Flash butt welding member, flash butt welding method, and component

Abstract

The present invention relates to a flash butt welding member, flash butt welding, and a component. An aspect of the present invention is to provide a flash butt welding member, flash butt welding, and a component.

Description

Flash butt weld member, flash butt welding method and part

[0001] The present invention relates to a flash butt weld member, a flash butt welding method, and a part.

[0002] Globally, as part of carbon neutrality policies aimed at environmental sustainability, lightweighting is becoming an essential requirement, particularly for improving fuel efficiency, alongside the expansion of eco-friendly vehicles. In the commercial vehicle market as well, the development of lightweight wheels to improve fuel efficiency is continuing in conjunction with the recent expansion of the aforementioned eco-friendly policies. Previously, wheels weighing 34 kg using 590 MPa steel were the main products in the market, but recently, wheels weighing 28 kg using 780 MPa steel have been developed, and their market adoption and application are expected to gradually expand.

[0003] Meanwhile, for ultra-lightweight wheels for commercial vehicles, lightweight component design is being pursued by maintaining the thickness of the wheel disc while primarily reducing the thickness of the wheel rim to ensure driving safety. Currently, the highest strength steel used for wheel rims is mainly 780 MPa grade, and using steel with a strength higher than that presents limitations in terms of reduced processability.

[0004] Meanwhile, it is possible to manufacture an ultra-lightweight wheel of 1.5 GPa by joining a base material of 590 MPa steel, processing it into a part shape, and then performing a final heat treatment. During the above joining process, a joint is formed using flash butt welding. However, under general welding thermal cycle conditions, the brittleness of the joint increases due to rapid cooling, which has a limitation in that processing cracks and hydrogen embrittlement sensitivity increase significantly during the part manufacturing process.

[0005] Accordingly, there is a need to develop flash-butt welded members with excellent processability and hydrogen embrittlement resistance, as well as parts utilizing them.

[0006] [Prior Art Literature]

[0007] (Patent Document 1) Korean Registered Patent Publication No. 10-2178723

[0008] One aspect of the present invention is to provide a flash butt weld member, a flash butt welding method, and a component.

[0009] A preferred aspect of the present invention is to provide a flash butt welded member, a flash butt welding method, and a part having excellent processability and resistance to hydrogen embrittlement.

[0010] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0011] One embodiment of the present invention provides a flash-butt welded member comprising: a base material; and a weld obtained by flash-butting the base material, wherein the base material comprises, in weight percent, C: 0.180~0.40%, Si: 1.0% or less (excluding 0%), Mn: 1.60% or less (excluding 0%), B: 0.010% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities, wherein the carbides distributed in the weld have an average diameter of 180 nm or less and an aspect ratio of 2.5 or less, and the weld has a Kernel Average Misorientation (KAM) value of 2.50 or less.

[0012] The above weldment may contain η-carbide, which is a metastable carbide.

[0013] The above weldment may have a maximum Goss Fiber Intensity of 10.0 or less.

[0014] The above weldment includes a coarse-grained heat-affected zone formed at the center in the width direction and a fine-grained heat-affected zone formed to surround the coarse-grained heat-affected zone, and the average size of the austenite grains in the coarse-grained heat-affected zone may be 20㎛ or more.

[0015] The above welded member may have a thickness of 2 to 20 mm.

[0016] The above weldment may have an average hardness of 200 to 400 Hv.

[0017] The above weldment may have a tensile strength of 1200 MPa or less and a yield strength of 620 MPa or less.

[0018] The above weldment may have an elongation of 4.0% or more.

[0019] The above weldment may have an average length of cracks of 310㎛ or less after immersion in a 0.1N HCl solution for 120 hours following 4-point bending at 150% or more of the base material yield strength.

[0020] Another embodiment of the present invention comprises the steps of: preparing a base material comprising, in weight percent, C: 0.180~0.40%, Si: 1.0% or less (excluding 0%), Mn: 1.60% or less (excluding 0%), B: 0.010% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; preheating the welding target surface of the base material such that the electrode travel length is 1.0~12.0 mm; flash heating the preheated welding target surface such that the flash speed is 5~25% and the electrode travel length is 2.0~6.0 mm; upset heating the flash-heated welding target surface such that the current is 20~40% of the short-circuit current and the electrode travel length is 2.0~16.0 mm, and forming a weld by flash-butt welding. The present invention provides a flashbutt welding method comprising the step of cooling the welded portion and then post-heating it for 0.5 to 2.5 seconds with a current of 5 to 25% of the short-circuit current.

[0021] Another embodiment of the present invention provides a component comprising, in weight percent, C: 0.180~0.40%, Si: 1.0% or less (excluding 0%), Mn: 1.60% or less (excluding 0%), B: 0.010% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; and a weld obtained by flash butt welding the base material and then post-heat treating, wherein the weld comprises a soft portion formed along the thickness direction, and the average hardness (H1) of the base material and the average hardness (H2) of the soft portion satisfy the following relationship 1.

[0022] [Equation 1] H1-H2 ≤ 110Hv

[0023] The carbon content of the above base material and the above soft part can satisfy the following relationship 2.

[0024] [Equation 2] α - β ≤ 0.1 wt%

[0025] (However, α is the carbon content (weight%) of the base material, and β is the carbon content (weight%) of the soft part.)

[0026] The above base material may have a tensile strength of 1000 to 2000 MPa.

[0027] The above soft part may have a width of 10% or less of the thickness of the base material.

[0028] According to one aspect of the present invention, a flash butt welding member, a flash butt welding method, and a component can be provided.

[0029] According to a preferred aspect of the present invention, a flash butt welded member, a flash butt welded method, and a part having excellent processability and resistance to hydrogen embrittlement can be provided.

[0030] Figure 1 is the result of confirming that the unit cell crystal structure of η-carbide, a metastable carbide, matches the unit cell crystal structure of the selected area diffraction (SAD or SAED) pattern by capturing bright field and dark field images of Invention Example 2 using a transmission electron microscope (TEM).

