CN114502761B

CN114502761B - Rail and manufacturing method thereof

Info

Publication number: CN114502761B
Application number: CN202080070379.7A
Authority: CN
Inventors: 德永和也; 安藤佳祐
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2019-10-11
Filing date: 2020-07-27
Publication date: 2024-01-09
Anticipated expiration: 2040-07-27
Also published as: CA3157401C; EP4023777A4; JPWO2021070452A1; EP4023777A1; CN114502761A; JP7063400B2; AU2020364505A1; US20230250505A1; CA3157401A1; AU2020364505B2; WO2021070452A1

Abstract

The rail (1) is provided with a foot (2), a waist (3) and a head (4), and the composition of the waist (3) contains C:0.70 to 1.20 mass% of Si:0.20 to 1.20 mass percent of Mn:0.20 to 1.50 mass percent, P:0.035 mass% or less, cr:0.20 to 2.50 mass% and the balance of Fe and unavoidable impurities, wherein the area ratio of pearlite at the waist (3) is 95% or more and the average size of pearlite blocks is 60 [ mu ] m or less.

Description

Rail and manufacturing method thereof

Technical Field

The present invention relates to a rail for railway having a foot portion, a waist portion and a head portion, and a method for manufacturing the same.

Background

For high axle weight railways with large loads, such as ore transportation, the axle of a truck is far more loaded than a passenger car, and the use environment of the steel rail is also severe. In addition, in order to increase the efficiency of railway transportation, the load of trucks is further increasing, and improvement of wear resistance, fatigue damage resistance and delayed fracture resistance is demanded.

Conventionally, various proposals have been made for controlling the material of a rail, performing special heat treatment in a manufacturing method, and the like, in order to improve the wear resistance of the rail (for example, refer to patent documents 1 to 8). Patent documents 1 and 2 disclose rails in which the abrasion resistance is improved by increasing the C content to more than 0.85 mass% and 1.20 mass% or less. Patent documents 3 and 4 disclose rails in which the C content is set to be greater than 0.85 mass% and 1.20 mass% or less, and the head portion of the rail is heat treated to increase the cementite fraction, thereby improving the wear resistance.

Patent document 5 proposes a steel rail in which the formation of eutectoid cementite is suppressed by adding Al and Si to improve fatigue damage resistance. Patent document 6 discloses a rail in which the vickers hardness in a depth range of at least 20mm from the corners of the head and the surface of the top of the head of the rail is set to Hv370 or more, thereby improving the service life of the rail.

Patent document 7 discloses a method in which a waist is quenched at a cooling rate of 15 ℃/sec or more, cooled to a temperature of 250 to 450 ℃ and stopped, and cooled to a temperature of Ms point or less when bainite transformation reaches 30% or more, whereby a martensitic structure is obtained, whereby the waist is made into a tempered martensitic structure with high toughness. In patent document 8, compressive residual stress is applied by cooling from the top of the head to the upper neck or waist with a high-pressure gas or an aqueous gas, thereby imparting an ability to suppress crack growth in the waist.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 8-109439

Patent document 2: japanese patent laid-open No. 8-144016

Patent document 3: japanese patent laid-open No. 8-246100

Patent document 4: japanese patent laid-open No. 8-246101

Patent document 5: japanese patent laid-open No. 2002-69585

Patent document 6: japanese patent laid-open No. 10-195601

Patent document 7: japanese patent laid-open No. 62-99438

Patent document 8: japanese patent laid-open No. 59-47326

Disclosure of Invention

Problems to be solved by the invention

According to the rails of patent documents 1 to 6, the wear resistance of the head portion of the rail, which is mainly in contact with the flange of the wheel, can be ensured. However, the control of the material quality of the waist portion of the rail is insufficient, and depending on the manufacturing method, cracks may develop in the waist portion.

In the case of patent document 7, it is necessary to maintain the temperature until the bainite transformation starts, and the manufacturing efficiency is lowered. In addition, in the technique disclosed in patent document 8, since it is most important to obtain the wear resistance and fatigue damage resistance of the head, a desired ability to suppress crack growth in the waist cannot be obtained, and depending on the manufacturing conditions, a martensitic structure having high crack sensitivity may be generated.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a rail and a method for manufacturing the same, which can improve manufacturing efficiency and suppress crack growth in a waist portion to suppress occurrence of breakage.

