Flash butt joint member and flash butt welding method

Abstract

An object of the present invention is to provide a flash-butt-joined member and welding method that are excellent in workability by controlling the difference in average effective grain size between the coarse-grained heat-affected zone and the fine-grained heat-affected zone that make up the joint, and by controlling the widths of the coarse-grained heat-affected zone and the fine-grained heat-affected zone that make up the joint, as a method for controlling the phase transformation structure of the joint when welding high-strength steel. [Solution] The present invention is characterized in that, in a joined component having a joint obtained by flash butt welding steel plates, the difference in average effective grain size between the coarse grain heat-affected zone and the fine grain heat-affected zone formed on either the left or right side of the joining line of the joint is within 10 μm.

Description

[Technical Field]

[0001] The present invention relates to a flash butt joint member and a flash butt welding method, and more particularly to a flash butt joint member and a flash butt welding method that ensure excellent workability of a joint obtained by flash butt welding, and in particular, that can ensure sufficient toughness of the joint. [Background technology]

[0002] In the automotive industry, technological research into lightweighting of vehicle bodies and components has emerged as a major challenge due to fuel efficiency regulations related to environmental protection, including issues such as global warming. In response to this trend, the use of high-strength steel materials to reduce weight is also required for chassis and wheel components, which are important for a vehicle's driving performance. In order to achieve lightweight components, it is essential to increase the strength of the material, and ensuring the workability of the joints is particularly important when processing is performed after welding to manufacture the components.

[0003] Flash butt welding, which is primarily used in manufacturing automobile wheels, is a process in which a flash arc causes molten spattering of the welded surfaces and upsets the molten zone, resulting in a joint. Therefore, optimal conditions must be selected to ensure the workability of the joint. In particular, for high-strength steels, the relatively high carbon equivalent can lead to the development of brittle structures due to hardening of the weld zone, which reduces workability. Therefore, it is necessary to develop process conditions that can control the phase transformation of the weld zone. In actual wheel manufacturing lines, the high incidence of processing cracks at the weld zone after welding makes mass production difficult. Therefore, it is important to develop a solution to address this issue. Meanwhile, when manufacturing high-strength, lightweight wheels for commercial vehicles, an optimal weld microstructure that can prevent not only the aforementioned weld brittleness but also softening, and the control of the welding process to achieve this are essential.

[0004] One example of a conventional technique for solving this problem is the invention described in Patent Document 1. Patent Document 1 proposes increasing the amount of upset during the joining stage to effectively remove oxide inclusions formed at high temperatures during flash butt welding. Patent Document 2 also proposes an oil application device for suppressing the formation of oxide inclusions during flash butt welding of high-tensile steel. Meanwhile, Patent Document 3 proposes a method for improving the toughness and workability of wheel rim joints by appropriately introducing a pre- / upset- / post-heat pattern during flash butt welding.

[0005] However, the prior art has limitations in that it only provides some solutions for controlling the microstructure of the flash butt welded joints of high strength steels, and does not provide any further solutions for improving the workability of the joints during flash butt welded manufacturing of components. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Korean Patent Registration No. 10-1281294 [Patent Document 2] Korean Patent Registration No. 10-0711459 [Patent Document 3] Korean Patent Registration No. 10-2178723 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a flash-butt-joined member and welding method that are excellent in workability by controlling the difference in average effective grain size between the coarse-grained heat-affected zone and the fine-grained heat-affected zone that make up the joint, and by controlling the widths of the coarse-grained heat-affected zone and the fine-grained heat-affected zone that make up the joint, as a method for controlling the phase transformation structure of the joint when welding high-strength steel.

[0008] Furthermore, the technical problems to be achieved by the present invention are not limited to the above-mentioned technical problems, 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 pertains from the following description. [Means for solving the problem]

[0009] The present invention provides a joined member having a joint obtained by flash butt welding steel plates, The joint has a coarse-grain heat-affected zone and a fine-grain heat-affected zone formed opposite to each other on the left and right sides of the joint line, The present invention relates to a joined member having excellent workability, in which the difference in average effective crystal grain size between the coarse grain heat-affected zone and the fine grain heat-affected zone formed on either the left or right side of the joined portion is within 10 μm.

