Press molding component

Abstract

To provide a blank (integrated blank) capable of suppressing breaking such as cracks when the integrated blank formed by partially stacking a plurality of blank parts is press-molded.SOLUTION: A method for manufacturing a press molding blank having a stacked part in which at least two blank parts are partially stacked, the press molding blank having been integrated by joining a plurality of blank parts made of steel plates, calculates a material flowing-in amount of a blank part press-molded (moving vector of point on each blank part) in the stacked part in advance, manufactures a press molding blank by joining the blank parts constituting the stacked part by only a part in which an absolute value of a difference between the material flowing-in amount (moving vector) of the blank parts is smaller than a predetermined limit value, and provides a desired press molding component by joining the other blank parts of the stacked part after hot press molding of the blank.SELECTED DRAWING: Figure 7

Description

Technical Field

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

Background Art

[0002] Centering around automobiles and the like, reduction of life cycle GHG (total emissions of greenhouse gases throughout the life cycle) is required, and there is an increasing demand for integration of parts and modules for the purpose of increasing the efficiency of manufacturing lines by reducing the number of parts and omitting processes. Therefore, with the introduction of optimized design, the demand for manufacturing parts by press forming a blank in which different types of steel plates are combined and integrated in a part, so-called a tailored welded blank (TWB), is increasing. Various press forming technologies for TWB have been proposed previously (for example, Patent Document 1).

[0003] A blank integrated with TWB (integrated blank) is usually manufactured by butting and welding blanks (partial blanks) for each part made of steel plates of different thicknesses and types. With the increase in the size of the integrated blank, a method has been proposed in which two partial blanks are partially overlapped and the overlapping part is spot welded to manufacture the integrated blank (Patent Document 2). In the trend of increasing the size of parts and modules and the accompanying improvement in the efficiency and cost reduction of part manufacturing, part manufacturing by batch press forming of integrated blanks is attracting increasing attention.

[0004] For example, Patent Document 3 describes that when manufacturing automotive structural parts, when manufacturing an inner front pillar, an inner center pillar, and an inner rear rail separately by hot pressing, a reinforcing blank is locally attached to the inner front pillar and the inner center pillar and hot pressed. However, the automotive structural parts disclosed in Patent Document 3 are integrated molding for each part in a broad sense, but not an integral molding of automotive structural parts (Patent Document 3 is an upper door ring part).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] When an integrated blank obtained by overlapping and joining two partial blanks (for example, spot welding, overlap welding, etc.) is press-formed, fractures such as cracks may be observed in the overlapping portion of the partial blanks. When fractures such as cracks occur in the parts, the quality of the parts deteriorates significantly and they are discarded. That is, the fracture of the press-formed product leads to problems not only in terms of quality but also in terms of productivity.

[0007] In hot press forming (hot stamping forming) that can tolerate a large amount of processing strain compared to cold working, such problems have not been recognized. In particular, with the requirements for complex shaping of parts and integral forming of large parts (such as automotive structural parts like door rings), when integral forming by hot pressing of an integrated blank using a tailored blank is required, fractures such as cracks may be observed in the overlapping portion of the partial blanks, and the problem has become apparent.

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

Means for Solving the Problems

[0009] The inventors of the present invention have intensively conducted research and development to achieve the above problems and 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 where at least two partial blanks are partially overlapped was manufactured, and the occurrence state of fracture was investigated and verified. As an example, a press-forming blank with the overlapping portion joined by spot welding was manufactured and the investigation and verification were carried out.

[0011] When observing the parts after press forming, it was found that fractures occurred in the overlapping portion, which is the portion where the partial blanks are overlapped. Taking the door ring of an automobile as an example, through analysis by experiment and simulation, it was found that fractures occurred in the overlapping portion of the A-pillar lower part (the lower part of the A-pillar and the L-shaped part) and the B-pillar lower part (the lower part of the B-pillar and the T-shaped part), in the portion where the flange extends and deforms and in the vicinity thereof. Conversely, it was found that fractures did not occur in the portion where the flange does not deform.

[0012] (B) Furthermore, through further analysis, it was found that the locations where fractures occur are (i) the portions where the material flow behavior of the partial blanks is significantly different when the overlapping portion deforms, and (ii) the portions where the material flow is restricted by joining (in the case of the investigation example, spot welding).

[0013] That is, when the difference in the amount of material flow between the partial blanks increases due to press forming, shear deformation occurs in the joined portion (spot welding portion), and it is considered that large deformation concentration occurs near the spot welding portion (factor (i)). When the material flow is restricted by joining (spot welding) there, deformation accompanied by material flow, such as flange extension deformation, does not occur, and it is considered that the deformation concentrates and fractures occur (factor (ii)). This has also become apparent in hot press forming (hot stamping forming), which allows a larger amount of processing strain compared to cold working, and in processing involving large deformations. With the increasing demand for complex part shapes, there is a need for an integrated blank and a hot press forming method that contribute to the hot press forming of an integrated blank using a tailored blank.

[0014] (c) To prevent breakage, the inventors analyzed the material flow behavior of partial blanks during hot press forming through simulation, divided it into parts with a small difference in material inflow and parts with a large difference in material inflow, and recalled that breakage during hot press forming can be prevented by joining the partial blanks only at the parts with a small difference in material inflow (spot welding, overlap welding, etc.), and then proceeded with the development.

[0015] As a result, it was confirmed that even at parts with a large difference in material inflow between partial blanks, such as the part where the flange deforms in elongation, breakage does not occur without joining. It was also confirmed that a press-formed part in which the partial blanks are integrated can be obtained by joining the unjoined parts of the overlapping portion by spot welding or the like after hot press forming. As described above, investigations were conducted using a press-forming blank in which the overlapping portions of the partial blanks were joined by spot welding. This is not limited to spot welding, and the same applies when the overlapping portions are joined by other joining methods (for example, overlap welding (arc welding, laser welding), fillet overlap welding (arc welding, laser welding), brazing, friction stir welding (FSW), friction pressure welding, etc.). It was confirmed that even an integrated blank (TWB) having a portion where a plurality of partial blanks are overlapped (overlapping portion) can be hot press formed without breaking, and a press-formed part having strength and rigidity can be obtained.

