A heat treatment method for reducing the rolling contact fatigue damage of a subway rail in a humid environment and a rail structure thereof

Abstract

The present application belongs to the field of rail material research and manufacturing, and specifically discloses a rail structure for a subway humid environment. According to the distribution of pearlite lamellar spacing, the rail head cross-section structure can be divided into cap layer 1, cap layer 2, cap layer 3 and base material layer. The pearlite lamellar spacing distribution of cap layer 1 is 130-150 nm, the pearlite lamellar spacing distribution of cap layer 2 is 90-120 nm, the pearlite lamellar spacing distribution of cap layer 3 is 140-170 nm, and the pearlite lamellar spacing distribution of the base material layer is 200-230 nm. The present application designs cap-shaped structures with different depths and different pearlite lamellar spacing in the rail head. By accelerating the cooling speed of the heat treatment of the hot-rolled rail in different stages, the reasonable gradient structure distribution of the pearlite structure of the rail material is realized, the expansion speed of the rolling contact fatigue crack of the rail in the humid environment to the internal material is effectively slowed down, the rolling contact fatigue damage is reduced, and thus the comprehensive performance and service life of the rail are improved.

Description

Technical Field

[0001] This invention relates to a heat treatment method for reducing rolling contact fatigue damage of subway rails in humid environments and the microstructure of the rails thereof, belonging to the field of rail material research and manufacturing. Background Technology

[0002] With the rapid urbanization and socio-economic development, urban rail transit construction has shown a booming trend. In the urban rail transit system, subways have become the main form of urban rail transit. Rolling contact fatigue damage is one of the most common types of damage to subway rails. Specifically, it refers to fatigue damage caused by the accumulation of plastic deformation of the rail material due to the cyclic contact stress between the wheel and rail under the repeated load of train wheels. Initially, it mainly manifests as surface fatigue cracks, which gradually evolve into spalling after a period of development, directly affecting the stability and safety of subway train operation.

[0003] Compared to national railway lines, subway lines are characterized by frequent train acceleration and deceleration, numerous small-radius curves, and high train operating frequency. Subway rails are more prone to microcracks due to accumulated surface plastic deformation. Furthermore, the lighter axle load of subway trains makes it difficult to grind away these microcracks on the rail surface, leading to more severe rolling contact fatigue damage. More importantly, because subway lines extend underground, especially in cities and regions with abundant groundwater, water seepage and seasonal dampness are unavoidable. Subway lines are enclosed spaces with less ventilation than exposed rails, resulting in a more humid environment. Extensive practical experience shows that compared to dry track environments, the development rate of rolling contact fatigue damage on rail surfaces increases significantly in humid environments. Rolling contact fatigue cracks on the rail surface rapidly develop into spalling damage, and in severe cases, even lead to rail fracture accidents. This necessitates frequent rail inspections, grinding, and replacement, severely impacting train safety and track maintenance costs. The development and control of rail fatigue damage in humid subway environments has become a key technical issue affecting the development of urban rail transit. Summary of the Invention

[0004] The technical problem this invention aims to solve is the rapid development of rolling contact fatigue damage in subway rails under humid conditions. It provides a heat treatment method and rail microstructure to reduce rolling contact fatigue damage in subway rails under humid conditions. This invention designs a cap-shaped microstructure with different depths and pearlite lamellar spacings at the rail head. By subjecting the hot-rolled rail to accelerated cooling heat treatment at different stages, a reasonable gradient distribution of pearlite microstructure in the rail material is achieved. This effectively slows down the propagation rate of rolling contact fatigue cracks into the material under humid conditions, thus reducing rolling contact fatigue damage.

[0005] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows: A rail microstructure for use in humid environments like subways, wherein the full cross-section of the rail has a microstructure consisting of lamellar pearlite and a small amount of ferrite, with the ferrite volume percentage being less than 10%. Based on the different distribution of pearlite lamellar spacing, the rail head cross-section microstructure can be divided into cap-shaped layer 1, cap-shaped layer 2, cap-shaped layer 3, and a substrate layer. Specifically, the pearlite lamellar spacing of the cap-shaped layer 1 is 130-150 nm (based on the average spacing of each pearlite lamellar layer, the same below) extending from the center line of the rail head surface downwards to a depth of 2-3 mm inside the rail head; the pearlite lamellar spacing of the cap-shaped layer 2 is 90-120 nm (excluding the cap-shaped layer 1) extending from the depth of the cap-shaped layer 1 to a depth of 7-8 mm inside the rail head; the pearlite lamellar spacing of the cap-shaped layer 3 is 140-170 nm (excluding the cap-shaped layer 2) extending from the depth of the cap-shaped layer 2 to a depth of 13-15 mm inside the rail head; the pearlite lamellar spacing of the substrate layer is 200-230 nm (excluding the cap-shaped layer 3) extending from the depth of the cap-shaped layer 3 to the substrate inside the rail head; and the pearlite lamellar spacing of the rail web and rail bottom is 180-220 nm.

