of the web is sufficient.

[0010] To solve the above-described problems advantageously, it is an aim of the present disclosure to provide a rail with excellent fracture resistance at the rail web, 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 hardness and three-point bending properties of the rail web. As a result, we discovered that excellent fracture resistance can be obtained by ensuring that the web surface layer hardness is equal to or greater than a certain value and by strictly controlling the variation in hardness at the aforementioned position.

[0012] 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 1.80 mass%, with a balance consisting of Fe and inevitable impurities, wherein when a Vickers hardness at a depth of 0.5 mm from a surface of a rail web is measured over a range of ±17.5 mm above and below a rail height center position, an average value of the Vickers hardness is Hv280 or more with a standard deviation of 5 or less. [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: in producing a rail by hot rolling a steel material having the chemical composition according to in [1] or [2], cooling after hot rolling is performed so that an average cooling rate for the rail web is 0.4 °C / s to 5.0 °C / s from a cooling start temperature of 750 °C or more to a cooling stop temperature of 450 °C to 600 °C at each position among the rail height center position, a position 20 mm above the rail height center position, and a position 20 mm below the rail height center position, and so that a difference in the average cooling rate at each position is within 0.5 °C / s. (Advantageous Effect)

[0013] According to the present disclosure, a rail with excellent fracture resistance at the rail web, 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. The method of producing a rail of the present disclosure enables stable production of the rail of the present disclosure and is thus industrially beneficial.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In the accompanying drawings: FIG. 1 is a rail cross-section; FIG. 2 is a diagram illustrating the position at which a test piece for Vickers hardness measurement of the rail web is collected; FIG. 3 is a schematic diagram illustrating a method of cooling the rail web; FIG. 4 is a diagram illustrating the positions at which test pieces for a three-point bend test of the rail web is collected; and FIG. 5 is a diagram illustrating the shape of a test piece for a three-point bend test of the rail web. DETAILED DESCRIPTION<Rail Parts>

[0015] 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. 1. In the rail 1 illustrated in FIG. 1, 11 indicates the rail head, 12 indicates the rail web, and 13 indicates the rail base. In the following, the rail head, rail web, and rail base are also referred to as the head, web, and base, respectively.<Chemical Composition of Rail>

[0016] 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 %

[0017] C is an essential element to ensure the strength of the pearlitic microstructure, i.e., the fracture resistance. If the C content is less than 0.70 %, it is difficult to obtain excellent fracture resistance. 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, leading to a reduction in fracture resistance. Although pro-eutectoid cementite is also present when the C content is 1.20 % or less, the amount produced is so small that its effect on fracture resistance is negligible. 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 %

[0018] In addition to its effect as a deoxidizer, Si is an element that contributes to the strengthening of the pearlitic microstructure, i.e., to the improvement in fracture resistance, 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. However, if the Si content exceeds 1.20 %, bainite and martensitic microstructures are likely to occur in the surface layer, which promotes variation in hardness and results in reduced fracture resistance. Furthermore, weldability is also degraded due to the increase in Si oxides. 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 %

[0019] Mn is an element that contributes to the strengthening of the pearlitic microstructure, i.e., to the improvement in fracture resistance, by decreasing the pearlite transformation temperature and by 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 %, bainite and martensitic microstructures are likely to occur in the surface layer, which promotes variation in hardness and results in reduced fracture resistance. 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 leads to a reduction in fracture resistance. 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

[0020] P in an amount exceeding 0.035 % degrades fracture resistance and 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

[0021] 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 fracture resistance and ductility deteriorate. 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 1.80 %

[0022] Cr is an element that contributes to the strengthening of the pearlitic microstructure, i.e., to the improvement in fracture resistance, by increasing the pearlite equilibrium transformation temperature and by reducing the lamellar spacing. If the Cr content is less than 0.05 %, the effect is insufficient. On the other hand, if the Cr content exceeds 1.80 %, the hardenability of the steel increases, and bainite and martensitic microstructures are likely to occur in the surface layer, which promotes variation in hardness and results in reduced fracture resistance. From these perspectives, the Cr content is set in a range of 0.05 % to 1.80 %. 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 %.

[0023] 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.

