rail

Abstract

A rail according to one embodiment of the present invention is configured such that: the metal structure at a depth of 25 mm from the outer contour surface of the head part contains a pearlite structure in an area ratio of 95% or more; the hardness of the rail is within the range of Hv 360 to 500 as measured at a depth of 25 mm from the outer contour surface of the head part; in a ferrite phase in the pearlite structure at a depth of 25 mm from the outer contour surface of the head part, the number density of nitrides having a particle diameter of 0.5 to 6.0 nm and containing Cr, Mn and V is within the range of 1.0 × 1017 to 5.0 × 1017 per 1 cm3; and with respect to the nitrides, the value obtained by rounding off the average of the ratios of the sum of the number (CA) of Cr atoms and the number (MA) of Mn atoms to the number (VA) of V atoms to the nearest whole number satisfies the relational expression 5 ≤ (CA + MA) / VA ≤ 100.

Description

[Technical Field]

[0001] This invention relates to rails. [Background technology]

[0002] In recent years, overseas freight railways transporting natural resources such as iron ore and coal, as well as grains, have been increasing the loading capacity of freight cars and becoming more crowded in order to improve transportation efficiency.

[0003] As a result of increased freight car loads and increased congestion, freight railways are subjected to even greater repeated loads, creating a track environment where fatigue damage occurs not only from the surface of the rail head but also from within the rail head (at a depth of 20-30 mm from the outer surface of the rail head).

[0004] Against this backdrop, there has been a growing need for the development of high-strength rails with improved wear resistance and resistance to internal fatigue damage.

[0005] To address the above issues, high-strength rails such as those shown in Patent Documents 1, 2, 3, or 4 have been proposed. The main features of these rails are that, in addition to improving wear resistance, they improve resistance to internal fatigue damage by either adding a small amount of alloy to control pearlite transformation, or by generating precipitates in the pearlite structure through alloy control or the addition of a small amount of alloy, thereby improving the hardness inside the head.

[0006] Specifically, Patent Document 1 discloses that by adding B to hypereutectoid steel (C: greater than 0.85 to 1.20%), the hardness inside the head can be improved by controlling the transformation temperature of the pearlite structure inside the head.

[0007] Next, Patent Document 2 discloses that the hardness inside the head is improved by adding V and N to hypereutectoid steel (C: greater than 0.85 to 1.20%) to precipitate carbonitrides of V in the pearlite structure.

[0008] In addition, Patent Document 3 discloses that, based on eutectoid steel (C: 0.73 to 0.85%), the hardness inside the head is improved by controlling the contents of Mn and Cr.

[0009] Furthermore, Patent Document 4 discloses that, by adding V to steel with C: 0.75 to 1.20% to form V nitrides containing Cr in ferrite in the pearlite structure, the hardness inside the head at a position 25 mm deep starting from the outer surface of the head is improved.

[0010] According to the techniques of Patent Documents 1, 2, 3, or 4, by controlling the pearlite transformation temperature inside the head and precipitation strengthening of the pearlite structure, the hardness inside the head can be improved, and the resistance to internal fatigue damage can be improved within a certain range.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0012] However, even in the high-strength rails disclosed in Patent Documents 1, 2, 3, and 4, with the increase in the loading capacity and densification of freight cars, in use in a track environment where they are subjected to an even greater repeated load, sufficient characteristics cannot be obtained, and further improvement of the resistance to internal fatigue damage has been an issue.

[0013] As mentioned above, high-strength rails with excellent resistance to internal fatigue damage, suitable for use in freight railways where the track environment has become subject to even greater repeated loads due to the increasing load capacity and congestion of freight cars, are not yet available.

[0014] This invention was devised in view of the above-mentioned problems, and aims to provide rails with superior resistance to internal fatigue damage compared to existing technologies for freight railways, which in recent years have seen increased load capacity and congestion, resulting in track environments subjected to even greater repeated loads. [Means for solving the problem]

[0015] The gist of this invention is as follows:

[0016] (1) A rail according to one aspect of the present invention has the following composition in unit mass%, C: 0.75~1.20%, Si: 0.10~2.00%, Mn: 0.05~2.00%, Cr: 0.05~2.00%, V: 0.005~0.100%, Al: 0.0010~1.0000%, N: 0.006~0.020%, P≦0.025%, S≦0.025%, Mo: 0~0.50%, Co: 0~1.00%, B: 0~0.0050%, Cu: 0~1.00%, Ni: 0~1.00%, Nb: 0~0.0500%, Ti: 0~0.0500%, Mg: 0~0.0200%, Ca: 0~ It contains 0.0200%, REM: 0~0.0500%, and Zr: 0~0.0200%, with the remainder being Fe and impurities, the metal structure at a depth of 25 mm from the outer surface of the head contains a pearlite structure with an area ratio of 95% or more, the hardness of the rail measured at the said position at a depth of 25 mm from the outer surface of the head is in the range of Hv360~500, and in the ferrite phase of the pearlite structure at the said position at a depth of 25 mm from the outer surface of the head, the number density of nitrides containing Cr, Mn, and V with a particle size of 0.5~6.0 nm is 1 cm 3 1.0 x 10 17 ~5.0×10 17In the nitride containing Cr, Mn, and V with a particle size of 0.5 to 6.0 nm, located in the ferrite phase of the pearlite structure at a depth of 25 mm from the outer surface of the head, the average value of the ratio of the sum of the number of atoms of Cr (CA) and the number of atoms of Mn (MA) to the number of atoms of V (VA), rounded to the first decimal place ((CA+MA) / VA), satisfies the following formula 1. 5 ≤ (CA + MA) / VA ≤ 100 ... Equation 1 (2) The rail described in (1) above may contain one or more of the following in unit mass%, in the following proportions: Mo: 0.01~0.50%, Co: 0.01~1.00%, B: 0.0001~0.0050%, Cu: 0.01~1.00%, Ni: 0.01~1.00%, Nb: 0.0010~0.0500%, Ti: 0.0030~0.0500%, Mg: 0.0005~0.0200%, Ca: 0.0005~0.0200%, REM: 0.0005~0.0500%, and Zr: 0.0001~0.0200%. [Effects of the Invention]

[0017] According to the above embodiment of the present invention, the resistance of rails to internal fatigue damage can be improved. Furthermore, such rails can significantly extend the service life of rails used in freight railways, where the track environment is subject to even greater repeated loads due to higher load capacities and increased density. [Brief explanation of the drawing]

[0018] [Figure 1] This figure shows the position (indicated by X) on the outer surface of the head where the temperature was measured during rolling and heat treatment in the experiments and examples described herein, and the position (indicated by Y) at a depth of 25 mm from the outer surface of the head where the metal structure, hardness, and precipitate state were evaluated. [Figure 2] This is a diagram illustrating the layout of a rolling fatigue testing machine. [Figure 3] This figure shows the effect of nitrides containing Cr, Mn, and V on rolling fatigue test results. [Figure 4]This figure shows the relationship between the number of nitrides containing Cr, Mn, and V, and the cumulative tonnage at which cracks occurred. [Figure 5] This diagram summarizes the relationship between the ratio of the sum of the number of atoms of Cr (CA) and Mn (MA) to the number of atoms of V (VA) ((CA+MA) / VA) and the number of microcracks generated. [Figure 6] This diagram shows the designations for the rail heads. [Figure 7] This diagram shows the position at a depth of 25 mm from the outer surface of the rail head. [Modes for carrying out the invention]

[0019] A rail with excellent resistance to internal fatigue damage according to one embodiment of the present invention (sometimes referred to as the rail according to this embodiment) will be described in detail below. Hereinafter, mass % in the composition will be simply indicated as %.

[0020] The rail according to this embodiment has the following features. (i) It has a predetermined chemical composition. (ii) The metallic structure at a depth of 25 mm from the outer surface of the head contains 95% or more pearlite by area, and the hardness of the rail measured at that location is in the range of Hv360 to 500. (iii) In the ferrite phase of the pearlite structure at a depth of 25 mm from the outer surface of the head, the number density of nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm is 1 cm 3 1.0 x 10 17 ~5.0×10 17 It is within the range of an individual. (iv) Furthermore, in the nitride containing Cr, Mn, and V with a particle size of 0.5 to 6.0 nm, located in the ferrite phase of the pearlite structure at a depth of 25 mm from the outer surface of the head, the average value of the ratio of the sum of the number of atoms of Cr (CA) and the number of atoms of Mn (MA) to the number of atoms of V (VA), rounded to the first decimal place ((CA+MA) / VA), satisfies the following equation 1. 5 ≤ (CA + MA) / VA ≤ 100 … Equation 1 In some cases, the average value of (CA+MA) / VA for V nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm, rounded to one decimal place, may simply be written as "(CA+MA) / VA".

[0021] The inventors have discovered a technique that can improve the internal fatigue damage resistance of rails compared to existing techniques by generating fine nitrides containing the aforementioned Cr, Mn, and V in the ferrite phase of the pearlite structure at a depth of 25 mm from the outer surface of the head. Pearlite is a layered aggregate of ferrite and cementite formed by the eutectoid transformation of austenite. The term "ferrite phase in pearlite structure" refers to the layered ferrite contained in pearlite. The term "ferrite phase in pearlite structure" is a concept that does not include ferrite that does not constitute pearlite, such as proterecution ferrite. The following describes some typical experimental procedures.

[0022] <Internal fatigue damage resistance of a rail in which nitrides containing Cr, Mn, and V were formed at a depth of 25 mm starting from the outer surface of the head (see Figure 3)> First, the inventors manufactured two types of rails, namely the "inventive steel rail" and the "comparative steel rail," using the chemical composition, rolling, and heat treatment conditions shown below, and investigated their resistance to internal fatigue damage.

[0023] [Chemical composition and manufacturing conditions of the inventive steel rail] ●Chemical components 0.85%C-0.60%Si-0.90%Mn-0.45%Cr-0.013%P-0.010%S-0.050%V-0.012%N (remainder: Fe and impurities) ● Rail shape 136 pounds (weight: 67 kg / m) ● Casting conditions Casting speed in the temperature range of 1300-1400°C: 0.80 m / min Heating rate of the cast slab: 5°C / min within the range of 1000-1200°C. Casting slab heating completion temperature: 1250℃ ●Rolling conditions (The rolling temperature was measured at the position marked with the symbol X in Figure 1, i.e., at the outer surface of the head.) Cross-sectional reduction rate of the outer surface of the head at a rolling temperature of 1020°C: 28% Cross-sectional reduction rate of the outer surface of the head at a rolling temperature of 980°C: 15% ● Heat treatment conditions (For temperature control of each cooling process, the temperature measured at the outer surface position of the head shown in Figure 1 was used.) Heat treatment conditions: After rolling, accelerated cooling + controlled cooling Accelerated cooling conditions (head outer surface): Accelerated cooling start temperature 790°C, accelerated cooling end temperature 580°C, and average cooling rate 6°C / sec Controlled cooling conditions (head outer surface): Holding temperature 600°C, rail surface temperature fluctuation range of 40°C during temperature holding, temperature holding time 80 sec, followed by accelerated cooling. Temperature maintenance during controlled cooling: Temperature is controlled by controlling the accelerated cooling rate, and further by repeatedly starting and stopping accelerated cooling, and by performing accelerated cooling in response to reheating from inside the rails. ●Metal structure (measured at the location marked with the symbol Y in Figure 1, i.e., at a depth of 25 mm from the outer surface of the head) Perlite ●Hardness (measured at a depth of 25 mm from the outer surface of the head shown in Figure 1) Hv400

[0024] [Chemical composition and manufacturing conditions of comparative steel rails] ●Chemical components 0.85%C-0.60%Si-0.90%Mn-0.45%Cr-0.013%P-0.010%S-0.050%V-0.004%N (remainder: Fe and impurities) ● Rail shape 136 pounds (weight: 67 kg / m) ● Casting conditions Casting speed in the temperature range of 1300-1400°C: 0.80 m / min Heating rate of the cast slab: 6°C / min within the range of 1000-1200°C. Casting slab heating completion temperature: 1250℃ ●Rolling conditions (The rolling temperature was measured at the outer surface position of the head shown in Figure 1) Cross-sectional reduction rate of the outer surface of the head at a rolling temperature of 1020°C: 27% Cross-sectional reduction rate of the outer surface of the head at a rolling temperature of 980°C: 16% ● Heat treatment conditions (For temperature control of each cooling process, the temperature measured at the outer surface position of the head shown in Figure 1 was used.) Heat treatment conditions: After rolling, accelerated cooling + controlled cooling Accelerated cooling conditions (head outer surface): Accelerated cooling start temperature 790°C, accelerated cooling end temperature 580°C, average cooling rate 6°C / sec Controlled cooling conditions (head outer surface): Holding temperature 600°C, rail surface temperature fluctuation range of 40°C during temperature holding, temperature holding time 80 sec, followed by accelerated cooling. Temperature maintenance during controlled cooling: Temperature is controlled by controlling the accelerated cooling rate, and further by repeatedly starting and stopping accelerated cooling, and by performing accelerated cooling in response to reheating from inside the rails. ● Metalloid structure (measured at a depth of 25 mm from the outer surface of the head shown in Figure 1) Perlite ●Hardness (measured at a depth of 25 mm from the outer surface of the head shown in Figure 1) Hv400

[0025] The inventive steel rail and the comparative steel rail were evaluated for internal fatigue damage resistance using the rolling fatigue testing machine shown in Figure 2. Details of the rolling fatigue test conditions and evaluation method are described below. In particular, to reproduce the track environment in recent freight railways, which are subjected to even greater repeated loads due to higher load capacities and increased congestion, the load was set higher than the conventional conditions (reference: International Publication No. WO2020 / 054339).

