Rail having excellent fatigue crack propagation resistance and method for manufacturing the same

Abstract

The present invention provides a rail having excellent fatigue damage resistance, particularly fatigue crack propagation resistance, and a preferred method for manufacturing the same. The rail has a composition comprising, by mass: C: 0.80 to 1.30%, Si: 0.10 to 1.20%, Mn: 0.20 to 1.80%, P: ≤0.035%, S: 0.0005 to 0.012%, Cr: 0.20 to 2.50%, and the balance consisting of Fe and unavoidable impurities, with CP = X / R A being set to 2500 or less (where X = {(10 x [%C]) + ([%Si] / 12) + ([%Mn] / 24) + ([%Cr] / 21)} 5 , and [%Y] is the content of element Y (mass %), R A is the prior austenite grain size (μm)). The method for manufacturing the rail includes: after heating a steel raw material to 1350°C or less, hot-rolling in such a way that the finish rolling temperature reaches 900°C or more.

Description

Technical Field

[0001] This invention relates to rails and methods for manufacturing the same, to rails with improved resistance to fatigue crack propagation, and to methods for manufacturing such rails that enable advantageous manufacturing of such rails. Background Technology

[0002] For high-axle-load railways, primarily used for transporting ore, the load on freight car axles is far greater than that on passenger cars, making the operating environment of the tracks more demanding. For tracks used in such environments, pearlitic steel has traditionally been used primarily due to its superior wear resistance. However, in recent years, to improve the efficiency of railway transportation, the load weight on freight cars is being further increased, requiring further improvements in the wear resistance and fatigue resistance of the tracks. It should be noted that high-axle-load railways refer to railways where the load weight of a single freight car in a train or freight train is large (e.g., around 150 tons or more).

[0003] Therefore, various studies have been conducted with the goal of further improving wear resistance. For example, in Patent Documents 1 and 2, the C content was increased to more than 0.85% by mass and less than 1.20% by mass. Furthermore, in Patent Documents 3 and 4, the C content was set to more than 0.85% by mass and less than 1.20% by mass, and heat treatment was performed on the track head. In these technologies, studies have been conducted on improving wear resistance by increasing the cementite fraction through increasing the C content.

[0004] On the other hand, for tracks in curved sections of high-axle-load railways, track wear becomes more severe due to the rolling stress from the wheels and the sliding force from centrifugal force, leading to fatigue damage caused by sliding. Therefore, Patent Document 5 proposes a technique to improve fatigue damage resistance by adding Al and Si to suppress the formation of proeutectoid cementite. Furthermore, Patent Document 6 proposes a technique to reduce the fatigue crack propagation rate by controlling the pearlite interlayer spacing within an appropriate range.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 8-109439

[0008] Patent Document 2: Japanese Patent Application Publication No. 8-144016

[0009] Patent Document 3: Japanese Patent Application Publication No. 8-246100

[0010] Patent Document 4: Japanese Patent Application Publication No. 8-246101

[0011] Patent Document 5: Japanese Patent Application Publication No. 2002-69585

[0012] Patent Document 6: Japanese Patent Application Publication No. 2010-185106 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] However, the aforementioned prior art still has the following unresolved problems.

[0015] As described in Patent Documents 1-4, when the C content is only between 0.85% and 1.20% by mass, a proeutectoid cementite structure is formed depending on the heat treatment conditions. Furthermore, the amount of brittle pearlitic lamellar cementite layers increases, thus failing to improve fatigue damage resistance. Additionally, in the technology described in Patent Document 5, the addition of Al generates oxides that become the initiation point for fatigue damage, making it difficult to suppress, in particular, the occurrence of fatigue cracks. Moreover, in the technology described in Patent Document 6, depending on the combination of composition and manufacturing conditions, a proeutectoid cementite structure may sometimes be formed, resulting in an increased fatigue crack propagation rate; therefore, it is difficult to consider material control to be adequate.

[0016] The present invention was made to advantageously solve the above-mentioned problems, and its object is to provide a track with excellent resistance to fatigue damage, especially fatigue crack propagation characteristics, and a preferred method for manufacturing the same.

