JP7364992B1 - Manufacturing method of flash butt welding rail - Google Patents

Abstract

A method for manufacturing a flash butt welded rail according to one aspect of the present invention includes an early flash process, a pulse process, a latter flash process, and an upset process, wherein the frequency in the pulse process is 1 to 10 Hz, and the maximum frequency in the pulse process is 1 to 10 Hz. The current is 2.33 to 6.98 A/mm2, the average current in the pulse step is 0.70 to 4.65 A/mm2, the final flash speed in the latter flash step is 0.2 to 3.4 mm/sec, and the pulse In the process, the upper limit of the integrated current in the unit kA sec/mm2 is (8 x frequency - 480 x C content + 1400) / 8600, and in the pulse process, the lower limit of the integrated current in the unit kA sec/mm2. is (3.5×frequency−180×C content+55×Mn content+45×Cr content+300)/8600.

Description

The present invention relates to a method of manufacturing a flash butt welded rail.

The parts of the rail that are most susceptible to damage and the most costly to maintain are the rail joints. In addition, joints are a major source of noise and vibration generated when trains pass. On the other hand, on freight railways, freight cars are increasingly loaded with heavier loads. Therefore, a technique for manufacturing long rails by making the rail joints having the above problems continuous by welding has become common.

There are various welding methods, but flash butt (FB) welding is the main one. The rails shipped from the rail manufacturing company are welded at the railway company's welding factory using a stationary type FB welding machine (fixed type FB welding machine), which allows long rails with a length of, for example, 200 m or more to be made. Manufactured. This long rail is transported to the installation site and further welded using a portable FB welding machine (mobile FB welding machine), thereby producing long rails of 1000 m or more in many cases.

As shown in FIGS. 2A and 2B, there are two main types of rail FB welding methods: a preheating flash method and a pulse flash method.

A fixed type FB welding machine has a large transformer capacity and can flow a large current. Therefore, in FB welding using a stationary FB welding machine, a preheating flash method that includes a preheating process in which a large current is passed is mainly adopted. The preheating flash method consists of an early flash process, a preheating process, a late flash process, and an upset process.

In the first flashing process, first, the end faces of two rails, which are to be welded base materials, are butted against each other with a gap provided while a voltage is applied. Here, "butting" means bringing two things close together, but facing each other while being separated. A distinction is made between the term "butting", which means bringing two facing objects into contact, and the term "butting". Next, the rail is moved so that the end surfaces of the rail are brought closer together. Usually one rail is advanced towards the other rail. As a result, a short circuit current flows locally at the end faces of the two rails.

Due to the resistance heat generation, the end surface of the rail is rapidly heated and melted. The butted rails are bridged with molten metal. An arc is generated at this bridge portion. As a result, a portion of the molten metal is scattered and the end face is heated by radiant heat. Splashing of molten metal is referred to as flash generation. In the early flash process, these phenomena are continuously repeated. Then, the end face of the rail is flattened by the first flashing process. The pre-flash process is sometimes referred to as a pre-flash or a planarization flash.

Since the rails are worn out due to the scattering of molten metal, it is necessary to move the rails closer to each other in the early flashing process and the latter flashing process described later. The relative movement speed of the rail at this time is called the flash speed. Also, the amount of relative movement of the rail is called the flash length. Furthermore, the length of the rail consumed during the flashing process is called the flashing cost. Usually, the flash length and the flash distance have substantially the same value.

In the preheating step, the entire end surfaces of the two rails are brought into contact, and after electricity is applied for about 2 to 5 seconds, the end surfaces of the rails are separated for about 1 to 2 seconds. Repeat contact and separation about 2 to 18 times. At this time, since the two end surfaces are entirely in contact, a large current flows through the rail. Thereby, the end surface of the rail is preheated prior to implementation of the latter flashing process.

In the latter flashing step, as in the first flashing step, first, the end surfaces of the two rails are opposed to each other with a gap provided while a voltage is applied. Next, the rail is moved so that the end surfaces of the rail are brought closer together. This creates a flash between the two end faces. The principle of the latter flush process is the same as that of the early flush process. However, unlike the early flash process, at the start of the latter flash process, the end faces of the rails are flattened and preheated. The flash rate in the late flash step, ie, the late flash rate, is generally faster than the flash rate in the early flash step, ie, the early flash rate. Further, the flash length in the latter flash process is longer than the flash length in the early flash process.

The latter flashing step brings the entire end face into a molten state. In the subsequent upsetting process, the end surfaces of the rails are butted against each other and a large rolling force is applied. The end surfaces of the rails are joined together by a large rolling force, producing a welded rail. Further, during the upset process, the molten material and oxides on the end face are pushed out from the welded part and become burrs. The flash is usually removed after welding is complete.

Mobile FB welding machines are designed with an emphasis on portability. Therefore, a mobile FB welding machine has a smaller transformer capacity and a smaller maximum current that can be passed than a fixed FB. Therefore, in the mobile FB welding machine, sufficient current cannot be passed in the above-mentioned preheating process. Therefore, in FB welding using a mobile FB welding machine, the pulse flash method is mainly adopted. The pulse flash method is characterized by a smaller maximum current than the preheat flash method.

Pulse flash type FB welding consists of an early flash process, a pulse process, a latter flash process, and an upset process. Pulse flash type FB welding can be considered to be when the preheating process in the preheating flash type FB welding described above is replaced with a pulse process. In the pulse process, the rail advances and retreats repeatedly at a rate of several times per second (several Hz), and is a welding method that can increase the current value as the contact area of the rail increases. Therefore, the pulse process can be said to be a type of rail preheating method. However, the preheating process in the preheating flash method and the pulse process in the pulse flash process differ in the presence or absence of a holding time.

In the preheating step, the pair of rails are forcibly brought into contact to create a short circuit, generating Joule heat, thereby heating the end surface. The contact state between the pair of rails is maintained for several seconds to several tens of seconds. Current is therefore applied to the rail for a few seconds to tens of seconds, as shown in FIG. 2A. The current waveform in the preheating process is sometimes referred to as a hat-shaped waveform. Furthermore, current may be applied intermittently while the pair of rails are in contact with each other.

On the other hand, in the pulse process, the platen is moved while superimposing forward and backward pulses of several Hz on the platen movement. At this time, the state of contact between the pair of rails is maintained only for a very short time. In the pulse process, the platen is controlled to pull a pair of rails apart as soon as they make contact. However, as a result of the pair of rails being crimped together, the rails may not be pulled apart quickly, and the pair of rails may remain in contact with each other in appearance. Flashing occurs when the pair of rails move forward and backward. The end surface of the rail is preheated by flashing. The current waveform in the pulse process becomes a peak type waveform as shown in FIG. 2B.

In addition, in general FB welding, one of the two rails is moved forward and backward toward the other, but both of the two rails may be moved toward and away from each other. Hereinafter, "advancing" a rail is a concept that includes moving two rails closer to each other, and "retreating" a rail is a concept that includes moving two rails apart from each other.

However, in the pulse process, even when the rails are moved forward to the maximum extent, the entire end surfaces may not come into contact with each other. Furthermore, even if you try to separate the rails from each other, the end faces may stick together and the rails may not be separated. Further, as described above, the current value in the pulse step of the pulse flash method is smaller than the current value in the preheating step of the preheating flash method. Therefore, the integrated current value in the pulse process is smaller than the integrated current value in the preheating process of the preheating flash method. Note that the integrated current value is the product of the average current value and the total energization time in the preheating process or pulse process, and is an index of the amount of heat input in the preheating process or pulse process. The total energization time in the pulse process is the pulse process time, that is, the time from the start to the end of the pulse process. The average current value in the pulse process is the value obtained by dividing the time integral value of the current value in the pulse process by the total energization time. The total energization time in the preheating process is the total value of the energization time in the preheating process. In the current waveform diagram of the preheating process shown in FIG. 2A, the time during which the current value is 0 is not included in the total energization time. The average current value in the preheating step is the value obtained by dividing the time integral value of the current value by the total energization time.

Flash butt welded rails are required to satisfy standards set by standards in bending tests. Examples of welding standards include AREMA (American Railway Engineering and Track Maintenance Association) standards. Here, the amount of deflection at break is determined to be 0.75 inches or more, and the bottom stress at break is determined to be 125,000 lb/in ² or more.

Flash butt welded rails welded by pulsed flash type FB welding need to satisfy standards regarding the amount of deflection and bottom stress at break, similar to preheated flash type FB welding.

Regardless of the welding method, the starting point at which a welded rail breaks during a bending test is often an oxide defect on the weld surface. In flash butt welding in which atmospheric welding is performed, oxides generated in the latter flash process before the upset process remain in the welded joint without being discharged during the upset process. This oxide causes oxide defects in the welded rail, causing it to fail to meet standards regarding bending tests.

The inventors have confirmed that the proportion of joints obtained by pulsed flash type FB welding that do not satisfy bending test standards is higher than that in preheated flash type FB welding. It is generally known that increasing the integrated current during welding is effective for increasing the amount of deflection at break and the bottom stress in a bending test. However, compared to the preheating flash method, it is difficult for joints obtained by flash butt welding using the pulse flash method to meet the standards in terms of deflection at break and bottom stress in bending tests, even if the integrated current is increased. There are challenges.

Furthermore, when performing FB welding using a pulse flash method using a mobile FB welding machine, there is a problem that the higher the integrated current in the pulse process, the more the end faces stick together and cannot be separated, making it impossible to maintain pulse control. In the pulse process, the phenomenon in which end faces stick together is called crimping.

Flash butt welding of rails has mainly been carried out by the preheating flash method. Therefore, many documents have been published regarding welding methods using the preheating flash method. On the other hand, there are very few cases in which welding methods using the pulse flash method have been studied.

Patent Document 1 discloses a welding method in which advance and retreat timings are determined by current values and power values in welding large cross-sectional area high-temperature materials using a pulse flash method, with the aim of obtaining an arc with a large heat input effect. ing. Patent Document 2 discloses a welding method in which the starting temperature of the entire pulse process and the amount of melting loss at the end of the pulse process are specified in welding using a pulse flash method for rails for the purpose of increasing production capacity. Patent Document 3 discloses a welding method in which the amount of heat input is specified in welding using a pulse flash method for the purpose of controlling the structure of R260 rail, which is a high Mn rail. Optimized welding conditions for pulse flashing of rails at a frequency of 7-9 Hz are disclosed in Non-Patent Document 1.

