WO2022004247A1

WO2022004247A1 - Rail having excellent fatigue crack propagation resistance characteristics, and method for producing same

Info

Publication number: WO2022004247A1
Application number: PCT/JP2021/020871
Authority: WO
Inventors: 佳祐安藤; 浩文大坪
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2020-06-29
Filing date: 2021-06-01
Publication date: 2022-01-06
Also published as: JPWO2022004247A1; US20230279517A1; CN115917019A; BR112022025557A2; CA3186612A1; AU2021302317A1; EP4174191A1; KR20220160695A; JP7173366B2; AU2021302317B2

Abstract

Provided are a rail that exhibits an excellent fatigue damage resistance and in particular excellent fatigue crack propagation resistance characteristics, and a preferred method for producing the rail. The rail has a component composition that contains, on a mass basis, C : 0.80-1.30%, Si : 0.10-1.20%, Mn : 0.20-1.80%, P ≤ 0.035%, S : 0.0005-0.012%, and Cr : 0.20-2.50%, with the balance being Fe and inevitable impurities, and has not more than 2500 for CP = X/R_A (wherein X = {(10 x [%C]) + ([%Si]/12) + ([%Mn]/24) + ([%Cr]/21)}⁵, [%Y] is the content (mass%) of element Y, and R_A is the prior austenite grain diameter (µm)). In the rail production method, a steel material is heated to not more than 1350°C and then hot rolled so as to provide a finishing temperature of at least 900°C.

Description

Rails with excellent fatigue crack propagation characteristics and their manufacturing methods

The present invention relates to a rail and a method for manufacturing the rail, and the present invention relates to a rail having improved fatigue crack propagation characteristics and a method for manufacturing the rail which can advantageously manufacture the rail.

In high-axle heavy railways that mainly transport ore, the load on the axles of freight cars is much higher than that of passenger cars, and the rail usage environment is harsh. As the rail used in such an environment, steel having a pearlite structure has been mainly used from the viewpoint of emphasizing wear resistance. However, in recent years, the weight loaded on freight cars has been further increased in order to improve the efficiency of transportation by rail, and rails are required to have further improvement in wear resistance and fatigue damage resistance. The high-axis heavy railway is a railway having a large load weight (for example, a load weight of about 150 tons or more) of one freight car such as a train or a freight car.

Therefore, various studies are being conducted with the aim of further improving wear resistance. For example, in Patent Document 1 and Patent Document 2, the C content is increased from 0.85% by mass to 1.20% by mass or less. Further, in Patent Document 3 and Patent Document 4, the C content is more than 0.85% by mass and 1.20% by mass or less, and the rail head is heat-treated. In these techniques, the wear resistance is improved by increasing the cementite fraction by increasing the C content.

On the other hand, the rails in the curved section of the high-axis heavy railway are subject to rolling stress due to the wheels and slipping force due to centrifugal force, so the rail wear becomes more severe and fatigue damage due to slipping occurs. Therefore, Patent Document 5 proposes a technique of suppressing the formation of proeutectoid cementite by adding Al and Si and improving fatigue damage resistance. Further, Patent Document 6 proposes a technique for reducing the fatigue crack propagation rate by controlling the lamellar spacing of pearlite within an appropriate range.

Japanese Unexamined Patent Publication No. 8-109439 Japanese Unexamined Patent Publication No. 8-144016 Japanese Unexamined Patent Publication No. 8-246100 Japanese Unexamined Patent Publication No. 8-246101 Japanese Unexamined Patent Publication No. 2002-69585 Japanese Unexamined Patent Publication No. 2010-185106

However, the above-mentioned conventional technique still has the following problems to be solved.
When the C content is simply more than 0.85% by mass and 1.20% by mass or less as in the techniques described in Patent Documents 1 to 4, an initial cementite structure is formed depending on the heat treatment conditions, and a brittle pearlite layered structure is formed. Since the amount of cementite layer is increased, improvement in fatigue damage resistance cannot be expected. Further, in the technique described in Patent Document 5, it is particularly difficult to suppress the occurrence of fatigue cracks because the addition of Al produces an oxide that is a starting point of fatigue damage. Further, in the technique described in Patent Document 6, the pro-eutectoid cementite structure may be formed depending on the combination of the components and the production conditions, and as a result, the fatigue crack propagation rate increases, so that the material control is sufficient. It's hard to say.

