Rail and method for manufacturing same

Abstract

A rail that can suppress breakage from the rail bottom is provided. The rail includes a chemical composition containing, by mass%, C: 0.60 % or more and less than 0.90 %, Si: 0.10 % or more and 1.20 % or less, Mn: 0.10 % or more and 1.50 % or less, Cr: 0.05 % or more and 2.00 % or less, Al: 0.0002 % or more and 0.005 % or less, P: 0 % or more and 0.035 % or less, S: 0 % or more and 0.020 % or less, V: 0 % or more and 0.30 % or less, Cu: 0 % or more and 1.0 % or less, Ni: 0 % or more and 1.0 % or less, Nb: 0 % or more and 0.05 % or less, Mo: 0 % or more and 0.5 % or less, B: 0 % or more and 0.0050 % or less, Ti: 0 % or more and 0.01 % or less, Mg: 0 % or more and 0.01 % or less, Ca: 0 % or more and 0.02 % or less, W: 0 % or more and 0.10 % or less, Sb: 0 % or more and 0.05 % or less, Sn: 0 % or more and 0.05 % or less, and Co: 0 % or more and 1.0 % or less, with the balance being Fe and incidental impurities. The 0.2 % proof stress at the rail bottom center is greater than 500 MPa and less than 1100 MPa, the residual stress in the longitudinal direction at the rail bottom center is less than 200 MPa, and the fatigue crack propagation rate is 5.0 × 10-8 m / cycle or less when a stress intensity factor range ΔK at the rail bottom center is 15 MPa m1 / 2.

Description

TECHNICAL FIELD

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

[0002] Compared to other means of transportation, rail transportation is highly efficient and environmentally friendly. Active efforts are thus being made to increase transport capacity by increasing operating speeds, increasing the load capacity of freight cars, and operating on tighter schedules. As a result, the number of wheels passing over the rails is increasing, and repeated tensile and compressive stresses are applied to the rails, causing them to break from the bottom. This has led to the problem of a gradual increase in the frequency of rail replacement. Demand is therefore growing for rail steel with reduced breakage from the rail bottom.

[0003] Therefore, various studies have been conducted to further improve breakage control from the rail bottom. For example, Patent Literature (PTL) 1 discloses a steel rail 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.

[0004] PTL 2 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 HB 320 or more.

[0005] PTL 3 discloses 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.CITATION LISTPatent Literature

[0006] PTL 1: JP 2006-57127 A PTL 2: JP H01-139724 A PTL 3: WO 2011 / 021582 SUMMARY(Technical Problem)

[0007] According to the rails in PTL 1 to 3, the microstructure of the rail bottom can be controlled and the rail bottom can be strengthened, but this alone is not sufficient to prevent cracks from occurring in the bottom of the rail or to reduce the speed of crack propagation due to tensile stress, and breakage may occur from the bottom of the rail.(Solution to Problem)

[0008] To control breakage from the bottom of the rail, it is effective to suppress the occurrence of cracks in the rail bottom, and if the rail bottom has a crack, to suppress propagation of the crack when mechanical loads are applied. The present disclosure was conceived to solve this problem, and we provide a rail, and a method of producing the same, that is capable of suppressing breakage from the bottom of the rail by controlling the chemical composition of the rail, while also controlling the 0.2 % proof stress at the rail bottom center, the residual stress in the longitudinal direction at the rail bottom center, and the crack propagation rate each to be within a predetermined range.

[0009] The primary features of the present disclosure are as follows. [1] A rail comprising: a chemical composition containing (consisting of), by mass%, C: 0.60 % or more and less than 0.90 %, Si: 0.10 % or more and 1.20 % or less, Mn: 0.10 % or more and 1.50 % or less, Cr: 0.05 % or more and 2.00 % or less, Al: 0.0002 % or more and 0.005 % or less, P: 0 % or more and 0.035 % or less, S: 0 % or more and 0.020 % or less, V: 0 % or more and 0.30 % or less, Cu: 0 % or more and 1.0 % or less, Ni: 0 % or more and 1.0 % or less, Nb: 0 % or more and 0.05 % or less, Mo: 0 % or more and 0.5 % or less, B: 0 % or more and 0.0050 % or less, Ti: 0 % or more and 0.01 % or less, Mg: 0 % or more and 0.01 % or less, Ca: 0 % or more and 0.02 % or less, W: 0 % or more and 0.10 % or less, Sb: 0 % or more and 0.05 % or less, Sn: 0 % or more and 0.05 % or less, and Co: 0 % or more and 1.0 % or less, with the balance being Fe and incidental impurities, wherein a 0.2 % proof stress at a rail bottom center is greater than 500 MPa and less than 1100 MPa, a residual stress in a longitudinal direction at the rail bottom center is less than 200 MPa, and a fatigue crack propagation rate is 5.0 × 10 -8< m / cycle or less when a stress intensity factor range ΔK at the rail bottom center is 15 MPa m 1 / 2< . [2] The rail according to [1], wherein the chemical composition further contains, by mass%, one or more selected from the group consisting of V: 0.001 % or more and 0.30 % or less, Cu: 0.001 % or more and 1.0 % or less, Ni: 0.001 % or more and 1.0 % or less, Nb: 0.001 % or more and 0.05 % or less, Mo: 0.001 % or more and 0.5 % or less, B: 0.0001 % or more and 0.0050 % or less, Ti: 0.001 % or more and 0.01 % or less, Mg: 0.0005 % or more and 0.01 % or less, Ca: 0.0005 % or more and 0.02 % or less, W: 0.001 % or more and 0.10 % or less, Sb: 0.001 % or more and 0.05 % or less, Sn: 0.001 % or more and 0.05 % or less, and Co: 0.001 % or more and 1.0 % or less. [3] A method of producing the rail according to [1] or [2], the method comprising: hot rolling a steel material having the chemical composition according to [1] or [2] and subsequently setting an average cooling rate during cooling of a rail bottom center from a cooling start point T 1 to a cooling stop point T 2 to 0.4 °C / s or more and 6.0 °C / s or less; wherein the average cooling rate of both rail bottom ends during cooling of the rail bottom center from the cooling start point T 1 to the cooling stop point T 2 is equal to or less than the average cooling rate of the rail bottom center, and T 1 is a temperature in a range of 650 °C or more and 800 °C or less, and T 2 is a temperature in a range of 400 °C or more and 600 °C or less.

