JP7173366B2 - RAIL EXCELLENT IN FATIGUE CRACK PROPAGATION RESISTANCE AND PRODUCTION METHOD THEREOF - Google Patents

Description

TECHNICAL FIELD The present invention relates to a rail and its manufacturing method, and more particularly to a rail with improved fatigue crack propagation resistance and a rail manufacturing method that can advantageously manufacture the rail.

In high-axle-load railways mainly used for transportation of ore, etc., the load applied to the axles of freight cars is much higher than that of passenger cars, and the environment in which the rails are used is severe. For rails used in such environments, steel having a pearlite structure has been conventionally used mainly from the viewpoint of wear resistance. However, in recent years, the load weight of freight cars has been increasing in order to improve the efficiency of railroad transportation, and rails are required to have further improvements in wear resistance and fatigue damage resistance. A high-axle-load railway is a railway in which a single freight car of trains and freight cars has a large loading weight (loading weight of about 150 tons or more, for example).

Therefore, various studies have been conducted with the aim of further improving wear resistance. For example, in Patent Documents 1 and 2, the C content is increased from 0.85% by mass to 1.20% by mass or less. Further, in Patent Documents 3 and 4, the C content is set to more than 0.85% by mass and 1.20% by mass or less, and the rail head is subjected to heat treatment. In these techniques, efforts are made to improve wear resistance by increasing the cementite fraction by increasing the C content.

On the other hand, rails in curved sections of high-axle-load railways are subject to rolling stress from wheels and slipping force due to centrifugal force. Therefore, Patent Document 5 proposes a technique for suppressing the formation of proeutectoid cementite by adding Al and Si to improve the fatigue damage resistance. Further, Patent Document 6 proposes a technique for reducing the fatigue crack propagation speed by controlling the lamellar spacing of pearlite within an appropriate range.

JP-A-8-109439 JP-A-8-144016 JP-A-8-246100 JP-A-8-246101 JP-A-2002-69585 JP 2010-185106 A

However, the conventional technique described above still has the following problems to be solved.
As in the techniques described in Patent Documents 1 to 4, if the C content is simply set to more than 0.85% by mass and 1.20% by mass or less, a proeutectoid cementite structure is generated depending on the heat treatment conditions, and a brittle pearlite layered structure is generated. Since the amount of cementite layer in the steel increases, no improvement in fatigue damage resistance can be expected. In addition, in the technique described in Patent Document 5, the addition of Al generates oxides that act as starting points for fatigue damage, so it is particularly difficult to suppress the occurrence of fatigue cracks. Furthermore, in the technique described in Patent Document 6, a proeutectoid cementite structure may be generated depending on the combination of components and manufacturing conditions, and as a result, the fatigue crack propagation speed increases, so material control is sufficient. Hard to say.

SUMMARY OF THE INVENTION It is an object of the present invention to advantageously solve the above-described problems, and to provide a rail excellent in fatigue damage resistance, particularly fatigue crack propagation resistance, together with a preferred manufacturing method thereof.

In order to solve the above problems, the inventors produced rails with varying contents of C, Si, Mn and Cr, and diligently investigated their structures and fatigue crack propagation resistance. As a result, the parameter CP was derived from the component parameter X corresponding to the amount of proeutectoid cementite and the prior austenite grain size _RA . They also found that by controlling the parameter CP within a predetermined range, excellent fatigue crack propagation resistance can be obtained even if a large amount of proeutectoid cementite is present.

