ELECTRIC RESISTANCE BUTT WELDING MEMBER AND ELECTRIC RESISTANCE BUTT WELDING METHOD FOR PROVIDING A WHEEL RIM WELD PART WITH EXCELLENT FORMABILITY

Abstract

A flash butt weld member and a flash butt welding method are provided to provide a wheel-edge welded part with excellent formability. The flash butt weld member of the present invention satisfies that a welded part has a hardness of 220-270 Hv, and d < 0.25T, where d is the nearest distance between the fine-grained heat-affected zones (HAZ) formed to the left and right of the welded part as they face each other, and T is the thickness of the welded base metal, and satisfies L / T > 2, where the overall length of a fitted part constituting the welded part is L.

Description

ELECTRIC RESISTANCE BUTT WELDING MEMBER AND ELECTRIC RESISTANCE BUTT WELDING METHOD FOR PROVIDING A WHEEL RIM WELDING PART WITH EXCELLENT FORMABILITY [TECHNICAL FIELD] The present description relates to the manufacture of a flash butt weld member capable of improving the strength and formability of a welded portion of a high-strength hot-rolled sheet having a tensile strength of 780 MPa or more and a thickness of 6 mm or less applied to a light steel automobile wheel or the like, and more particularly to a flash butt weld member and welding method capable of controlling the hardness of a welded portion to 220 Hv or more, 270 Hv or less by adopting pre- / upset / post-heat optimization patterns at the time of flash butt welding and further capable of ensuring formability through the improvement of the hardness of the welded portion at the time of part manufacturing by supplementing a region of a coarse-grained heat-affected zone, which degrades the hardness of the welded portion, with a fine-grained heat-affected zone.[Precedent Technique]. In the automotive field, research into technologies for reducing the weight of car bodies and parts has emerged as a major problem due to a policy for the LZnZ / q / YILI fuel economy regulation based on environmental protection, such as global warming or similar issues, necessitates the use of high-strength steel for weight reduction in chassis components that are crucial for vehicle performance. To achieve weight reduction, it is essential to increase material strength, and particularly, in the case of post-weld forming for manufacturing parts, ensuring the formability of the welded portion is a key factor. Flash butt welding, primarily used for assembling a vehicle wheel rim, is a process of joining a target joint surface by melting and dispersing heat and fixing the fusion zone with a flash arc. Selecting optimal conditions is crucial to ensure the formability of the welded portion. In the case of high-strength steel, due to its relatively high carbon equivalent, the development of a brittle structure based on joint work hardening is a factor that impairs formability. Therefore, it is necessary to develop process conditions to control the phase transformation of the joint. A high formation defect rate cracks the weld portion on the actual wheel rim manufacturing line, hindering mass production. Thus, finding a solution to this problem is essential. An example of a related technique for solving the problem includes the invention described in Korean Patent Registration No. 10-1281294. According to this related technique, increasing the amount of shim in a joining stage is proposed to effectively remove oxide inclusions formed at high temperatures during flash butt welding. Additionally, Korean Patent Registration No. 10-0711459 presents an oil application device for suppressing the formation of oxide inclusions during flash butt welding of high-tensile steel. However, the related technique does not present a method for controlling the structure of the flash-welded portion of high-strength steel as mentioned above, having a limitation in that it does not present a fundamental solution to improve the formability of the welded portion when manufacturing the parts. [Related technical document] [Patent Document] (Patent Document 1) Korean Patent No. of Registration 10-1281294 (Patent Document 2) Korean Patent No. Registration 10-0711459 RfrQZPn / LZnZ / q / YILI LZnZ / q / YILI [Description] [Technical Problem] One aspect of the present description may provide a flash-welded butt member that has excellent formability for the wheel-edge welded portion and a welding method capable of controlling the hardness of a welded portion to 220 Hv or more, 270 Hv or less by adopting pre- / set- / post-heat optimization patterns at the time of flash butt welding and capable of ensuring the hardness and formability of the welded portion by supplementing a region of a coarse-grained heat-affected zone, which degrades the hardness of the welded portion, with a fine-grained heat-affected zone, to prevent a degradation of the hardness of the welded portion according to the development of a martensitic phase with a Vickers hardness of 300 Hv or more.as a method for controlling a phase transformation structure at the time of welding of a high-strength hot-rolled steel having a high tensile strength of 780 MPa or more and a thickness of 6 mm or less applied to a light steel automobile wheel or the like. The technical objectives