Flash butt welding member and flash butt welding method

Abstract

Provided are a flash butt bonding member and a welding method. The present invention relates to a bonding member having a bonding portion obtained by flash-butt-welding a steel plate and, more particularly, to a bonding member having excellent processability and a welding method, characterized in that the difference in average effective crystal grain diameter between a coarse crystal grain thermally affected portion and a fine crystal grain thermally affected portion formed on one of the left and right sides with reference to a bonding line of the bonding portion is 10μm or less.

Description

Flashbutt joint member and flashbutt welding method

[0001] The present invention is intended to secure excellent workability of a joint obtained through flash butt welding, and particularly relates to a flash butt joining member and a flash butt welding method capable of securing sufficient toughness of the joint.

[0002] In the automotive industry, fuel efficiency regulations for environmental protection, including global warming, are driving research into lightweight technologies for car bodies and components. Chassis and wheel components, crucial to vehicle performance, are also seeing a growing need for high-strength steels for weight reduction. To achieve lightweight components, high-strength materials are essential. Furthermore, ensuring workability at joints is paramount, particularly when post-welding machining is required for component manufacturing.

[0003] In the case of flashbutt welding, which is mainly used in the manufacturing of automobile wheels, the joining is achieved through the melting scattering of the joining surface and the upset of the melt by the flash arc, so selecting optimal conditions to ensure the workability of the joint is important. In particular, in the case of high-strength steel, the development of brittle structure due to hardening of the joint is a factor in reducing the workability due to the relatively high carbon equivalent, so the development of process conditions that can control the phase transformation of the joint is necessary. In the actual wheel manufacturing line, if the processing crack defect of the joint after welding is high, it is difficult to apply it to mass production, so finding a solution for this is important. Meanwhile, in the manufacturing of high-strength and lightweight wheels for commercial vehicles, in addition to the brittleness of the joint described above, an optimal joint microstructure that can prevent softening of the joint and the welding process control to achieve this are essential.

[0004] As an example of a prior art to solve this problem, the invention described in Patent Document 1 can be cited. According to Patent Document 1, increasing the amount of upset during the joining stage is suggested to effectively discharge oxide inclusions formed at high temperatures during flashbutt welding. In addition, Patent Document 2 suggests an oil application device to suppress the formation of oxide inclusions during flashbutt welding of high-strength steel. Meanwhile, Patent Document 3 suggests a method to improve the toughness and workability of the wheel rim joint by appropriately introducing a pre- / upset- / post-heat pattern during flashbutt welding.

[0005] However, the prior art only presents some solutions for controlling the microstructure of the high-strength steel flashbutt joints mentioned above, and has the limitation of not presenting additional solutions for improving the workability of the joints during the flashbutt welding manufacturing of parts.

[0006] [Prior Art Literature]

[0007] [Patent Document]

[0008] (Patent Document 1) Republic of Korea Patent No. 10-1281294

[0009] (Patent Document 2) Republic of Korea Patent No. 10-0711459

[0010] (Patent Document 3) Republic of Korea Patent No. 10-2178723

[0011] The present invention is a method for controlling the phase transformation structure of a joint during welding of high-strength steel, and aims to provide a flashbutt joint member and welding method having excellent workability by controlling the average effective grain difference between a coarse grain heat-affected zone and a fine grain heat-affected zone forming the joint and the width of the coarse grain heat-affected zone and the fine grain heat-affected zone forming the joint.

[0012] In addition, the technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention belongs from the description below.

[0013] One aspect of the present invention is:

[0014] In a joining member having a joint obtained by flashbutt welding of steel plates,

[0015] The above joint has a coarse grain heat-affected zone and a fine grain heat-affected zone formed facing each other on the left and right based on the joint line,

[0016] The present invention relates to a joint member having excellent processability, wherein the average effective grain size difference between the coarse grain heat-affected zone and the fine grain heat-affected zone formed on either side of the joint is within 10 ㎛.

[0017] The above joint can satisfy the following relationship in which the width of the weld heat affected zone, which is composed of a coarse grain heat affected zone and a fine grain heat affected zone, is satisfied.

[0018] [Relationship]

[0019] W HAZ ≥ t × 1.77

[0020] (In the above relationship, W HAZ represents the width of the heat affected zone of the weld joint, and t represents the thickness of the steel plate.)

