JP2021063248A - rail - Google Patents

Abstract

To provide a rail in which cracks caused in a web section of the rail can be prevented from progressing, thereby preventing the rail from being broken.SOLUTION: A rail 1 comprises a foot section 2, a web section 3, and a head section 4. The component composition of the head section includes, by mass: 0.70 to 1.00% of C; 0.20 to 1.00% of Si; 0.50 to 1.50% of Mn; 0.035% or less of P; 0.0005 to 0.010% of S; 0.20 to 1.00% of Cr; 0.05 to 0.10% of V; 0.0015 to 0.0150% of N; and the balance of Fe and inevitable impurities, wherein the rail satisfies formula (1): [%Mn]/([%Si]+[%Cr])≥0.60, formula (2): ([%Si]+2[%Cr])-[%Mn]≥0.30, and formula (3): [%Mn]×[%S]≤0.004. In the formulae (1) to (3), [%X] expresses mass% of a component composition X. In the rail, the hardness from a surface layer of the head section to the depth of 25 mm is 380 to 480 HV, and a hardness difference between the surface layer of the head section and a position at a depth of 25 mm is less than 40 HV.SELECTED DRAWING: Figure 1

Description

The present invention relates to rails for railroads having feet, abdomen and head.

In a high-axle heavy railway with a large load weight, which mainly transports ore, the load applied to the axle of a freight car is much higher than that of a passenger car, and the rail usage environment becomes harsh. Further, in order to improve the efficiency of transportation by rail, the weight loaded on freight cars is being further increased, and it is required to improve wear resistance, fatigue damage resistance and delayed fracture resistance.

Conventionally, various proposals have been made to control the material of the rail or to perform a special heat treatment at the time of manufacturing method in order to improve the wear resistance of the rail (see, for example, Patent Documents 1 to 8). .. Patent Documents 1 and 2 disclose rails in which the amount of C is increased from more than 0.85% by mass to 1.20% by mass or less to improve wear resistance. Further, in Patent Documents 3 and 4, the amount of C is set to more than 0.85% by mass and 1.20% by mass or less, and the head of the rail is heat-treated to increase the cementite fraction and thereby wear resistance. Improved rails are disclosed.

On the other hand, the rails in the curved section of the high-axis heavy railway are subject to rolling stress due to the wheels and slipping force due to centrifugal force, so that the rails are more severely worn and fatigue damage due to slipping occurs. If the amount of C is simply more than 0.85% by mass and 1.20% by mass or less as described above, a proeutectoid cementite structure is formed depending on the heat treatment conditions, and the amount of the cementite layer of the brittle pearlite layered structure increases. No improvement in fatigue damage resistance is expected.

Therefore, Patent Document 5 proposes a rail in which the formation of proeutectoid cementite is suppressed by adding Al and Si, and the fatigue damage resistance is improved. Patent Document 6 discloses a rail in which the service life of the rail is improved by setting the Vickers hardness in a range of at least 20 mm from the corner portion of the head portion and the surface of the crown portion of the rail to Hv370 or more. ..

In Patent Document 7, by controlling the pearlite block, the hardness in the range of at least 20 mm in depth from the surface of the corner portion and the crown portion of the rail head is set to be in the range of Hv300 to 500. We are trying to improve the service life of the rail. Further, in Patent Document 8, in the cross section at the 25 mm position, ^{50 to 500 V carbonitrides having an average particle size of 5 to 20 nm are present per 1.0 μm 2 of the} test area, and the number of carbon atoms is CA and nitrogen. When the number of atoms is NA, CA / NA is 0.7 or less, the structure in the range up to a depth of 25 mm is 95% or more, and the Vickers hardness is in the range of Hv350 to Hv480. To extend the life of the rail.

