Press-molding method and press-molded component

Abstract

The present invention addresses the problem of suppressing cracks during bending processing even in the case of a patchwork blank in which a relatively large patchwork is joined, and the objective thereof is to provide such a press-molding method.　The present invention is a press-molding method for bending a patchwork blank obtained by overlaying a patchwork on the surface of a base blank. A press-formed component without cracks can be obtained by joining (spot welding) the patchwork and the base blank before press molding, only in one of two regions of the surface of the patchwork blank divided by the bent portion as a boundary in the case where there is one bent portion, and only in a region between two adjacent bent portions in the case where there are two or more bent portions, and, after press molding, joining (spot welding) the patchwork and the base blank in other regions.

Description

Press molding method and press molded part

[0001] The present invention relates to a press-forming method and a press-formed part.

[0002] There is a growing demand for reducing lifecycle GHG (total greenhouse gas emissions over the entire lifecycle) especially in automobiles, and there is an increasing demand for the integration of parts and modules with the aim of increasing the efficiency of production lines by reducing the number of parts and omitting processes. As a result, with the introduction of optimized design, there is an increasing demand for the manufacture of parts by press-forming so-called tailored welded blanks (TWBs), which are blanks made by combining and integrating different types of steel sheets within a part.

[0003] For example, a technology has been proposed for manufacturing parts with thickened portions by press-forming an integrated blank in which relatively smaller plate-shaped blanks are partially joined to a plate-shaped blank by welding or the like. This plate-shaped base blank is called a base blank, and the small blanks placed partially are called patchworks, and the integrated blank formed by stacking and joining them together is called a patchwork blank.

[0004] Patent document 1 proposes a method for press-forming a patchwork blank in which the part of the punch that comes into contact with the patchwork is recessed by the thickness of the patchwork, and the die side is made flush, so that the base blank and the patchwork are press-formed flush.

[0005] Patent Document 2 proposes bending only the thickened portion where the patchwork is joined, and bending the other portions separately from the thickened portion. It explains that this prevents stress from concentrating at the joint between the base work and the patchwork.

[0006] Patent Document 3 proposes that when bending a base blank made of patchwork joined at multiple joints, the patchwork blank is bent while being pressed through openings between the joints of the base blank. It explains that this makes it possible to offset the difference in actual bending length between the base blank and the patchwork.

[0007] Patent Document 4 proposes providing excess material to one of the blanks in the area to be bent when bending a laminated sheet made by joining two plate-shaped blanks together. It explains that this makes it possible to absorb distortion caused by the difference in wire length between the two blanks after bending, and prevents fractures from occurring at the welded area.

[0008] JP 2017-177115 A International Publication No. 2020 / 003767 JP 2021-049551 A JP 2023-077210 A

[0009] In conventional patchwork blanks, small patchwork pieces were mainly used to prevent cracks at the joints of the patchwork due to bending. Therefore, if you wanted to create thickened areas over a wide area, you had to arrange many small patchwork pieces or use a tailored blank (TWB) in which the base blank was divided into parts, which not only required time and effort but also limited the design itself.

[0010] In order to solve such problems, the present invention aims to provide a press-forming method that suppresses cracks caused by bending during press-forming, even in patchwork blanks made by joining relatively large patchwork pieces.

[0011] The present inventors have made extensive development efforts to achieve the above object and have obtained the following findings.

[0012] (A) Cracks that occur during bending of patchwork blanks originate from the part where the patchwork is joined to the base blank. This is thought to be due to the difference in material flow rate (material flow behavior) between the patchwork and the base blank, and the difference in material flow rate is concentrated at the welding point due to constraints at the welding point. Therefore, we realized that by releasing or relaxing the constraints at the welding point in the part where the difference in material flow rate is large, it would be possible to suppress the occurrence of cracks even in larger patchworks, and we proceeded with development.

[0013] (a) It was found that the difference in material flow behavior is mainly due to the difference in actual length at the bent portion. Therefore, it was discovered that when bending a patchwork blank, the difference in material flow behavior can be absorbed by fixing (joining) the patchwork and base blank in one region of the patchwork blank, with the bent portion as the boundary, and leaving the other region free (unjoined). Note that the bent portion targeted by this invention refers to the bent portion of the patchwork blank at the overlapping portion of the base blank and the patchwork.

[0014] (c) In the case of a bending pattern in which there is one bend on a plate-shaped patchwork blank (when there is one bend when viewed in cross section), i.e., when the cross section is L-shaped, it is advisable to join the patchwork and the base blank in either area, with the bend as the boundary.

[0015] Furthermore, if there are two bends on the patchwork blank, i.e., if the cross section is groove-shaped (C-shaped) or step-shaped (Z-shaped), it is advisable to join the patchwork and base blank in the area sandwiched between the bends (such as the part corresponding to the top surface of the groove).

[0016] Furthermore, when there are three or more bent portions, it is preferable to join the patchwork and the base blank in one of the areas sandwiched between two or more bent portions. For example, if a hat-shaped cross section (hat-shaped cross section) is composed of five surfaces, including flanges at both ends, vertical walls connected to the flanges, and a top surface in the middle, and the patchwork is stacked over all five surfaces, it is preferable to join the patchwork and the base blank in one of the areas corresponding to the top surface and the two vertical walls. Preferably, from the perspective of symmetry, it is preferable to join the patchwork and the base blank at the top surface, which is the center. Generally, part shapes are complex and diverse, so patchwork blanks often have three or more bent portions. Press-formed parts with three or more bent portions are particularly prone to cracking during bending. The inventors have discovered that even when there are three or more bent portions, it is effective to produce an integrated blank by joining the patchwork and the base blank in only one of the regions sandwiched between adjacent bent portions (there can be two or more such regions), and then press-forming the integrated blank. After press-forming this integrated blank, by joining the unjoined regions, a part with a complex shape can be obtained without cracking.

[0017] (d) After further investigation, it was found that even if the entire patchwork bundle was welded, it could be bent without cracking as long as the plastic strain near the weld was up to about 4%. In other words, it was found that the bending part should be one where the plastic strain exceeds 4%.

[0018] The present invention has been made based on the above findings, and its gist is as follows.