Means for solving the problems

The present inventors have completed the present invention in order to achieve the above object, and the gist thereof is as follows.

[1] A rail is provided with a leg portion, a waist portion and a head portion, wherein the composition of the waist portion contains C:0.70 to 1.20 mass% of Si:0.20 to 1.20 mass percent of Mn:0.20 to 1.50 mass percent, P:0.035 mass% or less, S:0.0005 to 0.012 mass%, cr:0.20 to 2.50 mass% and the balance of Fe and unavoidable impurities, wherein the area ratio of pearlite at the waist is 95% or more and the average size of pearlite blocks is 60 [ mu ] m or less.

[2] The steel rail according to [1], wherein the steel rail further comprises Cu:1.0 mass% or less, ni:1.0 mass% or less, nb:0.05 mass% or less, mo:1.0 mass% or less, V:0.005 to 0.10 mass%, W:1.0 mass% or less and B: 0.005% by mass or less of one or two or more kinds.

[3]Such as [1]]Or [2]]The steel rail, wherein the stressIntensity factor Δk=20 mpa·m ^1/2 The crack growth rate da/dN (m/cycle) of the waist is 8.0X10 ^-8 The following is given.

[4] A method for producing a steel rail from a steel sheet having a composition according to any one of [1] to [3], wherein the finish rolling is performed at a finish rolling temperature of 1000 ℃ or less and a reduction of area of a waist portion of 10% or more, and the waist portion after the finish rolling is cooled from a temperature of at least the pearlite transformation starting temperature to a temperature range of 400 to 600 ℃ at a cooling rate of 1 to 5 ℃/sec.

[5] The method of producing a steel rail according to [4], wherein the finish rolling of the waist portion is performed at a finish rolling temperature in the range of 800 to 900 ℃.

Effects of the invention

According to the present invention, the crack growth rate of the waist portion can be reduced, and the occurrence of crack growth and fracture of the waist portion can be suppressed.

Drawings

Fig. 1 is a perspective view showing a preferred embodiment of the rail of the present invention.

Fig. 2 is a top view showing a preferred embodiment of the rail of the present invention.

Fig. 3 is a schematic diagram showing an example of a rail manufacturing apparatus used in the rail manufacturing method of the present invention.

Fig. 4 is a schematic view showing an example of a test piece used for the crack growth test at the waist.

Detailed Description

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 of fig. 1 supports a load for a passenger railway or a freight railway, guides a railway vehicle in a traveling direction (arrow Y direction), and includes a leg portion (bottom portion) 2, a waist portion 3, and a head portion 4.

The leg portion 2 is placed on the sleeper and has a cross-sectional shape that expands in the width direction (arrow X direction). The waist portion 3 has a shape extending in the up-down direction (arrow Z direction) from the leg portion 2, and has a function of securing bending rigidity as a beam of the rail 1 itself. The head 4 is provided at the upper part of the waist 3, contacts with wheels of the train, and directly supports 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 portion 4 to the waist portion 3, and from the waist portion 3 to the foot portion 2.

Since the waist portion 3 is not a portion that directly contacts the wheel like the head portion 4, the wear resistance equivalent to that of the head portion 4 is not required. On the other hand, since the waist portion 3 transmits the wheel weight applied to the head portion 4 to the foot portion 2, when the wheel weight is applied eccentrically in the width direction from the center of the head portion 4, a bending stress may be generated in the waist portion 3, and a horizontal crack may be generated. Therefore, the waist portion 3 is required to have excellent crack propagation characteristics. Therefore, the waist portion 3 of the rail 1 has the following composition and steel structure.

Rail 1 contains C:0.70 to 1.20 mass% of Si:0.20 to 1.20 mass percent of Mn:0.20 to 1.50 mass percent, P:0.035 mass% or less, S:0.0005 to 0.012 mass%, cr:0.20 to 2.50 mass percent. The respective components will be described below.

C:0.70 to 1.20 mass percent

C is an essential element for securing fatigue damage resistance, which is the strength of the pearlite structure, and the fatigue damage resistance increases with increasing content. However, when the content is less than 0.70 mass%, it is difficult to obtain excellent fatigue damage resistance as compared with the conventional heat-treated pearlitic rail. When the content exceeds 1.20 mass%, a large amount of proeutectoid cementite is formed in austenite grain boundaries during pearlite transformation after hot rolling, and fatigue damage resistance is remarkably reduced. The pro-eutectoid cementite is present at 1.20 mass% or less, but the amount produced is small, so that the effect on fatigue damage resistance is slight. Accordingly, the amount of C is set to 0.70 to 1.20 mass%. Preferably 0.75 to 1.00 mass%. More preferably 0.75 to 0.85%.

