JP6601167B2 - Rail with excellent wear resistance - Google Patents

Description

  The present invention relates to a rail intended to improve wear resistance.

  Along with economic development, new development of natural resources such as coal is underway. Specifically, natural resources are being mined in areas that have been undeveloped until now and where the natural environment is severe. Along with this, the track environment has become extremely severe in overseas freight railroads that transport resources. For rails, higher wear resistance has been demanded. Against this background, development of rails with improved wear resistance has been demanded.

  In order to improve the wear resistance of rail steel, high-strength rails as shown in Patent Documents 1 and 2 have been developed. The main feature of these rails is to improve wear resistance, by reducing the pearlite lamella spacing by heat treatment, increasing the hardness of the steel, or increasing the carbon content of the steel, and cementite in the pearlite lamella. The volume ratio of the phase is increased.

  With the disclosed technology of Patent Document 1, the rail head that is excellent in wear resistance is provided by accelerating and cooling the re-heated rail head at a temperature of 850 to 500 ° C. at a rate of 1 to 4 ° C./second after rolling. can do.

  In the disclosed technique of Patent Document 2, a hypereutectoid steel (C: more than 0.85 to 1.20%) is used to increase the cementite volume ratio in the lamella in the pearlite structure, and the rail has excellent wear resistance. Can be provided.

  In the disclosed technologies of Patent Documents 1 and 2, the wear resistance within a certain range can be improved by increasing the hardness by refining the lamella spacing in the pearlite structure or increasing the volume ratio of the cementite phase in the pearlite structure lamella. I can plan. However, in overseas freight railways, in recent years, railway transportation has become more densely packed, rail head wear has progressed rapidly, and improved wear resistance inside the worn head has been demanded. .

  Therefore, development of a high-strength rail with improved wear resistance inside the head has been demanded. In order to solve this problem, high-strength rails as shown in Patent Documents 3 to 4 have been developed. The main feature of these rails is to improve the wear resistance inside the head, by adding a trace amount of alloy to control pearlite transformation or by depositing a trace amount of alloy in the pearlite structure. The internal hardness is improved.

  In the disclosed technology of Patent Document 3, by adding B to hypereutectoid steel (C: more than 0.85 to 1.20%), the transformation temperature of pearlite inside the head is controlled, and the hardness inside the head is hardened. It is improving.

  In the disclosed technique of Patent Document 4, V and N are added to hypereutectoid steel (C: more than 0.85 to 1.20%), and V carbonitride is precipitated in the pearlite structure. The internal hardness is improved.

  In the disclosed technologies of Patent Documents 3 and 4, by increasing the volume ratio of the cementite phase in the pearlite structure lamella, the wear resistance is improved, and at the same time, the control of the pearlite transformation temperature inside the head and the precipitation of the pearlite structure are performed. By strengthening, the hardness inside the head is improved, and the wear resistance inside the head where wear has progressed is improved. However, although improvement in the hardness inside the head can improve the service life slightly, it has not led to a drastic improvement in service life, and technical development to dramatically improve wear resistance has been an issue. .

JP-A-57-198216 JP-A-8-144016 International Publication No. 96/028581 JP 2000-345296 A JP 2002-226915 A

The present invention has been devised in view of the above-described problems, and it is an object of the present invention to provide a rail having dramatically improved wear resistance when compared with the same amount of carbon .

(1) By mass%, C: 0.75 to 1.20%, Si: 0.10 to 2.00%, Mn: 0.10 to 2.00%, P ≦ 0.0250%, S ≦ 0 In a steel rail containing 0.0250% and further containing 0.01 to 1.00% in total of either one or two of Ta and Hf, the balance being Fe and inevitable impurities, A rail having excellent wear resistance, characterized in that 95% or more of the range from the surface to a depth of 20 mm is a pearlite structure, and the hardness in the range is in the range of Hv 300 to 500.
(2) Furthermore, Ta carbide, Hf carbide, and composite carbide of Ta and Hf having a particle size of 0.5 to 4.0 μm are examined in a cross section at a depth of 2 to 20 mm starting from the outer surface of the head. The rail according to (1), wherein there are 100 to 1000 rails per 1000 μm ² .
(3) Moreover, the rail of said (1)-(2) can be made to selectively contain 1 group or 2 groups or more of a group-i group further by the mass%.
Group a: Cr: 0.10 to 2.00%, Mo: 0.01 to 0.50%, 1 type or 2 types,
b group: Co: 0.01-1.00%,
c group: B: 0.0001-0.0050%,
d group: Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, 1 type or 2 types,
e group: V: 0.01 to 0.50%, Nb: 0.0010 to 0.0500%, Ti: 0.0030 to 0.0500%, one or more,
f group: Mg: 0.0005-0.0200%, Ca: 0.0005-0.0200%, REM: 0.0005-0.0500%, one or more,
g group: Zr: 0.0001 to 0.0200%,
h group: Al: 0.0100 to 1.00%,
i group: N: 0.0050 to 0.0200%

  According to the present invention, the rail according to the present embodiment dramatically improves the wear resistance of the rail by controlling the alloy composition, structure, and head hardness of the rail steel, and greatly improves the service life. It becomes possible to make it. In particular, the rail according to the present invention is suitable as a rail used in overseas freight railways.

The figure which showed the relationship between the alloy element which can form a carbide | carbonized_material, and the amount of wear. The figure which showed the relationship between the particle size range of carbide | carbonized_material, and the amount of wear. The figure which showed the relationship between the number (number / 1000micrometer < ² >) of carbide | carbonized_materials with a particle size of 0.5-4.0 micrometers, and the amount of wear. The figure which showed the area | region where the structure | tissue containing the designation | designated in the head cross-section surface position of the rail excellent in abrasion resistance of this invention and a pearlite structure | tissue is required. The figure which showed the outline | summary of the abrasion test. The figure which showed the abrasion test piece collection position. The figure which showed the relationship between the carbon amount of steel, and the amount of wear about the rail of the example of this invention, and the rail of a comparative example. The figure which showed the relationship between the amount of carbon and the amount of wear of the steel of the rail of the example of the invention which controlled carbide in the rail of the example of the present invention, and the rail of the example of the invention which did not control carbide. The figure which showed the relationship between the number of the carbide | carbonized_material (particles / 1000micrometer < ² >) of the particle size of the rail of the example of this invention of 0.5-4.0 micrometers, and wear amount.

  Hereinafter, as an embodiment for carrying out the present invention, a rail having excellent wear resistance will be described in detail. Hereinafter, the mass% in the composition is simply described as%.

  First, the present inventors examined the minimum hardness required to ensure the wear resistance of a rail mainly composed of a pearlite structure. Using hypereutectoid steel (1.00% C-0.50% Si-0.70% Mn-0.150% P-0.120% S), the rail is hot-rolled and heat-treated, A prototype of a rail with varying head hardness was investigated, and the relationship between the hardness of the rail head and wear resistance was investigated. Specifically, a wear test was performed using a test piece collected from the rail head. As a result, in the rail head, in order to ensure wear resistance, the hardness of the metal structure mainly composed of pearlite structure in the range up to a depth of 20 mm starting from the outer surface of the head is in the range of Hv300 to 500. Confirmed that it needs to be controlled.

  In addition, as shown by the code | symbol 3 of FIG. 4, a rail head means the part above a part bundled in the center of the height direction of a rail when the rail is seen in cross section. In addition, the head outer surface refers to a surface of the rail head 3 that is a combination of the surface of the crown 1 and the surface of the head corner 2 that face upward when the rail is erected. The positional relationship between the parietal portion 1 and the head corner portion 2 is such that the parietal portion 1 is located approximately in the center in the width direction of the rail head portion, and the head corner portion 2 is located on both sides of the parietal portion 1.

  Next, the present inventors examined a method for dramatically improving the wear resistance of a rail mainly composed of a pearlite structure. In order to improve wear resistance, the application of carbide was examined. As alloy elements that form carbides, alloy elements of Ti, V, Zr, Nb, Hf, Ta, and W, which are considered to easily generate fine carbides, are selected, and steel containing these alloy elements is melted and cast. Then, hot rolling was performed, and a coarse carbide was generated by a heat treatment for maintaining a constant temperature, and the hardness of the carbide was investigated. The test conditions are as shown below.

