Post-welding heat treatment method for steel rail for exported American subway

Abstract

According to the post-welding heat treatment method for the steel rail for the export American subway, by controlling the cooling rate of the welding seam position of the welding joint after the steel rail is welded, generation of an abnormal martensitic structure at the welding seam position of the steel rail welding joint is effectively restrained, and the structure uniformity of the steel rail welding joint is improved; and the heavy-load steel rail is prevented from generating a crack source at the joint due to martensite in the joint during actual operation of the railway, so that the traffic safety of the railway line is seriously influenced.

Description

Technical Field

[0001] The invention belongs to the technical field of rail welding, and in particular relates to a post-welding heat treatment method for rails for export to American subways. Background Art

[0002] With the rapid development of the world economy, railways, as the main artery of the national economy, are also undergoing major changes. At present, various countries are constantly improving the transportation capacity, comfort and safety of railways through the transformation of existing lines, the construction of heavy-duty railways and the construction of seamless passenger dedicated lines. The rapid development and construction of rail transit systems such as light rails, urban subways and trams have greatly alleviated the pressure of urban traffic and provided more convenient conditions for people's daily life. At present, there are four main mature rail welding methods: flash welding, gas pressure welding, aluminum thermite welding, and arc welding, all of which can meet the requirements of high-speed railways and subways.

[0003] There are great differences between China and foreign countries in terms of rail flash welding technology. For example, the fixed flash welding used in Western Europe and China mostly adopts DC welding power supply and preheating flash process; while the fixed flash welding used in Eastern Europe mostly adopts AC welding power supply and pulsating flash process. North American countries also mainly use flash welding technology for rail welding. The factory flash welding used on North American railways is mainly used in laying new rails and overhauling rail replacement. The general operation process is to first

[0004] The 80-foot standard rail is welded into long rails, and then the connection between the long rails is made by a mobile flash welder. The test requirements are different from those in my country in that no heat treatment is performed after welding, and the joint performance inspection only performs static bending tests, not drop hammer tests. The biggest difference between my country and foreign countries is whether the welded joints are heat treated after welding. The width of the heat-affected zone of foreign flash welding joints is relatively uniform, and the heat-affected zone is relatively narrow. At present, whether it is base rail flash welding or on-site rail flash welding in China, basically a welding process with a wide heat-affected zone is used. The grains of the weld and the heat-affected zone are relatively coarse, and the comprehensive mechanical properties of the welded joints in the non-normalized state are poor, which is difficult to meet the requirements of drop hammer, hardness, etc. Therefore, the Chinese Iron Standard stipulates that normalizing treatment must be performed after welding. To this end, it is of great application value to explore welding processes that meet the requirements of domestic and foreign welded rail standards, study their performance, and compare and analyze the differences in performance and the reasons for the differences. Summary of the invention

[0005] The object of the present invention is to provide a post-welding heat treatment method for rails used in subways exported to the Americas. By controlling the cooling rate at the weld position of the welded joint after rail welding, the abnormal martensite structure generated at the weld position of the rail welded joint is effectively suppressed, the tissue uniformity at the rail welded joint is improved, and the crack source generated at this position due to the presence of martensite inside the joint during the actual operation of the heavy-duty rail is avoided, which seriously affects the driving safety of the railway line.

[0006] To solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A post-welding heat treatment method for rails used in subways exported to the Americas according to the present invention welds the rails, and after welding, slowly cools the heat-affected zone at a low cooling rate until the temperature drops to the first temperature threshold; when the joint temperature is less than 400 °C, appropriate accelerated cooling can be carried out. Cool to below 200 °C and then slowly cool at room temperature to ensure that complete phase transformation occurs at each position of the joint;

[0008] According to the mass percentages of the various elements in the adopted rail and the planned cooling rate, determine the corresponding martensite transformation start temperature K1 and martensite transformation end temperature K2; the determination method of the martensite transformation start temperature K1 and martensite transformation end temperature K2 is as follows: use a deformation thermo-mechanical phase transformation instrument to obtain the different phase transformation CTT curves of the adopted heavy-duty rail at different cooling rates, so as to obtain the martensite transformation start temperature and martensite transformation end temperature under the corresponding element composition, and then according to the heavy-duty rail composition and cooling rate, obtain the prediction formulas for the martensite transformation start temperature and martensite transformation end temperature; according to the mass percentages of the corresponding various elements in the adopted heavy-duty rail and the planned cooling rate, the prediction formulas are as follows:

