A heat treatment method for the heel end of a 60kg / m forged 50kg / m U75VH heat treated rail

Abstract

The application discloses a 60kg / m forged 50kg / m U75VH heat-treated rail heel end heat treatment method, wherein the 60kg / m forged 50kg / m U75VH heat-treated special-shaped rail heel end forging and pressing heat treatment area is heated by using a medium-frequency induction heating device, air blowing cooling is performed by using a self-designed air blowing cooling device, air blowing treatment is separately performed on a forming section, a transition section and a heat affected zone of the special-shaped rail, and the cooling speed of different positions (the forming section, the transition section and the heat affected zone) is controlled, so that the hardness of the forging and pressing area of the special-shaped rail heel end, the heat affected zone and the base material is matched, and the standard requirement is met.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of steel rail heat treatment, in particular to a 60kg / m forged 50kg / m U75VH heat treated rail heel end heat treatment method. BACKGROUND

[0002] At present, most of the domestic heavy load lines, passenger and freight mixed lines and the like use 50kg / m, 60kg / m or 75kg / m symmetric cross-section steel rails within the standard range. The transportation conditions, service environments and train axle loads are different between existing lines and new lines, and the economy of laying steel rails on the lines is fully considered, so there are different rail types laid between different lines, that is, they can be roughly divided into 50kg / m rail lines, 60kg / m rail lines and 75kg / m rail lines, and the connection between different rail type lines becomes a difficult and important part. At present, forged special-shaped steel rails need to be used to connect between different cross-section rail lines, and the forged special-shaped steel rail connection method has the advantages of safety and reliability, good mechanical properties, excellent organizational structure, high utilization rate, high cost performance and the like, which can ensure the safe and smooth operation of trains on different cross-section rail lines.

[0003] The special-shaped steel rail is manufactured by heating and forging at the rail end of the ordinary steel rail, realizing the transition from one rail cross-section to another rail cross-section. After the 60kg / m U75VH heat treated rail heel end is high-temperature forged, the metallographic structure is coarse lamellar pearlite, the strength and hardness are low, it cannot be matched with the base material, and it is difficult to meet the line service requirements. In order to refine the pearlite structure of the special-shaped steel rail and restore the performance of the rail heel end forging area, the rail heel end forging area needs to be re-induction normalized. The present application is directed to the invention of the 60kg / m forged 50kg / m U75VH heat treated rail heel end heat treatment after the 60kg / m U75VH heat treated rail is forged into 50kg / m, and the process of full cross-section heating and rail head top surface and rail head side surface air cooling treatment is adopted. The mechanical properties and metallographic structure of the 60kg / m forged 50kg / m U75VH heat treated special-shaped steel rail produced meet the standard requirements of TB / T2344.3-2018 Steel Rails Part 3: Special-shaped Steel Rails.

[0004] The patent with the application number 201310431374.4 discloses a steel rail profiled heel end section heat treatment device and heat treatment method, which mainly relates to a steel rail profiled heel end section heat treatment device and does not describe the heat treatment process or method of the steel rail profiled heel end section. The patent with the application number 201910277768.6 discloses a heat treatment method for a steel rail heel end forging section, which cools the rail head to 350-500 DEG C by air blowing after heating the rail head to 850-950 DEG C. The cooling temperature of the rail head by air blowing in the patent is between 350-500 DEG C, and the excessively low cooling temperature leads to problems such as abnormal organization of the steel rail and excessive hardness of the formed section, and the wide cooling range is not suitable for the promotion and application of the patent. The document "60AT1-U75V steel rail profiled heel end induction heat treatment" discusses the online heat treatment of 60AT1-U75V steel rail, and adopts medium-frequency induction preheating + medium-frequency induction heating + air cooling + spray cooling for the heel end. The document and the 60-50kg / m U75V hot-rolled steel rail standard requirement performance of the present application are different, and the process is not universal. The medium-frequency induction preheating and spray cooling process described in the document increases the production process, production cost and control difficulty of the special-shaped steel rail, and the spray cooling treatment of the rail head is prone to abnormal organization in the continuous production process. SUMMARY

[0005] In order to solve the above technical problems, the purpose of the present application is to provide a 60kg / m forged 50kg / m U75VH heat treatment steel rail heel end heat treatment method, which controls the cooling speed of different positions (formed section, transition section and heat affected zone) to match the hardness of the special-shaped steel rail heel end forging area, heat affected zone and base material, and meet the standard requirements.