[Carbide hardness test]
● Melting experiment Base steel composition: 1.00% C
Alloy elements: Ti, V, Zr, Nb, Hf, Ta, W
Alloy addition amount: 1.00% (each element)
● Hot rolling / heat treatment conditions Hot rolling conditions Reheating temperature: 1200 ° C, final rolling temperature: 1000 ° C
Heat treatment conditions Constant temperature holding: After holding at 900 to 1000 ° C. for 60 minutes after rolling, the film was naturally cooled.
● Hardness measurement conditions Equipment: Vickers hardness tester (load 0.098N)
Pre-treatment: After the diamond was polished, the nital etching was performed to reveal carbides.
● Carbide particle size measurement Measuring device: Scanning electron microscope Magnification: 1000-2000 times Particle size measurement: Individual carbides are analyzed by scanning microscope observation, and only Ta carbide, Hf carbide, and composite carbide of Ta and Hf are selected. Then, the area was obtained, and the particle diameter was calculated by the diameter of a circle corresponding to the area.
Carbide for which hardness measurement is performed: The hardness of a carbide having a particle size range of 5.0 to 10.0 μm was measured.

  As a result, it has been found that there is a difference in the hardness of the carbide depending on the type of alloy element to be added. Specifically, TiC: Hv3200, VC: Hv2900, ZrC: Hv2600, NbC: Hv2600, HfC: Hv2300, TaC: Hv2300, WC: Hv2000.

  Next, the present inventors evaluated the wear resistance of the pearlite structure containing these carbides. Based on hypereutectoid steel (1.00% C-0.50% Si-0.70% Mn-0.150% P-0.0120% S), alloy elements (Ti, V, Steels added with 0.50% each of Zr, Nb, Hf, Ta, and W) were melted and subjected to hot rolling and heat treatment to produce pearlite steel containing carbide, and the wear resistance was evaluated.

[Abrasion test of pearlite structure containing carbide]
Melting experiment Base steel component: 1.00% C-0.50% Si-0.70% Mn-0.150% P-0.0120% S
Alloy elements: Ti, V, Zr, Nb, Hf, Ta, W
Alloy addition amount: 0.50% (each element)
● Hot rolling / heat treatment conditions Hot rolling conditions Reheating temperature: 1250 ° C, final rolling temperature: 1000 ° C
Heat treatment conditions After hot rolling, cooling was performed under conditions of cooling rate: 10 ° C / sec, start temperature: 800 ° C, stop temperature: 580 ° C.
● Carbide particle size measurement Pre-treatment: After grinding the cross-section with diamond, the carbide was revealed by nital etching.
Measuring device: Scanning electron microscope Magnification: 1000-2000 times Particle size measurement: Individual carbides are analyzed by scanning microscope observation, and only Ta carbide, Hf carbide, and composite carbide of Ta and Hf are selected, and the area is obtained. The particle diameter was calculated from the diameter of a circle corresponding to the area.
● Carbide particle size Particle size range to be measured: 0.1-10.0 μm
Measurement result of the number of carbides per unit area in each steel: 100 to 3000 pieces / 1000 μm ² (targeting carbides in the range of 0.1 to 10.0 μm)
● Abrasion test tester: Nishihara type wear tester (see Fig. 5)
Specimen shape (rail material 4): disk-shaped specimen (outer diameter: 30 mm, thickness: 8 mm)
Mating material (wheel material 5): Pearlite steel (Hv380)
Test load: 686 N (contact surface pressure 640 MPa)
Slip rate: 20%
Atmosphere: In the air Cooling: Forced cooling by jetting compressed air from the cooling air nozzle 6 (flow rate: 100 Nl / min)
Repeat count: 700,000 times

  The wear test is an evaluation test that reproduces the actual track. As a result of elucidating the correlation between the wear test result of 1.0% carbon pearlite steel and the wear of the actual track, the wear amount of this test was 0.85gf or more of general pearlite steel rail. . Furthermore, when the amount of wear is reduced by about 10% in this test, it is confirmed that the wear resistance is improved on the actual track. Therefore, in this wear test, the presence or absence of improvement in wear resistance was determined using 0.75 gf, which reduces the wear amount by 10% or more (reference wear amount: 0.85 gf), as an evaluation criterion.

FIG. 1 shows the results of the wear test. The horizontal axis is an alloy element that forms carbide, and the vertical axis is the amount of wear. As a result, it was confirmed that the wear resistance varies greatly depending on the type of alloy. Specifically, the wear resistance of the pearlite steel in which carbides of Hf and Ta having a medium carbide hardness are dispersed was most excellent. As a result of elucidating the wear mechanism, it was confirmed that when the hardness of the carbide is high, the strength difference from the pearlite structure of the base becomes remarkably large, and cracks and the like are generated at the interface between the base and the carbide to promote wear. . Moreover, when the hardness of the carbide | carbonized_material was low, it was confirmed that the abrasion resistance of carbide | carbonized_material itself is low and there is little abrasion suppression effect. The number of carbides per unit area varies in the range of 100 to 3000/1000 μm ² for each steel. However, within this range, the influence of variation on the wear amount is small.

  Further, when the wear resistance of the composite carbide of Hf and Ta formed when Hf and Ta are added simultaneously is investigated, the composite carbide of Hf and Ta has the same effect as when Hf and Ta are added alone. It was recognized that there was.

  Thus, in order to reduce the amount of wear of the rail, it has been found that it is effective to add one or both of Hf and Ta to precipitate as carbides.

  Next, the reason for limiting the rail according to the present embodiment will be described in detail. Hereinafter, the mass% in the steel composition is simply described as%.

(1) Reason for limitation of chemical component of steel In the rail of this embodiment, the reason for limitation of the chemical component of steel will be described in detail.

  C is an effective element that promotes pearlite transformation and ensures wear resistance. When the C content is less than 0.75%, the present component system cannot maintain the minimum strength and wear resistance required for a rail mainly composed of a pearlite structure. In addition, a pro-eutectoid ferrite structure is formed, and the wear resistance is greatly reduced. On the other hand, when the C content exceeds 1.20%, a pro-eutectoid cementite structure is likely to be generated, and the wear resistance of the pearlite structure is lowered. For this reason, C addition amount was limited to 0.75 to 1.20%. In addition, in order to stabilize the production | generation of a pearlite structure | tissue, it is desirable to make C addition amount into 0.80-1.10%.

  Si is an element that dissolves in the ferrite phase of the pearlite structure, increases the hardness of the rail head, and improves the wear resistance. However, if the Si amount is less than 0.10%, these effects cannot be expected sufficiently. Moreover, when the amount of Si exceeds 2.00%, many surface defects are generated during hot rolling. Further, the hardenability is remarkably increased, a martensite structure is formed on the rail head, and the wear resistance is lowered. For this reason, Si addition amount was limited to 0.10 to 2.00%. In addition, in order to stabilize the production | generation of a pearlite structure | tissue and to improve abrasion resistance, it is desirable to make Si addition amount 0.20 to 1.50%.

  Mn is an element that enhances hardenability and stabilizes the pearlite transformation, and at the same time, refines the lamella spacing of the pearlite structure, secures the hardness of the pearlite structure, and further improves the wear resistance. However, if the amount of Mn is less than 0.10%, the effect is small, a soft pro-eutectoid ferrite structure is formed, and the wear resistance is lowered. On the other hand, if the amount of Mn exceeds 2.00%, the hardenability is remarkably increased, and a bainite structure or a martensite structure is generated on the rail head, resulting in a decrease in wear resistance. For this reason, Mn addition amount was limited to 0.10 to 2.00%. In addition, in order to stabilize the production | generation of a pearlite structure | tissue and to improve abrasion resistance, it is desirable to make Mn addition amount 0.20 to 1.50%.

  P is an element inevitably contained in steel. The content can be controlled by refining in a converter. If the P content exceeds 0.0250%, the pearlite structure becomes brittle, the ductility of the plastic deformation region decreases, peeling occurs on the worn surface, and the wear resistance decreases. For this reason, the P content is limited to 0.0250% or less. In addition, although the minimum of P addition amount is not limited, when the dephosphorization ability in a refining process is considered, about 0.0020% of P content will be the limit at the time of actually manufacturing.