The rail excellent in fatigue crack propagation resistance according to the present invention, which has been developed to solve the above problems and achieve the above objects, contains C: 0.80 to 1.30% by mass and Si: 0.10 to 1.0% by mass. 20% by mass, Mn: 0.20 to 1.80% by mass, P: 0.035% by mass or less, S: 0.0005 to 0.012% by mass, Cr: 0.20 to 2.50% by mass The balance has a component composition consisting of Fe and unavoidable impurities, and CP represented by the following formula (1) is 2500 or less, where [% Y] is the content of element Y (% by mass ), and _RA is the prior austenite grain size (μm).
CP=X/R _A (1)
X={(10×[%C])+([%Si]/12)+([%Mn]/24)+([%Cr]/21)} ⁵ (2)

Regarding the rail excellent in fatigue crack propagation resistance according to the present invention,
a. The component composition further includes V: 0.30% by mass or less, Cu: 1.0% by mass or less, Ni: 1.0% by mass or less, Nb: 0.05% by mass or less, and Mo: 2.0% by mass. Containing at least one selected from the following,
b. The component composition further includes Al: 0.07% by mass or less, W: 1.0% by mass or less, B: 0.005% by mass or less, Ti: 0.05% by mass or less, and Sb: 0.05% by mass. Containing at least one selected from the following,
etc. is considered to be a more preferable solution.

A method for manufacturing a rail having excellent fatigue crack propagation resistance according to the present invention, which has been developed to solve the above problems and achieve the above objects, comprises heating a steel material having any of the above chemical compositions to 1350° C. or less. A method of manufacturing a rail by performing hot rolling after subjecting the steel to hot rolling, characterized in that the hot rolling is performed so that the finishing temperature is 900° C. or higher.

Regarding the method for manufacturing a rail having excellent fatigue crack propagation resistance according to the present invention, accelerated cooling from 900 ° C. to 750 ° C. at a cooling rate in the range of 0.4 to 3 ° C./s after the hot rolling. However, it is considered that accelerated cooling from 750° C. to the cooling stop temperature of 400 to 600° C. at a cooling rate in the range of 1 to 10° C./s can be a more preferable solution.

INDUSTRIAL APPLICABILITY According to the rail and the manufacturing method thereof according to the present invention, it is possible to stably manufacture a fatigue damage resistant rail having excellent fatigue crack propagation resistance, thereby increasing the service life of rails for high axle load railways, It contributes to the prevention of accidents and has industrially beneficial effects.
In addition, by optimizing the heat treatment conditions after hot rolling, the resistance to fatigue damage can be improved, which is preferable.

It is a schematic diagram showing the effect of pro-eutectoid cementite on the fatigue crack propagation rate, (a) shows the case where the prior austenite grain size and the plastic region size are almost equal, and (b) shows the prior austenite grain size in the plastic region. Indicates the case of greater than dimension. It is a figure which shows the position which sampled the test piece for prior austenite grain size observation. FIG. 2 is a diagram showing the positions where fatigue crack propagation test pieces were taken; BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the test piece shape used for the fatigue crack propagation test, (a) represents a front view, (b) represents a side view, (c) represents a notch part enlarged front view. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the test piece shape used for the fatigue damage-resistant test, (a) represents a side view, (b) represents a front view. It is a figure which shows the position which extract|collected the fatigue-resistant damage test piece.

Embodiments of the present invention will be specifically described below. First, in the present invention, the reasons for limiting the chemical composition of the steel, which is the rail material, to the above range will be explained. Note that "%" in the following description represents "% by mass" unless otherwise specified.

C: 0.80-1.30%
C is an essential element for ensuring the strength of the pearlite structure, that is, the resistance to fatigue damage. However, if it is less than 0.80%, it is difficult to obtain excellent fatigue crack propagation resistance. On the other hand, if it exceeds 1.30%, a large amount of proeutectoid cementite is generated at the austenite grain boundary during cooling after hot rolling, resulting in an increase in fatigue crack propagation speed. Proeutectoid cementite is present even when the content is 1.30% or less, but its influence can be avoided by controlling the prior austenite grain size based on the relational expression described later. Therefore, the C content should be in the range of 0.80 to 1.30%. The upper limit of the C content is preferably 1.00%, more preferably 0.90%.