[0021] The above joint may have a difference between the minimum hardness of the weld heat affected zone, which is composed of a coarse grain heat affected zone and a fine grain heat affected zone formed on either side of the joint, and the maximum hardness of the steel plate base material, which may be within 30 in Vickers hardness.

[0022] The above joint may have a difference between the maximum hardness of the weld heat affected zone, which is composed of a coarse grain heat affected zone and a fine grain heat affected zone formed on either side of the joint, and the minimum hardness of the steel plate base material, which may be within 30 in Vickers hardness.

[0023] The average effective grain size of the coarse grain heat-affected zone formed on either side of the above joint may be 24 µm or less, and the average effective grain size of the fine grain heat-affected zone may be 15 µm or less.

[0024] The above steel plate may contain, in wt%, C: 0.04 to 0.18%, Si: 2.0% or less (including 0%), Mn: 0.5 to 3.0%, Cr: 2.0% or less (including 0%), Mo: 2.0% or less (including 0%), Al: 0.01 to 0.10%, P: 0.05% or less (excluding 0%), S: 0.05% or less (excluding 0%), the remainder being Fe and other unavoidable impurities.

[0025] Another aspect of the present invention is

[0026] The present invention relates to a flashbutt welding method having excellent joint workability, characterized in that, in a flashbutt welding method using an electrode to perform flash / upset heating and post-heating when flashbutt welding a steel plate, the surface to be joined of the steel plate is controlled to a flash speed of 5% or more and 20% or less during pre-heating and flash heating, the length of movement between electrodes is controlled to 2.0 mm or more and 10.0 mm or less during upset heating, and then post-heat treatment is performed to form a joint.

[0027] The above joint has a coarse grain heat-affected zone and a fine grain heat-affected zone formed facing each other on the left and right sides based on the joint line, and the average effective grain size difference between the coarse grain heat-affected zone and the fine grain heat-affected zone formed on either the left or right side of the joint can be controlled to within 10 µm.

[0028] The width of the weld heat affected zone, which is composed of a coarse grain heat affected zone and a fine grain heat affected zone of the above joint, can satisfy the following relationship.

[0029] [Relationship]

[0030] W HAZ ≥ t × 1.77

[0031] (In the above relationship, W HAZ represents the width of the heat affected zone of the weld joint, and t represents the thickness of the steel plate.)

[0032] The difference between the minimum hardness of the weld heat-affected zone, which is composed of a coarse grain heat-affected zone and a fine grain heat-affected zone formed on either side of the above-mentioned joint, and the maximum hardness of the steel plate base material may be within 30 in Vickers hardness.

[0033] The difference between the maximum hardness of the weld heat-affected zone, which is composed of a coarse grain heat-affected zone and a fine grain heat-affected zone formed on either side of the above-mentioned joint, and the minimum hardness of the steel plate base material may be within 30 in Vickers hardness.

[0034] The average effective grain size of the coarse grain heat-affected zone formed on either side of the above joint may be 24 µm or less, and the average effective grain size of the fine grain heat-affected zone may be 15 µm or less.

[0035] According to the present invention, when flashbutting high-strength hot-rolled steel having a tensile strength of 590 MPa or more, the microstructure of the joint is optimized to reduce embrittlement or softening of the joint, thereby improving toughness, thereby effectively improving the strength and workability of the joint of a lightweight steel wheel for automobiles. Therefore, the application of this technology to lightweight steel wheels for commercial vehicles can be expanded, resulting in the potential for expanding the application of high-strength steel.

[0036] FIG. 1(ac) shows, in an embodiment of the present invention, an optical photograph (a) of a cross-sectional structure of a steel plate base material and a flashbutt joint of Invention Example 1, a hardness measurement result (b) for these, and an optical photograph (c) of a cross-sectional structure (no bending cracks) after 180° bending of the joint.

[0037] Figure 2(ac) shows, in a comparative example of the present invention, an optical photograph (a) of a cross-sectional structure of a steel plate base material and a flashbutt joint of Comparative Example 1, the hardness measurement results (b) for these, and an optical photograph (c) of a cross-sectional structure (bending crack occurrence) after 180° bending of the joint.

[0038] Figure 3 is a result showing the difference in hardness distribution of Figure 1b in more detail.

[0039] Hereinafter, the present invention will be described.