[0031] Figure 2 is the result of confirming that Comparative Example 2 matches the unit cell crystal structure of cementite and metastable carbide η-carbide by capturing a bright field image using a transmission electron microscope (TEM) and analyzing the selected area diffraction (SAD or SAED) pattern.

[0032] Figure 3 is an image showing the optical photograph of the cross-sectional structure after hardness measurement of the flashbutt welded portion including the steel plate base material for Invention Example 2 before post-heat treatment, and the hardness distribution corresponding to the square box area as a color difference.

[0033] Figure 4 is an image showing the Goss Fiber Intensity measured in the width direction (TD) of the flash butt welds of Comparative Example 2 (left) and Invention Example 2 (right) before post-heat treatment.

[0034] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0035] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0036] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0037] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0038] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0039] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0040] Hereinafter, a flash butt welding member according to one embodiment of the present invention will be described.

[0041] A flash butt weld member according to one embodiment of the present invention comprises a base material and a weld obtained by flash butt welding the base material. The present invention does not specifically limit the shape of the weld member, but as an example, the weld member may have the shape of a steel pipe or a square pipe.

[0042] The above base material may consist of, in weight percent, C: 0.180~0.40%, Si: 1.0% or less (excluding 0%), Mn: 1.60% or less (excluding 0%), B: 0.010% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities.

[0043] C: 0.180~0.40%

[0044] The above C is an essential element for securing the strength of martensite, and as an element that increases the strength of heat-treated components and improves hardenability, it is an element that is essential for controlling the strength of steel sheets. If the content of the above C is less than 0.180%, hardenability is insufficient, so when the cooling rate is reduced, sufficient martensite is not secured and a ferrite structure develops, which may result in the disadvantage of inferior strength and impact resistance of parts made from steel sheets. If the content of the above C exceeds 0.40%, not only is the impact toughness of the slab reduced, but there may also be disadvantages such as an excessive increase in the strength of heat-treated components and reduced weldability. Therefore, it is advantageous for the content of the above C to be in the range of 0.180 to 0.40%. Regarding the lower limit of the above C content, it is more advantageous for it to be 0.190%, more advantageous for it to be 0.20%, and most advantageous for it to be 0.210%. The upper limit of the above C content is more advantageous at 0.39%, more advantageous at 0.38%, and most advantageous at 0.37%.

[0045] Si: 1.0% or less (excluding 0%)

[0046] The above Si is an element added not only as a deoxidizer in steelmaking but also as a solid solution strengthening element and a carbide formation inhibitor, which is effective for homogenizing the internal structure of the steel sheet, contributes to increasing the strength of heat-treated members, and is added for effective material homogenization. If the content of the above Si exceeds 1.00%, there may be a disadvantage in that Si-based oxides are excessively formed on the surface of the steel sheet, which can degrade surface quality and properties. Therefore, it is advantageous for the content of the above Si to be in the range of 1.0% or less. It is more advantageous for the above Si content to be 0.90% or less, more advantageous for it to be 0.80% or less, and most advantageous for it to be 0.70% or less. Meanwhile, since the above Si can have a sufficient effect even in trace amounts, there is no specific limit regarding its lower limit. However, as an example, the lower limit of the above Si content may be 0.10%.

[0047] Mn: 1.60% or less (excluding 0%)

[0048] The above Mn is an element added not only to secure the desired strength due to the solid solution strengthening effect but also to suppress unwanted ferrite or bainite transformations during heat treatment. If the content of the above Mn exceeds 1.60%, the manufacturing cost of the steel sheet may increase, and there may be disadvantages such as reduced impact resistance because band-like structures arranged in the rolling direction within the microstructure develop excessively, causing non-uniformity in the internal structure of the steel sheet. Therefore, it is advantageous for the content of the above Mn to be in the range of 1.60% or less. It is more advantageous for the content of the above Mn to be 1.50% or less, more advantageous for it to be 1.40% or less, and most advantageous for it to be 1.30% or less. Meanwhile, since the above Mn can have a sufficient effect even in small amounts, there is no specific limit regarding its lower limit. However, as an example, the lower limit of the above Mn content may be 0.10%.

[0049] B: 0.010% or less (excluding 0%)

[0050] The above B is an element added to suppress the formation of ferrite in the heat-treated member and to improve hardenability even with a small amount of addition, as well as to suppress brittleness of the heat-treated member caused by grain boundary segregation of P or S by segregating at the prior austenite grain boundaries. If the content of the above B exceeds 0.010%, not only is the effect saturated, but there may also be a disadvantage of significantly reducing the hot rolling properties of the slab because it causes brittleness by segregating at the grain boundaries. Therefore, it is advantageous for the content of the above B to be in the range of 0.010% or less. It is more advantageous for the above B content to be 0.0080% or less, more advantageous for it to be 0.0060% or less, and most advantageous for it to be 0.0040% or less. Meanwhile, since the above B can have a sufficient effect even in a small amount, there is no specific limit regarding its lower limit. However, as an example, the lower limit of the above B content may be 0.0010%.

