Heat treatment method for forging and pressing area of heel end of 60-50kg / m U71Mn steel rail

Abstract

The invention discloses a heat treatment method for a forging and pressing area of a heel end of a 60-50kg / mU71Mn steel rail. The heat treatment method comprises the following steps of: performing high-temperature die forging on the heel end of the 60-50kg / mU71Mn hot-rolled steel rail meeting component requirements to 50kg / m; performing air cooling to room temperature; induction heating; (3) heat preservation is conducted for 15-25 s at the target temperature; (4) the heel end is subjected to air spraying accelerated cooling treatment; 5) a molding section cooling process; (6) a transition section cooling process; 7) a heat affected zone cooling process; and (8) after the special-shaped steel rail reaches the highest temperature return temperature, air cooling to the room temperature. According to the 60-50 kg / mU71Mn steel rail heel end forging and pressing area heat treatment method, the cooling speed of different positions (a forming section, a transition section and a heat affected area) is controlled, so that the hardness of the heel end forging and pressing area and the hardness of the heat affected area of a special-shaped steel rail are matched with the hardness of a base material, and standard requirements are met.

Description

Technical Field

[0001] The present invention relates to the technical field of rail heat treatment, and in particular to a heat treatment method for a heel-end forging zone of a 60-50kg / mU71Mn rail. Background Art

[0002] At present, the mainline rail sections of domestic railway lines all use symmetrical rails with cross-sections of 50kg / m, 60kg / m or 75kg / m. Due to the different transportation conditions, service environments, and train axle loads between existing lines and newly built lines, and the economic efficiency of laying rails on the lines, different rail types are laid on different lines, which can be roughly divided into 50kg / m rail lines, 60kg / m rail lines, and 75kg / m rail lines. The connection between lines with different rail types has become a difficult and important part. The use of forged special-shaped rails to connect rails of different cross-sections has the advantages of safety, reliability, good mechanical properties, excellent organizational structure, high utilization rate, and high cost-effectiveness. It can ensure the safe and stable operation of trains through rail lines with different cross-sections.

[0003] The production of special-shaped rails presents numerous technical challenges, primarily the following: 1. Dimensional Accuracy Control: Special-shaped rails play a critical role as connectors and transitions within the rail system. Their accuracy directly impacts train stability and track life. For example, even slight dimensional deviations in rail head shape, waist thickness, and base width can lead to uneven wheel-rail contact, increasing wear and impact. During the production process, the heating, rolling, and cooling steps of the billet all impact dimensional accuracy, necessitating precise control of process parameters at each stage. 2. Metal Flow Control: Special-shaped rails have complex cross-sections, and their deformation process is characterized by significant asymmetric metal flow and temperature non-uniformity. Consequently, a systematic and accurate control model for metal flow and temperature has long been lacking. During the forging or rolling process, ensuring uniform metal flow to the desired locations and forming precise shapes and dimensions is a significant challenge. Uneven metal flow can lead to dimensional deviations, structural inhomogeneities, and even internal defects in the rail head, waist, and base. 3. Heat Treatment Process: Heat treatment is a crucial step in improving the performance of special-shaped rails. Through processes such as quenching, tempering, and normalizing, the microstructure and performance of the rails can be improved, enhancing their hardness, strength, and toughness. However, parameters such as temperature control and cooling rate during the heat treatment process significantly influence the rail's ultimate performance. For example, temperature uniformity before the rail begins cooling is a key factor affecting its microstructure uniformity after cooling. Rails with uneven temperatures can experience a hardness difference of up to 10-25 HB on the tread after cooling. Rails with uneven hardness are prone to premature corrugation during use, shortening the rail's service life. 4. Surface Quality Control: Special-shaped rails have high surface quality requirements. Forging defects such as cracks, overburning, localized pits, and folds are not permitted. Sharp corners must be longitudinally polished and smoothed. During the production process, factors such as the surface condition of the blank, scale formation during heating, and the surface condition of the molds during forging or rolling can all affect the rail's surface quality, requiring appropriate control measures.

[0004] Special-shaped rails are manufactured by using a method of heating and forging ordinary rails at the rail ends, which realizes the transition from one rail section to another. Since the metallographic structure of the heel end of the 60-50kg / mU71Mn hot-rolled special-shaped rail is coarse lamellar pearlite after high-temperature die forging, in order to refine the pearlite structure of the forming section, transition section and heat-affected zone and restore the performance of the heel end forging area of the rail, the heel end forging area of the rail needs to be re-induction normalized heat treated. The present invention is aimed at the heat treatment of the heel end forging area of the U71Mn hot-rolled rail after 60kg / m is forged into 50kg / m. The process of full-section heating and air jet cooling treatment on the top and sides of the rail head is adopted. The mechanical properties and metallographic structure of the produced 60-50kg / mU71Mn hot-rolled special-shaped rail meet the requirements.

