Method for manufacturing press-formed part, press-formed part, method for manufacturing blank for press forming, and blank for press forming

Abstract

The present invention addresses the problem of suppressing a facture such as a crack in press forming of an integral blank formed while a plurality of partial blanks partially overlap and has an objective to provide such a blank (integral blank).　The present invention provides a method for manufacturing an integral blank for press forming obtained by joining a plurality of partial blanks formed from steel sheets and having an overlap section where at least two partial blanks are partially overlapped. The method comprises obtaining, in advance, material inflow amounts (movement vectors of points on each partial blank) of the partial blanks due to press forming in the overlap section, manufacturing the blank for press forming by joining the partial blanks forming the overlap section only in a portion where the absolute value of a difference in material inflow amount (movement vector) between the partial blanks is smaller than a predetermined limit value, and, after hot press forming of this blank, joining another portion of the overlap section to obtain the desired press-formed part.

Description

Method for manufacturing press-molded parts, press-molded parts, method for manufacturing press-molded blanks, and press-molded blanks

[0001] The present invention relates to a method for manufacturing a press-molded part using a tailored blank, a press-molded part, a method for manufacturing a blank for press molding, and a blank for press molding.

[0002] There is a growing demand for reducing lifecycle GHG (total greenhouse gas emissions over the entire lifecycle), particularly in automobiles, and there is an increasing demand for component / module integration with the aim of improving the efficiency of production lines by reducing the number of parts and omitting processes. Accordingly, with the introduction of optimized design, there is an increasing demand for component manufacturing by press-forming so-called tailored welded blanks (TWBs), which are blanks in which different types of steel sheets are combined and integrated within a component. Various TWB press-forming technologies have been proposed (for example, see Patent Document 1).

[0003] A blank integrated by TWB (integrated blank) is usually manufactured by butt-welding separate blanks (partial blanks) made of steel sheets of different thicknesses and types. As integrated blanks become larger, a method for manufacturing an integrated blank by partially overlapping two partial blanks and spot welding the overlapping portion has been proposed (Patent Document 2). With the increasing size of parts and modules and the accompanying trend toward more efficient and cost-effective part manufacturing, part manufacturing by batch press forming of integrated blanks is attracting increasing attention.

[0004] For example, Patent Document 3 describes a method for manufacturing an automobile structural part in which an inner front pillar, an inner center pillar, and an inner back rail are separately manufactured by hot pressing, and a reinforcing blank is locally attached to the inner front pillar and the inner center pillar and then hot pressed. However, although the automobile structural part disclosed in Patent Document 3 is an integrally molded part in a broad sense, it is not an automobile structural part (Patent Document 3 is an upper door ring part) that is integrally molded.

[0005] International Publication No. 2020 / 059804 Special Publication No. 2021-528248 International Publication No. 2017 / 098427

[0006] When an integrated blank obtained by overlapping and joining two partial blanks (e.g., by spot welding, lap welding, etc.) is press-formed, fractures such as cracks may be observed in the overlapping portion of the partial blanks. If a fracture such as a crack occurs in a part, the part quality will be significantly deteriorated and the part will be scrapped. In other words, fractures in press-formed products will lead to problems not only in terms of quality but also in terms of productivity.

[0007] This problem has not been recognized in hot press forming (hot stamping), which can tolerate a larger amount of processing strain than cold working. In particular, with the demand for more complex shaped parts and the demand for integral forming of large parts (such as automotive structural parts such as door rings), there is a demand for integrated hot press forming of tailored blanks. However, cracks and other fractures may be observed in the overlapping areas of partial blanks, and this problem has become apparent.

[0008] An object of the present invention is to suppress breakage such as cracks when an integrated blank formed by partially overlapping a plurality of partial blanks is press-formed.

[0009] The present inventors have made extensive development efforts to achieve the above object and have obtained the following findings.

[0010] (A) First, when joining and integrating a plurality of partial blanks made of steel plates, a press-forming blank having an overlapping portion in which at least two partial blanks are partially overlapped was manufactured, and the state of fracture occurrence was investigated and verified. As an example, a press-forming blank in which the overlapping portion was joined by spot welding was manufactured, and the investigation and verification were carried out.

[0011] When the parts after press forming were observed, it was found that fractures of the partial blanks occurred at the overlapping portions where the partial blanks were overlapped. Using an automobile door ring as an example, analysis through experiments and simulations revealed that fractures occurred in the overlapping portions of the A-pillar lower (the L-shaped portion at the bottom of the A-pillar) and the B-pillar lower (the T-shaped portion at the bottom of the B-pillar) in and near the portions where stretch flange deformation occurred. Conversely, it was found that fractures did not occur in the portions where stretch flange deformation did not occur.

[0012] (i) Upon further analysis, it was found that the fracture occurred in (i) a part where the material flow behavior of the partial blank differed significantly when the overlapping portion deformed, and (ii) a part where the material flow was restricted by the joint (in the case of the investigated example, spot welding).

[0013] That is, when the difference in material flow rate between partial blanks increases due to press forming, shear deformation occurs at the joint (spot weld), resulting in large deformation concentration near the spot weld (factor (i)). The restriction of material flow by the joining (spot welding) prevents deformation accompanied by material flow, such as stretch flange deformation, and the deformation concentrates, resulting in fracture (factor (ii)). This problem is becoming apparent even in hot press forming (hot stamping), which can tolerate a larger amount of processing strain than cold working, when processing involves large deformation. With the demand for more complex shapes in parts, there is a need for integrated blanks and hot press forming methods that contribute to the hot press forming of integrated blanks using tailored blanks.

[0014] (c) In order to prevent breakage, the inventors analyzed the material flow behavior of partial blanks during hot press forming through simulation, divided them into parts with small differences in material flow rate and parts with large differences in material flow rate, and realized that breakage during hot press forming could be prevented by joining the partial blanks (by spot welding, lap welding, etc.) only in the parts with small differences in material flow rate, and proceeded with development.

[0015] As a result, it was confirmed that fractures do not occur unless the partial blanks are joined, even in areas where the difference in material flow rate between the partial blanks is large, such as areas where stretch flange deformation occurs. It was also confirmed that press-formed parts with integrated partial blanks can be obtained by joining the unjoined portions of the overlapping parts using spot welding or other methods after hot press forming. The above investigations and studies were conducted using press-formed blanks in which the overlapping parts of the partial blanks were joined by spot welding. However, this is not limited to spot welding; it is also true when the overlapping parts are joined by other joining methods (e.g., lap welding (arc welding, laser welding), lap fillet welding (arc welding, laser welding), brazing, friction stir welding (FSW), friction welding, etc.). It was confirmed that these methods can be used to hot press-form integrated blanks (TWBs) with overlapping parts (overlapped parts) of multiple partial blanks without fracture, and press-formed parts with strength and rigidity can be obtained.

