Fixed flash welding and joint heat treatment method for U75VH online heat treatment steel rail for heavy haul railway

Abstract

The invention discloses a fixed flash welding and joint heat treatment method for a U75VH online heat treatment steel rail for a heavy haul railway, which can ensure that a fixed flash welding joint of the U75VH online heat treatment steel rail for the heavy haul railway is a normal pearlite structure by optimally designing parameters of a joint fixed flash welding process and a joint heat treatment heating process. Abnormal structures such as bainite, martensite and secondary cementite do not exist, the strength, hardness and toughness of the joint are matched with the performance of a base material, and the requirement of a heavy haul railway for the smoothness of the steel rail joint can be met. And the risk that the steel rail is fractured due to'saddle-shaped 'abrasion caused by too large hardness difference of a welding area or a microscopic abnormal structure of a joint in the line service process is improved, and the line operation safety is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of rail welding, and particularly relates to a method for fixed flash welding and joint heat treatment of U75VH on-line heat-treated rails for heavy-haul railways. After using the flash welding joint process and joint heat treatment process of the present invention, it can be ensured that the fixed flash welding joints of U75VH on-line heat-treated rails for heavy-haul railways are all normal pearlite structures, and there are no abnormal structures such as bainite, martensite and secondary cementite; the present invention solves the problems of low hardness of high-strength and high-hardness rail joints, unqualified joint drop hammer performance and joint low collapse, and can effectively improve the performance matching between the rail joint and the base metal, improve the comprehensive mechanical properties and contact fatigue properties of the joint, thereby effectively improving the service performance of the joint. Background Art

[0002] The welding performance of steel is mainly affected by the chemical composition of the steel. Pearlite rails all belong to high-carbon steel. At the same time, by adding different contents of alloy strengthening elements such as silicon, manganese, chromium, and vanadium, the strength, hardness and toughness of the rails are ensured to meet the service requirements of different lines. The carbon content of U75VH on-line heat-treated rails is between 0.75 - 0.80%, the manganese content is between 0.95 - 1.05%, the tensile strength is between 1250 - 1300 MPa, and the hardness is between 360 - 380 HB. The rails have the characteristics of high carbon, high strength and high hardness, making their welding performance very poor, mainly manifested as: ① During the welding process, due to the burning loss of carbon and alloy and the action of the welding cycle, the grain size of the joint is significantly reduced compared with the base metal, resulting in a significant reduction in the comprehensive mechanical properties of the joint fusion line and the heat affected zone. Especially in the as-welded state, the strength, hardness and toughness of the joint are significantly different from those of the base metal, and cannot meet the service requirements of heavy-haul lines. ② During the production process of the rails, due to the segregation of carbon and manganese elements in the base metal, it is very easy to cause abnormal microstructures such as bainite or martensite in the joint under supercooled conditions during the welding process, and secondary cementite is easily formed near the heat affected zone, resulting in an increase in joint brittleness and an increase in the risk of joint rail breakage. ③ Normalizing the joint in the as-welded state can effectively improve the strength, hardness and toughness of the joint. However, an unreasonable heat treatment process is very likely to cause abnormal structures in the joint or the phenomenon that the joint performance does not meet the standard requirements, resulting in poor performance matching between the joint and the base metal. During the service process of the line, saddle wear, joint peeling and block removal or rail breakage will occur, seriously affecting the smoothness and safety of the seamless line. ④ The cross-section of U75VH on-line heat-treated rails is relatively large, and the cross-section can reach 95 cm 2, this makes it difficult to control the temperature during the flash welding process of rail, and it is easy to have the situation of local overheating or insufficient heating. Overheating will cause the grain to grow, reducing the mechanical properties of the welded joint; insufficient heating will lead to incomplete fusion of the weld, affecting the strength and toughness of the joint. ⑤ During the flash welding process of large-section rails, the rail end faces are in a high-temperature state and are prone to oxidation reaction with oxygen in the air, thus generating welding defects such as gray spots, which affect the quality of the weld. At the same time, the existence of oxidation products will hinder the mutual diffusion and combination of metal atoms on the rail end faces, and may generate defects such as pores and slag inclusions. ⑥ During the flash welding process of large-section rails, it is required that the flash welding equipment has sufficient power and pressure output to ensure that the rail end faces can be fully heated and upset, so as to ensure the joint quality. In addition, it is also necessary to accurately control welding parameters, such as flash current, flash time, upset pressure, upset time, etc., to ensure the stability of welding quality.

