High-toughness 60AT1 bainite steel rail heel end heat treatment method for heavy haul railway

Abstract

The invention discloses a heat treatment method for a heel end of a high-toughness 60AT1 bainite steel rail for a heavy haul railway. The heat treatment technological process comprises the steps of rail mounting, entering of the heel end part into the heating furnace, marking of the heating part, medium-frequency induction heating, stopping of heating, entering of the heel end part into the cooling device, marking of the air-jet cooling part, air-jet cooling of the steel rail, stopping of air-jet cooling, temperature returning and air cooling to the room temperature. According to the heat treatment method for the heel end of the high-toughness 60AT1 bainite steel rail for the heavy haul railway, by controlling the cooling speeds of different positions of a special-shaped steel rail, the hardness of the heel end area of a switch rail is matched with that of a base material; by combining with the method, various mechanical properties of the special-shaped part after the 60AT1 bainite steel rail heel end is forged into a 60kg / m or 75kg / m symmetrical section steel rail can be effectively improved, and the characteristics of high strength and high toughness of the bainite steel rail are fully exerted.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment of rails, and in particular relates to a heat treatment method for the heel end of a high-strength and tough 60AT1 bainite rail for heavy-duty railways. Background Art

[0002] Turnouts are an important part of railway tracks and system integration is a key equipment that affects the speed and safety of trains. As one of the main components of the turnout switch, the point rail is an important load-bearing part on the turnout, so its performance is crucial. The point rail of a heavy-duty railway turnout is made of asymmetric opposite 60AT1 rails, and the front end is cut and processed. When in use, it is closely attached to the base rail or wing rail to achieve smooth transition of the guided train; the heel end is usually forged and formed into a standard 60kg / m or 75kg / m section rail type to achieve connection with the standard section rail.

[0003] After the 60AT1 bainitic rail heel is forged at high temperature, the grain size in the forging area and heat-affected zone will recover and grow significantly, resulting in a significant reduction in the strength, hardness and toughness of the area, which cannot match the performance of the rail parent material and is difficult to meet the line service requirements. Therefore, it is necessary to perform normalizing heat treatment on the rail heel and heat-affected zone to achieve the purpose of refining the grains, improving the organization, and improving the hardness of the softening area and the comprehensive mechanical properties of the rail heel. After searching, most of the domestic patent technologies and documents are the process of heat treatment of the heel of pearlite rails. There is little research on the heat treatment method of the heel of bainitic rails in China, and there is no relevant standard support. The traditional heat treatment method cannot meet the performance and line service requirements of bainitic rails. It is necessary to adjust the heat treatment process of the heel of special-shaped rails based on the phase transformation characteristics of bainitic rails, combined with the CCT curve and rail production process parameters, and determine the heat treatment method of the heel of bainitic special-shaped rails to match the performance of the rail parent material.

[0004] The patent with application number 202410683261.1 discloses a full-section heat treatment process for bainitic rails after heel-end forging. The invention heats the U22SiMn bainitic rails to 940-980°C, cools the rail head to 280-300°C at a cooling rate of 0.8-1.5°C / s, and then performs a tempering treatment at 320-350°C. The obtained rails have a tensile strength of ≥1335MPa, a yield strength of ≥1074MPa, and a room temperature impact of ≥84J. The patent uses a cooling rate to cool the special-shaped rails to 280-300°C. In actual production applications, the hardness of the village forming section is too high or the hardness of the heat-affected zone is low. Adding the tempering process of the special-shaped rails will further increase the production costs of the enterprise and reduce the performance indicators of the rails.

