Flash butt welding components, flash butt welding methods, and parts

Abstract

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

Description

Technical Field

[0001] This disclosure relates to flash butt welding components, flash butt welding methods, and parts. Background Technology

[0002] Globally, as part of carbon neutrality policies aimed at environmental friendliness, and with the increasing prevalence of eco-friendly vehicles, weight reduction has become particularly important for improving fuel efficiency. Even in the commercial vehicle market, with the recent development of these environmentally friendly policies, the development of lighter wheels for improving fuel efficiency and other aspects continues. Conventionally, 34 kg wheels using 590 MPa grade steel are the main products on the market, but recently, 28 kg wheels using 780 MPa grade steel have been developed and are expected to gradually enter the market and be used more widely.

[0003] Meanwhile, in the case of ultralight wheels used in commercial vehicles, from a driving safety perspective, the thickness of the wheel disc is maintained, and the lightweight design of the components is mainly achieved by reducing the thickness of the wheel rim. Currently, the highest strength steel material mainly used as wheel rim material is 780 MPa grade steel, and it has the following limitation: when using steel materials with a strength higher than 780 MPa, the formability deteriorates.

[0004] Simultaneously, ultralight wheels with a pressure rating of 1.5 GPa can be manufactured by joining base materials of 590 MPa grade steel and then forming the joined base materials into a component shape, followed by final heat treatment. In the joining process, flash butt welding is used to form the joint, and under typical welding thermal cycling conditions, the following limitations exist: because the brittleness of the joint increases due to rapid cooling, the formation of cracks during the component manufacturing process and the susceptibility to hydrogen embrittlement are significantly increased.

[0005] Therefore, there is a need to develop flash butt welded components with excellent formability and resistance to hydrogen embrittlement, as well as components using these flash butt welded components.

[0006] [Related technical documents]

[0007] (Patent Document) Korean Patent Publication No. 10-2178723 Summary of the Invention

[0008] Technical issues

[0009] One aspect of this disclosure is to provide a flash butt welding component, a flash butt welding method, and a part.

[0010] A preferred aspect of this disclosure is to provide a flash butt weld member, flash butt weld method, and component that can achieve excellent formability and resistance to hydrogen embrittlement.

[0011] The purpose of this disclosure is not limited to the foregoing description. Those skilled in the art to which this disclosure pertains will not have difficulty understanding the additional purposes of this disclosure from the entire contents of this specification.

[0012] Technical solution

[0013] According to one aspect of this disclosure, a flash butt welding component includes: a substrate; and a welded portion obtained by flash butt welding the substrate, wherein the substrate contains, by weight, 0.180% to 0.40% C, 1.0% or less (excluding 0%) Si, 1.60% or less (excluding 0%) Mn, 0.010% or less (excluding 0%) B, and the balance Fe and unavoidable impurities, the carbides distributed in the welded portion have an average diameter of 180 nm or less and an aspect ratio of 2.5 or less, and the welded portion has a nucleus average orientation difference (KAM) value of 2.50 or less.

[0014] The welded portion may contain η-carbides, which are metastable carbides.

[0015] The welded portion can have a maximum Goss fiber strength value of 10.0 or less.

[0016] The welded portion may include a coarse-grained heat-affected zone formed at the center in the width direction and a fine-grained heat-affected zone formed around the coarse-grained heat-affected zone, and the coarse-grained heat-affected zone may have an average original austenite grain size of 20 μm or greater.

[0017] Welded components can have a thickness of 2 mm to 20 mm.

[0018] The welded portion can have an average hardness of 200 Hv to 400 Hv.

[0019] The welded portion can have a tensile strength of 1200 MPa or less and a yield strength of 620 MPa or less.

[0020] The welded portion can have an elongation of 4.0% or greater.

[0021] The welded portion may have cracks with an average length of 310 μm or less, which are generated after four-point bending at 150% or greater of the yield strength of the substrate and immersion in 0.1 N HCl solution for 120 hours.

[0022] According to another aspect of this disclosure, a flash butt welding method includes: preparing a substrate comprising, by weight, 0.180% to 0.40% C, 1.0% or less (excluding 0%) Si, 1.60% or less (excluding 0%) Mn, 0.010% or less (excluding 0%) B, and the balance Fe and unavoidable impurities; preheating the surfaces of the substrate to be welded such that the distance between the electrodes is 1.0 mm to 12.0 mm; flash heating the preheated surfaces to be welded such that the flash speed is 5% to 25% and the distance between the electrodes is 2.0 mm to 6.0 mm; and upsetting the flash-heated surfaces to be welded such that the current is 20% to 40% of the short-circuit current and the distance between the electrodes is 2.0 mm to 16.0 mm. mm, and form a welded part by flash butt welding; and after the welded part is cooled, the cooled welded part is post-heated for 0.5 to 2.5 seconds with a current of 5% to 25% of the short-circuit current.

[0023] According to another aspect of this disclosure, a component includes: a substrate comprising, by weight, 0.180% to 0.40% C, 1.0% or less (excluding 0%) Si, 1.60% or less (excluding 0%) Mn, 0.010% or less (excluding 0%) B, and the balance Fe and unavoidable impurities; and a weld portion obtained by flash butt welding the substrate and then subjecting the flash butt welded substrate to a post-heat treatment, wherein the weld portion includes a soft portion formed in the thickness direction, and the average hardness (H1) of the substrate and the average hardness (H2) of the soft portion satisfy the following relational expression 1.

[0024] [Relational Expression 1] H1-H2 ≤ 110 Hv

[0025] The carbon content of the substrate and the carbon content of the soft part can satisfy the following relationship expression 2.