[0010] Here, the 0.2 % proof stress at the rail bottom center, the residual stress in the longitudinal direction at the rail bottom center, and the fatigue crack propagation rate when a stress intensity factor range ΔK at the rail bottom center is 15 MPa m 1 / 2< can respectively be the values measured by the methods described in the Examples.(Advantageous Effect)

[0011] According to the present disclosure, it is possible to provide a rail, and a method of producing the same, that is capable of suppressing the occurrence of cracks in the rail bottom, and if the rail bottom has a crack, reducing the crack propagation rate when mechanical loads are applied, thereby suppressing breakage from the rail bottom.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In the accompanying drawings: FIG. 1 is a rail cross-section; FIG. 2 is a diagram illustrating the cross-sectional area and thickness of the bottom of a rail; FIG. 3 is a schematic diagram of an example of accelerated cooling in a method of producing a rail; FIG. 4 is a schematic diagram of another example of accelerated cooling in a method of producing a rail; FIG. 5 is a diagram illustrating a collection site of a tensile test piece; FIG. 6 is a diagram illustrating the dimensions of the tensile test piece; FIG. 7 is a diagram illustrating a collection site of a fatigue crack propagation test piece; FIG. 8 is a diagram illustrating the dimensions of the fatigue crack propagation test piece; and FIG. 9 is a diagram illustrating a method of measuring residual stress by a strain gauge cutting method. DETAILED DESCRIPTION<Rail Parts>

[0013] 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 is the head, 12 is the web, and 13 is the bottom (base).

[0014] When the width of the rail bottom surface is W, the rail bottom center (bottom center) is the widthwise center of the underside surface of the bottom and is a position 0.5 × W from each end.

[0015] The rail bottom end (bottom end) is a position 0.15 × W from each end in the width direction of the underside surface of the bottom.<Chemical Composition of Rail>

[0016] The chemical composition of the steel rail according to the present disclosure will be described. In the following description, "%" denotes "mass%" unless otherwise specified.C: 0.60 % or more and less than 0.90 %

[0017] C is an essential element for forming cementite in a pearlitic microstructure and ensuring the 0.2 % proof stress, and the 0.2 % proof stress improves as the C content increases. If the C content is less than 0.60 %, it is difficult to obtain an excellent 0.2 % proof stress and fracture toughness value. Therefore, the C content is set to 0.60 % or more. The C content is preferably 0.63 % or more. The C content is more preferably 0.70 % or more. At 0.90 % or more, pro-eutectoid cementite is formed at the austenite grain boundary during transformation after hot rolling, and crack propagation occurs more easily along the formed pro-eutectoid cementite, resulting in a significant increase in the crack propagation rate. Therefore, the C content is set to less than 0.90 %. The C content is preferably 0.89 % or less. The C content is more preferably 0.85 % or less.Si: 0.10 % or more and 1.20 % or less

[0018] Si is a deoxidizer and an element that strengthens the pearlitic microstructure, and in order to obtain the full effect of this element, 0.10 % or more is required. The Si content is preferably 0.15 % or more. The Si content is more preferably 0.20 % or more. If the Si content exceeds 1.20 %, a martensitic microstructure is likely to form, which ends up increasing the 0.2 % proof stress and the amount of Si oxides, resulting in a significant increase in the rate of crack propagation. Therefore, the Si content is set to 1.20 % or less. The Si content is preferably 1.10 % or less. The Si content is more preferably 1.00 % or less.Mn: 0.10 % or more and 1.50 % or less