[0024] The reasons for the above optional elements are described below.V: 0.30 % or less

[0025] V is an element that forms carbonitrides in the steel and disperses and precipitates in the matrix, thereby improving the fracture resistance. If the V content exceeds 0.30 %, the fracture resistance and ductility deteriorate, 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 fracture resistance. The range of the V content is more preferably 0.001 % to 0.15 %.Cu: 1.0 % or less

[0026] 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. The range of the Cu content is more preferably 0.001 % to 0.5 %.Ni: 1.0 % or less

[0027] 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 fracture resistance is reduced. From these perspectives, the Ni content is preferably 1.0 % or less in the case in which the chemical composition contains Ni. The range of the Ni content is more preferably 0.001 % to 0.5 %.Nb: 0.05 % or less

[0028] 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 fracture resistance and 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 properties 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

[0029] 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 increases, and the fracture resistance is reduced. 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

[0030] 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 fracture resistance and ductility of the steel are decreased. 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

[0031] 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 fracture resistance of the rail through strengthening by precipitation. If the W content exceeds 1.0 %, martensite is formed in the steel. As a result, the fracture resistance 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 fracture resistance. The range of the W content is more preferably 0.001 % to 0.5 %.Co: 1.0 % or less

[0032] 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 fracture resistance 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

[0033] 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 fracture resistance of the steel through strengthening by precipitation. If the B content exceeds 0.005 %, martensite is formed, and as a result, the fracture resistance 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 fracture resistance. The B content is more preferably 0.001 % to 0.003 %.Ti: 0.05 % or less

[0034] 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 fracture resistance 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 fracture resistance 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 fracture resistance. The range of the Ti content is more preferably 0.001 % to 0.03 %.Sb: 0.05 % or less

[0035] 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 fracture resistance and the toughness 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

[0036] 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 fracture resistance 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

[0037] 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 fracture resistance 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

[0038] 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 %.

[0039] 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>

[0040] The rail of the present disclosure is a pearlitic rail, and the microstructure of the rail is 95 % or more pearlite by area ratio. Residual microstructures other than pearlite are acceptable if the total area ratio is 5 % or less, since the fatigue crack propagation resistance is not significantly affected. Examples of the residual microstructure include ferrite, pro-eutectoid cementite, bainite, and martensite.<Vickers Hardness>

[0041] In the present disclosure, it is not sufficient merely for the chemical composition to satisfy the above ranges. To obtain excellent fracture resistance in the web, it is important to control the Vickers hardness Hv, at a depth of 0.5 mm from the surface of the rail web at a location in a range of 17.5 mm above and below a rail height center position (a total range of 35 mm), to be in a predetermined range. Specifically, the average value of the Vickers hardness at the aforementioned location is 280 or more, and the standard deviation thereof is 5 or less. From the perspective of obtaining a stable improvement in fracture resistance, the average Vickers hardness of the aforementioned location is preferably Hv300 or more, with a standard deviation of 4 or less.

[0042] In FIG. 2, the rail height is expressed as the length A from the bottom of the base to the top of the head, and the rail height center position is the center position of the rail height A (A / 2 position). The range of 17.5 mm above and below the rail height center position (total range of 35 mm) is within the shaded area in FIG. 2. The Vickers hardness of the present disclosure can, for example, be measured at a pressing load of 98 N, at a depth of 0.5 mm from the surface of the corresponding location, and at a pitch of 1 mm from the rail top to bottom of the rail in the range of 17.5 mm above and below the rail height center position (total range of 35 mm).

[0043] The hardness in the range of 17.5 mm above and below the rail height center position (total range of 35 mm) is set within a predetermined range for the following reasons.

[0044] As mentioned above, the passage of the wheels applies repetitive "bending stress" to the outer track web of the rail laid in curved sections. On the other hand, roll marks or engravings are imparted within a range of 17.5 mm above and below the rail height center in the rail web. We discovered that when "bending stress" is applied, a portion of this roll mark or engraving becomes a stress concentrator, and a fracture occurs from this area. The present disclosure is based on this finding and controls the hardness in the range of 17.5 mm above and below the rail height center in the rail web to be in a predetermined range.

[0045] Thus, in addition to strengthening the pearlitic microstructure of the rail web, strict control of the variation in hardness of the surface layer of the rail web makes it possible to suppress stress concentration in locally softened areas, which significantly improves the fracture resistance of the rail web. Excellent fracture resistance in the rail web is a property required for rails in general and is also effective in increasing the service life and preventing railroad accidents for rails used in straight sections and rails that do not have roll marks or engravings on the rail web, for example.<Shape of Rail>

[0046] 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>

[0047] 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 to a steel material having the chemical composition described above.

