Joining parts

Abstract

To provide a joined component in which hydrogen embrittlement cracking is suppressed. [Solution] A joined component according to one aspect of the present disclosure is a joined component comprising a first steel plate having a flat portion and a bent portion, and a second steel plate joined to the first steel plate at the flat portion by a spot weld, wherein the spot weld has a nugget and an indentation, and in a cross section passing through approximately the center of the indentation of the spot weld and perpendicular to the ridge line of the bent portion, an unpressed portion is formed near a first end portion, which is the end of the nugget on the opposite side to the bent portion, and a pressed portion is formed near a second end portion, which is the end of the nugget on the bent portion side, and a distance S2 between the first steel plate and the second steel plate at a position 1 mm away from the nugget starting from the second end portion is 0.25 mm or more in the cross section, and the diameter of the nugget is 3×(t 1 / 2 ) mm or more, where t indicates the thickness of the thinner of the first steel plate and the second steel plate.

Description

[Technical Field]

[0001] The present disclosure relates to joining components. [Background technology]

[0002] Hydrogen embrittlement of steel sheets is affected by three factors: the amount of hydrogen in the steel sheet, the hydrogen embrittlement susceptibility of the steel sheet, and the tensile stress applied to the steel sheet. When steel sheets are spot welded, hydrogen from rust-preventive oil and moisture present around the spot weld can penetrate the spot weld. In addition, the spot weld is quenched to form hard martensite. When high-strength steel sheets are spot welded, the hardness of this martensite increases, which tends to increase the susceptibility to hydrogen embrittlement. Furthermore, tensile stress is applied to the spot weld due to disturbances such as residual stress caused by the thermal history during spot welding and gaps between the spot-welded steel sheets.

[0003] Therefore, it can be said that spot welding of high-strength steel sheets is a joining process in which the above three factors are likely to overlap. In order to suppress hydrogen embrittlement cracking of steel sheets, it is necessary to implement measures to reduce any of the above three factors so that they do not overlap.

[0004] Conventionally, as described in Patent Document 1, a structure has been proposed for joined components having spot welds in which a filler is provided around the corona bond end of the spot weld to improve the filling rate and provide measures against hydrogen embrittlement cracking. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-095819 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present disclosure is to provide a joined component in which hydrogen embrittlement cracking is suppressed. [Means for solving the problem]

[0007] The gist of the present disclosure is as follows.

[0008] (1) A joined part according to one aspect of the present disclosure is a joined part including a first steel plate having a flat portion and a bent portion continuous with the flat portion, and a second steel plate having a flat portion overlapping the flat portion of the first steel plate and joined to the first steel plate at the flat portion of the first steel plate by a spot weld, wherein the spot weld has a nugget and an indentation, and in a cross section passing through an approximate center of the indentation of the spot weld and perpendicular to a ridge line of the bent portion, an unpressed portion is formed near a first end portion of the nugget that is an end portion opposite to the bent portion, and a press-welded portion is formed near a second end portion of the nugget that is an end portion of the nugget that is on the bent portion side, and a distance S2 between the first steel plate and the second steel plate at a position 1 mm away from the nugget starting from the second end portion is 0.25 mm or more in the cross section, and the diameter of the nugget is 3×(t 1 / 2 ) mm or more, where t indicates the thickness of the thinner of the first steel plate and the second steel plate. (2) Preferably, in the joined component described in (1) above, when the flat portion of the second steel plate is viewed from above, the flat portion of the second steel plate overlaps the bent portion of the first steel plate and extends beyond the bent portion of the first steel plate, and when a radius of curvature of the bent portion of the first steel plate measured in the cross section is R and a plate thickness of the first steel plate is t1, a distance D between an end of the bent portion of the first steel plate on the nugget side and the second end portion is 0.8−{(R+t1) 2 -(R+t1-0.08) 2} 1 / 2 ≦D≦1.2-{(R+t1) 2 -(R+t1-0.08) 2} 1 / 2 Meet the following. (3) Preferably, in the joining component described in (1) or (2) above, the first steel plate and the second steel plate each have an end face on the opposite side of the flat plate portion from the bent portion, and in the cross section, the distance E from the first end to the end face closer to the first end is 5 mm or more. (4) Preferably, in the joined component described in any one of (1) to (3) above, the average value of the indentation depth of the first steel plate and the indentation depth of the second steel plate in the cross section is 0.10 mm or more. (5) Preferably, in the joined component described in any one of (1) to (4) above, in the cross section, a distance S1 between the first steel plate and the second steel plate at a position 1 mm away from the nugget, starting from the first end of the nugget, is 0 mm or more and 0.25 mm or less. [Effects of the Invention]

[0009] According to the present disclosure, a joined component in which hydrogen embrittlement cracking is suppressed can be obtained. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view of a portion of a joining component according to an embodiment of the present disclosure. [Figure 2] 2 is an enlarged cross-sectional view showing a first end of a nugget formed in a spot weld of the joined components shown in FIG. 1. FIG. [Figure 3] 3 is an enlarged cross-sectional view showing a second end of a nugget formed in a spot weld of the joined components shown in FIG. 1. FIG. [Figure 4] FIG. 4 is a diagram illustrating the amount of sheet separation by changing the vertical and horizontal scales of the cross-sectional view of the joined part shown in FIG. [Figure 5] FIG. 5 is a partial cross-sectional view showing an example of a state in which stacked steel sheets are sandwiched between upper and lower electrodes, heated by electrical conduction, and spot-welded. [Figure 6] FIG. 6 is an explanatory diagram showing the state in which two steel sheets are spot-welded together by applying pressure to a position different from the electrodes using a mechanism other than the electrodes, as viewed from the surface of the steel sheets in the pressure direction of the electrodes. [Figure 7] FIG. 7 is a cross-sectional view showing an example of a joined component of a comparative example. [Figure 8] FIG. 8 is a cross-sectional view showing another example of a joining component of a comparative example. [Figure 9] FIG. 9 is an explanatory diagram showing a state in which two steel plates are spot-welded using electrodes in the example. [Figure 10] FIG. 10 is an explanatory diagram showing the criteria used to judge the press-contact portion in the examples. [Figure 11] FIG. 11 is an explanatory diagram showing the criteria used to judge the press-contact portion in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0011] Among welded components, the area around the nugget is particularly susceptible to hydrogen embrittlement cracking. The inventors attempted to form a spot weld that would suppress hydrogen embrittlement cracking in that area. As a result, they discovered that hydrogen embrittlement cracking can be suppressed when the periphery of the nugget is pressure welded.