[0016] The present invention was made based on the above findings, and its gist is as follows: [1-1] A method for manufacturing a press-molded part, comprising: a press-molding blank processing step for obtaining a press-molding blank formed by joining together a plurality of partial blanks made of steel plate, the press-molding blank having an overlapping portion formed by partially overlapping at least two of the partial blanks; a hot press-forming step for obtaining a press-molded product by hot press-forming the press-molding blank; and a post-press part joining step for joining parts of the press-molded product, wherein the press-molding blank processing step includes a step of determining in advance the amount of material flow for each of the partial blanks that constitute the overlapping portion formed by press-molding, and joining the partial blanks that constitute the overlapping portion only at a portion where the difference in material flow for the partial blanks is smaller than a predetermined limit value, and the post-press part joining step includes a step of joining the portions of the overlapping portion that were not joined in the press-molding blank processing step. The predetermined limit value is, for example, the difference in material flow rate caused by hot press forming (for example, the amount of misalignment between materials (between partial blanks) that would occur if there were no welding at a location corresponding to a welded part), and can be the amount of misalignment when the resulting part becomes less than the allowable tensile shear stress (TSS).[1-2] A method for manufacturing a press-molded part according to [1-1], in which the joining is spot welding, comprising: a press-molding blank processing step for obtaining a press-molding blank which is a press-molding blank obtained by joining together a plurality of partial blanks made of steel plate, the press-molding blank having an overlapping portion where at least two of the partial blanks are partially overlapping; a hot press-forming step for obtaining a press-molded product by hot press-forming the press-molding blank; and a post-press part joining step for spot-welding a portion of the press-molded product, wherein the press-molding blank processing step includes a step of determining in advance the amount of material flow for each of the partial blanks in the overlapping portion by press-molding, and spot-welding the partial blanks which constitute the overlapping portion only in portions where the difference in the material flow rates of the partial blanks is smaller than a predetermined limit value, and the post-press part joining step includes a step of spot-welding portions of the overlapping portion that were not spot-welded in the press-molding blank processing step. [1-3] The method for manufacturing a press-molded part according to the above [1] or [1-2], wherein the predetermined limit value is 1 mm, preferably 0.5 mm, in terms of an absolute value of the difference in the material flow rate of the partial blank by press molding. [1-4] The method for manufacturing a press-molded part according to any one of the above [1-1] to [1-3], wherein the overlapping portion of the press-molded part includes an L-shaped or T-shaped portion.[2-1] A press-formed part in which a plurality of partial blanks made of steel plate are joined together, and at least two of the partial blanks are joined at a plurality of joints at an overlapping portion formed by partially overlapping the other partial blanks, wherein, in a cross section perpendicular to the surface of the partial blank including the center of the joint of the outermost partial blank among the partial blanks, at a position 1 / 4 of the plate thickness from the surface of the partial blank where it contacts the other partial blanks, when the Vickers hardness at a position 15 mm or more away from the center of the joint and not joined is Hvm, for some of the plurality of joints, ΔHv, which is the difference between the maximum and minimum Vickers hardness in a range within 5 mm toward the base material from the end of the joint (or within 12 mm from the center of the joint), is less than 0.2 Hvm, preferably 0.1 Hvm or less, and for other joints of the plurality of joints (joints other than the some of the plurality of joints), the ΔHv is 0.2 Hvm or more, preferably 0.3 Hvm, 0.4 Hvm, or 0.5 Hvm or more. The outermost partial blank here means the partial blank on the surface side of the upper surface when the press-molded part is placed with the convex shape facing up. [2-2] The press-formed part according to [2-1], wherein the joining is spot welding, wherein a plurality of partial blanks made of steel plates are joined together, and at least two of the partial blanks are joined by a plurality of spot welds at overlapping portions where the partial blanks are partially overlapped, and in a cross section including the center of the spot weld of the outermost partial blank among the partial blanks, at a position that is 1 / 4 of the plate thickness from the surface of the partial blank, when the hardness at a position 15 mm or more away from the center of the spot weld and where the spot weld is not formed is Hvm, for some of the plurality of spot welds, ΔHv, which is the difference between the maximum hardness and the minimum hardness within a range within a radius of 12 mm from the center, is less than 0.2 Hvm, preferably 0.1 Hvm or less, and for other spot welds of the plurality of spot welds (spot welds other than the some of the spot welds), the ΔHv is 0.2 Hvm or more, preferably 0.3 Hvm, 0.4 Hvm, or 0.5 Hvm or more.The outermost partial blank here refers to the partial blank on the upper surface side when the press-formed part is placed with the convex shape facing up. [2-3] The press-formed part according to [2-1] or [2-2], wherein the overlapping portion is a portion where, when the material flow rate of the partial blank at the overlapping portion is calculated in advance by press forming, the absolute value of the difference in the material flow rate of the partial blank is smaller than a predetermined limit value. The specific analysis method is as follows: First, the press-formed part is three-dimensionally measured to create part shape data. From the created shape data, the part is developed using, for example, AUTOFORM R.10 manufactured by AUTOFORM, to create data on the blank to be used for press forming. Using the resulting blank data, the difference in material flow rate between each portion can be analyzed using a method similar to the analysis method for the difference in material flow rate between partial blanks performed in the press-formed part forming method described above. [2-4] The press-formed part according to [2-3], wherein the predetermined limit value is 1 mm, preferably 0.5 mm, in terms of the absolute value of the difference in material flow rate between the partial blanks during press forming. [2-5] The press-molded part according to any one of [2] to [2-4], wherein the other portion of the overlapping portion (the portion other than the portion of the overlapping portion) is a portion that undergoes stretch flange deformation during press forming. That is, the press-molded part according to any one of [2] to [2-4], wherein the other portion of the joints among the plurality of joints is set as a portion that undergoes stretch flange deformation during press forming. The portion that undergoes stretch deformation during press forming is a portion of the overlapping portion where, when the material flow rate of the partial blank at the overlapping portion during press forming is calculated in advance, the absolute value of the difference between the material flow rates of the partial blanks is greater than a predetermined limit value. The analysis method involves three-dimensionally measuring the shape of the press-molded part as described above, creating shape data for the part, and analyzing the shape data using, for example, AUTOFORM R.10 manufactured by AUTOFORM. This analysis method can identify the portion that undergoes stretch deformation. [2-6] The press-molded part according to any one of [2-1] to [2-5], wherein the overlapping portion of the press-molded part includes an L-shaped or T-shaped portion.[3-1] A method for manufacturing a press-forming blank obtained by joining together a plurality of partial blanks made of steel plate, the method having an overlapping portion formed by partially overlapping at least two of the partial blanks, the method comprising the steps of: determining in advance the amount of material flow through each of the partial blanks due to press forming at the overlapping portion; and joining the partial blanks that constitute the overlapping portion only at a portion where the difference in the material flow rates of the partial blanks is smaller than a predetermined limit value. [3-2] A method for manufacturing a press-forming blank according to [3-1], wherein the joining is by spot welding; A method for manufacturing a press-forming blank obtained by joining together a plurality of partial blanks made of steel plate, the method having an overlapping portion formed by partially overlapping at least two of the partial blanks, the method comprising the steps of determining in advance the amount of material flow through each of the partial blanks due to press forming at the overlapping portion; and spot welding the partial blanks that constitute the overlapping portion only at a portion where the difference in the material flow rates of the partial blanks is smaller than a predetermined limit value. [3-3] The method for manufacturing a press-forming blank according to [3-1] or [3-2] above, wherein the predetermined limit value is 1 mm, preferably 0.5 mm, in terms of the absolute value of the difference in the amount of material flowing into the partial blanks by press forming. [3-4] The method for manufacturing a press-forming blank according to any one of [3-1] to [3-3] above, wherein the overlapping portion of the press-forming blank includes an L-shaped or T-shaped portion. [4-1] A press-forming blank formed by joining together a plurality of partial blanks made of steel plate, the press-forming blank having an overlapping portion formed by partially overlapping at least two of the partial blanks, characterized in that the partial blanks constituting the overlapping portion are joined at a portion of the overlapping portion and not joined at other portions.[4-2] The press-forming blank according to [4-1], wherein the joining is by spot welding, and the press-forming blank is formed by joining together a plurality of partial blanks made of steel plates, and has an overlapping portion formed by partially overlapping at least two of the partial blanks, characterized in that the partial blanks constituting the overlapping portion are joined by spot welding at a portion of the overlapping portion and not spot welded at other portions. [4-3] The press-forming blank according to [4-1] or [4-2], wherein, when the material flow amounts of each of the partial blanks at the overlapping portion are previously calculated by press forming, the absolute value of the difference in material flow rates of the partial blanks is smaller than a predetermined limit value. [4-4] The press-forming blank according to [4-3], wherein the predetermined limit value is 1 mm, preferably 0.5 mm, in terms of the absolute value of the difference in material flow rates of the partial blanks due to press forming. [4-5] The press-molding blank according to any one of [4-1] to [4-4], wherein the other portion of the overlapping portion (the portion other than the part of the overlapping portion) is a portion that undergoes stretch flange deformation by press-molding. [4-6] The press-molding blank according to any one of [4-1] to [4-5], wherein the overlapping portion of the press-molding blank includes an L-shaped or T-shaped portion.

