Method for manufacturing tailor-welded blanks using hot-pressing steel sheets having an al-fe intermetallic compound alloy layer

Abstract

One embodiment of the present invention provides a method of manufacturing a tailor-welded blank using a hot press steel sheet having an Al-Fe intermetallic compound alloy layer, the method including the steps of: providing a plated steel sheet having a base steel sheet and an aluminum-based plating layer formed on one surface or both surfaces of the base steel sheet; heat-treating the plated steel sheet so that the aluminum-based plating layer becomes an Al-Fe intermetallic compound alloy layer; and subjecting the heat-treated plated steel sheet to butt welding, wherein a portion or all of the Al-Fe intermetallic compound alloy layer located in a region to be welded is removed before welding.

Description

Technical Field

[0001] This disclosure relates to a method for manufacturing welded blanks using hot-pressed steel plates, wherein an Al-Fe based intermetallic compound alloy layer is formed thereon. Background Technology

[0002] To ensure collision safety in the event of a vehicle collision, such as preventing intrusion, it is necessary to use steel materials with high mechanical strength due to their martensitic or bainitic-martensitic microstructure for components such as bumpers, door reinforcements, B-pillar reinforcements, and roof reinforcements.

[0003] Patent Document 1 discloses hot-rolled or cold-rolled steel sheets coated with aluminum or aluminum alloys. The aluminum- or aluminum alloy-coated steel sheets are heat-treated at an Ac1 temperature or higher and cooled under specific conditions to achieve a tensile strength exceeding 1500 MPa when forming parts of the desired shape. In this case, the coating forms a surface alloy layer during furnace heat treatment through interdiffusion between the steel and aluminum to prevent metal decarburization and oxidation, and allows for the omission of operations such as sandblasting during the manufacture of the final part.

[0004] Meanwhile, in the heat treatment of components made from steel plates coated with aluminum or aluminum alloys, if the heat treatment is not carried out under properly specified heating conditions, such as when rapid heating is applied, problems such as contamination and blockage may occur in conveying equipment, etc., due to the melting of the aluminum coated on its surface.

[0005] To address this issue, a technology has been proposed that allows for rapid heating of hot-rolled or cold-rolled steel sheets. While the final component exhibits similar performance to conventional components, it features an aluminum coating consisting solely of an intermetallic compound alloy layer. This allows for rapid heating during component manufacturing, thereby increasing manufacturing productivity and reducing costs.

[0006] Meanwhile, to reduce the weight of automobiles, components, such as blanks, have been developed that involve butt welding of steel materials with different alloy compositions or thicknesses. Components welded in this way are called "butt-welded blanks" or "butt-welded parts." Laser beam welding has been used as a preferred method for manufacturing such components, offering advantages in terms of process flexibility, quality, and productivity. The manufactured blanks, after cold pressing, can provide components with varying mechanical strength, pressing capacity, and damping properties within the component itself. Therefore, desired characteristics can be provided to suitable parts without incurring high costs.

[0007] The aluminum-coated steel sheet described above, consisting only of an aluminum-iron intermetallic compound alloy layer, is also manufactured as such a welded blank, hot-pressed and heat-quenched, so that it can be provided as a single part with different thicknesses, ideally meeting local load requirements and unique mechanical properties.

[0008] However, when laser welding is performed on steel sheets with a coating consisting solely of an aluminum-iron intermetallic compound alloy layer, the coating is incorporated into the molten zone formed during welding. Specifically, the Al component, which promotes ferrite formation, is concentrated due to the convection of the liquid metal, and the liquid component with the highest concentration segregates between dendrites during solidification. Therefore, when austenitic treatment (heating) is performed for the purpose of quenching the weld blank, the Al-rich region is then transformed into ferrite during cooling. In other words, the weld metal portion (molten portion) formed by laser welding has a mixed configuration of desired high hardness and strength martensite and undesirable soft ferrite. Consequently, when external forces, such as impacts, are applied during the use of the final component, fracture occurs in the weld zone, resulting in the loss of the component's intended function.

[0009] Therefore, there is a need for technology that can prevent the formation of ferrite, which is a source of fracture in welded components.

[0010] [Existing Technical Documents]

[0011] (Patent Document 1) EP 0971044 Summary of the Invention

[0012] Technical issues

[0013] One aspect of this disclosure is to provide a method for manufacturing welded blanks using hot-pressed steel sheets having an Al-Fe-based intermetallic compound alloy layer formed thereon.