Japanese Patent Application Publication No. 2002-205175 Japanese Patent Application Publication No. 57-50285 China Patent Application Publication No. 110616368

"ACCELERATION THE HEATING OF RAILS DURING FLASH WWELDING WITH PULSED FLASHING" S. I. KUCHUK-YATSENKO et al., Paton Welding Institute, Ukrainian Academy of Sciences, Avtom Svarka No. 4 Page. 45-47 (1977)

However, the above-mentioned prior art documents do not describe a method for preventing fracture at a low deflection amount and low bottom stress in a bending test of a flash butt welded rail, and a method for preventing crimping during a pulse process.

The object of the present invention is to (1) suppress fracture due to low deflection and low bottom stress during bending tests of flash butt welded rails by flash butt welding using the pulse flash method; It is an object of the present invention to provide a method for manufacturing a flash butt welded rail that can prevent the end face of the rail from being crimped during the process.

The gist of the invention is as follows.

(1) A method for manufacturing a flash butt welded rail according to one aspect of the present invention includes a first step in which a flash is generated between the end surfaces of a pair of rails arranged along the longitudinal direction, thereby flattening the end surfaces. a flash step; a pulse step of repeatedly bringing the end surfaces of the pair of rails into contact and separating them, thereby preheating the end surfaces; and generating a flash between the end surfaces of the pair of rails, thereby heating the end surfaces. a latter flashing step of melting the whole; and an upsetting step of abutting and pressurizing the end surfaces of the pair of rails, thereby joining the rails; the frequency of the pulse step is 1 to 10 Hz; The maximum current in the pulse step is 2.33 to 6.98 A/mm ² , the average current in the pulse step is 0.70 to 4.65 A/mm ² , and the final flash rate in the latter flash step is 0.2 to 6.98 A/mm 2 . 3.4 mm/sec, and the chemical components of the rail are C: 0.60 to 1.20%, Mn: 0.1 to 2.0%, and Cr: 0.01 to 2.00% in unit mass %. , Si: 0.10-2.00%, Al: 0.001-0.500%, P: 0.020% or less, S: 0.020% or less, N: 0.003-0.020%, V : 0-0.300%, Ti: 0-0.0500%, Nb: 0-0.050%, Cu: 0-1.000%, Ni: 0-1.00%, Ca: 0-0.0200 %, REM: 0 to 0.0500%, and Mo: 0 to 0.500%, with the remainder containing Fe and impurities, and in the pulse process, an integrated current that is the product of the average current and the energization time. The upper limit in the unit kA sec/mm ² is (8 x frequency - 480 x C content + 1400)/8600, and in the pulse process, the lower limit of the integrated current in the unit kA sec/mm ² The value is set as (3.5 x frequency - 180 x C content + 55 x Mn content + 45 x Cr content + 300)/8600.
(2) In the method for manufacturing a flash butt welded rail according to (1) above, preferably, the minimum current in the pulse step is 0.12 A/mm ² or more.
(3) In the method for manufacturing a flash butt welded rail according to (1) or (2) above, preferably, the flash length in the latter flash step is 5 to 50 mm.
(4) In the method for manufacturing a flash butt welded rail according to any one of (1) to (3) above, preferably, the chemical component of the rail is V: 0.001 to 0 in unit mass %. .300%, Ti: 0.0008-0.0500%, Nb: 0.001-0.050%, Cu: 0.005-1.000%, Ni: 0.01-1.00%, Ca: A type selected from the group consisting of 0.0005 to 0.0200%, REM: 0.0005 to 0.0500%, Al: 0.001 to 0.500%, and Mo: 0.002 to 0.500%. Contains the above.

According to the method for manufacturing a flash butt welded rail according to the present invention, (1) Pulse flash type flash butt welding can suppress fracture due to low deflection and low bottom stress during bending tests of flash butt welded rails. (2) It is possible to provide a method for manufacturing a flash butt welded rail that can prevent the end face of the rail from being crimped during the pulse process.

FIG. 3 is a schematic diagram of a first-stage flash process. It is a schematic diagram of a pulse process. FIG. 3 is a schematic diagram of a late flush process. It is a schematic diagram of an upset process. FIG. 3 is a comparison diagram of current value changes in flash butt welding using the preheating flash method. FIG. 3 is a comparison diagram of current value changes in flash butt welding using a pulse flash method. It is a graph showing the relationship between the maximum flat spot length and the bottom stress at the time of bending fracture. It is a figure which shows the temperature measurement location of an end surface, and the evaluation unit of a low temperature part. 4B is an enlarged view of a rectangular area surrounded by a broken line in FIG. 4A. FIG. It is a graph showing the relationship between C and the number of evaluation units in which the temperature difference between the average temperature and the minimum temperature of the end face immediately after the pulse process is 50° C. or more. It is a graph showing the relationship between Mn and the number of evaluation units in which the temperature difference between the average temperature and the minimum temperature of the end face immediately after the pulse process is 50° C. or more. It is a graph showing the relationship between Cr and the number of evaluation units in which the temperature difference between the average temperature and the minimum temperature of the end face immediately after the pulse process is 50° C. or more. It is a graph showing the relationship between the number of evaluation units having a temperature difference of 50° C. or more between the average temperature and the minimum temperature of the end face immediately after the pulse process and the maximum length of the flat spot. It is a graph showing the relationship between frequency and flat spot maximum length. It is a graph showing the relationship between maximum current and flat spot maximum length. It is a graph showing the relationship between average current and flat spot maximum length. It is a graph showing the relationship between the maximum pulse time ratio and the ratio at which it becomes impossible to retreat. It is a graph which shows the relationship between C and maximum pulse time ratio. It is a graph showing the relationship between frequency and maximum pulse time ratio. It is a graph showing the relationship between maximum current and maximum pulse time ratio. It is a graph showing the relationship between average current and maximum pulse time ratio. FIG. 2 is a schematic diagram showing a bending test method.

In the method for manufacturing a flash butt welded rail according to the present embodiment, the chemical components of the rails 1A and 1B, the frequency, maximum current, average current, and integrated current in the pulse step S2, and the final flash speed in the latter flash step S3 are set within the optimum range. Within. That is, as shown in FIGS. 1A to 1D, the method for manufacturing a flash butt welded rail according to the present embodiment includes the following steps:
(S1) A first flashing step of generating flash between the end faces of the pair of rails 1A and 1B arranged along the longitudinal direction, thereby flattening the end faces;
(S2) a pulse process of repeatedly contacting and separating the end surfaces of the pair of rails 1A and 1B, thereby preheating the end surfaces;
(S3) a late flashing step in which a flash is generated between the end faces of the pair of rails 1A and 1B, thereby melting the end faces as a whole;
(S4) an upsetting step in which the end surfaces of the pair of rails 1A and 1B are brought into contact and pressurized, thereby joining the rails;
Equipped with Furthermore, in the method for manufacturing a flash butt welded rail according to the present embodiment, the frequency in the pulse process is 1 to 10 Hz, the maximum current in the pulse process is 2.33 to 6.98 A/ ^mm2 , and the average current in the pulse process is 0.70 to 4.65 A/mm ² , the final flash speed in the latter flashing step is 0.2 to 3.4 mm/sec, and the chemical composition of the rail is C: 0.60 to 1.20 in unit mass %. %, Mn: 0.1 to 2.0%, Cr: 0.01 to 2.00%, Si: 0.10 to 2.00%, P: 0.020% or less, S: 0.020% or less, and N: 0.003 to 0.020%, the remainder includes Fe and impurities, and in the pulse process, the integrated current, which is the product of the average current and the energization time, in units of kA sec/mm ² The upper limit value is (8 x frequency - 480 x C content + 1400) / 8600, and in the pulse process, the lower limit value in the unit of integrated current kA sec/mm ² is (3.5 x frequency - 180 x C content +55×Mn content+45×Cr content+300)/8600.

Note that the welding conditions include planned values that are predetermined before the start of welding and actual values that are measured during welding. The above-mentioned welding conditions are based on actual values. Although not included in the above-mentioned welding conditions, the "maximum pulse time ratio" described later is an index indicating the deviation between the planned value and the actual value.

Further, FIGS. 1A to 1D illustrate a welding method in which an electrode is pressed against a column of a rail. The electrodes are arranged so as to sandwich the column part of the rail from both sides. In FIGS. 1A to 1D, only the electrodes arranged on the front side of the column are shown, and the electrodes arranged on the back side of the column are omitted. However, the location where the electrodes are installed is not particularly limited. Flash butt welding may be performed by pressing electrodes against the head and sole surfaces of the rail.

Thereby, in the manufacturing method according to the present embodiment, flash butt welding can be continued without crimping of the end faces during the pulse process. Furthermore, in the bending test of the joint obtained by the manufacturing method according to the present embodiment, that is, the flash butt welded rail, the welded portion does not break due to the low deflection amount and low bottom stress. Therefore, the bending performance of the flash butt welded rail obtained by the manufacturing method according to the present embodiment can satisfy the rail welding standard.

Below, first, the process of examining various conditions of the method for manufacturing the flash butt welded rail according to the present embodiment described above will be explained in detail using the drawings. However, those skilled in the art will easily understand that the present invention is not limited to the following description, and that the form and details thereof can be changed in various ways without departing from the spirit and scope of the present invention. Therefore, the present invention should not be interpreted as being limited to the contents described in the embodiments shown below.

<Definition of terms>
Before explaining the method for manufacturing a flash butt welded rail according to this embodiment, definitions of terms will be described.

Rail: means the rail before welding, which is the base material for welding.
Flash butt welded rail: means a rail manufactured by flash butt welding and obtained by welding two or more rails. The flash butt welded rail includes a base metal portion and a welded portion, and the base metal portion has the same chemical composition and structure as the rail before welding.
Crimp: During rail advancement and retraction control at a predetermined frequency, the end surfaces (welded surfaces) of two butted rails stick together and cannot be separated, or the end surfaces become difficult to separate, and it is the time required for the rails to move backward. It means that the time is longer compared to the predetermined time.

Oxide defect: In flash butt welding performed in the atmosphere, it refers to oxides generated in the later flash process that remain in the weld joint without being discharged in the upsetting process. It often becomes the starting point of fracture in bending tests and fatigue tests of welded joints.

Cumulative current: means the product of the average current and the total energization time in the pulse step of flash butt welding using a pulse flash method or the preheating step of flash butt welding using a preheating flash method. The integrated current is an index of the amount of heat input in the pulse process or preheating process.

1 pulse: means one advance and retreat of the rail in a pulse process.
Planned pulse time: means the planned value of the time required to perform one pulse, determined before welding.
Actual pulse time: means the actual value of the time required to perform one pulse, which is measured from the actual value of the current waveform during welding. In the pulse process, a pair of rails may be crimped together and the rails may not be separated quickly. Therefore, the planned pulse time and actual pulse time do not necessarily match.
Maximum actual pulse time: Maximum actual pulse time in a pulse process having multiple pulses.

