A production method for improving the straightness of rail ends after online heat treatment

Abstract

This invention relates to a production method for improving the straightness of rail ends after online heat treatment. After rolling, the rail enters an online heat treatment unit for cooling. The cooling process is divided into three stages: In the first stage, the rail is accelerated by supersonic jet cooling. When the rail head surface cools to 540℃~580℃ and the rail base surface cools to 600℃~650℃, it enters the second cooling stage. In the second stage, the rail continues to be accelerated cooled. When the rail head surface cools to 500℃~520℃ and the rail base surface cools to 560℃~600℃, it enters the third cooling stage. In the third stage, the rail is slowly cooled by subsonic jet cooling. When the rail head surface cools to 460℃~480℃ and the rail base surface cools to 500℃~520℃, the jet cooling stops, and the rail cools naturally in the air. When the rail temperature reaches 20℃~50℃, it enters a straightening unit for straightening. This invention effectively controls the cross-sectional temperature difference of the rail after heat treatment, improving the straightness of the rail ends.

Description

Technical Field

[0001] This invention relates to the field of steel materials technology, and in particular to a production method for improving the straightness of the ends of steel rails subjected to online heat treatment. Background Technology

[0002] Railway transportation, characterized by safety and efficiency, is widely used in passenger and freight transport, as well as urban rail transit, holding a central position in my country's transportation system and serving as the lifeline of the national economy. With the comprehensive completion of my country's high-speed railway network, higher requirements have been placed on key indicators such as rail performance, dimensional accuracy, and straightness. Rail straightness refers to the irregularity along the longitudinal direction of the rail and is one of the important indicators for measuring the physical quality of the rail. Rail straightness directly affects train speed and passenger comfort. Poor rail straightness leads to uneven welded joints, further affecting the smoothness of seamless railways, thus exacerbating wheel-rail impact and continuously deteriorating the stress state of the wheel and rail. This not only affects railway transport speed but also significantly reduces the service life of the rails. In more serious cases, it can lead to rail derailment and overturning, endangering train safety.

[0003] The current Chinese railway rail technical standard TB / T2344.1-2020 sets forth further requirements for the straightness of rail ends for high-speed railways with a speed of ≥200km / h. In actual production, due to differences in the amount of metal in the rail head and base, the cooling rates of the rail head and base differ significantly. This results in a noticeable temperature difference across the rail cross-section after online heat treatment, causing the rail ends to bend in different directions and to varying degrees due to temperature stress during cooling. Current straightening processes cannot stably control the straightness of the rail ends after straightening. Excessive straightening force can cause straightening defects during the straightening process, and in more serious cases, can lead to rail breakage.

[0004] Patent CN201811093074.9, "Online Heat Treatment Straightness Control Method for Rails," includes the following steps: 1) Bending treatment: After rolling, the hot rail is controlled by a bending machine to have a curvature of 0.4–1.0 mm / 1.5 m towards the rail base; 2) Heat treatment: The accelerated cooling process during rail heat treatment is divided into two stages. The first stage of accelerated cooling lasts for 60–100 seconds, with a flow rate ratio of 4–7:1 for the cooling medium applied to the rail head and rail base; the second stage of accelerated cooling lasts for 25–50 seconds, with a flow rate ratio of 1:3–6 for the cooling medium applied to the rail head and rail base; after heat treatment, the temperature of the rail head is 60–120°C lower than the temperature of the rail base. The straightness of the rail obtained by the online heat treatment straightness control method of this invention is 0.9–1.2 mm / 1.5 m. However, the straightness of the rail obtained by this invention is 0.9 to 1.2 mm / 1.5 m, which does not meet the requirements of the TB / T 2344.1-2020 technical standard, and it does not provide specific details on the straightness of the rail ends.

