Flash butt joint member with excellent processability and manufacturing method

Abstract

The present invention provides a flash butt joint member and welding method with excellent workability by controlling the difference in average hardness between the heat-affected zone of coarse crystal grains and the heat-affected zone of fine crystal grains that form the joint. [Solution] The present invention relates to a joint member having a joint obtained by flash butt welding a butt joint of steel plates using electrodes, wherein the average hardness difference between the heat-affected zone of coarse crystal grains and the heat-affected zone of fine crystal grains formed on either the left or right side of the joint line (joint interface) of the joint is 50 or less on a Vickers hardness scale, the joint further includes a band of soft phase having an average width of 100 μm or less, the band of soft phase is formed between the heat-affected zones of the coarse crystal grains, the average hardness difference between the heat-affected zone of the coarse crystal grains and the band of soft phase is 30 or less on a Vickers hardness scale, the average effective crystal grain size of the heat-affected zone of the coarse crystal grains is 17 μm or less, and the average effective crystal grain size of the heat-affected zone of the fine crystal grains is 7 μm or less.

Description

Technical Field

[0001] The present invention relates to a flash butt joint member having excellent workability and a manufacturing method thereof. More specifically, it is for ensuring excellent workability of a joint obtained by flash butt welding, and particularly relates to a flash butt joint member having excellent workability that can ensure sufficient toughness of the joint and a manufacturing method thereof.

Background Art

[0002] In the automotive field, due to policies on fuel consumption regulations for environmental protection such as the global warming problem, research on technologies for weight reduction of vehicle bodies and parts has emerged as a major issue. For chassis and wheel parts, which are important for vehicle driving performance, it is also necessary to apply high-strength steel materials for weight reduction under such circumstances. To achieve part weight reduction, it is essential to increase the strength of the material. Especially when post-welding processing is carried out for part manufacturing, ensuring the workability of the joint is the most important factor.

[0003] In the case of flash butt welding, which is mainly used in the manufacture of automotive wheels, it is important to select optimal conditions for ensuring the workability of the joint in the process where joining is performed through melting and scattering of the joining target surface and upsetting of the melted part by the flash arc. Especially in the case of high-strength steel, the development of brittle structures associated with the hardening of the joint due to a relatively high carbon equivalent becomes a factor in reducing workability, so it is necessary to develop process conditions that can control the phase transformation of the joint. When the defect rate of processing cracks in the joint after welding is high in an actual wheel manufacturing line, it is difficult to apply in mass production, so it is important to derive a solution for this. On the other hand, in the case of thick steel materials with a thickness of 10 mm or more used in the manufacture of high-strength and lightweight wheels for commercial vehicles or large mining trucks, not only the brittleness of the joint described above but also the control of the microstructure of the optimal joint that can prevent softening of the joint and the welding process for realizing this are always required.

[0004] As an example of prior art to address this issue, the invention described in Patent Document 1 can be cited. According to Patent Document 1, increasing the amount of upset during the joining stage has been proposed for the effective removal of oxidative inclusions formed at high temperatures during flash butt welding. Patent Document 2 also presents an oil application device for suppressing the formation of oxidative inclusions during flash butt welding of high-tensile steel. On the other hand, Patent Document 3 presents a method for improving the toughness and workability of the wheel rim joint by appropriately introducing a Pre- / Upset- / Post-heat pattern during flash butt welding.

[0005] However, while conventional technologies offer some solutions for controlling the microstructure of high-strength steel flash butt joints as described above, they lack specific control methods for optimally reducing the reduction in toughness of the joint, particularly regarding the difference in hardness distribution between the heat-affected zones of coarse grains and fine grains in the joint, the width of the soft phase band distributed between the heat-affected zones of coarse grains formed facing each other on the left and right sides of the joint, and the difference in hardness distribution between the heat-affected zones of the coarse grains and the soft phase band. Consequently, they have the limitation of not being able to offer additional solutions for improving the workability of the joint during the flash butt welding manufacturing of parts. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Korean Registered Patent Publication No. 10-1281294 [Patent Document 2] Korean Registered Patent Publication No. 10-0711459 [Patent Document 3] Korean Registered Patent Publication No. 10-2178723 [Overview of the project] [Problems that the invention aims to solve]

[0007] The present invention aims to provide a flash butt joint member and welding method with excellent workability by controlling the difference in average hardness between the heat-affected zone of coarse grains and the heat-affected zone of fine grains that make up the joint, as a method for controlling the phase transformation structure of the joint when welding high-strength hot-rolled steel materials.