[0040] The present invention, by introducing preheat / flash / upset heating and post-heat patterns during flashbutt welding, forms a sound joint while simultaneously controlling the difference in average effective grain size between the coarse grain heat-affected zone and the fine grain heat-affected zone formed facing each other on the left and right of the joint to within 10 μm. Accordingly, in a situation where tensile and bending stresses are applied to the joint members, the average effective grain size of the coarse grain heat-affected zone of the joint, which may have inferior material properties compared to the base material due to phase transformation, is not excessively large compared to the average effective grain size of the fine grain heat-affected zone, which has relatively good material properties, or the average effective grain size of the fine grain heat-affected zone of the joint, which has relatively inferior material properties, is not excessively large compared to the average effective grain size of the coarse grain heat-affected zone, which has relatively poor material properties, thereby providing a technology capable of improving the toughness and workability of the joint.

[0041] First, a joint member having excellent workability and having a joint obtained by flashbutting a steel plate, which is one aspect of the present invention, is described.

[0042] The present invention relates to a joint member having a joint obtained by flashbutt welding steel plates using an electrode, wherein the joint includes a coarse grain heat-affected zone and a fine grain heat-affected zone formed facing each other on the left and right with respect to a joint line. Here, the joint line refers to a joint interface formed along the weld line when flashbutt welding a butt joint of steel plates.

[0043] And the coarse grain heat-affected zone is a highly brittle weld heat-affected zone formed by heating to 1200℃ or higher, and the fine grain heat-affected zone is a weld heat-affected zone formed by heating to 850 to 1000℃, and has good mechanical properties such as toughness due to the grains being refined by recrystallization.

[0044] And, in the joint member having excellent processability of the present invention, the average effective grain size difference between the coarse grain heat-affected zone and the fine grain heat-affected zone formed on either side of the joint may be within 10 μm. If the average effective grain size difference between the coarse grain heat-affected zone and the fine grain heat-affected zone exceeds 10 μm, the material of the joint becomes non-homogeneous, and the toughness of the coarse grain heat-affected zone deteriorates, resulting in poor processability and an increased crack occurrence rate at the joint. On the other hand, if the average effective grain size difference is within 10 μm, the materials of the coarse grain heat-affected zone and the fine grain heat-affected zone become homogenized, thereby improving the toughness and processability of the joint.

[0045] Meanwhile, in the present invention, the average effective crystal grain size means the average size of crystal grains converted from the number of crystal grains per unit area.

[0046] In addition, in one embodiment of the present invention, the joint may have a width of a weld heat affected zone composed of a coarse grain heat affected zone and a fine grain heat affected zone that satisfies the following relationship.

[0047] [Relationship]

[0048] W HAZ ≥ t × 1.77

[0049] (In the above relationship, W HAZ represents the width of the heat affected zone of the weld joint, and t represents the thickness of the steel plate.)

[0050] By controlling the width of the above-mentioned heat-affected zone to a certain value or more, the average effective grain size of the coarse grain heat-affected zone of the joint, which may have inferior material properties compared to the base material due to phase transformation, can be prevented from being excessively large compared to the average effective grain size of the fine grain heat-affected zone, which has relatively good material properties. In addition, by preventing the average effective grain size of the fine grain heat-affected zone of the joint from being excessively large compared to the average effective grain size of the coarse grain heat-affected zone, which has relatively inferior material properties, the workability of the joint can be effectively improved. This means that even if the width of the heat-affected zone of the above-mentioned weld increases, the difference in hardness from the base material of the steel plate is small, and the difference in the average effective grain size between the coarse grain heat-affected zone, the heat-affected zone of the coarse grain, and the heat-affected zone of the fine grain is kept within a certain value, thereby uniformly controlling the effective grain size of the heat-affected zone of the weld, thereby further improving the toughness and workability of the joint.

[0051] In addition, in one embodiment of the present invention, by controlling the difference between the minimum hardness of the weld heat-affected zone composed of the coarse grain heat-affected zone and the fine grain heat-affected zone and the maximum hardness of the steel plate base material to within 30 in Vickers hardness, or by controlling the difference between the maximum hardness of the weld heat-affected zone and the minimum hardness of the steel plate base material to within 30 in Vickers hardness, a joint member having excellent workability can be provided. This is because, as described above, when the weld heat-affected zone of the joint is relatively excessively embrittled or softened compared to the material of the adjacent steel plate base material in various stress-added situations of the joint member, the joint member may become susceptible to processing cracks due to local stress concentration.

