Blank for hot forming and hot-formed member

Abstract

The present invention addresses the problem of obtaining a hot-formed member suitable as a material for a vehicle, and provides: a blank for hot forming, the blank having excellent weldability; and a hot-formed member obtained therefrom.

Description

Hot forming blanks and hot forming parts

[0001] The present invention relates to a blank for hot forming and a hot forming member.

[0002] Generally, hot-formed components utilize galvanized steel sheets as the steel sheets for hot forming. These galvanized steel sheets are formed into blanks, and the resulting blanks are heated, followed by forming and cooling in a hot state. Furthermore, to prevent the formation of excessive oxide films on the base material surface and decarburization of the surface layer during the blank heating process, the application of aluminum-based plating containing Si is widely used commercially.

[0003] Meanwhile, the above blank can be manufactured by cutting a galvanized steel sheet, and depending on the shape of the hot-formed member to be produced, the intended hot-formed member may also be manufactured by joining, i.e., welding, several blanks together and then hot-forming the welded blank (welded blank). In this case, laser welding, as one example, Tailor Welded Blank (TWB) laser welding, is used for welding.

[0004] However, when the galvanized steel sheet used to produce the blank is an aluminum-based galvanized steel sheet with aluminum as the main component, if the aluminum-based plating layer is incorporated into the weld zone during the welding process, brittle intermetallic compounds are formed, or a ferrite phase is formed in the steel sheet (base material) structure due to the ferrite stabilization effect, leading to a problem of degraded weld quality. In particular, to obtain high strength of 1300 MPa or more after hot forming, the structure of the steel sheet (base material) changes into martensite; however, as the areas where ferrite structures and intermetallic compounds are formed due to the aluminum-based plating layer incorporated into the weld zone remain without changing into martensite, problems such as fracture of the weld zone and, furthermore, fracture of the base material occur. Additionally, by increasing the cooling rate required for the steel sheet (base material) structure to change into martensite through rapid cooling in the mold after heating to a high temperature for hot forming, the structure of the weld zone does not completely change into martensite, resulting in reduced weldability.

[0005] To solve the aforementioned problem, a method has been proposed to ablate a portion of the area to be welded, specifically the portion to be melted by a laser, before welding the blank. As a related document, Patent Document 1 discloses a process of melting and removing a portion of an Al-Si plating layer using a laser.

[0006] However, as described above, the process of etching a portion of the Al-Si plating layer using a laser has the disadvantage of incurring significantly high costs because it requires precision laser processing equipment. Furthermore, since alignment and fixing must be precisely controlled for each blank during laser processing, there is a problem of significantly reduced productivity.

[0007] Furthermore, the Al-Si plating layer has high ductility, so it is easily deformed and elongated during the shearing process of the material, and incorporation occurs into the shear plane. As such, since the Al-Si plating layer incorporated into the surface of the steel sheet and the shear plane (the plane perpendicular to the rolling direction) formed by shearing is difficult to remove using laser methods, quality degradation may occur due to residual aluminum in the TWB weldment.

[0008] In addition, the method of mechanically removing the Al-Si plating layer is also economically disadvantageous because it requires frequent maintenance or replacement of the tool due to the high ductility of aluminum and the galling phenomenon where aluminum adheres to the tool. Furthermore, if a portion of the base material is polished together during the plating layer removal process, it becomes difficult to secure sufficient mechanical properties, which limits the ability to completely remove the plating layer mechanically.

[0009] Accordingly, in providing a welded blank for obtaining a hot-formed member, a method is required to improve weldability by preventing the incorporation of an aluminum-based plating layer (e.g., an Al-Si plating layer) into the shear surface, and a method to easily remove the Al-Si plating layer at the welding site.

[0010] (Patent Document 1) Republic of Korea Published Patent Application No. 10-2022-0102661

[0011] According to one aspect of the present invention, in order to obtain a hot-formed member suitable as a material for automobiles, the present invention aims to provide a hot-formed blank with excellent weldability and a hot-formed member obtained therefrom.

[0012] The problems of the present invention are not limited to those described above. A person skilled in the art will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0013] According to one aspect of the present invention, a blank for hot forming is provided, comprising: a base steel plate; a plating layer formed on at least one surface of the base steel plate; and a shear surface formed on the side of the base steel plate and the plating layer, wherein the plating layer has a crack density of 3.00% or more in a region up to 500 μm in a first direction from the shear surface.

[0014] In one embodiment of the present invention, the region from the shear plane to 800 μm in the first direction may include a peeling region.

[0015] In one embodiment of the present invention, the peeling area may include a first peeling area in which a base steel plate is exposed and a second peeling area in which a portion of the plating layer is peeled off.

[0016] In one embodiment of the present invention, the second peeling region may satisfy the following [Relationship 1].

[0017] [Relationship 1]

[0018] Y ≤ ((T / X)×3.09)×100

[0019] (In Equation 1, Y represents the area (%) of the second peeling region among the peeling regions, X represents the average thickness (㎛) of the plating layer of the second peeling region among the peeling regions, and T represents the thickness (mm) of the base steel sheet.)

[0020] In one embodiment of the present invention, Y of the above-mentioned relationship 1 may be greater than 0% and less than or equal to 60%, in another embodiment may be greater than 0% and less than or equal to 30%, and in yet another embodiment may be greater than 0% and less than or equal to 20%.

[0021] In one embodiment of the present invention, the plating layer may have an average Fe content of 30 weight% or more.

[0022] In one embodiment of the present invention, the plating layer may include an Fe-Al or Fe-Al-Si intermetallic compound, and the intermetallic compound may have a thickness of 10 μm or more.