[0017] Problem Solving Methods

[0018] To address the aforementioned problems, the inventors created orbitals by varying the contents of C, Si, Mn, and Cr, and conducted in-depth investigations into their microstructure and fatigue crack propagation characteristics. As a result, based on the compositional parameter X corresponding to the amount of proeutectoid cementite and the original austenite grain size R... A The parameter CP was derived. Furthermore, it was found that by controlling the parameter CP within a given range, excellent fatigue crack propagation resistance can be obtained even in the presence of a large amount of proeutectoid cementite.

[0019] The fatigue crack propagation resistance orbital of the present invention, developed to solve the above-mentioned problems and achieve the above-mentioned objectives, has the following composition: C: 0.80-1.30 wt%, Si: 0.10-1.20 wt%, Mn: 0.20-1.80 wt%, P: less than 0.035 wt%, S: 0.0005-0.012 wt%, Cr: 0.20-2.50 wt%, with the balance being Fe and unavoidable impurities. The CP of this orbital, expressed by the following formula (1), is less than 2500, where [%Y] is the content of element Y (mass %), R... A The original austenite grain size (μm) is given.

[0020] CP = X / R A ···(1)

[0021] X={(10×[%C])+([%Si] / 12)+([%Mn] / 24)+([%Cr] / 21)} 5 ···(2)

[0022] It should be noted that, for the track of the present invention with excellent fatigue crack propagation resistance,

[0023] a. The above-mentioned composition further contains at least one component selected from 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 or less;

[0024] b. The above-mentioned composition may further contain at least one selected from Al: less than 0.07% by mass, W: less than 1.0% by mass, B: less than 0.005% by mass, Ti: less than 0.05% by mass, and Sb: less than 0.05% by mass; etc., which may be a more preferred solution.

[0025] The present invention, developed to address the aforementioned issues and achieve the aforementioned objectives, provides a method for manufacturing a track with excellent fatigue crack propagation resistance. This method involves hot rolling a steel raw material having any of the aforementioned compositions after heating it to below 1350°C. The method includes hot rolling such that the finishing temperature reaches above 900°C.

[0026] It should be noted that, for the manufacturing method of the track with excellent fatigue crack propagation resistance of the present invention, after the above-mentioned hot rolling, accelerating the cooling from 900°C to 750°C at a cooling rate in the range of 0.4 to 3°C / second, and accelerating the 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 / second, can be considered a more preferred solution.

[0027] The effects of the invention

[0028] The track and manufacturing method of the present invention can stably manufacture fatigue-damage resistant tracks with excellent fatigue crack propagation resistance characteristics, which helps to extend the service life of high axle load railway tracks, prevent railway accidents, and bring beneficial effects to industry.

[0029] In addition, by properly controlling the heat treatment conditions after hot rolling, fatigue damage resistance can be improved, making it the preferred option. Attached Figure Description

[0030] Figure 1 This is a schematic diagram showing the effect of proeutectoid cementite on fatigue crack propagation rate. (a) indicates the case where the original austenite grain size is basically equal to the plastic zone size, and (b) indicates the case where the original austenite grain size is larger than the plastic zone size.

[0031] Figure 2 This diagram shows the location of the test pieces used for observing the original austenite grain size.

[0032] Figure 3 This is a diagram showing the location of the fatigue crack propagation test specimen.

[0033] Figure 4 The diagram illustrates the shape of the test piece used in fatigue crack propagation tests. (a) shows the front view, (b) shows the side view, and (c) shows the enlarged front view of the cut portion.

[0034] Figure 5 The diagram illustrates the shape of the test piece used for fatigue damage testing. (a) shows a side view, and (b) shows a front view.

[0035] Figure 6 This is a diagram showing the location where fatigue damage test pieces were collected.

[0036] Symbol Explanation

[0037] 1. Track Head

[0038] 11. Sample collection section for observing the original austenite grain size

[0039] 12. Gauge Corner (GC)

[0040] 13 Top of head

[0041] 14 Fatigue Damage Test Piece Collection Section

[0042] 15 Fatigue crack propagation test specimens

[0043] 16. Incision site

[0044] 17. Nishihara type wear test piece

[0045] 18 tire test pieces

[0046] 21 original austenite grains

[0047] 22 Plastic Zone

[0048] 23 Fatigue cracks

[0049] 24. Proeutectoid cementite

[0050] 25 Splitting Damage

[0051] 26. Increased fatigue crack propagation rate

[0052] 27. Fatigue crack propagation rate decreased

[0053] R A Original austenite grain size

[0054] R P Plastic zone size Detailed Implementation

[0055] The embodiments of the present invention will now be described in detail. First, the reasons for limiting the composition of the steel, which will become the raw material for the track, to the above-mentioned range will be explained in the present invention. It should be noted that, unless otherwise specified, "%" in the following description means "mass %".