  S is an element inevitably contained in steel. The content can be controlled by desulfurization in a hot metal ladle. When the S content exceeds 0.0250%, coarse MnS-based sulfide inclusions are likely to be generated, and coarse voids are generated due to stress concentration in the plastic deformation region due to contact, and peeling occurs on the wear surface. In addition, the wear resistance is reduced. For this reason, S content was limited to 0.0250% or less. In addition, although the minimum of S addition amount is not limited, when the desulfurization capability in a refining process is considered, about 0.0020% of S content will be the limit at the time of actually manufacturing.

  Hf and Ta are elements that both form carbides and finely disperse and precipitate in a matrix mainly composed of a pearlite structure, thereby further improving wear resistance. However, if the total amount of either one or two of Ta and Hf is less than 0.01%, it is difficult to maintain the particle size and generation amount of carbides, and it is difficult to improve wear resistance. Further, if the total amount of either one or two of Ta and Hf exceeds 1.00%, the carbide itself becomes coarse, and coarse voids are generated due to stress concentration in the plastic deformation region due to contact. It falls out of the pearlite structure and does not contribute to wear resistance. Further, the ductility of the plastic deformation region is lowered, peeling occurs on the worn surface, and the wear resistance is lowered. For this reason, the total addition amount of either one or two of Ta and Hf is limited to 0.01 to 1.00%. In addition, in order to ensure the particle size and production amount of carbide stably, it is desirable that the total addition amount of either one or two of Ta and Hf is 0.20 to 0.70%.

  Furthermore, the rail manufactured with the above-described composition has improved wear resistance and internal fatigue damage resistance due to increased hardness of the pearlite structure, improved toughness, prevention of softening of the weld heat affected zone, For the purpose of controlling the cross-sectional hardness distribution, a group Cr, Mo, b group Co, c group B, d group Cu, Ni, e group V, Nb, Ti, f group Mg, Ca, REM, g group Zr, h group Al, i group N element may be added as needed, or one or more groups may be added.

  The a group of Cr and Mo increases the equilibrium transformation point, refines the lamella spacing of the pearlite structure, and improves the hardness. Co of group b refines the lamellar structure of the worn surface and increases the hardness of the worn surface. B in group c reduces the dependency of the pearlite transformation temperature on the cooling rate and makes the hardness distribution of the rail head uniform. The d group of Cu and Ni dissolves in the ferrite in the pearlite structure, increases the hardness, and improves the toughness of the pearlite structure. V, Nb, and Ti in group e improve the fatigue strength of the pearlite structure by precipitation hardening of carbides and nitrides generated during hot rolling and subsequent cooling. In addition, carbides and nitrides are stably generated during reheating, and softening of the weld joint heat-affected zone is prevented. Group f Mg, Ca, and REM finely disperse MnS-based sulfides and reduce internal fatigue damage generated from inclusions. Zr in group g increases the equiaxed crystallization rate of the solidified structure, thereby suppressing the formation of a segregation zone at the center of the slab and suppressing the formation of a proeutectoid cementite structure and a martensite structure. The h group Al moves the eutectoid transformation temperature to the high temperature side, suppresses the formation of proeutectoid cementite structure, and improves the wear resistance of the pearlite structure. N in group i promotes pearlite transformation by segregating at austenite grain boundaries, improves toughness and promotes precipitation of V carbides and nitrides in the subsequent cooling step after hot rolling, and a pearlite structure The main purpose of addition is to improve the fatigue resistance.

<Group a>
Cr is an element that raises the equilibrium transformation temperature, increases the degree of supercooling, refines the lamella spacing of the pearlite structure, improves the hardness of the pearlite structure, and improves the wear resistance. However, if the Cr content is less than 0.10%, the effect is small, and the effect of improving the hardness of the rail steel is not seen at all. Further, if excessive addition exceeding the Cr amount of 2.00% is performed, the hardenability is remarkably increased, and a bainite structure or a martensite structure is formed on the rail head, and the wear resistance is lowered. For this reason, Cr addition amount was limited to 0.10 to 2.00%. In addition, in order to stabilize the production | generation of a pearlite structure | tissue and to improve abrasion resistance, it is desirable to make Cr addition amount 0.20 to 1.00%.

  Mo is an element that raises the equilibrium transformation temperature, refines the lamella spacing of the pearlite structure by increasing the degree of supercooling, improves the hardness of the pearlite structure, and improves the wear resistance. However, if the amount of Mo is less than 0.01%, the effect is small, and the effect of improving the hardness of the rail steel is not seen at all. Moreover, when the excessive addition over Mo amount exceeds 0.50%, a transformation rate will fall remarkably, a martensitic structure will produce | generate in a rail head part, and abrasion resistance will fall. For this reason, Mo addition amount was limited to 0.01 to 0.50%.

<Group b>
Co is an element that dissolves in the ferrite phase of the pearlite structure, refines the lamella structure of the pearlite structure immediately below the rolling surface, improves the hardness of the rolling surface, and improves the wear resistance. However, if the Co content is less than 0.01%, the refinement of the lamellar structure is not promoted, and the effect of improving the wear resistance cannot be expected. On the other hand, if the Co content exceeds 1.00%, the above effect is saturated, and the lamella structure cannot be refined according to the added amount. In addition, the economic efficiency decreases due to the increase in the alloy addition cost. For this reason, Co addition amount was limited to 0.01 to 1.00%.

<Group c>
B forms an iron boride (Fe ₂₃ (CB) ₆ ) at the austenite grain boundary, reduces the cooling rate dependence of the pearlite transformation temperature due to the effect of pearlite transformation, and is more uniform from the head surface to the inside. It is an element that imparts a hardness distribution to the rail and extends the life of the rail. However, if the amount of B is less than 0.0001%, the effect is not sufficient, and no improvement is observed in the hardness distribution of the rail head. On the other hand, if the amount of B exceeds 0.0050%, coarse ferrocarbon borides are generated, making brittle fracture redundant and reducing the toughness of the rail. For this reason, B addition amount was limited to 0.0001 to 0.0050%.

<Group d>
Cu is an element that dissolves in the ferrite phase of the pearlite structure, improves hardness by solid solution strengthening, and improves wear resistance. However, if it is less than 0.01%, the effect cannot be expected. On the other hand, if the amount of Cu exceeds 1.00%, a martensitic structure is generated in the rail head due to a significant improvement in hardenability, resulting in a decrease in wear resistance. For this reason, Cu addition amount was limited to 0.01 to 1.00%.

Ni is an element that improves the toughness of the pearlite structure and at the same time improves the hardness by solid solution strengthening and improves the wear resistance. Further, in the weld heat affected zone, Ni ₃ Ti intermetallic compound is finely precipitated in combination with Ti, and is an element that suppresses softening by precipitation strengthening. Moreover, it is an element which suppresses the embrittlement of a grain boundary in Cu addition steel. However, when the Ni content is less than 0.01%, these effects are remarkably small. When the Ni content exceeds 1.00%, a martensitic structure is generated in the rail head due to a marked improvement in hardenability. Abrasion is reduced. For this reason, Ni addition amount was limited to 0.01 to 1.00%.

<E group>
V is an element that increases the hardness (strength) of the pearlite structure and improves the fatigue damage resistance inside the head by precipitation hardening with V charcoal / nitride generated in the cooling process after hot rolling. However, if the amount of V is less than 0.01%, the number of fine carbon / nitrides precipitated in the ferrite phase of the pearlite structure is small, and no improvement in the hardness inside the head is observed. On the other hand, if the amount of V exceeds 0.50%, the number of fine V charcoal / nitrides becomes excessive, the pearlite structure becomes brittle, and the fatigue damage resistance decreases. For this reason, V addition amount was limited to 0.01 to 0.50%.

Nb is an element that increases the hardness of the pearlite structure and improves fatigue damage resistance inside the head by precipitation hardening with Nb carbide and Nb nitride generated in the cooling process after hot rolling. Also, in the heat-affected zone reheated to a temperature range below Ac ₁ point, Nb carbide and Nb nitride are stably generated from the low temperature range to the high temperature range to prevent softening of the heat affected zone of the weld joint. It is an effective element to do. However, if the Nb content is less than 0.0010%, these effects cannot be expected, and an improvement in the hardness of the pearlite structure is not recognized. On the other hand, if the Nb content exceeds 0.0500%, precipitation hardening of Nb carbides and nitrides becomes excessive, the pearlite structure itself becomes brittle, and fatigue damage resistance decreases. For this reason, Nb addition amount was limited to 0.0010 to 0.0500%.