Si:0.20 to 1.20 mass percent

Si is required to be 0.20 mass% or more as a deoxidizing agent and a strengthening element of a pearlite structure, but if it exceeds 1.20 mass%, the generation of surface defects of the rail is promoted. Therefore, the Si content is set to 0.20 to 1.20 mass%. Preferably 0.50 to 1.00 mass%.

Mn:0.20 to 1.50 mass percent

Mn has an effect of reducing the pearlite phase transition temperature and densifying the lamellar spacing, and is therefore an effective element for maintaining high hardness up to the inside of the rail 1. When the content is less than 0.20% by mass, a sufficient effect is not obtained. If the content exceeds 1.50 mass%, a martensitic structure is easily formed, hardening and embrittlement occur during heat treatment and welding, and the material is easily deteriorated. In addition, due to the high hardenability of Mn, a large amount of bainitic structure is formed in the surface layer of the internal high hardness rail, and the wear resistance is lowered. In addition, excessive addition lowers the equilibrium transformation temperature of pearlite, reduces supercooling, and coarsens the lamellar spacing. Therefore, the Mn content is set to 0.20 to 1.50 mass%. Preferably 0.40 to 1.20 mass%.

P:0.035 mass% or less

When the content of P exceeds 0.035%, ductility is deteriorated. Therefore, the amount of P is set to 0.035 mass% or less. Preferably 0.020% by mass or less. In order to be less than 0.001%, the steel-making cost has to be increased, and thus it is allowed to be contained at 0.001% or more.

S:0.0005 to 0.012 mass%

S exists mainly in the form of a-type inclusions in the steel, but if it exceeds 0.012 mass%, the amount of the inclusions increases significantly and coarse inclusions are formed, so that the cleanliness of the steel is deteriorated. In addition, if it is less than 0.0005 mass%, the steel-making cost increases. Therefore, the S content is set to 0.0005 to 0.012 mass%. Preferably 0.0005 to 0.010 mass%. More preferably 0.0005 to 0.008 mass%.

Cr:0.20 to 2.50 mass percent

Cr is an element that increases the pearlite equilibrium transformation temperature, contributes to the fine grain size of the lamellar spacing, and brings further high strength by solid solution strengthening. However, when the content is less than 0.20 mass%, sufficient internal hardness is not obtained. On the other hand, when the content exceeds 2.50 mass%, the hardenability becomes high, and a martensitic structure is easily formed. In addition, when the alloy is produced under the condition that the martensite structure is not formed, proeutectoid cementite is formed in the prior austenite grain boundary. Therefore, the abrasion resistance and the fatigue damage resistance are reduced. Therefore, the Cr content is set to 0.20 to 2.50 mass%. Preferably 0.60 to 1.30 mass%.

The composition of the rail of the present invention may contain Cu in addition to the above-mentioned composition: 1.0 mass% or less, ni:1.0 mass% or less, nb:0.05 mass% or less, mo:1.0 mass% or less, V:0.005 to 0.10 mass%, W:1.0 mass% or less and B: 0.005% by mass or less of one or two or more kinds. The respective components will be described below.

Cu:1.0 mass% or less

Cu is an element that can achieve further higher strength of steel by solid solution strengthening, similarly to Cr. However, when the content exceeds 1.0 mass%, cu cracks are likely to be generated. Therefore, when the component composition contains Cu, the Cu amount is preferably set to 1.0 mass% or less. More preferably 0.005 to 0.5 mass%.

Ni:1.0 mass% or less

Ni is an element that can achieve high strength of steel without deteriorating ductility. In addition, when the rail 1 contains Cu, ni is preferably added in combination with Cu because Cu cracking can be suppressed when Ni is added. However, when the Ni content exceeds 1.0 mass%, the hardenability of the steel is further increased, martensite is formed, and fatigue damage resistance tends to be lowered. Therefore, in the case of containing Ni, the Ni content is preferably set to 1.0 mass% or less. More preferably 0.005 to 0.5 mass%.