[0052] Furthermore, according to one embodiment of the present invention, the average effective grain size of the coarse-grained heat-affected zone and the average effective grain size of the fine-grained heat-affected zone may be similar to the average effective grain size of the base material. For example, when the average effective grain size of the base material is 4 µm or more and 10 µm or less, the average effective grain size of the coarse-grained heat-affected zone may be 15 µm or more and 24 µm or less, and the average effective grain size of the fine-grained heat-affected zone may be 15 µm or less. By forming the average effective grain size of the coarse-grained heat-affected zone and the average effective grain size of the fine-grained heat-affected zone not to be excessively large compared to the average effective grain size of the base material before welding, which is 4 µm or more and 10 µm or less, excellent workability of the joint can be secured. When the average effective grain size of the coarse grain heat-affected zone and the average effective grain size of the fine grain heat-affected zone are similar to the average effective grain size of the parent material, the probability of cracks occurring when tensile and bending stresses are applied to the bonded member decreases as the material between the parent material and the heat-affected zone becomes homogenized.

[0053] Meanwhile, the present invention does not specifically limit the alloy composition of the steel plate. However, as an example, the steel plate may contain, in wt%, C: 0.04 to 0.18%, Si: 2.0% or less (including 0%), Mn: 0.5 to 3.0%, Cr: 2.0% or less (including 0%), Mo: 2.0% or less (including 0%), Al: 0.01 to 0.10%, P: 0.05% or less (excluding 0%), S: 0.05% or less (excluding 0%), the remainder Fe, and other unavoidable impurities. In addition, the steel plate may optionally further include one or more of Ti: 0.20% or less (including 0%), Nb: 0.10% or less (including 0%), and Cu: 0.10% or less (including 0%). Additionally, the steel plate may have a thickness of 2.0 to 6.0 mm.

[0054] Next, another aspect of the present invention, a flashbutt welding method with excellent workability of the joint, is described.

[0055] The flashbutt welding method of the present invention, which has excellent workability of a joint, is a flashbutt welding method in which, when flashbutt welding the joint target surface of a butt joint of steel plates using an electrode, flash / upset heating and post-heating are performed, the joint target surface of the steel plates is preheated and flash-heated, the flash speed is controlled to be 5% or more and 20% or less, and the movement length between electrodes is controlled to be 2.0 mm or more and 10.0 mm or less during upset heating, and then post-heat treatment is performed to form a joint.

[0056] By using this welding process, the joint obtained can have improved toughness and workability of the joint. At this time, the preheating / preheating time / flash / flash (heating) length conditions are selected to be optimal so that the preheating and flash heating can occur at an appropriate level for welding, considering the thickness of the steel, and the flash speed is controlled to be 5% or more and 20% or less, and the movement length between the electrodes during upset heating is controlled to be 2.0 mm or more and 10.0 mm or less, thereby obtaining a joint with excellent workability.

[0057] If the flash speed of flash butt welding is insufficient, or the upset heating length or post-heat is insufficient compared to the appropriate level, the brittleness of the joint increases. Conversely, if the flash speed of flash butt welding is excessive, or the upset heating length or post-heat is excessive, the grain size of the heat-affected zone of the joint increases overall due to grain growth, which may become a factor in deterioration of the physical properties of the joint. Specifically, if the flash speed is less than 5%, problems such as non-welding defects may occur due to insufficient heat at the joint. Conversely, if the flash speed exceeds 20%, the grain size of the heat-affected zone of the weld may increase due to overheating of the joint, or defects such as oxide inclusions at the joint interface may become susceptible to formation. In addition, if the moving length between the electrodes is less than 2.0 mm, the molten portion formed during the flash heating process may not be sufficiently discharged from the joint interface and rapidly cool, which increases the brittleness of the joint. Conversely, if the moving length between the electrodes exceeds 10.0 mm, the grain size of the heat-affected zone of the weld may increase due to overheating of the joint, which may cause softening problems.

[0058] Meanwhile, in the present invention, the flash speed refers to the rate at which the initial inter-electrode spacing decreases per second.

