Pearlite rail

Abstract

The pearlite rail according to an embodiment of the present disclosure includes, in terms of mass%, C: 0.65 to 1.20%, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, Cr: 0.02 to 2.00%, N: 0.0020 to 0.0200%, V: 0.010 to 0.100%, Nb: 0% or more and less than 0.005%, Ti: 0% or more and less than 0.005%, Mo: 0 to 0.100%, and the like, and a balance including Fe and impurities, wherein a microstructure in a range from a head outer surface to a depth of 25 mm includes a pearlite structure in an area fraction of 95% or more; hardness at a position having a depth of 25 mm from a surface of a head top is at least HV350; and on a cementite interface of the pearlite structure at the position having a depth of 25 mm from the surface of the head top, nitride particles containing V and having a particle size of 4 nm or more are present in a number density of less than 1.0 × 1010 cm-2 per unit interface area.

Description

Pearlite Rail

[0001] The present disclosure relates to a pearlitic rail intended to simultaneously improve the wear resistance and ductility of the head of a rail used in heavy-duty railways.

[0002] Heavy-duty railways overseas are promoting the high density of freight in order to improve the efficiency of rail transport. In particular, rails with sharp curves are unable to ensure sufficient wear resistance in the gauge corner (G.C.) and head sections. This has led to a problem of reduced rail life due to wear. Given this background, there is a demand for the development of rails with wear resistance equal to or greater than that of currently used high-strength rails containing eutectoid carbon steel.

[0003] It is generally known that the higher the hardness of a rail, the better its wear resistance. However, as described in Non-Patent Document 1, it has been reported that wear resistance improves when the surface layer undergoes work hardening during use, increasing its hardness. Furthermore, as described in Non-Patent Document 2, it has been shown that pearlitic steel, which has excellent work hardening properties, has better wear resistance than martensitic steel or bainitic steel, which simply have high initial hardness. However, it has not been shown what microstructural structure of pearlitic steel has superior wear resistance, and there has been insufficient guidance for rail manufacturing.

[0004] Patent Document 1 describes that adding a certain amount of a carbonitride-forming element such as Ti, V, Nb, or Mo generates these carbonitride precipitate particles in the austenite phase, suppresses the grain growth of austenite during hot rolling, and makes the grains finer, thereby reducing the block size after pearlite transformation and improving ductility.

[0005] Furthermore, Patent Document 2 discloses a method for improving wear resistance by adding elements such as W, V, and Nb to precipitate carbonitrides in the ferrite phase after pearlite transformation, thereby strengthening the ferrite phase. These two documents suggest that adding the same type of elements can improve different properties of rails.

[0006] While these publications mainly use carbides as precipitates in pearlite, Patent Document 3 describes improving fatigue damage resistance and wear resistance by precipitating fine particles made of nitride in ferrite. Patent Document 4 also describes improving fatigue damage resistance and delayed fracture resistance by precipitating fine particles made of nitride in ferrite. These publications specify the particle size, number density, and chemical composition of the nitride particles precipitated in ferrite.

[0007] Patent Document 5 describes a steel rail containing, by mass%, C: 0.65 to 1.20%, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, Cr: 0.02 to 2.00%, and further containing at least 0.005% or more of two or more of V, Nb, Ti, and Mo, and containing V+Nb+Ti+Mo: 0.02 to 0.20%, with the balance being Fe and unavoidable impurities, and having a head hardness of at least 340 Hv, in which the number density of carbonitriding precipitate particles of 10 nm or less in the ferrite phase is 5×10 １５ cm －３ A pearlite rail is disclosed, characterized in that:

[0008] Patent Document 6 discloses a method for producing a high-toughness rail, in which, when steel containing 0.60 to 1.20% C is rolled into a rail, continuous finish rolling is performed in two or more passes at a temperature of 850 to 1000°C with a cross-sectional area reduction rate of 5 to 30% per pass and with a time between rolling passes of 8 seconds or less, followed by cooling to 800 to 950°C at a cooling rate of 0.5 to 50°C / s, and then natural cooling or accelerated cooling.

[0009] Ueda, M. et al., "Effect of Hardness and Carbon Content on Rolling Contact Wear of Pearlitic Steels," Iron and Steel, Japan, Iron and Steel Institute of Japan, 2001, Vol. 87, No. 4, pp. 190-197 Ueda, M. et al., "Effect of Metal Microstructure on Rolling Contact Wear of High Carbon Steels," Iron and Steel, Japan, Iron and Steel Institute of Japan, 2004, Vol. 90, No. 12, pp. 1023-1030

[0010] Japanese Patent Application Laid-Open No. 2010-1500 Japanese Patent Application Laid-Open No. 2007-51348 International Publication No. 2020 / 054339 Japanese Patent Application Laid-Open No. 2020-70495 Japanese Patent Application Laid-Open No. 2015-158006 Japanese Patent Application Laid-Open No. 2001-234238

[0011] In light of this background, there has been a demand for pearlitic rails for use in heavy-duty railways that have excellent head hardness, wear resistance, and ductility, as well as for methods for manufacturing such rails. The use of nitride particles, which has been practiced in recent years, is excellent from the viewpoints of removing unnecessary solute N that has remained as an impurity until now, and of rail hardening. However, knowledge regarding the location of precipitation of nitride particles in pearlite has not been sufficient. The present disclosure has been devised in view of the above-mentioned problems, and its purpose is to simultaneously improve the head wear resistance and ductility required for rails for heavy-duty railways.

[0012] The present inventors have conducted detailed microstructural studies on the surface hardness, internal hardness, and rolling wear surface of pearlitic rails. As a result, they have discovered that wear tests narrow the spacing of pearlite lamellae in the contact surface region (refining pearlite lamellae) and further microcrystallize them, resulting in a significant increase in hardness in the surface region, and that the greater the increase in hardness, the better the wear resistance. This increase in hardness due to thinning of lamellae and microcrystallization facilitates plastic deformation because the lamellar ferrite (ferrite phase) in pearlite is soft and ductile, making it easier for it to undergo plastic deformation, and the pearlite structure consisting of lamellar cementite and lamellar ferrite to become thinner and finer. It has now been discovered that any factors that hinder this process can lead to reduced wear resistance and fatigue damage.

[0013] In pearlitic rails utilizing nitride precipitate particles and clusters, it has been found that the precipitation positions of nitride particles in the lamellae of the pearlite structure 1 are mainly present in two locations: (1) the center of the lamellar ferrite 12, and (2) the cementite 11 / ferrite 12 interface (hereinafter referred to as the cementite interface 13), as shown in Figure 1. The nitride particles 22 distributed in the center of the ferrite 12 contribute to the hardening of the pearlite structure 1 through the hardening of the ferrite 12. On the other hand, the nitride particles 21 precipitated at the cementite interface 13 hardly contribute to hardening. Furthermore, because the nitride particles 21 precipitated at the cementite interface 13 are relatively large, they do not undergo plastic deformation or decompose even with repeated wear of the rail. Therefore, it has been found that the nitride particles 21 precipitated at the cementite interface 13 cause a decrease in the wear resistance and ductility of the rail. Therefore, it has been found that in a nitride-utilizing pearlite rail, it is necessary to reduce the number of nitride particles 21 on the cementite interface 13 and to make their size smaller.

[0014] On the other hand, the nitride particles 22 precipitated within the ferrite 12, rather than on the cementite interface 13, strengthen the ferrite 12, thereby increasing the hardness of the rail itself. In particular, when the nitride particles 22 precipitated within the ferrite 12 are fine and distributed at a high number density, they can be utilized for hardening without reducing the wear resistance or ductility.

[0015] The present disclosure has been made based on the above-mentioned new findings in order to solve the above-mentioned problems, and the gist of the disclosure is as follows.