[0006] This invention also provides a heat treatment method for reducing rolling contact fatigue damage to subway rails in humid environments, which can obtain rails with the aforementioned microstructure. The heat treatment method involves performing a phased accelerated cooling heat treatment on hot-rolled rails with a certain temperature, or on rails that have been reheated offline. The process involves four stages: First, the rail head area is cooled at a rate of 6-8℃ / s for 25-30 seconds. Second, the rail head area is cooled at a rate of 12-14℃ / s for 12-16 seconds, followed by a holding period of 8-10 seconds. Third, the rail head, web, and base areas are cooled at rates of 5-7℃ / s, 2-3℃ / s, and 3-4℃ / s for 30-35 seconds. Fourth, the rail head and base areas are cooled at rates of 1.5-3℃ / s and 0.5-1℃ / s for 50-60 seconds, followed by a cooling period of 50-60 seconds. After cooling, the rails are allowed to cool naturally to room temperature.

[0007] According to the above scheme, a humid environment is one where the surface of the rails comes into contact with a liquid medium. Subways are enclosed environments, and leaks, splashes, or condensation caused by excessive humidity will all form a water-based medium on the rail surface.

[0008] Preferably, the surface temperature of the rail at the start of accelerated cooling is 830-850℃. In the first cooling stage, the rail head area is accelerated at a rate of 6.5-7.5℃ / s for 26-28 seconds. In the second cooling stage, the rail head area is accelerated at a rate of 13-14℃ / s for 14-16 seconds, followed by a cooling stop and a holding period of 9-10 seconds. In the third cooling stage, the rail head, rail web, and rail... The rail head and rail base areas undergo accelerated cooling, with a cooling rate of 5.5-6.5℃ / s for the rail head, 2.2-2.8℃ / s for the rail web, and 3.4-3.8℃ / s for the rail base, for a cooling time of 32-35s. In the fourth cooling stage, the rail head and rail base areas are accelerated cooled separately, with a cooling rate of 2-3℃ / s for the rail head and 0.5-1℃ / s for the rail base, for a cooling time of 55-60s. After this, accelerated cooling is stopped, and the rails are allowed to cool naturally to room temperature.

[0009] Furthermore, the heat treatment method is applicable to the composition system of pearlitic steel rails widely used in subway lines. The inventors have also discovered that rails with a specific chemical composition exhibit less rolling contact fatigue damage compared to rails with other chemical compositions, and are more suitable for this production method. This rail with a specific chemical composition, based on the total weight of the rail, comprises, by mass percentage: C 0.70-0.82%, Si 0.55-0.80%, Mn 0.85-1.00%, V 0.05-0.15%, P≤0.025%, S≤0.025%, with the remainder being Fe and unavoidable impurities.

[0010] Currently, the mainstream rail materials used in national railway lines and urban rail transit lines are hypoeutectoid steel or eutectoid steel, with a microstructure consisting mainly of pearlite and a small amount of ferrite. To address the problem of rolling contact fatigue damage in rails, existing technologies mainly employ two approaches: one is alloying, which involves adding strengthening elements such as V and Cr to improve the strength and toughness of the rail material through solid solution strengthening and precipitation strengthening, thereby increasing the yield strength of the rail under wheel-rail contact stress to resist plastic deformation and contact fatigue damage; the other is heat treatment, which involves continuous or staged accelerated cooling of the rail head to refine the pearlite microstructure or control the mechanical properties of the rail head, improving material strength and resistance to rolling contact fatigue damage. Both approaches result in a rail head cross-section microstructure that is generally fine-grained pearlite, with little variation in the lamellar spacing from the surface to the core.