[0059] The present invention is not limited to the specific process conditions in the above process. According to one embodiment of the present invention, when the short-circuit current is 100%, the current can be applied in the ranges of 30 to 40% for preheating, 55 to 60% for flashing, 25 to 40% for upset heating, and 5 to 20% for post-heating. In addition, the preheating time can be 2.0 to 5.0 seconds, the flash (heating) length can be in the range of 2.0 to 4.0 mm, and the upset pressure can be in the range of 50 to 70 bar.

[0060] When the welding conditions described above are applied, the average effective grain size difference between the coarse grain heat-affected zone and the fine grain heat-affected zone formed on either side of the joint can be controlled to within 10 ㎛ through microstructural phase transformation control according to the appropriate heating and cooling rates of the joint during welding, thereby ensuring excellent workability through improved toughness of the joint.

[0061] In addition, by controlling the flash speed and upset heating length according to one embodiment of the present invention, the discharge of the molten portion can be optimized, thereby preventing defects such as non-bonding defects or formation of oxide inclusions in the bonded portion, and minimizing grain growth in the heat-affected zone of coarse grains.

[0062] Meanwhile, by minimizing the difference in average effective grain size between the coarse grain heat-affected zone and the fine grain heat-affected zone with relatively good material properties through post-heat control, cracks can be prevented from occurring at the joint during part processing.

[0063] As described above, in the present invention, by optimizing the flashbutt welding process and controlling the appropriate flash speed and upset heating length, the discharge of the molten portion can be optimized, thereby preventing defects such as non-bonding defects or formation of oxide inclusions in the joint. In addition, by minimizing the grain growth of the coarse grain heat-affected zone generated through the high-temperature heat history, and at the same time, by controlling the post-heat, the material of the joint becomes more homogeneous, thereby minimizing the difference in the average effective grain size between the fine grain heat-affected zone with relatively good material properties and the coarse grain heat-affected zone, thereby preventing the occurrence of cracks in the joint during component processing.

[0064] Hereinafter, the present invention will be described in detail through examples.

[0065] (Example)

[0066]

[0067] Two types of hot-rolled steel plate base materials having a tensile strength of 650 MPa / thickness of 4.5 mm / width of 300 mm (steel material 1) and a tensile strength of 650 MPa / thickness of 6.0 mm / width of 300 mm (steel material 2) and the compositions shown in Table 1 were prepared respectively. Afterwards, the base materials were prepared into a rim shape with a coiler, and then the facing surfaces were butt-joined, and flash butt welding was performed under the welding conditions shown in Table 2 below. At this time, the preheating / preheating time / flash / flash (heating) length conditions were selected as the optimal conditions so that appropriate preheating and flash heating could occur at a level where welding was possible, taking into consideration the steel thickness above. Microstructure analysis, hardness measurement, and tensile and bending tests were performed on the joints forming each of the obtained joining members, and the results are shown in Table 3 below. Meanwhile, in Table 2-3 below, invention examples 1-9 and comparative examples 1-8 are welding conditions and analysis / evaluation results for steel material 1 of Table 1, and invention examples 10-18 and comparative examples 9-22 are welding conditions and analysis / evaluation results for steel material 2 of Table 1.

[0068] In addition, in the case of the above microstructure analysis, cross-sectional tissue specimens were collected for the joints forming each joint member, then finely ground and etched with Nital solution, and then observed with an optical microscope. Meanwhile, the average effective grain size (G1) of the coarse grain heat-affected zone, the average effective grain size (G2) of the fine grain heat-affected zone, the difference in the average effective grain size (ΔG) between the coarse grain heat-affected zone and the fine grain heat-affected zone, and the width (W) of the weld heat-affected zone consisting of the coarse grain heat-affected zone and the fine grain heat-affected zone were measured. HAZ), the difference (ΔH1) between the minimum hardness of the weld heat-affected zone composed of the coarse grain heat-affected zone and the fine grain heat-affected zone and the maximum hardness of the steel plate base material, and the difference (ΔH2) between the maximum hardness of the weld heat-affected zone composed of the coarse grain heat-affected zone and the fine grain heat-affected zone and the minimum hardness of the steel plate base material were measured, and the results are shown in Table 3 below.

