75-60kg / m u75vh on-line heat treatment special-shaped rail for heavy haul railway and its heel end heat treatment method

Abstract

The application discloses a 75-60kg / m U75VH online heat treatment special-shaped rail for heavy haul railway and a heel end heat treatment method, wherein the heel end heat treatment method is provided for the 75kg / m forged 60kg / m U75VH heat treatment special-shaped rail, and the heat treatment cooling process of the heel end is controlled by controlling the heating temperature of the forged special-shaped rail at the heel end and controlling the air pressure, cooling speed and final cooling temperature of the rail head side at different positions (a forming section, a transition section and a heat affected zone), so that the structure and performance difference between the high-temperature forged heel end of the 75-60kg / m U75VH special-shaped rail and the base material can be effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of rail forging and heat treatment technology, specifically relating to a 75-60kg / mU75VH special-shaped rail for heavy-haul railways with online heat treatment and its heel end heat treatment method. Background Technology

[0002] Currently, most heavy-haul lines and mixed passenger-freight lines in China use symmetrical rails with a cross-section of 50kg / m, 60kg / m, or 75kg / m within the standard range. Due to differences in transportation conditions, service environments, and train axle loads between existing lines and newly built lines, and considering the economic efficiency of rail laying, different rail types are used for different lines, roughly categorized as 50kg / m, 60kg / m, and 75kg / m rail lines. Connecting lines with different rail types has become a challenging and crucial aspect. Currently, forged special-shaped rails are used to connect lines with different rail cross-sections. This method of connecting forged special-shaped rails offers advantages such as safety and reliability, good mechanical properties, excellent structural composition, high utilization rate, and high cost-effectiveness, ensuring safe and stable train operation across lines with different rail cross-sections.

[0003] Special-shaped rails are manufactured using a method of heating and forging ordinary rails at the rail ends, achieving a transition from one type of rail cross-section to another. Because the microstructure of the special-shaped rail heel end after high-temperature die forging is coarse lamellar pearlite, its strength and hardness are low and cannot match the base material, making it difficult to meet the service requirements of the railway line. To refine the pearlite structure of the special-shaped rail and restore the performance of the forged area at the rail heel end, it is necessary to re-induction normalizing the forged area.

[0004] Patent application number 201310431374.4 discloses a heat treatment device and method for the forming and pressing of rail heel end sections. This patent mainly relates to a heat treatment device for forming rail heel end sections, but does not describe the heat treatment process or method for forming rail heel end sections. Patent application number 201910277768.6 discloses a heat treatment method for forged rail heel end sections. After heating the rail to 850-950℃, the rail head is cooled to 350-500℃ by air jet cooling. The rail head air jet cooling temperature in this patent is between 350-500℃. The excessively low cooling temperature leads to abnormal rail structure and excessive hardness of the formed section rail, and the wide cooling range is not suitable for the promotion and application of the patent. Zhang Dawei et al. (Zhang Dawei et al. Induction heat treatment of the heel end of 60AT1-U75V steel rail [J]. Metal Heat Treatment, 2016, 41(2): 176-180.) discussed the online heat treatment of 60AT1-U75V steel rails, and adopted medium-frequency induction preheating + medium-frequency induction heating + air jet cooling + spray cooling for the heel end. However, the performance requirements of this literature are different from those of the 75-60kg / m U75V hot-rolled steel rail standard of this invention, and the process is not universal. Moreover, the medium-frequency induction preheating and spray cooling process described in this literature increases the production process, production cost and control difficulty of special-shaped steel rails, and the spray cooling treatment of the rail head is prone to abnormal structure during continuous production.

[0005] Although patent document CN117126997A (hereinafter referred to as Document 1) has developed a method for heat treatment of the heel end of 60kg / m forged 50kg / m U75VH heat-treated steel rails, the standard performance requirements for the 75-60kg / m U75V hot-rolled steel rails of this invention are different from those of the 60-50kg / m U75VH heat-treated steel rails disclosed in Document 1. When the heel end heat treatment method disclosed in Document 1 is used to perform heel end heat treatment on 75-60kg / m U75V hot-rolled steel rails, it cannot meet the requirement that the hardness of the forged area and heat-affected zone at the heel end of the special-shaped steel rail matches that of the base material. Therefore, it is necessary to develop a heel end heat treatment method suitable for online heat treatment of 75-60kg / m U75VH special-shaped steel rails for heavy-haul railways. Summary of the Invention

