Press forming blanks

Abstract

To suppress fracture including cracking of an integrated blank formed by partially overlapping a plurality of partial blanks, in performing press-molding.SOLUTION: A blank for press-molding, which is integrated by joining a plurality of partial blanks made of steel plates, has an overlapped part at which at least two partial blanks are partially overlapped with each other. Material inflow (movement vectors of points on the partial blanks) of the partial blanks through press-molding at the overlapped part is determined in advance. The partial blanks constituting the overlapped part are joined to each other, through only portions at which an absolute value of a difference in the material inflow (the movement vectors) between the partial blanks is smaller than a predetermined limit value, so as to obtain the blank for press-molding.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[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. [Background technology]

[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 manufacturing 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 some time (for example, Patent Document 1).

[0003] Blanks integrated by TWB (integrated blanks) are typically manufactured by butt-welding separate blanks (partial blanks) made of steel plates of different thicknesses and types. As integrated blanks become larger, a method has been proposed for manufacturing integrated blanks by partially overlapping two partial blanks and spot welding the overlapping portion (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 the manufacture of automotive structural parts, in which an inner front pillar, an inner center pillar, and an inner back rail are separately manufactured by hot pressing, and reinforcing blanks are locally attached to the inner front pillar and the inner center pillar and then hot pressed. However, although the automotive structural part disclosed in Patent Document 3 is integrally molded for each part in a broad sense, it is not an integrally molded automotive structural part (Patent Document 3 is an upper door ring part). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2020 / 059804 [Patent Document 2] Special Publication No. 2021-528248 [Patent Document 3] International Publication No. 2017 / 098427 Summary of the Invention [Problem to be solved by the invention]

[0006] When two partial blanks are overlapped and joined (for example, by spot welding or lap welding) to form an integrated blank, cracks or other fractures may be observed in the overlapping area of ​​the partial blanks. If cracks or other fractures occur in the 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 had 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 shapes for parts and the demand for integral forming of large parts (such as automotive structural parts like door rings), there has been a demand for integrated hot press forming of tailored blanks. However, cracks and other fractures have been observed in 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. [Means for solving the problem]

[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 multiple partial blanks made of steel plates together, we investigated and verified the occurrence of fractures by manufacturing a press-forming blank with an overlapping section where at least two partial blanks are partially overlapped. As an example, we manufactured a press-forming blank with the overlapping section joined by spot welding and proceeded with the investigation and verification.

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

[0012] (stomach) Further analysis revealed that the fracture occurred in (i) the area where the material flow behavior of the partial blank differed significantly when the overlapping portion deformed, and (ii) the area where material flow was restricted by the joint (spot welding in the case of the investigated example).

[0013] In other words, when the difference in the amount of material flow between the partial blanks increases due to press forming, shear deformation occurs at the joint (spot weld), and it is thought that large concentrations of deformation occur near the spot weld (factor (i)). The joining (spot welding) restricts the flow of material, preventing deformation accompanied by material flow, such as stretch flange deformation, and this concentration of deformation leads to 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 parts with more complex shapes, there is a demand for integrated blanks and hot press forming methods that contribute to the hot press forming of integrated blanks using tailored blanks.

[0014] (cormorant) In order to prevent breakage, the inventors analyzed the material flow behavior of partial blanks during hot press forming using simulations, divided them into areas with small differences in material flow rate and areas 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 areas with small differences in material flow rate, and proceeded with development.

[0015] As a result, it was confirmed that no fracture would occur if no joining was performed, even in areas where there was a large difference in the amount of material flow between partial blanks, such as areas that would undergo stretch flange deformation. It was also confirmed that the unjoined portions of the overlapping sections can be joined by spot welding or other methods after hot press forming to obtain press-molded parts with integrated partial blanks. The above investigation and study was carried out using blanks for press forming in which the overlapping parts of partial blanks were joined by spot welding, but this is not limited to spot welding; the same applies when overlapping parts are joined by other joining methods (for example, 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 enable hot press forming of one-piece blanks (TWBs) with overlapping sections (overlapped sections) without fracture, and that press-formed parts with strength and rigidity can be obtained.