[0012] The reason why hydrogen embrittlement cracking is suppressed is thought to be that when the steel plates are pressed together, the shape of the edge of the nugget is no longer notched, and stress concentration around the nugget is reduced.

[0013] However, in the past, it was not possible to freely control the pressure welding state of spot welds. Increasing the heat input during spot welding to increase the plastic deformation of the steel sheets makes it easier for the steel sheets to be pressure welded together. However, increasing the heat input also increases the gap between the overlapping steel sheets, which results in poor dimensional accuracy of the joined parts.

[0014] As a result of various studies, the inventors have found that by varying the spacing between steel sheets around the position where the nugget of the spot weld is formed, it is possible to selectively plastically deform the areas with larger spacing, making it easier to press-weld the steel sheets. In other words, by making it easier to press-weld specific areas, it is possible to make it easier to press-weld areas that are more likely to be subjected to stress, and it has been found that hydrogen embrittlement cracking can be suppressed.

[0015] A joined component according to one aspect of the present disclosure, obtained based on the above findings, comprises a first steel plate having a flat portion and a bent portion continuous with the flat portion, and a second steel plate joined to the first steel plate at the flat portion by a spot weld, the spot weld having a nugget and an indentation, and in a cross section passing through approximately the center of the indentation of the spot weld and perpendicular to the ridge line of the bent portion, an unpressed portion is formed near a first end portion which is the end of the nugget on the opposite side to the bent portion, and a press-welded portion is formed near a second end portion which is the end of the nugget on the bent portion side, and in the cross section, a distance S1 between the first steel plate and the second steel plate at a position 1 mm away from the nugget starting from the second end portion is 0.25 mm or more, and in the cross section, a diameter of the nugget is 3×(t 1 / 2 ) mm or more. Hereinafter, a preferred embodiment of the joining component according to the present embodiment will be described with reference to the drawings.

[0016] In the drawings used in the following description of the embodiments, characteristic portions may be enlarged or otherwise emphasized for the sake of clarity. In addition, common components in the description of the embodiments are given the same reference numerals, and descriptions of components with the same reference numerals may be simplified or omitted.

[0017] Fig. 1 is a diagram showing a cross section of a joined component according to an embodiment of the present disclosure. As will be described later, in this embodiment, the term "cross section" refers to a cross section passing through approximately the center of the indentation of a spot welded portion 4 and perpendicular to the ridge line of a bent portion. As shown in the cross section of Fig. 1, a joined component A of this embodiment is a joined component in which a first steel plate 1 and a second steel plate 2 are joined by spot welding. It is possible that the ridgeline of the bent portion 1B is not a straight line, but in this case, it is possible to draw a perpendicular line and therefore define the cross section described above. Furthermore, since the ridgeline of the bent portion is approximately parallel to the surface of the first steel plate, the cross section perpendicular to the ridgeline of the bent portion will necessarily be perpendicular to the surface of the first steel plate.

[0018] (First steel plate 1 and second steel plate 2) As shown in the cross section of Figure 1, the first steel plate 1 has a flat portion 1A and a bent portion 1B continuous with the flat portion. The bent portion 1B is, for example, the portion between the vertical wall portion and the flange portion of a hat-shaped member. The flat portion 1A is, for example, the flange portion of a hat-shaped member.

[0019] The second steel plate 2 has at least a flat plate portion 2A that is joined to the flat plate portion 1A of the first steel plate 1. The flat plate portion 2A of the second steel plate 2 is overlapped on the flat plate portion 1A of the first steel plate 1 and joined by spot welds. However, unlike the first steel plate 1, the second steel plate 2 does not have to have a bent portion. In the joined component exemplified in FIG. 1 , the second steel plate 2 consists of a flat plate portion 2A. On the other hand, the second steel plate 2 may have a flat plate portion and a bent portion, similar to the first steel plate 1.

[0020] In joined components, the first steel plate and the second steel plate are distinguished as follows. For example, in a joined component that includes a steel plate with a bent portion and a steel plate without a bent portion, such as a hat-shaped component, the steel plate with a flat portion and a bent portion is considered to be the first steel plate, and the steel plate without a bent portion is considered to be the second steel plate. Also, in a joined component that includes two or more steel plates with bent portions, the steel plate with the bent portion closest to the nugget is considered to be the first steel plate, and the other steel plates are considered to be the second steel plates. When a joined component includes multiple steel plates with the same distance between the bent portion and the nugget, any steel plate can be considered to be the first steel plate.

[0021] The flat plate portion 1A of the first steel plate 1 and the flat plate portion 2A of the second steel plate 2 are overlapped with each other. The flat plate portions 1A and 2A are joined by spot welds 4.

[0022] (Spot weld 4) The spot welded portion 4 has a nugget 3 and an indentation. The nugget 3 is a molten and solidified portion that occurs in the spot welded portion 4 during spot welding. The nugget 3 is formed on the mating surface that is the boundary between the flat plate portion 1A and the flat plate portion 2A. The nugget 3 is depicted as an oval in the cross section of Figure 1, but it may have other shapes. The indentation is a depression on the surface of the base material caused by the electrode tip as a result of spot welding. An indentation 1C is formed on the first steel plate 1, and an indentation 2C is formed on the second steel plate 2.

[0023] In Figure 1, the flat portion 2A of the second steel plate 2 is placed on the bottom, and the flat portion 1A of the first steel plate 1 is placed on top, with the flat portion 1A and flat portion 2A oriented horizontally. In addition, in the first steel plate 1 in Figure 1, the bent portion 1B is placed so as to bend upward on the left side of the flat portion 1A. In other words, the bent portion 1B is inclined so that the gap between it and the second steel plate 2 gradually widens as it goes to the left side in Figure 1.