[0006] To address the problems existing in the prior art, one aspect of the present invention provides a method for heat treatment of the heel end of a 75-60kg / mU75VH online heat-treated special-shaped steel rail for heavy-haul railways, comprising the following process steps: blanking → forging → heat treatment; wherein:

[0007] In the heat treatment process, the forged special-shaped rail at the heel end is heated from room temperature to 960±10℃, and after heat preservation, the heated part of the special-shaped rail at the heel end is subjected to accelerated cooling treatment by air jetting, including:

[0008] Cooling process for the forming section: The air pressure on the top surface of the rail head is 0.15-0.25MPa, the air pressure on the side surface of the rail head is 0.20-0.25MPa, the cooling rate of the rail head is 2.0-4.0℃ / s, and the forming section of the rail head is cooled from 960±10℃ to 500-530℃, and the return temperature is controlled at 550-580℃.

[0009] Transition section cooling process: The air pressure on the top surface of the rail head is 0.20-0.30MPa, the air pressure on the side of the rail head is 0.20-0.25MPa, the rail head cooling rate is 2.5-4.5℃ / s, and the rail head of the transition section is cooled from 960±10℃ to 470-490℃, and the return temperature is controlled at 540-580℃;

[0010] Cooling process for the heat-affected zone: The air pressure on the top surface of the rail head is 0.30-0.50MPa, the air pressure on the side surface of the rail head is 0.25-0.40MPa, the cooling rate of the rail head is 3.0-5.0℃ / s, and the rail head in the heat-affected zone is cooled from 960±10℃ to 440-490℃, and the return temperature is controlled at 490-540℃.

[0011] In some embodiments, during the forging process, the heel end of a 75kg / m section U75VH online heat-treated steel rail is die-forged at high temperature 2-3 times to form a 60kg / m rail, thereby creating a 75-60kg / m U75VH heat-treated special-shaped rail.

[0012] In some embodiments, the chemical composition of the U75VH online heat-treated steel 75kg / m section rail, by weight percentage, is C: 0.71-0.80%, Si: 0.50-0.80%, Mn: 0.75-1.05%, V: 0.04-0.12%, P≤0.025%, S≤0.025%, Al: ≤0.010%, with the remainder being Fe.

[0013] In some embodiments, heating the heel-end forged special-shaped rail from room temperature to 960±10℃ involves using a medium-frequency induction heater to induction heat the forging area and affected area of ​​the forged 75-60kg / m U75VH heat-treated special-shaped rail. The length of the heating zone after heat treatment of the forged special-shaped rail is greater than the length of the forging area, and the distance between the heating zone and the rail end of the forming section is controlled between 750-850mm.

[0014] In some embodiments, the medium-frequency induction heating frequency of the medium-frequency induction heater is between 1600-2100Hz, and the medium-frequency induction heating power is between 50-100kW, heating the forged special-shaped steel rail from room temperature to 960±10℃, with the heating time controlled between 220-300s.

[0015] In some embodiments, the heat preservation treatment lasts for 10-30 seconds.

[0016] In some embodiments, the area subjected to the accelerated cooling treatment by the air jet extends more than 70 mm beyond the area subjected to the normalizing heating.

[0017] In some implementations, the forged 75-60kg / m U75VH heat-treated special-shaped steel rails are cooled to the target temperature at different locations and then air-cooled to room temperature.

[0018] In some embodiments, the heel heat treatment method further includes the following process steps: shot blasting → straightening → milling → fine adjustment → length setting → drilling → flaw detection → finished product.

[0019] Another aspect of the present invention provides a 75-60kg / m U75VH online heat-treated special-shaped rail for heavy-haul railways, which is obtained by the above-described method after end heat treatment.

[0020] In some embodiments, the tensile strength Rm of the formed section of the heavy-haul railway 75-60kg / m U75VH online heat-treated special-shaped rail after end heat treatment is ≥1267MPa, and the elongation after fracture A is ≥12.0%; the hardness of the rail top surface of the formed section, transition section and heat-affected zone is between 340-370HBW; the hardness of the cross section 3mm from the surface of the formed section and transition section (A1, B1, C1) is ≥37.0HRC, and the hardness of the cross section 15mm from the surface (A5, B5, C5) is ≥36.0HRC.