[0056] C: 0.80~1.30%

[0057] Carbon (C) is an essential element for ensuring the strength of pearlitic microstructure, i.e., its resistance to fatigue damage. However, when the C content is below 0.80%, it is difficult to obtain excellent fatigue crack propagation characteristics. Furthermore, when the C content exceeds 1.30%, a large amount of proeutectoid cementite is generated at the austenite grain boundaries during cooling after hot rolling, leading to an increase in the fatigue crack propagation rate. It should be noted that even when the C content is below 1.30%, although proeutectoid cementite is present, its influence can be avoided by controlling the proto-austenite grain size based on the relationship described later. Therefore, the C content is set in the range of 0.80% to 1.30%. It should be noted that the upper limit of the C content is preferably 1.00%, and more preferably 0.90%.

[0058] Si: 0.10–1.20%

[0059] Besides acting as a deoxidizer, silicon (Si) also helps reduce the fatigue crack propagation rate by raising the equilibrium phase transformation temperature of pearlite and making the interlayer spacing finer. Therefore, a content of 0.10% or more is required, but when it exceeds 1.20%, the weldability deteriorates due to the high binding force of Si with oxygen. Furthermore, Si has the effect of shifting the eutectoid point towards the lower carbon side; therefore, excessive addition will promote the formation of proeutectoid cementite, leading to an increase in the fatigue crack propagation rate. Therefore, the Si content is set in the range of 0.10% to 1.20%. It should be noted that the lower limit of the Si content is preferably 0.20%, and the upper limit is preferably 0.80%, more preferably 0.60%.

[0060] Mn: 0.20~1.80%

[0061] Manganese (Mn) helps reduce fatigue crack propagation rate by lowering the pearlite phase transformation temperature and making the interlayer spacing finer. However, sufficient effect cannot be obtained when the Mn content is less than 0.20%. On the other hand, when the Mn content exceeds 1.80%, martensite structure is easily formed, which hardens and becomes embrittled during heat treatment and welding of the track, leading to material deterioration. In addition, Mn has the effect of shifting the eutectoid point towards the lower carbon side; therefore, excessive addition will promote the formation of proeutectoid cementite, resulting in an increase in fatigue crack propagation rate. Therefore, the Mn content is set in the range of 0.20% to 1.80%. It should be noted that the lower limit of Mn content is preferably 0.30%, and the upper limit of Mn content is preferably 1.00%, more preferably 0.60%.

[0062] P: below 0.035%

[0063] The presence of phosphorus (P) exceeding 0.035% will worsen the ductility. Therefore, the P content is set to 0.035% or less, preferably 0.020% or less. On the other hand, there is no particular limitation on the lower limit of the P content, which can be 0%, but it is common practice in industry to exceed 0%. It should be noted that excessively reducing the P content will lead to an increase in refining costs; therefore, from an economic point of view, it is preferable to set the P content to 0.001% or more.

[0064] S: 0.0005~0.012%

[0065] Sulfur (S) mainly exists in steel as type A inclusions (obtained through viscous deformation during processing). However, when its content exceeds 0.012%, the amount of these inclusions increases significantly, resulting in the formation of large inclusions and consequently, a decrease in the cleanliness of the steel. Furthermore, keeping the S content below 0.0005% leads to increased refining costs. Therefore, the S content is set within the range of 0.0005% to 0.012%. It should be noted that the upper limit of the S content is preferably 0.010%, and more preferably 0.008%.

[0066] Cr: 0.20–2.50%

[0067] Chromium (Cr) contributes to reducing the fatigue crack propagation rate by increasing the equilibrium phase transformation temperature of pearlite and making the interlayer spacing finer. However, when the Cr content is less than 0.20%, fatigue crack propagation cannot be sufficiently suppressed. On the other hand, when the Cr content exceeds 2.50%, the hardenability of the steel increases, and martensite is easily formed. Furthermore, when manufacturing under conditions that do not produce martensite, proeutectoid cementite is formed at the original austenite grain boundaries. Therefore, the fatigue crack propagation rate increases. Therefore, the Cr content is set in the range of 0.20% to 2.50%. It should be noted that 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%, more preferably 1.00%.