[0024] The radius of curvature R of the bent portion 1B can be selected from approximately 3 to 12 mm, preferably approximately 5 to 10 mm. The radius of curvature R is the radius of a circle measured at the cross section of the bent portion, passing through the first end position H1 of the bent portion, the second end position H2 of the bent portion, and the intermediate position H3 between H1 and H2. Here, H1, H2, and H3 are all located on the outer surface of the first steel plate, i.e., the surface opposite the second steel plate. The first end position H1 of the bent portion 1B refers to the boundary between the surface of the bent portion 1B and the surface of the flat portion 1A of the first steel plate 1. The second end position H2 of the bent portion 1B refers to the boundary between the surface of the bent portion 1B and the surface of another flat portion formed continuously with the bent portion 1B. The intermediate position H3 between H1 and H2 refers to the midpoint of H1 and H2 along the surface of the bent portion 1B. As described above, the cross section refers to a cross section that passes through approximately the center of the indentation of the spot weld and is perpendicular to the ridge line of the bent portion.

[0025] The other flat plate portion provided contiguous to the second end position H2 of the bent portion is, for example, a vertical wall portion of a hat-shaped member. In the embodiment shown in Fig. 1, the portion of the first steel plate 1 above the bent portion 1B constitutes, as an example, the vertical wall portion of the joining part A, but the portion of the first steel plate 1 above the bent portion 1B is not shown. Note that the bent portion 1B is not limited to a circular cross section with a fixed radius of curvature, and various shapes can be used.

[0026] The nugget 3 is a molten solidified portion formed on the mating surface of the flat plate portion 1A and the flat plate portion 2A. The nugget is formed to a size such that it extends from the mating surface to a fraction of the thickness of the flat plate portion 1A on the flat plate portion 1A side, and also extends from the mating surface to a fraction of the thickness of the flat plate portion 2A on the flat plate portion 2A side. In FIG. 1, the first steel plate 1 and the second steel plate 2 are joined via a spot weld 4 formed by the nugget 3 and its surrounding area.

[0027] A flange end 1D, which is the end on the leading edge side of the flat plate portion 1A of the first steel plate 1, may be aligned with an end (flange end 2D) of the flat plate portion 2A of the second steel plate 2. For example, in the joined component of FIG. 1 , an end face 1E of the flange end 1D, which is the end on the leading edge side of the flat plate portion 1A, and an end face 2E of the end (flange end 2D) of the flat plate portion 2A are aligned so as to be flush with each other. An example of a cross section in which the end face 1E of the first steel plate 1 and the end face 2E of the second steel plate 2 are aligned flush with each other is a cross section obtained by cutting the first steel plate 1 and the second steel plate 2 along a plane perpendicular to the surfaces of the flat plate portions 1A and 2A and perpendicular to the bending ridgeline.

[0028] In this example, the end surface 1E of the first steel plate 1 is aligned flush with the end surface 2E of the second steel plate 2, but the position of the flange end 1D of the first steel plate 1 does not have to be aligned with the end (flange end 2D) of the second steel plate 2. That is, the right end of the second steel plate 2 shown in FIG. 1 may extend further to the right in FIG. 1. The left end of the second steel plate 2 shown in FIG. 1 may also extend leftward from the position shown in FIG. 1. The upper side of the bent portion 1B of the first steel plate may also be extended further to form another shape. FIG. 1 can be regarded as a structural example showing a cross section of a portion of a structural member, such as that of an automobile.

[0029] The first steel sheet 1 and the second steel sheet 2 are joined by spot welding using a welding electrode (not shown) that is pressed with a predetermined pressure so as to sandwich them. For this reason, in the second steel sheet 2 shown in Fig. 1, an indentation 2C caused by pressing the electrode tip, which is the tip of the welding electrode, is formed on the outer surface of the second steel sheet 2, below the nugget 3. Similarly, in the first steel sheet 1, an indentation 1C caused by pressing the welding electrode is formed on the outer surface of the first steel sheet 1, above the nugget 3.

[0030] As an example of a welding electrode, a welding electrode having a flat tip and a tapered shape that gradually reduces in diameter toward the tip can be used, so that after welding, indentations 1C and 2C of the shapes shown in Figure 1 are formed.

[0031] In this specification, the depth of indentation 1C formed in first steel plate 1 can be referred to as the indentation depth of first steel plate 1. Similarly, the depth of indentation 2C formed in second steel plate 2 can be referred to as the indentation depth of second steel plate 2. When first steel plate 1 and second steel plate 2 are steel plates with the same composition and the same strength and hardness, the indentation depths of each will be approximately the same. Furthermore, in this specification, when the indentation depths of first steel plate 1 and second steel plate 2 can be referred to without any particular distinction, they will be simply referred to as the indentation depth of the steel plate.

[0032] (Pressure-welded and non-pressure-welded parts) The joined part A has an unpressurized portion 6 of the first steel plate 1 and the second steel plate 2 near the end of the nugget 3 on the side opposite to the bent portion 1B shown in the cross section of Figure 1. Furthermore, the joined part A has a pressurized portion 7 of the first steel plate 1 and the second steel plate 2 near the end of the nugget 3 on the bent portion side.

[0033] In this disclosure, the end of the nugget opposite the bent portion is referred to as the "first end 3a." The end of the nugget on the bent portion side is referred to as the "second end 3b." "Via the end" refers to the range up to 0.3 mm away from the end of the nugget. The pressure-welded portion 7 refers to the portion where the interface between the first steel plate 1 and the second steel plate 2 is pressed together while remaining solid during spot welding without melting. The non-pressure-welded portion 6 refers to the portion where the interfaces of the first steel plate 1 and the second steel plate 2 face each other with a small gap between them, or where the interfaces of the first steel plate 1 and the second steel plate 2 are in contact but not pressed together.