[0019] [1] A press-forming method for bending a patchwork blank, which has a patchwork member made of steel plate superimposed on the surface of a base blank made of steel plate, at a bent portion on the surface of the patchwork blank, the press-forming method comprising: press-forming the patchwork blank in which the patchwork member and the base blank are joined together in only one region of the surface of the patchwork blank, separated by the bent portion, if there is only one bent portion; or press-forming the patchwork blank in which the patchwork member and the base blank are joined together in only one region of the surface of the patchwork blank, separated by the bent portion, if there is two or more bent portions; and then joining the patchwork member and the base blank together in another region of the patchwork blank. [2] The press-forming method according to [1], which comprises: press-forming the patchwork blank in which the patchwork member and the base blank are joined together in only one region of the region between two adjacent bent portions on the surface of the patchwork blank, if there are two or more bent portions and there are three or more bent portions, and then joining the patchwork member and the base blank together in another region of the patchwork blank. [3] The press-forming method according to [1] or [2], wherein the bent portion is a bent portion that generates a portion where, assuming that the patchwork and the base blank are joined, plastic strain of the base blank due to bending of the bent portion is 4% or more. Alternatively, the press-forming method according to [1], wherein the following formula 1 is satisfied when R is the radius of curvature of the bent portion in the base blank, tp is the thickness of the patchwork, and tb is the thickness of the base blank: R<12.5×(tp+tb) Formula 1 [4] The press-forming method according to any one of [1] to [3], wherein, when the number of bent portions is three or more and is an odd number, the patchwork blank in which the patchwork and the base blank are joined is press-formed only in a central region.[5] The press-forming method according to any one of [1] to [3], wherein when the cross section of the patchwork blank after press-forming is groove-shaped or hat-shaped, the patchwork blank and the base blank are joined only in a region corresponding to a top surface, and the patchwork blank and the base blank are joined at a joint. [6] A press-formed part having a bent part, in which a patchwork made of steel plate is overlapped on the surface of a base blank made of steel plate and joined at a joint, the joint being characterized in that the difference between the maximum hardness within 2 mm outward from the outer edge of the joint on the surface of the base blank and the hardness of the base blank base material is 7% or more of the hardness of the base blank base material, and the joint is present only on one side of the bent part on the surface of the base blank, or only in a region between two adjacent bent parts on the surface of the base blank, if there is one bent part. [7] The press-formed part according to [6], wherein the joint is a spot weld. That is, a press-molded part having a bent portion, in which a patchwork of steel plates is superimposed on the surface of a base blank made of steel plate and joined at the joining points by spot welding, wherein the joint, in which the difference between the maximum hardness at a distance of 5 mm from the center of the joining point on the surface of the base blank and the hardness of the base material of the base blank is 7% or more of the hardness of the base material of the base blank, exists in only one region of the surface of the base blank, either side of the bent portion, if there is one bent portion; or in only the region between two adjacent bent portions on the surface of the base blank, if there are two or more bent portions. [8] When there are two or more bent portions and three or more bent portions, in only one of the regions between two adjacent bent portions on the surface of the base blank, there is a joint in which the difference between the maximum hardness within 2 mm outward from the outer edge of the joint on the surface of the base blank and the hardness of the base material of the base blank is 7% or more of the hardness of the base material of the base blank.

[0020] According to the present invention, cracks can be suppressed during bending even in patchwork blanks formed by joining relatively large patchwork pieces. Large patchwork blanks can be press-formed in one go without cracks, which not only improves quality and productivity but also significantly reduces costs by reducing the number of press-forming dies. The effects of the present invention are particularly pronounced when the present invention is applied to parts with complex shapes, such as patchwork blanks with three or more bends, or when the present invention is applied to hot working (hot press forming (hot stamping) and the like).

[0021] 1A and 1B are diagrams for explaining the present invention, and are schematic diagrams showing an example of a press-formed part (hat-shaped part) with a hat-shaped cross section. FIG. 1A shows an external view thereof, and FIG. 1B shows a cross-sectional view thereof. A schematic diagram showing an example of a blank (patchwork blank) used in press-forming the hat-shaped part of FIG. 1. A schematic diagram showing an example of a cross-section of the hat-shaped part of FIG. 1, which has a recess in the top surface thereof. A schematic diagram showing an example of a blank (patchwork blank) used in press-forming the hat-shaped part of FIG. 3, which has a recess in the top surface thereof. A diagram for explaining an example of a hat-shaped part to which a conventional manufacturing method is applied. FIG. 5A shows an external view thereof, FIG. 5B shows a cross-sectional view thereof, and FIG. 5C shows a schematic diagram of an example of a patchwork blank used in press-forming the hat-shaped part of FIG. 5A. A diagram for explaining an example of a hat-shaped part according to one embodiment of the present invention. FIGS. 6A to 6C show examples joined by spot welding. FIG. 6( a ) is an external view, FIG. 6( b ) is a cross-sectional view, and FIG. 6( c ) is a schematic diagram showing an example of a patchwork blank used in press-forming the hat-shaped part of FIG. 6( a ). FIGS. 6( d ) to 6 ( f ) are schematic diagrams showing examples of patchwork blanks in the same hat-shaped part when other joining methods are used instead of spot welding. FIG. 6( d ) shows an example in which lap fillet welding is used, FIG. 6( e ) shows an example in which lap welding is used, and FIG. 6( f ) shows an example in which adhesive is applied before press-forming and laser spot welding is used after press-forming. This figure is for explaining an example of a hat-shaped part according to one embodiment of the present invention, and is a schematic diagram showing an example in which the patchwork of the hat-shaped part shown in FIG. 6( a ) is extended to the flange. This figure is for explaining an example of a hat-shaped part according to one embodiment of the present invention. FIG. 8( a ) is a schematic diagram showing an example in which there is no patchwork on one side of the vertical wall of the hat-shaped part shown in FIG. 6( a ). Fig. 8(b) is a schematic diagram showing an example of a hat-shaped part shown in Fig. 7 when there is no patchwork on one side of the standing wall. Fig. 9(a) is a schematic diagram showing an example of a hat-shaped part according to one embodiment of the present invention, where Fig. 9(b) is a schematic diagram showing an example of a hat-shaped part according to one embodiment of the present invention when the top surface has a recess, Fig. 9(b) is a schematic diagram showing an example of a hat-shaped part according to one embodiment of the present invention when the top surface has a protrusion, and Fig. 9(c) is a schematic diagram showing another example of a hat-shaped part according to one embodiment of the present invention when the top surface has a protrusion.

[0022] The present invention will be described as an example of a hat-shaped cross-section press-formed product, which is one embodiment of the present invention (hereinafter simply referred to as the present invention). Figure 1 shows a schematic diagram of a hat-shaped cross-section press-formed product 1 used in the description, with Figure 1(a) showing an external view and Figure 1(b) showing a cross-sectional view. The hat-shaped cross-section press-formed product (hereinafter simply referred to as a hat-shaped part) is composed of a top surface 2, vertical walls 3 connected to both sides of the top surface 2, and flanges 4 connected to the vertical walls 3. The hat-shaped part 1 in Figure 1 has a patchwork 12 bonded to its interior, which is configured to reinforce the top surface 2 and the vertical walls 3 connected to it.