[0011] Compared to national railway lines, subway lines are characterized by frequent train acceleration and deceleration, numerous small-radius curves, and high train operating frequency. Subway rails are more prone to microcracks due to accumulated surface plastic deformation. Furthermore, subway trains have lighter axle loads, and in humid environments, the persistent liquid medium at the wheel-rail contact interface reduces the wheel-rail friction coefficient, making surface fatigue microcracks on the rails difficult to remove, resulting in more severe rolling contact fatigue damage to subway rails. Numerous studies have shown that under wheel-rail stress, rolling contact fatigue damage to rails initially manifests as surface fatigue cracks. These cracks initially propagate inwards at a large angle, typically accelerating further at a smaller angle at a depth of 2-3 mm from the surface. Once the cracks have reached a certain length and depth, they rapidly turn towards the surface, causing metal spalling in that area, forming spalling damage. In the humid environment of subway lines, the liquid medium can fill the fatigue crack tip. Because liquids are incompressible, under the squeezing action of wheel-rail stress, the liquid medium at the crack tip can generate an explosive "oil wedge" effect, causing the internal crack tip to propagate rapidly, resulting in severe rolling contact fatigue damage. Under current technology, further refining the pearlitic microstructure of the rail head to increase rail strength can prevent sufficient wheel-rail break-in and instead promote the initiation of surface fatigue cracks, increasing the number of cracks. While appropriately increasing the pearlitic lamellar spacing of the rail head can slow down the formation of some surface fatigue cracks, it is difficult to suppress the rapid propagation of internal fatigue cracks under the influence of liquid media, causing fatigue cracks to quickly develop into more severe spalling. Existing rail microstructures are insufficient to meet the application requirements of subways in humid environments. To address these issues, this invention designs a cap-shaped microstructure with different depths and pearlitic lamellar spacings in the rail head. Through a reasonable heat treatment process, a rational gradient distribution of the pearlitic microstructure is achieved, slowing the propagation rate of rolling contact fatigue cracks into the material interior under humid conditions and reducing rolling contact fatigue damage.

[0012] In this invention, the surface temperature of the rail at the start of accelerated cooling is 820-860℃. This is to avoid excessively high temperatures leading to coarse austenite grains, while ensuring that the entire rail material remains within the austenite temperature range, providing kinetic conditions for the transformation of supercooled austenite into pearlite. In the first cooling stage, the rail head area is acceleratedly cooled at a rate of 6-8℃ / s for 25-30s. The main purpose of this stage is to remove heat from the surface of the rail head by controlling the cooling rate. The material from the surface to a depth of 2-3mm inside the rail head transforms from supercooled austenite into a pearlite structure with appropriate lamellar spacing, forming a cap-shaped layer 1. The pearlite lamellar spacing is 130-150nm, giving the rail surface a reasonable wear rate. While ensuring the normal use of the rail, this also removes some of the fatigue cracks that have started on the surface, reducing the length of surface fatigue cracks and lowering the risk of fatigue cracks extending into the material.

[0013] In the second cooling stage, the rail head area is accelerated and cooled at a rate of 12-14℃ / s for 12-16s. Cooling is then stopped, and the area is held for 8-10s. After the pearlitic phase transformation of the cap layer 1 is completed, the undercooled austenite deeper in the rail head needs to exchange heat with the outside through the pearlitic structure of the cap layer 1. At this time, the cooling rate of the pearlitic and austenitic transformation interface is slightly lower than that of the surface. As the phase transformation progresses into the interior of the rail head, the cooling rate needs to be increased accordingly. Another important function of increasing the cooling rate is to ensure that the applied cooling rate is greater than the critical cooling rate of the pearlitic phase transformation, and to avoid the high-temperature phase transformation of pearlite during the cooling process. This allows the undercooled austenite in a certain depth range to be reduced to the low-temperature transformation temperature range of pearlite. Then, the area is held for 8-10s to allow the material of the cap layer 2 to undergo a full pearlitic isothermal phase transformation, improve the uniformity of the pearlitic lamellar structure, and form a pearlitic cap layer 2 with extremely fine lamellar spacing. Numerous studies have shown that after rolling contact fatigue cracks in rails propagate to a depth of 2-3 mm, they accelerate further along small angles. The liquid medium in humid environments exacerbates crack tip propagation, ultimately leading to fatigue damage. Therefore, the key to controlling rolling contact fatigue damage in humid environments is to reduce the propagation rate of fatigue crack tips at the rail head. Through the cap-shaped layer 2 formed in the second cooling stage, from the depth of cap-shaped layer 1 to a depth of 7-8 mm inside the rail head, the pearlite lamellar spacing is distributed at 90-120 nm. Within this depth range, extremely fine pearlite lamellars are formed. When fatigue cracks propagate to this depth, the refined pearlite lamellar structure increases the number of phase interfaces per unit length of the crack propagation path. Each time the crack crosses a phase interface, it consumes additional energy, increasing crack propagation resistance and effectively reducing the fatigue crack propagation rate.