[0026] [Fatigue test conditions for rolling] ● Test conditions Testing machine: Rolling fatigue testing machine (see Figure 2) Test specimen shape: Rail: 136 lb rail x 2 m Wheels: AAR type (920mm diameter) Load: Radial: 290-340KN, Thrust: 60-90KN (Reproduces load conditions on tracks with higher loads than conventional tracks) Lubrication: oil lubrication Cumulative tonnage passed through: Until crack initiation (maximum 200 MGT*) *MGT: Million Gloss Tonnage, the total weight of the freight cars running on the rails. In this test, it was evaluated as twice the passing weight acting on the wheels. ●Evaluation An ultrasonic flaw detection device was used to investigate the presence of cracks inside the rail head along its entire length. Cracks with a length of 0.5 mm or more were considered damaged, and the cumulative tonnage passed through the rail until crack initiation was used as an evaluation index for internal fatigue damage resistance. Three evaluation units were used for each test.

[0027] As a result, as shown in Figure 3, it was confirmed that the inventive steel rail has superior resistance to internal fatigue damage in high load ranges compared to the comparative steel rail.

[0028] The microstructure of the inventive steel rail and the comparative steel rail, which underwent rolling fatigue testing, was investigated in detail at a depth of 25 mm from the outer surface of the rail head (see Figure 1). As a result, it was confirmed that nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm were formed in the ferrite phase of the pearlite structure inside the rail head of the inventive steel rail. On the other hand, nitrides containing Cr, Mn, and V were not formed in the ferrite phase of the pearlite structure inside the rail head of the comparative steel rail. From this, it was found that by generating fine nitrides containing Cr, Mn, and V in the ferrite phase of the pearlite structure inside the rail head at a depth of 25 mm from the outer surface of the rail head, excellent resistance to internal fatigue damage can be imparted to the rail.

[0029] The above effects are considered to be obtained by generating nitrides containing Cr, Mn, and V with a particle size of 0.5 to 6.0 nm in the ferrite phase in the pearlite structure, suppressing the generation of microscopic softening parts in the ferrite in the pearlite structure, and making the material strength uniform within the cross-section inside the head. By making the material strength uniform within the cross-section inside the head and eliminating the microscopic softening parts of the ferrite phase in the pearlite structure, it is considered that the concentration of strain on the microscopic softening parts of the ferrite phase inside the head and the generation of fatigue cracks, which occur when the rail contacts the wheel, are suppressed.

[0030] Regarding the reason why the nitrides containing Cr, Mn, and V exhibit high resistance to internal fatigue damage, the nitrides containing Cr, Mn, and V are considered to have higher stability against heat and stress than carbides and simple V nitrides, suppress the microscopic softening of the ferrite phase in the pearlite structure inside the head, and can stably improve the hardness of the ferrite phase in the pearlite structure.

[0031] <Reason for limiting the particle size and number density of nitrides containing Cr, Mn, and V at a position 25 mm deep from the head outer surface (see Fig. 4)> Next, in the cross-section at a position 25 mm deep from the head outer surface, the number density of nitrides containing Cr, Mn, and V with a particle size of 0.5 to 6.0 nm is 1 cm 3 per 1.0×10 17 ~5.0×10 17 The reason for limiting it to the range of particles will be explained.

[0032] The inventors investigated the generation status of nitrides containing Cr, Mn, and V at a position 25 mm deep from the head outer surface in rails showing excellent resistance to internal fatigue damage.

[0033] Steel with a chemical composition of 0.85%C-0.60%Si-0.90%Mn-0.45%Cr-0.013%P-0.010%S-0.080%V-0.012%N (by mass%) was prepared as steel rails under the casting, rolling, and heat treatment conditions shown below. Next, the number of nitrides containing Cr, Mn, and V at a depth of 25 mm starting from the outer surface of the head was varied, and the effect on resistance to internal fatigue damage was investigated. The composition, casting conditions, rolling conditions, and heat treatment conditions of each rail used in the test are shown below. The resistance to internal fatigue damage was evaluated using a rolling fatigue tester, and the test conditions were those shown in [Rolling Fatigue Test Conditions] above.

[0034] [Manufacturing conditions for rails that investigated the relationship between the number of nitrides containing Cr, Mn, and V at a depth of 25 mm from the outer surface of the head and resistance to internal fatigue damage.] ●Chemical components 0.85%C-0.60%Si-0.90%Mn-0.45%Cr-0.013%P-0.010%S-0.080%V-0.012%N (remainder: Fe and impurities) ● Rail shape 136 pounds (weight: 67 kg / m) ● Casting conditions Casting speed in the temperature range of 1300-1400°C: 0.80 m / min The casting speed for each steel rail was kept the same. Heating rate of the cast slab: 7°C / min within the range of 1000-1200°C. Casting slab heating completion temperature: 1260℃ ●Rolling conditions (The rolling temperature was measured at the outer surface position of the head shown in Figure 1) Maximum reduction rate of the head outer surface at a rolling temperature of 1030°C: 26% Cross-sectional reduction rate of the outer surface of the head at a rolling temperature of 980°C: 18% The rolling conditions for each steel rail were all the same. ● Heat treatment conditions (For temperature control of each cooling process, the temperature measured at the outer surface position of the head shown in Figure 1 was used.) Heat treatment conditions: After rolling, accelerated cooling + controlled cooling Accelerated cooling conditions (head outer surface): Accelerated cooling start temperature 800°C, accelerated cooling end temperature 580°C, and average cooling rate 5°C / sec Controlled cooling conditions (head outer surface): Holding temperature of 600-660°C, rail surface temperature fluctuation range of 20-40°C during temperature holding, temperature holding time of 2-180 seconds, followed by accelerated cooling. Temperature maintenance during controlled cooling: Temperature was controlled by controlling the accelerated cooling rate, and further by repeatedly starting and stopping accelerated cooling, and performing accelerated cooling in response to reheating from inside the rails.

[0035] By significantly changing the holding temperature and holding time during controlled cooling, the number of nitrides containing Cr, Mn, and V at a depth of 25 mm starting from the outer surface of the head was greatly altered.

[0036] By keeping the casting and rolling conditions the same, the segregation of Cr, Mn, and V during final solidification, as well as the introduction of dislocations in the austenite during rolling, were made identical. As a result, the average value of the ratio of the sum of the number of atoms of Cr (CA) and the number of atoms of Mn (MA) to the number of atoms of V (VA) in nitrides containing Cr, Mn, and V, rounded to the first decimal place, "(CA+MA) / VA", was made to be approximately the same within the range of 19 to 21. By significantly changing the stop temperature during accelerated cooling and the holding temperature and time during controlled cooling, the number of nitrides containing Cr, Mn, and V at a depth of 25 mm starting from the outer surface of the head was significantly changed.

[0037] The investigation method for nitrides containing Cr, Mn, and V is as follows.

[0038] [Method for investigating nitrides containing Cr, Mn, and V] ●Sample collection location: Inside the head (25 mm deep, starting from the outer surface of the head as shown in Figure 1) ●Pre-treatment: Three needle samples with a radius of curvature of 30-80 nm are prepared using the FIB (Focused Ion Beam) method. The needle sample was prepared so that the tip of the needle formed a pearlite structure. A three-dimensional elemental map of the tip was created, and areas with relatively low carbon concentrations were identified as ferrite. These ferrite portions were then subjected to analysis. ●Measurement device: 3D Atom Probe (3DAP) method ●Measurement method A DC voltage is applied to the needle sample, and then a pulsed voltage is applied, or the needle sample is irradiated with a pulsed laser, causing ions of the constituent atoms to evaporate from the needle tip. These ions are detected by a coordinate detector. The type of element is identified by the ion time of flight. Based on the detected coordinates and measurement order, the three-dimensional position and number of atoms of the elements are determined. Voltage: DC, voltage pulse (pulse ratio 15% or more), or laser pulse (40 pJ) Sample temperature: 40K to 70K

[0039] ● Method for determining and counting nitrides containing Cr, Mn, and V The measurement data was analyzed using IVAS software (CAMECA). In the mass-to-charge ratio spectrum, the peaks at 25, 26, and 26.5 were identified as Cr 2+ It was identified as such, and the peak at 27.5 was Mn 2+ It was identified as such, and the peak at 25.5 Da was V 2+ It was identified as NN. + The peak is Fe 2+ Since it overlaps with the main peak, it cannot be directly recognized in the chemical composition of the rail according to this embodiment. Therefore, NV appearing at 32.5Da 2+ The peak was identified as N. This means that the ion corresponding to this peak contains an equivalent amount of V to N.

[0040] After obtaining a 3D elemental map based on the coordinates and measurement order of the detected ions, nitride precipitates are identified using atomic position data of Cr, Mn, V, and N. For this, the Maximum Separation Method included in IVAS is used, for example. This method separates groups of Cr, Mn, V, and N atoms whose distance from each other is less than a certain value from the matrix and recognizes them as precipitates. In this experiment, 1 nm was used as the "certain value".

[0041] After identifying precipitates using the method described above, the number of precipitates identified as nitrides containing Cr, Mn, and V in the ferrite phase of the pearlite structure within the measurement area is counted using IVAS software.

[0042] Furthermore, the pearlite structure contains both a ferrite phase and a cementite phase. In the rail according to this embodiment, nitrides containing Cr, Mn, and V are used to strengthen the ferrite phase in the pearlite structure; therefore, in this experiment, only those present in the central part of the ferrite phase in the pearlite structure were evaluated. The separation of the cementite phase and the ferrite phase in the measurement area can be determined from the C distribution. In the cementite phase, the C concentration is 25% in terms of atomic ratio.

[0043] ●Method for measuring the number density of nitrides containing Cr, Mn, and V The number density of nitrides containing Cr, Mn, and V, determined by the method described above, is measured as follows.

[0044] The volume of the analysis region is estimated from the number of atoms contained in the analysis region measured by 3DAP. In the case of general steel, alloying elements other than iron are very few. Therefore, even if we assume that all atoms constituting the analysis region are iron atoms and calculate the volume of the analysis region from the number of elements in the analysis region, it is thought that there will be no significant difference from the true value. Therefore, the number of iron atoms is corrected by the detection rate of the ion detector, and that value is used for the atomic density of Fe (85 atoms / nm). 3 The value obtained by dividing by the volume of the measurement site (nm) 3) can be considered as follows. The detection rate varies depending on the instrument, but the instrument used in this experiment had a detection rate of 35%, so the number of atoms included in the analysis area was estimated by dividing the number of detected atoms by 0.35.

[0045] By dividing the number of precipitates in the central region of the ferrite phase where the precipitates are distributed by the volume of the extracted region, the number density of nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm in the ferrite phase of the pearlite structure can be determined. For example, if one precipitate is observed in the measurement of the volume corresponding to 30 million iron atoms in the ferrite phase, the volume of the analysis region is 3 x 10 7 / 0.35 / 85=1.0×10 6 nm 3 Therefore, the number density is 1 nm 3 1.0 x 10 -6 The result is [number of particles]. Note that "0.35" in the above formula refers to the detection rate of the ion detector mentioned above, and "85" refers to the atomic density of Fe (unit: number of particles / nm) mentioned above. 3 ) The unit is 1 cm. 3 To convert this value to the number of items per unit, add 10 to this value. 21 You just need to multiply by , and in the above case, the number density is 1cm 3 1.0 x 10 17 The number of needles was determined by taking the average of the number densities in three needle samples and using that as the number density for the rail.

[0046] ●Method for measuring the particle size of nitrides containing Cr, Mn, and V In this experiment, only the number density of nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm was measured. This was because nitrides containing Cr, Mn, and V with particle sizes less than 0.5 nm or greater than 6.0 nm were not considered to contribute to improving the rail properties. Therefore, in evaluating nitrides containing Cr, Mn, and V, only those nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm were extracted, and their number was counted.

[0047] The method for measuring the particle size of each nitride containing Cr, Mn, and V is as follows. First, the total number of atoms of Cr, Mn, and V constituting the nitride containing Cr, Mn, and V was determined. Assuming that the same number of N atoms as this total number of atoms are present in the precipitate, and that the crystal structure is of the B1(NaCl) type in which Mn atoms and V atoms substitute for the Cr atoms in CrN, the volume of each precipitate was estimated. Using the literature values ​​of 0.415 nm and 0.413 nm for the lattice constants of CrN and VN of the B1(NaCl) type, and assuming the lattice constant of the nitride containing Cr, Mn, and V is 0.414 nm, then 1 nm 3 The number of atoms contained within is approximately 113. Note that elemental Mn4N and Mn2N were not used as reference for the lattice constant. This is because elemental Mn4N and Mn2N have different crystal structures than the B1(NaCl) type, resulting in different lattice constants, and also because Mn is substituted for Cr. The volume of the precipitate can be estimated based on the number of atoms contained within it. Here, the nitride containing Cr, Mn, and V was assumed to be a sphere, and the diameter of this sphere was taken as the particle size of the nitride containing Cr, Mn, and V. That is, the equivalent diameter of the sphere of the nitride containing Cr, Mn, and V was determined.

[0048] Figure 4 shows the results of rolling fatigue tests conducted on steel rails in which the number of nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm was greatly varied at a depth of 25 mm starting from the outer surface of the rail head. The results are summarized in the relationship between the number of nitrides containing Cr, Mn, and V at a depth of 25 mm and the cumulative tonnage at which cracks occurred.

[0049] As shown in Figure 4, the number of nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm is 1 cm². 3 1.0 x 10 17 In steel rails with fewer than 1 cm³ of nitrides, fatigue cracks occurred at a low cumulative tonnage. This was because the number of nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm was 1 cm³. 3 1.0 x 10 17In cases with fewer than [number] units, it is thought that the microscopic softening of the ferrite phase in the pearlite structure within the head (at a depth of 25 mm from the outer surface of the head) was insufficient, resulting in no improvement in internal fatigue damage resistance.

[0050] On the other hand, the number of nitrides containing Cr, Mn, and V with a particle size of 0.5 to 6.0 nm is 1 cm 3 5.0 per unit x 10 17 Even in steel rails exceeding 1 cm², fatigue cracks occurred at a low cumulative tonnage. This is because the number of nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm is 1 cm². 3 5.0 per unit x 10 17 It is thought that when the number exceeds a certain limit, the number density of precipitates becomes excessive, the pearlite structure inside the head becomes brittle, and crack initiation is promoted, leading to a decrease in resistance to internal fatigue damage.