[0016] The present invention was made based on the above findings, and the gist of the present invention is as follows. [1-1] a press-molding blank processing step for obtaining a press-molding blank which is an integrated press-molding blank obtained by joining a plurality of partial blanks made of steel plates, the press-molding blank having an overlapping portion formed by partially overlapping at least two of the partial blanks; a hot press forming step of obtaining a press-formed product by hot press forming the press-forming blank; a post-press part joining step of joining a part of the press-molded product; the press-molding blank processing step includes a step of determining in advance the amount of material flowing into each of the partial blanks constituting the overlapping portion by press molding, and joining the partial blanks constituting the overlapping portion only at portions where the difference in the amount of material flowing into the partial blanks is smaller than a predetermined limit value; The post-press part joining step includes a step of joining a portion of the overlapping portion that has not been joined in the press-molding blank processing step. A method for manufacturing a press-molded part. 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 (partial blanks) that would occur at a location corresponding to a welded part if there was no welding), and can be the amount of misalignment when the resulting part has an allowable tensile shear stress (TSS) or less. [1-2] The method for manufacturing a press-molded part according to [1-1], wherein the joining is spot welding, a press-molding blank processing step for obtaining a press-molding blank obtained by joining together a plurality of partial blanks made of steel plates, the press-molding blank having an overlapping portion where at least two of the partial blanks are partially overlapped; a hot press forming step of obtaining a press-formed product by hot press forming the press-forming blank; a post-press part joining step of spot welding a part of the press-molded product; the press-molding blank processing step includes a step of determining in advance the amount of material flowing into each of the partial blanks in the overlapping portion by press molding, and spot-welding the partial blanks constituting the overlapping portion only in a portion where the difference in the amount of material flowing into the partial blanks is smaller than a predetermined limit value; the post-press component joining step includes a step of spot welding a portion of the overlapping portion that has not been spot welded in the press-molding blank processing step, A method for manufacturing a press-molded part. [1-3] The method for manufacturing a press-molded part according to [1] or [1-2], wherein the predetermined limit value is 1 mm, preferably 0.5 mm, in terms of the absolute value of the difference in the material flow rate of the partial blank by press molding. [1-4] The method for producing a press-molded part according to any one of [1-1] to [1-3] above, wherein the overlapping portion of the press-molded part includes an L-shaped or T-shaped portion. [2-1] A plurality of partial blanks made of steel plates are joined together, At least two of the partial blanks are joined at a plurality of joining portions at an overlapping portion configured by partially overlapping each other, In a cross section perpendicular to the surface of the partial blank including the center of the joining portion 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 in contact with 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, A part of the plurality of bonding portions has a Vickers hardness difference ΔHv, which is the difference between the maximum hardness and the minimum hardness within a range of 5 mm from the end of the bonding portion toward the base material (or within 12 mm from the center of the bonding portion), of less than 0.2 Hvm, preferably 0.1 Hvm or less, A press-molded part characterized in that the ΔHv of other joints (joints other than the one) of the plurality of joints 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-molded part according to [2-1], wherein the joining is spot welding, A plurality of partial blanks made of steel plates are joined together, 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, In a cross section including the center of the spot weld 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, When the hardness at a position 15 mm or more away from the center of the spot weld and not spot welded is Hvm, A part of the plurality of spot welds has a difference ΔHv between the maximum hardness and the minimum hardness within a range of 12 mm radius from the center of the spot weld, which is less than 0.2 Hvm, preferably 0.1 Hvm or less, The ΔHv of the other spot welds (spot welds other than the part) of the plurality of spot welds is 0.2 Hvm or more, preferably 0.3 Hvm, 0.4 Hvm, or 0.5 Hvm or more. A press-molded part characterized by: 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. [2-3] A press-molded part according to [2-1] or [2-2], wherein a part of 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 molding, 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-molded part is subjected