[0048] The steel material used as a rail material has the chemical composition of the rail described above and can be produced by any method. In general, the steel material is preferably produced by casting, particularly continuous casting.(1) Hot Rolling

[0049] The steel material can be heated and then hot rolled into the shape of a rail.(Heating Temperature)

[0050] In heating the steel material prior to hot rolling, the heating temperature is preferably 1350 °C or less. If the heating temperature exceeds 1350 °C, the steel material may partially melt due to excessive temperature increase, resulting in defects inside the rail. On the other hand, 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 Finishing Temperature)

[0051] The hot rolling is preferably performed at a rolling finish temperature of 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, and processing strain is introduced into austenite crystal grains, which can lead to variations in the hardness of the pearlitic microstructure formed by accelerated cooling. Therefore, the rolling finishing temperature is preferably 850 °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 fracture resistance 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 web at the entry side of the final rolling mill and can be measured with a radiation thermometer.

[0052] The other conditions for hot rolling are not limited.(2) Accelerated Cooling

[0053] The rail can be obtained by cooling after hot rolling. During the cooling, the rail web is subjected to accelerated cooling. At this time, the average cooling rate is controlled to be 0.4 °C / s to 5.0 °C / s from a cooling start temperature of 750 °C or more to a cooling stop temperature of 450 °C to 650 °C at each position among the rail height center position (A / 2 position), a position 20 mm above the rail height center position (upper 20 mm position), and a position 20 mm below the rail height center position (lower 20 mm position), where the rail height is A, and the difference in the average cooling rate at these three positions is controlled to be within 0.5 °C / s.

[0054] 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.

[0055] In this accelerated cooling, if the average cooling rate at any of the rail height center position (A / 2 position) or the positions 20 mm above and below the rail height center position (upper 20 mm position and lower 20 mm position) is less than 0.4 °C / s, then the lamellar spacing in the web surface layer will coarsen, and pro-eutectoid cementite will also be more likely to form, resulting in a decrease in fracture resistance. In addition, the increased cooling time at low temperatures may reduce productivity and increase rail production costs. Therefore, the average cooling rate at each of the above three positions is set to 0.4 °C / s or more. The average cooling rate is preferably 1.0 °C / s or more. On the other hand, if the average cooling rate at any of the above three positions exceeds 5.0 °C / s, bainite and martensitic microstructures will form, decreasing the fracture resistance. Therefore, each average cooling rate in the above range is set to 5.0 °C / s or less. The average cooling rate is preferably 4.0 °C / s or less.

[0056] Furthermore, if the difference in average cooling rate at the aforementioned three positions exceeds 0.5 °C / s, the variation in hardness of the rail web surface layer increases, making stress concentration more likely to occur at the locally softened areas during bending stress loading and thereby decreasing the fracture resistance. Therefore, the difference in average cooling rate at the above three positions is within 0.5 °C / s. The difference in average cooling rate is preferably within 0.3 °C / s. The difference between the average cooling rate at the three positions is the difference between the largest average cooling rate and the smallest average cooling rate among the average cooling rates at the three positions.

[0057] The control of the average cooling rate at the rail height center position (A / 2 position), upper 20 mm position, and lower 20 mm position is now described. With respect to heat inflow, the upper and lower sides of the A / 2 position are more susceptible than the A / 2 position to radiation heat and heat conduction from the rail head and base, respectively. If the average cooling rate at the three positions, i.e., the A / 2 position, upper 20 mm position, and lower 20 mm position, is controlled within an appropriate range (0.4 °C / s to 5.0 °C / s), and the difference in average cooling rate at the three positions is controlled within a certain range (within 0.5 °C / s), the average cooling rate in a range of 20 mm above and below the A / 2 position (within a total range of 40 mm) is estimated to be appropriate and uniform.

[0058] By appropriately enhancing the accelerated cooling on the upper side of the A / 2 position and the lower side of the A / 2 position relative to the A / 2 position, the difference in average cooling rates at the above three positions can be stably kept within 0.5 °C / s.