[0023] The top surface 2, the vertical wall 3, and the flange 4 of the hat-shaped part 1 are formed from a single steel plate, and this single original steel plate is called a base blank 11. Here, the base blank 11 is described as being made from a single steel plate, but the base blank itself may be made from multiple steel plates (each of these steel plates may be called a partial blank because they constitute part of the blank). In other words, the base blank may be a tailored blank (TWB) made by joining partial blanks.

[0024] The patchwork 12 joined to the inside of the hat-shaped part 1 will be described as being formed from a single steel plate. Of course, the patchwork may also be a tailored blank (TWB) made up of multiple partial blanks, and there are no limitations on how the patchwork is arranged.

[0025] The patchwork 12 is joined to the base blank 11 of the hat-shaped part 1, but the joining method 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, brazing, adhesives, and in some cases mechanical joining such as rivets. Joining by welding is preferred in terms of press formability and joining workability. The welding method is also not particularly limited, but it is preferable to spot weld, lap fillet weld, or lap weld the overlapping portion, for example. Spot welding is typically used, and in this description, the base blank 11 and the patchwork 12 are joined by spot welding as an example. By replacing spot welding in the description with other joining methods, the application of other joining methods can be understood.

[0026] FIG. 2 shows a schematic diagram of a blank used in press-forming to obtain the hat-shaped part 1. The steel plate constituting the main body of the hat-shaped part is the base blank 11, and a steel plate constituting the patchwork part 12 is placed on its surface. The blank formed by overlaying the patchwork part 12 on the base blank 11 is called the patchwork blank 10. The patchwork blank 10 is press-formed to produce the hat-shaped part 1. The patchwork blank 10 is bent using a combination of punches and dies in a press-forming machine to form ridges between the flange 4 and the vertical wall 3 and between the vertical wall 3 and the top surface 2, thereby obtaining the hat-shaped part. When the completed hat-shaped part is unfolded and returned to its original plate-like patchwork blank, the four ridges of the hat-shaped part become bent portions 13, 14, 15, and 16. The bent portions 13 and 16 are located between the flange 4 and the vertical wall 3, and the bent portions 14 and 15 are located between the vertical wall 3 and the top surface 2. In the case of the hat-shaped part shown in Figure 1, the bent portion is linear, but the shape of the bent portion is not particularly limited. To obtain a press-molded part with a complex shape, the bent portion may be curved or may intersect.

[0027] To manufacture the patchwork blank 10, the patchwork 12 is joined to the base blank 11 by spot welding or the like (as mentioned above, for convenience of explanation, spot welding will be used as an example). However, as mentioned above, if the patchwork 12 is spot welded to the base blank 11 over the entire surface across the bend, a difference in actual length will occur between the inside and outside of the bend due to press forming, and this difference will cause plastic strain to accumulate at joints (portions where the material flow behavior is constrained) such as the spot welds of the base blank 11 and the patchwork 12, which is thought to cause cracks.

[0028] Therefore, when joining the base blank 11 and the patchwork 12, it is advisable to avoid the bent portion and join them by spot welding or the like only in one area of ​​the bent portion. If there is only one bent portion on the patchwork blank 10, for example in the case of bending that results in an L-shaped cross section, in such a case where there is only one bent portion, it is advisable to join the base blank 11 and the patchwork 12 only in one area of ​​the surface of the patchwork blank 10, with the bent portion as the boundary.

[0029] If there are two bends on the patchwork blank, i.e., if the cross section is groove-shaped (C-shaped) or step-shaped (Z-shaped), it is recommended to join the patchwork and base blank in the area sandwiched between the bends (the part corresponding to the top surface of the groove).

[0030] If there are two or more (i.e., multiple) bends on the patchwork blank 10, such as in the case of the hat-shaped part 1 shown in Figure 1, it is preferable to join the base blank 11 and the patchwork 12 only in the area between two adjacent bends on the surface of the patchwork blank 10.

[0031] Generally, part shapes are complex and diverse, so patchwork blanks often have three or more bends. In press-formed parts with two or more bends, particularly those with three or more bends (three or more locations), the material flow behavior becomes complex, making cracks particularly likely to occur during bending. Even in cases with three or more bends, the present inventors propose that an integrated blank be produced by joining the patchwork blank and the base blank in only one of the regions sandwiched between adjacent bends (since there are three or more bends, there will be two or more regions sandwiched between the bends). After press-forming the integrated blank produced in this manner, the unjoined regions can be joined to produce a part with a complex shape without cracks.

[0032] If the number of bent portions is odd, it is preferable to join the patchwork 12 and the base blank 11 only in the central region from the standpoint of symmetry. For example, when the cross section of the patchwork blank after press forming is groove-shaped or hat-shaped, it is preferable to join the patchwork 12 and the base blank 11 only in the region corresponding to the top surface to form the patchwork blank 10. Furthermore, for example, if the bent portions are arranged asymmetrically, it is preferable to join only in the region close to the center of the patchwork blank (e.g., the center of gravity of the surface). In any case, it is preferable to join the base blank 11 and the patchwork 12 in only one region between two adjacent bent portions.

[0033] For example, in the case of the hat-shaped part 1 in Fig. 1, there are four bent portions as shown in Fig. 2. As can be seen from Fig. 2, of these four bent portions, the patchwork 12 is connected to two bent portions, 14 and 15. Therefore, the two adjacent bent portions are bent portions 14 and 15, and the region between these two adjacent bent portions is region 23, which corresponds to the top surface. Therefore, in the case of the patchwork blank 10 shown in Fig. 2, it is preferable to manufacture the patchwork blank 10 by spot welding the patchwork 12 and the base blank 11 only in region 23, and leaving the other regions, regions 22 and 24, unjoined.

[0034] For example, in the case of a part 1 having the cross-sectional shape shown in FIG. 3 , there are eight bends as shown in FIG. 4 . As can be seen from FIG. 4 , the patchwork 12 is applied to six of these eight bends: bends 14, 31, 32, 33, 34, and 15. Therefore, two adjacent bends are bends 14 and 31, bends 31 and 32, bends 32 and 33, bends 33 and 34, and bends 34 and 15. The areas between two adjacent bends are five areas sandwiched between these bends. It is preferable to join (e.g., spot weld) the patchwork 12 and the base blank 11 in only one of the areas between these five bends, while leaving the other areas unjoined to produce the patchwork blank 10. The selection of the areas is not particularly limited. The areas to be joined prior to press forming can be determined by taking into account the material flow behavior during press forming.