[0034] The presence or absence of an unpressurized portion near the first end 3a can be determined by the following procedure. First, the spot welded portion 4 is cut along a plane that passes through the approximate center of the indentation of the spot welded portion 4 and is perpendicular to the ridgeline of the bent portion. If the shape of the indentation is not circular, the center of gravity of the indentation is considered to be the center of the indentation. The cross section does not need to be formed so that it passes exactly through the center of the indentation. For example, the distance between the center of the indentation and the cross section may be approximately 10% of the diameter of the indentation or less. Next, the cross section is mirror-polished. The mating surfaces of the first steel sheet 1 and the second steel sheet 2 are then observed using an SEM. The observation magnification is 500x to 5000x. During this process, inclusions 41, voids, and peelings 40 present on the mating surfaces can be identified, as shown in the cross-sectional schematic diagrams of FIGS. 10 and 11. Furthermore, the total lengths of the inclusions, voids, and peelings are measured within a range from the first end 3a of the nugget to a position 0.3 mm away. These lengths are measured along the mating surfaces. If the total length of inclusions, voids, and peelings present on the mating surfaces of the steel sheets in the range from the first end of the nugget to a position 0.3 mm away, measured along the mating surfaces of the steel sheets, divided by 0.3 mm, is more than 30%, it is considered that an unpressurized joint has formed near the first end.

[0035] Whether or not a pressure-welded portion is formed near the second end 3b is determined by the following procedure. The cross section is observed using an SEM according to the procedure described above. Then, the total length of inclusions, voids, and peeling is measured within a range from the second end of the nugget to a position 0.3 mm away. If the total length of inclusions, voids, and peeling present on the mating surfaces of the steel sheets within a range from the second end of the nugget to a position 0.3 mm away, measured along the mating surfaces of the steel sheets, is divided by 0.3 mm and is 30% or less, it is considered that a pressure-welded portion is formed near the second end.

[0036] In addition, JIS Z 3001-6:2013 defines the technical term "corona bond" as "a solid-phase welded ring-shaped portion that occurs around the nugget in lap resistance welding." The pressure-welded portion of the joined parts according to this embodiment is similar to a corona bond, but its shape differs from that of a corona bond. Not only a pressure-welded portion but also an unpressurized portion is formed around the nugget. Therefore, the shape of the pressure-welded portion is not ring-shaped like the corona bond of conventional spot welds.

[0037] Fig. 2 is a partial cross-sectional schematic view showing an enlarged cross section of the first end 3a of the nugget 3 and its vicinity in the spot welded portion 4 of the joined parts A shown in Fig. 1. Fig. 3 is a partial cross-sectional schematic view showing an enlarged cross section of the second end 3b of the nugget 3 and its vicinity in the spot welded portion 4 of the joined parts A shown in Fig. 1.

[0038] 2, when the cross section of the first end 3a of the nugget 3 and its surrounding area is viewed in an enlarged manner, there is a small gap G1 between the first steel plate 1 and the second steel plate 2. Furthermore, an unpressurized portion 6 is formed between the first steel plate 1 and the second steel plate 2. It is acceptable for the unpressurized portion 6 to be formed over a longer range than the range of 0.3 mm.

[0039] 3, when the cross section of the second end 3b of the nugget 3 and its surrounding area is viewed enlarged, there is no gap between the first steel plate 1 and the second steel plate 2. The mating surfaces of the first steel plate 1 and the second steel plate 2 are solid-state welded to form a pressure-welded portion 7. It is acceptable for the pressure-welded portion 7 to be formed over a range longer than 0.3 mm.

[0040] In the cross sections shown in Figures 1 to 4, during spot welding, welding electrodes are used to apply pressure to the first steel plate 1 and the second steel plate 2 from above and below while heating them with electricity, resulting in the boundary between the first steel plate 1 and the second steel plate 2 melting and solidifying to form a nugget 3.

[0041] In the region surrounding the second end 3b of the nugget 3 shown in the cross section in Figure 3, the first steel sheet 1 and the second steel sheet 2 are pressurized from above and below by the welding electrodes while softening and remaining in a solid, unmelted state. As a result, portions of the first steel sheet 1 and the second steel sheet 2 are plastically deformed in a direction perpendicular to the direction of pressure, forming bulges 1a and 2a. In the cross section shown in Figure 3, the pressure-welded portion 7 is depicted as a straight line extending from the second end 3b of the nugget 3 to the center of the tips of the bulges 1a and 2a. A gap G2 is formed to the right of the bulges 1a and 2a in Figure 3. A sheet separation amount S2, described below, is defined to the left of this gap G2.

[0042] (Sheet separation amount S2 on the second end side) 1, the distance S2 between the first steel plate and the second steel plate at a position 1 mm away from the nugget, starting from the second end, is 0.25 mm or more. In the present disclosure, the distance S2 between the first steel plate and the second steel plate at a position 1 mm away from the nugget, starting from the second end, is referred to as the sheet separation amount S2.

[0043] The sheet separation amount S2 cannot be adequately shown in the cross section of Fig. 1, so the following description will also refer to Fig. 4, which shows an example of an equivalent cross section with slightly altered vertical and horizontal scales. Note that Fig. 4 omits the depiction of indentations related to the indentation depth, which will be described later.

[0044] The sheet separation amount S2 at a position 1 mm away from the second end 3b is 0.25 mm or more. Preferably, the sheet separation amount S2 is 0.30 mm or more, 0.32 mm or more, or 0.35 mm or more. There is no particular upper limit to the sheet separation amount S2, but for example, the sheet separation amount S2 can be 0.55 mm or less. Preferably, the sheet separation amount S2 is 0.52 mm or less, 0.50 mm or less, or 0.45 mm or less.

[0045] The larger the sheet separation amount S2 on the side of the second end 3b, the more likely it is that a pressure-welded portion will be formed during spot welding. On the other hand, if the sheet separation amount S2 is too large, the adverse effect on the shape accuracy of the joined parts A will be greater. If the sheet separation amount S2 is too large, expulsion will also be more likely to occur during spot welding. If the sheet separation amount S2 is too small, it will be difficult to form a pressure-welded portion, and stress concentration will increase. Note that it is preferable that the sheet separation amount S1 on the side of the first end 3a of the nugget 3 is small, as will be described later.

