[0001]The present invention relates to a rail having excellent damage resistance used in a freight railway.
　This application claims priority based on Japanese Patent Application No. 2019-048809 filed in Japan on March 15, 2019, the contents of which are incorporated herein by reference.
Background technology
[0002]
　With economic development, new development of natural resources such as coal is underway. Specifically, mining is being promoted in areas where the natural environment is harsh, which was previously undeveloped. Along with this, the track environment of freight railways that transport resources has become extremely severe. Therefore, rails are required to have higher wear resistance than ever before. Against this background, the development of rails with improved wear resistance has been required.
[0003]
　In recent years, it has been pointed out that rail transportation may become more overcrowded and fatigue damage may occur from the rail columns. Therefore, in order to further improve the service life of the rail, it has been required to improve the fatigue damage resistance of the column portion in addition to the wear resistance of the head. Such a requirement is particularly noticeable for rails used in curved sections. In the curved section, it is clear from recent research that stress damage toward the outside of the curve is applied to the rail head, and bending stress is applied to the rail column, so that fatigue damage starting from the column is likely to occur. Because it became.
[0004]
　In order to improve the wear resistance of rail steel, for example, high-strength rails as shown in Patent Documents 1 and 2 have been developed. The main feature of these rails is that heat treatment reduces the pearlite lamella spacing at the rail head to increase the hardness of the steel, or increases the carbon content of the steel to improve the wear resistance of the rail. It is to increase the volume ratio of the cementite phase in the pearlite lamella of the head.
[0005]
　According to Patent Document 1, wear resistance is obtained by accelerating and cooling the rail head after rolling or reheating from the temperature in the austenite region to 850 to 500 ° C. at a cooling rate of 1 to 4 ° C./sec. It is disclosed that excellent rails can be obtained.
[0006]
　Further, in Patent Document 2, hypereutectoid steel (C: more than 0.85 to 1.20%) is used to increase the cementite volume ratio in the lamellar in the pearlite structure of the rail head, thereby making it resistant. It is disclosed that a rail having excellent wear resistance can be obtained.
[0007]
　In the disclosed techniques of Patent Documents 1 and 2, the wear resistance of the rail head is increased by increasing the hardness by reducing the lamellar spacing in the pearlite structure of the rail head and increasing the volume ratio of the cementite phase in the pearlite structure lamellar. The properties have been improved and the service life has been improved to a certain extent. However, in the rails disclosed in Patent Documents 1 and 2, no study has been made on the fatigue damage resistance to prevent fatigue damage of the rail column portion.
[0008]
　Further, for example, Patent Document 3 discloses that a rail having improved toughness of the rail column portion can be obtained by controlling the amount of proactive cementite structure formed in the rail column portion.
[0009]
　In the technique disclosed in Patent Document 3, by controlling the amount of cementite structure produced in the pearlite structure, the toughness of the rail column portion is improved, the rail breakage is suppressed, and the service life is improved to a certain extent. However, in the rail disclosed in Patent Document 3, no study has been made on the fatigue damage resistance to prevent fatigue damage of the rail column portion.
[0010]
　Further, for example, Patent Document 4 discloses that a rail having improved fatigue characteristics of a rail column portion can be obtained by reducing residual stress by cooling the rail welded joint portion immediately after welding. There is.
[0011]
　In the disclosed technique of Patent Document 4, by controlling the residual stress of the rail welded joint portion, the fatigue characteristic of the rail column portion is improved, the rail breakage is suppressed, and the service life is improved to a certain extent. However, in the rail disclosed in Patent Document 4, the rail welded joint is targeted, and the prevention of fatigue damage of the rail base material has not been studied at all. Further, the technique disclosed in Patent Document 4 controls residual stress, and the relationship between the material and hardness of the rail column portion and the fatigue characteristics has not been studied in Patent Document 4.
[0012]
　Further, in the technique disclosed in Patent Document 5, the hardness of the rail column portion is defined in order to ensure toughness in the heat treatment method for the rail. However, in the rail disclosed in Patent Document 5, the prevention of fatigue damage of the rail column portion has not been studied at all. Further, Patent Document 5 only shows the range of the average value of the hardness of the column portion, and no study has been made on the hardness distribution that affects the suppression of fatigue damage of the rail column portion.
Prior art literature
Patent documents
[0013]
Patent Document 1: Japanese
Patent Application Laid-Open No. 63-023244 Patent Document 2: Japanese Patent Application Laid-Open No. 8-144016
Patent Document 3: Japanese Patent Application Laid-Open No. 2004-43863
Patent Document 4: Japanese Patent Application Laid-Open No. 4819183
Patent Document 5: Japanese Patent Application Laid-Open No. 8-170120
Patent Document 6: Japanese Patent Application Laid-Open No. 2002-226915
Patent Document 7: Japanese Patent Application Laid-Open No. 8-246100
Outline of the invention
Problems to be solved by the invention
[0014]
　The present invention has been made in view of the above problems. An object of the present invention is to provide a rail having excellent fatigue breakage resistance, which is required for a rail of a freight railroad and can suppress the occurrence of fatigue damage from a pillar portion. In particular, it is an object of the present invention to provide a rail capable of suppressing the occurrence of fatigue damage even when applied to a curved track where fatigue breakage is likely to occur.
Means to solve problems
[0015]
　The gist of the present invention is as follows.
(1) The rail according to one aspect of the present invention has a mass% of C: 0.75 to 1.20%, Si: 0.10 to 2.00%, Mn: 0.10 to 2.00%, Cr: 0 to 2.00%, Mo: 0 to 0.50%, Co: 0 to 1.00%, B: 0 to 0.0050%, Cu: 0 to 1.00%, Ni: 0-1 .00%, V: 0 to 0.50%, Nb: 0 to 0.050%, Ti: 0 to 0.0500%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, REM : 0 to 0.0500%, Zr: 0 to 0.0200%, N: 0 to 0.0200%, Al: 0 to 1.00%, P: 0.0250% or less, S: 0.0250% or less , The balance has a steel component composed of Fe and impurities, 90 area% or more of the metal structure of the cross section of the rail column portion is a pearlite structure, and the minimum value of the hardness of the cross section of the rail column portion. Is Hv300 or more, and the difference between the maximum value and the minimum value of the hardness of the cross section of the rail column portion is Hv40 or less.
(2) In the rail described in (1) above, the difference between the maximum value and the minimum value of the hardness of the cross section of the rail column portion may be Hv20 or less.
(3) In the rail according to any one of (1) to (2) above, the steel component is, in terms of mass%, Cr: 0.01 to 2.00%, Mo: 0.01 to 0.50%. , Co: 0.01-1.00%, B: 0.0001-0.0050%, Cu: 0.01-1.00%, Ni: 0.01-1.00%, V: 0.005 ~ 0.50%, Nb: 0.0010 to 0.050%, Ti: 0.0030 to 0.0500%, Mg: 0.0005 to 0.0200%, Ca: 0.0005 to 0.0200%, Selected from the group consisting of REM: 0.0005 to 0.0500%, Zr: 0.0001 to 0.0200%, N: 0.0060 to 0.0200%, Al: 0.0100 to 1.00%. It may contain one kind or two or more kinds.
The invention's effect
[0016]
　According to the above aspect of the present invention, it is possible to provide a rail having excellent fatigue damage resistance required for a pillar portion of a rail applied to a curved section of a freight railway.
A brief description of the drawing
[0017]
[Fig. 1] Fig. 1 is a diagram showing a measurement position of hardness of a cross section of a rail column portion.
[Fig. 2] Fig. 2 is a diagram showing an outline of a rail fatigue test.
[Fig. 3] Fig. 3 is a graph showing the relationship between the difference between the maximum and minimum hardness of the cross section of a rail column and the number of repetitions when a crack occurs in a rail fatigue test.
FIG. 4 is a schematic cross-sectional view of a rail according to the present embodiment.
[Fig. 5] This is the required range of the pearlite structure of the pillar.
Embodiment for carrying out the invention
[0018]
　Hereinafter, a rail having excellent fatigue damage resistance of the column portion (sometimes referred to as a rail according to the present embodiment) according to an embodiment of the present invention will be described in detail. Hereinafter,% in the composition is mass%.