[0051] Based on the above, the number density of V nitrides containing Cr with a particle size of 0.5 to 6.0 nm is found to be 1 cm² inside the head (at a depth of 25 mm from the outer surface of the head). 3 1.0 x 10 17 ~5.0×10 17 By controlling the process within a specific range, we confirmed that the microscopic softened areas were reduced, and the resistance to internal fatigue damage was stably improved.

[0052] The reason for limiting the particle size of nitrides containing Cr, Mn, and V to the range of 0.5 to 6.0 nm for controlling the number density is that this numerical range is the most effective size for reducing the microscopic softening areas in the pearlite structure and achieving uniform hardness when nitrides containing Cr, Mn, and V precipitate in the ferrite phase of the pearlite structure. Nitrides containing Cr, Mn, and V with particle sizes less than 0.5 nm or greater than 6.0 nm do not contribute to improving the rail properties, so it is considered better to have a low content of them. However, as long as the number density of nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm is kept within the specified range, the magnitude of these number densities is not considered to affect the rail properties. When evaluating nitrides containing Cr, Mn, and V, those with particle sizes less than 0.5 nm or greater than 6.0 nm are ignored.

[0053] Therefore, in the ferrite phase of the pearlite structure at a depth of 25 mm from the outer surface of the head, the number density of nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm is 1 cm³. 3 1.0 x 10 17 ~5.0×10 17 The range is defined as 1 cm². Furthermore, the 1 cm² of nitride containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm is specified. 3 The preferred lower limit for the number density per unit is 1.2 × 10⁻⁶. 17 pieces, 1.5×10 17 pieces, or 1.8 × 10 17 It is a 1cm³ nitride containing Cr, Mn, and V with a particle size of 0.5-6.0 nm. 3 The preferred upper limit for the number density per unit is 4.5 × 10⁻⁶. 17 pieces, 4.0×10 17 1, or 3.5 × 10 17 For example, to improve the microscopic softening of the ferrite phase in the pearlite structure and stably improve resistance to internal fatigue damage, the number density of nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm should be 1 cm³. 3 1.5 x 10 17 ~4.0×10 17 It is desirable to control it within a range of individual values.

[0054] <Reason for controlling the number of Cr atoms (CA), the number of Mn atoms (MA), and the number of V atoms (VA) to satisfy the following formula 1 (see Fig. 5)> Next, the inventors will explain the reason for limiting the ratio of the number of atoms of Cr, Mn, and V in the nitride containing Cr, Mn, and V in order to stably improve the internal fatigue damage resistance of the rail.

[0055] The microscopic softening of the ferrite phase of the pearlite structure inside the head cannot be sufficiently suppressed only by controlling the amount and hardness of the pearlite structure. However, as described above, by setting the number density of nitrides containing Cr, Mn, and V having a predetermined particle size within a predetermined range at a predetermined location, the microscopic softening of the ferrite phase of the pearlite structure inside the head can be suppressed, and the internal fatigue damage resistance can be improved. As a result, the internal fatigue damage resistance of the rail according to the present embodiment can be sufficiently enhanced. However, from the viewpoint of further enhancing safety, the inventors considered improvement measures for characteristics during long-term use.

[0056] As a result of detailed observation of the rail on which the above rolling fatigue test was conducted, it was confirmed that minute cracks (less than 0.5 mm in length) may be generated around the nitrides containing Cr, Mn, and V. The inventors considered a method to eliminate these minute cracks.

[0057] Therefore, the inventors investigated in detail the relationship between the composition of the nitrides containing Cr, Mn, and V and the minute cracks generated around them. The following shows the components, casting conditions, rolling conditions, and heat treatment conditions of the steel rail used in the test.

[0058] [Rail in which the relationship between the composition of nitrides containing Cr, Mn, and V and the minute cracks generated around them was investigated] ● Chemical composition 0.85%C - 0.60%Si - 0.90%Mn - 0.45%Cr - 0.013%P - 0.010%S - 0.100%V - 0.010%N (balance Fe and impurities) ● Rail shape 136 pounds (weight: 67 kg / m) ● Casting conditions Casting speed in the temperature range of 1300-1400°C: 0.3-1.2 m / min Heating rate of cast slab: 3°C / min within the range of 1000-1200°C. Casting slab heating completion temperature: 1240℃ The segregation of V, Cr, and Mn in the alloy during final solidification was altered to significantly change the composition of the nitride containing Cr, Mn, and V. ●Rolling conditions (The rolling temperature was measured at the outer surface position of the head shown in Figure 1) Rolling temperature of the outer surface of the head: Cross-sectional reduction rate at 1000~1050℃: 1~50% Rolling temperature of the outer surface of the head: Cross-sectional reduction rate at 800~1000℃: 1~30% During the hot rolling of each steel rail, the reduction rate of cross-sectional area of ​​the outer surface of the top rail in the above rolling temperature range was significantly altered, causing a large change in the composition of nitrides containing Cr, Mn, and V. ● Heat treatment conditions (For temperature control of each cooling process, the temperature measured at the outer surface position of the head shown in Figure 1 was used.) Heat treatment conditions: After rolling, accelerated cooling + controlled cooling Accelerated cooling conditions (head outer surface): Accelerated cooling start temperature 800°C, accelerated cooling end temperature 630°C, and average cooling rate 4°C / sec Controlled cooling conditions (head outer surface): Holding temperature range of 630-640°C, rail surface temperature fluctuation range of 10°C during temperature holding, temperature holding time of 50-55 seconds, followed by accelerated cooling. Temperature maintenance during controlled cooling: Temperature was controlled by controlling the accelerated cooling rate, and further by repeatedly starting and stopping accelerated cooling, and performing accelerated cooling in response to reheating from inside the rails. By keeping the stop temperature during accelerated cooling and the holding temperature and time during controlled cooling under nearly identical conditions, the number of nitrides containing Cr, Mn, and V at a depth of 25 mm starting from the outer surface of the head was made nearly identical.

[0059] By significantly changing the casting and rolling conditions, the composition of nitrides containing Cr, Mn, and V was greatly altered. By controlling the heat treatment conditions, the number density of V nitrides containing Cr with a particle size of 0.5 to 6.0 nm was increased to 1 cm³. 32.0 x 10 17 ~2.2×10 17 Steel rails within a specified range were manufactured and subjected to rolling fatigue tests. The test conditions for the rolling fatigue tests were those specified in [Rolling Fatigue Test Conditions] above. After the rolling fatigue tests, the formation of microcracks inside the head (at a depth of 25 mm from the outer surface of the head) was investigated. The investigation method is as follows.

[0060] [Methods for investigating microcracks] ● Sample preparation The rail was cut, and a sample was prepared from the inside of the head (at a depth of 25 mm from the outer surface of the head shown in Figure 1). ●Pre-treatment: Diamond polishing of the cross-section. ● Observation method Equipment: Scanning electron microscope Magnification: 100,000 Evaluation: Microcracks were counted.

[0061] We focused on the composition of nitrides containing Cr, Mn, and V as a factor in the occurrence of microcracks, and the investigation method is as follows.

[0062] [Method for investigating the composition of nitrides containing Cr, Mn, and V] The sampling location, pretreatment, measuring instrument, measurement method, and method for identifying nitrides containing Cr, Mn, and V were all determined using the same three-dimensional atom probe (3DAP) method as described in the "Method for Investigating Nitrides Containing Cr, Mn, and V" above. ●Sample collection location: Inside the head (25 mm deep, starting from the outer surface of the head as shown in Figure 1) This is the location where crack formation around nitrides containing Cr, Mn, and V was investigated. ●Pre-treatment: Three needle samples with a radius of curvature of 30-80 nm are prepared using the FIB (Focused Ion Beam) method. ●Measurement device: 3D Atom Probe (3DAP) method

[0063] ● Calculation of the number of atoms and compositional ratios of Cr, Mn, and V Using the method described above, nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm, located in the ferrite phase of the pearlite structure at a depth of 25 mm starting from the outer surface of the head, are determined to be subjected to detailed analysis using the following procedure.

[0064] For each nitride, the number of Cr, Mn, and V atoms was counted, and the ratio of the sum of the number of Cr atoms (CA) and Mn atoms (MA) to the number of V atoms (VA) was calculated. For each needle sample, 50 precipitates were randomly selected from nitrides containing Cr, Mn, and V with a size of 0.5 to 6.0 nm. For each of these 50 nitrides containing Cr, Mn, and V, the ratio of the sum of the number of Cr atoms (CA) and Mn atoms (MA) to the number of V atoms (VA) was calculated. Then, the 10 values ​​that are in the middle of these 50 values ​​(i.e., the 21st to 30th largest values) were extracted. Note that the number of nitrides in one needle sample is about several hundred. According to the inventors' experimental results, these nitrides are generally homogeneous, and it is presumed that the method of selecting nitrides does not affect the measurement results of the nitrides.

[0065] The above procedure was performed on three needle samples, and the ratio of the sum of the number of Cr atoms (CA) and Mn atoms (MA) to the number of V atoms (VA) in a total of 30 nitrides containing Cr, Mn, and V was calculated. The average value of this ratio, rounded to the first decimal place, was used as the representative value for each steel rail. Hereinafter, the average value of the ratio of the sum of the number of Cr atoms (CA) and Mn atoms to the number of V atoms in nitrides containing Cr, Mn, and V with grain sizes of 0.5 to 6.0 nm in the ferrite phase of the pearlite structure at a depth of 25 mm from the outer surface of the head, rounded to the first decimal place, will be written as "(CA+MA) / VA".

[0066] Figure 5 shows a diagram summarizing the relationship between (CA+MA) / VA and the number of microcracks generated. As a result of detailed investigation, as shown in Figure 5, it was found that there is an appropriate range for (CA+MA) / VA in order to suppress the generation of microcracks with a length of less than 0.5 mm to zero. When (CA+MA) / VA is less than 5, the hardness of the nitride containing Cr, Mn, and V increases significantly due to the increase in the amount of V in the nitride, and the amount of microcracks (less than 0.5 mm) generated increases, thus reducing resistance to internal fatigue damage. On the other hand, when (CA+MA) / VA exceeds 100, the hardness of the nitride decreases, and the amount of microcracks (less than 0.5 mm) generated increases, thus reducing resistance to internal fatigue damage.

[0067] Therefore, the (CA+MA) / VA value of nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm in the ferrite phase of the pearlite structure at a depth of 25 mm from the outer surface of the head needs to be kept within the range of Equation 1 below. Furthermore, nitrides containing Cr, Mn, and V with particle sizes less than 0.5 nm or greater than 6.0 nm are not considered to contribute to improving the internal fatigue damage resistance of the rail, and are therefore excluded from the measurement of (CA+MA) / VA. 5 ≤ (CA + MA) / VA ≤ 100 … Equation 1 A preferred upper limit for (CA+MA) / VA is, for example, 90, 80, or 60. A preferred lower limit for (CA+MA) / VA is, for example, 8, 10, or 20.

[0068] These results indicate that, in order to suppress and prevent the formation of cracks and microcracks inside the rail head, thereby improving the rail's resistance to internal fatigue damage and ensuring safety, it is necessary to control not only the particle size and number density of nitrides containing Cr, Mn, and V, but also the composition of nitrides containing Cr, Mn, and V that serve as crack initiation sites. Furthermore, increasing the amount of Cr in the V nitride does not necessarily lead to an improvement in internal fatigue damage resistance. The inventors have found that as the number of Cr atoms increases, the hardness of the Cr-containing V nitride increases significantly, and the amount of microcracks generated in the surrounding matrix tends to increase (see Patent Document 4). Patent Document 4 also discloses experimental results suggesting that when the ratio of the number of Cr atoms CA to the number of V atoms VA in the Cr-containing V nitride, CA / VA, exceeds 0.70, the internal fatigue damage resistance decreases slightly.

[0069] In the V nitride contained in the rail according to this embodiment, the amount of Cr is very high. For reference, the inventors measured the CA / VA in rails where the ((CA+MA) / VA) of the nitride was between 5 and 100, and it was usually 2 or higher. Considering the findings described in Patent Document 4, it is expected that such nitride may impair the internal fatigue damage resistance of the rail.

[0070] Contrary to expectations, the rails according to this embodiment have enhanced resistance to internal fatigue damage due to the V nitride containing a large amount of Cr. This is presumed to be because the V nitride contains Mn in addition to Cr. The reason why the V nitride contains Mn in the rails according to this embodiment is, (1) By optimizing the casting rate at temperatures of 1300-1400°C, the formation of coarse nitrides during solidification was reduced, and dissolved nitrogen was secured, and, (2) By performing rolling under appropriate rolling conditions, the dislocations introduced into the austenite remain after the pearlite transformation, which increases the driving force for Mn nitride formation. It is presumed that the reason Mn improves the properties of V nitrides is due to solid solution strengthening of the nitrides by Mn.

[0071] In this embodiment, the rail has a defined ((CA+MA) / VA) ratio in the nitride, and satisfying this ratio enhances resistance to internal fatigue damage. Therefore, it is not necessary to define the CA / VA ratio in the nitride. According to the inventors' experimental results, when ((CA+MA) / VA) satisfies Equation 1, CA / VA is generally in the range of 2 to 70.

[0072] <Reasons for limiting the required range of metallic and pearlite structures> In the rail according to this embodiment, from the viewpoint of ensuring wear resistance, it is necessary that the metallic structure at a depth of 25 mm from the outer surface of the head be composed of pearlite or more in terms of area ratio. First, the reason for making the pearlite structure comprise 95% or more of the area ratio will be explained.