[0008] Furthermore, the technical problems that this invention aims to solve are not limited to those mentioned above, and any other technical problems not mentioned can be clearly understood by a person with ordinary skill in the art to which this invention belongs from the following description. [Means for solving the problem]

[0009] The joint member of the present invention, which has excellent processability, In a joint member having a joint obtained by flash butt welding of a butt joint of steel plates using electrodes, The above-mentioned joint has a heat-affected zone of coarse crystal grains and a heat-affected zone of fine crystal grains, which are formed facing each other on the left and right sides with respect to the joint line (joint interface). The characteristic feature is that the average hardness difference between the heat-affected zone of the coarse crystal grains and the heat-affected zone of the fine crystal grains, formed on either the left or right side of the joint, is 50 or less on a Vickers hardness scale.

[0010] In the above-mentioned joint, a band of soft phase with an average width of 100 μm or less can be distributed between the heat-affected zones of the coarse crystal grains.

[0011] The above-mentioned joint can have an average hardness difference of 30 or less on the Vickers hardness scale between the heat-affected zone of the coarse crystal grains formed on either the left or right side of the joint and the band of the soft phase.

[0012] In the above-mentioned joint, the average effective grain size of the heat-affected zone of the coarse grains formed on either the left or right side of the joint may be 17 μm or less, and the average effective grain size of the heat-affected zone of the fine grains may be 7 μm or less.

[0013] The above steel sheet contains, by weight percent, C: 0.04~0.18%, Si: 2.0% or less (including 0%), Mn: 0.5~3.0%, Cr: 2.0% or less (including 0%), Mo: 2.0% or less (including 0%), Al: 0.01~0.10%, P: 0.05% or less (excluding 0%), S: 0.05% or less (excluding 0%), with the remainder being Fe and other unavoidable impurities.

[0014] Furthermore, the flash butt welding method of the present invention, which has excellent joint machinability, In a flash butt welding method in which electrodes are used to upset heat the joining surfaces of a butt joint of steel plates during flash butt welding, the joining surfaces of the steel plates are preheated, flash heated, and then the distance traveled between electrodes is controlled to 5.0 mm or more and less than 15.0 mm during upset heating to form the joint.

[0015] The above-mentioned joint has heat-affected zones for coarse grains and fine grains, respectively, formed facing each other on the left and right sides with respect to the joint line (joint interface), and the average hardness difference between the heat-affected zone for coarse grains and the heat-affected zone for fine grains formed on either the left or right side of the above-mentioned joint can be controlled to within 50 on the Vickers hardness scale.

[0016] In the above-mentioned joint, a band of soft phase with an average width of 100 μm or less can be distributed between the heat-affected zones of the coarse crystal grains.

[0017] The average hardness difference between the heat-affected zone of the coarse crystal grains formed on either the left or right side of the joint and the band of the soft phase can be 30 or less on a Vickers hardness scale.

[0018] In the above-mentioned joint, the average effective grain size of the heat-affected zone of the coarse grains formed on either the left or right side of the joint may be 17 μm or less, and the average effective grain size of the heat-affected zone of the fine grains may be 7 μm or less. [Effects of the Invention]

[0019] According to the present invention, during the flash butt welding of high-strength hot-rolled steel sheets having a tensile strength of 590 MPa or more, it is possible to improve the toughness by reducing embrittlement or softening of the joint through optimization of the microstructure of the joint, and effectively improve the strength and workability of the joint of an automotive lightweight steel wheel. Therefore, there is an effect that the application of high-strength steel can be expanded by applying this technology to commercial vehicle lightweight steel wheels and the like.

Brief Description of the Drawings

[0020] [Figure 1] In an embodiment of the present invention, (a) is an optical micrograph of the cross-sectional structure of the flash butt joint of Invention Example 1, and (b) is a SEM micrograph of the joint interface enlarged. [Figure 2] In an embodiment of the present invention, (a) is an optical micrograph of the cross-sectional structure of the flash butt joint of Invention Example 1 after hardness measurement, and (b) is an image showing the hardness distribution corresponding to the square box region of the joint in terms of color difference. [Figure 3] In an embodiment of the present invention, (a) is an IQ (Image Quality) micrograph of the heat-affected zone of the coarse crystal grains of Invention Example 1 observed by EBSD, and (b) is an IQ (Image Quality) micrograph of the heat-affected zone of the coarse crystal grains of Comparative Example 1 observed by EBSD.