[0009] K1 = 399.8 - 11.5×(7.37x)V1

[0010] K2 = 259.5 - 14.1×(6.76x)V2

[0011] Wherein, in the above formulas, x = C·Mn / 12 + Cr / 35 + Ni / 21 + Mo / 20 + Si / 40 - V / 3 - Al / 21. In the calculation formula of x, the unit of each element is its mass percentage of the total element content; V1 is the cooling rate for cooling to the martensite transformation start temperature K1 after welding, and V1 is 4 - 8 K / s;

[0012] V2 is the cooling rate from the martensite transformation start temperature K1 to the martensite transformation end temperature K2, and V2 < 2.0 K / s.

[0013] Further, the rail is an IH rail for subways exported to the Americas.

[0014] Further, the steel rail uses C, Si, Mn, and Cr as the main alloying elements. The weight percentages of the chemical components of the steel rail are as follows: C: 0.72 - 0.82%, Si: 0.40 - 0.60%, Mn 0.90 - 1.20%, 0.4% ≤ Cr ≤ 0.7%, P ≤ 0.02%, S ≤ 0.02%, and the balance is Fe and inevitable impurities.

[0015] Further, according to the component requirements, the steel rail is produced through smelting, continuous casting, slow cooling of the steel billet, rolling. The size of the continuous casting billet of the steel rail is 280mm × 380mm, the rolling section is 60N, the rolling reduction ratio of the steel rail is greater than or equal to 11:1, the finish rolling temperature of the steel rail should not be higher than 980°C. The high-temperature welded joint is air-cooled to below 400°C, and the cooling is appropriately accelerated to ensure that complete phase transformation occurs at each position of the joint. When the joint temperature is less than 200°C, room temperature cooling is carried out.

[0016] Further, the weld area of the welded joint of the 60kg / m medium-strength steel rail is heated to 895°C by high-frequency heating and then the heating is stopped. The heating time is 120s. After flash welding is completed, the upsetting process should be automatically completed, and the time from upsetting to the completion of upsetting is 18s. After stopping heating, when the temperature of the rail joint tread drops below 400°C, the cooling rate of the joint is 1.5°C / s. Ensure that complete phase transformation occurs at each position of the joint. When the joint temperature is less than 200°C, room temperature cooling can be carried out.

[0017] Further, the ratio of the average hardness HJ of the post-weld heat-treated joint of the 60kg / m medium-strength steel rail to the average hardness HP of the base metal is 0.90, and the ratio of the average hardness HJ1 of the soft spot of the joint to the average hardness HP of the base metal is 0.93.

[0018] Further, the width of the softened area on the left side of the joint is 12.0mm, and the width of the softened area on the right side of the joint is 11.0mm.

[0019] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0020] The present invention aims at the welding of IH hot-rolled steel rails exported to the Americas. Based on the formula proposed by the present invention, the martensite transformation start temperature (K1) and the martensite transformation end temperature (K2) of the weld position in the welding joint during the cooling process of the steel rail welding joint are determined, revealing the influence of the element composition and cooling rate of the steel rail after welding on the abnormal martensite structure at the weld position of the welding joint. An appropriate post-weld cooling system can be effectively applied to the formation temperature range of this abnormal structure. By controlling the cooling rate of the weld position in the steel rail welding joint after welding, the generation of abnormal martensite structure at the weld position of the steel rail welding joint is effectively inhibited, the tissue homogeneity at the steel rail welding joint is improved, and it is avoided that cracks are generated at this place due to the presence of martensite inside the joint during the actual operation of the heavy-haul steel rail, seriously affecting the driving safety of the railway line. Brief Description of the Drawings

[0021] Figure 1 It is for the longitudinal section hardness analysis of medium-strength rail weld joints. Detailed Implementation Manner

[0022] 1) The present invention discloses a heat treatment method for welded joints of high-hardness hot-rolled subway rails: The flash-welded rails use the IH rails for subways exported to the Americas as the base material, and C, Si, Mn, and Cr are the main alloying elements. The weight percentages of the chemical components of the rails are: C: 0.72 - 0.82%, Si: 0.40 - 0.60%, Mn 0.90 - 1.20%, 0.4% ≤ Cr ≤ 0.7%, P ≤ 0.02%, S ≤ 0.02%, and the balance is Fe and unavoidable impurities.