[0019] Mn is an element that strengthens the pearlitic microstructure, and in order to obtain the full effect of this element, 0.10 % or more is required. The Mn content is preferably 0.20 % or more. The Mn content is more preferably 0.30 % or more. If the Mn content exceeds 1.50 %, a martensitic microstructure is likely to form due to the high hardenability of Mn, which ends up increasing the 0.2 % proof stress, resulting in a significant increase in the rate of crack propagation. Therefore, the Mn content is set to 1.50 % or less. The Mn content is preferably 1.40 % or less. The Mn content is more preferably 1.30 % or less.P: 0 % or more and 0.035 % or less

[0020] If P is contained in an amount exceeding 0.035 %, the ductility, which is a property of the rail itself, will deteriorate. Therefore, the P content is set to 0.035 % or less. The P content is preferably 0.020 % or less. No lower limit is placed on the P content, which may be 0 %, but setting the content to less than 0.001 % would inevitably increase the steelmaking costs. Hence, the P content may be set to 0.001 % or more.S: 0 % or more and 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 cleanliness of the rail steel deteriorates, and the crack propagation rate in the rail increases significantly. 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 lower limit is placed on the S content, which may be 0 %, but setting the content to less than 0.0005 % would inevitably increase the steelmaking costs. Hence, the S content may be set to 0.0005 % or more.Cr: 0.05 % or more and 2.00 % or less

[0022] Cr is an element that increases the pearlite equilibrium transformation temperature and contributes to the refinement of the lamellar spacing. At the same time, the inclusion of Cr can further increase the 0.2 % proof stress through solid solution strengthening. The Cr content is set to 0.05 % or more, since a sufficient 0.2 % proof stress cannot be obtained if the Cr content is less than 0.05 %. The Cr content is preferably 0.10 % or more. The Cr content is more preferably 0.15 % or more. If the Cr content exceeds 2.00 %, the hardenability increases, and a martensitic microstructure is likely to form, which ends up increasing the 0.2 % proof stress, resulting in a significant increase in the rate of crack propagation. Therefore, the Cr content is set to 2.00 % or less. The Cr content is preferably 1.60 % or less. The Cr content is more preferably 1.40 % or less.Al: 0.0002 % or more and 0.005 % or less

[0023] Al is a deoxidizer and an element that strengthens the pearlitic microstructure, and in order to obtain the full effect of this element, 0.0002 % or more is required. If the Al content exceeds 0.005 %, the 0.2 % proof stress and the amount of Al oxides end up increasing, resulting in a significant increase in the rate of crack propagation. Therefore, the Al content is set to 0.0002 % or more and 0.005 % or less.

[0024] In addition to the aforementioned basic components, the chemical composition of the rail in the present disclosure may optionally contain one or more selected from the group consisting of V: 0 % or more and 0.30 % or less, Cu: 0 % or more and 1.0 % or less, Ni: 0 % or more and 1.0 % or less, Nb: 0 % or more and 0.05 % or less, Mo: 0 % or more and 0.5 % or less, B: 0 % or more and 0.0050 % or less, Ti: 0 % or more and 0.01 % or less, Mg: 0 % or more and 0.01 % or less, Ca: 0 % or more and 0.02 % or less, W: 0 % or more and 0.10 % or less, Sb: 0 % or more and 0.05 % or less, Sn: 0 % or more and 0.05 % or less, and Co: 0 % or more and 1.0 % or less.

[0025] Since these are optional components and are not necessarily included, the lower limit is 0 %. On the other hand, when these elements are included, the preferred contents are as follows.V: 0.001 % or more and 0.30 % or less

[0026] V is an element that forms carbonitrides in the steel, disperses and precipitates in the matrix, and improves the 0.2 % proof stress, thereby improving the fracture toughness value. Since this effect is achieved when the V content is 0.001 % or more, the V content is preferably 0.001 % or more in the case of including V. The V content is even more preferably 0.005 % or more. On the other hand, if the V content exceeds 0.30 %, the 0.2 % proof stress increases, and the fracture toughness value ends up decreasing. The alloy cost also increases, resulting in an increased cost of the rail steel. Therefore, in the case of including V, the V content is preferably 0.30 % or less. The V content is more preferably 0.29 % or less.Cu: 0.001 % or more and 1.0 % or less

[0027] Cu is an element capable of achieving further strengthening by solid solution strengthening, as with Cr. If the Cu content is less than 0.001 %, this effect is small, whereas if the Cu content exceeds 1.0 %, Cu cracking is likely to occur. Therefore, in the case of including Cu, the Cu content is preferably 0.001 % or more. The Cu content is preferably 1.0 % or less.Ni: 0.001 % or more and 1.0 % or less

[0028] Ni is an element that can increase the strength without deteriorating the ductility. In addition, in the case of adding Cu, it is preferable also to add Ni, since the addition of Ni in combination with Cu suppresses Cu cracking. A Ni content below 0.001 % has little effect, whereas a Ni content above 1.0 % increases hardenability and leads to martensite formation, which tends to increase the rate of crack propagation significantly. Therefore, when Ni is included, the Ni content is preferably 0.001 % or more. The Ni content is preferably 1.0 % or less.Nb: 0.001 % or more and 0.05 % or less