[0016] (1) A pearlite rail according to an embodiment of the present disclosure contains, in mass%, C: 0.65 to 1.20%, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, Cr: 0.02 to 2.00%, N: 0.0020 to 0.0200%, V: 0.010 to 0.100%, Nb: 0% or more and less than 0.005%, Ti: 0% or more and less than 0.005%, Mo: 0 to 0.100%, B: 0 to 0.0050%, Co: 0 to 1.00%, Ni: 0 to 1.00%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, Cu: 0 to 1.00%, REM: 0 to 0.0500%, and Zr: 0 The head has a structure ranging from the outer surface of the head to a depth of 25 mm, and contains a pearlite structure with an area ratio of 95% or more. The hardness at a position 25 mm deep from the surface of the top of the head is at least HV350. The number density per unit interface area of ​​V-containing nitride particles having a particle size of 4 nm or more and present on the cementite interface of the pearlite structure at the position 25 mm deep from the surface of the top of the head is 1.0 × 10 １０ pieces cm －２ (2) In the pearlitic rail according to the above (1), preferably, nitride particles containing V and having a particle size of 0.5 to 4 nm, which are separated from the cementite interface in the ferrite of the pearlitic structure, have a number density per unit volume of less than 4.0 × 10 １６ ~4.0 x 10 １７ pieces cm －３ (3) Preferably, the pearlitic rail according to (1) or (2) above further contains, in mass%, one or more elements selected from the group consisting of B: 0.0001 to 0.0050%, Co: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Mg: 0.0005 to 0.0200%, Ca: 0.0005 to 0.0200%, Cu: more than 0% and not more than 1.00%, REM: 0.0005 to 0.0500%, Zr: 0.0001 to 0.0200%, and Al: more than 0% and not more than 1.000%.

[0017] According to the present disclosure, it is possible to provide a pearlitic steel rail that is excellent in wear resistance and ductility. That is, the present disclosure presents a method for producing a rail with improved wear resistance and ductility by suppressing the deterioration of wear resistance due to nitride particles precipitated at the cementite / ferrite interface.

[0018] FIG. 1 is a schematic diagram showing the distribution of precipitated particles in the pearlite structure of a rail; FIG. 2 is a diagram showing the relationship between the number density of nitride particles on the cementite interface and the wear amount; FIG. 3 is a diagram showing the designations of rail heads; FIG. 4 is an example of a three-dimensional element map of Fe (iron) in a V-added rail; FIG. 5 is an example of a three-dimensional element map of C (carbon) in a V-added rail; ２＋ This is an example of a three-dimensional element map of V (vanadium) detected by ions. ２＋ 4D is an example of a three-dimensional elemental map of V (vanadium) detected by ions. 4D is an example of a three-dimensional elemental map of C, which is an enlargement of the boxed region X in FIG. 4D. 4D is an example of a three-dimensional elemental map of V, which is an enlargement of the boxed region X in FIG. 4D. 4D is an example of a three-dimensional elemental map of Cr, which is an enlargement of the boxed region X in FIG. 4D. 4D is an example of a three-dimensional elemental map of Mn, which is an enlargement of the boxed region X in FIG.

[0019] The present inventors conducted rolling wear tests on pearlitic steels with various compositions and manufacturing methods to investigate what factors strongly affect the wear resistance of pearlitic rails that utilize nitrides. The inventors then observed the microstructures before and after the wear tests using a transmission electron microscope (TEM) to investigate in detail the relationship between the microstructural changes and the wear resistance properties. Furthermore, the present inventors used a three-dimensional atom probe to investigate the location, number density, and composition of fine nitride particles (including even finer clusters) in the pearlitic steel and to investigate their relationship with the wear resistance properties of pearlitic rails.

[0020] FIG. 1 is a diagram showing the distribution of nitride precipitates and clusters in a pearlite structure 1 of a rail. Generally, pearlite lamellae are composed of ferrite lamellae 12 with a width of 80 to 200 nm and cementite lamellae 11 with a width of 10 to 20 nm. To increase the hardness of a rail with the same carbon content, the pearlite lamellar spacing can be reduced (strengthening by refinement) or the hardness of ferrite 12 can be increased. While increasing the hardness of cementite 11 is also possible, the hardness of cementite 11 is already quite high, and it is practically difficult to intentionally increase this hardness further. Furthermore, simply significantly increasing the hardness of ferrite 12 would make the rail surface less susceptible to plastic deformation during use, preventing the increase in hardness of the wear surface due to thinner and finer pearlite lamellae. On the other hand, when hardening is achieved with very fine nitride particles, plastic deformation is not completely suppressed, and it has been found that the fine nitride particles decompose during wear and do not become the starting points for cracks or voids, but form solute atoms or dipoles of N with V or Cr, etc., which contribute to hardness and wear resistance.

[0021] Detailed structural observations revealed that nitride particle precipitation occurs mainly at two locations: the cementite interface 13 (i.e., the interface between the lamellar cementite 11 and the lamellar ferrite 12) and the central portion of the lamellar ferrite 12, which is distant from the cementite interface 13. Here, the central portion of the lamellar ferrite 12 refers to a region 5 nm or more away from the cementite interface 13. It was thought that, among the cementite interface 13 and the central portion of the ferrite 12, the nitride particles 22 distributed in the central portion of the ferrite 12 contribute to hardening, while the nitride particles 21 precipitated on the cementite interface 13 do not contribute to hardening. Furthermore, since slightly larger nitride particles do not deform or decompose due to wear, strain concentrates around the nitride particles, which are thought to become the origin of voids and cracks, thereby causing a decrease in wear resistance, ductility, and the like. Therefore, it is necessary to make the nitride particles 21 on the cementite interface 13 smaller, and to reduce the number density of larger ones. In addition, it was thought that by precipitating the nitride particles 22 in the ferrite 12 finely and in high number density, they could contribute to hardening without reducing ductility or wear resistance.

[0022] By devising chemical compositions and heat treatments, the inventors have produced pearlite rails in which the hardness (strength) of the initial pearlite is within a certain range, but the size and distribution of nitride particles in the pearlite are different. The relationship between nitride particle distribution and wear resistance was investigated through structural analysis and wear tests of these pearlite rails. A Nishihara wear tester was used to quantitatively evaluate wear resistance. In the wear tests, the wear amounts were compared after 700,000 cycles at a contact pressure of 640 MPa and a sliding ratio of 20%.

[0023] Figure 2 shows (1) the relationship between the number density per unit interfacial area of ​​nitride particles 4 nm or larger observed at the cementite interface and the wear loss for each pearlite rail sample, and (2) the relationship between the number density per unit interfacial area of ​​nitride particles 0.5 nm or larger but smaller than 4 nm at the cementite interface and the wear loss for a rail sample that does not contain nitride particles 4 nm or larger. Rail samples used here had a Vickers hardness of 380 to 400 Hv at a depth of 25 mm from the surface. The particle size and number density of the precipitated particles were investigated using a three-dimensional atom probe.

[0024] According to the investigation results shown in Fig. 2, the number density of nitride particles with a diameter of 4 nm or more on the cementite interface was 1.0 × 10 １０ pieces cm －２ It can be seen that the wear amount increases when the number density of nitride particles with a diameter of 4 nm or more on the cementite interface is 1.0 × 10 １０ pieces cm －２ If the diameter of the nitride particles is less than 0.5 nm, the wear amount of the rail is not much different from that of a rail containing substantially no precipitates. Furthermore, when nitride particles with a diameter of 0.5 nm or more and less than 4 nm are plotted on this graph, it is found that there is no correlation with the wear amount. In other words, even if relatively small nitride particles precipitate on the cementite interface, their effect on the deterioration of wear resistance is small.

[0025] The presence of relatively large nitride particles on the cementite interface not only does not contribute to improving rail hardness, but also causes strain accumulation at the particle position of the cementite interface during wear tests, leading to the formation of voids and cracks, which can cause fracture. Therefore, the presence of many relatively large nitride particles on the cementite interface is interpreted as an increase in the amount of wear. However, if the nitride particles are very fine, they will decompose during wear. Therefore, strain does not accumulate around the fine nitride particles, and this does not lead to a decrease in wear resistance.

[0026] From the above basic experiments, in order to improve the wear resistance of pearlitic rail steel that utilizes nitride particles for hardening, it is desirable to make the size of the nitride particles on the cementite interface very small or to reduce the number density of large nitride particles on the cementite interface.