achieved by the present invention are not limited to those mentioned above, and other technical objectives not described herein may be readily understood by a person of ordinary skill in the art to which the present invention belongs from the following description. [Technical Solution] According to one aspect of the present description, a welded member having excellent formability for a wheel-edge welded portion, including a welded portion obtained by flash butt welding two steel plates using an electrode, wherein the hardness of the welded portion is 220 to 270 Hv, the welded portion satisfies d < 0.25 T in which d is a shorter distance between fine-grained heat-affected zones (HAZs) formed to the left and right of the welded portion in an opposing manner and T is a thickness of a target welding material, and the welded portion satisfies L / T > 2 in which L is a total length of a fitted part forming the welded portion. The welded portion can satisfy d <0.13T. The welded portion can satisfy 3 <L / T<4. An average grain diameter of a coarse-grained HAZ portion forming the welded portion may be 100 pm or less, and an average grain diameter of the fine-grained HAZ portion may be 10 pm or less. According to another aspect of the present description, in a flash butt welding method, a target surface is welded to a flash butt welding surface. RfrQZPn / LZnZ / q / YILI welded two steel plates using one electrode and subsequently adjusted heating, the flash butt welding method may include: preheating the target weld surface of the steel plate before flash butt welding, cooling the adjusted heated weld portion, and subsequently post-heating the weld portion. The welded portion obtained by post-heating can have a hardness of 220 to 270 Hv, satisfy d < 0.25 T, where d is the shortest distance between the fine-grained heat-affected zones (HAZs) formed to the left and right of the welded portion in an opposing manner, and T is the thickness of a target weld material, and satisfy L / T > 2, where L is the total length of a fitted part forming the welded portion. [Advantageous Effects] According to exemplary methods described herein, the hardness of the welded portion can be improved by optimizing the microstructure of the flash-welded portion of high-strength hot-rolled steel with a tensile strength of 780 MPa or more and a thickness of 6 mm or less. This effectively enhances the strength and formability of the welded portion in the manufacture of lightweight steel automotive wheel rims. Therefore, this solution creates new demand for steel and expands its applications. RfrQZPn / LZnZ / q / YILI sales, when applied to light steel wheel rims for commercial vehicles. [Description of the Drawings] FIG. 1 is a diagram that schematically illustrates the flash butt welding patterns (preheating, heating adjustment and post-heating stages) according to an exemplary modality in the present description. FIG. 2 is a cross-sectional view of a welded portion manufactured according to an exemplary modality described herein. FIGS. 3(a) and 3(b) are photographs illustrating a cross-sectional structure of a flash butt weld, wherein FIG. 3(a) is Comparative Example 5 and FIG. 3(b) is Inventive Example 1. FIG. 4 is a photograph illustrating the stress of a flash-welded butt portion of a complex-phase steel having a grade tensile strength of 780 MPa and a three-point bend (4R) resulting for Inventive Example 2 in the exemplary form in the present description. FIG. 5 is a photograph illustrating the stress on a flash-welded portion of a dual-phase steel having a grade tensile strength of 780 MPa and a three-point bend (4R) resulting for Example RfrQZPn / LZnZ / q / YILI Inventive 3 in the exemplary form in the present description. [Best Mode] Next, the present description will be described. The present description provides a technology capable of controlling the hardness of a welded portion at 220 Hv or more, 270 Hv or less by adopting pre- / adjustment / post-heating patterns at the time of flash butt welding, and capable of improving the hardness of the welded portion by supplementing a region of a coarse-grained heat-affected zone, which degrades the hardness of the welded portion, with a fine-grained heat-affected zone. Therefore, the present description provides a welded member that includes a welded portion obtained by flash butt welding two steel plates using an electrode, wherein the hardness of the welded portion is 220 to 270 Hv, the welded portion satisfies d < 0.25 T wherein d is a shorter distance between fine-grained heat-affected zones (HAZs) formed on the left and right in the welded portion in an opposing manner and T is a thickness of a target welding material, and the welded portion satisfies L / T > 2 wherein L is a total length of a fitted part forming the welded portion. RfrQZPn / LZnZ / q / YILI FIG. 1 is a diagram that schematically illustrates the flash butt welding patterns (preheating, heating adjustment and post-heating stages) according to an exemplary modality in the present description. As shown in FIG. 1, a flash butt welding process of the present description includes 1) burning (melting and dispersing a joint target surface of a material), 2) preheating (preheating