[0006] In order to solve the above technical problems, the present application adopts the following technical scheme:

[0007] The present application is a 60kg / m forged 50kg / m U75VH heat treatment steel rail heel end heat treatment method, and the process flow of the 60kg / m forged 50kg / m U75VH heat treatment special-shaped steel rail is: blanking → forging → heat treatment → shot blasting → straightening → milling and grinding → fine adjustment → sizing → drilling → flaw detection → finished product; and the features are as follows:

[0008] 1) The 60kg / m U75VH heat treatment steel rail heel end is forged into a 50kg / m steel rail at high temperature twice, forming a 60-50kg / m U75VH heat treatment special-shaped steel rail;

[0009] 2) The forged 60-50kg / m U75VH hot treatment special-shaped rail heel end forging and affecting area is placed to the medium frequency induction heater for induction heating, the heating area length of the forged special-shaped rail after heat treatment should be greater than the forging area length, and the distance between the formed section rail end and the heating area is controlled to be 710-800mm;

[0010] 3) The medium frequency induction heating frequency is between 1500-2000Hz, the medium frequency induction heating power is between 50-100kW, the forged special-shaped rail heel end is heated from room temperature to 900±20℃, and the heating time is controlled to be 200-300s; the medium frequency heating can ensure that the high temperature die forging area surface and the rail head core are heated to the temperature range required for rail austenitization;

[0011] 4) After the special-shaped rail heel end is heated to the target temperature of 900±10℃, the medium frequency induction heating power is reduced, the heel end is kept at the target temperature for 20-30s; the heel end heating part is kept at the target temperature, and the uniformity of the temperature of the rail head core and surface is improved;

[0012] 5) The high-temperature special-shaped rail heel end heating part enters the cooling device, and the heel end is subjected to air blast accelerated cooling treatment, mainly for air blast cooling of the rail head top surface and the rail head side surface, and the air blast cooling part is more than 60mm above the normalizing heating part;

[0013] 6) Formed section cooling process: the rail head top surface air pressure is 0.10-0.20MPa, the rail head side surface air pressure is 0.10-0.15MPa, and the rail head cooling speed is 2.0-4.0℃ / s, the formed section rail head is controlled to be cooled from 900±10℃ to 500-530℃, and the reheat temperature is controlled to be 550-590℃;

[0014] 7) Transition section cooling process: the rail head top surface air pressure is 0.15-0.22MPa, the rail head side surface air pressure is 0.10-0.18MPa, and the rail head cooling speed is 2.0-4.5℃ / s, the transition section rail head is controlled to be cooled from 900±10℃ to 490-520℃, and the reheat temperature is controlled to be 540-580℃;

[0015] 8) Heat affected zone cooling process: the rail head top surface air pressure is 0.15-0.25MPa, the rail head side surface air pressure is 0.15-0.20MPa, and the rail head cooling speed is 2.5-5.0℃ / s, the heat affected zone rail head is controlled to be cooled from 900±10℃ to 460-500℃, and the reheat temperature is controlled to be 500-560℃;

[0016] 9) After the special-shaped rail different positions are controlled to reach the target temperature, air cooling is performed to room temperature.

[0017] Further, the 60kg / m U75VH heat-treated steel rail raw material takes C, Si, Mn and V as the main strengthening elements, and the weight percentage of the chemical composition of the steel rail is as follows: 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 rest is Fe.

[0018] Further, the obtained 60kg / m forged 50kg / m U75VH heat-treated special-shaped steel rail has the following properties: the tensile strength Rm of the forming section is greater than or equal to 1238MPa, the elongation A after fracture is greater than or equal to 12.0%; the hardness of the rail top surface of the forming section, the transition section and the heat-affected zone is between 340-370HBW; the hardness of the cross section of the forming section, the transition section and the heat-affected zone is greater than or equal to 37.0HRC at a distance of 3mm (A1, B1, C1) from the surface, and greater than or equal to 36.0HRC at a distance of 15mm (A5, B5, C5) from the surface.

