KR20010056389A - A Method of Flash Butt Welding for High Carbon Steel - Google Patents

Abstract

PURPOSE: A method for welding flash butt of high carbon steel is provided to improve weldability and form the sound welding part by properly controlling conditions of flash and upset processes during flash butt welding of high carbon steel, and heat treating the flash butt by the proper current conditions after welding. CONSTITUTION: In a method for welding a flash butt with a material consisting of high carbon steel having a carbon content of 0.45 wt.% or more contacted face to face, the method comprises the processes of performing the welding under the conditions that a total length of the material disappeared by welding is 15 to 20 mm, a first flash time is 20 to 25 seconds, a moving distance by the second flash is 5 to 8 mm, and a moving distance by upset is 3 to 5 mm; and conducting a current of 2500 to 3000 A to the welding part for more than 6 seconds.

Description

A Method of Flash Butt Welding for High Carbon Steel}

The present invention relates to a flash butt welding method that can be applied to fields such as coil welding of hot rolled high carbon steel production lines.

In general, flash butt welding is a typical butt welding method for resistance welding. By generating localized contact portions at appropriate speeds in the cross sections for welding with each other, the contact portions generate resistance heat due to the high current density short-circuit current. As a result, the contact portion melts and the arc is generated and the vicinity thereof is heated. The molten metal is scattered by this arc. This is called a flash, and this flash is generated continuously to heat the weld cross section. The weld end face is pressurized in a uniformly heated state, which is called an upset process.

On the other hand, the oxide formed during the flash is scattered with the flash, or is released to the outside with the molten metal in the upset process. Thus, both parts heated in the flash process form a healthy weld. However, high carbon steel containing about 0.45% or more of carbon has a problem in that the welded part becomes very vulnerable because the heat-affected part, that is, the heat affected part, is transformed into a brittle structure having high hardness. Such high hardness welds act as a major cause of fracture.

Accordingly, the present inventors have repeatedly conducted research and experiments to solve the problem of weakening the welded part and propose the present invention based on the results, and the present invention provides a flash process and an upset process during the flash welding of high carbon steel. It is an object of the present invention to provide a welding method in which weldability is improved and a healthy welded portion is formed by appropriately controlling the conditions and performing heat treatment by a current of appropriate conditions after welding.

Figure 1 is a graph showing the crack ratio of the weld portion appearing before and after the heat treatment bending test

Figure 2 is a graph showing the hardness before and after heat treatment of the weld portion by flash butt welding

In order to achieve the above object, the present invention provides a method for butt welding a material composed of high carbon steel having a carbon content of 0.45% or more, wherein the total length of the material to be extinguished by welding is 15-20 mm, and The welding time is 20-25 seconds, the travel distance by the secondary flash is 5-8mm, the travel distance by the upset is 3-5mm, and the welding is made with a current of 2500-3000A for 6 seconds or more. It relates to a high carbon steel flash butt welding method characterized in that the current.

Hereinafter, the present invention will be described in detail. First, welding conditions by flash and upset will be described, and then heat treatment conditions will be described.

In general, in the flash butt welding, in order to suppress oxide formation and obtain a complete weld, the heating degree of the welding section must be uniform during the flash process, and an upset pressure or distance should be appropriately applied. The distance that the weld base material melts away by flash and upset is called the unclamp distance. The flash process needs to be divided into 1st and 2nd stages to generate a fine flash in the 1st flash process to make the molten layer uniform, and the 2nd flash process becomes the step before the upset by the acceleration.

On the other hand, it must be pressurized at high speed during upset. Otherwise, oxides are formed on the welded end face, which is a problem. In order to suppress oxide formation during pressurization immediately after the flash, the initial upset is carried out at the same acceleration as the secondary flash process and finally pressurized instantaneously with high current.

In the present invention, the total length of the material disappeared by welding, that is, the distance between the unclamps is preferably 15-20 mm.

If the unclamp distance is less than 15mm, the amount of heat dissipation is small, so there is a problem that it is not sufficiently pressurized and results in cold welding. If the distance exceeds 20mm, a large amount must be dissipated. However, when pressurized, the heated / melted part is not completely discharged, and there is a high possibility of causing a defect due to molten metal remaining in the welded part.

In the present invention, it is preferable to set the primary flash time to 20-25 seconds.

The primary flash is a uniformly molten layer. If the time is less than 20 seconds, the primary flash is not sufficiently heated. If it is more than 25 seconds, the heating zone is too large to be heated and pushed to act as a weld defect. Therefore, 20-25 seconds.

In the present invention, it is preferable that the moving distance by the secondary flash is 5-8 mm.

The secondary flash is a process performed by acceleration, and when the moving distance is less than 5 mm or more than 8 mm, the movement appropriate to the heated area is not performed.