[0016] The present invention has been made based on the above findings, and the gist thereof is as follows. [1-1] A press-forming blank obtained by joining and integrating a plurality of partial blanks made of steel plates, and a press-forming blank processing step of obtaining the press-forming blank having an overlapping portion formed by at least two of the partial blanks being partially overlapped, A hot press-forming step of obtaining a press-formed product by hot press-forming the press-forming blank, A post-press component joining step of joining a part of the press-formed product, In the press-forming blank processing step, the material inflow amount of each of the partial blanks constituting the overlapping portion by press-forming is obtained in advance, and the overlapping portion is formed only by the portions where the difference in the material inflow amount of the partial blanks is smaller than a predetermined limit value. The step includes joining the partial blanks that make up the overlapping portion, The post-press component joining step includes a step of joining a portion of the overlapping portion that was not joined in the press-forming blank processing step, A method for manufacturing a press-formed component, characterized by the above. The predetermined limit value is, for example, the difference in material inflow amount caused by hot press-forming (for example, the displacement amount between materials (between partial blanks) when welding is not performed at a location corresponding to the welded portion), and can be set as the displacement amount when the tension shear stress (TSS) allowed when the component is formed is reached or less. [1-2] The method for manufacturing a press-formed component according to [1―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, and a press-forming blank processing step of obtaining the press-forming blank having an overlapping portion formed by at least two of the partial blanks being partially overlapped, A hot press-forming step of obtaining a press-formed product by hot press-forming the press-forming blank, A post-press component joining step of spot welding a part of the press-formed product, The press forming blank processing step includes a step of obtaining in advance the material inflow amount of each of the partial blanks of the overlapping portion by press forming, and spot welding the partial blanks constituting the overlapping portion only for the portions where the difference in the material inflow amount of the partial blanks is smaller than a predetermined limit value. The press rear part joining step includes a step of spot welding the portions of the overlapping portion that were not spot welded in the press forming blank processing step. A method for manufacturing a press formed part, characterized in that. [1-3] The method for manufacturing a press formed 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 inflow amount of the partial blanks by press forming. [1-4] The method for manufacturing a press formed part according to any one of [1-1] to [1-3], wherein the overlapping portion of the press formed part includes an L-shaped or T-shaped portion. [2-1] A plurality of partial blanks made of steel plates are joined. At least two of the partial blanks are joined at a plurality of joining portions in an overlapping portion formed by partially overlapping them. 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 in contact with the other partial blanks of the partial blank. When the Vickers hardness at a position more than 15 mm away from the center of the joining portion and where joining is not performed is Hvm. For a part of the plurality of joining portions, the difference ΔHv between the maximum hardness and the minimum hardness in terms of the Vickers hardness within a range of 5 mm (or within 12 mm from the center of the joining portion) from the end of the joining portion to the base material side is less than 0.2 Hvm, preferably 0.1 Hvm or less. The other joint parts (joint parts other than the above-mentioned part) of the plurality of joint parts are characterized in that the △Hv is 0.2Hvm or more, preferably 0.3Hvm, 0.4Hvm, or 0.5Hvm or more, in a press-formed part. Here, the outermost partial blank means the partial blank on the surface side of the upper surface when the press-formed part is placed with the convex shape facing upward. [2-2] The press-formed part according to [2-1], wherein the joint is spot welding, A plurality of partial blanks made of steel plates are joined, At least two of the partial blanks are joined by a plurality of spot welds at an overlapping part 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 distance from the center of the spot weld is more than 15 mm and the hardness at a position where the spot weld is not performed is defined as Hvm, For a part of the plurality of spot welds, the difference △Hv between the maximum hardness and the minimum hardness within a range of a radius of 12 mm from the center is less than 0.2Hvm, preferably 0.1Hvm or less, For the other spot welds (spot welds other than the above-mentioned part) of the plurality of spot welds, the △Hv is 0.2Hvm or more, preferably 0.3Hvm, 0.4Hvm, or 0.5Hvm or more A press-formed part characterized by this. Here, the outermost partial blank means the partial blank on the surface side of the upper surface when the press-formed part is placed with the convex shape facing upward. [2-3] A part of the overlapping part is a part where, when obtaining the material inflow amount of the partial blank in the overlapping part by press forming in advance, the absolute value of the difference in the material inflow amount of the partial blank is smaller than a predetermined limit value, in the press-formed part according to [2-1] or [2-2]. The specific analysis method is as follows. First, the press-formed part is measured in three-dimensional shape to create the shape data of the part. From the created shape data, for example, using AUTOFORM R.10 manufactured by AUTOFORM, the part is developed to create the data of the blank to be used for press forming. Using the obtained blank data, the difference in material flow rate of each part can be analyzed by the same method as the analysis method of the difference in material flow rate of the partial blank performed by the above-described forming method of the press-formed part. [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 the material inflow amount of the partial blank by press forming. [2-5] The press-formed part according to any one of [2] to [2-4], wherein the other part of the overlapping part (the part other than the one part of the overlapping part) is a part that undergoes elongation flange deformation by press forming. That is, the press-formed part according to any one of [2] to [2-4], wherein the joining parts other than the one part among the plurality of joining parts are set as parts that undergo elongation deformation by press forming. Note that the part that undergoes elongation deformation by press forming means that when the material inflow amount of the partial blank in the overlapping part by press forming is obtained in advance for a part of the overlapping part, the absolute value of the difference in the material inflow amount of the partial blank is a part larger than the predetermined limit value. And the analysis method is to measure the three-dimensional shape of the press-formed part as described above, create the shape data of the part, and analyze using, for example, AUTOFORM R.10 manufactured by AUTOFORM from the shape data, whereby the part that undergoes elongation deformation can be discriminated. [2-6] The press-formed part according to any one of [2-1] to [2-5], wherein the overlapping part of the press-formed part includes an L-shaped or T-shaped part. [3-1] In a press-forming blank formed by joining and integrating a plurality of partial blanks made of steel plates, a method for manufacturing the press-forming blank having an overlapping portion formed by partially overlapping at least two of the partial blanks, The method includes determining in advance the material inflow amount of each of the partial blanks by press forming in the overlapping portion, and joining the partial blanks constituting the overlapping portion only in a portion where the difference in the material inflow amount of the partial blanks is smaller than a predetermined limit value. A method for manufacturing a press-forming blank, characterized by this. [3-2] The method for manufacturing a press-forming blank according to [3-1] above, wherein the joining is spot welding, In a press-forming blank formed by joining and integrating a plurality of partial blanks made of steel plates, a method for manufacturing the press-forming blank having an overlapping portion formed by partially overlapping at least two of the partial blanks, The method includes determining in advance the material inflow amount of each of the partial blanks by press forming in the overlapping portion, and spot welding the partial blanks constituting the overlapping portion only in a portion where the difference in the material inflow amount of the partial blanks is smaller than a predetermined limit value. A method for manufacturing a press-forming blank, characterized by this. [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 material inflow amount of 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 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 not joined at other parts, and the blank for press forming is characterized in that. [4-2] The blank for press forming according to [4-1], wherein the joining is spot welding, In a blank for press forming in which a plurality of partial blanks made of steel plates are joined and integrated, the blank for press forming having an overlapping portion formed by partially overlapping at least two of the partial blanks, The partial blanks constituting the overlapping portion are joined by spot welding at a part of the overlapping portion and not spot welded at other parts, and the blank for press forming is characterized in that. [4-3] A part of the overlapping portion is a part where the absolute value of the difference in the material inflow amount of each of the partial blanks in the overlapping portion by press forming is smaller than a predetermined limit value when the material inflow amount of each of the partial blanks in the overlapping portion by press forming is obtained in advance, and the blank for press forming according to [4-1] or [4-2]. [4-4] The predetermined limit value is 1 mm, preferably 0.5 mm, which is the absolute value of the difference in the material inflow amount of the partial blanks by press forming, and the blank for press forming according to [4-3]. [4-5] The other part of the overlapping portion (the part other than the one part of the overlapping portion) is a part that undergoes elongation flange deformation by press forming, and the blank for press forming according to any one of [4-1] to [4-4]. [4-6] The overlapping portion of the blank for press forming includes an L-shaped or T-shaped part, and the blank for press forming according to any one of [4-1] to [4-5].