Modes for Carrying Out the Invention

[0021] Hereinafter, the present invention will be described.

[0022] The present invention provides a technology that improves the toughness and workability of a joint by introducing a preheating / flash / upset heating pattern during flash butt welding, thereby forming a sound joint, and simultaneously controlling the difference in average Vickers hardness between the heat-affected zones of coarse grains and fine grains formed opposite each other on the left and right sides of the joint to within 50. This prevents the average hardness of the heat-affected zone of coarse grains in the joint from becoming excessively (brittle) or insufficiently (softened) compared to the heat-affected zone of fine grains, which have relatively good material properties, when tensile and bending stresses are applied to the joined members and the material properties of the base material may deteriorate due to phase transformation.

[0023] Accordingly, the present invention provides a joint member having a joint obtained by flash butt welding a butt joint of steel plates using electrodes, wherein the joint includes a heat-affected zone of coarse crystal grains and a heat-affected zone of fine crystal grains formed facing each other on the left and right sides with respect to the joint line (joint interface), and the average hardness difference between the heat-affected zone of coarse crystal grains and the heat-affected zone of fine crystal grains formed on either the left or right side of the joint is 50 or less on a Vickers hardness scale, thereby providing a joint member with excellent workability.

[0024] In other words, the present invention provides a flash butt welding method in which electrodes are used to upset heat the joining surfaces of a butt joint of steel plates during flash butt welding, characterized in that the joining surfaces of the steel plates are preheated, flash heated, and then the distance traveled between electrodes is controlled to 5.0 mm or more and less than 15.0 mm during upset heating to form the joint, thereby providing a flash butt welding method with excellent workability of the joint.

[0025] The joint obtained using such a welding process can have improved toughness and workability. In this case, the conditions for preheating / preheating time / flash / flash (heating) length are selected to be optimal, taking into account the thickness of the steel material, so that appropriate preheating and flash heating occur to a level that allows welding. By controlling the movement length between electrodes during upset heating to 5.0 mm or more and less than 15.0 mm, a joint with excellent workability can be obtained. The present invention is not limited to the specific process conditions in the above process. However, when the short-circuit current is set to 100%, it is preferable to apply current in the range of 30-40% for preheating, 55-60% for flash, and 25-40% for upset heating. Furthermore, it is preferable to control the preheating time to 8.0-18.0 seconds, the flash (heating) length to 5.0-10.0 mm, and the upset pressure to 50-70 bar.

[0026] On the other hand, the present invention does not particularly limit the alloy composition of the steel sheet. However, as an example, the steel sheet contains, by weight %, C: 0.04~0.18%, Si: 2.0% or less (including 0%), Mn: 0.5~3.0%, Cr: 2.0% or less (including 0%), Mo: 2.0% or less (including 0%), Al: 0.01~0.10%, P: 0.05% or less (excluding 0%), S: 0.05% or less (excluding 0%), with the remainder being Fe and other unavoidable impurities. Furthermore, the steel sheet may selectively further contain 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%). In addition, the steel sheet may have a thickness of 2.0~20.0 mm.

[0027] When applying the welding conditions described above, by controlling the phase transformation of the microstructure according to the appropriate heating and cooling rates of the joint during welding, a joint is formed having heat-affected zones of coarse grains and fine grains facing each other on the left and right sides, respectively. Furthermore, the average hardness difference between the heat-affected zones of coarse grains and fine grains formed on either the left or right side of the joint can be controlled to within 50 on the Vickers hardness scale, thereby ensuring excellent workability by improving the toughness of the joint.

[0028] In this invention, by optimizing the flash butt welding process and controlling the appropriate upset heating length, the discharge of the molten portion is optimized, preventing defects such as unjointed areas or the formation of oxidative inclusions at the joint. This minimizes grain growth in the heat-affected zone of coarse grains generated by high-temperature thermal history, while simultaneously minimizing the hardness difference between the heat-affected zone of fine grains, which have relatively good material properties, thus preventing crack formation at the joint during part processing. On the other hand, in this invention, the heat-affected zone of coarse grains is a highly brittle heat-affected zone formed by heating to 1200°C or higher, while the heat-affected zone of fine grains is a heat-affected zone formed by heating to 850-1000°C. In both cases, the grains are refined through recrystallization, resulting in good mechanical properties such as toughness.

[0029] On the other hand, in the present invention, the joint portion may include a band of soft phase between the heat-affected zones of the coarse crystal grains.