to 3D shape measurement 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 blank data to be used for press molding. Using the resulting blank data, the difference in material flow rate between each part can be analyzed using a method similar to the analysis method for analyzing the difference in material flow rate between partial blanks performed in the above-mentioned press-molded part forming method. [2-4] The press-molded part according to [2-3], wherein the predetermined limit value is 1 mm, preferably 0.5 mm, in terms of absolute value of the difference in the amount of material flow into the partial blank by press molding. [2-5] The press-molded part according to any one of [2] to [2-4], wherein the other part of the overlapping portion (the part other than the part of the overlapping portion) is a part that undergoes stretch flange deformation due to press-molding. That is, a press-molded part according to any one of [2] to [2-4], wherein the joints other than the part among the plurality of joints are set in areas that are elongated and deformed by press molding. The portion that undergoes elongation deformation due to press forming is a portion of the overlapping portion where, when the material flow rate of the partial blank at the overlapping portion due to press forming is calculated in advance, the absolute value of the difference in the material flow rate of the partial blank is greater than a predetermined limit value. The analysis method involves measuring the three-dimensional shape of the press-molded part as described above, creating shape data of the part, and analyzing the shape data using, for example, AUTOFORM R.10 manufactured by AUTOFORM, thereby determining the portion that undergoes elongation 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 in which a plurality of partial blanks made of steel plates are joined together to form an integrated blank, the press-forming blank having an overlapping portion formed by partially overlapping at least two of the partial blanks, A method for manufacturing a blank for press molding, characterized in that it includes a step 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 make up the overlapping portion only in the portion where the difference in the material flowing amounts of the partial blanks is smaller than a predetermined limit value. [3-2] The method for manufacturing a press-forming blank according to [3-1], wherein the joining is spot welding, A method for manufacturing a press-forming blank in which a plurality of partial blanks made of steel plates are joined together to form an integrated blank, the press-forming blank having an overlapping portion formed by partially overlapping at least two of the partial blanks, A method for manufacturing a blank for press molding, characterized in that it includes a step of determining in advance the amount of material flowing into each of the partial blanks due to press molding at the overlapping portion, and spot welding the partial blanks that make up the overlapping portion only in the portion where the difference in the material flowing amounts of the partial blanks is smaller than a predetermined limit value. [3-3] A method for manufacturing a blank for press molding according to [3-1] or [3-2], 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 blank by press molding. [3-4] The method for producing a press-molding blank according to any one of [3-1] to [3-3] above, wherein the overlapping portion of the press-molding blank includes an L-shaped or T-shaped portion. [4-1] A press-forming blank obtained by joining and integrating a plurality of partial blanks made of steel plates, the press-forming blank having an overlapping portion formed by partially overlapping at least two of the partial blanks, A blank for press molding, characterized in that the partial blanks constituting the overlapping portion are joined at a portion of the overlapping portion and are not joined at other portions. [4-2] The press-forming blank according to [4-1], wherein the joining is spot welding, A press-forming blank obtained by joining and integrating a plurality of partial blanks made of steel plates, the press-forming blank having an overlapping portion formed by partially overlapping at least two of the partial blanks, A blank for press molding, characterized in that the partial blanks constituting the overlapping portion are joined by spot welding in one portion of the overlapping portion and not spot welded in other portions. [4-3] A blank for press molding according to [4-1] or [4-2], wherein a portion of the overlapping portion is a portion where, when the material flow rate of each of the partial blanks 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 blanks is smaller than a predetermined limit value. [4-4] A press-molding blank according to [4-3] above, wherein the predetermined limit value is 1 mm, preferably 0.5 mm, in terms of absolute value of the difference in material flow rate due to press molding of the partial blank. [4-5] A press-molding blank according to any one of [4-1] to [4-4], wherein the other part of the overlapping portion (the part other than the part of the overlapping portion) is a part 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. [Effects of the Invention]