The present invention has been made to advantageously solve the above-mentioned problems, and an object of the present invention is to provide a rail having excellent fatigue damage resistance, particularly fatigue crack propagation characteristics, together with a preferable manufacturing method thereof.

In order to solve the above problems, the inventors have manufactured rails having different contents of C, Si, Mn and Cr, and have diligently investigated the structure and fatigue crack propagation characteristics. As a result, the parameter CP was derived from the component parameter X corresponding to the amount of pro-eutectoid cementite and the old austenite particle size _RA. Then, they have found that by controlling the parameter CP within a predetermined range, excellent fatigue-resistant crack propagation characteristics can be obtained even if a large amount of pro-eutectoid cementite is present.

The rails having excellent fatigue crack propagation characteristics according to the present invention developed to solve the above problems and realize the above objects have C: 0.80 to 1.30% by mass and Si: 0.10 to 1. Contains 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. However, the balance has a component composition consisting of Fe and unavoidable impurities, and the CP represented by the following formula (1) is 2500 or less, where [% Y] is the content (mass%) of the element Y. ), And _RA is characterized by having an old austenite particle size (μm).
CP = X / _RA ... (1)
X = {(10 × [% C]) + ([% Si] / 12) + ([% Mn] / 24) + ([% Cr] / 21)} ⁵ ... (2)

Regarding the rail having excellent fatigue crack propagation characteristics according to the present invention,
a. Further, the component composition is 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. may be a more preferable solution.

The method for manufacturing a rail having excellent fatigue-resistant crack propagation characteristics according to the present invention, which has been developed to solve the above problems and realize the above object, is to heat a steel material having any of the above component compositions to 1350 ° C. or lower. This is a method for manufacturing a rail by hot rolling after performing hot rolling, and is characterized by hot rolling so that the finishing temperature becomes 900 ° C. or higher.

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

According to the rail and the manufacturing method thereof according to the present invention, it is possible to stably manufacture a fatigue-resistant damage-resistant rail having excellent fatigue-resistant crack propagation characteristics, and it is possible to extend the life of rails for high-axis heavy railways and railways. It contributes to accident prevention and has an industrially beneficial effect.
In addition, it is preferable to optimize the heat treatment conditions after hot rolling because the fatigue damage resistance can be improved.

It is a schematic diagram which shows the influence of the analytic cementite on the fatigue crack propagation rate, (a) shows the case where the former austenite grain size and the plastic region dimension are almost equal, and (b) is the former austenite particle diameter in the plastic region. The case where it is larger than the dimension is shown. It is a figure which shows the position where the test piece for observing the old austenite particle size was collected. It is a figure which shows the position where the fatigue crack propagation test piece was collected. It is a figure explaining the test piece shape used for the fatigue crack propagation test, (a) shows the front view, (b) shows the side view, (c) shows the notch part enlarged front view. It is a figure explaining the shape of the test piece used for the fatigue-resistant damage test, (a) shows the side view, and (b) shows the front view. It is a figure which shows the position where the fatigue-resistant damage test piece was collected.

Hereinafter, embodiments of the present invention will be specifically described. First, in the present invention, the reason why the composition of steel as a rail material is limited to the above range will be described. In addition, "%" in the following description shall represent "mass%" unless otherwise specified.

C: 0.80 to 1.30%
C is an essential element for ensuring the strength of the pearlite structure, that is, the fatigue damage resistance. However, if it is less than 0.80%, it is difficult to obtain excellent fatigue-resistant crack propagation characteristics. On the other hand, if it exceeds 1.30%, a large amount of proeutectoid cementite is generated at the austenite grain boundaries during cooling after hot rolling, which causes an increase in the fatigue crack propagation rate. Although the proeutectoid cementite is present even when it is 1.30% or less, its influence can be avoided by controlling the old austenite particle size based on the relational expression described later. Therefore, the C content is 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 to 1.20%
In addition to its effect as a deoxidizing agent, Si contributes to a decrease in the fatigue crack propagation rate by increasing the pearlite equilibrium transformation temperature and making the lamellar interval finer. Therefore, 0.10% or more is required, but if it exceeds 1.20%, the weldability deteriorates due to the high bonding force of Si with oxygen. Further, since Si has an action of moving the eutectic point to the low C side, excessive addition promotes the formation of eutectic cementite and causes an increase in the fatigue crack propagation rate. Therefore, the Si content is in the range of 0.10 to 1.20%. The lower limit of the Si content is preferably 0.20%, the upper limit of the Si content is preferably 0.80%, and even more preferably 0.60%.

