Press molding method and press molded part

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

[Technical Field]

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

[0002] There is a growing demand for reducing lifecycle GHG (total greenhouse gas emissions over the entire lifecycle), particularly in the automotive industry, and there is an increasing demand for component / module integration with the aim of increasing the efficiency of manufacturing 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 component manufacturing by press-forming so-called tailored welded blanks (TWB), which are blanks made by combining and integrating different types of steel sheets within a component.

[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 overlapping and joining plate-shaped blanks. 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 in the welded area. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-177115 [Patent Document 2] International Publication No. 2020 / 003767 [Patent Document 3] Patent Publication No. 2021-049551 [Patent Document 4] Japanese Patent Publication No. 2023-077210 Summary of the Invention [Problem to be solved by the invention]

[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 use many small patchwork pieces or a tailored blank (TWB), which is made by dividing the base blank. This not only took 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. [Means for solving the problem]

[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 areas where the difference in material flow rate is large, it would be possible to suppress cracks even in larger patchworks, and we proceeded with development.

[0013] (stomach) 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, separated by the bent portion, 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 where the base blank and patchwork overlap.

[0014] (cormorant) As for the bending pattern, when there is one bend on a plate-shaped patchwork blank (when there is one bend when viewed in cross section), that is, when the cross section is L-shaped, it is advisable to join the patchwork and 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, when 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 to be layered 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 viewpoint of symmetry, it is preferable to join the patchwork and the base blank at the top surface, which is in the middle. Generally, part shapes are complex and diverse, so patchwork blanks often have three or more bent sections. Press-formed parts with three or more bent sections are particularly susceptible to cracking during bending. The inventors have discovered that even when there are three or more bent sections, it is effective to produce an integrated blank by joining the patchwork and base blank in only one of the regions sandwiched between adjacent bent sections (there can be two or more of these regions), and then press-form the blank. After press-forming this integrated blank, joining the unjoined regions allows parts with complex shapes to be obtained without cracking.

[0017] (workman) Further investigation revealed that even if the entire patchwork bundle is welded, it can be bent without cracking as long as the plastic strain near the weld is 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 is formed by superimposing a patchwork made of steel plate on a surface of a base blank made of steel plate, at a bending portion on a surface of the patchwork blank, When there is one bent portion, the bent portion is separated from the patchwork blank by only one region on either side of the surface of the patchwork blank. When there are two or more bent portions, only in the area between two adjacent bent portions on the surface of the patchwork blank, Press-forming the patchwork blank in which the patchwork and the base blank are joined, A press-forming method characterized by then joining the patchwork and the base blank in other areas of the patchwork blank. [2] When there are two or more bent portions, and when there are three or more bent portions, in only one of the areas between two adjacent bent portions on the surface of the patchwork blank that exist, The press-forming method described in [1], wherein the patchwork blank in which the patchwork and the base blank are joined is press-formed, and then the patchwork and the base blank are joined in another area of ​​the patchwork blank. [3] The press forming method according to [1] or [2], wherein the bent portion is a bent portion where, assuming that the patchwork and the base blank are joined, a portion is generated in which the plastic strain of the base blank due to bending of the bent portion is 4% or more. Alternatively, 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, the press forming method described in [1] satisfies the following formula 1. R<12.5×(tp+tb)...Equation 1 [4] The bent portion The area betweenWhen the number is 3 or more and is an odd number, the patchwork blank in which the patchwork and the base blank are joined only in the central region is press-molded.