[0014] In the third cooling stage, accelerated cooling is applied to the rail head, rail web, and rail base regions. The cooling rate for the rail head is 5-7℃ / s, for the rail web it is 2-3℃ / s, and for the rail base it is 3-4℃ / s, with a cooling time of 30-35s. To ensure the rail possesses a certain degree of toughness, the pearlitic microstructure in the rail head core cannot be too refined. After the pearlitic phase transformation of the cap-shaped layer 2, the phase transformation continues deeper into the rail head. To ensure the uniformity of the microstructure gradient in the rail head and avoid performance gradient fluctuations, a microstructure transition layer is designed between the cap-shaped layer 2 and the core, with the pearlitic lamellar spacing appropriately reduced. In this cooling stage, a lower cooling rate and a reasonable cooling time are used for the rail head, forming a cap-shaped layer 3 below the cap-shaped layer 2. The depth ranges from the depth of the cap-shaped layer 2 to the interior of the rail head, 13-15mm, with a pearlitic lamellar spacing distribution of 140-170nm. During the accelerated cooling process of the rail head, the temperature of the rail head drops faster than that of the rail web and rail base. In order to avoid the temperature inconsistency of the rail cross section and the resulting temperature stress that leads to rail deformation, a certain cooling rate needs to be applied to the rail web and rail base as well. Based on the distribution of the metal heat capacity of the rail head, rail web and rail base, a lower cooling rate is applied to the rail web and rail base in the third cooling stage. The pearlite lamellar spacing of the rail web and rail base is distributed in the range of 180-220nm, which gives the rail web and rail base a certain degree of toughness. At the same time, it can also balance the temperature distribution of the rail cross section and reduce the deformation and bending of the rail during the heat treatment process.

[0015] In the fourth cooling stage, accelerated cooling is applied to both the rail head and rail base regions. The cooling rate for the rail head is 1.5-3℃ / s, and the cooling rate for the rail base is 0.5-1℃ / s, with a cooling time of 50-60s. Accelerated cooling is then stopped, and the rail is allowed to cool naturally to room temperature. In this stage, the cooling rate for the rail head is further reduced, and the cooling time is appropriately extended to ensure that the material in the depth of the cap-shaped layer 3, extending into the internal matrix, undergoes sufficient pearlitic phase transformation to form the substrate layer. The pearlitic lamellar spacing is distributed at 200-230nm, giving the rail head a certain degree of toughness. At this stage, the pearlitic phase transformation has been completed in the rail web and rail base. Since the rail web contains less metal than the rail base, only a smaller cooling rate needs to be applied to the rail base in this stage to balance the temperature distribution during the rail heat treatment process.

[0016] The rails obtained by the method of this invention have a full-section room-temperature microstructure consisting of pearlite and a very small amount of ferrite, without harmful structures such as bainite and martensite, thus meeting the requirements for use in subway lines. Compared with traditional hot-rolled subway rail materials, the rail materials treated by the method of this invention show a reduction of more than 30% in the length and propagation depth of rolling contact fatigue cracks in humid environments, improving the overall performance and service life of the rails.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a rail microstructure and heat treatment method for use in humid environments in subways. Based on the influence mechanism of liquid media on rolling contact fatigue damage in subway rails, a cap-shaped microstructure with different depths and pearlite lamellar spacings is designed at the rail head. By performing staged accelerated cooling heat treatment on the rail within the austenitizing temperature range, and rationally allocating the cooling rate and time at different stages for the rail head, rail web, and rail base, a reasonable gradient distribution of pearlite lamellar spacing is achieved in different parts of the rail cross-section. This effectively slows down the propagation rate of rolling contact fatigue cracks into the material in humid environments. Compared with traditional hot-rolled subway rail materials, the rail material treated by this invention shows a reduction of over 30% in both the length and depth of rolling contact fatigue cracks in humid environments, thereby improving the overall performance and service life of the rail, reducing subway line maintenance costs, and enhancing the safety assurance capability of subway trains. Attached Figure Description

[0018] Figure 1 This is a schematic cross-sectional view of the rail head, rail web, and rail bottom of the rail described in this invention.

[0019] Figure 2 This is a schematic diagram of the rail head cap-shaped layer and the substrate layer of the present invention.

[0020] Figure 3 This is an assembly drawing of a wheel-rail rolling contact fatigue friction and wear test specimen.