Mn: 0.20 to 1.80%
Mn contributes to a decrease in the fatigue crack propagation rate by lowering the pearlite transformation temperature and making the lamellar interval finer. However, if the Mn content is less than 0.20%, a sufficient effect cannot be obtained. On the other hand, when the Mn content exceeds 1.80%, a martensite structure is likely to be formed, and the material is likely to be deteriorated due to hardening or embrittlement during heat treatment and welding of the rail. Further, since Mn has an action of moving the eutectic point to the low C side, excessive addition promotes the formation of eutectic cementite and causes an increase in the fatigue crack propagation rate. Therefore, the Mn content is in the range of 0.20 to 1.80%. The lower limit of the Mn content is preferably 0.30%, the upper limit of the Mn content is preferably 1.00%, and even more preferably 0.60%.

P: 0.035% or less The content of P exceeding 0.035% deteriorates ductility. Therefore, the P content is 0.035% or less. It is preferably 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 more than 0%. It should be noted that excessively reducing the P content leads to an increase in refining cost, and therefore, from the viewpoint of economic efficiency, it is preferable to set the P content to 0.001% or more.

S: 0.0005 to 0.012%
S is mainly present in steel in the form of A-based inclusions (those that undergo viscous deformation due to processing), but when the content exceeds 0.012%, the amount of these inclusions increases remarkably, and at the same time, it is coarse. Since inclusions are generated, the cleanliness of the steel material deteriorates. Further, if the S content is less than 0.0005%, the refining cost will increase. Therefore, the S content is 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 to 2.50%
Cr contributes to a decrease in the fatigue crack propagation rate by increasing the pearlite equilibrium transformation temperature and making the lamellar spacing finer. However, if the Cr content is less than 0.20%, fatigue crack growth cannot be sufficiently suppressed, while if the Cr content exceeds 2.50%, the hardenability of steel becomes high and martensite. Is easy to generate. Further, when manufactured under the condition that martensite is not formed, proeutectoid cementite is formed at the old austenite grain boundaries. Therefore, the fatigue crack propagation rate increases. Therefore, the Cr content is in the range of 0.20 to 2.50%. The lower limit of the Cr content is preferably 0.40%, more preferably 0.50%, and the upper limit of the Cr content is preferably 1.50%, further preferably 1.00%.

Furthermore, in the present invention, it is not sufficient for each element to simply satisfy the above range, and it is derived from the component parameter X corresponding to the amount of proeutectoid cementite represented by the following equation (2) and the former austenite particle size _RA. It is important to control the CP value represented by the following equation (1) to 2500 or less.
CP = X / _RA ... (1)
X = {(10 × [% C]) + ([% Si] / 12) + ([% Mn] / 24) + ([% Cr] / 21)} ⁵ ... (2)
However, [% Y]: content of element Y (mass%),
_RA : Old austenite particle size (μm)
Represents.

The inventors investigated the cause of the increase in fatigue crack propagation rate due to the presence of proeutectoid cementite. As a result, as shown in the schematic diagram of FIG. 1 (a), the brittle fracture of the proactive cementite 24 at the tip of the fatigue crack 23 causes the increase in the fatigue crack propagation rate 26. I got the finding that it is. Furthermore, by adjusting the particle size of the old austenite, which is the formation site of the structure, according to the amount of proactive cementite produced, the frequency of encounters between the plastic region 22 formed at the tip of the fatigue crack and the proactive cementite is reduced. It was found that brittle crack growth can be suppressed. Specifically, even when a large amount of pro-eutectoid cementite is present, as shown in FIG. 1 (b), the austenite grains 21 are sufficiently coarsened to be larger than the size of the plastic region 22 at the crack tip. This makes it possible to control the CP value to 2500 or less. As a result, the above-mentioned effect of suppressing the fatigue crack propagation rate can be stably obtained. The CP value is preferably 2000 or less.