Further, as a technique for preventing delayed fracture of rails, it is known that control of the form and amount of A-based inclusions as disclosed in Patent Documents 9 to 12, for example, is effective. In particular, in Patent Document 10, the toughness and ductility of the rail are improved by setting the size of the A-based inclusions to 0.1 to 20 μm and ^{controlling the number of A-based inclusions to 25 to 11000 per 1 mm 2.} There is.

Japanese Unexamined Patent Publication No. 8-109439 Japanese Unexamined Patent Publication No. 8-144016 Japanese Unexamined Patent Publication No. 8-246100 Japanese Unexamined Patent Publication No. 8-246101 Japanese Unexamined Patent Publication No. 2002-69585 Japanese Unexamined Patent Publication No. 10-195601 Japanese Unexamined Patent Publication No. 2003-293086 Japanese Patent No. 6341298 Japanese Unexamined Patent Publication No. 2000-328190 Japanese Unexamined Patent Publication No. 6-279928 Japanese Patent No. 3323272 Japanese Unexamined Patent Publication No. 6-279929

However, in the case of the rails of Patent Documents 1 to 8, although the strength of the rail can be increased, the effect of preventing delayed fracture is insufficient. Further, in the case of Patent Documents 9 to 12, the toughness and ductility of the rail can be improved by controlling the form and amount of the A-based inclusions. However, even in this case, good delayed fracture resistance is not always obtained.

The present invention has been made in view of the above circumstances, and provides a rail capable of improving delayed fracture resistance while satisfying wear resistance and fatigue damage resistance of the rail head. With the goal.

The present inventors have been made to achieve the above object, and the gist thereof is as follows.
[1] A rail provided with a foot, an abdomen, and a head.
The composition of the head component is
C: 0.70 to 1.00% by mass,
Si: 0.25 to 1.00% by mass,
Mn: 0.50 to 1.50% by mass,
P: 0.035% by mass or less,
S: 0.0005 to 0.010% by mass,
Cr: 0.25 to 1.00% by mass,
V: 0.005 to 0.10% by mass,
N: 0.0015 to 0.0150% by mass,
The balance is composed of Fe and unavoidable impurities, and the following formulas (1) to (3) are satisfied.
The Vickers hardness of the region from the surface layer of the head to the depth position of 25 mm is Hv380 or more and less than Hv480.
A rail characterized in that the hardness difference between the hardness of the surface layer of the head and the hardness at a depth of 25 mm from the surface layer is less than Hv40.
[% Mn] / ([% Si] + [% Cr]) ≧ 0.60 ・ ・ ・ (1)
([% Si] + 2 [% Cr])-[% Mn] ≥ 0.30 ... (2)
[% Mn] x [% S] ≤ 0.004 ... (3)
However, [% X] represents the mass% of the component composition X.
[2] The head further includes Cu: 1.0% by mass or less, Ni: 1.0% by mass or less, Nb: 0.05% by mass or less, Mo: 1.0% by mass or less, and V: 0.005. The first aspect of claim 1, wherein any one or more of the component compositions of ~ 0.10% by mass, W: 1.0% by mass or less, and B: 0.005% by mass or less are contained. rail.

According to the present invention, it is possible to improve the delayed fracture resistance while satisfying the wear resistance and fatigue damage resistance of the rail head.

It is a perspective view which shows the preferable embodiment of the rail of this invention. It is a schematic diagram which shows an example of the measurement position of the Vickers hardness in the head of a rail. It is a schematic diagram which shows the collection place of two wear test pieces in the head of a rail. It is a schematic diagram which shows an example of the wear test apparatus. It is a schematic diagram which shows an example of the fatigue resistance damage test apparatus. It is a schematic diagram which shows the collection place of the SSRT (Slow Straight Rate Technique) test piece in the head of a rail. It is a schematic diagram which shows an example of the dimension and shape of the SSRT test piece TP.

