Title of Invention: Flashbutt welding member and flashbutt welding method with excellent formability for wheel rim welding technical field [One] The present invention relates to the manufacture of a flashbut welding member that can improve the weld strength and formability of high-strength hot-rolled steel with a tensile strength of 780 MPa or more and a steel thickness of 6 mm or less applied to a lightweight steel wheel of an automobile, etc. More specifically, Flash By introducing the Pre-/Upset-/Post-heat optimization pattern during butt welding, the hardness of the welded part can be controlled within 220Hv or more and within 270Hv, and furthermore, the coarse-grained heat-affected zone (Coarse-Grained Heat Affected Zone), which is a factor of deterioration of the toughness of the welded part. ) area is supplemented with a fine-grained heat-affected zone, and it relates to a flashbutt welding member and a welding method that can secure formability by improving the toughness of the welded part when manufacturing parts. background [2] In the automobile sector, research on light-weight technology for car bodies and parts is emerging as a major issue due to fuel economy regulation policies for environmental protection such as global warming. Chassis parts, which are important for vehicle driving performance, also require the application of high-strength steel for weight reduction in accordance with this principle. In order to achieve weight reduction of parts, it is essential to increase the strength of the material. In particular, when forming after welding for manufacturing parts, securing the formability of the welded part is an important factor. [3] In the case of flashbut welding, which is mainly used in the assembly of automobile wheel rims, it is important to select the optimal conditions for securing the formability of the welded part as it is a process in which the joining is made through the melt scattering of the surface to be joined by the flash arc and the upset of the molten part. In particular, in the case of high-strength steel, the development of a brittle structure due to hardening of the joint due to a relatively high carbon equivalent is a factor of deterioration of the formability, so it is necessary to develop process conditions that can control the phase transformation of the joint. In the actual wheel rim manufacturing line, it is important to derive a solution for this because it is difficult to apply to mass production when the forming crack defect of the welding part is high. [4] [5] As an example of the prior art for solving this problem, the invention described in Patent Document 1 is exemplified. According to Patent Document 1, an increase in the amount of upset in the bonding step has been suggested for effective discharge of oxide inclusions formed at high temperatures during flashbutt welding. In addition, in Patent Document 2, an oil coating device for suppressing the formation of oxidative inclusions during flashbut welding of high-tensile steel was proposed. [6] However, the prior art has a limitation in that it does not provide a fundamental solution for improving the formability of the welded part when manufacturing parts because the method for controlling the structure of the above-described high-strength steel flashbutt weld is not presented. [7] [Prior art literature] [8] [Patent Literature] [9] (Patent Document 1) Republic of Korea Patent No. 10-1281294 [10] (Patent Document 2) Republic of Korea Patent No. 10-0711459 DETAILED DESCRIPTION OF THE INVENTION technical challenge [11] One aspect of the present invention is a method of controlling the phase transformation structure during welding of high-strength hot-rolled steel with a tensile strength of 780 MPa or more and a steel thickness of 6 mm or less applied to a lightweight steel wheel of an automobile, etc. In order to prevent this, by introducing the Pre-/Upset-/Post-heat pattern during flash butt welding, the hardness of the weld can be controlled within 220Hv or higher and within 270Hv, and coarse grain heat-reducing zone, which is a factor of deterioration of the toughness of the welded part. Flashbutt welding member and welding with excellent formability of wheel rim welds that can secure the toughness and formability of the weld by supplementing the (Coarse-Grained Heat Affected Zone) area with the Fine-Grained Heat Affected Zone. The purpose is to provide a method. [12] 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 are clearly understood by those of ordinary skill in the art to which the present invention belongs from the description below. it could be means of solving the problem [13] The present invention for achieving the above object, [14] A welding member having a welded portion obtained by flashbutt welding two steel plates using an electrode, the welding member comprising: [15] The hardness of the welding part is 220 ~ 270 Hv, [16] When the nearest distance between fine-grained HAZ parts formed facing the left and right sides of the welding part is d and the thickness of the welding base material is T, d <0.25T is satisfied, and [17] When the total length of the upset portion constituting the welding portion is L, it relates to a welding member having excellent formability of the wheel rim welding portion, characterized in that it satisfies L/T >2. [18] [19] It is preferable that the welding part satisfies d < 0.13T. [20] [21] Preferably, the welding part satisfies 32 may be satisfied. Effects of the Invention [29] According to the present invention of the above configuration, it is possible to improve the toughness of the welded part through optimization of the microstructure of the flash butt weld of high-strength hot-rolled steel with a tensile strength of 780 MPa or more and a steel thickness of 6 mm or less, so that the weld strength and formability are improved when manufacturing lightweight steel wheel rims for automobiles. can be effectively improved. Therefore, this solution has the effect of creating new demand for steel and expanding sales by applying it to light steel wheel rims for commercial vehicles. Brief description of the drawing [30] 1 is a diagram schematically showing a flash butt welding pattern (Pre-heat, Upset-heat, Post-heat steps) according to an embodiment of the present invention. [31] 2 is a diagram showing a cross-sectional shape of a welded portion manufactured according to an embodiment of the present invention. [32] Figure 3 (ab) is a photograph showing the cross-sectional structure of the flash butt