[0035] The material flow behavior refers to the difference in material flow rates between the base blank and the patchwork blank that make up the patchwork blank. The difference in material flow rates between the base blank and the patchwork blank is the absolute value of the difference in movement vectors obtained as a result of the press-forming movement of a point (position) on each blank corresponding to an arbitrary point (position) in the overlapping portion of the base blank and the patchwork blank before forming. That is, the movement vector of a point (position) on each blank corresponding to an arbitrary point (position) in the overlapping portion of the base blank and the patchwork blank is calculated from the point (position) to which the point moved after press-forming, and the absolute value of the difference in movement vectors of the points (positions) on each partial blank (i.e., the distance between the points (positions) after each movement) is calculated as the difference in material flow rate at that point (any point in the overlapping portion).

[0036] A specific simulation example is shown below. While spot welding is used as an example, other joining methods can be applied in a similar manner. For example, assuming that the base blank and patchwork are fixed (referring to a state where they are joined by spot welding or the like) at at least one point (e.g., the center of gravity of the overlapping portion) at which the overlapping portion of the base blank and patchwork are pressed together during press forming, a press forming analysis (simulation analysis using FEM, etc.) of the integrated blank is performed. In this case, the movement vectors of each point on the base blank and patchwork corresponding to any point in the overlapping portion are calculated from their positions before and after movement. The absolute value of the difference between the obtained movement vectors of each point is calculated as the material inflow difference for that point. If the calculated material inflow difference is smaller than a preset limit value, the material inflow difference is small, and the point (position) can be spot welded before press forming. On the other hand, if the calculated material inflow difference is larger than the preset limit value, the material inflow difference is large, and spot welding should not be performed before press forming, but should be performed after press forming.

[0037] In fact, a joint made by overlapping and spot-welding 1.5 GPa-class steel plates (1.2 mm thick) was heated to 900 ° C and held for 1 minute, then pulled at 740 ° C to displace the steel plates, and then quenched. The steel plates were pulled at 1 mm / s at room temperature, and the amount of displacement 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 amount of displacement of the steel plates during hot rolling was 0 to 1.0 mm, the TSS of the joint at room temperature was equivalent to that of a joint with a displacement of 0 mm (i.e., not hot-deformed). On the other hand, when the amount of displacement exceeded 1.0 mm, the TSS tended to decrease. Specifically, under these test conditions, the TSS was confirmed to decrease to 102% of the TSS at a misalignment of 0.1 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. That is, it was discovered for the first time that even in hot press forming (hot stamping), even if a material flow rate difference of 0.0 mm or less (corresponding to the above-mentioned misalignment) occurs at the spot welding point, it is possible to ensure tensile shear strength equivalent to or greater than that when no deformation occurs. Based on this finding, in the case of hot press forming, the limit value of the material flow rate difference is preset to 1.0 mm, and joining is performed at points (or regions including these points) where the material flow rate difference at each point is below this preset limit value through simulation (or a region including these points). The limit value of the difference in material flow rate is preferably 0.9 mm, 0.8 mm, 0.7 mm, 0.6 mm, or 0.5 mm.

[0038] In cold press forming, the tensile shear strength (TSS) generated relative to the amount of misalignment is larger than in hot press forming. Therefore, the limit value of the material inflow amount is smaller than in hot press forming, at about 0.5 mm. Therefore, in cold press forming, the limit value of the difference in material inflow amount is preferably set in advance to 0.5 mm, 0.4 mm, 0.3 mm, or 0.2 mm, and the joining area can be determined by simulation.

[0039] The point (position) at which the difference in material inflow rate is calculated does not need to be particularly limited. For example, the difference in material inflow rate may be analyzed at a point (position) set as the final spot welding position. Alternatively, for example, the analysis mesh may be refined to determine a region where the difference in material inflow rate is equal to or less than a threshold value. In this case, it is advisable to set the pre-press forming spot welding point (position) within a region where the difference in material inflow rate is analyzed to be small.

[0040] 3 and 4, in the case of parts that are nearly axisymmetric, it is advisable to join the patchwork 12 and the base blank 11 only in the region between the bent portion 32 and the bent portion 33, which corresponds to the region where the axis of symmetry exists (the central region in the case of FIG. 3), from the viewpoint of symmetry. This is because, in parts that are nearly axisymmetric, the material flow also occurs axisymmetrically, and the difference in the amount of material flow around the axis of axis symmetry becomes small.

[0041] The patchwork blank 10 thus obtained is bent by press forming to obtain a press-formed part having a predetermined shape. The press forming method is not particularly limited, and a conventional press forming method can be applied. For example, it may be cold press forming or hot press forming. Bending other than press forming, such as simple bending, may also be used. Regardless of the forming method, as mentioned above, in the case of hot forming (such as hot press forming (hot stamping)), the limit value of the material flow rate difference can be set larger than in cold forming, so when the present invention is applied to hot forming, the effects of the present invention can be more fully exerted.

[0042] After press forming, the base blank 11 and the patchwork 12 may be joined in an area of ​​the patchwork blank 10 other than the area where the base blank 11 and the patchwork 12 are joined. The joining method here is not particularly limited, but may be the same as the joining method used before press forming. In the example of the hat-shaped part described above, the base blank 11 and the patchwork 12 are joined by spot welding in the area 23 corresponding to the top surface 2 before press forming, so the base blank 11 and the patchwork 12 may be joined by spot welding in the areas 22 and 24 corresponding to the standing wall 3, which are areas that are not joined after press forming.

[0043] Next, we investigated the allowable curvature of the bent part even for a patchwork blank in which the base blank 11 and patchwork 12 are integrated. Even with the same bending process, the difference in the amount of material flow (amount of material movement) between the two blanks (the actual difference in length when viewed in cross section) differs depending on whether the radius of curvature of the bent part is large or small, and when the curvature is large, the difference in the amount of material flow is small, and cracks due to the bending process may not occur.

[0044] For example, in the case of automotive parts, where press forming using patchwork blanks is often used, high-tensile steel sheets (high-strength steel sheets with a tensile strength of 980 MPa or more) may be used for the base blank and patchwork. For example, it has been confirmed that 1470 MPa-class high-tensile steel sheets can be formed without cracking with a plastic strain of approximately 4%. Therefore, it is believed that cracking will not occur in the blank if the plastic strain generated by bending during press forming is 4% or less. That is, the strain is calculated from the difference in actual length between the bent portions of the base blank 11 and the patchwork 12, and bent portions with this difference of 4% or less can be excluded from determining the area where the patchwork 12 can be joined. In other words, when determining the area where the patchwork can be joined, it is best to determine the bent portions where the plastic strain calculated from the difference in actual length between the bent portions of the base blank 11 and the patchwork 12 exceeds 4%.

