CN110480142B

CN110480142B - Process for controlling flash welding joint structure of steel rail with middle and upper limit Mn content R350HT

Info

Publication number: CN110480142B
Application number: CN201910790177.9A
Authority: CN
Inventors: 陆鑫; 李大东; 白威; 徐飞翔
Original assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Current assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Priority date: 2019-08-26
Filing date: 2019-08-26
Publication date: 2021-07-13
Anticipated expiration: 2039-08-26
Also published as: CN110480142A; AU2020210190B2; AU2020210190A1

Abstract

The invention discloses a process for controlling the structure of a flash welding joint of a steel rail with the middle and upper limit Mn content of R350HT, belonging to the technical field of flash welding of steel rails. The invention provides a process for controlling the structure of a R350HT steel rail flash welding joint, which aims to solve the technical problem that the R350HT steel rail flash welding joint is easy to generate a martensite structure in the prior art, and comprises a welding process and post-welding treatment; in the welding process, the welding heat input is controlled to be 7.0-8.5 MJ, the welding is performed by pulsation or preheating flash welding, and the welding upset forging amount is 14.5-15.5 mm; in the post-welding treatment, the joint is naturally cooled to 525-560 ℃, then is cooled to be less than or equal to 250 ℃ through heat preservation, and finally is naturally cooled to the room temperature. By comprehensively controlling the chemical components of the steel rail, the hot rolling process, the welding process and the cooling after welding, the flash welding joint of the steel rail is ensured to have no abnormal structures such as martensite and the like, and the mechanical property of the joint is ensured.

Description

Process for controlling flash welding joint structure of steel rail with middle and upper limit Mn content R350HT

Technical Field

The invention belongs to the technical field of steel rail flash welding, and particularly relates to a process for controlling the structure of a middle and upper limit Mn content R350HT steel rail flash welding joint.

Background

With the rapid development of the seamless track technology in the fields of passenger transport, freight transport, high-speed and heavy-load railway construction in the world, the quality of the steel rail joint attracts more and more attention of related departments. The railway line is used as a direct carrier for train operation, and the reliability of the quality of the railway line is the key of the safe operation of the train. The flash welding joint of the steel rail belongs to a weak link of the whole line, and the quality of the flash welding joint can directly influence the safety of the railway. In the base body of the steel rail flash welding head, the martensite structure belongs to a hard brittle phase, has high hardness and extremely low toughness, is a special-shaped structure which has serious influence on a base metal and a joint in the steel rail welding production process, is very easy to cause the phenomena of steel rail breakage, block falling, even cover uncovering and the like on a line, and seriously influences the safe operation of a train.

At present, the shape and the content of martensite in a joint are regulated to different degrees in the mainstream steel rail flash welding standard and enterprise technical conditions at home and abroad. The European standard BS EN14587-2:2009 Rail way applications-Track-Flash welding of rails, part 2: New R220R 260R 260Mn and R350HT grade rails by mobile welding machines at sites other than a fixed point, observing by using an optical microscope under the magnification of 100 x, wherein the harmful tissues such as martensite or bainite cannot appear, the sampling position is a steel Rail vertical central plane, the Rail head and the Rail low are sampled, the Rail head position is 1, the sample size is 20 x 20mm, the two Rail feet are respectively 1, and the sample size is 10 x 20 mm; the European standard R350HT steel Rail is a heat-treated steel Rail specified in EN 13674-1-2011 < railways applications-Track-Rail Part 1: Vignole railways 46kg/m and above >, the C content is 0.70-0.82%, the Mn content is 0.65-1.25%, the Si content is 0.13-0.60%, the Cr content is less than or equal to 0.15%, the mechanical property is that Rm is greater than or equal to 1175MPa, the A is greater than or equal to 9%, the tread hardness is greater than or equal to 350HB, and the steel Rail is widely applied to foreign iron and subway lines. Moreover, the shape and the content of martensite in the rail flash welding joint are extremely high in various countries in the world. How to inhibit and eliminate the martensite in the steel rail flash welding head through the welding process and the post-welding treatment process is an important factor for judging whether the quality of the steel rail flash welding head reaches the standard or not.

However, the component range of the R350HT steel rail of the EN standard is wide, the chemical components of steel are designed by using high-Mn alloy elements, the condition that the mechanical property of the steel rail after hot rolling meets the EN standard is only considered, and the change of welding acceptance standard of a user in the using process is not considered, so that the metallographic microstructure inspection of the central plane under the tread of the flash welding head always has a martensite structure, and the martensite structure cannot pass the welding type inspection.