Nb:0.05 mass% or less

Nb is bonded to C in steel and precipitates as carbide during and after hot rolling for forming a rail, and thus effectively contributes to the reduction of the prior austenite grain size. As a result, the wear resistance, fatigue damage resistance, and ductility are greatly improved, which greatly contributes to the long life of the rail. However, even if the Nb content exceeds 0.05 mass%, the effect of improving the wear resistance and fatigue damage resistance is saturated, and the effect corresponding to the increase in the content is not obtained. Therefore, nb may be contained by setting the upper limit of the content to 0.05 mass%. When the Nb content is less than 0.001 mass%, it is difficult to obtain a sufficient effect for the long life of the rail. When the content is 0.001 mass% or more, the effect of prolonging the lifetime can be obtained. Therefore, in the case of containing Nb, the Nb content is preferably 0.001 mass% or more. More preferably 0.001 to 0.03 mass%.

Mo:1.0 mass% or less

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

V:0.005 to 0.10 mass%

V is an element that forms carbonitrides, disperses and precipitates in the matrix, and improves fatigue damage resistance and delayed fracture resistance. When the V content is less than 0.005% by mass, the effect is small. On the other hand, when the V content exceeds 0.10 mass%, workability is deteriorated and alloy cost is increased, so that manufacturing cost of the rail material is increased. Accordingly, the V content is set to 0.005 to 0.10 mass% or less. Preferably 0.01 to 0.08 mass%.

W:1.0 mass% or less

W is an element that precipitates as carbide after hot rolling and hot rolling to form a rail shape, and enhances the strength and ductility of the rail by precipitation strengthening. However, when the W content exceeds 1.0 mass%, martensite is formed in the steel, and as a result, ductility is lowered. Therefore, when W is added, the W amount is preferably set to 1.0 mass% or less. On the other hand, the lower limit of the W amount is not particularly limited, but is preferably set to 0.001 mass% or more in order to exhibit the above-described effect of improving strength and ductility. More preferably 0.005 to 0.5 mass%.

B: less than 0.005 mass percent

B is an element that improves hardenability and strength of the steel rail by segregation in the prior austenite grain boundaries. However, when the B content exceeds 0.005 mass%, a martensitic structure is formed, and as a result, the wear resistance and fatigue damage resistance are lowered. Therefore, in the case of containing B, the B content is preferably set to 0.005 mass% or less. On the other hand, the lower limit of the amount of B is not particularly limited, but is preferably set to 0.001 mass% or more in order to exhibit the above-described effect of improving strength and ductility. More preferably 0.001 to 0.003 mass%.

In the rail 1, the balance of the above-described constituent components contains Fe and unavoidable impurities. The unavoidable impurities refer to impurities as follows: the material is an originally unnecessary material which is present in the raw material or which is inevitably mixed in the production process, but is contained in a trace amount and has no influence on the characteristics, and therefore, is allowed to be contained. As the unavoidable impurities, for example, N, O and the like are cited, N may be allowed to 0.0080 mass% and O may be allowed to 0.004 mass%. Further, ti forms an oxide, which causes a decrease in fatigue damage resistance as a basic characteristic of the rail, and thus is preferably controlled to 0.0010 mass% or less.

< Steel Structure >)

The waist portion 3 of the rail 1 contains a pearlite structure having an area ratio of 95% or more. The waist portion 3 of the rail 1 may contain a trace amount of a bainite structure, a martensite structure, a proeutectoid cementite structure, or a proeutectoid ferrite structure, which are 5% or less in total. The pearlite structure (pearlite block) is a lamellar structure in which ferrite and cementite are arranged in layers, and the pearlite block is composed of clusters of pearlite grains having the same orientation. There is a correlation between the pearlite structure and crack growth, and the pearlite grain boundaries have a function of becoming a barrier to crack growth. When the area ratio of the pearlite structure in the waist portion 3 is less than 95%, the pearlite grain boundaries that serve as barriers for crack propagation are insufficient. Therefore, the waist portion 3 of the rail 1 contains a pearlite structure having an area ratio of 95% or more.

The average size of the pearlite block is 60 μm or less. The size of the pearlite block also has a correlation with fatigue crack growth. As described above, the pearlite grain boundary serves as a barrier against crack growth, and has a function of inhibiting crack growth. Therefore, when the size of the pearlite block is made finer, the probability of passing through the grain boundary having the crack growth suppressing effect becomes high, and as a result, crack growth is suppressed. If the average size of the pearlite block is greater than 60 μm, the crack growth suppression effect cannot be sufficiently obtained. Therefore, the average size of the pearlite block is 60 μm or less, preferably 40 μm or less.