Effect of the Invention

[0017] According to the present invention, in a press-forming blank formed by joining and integrating a plurality of partial blanks, when press-forming (hot press-forming) an integrated blank (TWB) having an overlapping portion where at least two partial blanks are partially overlapped, a press-formed product with suppressed breakage such as cracks can be obtained. Thereby, even for large parts with complex shapes, they can be manufactured efficiently and with high productivity. Furthermore, parts with excellent impact resistance can be obtained.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4

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Figure 10

Figure 11

Embodiments for Carrying Out the Invention

[0019] Regarding the present invention, an automotive door ring, which is an embodiment of the present invention (hereinafter simply referred to as the present invention), will be described as an example. FIG. 1 shows an external view of a single door ring 1 as an example of an automotive door ring. Even this single door ring 1 is usually composed of a combination of multiple types of steel plates. For example, the A-pillar 2 (also called the front pillar) and the B-pillar 3 (also called the center pillar) of the door link 1 may be combined with a high-strength steel plate of 2.0 GPa class at the upper part and a high-strength steel plate of 1.3 GPa class at the lower part that joins the rocker 4 to ensure workability and toughness from the viewpoint of ensuring the vehicle interior space during a collision. When manufacturing such parts by press forming, as a press blank material, partial blanks for each part are joined to manufacture an integrated blank (TWB: Tailored Blank), and this integrated blank is press formed to manufacture the parts constituting the door ring. The parts thus obtained are further welded to manufacture the door ring.

[0020] FIG. 2 shows an outline of the manufacturing process of parts (press-formed parts) by press forming of a general TWB.

[0021] Partial blank processing step: This is a step of manufacturing partial blanks that are parts of the integrated blank. The partial blanks are cut out (blanking) from a predetermined steel plate and refined by laser trimming or the like to manufacture the partial blanks.

[0022] Press blank processing step: This is a process of manufacturing a blank for press forming (integrated blank) by joining the obtained partial blanks. The method of joining the partial blanks is not particularly limited. For example, when butting and welding the partial blanks, it can be carried out by laser welding or arc welding. Also, when overlapping the partial blanks, the joining of the overlapping part can be achieved by, for example, spot welding (resistance spot welding, laser spot welding, etc.), overlap welding (arc welding, laser welding), fillet welding (arc welding, laser welding), brazing, friction stir welding (FSW), friction press welding, etc. By combining and joining predetermined partial blanks to integrate them, a blank for press forming can be obtained.

[0023] Hot press forming process: This is a process of hot press forming the obtained blank for press forming (integrated blank). By press forming the blank for press forming, a part or a part with a shape close to that of the part (near-net shape) (the part obtained after the hot press process is called a press-formed product) can be obtained. The press forming method is not particularly limited, but generally, when press forming a blank using a high-strength steel sheet (for example, a steel sheet having a tensile strength exceeding 590 MPa), hot press forming is preferably carried out. Hot press forming is also called hot stamping (hot stamp process). It is a press forming method in which the blank (steel sheet) is heated to the austenite temperature range of about 900 °C and then press formed while being rapidly cooled to perform quenching by martensite transformation. Hot press forming has the characteristics that since it is formed at a high temperature, the press load can be reduced, and after forming, it has high strength and excellent shape freezing property due to martensite transformation. Therefore, it is widely used for press forming high-strength steel sheets.

[0024] Trimming process: This is a process of purifying a hot press-formed product (including those with a component shape and near-net shape components). The method of purifying the press-formed product is not particularly limited. For example, it includes processes such as removing burrs generated at the ends of the press-formed product with a laser to shape it into a predetermined shape (laser trimming). Also, in the case of a near-net shape press-formed product, it includes the process of machining to obtain the final component shape. If a press-formed product with the final shape is obtained by hot press forming, this trimming process can be omitted.

[0025] Post-press component joining process: This is a process of joining additional components to the obtained press-formed product when necessary. The method of joining the other components is not particularly limited. For example, it may be joined by joining methods such as spot welding, arc welding, laser welding, brazing, etc. Also, there may be cases of attaching partial reinforcing materials to the press-formed product or joining components that cannot be simultaneously formed by press forming. Of course, if there is no need to join other components, this post-press component joining process can be omitted.

[0026] By going through these processes, the ultimately desired component (press-formed component) can be obtained. Note that the manufacturing process of components by press forming of TWB is not limited to the processes described above. Other necessary processes can be added.

[0027] Recently, there has been a demand for improving the efficiency and reducing the cost of component production, and the enlargement of components and modules (components formed by further combining small components) has been pursued. For example, in the case of an automotive door ring, instead of manufacturing and assembling the aforementioned A-pillar and B-pillar separately, it is required to manufacture the door ring by batch press forming. Therefore, a press-forming blank for batch press forming of the door ring is needed.

[0028] FIG. 3 shows an example of a blank 30 for press forming a door ring 1 (FIG. 3 is an example of a single door ring). The press forming blank 31 in FIG. 3 is composed of an upper A-pillar 31 (also called A-pillar upper), a lower A-pillar 32 (also called A-pillar lower), an upper B-pillar 33 (also called B-pillar upper), a lower B-pillar 34 (also called B-pillar lower), and a partial blank corresponding to a rocker 35 joined together. Each partial blank may have the same or different steel grades and sheet thicknesses. In the example of FIG. 3, for example, the A-pillar lower 32 is a 1.5 GPa grade steel sheet with a thickness of 1.4 mm, the B-pillar upper 33 is a 2.0 GPa grade steel sheet with a thickness of 1.4 mm, the B-pillar lower 34 is a 1.0 GPa grade steel sheet with a thickness of 1.2 mm, the rocker 35 is a 1.5 GPa grade steel sheet with a thickness of 1.2 mm, and the A-pillar upper 31 is composed of a 2.0 GGPa grade steel sheet with a thickness of 1.4 mm. A blank 30 for press forming (hereinafter sometimes referred to as an integrated blank) is manufactured by joining these partial blanks together and integrating them.