[0051] The remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0052] The carbides distributed in the weldment may have an average diameter of 180 nm or less and an aspect ratio of 2.5 or less. If the average diameter of the carbides exceeds 180 nm, the toughness of the joint is reduced, making it susceptible to processing cracks; furthermore, there may be a disadvantage in that the resistance to hydrogen embrittlement is also relatively inferior due to the reduced toughness of the joint. It is more advantageous for the average diameter of the carbides to be 165 nm or less, more advantageous for 145 nm or less, and most advantageous for 125 nm or less. Since a smaller average diameter of the carbides is more advantageous, the present invention does not specifically limit the lower limit thereof. However, as an example, the lower limit of the average diameter of the carbides may be 50 nm. If the aspect ratio of the carbides exceeds 2.5, as described above, there may be a disadvantage in that the toughness of the joint is reduced, making it susceptible to processing cracks, and furthermore, the resistance to hydrogen embrittlement is also relatively inferior due to the reduced toughness of the joint. This is because the carbides distributed at the joint during the processing process have a shape that makes it relatively difficult for stress to be dispersed, making it easy for processing cracks to be induced at localized stress concentration points. In fact, if the fracture surface of the joint where processing cracks have occurred is observed in detail at high magnification using a scanning electron microscope (SEM), it can be confirmed that numerous cracks occur at the interface between the acicular cementite, which has a relatively large aspect ratio value, and the matrix. An aspect ratio of the carbide of 2.4 or less is more advantageous, 2.3 or less is more advantageous, and 2.2 or less is most advantageous. Since a smaller aspect ratio of the carbide is more advantageous, the present invention does not specifically limit its lower limit. However, as an example, the lower limit of the aspect ratio of the carbide may be 1.0. Meanwhile, the present invention does not specifically limit the type of the carbide, but as an example, it may be cementite.

[0053] The above weldment may have a Kernel Average Misorientation (KAM) value of 2.50 or less. If the KAM value exceeds 2.50, the dislocation density within the microstructure of the joint is relatively high, which degrades the machinability of the joint. Furthermore, since hydrogen is not strongly held by dislocations compared to fine carbide precipitates and can move easily, there may be a disadvantage in that the resistance to hydrogen embrittlement of the joint is reduced. A KAM value of 2.4 or less is more advantageous, 2.3 or less is more advantageous, and 2.2 or less is most advantageous. Since a smaller KAM value is more advantageous, the present invention does not specifically limit its lower limit. However, as an example, the lower limit of the KAM value may be 0.5. Meanwhile, the dislocation density within the grain can be evaluated by the Kernel Average Misorientation (KAM) value. The above KAM value is the average value of the crystal rotation amount (crystal orientation difference) between the target measurement point and surrounding measurement points; the larger this value, the more deformation exists within the crystal and the higher the dislocation density.

[0054] The above weldment may include η-carbide, which is a metastable carbide. By including η-carbide, which is a fine spherical metastable carbide compared to conventional cementite, the above weldment can more effectively disperse stress during processing and exhibit the effect of strengthening the matrix, thereby improving the toughness of the joint and consequently improving the resistance to hydrogen embrittlement of the joint.

[0055] The maximum value of the Goss Fiber Intensity of the above weldment may be 10.0 or less. If the maximum value of the Goss Fiber Intensity exceeds 10.0, a texture with an orientation unfavorable to machinability may develop, which may result in a disadvantage of reduced machinability of the joint. It is more advantageous for the maximum value of the Goss Fiber Intensity to be 9.9 or less, more advantageous for it to be 9.8 or less, and most advantageous for it to be 9.7 or less. Since a smaller maximum value of the Goss Fiber Intensity is more advantageous, the present invention does not specifically limit its lower limit. However, as an example, the lower limit of the maximum value of the Goss Fiber Intensity may be 2.50. Meanwhile, the above Goss Fiber Intensity refers to the relative volume fraction of grains having specific orientations of Euler angles (90˚, 90˚, 45˚) in the Orientation Distribution Function (ODF), which is the orientation distribution function of the texture, compared to the case of having a texture without orientation. For example, if this value is 2.0, it means that the Goss Fiber texture has developed twice as much compared to the case of having the aforementioned texture without orientation.

[0056] The above weldment includes a coarse-grain heat-affected zone (CGHAZ) formed at the center in the width direction and a fine-grain heat-affected zone (FGHAZ) formed to surround the coarse-grain heat-affected zone, wherein the average size of the austenite grains in the coarse-grain heat-affected zone may be 20 μm or more. If the average size of the austenite grains in the coarse-grain heat-affected zone is less than 20 μm, there may be a disadvantage in that the toughness of the joint becomes relatively insufficient and brittleness increases, resulting in inferior machinability. Additionally, as the grain size decreases, the boundary areas within the grains where hydrogen may be trapped are relatively reduced, which may result in a disadvantage in that the resistance to hydrogen embrittlement of the joint is also inferior. It is more advantageous for the average size of the austenite grains in the coarse-grain heat-affected zone to be 22 μm or more, more advantageous for it to be 24 μm or more, and more advantageous for it to be 26 μm or more. Since it is advantageous for the average size of the coarse-grained heat-affected zone's austenite grains to be larger, the present invention does not specifically limit the upper limit thereof. However, as an example, the upper limit of the average size of the coarse-grained heat-affected zone's austenite grains may be 40 μm.

[0057] The flashbutt welded member of the present invention may have a thickness of 2 to 20 mm. If the thickness of the welded member is less than 2 mm, there may be a disadvantage in that it is difficult to secure sufficient strength and durability due to excessive thinning of the heat-treated member. If the thickness of the welded member exceeds 20 mm, there may be a disadvantage in that it is difficult to secure sufficient hardenability of the heat-treated member. The lower limit of the thickness of the welded member is more advantageous at 2.2 mm, more advantageous at 2.6 mm, and most advantageous at 3.0 mm. The upper limit of the thickness of the welded member is more advantageous at 18 mm, more advantageous at 16 mm, and most advantageous at 10 mm.

[0058] The flash butt welded member of the present invention may have an average hardness of 200 to 400 Hv of the welded portion. The welded portion may have a tensile strength of 1200 MPa or less and a yield strength of 620 MPa or less. The welded portion may have an elongation of 4.0% or more. In the present invention, the lower limit of the tensile strength is not specifically limited, but as an example, the lower limit may be 700 MPa. In the present invention, the lower limit of the yield strength is not specifically limited, but as an example, the lower limit may be 500 MPa. In the present invention, the upper limit of the elongation is not specifically limited, but as an example, the upper limit may be 10%.