[0023] In one embodiment of the present invention, the plating layer comprises a first intermetallic compound and a second intermetallic compound, and a crack may be formed at the interface between the first intermetallic compound and the second intermetallic compound.

[0024] In one embodiment of the present invention, the plating layer has an average fracture toughness of 2 MPa / m 0.5 It may be less than or equal to, and the average hardness may be 800.0 Hv or higher.

[0025] As such, a hot forming blank according to one aspect of the present invention can minimize the incorporation of the plating layer into the shear surface during shearing as the plating layer is alloyed, and when such a hot forming blank is welded, the welding quality can be improved and furthermore, a hot forming member having intended physical properties can be provided.

[0026] According to another aspect of the present invention, a hot forming member is provided comprising: a first hot forming blank and a second hot forming blank; and a weld bead disposed between the first hot forming blank and the second hot forming blank.

[0027] In one embodiment of the present invention, the crack density in the region up to 300 μm from the end of the weld bead in the direction of the first hot forming blank or the second hot forming blank may be 1.15% or more.

[0028] In one embodiment of the present invention, one or more of the first hot forming blank or the second hot forming blank may include: a base steel plate; a plating layer formed on at least one surface of the base steel plate; and a shear surface formed on the side of the base steel plate and the plating layer.

[0029] In one embodiment of the present invention, the crack density in the region up to 300 μm from the end of the weld bead in the direction of the first hot forming blank or the second hot forming blank may be less than 3.00%.

[0030] In one embodiment of the present invention, the crack density in the region up to 300 μm from the end of the weld bead toward the first hot forming blank or the second hot forming blank may be greater than the crack density in the region up to 300 μm inward from the end surface of the hot forming member, which is not on the weld bead side.

[0031] In one embodiment of the present invention, the plating amount in the region up to 300 μm from the end of the weld bead toward the first hot forming blank or the second hot forming blank may be smaller than the plating amount in the region up to 300 μm inward from the end surface of the hot forming member, which is not on the weld bead side.

[0032] In one embodiment of the present invention, the plating layer may have a crack density of 3.00% or more in a region up to 500 μm in a first direction from the shear plane.

[0033] In one embodiment of the present invention, the Al content of the shear surface may be less than 0.5 weight%.

[0034] In one embodiment of the present invention, the Al area fraction of the weld bead cross-section may be less than 0.5%.

[0035] According to the present invention, a method can be provided that prevents the incorporation of a plating layer into the shear surface (a surface created perpendicular to the rolling direction by shearing) during the shearing process of a plated steel sheet, and enables the peeling of the plating layer during the shearing process without a separate additional process for removing the plating layer.

[0036] In addition, a hot forming blank manufactured therefrom can be provided, and this hot forming blank can ensure excellent quality without defects such as fracture at the weld joint during laser welding.

[0037] FIG. 1 schematically illustrates a cross-section of a blank for hot forming according to one embodiment of the present invention, showing a first peeling region, a second peeling region, and a first direction of one example (wherein the intermetallic compound (layer) is omitted).

[0038] FIG. 2 shows a photograph of the surface of a blank for hot forming according to one embodiment of the present invention, and is a photograph of the surface observed in the longitudinal direction (first direction) of the plating layer from the shear plane of the blank.

[0039] FIG. 3 shows a photograph of the plating layer of a hot forming blank according to one embodiment of the present invention, which is a cross-section in the thickness direction taken as an SEM BS image.

[0040] FIG. 4 shows a photograph of a weld bead of a hot-formed member according to one embodiment of the present invention, showing an example of a weld bead and a certain area from the weld bead in the direction of the blank based on the shear plane.

[0041] FIG. 5 shows a photograph of a weld bead of a hot-formed member according to one embodiment of the present invention and a crack that occurred in a certain area from the weld bead in the direction of the blank (the white part is the crack).

[0042] Preferred embodiments of the present invention will be described below with reference to the attached drawings. 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.

[0043] 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.

[0044] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0045] 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.

[0046] 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.

[0047] In addition, in the present invention, the term "steel plate" refers to a coil or sheet material that has not yet been processed into a specific shape, and the term "member" refers to a material that has been processed into a non-plate shape through a forming process. Furthermore, the term "plating layer" in the present invention refers to a layer of metal, alloy, or intermetallic compound formed in contact with a base steel plate.

[0048] It should be noted that in the present invention, when expressing the content of each element, the reference is based on weight (mass%) unless otherwise specifically determined. Additionally, the proportion of crystals or structures is based on area (area%) unless otherwise specifically expressed, and the gas content is based on volume unless otherwise specifically expressed.

[0049] 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.

[0050] The present invention will be described in detail below.

[0051] The inventors of the present invention have conducted in-depth research on methods to improve the weldability of a welded black while obtaining a hot-formed member using an aluminum-based plated steel sheet as a hot-formed steel sheet. In particular, the inventors intended to provide a method to prevent the plating layer corresponding to the welded area from being incorporated into the shear plane while simultaneously efficiently removing the plating layer when manufacturing the welded black.

[0052] The present invention is designed to solve the aforementioned problems and can provide a welded blank with improved weldability. Furthermore, in manufacturing this welded blank, the invention has technical significance in providing a hot forming blank and a hot forming member obtained therefrom, as well as a method for manufacturing such a blank, which prevents or minimizes the incorporation of an aluminum-based plating layer that degrades the quality of the welded part, and in particular, ensures the quality of the welded part without a separate process for removing the plating layer.

[0053] In one aspect of the present invention, a blank for hot forming is provided.

