A method for low residual stress straightening of high strength hot rolled rails

Abstract

The application discloses a high-strength hot-rolled rail low-residual stress straightening method and belongs to the technical field of rail production processes. The method realizes effective reduction of the bending degree of the rail before straightening and precise control of uneven deformation in the straightening process by implementing the precise reverse pre-bending process on the rail cooling stage, cooperating with the optimized straightening machine horizontal roll reduction parameters and the straightening speed control, finally controls the rail bottom residual stress of the 60kg / m (60, 60N) high-strength hot-rolled rail below 250MPa, and reduces 37-46MPa compared with the traditional process, significantly improves the rail straightness and use safety, and enhances the product market competitiveness.

Description

Technical Field

[0001] This invention belongs to the field of rail manufacturing technology, and particularly relates to a method for straightening high-strength hot-rolled rails with low residual stress. Background Technology

[0002] With the development of the railway transportation industry, higher requirements have been placed on the strength and safety of rails. High-strength hot-rolled rails, with a tensile strength Rm≥1080MPa, are more than 100MPa higher than traditional U75V rails, making them the preferred material for railway tracks. However, while the strength and hardness of rails have increased, the residual stress at the rail base has also increased. Furthermore, due to factors such as asymmetrical rolling and uneven cooling during rail production, residual stress is inevitably generated internally. Excessive residual tensile stress at the rail base can easily lead to early rail failure. During operation, the superposition of traffic stress and residual stress can also trigger crack initiation and propagation, seriously threatening railway transportation safety. Current rail straightening processes only focus on ensuring rail straightness, without optimizing straightening parameters for the characteristics of high-strength rails, and without precise control over bending deformation during the rail cooling process. This results in large deformation during straightening, severe uneven deformation in different parts of the rail, and ultimately, high residual stress at the rail base after straightening, failing to meet the industry standard TB / T2344-2012 requirement of residual stress ≤250MPa. Therefore, there is an urgent need for a low residual stress straightening method suitable for 60kg / m (60, 60N) high-strength hot-rolled steel rails, which can effectively control residual stress through process optimization, taking into account both rail straightness and safety in use. Summary of the Invention

[0003] To address the technical problems of excessive residual stress at the rail base and the tendency to cause early failure in the existing straightening process for 60kg / m (60, 60N) high-strength hot-rolled rails, the present invention aims to provide a low residual stress straightening method for high-strength hot-rolled rails. Through the synergistic optimization of pre-bending and straightening processes, the residual stress of the rail can be precisely controlled while ensuring the straightness of the rail.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] This invention discloses a method for straightening high-strength hot-rolled steel rails with low residual stress, the specific steps of which are as follows:

[0006] The reverse pre-bending process for rails is designed for 60kg / m high-strength hot-rolled rails. At the front of the cooling bed entrance, 36-39 pre-bending cars are used to perform reverse pre-bending operations. The forward speed of the middle pre-bending cars is controlled to be faster than that of the side pre-bending cars, causing the rail to bend in the opposite direction towards the rail base. A trapezoidal pre-bending line with a chord height of 1.6m is used as the pre-bending reference to counteract the bending deformation towards the rail head caused by differences in cooling and phase transformation during the natural cooling process of the rail. The pre-straightening curvature of a 100m rail is strictly controlled within 500mm, achieving basic straightness of the rail before straightening. This lays the foundation for reducing deformation and residual stress in subsequent straightening processes.

[0007] The precision rail straightening process involves feeding the pre-bent and cooled rails into the SMS straightening machine for straightening. It employs a gradient pressing + low-speed straightening control method. Specific parameters and operating requirements are as follows:

[0008] (1) Horizontal roll pressing parameters: The pressing amount is set for the four active horizontal rolls HR2, HR4, HR6 and HR8 of the straightening machine respectively. The rail is first subjected to moderate plastic deformation by gradient pressing, and then gradually fine-tuned to achieve straightness.

[0009] (2) Control of the difference in reduction amount: reduce the difference in reduction amount between adjacent horizontal rollers, reduce the drastic fluctuations in the degree of deformation during the straightening of rails, and reduce the residual stress caused by uneven deformation;

[0010] (3) Straightening speed control: The straightening speed is reduced to 2.08-2.12 m / s. Compared with the traditional straightening speed of 2.3 m / s, the deformation rate is reduced, the occurrence and development time of dislocation movement during plastic deformation is increased, the deformation resistance of the rail is improved, and the rail distortion is reduced.

[0011] (4) Straightening deformation control: HR2 and HR4 are used to achieve basic plastic deformation of the rails, so that rails from different materials form a consistent deformation trend. Then, the small deformation of HR6 and HR8 is used for fine adjustment to ensure the straightness of the rails and avoid excessive deformation leading to an increase in residual stress.

[0012] Furthermore, the straightening speed is 2.0 m / s.

[0013] Furthermore, the difference in the amount of pressure applied to adjacent rollers should be controlled to not exceed 8 mm.

[0014] Furthermore, the difference in the amount of pressure applied to adjacent rollers should be controlled to be ≤7.5mm.