[0005] The standard requirements of "TB / T2344.3-2018 Rails Part 3: Special-shaped Rails".

[0006] Patent application number 201310431374.4 discloses a heat treatment device and method for the profiling heel and end sections of rails. The patent mainly involves a heat treatment device for the profiling heel and end sections of rails, but does not describe the heat treatment process or method for the profiling heel and end sections of rails.

[0007] Patent application number 201910277768.6 discloses a heat treatment method for the heel-end forging section of a rail. After heating the heel-end forging section to 850-950°C, the rail head is cooled by air jet to 350-500°C. The patent specifies an air jet cooling temperature between 350-500°C. Excessively low cooling temperatures can lead to problems such as abnormal microstructure and excessive hardness in the formed section. Furthermore, the wide cooling range is not suitable for the patent's promotion and application. The document "Induction Heat Treatment of the Heel-End of 60AT1-U71Mn Rails" discusses online heat treatment of 60AT1-U71Mn rails. The heel-end is treated using medium-frequency induction preheating, medium-frequency induction heating, air jet cooling, and spray cooling. This document differs from the cross-section of the 60-50kg / mU71Mn hot-rolled rails of the present invention, and the process is not universal. The medium frequency induction preheating and spray cooling process described in the literature increases the production process, production cost and control difficulty of special-shaped rails, and the spray cooling treatment of the rail head is very likely to cause abnormal structure during continuous production. Summary of the Invention

[0008] The purpose of the present invention is to provide a heat treatment method for the heel-end forging area of a 60-50kg / mU71Mn rail. By controlling the cooling rate at different positions (forming section, transition section and heat-affected zone), the hardness of the heel-end forging area and the heat-affected zone of the special-shaped rail can be matched with the hardness of the base material and meet the standard requirements.

[0009] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0010] The present invention provides a heat treatment method for the heel-end forging area of a 60-50 kg / m U71Mn rail, comprising:

[0011] 1) The heel end of a 60kg / m U71Mn hot-rolled rail meeting the composition requirements is die-forged at high temperature to a 50kg / m rail; and then air-cooled to room temperature;

[0012] 2) Place the heel-end forging area and the affected area of the 60-50kg / mU71Mn hot-rolled special-shaped rail on a medium-frequency induction heater for induction heating; heat the heel-end forged special-shaped rail from room temperature to 900±10°C for 250-300s, wherein: the medium-frequency induction heating frequency is between 1500-2000Hz, and the medium-frequency induction heating power is between 50-100kW;

[0013] 3) After the heel end of the special-shaped rail is heated to the target temperature of 900±10℃, reduce the medium-frequency induction heating power to keep the heel end at the target temperature for 15-25 seconds;

[0014] 4) The high-temperature special-shaped rail heel end heated part enters the cooling device, and the heel end is subjected to air jet accelerated cooling treatment, mainly for the rail head top surface and rail head side surface. The air jet cooling part is more than 60mm beyond the normalizing heating part;

[0015] 5) Cooling process of forming section: The wind pressure on the top of the rail head is 0.1-0.15MPa, the cooling rate is 3.5-5.0℃ / s, the wind pressure on the side of the rail head is 0.1-0.15MPa, the cooling rate is 2.0-3.0℃ / s, and the rail head in the forming section is controlled to be cooled from 900±10℃ to 490-510℃;

[0016] 6) Transition section cooling process: The wind pressure on the top of the rail head is 0.13-0.18MPa, the cooling rate is 3.5-5.0℃ / s, the wind pressure on the side of the rail head is 0.13-0.18MPa, the cooling rate is 2.5-3.5℃ / s, and the forming section rail head is controlled to cool from 900±10℃ to 470-500℃;

[0017] 7) Heat-affected zone cooling process: The wind pressure on the top of the rail head is 0.15-0.20MPa, the cooling rate is 3.5-5.0℃ / s, the wind pressure on the side of the rail head is 0.15-0.20MPa, the cooling rate is 3.5-5.0℃ / s, and the forming section of the rail head is controlled to cool from 900±10℃ to 430-470℃;

[0018] 8) After the special-shaped rail reaches the highest return temperature, it is air-cooled to room temperature.

