JP7620259B1 - Pearlite Rail - Google Patents

Abstract

A pearlite rail according to one embodiment of the present disclosure contains, 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%, 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 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 at 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 a position 25 mm deep from the surface of the top of the head is less than 1.0 x ¹⁰¹⁰ particles cm ^-2 .

Description

This disclosure relates to pearlitic rails intended to simultaneously improve the wear resistance and ductility of the head of rails used on heavy-duty railways.

Heavy-duty railways overseas are promoting the high concentration of freight in order to improve the efficiency of railway transport. In particular, in rails with sharp curves, the wear resistance of the G.C. (gauge corner) and head side sections cannot be sufficiently ensured. This has led to a problem of reduced rail life due to wear. Against 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.

It is generally known that the harder the rail, the better its wear resistance. However, as described in Non-Patent Document 1, it has been reported that wear resistance improves as the surface layer hardens during use, increasing its hardness. Also, as described in Non-Patent Document 2, it has been shown that pearlite steel, which has excellent work hardening characteristics, is more wear-resistant than martensitic steel or bainitic steel, which simply have high initial hardness. However, it has not been shown what type of microstructural structure of pearlite steel has superior wear resistance, and there have been insufficient guidelines for rail manufacturing.

Patent Document 1 describes that by adding a certain amount of carbonitride-forming elements such as Ti, V, Nb, and Mo, these carbonitride precipitate particles are formed in the austenite phase, and the grain growth of austenite during hot rolling is suppressed and the grains are made fine, thereby refining the block size after pearlite transformation and improving ductility.

On the other hand, Patent Document 2 discloses a method of improving wear resistance by adding W, V, Nb, etc. to precipitate carbonitrides in the ferrite phase after pearlite transformation and strengthen the ferrite phase. These two documents suggest that adding the same type of element improves different properties of rails.

While these mainly use carbides as precipitates in pearlite, Patent Document 3 describes how fine particles of nitride are precipitated in ferrite to improve fatigue damage resistance and wear resistance. Patent Document 4 describes how fine particles of nitride are precipitated in ferrite to improve fatigue damage resistance and delayed fracture resistance. These documents specify the particle size, number density, and component composition of the nitride particles precipitated in ferrite.

Patent Document 5 discloses a pearlite rail which contains, by mass%, 0.65-1.20% C, 0.05-2.00% Si, 0.05-2.00% Mn, 0.02-2.00% Cr, at least 0.005% or more of two or more of V, Nb, Ti and Mo, and 0.02-0.20% V+Nb+Ti+Mo, with the balance being Fe and unavoidable impurities, and has a head hardness of at least 340 Hv, and in which the number density of carbonitriding precipitate particles of 10 nm or less in the ferrite phase is 5×10 ¹⁵ cm ^-3 or less.

Patent Document 6 discloses a method for producing high-toughness rails 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 850 to 1000°C with a cross-sectional area reduction rate of 5 to 30% per pass and with 8 seconds or less between passes, followed by cooling to 800 to 950°C at a cooling rate of 0.5 to 50°C/s, and then allowing to cool or allowing to cool with acceleration.

Ueda, Masaharu 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 Masaharu Ueda 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, P1023-1030

JP 2010-1500 A JP 2007-51348 A International Publication No. 2020/054339 JP 2020-70495 A JP 2015-158006 A JP 2001-234238 A

In light of this background, there has been a demand for pearlite rails for use in heavy-duty railways, which have excellent head hardness, wear resistance, and ductility, and for methods of manufacturing the same. The use of nitride particles, which has been practiced in recent years, is excellent in terms of removing unnecessary dissolved N that has remained as an impurity until now, and in terms of rail hardening. However, knowledge regarding the location at which nitride particles should be precipitated in pearlite has been insufficient. The present disclosure has been devised in light of the above-mentioned problems, and its purpose is to simultaneously improve the wear resistance and ductility of the head portion required for rails of heavy-duty railways.

The inventors have conducted detailed structural studies on the surface hardness and internal hardness of pearlite rails, as well as the rolling wear surface. As a result, the inventors have found that the wear test narrows the spacing of the pearlite lamellae in the contact surface area (refining of pearlite lamellae), and they further become microcrystallized, resulting in a significant increase in the hardness of the surface area, and the greater the increase in hardness, the better the wear resistance. In this hardness increase due to thinning of lamellae and microcrystallization, the lamellar ferrite (ferrite phase) in the pearlite is soft and ductile, making plastic deformation easier, and the pearlite structure consisting of lamellar cementite and lamellar ferrite is easily thinned and refined. This time, it was found that if there is a factor that inhibits this, it will cause a decrease in wear resistance and the occurrence of fatigue damage.

In pearlite rails utilizing nitride precipitate particles and clusters, the precipitation positions of nitride particles in the lamella of the pearlite structure 1 are mainly as shown in FIG.
(1) The center of the lamellar ferrite 12 and (2) the cementite 11/ferrite 12 interface (hereinafter referred to as the cementite interface 13).
It was found that the nitride particles 22 distributed in the center of the ferrite 12 contribute to hardening of the pearlite structure 1 through hardening of the ferrite 12. On the other hand, the nitride particles 21 precipitated at the cementite interface 13 hardly contribute to hardening. In addition, since the nitride particles 21 precipitated at the cementite interface 13 are relatively large, they do not undergo plastic deformation or decompose even when the rail is repeatedly worn. Therefore, it was 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 was found that in a pearlite rail using nitrides, it is necessary to reduce the number of nitride particles 21 on the cementite interface 13 and to reduce their size.

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 in high number density, they can be utilized for hardening without reducing wear resistance or ductility.