[0003] The flash welding process and joint heat treatment process of rails directly affect the joint microstructure properties and service state. Improper flash welding process of the joint will cause defects such as joint overburning, non-welded joint, excessive gray spots, liquefaction cracks, etc. If the joint heat treatment process is improper, harmful microstructures such as coarse grains and martensite are likely to appear in the joint. That is, when the cooling rate is too fast, hard and brittle harmful microstructures such as martensite may be formed, resulting in hardening and embrittlement phenomena at the joint. When the cooling rate is too slow, it may lead to coarse grains, reducing the strength and toughness of the joint. When the joint is unevenly heated or cooled, due to temperature changes or internal stress effects, the rail size may change, resulting in joint deformation, which affects the smoothness and service performance of the track.

[0004] Patent document CN201810708275.9 discloses a heat treatment method for flash welded rail joints. This method is for dissimilar rail welded joints of hypereutectoid rails and eutectoid rails. After welding, the high-temperature welded joints are directly subjected to three-stage controlled cooling. In the first cooling stage, the temperature is reduced from 1000 - 1400 °C to 650 - 720 °C, in the second cooling stage, the temperature is reduced to 350 - 410 °C, and in the third cooling stage, the temperature is reduced to 10 - 30 °C. This patent is an invention for welding with a rail moving flash welding machine, and the cooling medium can be a water mist mixture. According to the requirements of TB / T 1632.2 standard, the drop hammer requirement for moving flash welded joints is that 15 consecutive pieces are unbroken, and only compressed air can be used for joint cooling treatment, which does not meet the requirements of the standard for the joint heat treatment process, and this invention also does not meet the requirement that 25 consecutive pieces of fixed flash welded joints are unbroken.

[0005] Patent document CN202010886023.2 discloses a post-weld heat treatment method for 1300MPa grade low-alloy heat-treated steel rails. In this method, the steel rail welded joint with a residual temperature of 900 - 1100°C formed in the as-welded state is cooled in three stages. In the first stage, it is naturally cooled to 650 - 720°C, in the second stage, it is controlled to cool to 480 - 550°C, and in the third stage, it is controlled to cool to 10 - 30°C. This patent is an invention for the welding of steel rails by a moving flash welding machine. The cooling medium can be a water mist mixture, and the hardness of the joint is detected by Vickers hardness, which does not meet the requirements of the heat treatment process and performance inspection of the fixed flash welded joint in TB / T 1632.2 standard. Moreover, a certain proportion of abnormal martensite-like structures appear in the joints of this invention, which not only fails to meet the standard requirements but also seriously affects the safe operation of the joint on the line.

[0006] Patent document CN201510319185.7 discloses a post-weld heat treatment method for hypereutectoid steel rail welded joints. In this method, for hypereutectoid steel rail flash welded joints or gas pressure welded joints, after the weld area is rapidly heated to 900 - 920°C by medium frequency or oxyacetylene flame, the joint is cooled at a cooling rate of 2.2 - 2.8°C / s to 430 - 450°C, and then air-cooled to room temperature. The Aku of the full-section weld of this patent is ≥9J, although it meets the requirement of the standard ≥6.5J, but the impact performance index is relatively low compared with that of the base metal; the hardness of the joint after heat treatment reaches 105% of the base metal. The excessive hardness of the hypereutectoid steel rail welded joint will inevitably lead to abnormal structures in the joint, with high hardness and low toughness of the joint, which seriously affects the service performance of the joint and the safe operation of the line. Summary of the Invention