Si: 0.10-1.20%
In addition to its effect as a deoxidizing agent, Si raises the pearlite equilibrium transformation temperature and narrows the lamellar spacing, thereby contributing to the reduction of the fatigue crack propagation speed. Therefore, 0.10% or more is necessary, but if it exceeds 1.20%, weldability deteriorates due to the high bonding strength of Si with oxygen. Furthermore, since Si has the effect of moving the eutectoid point to the low C side, excessive addition promotes the formation of proeutectoid cementite, resulting in an increase in the fatigue crack propagation rate. Therefore, the Si content should be in the range of 0.10 to 1.20%. The lower limit of the Si content is preferably 0.20%, and the upper limit of the Si content is preferably 0.80%, more preferably 0.60%.

Mn: 0.20-1.80%
Mn lowers the pearlite transformation temperature and narrows the lamellar spacing, thereby contributing to the reduction of the fatigue crack propagation rate. However, if the Mn content is less than 0.20%, sufficient effects cannot be obtained. On the other hand, if the Mn content exceeds 1.80%, a martensitic structure tends to occur, and hardening and embrittlement occur during heat treatment and welding of the rail, and the material tends to deteriorate. Furthermore, since Mn has the effect of moving the eutectoid point to the low C side, excessive addition promotes the formation of proeutectoid cementite, resulting in an increase in the fatigue crack propagation rate. Therefore, the Mn content should be in the range of 0.20 to 1.80%. The lower limit of the Mn content is preferably 0.30%, and the upper limit of the Mn content is preferably 1.00%, more preferably 0.60%.

P: 0.035% or less A content of P exceeding 0.035% deteriorates ductility. Therefore, the P content should be 0.035% or less. Preferably, it is 0.020% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0%, but industrially it is usually over 0%. Note that an excessive reduction in the P content leads to an increase in refining costs, so from the viewpoint of economy, the P content is preferably 0.001% or more.

S: 0.0005-0.012%
S exists in steel mainly in the form of A-based inclusions (those subject to viscous deformation by working). Since inclusions are generated, the cleanliness of the steel deteriorates. Also, if the S content is less than 0.0005%, the refining cost will increase. Therefore, the S content should be in the range of 0.0005 to 0.012%. The upper limit of the S content is preferably 0.010%, more preferably 0.008%.

Cr: 0.20-2.50%
Cr raises the pearlite equilibrium transformation temperature and refines the lamellar spacing, thereby contributing to the reduction of the fatigue crack propagation rate. However, if the Cr content is less than 0.20%, fatigue crack propagation cannot be sufficiently suppressed, while if the Cr content exceeds 2.50%, the hardenability of the steel increases, and martensite is easier to generate. In addition, when the steel is manufactured under conditions in which martensite is not generated, proeutectoid cementite is generated at the prior austenite grain boundaries. Therefore, the fatigue crack propagation rate increases. Therefore, the Cr content should be in the range of 0.20 to 2.50%. The lower limit of Cr content is preferably 0.40%, more preferably 0.50%, and the upper limit of Cr content is preferably 1.50%, more preferably 1.00%.

Furthermore, in _the present invention, it is not sufficient for each element to simply satisfy the above range. It is important to control the CP value represented by the following formula (1) to 2500 or less.
CP=X/R _A (1)
X={(10×[%C])+([%Si]/12)+([%Mn]/24)+([%Cr]/21)} ⁵ (2)
However, [% Y]: content of element Y (% by mass),
R _A : Prior austenite grain size (μm)
represents

The inventors investigated the cause of the increase in fatigue crack propagation rate due to the presence of pro-eutectoid cementite. As a result, as shown in the schematic diagram of FIG. 1( a ), the proeutectoid cementite 24 precedes the brittle fracture at the tip of the fatigue crack 23 , which causes the fatigue crack propagation speed increase 26 . I got the knowledge that Furthermore, by adjusting the grain size of prior austenite, which is the formation site of the structure, according to the amount of pro-eutectoid cementite generated, the frequency of encounter between the plastic region 22 formed at the tip of the fatigue crack and the pro-eutectoid cementite is reduced. It was found that brittle crack propagation can be suppressed. Specifically, even when a large amount of proeutectoid cementite exists, as shown in FIG. This makes it possible to control the CP value to 2500 or less. As a result, the effect of suppressing the fatigue crack propagation rate described above can be stably obtained. The CP value is preferably 2000 or less.