At present, the method for inhibiting and eliminating the martensite structure of the flash welded joint of the steel rail commonly used at home and abroad is postweld heat treatment, namely, the joint which is cooled to a lower temperature after being welded is reheated to a certain temperature by adopting an electromagnetic induction coil or a flame heating device, and then the heated welded joint area is cooled at a certain cooling rate. For pearlitic rails, the specification of the iron standard TB/T1632 states that the post-weld heat treatment includes normalizing and post-weld under-hardening used for restoring the hardness of the rail head; the welding joint of the fixed flash welding is heated in a medium-frequency induction mode, and the welding joint of the movable flash welding is heated in a medium-frequency induction mode or a flame heater swinging mode; when the medium-frequency induction heating mode is adopted, the initial temperature of the surface of the rail head is lower than 500 ℃, and the heating temperature is preferably 900 +/-20 ℃; when the flame heater is used for heating in a swinging mode, the initial temperature of the surface of the rail head is lower than 500 ℃, the heating width is 50 +/-10 mm, and the heating temperature is preferably 850-950 ℃; the rail head under-speed quenching adopts the cooling of the jet compressed air. The methods adopted by other scholars in China are postweld heat treatment, namely different purposes are achieved by controlling the heating rate, the peak temperature of heating, the cooling rate and the final temperature of forced cooling. The method only aims at the rail head strengthening heat treatment rail flash welding joint abroad, and forcibly cools a welding seam area by using compressed air in the process of naturally cooling the welded joint to achieve the purpose of improving the hardness of the joint; and aiming at the partially heat-treated steel rail and all heat-treated steel rail varieties, no post-welding treatment measure is adopted, namely only natural cooling is carried out after welding.

Disclosure of Invention

The invention solves the technical problem that the welding and heat treatment process in the prior art easily causes the R350HT steel rail flash welding joint to have a martensite structure.

The technical scheme for solving the technical problems is to provide a process for controlling the structure of a medium and upper limit Mn content R350HT steel rail flash welding joint, which is characterized by comprising the following steps: comprises a welding process and post-welding treatment; wherein the Mn content of the R350HT steel rail base metal is 1.05-1.15 wt%; in the welding process, the welding heat input is controlled to be 7.0-8.5 MJ, the welding is performed by pulsation or preheating flash welding, and the welding upset forging amount is 14.5-15.5 mm.

In the process for controlling the structure of the steel rail flash welding joint with the upper limit Mn content R350HT, after flash welding, the welding joint is naturally cooled to 525-560 ℃ of the central position of the welding line of the driving surface, then is cooled to 250 ℃ or below by heat preservation, and finally is naturally cooled to room temperature in post-welding treatment.

In the process for controlling the structure of the steel rail flash welding joint with the upper limit Mn content R350HT, the center of the welding line is an area with the length of 150-200 mm on each side of a joint fusion line.

In the process for controlling the steel rail flash welding joint structure with the upper limit Mn content R350HT, the thermal insulation and cooling adopt aluminous acid fiber cotton with the thickness of more than or equal to 30mm as a thermal insulation material.

In the process for controlling the flash welded joint structure of the steel rail with the medium and upper limit Mn content of R350HT, the C content of the R350HT steel rail base metal is in the medium and lower limit, and the Si content is less than or equal to 0.30 wt%.

In the process for controlling the flash welded joint structure of the steel rail with the upper limit Mn content of R350HT, the R350HT steel rail base metal is produced by adopting a universal rolling line and an online heat treatment line, and the mechanical property of the R350HT steel rail base metal meets the requirements that Rm is not less than 1175MPa, A is not less than 9 percent, and tread hardness is not less than 350 HB.

The invention has the beneficial effects that:

according to the invention, through comprehensive control of chemical components, hot rolling process, welding process and post-welding cooling of the R350HT steel rail with the middle and upper limit Mn content, under the condition that the Mn content of the R350HT steel rail component is 1.05-1.15 wt%, the joint does not need to be reheated, so that abnormal structures such as martensite and the like in a steel rail flash welding joint are avoided, and meanwhile, the hardness, static bending and fatigue performance of the joint are ensured to meet the technical requirements of EN14587-2:2009 standard; the invention adopts a simple heat preservation device to carry out heat preservation and cooling control, and the cooling speed after welding is well controlled; the invention has simple control process, is applicable to both fixed flash welding and movable flash welding, can solve the major welding technical problem of R350HT steel rail popularization and application on roads in a matching way, and has good popularization and application prospect.