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:

(1) A pearlite rail according to an embodiment of the present disclosure has, 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. up to 0.0200%, P: 0.025% or less, S: 0.025% or less, Al: 0-1.000%, O: 0.0040% or less, with the balance being Fe and impurities, the structure in the range from the head outer surface as the starting point to a depth of 25 mm contains a pearlite structure at 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 less than 1.0 x ¹⁰¹⁰ particles cm ^-2 .
(2) In the pearlitic rail described in (1) above, preferably, V-containing nitride particles having a particle size of 0.5 to 4 nm are present in the ^ferrite of the pearlitic structure, separated from the cementite interface, in a number density per unit volume range of 4.0 x ¹⁰ to 4.0 x ¹⁰ particles cm.
(3) Preferably, the pearlitic rail described in the above (1) or (2) 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%, Al: more than 0% and not more than 1.000%.

According to the present disclosure, it is possible to provide a steel rail having excellent wear resistance and ductility in a pearlitic structure. In other words, the present disclosure presents a method for manufacturing a rail having improved wear resistance and ductility by suppressing the decrease in wear resistance caused by nitride particles precipitated at the cementite/ferrite interface.

FIG. 2 is a schematic diagram showing the distribution of precipitate particles in a rail pearlite structure. FIG. 1 is a diagram showing the relationship between the number density of nitride particles on a cementite interface and the amount of wear. FIG. 2 is a diagram showing the designations of the rail heads. 1 is an example of a three-dimensional element map of Fe (iron) of a V-added rail. 1 is an example of a three-dimensional element map of C (carbon) of a V-doped rail. 1 is an example of a 3D elemental map of V (vanadium) detected by V ²⁺ ions for a V-doped rail. 1 is an example of a 3D elemental map of V (vanadium) detected with ^VN ions for a V-doped rail. FIG. 4C is an example of a three-dimensional elemental map of C, enlarging the boxed region X in FIG. 4D. 4D is an example of a 3D elemental map of V, zoomed in on the boxed region X in FIG. 4D. FIG. 4E is an example of a three-dimensional elemental map of Cr, enlarging the boxed region X in FIG. 4D. FIG. 4E is an example of a three-dimensional elemental map of Mn, enlarging the boxed region X in FIG. 4D.

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

Figure 1 is a diagram showing the distribution of nitride precipitates and clusters in the pearlite structure 1 of a rail. In general, pearlite lamellae are composed of lamellar ferrite 12 with a width of 80 to 200 nm and lamellar cementite 11 with a width of 10 to 20 nm. The means for increasing the hardness of a rail with the same C content are either to reduce the pearlite lamellar spacing (refining and strengthening) or to increase the hardness of ferrite 12. Although a method of increasing the hardness of cementite 11 is also conceivable, the hardness of cementite 11 is already quite high, and it is practically difficult to intentionally increase this hardness further. In addition, if the hardness of ferrite 12 is simply increased significantly, the rail surface becomes less susceptible to plastic deformation during use, and the increase in hardness of the wear surface due to the thinning and refinement of the pearlite lamellae does not occur. 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 of cracks or voids, but form solid solution atoms or dipoles of N with V or Cr, etc., thereby contributing to hardness and wear resistance.

This detailed structural observation revealed that the nitride particles are mainly precipitated at two locations: the cementite interface 13 (i.e., the interface between the lamellar cementite 11 and the lamellar ferrite 12) and the central part of the lamellar ferrite 12, which is distant from the cementite interface 13. Here, the central part of the lamellar ferrite 12 refers to a region that is 5 nm or more away from the cementite interface 13. Of the cementite interface 13 and the central part of the ferrite 12, the nitride particles 22 distributed in the central part of the ferrite 12 contribute to hardening, but the nitride particles 21 precipitated on the cementite interface 13 are thought not to contribute to hardening. In addition, since the nitride particles that are slightly larger in size do not deform or decompose due to wear, strain is concentrated around the nitride particles, and the nitride particles become the starting points of voids and cracks, which are thought to cause a decrease in wear resistance and ductility. Therefore, it is necessary to make the nitride particles 21 on the cementite interface 13 smaller, and to reduce the number density of the larger ones. In addition, it was thought that by precipitating the nitride particles 22 in the ferrite 12 finely and in a high number density, they could contribute to hardening without reducing ductility or wear resistance.

By devising chemical components and heat treatment, 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 testing of these pearlite rails. A Nishihara-type wear tester was used to quantitatively evaluate wear resistance. In the wear testing, the amount of wear was compared after 700,000 repetitions at a contact pressure of 640 MPa and a sliding ratio of 20%.

Figure 2 shows (1) the relationship between the number density per unit interfacial area of nitride particles of 4 nm or more observed at the cementite interface and the wear volume for each pearlite rail sample, and (2) the relationship between the number density per unit interfacial area of nitride particles of 0.5 nm or more and less than 4 nm at the cementite interface and the wear volume for rail samples that do not contain nitride particles of 4 nm or more. 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.

According to the survey results shown in Fig. 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 higher than 1.0 x 10 ¹⁰ particles cm ^-2. If the number density of nitride particles with a diameter of 4 nm or more on the cementite interface is less than 1.0 x 10 ¹⁰ particles cm ^-2 , the wear amount of the rail is not so different from that in the case where there are substantially no precipitates. In addition, when nitride particles with a diameter of 0.5 nm or more and less than 4 nm are plotted on this figure, it can be seen that there is no correlation with the wear amount. In other words, this shows that even if relatively small nitride particles precipitate on the cementite interface, the effect on the deterioration of wear resistance is small.

If relatively large nitride particles are present on the cementite interface, not only do they not contribute to improving rail hardness, but they also cause strain to accumulate at the particle position of the cementite interface during wear tests, forming voids and cracks and causing destruction. 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.

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

On the other hand, for nitride precipitation in pearlite, the cementite interface is the preferred nucleation site. For this reason, nitride precipitation at the cementite interface is more likely to occur than nitride precipitation within ferrite grains, which has a hardening effect. Therefore, when precipitating nitride particles 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 manufacturing conditions for the ingredients and thermomechanical treatment.