[0017] According to the present invention, a press-formed product can be obtained in which fractures such as cracks are suppressed when a press-formed integrated blank (TWB) is press-formed (hot press-formed) by joining a plurality of partial blanks together, the integrated blank having an overlapping portion where at least two partial blanks are partially overlapped. This makes it possible to efficiently and productively manufacture even large parts with complex shapes. Furthermore, parts with excellent crash resistance can be obtained.

[0018] 4(a) is a schematic diagram showing the appearance of an automobile single door ring. 4(b) is a schematic diagram explaining a part manufacturing process by press-forming a conventional general TWB. 4(c) is a schematic diagram explaining the configuration of a blank for press-forming a door ring. 4(d) is a conceptual diagram showing the results of stress analysis of the overlapping portion of the A-pillar lower. 4(a) is a conceptual diagram showing the stress state of the A-pillar lower as viewed from the outside, and 4(b) is a conceptual diagram showing the stress state of the rocker as viewed from the inside, in contour diagrams. 4(c) is a conceptual diagram showing an example of spot welding points at the overlapping portion of the A-pillar lower. 5(a) is an example of spot welding point locations at the overlapping portion of the A-pillar lower, and 5(b) is a conceptual diagram for explanation, in which parts at risk of fracture are surrounded by solid lines and parts at no risk of fracture are surrounded by dotted lines. 5(d) is a conceptual diagram showing spot welding points at the overlapping portion determined by simulation analysis. FIG. 6( a) shows an example of spot welding at the overlapping portion (L-shaped) of the A-pillar lower section and the overlapping portion (T-shaped) of the B-pillar lower section, as viewed from the outside, and FIG. 6( b) is a conceptual diagram showing the same as viewed from the inside. This is a conceptual diagram showing the stress state after hot pressing the integrated blank of FIG. 6, determined by FEM simulation. FIG. 7( a) shows the stress state at the overlapping portion (L-shaped) of the A-pillar lower section and the overlapping portion (T-shaped) of the B-pillar lower section, as viewed from the outside, and FIG. 7( b) is a conceptual diagram showing the same as viewed from the inside. This is a diagram explaining a part manufacturing process by press forming the TWB according to the present invention. This is an explanatory diagram for explaining a HAZ softened portion caused by spot welding. This is a conceptual diagram for explaining an example of application of the present invention to an automobile floor module. This is a conceptual diagram showing the overlap welding position of the overlapping portion determined by simulation analysis, and showing an example in which lap welding is performed instead of the spot welding of FIG. Figure 11(a) shows an example of lap welding at the overlapping portion of the A-pillar lower (L-shaped) and the B-pillar lower (T-shaped) as viewed from the outside, and Figure 11(b) is a conceptual diagram showing the same thing as viewed from the inside.

[0019] The present invention will be described below by way of an embodiment, namely, an automobile door ring, which is an embodiment of the present invention (hereinafter simply referred to as the present invention). FIG. 1 shows an external view of a single door ring 1 as an example of an automobile door ring. This door ring 1 alone is typically constructed by combining multiple types of steel sheets. For example, the A-pillar 2 (also called a front pier) and B-pillar 3 (also called a center pillar) of the door link 1 may be constructed with a 2.0 GPa-class high-strength steel sheet in the upper portion to ensure interior space in the event of a collision, and a 1.3 GPa-class high-strength steel sheet in the lower portion, which is joined to the rocker 4, to ensure workability and toughness. When manufacturing such parts by press forming, partial blanks that will become the respective parts are joined to form an integrated blank (TWB: tailored blank) as a press blank material, and this integrated blank is then press-formed to produce the components that make up the door ring. The resulting parts are then welded to manufacture the door ring.

[0020] FIG. 2 shows an outline of the manufacturing process for a typical TWB press-molded part (press-molded part).

[0021] Partial blank processing process: This is the process of manufacturing partial blanks that will become parts of the integrated blank. Partial blanks are cut out (blanking) from a specified steel plate and refined using laser trimming, etc. to manufacture partial blanks.

[0022] Press blank processing step: This is a step in which the obtained partial blanks are joined to produce a press-molding blank (integrated blank). The method for joining the partial blanks is not particularly limited. For example, when the partial blanks are butt-welded, this can be done by laser welding or arc welding. Furthermore, the partial blanks can be overlapped and the overlapping portions can be joined by, for example, spot welding (resistance spot welding, laser spot welding, etc.), lap welding (arc welding, laser welding), lap fillet welding (arc welding, laser welding), brazing, friction stir welding (FSW), friction welding, etc. A press-molding blank can be obtained by combining and joining predetermined partial blanks to form an integrated unit.

[0023] Hot press forming process: This is a process in which the obtained press-forming blank (integrated blank) is hot-press-formed. By press-forming the press-forming blank, a part or a part with a shape similar to the part (near-net shape) can be obtained (the part obtained after the hot press process is called a press-formed product). The press-forming method is not particularly limited, but hot press forming is generally preferred when press-forming a blank made of high-strength steel plate (e.g., steel plate with a high tensile strength of over 590 MPa). Hot press forming, also known as hot stamping, is a press-forming method in which a blank (steel plate) is heated to an austenite temperature range of approximately 900°C, and then press-formed and rapidly cooled simultaneously to perform quenching via martensitic transformation. Hot press forming is characterized by reduced press load due to forming at high temperatures, and excellent shape fixability due to martensitic transformation after forming, while maintaining high strength. For this reason, it is widely used in press-forming of high-strength steel plate.

[0024] Trimming process: This is a process for refining press-molded products (including those that have been formed into a part shape or near-net-shape parts) that have been hot-press-molded. The method for refining press-molded products is not particularly limited. For example, it includes a process (laser trimming) in which burrs on the edges of the press-molded product are removed with a laser to shape the product into a predetermined shape. In the case of near-net-shape press-molded products, it also includes processing to form the product into the final part shape. If a press-molded product with the final shape is obtained by hot-press-molding, this trimming process can be omitted.

[0025] Post-press part joining process: This is a process for joining other parts to the obtained press-molded product if necessary. The method for joining the other parts is not particularly limited. For example, joining may be done by joining methods such as spot welding, arc welding, laser welding, brazing, etc. In addition, there are cases where partial reinforcement materials are attached to the press-molded product, or where parts that cannot be formed simultaneously in the press molding are joined. Of course, if there is no need to join other parts, this post-press part joining process can be omitted.