[0029] In the example of FIG. 3, in the case of the door ring 1, the lower A-pillar (A-pillar lower) 32 and the rocker 35, the lower B-pillar (B-pillar lower) 34 and the rocker 35, and further the upper B-pillar (B-pillar upper) 33 and the upper A-pillar (A-pillar upper) 31 are overlapped and joined to manufacture the door ring 1. Therefore, in order to manufacture such a door ring by batch press forming, a partial blank corresponding to the A-pillar lower 32 and a partial blank corresponding to the rocker 35 are overlapped to manufacture the 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 of the press forming blank of this door ring. The portion where the partial blanks are overlapped (the overlapping portion) is shown hatched in FIG. 3. The overlapping portion 36 between the A-pillar lower 32 and the rocker 35 is L-shaped, the overlapping portion 37 between the B-pillar lower 34 and the rocker 35 is T-shaped, and the 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 partial blanks to be overlapped is not limited. For example, three or more partial blanks may be overlapped. It may be determined according to the required characteristics, shape, etc. of the parts to be finally manufactured.

[0031] In this way, those with a shape generally L-shaped when viewed vertically above the part surface are called L-shaped, and those generally T-shaped are called T-shaped. In the case of a three-dimensional structure such as a hat-shaped cross-sectional shape like the overlapping portion 36 of the A-pillar lower 32 and the rocker 35, which becomes a complex shape such as L-shaped or T-shaped when viewed from above, the material flow behavior at the corner portion becomes complex. Therefore, cracks are likely to occur during the press forming of the integrated blank.

[0032] [Overlapping portion] When the entire overlapping portion is joined to manufacture an integrated blank, breakage (such as cracks and fractures) may occur in the overlapping portion after press forming. Observation of the broken portion revealed that breakage often occurs where the sheet thickness reduction is large due to press forming. Therefore, an analysis was conducted on the cause of the breakage, leading to the present invention.

[0033] Note that the joining method for the overlapping portion of the partial blanks or the joining after press forming of the parts is not particularly limited. As the joining method, for example, resistance welding (resistance spot welding, projection welding, seam welding, etc.), arc welding (overlay arc welding, fillet arc welding), laser welding (overlay laser welding, fillet laser welding, laser spot welding), friction stir welding (FSW), friction pressure welding, brazing, etc. can be applied. In addition, in the joining of parts after press forming, a mechanical fastening method can also be applied. Hereinafter, in the description of the present invention, the case where spot welding (resistance spot welding) is applied as the joining method will be described as an example. The present invention is not limited to the forms described below. In particular, the joining method is not limited to spot welding, and the above-described various joining methods and joining methods similar thereto can be applied. By replacing the spot welding in the description with other joining methods, the application of other joining methods can be understood.

[0034] The analysis of the cause of fracture can be performed by simulation methods such as the finite element method (FEM: Finite Element Method). The inventors of the present invention performed an analysis by FEM. Fig. 4 shows the analysis results at the L-shaped overlapping portion 36 of the A-pillar lower 32 and the rocker 35. Fig. 4(a) shows the stress state of the A-pillar lower 32 as viewed from the outside, and Fig. 4(b) shows the stress state of the rocker 35 as viewed from the inside in the form of a contour map. The portion where the fracture actually occurred is the portion surrounded by an ellipse and corresponds to the portion where the contour map becomes darker. By comparing these, it was confirmed that fracture actually occurred at the portion where the shear stress became high and the fracture state was shown by simulation.

[0035] From this analysis result, it was found that (i) when the overlapping portion is deformed, the material inflow behavior of the partial blank is significantly different, and (ii) the material inflow behavior is restricted by spot welding, which are affecting factors. Here, material inflow means that the material is deformed (moved) by press forming, and the material inflow behavior refers to the deformation behavior of how far and in which direction the material at a specific position is deformed (moved) before and after press forming.

[0036] When the difference in the amount of material inflow between the partial blanks becomes large (factor (i)), shear deformation occurs in the spot weld portion, and it is considered that large deformation concentration occurs in the vicinity of the spot weld portion. Due to the restriction of material flow there (factor (ii)), deformation accompanied by material inflow, such as elongation flange deformation, does not occur. As a result, it is considered that deformation is likely to concentrate and fracture occurs.

[0037] Therefore, in order to suppress the breakage of the integrated blank during press forming, the inventors of the present invention focused on the material inflow behavior of the partial blanks during hot press forming. That is, the material inflow behavior of each partial blank constituting the overlapping portion was grasped in advance by simulation, and the partial blanks were divided into a portion with a small difference in the material inflow amount (the material inflow amount difference between the partial blanks) and a portion with a large difference in the inflow amount. The inventors recalled that by spot-welding only the partial blanks in the portion with a small difference in the material inflow amount and not spot-welding the other portions, breakage during press forming could be suppressed, and thus the development was advanced.

[0038] As a result, it was confirmed that even in a portion where the material inflow amount difference of the partial blank, such as the portion where the flange deforms, is large, no breakage occurs without spot-welding, and hot press forming can be performed. That is, even if the factor (i) that the material inflow behavior of the partial blanks varies greatly when the overlapping portion deforms remains, it was confirmed that the occurrence of breakage can be suppressed by removing the factor (ii) that the material inflow behavior is restricted by spot-welding.

[0039] The material inflow amount difference between the partial blanks is the absolute value of the difference in the movement vectors obtained as a result of the movement of the respective points (positions) corresponding to an arbitrary point (position) of the overlapping portion before forming in the partial blanks constituting the overlapping portion by press forming. That is, for each point (position) of the partial blanks corresponding to an arbitrary point (position) of the overlapping portion, the movement vector of that point is obtained from the point (position) to which it has moved after press forming, and the absolute value of the difference in the movement vectors of the points (positions) of the respective partial blanks (the distance between the respective points (positions) after movement) is obtained as the material inflow amount difference (deviation amount) of that point (an arbitrary point of the overlapping portion).