[0019] Furthermore, the 60-50kg / mU71Mn hot-rolled special-shaped rail uses C, Si, and Mn as main strengthening elements, and the weight percentage of the rail chemical composition is C: 0.70-0.75%, Si: 0.30-0.50%, Mn: 0.90-1.00%, P≤0.015%, S≤0.005%, Al: ≤0.010%, and the rest is Fe and impurities.

[0020] Furthermore, the medium frequency induction heating power was reduced so that the heel end was kept at the target temperature for 20 seconds.

[0021] Furthermore, the obtained 60-50kg / mU71Mn hot-rolled special-shaped rail has a tensile strength Rm of 940 MPa or greater in the formed section and an elongation after fracture A of 12% or greater.

[0022] Furthermore, the hardness of the rail top surface in the forming section, transition section and heat-affected zone is between 260-310 HB.

[0023] Furthermore, the cross-sectional hardness of the forming section and the transition section at 3 mm and 15 mm from the surface both meet ≥25HRC.

[0024] Furthermore, the forming section and transition section are typical pearlite structures, without any abnormal structures or overburning, and the depth of the decarburized layer in the transition section meets the requirement of ≤0.3 mm.

[0025] Compared with the prior art, the present invention has the following beneficial technical effects:

[0026] Based on the above invention, the obtained 60-50kg / mU71Mn hot-rolled special-shaped rail has the following characteristics: the tensile strength Rm of the forming section is ≥940MPa, the elongation after fracture A is ≥12%; the rail top surface hardness of the forming section, transition section and heat-affected zone is between 260-310HB, and the cross-sectional hardness of the forming section and transition section at 3mm (A1, B1, C1) and 15mm (A5, B5, C5) from the surface is ≥25HRC; the forming section and transition section have typical pearlite structure, no abnormal structure, no overburning phenomenon, and the depth of the decarburized layer in the transition section is ≤0.3mm.

[0027] The present invention is aimed at the heat treatment cooling process of 60-50kg / m U71Mn hot-rolled special-shaped rails. The forming section, transition section and heat-affected zone are subjected to separate air spray treatment. By controlling the cooling rate and final cooling temperature of different positions (forming section, transition section and heat-affected zone), the problems of low hardness of the top surface of the forming section, transition section and heat-affected zone of the U71Mn hot-rolled rail head after forging from 60kg / m to 50kg / m, excessive width of the S zone, easy occurrence of abnormal structure and excessive decarburization layer are solved. The special-shaped rail heel end forging area and heat-affected zone are matched with the strength, hardness and toughness of the base material. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will be further described below with reference to the accompanying drawings.

[0029] Figure 1 It is the CCT curve of U71Mn rail. DETAILED DESCRIPTION

[0030] A heat treatment method for the heel-end forging area of a 60-50kg / m U71Mn rail comprises:

[0031] 1) 60-50kg / mU71Mn hot-rolled special-shaped rails use C, Si, and Mn as the main strengthening elements. The chemical composition of the rails is C: 0.70-0.75%, Si: 0.30-0.50%, Mn: 0.90-1.00%, P≤0.015%, S≤0.005%, Al: ≤0.010%, and the rest is Fe and unavoidable impurities.

[0032] 2) The heel end of the 60kg / m U71Mn hot rolled rail is die-forged at high temperature to 50kg / m and air-cooled to room temperature.

[0033] 3) For manufacturing 60-50kg / m U71Mn hot-rolled special-shaped rails, the heat treatment process flow of the heel forging area is as follows: rail loading → heel end enters the heating furnace → heating area marking → medium frequency induction heating → heating stop → heel end enters the cooling device → air jet cooling area marking → rail air jet cooling → air jet cooling stop → return to temperature → air cooling to room temperature.

[0034] 4) Place the heel-end forging area and the affected area of a 60-50 kg / m U71Mn hot-rolled profiled rail onto a medium-frequency induction heater for induction heating. The heel-end forged profiled rail is heated from room temperature to 900 ± 10°C for 250-300 seconds. The medium-frequency induction heating frequency is between 1500-2000 Hz and the power is between 50-100 kW. Medium-frequency heating ensures that the surface of the high-temperature die forging area and the core of the rail head are heated to the temperature range required for austenitization of the rail.

[0035] 5) After the heel end of the special-shaped rail is heated to the target temperature of 900±10℃, the medium-frequency induction heating power is reduced to keep the heel end at the target temperature for 20 seconds.