[0033] The press-molding method according to any one of [1] to [3] above. [5] A 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 in which the patchwork and the base blank are joined is press-formed only in the area corresponding to the top surface. [6] A press-formed part having a bent portion, in which a patchwork made of steel plates is superimposed on a surface of a base blank made of steel plates and joined at a joint, The difference between the maximum hardness within 2 mm 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. Department but, When there is one bent portion, the bent portion is located in only one region of the surface of the base blank, When there are two or more bent portions, only in the region between two adjacent bent portions on the surface of the base blank, 1. A press-formed part characterized in that: [7] The press-molded part according to [6], wherein the joining is spot welding. That is, a press-formed part having a bent portion, in which a patchwork made of steel plates is superimposed on the surface of a base blank made of steel plates and joined at joining points by spot welding, The difference between the maximum hardness at a position 5 mm away 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. When there is one bent portion, the bent portion is located in only one region of the surface of the base blank, When there are two or more bent portions, only in the region between two adjacent bent portions on the surface of the base blank, 1. A press-formed part characterized in that: [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 that are two or more, The press-molded part according to [6] or [7], 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 material of the base blank is 7% or more of the hardness of the base material of the base blank. [Effects of the Invention]

[0020] According to the present invention, cracks can be suppressed during bending even in patchwork blanks made 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 reduces the number of press-forming dies, resulting in significant cost savings. In particular, the effects of the present invention are more pronounced when applied to parts with complex shapes, such as patchwork blanks with three or more bends, or when applied to hot processing (hot press forming (hot stamping) and the like). [Brief explanation of the drawings]

[0021] [Figure 1] 1A and 1B are diagrams for explaining the present invention, each showing a schematic diagram of an example of a press-formed part having a hat-shaped cross section (a hat-shaped part), in which Fig. 1A shows an external view and Fig. 1B shows a cross-sectional view. [Figure 2] FIG. 2 is a schematic diagram showing an example of a blank (patchwork blank) to be used for press-forming the hat-shaped part of FIG. 1. [Figure 3] 2 is a schematic diagram showing an example of a cross section of the hat-shaped part of FIG. 1 having a recessed portion on the top surface thereof. FIG. [Figure 4]FIG. 4 is a schematic diagram showing an example of a blank (patchwork blank) to be used for press-forming the hat-shaped part having a recess in the top surface portion of FIG. 3. [Figure 5] 5(a) and 5(b) are diagrams illustrating an example of a hat-shaped part manufactured by a conventional manufacturing method, in which Fig. 5(a) shows an external view, Fig. 5(b) shows a cross-sectional view, and Fig. 5(c) is a schematic diagram showing an example of a patchwork blank used in press-forming the hat-shaped part of Fig. 5(a). [Figure 6] 6(a) to 6(c) are diagrams illustrating an example of a hat-shaped part according to one embodiment of the present invention. Figs. 6(a) to 6(c) are examples of joining 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 an example of a patchwork blank for the same hat-shaped part, in which a different joining method is 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 applied after press-forming. [Figure 7] 6(a) is a diagram 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. [Figure 8] 8(a) and 8(b) are diagrams illustrating an example of a hat-shaped part according to an embodiment of the present invention. Fig. 8(a) is a schematic diagram illustrating an example of the hat-shaped part shown in Fig. 6(a) when there is no patchwork on one side of the standing wall. Fig. 8(b) is a schematic diagram illustrating an example of the hat-shaped part shown in Fig. 7 when there is no patchwork on one side of the standing wall. [Figure 9] 9A and 9B are diagrams for explaining an example of a hat-shaped part which is one embodiment of the present invention, in which FIG. 9A is a schematic diagram showing an example of a case where a concave portion is provided on the top surface, FIG. 9B is a schematic diagram showing an example of a case where a convex portion is provided on the top surface, and FIG. 9C is a schematic diagram showing another example of a case where a convex portion is provided on the top surface. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention will be described below with reference to an embodiment of the present invention (hereinafter simply referred to as the present invention), namely, a hat-shaped cross-section press-formed product. FIG. 1 shows a schematic diagram of a hat-shaped cross-section press-formed product 1 used in the description, with FIG. 1(a) showing an external view and FIG. 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 FIG. 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, vertical wall 3, and 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, as 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 inside 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 another joining method, 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 hat-shaped part 1. The steel plate that forms the main body of the hat-shaped part is called base blank 11, and a steel plate that forms patchwork 12 is placed on its surface. The blank formed by overlaying patchwork 12 on base blank 11 is called patchwork blank 10. Patchwork blank 10 is press-formed to produce 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 flange 4 and vertical wall 3, and between vertical wall 3 and top surface 2, to obtain 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 flange 4 and vertical wall 3, and the bent portions 14 and 15 are located between vertical wall 3 and top surface 2. In the case of the hat-shaped part shown in Figure 1, the bent part is linear, but the shape of the bent part is not particularly limited. When obtaining a press-molded part with a complex shape, the bent part may be curved. The bent part may also 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 the sake of convenience, 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 the joints (areas 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 be the cause of 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 the hat-shaped part 1 shown in Figure 1, when there are two or more bends, it is advisable 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 where there are three or more bends, the present inventors have found that an integrated blank can be produced by joining the patchwork and 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 way, the unjoined regions can be joined to obtain a part with a complex shape without cracks.