[0029] Nb is an element that precipitates as a carbide during and after rolling in combination with C in the steel, thereby improving the fracture toughness value through an increase in 0.2 % proof stress. However, a Nb content below 0.001 % has little effect, whereas if the Nb content is above 0.05 %, an effect commensurate with the content cannot be obtained. Therefore, when Nb is included, the Nb content is preferably 0.001 % or more. The Nb content is preferably 0.05 % or less.Mo: 0.001 % or more and 0.5 % or less

[0030] Mo is an element capable of achieving further strengthening by solid solution strengthening. However, a Mo content below 0.001 % has little effect, whereas a Mo content above 0.5 % increases hardenability and leads to martensite formation, which tends to increase the rate of crack propagation significantly. Therefore, when Mo is included, the Mo content is preferably 0.001 % or more. The Mo content is preferably 0.5 % or less.B: 0.0001 % or more and 0.0050 % or less

[0031] B is an element that precipitates as a nitride and is capable of further strengthening the rail through strengthening by precipitation. A B content of less than 0.0001 % has little effect, whereas a B content exceeding 0.0050 % leads to an increased alloy cost. Therefore, when B is included, the B content is preferably 0.0001 % or more. The B content is preferably 0.0050 % or less.Ti: 0.001 % or more and 0.01 % or less

[0032] Ti is an element that precipitates as a carbide, nitride, or carbonitride and is capable of further strengthening the rail through strengthening by precipitation. A Ti content of less than 0.001 % has little effect, whereas a Ti content exceeding 0.01 % leads to an increased alloy cost. Therefore, when Ti is included, the Ti content is preferably 0.001 % or more. The Ti content is preferably 0.01 % or less.Mg: 0.0005 % or more and 0.01 % or less

[0033] Mg is an element that combines with oxygen to precipitate as MgO, thereby further enhancing strength. If the Mg content is less than 0.0005 %, the effect is small, whereas if the Mg content exceeds 0.01 %, the rate of crack propagation tends to increase significantly due to the increase in MgO. Therefore, when Mg is included, the Mg content is preferably 0.0005 % or more. The Mg content is preferably 0.01 % or less.Ca: 0.0005 % or more and 0.02 % or less

[0034] Ca is an element that combines with oxygen to precipitate as CaO, thereby further enhancing strength. If the Ca content is less than 0.0005 %, the effect is small, whereas if the Ca content exceeds 0.02 %, the rate of crack propagation tends to increase significantly due to the increase in CaO. Therefore, when Ca is included, the Ca content is preferably 0.0005 % or more. The Ca content is preferably 0.02 % or less.W: 0.001 % or more and 0.10 % or less

[0035] W is an element that precipitates as a carbide and is capable of further strengthening the rail through strengthening by precipitation. A W content of less than 0.001 % has little effect, whereas a W content exceeding 0.10 % leads to an increased alloy cost. Therefore, when W is included, the W content is preferably 0.001 % or more. The W content is preferably 0.10 % or less.Sb: 0.001 % or more and 0.05 % or less

[0036] Sb has a remarkable effect of preventing the decarburization of the steel during reheating of the 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 Sb is included, the Sb content is preferably 0.05 % or less. On the other hand, in the case in which Sb is included, the Sb content is preferably 0.001 % or more in order to exert the effect of reducing the decarburized layer.Sn: 0.001 % or more and 0.05 % or less

[0037] Sn is an element that has a remarkable effect of preventing the decarburization of the steel during reheating of the 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 Sn is included, the Sn content is preferably 0.05 % or less. On the other hand, in the case in which Sn is included, the Sn content is preferably 0.001 % or more in order to exert the effect of reducing the decarburized layer.Co: 0.001 % or more and 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 %.

[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 microstructure of the rail of the present disclosure is 95 % or more pearlite by area ratio at the rail bottom. Residual microstructures other than pearlite are acceptable if the total area ratio is 5 % or less, since the characteristics of the present disclosure are not significantly affected. Examples of residual microstructures include ferrite and bainite. Microstructures can be identified by the method described in the Examples.<0.2 % Proof Stress at Rail Bottom Center Greater Than 500 MPa and Less Than 1100 MPa>

[0041] In the rail of the present disclosure, the 0.2 % proof stress at the rail bottom center is greater than 500 MPa and less than 1100 MPa. If the 0.2 % proof stress is 500 MPa or less, the resistance to cracking is weak when a wheel passes over the rail, and cracks are more likely to occur at the rail bottom due to stress generated within the rail. On the other hand, if the 0.2 % proof stress exceeds 1100 MPa, the strength of the rail bottom increases, which ends up increasing the susceptibility to crack growth after crack initiation, making the rail more susceptible to breakage. Therefore, the 0.2 % proof stress at the rail bottom center is set greater than 500 MPa. The 0.2 % proof stress is preferably 550 MPa or more. The 0.2 % proof stress at the rail bottom center is set to less than 1100 MPa. The 0.2 % proof stress is preferably 1000 MPa or less.<Fatigue Crack Propagation Rate of 5.0 × 10 -8< m / cycle or Less When Stress Intensity Factor Range ΔK at Rail Bottom Center is 15 MPa m 1 / 2< >