[0005] Patent CN202011475289.4, "Method for Improving the Straightness of the Rail Front End Before Straightening," describes a method that improves the straightness of the rail front end by appropriately reducing the rolling bite speed of the finishing mill. This results in more uniform temperature at each end of the rolled piece, reduces residual stress within the piece, and creates a more stable incoming material curvature for pre-bending. This improves the straightness of the rail front end before straightening, achieving elasto-plastic deformation, shortening the pressing time, and reducing the inverse bending rate. Furthermore, the reduced rolling speed on the finishing mill effectively reduces high-speed rolling collisions, lowers the motor load, and improves the ability of the pressing system to perform segmented speed-up rolling. Practical experience shows that this invention can solve the problem of steel bending at the finishing mill, which affects the straightness of the end, while still meeting standard requirements for specifications and residual stress. However, this invention only controls the straightness during the rail rolling process and is not applicable to rails subjected to online heat treatment.

[0006] Patent CN202110037685.7, "A Production Control Method for Improving the Straightness of Rails After Online Heat Treatment and Quenching," describes a method that uses water mist cooling to rationally control the cooling intensity and temperature difference between the rail head and bottom, ensuring that the bending of the rail after heat treatment is controlled within an ideal range. After cooling, the rail is straightened using light pressure, and the final mechanical properties and straightness of the rail meet the technical requirements of TB / T2344-2012 standard. However, this invention does not specify whether the rails obtained through this method meet the technical requirements of TB / T2344.1-2020 regarding the end straightness of rails for high-speed railways with a speed ≥200km / h, nor does it provide specific details on the straightness performance after rail straightening. Furthermore, this invention uses a mixed medium for cooling, which has poor cooling uniformity and is sensitive to the surface condition of the rail. In cases where the rail surface condition is poor, it can easily lead to low quenching hardness or the formation of harmful structures such as martensite, making production management difficult.

[0007] Patent CN202211110868.8, "A Production Control Method for Improving the Straightness of Turnout Rails After Quenching in Online Heat Treatment," applies a certain cooling rate to different parts of the rail to maintain a certain temperature difference range. This ensures that the rail maintains ideal straightness both during and after heat treatment, and also maintains good straightness after slow cooling to room temperature. This invention, by rationally controlling the cooling intensity and temperature difference at the rail head, rail base, and left and right ends, keeps the bending of the turnout rail within an ideal range after heat treatment. Light pressure straightening is then applied after cooling, and the final mechanical properties and straightness of the rail meet the technical requirements of TB / T2344-2012 standard. However, this invention applies different cooling rates to the two sides of the rail head, which can easily lead to uneven microstructure and properties after heat treatment, making it unsuitable for the production of rails with online heat treatment.

[0008] Patent CN202210606235.X, entitled "A Method for Controlling Local Lateral Bending of Rail Head After Online Heat Treatment," employs a heat treatment unit equipped with several sets of side guide wheels. Some sets of side guide wheels at the outlet of the heat treatment unit are staggered left and right, with one set of non-staggered side guide wheels spaced between adjacent sets of staggered side guide wheels. This method utilizes existing roller conveyor side guide wheels to implement a staggered arrangement, applying bending deformation to the rail to correct local lateral bending of the rail head. This method can effectively control local lateral bending of the rail head after online heat treatment without increasing additional costs, ensuring that the maximum local lateral bending of the rail head after exiting the heat treatment unit is less than 30mm. This effectively avoids the rail deviating from the transverse roller conveyor and thus prevents rejection, improving the rail yield. However, this invention only controls the lateral bending of the rail and does not address the straightness of the rail end. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a production method for improving the end straightness of online heat-treated rails, so that the produced 100-meter online heat-treated rails have good end straightness.