[0010] In the above-mentioned joint, the width of the welding heat affected zone consisting of the coarse grain heat affected zone and the fine grain heat affected zone can satisfy the following relational expression:

[0011] [Equation] W HAZ ≧t×1.77 (In the above equation, W HAZ indicates the width of the weld heat affected zone of the joint, and t indicates the thickness of the steel plate.)

[0012] The weld may have a Vickers hardness difference of 30 or less between the minimum hardness of a weld heat-affected zone formed on either the left or right side, the minimum hardness of which is composed of a coarse-grained heat-affected zone and a fine-grained heat-affected zone, and the maximum hardness of the steel plate base material.

[0013] The weld may have a difference in maximum hardness of a weld heat-affected zone formed on either the left or right side, the maximum hardness of which is comprised of a coarse-grained heat-affected zone and a fine-grained heat-affected zone, and the minimum hardness of the steel plate base material, of 30 or less in Vickers hardness.

[0014] The coarse grain heat-affected zone formed on either the left or right side of the joint may have an average effective grain size of 24 μm or less, and the fine grain heat-affected zone may have an average effective grain size of 15 μm or less.

[0015] The steel sheet may contain, 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.

[0016] The present invention also provides This flash butt welding method involves flash / upset heating and post-heating when flash butt welding steel plates using electrodes, and is characterized by controlling the flashing rate during pre-heating and flash heating of the joining surfaces of the steel plates to be joined at 5% to 20%, controlling the inter-electrode travel length during upset heating at 2.0 mm to 10.0 mm, and then performing post-heat treatment to form a joint, thereby achieving excellent joint workability.

[0017] The bonded portion has a coarse grain heat-affected zone and a fine grain heat-affected zone formed on opposite sides of the bond line, and the difference in average effective grain size between the coarse grain heat-affected zone and the fine grain heat-affected zone formed on either the left or right side of the bonded portion can be controlled to within 10 μm.

[0018] The width of the welding heat affected zone consisting of the coarse grain heat affected zone and the fine grain heat affected zone of the above-mentioned joint can satisfy the following relational expression.

[0019] [Equation] W HAZ ≧t×1.77 (In the above equation, W HAZ indicates the width of the weld heat affected zone of the joint, and t indicates the thickness of the steel plate.)

[0020] The difference between the minimum hardness value of the weld heat-affected zone, which is formed on either the left or right side of the weld and is composed of a coarse-grained heat-affected zone and a fine-grained heat-affected zone, and the maximum hardness value of the steel plate base material may be within 30 in Vickers hardness.

[0021] The difference between the maximum hardness value of the weld heat-affected zone, which is formed on either the left or right side of the weld and is composed of a coarse-grained heat-affected zone and a fine-grained heat-affected zone, and the minimum hardness value of the steel plate base material may be within 30 in Vickers hardness.

[0022] The coarse grain heat-affected zone formed on either the left or right side of the joint may have an average effective grain size of 24 μm or less, and the fine grain heat-affected zone may have an average effective grain size of 15 μm or less. [Effects of the Invention]

[0023] According to the present invention, when flash butt welding high-strength hot-rolled steel with a tensile strength of 590 MPa or more, the microstructure of the weld can be optimized to reduce embrittlement or softening of the weld, thereby improving toughness and effectively improving the strength and workability of the welded joint of lightweight steel wheels for automobiles. Therefore, by applying this technology to lightweight steel wheels for commercial vehicles, the application of high-strength steel can be expanded. [Brief explanation of the drawings]

[0024] [Figure 1] (a) to (c) show an optical photograph (a) of the cross-sectional structure of the steel plate base material and flash butt joint of Example 1 in the examples of the present invention, the results of hardness measurements for these (b), and an optical photograph (c) of the cross-sectional structure of the joint after bending 180° (no bending cracks have occurred). [Figure 2] (a) to (c) show an optical photograph (a) of the cross-sectional structure of the steel plate base material and flash butt joint of Comparative Example 1 in Comparative Examples of the present invention, the results of hardness measurements for these (b), and an optical photograph (c) of the cross-sectional structure (bending crack occurrence) of the joint after bending 180°. [Figure 3]This shows the difference in hardness distribution in Figure 1(b) in more detail. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will be described below.

