Press-formed article and method for producing press-formed article

Abstract

A press-formed article according to one aspect disclosed herein comprises: a plurality of press-formed parts that are made of steel and partially overlap each other; and spot weld parts that are disposed in overlapping sections of the plurality of press-formed parts and have nuggets for joining the plurality of press-formed parts. The Vickers hardness of base material sections of one or more sheets of the press-formed parts is 410 HV or more, the lowest Vickers hardness is at least 80% of the Vickers hardness of the base material section of the hardest press-formed part, one or more through-holes extending in the thickness direction of the overlapping sections are disposed in the overlapping sections, and the press-formed article further comprises nuts fixed to first ends of the through-holes.

Description

Press-molded article and method for producing press-molded article

[0001] The present disclosure relates to a press-molded article and a method for producing a press-molded article. This application claims priority based on Japanese Patent Application No. 2025-053246 filed in Japan on March 27, 2025, the content of which is incorporated herein by reference.

[0002] In automotive frame components such as A-pillars and B-pillars, there are cases where a high-strength steel sheet is used as the material on the upper side of the vehicle body, and a low-strength steel sheet is used as the material on the lower side of the vehicle body. In some cases, the thickness of the material is changed depending on the position of the frame component. This makes it possible to select a material according to the structure and purpose of the component.

[0003] When manufacturing a single frame component using a plurality of steel sheets with different thicknesses and strengths, usually, first, a sub-component constituting an upper portion of the vehicle body of the frame component and a sub-component constituting a lower portion of the vehicle body are manufactured. The sub-component is a press-molded article manufactured by press-forming a steel sheet. Next, the sub-components are assembled by spot welding to obtain the frame component.

[0004] Patent Documents 1 to 3 disclose various aspects of automotive components or technologies applicable thereto.

[0005] Japanese Patent Application Laid-Open No. 2021-91311 Japanese Patent Application Laid-Open No. 2021-195005 Japanese Patent Application Laid-Open No. 2022-129107

[0006] Generally, high-strength steel sheets increase the carbon content to enhance the base material strength. When such a high-strength steel sheet is spot-welded, local hardness reduction, that is, a softened portion may be formed in the heat-affected zone (HAZ) around the nugget due to the influence of welding heat. Such a softened portion can serve as a fracture initiation point of the spot welded portion, so there is a risk that the bonding strength may decrease.

[0007] Furthermore, some automotive parts require rigidity. For example, the door mounting points on the A-pillar and B-pillar require rigidity. To ensure rigidity, the steel plates that make up the parts need to be sufficiently thick. In areas where rigidity is required, additional parts such as reinforcements may be used to ensure sufficient thickness. The use of additional parts increases the manufacturing cost of automotive parts.

[0008] In view of the above circumstances, the present disclosure aims to provide a press-formed product and a method for manufacturing the same that do not require separate parts to increase rigidity and can easily ensure excellent joint strength.

[0009] The gist of this disclosure is as follows:

[0010] (1) A press-formed article according to one aspect of the present disclosure comprises a plurality of press-formed parts made of steel, which are partially overlapped with each other, and a spot welded part disposed in the overlapping portion of the plurality of press-formed parts and having a nugget for joining the plurality of press-formed parts, wherein the Vickers hardness of the base material of one or more of the press-formed parts is 410 HV or more, and the minimum Vickers hardness determined by measuring the distribution of Vickers hardness in a region of 0.2 mm to 2.0 mm outside the nugget from the molten boundary along a hardness measuring line disposed in the hardest press-formed part and 0.2 mm away from the mating surface between the hardest press-formed part and the press-formed part in contact therewith, in the cross section of the spot welded part, is 80% or more of the Vickers hardness of the base material of the hardest press-formed part, and the overlapping portion has one or more through holes extending in the thickness direction of the overlapping portion, and the press-formed article further comprises a nut fixed to the first end of the through hole. (2) Preferably, in the press-formed product described in (1) above, the maximum overlap of the overlapping portion is 30 mm or more and 280 mm or less. (3) Preferably, the press-formed product described in (1) or (2) above further comprises a bolt inserted into the through hole and the nut, and a component disposed at the second end of the through hole, wherein the bolt and the nut fasten the component to the press-formed portion where the second end of the through hole is formed. (4) Preferably, in the press-formed product described in (3) above, the component is a door hinge and / or an additional press-formed portion. (5) Preferably, the press-formed product described in any one of (1) to (4) above has a plurality of overlapping portions, and each of the plurality of overlapping portions has a spot weld, a through hole, and a nut. (6) Preferably, the press-formed product described in any one of (1) to (5) above is an A-pillar, B-pillar, or door ring of an automobile. (7) Preferably, in the press-formed product described in (6) above, the overlapping portion is located on the mounting portion of the automobile door.(8) Preferably, in the press-formed product described in any one of (1) to (7) above, the means for fixing the nut in the through hole is a projection weld or an arc weld, and the hardness of the base material of the press-formed part to which the nut is fixed is 455 HV or more.

[0011] (9) A method for manufacturing a press-formed product according to another aspect of the present disclosure comprises the steps of: partially overlapping a plurality of steel plates; spot welding the overlapping portion of the steel plates to obtain a tailored blank; heating the tailored blank to 800°C or higher and press-forming it; forming through holes in the overlapping portion after press-forming; and fixing nuts to the first ends of the through holes.

[0012] (10) A method for manufacturing a press-formed product according to another aspect of the present disclosure comprises the steps of: partially overlapping a plurality of steel plates; spot welding the overlapping portion of the steel plates to obtain a tailored blank; forming through holes in the overlapping portion of the steel plates; heating the tailored blank having the through holes to 800°C or higher and press-forming it; and fixing a nut to the first end of the through hole.

[0013] (11) A method for manufacturing a press-formed product according to another aspect of the present disclosure comprises the steps of partially overlapping a plurality of steel plates, spot welding the overlapping portion of the steel plates to obtain a tailored blank, heating the tailored blank to 800°C or higher and press-forming it, and fixing a nut to the first end of a through hole, wherein the press-forming die has a perforating means, and the through hole is formed in the overlapping portion using the perforating means during press-forming.