the joint target surface uniformly), 3) flashing (melting and dispersing the joint target surface with high arc heat), 4) fitting (discharging and joining a fusion zone by pressing the joint target surface with a predetermined force [here, rapid cooling of a joint part is relieved by introducing fitting heat]), and 5) post-heating (post-heating treatment to temper a low-temperature transformation brittle structure after the phase transformation of the joint part). That is, the present description provides a flash butt welding method that has excellent formability for a wheel edge welded portion, in the flash butt welding method for welding a welded target surface of two steel plates using one electrode and adjusted heating, wherein the welded target surface of the steel plate is preheated before the flash butt welding, the heated welded portion is cooled, and then the welded portion is post-heated. As such, the welded portion obtained using the welding process can also be improved in hardness and formability. That is, a welded portion with excellent hardness and formability can be obtained by sequentially using preheating, flash, heat adjustment, postheating, etc. In this case, the present description is not limited to the specific process conditions. However, when a short-circuit current is 100%, it preferably applies a current in the range of 30 to 40% for preheating, 60 to 70% for flashing, 30 to 40% for adjustment heat, and 10 to 15% for post-heating. Additionally, preferably, the preheating time is handled in the range of 1.5 to 4.0 seconds, the adjusted warm-up time is handled in the range of 0.17 to 0.3 seconds, and the post-warm-up time is handled in the range of 0.1 to 0.3 seconds. In the case of adjusted heating, an adjustment force is in the range of 7.0 to 11.0 tons and an adjustment length is in the range of 7.0 to 7.5 mm. When the welding conditions as described above apply, the appropriate values ​​of dy L / T of RfrQZPn / LZnZ / q / YILI the welded portion can be satisfied, and at the same time, it is possible to ensure excellent formability through the improvement of the hardness of the welded portion by controlling the hardness of a final continuous cooling phase transformation structure to 200 Hv or more and 270 Hv or less through appropriate control of the heating and cooling rate of the welded portion. In this description, the coarse-grained heat-affected zone (CGHAZ) generated through high-temperature heating is minimized by optimizing the melt flow through proper welding process optimization. Simultaneously, cracking during machining is prevented by minimizing the distance between the fine-grained heat-affected zones (FGHAZs) formed on either side of the target joint surface. Herein, CGHAZ refers to a highly brittle, coarse-grained heat-affected zone formed at 1200°C or higher, while FGHAZ refers to a fine-grained heat-affected zone formed at 850 to 1000°C, which is refined by recrystallization and possesses good mechanical properties such as hardness. Here, in the present description, when the shortest distance between the fine-grained HAZ portions formed at the RfrQZPn / LZnZ / q / YILI left and right of the weld portion in a facing manner is dy and a thickness of a target weld material is T, it is satisfied that d < 0.25T. If the value of d exceeds 0.25T, the hardness and formability of the weld portion cannot be guaranteed. FIG. 2 is a cross-sectional view of a weld portion manufactured according to an exemplary modality in the present description. Preferably, the welded portion satisfies d<0.13T. Furthermore, in the present description, when the total length of the fitted part that constitutes the welded portion is L, L / T > 2 is satisfied. If the value L / T is less than 2, the fusion and oxide portion formed at a joint interface cannot be sufficiently discharged, which in turn causes a decrease in the bond strength, and, moreover, d<0.25T cannot be satisfied, so it is not possible to ensure the excellent hardness and formability of the welded portion. Preferably, the welded portion satisfies 3 <l t<4.Furthermore, in the present description, preheating, heating adjustment, and post-heating are appropriately adopted to control the hardness to 220 Hv or higher and 270 Hz or lower after the phase transformation of the welding portion, thereby preventing the formation RfrQZPn / LZnZ / q / YILI of cracking due to brittleness. Furthermore, in this description, the average grain diameter of the coarse-grained HAZ portion forming the welded portion is preferably 100 µm or less, and the average grain diameter of the fine-grained HAZ portion is preferably 10 µm or less. The average grain diameter of the base metal before welding is approximately 4 µm, so this is the grain diameter of the welded portion required to ensure excellent formability of the welded portion obtained through the technical concept described herein. If the amount of heat for flash butt welding is excessive, the grain diameter of the HAZ portion, in accordance with grain growth, increases overall, which causes a decrease in the mechanical properties of the welded portion. [Mode for the Invention] Next, the present description will be described in detail through