[0040] A specific example of the simulation will be described. As described above, spot welding will be used as an example, but other joining methods may be applied in accordance with this example. For example, assuming that the partial blank is fixed (referring to a state of being joined by spot welding or the like) at at least one point (for example, the center of gravity of the overlapping part or the center of gravity of the top surface part, etc.) of the part that will become the top surface part (part pressed by a pad or die) when press-molded at the overlapping part, press molding (hot press molding) analysis (simulation analysis by FEM, etc.) of the integrated blank is performed. At that time, the movement vector of each point of the partial blank corresponding to any point of the overlapping part is obtained from the positions before and after the movement of each point. The absolute value of the difference between the movement vectors of the corresponding points for each partial blank obtained is obtained as the material inflow amount difference (shift amount) of that point. If the obtained material inflow amount difference (shift amount) is smaller than a preset limit value (described later, for example, 1 mm, preferably 0.5 mm), the material inflow amount difference is small, so it can be the point (position) to be spot-welded before press molding. On the other hand, if the calculated difference in the amount of material flow (deviation amount) is larger than a preset limit value, the difference in the amount of material flow is large, so spot welding is not performed before press forming, but rather 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 0-1 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). Although the technical reason for this phenomenon has not been elucidated, 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] An example of a specific test will be described. A joint (test piece) obtained by spot welding stacked 1.5 GPa-class steel plates (plate thickness: 1.2 mm) was heated to 900 °C and held for 1 minute, and then the steel plates were pulled at 740 °C so as to be displaced by a predetermined length (displacement amount), and then the quenched test piece was prepared. Thereafter, the test piece was pulled at 1 mm / s at room temperature, and the displacement amount between 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 amount of the steel plate during hot working was 0 to 1.0 mm, the TSS of the joint at room temperature exhibited a TSS equal to or higher than that when the displacement amount was 0 mm (i.e., not hot-worked). On the other hand, when the displacement amount exceeded 1.0 mm, the TSS tended to decrease. Specifically, under the present test conditions, the TSS was 102% compared to the TSS at 0 mm of displacement amount at 0.1 mm of displacement amount, 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. That is, for the first time, it was found that even when a material inflow difference (displacement amount) of 1.0 mm or less but more than 0.0 mm occurs at the point of spot welding in hot press forming (hot stamping forming), a tensile shear strength equal to or higher than that in the case where no deformation occurs can be ensured.

[0043] From this new finding, when hot press forming an integrated blank with overlapping partial blanks, it is advisable to set the limit value of the material inflow difference (displacement amount) to 1.0 mm, preferably preset it to 0.9 mm, 0.8 mm, 0.7 mm, 0.6 mm, or 0.5 mm.

[0044] The point (position) for obtaining the material inflow difference does not need to be particularly limited. For example, the material inflow difference at the point (position) set as the position for finally performing spot welding may be analyzed. Further, for example, the analysis mesh may be made finer to obtain a region where the material inflow difference is below the limit value. In this case, it is advisable to set the spot welding point (position) before press forming within the region where the material inflow difference is analyzed to be small.

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

[0046] Next, assuming that spot welding is performed only at the double-circle point (the point with a black circle inside a white circle) approximately in the center of the overlapping portion 36 shown in FIG. 5(a) and spot welding is not performed at all other welding point positions (the points indicated only by black circles (●)), press forming was simulated by FEM, and the material inflow amounts (movement vectors) of the A-pillar lower 32 and the rocker 35 were obtained. Also, for each node of the FEM, the material inflow amounts (movement vectors) at the corresponding points of the partial blanks constituting the overlapping portion 36 were obtained, and the difference in the material inflow amounts (material inflow amount difference) of each partial blank was obtained. The limit value of the absolute value of the difference in the material inflow amounts at the corresponding nodes (sometimes simply referred to as the limit value of the material inflow amount difference) was set to 1.0 mm as described above, and the portion with a material inflow amount difference greater than that was regarded as the portion with a fracture risk. As a result, the portion 40 surrounded by the solid line in FIG. 5(b) was determined to be the portion with a fracture risk when spot welding is performed and press forming is carried out.

[0047] The method for obtaining the region where the material inflow amount difference is small is not particularly limited. It may be set based on analysis by simulation such as FEM as described above, or from previous offline tests or actual performance values in actual pressing.

[0048] In addition to the evaluation based on the difference in material inflow, the sheet thickness reduction rate may be evaluated. For example, in the case of simulation by FEM, the sheet thickness reduction rate of each element may be obtained and taken into account in the above-mentioned difference in material inflow. This is because in the part where the sheet thickness reduction rate is large, deformation is more likely to concentrate on the spot welding part, and thus the risk of fracture is likely to increase accordingly. In this case, it is advisable to determine a limit value based on the sheet thickness reduction rate. The limit value is set as an absolute value because there are cases where the sheet thickness increases (for example, when wrinkles occur), and in this case, the sheet thickness reduction rate is represented as a negative value. It is advisable to set the absolute value of the sheet thickness reduction rate as the limit value at 10%, and consider the part with a sheet thickness reduction rate exceeding this value as the part with a high fracture risk. The limit value of the sheet thickness reduction rate is preferably 9%, 8%, 7%, 6%, or 5%. Since the sheet thickness reduction rate can be directly obtained by FEM analysis or the like, it is also a relatively easy-to-grasp index.

[0049] In Fig. 5(b), the portion enclosed by the solid line indicates the portion 40 with a fracture risk. On the other hand, the portion enclosed by the thick dotted line indicates the portion 41 with a low fracture risk. Spot welding was performed only at the spot welding points that enter the portion with a low fracture risk (the portion where the sheet thickness reduction rate is below the limit value) 41 by this press forming to manufacture an integrated blank. Needless to say, the integrated blank was manufactured without performing spot welding at the points that enter the other portions (the portion 40 with a fracture risk enclosed by the solid line in Fig. 5(b) as the portion with a fracture risk).

[0050] In the same manner, the overlapping portions 37 of the B-pillar lower 34 and the rocker 35, and the overlapping portion 38 of the B-pillar upper 33 and the A-pillar upper 31 were determined for the parts to be spot-welded and other parts (parts not to be spot-welded), and the blank 30 for press forming was manufactured. FIG. 6 is a conceptual diagram showing the spot-welding positions of the overlapping portions 36 and 37 of the A-pillar lower 34 and the B-pillar lower 34 and the rocker 35 determined by simulation analysis. FIG. 6(a) shows the spot-welding positions at the overlapping portion 36 (L-shaped) of the A-pillar lower 32 and the rocker 35 and the overlapping portion 37 (T-shaped) of the B-pillar lower 34 and the rocker 35 when the rocker 35 is viewed from the outside. FIG. 6(b) shows the same when viewed from the inside, and the spot-welding positions at the overlapping portions 36 and 37 of the rocker 35 can be seen. In this way, the integrated blank obtained by joining the respective part blanks was hot-pressed to manufacture a press-formed part that becomes a door ring.

[0051] FIG. 7 schematically shows the stress state after hot-pressing the integrated blank 30 shown in FIG. 6 obtained by FEM simulation. FIG. 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 when viewed from the outside. In FIG. 7(a), the thick dotted line indicates the portion 41 with less fracture risk, and spot welding is performed only at this portion. FIG. 7(b) shows the same when viewed from the inside, and the stress state at the overlapping portions 36 and 37 of the rocker 35 can be seen. No fracture was observed at either overlapping portion in the actually manufactured press-formed part that becomes a door ring after hot-press forming of the integrated blank.