[0068] Furthermore, in this invention, it is insufficient to merely satisfy the above-mentioned ranges for each element; it is important to control the CP value, expressed by the following formula (1), to be below 2500. This CP value is determined by the compositional parameter X corresponding to the amount of proeutectoid cementite expressed by the following formula (2) and the original austenite grain size R. A Exported.

[0069] CP = X / R A ···(1)

[0070] X={(10×[%C])+([%Si] / 12)+([%Mn] / 24)+([%Cr] / 21)} 5 ···(2)

[0071] In the formula, [%Y] represents the content of element Y (mass%).

[0072] R A This indicates the original austenite grain size (μm).

[0073] The inventors investigated the reason why the fatigue crack propagation rate increases due to the presence of proeutectoid cementite. The results yielded the following insights: Figure 1As shown in the schematic diagram (a), brittle fracture occurs first in the proeutectoid cementite 24 at the tip of fatigue crack 23, which is the main reason for the increased fatigue crack propagation rate 26. Furthermore, it is known that by adjusting the grain size of the proto-austenite at the formation site of the structure in accordance with the amount of proeutectoid cementite, the frequency of encounters between the plastic zone 22 formed at the fatigue crack tip and the proeutectoid cementite is reduced, thus suppressing brittle crack propagation. Specifically, even in the case of a large amount of proeutectoid cementite, such as Figure 1 As shown in (b), by sufficiently coarsening the original austenite grains 21 compared to the size of the plastic zone 22 at the crack tip, the CP value can also be controlled to be below 2500. This allows for a stable suppression effect on the fatigue crack propagation rate described above. It should be noted that the CP value is preferably set to 2000 or below.

[0074] In addition to the components described above, the orbitals used in this invention may also contain any one or both of the components selected from group A and group B.

[0075] Group A: V: below 0.30%, Cu: below 1.0%, Ni: below 1.0%, Nb: below 0.05%, and Mo: below 2.0%.

[0076] Group B: Al: less than 0.07%, W: less than 1.0%, B: less than 0.005%, Ti: less than 0.05%, and Sb: less than 0.05%.

[0077] The reasons for limiting the content of elements belonging to Group A and Group B are explained below.

[0078] V: Below 0.30%

[0079] Vanadium (V) forms carbonitrides in steel and disperses into the base steel plate, improving the steel's wear resistance. However, when its content exceeds 0.30%, processability deteriorates and manufacturing costs increase. Furthermore, when the V content exceeds 0.30%, alloy costs increase, thus increasing the cost of internal high-hardness track types. Therefore, it is preferable to contain V with an upper limit of 0.30%. It should be noted that, to exhibit the aforementioned effect of improving wear resistance, a V content of 0.001% or more is preferred. It should also be noted that an upper limit of 0.15% is more preferable for the V content.

[0080] Cu: below 1.0%

[0081] Like Cr, Cu (copper) is an element that can further enhance the strength of steel through solid solution strengthening. However, when its content exceeds 1.0%, Cu cracking is likely to occur. Therefore, when Cu is present in the composition, the Cu content is preferably set to 1.0% or less. It should be noted that the lower limit of Cu content is more preferably 0.005%, and the upper limit of Cu content is more preferably 0.5%.

[0082] Ni: below 1.0%

[0083] Ni (Ni) is an element that can achieve high strength in steel without compromising its ductility. Furthermore, by adding it in combination with Cu, Cu cracking can be suppressed; therefore, when Cu is present in the composition, Ni is also preferred. However, when the Ni content exceeds 1.0%, the hardenability of the steel further increases, the formation of martensite and bainite increases, and wear resistance and fatigue damage resistance tend to decrease. Therefore, when Ni is present, the Ni content is preferably set to 1.0% or less. It should be noted that 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%.