[0015] Furthermore, the reduction amounts of the four active horizontal rollers HR2, HR4, HR6, and HR8 are as follows: HR2 reduces by 20mm, HR4 reduces by 15mm, HR6 reduces by 7.5mm, and HR8 reduces by 2mm.

[0016] Furthermore, this method reduces the drastic fluctuations in the degree of deformation during rail straightening by controlling the difference in the amount of pressure applied to adjacent horizontal rollers of the straightening machine. At the same time, it increases the occurrence and development time of dislocation movement during plastic deformation by reducing the straightening speed, thereby improving deformation resistance and reducing rail distortion.

[0017] Furthermore, the 60kg / m high-strength hot-rolled steel rail has a tensile strength ≥1100MPa and a Brinell hardness ≥310HB.

[0018] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0019] (1) This invention implements reverse pre-bending through a trapezoidal pre-bending line with a chord height of 1.6m, which accurately counteracts the bending deformation of the rail during natural cooling, and controls the pre-straightening curvature of 100m rail to within 500mm, significantly reducing the deformation of subsequent straightening processes and reducing the generation of residual stress from the source; (2) This invention optimizes and determines the optimal pressing parameters and straightening speed of the horizontal rollers of the SMS straightening machine for the characteristics of 60kg / m (60, 60N) high-strength hot-rolled rails. Through gradient pressing, reducing the difference in pressing amount between adjacent rollers, and coordinated control of low-speed straightening, it effectively reduces the uneven deformation of various parts of the rail during the straightening process and solves the technical problem of excessive residual tensile stress at the bottom of high-strength rails; (3) Using the method of this invention After straightening by the method, the maximum residual stress of the 60kg / m (60, 60N) high-strength hot-rolled steel rails was tested by the stress release method. The maximum residual stress of the rail bottom of the six samples was 192, 160, 218, 190, 202 and 204 MPa, respectively, with an average value of 194 MPa. This is 37 to 46 MPa lower than that of the traditional process, and the maximum residual stress is ≤250 MPa, which fully meets the requirements of the TB / T2344-2012 industry standard. (4) The method of the present invention takes into account both the straightness of the rail and the requirements of low residual stress, which significantly improves the safety of the use of 60kg / m (60, 60N) high-strength hot-rolled steel rails, reduces the early damage of the rails, and enhances the market competitiveness of the products. It is suitable for large-scale industrial production. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to specific embodiments. In this embodiment, a 60kg / m (60, 60N) H370 high-strength hot-rolled steel rail is used as the treatment object. The tensile strength of the steel rail is ≥1100MPa and the Brinell hardness is ≥310HB.

[0021] Comparative Example 1:

[0022] Comparative Example 1 describes a low residual stress straightening process for 60kg / m (60N) high-strength hot-rolled steel rails. The specific operation is as follows: The high-strength hot-rolled steel rail, after reverse pre-bending, is fed into the SMS straightening machine. The horizontal roll reduction of the straightening machine is set as follows: HR2 = 25mm, HR4 = 12.5mm, HR6 = 8.5mm, and HR8 = 3.5mm. The straightening speed is controlled at 2.3m / s, and other straightening conditions remain consistent. After implementation, the straightened rail straightness meets the standard. The average residual stress at the rail base center is 279MPa, which does not meet the standard requirement of ≤250MPa. The residual stress value is too high, and there is still room for optimization.

[0023] Comparative Example 2:

[0024] Comparative Example 2 describes the No. 2 process for straightening 60kg / m (60N) high-strength hot-rolled steel rails with low residual stress. The specific operation is as follows: The high-strength hot-rolled steel rails that have undergone reverse pre-bending are fed into the SMS straightening machine. The horizontal roll reduction of the straightening machine is set as follows: HR2 = 21mm, HR4 = 13mm, HR6 = 7mm, and HR8 = 3mm. The straightening speed is controlled at 2.3m / s, and the remaining straightening conditions are consistent with Example 1. After the process is implemented, the straightness of the rails after straightening meets the standard. The average residual stress at the center of the rail base is 265MPa, which is lower than that of the No. 1 scheme. However, it still does not meet the standard requirements, but the process optimization effect is initially apparent.

[0025] Example 1 A method for straightening a 60kg / m high-strength hot-rolled steel rail with low residual stress, the specific steps are as follows: (1) Reverse pre-bending: The high-strength steel rail after rolling is pre-bent in reverse by 39 sets of pre-bending cars. The pre-bending line is a trapezoid with a chord height of 1.6m. After cooling, the pre-straightening curvature of 100m of steel rail is 450mm; (2) Precise straightening: The horizontal roll reduction of the SMS straightening machine is set to HR2=20mm, HR4=15mm, HR6=7.5mm, HR8=2mm, the straightening speed is 2.0m / s, and the difference in reduction between adjacent rolls is controlled to be ≤7.5mm; (3) Residual stress detection: The residual stress at the bottom of the rail is detected by stress release method. The average residual stress is 191MPa, the maximum residual stress is 215MPa, which is 46MPa lower than the traditional process, meeting the industry standard requirements.