[0027] On the other hand, for nitride precipitation in pearlite, the cementite interface serves as a preferred nucleation site. Therefore, nitride precipitation at the cementite interface is more likely than nitride precipitation within ferrite grains, which acts to harden the steel. Therefore, when nitride particles are precipitated within ferrite grains, it is difficult to completely suppress precipitation at the cementite interface. It is necessary to reduce the precipitation of nitride particles at the cementite interface by narrowing the range of chemical composition and thermomechanical processing conditions.

[0028] The reasons for the limited ranges of the pearlitic rail according to this embodiment are explained below. The head of a pearlitic rail is usually worn during use due to contact with wheels. Therefore, from the perspective of extending its service life, pearlitic rails are required to have properties suitable for railway rails not only on the surface of the head vertex but also at a depth of 25 mm from the head vertex. Furthermore, pearlite lamellae are more likely to be refined at the surface of a pearlitic rail than at a depth of 25 mm, resulting in higher hardness, making it easier to ensure wear resistance. A pearlitic rail that has appropriate properties at a depth of 25 mm from the head vertex surface will naturally also have appropriate properties at the surface of the head vertex. Taking the above into consideration, in the pearlitic rail according to this embodiment, the particle size, number density, and hardness of V-containing nitrides are limited at a depth of 25 mm from the head vertex surface. Furthermore, in the pearlitic rail according to this embodiment, the amount of pearlite structure is limited within a range from the outer surface of the head vertex to a depth of 25 mm.

[0029] (Regarding V-Containing Nitrides) The pearlite rail according to this embodiment specifies V-containing nitride particles. V-containing nitride particles refer to nitride particles with a V content of 1 atomic % or more. Generally, nitrides are crystalline phases composed of N and metal elements. They include nitrides composed of a single metal element, such as VN, TiN, NbN, and CrN, as well as nitrides composed of multiple metal elements, such as (V,Cr)N and (Ti,V)N. (V,Cr)N refers to a nitride containing V and Cr, and (Ti,V)N refers to a nitride containing Ti and V. The elements in parentheses are listed in descending order of atomic number. Generally, fine particles that are compatible with the ferrite matrix have a NaCl-type crystal structure. On the other hand, carbides are crystalline phases in which N is replaced by C, and may contain small amounts of N as carbonitrides. The nitride particles described in this disclosure do not contain C. "Not contained" means that the concentration is equal to or less than the concentration of dissolved carbon in the matrix (ferrite phase) in which the nitride particles exist. Nitrides can precipitate finer than carbides and are often called clusters. In this specification, such fine particles (0.5 to 4 nm) are also referred to as nitride particles.

[0030] (1) Particle size and number density of V-containing nitrides on the cementite interface in the pearlite structure at a depth of 25 mm from the outer surface of the head. The number density of nitride particles on the cementite interface has almost no effect on the strength of the ferrite phase. This is because the nitride particles on the cementite interface do not hinder the movement of dislocations in the ferrite. However, the nitride particles on the cementite interface did affect the wear resistance. In particular, the wear resistance decreased as the number density of nitride particles on the cementite interface increased beyond a certain size. This is because large nitride particles do not deform or decompose, resulting in the accumulation of strain around them, which becomes the source of voids and cracks.

[0031] Therefore, in order to maintain sufficient wear resistance, the number density per unit interface area of ​​V-containing nitride particles with a particle size of 4 nm or more present on the cementite interface must be 1.0 × 10 １０ pieces cm －２ If the size of the nitride particles on the cementite interface is small, they have almost no effect on the deterioration of wear resistance, so there is no particular limitation on the number density as long as the nitride particles are smaller than 4 nm.

[0032] (2) Particle size and number density of V-containing nitrides in ferrite in the pearlite structure at a depth of 25 mm from the outer surface of the head. In the ferrite, V-containing nitride particles with a particle size of 0.5 to 4 nm separated from the cementite interface were found to have a number density per unit volume of 4.0 × 10 １６ ~4.0 x 10 １７ pieces cm －３ The reason why it is preferable that the range of is included is that the lamellar ferrite is strengthened to increase the rail hardness and improve the wear resistance. By setting the particle size of the V-containing nitride in the ferrite to be within the particle size range of 0.5 to 4 nm, the effect of strengthening the lamellar ferrite can be obtained. In addition, the number density is set to 6.0 × 10 １６ ~5.0 x 10 １７ pieces cm －３ By setting the number density of the V-containing nitride particles having a particle size of 0.5 to 4 nm in the ferrite to within the range of 8.0×10, a further effect of strengthening the lamellar ferrite can be obtained. １６~2.0 x 10 １７ pieces cm －３ The range is.

[0033] (3) Steel Chemical Composition The chemical composition of the pearlitic rail according to the present embodiment may be limited for the following reasons: Note that "%" shown below means "mass %" unless otherwise specified.

[0034] C: 0.65 to 1.20% C is an effective element for promoting pearlite transformation and ensuring wear resistance. If the C content is less than 0.65%, the minimum strength and wear resistance required for a rail cannot be maintained. Furthermore, if the C content exceeds 1.20%, a large amount of coarse pro-eutectoid cementite structure is formed, resulting in reduced wear resistance and ductility. For this reason, the C content is limited to 0.65% to 1.20%. Note that if the C content is 0.70% or more, 0.80% or more, or 0.90% or more, wear resistance is further improved and the service life of the rail is further improved. The C content may also be 1.10% or less, 1.00% or less, or 0.95% or less.

[0035] Si: 0.05 to 2.00% Si is an essential component as a deoxidizer. Si also improves the hardness (strength) of a railhead by solid solution strengthening in the ferrite phase in the pearlite structure. Furthermore, Si suppresses the formation of pro-eutectoid cementite in hypereutectoid steel, suppressing the decrease in ductility. However, if the Si content is less than 0.05%, these effects cannot be fully expected. Furthermore, if the Si content exceeds 2.00%, many surface defects are formed during hot rolling, and weldability is reduced due to the formation of oxides. Furthermore, if the Si content exceeds 2.00%, hardenability increases significantly, and martensite structures that are harmful to the wear resistance and ductility of the rail are formed. For these reasons, the Si content is limited to 0.05 to 2.00%. The Si content may be 0.10% or more, 0.20% or more, or 0.50% or more. The Si content may be 1.80% or less, 1.60% or less, or 1.20% or less.

[0036] Mn: 0.05 to 2.00% Mn is an element that improves hardenability and refines the pearlite lamellar spacing, thereby ensuring the hardness of the pearlite structure and improving wear resistance. However, if the Mn content is less than 0.05%, this effect is small, making it difficult to ensure the wear resistance required for rails. Furthermore, if the Mn content exceeds 2.00%, hardenability increases significantly, making it easier for martensite structures, which are harmful to wear resistance and ductility, to form. For this reason, the Mn content is limited to 0.05 to 2.00%. The Mn content may be 0.10% or more, 0.20% or more, or 0.50% or more. The Mn content may also be 1.80% or less, 1.60% or less, or 1.20% or less.

[0037] Cr: 0.02 to 2.00% Cr increases the equilibrium transformation temperature, resulting in finer ferrite and pearlite structures, contributing to increased hardness. Furthermore, Cr is an element that forms nitrides with V and improves the hardness (strength) of pearlite structures through precipitation hardening. However, if the Cr content is less than 0.02%, this effect is small and the effect of improving the hardness of rail steel is not observed. Furthermore, if the Cr content exceeds 2.00%, hardenability increases, martensite structures are formed, spalling damage originating from the martensite structures occurs in the head corners and apex, and surface damage resistance decreases. For this reason, the Cr content is limited to 0.02 to 2.00%. The Cr content may be 0.10% or more, 0.20% or more, or 0.50% or more. The Cr content may be 1.80% or less, 1.60% or less, or 1.20% or less.