[0014] (12) A method for manufacturing a press-formed product according to another aspect of the present disclosure comprises the steps of: forming through holes in each of a plurality of steel plates; partially overlapping the plurality of steel plates with the through holes aligned; spot welding the overlapping portion of the steel plates to obtain a tailored blank; heating the tailored blank to 800°C or higher and press-forming it; and fixing a nut to the first end of the through hole in the overlapping portion.

[0015] According to this disclosure, it is possible to provide a press-formed product and a method for manufacturing the same that do not require separate parts to increase rigidity and can easily ensure excellent joint strength.

[0016] This is a plan view of an example of a press-formed product according to this embodiment. This is a cross-sectional view of the press-formed product in Figure 1 along line II-II. This is an enlarged cross-sectional view of a spot weld. This is a schematic graph of the hardness distribution around the molten boundary of a hot-press-formed spot weld. This is a schematic graph of the hardness distribution around the molten boundary of a spot weld that was not hot-press-formed. This is a cross-sectional view of a press-formed product in which a door hinge and an additional press-formed part are fastened with bolts. This is a cross-sectional view of a front floor module in which a floor panel is fastened with bolts. This is a side view of the body frame of an automobile. This is a flowchart of a first example of a method for manufacturing a press-formed product. This is a flowchart of a second example of a method for manufacturing a press-formed product. This is a flowchart of a third example of a method for manufacturing a press-formed product. This is a flowchart of a fourth example of a method for manufacturing a press-formed product. This is a schematic diagram of hot press forming using a die with a perforating means. This is a perspective view of an example of a B-pillar.

[0017] A press-formed product 1 according to one aspect of the present disclosure, as shown in Figures 1 to 3, comprises a plurality of steel press-formed parts 11 that are partially overlapping each other, and a spot welded part 13 having a nugget 131 that is positioned in the overlapping portion 12 of the plurality of press-formed parts 11 and joins the plurality of press-formed parts 11, wherein the Vickers hardness of the base material portion 14 of one or more press-formed parts 11 is 410 HV or higher, and in the cross-section of the spot welded part 13, it is positioned at the hardest press-formed part 11 and the press-formed part 1 that is in contact with the hardest press-formed part 11. The minimum Vickers hardness HVmin, determined by measuring the distribution of Vickers hardness in a region of 0.2 mm to 2.0 mm outside the nugget 131 from the melting boundary 133 along a hardness measurement line L located 0.2 mm away from the mating surface 134 with 1, is 80% or more of the Vickers hardness of the hardest press-formed part 11 base material 14. The overlapping part 12 is provided with one or more through holes 15 extending in the thickness direction of the overlapping part 12, and the press-formed product 1 further includes a nut 16 fixed to the first end of the through hole 15. If there is a gap between the pair of press-formed parts 11 at the mating surface 134, the hardness measurement line L is considered to be a position 0.2 mm away from the surface of the press-formed part 11 facing the gap.

[0018] The most basic embodiment of the press-formed product 1 according to this embodiment will be described below with reference to Figures 1 to 5. Figure 1 is a plan view of an example of the press-formed product 1. Figure 2 is a cross-sectional view of the press-formed product 1 of Figure 1 along line II-II. Figure 3 is an enlarged cross-sectional view of the spot weld. Figures 4 and 5 are schematic graphs of the hardness distribution around the molten boundary.

[0019] (Press-formed parts 11) The press-formed product 1 according to this embodiment has a plurality of press-formed parts 11 made of steel. In this disclosure, a press-formed part 11 is defined as a part of the press-formed product 1 that is made from a single steel sheet. The press-formed part 11 is obtained by press-forming the steel sheet. In the press-formed product 1 illustrated in Figure 1, the press-formed part 11 on the left side of the page has a flat plate shape. However, naturally, the press-formed part 11 can have a bent portion and form a three-dimensional shape. The press-formed part 11 on the right side of the page in Figure 1 has a serrated portion. A serrated portion is a step provided on one of the members of an lap joint. The serrated portion causes the base material surfaces of the lap joint to be arranged on substantially the same plane. Furthermore, the press-formed part 11 may have various shapes, such as a hat-shaped member.

[0020] (Overlapping portion 12) The multiple press-formed portions 11 are partially overlapped with each other. That is, as shown in the plan view of Figure 1 and the cross-sectional view of Figure 2, one press-formed portion 11 covers only a portion of the other press-formed portion 11, and the other press-formed portion 11 covers only a portion of the one press-formed portion 11. The portion in which the press-formed portions 11 overlap is referred to as the overlapping portion 12 in this disclosure. The position, size, and shape of the overlapping portion 12 are not particularly limited. For example, the overlapping portion 12 can be placed in the mounting portion of an automobile door. Details of the configuration of the overlapping portion 12 will be described later.

[0021] In some cases, reinforcing parts called reinforcements may be placed on the press-formed product 1. In such cases, the entire reinforcing part is superimposed on another press-formed part 11. A part that is entirely superimposed on another press-formed part 11 is considered to be different from a press-formed part 11 that is partially superimposed on another press-formed part 11, as shown in Figure 1. The superimposed portion 12 formed by a reinforcement that is entirely superimposed on another press-formed part 11 is not considered to be the superimposed portion 12 of the press-formed product 1 according to this embodiment. While it is permissible for the press-formed product 1 according to this embodiment to have such reinforcing parts, it is preferable to omit reinforcing parts as much as possible from the viewpoint of reducing the number of parts and the vehicle weight. In the press-formed product 1 according to this embodiment, only the parts where strength is required can be locally reinforced. Also, unlike reinforcements, in the press-formed product 1 according to this embodiment, the entire press-formed part 11 is not superimposed on another press-formed part 11, so the rigidity of the press-formed product 1 can be ensured while suppressing unnecessary weight increase and material waste.

[0022] (Spot Welded Section 13) The press-formed product 1 has a spot welded section 13 located in the overlapping section 12. Figure 3 shows an enlarged cross-sectional view of the spot welded section 13. The spot welded section 13 has a nugget 131. The nugget 131 is metal that has melted and solidified during spot welding. The outer edge of the nugget 131 is called the molten boundary 133.

[0023] The nugget 131 of the spot weld 13 joins multiple press-formed parts 11 at the overlapping section 12. The overlapping section 12 of the press-formed product 1 according to this embodiment is an overlap joint. The nugget 131 can be identified by etching the cross-section of the spot weld 13 to reveal the metallic structure.