  Ti is an element that increases the hardness of the pearlite structure by precipitation hardening by Ti carbide and Ti nitride generated in the cooling process after hot rolling and improves fatigue damage resistance inside the head. In addition, by utilizing the fact that Ti carbide and Ti nitride precipitated during reheating during welding do not dissolve, the structure of the heat affected zone heated to the austenite region is refined and the weld joint becomes brittle. It is an effective ingredient to prevent However, when the Ti content is less than 0.0030%, these effects are small. On the other hand, if the Ti content exceeds 0.0500%, coarse Ti carbides and Ti nitrides are formed, and fatigue cracks are generated due to stress concentration, thereby reducing fatigue damage resistance. For this reason, Ti addition amount was limited to 0.0030-0.0500%.

<Group f>
Mg is an element that combines with S to form fine sulfides, and MgS finely disperses MnS, relieves stress concentration, and improves fatigue damage resistance. However, if the amount is less than 0.0005%, the effect is weak, and if added over 0.0200%, a coarse oxide of Mg is generated, fatigue cracks are generated due to stress concentration, and the fatigue damage resistance is reduced. . For this reason, the amount of Mg was limited to 0.0005 to 0.0200%.

  Ca is an element that has a strong bonding force with S and forms a sulfide as CaS. Further, CaS finely disperses MnS, relaxes stress concentration, and improves fatigue damage resistance. However, if the amount is less than 0.0005%, the effect is weak, and if added over 0.0200%, a coarse oxide of Ca is generated, fatigue cracks are generated due to stress concentration, and the fatigue damage resistance is reduced. . For this reason, the amount of Ca was limited to 0.0005 to 0.0200%.

REM is a deoxidation / desulfurization element. When added, REM generates REM oxysulfide (REM ₂ O ₂ S), which serves as a nucleus of Mn sulfide inclusions. In addition, since the melting point of oxysulfide (REM ₂ O ₂ S) as the nucleus is high, stretching of Mn sulfide inclusions after rolling is suppressed. As a result, MnS is finely dispersed, stress concentration is relaxed, and fatigue damage resistance is improved. However, if the amount of REM is less than 0.0005%, the effect is small, and it is insufficient as a production nucleus of MnS-based sulfide. On the other hand, when the amount of REM exceeds 0.0500%, hard REM oxysulfide (REM ₂ O ₂ S) is generated, fatigue cracks are generated due to stress concentration, and fatigue damage resistance is reduced. For this reason, the amount of REM added is limited to 0.0005 to 0.0500%.

  Note that REM is a rare earth metal such as Ce, La, Pr, or Nd. The above addition amount is limited to the addition amount of all these REMs. As long as the total sum of the total addition amounts is within the above range, the same effect can be obtained regardless of whether the form is single or composite (two or more types).

<G group>
Zr has good lattice matching with ZrO ₂ inclusions and γ-Fe, so γ-Fe becomes a solidification nucleus of high-carbon rail steel, which is a solidified primary crystal, and increases the equiaxed crystallization rate of the solidified structure. An element that suppresses the formation of a segregation zone at the center of a slab and suppresses the formation of a martensite structure generated in a rail segregation portion. However, if the amount of Zr is less than 0.0001%, the number of ZrO ₂ -based inclusions is small and does not exhibit a sufficient effect as a solidification nucleus. As a result, a martensite structure is likely to be generated in the segregated portion, and improvement in fatigue damage resistance of the rail cannot be expected. On the other hand, if the amount of Zr exceeds 0.0200%, a large amount of coarse Zr-based inclusions are generated, fatigue cracks are generated due to stress concentration, and fatigue damage resistance is reduced. For this reason, the Zr addition amount is limited to 0.0001 to 0.0200%.

<Group h>
Al is a component that functions as a deoxidizer. Moreover, it is an element that moves the eutectoid transformation temperature to the high temperature side, and is an element that suppresses the formation of a proeutectoid cementite structure and improves the wear resistance of the pearlite structure. However, when the Al content is less than 0.0100%, the effect is weak. Further, if the Al content exceeds 1.00%, it becomes difficult to make a solid solution in the steel, coarse alumina inclusions are generated, fatigue cracks are generated from the coarse precipitates, and fatigue damage resistance is caused. Sex is reduced. Furthermore, oxides are generated during welding, and weldability is significantly reduced. For this reason, Al addition amount was limited to 0.0100 to 1.00%.

<Group i>
N is an element effective for improving the toughness by promoting the pearlite transformation from the austenite grain boundary by segregating at the austenite grain boundary and mainly by reducing the pearlite block size. Further, when N is added simultaneously with V, the precipitation of V carbonitride is promoted in the cooling process after hot rolling, the hardness of the pearlite structure is increased, and the fatigue resistance is improved. However, when the N content is less than 0.0050%, these effects are weak. On the other hand, if the N content exceeds 0.0200%, it becomes difficult to make a solid solution in the steel, and bubbles that become the starting point of fatigue damage are generated, and fatigue damage is likely to occur. For this reason, N addition amount was limited to 0.0050-0.0200%.

  Rail steel composed of the above component composition is melted in a commonly used melting furnace such as a converter, electric furnace, etc., and this molten steel is ingot / bundled or continuous casting, It is manufactured as a rail through hot rolling. Further, heat treatment is performed as necessary to control the metal structure and hardness of the rail head, the particle size range of carbide, and the number per unit area.

(2) Reason for Limiting Necessary Range of Metallic Structure and Pearlite Structure Next, in this embodiment, 95% (area ratio) or more of the range of depth of at least 20 mm starting from the outer surface of the head is limited to the pearlite structure. Will be described in detail.
First, the reason for limiting to the pearlite structure will be described.
It is most important to ensure wear resistance in the rail head that comes into contact with the wheels. As a result of investigating the relationship between the metal structure and the wear resistance, it was confirmed that the pearlite structure was the best. Moreover, the pearlite structure is a low alloy and is easy to obtain hardness. Therefore, the pearlite structure is limited for the purpose of improving the wear resistance.

  Next, the reason why the necessary range of the metal structure (the structure including the pearlite structure) including the pearlite structure in an area ratio of 95% or more is limited to a range of at least 20 mm from the head outer surface as a starting point will be described. To do.

  If the necessary range of the tissue including the pearlite structure is less than 20 mm starting from the outer surface of the head, it is too small as a necessary area for wear resistance required for the rail head, and it is difficult to ensure a sufficient rail service life. Become. Further, in order to further improve the wear resistance, it is desirable to make the structure including the pearlite structure up to a depth of about 30 mm starting from the outer surface of the head.

  Here, FIG. 4 shows the designation of the rail head section surface position excellent in wear resistance according to the present embodiment and the region where the structure including the pearlite structure is necessary. The rail head portion 3 includes a top portion 1 and head corner portions 2 located at both ends of the top portion 1. One of the head corner portions 2 is a gauge corner (GC) portion that mainly contacts the wheel.

  A range from the surface of the head corner portion 2 and the top of the head 1 to a depth of 20 mm is referred to as a head surface portion (3a, hatched portion). As shown in FIG. 4, if a structure including a pearlite structure of a predetermined hardness is arranged on the head surface part 3 a up to a depth of 20 mm starting from the surfaces of the head corner part 2 and the top of the head part 1, Abrasion resistance can be ensured.

  Therefore, it is desirable that the structure including the pearlite structure is disposed on the head surface portion 3a where the wheel and the rail are mainly in contact with each other, and wear resistance is required, and other parts where these characteristics are not required are other than the pearlite structure. It may be a metal structure.

  In addition, the metal structure of the head surface portion 3a of the rail according to the present embodiment is desirably a structure including a pearlite structure as described above. However, depending on the component system of the rail and the heat treatment manufacturing method, in addition to the pearlite structure, a small amount of pro-eutectoid ferrite phase, pro-eutectoid cementite phase, bainite structure and martensite structure with an area ratio of 5% or less are included in these structures. May be mixed. However, even if these structures are mixed, since it does not have a great adverse effect on the wear resistance of the head surface, the rail structure of this embodiment has a very small amount of pro-eutectoid ferrite phase of 5% or less. It also contains a mixture of cementite phase, bainite structure, and martensite structure. In other words, the metal structure of the rail head of the rail according to the present embodiment may be a pearlite structure with an area ratio of 95% or more. To sufficiently secure wear resistance, the metal structure of the rail head is 98. % Or more is preferably a pearlite structure.