[0019]
　First, the present inventors investigated in more detail the cause of fatigue damage from rail columns in the current freight railway. As a result of detailed investigation of the rail of the pearlite structure where fatigue damage occurred, it was found that there is a correlation between the cross-sectional hardness of the column and the fatigue damage of the rail. In the cross section of the rail column portion, it was confirmed that fatigue damage was generated from the rail column portion in the rail having a region where the hardness was less than Hv300.
[0020]
　In addition, we investigated the rails in which fatigue damage occurred in more detail. As a result, it was confirmed that there are cases where fatigue damage occurs from the column portion even in a rail where there is no region where the hardness is less than Hv300 in the cross section of the rail column portion in the curved section where the usage environment is harsh.
[0021]
　Therefore, the present inventors have investigated in detail the cause of fatigue damage from the column portion by trial production evaluation of the actual rail even if the rail does not have a region where the hardness is less than Hv300 in the cross section of the rail column portion. bottom.
[0022]
　Here, the present inventors decided to perform a fatigue damage test that simulates a curved section in the trial production evaluation. This is because there is a peculiar situation that bending stress is easily applied to the column portion in the curved section. As shown in FIG. 4, the rail has a rail column portion 1, a rail head portion 2, and a rail foot portion 3. Since the rail column portion 1 does not come into contact with the wheels, it has not always been regarded as important in the prior art. However, in the curved section, when the train passes, the stress toward the outside of the curved section is applied to the rail head 2, so that the bending stress is applied to the rail column 1. The present inventors presume that the rail column portion 1 is likely to be fatigue-damaged in the curved section due to the repeated occurrence of this bending stress, and the fatigue damage test should be carried out so as to reproduce the above-mentioned bending stress. I thought there was. The details of the prototype evaluation method are shown below.
[0023]
　Rail rolling and heat treatment under various conditions were performed on a steel material (hypereutectoid steel) having the following steel components, and rails with various rail column cross-sectional hardness were prototyped and their fatigue damage resistance was evaluated. .. Then, the relationship between the cross-sectional hardness of the rail column and the fatigue damage resistance was investigated. The rail rolling conditions, heat treatment conditions, and fatigue test conditions are as shown below. In order to change the cross-sectional hardness of the rail column, controlled cooling was performed on the column.
[0024]
[Actual rail rolling, heat treatment conditions]
● Steel component
　0.90% C-0.50% Si-0.70% Mn-0.0150% P-0.0120% S (remaining Fe and impurities)
● Rail shape
　141 Pound (weight: 70 kg / m).
● Rolling / heat treatment conditions
　Final rolling temperature (column outer surface): 900 ° C.
　Heat treatment conditions: Rolling → Accelerated cooling
　control Cooling conditions (column outer surface): Accelerated cooling in the temperature range from 800 ° C to 500 ° C at an average cooling rate of 0.5 to 5 ° C / sec, or 800 ° C to 580 to 580. Accelerated cooling to 680 ° C. Then, after the heat was raised by the generation of reheat and the temperature was maintained, accelerated cooling was performed again.
　Accelerated cooling was carried out by injecting a refrigerant such as air or cooling water onto either or both of the rail head surface and the column surface. In addition, the heat rise and temperature maintenance due to the generation of reheat were controlled by repeating minute accelerated cooling according to the amount of temperature rise.
[0025]
[Measuring method of cross-sectional hardness of rail column, measurement conditions and method of organizing hardness]
● Measuring device and method
　Equipment: Vickers hardness tester (load 98N)
　Measurement test piece collection: Sample from the cross section of the rail column Cut out.
　Pretreatment: The cross section is polished with diamond abrasive grains with an average particle size of 1 μm.
　Measurement method: Measured according to JIS Z 2244: 2009.
● Measurement position
　A cross section within a range of ± 15 mm in the vertical direction of the rail from the middle line between the bottom of the rail and the top of the rail (see Fig. 1).
　The hardness distribution was measured by continuously indenting in the thickness direction of the column portion at a pitch of 1.0 mm starting from a position at a depth of 1.0 mm from the outer surface of the column portion. Hardness measurements were made on at least 5 lines.
　In addition, in order to eliminate the mutual influence of indentations, a distance of 1.0 mm or more was provided between each measurement line.
● Hardness arrangement method
　The minimum and maximum measured hardness were set as the minimum and maximum cross-sectional hardness of the rail column, respectively.
[Hardness characteristics of test rail]
● Range of minimum cross-sectional hardness of rail column: Hv300 to 500
● Difference between minimum and maximum cross-sectional hardness of rail column: Hv10 to 80
[0026]
[Rail column fatigue test method and test conditions]
● Rail fatigue test
　test method: 3-point bending of the actual rail (span length: 650 mm, see Fig. 2).
　Load conditions: Variable in the range of 2 to 20 tons.
　Load Frequency of load fluctuation: 5Hz.
　Test posture: An eccentric load was applied to the rail head. The position where the load was applied was set to a position deviated from the center of the rail head in the width direction of the rail by 1/3 of the width of the rail head (see FIG. 2).
　Stress measurement: Measured with a strain gauge attached to the rail column.
　Number of repeated load fluctuations: Up to 3 million times (no cracks) or up to cracks.
　Crack judgment: The test is stopped periodically, and the presence or absence of cracks on the surface of the rail column is confirmed by detecting magnetic particles on the surface of the rail column.
　Pass judgment: A rail that repeats load fluctuations up to the occurrence of cracks 2 million times or more, or that does not generate cracks until the end of the test (load fluctuation 3 million times) is judged to be a rail with excellent fatigue breakage resistance.
[0027]
　As shown in FIG. 2, when an eccentric load that fluctuates in a fixed cycle is applied to the rail, bending stress is applied to the rail column portion in a fixed cycle. This makes it possible to simulate the bending stress applied to the rail column portion by the centrifugal force of the train passing through the curved section (the vertical tensile stress applied to the side of the rail column portion that hits the outside of the curve). ..
　As a result of detailed investigation of the rail pillars where cracks occurred before the number of repeated load fluctuations reached 2 million times, the cracks had markedly non-uniform cross-sectional hardness (that is, the maximum value of cross-sectional hardness). It was confirmed that it occurred on the rail (the difference between the minimum value and the minimum value is large). From this result, the present inventors have found that the occurrence of cracks is caused by the local concentration of strain in the cross section of the column portion due to the remarkable non-uniformity of the cross-sectional hardness.
[0028]
　FIG. 3 shows the rail fatigue test results. FIG. 3 shows the relationship between the difference between the maximum value and the minimum value of the cross-sectional hardness of the rail column portion and the number of repetitions of the load fluctuation until the crack occurs in the fatigue test. As can be seen from the result of FIG. 3, there is a correlation between the difference between the maximum value and the minimum value of the cross-sectional hardness and the number of repetitions of the load fluctuation until the crack occurs in the fatigue test, and the maximum value of the cross-sectional hardness As the difference between the minimum values ​​becomes smaller, the number of repetitions of load fluctuations until cracks occur tends to increase. In particular, when the difference between the maximum value and the minimum value of the cross-sectional hardness is Hv40 or less, the number of repeated load fluctuations reaches 2 million times, cracks do not occur, and the damage resistance of the column is greatly improved. The inventors confirmed.
[0029]
　Furthermore, when the difference between the maximum value and the minimum value of the cross-sectional hardness of the column part is Hv20 or less, the number of repetitions of load fluctuation until the occurrence of cracks further increases, and cracks do not occur up to 3 million times, and the column part The present inventors have confirmed that the damage resistance is further improved.
[0030]
　It is said that increasing the hardness (hardening) of a material is effective in preventing fatigue fracture of the material. However, in order to suppress the fatigue damage generated in the rail column in addition to increasing the hardness of the rail column, the present inventors set the maximum and minimum hardness in the cross section of the rail column. It was newly found that it is necessary to suppress the difference between the rails and the concentration of strain in the cross section of the rail column.
[0031]
　FIG. 4 is a schematic view of a rail cross section according to the present embodiment. With reference to FIG. 4, the rail column portion (rail column portion 1) according to the present embodiment will be described again.