[0024] Furthermore, an indentation 132 is formed in the spot welded area 13. An indentation 132 is a depression on the surface of the base material caused by the electrode tip as a result of welding in lap resistance welding.

[0025] Furthermore, a heat-affected zone (HAZ) may be formed around the nugget 131 in the spot weld 13. The HAZ is the unmelted portion of the base material whose structure, metallurgical properties, and mechanical properties have been altered by the welding heat. The HAZ is sometimes referred to as the heat-affected zone. The nugget 131 can be identified by etching the cross-section of the spot weld 13 to reveal the metal structure. However, the HAZ may disappear if the spot weld 13 is heat-treated. Therefore, in the spot weld 13 of the press-formed product 1 according to this embodiment, the presence of a HAZ may not be observed.

[0026] (Vickers hardness of the base material portion 14 of the press-formed portion 11) One or more of the multiple press-formed portions 11 joined by the spot welds 13 are designated as high-strength press-formed portions 111. A high-strength press-formed portion 111 is a press-formed portion 11 in which the Vickers hardness of the base material portion 14 is 410 HV or higher. The base material portion 14 is a region that is 4 mm or more away from the weld metal of a weld such as a spot weld 13.

[0027] The hardness of the base material portion 14 of the press-formed portion 11 is preferably 420 HV or higher, 440 HV or higher, or 450 HV or higher. There is no particular upper limit to the hardness of the base material portion 14 of the press-formed portion 11. For example, the hardness of the base material portion 14 of the press-formed portion 11 may be 740 HV or lower, 700 HV or lower, or 650 HV or lower.

[0028] Furthermore, press-formed portions 11 of the base material 14 with a Vickers hardness of less than 410 HV are referred to as low-strength press-formed portions 112. All press-formed portions 11 included in the overlapping portion 12 may be high-strength press-formed portions 111. On the other hand, the overlapping portion 12 may include one or more low-strength press-formed portions 112.

[0029] The Vickers hardness of the press-formed section 11 shall be measured in accordance with JIS Z 2244-1:2024 "Vickers hardness test - Part 1: Test method". The test force shall be 0.5 kgf (HV 0.5).

[0030] The Vickers hardness test is performed on a polished cross section approximately perpendicular to the surface of the press-formed part 11. The method for preparing the cross section also conforms to JIS Z 2244-1:2024. First, the cross section is polished using silicon carbide sandpaper ranging from #600 to #1500. Next, the cross section is finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 μm to 6 μm is dispersed in a diluent such as alcohol or pure water. The polished cross section is then etched using an etching solution. This reveals the plastic flow state of the metal (metal flow), and the molten boundary 133 is identified.

[0031] The Vickers hardness test of the base material portion 14 of the press-formed section 11 is performed on both sides of the nugget 131 of the spot weld 13. In the Vickers hardness test, the positions of the indentations in the cross-section are set as follows: (1) 0.2 mm deep from the surface of the press-formed section 11 facing the mating surface, and (2) 4.0 mm, 4.2 mm, and 4.4 mm away from the molten boundary 133 along the surface of the press-formed section 11 in the direction away from the nugget 131. Therefore, there are a total of six measurement positions. When measuring the hardness of the base material portion 14 of the high-strength press-formed section 111, the hardness measurement positions will be set on the hardness measurement line L described later. The average value of the six hardness measurements is considered to be the hardness of the base material portion 14 of the press-formed section 11.

[0032] (Minimum Vickers Hardness) In the press-formed product 1 according to this embodiment, the minimum Vickers hardness HVmin is 80% or more of the Vickers hardness of the hardest press-formed part 11. The minimum Vickers hardness HVmin is a value determined by measuring the hardness distribution at a predetermined position around the molten boundary 133 along a predetermined hardness measurement line L of the hardest press-formed part 11. The minimum Vickers hardness HVmin will be explained below with reference to the schematic cross-sectional view of the spot welded part 13 illustrated in Figure 3. Note that the dimensions shown in Figure 3 are not accurate from the viewpoint of ensuring visibility. The measurement intervals described below do not match the dimensions shown in Figure 3.

[0033] The minimum Vickers hardness is determined by a Vickers hardness test of the cross-section of the spot weld 13. The cross-section passes approximately through the center of the indentation 132 and is approximately perpendicular to the surface of the overlap 12. The method for preparing the cross-section and the Vickers hardness test method are the same as the method for measuring the Vickers hardness of the base material 14 described above.

[0034] The hardness measurement line L is positioned at the hardest press-formed section 11 in the overlapping section 12. In Figure 3, the press-formed section 11 on the upper side of the paper is the high-strength press-formed section 111, and the press-formed section 11 on the lower side of the paper is the low-strength press-formed section 112. Therefore, the press-formed section 11 on the upper side of the paper is the hardest press-formed section 11. The hardest press-formed section 11 is determined based on the hardness of the base material section 14 as described above. If multiple press-formed sections have the same maximum hardness, the side on which the nut is fixed is considered the hardest press-formed section.

[0035] Furthermore, the hardness measurement line L is positioned 0.2 mm away from the mating surface 134 between the hardest press-formed portion 11 and the press-formed portion 11 in contact with it. If another press-formed portion 11 is in contact with both the first and second surfaces of the hardest press-formed portion 11, the hardness measurement line L is positioned 0.2 mm away from the mating surface 134 between the harder of the two portions of base material 14 and the hardest press-formed portion 11. Note that a gap may occur between the pair of press-formed portions 11 at the mating surface 134. Such a gap is called sheet separation. If a gap exists between the pair of press-formed portions 11 at the mating surface 134, the Vickers hardness distribution is measured along the hardness measurement line L, which is positioned 0.2 mm away from the surface of the press-formed portion 11 facing the gap. For example, in the press-formed product 1 illustrated in Figure 3, if there is a gap between the high-strength press-formed portion 111 on the upper side of the paper and the low-strength press-formed portion 112 on the lower side of the paper, a virtual line that is 0.2 mm away from the surface of the high-strength press-formed portion 111 facing the gap, i.e., the surface of the high-strength press-formed portion 111 on the lower side of the paper, and that runs along that surface, is considered to be the hardness measurement line L.