Maximum pulse time ratio: This is the value obtained by dividing the maximum actual pulse time by the planned pulse time, and is an index indicating the deviation between the planned value and the actual value. When the maximum pulse time ratio is 100%, it indicates that the rail moves forward and backward in a predetermined period of time.

<Explanation of welding method>
When considering the manufacturing method of the flash butt welded rail according to this embodiment, welding was performed using a pulse flash type mobile FB welding machine. The welding machine used was a mobile FB welding machine manufactured by Plasser. However, this does not limit the type of welding machine for implementing the manufacturing method according to this embodiment.

As mentioned above, flash butt welding using the pulse flash method includes an early flash process S1, a pulse process S2, a latter flash process S3, and an upset process S4.

In the first flash step S1 shown in FIG. 1A, a gap is provided between the end surfaces of a pair of rails arranged in the longitudinal direction, and the two rails are moved closer to each other while a voltage is applied. Create a flash between the end faces. At the start of the pre-flash process, the end face is typically not flat. Therefore, in the first flashing process, a short circuit current locally flows between the end faces, which rapidly heats up due to resistance heat generation, leading to melting. Then, the end faces are bridged with molten metal. An arc is generated at this bridge portion. The arc scatters a portion of the molten metal and heats the end face with radiant heat. In the early flash process, these phenomena are continuously repeated. This flattens the end surface of the rail.

Since the rails are worn out by the scattering of molten metal, it is necessary to move the rails closer together. The forward speed of the rail at this time is called the flash speed, and the amount of movement is called the flash length. If only one of the rails is moved, the amount of movement of this rail is considered to be the flash length. Furthermore, when both rails are moved closer to each other, the total value of the amount of movement of both rails is regarded as the flash length. Also in the pulse step S2, the latter flush step S3, and the upset step S4, the amount of movement of the rail is specified using the above-described method. Hereinafter, for convenience, moving the end surfaces of the rails closer to each other will be referred to as "advancing," and moving the end surfaces of the rails away from each other will be referred to as "retreating."

In the pulse step S2 shown in FIG. 1B, the rail is moved forward and backward repeatedly at a frequency of several Hz. As a result, the end surfaces of the pair of rails are brought into contact and separated repeatedly. This preheats the end face. When the rail is moved forward, the contact area of the end surface becomes maximum, and when the rail is moved backward, the contact area of the end surface becomes 0 or a small value close to this value. The current increases in proportion to the contact area of the end faces. Therefore, in the pulse process, the current increases and decreases repeatedly. If the two end faces are completely separated, the current will be zero.

In the latter flash step S3 shown in FIG. 1C, a flash is generated between the end faces of the pair of rails. The principle of the latter flush process is the same as that of the early flush process. However, the speed at which the rail is advanced in the later flash step, ie, the later flash speed, is generally faster than the flash speed in the early flash step, ie, the earlier flash speed. Also, the late s flash length is generally longer than the early flash step. Further, in the first flash process, the end face is flattened and then the next pulse process is started, but in the latter flash process, the end face is completely melted and then the next upset process is started.

In the upset step S4 shown in FIG. 1D, a large load is applied to the rail after the entire end surface of the rail is melted in the latter flash step S3. This joins the end faces of the rails and creates a flash butt welded rail. Further, in the upset step S4, the molten metal on the end face is discharged to the outside of the welded part and becomes a burr 3. The manufacturing method according to the present embodiment may further include a step of removing the burr 3.

<About rail material>
The rail material is generally a eutectoid or hypereutectoid rail steel containing 0.60 to 0.86 mass% of C, as specified in AREMA Chapter 4 “Rail”, UIC860-R. be. Moreover, recently, in order to further improve the wear resistance of flash butt welded rails, rail steels with even higher C content are becoming popular. Further, the cross-sectional size of the flash butt welded rail is selected depending on the weight of the freight cars on the route where the flash butt welded rail is used. In other words, in sections where heavy freight cars pass, rails with high rigidity and a large cross-sectional size are used.

<Causes of low deflection and low bottom stress in bending tests and oxide formation mechanism>
The fracture surface formed by performing a bending test on a flash butt welded rail has one or more areas with no unevenness on the surface (referred to as "flat spots"), which is clearly different from a brittle fracture surface. , this can be confirmed visually. Analysis of the flat spot reveals that it contains aggregates of oxides such as Mn and Cr. Therefore, this flat spot is a type of oxide defect. It is known that flat spots cause a decrease in the amount of deflection and bottom stress in bending tests.

When a flash butt welded rail fractures with a low deflection amount and low bottom stress, the origin of the fracture is often an oxide defect on the weld surface, regardless of the welding method. In flash butt welding, which performs atmospheric welding, if oxides generated in the latter flash process before the upset process remain in the weld joint without being discharged during the upset process, this oxide becomes an oxide defect. The oxide defects cause the amount of deflection and bottom stress to fail to meet the standards in bending tests.

<Relationship between maximum flat spot length and bottom stress during bending: explanation of the cause of decrease in bending rupture stress>
Figure 3 shows the relationship between the maximum flat spot length and the bottom stress during bending. The bottom stress is affected by the maximum length of the flat spot or spots. As the maximum length of the flat spot increases, the bending fracture stress of the flash butt welded rail shows a tendency to decrease. The present inventors have revealed that when the maximum length of the flat spot exceeds 12 mm, the bottom breaking stress during bending decreases significantly and the welding standard is no longer satisfied. Therefore, it was clarified that the maximum flat spot length must be 12 mm or less in order for the bottom stress during bending of flash butt welded rails to satisfy the standard. Although this time we presented the relationship between the maximum flat spot length and the bottom stress during bending, similar results were obtained regarding the relationship between the maximum flat spot length and the amount of deflection.

<General concept of measures to improve bending performance and results of applying these measures to flash butt welding using the pulse flash method>
In the flash butt welding rail bending test, in order to increase the amount of deflection at break and the bottom stress, when using a fixed FB welder, increase the cumulative current during welding and It is known that increasing the temperature of the entire end face is effective. Thereby, it is possible to easily discharge the sub-acid substances generated in the latter flushing process in the upset process. However, in flash butt welding using the pulse flash method, even if the integrated current is increased, the amount of deflection at break and the bottom stress may not meet the standards.

<Assumed reason why general bending performance improvement measures were not effective in pulse flash type flash butt welding>
As a result of repeated studies, the inventors investigated the reason why fracture occurs at low deflection and low bottom stress even when the integrated current is increased in pulsed flash type flash butt welding. Specifically, we investigated the generation mechanism of flat spots in pulsed flash type flash butt welding. As a result, the following findings were obtained.

Even if the integrated current is increased in flash butt welding using the pulse flash method, the amount of deflection at break in the bending test and the bottom stress may not meet the standards. This is because there is. To explain in more detail, if the chemical components of the rail include many alloy components that are likely to generate oxides, oxides are likely to be generated on the end face during the pulse process. At the site where the oxide is generated, it is extremely difficult for current to flow and the temperature to rise is difficult. That is, the portion of the end face where the oxide is generated cannot be sufficiently preheated even by the pulse process. Therefore, the temperature of the region where oxides are generated in the pulse process does not rise sufficiently even in the latter flash process. In addition, the oxides generated in the latter flashing process are difficult to be discharged in the upset process. As a result, portions where oxides are generated during the pulse process tend to become oxide defects (flat spots). As a result, the inventors believe that in a bending test of a flash butt welded rail, fracture occurs at low deflection and low bottom stress. The present inventors believe that most of the oxides generated in the pulse process are scattered together with the molten steel in the subsequent latter flash process. Therefore, it is presumed that the oxide that forms the flat spot is not an oxide produced in the pulse process, but an oxide produced in the latter flash process.

<Measures against fractures caused by low deflection and low bottom stress in bending tests caused by oxides>
In order for the amount of deflection and bottom stress to satisfy the standards in the bending test of flash butt welded rails obtained by flash butt welding using the pulse flash method, it is necessary to The inventors of the present invention thought that it would be effective to clarify the relationship between flat spots and flat spots. Heat input influencing factors include integrated current, maximum current, average current, frequency, etc. Furthermore, the inventors believed that it would be effective to clarify the relationship between the rail component and the flat spot in order to prevent a temperature drop in the portion of the end face where oxides were generated.

As a result of studies, the inventors have now obtained the following knowledge as a countermeasure against fracture due to low deflection and low bottom stress in flash butt welded rails obtained by pulse flash type flash butt welding. The study to obtain the following knowledge was conducted under the following conditions. A rail having chemical components of 0.9% by mass of C, 1.0% by mass of Mn, and 0.5% by mass of Cr and weighing 67 kg per 1 m was used as the welding base material. The cross-sectional area of the rail was 8600 ^mm2 . Under the basic conditions, the integrated current is 500 kA・sec (0.06 kA・sec/mm ² ), the maximum current is 30000 A (3.49 A/mm ² ), the average current is 18000 A (2.09 A/mm ² ), and the frequency is 5 Hz. , flash butt welding using the pulse flash method was performed.

<Explanation of oxide suppression mechanism depending on pulse conditions and component conditions>
(About the individual effects of ingredients)
The inventors investigated the bending performance of flash butt welded rails and the influence of the chemical components of the rails on the maximum flat spot length, and obtained the following findings. When oxides are generated on the end face during the pulse process due to the influence of chemical components, the temperature of the area where the oxides are generated is low. On the other hand, the temperature of the parts where oxides are not produced is high. Therefore, a temperature difference occurs within the end face. The influence of this temperature difference continues in the later flash process and the upset process, and the oxides generated in the latter flash process are difficult to be discharged in the upset process in areas where the temperature is low after the completion of the pulse process. This causes flat spots (oxide defects) to be generated. Then, in the bending test, rupture occurs at low bottom stress. Therefore, in order to evaluate the influence of chemical components on the bending test, "the number of low-temperature parts on the end face after the end of the pulse process" was used as an index. The number of low-temperature parts on the end face after the end of the pulse process was measured by separating the end face immediately after the end of the pulse process. It is also believed that the oxides generated on the end face during the pulse process are discharged to the outside along with the scattering of molten steel during the latter flash process.