[0032]
　When viewed in the vertical cross section in the length direction of the rail, there is a part where the width of the rail is constricted in the center in the height direction of the rail. This constricted part is called the rail pillar part 1. A portion having a width larger than the width of the constricted portion and located below the constricted portion is referred to as a rail foot portion 3, and a portion located above the constricted portion is referred to as a rail head 2. The rail pillar portion 1 is a region sandwiched between the rail head portion 2 and the rail foot portion 3.
[0033]
(1) Reason for limiting the chemical component (steel component) of rail steel The reason for limiting the chemical component (steel component) of
　steel in the rail according to the present embodiment will be described in detail.
[0034]
　C: 0.75 to 1.20%
　C is an element that promotes pearlite transformation and contributes to improvement of fatigue resistance. However, if the amount of C is less than 0.75%, the minimum strength and fatigue damage resistance required for the rail cannot be ensured. Further, when the amount of C is less than 0.75%, a soft proeutectoid ferrite structure is likely to be generated in the rail column portion, the hardness difference in the cross section of the rail column portion becomes large, and the fatigue damage resistance is lowered. .. On the other hand, when the amount of C exceeds 1.20%, a hard proeutectoid cementite structure is likely to be formed on the rail column portion, the hardness difference in the cross section of the rail column portion becomes large, and the fatigue damage resistance is lowered. Therefore, in order to promote the formation of pearlite structure and ensure fatigue damage resistance, the amount of C is set to 0.75 to 1.20%. In order to further stabilize the formation of the pearlite structure and further improve the fatigue damage resistance, it is desirable that the amount of C is 0.80% or more, 0.85% or more, or 0.90% or more. For the same reason, it is desirable that the amount of C is 1.15% or less, 1.10% or less, or 1.05% or less.
[0035]
　Si: 0.10 to 2.00%
　Si is an element that dissolves in the ferrite phase in the pearlite structure, increases the cross-sectional hardness (strength) of the rail column, and improves fatigue damage resistance. Further, Si is also an element that suppresses the formation of an eutectoid cementite structure, suppresses the difference in hardness in the cross section of the rail column portion, and improves fatigue damage resistance. However, if the amount of Si is less than 0.10%, these effects cannot be sufficiently obtained. On the other hand, when the amount of Si exceeds 2.00%, many surface defects are generated during hot rolling. Further, when the amount of Si exceeds 2.00%, the hardenability is remarkably increased, a hard martensite structure is easily generated in the rail column portion, the hardness difference in the cross section of the rail column portion becomes large, and fatigue resistance is increased. Damage is reduced. Therefore, in order to promote the formation of a pearlite structure and secure fatigue damage resistance and toughness, the amount of Si is set to 0.10 to 2.00%. In order to further stabilize the formation of pearlite structure and further improve fatigue damage resistance and toughness, it is desirable that the amount of Si is 0.15% or more, 0.20% or more, or 0.40% or more. For the same reason, it is desirable that the amount of Si is 1.80% or less, 1.50% or less, or 1.30% or less.
[0036]
　Mn: 0.10 to 2.00%
　Mn enhances hardenability, suppresses the formation of a soft proeutectoid ferrite structure, stabilizes the pearlite transformation, and at the same time, makes the lamellar spacing of the pearlite structure finer and pearlite. It is an element that improves fatigue damage resistance by ensuring the hardness of the structure. However, when the amount of Mn is less than 0.10%, the effect is small, a soft proeutectoid ferrite structure is likely to be formed on the rail column portion, the hardness difference in the cross section of the rail column portion becomes large, and fatigue damage resistance is increased. Decreases. On the other hand, when the amount of Mn exceeds 2.00%, the hardenability is remarkably increased, a hard martensite structure is easily generated in the rail column portion, the hardness difference in the cross section of the rail column portion becomes large, and fatigue resistance is increased. Damage is reduced. Therefore, in order to promote the formation of a pearlite structure and secure fatigue damage resistance and toughness, the amount of Mn is set to 0.10 to 2.00%. In order to stabilize the formation of pearlite structure and further improve fatigue damage resistance and toughness, it is desirable that the amount of Mn is 0.20% or more, 0.30% or more, or 0.40% or more. For the same reason, it is desirable that the amount of Mn is 1.80% or less, 1.50% or less, or 1.20% or less.
[0037]
　P: 0.0250% or less
　P is an impurity element contained in steel. It is possible to control the content by refining in a converter. It is preferable that the amount of P is small, but especially when the amount of P exceeds 0.0250%, the concentration of P in the segregation zone of the rail column is promoted, the hardness of the segregation increases, and the cross section of the rail column is cross-sectioned. The difference in hardness inside becomes large, and the resistance to fatigue damage decreases. Therefore, the amount of P is limited to 0.0250% or less. In order to stably secure the fatigue damage resistance of the rail column portion, it is desirable that the P amount is 0.0200% or less, 0.0180% or less, or 0.0150% or less. Since P does not contribute to solving the problem of the invention, it is not necessary to limit the lower limit of the amount of P, and it may be set to 0%, for example. However, considering the dephosphorization ability in the refining process, it is economically advantageous to set the lower limit of the amount of P to about 0.0050%.
[0038]
　S: 0.0250% or less
　S is an impurity element contained in the steel. It is possible to control the content by desulfurizing in a hot metal pan. It is preferable that the amount of S is small, but when the amount of S exceeds 0.0250%, the formation of MnS-based sulfide is promoted and the Mn concentration in the steel is lowered. As a result, a negative segregation portion is generated, the hardness of the negative segregation portion decreases, the hardness difference in the cross section of the rail column portion increases, and the fatigue damage resistance decreases. Therefore, the amount of S was limited to 0.0250% or less. In order to more stably secure the fatigue damage resistance of the rail column portion, it is desirable that the S amount is 0.0200% or less, 0.0180% or less, or 0.0150% or less. Since S does not contribute to solving the problem of the invention, it is not necessary to limit the lower limit of the amount of S, and it may be set to 0%, for example. However, considering the desulfurization capacity in the refining process, it is economically advantageous to set the lower limit of the S amount to about 0.0030%.
[0039]
　The rail according to this embodiment basically contains the above-mentioned chemical components, and the balance is composed of Fe and impurities. However, instead of a part of the remaining Fe, if necessary, the purpose is to improve the fatigue damage resistance by increasing the hardness (strength) of the pearlite structure, and in particular, to control the cross-sectional hardness distribution of the rail column. Then, one kind or two or more kinds selected from the group consisting of Cr, Mo, Co, B, Cu, Ni, V, Nb, Ti, Mg, Ca, REM, Zr, N and Al can be selected in the range described later. It may be contained. Specifically, Cr and Mo reduce the lamellar spacing and improve the hardness of the pearlite structure. Co refines the lamellar structure and increases the hardness of the pearlite structure. B reduces the cooling rate dependence of the pearlite transformation temperature and makes the hardness distribution in the cross section of the rail column uniform. Cu dissolves in ferrite in the pearlite structure and increases the hardness of the pearlite structure. Ni dissolves in ferrite in the pearlite structure and improves the hardness of the pearlite structure. V, Nb, and Ti increase the hardness of the pearlite structure by precipitation hardening of carbides and nitrides generated in the hot rolling and subsequent cooling process. Mg, Ca and REM finely disperse MnS-based sulfides and promote pearlite transformation. Zr suppresses the formation of segregation zones in the center of the slab by increasing the equiaxed crystallization rate of the solidified structure, suppresses the formation of proeutectoid ferrite structure and proeutectoid cementite structure, and promotes pearlite transformation. N promotes pearlite transformation by segregating at austenite grain boundaries. Al shifts the eutectic transformation temperature to the high temperature side and improves the hardness of the pearlite structure. Therefore, in order to obtain the above effects, these elements may be contained in the range described later. Even if these elements are contained within the range described later, they do not impair the characteristics of the rail according to the present embodiment. Further, since these elements do not necessarily have to be contained, the lower limit thereof is 0%.