[0026] By going through these steps, the final target part (press-molded part) can be obtained. Note that the manufacturing process for parts by press-molding a TWB is not limited to the steps described above. Other necessary steps can be added.

[0027] Recently, there has been a demand for more efficient and cost-effective parts production, leading to a trend toward larger parts and modules (parts that combine smaller parts). For example, in the case of automobile door rings, there is a demand to manufacture door rings by batch press forming, rather than separately manufacturing and assembling the aforementioned A-pillar and B-pillar. This has led to a need for press-forming blanks for use in batch press forming of door rings.

[0028] Figure 3 shows an example of a press-molding blank 30 for a door ring 1 (Figure 3 shows an example of a single door ring). The press-molding blank 31 in Figure 3 is formed by joining partial blanks corresponding to an upper A-pillar 31 (also called an A-pillar upper), a lower A-pillar 32 (also called an A-pillar lower), an upper B-pillar 33 (also called a B-pillar upper), a lower B-pillar 34 (also called a B-pillar lower), and a rocker 35. The respective partial blanks may be made of the same or different steel types and thicknesses. In the example of Figure 3, for example, the A-pillar lower 32 is made of a 1.5 GPa-class steel plate with a thickness of 1.4 mm, the B-pillar upper 33 is made of a 2.0 GPa-class steel plate with a thickness of 1.4 mm, the B-pillar lower 34 is made of a 1.0 GPa-class steel plate with a thickness of 1.2 mm, the rocker 35 is made of a 1.5 GPa-class steel plate with a thickness of 1.2 mm, and the A-pillar upper 31 is made of a 2.0 GPa-class steel plate with a thickness of 1.4 mm. These partial blanks are joined together to produce an integrated press-forming blank 30 (hereinafter sometimes referred to as an integrated blank).

[0029] In the example of FIG. 3 , the door ring 1 is manufactured by overlapping and joining a lower A-pillar (A-pillar lower) 32 and a rocker 35, a lower B-pillar (B-pillar lower) 34 and a rocker 35, and an upper B-pillar (B-pillar upper) 33 and an upper A-pillar (A-pillar upper) 31. Therefore, to manufacture such a door ring by one-time press molding, a partial blank corresponding to the A-pillar lower 32 and a partial blank corresponding to the rocker 35 are overlapped to manufacture an integrated blank 30. Similarly, the partial blanks constituting the B-pillar lower 34 and the rocker 35, and the B-pillar upper 33 and the A-pillar upper 31 are overlapped to manufacture the integrated blank 30. FIG. 3 shows an outline of the configuration of the partial blanks for press molding this door ring. The overlapping portions (overlapped portions) of the partial blanks are indicated by hatching in FIG. 3 . An overlapping portion 36 between the A-pillar lower 32 and the rocker 35 is L-shaped, an overlapping portion 37 between the B-pillar lower 34 and the rocker 35 is T-shaped, and an overlapping portion 38 between the B-pillar upper 33 and the A-pillar upper 31 is also T-shaped.

[0030] In the example of Fig. 3, the overlapping portion is composed of two partial blanks, but the number of overlapping partial blanks is not limited. For example, three or more partial blanks may be overlapped. The number may be determined depending on the required characteristics and shape of the final part to be manufactured.

[0031] When the part surface has a roughly L-shaped configuration when viewed vertically from above, it is called an L-shape, and when it has a roughly T-shaped configuration, it is called a T-shape. When the cross-sectional shape is a three-dimensional structure such as a hat shape, such as the overlapping portion 36 of the A-pillar lower 32 and rocker 35, and the resulting shape is complex, such as an L-shape or a T-shape when viewed from above, the material flow behavior at the corners becomes complex. This makes it easy for cracks to occur during press forming of the integrated blank.

[0032] [Overlapped portion] When an integrated blank is manufactured by joining the entire overlapped portion, fractures (cracks, breaks, etc.) may occur in the overlapped portion after press forming. When observing the fractured portion, it was found that fractures often occur in areas where the plate thickness is significantly reduced by press forming. Therefore, the present invention was achieved by analyzing the cause of fractures.

[0033] The joining method for joining overlapping portions of partial blanks or joining parts after pressing is not particularly limited. Examples of joining methods that can be used include resistance welding (resistance spot welding, projection welding, seam welding, etc.), arc welding (lap arc welding, lap fillet arc welding), laser welding (lap laser welding, lap fillet laser welding, laser spot welding), friction stir welding (FSW), friction welding, and brazing. Furthermore, mechanical fastening methods can also be used for joining parts after pressing. In the following description of the present invention, spot welding (resistance spot welding) will be used as an example of the joining method. The present invention is not limited to the following embodiments. The joining method is not limited to spot welding, and the various joining methods described above or similar joining methods can be used. By replacing spot welding in the description with another joining method, the application of other joining methods can be understood.

[0034] The cause of fracture can be analyzed using simulation techniques such as the finite element method (FEM). The inventors conducted an FEM analysis. Figure 4 shows the analysis results for the L-shaped overlapping portion 36 of the A-pillar lower 32 and rocker 35. Figure 4(a) shows the stress state of the A-pillar lower 32 as viewed from the outside, and Figure 4(b) shows the stress state of the rocker 35 as viewed from the inside, using contour diagrams. The area where fracture actually occurred is the area surrounded by an ellipse, which corresponds to the darkened area on the contour diagram. Comparing these figures, it was confirmed that fracture actually occurred in the area where the simulation indicated high shear stress and fracture.

[0035] The analysis results revealed that the fracture was caused by (i) the significant difference in material flow behavior of the partial blank when the overlapping portion deformed, and (ii) the restriction of material flow behavior due to spot welding. Here, material flow refers to the deformation (movement) of material due to press forming, and material flow behavior refers to the deformation behavior of the material at a specific position, i.e., the direction and distance it deforms (moves) before and after press forming.

[0036] When the difference in material flow rate between the partial blanks becomes large (factor (i)), shear deformation occurs in the spot weld, and it is thought that large deformation concentration occurs near the spot weld. Because material flow is restricted there (factor (ii)), deformation accompanied by material flow, such as stretch flange deformation, does not occur. As a result, it is thought that deformation tends to concentrate, causing fracture.

[0037] Therefore, in order to prevent breakage of the integrated blank during press forming, the inventors focused on the material flow behavior of the partial blanks during hot press forming. That is, by understanding the material flow behavior of each partial blank that constitutes the overlapping portion through simulation in advance, and dividing each partial blank into parts where the difference in material flow rate (the difference in material flow rate between each partial blank) is small and parts where the difference in material flow rate is large, the partial blanks are spot welded together only in the parts where the difference in material flow rate is small, and other parts are not spot welded, it was realized that breakage during press forming can be prevented, and development was carried out based on this idea.

[0038] As a result, it was confirmed that even in areas where the difference in material flow rate of the partial blank is large, such as areas where stretch flange deformation occurs, no fracture occurs and hot press forming is possible if spot welding is not performed. In other words, even if the above-mentioned factor (i) that the material flow behavior of the partial blank differs significantly when the overlapping portion deforms remains, it was confirmed that fracture can be prevented by eliminating factor (ii) that the material flow behavior is restricted by spot welding.