[0030] The soft phase band is a structure that has been softened relative to the heat-affected zone of the joint and has the characteristic of being relatively low in hardness. If the width of the soft phase band is too large, it may cause a decrease in the strength of the joint. According to the present invention, the soft phase band can have an average width of 100 μm or less. In this case, toughness can be ensured without a decrease in the strength of the joint, thereby improving the workability of the joined members. In other words, if the width of the soft phase band exceeds an average width of 100 μm, the stress may concentrate in the soft phase distributed at the joint interface when tensile and bending stresses are applied to the joined members, which may reduce the toughness and workability of the joint.

[0031] The soft phase band is formed when alloying elements such as C and Mn, which improve hardening ability (austenite stabilization), rapidly diffuse out from the molten area formed at the joint during the welding process. This occurs when the upsetting heating length is too short during the upsetting process, resulting in insufficient removal of the molten area and rapid cooling, or conversely, when the upsetting heating length is too long, resulting in a large molten area. As the depletion region of the alloying elements such as C and Mn expands, a band-shaped soft phase due to ferrite transformation can develop and be distributed at the joint interface.

[0032] In this case, if the width of the soft phase band exceeds an average of 100 μm, the tensile and bending stresses applied to the joint members concentrate in the soft phase distributed at the joint interface, reducing the toughness and workability of the joint.

[0033] Furthermore, by controlling the average hardness difference between the heat-affected zone of coarse grains and the soft phase band to within 30 on the Vickers hardness scale, it is possible to provide a joint member with excellent workability. This is because, as mentioned above, under various stress conditions in the joint member, when the soft phase band becomes relatively excessively softer than the material of the heat-affected zone of the adjacent coarse grains, processing cracks in the joint due to localized stress concentration may become more sensitive.

[0034] Furthermore, according to the present invention, the average effective grain size of the heat-affected zone of coarse grains can be 17 μm or less, and the average effective grain size of the heat-affected zone of fine grains can be 7 μm or less. By forming the average effective grain size of the heat-affected zone of coarse grains and the average effective grain size of the heat-affected zone of fine grains to a level similar to the average effective grain size of the base material of steel materials 1 and 2 of the present invention before welding, which is 4 μm to 17 μm, excellent workability of the joint can be ensured. If the upset heating length of flash butt welding is too short compared to the appropriate level, the brittleness of the joint will increase, and conversely, if the upset heating length of flash butt welding is too long, the grain size of the heat-affected zone of the joint will increase overall due to grain growth, which may cause a decrease in the physical properties of the joint. [Examples]

[0035] The present invention will be described in detail below with reference to examples. (Examples) [Table 1]

[0036] Two types of hot-rolled steel sheets, each with a tensile strength of 590 MPa, a thickness of 18 mm, and a width of 280 mm (steel material 1), and a tensile strength of 650 MPa, a thickness of 11 mm, and a width of 250 mm (steel material 2), were prepared as shown in Table 1 above. After preparing the base materials with a coiler to form a rim shape, the opposing surfaces were joined together and flash butt welding was performed under the welding conditions shown in Table 2 below. The preheating / preheating time / flash / flash (heating) length conditions were selected to ensure that appropriate preheating and flash heating occurred at a level suitable for welding, taking into account the steel material thickness. Microstructural analysis, hardness measurement, and tensile and bending tests were performed on the joints forming each of the resulting joined members, and the results are shown in Table 3 below. On the other hand, in Tables 2-3 below, Invention Examples 1-9 and Comparative Examples 1-8 are welding conditions and analysis / evaluation results for steel material 1 in Table 1, while Invention Examples 10-18 and Comparative Examples 9-22 are welding conditions and analysis / evaluation results for steel material 2 in Table 1.

[0037] Furthermore, in the case of the microstructural analysis described above, after taking test specimens of the cross-sectional structure from the joints forming each joint member, they were finely polished, etched with Nital solution, and then observed with an optical microscope. On the other hand, the average hardness difference (ΔH1) between the heat-affected zone (HZ) of coarse grains and the heat-affected zone (HZ) of fine grains formed facing each other on either the left or right side of the joint, the average hardness difference (ΔH2) between the bands of soft phase distributed between the heat-affected zone of the coarse grains on either the left or right side and the heat-affected zones of the left and right coarse grains, the width of the bands of the soft phase (Ws), and the average effective grain size (G1) of the heat-affected zone of the coarse grains and the average effective grain size (G2) of the heat-affected zone of the fine grains were measured, and the results are shown in Table 3 below.