[0026] This invention provides a technology for forming a sound joint by introducing preheating / flash / upset heating and post-heating patterns during flash butt welding, while controlling the difference in average effective grain size between the coarse-grained HAZ and the fine-grained HAZ formed on opposite sides of the joint to within 10 μm. This prevents the average effective grain size of the coarse-grained HAZ of the joint, which may have inferior material properties compared to the base metal due to phase transformation under conditions where tensile and bending stresses are applied to the joined parts, from becoming excessively large compared to the average effective grain size of the fine-grained HAZ, which has relatively good material properties, or prevents the average effective grain size of the fine-grained HAZ of the joint from becoming excessively small compared to the coarse-grained HAZ, which has relatively poor material properties.

[0027] First, a description will be given of a joint member having excellent workability and a joint portion obtained by flash butt welding the steel plates of the present invention.

[0028] The present invention relates to a joined member having a joint obtained by flash butt welding steel plates using an electrode, the joint including a coarse-grained heat-affected zone and a fine-grained heat-affected zone formed on opposite sides of a weld line, where the weld line refers to a joint interface formed along the weld line when butt-welding the steel plates.

[0029] The coarse grain heat-affected zone is a highly brittle weld heat-affected zone formed by heating to 1200°C or higher, while the fine grain heat-affected zone is a weld heat-affected zone formed by heating to 850-1000°C, and has the characteristic of having fine grains due to recrystallization, resulting in good mechanical properties such as toughness.

[0030] Furthermore, in the bonded member of the present invention with excellent workability, the difference in average effective grain size between the coarse-grained heat-affected zone and the fine-grained heat-affected zone formed on either the left or right side of the bonded portion may be within 10 μm. If the difference in average effective grain size between the coarse-grained heat-affected zone and the fine-grained heat-affected zone exceeds 10 μm, the material of the bonded portion becomes non-uniform, the toughness of the coarse-grained heat-affected zone decreases, resulting in poor workability and an increased rate of cracking at the bonded portion. On the other hand, if the difference in average effective grain size is within 10 μm, the material of the coarse-grained heat-affected zone and the fine-grained heat-affected zone becomes homogenous, improving the toughness and workability of the bonded portion.

[0031] On the other hand, 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.

[0032] In the present invention, the width 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 can satisfy the following relational expression:

[0033] [Equation] W HAZ ≧t×1.77 (In the above equation, W HAZ indicates the width of the weld heat affected zone of the joint, and t indicates the thickness of the steel plate.)

[0034] By controlling the width of the weld heat-affected zone to a certain value or more, the average effective grain size of the coarse-grained heat-affected zone of the joint, which may have inferior material properties compared to the base metal due to phase transformation, can be prevented from becoming excessively large compared to the average effective grain size of the fine-grained heat-affected zone, which has relatively good material properties. Furthermore, by preventing the average effective grain size of the fine-grained heat-affected zone of the joint from becoming too small compared to the average effective grain size of the coarse-grained heat-affected zone, which has relatively poor material properties, the workability of the joint can be effectively improved. This is because the difference in hardness between the steel plate base material and the coarse-grained heat-affected zone is small even when the width of the weld heat-affected zone is increased. By keeping the difference in average effective grain size between the coarse-grained heat-affected zone and the fine-grained heat-affected zone within a certain value and uniformly controlling the effective grain size of the weld heat-affected zone, the toughness and workability of the joint can be further improved.

[0035] Furthermore, in the present invention, a joined member with excellent workability can be provided by controlling the difference in minimum hardness of the weld heat-affected zone, which is composed of the coarse-grained heat-affected zone and the fine-grained heat-affected zone, and the maximum hardness of the base steel sheet to within 30 Vickers hardness, or by controlling the difference in maximum hardness of the weld heat-affected zone and the minimum hardness of the base steel sheet to within 30 Vickers hardness. This is because, as described above, when the weld heat-affected zone of the joint becomes relatively excessively embrittled or softened compared to the material of the adjacent base steel sheet under various stress application conditions of the joined member, there is a possibility that the joint will become sensitive to processing cracks due to local stress concentration.