Hereinafter, embodiments of the present invention will be described. FIG. 1 is a schematic view showing a preferred embodiment of the rail of the present invention. The rail 1 in FIG. 1 supports a load for a passenger railroad or a freight railroad, and guides a railroad vehicle in the traveling direction (arrow Y direction), and has a foot (bottom) 2, an abdomen 3, and a head. A part 4 is provided.

The foot portion 2 is placed on a sleeper and has a cross-sectional shape extending in the width direction (arrow X direction). The abdomen 3 has a shape extending from the foot 2 in the vertical direction (arrow Z direction), and has a function of ensuring bending rigidity of the rail 1 itself as a beam. The head 4 is provided on the upper part of the abdomen 3 and comes into contact with the wheels of the train to directly support the load of the train. When the train travels on the rail 1, the load from the wheels of the train is transmitted from the head 4 to the abdomen 3 and from the abdomen 3 to the foot 2.

Since the head 4 is a portion where the wheels come into direct contact with each other, wear resistance is important, and delay fracture resistance must be satisfied while satisfying wear resistance and fatigue damage resistance. Therefore, the head 4 of the rail 1 has the following composition and steel structure.

Rail 1 has C: 0.70 to 1.00% by mass, Si: 0.25 to 1.00% by mass, Mn: 5.00 to 1.50% by mass, P: 0.035% by mass or less, S. : 0.0005 to 0.010% by mass, Cr: 0.25 to 1.00% by mass, V: 0.005 to 0.10% by mass, N: 0.0015 to 0.0150% by mass. ing. Hereinafter, each composition component will be described separately.

C: 0.70 to 1.00 mass%
C is an essential element for ensuring the strength of the pearlite structure, that is, the fatigue damage resistance, and the fatigue damage resistance improves as the content increases. However, if the amount of C is less than 0.70% by mass, it is difficult to obtain excellent fatigue damage resistance as compared with the conventional heat-treated pearlite steel rail. Further, when the amount of C exceeds 1.00% by mass, proeutectoid cementite is generated at the austenite grain boundaries during the pearlite transformation after hot rolling, and the fatigue damage resistance is remarkably lowered. Therefore, the amount of C is set to 0.70 to 1.00% by mass. It is preferably 0.75 to 0.85% by mass.

Si: 0.25 to 1.00 mass%
Si is required to be 0.20% by mass or more as an oxygen scavenger and a strengthening element of the pearlite structure. However, when the amount of Si exceeds 1.00% by mass, Si stabilizes ferrite, so that the slope of the TTT curve (Time-Temperature-Transformation: constant temperature transformation curve) when transforming from austenite to pearlite becomes large. Metamorphosis occurs at once in a wide range from the surface. As a result, the internal cooling rate decreases due to the transformation heat generation, and the hardness also decreases. Therefore, the amount of Si is set to 0.25 to 1.00% by mass. It is preferably 0.50 to 0.80% by mass.

Mn: 0.50 to 1.50% by mass
Mn needs to be added in an amount of 0.50% by mass or more in order to improve the strength and reduce the difference in hardness between the surface and the inside. The reason why the hardness difference becomes small is that Mn stabilizes austenite, so that the slope of the TTT curve becomes small. As a result, the transformation temperature difference between the surface and the inside becomes small. However, if the amount of Mn exceeds 1.50% by mass, a martensite structure is likely to be formed, and the material is likely to be deteriorated due to hardening or embrittlement during heat treatment and welding of the rail. Further, since excessive addition of Mn lowers the equilibrium transformation temperature of pearlite, it becomes difficult to obtain a desired hardness due to the coarsening of the lamellar interval. Therefore, the Mn content is set to 0.50 to 1.50% by mass. It is preferably 0.70 to 1.00% by mass.

P: 0.035% by mass or less The content of P in which the amount of P exceeds 0.035% by mass deteriorates ductility. Therefore, the amount of P is set to 0.035% by mass or less. It is preferably 0.020% by mass or less. In addition, in order to make it less than 0.001% by mass, the steelmaking cost must be increased, so that the content of 0.001% by mass or more is allowed.