Rail manufacturing device and manufacturing method

Fig. 3 is a schematic view showing an example of the rail manufacturing apparatus. The rail manufacturing apparatus 10 of fig. 3 has a BD (bloom) rolling mill 11, roughing mills 12, 13, finishing mill 14, and cooling equipment 15. The BD rolling mill 11, roughing mills 12, 13, and finishing mill 14 hot-roll the steel sheet, and the cooling device 15 cools the hot-rolled steel sheet. The finishing mill 14 rolls by, for example, a Kong Xingga method, and has a hole pattern corresponding to a desired cross-sectional shape disposed in the upper and lower rolls, and directly applies pressure to the waist portion 3, the head portion 4, and the leg portion 2. The rolling reduction of the waist portion 3, the head portion 4, and the leg portion 2 is controlled by adjusting the shape of the hole pattern provided in the upper and lower rollers.

Next, a method of manufacturing the rail will be described with reference to fig. 3. First, a steel sheet SS (raw billet) reheated by a heating furnace is rolled into a rough shape of the rail 1 in a BD (bloom) rolling mill 11. The steel sheet SS rolled by the BD rolling mill 11 is hot rolled by the roughing mills 12 and 13. In this way, austenite grains coarsened by heating are repeatedly rolled and recrystallized in the BD rolling mill 11 and roughing mills 12 and 13 in the recrystallization temperature range to be refined.

Then, in finish rolling by the finishing mill 14, hot rolling is performed so that the finish rolling temperature of the lap portion 3 is 1000 ℃ or lower and the reduction of area is 10% or higher. The finish rolling temperature here refers to the surface temperature of the lap 3 at the time of finish rolling, but the surface temperature of the head 4 may be regarded as the finish rolling temperature of the lap 3.

If the steel sheet SS is rolled in a non-recrystallization temperature range (low temperature range) where recrystallization is difficult to occur at 1000 ℃ or less, austenite grains are elongated without recrystallization, and deformed bands are formed in the grains. In addition, in the transformation from austenite to pearlite, the deformation zone in the crystal grain acts as a nucleation site for pearlite transformation together with the austenite grain boundary. Thus, the pearlite grains are refined. When the finish rolling temperature is higher than the recrystallization temperature range, recovery due to recrystallization occurs, and therefore, the pearlite block cannot be miniaturized to an average size of 60 μm or less. Therefore, in order to refine the crystal grains by rolling in the unrecrystallized temperature range (low temperature range), the finish rolling temperature at the time of finish rolling is set to 1000 ℃ or less as the unrecrystallized temperature range (low temperature range). When the finish rolling temperature is lower than 800 ℃, the load on the rolls during rolling becomes extremely large. Further, since rolling is performed in the austenite low temperature range, a significant working strain is introduced into austenite grains, and as a result, a desired crack growth suppression effect cannot be obtained. Therefore, it is preferable to finish-roll at a finish-roll temperature of 800 to 900 ℃.

In order to miniaturize the pearlite block, the waist portion 3 needs to be depressed to apply strain. Accordingly, finish rolling is performed in the finishing mill 14 so that the reduction of area of the waist portion 3 is 10% or more. When the sectional area before finish rolling is A1 and the sectional area after finish rolling is A2, the reduction of area is represented by a reduction of area (%) = ((A1-A2)/A1) ×100. When the reduction of area is less than 10%, the pearlite block cannot be miniaturized to an average size of 60 μm or less, and the crack growth suppressing effect cannot be obtained. More preferably, the reduction of area is 30% or more.

After finish rolling by the finishing mill 14, the waist portion 3 of the rail is accelerated cooled from a temperature equal to or higher than the pearlite transformation starting temperature to a temperature range of 400 to 600 ℃ in the cooling equipment 15 at a cooling rate of 1 to 5 ℃/sec. Here, the cooling stop temperature refers to, for example, a surface temperature at the time of measuring the central portion of the rail web 3 by a radiation thermometer at the time of cooling stop. The cooling rate is a cooling rate (c/s) obtained by converting a temperature change from the start of cooling to the stop of cooling into a value per unit time(s).