[0032] Bent section The area between If the number of bends 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. Also, for example, if the bends are arranged asymmetrically, it is preferable to join only in the region close to the center of the patchwork blank (for example, 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 bends.

[0033] For example, in the case of the hat-shaped part 1 in Figure 1, there are four bends as shown in Figure 2. As can be seen from Figure 2, of these four bends, the patchwork 12 is placed over two bends, bend 14 and bend 15. Therefore, the two adjacent bends are bend 14 and bend 15, and the area between these two adjacent bends is area 23, which corresponds to the top surface. Therefore, in the case of the patchwork blank 10 shown in Figure 2, it is advisable to manufacture the patchwork blank 10 by spot welding the patchwork 12 and the base blank 11 only in area 23, and leaving the other areas, areas 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 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 may be determined before press forming, taking into account the material flow behavior during press forming.

[0035] Material flow behavior refers to the difference in material flow rates between the base blank and patchwork blank that make up the patchwork blank. The difference in material flow rates between the base blank and patchwork blank is the absolute value of the difference in movement vectors obtained as a result of press-forming movement of a point (position) on each blank corresponding to an arbitrary point (position) in the overlapping area between the base blank and patchwork blank before forming. In other words, the movement vector of a point (position) on each blank corresponding to an arbitrary point (position) in the overlapping area between the base blank and patchwork blank is calculated from the point (position) to which it 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 area).

[0036] A specific simulation example is presented. 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 (joined by spot welding or other methods) at at least one point (e.g., the center of gravity of the overlapping portion) at the overlapping portion of the base blank and patchwork during press forming, the press forming analysis of the integrated blank (simulation analysis using FEM, etc.) is performed. The movement vectors of each point are calculated from the positions of each point on the base blank and patchwork corresponding to any point in the overlapping portion 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 difference in material inflow 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 difference in material inflow 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 spot-welded joint made by overlapping 1.5 GPa-class steel plates (1.2 mm thick) was heated to 900°C and held for one minute, then pulled at 740°C with the steel plates sheared, then quenched and pulled at 1 mm / s at room temperature, and the amount of shear between the two steel plates and the tensile shear strength (TSS) of the joint (spot weld) at room temperature were observed. The results showed that when the amount of shear between the steel plates during hot working was 0 to 1.0 mm, the TSS of the joint at room temperature was equivalent to that of a joint with a shear of 0 mm (i.e., not hot-deformed). However, when the shear exceeded 1.0 mm, the TSS tended to decrease. Specifically, under these test conditions, it was confirmed that the TSS was 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. In other words, it was discovered for the first time that even in hot press forming (hot stamping), even if a difference in material flow rate at the spot welding point (equivalent to the above-mentioned misalignment) of less than 1.0 mm from 0.0 mm can occur, it is possible to ensure tensile shear strength equivalent to or greater than that when no deformation occurs. Based on this knowledge, in the case of hot pressing, the limit value of the difference in material flow rate is preset to 1.0 mm, and joining is performed at points (or regions including these points) where the difference in material flow rate at each point is below this preset limit value through simulation. 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 by the misalignment is larger than in hot press forming. Therefore, the limit value for the amount of material inflow is smaller than in hot press forming, at about 0.5 mm. Therefore, in cold press forming, the limit value for the difference in material inflow 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 the region where the difference in material inflow rate is equal to or less than a threshold value. In this case, it is advisable to set the pre-press forming spot welding point (position) within the region where the analysis results indicate that the difference in material inflow rate is small.