S: 0.0005 to 0.010 mass%
S is mainly present in the steel material in the form of A-based inclusions, but when it exceeds 0.010% by mass, the amount of these inclusions increases remarkably, and at the same time, coarse inclusions are generated, so that the cleanliness of the steel material deteriorates. To do. Further, when the amount of S is less than 0.0005% by mass, the steelmaking cost increases. Therefore, the amount of S is set to 0.0005 to 0.010% by mass. It is preferably 0.0005 to 0.008% by mass.

Cr: 0.20 to 1.00% by mass or less Cr needs to be added in an amount of 0.20% by mass or more in order to improve the pearlite structure strength. However, if the amount of Cr exceeds 1.00% by mass, a martensite structure is likely to be formed, and the material is likely to be deteriorated due to hardening or embrittlement during heat treatment and welding of the rail. Further, since Cr stabilizes ferrite, the inclination of the line connecting the start point Ps of the pearlite transformation and the end point Pf of the pearlite transformation in the TTT curve becomes large, and the transformation occurs at once in a wide range from the surface. As a result, the internal cooling rate decreases due to the transformation heat generation, and the hardness also decreases. Therefore, the amount of Cr is set to 0.25 to 1.00% by mass. It is preferably 0.60 to 0.80% by mass.

V: 0.005 to 0.10% by mass
V forms a carbonitride and is dispersed and precipitated in the matrix to improve fatigue damage resistance and delayed fracture resistance. However, if the amount of V is less than 0.005% by mass, the effect is reduced. On the other hand, if the amount of V exceeds 0.10% by mass, the workability deteriorates and the alloy cost also increases, so that the manufacturing cost of the rail material increases. Therefore, the amount of V is set to 0.005 to 0.10% by mass. It is preferably 0.01 to 0.08% by mass.

N: 0.0015 to 0.0150% by mass
N forms a nitride and is dispersed and precipitated in the matrix to improve fatigue damage resistance and delayed fracture resistance. However, if the amount of N is less than 0.0015% by mass, the effect is small. On the other hand, when the amount of N exceeds 0.0150% by mass, coarse nitrides are formed in the rail, and fatigue damage resistance and delayed fracture resistance are lowered. Therefore, the amount of N is set to 0.0015 to 0.0150% by mass. It is preferably 0.0030 to 0.0100% by mass.

Further, the component composition of the head 4 satisfies the following formulas (1) to (3). In the formulas (1) to (3), [% X] represents the mass% of the component composition X.

[% Mn] / ([% Si] + [% Cr]) ≧ 0.60 ・ ・ ・ (1)
Increasing the amount of Mn reduces the inclination of the TTT curve, which is advantageous for reducing the difference in hardness between the surface and the inside of the head 4. On the other hand, the increase in the amount of Si and the amount of Cr increases the slope between the start point Ps and the end point Pf of the pearlite transformation of the TTT curve, and acts in the direction of increasing the hardness difference. Therefore, the balance of the addition amounts of Mn, Si, and Cr is important from the viewpoint of reducing the difference in hardness. If [% Mn] / ([% Si] + [% Cr]) is less than 0.60, the hardness difference between the surface layer of the head 4 and the 25 mm depth position becomes large, and high hardness cannot be achieved to the inside. .. Therefore, the value of [% Mn] / ([% Si] + [% Cr]) is set to 0.60 or more in order to suppress the decrease in hardness. It is preferably 0.80 or more.