[0036] A Vickers hardness test is performed continuously along the hardness measurement line L. The measurement interval is 0.2 mm. The Vickers hardness test is performed in an area of ​​at least 0.2 mm to 2.0 mm outside the nugget from the molten boundary along the hardness measurement line. That is, the Vickers test is performed in an area between (A) a position 2.0 mm away from the molten boundary 133 outside the nugget 131 and (B) a position 0.2 mm away from the molten boundary 133 outside the nugget 131 on the hardness measurement line L. The area from position A to position B corresponds to the predetermined position around the molten boundary 133 as described above. Since the distance from position A to position B is 1.8 mm, if hardness measurements are taken at 0.2 mm intervals in the area from position A to position B, the number of hardness measurement points will be 10 (= 1.8 ÷ 0.2 + 1). In Figure 3, the number of hardness measurement points is set to 5 to ensure visibility. The Vickers hardness measurement position is set to avoid welding defects such as shrinkage cavities, blowholes, and cracks. The hardness measurement line L determined by the above means is, in principle, set at a position 0.2 mm away from the mating surface 134. Only if microscopic observation confirms the presence of a welding defect at that position and makes hardness measurement impossible, the hardness measurement line L may be set at a position 0.4 mm away from the mating surface 134. If hardness measurement at a position 0.4 mm away from the mating surface 134 is confirmed to be impossible, it may be set at a position 0.6 mm away from the mating surface 134. Regardless of whether the distance between the hardness measurement line L and the mating surface 134 is 0.2 mm, 0.4 mm, or 0.6 mm, there is no substantial effect on the hardness measurement value. If there is a gap in the mating surface 134, the hardness measurement line L may be set at a position 0.2 mm away, 0.4 mm away, or 0.6 mm away from the surface of the high-strength press-formed part 111 that faces the gap. These considerations apply not only to the measurement of the minimum Vickers hardness but also to the measurement of the Vickers hardness of the base material as described above.

[0037] When Vickers hardness tests are performed continuously using the procedure described above, a hardness distribution curve like those exemplified in Figures 4 and 5 is obtained. The minimum Vickers hardness in the region between dashed line A and dashed line B in the hardness distribution curve is the minimum Vickers hardness HVmin. The minimum Vickers hardness refers to the lowest Vickers hardness in the vicinity of the nugget in the spot weld 13. The black data points in Figures 4 and 5 represent the Vickers hardness in the region between 0.2 mm and 2.0 mm outside the nugget from the molten boundary. The white data points in Figures 4 and 5 are for reference only and represent the Vickers hardness outside that region. Also, Figures 4 and 5 are schematic diagrams to explain the trend, not actual measured Vickers hardness values.

[0038] The minimum Vickers hardness HVmin is set to 80% or more of the Vickers hardness of the hardest press-formed portion 11 base material 14. Preferably, the minimum Vickers hardness is 82% or more, 85% or more, 88% or more, 90% or more, or 95% or more of the Vickers hardness of the hardest press-formed portion 11 base material 14.

[0039] (Through-hole 15) As shown in Figure 2, the overlapping portion 12 has one or more through-holes 15 in addition to the spot welds 13. The through-holes 15 extend in the thickness direction of the overlapping portion 12 and penetrate all the press-formed portions 11 in the overlapping portion 12. The shaft portion of a bolt 17, etc., can be passed through the overlapping portion 12 through the through-holes 15. For convenience in this disclosure, one end of the through-hole 15 is referred to as the first end and the other end as the second end.

[0040] (Nut 16) The press-formed product 1 according to this embodiment further comprises a nut 16. The nut 16 is fixed to the first end of the through hole 15. The hole in the nut 16 is also aligned with the through hole 15. The shaft portion of the bolt 17 can be inserted through the through hole 15 into the nut 16.

[0041] The nut 16 may be disposed at either end of the through-hole 15, but is preferably disposed at the end on the vehicle interior side of an automobile. The means for fixing the nut 16 to the through-hole 15 is not particularly limited, and preferred examples include a projection weld, an arc weld, and a caulked portion. The caulked portion refers to a caulking means provided on a caulking nut. A caulking nut is also referred to as a press-fit nut. Examples of caulking nuts include rivet nuts and clinching nuts.

[0042] (Functions and Effects) In the press-formed product 1 according to the present embodiment, a plurality of press-formed portions 11 are joined by spot welded portions 13. In the spot welded portion 13, as schematically shown in Fig. 4, the minimum Vickers hardness is set to 80% or more of the Vickers hardness of the base material portion 14. This increases the joint strength of the spot welded portion 13.

[0043] As schematically shown in Fig. 5, a conventional spot welded portion 13 has a marked softened portion S in the vicinity of the nugget 131. The softened portion S serves as a fracture starting point of the spot welded portion 13 and reduces the joint strength of the spot welded portion 13. However, by removing the softened portion S and setting the minimum Vickers hardness to 80% or more of the Vickers hardness of the base material portion 14, the joint strength of the spot welded portion 13 can be increased. A method for removing the softened portion S will be described later.

[0044] Furthermore, in the press-formed product 1 according to the present embodiment, the overlapping portion 12 is provided with the through-hole 15 and the nut 16. That is, in addition to the spot welded portion 13, a bolt fastening portion can be additionally provided in the overlapping portion 12. When the overlapping portion 12 is fastened by a bolt and a nut, the rigidity of the press-formed product 1 is improved. Therefore, the press-formed product 1 according to the present embodiment can extremely easily reinforce the overlapping portion 12 using the bolt 17.

[0045] In addition, the press-formed product 1 according to the present embodiment has the overlapping portion 12 at an arbitrary position. The overlapping portion 12 has a large thickness and high rigidity. Therefore, by arranging the overlapping portion 12 at a location where rigidity is required, additional components for improving rigidity can be omitted.

[0046] Heretofore, the most basic aspect of the press-formed product 1 according to the present embodiment has been described. Hereinafter, more preferable aspects will be described.