[0038] V: 0.010 to 0.100% V is an important element in the steel according to the present disclosure. V suppresses and refines austenite grain growth during the production of pearlitic rails. Cooling the rail with refined austenite grains forms fine pearlite, improving the wear resistance of the pearlitic rail. Furthermore, V may bond with N atoms to form nitride nuclei and form nitride particles or clusters. When nitrides containing V precipitate in ferrite, they contribute to increasing the hardness of pearlite by suppressing dislocation movement, which is even more preferable.

[0039] If the V content is less than 0.010%, the effect cannot be fully expected, and no increase in hardness or improvement in wear resistance is observed. On the other hand, if the V content is excessive, large nitride particles precipitate on the cementite interface. The coarse nitrides on the cementite interface reduce plastic deformability and wear resistance. Furthermore, if the V content is excessive, V carbides are more likely to form rather than V nitrides. Therefore, the upper limit of the V content is set to 0.100%. More preferably, the V content is in the range of 0.020 to 0.050%. The V content may be 0.015% or more, 0.020% or more, or 0.030% or more. The V content may be 0.090% or less, 0.080% or less, or 0.070% or less.

[0040] N: 0.0020 to 0.0200% N, together with V, acts as a nitride nucleus to form nitride particles. Generally, about 0.0010% N is contained in steel as an impurity. However, a N content of 0.0020% or more is preferable because it allows nitride particles to form inside ferrite. On the other hand, if the N content exceeds 0.0200%, the nitride particles become large and also form on the cementite interface. This reduces wear resistance. Furthermore, if solute N remains in high concentrations, it can cause a decrease in ductility itself. Therefore, the N content is limited to 0.0020 to 0.0200%. The N content may be 0.0025% or more, 0.0030% or more, or 0.0050% or more. The N content may be 0.0180% or less, 0.0150% or less, or 0.0100% or less.

[0041] P: 0.025% or less P is an element that deteriorates the ductility of rail steel. If the P content exceeds 0.025%, its influence cannot be ignored. Therefore, the P content is set to 0.025% or less. The P content may be 0.024% or less, 0.023% or less, or 0.022% or less. The lower limit of the P content is not particularly limited and may be 0%. On the other hand, from the viewpoint of reducing refining costs, the P content may be set to 0.010% or more, 0.011% or more, or 0.012% or more.

[0042] S: 0.025% or less S is an element that exists in steel mainly in the form of inclusions (such as MnS) and causes embrittlement of steel. In particular, if the S content exceeds 0.025%, the adverse effect on embrittlement cannot be ignored. Therefore, the S content is set to 0.025% or less. The S content may be 0.024% or less, 0.023% or less, or 0.022% or less. The lower limit of the S content is not particularly limited and may be 0%. On the other hand, from the viewpoint of reducing refining costs, the S content may be set to 0.008% or more, 0.009% or more, or 0.010% or more.

[0043] O: 0.0040% or less O is an impurity. There is no lower limit for the O content, but from the viewpoint of production costs, it may be 0.0001%. On the other hand, if the O content is excessive, coarse oxides are generated, causing a decrease in the ductility and toughness of the steel. Preferably, the O content is 0.0030% or less, and more preferably, the O content is 0.0020% or less.

[0044] Nb: 0% or more but less than 0.005% Ti: 0% or more but less than 0.005% Mo: 0 to 0.100% Furthermore, rails manufactured with the above chemical composition may contain one or more of Nb, Ti, and Mo to refine the pearlite structure. Nb, Ti, and Mo are elements that form carbonitrides at relatively high temperatures in heat-affected zones reheated to a temperature range below the Ac1 point, thereby effectively preventing softening and embrittlement of the heat-affected zone of welded joints. In particular, Mo, like Cr, not only forms carbonitrides but also raises the equilibrium transformation temperature, thereby refining the ferrite and pearlite structures and contributing to high hardness. At the same time, Mo strengthens the cementite phase and improves the hardness (strength) of the pearlite structure. However, these elements tend to form large nitrides, unnecessarily consuming N. Therefore, Nb is preferably less than 0.0050%. Ti is preferably less than 0.0050%. Mo is preferably 0.100% or less. Mo is more preferably 0.005% or less. Nb, Ti, and Mo may each be 0.0040% or less, 0.0035% or less, or 0.0030% or less. Note that it is not essential that Nb, Ti, and Mo are contained in the rail according to this embodiment. Therefore, Nb, Ti, and Mo may each be 0%.

[0045] Furthermore, rails manufactured with the above-mentioned chemical composition may contain the elements B, Co, Ni, Mg, Ca, Al, and Cu as necessary for the purposes of refining the pearlite structure, increasing strength, improving ductility, preventing softening of the weld heat affected zone, and controlling the cross-sectional hardness distribution inside the rail head. However, these elements do not necessarily have to be contained in pearlite rails. The lower limit of the content of each of B, Co, Ni, Mg, Ca, Al, and Cu is 0%.

[0046] B: 0 to 0.0050% B is an iron boride (Fe ２３ (CB) ６B is an element that forms a B-layer (B-layer) and accelerates pearlite transformation, thereby reducing the cooling rate dependency of the pearlite transformation temperature and imparting a more uniform hardness distribution to the rail from the head surface to the interior, thereby extending the rail's service life. By setting the B content to 0.0001% or more, this effect is fully exerted, and the hardness distribution of the rail head can be improved. Furthermore, setting the B content to 0.0050% or less suppresses the formation of coarse iron boron dioxide particles and improves ductility and toughness. Therefore, the B content may be set to 0.0001 to 0.0050%. The B content may also be set to 0.0002% or more, 0.0003% or more, or 0.0005% or more. The B content may also be set to 0.0040% or less, 0.0020% or less, or 0.0010% or less.

[0047] Co: 0 to 1.00% Co is an element that has almost no effect on solid solution strengthening, but further refines the fine ferrite structure formed by contact with the wheel on the wear surface of the rail head, thereby improving wear resistance. A Co content of 0.01% or more refines the lamellar structure and ferrite grain size, thereby improving wear resistance. Furthermore, a Co content of 2.00% or less can improve the ductility of the pearlite structure. Furthermore, a Co content of 1.00% or less can reduce alloy costs and improve economy. Therefore, the Co content may be limited to 0.01 to 1.00%. The Co content may be 0.02% or more, 0.05% or more, or 0.10% or more. The Co content may be 0.80% or less, 0.50% or less, or 0.10% or less.

[0048] Ni: 0 to 1.00% Ni improves ductility in the pearlite structure and suppresses the decrease in ductility due to Co. At the same time, Ni is an element that increases the hardness (strength) of rails through solid solution strengthening. If the Ni content is 0.01% or more, this effect can be fully obtained. Furthermore, if the Ni content is 1.00% or less, the ductility of the ferrite phase in the pearlite structure can be increased, and the wear resistance of the pearlite structure can be improved. Therefore, the Ni content may be 0.01 to 1.00%. The Ni content may be 0.02% or more, 0.05% or more, or 0.08% or more. The Ni content may be 0.80% or less, 0.50% or less, or 0.10% or less.

[0049] Mg: 0 to 0.0200% Mg combines with O, S, or Al to form fine oxides. This effectively inhibits grain growth during reheating during rail rolling, refines austenite grains, and improves the ductility of ferrite and pearlite structures. Furthermore, MgO and MgS formed from Mg finely disperse MnS, forming a thin Mn-dilute layer around the MnS and contributing to the formation of ferrite and pearlite transformation. As a result, pearlite block size is primarily refined. Therefore, Mg is an effective element for improving the ductility of pearlite structures. A Mg content of 0.0005% or more can fully achieve this effect. Furthermore, a Mg content of 0.0200% or less suppresses the formation of coarse Mg oxides, improving rail toughness while simultaneously suppressing fatigue damage originating from coarse precipitates. Therefore, the Mg content may be limited to 0.0005 to 0.0200%. The Mg content may be set to 0.0008% or more, 0.0010% or more, or 0.0012% or more. The Mg content may be set to 0.0180% or less, 0.0100% or less, or 0.0030% or less.