  In the above-mentioned observation of the range of at least 20 mm depth starting from the outer surface of the head, the metal structure is observed with a 200 × optical microscope, and the area of each metal structure is determined by determining the area of each metal structure. The rate can be determined. Moreover, 10 visual fields (10 places) or more can be used as the visual field of the optical microscope, and the average value of the area ratio can be used as the area ratio of the observation site.

(3) Reason for limiting the hardness of the structure including the pearlite structure Next, the reason why the hardness of the structure including the pearlite structure is limited to a range of Hv 300 to 500 in the present embodiment will be described. When the hardness of the structure including the pearlite structure is less than Hv300, the wear proceeds and it is difficult to ensure the wear resistance required for the rail head. In addition, when the hardness of the pearlite structure exceeds Hv500, a micro crack is generated on the head surface in contact with the wheel on the head surface due to the embrittlement of the structure including the pearlite structure, and the surface damage resistance is ensured. It becomes difficult. For this reason, the hardness of the structure | tissue containing a pearlite structure | tissue was limited to the range of Hv300-500.

  In addition, the measurement of the hardness of the tissue including the pearlite tissue is performed by measuring 10 points (10 points) or more at a measurement place (for example, a position having a depth of 2 to 20 mm starting from the outer surface of the head), and calculating an average value. It is desirable to adopt it as a value. In the present invention, the pearlite structure accounts for 95% or more in area ratio, but other structures (proeutectoid cementite phase, proeutectoid ferrite phase, martensite structure, bainite structure, etc.) exist within a range of 5% or less. This is because the hardness of the structure including the pearlite structure may not be representative in this measurement.

(4) Reasons for Limiting the Particle Size Range of Ta Carbide, Hf Carbide, and Ta and Hf Composite Carbide Next, in this embodiment, in the cross section at a depth of 2 to 20 mm from the head outer surface, The reason why the particle size of Ta carbide, Hf carbide, and composite carbide of Ta and Hf contributing to wear is limited to the range of 0.5 to 4.0 μm will be described.

  The present inventors examined a method for further improving the wear resistance of pearlite steel in which carbides of Hf and Ta and composite carbides of Hf and Ta are dispersed. Based on hypereutectoid steel (1.00% C-0.50% Si-0.70% Mn-0.150% P-0.0120% S), 0.50% Hf and Ta were added respectively. Steel, steel with 0.50% added in combination with Hf and Ta was melted, hot rolled, heat treatment conditions were adjusted, pearlite steel with varying carbide particle size range was produced, and wear resistance was evaluated. .

[Carbide wear test]
Melting experiment Base steel component: 1.00% C-0.50% Si-0.70% Mn-0.150% P-0.0120% S
Alloy elements: Hf, Ta
Alloy addition amount: 0.50%
● Hot rolling / heat treatment conditions Hot rolling conditions Reheating temperature: 1100 to 1350 ° C, Final rolling temperature: 800 to 1100 ° C
Heat treatment conditions After hot rolling, cooling was performed under the conditions of cooling rate: 2 to 20 ° C / sec, start temperature: 700 to 900 ° C, and stop temperature: 550 to 650 ° C.
● Measurement of carbide particle size Pretreatment: After grinding the diamond, the carbide was exposed by nital etching.
Measuring device: Scanning electron microscope Magnification: 1000-2000 times Particle size measurement: Individual carbides are analyzed by scanning microscope observation, and only Ta carbide, Hf carbide, and composite carbide of Ta and Hf are selected, and the area is obtained. The particle diameter was calculated from the diameter of a circle corresponding to the area.
● Carbide particle size Particle size range to be measured: 0.1-10.0 μm
● Abrasion test tester: Nishihara type wear tester (see Fig. 5)
Specimen shape (rail material 1): disk-shaped specimen (outer diameter: 30 mm, thickness: 8 mm)
Mating material (wheel material 2): Pearlite steel (Hv380)
Test load: 686 N (contact surface pressure 640 MPa)
Slip rate: 20%
Atmosphere: In the air Cooling: Forced cooling by jetting compressed air from the cooling air nozzle 3 (flow rate: 100 Nl / min)
Repeat count: 700,000 times

  This wear test is an evaluation test that reproduces the actual track. As a result of elucidating the correlation between the wear test result of 1.0% carbon pearlite steel and the wear of the actual track, the wear amount of this test was 0.85gf or more of general pearlite steel rail. . In this test, it has been confirmed that when the amount of wear is reduced by about 15%, the wear resistance on the actual track is greatly improved. In this wear test, whether or not there is further improvement in wear resistance when the particle size range of the composite carbide is controlled using 0.70 gf, which reduces the wear amount by 15% or more (standard wear amount: 0.85 gf), as an evaluation criterion. It was judged.

  FIG. 2 shows the results of the wear test. The horizontal axis is the carbide particle size range, and the vertical axis is the wear amount. As a result, it was confirmed that the wear resistance of the carbides of Hf and Ta and the composite carbides of Hf and Ta greatly vary depending on the particle size range of the carbides.

  Specifically, it has become clear that there is a suitable range for the particle size range of the carbide. It has also been clarified that the wear resistance is slightly lowered when the carbide particle size range is smaller than the preferred range or when the carbide particle size range is larger than the preferred range.

  As a result of elucidating the wear mechanism, it is clear that when the particle size is 0.5 μm or more, plastic deformation due to wheel contact is suppressed, and carbide on the wear surface is not buried in the pearlite structure of the base and wear resistance is improved. It became. On the other hand, when the particle size is 4.0 μm or less, coarse voids due to stress concentration are less likely to be generated around the carbide in the plastic deformation region due to the wheel contact, and the carbide does not fall out of the base pearlite structure and wear resistance is improved. It became clear that it improved. For this reason, the particle size of Ta carbide, Hf carbide, and composite carbide of Ta and Hf is preferably limited to a range of 0.5 to 4.0 μm.

  The preferred particle size range is 0.5 to 4.0 μm regardless of the type of Ta carbide, Hf carbide, or composite carbide of Ta and Hf, so that the wear resistance can be further improved. Become.

(5) Reason for limiting the number per unit area of Ta carbide, Hf carbide, and composite carbide of Ta and Hf having a particle size of 0.5 to 4.0 μm Next, in this embodiment, the depth is determined from the outer surface of the head as a starting point. Reason why the number per unit area of Ta carbide, Hf carbide, composite carbide of Ta and Hf contributing to wear resistance is limited to 100 to 1000 per 1000 μm ² in the cross section at a position of 2 to 20 mm in thickness Will be described.

  The present inventors have studied a method for stably improving the wear resistance of pearlite steel in which Hf, Ta carbide, and Hf and Ta composite carbide are dispersed. Based on hypereutectoid steel (1.00% C-0.50% Si-0.70% Mn-0.150% P-0.0120% S), 0.50% Hf and Ta were added respectively. Steel, steel with 0.50% added in combination with Hf and Ta is melted, hot rolling and heat treatment conditions are adjusted, and the number of carbides (particle size range 0.5 to 4.0 μm) per unit area is changed The pearlite steel made was manufactured, and abrasion resistance was evaluated by the same method as the above (4).

[Carbide wear test]
Melting experiment Base steel component: 1.00% C-0.50% Si-0.70% Mn-0.150% P-0.0120% S
Alloy elements: Hf, Ta
Alloy addition amount: 0.50%
● Hot rolling / heat treatment conditions Hot rolling conditions Reheating temperature: 1100 to 1350 ° C, Final rolling temperature: 800 to 1100 ° C
Heat treatment conditions After hot rolling, cooling was performed under the conditions of cooling rate: 2 to 20 ° C / sec, start temperature: 700 to 900 ° C, and stop temperature: 550 to 650 ° C.
● Measurement of carbide particle size Pretreatment: After grinding the diamond, the carbide was exposed by nital etching.
Measuring apparatus: Scanning electron microscope Magnification: 1000 to 2000 times Particle size measurement: Individual carbides are analyzed by observation, only Ta carbide, Hf carbide, and composite carbide of Ta and Hf are selected, and the area is obtained. The particle size was calculated from the diameter of the corresponding circle.
● Carbide particle size Particle size range to be measured: 0.5-4.0μm
Measurement result of the number of carbides per unit area in each steel: 30 to 1300 pieces / 1000 μm ² (targeting carbides in the range of 0.5 to 4.0 μm)

  This wear test is an evaluation test that reproduces the actual track. As a result of elucidating the correlation between the wear test result of 1.0% carbon pearlite steel and the wear of the actual track, the wear amount of this test was 0.85gf or more of general pearlite steel rail. . In this test, it has been confirmed that when the amount of wear is reduced by about 15%, the wear resistance on the actual track is greatly improved. In this test, whether or not the wear resistance is further improved when the number of composite carbides per unit area is controlled using 0.70 gf, which reduces the wear amount by 15% or more (reference wear amount: 0.85 gf). Judged.