[0007] The object of the present invention is to provide a method for fixed flash welding and joint heat treatment of U75VH on-line heat-treated steel rails for heavy-haul railways, which includes:

[0008] 1) Grinding and rust-removing the end faces and electrode clamping ends of two U75VH on-line heat-treated steel rails for heavy-haul railways with the same cross-section. The U75VH on-line heat-treated steel rails for heavy-haul railways have C, Si, Mn, and V as the main alloying elements, and their chemical compositions are as follows by mass percentage: C: 0.75 - 0.80%, Si: 0.60 - 0.80%, Mn: 0.95 - 1.05%, V: 0.06 - 0.09%, P ≤ 0.015%, S ≤ 0.003%, Al: ≤ 0.004%, and the rest is Fe and inevitable impurities;

[0009] 2) Using a fixed flash welding machine for steel rail welding: controlling the oil temperature of the welding machine at 39 - 41°C, the water temperature at 19 - 21°C, and the grid voltage between 420 - 425V;

[0010] 3) Fixed flash welding process for joints: The joint closing amount of the rail weld is controlled between 10 and 12 mm, the arching amount is controlled between 0.60 and 0.65 mm, the preheating current is controlled between 61 and 67 KA, the number of preheating times for flash welding is between 12 and 14 times, the upsetting amount for flash welding is controlled between 16 and 19 mm, the upsetting force ≥ 530 KN, the welding time is controlled between 120 and 125 s, the holding pressure and upsetting time are controlled ≤ 35 s, the initial melting speed is controlled between 0.30 and 0.40 mm / s, and the final melting speed is controlled between 2.40 and 2.60 mm / s; This welding process can ensure that the fixed flash joints of the welded U75VH online heat-treated rails have excellent internal quality and will not appear welding defects such as overburning, lack of fusion, excessive gray spots, and liquefaction cracks;

[0011] 4) Air-cool the high-temperature welded joint to below 200 °C to ensure complete phase transformation inside and outside the joint;

[0012] 5) Joint heat treatment heating process: Use a dual-frequency normalizing machine for the joint for induction heating treatment. First, heat the joint from 0 to 200 °C to 830 - 850 °C with a low-frequency power of 67 - 75 kW, and the low-frequency heating time is controlled within 85 - 100 s. Selecting low-frequency heating ensures that the core of the rail head can be heated to the required temperature; After the joint reaches the target temperature during low-frequency heating, immediately perform high-frequency induction heating on the joint. Heat the joint from 830 - 850 °C to 890 - 910 °C with a high-frequency power of 50 - 65 kW, and the low-frequency heating time is controlled within 60 - 80 s. High-frequency heating ensures that the temperature within the range of 0 - 15 mm below the tread of the rail head can be heated to the target temperature, and the total joint heating time is controlled between 140 and 180 s;

[0013] 6) Joint heat treatment controlled cooling process: After the joint reaches the target temperature of 890 - 910 °C, stop heating and immediately perform air-blowing accelerated cooling treatment on the joint and the rail bottom. The air-blowing nozzle for air-blowing cooling is 20 - 30 mm away from the rail top surface, 20 - 30 mm away from the side of the rail head, and 50 - 60 mm away from the rail bottom; The air-blowing wind pressure is 0.25 - 0.30 MPa, and the air-blowing time is 110 - 130 s; Stop air-blowing cooling when the temperature of the rail bottom surface reaches between 420 and 440 °C, and the joint naturally cools to room temperature; The air-blowing treatment for the rail bottom of the rail flash welding joint can effectively inhibit the precipitation of cementite, improve the microstructure of the joint, and improve the strength, hardness, and toughness of the joint.