[0010] To achieve the above objectives, the present invention employs the following technical solution:

[0011] A production method for improving the straightness of rail ends after online heat treatment, comprising the following rail production steps: vacuum smelting → continuous casting billet → walking beam furnace → high-pressure water descaling → universal rolling of rails → online heat treatment → cooling bed → straightening → flaw detection → online inspection → warehousing and delivery; wherein:

[0012] 1) Online heat treatment process

[0013] After rolling, the rails enter an online heat treatment unit for cooling. The cooling process is divided into three stages:

[0014] In the first stage, the rails are accelerated by supersonic jet airflow. When the rails enter the cooling unit, their temperature is 720-800℃. The roller speed of the cooling unit is 1.2-1.8m / s, the jet velocity is 1.0-2.0 Mach, the rail head cooling rate is 5.0-7.0℃ / s, and the rail bottom cooling rate is 40%-60% of the rail head cooling rate. When the rail head surface is cooled to 540-580℃ and the rail bottom surface is cooled to 600-650℃, the second stage begins.

[0015] In the second stage, the rails continue to be cooled by supersonic jet air. The cooling rate of the rail head is 3.0 to 4.0℃ / s, and the cooling rate of the rail bottom is 80% to 120% of the cooling rate of the rail head. When the surface of the rail head is cooled to 500℃ to 520℃ and the surface of the rail bottom is cooled to 560℃ to 600℃, the third stage begins.

[0016] In the third stage, the rails are slowly cooled by subsonic jet airflow with a jet velocity of 0.1 to 0.5 Mach. The rail head cooling rate is 2.0 to 3.0℃ / s, and the rail bottom cooling rate is 100% to 150% of the rail head cooling rate. When the rail head surface cools to 460℃ to 480℃ and the rail bottom surface cools to 500℃ to 520℃, the airflow cooling is stopped, and the rails are allowed to cool naturally in the air.

[0017] 2) Straightening process

[0018] The rails are straightened when their temperature is between 20℃ and 50℃.

[0019] Step 1) In the online heat treatment process, the first and second stages of accelerated cooling adopt supersonic jet air-cooled units.

[0020] Step 1) In the third stage of the online heat treatment process, slow cooling is achieved using a subsonic jet air-cooled unit.

[0021] Step 2) In the straightening process, the straightening unit is an 8+1 horizontal-vertical composite straightening unit with a straightening speed of 1.6-2.2 m / s. The 8+1 horizontal-vertical composite straightening unit has a total of 9 horizontal rollers, numbered 1 to 9. Among them, rollers 2, 4, 6, and 8 are downward straightening rollers. The downward pressure of roller 2 is 25-21 mm, that of roller 4 is 16-12 mm, that of roller 6 is 10-7 mm, and that of roller 8 is 6-4 mm.

[0022] Compared with existing technologies, the beneficial effects of this invention are:

[0023] This invention employs supersonic jet cooling to increase the cooling rate of the rail head during online heat treatment. Simultaneously, it adjusts the cooling rate of the rail base, effectively controlling the cross-sectional temperature difference of the rail after heat treatment. Furthermore, it adjusts the process parameters for subsequent straightening, ensuring that the vertical straightness of the 0-1.5m and 1-2.5m sections of the 100-meter online heat-treated rail produced by this method is ≤0.2mm / 1.5m. Its mechanical properties and straightness meet the technical requirements of the TB / T2344.1-2020 standard. By improving the straightness of the rail ends, this invention increases the production efficiency of online heat-treated rails, enhances product competitiveness, and improves the safety of rail use, resulting in significant economic and social benefits. Attached Figure Description

[0024] Figure 1 Metallographic image of U71Mn steel rail (x200).