([% Si] + 2 [% Cr])-[% Mn] ≥ 0.30 ... (2)
Increasing the amount of Mn is advantageous for reducing the difference in hardness between the surface and the inside. On the other hand, since Mn is an element that reduces the degree of supercooling, it may cause a decrease in hardness. Since Si and Cr are elements that increase the degree of supercooling, they are elements necessary for increasing the hardness. Therefore, the balance of the addition amounts of Mn, Si, and Cr is important from the viewpoint of high hardness. ([% Si] + 2 [% Cr])-If [% Mn] is less than 0.30, the hardness is low, and the Vickers hardness in the region from the upper surface (surface layer) of the rail head 4 to the position of 25 mm in depth. Cannot achieve Hv380 or more and less than Hv480. Therefore, in order to secure the supercooling degree, the value of ([% Si] + 2 [% Cr])-[% Mn] is set to 0.30 or more. It is preferably 0.6 or more.

[% Mn] x [% S] ≤ 0.004 ... (3)
Mn and S are important elements for forming Mn-based sulfides that serve as hydrogen trap sites, and the amount of Mn and S added must be controlled in order to form Mn-based sulfides. When [% Mn] × [% S] is larger than 0.004, Mn-based sulfides are often formed, hydrogen trap sites increase, and delayed fracture resistance deteriorates. Therefore, the value of [% Mn] × [% S] is set to 0.004 or less in order to suppress delayed fracture.

In addition to the above composition components, the component composition of the rail of the present invention is Cu: 1.0% by mass or less, Ni: 1.0% by mass or less, Nb: 0.05% by mass or less, Mo: 1.0% by mass. It may contain any one or more of% or less. Hereinafter, each composition component will be described separately.

Cu: 1.0% by mass or less Cu is an element that can further increase the strength of steel by solid solution strengthening like Cr. However, if the content exceeds 1.0% by mass, Cu cracking is likely to occur. Therefore, when the component composition contains Cu, the amount of Cu is preferably 1.0% by mass or less. More preferably, it is 0.005 to 0.5% by mass.

Ni: 1.0% by mass or less Ni is an element that can increase the strength of steel without deteriorating ductility. Further, since Cu cracking can be suppressed by compound addition with Cu, it is desirable that Ni is also contained when the component composition contains Cu. However, when the Ni content exceeds 1.0% by mass, the hardenability of the steel is further increased and martensite is generated, and the fatigue damage resistance and the fatigue damage resistance tend to be lowered. Therefore, when Ni is contained, the Ni content is preferably 1.0% by mass or less. More preferably, it is 0.005 to 0.5% by mass.

Nb: 0.05% by mass or less Nb is combined with C in steel to form a rail, so that it is precipitated as carbide during hot rolling and after hot rolling, and effectively acts on the miniaturization of the old austenite particle size. As a result, fatigue damage resistance, fatigue damage resistance, and ductility are greatly improved, which greatly contributes to extending the life of the rail. However, even if the amount of Nb exceeds 0.05% by mass, the effect of improving wear resistance and fatigue damage resistance is saturated, and an effect commensurate with the increase in content cannot be obtained. Therefore, Nb may be contained with the upper limit of its content being 0.05% by mass. The above effect is exhibited when the amount of Nb is 0.001% by mass or more. Therefore, when Nb is contained, the Nb content is preferably 0.001% by mass or more. More preferably, it is 0.001 to 0.03% by mass.

Mo: 1.0% by mass or less Mo is an element that can improve hardenability and further increase the strength of steel by solid solution strengthening. However, if it exceeds 1.0% by mass, martensite is formed in the steel, and the wear resistance and fatigue damage resistance tend to decrease. Therefore, when the component composition of the rail contains Mo, the Mo content is preferably 1.0% by mass or less. More preferably, it is 0.005 to 0.5% by mass.

In the rail 1, Fe and unavoidable impurities are contained in the remainder of the composition component. Inevitable impurities are those that are present in the raw material or are inevitably mixed in the manufacturing process, and are originally unnecessary, but they are allowed to be contained because they are in trace amounts and do not affect the characteristics. Means impurities. Examples of the unavoidable impurities include O and the like, and O can be allowed up to 0.004% by mass. Further, in the present invention, Ti mixed as an impurity forms an oxide and causes a decrease in fatigue damage resistance, which is a basic characteristic of the rail. Therefore, it is preferable to control it to 0.0010% by mass or less.