[0026] [Relational Expression 2] α - β ≤ 0.1 wt%

[0027] (Where α is the carbon content of the substrate (wt%), and β is the carbon content of the soft portion (wt%).)

[0028] The substrate can have a tensile strength of 1000 MPa to 2000 MPa.

[0029] The width of the soft portion can be 10% or less of the thickness of the substrate.

[0030] Beneficial effects

[0031] According to one aspect of this disclosure, a flash butt welding component, a flash butt welding method, and a part can be provided.

[0032] According to a preferred aspect of this disclosure, a flash butt welding component, flash butt welding method, and part can be provided that can achieve excellent formability and resistance to hydrogen embrittlement. Attached Figure Description

[0033] Figure 1 The results are shown by taking bright-field and dark-field images of Example 2 of the invention using a transmission electron microscope (TEM) and confirming by selected area diffraction (SAD or SAED) pattern analysis that the cell crystal structure is consistent with the cell crystal structure of η-carbide as a metastable carbide.

[0034] Figure 2 The results are shown by taking bright-field images of Comparative Example 2 using transmission electron microscopy (TEM) and confirming that the cell crystal structure is consistent with the cell crystal structure of cementite and η-carbide as metastable carbides by selected area diffraction (SAD or SAED) pattern analysis.

[0035] Figure 3 The image shown is as follows: This image, for Example 2 of the invention prior to post-heat treatment, shows a cross-sectional optical micrograph of the flash butt weld portion of the steel plate substrate after hardness measurement, and the hardness distribution corresponding to the rectangular frame region, represented by color difference.

[0036] Figure 4 The image below shows the Gaussian fiber strength measured in the transverse direction (TD) of the flash butt weld portions of Comparative Example 2 (left) and Invention Example 2 (right) before post-heat treatment. Detailed Implementation

[0037] The best mode of the invention

[0038] Preferred embodiments of this disclosure will be described below. However, the embodiments of this disclosure can be modified in various other ways, and the scope of this disclosure is not limited to the embodiments described below.

[0039] In addition, embodiments of this disclosure are provided to describe this disclosure more fully to those skilled in the art.

[0040] In describing embodiments of this disclosure, detailed descriptions of known technologies related to this disclosure will be omitted where it is determined that such detailed descriptions may unnecessarily obscure the main points of this disclosure. Furthermore, the terminology described below is defined in consideration of the functionality of this disclosure, and these terms may vary depending on the intent or habit of the user or operator. Therefore, the definitions of these terms should be understood based on the entirety of this specification. The terminology used in the detailed description is for the purpose of describing embodiments of this disclosure only and should not be construed as limiting. Unless explicitly stated otherwise, singular expressions include plural expressions.

[0041] In this specification, expressions such as “comprising” and “including” are intended to refer to certain features, numbers, steps, operations, elements, parts thereof, or combinations thereof, and should not be construed as excluding the presence or possibility of one or more other features, numbers, steps, operations, elements, parts thereof, or combinations thereof in addition to the described features, numbers, steps, operations, elements, parts thereof, or combinations thereof.

[0042] Unless otherwise specifically defined in this disclosure, % means wt%.

[0043] The present disclosure will be described in detail below through each embodiment or example thereof. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, and may be combined with other embodiments or examples. Therefore, references to claims in the claims are merely examples of embodiments, and the technical concept of the present disclosure should not be construed as being limited only to combinations with the referenced claims, and combinations with various claims are also included within the scope of the technical concept of the present disclosure.

[0044] In the following, a flash butt welding component according to an embodiment of the present disclosure will be described.

[0045] The flash butt welding member according to the embodiments of this disclosure includes a substrate and a welded portion obtained by flash butt welding the substrate. In this disclosure, the form of the welded member is not particularly limited; however, as an example, the welded member may be in the form of a steel pipe or a rectangular pipe.

[0046] The substrate may contain, by wt%, 0.180% to 0.40% C, 1.0% or less (excluding 0%) Si, 1.60% or less (excluding 0%) Mn, 0.010% or less (excluding 0%) B, and the balance Fe and unavoidable impurities.

[0047] C: 0.180% to 0.40%

[0048] Carbon (C) is an essential element for ensuring the strength of martensite and is necessary to control the strength of steel plates because it increases the strength of heat-treated components and improves hardenability. When the C content is less than 0.180%, insufficient hardenability at reduced cooling rates prevents the formation of sufficient martensite, potentially leading to ferrite structures, which in turn reduces the strength and impact resistance of components made from the steel plate. When the C content exceeds 0.40%, the impact toughness of the slab may decrease, the strength of heat-treated components may become excessive, and weldability may deteriorate. Therefore, the C content is advantageously in the range of 0.180% to 0.40%. The lower limit of the C content is more advantageously 0.190%, even more advantageously 0.20%, and most advantageously 0.210%. The upper limit of the C content is more advantageously 0.39%, even more advantageously 0.38%, and most advantageously 0.37%.

[0049] Si: 1.0% or less (excluding 0%)

[0050] The addition of Si—a solid solution strengthening element and a carbide formation inhibitor—serves not only as a deoxidizer in steelmaking but also as a means to effectively homogenize the internal structure of steel plates and to increase the strength of heat-treated components. When the Si content exceeds 1.00%, excessive Si-based oxides may form on the surface of the steel plate, leading to deterioration in surface quality and properties. Therefore, the Si content is advantageously in the range of 1.0% or less. More advantageously, the Si content is 0.90% or less, even more advantageously, 0.80% or less, and most advantageously, 0.70% or less. Meanwhile, because Si can be sufficiently effective even in trace amounts, the lower limit for Si is not particularly limited. However, by way of example, the lower limit for Si content can be 0.10%.