[0050] Ca: 0 to 0.0200% Ca is an element that bonds strongly with S and forms CaS (sulfide). This CaS finely disperses MnS, alleviating stress concentration and improving the internal fatigue damage resistance of the rail. When the Ca content is 0.0005% or more, this effect can be fully achieved. When the Ca content is 0.0200% or less, the formation of coarse Ca oxides can be suppressed. Coarse Ca oxides are prone to stress concentration that causes fatigue cracks. However, by suppressing the formation of coarse Ca oxides, the occurrence of fatigue cracks can be suppressed and internal fatigue damage resistance can be improved. Therefore, the Ca content may be limited to 0.0005 to 0.0200%. The Ca content may also be 0.0008% or more, 0.0010% or more, or 0.0012% or more. The Ca content may be 0.0180% or less, 0.0150% or less, or 0.0100% or less.

[0051] Al: 0 to 1.000% Al has a deoxidizing effect. Furthermore, Al is an element that shifts the eutectoid transformation temperature toward the higher side and contributes to increasing the hardness (strength) of the pearlite structure. When the Al content is 0.025% or more, this effect can be fully obtained. Furthermore, when the Al content is 1.000% or less, it becomes easier to dissolve Al in the steel, reducing the number of coarse alumina-based inclusions, thereby improving the toughness of the rail and preventing fatigue damage from occurring due to coarse precipitates. Furthermore, when the Al content is 1.000% or less, oxide formation in the weld zone can be prevented, significantly improving weldability. Therefore, the Al content may be limited to 0.025 to 1.000%. The Al content may be more than 0%, 0.028% or more, 0.030% or more, or 0.035% or more. The Al content may be 0.900% or less, 0.600% or less, or 0.400% or less.

[0052] Cu: 0 to 1.00% Cu is an element that dissolves or precipitates in ferrite and pearlite structures, improving the hardness (strength) of the pearlite structure through solid solution strengthening or precipitation strengthening. Because Cu precipitate particles are soft, they do not adversely affect wear resistance when precipitated in the ferrite phase. However, when the Cu content is high, the amount of Cu in solid solution also increases, resulting in reduced wear resistance of the rail. On the other hand, from the perspective of recycling steel scrap, rails are likely to contain Cu. Furthermore, Cu may be intentionally added to rails for use as a solid solution strengthening element or to improve surface damage resistance. In this sense, the Cu content may be 1.00% or less. The Cu content may be more than 0%, 0.01% or more, 0.05% or more, or 0.010% or more. It may also be 0.20% or less, 0.10% or less, or 0.05% or less.

[0053] REM: 0 to 0.0500% REM is a deoxidizing and desulfurizing element. When contained, REM oxysulfides (REM) become the nuclei for the formation of Mn sulfide-based inclusions. ２ O ２ S) is produced. ２ O ２S) has a high melting point. Therefore, oxysulfides suppress the elongation of Mn sulfide-based inclusions after rolling. As a result, the inclusion of REM finely disperses MnS, alleviating stress concentration and improving the internal fatigue damage resistance of the rail. When the REM content is 0.0005% or more, sufficient nuclei for the formation of MnS-based sulfides are formed, and the above-mentioned effects are obtained. On the other hand, when the REM content is 0.0500% or less, hard REM oxysulfides (REM ２ O ２ The excessive generation of REM (S) can be suppressed. In this case, the generation of fatigue cracks due to stress concentration is suppressed, and internal fatigue damage resistance is further improved. Therefore, when REM is contained in a rail, the REM content is preferably set to 0.0005 to 0.0500%. More preferred lower limit values ​​of the REM content are 0.0010%, 0.0012%, or 0.0015%. More preferred upper limit values ​​of the REM content are 0.0400%, 0.0300%, or 0.0100%.

[0054] REM refers to a rare earth metal selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The REM content is the total content of the rare earth metal elements. As long as the total content falls within the above range, the same effect can be obtained whether the rail contains one type of rare earth metal element or two or more types.

[0055] Zr: 0 to 0.0200% Zr combines with O to form ZrO ２ This ZrO ２ The inclusions and γ-Fe have good lattice matching. ２ These inclusions act as solidification nuclei in high-carbon rail steels in which γ-Fe is the primary solidification crystal, increasing the equiaxed crystallization rate of the solidification structure and refining the solidification structure. This results in fine dispersion of MnS, mitigating stress concentration and improving the internal fatigue damage resistance of the rail. Furthermore, Zr suppresses the formation of a segregation zone in the center of the cast slab, thereby suppressing the formation of martensite structures in the segregation zones of the rail. When the Zr content is 0.0001% or more, the number of ZrO particles is sufficient to act as solidification nuclei. ２Zr-based inclusions are formed. On the other hand, if the Zr content is 0.0200% or less, the formation of large amounts of coarse Zr-based inclusions is suppressed, fatigue cracks due to stress concentration are prevented, and the internal fatigue damage resistance of the rail can be further improved. Therefore, when Zr is contained, the Zr content is preferably set to 0.0001 to 0.0200%. The more preferable lower limit of the Zr content is 0.0005%, 0.0010%, or 0.0012%. The more preferable upper limit of the Zr content is 0.0100%, 0.0050%, or 0.0020%.

[0056] The balance of the chemical components of rail steel is mainly Fe. In addition to the above components, rail steel also contains impurities.

[0057] (4) Area Ratio of Pearlite Structure The structure of the pearlitic rail according to this embodiment in the range from the outer surface of the head to a depth of 25 mm (head surface portion 3a) is mainly pearlite. However, trace amounts of pro-eutectoid ferrite, pro-eutectoid cementite, bainite, and martensite, in terms of area ratio, of 5% or less, may be mixed into the structure of the pearlitic rail in the range from the outer surface of the head to a depth of 25 mm. However, even if these structures are mixed, this does not significantly affect the wear resistance and ductility of the rail head. Therefore, trace amounts of pro-eutectoid ferrite, pro-eutectoid cementite, bainite, and martensite, in terms of area ratio, of 5% or less, may be mixed into the structure of the pearlitic rail in the range from the outer surface of the head to a depth of 25 mm. In other words, it is sufficient that 95% or more of the structure of the pearlitic rail according to this embodiment in the range from the outer surface of the head to a depth of 25 mm is pearlite. In order to ensure sufficient wear resistance and ductility, it is desirable that 98% or more of the range be pearlite structure.

[0058] The meaning of the term "head outer surface" and other aspects of the shape of a pearlite rail are explained below. The head 3 refers to the portion above the centrally constricted portion in the height direction of the pearlite rail when viewed in cross section as shown in Figure 3. The head 3 has a top 31 and head corners 32 located at both ends of the top 31. One of the head corners 32 is a gauge corner (G.C.) portion that primarily comes into contact with the wheel. The head outer surface refers to the surface of the head 3 formed by combining the surface of the top 31 and the surface of the head corner 32, which face upward when the rail is upright. The positional relationship between the top 31 and the head corners 32 is such that the top 31 is located approximately in the center of the rail head width direction, and the head corners 32 are located on both sides of the top 31.

[0059] The area extending from the surface (outer head surface) of the head corners 32 and the head top 31 to a depth of 25 mm is called the head surface area (3a, shaded area). As shown in Figure 6, in order to improve the wear resistance and internal fatigue damage resistance of a rail, it is necessary for the head surface area 3a extending from the surface (outer head surface) of the head corners 32 and the head top 31 to a depth of 25 mm to have a metal structure containing pearlite with a predetermined hardness (a metal structure containing pearlite at an area ratio of 95% or more).

[0060] (5) Hardness at a Depth of 25 mm from the Surface of the Rail Top 31 The reason for limiting the hardness at a depth of 25 mm from the surface of the rail top 31 to at least 350 Hv will be explained. If the hardness at a depth of 25 mm from the surface of the rail top 31 is less than 350 Hv within the above-described chemical composition range, flaking damage due to plastic deformation will occur on the rolling surface of the rail top 31. Furthermore, when pearlitic rails are used in heavy-duty railways, if the hardness at a depth of 25 mm from the surface of the rail top 31 is less than 350 Hv, it will be difficult to ensure wear resistance, thereby shortening the service life of the rail. For this reason, the hardness at a depth of 25 mm from the surface of the rail top 31 is limited to at least 350 Hv. The hardness at a depth of 25 mm from the surface of the rail top 31 may also be set to 360 HV or more, 380 HV or more, or 400 HV or more. Generally, the higher the hardness, the better the wear resistance. Therefore, there is no particular upper limit for the hardness at this position, but it may be, for example, 600 HV or less, 550 HV or less, or 500 HV or less.