[0017] According to the present invention, a press-formed product can be obtained in which fractures such as cracks are suppressed when a one-to-one blank (TWB), which is a blank for press-forming formed by joining and integrating a plurality of partial blanks and has an overlapping portion where at least two partial blanks are partially overlapped, is press-formed (hot press-formed). As a result, even large parts with complex shapes can be manufactured efficiently and with high productivity. Furthermore, parts with excellent crash resistance can be obtained. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic diagram showing the appearance of a single door ring of an automobile. [Figure 2] FIG. 1 is a diagram illustrating a conventional part manufacturing process using a general TWB press molding method. [Figure 3] FIG. 1 is a schematic diagram for explaining the configuration of a blank for press-molding a door ring. [Figure 4] These are conceptual diagrams showing the results of stress analysis of the overlapping part of the A-pillar lower. Figure 4(a) is a conceptual diagram showing the stress state of the A-pillar lower part as viewed from the outside, and Figure 4(b) is a conceptual diagram showing the stress state of the rocker as viewed from the inside, in a contour diagram. [Figure 5] 5A is a conceptual diagram showing an example of the spot welding points at the overlapping portion of the A-pillar lower. Fig. 5A shows an example of the spot welding point positions at the overlapping portion of the A-pillar lower, and Fig. 5B is an explanatory conceptual diagram showing, among the spot welding point positions, the parts at risk of fracture surrounded by solid lines and the parts at no risk of fracture surrounded by dotted lines. [Figure 6] This is a conceptual diagram showing the spot welding positions at overlapping parts determined through simulation analysis. Figure 6(a) shows an example of spot welding at the overlapping parts (L-shaped) of the A-pillar lower and the B-pillar lower (T-shaped) when viewed from the outside, and Figure 6(b) is a conceptual diagram showing the same thing as viewed from the inside. [Figure 7]This is a conceptual diagram obtained by FEM simulation of the stress state after hot pressing the integrated blank in Figure 6. Figure 7(a) is a conceptual diagram showing the stress state at the overlapping part (L-shaped) of the lower pillar A and the overlapping part (T-shaped) of the lower pillar B when viewed from the outside, and Figure 7(b) is a conceptual diagram showing the same thing when viewed from the inside. [Figure 8] 1A to 1C are diagrams illustrating a part manufacturing process by press molding using the TWB according to the present invention. [Figure 9] FIG. 2 is an explanatory diagram for explaining a HAZ softened portion caused by spot welding. [Figure 10] FIG. 1 is a conceptual diagram for explaining an example of application of the present invention to a floor module of an automobile. [Figure 11] This is a conceptual diagram showing the overlap welding positions of the overlapping parts determined by simulation analysis, and showing an example of lap welding instead of the spot welding shown in Figure 6. Figure 11(a) shows an example of lap welding at the overlapping parts (L-shaped) of the A-pillar lower and the B-pillar lower (T-shaped) as seen from the outside, and Figure 11(b) is a conceptual diagram showing the same thing as seen from the inside. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be described below by way of an example of an automobile door ring, which is one 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 for the upper portion to ensure interior space in the event of a collision, and a 1.3 GPa-class high-strength steel sheet for the lower portion, which is joined to the rocker 4, to ensure workability and toughness. When manufacturing such parts by press forming, a tailored blank (TWB) is manufactured by joining partial blanks that will become the respective parts as press blanks. This tailored 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] Figure 2 shows an overview of the manufacturing process for a typical TWB press-formed part (press-formed part).