[0042] In the rail of the present disclosure, the fatigue crack propagation rate is 5.0 × 10 -8< m / cycle or less when the stress intensity factor range ΔK at the rail bottom center is 15 MPa m 1 / 2< . If the fatigue crack propagation rate when the stress intensity factor range ΔK is 15 MPa m 1 / 2< (hereinafter also referred to as fatigue crack propagation rate) exceeds 5.0 × 10 -8< m / cycle, the crack propagation rate is too fast for a crack at the rail bottom, and the possibility of rail breakage before periodic rail inspection or replacement increases. Therefore, the fatigue crack propagation rate at the rail bottom center is set to 5.0 × 10 -8< m / cycle or less. The fatigue crack propagation rate is preferably 4.8 × 10 -8< m / cycle or less. No lower limit is placed on the fatigue crack propagation rate at the rail bottom center, but the fatigue crack propagation rate can be 1.0 × 10 -8< m / cycle or more from the perspective of the lamellar spacing that can be industrially refined.<Residual Stress in Longitudinal Direction at Rail Bottom Center of Less Than 200 MPa>

[0043] In the rail of the present disclosure, the residual stress in the longitudinal direction at the rail bottom center is less than 200 MPa. If the residual stress at the rail bottom center is less than 200 MPa, it is easier to avoid a situation in which a large tensile residual stress acts on the rail bottom and accelerates the fatigue crack propagation rate, resulting in rail breakage. The residual stress at the rail bottom center is more preferably 150 MPa or less. On the other hand, strong compressive residual stress can be obtained by cooling the bottom of the rail to a lower temperature, but this ends up reducing production efficiency. Therefore, the residual stress at the rail bottom center is preferably -100 MPa or more.<Shape of Rail>

[0044] 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>

[0045] A method of producing the rail of the present disclosure is now described. For the rail of the present disclosure, a steel material (bloom) having the above-described chemical composition is hot rolled, after which the average cooling rate during cooling of the rail bottom center from a cooling start point T 1 to a cooling stop point T 2 is set to 0.4 °C / s or more and 6.0 °C / s or less, and the average cooling rate of both rail bottom ends during cooling of the rail bottom center from the cooling start point T 1 to the cooling stop point T 2 is equal to or less than the average cooling rate of the rail bottom center.

[0046] Here, the cooling start point T 1 is a temperature in a range of 650 °C or more and 800 °C or less.

[0047] The cooling start point T 1 and the cooling stop point T 2 are the surface temperatures at the rail bottom center measured by a radiation thermometer, and the average cooling rate of the rail bottom center is the difference between the cooling start point T 1 and cooling stop point T 2 at the bottom center divided by the time required for cooling.

[0048] The average cooling rate of the rail bottom ends is calculated by dividing the difference between the surface temperatures at each rail bottom end, measured with a radiation thermometer at the points in time corresponding to the cooling start point T 1 and cooling stop point T 2 at the rail bottom center, by the time required to cool the rail bottom center from the cooling start point T 1 to the cooling stop point T 2 .

[0049] The two rail bottom ends are usually cooled so that the thermal hysteresis is the same. The temperature of each rail bottom end corresponding to the cooling start point T 1 and cooling stop point T 2 at the rail bottom center is usually the same, and the average cooling rate of each rail bottom end is also usually the same.(Hot Rolling)

[0050] The steel material (bloom) used as rail material has the chemical composition of the rail described above and can be produced by any method. The steel material can be produced by casting, particularly continuous casting. For example, steel can be melted in a converter or electric furnace, and the chemical composition of the steel can be adjusted to the above ranges through secondary refining such as degassing if necessary. The melted steel can then be continuously cast into a steel material (bloom).

[0051] It is preferable to heat the continuous-cast steel before subjecting it to hot rolling. For example, the steel material can be heated in a heating furnace to 1200 °C or more and 1350 °C or less and then hot rolled. In the hot rolling process, hot rolling can be performed by a breakdown mill, rougher, and finishing mill. In this hot rolling process, the microstructure of austenite grains coarsened by heating can be refined by rolling-recrystallization in the recrystallization temperature range, or the microstructure after pearlite transformation can be refined by introducing strain in the non-recrystallization temperature range.

[0052] In order to refine the microstructure after pearlite transformation, the rolling finish temperature of the bottom (base) during hot rolling is preferably 800 °C or more. The rolling finish temperature is preferably 1000 °C or less.

[0053] Furthermore, hot rolling is preferably performed so that the reduction in area at the bottom during rolling at 1050 °C or less is 11 % or more. The reduction in area is more preferably 13 % or more. The reduction in area can be 20 % or less. The reduction in area is preferably 18 % or less.

[0054] The reduction in area at 1050 °C or less is the value calculated as reduction in area (%) = {(S 0 - S 1 ) / S 0 } × 100, where S 0 is the cross-sectional area of the bottom at the point when the temperature reaches 1050 °C or less, and S 1 is the cross-sectional area of the bottom after finish rolling.