[0040]
　Cr: 0 to 2.00%
　Cr raises the equilibrium transformation temperature and increases the degree of supercooling, thereby reducing the lamellar spacing of the pearlite structure and improving the hardness (strength) of the pearlite structure. Further, Cr is an element that improves hardenability, suppresses the formation of a soft proeutectoid ferrite structure, and stabilizes the pearlite transformation to improve fatigue damage resistance. In order to obtain this effect, the amount of Cr is preferably 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, when the amount of Cr exceeds 2.00%, the hardenability is remarkably increased, a hard martensite structure is easily generated in the rail column portion, the hardness difference in the cross section of the rail column portion becomes large, and fatigue resistance is damaged. There is a risk of deterioration. Therefore, when it is contained, it is preferable that the amount of Cr is 2.00% or less, 1.80% or less, or 1.50% or less.
[0041]
　Mo: 0 to 0.50%
　Mo raises the equilibrium transformation temperature and increases the degree of supercooling in the same manner as Cr, thereby reducing the lamellar spacing of the pearlite structure and improving the hardness (strength) of the pearlite structure. .. In particular, Mo is an element that increases the hardness of the soft pearlite structure of the rail column, reduces the difference in hardness of the pearlite structure, and improves the fatigue damage resistance of the rail column. In order to obtain this effect, the amount of Mo is preferably 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, when the amount of Mo exceeds 0.50%, the transformation rate is remarkably lowered, a hard martensite structure is likely to be generated in the rail column portion, the hardness difference in the cross section of the rail column portion becomes large, and the fatigue resistance damage resistance is increased. There is a risk of deterioration. Therefore, when it is contained, it is preferable that the amount of Mo is 0.50% or less, 0.40% or less, or 0.30% or less.
[0042]
　Co: 0 to 1.00%
　Co refines the lamellar structure of the pearlite structure and improves the hardness (strength) of the pearlite structure. In particular, Co is an element that increases the hardness of the soft pearlite structure of the rail column, reduces the difference in hardness of the pearlite structure, and improves the fatigue damage resistance of the rail column. In order to obtain this effect, the amount of Co is preferably 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, if the amount of Co exceeds 1.00%, the above effects are saturated and the cost of adding the alloy may increase, resulting in a decrease in economic efficiency. Therefore, when it is contained, it is preferable that the amount of Co is 1.00% or less, 0.80% or less, or 0.50% or less.
[0043]
　B: 0 to 0.0050%
　B forms an iron charcoal boride (Fe 23 (CB) 6 ) at the austenite grain boundaries and promotes pearlite transformation, thereby reducing the cooling rate dependence of the pearlite transformation temperature. It is an element. When the cooling rate dependence of the pearlite transformation temperature is reduced, the hardness distribution in the cross section of the rail column portion becomes uniform, and the fatigue damage resistance is improved. In order to obtain this effect, the amount of B is preferably 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the amount of B exceeds 0.0050%, coarse iron-carbon boride is generated, and there is a possibility that fatigue damage is likely to occur in the rail column portion due to stress concentration. Therefore, when it is contained, the amount of B is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less.
[0044]
　Cu: 0 to 1.00%
　Cu dissolves in the ferrite phase of the pearlite structure, and the hardness (strength) is improved by strengthening the solid solution. In particular, Cu is an element that increases the hardness of the soft pearlite structure of the rail column portion, reduces the difference in hardness of the pearlite structure, and improves the fatigue damage resistance of the rail column portion. In order to obtain this effect, the amount of Cu is preferably 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, when the amount of Cu exceeds 1.00%, a hard martensite structure is likely to be generated in the rail column portion due to the remarkable improvement in hardenability, the hardness difference in the cross section of the rail column portion becomes large, and the fatigue damage resistance becomes large. May decrease. Therefore, when it is contained, it is preferable that the amount of Cu is 1.00% or less, 0.80% or less, or 0.50% or less.
[0045]
　Ni: 0 to 1.00%
　Ni improves the toughness of the pearlite structure and at the same time improves the hardness (strength) by strengthening the solid solution. In particular, Ni is an element that increases the hardness of the soft pearlite structure of the rail column, reduces the difference in hardness of the pearlite structure, and improves the fatigue damage resistance of the rail column. In order to obtain this effect, the amount of Ni is preferably 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, when the amount of Ni exceeds 1.00%, a hard martensite structure is likely to be generated in the rail column due to the remarkable improvement in hardenability, the difference in hardness in the cross section of the rail column becomes large, and the fatigue damage resistance is increased. May decrease. Therefore, when it is contained, it is preferable that the amount of Ni is 1.00% or less, 0.80% or less, or 0.50% or less.
[0046]
　V: 0 to 0.50%
　V increases the hardness (strength) of the pearlite structure by precipitation hardening with V carbides and V nitrides generated in the cooling process after hot rolling. In particular, V is an element that increases the hardness of the soft pearlite structure of the rail column portion, reduces the difference in hardness of the pearlite structure, and improves the fatigue damage resistance of the rail column portion. In order to obtain this effect, the amount of V is preferably 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, if the amount of V exceeds 0.50%, precipitation hardening due to carbides and nitrides of V becomes excessive, the pearlite structure becomes brittle, and the fatigue damage resistance of the rail column portion may decrease. Therefore, when it is contained, it is preferable that the amount of V is 0.50% or less, 0.40% or less, or 0.30% or less.
[0047]
　Nb: 0 to 0.050%
　Nb, like V, increases the hardness (strength) of the pearlite structure by precipitation hardening with Nb carbides and Nb nitrides produced in the cooling process after hot rolling. In particular, Nb is an element that increases the hardness of the soft pearlite structure of the rail column portion, reduces the difference in hardness of the pearlite structure, and improves the fatigue damage resistance of the rail column portion. In order to obtain this effect, the amount of Nb is preferably 0.0010% or more, 0.0050% or more, or 0.010% or more. Further, if the amount of Nb exceeds 0.050%, precipitation hardening of carbides and nitrides of Nb becomes excessive, the pearlite structure becomes brittle, and the fatigue damage resistance of the rail column portion may decrease. Therefore, when it is contained, the amount of Nb is preferably 0.050% or less, 0.040% or less, or 0.030% or less.
[0048]
　Ti: 0 to 0.0500%
　Ti precipitates as Ti carbides and Ti nitrides produced in the cooling process after hot rolling, and the hardness (strength) of the pearlite structure is increased by precipitation hardening. In particular, Ti is an element that increases the hardness of the soft pearlite structure of the rail column, reduces the difference in hardness of the pearlite structure, and improves the fatigue damage resistance of the rail column. In order to obtain this effect, the Ti amount is preferably 0.0030% or more, 0.0100% or more, or 0.0150% or more. On the other hand, if the amount of Ti exceeds 0.0500%, coarse Ti carbides and Ti nitrides are generated, and stress concentration may easily cause fatigue damage to the rail column portion. Therefore, when it is contained, it is preferable that the amount of Ti is 0.0500% or less, 0.0400% or less, or 0.0300% or less.
[0049]
　Mg: 0 to 0.0200%
　Mg is an element that combines with S to form fine sulfide (MgS). MgS finely disperses MnS. In addition, this finely dispersed MnS becomes the core of the pearlite transformation, promotes the pearlite transformation, suppresses the formation of proeutectoid ferrite and proeutectoid cementite structure formed on the rail column, and reduces the difference in hardness of the pearlite structure. And improve the fatigue damage resistance of the rail column. In order to obtain this effect, the amount of Mg is preferably 0.0005% or more, 0.0010% or more, or 0.0050% or more. On the other hand, if the Mg content exceeds 0.0200%, a coarse oxide of Mg is generated, and stress concentration may easily cause fatigue damage to the rail column portion. Therefore, when it is contained, it is preferable that the amount of Mg is 0.0200% or less, 0.0150% or less, or 0.0100% or less.
[0050]
　Ca: 0 to 0.0200%
　Ca is an element that has a strong binding force with S and forms a sulfide (CaS). This CaS finely disperses MnS. Fine MnS becomes the core of pearlite transformation, promotes pearlite transformation, suppresses the formation of proeutectoid ferrite and proeutectoid cementite structure generated in the rail column, reduces the difference in hardness of the pearlite structure, and reduces the difference in hardness of the rail column. Improves fatigue damage resistance. In order to obtain this effect, the Ca amount is preferably 0.0005% or more, 0.0010% or more, or 0.0050% or more. On the other hand, if the Ca content exceeds 0.0200%, a coarse oxide of Ca is generated, and there is a possibility that fatigue damage is likely to occur due to stress concentration. Therefore, when it is contained, it is preferable that the Ca amount is 0.0200% or less, 0.0150% or less, or 0.0100% or less.