The reasons for the limited range of the pearlitic rail according to this embodiment will be described below.
The head of a pearlite rail is usually worn during use due to contact with the wheels. Therefore, from the viewpoint of extending the life of the pearlite rail, it is required that the pearlite rail has properties suitable for railway rails not only on the surface of the head but also at a depth of 25 mm from the head. In addition, the pearlite lamellae are more likely to be finer and harder on the surface of the pearlite rail than at a depth of 25 mm, so that wear resistance is easily ensured. A pearlite rail having suitable properties at a depth of 25 mm from the surface of the head naturally has suitable properties on the surface of the head. In consideration of the above circumstances, in the pearlite rail according to this embodiment, the particle size, number density, and hardness of the V-containing nitride are limited at a depth of 25 mm from the surface of the head. In addition, in the pearlite rail according to this embodiment, the amount of pearlite structure is limited to a depth of 25 mm from the surface of the outer shell of the head.

(Regarding V-containing nitrides)
In the pearlite rail according to the present embodiment, the nitride particles containing V are specified. The nitride particles containing V are nitride particles having a V content of 1 atomic % or more. Generally, nitrides are crystal phases composed of N and metal elements, and include those composed of one type of metal element such as VN, TiN, NbN, and CrN, and those composed of multiple types of metal elements such as (V, Cr)N and (Ti, V)N. (V, Cr)N is a nitride in which V and Cr are mixed, and (Ti, V)N is a nitride in which Ti and V are mixed. The elements in parentheses are written in order of the number of atoms. In general, fine ones that are consistent with the ferrite matrix have a NaCl type crystal structure. On the other hand, carbides are a crystal phase in which N is replaced with C, and may contain a small amount of N as carbonitride. The nitride particles described in this disclosure do not contain C. Not containing means that the concentration is equal to or less than the C solid solution concentration 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.

(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 head outer surface 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 have an effect on the wear resistance. In particular, when the number density of nitride particles on the cementite interface larger than a specific size increases, the wear resistance decreases. This is because large nitride particles do not deform or decompose, so that strain accumulates around them, becoming the source of voids and cracks.

Therefore, in order to maintain sufficient wear resistance, the number density per unit interface area of V-containing nitride particles having a particle size of 4 nm or more present on the cementite interface is made less than 1.0×10 ¹⁰ particles cm ^-2 . Since the nitride particles on the cementite interface have almost no effect on the deterioration of wear resistance when they are small in size, there are no particular limitations on the number density as long as they are nitride particles smaller than 4 nm.

(2) Particle size and number density of V-containing nitrides in ferrite in pearlite structure at a depth of 25 mm from the head outer surface The reason why the V-containing nitride particles separated from the cementite interface in the ferrite preferably have a number density per unit volume of 4.0×10 ¹⁶ to 4.0×10 ¹⁷ cm ⁻³ is that the lamellar ferrite is strengthened to increase rail hardness and improve wear resistance. By setting the particle size of the V-containing nitrides in the ferrite within the particle size range of 0.5 to 4 nm, the effect of strengthening the lamellar ferrite is obtained. In addition, by setting the number density within the range of 6.0×10 ¹⁶ to 5.0×10 ¹⁷ cm ⁻³ , the effect of strengthening the lamellar ferrite is further improved. More preferably, the number density of the V-containing nitride particles with a particle size of 0.5 to 4 nm in the ferrite is in the range of 8.0×10 ¹⁶ to 2.0×10 ¹⁷ cm ⁻³ .

(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.

C: 0.65-1.20%
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 rails cannot be maintained. Furthermore, if the C content exceeds 1.20%, a large amount of coarse pro-eutectoid cementite structure is generated, which reduces the wear resistance and ductility. When the C content is 0.70% or more, 0.80% or more, or 0.90% or more, the wear resistance is further improved, and the service life of the rail is further improved. The amount may be 1.10% or less, 1.00% or less, or 0.95% or less.

Si: 0.05-2.00%
Silicon is an essential component as a deoxidizer. Silicon is also an element that improves the hardness (strength) of the rail head by solid solution strengthening in the ferrite phase in the pearlite structure. In eutectoid steel, Si is an element that suppresses the formation of proeutectoid cementite structure and suppresses 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 generated during hot rolling, and the weldability is reduced due to the generation of oxides. The hardenability increases significantly and martensite structure that is harmful to the wear resistance and ductility of rails is formed. For this reason, the amount of Si is limited to 0.05 to 2.00%. The amount of Si is 0.10% or more. The Si content may be 1.80% or less, 1.60% or less, or 1.20% or less.

Mn: 0.05-2.00%
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 not achieved. In addition, if the Mn content exceeds 2.00%, the hardenability increases significantly, and martensite, which is harmful to wear resistance and ductility, is generated. Therefore, 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 be 1.80% or less, 1.60% or less, or 1.20% or less.

Cr:0.02~2.00%
Cr increases the equilibrium transformation temperature, which results in finer ferrite and pearlite structures, contributing to high hardness. Furthermore, Cr forms nitrides together with V, which increases the hardness (strength) of the pearlite structure through precipitation hardening. However, if the Cr content is less than 0.02%, the effect is small and the effect of improving the hardness of the rail steel is not observed. Also, if the Cr content exceeds 2.00%, the hardenability As a result, martensite structure is formed, spalling damage originating from the martensite structure occurs at the head corners and top, and surface damage resistance decreases. The Cr content is limited to 2.00%. The Cr content may be set to 0.10% or more, 0.20% or more, or 0.50% or more. The Cr content may be set to 1.80% or less, 1.60% or less, or 1. It may be set to 20% or less.

V:0.010~0.100%
V is an important element in the steel according to the present disclosure. V suppresses and refines austenite grain growth during the manufacture of pearlitic rails. By cooling the rail with refined austenite grains, the grains are refined. This leads to the formation of fine pearlite, improving the wear resistance of the pearlite rail. Furthermore, V may bond with N atoms to become the nucleus of nitrides, forming nitride particles or clusters. Nitrides containing V When precipitated in ferrite, it is more preferable because it inhibits the movement of dislocations and thereby contributes to an increase in the hardness of pearlite.

If the V content is less than 0.010%, the effect cannot be expected sufficiently, 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 are precipitated on the cementite interface. The coarse nitrides on the cementite interface reduce the plastic deformability and cause a decrease in wear resistance. In addition, if the V content is excessive, V carbides are more likely to be formed instead of 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.