weld, (a) is for Comparative Example 5, (b) is for Inventive Example 1. [33] 4 is a photograph showing the tensile strength of the 780 MPa grade composite structure steel flash butt weld zone and the three-point bending bending (4R) results for Inventive Example 2 in the embodiment of the present invention. [34] 5 is a photograph showing the tensile strength of 780 MPa grade composite structure steel flash butt welding part tensile and three-point bending bending (4R) results for Inventive Example 3 in an embodiment of the present invention. Best mode for carrying out the invention [35] Hereinafter, the present invention will be described. [36] In the present invention, by introducing a Pre-/Upset-/Post-heat pattern during flash butt welding, the hardness of the weld can be controlled within 220Hv or more and within 270Hv, and coarse grains that are a factor of deterioration of the toughness of the weld zone (Coarse) - It is characterized by providing a technology to improve the toughness of the weld zone by supplementing the area of ​​the grained heat affected zone with a fine-grained heat affected zone. [37] Therefore, in the present invention, in a welding member having a welding portion obtained by flashbut welding two steel sheets using an electrode, the hardness of the welding portion is 220 to 270 Hv, and fine grain HAZ portions formed facing left and right of the welding portion When the nearest distance between -grained HAZs) is d, and the thickness of the welding base material is T, d < 0.25T is satisfied, and when the total length of the upset part forming the welding part is L, L/T > It provides a welding member characterized in that it satisfies 2. [38] [39] 1 is a diagram schematically showing a flash butt welding pattern (Pre-heat, Upset-heat, Post-heat steps) according to an embodiment of the present invention. [40] As shown in Fig. 1, the flash butt welding process of the present invention is largely 1) Burn off (melting and scattering the surface to be joined), 2) Preheat (preheating the surface to be joined evenly), 3) Flash (surface to be joined) melt scattering with high arc heat), 4) Upset (discharge and join the molten part by pressing the surface to be joined with a certain force [at this time, rapid cooling of the joint is relieved through the introduction of upset heat]) and 5) Post-heat (the joint It is composed including post-heat treatment for tempering of low-temperature transformation brittle tissue after phase transformation). [41] [42] That is, the present invention relates to a flash-butt welding method in which the welding target surfaces of two steel sheets are flash-butt welded using an electrode and then subjected to upset heating. Before the flash-butt welding, the welding target surfaces of the steel sheets are preheated. And, after cooling the upset-heated welded part, there is provided a flashbut welding method having excellent formability of the wheel rim weld, characterized in that the welded part is post-heated. [43] [44] A weld obtained by using such a welding process can be further improved in its toughness and formability. That is, by sequentially using Pre-heat, Flash, Upset-heat, Post-heat, etc., it is possible to obtain a weld with excellent toughness and formability. At this time, the present invention is not limited to specific process conditions in the process. [45] However, when the short-circuit current is 100%, it is preferable to apply the current in the range of 30 to 40% preheating, 60 to 70% flash, 30 to 40% upset heat, and 10 to 15% after heat. [46] In addition, it is desirable to manage the preheating time in the range of 1.5 to 4.0 seconds, upset heating time 0.17 to 0.3 seconds, and postheating time 0.1 to 0.3 seconds. [47] And, when heating upset, it is preferable to set the upset force to 7.0 to 11.0 tons, and the upset length to be in the range of 7.0 to 7.5 mm. [48] When the welding conditions as described above are applied, the appropriate values ​​of d and L/T of the weld can be satisfied, and the hardness of the final continuous cooling phase transformation structure is 200 Hv or more and 270 Hv through appropriate control of the heating and cooling rate of the weld. It is possible to secure excellent formability by improving the toughness of the welded part by controlling it within the [49] [50] In the present invention, the area of ​​the coarse-grained heat-affected zone (CGHAZ) generated through high-temperature heat history is optimized by optimizing the discharge of the molten part with an appropriate upset through the optimization of the flash welding process. By minimizing the distance between the regions of the Fine-Grained Heat Affected Zone (FGHAZ) formed on both sides of the surface, it is possible to prevent cracks during part processing. Meanwhile, in the present invention, the coarse grain heat-affected zone (CGHAZ) refers to a coarse grain heat-affected zone with large brittleness formed by heating to 1200° C. or higher, and the fine grain heat-affected zone (FGHAZ) is a fine grain formed by heating to 850 to 1000° C. It refers to the heat-affected zone of crystal grains, which is refined by recrystallization and has good mechanical properties such as toughness. 　　　　　　　　　　　　　 [51] At this time, in the present invention, when the nearest distance between the fine-grained HAZ parts formed facing the left and right of the welding part is d, and the thickness of the welding base material is T, d <0.25T is satisfied characterized. If the d value exceeds 0.25T, the toughness and formability of the weld cannot be guaranteed. 2 is a diagram schematically illustrating a cross-sectional shape of a welded portion manufactured according to an embodiment of the present invention. [52] More preferably, the welding portion satisfies d < 0.13T. [53] Furthermore, in the present invention, when the total length of the upset portion constituting the welding portion is L, it is characterized in that L/T >2 is satisfied. If the L/T value is 2 or less, sufficient discharge of the molten portion and oxide formed at the bonding interface is insufficient, which causes a decrease in bonding strength. There is a problem that cannot be secured. [54] More preferably, the welding part satisfies 32 is satisfied Welding member with excellent formability of wheel rim welding part. [Claim 2] [2] The welding member according to claim 1, wherein the welding portion satisfies d <0.13T. [Claim 3] The welding member according to claim 1, wherein the welding part satisfies 32 is satisfied. Flashbut welding method.