[0069] At this time, the hardness measurement was performed by measuring the Vickers hardness at 0.2 mm intervals under a load of 300 gf for each of the above-mentioned areas, and a minimum of 1,000 and a maximum of 1,500 hardness values ​​were obtained. In addition, the width (W) of the weld heat-affected zone, which is composed of the coarse grain heat-affected zone and the fine grain heat-affected zone of the joint, was measured. HAZ ) is a region where the heat-affected zone, which has a relatively low temperature compared to the coarse grain heat-affected zone and fine grain heat-affected zone during flashbutt welding, is subjected to work hardening by the upset pressure, excluding the region where the hardness increases compared to the steel plate base material (strain-hardening region of Fig. 1-3).

[0070] In addition, as a method for measuring the average effective grain size (G1) of the coarse grain heat-affected zone of the joint and the average effective grain size (G2) of the fine grain heat-affected zone, the Kikuchi pattern was analyzed through EBSD (Electron Backscattered Diffraction) to obtain IQ (Image Quality) and IPF (Inverse Pole Figure) Maps that visualize grain boundary and grain orientation information. After that, the grains were distinguished by referring to the IQ and IPF Maps of EBSD together with the microstructure photographs observed with the optical microscope described above, and the average size of the grains converted from the number of grains per unit area was calculated to measure each average effective grain size.

[0071] Here, the above G1 and G2 were measured only in the area corresponding to 50% of the thickness of the steel plate base material on either side from the joint line (joining interface), taking into account the area that affects the workability of the joint.

[0072] In addition, for each joint member having the joint obtained as described above, a tensile test was performed to evaluate the location where the fracture occurred, as shown in Table 3 below, and a 3-point bending test (4R) was performed to evaluate the occurrence of bending cracks. For the test, tensile specimens were prepared with a size of 50 mm wide and 220 mm long from the joint member corresponding to each welding condition, and 5 specimens were evaluated at a tensile speed of 10 mm / min to verify reproducibility, and bending specimens were prepared with a size of 30 mm wide and 100 mm long, and 8 specimens were evaluated at a 180° bend to verify reproducibility. Afterwards, each specimen was visually observed, and if the tensile fractured portion was the base material, it was considered pass (○), if it was the joint, it was considered fail (X), and if no bending crack occurred, it was considered pass (○), and if a bending crack occurred, it was considered fail (X).

[0073]

[0074] *In Table 2 above, preheat / flash / upset heating / postheat current% is the ratio of short-circuit current, and upset heating length means the distance traveled between electrodes during upset heating.

[0075]

[0076] As shown in Table 2-3 above, by sequentially using the preheat / flash / upset heating processes to perform flash welding under various welding conditions, the flash speed of the obtained joint satisfies 5% to 20%, and the upset heating length satisfies 2.0 mm to 10.0 mm, so that the value of ΔG satisfies 10㎛ or less. In addition, it can be seen that in the case of Invention Example 1-9 (steel material 1) and Invention Example 10-18 (steel material 2), fracture occurs in the base material during a tensile test, and bending cracks do not occur during a bending test.

[0077] In contrast, when the flash speed of the joint obtained under the above-mentioned various welding conditions deviates from the range of 5% to 20%, the upset heating length deviates from the range of 2.0 mm to 10.0 mm, and the value of ΔG may exceed the range within 10 μm. In addition, accordingly, in the case of Comparative Example 1-8 (steel material 1) and Comparative Example 9-22 (steel material 2) that are outside the predetermined range, it can be seen that tensile fracture and / or bending cracks occurred at the joint. This is because, as described above, when tensile and bending stresses are applied to the joint member, if the average effective grain size of the coarse grain heat-affected zone, which has inferior material properties compared to the base material due to phase transformation, is embrittled or softened to a level exceeding 10 μm compared to the average effective grain size of the fine grain heat-affected zone, which has relatively good material properties, the difference in toughness or strength between the coarse grain heat-affected zone and the fine grain heat-affected zone becomes large, making the area vulnerable.

[0078] FIG. 1(ac) shows, in an embodiment of the present invention, an optical photograph (a) of a cross-sectional structure of a steel plate base material and a flashbutt joint of Invention Example 1, a hardness measurement result (b) for these, and an optical photograph (c) of a cross-sectional structure (no bending cracks) after 180° bending of the joint.

[0079] Here, as described above, in Invention Example 1, the flash speed and upset heating length are suitable, and the value of ΔG satisfies the range within 10㎛, so it can be seen that no cracks occur after 180° bending of the joint.