N:0.0020~0.0200%
N, together with V, becomes the nucleus of nitrides and forms nitride particles. Generally, about 0.0010% of N is contained in steel as an impurity. However, by making the N content 0.0020% or more, the ferrite On the other hand, if the N content exceeds 0.0200%, the nitride particles become large and are also formed on the cementite interface. This will reduce the wear resistance. Furthermore, if solute N remains in high concentration, it will cause a decrease in ductility itself. Therefore, the amount of N 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. It's fine.

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.

S: 0.025% or less S is an element that exists mainly in the form of inclusions (MnS, etc.) in steel and causes embrittlement of steel. In particular, when 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.

O: 0.0040% or less O is an impurity. The lower limit of the O content is not limited, but may be 0.0001% from the viewpoint of manufacturing costs. 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.

Nb: 0% or more and less than 0.005% Ti: 0% or more and less than 0.005% Mo: 0 to 0.100%
In addition, the rail manufactured with the above-mentioned composition may contain one or more of Nb, Ti, and Mo to refine the pearlite structure. Nb, Ti, and Mo are elements that are effective in preventing softening and embrittlement of the heat-affected zone of the welded joint by generating carbonitrides in a relatively high temperature range in the heat-affected zone reheated to a temperature range below the Ac1 point. In particular, Mo, in addition to generating carbonitrides, increases the equilibrium transformation temperature, as with Cr, and as a result, refines the ferrite structure and pearlite structure, contributing to high hardness, and at the same time, strengthens the cementite phase and improves the hardness (strength) of the pearlite structure. However, these elements are elements that tend to generate large nitrides, and N is unnecessarily consumed. Therefore, Nb is preferably less than 0.0050%. Ti is preferably less than 0.0050%. Mo is preferably 0.100% or less. It is even more preferable that Mo is 0.005% or less. Nb, Ti, and Mo may each be 0.0040% or less, 0.0035% or less, or 0.0030% or less. It is not essential that Nb, Ti, and Mo are contained in the rail according to the present embodiment. Thus, Nb, Ti, and Mo may each be 0%.

In addition, rails manufactured with the above-mentioned composition may contain the elements B, Co, Ni, Mg, Ca, Al, and Cu as necessary for the purpose of refining the pearlite structure, increasing strength, improving ductility, preventing softening of the welded heat-affected zone, and controlling the cross-sectional hardness distribution inside the rail head. However, these elements do not 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%.

B: 0-0.0050%
B forms iron boride (Fe ₂₃ (CB) ₆ ) at the austenite grain boundaries, and by promoting the pearlite transformation, it reduces the cooling rate dependency of the pearlite transformation temperature and provides a more uniform hardness from the head surface to the inside. It is an element that gives rails a uniform hardness distribution and extends the life of the rails. By making the B content 0.0001% or more, the effect of this can be fully exerted and the hardness distribution of the rail head can be improved. Furthermore, if the B content is 0.0050% or less, the formation of coarse iron borocarbides is suppressed, and ductility and toughness can be improved. For this reason, the B content is set to 0.0001 to 0.0050%. The B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. The B content may be 0.0040% or less, 0.0020% or less, or 0.0010% or less. It is also possible to use the following.

Co: 0-1.00%
Co is an element that has almost no effect on solid solution strengthening, but further refines the fine ferrite structure formed on the wear surface of the rail head due to contact with the wheel, thereby improving wear resistance. When the amount of Co is 0.01% or more, the lamellar structure and the ferrite grain size are refined, and the wear resistance is improved. Furthermore, when the Co content is 1.00% or less, the alloy cost can be suppressed and the economy can be improved. For this reason, the Co content is 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. It is also possible to use the following.

Ni: 0-1.00%
Ni improves the ductility in the pearlite structure and suppresses the decrease in ductility caused by Co. At the same time, Ni is an element that increases the hardness (strength) of the rail through solid solution strengthening. If the Ni content is 1.00% or less, the effect can be sufficiently obtained. On the other hand, 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 increased. Therefore, the Ni content may be set to 0.01 to 1.00%. The Ni content may be set to 0.02% or more, 0.05% or more, or 0.08% or more. It may be 80% or less, 0.50% or less, or 0.10% or less.

Mg: 0-0.0200%
Mg combines with O, S, and Al to form fine oxides, which inhibit the growth of crystal grains during reheating during rail rolling, refine the austenite grains, and improve the ductility of the ferrite and pearlite structures. Furthermore, MgO and MgS formed from Mg disperse MnS finely and form a thin Mn layer around MnS, which contributes to the formation of ferrite and pearlite transformation. As a result, mainly the pearlite block size becomes finer. Therefore, Mg is an effective element for improving the ductility of the pearlite structure. If the Mg content is 0.0005% or more, this effect is enhanced. Furthermore, if the Mg content is 0.0200% or less, the generation of coarse Mg oxides is suppressed, the toughness of the rail is increased, and at the same time, fatigue damage originating from coarse precipitates is prevented. Therefore, the Mg content may be limited to 0.0005 to 0.0200%. The Mg content may be 0.0008% or more, 0.0010% or more, or 0.0012% or more. The Mg content may be 0.0180% or less, 0.0100% or less, or 0.0030% or less.

Ca: 0-0.0200%
Ca is an element that has a strong bond with S to form CaS (sulfide). This CaS finely disperses MnS, mitigating stress concentration and improving the internal fatigue damage resistance of the rail. When the Ca content is 0.0005% or more, the effect can be sufficiently obtained. When the Ca content is 0.0200% or less, the generation of coarse Ca oxides can be suppressed. In the coarse oxides, stress concentration that causes fatigue cracks is likely to occur. However, by suppressing the generation of coarse Ca oxides, the occurrence of fatigue cracks can be suppressed and resistance to internal fatigue damage can be improved. Therefore, the Ca content may be limited to 0.0005 to 0.0200%. The Ca content may be 0.0008% or more, 0.0010% or more, or 0.0012% or more. It may be 0.0180% or less, 0.0150% or less, or 0.0100% or less.