Drawings

FIG. 1 is a metallographic structure diagram of a flash welded joint of R350HT steel rail according to the invention.

Detailed Description

Specifically, the process for controlling the steel rail flash welding joint structure with the middle and upper limit Mn content of R350HT comprises the following steps: comprises a welding process and post-welding treatment; wherein the Mn content of the R350HT steel rail base metal is 1.05-1.15 wt%; in the welding process, the welding heat input is controlled to be 7.0-8.5 MJ, the welding is performed by pulsation or preheating flash welding, and the welding upset forging amount is 14.5-15.5 mm.

In the flash welding process of the steel rail, firstly, the lintel blasting in the flash leveling stage is adopted to enable the section of the steel rail to be welded to be flat and clean, so that favorable conditions of relatively uniform leveling are provided for subsequent flash and heating; then accumulating heat through a high-current and low-voltage flashing process in a preheating stage, forming a certain temperature gradient in the longitudinal direction of the steel rail, heating the end face of the steel rail to a sufficient temperature and laying conditions for accelerating the homogenization process of flashing; and finally, carrying out final-stage accelerated flash firing, forming protective atmosphere for preventing end surface oxidation in the whole welding area, and finally forming proper temperature field distribution to provide conditions for upsetting. The control of the welding heat input is mainly to control the heat accumulated in the high-current and low-voltage flashing process in the preheating stage, and the heat directly influences the temperature gradient of the end face of the steel rail.

After a suitable temperature gradient is obtained, the two rails are brought together by applying a force on the rails in the longitudinal direction of the rails. At this time, the rail having plastic deformability is crushed and deformed. The amount of metal consumed by the front section of the rail before and after upsetting is the amount of upsetting. Generally, when the heat input is constant, the larger the upsetting force is, the larger the upsetting amount is. Tests show that when the Mn content of the R350HT steel rail is at the middle upper limit (1.05-1.15 wt%), the welding heat input is 7.0-8.5 MJ, and the welding upset forging amount is 14.5-15.5 mm, the optimal matching can be achieved, the bonding strength of the steel rail joint is the highest, and the joint quality is the most stable.

In the process of smelting the steel rail, Mn element and S element are easy to form manganese sulfide, and the manganese sulfide is a good deoxidizer and desulfurizer, and can reduce the harm of sulfur in steel to a certain extent. Meanwhile, Mn is dissolved in ferrite to cause solid solution strengthening, and the hardness and the strength of the hot rolled steel are improved. However, when the content of Mn is high, the temper brittleness phenomenon is obvious, and Mn has the function of promoting the growth of crystal grains, so that the steel is sensitive to overheating, and when the mass fraction of Mn exceeds 1%, the welding performance of the steel is reduced. In the process of cooling the flash welding joint of the steel rail, martensite which is distributed in a point or strip shape is easily formed in a micro segregation area of the Mn element, and the joint performance is seriously influenced. Generally, the higher the average mass fraction of Mn element in steel, the higher the probability of occurrence of segregation of Mn, that is, the higher the Mn content, the more easily the martensite structure is generated.

The martensitic nature is a solid solution of interstitial spaces supersaturated with carbon in α -Fe. The martensite has two main structural forms: one is lath martensite, and the other is sheet martensite (the martensite referred to in the present invention is mainly sheet martensite). The sheet martensite generally appears in high carbon steel with a C content of more than 0.6%, and is sometimes needle-shaped or bamboo leaf-shaped under an optical microscope due to a sample phase, so the sheet martensite is also called needle-shaped martensite or bamboo leaf-shaped martensite. Another important characteristic of the sheet martensite is that a large number of microcracks exist, the brittleness of the high-carbon steel part is increased due to the existence of the microcracks, and the microcracks gradually expand into macrocracks under the action of internal stress, so that joint cracking or joint fatigue life is obviously reduced. Like other solid-state transformations, martensitic transformation requires a large enough supercooling degree to make the driving force of transformation greater than the resistance to transformation to cause transformation from austenite to martensite, but differs from the large supercooling degree required by the transformation, which must be supercooled to below the Ms point of T0; martensite is carried out under the condition of no diffusion, the phase change speed is extremely high at quite low temperature, and when austenite in steel is converted into martensite, the martensite is only reformed into a body-centered cubic lattice from a face-centered cubic lattice without component change; meanwhile, the martensite transformation is performed in a certain temperature range. The martensite transformation in the R350HT rail steel is carried out in the continuous (temperature-changing) cooling process, austenite is cooled below the Ms point at a speed higher than the critical quenching speed, a certain amount of martensite is formed immediately, a certain amount of martensite is formed along with the reduction of the temperature, the martensite formed first does not grow any more, and the martensite transformation quantity is gradually increased along with the reduction of the temperature until the martensite transformation quantity is lower than the Mf point.