[0019] Further, the microstructure of the forming section and the transition section is consistent with the type of the base material, and is a typical pearlite plus a small amount of ferrite structure, without abnormal structure and overburning phenomenon; the decarburization layer depth of the rail head of the forming section is about 0.20mm, and the decarburization layer depth of the rail head of the transition section is about 0.15mm, and all the mechanical properties and the microstructure meet the standard requirements.

[0020] Compared with the prior art, the beneficial technical effects of the present application are as follows:

[0021] The present application is directed to the 60kg / m forged 50kg / m U75VH heat-treated special-shaped steel rail, and the forming section, the transition section and the heat-affected zone are respectively treated by air blowing during the heat treatment cooling process of the heel end, and the cooling speed and the final cooling temperature of different positions (the forming section, the transition section and the heat-affected zone) are controlled, so as to effectively improve the difference in the microstructure and the performance between the heel end of the special-shaped steel rail after high-temperature die forging and the base material, thereby solving the problems of low rail head top surface hardness of the forming section, the transition section and the heat-affected zone, excessive S zone width, easy abnormal structure and excessive decarburization layer of the U75VH heat-treated 60kg / m forged 50kg / m steel rail, and matching the strength, hardness and toughness of the heel end forging and pressing area, the heat-affected area and the base material.

[0022] Based on the above invention, the 60kg / m forged 50kg / m U75VH heat-treated special-shaped rail is obtained: the tensile strength Rm of the forming section is ≥1238MPa, the elongation A after breaking is ≥12.0%; the rail top surface hardness of the forming section, the transition section and the heat-affected zone is between 340-370HBW; the cross-section hardness of the forming section, the transition section 3mm (A1, B1, C1) away from the surface is ≥37.0HRC, and the cross-section hardness of the forming section, the transition section 15mm (A5, B5, C5) away from the surface is ≥36.0HRC; the microstructure of the forming section and the transition section is consistent with the type of the base material, which is typical pearlite plus a small amount of ferrite structure, there is no abnormal structure, and there is no overburning phenomenon; the decarburization layer depth of the forming section rail head is about 0.20mm, and the decarburization layer depth of the transition section rail head is about 0.15mm, and all the mechanical properties and microstructure meet the standard requirements. BRIEF DESCRIPTION OF DRAWINGS

[0023] The application will be further described below in combination with the drawings.

[0024] Figure 1 The CCT curve of the U75V rail;

[0025] Figure 2 The rail top surface hardness curve result of Example 1;

[0026] Figure 3 The rail top surface hardness curve result of Example 2;

[0027] Figure 4 The rail top surface hardness curve result of Example 3;

[0028] Figure 5 The microstructure (500X) of the special-shaped rail at different positions of Example 3;

[0029] Figure 6 The decarburization layer (100X) of the special-shaped rail at different positions of Example 3. DETAILED DESCRIPTION

[0030] A 60kg / m forged 50kg / m U75VH heat-treated rail heel end heat treatment method, specifically comprising:

[0031] 1) The raw material of the 60kg / m U75VH heat-treated rail takes C, Si, Mn and V as the main strengthening elements, and 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 rest is Fe.

[0032] 2) The above component 60 kg / m U75VH heat-treated rail heel is twice forged at high temperature to form a 50 kg / m rail, forming a 60-50 kg / m U75VH heat-treated special-shaped rail.

[0033] 3) The forged 60-50 kg / m U75VH heat-treated special-shaped rail heel forging area and the affected area are placed in a medium-frequency induction heater for induction heating. The length of the heating zone of the heat-treated special-shaped rail after forging and the distance from the rail end of the forming section are controlled to be between 710-800 mm.

[0034] 4) Adjust the medium-frequency induction heating frequency to be between 1500-2000 Hz, and the medium-frequency induction heating power to be between 50-100 kW, heat the forged special-shaped rail heel from room temperature to 900±20℃, and control the heating time to be between 200-300 s.

[0035] 5) After the special-shaped rail heel is heated to the target temperature of 900±10℃, reduce the medium-frequency induction heating power, and keep the heel at the target temperature for 20-30 s.

[0036] 6) The high-temperature special-shaped rail heel heating part is put into a cooling device for air blast accelerated cooling treatment, mainly for air blast cooling of the rail head top surface and the rail head side surface, and the air blast cooling part is more than 60 mm above the normalizing heating part.