examples. (Example) A complex-phase steel, the target welding material, was prepared with a thickness of 4.2 mm and a tensile strength of 780 MPa. The hardness of a welded portion, comprising each welded member, was then determined after flash butt welding. RfrQZPn / LZnZ / q / YILI was measured in the same manner as above, according to the welding conditions shown in Table 1 below, and The results are shown in Table 1 below. [Table 1] Classification Preheating (%) Preheating time (sec) Flash (%) Adjusted heat (%) Adjusting heat time (sec) Adjustment force (Ton) Adjustment length (mm) Postheating (%) Postheating time (sec) Hardness of welded portion (Hv) 1 Inventive example 30 4.0 60 30 0.25 7.0 7.2 15 0.3 220-245 2 Inventive example 30 3.0 70 40 0.17 11.0 7.0 10 0.1 220-265 3 Inventive example 40 1.5 70 35 0.30 11.0 7.5 12 0.2 220-255 4 Comparative example 0 0 60 30 0.33 10.0 6.0 0 0.0 275-300 5 Comparative example 25 2.0 5 5 10 0.25 7.0 1.0 0 0.0 277-305 6 Comparative example 0 0 60 30 0.17 10.0 3.0 0 0.0 285-310 7 Comparative example 25 1.6 5 5 10 0.08 7.0 6.0 0 0.0 280-307 * % of current in Table 1 is a ratio of the short-circuit current Furthermore, with respect to the welded portion that constitutes each of the obtained welded members, the shortest distance d between the fine-grained HAZ portions formed to the left and right of the welded portion in an opposing manner and L (total length of the fitting part 5 that forms the welded portion) / T (thickness of the welded base metal) were measured and the results thereof are shown in Table 2. In addition, a tensile test was carried out on each welded member that has the welded portion obtained as described above to evaluate a position where fracture occurred, and the presence or absence of bend cracking was evaluated by performing the 3-point bend test (4R) as shown in Table 2. RfrQZPn / LZnZ / q / YILI [Table 2] L / T Classification Tensile Fracture Position Bend Crack 1 Inventive Example 0.12 2.8 Base Metal X 2 Inventive Example 0.23 3.6 Base Metal X 3 Inventive Example 0.24 3.4 Base Metal X 4 Comparative Example 0.31 1.9 Welded Portion 0 5 Comparative Example 0.46 2.0 Welded Portion 0 6 Comparative Example 0.95 1.8 Welded Portion 0 7 Comparative Example 0.92 1.7 Welded Portion 0 As shown in Table 1 and Table 2, it can be observed that, in Inventive Examples 1 to 3, in which all obtained d values ​​of the welded portion and L / T values ​​satisfy the predetermined range of flash butt welding by sequentially using the preheating, heat setting, and post-heating processes, fracture occurs in the base metal in the tensile test and bend cracking does not occur in the bend test. Conversely, it can be observed that in Comparative Examples 4 through 7, where preheating, heat setting, setting force, and setting length are not appropriate, or where a post-heating treatment process, etc., is not used, all the obtained d values ​​of the welded portion and the L / T values ​​are outside the range described herein. Fracture occurs in the welded portion during the tensile test, and bend cracking occurs during the bend test. That is, as in Comparative Example 5, it can be observed that even with preheating, if the setting length is 1.0 mm outside the appropriate range, the d and L / T values ​​are outside the appropriate range proposed herein. Furthermore, since a post-heating process is absent, the maximum hardness of the welded portion reaches up to 300 Hv to increase the RfrQZPn / LZnZ / q / YILI fragility. Figures 3(a) and 3(b) are photographs illustrating a cross-sectional structure of a flash-welded butt portion, where Figure 3(a) is Comparative Example 5, and Figure 3(b) is Inventive Example 1. As shown in Figure 3, it can be observed that when the d value of the welded portion (distance between fine-grained HAZs) is within 13% of the base metal thickness T and the L / T exceeds 2, the occurrence of tensile fracture of the welded portion is prevented and bend cracking does not occur. Meanwhile, FIG. 4 is a photograph illustrating the tensile result of a flash-welded portion of a complex-phase steel having a tensile strength of 780 MPa and a three-point bend (4R) result for Inventive Example 2 in the exemplary embodiment described herein, and FIG. 5 is a photograph illustrating the tensile result of a flash-welded portion of a complex-phase steel having a tensile strength of 780 MPa and a three-point bend (4R) result for Inventive Example 3 in the exemplary embodiment described herein. It can be observed that the d-values ​​are less than 25% of the T-values ​​in both cases of FIGS. 4 and 5, and thus the tensile characteristics and characteristics RfrQZPn / LZnZ / q / YILI folding are excellent. Exemplary embodiments of the present invention have been described in the detailed description of the present invention. However, various modifications may be made by a person skilled in the art to which the description belongs without departing from the scope of the present description. The technical concepts of the present invention should not be determined to be limited to the exemplary embodiments described herein, but rather are determined by the claims and their equivalents, as well as the claims. RfrQZrñ / LZnZ / q / YILI< / l>