[0054] In one embodiment of the present invention, a hot forming blank is a material obtained by processing a hot forming steel sheet to obtain a hot forming member. As a non-limiting example, a hot forming blank can be obtained by performing processing such as cutting, punching, or shearing on a hot forming steel sheet, among which shearing is mainly performed.

[0055] In one embodiment of the present invention, the hot forming steel sheet for obtaining a hot forming blank may, as one example, be a base steel sheet and a plated steel sheet having a plating layer on at least one surface of the base steel sheet. As a preferred example, the hot forming steel sheet may be an aluminum-based plated steel sheet, specifically a plated steel sheet having an Al-Si plating layer.

[0056] According to one embodiment of the present invention, in order to ensure excellent weld properties when producing a welded blank from a hot forming blank obtained by processing, the steel sheet for hot forming prior to processing into the hot forming blank may have an Al-Si plating layer alloyed therein. At this time, although the alloying conditions will be explained in detail later, an aluminum-plated steel sheet having an Al-Si plating layer may be alloyed at a certain temperature.

[0057] By means of the alloying treatment described above, Fe in the substrate iron present directly beneath the plating layer diffuses into the plating layer, thereby allowing the plating layer of the hot forming blank according to one embodiment of the present invention to be composed of an Fe-Al alloy. More specifically, the plating layer may be composed of an intermetallic compound of highly brittle Fe(Al) (e.g., FeAl3, Fe2Al5, etc.) or an intermetallic compound of Fe-Al-Si. As a result, the average Fe in the plating layer may be 30% or more.

[0058] In one embodiment of the present invention, the intermetallic compound may exist as a layer within the plating layer, and as one example, may exist with an average thickness of 10 μm or more based on the thickness direction of the plating layer. The thickness of the intermetallic compound may correspond to the thickness of the plating layer before the alloying treatment, and accordingly, the thickness of the intermetallic compound and the thickness of the plating layer before the alloying treatment may be the same. As another example, the thickness of the intermetallic compound may be up to 30 μm. Meanwhile, if the plating layer is not fully alloyed with the intermetallic compound after the alloying treatment and remains in the state of a plating layer, the objective, such as preventing the incorporation of the plating layer into the shear surface mentioned above, cannot be achieved due to the high ductility characteristics of the aluminum-based plating layer.

[0059] Meanwhile, in one embodiment of the present invention, the plating layer may include a first intermetallic compound and a second intermetallic compound, and a crack may be formed at the interface between the first intermetallic compound and the second intermetallic compound. As one example, referring to the drawings, it can be seen that a crack has occurred at the interface between the FeAl3 intermetallic compound and the Fe2Al5 intermetallic compound, as shown in FIG. 3.

[0060] Thus, a hot forming blank according to one embodiment of the present invention has a plating layer composed of an intermetallic compound, and such plating layer has an average fracture toughness of 2 MPa / m 0.5 It may be less than or equal to, and the average hardness may be 800.0 Hv or higher. In this case, the fracture toughness and average hardness of the plating layer may be measured by the following methods. However, the measurement methods are not limited to these. As a non-limiting example, the fracture toughness of the above plating layer may be measured using a CT (compact tension) test specimen K in accordance with the standards of ASTM E399 and ASTM E1820. IC (Fracture toughness value) can be obtained. Additionally, as another non-limiting example, the average hardness of the plating layer can be obtained by cutting the alloyed plating layer into a certain size to prepare a test specimen, applying a load perpendicularly to the surface of the plating layer using a micro-Vickers hardness tester, and then calculating the Vickers hardness value by dividing the surface area of ​​the mark formed on the surface of the plating layer by the indentation load, or by calculating the hardness value from the load-displacement curve using nanoindentation. Here, the indentation load for measuring the average hardness is 5g, and the average value can be taken after 10 measurements.

[0061] Meanwhile, a hot forming blank according to one embodiment of the present invention can be obtained by processing and may have a shear surface formed on the side of the plating layer.

[0062] In one embodiment of the present invention, a blank for hot forming may have cracks on its surface, and specifically, the crack density in the region up to 500 μm from the shear plane in the first direction of the plating layer may be 3.00% or more. Here, the first direction means the direction inward of the blank from the shear plane along the length direction of the plating layer. As one example, referring to the drawings, as shown in FIG. 1, it may mean the direction from the shear plane to the region where the plating layer has peeled off.

[0063] In this invention, the term "crack density" is used to express the area of ​​cracks existing per unit area, representing the ratio of crack areas existing per unit area. It should be noted that this is an extension of the concept of density (mass per volume) and is used for an objective evaluation of the level of crack generation and propagation.

[0064] In one embodiment of the present invention, the surface for measuring crack density refers to the surface of a plating layer, and can be measured based on a surface area extending from any point where shearing begins to 500 μm in the first direction, for example, an area corresponding to 1 mm (width direction) × 500 μm (length direction). At this time, the method for measuring crack density is not specifically limited, but as a non-limiting example, a scanning electron microscope (SEM) may be used to capture a SEM BS image of the area and the area fraction (%) of the cracks may be obtained through image analysis. As an example, as shown in FIG. 5, the parts observed as white in the image analysis represent cracks, and a value can be obtained by calculating the density per unit area for these cracks.

[0065] As described above, cracks present on the surface of the plating layer can be generated during the shearing process, and the density of these cracks tends to decrease as they move inward from the point where shearing begins (i.e., the shear plane) toward the interior of the plating layer.