[0025] Figure 2 X500 image of the metallographic structure of U71Mn steel rail. Detailed Implementation

[0026] It should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used herein in the specification is for the purpose of describing particular embodiments only and is not intended to limit the scope of the application. The reference to "embodiment" herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0027] A production method for improving the straightness of rail ends after online heat treatment, comprising the following rail production steps: vacuum smelting → continuous casting billet → walking beam furnace → high-pressure water descaling → universal rolling of rails → online heat treatment → cooling bed → straightening → flaw detection → online inspection → warehousing and delivery; wherein:

[0028] 1) Online heat treatment process

[0029] After rolling, the rails enter an online heat treatment unit for cooling. The cooling process is divided into three stages:

[0030] The first stage involves accelerated cooling of the rails using supersonic jet airflow. Upon entering the cooling unit, the rail temperature is 720–800℃. The roller speed of the cooling unit is 1.2–1.8 m / s, the jet velocity is Mach 1.0–2.0, the rail head cooling rate is 5.0–7.0℃ / s, and the rail base cooling rate is 40%–60% of the rail head cooling rate. When the rail head surface cools to 540℃–580℃ and the rail base surface cools to 600℃–650℃, the second cooling stage begins. The purpose of the first stage, using supersonic jet airflow cooling, is to increase the surface cooling rate of the rails, obtain a fine lamellar pearlite microstructure, and improve the tensile, hardness, and other mechanical properties of the rails. The microstructure type is fine lamellar pearlite.

[0031] In the second stage, the rails continue to be cooled using supersonic jet airflow. The cooling rate of the rail head is 3.0–4.0℃ / s, and the cooling rate of the rail base is 80%–120% of the rail head cooling rate. When the rail head surface cools to 500℃–520℃ and the rail base surface cools to 560℃–600℃, the third cooling stage begins. The purpose of the accelerated cooling in the second stage is that the large cooling rate applied to the rail surface in the first stage results in a large temperature difference between the interior and the surface, causing a significant temperature rebound on the rail surface. The second stage further accelerates the cooling of the rails to facilitate effective control of the temperature distribution across the rail cross-section in the third stage.

[0032] In the third stage, the rails are slowly cooled using subsonic jet cooling at a velocity of 0.1–0.5 Mach. The rail head cooling rate is 2.0–3.0 °C / s, and the rail base cooling rate is 100%–150% of the rail head cooling rate. Cooling is stopped when the rail head surface reaches 460–480 °C and the rail base surface reaches 500–520 °C, at which point the jet cooling stops, and the rails are allowed to cool naturally in the air. The purpose of this third stage cooling is to adjust the cross-sectional temperature distribution of the rails after heat treatment. Due to the different metal content at the rail head and base, the different cooling rates during natural air cooling after the heat treatment unit cause the rails to bend under thermal stress, affecting their straightness. By controlling the cross-sectional temperature distribution, the straightness of the rails before straightening can be effectively improved.

[0033] 2) Straightening process

[0034] The rails are straightened when their temperature is between 20℃ and 50℃.

[0035] Step 1) In the online heat treatment process, the first and second stages of accelerated cooling adopt supersonic jet air-cooled units.

[0036] Step 1) In the third stage of the online heat treatment process, slow cooling is achieved using a subsonic jet air-cooled unit.

[0037] Step 2) In the straightening process, the straightening unit is an 8+1 horizontal-vertical composite straightening unit with a straightening speed of 1.6–2.2 m / s. The 8+1 horizontal-vertical composite straightening unit consists of 9 horizontal rollers, numbered 1 to 9. Among them, rollers 2, 4, 6, and 8 are downward straightening rollers. The downward pressure of roller 2 is 25–21 mm, that of roller 4 is 16–12 mm, that of roller 6 is 10–7 mm, and that of roller 8 is 6–4 mm. The downward pressure of rollers 2, 4, 6, and 8 gradually decreases. The straightening process adopts a "large deformation + small deformation" scheme. Rollers 2 and 4 apply a large amount of pressure to quickly reduce the original curvature difference of the rail, while rollers 6 and 8 apply small deformation to eliminate residual curvature, thereby improving the straightness of the straightened rail. The large deformation + small deformation straightening scheme involves applying a large reduction amount to rollers 2 and 4 to rapidly reduce the original curvature difference of the rail, followed by applying a small deformation to rollers 6 and 8 to ensure the straightened straightness. Increasing the reduction amount of straightening rollers 2 and 4 causes sufficient plastic deformation in the rail, which can rapidly reduce the original curvature differences of different directions, positions, and sizes at the rail ends. The small deformation applied by rollers 6 and 8 eliminates residual curvature, thereby improving the straightness of the straightened rail.