[0059] The above weldment may have an average length of cracks of 310 μm or less after immersion in a 0.1 N HCl solution for 120 hours following 4-point bending at 150% or more of the yield strength of the base material. In the present invention, the lower limit of the average length of the cracks is not specifically limited, but as an example, the lower limit may be 10 μm.

[0060] Hereinafter, a flash butt welding method according to one embodiment of the present invention will be described.

[0061] First, a base material satisfying the aforementioned alloy composition is prepared. In the present invention, the process for preparing the base material is not specifically limited, and any method used in the relevant technical field may be utilized.

[0062] Subsequently, the welding target surface of the above base material is preheated. The preheating step can be controlled so that the travel length between electrodes is 1.0 to 12.0 mm. If the travel length between electrodes during preheating is less than 1.0 mm, there may be a disadvantage in that the preheating of the steel plate base material is insufficient, making the joint brittleness sensitive to rapid cooling. If the travel length between electrodes during preheating exceeds 5.0 mm, the preheating of the steel plate base material is excessive, which may result in disadvantages such as a decrease in joint properties caused by grain growth in the microstructure of the joint. The lower limit of the travel length between electrodes during preheating is more advantageous at 1.2 mm, more advantageous at 1.6 mm, and most advantageous at 1.8 mm. The upper limit of the travel length between electrodes during preheating is more advantageous at 11.5 mm, more advantageous at 11.0 mm, and most advantageous at 10.5 mm.

[0063] Subsequently, the preheated welding target surface is flash-heated. The flash-heating step can be controlled such that the flash speed is 5–25% and the travel length between electrodes is 2.0–6.0 mm. If the flash speed is less than 5%, there may be a disadvantage in that the flash heating speed in the flash section is too slow, resulting in an excessive heat input to the joint. If the flash speed exceeds 25%, there may be a disadvantage in that the flash heating speed in the flash section becomes too fast, resulting in an insufficient heat input to the joint. The lower limit of the flash speed is more advantageous at 6%, more advantageous at 8%, and most advantageous at 10%. The upper limit of the flash speed is more advantageous at 24%, more advantageous at 22%, and most advantageous at 20%. Meanwhile, the flash speed refers to the rate at which the gap between electrodes decreases per second compared to the initial gap between electrodes in the flash section. When the distance between electrodes during the above flash heating is less than 2.0 mm, there may be a disadvantage in that the strength of the joint is insufficient due to insufficient flash heating of the steel plate base material. When the distance between electrodes during the above flash heating exceeds 6.0 mm, there may be a disadvantage in that the physical properties of the joint are degraded due to excessive flash heating of the steel plate base material, such as grain growth in the microstructure of the joint. The lower limit of the distance between electrodes during the above flash heating is more advantageous at 2.2 mm, more advantageous at 2.4 mm, and most advantageous at 2.6 mm. The upper limit of the distance between electrodes during the above flash heating is more advantageous at 5.8 mm, more advantageous at 5.6 mm, and most advantageous at 5.4 mm.

[0064] Subsequently, the flash-heated welding target surface is upset-heated and flash-butt welded to form a weld. The upset-heating step is performed with a current of 20 to 40% of the short-circuit current, and the travel length between electrodes can be controlled to be 2.0 to 16.0 mm. If the current during upset-heating is less than 20% of the short-circuit current, rapid cooling of the joint becomes relatively sensitive, resulting in the disadvantage that the discharge of the molten part is not maximized and the brittleness of the joint becomes sensitive. If the current during upset-heating exceeds 40% of the short-circuit current, the cooling rate of the joint increases excessively, which may result in disadvantages such as deterioration of the joint's physical properties due to grain growth in the microstructure of the joint. The lower limit of the current during upset-heating is more advantageous at 22% of the short-circuit current, more advantageous at 24% of the short-circuit current, and most advantageous at 26% of the short-circuit current. When performing the upset heating above, the upper limit of the current is more advantageous at 38% of the short-circuit current, more advantageous at 36% of the short-circuit current, and most advantageous at 34% of the short-circuit current. When performing the upset heating above, if the travel length between electrodes is less than 2.0 mm, the upset heating and the amount of upset may be insufficient, resulting in a disadvantage where the strength of the joint is insufficient. When performing the upset heating above, if the travel length between electrodes exceeds 16.0 mm, the upset heating and the amount of upset may be excessive, resulting in a disadvantage where the processability of the joint is inferior. When performing the upset heating above, the lower limit of the travel length between electrodes is more advantageous at 2.2 mm, more advantageous at 2.4 mm, and most advantageous at 2.6 mm. When performing the upset heating above, the upper limit of the travel length between electrodes is more advantageous at 15.8 mm, more advantageous at 15.6 mm, and most advantageous at 15.4 mm.

[0065] Afterward, the welded joint is cooled and then post-heated. The post-heating can be performed for 0.5 to 2.5 seconds with a current of 5 to 25% of the short-circuit current. If the current during post-heating is less than 5% of the short-circuit current, the amount of post-heating of the joint is insufficient, which may result in a disadvantage where the effect of mitigating brittleness of the joint due to post-heating is insufficient. If the current during post-heating exceeds 25% of the short-circuit current, the amount of post-heating of the joint is excessive, which may result in disadvantages such as insufficient strength due to softening of the joint due to post-heating. The lower limit of the current during post-heating is more advantageous at 6% of the short-circuit current, more advantageous at 8% of the short-circuit current, and most advantageous at 10% of the short-circuit current. The upper limit of the current during post-heating is more advantageous at 24% of the short-circuit current, more advantageous at 22% of the short-circuit current, and most advantageous at 20% of the short-circuit current. If the post-heating time is less than 0.5 seconds, there is a disadvantage that the effect of post-heating is insufficient, resulting in a lack of improvement in the toughness of the joint. If the post-heating time exceeds 2.5 seconds, there is a disadvantage that the post-heating is excessive, leading to insufficient strength of the joint or, conversely, an increase in the brittleness of the joint. The lower limit of the post-heating time is more advantageous at 0.6 seconds, more advantageous at 0.7 seconds, and most advantageous at 0.8 seconds. The upper limit of the post-heating time is more advantageous at 2.4 seconds, more advantageous at 2.2 seconds, and most advantageous at 2.0 seconds.