[0036] Furthermore, according to 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 to 10 μm, the average effective grain size of the coarse-grained heat-affected zone may be greater than 15 μm and less than 24 μm, and the average effective grain size of the fine-grained heat-affected zone may be 15 μm or less. By ensuring that the average effective grain sizes of the coarse-grained heat-affected zone and the fine-grained heat-affected zone are not excessively larger than the average effective grain size of the base material before welding, which is 4 μm to 10 μm, excellent workability of the joint can be ensured. When the average effective grain sizes of the coarse-grained heat-affected zone and the fine-grained heat-affected zone are similar to the average effective grain size of the base material, the material between the base material and the heat-affected zone is homogenized, reducing the probability of cracking under conditions in which tensile and bending stresses are applied to the joined components.

[0037] Meanwhile, the present invention does not particularly limit the alloy composition of the steel sheet. However, as an example, the steel sheet may contain, by weight, 0.04 to 0.18% C, 2.0% or less (including 0%) Si, 0.5 to 3.0% Mn, 2.0% or less (including 0%) Cr, 2.0% or less (including 0%) Mo, 0.01 to 0.10% Al, 0.05% or less (excluding 0%) P, and 0.05% or less (excluding 0%) S, with the balance being Fe and other unavoidable impurities. The steel sheet may optionally further contain one or more of 0.20% or less (including 0%) Ti, 0.10% or less (including 0%) Nb, and 0.10% or less (including 0%) Cu. The steel sheet may have a thickness of 2.0 to 6.0 mm.

[0038] Next, a flash butt welding method according to the present invention, which provides excellent workability of the joint, will be described.

[0039] The flash butt welding method of the present invention, which provides excellent workability of the joint, is a flash butt welding method that involves flash / upset heating and post-heating when flash butt welding the surfaces to be joined of a butt joint of steel plates using electrodes, and when pre-heating and flash heating the surfaces to be joined of the steel plates, controls the flashing rate to between 5% and 20%, and controls the inter-electrode travel length during upset heating to between 2.0 mm and 10.0 mm, and then performs post-heat treatment to form the joint.

[0040] The joints obtained by using this welding process can have improved toughness and workability. The conditions for preheating, preheating time, flashing, and flash (heating) length are selected based on the thickness of the steel material to ensure proper preheating and flash heating to a level that allows welding. By controlling the flashing rate to 5% to 20% and the electrode travel length during upset heating to 2.0 mm to 10.0 mm, joints with excellent workability can be obtained.

[0041] If the flash velocity during flash butt welding is too low, or if the upset heating length or post-heating is too short compared to the appropriate level, the brittleness of the joint increases. Conversely, if the flash velocity during flash butt welding is too high, or if the upset heating length or post-heating is too long, the grain size of the heat-affected zone (HAZ) of the joint increases overall due to grain growth, which can lead to a deterioration in the physical properties of the joint. Specifically, if the flash velocity is less than 5%, insufficient heat can be generated in the joint, resulting in problems such as poor welding. Conversely, if the flash velocity is more than 20%, overheating of the joint can increase the grain size of the HAZ or increase the susceptibility to defects such as the formation of oxide inclusions at the joint interface. Furthermore, if the electrode travel length is less than 2.0 mm, the molten zone formed during the flash heating process cannot be sufficiently expelled from the joint interface before it is rapidly cooled, increasing the brittleness of the joint. Conversely, if the electrode travel length is more than 10.0 mm, overheating of the joint can increase the grain size of the HAZ, resulting in softening.

[0042] In the present invention, the flash rate refers to the rate at which the distance between the electrodes decreases per second relative to the initial distance between the electrodes.

[0043] The present invention is not limited to specific process conditions for the above steps. According to the present invention, when the short-circuit current is 100%, current can be applied in the ranges of preheating 30-40%, flash 55-60%, upset heat 25-40%, and post-heat 5-20%. The preheating time may be 2.0-5.0 seconds, the flash (heating) length may be 2.0-4.0 mm, and the upset pressure may be 50-70 bar.