[0005] Patent application number 201310431374.4 discloses a heat treatment device and method for the profiling heel and end sections of a rail. The patent mainly involves a heat treatment device for the profiling heel and end sections of a rail, but does not describe the heat treatment process or method for the profiling heel and end sections of the rail. Moreover, the patent is for pearlite rails. There are significant differences in composition, structure, and performance between pearlite rails and bainite rails, and the process is not universal. Summary of the invention

[0006] The purpose of the present invention is to provide a high-strength and tough 60AT1 bainite rail heel end heat treatment method for heavy-duty railways, by controlling the cooling rate of different positions of the special-shaped rail, so as to achieve the matching of the hardness of the tip rail heel end area with that of the base material; combined with this method, the mechanical properties of the special-shaped part of the 60AT1 bainite rail heel end after forging into a 60kg / m or 75kg / m symmetrical section rail can be effectively improved, and the high strength and high toughness characteristics of the bainite rail can be fully utilized.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] The present invention discloses a method for heat treatment of the heel end of a high-strength and tough 60AT1 bainite rail for heavy-duty railways. The heat treatment process is as follows: rail loading → heel end portion enters a heating furnace → heating portion marking → medium frequency induction heating → heating stop → heel end portion enters a cooling device → air jet cooling portion marking → rail air jet cooling → air jet cooling stop → temperature return → air cooling to room temperature; wherein:

[0009] The asymmetric opposite 60AT1 bainite rail heels are forged 2-3 times at high temperature into standard 60kg / m or 75kg / m cross-section rails;

[0010] The initial forging heating temperature of special-shaped rails is controlled at 900-1000℃ and cannot exceed 1150℃; the fine forging heating temperature is controlled at 1000-1100℃ and cannot exceed 1150℃;

[0011] Place the forging area and affected area of ​​the 60AT1 point rail heel end on the medium frequency induction heater for induction heating, with the heating length ≥800mm from the end of the special-shaped rail;

[0012] Heat the forging area of ​​the heel end of the 60AT1 rail from room temperature to 920-950℃, and control the heating time to 250-350s; after the heel end of the rail is heated to the target temperature, reduce the heating power and keep the rail at the target temperature for 10-20s;

[0013] After the heating and insulation of the special-shaped rails meet the design requirements, the high-temperature heel end of the 60AT1 point rail is immediately placed in the cooling device for controlled air jet cooling. The air jet cooling area is at least 100mm beyond the normalizing heating area.

[0014] The heat treatment of special-shaped rails adopts zone controlled cooling treatment, that is, the air jet controlled cooling treatment needs to be carried out on the top surface of the rail head, the side surface of the rail head and the bottom of the rail;

[0015] Cooling process of forming section: the wind pressure on the top of the rail head is 7-10KPa, the wind pressure on the side of the rail head is 7-10KPa, the cooling rate of the rail head is 3.0-3.3℃ / s, the wind pressure on the bottom of the rail is 10-13KPa, and the rail head of the forming section is controlled to be cooled from 920-950℃ to 350-380℃, and the return temperature is ≤450℃;

[0016] Transition section cooling process: The wind pressure on the top of the rail head is 9-12KPa, the wind pressure on the side of the rail head is 9-12KPa, the cooling rate of the rail head is 3.3-3.5℃ / s, the wind pressure on the bottom of the rail is 10-15KPa, and the rail head of the forming section is controlled to be cooled from 920-950℃ to 330-350℃, and the return temperature is ≤430℃;

[0017] Heat affected zone cooling process: the wind pressure on the top of the rail head is 12-16KPa, the wind pressure on the side of the rail head is 12-16KPa, the rail head cooling rate is 3.4-3.7℃ / s, the wind pressure on the bottom of the rail is 10-15KPa, the forming section rail head is controlled to cool from 920-950℃ to 300-330℃, and the return temperature is ≤400℃.

[0018] Furthermore, the raw material of the asymmetric opposite 60AT1 bainitic rail has C, Si, Mn, Cr, Ni, Mo, Nb and V as main strengthening elements.

[0019] Furthermore, the chemical composition of the rail is C: 0.16-0.25%, Si: 0.70-1.20%, Mn: 1.60-2.45%, Cr: 0.60-1.20%, Ni: ≤0.70%, Mo: 0.15-0.60%, Nb: ≤0.06%, V: ≤0.09%, P≤0.02%, S≤0.01%, Al: ≤0.01%, and the rest is Fe and impurities.