[0066] Hereinafter, a component according to an embodiment of the present invention will be described. Although the present invention is not particularly limited, as an example, the component may be manufactured using a welded member having the aforementioned characteristics.

[0067] A component according to one embodiment of the present invention comprises, in weight percent, a base material comprising C: 0.180~0.40%, Si: 0.60% or less (excluding 0%), Mn: 1.40% or less (excluding 0%), B: 0.010% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities, and a weldment obtained by performing flash butt welding on the base material followed by post-heat treatment. The post-heat treatment is distinguished from the post-heat treatment in the flash butt welding method mentioned above.

[0068] The above-mentioned weldment may include a soft portion formed along the thickness direction. The soft portion is an area adjacent to the molten portion formed during the manufacture of the welded member. The molten portion is discharged to the outside during the upset process while having a relatively high carbon solubility. At this time, as carbon from the area adjacent to the molten portion escapes into the molten portion, the carbon content may become relatively lower, and consequently, the hardenability may be insufficient compared to the base material. Furthermore, the durability and impact resistance of a part manufactured from a welded member having a soft portion with inferior hardenability may also be inferior.

[0069] However, according to the manufacturing process of the present invention, the average hardness (H1) of the base material and the average hardness (H2) of the soft part may satisfy the following relationship 1. It is more advantageous for H1-H2 to be 108 Hv or less, more advantageous for it to be 104 Hv or less, and most advantageous for it to be 100 Hv or less. Since it is more advantageous for H1-H2 to be smaller, the present invention does not specifically limit its lower limit. However, as an example, the lower limit of H1-H2 may be 10 Hv. In addition, H1 may be greater than H2.

[0070] [Equation 1] H1-H2 ≤ 110Hv

[0071] That is, by controlling the average hardness difference between the base material and the soft part to a specific value or less, hardenability, durability, and impact resistance equivalent to those of the base material can be secured.

[0072] The carbon content of the above base material and the above soft part can satisfy the following relationship Equation 2. By controlling the difference in carbon content between the base material and the soft part to be small in this way, the difference in average hardness between the base material and the soft part can be reduced.

[0073] [Equation 2] α - β ≤ 0.1 wt%

[0074] (However, α is the carbon content (weight%) of the base material, and β is the carbon content (weight%) of the soft part.)

[0075] Meanwhile, although not specifically limited in the present invention, as an example, the soft part may have a width of 10% or less relative to the thickness of the base material and may include a bond line.

[0076] The above part may be a wheel rim, but is not limited thereto.

[0077] The above-mentioned part is manufactured by post-heat treating the above-mentioned welded member, and the post-heat treatment process may be a normalizing step, an austenitizing step, a quenching step, and a tempering step.

[0078] The above normalizing step can be performed at 850 to 950°C for 5 to 7 minutes.

[0079] The above austenitizing step can be performed at 850 to 950°C for 5 to 7 minutes.

[0080] The above quenching step can be performed at 200℃ or lower.

[0081] The above tempering step can be performed at 150 to 600°C for 30 to 60 minutes.

[0082] The tensile strength of the base material of the part after the above post-heat treatment process is 1000 MPa to 2000 MPa, which can be increased compared to the bonded member after the above flash butt bonding. Accordingly, it is possible to make the part thinner and lighter.

[0083] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0084] (Example)

[0085] After preparing a base material satisfying the alloy composition listed in Table 1 below, a flash butt welded member was manufactured by preheating, flash heating, upset heating / flash butt welding, and cooling / postheating under the conditions listed in Table 2 below. For the welded member manufactured in this manner, the average diameter and aspect ratio of carbides distributed in the weld zone, the presence of η-carbide in the weld zone, the Kernel Average Misorientation (KAM) value of the weld zone, the maximum value of Goss Fiber Intensity of the weld zone, the average size of coarse austenite grains in the heat-affected zone, mechanical properties, and resistance to hydrogen embrittlement were measured, and the results are shown in Tables 3 and 4 below. In addition, the welded member manufactured above was machined into a part shape and subjected to post-heat treatment to manufacture a part, and the average hardness of the base material and the average hardness of the soft zone were measured, and the results are shown in Table 4 below.

[0086] The average diameter and aspect ratio of the carbides distributed in the weldment were determined by preparing the weldment as a thin foil sample using a jet polisher, confirming the presence of carbides through selective area diffraction (SAD or SAED) pattern analysis using a transmission electron microscope (TEM), measuring the major and minor axes of each carbide distributed in the TEM image through image analysis, calculating the average value of these values ​​as the diameter of the carbides, and calculating the average diameter of the entire carbide. The ratio of the major and minor axes of each of the aforementioned carbides was calculated as the aspect ratio, and the average aspect ratio of the entire carbide was calculated.

[0087] The presence of η-carbide in the weldment was confirmed by taking bright field and dark field images using the transmission electron microscope (TEM) described above and analyzing the selected area diffraction (SAD or SAED) pattern to determine whether it matched the unit cell crystal structure of η-carbide.

[0088] The Kernel Average Misorientation (KAM) value of the weld zone is defined as the average difference between the crystal orientation at a specific point and the surrounding crystal orientations, and was measured using Electron Backscatter Diffraction (EBSD). In this context, a higher KAM value indicates a higher dislocation density due to deformation, and in particular, the density of Geometrically Necessary Dislocations (GND) is proportional to the KAM value.