[0052] As an example of joining other than spot welding, an example of lap laser welding is shown in Fig. 11. The example in Fig. 11 is the same integrated blank as in Fig. 6, but lap laser welding is performed instead of spot welding in Fig. 6. Similar to Fig. 6, Fig. 11(a) shows the welding positions (the positions of the welding lines 42 of the lap welding) at the overlapping portion 36 (L-shaped) of the A-pillar lower 32 and the rocker 35 and the overlapping portion 37 (T-shaped) of the B-pillar lower 34 and the rocker 35 when viewing the rocker 35 from the outside. In Fig. 11(a), the thick dotted line indicates the portion 41 with less fracture risk, and lap welding is performed only at this portion. Fig. 11(b) shows the same when viewed from the inside, and the welding positions (the positions of the welding lines 42) at the overlapping portions 36 and 37 of the rocker 35 can be seen. In this way, the integrated blank formed by joining the partial blanks is hot-pressed to manufacture a press-formed part that becomes a door ring. Also in this case, lap laser welding is performed only at the portion 41 with less fracture risk to manufacture the integrated blank, but the method of drawing the welding line 42 of the lap welding is not particularly limited. Similar to determining the spot welding points, it is advisable to join at the ends or the center of the overlapping portion of the partial blanks, or both. Fig. 11 also shows the stress state after hot-pressing the integrated blank 30 manufactured in this way in a contour plot. A stress state almost equivalent to that in the case of manufacturing the integrated blank by spot welding (Fig. 7) is obtained, and no fracture is observed at the overlapping portion of the partial blanks.

[0053] In summary, the material inflow amount (movement vector) of each partial blank in the overlapping part by pre-pressing (beforehand) is obtained in advance, and the overlapping part is formed only by the partial blanks in the part where the absolute value of the difference in the material inflow amount at the corresponding positions of each partial blank (the absolute value of the difference in the movement vector) is smaller than a predetermined limit value. The method for obtaining the difference in the material inflow amount of the partial blank by simulation in advance is not particularly limited. For example, the deformation behavior of the material may be obtained by FEM, or it may be obtained by conducting a press forming test using a test piece simulating the actual overlapping part. Further, for example, the evaluation may be further considered by taking into account the sheet thickness reduction rate before and after pressing at a specific position of the partial blank.

[0054] Whether the difference in the material inflow amount is large can be evaluated based on a predetermined limit value. The method for setting the predetermined limit value is not particularly limited. It is preferable to set it based on analysis by simulation such as FEM in advance or actual performance values. When evaluating by the sheet thickness reduction rate, the limit value may be determined by the absolute value of the sheet thickness reduction rate of the overlapped partial blanks. The limit value is set as an absolute value because the sheet thickness may increase (for example, when wrinkles occur).

[0055] In particular, as described above, L-shaped parts such as the overlapping part 36 of the A-pillar lower 32 and the rocker 35, and T-shaped parts such as the overlapping part 37 of the B-pillar lower 34 and the rocker 35 are parts accompanied by complex deformation (complex material inflow behavior). Therefore, when the present invention is applied, the effect is remarkable.

[0056] [Joining in the post-press part joining process] The parts that are not joined (such as spot welding) at the overlapping part of the integrated blank are not joined even after press forming, so there may be a case where the strength and rigidity are insufficient and the performance as designed cannot be obtained. Therefore, after press forming (after the press process), it is preferable to join (such as spot welding) the parts that are not joined in the blank processing process for press forming at the overlapping part.

[0057] Generally, after press-forming an integrated blank 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 process of joining (such as spot welding) the parts that are not joined at the overlapping part (spot welding in the above example) is included in the post-press part joining process. This is because it can be efficiently realized without adding a new process by joining the unjoined parts of the overlapping part as an extension of the conventional post-press part joining process. Press-formed parts can be obtained by going through these processes.

[0058] Based on the above description, the outline of the manufacturing process of press-formed parts by TWB according to the present invention is shown in FIG. 8. It differs from the conventional manufacturing process (FIG. 2) in the following points. In the press blank processing step, the material flow behavior of the partial blank at the overlapping part is analyzed in advance, the part exceeding a predetermined limit value is determined as the fracture risk area, and a joining process is included only for a part of the overlapping part that is outside the fracture risk area of the overlapping part. Further, the process of joining the unjoined part (the remaining part) of the overlapping part in the post-press part joining process is included.

[0059] [Press-formed parts] The component (press-formed component) obtained by the method described above is an integrated blank in which a plurality of partial blanks made of steel plates are joined. In the overlapping portion where at least two partial blanks are partially overlapped, the partial blanks constituting the overlapping portion are joined at a plurality of joining portions, which is a press-formed component. Among the plurality of joining portions in the overlapping portion, some are applied before hot press forming, and the remaining ones (joining portions other than some) are applied after hot press forming. Therefore, in the case of joining by welding such as spot welding or seam welding, friction stir welding, friction pressure welding, etc., the HAZ softening portion generated before hot press disappears due to the heat treatment during hot press. On the other hand, in the joining portions such as welding or friction stir welding applied after hot press forming, since they are not heat-treated by hot press, the HAZ softening portion remains. That is, in the obtained component (press-formed component), the joining portion of the overlapping portion has no HAZ softening portion in a part of the overlapping portion, and has a HAZ softening portion in other parts. Hereinafter, spot welding will be described as an example. As described above, it is needless to say that the joining method is not limited to spot welding.

[0060] The HAZ softening portion refers to the phenomenon that the heat affected zone (HAZ) in the base material immediately outside the outer edge of the weld metal in spot welding nuggets, arc welding, etc. is annealed and softened compared to the base material. Similarly, in friction stir welding, friction pressure welding, and brazing, the softening portion generated in the heat affected zone (HAZ) of the base material immediately outside the outer edge of the joining portion is the HAZ softening portion. Hereinafter, spot welding will be described as an example.

[0061] Fig. 9 shows an example of the correspondence between the cross-sectional investigation results of the spot welding test piece 90 and the hardness distributions of the spot welded portion 91 (near the spot welding dot) and the base material 92 (corresponding to the partial blank). Unless otherwise specified, the hardness refers to the Vickers hardness. As can be seen from Fig. 9, since the spot welded portion including the joint portion (nugget) is quenched, it has a hardness of about Hv500 (since the hardness within the spot welding dot is almost the same, it is advisable to use the hardness at the center of the spot welding dot as the representative value). On the other hand, it can be seen that the hardness becomes about Hv300 and softens at a point about 1 mm away from the end (outer edge) of the nugget 93 of the spot welding (near the outer edge of the spot welded portion). This softened portion is the HAZ softened portion. Usually, the HAZ softened portion occurs in a region within 5 mm even far from the end (outer edge) of the joint portion (nugget or weld metal).