[0066] As another embodiment of the present invention, the crack density in the region up to 500 μm in the first direction from the shear plane may be 4.00% or more and 5.00% or more. However, if the crack density exceeds 12.50%, as the crack density in the region exceeding 500 μm in the first direction from the shear plane increases to 4.00% or more, a decrease in plating adhesion and plating quality may occur in the region other than the weld (e.g., TWB weld) during subsequent welding. Accordingly, the crack density may be limited to 12.50% or less.

[0067] In one embodiment of the present invention, a crack formed on the surface of the plating layer during the shearing process affects the peeling of the plating layer, and thus, an area up to 800 μm in the first direction from the shearing surface may include a peeling area.

[0068] In one embodiment of the present invention, the peeling area may include a first peeling area in which the base steel plate is exposed and a second peeling area in which a portion of the plating layer is peeled off (see FIG. 1). Here, the exposure of the base steel plate means that the plating layer of the entire thickness is peeled off (detached) in the area, and the peeling off of a portion of the plating layer means that the base steel plate is not exposed. As one example, referring to the drawings, as shown in FIG. 2, the portion in which Fe is observed as is corresponds to the first peeling area, and the portion in which a portion of the plating layer (intermetallic compound layer) is observed corresponds to the second peeling area.

[0069] In order to obtain a welded blank from a hot forming blank, it is advantageous for the plating layer at the area to be welded to be completely peeled off. However, according to the present invention, by alloying the plating layer to form a highly brittle Fe-Al alloy (specifically, an Fe-Al or Fe-Al-Si intermetallic compound), the generation and propagation of cracks in the plating layer can be induced using the stress and impact applied to the material during the shearing process. In this case, during the process of peeling off the plating layer, not only is complete peeling caused by cracks at the interface between the material and the plating layer occurring, but in some areas, cracks are generated and propagated within the plating layer, creating an area where only a part of the plating layer is peeled off, i.e., a second peeling area.

[0070] If such a second peeling region exists in part, a small amount of aluminum may be incorporated into the weld during the subsequent TWB welding process. Since the effect on weldability is negligible when the amount of aluminum incorporated into the weld is significantly small, the present invention allows for the partial existence of the second peeling region.

[0071] Specifically, a peeling region according to one embodiment of the present invention may satisfy the following [Equation 1]. Here, Y represents the area (%) of the second peeling region among the peeling regions, X represents the average thickness (μm) of the plating layer of the second peeling region among the peeling regions, and T represents the thickness (mm) of the substrate steel plate.

[0072] [Relationship 1]

[0073] Y ≤ ((T / X)×3.09)×100

[0074] In one embodiment of the present invention, the Y value of [Equation 1] may be greater than 0% and less than or equal to 60%. In another embodiment, the Y value may be greater than 0% and less than or equal to 30%, and in yet another embodiment, the Y value may be greater than 0% and less than or equal to 20%.

[0075] In this way, when a region where the plating layer is peeled off exists within the welded portion of a hot-forming blank, specifically within a region corresponding to a specific point in the first direction of the plating layer from the shear plane, a region where the plating layer is not completely removed but partially remains may exist to satisfy the aforementioned [Equation 1]. At this time, the allowable area of ​​the second peeled-off region may change depending on the thickness of the material (base steel sheet). This is because as the thickness of the material increases, the total weld area increases, and thus the allowable amount of aluminum incorporated (incorporated area) may also increase.

[0076] Thus, a hot forming blank according to one embodiment of the present invention, particularly a hot forming blank having the aforementioned peeling region, may have an Al content of less than 0.5 weight% in the shear surface. If the Al content in the shear surface is 0.50% or more, it may be difficult to secure the desired ultra-high strength during the process of manufacturing a subsequent hot forming member after welding (e.g., laser welding), and welding quality cannot be secured as the welded part is easily fractured.

[0077] In another embodiment, the Al content on the shear surface may be 0.10 weight% or less, provided that the Al concentration is 0%.

[0078] Generally, when producing a blank from a steel sheet for hot forming (e.g., galvanized steel sheet), unlike the phenomenon where the plating layer at the point where processing begins is incorporated into the processed surface (e.g., shear surface) formed in the thickness direction, the incorporation of the plating layer into the shear surface can be suppressed according to the present invention. As a result, the increase in the cooling rate required for the structure of the steel sheet (base material) to transform into martensite during the rapid cooling process following high-temperature heating for subsequent hot forming can be prevented. Consequently, it is easy to secure a martensite structure during hot forming, and the formation of ferrite or intermetallic compounds can be prevented. Furthermore, when welding such a blank, a weld with improved welding quality can be obtained.

[0079] As described above, according to the present invention, not only can the incorporation of the plating layer into the shearing surface be effectively prevented during the shearing process, but the peeling of the plating layer can also be induced solely through the shearing process. This ultimately means that a separate process for removing the plating layer, such as a process of artificially inducing the peeling of the plating layer using a laser, can be omitted.

[0080] As mentioned above, according to one embodiment of the present invention, the plating layer is composed of an intermetallic compound of an Al-Fe alloy, and the average fracture toughness of the plating layer is 2 MPa / m 0.5 Below, it may have characteristics with an average hardness of 800.0 Hv or higher. A plating layer with these characteristics can prevent deformation and elongation of the plating layer during the shearing process and prevent the incorporation of the plating layer into the shearing surface, thereby enabling the securing of good quality for the TWB weld even after subsequent hot forming.