[0014] In some embodiments, the fixed flash welding and joint heat treatment method further includes the following processes: failure treatment, straightening, and flaw detection, and then subsequent shipping and track laying can be carried out.

[0015] In some embodiments, the tensile strength Rm of the joint obtained by the fixed flash welding and joint heat treatment method is ≥1150 MPa, the joint elongation A is ≥10%, the joint impact KU2 is ≥17 J, and the average hardness H of the joint J and the average hardness H of the base metal P The ratio satisfies: H J / H P ≥0.93, the average hardness H of the soft spots of the joint J1 and the average hardness H of the base metal P The ratio: H J1 / H P ≥0.83, the softening zone W ≤ 18 mm, and the weld and heat affected zone are typical pearlite structures.

[0016] In some embodiments, the fixed flash welding machine is a GAAS80 / 580 fixed flash welding machine.

[0017] In some embodiments, the U75VH on-line heat-treated rail for heavy-haul railways is produced through smelting, continuous casting, slow cooling of the billet, rolling, and heat treatment processes. Among them, the size of the rail continuous casting billet is 280 mm × 380 mm, the rolling section is 75 kg / m (75, 75N), the rolling reduction ratio of the rail is greater than or equal to 11:1, the finish rolling temperature of the rail is not higher than 950 °C, and the final cooling temperature of the on-line heat treatment of the rail is not higher than 520 °C, so as to ensure the grain size and pearlite structure lamellar spacing of the rail.

[0018] Another object of the present invention also provides a welded rail with a heat-treated joint, which is obtained by the above method.

[0019] Based on the above invention content, the tensile strength Rm of the joint obtained by the method of the present invention is ≥1150 MPa, the joint elongation A is ≥10%, the joint impact KU2 is ≥17 J, and the average hardness H of the joint J and the average hardness H of the base metal P The ratio satisfies: H J / H P ≥0.93, the average hardness H of the soft spots of the joint J1 and the average hardness H of the base metal P The ratio: H J1 / H P ≥0.83, the softening zone W ≤ 18 mm, and the weld and heat affected zone are typical pearlite structures. All properties meet the standard requirements. The fixed flash welding joint strength, hardness, and toughness of the U75VH on-line heat-treated rail for heavy-haul railways obtained are matched with the properties of the base metal, which can meet the requirements of the heavy-haul railway for the smoothness of the rail joint, improve the risk of "saddle-shaped" wear caused by excessive hardness difference in the welding area during the service of the rail on the line or fracture caused by abnormal microstructures at the joint, and ensure the safety of line operation. Brief Description of the Drawings

[0020] Figure 1 It is the test result of the longitudinal section of the joint (test line 1) in Example 3. Detailed Implementation Manner

[0021] The present invention provides a method for fixed flash welding and joint heat treatment of U75VH on-line heat-treated steel rails for heavy-haul railways, which specifically includes the following operation steps:

[0022] 1). The U75VH on-line heat-treated steel rails for heavy-haul railways take C, Si, Mn, and V as the main alloying elements. The weight percentages of the chemical components of the steel rails are C: 0.75 - 0.80%, Si: 0.60 - 0.80%, Mn: 0.95 - 1.05%, V: 0.06 - 0.09%, P≤0.015%, S≤0.003%, Al: ≤0.004%, and the rest are Fe and inevitable impurities.

[0023] 2). The production of steel rails is carried out through smelting, continuous casting, slow cooling of the steel billet, rolling, heat treatment, etc. The size of the continuous casting billet of the steel rail is 280mm×380mm, the rolling section is 75kg / m (75, 75N), the rolling reduction ratio of the steel rail is greater than or equal to 11:1, the final rolling temperature of the steel rail should not be higher than 950°C, and the final cooling temperature of the on-line heat treatment of the steel rail should not be higher than 520°C, so as to ensure the grain size, pearlite lamellar spacing, strength, hardness and toughness of the steel rail.

[0024] 3). Grind and remove rust from the end faces and electrode clamping ends of two steel rails with the same cross-section.