[0051]
　REM: 0 ~
　0.0500% REM is a deoxidizing and desulfurization elements, REM of oxysulfide (REM by incorporating 2 O 2 to generate the S), and the nuclei for Mn sulfide-based inclusions. Further, oxysulfides is this nucleus (REM 2 O 2 because the melting point is high S), As a result, due to the inclusion of REM, MnS is finely dispersed, MnS becomes the core of the pearlite transformation, and the pearlite transformation is promoted. As a result, the formation of proeutectoid ferrite and proeutectoid cementite structure generated in the rail column portion is suppressed, the difference in hardness of the pearlite structure is reduced, and the fatigue damage resistance of the rail column portion is improved. In order to obtain this effect, the REM amount is preferably 0.0005% or more, 0.0010% or more, or 0.0050% or more. On the other hand, when the REM content exceeds 0.0500%, coarse REM oxysulfide (REM 2 O 2 S) is generated by the stress concentration, fatigue damage of the rail pillar portion may become more likely to occur. Therefore, when it is contained, the REM amount is preferably 0.0500% or less, 0.0400% or less, or 0.0300% or less.
[0052]
　Here, REM is a rare earth metal such as Ce, La, Pr or Nd. The above content limits the total content of all these REMs. As long as the total content of all REM elements is within the above range, the same effect can be obtained regardless of whether the contained REM is in the form of a single element or in the form of a plurality of elements.
[0053]
　Zr: 0 ~
　0.0200% Zr combines with O ZrO 2 produces inclusions. Since this ZrO 2 inclusion has good lattice consistency with γ-Fe, γ-Fe becomes a solidified nucleus of high carbon rail steel which is a solidified primary crystal, and by increasing the equiaxed crystallization rate of the solidified structure, Suppresses the formation of segregation zones in the center of the slab, suppresses the formation of martensite and proeutectoid cementite structure generated in the rail column, reduces the difference in hardness of the pearlite structure, and resists fatigue damage in the rail column. Improve sex. In order to obtain this effect, the amount of Zr is preferably 0.0001% or more, 0.0010% or more, or 0.0050% or more. On the other hand, if the amount of Zr exceeds 0.0200%, a large amount of coarse Zr-based inclusions may be generated, and fatigue damage may easily occur in the rail column portion due to stress concentration. Therefore, when it is contained, it is preferable that the amount of Zr is 0.0200%, 0.0150%, or 0.0100%.
[0054]
　N: 0 to 0.0200%
　N promotes pearlite transformation from austenite grain boundaries by segregating into austenite grain boundaries, and suppresses the formation of proeutectoid ferrite and proeutectoid cementite structures formed on rail columns. , The difference in hardness of the pearlite structure is reduced, and the fatigue damage resistance of the rail column is improved. Further, when N is contained at the same time as V, the precipitation of carbonitride of V is promoted in the cooling process after hot rolling, the hardness (strength) of the pearlite structure is increased, and the fatigue damage property of the rail column portion is improved. .. In order to obtain this effect, the amount of N is preferably 0.0060% or more, 0.0080% or more, or 0.0100% or more. On the other hand, if the N content exceeds 0.0200%, it may be difficult to dissolve N in the steel. In this case, air bubbles that are the starting point of fatigue damage may be generated, and fatigue damage of the rail column portion may easily occur. Therefore, when it is contained, the amount of N is preferably 0.0200% or less, 0.0180% or less, or 0.0150% or less.
[0055]
　Al: 0 to 1.00%
　Al is a component that functions as a deoxidizing material. Al is an element that shifts the eutectic transformation temperature to the high temperature side, contributes to increasing the hardness (strength) of the pearlite structure, and further increases the hardness of the soft pearlite structure of the rail column, and pearlite. It is an element that reduces the difference in the hardness of the structure and improves the fatigue damage resistance of the rail column. In order to obtain this effect, the Al amount is preferably 0.0100% or more, 0.0500% or more, or 0.1000% or more. On the other hand, if the amount of Al exceeds 1.00%, it may be difficult to dissolve Al in the steel. In this case, coarse alumina-based inclusions are generated, fatigue cracks are generated from the coarse precipitates, and fatigue damage may easily occur on the rail column portion. Further, in this case, an oxide may be generated when the rail is welded, and the weldability may be significantly deteriorated. Therefore, when it is contained, it is preferable that the Al amount is 1.00% or less, 0.80% or less, or 0.60% or less.
[0056]
(2) Metal structure In the
　rail according to the present embodiment, the reason for limiting 90 area% or more of the metal structure of the column cross section to the pearlite structure will be described in detail. The “metal structure of the cross section of the rail column” means the cross section of the column and the metal structure within a range of ± 15 mm in the vertical direction of the rail from the intermediate line between the bottom of the rail and the top of the rail.
[0057]
　First, the reason why 90 area% or more is limited to the pearlite structure will be described.
　The pearlite structure is an advantageous structure for improving fatigue damage resistance because strength (hardness) can be easily obtained even if the amount of alloying elements is low. Further, the pearlite structure has easy control of strength (hardness). Therefore, in order to improve the fatigue damage resistance of the rail column cross section, the amount of pearlite structure is limited to a predetermined amount or more.
[0058]
　Further, the region for controlling the metal structure of the cross section of the rail column is a portion of the rail column where fatigue damage resistance is required. FIG. 5 shows the required range of the pearlite structure of the pillar portion. At least 90 area% or more of the metal structure in the range of ± 15 mm in the vertical direction of the rail from the intermediate line between the bottom of the rail and the top of the rail may be a pearlite structure.
[0059]
　The metal structure of the cross section of the pillar portion of the rail according to the present embodiment is preferably a pearlite structure as described above, but depending on the component system of the rail and the heat treatment manufacturing method, the area ratio in the pearlite structure is 10% or less. A small amount of proeutectoid ferrite structure, proeutectoid cementite structure, bainite structure and martensite structure may be mixed. However, even if these structures are mixed, if the amount is small, the hardness of the rail column portion is not significantly affected, and the fatigue damage resistance of the rail column portion is not significantly adversely affected. Therefore, as the structure of the column portion of the rail having excellent fatigue damage resistance, a mixture of a trace amount of proeutectoid ferrite structure, proeutectoid cementite structure, bainite structure, and martensite structure of 10 area% or less is allowed. In other words, the metal structure of the cross section of the pillar portion of the rail according to the present embodiment may be 90 area% or more as long as it has a pearlite structure. In order to sufficiently improve the fatigue damage resistance, it is desirable that 92 area% or more, 95 area% or more, or 98 area% or more of the metal structure of the column cross section is a pearlite structure.
[0060]
　The method for observing and quantifying the cross section of the rail column is as follows.
[Method of observing and quantifying the structure of the cross section of the rail column]
● Observation method
　Device: Sample
　piece for optical microscope observation: Cross section in the range of ± 15 mm in the vertical direction of the rail from the intermediate line between the bottom of the rail and the top of the rail. Cut out a sample from (see Fig. 5).
　Pretreatment: The cross section is polished with diamond abrasive grains with an average particle size of 1 μm, and night-game etching is performed.
　Observation magnification: 200 times
● Observation position
　position: 1.0 mm from the outer surface of the rail column, and the position of the center of the thickness of the column.
●
　　Number of quantified observations of tissue : 5 or more visual fields each at a position 1.0 mm from the outer surface and a position at the center of the thickness of the pillar.
　　Quantification: The value obtained by averaging the area ratio of pearlite (total of 10 or more fields of view) at the position 1.0 mm from the outer surface (5 or more fields of view) and the position of the center of the thickness of the column (5 or more fields of view) is the rail column. It is the area ratio of pearlite contained in the metal structure of the cross section of the part.
[0061]
(3) Reason for limiting the minimum value of the cross-sectional hardness of the column portion In the
　rail according to the present embodiment, the reason why the minimum value of the cross-sectional hardness of the rail column portion is limited to the range of Hv300 or more will be described. The "minimum value of the hardness of the cross section of the rail column" is the cross section of the column, and the minimum hardness in the range of ± 15 mm in the vertical direction of the rail from the intermediate line between the bottom of the rail and the top of the rail. Means a value.