[0045] For example, it may be determined by a forming simulation using a patchwork blank. A forming analysis (bending analysis) is performed using a patchwork blank, in which a patchwork piece is spot-welded to a base blank at a position that is a candidate for a spot welding point (i.e., the position of the joining portion (joint)), to determine the maximum plastic strain around the patchwork. If the plastic strain exceeds 4%, the formed (bent) bent portion is considered to be a bent portion that should be considered for determining the weldable region. On the other hand, if the plastic strain is 4% or less, the bent portion is considered to be a bent portion that may be excluded when determining the weldable region. The forming analysis method is not particularly limited. Numerical simulations such as the finite element method (FEM) may also be applied. The maximum plastic strain is defined as the maximum value of plastic strain within a range of 2 mm from the outer edge (end) of the joined portion to the outside (the blank base material side opposite the weld center). In the case of spot welding, the maximum plastic strain may be the maximum value of the plastic strain obtained by FEM analysis within a range of 2 mm outside the spot weld or at a position 5 mm away from the joining point (spot weld point). Also, in the case of lap welding, for example, the maximum plastic strain may be the maximum value of the plastic strain obtained by FEM analysis in a region within 2 mm from the outer edge of the weld, i.e., the boundary between the weld metal and the blank surface, to the blank side (opposite the weld metal).

[0046] For example, in the case of the hat-shaped part shown in Figure 1, consider the case of a bent portion consisting of a top surface portion 2 and a vertical wall 3. As shown in Figure 1(b), the thickness of the base blank 11 is tb, the thickness of the patchwork 12 is tp, the radius of curvature of the bent portion of the base blank 11 is R (the radius of curvature at the center line of the thickness of the base blank 11), and the central angle of the bend is Θ (radians). In this case, the length of the bent portion of the base blank 11 is RΘ, and the length of the bent portion of the patchwork 12 is {R - 0.5 (tb + tp)}Θ. From these relationships, a bent portion with a radius of curvature that satisfies the following formula 1 corresponds to a case where the plastic strain exceeds 4%, and is the bent portion that should be considered for determining the weldable region. In other words, a bent portion that satisfies the following formula 1 can be considered a bent portion.

[0047] R<12.5×(tp+tb) ...Equation 1 R: Radius of curvature of the bent portion of the base blank tb: Plate thickness of the base blank tp: Plate thickness of the patchwork

[0048] Formula 1 applies when the plastic strain of the bent portion exceeds 4%, so satisfying Formula 1 does not necessarily mean that cracks will immediately occur. When applying the present invention, it is more effective to apply it to bent portions where the expected plastic strain is large. Preferably, the present invention can be applied by replacing formula 1 with the following: Plastic strain over 6%: R<8.33×(tp+tb), Plastic strain over 8%: R<6.25×(tp+tb), Plastic strain over 10%: R<5.00×(tp+tb), Plastic strain over 12%: R<4.17×(tp+tb), Plastic strain over 15%: R<3.33×(tp+tb), Plastic strain over 20%: R<2.50×(tp+tb), or Plastic strain over 25%: R<2.00×(tp+tb). In particular, when the present invention is applied to hot working (such as hot stamping), it is preferable to replace formula 1 with the formula corresponding to a plastic strain of more than 20%.

[0049] Consider, for example, a case where the top surface 2 of the hat-shaped part 1 has a recess or protrusion. Figure 3 shows a cross-sectional view of the hat-shaped part 1 of Figure 1 with a recess 30 on the top surface 2. There are four bent portions 31, 32, 33, and 34 that form the recess 30 in the top surface 2. However, if the radius of curvature of each bent portion does not satisfy Equation 1 (or an alternative interpretation of Equation 1), the four bent portions 31, 32, 33, and 34 can be excluded when determining the joining region of the patchwork 12. Therefore, in this case, the consideration is the same as for the patchwork blank 10 of Figure 2, as described above.

[0050] On the other hand, if the radii of curvature of bent portions 31, 32, 33, and 34 satisfy Equation 1 (or a modified version of Equation 1), the joining regions of patchwork 12 must be determined taking into account each of bent portions 31, 32, 33, and 34, as shown in FIG. 4 . In this case, there are a total of six bent portions in patchwork 12 (bent portions 14, 31, 32, 33, 34, and 15). Since the patchwork is joined only in the regions between two adjacent bent portions, five candidate regions are identified: the region between bent portions 14 and 31, the region between bent portions 31 and 32, the region between bent portions 32 and 33, the region between bent portions 33 and 34, and the region between bent portions 34 and 15. One of these five regions can be selected and joined (e.g., spot welded) in that region only. However, considering the symmetry of the bending process during press forming, it is preferable to join in a region as close to the center as possible. This is because the processing load during press forming is more easily applied evenly, improving the shape accuracy after press forming. In the case of Figure 4, the area between bent portions 32 and 33 is in contact with the center of patchwork blank 10, so it is preferable to join in this area.

[0051] For example, if the cross section of the press-molded part is hat-shaped or groove-shaped (sometimes also called U-shaped or C-shaped), the patchwork 12 and base blank 11 can be joined only in the area corresponding to the top surface to produce a patchwork blank 10, and press-molding involving bending can be performed.

[0052] [Press-Molded Part] In a press-molded part obtained using the press-molding method of the present invention, a portion of the patchwork is joined before press-molding, and the remaining portion is joined after press-molding. Therefore, the portion joined after press-molding naturally does not experience strain due to press-molding (bending). Furthermore, the portion joined before press-molding is also less susceptible to bending during press-molding, and even if bending is performed, the portion experiences a plastic strain of 4% or less. Therefore, even when processed with bending, the press-molded part having the patchwork of the present invention has a maximum base blank strain of 4% or less around the joint (spot welding point) or joint (the portion where the base blank and the patchwork are joined, including other joining methods). Here, the periphery of the joint or joint refers to a region within 2 mm from the outer edge of the joint (the outer edge of the weld metal in the case of welding, the outer edge of the adhesive application area in the case of adhesive, or the outer edge of the joint in the case of friction stir welding, etc.) toward the blank (the opposite side of the joint center). In the case of spot welding, since no nugget appears on the blank surface, the welding may be performed at a position 5 mm away from the joining point on the blank surface (the center of the spot welding point).