Next, the hardness of the head 4 of the rail 1 will be described. The Vickers hardness of the region from the surface layer (surface) of the head 4 of the rail 1 to the depth of 25 mm is Hv380 or more and less than Hv480. The domain of the internal hardness of the head 4 is defined as a region up to a depth of 25 mm because the wear resistance is lowered and the service life is shortened. Further, when the Vickers hardness is less than Hv380, the wear resistance is lowered and the service life of the rail is shortened. On the other hand, when the Hv is 480 or higher, martensite is generated and the fatigue damage resistance of the steel is lowered. Therefore, the Vickers hardness of the above region of the head 4 is Hv380 or more and less than Hv480. It is preferably Hv400 or more and less than Hv460.

Further, the hardness difference between the hardness of the surface layer of the head 4 and the hardness at a depth of 25 mm from the surface layer is less than Hv40. This is because when the hardness difference is Hv40 or more, the wear resistance improvement allowance and the fatigue damage resistance improvement allowance deteriorate as the wear progresses due to use.

Hereinafter, the constitution and the action and effect of the present invention will be described more specifically according to Examples. The present invention is not limited by the following examples, and can be appropriately modified within a range that can be adapted to the gist of the present invention, all of which are included in the technical scope of the present invention. Is done.

<Preparation of test piece>
First, steels A0, B1 to B27, and C1 to C14 having different composition components are produced. Table 1 below shows the components of steels A0, B1 to B27, and C1 to C14. It should be noted that the blank portion in Table 1 is not contained, or the content is in the category of unavoidable impurities and can be ignored.

Next, rail 1 was manufactured using steels A0, B1 to B27, and C1 to C14 containing the composition components shown in Table 1, respectively. At this time, the steel materials having the component compositions shown in Table 1 are hot-rolled at a rolling finish temperature of 850 to 950 ° C., and then the surface of the head 4 of the rail 1 is heated to 400 to 600 ° C. from a temperature equal to or higher than the pearlite transformation start temperature. The temperature range was accelerated and cooled at a cooling rate of 1 to 5 ° C./sec. Cooling was performed only on the head 4 of the rail 1, and after the cooling was stopped, the material was allowed to cool. As the rolling finish temperature, the value obtained by measuring the surface temperature of the side surface of the head 4 of the rail 1 on the entry side of the final rolling mill with a radiation thermometer is shown as the rolling finish temperature. As for the cooling stop temperature, the value obtained by measuring the temperature of the surface layer on the side surface of the rail head at the time of cooling stop with a radiation thermometer is shown as the cooling stop temperature. The cooling rate was defined as the cooling rate (° C./sec) by converting the temperature change from the start of cooling to the stop of cooling per unit time (seconds).

The obtained rails were evaluated for head hardness, wear resistance, fatigue damage resistance, and delayed fracture resistance. Table 2 shows the evaluation results of the rail head. In Tables 1 and 2, numerical values outside the scope of the present invention are underlined.

<Surface hardness of the head, hardness at a depth of 25 mm and hardness difference ΔHv>
For the central portion of the head 4 of the rail 1 shown in FIG. 2 in the width direction (arrow X direction), the Vickers hardness in the range of 1.0 mm to 25 mm depth of the surface layer is measured along the depth direction (arrow Z direction). It was measured at a load of 98 N and a pitch interval of 1.0 mm. Of the plurality of measured hardnesses, the hardness of the surface layer of the central line tends to be the maximum value, and the hardness of the depth position 25 mm in the central line tends to be the minimum value. The difference between the hardness of the surface layer and the hardness of the depth position of 25 mm was calculated as the hardness difference ΔHv.

<Organization>
“P” in Table 2 refers to the case where the area ratio of the pearlite structure is 95% or more and a total of 5% or less contains a trace amount of bainite structure, martensite structure, proeutectoid cementite structure, and proeutectoid ferrite structure. means.