[0084] Nb: below 0.05%

[0085] Niobium (Nb) combines with carbon (C) in steel during and after hot rolling for rail forming, precipitating as carbides and effectively contributing to the refinement of pearlite cluster size. This results in significantly improved wear resistance, fatigue damage resistance, and ductility, significantly contributing to the long service life of internally high-hardness rails. However, even when the Nb content exceeds 0.05%, the improvement in wear resistance and fatigue damage resistance becomes saturated, and the effect cannot be obtained commensurate with the increase in content. Therefore, the upper limit of its content can be set at 0.05%. It should be noted that when the Nb content is less than 0.001%, it is difficult to obtain sufficient effect in terms of extending the rail's service life. Therefore, when Nb is present, the Nb content is preferably 0.001% or more. It should be noted that the upper limit of the Nb content is more preferably 0.03%.

[0086] Mo: 2.0% or less

[0087] Mo (Mo) is an element that can further increase the strength of steel through solid solution strengthening. In addition, Mo has the effect of shifting the eutectoid point towards the higher carbon content, thus also inhibiting the formation of proeutectoid cementite. However, when the content exceeds 2.0%, the amount of bainite formed in the steel increases, reducing wear resistance. Therefore, when the composition of the rail contains Mo, the Mo content is preferably set to 2.0% or less. It should be noted that 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%.

[0088] Al: below 0.07%

[0089] Al (aluminum) is an element that can be added as a deoxidizer. However, when the Al content exceeds 0.07%, due to Al's high affinity for oxygen, a large number of oxide inclusions are formed in the steel, resulting in reduced ductility. Therefore, the Al content is preferably set to 0.07% or less. On the other hand, there is no particular limitation on the lower limit of the Al content, but for deoxidation purposes, it is preferably set to 0.001% or more. It should be noted that the upper limit of the Al content is more preferably 0.03%.

[0090] W: Below 1.0%

[0091] Tungsten (W) precipitates as carbides during and after hot rolling to form a rail shape, improving the strength and ductility of the rail through precipitation strengthening. However, when the W content exceeds 1.0%, martensite forms in the steel, resulting in reduced ductility. Therefore, when adding W, it is preferable to set the W content to 1.0% or less. On the other hand, there is no particular limitation on the lower limit of the W content, but to exhibit the aforementioned effects of improving strength and ductility, it is preferable to set it to 0.001% or more. It should be noted that 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%.

[0092] B: Below 0.005%

[0093] Boron (B) precipitates in steel as nitrides during and after hot rolling to form a track shape, thereby improving the steel's strength and ductility through precipitation strengthening. However, when the B content exceeds 0.005%, martensite is formed, resulting in a decrease in the steel's ductility. Therefore, when B is present, it is preferable to set the B content to 0.005% or less. On the other hand, there is no particular limitation on the lower limit of the B content, but to exhibit the aforementioned effect of improving strength and ductility, it is preferable to set it to 0.001% or more. It should be noted that the upper limit of the B content is more preferably 0.003%.

[0094] Ti: below 0.05%

[0095] Titanium (Ti) precipitates in steel as carbides, nitrides, or carbonitrides during and after hot rolling to form a track shape, thereby improving the steel's strength and ductility through precipitation strengthening. However, when the Ti content exceeds 0.05%, coarse carbides, nitrides, or carbonitrides are formed, resulting in reduced ductility of the steel. Therefore, when Ti is present, it is preferable to set the Ti content to 0.05% or less. On the other hand, there is no particular limitation on the lower limit of the Ti content, but to exhibit the aforementioned effect of improving strength and ductility, it is preferable to set it to 0.001% or more. It should be noted that 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%.

[0096] Sb: below 0.05%

[0097] Antimony (Sb) has a significant effect in preventing decarburization of rail steel raw materials during reheating in a furnace before hot rolling. However, when the Sb content exceeds 0.05%, it adversely affects the ductility and toughness of the steel. Therefore, in cases where Sb is present, it is preferable to set the Sb content to 0.05% or less. On the other hand, there is no particular limitation on the lower limit of the Sb content, but to exhibit the effect of reducing the decarburized layer, it is preferable to set it to 0.001% or more. It should be noted that 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%.

[0098] The steel raw material used as the material for the track of the present invention comprises the above-mentioned components, with the balance being Fe and unavoidable impurities. Tracks obtained by containing other trace elements to replace a portion of the balance Fe in the composition of the present invention, within a range that does not substantially affect the effect of the present invention, are also considered part of the present invention. Among these unavoidable impurities, examples include N and O, with N permitted to be no more than 0.008% and O no more than 0.004%.