[0047] (Overlap Margin) The width of the overlapping portion 12 is referred to as an overlap margin. The overlap margin is preferably, for example, not less than 30 mm and not more than 280 mm. This enables the rigidity of the press-formed product 1 to be further enhanced. The overlap margin may be, for example, not less than 40 mm, not less than 50 mm, or not less than 70 mm. Increasing the overlap margin further improves the rigidity of the press-formed product. On the other hand, the overlap margin may be, for example, not more than 250 mm, not more than 200 mm, or not more than 170 mm. Reducing the overlap margin reduces the weight of the press-formed product. In the B-pillar 22 illustrated in FIG. 14, the width of the overlapping portion 221 is not constant along the vehicle body traveling direction. In such a case, it is preferable that the maximum value of the overlap margin of the overlapping portion, that is, the maximum value of the width of the overlapping portion measured along the extending direction of the component, is within the numerical range exemplified above.

[0048] (Fastening of Another Component Using Bolt 17) Preferably, the press-formed product 1 includes a bolt 17 as illustrated in FIGS. 2, 6, and 7. A shaft portion of the bolt 17 is inserted into the through-hole 15 and a nut 16. Naturally, the shaft portion of the bolt 17 is screwed to the nut 16. More preferably, the press-formed product 1 includes an additional component separate from the press-formed portion 11 described above. The component is disposed at a second end of the through-hole 15. The second end of the through-hole 15 is the end of the through-hole 15 on the opposite side to a first end where the nut 16 is disposed. The component is clamped between the press-formed portion 11 disposed on the second end side of the through-hole 15 and a head portion of the bolt 17. The bolt 17 and the nut 16 fasten the component to the press-formed portion 11 in which the second end of the through-hole 15 is formed.

[0049] The type of the component is not particularly limited. A preferable example of the component is a door hinge 18. As illustrated in FIG. 6, fastening the door hinge 18 to the press-formed product 1 via the through-hole 15 makes it possible to secure the rigidity of the door attachment portion without an additional component.

[0050] On the other hand, the part fastened by the bolt 17 and nut 16 can also be a separate press-formed part. This separate press-formed part is referred to as an additional press-formed part 19. As illustrated in Figure 6, both the additional press-formed part 19 and the aforementioned door hinge 18 may be fastened by the bolt 17 and nut 16. The additional press-formed part 19 is preferably part of the outer side panel.

[0051] The two additional press-formed sections 19 may be fastened together by bolts 17 and nuts 16. An example of a location where such a configuration can be applied is the front floor module. As illustrated in Figure 7, a portion of the front side member 41 and the floor side member lower 42 of the front floor module can be overlapped and spot-welded. Through holes 15 and nuts 16 can be provided in the overlapping portion 12 of the front side member 41 and the floor side member lower 42. Then, the floor panel 43 and the floor side member upper 44 can be fastened to the overlapping portion 12 of the front side member 41 and the floor side member lower 42. In this case, the floor panel 43 and the floor side member upper 44 correspond to the additional press-formed sections 19 that are fastened to the press-formed product 1 by bolts 17 and nuts 16.

[0052] (Multiple lap joints) As described above, in the press-formed product 1 according to this embodiment, the overlapping portion 12 of the press-formed portion 11 forms a lap joint. The press-formed portion 11 may have multiple lap joints. That is, the press-formed product 1 may have multiple overlapping portions 12, and each of the multiple overlapping portions 12 may have a spot welded portion 13, a through hole 15, and a nut 16.

[0053] Figure 8 shows a preferred example of a press-formed product 1 having multiple overlapping joints. Figure 8 is a side view of the body frame 2 of an automobile. The column located in front of the front seats is called the A-pillar 21, and the column located between the front and rear seats is called the B-pillar 22. The part connecting the upper end of the A-pillar 21 and the upper end of the B-pillar 22 is called the roof rail 23. The part connecting the lower end of the A-pillar 21 and the lower end of the B-pillar 22 is called the side sill 24. The A-pillar 21, B-pillar 22, roof rail 23, and side sill 24 may constitute an integrated automobile part. This integrated part is called a door ring.

[0054] Each of the A-pillar 21 and B-pillar 22 illustrated in Figure 8 has two overlapping sections. In the upper overlapping section 211 of the A-pillar 21, the two press-formed sections 11 that make up the A-pillar 21 are overlapped. Note that the side panel of the outer body is omitted in the description of Figure 8. In the lower overlapping section 211 of the A-pillar 21, the two press-formed sections 11 that make up the A-pillar 21 are overlapped. Furthermore, the press-formed section 11 at the bottom of the A-pillar and the side sill are joined together by overlapping. In the upper overlapping section 221 of the B-pillar 22, the two press-formed sections 11 that make up the B-pillar 22 are overlapped. In the lower overlapping section 221 of the B-pillar 22, the two press-formed sections 11 that make up the B-pillar 22 are overlapped. Furthermore, the press-formed section 11 at the bottom of the B-pillar and the side sill are joined together by overlapping.

[0055] All overlapping sections are spot-welded and further provided with through-holes. The overlapping section illustrated in Figure 8 is located at the door mounting section of an automobile. Door hinges, door strikers, and door checkers can be fixed to the through-holes in the overlapping section. By using the overlapping section as the door mounting section, the rigidity of the door mounting section can be ensured without using additional parts such as reinforcements. Reducing the number of additional parts can reduce the manufacturing cost of the automobile. Although Figure 8 shows an example of A-pillars and B-pillars having two overlapping sections, the A-pillars and B-pillars may also have only one overlapping section 221 and one or more butt welds. Examples of butt welds include laser welds, plasma welds, and mash seam welds. Furthermore, in the A-pillar and / or B-pillar, if there is only one overlapping section 221 and the dimension of the overlapping section 221 along the vertical direction of the vehicle body is large, two or more hinges can be attached to the single overlapping section 221.

[0056] (Vickers hardness of the base material of the press-formed part to which the nut is welded) As described above, the means for fixing the nut 16 to the through hole 15 is not particularly limited, but preferred examples include projection welding and arc welding. In this case, the Vickers hardness of the base material 14 of the press-formed part 11 to which the nut 16 is fixed may be 455 HV or higher.

[0057] Here, the base material portion 14 refers to the area of ​​the weld metal of a welded portion such as a spot weld 13 that is separated by 4 mm or more. The Vickers hardness test method for the base material portion 14 is as described above.