[0039] The difference in material flow rate for each partial blank is the absolute value of the difference in movement vectors obtained as a result of the movement, by press forming, of each point (position) of the partial blanks that make up the overlapping portion, which corresponds to an arbitrary point (position) of the overlapping portion before forming. That is, the movement vector of each point (position) of the partial blank that corresponds to an arbitrary point (position) of the overlapping portion is calculated from the point (position) to which that point moved after press forming, and the absolute value of the difference in movement vectors of the points (positions) of the partial blanks (the distance between the points (positions) after each movement) is calculated as the difference in material flow rate (deviation) for that point (any point in the overlapping portion).

[0040] A specific example of the simulation will be described. As mentioned above, spot welding will be used as an example, but other joining methods may be applied in a similar manner. For example, assuming that the partial blank is fixed (referring to a state where it is joined by spot welding or the like) at at least one point (e.g., the center of gravity of the overlapping portion or the center of gravity of the top surface) of the overlapping portion that will become the top surface (the portion held down by a pad or die) when press-formed, a press-forming (hot press-forming) analysis (simulation analysis using FEM, etc.) of the integrated blank is performed. In this case, the movement vector of each point of the partial blank corresponding to any point in the overlapping portion is calculated from the positions before and after the movement of each point. The absolute value of the difference between the obtained movement vectors of corresponding points for each partial blank is calculated as the material flow difference (deviation amount) at that point. If the calculated material flow difference (deviation amount) is smaller than a preset limit value (e.g., 1 mm, preferably 0.5 mm, as described below), the material flow difference is small, and the point (position) can be spot-welded before press-forming. On the other hand, if the calculated difference in material flow rate (deviation amount) is greater than a preset limit value, the difference in material flow rate is large, so spot welding should not be performed before press forming, but should be performed after press forming.

[0041] The inventors investigated the relationship between the difference in material flow rate (misalignment) in hot-pressed joints and the tensile shear strength (TSS) at room temperature, and found that if the difference in material flow rate (misalignment) in hot-pressed joints is 0 to 1 mm (greater than 0 mm and 1 mm or less), the tensile shear strength (TSS) at room temperature is equal to or greater than the original TSS (TSS when the misalignment is 0 mm). The technical reason for this phenomenon has not been elucidated, but it is speculated that when the misalignment is small (1 mm or less), a synergistic effect is obtained between the processing strain introduced by hot-pressing and the hardening caused by quenching.

[0042] A specific test example will be described. A joint (test specimen) made by overlapping and spot-welding 1.5 GPa-class steel plates (1.2 mm thick) was heated to 900 °C and held there for 1 minute. The steel plates were then pulled at 740 °C to shift a predetermined length (displacement), and then quenched to prepare a test specimen. The test specimen was then pulled at 1 mm / s at room temperature, and the displacement of the two steel plates and the tensile shear strength (TSS) of the joint (spot weld) at room temperature were observed. As a result, when the displacement of the steel plates during hot rolling was 0 to 1.0 mm, the TSS of the joint at room temperature was equal to or greater than that of a joint with a displacement of 0 mm (i.e., not hot-deformed). On the other hand, when the displacement exceeded 1.0 mm, the TSS tended to decrease. Specifically, under these test conditions, it was confirmed that the TSS at a displacement of 0.1 mm was 102% of the TSS at 0 mm, 105% at 0.3 mm, 107% at 0.5 mm, 102% at 0.7 mm, 100% at 1.0 mm, 90% at 1.2 mm, 86% at 1.5 mm, and 71% at 2.0 mm. In other words, it was discovered for the first time that even in hot press forming (hot stamping), even if a difference in material flow (displacement) of 0.0 mm to 1.0 mm or less occurs at the spot welding point, it is possible to ensure tensile shear strength that is equal to or greater than that when no deformation occurs.

[0043] Based on this new finding, when hot press forming an integrated blank made up of overlapping partial blanks, the limit value for the difference in material flow rate (deviation amount) should be set to 1.0 mm, and preferably should be preset to 0.9 mm, 0.8 mm, 0.7 mm, 0.6 mm, or 0.5 mm.

[0044] The point (position) at which the difference in material inflow rate is calculated does not need to be particularly limited. For example, the difference in material inflow rate may be analyzed at a point (position) set as the final spot welding position. Alternatively, for example, the analysis mesh may be refined to determine a region where the difference in material inflow rate is equal to or less than a threshold value. In this case, it is advisable to set the pre-press forming spot welding point (position) within a region where the difference in material inflow rate is analyzed to be small.

[0045] Figure 5(a) shows an example of the locations of spot welds (marked with black circles (●) in the figure) at the overlapping portion 36 of the A-pillar lower 32 and rocker 35. When spot welding was performed at all of the locations before press forming, stress concentration was confirmed at the actual fracture locations (areas surrounded by ellipses in Figures 4(a) and 4(b)), as shown in Figures 4(a) and 4(b)).

[0046] Next, assuming that only the double circle (black circle within a white circle) located approximately in the center of the overlapping portion 36 shown in FIG. 5( a) was spot welded, and that all other spot welding locations (points indicated only by black circles (●)) were not spot welded, a FEM simulation was performed to determine the material inflow (movement vector) of the A-pillar lower 32 and the rocker 35. Additionally, the material inflow (movement vector) at each FEM node of the partial blank constituting the overlapping portion 36 was calculated, and the difference (material inflow difference) between the material inflows (movement vectors) of each partial blank was calculated. The absolute limit of the difference in material inflow at the corresponding node (sometimes simply referred to as the limit of the material inflow difference) was set to 1.0 mm, as described above, and any portion with a material inflow difference greater than this was determined to be at risk of fracture. As a result, the portion 40 surrounded by a solid line in FIG. 5( b) was determined to be at risk of fracture if spot welded and press-formed.

[0047] The method for determining the region where the difference in the amount of material inflow is small is not particularly limited. As described above, it is preferable to set the region based on analysis using simulation such as FEM, or based on results of a prior offline test or an actual press.

[0048] In addition to evaluating the difference in material inflow rate, the thickness reduction rate may also be evaluated. For example, in the case of an FEM simulation, the thickness reduction rate of each element may be calculated and added to the difference in material inflow rate. This is because areas with a large thickness reduction rate are more likely to experience concentrated deformation at spot welds, increasing the risk of fracture. In this case, it is advisable to determine the limit value based on the thickness reduction rate. The reason for using an absolute limit value is that there are cases where the thickness increases (e.g., when wrinkles occur), in which case the thickness reduction rate is expressed as a negative value. It is advisable to set the limit value for the absolute value of the thickness reduction rate at 10%, and areas with a thickness reduction rate above this value are considered to be areas with a high fracture risk. The limit value for the thickness reduction rate is preferably 9%, 8%, 7%, 6%, or 5%. The thickness reduction rate is a relatively easy indicator to grasp because it can be directly determined using FEM analysis, etc.

[0049] In Figure 5(b), the area surrounded by a solid line indicates a portion 40 with a high risk of fracture. In contrast, the portion surrounded by a thick dotted line indicates a portion 41 with a low risk of fracture. By this press forming, spot welding was performed only at spot welding points falling within the portion 41 with a low risk of fracture (a portion where the sheet thickness reduction rate is below a limit value), and an integrated blank was manufactured. Needless to say, the integrated blank was manufactured without spot welding at welding points falling within other portions (portions with a high risk of fracture, i.e., portion 40 with a high risk of fracture surrounded by a solid line in Figure 5(b)).