[0099] It should be noted that the microstructure of the track in this invention, excluding pearlite, is not particularly limited. When the total area fraction is less than 5%, it will not significantly affect the fatigue crack propagation resistance; therefore, other microstructures are permissible. Examples of such other microstructures include ferrite, proeutectoid cementite, bainite, and martensite.

[0100] Next, the manufacturing method of the track of the present invention described above will be explained. The track of the present invention can be manufactured by sequentially performing the following processes (1) to (3) on steel raw materials having the above-described composition.

[0101] (1) Hot rolling

[0102] (2) Primary cooling

[0103] (3) Secondary cooling

[0104] The steel raw materials used as rail raw materials can be manufactured by any method, but casting, especially continuous casting, is generally preferred.

[0105] (1) Hot rolling

[0106] First, the aforementioned steel raw material is hot-rolled to form a track shape. In this invention, by controlling the finishing rolling temperature during the hot rolling process, the original austenite grain size of the final track can be controlled. Therefore, the hot rolling method is not particularly limited and can be carried out by any method.

[0107] Heating temperature: below 1350℃

[0108] In the heating of steel raw materials prior to hot rolling, the heating temperature needs to be set below 1350°C. If the heating temperature exceeds the upper limit, the steel raw material may partially melt due to excessive heating, posing a potential risk of defects forming inside the rolling mill. On the other hand, there is no particular restriction on the lower limit of the heating temperature; however, to reduce deformation resistance during rolling, it is preferable to set it above 1150°C.

[0109] Finishing rolling temperature: above 900℃

[0110] When the finishing temperature in the aforementioned hot rolling process is below 900°C, rolling within the low-temperature range of austenite does not introduce processing strain into the austenite grains, and the elongation of the austenite grains becomes significant. The increase in austenite grain boundary area increases the nucleation sites for proeutectoid cementite, resulting in a decrease in fatigue crack propagation resistance. Therefore, the finishing temperature is set above 900°C. On the other hand, there is no particular limit to the upper limit of the finishing temperature; however, if the original austenite grain size becomes extremely coarse, ductility and toughness decrease. Therefore, it is preferable to set it below 1050°C. It should be noted that the finishing temperature described here refers to the temperature on the side of the track head at the entry point of the final rolling mill, which can be measured using a radiation thermometer.

[0111] (2) Primary cooling

[0112] Average cooling rate from 900℃ to 750℃: 0.4~3℃ / second

[0113] Next, accelerated cooling is performed. During this initial cooling, if the average cooling rate within the preeutectoid cementite formation temperature range (900°C to 750°C) is less than 0.4°C / second, the amount of preeutectoid cementite increases. Therefore, the preeutectoid cementite structure is prone to fracture, potentially reducing the fatigue resistance of the track. Therefore, the lower limit of the average cooling rate for this initial cooling is preferably set to 0.4°C / second, more preferably 0.7°C / second. On the other hand, if the average cooling rate for this initial cooling exceeds 3°C / second, martensite is formed, potentially reducing ductility and fatigue resistance. Therefore, the upper limit of the average cooling rate for this initial cooling is preferably set to 3°C / second, more preferably 2°C / second.

[0114] (3) Secondary cooling

[0115] Average cooling rate over the temperature range of 750℃ to 400~600℃: 1~10℃ / second

[0116] A second cooling process is performed after the first cooling is completed. When the average cooling rate from the second cooling start temperature (750°C to 400-600°C) to the cooling stop temperature is less than 1°C / second, the interlayer spacing of the pearlite structure becomes coarser. Therefore, there is a risk of reduced hardness of the pearlite structure and decreased fatigue resistance of the track. Furthermore, the increased cooling time in the low-temperature range leads to reduced productivity and increased manufacturing costs for the track. On the other hand, when the average cooling rate of the second cooling exceeds 10°C / second, martensite is formed, which reduces ductility and fatigue resistance. Therefore, the average cooling rate of the second cooling is preferably set to a range of 1-10°C / second. It should be noted that the upper limit of the average cooling rate of the second cooling is more preferably 5°C / second.

[0117] It should be noted that in the primary and secondary cooling processes described above, the temperature used to determine the average cooling rate is the surface temperature of the track head side, which can be measured using a radiation thermometer. Here, the cooling stop temperature during secondary cooling is defined as the temperature obtained by measuring the temperature of the track head side after accelerated cooling has stopped (before reheating) using a radiation thermometer.