[0073] Ensuring wear resistance is paramount in the rail head, where it comes into contact with the wheels. The inventors investigated the relationship between metal structure and wear resistance and found that pearlite structure exhibits the best wear resistance. Furthermore, pearlite structure easily achieves hardness (strength) even with a low alloying element content and also exhibits excellent resistance to internal fatigue damage. Therefore, to improve wear resistance and resistance to internal fatigue damage, the area ratio of pearlite structure was limited to 95% or more. If the area ratio of pearlite structure is less than 95%, wear resistance and resistance to internal fatigue damage are not sufficiently improved. To ensure sufficient wear resistance, it is desirable that the metal structure of the rail head be 96% or more, 97% or more, or 98% or more. The area ratio of pearlite structure may be 100%, but it may also be specified as, for example, 100% or less, 99% or less, or 98% or less.

[0074] Next, we will explain why the required range for metallic structures containing 95% or more of pearlite (metallic structures containing pearlite) was limited to a depth of 25 mm, starting from the outer surface of the head (the surface of the head corners and crown).

[0075] If the location defining the metallic structure including the pearlite structure is at a depth of less than 25 mm from the outer surface of the rail head, considering wear during use, this area is insufficient to meet the requirements for wear resistance and resistance to internal fatigue damage of the rail head. As a result, it becomes difficult to sufficiently improve wear resistance and resistance to internal fatigue damage, and consequently, it becomes difficult to sufficiently improve the rail's service life. To further improve wear resistance and resistance to internal fatigue damage, it is desirable to have a metallic structure including the pearlite structure at a depth of 25 mm from the outer surface of the rail head. When the area ratio of the pearlite structure at a depth of 25 mm from the outer surface of the rail head is 95% or more, it is usually the case that the area ratio of the pearlite structure is 95% or more throughout the entire range from the outer surface of the rail head to that location.

[0076] Here, Figure 6 shows the designation of the rail head according to this embodiment, and the region where the metallic structure including the pearlite structure is required. First, the rail head, as indicated by reference numeral 3 in Figure 6, refers to the portion above the constricted portion in the center of the rail in the height direction when the rail is viewed in cross-section. The rail head 3 has a top portion 1 and head corner portions 2 located at both ends of the top portion 1. One of the head corner portions 2 is the gauge corner (GC) portion that mainly contacts the wheel. The outer surface of the head refers to the surface of the rail head 3 that faces upward when the rail is upright, combined with the surface of the head corner portions 2. The positional relationship between the top portion 1 and the head corner portions 2 is such that the top portion 1 is located approximately in the center in the width direction of the rail head, and the head corner portions 2 are located on both sides of the top portion 1.

[0077] The area extending from the surface of the head corner 2 and the top 1 (outer surface of the head) to a depth of 25 mm is called the head surface (3a, shaded area). As shown in Figure 6, it is necessary for the head surface 3a, extending from the surface of the head corner 2 and the top 1 (outer surface of the head) to a depth of 25 mm, to contain a metallic structure with a predetermined hardness (a metallic structure in which pearlite material accounts for 95% or more of the area) in order to improve the wear resistance and resistance to internal fatigue damage of the rail.

[0078] Therefore, it is desirable that the metallic structure containing the pearlite structure be located on the top surface 3a, where the wheel and rail are in main contact and wear resistance and resistance to internal fatigue damage are required. In parts other than the top surface where these properties are not required, the area ratio of the pearlite structure may be 95% or more, but it does not have to be 95% or more.

[0079] Furthermore, in the metal structure of the rail according to this embodiment, starting from the surface (outer surface of the head) to a depth of 25 mm, if the area ratio of the pearlite structure is 95% or more, trace amounts of proterite ferrite, proterite cementite, bainite, martensite, etc., in addition to the pearlite structure, in amounts of less than 5% of the area ratio, may be mixed in. Even if these structures are mixed in, if they are less than 5%, they will not have a significant adverse effect on the internal fatigue damage resistance inside the head. In other words, in the metal structure of the rail head of the rail according to this embodiment, it is sufficient if 95% or more of the area ratio of the head surface is pearlite, and in order to sufficiently improve the internal fatigue damage resistance, it is desirable that 98% or more of the metal structure of the head surface of the rail head be pearlite. The area ratio of the pearlite structure may be 100%.

[0080] The area percentage of the pearlite structure is measured using the following procedure: A sample is cut from the cross section of the rail head. Each sample is diamond polished and then etched with 3% nital. The area percentage of the pearlite structure can be determined by observing the structure of the polished and etched cross section using an optical microscope (200x magnification). The measurement field of view is any 10 fields of view at a depth of 25 mm from the outer surface of the rail head (see Figure 1). The average value of the area percentage of the pearlite structure in any 10 fields of view at a depth of 25 mm from the outer surface of the rail head is considered as the "area percentage of the pearlite structure at the 25 mm position".

[0081] <Reasons for limiting the rail hardness> In the rail according to this embodiment, the hardness of the rail containing the pearlite structure, measured at a depth of 25 mm from the outer surface of the head, must be limited to the range of Hv360 to 500. Next, the reason for limiting the hardness of the rail containing the pearlite structure to the range of Hv360 to 500 in the rail according to this embodiment will be explained.

[0082] It was found that when the hardness of rails containing pearlite is less than Hv360, the wear on the surface of the rail head reaches 25 mm with a small cumulative tonnage, making it difficult to ensure the required wear resistance of the rail head as wear progresses. Furthermore, it was found that when the hardness of rails containing pearlite is less than Hv360, fatigue cracks occur and propagate inside the rail head with a small cumulative tonnage, reducing resistance to internal fatigue damage.

[0083] Furthermore, it was found that when the hardness of the pearlite structure exceeds Hv500, the metallic structure including the pearlite structure becomes brittle, causing fatigue cracks to occur and propagate within the head with a small cumulative passage of tons, resulting in a decrease in resistance to internal fatigue damage.

[0084] Therefore, the hardness of the rail containing the pearlite structure was limited to the range of Hv360 to 500. Furthermore, in order to ensure wear resistance and stably improve resistance to internal fatigue damage, it is desirable to control the hardness of the rail containing the pearlite structure located within a 25mm depth range from the outer surface of the head to the range of Hv380 to 480. The preferred lower limit for the hardness of the rail containing the pearlite structure located within a 25mm depth range from the outer surface of the head is Hv390, Hv400, or Hv410. The preferred upper limit for the hardness of the rail containing the pearlite structure located within a 25mm depth range from the outer surface of the head is Hv470, Hv460, or Hv450.

[0085] Furthermore, since the primary objective of the rail according to this embodiment is to improve the internal fatigue damage resistance of the head, the effects of the rail according to this embodiment will be realized if, as a representative example, the metal structure and hardness at a depth of 25 mm from the outer surface of the head, the number density of nitrides containing Cr, Mn, and V, and the nitride composition ((CA+MA) / VA) are within the above range. The position at a depth of 25 mm from the outer surface of the rail head is the position indicated by the dashed line 3b in Figure 7. Also, if the hardness of the rail at a depth of 25 mm from the outer surface of the head is Hv380 to 480, then the hardness of the rail will typically be Hv380 to 480 throughout the entire range from the outer surface of the head to that position.

[0086] The hardness of rails containing pearlite is measured using the following procedure: A sample is cut from the cross-section of the rail head. The portion of each sample corresponding to the rail cross-section is polished with diamond abrasive grains with an average particle size of 1 μm. Then, the hardness is measured on the polished cut surface using a Vickers hardness tester (load 98 N) in accordance with JIS Z 2244. The measurement position is 25 mm deep from the outer surface of the rail head (see Figure 1). The number of measurement points is 20. The measurement interval is 1 mm. Furthermore, the midpoint of the linear measurement area consisting of the 20 measurement points is positioned at the center of the rail width direction of the C cross-section of the rail. The average value of the hardness measurement is considered as the "hardness at the 25 mm position". In the rail according to this embodiment, pearlite accounts for more than 95% of the area, but other structures (proeutectoid cementite, proeutectoid ferrite, martensite, bainite, etc.) are present in a range of 5% or less, so the hardness of the rail containing pearlite may not be representative with a single measurement.

[0087] <Reasons for limiting the chemical composition of the rails> In this embodiment, the reasons for limiting the chemical composition of the rail steel (the steel material used for the rail) will be explained in detail. Hereinafter, the unit "%" indicating the content of each element means "mass%".

[0088] C: 0.75~1.20% Carbon (C) is an effective element for promoting pearlite transformation and ensuring wear resistance. If the C content is less than 0.75%, the minimum strength and wear resistance required for rails cannot be maintained in this composition system. Furthermore, if the C content is less than 0.75%, a soft proterecution ferrite structure that is prone to fatigue crack formation is formed inside the head, reducing resistance to internal fatigue damage. On the other hand, if the C content exceeds 1.20%, a proterecution cementite structure is more likely to form inside the head, and fatigue cracks occur at the interface between the pearlite structure and the proterecution cementite structure, reducing resistance to internal fatigue damage. For this reason, the C content should be between 0.75% and 1.20%. The preferred lower limit for the C content is 0.77%, 0.80%, 0.85%, or 0.90%. The preferred upper limit for the C content is 0.77%, 0.80%, 0.85%, or 0.90%. For example, to stabilize the formation of the pearlite structure and improve resistance to internal fatigue damage, it is desirable to have a carbon content of 0.80 to 1.10%.

[0089] Si: 0.10~2.00% Si is an element that dissolves in the ferrite phase of the pearlite structure, increasing the hardness (strength) of the rail head and improving wear resistance and resistance to internal fatigue damage. However, if the Si content is less than 0.10%, solid solution strengthening is insufficient, the pearlite becomes soft, fatigue cracks are more likely to occur, and resistance to internal fatigue damage decreases. On the other hand, if the Si content exceeds 2.00%, many surface defects are generated during hot rolling of the rail. Furthermore, if the Si content exceeds 2.00%, hardenability increases significantly, a martensitic structure is formed in the rail head, fatigue cracks are more likely to occur at the boundary between the martensitic and pearlite structures, and resistance to internal fatigue damage decreases. For this reason, the Si content should be between 0.10% and 2.00%. The preferred lower limit of the Si content is 0.20%, 0.50%, or 0.80%. The preferred upper limit of the Si content is 1.80%, 1.50%, or 1.20%. For example, to stabilize the hardness increase of the pearlite structure and improve wear resistance and resistance to internal fatigue damage, it is desirable to set the Si content to 0.20 to 1.20%.

[0090] Mn: 0.05~2.00% Mn is an element that enhances hardenability, stabilizes pearlite transformation, refines the lamellar spacing of the pearlite structure, increases the hardness of the pearlite structure, and further improves the internal fatigue damage resistance of the rail head by suppressing microscopic softening of the ferrite phase in the pearlite structure inside the rail head through precipitation strengthening by the formation of fine Cr, Mn, and V-containing nitrides in the ferrite phase of the pearlite structure. However, if the Mn content is less than 0.05%, the effect is small, the number of fine Cr, Mn, and V-containing nitrides precipitated in the ferrite phase of the pearlite structure decreases, and the improvement of microscopic softening of the ferrite phase in the pearlite structure is insufficient, resulting in no improvement in internal fatigue damage resistance. Furthermore, if the Mn content is less than 0.05%, a soft protereminate ferrite structure prone to fatigue crack formation is likely to form inside the rail head. In addition, the number density of nitrides containing Cr, Mn, and V with a size of 0.5-6.0 nm precipitated in the ferrite phase of the pearlite structure is insufficient, resulting in inadequate improvement of the microscopic softening of the ferrite phase in the pearlite structure, thus failing to improve internal fatigue damage resistance. On the other hand, if the Mn content exceeds 2.00%, hardenability increases significantly, a martensitic structure is formed in the rail head, and fatigue cracks are more likely to occur from the boundary between the martensitic and pearlite structures. In addition, the number density of nitrides containing Cr, Mn, and V with a size of 0.5-6.0 nm becomes excessive, causing the pearlite structure inside the rail head (at a depth of 25 mm from the outer surface of the rail head) to become brittle, accelerating fatigue crack initiation and reducing the rail's internal fatigue damage resistance. For this reason, the Mn content should be set to 0.05-2.00%. The preferred lower limit for Mn content is 0.10%, 0.20%, or 0.40%. The preferred upper limit for Mn content is 1.80%, 1.50%, or 1.20%. For example, to stabilize the formation of pearlite structure, stably produce nitrides containing Cr, Mn, and V, and improve the internal fatigue damage resistance of the rail head, it is desirable to have a Mn content of 0.20 to 1.50%.

[0091] Cr: 0.05~2.00% Cr is an element that increases the equilibrium transformation temperature of steel, and by increasing the degree of supercooling, it refines the lamellar spacing of the pearlite structure, increases the hardness of the pearlite structure, and further improves the internal fatigue damage resistance of the rail head by suppressing microscopic softening of the ferrite phase in the pearlite structure inside the rail head through precipitation strengthening by the formation of fine Cr, Mn, and V-containing nitrides in the ferrite phase of the pearlite structure. However, if the Cr content is less than 0.05%, the effect is small, the number density of 0.5-6.0 nm Cr, Mn, and V-containing nitrides precipitated in the ferrite phase of the pearlite structure is insufficient, and the improvement of microscopic softening of the ferrite phase in the pearlite structure is insufficient, so the internal fatigue damage resistance does not improve. On the other hand, if the Cr content exceeds 2.00%, hardenability increases significantly, bainite or martensite structures are formed in the rail head, and fatigue cracks are more likely to occur at the boundary between the bainite or martensite structure and the pearlite structure. In addition, the number density of nitrides containing Cr, Mn, and V in the range of 0.5 to 6.0 nm becomes excessive, causing the pearlite structure inside the rail head (at a depth of 25 mm from the outer surface of the head) to become brittle and promoting fatigue crack initiation, thus reducing the rail's resistance to internal fatigue damage. For this reason, the Cr content should be 0.05 to 2.00%. The preferred lower limit for the Cr content is 0.10%, 0.20%, or 0.50%. The preferred upper limit for the Cr content is 1.80%, 1.50%, or 1.20%. For example, to stabilize the formation of pearlite structure, stably produce nitrides containing Cr, Mn, and V, and improve the internal fatigue damage resistance of the rail head, it is desirable to set the Cr content to 0.20-1.50%.