FIG. 3 shows the results of the wear test. The horizontal axis is the number of carbides per unit area (pieces / 1000 μm ² ), and the vertical axis is the wear amount. As a result, it was confirmed that the wear resistance of the carbides of Hf and Ta and the composite carbides of Hf and Ta greatly vary depending on the number of carbides per unit area.

  Specifically, when the number of carbides per unit area is small, when the number of carbides per unit area is large, the wear resistance decreases, and in order to further improve the wear resistance, the unit area of carbides It became clear that there was an optimal range for the number of hits.

As a result of elucidating the wear mechanism, when the amount of carbide generated was 100 or more per 1000 μm ^{2 of the} test area, the amount of carbide increased and the wear resistance was further improved. Further, when the amount of carbide generated was 1000 or less per 1000 μm ^{2 of the} test area, the ductility of the plastic deformation region due to the wheel contact was improved, the wear surface was not peeled off, and the wear resistance was improved. As described above, the number of particles per unit area of Ta carbide, Hf carbide, and composite carbide of Ta and Hf having a particle diameter of 0.5 to 4.0 μm is set to 100 to 1000 per 1000 μm ^{2 of the} test area. It has been found that the performance can be further improved.

The preferred number density range of carbides is Hf, Ta carbide, and Hf and Ta composite carbides, regardless of the type of carbide, control to 100-1000 pieces / 1000 μm ² to further improve wear resistance. It becomes possible to make it.

As described above, in order to improve the wear resistance of the pearlite structure rail, Hf and Ta are added, and Hf carbide, Ta carbide, and composite carbide of Ta and Hf are dispersed in the pearlite structure. In order to further improve the wear resistance, it is preferable to control the particle size range and the number per unit area of Ta carbide, Hf carbide, and composite carbide of Ta and Hf.
That is, the rail according to this embodiment controls the alloy composition, structure, and hardness of the head of the rail steel, and further, the particle size range and unit area of Ta carbide, Hf carbide, and Ta and Hf composite carbide. By controlling the number, it is possible to dramatically improve the wear resistance of the rail when used in overseas freight railways and greatly improve the service life.

By making the number per unit area of Ta carbide, Hf carbide, and composite carbide of Ta and Hf with a particle size of 0.5 to 4.0 μm to 100 to 1000 per 1000 μm ^{2 of} test area, wear resistance is further improved. Can be improved. When the amount of carbide generated is 100 or more per 1000 μm ^{2 of the} test area, the abundance of the carbide itself becomes sufficient, and the wear resistance can be greatly improved. In addition, when the amount of carbide generated is 1000 or less per 1000 μm ^{2 of the} test area, the ductility of the plastic deformation region due to wheel contact is improved, peeling is prevented on the wear surface, and wear resistance is improved.

In order to further improve the wear resistance, the number per unit area of Ta carbide having a particle size of 0.5 to 4.0 μm, Hf carbide, and composite carbide of Ta and Hf is set to 400 to 800 per 1000 μm ^{2 of the} test area. It is desirable to control them individually.
In the above limitation, the wear resistance is further improved by controlling the number of carbides per unit area within the optimum range regardless of the type of Ta carbide, Hf carbide, and composite carbide of Ta and Hf.

In addition, the following method should just be used for calculation of a particle size and a number.
[Calculation method of particle size and number of carbides]
Measuring device: Scanning electron microscope Magnification: 1000 to 2000 times Measurement position: Any point having a depth of 2 to 20 mm starting from the outer surface of the head.
Particle size measurement: Individual carbides are analyzed by scanning microscope observation, only Ta carbide, Hf carbide, and composite carbide of Ta and Hf are selected, the area is obtained, and the particle diameter is calculated by the diameter of the circle corresponding to the area. .
Calculation of the number: Observe 20 fields of view, count the number of Ta carbide, Hf carbide, composite carbide of Ta and Hf, convert to the number per 1000 μm ² , and obtain the average value.
Final analysis: From the results of particle size measurement and number calculation, the particle size range and number of carbides are analyzed, and Ta carbide, Hf carbide, and composite carbide of Ta and Hf with a particle size of 0.5 to 4.0 μm are analyzed. Calculate the number per unit area.

(6) Manufacturing method of rail of this embodiment The rail of this embodiment casts the molten steel after component adjustment into a bloom, and hot-rolls the bloom into a rail shape. Furthermore, if necessary, heat treatment is performed after hot rolling for the purpose of controlling the metal structure of the rail head, controlling the hardness, and controlling the particle size range of carbide and the number per unit area.

(6-1) Hot rolling conditions and heat treatment conditions About the hot rolling conditions and heat treatment conditions, in order to maintain the pearlite structure and control the hardness of the rail head, the rail manufacturing shown below is performed as necessary. It is desirable to carry out within a general range of conditions.

First, hot rolling conditions will be described.
First, the bloom reheating temperature will be described. When the bloom reheating temperature is less than 1000 ° C., rolling molds that can ensure hot formability in rail rolling are generated, and rail manufacture becomes difficult. If the reheating temperature exceeds 1400 ° C., the melting point decreases depending on the amount of carbon and alloy of the steel, the steel melts, and rail production becomes difficult, so the bloom reheating temperature is 1000 to 1400 ° C. The range of is preferable.

Next, the final rolling temperature will be described.
When the final rolling temperature is less than 750 ° C., pearlite transformation starts immediately after rolling, and the hardness cannot be increased in the subsequent heat treatment, so that the wear resistance cannot be ensured. In addition, when the final rolling temperature exceeds 1100 ° C., the austenite grains after rolling are coarsened, the hardenability is greatly increased, and a bainite structure harmful to wear resistance is generated on the rail head, resulting in a decrease in wear resistance. To do. Moreover, since the minimum ductility required for a rail cannot be ensured, the final rolling temperature is preferably in the range of 750 to 1100 ° C.

Next, heat treatment conditions will be described.
First, the cooling rate will be described.
When the cooling rate is less than 1 ° C./sec, the pearlite transformation temperature rises, the hardness cannot be increased in the subsequent heat treatment, and the wear resistance cannot be ensured. In addition, when the cooling rate exceeds 20 ° C./sec, in this component system, a bainite structure or a martensite structure is generated in the rail head, and the wear resistance is lowered. preferable.
Next, the cooling start temperature will be described.

  When the cooling start temperature is less than 700 ° C., in this component system, a pearlite structure is generated in a high temperature range before accelerated cooling, the hardness cannot be increased, and the wear resistance cannot be ensured. In addition, in a steel having a relatively large amount of carbon, a pro-eutectoid cementite structure is formed and wear resistance is reduced. Further, when the starting temperature exceeds 900 ° C., the hardenability is greatly increased, and a bainite structure harmful to the wear resistance is generated on the rail head, and the wear resistance is lowered. .

Next, the cooling stop temperature will be described.
When the cooling stop temperature exceeds 650 ° C., in this component system, pearlite transformation starts in a high temperature range immediately after cooling, and many pearlite structures with low hardness are generated. As a result, the hardness of the head cannot be ensured, and it becomes difficult to ensure the wear resistance necessary for the rail. In addition, when accelerated cooling is performed to below 500 ° C., in this component system, a lot of bainite structure that is harmful to wear resistance is generated immediately after cooling. As a result, it becomes difficult to ensure the wear resistance necessary for the rail, and therefore a range of 500 to 650 ° C. is preferable.

(6-2) Hot rolling conditions for controlling the particle size range of Ta carbide, Hf carbide, Ta and Hf composite carbide and the number per unit area Further, grains of Ta carbide, Hf carbide, Ta and Hf composite carbide In order to control the diameter range and the number per unit area, it is desirable to change the hot rolling and heat treatment conditions to the following ranges in accordance with the addition amount of Ta and Hf.