The component composition of the rail used in the present invention includes, in addition to the components described above, at least one selected from the following group A and at least one selected from the following group B, or both. may be contained in
Group A: V: 0.30% or less, Cu: 1.0% or less, Ni: 1.0% or less, Nb: 0.05% or less and Mo: 2.0% or less Group B: Al: 0.07 % or less, W: 1.0% or less, B: 0.005% or less, Ti: 0.05% or less and Sb: 0.05% or less

The reason for specifying the contents of the elements belonging to the above groups A and B will be described below.
V: 0.30% or less V forms carbonitrides in the steel and precipitates dispersedly in the matrix to improve the wear resistance of the steel. However, when the content exceeds 0.30%, workability deteriorates and manufacturing costs increase. Also, if the V content exceeds 0.30%, the cost of the alloy increases, so the cost of the internal high-hardness rail increases. Therefore, V preferably contains 0.30% as the upper limit. In addition, in order to exhibit the above-mentioned effect of improving the wear resistance, V is preferably contained in an amount of 0.001% or more. Note that the upper limit of the V content is more preferably 0.15%.

Cu: 1.0% or less Cu, like Cr, is an element capable of further increasing the strength of steel through solid-solution strengthening. However, when the content exceeds 1.0%, Cu cracks are likely to occur. Therefore, when Cu is contained in the component composition, the Cu content is preferably 1.0% or less. The lower limit of Cu content is more preferably 0.005%, and the upper limit of Cu content is more preferably 0.5%.

Ni: 1.0% or less Ni is an element capable of increasing the strength of steel without deteriorating ductility. Moreover, since Cu cracking can be suppressed by adding Cu in combination, it is desirable to also contain Ni when Cu is contained in the composition. However, if the Ni content exceeds 1.0%, the hardenability of the steel increases, the amount of martensite and bainite increases, and wear resistance and fatigue damage resistance tend to decrease. Therefore, when Ni is contained, the Ni content is preferably 1.0% or less. The lower limit of the Ni content is more preferably 0.005%, and the upper limit of the Ni content is more preferably 0.5%.

Nb: 0.05% or less Nb binds with C in steel and precipitates as carbide during and after hot rolling for forming rails, and acts effectively to refine pearlite colony size. As a result, the wear resistance, fatigue damage resistance, and ductility are greatly improved, which greatly contributes to the extension of the service life of the internal high-hardness rail. However, even if the Nb content exceeds 0.05%, the effect of improving wear resistance and fatigue damage resistance is saturated, and the effect corresponding to the increase in content cannot be obtained. Therefore, Nb may be contained with the upper limit of the content being 0.05%. If the Nb content is less than 0.001%, it is difficult to obtain a sufficient effect for extending the life of the rail. Therefore, when Nb is contained, the Nb content is preferably 0.001% or more. Note that the upper limit of the Nb content is more preferably 0.03%.

Mo: 2.0% or less Mo is an element capable of further increasing the strength of steel through solid-solution strengthening. In addition, since Mo has the effect of moving the eutectoid point to the high C side, it also has the effect of suppressing the formation of proeutectoid cementite. However, if it exceeds 2.0%, the amount of bainite generated in the steel increases and wear resistance decreases. Therefore, when the rail composition contains Mo, the Mo content is preferably 2.0% or less. The lower limit of Mo content is more preferably 0.005%, and the upper limit of Mo content is more preferably 1.0%.

Al: 0.07% or less Al is an element that can be added as a deoxidizing agent. However, when the Al content exceeds 0.07%, a large amount of oxide-based inclusions are formed in the steel due to the high bonding strength of Al with oxygen, resulting in a decrease in ductility of the steel. Therefore, the Al content is preferably 0.07% or less. On the other hand, although the lower limit of the Al content is not particularly limited, it is preferably 0.001% or more for deoxidation. In addition, the upper limit of Al content is more preferably 0.03%.