[0021] In some embodiments, the microstructure of the forming section and transition section of the heavy-haul railway 75-60kg / m U75VH online heat-treated special-shaped rail after end heat treatment is consistent with the type of the base material, which is a typical pearlite structure with a small amount of ferrite. There is no abnormal structure and no overheating phenomenon is observed. The decarburized layer depth of the rail head in the forming section and transition section is about 0.20-0.35mm.

[0022] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0023] This invention addresses the issue of separate air spraying treatment of the forming section, transition section, and heat-affected zone during the heat treatment and cooling process of the heel end of a 75kg / m forged 60kg / m U75VH heat-treated profiled rail. By controlling the heating temperature of the forged profiled rail at the heel end, as well as the air pressure, cooling rate, and final cooling temperature at different locations (forming section, transition section, and heat-affected zone), the difference in microstructure and properties between the profiled rail heel end and the base material after high-temperature die forging is effectively improved. This solves the problems of low hardness of the top surface of the rail head in the forming section, transition section, and heat-affected zone after U75VH heat-treated rail is forged from 75kg / m to 60kg / m, excessive width of the S-zone, and easy occurrence of abnormal microstructure and excessive decarburization layer. It ensures that the strength, hardness, and toughness of the forged area and heat-affected zone of the profiled rail heel end match those of the base material.

[0024] Based on the above invention, the obtained 75kg / m forged 60kg / m U75VH heat-treated special-shaped steel rail has the following characteristics: tensile strength Rm ≥ 1267MPa in the forming section, elongation after fracture A ≥ 12.0%; the hardness of the rail top surface in the forming section, transition section, and heat-affected zone is between 340-370HBW; the hardness of the cross sections 3mm from the surface (A1, B1, C1) in the forming section and transition section is ≥ 37.0HRC, and the hardness of the cross sections 15mm from the surface (A5, B5, C5) is ≥ 36.0HRC; the microstructure of the forming section and transition section is consistent with the base material, both being typical pearlite with a small amount of ferrite, without any abnormal structure or overheating; the decarburized layer depth of the rail head in the forming section and transition section is approximately 0.20-0.35mm, and all mechanical properties and metallographic structures meet the standard requirements. Detailed Implementation

[0025] This invention aims to provide a method for heat treatment of the heel end of 75-60kg / m U75VH special-shaped steel rails for heavy-haul railways using online heat treatment, which specifically includes:

[0026] 1) The raw material of U75VH heat-treated steel rail with a cross section of 75kg / m uses C, Si, Mn and V as the main strengthening elements. The weight percentage of the chemical composition of the rail is C: 0.71-0.80%, Si: 0.50-0.80%, Mn: 0.75-1.05%, V: 0.04-0.12%, P≤0.025%, S≤0.025%, Al: ≤0.010%, and the remainder is Fe.

[0027] 2) The U75VH online heat-treated steel with the above composition is used to form a 75kg / m cross-section rail. The rail end is forged at high temperature 2-3 times to form a 60kg / m rail, thus forming a 75-60kg / m U75VH heat-treated special-shaped rail.

[0028] 3) Place the forging area and affected area of ​​the forged 75-60kg / m U75VH heat-treated special-shaped steel rail at the heel end into a medium-frequency induction heater for induction heating. The length of the heating zone for the heat treatment of the special-shaped steel rail after forging should be greater than the length of the forging area, and the distance between the heating zone and the rail end of the forming section should be controlled between 750-850mm.

[0029] 4) Adjust the medium-frequency induction heating device, with the heating frequency between 1600-2100Hz and the medium-frequency induction heating power between 50-100kW, to heat the forged special-shaped rail from room temperature to 960±10℃, with the heating time controlled between 220-300s. Medium-frequency heating can ensure that the surface of the high-temperature die-forged area and the core of the rail head are heated to the temperature range required for austenitization of the rail.

[0030] 5) After heating the heel end of the irregular-shaped rail to the target temperature of 960±10℃, reduce the power of the medium-frequency induction heating and keep the heel end at the target temperature for 10-30 seconds. Insulate the heated part of the heel end to improve the temperature uniformity of the rail head core and surface.