[0025] 4). Use a GAAS80 / 580 fixed flash welding machine to weld the steel rails. The oil temperature of the welding machine is controlled at about 40°C (for example, 39 - 41°C), the water temperature is controlled at about 20°C (for example, 19 - 21°C), and the grid voltage is between 420 - 425V.

[0026] 5). Carry out fixed flash welding of the steel rails. The seam closing amount of the steel rail weld is controlled between 10 - 12mm, the arching amount is controlled between 0.60 - 0.65mm, the preheating current is controlled between 61 - 67KA, the number of preheating times for flash welding is between 12 - 14 times, the upsetting amount for flash welding is controlled between 16 - 19mm, the upsetting force ≥530KN, the welding time is controlled between 120 - 125s, the pressure holding and upsetting time is controlled ≤35s, the initial melting speed is controlled between 0.30 - 0.40mm / s, and the final melting speed is controlled between 2.40 - 2.60mm / s.

[0027] 6) The time from the completion of upset for flash welding to upset removal shall be controlled within 15 s. The maximum allowable upset removal for the rail head is 2.0 mm, for the lower jaw and web of the rail head is 2.0 mm, and for the rail base is 1.5 mm.

[0028] 7) Air-cool the high-temperature as-welded joint to below 200 °C to ensure complete phase transformation at each position of the joint. After the joint cools, grind the weld bead of the joint flat, that is, grind the upset removal at the rail head, web, and base of the rail flat.

[0029] 8) Use a dual-frequency normalizing machine for induction heating treatment of the joint. First, heat the joint from 0 - 200 °C to 830 - 850 °C with a low-frequency power of 67 - 75 kW, and control the low-frequency heating time within 85 - 100 s.

[0030] 9) After the joint reaches the target temperature during low-frequency heating, immediately perform high-frequency induction heating on the joint. Heat the joint from 830 - 850 °C to 890 - 910 °C with a high-frequency power of 50 - 65 kW, control the low-frequency heating time within 60 - 80 s, and control the total heating time of the joint within 140 - 180 s.

[0031] 10) When the joint reaches the target temperature of 890 - 910 °C, stop heating and immediately perform air-blowing accelerated cooling treatment on the joint and the rail base. The air-blowing pressure is 0.25 - 0.30 MPa, and the air-blowing time is 110 - 130 s. Stop air-blowing cooling when the temperature of the rail base surface reaches between 420 - 440 °C, and let the joint cool naturally to room temperature.

[0032] 11) The air-blowing nozzle for the air-blowing cooling is 20 - 30 mm away from the top surface of the rail, 20 - 30 mm away from the side surface of the rail head, and 50 - 60 mm away from the rail base.

[0033] 12) The flash-welded joint of U75VH on-line heat-treated rail for heavy-haul railways obtained by the above welding and heat treatment methods can be subjected to subsequent shipping and track laying after processes such as failure treatment, straightening, and flaw detection.

[0034] The content of the present invention is described in detail below through specific embodiments. The embodiments are intended to help understand the present invention and are not intended to limit the content of the present invention.

[0035] The U75VH on-line heat-treated rail for heavy-haul railways belongs to high-carbon steel. At the same time, by adding alloy strengthening elements such as silicon, manganese, chromium, vanadium, etc. with different contents, due to the segregation of elements such as C and Mn in the base metal, abnormal structures and micro-defects are likely to occur in the heat-affected zone during the fixed flash welding process. An invention is made for the rail welding process and the heat treatment process parameters of the joint. By repeatedly adjusting key parameters such as the preheating current, the number of preheating times, the top amount, the initial melting speed, the tail burning rate, the normalizing heating method of the joint, the normalizing temperature, the final cooling temperature, the air spraying pressure, etc., and through comparative analysis of the mechanical properties of the joint, the drop hammer test results, etc., the flash welding process of the U75VH on-line heat-treated rail and the heat treatment process of the joint are determined.