W: 1.0% or less W precipitates as carbide during and after hot rolling for shaping into a rail shape, and improves the strength and ductility of the rail by precipitation strengthening. However, if the W content exceeds 1.0%, martensite is formed in the steel, resulting in reduced ductility. Therefore, when adding W, the W content is preferably 1.0% or less. On the other hand, the lower limit of the W content is not particularly limited, but it is preferably 0.001% or more in order to exhibit the effect of improving the strength and ductility. The lower limit of the W content is more preferably 0.005%, and the upper limit of the W content is more preferably 0.5%.

B: 0.005% or less B precipitates as a nitride in steel during and after hot rolling for forming into a rail shape, and improves the strength and ductility of steel by precipitation strengthening. However, when the B content exceeds 0.005%, martensite is formed and as a result the ductility of the steel is reduced. Therefore, when B is contained, the B content is preferably 0.005% or less. On the other hand, the lower limit of the B content is not particularly limited, but it is preferably 0.001% or more in order to exhibit the effect of improving the strength and ductility. Incidentally, the upper limit of the B content is more preferably 0.003%.

Ti: 0.05% or less Ti precipitates in the steel as carbides, nitrides or carbonitrides during and after hot rolling for forming into a rail shape, and increases the strength and ductility of the steel by precipitation strengthening. Improve. However, if the Ti content exceeds 0.05%, coarse carbides, nitrides or carbonitrides are formed, resulting in a decrease in ductility of the steel. Therefore, when Ti is contained, the Ti content is preferably 0.05% or less. On the other hand, the lower limit of the Ti content is not particularly limited, but it is preferably 0.001% or more in order to exhibit the effect of improving the strength and ductility. The lower limit of the Ti content is more preferably 0.005%, and the upper limit of the Ti content is more preferably 0.03%.

Sb: 0.05% or less Sb has a remarkable effect of preventing decarburization of the steel during reheating when the rail steel material is reheated in a heating furnace before hot rolling. However, if the Sb content exceeds 0.05%, the ductility and toughness of the steel are adversely affected. On the other hand, the lower limit of the Sb content is not particularly limited, but it is preferably 0.001% or more in order to exhibit the effect of reducing the decarburized layer. The lower limit of the Sb content is more preferably 0.005%, and the upper limit of the Sb content is more preferably 0.03%.

The chemical composition of the steel material, which is the material for the rail of the present invention, contains the above components, the balance of Fe, and unavoidable impurities. The present invention also includes rails containing other minor constituent elements in place of part of the balance Fe in the composition according to the present invention within a range that does not substantially affect the effects of the present invention. Here, the unavoidable impurities include N, O, etc., and up to 0.008% of N and up to 0.004% of O are permissible.

The structure other than pearlite in the microstructure of the rail according to the present invention is not particularly limited. If the total area ratio is 5% or less, the presence of other structures is allowed because the fatigue crack propagation resistance is not significantly affected. Said other structures include, for example, ferrite, proeutectoid cementite, bainite and martensite.

Next, a method for manufacturing the rail according to the present invention explained above will be explained. The rail of the present invention can be manufactured by sequentially subjecting a steel material having the chemical composition described above to the following treatments (1) to (3).
(1) Hot rolling (2) Primary cooling (3) Secondary cooling The steel material used as the rail material can be produced by any method, but generally the steel material is produced by casting, particularly continuous casting. is preferred.

(1) Hot Rolling First, the steel material is hot rolled into a rail shape. In the present invention, since the prior austenite grain size of the finally obtained rail can be controlled by controlling the rolling finish temperature in the hot rolling, the method of the hot rolling is not particularly limited, and any method can be used. be able to.
Heating temperature: 1350°C or less In heating the steel material prior to hot rolling, the heating temperature must be 1350°C or less. If the heating temperature exceeds the upper limit, the excessive temperature rise may partially melt the steel material and cause defects inside the rail. On the other hand, the lower limit of the heating temperature is not particularly limited, but is preferably 1150° C. or higher in order to reduce deformation resistance during rolling.