[0046] (nugget diameter) In the joined part A, the diameter d of the nugget 3 (the distance between the first end 3a and the second end 3b in the cross section of FIGS. 1 and 4: nugget diameter) is 3×(t 1 / 2 ) or more. In this relational expression, t indicates the thinner of the first steel plate and the second steel plate. If the diameter d of the nugget 3 is too large, there is a risk of expulsion during spot welding. On the other hand, if the diameter d of the nugget 3 is too small, it will have a negative impact on the formation of the pressure weld. The first end 3a and second end 3b of the nugget are the outer edges of the nugget, i.e., the intersections between the fusion interface and the mating surfaces of the steel plates.

[0047] (Radius of curvature of bent part) For example, as shown in Fig. 1, when the flat portion 2A of the second steel plate 2 is viewed from above, it is preferable that the flat portion 2A of the second steel plate 2 overlaps with the bent portion 1B of the first steel plate 1 and extends beyond the bent portion 1B of the first steel plate 1. In other words, when the flat portion 2A is viewed from above, it is preferable that the entire bent portion 1B of the first steel plate overlaps with the flat portion 2A of the second steel plate 2. This further stabilizes the welding at the second end 3b of the nugget and its surrounding area.

[0048] Furthermore, when the flat portion 2A of the second steel plate 2 extends beyond the rear end of the bent portion 1B of the first steel plate 1, in the joined part A, when the radius of curvature of the bent portion measured in the cross section is R and the plate thickness of the first steel plate is t1, the distance D between the nugget side end of the bent portion and the second end 3b is 0.8-{(R+t1)2 -(R+t1-0.08) 2} 1 / 2 ≦D≦1.2-{(R+t1) 2 -(R+t1-0.08) 2} 1 / 2 It is preferable that the following condition is satisfied. The definition of the radius of curvature R of the bent portion is as described above. The nugget side end of the bent portion is the first end position H1 of the bent portion 1B, which was explained when defining the radius of curvature R. The thickness t1 of the first steel plate indicates the thickness of the flat portion of the first steel plate 1. The distance D is the distance between H1 and 3b measured along the mating surface.

[0049] Assuming that the radius of curvature R is in the range of 3 to 12 mm, expulsion is less likely to occur by setting the aforementioned distance D to a predetermined value or greater. Setting the aforementioned distance D to a predetermined value or less prevents the sheet separation amount S2 on the second end 3b side from becoming too large, ensures the effect of providing sheet separation between the first steel sheet 1 and the second steel sheet 2, and promotes the creation of the pressure-welded portion 7 that should be created on the bent portion 1B side.

[0050] 1 to 4, it is preferable to assume that the radius of curvature R of the bent portion 1B is in the range of 3 to 10 mm, and to set the distance D between the first end position H1 of the bent portion 1B and the second end 3b of the nugget 3 within the appropriate range described above. This prevents expulsion, promotes pressure welding, and more reliably forms the pressure-welded portion 7 on the second end 3b side of the nugget 3.

[0051] (Distance E between the nugget and the flange end face) The first steel plate 1 may have an end face 1E on its flat plate portion. The second steel plate 2 may also have an end face 2E on its flat plate portion. The end faces 1E and 2E of the first steel plate 1 are provided on the opposite side of the nugget 3 from the bent portion 1B. Preferably, in the cross section, the distance E from the first end 3a of the nugget to the end face closer to the first end 3a is 5 mm or more.

[0052] 1 to 4, for example, the end face 1E of the flange end 1D of the first steel plate 1 and the end face 2E of the right end (flange end 2D) of the flat plate portion 2A of the second steel plate 2 are aligned so as to be flush with each other. However, the end face 2E of the right end (flange end 2D) of the flat plate portion 2A of the second steel plate 2 may be located closer to the nugget than the end face 1E of the flange end 1D of the first steel plate 1. In this case, the distance from the first end 3a of the nugget 3 closer to the flange end 1D to the end face 2E of the right end (flange end 2D) of the flat plate portion 2A is defined as E.

[0053] If the distance E is set to a predetermined value or more, expulsion is less likely to occur during spot welding. Furthermore, if the distance E is set to a predetermined value or more, deformation due to the influence of heat during spot welding is suppressed, preventing the joined part A from deviating from the desired dimensions and improving shape accuracy. Since the flange end side is not pressure-welded, separation is minimized and shape accuracy can be improved.

[0054] (Indentation depth) In the joined part A, the average value of the indentation depth of the first steel plate 1 and the indentation depth of the second steel plate 2 is preferably 0.10 mm or more. The indentation depth may be 0.11 mm or more, 0.12 mm or more, or 0.15 mm or more.

[0055] The average depth of the indentation on the steel plate is related to the amount of sheet separation S2 on the second end 3b side. If the average depth of the indentation is set to a predetermined value or less, in other words, if the pressing pressure of the welding electrode is reduced, the impact on the shape of the joined part A can be reduced. On the other hand, if the average depth of the indentation is set to a predetermined value or more, the pressing pressure of the welding electrode increases, promoting pressure welding. It is better to reduce the amount of sheet separation on the first end side of the nugget. By arranging an unpressed portion on the first end side of the nugget, separation can be minimized and the shape precision of the joined part A can be improved.

[0056] On the other hand, the average indentation depth may be set to a value of 0.10 mm or less. By reducing the average indentation depth, the strength of the spot weld can be improved. For example, the average indentation depth can be reduced by changing the tip diameter of the welding electrode, while adjusting the current flow time to maintain a high temperature near the nugget edge, thereby forming a pressure weld.

[0057] When spot welding is performed, a raised portion c may occur around the periphery of the indentation 1C and the indentation 2C. When a raised portion c occurs, the average height from the top of the raised portion c to the bottom of the recess is determined as the indentation depth. The indentation depth is measured based on the surface of the steel sheet on the first end side of the nugget. In Figure 1, the area indicated by the symbol I is the indentation depth of the second steel sheet 2.