[0081] Meanwhile, the base steel sheet forming the hot forming blank according to one embodiment of the present invention may include elements that can typically be added to steel, and the types and contents thereof are not specifically limited. However, non-limiting examples of elements that may be added to a base steel sheet according to one embodiment of the present invention are, in weight%, carbon (C): 0.02~0.60%, silicon (Si): 0.001~2.000%, aluminum (Al): 0.001~1.000%, manganese (Mn): 0.10~4.00%, phosphorus (P): 0.050% or less, sulfur (S): 0.0200% or less, nitrogen (N): 0.0200% or less, titanium (Ti): 0~1.0000%, niobium (Nb): 0~1.0000%, vanadium (V): 0~1.0000%, boron (B): 0~0.0100%, chromium (Cr): 0~1.00%, molybdenum (Mo): 0~1.00%, tungsten (W): It may contain 0~1.00%, copper (Cu): 0~1.0%, nickel (Ni): 0~1.0%, tin (Sn): 0~1.00%, antimony (Sb): 0~1.00%, calcium (Ca): 0~0.10%, magnesium (Mg): 0~0.10%, cobalt (Co): 0~1.00%, arsenic (As): 0~1.00%, zirconium (Zr): 0~1.00%, bismuth (Bi): 0~1.00%, rare earth elements (REM): 0~0.3%, and the remainder being Fe and other unavoidable impurities.

[0082] Among the alloy compositions described above, C, Mn, etc., may be added to ensure the strength of the steel; Si is effective not only for its deoxidation effect but also for reducing the segregation of Mn, etc., within the base steel sheet, and Al has a deoxidation effect. It should be noted that P, S, N, etc., may be elements inevitably introduced during the steel manufacturing process, but are not limited to these. Furthermore, it will be obvious to those skilled in the art that, in addition to the aforementioned composition, Ti, B, Cu, Mo, Cr, Ni, V, Ca, Nb, Sn, W, Sb, Mg, Co, As, Zr, Bi, REM, etc., may be additionally included in consideration of the target physical properties of the final product.

[0083] Hereinafter, a method for manufacturing a hot forming blank according to another aspect of the present invention will be described. However, it should be noted that the following method is merely one example for manufacturing a hot forming blank, and that a hot forming blank according to one embodiment of the present invention must not necessarily be manufactured by this manufacturing method. Furthermore, any manufacturing method that satisfies the claims of the present invention may be used without issue to implement each embodiment of the present invention.

[0084] According to one embodiment of the present invention, a blank for hot forming can be manufactured by the steps of: preparing a base steel plate; manufacturing an aluminum-plated steel plate by plating the base steel plate with an aluminum-based plate; alloying the aluminum-plated steel plate; and shearing the alloyed aluminum-plated steel plate to form a shear surface.

[0085] In one embodiment of the present invention, the base steel sheet for obtaining a hot-formed blank may be the base steel sheet mentioned above, and it is noted that there are no particular restrictions on its composition and that it is replaced by the aforementioned details.

[0086] In one embodiment of the present invention, the prepared steel sheet can be plated with an aluminum-based material to produce an aluminum-based plated steel sheet having an aluminum-based plating layer on at least one surface of the steel sheet. As one example, the aluminum-based plating may be performed by a molten aluminum plating method in which the steel sheet is immersed in a molten aluminum plating bath. At this time, the temperature of the molten aluminum plating bath may be set to a temperature range for normal aluminum plating, and as a non-limiting example, it may be performed in a temperature range of 620 to 680°C.

[0087] In one embodiment of the present invention, the molten aluminum plating bath is a plating bath containing aluminum (Al) as a main component, and as one example, the plating bath may contain silicon (Si) in an amount of 6.0 to 15.0 weight%. In this way, by adding Si, effects such as improving the fluidity of the molten bath and suppressing the diffusion of Al into the substrate steel sheet during plating can be obtained.

[0088] As another example, in addition to the above Si, iron (Fe) may be included in an amount of 10.0% or less, preferably 0.1 to 2.0%, and furthermore, one or more selected from Mg, Mn, Cr, Cu, Mo, Ni, Sb, Sn, Ti, Ca, and Sr may be included.

[0089] In one embodiment of the present invention, an aluminum-based plated steel sheet produced by the aluminum plating can be subjected to alloying heat treatment. The step of alloying heat treatment is not particularly limited, but can be performed by on-line heating in which the plated steel sheet (aluminum-based plated steel sheet) obtained by the molten aluminum plating is heated while in motion.

[0090] As a non-limiting example, the alloying treatment may be performed continuously after the aluminum-based plating, known as diffusion annealing heat treatment. Diffusion annealing heat treatment is a process of performing heat treatment at a specific temperature to cause mutual diffusion between the base steel sheet and the plating layer. In one embodiment of the present invention, the diffusion annealing heat treatment may be a process of heating to a temperature range of 650 to 850°C and maintaining that temperature for 3 to 20 seconds.

[0091] By the alloying treatment described above, the plating layer of the aluminum-based plated steel sheet may be composed of an Al-Fe alloy. As one example, the average Fe content in the plating layer may be 30% or more, and the plating layer may be composed of an Fe-Al intermetallic compound or an Fe-Al-Si intermetallic compound. The description of the intermetallic compound is replaced with the previously explained content.

[0092] In one embodiment of the present invention, an aluminum-based plated steel sheet can be sheared after alloying treatment to form a shear surface. At this time, since the shearing can be performed from the surface of the plating layer of the aluminum-based plated steel sheet toward the thickness direction, the shear surface can also be formed in the thickness direction from the surface of the plating layer toward the base steel sheet.

[0093] In one embodiment of the present invention, it is noted that the shearing conditions are not specifically limited and general conditions used for producing hot-forming blanks may be applied. However, it is noted that the shearing speed, shearing angle, and gap between the punch and die may be adjusted to increase the stress applied to the shearing section within a range where the quality of the shearing surface does not cause problems for welding, for example, laser welding, and that these conditions may also be applied under normal conditions.