Claims

1. A welded member having excellent formability for a wheel-edge welded portion, including a welded portion obtained by flash butt welding two steel plates using an electrode: characterized in that the hardness of the welded portion is 220 to 270 Hv, the welded portion satisfies d < 0.25 T in which d is a shorter distance between fine-grained heat-affected zones (HAZs) formed to the left and right of the welded portion in an opposing manner and T is a thickness of a target welding material, and the welded portion satisfies L / T > 2 in which L is a total length of a fitted part forming the welded portion.

2. The welding element according to claim 1, characterized in that the welded portion satisfies d<0.13T.

3. The welded member according to claim 1, characterized in that the welded portion satisfies 3 <L / T<4.

4. The welded member according to claim 1, characterized in that an average grain diameter of a coarse-grained HAZ portion forming the welded portion is 100 pm or less, and an average grain diameter of the fine-grained HAZ portion is 10 pm or less.

5. A flash butt welding method for a flash butt weld of a target weld surface of two steel plates using an electrode and subsequently the heating setting, the flash butt welding method characterized in that it comprises: pre-heating the target weld surface of the steel plate prior to the flash butt welding, cooling the heated weld portion set and subsequently post-heating the weld portion.

6. The flash butt welding method according to claim 5, characterized in that the welded portion obtained by post-heating has a hardness of 220 to 270 Hv, the welded portion satisfies d < 0.25 T wherein d is a shorter distance between the fine-grained heat-affected zones (HAZs) formed to the left and right of the welded portion in an opposing manner and T is a thickness of a target welding material, and the welded portion satisfies L / T > 2 wherein L is a total length of a fitted part forming the welded portion.