[0014] Technical solution

[0015] According to one embodiment of this disclosure, a method is provided for manufacturing a weldable blank using a hot-pressed steel sheet having an Al-Fe based intermetallic compound alloy layer formed thereon, the method comprising the steps of: providing a coated steel sheet having a base steel sheet and an aluminum-based coating formed on one or both surfaces of the base steel sheet; heat-treating the coated steel sheet such that the aluminum-based coating becomes an Al-Fe based intermetallic compound alloy layer; and subjecting the heat-treated coated steel sheet to butt welding, wherein, prior to welding, a portion or all of the Al-Fe based intermetallic compound alloy layer located in the area to be welded is removed.

[0016] Beneficial effects

[0017] As described above, according to one aspect of this disclosure, a method can be provided for manufacturing welded blanks using hot-pressed steel sheets having an Al-Fe-based intermetallic compound alloy layer formed thereon. Attached Figure Description

[0018] Figure 1 This is a schematic diagram showing how an aluminum-based coating formed on a base steel plate is transformed into an Al-Fe-based intermetallic compound alloy layer through heat treatment.

[0019] Figure 2 This is a schematic diagram showing the removal of a portion of the Al-Fe based intermetallic compound alloy layer formed on the base steel plate.

[0020] Figure 3 This is a schematic diagram showing a base steel plate after butt welding, with a portion of the Al-Fe based intermetallic compound alloy layer removed.

[0021] Figure 4 This is a photograph of the microstructure of a plated steel sheet having an Al-Fe based intermetallic compound alloy layer manufactured according to one embodiment of the present disclosure, as observed using an optical microscope.

[0022] Best practice

[0023] The following describes a method for manufacturing a welded blank using a hot-pressed steel sheet having an Al-Fe based intermetallic compound alloy layer formed thereon, according to one embodiment of the present disclosure.

[0024] First, a coated steel sheet is provided for use, having a base steel sheet and an aluminum-based coating formed on one or both surfaces of the base steel sheet. The coated steel sheet can be obtained by immersing the base steel sheet in an aluminum-based coating bath. In this disclosure, the type of base steel sheet is not particularly limited, and all types of steel sheets used in the art can be used. However, preferably, a steel sheet with a high strength of 1500 MPa or greater after hot forming can be used. Furthermore, in this disclosure, the composition of the aluminum-based coating is not particularly limited, and all types of aluminum-based coatings used in the art can be used. Simultaneously, when the base steel sheet is immersed in the aluminum-based coating bath, intermetallic compounds of several μm crystallize on the surface of the base steel sheet through a very rapid reaction. However, since these intermetallic compounds are relatively brittle, the aluminum-based coating bath may also contain Si as an inhibitor to suppress the growth of intermetallic compounds. That is, the aluminum-based coating may also contain Si, and in this case, the Si content may be from 8% to 12% by weight.

[0025] Subsequently, the coated steel sheet is heat-treated to transform the aluminum-based coating into an Al-Fe-based intermetallic compound alloy layer. Figure 1 This is a schematic diagram illustrating how an aluminum-based plating layer formed on a base steel plate transforms into an Al-Fe-based intermetallic compound alloy layer through heat treatment. (See diagram for reference.) Figure 1 As shown, when Fe diffuses from the base steel plate through heat treatment, the aluminum-based plating layer 20 formed on the base steel plate 10 becomes an Al-Fe based intermetallic compound alloy layer 30. Simultaneously, the Al-Fe based intermetallic compound alloy layer may sometimes include a first layer and a second layer, wherein the first layer may contain at least one of FeAl and Fe3Al, and the second layer may contain Fe2Al5. The second layer may exist in the form of layers or islands. Furthermore, when the aluminum-based plating layer contains Si and the Al-Fe based intermetallic compound alloy layer also contains Si, the second layer may also contain Al2Fe3Si3. The difference in composition between the first and second layers is due to the difference in the amount of Fe diffused from the base steel plate. That is, the first layer, which is closer to the base steel plate, has a higher Fe content than the second layer because Fe diffusion is more likely to occur in the first layer than in the second layer, which is relatively farther from the base steel plate. The thickness or composition of the first and second layers can vary depending on the heat treatment temperature and time, but this disclosure is not particularly limited to specific conditions. However, the temperature during heat treatment is preferably lower than Ac1, but when the temperature during heat treatment is higher than Ac1, problems may occur during processing in coil form due to changes in the microstructure of the steel sheet. Meanwhile, compared to aluminum-based plating layers formed solely by plating without heat treatment, Al-Fe-based intermetallic compound alloy layers formed by heat treatment have the advantage of rapid heating during hot forming as a post-processing step. Furthermore, since the total thickness of the plating layer is increased by forming the Al-Fe-based intermetallic compound alloy layer through heat treatment in this disclosure, the thickness of the aluminum-based plating layer can be reduced. That is, the amount of Al plating can be reduced, and problems such as welding defects can be improved because the aluminum content is lower compared to existing Al-Si plating layers. Furthermore, the existing Al-Si coating intermetallic compound alloy layer is FeAl3, which has a melting point of 1160°C. In contrast, the first layer of the Al-Fe based intermetallic compound alloy layer disclosed herein is either FeAl or Fe3Al and has a higher melting point of 1310°C, which results in superior corrosion resistance in the heat-affected zone during welding.