[0040] 3 and 4, from the viewpoint of symmetry, 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). When parts are nearly axisymmetric, the material flow also occurs axisymmetrically, so the difference in the amount of material flow becomes small around the axis of symmetry.

[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 forming)), 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 effectively exhibited.

[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 the joining may be performed using the same method 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 high-tensile steel sheets of 1470 MPa class can be formed without cracking with a plastic strain of approximately 4%. Therefore, it is believed that cracks will not occur in the blank if the plastic strain generated by bending during press forming is 4% or less. In other words, 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 can be determined by a forming simulation using a patchwork blank. A forming analysis (bending analysis) is performed using a patchwork blank, in which patchwork is spot-welded to a base blank at candidate locations for spot welding (i.e., locations of the joining portions (joints)), 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 to 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 can be excluded when determining the weldable region. The forming analysis method is not particularly limited. Numerical simulations such as the finite element method (FEM) can 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 welded portion to the outside (the blank base material side opposite the weld center). In the case of spot welding, the maximum plastic strain should 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 joint (spot weld point).Also, for example, in the case of lap welding, the maximum plastic strain should be the maximum value of the plastic strain obtained by FEM analysis in the area within 2 mm on the blank side (opposite the weld metal) from the outer edge of the weld, that is, the boundary between the weld metal and the blank surface.

[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 part of the base blank tb: Base blank thickness tp: patchwork thickness

[0048] Formula 1 applies when the plastic strain of the bent portion exceeds 4%, so just because Formula 1 is satisfied does not 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 will be large. Preferably, 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 of more than 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 read Equation 1 as an equation corresponding to a plastic strain of more than 20%.

[0049] For example, consider 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, where the top surface 2 has a recess 30. 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 Equation 1 rephrased), the four bent portions 31, 32, 33, and 34 can be excluded when determining the joining area 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 curvature radii of bends 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 bends 31, 32, 33, and 34, as shown in FIG. 4. In this case, patchwork 12 has a total of six bends (bends 14, 31, 32, 33, 34, and 15). Because the patchwork is joined only in the region between two adjacent bends, five candidate regions are identified: the region between bends 14 and 31, the region between bends 31 and 32, the region between bends 32 and 33, the region between bends 33 and 34, and the region between bends 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 parts] In the press-formed part obtained by the press-forming method of the present invention, a portion of the patchwork is joined before press-forming, and the remaining portion is joined after press-forming. Therefore, the portion joined after press-forming naturally does not experience strain due to press-forming (bending). Furthermore, the portion joined before press-forming is also less susceptible to bending during press-forming, and even if bending is performed, the region is one in which plastic strain is 4% or less. Therefore, even if the press-formed part having the patchwork of the present invention is processed with bending, the maximum strain of the base blank around the joint (spot welding point) or joint (the area where the base blank and patchwork are joined, including other joining methods) is kept to 4% or less. Here, the periphery of the joint or joint refers to the area 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 point on the blank surface may be 5 mm away from the joining point (the center of the spot welding point).