(Method of measuring temperature at end surface)
An example of a device used to measure temperature is a thermoviewer (a thermometer that measures the temperature distribution on the surface of an object without contact). The required number of detection pixels of the thermoviewer is 320 pixels or more in the horizontal direction and 240 pixels or more in the vertical direction. The measurement target is the area below the neutral line in the height direction of the rail. In bending tests of welded rails, tensile stress acts below the neutral line, so it is considered necessary to measure the temperature in that range to evaluate the oxides present in that range. Ta.

However, the outer periphery of the end face and the 2 mm inside thereof are excluded from the evaluation of the low temperature part. This is because the outer periphery of the end face and the area within 2 mm inside thereof are cooled by the surrounding air, so the temperature is low regardless of whether oxides are produced or not. In addition, the molten metal on the outer periphery of the end face and its vicinity is likely to be discharged to the outside during the upsetting process, so even if oxides are generated in this area during the later flashing process, they will be discharged and rendered harmless during the upsetting process. it was thought.

Temperature measurement was carried out by separating the end face immediately after the end of the pulse process and measuring the temperature of the end face in the range below the neutral line in the height direction using a thermoviewer. At this time, the distance between the thermoviewer and the end face was adjusted so that the length of the area corresponding to one pixel on the end face was 0.5 mm or less in both the horizontal and vertical directions. Specifically, the distance was adjusted to match the width of the lateral field of view of the thermoviewer and the width of the foot of the rail. Note that it is preferable that the bottom of the rail and the bottom of the thermoviewer image are parallel.

(Evaluation method for low temperature section)
As shown in FIGS. 4A and 4B, in the evaluation of the temperature data measured by the thermoviewer, a range of 10 mm in the width direction x 2 mm in the height direction was taken as one evaluation unit. Then, this evaluation unit was placed at a plurality of predetermined positions, and the evaluation was performed using temperature data at each location. FIG. 4A is a schematic diagram of the rail end surface, and FIG. 4B is a schematic diagram of the range below the neutral line in the height direction of the rail end surface. The rectangular area surrounded by the broken line in FIG. 4A is shown in FIG. 4B. In FIGS. 4A and 4B, the dashed-dotted line marked with the symbol A is the center in the width direction of the rail end surface, and the solid line marked with the symbol B is the lower end of the rail end surface, that is, the sole surface of the rail, and the symbol O is marked with the solid line. The excluded area is the measurement exclusion area. In FIG. 4B, the cross marked with the symbol C1 is the center of the first evaluation unit, and the rectangular area with a width of 10 mm and a height of 2 mm surrounded by the broken line marked with the symbol E1 is the center of the first evaluation unit. It is a unit of evaluation. Symbol E2 and symbol C2 are the second evaluation unit and its center.
As described above, the shape of the evaluation unit was a rectangular shape of 10 mm along the width direction and 2 mm along the height direction of the end surface of the rail.
The location of the evaluation unit was as follows. First, the center C1 of the first evaluation unit E1 was placed at a position above the width direction center A of the end surface of the rail and 3 mm above the rail sole surface B. Next, with the first center C1 as a reference, each point at a pitch of 2 mm in the width direction and a pitch of 2 mm in the height direction was set as the center position of the second and subsequent evaluation units. The x marks other than C1 shown in FIG. 4B are the center positions of the second and subsequent evaluation units. For reference, the second evaluation unit E2 is also shown in FIG. 4B. However, evaluation units after the third one were omitted. Note that the evaluation units overlap each other at the ends in the width direction. Further, in FIG. 4B, only nine center positions of evaluation positions are shown, and descriptions of the tenth and subsequent positions are omitted.

Cut out the temperature measurement results using the thermoviewer for each evaluation unit. Then, the temperature difference between the average temperature and the lowest temperature within each evaluation unit was determined. However, if the evaluation unit includes conditions to be excluded from the above-mentioned measurement target (the outer peripheral part of the rail and 2 mm inside thereof), that evaluation unit is excluded from the evaluation target.

In this way, the average temperature and minimum temperature in each evaluation unit were calculated, and evaluation units with a temperature difference of more than 50°C were regarded as "low-temperature parts on the end face after the end of the pulse process" and the number of them was counted. . High-temperature oxides are presumed to have properties close to nonconductors, and it is extremely difficult for current to flow through them. Therefore, it is considered that the temperature of the region where the oxide exists is difficult to rise. Therefore, if the difference between the average temperature and the lowest temperature exceeded 50° C. in each evaluation unit, it was determined that an oxide defect had occurred in the evaluation unit. In addition, taking into consideration data variations, welding was performed three times using rails with the same chemical composition, and the score of the evaluation unit in which the temperature difference between the average temperature and the lowest temperature exceeded 50°C was the highest. The data were employed in various evaluations described below.

(Relationship between the amount of C in the rail and the number of evaluation units with a temperature difference of 50°C or more between the average temperature and the minimum temperature of the end face immediately after the pulse process: b)
Rails to which various amounts of C were applied were welded, and as described above, the end faces were pulled apart immediately after the completion of the pulse process, and the temperature of the end face was measured using a thermoviewer, and the average temperature and minimum temperature in each evaluation unit were calculated. I looked for the difference. FIG. 5 shows the relationship between the amount of C at this time and the number of evaluation units in which the temperature difference was 50° C. or more. In a range where the C amount of the rail is 0.6% by mass or more, the number of evaluation units with a temperature difference of 50° C. or more increases as the C amount decreases, but the rate of increase is small. From this, the C content of the rail is set to 0.6% by mass or more. In order to further improve the bending performance, the C content of the rail is preferably 0.7% by mass or more.

On the other hand, the present inventors have revealed that when the C content of the rail is less than 0.6% by mass, the number of evaluation units with a temperature difference of 50° C. or more increases significantly. The reason why the number of evaluation units with a temperature difference of 50°C or more increases when the C content of the rail is less than 0.6% by mass is that C is a highly reducing element, and as the C content decreases, oxides are generated. This is presumed to be because the number of oxides generated increases. Furthermore, as mentioned above, it is extremely difficult for current to flow through oxides, so it is easy for a temperature difference to occur between a part where no oxide is formed and a part where an oxide is formed, and the number of evaluation units with a large temperature difference has increased. I think so. It is considered that this tendency became noticeable in the range where the amount of C was less than 0.6%.

(Relationship between the amount of Mn in the rail and the number of evaluation units with a temperature difference of 50°C or more between the average temperature and the minimum temperature of the end face immediately after the pulse process: c)
Rails with various amounts of Mn applied were welded, and as described above, the end faces were separated immediately after the completion of the pulse process, the temperature of the end face was measured using a thermoviewer, and the average temperature and minimum temperature in each evaluation unit were calculated. I looked for the difference. FIG. 6 shows the relationship between the amount of Mn at this time and the number of evaluation units in which the temperature difference was 50° C. or more. In a range where the Mn content of the rail is 2.0% by mass or less, the number of evaluation units with a temperature difference of 50° C. or more increases as the Mn content increases, but the rate of increase is small. On the other hand, the present inventors revealed that when the amount of Mn exceeds 2.0% by mass, the number of evaluation units with a temperature difference of 50° C. or more increases significantly. From this, the amount of Mn in the rail is set to 2.0% by mass or less. In order to further improve the bending performance, the Mn content of the rail is preferably 1.5% by mass or less.

The reason why the number of evaluation units with a temperature difference of 50°C or more increases when the Mn content of the rail exceeds 2.0 mass% is that Mn is a highly oxidizing element, and as the Mn content increases, oxides are generated. This is presumed to be because the number of oxides generated increases. Furthermore, as mentioned above, it is extremely difficult for current to flow through oxides, so it is easy for a temperature difference to occur between a part where no oxide is formed and a part where an oxide is formed, and the number of evaluation units with a large temperature difference has increased. I think so. It is considered that this tendency became noticeable in the range where the Mn content exceeded 2.0%.

(Relationship between the amount of Cr in the rail and the number of temperature differences of 50°C or more between the average temperature and the minimum temperature of the end face immediately after the pulse process: d)
Rails with various amounts of Cr applied were welded, and as described above, the end faces were separated immediately after the completion of the pulse process, the temperature of the end face was measured using a thermoviewer, and the average temperature and minimum temperature in each evaluation unit were calculated. I looked for the difference. FIG. 7 shows the relationship between the amount of Cr at this time and the number of evaluation units in which the temperature difference was 50° C. or more. In a range where the Cr content of the rail is 2.0% by mass or less, the number of evaluation units with a temperature difference of 50° C. or more increases as the Cr content increases, but the rate of increase is small. On the other hand, the present inventors have revealed that when the Cr content exceeds 2.0% by mass, the number of evaluation units with a temperature difference of 50° C. or more increases significantly. From this, the amount of Cr in the rail is set to 2.0% by mass or less. In order to further improve the bending performance, the Cr content of the rail is preferably 1.3% by mass or less.

The reason why the number of evaluation units with a temperature difference of 50°C or more increases when the Cr content of the rail exceeds 2.0 mass% is that Cr is a highly oxidizing element, and as the Cr content increases, oxides are generated. This is presumed to be because the number of oxides generated increases. Furthermore, as mentioned above, it is extremely difficult for current to flow through oxides, so it is easy for a temperature difference to occur between a part where no oxide is formed and a part where an oxide is formed, and the number of evaluation units with a large temperature difference has increased. I think so. It is believed that this tendency became noticeable in the range where the Cr content exceeded 2.0%.

(Relationship between the number of evaluation units with a temperature difference of 50°C or more between the average temperature of the end face and the minimum temperature immediately after the pulse process and the maximum length of the flat spot: o)
In the evaluation of the relationship between the chemical composition and the number of evaluation units with a temperature difference of 50°C or more shown in Figures 5 to 7, the temperature was measured after the pulse process was completed, so the latter flash process and upset process were not performed. It wasn't done. In order to confirm the influence of the number of evaluation units with a temperature difference of 50°C or more on flash butt welded rails, the present inventors used the welding conditions in the above evaluation and conducted a late flash process, and an upsetting process to manufacture various flash butt welded rails. No heat treatments such as accelerated cooling and reheating were performed after welding. Then, the maximum length of the flat spot in various joints was measured, and thereby the relationship between the number of evaluation units with a temperature difference of 50° C. or more and the maximum length of the flat spot was evaluated. The evaluation results are shown in FIG. When the number of evaluation units with a temperature difference of 50° C. or more between the average temperature and the lowest temperature was less than 10, the maximum flat spot length was a small value of about 3 mm. On the other hand, when the number of evaluation units with a temperature difference of 50° C. or more was 10 or more, the maximum flat spot length suddenly increased. Furthermore, when the number of evaluation units with a temperature difference of 50° C. or more between the average temperature and the lowest temperature exceeded 40, the maximum flat spot length tended to be saturated at about 21 mm.