[0059] For example, nozzles can be installed in three stages in the height direction so as to discharge directly towards the A / 2 position, the upper side of the A / 2 position, and the lower side of the A / 2 position, respectively, and the amount of spray and type of coolant at the three positions can be varied according to the temperature and shape of the rail. For example, as illustrated in FIG. 3, the air nozzles can be installed in three stages, with the highest injection volume set at the upper 20 mm position, the second highest injection volume at the lower 20 mm position, and the lowest injection volume at the A / 2 position.

[0060] In accelerated cooling, the temperatures used in determining the average cooling rate are representative of the surface temperatures at the A / 2 position, upper 20 mm position, and lower 20 mm position. These can be measured with a radiation thermometer. Here, the cooling start temperature is the surface temperature of the rail web at the start of accelerated cooling, measured with a radiation thermometer, and the cooling stop temperature is the surface temperature of the rail web after accelerated cooling is stopped (before heat recuperation), measured with a radiation thermometer.

[0061] In the production method of the present disclosure, it is important to cool the rail after hot rolling so as to satisfy the above set of conditions with respect to the surface temperature of the rail web, and if this set of conditions is satisfied, the method of cooling other parts of the rail (such as the rail head) is not limited. The rail head and base may be allowed to cool naturally or subjected to accelerated cooling.(3) Other Treatments

[0062] After cooling, the rail material may be subjected to known treatments, such as cold roller straightening.EXAMPLES

[0063] 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.

[0064] 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. The average cooling rate (°C / s) is obtained by converting the temperature change from the start of cooling to the stop of cooling into a value per unit time (seconds). After cooling was stopped, the rail materials were allowed to cool naturally.

[0065] In the Examples, accelerated cooling of the rail web was performed using the air nozzles illustrated in FIG. 3, and the injected air content was changed as needed to vary the cooling rate. In the Comparative Example test No. 34, accelerated cooling was performed using only the one set of air nozzles installed at the A / 2 height among the air nozzles illustrated in FIG. 3.

[0066] At the start and stop points of accelerated cooling, the surface temperatures at the A / 2 position, upper 20 mm position, and lower 20 mm position were measured using a two-dimensional radiation thermometer capable of measuring the temperature distribution in the height direction of the rail web, and the average cooling rate at these three positions was calculated. [Table 2]

[0067] Table 2Test No.Steel No.Production conditionsInvestigation resultsNotesHeating temperature [°C]Rolling finish temperature [°C]Cooling start temperature [°C]Cooling stop temperature [°C]Average cooling rate [°C / s]Difference in average cooling rate [°C s]Vickers hardness HvThree-point bend test *2< A / 2 position *1< 20 mm upper position20 mm lower positionMeanStandard deviation1112508807505303.43.53.20.33853excellent2213009108105601.71.51.70.23783excellent3312758607805200.60.50.60.13842excellent4412509208305402.22.32.00.34124excellent5513008708005601.81.81.60.23072excellent6612508907704502.82.82.10.53595excellent7712009708406504.24.04.20.23402excellent8812758907605901.31.31.30.03271excellent9913008707805402.12.21.90.34034excellent101012509108005604.74.84.40.43655excellent111112258507605301.11.11.00.12822excellent121212509008005403.53.23.50.33743excellent131312759107905301.51.61.50.13963excellent141413009307805502.42.52.30.24103excellent151512759008005601.21.21.10.13841excellent161612508908105202.02.11.90.23952excellent171712258607705802.21.32.00.34034excellent181812508908006000.90.90.90.04152excellent191912759208106101.41.51.40.14342excelent202012509007905603.63.73.40.34293excellent212112259708206000.90.90.80.13802excellent222212509208305803.33.53.10.44485excellentExample232312759007706301.01.11.00.14362excellent242412509307805401.71.81.50.33463excelent252513008808005502.22.3220.13672excellent262612509208105701.31.31.30.03101excellent272712759107906002.22.32.00.34093excellent282812509007805801.82.01.70.33883excellent292912509208005702.22.12.0024052excellent303012258607705901.21.41.00.44215excellent313111759008005503.23.32.90.44004excelent323212509308405701.61.81.50.33793excelent333313258908105302.52.52.10.22992excellent343412508607505500.90.90.80.13372excellent353512509108405603.13.02.90.23852excellent363612759007805901.61.61.40.23123excellent373712508807605501.91.91.83492excellent383813008907705302.52.52.30.23012excellent393912509007905802.32.42.00.43264excellent404012258607906200.80.80.60.22462poor414112509408305003.83.93.40.54629poor424212509008006401.31.11.20.22532poor434313009708504904.44.54.10.44387poor444412258507605501.41.41.30.12552poor454512509508405003.43.63.10.54498poorComparative Example464612009007705602.12.01.80.33602poor474713009207905502.42.52.20.33574poor484812508808006000.50.50.50.02441poor494912759008105104.24.04.20.24426poor50512509007806000.40.30.40.12782poor51312259207705005.04.95.20.34207poor52812758807905302.21.81.60.63496poorNote Underlining indicates a value outside the applicable range. Note 1 A indicates rail height (A / 2 corresponds to the rail height center position). Note 2 Excellent: neither piece fractured; poor: at least one piece fractured.