[0062]
　If the minimum cross-sectional hardness of the column is less than Hv300, fatigue cracks will occur from the column in the usage environment of heavy-duty railways, fatigue strength cannot be secured, and fatigue damage resistance of the rail column will decrease. .. Therefore, the minimum value of the cross-sectional hardness of the column portion is limited to the range of Hv300 or more. In order to stably secure the fatigue damage resistance of the rail column portion, it is desirable that the minimum value of the cross-sectional hardness of the column portion is Hv320 or more, Hv340 or more, or Hv360 or more. The maximum value of the cross-sectional hardness of the column is not particularly limited as long as the requirements for the hardness difference described later are satisfied, but in order to prevent the deterioration of the toughness of the rail column, the minimum value of the cross-sectional hardness of the column is Hv450. Hereinafter, it is desirable that the content is Hv420 or less, or Hv400 or less.
[0063]
(4) Reason for limiting the difference between
　the maximum value and the minimum value of the hardness of the cross section of the rail column portion In the rail according to the present embodiment , the difference between the maximum value and the minimum value of the hardness of the cross section of the rail column portion is Hv40. The reason for limiting the scope to the following will be explained. The "difference between the maximum and minimum hardness of the cross section of the rail column" is the cross section of the column, which is ± 15 mm in the vertical direction of the rail from the intermediate line between the bottom of the rail and the top of the rail. It means the difference between the maximum and minimum hardness in the range.
[0064]
　When the difference between the maximum value and the minimum value of the cross-sectional hardness of the column portion exceeds Hv40, the strain of the column portion acting on the rail column portion is concentrated in the part where the hardness is not uniform in the heavy load railway. The occurrence of cracks reduces the fatigue damage resistance of the rail column. Therefore, the difference between the maximum value and the minimum value of the cross-sectional hardness of the rail column portion is limited to the range of Hv40 or less.
[0065]
　Further, in order to further improve the fatigue damage resistance of the rail column portion, the difference between the maximum value and the minimum value of the cross-sectional hardness of the rail column portion should be limited to the range of Hv30 or less, Hv20 or less, or Hv15 or less. Is desirable. The lower limit of the difference between the maximum value and the minimum value of the cross-sectional hardness of the rail column portion does not need to be limited and may be Hv0, but the maximum value and the minimum value of the cross-sectional hardness of the rail column portion. The difference from is usually Hv10 or more.
[0066]
　The cross-sectional hardness of the rail column is measured under the following conditions.
[Measuring method of cross-sectional hardness of rail column, measurement conditions and method of organizing hardness]
● Measuring device and method
　Equipment: Vickers hardness tester (load 98N)
　Measurement test piece collection: Sample from the cross section of the rail column Cut out.
　Pretreatment: The cross section is polished with diamond abrasive grains with an average particle size of 1 μm.
　Measurement method: Measured according to JIS Z 2244: 2009.
● Measurement position
　A cross section within a range of ± 15 mm in the vertical direction of the rail from the middle line between the bottom of the rail and the top of the rail (see Fig. 1).
　Starting from a position at a depth of 1.0 mm from the outer surface of the column, indentation is continuously performed in the thickness direction of the column at a pitch of 1.0 mm, and the hardness is measured. Hardness measurement is performed on at least 5 lines.
　In addition, in order to eliminate the mutual influence of indentations, a distance of 1.0 mm or more is provided between each measurement line.
● Hardness arrangement method
　The minimum and maximum measured hardness shall be the minimum and maximum cross-sectional hardness of the rail column.
[0067]
(5) Method for controlling the cross-sectional hardness of the rail column The cross-sectional hardness of the
　rail column can be controlled by, for example, adjusting the rolling conditions, the head after rolling, and the control cooling conditions of the column. ..
[0068]
　The rail according to the present embodiment is provided with the above-mentioned components, metal structure, and hardness, so that the effect can be obtained regardless of the manufacturing method. However, for example, a rail steel having the above-mentioned composition is melted in a commonly used melting furnace such as a converter or an electric furnace, and the molten steel is formed by an ingot / split method or a continuous casting method. Next, in order to control the cross-sectional hardness of the rail column portion by hot rolling, it can be obtained by performing controlled cooling on the surface of the rail column portion.
[0069]
　For example, in the rail manufacturing method according to the present embodiment, molten steel after component adjustment is cast into bloom, the bloom is heated to 1250 to 1300 ° C., and hot-rolled to form a rail shape. Then, the rail according to the present embodiment can be obtained by controllingly cooling the surface of the rail column portion after rolling, or by controllingly cooling the surface of the rail column portion after hot rolling and allowing to cool and then reheating.
[0070]
　In these series of steps, in order to adjust the cross-sectional hardness of the column portion, hot rolling conditions, controlled cooling conditions after hot rolling, reheating conditions after hot rolling, and controlled cooling conditions after reheating are manufactured. The conditions may be controlled. The manufacturing temperature conditions described below should all be applied to the surface of the rail column (the outer surface of the rail column). Even if these manufacturing temperature conditions are applied to the surface of the rail head, it is considered that the thermal history of the surface of the rail column is not preferably controlled. Since the rail head and the rail pillar portion have different thicknesses, for example, the degree of heat recovery during cooling differs. Therefore, the heat history of the surface of the rail head and the rail column is inevitably different.
[0071]
● Preferred hot rolling conditions and reheating conditions In
　order to secure the cross-sectional hardness of the rail column, the final rolling temperature of the column is set to 750 to 1000 ° C (the temperature of the outer surface of the rail column). It is possible to secure the minimum value of the cross-sectional hardness of.
[0072]
　As the hot rolling method, for example, a steel piece is roughly rolled with reference to the method described in Patent Document 6 and the like. After that, it is desirable to perform intermediate rolling by a reverse rolling mill over a plurality of passes, and then perform finish rolling by a continuous rolling mill in two or more passes.
[0073]
　When the rail is once cooled and then reheated after hot rolling, the reheating conditions are, for example, in the range of the reheating temperature of the rail column portion of 800 to 1100 ° C. (the outer surface of the rail column portion). By reheating, it is possible to secure the minimum cross-sectional hardness of the rail column.
[0074]
● Preferred control cooling conditions after hot rolling and control cooling conditions after reheating
　The control cooling means of the rail column is not particularly limited. In order to impart fatigue damage resistance and control the cross-sectional hardness, air injection cooling, mist cooling, mixed injection cooling of water and air, or a combination of these is used to perform controlled cooling of the rail column during heat treatment. Just do it. The cooling rate and the cooling temperature range in the controlled cooling are controlled based on the temperature of the outer surface of the rail column as described above.
[0075]
　Control cooling is performed for the purpose of making the cross-sectional hardness of the rail column portion uniform. A segregation zone exists in the column portion, and non-uniformity of hardness is likely to occur. Therefore, in controlled cooling, in order to suppress the increase in hardness of the segregation zone, accelerated cooling is temporarily stopped after the accelerated cooling of the first stage, and the temperature rise due to the internal double heat and a small amount of accelerated cooling are used. Maintains the temperature and suppresses the increase in hardness of the segregated portion. Specifically, by spraying the refrigerant, the temperature rise on the outer surface of the pillar due to the reheat is balanced with the temperature drop on the outer surface of the pillar due to the spraying of the refrigerant, so that the temperature on the outer surface of the pillar becomes substantially constant. Performs a small amount of accelerated cooling (controlled cooling). After the temperature is maintained, the second stage of accelerated cooling is performed to secure the hardness. The preferred cooling condition range is as shown below. The average cooling rate for accelerated cooling is the average cooling rate during refrigerant spraying, that is, the value obtained by dividing the difference between the refrigerant spraying start temperature and the refrigerant spraying end temperature by the refrigerant spraying time.