[0053] On the other hand, parts joined before press forming (by spot welding, etc.) are work-hardened, even though the maximum strain is less than 4%, which can improve the component's shear deformation resistance. In particular, in the case of high-tensile steel, it has been confirmed that the Vickers hardness (HV) of parts near parts joined before press forming (by spot welding, etc.) is 7% or more harder than the base material due to work hardening. In other words, if the base material hardness is Hvm, then the difference △Hv between the maximum hardness around the joint (maximum hardness of the work-hardened part) Hvmax and the base material hardness Hvm will be 7% or more of the base material hardness Hvm (see the formula below). △Hv=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 outward (opposite the center of the joint) 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 area around the joint. In the case of spot welding, measurements can be 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 area around the joint. The base material hardness of the base blank is the hardness (Vickers hardness) of the part that is 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 made 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 the joint exists only in one region of the surface of the base blank (patchwork blank) on either side of the bent portion if there is one bent portion, and only in the region between two adjacent bent portions on the surface of the base blank (patchwork blank) if there are two or more bent portions.

[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 best to check the bent portions when viewing a cross section of the component (a cross section perpendicular to the longitudinal direction of the component). [Example]

[0057] [Example 1] Using the hat-shaped part shown in Figure 1 as a base, a hat-shaped part was manufactured in which patchwork was arranged from the top surface to the vertical wall in the center of the hat-shaped part's longitudinal direction. 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: 100mm Vertical wall length: 100mm Flange width: 30mm Longitudinal length: 300mm Top shoulder R (base blank): 10mm Flange-to-wall R (base blank): 15 mm

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

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

[0061] <Joining method> Spot welding diameter: 5.5 mm (= 5√thickness of thinner plate (1.2 mm)) Spot welds: Top surface: 4 locations (2 locations across x 2 rows across) Vertical wall: 4 locations x 2 (2 locations widthwise x 2 rows lengthwise)

[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 of a hat-shaped part manufactured using a conventional manufacturing method (comparative example). Figure 5 shows an example in which all spot welding was performed before press forming due to the application of the 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 that spot welding was 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 that spot welding was 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), the black circles (●) indicate the locations of spot welds applied 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) on the lower side of the vertical wall (flange side) out of the eight spot welds on the vertical wall.

[0064] Furthermore, a FEM simulation analysis was performed 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) at the bottom of the vertical wall is 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 circle (●) spot welding indicates spot welding that was performed before press forming, while the white circle (◯) spot welding indicates spot welding that was performed after press forming.

[0067] Figure 6(b) shows a cross section of the hat-shaped part in 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 spot welds in Figure 6(b) indicate welds that were spot welded before press forming. On the other hand, the white spot welds in Figure 6(b) indicate welds that were spot welded after press forming. The black circles in Figure 6(a) correspond to the black squares in Figure 6(b) and indicate welds that were spot welded before press forming, while the white circles in Figure 6(a) correspond to the white squares in Figure 6(b) and indicate welds that were spot welded after press forming.

[0068] Figure 6(c) shows the patchwork blank for the hat-shaped part 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), the black circles (●) indicate locations where spot welding was performed before press forming, i.e., when manufacturing the patchwork blank, while the white circles (◯) indicate locations where spot welding was performed after press forming. In other words, the patchwork blank is spot welded only at the locations indicated by the 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 sections 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 checked, centered around the spot welds. As a result, no cracks were found in the entire hat-shaped part (including the patchwork).

[0070] Furthermore, an FEM simulation analysis of strain during press forming of the patchwork blank in Figure 6(c) was performed, and it was confirmed 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 that the maximum plastic strain was 1.2%. It was confirmed that the maximum plastic strain at the spot weld point below the vertical wall, where cracking occurred in the hat-shaped part in Figure 5(a), was nearly 0%. The maximum plastic strain around the spot weld point (joint point) on the top surface was also about 0.8%.

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

[0072] On the other hand, when the hardness around the spot welds in 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 mentioned above, the hardness (surface hardness) of the base blank around the spot weld point was measured at five or more points 5 mm away from the center of the spot weld point, 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 point and 30 mm or more away from the outer edge of the bent part, and using the arithmetic average of these measurements.

[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 joining states are shown in Figures 6(d), 6(e), and 6(f).

[0076] Figure 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 sections 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] Figure 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 sections 14 and 15 that was joined by lap welding before press forming. The white band indicates the area in the vertical wall of the hat-shaped part where the patchwork and base blank are further joined by lap fillet arc welding after press forming.