The martensite structure related in the invention mainly appears in 5mm of two sides of a fusion line along a vertical central plane of a steel rail, because C, Mn is a segregation element, when the Mn content in the steel rail is increased to 1.05-1.15 wt%, a steel rail base metal with high Mn content tends to cause Mn micro-region segregation, and a CCT curve is seriously shifted to the right due to high chemical components in a segregation region, so that the punctiform martensite structure appears in the air cooling process of a joint after welding. Therefore, after flash welding is finished, the welding joint is naturally cooled to the central position of the welding line of the driving surface at the temperature of 525-560 ℃, then is cooled to the central position of the welding line of the driving surface at the temperature of less than or equal to 250 ℃ in a heat preservation way, and finally is naturally cooled to room temperature, the cooling speed in the cooling process of the steel rail joint is controlled to be in a proper range, so that the cooling speed is lower than the critical cooling speed, and the excellent performance of the joint is ensured while no martensite phase change occurs.

The specific heat treatment steps are as follows: after flash welding is finished, firstly, an infrared thermometer is adopted to measure the temperature of the center position of a welding line of a running surface of a naturally cooled steel rail joint, and when the temperature is reduced to 525-560 ℃, a heat insulation device is immediately adopted to preserve the heat of a welding line area; continuously monitoring the temperature of the joint in the process, removing the device when the temperature of the center position of the welding line of the driving surface is reduced to be less than or equal to 250 ℃, and finally placing the joint in air to naturally cool to room temperature; wherein the center of the welding line is an area of 150-200 mm on each side of the joint fusion line; the heat preservation and cooling adopt the aluminosilicate fiber cotton with the thickness more than or equal to 30mm as the heat insulation material.

The natural cooling speed of the high-temperature section is high, the cooling speed is about 3-5 ℃/s, the natural cooling speed of the low-temperature section is low, and the cooling speed is less than 0.2 ℃/s; by controlling the heat preservation initial temperature, the heat preservation device and the like, the rate of heat preservation and cooling is 1-2 ℃/s.

The invention is suitable for R350HT steel rails with C0.70-0.82 wt% and Si 0.13-0.60 wt%; in practice, the inventor finds that when the C content of the R350HT steel rail base metal is in the middle-lower limit (the C content is generally 0.72-0.78 wt%) and the Si is less than or equal to 0.30 wt%, the metallographic structure of the joint can be better controlled by adopting the method.

The R350HT steel rail adopted by the invention is produced by adopting a universal rolling line and an online heat treatment line, and the mechanical property of the R350HT steel rail meets the requirements that Rm is more than or equal to 1175MPa, A is more than or equal to 9 percent, and the tread hardness is more than or equal to 350 HB.

In the invention, the contents are all mass percent.

The present invention is further illustrated by the following examples, which are not intended to limit the scope of the invention.

In the embodiment, the steel rail static bending test is the most widely applied steel rail joint overall performance evaluation method at home and abroad at present, all steel rail joint inspection standards are specified, and two indexes, namely load and deflection, are mainly included.

The static bending test is to apply load to the steel rail joint at a certain loading rate by adopting a three-point or four-point supporting method, and when the load reaches a standard specified value and the maximum deflection is greater than the standard specified value, the static bending performance of the joint is judged to be qualified, and different rail types correspond to different load and deflection values. In the standards of various countries, the Russian STO RZD 1.08.002:2009 standard has the highest quality requirement, the lowest load and deflection of a joint are required to be not less than 2100kN and 30mm (65kg/m steel rail and head of the rail are compressed), and the lowest load when the head of the 60kg/m steel rail is compressed is 1907kN and 30 mm. European standard BS EN14587-2:2009 requires that the minimum load and deflection of the joint is not less than 1600KN and 20mm respectively (60kg/m rail, head compressed). The Australian Standard AS1085.20-2012 requires that the minimum stress of the joint rail bottom is 900MPa, and the converted minimum load and deflection are not less than 1670KN and 20mm respectively (60kg/m rail, the head of the rail is pressed). The standard requirements of China on static bending are that the breaking load is not less than 1450kN (60kg/m, the railhead is pressed), and no requirement is made on the deflection.