[0038] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0039] Example

[0040] The production process of 60kg / m U71Mn steel rail is as follows: vacuum smelting → continuous casting billet → walking beam furnace → high-pressure water descaling → universal rolling of steel rail → online heat treatment → cooling bed → straightening → flaw detection → online inspection → warehousing and delivery.

[0041] The parameters for the online heat treatment process in the embodiment are shown in Table 1, the parameters for the straightening process in the embodiment are shown in Table 2, and the performance of the rail in the embodiment is shown in Table 3.

[0042] Table 1. Online heat treatment process parameters for the rails in the embodiment.

[0043]

[0044] Table 2. Straightening process parameters for the rails in the embodiments.

[0045]

[0046]

[0047] Table 3 Performance of the Rails in Examples

[0048]

[0049] As can be seen from the examples, the vertical straightness of the 0-1.5m and 1-2.5m ends of the 100-meter online heat-treated steel rail produced by the present invention is ≤0.2mm, and its mechanical properties and end straightness meet the technical requirements of TB / T2344.1-2020 standard.

[0050] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A production method for improving the straightness of rail ends after online heat treatment, characterized in that, The rail production process is as follows: vacuum smelting → continuous casting billet → walking beam furnace → high-pressure water descaling → universal rolling of rails → online heat treatment → cooling bed → straightening → flaw detection → online inspection → warehousing and delivery; among which: 1) Online heat treatment process After rolling, the rails enter an online heat treatment unit for cooling. The cooling process is divided into three stages: In the first stage, the rails are accelerated and cooled using supersonic jet airflow at a velocity of 1.0–2.0 Mach. When the rails enter the cooling unit, their temperature is 720–800℃. The roller speed of the cooling unit is 1.2–1.8 m / s, the rail head cooling rate is 5.0–7.0℃ / s, and the rail bottom cooling rate is 40%–60% of the rail head cooling rate. When the rail head surface cools to 540℃–580℃ and the rail bottom surface cools to 600℃–650℃, the second stage of cooling begins. In the second stage, the rails continue to be cooled by supersonic jet airflow at a speed of 1.0 to 2.0 Mach. The rail head cooling rate is 3.0 to 4.0 °C / s, and the rail bottom cooling rate is 80% to 120% of the rail head cooling rate. When the rail head surface cools to 500 °C to 520 °C and the rail bottom surface cools to 560 °C to 600 °C, the third stage of cooling begins. In the third stage, the rails are slowly cooled by subsonic jet airflow with a jet velocity of 0.1 to 0.5 Mach. The rail head cooling rate is 2.0 to 3.0℃ / s, and the rail bottom cooling rate is 100% to 150% of the rail head cooling rate. When the rail head surface cools to 460℃ to 480℃ and the rail bottom surface cools to 500℃ to 520℃, the airflow cooling is stopped, and the rails are allowed to cool naturally in the air. 2) Straightening process The rails are straightened when their temperature is between 20℃ and 50℃.

2. The production method for improving the straightness of rail ends after online heat treatment according to claim 1, characterized in that, Step 2) In the straightening process, the straightening unit is an 8+1 horizontal-vertical composite straightening unit with a straightening speed of 1.6-2.2 m / s. The 8+1 horizontal-vertical composite straightening unit has a total of 9 horizontal rollers, numbered 1 to 9. Among them, rollers 2, 4, 6, and 8 are downward straightening rollers. The downward pressure of roller 2 is 25-21 mm, that of roller 4 is 16-12 mm, that of roller 6 is 10-7 mm, and that of roller 8 is 6-4 mm.

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