<Abrasion resistance>
Regarding wear resistance, it is most desirable to actually lay the rail and evaluate it, but the test takes a long time. Therefore, using a wear tester that can evaluate the wear resistance in a short time, the amount of wear and the wear resistance improvement allowance are measured by a comparative test that simulates the actual contact conditions between the rail and the wheel. Abrasion resistance was evaluated.

First, as shown in FIG. 3, Nishihara-type wear test pieces TP1 and TP2 having an outer diameter of 30 mm were collected from two places on the head 4 of the rail 1. The Nishihara-type wear test piece TP1 was collected from the surface layer of the rail head 4, and the Nishihara-type wear test piece TP2 was collected from a depth of 24 to 26 mm (average value 25 mm) from the upper surface of the head 4. ..

FIG. 4 is a schematic view showing an example of a wear test apparatus. As shown in FIG. 4, the wear test is performed by rotating the test piece TP1 (or the test piece TP2) in contact with the tire test piece 10. The tire test piece 10 is obtained by collecting a round bar having a diameter of 32 mm from the head of the rail described in JIS standard E1101. The tire test piece 10 has a Vickers hardness (load 98N) of Hv390, and is processed into a cylindrical shape after being heat-treated so that the structure becomes a tempered martensite structure. The arrows in FIG. 4 indicate the rotation directions of the Nishihara-type wear test piece TP1 (TP2) and the tire test piece 10, respectively.

The test environment condition in FIG. 4 is a dry state, and after 100,000 rotations under the conditions of contact pressure: 1.2 GPa, slip ratio: -10%, rotation speed: 675 times / min (tire test piece is 750 times / min). The amount of wear was measured. When comparing the magnitude of the amount of wear, the evaluation standard was the standard steel A0, which is a heat-treated pearlite steel rail, and it was determined that the wear resistance was improved when the amount of wear was 15% or more smaller than that of the standard steel A0. The wear resistance improvement allowance was calculated by {(wear amount of reference steel A0-wear amount of test material) / (wear amount of reference steel A0)} × 100 for each of the Nishihara-type wear test pieces TP1.

<Fatigue damage resistance>
FIG. 5 is a schematic view showing an example of a fatigue resistance damage test apparatus. The sampling positions and dimensions of the test pieces TP11, TP12 and the tire test piece 10 used in the fatigue resistance test piece of FIG. 5 are the same as those of FIG. However, the contact surfaces of the test pieces TP11 and TP12 are curved with a radius of curvature of 15 mm. The arrows in FIG. 5 indicate the rotation directions of the Nishihara-type wear test piece TP1 (TP2) and the tire test piece 10, respectively.

The test environment in FIG. 5 is an oil-lubricated condition, the contact pressure is 2.2 GPa, the slip ratio is -20%, the rotation speed is 600 rpm (the tire test piece is 750 rpm), and the surface of the test piece is observed every 25,000 times. The number of rotations at the time when a crack of 0.5 mm or more was generated was defined as the fatigue damage life. When comparing the magnitude of fatigue damage life, the evaluation standard is set to the standard steel A0 as in the above evaluation of wear resistance, and the fatigue damage resistance is improved when the fatigue damage time is 10% or more longer than that of the standard steel A0. It was judged that it was done. The fatigue damage resistance improvement allowance for each of the Nishihara-type wear test pieces TP11 and TP12 is [{(rotation speed until fatigue damage occurs in the test material)-(rotation speed until fatigue damage occurs in the reference steel A0). } / (Rotation speed until fatigue damage of reference steel A0 occurs)] × 100 was calculated.