[0044] When the above-mentioned welding conditions are applied, the difference in average effective grain size between the coarse-grained heat-affected zone and the fine-grained heat-affected zone formed on either the left or right side of the weld can be controlled to within 10 μm by controlling the phase transformation of the microstructure according to the appropriate heating and cooling rates of the weld joint during welding, thereby improving the toughness of the weld joint and ensuring excellent workability.

[0045] Furthermore, by controlling the flashing velocity and upset heating length of the present invention, the discharge of the molten zone can be optimized, which can prevent defects such as poor bonding or the formation of oxide inclusions in the bonded zone, and minimize grain growth in the coarse-grained heat-affected zone.

[0046] Meanwhile, post-heat control minimizes the difference in average effective grain size between the coarse grain heat-affected zone and the fine grain heat-affected zone, which has relatively good material properties, thereby preventing cracks from occurring at the joint during part processing.

[0047] As described above, in the present invention, by optimizing the flash butt welding process and controlling the appropriate flash velocity and upset heating length, the discharge of the molten zone can be optimized, preventing defects such as poor welding or the formation of oxide inclusions in the weld. Furthermore, grain growth in the coarse-grained heat-affected zone (HAZ) formed by high-temperature thermal history can be minimized, and post-heat control can be used to further homogenize the material of the weld. This minimizes the difference in average effective grain size between the fine-grained HAZ, which has relatively good material properties, and the coarse-grained HAZ, thereby preventing cracks from occurring in the weld during part processing. [Example]

[0048] The present invention will be described in detail below with reference to examples.

[0049] (Example) [Table 1]

[0050] Two types of hot-rolled steel sheet base materials were prepared, each with a tensile strength of 650 MPa, a thickness of 4.5 mm, and a width of 300 mm (Steel Material 1) and a tensile strength of 650 MPa, a thickness of 6.0 mm, and a width of 300 mm (Steel Material 2), and the chemical compositions shown in Table 1 were prepared. The base materials were then coiled into a rim shape, and the opposing surfaces were butt-jointed and flash-butt welded under the welding conditions shown in Table 2. The preheating, preheating time, flash, and flash (heating) length conditions were optimized to ensure adequate preheating and flash heating to a level that would enable welding, taking into account the steel thickness. The resulting joints were subjected to microstructural analysis, hardness measurement, and tensile and bending tests. The results are shown in Table 3. Meanwhile, in Tables 2-3 below, invention examples 1-9 and comparative examples 1-8 are the welding conditions and analysis / evaluation results for steel material 1 in Table 1 above, and invention examples 10-18 and comparative examples 9-22 are the welding conditions and analysis / evaluation results for steel material 2 in Table 1 above.

[0051] In the case of the above-mentioned microstructural analysis, cross-sectional microstructure specimens were taken from the joints of each joining member, finely polished, etched with Nital solution, and then observed under an optical microscope. Meanwhile, the average effective grain size (G1) of the coarse-grained heat-affected zone, the average effective grain size (G2) of the fine-grained heat-affected zone, the difference in average effective grain size between the coarse-grained heat-affected zone and the fine-grained heat-affected zone (ΔG), the width (W) of the weld heat-affected zone consisting of the coarse-grained heat-affected zone and the fine-grained heat-affected zone, HAZ ), the difference (ΔH1) between the minimum hardness value of the weld heat affected zone consisting of the coarse grain heat affected zone and the fine grain heat affected zone and the maximum hardness value of the steel plate base material, and the difference (ΔH2) between the maximum hardness value of the weld heat affected zone consisting of the coarse grain heat affected zone and the fine grain heat affected zone and the minimum hardness value of the steel plate base material were measured, and the results are shown in Table 3 below.