[0058] (Method of applying pressure) In normal spot welding, overlapping steel sheets are pressed with a pair of welding electrodes while current is applied. In this case, the pressure position and the current application position are the same. However, when manufacturing a joined part according to this embodiment, in addition to the pressure position applied by the electrodes, pressure can also be applied to a position different from the electrodes.

[0059] 5 and 6 show an example of the positional relationship between the pressure application position and the welding electrodes for applying current. In spot welding, a first steel sheet 1 and a second steel sheet 2 are overlapped, and welding electrodes 20, 21 are positioned so as to sandwich the first steel sheet 1 and the second steel sheet 2. Then, a pressure mechanism separate from the electrodes applies pressure to a position closer to the flange end than the electrodes. Then, electricity is applied to heat the electrodes 20, 21, and the first steel sheet 1 and the second steel sheet 2 are spot welded together.

[0060] When the nugget diameter is large, there is a risk of excessive sheet separation at the flange end. However, by applying pressure to the base steel sheet using a pressure mechanism other than an electrode, a spot weld that can solve this problem can be obtained even with a large nugget diameter. The pressure mechanism may be provided on the welding gun. Alternatively, the first steel sheet and the second steel sheet may be clamped and pressure applied using a separate member such as a clamp.

[0061] An example of a suitable pressure position when pressing the base steel sheet using a pressure mechanism other than the welding electrode is described below. First, as shown in FIG. 6, a first imaginary line is assumed to be perpendicular to the ridge line of the bent portion 1B and to pass through the welding point. The welding point is the position where the center of the tip of the welding electrode contacts the base steel sheet. A second imaginary line is also assumed to connect the pressure position and the welding point. It is preferable that the angle θ formed by the first imaginary line and the second imaginary line is in the range of 0° to 60°. Note that, as shown in FIG. 6, when the angle θ formed by the first imaginary line and the second imaginary line is 0°, this means that the pressure position is on the first imaginary line and is closer to the flange end than the welding point. It is also preferable that the distance between the welding point and the pressure position is approximately 4 to 15 mm.

[0062] The distance from the welding point to the pressure position is more preferably 4 to 10 mm. Also, the angle θ is more preferably within the range of 0° or more and 45° or less. The pressure can be set to 0.1 kN or more. The upper limit of the pressure is set to about 1 kN. If a pressure exceeding the upper limit is applied, current may be applied to a pressure portion that is not the target position for welding, which may result in unstable welding.

[0063] In the example shown in Figure 5, the tips of the electrodes 20, 21 have a shape that gradually reduces in diameter, and have flat ends 20a, 21a at their tips. Because the diameter of the tips of the electrodes 20, 21 is gradually reduced and they have flat ends 20a, 21a at their tips, the above-mentioned indentations 1C and 2C are formed on the first steel sheet 1 and the second steel sheet 2 during welding. The presence of these indentations 1C and 2C determines the above-mentioned indentation depth.

[0064] (Jointed parts with three or more steel plates) In the examples of Figures 1 to 4, a joined component in which a first steel plate 1 and a second steel plate 2 are joined by spot welding is described, but the present disclosure may also be applied to a joined component in which three or more steel plates are stacked and spot welded together.

[0065] When the joining components include three or more steel plates, the number of mating surfaces of the steel plates is two or more. In this case, it is preferable that the above-mentioned requirements be met at least on one or more of the same mating surfaces. For example, when the joining components include two overlapping thick steel plates and a thin steel plate overlapping the thick steel plates, it is preferable that the pressure-welded portion and the non-pressure-welded portion are formed as described above on the mating surfaces of the two thick steel plates, and that the nugget diameter is within the above-mentioned range. In this case, one of the two thick steel plates is considered to be the first steel plate, and the other is considered to be the second steel plate. In addition, in this case, it is preferable that the lower limit of the indentation amount is 0.1 × (n-1) mm, where n is the number of overlapping steel plates.

[0066] For example, when calculating the upper and lower limits of the distance D between the nugget-side end of the bent portion and the second end, of the two steel plates that make up any mating surface, the steel plate having the bent portion can be regarded as the first steel plate, and its thickness can be regarded as t1. Even when multiple steel plates are stacked and spot welded, the present application can be applied as long as at least one steel plate has a bent portion, a flat end, and a flange end, and the flat portion of the steel plate including the flange end is stacked on another steel plate.

[0067] (Sheet separation amount S1 on the first end 3a side) The distance between the first steel plate and the second steel plate measured on the cross section at a position 1 mm away from the nugget starting from the first end 3a, i.e., the sheet separation amount S1, is preferably in the range of 0 mm or more and 0.25 mm or less.

[0068] (Action and effect) The joined component A illustrated in FIGS. 1 to 4 is formed by spot welding a first steel plate 1 and a second steel plate 2 together, and has an unpressurized portion 6 near the end of the nugget 3 opposite the bent portion, i.e., the first end 3a, and a press-welded portion 7 near the end of the nugget 3 on the bent portion side, i.e., the second end 3b. In the joined component A formed by spot welding the first steel plate 1 and the second steel plate 2 together, stress is likely to act on the bent portion 1B side of the first steel plate 1. As stress is likely to act on the region close to the bent portion 1B of the first steel plate 1, the press-welded portion 7 is provided on the second end 3b side of the nugget 3, which is close to that region. Because the press-welded portion 7 is located in a region susceptible to stress, it is possible to provide a joined component A with low susceptibility to hydrogen embrittlement, even if a large amount of stress acts on the first steel plate 1 on the bent portion 1B side.

[0069] Sheet separation S2 occurs between the bent portion 1B of the first steel plate 1 and the second steel plate 2, but by providing a pressure-welded portion 7 and an unpressurized portion 6, excessive deformation can be suppressed, and a joined part A with improved shape accuracy can be provided.

[0070] Furthermore, by adopting an appropriate nugget diameter d, it is possible to prevent the occurrence of expulsion and provide a joined component A having a desirable spot weld.