From FIG. 8, it was estimated that the maximum flat spot length was 12 mm when the number of evaluation units in which the temperature difference between the average temperature and the minimum temperature was 50° C. or more was 20. Therefore, in order to satisfy the standards during the bending test, as shown in Figs. It is necessary to select the contents of C, Mn, and Cr. Specifically, the range of the amount of C is 0.6% by mass or more, and the range of the amounts of Mn and Cr is 2.0% by mass or less. In order to further improve the bending performance, the C content is preferably 0.7% by mass or more, the Mn content is 1.5% by mass or less, and the Cr content is 1.3% by mass or less.

(Relationship between pulse process frequency and maximum flat spot length: e)
If the frequency in the pulse process is too high, the energization time per pulse (one advance or retreat of the rail) becomes short. Therefore, the amount of temperature rise per pulse becomes small, and the temperature of the end face becomes low. Furthermore, the temperature is not sufficiently high even in the latter flashing step, making it difficult for oxides generated in the latter flashing step to be discharged in the upset step, and oxide defects (flat spots) are likely to be formed. Therefore, we believe that if the frequency in the pulse process is too high, fracture will occur with low deflection and low bottom stress in the bending test of flash butt welded rails.

FIG. 9 shows the relationship between frequency and maximum flat spot length. With the increase of frequency, the maximum flat spot length showed an increasing trend. It was found that the maximum flat spot length was 12 mm at a frequency of 10 Hz. Therefore, the frequency range is set to 10 Hz or less. In order to further improve bending performance, the frequency range is preferably 8 Hz or less, 7 Hz or less, or 6 Hz or less. The cross-sectional area of the rail at this time is 8,600 mm ² , the cumulative current is 500 kA・sec (0.06 kA・sec/mm ² ), the maximum current is 30,000 A (3.49 A/mm ² ), and the average current is 18,000 A (2 .09A/mm ² ), the amount of C is 0.9% by mass, the amount of Mn is 1.0% by mass, and the amount of Cr is 0.5% by mass.

(Relationship between the integrated current and frequency of the pulse process, the chemical composition of the rail, and the flat spot: f)
Previous studies have revealed that the maximum flat spot length is strongly influenced by the amount of C, the amount of Mn, and the amount of Cr. We also found that it is strongly influenced by frequency. Furthermore, according to the inventors' research, it was revealed that the flat spot length changes by controlling the integrated current according to the C content, Mn content, Cr content, and frequency. The range in which the maximum length was 12 mm or less was determined using the integrated current equation (2).
Integrated current lower limit = 3.5 x frequency - 180 x C% + 55 x Mn% + 45 x Cr% + 300...Formula (2)
Note that in formula (2), C%, Mn%, and Cr% are the C content, Mn content, and Cr content of the rail in unit mass %, respectively. In formula (2), the C content is 0.6% by mass or more, the Mn content is 2.0% by mass or less, the Cr content is 2.0% by mass or less, and the frequency is in the range of 1Hz or more.

(Relationship between maximum current of pulse process and maximum flat spot length: g)
FIG. 10 shows the relationship between the maximum current in the pulse process and the maximum flat spot length. As the maximum current decreased, the maximum flat spot length tended to increase. It was found that the maximum flat spot length was 12 mm when the maximum current was 2.33 A/mm ² . Therefore, the maximum current in the pulse process is set to 2.33 A/mm ² or more. In order to further improve the bending performance, the maximum current in the pulse process is preferably 2.91 A/mm ² or more.

The reason why the maximum length of the flat spot increases as the maximum current decreases is that, as mentioned above, the integrated current in the pulse process is small, so the temperature of the end face does not become high enough in the latter flash process, and the flat spot is generated in the latter flash process. We believe this is because oxides were difficult to discharge during the upset process. The cross-sectional area of the rail at this time is 8,600 mm ² , the cumulative current is 500 kA・sec (0.06 kA・sec/mm ² ), the average current is 18,000 A (2.09 A/mm ² ), and the amount of C is 0. .9% by mass, the amount of Mn is 1.0% by mass, and the amount of Cr is 0.5% by mass.

(Relationship between the average current of the pulse process and the maximum length of the flat spot: h)
FIG. 11 shows the relationship between the average current of the pulse process and the maximum flat spot length. As the average current decreased, the maximum flat spot length tended to increase. It was found that the maximum flat spot length was 12 mm at an average current of 6,000 A (0.70 A/mm ² ). Therefore, the range of the average current in the pulse process is set to 0.70 A/mm ² or more. In order to further improve the bending performance, the average current range of the pulse process is preferably 1.74 A/mm ² or more.

The reason why the maximum length of the flat spot increases as the average current decreases in the pulse process is that, as mentioned above, the integrated current in the pulse process is small, so the temperature of the end face does not become high enough in the latter flash process, This is thought to be because the oxides generated during the upset process were difficult to discharge. The cross-sectional area of the rail at this time is 8,600 mm ² , the cumulative current is 500 kA・sec (0.06 kA・sec/mm ² ), the maximum current is 30,000 A (3.49 A/mm ² ), and C is 0.9 % by mass, Mn is 1.0% by mass, and Cr is 0.5% by mass.

<About the crimp generation mechanism in the pulse process>
In controlling the forward and backward movement of rails at a predetermined frequency, the time required for the end surfaces (welded surfaces) of two butted rails to stick together and not separate, or for the end surfaces to become difficult to separate and move backward. The fact that the time becomes longer than the predetermined time is called crimping. Generally, if the integrated current to the end face is excessively large and the temperature of the end face becomes significantly high, crimping is likely to occur.

In the case of preheating flash type flash butt welding performed using a stationary FB welding machine, crimp did not occur in the preheating process even if the integrated current in the preheating process was increased. In the preheating process using a stationary FB welder, the entire surface is brought into contact and a large current is applied. Therefore, the unevenness between the end faces is crushed and becomes flat or small. As a result, the entire end face was heated approximately evenly, no local overheating occurred, and therefore no crimping occurred. It is believed that the phenomenon in which the entire end face is heated approximately uniformly occurs in the same manner regardless of the magnitude of the integrated current.

The inventors obtained the following findings regarding the occurrence of crimping during the pulse process.
- In the pulse process, when the integrated current (maximum current, average current) is large, the amount of heat input to the end face becomes large. As a result of the increased temperature of the end face, crimping is likely to occur.
- If the frequency is low in the pulse process, the time at which the temperature reaches a high temperature during one advance becomes longer, and as a result, the amount of temperature rise on the end face becomes large, and as a result, crimping is likely to occur.
・Unlike the preheating process of flash butt welding using the preheating flash method, in the pulse process of flash butt welding using the pulse flash method, as the rail moves forward and backward repeatedly, all end faces come into contact even when the rail moves forward to the maximum extent possible. However, it is only a partial contact. This greatly affects the formation of crimp. In particular, if a portion with high electrical resistance exists in the end face, the Joule heat generation in the portion increases, and the temperature of the portion and its surrounding area increases. As a result, crimping tends to occur.

Specifically, the end face in the pulse process is affected by the scattering of molten metal before the pulse process, and is not completely flat, but has numerous irregularities. As the rail moves forward, the protrusions first come into contact and are heated by Joule heat generation, resulting in a high temperature. Therefore, the temperature of the end face is non-uniform, and there is a region of the end face that is partially melted. When the rail retreats, the molten portion solidifies as the temperature decreases. It is known that just before solidification, alloying elements are expelled from the solid phase into the liquid phase, creating a region of enrichment of the alloying elements. For example, C has a small equilibrium distribution coefficient of 0.2. During the pulse process, when a portion of the end face that has partially become a liquid phase solidifies and becomes a solid phase, C is discharged from the solid phase into the liquid phase, causing concentration of C. In the case of a 0.9% C rail, it is assumed that C is concentrated to about 4.5% by mass in a portion of the end face and precipitated as graphite.

It is known that the electrical resistivity of graphite at high temperatures, for example 1200°C, is about 8.4 μΩ・m (Toyo Tanso Co., Ltd. Nishimoto et al. "High-temperature resistivity change rate measurement device for graphite materials by direct energization") , 2015). On the other hand, the electrical resistivity of rail steel at high temperatures, for example 1200° C., is about 2.1 μΩ·m. The inventors have found that this is about 1/4 of the electrical resistivity of graphite. As described above, the electrical resistance of graphite at high temperatures is extremely large compared to that of rail steel. Therefore, the Joule calorific value in the graphite part is larger than that in rail steel, and as a result of the increased temperature in the graphite part and its vicinity, it is thought that crimping is likely to occur.

Note that even in the preheating process of flash butt welding using a preheating flash method using a fixed FB welding machine, the end face may melt and the alloying elements may be discharged to the end face, resulting in an increase in electrical resistance. However, in the case of the preheating flash method, the entire end surfaces come into contact and melt. Therefore, concentration occurs over the entire end face due to the discharge of alloying elements during solidification. Further, when the rail moves forward for the next preheating and the end surfaces come into contact, the molten steel is scattered and the concentrated portion is also discharged to the outside. Therefore, it is estimated that the effect of concentration is small in flash butt welding using the preheating flash method. Therefore, the inventors thought that crimping is less likely to occur in the preheating step of the preheating flash method than in the pulse step of the pulse flash method.

<Thoughts on crimp countermeasures in pulse process>
In pulse flash type flash butt welding, where the force (hydraulic pressure) used to control the forward and backward movement of the rail is smaller than that of a fixed FB welding machine, measures specific to the pulse flash type are required to prevent crimping during the pulse process. The inventors thought this. Specifically, as a countermeasure for crimping, in order to prevent excessive temperature rise at the end face, we will clarify the relationship between heat input influencing factors (maximum current, average current, integrated current, frequency, etc.) in the pulse process and crimping. Furthermore, the inventors believed that it would be effective to clarify the relationship between the chemical components of the rail components and crimping in order to prevent excessive temperature rises in areas with high electrical resistance.

<Explanation of crimp suppression mechanism based on pulse conditions and component conditions>
FIG. 12 shows the relationship between the maximum pulse time ratio and the ratio at which it becomes impossible to move backward after moving forward in the pulse process. As described above, the maximum pulse time is the value obtained by dividing the maximum actual pulse time, which is the maximum value of the actual pulse times, by the planned pulse time, which is the planned value of the pulse time determined before welding. The maximum pulse time is an index indicating the deviation between the planned value and the actual value.

When the maximum pulse time ratio exceeds 300%, the ratio of inability to retreat increases significantly. Therefore, the maximum pulse time ratio needs to be 300% or less. It is important to reduce the range of the maximum pulse time ratio to 300% or less by optimizing the pulse conditions and component conditions to suppress crimping.