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

[0022] Press blank processing process: This is a process in which the obtained partial blanks are joined to produce a blank for press molding (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 blank for press molding can be obtained by combining and joining predetermined partial blanks to form an integrated unit.

[0023] Hot press forming process: This is the process of hot press-forming the obtained press-forming blank (integrated blank). 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). While the press-forming method is not particularly limited, 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 the austenite temperature range of approximately 900°C, press-formed, and simultaneously quenched by martensitic transformation. Hot press-forming is characterized by reduced press load due to the high temperature at which it is formed, and excellent shape fixability due to the martensitic transformation that results in high strength after forming. For these reasons, it is widely used for 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. There are no particular restrictions on the method for refining press-molded products. 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 when they need to be joined. 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 a partial reinforcing material is attached to the press-molded product, or where parts that cannot be simultaneously molded by 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 TWB press molding 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 pursuit of 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 manufacturing and assembling the aforementioned A-pillar and B-pillar separately. This has led to a need for press-forming blanks to be used for batch press forming of door rings.

[0028] Fig. 3 shows an example of a press-molding blank 30 for a door ring 1 (Fig. 3 shows an example of a single door ring). The press-molding blank 31 in Fig. 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 manufacture an integrated press-forming blank 30 (hereinafter sometimes referred to as an integrated blank).

[0029] In the example of FIG. 3, in the case of door ring 1, 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, in order to manufacture such a door ring by one-time press forming, a partial blank corresponding to A-pillar lower 32 and a partial blank corresponding to rocker 35 are overlapped to manufacture integrated blank 30. Similarly, integrated blank 30 is manufactured by overlapping partial blanks constituting B-pillar lower 34 and rocker 35, and B-pillar upper 33 and A-pillar upper 31, respectively. FIG. 3 shows an overview of the configuration of the partial blanks for press forming this door ring. The overlapping portions of the partial blanks (overlapped portions) 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 Figure 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 can be determined based on the required characteristics and shape of the final part to be manufactured.

[0031] When viewed from above, a part surface that is roughly L-shaped is called an L-shape, and when it is roughly T-shaped, 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 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 part] When an integrated blank is manufactured by joining the entire overlapping portion, fractures (cracks, breaks, etc.) may occur in the overlapping 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 developed after analyzing the cause of fractures.

[0033] The joining method for joining overlapping portions of partial blanks and 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, a case where spot welding (resistance spot welding) is used as a joining method will be described as an example. The present invention is not limited to the embodiment described below. In particular, the joining method is not limited to spot welding, and the various joining methods described above or similar joining methods can be applied. 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 analysis using FEM. 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 section 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 at 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 and fracture occurs.

[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, they realized that breakage during press forming could be prevented by understanding the material flow behavior of each partial blank that makes up the overlapping part through simulations in advance, dividing each partial blank into parts where the difference in material flow rate (difference in material flow rate between each partial blank) is small and parts where the difference in material flow rate is large, and spot welding the partial blanks together only in the parts where the difference in material flow rate is small, and not spot welding the other parts, and proceeded with development based on this idea.

[0038] As a result, it was confirmed that even in areas where the material flow rate difference in 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 aforementioned factor (i) that the material flow behavior of the partial blank differs significantly when the overlapping part deforms remains, it was confirmed that fracture can be prevented by removing 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, due to press forming, of each point (position) in the partial blanks that make up the overlapping portion, which corresponds to an arbitrary point (position) in the overlapping portion before forming. In other words, the movement vector of each point (position) of each partial blank that corresponds to an arbitrary point (position) in 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 each partial blank (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 simulation example will be described. As mentioned above, spot welding will be used as an example, but other joining methods can also 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 or the like) 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 (as described below, for example, 1 mm, preferably 0.5 mm), 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 less than 1 mm), 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 (less than 1 mm), 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) consisting of overlapping 1.5 GPa-class steel plates (1.2 mm thick) spot-welded was heated to 900°C and held there for 1 minute. The steel plates were then stretched at 740°C to shift the specified distance (displacement), and the specimen was then quenched. The test specimen was then stretched at 1 mm / s at room temperature, and the tensile shear strength (TSS) of the joint (spot weld) and the displacement of the two steel plates were observed. The results showed that when the steel plates were displaced from 0 to 1.0 mm during hot welding, 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). However, 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 misalignment 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 (misalignment) of 0.0 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 the 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 the region where the analysis results indicate that the difference in material inflow rate is 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 points before press forming, stress concentration was confirmed at the actual fracture location (the area surrounded by the ellipse in Figures 4(a) and (b)), as shown in Figures 4(a) and (b).