[0071] For the joined parts A shown in Figures 1 to 4, Figure 7 shows a cross section of a weld when the diameter of the nugget 300 is significantly small, and Figure 8 shows a cross section of a weld when the diameter of the nugget 300 is significantly large. When the diameter of the nugget 300 is significantly small as shown in Figure 7, there is a tendency for non-pressure-welded portions 1000 and 1100 to occur at both ends of the nugget 300. When the diameter of the nugget 300 is significantly large as shown in Figure 8, expulsion is likely to occur in the gap between the first steel plate 100 and the second steel plate 200 located outside the bent portion 100B during spot welding. Explosion is also likely to occur on the outside of the flange end 200D. [Example]

[0072] Two cold-rolled steel sheets (unplated, 1.6 mm thick) with a tensile strength of 1500 MPa were stacked together and spot-welded to produce joined parts. The first steel sheet was used as the upper sheet, and several sheets were prepared for the spot-welding test by bending them 90° to form bent sections with curvature radii R (R: 5 mm, 7 mm, 10 mm) as shown in Table 1 below. Several flat steel sheets with the same tensile strength and composition as the first steel sheet were prepared as the second steel sheet and used for the spot-welding test.

[0073] As shown in Figure 9, a spot welding test was carried out by overlapping a first steel plate 31 after 90° bending with a flat second steel plate 32, and applying pressure from above and below with electrodes 33, 34 while passing an electric current. Spot welding was carried out with the end of the second steel plate 32 overlapping the flat portion 31a on one end of the first steel plate 31 that had been bent 90°.

[0074] The electrodes used in the welding tests had the diameter and tip diameter shown in Table 1. Samples Nos. 7 and 8 shown in Tables 1 and 2 were manufactured by applying pressure with a clamp in addition to pressure with an electrode. Specifically, samples Nos. 7 and 8 were manufactured by spot welding while applying pressure with a clamp at a position 9 mm away from the welding point. The pressure applied by the clamp was set to 0.3 kN. The pressure position was set to a position where the θ was 0°.

[0075] When spot welding is performed while applying pressure to the steel sheet with the electrode, a protrusion c may occur on the surface of the steel sheet around the edge of the recess, as shown in Figure 1. When a protrusion c is formed, the impression depth was measured as the average depth (height) from the apex of the protrusion c to the bottom of the recess.

[0076] Joined parts were produced under the conditions of each test example shown in numbers 1 to 12 in Table 1. The size of each part of each of the resulting joined parts was measured, and the pressure-welded state of each joined part was evaluated.

[0077] The measurement items were as follows: Nugget diameter d (mm) on a cross section that intersects the ridge of the bent part at 90°, passes through the center of the indentation, and is perpendicular to the steel plate surface (see the "d" column in Table 1). Nugget diameter d (t) 1 / 2 (See the "√t" column in Table 1) Distance D (mm) between the nugget end of the bend and the second end of the nugget Distance E (mm) from the first edge of the nugget to the edge of the flange (see column "E" in Table 1) The sheet separation distance measured from the second edge of the nugget at a position 1 mm away from the nugget (see the "R side" column in Table 2). The sheet separation distance measured from the first edge of the nugget to a position 1 mm away from the nugget (see the "Flange End Side" column in Table 2). Indentation depth (mm)

[0078] The evaluation items were whether the two steel plates were pressed together or not in the range of 0.3 mm from the nugget edge closest to the bend in the cross section mentioned above (see the "R side" column in Table 2), and whether the two steel plates were pressed together or not in the range of 0.3 mm from the nugget edge closest to the flange edge toward the flange edge (see the "Flange side" column in Table 2).

[0079] The nugget diameter d was measured in accordance with JIS Z 3139:2009, Section 6.1, Macroscopic Test. First, the spot weld was cut along a plane passing through the approximate center of the indentation in the spot weld and perpendicular to the ridge of the bent portion. If the shape of the indentation was not circular, the center of gravity of the indentation was considered to be the center of the indentation. Next, a picric acid corrosion test was performed on the cross section. The nugget diameter d was then determined by observing the cross section after the corrosion test.

[0080] The distance D between the nugget-side end of the bent portion and the second end of the nugget was measured on the cross section used to measure the nugget diameter d. The nugget-side end of the bent portion was identified based on the shape of the outer surface of the first steel plate. The second end of the nugget was identified by observing the nugget revealed by a picric acid corrosion test. The first and second ends of the nugget are the outer edges of the nugget, i.e., the intersections between the fusion interface and the mating surfaces of the steel plates.

[0081] The distance E from the first end of the nugget to the end of the flange was also measured at the cross section used to measure the nugget diameter d. The first end of the nugget was identified by observing the nugget revealed by the picric acid corrosion test.

[0082] The amount of sheet separation at a position 1 mm away from the nugget starting from the second end of the nugget, and the amount of sheet separation at a position 1 mm away from the nugget starting from the first end of the nugget were also measured on the cross section used to measure the nugget diameter d. The mating surfaces of the steel sheets were observed under a microscope to identify the shape of the sheet separation and measure its amount.

[0083] The indentation depth was also measured on the cross section used to measure the nugget diameter d. The indentation measurement method complies with 6.1.4 of JIS Z 3139:2009.

[0084] The state of pressure welding near the nugget was also measured on the cross section used to measure the nugget diameter d. The cross section was mirror-polished and subjected to the SEM observation described above to identify whether the area near the edge of the nugget was a pressure-welded area or an unpressure-welded area.

[0085] In addition, when the indentation formed by pressing the welding electrode reaches the end position of the flat portion of the bent portion, the end position of the bent portion was calculated by extrapolating the numerical value. When the distance D between the end position of the flat portion of the bent portion and the end of the nugget is a negative value, it means that the end of the nugget is closer to the bent portion than the end position. Because the steel plate undergoes plastic deformation during spot welding, a negative distance D does not necessarily mean that expulsion has occurred.

[0086] The measurement results of the above-mentioned measurement items and the above-mentioned evaluation results are shown in Tables 1 and 2 below.