[0055] The rolling reduction (thickness reduction) in the thickness direction of the bottom during the finish rolling, which is the final rolling pass, is preferably the same as or greater than the reduction in area during this pass. For example, if the reduction in area in finish rolling is set to 11 %, the thickness reduction at the bottom center in finish rolling may be set to 15 % or more. This can introduce a large strain in the bottom, which can further promote the refinement of the microstructure after pearlite transformation and effectively increase the fatigue strength of the underside surface of the bottom. By designing the caliber so that the width dimension of the bottom is widened during this finish rolling, rolling with relatively large thickness reduction in the finish rolling compared to the reduction in area in the finish rolling can be performed stably.

[0056] The difference between the thickness reduction and the reduction in area in the finish rolling (thickness reduction - reduction in area) is preferably 4 % or more, and more preferably 5 % or more. The difference is preferably 8 % or less, and more preferably 7 % or less.

[0057] Here, the thickness reduction in the finish rolling 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. Here, finish rolling refers to one pass of final rolling, performed using the final rolling caliber of one final rolling mill.

[0058] Finishing rolling under these conditions makes it easier to produce a rail with a 0.2 % proof stress of greater than 500 MPa and less than 1100 MPa at the bottom and a fatigue crack propagation rate of 5.0 × 10 -8< m / cycle or less when the stress intensity factor range ΔK is 15 MPa m 1 / 2< at the rail bottom. This set of conditions is also effective in achieving a residual stress in the longitudinal direction at the widthwise central portion of the rail bottom surface of less than 200 MPa.

[0059] Here, the cross-sectional area of the bottom of the rail is the area of the shaded portion marked S in the cross-sectional view of the rail in FIG. 2, and the thickness of the bottom of the rail is the height marked H in the same drawing.

[0060] The other conditions for hot rolling are not limited.(Accelerated Cooling)

[0061] In the method of production of the present disclosure, cooling after hot rolling is performed as accelerated cooling to achieve a predetermined average cooling rate of the rail bottom center and each rail bottom end.

[0062] The cooling start point T 1 of the accelerated cooling is a temperature in a range of 650 °C or more and 800 °C or less. If the cooling start temperature is less than 650 °C, the pearlite transformation temperature increases, and the 0.2 % proof stress decreases. On the other hand, when cooling from above 800 °C, martensite tends to form at the bottom of the rail, which tends to increase the crack propagation rate due to an increase in the 0.2 % proof stress. The cooling start temperature is therefore 650 °C or more and 800 °C or less.

[0063] The cooling stop point T 2 is a temperature in the range of 400 °C or more and 600 °C or less. If the cooling stop temperature is less than 400 °C, cooling is necessary even after completion of pearlite transformation, which increases cooling time and reduces productivity. On the other hand, at temperatures above 600 °C, cooling is stopped when the pearlite transformation is not yet complete, resulting in a reduction in the 0.2 % proof stress. The cooling stop temperature is therefore 400 °C or more and 600 °C or less.

[0064] The average cooling rate from the cooling start point T 1 to the cooling stop point T 2 is 0.4 °C / s or more and 6.0 °C / s or less. If the average cooling rate is less than 0.4 °C / s, the 0.2 % proof stress at the rail bottom decreases and the tensile residual stress increases, leading to an increase in the fatigue crack propagation rate. On the other hand, if the temperature exceeds 6.0 °C / s, the 0.2 % proof stress at the rail bottom ends up increasing, which leads to an increase in the fatigue crack propagation rate. Therefore, the average cooling rate during accelerated cooling is set to 0.4 °C / s or more and 6.0 °C / s or less. The average cooling rate is preferably 2.9 °C / s or less.

[0065] Since the rail bottom center is thicker than other parts of the rail bottom and is prone to heat accumulation, a large residual stress representing a tensile stress in the longitudinal direction is relatively likely to occur in the rail bottom center when room temperature is reached after cooling is completed, which may increase the fatigue crack propagation rate in the rail bottom. To suppress this, it is effective in the production method of the present disclosure to cool the rail bottom center more intensively than the ends, which are thinner than the rest of the rail bottom, and to set the average cooling rate of the rail bottom center to be equal to or greater than the corresponding average cooling rate of each rail bottom end. From this perspective, the average cooling rate of both rail bottom ends during cooling of the rail bottom center from the cooling start point T 1 to the cooling stop point T 2 is set to be equal to or less than the average cooling rate of the rail bottom center. The average cooling rate of each rail bottom end is preferably 0.4 °C / s or more. The average cooling rate of each rail bottom end is preferably 2.5 °C / s or less. The average cooling rate of each rail bottom end is preferably (V 1 - 0)°C / s to (V 1 - 1.5)°C / s, where V 1 is the average cooling rate of the rail bottom center.

[0066] The method of accelerated cooling is not limited and can be performed by, for example, a cooling method 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.