[0089] The maximum value of the Goss Fiber Intensity of the weldment was measured by defining it as the relative volume fraction of grains having specific orientations at Euler angles (90˚, 90˚, 45˚) in the Orientation Distribution Function (ODF), which is the orientation distribution function of the texture, through electron backscatter diffraction (EBSD). At this time, the above value was measured in the width direction (TD) and thickness direction (ND), and the maximum value among them was taken.

[0090] The average size of the coarse grain heat-affected zone of the weldment was calculated by observing the microstructure etched with Nital solution using a scanning electron microscope (SEM) to distinguish the grain boundaries, measuring the major and minor axes of each grain through image analysis to obtain the average value, and then calculating the overall average grain size.

[0091] The hardness of the base material and weldment of the welded member was calculated by measuring Vickers hardness at 0.2 mm intervals under a load of 300 gf to obtain 1,000 hardness values ​​and then calculating the average value.

[0092] In particular, when measuring the hardness of the base material and weldment of a part, in the case of the soft part distributed along the part bond line, the microstructure etched with Nital solution is observed using a scanning electron microscope (SEM) to distinguish the base material and the bond line, and then at least 10 and up to 50 hardness measurements are taken at 0.2 mm intervals along the base material thickness direction corresponding to the entire length of the bond at a load of 200 gf, and the average hardness value is calculated and used as the average hardness (H1) of the heat-treated part base material and the average hardness (H2) of the soft part.

[0093] The tensile strength, yield strength, and elongation of the weldment were measured by taking a tensile specimen with a width of 1.6 mm, a length of 20 mm, and a thickness of 0.5 mm from the center of the weldment and performing a tensile test.

[0094] The hydrogen embrittlement resistance of the weldment was determined by taking a specimen with a width of 30 mm and a length of 180 mm with the weldment positioned in the center while containing the base material, bending it at four points to more than 150% of the yield strength of the base material, and then immersing it in a 0.1 N HCl solution for 120 hours. Subsequently, the distribution of cracks was measured using computed tomography (CT), and the average value of the crack lengths distributed in the weldment was calculated and compared.

[0095] Base Material No. Alloy Composition (Weight%) CsiMnB Balance 10.35 0.5 1.4 0.00 95Fe 20.22 0.3 0.9 0.00 25Fe 30.31 0.6 1.2 0.00 55Fe 40.38 0.9 1.5 0.00 30Fe 50.18 0.8 1.6 0.00 10Fe

[0096] Classification Base Material No. Preheating Flash Heating Upset Heating Postheating Travel Length Between Electrodes (mm) Flash Speed ​​(%) Travel Length Between Electrodes (mm) Current Relative to Short-circuit Current (%) Travel Length Between Electrodes (mm) Relative to Short-circuit Current Current (%) Time (sec) Invention Example 1 1.0 20 2.0 20 2.0 100.5 Invention Example 2 23.5 15 4.0 30 4.0 15 1.0 Invention Example 3 34.0 10 4.5 35 4.5 5 2.0 Invention Example 4 45.0 25 5.0 40 10.0 10 1.5 Invention Example 5 5 12.0 5 6.0 35 15.0 25 2.5 Comparative Example 1 1.0 20 2.0 10 2.0 100.4 Comparative Example 2 23.5 15 4.0 30 4.0 30 0.5 Comparative Example 3 34.0 30 4.5 35 4.5 5 2.0 Comparative Example 4 4 ​​13.0 25 5.0 40 10.0 10 1.5 Comparative Example 5 5 12.0 5 7.0 35 15.0 252.5 Comparative Example 6 11.0 202.0 202.0 -- Comparative Example 7 23.5 154.0 504.0 151.0 Comparative Example 8 34.0 101.5 354.5 52.0 Comparative Example 9 45.0 35.0 40 10.0 101.5 Comparative Example 10 512.0 56.0 3517.0 252.5 Comparative Example 11 11.0 202.0 202.0 270.1 Comparative Example 12 23.5 351.5 304.0 151.0 Comparative Example 13 34.0 104.5 151.8 52.0 Comparative Example 14 412.5 25.0 40 10.0 101.5 Comparative Example 15 512.0 58.5 3515.0 252.7

[0097] (In Table 2 above, the current ratio relative to the short-circuit current of flash heating was fixed at 55%.)

[0098] Classification Base Material No. Average Carbide Diameter (nm) Carbide Aspect Ratio Presence of η-carbide KAM Value Maximum Value of Goss Fiber Intensity Coarse Grains Heat-Affected Zone Average Austenitic Grains Size (㎛) Invention Example 1 105 1.5 ○ 2.5 10.0 20.0 Invention Example 2 21 30 1.8 ○ 2.1 7.4 26.5 Invention Example 3 31 47 2.2 ○ 1.8 7.2 28.2 Invention Example 4 41 54 2.0 ○ 1.9 6.7 30.2 Invention Example 5 51 78 2.5 ○ 1.6 6.1 32.6 Comparative Example 1 19 41.3 × 2.7 11.3 18.7 Comparative Example 2 21 87 3.3 ○ 1.7 6.9 32.5 Comparative Example 3 31 44 2.1 ○ 2.6 8.6 27.1 Comparative Example 4 42 26 3.5 × 1.4 6.5 33.4 Comparative Example 5 51 82 2.7 ○ 1.5 5.9 33.5 Comparative Example 6 187 1.1×2.9 12.3 17.7 Comparative Example 7 2 181 2.3○2.0 7.1 28.9 Comparative Example 8 3 137 1.9○2.7 8.8 26.5 Comparative Example 9 4 218 3.2×1.6 6.7 32.8 Comparative Example 10 5 180 2.6○1.7 6.3 33.1 Comparative Example 11 19 11.2×2.8 11.9 18.2 Comparative Example 12 21 24 1.6○2.6 9.8 24.3 Comparative Example 13 31 55 2.6○2.2 8.1 27.5 Comparative Example 14 4 237 3.8×1.2 6.2 35.2 Comparative Example 15 5 24 14.3○1.0 5.7 37.3