[0021] Figure 4 This is an image showing the pearlite lamellar spacing of the rail head cap-shaped layer 1 obtained in Embodiment 1 of the present invention.

[0022] Figure 5 This is an image showing the pearlite lamellar spacing of the rail head cap-shaped layer 2 obtained in Embodiment 1 of the present invention.

[0023] Figure 6 This is an image showing the pearlite layer spacing of the rail head cap-shaped layer 3 obtained in Embodiment 1 of the present invention.

[0024] Figure 7 This is an image showing the spacing between pearlite layers in the rail head substrate layer obtained in Embodiment 1 of the present invention.

[0025] Figure 8 The image shows the rolling contact fatigue crack of the rail obtained in Embodiment 1 of the present invention.

[0026] Figure 9 The image shows the rolling contact fatigue crack of the rail obtained in Comparative Example 1 of this invention. Detailed Implementation

[0027] In this invention, the rail web region refers to the vertical region in the middle of the "I"-shaped cross-section of the rail, the rail head region refers to the region above the rail web of the "I"-shaped cross-section of the rail, and the rail bottom region refers to the region below the rail web of the "I"-shaped cross-section of the rail.

[0028] In this invention, the rail surface temperature refers to the temperature at the center line position of the upper surface of the rail head.

[0029] In this invention, the accelerated cooling of the rail head area specifically refers to the accelerated cooling of the upper surface of the rail head and the left and right vertical sides of the rail head, while the lower jaw of the rail head is not accelerated; the accelerated cooling of the rail web area specifically refers to the accelerated cooling of the left and right sides of the rail web; and the accelerated cooling of the rail bottom area specifically refers to the accelerated cooling of the upper and lower surfaces of the rail bottom.

[0030] In this invention, the accelerated cooling medium used is a commonly used cooling medium in this technical field, including but not limited to water, polymer solution, oil, compressed air, water mist or oil mist mixture, and any substance that can ensure a uniform and stable cooling rate.

[0031] The production method of the rails described in this invention is not particularly limited; conventional industrial rail production processes can be followed. The main production steps include: hot metal desulfurization, converter smelting, LF refining, vacuum treatment, continuous casting, billet heating, and universal rolling. For example, a top-and-bottom blowing converter is used to smelt the steel; the casting process should be carried out under full protection to prevent contact with air; the cast billets should undergo slow cooling; a walking beam furnace is used to heat the billets and perform heat preservation treatment; and a universal rolling production line is used for rolling, etc.

[0032] To better understand the present invention, the following description, in conjunction with the accompanying drawings and embodiments, further clarifies the content of the present invention. However, it should be understood that the specific embodiments and comparative examples described herein are only for explaining the present invention and do not limit the present invention.

[0033] Examples 1-5 and Comparative Examples 1-4 of this invention use industrially produced U75V finished steel rails with the same chemical composition. These rails are currently the most widely used pearlitic steel rails in the domestic railway industry. The main production processes for the rails include: hot metal desulfurization, converter smelting, LF refining, vacuum treatment, continuous casting, billet heating, and rolling. Based on the total weight of the rails, the chemical composition is 0.75 wt% C, 0.71 wt% Si, 0.88 wt% Mn, 0.07 wt% V, 0.012 wt% P, and 0.008 wt% S, with the remainder being Fe and unavoidable impurities.

[0034] Example 1 After desulfurization, smelting, LF refining, vacuum treatment, and continuous casting, molten iron is used to obtain a billet. After the billet is heated, rough rolled, and universal rolled, the hot-rolled rail is subjected to accelerated cooling heat treatment. The surface temperature of the rail at the start of accelerated cooling is 820℃. The first cooling stage involves accelerated cooling of the rail head area at a rate of 6℃ / s for 25 seconds. The second cooling stage involves accelerated cooling of the rail head area at a rate of 13℃ / s for 14 seconds, followed by a 10-second holding period. The third cooling stage involves accelerated cooling of the rail head, rail web, and rail base areas separately, with a cooling rate of 6℃ / s for the rail head, 3℃ / s for the rail web, and 3℃ / s for the rail base, for a total of 30 seconds. The fourth cooling stage involves accelerated cooling of the rail head and rail base areas separately, with a cooling rate of 2℃ / s for the rail head and 0.8℃ / s for the rail base, for a total of 55 seconds, followed by a complete stop to allow natural cooling to room temperature.