[0067] The rail bottom center can be cooled more aggressively than each rail bottom end to ensure that the average cooling rates of the rail bottom center and each rail bottom end satisfy the predetermined conditions. For example, as illustrated in FIG. 3, a nozzle 21 can be used to directly spray coolant onto an area that is 1 / 5 of the rail bottom width W (0.2 W) centered on the rail bottom center, while other parts of the bottom, such as the rail bottom ends, are cooled by the coolant flowing along the bottom surface. In FIG. 3, the range for direct coolant spraying is set to 1 / 5 of the rail bottom width W, but the range for direct coolant spraying can be changed to about 0.1 to 0.5 times W by selecting nozzles. Another effective method is to use nozzles 22 to inject coolant from the rail web side toward the rail bottom center, as illustrated in FIG. 4.

[0068] Cooling of other parts of the rail (such as the rail head) is not limited, and these parts may be allowed to cool naturally or may be subjected to accelerated cooling.(Other Conditions)

[0069] After cooling, the rail material may be subjected to known treatments, such as cold roller straightening.EXAMPLES

[0070] The present disclosure will be described more specifically by means of examples below, but the present disclosure is not limited by the examples.

[0071] Slabs with the chemical compositions listed in Table 1 were heated to 1250 °C and then hot rolled into a rail shape at the rolling finish temperatures listed in Table 2. The shape of the rail was that of a 60 kg rail as described in JIS E 1101:2001. At this time, the reduction in area at the bottom during rolling at temperatures of 1050 °C or less was set to satisfy the values listed in Table 2. The rolling reduction (thickness reduction) in the thickness direction of the bottom during the finish rolling, which is the final rolling pass, was set to 15 % or more, which is a value greater than the reduction in area during this pass, as described above.

[0072] The resulting rails were then transported to an on-line heat treatment facility, where the rail bottom center was cooled at the average cooling rates listed in Table 2. The rails were then transported to a cooling bed and allowed to cool naturally to room temperature.

[0073] Here, for the cooling of the base, the air nozzles were selected as appropriate and the cooling was performed with a concentrated injection of coolant over a range of 0.1 × W to 0.5 × W relative to the bottom width W, centered on the rail bottom center, as illustrated in FIG. 3. However, for Test Nos. 31, 41, and 46, the coolant was injected over almost the entire surface of the bottom. For Test Nos. 3, 4, 9, 10, 11, 20, 21 and 47, cooling was also performed from the web side, as illustrated in FIG. 4.

[0074] Here, the cooling start point and cooling stop point in Table 2 are the surface temperatures at the corresponding position measured using a two-dimensional radiation thermometer capable of measuring the temperature distribution at the rail bottom center, and the average cooling rate is the average cooling rate between these temperatures.

[0075] The average cooling temperature of the rail ends is calculated by measuring the temperature of one rail bottom end (at a position 0.15 × W from one end) with a spot thermometer and dividing by the accelerated cooling time. Since the cooling conditions for the rail ends are uniform on both sides of the rail center, the measured values at one rail bottom end were used.

[0076] A tensile test, fatigue crack propagation test, residual stress test, and fatigue test were conducted in the following manner on the rails obtained as described above.<Tensile Test>

[0077] As illustrated in FIG. 5, a tensile test piece was collected from a position including the rail bottom center, and a tensile test was conducted according to JIS Z2241. FIG. 6 illustrates the shape of the test piece. After tensile testing, the 0.2 % proof stress was determined.<Fatigue Crack Propagation Test>

[0078] As illustrated in FIG. 7, a test piece for a fatigue crack propagation test was collected from a position including the rail bottom center. FIG. 8 illustrates the shape of the test piece. In FIG. 8, the test piece is, for example, a plate with a width W of 20 mm, a height H of 100 mm, and a thickness B of 5 mm, with a notch in the middle of the height H (H / 2), a length L of the notch of 2 mm, a width C of 0.2 mm, and a curvature R of 0.1 mm at the edge of the notch. A fatigue crack propagation test was conducted on this test piece under a set of conditions including a stress ratio R of 0.1, and the fatigue crack propagation rate da / dN (m / cycle) at ΔK = 15 MPa m 1 / 2< was measured.<Residual Stress Test>

[0079] Residual stress in the rail was measured by a strain gauge cutting method. Specifically, a 2 m long rail was collected, and at the 1 m position in the longitudinal center, a strain gauge with a gauge length of 3 mm, resistance of 120 Ω, and gauge factor of 2 was affixed to the widthwise center of the rail bottom surface illustrated in FIG. 9 to measure the initial strain. Next, saw cutting was performed at the position illustrated in FIG. 9, and the strain after sawing was measured. The residual stress in the longitudinal direction of the rail was calculated from the change in strain before and after sawing. If the residual stress in the longitudinal direction of the rail is less than 200 MPa, the residual stress can be considered good.<Fatigue Test>

[0080] A fatigue test was evaluated as follows. The obtained rail was cut to 1500 mm, and a three-point bending test was performed repeatedly in a head-up position with the rail head up and the rail bottom down, using a "Pulsator 250 PUS" produced by Maekawa Testing Machine MFG. Co. Specifically, three-point bending was performed 3.5 million times under a set of conditions including a test load of 1000 kN, a repetition rate of 600 rpm, and a bending support interval of 1000 mm, and the presence or absence of fracture was visually confirmed. For rails that did not fracture (unfractured rails), the presence or absence of cracks at the rail bottom was visually confirmed after the fatigue test.