[0058] In conventional press-formed products, the base material of the press-formed part to which the nut is welded is of low strength. For example, in the prior art, it is believed that welding a nut to a high-strength press-formed part with a strength of more than 1.5 GPa may cause cold cracking. However, in the press-formed product according to this embodiment, the joint strength of the press-formed product is increased by fastening a bolt to the nut. On the other hand, by increasing the hardness of the base material of the press-formed part to which the nut is welded, the strength of the press-formed product is further increased, and buckling and local deformation around the nut are suppressed. Therefore, it is preferable that the Vickers hardness of the base material 14 of the press-formed part 11 to which the nut 16 is fixed be 455 HV or higher. Note that the tensile strength of steel with a Vickers hardness of 455 HV is approximately 1.5 GPa. More preferably, the Vickers hardness of the base material 14 of the press-formed part 11 to which the nut 16 is fixed is 460 HV or higher, 470 HV or higher, or 490 HV or higher.

[0059] Next, an example of a method for manufacturing a press-formed product 1 according to another embodiment of the present disclosure will be described. The method for manufacturing a press-formed product 1 according to this embodiment is suitable for manufacturing the press-formed product 1 according to the present embodiment described above. Therefore, the preferred embodiment of the press-formed product 1 according to the present embodiment described above can be applied to the method for manufacturing a press-formed product 1.

[0060] However, the examples of manufacturing methods described below are not limited to the press-formed product 1 according to this embodiment. Any press-formed product 1 that satisfies the above-mentioned requirements is considered to be the press-formed product 1 according to this embodiment, regardless of its manufacturing method.

[0061] A first example of the manufacturing method for the press-formed product 1 according to this embodiment is as shown in the flowchart of Figure 9, and comprises: (S11) a step of partially overlapping a plurality of steel plates; (S12) a step of spot welding the overlapping portion 12 of the steel plates to obtain a tailored blank; (S13) a step of press-forming the tailored blank by heating it to 800°C or higher; (S14) a step of forming a through hole 15 in the overlapping portion 12 after press-forming; and (S15) a step of fixing a nut 16 to the first end of the through hole 15.

[0062] First, multiple steel plates are partially overlapped. This forms an overlapping section 12. Next, the overlapping section 12 is spot-welded. This creates a single press-formed blank made from multiple steel plates. A blank formed by joining multiple steel plates is called a tailored blank.

[0063] Next, the tailored blank is hot-pressed. Hot-pressing is also called hot stamping or hot pressing. In hot-pressing, the tailored blank is first heated. This softens the blank and reduces its resistance to deformation during press forming. Next, the heated tailored blank is press-formed using a die. This applies the desired shape to the tailored blank. Furthermore, the tailored blank is rapidly cooled. The die is cooled using a coolant circulating inside the die. After press forming, the tailored blank is kept in place in the die, allowing heat to transfer from the blank to the die. This hardens the steel that makes up the tailored blank.

[0064] The heating temperature of the tailored blank in hot press forming shall be above the temperature at which austenite transformation occurs in the steel sheet contained in the tailored blank. The temperature at which austenite transformation occurs is, for example, the Ac3 point. The heating temperature of the tailored blank can be, for example, 800°C or higher, 840°C or higher, 860°C or higher, or 880°C or higher. There is no upper limit to the heating temperature, but for example, to reduce heating costs or protect the plating layer, the heating temperature of the tailored blank may be set to 1000°C or lower, 980°C or lower, or 960°C or lower. Heating causes austenite transformation to occur in both the base material portion 14 and the spot weld portion 13 of the tailored blank. By rapidly cooling the tailored blank, which has been austenitized overall, both the base material portion 14 and the spot weld portion 13 are hardened. As a result, the formation of softened areas S in the spot weld portion is suppressed. As a result, the minimum Vickers hardness is 80% or more of the Vickers hardness of the hardest press-formed part 11, which is the base material 14.

[0065] Then, a through hole 15 is formed in the overlapping portion 12 of the press-formed product 1, which is formed by hot press forming. A preferred method for forming the through hole 15 is laser cutting. Furthermore, a nut 16 is fixed to the first end of the through hole 15. Preferred methods for fixing the nut 16 are projection welding, riveting, and arc welding. This yields the press-formed product 1 according to this embodiment.

[0066] A second example of the method for manufacturing the press-formed product 1 according to this embodiment is as shown in the flowchart of Figure 10, and comprises: (S21) a step of partially overlapping a plurality of steel plates; (S22) a step of spot welding the overlapping portion 12 of the steel plates to obtain a tailored blank; (S23) a step of forming through holes 15 in the overlapping portion 12 of the steel plates; (S24) a step of press-forming the tailored blank with the through holes 15 formed therein by heating it to 800°C or higher; and (S25) a step of fixing a nut 16 to the first end of the through hole 15.

[0067] The second example of the manufacturing method is similar to the first example. However, the second example differs from the first example in that the through-hole 15 is formed before hot press forming. In this case, it is desirable that the position of the through-hole 15 be determined considering the amount of strain during hot press forming. A through-hole 15 formed in a location with significantly large strain will be deformed by hot pressing. This may prevent the bolt 17 from being inserted into the through-hole 15. Therefore, it is preferable to provide the through-hole 15 in a location with small strain in the overlapping portion 12.

[0068] A third example of the method for manufacturing the press-formed product 1 according to this embodiment, as shown in the flowchart of Figure 11, comprises: (S31) a step of partially overlapping a plurality of steel plates; (S32) a step of spot welding the overlapping portion 12 of the steel plates to obtain a tailored blank; (S33) a step of press-forming the tailored blank by heating it to 800°C or higher; and (S34) a step of fixing a nut 16 to the first end of the through hole 15. The die for press forming has a perforating means. Furthermore, during press forming, the perforating means is used to form the through hole 15 in the overlapping portion 12.

[0069] A third example of the manufacturing method is similar to the first example. However, the third example differs from the first example in that the through-hole 15 is formed during hot press forming. In the third example, the mold for press forming has a perforating means. An example of a perforating means is a punch provided in the mold.

[0070] Figure 13 shows a preferred example of a die. The first die 31, located on the upper side of Figure 13, is provided with a punch 33 for perforation. The second die 32, located on the lower side of Figure 13, has a conduit 34 for passing the material cut off from the press forming section 11. The punch 33 and conduit 34 are located in the overlapping section 12 of the tailored blank. When the tailored blank is press-formed using the first die 31 and the second die 32, the punch 33 provided in the die forms a through hole 15 in the press forming section 11. The material cut off from the press forming section 11 falls through the conduit 34.