[0037] 7) Forming section cooling process: the rail head top surface air pressure is 0.10-0.20 MPa, the rail head side surface air pressure is 0.10-0.15 MPa, and the rail head cooling speed is 2.0-4.0℃ / s, which controls the rail head of the forming section to be cooled from 900±10℃ to 500-530℃, and the reheat temperature is controlled to be 550-590℃.

[0038] 8) Transition section cooling process: the rail head top surface air pressure is 0.15-0.22 MPa, the rail head side surface air pressure is 0.10-0.18 MPa, and the rail head cooling speed is 2.0-4.5℃ / s, which controls the rail head of the transition section to be cooled from 900±10℃ to 490-520℃, and the reheat temperature is controlled to be 540-580℃.

[0039] 9) Heat-affected zone cooling process: the rail head top surface air pressure is 0.15-0.25 MPa, the rail head side surface air pressure is 0.15-0.20 MPa, and the rail head cooling speed is 2.5-5.0℃ / s, which controls the rail head of the heat-affected zone to be cooled from 900±10℃ to 460-500℃, and the reheat temperature is controlled to be 500-560℃.

[0040] 10) After the special-shaped rail at different positions is controlled to reach the target temperature, air cooling is performed to room temperature.

[0041] Process comparison during implementation:

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

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

[0044]

[0045] According to "TB / T2344.3-2018 Steel Rails Part 3: Special-shaped Rails", the Brinell hardness of the rail head top surface is tested. The rail head top surface is ground to 0.5 mm and above. The distance between the measuring points in the forming section and the transition section is 100 mm. The distance between the measuring points from the special-shaped part to the base material is 10 mm. The rail top surface hardness test results of the steel rails of Example 1-Example 3 are shown in Table 2. Figures 1-3 .

[0046] As can be seen from Table 2, Figure 1 the rail top surface hardness of the forming section, the transition section and the heat affected zone of the special-shaped rail of Example 1 is less than 340HBW, which does not meet the standard requirement that the hardness of at most 5 measuring points is less than 340HBW. The hardness of the special-shaped rail of Example 1 is significantly lower than that of the base material, which cannot match the hardness of the base material and affects the smoothness and service performance of the train on the line. The high final cooling temperature of the special-shaped rail cannot guarantee the hardness of the forming section, the transition section and the heat affected zone.

[0047] As can be seen from Table 2, Figure 2 the rail top surface hardness of the forming section and the transition section of the special-shaped rail of Example 2 meets the standard requirement, but the hardness is close to the lower limit of the standard hardness, the production window is small, and it is very easy to produce unqualified products in the continuous production process. The hardness of the heat affected zone of Example 2 (including S zone) is less than 340HBW for 7 measuring points, which does not meet the standard requirement that the hardness of at most 5 measuring points is less than 340HBW. The difference between the hardness of the forming section, the heat affected zone of the special-shaped rail and the base material is large, which may cause abnormal wear or "saddle type" wear during the service of the rail, and affects the smoothness of the line. The final cooling temperature of the special-shaped rail needs to be further reduced to improve the hardness of the rail.

[0048] As can be seen from Table 2, Figure 3 the rail top surface hardness of the forming section, the transition section and the heat affected zone of the special-shaped rail of Example 3 is between 340-370HBW, which meets the hardness requirement of the standard for 60kg / m forged 50kg / m U75VH heat treated special-shaped rail. The rail top surface hardness of Example 3 meets the line service requirement and can ensure the safe and smooth operation of the train between different section rails.

[0049] According to the test methods in TB / T2344.3-2018 Steel Rails Part 3: Special-shaped Steel Rails, the special-shaped steel rail of Example 3 was analyzed for rail head cross-sectional hardness, tensile properties, and microstructure. The rail head cross-sectional hardness is shown in Table 2, the tensile properties of the formed section and the base material are shown in Table 3, and the microstructure of the formed section, transition section, and base material are shown in Table 3. Figure 4 .