In addition to the components described above, the component composition of the rail used in the present invention may be any one or both of at least one selected from the following group A and at least one selected from group B below. It 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

Hereinafter, the reason for specifying the content of the elements belonging to the above groups A and B will be described.
V: 0.30% or less V forms a carbonitride in the steel and disperses and precipitates in the substrate, improving the wear resistance of the steel. However, if the content exceeds 0.30%, the processability deteriorates and the manufacturing cost increases. Further, when the V content exceeds 0.30%, the alloy cost increases, so that the cost of the internal high hardness type rail increases. Therefore, V is preferably contained up to 0.30%. In addition, in order to exhibit the above-mentioned effect of improving wear resistance, V is preferably contained in an amount of 0.001% or more. The upper limit of the V content is more preferably 0.15%.

Cu: 1.0% or less Cu is an element that can further increase the strength of steel by solid solution strengthening like Cr. However, if the content exceeds 1.0%, Cu cracking is 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 the Cu content is more preferably 0.005%, and the upper limit of the Cu content is more preferably 0.5%.

Ni: 1.0% or less Ni is an element that can increase the strength of steel without deteriorating ductility. Further, since Cu cracking can be suppressed by compound addition with Cu, it is desirable that Ni is also contained when Cu is contained in the component composition. However, when the Ni content exceeds 1.0%, the hardenability of the steel is further increased, the amount of martensite and bainite produced is increased, and the wear resistance and the fatigue damage resistance tend to be lowered. 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 is combined with C in steel and precipitated as carbide during hot rolling for forming a rail and after hot rolling, and effectively acts on the miniaturization of pearlite colony size. As a result, wear resistance, fatigue damage resistance, and ductility are greatly improved, which greatly contributes to extending the life of the internal high hardness type rail. However, even if the Nb content exceeds 0.05%, the effect of improving wear resistance and fatigue damage resistance is saturated, and an effect commensurate with the increase in content cannot be obtained. Therefore, Nb may be contained with the upper limit of its content being 0.05%. If the Nb content is less than 0.001%, it is difficult to obtain a sufficient effect on extending the life of the rail. Therefore, when Nb is contained, the Nb content is preferably 0.001% or more. The upper limit of the Nb content is more preferably 0.03%.

Mo: 2.0% or less Mo is an element that can further increase the strength of steel by solid solution strengthening. Further, since Mo has an action of moving the eutectic point to the high C side, it also has an action of suppressing the formation of eutectic cementite. However, if it exceeds 2.0%, the amount of bainite generated in the steel increases and the wear resistance deteriorates. Therefore, when the component composition of the rail contains Mo, the Mo content is preferably 2.0% or less. The lower limit of the Mo content is more preferably 0.005%, and the upper limit of the 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 generated in the steel due to the high binding force of Al with oxygen, and as a result, the ductility of the steel is lowered. Therefore, the Al content is preferably 0.07% or less. On the other hand, the lower limit of the Al content is not particularly limited, but is preferably 0.001% or more for deoxidation. The upper limit of the Al content is more preferably 0.03%.

W: 1.0% or less W is precipitated as carbide during and after hot rolling for forming into a rail shape, and the strength and ductility of the rail are improved by strengthening the precipitation. However, when the W content exceeds 1.0%, martensite is formed in the steel, and as a result, the ductility is lowered. Therefore, when W is added, 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 above-mentioned action of improving 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 is precipitated as nitride in steel during and after hot rolling for forming into a rail shape, and the strength and ductility of the steel are improved by precipitation strengthening. However, if the B content exceeds 0.005%, martensite is formed, and as a result, the ductility of the steel is lowered. 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 above-mentioned action of improving strength and ductility. The upper limit of the B content is more preferably 0.003%.