(Relationship between rail C amount and maximum pulse time ratio: j)
FIG. 13 shows the relationship between the amount of C in the rail and the maximum pulse time ratio in the pulse process. The maximum pulse time ratio shows a tendency to increase as the amount of C increases. The maximum pulse time ratio is 300% when the C amount is 1.2% by mass. Therefore, the amount of C is set to 1.2% by mass or less. In order to further suppress the occurrence of crimping, the C content range is preferably 1.1% by mass or less.

The reason why the maximum pulse time ratio increases when the amount of C is high is considered to be as follows. As mentioned above, when the rail moves forward, the end face may become partially heated and melt. As the rail retreats, the temperature at the end surface decreases and the molten part solidifies. Immediately before this solidification, C is discharged from the solid phase to the liquid phase, a C concentration region is partially generated, and C is precipitated as graphite. As mentioned above, the electrical resistance of graphite is higher than that of rail steel. Therefore, the Joule calorific value in the graphite precipitated part, that is, in the graphite part, is larger than that in the surrounding area. The inventors believe that because the graphite part and its vicinity become high temperature, the end faces tend to stick together. Furthermore, as the amount of C increases, the range in which C is concentrated and graphite is generated becomes wider. Therefore, the inventors thought that the maximum pulse time ratio increased as the amount of C increased. The cross-sectional area of the rail at this time is 8,600 mm ² , the cumulative current is 500 kA・sec (0.06 kA・sec/mm ² ), the maximum current is 30,000 A (3.49 A/mm ² ), and the average current is 18,000 A ( 2.09 A/mm ² ), the frequency is 5 Hz, the amount of Mn is 1.0% by mass, and the amount of Cr is 0.5% by mass.

(Relationship between frequency in pulse process and maximum pulse time ratio: k)
FIG. 14 shows the relationship between the frequency in the pulse process and the maximum pulse time ratio. It was found that the maximum pulse time ratio tends to increase as the frequency decreases, and the maximum pulse time ratio reaches 300% when the frequency is 1 Hz. Therefore, the frequency range is set to 1 Hz or more. In order to further suppress the occurrence of crimping, the frequency range is preferably 3 Hz or higher.

The reason why the maximum pulse time ratio increases as the frequency decreases is that, as mentioned above, when the frequency is low, the total time for advancing and retracting one pulse is longer, the amount of heat input to the end face increases, and the temperature of the end face becomes higher. This is thought to be because the end surfaces became more likely to stick together as a result. The cross-sectional area of the rail at this time is 8600 mm ² , the cumulative current is 500 kA・sec (0.06 kA・sec/mm ² ), the maximum current is 30,000 A (3.49 A/mm ² ), and the average current is 18,000A (2.09A/mm ² ), the amount of C is 0.9% by mass, the amount of Mn is 1.0% by mass, and the amount of Cr is 0.5% by mass.

(The relationship between the integrated current and frequency in the pulse process, the chemical composition of the rail, and crimping: l)
Previous studies have revealed that the occurrence of crimping is strongly influenced by the amount of C and frequency. Furthermore, according to the inventors' research, it has been revealed that controlling the integrated current according to the C amount and frequency has a strong influence on the occurrence of crimping. Then, as a function of these, the range of cumulative current (unit: kA sec/mm ² ) where no crimping occurs was determined using formula (3). Maximum cumulative current = (8 x frequency - 480 x C mass % + 1400) / 8600...Formula (3)
In addition, in formula (3), the amount of C is 1.2% by mass or less, and the frequency is in the range of 1Hz or more.

(Relationship between maximum current in pulse process and maximum pulse time ratio: m)
FIG. 15 shows the relationship between the maximum current in the pulse process and the maximum pulse time ratio. It was found that the maximum pulse time ratio showed a tendency to increase as the maximum current increased, and the maximum pulse time ratio reached 300% when the maximum current was 6.98 A/mm ² . Therefore, the maximum current range is 6.98 A/mm ² or less. In order to further suppress the occurrence of crimping, the maximum current range is preferably 5.23 A/mm ² or less. The cross-sectional area of the rail at this time is 8600 mm ² , the cumulative current is 500 kA・sec (0.06 kA・sec/mm ² ), the average current is 18,000 A (2.09 A/mm ² ), and the amount of C is The amount of Mn is 1.0% by mass, the amount of Cr is 0.5% by mass, and the frequency is 5Hz. The reason why the maximum pulse time increases as the maximum current increases is that, as mentioned above, when the maximum current is large, the amount of heat input to the end face increases, and as a result, the end face becomes more likely to stick together as a result of the higher temperature of the end face. .

(Relationship between average current in pulse process and maximum pulse time ratio: n)
FIG. 16 shows the relationship between the average current in the pulse process and the maximum pulse time ratio. It was found that the maximum pulse time ratio showed a tendency to increase as the average current increased, and the maximum pulse time ratio reached 300% when the average current was 4.65 A/mm ² . Therefore, the average current range is set to 4.65 A/mm ² or less. In order to further suppress the occurrence of crimping, the average current range is preferably 3.26 A/mm ² or less.

At this time, the cross-sectional area of the rail is 8600 mm ² , the cumulative current is 500 kA・sec (0.06 kA・sec/mm ² ), the maximum current is 30,000 A (3.49 A/mm ² ), and the amount of C is 0. The amount of Mn is 1.0% by mass, the amount of Cr is 0.5% by mass, and the frequency is 5Hz. The reason why the maximum pulse time increases as the average current increases is that, as mentioned above, when the average current is large, the amount of heat input to the end face increases, and as a result, the end face becomes more likely to stick together as a result of the increased temperature of the end face.

The conditions of the method for manufacturing a flash butt welded rail according to this embodiment, which was completed through the above study process, will be explained below. Furthermore, further preferred aspects of the manufacturing method according to this embodiment will also be described below.

<Frequency (f) in pulse process: 1 to 10 Hz>
From the viewpoint of preventing crimping as described in item k above, the lower limit of the frequency in the pulse process is set to 1 Hz. In order to further suppress the occurrence of crimping, the frequency range is preferably 2 Hz or more, 3 Hz or more, or 4 Hz or more. Further, from the viewpoint of suppressing oxide defects as described in item e above, the upper limit of the frequency in the pulse process is set to 10 Hz. From the viewpoint of further suppressing oxides, the upper limit of the frequency in the pulse step is preferably 9 Hz or less, 8 Hz or less, or 6 Hz or less. The most preferable frequency range is 3 Hz or more and 6 Hz or less.

<Maximum current in pulse process: 2.33 to 6.98 A/mm ² >
From the viewpoint of suppressing oxide defects as described in item g above, the lower limit of the maximum current in the pulse process is set to 2.33 A/mm ² . In order to further improve the bending performance, the maximum current in the pulse process is preferably 2.50 A/mm ² or more, 2.91 A/mm ² or more, or 3.20 A/mm ² . Furthermore, from the viewpoint of preventing crimping as described in item m above, the upper limit of the maximum current in the pulse process is set to 6.98 A/mm ² . In order to further suppress the occurrence of crimping, the maximum current range is preferably 6.00 A/mm ² or less, 5.23 A/mm ² or less, or 4.80 A/mm ² or less. The most preferable range of maximum current in the pulse process is 2.91 A/mm ² or more and 5.23 A/mm ² or less.

<Average current in pulse process: 0.70 to 4.65 A/mm ² >
From the viewpoint of suppressing oxide defects as described in item h above, the lower limit of the average current in the pulse process is set to 0.70 A/mm ² . Preferably, the range of the average current of the pulse step is 1.00 A/mm ² or more, 1.74 A/mm ² or more, or 2.00 A/mm ² or more. Further, from the viewpoint of preventing crimping as described in item n above, the upper limit of the average current in the pulse process is set to 4.65 A/mm ² . In order to further suppress the occurrence of crimping, the average current range is preferably 4.00 A/mm ² or less, 3.26 A/mm ² or less, or 2.80 A/mm ² or less. The most preferable range of average current in the pulse process is 1.74 A/mm ² or more and 3.26 A/mm ² or less.

<Final flash speed in late flash process: 0.2 to 3.4 mm/sec>
The final flash rate in the late flash process means the average value of the flash rate during the last 5 seconds of the late flash process. That is, the final flash speed in the latter flash step is the value obtained by dividing the amount of rail movement in the last 5 seconds of the latter flash step by 5 seconds. However, if the time of the latter flash process is less than 5 seconds, the final flash speed in the latter flash process is the average value of the flash speed from the start to the end of the latter flash process, that is, the amount of rail movement in the latter flash process. It means the value obtained by dividing the late flash length, which is , by the time of the late flash step.

The faster the flash speed in the latter flash step, that is, the later flash speed, the higher the current, and the higher the temperature of the end surface immediately before the upset step. As a result, the oxides generated in the latter flashing process can be easily discharged to the outside of the weld zone in the upsetting process. However, even if the latter flash rate is increased only immediately before the upset process, the effect of increasing the temperature at the end face is limited. Therefore, in the latter flashing process, the lower limit of the final flashing speed is set to 0.2 mm/sec. The final flash rate is preferably at least 0.5 mm/sec, at least 0.8 mm/sec, or at least 1.0 mm/sec.

On the other hand, if the flashing speed in the latter flashing is too fast, an abnormal phenomenon (crimping in the flashing process) occurs in which the end faces come into contact and the flash disappears. Therefore, the upper limit of the final flash speed is set to 3.4 mm/sec. The final flash rate is preferably 3.0 mm/sec or less, 2.5 mm/sec or less, or 2.0 mm/sec or less. The most preferred range of final flash speed is 0.6 mm/sec or more and 2.8 mm/sec or less.

<C: 0.60 to 1.20% by mass>
From the viewpoint of suppressing oxide defects described in the above-mentioned items b and o, the lower limit of the amount of C is 0.60% by mass. The amount of C is preferably 0.65% by mass or more, 0.70% by mass or more, or 0.80% by mass or more. On the other hand, from the viewpoint of preventing crimping as described in item j above, the upper limit of the amount of C is 1.20% by mass. The amount of C is preferably 1.10% by mass or less, 1.00% by mass or less, or 0.90% by mass or less. The most preferable range of the amount of C is 0.70% by mass or more and 1.10% by mass or less.