[0092] V: 0.005~0.100% V is an element that, during the cooling process after hot rolling of rails, generates fine nitrides containing Cr, Mn, and V within the ferrite phase of the pearlite structure. This precipitation strengthening suppresses microscopic softening of the ferrite phase within the pearlite structure inside the rail head, thereby improving the rail's resistance to internal fatigue damage. However, if the V content is less than 0.005%, the number density of nitrides containing Cr, Mn, and V with a size of 0.5-6.0 nm precipitated within the ferrite phase of the pearlite structure is low, resulting in insufficient improvement of the microscopic softening of the ferrite phase within the pearlite structure inside the rail head, and thus the rail's resistance to internal fatigue damage does not improve. On the other hand, if the V content exceeds 0.100%, the number density of nitrides containing Cr, Mn, and V with a size of 0.5-6.0 nm becomes excessive, causing the pearlite structure inside the rail head (at a depth of 25 mm from the outer surface of the head) to become brittle, promoting crack initiation and reducing the rail's resistance to internal fatigue damage. Therefore, the V content should be 0.005 to 0.100%. The preferred lower limit for the V content is 0.010%, 0.020%, or 0.040%. The preferred upper limit for the V content is 0.090%, 0.080%, or 0.060%. For example, to stably produce nitrides containing Cr, Mn, and V and improve the internal fatigue damage resistance of rails, it is desirable to have a V content of 0.010 to 0.080%.

[0093] Al: 0.0010~1.0000% Al (Al) is an element that acts as a deoxidizing agent. Furthermore, Al shifts the eutectoid transformation temperature to a higher temperature, contributing to increased hardness in the pearlite structure and improving resistance to internal fatigue damage. However, this effect is weak when the Al content is less than 0.0010%. On the other hand, when the Al content exceeds 1.0000%, it becomes difficult to solid-solve Al in the steel, resulting in the formation of coarse alumina-based inclusions. These coarse Al-based inclusions can become the initiation points for fatigue cracks, potentially reducing the rail's resistance to internal fatigue damage. Therefore, the Al content should be between 0.0010% and 1.0000%. The preferred lower limit for Al content is 0.0020%, 0.0050%, or 0.0100%. The preferred upper limit for Al content is 0.9000%, 0.8000%, or 0.6000%.

[0094] N: 0.006~0.020% N, when included simultaneously with Cr and V, is an element that promotes the formation of nitrides containing Cr, Mn, and V in the ferrite phase of the pearlite structure during the cooling process after hot rolling of the rail. When fine nitrides containing Cr, Mn, and V are formed, the microscopic softening of the ferrite phase in the pearlite structure inside the rail head is suppressed, improving the rail's resistance to internal fatigue damage. However, if the N content is less than 0.006%, the number density of nitrides containing Cr, Mn, and V with a size of 0.5 to 6.0 nm formed in the ferrite phase of the pearlite structure is low, resulting in insufficient improvement of the microscopic softening of the ferrite phase in the pearlite structure inside the rail head, and thus the rail's resistance to internal fatigue damage does not improve. Generally, N is considered a harmful impurity element, and the N content of rails manufactured from blast furnace steel is often kept below 0.005%. However, in the rail according to this embodiment, the N content is set to a higher value than usual in order to promote the formation of nitrides. On the other hand, if the N content exceeds 0.020%, the number density of nitrides containing Cr, Mn, and V with a size of 0.5 to 6.0 nm becomes excessive, causing the pearlite structure inside the rail head (at a depth of 25 mm from the outer surface of the head) to become brittle, promoting crack initiation and reducing the rail's resistance to internal fatigue damage. Furthermore, if the N content exceeds 0.0200%, it becomes difficult to solid-solve N in the steel, leading to the formation of bubbles that serve as initiation points for fatigue damage, making internal fatigue damage more likely. For this reason, the N content should be set to 0.006 to 0.020%. The preferred lower limit for the N content is 0.007%, 0.008%, or 0.010%. The preferred upper limit for the N content is 0.019%, 0.018%, or 0.015%. For example, to stably generate nitrides containing Cr, Mn, and V and improve resistance to internal fatigue damage, it is desirable to set the N content to 0.008 to 0.020%. More preferably, it is 0.008 to 0.018%.

[0095] P:0.025% or less P is an impurity element contained in steel, and its content can be controlled by refining in a converter. A lower P content is preferable, but if the P content exceeds 0.025%, the pearlite structure becomes brittle, brittle cracks occur inside the head, and the internal fatigue damage resistance of the rail decreases. For this reason, the P content is limited to 0.025% or less. Preferably, the P content is 0.020% or less, 0.018% or less, or 0.016% or less. There is no lower limit for the P content, but considering the dephosphorization capacity in the refining process, it is thought that the P content will be limited to around 0.005% in actual manufacturing.

[0096] S: 0.025% or less S is an impurity element contained in steel, and its content can be controlled by desulfurization in the molten iron ladle. A lower S content is preferable, but if the S content exceeds 0.025%, coarse MnS-based sulfide inclusions are more likely to form. In the head, stress concentration around these inclusions can cause fatigue cracks to form, reducing the rail's resistance to internal fatigue damage. Therefore, the S content is limited to 0.025% or less. Preferably, the S content is 0.020% or less, 0.018% or less, or 0.016% or less. Although there is no lower limit for the S content, considering the desulfurization capacity in the refining process, it is thought that the S content will be around 0.001% in actual manufacturing.

[0097] The rail according to this embodiment basically contains the above chemical components, with the remainder being Fe and impurities. Impurities are components that are mixed in during the industrial production of steel materials due to raw materials such as ore or scrap, or various factors in the manufacturing process, and are permissible within a range that does not adversely affect the rail according to this embodiment. However, in place of a portion of the remaining Fe, if necessary, one or more elements selected from the group consisting of Mo, Co, B, Cu, Ni, Nb, Ti, Mg, Ca, REM, and Zr may be included within the range described later, for the purpose of increasing the hardness (strength) of the pearlite structure to improve wear resistance and resistance to internal fatigue damage, improving toughness, preventing softening of the heat-affected zone during welding, and controlling the cross-sectional hardness distribution inside the head. Specifically, the function of each arbitrary element is as follows.

[0098] (a) Mo raises the equilibrium transformation point, refines the lamellar spacing in the pearlite structure, and improves the hardness of the rail. (b) Co refines the lamellar structure of the wear surface and increases the hardness of the wear surface. (c) Group B reduces the cooling rate dependence of the pearlite transformation temperature and makes the hardness distribution of the rail head uniform. (Group d) Cu dissolves into the ferrite phase in the pearlite structure, increasing the hardness of the rail. Ni improves the toughness and hardness of the pearlite structure, and at the same time prevents softening of the heat-affected zone of the welded joint. (e) Nb and Ti improve the fatigue strength of the pearlite structure through precipitation hardening of carbides and nitrides generated during hot rolling and subsequent cooling processes. In addition, Nb and Ti stably generate carbides and nitrides during reheating, preventing softening of the heat-affected zone of welded joints. (Group f) Mg, Ca, and REM finely disperse MnS-based sulfides and reduce internal fatigue damage generated from inclusions. (g group) Zr suppresses the formation of segregation zones in the center of the cast slab by increasing the equiaxed crystallization rate of the solidification structure, thereby suppressing the formation of proterite cementite and martensite structures.

[0099] Therefore, these elements may be included to obtain the above-mentioned effects. Even if these elements are included in amounts below the range described later, it will not impair the properties of the rail according to this embodiment. Since it is not necessary to include these elements, the lower limit is 0%.

[0100] Mo: Preferably 0.01-0.50% Mo is an element that increases the equilibrium transformation temperature, and by increasing the degree of supercooling, it refines the lamellar spacing of the pearlite structure, improves the hardness of the pearlite structure, and consequently improves the internal fatigue damage resistance of the rail. However, if the Mo content is less than 0.01%, the effect is small, and no improvement in the hardness of the rail steel is obtained. On the other hand, if the Mo content exceeds 0.50%, the transformation rate decreases significantly, a martensitic structure is formed in the rail head, and fatigue cracks are more likely to occur at the boundary between the martensitic and pearlite structures, which may reduce the internal fatigue damage resistance. For this reason, when including Mo, it is preferable to set the Mo content to 0.01 to 0.50%. The even more preferable lower limit for the Mo content is 0.02%, 0.05%, or 0.10%. The even more preferable upper limit for the Mo content is 0.40%, 0.20%, or 0.15%.

[0101] Co: Preferably 0.01-1.00% Co dissolves in ferrite within the pearlite structure, improving the hardness of the pearlite structure through solid solution strengthening and enhancing the rail's resistance to internal fatigue damage. However, if the Co content is less than 0.01%, the refinement of the lamellar structure is not promoted, and the improvement in wear resistance and resistance to internal fatigue damage cannot be obtained. On the other hand, if the Co content exceeds 1.00%, the above effect saturates, and it may not be possible to refine the lamellar structure in proportion to the Co content. Furthermore, if the Co content exceeds 1.00%, the cost of adding alloy may increase, reducing economic viability. For this reason, when including Co, it is preferable to set the Co content between 0.01% and 1.00%. The even more preferable lower limit for the Co content is 0.02%, 0.05%, or 0.10%. The even more preferable upper limit for the Co content is 0.80%, 0.60%, or 0.20%.

[0102] B: Preferably 0.0001 to 0.0050% B has iron carbon borides (Fe) at the austenite grain boundaries. 23 (CB)6) is an element that forms pearlite transformation and reduces the cooling rate dependence of the pearlite transformation temperature by promoting pearlite transformation. Furthermore, B improves the internal fatigue damage resistance of rails by providing a uniform hardness distribution to the rail through the above effect, thereby suppressing the weakest parts (low hardness parts) that become the source of fatigue cracks. However, if the B content is less than 0.0001%, the effect is not sufficient, and no improvement is observed in the hardness distribution of the rail head. On the other hand, if the B content exceeds 0.0050%, coarse iron carbon borides are formed, and stress concentration can lead to fatigue crack formation, which may reduce the internal fatigue damage resistance of rails. For this reason, when including B, it is preferable to set the B content to 0.0001 to 0.0050%. More preferable lower limits for the B content are 0.0005%, 0.0010%, or 0.0015%. A more preferable upper limit for the B content is 0.0040%, 0.0030%, or 0.0020%.

[0103] Cu: Preferably 0.01-1.00% Cu is an element that improves hardness through solid solution strengthening by solid solution hardening in the ferrite phase of the pearlite structure, thereby improving the rail's resistance to internal fatigue damage. However, this effect is not obtained if the Cu content is less than 0.01%. On the other hand, if the Cu content exceeds 1.00%, a significant improvement in hardenability can lead to the formation of a martensitic structure in the rail head, making it easier for fatigue cracks to occur at the boundary between the martensitic and pearlite structures, which can reduce resistance to internal fatigue damage. For this reason, when including Cu, it is preferable to set the Cu content to 0.01 to 1.00%. A more preferable lower limit for the Cu content is 0.02%, 0.03%, or 0.05%. A more preferable upper limit for the Cu content is 0.80%, 0.60%, or 0.30%.

[0104] Ni: Preferably 0.01-1.00% Ni is an element that improves the toughness of the pearlite structure and, at the same time, improves hardness through solid solution strengthening, thereby improving the internal fatigue damage resistance of the rail. Furthermore, in the heat-affected zone of welding, Ni, in combination with Ti, finely precipitates an intermetallic compound of Ni3Ti, and suppresses softening through precipitation strengthening. In addition, Ni is an element that suppresses grain boundary embrittlement in Cu-containing steel. However, these effects are significantly small when the Ni content is less than 0.01%. On the other hand, when the Ni content exceeds 1.00%, a significant improvement in hardenability can lead to the formation of a martensitic structure in the rail head, making it easier for fatigue cracks to occur at the boundary between the martensitic and pearlite structures, which may reduce internal fatigue damage resistance. For this reason, when Ni is included, it is preferable to set the Ni content to 0.01 to 1.00%. The even more preferable lower limit for Ni content is 0.02%, 0.03%, or 0.05%. The even more preferable upper limit for Ni content is 0.80%, 0.60%, or 0.30%.

[0105] Nb: Preferably 0.0010~0.0500% Nb precipitates as Nb carbides and / or Nb nitrides during the cooling process after hot rolling, and through precipitation hardening, it increases the hardness of the pearlite structure and improves the internal fatigue damage resistance of rails. Furthermore, Nb is an effective element in preventing softening of the heat-affected zone of welded joints by stably generating Nb carbides and Nb nitrides from low to high temperatures in the heat-affected zone reheated to a temperature range below the Ac1 point. However, if the Nb content is less than 0.0010%, these effects are not obtained, and no improvement in the hardness (strength) of the pearlite structure is observed. On the other hand, if the Nb content exceeds 0.0500%, the precipitation hardening of Nb carbides and nitrides becomes excessive, the pearlite structure itself becomes brittle, and the internal fatigue damage resistance of rails may decrease. For this reason, when including Nb, it is preferable to have an Nb content of 0.0010 to 0.0500%. A more preferable lower limit for the Nb content is 0.0015%, 0.0020%, or 0.0025%. A more preferable upper limit for the Nb content is 0.0400%, 0.0300%, or 0.0100%.