Ti: 0.05% or less Ti precipitates in steel as carbides, nitrides or carbonitrides during and after hot rolling to form a rail shape, and the strength and ductility of the steel are enhanced by precipitation strengthening. Improve. However, if the Ti content exceeds 0.05%, coarse carbides, nitrides or carbonitrides are formed, resulting in a decrease in steel ductility. 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 above-mentioned action of improving 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 the 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. Therefore, when the Sb is contained, the Sb content is preferably 0.05% or less. 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 composition of the steel material used as the material for the rail of the present invention contains the above components, the balance Fe, and unavoidable impurities. A rail that contains other trace component elements in place of a part of the balance Fe in the composition according to the present invention within a range that does not substantially affect the action and effect of the present invention also belongs to the present invention. Here, examples of the unavoidable impurities include N, O, etc., and N can be allowed up to 0.008% and O can be allowed up to 0.004%.

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 fatigue crack propagation characteristics are not significantly affected, so that other tissues are allowed to be present. Examples of the other structures include ferrite, proeutectoid cementite, bainite and martensite.

Next, the rail manufacturing method according to the present invention described above will be described. The rail of the present invention can be manufactured by sequentially applying the following treatments (1) to (3) to a steel material having the above-mentioned composition.
(1) Hot rolling (2) Primary cooling (3) Secondary cooling The steel material used as the rail material can be manufactured by any method, but in general, the steel material is manufactured by casting, especially continuous casting. Is preferable.

(1) Hot rolling First, the steel material is hot-rolled to form a rail shape. In the present invention, since the old austenite grain size of the rail finally obtained can be controlled by controlling the rolling finish temperature in the hot rolling, the hot rolling method is not particularly limited and is performed by any method. be able to.
Heating temperature: 1350 ° C. or lower In heating the steel material to be applied prior to hot rolling, the heating temperature must be 1350 ° C. or lower. If the heating temperature exceeds the upper limit, the steel material may be partially melted due to excessive temperature rise, and defects may occur 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 the deformation resistance during rolling.

Rolling finish temperature: 900 ° C or higher When the rolling finish temperature in the above hot rolling is lower than 900 ° C, rolling is performed in the austenite low temperature range, and not only processing strain is introduced into the austenite crystal grains but also austenite is introduced. The elongation of the crystal grains becomes remarkable. Increasing the austenite grain boundary area increases the nucleation sites of proeutectoid cementite, resulting in a decrease in fatigue-resistant crack propagation characteristics. Therefore, the rolling finish temperature is set to 900 ° C. or higher. On the other hand, the upper limit of the rolling finish temperature is not particularly limited, but if the old austenite particle size becomes extremely coarse, the ductility and toughness will decrease, so the temperature is preferably 1050 ° C. or lower. The rolling finish temperature referred to here is the temperature of the side surface of the rail head on the entry side of the final rolling mill, and can be measured by a radiation thermometer.

(2) Primary cooling Average cooling rate from 900 ° C to 750 ° C: 0.4 to 3 ° C / s
Next, accelerated cooling is performed. At that time, as the primary cooling, if the average cooling rate in the temperature range of 900 ° C. to 750 ° C., which is the temperature range for producing the proeutectoid cementite, is less than 0.4 ° C./s, the amount of the proeutectoid cementite increases. Therefore, cracks are likely to occur in the proactive cementite structure, which may reduce the fatigue damage resistance of the rail. 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, when the average cooling rate of the primary cooling exceeds 3 ° C./s, a martensite structure may be formed, and ductility and fatigue damage resistance may decrease. 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 to 600 ° C: 1 to 10 ° C / s
After the primary cooling is stopped, the secondary cooling is performed. When the average cooling rate from the secondary cooling start temperature of 750 ° C. to the cooling stop temperature of the secondary cooling in the temperature range of 400 to 600 ° C. is less than 1 ° C./s, the lamellar spacing of the pearlite structure becomes coarse. Therefore, the hardness of the pearlite structure may decrease, and the fatigue damage resistance of the rail may decrease. In addition, the increased cooling time in the low temperature region may reduce productivity and increase rail manufacturing costs. On the other hand, if the average cooling rate of the secondary cooling exceeds 10 ° C./s, a martensite structure may be formed, and ductility and fatigue damage resistance may decrease. Therefore, the average cooling rate of the secondary cooling is preferably in the range of 1 to 10 ° C./s. The upper limit of the average cooling rate of the secondary cooling is more preferably 5 ° C./s.