Al: 0-1.000%
Al has a deoxidizing effect. In addition, Al is an element that shifts the eutectoid transformation temperature to the high temperature side, and is an element that contributes to increasing the hardness (strength) of the pearlite structure. The Al content is 0.025%. When the Al content is 1.000% or less, the effect can be sufficiently obtained. When the Al content is 1.000% or less, Al can be easily dissolved in the steel, and the number of coarse alumina-based inclusions can be reduced. By reducing the amount of Al, it is possible to increase the toughness of the rail and at the same time prevent fatigue damage caused by coarse precipitates. Furthermore, if the amount of Al is 1.000% or less, oxides in the welded portion tend to form. The formation of Al can be prevented, and the weldability can be significantly improved. Therefore, the Al content may be limited to 0.025 to 1.000%. The Al content may be 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.

Cu: 0-1.00%
Cu is an element that dissolves or precipitates in the ferrite structure or pearlite structure, and improves the hardness (strength) of the pearlite structure by solid solution strengthening or precipitation strengthening. Cu precipitate particles are soft, so they can be easily cooled in the ferrite phase. When Cu precipitates in the steel, it does not adversely affect the wear resistance. However, when the Cu content is high, the amount of Cu in solid solution also increases, and Cu reduces the wear resistance of the rail. From the viewpoint of recycling scrap, there is a high possibility that rails contain Cu. Furthermore, in the case where Cu is intentionally contained in rails from the viewpoint of utilization as a solid solution strengthening element and surface damage resistance, etc. 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 be 20% or less, 0.10% or less, or 0.05% or less.

REM: 0~0.0500%
REM is a deoxidizing and desulfurizing element, and when contained, it generates REM oxysulfide (REM ₂ O ₂ S), which becomes the nucleus for the formation of Mn sulfide-based inclusions. This oxysulfide (REM ₂ O ₂ S ) has a high melting point. Therefore, oxysulfides suppress the elongation of Mn sulfide inclusions after rolling. As a result, the inclusion of REM causes MnS to be finely dispersed, mitigating stress concentration and improving the durability of the rail. When the REM content is 0.0005% or more, nuclei for the formation of MnS-based sulfides are sufficiently formed, and the above-mentioned effects are obtained. %, excessive generation of hard REM oxysulfide (REM ₂ O ₂ S) can be suppressed. In this case, generation of fatigue cracks due to stress concentration can be suppressed, and resistance to internal fatigue damage can be improved. will be further improved. For this reason, when REM is contained in a rail, the REM content is preferably set to 0.0005 to 0.0500%. More preferable lower limits of the REM content are 0.0010%, 0.0012%, and More preferably, the upper limit of the REM content is 0.0400%, 0.0300%, or 0.0100%.

Note that REM refers to the group of rare earth metals 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 is 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.

Zr: 0-0.0200%
Zr combines with O to form _ZrO2- based inclusions. These _ZrO2 inclusions have good lattice matching with γ-Fe. Therefore, ZrO2 _- based inclusions are formed when γ-Fe is the initial solidification crystal. It acts as a solidification nucleus for certain high carbon rail steels, increasing the equiaxed crystallization rate of the solidification structure and refining the solidification structure. This finely disperses MnS, mitigating stress concentration and improving the resistance to internal fatigue damage of rails. Furthermore, Zr suppresses the formation of a segregation band in the center of the cast slab, thereby suppressing the formation of martensite structures in the segregated parts of the rail. When the Zr content is 0.0001% or more On the other hand, when the Zr content is 0.0200% or less, a large amount of coarse Zr _- based inclusions are generated. This suppresses fatigue cracks caused by stress concentration, thereby further improving the rail's resistance to internal fatigue damage. Therefore, when Zr is contained, the Zr content is preferably 0.0001 to 0.0200%. More preferably, the lower limit of the Zr content is 0.0005%, 0.0010%, or 0. The upper limit of the Zr content is more preferably 0.0100%, 0.0050%, or 0.0020%.

The remainder of the chemical composition of rail steel is mainly Fe. In addition to the above components, rail steel also contains impurities.

(4) Area ratio of pearlite structure The structure in the range from the head outer surface to a depth of 25 mm (head surface portion 3a) of the pearlite rail according to this embodiment is mainly pearlite structure. However, the structure in the range from the head outer surface to a depth of 25 mm may contain a trace amount of pro-eutectoid ferrite structure, pro-eutectoid cementite structure, bainite structure, or martensite structure of 5% or less in area ratio. However, even if these structures are mixed, it does not have a significant effect on the wear resistance and ductility of the rail head. Therefore, the structure in the range from the head outer surface to a depth of 25 mm of the pearlite rail may contain a trace amount of pro-eutectoid ferrite structure, pro-eutectoid cementite structure, bainite structure, or martensite structure of 5% or less. In other words, it is sufficient that the structure in the range from the head outer surface to a depth of 25 mm of the pearlite rail according to this embodiment is 95% or more pearlite structure. In order to ensure sufficient wear resistance and ductility, it is desirable for 98% or more of this range to be pearlite structure.

The meaning of the term "head outer surface" and other shapes of pearlite rails are explained below. The head 3 refers to the part above the part that is narrowed in the height direction center when the pearlite rail is 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.) part that mainly contacts the wheel. The head outer surface refers to the surface of the head 3 that faces upward when the rail is upright, combined with the surface of the top 31 and the head corners 32. The positional relationship between the top 31 and the head corners 32 is such that the top 31 is located almost in the center of the width direction of the rail head, and the head corners 32 are located on both sides of the top 31.

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

(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. In the present composition range, if the hardness at a depth of 25 mm from the surface of the rail top 31 is less than 350 Hv, flaking damage due to plastic deformation will occur on the rolling surface of the rail top 31. In addition, when pearlitic rails are used in heavy load railways, if the hardness at a depth of 25 mm from the surface of the rail top 31 is less than 350 Hv, it becomes difficult to ensure wear resistance, and the service life of the rail will be reduced. 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 be 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 that position, but it may be, for example, 600 HV or less, 550 HV or less, or 500 HV or less.