[0080] Figure 2(ac) shows, in a comparative example of the present invention, an optical photograph (a) of a cross-sectional structure of a steel plate base material and a flashbutt joint of Comparative Example 1, the hardness measurement results (b) for these, and an optical photograph (c) of a cross-sectional structure (bending crack occurrence) after 180° bending of the joint.

[0081] Here, in Comparative Example 1, as described above, the flash speed and upset heating length were not suitable, and as the value of ΔG was outside the range of 10 ㎛, it was found that a crack occurred after 180° bending of the joint.

[0082] Figure 3 is a result showing the difference in hardness distribution of Figure 1b in more detail.

[0083] Here, FG~CGHAZ in FIGS. 1b, 2b, and 3 represent fine-grained and coarse-grained heat-affected zones, and strain-hardening represents a region in which the heat-affected zone, which has a relatively low temperature compared to FG~CGHAZ during flashbutt welding, undergoes work hardening due to upset pressure, thereby increasing the hardness compared to the steel plate base material. The strain-hardening region is outside the region that actually affects the workability of the joint, and therefore was not considered a heat-affected zone that directly affects the workability of the joint.

[0084] As described above, the detailed description of the present invention has described preferred embodiments of the present invention. However, it will be apparent to those skilled in the art that various modifications may be made without departing from the scope of the present invention. Therefore, the scope of the rights of the present invention should not be limited to the described embodiments, but should be determined not only by the claims described below but also by equivalents thereof.

Claims

1. In a joining member having a joint obtained by flash welding of steel plates, The above joint includes a coarse grain heat-affected zone and a fine grain heat-affected zone formed facing each other on the left and right based on the joint line, A joint member having excellent processability, wherein the average effective grain size difference between the coarse grain heat-affected zone and the fine grain heat-affected zone formed on either side of the joint is within 10 ㎛.

2. In the first paragraph, the joint is a joint member having excellent processability, wherein the width of the weld heat affected zone, which is composed of a coarse grain heat affected zone and a fine grain heat affected zone, satisfies the following relationship. [Relationship] W HAZ ≥ t × 1.77 (In the above relationship, W HAZ represents the width of the heat affected zone of the weld joint, and t represents the thickness of the steel plate.) 3. In the first paragraph, the joint is a joint member having excellent workability, wherein the difference between the minimum hardness of the weld heat-affected zone, which is composed of a coarse grain heat-affected zone and a fine grain heat-affected zone, and the maximum hardness of the steel plate base material is within 30 in Vickers hardness.

4. In the first paragraph, the joint is a joint member having excellent workability, wherein the difference between the maximum hardness of the weld heat-affected zone, which is composed of a coarse grain heat-affected zone and a fine grain heat-affected zone, and the minimum hardness of the steel plate base material is within 30 in Vickers hardness.

5. A bonding member having excellent processability, wherein the average effective grain size of the heat-affected zone of the coarse grains is 24 µm or less, and the average effective grain size of the heat-affected zone of the fine grains is 15 µm or less in the first paragraph.

6. In the first paragraph, the steel plate is a joining member having excellent workability, which contains, in weight %, C: 0.04 to 0.18%, Si: 2.0% or less (including 0%), Mn: 0.5 to 3.0%, Cr: 2.0% or less (including 0%), Mo: 2.0% or less (including 0%), Al: 0.01 to 0.10%, P: 0.05% or less (excluding 0%), S: 0.05% or less (excluding 0%), the remainder being Fe and other unavoidable impurities.

7. In the flashbutt welding method of flash / upset heating and post-heating when flashbutt welding a steel plate using an electrode, A flashbutt welding method having excellent joint workability characterized by controlling the flash speed to 5% or more and 20% or less during preheating and flash heating of the joint target surface of the above steel plate, controlling the length of movement between electrodes to 2.0 mm or more and 10.0 mm or less during upset heating, and then performing post-heat treatment to form a joint.

8. A flashbutt welding method with excellent joint workability, characterized in that, in paragraph 7, when the short-circuit current of the joint is 100%, the post-heat is controlled to be 5% or more and 20% or less.

9. In paragraph 7, the joint includes a coarse grain heat-affected zone and a fine grain heat-affected zone formed facing each other on the left and right based on the joint line, A flashbutt welding method having excellent joint processability, wherein the average effective grain size difference between the coarse grain heat-affected zone and the fine grain heat-affected zone formed on either side of the joint is within 10 ㎛.

10. A component having a joint described in any one of clauses 1 to 6.

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