In the above-mentioned primary and secondary cooling, the temperature for determining the average cooling rate is the surface temperature of the side surface of the rail head, and can be measured by a radiation thermometer. Here, the cooling stop temperature at the time of secondary cooling is the temperature measured by a radiation thermometer after the acceleration cooling stop (before reheating) on the side surface of the rail head.

Hereinafter, the configuration and action / effect 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 appropriately modified within a range that can be adapted to the gist of the present invention, all of which are included in the technical scope of the present invention. Will be.

The steel material having the composition shown in Table 1 was hot-rolled under the conditions shown in Table 2 and accelerated cooling after the hot-rolling to produce a rail material. Accelerated cooling was performed only on the rail head, and after cooling was stopped, it was allowed to cool. The rolling finish temperature in Table 2 is a value obtained by measuring the temperature of the surface of the side surface of the rail head on the entry side of the final rolling mill with a radiation thermometer as the rolling finish temperature. As the cooling stop temperature in Table 2, the value measured by the radiation thermometer for the temperature of the surface layer on the side surface of the rail head when the cooling of the secondary cooling is stopped is shown as the cooling stop temperature. The cooling rate was defined as the cooling rate (° C./s) by converting the temperature change from the start of cooling to the stop of cooling per unit time (seconds) for the primary cooling and the secondary cooling.

The obtained rails were evaluated for the old austenite particle size _RA , fatigue crack propagation characteristics, and fatigue damage resistance. Each evaluation content will be described in detail below.
<Old austenite particle size _RA >
After cutting the tip of the rail after hot finish rolling, the cut material was immediately water-cooled. With respect to the obtained water-cooled material, a test piece for microstructure observation was taken from the rolling longitudinal direction at a depth of 5 mm from the surface of the rail head 1 shown in FIG. The obtained test piece was subjected to γ-grain etching after mirror polishing, and a cross-sectional observation of 200 times was performed using an optical microscope. The old austenite particle size _RA was evaluated by measuring the particle size of 400 or more by tracing work using image analysis software and calculating the average value.

<Fatigue crack propagation characteristics>
Fatigue crack propagation test pieces were collected from two locations, the top of the rail and the gauge corner (GC) shown in FIG. 3, and the fatigue crack propagation test was performed. 4A and 4B are schematic views showing an example of a test piece, FIG. 4A shows a front view, FIG. 4B shows a side view, and FIG. 4C shows an enlarged front view of a notch portion. In FIG. 4, the test piece is, for example, a plate having a width W = 20 mm, a height H = 100 mm, and a thickness B = 5 mm, and has a notch at one width end of the central H / 2 portion of the height H. Is formed. The notch portion has a length L = 2 mm and a width C = 0.2 mm, and the end portion of the notch portion is formed with a curvature R = 0.1 mm. The stress ratio (R ratio = minimum stress / maximum stress) is 0.1, and the fatigue crack propagation velocity da / dN (m / cycle) ^{at the stress intensity factor ΔK = 20 MPa · m 1/2 is measured to withstand fatigue.} The crack propagation characteristics were evaluated. When the value of da / dN was 8.0 × ^10-8 or less, it was evaluated as having the fatigue crack propagation suppressing performance.