(Production method)
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.

An example of a preferred method for manufacturing a rail according to the present disclosure will be described. According to this manufacturing method, it is possible to obtain a pearlite rail in which the number density of nitride particles having a grain size of 4 nm or more on the cementite interface is less than 1.0×10 ¹⁰ particles cm ^-2 , the hardness of the top portion is at least 350 Hv, and preferably the number density of nitride particles having a grain size of 0.5 to 4 nm in the ferrite is 4.0×10 ¹⁶ particles cm ^-3 or more. However, the example of the manufacturing method described below does not limit the manufacturing method of the pearlite rail according to the present embodiment described above.

In order to make the number density of nitride precipitates of 4 nm or more on the cementite interface in ferrite less than 1.0×10 ¹⁰ cm ⁻² , it is necessary to suppress the generation of nitrides on the cementite interface. Nitrides are likely to precipitate in the ferrite after pearlite transformation at a location where solute V is present at a high concentration. Since the nucleus of the nitride is VN, in order to suppress the precipitation of nitrides on the cementite interface, V is distributed from the cementite interface to the inner side of the cementite during pearlite transformation. This makes it possible to reduce the solute V concentration in the ferrite near the interface.

One example of a means for distributing V to the inner side of the cementite is the optimization of cooling conditions. V has a small distribution coefficient at high temperatures and a slow diffusion at low temperatures, so it is difficult to distribute to the inner side of the cementite. Therefore, a temperature intermediate between these is preferable, and it is necessary to find the optimal cooling rate according to the steel composition. In the manufacture of pearlite rails, it is preferable to achieve optimal temperature control.

In order to precipitate 4.0×10 ¹⁶ particles cm ^-3 or more of nitride particles with a particle size of 0.5 to 4 nm in ferrite, it is preferable to leave a sufficient amount of dissolved V in the center of the lamellar ferrite immediately after pearlite transformation. For this purpose, it is necessary to appropriately prevent the lamellar ferrite width from being too narrow and to optimize the cooling rate so that V is not excessively distributed to cementite.

There is no specific upper limit for the heating temperature, but if the temperature is too high, a liquid phase will appear and the austenite phase will become unstable. Therefore, the actual upper limit for the temperature is 1350°C. As for the lower limit for the heating temperature, it is preferable to carry out the process at a high temperature of 1180°C or higher, since it is necessary to dissolve nitrides such as VN once.

In order to completely transform austenite into pearlite, the temperature range of the rail head immediately after rolling is set to the Ar1 point or higher. If the rolled rail is cooled and then reheated to manufacture the rail, the temperature of the reheated rail head is set to the Ac1 point + 30°C or higher. Finish rolling is preferably performed at 900 to 950°C.

Cooling of rails immediately after rolling, or rails that have been cooled and reheated after rolling, is preferably started at a temperature in the range of 850 to 900°C. The time at which cooling starts refers to the time at which the coolant starts to be sprayed onto the rail. In other words, it is preferable to start spraying the coolant onto the head of the rail when the temperature of the head of the rail is in the range of 850 to 900°C.

It is preferable that the cooling end temperature is 790 to 820°C. The cooling end temperature is the point at which spraying of the coolant onto the rail is stopped. In other words, it is preferable to stop spraying of the coolant onto the head of the rail when the head of the rail is in the temperature range of 790 to 820°C.

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 was 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.

By rapidly cooling the head of the rail under the above conditions, the grain growth of austenite is suppressed. If the cooling rate immediately after hot rolling is too high, the precipitation of carbonitrides on the high temperature side may be completely suppressed, and the driving force for nitride precipitation at the cementite interface after pearlite transformation may increase. In this case, nitrides containing V tend to precipitate at the cementite interface. In addition, it is technically difficult to achieve a cooling rate greater than 50°C/s. At a cooling rate of less than 20°C/s, the effect of suppressing grain growth is small.

Following rapid cooling using a refrigerant, the material is slowly cooled at 2-5°C/s to 740-780°C, the temperature range just before the start of pearlite transformation (pre-controlled cooling). The start of pre-controlled cooling refers to the end of the spraying of the refrigerant as described above. 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 moves the pearlite transformation temperature slightly higher.

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

If the cooling rate of controlled cooling is too high, the transformation temperature drops significantly. In this case, distribution of V solid solution atoms does not occur sufficiently during pearlite transformation, and the V solid solution concentration in ferrite near the cementite interface does not decrease. As a result, large nitrides are likely to form on the cementite interface after pearlite transformation. Furthermore, 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.

In reality, the precipitation driving force differs depending on the V, Cr and N content, so the correspondence with the preferred manufacturing method is not simple and varies. If the V, Cr and N content is within the range of this embodiment, the above manufacturing conditions can be applied to suppress the precipitation and enlargement of nitrides on the cementite interface. Regarding the method of controlled cooling, for example, a refrigerant medium such as air or air mainly with mist added, or a combination of these, can be used to obtain a predetermined cooling rate.

The manufacturing method described here is one 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 of this embodiment.

Next, an embodiment of the present disclosure will be described. However, the conditions in the embodiment are merely one example of conditions adopted to confirm the feasibility and effects of the present disclosure. The present disclosure is not limited to this one example of conditions. Various conditions may be adopted for the present disclosure as long as they do not deviate from the gist of the disclosure and achieve its purpose.

Tables 1A and 1B show the chemical compositions of the test rail steels. Tables 2A and 2B show the materials and manufacturing conditions of the test rail steels (Test Nos. 1 to 28) made of 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 characteristics and the results of the wear test. As the manufacturing conditions, the cooling rate in each temperature range is shown. As the rail characteristics, the head hardness (Hv) (test load 98N) and the total elongation in the tensile test are shown. As the wear test, the wear amount (g) in the wear test of 700,000 times is shown. The balance of the chemical compositions listed in Tables 1A and 1B is iron and impurities.
In Tables 1A and 1B, the symbol "-" in the column of the content of an element means that the element with that symbol is not intentionally added to the pearlite rail. Also, in Tables 1A and 1B, "<0.005" in the column of the content of an element means less than 0.005 mass%.