[0099] Classification Base Material No. Welded Part Thickness (mm) Average Weld Hardness (Hv) Weld Tensile Strength (MPa) Weld Yield Strength (MPa) Weld Elongation (%) Average Crack Length (㎛) Base Material Average Hardness (H1) (Hv) Soft Part Average Hardness (H2) (Hv) H1-H2 Invention Example 1 12.4400 1200 6204.03 106 1059 515 Invention Example 2 23.235 5105 06 104.230 558 054 733 Invention Example 3 34.53159405854.530155047575 Invention Example 4411.03059155624.629853243399 Invention Example 5518.02016034936.7104502392110 Comparative Example 112.441212376363.9317---Comparative Example 223.23299865934.3312---Comparative Example 334.53179525914.4311---Comparative Example 4411.02998975544.7324532419113Comparative Example 5518.01985944916.8115502388114Comparative Example 612.441412436243.7319---Comparative Example 723.234510365984.3313---Comparative Example 834.53209595954.1314---Comparative Example 9411.03019025604.43215324211 11 Comparative Example 10518.01996014966.6117502390112 Comparative Example 1112.441312396303.8318---Comparative Example 1223.235910776143.9315---Comparative Example 1334.53169485894.3312---Comparative Example 14411.02958845464.8321---Comparative Example 15518.01945824886.7334---

[0100] (In Table 4 above, '-' indicates cases where measurement could not be taken because crack defects occurred in the weld joint during or after flash butt welding, preventing production into a final part.)

[0101] Figure 1 is the result of confirming that the unit cell crystal structure of η-carbide, a metastable carbide, matches the unit cell crystal structure of the selected area diffraction (SAD or SAED) pattern by capturing bright field and dark field images of Invention Example 2 using a transmission electron microscope (TEM).

[0102] Figure 2 is the result of confirming that Comparative Example 2 matches the unit cell crystal structure of cementite and metastable carbide η-carbide by capturing a bright field image using a transmission electron microscope (TEM) and analyzing the selected area diffraction (SAD or SAED) pattern.

[0103] Figure 3 is an image showing the optical photograph of the cross-sectional structure after hardness measurement of the flashbutt welded portion including the steel plate base material for Invention Example 2 before post-heat treatment, and the hardness distribution corresponding to the square box area as a color difference.

[0104] Figure 4 is an image showing the Goss Fiber Intensity measured in the width direction (TD) of the flash butt welds of Comparative Example 2 (left) and Invention Example 2 (right) before post-heat treatment.

[0105] As can be seen from FIGS. 1 to 4 and Tables 1 to 4 above, in the case of Invention Examples 1 to 5 satisfying the alloy composition and manufacturing conditions proposed by the present invention, it can be seen that the mechanical properties and hydrogen embrittlement resistance are excellent as a result of securing the average diameter and aspect ratio of carbides distributed in the weld zone proposed by the present invention, the presence or absence of η-carbide in the weld zone, the Kernel Average Misorientation (KAM) value of the weld zone, the maximum value of Goss Fiber Intensity of the weld zone, and the average size of the austenite grains in the heat-affected zone of the coarse grains.

[0106] In the case of Comparative Example 1, which does not satisfy the upset heating current and post-heat treatment time proposed by the present invention, it can be seen that the mechanical properties and hydrogen embrittlement resistance are insufficient because the inclusion of η-carbide in the weld zone, the Kernel Average Misorientation (KAM) value of the weld zone, the maximum Goss Fiber Intensity of the weld zone, and the average size of the austenite grains in the heat-affected zone of the coarse grains are not satisfied.

[0107] In the case of Comparative Example 2, which does not satisfy the post-heat treatment current proposed by the present invention, it can be seen that the resistance to hydrogen embrittlement is insufficient as the average diameter and aspect ratio of the carbides distributed in the weldment are not satisfied. Furthermore, it could not be manufactured into a component.

[0108] In the case of Comparative Example 3, which does not satisfy the flash rate proposed by the present invention, it can be seen that the resistance to hydrogen embrittlement is insufficient as the Kernel Average Misorientation (KAM) value of the weldment is not satisfied. Furthermore, it could not be manufactured into a component.

[0109] In the case of Comparative Example 4, which does not satisfy the electrode travel length during preheating proposed by the present invention, it can be seen that the hydrogen embrittlement resistance is insufficient as it does not satisfy the average diameter and aspect ratio of carbides distributed in the weldment and whether η-carbide is included in the weldment. In addition, it can be seen that the H1-H2 of the part exceeds 110 Hv.

[0110] In the case of Comparative Example 5, which does not satisfy the distance between electrodes during flash heating proposed by the present invention, it can be seen that the average hardness of the weldment is insufficient as the average diameter and aspect ratio of the carbides are not satisfied. In addition, it can be seen that the H1-H2 of the part exceeds 110 Hv.

[0111] In the case of Comparative Example 6, which did not undergo the post-heat treatment proposed by the present invention, it can be seen that the mechanical properties and hydrogen embrittlement resistance are insufficient because the inclusion of η-carbide, the Kernel Average Misorientation (KAM) value of the weld zone, the maximum Goss Fiber Intensity of the weld zone, and the average size of the austenite grains in the heat-affected zone of the coarse grains are not satisfied. Furthermore, it could not be manufactured into a part.

[0112] In the case of Comparative Example 7, which does not satisfy the travel length between electrodes during upset heating proposed by the present invention, it can be seen that the hydrogen embrittlement resistance is insufficient as the average diameter of the carbide is not satisfied. Furthermore, it could not be manufactured into a component.

[0113] In the case of Comparative Example 8, which does not satisfy the electrode travel length during flash heating proposed by the present invention, it can be seen that the hydrogen embrittlement resistance is insufficient as the KAM (Kernel Average Misorientation) value of the weldment is not satisfied. Furthermore, it could not be manufactured into a component.