[0118] Example

[0119] The structure and effects of the present invention will be described in more detail below with reference to embodiments. However, the present invention is not limited to the following embodiments, and appropriate modifications can be made within the scope of the present invention, all of which are included in the technical scope of the present invention.

[0120] Rail material was manufactured by hot rolling and accelerated cooling after hot rolling of steel raw materials with the composition shown in Table 1, under the conditions shown in Table 2. Accelerated cooling was performed only on the rail head, followed by natural cooling after cooling was stopped. The finishing rolling temperature in Table 2 refers to the temperature of the side surface of the rail head at the final mill entry side, measured with a radiation thermometer. The cooling stop temperature in Table 2 refers to the temperature of the side surface of the rail head at the end of the secondary cooling, measured with a radiation thermometer. For the cooling rate, the temperature change from the start to the stop of cooling during the primary and secondary cooling processes is converted into an average unit time (seconds) and expressed as the cooling rate (°C / second).

[0121]

[0122]

[0123]

[0124]

[0125] The obtained orbitals were used to evaluate the original austenite grain size R. A The evaluation criteria include fatigue crack propagation characteristics and fatigue damage resistance. The following sections provide a detailed explanation of each evaluation item.

[0126] < Original austenite grain size R A >

[0127] After the front end of the hot-rolled and precision-rolled rail is cut off, the cut material is immediately subjected to water cooling treatment. The resulting water-cooled material is then... Figure 2 Test pieces for microstructure observation were collected along the rolling length from a depth of 5 mm on the surface of track head 1. After mirror polishing, the obtained test pieces underwent gamma-grain etching, and cross-sectional observation was performed using an optical microscope at 200x magnification. By using image analysis software for tracking, over 400 grain sizes were measured, and their average values ​​were calculated. This was used to determine the original austenite grain size R. A An evaluation was conducted.

[0128] <Fatigue Crack Propagation Characteristics>

[0129] from Figure 3 Fatigue crack propagation test specimens were collected from the top of the track head and the transition rounded corner (GC) section, as shown, and fatigue crack propagation tests were conducted. Figure 4 This is a schematic diagram showing an example of a test piece. Figure 4 (a) Shows a front view. Figure 4 (b) shows a side view. Figure 4(c) shows an enlarged front view of the cut. Figure 4 In this test specimen, for example, it is a plate with a width W = 20 mm, a height H = 100 mm, and a thickness B = 5 mm, and a notch is formed at one end of the width portion H / 2 of the height H. The length L = 2 mm and the width C = 0.2 mm of the notch, and the end of the notch is formed with a curvature R = 0.1 mm. The stress ratio (R ratio = minimum stress / maximum stress) is set to 0.1, and the stress intensity factor ΔK = 20 MPa·m is used. 1 / 2 The fatigue crack propagation rate da / dN (m / cycle) was measured to evaluate the fatigue crack propagation resistance. The value of da / dN was 8.0 × 10⁻⁶. -8 The following conditions are considered to indicate that it has fatigue crack propagation suppression performance.

[0130] <Resistance to fatigue damage>

[0131] Regarding fatigue damage resistance, the optimal method is to evaluate it by actually laying the track, but such tests take a long time. Therefore, the Nishihara wear testing machine, which can evaluate fatigue damage resistance in a short time, was used. Here, fatigue damage resistance was evaluated through a comparative test simulating the actual contact conditions between the track and the wheel. Specifically, the contact surface was set as a curved surface with a radius of curvature R = 15 mm, and a Nishihara wear test piece 17 with a diameter of 30 mm was collected from the track head 1. Figure 5 The test was conducted by contacting and rotating the wheel test piece 18. First, for the wheel test piece 18, a 32mm diameter round bar was taken from the head of a standard rail as described in JIS standard E1101:2012. Then, the round bar was heat-treated to achieve a Vickers hardness (load 98N) of Hv390 and a tempered martensitic microstructure. It was then machined into a 30mm diameter cylinder for testing. It should be noted that for the Nishihara-type wear test piece 17, as... Figure 6 As shown, the fatigue damage test piece was collected from the surface of the track head 1 by the sampling section 14. Figure 5 The arrows in (a) indicate the rotation directions of the Nishihara wear test piece 17 and the wheel test piece 18, respectively. The test environment was set to oil sliding conditions, with a contact pressure of 1.8 GPa, a slip ratio of -20%, and a rotation speed of 600 rpm (750 rpm for the wheel test piece), every 2.5 × 10 4 The fatigue damage life was determined by observing the surface of the test piece and counting the number of revolutions at which cracks larger than 0.5 mm appeared. This value was 8 × 10⁻⁶. 5 When the damage occurs more than once, it is determined to have fatigue resistance.