Rolling finishing temperature: 900 ° C. or higher When the rolling finishing temperature in the hot rolling is lower than 900 ° C., rolling is performed in the austenite low temperature region, and not only is processing strain introduced into the austenite crystal grains, but also austenite Elongation of crystal grains becomes remarkable. An increase in the austenite grain boundary area increases the nucleation sites for proeutectoid cementite, resulting in a decrease in fatigue crack propagation resistance. Therefore, the rolling finish temperature is set to 900° C. or higher. On the other hand, there is no particular upper limit for the finishing temperature of rolling, but if the grain size of the prior austenite is excessively coarsened, the ductility and toughness will decrease, so it is preferably 1050° C. or less. The rolling finish temperature referred to here is the temperature of the rail head side surface on the entry side of the final rolling mill, and can be measured with a radiation thermometer.

(2) Average cooling rate from 900°C to 750°C for primary cooling: 0.4-3°C/s
Next, accelerated cooling is performed. At this time, when the average cooling rate in the primary cooling in the temperature range of 900° C. to 750° C. where pro-eutectoid cementite is generated is less than 0.4° C./s, the amount of pro-eutectoid cementite increases. As a result, cracks are likely to occur in the pro-eutectoid cementite structure, and the fatigue damage resistance of the rail may deteriorate. Therefore, the lower limit of the average cooling rate of the primary cooling is preferably 0.4°C/s, more preferably 0.7°C/s. On the other hand, if the average cooling rate of the primary cooling exceeds 3° C./s, a martensite structure may be generated, and ductility and fatigue damage resistance may be lowered. Therefore, the upper limit of the average cooling rate of the primary cooling is preferably 3°C/s, more preferably 2°C/s.

(3) Secondary cooling Average cooling rate from 750°C to 400-600°C: 1-10°C/s
After stopping the primary cooling, secondary cooling is performed. When the average cooling rate from the secondary cooling start temperature of 750° C. to the secondary cooling stop temperature in the temperature range of 400 to 600° C. is less than 1° C./s, the lamellar spacing of the pearlite structure becomes rough. As a result, the hardness of the pearlite structure is lowered, and the fatigue damage resistance of the rail may be lowered. In addition, since the cooling time in the low temperature range increases, the productivity may decrease and the manufacturing cost of the rail may increase. On the other hand, if the average cooling rate of the secondary cooling exceeds 10° C./s, a martensite structure may be generated, resulting in deterioration of ductility and fatigue damage resistance. Therefore, the average cooling rate of the secondary cooling is preferably in the range of 1 to 10°C/s. In addition, the upper limit of the average cooling rate of the secondary cooling is more preferably 5°C/s.

In the primary and secondary cooling described above, the temperature used to determine the average cooling rate is the surface temperature of the side surface of the rail head, which can be measured with a radiation thermometer. Here, the cooling stop temperature during secondary cooling is the temperature obtained by measuring the temperature of the side surface of the rail head after accelerated cooling is stopped (before reheating) with a radiation thermometer.

Hereinafter, the configuration and effects of the present invention will be described more specifically according to examples. However, the present invention is not limited by the following examples, and can be modified as appropriate within the scope of the gist of the present invention, and any of these are included in the technical scope of the present invention. be

Steel materials having the chemical compositions shown in Table 1 were subjected to hot rolling under the conditions shown in Table 2 and accelerated cooling after hot rolling to produce rail materials. Accelerated cooling was performed only on the rail head, and after the cooling was stopped, it was allowed to cool. The rolling finish temperature in Table 2 indicates the value obtained by measuring the temperature of the rail head side surface on the entry side of the final rolling mill with a radiation thermometer as the rolling finish temperature. The cooling stop temperature in Table 2 indicates the value obtained by measuring the temperature of the rail head side surface layer at the time of cooling stop of the secondary cooling with a radiation thermometer. As for the cooling rate, the cooling rate (°C/s) was obtained by converting the temperature change from the start of cooling to the stop of cooling into a unit time (second) for primary cooling and secondary cooling.