[0094] According to one embodiment of the present invention, cracks may be formed on the surface of the plating layer during the shearing process. Specifically, cracks may be formed on the surface of the plating layer in the longitudinal direction (first direction) from the shearing surface formed after the shearing. In one embodiment of the present invention, the crack density in the region up to 500 μm in the longitudinal direction of the plating layer may be 3.00% or more. At this time, the crack density may also increase as the shear stress increases.

[0095] In addition, peeling of the plating layer may occur in the area where a crack is generated during the above shearing process, and at this time, there may be a first peeling area where the base steel plate is exposed and a second peeling area where a part of the plating layer is peeled off.

[0096] Meanwhile, after forming the above-mentioned shear plane, the method may further include a step of artificially removing (peeling off) the plating layer in which cracks have formed. As a non-limiting example, this step may involve removing the plating layer by a mechanical method using a brush, abrasive stone, etc. A plating layer according to one embodiment of the present invention has an average fracture toughness of 2 MPa / m² depending on alloying. 0.5 The plating layer can be easily removed as numerous cracks are formed during the shearing process. As an example, under conditions where no artificial scratches occur on the surface of the material, the plating layer can be removed using a nylon brush. Additionally, the area of ​​the plating layer to be removed may be greater than the width of the welding area (meaning the area in the direction perpendicular to the shear plane) where welding is performed during subsequent welding. Specifically, the plating layer can be removed up to a point of 500 to 1500 μm in the direction perpendicular to the shear plane.

[0097] Hereinafter, a hot-formed member and a method for manufacturing the same according to another aspect of the present invention will be described in detail.

[0098] Generally, a hot-formed member can be obtained by hot-forming a steel plate for hot forming, but in one embodiment of the present invention, a hot-formed blank can be obtained by welding it with another material and then hot-forming the welded blank.

[0099] A hot forming member according to one embodiment of the present invention may include a weld bead disposed between a first hot forming blank and a second hot forming blank.

[0100] In one embodiment of the present invention, one or more of the first hot forming blank or the second hot forming blank may comprise: a base steel plate; a plating layer formed on at least one surface of the base steel plate; and a shear surface formed on the side of the base steel plate and the plating layer. In this case, the plating layer may have a crack density of 3.00% or more in the region up to 500 μm in the longitudinal direction (first direction) of the plating layer from the shear surface, and the Al content of the shear surface may be less than 0.5 weight%.

[0101] As an example, referring to the drawings, as shown in FIG. 4, a weld bead can be formed up to a certain area in the left and right inner directions centered on the shear plane. At this time, the certain area may be up to 500 μm in the inner direction relative to the shear plane. At this time, the Al area fraction of the weld bead may be less than 0.5%.

[0102] In this way, by using a hot forming blank including a plating layer and a shear plane on the side of the plating layer according to one embodiment of the present invention, the hot forming member according to one embodiment of the present invention may have a crack density of 1.15% or more in the region up to 300 μm from the end of the weld bead in the direction of the first hot forming blank or the second hot forming blank.

[0103] As an example, referring to the drawing, as shown in Fig. 5, it can be seen that cracks (white lines) exist in the area extending 300 μm in the left and right inner directions from the end of the weld bead. However, Fig. 5 shows only one direction (e.g., left direction).

[0104] In another embodiment of the present invention, the crack density in the region up to 300 μm from the end of the weld bead in the direction of the first hot forming blank or the second hot forming blank may be less than 3.00%.

[0105] In one embodiment of the present invention, at least one of the first hot forming blank or the second hot forming blank has a shear plane, and since a crack may be formed in the length direction (first direction) of the plating layer from the shear plane of the blank having such a shear plane, the area around the weld bead obtained by welding the shear plane may also have a certain crack density. Accordingly, the crack density around the weld bead may be higher compared to the plating layer area where welding is not performed in the hot forming member. As one example, the crack density in the area up to 300 μm from the end of the weld bead toward the first hot forming blank or the second hot forming blank may be greater than the crack density in the area up to 300 μm inward from the end surface of the hot forming member, which is not on the weld bead side.

[0106] Furthermore, in one embodiment of the present invention, the plating amount in the region extending 300 μm from the end of the weld bead toward the first hot forming blank or the second hot forming blank may be smaller than the plating amount in the region extending 300 μm inward from the end surface of the hot forming member, rather than on the weld bead side. As previously explained, the region extending 300 μm from the end of the weld bead toward the first hot forming blank or the second hot forming blank has a certain crack density, and having such a crack density may mean that a portion of the plating layer has peeled off. Additionally, the plating amount around the weld bead may be lower compared to the plating layer region where welding was not performed in the hot forming member, for example, a region located at least 2 mm away from the shear surface. Here, the plating amount is the plating adhesion amount (mg / m²). 2 It can be compared as ).

[0107] Meanwhile, among the first and second hot forming blanks mentioned above, the remaining blanks, excluding one of the hot forming blanks, may be blanks of the same material or blanks of a different material. Here, "different material" refers to a material with different physical properties such as strength, ductility, and toughness, or a material with a different alloy composition of the base steel sheet constituting the blank or a different composition of the plating layer. In other words, it is acceptable to have any material that can be called a hot forming blank, so the material conditions are not specifically limited.