[0051] Manganese: 1.60% or less (excluding 0%)

[0052] Mn is added not only to ensure the required strength due to the solid solution strengthening effect, but also to suppress unwanted ferrite or bainite transformations during heat treatment. When the Mn content exceeds 1.60%, the manufacturing cost of the steel sheet increases, and the banded structure aligned in the rolling direction develops excessively in the microstructure, leading to inhomogeneity in the internal structure of the steel sheet and deteriorating its impact resistance. Therefore, the Mn content is advantageously in the range of 1.60% or less. More advantageously, the Mn content is 1.50% or less, even more advantageously, 1.40% or less, and most advantageously, 1.30% or less. Meanwhile, because Mn can have sufficient effect even in trace amounts, the lower limit of Mn content is not particularly limited. However, as an example, the lower limit of Mn content can be 0.10%.

[0053] B: 0.010% or less (excluding 0%)

[0054] Boron (B) is an element added to suppress ferrite formation in heat-treated components, improve hardenability even in small amounts, and suppress brittleness caused by grain boundary segregation of P or S by segregation at the original austenite grain boundaries. When the B content exceeds 0.010%, the effect of B may become saturated, and B may segregate at grain boundaries and lead to brittleness, thus significantly deteriorating the hot-rolling properties of the slab. Therefore, the B content is advantageously in the range of 0.010% or less. More advantageously, the B content is 0.0080% or less, even more advantageously, 0.0060% or less, and most advantageously, 0.0040% or less. Meanwhile, because B can have sufficient effect even in trace amounts, the lower limit of B is not particularly limited. However, by way of example, the lower limit of the B content can be 0.0010%.

[0055] The remaining component is iron (Fe). However, in typical manufacturing processes, undesirable impurities may inevitably be introduced from raw materials or the surrounding environment, and therefore cannot be eliminated. Since these impurities are readily understood by those skilled in the art during typical manufacturing processes, not all details of these impurities are specifically mentioned in this specification.

[0056] The carbides distributed in the welded portion can have an average diameter of 180 nm or less and an aspect ratio of 2.5 or less. When the average diameter of the carbides exceeds 180 nm, the toughness of the joint decreases, making the joint susceptible to forming cracks, and the resistance to hydrogen embrittlement is also relatively poor due to the reduced toughness of the joint. The average diameter of the carbides is advantageously 165 nm or less, more advantageously 145 nm or less, and most advantageously 125 nm or less. Because a smaller average diameter of the carbides is more advantageous, the lower limit of the carbides is not particularly limited in this disclosure. However, as an example, the lower limit of the average diameter of the carbides can be 50 nm. When the aspect ratio of the carbides exceeds 2.5, as mentioned above, the toughness of the joint decreases, making the joint susceptible to forming cracks, and the resistance to hydrogen embrittlement is also relatively poor due to the reduced toughness of the joint. This is because the carbides distributed in the joint have a shape that makes stress relatively difficult to disperse during the forming process, thereby making forming cracks more likely to occur at locations of localized stress concentration. In fact, when the fractured surface of the joint, in which shaped cracks have already formed, is carefully observed at high magnification using a scanning electron microscope (SEM), it can be confirmed that numerous cracks occur at the interface between the needle-like cementite and the matrix, which has a relatively large aspect ratio. The aspect ratio of the carbide is advantageously 2.4 or less, more advantageously 2.3 or less, and most advantageously 2.2 or less. Because a lower aspect ratio of the carbide is more advantageous, the lower limit of the carbide is not particularly limited in this disclosure. However, as an example, the lower limit of the aspect ratio of the carbide can be 1.0. Meanwhile, the type of carbide is not particularly limited in this disclosure; however, as an example, the carbide can be cementite.

[0057] The welded portion can have a nucleus average orientation difference (KAM) value of 2.50 or less. When the KAM value exceeds 2.50, the dislocation density in the microstructure of the joint is relatively high, leading to a deterioration in the formability of the joint. Additionally, because dislocations do not trap hydrogen as strongly as fine carbide precipitates, and therefore hydrogen may move more easily, the resistance to hydrogen embrittlement of the joint may deteriorate. Advantageously, the KAM value is 2.4 or less, more advantageously 2.3 or less, and most advantageously 2.2 or less. Because lower KAM values ​​are more advantageous, the lower limit of the KAM value is not particularly limited in this disclosure. However, as an example, the lower limit of the KAM value can be 0.5. Meanwhile, the dislocation density in the grain can be evaluated by the nucleus average orientation difference (KAM) value. The KAM value refers to the average amount of crystal rotation (crystal orientation difference) between the target measurement point and the measurement points around that target measurement point, and a larger value indicates more deformation and a higher dislocation density in the crystal.

[0058] The welded portion may contain η-carbides, which are metastable carbides. By including η-carbides in the form of fine spheres compared to ordinary cementite, stress can be more effectively dispersed during the formation process and the matrix can be strengthened, thereby improving the toughness of the joint and thus its resistance to hydrogen embrittlement.

[0059] The welded portion may have a maximum Gaussian fiber strength value of 10.0 or less. When the maximum Gaussian fiber strength value exceeds 10.0, a texture with orientation unfavorable to formability may be formed, leading to a deterioration in the formability of the joint. The maximum Gaussian fiber strength value is advantageously 9.9 or less, more advantageously 9.8 or less, and most advantageously 9.7 or less. Because a smaller maximum Gaussian fiber strength value is more advantageous, the lower limit of the maximum Gaussian fiber strength value is not particularly limited in this disclosure. However, as an example, the lower limit of the maximum Gaussian fiber strength value can be 2.50. Meanwhile, compared to the case of no orientation in the texture, Gaussian fiber strength refers to the relative volume fraction of grains with a specific Euler angle (90°, 90°, 45°) orientation in the orientation distribution function (ODF)—the orientation distribution function of the texture. For example, when this value is 2.0, it means that the Gaussian fiber texture has developed to twice the extent described above in the case of no orientation in the texture.

