Optimization of Standard Rail Rolling Process and Track Connection Accuracy
What are the core process control parameters of fixed-length rolling of national standard rails?
The core process control parameters of fixed-length rolling of national standard rails focus on rolling speed, flying shear cutting accuracy, cold bed cooling rate and straightening accuracy, which jointly ensure the fixed-length accuracy and shape quality of rails. The rolling speed is adjusted according to the rail type: the finishing rolling speed of 60kg/m rail is controlled at 8-10m/s, and that of 50kg/m rail is controlled at 10-12m/s. Excessively slow speed will cause overheating and oxidation of the rail head, while excessively fast speed makes it difficult to control the rolling size. Flying shear cutting accuracy is the core of fixed length, the rail running speed during cutting is synchronized with the flying shear speed with a synchronization error ≤0.1m/s, the fixed-length cutting deviation is controlled within ±5mm, and the verticality deviation of the cutting notch is ≤0.5° to avoid the influence of notch skew on docking accuracy. The cold bed cooling adopts stepped rate cooling, the cooling rate of the rail when entering the cold bed is 15-20℃/h, and when the temperature drops below 400℃, the cooling rate is reduced to 5-8℃/h to prevent the rail from warping and deformation due to too fast cooling speed and ensure the straightness of the rail. In terms of straightening accuracy control, a multi-roll straightener is used to straighten the cooled rail, the reduction of the straightening roll is adjusted according to the rail deformation, the straightening reduction of 60kg/m rail is 0.3-0.5mm, after straightening, the straightness deviation of the rail is ≤0.5mm/m, and the total length bending is ≤10mm, laying the foundation for subsequent fixed-length use. In addition, the cross-sectional dimension deviation of the rail should be controlled within ±0.3mm to ensure the cross-sectional consistency of the same fixed-length rail.
What are the adaptive line working conditions of 25m and 50m fixed-length national standard rails?
The adaptive line working conditions of 25m and 50m fixed-length national standard rails are mainly divided according to the line operation speed, axle load and laying scene, matching the smoothness and construction convenience requirements of different working conditions. 25m fixed-length rail is the basic fixed-length specification, adapted to ordinary speed railways, urban rail transit and railway station yard lines. Such lines have an operation speed ≤160km/h and small axle load, the influence of the number of butt joints is relatively limited. In addition, the single weight of 25m rail is moderate, which is convenient for on-site transportation, hoisting and laying, with high construction flexibility, and can adapt to the complex laying scene with many curves and joints in station yard lines. 50m fixed-length rail is a large fixed-length specification, adapted to the main lines of high-speed railways and heavy-haul railways. High-speed railways have an operation speed ≥250km/h and extremely high requirements for line smoothness. 50m fixed-length can reduce the number of line butt joints by 50% compared with 25m, greatly reducing the impact of joints on train operation. Heavy-haul railways have large axle load, 50m fixed-length can reduce stress concentration at joints and improve the overall bearing capacity of the line. In addition, 50m fixed-length rail is more suitable for the welding construction of seamless lines, reducing the number of welding joints and improving the integrity of seamless lines, while 25m fixed-length rail is more suitable as an auxiliary rail for seamless lines due to many welding joints, used in special parts such as curve sections and turnout areas.
What are the line laying differences between fixed-length rolled national standard rails and non-fixed-length rails?