If the cooling rate is higher than 5 ℃/sec, the area ratio of the pearlite structure decreases, the area ratio of the martensite structure increases, and the area ratio of the pearlite structure cannot be 95% or more. On the other hand, when accelerated cooling is performed at a cooling rate of 1 to 5 ℃/sec, the waist portion 3 containing pearlite having a pearlite structure with an area ratio of 95% or more can be formed. Further, since it is not necessary to maintain the temperature until the bainite transformation starts as in the conventional method, the manufacturing efficiency can be improved.

Example 1

Hereinafter, the constitution and the working effects of the present invention will be described more specifically with reference to examples. The present invention is not limited to the following examples, and may be modified appropriately within the scope of the gist of the present invention, and these are included in the technical scope of the present invention.

< preparation of test piece >)

First, steels A1 to a15 and B1 to B6 having different compositions were produced. The compositions of steel A1 to A15 and steel B1 to B6 are shown in Table 1 below. In table 1, the blank portion refers to a region that does not contain or contains unavoidable impurities and is negligible.

Next, with the rail manufacturing apparatus 10 of fig. 3, a plurality of rails 1 (nos. 1 to 25 of table 2 below) are manufactured under predetermined manufacturing conditions using the steels A1 to a15 and B1 to B6 containing the components of table 1, respectively. Then, a steel material was extracted from the waist portion 3 of the manufactured rail 1 to manufacture a test piece. Fig. 4 is a schematic diagram showing an example of a test piece. In fig. 4, the test piece is a plate-shaped test piece having a width w=20 mm, a height h=100 mm, and a thickness b=5 mm, for example, and a notch is formed at a center H/2 portion of the height H. The length l=2 mm and the width c=0.2 mm of the notch portion, and the end portion of the notch portion is formed with a curvature r=0.1 mm.

The production conditions and test results of the rails are shown in table 2.

< evaluation method >)

Using the test piece of fig. 4, the stress ratio r=Fatigue crack propagation test was performed under a condition of 0.1, and the stress intensity factor Δk=20mpa·m was measured ^1/2 Fatigue crack propagation rate da/dN (m/cycle) at the time of the fatigue crack was evaluated for the surface damage resistance of the waist. This number is 8.0X10 ^-8 Hereinafter, it was evaluated as having crack propagation inhibition performance.

In the production conditions shown in table 2, the finish rolling temperature is a value obtained by measuring the surface temperature of the lap 3 at the inlet side of the finishing mill 14 with a radiation thermometer, and the cooling stop temperature is a value obtained by measuring the surface temperature of the lap 3 at the time of cooling stop with a radiation thermometer.

The sizes of the pearlite blocks in table 2 are the values as follows: a test piece for microscopic L section observation was cut from the center of the rail waist, embedded, mirror polished, and then subjected to orientation analysis by EBSD (electron back scattering diffraction) (Electron backscatter diffraction pattern), and the particle size of the pearlite grains was measured as an equivalent circle for each orientation, and the thus obtained value was taken as the size of the pearlite block. The grain boundaries having a difference in orientation between adjacent crystal orientations of 5 ° or more were determined to be other pearlite blocks. The measurement area was set to 300 μm square, the measurement steps were set to 0.3 μm intervals, and measurement points having a Confidence coefficient (Confidence index) of 0.1 or less, which indicates the reliability of the measurement orientation, were excluded from the measurement. The crystal grains at the end of the measurement region are also excluded from the measurement object.

The term "P" in Table 2 refers to the case where the area ratio of the pearlite structure is 95% or more and contains a trace amount of bainite structure, martensite structure, proeutectoid cementite structure, proeutectoid ferrite structure in total of 5% or less. In the measurement of the area ratio of the pearlite structure, a known technique may be used, for example, the cut test piece is polished and then etched with an aqueous solution of nitric acid and ethanol, the type of the structure is identified by observation of a 400-fold cross section using an optical microscope, and the area ratio of the pearlite structure is calculated by image analysis.