The obtained rail was evaluated for prior austenite grain size _RA , fatigue crack propagation resistance, and fatigue damage resistance. Details of each evaluation are described below.
<Previous austenite grain size _RA >
After cutting the tip of the rail after hot finish rolling, the cut material was immediately water-cooled. From the obtained water-cooled material, a test piece for structure observation was taken from the rolling direction at a depth of 5 mm from the surface of the rail head 1 shown in FIG. The obtained test piece was mirror-polished and then subjected to γ-grain etching, and its cross section was observed with an optical microscope at a magnification of 200 times. Prior austenite grain size _RA was evaluated by measuring 400 or more grain sizes by tracing work using image analysis software and obtaining the average value.

<Fatigue crack propagation resistance>
Fatigue crack propagation test specimens were taken from two locations, the top of the rail and the gauge corner (GC) shown in FIG. 3, and a fatigue crack propagation test was performed. FIG. 4 is a schematic diagram showing an example of a test piece, FIG. 4(a) showing a front view, FIG. 4(b) showing a side view, and FIG. 4(c) showing an enlarged front view of a notch portion. In FIG. 4, the test piece is, for example, a plate having a width of W = 20 mm, a height of H = 100 mm, and a thickness of B = 5 mm. is formed. The length L of the notch portion is 2 mm, the width C is 0.2 mm, and the end portion of the notch portion is formed with a curvature R of 0.1 mm. The stress ratio (R ratio = minimum stress / maximum stress) is 0.1, and the fatigue crack propagation speed da / dN (m / cycle) is measured at the stress intensity factor ΔK = 20 MPa m ^1/2 , and the fatigue resistance Crack propagation properties were evaluated. If the value of da/dN was 8.0×10 ⁻⁸ or less, it was evaluated that there was fatigue crack propagation inhibition performance.

<Fatigue damage resistance>
As for fatigue damage resistance, it is most desirable to evaluate by actually laying rails, but it takes a long time to test. Therefore, we used a Nishihara wear tester that can evaluate fatigue damage resistance in a short time. Here, fatigue damage resistance was evaluated by a comparative test simulating actual contact conditions between rails and wheels. Specifically, a Nishihara wear test piece 17 with a diameter of 30 mm was collected from the rail head 1 with the contact surface being a curved surface with a curvature radius R = 15 mm. was tested. For the wheel test piece 18, first, a round bar with a diameter of 32 mm was obtained from the head of a normal rail described in JIS E1101:2012. Then, the round bar was heat-treated so that the Vickers hardness (load 98 N) was Hv390 and the structure was a tempered martensitic structure, and then processed into a cylindrical shape with a diameter of 30 mm and subjected to a test. As shown in FIG. 6, the Nishihara wear test piece 17 was taken from the fatigue damage resistance test piece sampling portion 14 of the surface layer of the rail head 1 . The arrows in FIG. 5(a) indicate the directions of rotation of the Nishihara wear test piece 17 and the wheel test piece 18, respectively. The test environment was oil lubrication conditions, contact pressure: 1.8 GPa, slip ratio: -20%, rotation speed: 600 rpm (wheel test piece: 750 rpm), and the test piece surface was observed every 2.5 × 10 ⁴ times. , and the number of revolutions at which a crack of 0.5 mm or more was generated was taken as the fatigue damage life. When this numerical value was 8×10 ⁵ times or more, it was judged that the fatigue damage resistance was present.