[0026] Example 2:

[0027] A method for straightening 60kg / m (60N) high-strength hot-rolled steel rail with low residual stress, the specific steps are as follows: (1) Reverse pre-bending: When the rolled high-strength hot-rolled steel rail is transported to the front end of the cooling bed entrance, 38 sets of pre-bending cars are started, the forward speed of the middle pre-bending car is controlled to be faster than that of the sides, and a trapezoidal pre-bending line with a chord height of 1.6m is used to perform reverse pre-bending of the rail towards the bottom of the rail. After cooling, the pre-straightening curvature of 100m rail is 480mm, which meets the control requirement of ≤500mm; (2) Precise straightening: The pre-bent rail is sent into the SMS straightening machine, and the horizontal roller pressing parameters are set as follows: HR2=20mm, HR4=15mm, HR6=7.5mm, HR8=2mm, straightening speed is adjusted to 2.0m / s, and the difference in the amount of pressure of adjacent rollers is controlled to not exceed 8mm. The plastic deformation and fine-tuning of the rail are completed in sequence; (3) Residual stress detection: According to the TB / T2344-2012 standard, the stress release method is used to sample and test the straightened rail. The maximum residual stress of the rail bottom of the 6 samples is 192, 160, 218, 190, 202 and 204MPa respectively, with an average value of 194MPa. None of them exceed the standard limit of 250MPa, and the average residual stress is reduced by 42MPa compared with the traditional process.

[0028] The embodiments described above are merely preferred embodiments 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 method for straightening high-strength hot-rolled steel rails with low residual stress, characterized in that: Specifically, it includes: The reverse pre-bending process for rails is designed for 60kg / m high-strength hot-rolled rails. At the front of the cooling bed entrance, 36-39 pre-bending cars are used to perform reverse pre-bending operations. The forward speed of the middle pre-bending cars is controlled to be faster than that of the side pre-bending cars, causing the rail to bend in the opposite direction towards the rail base. A trapezoidal pre-bending line with a chord height of 1.6m is used as the pre-bending reference to counteract the bending deformation towards the rail head caused by differences in cooling and phase transformation during the natural cooling process of the rail. The pre-straightening curvature of a 100m rail is strictly controlled within 500mm, achieving basic straightness of the rail before straightening. This lays the foundation for reducing deformation and residual stress in subsequent straightening processes. The precision rail straightening process involves feeding the pre-bent and cooled rails into the SMS straightening machine for straightening. It employs a gradient pressing + low-speed straightening control method. Specific parameters and operating requirements are as follows: (1) Horizontal roll pressing parameters: The pressing amount is set for the four active horizontal rolls HR2, HR4, HR6 and HR8 of the straightening machine respectively. The rail is first subjected to moderate plastic deformation by gradient pressing, and then gradually fine-tuned to achieve straightness. (2) Control of the difference in reduction amount: reduce the difference in reduction amount between adjacent horizontal rollers, reduce the drastic fluctuations in the degree of deformation during the straightening of rails, and reduce the residual stress caused by uneven deformation; (3) Straightening speed control: The straightening speed is reduced to 2.08-2.12 m / s. Compared with the traditional straightening speed of 2.3 m / s, the deformation rate is reduced, the occurrence and development time of dislocation movement during plastic deformation is increased, the deformation resistance of the rail is improved, and the rail distortion is reduced. (4) Straightening deformation control: HR2 and HR4 are used to achieve basic plastic deformation of the rails, so that rails from different materials form a consistent deformation trend. Then, the small deformation of HR6 and HR8 is used for fine adjustment to ensure the straightness of the rails and avoid excessive deformation leading to an increase in residual stress.

2. The method for straightening high-strength hot-rolled steel rails with low residual stress according to claim 1, characterized in that: Straightening speed: 2.0 m / s.

3. The method for straightening high-strength hot-rolled steel rails with low residual stress according to claim 1, characterized in that: Control the difference in the amount of pressure applied to adjacent rollers to not exceed 8mm.

4. The method for straightening high-strength hot-rolled steel rails with low residual stress according to claim 1, characterized in that: Control the difference in pressure reduction between adjacent rollers to ≤7.5mm.

5. The method for straightening high-strength hot-rolled steel rails with low residual stress according to claim 1, characterized in that: The pressing amounts of the four active horizontal rollers HR2, HR4, HR6 and HR8 are as follows: HR2 presses down 20mm, HR4 presses down 15mm, HR6 presses down 7.5mm and HR8 presses down 2mm.

6. The method for straightening high-strength hot-rolled steel rails with low residual stress according to claim 1, characterized in that: This method reduces the drastic fluctuations in the degree of deformation during rail straightening by controlling the difference in the amount of pressure applied to adjacent horizontal rollers of the straightening machine. At the same time, it increases the occurrence and development time of dislocation movement during plastic deformation by reducing the straightening speed, thereby improving deformation resistance and reducing rail distortion.

7. The method for straightening high-strength hot-rolled steel rails with low residual stress according to claim 1, characterized in that: The 60kg / m high-strength hot-rolled steel rail has a tensile strength ≥1100MPa and a Brinell hardness ≥310HB.