First, hot rolling conditions will be described.
First, the bloom reheating temperature will be described.
When the bloom reheating temperature is less than 1200 ° C., the solid solution amount of Ta and Hf in the austenite can be secured. During the subsequent rolling, the amount of carbide generated decreases, and Ta carbide, Hf carbide, Ta in the pearlite structure. It is difficult to ensure the amount of composite carbide of Hf and Hf. As a result, the wear resistance is not improved. In addition, when the reheating temperature exceeds 1300 ° C., the amount of Ta and Hf dissolved in the austenite becomes excessive, the amount of carbide generated increases during the subsequent rolling, and the particle size of the carbide also increases. . As a result, a coarse void accompanying separation and stress concentration is generated in the plastic deformation region due to wheel contact, and the wear resistance is not improved. Therefore, the range of 1200 to 1300 ° C. is preferable.

Next, the final rolling temperature will be described.
When the final rolling temperature is less than 850 ° C., the production of Ta carbide, Hf carbide, and composite carbide of Ta and Hf is promoted immediately after rolling, and the amount of carbide produced increases. As a result, the ductility of the plastic deformation region due to wheel contact is reduced, peeling occurs, and wear resistance is not improved. In addition, when the final rolling temperature exceeds 1000 ° C., the grain size of Ta carbide, Hf carbide, and composite carbide of Ta and Hf becomes coarse after rolling, and coarse due to stress concentration around the carbide in the plastic deformation region due to wheel contact. In this case, a range of 850 to 1000 ° C. is preferable because a void is generated and the carbide falls out of the matrix pearlite structure and does not contribute to wear resistance.

Next, heat treatment conditions will be described.
First, the cooling rate will be described.
When the cooling rate is less than 3 ° C./sec, the growth of Ta carbide, Hf carbide, and composite carbide of Ta and Hf is promoted during cooling, and the particle size of the carbide becomes coarse. As a result, in the plastic deformation region due to the wheel contact, coarse voids are generated around the carbide due to stress concentration, and the carbide falls out of the base pearlite structure and does not contribute to wear resistance. When the cooling rate exceeds 15 ° C./sec, the growth of Ta carbide, Hf carbide, and Ta and Hf composite carbide becomes insufficient during cooling. As a result, the particle size of the carbide is reduced, and the carbide on the wear surface is buried in the pearlite structure of the base due to plastic deformation due to wheel contact, so that it does not contribute to wear resistance.

Next, the cooling start temperature will be described.
When the cooling start temperature is less than 750 ° C., the growth of Ta carbide, Hf carbide, and composite carbide of Ta and Hf is promoted before the start of cooling, and the particle size of the carbide becomes coarse. As a result, in the plastic deformation region due to the wheel contact, coarse voids are generated around the carbide due to stress concentration, and the carbide falls out of the base pearlite structure and does not contribute to wear resistance. On the other hand, when the start temperature exceeds 850 ° C., Ta carbide, Hf carbide, and composite carbide of Ta and Hf are not sufficiently generated before the start of cooling. As a result, the amount of carbide is insufficient and does not contribute to wear resistance, so a range of 750 to 850 ° C. is preferable.

Next, the cooling stop temperature will be described.
When the cooling stop temperature exceeds 650 ° C., in this component system, pearlite transformation starts in a high temperature range immediately after cooling, and many pearlite structures with low hardness are generated. As a result, the hardness of the head cannot be ensured, and it becomes difficult to ensure the wear resistance necessary for the rail. In addition, when accelerated cooling is performed to below 500 ° C., in this component system, a lot of bainite structure that is harmful to wear resistance is generated immediately after cooling. As a result, it becomes difficult to ensure the wear resistance necessary for the rail, and therefore a range of 500 to 650 ° C. is preferable.

  There are no particular limitations on the rolling conditions of the rail. In order to ensure the hardness of the rail head, it is sufficient to control the final rolling temperature by the normal rail hole rolling. In addition, what is necessary is just to make a pearlite structure | tissue mainly obtain a rolling method with reference to the method described in patent document 5, etc., for example. That is, after roughly rolling a steel slab, intermediate rolling by a reverse rolling mill is performed over a plurality of passes, and then finish rolling by a continuous rolling mill is performed by two or more passes. What is necessary is just to control to said temperature range at the time of final rolling of finish rolling.

  Moreover, it does not specifically limit about the heat processing refrigerant | coolant of a rail. In order to provide wear resistance, in order to control the hardness, it is desirable to control the cooling rate of the rail during heat treatment by air jet cooling, mist cooling, water / air mixed jet cooling, or a combination thereof. .

Next, examples of the present invention will be described.
Tables 1A to 1D show chemical components and properties of the rail of the present invention. Tables 1A to 1D show chemical component values, head microstructure, head hardness, Ta carbide, Hf carbide, and composite carbides of Ta and Hf. Furthermore, the result of the abrasion test conducted by the method shown in FIG. 5 is also shown. In the case of a pearlite structure, the microstructure of the head includes a mixture of a small amount of pro-eutectoid ferrite structure, pro-eutectoid cementite structure, bainite structure and martensite structure with an area ratio of 5% or less.

  Tables 2A to 2B show the chemical components and various properties of the comparative rail. Tables 2A to 2B show chemical component values, head microstructure, head hardness, Ta carbide, Hf carbide, and composite carbides of Ta and Hf. Furthermore, the result of the abrasion test conducted by the method shown in FIG. 5 is also shown. In the case of a pearlite structure, the microstructure of the head includes a mixture of a very small amount of pro-eutectoid ferrite, pro-eutectoid cementite, bainite structure and martensite structure with an area ratio of 5% or less.

  The outlines of the production steps and production conditions of the inventive examples and comparative examples shown in Tables 1A to 1D and 2A to 2B are as shown below.

● Overall process Molten steel → Component adjustment → Casting (bloom) → Reheating → Hot rolling → Heat treatment.
Moreover, the outline of the manufacturing conditions of the examples of the present invention and comparative examples is as shown below.
-Hot rolling conditions Reheating temperature: 1000-1400 degreeC, Final rolling temperature: 750-1100 degreeC
Heat treatment conditions: hot rolling → cooling Cooling rate: 1-20 ° C./sec, cooling start temperature: 700-900 ° C., cooling stop temperature: 500-650 ° C.
Further, in order to control the particle size range and the number per unit area of Ta carbide, Hf carbide, and composite carbide of Ta and Hf, the above hot rolling and heat treatment were performed under the following conditions.
● Hot rolling conditions Reheating temperature: 1200-1300 ° C, final rolling temperature: 750-1100 ° C
● Heat treatment conditions: hot rolling → cooling Cooling rate: 3-15 ° C./sec, cooling start temperature: 750-850 ° C., cooling stop temperature: 500-650 ° C.

  Tables 3A and 3B show some of the manufacturing conditions for the rails of the present invention (Tables 1A to 1D). By optimizing the manufacturing conditions, it is possible to manufacture rails having the characteristics described in Tables 1A to 1D. In addition, by further optimizing the manufacturing conditions, it is possible to control the particle size range and the number per unit area of Ta carbide, Hf carbide, and composite carbide of Ta and Hf, which can further improve wear resistance. is there.

  Details of the rails of the examples of the present invention and the rails of the comparative examples shown in Tables 1A to 1D and 2A to 2B are as follows.

(1) Invention rail (60)
Invention Examples A1 to A13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59: A rail in which the chemical composition, the microstructure of the head, and the hardness of the head are confirmed to be within the scope of the present invention.
Invention examples other than the above: Rails in which the chemical components, the microstructure of the head, the hardness of the head, Ta carbide, Hf carbide, and the composite state of Ta and Hf are within the scope of the present invention.

(2) Comparison rail (22)
Comparative Examples B1 to B8 (eight): Rails whose added values of C, Si, Mn, P, and S are outside the scope of the present invention. Some rails had a head microstructure or a head hardness outside the scope of the present invention.
Comparative examples B9 to B22 (14 pieces): A rail whose total addition amount of any one or two of Hf and Ta is outside the scope of the present invention.