[0060] The welded portion may include a coarse-grained heat-affected zone (CGHAZ) formed at its central portion in the width direction and a fine-grained heat-affected zone (FGHAZ) formed surrounding the coarse-grained heat-affected zone, and the coarse-grained heat-affected zone may have an average original austenite grain size of 20 μm or greater. When the average original austenite grain size of the coarse-grained heat-affected zone is less than 20 μm, the toughness of the joint may be relatively insufficient and the brittleness may increase, resulting in a deterioration in formability. In addition, as the grain size decreases, the number of boundaries within the grains where hydrogen may be trapped decreases relatively, resulting in a deterioration in the joint's resistance to hydrogen embrittlement. The average original austenite grain size of the coarse-grained heat-affected zone is advantageously 22 μm or greater, more advantageously 24 μm or greater, and even more advantageously 26 μm or greater. Because a larger average original austenite grain size of the coarse-grained heat-affected zone is more advantageous, the upper limit of the average original austenite grain size is not particularly limited in this disclosure. However, as an example, the upper limit of the average original austenite grain size in the coarse-grained heat-affected zone can be 40 μm.

[0061] The flash butt weld members disclosed herein can have a thickness from 2 mm to 20 mm. When the thickness of the weld member is less than 2 mm, it may be difficult to ensure sufficient strength and durability due to excessive thinning of the heat-treated member. When the thickness of the weld member exceeds 20 mm, it may be difficult to ensure sufficient hardenability of the heat-treated member. The lower limit of the thickness of the weld member is advantageously 2.2 mm, more advantageously 2.6 mm, and most advantageously 3.0 mm. The upper limit of the thickness of the weld member is advantageously 18 mm, more advantageously 16 mm, and most advantageously 10 mm.

[0062] The welded portion of the flash butt welded component of this disclosure can have an average hardness of 200 Hv to 400 Hv. The welded portion can have a tensile strength of 1200 MPa or less and a yield strength of 620 MPa or less. The welded portion can have an elongation of 4.0% or greater. In this disclosure, the lower limit of the tensile strength is not particularly limited; however, as an example, the lower limit can be 700 MPa. In this disclosure, the lower limit of the yield strength is not particularly limited; however, as an example, the lower limit can be 500 MPa. In this disclosure, the upper limit of the elongation is not particularly limited; however, as an example, the upper limit can be 10%.

[0063] The welded portion may have cracks with an average length of 310 μm or less, which are generated after four-point bending at 150% or greater of the yield strength of the substrate and immersion in a 0.1 N HCl solution for 120 hours. In this disclosure, there is no particular limitation on the lower limit of the average crack length; however, by way of example, the lower limit may be 10 μm.

[0064] The flash butt welding method according to embodiments of the present disclosure will be described below.

[0065] First, a substrate satisfying the above alloy composition is prepared. In this disclosure, the preparation process of the substrate is not particularly limited, and any method used in the art can be employed.

[0066] Subsequently, the surfaces of the substrate to be welded are preheated. Preheating can be controlled such that the distance between the electrodes is between 1.0 mm and 12.0 mm. When the distance between the electrodes is less than 1.0 mm during preheating, the preheating of the steel substrate may be insufficient, increasing the brittleness of the joint due to rapid cooling. When the distance between the electrodes exceeds 5.0 mm during preheating, the preheating of the steel substrate may be overheated, potentially deteriorating the performance of the joint due to grain growth in the joint's microstructure. The lower limit of the distance between the electrodes during preheating is advantageously 1.2 mm, more advantageously 1.6 mm, and most advantageously 1.8 mm. The upper limit of the distance between the electrodes during preheating is advantageously 11.5 mm, more advantageously 11.0 mm, and most advantageously 10.5 mm.

[0067] Subsequently, the preheated surface to be welded undergoes flash heating. Flash heating can be controlled such that the flash rate is 5% to 25% and the distance between the electrodes is 2.0 mm to 6.0 mm. When the flash rate is less than 5%, the flash heating rate in the flash section may be too slow, resulting in excessive heat being input to the joint. When the flash rate exceeds 25%, the flash heating rate in the flash section may become too fast, resulting in insufficient heat being input to the joint. The lower limit of the flash rate is advantageously 6%, more advantageously 8%, and most advantageously 10%. The upper limit of the flash rate is advantageously 24%, more advantageously 22%, and most advantageously 20%. Meanwhile, flash rate refers to the rate at which the gap between the electrodes decreases per second relative to the initial gap between the electrodes in the flash section. When the distance between the electrodes during flash heating is less than 2.0 mm, the flash heating of the steel substrate may be insufficient, resulting in insufficient strength of the joint. When the movement distance between electrodes exceeds 6.0 mm during flash heating, the flash heating of the steel substrate may be excessive, potentially degrading the performance of the joint due to grain growth in the joint's microstructure. The lower limit of the movement distance between electrodes during flash heating is advantageously 2.2 mm, more advantageously 2.4 mm, and most advantageously 2.6 mm. The upper limit of the movement distance between electrodes during flash heating is advantageously 5.8 mm, more advantageously 5.6 mm, and most advantageously 5.4 mm.