[0106] Ti: Preferably 0.0030~0.0500% Ti (Ti) precipitates as Ti carbides and / or Ti nitrides during the cooling process after hot rolling, and through precipitation hardening, it increases the hardness of the pearlite structure, improving the internal fatigue damage resistance of rails. Furthermore, during reheating in welding, Ti is an effective component for refining the metal structure of the heat-affected zone heated to austenite temperatures by utilizing the fact that the precipitated Ti carbides and Ti nitrides do not dissolve, thereby preventing embrittlement of welded joints. However, these effects are minimal when the Ti content is less than 0.0030%. On the other hand, if the Ti content exceeds 0.0500%, coarse Ti carbides and Ti nitrides are formed, which can lead to stress concentration, fatigue crack formation, and a decrease in internal fatigue damage resistance. For this reason, when including Ti, it is preferable to set the Ti content to 0.0030 to 0.0500%. More preferable lower limits for the Ti content are 0.0040%, 0.0050%, or 0.0080%. A more preferable upper limit for the Ti content is 0.0400%, 0.0300%, or 0.0100%.

[0107] Mg: Preferably 0.0005 to 0.0200% Mg is an element that combines with S to form fine sulfides. These Mg sulfides finely disperse MnS, alleviate stress concentration, and improve the rail's resistance to internal fatigue damage. However, the effect is weak when the Mg content is less than 0.0005%. On the other hand, if the Mg content exceeds 0.0200%, coarse Mg oxides are formed, and stress concentration can lead to fatigue crack formation, potentially reducing the rail's resistance to internal fatigue damage. Therefore, when including Mg, it is preferable to set the Mg content between 0.0005% and 0.020%. The even more preferable lower limits for the Mg content are 0.0010%, 0.0015%, or 0.0020%. The even more preferable upper limits for the Mg content are 0.0100%, 0.0050%, or 0.0025%.

[0108] Ca: Preferably 0.0005 to 0.0200% Ca has a strong bonding force with S, forming CaS (sulfide). This CaS finely disperses MnS, easing stress concentration and improving the rail's resistance to internal fatigue damage. However, this effect is weak when the Ca content is less than 0.0005%. On the other hand, if the Ca content exceeds 0.0200%, coarse oxides of Ca are formed, and stress concentration can lead to fatigue crack formation, potentially reducing resistance to internal fatigue damage. Therefore, when including Ca, it is preferable to set the Ca content between 0.0005% and 0.0200%. The even more preferable lower limit for Ca content is 0.0010%, 0.0012%, or 0.0015%. The even more preferable upper limit for Ca content is 0.0150%, 0.0100%, or 0.0050%.

[0109] REM: Preferably 0.0005 to 0.0500% REM is a deoxidizing and desulfurizing element, and when present, it generates REM oxysulfide (REM2O2S), which acts as a nucleus for the formation of Mn sulfide inclusions. Because this oxysulfide (REM2O2S) has a high melting point, it suppresses the elongation of Mn sulfide inclusions after rolling. As a result, the presence of REM allows for fine dispersion of MnS, easing stress concentration and improving the internal fatigue damage resistance of the rail. However, if the REM content is less than 0.0005%, it is insufficient as a nucleus for the formation of MnS sulfides, and its effect is small. On the other hand, if the REM content exceeds 0.0500%, excessive hard REM oxysulfide (REM2O2S) is generated, which can lead to stress concentration, fatigue crack formation, and a decrease in internal fatigue damage resistance. Therefore, when including REM, it is preferable to keep the REM content between 0.0005% and 0.0500%. A more preferable lower limit for the REM content is 0.0010%, 0.0012%, or 0.0015%. A more preferable upper limit for the REM content is 0.0400%, 0.0300%, or 0.0100%.

[0110] REM refers to rare earth metals such as Ce, La, Pr, or Nd. The REM content is the total content of all these REMs. As long as the sum of the total content is within the above range, the same effect can be obtained whether used individually or in combination (two or more types).

[0111] Zr: Preferably 0.0001~0.0200% Zr combines with oxygen to form ZrO2 inclusions. These ZrO2 inclusions have good lattice compatibility with γ-Fe, so they act as solidification nuclei in high-carbon rail steel where γ-Fe is the primary solidification crystal. By increasing the equiaxed crystallization rate of the solidification structure, they refine the structure, finely disperse MnS, alleviate stress concentration, and improve the rail's resistance to internal fatigue damage. Furthermore, Zr suppresses the formation of segregation zones in the center of the cast slab, thereby inhibiting the formation of martensitic structures in the rail's segregated areas. However, if the Zr content is less than 0.0001%, the number of ZrO2 inclusions formed is small, and they do not act sufficiently as solidification nuclei. On the other hand, if the Zr content exceeds 0.0200%, a large number of coarse Zr inclusions are formed, which can lead to stress concentration, fatigue crack formation, and a decrease in the rail's resistance to internal fatigue damage. Therefore, when Zr is included, it is preferable that the Zr content be between 0.0001% and 0.0200%. The even more preferable lower limit for the Zr content is 0.0005%, 0.0010%, or 0.0012%. The even more preferable upper limit for the Zr content is 0.0100%, 0.0050%, or 0.0020%.

[0112] The rails according to this embodiment control the chemical composition, microstructure, internal hardness of the rail head, and the number density of nitrides containing Cr, Mn, and V in the range of 0.5 to 6.0 nm. Furthermore, by controlling the composition of the nitrides containing Cr, Mn, and V ((CA+MA) / VA), it is possible to improve the internal fatigue damage resistance of rails used in freight railways, which have recently been subjected to much larger repeated loads due to increased load capacity and congestion, and to significantly extend their service life.

[0113] Next, a preferred method for manufacturing the rail according to this embodiment will be described. The rail according to this embodiment can achieve the above-mentioned effects regardless of the manufacturing method, provided it possesses the above-mentioned components, metallic structure, etc. However, a manufacturing method including the steps shown below is preferable because it allows for the stable production of the rail according to this embodiment.

[0114] The rail manufacturing method according to this embodiment involves casting molten steel having the chemical composition of the rail according to this embodiment into a steel billet by a continuous casting method, heating the steel billet, hot rolling the heated steel billet to form a rail, and then accelerating and controlling the cooling of the rail. Preferred manufacturing conditions are shown in Table 1, and the specific reasons for these are explained below. Note that the temperatures shown as rolling temperature and heat treatment conditions (excluding the steel billet temperature) refer to the temperature of the outer surface of the rail head. In the rail according to this embodiment, the metal structure, hardness, and nitrides containing Cr, Mn, and V at a depth of 25 mm starting from the outer surface of the rail head need to be controlled, but the composition of other parts is not particularly limited, so the rolling temperature and heat treatment conditions are also determined with respect to the outer surface of the rail head.

[0115] [Table 1]

[0116] The rails of this embodiment are manufactured by melting molten steel in a conventional melting furnace such as a converter or electric furnace to adjust its composition to the above-mentioned range, casting it into a steel billet (bloom or slab) using a continuous casting method, reheating the steel billet and hot-rolling it to form a rail shape, and then heat-treating it after hot-rolling.

[0117] In the rails of this embodiment, to control the number density and composition (5 ≤ (CA + MA) / VA ≤ 100) of nitrides containing Cr, Mn, and V in the range of 0.5 to 6.0 nm, it is desirable to control the conditions during casting, the heating conditions during the heating of the steel billet before hot rolling, the rolling conditions during hot rolling, and the heat treatment conditions after rolling. Furthermore, to control the hardness and microstructure of the rail head, it is necessary to control the hot rolling conditions and the heat treatment conditions after rolling.

[0118] When casting steel billets (blooms or slabs) using the continuous casting method, if the solid fraction remains high for a long time, i.e., if the casting rate is too low, coarse nitrides containing alloying elements such as V, Cr, and Mn are formed (crystallized) in the molten steel where these elements are concentrated in the final solidification region. These coarse nitrides formed in the molten steel tend to remain as undissolved nitrides during the heating process of the hot rolling process. In other words, in the ferrite after pearlite transformation, there are fewer V, Cr, and Mn to form nitrides containing fine Cr, Mn, and V, and the precipitation strengthening by nitrides containing fine Cr, Mn, and V is reduced. As a result, the effect of suppressing the microscopic softening of the ferrite phase in the pearlite structure inside the rail head and improving the internal fatigue damage resistance inside the head is reduced. Also, if a large amount of Cr and Mn segregates during alloy segregation, V becomes excessive. Therefore, the (CA+MA) / VA value becomes less than 5, and microcracks occur around the nitride containing Cr, Mn, and V, so the resistance to internal fatigue damage does not improve. If there is a large amount of V segregation, the number of Cr and Mn atoms becomes excessive, and the (CA+MA) / VA value exceeds 100, and microcracks occur around the nitride containing Cr, Mn, and V, so the resistance to internal fatigue damage does not improve. On the other hand, if the casting speed is too fast, manufacturing problems such as slab cracking and powder inclusion are likely to occur. Therefore, in order to stably suppress the enrichment of V, Cr, and Mn, the casting speed should be 0.60 to 0.90 m / min in the temperature range of 1300 to 1400°C.

[0119] Next, we will explain the control of heating conditions in the heating of steel billets before rolling. Heating the steel billet is the most important step in stably generating fine V nitrides containing Cr during rail heat treatment. Since controlled cooling is not performed during steel billet production, the nitrides containing Cr, Mn, and V become coarser at the steel billet stage. Therefore, in order to stably generate fine V nitrides containing Cr after rail heat treatment, it is necessary to remelt the coarse nitrides containing Cr, Mn, and V in the steel billet before rolling. For this reason, it is necessary to control the heating conditions of the steel billet in the temperature range (1000~1200°C) in which nitrides containing Cr, Mn, and V remelt.

[0120] The following heating conditions for the steel billet are preferable. Heating rate: 1~8℃ / min Speed ​​control temperature range: 1000~1200℃ The above temperatures are the temperature conditions for the steel billet, and it is desirable to control the temperature of the heating furnace to match these heating conditions. Furthermore, it should be noted that the heating rate of the steel billet before hot rolling is not the average heating rate. That is, within the temperature range of 1000 to 1200°C, the time derivative of the steel billet's temperature must always be within the range of 1 to 8°C / min.

[0121] First, let's explain why it is preferable to set the heating rate of the steel billet in the range of 1 to 8°C / min. If the heating rate is less than 1°C / min, the nitrides containing Cr, Mn, and V that were coarsened during casting will remelt, but they will precipitate again during heating, causing the nitrides containing Cr, Mn, and V to coarse again and making it difficult to melt them. This can make it difficult to stably generate nitrides containing Cr, Mn, and V during rail heat treatment. Furthermore, if the heating rate is less than 1°C / min, the steel billet will be overheated, leading to decarburization of the steel billet surface and the occurrence of cracks in the steel billet, which may prevent the quality of the rail product after hot rolling and heat treatment from being ensured. In addition, if the heating rate is less than 1°C / min, a large amount of heating fuel will be used, which may reduce economic efficiency.

[0122] On the other hand, if the heating rate exceeds 8°C / min, it becomes difficult to remelt the nitrides containing Cr, Mn, and V that have been coarsened during casting, resulting in residual nitrides containing coarse Cr, Mn, and V. Furthermore, it may become difficult to stably generate nitrides containing Cr, Mn, and V during rail heat treatment. For this reason, it is preferable to set the heating rate in the range of 1 to 8°C / min.

[0123] As mentioned above, this heating rate represents the time derivative of the steel billet's temperature during heating. By consistently controlling the time derivative of the steel billet's temperature within the above range, it becomes possible to stably generate nitrides containing Cr, Mn, and V during the heat treatment of rails obtained by hot rolling the steel billet. The heating rate after the steel billet temperature exceeds 1200°C is not particularly limited. Furthermore, the temperature at which heating of the steel billet is stopped can be any value above 1200°C.

[0124] Next, the hot rolling conditions for the rails will be explained. In the rails of this application, when a nitride containing fine Cr, Mn, and V is formed in the ferrite after pearlite transformation, and the composition of the precipitate is defined as the ratio of the sum of the number of atoms of Cr (CA) and the number of atoms of Mn (MA) to the number of atoms of V (VA) (CA+MA) / VA, it is desirable to control the hot rolling process in order to ensure that this value stably stays within the range of 5 ≤ (CA+MA) / VA ≤ 100. In order to stably form a nitride containing Cr, Mn, and V with the aforementioned fine and specific composition in the ferrite of the pearlite structure, it is sufficient to create conditions during hot rolling that facilitate the formation of precipitates in the subsequent cooling process. In the hot rolling process, if dislocations introduced into the austenite during reduction remain after pearlite transformation, it is thought that V present in the ferrite of the pearlite structure can easily move to the dislocations, thus becoming so-called precipitation nuclei that facilitate the formation of nitrides containing Cr, Mn, and V. Therefore, by performing appropriate reduction so that dislocations remain in the austenite, the formation of nitrides containing Cr, Mn, and V in the pearlite during the subsequent cooling process is promoted. For this reason, it is desirable to perform the following reduction during hot rolling. (1) Hot rolling at a temperature of 1000-1050°C on the outer surface of the head, performing a reduction of 3-30% in cross-sectional area in one or more passes. (2) Hot rolling of the head outer surface at a temperature of 800-1000°C, performing one or more passes to reduce the cross-sectional area by 3-20%.

[0125] If the reduction in cross-sectional area exceeds the desirable range at each temperature, an excess of dislocations will remain after pearlite transformation, resulting in a large amount of V being present in the nitride during formation, causing the (CA+MA) / VA value to fall below 5. This significantly increases the hardness of the nitride containing Cr, Mn, and V, and increases the amount of microcracks (less than 0.5 mm) in the surrounding matrix, potentially reducing internal fatigue damage resistance. Since dislocations introduced during reduction tend to recover more easily at higher temperatures, a higher reduction in cross-sectional area is permissible for the maximum value of the reduction in cross-sectional area at 1000-1050°C under condition (1) than under condition (2) in order to retain the dislocations in the austenite. On the other hand, if the cross-sectional reduction rate falls below the desirable range at each temperature, dislocations disappear immediately due to the recovery phenomenon, making it impossible to retain dislocations in the austenite. This results in insufficient precipitation nucleation effect, making it difficult for V to be incorporated during the formation of nitrides containing Cr, Mn, and V. As a result, the (CA+MA) / VA value exceeds 100, the hardness of the nitride decreases, and the amount of microcracks (less than 0.5 mm) in the surrounding matrix increases, potentially reducing resistance to internal fatigue damage.