[0068] The resulting rails are pearlitic rail. The Vickers hardness and three-point bending properties of the web were evaluated for each rail. Table 2 lists the test results. The following describes the details of each evaluation.<Vickers Hardness>

[0069] After cutting the rail lead end after completion of accelerated cooling, a portion of the rail web, in a range of 20 mm above and below the rail height center position (A / 2 position) illustrated in FIG. 2 (within a total range of 40 mm), was collected so as to include the rail web the rail web surface and was used as a test piece for Vickers hardness measurement. The test piece was embedded in resin and mirror polished, and the Vickers hardness at a depth of 0.5 mm from the rail web surface was measured at 36 points at a pitch of 1 mm from the upper to lower portion of the rail at a load of 98 N over a range of 17.5 mm on either side in the vertical direction (total range of 35 mm) from the rail height center position (A / 2 position). The mean and standard deviation were then calculated from each obtained Vickers hardness.<Three-Point Bending Properties>

[0070] Test pieces were collected from the rail web from two locations, a position 17.5 mm above and a position 17.5 mm below the rail height center position (A / 2 position), so as to include the rail web surface, as illustrated in FIG. 4. Each test piece had the shape illustrated in FIG. 5, with a length L of 100 mm, width H of 10 mm, and thickness B of 10 mm. A notch was formed in the middle L / 2 portion of the length L on the surface of the test piece corresponding to the rail web surface. The depth N of the notch was 0.5 mm, the width C of the notch was 1.0 mm, and the bottom of the notch had a curvature radius R of 0.5 mm.

[0071] The test piece was placed with the notch facing down on supports with a support distance of 50 mm and a curvature radius of 17 mm, and a bending strain was applied from the opposite side of the notch using an indenter with a curvature radius of 16 mm at a pressing speed of 0.1 mm / s. The test pieces collected from the above two locations were tested and evaluated as having excellent fracture resistance at the rail web if neither test piece was fractured at a displacement of 1.0 mm.

[0072] As illustrated in Table 2, the test results for the rail materials (test Nos. 1 to 39 in Table 2) of the Examples exhibited a small variation in hardness at the surface layer of the rail web, and all rail materials exhibited good three-point bending properties. On the other hand, the Comparative Examples (test Nos. 40 to 52 in Table 2), for which the chemical composition of the rail material did not satisfy the set of conditions of the present disclosure or for which a production method outside the scope of the present disclosure was applied, fractured before reaching a predetermined displacement in the three-point bend test.INDUSTRIAL APPLICABILITY

[0073] According to the present disclosure, a rail with excellent fracture resistance at the rail web, 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. The method of producing a rail of the present disclosure enables stable production of the rail of the present disclosure and is thus industrially beneficial.REFERENCE SIGNS LIST

[0074] 1Rail 11Rail head (head) 12Rail web (web) 13Rail base (base)

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 1.80 mass%, with a balance consisting of Fe and inevitable impurities, wherein when a Vickers hardness at a depth of 0.5 mm from a surface of a rail web is measured over a range of ±17.5 mm above and below a rail height center position, an average value of the Vickers hardness is Hv280 or more with a standard deviation of 5 or less.

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: in producing a rail by hot rolling a steel material having the chemical composition according to claim 1 or 2, cooling after hot rolling is performed so that an average cooling rate for the rail web is 0.4 °C / s to 5.0 °C / s from a cooling start temperature of 750 °C or more to a cooling stop temperature of 450 °C to 650 °C at each position among the rail height center position, a position 20 mm above the rail height center position, and a position 20 mm below the rail height center position, and so that a difference in the average cooling rate at each position is within 0.5 °C / s.

Images