[0076]
(1) When performing controlled cooling after rolling
Control part:
Accelerated cooling of the outer surface of the rail column 1st stage Average cooling rate: 0.5 to 5.0 ° C / sec
　　　　　　　　　Cooling stop temperature range: 580 to 680 ° C
Temperature retention: 20 to 200 sec in the range of 580 to 680 ° C (a small amount of accelerated cooling is performed) Accelerated cooling of
the second stage Average cooling rate: 2.0 to 5.0 ° C / sec
　　　　　　　　　Cooling stop temperature range: 500 ° C or less
[0077]
(2) When performing controlled cooling after reheating
Control part:
Accelerated cooling of the outer surface of the rail column 1st stage Average cooling rate: 1.0 to 6.0 ° C / sec
　　　　　　　　　Cooling stop temperature range: 580 to 680 ° C
temperature maintenance : 20 to 200 sec in the range of 580 to 680 ° C (a small amount of accelerated cooling is performed) Accelerated cooling of
the second stage Average cooling rate: 2.0 to 5.0 ° C / sec
　　　　　　　　　Cooling stop temperature range: 500 ° C or less
[0078]
　Controlled cooling of the column was carried out by injecting a refrigerant such as air or cooling water onto the surface of the rail column, the surface of the head, or both. Further, as described above, the temperature holding can be controlled by repeating minute accelerated cooling according to the amount of temperature rise due to the generation of reheat generation.
　The part to be controlled and cooled is the rail pillar, but when the rail is erected (head up) and cooled, the refrigerant flows to the surface of the rail pillar by injecting the refrigerant onto the surface of the rail head, and the cooling is performed. It is not always necessary to directly cool the surface of the rail column as described above. However, it goes without saying that even when the refrigerant is injected onto the surface of the rail head, the control target is the temperature of the outer surface of the column portion.
[0079]
● Preferred rail head, foot material and manufacturing conditions The
　rail head and rail foot materials are not particularly limited. It is desirable to have a structure in which strength (hardness) can be easily obtained even with a low amount of alloying elements, and wear resistance and fatigue damage resistance are ensured.
　For the rail head, a pearlite structure having a hardness of Hv340 or higher is desirable in order to ensure wear resistance.
　For the rail foot, a metal structure having a hardness of Hv300 or higher is desirable in order to ensure fatigue damage resistance. Since it is not necessary to ensure wear resistance, the foot portion may be not limited to a pearlite structure but may be a metal structure such as bainite having an excellent balance of strength and ductility.
　Further, in order to secure the hardness of the rail head, it is desirable to perform heat treatment after rolling or reheating. The hardness of the rail head can be ensured by performing accelerated cooling by the method described in Patent Document 1, Patent Document 7, and the like. It is desirable to perform accelerated cooling similar to the rail head in order to secure the hardness of the rail foot, balance it with the head during heat treatment, and suppress bending.
[0080] [0080]
　By utilizing the above-mentioned method for controlling the hardness of the rail head in combination with the new knowledge obtained by the present inventors, it is possible to manufacture the rail according to the present embodiment.
Example
[0081]
　Next, examples of the present invention will be described.
　Table 1 shows the chemical composition (steel composition) of the rail which is an example of the present invention. In Table 1, the balance of the chemical components is iron and impurities, and the content of the element not intentionally added is described as "-".
　Table 3 shows the pearlite fraction (area%) of the cross-section of the column, the minimum value of the cross-sectional hardness of the column (Hv), and the difference between the maximum and minimum values ​​of the cross-sectional hardness of the column (Hv). .. Further, Table 3 also shows the results of the fatigue test conducted by the method shown in FIG. When the pearlite fraction in the cross section of the column is described as 90%, the area ratio of the pearlite structure in the cross section of the rail column is 90%, and the area ratio is 10%. It also includes those containing one or more types of martensite tissue.
[0082]
　On the other hand, Table 2 shows the chemical composition of the rail as a comparative example. In Table 2, the balance of the chemical components is iron and impurities, and the content of the element not intentionally added is described as "-".
　Table 4 shows the pearlite fraction (area%) of the cross-section of the column, the minimum value of the cross-sectional hardness of the column (Hv), and the difference between the maximum and minimum values ​​of the cross-sectional hardness of the column (Hv). .. Further, Table 4 also shows the results of the fatigue test conducted by the method shown in FIG. When the pearlite fraction in the cross section of the column is described as 86%, the area ratio of the pearlite structure in the cross section of the rail column is 86%, and the area ratio is 14%. It also includes those containing one or more of martensite tissues.
[0083]
　The outline of the manufacturing process and manufacturing conditions of the rail of the present invention and the comparison rail shown in Tables 1 to 4 is as follows.
[0084]
[Manufacturing process of the rail of the present invention]
● Basic conditions (directly controlled cooling is performed without cooling and reheating after rolling)
　Molten steel → component adjustment → casting (bloom) → reheating (1250-1300 ° C) → heat Rolling → controlled cooling.
●
　Reheating conditions Molten steel → component adjustment → casting → reheating → hot rolling → cooling → reheating (rail) → controlled cooling.
[0085]
　The outline of the manufacturing conditions of the rail of the present invention shown in Tables 1 and 3 is as shown below. Regarding the manufacturing conditions of the comparative rails in Tables 2 and 4, Comparative Examples D to K are manufactured under the following basic conditions of the rail of the present invention (controlled cooling after rolling), and Comparative Examples A to C are of the rail of the present invention. Manufactured under conditions that did not meet any of the manufacturing conditions.
[0086]
[Manufacturing conditions of the rail of the present invention]
● Basic conditions (controlled cooling after rolling)
　Rolling conditions
Control part: Rail column outer shell surface
Final rolling temperature: 750 to 1000 ° C
control Cooling condition
Control part: Rail pillar outer shell surface
1st stage Accelerated cooling Average cooling rate: 0.5 to 5.0 ° C / sec
　　　　　　　　　Cooling stop temperature range: 580 to 680 ° C
Temperature retention: 20 to 200 sec within the range of 580 to 680 ° C (a small amount of accelerated cooling is performed)
Second stage Accelerated cooling Average cooling rate: 2.0 to 5.0 ° C / sec
　　　　　　　　　Cooling stop temperature range: 500 ° C or less
● Reheating conditions (controlled cooling after
　reheating ) Heating conditions
Control part: Rail column outer shell surface
heating temperature 800 to 1100 ° C
control Cooling condition
Control part:
Accelerated cooling of the outer surface of the rail column 1st stage Average cooling rate: 1.0 to 6.0 ° C / sec
　　　　　　　　　Cooling stop temperature range: 580 to 680 ° C
Temperature retention: Range of 580 to 680 ° C 20-200 sec
(minimal accelerated cooling)
Second stage accelerated cooling Average cooling rate: 2.0 to 5.0 ° C / sec
　　　　　　　　　Cooling stop temperature range: 500 ° C or less
[0087]
　The details of the rail of the present invention and the comparison rail shown in Tables 1 to 4 are as shown below.
[0088]
(1) Rails of the present invention (37)
　Invention Examples 1 to 37: Chemical composition value, pearlite fraction of column cross section, minimum cross section hardness of column, maximum and minimum cross section hardness of column. The difference between the rails is within the scope of the present invention.
　Inventive Examples 1 to 18 and 23 to 37 are rails manufactured under basic conditions (directly controlled cooling is performed after rolling), and Invention Examples 19 to 22 are rails manufactured under reheating conditions.
[0089]
(2) Comparison rails (11)
　Comparative examples A to C (3): Pearlite fraction of the cross section of the column, the minimum value of the cross-sectional hardness of the column, and the maximum and minimum values ​​of the cross-sectional hardness of the column. A rail whose difference is outside the scope of the present invention.
　Here, the comparative example rail A had an average cooling rate of 0.2 ° C./sec in the accelerated cooling of the first stage, but other than that, the same manufacturing conditions as the rail of the present invention were used. The final rolling temperature of the comparative example rail B was 700 ° C., but the same manufacturing conditions as those of the rail of the present invention were used except for the rail B. The temperature holding time of the comparative example rail C was 10 seconds, but other than that, the same manufacturing conditions as those of the rail of the present invention were used. Further, all of the comparative example rails A to C were directly subjected to controlled cooling after rolling.
　Comparative Examples D to K (8 rails): Rails in which the content of any of C, Si, Mn, P, and S is outside the scope of the present invention. Comparative Examples All of the rails D to K were directly subjected to controlled cooling after rolling.
[0090]
　The method of observing the structure of the cross section of the rail column is as follows.