[0068] Subsequently, the flash-heated surfaces to be welded are subjected to upsetting heating, and flash butt welding is performed to form a welded portion. Upsetting heating can be performed with a current equal to 20% to 40% of the short-circuit current, and can be controlled such that the movement distance between the electrodes is 2.0 mm to 16.0 mm. When the current during upsetting heating is less than 20% of the short-circuit current, the joint may be relatively sensitive to rapid cooling, which may prevent the molten portion from being maximized, and the brittleness of the joint may increase. When the current during upsetting heating exceeds 40% of the short-circuit current, the cooling rate of the joint will increase excessively, causing the performance of the joint to deteriorate due to grain growth in the microstructure of the joint. The lower limit of the current during upsetting heating is advantageously 22% of the short-circuit current, more advantageously 24% of the short-circuit current, and most advantageously 26% of the short-circuit current. The upper limit of the current during upsetting heating is advantageously 38% of the short-circuit current, more advantageously 36% of the short-circuit current, and most advantageously 34% of the short-circuit current. When the movement distance between the electrodes during upsetting heating is less than 2.0 mm, the upsetting heating and upsetting amount may be insufficient, resulting in insufficient strength of the joint. When the movement distance between the electrodes during upsetting heating exceeds 16.0 mm, the upsetting heating and upsetting amount may be excessive, resulting in deterioration of the formability of the joint. The lower limit of the movement distance between the electrodes during upsetting heating is advantageously 2.2 mm, more advantageously 2.4 mm, and most advantageously 2.6 mm. The upper limit of the movement distance between the electrodes during upsetting heating is advantageously 15.8 mm, more advantageously 15.6 mm, and most advantageously 15.4 mm.

[0069] After the welded portion has cooled, it is then subjected to post-heating. Post-heating can be performed for 0.5 to 2.5 seconds using a current of 5% to 25% of the short-circuit current. When the current during post-heating is less than 5% of the short-circuit current, the amount of post-heating applied to the joint may be insufficient, resulting in insufficient reduction of the joint's brittleness. When the current during post-heating exceeds 25% of the short-circuit current, the amount of post-heating applied to the joint may be excessive, resulting in insufficient joint strength due to softening by post-heating. The lower limit of the current during post-heating is advantageously 6% of the short-circuit current, more advantageously 8%, and most advantageously 10%. The upper limit of the current during post-heating is advantageously 24% of the short-circuit current, more advantageously 22%, and most advantageously 20%. When the post-heating period is less than 0.5 seconds, the effect of post-heating may be insufficient, resulting in insufficient improvement of the joint's toughness. When the post-heating period exceeds 2.5 seconds, overheating may occur, potentially leading to insufficient strength or increased brittleness of the joint. The lower limit for the post-heating period is advantageously 0.6 seconds, more advantageously 0.7 seconds, and most advantageously 0.8 seconds. The upper limit for the post-heating period is advantageously 2.4 seconds, more advantageously 2.2 seconds, and most advantageously 2.0 seconds.

[0070] In the following, a component according to an embodiment of the present disclosure will be described. While not particularly limited in the present disclosure, by way of example, the component can be manufactured using a welded member having the features described above.

[0071] A component according to an embodiment of the present disclosure includes: a substrate comprising, by weight, 0.180% to 0.40% C, 0.60% or less (excluding 0%) Si, 1.40% or less (excluding 0%) Mn, 0.010% or less (excluding 0%) B, and the balance being Fe and unavoidable impurities; and a welded portion obtained by subjecting the substrate to a post-heat treatment after flash butt welding. The post-heat treatment differs from the post-heating in the flash butt welding method described above.

[0072] The welded portion may include a soft portion formed in the thickness direction. The soft portion is the region adjacent to the molten portion formed during the manufacture of the welded component. The molten portion is discharged during the upsetting process in a state with relatively high carbon solubility. At this time, as carbon escapes from the region adjacent to the molten portion into the molten portion, the carbon content may become relatively low, and ultimately, the hardenability may be insufficient compared to the base material. Furthermore, the durability and impact resistance of parts manufactured from the welded component—a soft portion with low hardenability—may also deteriorate.

[0073] However, according to the manufacturing process of this disclosure, the average hardness (H1) of the substrate and the average hardness (H2) of the soft portion can satisfy the following relationship expression 1. H1-H2 is advantageously 108 Hv or lower, more advantageously 104 Hv or lower, and most advantageously 100 Hv or lower. Because a smaller H1-H2 is more advantageous, the lower limit of H1-H2 is not particularly limited in this disclosure. However, as an example, the lower limit of H1-H2 can be 10 Hv. In addition, H1 may be greater than H2.

[0074] [Relational Expression 1] H1-H2 ≤ 110 Hv

[0075] In other words, by controlling the average hardness difference between the substrate and the soft part to a specific value or less, it is possible to ensure that the hardenability, durability and impact resistance are at a level comparable to the hardenability, durability and impact resistance of the substrate.

[0076] The carbon content of the substrate and the carbon content of the soft part can satisfy the following relationship expression 2. Therefore, by controlling the difference in carbon content between the substrate and the soft part to be small, the average hardness difference between the substrate and the soft part can be reduced.

[0077] [Relational Expression 2] α - β ≤ 0.1 wt%

[0078] (Where α is the carbon content of the substrate (wt%), and β is the carbon content of the soft portion (wt%).)

[0079] Meanwhile, although not particularly limited in this disclosure, by way of example, the width of the soft portion may be 10% or less of the substrate thickness and may include bonding lines.

[0080] The component can be a wheel rim, but is not limited to this.

[0081] The components are manufactured by subjecting the welded parts to post-heat treatment, and the post-heat treatment process may include normalizing, austenitizing, quenching and tempering steps.

[0082] The normalizing step can be performed at 850°C to 950°C for 5 to 7 minutes.

[0083] The austenitizing step can be performed at 850°C to 950°C for 5 to 7 minutes.

[0084] The quenching process can be performed at 200°C or lower.