<Mn: 0.10 to 2.00% by mass>
From the viewpoint of suppressing oxide defects described in the above-mentioned items c and o, the upper limit of the amount of Mn is 2.00% by mass. The amount of Mn is preferably 1.80% by mass or less, 1.50% by mass or less, or 1.20% by mass or less. On the other hand, Mn is an element that exhibits the effect of increasing hardness by improving hardenability. If the amount of Mn is insufficient, this effect cannot be obtained. Therefore, the lower limit of the amount of Mn is 0.10% by mass. The amount of Mn is preferably 0.20% by mass or more, 0.50% by mass or more, or 0.80% by mass or more. The most preferable range of the Mn amount is 0.20% by mass or more and 1.80% by mass or less.

<Cr: 0.01 to 2.00% by mass>
From the viewpoint of suppressing oxide defects described in the above-mentioned items d and o, the upper limit of the Cr content is set to 2.00% by mass. The amount of Cr is preferably 1.80% by mass or less, 1.50% by mass or less, or 1.20% by mass or less. On the other hand, if the amount of Cr is too small, the hardness of the flash butt welded rail cannot be obtained. Therefore, the lower limit of the amount of Cr is set to 0.01% by mass. The amount of Cr is preferably 0.20% by mass or more, 0.50% by mass or more, or 0.80% by mass or more. The most preferable range of the amount of Cr is 0.20 mass% or more and 1.30 mass% or less.

<In the pulse process, the upper limit of the integrated current is (8 x frequency - 480 x C mass % + 1400)/8600 in kA sec/ ^mm2 >
From the viewpoint of preventing crimping as described in item 1 above, the upper limit value of the integrated current was defined as a function of the frequency in the pulse process and the C amount of the rail.

<In the pulse process, the lower limit of the integrated current is (3.5 x frequency - 180 x C% + 55 x Mn% + 45 x Cr% + 300) / 8600 in kA sec/ ^mm2 >
From the viewpoint of suppressing oxide defects as described in item f above, the lower limit value of the integrated current was defined as a function of the frequency in the pulse process and the C content, Mn content, and Cr content of the rail.

<Preferably, the minimum current in the pulse process is 0.12 A/mm ² or more>
In the pulse process, when the rail is retreated and the two end faces are completely separated, the current value is 0 A/mm ² . Therefore, the minimum value of current in the pulse process is 0A. However, if the rail stops retreating and is moved forward before the rail is completely separated, the current value may not drop to 0 A/mm ² .

In the manufacturing method according to this embodiment, it is necessary to avoid a phenomenon in which the end surfaces become stuck together and cannot be separated from each other, that is, crimping must be avoided. In this case, the crimped end face must be cut and flash butt welding must be restarted from the beginning. On the other hand, in the pulse process, it is not necessary to completely separate the end faces for each pulse. Rather, it is considered that oxides are less likely to form on the end surfaces unless the rails that abut each other are completely separated when the rails retreat during the pulse process. Further, as long as the above-mentioned conditions are satisfied, even if the rails that abut against each other are not completely separated when the rails retreat, the maximum pulse time ratio will not increase and the end faces will not be crimped.

When the end faces are completely separated, the current will be zero. Therefore, the minimum current may be set to 0.12 A/mm ² as an indicator to ensure that the end faces are not completely separated. By doing so, it is difficult to form oxides on the end face, and further improvement in bending performance is expected. A more preferable range of the minimum current in the pulse process is 0.1812 A/mm ² or more, 0.23 A/mm ² or more, or 0.25 A/mm ² or more.

<Preferably, the flash length in the latter flash step is 5 to 50 mm>
The flash length of the latter flash process, ie, the latter flash length, is the amount of movement of the rail from the start to the end of the latter flash process. As mentioned above, there is an optimal range for the late flash rate in the late flash step. However, by increasing the latter flash length, the welding time (flash length/flash speed) becomes longer, the temperature of the end face becomes higher, and the effect of discharging generated oxides to the outside in the upsetting process is further enhanced. Therefore, it is preferable that the lower limit of the latter flash length is 5 mm. It is more preferable that the latter flash length is 6 mm or more, 8 mm or more, or 10 mm or more.

On the other hand, in welding using a mobile FB welding machine that welds on-site, by shortening the flash length (rail wear amount) in the latter stage, it is necessary to pull the rail or prepare another rail to fill the gap that occurs in the longitudinal direction. This eliminates the need for additional work. Therefore, suppressing the late flash length leads to improved rail construction workability. Therefore, the upper limit of the latter flash length is preferably 50 mm. It is more preferable that the latter flash length is 45 mm or less, 40 mm or less, or 30 mm or less. A more preferable range of the latter flash length is 10 mm or more and 40 mm or less.

<Si: 0.10 to 2.00% by mass>
Si is an element that solid-solution strengthens the ferrite phase in the pearlite structure and contributes to increasing the strength of the base material. In order to obtain this effect, the rail contains 0.10% or more and 2.00% or less of Si. The amount of Si is preferably 1.80% by mass or less, 1.50% by mass or less, or 1.20% by mass or less. If the Si content is less than 0.10%, this effect cannot be obtained, and if it exceeds 2.00%, the material tends to become brittle. The amount of Si is preferably 0.20% by mass or more, 0.50% by mass or more, or 0.80% by mass or more. A more preferable range of the amount of Si is 0.20% or more and 1.50% or less.

<Al: 0.001 to 0.500% by mass>
Al is included in the rail as a deoxidizing element. Furthermore, Al is an element that reduces the difference in hardness of the pearlite structure and improves the fatigue strength of the rail column. Therefore, Al content is 0.001% or more. The amount of Al is preferably 0.100% by mass or more, 0.120% by mass or more, or 0.150% by mass or more. On the other hand, from the viewpoint of suppressing the formation of coarse oxides and coarse martensite and further improving fatigue strength, Al is set to 0.500% or less. The amount of Al is preferably 0.450% by mass or less, 0.400% by mass or less, or 0.300% by mass or less. A more preferable range of Al is 0.005% or more and 0.100% or less.

<P: 0.020% by mass or less>
<S: 0.020% by mass or less>
<N: 0.003 to 0.020% by mass>
P and S are impurities in steel. When P and S are large, the toughness of the steel decreases and the fatigue strength of the flash butt welded rail decreases. Therefore, in order to increase the fatigue strength of the welded rail, P is set to 0.020% or less and S is set to 0.020% or less. The preferred range for P is 0.010% or more and 0.018% or less, and the preferred range for S is 0.008% or more and 0.018% or less. Note that the lower limits of the amount of P and the amount of S are not particularly limited, and may be 0%. On the other hand, in order to improve the refining cost, the amount of P and the amount of S may each be set to 0.001% by mass or more, 0.001% by mass or more, or 0.010% by mass or more.

N is an element that equalizes the hardness of the pearlite structure and improves the fatigue strength of the rail column. Therefore, if necessary, the rail may contain 0.003% or more and 0.020% or less of N. When the amount of N exceeds 0.020%, coarse nitrides are formed, which may reduce fatigue strength. Therefore, the preferable range of the amount of N is 0.008% or more and 0.018% or less.

<Preferably V: 0.001 to 0.300% by mass>
<Preferably Ti: 0.0008 to 0.0500% by mass>
<Preferably Nb: 0.001 to 0.050% by mass>
V, Ti, and Nb are not essential in the rail used as a welding base material. Therefore, the amount of V, the amount of Ti, and the amount of Nb may be 0%. On the other hand, V, Ti, and Nb are elements that increase the hardness of steel through precipitation strengthening and improve the fatigue strength of flash butt welded rails. Therefore, if necessary, the rail may contain V from 0.001% to 0.300%, Ti from 0.0008% to 0.0500%, and Nb from 0.001% to 0.050%. You may let them. If V is less than 0.001%, Ti is less than 0.0008%, and Nb is less than 0.001%, the effect of increasing hardness and fatigue strength cannot be obtained. In addition, from the viewpoint of suppressing the formation of coarse nitrides and further increasing the fatigue strength of flash butt welded rails, V is set to 0.300% or less, Ti is set to 0.0500% or less, and Nb is set to 0.050% or less. It is preferable. More preferable ranges are 0.005% to 0.200% for V, 0.0030% to 0.0400% for Ti, and 0.003% to 0.030% for Nb.

<Cu: Preferably 0.005 to 1.000% by mass>
<Ni: Preferably 0.01 to 1.00% by mass>
Cu and Ni are not essential in the rail of the manufacturing method according to this embodiment. Therefore, the amount of Cu and the amount of Ni may be 0%. On the other hand, Cu and Ni are elements that increase the hardness of steel through solid solution strengthening and improve the fatigue strength of flash butt welded rails. Therefore, if necessary, the rail may contain Cu from 0.005% to 1.000% and Ni from 0.01% to 1.00%. If Cu is less than 0.005% and Ni is less than 0.01%, the effect of increasing hardness and fatigue strength cannot be obtained. Further, from the viewpoint of suppressing embrittlement due to excessive solid solution strengthening and further increasing the ductility of the rail steel, it is preferable that Cu be 1.000% or less and Ni be 1.00% or less. A more preferable range is that Cu is 0.010% or more and 0.600% or less, and Ni is 0.02% or more and 0.60% or less.

<Preferably Ca: 0.0005 to 0.0200% by mass>
<Preferably REM: 0.0005 to 0.0500% by mass>
Ca and REM are not essential in the rail of the manufacturing method according to this embodiment. Therefore, the amount of Ca and the amount of REM may be 0%. On the other hand, Ca has a strong binding force with S and forms sulfide. This sulfide finely disperses MnS, thereby further improving the fatigue strength of the rail. REM acts as a generation nucleus for oxides and MnS. These finely disperse MnS, thereby further improving the fatigue strength of the rail. Therefore, if necessary, the rail may contain Ca in an amount of 0.0005% or more and 0.0200% or less, and REM in an amount of 0.0005% or more and 0.0500% or less. If Ca is less than 0.0005% and REM is less than 0.0005%, the effect of improving fatigue strength cannot be obtained. Further, from the viewpoint of suppressing the generation of coarse oxides and coarse oxysulfides and further improving fatigue strength, it is preferable that Ca is 0.0200% or less and REM is 0.0500% or less. More preferred ranges are 0.0008% to 0.0180% for Ca and 0.0010% to 0.0450% for REM. Note that REM is a rare earth element such as Ce, La, Pr, or Nd. The above upper and lower limits of REM content limit the total content of all these REM elements.