[0036] 6) The high-temperature special-shaped rail heel end heating part enters the cooling device, and the heel end is subjected to air jet accelerated cooling treatment, mainly for the rail head top surface and rail head side surface air jet cooling, and the air jet cooling part exceeds the normalizing heating part by more than 60mm.

[0037] 7) Cooling process of forming section: The wind pressure on the top of the rail head is 0.1-0.15MPa, the cooling rate is 3.5-5.0℃ / s, the wind pressure on the side of the rail head is 0.1-0.15MPa, the cooling rate is 2.0-3.0℃ / s, and the forming section rail head is controlled to cool from 900±10℃ to 490-510℃.

[0038] 8) Transition section cooling process: The wind pressure on the top of the rail head is 0.13-0.18MPa, the cooling rate is 3.5-5.0℃ / s, the wind pressure on the side of the rail head is 0.13-0.18MPa, the cooling rate is 2.5-3.5℃ / s, and the forming section rail head is controlled to cool from 900±10℃ to 470-500℃.

[0039] 9) Heat affected zone cooling process: The wind pressure on the top of the rail head is 0.15-0.20MPa, the cooling rate is 3.5-5.0℃ / s, the wind pressure on the side of the rail head is 0.15-0.20MPa, the cooling rate is 3.5-5.0℃ / s, and the formed section rail head is controlled to cool from 900±10℃ to 430-470℃.

[0040] 10) After the special-shaped rail reaches the highest return temperature, it is air-cooled to room temperature.

[0041] Process comparison during implementation:

[0042] During the implementation process, the focus is on the heel-end forging area of the special-shaped rail. The hardness of the rail head top surface corresponding to different positions and different heat treatment processes is compared and analyzed to determine the optimal heat treatment implementation process. During the implementation process, the main adjustment parameters of the heat treatment process for the heel-end forging area of the special-shaped rail are shown in Table 1.

[0043] Table 1 Comparison of different heat treatment processes during implementation

[0044]

[0045] The Brinell hardness test of the rail head top surface was conducted in accordance with "TB / T2344.3-2018 Steel Rail Part 3: Special-shaped Rail". The top surface of the rail head was ground off 0.5 mm or more. The distance between the measuring points in the forming section and the transition section was 100 mm. The distance between the measuring points from the special-shaped part to the parent material was 10 mm. The test results of the top surface hardness of the rails of Examples 1 to 3 are shown in Tables 2 to 4.

[0046] Table 2 Rail top surface hardness of Example 1

[0047]

[0048]

[0049] Table 3 Rail top surface hardness of Example 2

[0050]

[0051] Table 4 Rail top surface hardness of Example 3

[0052]

[0053] In Example 1, the hardness of the heat-affected zone (including the S zone) of the special-shaped rail was less than 260 HB at 9 measurement points, which does not meet the standard requirement that a maximum of 5 measurement points with a hardness less than 260 HB are allowed. In Example 2, the hardness of the heat-affected zone (including the S zone) of the special-shaped rail was less than 260 HB at 7 measurement points, which does not meet the standard requirement that a maximum of 5 measurement points with a hardness less than 260 HB are allowed. In Example 3, the hardness of the top surface of the forming section, transition section, and heat-affected zone of the special-shaped rail was between 260 and 274 HB, all meeting the standard requirement that the rail hardness be between 260 and 310 HB. The hardness of the heat-affected zone (including the S zone) was greater than 260 HB.

[0054] According to the test methods in TB / T 2344.3-2018 Steel Rails, Part 3: Special-Shaped Rails, the special-shaped rails of Example 3 were tested and analyzed for cross-sectional hardness and tensile properties. The cross-sectional hardness of the rail head is shown in Table 5, and the tensile properties are shown in Table 6.

[0055] Table 5 Hardness of cross-section hardened layer of Example 3 (HRC)

[0056]

[0057] Table 6 Tensile properties of Example 3

[0058]

[0059] As shown in Tables 5 and 6, the cross-sectional hardness of the forming section and transition section of the special-shaped rail in Example 3 at 3 mm (A1, B1, C1) and 15 mm (A5, B5, C5) from the surface is ≥25 HRC, meeting the standard requirement of ≥23.0 HRC. The tensile strength Rm of the forming section of the special-shaped rail in Example 3 is ≥940 MPa, and the elongation after fracture A is ≥12%, which also meet the standard requirements.