[0023] 2) The above components are used for rail production through smelting, continuous casting, slow cooling of the steel billet, rolling. The size of the continuous casting billet of the rail is 280mm×380mm, the rolling section is 60N, the rolling reduction ratio of the rail is greater than or equal to 11:1, the finish rolling temperature of the rail should not be higher than 980°C, the high-temperature welded joint is air-cooled to below 400°C, and the cooling is appropriately accelerated to ensure that complete phase transformation occurs at each position of the joint. When the joint temperature is less than 200°C, room temperature cooling can be carried out.

[0024] Process comparison during implementation:

[0025] The carbon content of the IH hot-rolled rails according to the American standard can reach about 0.80%, and the alloy content of the rails is also relatively high. During the fixed flash welding process, segregation of elements such as C and Mn, microstructural defects, and secondary cementite are likely to occur in the fusion line and the overheated zone. Therefore, an invention is made for the welding process of high-hardness rails according to the American standard. Through the control of key parameters such as the cooling rate and the final cooling temperature, and the comparative analysis of the mechanical properties of the joint, the drop hammer results, etc., the heat treatment process of the flash welding process for high-hardness IH rails according to the American standard is determined.

[0026] Example 1:

[0027] The weld area of the welded joint of 60 kg / m medium-strength steel rail is heated to 895 °C by high-frequency heating and then the heating is stopped. The heating time is 120 s. After flash welding is completed, the upsetting process should be automatically completed, and the time from upsetting to the completion of upsetting is 18 s. After the heating is stopped, wait until the temperature of the rail joint tread drops below 400 °C, and the cooling rate of the joint is 1.5 °C / s. Ensure that complete phase transformation occurs at each position of the joint. When the joint temperature is less than 200 °C, room temperature cooling can be carried out. Referring to the requirements of TB / T 1632.2-2014 standard, an HR-150A Rockwell hardness tester is used to measure the Rockwell hardness 5 mm below the tread of the rail head on the longitudinal section. In this embodiment, the ratio of the average hardness HJ of the post-weld heat-treated joint of 60 kg / m medium-strength steel rail to the average hardness HP of the base metal is 0.90, the ratio of the average hardness HJ1 of the soft spot of the joint to the average hardness HP of the base metal is 0.93, the width of the softened area on the left side of the joint is 12.0 mm, and the width of the softened area on the right side of the joint is 11.0 mm, meeting the standard requirements.

[0028] Table 1 Test results of the longitudinal section (test line 1) of the joint in Example 1

[0029]

[0030]

[0031] Table 2 Tensile properties of the flash-welded joint in Example 1

[0032]

[0033] Table 3 Impact properties of the flash-welded joint in Example 2

[0034]

[0035] As can be seen from Table 1 to Table 3, the tensile properties and impact properties of the flash-welded joint in Example 1 meet the standard requirements, and the joint has a large performance margin. The present invention provides a post-weld heat treatment method for American standard steel rails, which improves the strength, hardness and toughness of the rail joint and makes its performance match that of the base metal by controlling the cooling process and cooling rate.

[0036] The present invention is directed to the welding of IH hot-rolled steel rails for export to the Americas. Based on the formula proposed by the present invention, the martensite transformation start temperature (K1) and the martensite transformation end temperature (K2) of the weld position at the welded joint of the steel rail during the cooling process are determined. It reveals the influence of the element composition and cooling rate of the steel rail after welding on the abnormal martensite structure at the weld position of the welded joint. It is possible to effectively apply a suitable post-weld cooling regime to the temperature range where this abnormal structure forms. By controlling the cooling rate at the weld position of the welded joint after the steel rail is welded, the generation of abnormal martensite structure at the weld position of the steel rail welded joint can be effectively suppressed, the tissue uniformity at the steel rail welded joint can be improved, and the occurrence of crack sources at this location due to the presence of martensite inside the joint during the actual operation of the heavy-haul steel rail can be avoided, which seriously affects the safety of railway line operation.