[0087] [Table 1]

[0088] [Table 2]

[0089] From the results shown in Table 1, it is considered preferable that the sheet separation amount between the steel sheets at a position 1 mm away from the second end of the nugget in the direction closer to the bent portion is 0.25 mm or more. Also, in the column in Table 1 where the nugget diameter is expressed as √t, for example, 2.80 means that the nugget diameter is 2.80√t. From the results shown in Table 1, it is considered that the nugget diameter is 3×(t 1 / 2 ) or more is considered preferable.

[0090] A sheet separation of more than 0.25 mm reduces stress concentration because the shape of the pressure-welded portion becomes obtuse. However, if the sheet separation is too large, it adversely affects the shape of the weld.

[0091] If the nugget diameter d is too small, the joining strength will be insufficient, and sufficient plastic deformation will not occur around the nugget, which may prevent the bent portion from being pressure welded.

[0092] If the nugget diameter d is too large, the sheet separation at the flange end side will be excessive, but by applying pressure with something other than the electrode, it is possible to obtain a spot weld that can solve this problem even with a large nugget diameter.

[0093] From the results shown in Table 1, it can be seen that the distance D between the end position of the bent part on the flat part side and the second end of the nugget is 0.8-{(R+t1) 2 -(R+t1-0.08) 2} 1 / 2 ≦D≦1.2-{(R+t1) 2 -(R+t1-0.08) 2} 1 / 2 It is considered preferable that the distance D is in the range of

[0045] . Samples No. 10 and No. 11 are samples in which the distance D is outside the above range.

[0094] From the results shown in Table 1, it is considered preferable that the distance E from the first end of the nugget to the end face of the flange end be 5 mm or more. In sample No. 12, the distance E was less than 5 mm, so a pressure-welded portion was also formed on the flange end side.

[0095] From the results shown in Table 1, it is considered preferable that the indentation depth of the steel sheet is 0.10 mm or more. Samples 4 and 9 had indentation depths of less than 0.10 mm, resulting in unwelded areas on both sides of the nugget. However, by changing the shape of the electrode tip, it is possible to form a welded area while keeping the indentation depth less than 0.10 mm.

[0096] From the results shown in Table 1, it is considered preferable that the sheet separation amount between the steel sheets at a position 1 mm away from the first end of the nugget in the direction closer to the flange end be 0 mm or more and 0.25 mm or less. In the case of sample No. 12, the sheet separation amount was large, exceeding 0.25 mm, and therefore a pressure-welded portion was also formed on the flange end side.

[0097] Comparative Example No. 6 is an example in which expulsion occurred because the target nugget diameter was large and the welding current was large. Due to the expulsion, a pressure weld was not formed in Comparative Example No. 6.

[0098] Examples numbered 7 and 8 in Tables 1 and 2 are samples that were spot-welded while applying pressure to the flange end with a clamp. By spot-welding while applying pressure by a means other than electrode pressure, it was possible to stably produce nuggets even with large diameters. [Explanation of symbols]

[0099] A Joining parts 1. First Steel Plate 1A flat plate part 1B Bending section 1C impression 1D flange end 1E End face 2. Second Steel Plate 2A flat plate part 2C Indentation 2D flange end 2E End face 3. Nuggets 3a First end 3b Second end 4 Spot welds 6 Unwelded area 7 Pressure welding part 100 First Steel Plate 100A flat plate part 100B bending part 100C impression 100D flange end 100E end face 1000 Unwelded part 1100 Unwelded part 200 Second Steel Plate 200A flat plate part 200B bending section 200C impression 200D flange end 200E end face 300 nuggets D: Distance between the end of the bend and the other end of the nugget d nugget diameter E: Distance from the first edge of the nugget to the edge of the first steel plate H1 First end position of bend H2 Second end position of bend H3: Midway between H1 and H2 I. Indentation depth of second steel plate S1: Distance between the first steel sheet and the second steel sheet at a position 1 mm away from the nugget starting from the first end (sheet separation amount on the first end side) S2: Distance between the first steel sheet and the second steel sheet at a position 1 mm away from the nugget starting from the second end (sheet separation amount on the second end side)

Claims

1. A first steel plate having a flat portion and a bent portion continuous with the flat portion; and a second steel plate having a flat portion overlapping the flat portion of the first steel plate and joined to the first steel plate at the flat portion of the first steel plate by spot welds; A joining component comprising: the spot weld has a nugget and an indentation; In a cross section passing substantially through the center of the indentation of the spot welded portion and perpendicular to the ridge line of the bent portion, an unpressurized portion is formed in the vicinity of a first end portion, which is an end portion of the nugget opposite to the bent portion, and a press-welded portion is formed in the vicinity of a second end portion, which is an end portion of the nugget on the bent portion side, In the cross section, a distance S2 between the first steel plate and the second steel plate at a position 1 mm away from the nugget starting from the second end is 0.25 mm or more, In the cross section, the diameter of the nugget is 3 × (t １／２ ) mm or more A joining component characterized by: Here, the t indicates the thickness of the thinner of the first steel plate and the second steel plate.

2. When the flat portion of the second steel plate is viewed in plan, the flat portion of the second steel plate overlaps the bent portion of the first steel plate and extends beyond the bent portion of the first steel plate, The radius of curvature of the bent portion of the first steel plate measured at the cross section is R, and the plate thickness of the first steel plate is t １ In this case, the distance D between the nugget side end of the bent portion of the first steel plate and the second end is 0.8-{(R+t １ ) ２ −(R+t １ -0.08) ２ } １／２ ≦D≦1.2−{(R+t １ ) ２ −(R+t １ -0.08) ２ } １／２ fulfill The joining component according to claim 1 .

3. The first steel plate and the second steel plate each have an end surface on an opposite side of the flat plate portion from the bent portion, In the cross section, a distance E from the first end to the end face closer to the first end is 5 mm or more. The joining component according to claim 1 or 2.

4. In the cross section, the average value of the indentation depth of the first steel plate and the indentation depth of the second steel plate is 0.10 mm or more. The joining component according to claim 1 or 2.

5. In the cross section, a distance S1 between the first steel plate and the second steel plate at a position 1 mm away from the nugget, starting from the first end of the nugget, is 0 mm or more and 0.25 mm or less. The joining component according to claim 1 or 2.

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