[0061] (Manufacturing Method) The rail steel having the above-mentioned chemical composition is produced by melting in a commonly used melting furnace such as a converter or an electric furnace, and the molten steel is then subjected to an ingot-making / blooming method or a continuous casting method, and further subjected to hot rolling to produce a rail.

[0062] An example of a preferred method for manufacturing a rail according to the present disclosure will be described. According to this manufacturing method, the number density of nitride particles with a diameter of 4 nm or more on the cementite interface is 1.0 × 10 １０ pieces cm －２ The hardness of the top of the ferrite is at least 350 Hv, and preferably the number density of nitride particles having a particle size of 0.5 to 4 nm in the ferrite is 4.0 × 10 １６ pieces cm －３ However, the example of the manufacturing method described below does not limit the manufacturing method of the pearlitic rail according to the present embodiment.

[0063] The number density of nitride precipitates of 4 nm or more on the cementite interface in ferrite is 1.0 × 10 １０ pieces cm －２To achieve a concentration of less than 10 ...

[0064] One example of a means for distributing V to the inner side of cementite is to optimize the cooling conditions. V has a small distribution coefficient at high temperatures and a slow diffusion rate at low temperatures, making it difficult for V to distribute to the inner side of cementite. Therefore, a temperature intermediate between these two is preferable, and it is necessary to find an optimal cooling rate in accordance with the steel composition. In the production of pearlitic rails, it is preferable to achieve optimal temperature control.

[0065] In addition, nitride particles with a particle size of 0.5 to 4 nm were added to the ferrite at 4.0 × 10 １６ pieces cm －３ To achieve this, it is preferable to leave a sufficient amount of solute V in the center of the lamellar ferrite immediately after pearlite transformation. To achieve this, it is necessary to appropriately prevent the lamellar ferrite width from being too narrow and to optimize the cooling rate so that excessive V distribution to cementite is not achieved.

[0066] Although there is no particular upper limit to the heating temperature, if the temperature is too high, a liquid phase appears and the austenite phase becomes unstable. Therefore, the temperature is practically limited to 1350°C. As for the lower limit of the heating temperature, it is preferable to carry out the heating at a high temperature of 1180°C or higher, since nitrides such as VN must be dissolved once.

[0067] To achieve complete transformation from austenite to pearlite, the rail head immediately after rolling is kept at a temperature above the Ar1 point. When a rolled rail is cooled and then reheated to produce a rail, the reheated rail head is kept at a temperature above the Ac1 point + 30°C. Finish rolling is preferably carried out at a temperature of 900 to 950°C.

[0068] Cooling of a rail immediately after rolling, or a rail that has been cooled and reheated after rolling, is preferably initiated in the temperature range of 850 to 900°C. The time at which cooling begins refers to the time at which spraying of a coolant onto the rail begins. In other words, it is preferable to start spraying of a coolant onto the rail head when the temperature of the rail head is in the temperature range of 850 to 900°C.

[0069] The cooling end temperature is preferably 790 to 820°C. The cooling end temperature refers to the point in time when spraying of the coolant onto the rail is terminated. In other words, it is preferable to terminate spraying of the coolant onto the rail head when the rail head is in the temperature range of 790 to 820°C.

[0070] The cooling rate is preferably 20 to 50°C / s. The cooling rate is the difference between the cooling start temperature (the temperature of the rail head when the spraying of the coolant begins) and the cooling end temperature (the temperature of the rail head when the spraying of the coolant ends) divided by the time the coolant is sprayed. The temperature of the rail head is measured by a radiation thermometer. The temperature is measured at the outer surface of the rail head.

[0071] Rapid cooling of the rail head under the above conditions suppresses austenite grain growth. If the cooling rate immediately after hot rolling is too high, carbonitride precipitation at the high temperature side may be completely suppressed, increasing the driving force for nitride precipitation at the cementite interface after pearlite transformation. In this case, V-containing nitrides are likely to precipitate at the cementite interface. Furthermore, it is technically difficult to achieve a cooling rate higher than 50°C / s. A cooling rate lower than 20°C / s has little effect on suppressing grain growth.

[0072] Following rapid cooling using a refrigerant, the material is slowly cooled at a rate of 2 to 5°C / s to 740 to 780°C, which is the temperature range immediately before the start of pearlite transformation (pre-controlled cooling). The start of pre-controlled cooling refers to the end of the above-mentioned refrigerant spraying. The end of pre-controlled cooling refers to the start of controlled cooling, which will be described later. The cooling rate in pre-controlled cooling is the difference between the rail head temperature at the start of pre-controlled cooling and the rail head temperature at the end of pre-controlled cooling, divided by the time required for pre-controlled cooling. Pre-controlled cooling shifts the pearlite transformation temperature slightly higher.

[0073] Following the slow cooling described above, the rail head is subjected to controlled cooling from the austenite temperature range to preferably 600 to 480°C at a cooling rate of preferably 5 to 12°C / s. That is, the controlled cooling end temperature is within the range of 600 to 480°C. Preferably, the controlled cooling end temperature is 580 to 480°C. This results in a complete pearlite structure. The start of controlled cooling refers to the start of refrigerant spraying. The end of controlled cooling refers to the end of refrigerant spraying. The cooling rate in controlled cooling is the difference between the rail head temperature at the start and end of controlled cooling divided by the time required for controlled cooling.

[0074] If the cooling rate of controlled cooling is too high, the transformation temperature drops significantly. In this case, the distribution of solute V atoms during pearlite transformation is insufficient, and the solute V concentration in ferrite near the cementite interface does not decrease. As a result, large nitrides tend to form on the cementite interface after pearlite transformation. If the cooling rate is too low or the end temperature is too high, the pearlite transformation proceeds at a higher temperature, resulting in coarse pearlite and less hardening due to lamellar refinement.

[0075] In reality, the precipitation driving force differs depending on the V, Cr, and N contents, so the correspondence with the preferred manufacturing method is not simple and varies. As long as the V, Cr, and N contents are within the ranges of this embodiment, applying the above manufacturing conditions can suppress the precipitation and enlargement of nitrides on the cementite interface. Regarding the controlled cooling method, for example, a predetermined cooling rate can be obtained by using air, a refrigerant medium mainly consisting of air with mist added, or a combination of these.

[0076] The manufacturing method described here is an example and does not limit the pearlite rail having the structure characteristics described in the claims. Pearlite rails having a similar structure obtained by other manufacturing methods also belong to the pearlite rail according to this embodiment.

[0077] Next, examples of the present disclosure will be described. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present disclosure. The present disclosure is not limited to these examples of conditions. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the disclosure and achieve the purpose.

[0078] Tables 1A and 1B show the chemical compositions of the test rail steels. Tables 2A and 2B show the materials and manufacturing conditions for the test rail steels (Test Nos. 1 to 28) made from any of the steel types A to P disclosed in Tables 1A and 1B. Tables 3A and 3B show the state of precipitates in the ferrite phase. Tables 4A and 4B show the rail properties and wear test results. The manufacturing conditions include the cooling rate in each temperature range. The rail properties include the head hardness (Hv) (test load 98 N) and the total elongation in the tensile test. The wear test results include the wear loss (g) in a 700,000-cycle wear test. The remainder of the chemical compositions listed in Tables 1A and 1B are iron and impurities. In Tables 1A and 1B, the symbol "-" in the element content column indicates that the element with that symbol was not intentionally added to the pearlitic rail. In Tables 1A and 1B, "<0.005" in the element content column means less than 0.005 mass%.

[0079] The manufacturing conditions not listed in the table are as follows. These steel rails were prepared by adjusting the composition in a converter, and then heating billets for rail rolling, which were cast by a continuous casting method, at a heating temperature of 1250°C for at least one hour. Hot rolling after heating was carried out with an inter-pass time of 11 s. Finish rolling was carried out at 910°C with a cross-sectional area reduction rate of 10% per pass.