[0078] Figure 6(f) shows an example in which the patchwork was bonded to the base blank with adhesive before press forming. The area (top surface) between bent sections 14 and 15 before press forming is shown as a checkered pattern (checkered square area). After press forming, the patchwork and base blank were joined by laser spot welding in the vertical wall of the hat-shaped part. The joints (joint points) created by laser spot welding after press forming are shown as 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 material of the base blank.The hardness of the base material 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 in the hat-shaped part shown in Example 1 (Figure 6). In this case, spot welding was also performed on the flange in two places on each side after press forming. In Example 2, as in Example 1, no cracks were observed in the entire manufactured hat-shaped part (including the patchwork). Note that in Figures 7 to 9, as in Figure 6, black squares (filled squares) indicate spot welding that was performed before press forming, and white squares (□) indicate spot welding that was 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) where there is no patchwork on one of the standing walls. Figure 8(a) shows an example where patchwork is arranged on the top surface and one of the standing walls, and Figure 8(b) shows an example where 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 with a convex portion on the top surface. It has a shape like the inverted concave portion of Figure 9(a), with a height of 1 mm and a curvature radius (R) of the corner of the convex portion 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] Figure 9(c) shows an example similar to Figure 9(b), in which the top surface has a convex portion, with a height of 1 mm and a curvature radius (R) of 35 mm at the corner of the convex portion. As can be seen from the cross-sectional view of Figure 9(c), spot welding was performed on the top surface in three locations in two rows, for a total of six locations. As with Figure 9(b), spot welding was performed on this inner convex portion in two locations in two rows. In the example of Figure 9(c), the convex corner R 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 Figure 9(c), regardless of whether or not there is a convex portion, spot welding was performed only in the area 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)(c)), as in Example 1, no cracks were found in the entire hat-shaped part (including the patchwork) that was manufactured.

[0089] [Example 5] This shows an example (Example 5) of a part with a hat-shaped cross section similar to that of Figure 3 (same as Figure 9(a)) and a recess on the top surface, but with a small radius of curvature at 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. <Hat-shaped part top surface dimensions (base blank)> Top width: 200mm Vertical wall length: 100mm Flange width: 30mm Longitudinal length: 300mm Top shoulders (left and right shoulders of the top in Figure 3) R: 4mm Top surface bends (bends 31, 32, 33, and 34 in Figure 3) R: 5 mm Top recess depth x width: 20mm x 100mm Flange-to-wall R (base blank): 15 mm

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

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

[0092] <Joining method> Spot welding diameter: 5.5 mm (= 5√thickness of thinner plate (1.2 mm)) Spot welding points ·Pre-welding before press forming Top surface: Bending sections 32-33: 4 locations (2 locations across x 2 rows across) · Welding after press forming Top surface: Left shoulder of Figure 3 to bend 31: 2 locations (1 location in the width direction x 2 rows in the length direction) : Bending section 31-32: 2 places (1 place in the width direction x 2 rows in the length direction) : Bending section 33-34: 2 places (1 place in the width direction x 2 rows in the length direction) : Bending section 34 - Right shoulder of 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 across the width x 2 rows across the length) 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 explained 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). [Industrial Applicability]

[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. [Explanation of symbols]

[0096] 1 hat-shaped part 2 Top section 3 Standing Wall 4 flanges 10 Patchwork Blanks 11 Base Blank 12 Patchwork 13 Bending section (bending section between flange and vertical wall) 14 Bending section (bending section between the vertical wall and the top surface) 15 Bending section (bending section between the vertical wall and the top surface) 16 Bending section (bending section between flange and vertical wall) 21 area (area corresponding to flange) Area 22 (area corresponding to the vertical wall) 23 area (area corresponding to the top surface) 24 area (area corresponding to the vertical wall) 25 area (area corresponding to flange) 30 recess 31~34 Concave bend

Claims

1. A press forming method for bending a patchwork blank, which is formed by superimposing a patchwork made of steel plate on a surface of a base blank made of steel plate, at a bending portion located at a portion where the patchwork is superimposed on the surface of the patchwork blank, comprising: There are five or more bent portions, and only in the area between two adjacent bent portions on the surface of the patchwork blank, Press-forming the patchwork blank in which the patchwork and the base blank are joined, A press-forming method characterized by then joining the patchwork and the base blank in other areas of the patchwork blank.