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

[0047] There are no particular restrictions on the method for determining the region where the difference in material inflow rate is small. As mentioned above, it is advisable to set it based on analysis using simulations such as FEM, or on results from prior offline tests or actual press performance.

[0048] In addition to evaluating the difference in material inflow, the thickness reduction rate can also be evaluated. For example, in the case of FEM simulations, the thickness reduction rate for each element can be calculated and added to the difference in material inflow. 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 thickness can increase (e.g., wrinkles), 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 at a high risk of fracture. The thickness reduction rate limit is preferably set at 9%, 8%, 7%, 6%, or 5%. The thickness reduction rate is a relatively easy indicator to grasp, as it can be directly calculated using FEM analysis.

[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 the spot welding points that fell within the portion 41 with a low risk of fracture (the portion where the sheet thickness reduction rate is below the limit value), and an integrated blank was manufactured. Needless to say, the integrated blank was manufactured without spot welding at the welding points that fell within other portions (portions with a high risk of fracture, such as 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-forming 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 at 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, showing 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 way was hot-pressed to produce the press-formed part that would become the door ring.

[0051] Figure 7 is 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 32 and the overlapping portion 37 (T-shaped) of the B-pillar lower 34, as viewed from the outside. The thick dotted line in Figure 7(a) indicates the area 41 with low risk of fracture, and spot welding was performed only in this area. Figure 7(b) shows the same view from the inside, showing the stress state at the overlapping portions 36 and 37 of the rocker 35. After hot pressing of 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] Figure 11 shows an example of overlap laser welding as an example of joining other than spot welding. The example in Figure 11 shows the same integrated blank as Figure 6, but with overlap laser welding instead of the spot welding shown in Figure 6. Similar to Figure 6, Figure 11(a) shows the weld positions (positions of overlap welding lines 42) at the overlapping portions 36 (L-shaped) between the A-pillar lower 32 and the rocker 35 and at the overlapping 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 lines indicate the areas 41 with low fracture risk, and overlap welding was performed only in these areas. Figure 11(b) shows the same view from the inside, showing the weld positions (positions of weld lines 42) at the overlapping portions 36 and 37 of the rocker 35. The integrated blank, with each partial blank joined in this way, was hot-pressed to produce the press-formed part that would become the door ring. In this case, the integrated blank is also produced by overlap laser welding only in the area 41 where there is little risk of fracture, but there are no particular restrictions on how the weld line 42 for the overlap welding is drawn. As with determining the welding points for spot welding, it is advisable to join the ends or center of the overlapping area of ​​the partial blanks, 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 roughly the same as when the integrated blank was produced by spot welding (Figure 7), and no fractures were observed in the overlapping areas 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 the partial blanks that make up the overlapping portion only in the portions 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 also 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 material inflow rate is large can be evaluated using a predetermined limit value. There are no particular restrictions on how to set the predetermined limit value. It can be set based on prior analysis using simulations such as FEM or actual values. When evaluating based on the thickness reduction rate, it is recommended to determine the limit value as the absolute value of the thickness reduction rate of the overlapping partial blanks. The limit value is set to an absolute value because there are cases where the thickness increases (for example, when wrinkles occur).

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

[0056] [Joining in the part joining process after pressing] The overlapping portions of the integrated blank that are not joined (by spot welding, etc.) remain unjoined after press forming, and as a result, the strength and rigidity may be insufficient, and the designed performance may not be achieved. Therefore, after press forming (after the pressing process), it is recommended to join (by spot welding, etc.) the overlapping portions that were not joined during the press forming blank processing process.