[0020] Furthermore, after the 60AT1 tip rail heel end heat treatment area is cooled at different locations to reach the target temperature, it is air-cooled to room temperature.

[0021] Compared with the prior art, the beneficial technical effects of the present invention are:

[0022] Based on the above invention, the heel end of the obtained 60AT1 bainite special-shaped rail has the following characteristics: tensile strength Rm≥1360MPa, yield strength Rp0.2≥1100MPa, elongation after fracture A≥14.0%, normal temperature impact KU2≥100J, rail top surface hardness≥410HBW, cross-section hardness≥40.0HRC; rail top surface hardness ≥410HBW, cross-section hardness ≥40.0HRC in transition section; rail top surface hardness ≥400HBW, cross-section hardness ≥39.5HRC in heat-affected zone. The microstructure of the forming section and transition section is consistent with the parent material type, both of which are typical bainite and martensite complex phase structures.

[0023] The present invention aims at the special-shaped rail heel end of a 60AT1 bainitic rail for heavy-duty railway after forging to 60kg / m or 75kg / m. During the heat treatment cooling process of the special-shaped rail heel end, the forming section, the transition section and the heat-affected zone are subjected to separate air spray treatment respectively. By controlling the cooling speed of different positions of the 60AT1 pointed rail heel end, the performance difference between the special-shaped rail heel end at different positions and the parent material after the 60AT1 bainitic rail is subjected to high-temperature die forging is effectively improved, so that the forged special-shaped rail heel end area matches the strength, hardness and toughness of the parent material, and meets the requirements of line service. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The present invention will be further described below in conjunction with the accompanying drawings.

[0025] Figure 1 This is the transition section microstructure of Example 3. DETAILED DESCRIPTION

[0026] A method for heat treatment of the heel end of a high-strength and tough 60AT1 bainite rail for heavy-duty railways, comprising:

[0027] 1) The raw material of the asymmetric opposite 60AT1 bainitic rail uses C, Si, Mn, Cr, Ni, Mo, Nb and V as main strengthening elements. The weight percentage of the chemical composition of the rail is C: 0.16-0.25%, Si: 0.70-1.20%, Mn: 1.60-2.45%, Cr: 0.60-1.20%, Ni: ≤0.70%, Mo: 0.15-0.60%, Nb: ≤0.06%, V: ≤0.09%, P≤0.02%, S≤0.01%, Al: ≤0.01%, and the rest is Fe.

[0028] 2) The process flow of heat treatment of the heel end of 60AT1 point rail is as follows: rail mounting → heel end enters heating furnace → heating part marking → medium frequency induction heating → heating stop → heel end enters cooling device → air jet cooling part marking → rail air jet cooling → air jet cooling stop → return to temperature → air cooling to room temperature.

[0029] 3) The asymmetric opposite 60AT1 bainite rail heels are forged at high temperature for 2-3 times into standard 60kg / m or 75kg / m cross-section rail types.

[0030] 4) The initial forging heating temperature of special-shaped rails is controlled at 900-1000℃ and cannot exceed 1150℃; the fine forging heating temperature is controlled at 1000-1100℃ and cannot exceed 1150℃.

[0031] 5) Place the forging area (including the forming section and transition section) and the affected area of ​​the 60AT1 point rail heel end on a medium frequency induction heater for induction heating, and the heating length is ≥800mm from the end of the special-shaped rail.

[0032] 6) Heat the forging area of ​​the heel end of the 60AT1 point rail (including the forming section and the transition section) from room temperature to 920-950°C, and control the heating time to 250-350s; after the heel end of the rail is heated to the target temperature, reduce the heating power and keep the rail at the target temperature for 10-20s.

[0033] 7) After the heating and insulation of the special-shaped rails meet the design requirements, the high-temperature heel end of the 60AT1 pointed rail is immediately placed in the cooling device for controlled air jet cooling. The air jet cooling area should be more than 100mm beyond the normalizing heating area.

[0034] 8) The heat treatment of special-shaped rails adopts zone controlled cooling treatment, that is, the air jet controlled cooling treatment needs to be carried out on the top surface of the rail head, the side surface of the rail head and the bottom of the rail.