[0081] Unfractured rails can be judged to have good fatigue resistance, indicating that the rail is capable of suppressing breakage from the rail bottom. Among these rails, rails that are free of visible cracks have excellent properties.<Microstructure>

[0082] A test piece for microstructure observation was cut from a position including the rail bottom center of the produced rail at a depth of 1 mm from the surface. After mirror polishing, the test piece was corroded with 1 % alcohol nitrate (1 % nital), and the microstructure was observed by optical microscopy at 100x magnification (1200 µm × 950 µm per field of view; number of fields of view: 10). If there was a microstructure other than pearlite, the microstructure was photographed, the structures other than the pearlitic microstructure were traced in the obtained micrograph, and the microstructure proportion other than the pearlitic microstructure was calculated based on the following formula using the image interpretation software Image-Pro (Nippon Roper, K.K.).

[0083] It is clear from Table 2 that the rails of the Examples all have good properties, with the 0.2 % proof stress at the rail bottom center being greater than 500 MPa and less than 1100 MPa, the residual stress in the longitudinal direction at the rail bottom center being less than 200 MPa, and the fatigue crack propagation rate being 5.0 × 10 -8< m / cycle or less when the stress intensity factor range ΔK at the rail bottom center is 15 MPa m 1 / 2< .INDUSTRIAL APPLICABILITY

[0084] According to the present disclosure, it is possible to provide a rail, and a method of producing the same, that is capable of suppressing the occurrence of cracks in the rail bottom, and if the rail bottom has a crack, reducing the crack propagation rate when mechanical loads are applied, thereby suppressing breakage from the rail bottom.REFERENCE SIGNS LIST

[0085] 1 Rail 11 Head 12 Web 13 Bottom (base) 21 Nozzle 22 Nozzle Pcenter Rail bottom center Pend Rail bottom end

Claims

1. A rail comprising: a chemical composition containing, by mass%, C: 0.60 % or more and less than 0.90 %, Si: 0.10 % or more and 1.20 % or less, Mn: 0.10 % or more and 1.50 % or less, Cr: 0.05 % or more and 2.00 % or less, Al: 0.0002 % or more and 0.005 % or less, P: 0 % or more and 0.035 % or less, S: 0 % or more and 0.020 % or less, V: 0 % or more and 0.30 % or less, Cu: 0 % or more and 1.0 % or less, Ni: 0 % or more and 1.0 % or less, Nb: 0 % or more and 0.05 % or less, Mo: 0 % or more and 0.5 % or less, B: 0 % or more and 0.0050 % or less, Ti: 0 % or more and 0.01 % or less, Mg: 0 % or more and 0.01 % or less, Ca: 0 % or more and 0.02 % or less, W: 0 % or more and 0.10 % or less, Sb: 0 % or more and 0.05 % or less, Sn: 0 % or more and 0.05 % or less, and Co: 0 % or more and 1.0 % or less, with the balance being Fe and incidental impurities, wherein a 0.2 % proof stress at a rail bottom center is greater than 500 MPa and less than 1100 MPa, a residual stress in a longitudinal direction at the rail bottom center is less than 200 MPa, and a fatigue crack propagation rate is 5.0 × 10-8 m / cycle or less when a stress intensity factor range ΔK at the rail bottom center is 15 MPa m1 / 2.

2. The rail according to claim 1, wherein the chemical composition further contains, by mass%, one or more selected from the group consisting of V: 0.001 % or more and 0.30 % or less, Cu: 0.001 % or more and 1.0 % or less, Ni: 0.001 % or more and 1.0 % or less, Nb: 0.001 % or more and 0.05 % or less, Mo: 0.001 % or more and 0.5 % or less, B: 0.0001 % or more and 0.0050 % or less, Ti: 0.001 % or more and 0.01 % or less, Mg: 0.0005 % or more and 0.01 % or less, Ca: 0.0005 % or more and 0.02 % or less, W: 0.001 % or more and 0.10 % or less, Sb: 0.001 % or more and 0.05 % or less, Sn: 0.001 % or more and 0.05 % or less, and Co: 0.001 % or more and 1.0 % or less.

3. A method of producing the rail according to claim 1 or 2, the method comprising: hot rolling a steel material having the chemical composition according to claim 1 or 2 and subsequently setting an average cooling rate during cooling of a rail bottom center from a cooling start point T1 to a cooling stop point T2 to 0.4 °C / s or more and 6.0 °C / s or less; wherein the average cooling rate of both rail bottom ends during cooling of the rail bottom center from the cooling start point T1 to the cooling stop point T2 is equal to or less than the average cooling rate of the rail bottom center, and T1 is a temperature in a range of 650 °C or more and 800 °C or less, and T2 is a temperature in a range of 400 °C or more and 600 °C or less.

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