[0050] Similarly, the overlapping portion 37 between the B-pillar lower section 34 and the rocker 35 and the overlapping portion 38 between the B-pillar upper section 33 and the A-pillar upper section 31 were also determined to determine the areas to be spot welded and the other areas (areas not to be spot welded), and a press-molded blank 30 was manufactured. Figure 6 is a conceptual diagram showing the spot welding positions at the overlapping portions 36, 37 between the A-pillar lower section 34 and the rocker 35, as determined through simulation analysis. Figure 6(a) shows the spot welding positions at the overlapping portion 36 (L-shaped) between the A-pillar lower section 32 and the rocker 35 and the overlapping portion 37 (T-shaped) between the B-pillar lower section 34 and the rocker 35, as viewed from the outside. Figure 6(b) shows the same view from the inside, revealing the spot welding positions at the overlapping portions 36, 37 of the rocker 35. The integrated blank formed by joining the partial blanks in this manner was hot-pressed to produce a press-molded part that would become a door ring.

[0051] Figure 7 shows a schematic representation of the stress state after hot pressing the integrated blank 30 shown in Figure 6, determined by FEM simulation. Figure 7(a) shows the stress state at the overlapping portion 36 (L-shaped) of the A-pillar lower section 32 and the overlapping portion 37 (T-shaped) of the B-pillar lower section 34, as viewed from the outside. The thick dotted line in Figure 7(a) indicates the area 41 with low fracture risk, and only this area was spot welded. Figure 7(b) shows the same view from the inside, showing the stress state at the overlapping portions 36, 37 of the rocker 35. After hot pressing the actual integrated blank, i.e., in the press-formed part that would become the door ring, no fractures were observed at either overlapping portion.

[0052] As an example of joining other than spot welding, an example of overlap laser welding is shown in Figure 11 . In the example of Figure 11 , overlap laser welding was used instead of the spot welding of Figure 6 when manufacturing the same integrated blank as Figure 6 . Similar to Figure 6 , Figure 11 (a) shows the weld positions (positions of the overlap weld lines 42) at the overlap portion 36 (L-shaped) between the A-pillar lower 32 and the rocker 35 and the overlap portion 37 (T-shaped) between the B-pillar lower 34 and the rocker 35, as viewed from the outside. In Figure 11 (a), the thick dotted line indicates the area 41 with low fracture risk, and overlap welding was performed only in this area. Figure 11 (b) shows the same view from the inside, showing the weld positions (positions of the weld lines 42) at the overlap portions 36 and 37 of the rocker 35. The integrated blank, in which the partial blanks were joined in this manner, was hot-pressed to produce a press-formed part that would become a door ring. In this case, the integrated blank is also produced by laser overlap welding only in the area 41 where there is little risk of fracture, but the method of drawing the weld line 42 for the overlap welding is not particularly limited. As with determining the welding points for spot welding, the overlapping portions of the partial blanks may be joined at the ends or the center, or both. Figure 11 also shows, in a contour diagram, the stress state after hot pressing the integrated blank 30 produced in this manner. The stress state obtained was approximately the same as when the integrated blank was produced by spot welding (Figure 7), and no fracture was observed in the overlapping portions of the partial blanks.

[0053] In summary, it is advisable to determine the material flow rate (movement vector) of each partial blank at the overlapping portion during press forming in advance, and join only the portions of the partial blanks that constitute the overlapping portion where the absolute value of the difference in material flow rate at corresponding positions of each partial blank (absolute value of the difference in movement vectors) is smaller than a predetermined limit value. There are no particular limitations on the method for determining the difference in material flow rate of partial blanks by simulation in advance. For example, the deformation behavior of the material may be determined using FEM, or it may be determined by conducting a press forming test using a test piece that simulates an actual overlapping portion. Furthermore, for example, the evaluation may further take into account the rate of thickness reduction before and after pressing at a specific position of the partial blank.

[0054] Whether the difference in the amount of material flow is large can be evaluated using a predetermined limit value. There are no particular limitations on how to set the predetermined limit value. It may be set based on a prior analysis using simulations such as FEM or actual values. When evaluating based on the thickness reduction rate, it is advisable to determine the limit value as the absolute value of the thickness reduction rate of the overlapping partial blanks. The limit value is set as an absolute value because there are cases where the thickness increases (for example, when wrinkles occur).

[0055] As explained above, L-shaped portions such as the overlapping portion 36 of the A-pillar lower 32 and rocker 35, and T-shaped portions such as the overlapping portion 37 of the B-pillar lower 34 and rocker 35, are areas that involve complex deformation (complex material flow behavior), so the effects of applying this invention are particularly pronounced.

[0056] [Joining in the post-press part joining process] Portions of the overlapping portions of the integrated blank that are not joined (by spot welding, etc.) remain unjoined after press forming, and may therefore lack strength and rigidity, preventing the performance as designed. Therefore, after press forming (after the pressing process), it is advisable to join (by spot welding, etc.) the overlapping portions that were not joined in the press-forming blank processing process.

[0057] Generally, after an integrated blank is press-formed to obtain a press-formed product, additional components are often joined to the press-formed product (post-press part joining process). Therefore, it is preferable to include a process of joining (by spot welding, etc.) the unjoined portions of the overlapping portions (spot welding in the above example) in the post-press part joining process. This is because joining the unjoined portions of the overlapping portions as an extension of the conventional post-press part joining process can be efficiently achieved without adding a new process. A press-formed part can be obtained by going through these processes.

[0058] Based on the above explanation, an overview of the manufacturing process for press-formed parts using TWB according to the present invention is shown in Figure 8. Compared to the conventional manufacturing process (Figure 2), it differs in the following respects: The press blank processing step includes a step of analyzing the material flow behavior of the partial blank at the overlapping portion in advance, determining that the portion exceeding a predetermined limit value is a fracture risk region, and joining only a portion of the overlapping portion that does not fall within the fracture risk region. Furthermore, the post-press part joining step includes a step of joining the unjoined portion (the remainder) of the overlapping portion.

[0059] [Press-Molded Part] The part (press-molded part) obtained by the above-described method is an integrated blank formed by joining multiple partial blanks made of steel plates. At the overlapping portion where at least two partial blanks are partially overlapped, the partial blanks constituting the overlapping portion are joined at multiple joints. Some of the joints at the overlapping portion are formed before hot press forming, while the remaining joints (the remaining joints) are formed after hot press forming. Therefore, in the case of joining by welding such as spot welding or lap welding, friction stir welding, or friction welding, the HAZ softened portion formed before hot press forming disappears due to heat treatment in the hot press. On the other hand, in the joints formed by welding or friction stir welding after hot press forming, the HAZ softened portion remains because it is not heat treated in the hot press. That is, the resulting part (press-molded part) has no HAZ softened portion in some overlapping portions, but has a HAZ softened portion in other overlapping portions. The following explanation uses spot welding as an example. As mentioned above, the joining method is not limited to spot welding.

[0060] A HAZ-softened zone is a phenomenon in which the heat-affected zone (HAZ) in the base metal located just outside the outer edge of the weld metal in spot welding nuggets, arc welding, etc., is tempered and becomes softer than the base metal. Similarly, in friction stir welding, friction welding, and brazing, the softened zone that occurs in the heat-affected zone (HAZ) of the base metal that occurs just outside the outer edge of the joint is a HAZ-softened zone. The following explanation will use spot welding as an example.