<Preferably Mo: 0.002 to 0.500% by mass>
Mo is not essential in the rail of the manufacturing method according to this embodiment. Therefore, the amount of Mo may be 0%. On the other hand, Mo is an element that increases the hardness of steel by making the lamella spacing (the spacing between the ferrite phase and cementite phase) of the pearlite structure finer, thereby improving the fatigue strength of the flash butt welded rail. Therefore, if necessary, Mo may be contained in an amount of 0.002% or more and 0.500% or less. When Mo is less than 0.002%, the effect of increasing hardness and fatigue strength cannot be obtained. On the other hand, from the viewpoint of suppressing the formation of coarse oxides and coarse martensite and further improving fatigue strength, Mo is preferably 0.500% or less. A more preferable range of Mo is 0.010% or more and 0.300% or less.

Note that the remainder of the chemical components of the rail include iron and impurities. In addition, as long as C, Mn, and Cr, which can adversely affect the bending test results of flash butt welded rails and welding workability, are within the above range, alloying elements other than those exemplified above may be added to the rail. May be contained. It is also acceptable for the two rails to be welded to have slightly different chemical compositions. In this case, it is sufficient that the average value of the chemical components of the two rails satisfies the above-mentioned conditions.

Impurities in the chemical components of the rail are components that are mixed in from raw materials such as ore or scrap, or various factors in the manufacturing process, when manufacturing steel materials industrially, for example, and are components that are mixed in by various factors in the manufacturing process. It means what is permissible within the range that does not adversely affect the flash butt welded rail obtained by the method.

<Other conditions>
In the method for manufacturing a flash butt welded rail according to this embodiment, various welding conditions can be employed as long as the above-mentioned conditions are satisfied. For example, in the first flash process, normal conditions that can flatten the end face of the rail to a level suitable for the pulse process can be appropriately employed. Also in the upset process, normal conditions that allow a normal weld to be formed can be appropriately employed. For example, the pressurizing force in the upset step is not particularly limited, but from the viewpoint of further improving the ability to discharge oxides, it is preferably 55 MPa or more, 58 MPa or more, or 60 MPa or more. Furthermore, in consideration of equipment capacity, it is preferable that the pressurizing force in the upset process be 120 MPa or less, 100 MPa or less, or 82 MPa or less. Also in the pulse process and the latter flash process, normal conditions can be appropriately adopted for welding conditions not mentioned above. The method for manufacturing a flash butt welded rail may include additional steps such as removing burrs and subjecting the weld to post-heat treatment.

<Bending test procedure>
The bending test is performed in accordance with the American Railway Engineering and Maintenance-of-Way Association standard: AREMA-sec. 2.3.3.6. That is, as shown in FIG. 17, the welded rail is supported with a distance of 1200 mm between the supporting points centering on the welded part. Then, a load is applied to the welding rail from above the welding rail at intervals of 300 mm centering on the welded portion. Increase the load until the welded rail breaks. Then, the longitudinal stress (bottom stress) at the bottom of the welding rail at the time of breakage is measured based on the amount of deflection of the welded rail at the time of breakage and the load at the time of breakage. A deflection amount at break of 0.75 inch or more and a bottom stress of 125 kbp/inch ² or more are acceptable. The flash butt welded rail obtained by the manufacturing method according to the present embodiment can extremely effectively suppress the formation of flat spots in the welded portion, and therefore can satisfy the above-mentioned pass/fail criteria.

EXAMPLES The effects of one embodiment of the present invention will be explained in more detail with reference to Examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this one example condition. The present invention may adopt various conditions as long as the objectives of the present invention are achieved without departing from the gist of the present invention.

Flash butt welding using a pulse flash method was performed on two rails of various types having the chemical compositions listed in Table 1A, Table 1B, Table 2A, and Table 2B. The flash butt welding conditions were as shown in Table 3A, Table 3B, Table 4A, and Table 4B. The cross-sectional area of the rail was 8600 ^mm2 .

Note that in Table 2A and Table 2B, the symbol "-" indicates that the element with the symbol is not intentionally added to the rail. The "maximum current" listed in Tables 3A and 3B is the maximum value of the current applied to the rail during the pulse process. The "maximum current per cross-sectional area" is the value obtained by dividing the "maximum current" by the cross-sectional area of the rail, which is 8600 ^mm2 . Similarly, the "average current" listed in Tables 3A and 3B is the average value of the current applied to the rail during the pulse process. The "average current per cross-sectional area" is the value obtained by dividing the "average current" by the cross-sectional area of the rail, which is 8600 ^mm2 . The "minimum current value of the pulse process" described in Tables 4A and 4B is the minimum value of the current applied to the rail in the pulse process. The "minimum current value per cross-sectional area in the pulse process" is the value obtained by dividing the "minimum current value in the pulse process" by 8600 mm ² which is the cross-sectional area of the rail.

The cumulative current upper limit and cumulative current lower limit listed in Table 3A and Table 3B are values calculated using the following formula.
Accumulated current upper limit: 8 x frequency - 480 x C mass% + 1400
Cumulative current lower limit: 3.5 x frequency - 180 x C mass % + 55 x Mn mass % + 45 x Cr mass % + 300
Here, the frequency is the frequency in the pulse process, and the C mass %, Mn mass %, and Cr mass % are the C, Mn, and Cr contents in the chemical components of the rail in unit mass %. Regarding the integrated current and its upper and lower limit values, the description of the values taking into consideration the cross-sectional area is omitted.

In the various flash butt welded rails thus obtained, the maximum pulse time ratio and maximum flat spot length were measured by the above-mentioned means. The measurement results are listed in Table 4A and Table 4B. The evaluation criteria were as follows.
(Maximum pulse time ratio)
120% or less: A
More than 120% and less than 300%: B
Over 300%:X
Examples in which the maximum pulse time ratio was A or B were determined to be examples in which crimping of the end face during welding was suppressed.
(Flat spot maximum length)
5mm or less: A
More than 5mm and less than 8mm: B
More than 8 mm and less than 12 mm: C
More than 12mm:X
In the evaluation of flat spots, flat spots with a length of less than 1.0 mm were not included in the evaluation. Examples in which the maximum flat spot length was A, B, or C were determined to be examples in which the flash butt welded rail met the bending test standard.

In Example 9, the frequency of the pulse step was excessive. As a result, in Example 9, the maximum length of the flat spot was excessive.

In Example 12, the frequency of the pulse process was insufficient. As a result, in Example 12, the maximum pulse time ratio became excessive.

In Example 13, the maximum current of the pulse step was excessive. As a result, in Example 13, the maximum pulse time ratio became excessive.

In Example 16, the maximum current in the pulse process was insufficient. As a result, in Example 16, the maximum length of the flat spot was excessive.

In Example 17, the average current of the pulse step was excessive. As a result, in Example 17, the maximum pulse time ratio became excessive.

In Example 20, the average current in the pulse process was insufficient. As a result, in Example 20, the maximum length of the flat spot was excessive.

In Examples 21, 25, 29, 33, 37, 41, 45, 49, and 53, the cumulative current in the pulse process was excessive. In these examples, the maximum pulse time ratio was excessive.

In Examples 24, 28, 32, 36, 40, 44, 48, 52, and 56, the integrated current in the pulse process was insufficient. In these examples, the flat spot maximum length was excessive.

In Example 59, the final flash rate in the late flash step was insufficient. As a result, in Example 59, the maximum length of the flat spot was excessive.

In Example 64, the amount of C was excessive. As a result, in Example 64, the maximum pulse time ratio became excessive.

In Example 67, the amount of C was insufficient. As a result, in Example 67, the maximum length of the flat spot was excessive.

In Example 68, the amount of Mn was excessive. As a result, in Example 68, the maximum length of the flat spot was excessive.

In Example 71, the amount of Cr was excessive. As a result, in Example 71, the maximum length of the flat spot was excessive.

On the other hand, in the example satisfying all the conditions of the flash butt welded rail manufacturing method according to the present embodiment, both the maximum pulse time ratio and the maximum flat spot length were suppressed. That is, these examples are as follows: (1) Flash butt welding using the pulse flash method can suppress fracture due to low deflection and low bottom stress during bending tests of flash butt welded rails, and (2) Pulse process This was a method of manufacturing flash-butt welded rails that could prevent the end faces of the rails from being crimped during the process.

1A, 1B Rail 2 Electrode 3 Burr

Claims (4)

an initial flashing step of generating flash between the end surfaces of a pair of rails arranged along the longitudinal direction, thereby flattening the end surfaces;
a pulse step of repeatedly contacting and separating the end surfaces of the pair of rails, thereby preheating the end surfaces;
a latter flashing step of causing flash between the end faces of the pair of rails, thereby melting the end faces as a whole;
an upsetting step of abutting and pressurizing the end surfaces of the pair of rails, thereby joining the rails;
Equipped with
The frequency in the pulse step is 1 to 10 Hz,
The maximum current in the pulse step is 2.33 to 6.98 A/mm ² ,
The average current in the pulse step is 0.70 to 4.65 A/mm ² ,
The final flash speed in the latter flash step is 0.2 to 3.4 mm/sec,
The chemical component of the rail is C: 0.60 to 1.20% in unit mass %,
Mn: 0.1 to 2.0%,
Cr: 0.01-2.00%,
Si: 0.10-2.00%,
Al: 0.001-0.500%,
P: 0.020% or less S: 0.020% or less,
N: 0.003-0.020%,
V: 0-0.300%,
Ti: 0 to 0.0500%,
Nb: 0-0.050%
Cu: 0 to 1.000%,
Ni: 0 to 1.00%,
Ca: 0-0.0200%,
REM: 0 to 0.0500%, and Mo: 0 to 0.500%
The remainder contains Fe and impurities,
In the pulse step, the upper limit of the integrated current, which is the product of the average current and the current application time, in the unit kA sec/mm ² is (8 x frequency - 480 x C content + 1400) / 8600,
In the pulse step, the lower limit of the integrated current in the unit kA sec/mm ² is (3.5 x frequency - 180 x C content + 55 x Mn content + 45 x Cr content + 300) / 8600. Method of manufacturing flash butt welded rails. The method for manufacturing a flash butt welding rail according to claim 1, characterized in that the minimum current in the pulse step is 0.12 A/mm ² or more. The method for manufacturing a flash butt welded rail according to claim 1 or 2, characterized in that the flash length in the latter flash step is 5 to 50 mm. The chemical component of the rail is V: 0.001 to 0.300% in unit mass %,
Ti: 0.0008 to 0.0500%,
Nb: 0.001 to 0.050%,
Cu: 0.005-1.000%,
Ni: 0.01-1.00%,
Ca: 0.0005-0.0200%,
REM: 0.0005-0.0500%,
Al: 0.001 to 0.500%, and Mo: 0.002 to 0.500%
The method for manufacturing a flash butt welded rail according to claim 1 or 2, characterized in that it contains one or more selected from the group consisting of:

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