[0053] On the other hand, the parts joined (by spot welding, etc.) before press forming are work-hardened, even though the maximum strain is 4% or less, and this improves the strength of the parts against shear deformation. It has been confirmed that, particularly in the case of high-tensile steel, there are parts near the parts joined (by spot welding, etc.) before press forming that have hardened by Vickers hardness (HV) by 7% or more of the base material hardness due to work hardening. In other words, if the base material hardness is Hvm, then the difference ΔHv between the maximum hardness around the joint (maximum hardness of the work-hardened part) Hvmax and the base material hardness Hvm is 7% or more of the base material hardness Hvm (see the formula below). ΔHv = Hvmax - Hvm ≧ 0.07 × Hvm

[0054] The Vickers hardness measurement position around the joint is at least five points within a range of 2 mm from the outer edge of the joint (the outer edge of the weld metal in the case of welding, the outer edge of the adhesive application area in the case of adhesive, and the outer edge of the joint in the case of friction stir welding, etc.), and the maximum of these measurements is taken as the hardness of the work-hardened zone around the joint. In the case of spot welding, measurements are taken at five points or more 5 mm away from the center of the joint (the spot weld point), and the maximum of these measurements can also be taken as the hardness of the work-hardened zone around the joint. The base material hardness of the base blank is the hardness (Vickers hardness) of the part not affected by joining such as spot welding or bending. Of course, the base material hardness can also be measured before joining such as press forming or spot welding.

[0055] In other words, the press-formed product of the present invention is a press-formed part having a bent portion, joined at a joint (including a joint point by spot welding) where a patchwork of steel plate is superimposed on the surface of a base blank made of steel plate, and wherein the difference between the maximum hardness within a range of 2 mm from the outer edge of the joint on the surface of the base blank (in the case of spot welding, it may be a position 5 mm away from the center of the joint (center of the welding point)) and the hardness of the base material of the base blank is 7% or more of the hardness of the base material of the base blank, and when there is one bent portion, this joint exists only in one of the regions on the surface of the base blank (patchwork blank) separated by the bent portion, and when there are two or more bent portions, this joint exists only in the region between two adjacent bent portions on the surface of the base blank (patchwork blank).

[0056] As mentioned above, the present invention has been described based on one example of an embodiment of the present invention. The components illustrated in the drawings have been described as having bent portions that are substantially parallel when the component surface is viewed vertically from above, but the arrangement of the bent portions is not particularly limited. It is advisable to check the bent portions when viewing a cross section of the component (a cross section perpendicular to the longitudinal direction of the component).

[0057] [Example 1] Using the hat-shaped part shown in Figure 1 as a base, a hat-shaped part was manufactured in which a patchwork was arranged from the top surface part to the vertical wall in the center of the longitudinal direction of the hat-shaped part. The steel plates used for the hat-shaped part and the dimensions of each part are as follows.

[0058] <Hat-shaped part dimensions (base blank)> Top width: 100 mm Wall length: 100 mm Flange width: 30 mm Longitudinal length: 300 mm Top shoulder R (base blank): 10 mm Flange-wall R (base blank): 15 mm

[0059] <Base blank> Steel plate: Tensile strength 1180 MPa steel plate, plate thickness 1.4 mm Length: 300 mm Width: 360 mm

[0060] <Patchwork> Steel plate: Tensile strength 1470 MPa steel plate, plate thickness 1.2 mm Length (longitudinal length of hat-shaped part): 100 mm Width: 280 mm (length equivalent to top surface: 100 mm) (length equivalent to vertical wall: 90 mm)

[0061] <Joining method> ・Spot welding point diameter: 5.5 mm (= 5√thickness of thinner plate (1.2 mm)) ・Spot welding locations: Top surface: 4 locations (2 locations in width direction x 2 rows in length direction) Vertical wall: 4 locations x 2 (2 locations in width direction x 2 rows in length direction)

[0062] <Press molding conditions> Press machine: 500t mechanical press Pad pressure: 20t Average press speed: 200mm / sec Molding pressure: 100t

[0063] Figure 5 shows an example (comparison example) of a hat-shaped part manufactured using a conventional manufacturing method. Figure 5 shows an example in which all spot welding was performed before press forming due to the application of a conventional manufacturing method. Figure 5(a) shows an external view of the manufactured hat-shaped part. The black circles (●) in the figure indicate locations where spot welding was performed before press forming. The black circle spot welding indicates spot welding performed before press forming. Figure 5(b) shows a cross section of the hat-shaped part in Figure 5(a) cut through the spot welds. The black squares (filled black rectangles) in Figure 5(b) indicate areas where spot welding was performed before press forming. The black circles (●) in Figure 5(a) correspond to the black squares (filled black rectangles) in Figure 5(b), indicating spot welding performed before press forming. Figure 5(c) shows a patchwork blank for the hat-shaped part in Figure 5(a). Patchwork 12 is placed on base blank 11, and the two are joined by spot welding. As in Figure 5(a), black circles (●) indicate the locations of spot welds made before press forming, i.e., when manufacturing the patchwork blank. Also, in Figure 5(c), bends 13, 14, 15, and 16 indicate bends 13 and 16 between the flange and the vertical wall, and bends 14 and 15 between the vertical wall and the top surface. The patchwork blank shown in Figure 5(c) was press-formed to manufacture the hat-shaped part shown in Figure 5(a), and the presence or absence of cracks was then visually inspected, focusing on the spot welds. As a result, cracks were confirmed in four locations (two locations on each side) below the vertical wall (on the flange side) out of the eight spot welds on the vertical wall.

[0064] Furthermore, a simulation analysis was performed using FEM to analyze the strain that occurs when the patchwork blank shown in Figure 5(c) is press-formed, and it was confirmed that the maximum plastic strain around the spot welding point (joint point) below the vertical wall was 4.5%.

[0065] An example of a hat-shaped part to which the manufacturing method according to the present invention is applied is shown in Fig. 6. The hat-shaped part shown in Fig. 6 has the same shape as the hat-shaped part shown in Fig. 5, but the timing of spot welding is different.

[0066] Figure 6(a) shows the external view of the manufactured hat-shaped part. The black circles (●) and white circles (◯) in the figure indicate the locations of spot welding. As in Figure 5, the black circles (●) indicate spot welding that was performed before press forming, while the white circles (◯) indicate spot welding that was performed after press forming.

[0067] Figure 6(b) shows a cross section of the hat-shaped part of Figure 6(a) taken through the spot welds. The black and white squares in the figure indicate spot welds. As in Figure 6(a), the black and white squares in Figure 6(b) indicate spot welds that were applied before press forming. Meanwhile, the white squares in Figure 6(b) indicate spot welds that were applied after press forming. The black circles in Figure 6(a) correspond to the black squares in Figure 6(b) and indicate spot welds that were applied before press forming. The white circles in Figure 6(a) correspond to the white squares in Figure 6(b) and indicate spot welds that were applied after press forming.

[0068] Figure 6(c) shows a patchwork blank for the hat-shaped part shown in Figure 6(a). The patchwork 12 is placed on the base blank 11, and the two are joined by spot welding. As with Figure 5(a), black circles (●) indicate locations where spot welding was performed before press forming, i.e., when manufacturing the patchwork blank, while white circles (◯) indicate locations where spot welding was performed after press forming. In other words, in the patchwork blank, spot welding was performed only at the locations indicated by black circles. Also, in Figure 6(c), the bends 13, 14, 15, and 16 indicate the bends 13 and 16 between the flange and the vertical wall, and the bends 14 and 15 between the vertical wall and the top surface.