[0132] The results of the above investigation are also shown in Table 2. The fatigue crack propagation rates of the rail materials (Tests No. 1 to 20, ... in Table 2) prepared using suitable steel with a composition satisfying the present invention and a CP of 2500 or less, and manufactured by the manufacturing method (heating temperature, finishing rolling temperature) within the scope of the present invention, all satisfy ΔK = 20 MPa·m. 1 / 2 The fatigue crack propagation rate da / dN (m / cycle) is 8.0 × 10⁻⁶. -8 The following are examples. Furthermore, tests No. 1 to 20, where the primary and secondary cooling conditions are within the preferred range, all satisfy a fatigue crack propagation rate da / dN (m / cycle) of 8.0 × 10⁻⁶. -8 The following, and the fatigue damage life is 8×10 5 More than once. On the other hand, comparative examples (tests No. 21-28, 30 in Table 2) whose track material composition does not meet the conditions of this invention or whose manufacturing methods do not apply the scope of this invention have a CP exceeding 2500 and a fatigue crack propagation rate da / dN (m / cycle) exceeding 8.0 × 10⁻⁶. -8 Or fatigue damage lifespan less than 8×10 5 It should be noted that the heating temperature in Test No. 29 was too high, causing some of the steel raw material to melt during heating. Therefore, there is a risk of breakage during rolling, making it unsuitable for rolling and unable to undergo property evaluation.

[0133] Industrial applicability

[0134] The track and manufacturing method of the present invention can stably manufacture fatigue-damage resistant tracks with excellent fatigue crack propagation resistance characteristics, which helps to extend the service life of high axle load railway tracks, prevent railway accidents, and bring beneficial effects to industry.

Claims

1. A track with excellent fatigue crack propagation resistance, comprising the following components: C: 0.80~1.30 wt%, Si: 0.10~1.20 wt%, Mn: 0.20~1.80 wt%, P: less than 0.035 wt%, S: 0.0005~0.012 wt%, Cr: 0.20~2.50 wt%, with the balance consisting of Fe and unavoidable impurities. The microstructure of the orbital, excluding pearlite, comprises less than 5% of the total area. The CP of this orbital, expressed by the following formula (1), is 2500 or less. CP=X / R A ···(1) X={(10×[%C])+([%Si] / 12)+([%Mn] / 24)+([%Cr] / 21)} 5 ···(2) In the formula, [%Y] represents the content (mass%) of element Y. R A This indicates the original austenite grain size (μm).

2. The track with excellent fatigue crack propagation resistance according to claim 1, wherein, The composition also contains at least one element selected from the following: V: less than 0.30% by mass, Cu: less than 1.0% by mass, Ni: less than 1.0% by mass, Nb: less than 0.05% by mass, and Mo: less than 2.0% by mass.

3. The track with excellent fatigue crack propagation resistance according to claim 1 or 2, wherein, The composition also contains at least one element selected from the following: Al: less than 0.07% by mass, W: less than 1.0% by mass, B: less than 0.005% by mass, Ti: less than 0.05% by mass, and Sb: less than 0.05% by mass.

4. A method for manufacturing a track with excellent fatigue crack propagation resistance, as described in any one of claims 1 to 3, comprising: When hot rolling is performed on steel raw materials having the aforementioned composition after heating them to 1150~1350°C to manufacture rails, Hot rolling is performed at a finishing temperature of 900~1050℃, and the CP represented by the formula (1) is controlled to be below 2500.

5. The method for manufacturing a track with excellent fatigue crack propagation resistance according to claim 4, wherein, After hot rolling, the temperature is accelerated from 900°C to 750°C at a cooling rate in the range of 0.4 to 3°C / second, and then accelerated 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 / second.

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