[0126] As long as the above conditions are met, there are no particular limitations on other rolling conditions for the rail head. That is, after rough rolling of the steel billet, intermediate rolling is performed over multiple passes using a reverse rolling mill, finish rolling is performed over two or more passes using a continuous rolling mill within the above temperature range and within the cross-sectional reduction rate limited for each temperature range, and the temperature should be controlled to the above temperature range during the final rolling.

[0127] Next, we will explain how to control the heat treatment conditions after rolling. To control the hardness and microstructure of the rail head, it is necessary to control the heat treatment conditions after rolling. Furthermore, to control the number density and particle size of nitrides containing Cr, Mn, and V, it is necessary to control the heat treatment conditions after rolling. It is desirable to perform the heat treatment conditions after rolling within the following range. Note that accelerated cooling is a cooling method performed by spraying a coolant such as water onto the rail surface. The start and end points of accelerated cooling are the start and end points of coolant spraying. Also, the cooling rate during accelerated cooling refers to the average cooling rate, and specifically, it is the value obtained by dividing the difference in rail surface temperature between the start and end points of accelerated cooling by the elapsed time between the start and end points of accelerated cooling.

[0128] ● Heat treatment conditions after hot rolling (outer surface of the head): After rolling, accelerated cooling and controlled cooling are performed. Accelerated cooling (outer surface of the head) Average cooling rate: 2~30℃ / sec Accelerated cooling start temperature: 750℃ or higher Accelerated cooling stop temperature: 580~660℃ Controlled cooling (head outer surface) After halting accelerated cooling, the temperature of the head's outer surface is maintained in the range of 580-660°C for 5-150 seconds, followed by natural cooling and then accelerated cooling. Temperature maintenance: Temperature is controlled by controlling the accelerated cooling rate, and further by repeatedly starting and stopping accelerated cooling, and by performing accelerated cooling in response to reheating from inside the rails.

[0129] Next, we will explain why it is preferable to set the average cooling rate of accelerated cooling (head outer surface) to 2-30°C / sec. If the average cooling rate falls below 2°C / sec, pearlite transformation begins in the high-temperature region during accelerated cooling. As a result, in the rail composition system according to this embodiment, areas with a hardness of less than Hv360 may occur on the rail head surface, making it difficult to ensure the wear resistance and internal fatigue damage resistance required for the rail. On the other hand, if the average cooling rate exceeds 30°C / sec, the hardness of the pearlite structure increases significantly in the rail composition system according to this embodiment, resulting in areas with a hardness exceeding Hv500. Furthermore, bainite and martensitic structures may form on the rail head surface, raising concerns about a decrease in the rail's wear resistance and internal fatigue damage resistance. For this reason, it is preferable to set the average cooling rate in accelerated cooling to 2 to 30°C / sec.

[0130] Next, we will explain why it is preferable to set the starting temperature for accelerated cooling (i.e., the rail temperature when refrigerant spraying begins) to 750°C or higher, and the stopping temperature (i.e., the rail temperature when refrigerant spraying ends) to be in the range of 580 to 660°C.

[0131] If the accelerated cooling start temperature of the rail head outer surface is below 750°C, a low-hardness pearlite structure may form in the high-temperature range before accelerated cooling. In this case, the required hardness (Hv360 or higher) cannot be obtained, making it difficult to ensure the wear resistance and surface damage resistance required for rails. Furthermore, in the above case, with steels that have a relatively high carbon content, a protruding cementite structure may form, the pearlite structure becomes brittle, and fatigue cracks are more likely to form, raising concerns that the rail's resistance to internal fatigue damage will decrease. For this reason, it is preferable to set the temperature of the rail head outer surface at 750°C or higher when accelerated cooling is started. Considering the final rolling temperature mentioned above, it is considered necessary to start accelerated cooling within 180 seconds after the completion of hot rolling in order to set the accelerated cooling start temperature at 750°C or higher.

[0132] Furthermore, if the accelerated cooling stop temperature exceeds 660°C, pearlite transformation begins in the high-temperature region immediately after cooling, resulting in the formation of a large amount of low-hardness pearlite structure. As a result, the hardness of the rail head (Hv360 or higher) cannot be ensured, making it difficult to ensure the wear resistance and internal fatigue damage resistance required for the rail. On the other hand, if the accelerated cooling stop temperature is below 580°C, a large amount of bainite structure is formed in the rail head immediately after cooling, making it difficult to ensure the wear resistance required for the rail. In addition, fatigue cracks are more likely to occur at the boundary between the bainite and pearlite structures, which may reduce internal fatigue damage resistance. For this reason, it is preferable to set the accelerated cooling stop temperature in the range of 580 to 660°C.

[0133] There are no particular limitations on the heat treatment refrigerant used for the rail head during accelerated cooling. To control the hardness within a predetermined range in order to impart wear resistance and resistance to internal fatigue damage to the rail, it is desirable to control the cooling rate of the rail head during heat treatment by air injection cooling, mist cooling, mixed water and air injection cooling, or a combination thereof.

[0134] Next, the preferred conditions for controlled cooling performed after accelerated cooling will be explained, along with the reasons for their limitations. This process significantly affects the number density of nitrides containing Cr, Mn, and V in the range of 0.5 to 6.0 nm. In the rail manufacturing method according to this embodiment, controlled cooling involves injecting a refrigerant according to the degree of reheating to maintain the rail temperature within a certain range for a certain period of time, and then lowering the rail temperature. In other words, the controlled cooling process can be described as a combination of a temperature holding process and a subsequent cooling process.

[0135] An example of a controlled cooling method is described below. In the rail manufacturing method according to this embodiment, the accelerated cooling described above is first terminated. The point at which accelerated cooling ends is set as the point at which temperature holding in controlled cooling begins. Upon termination of accelerated cooling, reheating occurs in the rail, and the surface temperature of the rail usually rises. Once the surface temperature of the rail has risen to a certain extent due to reheating, the surface temperature of the rail is lowered again by injecting a refrigerant into the rail. Once the surface temperature of the rail has dropped to a certain extent due to the injection of the refrigerant, the injection of the refrigerant into the rail is stopped, causing the surface temperature of the rail to rise again. In other words, temperature holding in controlled cooling of the rail is usually achieved by repeatedly raising the temperature due to reheating and processing the temperature by cooling. Thus, it is desirable to stop accelerated cooling on the lower end of the temperature range in which temperature holding is to be performed, start cooling in anticipation of the reheating generated from inside the rail head, and stop cooling before reaching the lower limit of the predetermined temperature range. Furthermore, in order to control the holding time, it is desirable to repeat this temperature control. If the amount of reheating is small, heating with an IH coil or the like is also effective. However, if the degree of reheating is small, the temperature fluctuation of the rail surface may be kept within a certain range even without injecting a refrigerant. In this case, the temperature can be maintained simply by leaving the rails undisturbed.

[0136] In the temperature maintenance of the controlled cooling described above, it is preferable to keep the rail surface temperature in the range of 580 to 660°C, keep the temperature fluctuation range of the rail surface within 60°C, and keep the temperature maintenance time in the range of 5 to 150 seconds.

[0137] First, we will explain why it is preferable to set the holding temperature after accelerated cooling in the range of 580 to 660°C and to keep the temperature fluctuation range of the rail surface within 60°C.

[0138] If the holding temperature exceeds 660°C, in the rail composition system according to this embodiment, the formation of nitrides containing Cr, Mn, and V is promoted inside the rail head, and the number density of nitrides containing Cr, Mn, and V with a particle size of 0.5 to 6.0 nm increases excessively. As a result, the pearlite structure inside the rail head becomes brittle, crack initiation is promoted, and there is a concern that the resistance to internal fatigue damage will decrease. On the other hand, if the holding temperature is below 580°C, the formation and growth of nitrides containing Cr, Mn, and V inside the rail head is suppressed, and the number density of nitrides containing Cr, Mn, and V with a particle size of 0.5 to 6.0 nm becomes insufficient. As a result, the microscopic softening of the ferrite phase in the pearlite structure may not be sufficiently improved, and an improvement in the rail's resistance to internal fatigue damage may not be observed. For this reason, it is preferable to set the holding temperature after accelerated cooling in the range of 580 to 660°C.

[0139] If the temperature fluctuation range due to reheating of the rail surface exceeds 60°C, the formation and growth of nitrides containing Cr, Mn, and V within the rail head may be accelerated during the holding time, potentially leading to an excessive number density. As a result, the pearlite structure within the rail head may become brittle, crack initiation may be accelerated, and there is a concern that the resistance to internal fatigue damage may decrease. For this reason, it is preferable to keep the temperature fluctuation range due to reheating of the rail surface below 60°C.

[0140] Next, we will explain why it is preferable to set the holding time in the range of 5 to 150 seconds. Note that when temperature is maintained by a combination of reheating and refrigerant spraying, the holding time refers to the period from the end of the accelerated cooling described above to the end of the final reheating (the point at which the rail temperature begins to decrease naturally, or the point at which refrigerant spraying begins). When temperature is maintained by reheating or transformation exothermic heat alone, the holding time refers to the period from the end of the accelerated cooling described above to the end of the reheating or transformation exothermic heat (the point at which the rail temperature begins to decrease naturally, or the point at which refrigerant spraying begins).

[0141] If the holding time exceeds 150 seconds, nitrides containing Cr, Mn, and V grow inside the rail head, increasing their particle size. As a result, the number density of nitrides containing Cr, Mn, and V with a particle size of 0.5 to 6.0 nm decreases, and improvement in the microscopic softening of the ferrite phase in the pearlite structure cannot be expected. On the other hand, if the holding time is less than 5 seconds, the formation and growth of nitrides containing Cr, Mn, and V are suppressed, and the particle size decreases. As a result, the number density of nitrides containing Cr, Mn, and V with a particle size of 0.5 to 6.0 nm is insufficient, the microscopic softening of the ferrite phase in the pearlite structure does not improve, and improvement in resistance to internal fatigue damage cannot be expected. For this reason, it is preferable to set the temperature holding time after accelerated cooling to 5 to 150 seconds.

[0142] There are no particular limitations on the method of maintaining temperature during controlled cooling. It is desirable to perform cooling that controls the regeneration heat generated from inside the rail head by repeatedly cooling and stopping the outer surface of the rail head using a refrigerant such as air injection cooling, mist cooling, mixed water and air injection cooling, or a combination thereof.

[0143] After the isothermal holding described above, the rail is cooled by air cooling and accelerated cooling. If the cooling rate of the rail after isothermal holding is too low, the pearlite structure may be tempered during holding, similar to when isothermal holding is continued for a long time. This may result in insufficient hardness on the surface and inside of the rail head, and a decrease in the number density of nitrides containing fine Cr, Mn, and V. Therefore, to prevent this, it is considered necessary to maintain a cooling rate of 0.5°C / sec or higher up to at least 200°C. Such cooling conditions can be achieved by leaving the rail in ambient air at room temperature or by accelerated cooling after the temperature holding described above. [Examples]

[0144] To confirm the effects of the present invention, an experiment was conducted according to the following procedure.

[0145] Tables 2-1, 2-2, and 2-3 show the chemical composition of the steel billets used in the experiment. Tables 3-1, 3-2, and 3-3 show the manufacturing conditions for the steel billets having the chemical composition described in Tables 2-1, 2-2, and 2-3. Tables 4-1, 4-2, and 4-3 show the results of the characterization evaluation of the manufactured rails. In these tables, values ​​outside the scope of the invention are underlined. The manufacturing conditions include hot rolling conditions, accelerated cooling conditions, and controlled cooling conditions after accelerated cooling.

[0146] The manufacturing conditions not listed in Tables 3-1 and 3-2 were as follows. During rolling and heat treatment (accelerated cooling, controlled cooling), the temperature at the outer surface position of the head shown in Figure 1 was measured to control the rolling and heat treatment. ●Casting rate in the temperature range of 1300~1400℃: 0.70℃ / min (0.50℃ / min for No. 58 only) ● Heating rate of steel slab: 3°C / min within the range of 1000-1200°C ●End temperature for heating the steel billet: 1250℃ ●Cooling after temperature maintenance: Cool to room temperature by leaving it in ambient air at room temperature. For No. 58 only, the casting rate in the temperature range of 1300-1400°C was set to 0.40°C / min.

[0147] The characteristic evaluation is (1) Area ratio of perlite tissue at a depth of 25 mm, starting from the outer surface of the head. (2) Hardness at a depth of 25 mm from the outer surface of the head, (3) State of precipitates (number density of nitrides containing Cr, Mn, and V with particle sizes of 0.5 to 6.0 nm, and (CA+MA) / VA), and (4) Resistance to internal fatigue damage The CA / VA was also evaluated and included in the table as reference information. These evaluations were performed using the following procedure.

[0148] (1) The area ratio of the pearlite tissue was measured by cutting a sample from the cross section of each rail head, diamond polishing each sample, and then etching it with 3% nital, followed by microscopic observation (200x magnification). The measurement field of view was set to 10 arbitrary fields of view at a depth of 25 mm from the outer surface of the rail head (see Figure 1). The average value of the area ratio of the pearlite tissue in the 10 arbitrary fields of view at a depth of 25 mm from the outer surface of the rail head was defined as the "area ratio of the pearlite tissue at the 25 mm position".

[0149] (2) Hardness was determined by cutting a sample from the cross-section of each rail head, polishing the portion of each sample corresponding to the rail cross-section with diamond abrasive grains with an average particle size of 1 μm, and then measuring the hardness using a Vickers hardness tester (load 98 N) in accordance with JIS Z 2244. Twenty measurements were taken at a depth of 25 mm from the outer surface of the rail head (see Figure 1), and the average value was taken as the "hardness at the 25 mm position". The measurement interval was 1 mm. Furthermore, the midpoint of the linear measurement area consisting of the 20 measurement points was positioned at the center of the rail width direction of the C cross-section of the rail.