[0035] Example 2 The production method of Example 1 was followed, except that: the surface temperature of the rail at the start of accelerated cooling was 855°C; in the first cooling stage, the rail head area was accelerated to cool at a rate of 6.5°C / s for 27s; in the second cooling stage, the rail head area was accelerated to cool at a rate of 12.5°C / s for 15s, after which cooling was stopped and the rail was held at that temperature for 8s; in the third cooling stage, the rail head, rail web, and rail base areas were accelerated to cool at a rate of 7°C / s for the rail head, 2.5°C / s for the rail web, and 4°C / s for the rail base for 32s; in the fourth cooling stage, the rail head and rail base areas were accelerated to cool at a rate of 1.5°C / s for the rail head and 0.6°C / s for the rail base for 52s, after which accelerated cooling was stopped and the rails were allowed to cool naturally to room temperature.

[0036] Example 3 The production method of Example 1 was followed, except that: the surface temperature of the rail at the start of accelerated cooling was 830°C; in the first cooling stage, the rail head area was accelerated to cool at a rate of 8°C / s for 28s; in the second cooling stage, the rail head area was accelerated to cool at a rate of 14°C / s for 12s, after which cooling was stopped and the rail was held at that temperature for 9s; in the third cooling stage, the rail head, rail web, and rail base areas were accelerated to cool at a rate of 7°C / s for the rail head, 2.5°C / s for the rail web, and 3°C / s for the rail base for 35s; in the fourth cooling stage, the rail head and rail base areas were accelerated to cool at a rate of 2°C / s for the rail head and 0.5°C / s for the rail base for 58s, after which accelerated cooling was stopped and the rails were allowed to cool naturally to room temperature.

[0037] Example 4 The production method was carried out according to Example 1, except that: after hot rolling, the rails were cooled to room temperature, and then uniformly heated to 900°C using offline heating, followed by accelerated cooling heat treatment. The surface temperature of the rails at the start of accelerated cooling was 840°C. In the first cooling stage, the rail head area was accelerated cooled at a rate of 7°C / s for 30s. In the second cooling stage, the rail head area was accelerated cooled at a rate of 12°C / s for 13s, after which cooling was stopped and the rails were held at that temperature for 8s. In the third cooling stage, the rail head, rail web, and rail base areas were accelerated cooled at a rate of 5°C / s for the rail head, 2°C / s for the rail web, and 3.5°C / s for the rail base, for 34s. In the fourth cooling stage, the rail head and rail base areas were accelerated cooled at a rate of 3°C / s for the rail head and 1°C / s for the rail base, for 54s, after which accelerated cooling was stopped and the rails were allowed to cool naturally to room temperature.

[0038] Example 5 The production method is carried out according to Example 1, except that the hot-rolled rail is cooled to room temperature, and then the rail is uniformly heated to 900°C using an offline heating method, followed by accelerated cooling heat treatment. The surface temperature of the rail at the start of accelerated cooling is 850℃. The first cooling stage involves accelerated cooling of the rail head area at a rate of 6.5℃ / s for 28 seconds. The second cooling stage involves accelerated cooling of the rail head area at a rate of 14℃ / s for 15 seconds, followed by a 10-second holding period. The third cooling stage involves accelerated cooling of the rail head, rail web, and rail base areas separately, with a cooling rate of 6℃ / s for the rail head, 2℃ / s for the rail web, and 3℃ / s for the rail base, for a total of 32 seconds. The fourth cooling stage involves accelerated cooling of both the rail head and rail base areas separately, with a cooling rate of 2.5℃ / s for the rail head and 0.6℃ / s for the rail base, for a total of 50 seconds, followed by a complete stop to allow natural cooling to room temperature.

[0039] Comparative Example 1 The production method is carried out according to Example 1, except that the rails do not undergo accelerated cooling heat treatment and are naturally air-cooled to room temperature after hot rolling.

[0040] Comparative Example 2 The production method of Example 1 is followed, except that the surface temperature of the rail is 820°C when the accelerated cooling begins, the rail head is subjected to continuous accelerated cooling heat treatment at a cooling rate of 8°C / s, and the accelerated cooling is stopped when the rail head temperature drops to 300°C, and the rail head is allowed to cool naturally to room temperature.

[0041] Comparative Example 3 The production method of Example 1 is followed, except that the surface temperature of the rail is 830°C when the accelerated cooling begins. The rail head is subjected to accelerated cooling heat treatment in stages. The cooling rate is 2°C / s in the first stage, 4°C / s in the second stage, 6°C / s in the third stage, 4°C / s in the fourth stage, and 3°C / s in the fifth stage. The cooling time for each stage is 10s. When the rail head temperature drops to 350-380°C, the accelerated cooling is stopped and the rail head is allowed to cool naturally to room temperature.