Table 2 also shows the results of the above investigation. Test results (Test Nos. 1 to 1 in Table 2) of rail materials manufactured by the manufacturing method (heating temperature, finishing temperature of rolling) within the range of the present invention using compatible steel satisfying the composition of the present invention and CP of 2500 or less. 20, . . . ) satisfied the fatigue crack propagation speed da/dN (m/cycle) at ΔK=20 MPa·m ^1/2 of 8.0×10 ⁻⁸ or less. In addition, test No. 1 in which the primary cooling and secondary cooling conditions were in the suitable range. 1 to 20 satisfied both the fatigue crack propagation speed da/dN (m/cycle) of 8.0×10 ⁻⁸ or less and the fatigue damage life of 8×10 ⁵ times or more. On the other hand, in comparative examples (Test Nos. 21 to 28 and 30 in Table 2) in which the chemical composition of the rail material did not satisfy the conditions of the present invention, or in which the production method within the scope of the present invention was not applied, CP was It exceeded 2500 and the fatigue crack propagation speed da/dN (m/cycle) exceeded 8.0×10 ⁻⁸ , or the fatigue damage life remained below 8×10 ⁵ cycles. In Test No. 29, the heating temperature was too high, so part of the steel material melted during heating. For this reason, it was not possible to use it for rolling due to the fear of breakage during rolling, and it was not possible to evaluate the characteristics.

INDUSTRIAL APPLICABILITY According to the rail and the manufacturing method thereof according to the present invention, it is possible to stably manufacture a fatigue damage resistant rail having excellent fatigue crack propagation resistance, thereby increasing the service life of rails for high axle load railways, It contributes to the prevention of accidents and has industrially beneficial effects.

1 Rail head 11 Old austenite grain size observation test piece sampling part 12 Gauge corner (GC) part 13 Head top part 14 Fatigue damage resistance test piece sampling part 15 Fatigue crack propagation test piece 16 Notch part 17 Nishihara type wear test piece 18 Tire test piece 21 Prior austenite grain 22 Plastic region 23 Fatigue crack 24 Proeutectoid cementite 25 Cleavage fracture 26 Fatigue crack propagation speed increase 27 Fatigue crack propagation speed decrease _RA Prior austenite grain size _RP Plastic region dimension

Claims (5)

C: 0.80 to 1.30% by mass, Si: 0.10 to 1.20% by mass, Mn: 0.20 to 1.80% by mass, P: 0.035% by mass or less, S: 0.0005 ~ 0.012% by mass, Cr: 0.20 to 2.50% by mass, the balance being Fe and unavoidable impurities, CP represented by the following formula (1) is 2500 or less A rail having excellent fatigue crack propagation resistance, characterized by:
CP=X/R _A (1)
X={(10×[%C])+([%Si]/12)+([%Mn]/24)+([%Cr]/21)} ⁵ (2)
However, [% Y]: content of element Y (% by mass),
R _A : Prior austenite grain size (μm)
represents The component composition further includes V: 0.30% by mass or less, Cu: 1.0% by mass or less, Ni: 1.0% by mass or less, Nb: 0.05% by mass or less, and Mo: 2.0% by mass. 2. A rail excellent in fatigue crack propagation resistance according to claim 1, characterized by containing at least one selected from the following. The component composition further includes Al: 0.07% by mass or less, W: 1.0% by mass or less, B: 0.005% by mass or less, Ti: 0.05% by mass or less, and Sb: 0.05% by mass. 3. A rail excellent in fatigue crack propagation resistance according to claim 1 or 2, characterized by containing at least one selected from the following. A method for manufacturing a rail having excellent fatigue crack propagation resistance according to any one of claims 1 to 3,
A steel material having the above chemical composition is heated to 1350° C. or less and then hot rolled to manufacture a rail. A method for manufacturing a rail excellent in fatigue crack propagation resistance, characterized in that CP represented by the formula is 2500 or less. After the hot rolling, accelerated cooling is performed from 900 ° C. to 750 ° C. at a cooling rate in the range of 0.4 to 3 ° C./s, and from 750 ° C. to the cooling stop temperature of 400 to 600 ° C. is 1 to 10 ° C./s. 5. The method of manufacturing a rail having excellent fatigue crack propagation resistance according to claim 4, wherein accelerated cooling is performed at a cooling rate in the range of .

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