[0061] FIG. 9 shows an example of the correspondence between the results of a cross-sectional examination of a spot-welded test piece 90 and the hardness distribution of a spot weld 91 (near the spot weld point) and a base material 92 (corresponding to a partial blank). Note that, unless otherwise noted, hardness refers to Vickers hardness. As can be seen in FIG. 9 , the spot weld, including the joint (nugget), is quenched and has a hardness of approximately Hv 500 (since the hardness is roughly the same within the spot weld point, the hardness at the center of the spot weld point can be used as a representative value). Meanwhile, the hardness at a point approximately 1 mm away from the edge (outer edge) of the spot weld nugget 93 (near the outer edge of the spot weld) is approximately Hv 300, indicating softening. This softened portion is the HAZ softened portion. Typically, the HAZ softened portion occurs within a region no farther than 5 mm from the edge (outer edge) of the joint (nugget or weld metal).

[0062] Further away from the welded portion (nugget 93), the hardness converges to that of the base material 92 (FIG. 9 shows that the hardness converges to just under Hv 500). The hardness of the HAZ-softened portion relative to the center of the spot weld point 91 is, for example, smaller by Hv 50 or more when the base material 92 is a 1.0 GPa-class steel plate, smaller by Hv 100 or more when the base material is a 1.5 GPa-class steel plate, and smaller by Hv 150 or more when the base material is a 2.0 GPa-class steel plate. Generally speaking, if the hardness of the base metal 92, that is, the hardness of the base metal 92 in a portion not affected by the spot weld, is defined as Hvm, the maximum value of the hardness measured within a radius of 12 mm from the center of the spot weld 91 (or within 5 mm from the edge of the joint to the outside (base metal side)) is defined as the maximum hardness, the minimum value is defined as the minimum hardness, and the difference between the maximum and minimum hardness is defined as ΔHv, then if the HAZ is not softened, ΔHv should be less than 0.2 Hvm, preferably 0.1 Hvm or less. Conversely, if the HAZ is softened, ΔHv should be 0.2 Hvm or more, preferably 0.3 Hvm, 0.4 Hvm, or 0.5 Hvm or more.

[0063] The hardness distribution of the HAZ-softened zone can be determined by measuring the hardness (Vickers hardness) of the outermost partial blank at a thickness cross section including the center of the joint (the center of the spot weld) of the outermost partial blank, at a position 1 / 4 of the thickness from the surface where the partial blank contacts the other partial blanks, along a straight line (hardness measurement line) parallel to the surface of the partial blank, from the center of the spot weld outward. For example, the hardness distribution can be measured in the thickness cross section of the outermost partial blank that constitutes the overlapping portion. First, the hardness of the base material (the hardness of the base material in a portion not affected by the spot weld) is measured at a position 15 mm or more away from the center of the spot weld, where the spot weld is not present, and this hardness is designated Hvm. Note that the outermost partial blank here refers to the partial blank on the upper surface side when the press-formed part is placed with the convex shape facing up.

[0064] Next, hardness measurements are made from the outer edge of the spot weld outward, along the centerline of the partial blank's plate thickness, to a range of 12 mm from the center of the spot weld, to determine ΔHv. In particular, measurements should be made at intervals (pitch) of 0.1 to 0.2 mm in the range from 0.5 to 1 mm inside the outer edge of the spot weld to 2 to 3 mm outside the outer edge. This is because the minimum hardness value of the HAZ softened by spot welding often exists within this range.

[0065] The spot welds without HAZ softening were applied before hot press forming. That is, they are spot welds in areas where the difference in material flow rate between the partial blanks at the overlapping portion is small. On the other hand, the spot welds with HAZ softening were applied after hot press forming, and are therefore spot welds in areas where the difference in material flow rate between the partial blanks is large, such as areas that undergo stretch flange deformation during press forming. That is, in the press-formed part according to the present invention, some of the multiple spot welds (at least one spot weld that was joined before hot press forming) have a ΔHv of less than 0.2 Hvm. On the other hand, the other spot welds (spot welds other than the above-mentioned part, at least one other spot weld that was joined after hot press forming) have a ΔHv of 0.2 Hvm or more because they have a HAZ softening portion.

[0066] As explained in the press-forming method, the press-formed part according to the present invention is a press-formed part that is joined by spot welding or the like before press-forming in a portion where the absolute value of the difference in material flow rate between the partial blanks is smaller than a predetermined limit value, by determining the material flow rate of each partial blank in advance of press-forming. That is, some of the above-mentioned spot welds (spot welds where ΔHv is less than 0.2 Hvm) are the corresponding portions, i.e., the portions where the absolute value of the difference in material flow rate is smaller than the predetermined limit value.

[0067] From the shape of a press-formed part, it is also possible to determine whether spot welding was performed before press forming only in areas where the absolute value of the difference in material flow rate between each partial blank is smaller than a threshold value. For example, analysis can be performed by measuring the 3D shape of the press-formed part, centered on the overlapping portion of the partial blank, to obtain shape data for the part. From the obtained 3D shape data, blank data for the overlapping portion before press forming can be created. For example, by using a device such as the AUTOFORM R.10 manufactured by AUTOFORM, data on the blank to be used for press forming can be obtained from the shape data of the press-formed part. Using the obtained blank data, the difference in material flow rate between each partial blank due to press forming can be analyzed using FEM or other methods, as described in the manufacturing method of press-formed parts above. This allows for determining whether spot welds without HAZ softening (spot welds performed before press forming) belong to areas where the difference in material flow rate due to press forming is smaller than a threshold value. Similarly, it allows for determining whether spot welds with HAZ softening (spot welds performed after press forming) belong to areas where the difference in material flow rate due to press forming is larger than a threshold value.

[0068] The above description has been given using spot welding as an example. Even with joining methods other than spot welding, such as arc welding and laser welding, in which the base steel plate is heated and melted to join, or friction stir welding, friction welding, brazing, and other methods in which the base steel plate is heated but not melted to join, a HAZ-softened zone similarly occurs in the joined portion after hot press forming. For example, in the case of arc welding, a HAZ-softened zone occurs in the base material outside the outer edge of the weld metal. For example, in the case of friction stir welding, a HAZ-softened zone also occurs in the base material outside the outer edge of the joined portion. Even with joining methods other than spot welding, as with spot welding, it is advisable to measure the Vickers hardness along a hardness measurement line located 1 / 4 of the plate thickness from the surface of the partial blank where the partial blank contacts the other partial blank in a cross section that includes the center of the joined portion of the outermost partial blank and is perpendicular to the surface of the partial blank.

[0069] For example, in the case of lap fillet welding or lap welding by arc welding or laser welding, it is advisable to measure the hardness distribution by setting a hardness measurement line on a cross section perpendicular to the weld line. In this case, the center of the joint should be the center of the hardness measurement line in the weld metal. In the case of friction stir welding, friction welding, and brazing, it is advisable to measure the hardness distribution in the same way as in spot welding.

[0070] The hardness distribution measurement procedure is similar to that for spot welding, and therefore can be performed in accordance with the same measurement procedure. Specifically, when the Vickers hardness at a non-welded position 15 mm or more away from the center of the welded portion (e.g., weld metal) in the target partial blank is defined as Hvm, the difference between the maximum and minimum Vickers hardness within a range of 5 mm outward from the edge of the welded portion is defined as ΔHv. Since the HAZ softening is eliminated in a portion of the welded portion (those hot-pressed after welding), ΔHv is less than 0.2 Hvm, preferably 0.1 Hvm or less. Since the HAZ softening is eliminated in the other welded portions (those other than the portion, e.g., those welded after hot-pressing), ΔHv is 0.2 Hvm or more, preferably 0.3 Hvm, 0.4 Hvm, or 0.5 Hvm or more.