[0071] A fourth example of the method for manufacturing the press-formed product 1 according to this embodiment is as shown in the flowchart of Figure 12, and comprises: (S41) the step of forming through holes in each of a plurality of steel plates; (S42) the step of partially overlapping the plurality of steel plates with the through holes aligned; (S43) the step of spot welding the overlapping portion 12 of the steel plates to obtain a tailored blank; (S44) the step of press-forming the tailored blank by heating it to 800°C or higher; and (S45) the step of fixing a nut to the first end of the through hole 15 of the overlapping portion 12.

[0072] The fourth example of the manufacturing method is similar to the first example. However, the fourth example differs from the first example in that the through-holes 15 are formed before the spot welding of the overlapping sections. Naturally, the through-holes provided in each of the multiple steel plates need to be aligned in the final press-formed product. The position for forming the through-holes is determined while considering the overlapping positions of the multiple steel plates. This ensures that the through-holes are aligned when the multiple steel plates are overlapped.

[0073] While embodiments of the present disclosure have been described above, the present disclosure is not limited thereto and can be modified as appropriate without departing from the technical spirit. Further preferred examples of the press-formed article 1 and its manufacturing method according to the present embodiment are described below. Unless otherwise specified, the configurations exemplified below are applicable to both the press-formed article 1 and its manufacturing method.

[0074] (Applications of Press-Formed Product 1) The applications of press-formed product 1 are not limited. Examples of suitable applications include the A-pillar 21, B-pillar 22, and door ring described above. Furthermore, press-formed product 1 according to this embodiment can also be applied to parts such as the rear frame, front frame, tunnel reinforcement, and battery case.

[0075] (Steel Plate Composition) The steel plate used as the material for the press-formed product 1 is preferably a hot-stamping steel plate. The thickness of the hot-stamping steel plate is preferably, for example, 0.7 to 2.6 mm. The hot-stamping steel plate may have a surface treatment layer. Suitable examples of the surface treatment layer are aluminum-based plating and zinc-based plating. Aluminum-based plating is the most suitable. Naturally, the above-described steel plate composition can be applied to the press-formed part 11. The hardness of the base material of the hot-stamping steel plate after hot press forming is preferably 130 HV or more and 730 HV or less.

[0076] (Configuration of the overlapping section 12) The thickness of the overlapping section 12 is not particularly limited. The number of steel plates or press-formed sections 11 arranged in the overlapping section 12 is also not limited. The hardness and thickness of the steel plates in the overlapping section 12 may be the same or different. For example, in the A-pillar and B-pillar, the hardness of the steel plate on the upper side of the vehicle body may be increased, and the hardness of the steel plate on the lower side of the vehicle body may be decreased.

[0077] The diameter of the nugget 131 formed in the overlapping portion 12 is not limited, but is preferably, for example, 3√t to 8√t. The diameter of the nugget 131 is a value measured along the overlapping surface of the steel plate or press-formed portion 11 in a cross-section passing approximately through the center of the indentation 132. The symbol t is the thickness of the thinner of the two steel plates (or press-formed portion 11) arranged on the overlapping surface.

[0078] The spot welding conditions are not particularly limited. Various conditions can be adopted depending on the composition, strength, and thickness of the steel plates constituting the overlap 12. Preferably, spot welding conditions that can achieve the diameter of the nugget 131 described above are adopted.

[0079] The number of spot welds 13 arranged in one overlapping section 12 can be any value of one or more. Preferably, the number of spot welds 13 arranged in one overlapping section 12 is two or more, or four or more. The position of the spot welds 13 is not particularly limited, but it is preferable to provide them on both sides of the through hole 15, for example. In other words, it is preferable to provide the through hole 15 in the middle of two spot welds 13. This prevents misalignment of the through holes 15 of each of the multiple press-formed sections 11.

[0080] The shape of the overlapping portion 12 is not particularly limited. In the schematic diagram of Figure 1, the overlapping portion 12 has a strip-like shape that extends perpendicular to the extending direction of the press-formed product 1. The overlap of the overlapping portion 12 in Figure 1 is uniform along the width direction of the press-formed product 1. On the other hand, the overlapping portion may have a complex shape as shown in Figure 14.

[0081] Figure 14 is a perspective view of an example of a B-pillar 22. The B-pillar upper 223 and B-pillar lower 222 are overlapped at the overlapping section 221. At the overlapping section 221, the B-pillar upper 223 is positioned towards the viewer's perspective, and the B-pillar lower 222 is positioned towards the viewer's perspective. Spot welds 13 and through holes 15 are provided at the overlapping section. In the overlapping section 221 of Figure 14, the overlap is small on both sides of the B-pillar 22 and large in the center of the B-pillar 22. In the center of the B-pillar 22, the overlap of the overlapping section 221 is designed to be large in order to enhance safety against side collisions. On the other hand, on both sides of the B-pillar 22, the overlap of the overlapping section 221 is designed to be small in order to control the deformation mode during a side collision.

[0082] Furthermore, in the case of the B-pillar 22, it is desirable that the B-pillar upper 223 have a plate thickness of 1.0 mm or more and 2.6 mm or less, and the hardness of the base material be 410 HV or more and 730 HV or less. Also, the B-pillar lower 222 may have a plate thickness and hardness lower than the B-pillar upper 223, with a plate thickness of 0.9 mm or more and 2.4 mm or less, and the hardness of the base material be 130 HV or more and 550 HV or less.

[0083] The diameter of the through-hole 15 provided in the overlapping portion 12 is preferably, for example, 6.0 to 17.0 mm. The diameters of the through-holes 15 in each of the multiple steel plates (or press-formed portions 11) of the overlapping portion 12 may be the same or different. For example, when the through-holes 15 are formed by laser cutting, it may be difficult to form through-holes 15 of the same diameter in all the steel plates (or press-formed portions 11). Before hot press forming, a steel plate in which through-holes 15 have been formed by punching or laser cutting can be used to form through-holes 15 smaller than those previously formed in the other steel plate of the overlapping portion 12 during or after hot press forming.