[0080] The rails were rapidly cooled by water cooling or the like from 900 to 800°C at a cooling rate of 20 to 50°C / s from the hot rolling finish temperature, then slowly cooled to 760°C at a rate of 2 to 10°C / s, and subsequently subjected to controlled cooling at a cooling rate of 4 to 15°C / s until the temperature of the rail head reached 610 to 450°C. The characteristics of these rail samples were evaluated by the following methods.

[0081] A Nishihara abrasion test was conducted to evaluate rail head wear. The test conditions were as follows: Testing machine: Nishihara abrasion tester; Test specimen shape: disc-shaped test specimen (outer diameter: 30 mm, thickness: 8 mm); Test specimen collection position: 5 mm below the rail head surface; Test load: 684 N (contact pressure: 640 MPa); Slip ratio: 20%; Counterpart material: pearlitic steel (360 Hv); Atmosphere: air; Cooling: forced cooling with compressed air (flow rate: 100 Nl / min); Number of repetitions: 700,000; The amount of wear was calculated as the loss in mass of the test specimen after the test. A wear amount of less than 1.00 g was considered good wear resistance. A wear amount of less than 0.80 g was considered even better.

[0082] Furthermore, test specimens were cut out from the rail head and the total elongation was estimated from tensile tests.

[0083] Testing machine: Universal small tensile testing machine Test piece shape: Similar to JIS No. 4 Parallel portion length: 30 mm Parallel portion diameter: 6 mm Distance between elongation measurement rating points: 25 mm Test piece collection position: 5 mm below the surface of the rail head Tensile speed: 10 mm / min Test temperature: Room temperature The tensile test was carried out in accordance with JIS Z 2241:2011. A total elongation value of 10.0% or more was considered good.

[0084] The hardness of the top of the head was determined by Vickers hardness measurement using a load of 1 kg. The location was 25 mm below the surface of the top of the head. Due to variations, the average value of the measurement results at five points is listed in the table. A value of 350 Hv or more was considered good.

[0085] The pearlite fraction was determined by nital etching of the polished sample surface. When the area fraction of a second phase other than pearlite exceeded 5%, the primary second phase was listed in the table. The pearlite fraction was measured from the outer surface of the head to a depth of 25 mm. Specifically, samples were cut from the cross section of each railhead. Each sample was diamond polished, etched with 3% nital, and then observed under an optical microscope (200x magnification). The white areas observed under 200x magnification are pro-eutectoid ferrite. Pro-eutectoid cementite can be identified by the different contrast along the grain boundaries. Martensite and bainite can be identified from the lath structure. The remaining structure is massive pearlite, identified by the contrast that varies depending on the lamellar direction. If the structure is unknown, the pearlite lamellar structure can be identified by high-magnification observation using a scanning electron microscope (SEM). The measurement fields were 10 arbitrary fields at a depth of 2 mm from the head outer shell surface and 10 arbitrary fields at a depth of 25 mm from the head outer shell surface. The average value of the area fraction of pearlite structures in 10 arbitrary fields at a depth of 2 mm from the head outer shell surface was defined as the "surface pearlite fraction," and the average value of the area fraction of pearlite structures in 10 arbitrary fields at a depth of 25 mm from the head outer shell surface was defined as the "25 mm position pearlite fraction." A rail in which both were 95 area % or more was determined to be a rail in which the structure in the range from the head outer shell surface to a depth of 25 mm contained pearlite structures in an area fraction of 95% or more.

[0086] Precipitates in the pearlite structure at a depth of 25 mm from the outer surface of the head were investigated by 3D atom probe observation using a CAMECA LEAP4000XHR. A needle sample with a curvature radius of 30 to 80 nm was fabricated from the pearlite structure at a depth of 25 mm from the outer surface of the head of the sample pearlite rail by focused ion beam (FIB) processing. Measurements of 20 million atoms were performed multiple times using the 3D atom probe. From this data, a 3D elemental map was obtained using, for example, IVAS software, a 3D construction software. V is the mass-to-charge ratio, and a peak appears at 25.5 Da. ２+ ion and a peak at 32.5 Da. ２＋ ions are detected. ２＋ Since the ions are composed of V and N, they indicate the presence of nitride particles containing V. Areas where C is concentrated at 20 at% or more become cementite lamellae. There are two types of precipitates: particles present on the cementite interface, and particles that are distributed within ferrite grains away from the cementite interface (see Figure 1).

[0087] 4A to 4D show an example of a three-dimensional elemental map of a V-doped rail. These are atomic maps of Fe, C, and V in the same rectangular parallelepiped region with a height of 77 nm, a width of 78 mm, and a length of 232 nm. Each point in the map represents one atom. For V, V ２＋ Ion map and VN ２＋ The ion map is shown. ２＋ Since the ion is a pair of N and V, VN ２＋ The areas where the dots representing ions are concentrated correspond to nitride particles containing V. The C atom map shows lamellar cementite and lamellar ferrite. In the figure, lamellar cementite is written as "cementite" and lamellar ferrite is written as "ferrite." VN ２＋ In the map, fine nitride particles are observed in the ferrite, and the slightly larger particles enclosed in the square box marked with an X are nitride particles on the cementite interface.

[0088] 5A to 5D show the VN of FIG. ２＋The three-dimensional elemental map of the square box region X set on the map is shown. This box has a cubic shape with each side being 10 nm. Figures 5A to 5D show atomic maps of C, V, Cr, and Mn. Here, the atomic map of V is VN ２＋ and V ２＋ The carbon atom map is a combination of the carbon atoms and ferrite atoms. The interface between cementite and ferrite is indicated by a dashed line in the carbon atom map. It shows that nitride particles containing V and Cr are in contact with cementite, that is, they are present on the cementite interface. Also, Figures 5A to 5D show that carbon atoms are not contained in the nitride particles.

[0089] The type, number density, and size of nitride particles on the cementite interface were estimated as follows: The number n of nitride particles in contact with the cementite interface in the element map was calculated, and this was used to calculate the total surface area S nm of the cementite within the field of view. ２ By dividing by , the number density of nitride particles per cementite interface area n / S nm ―２ When lamellar cementite crosses the map, the area must be calculated by adding up both sides. Also, as shown in Table 3A and Table 3B, the unit is cm －３ When converting to １４ Just multiply it by .

[0090] The type, number density, and size of nitride particles in ferrite were estimated as follows. For example, in the case of measuring 20 million atoms, the value was divided by the detection rate (up to 0.38) of the ion detector in the device, and the resulting value was used to estimate the Fe atom density (up to 85 nm －３ ) is the measured volume (nm ３ ) This detection rate does not change depending on the number of atoms or the type of element, but is determined by the device. In order to determine the number density in the ferrite phase, if the cementite phase is included in the measurement field of view, this volume must be excluded from the measured volume V of the ferrite phase. ｆ (nm ３ If n precipitates are observed in this measurement volume, the number density is calculated as n / V ｆ nm ー３ As shown in Table 2A and Table 2B, the unit is cm －３When converting to ２１ Just multiply it by .

[0091] To improve accuracy, multiple measurements were taken across the cementite lamella. For example, if one precipitate was observed in a measurement of a volume equivalent to 30 million atoms in ferrite, the number of precipitates was approximately 1.0 × 10 １５ pieces cm －３ In addition, when no precipitates were observed by such a measurement, it was indicated as "-". Therefore, when no precipitates were observed, the number density was 1.0 × 10 １５ pieces cm －３ For example, if the area of ​​the cementite interface in the 3D element map is 100 nm × 50 nm × 2 = 10,000 nm ２ When one nitride particle is observed on the cementite interface, the １０ pieces cm －２ In cases where such cementite could not be observed even after multiple measurements, it is indicated as "-" in the table.