[0036] The chemical composition of the U75VH on-line heat-treated rail for heavy-haul railways involved in the following examples is: C: 0.78%, Si: 0.73%, Mn: 0.95%, V: 0.08%, P: 0.014%, S: 0.003%, Al: 0.004%, and the rest is Fe and unavoidable impurities.

[0037] During the implementation process, the main adjusted parameters of the welding process and the corresponding joint hardness and drop hammer test results are shown in Table 1.

[0038] Table 1: The joint welding process and heat treatment process corresponding to different examples

[0039]

[0040]

[0041] For the flash-welded joints after normalizing treatment in Examples 1 - 4, according to "TB / T 1632.2 - 2014 Rail Welding Part 2: Flash Welding", a comparative analysis of the Rockwell hardness performance of the longitudinal section of the rail head is carried out. The comparative analysis results are shown in Tables 2 - 5. Figure 1 The test results of the longitudinal section (test line 1) of the joint in Example 3 are shown.

[0042] Table 2: Test results of the longitudinal section of the joint in Example 1

[0043]

[0044] Table 3: Test results of the longitudinal section of the joint in Example 2

[0045]

[0046] Table 4: Test results of the longitudinal section of the joint in Example 3

[0047]

[0048]

[0049] Table 5: Results of the longitudinal section test of the joint in Example 4

[0050]

[0051] From Tables 1 - 5 and Figure 1 it can be seen that for the welded joints in Example 1, Example 2, and Example 4, the average ratio H of the hardness of the welded joint to the average hardness of the base material J / H P is less than 0.90, and the width W of the softened zone in Example 2 is greater than 20 mm, not meeting the standard requirements for heat-treated joints in "TB / T 1632.2 - 2014 Rail Welding - Part 2: Flash Welding"; the joint hardness and the width of the softened zone in Example 3 both meet the standard requirements, and the joint and the base material achieve excellent hardness matching.

[0052] According to the sampling method in "TB / T 1632.2 - 2014 Rail Welding - Part 1: General Technical Conditions", the tensile and impact properties of the flash-welded joint in Example 3 were inspected. The specific results are shown in Table 6 and Table 7.

[0053] Table 6: Tensile properties of the flash-welded joint in Example 3

[0054]

[0055] Table 7: Impact properties of the flash-welded joint in Example 3

[0056]

[0057] From Table 6 and Table 7, it can be seen that the tensile and impact properties of the flash-welded joint in Example 3 both meet the standard requirements, and the joint has a large performance margin.

[0058] From the process comparison during the implementation, it can be known that for the joint in Example 3, the tensile strength Rm of the joint ≥ 1150 MPa, the elongation A of the joint ≥ 10%, the impact KU2 of the joint ≥ 17 J, the average ratio of the hardness of the joint H J to the average hardness of the base material H P satisfies: H J / H P ≥ 0.93, and the average ratio of the hardness of the soft spots of the joint H J1 to the average hardness of the base material H P is: H J1 / H P≥0.83, the softening zone W ≤ 18 mm, and all performances meet the standard requirements. The fixed flash welded joint of the U75VH on-line heat-treated rail for heavy-haul railways obtained in Example 3 has its strength, hardness, and toughness matched with those of the base metal, which can meet the requirements of the heavy-haul railway for the smoothness of the rail joint, improve the risk of "saddle-shaped" wear caused by excessive hardness difference in the welding area during the service of the rail on the line or fracture caused by abnormal microstructure of the joint, and ensure the safety of line operation.