[0052] At this time, the Vickers hardness was measured at intervals of 0.2 mm for each of the above-mentioned regions with a load of 300 gf, and the hardness values ​​were obtained for a minimum of 1,000 or more and a maximum of 1,500. In addition, the width (W) of the weld heat affected zone consisting of the coarse grain heat affected zone and the fine grain heat affected zone of the above-mentioned joint was measured. HAZ ) is the area excluding the region where the heat-affected zone, which is relatively cooler than the coarse-grained heat-affected zone and the fine-grained heat-affected zone during flash butt welding, is work-hardened by the upset pressure and has increased hardness compared to the steel plate base material (strain-hardening region in Figure 1-3), as shown in Figure 1-3.

[0053] In addition, the average effective grain size (G1) of the coarse-grained heat-affected zone of the joint and the average effective grain size (G2) of the fine-grained heat-affected zone were measured by analyzing the Kikuchi pattern using EBSD (Electron Backscattered Diffraction) to obtain IQ (Image Quality) and IPF (Inverse Pole Figure) maps that visualize the orientation information of the grain boundaries and grains. After that, the grains were classified using the EBSD IQ and IPF maps along with the microstructure photographs observed with the optical microscope mentioned above, and the average effective grain size of each was measured by calculating the average grain size converted from the number of grains per unit area.

[0054] Here, taking into consideration the area that affects the workability of the joint, the measurement targets for G1 and G2 were limited to the area from the joint line (joint interface) to either the left or right side up to 50% of the thickness of the steel plate base material.

[0055] In addition, for each of the bonded components having the bonded joints obtained as described above, a tensile test was performed to evaluate the location of fracture, as shown in Table 3 below. A three-point bending test (4R) was also performed to evaluate the occurrence of bending cracks. For the above tests, for the bonded components corresponding to each welding condition, tensile test specimens were prepared with a width of 50 mm and a length of 220 mm. Five specimens were evaluated at a tension speed of 10 mm / min to verify reproducibility. Bending test specimens were prepared with a width of 30 mm and a length of 100 mm. Eight specimens were evaluated at a 180° bend to verify reproducibility. Each test specimen was then visually inspected, and a pass (○) was given if the tensile fracture occurred in the base material, a fail (X) if the fracture occurred in the bonded joint, a pass (○) if no bending cracks occurred, and a fail (X) if bending cracks occurred.

[0056] [Table 2] TIFF2025537535000004.tif133167*In Table 2 above, the current % for preheating / flash / upset heating / postheating is the percentage of short-circuit current, and the upset heating length means the distance traveled between electrodes during upset heating.

[0057] [Table 3] TIFF2025537535000006.tif197165

[0058] As shown in Table 2-3 above, by performing flash butt welding under various welding conditions using the preheating / flash / upset heating processes in sequence, the flashing rate of the resulting joint can be between 5% and 20%, and the upset heating length can be between 2.0mm and 10.0mm, which allows the ΔG value to be within 10μm. Furthermore, in the case of Invention Examples 1-9 (Steel 1) and Invention Examples 10-18 (Steel 2), fracture occurs in the base metal during tensile testing, but no bending cracks occur during bending testing.

[0059] In contrast, if the flashing velocity of the joint obtained under the various welding conditions described above deviates from the range of 5% to 20%, the upset heat length may be outside the range of 2.0 mm to 10.0 mm, and the ΔG value may exceed the range of 10 μm. Furthermore, in Comparative Examples 1-8 (Steel 1) and 9-22 (Steel 2), which are outside the specified range, tensile fracture and / or bending cracks occurred in the joint. This is because, as described above, when tensile and bending stresses are applied to the joined parts, if the average effective grain size of the coarse-grained heat-affected zone, which has poor material properties relative to the base metal, becomes embrittled or softened to a level exceeding 10 μm compared to the average effective grain size of the fine-grained heat-affected zone, which has relatively good material properties, due to phase transformation, the difference in toughness or strength between the coarse-grained heat-affected zone and the fine-grained heat-affected zone becomes large, making the corresponding region brittle.

[0060] Figure 1 (a) to (c) show an optical photograph of the cross-sectional structure of the steel plate base material and flash butt joint of Example 1 (a), the results of hardness measurements for these (b), and an optical photograph of the cross-sectional structure of the joint after bending 180° (no bending cracks have occurred) (c) in an embodiment of the present invention.