[0108] As will be described in detail later, the base steel sheet of the hot-formed member according to one embodiment of the present invention may have a hard structure by undergoing a series of heating and holding processes followed by cooling and forming. As one example, the base steel sheet may have a microstructure with an area fraction of 90% or more by combining martensite and bainite phases, and may also include pearlite, ferrite, etc. as other structures. However, it is not limited thereto. Accordingly, the hot-formed member according to one embodiment of the present invention may have ultra-high strength, and as an example, may have a tensile strength of 1800 MPa or more.

[0109] The method for manufacturing a hot-formed member according to one embodiment of the present invention is not specifically limited, and can be manufactured through a process of heating a steel plate for hot forming to a temperature above the austenitizing temperature (Ac3), maintaining it, and then forming (pressing) it simultaneously with rapid cooling, as is widely known in the past.

[0110] However, as one example of the present invention, a first hot forming blank and a second hot forming blank are prepared, and then these blanks are joined together and welded so that a weld bead is formed at the joint, thereby obtaining a welded hot forming blank. Subsequently, the welded hot forming blank may be heated to a temperature range of 860 to 970°C and maintained for 2 to 15 minutes. The blank heated and maintained in this manner may be formed (pressed) in a hot state, and a step of cooling at a cooling rate greater than or equal to the critical cooling rate may be performed to manufacture an intended hot forming member. As a non-limiting example, the cooling may be performed at a cooling rate of 30°C / s or more.

[0111] In one embodiment of the present invention, the welding may be a Tailed Welded Blank (TWB) laser welding.

[0112] 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.

[0113] (Example 1)

[0114] Carbon steel containing, in weight percent, C: 0.22%, Si: 0.25%, Mn: 1.15%, Cr: 0.20%, B: 0.0050%, and the remainder being Fe and other unavoidable impurities was prepared as a base steel sheet, and then immersed in an aluminum-based plating bath to plating, thereby producing a plated steel sheet having an Al-Si plating layer on at least one surface. The plated steel sheet was subjected to an alloying treatment so that the plating layer was alloyed to form an Fe-Al or Fe-Al-Si intermetallic compound, and then the plated steel sheet was sheared to produce a hot forming blank having a shear surface.

[0115] At this time, the plating was performed using an aluminum-based plating bath with a composition containing silicon (Si): 9.5%, iron (Fe): 1.5%, and the remainder being Al and other unavoidable impurities in weight percent, and the alloying treatment was performed by diffusion annealing heat treatment at 800–830°C for 10–15 seconds after plating. After the diffusion annealing heat treatment, the Fe content of the plating layer was 33–39%. Meanwhile, the shear conditions are shown in Table 1 below.

[0116] The Fe content in the plating layer and the thickness of the intermetallic compound layer after the alloying treatment were measured using SEM based on the cross-section in the thickness direction of the plating layer. In addition, the fracture toughness and hardness of the plating layer were measured. At this time, the hardness of the plating layer was measured using the Vickers hardness method with a digital microhardness tester, and the fracture toughness of the plating layer was measured using the indentation method. Measurements were taken at three different points on each siften, and the average value was calculated and presented.

[0117] After completing the shearing, the crack density was measured in the region extending 500 µm in the longitudinal direction (first direction) of the plating layer from the shear plane. The crack density was measured by taking a SEM-BS (Back-scattering) image of the shear plane of each specimen at a magnification of 100x, and then calculating the crack density using an image analyzer. At this time, the reference area for measuring crack density was set to 1 mm along the shear plane and the region extending 500 µm in the first direction from the shear plane. To increase accuracy during measurement, the average density can be calculated after three measurements. In addition, the first and second delamination regions of the plating layer on the surface were identified and their areas measured in the same manner. Specifically, the cross-section of the plating layer at any point in the 1 mm × 500 µm area (transverse plane direction × longitudinal direction of the plating layer from the transverse plane (first direction)) was observed with SEM, and the thickness of the cross-section obtained through FIB analysis at any 10 points in the second peeling area was measured and the average value was taken, and then [Equation 1] was calculated.

[0118] Steel Type Thickness (mm) Shear Plating Layer Shear Pre-shear Post-shear Force (MPa) Fe Content (weight%) Intermetallic Compound Layer Thickness (㎛) Fracture Toughness (MPa / m² 0.5) Average Hardness (Hv) Crack Density (%) Shear Plane Average Plating Thickness (㎛) Second Delamination Area (%) Equation 1 A 1.0 4 1 4.9 3 8.1 2 3.4 1.5 8 6 1.7 5.2 3 7.9 7 3 8.8 Satisfied B 1.2 4 9 6.3 3 6.2 2 5.7 1.8 8 0 3.4 6.7 8 12.1 2 3 0.6 Satisfied C 1.5 3 7 1.6 3 4.7 2 2.8 1.4 8 0 0.0 4.5 6 7.9 5 8.0 Satisfied D 1.8 5 12.6 3 5.4 2 6.1 1.9 8 2 6.3 7.1 2 13.3 4 4 1.7 Satisfied E 2.0 4 4 9.0 3 3.9 2 4.5 1.6 8 0 8.9 5.8 9 15.0 2 4 1.1 Satisfied In Table 1, Equation 1 is [Y ≤ It refers to the value of the right side of [(T / X)×3.09)×100]. In Table 1, 'average plating thickness of the shear plane' corresponds to X in Equation 1, and 'area of ​​the second peeling region' corresponds to Y in Equation 1.

[0119] (Example 2)

[0120] After additionally preparing the base steel plate (second hot forming blank) used in Example 1, the cross-sectional surface of this base steel plate and the hot forming blank (first hot forming blank) manufactured according to Example 1 was joined. Then, the joint was laser welded to form a weld bead. Afterward, the joined blank with the weld bead formed was heated to 900°C and maintained for 5 minutes, and then hot-formed using a flat plate mold to manufacture a hot-formed member.