[0026] Figure 2 This is a schematic diagram showing the removal of a portion of the region where an Al-Fe based intermetallic compound alloy layer formed on a base steel plate has been removed. Figure 3 This is a schematic diagram showing the butt welding of a portion of the base steel plate after the Al-Fe based intermetallic compound alloy layer has been removed. Figure 2 and Figure 3As shown in this disclosure, prior to welding, part or all of the Al-Fe based intermetallic compound alloy layer 30 located in the area to be welded is removed. This prevents the Al contained in the Al-Fe based intermetallic compound alloy layer from being incorporated into the molten zone during welding, ensuring that the microstructure of the final product does not contain ferrite and thus guarantees high strength. The Al-Fe based intermetallic compound alloy layer is preferably completely removed; however, even if the Al-Fe based intermetallic compound alloy layer remains after partial removal, it will not significantly affect the physical properties of the final product. However, after partial removal of the Al-Fe based intermetallic compound alloy layer, the thickness of the remaining Al-Fe based intermetallic compound alloy layer is preferably 10 μm or less. If its thickness exceeds 10 μm, the mechanical properties of the weld zone of the final part after hot forming and quenching may deteriorate due to the large amount of Al component being incorporated into the weld metal zone during welding. In this disclosure, there are no particular limitations on the method for removing the Al-Fe based intermetallic compound alloy layer; for example, methods such as brushing, grinding, lapping, laser irradiation, and chemical solution impregnation can be used. The removal of the Al-Fe based intermetallic compound alloy layer can be performed as a preparatory operation before welding, or it can be performed during welding and immediately before welding. The width W to be removed from the Al-Fe based intermetallic compound alloy layer can range from Wb / 2 to Wb (mm). Here, Wb refers to the average width of the weld metal zone 40 of the welded blank obtained after welding. When the width to be removed from the Al-Fe based intermetallic compound alloy layer is less than Wb / 2, the Al component of the Al-Fe based intermetallic compound alloy layer can be mixed into the weld metal zone; however, when its width is greater than Wb, excessive removal of the Al-Fe based intermetallic compound alloy layer may not adequately guarantee corrosion resistance. Furthermore, when welding steel plates of different thicknesses, the molten zones of the steel plates may differ, and therefore, the width of the Al-Fe based intermetallic compound alloy layer removed from the steel plate may also differ.

[0027] Subsequently, a welded blank can be manufactured by butt welding the heat-treated coated steel sheet. In this disclosure, there are no particular limitations on the welding method, and any method can be used as long as it meets the productivity and quality requirements. For example, laser beam welding, GTAW, plasma welding, GMAW, etc., can be used. According to this disclosure, when the Al-Fe based intermetallic compound alloy layer present in the area to be welded is removed in advance during welding, since the Al component is not mixed into the molten portion, it has the advantage of excellent physical properties of the welded metal portion of the final product obtained through post-treatment hot forming (austenitic heat treatment, forming / quenching).

[0028] At the same time, after butt welding, the obtained welded blanks can be subjected to a hot pressing forming process, so that the final product can be preferentially used in automotive parts, etc. Detailed Implementation

[0029] The present invention will be described in more detail below through embodiments. However, the following embodiments are merely for illustrating the present invention in more detail and do not limit the scope of the present invention.

[0030] (Example)

[0031] Cold-rolled coated steel sheets are obtained by applying a 10% Si-Al coating to 22MnB5 steel (boron steel with a tensile strength of 1500 MPa), which is widely used as collision components in vehicle bodies. The cold-rolled coated steel sheets are then heat-treated at 500°C to form... Figure 4 The diagram shows an Al-Fe based intermetallic compound alloy layer comprising FeAl, Fe3Al, Fe2Al5, and Al2Fe3Si3. In this case, the thickness of the Al-Fe based intermetallic compound alloy layer is approximately 40 μm on one side. The cold-rolled coated steel sheet was then cut into rectangular blanks measuring 500 mm × 120 mm. A portion of the Al-Fe based intermetallic compound alloy layer was removed using various methods described below, and butt welding was performed using laser beam welding to manufacture welded blanks. The welded blanks were then heated to 920°C, held for 4 minutes, and subjected to hot forming and quenching. The tensile strength, elongation, and fracture rate of the weld zone of the welded blanks manufactured as described above were measured, and the results are shown in Table 1 below. The average tensile strength of the manufactured welded blanks in the region corresponding to the base steel sheet was 1470 MPa.