[0061] Here, as described above, in Example 1 of the present invention, the flashing velocity and upset heating length were appropriate, and the ΔG value was within the range of 10 μm, so no cracks occurred after bending the joint 180°.

[0062] Figure 2 (a) to (c) show optical photographs of the cross-sectional structure of the steel plate base material and flash butt joint of Comparative Example 1 in Comparative Examples of the present invention (a), the results of hardness measurements for these (b), and an optical photograph (c) of the cross-sectional structure of the joint after bending 180° (bending cracks occurred).

[0063] Here, as described above, in Comparative Example 1, the flashing velocity and upset heating length were not suitable, so the ΔG value was outside the range of 10 μm, and cracks occurred after bending the joint 180°.

[0064] FIG. 3 shows the results of the hardness distribution difference in FIG. 1(b) in more detail.

[0065] Here, FG to CGHAZ in Figure 1(b), Figure 2(b), and Figure 3 indicate fine-grained and coarse-grained heat-affected zones, and strain-hardening refers to the region where the heat-affected zone, which is relatively cooler than the FG to CGHAZ during flash butt welding, is work-hardened by the upset pressure and has increased hardness compared to the base steel. The strain-hardening region is outside the region that affects the actual workability of the joint, and was not considered as a heat-affected zone that directly affects the workability of the joint.

[0066] As described above, the detailed description of the present invention has been given with reference to the preferred embodiment of the present invention, but it is obvious that various modifications can be made by a person skilled in the art without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiment, but should be defined by the claims below as well as their equivalents.

Claims

1. A joining member having a joining portion obtained by flash butt welding steel plates, The joint includes a coarse-grained heat-affected zone and a fine-grained heat-affected zone formed opposite to each other on the left and right sides of the joint line, A bonded member with excellent workability, characterized in that the difference in average effective grain size between the coarse grain heat-affected zone and the fine grain heat-affected zone formed on either the left or right side of the bonded portion is within 10 μm.

2. 2. A joint member with excellent workability according to claim 1, characterized in that the width of the welding heat affected zone consisting of a coarse grain heat affected zone and a fine grain heat affected zone satisfies the following relational expression. [Relationship] W ＨＡＺ ≧t×1.77 (In the above relation, W ＨＡＺ indicates the width of the weld heat affected zone of the joint, and t indicates the thickness of the steel plate.)

3. 2. A joining member having excellent workability as described in claim 1, characterized in that the difference between the minimum hardness value of the welding heat-affected zone consisting of a coarse-grained heat-affected zone and a fine-grained heat-affected zone and the maximum hardness value of the steel plate base material is within 30 in Vickers hardness.

4. 2. A joining member having excellent workability as described in claim 1, characterized in that the difference between the maximum hardness value of the welding heat-affected zone consisting of a coarse-grained heat-affected zone and a fine-grained heat-affected zone and the minimum hardness value of the steel plate base material is within 30 in Vickers hardness.

5. 2. A joining member with excellent workability as described in claim 1, characterized in that the coarse grain heat-affected zone has an average effective grain size of 24 μm or less, and the fine grain heat-affected zone has an average effective grain size of 15 μm or less.

6. The steel plate contains, by 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%), and the balance being Fe and other unavoidable impurities. The joining member having excellent workability according to claim 1, characterized in that

7. A flash butt welding method for flash / upset heating and post-heating when flash butt welding steel plates using an electrode, comprising: A flash butt welding method with excellent joint workability, characterized in that the flashing speed when preheating and flash heating the surfaces of the steel plates to be joined is controlled to be 5% or more and 20% or less, the inter-electrode travel length during upset heating is controlled to be 2.0 mm or more and 10.0 mm or less, and then post-heat treatment is performed to form a joint.

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

9. The joint includes a coarse-grained heat-affected zone and a fine-grained heat-affected zone formed opposite to each other on the left and right sides of the joint line, 8. The flash butt welding method according to claim 7, wherein the difference in average effective grain size 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 is within 10 μm.

10. A component comprising a joint according to any one of claims 1 to 6.

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