[0031] 6) The heated part of the high-temperature special-shaped rail is put into the cooling device and the heel is cooled by air spraying. The air spraying is mainly used to cool the top surface and side surface of the rail head. The air spraying part extends more than 70mm beyond the normalizing heated part.

[0032] 7) Cooling process of the forming section: The air pressure on the top surface of the rail head is 0.15-0.25MPa, the air pressure on the side of the rail head is 0.20-0.25MPa, the cooling rate of the rail head is 2.0-4.0℃ / s, and the forming section of the rail head is cooled from 960±10℃ to 500-530℃, and the return temperature is controlled at 550-580℃.

[0033] 8) Transition section cooling process: The air pressure on the top surface of the rail head is 0.20-0.30MPa, the air pressure on the side of the rail head is 0.20-0.25MPa, the rail head cooling rate is 2.5-4.5℃ / s, and the transition section rail head is cooled from 960±10℃ to 470-490℃, and the return temperature is controlled at 540-580℃.

[0034] 9) Cooling process for heat-affected zone: The air pressure on the top surface of the rail head is 0.30-0.50MPa, the air pressure on the side surface of the rail head is 0.25-0.40MPa, the cooling rate of the rail head is 3.0-5.0℃ / s, and the rail head in the heat-affected zone is cooled from 960±10℃ to 440-490℃, and the return temperature is controlled at 490-540℃.

[0035] 10) After the forged 75-60kg / m U75VH heat-treated special-shaped steel rails reach the target temperature by controlled cooling at different positions, they are air-cooled to room temperature.

[0036] The present invention will be described in detail below through specific embodiments. These embodiments are intended to help understand the present invention and are not intended to limit the scope of the present invention.

[0037] Example

[0038] During implementation, the focus was on the forging area at the heel end of the irregular-shaped rail. A comparative analysis of the hardness of the rail head top surface was conducted for different locations and different heat treatment processes. The main adjustment parameters of the heat treatment process for the forging area at the heel end of the irregular-shaped rail during implementation are shown in Table 1.

[0039] Table 1: Comparison of different heat treatment processes during implementation

[0040]

[0041] Table 2: Tensile strength and hardness of different examples

[0042]

[0043] Table 3: Hardness (HRC) of the hardened layer in cross-sections of different examples

[0044]

[0045]

[0046] Tables 1 to 3 show that: In Example 1, the hardness of the top surface of the special-shaped rail in the forming section, transition section, and heat-affected zone is less than 340 HBW, which does not meet the requirement that the rail hardness be within the range of 340-420 HBW. Furthermore, the tensile strength and cross-sectional hardness of Example 1 also do not meet the standard requirements. The strength and hardness of the special-shaped rail in Example 1 are significantly lower than those of the base material, failing to match the hardness of the base material, thus affecting the smoothness of train operation and service performance. In Examples 2 and 4, the hardness of the top surface of the rail in the heat-affected zone is less than 340 HBW, failing to meet the rail hardness requirements. Moreover, the cross-sectional hardness of Examples 2 and 4 also does not meet the standard requirements. Therefore, the strength of the heat-affected zone and the cross-sectional hardness of the forming section of the special-shaped rail in Examples 2 and 4 need further improvement. Example 3: The tensile strength Rm of the formed section of the special-shaped rail is ≥1267MPa, and the elongation after fracture A is ≥12.0%. The hardness of the rail top surface of the formed section, transition section, and heat-affected zone is between 340-370HBW. The hardness of the cross section 3mm from the surface of the formed section and transition section (A1, B1, C1) is ≥37.0HRC, and the hardness of the cross section 15mm from the surface (A5, B5, C5) is ≥36.0HRC. The microstructure of the formed section and transition section is consistent with the base material, which is a typical pearlite structure with a small amount of ferrite. There are no abnormal structures and no overheating phenomenon is observed. The decarburized layer depth of the rail head of the formed section and transition section is about 0.20-0.35mm. All mechanical properties and metallographic structures meet the standard requirements.