The line laying differences between fixed-length rolled national standard rails and non-fixed-length rails are mainly reflected in four aspects: number of joints, docking accuracy, construction efficiency and line smoothness. Fixed-length rails (25m/50m) are produced according to fixed length, the length deviation of rails in the same batch is extremely small, the number of butt joints is greatly reduced during laying, the number of joints of 50m fixed-length rail is only 1/4 of that of non-fixed-length rail, and 25m fixed-length is 1/2, while the length of non-fixed-length rail is uneven with a deviation of up to 1-2m, requiring a lot of cutting and splicing during laying and resulting in a large number of joints. In terms of docking accuracy, fixed-length rails have high notch verticality and cross-sectional dimension consistency, the height deviation and left-right misalignment during docking can be controlled within 0.1mm with excellent docking accuracy, while non-fixed-length rails have large notch skew and cross-sectional deviation, the installation deviation during docking often exceeds 0.5mm, affecting the docking quality. In terms of construction efficiency, fixed-length rails do not require a lot of on-site cutting and can be directly hoisted and docked, the laying efficiency is more than 60% higher than that of non-fixed-length rails, while non-fixed-length rails require measuring, cutting and grinding one by one, with tedious construction procedures and time-consuming and labor-intensive. In terms of line smoothness, fixed-length rails have few joints and high line continuity, the wheel-rail impact during train operation is small, and the smoothness index meets the requirements of high-speed and heavy-haul lines, while non-fixed-length rails have many joints and frequent wheel-rail impact, resulting in poor line smoothness, easy to cause train jolting and increase wheel-rail wear.
What is the influence of cold bed cooling process of fixed-length rolling of national standard rails on rail performance?
The cold bed cooling process of fixed-length rolling of national standard rails directly affects the metallographic structure, mechanical properties and dimensional stability of rails, and is a key link in the quality control of fixed-length rails. The fixed-length rail adopting stepped cooling process has a uniform metallographic structure of pearlite + a small amount of ferrite with a grain size of grade 7-8, uniform surface hardness of HRC30-35, core hardness of HRC28-30, and good matching of strength and toughness. However, rapid cooling will lead to brittle phases such as martensite in the rail metallographic structure, increasing the brittleness of the rail and easily causing cracks, while slow cooling will lead to coarse grains and reduce the strength and wear resistance of the rail. The uniformity of cooling rate can effectively prevent the rail from warping and deformation, the stepped cooling keeps the cooling speed of the upper and lower surfaces, head and bottom of the rail consistent with a temperature difference controlled within 5℃, avoiding internal stress caused by uneven temperature and leading to rail bending. The straightness of fixed-length rail is the basis for ensuring line laying accuracy, if the rail is deformed, it cannot meet the fixed-length docking requirements. In addition, a reasonable cold bed cooling process can reduce the formation of oxide scale on the rail surface, water mist cooling is used to assist during cooling, so that the thickness of oxide scale on the rail surface is controlled within 0.1mm, reducing the subsequent grinding workload and improving the surface quality of the rail, while improper cooling will lead to too thick oxide scale and even surface pitting, affecting the service performance of the rail.
What are the key construction process points of on-site docking of fixed-length national standard rails?
The key construction process points of on-site docking of fixed-length national standard rails focus on pre-treatment before docking, docking positioning and joint fixing to ensure docking accuracy and joint quality, matching the laying requirements of fixed-length rails. Before docking, the docking end face of the rail needs to be pretreated, the notch is polished with a special sander to remove oxide scale and burrs, the flatness deviation of the polished end face is ≤0.05mm, the verticality deviation is ≤0.1°, and the impurities on the top and side of the rail are cleaned at the same time to ensure the cleanliness of the docking surface. Docking positioning adopts special positioning tooling to align the center lines of two fixed-length rails with a center line deviation ≤0.1mm, adjust the height and left-right position of the rails so that the height deviation ≤0.1mm and left-right misalignment ≤0.1mm, the positioning tooling must be firmly fixed to prevent rail displacement during the docking process. Joint fixing is divided into temporary fixing and permanent fixing, temporary fixing uses clamps to clamp the rail joint with uniform clamping force to avoid rail deformation, permanent fixing is selected according to the line type: ordinary speed lines are connected with fish plates, the tightening torque is controlled at 400-450N·m, high-speed and heavy-haul lines adopt flash butt welding, after welding, the weld is precision ground to make the weld area flush with the rail base material, the surface roughness Ra≤0.8μm. After the docking is completed, the joint must be inspected, the height deviation is detected with a level gauge, and the left-right misalignment is detected with a caliper, unqualified joints need to be readjusted and ground. In addition, after the docking of fixed-length rails of seamless lines, stress relief treatment must be carried out, and the locked rail temperature is controlled at 20-25℃ to ensure the stability of the seamless line.