2. When the cross section of the patchwork blank after press forming is groove-shaped or hat-shaped and there are four or more bent portions in the region corresponding to the top surface, in only one of the three or more regions between two adjacent bent portions on the surface of the patchwork blank, The press forming method according to claim 1 , wherein the patchwork and the base blank are joined together.

3. The press forming method according to claim 1, wherein the following formula 1 is satisfied when the radius of curvature of the bent portion in the base blank is R, the plate thickness of the patchwork is tp, and the plate thickness of the base blank is tb. R<12.5×(tp+tb)...Formula 1

4. A press forming method for bending a patchwork blank, which is formed by superimposing a patchwork made of steel plate on a surface of a base blank made of steel plate, at a bending portion on a surface of the patchwork blank, The bent portion satisfies the following formula 1 when the radius of curvature of the base blank is R, the plate thickness of the patchwork is tp, and the plate thickness of the base blank is tb: When there is one bent portion, the bent portion is located only in one area of ​​the surface of the patchwork blank, and when there are two or more bent portions, the bent portion is located only in the area between two adjacent bent portions on the surface of the patchwork blank. Press-forming the patchwork blank in which the patchwork and the base blank are joined, A press-forming method characterized by then joining the patchwork and the base blank in other areas of the patchwork blank. R<12.5×(tp+tb)...Formula 1

5. When there are two or more bent portions, and when there are three or more bent portions, in only one of the areas between two adjacent bent portions on the surface of the patchwork blank, The press-forming method according to claim 4, wherein the patchwork blank in which the patchwork and the base blank are joined is press-formed, and then the patchwork and the base blank are joined in another region of the patchwork blank.

6. When the number of regions between the bent portions is 3 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. The press forming method according to any one of claims 1 to 5.

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

8. A press-formed part in which a patchwork made of steel plates is superimposed on a surface of a base blank made of steel plates and joined at a joint, and a bent portion is formed in the overlapping portion of the patchwork, The joint has a Vickers hardness of 7% or more of the hardness of the base blank's base material, where the difference between the maximum hardness within 2 mm 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 There are five or more bent portions, and only in the region between two adjacent bent portions on the surface of the base blank, 1. A press-formed part characterized in that:

9. A press-molded part as described in claim 8, wherein when the cross section is groove-shaped or hat-shaped and there are four or more bent portions in the region corresponding to the top surface, the joint has a hardness of 7% or more of the base material of the base blank in only one of the three or more regions between two adjacent bent portions on the surface of the base blank.

10. A press-formed part having a bent portion, in which a patchwork made of steel plates is superimposed on a surface of a base blank made of steel plates and joined at a joint, 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 in Vickers hardness. The bent portion satisfies the following formula 1 when the radius of curvature of the base blank is R, the plate thickness of the patchwork is tp, and the plate thickness of the base blank is tb: When there is one bent portion, in only one region of the surface of the base blank, with the bent portion as a boundary, When there are two or more bent portions, only in the region between two adjacent bent portions on the surface of the base blank, 1. A press-formed part characterized in that: R<12.5×(tp+tb)...Formula 1

11. When there are two or more bent portions, and when there are three or more bent portions, in only one region between two adjacent bent portions on the surface of the base blank, there are two or more bent portions, 11. The press-molded part according to claim 10, 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 material of the base blank is 7% or more of the hardness of the base material of the base blank in Vickers hardness.

12. The press-molded part according to any one of claims 8 to 11, wherein the joining is by spot welding.

Images