[0085] The tempering process can be performed at 150°C to 600°C for 30 to 60 minutes.

[0086] In components that have undergone post-heat treatment, the tensile strength of the substrate can range from 1000 MPa to 2000 MPa, and this tensile strength can be increased compared to that of components that have undergone flash butt bonding. Therefore, thinning and weight reduction of the components are possible.

[0087] The mode of the present invention

[0088] The present disclosure will be described in detail below with reference to examples. However, it should be noted that the examples described below are for illustration purposes only and are not intended to limit the scope of the disclosure. This is because the scope of the disclosure is determined by the content listed in the claims and the content reasonably inferred from the claims.

[0089] (Example)

[0090] After preparing a substrate with alloy composition as shown in Table 1, flash butt welded components were manufactured by subjecting the substrate to preheating, flash heating, upsetting heating / flash butt welding, and cooling / post-heating under the conditions shown in Table 2. For the thus manufactured welded components, the following were measured: the average diameter and aspect ratio of carbides distributed in the weld portion, the presence of η-carbides in the weld portion, the nucleus average orientation difference (KAM) value of the weld portion, the maximum Gaussian fiber strength value of the weld portion, the average original austenite grain size of the coarse-grained heat-affected zone, mechanical properties, resistance to hydrogen embrittlement, etc. The results are shown in Tables 3 and 4. In addition, the manufactured welded components were formed into part shapes, and then subjected to post-heat treatment to manufacture parts, and the average hardness of the substrate and the average hardness of the soft portion were measured. The results are shown in Table 4.

[0091] The average diameter and aspect ratio of the carbides distributed in the welded portion were measured as follows. The welded portion was prepared as a foil sample using an electropolishing apparatus (jet polisher), and the presence of carbides was confirmed by selected area diffraction (SAD or SAED) pattern analysis using transmission electron microscopy (TEM). Through image analysis, the major and minor axes of each carbide distributed in the TEM image were measured, and the average value of the major and minor axes was determined as the diameter of the carbide. The average diameter of all carbides was then calculated. Furthermore, the ratio of the major to minor axis of each of the aforementioned carbides was determined as the aspect ratio, and the average aspect ratio of all carbides was calculated.

[0092] The presence of η-carbide in the welded portion was confirmed by the following method: bright-field and dark-field images were captured using a transmission electron microscope (TEM) as described above, and the cell structure was determined to be consistent with the cell structure of η-carbide by selected area diffraction (SAD or SAED) pattern analysis.

[0093] The nucleus average orientation difference (KAM) value of the welded portion is defined as the average difference between the orientation at a given point and the orientation around that point, and the KAM value of the welded portion is measured using electron backscatter diffraction (EBSD). Here, a point with a higher KAM value indicates a higher dislocation density due to deformation, and in particular, the density of geometrically necessary dislocations (GNDs) is proportional to the KAM value.

[0094] The maximum Gaussian fiber strength of the welded portion was measured by electron backscatter diffraction (EBSD) and defined as the relative volume fraction of grains with specific Euler angle (90°, 90°, 45°) orientations in the orientation distribution function (ODF)—the orientation distribution function of the texture. The values ​​were measured in both the transverse (TD) and normal (ND) directions, and the maximum value was taken.

[0095] The average original austenite grain size of the coarse-grained heat-affected zone of the welded portion was calculated as follows: the microstructure etched with nitric acid solution was observed using a scanning electron microscope (SEM) to distinguish grain boundaries; the long axis and short axis of each grain were measured by image analysis and the average value of the long axis and short axis was obtained; and then the average size of all grains was calculated from this.

[0096] For welded components, the hardness of each of the base material and the welded part is determined by measuring Vickers hardness at 0.2 mm intervals under a load of 300 gf; obtaining 1000 hardness values; and calculating the average of the 1000 hardness values.

[0097] Specifically, when measuring the hardness of the substrate and welded parts of the component, the soft parts distributed at the component joint line are distinguished from the substrate by observing the microstructure etched with nitric acid solution using a scanning electron microscope (SEM). Then, under a load of 200 gf, at least 10 and at most 50 hardness values ​​are measured along the thickness direction of the substrate at intervals of 0.2 mm over the entire length of the joint. The average hardness value is then calculated and used as the average hardness (H1) of the substrate and the average hardness (H2) of the soft parts of the heat-treated component, respectively.

[0098] After taking a tensile sample with a width of 1.6 mm, a length of 20 mm, and a thickness of 0.5 mm from the center of the welded part, the tensile strength, yield strength, and elongation of the welded part were measured by tensile testing.

[0099] The resistance to hydrogen embrittlement of the welded portion was measured as follows: a sample with a width of 30 mm and a length of 180 mm was collected, with the welded portion located at the center, and the sample included the substrate; a four-point bend was performed at 150% or greater of the yield strength of the substrate; the sample was immersed in a 0.1 N HCl solution for 120 hours; the distribution of cracks generated thereafter was measured using computed tomography (CT); and the average crack lengths distributed in the welded portion were then calculated and compared.

[0100] [Table 1]

[0101]

[0102] [Table 2]

[0103]

[0104] (In Table 2, the ratio of flash heating current to short-circuit current is fixed at around 55%.)

[0105] [Table 3]

[0106]

[0107] [Table 4]

[0108]

[0109] (In Table 4, "-" indicates the following situation: in which crack defects occur in the welded part during forming after flash butt welding or after forming, and therefore the final part cannot be manufactured and measurements cannot be performed.)

[0110] Figure 1 The results are shown by taking bright-field and dark-field images of Example 2 of the invention using a transmission electron microscope (TEM) and confirming by selected area diffraction (SAD or SAED) pattern analysis that the cell crystal structure is consistent with the cell crystal structure of η-carbide as a metastable carbide.