[0042] Comparative Example 4 The production method of Example 1 was followed, except that the surface temperature of the rail was 820°C when it began to be accelerated cooled. The rail head was subjected to accelerated cooling heat treatment in stages. The first stage cooling rate was 10°C / s and the cooling time was 12s. The second stage cooling rate was 5°C / s and the cooling time was 45s. In the third stage, the rail head was not accelerated cooled, and the rail bottom was accelerated cooled at a rate of 2°C / s and the cooling time was 20s. Then the accelerated cooling was stopped and the rail was allowed to cool naturally to room temperature.

[0043] The pearlite lamellar spacing at different depths in the cross-sections of the rails obtained in the examples and comparative examples was observed using scanning electron microscopy. The total thickness of the pearlite lamellar spacing under the field of view was measured by the plumb line method and the average value was taken. The results are shown in Table 1.

[0044] Table 1. Pearlite lamellar spacing at different depths in rail cross-sections obtained from the examples and comparative examples.

[0045] To verify the performance of the rail obtained in this invention under humid conditions, a rolling contact friction and wear testing machine (M-2000 type) was used to conduct rolling contact fatigue wear tests on the rails of the embodiments and comparative examples under the same test conditions. The experiment involved relative rolling of circular specimens. The upper specimen was taken from the rail head cap layer 2 region (3-8 mm below the rail head surface) of both the embodiments and comparative examples, with the circular specimen taken longitudinally along the centerline. The lower specimen was taken from a CL60 type wheel. A schematic diagram of the experiment is shown below. Figure 3 As shown. Pure water was continuously dripped onto the wheel-rail friction interface at a certain frequency to keep the friction interface moist. The experimental conditions were as follows: sample size: ring thickness 4mm, inner diameter 6mm, outer diameter 15mm; experimental load: 260N; rotation speed: upper sample 180r / min, lower sample 200r / min; slip rate: 10%; experimental time: 20h.

[0046] After the rolling contact fatigue wear test, the rail specimen was cut perpendicular to the rolling direction to obtain a small specimen. The specimen was ultrasonically cleaned, and the fatigue crack propagation of the cross section was observed using a scanning electron microscope. The length and depth of the rolling contact fatigue crack propagation were measured. The results are shown in Table 2.

[0047] Table 2. Contact fatigue crack measurement results of the examples and comparative examples.

[0048] It can be seen that in Examples 1-5, the pearlite lamellar spacing of each cap layer and base layer of the rail head is stably controlled, and a reasonable gradient distribution is shown between each cap layer. The pearlite lamellar spacing of the rail web and rail base is uniformly distributed. Comparative Example 1 is a rail obtained using a conventional hot-rolling process, which is currently the most widely used in subway lines; Comparative Example 2 is a rail obtained using the current mainstream heat treatment process; and Comparative Examples 3-4 are rails obtained using other staged accelerated cooling processes. Comparative Example 1 uses natural cooling, and the overall pearlite lamellar spacing of the rail head is relatively large. Comparative Examples 2-4 involve continuous or staged heat treatment of the rail head, and the overall distribution of pearlite lamellar spacing is relatively consistent, without a clear gradient distribution or a reasonable cap layer.

[0049] Based on the results of rolling contact friction and wear tests under simulated humid conditions, the rails obtained in Examples 1-5 exhibited relatively mild rolling contact fatigue crack lengths and depths, with crack lengths ranging from approximately 40-60 μm and crack propagation depths from approximately 8-11 μm. The rails obtained in Comparative Example 1 showed a larger overall pearlite lamellar spacing at the rail head, resulting in lower resistance to fatigue crack tip propagation. Under the influence of liquid media, their rolling contact fatigue crack propagation was most severe, with a crack length of 104 μm and a crack propagation depth reaching 16 μm. The rails obtained in Comparative Examples 2-4 showed relatively larger pearlite lamellar spacing within a 3-8 mm depth range at the rail head, leading to more severe rolling contact fatigue crack propagation lengths and depths compared to the examples. Overall, compared to the most widely used hot-rolled rails in current subway lines, the rail material treated by the method of this invention exhibits a reduction of over 30% in both rolling contact fatigue crack length and propagation depth under humid conditions. This effectively improves the comprehensive performance and service life of the rails, reduces subway line maintenance costs, and enhances the safety assurance capability of subway trains.