[0047] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended 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 described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for heat treatment of the heel end of a 75-60 kg / m U75VH special-shaped steel rail for heavy-haul railways, comprising the following steps: blanking → forging → heat treatment; characterized in that: In the forging process, the heel end of the 75 kg / m section U75VH online heat-treated steel rail is die-forged at high temperature 2-3 times to form a 60 kg / m rail, thus forming a 75-60 kg / m U75VH heat-treated special-shaped rail. The chemical composition of the 75 kg / m section U75VH online heat-treated steel rail, by weight percentage, is: C: 0.71-0.80%, Si: 0.50-0.80%, Mn: 0.75-1.05%, V: 0.04-0.12%, P≤0.025%, S≤0.025%, Al: ≤0.010%, with the remainder being Fe. In the heat treatment process, the forged special-shaped rail at the heel end is heated from room temperature to 960±10℃, and after heat preservation, the heated portion of the high-temperature special-shaped rail at the heel end is subjected to accelerated cooling treatment by air jetting. The portion subjected to accelerated cooling by air jetting extends at least 70mm beyond the normalizing heated portion, including: Cooling process for the forming section: The air pressure on the top surface of the rail head is 0.15-0.25MPa, the air pressure on the side surface of the rail head is 0.20-0.25MPa, the cooling rate of the rail head is 2.0-4.0℃ / s, and the forming section of the rail head is cooled from 960±10℃ to 500-530℃, and the return temperature is controlled at 550-580℃. Transition section cooling process: The air pressure on the top surface of the rail head is 0.20-0.30MPa, the air pressure on the side of the rail head is 0.20-0.25MPa, the rail head cooling rate is 2.5-4.5℃ / s, and the rail head of the transition section is cooled from 960±10℃ to 470-490℃, and the return temperature is controlled at 540-580℃; Cooling process for heat-affected zone: The air pressure on the top surface of the rail head is 0.30-0.50MPa, the air pressure on the side surface of the rail head is 0.25-0.40MPa, the cooling rate of the rail head is 3.0-5.0℃ / s, and the rail head in the heat-affected zone is cooled from 960±10℃ to 440-490℃, and the return temperature is controlled at 490-540℃. The heat-treated profiled rail forming section has a tensile strength Rm ≥ 1267 MPa and an elongation at break A ≥ 12.0%; the hardness of the rail top surface of the forming section, transition section, and heat-affected zone is between 340-370 HBW; the hardness of the cross section 3 mm from the surface of the forming section and transition section is ≥ 37.0 HRC, and the hardness of the cross section 15 mm from the surface is ≥ 36.0 HRC; and The microstructure of the formed section and transition section of the heat-treated special-shaped rail is consistent with that of the base material, which is a typical pearlite structure with a small amount of ferrite. There is no abnormal structure and no overheating phenomenon is observed. The decarburization layer depth of the rail head in the formed section and transition section is 0.20-0.35mm.

2. The heel end heat treatment method according to claim 1, characterized in that, The process of heating the forged special-shaped rail from room temperature to 960±10℃ involves using a medium-frequency induction heater to induction heat the forging area and affected area of ​​the 75-60kg / m U75VH heat-treated special-shaped rail. The length of the heating zone after heat treatment of the forged special-shaped rail is greater than the length of the forging area, and the distance between the heating zone and the rail end of the forming section is controlled between 750-850mm.

3. The heel end heat treatment method according to claim 2, characterized in that, The medium-frequency induction heater has a medium-frequency induction heating frequency between 1600-2100Hz and a medium-frequency induction heating power between 50-100kW. It heats the forged special-shaped steel rail from room temperature to 960±10℃, and the heating time is controlled between 220-300s.

4. The heel end heat treatment method according to claim 1, characterized in that, The heat preservation treatment time is 10-30 seconds.

5. The heel end heat treatment method according to claim 1, characterized in that, After the forged 75-60 kg / m U75VH heat-treated special-shaped steel rails reach the target temperature through controlled cooling at different locations, they are air-cooled to room temperature.

6. The heel end heat treatment method according to claim 1, characterized in that, The heel end heat treatment method also includes the following process steps: shot blasting → straightening → milling → fine adjustment → length setting → drilling → flaw detection → finished product.

7. A 75-60 kg / m U75VH online heat-treated special-shaped rail for heavy-haul railways, obtained by the method of any one of claims 1-6, after end heat treatment.

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