[0111] Figure 2 The results are shown by taking bright-field images of Comparative Example 2 using transmission electron microscopy (TEM) and confirming that the cell crystal structure is consistent with the cell crystal structure of cementite and η-carbide as metastable carbides by selected area diffraction (SAD or SAED) pattern analysis.

[0112] Figure 3The image shown is as follows: This image, for Example 2 of the invention prior to post-heat treatment, shows a cross-sectional optical micrograph of the flash butt weld portion of the steel plate substrate after hardness measurement, and the hardness distribution corresponding to the rectangular frame region, represented by color difference.

[0113] Figure 4 The image below shows the Gaussian fiber strength measured in the transverse direction (TD) of the flash butt weld portions of Comparative Example 2 (left) and Invention Example 2 (right) before post-heat treatment.

[0114] As from Figures 1 to 4 As can be seen from Tables 1 to 4, in Invention Examples 1 to 5, which satisfy the alloy composition and manufacturing conditions proposed in this disclosure, the following features proposed in this disclosure are achieved: the average diameter and aspect ratio of carbides distributed in the welded portion, the presence of η-carbides in the welded portion, the nucleus average orientation difference (KAM) value of the welded portion, the maximum Gaussian fiber strength value of the welded portion, and the average original austenite grain size of the coarse-grained heat-affected zone. As a result, excellent mechanical properties and resistance to hydrogen embrittlement are exhibited.

[0115] In Comparative Example 1—which does not meet the upsetting heating current and post-heating time requirements proposed in this disclosure—it was found that the mechanical properties and resistance to hydrogen embrittlement were insufficient. This is because the presence of η-carbides in the weld portion, the nucleus average orientation difference (KAM) value of the weld portion, the maximum Gaussian fiber strength value of the weld portion, and the average original austenite grain size of the coarse-grained heat-affected zone do not meet the requirements proposed in this disclosure.

[0116] In Comparative Example 2—which does not meet the post-heating current requirements of this disclosure—hydrogen embrittlement resistance was found to be insufficient because the average diameter and aspect ratio of the carbides distributed in the weld portion did not meet the average diameter and aspect ratio of the carbides proposed in this disclosure. Furthermore, it was impossible to manufacture the component.

[0117] In Comparative Example 3—which does not meet the flash speed requirements of this disclosure—hydrogen embrittlement resistance was found to be insufficient because the nucleus average orientation difference (KAM) value of the weld portion does not meet the KAM value of the weld portion as specified in this disclosure. Furthermore, it is impossible to manufacture the component.

[0118] In Comparative Example 4—which does not meet the requirements for the movement distance between electrodes during preheating as specified in this disclosure—hydrogen embrittlement resistance was found to be insufficient. This is because the average diameter and aspect ratio of the carbides distributed in the weld portion, as well as the presence of η-carbides in the weld portion, do not meet the requirements for the average diameter and aspect ratio of the carbides distributed in the weld portion, as specified in this disclosure, and the presence of η-carbides in the weld portion. Furthermore, the H1-H2 of the component was found to exceed 110 Hv.

[0119] In Comparative Example 5—which does not meet the required movement distance between electrodes during flash heating as described in this disclosure—the average hardness of the welded portion was found to be insufficient because the average diameter and aspect ratio of the carbides did not meet the requirements for the average diameter and aspect ratio of the carbides as described in this disclosure. Additionally, the H1-H2 of the component was found to exceed 110 Hv.

[0120] In Comparative Example 6—without the post-heat treatment proposed in this disclosure—the mechanical properties and resistance to hydrogen embrittlement were found to be insufficient. This is because the presence of η-carbides, the nucleus average orientation difference (KAM) value of the weld portion, the maximum Gaussian fiber strength value of the weld portion, and the average original austenite grain size of the coarse-grained heat-affected zone do not meet the requirements proposed in this disclosure. Furthermore, it is impossible to manufacture the component.

[0121] In Comparative Example 7—which does not meet the required movement distance between electrodes during upsetting heating as described in this disclosure—hydrogen embrittlement resistance was found to be insufficient because the average diameter of the carbides did not meet the requirements of the average diameter of the carbides described in this disclosure. Furthermore, it was impossible to manufacture the component.

[0122] In Comparative Example 8—which does not meet the required movement distance between electrodes during flash heating as specified in this disclosure—hydrogen embrittlement resistance was found to be insufficient because the nucleus average orientation difference (KAM) value of the weld portion did not meet the KAM value of the weld portion specified in this disclosure. Furthermore, it was impossible to manufacture the component.

[0123] In Comparative Example 9—which does not meet the flash speed requirements outlined in this disclosure—hydrogen embrittlement resistance was found to be insufficient. This is because the average diameter and aspect ratio of the carbides distributed in the weld portion, as well as the presence of η-carbides in the weld portion, do not meet the requirements outlined in this disclosure. Furthermore, the component's H1-H2 values ​​were found to exceed 110 Hv.

[0124] In Comparative Example 10—which does not meet the required movement distance between electrodes during upsetting heating as described in this disclosure—the average hardness of the welded portion was found to be insufficient because the average diameter and aspect ratio of the carbides did not meet the requirements for the average diameter and aspect ratio of the carbides as described in this disclosure. Furthermore, it is impossible to manufacture the part.

[0125] In Comparative Example 11—which does not meet the current and time requirements for post-heat treatment as proposed in this disclosure—it was found that the mechanical properties and resistance to hydrogen embrittlement were insufficient. This is because the presence of η-carbides in the weld portion, the nucleus average orientation difference (KAM) value of the weld portion, the maximum Gaussian fiber strength value of the weld portion, and the average original austenite grain size of the coarse-grained heat-affected zone do not meet the requirements proposed in this disclosure. Furthermore, it is impossible to manufacture the component.