[0071] [Other Examples] Figure 10 is a schematic diagram of an example application to an automobile floor module 100. Conventionally, floor modules are manufactured by manufacturing components separately, overlapping them, and joining them (by spot welding, for example). However, by applying the blank manufacturing method according to the present invention, an integrated blank (blank for press molding) for the front module 100 is manufactured, and this is then hot-pressed in one go to manufacture the floor module 100. In this case, the blank and blank manufacturing method according to the present invention were applied to the joining (by spot welding, for example) of the six overlapping portions 101. As a result, even when hot-pressed in one go, the floor module 100 was obtained without fracture at any of the overlapping portions 101.

[0072] As described above, the parts (press-formed parts) obtained by the present invention are free from cracks and have excellent crash resistance. Compared to conventional spot-welded assemblies in which all spot welding is performed either before or after hot press forming, the press-formed parts of the present invention have the following advantages:

[0073] Compared to conventional parts in which all spot welding is performed before hot press forming, press-formed parts according to the present invention have improved formability. This allows for greater part depth and steeper vertical wall angles, thereby increasing the total plastic bending moment at each cross section. Furthermore, excessive thickness reduction at joints with high material flow (such as spot welds) can be eliminated, improving the crashworthiness of the module.

[0074] Furthermore, the press-molded part according to the present invention eliminates the HAZ softening portion, mainly on the top surface (for example, the top surface of a part with a hat-shaped cross section), improving the bonding strength between the parts and enabling improved crashworthiness of the module.

[0075] The press-molding blank, the method for manufacturing the press-molding blank, the press-molded part, and the method for manufacturing the press-molded part according to the present invention have been described above using an automobile door ring and floor module as examples. The blank, part, and the method for manufacturing the same according to the present invention are not limited to the examples used in the above description. The present invention can be applied to any TWB having an overlapping portion, regardless of its type or structure.

[0076] The present invention can be widely used in industrial fields such as the automobile and other transportation machinery industry, the general machinery industry, and electrical equipment industry.

[0077] 1 Door ring 2 A pier (front pillar) 3 B pillar (center pillar) 4 Rocker 30 Press forming blank 31 A pillar upper (upper A pillar) 32 A pillar lower (lower A pillar) 33 B pillar upper (upper B pillar) 34 B pillar lower (lower B pillar) 35 Rocker 36 Overlapped part of A pillar lower and rocker 37 Overlapped part of B pillar lower and rocker 38 Overlapped part of B pillar upper and A pillar upper 40 Part with fracture risk 41 Part with low fracture risk 42 Weld line of overlap welding 90 Spot welded test piece 91 Spot welded part (spot weld point) 92 Base material 93 Nugget

Claims

1. A method for manufacturing a press-molded part, comprising: a press-molding blank processing step for obtaining a press-molding blank which is a blank for press molding formed by joining together a plurality of partial blanks made of steel plate, the blank having an overlapping portion formed by partially overlapping at least two of the partial blanks; a hot press forming step for obtaining a press-molded product by hot press forming the blank for press molding; and a post-press part joining step for joining parts of the press-molded product, wherein the press-molding blank processing step includes a step of determining in advance the amount of material flow for each of the partial blanks which constitute the overlapping portion formed by press molding, and joining the partial blanks which constitute the overlapping portion only at portions where the difference in the amount of material flow for the partial blanks is smaller than a predetermined limit value, and the post-press part joining step includes a step of joining portions of the overlapping portion which were not joined in the press-molding blank processing step.

2. The method for producing a press-molded part according to claim 1, wherein the joining is by spot welding.

3. A method for manufacturing a press-molded part according to claim 1 or 2, wherein the predetermined limit value is 1 mm in absolute value of the difference in the amount of material flow into the partial blank by press molding.

4. A method for manufacturing a press-molded part described in any one of claims 1 to 3, wherein the overlapping portion of the press-molded part includes an L-shaped or T-shaped portion.

5. A press-formed part in which a plurality of partial blanks made of steel plates are joined, at least two of the partial blanks are joined at a plurality of joints at an overlapping portion formed by partially overlapping the other partial blanks, wherein, in a cross section perpendicular to the surface of the partial blank including the center of the joint of the outermost partial blank among the partial blanks, at a position 1 / 4 of the plate thickness from the surface of the partial blank that contacts the other partial blanks, when the Vickers hardness at a position 15 mm or more away from the center of the joint and not joined is Hvm, for some of the plurality of joints, ΔHv, which is the difference between the maximum and minimum Vickers hardness within a range of 5 mm toward the base material from the end of the joint, is less than 0.2 Hvm, preferably 0.1 Hvm or less, and for the other joints of the plurality of joints, the ΔHv is 0.2 Hvm or more.

6. The press-formed part of claim 5, wherein said joint is a spot weld.

7. A press-molded part as described in claim 5 or 6, wherein a portion of the multiple joints is set to a portion where, when the material flow rate of the partial blank at the overlapping portion is calculated in advance by press molding, the absolute value of the difference in the material flow rate of the partial blank is smaller than a predetermined limit value.

8. The press-molded part according to claim 7, wherein the predetermined limit value is 1 mm in absolute value of the difference in the amount of material flow of the partial blank by press molding.

9. A press-molded part according to any one of claims 5 to 8, wherein the joints other than the portion of the plurality of joints are set in a portion that undergoes stretch flange deformation due to press molding.

10. The press-molded part according to any one of claims 5 to 9, wherein the overlapping portion of the press-molded part includes an L-shaped or T-shaped portion.

11. A method for manufacturing a press-molding blank in which a plurality of partial blanks made of steel plate are joined together to form an integrated blank, the blank having an overlapping portion formed by partially overlapping at least two of the partial blanks, the method comprising the steps of: determining in advance the amount of material flowing into each of the partial blanks due to press molding at the overlapping portion; and joining the partial blanks that form the overlapping portion only in portions where the difference in the amount of material flowing into the partial blanks is smaller than a predetermined limit value.

12. The method for producing a blank for press molding according to claim 11, wherein the joining is by spot welding.

13. A method for manufacturing a blank for press molding according to claim 11 or 12, wherein the predetermined limit value is 1 mm in absolute value of the difference in the amount of material flow into the partial blank by press molding.

14. A method for producing a press-molding blank according to any one of claims 11 to 13, wherein the overlapping portion of the press-molding blank includes an L-shaped or T-shaped portion.

15. A press-molding blank formed by joining together a plurality of partial blanks made of steel plate, the press-molding blank having an overlapping portion formed by partially overlapping at least two of the partial blanks, characterized in that the partial blanks constituting the overlapping portion are joined at a portion of the overlapping portion and not joined at other portions.

16. The press forming blank according to claim 15, wherein said joints are spot welds.

17. A blank for press molding as described in claim 15 or 16, wherein a portion of the overlapping portion is a portion in which, when the material flow rate of each of the partial blanks at the overlapping portion is previously determined by press molding, the absolute value of the difference in the material flow rate of the partial blanks is smaller than a predetermined limit value.

18. A press-molding blank according to claim 17, wherein the predetermined limit value is 1 mm in absolute value of the difference in the amount of material flow caused by press-molding the partial blank.

19. A blank for press molding according to any one of claims 15 to 18, wherein the other portion of the overlapping portion is a portion that undergoes stretch flange deformation due to press molding.

20. A blank for press molding according to any one of claims 15 to 19, wherein the overlapping portion of the blank for press molding includes an L-shaped or T-shaped portion.

Images