[0121] The tensile properties of the above hot-formed member were measured, and a tensile test was performed on the weld bead. At this time, the tensile test was conducted using a universal testing machine after preparing a specimen according to the DIN EN SIO 6892-1 standard, and the specimen was prepared so that the weld bead was positioned at the center of the tensile test specimen. Then, the crack density in the region from the end of the weld bead to the first hot-formed blank direction up to 300 μm was measured using the same method applied in Example 1.

[0122] Steel Grade Thickness (mm) TS (MPa) YS (MPa) El (%) Weld Crack Density (%) A 1.01 47 4.4 107 0.2 6.95 Good 2.62 B 1.21 47 7.5 106 5.0 7.42 Good 2.12 C 1.51 48 5.2 107 4.2 7.79 Good 1.73 D 1.81 47 2.9 105 9.5 7.57 Good 1.36 E 2.01 45 2.1 104 1.5 6.82 Good 1.15

[0123] As shown in Table 2 above, all steel grades show good weld properties, which means that during the tensile test, fracture did not occur in the weld but occurred first on the base metal side.

[0124] In this way, a hot-formed member obtained from a hot-forming blank according to one embodiment of the present invention can have the target ultra-high strength while ensuring weld quality.

Claims

1. Base steel plate; A plating layer formed on at least one surface of the above-mentioned base steel plate; and It includes a shear surface formed on the side of the above-mentioned base steel plate and the above-mentioned plating layer, and The above plating layer is a hot forming blank having a crack density of 3.00% or more in a region up to 500 μm in the first direction from the shear plane.

2. In Paragraph 1, A hot forming blank, wherein the region from the above shear plane to 800 μm in the first direction includes a peeling region.

3. In Paragraph 1 or 2, A blank for hot forming, wherein the above peeling area includes a first peeling area where the base steel plate is exposed and a second peeling area where a portion of the plating layer is peeled off.

4. In any one of paragraphs 1 to 3, The above second peeling region is a hot forming blank satisfying the following [Relationship 1]. [Relationship 1] Y ≤ ((T / X)×3.09)×100 (In Equation 1, Y represents the area (%) of the second peeling region among the peeling regions, X represents the average thickness (㎛) of the plating layer of the second peeling region among the peeling regions, and T represents the thickness (mm) of the base steel sheet.) 5. In any one of paragraphs 1 through 4, A hot forming blank in which Y in the above relationship 1 is greater than 0% and less than or equal to 60%.

6. In any one of paragraphs 1 through 5, A hot forming blank in which Y in the above relationship 1 is greater than 0% and less than or equal to 30%.

7. In any one of paragraphs 1 through 6, A hot forming blank in which Y in the above relationship 1 is greater than 0% and less than or equal to 20%.

8. In any one of paragraphs 1 through 7, A hot forming blank having an Al content of less than 0.5 weight% in the shear surface.

9. In any one of paragraphs 1 through 8, The above plating layer is a hot forming blank having an average Fe content of 30 weight% or more.

10. In any one of paragraphs 1 through 9, A blank for hot forming, wherein the plating layer comprises an Fe-Al or Fe-Al-Si intermetallic compound.

11. In any one of paragraphs 1 through 10, The above intermetallic compound is a hot forming blank having a thickness of 10 μm or more.

12. In any one of paragraphs 1 through 11, The above plating layer comprises a first intermetallic compound and a second intermetallic compound, and A hot forming blank comprising a crack at the interface between the first intermetallic compound and the second intermetallic compound.

13. In any one of paragraphs 1 through 12, The above plating layer has an average fracture toughness of 2 MPa / m 0.5 Lee Ha-in, hot forming blank.

14. In any one of paragraphs 1 through 13, The above plating layer is a hot forming blank having an average hardness of 800.0 Hv or more.

15. First hot forming blank and second hot forming blank; and It includes a weld bead disposed between the first hot forming blank and the second hot forming blank, and One or more of the first hot forming blank or the second hot forming blank comprises: a base steel plate; a plating layer formed on at least one surface of the base steel plate; and a shear surface formed on the side of the base steel plate and the plating layer. A hot-formed member having a crack density of 1.15% or more in an area up to 300 μm from the end of the weld bead in the direction of the first hot-forming blank or the second hot-forming blank.

16. In Paragraph 15, A hot-formed member having a crack density of less than 3.00% in an area up to 300 μm from the end of the weld bead in the direction of the first hot-forming blank or the second hot-forming blank.

17. In Paragraph 15 or 16, A hot-formed member in which the crack density in the region up to 300 μm from the end of the weld bead in the direction of the first hot-formed blank or the second hot-formed blank is greater than the crack density in the region up to 300 μm inward from the end surface of the hot-formed member, not on the weld bead side.

18. In any one of paragraphs 15 through 17, A hot-formed member in which the plating amount in the region up to 300 μm from the end of the weld bead in the direction of the first hot-formed blank or the second hot-formed blank is smaller than the plating amount in the region up to 300 μm inward from the end surface of the hot-formed member, which is not on the weld bead side.

19. In any one of paragraphs 15 through 18, The above plating layer is a hot-formed member having a crack density of 3.00% or more in a region up to 500 μm in a first direction from the shear plane.

20. In any one of paragraphs 15 through 19, A hot-formed member having an Al content of less than 0.5 weight% in the shear plane above.

21. In any one of paragraphs 15 through 20, A hot-formed member having an Al area fraction of less than 0.5% in the cross-section of the weld bead.

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