[0038] In this case, as shown in Figure 2, the hardness was measured for each of the above-mentioned regions at 0.2 mm intervals with a load of 300 gf, and a minimum of 1,500 hardness values ​​and a maximum of 2,000 hardness values ​​were obtained. In particular, in the case of the soft phase band distributed between the heat-affected zones of the coarse crystal grains, the hardness was measured at 0.2 mm intervals with a load of 200 gf, with a minimum of 50 hardness values ​​and a maximum of 100 hardness values ​​along the base material thickness direction corresponding to the total length of the joint, and then the average hardness value was calculated and used as the values ​​of ΔH1 and ΔH2.

[0039] On the other hand, the width (Ws) of the soft phase bands was determined by obtaining images of the soft phase bands, which are continuously or discontinuously distributed at the joint interface, using a scanning electron microscope (SEM), and by using energy-dispersive X-ray spectroscopy (EDS) to confirm that the depletion of alloy components that improve hardening ability (austenite stabilization), such as C and Mn, had occurred (primarily resulting in ferrite phase transformation). After that, the width of each soft phase band was measured and the average width was calculated.

[0040] Furthermore, to measure the average effective grain size (G1) of the heat-affected zone of coarse grains at the junction and the average effective grain size (G2) of the heat-affected zone of fine grains, Kikuchi patterns were analyzed via EBSD (Electron Backscattered Diffraction) to obtain IQ (Image Quality) and IPF (Inverse Pole Figure) maps, which visualize grain boundary and grain orientation information. Subsequently, the grains were classified using the EBSD IQ and IPF maps along with microstructural photographs observed with the aforementioned optical microscope. The average effective grain diameter of each grain was then measured by calculating the average grain size based on the number of grains per unit area.

[0041] In addition, for each joint member having the joint obtained as described above, a tensile test was performed to evaluate the location of fracture, as shown in Table 3 below, and a three-point bending test (8R) was also performed to evaluate whether or not bending cracks occurred. For the above tests, tensile test specimens were prepared for each welding condition, with dimensions of 50 mm in width and 350 mm in length, and five specimens were evaluated at a tensile speed of 10 mm / min to verify reproducibility. Bending test specimens were prepared with dimensions of 30 mm in width and 150 mm in length, and eight specimens were evaluated at 90-degree bending to verify reproducibility. After this, each test specimen was visually inspected, and it was judged as pass (○) if the tensile fracture was in the base material, fail (×) if it was in the joint, pass (○) if no bending cracks occurred, and fail (×) if bending cracks occurred.

[0042] [Table 2] *The current percentages for preheating / flashing / upset heat in Table 2 above represent the proportion of the short-circuit current.

[0043] [Table 3] *In Table 3 above, "-" indicates that there is no applicable item.

[0044] As shown in Tables 2-3 above, by sequentially utilizing the preheating / flash / upset heating process and performing flash butt welding under various welding conditions, in the cases of Invention Examples 1-9 (Steel Material 1) and Invention Examples 10-18 (Steel Material 2), where the upset heating length of the resulting joint satisfies the requirement of 5.0 mm or more and less than 15.0 mm, and the value of ΔH1 satisfies the predetermined range, it can be seen that in all cases fracture occurs in the base material during tensile testing, and bending cracks do not occur during bending testing. Through research on the present invention, it was discovered that this is the result of controlling the average hardness difference between the heat-affected zone of coarse crystal grains, whose material properties have deteriorated due to phase transformation, and the heat-affected zone of fine crystal grains, which have relatively good material properties, to within 50 Vickers hardness when tensile and bending stresses are applied to the above-mentioned joint member.

[0045] On the other hand, as shown in Table 3, in the case of Invention Example 3 (Steel Material 1) and Invention Example 12 (Steel Material 2), under the conditions shown in Table 2, the molten portion was sufficiently discharged before rapid cooling due to the optimization of the upset heating length during flash butt welding, and no band of soft phase was formed at the joint. As a result, it can be seen that tensile fracture and bending cracks did not occur at the joint.

[0046] In contrast, in Comparative Examples 1-8 (Steel Material 1) and 9-22 (Steel Material 2), where the upset heating length of the joint obtained under the various welding conditions described above fell outside the range of 5.0 mm to less than 15.0 mm, and the value of ΔH1 also fell outside the specified 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 joined members, if the average Vickers hardness of the heat-affected zone of coarse grains, whose material properties have deteriorated relative to the base material due to phase transformation, is excessively high (embrittlement) or low (softening) to a level exceeding 50 compared to the heat-affected zone of fine grains, which have relatively good material properties, the difference in toughness or strength between the heat-affected zone of coarse grains and the heat-affected zone of fine grains becomes large, causing the affected area to become brittle.