[0069] As can be seen in Figure 6(c), the spot welding on the patchwork blank was performed only in the area between two adjacent bent portions on the surface of the patchwork blank (in this case, the area corresponding to the top surface). The patchwork blank shown in Figure 6(c) was press-formed to produce the hat-shaped part shown in Figure 6(a), and then the presence or absence of cracks was visually confirmed, centered around the spot welds. As a result, no cracks were found in the entire hat-shaped part (including the patchwork).

[0070] Furthermore, a FEM simulation analysis of strain during press forming of the patchwork blank shown in Figure 6(c) revealed that the maximum plastic strain occurred at the outer edge of the bend between the top surface of the vertical wall and the vertical wall, and was confirmed to be 1.2%. The maximum plastic strain at the spot weld below the vertical wall, where cracking occurred in the hat-shaped part shown in Figure 5(a), was confirmed to be nearly 0%. The maximum plastic strain around the spot weld (joint) on the top surface was also approximately 0.8%.

[0071] When the hardness of an actual molded product was measured around the spot weld (spot weld before press molding) on ​​the base blank side of the top surface and the maximum hardness was determined, the difference between the maximum hardness and the hardness of the general part (base material) of the base blank was 7% of the hardness of the general part (base material) of the base blank.

[0072] On the other hand, when the hardness around the spot welds on the vertical wall (spot welds after press forming) was measured, it was found to be the same as the hardness of the general part (base material) of the base blank.

[0073] As described above, the hardness (surface hardness) of the base blank around the spot weld was measured at five or more points 5 mm away from the center of the spot weld, and the maximum value was taken as the maximum hardness, which was then compared with the hardness of the base blank's base material. The hardness of the base blank's base material was determined by measuring the hardness (Vickers hardness) at five points 30 mm or more away from the center of the spot weld and 30 mm or more away from the outer edge of the bent portion, and using the arithmetic mean value.

[0074] From the above, it was confirmed that even for hat-shaped parts having patchwork of the same shape, cracks do not occur when the press molding method of the present invention is applied, and the strength of the parts at the joint before press molding is improved.

[0075] Next, in Example 1, the base blank and the patchwork were joined before pressing using lap fillet arc welding, lap arc welding, and adhesive instead of spot welding, to produce a hat-shaped part. Schematic diagrams of the respective joined states are shown in Figures 6(d), 6(e), and 6(f).

[0076] 6(d) shows an example in which the patchwork is joined to the base blank by lap fillet arc welding. The area (top surface) between bent portions 14 and 15 before press forming is shown as a black band (black rectangular band). The area in the vertical wall of the hat-shaped part after press forming, where the patchwork and base blank are further joined by lap fillet arc welding, is shown as a white band (white rectangular band).

[0077] 6(e) shows an example in which the patchwork is joined to the base blank by lap arc welding. The black band indicates the area (top surface) between bent portions 14 and 15 before press forming, where the lap welding was performed. The white band indicates the area in the vertical wall of the hat-shaped part after press forming, where the patchwork and base blank are further joined by lap fillet arc welding.

[0078] Figure 6(f) shows an example in which the patchwork was bonded to the base blank with adhesive before press forming. The checkered pattern (checkered rectangular area) shows the adhesive bonded area in the area (top surface) between bent portions 14 and 15 before press forming. After press forming, the patchwork and base blank were joined by laser spot welding in the vertical wall portion of the hat-shaped part. The laser spot welding joints (joint points) after press forming are shown by white circles (◯).

[0079] As in the example using spot welding, the hardness of an actual molded product was measured around the joint on the base blank side of the top surface to determine the maximum hardness.The difference between this maximum hardness and the hardness of the general part (base material) of the base blank was 7% of the hardness of the general part (base material) of the base blank.

[0080] On the other hand, when the hardness of the periphery of the joint of the vertical wall portion was measured, it was found to be the same as the hardness of the general portion (base material) of the base blank.

[0081] In this case, the hardness (surface hardness) of the base blank around the joint was measured as described above by measuring at least five points within 2 mm from the outer edge of the joint (the outer edge of the weld metal in the case of lap fillet arc welding and lap arc welding, and the outer edge of the adhesive application area in the case of adhesive), taking the maximum value as the maximum hardness, and comparing it with the hardness of the base blank. The hardness of the base blank was determined by measuring the hardness (Vickers hardness) at five points 30 mm or more away from the center of the joint and 30 mm or more away from the outer edge of the bent part, and taking the arithmetic mean value of the measurements.

[0082] In both cases, as with spot welding, FEM analysis confirmed that the maximum plastic strain around the joint was reduced, and no cracks occurred during actual press forming, confirming that the strength of the components at the joint was improved.

[0083] [Example 2] Figure 7 shows an example (Example 2) in which the patchwork was extended to the flange of the hat-shaped part shown in Example 1 (Figure 6). In this case, spot welding was performed on two locations on each side of the flange after press forming. In Example 2, as in Example 1, no cracks were observed in the entire hat-shaped part (including the patchwork). Note that in Figures 7 to 9, as in Figure 6, black squares (filled squares) indicate spot welding performed before press forming, and white squares (□) indicate spot welding performed after press forming.

[0084] [Example 3] Figure 8 shows an example (Example 3) of the hat-shaped part shown in Example 1 (Figure 6) in which there is no patchwork on one of the standing walls. Figure 8(a) shows an example in which patchwork is arranged on the top surface and one of the standing walls, and Figure 8(b) shows an example in which the patchwork in Figure 8(a) extends to the flange. In Example 3, as in Example 1, no cracks were found in the entire hat-shaped part (including the patchwork).

[0085] [Example 4] Figure 9 shows an example (Example 4) of the hat-shaped part shown in Example 1 (Figure 6) with a recess or protrusion on the top surface. Figure 9(a) shows an example with a recess on the top surface. The recess has a depth of 1 mm, a width of 50 mm, and a radius of curvature (R) of the corner of the recess of 35 mm. As shown in Figure 9(a), spot welding was performed in the area corresponding to the recess before press forming, and other spot welding was performed after press forming.

[0086] Figure 9(b) shows an example having a convex portion on the top surface. The shape is like the inverted concave portion of Figure 9(a), and the height of the convex portion is 1 mm, and the radius of curvature (R) of the corner of the convex portion is set to 35 mm. As shown in Figure 9(b), spot welding was performed in the area corresponding to the convex portion before press forming, and other spot welding was performed after press forming.