[Method of observing the structure of the cross section of the rail column]
● Observation method
　Device: Sample
　piece for optical microscope observation: Cross section in the range of ± 15 mm in the vertical direction of the rail from the intermediate line between the bottom of the rail and the top of the rail (see Fig. 5). Cut out a sample from.
　Pretreatment: The cross section is polished with diamond abrasive grains with an average particle size of 1 μm, and night-game etching is performed.
　Observation magnification: 200 times
● Observation position
position: 1.0 mm from the outer surface of the rail column, and the position of the center of the thickness of the column.
● Quantification of tissue
　　Number of observations: More than 5 visual fields each at the position 1.0 mm from the outer surface of the rail column and the position of the center of the thickness of the column.
　　Quantification: Average value of the area ratio of pearlite (total of 10 or more fields of view) at the position 1.0 mm from the outer surface of the rail column (4 fields of view or more) and the position of the center of the thickness of the column (5 fields of view or more). Is the area ratio of pearlite contained in the metal structure of the cross section of the rail column portion.
[0091]
　The method and conditions for measuring the hardness of the cross section of the rail column are as follows.
[Measuring method of cross-sectional hardness of rail column, measurement conditions and method of organizing hardness]
● Measuring device and method
　Equipment: Vickers hardness tester (load 98N)
　Measurement test piece collection: Sample from the cross section of the rail column Cut out.
　Pretreatment: The cross section is polished with diamond abrasive grains with an average particle size of 1 μm.
　Measurement method: Measured according to JIS Z 2244: 2009.
● Measurement position
　A cross section within a range of ± 15 mm in the vertical direction of the rail from the middle line between the bottom of the rail and the top of the rail (see Fig. 1).
　Starting from a position at a depth of 1.0 mm from the outer surface of the column, indentation is continuously performed in the thickness direction of the column at a pitch of 1.0 mm, and the hardness is measured. Hardness measurement is performed on at least 5 lines.
　In addition, in order to eliminate the mutual influence of indentations, a distance of 1.0 mm or more is provided between each measurement line.
● Hardness arrangement method
　The minimum and maximum measured hardness shall be the minimum and maximum cross-sectional hardness of the rail column.
[0092]
　The rail fatigue test conditions are as follows.
[Rail fatigue test (see Fig. 2)]
　Test method: Three-point bending of the actual rail (span length: 650 mm).
　Load conditions: Variable in the range of 2 to 20 tons.
　Load Frequency of load fluctuation: 5Hz.
　Test posture: An eccentric load was applied to the rail head. The position where the load was applied was set to a position deviated from the center of the rail head in the width direction of the rail by 1/3 of the width of the rail head (see FIG. 2). (Tensile stress is applied to the rail column to reproduce the curved track).
　Stress measurement: Measured with a strain gauge attached to the rail column.
　Number of repeated load fluctuations: Up to 3 million times (no cracks) or up to cracks.
　Crack judgment: The test is stopped periodically, and the presence or absence of cracks on the surface of the rail column is confirmed by detecting magnetic particles on the surface of the rail column.
　Pass judgment: A rail that repeats load fluctuations up to the occurrence of cracks 2 million times or more, or that does not generate cracks until the end of the test (load fluctuation 3 million times) is judged to be a rail with excellent fatigue breakage resistance.
[0093]
　The test results are organized as follows.
　　● Passed material
　　　evaluation S: No cracks occurred up to 3 million times after the test was completed.
　　　Evaluation A: The number of cracks generated is 2.5 million or more and less than 3 million.
　　　Evaluation B: The number of cracks generated is 2.3 million or more and less than 2.5 million.
　　　Evaluation C: The number of crack occurrences is 2 million or more and less than 2.3 million.
　　●
　　　Evaluation of rejected material X: The number of cracks is less than 2 million.
　The evaluation results of the invention examples are shown in Table 3, and the evaluation results of the comparative examples are shown in Table 4.
[0094]
[table 1]

[0095]
[Table 2]

[0096]
[Table 3]

[0097]
[Table 4]

[0098]
　As shown in Tables 1 to 4, the rails of the present invention (Invention Examples 1 to 37) were evaluated as rails having excellent fatigue breakage resistance, capable of suppressing the occurrence of fatigue damage from the pillars.
　Specifically, the rails of the present invention (Invention Examples 1 to 12) contain the contents of C, Si, Mn, P, and S of steel within a limited range as compared with the comparative rails (Comparative Examples D to K). Furthermore, by controlling the pearlite fraction of the column cross section, the minimum value of the column hardness, and the difference between the maximum and minimum values ​​of the column hardness, the fatigue strength of the rail column is improved. The resistance to fatigue damage of the rail is improved.
　Further, the rail of the present invention (Invention Examples 13 to 22) has a pearlite component in the cross section of the column portion by appropriately controlling the rolling conditions and the heat treatment conditions of the rail column portion as compared with the comparative rails (Comparative Examples A to C). By controlling the rate, the minimum value of the cross-sectional hardness of the column, and the difference between the maximum and minimum values ​​of the cross-sectional hardness of the column, the fatigue strength of the rail column is improved and the fatigue damage resistance of the rail is improved. doing.
　Further, in the rail of the present invention (Invention Examples 16 to 18 and 20 to 22), the difference between the maximum value and the minimum value of the cross-sectional hardness of the column portion is further increased by more appropriately controlling the control cooling conditions of the rail column portion. Reduce. As a result, the fatigue strength of the rail column portion is improved, and the fatigue damage resistance of the rail is further improved.
[0099]
　On the other hand, in Comparative Examples Rails A to K, the chemical composition, the metal structure of the cross section of the rail column, the minimum value of the hardness of the cross section of the rail column, and the maximum and minimum values ​​of the hardness of the cross section of the rail column are included. Any one or more of the differences became inappropriate, and the fatigue damage property was impaired.
Industrial applicability
[0100]
　According to the present invention, the composition of the rail steel as the material of the rail is controlled, the metal structure of the rail column portion, the minimum value of the hardness of the rail column portion, and the maximum value of the hardness in the cross section thereof. By suppressing the difference in the minimum value, it is possible to suppress the concentration of strain in the cross section of the rail column portion, and to provide a rail having excellent fatigue damage resistance required for the rail column portion used in the curved section of the freight railroad.
Code description
[0101]
1: Rail pillar
2: Rail head
3: Rail foot

WE CLAIMS

[Claim 1]By mass%,
C: 0.75 to 1.20%,
Si: 0.10 to 2.00%,
Mn: 0.10 to 2.00%,
Cr: 0 to 2.00%,
Mo: 0 to 0.50%,
Co: 0 to 1.00%,
B: 0 to 0.0050%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
V: 0 to 0.50%,
Nb: 0 to 0.050%,
Ti: 0 to 0.0500%,
Mg: 0 to 0.0200%,
Ca: 0 to 0.0200%,
REM: 0 to 0.0500%,
Zr: 0 to 0 .0200%,
N: 0 to 0.0200%,
Al: 0 to 1.00%,
P: 0.0250% or less,
S: 0.0250% or less,
and the balance is Fe and impurities.
　90 area% or more of the metal structure of the cross section of the rail column portion having a component is a pearlite structure, and
　the minimum value of the hardness of the cross section of the rail column portion is Hv300 or more.
　A rail
characterized in that the difference between the maximum value and the minimum value of the hardness of the cross section of the rail column portion is Hv40 or less .
[Claim 2]
　The rail according
to claim 1, wherein the difference between the maximum value and the minimum value of the hardness of the cross section of the rail column portion is Hv20 or less .
[Claim 3]
　In
terms of mass%, the steel component is Cr: 0.01 to 2.00%,
Mo: 0.01 to 0.50%,
Co: 0.01 to 1.00%,
B: 0.0001 to 0. 0050%,
Cu: 0.01 to 1.00%,
Ni: 0.01 to 1.00%,
V: 0.005 to 0.50%,
Nb: 0.0010 to 0.050%,
Ti: 0 .0030 to 0.0500%,
Mg: 0.0005 to 0.0200%,
Ca: 0.0005 to 0.0200%,
REM: 0.0005 to 0.0500%,
Zr: 0.0001 to 0.0200 %,
N: 0.0060 to 0.0200%,
Al: 0.0100 to 1.00%,
claim 1 or 2 comprising one or more selected from the group. The rails listed in.