The static bending load mainly reflects the indexes of the joint strength, the joint appearance and the internal defects, and the deflection mainly reflects the indexes of the joint toughness. If the welding process of the joint is poor, the joint is likely to break before the joint does not reach the load value specified by the standard due to defects of dust spots, incomplete fusion or overburning, or the joint is hard or soft due to improper matching of heat input and upsetting amount of the joint and improper treatment method after welding, so that the deflection of the joint does not reach the standard requirement.

The fatigue performance of the steel rail is similar to that of a static bending test, a three-point or four-point supporting method is adopted, alternating load is applied to the steel rail joint at a certain frequency and amplitude, the static bending performance of the joint is judged to be qualified until the cycle times reach a standard specified value, and different rail types correspond to different rail bottom maximum stress.

Example 1

A flash welding process with the heat input of 7.0MJ and the upsetting amount of 14.5mm is adopted to carry out flash welding on the European standard R350HT heat-treated steel rail with the Mn mass fraction of 1.05% (the C content is in the middle and lower limits, the Si content is less than or equal to 0.30 wt%, the Rm is greater than or equal to 1175MPa, the A is greater than or equal to 9%, and the tread hardness is greater than or equal to 350 HB). After flash welding is finished, firstly, an infrared thermometer is adopted to measure the temperature of the center position of a welding line of a running surface of a naturally cooled steel rail joint, and when the temperature is reduced to 550 ℃, an aluminosilicate fiber cotton heat insulation device with the thickness of more than or equal to 30mm is immediately adopted to insulate the heat of 200mm areas on the two sides of a joint fusion line; and continuously monitoring the temperature of the joint in the process, removing the device when the temperature of the center position of the welding line of the driving surface is lower than 250 ℃, and finally placing the joint in the air to naturally cool to the room temperature.

The joint obtained by adopting the welding process and the post-welding treatment method is inspected and analyzed according to EN 14587.2, the microstructures of the rail head and the rail foot position of the central plane of the joint are pearlite, and harmful structures such as martensite and the like are not seen; the hardness of the joints except the softening zone is in the range of-30 HV to +60HV of the average hardness of the parent metal; the three-point bending static bending test is continuous at 2000kN, and the maximum deflection is 26.8 mm; three-point support physical fatigue test is continued for 500 ten thousand times, and each index meets EN standard requirements.

Example 2

A flash welding process with the heat input of 7.9MJ and the upsetting amount of 14.8mm is adopted to carry out flash welding on the European standard R350HT heat-treated steel rail with the Mn mass fraction of 1.13% (the C content is in the middle and lower limits, the Si content is less than or equal to 0.30 wt%, the Rm is greater than or equal to 1175MPa, the A is greater than or equal to 9%, and the tread hardness is greater than or equal to 350 HB). After flash welding is finished, firstly, an infrared thermometer is adopted to measure the temperature of the center position of a welding line of a running surface of a naturally cooled steel rail joint, and when the temperature is reduced to 560 ℃, an aluminosilicate fiber cotton heat insulation device with the thickness of more than or equal to 30mm is immediately adopted to insulate the heat of 200mm areas on the two sides of a joint fusion line; and continuously monitoring the temperature of the joint in the process, removing the device when the temperature of the center position of the welding line of the driving surface is lower than 250 ℃, and finally placing the joint in the air to naturally cool to the room temperature.

The joint obtained by adopting the welding process and the post-welding treatment method is inspected and analyzed according to EN 14587.2, the microstructures of the rail head and the rail foot position of the central plane of the joint are pearlite, and harmful structures such as martensite and the like are not seen; the hardness of the joints except the softening zone is in the range of-30 HV to +60HV of the average hardness of the parent metal; the three-point bending static bending test is continuous at 2000kN, and the maximum deflection is 26.9 mm; three-point support physical fatigue test is continued for 500 ten thousand times, and each index meets EN standard requirements.