Rails No.1 to 7 and 24 are produced by satisfying the finish rolling conditions and cooling using a suitable steel satisfying the mass% of the constituent composition of the present inventionA steel rail manufactured by the manufacturing method of the condition. Thus, the area ratio of pearlite in the waist portion 3 is 95% or more, and the average size of pearlite blocks is 60 μm or less. As a result, the stress intensity factor Δk=20mpa·m ^1/2 The crack growth rate da/dN (m/cycle) was 8.0X10 ^-8 The following is given. Further, as shown in Nos. 17 to 23 (steels Nos. A8 to A14), even when at least one of Cu, ni, nb, mo, V, W, B is contained in a predetermined mass%, the crack growth rate da/dN (m/cycle) of the waist portion 3 can be set to 8.0X10 ^-8 The following is given.

On the other hand, as shown in No.8 and No.9, in the case where the reduction of area in finish rolling is less than 10%, the average size of pearlite block is more than 60 μm. As a result, the crack growth rate da/dN (m/cycle) of the waist 3 did not satisfy 8.0X10 ^-8 The following is given.

As shown in No.10, when the cooling rate after finish rolling is greater than 5 ℃/sec, the area ratio of the martensite structure increases, and the area ratio of the pearlite structure in the waist portion 3 is less than 95%. As a result, the crack growth rate da/dN (m/cycle) did not satisfy 8.0X10 ^-8 The following is given.

As shown in nos. 11 to 16 and 25, in the case of using the steel other than the mass% of the component composition of the present invention, even if the temperature condition and the reduction of area and the cooling rate of finish rolling satisfy the conditions specified in the present invention, the area ratio of the pearlite structure of the waist portion 3 is less than 95%, or the average size of the pearlite block is more than 60 μm. As a result, the crack growth rate da/dN (m/cycle) of the waist portion did not satisfy 8.0X10 ^-8 The following is given.

As described above, according to the present invention, by controlling the composition, finish rolling conditions, and cooling conditions of the steel, the structure of the waist portion 3 of the rail 1 can be controlled to reduce the crack growth rate of the waist portion 3 of the rail 1, and the occurrence of crack growth and breakage of the waist portion 3 can be suppressed.

The embodiments of the present invention are not limited to the above embodiments, and various modifications may be applied. For example, in the above embodiment, the manufacturing conditions of the waist portion 3 are exemplified, but when the waist portion 3 is hot-rolled, the leg portion 2 and the head portion 4 are also hot-rolled at the same time. Therefore, for example, it is also possible to manufacture a steel rail 1 satisfying both the crack growth suppression characteristics of the waist portion 3 and the wear resistance of the head portion 4 by hot-rolling and cooling the waist portion 3 and the head portion 4 under different conditions using steel sheets having components satisfying the performance requirements of both the waist portion 3 and the head portion 4.

Symbol description

1. Rail for rail

2. Foot (bottom)

3. Waist part

4. Head part

10. Rail manufacturing device

11 BD rolling mill

12. 13 roughing mill

14. Finishing mill

15. Cooling apparatus

SS steel sheet

Claims

1. A rail comprising a foot portion, a waist portion and a head portion, wherein,

the composition of the waist part comprises C:0.70 to 1.20 mass% of Si:0.20 to 1.20 mass percent of Mn:0.20 to 1.50 mass percent, P:0.035 mass% or less, S:0.0005 to 0.012 mass%, cr:0.20 to 2.50 mass% and the balance of Fe and unavoidable impurities,

the area ratio of pearlite at the waist is more than 95%,

the average size of the pearlite block is 30 μm or less.

2. The steel rail of claim 1, further comprising Cu:1.0 mass% or less, ni:1.0 mass% or less, nb:0.05 mass% or less, mo:1.0 mass% or less, V:0.005 to 0.10 mass%, W:1.0 mass% or less and B: 0.005% by mass or less of one or two or more kinds.

3. A rail according to claim 1 or claim 2, wherein the stress intensity factor Δk = 20 MPa-m ^1/2 The crack growth rate da/dN of the waist is 6.0x10 ^-8 The crack growth rate da/dN is as followsThe bits are m/cycles.

4. A method for producing a rail according to any one of claims 1 to 3, wherein,

finish rolling is performed at a finish rolling temperature of 1000 ℃ or lower and a reduction of area of the waist of 32% or higher,

and cooling the lap after finish rolling at a cooling rate of 1 to 5 ℃/sec from a temperature equal to or higher than the pearlite transformation starting temperature to a temperature range of 400 to 600 ℃.

5. A method of producing a steel rail according to claim 4, wherein the finish rolling of the waist portion is performed at a finish rolling temperature in the range of 800 to 900 ℃.