[0062] Furthermore, as the distance from the joint portion (nugget 93) increases, the hardness converges to the hardness of the base material 92 (Fig. 9 shows that it converges to a hardness of less than Hv500). For example, with respect to the center of the spot welding dot 91, the hardness of the HAZ softened portion becomes less than Hv50 when the base material 92 is a steel plate of the 1.0 GPa class, less than Hv100 when it is a 1.5 GPa class steel plate, and less than Hv150 when it is a 2.0 GPa class steel plate. Generally speaking, taking the hardness of the base material 92, that is, the hardness at the portion of the base material 92 not affected by the spot welding as Hvm, the maximum value among the hardnesses measured in the range within a radius of 12 mm from the center of the spot welded portion 91 (or within 5 mm from the end of the joint portion to the outside (base material side)) as the maximum hardness, the minimum value as the minimum hardness, and the difference between the maximum hardness and the minimum hardness as △Hv, when there is no HAZ softening, △Hv is preferably less than 0.2Hvm, preferably 0.1Hvm or less. Conversely, when there is HAZ softening, △Hv is preferably 0.2Hvm or more, preferably 0.3Hvm, 0.4Hvm, or 0.5Hvm or more.

[0063] The hardness distribution of the HAZ softened portion can be grasped by measuring the hardness (Vickers hardness) along a straight line (hardness measurement line) parallel to the surface of the partial blank from the center of the spot weld towards the outside at the 1 / 4 position of the thickness from the surface in contact with other partial blanks of the partial blank in the thickness direction cross-section including the center of the joint portion of the outermost partial blank among the partial blanks (the center of the spot weld). For example, it is advisable to measure the hardness distribution in the thickness direction cross-section of the outermost partial blank among the partial blanks constituting the overlapping portion. First, as the base material hardness (hardness at a position not affected by the spot weld of the base material), measure the hardness at a position more than 15 mm away from the center of the spot weld and where the spot weld is not performed, and let the hardness be Hvm. Here, the outermost partial blank means the partial blank on the surface side of the upper surface when the press-formed part is placed with the convex shape facing up.

[0064] Next, from the outer edge of the spot weld towards the outside of the weld, measure the hardness along the center line of the plate thickness of the partial blank in a straight line up to a range of 12 mm from the center of the spot weld to grasp △Hv. In particular, for 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, it is advisable to measure at a measurement interval (pitch) of 0.1 to 0.2 mm. This is because within this range, the minimum value of the hardness of the HAZ softened portion due to the spot weld often exists.

[0065] Spot welding without a HAZ softening part is performed before hot press forming. That is, it is spot welding at a part where the difference in the material inflow amount of the partial blanks in the overlapping part is small. On the other hand, since spot welding with a HAZ softening part is performed after hot press forming, it is spot welding at a part where the difference in the material inflow amount of the partial blanks is large. For example, it corresponds to a part where the elongation flange is deformed by press forming. That is, in the press-formed part according to the present invention, a part of a plurality of spot welds (at least one spot weld joined before hot press forming) has △Hv less than 0.2Hvm. On the other hand, the other spot welds (spot welds other than the above part, at least one other spot weld joined after hot press forming) have a HAZ softening part, so △Hv is 0.2Hvm or more.

[0066] Also, as described in the above press forming method, the press-formed part according to the present invention obtains the material inflow amount of each partial blank in the overlapping part by press forming in advance, and the absolute value of the difference in the material inflow amount between each partial blank is less than a predetermined limit value. It is a press-formed part joined by spot welding or the like before press forming. That is, a part corresponding to a part of the above plurality of spot welds (spot welds with △Hv less than 0.2Hvm), that is, a part where the absolute value of the difference in the material inflow amount is less than a predetermined limit value.

[0067] From the shape of the press-formed part, it is also possible to confirm whether spot welding was performed before press forming only in the part where the absolute value of the difference in the amount of material flow between the partial blanks is smaller than the limit value. For example, it can be analyzed by performing three-dimensional shape measurement centering on the overlapping portion of the partial blanks in the press-formed part to obtain the shape data of the part. The data of the blanks in the overlapping portion before press forming can be created from the obtained three-dimensional shape data. For example, by using an apparatus such as AUTOFORM R.10 manufactured by AUTOFORM, the data of the blanks to be used for press forming can be obtained from the shape data of the press-formed part. Using the obtained blank data, as described in the above manufacturing method of the press-formed part, the difference in the amount of material flow of each partial blank due to press forming can be analyzed by FEM or the like. Thereby, it is possible to determine whether the spot welding without a HAZ softening part (spot welding performed before press forming) belongs to the part where the difference in the amount of material flow is smaller than the limit value due to press forming. Similarly, it is possible to determine whether the spot welding with a HAZ softening part (spot welding performed after press forming) belongs to the part where the difference in the amount of material flow is larger than the limit value due to press forming.

[0068] As described above, spot welding has been described as an example. Also in joining methods other than spot welding, when joining by heating and melting the steel plate as the base material such as arc welding or laser welding, and also when joining by heating the steel plate as the base material without melting such as friction stir welding, friction pressure welding, brazing, etc., similarly, a HAZ softening part is generated in the joined part after hot press forming. For example, in the case of arc welding, a HAZ softening part is generated in the base material outside the outer edge of the weld metal. For example, also in the case of friction stir welding, a HAZ softening part is generated in the base material outside the outer edge of the joined part. Also in joining methods other than spot welding, similar to the case of spot welding, including the center of the joined part of the outermost partial blank, in a cross section perpendicular to the surface of the partial blank, the Vickers hardness may be measured along the hardness measurement line at a position of 1 / 4 of the plate thickness from the surface in contact with the other partial blanks of the partial blank.

[0069] For example, in the case of lap fillet welding or lap welding by arc welding or laser welding, a hardness measurement line should be set on a cross section perpendicular to the weld line to measure the hardness distribution. 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, the hardness distribution should be measured in the same way as in spot welding.

[0070] The method for measuring the hardness distribution is the same as that for spot welding, and therefore the method for measuring the hardness distribution may be followed. That is, when the Vickers hardness at a position not joined and 15 mm or more away from the center of the joint (weld metal, etc.) in the partial blank to be measured is Hvm, the difference between the maximum hardness and the minimum hardness in the Vickers hardness within a range of 5 mm outward from the end of the joint is ΔHv. Since the HAZ softening is eliminated in a part of the joint (which is hot pressed after joining), ΔHv is less than 0.2 Hvm, preferably 0.1 Hvm or less, and since the HAZ softening is eliminated in the other joints (the joints other than the part, for example, which are 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 applied to an automobile floor module 100. Conventionally, floor modules have been manufactured by manufacturing parts separately, overlapping them and joining them (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 can be manufactured by hot pressing it all at once to manufacture the floor module 100. At this time, the blank and blank manufacturing method according to the present invention were applied to the joining (spot welding, etc.) of the six overlapping parts 101. As a result, even when hot pressing all at once was performed, the floor module 100 could be obtained without fracture at any of the overlapping parts 101.