[0059] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for fixing flash welding and heat treatment of joints of U75VH online heat-treated rails for heavy-duty railways, comprising: 1) grinding and removing rust from the rail end faces and electrode clamping ends of two U75VH online heat-treated rails for heavy-duty railways with the same cross-section, wherein the chemical composition of the U75VH online heat-treated rails for heavy-duty railways is as follows by mass percentage: C: 0.75-0.80%, Si: 0.60-0.80%, Mn: 0.95-1.05%, V: 0.06-0.09%, P≤0.015%, S≤0.003%, Al: ≤0.004%, and the rest is Fe and unavoidable impurities; 2) Use a fixed flash welder for rail welding: the welding machine oil temperature is controlled at 39-41°C, the water temperature is controlled at 19-21°C, and the grid voltage is between 420 and 425V; 3) Joint fixed flash welding process: the seam of the rail weld is controlled between 10 and 12 mm, the arch is controlled between 0.60 and 0.65 mm, the preheating current is controlled between 61 and 67 KA, the number of flash welding preheating times is between 12 and 14 times, the flash welding upsetting amount is controlled between 16 and 19 mm, the upsetting force is ≥530 KN, the welding time is controlled between 120 and 125 s, the holding pressure and upsetting time is controlled ≤35 s, the initial melting velocity is controlled between 0.30 and 0.40 mm / s, and the final melting velocity is controlled between 2.40 and 2.60 mm / s; 4) Air cool the high-temperature welded joint to below 200°C to ensure complete phase change inside and outside the joint; 5) Joint heat treatment heating process: Use the joint dual-frequency normalizing machine for induction heating treatment. First, use 67-75kW low-frequency power to heat the joint from 0-200℃ to 830-850℃. The low-frequency heating time is controlled at 85-100s. Low-frequency heating is used to ensure that the core of the rail head can be heated to the required temperature. After the joint reaches the target temperature through low-frequency heating, the joint is immediately subjected to high-frequency induction heating. Use 50-65kW high-frequency power to heat the joint from 830-850℃ to 890-910℃. The low-frequency heating time is controlled at 60-80s. High-frequency heating ensures that the temperature within 0-15mm below the tread of the rail head can be heated to the target temperature. The total joint heating time is controlled between 140 and 180s. 6) Joint heat treatment controlled cooling process: After the joint reaches the target temperature of 890-910°C, stop heating and immediately perform air jet accelerated cooling on the joint and rail bottom. The air jet cooling nozzle is 20-30mm away from the top surface of the rail, 20-30mm away from the side of the rail head, and 50-60mm away from the rail bottom; the air jet pressure is 0.25-0.30MPa, and the air jet time is 110-130s; when the rail bottom surface temperature reaches between 420-440°C, stop air jet cooling and let the joint cool naturally to room temperature.

2. The fixed flash welding and joint heat treatment method according to claim 1 further comprises the following steps: failure treatment, straightening and flaw detection.

3. The method for fixed flash welding and joint heat treatment according to claim 1 or 2, wherein the obtained joint tensile strength Rm ≥ 1150MPa, the joint extension A ≥ 10%, the joint impact KU2 ≥ 17J, and the average hardness H of the joint J Average hardness of base material H P The ratio satisfies: H J / H P ≥0.93, average hardness of joint soft spot H J1 Average hardness of base material H P Ratio: H J1 / H P ≥0.83, softening area W≤18mm, weld and heat affected zone are typical pearlite structure.

4. The fixed flash welding and joint heat treatment method according to any one of claims 1 to 3, wherein the fixed flash welder is a GAAS80 / 580 fixed flash welder.

5. The fixed flash welding and joint heat treatment method according to any one of claims 1 to 4, wherein the U75VH online heat treatment rail for heavy-duty railway is produced through smelting, continuous casting, slow cooling of steel billets, rolling and heat treatment processes, wherein the size of the rail continuous casting billet is 280 mm×380 mm, the rolling section is 75 kg / m (75, 75N), the rolling compression ratio of the rail is greater than or equal to 11:1, the final rolling temperature of the rail is not higher than 950°C, and the final cooling temperature of the online heat treatment of the rail is not higher than 520°C.

6. A welded steel rail with heat-treated joints, obtained by the method according to any one of claims 1 to 5.

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