[0035] 9) Cooling process of forming section: The wind pressure on the top of the rail head is 7-10KPa, the wind pressure on the side of the rail head is 7-10KPa, the cooling rate of the rail head is 3.0-3.3℃ / s, the wind pressure on the bottom of the rail is 10-13KPa, and the rail head of the forming section is controlled to be cooled from 920-950℃ to 350-380℃, and the return temperature is ≤450℃.

[0036] 10) Transition section cooling process: The wind pressure on the top of the rail head is 9-12KPa, the wind pressure on the side of the rail head is 9-12KPa, the rail head cooling rate is 3.3-3.5℃ / s, the wind pressure on the bottom of the rail is 10-15KPa, and the forming section rail head is controlled to be cooled from 920-950℃ to 330-350℃, and the return temperature is ≤430℃.

[0037] 11) Heat affected zone cooling process: The wind pressure on the top of the rail head is 12-16KPa, the wind pressure on the side of the rail head is 12-16KPa, the rail head cooling rate is 3.4-3.7℃ / s, the wind pressure on the bottom of the rail is 10-15KPa, and the forming section rail head is controlled to cool from 920-950℃ to 300-330℃, and the return temperature is ≤400℃.

[0038] 12) After the 60AT1 point rail heel end heat treatment area is cooled at different locations to reach the target temperature, it is air-cooled to room temperature.

[0039] Process comparison during implementation:

[0040] During the implementation process, different heat treatment process tests and performance comparison analyses were conducted on the heel end of 60AT1 bainitic rail for heavy-duty railways. The processes corresponding to different embodiments are shown in Table 1, and the performances corresponding to different embodiments are shown in Table 2.

[0041] Table 1 Different heat treatment processes during implementation

[0042]

[0043]

[0044] Table 2 Tensile properties of the forming section corresponding to different heat treatment processes during implementation (mean)

[0045]

[0046] Table 3 Rail hardness corresponding to different heat treatment processes during implementation (mean)

[0047]

[0048] From Table 1 to Table 3, Figure 1 It can be seen that Example 1 adopts the heat treatment method of normalizing + air cooling, and the strength, hardness and toughness of the special-shaped rail are low, which cannot achieve a reasonable match with the performance of the rail base material; Example 2 further reduces the heating temperature, and adopts the same controlled cooling process for different positions of the special-shaped rail, which further reduces the final cooling temperature and increases the cooling rate compared with Example 1. Compared with Example 1, the strength and hardness of the special-shaped rail are significantly improved, but the target requirements are still not met; Example 3 further reduces the heating temperature, and adopts the same controlled cooling process for different positions of the special-shaped rail, which further reduces the final cooling temperature and increases the cooling rate compared with Example 2. Compared with Example 2, the strength and hardness of the special-shaped rail are further improved, but the improvement is low, and the hardness of the heat-affected zone is still low; during the heat treatment cooling process of the heel end of the special-shaped rail in Example 4, the forming section, transition section and heat-affected zone are separately subjected to air spray treatment, and the final cooling temperature of the transition section and the heat-affected zone is further reduced, and the cooling rate is increased. Compared with Example 2, the hardness of the transition section and the heat-affected zone of the special-shaped rail is significantly improved, and the mechanical properties of the forming section rail are also improved to a certain extent, ensuring that the forged special-shaped rail heel end forming section, transition section and heat-affected zone match the strength, hardness and toughness of the parent material, and meet the requirements of line service.

[0049] The heel end of the 60AT1 bainite special-shaped rail obtained in Example 4 has the following characteristics: tensile strength Rm≥1360MPa, yield strength Rp0.2≥1100MPa, elongation after fracture A≥14.0%, normal temperature impact KU2≥100J, rail top surface hardness≥410HBW, cross-section hardness≥40.0HRC; rail top surface hardness≥410HBW, cross-section hardness≥40.0HRC in the transition section; rail top surface hardness≥400HBW, cross-section hardness≥39.5HRC in the heat-affected zone. The microstructures of the forming section and transition section are consistent with the parent material type, both of which are typical bainite and martensite multiphase structures.