In addition, in the present invention, it is preferable that the moving distance by the upset is 3-5 mm.

The upset is pressurized at a high speed. In order to suppress the formation of oxide during the pressurization, the initial upset is accelerated and finally pressurized with a high current. At this time, the upset is preferably carried out with a moving distance of 3-5mm, the initial upset is preferably 1-2mm, after 2-3mm is preferred.

In general, high carbon steel is a breakage due to the fragility of the heat affected zone, causing a lot of trouble in the operation. For example, in the process of hot rolling high carbon steel having a carbon content of 0.45% or more, removing oxides from the surface, and coating the film, flash butt welding is performed for continuous operation. . Therefore, in the present invention, the heat treatment after welding is to reduce the fragility of such a heat affected zone.

In the present invention, after the upset is completed in the welding process, it is unclamped (clamped) and clamped again (clamping) to energize the current of 2500-3000A for 6 seconds or more.

If the current is less than 2500A, there is a problem that the heat treatment temperature cannot be reached. If the current is more than 3000A, there is a problem that the temperature becomes too high and deteriorates. Therefore, the current is preferably in the range of 2500-3000A.

The current is preferably energized for 6 seconds or more, because in the case of less than 6 seconds, the heat treatment effect is insignificant and the ductility cannot be sufficiently recovered.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

High-carbon steels with a carbon content of 0.46% were subjected to two flash-butt welding, respectively, under welding conditions as shown in Table 1 below, to prepare two test specimens for each welding condition.

Comparative Example 1 Comparative Example 2 Inventive Example 3 Comparative Example 4 Comparative Example 5 Jump (seconds) 10 10 10 10 10 Preliminary Flash (sec) One One One One One 1st flash (sec) 25 15 25 20 15 Secondary flash (mm) 6 8 8 12 12 Primary upset (mm) One One 2 One One Secondary upset (mm) 2 2 3 3 3 Unclamp (mm) 14 14 18 22 22

After welding under the conditions of Table 2, one of the obtained test pieces was subjected to heat treatment by applying a current of 2700 A for 10 seconds, and the other one was not subjected to heat treatment. A bending test was conducted to evaluate the integrity and formability of the welded part of the obtained test piece. As a result, the length of the crack generated in the welded part was measured by a ratio to the width of the bend test piece, and the result is shown in FIG. 1.

As can be seen in Figure 1, all the test pieces that were not subjected to heat treatment were not only broken by the oxides present in the weld, but also 100% of the fracture test results after welding due to the hardenability of the heat affected zone. This recovery did not cause such breakage, but only when the welding conditions were outside the scope of the present invention (Comparative Examples 1, 2, 4, 5), cracks were caused by oxides in the welded portions.

Example 2

After welding under the conditions of Inventive Example (3) in Example 1, the current of 2700A was subjected to a heat treatment with a current carrying time as shown in Table 2 below.

Tensile strength and maximum hardness (Hv) of the obtained welds were measured, and the results are shown in Table 2 below.

Processing time (seconds) Tensile Strength (kg / mm ² ) Hardness (Hv) Comparative material 1 2 63.2 595 Comparative material 2 4 63.7 534 Invention 3 6 58.6 447 Invention 4 8 52 381 Invention 5 10 52.5 323

As can be seen in Table 2, in the case of the invention material (3-5) which performed the energization time for 6 seconds or more, the maximum hardness value was less than 500 Hv, which means that recovery occurred and sufficient ductility was recovered. .

On the contrary, in the case of the comparative material (1-2) having the energization time of less than 6 seconds, the maximum hardness showed a value of 500 Hv or more.

In the welded part by the said invention material 5, and the welded part which did not heat-process, the hardness according to the distance from the welding center line was measured, respectively, and the result is shown in FIG.

As can be seen from Figure 2, by performing the heat treatment by the current of the present invention, the toughness of the weld portion is recovered, in particular, the recovery of the weld center portion was excellent.

On the other hand, an experiment was performed for 6 seconds at a current of less than 2500 A for 6 seconds and more than 3000 A for 6 seconds. When the temperature was less than 2500 A, the heat treatment temperature was lower than 550 ° C., and the effect was insignificant. There was a problem.

According to the present invention as described above, the weld defect ratio is 10 times lower than that of the welding under the existing conditions, the heat treatment is excellent in the formability of the weld can be obtained.

Claims (1)

In the method of butt welding the material consisting of high carbon steel with a carbon content of 0.45% or more, The total length of the material disappeared by welding is 15-20 mm, the first flash time is 20-25 seconds, the travel distance by the secondary flash is 5-8 mm, and the travel distance by upset is 3- After welding to 5mm, high-carbon steel flash butt welding method characterized in that the welding part is energized for 2500 seconds or more current of 2500-3000A