[0092] The composition of the precipitate particles was determined by determining the constituent elements and their atomic numbers of the nitride particles using a particle analysis program such as the Maximum Separation Method included in IVAS software, a 3D construction software manufactured by CAMECA. Alternatively, the composition can be determined by directly drawing a box on the 3D element map. The IVAS software version was 3.6.8. However, the software version does not affect the measurement results. The size of the precipitate particles was calculated as the spherical equivalent diameter as follows: The number of constituent atoms other than nitrogen was calculated using the above method. This is because current atom probes cannot accurately measure all nitrogen atoms. Assuming a stoichiometric composition, in an NaCl-type crystal structure, the number of constituent atoms of the nitride particles, including nitrogen, is twice the number of constituent atoms other than nitrogen, such as V, Cr, and Mn. This value was then divided by the atom probe's ion detection rate (0.38 in this case) to obtain the actual number of constituent atoms, N. Finally, assuming a sphere of diameter D and the atomic density of the precipitate a, D = (6N / πa) １／３The particle diameter D can be calculated from the equation: Here, the atomic density is a = 113 nm －３ The number density of V-containing nitride particles with a particle size of 4 nm or more on the cementite interface and the number density of nitride particles with a particle size of 0.5 to 4 nm in ferrite, which were estimated using this method, are shown in Tables 3A and 3B.

[0093]

[0094]

[0095]

[0096]

[0097]

[0098]

[0099]

[0100]

[0101] In Test No. 1 (Steel Type A), the amount of C was insufficient. In Test No. 1, pro-eutectoid ferrite was mixed into the structure ranging from the outer surface of the head to a depth of 25 mm, and the area ratio of pearlite structure was less than 95%. In Test No. 1, the hardness at a depth of 25 mm from the surface of the top of the head was insufficient, resulting in insufficient wear resistance.

[0102] Test No. 2 (steel type B) had an insufficient amount of N. In Test No. 2, the hardness at a depth of 25 mm from the surface of the top of the head was insufficient, and wear resistance was impaired.

[0103] In Test No. 3 (steel type C), the amount of V was insufficient. In Test No. 3, the hardness at a position 25 mm deep from the surface of the top of the head was insufficient, and the wear resistance was insufficient.

[0104] In Test No. 11, the cooling rate in the accelerated cooling immediately after hot rolling was excessive. In Test No. 11, the number density of nitride particles present on the cementite interface became excessive, and wear resistance was impaired.

[0105] In Test No. 12, the cooling rate in the controlled cooling was excessive. In Test No. 12, the number density of nitride particles present on the cementite interface became excessive, which impaired the wear resistance and ductility.

[0106] In Test No. 13, the cooling rate in the controlled cooling was excessive. In Test No. 13, bainite was mixed into the structure in the range from the outer surface of the head to a depth of 25 mm, and the area ratio of the pearlite structure was less than 95%. In Test No. 13, the wear resistance was impaired.

[0107] In Test No. 14, the cooling rate in the controlled cooling was insufficient, and the number density of nitride particles present on the cementite interface became excessive, resulting in a loss of wear resistance.

[0108] In Test No. 15, the end temperature of the controlled cooling was too low. In Test No. 15, bainite was mixed into the structure ranging from the outer surface of the head to a depth of 25 mm, and the area ratio of the pearlite structure was less than 95%. In Test No. 15, the wear resistance and ductility were impaired.

[0109] In Test No. 16, the end temperature of the controlled cooling was too high, and the number density of nitride particles present on the cementite interface became excessive, resulting in a loss of wear resistance.

[0110] In Test No. 17, the cooling rate in the accelerated cooling immediately after hot rolling was insufficient, and the number density of nitride particles present on the cementite interface became excessive, resulting in a loss of ductility.

[0111] In Test No. 18, the cooling rate in the cooling before controlled cooling was excessive. In Test No. 18, the number density of nitride particles present on the cementite interface became excessive, and wear resistance was impaired.

[0112] In Test No. 19, the cooling rate in the cooling before controlled cooling was insufficient, and the number density of nitride particles present on the cementite interface became excessive, resulting in a loss of wear resistance.

[0113] Test No. 20 (steel type H) contained an excessive amount of N. In Test No. 20, the number density of nitride particles present on the cementite interface became excessive, and ductility was impaired.

[0114] In Test No. 21 (Steel Type I), the amount of V was excessive, and the wear resistance of Test No. 21 was impaired.

[0115] In Test No. 22 (Steel Type J), ​​the C content was excessive. In Test No. 22, pro-eutectoid cementite was mixed into the structure ranging from the outer surface of the head to a depth of 25 mm, and the area ratio of pearlite structure was less than 95%. In Test No. 22, wear resistance and ductility were impaired.

[0116] In Test No. 23 (Steel Type K), the Si content was excessive. In Test No. 24 (Steel Type L), the Mn content was excessive. In Test No. 25 (Steel Type M), the Cr and Mo contents were excessive. In Test Nos. 23 to 25, martensite was mixed into the structure ranging from the outer surface of the head to a depth of 25 mm, and the area ratio of the pearlite structure was less than 95%. In Test Nos. 23 to 25, wear resistance and ductility were impaired.

[0117] In Test No. 26 (steel type N), the Nb content was excessive. In Test No. 26, the number density of nitride particles present on the cementite interface became excessive, and ductility and wear resistance were impaired.

[0118] In Test No. 27 (steel type O), the Ti content was excessive. In Test No. 27, the number density of nitride particles present on the cementite interface became excessive, and wear resistance was impaired.

[0119] In Test No. 28 (Steel Type P), the Ti content was excessive. In Test No. 28, the number density of nitride particles present on the cementite interface became excessive, and ductility and wear resistance were impaired.

[0120] On the other hand, in Test Nos. 4 to 10 and 29 to 36, in which the chemical composition, hardness, pearlite area ratio, and state of nitrides on the cementite interface were all appropriate, both ductility and wear resistance were excellent.

[0121] DESCRIPTION OF SYMBOLS 1 Pearlite structure 11 Cementite (lamellar cementite) 12 Ferrite (lamellar ferrite) 13 Cementite interface 21 Nitride particles present on the cementite interface 22 Nitride particles in ferrite separated from the cementite interface 3 Head 31 Head top 32 Head corner 3a Head surface (range starting from the outer surface of the head to a depth of 25 mm)

Claims

1. In mass%, C: 0.65 to 1.20%, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, Cr: 0.02 to 2.00%, N: 0.0020 to 0.0200%, V: 0.010 to 0.100%, Nb: 0% or more and less than 0.005%, Ti: 0% or more and less than 0.005%, Mo: 0 to 0.100%, B: 0 to 0.0050%, Co: 0 to 1.00%, Ni: 0 to 1.00%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, Cu: 0 to 1.00%, REM: 0 to 0.0500%, and Zr: 0 to 0.0200%, P: 0.025% or less, S: 0.025% or less, Al: 0 to 1.000%, O: 0.0040% or less, with the balance being Fe and impurities, the structure in the range from the outer surface of the head to a depth of 25 mm contains a pearlite structure with an area ratio of 95% or more, the hardness at a position 25 mm deep from the surface of the top of the head is at least HV350, and the number density per unit interface area of ​​V-containing nitride particles having a particle size of 4 nm or more and present on the cementite interface of the pearlite structure at the position 25 mm deep from the surface of the top of the head is 1.0 x 10 １０ pieces cm －２ A pearlite rail characterized in that:

2. In the ferrite of the pearlite structure, nitride particles containing V having a particle size of 0.5 to 4 nm and separated from the cementite interface are present at a number density per unit volume of 4.0 x 10 １６ ~4.0 x 10 １７ pieces cm －３ 2. The pearlitic rail according to claim 1, wherein the pore size is in the range of 0.1 to 1.0 μm.

3. A pearlitic rail according to claim 1 or 2, further comprising, in mass%, one or more elements selected from the group consisting of: B: 0.0001 to 0.0050%, Co: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Mg: 0.0005 to 0.0200%, Ca: 0.0005 to 0.0200%, Cu: more than 0% and not more than 1.00%, REM: 0.0005 to 0.0500%, Zr: 0.0001 to 0.0200%, and Al: more than 0% and not more than 1.000%.