[0057] Generally, after an integrated blank is press-formed to obtain a press-formed product, additional parts are often joined to the press-formed product (post-press part joining process). Therefore, it is preferable that the post-press part joining process includes a process of joining (by spot welding, etc.) the unjoined parts of the overlapping portions (spot welding in the above example). This is because joining the unjoined parts of the overlapping portions can be achieved efficiently without adding a new process, as an extension of the conventional post-press part joining 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-molded 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 process includes a step of analyzing the material flow behavior of the partial blank at the overlapping portion in advance, determining that any portion exceeding a predetermined limit value is a fracture risk region, and joining only a portion of the overlapping portion that is not in the fracture risk region. Furthermore, the post-press part joining process includes a step of joining the unjoined portion (remaining portion) of the overlapping portion.

[0059] [Press-molded parts] The part (press-formed part) obtained by the above-described method is an integrated blank formed by joining multiple partial blanks made of steel plates. At a joint 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 were formed before hot press forming, while the remaining joints (the remaining joints) were 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, in the resulting part (press-formed part), the joints at the overlapping portion do not have a HAZ softened portion in some parts of the overlapping portion, but have a HAZ softened portion in other parts. Hereinafter, spot welding will be used 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 or arc welding 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 uses spot welding as an example.

[0061] Figure 9 shows an example of the cross-sectional survey results of a spot-welded test piece 90 and the corresponding hardness distributions of a spot weld 91 (near the spot weld point) and a base material 92 (corresponding to a partial blank). Unless otherwise noted, hardness refers to Vickers hardness. As can be seen in Figure 9, the spot weld, including the joint (nugget), is quenched and has a hardness of approximately Hv500. (Since the hardness is roughly uniform 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 Hv300, indicating softening. This softened area is the HAZ (heat-affected zone). Typically, a HAZ 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 Hv500). The hardness of the HAZ-softened portion relative to the center of the spot weld point 91 is, for example, Hv50 or more less when the base material 92 is a 1.0 GPa-class steel plate, Hv100 or more less when it is a 1.5 GPa-class steel plate, and Hv150 or more less when it 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 hardness among the hardnesses 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 toward the outside (base metal side)) is defined as the maximum hardness, the minimum hardness is defined as the difference between the maximum and minimum hardnesses, and ΔHv is defined as the difference between the maximum and minimum hardnesses. If there is no HAZ softening, ΔHv should be less than 0.2 Hvm, preferably 0.1 Hvm or less. Conversely, if there is HAZ softening, 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 one-quarter of the thickness from the surface where the partial blank contacts the other partial blank, 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, it is effective to measure the hardness distribution 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 at least 15 mm 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 the spot weld often exists within this range.

[0065] The spot welds without HAZ-softened regions were applied before hot press forming. That is, they are spot welds in areas where the difference in material flow rates between the partial blanks at the overlapping portions is small. On the other hand, the spot welds with HAZ-softened regions were applied after hot press forming, and are therefore spot welds in areas where the difference in material flow rates 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 some, 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-softened region.