Example 3

A flash welding process with the heat input of 8.5MJ and the upsetting amount of 15.5mm is adopted to carry out flash welding on the European standard R350HT heat-treated steel rail with the Mn mass fraction of 1.15% (the C content is in the middle and lower limits, the Si content is less than or equal to 0.30 wt%, the Rm is greater than or equal to 1175MPa, the A is greater than or equal to 9%, and the tread hardness is greater than or equal to 350 HB). After flash welding is finished, firstly, an infrared thermometer is adopted to measure the temperature of the center position of a welding line of a running surface of a naturally cooled steel rail joint, and when the temperature is reduced to 525 ℃, an aluminosilicate fiber cotton heat insulation device with the thickness of more than or equal to 30mm is immediately adopted to insulate the heat of 200mm areas on the two sides of a joint fusion line; and continuously monitoring the temperature of the joint in the process, removing the device when the temperature of the center position of the welding line of the driving surface is lower than 250 ℃, and finally placing the joint in the air to naturally cool to the room temperature.

The joint obtained by adopting the welding process and the post-welding treatment method is inspected and analyzed according to EN 14587.2, the microstructures of the rail head and the rail foot position of the central plane of the joint are pearlite, and harmful structures such as martensite and the like are not seen; the hardness of the joints except the softening zone is in the range of-30 HV to +60HV of the average hardness of the parent metal; the three-point bending static bending test is continuous at 2000kN, and the maximum deflection is 27.1 mm; three-point support physical fatigue test is continued for 500 ten thousand times, and each index meets EN standard requirements.

Example 4

A flash welding process with the heat input of 8.4MJ and the upsetting amount of 15.0mm is adopted to carry out flash welding on the European standard R350HT heat-treated steel rail with the Mn mass fraction of 1.08% (the C content is in the middle and lower limits, the Si content is less than or equal to 0.30 wt%, the Rm is greater than or equal to 1175MPa, the A is greater than or equal to 9%, and the tread hardness is greater than or equal to 350 HB). After flash welding is finished, firstly, an infrared thermometer is adopted to measure the temperature of the center position of a welding line of a running surface of a naturally cooled steel rail joint, and when the temperature is reduced to 540 ℃, an aluminosilicate fiber cotton heat insulation device with the thickness of more than or equal to 30mm is immediately adopted to insulate the heat of 200mm areas on the two sides of a joint fusion line; and continuously monitoring the temperature of the joint in the process, removing the device when the temperature of the center position of the welding line of the driving surface is lower than 250 ℃, and finally placing the joint in the air to naturally cool to the room temperature.

The joint obtained by adopting the welding process and the post-welding treatment method is inspected and analyzed according to EN 14587.2, the microstructures of the rail head and the rail foot position of the central plane of the joint are pearlite, and harmful structures such as martensite and the like are not seen; the hardness of the joints except the softening zone is in the range of-30 HV to +60HV of the average hardness of the parent metal; the three-point bending static bending test is continuous at 2000kN, and the maximum deflection is 27.2 mm; three-point support physical fatigue test is continued for 500 ten thousand times, and each index meets EN standard requirements.

Claims

1. The process for controlling the steel rail flash welding joint structure with the middle and upper limit Mn content of R350HT is characterized by comprising the following steps: comprises a welding process and post-welding treatment; wherein the Mn content of the R350HT steel rail base metal is 1.05-1.15 wt%; in the welding process, the welding heat input is controlled to be 7.0-8.5 MJ, the welding is performed by pulsation or preheating flash welding, and the welding upset forging amount is 14.5-15.5 mm; in the post-welding treatment, after flash welding is finished, naturally cooling a welding joint to the temperature of 525-560 ℃ of the center position of a welding line of the driving surface, then preserving heat and cooling to the temperature of less than or equal to 250 ℃ of the center position of the welding line of the driving surface, and finally naturally cooling to room temperature; the content of C in the R350HT steel rail base metal is 0.72-0.78%, and Si is less than or equal to 0.30 wt%.

2. A process for controlling the structure of a flash welded joint of a steel rail with the upper limit Mn content of R350HT according to claim 1, wherein: the center of the welding line is an area with the length of 150-200 mm on each side of the joint fusion line.

3. A process for controlling the structure of a flash welded joint of a steel rail with the upper limit Mn content of R350HT according to claim 1, wherein: the heat preservation and cooling adopts the aluminosilicate fiber cotton with the thickness more than or equal to 30mm as the heat insulation material.

4. A process for controlling the structure of a flash welded joint of a steel rail with a medium-high limit Mn content R350HT according to any one of claims 1 to 3, which comprises the following steps: the R350HT steel rail base metal is produced by adopting a universal rolling line and an online heat treatment line, and the mechanical property of the R350HT steel rail base metal meets the condition that Rm is more than or equal to 1175MPa, A is more than or equal to 9 percent, and the tread hardness is more than or equal to 350 HB.