[0050] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for heat treatment of the heel end of a high-strength and tough 60AT1 bainite rail for heavy-duty railway, characterized in that: The heat treatment process is as follows: rail loading → heel end enters heating furnace → heating part marking → medium frequency induction heating → stop heating → heel end enters cooling device → air jet cooling part marking → rail air jet cooling → stop air jet cooling → return to temperature → air cooling to room temperature; where: The asymmetric opposite 60AT1 bainite rail heels are forged 2-3 times at high temperature into standard 60kg / m or 75kg / m cross-section rails; The initial forging heating temperature of special-shaped rails is controlled at 900-1000℃ and cannot exceed 1150℃; the fine forging heating temperature is controlled at 1000-1100℃ and cannot exceed 1150℃; Place the forging area and affected area of ​​the 60AT1 point rail heel end on the medium frequency induction heater for induction heating, with the heating length ≥800mm from the end of the special-shaped rail; Heat the forging area of ​​the heel end of the 60AT1 rail from room temperature to 920-950℃, and control the heating time to 250-350s; after the heel end of the rail is heated to the target temperature, reduce the heating power and keep the rail at the target temperature for 10-20s; After the heating and insulation of the special-shaped rails meet the design requirements, the high-temperature heel end of the 60AT1 point rail is immediately placed in the cooling device for controlled air jet cooling. The air jet cooling area is at least 100mm beyond the normalizing heating area. The heat treatment of special-shaped rails adopts zone controlled cooling treatment, that is, the air jet controlled cooling treatment needs to be carried out on the top surface of the rail head, the side surface of the rail head and the bottom of the rail; Cooling process of forming section: the wind pressure on the top of the rail head is 7-10KPa, the wind pressure on the side of the rail head is 7-10KPa, the cooling rate of the rail head is 3.0-3.3℃ / s, the wind pressure on the bottom of the rail is 10-13KPa, and the rail head of the forming section is controlled to be cooled from 920-950℃ to 350-380℃, and the return temperature is ≤450℃; Transition section cooling process: The wind pressure on the top of the rail head is 9-12KPa, the wind pressure on the side of the rail head is 9-12KPa, the cooling rate of the rail head is 3.3-3.5℃ / s, the wind pressure on the bottom of the rail is 10-15KPa, and the rail head of the forming section is controlled to be cooled from 920-950℃ to 330-350℃, and the return temperature is ≤430℃; Heat affected zone cooling process: the wind pressure on the top of the rail head is 12-16KPa, the wind pressure on the side of the rail head is 12-16KPa, the rail head cooling rate is 3.4-3.7℃ / s, the wind pressure on the bottom of the rail is 10-15KPa, the forming section rail head is controlled to cool from 920-950℃ to 300-330℃, and the return temperature is ≤400℃.

2. The method for heat treatment of the heel end of high-strength and tough 60AT1 bainite rail for heavy-duty railway according to claim 1, characterized in that: The raw materials of asymmetric opposite 60AT1 bainitic rails contain C, Si, Mn, Cr, Ni, Mo, Nb and V as main strengthening elements.

3. The method for heat treatment of the heel end of high-strength and tough 60AT1 bainite rail for heavy-duty railway according to claim 2, characterized in that: The weight percentage of the chemical composition of the rail is C: 0.16-0.25%, Si: 0.70-1.20%, Mn: 1.60-2.45%, Cr: 0.60-1.20%, Ni: ≤0.70%, Mo: 0.15-0.60%, Nb: ≤0.06%, V: ≤0.09%, P≤0.02%, S≤0.01%, Al: ≤0.01%, and the rest is Fe and impurities.

4. The method for heat treatment of the heel end of high-strength and tough 60AT1 bainite rail for heavy-duty railway according to claim 1, characterized in that: After the 60AT1 point rail heel end heat treatment area is cooled at different locations to reach the target temperature, it is air-cooled to room temperature.

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