[0047] Figure 1 shows an optical photograph (a) of the cross-sectional structure of the flash butt joint of Invention Example 1 and an SEM photograph (b) of the joint interface magnified, in an embodiment of the present invention.

[0048] Here, we show the heat-affected zones of coarse crystal grains, fine crystal grains, and a band of soft phase distributed between the heat-affected zones of coarse crystal grains, which are formed opposite each other on the left and right sides of the joint portion of the joining member. As described above, in Invention Example 1, it can be confirmed that the upset heating length was appropriate and a sound joint was obtained without any defects such as unbonded areas or oxidation inclusions in the joint portion.

[0049] Figure 2 shows an optical photograph of the cross-sectional structure of the flash butt joint of Invention Example 1 after hardness measurement (a) and an image showing the hardness distribution corresponding to the square box region of the joint using color difference (b).

[0050] Here, as described above, it can be confirmed that the upset heating length of Invention Example 1 is suitable, the difference in average Vickers hardness between the heat-affected zone of coarse grains and the heat-affected zone of fine grains formed opposite each other on the left and right sides of the joint is within 50, and the difference in average Vickers hardness between the heat-affected zone of coarse grains and the soft phase band is within 30, which is good.

[0051] Figure 3 shows an IQ (Image Quality) photograph (a) of the heat-affected zone of coarse crystal grains in Invention Example 1 observed by EBSD, and an IQ (Image Quality) photograph (b) of the heat-affected zone of coarse crystal grains in Comparative Example 1 observed by EBSD.

[0052] Here, as described above, it can be confirmed that the upset heating length is suitable for Invention Example 1, and that the crystal grains in the heat-affected zone of the coarse crystal grains are relatively finer compared to Comparative Example 1.

[0053] As described above, preferred embodiments of the present invention have been described in the detailed description of the present invention, but it goes without saying that various modifications are possible within the scope of the present invention for those who have ordinary skill in the art to which the present invention pertains. Therefore, the scope of the present invention should not be limited to the embodiments described, but should be defined not only by the claims described later, but also by equivalents thereof.

Claims

1. A joint member having a joint obtained by flash butt welding of butt joints of steel plates, The aforementioned joint includes a heat-affected zone of coarse crystal grains and a heat-affected zone of fine crystal grains, which are formed facing each other on the left and right sides with respect to the joint line (joint interface). A joint member with excellent workability, characterized in that the average hardness difference between the heat-affected zone of the coarse crystal grains and the heat-affected zone of the fine crystal grains formed on either the left or right side of the joint is 50 or less on a Vickers hardness scale.

2. The joint member with excellent processability according to claim 1, characterized in that the joint portion further includes a band of a soft phase having an average width of 100 μm or less.

3. The joint member with excellent workability according to claim 2, characterized in that the band of the soft phase is formed between the heat-affected zones of the coarse crystal grains.

4. The joint member with excellent workability according to claim 2, characterized in that the average hardness difference between the heat-affected zone of the coarse crystal grains and the band of the soft phase is 30 or less on a Vickers hardness scale.

5. The joint member with excellent processability according to claim 1, characterized in that the average effective grain size of the heat-affected zone of the coarse grains in the joint is 17 μm or less, and the average effective grain size of the heat-affected zone of the fine grains is 7 μm or less.

6. The steel sheet contains, by weight percent, 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%), with the remainder being Fe and other unavoidable impurities, as described in claim 1, which is a joint member with excellent workability.

7. A flash butt welding method that uses electrodes to upset-heat the joining surfaces of a butt joint of steel plates during flash butt welding, A flash butt welding method with excellent joint workability, characterized by preheating the joining surface of the steel plate, flash heating, and then controlling the distance between electrodes to 5.0 mm or more and less than 15.0 mm during upset heating to form the joint.

8. The aforementioned joint includes a heat-affected zone of coarse crystal grains and a heat-affected zone of fine crystal grains, which are formed facing each other on the left and right sides with respect to the joint line (joint interface). The flash butt welding method for a joint with excellent workability according to claim 7, characterized in that the difference in average hardness between the heat-affected zone of the coarse crystal grains and the heat-affected zone of the fine crystal grains formed on either the left or right side of the joint is 50 or less on a Vickers hardness scale.

9. A component characterized by comprising a joint according to any one of claims 1 to 6.

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