[0050] The embodiments described above are merely specific implementations of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A rail structure for use in humid environments like subways, characterized in that, The metallographic structure of the entire rail cross section is mainly composed of lamellar pearlite. Based on the different distribution of pearlite lamellar spacing, the cross-sectional structure of the rail head is divided into cap-shaped layer 1, cap-shaped layer 2, cap-shaped layer 3, and substrate layer. Among them, cap-shaped layer 1 extends from the center line of the rail head surface downwards to a depth of 2-3 mm from the rail head surface to the interior of the rail head, with a pearlite lamellar spacing of 130-150 nm; cap-shaped layer 2 extends from the depth of cap-shaped layer 1 to a depth of 7-8 mm from the interior of the rail head, with a pearlite lamellar spacing of 90-120 nm; cap-shaped layer 3 extends from the depth of cap-shaped layer 2 to a depth of 13-15 mm from the interior of the rail head, with a pearlite lamellar spacing of 140-170 nm; and substrate layer extends from the depth of cap-shaped layer 3 to the interior matrix of the rail head, with a pearlite lamellar spacing of 200-230 nm.

2. The rail structure for use in a humid environment in a subway, as described in claim 1, is characterized in that, The spacing between pearlite lamellars in the web and bottom of the rail is 180-220 nm.

3. A heat treatment method for reducing rolling contact fatigue damage to subway rails in humid environments, characterized in that, For hot-rolled rails with a certain temperature after rolling, or rails that have been reheated offline, a phased accelerated cooling heat treatment is performed. The initial surface temperature of the rail at the start of accelerated cooling is 820-860℃. In the first cooling stage, the rail head area is accelerated at a rate of 6-8℃ / s for 25-30 seconds. In the second cooling stage, the rail head area is accelerated at a rate of 12-14℃ / s for 12-16 seconds, after which cooling is stopped and the rail is held at that temperature for 8-10 seconds. The third cooling stage involves accelerated cooling of the rail head, rail web, and rail base areas, with a cooling rate of 5-7℃ / s for the rail head, 2-3℃ / s for the rail web, and 3-4℃ / s for the rail base, for a cooling time of 30-35s. The fourth cooling stage involves accelerated cooling of the rail head and rail base areas, with a cooling rate of 1.5-3℃ / s for the rail head and 0.5-1℃ / s for the rail base, for a cooling time of 50-60s. After this, accelerated cooling is stopped, and the rails are allowed to cool naturally to room temperature.

4. The heat treatment method for reducing rolling contact fatigue damage of subway rails in humid environments according to claim 3, characterized in that, The surface temperature of the rail when it begins to accelerate cooling is 830-850℃.

5. The heat treatment method for reducing rolling contact fatigue damage of subway rails in humid environments according to claim 3, characterized in that, In the first cooling stage, the rail head area is accelerated and cooled at a rate of 6.5-7.5℃ / s for 26-28s.

6. The heat treatment method for reducing rolling contact fatigue damage of subway rails in humid environments according to claim 3, characterized in that, In the second cooling stage, the rail head area is accelerated and cooled at a rate of 13-14℃ / s for 14-16s. Cooling is then stopped and the area is held at that temperature for 9-10s.

7. The heat treatment method for reducing rolling contact fatigue damage of subway rails in humid environments according to claim 3, characterized in that, In the third cooling stage, the rail head, rail web, and rail base areas are cooled at an accelerated rate of 5.5-6.5℃ / s, the rail web at 2.2-2.8℃ / s, and the rail base at 3.4-3.8℃ / s, with a cooling time of 32-35s.

8. The heat treatment method for reducing rolling contact fatigue damage of subway rails in humid environments according to claim 3, characterized in that, In the fourth cooling stage, the rail head and rail base areas are accelerated for cooling. The cooling rate of the rail head is 2-3℃ / s, and the cooling rate of the rail base is 0.5-1℃ / s. The cooling time is 55-60s. Then, the accelerated cooling is stopped, and the rails are allowed to cool naturally to room temperature.

9. A heat treatment method for reducing rolling contact fatigue damage of subway rails in humid environments according to claim 3, characterized in that, The heat treatment method is applicable to the composition system of pearlitic rails.

10. A heat treatment method for reducing rolling contact fatigue damage of subway rails in humid environments according to claim 3, characterized in that, Based on the total weight of the rail, the chemical composition of the rail, by mass percentage, includes: C 0.70-0.82%, Si 0.55-0.80%, Mn 0.85-1.00%, V 0.05-0.15%, P≤0.025%, S≤0.025%, with the remainder being Fe and unavoidable impurities.

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