<Fatigue damage resistance>
Regarding fatigue damage resistance, it is most desirable to actually lay the rail and evaluate it, but it takes a long time to test. Therefore, we used a Nishihara-type wear tester that can evaluate fatigue resistance in a short time. Here, the fatigue damage resistance was evaluated by a comparative test simulating the actual contact conditions between the rail and the wheel. Specifically, the Nishihara-type wear test piece 17 having a diameter of 30 mm is collected from the rail head 1 with the contact surface as a curved surface having a radius of curvature R = 15 mm, and is brought into contact with the wheel test piece 18 and rotated as shown in FIG. Was tested. For the wheel test piece 18, first, a round bar having a diameter of 32 mm was taken from the head of an ordinary rail described in JIS standard E1101: 2012. Then, the round bar was heat-treated so that the Vickers hardness (load 98N) was Hv390 and the structure became a tempered martensite structure, and then the round bar was processed into a cylinder having a diameter of 30 mm and used for the test. As shown in FIG. 6, the Nishihara-type wear test piece 17 was collected from the fatigue-resistant damage test piece collecting section 14 on the surface layer of the rail head 1. The arrows in FIG. 5A indicate the rotation directions of the Nishihara-type wear test piece 17 and the wheel test piece 18, respectively. The test environment is oil lubrication conditions, contact pressure: 1.8 GPa, slip rate: -20%, rotation speed: 600 rpm (wheel test piece is 750 rpm), and observe the surface of the test piece every ^{2.5 x 10 4 times.} The number of revolutions at the time when a crack of 0.5 mm or more was generated was defined as the fatigue damage life. If this value is 8 × 10 ⁵ times or more, it is determined that there is fatigue damage resistance.

Table 2 also shows the results of the above survey. Test results of rail materials produced by the manufacturing method (heating temperature, rolling finish temperature) within the scope of the present invention using compatible steel satisfying the composition of the present invention and CP of 2500 or less (Test No. 1 to No. 1 in Table 2). The fatigue crack propagation rate of 20, ...) Satisfied with the fatigue crack propagation rate da / dN (m / cycle) of 8.0 × ^10-8 ^{or less at ΔK = 20 MPa · m 1/2.} In addition, the test No. 1 in which the primary cooling and secondary cooling conditions are in a suitable range. In 1 to 20, the fatigue crack propagation speed da / dN (m / cycle) was 8.0 × ^10-8 or less, and the fatigue damage life was 8 × 10 ⁵ times or more. On the other hand, in the comparative example (test Nos. 21 to 28 and 30 in Table 2) in which the component composition of the rail material did not satisfy the conditions of the present invention or was not applied to the production method within the scope of the present invention, CP was used. It exceeded 2500 and the fatigue crack propagation velocity da / dN (m / cycle) ^{exceeded 8.0 × 10-8} , or the fatigue damage life was less than ^{8 × 10 5 times.} In Test No. 29, the heating temperature was too high, so that a part of the steel material melted during heating. For this reason, it was not possible to perform rolling because there was a concern about breakage during rolling, and it was not possible to evaluate the characteristics.

According to the rail and the manufacturing method thereof according to the present invention, it is possible to stably manufacture a fatigue-resistant damage-resistant rail having excellent fatigue-resistant crack propagation characteristics, and it is possible to extend the life of rails for high-axis heavy railways and railways. It contributes to accident prevention and has an industrially beneficial effect.

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

Claims

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 It contains ~ 0.012% by mass, Cr: 0.20 ~ 2.50% by mass, has a component composition in which the balance is composed of Fe and unavoidable impurities, and has a CP represented by the following formula (1) of 2500 or less. A rail with excellent fatigue crack propagation characteristics.
CP = X / RA ... (1)
X = {(10 × [% C]) + ([% Si] / 12) + ([% Mn] / 24) + ([% Cr] / 21)} 5 ... (2)
However, [% Y]: content of element Y (mass%),
RA : Old austenite particle size (μm)
Represents.
Further, the component composition is 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. The rail having excellent fatigue crack propagation characteristics according to claim 1, which contains 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. The rail having excellent fatigue crack propagation characteristics according to claim 1 or 2, which contains at least one selected from the following.
A method for manufacturing a rail by heating a steel material having the component composition according to any one of claims 1 to 3 to 1350 ° C. or lower and then hot rolling to produce a rail, wherein the finishing temperature is 900. A method for manufacturing a rail having excellent fatigue-resistant crack propagation characteristics, which is characterized by hot rolling to a temperature of ℃ or higher.
After the hot rolling, acceleration cooling is performed from 900 ° C. to 750 ° C. at a cooling rate in the range of 0.4 to 3 ° C./s, and cooling stop temperature from 750 ° C. to 400 to 600 ° C. is 1 to 10 ° C./s. The method for manufacturing a rail having excellent fatigue crack propagation characteristics according to claim 4, wherein the rail is accelerated and cooled at a cooling rate in the range of.