[0084] The number of through holes 15 provided in one overlapping section 12 can be any value of one or more. When the through holes 15 are used for attaching a door, it is preferable that the number of through holes 15 provided in one overlapping section 12 be two or more.

[0085] The form of the nut 16 fixed to the through hole 15 is not particularly limited. It is preferable to use an inexpensive weld nut or a crimp nut. Examples of the type of nut 16 include square nuts, hexagonal nuts, flanged nuts, etc. Square nuts are preferred because they are inexpensive. Flanged nuts are also preferred because they make it easier to ensure bonding strength, torsional strength, and fatigue strength with the press-formed part 11.

[0086] The size of the nut 16 is, for example, M5 to M14. Preferably, the size of the nut 16 is M6 to M10. The larger the size of the nut 16, the greater the effect of suppressing the breakage of the overlapping portion 12. On the other hand, the smaller the size of the nut 16, the lighter the vehicle body can be. The number of nuts 16 is not particularly limited, but when the nuts 16 are used to attach the door, it is preferable that the number of nuts 16 provided in one overlapping portion 12 be two or more.

[0087] The welding conditions when projection welding the nut 16 to the press-formed part 11 are not particularly limited. For example, the pressing force can be 200 kgf to 800 kgf, the energizing time can be 0.08 seconds to 0.5 seconds, and the welding current value can be 6 kA to 16 kA. Projection welding is performed with two or more press-formed parts 11 overlapping.

[0088] The size of the bolts 17 placed in the overlapping section 12 shall be a value that matches the nut 16. That is, the size of the bolts 17 is, for example, M5 to M14. Preferably, the size of the bolts 17 is M6 to M10. The bolts 17 can be, for example, flanged bolts 17. The strength class of the bolts 17 is preferably, for example, 4.8, 8.8, 10.9, or 12.9. The strength class of the bolts 17 is standardized in JIS B 1051:2014. The number of bolts 17 is not particularly limited, but when the bolts 17 are used to attach the door, it is preferable that the number of bolts 17 provided in one overlapping section 12 be two or more.

[0089] 1 Press-formed product 11 Press-formed section 111 High-strength press-formed section 112 Low-strength press-formed section 12 Overlap section 13 Spot-welded section 131 Nugget 132 Indentation 133 Melting boundary 134 Joint surface 14 Base material section 15 Through hole 16 Nut 17 Bolt 18 Door hinge 19 Additional press-formed section 2 Automobile body frame 21 A-pillar 211 Overlap section of A-pillar 22 B-pillar 221 Overlap section of B-pillar 23 Roof rail 24 Side sill 31 First die 32 Second die 33 Punch 34 Conduit 41 Front side member 42 Lower floor side member 43 Floor panel 44 Upper floor side member L Hardness measurement line A Position 2.0 mm away from the melting boundary and outside the nugget B S Softened area: 0.2 mm away from the melting boundary, outside the nugget.

Claims

1. A press-formed product comprising: a plurality of press-formed parts made of steel, partially overlapping each other; and a spot welded part disposed in the overlapping portion of the plurality of press-formed parts and having a nugget for joining the plurality of press-formed parts, wherein the Vickers hardness of the base material of one or more of the press-formed parts is 410 HV or more; the minimum Vickers hardness determined by measuring the distribution of Vickers hardness in a region of 0.2 mm to 2.0 mm outside the nugget from the melting boundary along a hardness measurement line disposed in the hardest press-formed part and 0.2 mm away from the mating surface between the hardest press-formed part and the press-formed part in contact therewith, in the cross-section of the spot welded part, is 80% or more of the Vickers hardness of the base material of the hardest press-formed part; the overlapping portion has one or more through holes extending in the thickness direction of the overlapping portion; and the press-formed product further comprises a nut fixed to the first end of the through hole.

2. The press-formed product according to claim 1, characterized in that the maximum overlap amount of the overlapping portion is 30 mm or more and 280 mm or less.

3. The press-formed product according to claim 1, further comprising: a bolt inserted into the through hole and the nut; and a component disposed at the second end of the through hole, wherein the bolt and the nut fasten the component to the press-formed portion where the second end of the through hole is formed.

4. The press-formed article according to claim 3, characterized in that the component is a door hinge and / or an additional press-formed part.

5. The press-formed product according to claim 1, characterized in that it has a plurality of overlapping portions, each of which has the spot weld portion, the through hole, and the nut.

6. A press-formed article according to any one of claims 1 to 5, characterized in that it is an A-pillar, B-pillar, or door ring of an automobile.

7. The press-formed product according to claim 6, characterized in that the overlapping portion is arranged in the mounting portion of the automobile door.

8. The press-formed article according to any one of claims 1 to 5, characterized in that the means for fixing the nut to the through hole is a projection weld or an arc weld, and the hardness of the base material of the press-formed part to which the nut is fixed is 455 HV or more.

9. A method for manufacturing a press-formed product, comprising the steps of: partially overlapping a plurality of steel plates; spot welding the overlapping portion of the steel plates to obtain a tailored blank; heating the tailored blank to 800°C or higher and press-forming it; forming through holes in the overlapping portion after press-forming; and fixing nuts to the first ends of the through holes.

10. A method for manufacturing a press-formed product, comprising the steps of: partially overlapping a plurality of steel plates; spot welding the overlapping portion of the steel plates to obtain a tailored blank; forming through holes in the overlapping portion of the steel plates; heating the tailored blank having the through holes to 800°C or higher and press-forming it; and fixing a nut to the first end of the through hole.

11. A method for manufacturing a press-formed product, comprising the steps of: partially overlapping a plurality of steel plates; spot welding the overlapping portion of the steel plates to obtain a tailored blank; heating the tailored blank to 800°C or higher and press-forming it; and fixing a nut to the first end of a through hole, wherein the press-forming die has a perforating means, and the through hole is formed in the overlapping portion using the perforating means during press-forming.

12. A method for manufacturing a press-formed product, comprising the steps of: forming through holes in each of a plurality of steel plates; partially overlapping the plurality of steel plates with the through holes aligned; spot welding the overlapping portion of the steel plates to obtain a tailored blank; heating the tailored blank to 800°C or higher and press-forming it; and fixing a nut to the first end of the through hole in the overlapping portion.

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