The manufacturing conditions not listed in the table are as follows. After the components of these steel rails were adjusted in a converter, the steel billets for rail rolling were cast by continuous casting, and heated at a heating temperature of 1250°C for more than one hour. After holding at heat, hot rolling was performed with an interval between passes of 11 s. Finish rolling was performed at 910°C with a cross-sectional area reduction rate of 10% per pass.

After rapid cooling from 900°C to 800°C at a cooling rate of 20-50°C/s from the hot rolling end temperature by water cooling or the like, it was slowly cooled to 760°C at a cooling rate of 2-10°C/s, and then continuously cooled under controlled conditions at a cooling rate of 4-15°C/s until the temperature of the rail head reached 610-450°C. The characteristics of this rail sample were evaluated by the following methods.

The wear of the rail head was evaluated by a Nishihara type wear test under the following test conditions:
Testing machine: Nishihara type abrasion testing machine Test piece shape: Disc-shaped test piece (outer diameter: 30 mm, thickness: 8 mm)
- Test piece collection location: 5 mm below the rail head surface
Test load: 684N (contact pressure 640MPa)
Slip rate: 20%
・Mating material: Pearlite steel (360Hv)
Atmosphere: Air Cooling: Forced cooling with compressed air (flow rate: 100 Nl/min)
Number of repetitions: 700,000 times. The amount of wear was calculated as the mass loss of the test piece after the test. When the amount of wear was less than 1.00 g, the wear resistance was considered to be good. When the amount of wear was less than 0.80 g, the wear resistance was considered to be even better.

In addition, test pieces were cut from the rail head and the total elongation was estimated from tensile tests. The conditions are as follows:

・Testing machine: Universal small tensile testing machine ・Test piece shape: JIS No. 4 similar ・Parallel part length: 30 mm
・Parallel diameter: 6mm
Distance between elongation measurement points: 25 mm
- Test piece collection location: 5 mm below the rail head surface
Pull speed: 10 mm/min
Test temperature: room temperature The tensile test was performed in accordance with JIS Z 2241: 2011. A total elongation value of 10.0% or more was rated as good.

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. Because there was some variability, the average values of the measurement results from five points are listed in the table. A value of 350 Hv or more was considered good.

The pearlite fraction was examined by nital etching of the polished sample surface. When the area ratio of the second phase other than the pearlite phase exceeded 5%, the main second phase was listed in the table. The measurement position of the pearlite fraction was within a range of 25 mm depth from the head outer surface. Specifically, samples were cut from the cross section of each rail head, and each sample was diamond polished and etched with 3% nital, and then measured by observing the structure using an optical microscope (200x). In the structure observation at 200x magnification, the white parts are pro-eutectoid ferrite. Pro-eutectoid cementite can be determined as different contrasts along the grain boundaries. Martensite and bainite can be determined from the lath structure. The other structures are massive pearlite, and are determined from the contrasts that differ depending on the lamellar direction. If the structure is unknown, the pearlite lamellar structure can be determined 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 surface, and 10 arbitrary fields at a depth of 25 mm from the head outer surface. The average value of the area ratio of pearlite structure in 10 arbitrary fields at a depth of 2 mm from the head outer surface was defined as the "surface pearlite ratio", and the average value of the area ratio of pearlite structure in 10 arbitrary fields at a depth of 25 mm from the head outer surface was defined as the "25 mm position pearlite ratio". 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 surface to a depth of 25 mm contained pearlite structure at an area ratio of 95% or more.

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

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 region with a height of 77 nm, a width of 78 mm, and a length of 232 nm. A point in the map represents one atom. For V, a map of V ²⁺ ions and a map of VN ²⁺ ions are shown. Since VN ²⁺ ions are ions consisting of a pair of N and V, the area where the points representing VN ²⁺ ions are concentrated corresponds to nitride particles containing V. In the C atomic map, lamellar cementite and lamellar ferrite are shown. In the figures, lamellar cementite is written as "cementite" and lamellar ferrite is written as "ferrite". In the VN ²⁺ map, fine nitride particles are observed in the ferrite, and the slightly larger particles surrounded by a square box marked with a symbol X are nitride particles on the cementite interface.

5A to 5D show three-dimensional element maps of a square box region X set in the VN ²⁺ map of FIG. 4. This box has a cubic shape with one side of 10 nm. Atomic maps of C, V, Cr, and Mn are shown in FIG. 5A to 5D. Here, the atomic map of V is a combination of VN ²⁺ and V ²⁺ . In the C atomic map, the position of the interface between cementite and ferrite is shown by a dashed line. It is shown that nitride particles containing V and Cr are in contact with cementite, that is, present on the cementite interface. It is also shown in FIG. 5A to 5D that C atoms are not included in the nitride particles.

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 is calculated and divided by the total surface area S ^nm2 of the cementite in the field of view to determine the number density n/S nm ^-2 of nitride particles per cementite interface area. When lamellar cementite crosses the map, the areas of both sides must be added together to determine the area. In addition, to convert the unit to cm ^-3 as in Tables 3A and 3B, this value should be multiplied by ¹⁰¹⁴ .

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 measured volume (nm ³ ) can be obtained by dividing the value by the detection rate (up to 0.38) of the ion detector of the device and dividing the result by the atomic density of Fe (up to 85 nm ⁻³ ). This detection rate does not change depending on the number of atoms or the element type, but is determined by the device. In order to obtain the number density in the ferrite phase, if the cementite phase is included in the measurement field of view, the measured volume V _f (nm ³ ) of the ferrite phase is obtained by excluding this volume. If n precipitates are observed in this measurement volume, the number density is obtained as n/V _f nm ⁻³ . As in Tables 2A and 2B, this value can be multiplied by 10 ²¹ to convert the unit to cm ⁻³ .