[0060] The present invention adopts segmented and separate control of key heat treatment parameters such as the heating temperature, final cooling temperature, cooling rate, and cooling air pressure of the forming section, transition section, and heat-affected zone of the 60-50 kg / m U71Mn hot-rolled special-shaped rail during the heat treatment process of the heel-end forging area. This allows the strength, hardness, and microstructure of the special-shaped portion and heat-affected zone of the rail to meet standard requirements while achieving optimal mechanical properties and microstructure matching with the rail base material. This effectively improves the organizational and performance differences between the heel-end of the special-shaped rail and the base material after high-temperature die forging. The present invention ensures the safe and stable service of the 60-50 kg / m U71Mn hot-rolled special-shaped rail on the line.

[0061] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for heat treatment of the heel-end forging zone of a 60-50 kg / m U71 Mn rail, characterized by: include: 1) The heel end of 60kg / m U71 Mn hot-rolled rail meeting the composition requirements is forged into 50kg / m through high temperature die forging; and air-cooled to room temperature; 2) Place the heel-end forging area and the affected area of a 60-50kg / mU71 Mn hot-rolled special-shaped rail on a medium-frequency induction heater for induction heating; heat the heel-end forged special-shaped rail from room temperature to 900±10°C for 250-300 seconds, with a medium-frequency induction heating frequency between 1500-2000Hz and a medium-frequency induction heating power between 50-100kW; 3) After the heel end of the special-shaped rail is heated to the target temperature of 900±10℃, reduce the medium-frequency induction heating power to keep the heel end at the target temperature for 15-25 seconds; 4) The high-temperature special-shaped rail heel end heated part enters the cooling device, and the heel end is subjected to air jet accelerated cooling treatment, mainly for the rail head top surface and rail head side surface. The air jet cooling part is more than 60mm beyond the normalizing heating part; 5) Cooling process of forming section: The wind pressure on the top of the rail head is 0.1-0.15MPa, the cooling rate is 3.5-5.0℃ / s, the wind pressure on the side of the rail head is 0.1-0.15MPa, the cooling rate is 2.0-3.0℃ / s, and the rail head in the forming section is controlled to be cooled from 900±10℃ to 490-510℃; 6) Transition section cooling process: The wind pressure on the top of the rail head is 0.13-0.18MPa, the cooling rate is 3.5-5.0℃ / s, the wind pressure on the side of the rail head is 0.13-0.18MPa, the cooling rate is 2.5-3.5℃ / s, and the forming section rail head is controlled to cool from 900±10℃ to 470-500℃; 7) Heat-affected zone cooling process: The wind pressure on the top of the rail head is 0.15-0.20MPa, the cooling rate is 3.5-5.0℃ / s, the wind pressure on the side of the rail head is 0.15-0.20MPa, the cooling rate is 3.5-5.0℃ / s, and the forming section of the rail head is controlled to cool from 900±10℃ to 430-470℃; 8) After the special-shaped rail reaches the highest return temperature, it is air-cooled to room temperature.

2. The method for heat treatment of the heel-end forging area of a 60-50 kg / m U71 Mn rail according to claim 1, characterized in that: 60-50kg / mU71 Mn hot-rolled special-shaped rail uses C, Si and Mn as main strengthening elements. The chemical composition of the rail is C: 0.70-0.75%, Si: 0.30-0.50%, Mn: 0.90-1.00%, P≤0.015%, S≤0.005%, Al: ≤0.010%, and the rest is Fe and impurities.

3. The heat treatment method for the heel-end forging area of a 60-50 kg / m U71 Mn rail according to claim 1, characterized in that: Reduce the medium frequency induction heating power so that the heel end is kept at the target temperature for 20 seconds.

4. The method for heat treatment of the heel-end forging area of a 60-50 kg / m U71 Mn rail according to claim 1, characterized in that: The obtained 60-50kg / mU71 Mn hot-rolled special-shaped steel rail has the following characteristics: the tensile strength Rm of the formed section is ≥940 MPa, and the elongation after fracture A is ≥12%.

5. The method for heat treatment of the heel-end forging area of a 60-50 kg / m U71 Mn rail according to claim 1, characterized in that: The hardness of the rail top surface in the forming section, transition section and heat affected zone is between 260-310HB.

6. The method for heat treatment of the heel-end forging area of a 60-50 kg / m U71 Mn rail according to claim 1, characterized in that: The cross-section hardness of the forming section and transition section 3mm from the surface and 15mm from the surface shall meet ≥25HRC.

7. The method for heat treatment of the heel-end forging area of a 60-50 kg / m U71 Mn rail according to claim 1, characterized in that: The forming section and transition section are typical pearlite structures, there is no abnormal structure, no overburning phenomenon, and the decarburization layer depth in the transition section is ≤0.3mm.

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