[0150] (3) The state of the precipitate was determined by taking three needle samples with a radius of curvature of 30-80 nm from the ferrite phase in the pearlite structure at a depth of 25 mm from the outer surface of the head (see Figure 1) using the FIB (Focused Ion Beam) method, and evaluating these using the 3D Atom Probe (3DAP) method. The details of the evaluation conditions are as described above. The average number density of nitrides containing Cr, Mn, and V with particle sizes of 0.5-6.0 nm in the ferrite phase in the pearlite structure at a depth of 25 mm from the outer surface of the head (see Figure 1) in each needle sample obtained in this way was defined as the "number density of nitrides containing Cr, Mn, and V with particle sizes of 0.5-6.0 nm". Furthermore, for each needle sample, 10 nitrides containing Cr, Mn, and V between 0.5 and 6.0 nm were randomly selected. For each nitride, the number of Cr, Mn, and V atoms was counted, and the ratio of the sum of the number of Cr atoms (CA) and Mn atoms (MA) to the number of V atoms (VA) was calculated. This measurement was performed for three needle samples, and the average value of the ratio of the sum of the number of Cr atoms (CA) and Mn atoms (MA) to the number of V atoms (VA) for a total of 30 nitrides containing Cr, Mn, and V was rounded to the first decimal place and defined as "(CA+MA) / VA". In addition, the average value of the ratio of the number of Cr atoms (CA) to the number of V atoms (VA) for a total of 30 nitrides containing Cr, Mn, and V was defined as "CA / VA".

[0151] (4) The internal fatigue damage resistance was evaluated using a rolling fatigue testing machine as shown in Figure 2. The test specimen was a 2m 136-pound rail, and the wheel in contact with it was an AAR type (diameter 920mm). The load applied to the wheel was radial: 290-340KN and thrust: 60-90KN. Oil lubrication was used.

[0152] During the evaluation of internal fatigue damage resistance, an ultrasonic flaw detector was used to check for cracks inside the rail head. Cracks longer than 2 mm were considered damage, and the above test was repeated five times until damage occurred. If no damage occurred, the test was stopped at 200 MGT (Million Gloss Tonnage), and the cumulative tonnage passed until damage occurred was considered to be 200 MGT. The average value of the cumulative tonnage passed until damage occurred was used as an index for evaluating the rail's internal fatigue damage resistance. The evaluation criteria were as follows. Rails judged as A to C rank in the evaluation criteria below were judged to be rails with excellent internal fatigue damage resistance. A rank of X meant that the rail failed internal fatigue damage resistance. A: The cumulative tonnage passed through the area at the time of damage is between 175 and 200 MGT. B: The cumulative tonnage passed at the time of damage is between 150 and 175 MGT. C: Cumulative tonnage at the time of damage is between 100 and 150 MGT. X: The cumulative tonnage passed through at the time of damage is 100 MGT or less.

[0153] [Table 2-1]

[0154] [Table 2-2]

[0155] [Table 2-3]

[0156] [Table 3-1]

[0157] [Table 3-2]

[0158] [Table 3-3]

[0159] [Table 4-1]

[0160] [Table 4-2]

[0161] [Table 4-3]

[0162] As shown in Tables 4-1, 4-2, and 4-3, rails whose chemical composition, area ratio of the pearlite structure at the 25 mm position, hardness at the 25 mm position, number density of nitrides containing Cr, Mn, and V in the range of 0.5 to 6.0 nm, and (CA+MA) / VA were within the range of the present invention exhibited excellent resistance to internal fatigue damage. Furthermore, rails with the addition of Mo, Co, B, Cu, Ni, Nb, Ti, Mg, Ca, REM, Zr, and Al as optional additives (No. 24 to 36) showed even greater resistance to internal fatigue damage due to effects such as strengthening of the pearlite structure through solid solution strengthening and precipitation strengthening, and reduction of stress concentration due to fine dispersion of MnS, i.e., suppression of fatigue crack formation.

[0163] On the other hand, comparative steel rails in which one or more of the following were outside the scope of the present invention—chemical composition, area ratio of pearlite structure at the 25 mm position, hardness at the 25 mm position, number density of nitrides containing Cr, Mn, and V in the range of 0.5 to 6.0 nm, and (CA+MA) / VA—failed to pass the internal fatigue damage resistance test.

[0164] No. 6 exhibited impaired resistance to internal fatigue damage. This is thought to be because the excess Mn led to the formation of a large amount of martensitic structure, resulting in insufficient pearlite structure and excessive hardness. This made fatigue cracks more likely to occur at the boundary between the martensitic and pearlite structures. In addition, the excessive number density of nitrides containing Cr, Mn, and V in the 0.5-6.0 nm range caused the pearlite structure inside the rail head (at a depth of 25 mm from the outer surface of the head) to become brittle, accelerating fatigue crack initiation and thus impairing resistance to internal fatigue damage.

[0165] No. 9 exhibited impaired resistance to internal fatigue damage. This is thought to be due to a deficiency in Mn, which led to the formation of a large amount of protereminate ferrite, resulting in insufficient quantity and hardness of the pearlite structure. Furthermore, the number density of nitrides containing Cr, Mn, and V with a size of 0.5-6.0 nm precipitated within the ferrite phase of the pearlite structure was insufficient, resulting in inadequate improvement of the microscopic softened areas of the ferrite phase within the pearlite structure.

[0166] No. 10 exhibited impaired resistance to internal fatigue damage. This is thought to be because the excess Cr led to the formation of a large amount of martensite, resulting in insufficient pearlite and excessive hardness. This made fatigue cracks more likely to occur at the boundary between the martensite and pearlite structures. In addition, the excessive number density of nitrides containing Cr, Mn, and V in the 0.5-6.0 nm range caused the pearlite structure inside the rail head (at a depth of 25 mm from the outer surface of the head) to become brittle, accelerating fatigue crack initiation and thus impairing resistance to internal fatigue damage.

[0167] No. 13 exhibited impaired resistance to internal fatigue damage. This is thought to be because the lack of Cr softened the pearlite structure, and the insufficient number density of nitrides containing Cr, Mn, and V in the 0.5-6.0 nm range prevented the suppression of localized softening of the ferrite phase within the pearlite structure.

[0168] No. 14 exhibited impaired resistance to internal fatigue damage. This is thought to be because the excess V resulted in an excessive number density of nitrides containing Cr, Mn, and V in the 0.5-6.0 nm range, leading to embrittlement of the pearlite structure.

[0169] No. 17 exhibited impaired resistance to internal fatigue damage. This is thought to be because the deficiency of V resulted in insufficient number density of nitrides containing Cr, Mn, and V in the 0.5-6.0 nm range, preventing suppression of localized softening of the ferrite phase in the pearlite structure.

[0170] No. 18 exhibited impaired resistance to internal fatigue damage. This is thought to be because the excess nitrogen led to an excessive number density of nitrides containing Cr, Mn, and V in the 0.5-6.0 nm range, resulting in embrittlement of the pearlite structure.

[0171] No. 21 exhibited impaired resistance to internal fatigue damage. This is thought to be because the lack of nitrogen resulted in insufficient number density of nitrides containing Cr, Mn, and V in the 0.5-6.0 nm range, preventing suppression of localized softening of the ferrite phase in the pearlite structure.

[0172] No. 40 exhibited impaired resistance to internal fatigue damage. This is thought to be because, during controlled cooling performed after accelerated cooling, the holding temperature exceeded 660°C, promoting the formation of nitrides containing Cr, Mn, and V. This resulted in an excessive number density of nitrides containing Cr, Mn, and V in the 0.5-6.0 nm range, leading to embrittlement of the pearlite structure.

[0173] No. 41 exhibited impaired resistance to internal fatigue damage. This is thought to be because, during controlled cooling performed after accelerated cooling, the temperature fluctuation range due to surface reheating exceeded 60°C. This promoted the formation and growth of nitrides containing Cr, Mn, and V within the rail head during the holding time, resulting in an excessive number density of nitrides containing Cr, Mn, and V in the 0.5-6.0 nm range, which embrittled the pearlite structure.

[0174] No. 47 exhibited impaired resistance to internal fatigue damage. This is thought to be because, during controlled cooling performed after accelerated cooling, the holding temperature was below 580°C, which suppressed the formation and growth of nitrides containing Cr, Mn, and V. As a result, the number density of nitrides containing Cr, Mn, and V in the 0.5-6.0 nm range was insufficient, and localized softening of the ferrite phase in the pearlite structure was not suppressed.

[0175] No. 48 exhibited impaired resistance to internal fatigue damage. This is thought to be because, during controlled cooling performed after accelerated cooling, the holding time exceeded 150 seconds, causing nitrides containing Cr, Mn, and V to grow inside the rail head. As a result of this rapid increase in particle size, the number density of nitrides containing Cr, Mn, and V in the 0.5-6.0 nm range decreased, and local softening of the ferrite phase in the pearlite structure was not suppressed.

[0176] No. 49 exhibited impaired resistance to internal fatigue damage. This is thought to be because, during controlled cooling performed after accelerated cooling, the holding time was less than 5 seconds, which suppressed the formation and growth of nitrides containing Cr, Mn, and V. As a result, the number density of nitrides containing Cr, Mn, and V in the 0.5-6.0 nm range was insufficient, and localized softening of the ferrite phase in the pearlite structure was not suppressed.

[0177] No. 50 exhibited impaired resistance to internal fatigue damage. This is thought to be because, during hot rolling, the reduction in cross-sectional area during rolling at 1020°C was too low, causing the (CA+MA) / VA value to exceed 100. This reduced the hardness of the nitride containing Cr, Mn, and V, leading to an increase in the formation of microcracks (less than 0.5 mm) in the surrounding matrix, and consequently, a decrease in resistance to internal fatigue damage.

[0178] No. 51 exhibited impaired resistance to internal fatigue damage. This is thought to be because, during hot rolling, the reduction in cross-sectional area during rolling at 920°C was too low, causing the (CA+MA) / VA value to exceed 100. This reduced the hardness of the nitride containing Cr, Mn, and V, leading to an increase in the formation of microcracks (less than 0.5 mm) in the surrounding matrix, and consequently, a decrease in resistance to internal fatigue damage.

[0179] No. 56 exhibited impaired resistance to internal fatigue damage. This is thought to be because the reduction in cross-sectional area during hot rolling at 1040°C was too high, resulting in a (CA+MA) / VA value of less than 5. This significantly increased the hardness of the nitride containing Cr, Mn, and V, leading to a large amount of microcracks (less than 0.5 mm) in the surrounding matrix, and consequently reducing resistance to internal fatigue damage.

[0180] No. 57 exhibited impaired resistance to internal fatigue damage. This is thought to be because the reduction in cross-sectional area during hot rolling at 900°C was too high, resulting in a (CA+MA) / VA value of less than 5. This significantly increased the hardness of the nitride containing Cr, Mn, and V, leading to a large amount of microcracks (less than 0.5 mm) in the surrounding matrix, and consequently reducing its resistance to internal fatigue damage.

[0181] No. 58 exhibited impaired resistance to internal fatigue damage. This is thought to be because the casting rate was too low, causing excessive segregation of V and the formation (crystallization) of coarse V nitrides in the molten steel. Specifically, the coarse nitrides formed in the molten steel remained as undissolved nitrides during the heating process of the hot rolling process. When fine nitrides containing Cr, Mn, and V were formed in the ferrite after pearlite transformation, the number of Cr and Mn atoms became excessive, causing the (CA+MA) / VA value to exceed 100. This reduced the hardness of the nitrides containing Cr, Mn, and V, leading to an increase in the formation of microcracks (less than 0.5 mm) in the surrounding matrix, and consequently, a decrease in resistance to internal fatigue damage.

Claims

1. In unit mass%, C: 0.75-1.20%, Si: 0.10-2.00%, Mn: 0.05-2.00%, Cr: 0.05-2.00%, V: 0.005-0.100%, Al: 0.0010-1.0000%, N: 0.006-0.020%, P ≤ 0.025%, S ≤ 0.025%, Mo: 0 to 0.50%, Co: 0-1.00%, B: 0 to 0.0050%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Nb: 0 to 0.0500%, Ti: 0 to 0.0500%, Mg: 0 to 0.0200%, Ca: 0-0.0200%, REM: 0-0.0500%, and Zr: 0-0.0200% It contains, with the remainder being Fe and impurities. Starting from the outer surface of the head and at a depth of 25 mm, the metallic structure contains pearlite tissue accounting for more than 95% of the area. The hardness of the rail, measured at the aforementioned position 25 mm deep from the outer surface of the head, is in the range of Hv360 to 500. At the aforementioned position, 25 mm deep from the outer surface of the head, the number density of nitrides containing Cr, Mn, and V with a particle size of 0.5 to 6.0 nm in the ferrite phase of the pearlite structure is 1 cm ３ 1.0 x 10 １７ ~5.0 x 10 １７ It is within the range of an individual, In the nitride containing Cr, Mn, and V with a particle size of 0.5 to 6.0 nm, located at a depth of 25 mm from the outer surface of the head, the average value of the ratio of the sum of the number of atoms of Cr (CA) and the number of atoms of Mn (MA) to the number of atoms of V (VA), rounded to the first decimal place ((CA + MA) / VA), satisfies the following formula 1. A rail characterized by its features. 5≦(CA+MA) / VA≦100 … Formula 1

2. In unit mass%, Mo: 0.01-0.50%, Co: 0.01 to 1.00%, B: 0.0001 to 0.0050%, Cu: 0.01 to 1.00%, Ni: 0.01-1.00%, Nb: 0.0010-0.0500%, Ti: 0.0030-0.0500%, Mg: 0.0005-0.0200%, Ca: 0.0005-0.0200%, REM: 0.0005 to 0.0500%, and Zr: 0.0001-0.0200% Contains one or more of the following: The rail according to feature 1.

Images