In order to improve accuracy, multiple measurements were taken across the cementite lamella. For example, when one precipitate was observed in the measurement of a volume equivalent to 30 million atoms in ferrite, the number density was about 1.0×10 ¹⁵ cm ⁻³ . Furthermore, when no precipitate was observed by such a measurement, it was indicated as “-”. Therefore, when no precipitate was observed, it may be considered that the number density is less than 1.0×10 ¹⁵ cm ⁻³ . Furthermore, for example, when the area of the cementite interface in the 3D element map is 100 nm×50 nm×2=10000 nm ² and one nitride particle was observed on this cementite interface, it is 1.0×10 ¹⁰ cm ⁻² . When such cementite was not observed even after multiple measurements, it was indicated as “-” in the table.

The composition of the precipitate particles was determined by determining the constituent elements and the number of atoms of the nitride particles using a particle analysis program such as the maximum separation method included in IVAS software, which is a three-dimensional construction software made by CAMECA. It may also be determined by cutting a box directly on the 3D element map. The version of the IVAS software was 3.6.8. However, the software version does not affect the measurement results.
The size of the precipitate particles was calculated as a sphere equivalent diameter as follows. The number of constituent atoms other than nitrogen was calculated by the above method. This is because the current atom probe cannot measure all nitrogen atoms correctly. Assuming a stoichiometric composition, in a NaCl type crystal structure, the number of constituent atoms of the nitride particles including nitrogen is twice the number of constituent atoms such as V, Cr, Mn, etc. other than nitrogen. Furthermore, this value was divided by the ion detection rate of the atom probe (0.38 in this case) to obtain the actual number of constituent atoms N. Finally, assuming a sphere with a diameter D and the atomic density of the precipitate as a, the particle diameter D can be calculated from the formula D = (6N / πa) ^1/3 . Here, the value of a = 113 nm ^-3 was used as the atomic density. The number density of nitride particles containing V with a particle diameter of 4 nm or more on the cementite interface and the number density of nitride particles with a particle diameter of 0.5 to 4 nm in ferrite, which were estimated in this way, are shown in Tables 3A and 3B.

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 in the range from the head outer surface 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, and wear resistance was insufficient.

In test No. 2 (steel type B), the amount of N was insufficient. 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.

In Test No. 3 (steel type C), the amount of V was insufficient. In Test No. 3, the hardness at a depth of 25 mm from the surface of the top of the head was insufficient, resulting in insufficient wear resistance.

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.

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, and wear resistance and ductility were impaired.

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 head outer surface to a depth of 25 mm, and the area ratio of pearlite structure was less than 95%. In Test No. 13, wear resistance was impaired.

In Test No. 14, the cooling rate in the controlled cooling was insufficient. In Test No. 14, the number density of nitride particles present on the cementite interface became excessive, and wear resistance was impaired.

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

In Test No. 16, the end temperature of the controlled cooling was too high. In Test No. 16, the number density of nitride particles present on the cementite interface became excessive, and wear resistance was impaired.

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

In Test No. 18, the cooling rate in the pre-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.

In Test No. 19, the cooling rate in the pre-controlled cooling was insufficient. In Test No. 19, the number density of nitride particles present on the cementite interface became excessive, and wear resistance was impaired.

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

In test No. 21 (steel type I), the amount of V was excessive. In test No. 21, wear resistance was impaired.

In Test No. 22 (steel type J), the C content was excessive. In Test No. 22, pro-eutectoid cementite was mixed into the structure in the range from the head outer surface 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.

In Test No. 23 (steel type K), the amount of Si was excessive. In Test No. 24 (steel type L), the amount of Mn was excessive. In Test No. 25 (steel type M), the amounts of Cr and Mo were excessive. In Test Nos. 23 to 25, martensite was mixed into the structure in the range from the head outer surface to a depth of 25 mm, and the area ratio of pearlite structure was less than 95%. In Test Nos. 23 to 25, wear resistance and ductility were impaired.

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.

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

In Test No. 28 (steel type P), the amount of Ti 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.

On the other hand, in Tests No. 4 to No. 10 and Tests No. 29 to No. 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.

1 Pearlite structure 11 Cementite (lamellar cementite)
12 Ferrite (lamellar ferrite)
13 Cementite interface 21 Nitride particle present on the cementite interface 22 Nitride particle 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 (3)

In mass percent,
C: 0.65-1.20%,
Si: 0.05-2.00%,
Mn: 0.05-2.00%,
Cr: 0.02-2.00%,
N: 0.0020-0.0200%,
V: 0.010-0.100%,
Nb: 0% or more and less than 0.005%;
Ti: 0% or more and less than 0.005%
Mo: 0-0.100%,
B: 0 to 0.0050%,
Co: 0-1.00%,
Ni: 0 to 1.00%,
Mg: 0 to 0.0200%,
Ca: 0-0.0200%,
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: 0-1.000%,
O: 0.0040% or less,
and the balance being Fe and impurities,
The structure in the range from the head outer surface to a depth of 25 mm contains a pearlite structure in an area ratio of 95% or more,
The hardness at a depth of 25 mm from the surface of the top of the head is at least HV350;
A pearlite rail characterized in that the number density per unit interface area of V-containing nitride particles having a particle size of 4 nm or more and present on a cementite interface of the pearlite structure at the position 25 mm deep from the surface of the top portion is less than 1.0 x ¹⁰¹⁰ particles cm ^-2 . 2. The pearlite rail according to claim 1, wherein V-containing nitride particles having a particle size of 0.5 to 4 nm are present in the ferrite of the pearlite structure, separated from the cementite interface, in a range of 4.0×10 ¹⁶ to 4.0×10 ¹⁷ particles cm ^-3 in terms of number density per unit volume. Further, in mass %,
B: 0.0001 to 0.0050%,
Co: 0.01 to 1.00%,
Ni: 0.01-1.00%,
Mg: 0.0005-0.0200%,
Ca: 0.0005-0.0200%,
Cu: more than 0% but not more than 1.00% REM: 0.0005 to 0.0500%
Zr: 0.0001-0.0200%
Al: more than 0% but not more than 1.000%,
3. The pearlite rail according to claim 1, further comprising at least one selected from the group consisting of:

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