In Tables 1A to 1D, 2A to 2B, and 3A to 3B, what is described as “Δ” in the column of the state of Ta carbide, Hf carbide, and composite carbide of Ta and Hf In addition, Ta carbide, Hf carbide, and composite carbide of Ta and Hf, which are basically necessary for improving wear resistance, are within a preferable range. “Particle size range: 0.1 to 10.0 μm, number per unit area: 100 to 3000/1000 .mu.m ² "is but, still more preferably in the range within the" particle size: 0.5～4.0Myuemu, number per unit area: 100 to 1000/1000 .mu.m ² "means not satisfied.
Further, in the column of the state of formation of Ta carbide, Hf carbide, and composite carbide of Ta and Hf, what is described as “◯” is Ta carbide, Hf carbide, Ta and Hf necessary for further improvement of wear resistance. This means that the composite carbide of the composite carbide is within the preferred range of “particle size: 0.5 to 4.0 μm, number per unit area: 100 to 1000/1000 μm ² ”, and the number per unit area is Also written.

[Abrasion test]
Testing machine: Nishihara type abrasion testing machine (see Fig. 5)
Specimen shape (rail material 4): disk-shaped specimen (outer diameter: 30 mm, thickness: 8 mm)
Mating material (wheel material 5): Pearlite steel (Hv380)
Specimen sampling position: Position corresponding to 2 mm below the head outer surface (head surface, see FIG. 6)
Test load: 686 N (contact surface pressure 640 MPa)
Slip rate: 20%
Atmosphere: In the air Cooling: Forced cooling by jetting compressed air from the cooling air nozzle 6 (flow rate: 100 Nl / min)
Repeat count: 700,000 times

[Method for measuring cross-section hardness of rail head]
Measuring device: Vickers hardness tester (load 98N)
Sample collection for measurement: Sample cutting from the cross section of the rail head Pretreatment: Polishing the cross section with diamond abrasive grains having a particle size of 1 μm Measurement method: Measured according to JIS Z 2244 Calculation of hardness:
2 mm below the head surface: 20 points were measured in an arbitrary cross section of the surface of the site shown in FIG. 4 (2 mm below the surface), and the average value was defined as the hardness of the site.
20 mm below the head surface: 20 points were measured in an arbitrary cross section of the surface of the site shown in FIG. 4 (20 mm below the surface), and the average value was defined as the hardness of the site.

As shown in Tables 1A to 1D and 2A to 2B, the rails (A1 to A60) of the examples of the present invention are C, Si, Mn, and P of steel compared to the rails (B1 to B8) of the comparative examples. , S was kept within a limited range, thereby suppressing generation of pro-eutectoid ferrite phase, pro-eutectoid cementite phase, bainite structure, and martensite structure, and the head was made to be pearlite structure. Furthermore, compared with the rails (B9 to B22) of the comparative example, the rails (A1 to A60) of the present invention are within a limited range of the total addition amount of either Hf or Ta of steel or two types The wear resistance was improved by storing. Since the total amount of Hf and Ta deviated from the preferable range (particle size range: 0.1 to 10.0 μm, number per unit area: 100 to 3000/1000 μm ² ), the rails B9 to B22 were wear resistant. Therefore, it is difficult to maintain the particle size and the amount of the carbide effective for the wear, and the wear resistance is lowered.

  “Carbide coarsening” in Tables 2A to 2B means that the carbide is coarsened due to an increase in the amount of addition, and as a result, the total amount of Hf and Ta is out of the preferred range, and stress is applied in the plastic deformation region due to wheel contact. Since coarse voids are generated as a result of concentration, the wear resistance is reduced. “Carbide finer / smaller amount” means that the amount of carbide is reduced and further reduced due to a decrease in the amount added. As a result, the total amount of Hf and Ta is out of the preferred range, which is effective for wear resistance. Carbides cannot be maintained and wear resistance is reduced.

In Tables 2A to 2B, Ta carbides, Hf carbides, Ta and Hf composite carbides in the column of “X” are basically required to improve wear resistance. This means that Ta carbide, Hf carbide, and composite carbide of Ta and Hf deviated from the preferable range “particle size range: 0.1 to 10.0 μm, number per unit area: 100 to 3000/1000 μm ² ”. .

  In FIG. 7, the rails (A1 to A60) of the example of the present invention show the wear test results of the rails (B1 to B22) of the comparative example in relation to the carbon content of steel. In FIG. 7, the lower limit of the relationship between the amount of carbon and the amount of wear is also shown based on the result of a general pearlite steel rail wear test in this test.

When compared with the same amount of carbon, it can be seen that the rail of the example of the present invention has less wear and improved wear resistance compared to the rail of the comparative example.
It can be seen that it has an effect of improving the wear resistance by 10% or more as compared with a pearlite steel rail (standard) having a carbon content of 0.75 to 1.20%.

Furthermore, FIG. 8 shows the relationship between the amount of carbon and the amount of wear of the steel of the rail of the invention example in which the carbide is controlled in the rail (A13 to 60) of the invention example and the rail of the invention example in which the carbide is not controlled.
Inventive rail with controlled carbide (A14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54 56, 58, 60) is a rail (A13) in which the carbide is not controlled by controlling the particle size range of Ta carbide, Hf carbide, Ta and Hf composite carbide and the number per unit area thereof. 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59) It can be seen that the wear resistance can be further improved.

FIG. 9 shows the relationship between the number of carbides having a particle size of 0.5 to 4.0 μm and the amount of wear in the rails (A38, 40, 42, 44, 46, 48, 50) of the present invention. It can be seen that the wear resistance can be further improved by controlling the number of Ta carbide, Hf carbide, and composite carbide of Ta and Hf to 400 to 800 per 1000 μm ^{2 of the} test area.

  The wear test is an evaluation test that reproduces the actual track. As a result of elucidating the correlation between the wear test result of 1.0% carbon pearlite steel and the wear of the actual track, the wear amount of this test was 0.85gf or more of general pearlite steel rail. . Here, using 0.62 gf, which reduces the wear amount by 25% or more (reference wear amount: 0.85 gf) as an evaluation criterion, it was determined whether or not the wear resistance was further improved when the number of composite carbides was strictly controlled. .

  Further, as shown in Tables 3A to 3B, the head is hot-rolled and heat-treated under certain conditions to suppress the formation of proeutectoid cementite phase, bainite structure and martensite structure, and the hardness of the pearlite structure It can be seen that the wear resistance can be improved by ensuring the above. Furthermore, by controlling the conditions of hot rolling and heat treatment, the particle size range of Ta carbide, Hf carbide, composite carbide of Ta and Hf and the number per unit area thereof can be controlled, and the wear resistance can be further improved. I understand.

  1: head portion, 2: head corner portion, 3: rail head portion, 3a: head surface portion (range from head corner portion and top surface to depth of 20mm, hatched portion), 4: rail Material, 5: Wheel material, 6: Air nozzle for cooling.

Claims (3)

  In mass%, C: 0.75 to 1.20%, Si: 0.10 to 2.00%, Mn: 0.10 to 2.00%, P ≦ 0.0250%, S ≦ 0.0250% In addition, 0.01 to 1.00% of the total of either one or two of Ta and Hf is contained, and the remainder of the steel rail is composed of Fe and inevitable impurities. A rail having excellent wear resistance, wherein 95% or more of the range up to a depth of 20 mm is a pearlite structure, and the hardness in the range is in the range of Hv 300 to 500. Furthermore, Ta carbide, Hf carbide, and composite carbide of Ta and Hf having a particle size of 0.5 to 4.0 μm per 1000 μm ^{2 of the} test area in the cross section at a depth of 2 to 20 mm starting from the outer surface of the head The rail having excellent wear resistance according to claim 1, wherein there are 100 to 1000 rails. In mass%,
Group a: Cr: 0.10 to 2.00%, Mo: 0.01 to 0.50%, 1 type or 2 types,
b group: Co: 0.01-1.00%,
c group: B: 0.0001-0.0050%,
d group: Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, 1 type or 2 types,
e group: V: 0.01 to 0.50%, Nb: 0.0010 to 0.0500%, Ti: 0.0030 to 0.0500%, one or more,
f group: Mg: 0.0005-0.0200%, Ca: 0.0005-0.0200%, REM: 0.0005-0.0500%, one or more,
g group: Zr: 0.0001 to 0.0200%,
h group: Al: 0.0100 to 1.00%,
i group: N: 0.0050 to 0.0200%
The rail having excellent wear resistance according to any one of claims 1 and 2, characterized by containing at least one group.

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