[0037] The embodiments described above are only descriptions of the preferred embodiments of the present invention and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A post-welding heat treatment method for rails used in subways exported to the Americas, characterized in that: Weld the rail. After welding, slowly cool the heat-affected zone at a low cooling rate until the temperature drops to the first temperature threshold; when the joint temperature is less than 400 °C, appropriate accelerated cooling can be carried out. Cool to below 200 °C and then carry out slow cooling at room temperature to ensure that complete phase transformation occurs at each position of the joint; According to the mass percentages of the various elements in the rail used and the cooling rate to be adopted, determine the corresponding martensite transformation start temperature K1 and martensite transformation end temperature K2; the method for determining the martensite transformation start temperature K1 and martensite transformation end temperature K2 is as follows: use a deformation thermal expansion phase transformation instrument to obtain the different phase transformation CTT curves of the heavy-haul rail used at different cooling rates, so as to obtain the martensite transformation start temperature and martensite transformation end temperature under the corresponding element composition. Then, according to the composition of the heavy-haul rail and the cooling rate, obtain the prediction formulas for the martensite transformation start temperature and martensite transformation end temperature; according to the mass percentages of the corresponding various elements in the heavy-haul rail used and the cooling rate to be adopted, the prediction formulas are as follows: K1 = 399.8 - 11.5×(7.37x)V1 K2 = 259.5 - 14.1×(6.76x)V2 Among them, in the above formulas, x = C·Mn / 12 + Cr / 35 + Ni / 21 + Mo / 20 + Si / 40 - V / 3 - Al / 21. In the calculation formula of x, the unit of each element is its mass percentage of the total element content; V1 is the cooling rate for cooling the martensite transformation start temperature K1 after welding is completed, and V1 is 4 - 8 K / s; V2 is the cooling rate from the martensite transformation start temperature K1 to the martensite transformation end temperature K2, and V2 < 2.0 K / s.

2. The post-welding heat treatment method for rails used in subways exported to the Americas according to claim 1, characterized in that: The rail is the IH rail for subways exported to the Americas.

3. The post-welding heat treatment method for steel rails used in subways exported to the Americas according to claim 2, characterized in that: The rail uses C, Si, Mn, and Cr as the main alloying elements. The weight percentages of the chemical components of the rail are C: 0.72 - 0.82%, Si: 0.40 - 0.60%, Mn 0.90 - 1.20%, 0.4% ≤ Cr ≤ 0.7%, P ≤ 0.02%, S ≤ 0.02%, and the balance is Fe and unavoidable impurities.

4. The post-welding heat treatment method for steel rails used in subways exported to the Americas according to claim 2 or 3, characterized in that: According to the composition requirements, carry out rail production through smelting, continuous casting, slow cooling of the billet, rolling. The size of the continuous casting billet of the rail is 280 mm × 380 mm, the rolling section is 60N, the rolling reduction ratio of the rail is greater than or equal to 11:1, the final rolling temperature of the rail should not be higher than 980 °C. Air-cool the high-temperature welded joint to below 400 °C, and appropriately accelerate cooling to ensure that complete phase transformation occurs at each position of the joint. When the joint temperature is less than 200 °C, carry out cooling at room temperature.

5. The post-welding heat treatment method for rails used in subways exported to the Americas according to claim 1, characterized in that: Adopt high-frequency heating to heat the weld area of the 60 kg / m medium-strength rail joint to 895 °C and then stop heating. The heating time is 120 s; after flash welding is completed, the upsetting process should be automatically completed, and the time from upsetting to upsetting completion is 18 s; after stopping heating, when the temperature of the rail joint tread drops below 400 °C, the joint cooling rate is 1.5 °C / s; ensure that complete phase transformation occurs at each position of the joint. When the joint temperature is less than 200 °C, cooling at room temperature can be carried out.

6. The post-welding heat treatment method for the rail used in the subway exported to the Americas according to claim 5, characterized in that: The ratio of the average hardness HJ of the post-weld heat treatment joint of the 60 kg / m medium-strength rail to the average hardness HP of the base metal is 0.90, and the ratio of the average hardness HJ1 of the soft spot of the joint to the average hardness HP of the base metal is 0.

93.

7. The post-welding heat treatment method for rails used in subways exported to the Americas according to claim 5 or 6, characterized in that: The width of the softened zone on the left side of the joint is 12.0 mm, and the width of the softened zone on the right side is 11.0 mm.

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