<Delay resistance>
As shown in FIG. 6, the SSRT (Slow Strain Rate Technology) test piece TP20 is collected from the upper surface of the head 4 of the rail 1 at a depth position of 25 mm. FIG. 7 is a schematic view showing an example of the dimensions and shape of the SSRT test piece TP20. As shown in FIGS. 6 and 7, the parts other than the screw portion 21 and the R portion are finished with ▽▽▽ finish (micro mirror finish), and the parallel portion 22 is emery-polished to # 600. The SSRT test piece TP20 was mounted on a test device (not shown), and the ^{SSRT test was performed at a strain rate of 3.3 × 10-6} / sec at 25 ° C. in the atmosphere, and the elongation E0 of the SSRT test piece in the atmosphere was measured. .. Further, an SSRT test was carried out at 25 ° C. in a 20 mass% ammonium thiocyanate aqueous solution at a strain rate of 3.3 × ^10-6 / sec, and the elongation E1 of the SSRT test piece in the ammonium thiocyanate aqueous solution was measured.

Then, the delayed fracture sensitivity (that is, DF), which is an index for evaluating the delayed fracture resistance, was calculated by DF (%) = 100 × (1-E1 / E0). Then, it was determined that the delayed fracture resistance was improved when the reference material A0 (that is, the heat-treated pearlite steel rail having a C amount of 0.68 mass%) had an improvement allowance of 10% or more with respect to the delayed fracture sensitivity. The delay fracture susceptibility improvement allowance was calculated by [{(delayed fracture susceptibility of test material)-(delayed fracture susceptibility of reference steel A0)} / (delayed fracture susceptibility of reference steel A0)] × 100.

As shown in Table 2, the test results of the rail material (test Nos. B1 to B27 in Table 2) produced by using the conforming steel satisfying the component composition of the present invention show the wear resistance characteristics with respect to the reference material. The fatigue damage resistance was improved by 15% or more, the delayed fracture resistance was improved by 10% or more.

On the other hand, in Comparative Examples (Test Nos. C1 to C14 in Table 2), the component composition of the rail material did not satisfy the conditions of the present invention, and as a result, the steel structure of the present invention was not satisfied. The improvement allowance for any of the wear resistance, fatigue damage resistance, and delayed fracture resistance with respect to the reference material was lower than that of the invention example, or some of them contained a part of martensite structure.

As described above, according to the present invention, by controlling the composition component of the head 4 and the hardness in the head 4, the delayed fracture resistance while satisfying the wear resistance and the fatigue damage resistance of the head 4 of the rail 1. Can be improved.

1 Rail 2 Foot 3 Abdominal 4 Head 10 Tire test piece TP1, TP2, TP11, TP12 Nishihara type wear test piece TP20 SSRT test piece

Claims (2)

A rail with legs, abdomen, and head
The composition of the head component is
C: 0.70 to 1.00% by mass,
Si: 0.25 to 1.00% by mass,
Mn: 0.50 to 1.50% by mass,
P: 0.035% by mass or less,
S: 0.0005 to 0.010% by mass,
Cr: 0.25 to 1.00% by mass,
V: 0.005 to 0.10% by mass,
N: 0.0015 to 0.0150% by mass,
The balance is composed of Fe and unavoidable impurities, and the following formulas (1) to (3) are satisfied.
The Vickers hardness of the region from the surface layer of the head to the depth position of 25 mm is Hv380 or more and less than Hv480.
A rail characterized in that the hardness difference between the hardness of the surface layer of the head and the hardness at a depth of 25 mm from the surface layer is less than Hv40.
[% Mn] / ([% Si] + [% Cr]) ≧ 0.60 ・ ・ ・ (1)
([% Si] + 2 [% Cr])-[% Mn] ≥ 0.30 ... (2)
[% Mn] x [% S] ≤ 0.004 ... (3)
However, [% X] represents the mass% of the component composition X. The head is further one or more of Cu: 1.0% by mass or less, Ni: 1.0% by mass or less, Nb: 0.05% by mass or less, Mo: 1.0% by mass or less. The rail according to claim 1, wherein the rail contains the component composition of.

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