[0087] FIG. 9( c ) shows an example similar to FIG. 9( b ), in which the top surface has a convex portion, the height of the convex portion is 1 mm, and the radius of curvature (R) of the corner of the convex portion is set to 35 mm. As can be seen from the cross-sectional view of FIG. 9( c ), spot welding was performed on the top surface at three locations in two rows, for a total of six locations. As with FIG. 9( b ), spot welding was performed on this inner convex portion at two locations in two rows. In the example of FIG. 9( c ), the corner R of the convex portion is 35 mm, which does not satisfy Equation 1. Therefore, when manufacturing a patchwork blank, the spot welding locations can be determined without considering the bends corresponding to the corners of the convex portion. In other words, in the case of FIG. 9( c ), regardless of whether or not there is a convex portion, spot welding was performed only in the region corresponding to the top surface before press forming, and other spot welding was performed after press forming.

[0088] In Example 4 (FIGS. 9(a), (b), and (c)), similarly to Example 1, no cracks were observed in the entire hat-shaped part (including the patchwork) produced.

[0089] [Example 5] This example (Example 5) shows a part with a hat-shaped cross section similar to that of Figure 3 (same as Figure 9(a)) and a recess in the top surface, but with a small radius of curvature in the bent portion, unlike Example 4. The specifications of the top surface are shown below. The part shape and manufacturing conditions (pressing conditions, etc.) other than the top surface are the same as Example 1. <Top surface dimensions of hat-shaped part (base blank)> Overall width of top surface: 200 mm Vertical wall length: 100 mm Flange width: 30 mm Longitudinal length: 300 mm Top surface shoulder (left and right shoulders of the top surface in Figure 3) R: 4 mm Top surface bend (bends 31, 32, 33, 34 in Figure 3) R: 5 mm Top surface recess depth x width: 20 mm x 100 mm Flange-to-vertical wall R (base blank): 15 mm

[0090] <Base blank> Steel plate: Tensile strength 1180 MPa steel plate, plate thickness 1.4 mm Length: 300 mm Width: 480 mm

[0091] <Patchwork> Steel plate: Tensile strength 1470 MPa steel plate, plate thickness 1.2 mm Length (longitudinal length of hat-shaped part): 100 mm Width: 400 mm

[0092] <Joining method> ・Spot welding point diameter: 5.5 mm (= 5√thickness of thinner plate (1.2 mm)) ・Spot welding locations ・Welding before press forming Top surface: Bent sections 32-33: 4 locations (2 locations in the width direction x 2 rows in the length direction) ・Welding after press forming Top surface: From left shoulder to bent section 31 in Figure 3: 2 locations (1 location in the width direction x 2 rows in the length direction) : Bent sections 31-32: 2 locations (1 location in the width direction x 2 rows in the length direction) : Bent sections 33-34: 2 locations (1 location in the width direction x 2 rows in the length direction) : Bent section 34 to right shoulder in Figure 3: 2 locations (1 location in the width direction x 2 rows in the length direction) Standing wall: 4 locations on one side (2 locations in the width direction x 2 rows in the length direction) x both sides

[0093] <Press molding conditions> Press machine: 500t mechanical press Pad pressure: 20t Average press speed: 200mm / sec Molding pressure: 100t

[0094] As described above, the spot welding before press forming was performed in the central region of the region corresponding to the recess, and the other spot welding was performed after press forming. In Example 5, as in Example 1, no cracks were observed in the entire hat-shaped part (including the patchwork).

[0095] The present invention can be used in all industries that manufacture parts by press forming, such as the transportation equipment manufacturing industry, including automobiles, and machinery manufacturing industry.

[0096] 1 Hat-shaped part 2 Top surface portion 3 Standing wall 4 Flange 10 Patchwork blank 11 Base blank 12 Patchwork 13 Bent portion (bent portion between flange and standing wall) 14 Bent portion (bent portion between standing wall and top surface portion) 15 Bent portion (bent portion between standing wall and top surface portion) 16 Bent portion (bent portion between flange and standing wall) 21 Area (area corresponding to flange) 22 Area (area corresponding to standing wall portion) 23 Area (area corresponding to top surface portion) 24 Area (area corresponding to standing wall portion) 25 Area (area corresponding to flange) 30 Recess 31 to 34 Bent portion of recess

Claims

1. A press forming method in which a patchwork blank, which has a patchwork made of steel plate superimposed on the surface of a base blank made of steel plate, is bent at a bent portion on the surface of the patchwork blank, wherein the patchwork blank in which the patchwork and the base blank are joined is press-formed in only one area of ​​the surface of the patchwork blank, separated by the bent portion, when there is only one bent portion, or in only an area between two adjacent bent portions on the surface of the patchwork blank, when there is two or more bent portions, and then the patchwork and the base blank are joined in another area of ​​the patchwork blank.

2. A press forming method as described in claim 1, in which, when there are two or more bent portions and three or more bent portions, the patchwork blank in which the patchwork and the base blank are joined is press-formed in only one of the areas between two adjacent bent portions on the surface of the patchwork blank, and then the patchwork and the base blank are joined in another area of ​​the patchwork blank.

3. The press forming method according to claim 1 or 2, wherein the following formula 1 is satisfied when the radius of curvature of the bent portion in the base blank is R, the thickness of the patchwork is tp, and the thickness of the base blank is tb: R<12.5×(tp+tb) Formula 1 4. A press forming method according to any one of claims 1 to 3, wherein when the number of bent portions is three or more and is an odd number, the patchwork blank in which the patchwork and the base blank are joined is press formed only in the central region.

5. A press molding method according to any one of claims 1 to 3, in which when the cross section of the patchwork blank after press molding is groove-shaped or hat-shaped, the patchwork blank in which the patchwork and base blank are joined is press-molded only in the area corresponding to the top surface.

6. A press-molded part having a bent portion, in which a patchwork of steel plates is superimposed on the surface of a base blank made of steel plate and joined at a joint, wherein the difference between the maximum hardness within 2 mm outward from the outer edge of the joint on the surface of the base blank and the hardness of the base blank's base material is 7% or more of the hardness of the base blank's base material, the joint being present only in one of the regions on either side of the bent portion on the surface of the base blank when there is one bent portion, and only in the region between two adjacent bent portions on the surface of the base blank when there are two or more bent portions.

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

8. A press-molded part as described in claim 6 or 7, wherein when there are two or more bent portions and when there are three or more bent portions, in only one of the regions between two adjacent bent portions on the surface of the base blank, the difference between the maximum hardness within 2 mm outward from the outer edge of the joint on the surface of the base blank and the hardness of the base blank's base material is 7% or more of the hardness of the base blank's base material.

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