[0072] As described above, the component (press-formed component) obtained by the present invention is free of cracks and has excellent impact resistance performance. Compared with components in which all spot welds are performed either before or after hot press forming as in conventional spot-welded assembled components, the press-formed component according to the present invention can have the following advantages.

[0073] Compared with components in which all spot welds are performed before hot press forming as in the conventional method, the formability of the press-formed component according to the present invention is improved. Therefore, the depth of the component can be made deeper, the angle of the vertical wall portion can be made steeper, and the full plastic bending moment of each cross-section can be increased. Furthermore, since excessive reduction in sheet thickness introduced at the joint portion (such as the spot weld portion) where the material flow is large can be eliminated, it is possible to improve the impact resistance performance as a module.

[0074] Furthermore, in the press-formed component according to the present invention, the HAZ softening portion on the main top surface portion (for example, the top surface portion of a component having a hat-shaped cross-section) is eliminated, the bonding force between members can be improved, and the impact resistance performance as a module can be improved.

[0075] As described above, the blank for press forming, the method for manufacturing the blank for press forming, the press-formed component, and the method for manufacturing the press-formed component according to the present invention have been described by taking the door ring and floor module of an automobile as examples. The blanks, components, and their manufacturing methods according to the present invention are not limited to the embodiments used in the above description. The present invention can be applied without being limited to the type and structure as long as it is a TWB having an overlapping portion.

Industrial Applicability

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

Explanation of Reference Numerals

[0077] 1 Door ring 2 A-pillar (front pillar) 3 B-pillar (center pillar) 4 Locker 30 Blank for press forming 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 Locker 36 Overlap part of A-pillar lower and locker 37 Overlap part of B-pillar lower and locker 38 Overlap part of B-pillar upper and A-pillar upper 40 Part with break risk 41 Part with low break risk 42 Welding line of overlap welding 90 Spot welding test piece 91 Spot welding part (spot welding dots) 92 Base material 93 Nugget

Claims

1. A plurality of partial blanks made of steel plates are joined, at least two of the partial blanks are joined at a plurality of joining portions in an overlapping portion formed by being partially overlapped, 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 in contact with the other partial blanks of the partial blank, when the distance from the center of the joining portion is more than 15 mm and the Vickers hardness at a position where joining is not performed is Hvm, a part of the plurality of joining portions has a ΔHv, which is the difference between the maximum hardness and the minimum hardness in terms of Vickers hardness within a range of 5 mm or less from the end of the joining portion toward the base material side, of less than 0.2 Hvm, the other joining portions of the plurality of joining portions have the ΔHv of 0.2 Hvm or more A press-formed part, characterized in that.

2. The press-formed part according to claim 1, wherein the joining is spot welding.

3. A part of the plurality of joining portions is set in a portion where the absolute value of the difference in the material inflow amount of the partial blank in the overlapping portion by press forming is smaller than a predetermined limit value when the material inflow amount of the partial blank in the overlapping portion by press forming is obtained in advance. The press-formed part according to claim 1.

4. The press-formed part according to claim 3, wherein the predetermined limit value is 1 mm in terms of the absolute value of the difference in the material inflow amount of the partial blank by press forming.

5. The press-formed part according to any one of claims 1 to 4, wherein the joining portions other than the above-mentioned part among the plurality of joining portions are set in portions that undergo elongation flange deformation by press forming.

6. The press-formed part according to any one of claims 1 to 4, wherein the overlapping portion of the press-formed part includes an L-shaped or T-shaped portion.

7. The press-formed part according to claim 5, wherein the overlapping portion of the press-formed part includes an L-shaped or T-shaped portion.

8. A part of the plurality of joining portions is set in a portion where the absolute value of the difference in the material inflow amount of the partial blank in the overlapping portion by press forming is smaller than a predetermined limit value when the material inflow amount of the partial blank in the overlapping portion by press forming is obtained in advance. The press-formed part according to claim 2.

9. The press-formed part according to claim 8, wherein the predetermined limit value is 1 mm in terms of the absolute value of the difference in the material inflow amount of the partial blank by press forming.

10. The press-formed part according to claim 8 or 9, wherein among the plurality of joint portions, the joint portions other than the part are set to portions that undergo elongation flange deformation by press forming.

11. The press-formed part according to claim 8 or 9, wherein the overlapping portion of the press-formed part includes an L-shaped or T-shaped portion.

12. The press-formed part according to claim 10, wherein the overlapping portion of the press-formed part includes an L-shaped or T-shaped portion.

13. A blank for press forming formed by joining and integrating a plurality of partial blanks made of steel sheets, the blank for press forming having an overlapping portion formed by partially overlapping at least two of the partial blanks, The blank for press forming, wherein the partial blanks constituting the overlapping portion are joined at a part of the overlapping portion and are not joined at other parts.

14. The blank for press forming according to claim 13, wherein the joining is spot welding.

15. The blank for press forming according to claim 13, wherein a part of the overlapping portion is a part where, when obtaining the material inflow amounts of the respective partial blanks in the overlapping portion by press forming in advance, the absolute value of the difference in the material inflow amounts of the partial blanks is smaller than a predetermined limit value.

16. The blank for press forming according to claim 15, wherein the predetermined limit value is 1 mm in terms of the absolute value of the difference in the material inflow amounts of the partial blanks by press forming.

17. The blank for press forming according to any one of claims 13 to 16, wherein the other part of the overlapping portion is a part that undergoes elongation flange deformation by press forming.

18. The blank for press forming according to any one of claims 13 to 16, wherein the overlapping portion of the blank for press forming includes an L-shaped or T-shaped portion.

19. The blank for press forming according to claim 17, wherein the overlapping portion of the blank for press forming includes an L-shaped or T-shaped portion.

20. The blank for press forming according to claim 14, wherein a part of the overlapping portion is a part where, when obtaining the material inflow amounts of the respective partial blanks in the overlapping portion by press forming in advance, the absolute value of the difference in the material inflow amounts of the partial blanks is smaller than a predetermined limit value.

21. The blank for press forming according to claim 20, wherein the predetermined limit value is 1 mm in terms of the absolute value of the difference in the material inflow amount due to press forming of the partial blank.

22. The blank for press forming according to claim 20 or 21, wherein the other part of the overlapping portion is a part that undergoes an elongation flange deformation by press forming.

23. The blank for press forming according to claim 20 or 21, wherein the overlapping portion of the blank for press forming includes an L-shaped or T-shaped part.

24. The blank for press forming according to claim 22, wherein the overlapping portion of the blank for press forming includes an L-shaped or T-shaped part.

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