[0032] Tensile strength and elongation were measured by taking a section of the weld zone. Five tensile test specimens were prepared according to KS standards, and for each specimen, tensile tests were performed, with the results expressed as an average. In this case, the tensile test speed was 10 mm / min.

[0033] The fracture rate of the weld zone is expressed as the percentage of samples that fractured during the tensile test compared to the number of tensile samples.

[0034] In Invention Example 1, a Q-switched pulsed laser was used to remove the Al-Fe based intermetallic compound alloy layer from both surfaces of the area to be welded, with a removal width of 1 mm. Subsequently, butt welding was performed using laser beam welding.

[0035] In Invention Example 2, a portion of the Al-Fe based intermetallic compound alloy layer present on both surfaces of the area to be welded was removed using a Q-switched pulsed laser. The removal width of the Al-Fe based intermetallic compound alloy layer was 1 mm, and its remaining thickness was 5 μm. Afterwards, butt welding was performed using laser beam welding.

[0036] In Invention Example 3, a grinding device was used to remove the Al-Fe based intermetallic compound alloy layer present on both surfaces of the area to be welded, with the removal width of the Al-Fe based intermetallic compound alloy layer being 1 mm. Afterwards, butt welding was performed using laser beam welding.

[0037] In Comparative Example 1, butt welding was performed using laser beam welding without removing the Al-Fe based intermetallic compound alloy layer.

[0038] During laser beam welding, a CO2 laser welding machine with a beam diameter of approximately 0.4 mm at the focal length was used, with 20 liters of He gas per minute as the shielding gas, the laser power was set to 6 kW, and the welding speed was set to 6 mpm.

[0039] [Table 1]

[0040]

[0041] As can be seen from Table 1, in the cases of Invention Examples 1 to 3 manufactured to meet the conditions of this disclosure, strength and elongation at a level similar to that of the base steel plate are shown. On the other hand, in the case of Comparative Example 1, which does not meet the conditions of this disclosure, both strength and elongation are poor.

[0042] [Explanation of reference numerals in the attached figures]

[0043] 10, 10': Foundation steel plate

[0044] 20: Aluminum-based coatings

[0045] 30, 30': Al-Fe based intermetallic compound alloy layer

[0046] 40: Weld metal zone

Claims

1. A method for manufacturing welded blanks from hot-pressed steel sheets having an Al-Fe based intermetallic compound alloy layer formed thereon, the method comprising the following steps: A coated steel sheet having a base steel plate and an aluminum-based coating formed on one or both surfaces of the base steel plate is provided for use. The coated steel sheet is heat-treated to transform the aluminum-based coating into an Al-Fe-based intermetallic compound alloy layer; and This allows the heat-treated coated steel sheet to undergo butt welding. Prior to the welding, part or all of the Al-Fe based intermetallic compound alloy layer located in the area to be welded is removed, and The temperature during the heat treatment is lower than Ac1. The Al-Fe based intermetallic compound alloy layer comprises a first layer and a second layer. The first layer contains at least one of FeAl and Fe3Al. The second layer contains Fe2Al5.

2. The method for manufacturing welded blanks from hot-pressed steel sheets having an Al-Fe-based intermetallic compound alloy layer formed thereon, according to claim 1, wherein the aluminum-based coating further comprises Si.

3. The method for manufacturing welded blanks from a hot-pressed steel sheet having an Al-Fe-based intermetallic compound alloy layer formed thereon, according to claim 2, wherein the Al-Fe-based intermetallic compound alloy layer comprises a first layer and a second layer. The first layer contains at least one of FeAl and Fe3Al. The second layer also contains Al2Fe3Si3.

4. The method for manufacturing welded blanks from a hot-pressed steel sheet having an Al-Fe-based intermetallic compound alloy layer formed thereon according to claim 1, wherein after removing a portion of the Al-Fe-based intermetallic compound alloy layer, the remaining Al-Fe-based intermetallic compound alloy layer has a thickness of 10 μm or less.

5. The method for manufacturing welded blanks from a hot-pressed steel sheet having an Al-Fe-based intermetallic compound alloy layer formed thereon, according to claim 1, wherein the width of the removed Al-Fe-based intermetallic compound alloy layer is from Wb / 2 to Wb, in mm. in, Wb refers to the average width of the weld metal zone of the welded blank obtained after welding.

6. The method of manufacturing a tailor-welded blank using a hot-pressing steel sheet having an Al-Fe-based intermetallic compound alloy layer formed thereon according to claim 1, wherein, After the butt welding, a hot pressing process is performed.

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