[0066] As explained in the press-forming method above, 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, after determining the material flow rate of each partial blank in advance by press-forming. That is, some of the above-mentioned multiple 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] The shape of a press-formed part can also be used to determine whether spot welding was performed before press forming only in areas where the absolute difference in material flow between each partial blank is less than a critical value. For example, analysis can be performed by measuring the 3D shape of the overlapping portion of the partial blank in a press-formed part 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, 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 between each partial blank due to press forming can be analyzed using FEM or other methods, as described in the manufacturing method for press-formed parts above. This allows us to determine whether spot welds without HAZ softening (spot welds performed before press forming) belong to areas where the difference in material flow due to press forming is less than a critical value. Similarly, it allows us to determine whether spot welds with HAZ softening (spot welds performed after press forming) belong to areas where the difference in material flow due to press forming is greater than a critical 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, which heat and melt the base steel sheets to join, or friction stir welding, friction welding, brazing, and other methods in which the base steel sheets are heated but not melted, HAZ softening occurs in the joined portion after hot press forming. For example, in the case of arc welding, a HAZ softening occurs in the base material outside the outer edge of the weld metal. For example, in the case of friction stir welding, a HAZ softening 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 outermost partial blank is in contact with 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 should follow the same measurement procedure. Specifically, when the Vickers hardness at a non-welded location 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 5 mm radius from the edge of the welded portion should be defined as ΔHv. Since HAZ softening is eliminated in a portion of the welded portion (those hot-pressed after joining), ΔHv is less than 0.2 Hvm, preferably 0.1 Hvm or less. Since HAZ softening is eliminated in the remaining welded portions (those other than the portion, e.g., those joined 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] FIG. 10 shows a schematic diagram of an embodiment in which the present invention is applied to an automobile floor module 100. Conventionally, floor modules are manufactured by manufacturing components separately, then overlapping and joining them (by spot welding, etc.). 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 all at once 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, etc.) of the six overlapping portions 101. As a result, even when hot-pressed all at once, the floor module 100 could be obtained without fracture at any of the overlapping portions 101.

[0072] As described above, the parts (press-molded 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-molded 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 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 their manufacturing methods 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 that has an overlapping portion, regardless of its type or structure. [Industrial Applicability]

[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. [Explanation of symbols]

[0077] 1 door ring 2 A Pier (Front Pillar) 3 B pillar (center pillar) 4 Lockers 30 Press forming blanks 31 Upper A-pillar (upper A-pillar) 32 Lower A-pillar (lower A-pillar) 33 Upper B-pillar (upper B-pillar) 34 Lower B-pillar (lower B-pillar) 35 Locker 36 A-pillar lower and rocker overlap 37 B-pillar lower and rocker overlap 38 Overlapped area of ​​upper B pillar and upper A pillar 40 Parts at risk of fracture 41 Areas with low risk of breakage 42 Lap welding seam 90 Spot-welded test specimens 91 Spot welds (spot welding points) 92 Base material 93 Nuggets

Claims

1. A press-forming blank obtained by joining and integrating a plurality of partial blanks made of steel plates, the press-forming blank having an overlapping portion formed by partially overlapping at least two of the partial blanks, The partial blanks constituting the overlapping portion are joined at a part of the overlapping portion and are not joined at other parts; A blank for press molding, characterized in that a portion of the overlapping portion is a portion where, when the material flow rate of each of the partial blanks 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 blanks is smaller than a predetermined limit value.

2. 2. The press-forming blank according to claim 1, wherein the joining is by spot welding.

3. 2. The press-forming blank according to claim 1, wherein the predetermined limit value is 1 mm in absolute value of the difference in the amount of material flow caused by press-forming the partial blank.

4. The press-forming blank according to any one of claims 1 to 3, wherein the other portion of the overlapping portion is a portion that undergoes stretch flange deformation during press forming.

5. The press-molding blank according to any one of claims 1 to 3, wherein the overlapping portion of the press-molding blank includes an L-shaped or T-shaped portion.

6. The press-molding blank according to claim 4 , wherein the overlapping portion of the press-molding blank includes an L-shaped or T-shaped portion.

7. 3. The press-forming blank according to claim 2, wherein the predetermined limit value is 1 mm in absolute value of the difference in the amount of material flow caused by press-forming the partial blank.

8. 8. The blank for press forming according to claim 7, wherein the other portion of the overlapping portion is a portion that undergoes stretch flange deformation during press forming.

9. The press-molding blank according to claim 7 , wherein the overlapping portion of the press-molding blank includes an L-shaped or T-shaped portion.

10. The press-molding blank according to claim 8 , wherein the overlapping portion of the press-molding blank includes an L-shaped or T-shaped portion.

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