[0114] In the case of Comparative Example 9, which does not satisfy the flash rate proposed by the present invention, it can be seen that the resistance to hydrogen embrittlement is insufficient as it does not satisfy the average diameter and aspect ratio of carbides distributed in the weldment and the presence of η-carbide in the weldment. In addition, it can be seen that the H1-H2 of the part exceeds 110 Hv.

[0115] In the case of Comparative Example 10, which does not satisfy the electrode travel length during upset heating proposed by the present invention, it can be seen that the average hardness of the weldment is insufficient as the average diameter and aspect ratio of the carbides are not satisfied. Furthermore, it could not be manufactured into a component.

[0116] In the case of Comparative Example 11, which does not satisfy the post-heat treatment current and time proposed by the present invention, it can be seen that the mechanical properties and hydrogen embrittlement resistance are insufficient because the inclusion of η-carbide in the weld zone, the Kernel Average Misorientation (KAM) value of the weld zone, the maximum Goss Fiber Intensity of the weld zone, and the average size of the austenite grains in the heat-affected zone of the coarse grains are not satisfied. Furthermore, it could not be manufactured into a component.

[0117] In the case of Comparative Example 12, which does not satisfy the flash speed and the distance traveled between electrodes during flash heating proposed by the present invention, it can be seen that the resistance to hydrogen embrittlement is insufficient as the Kernel Average Misorientation (KAM) value of the weldment is not satisfied. Furthermore, it could not be manufactured into a component.

[0118] In the case of Comparative Example 13, which does not satisfy the current and the travel length between electrodes during upset heating proposed by the present invention, it can be seen that the resistance to hydrogen embrittlement is insufficient as the aspect ratio of the carbide is not satisfied. Furthermore, it could not be manufactured into a component.

[0119] In the case of Comparative Example 14, which does not satisfy the electrode travel length and flash rate during preheating proposed by the present invention, it can be seen that the hydrogen embrittlement resistance is insufficient as it does not satisfy the average diameter and aspect ratio of the carbides and the inclusion of η-carbide in the weldment. Furthermore, it could not be manufactured into a component.

[0120] In the case of Comparative Example 15, which does not satisfy the electrode travel length and post-heat treatment time during flash heating proposed by the present invention, it can be seen that the average diameter and aspect ratio of the carbides are not satisfied, resulting in insufficient average hardness of the weldment and resistance to hydrogen embrittlement. Furthermore, it could not be manufactured into a component.

Claims

Base material; and It includes a weld obtained by flash butt welding the above base material, and The above base material comprises, in weight percent, C: 0.180~0.40%, Si: 1.0% or less (excluding 0%), Mn: 1.60% or less (excluding 0%), B: 0.010% or less (excluding 0%), and the remainder consists of Fe and other unavoidable impurities. The carbides distributed in the above weldment have an average diameter of 180 nm or less and an aspect ratio of 2.5 or less, and The above weld is a flashbutt weld member with a KAM (Kernel Average Misorientation) value of 2.50 or less. In paragraph 1, The above weld is a flashbutt weld member containing η-carbide, which is a metastable carbide. In paragraph 1, The above weld is a flashbutt weld member in which the maximum value of Goss Fiber Intensity is 10.0 or less. In paragraph 1, The above weldment includes a coarse-grained heat-affected zone formed at the center in the width direction and a fine-grained heat-affected zone formed to surround the coarse-grained heat-affected zone. The above coarse-grained heat-affected zone is a flash-butt welded member in which the average size of the austenite grains is 20㎛ or more. In paragraph 1, The above welded member is a flash butt welded member with a thickness of 2 to 20 mm. In paragraph 1, The above weld is a flash butt weld member having an average hardness of 200 to 400 Hv. In paragraph 1, The above weld is a flash-butt welded member having a tensile strength of 1200 MPa or less and a yield strength of 620 MPa or less. In paragraph 1, The above weld is a flash-butt welded member with an elongation of 4.0% or more. In paragraph 1, The above weld is a flash-butt welded member in which the average length of cracks generated after 4-point bending at 150% or more of the base material yield strength and immersion in a 0.1N HCl solution for 120 hours is 310㎛ or less. A step of preparing a base material comprising, in weight percent, C: 0.180~0.40%, Si: 1.0% or less (excluding 0%), Mn: 1.60% or less (excluding 0%), B: 0.010% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; A step of preheating the welding target surface of the above base material such that the travel length between electrodes is 1.0 to 12.0 mm; A step of flash heating the preheated welding target surface such that the flash speed is 5~25% and the travel length between electrodes is 2.0~6.0mm; A step of upset heating the flash-heated welding target surface by setting the current to 20~40% of the short-circuit current and the travel length between electrodes to 2.0~16.0mm, and forming a weld by flash-butt welding; and A flashbutt welding method comprising the step of cooling the welded portion and then post-heating it for 0.5 to 2.5 seconds with a current of 5 to 25% of the short-circuit current. A base material comprising, in weight%, C: 0.180~0.40%, Si: 1.0% or less (excluding 0%), Mn: 1.60% or less (excluding 0%), B: 0.010% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; and It includes a weld obtained by performing flash butt welding on the above base material followed by post-heat treatment, and The above weldment includes a soft portion formed along the thickness direction, and A part in which the average hardness (H1) of the above base material and the average hardness (H2) of the above soft part satisfy the following relationship 1. [Equation 1] H1-H2 ≤ 110Hv In Paragraph 11, A part in which the carbon content of the above base material and the above soft part satisfies the following relationship 2. [Equation 2] α - β ≤ 0.1 wt% (However, α is the carbon content (weight%) of the base material, and β is the carbon content (weight%) of the soft part.) In Paragraph 11, The above base material is a part having a tensile strength of 1000 to 2000 MPa. In Paragraph 11, The above soft part is a part having a width of 10% or less of the thickness of the base material.

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