[0126] In Comparative Example 12—which does not meet the flash speed and electrode movement distance during flash heating as specified in this disclosure—hydrogen embrittlement resistance was found to be insufficient because the nucleus average orientation difference (KAM) value of the weld portion does not meet the KAM value of the weld portion specified in this disclosure. Furthermore, it is impossible to manufacture the component.

[0127] In Comparative Example 13—which does not meet the requirements for current and electrode movement distance during upsetting heating as specified in this disclosure—it was found that the resistance to hydrogen embrittlement was insufficient because the aspect ratio of the carbide did not meet the requirements for the aspect ratio of the carbide as specified in this disclosure. Furthermore, it was impossible to manufacture the component.

[0128] In Comparative Example 14—which does not meet the requirements of the inter-electrode movement distance and flash speed during preheating as specified in this disclosure—hydrogen embrittlement resistance was found to be insufficient. This is because the average diameter and aspect ratio of the carbides, as well as the presence of η-carbides in the weld portion, do not meet the requirements of the average diameter and aspect ratio of the carbides, as well as the presence of η-carbides in the weld portion, as specified in this disclosure. Furthermore, it is impossible to manufacture the component.

[0129] In Comparative Example 15—which does not meet the requirements of the movement distance between electrodes during flash heating and the post-heat treatment time as proposed in this disclosure—the average hardness and resistance to hydrogen embrittlement of the welded portion were found to be insufficient. This is because the average diameter and aspect ratio of the carbides do not meet the requirements of the average diameter and aspect ratio of the carbides as proposed in this disclosure. Furthermore, it is impossible to manufacture the component.

Claims

1. A flash butt welding component, comprising: Substrate; as well as The welded portion is obtained by flash butt welding the substrate. The substrate comprises, by weight, 0.180% to 0.40% C, 1.0% or less (excluding 0%) Si, 1.60% or less (excluding 0%) Mn, 0.010% or less (excluding 0%) B, and the balance being Fe and unavoidable impurities. The carbides distributed in the welded portion have an average diameter of 180 nm or less and an aspect ratio of 2.5 or less, and The welded portion has a nuclear average orientation difference (KAM) value of 2.50 or less.

2. The flash butt welding component according to claim 1, wherein, The welded portion contains η-carbides, which are metastable carbides.

3. The flash butt welding component according to claim 1, wherein, The welded portion has a maximum Gaussian fiber strength value of 10.0 or less.

4. The flash butt welding component according to claim 1, wherein, The welded portion includes a coarse-grained heat-affected zone formed at its center in the width direction and a fine-grained heat-affected zone formed around the coarse-grained heat-affected zone. The coarse-grained heat-affected zone has an average original austenite grain size of 20 μm or larger.

5. The flash butt welding component according to claim 1, wherein, The welded component has a thickness of 2 mm to 20 mm.

6. The flash butt welding component according to claim 1, wherein, The welded portion has an average hardness of 200 Hv to 400 Hv.

7. The flash butt welding component according to claim 1, wherein, The welded portion has a tensile strength of 1200 MPa or less and a yield strength of 620 MPa or less.

8. The flash butt welding component according to claim 1, wherein, The welded portion has an elongation of 4.0% or greater.

9. The flash butt welding component according to claim 1, wherein, The welded portion has cracks with an average length of 310 μm or less, which are generated after being subjected to four-point bending at 150% or greater of the yield strength of the substrate and immersed in 0.1 N HCl solution for 120 hours.

10. A flash butt welding method, comprising: Prepare a substrate comprising, by weight, 0.180% to 0.40% C, 1.0% or less (excluding 0%) Si, 1.60% or less (excluding 0%) Mn, 0.010% or less (excluding 0%) B, and the balance Fe and unavoidable impurities. The surface of the substrate to be welded is preheated so that the movement distance between the electrodes is 1.0 mm to 12.0 mm; The preheated surface to be welded is subjected to flash heating, such that the flash rate is 5% to 25% and the movement distance between the electrodes is 2.0 mm to 6.0 mm; The surfaces to be welded, which have been flash-heated, are upsetting heated such that the current is 20% to 40% of the short-circuit current and the movement distance between the electrodes is 2.0 mm to 16.0 mm, and the welded portion is formed by flash butt welding; as well as After the welded portion is cooled, it is post-heated for 0.5 to 2.5 seconds with a current of 5% to 25% of the short-circuit current.

11. A component comprising: The substrate, by weight, comprises 0.180% to 0.40% C, 1.0% or less (excluding 0%) Si, 1.60% or less (excluding 0%) Mn, 0.010% or less (excluding 0%) B, and the balance Fe and unavoidable impurities; and The welded portion is obtained by flash butt welding the substrate and then subjecting the flash-butt welded substrate to post-heat treatment. The welded portion includes a soft portion formed in the thickness direction, and The average hardness (H1) of the substrate and the average hardness (H2) of the soft portion satisfy the following relationship expression 1: [Relational Expression 1] H1-H2 ≤ 110 Hv.

12. The component according to claim 11, wherein, The carbon content of the substrate and the carbon content of the soft part satisfy the following relationship expression 2: [Relational Expression 2] α - β ≤ 0.1 wt% Wherein, α is the carbon content (wt%) of the substrate, and β is the carbon content (wt%) of the soft portion.

13. The component according to claim 11, wherein, The substrate has a tensile strength of 1000 MPa to 2000 MPa.

14. The component according to claim 11, wherein, The width of the soft portion is 10% or less of the thickness of the substrate.

Images