[0050] Table 2. Hardness (HRC) of the hardened layer in the cross section of Example 3

[0051]

[0052] Table 3 Room temperature tensile properties of the molding section and base material in Example 3

[0053]

[0054] From Table 2, Table 3 and Figure 4 It can be seen that the cross-sectional hardness of the special-shaped rail in Example 3, 3mm from the surface (A1, B1, C1), is ≥37.0HRC, meeting the standard requirement of ≥36.0HRC; the cross-sectional hardness of the special-shaped rail in Example 3, 15mm from the surface (A5, B5, C5), is ≥36.0HRC, meeting the standard requirement of ≥34.0HRC; the tensile strength Rm of the formed section of the special-shaped rail in Example 3 is ≥1238MPa, and the elongation after fracture A is ≥12.0%, meeting the standard requirements; the microstructure of the formed section and transition section of the special-shaped rail in Example 3 is consistent with the base material, both being typical pearlite with a small amount of ferrite, without harmful structures such as high-carbon martensite and Widmanstätten, and no overheating phenomenon was observed in the formed section and transition section of the rail.

[0055] Example 3: Decarburized layer samples were taken from the rail head of the irregular-shaped rail forming section and transition section. The microstructure of the decarburized layer is as follows: Figure 5 As shown, the decarburized layer depth of the rail head in the formed section is approximately 0.20 mm, and the decarburized layer depth of the rail head in the transition section is approximately 0.15 mm, meeting the standard requirement of a decarburized layer depth ≤ 0.50 mm.

[0056] 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 heat treatment method for a 60kg / m forged 50kg / m U75VH heat treated rail foot end, the 60kg / m forged 50kg / m U75VH heat treated special-shaped rail special-shaped rail process flow being: blanking → forging → heat treatment → shot blasting → straightening → milling and grinding → fine adjustment → sizing → drilling → flaw detection → finished product; characterized in that: Specifically comprising: 1) 60kg / m U75VH heat-treated rail heel end is forged into 50kg / m rail twice at high temperature, forming 60-50kg / m U75VH heat-treated special-shaped rail; 2) The forged 60-50kg / m U75VH heat-treated special-shaped rail heel end forging and affecting area is placed into the medium-frequency induction heater for induction heating, the heating area length of the forged special-shaped rail heat treatment should be greater than the forging area length, and the distance between the formed section rail end and the heating area is controlled to be between 710-800mm; 3) The medium-frequency induction heating frequency is between 1500-2000Hz, the medium-frequency induction heating power is between 50-100kW, the forged special-shaped rail heel end is heated from room temperature to 900±20℃, and the heating time is controlled to be between 200-300s; The medium-frequency heating can ensure that the high-temperature forging area surface and the rail head core are heated to the temperature range required for rail austenitization; 4) After the special-shaped rail heel end is heated to the target temperature of 900±10℃, the medium-frequency induction heating power is reduced, the heel end is kept at the target temperature for 20-30s; the heel end heating part is kept at the target temperature for 20-30s; the heel end heating part is kept at the target temperature for 20-30s; the heel end heating part is kept at the target temperature for 20-30s; the heel end heating part is kept at the target temperature for 20-30s; the heel end heating part is kept at the target temperature for 20-30s; the heel end heating part is kept at the target temperature for 20-30s; the heel end heating part is kept at the target temperature for 20-30s; the heel end heating part is kept at the target temperature for 20-30s; the heel end heating part is kept at the target temperature for 20-30s; 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The method of heat treating a 60 kg / m forged 50 kg / m U75VH heat treated rail foot end according to claim 1, characterized in that: ​ ​ 3. The method of heat treating a 60 kg / m forged 50 kg / m U75VH heat treated rail foot end according to claim 2, characterized in that: The obtained 60kg / m forged 50kg / m U75VH heat-treated special-shaped rail: the tensile strength Rm of the forming section is greater than or equal to 1238MPa, the elongation A after fracture is greater than or equal to 12.0%; the rail head surface hardness of the forming section, the transition section and the heat-affected zone is between 340-370HBW; the cross-section hardness of the forming section and the transition section 3mm away from the surface is greater than or equal to 37.0HRC, and the cross-section hardness 15mm away from the surface is greater than or equal to 36.0HRC.

4. The method of heat treating a 60 kg / m forged 50 kg / m U75VH heat treated rail foot end according to claim 2, characterized in that: The microstructure of the forming section and the transition section is consistent with the type of the parent material, which is typical pearlite plus a small amount of ferrite structure, there is no abnormal structure, and there is no overburning phenomenon; The decarburization layer depth of the forming section rail head is 0.20mm, the decarburization layer depth of the transition section rail head is 0.15mm, and all the mechanical properties and the microstructure meet the standard requirements.

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