Spot weld joint

Abstract

To provide a spot weld joint which has a small nugget diameter but is unlikely to cause hydrogen embrittlement cracking.SOLUTION: A spot weld joint (1) has a nugget (3) on a superimposed surface of two or more steel plates (21, 22). A nugget diameter DN of the nugget (3) is 4.0√t or less relative to plate thickness t of the steel plate, and size of a seat separation h at a position away for 0.5DN from ends (3E) of the nugget 3 in a superimposed surface direction DL of the steel plate is 0.60 mm or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a spot welded joint. [Background technology]

[0002] In recent years, efforts have been made to increase the strength of steel sheets used in various fields such as automobiles, home appliances, and building materials. For example, in the automobile field, the use of thin, high-strength steel sheets has been increasing in order to reduce the weight of vehicle bodies and parts and improve fuel efficiency.

[0003] On the other hand, automobile body assembly and component installation are mainly performed by spot welding from the viewpoints of cost, manufacturing efficiency, etc. However, when spot welding is performed using the above-mentioned high-strength steel sheets, hydrogen embrittlement cracking is likely to occur in the welded portion of the resulting welded joint. Hydrogen embrittlement cracking is a phenomenon in which a steel member subjected to high stress under usage conditions suddenly fractures due to hydrogen that penetrates into the steel from the surrounding environment. This phenomenon is also called delayed fracture due to the fracture mode. It is generally known that hydrogen embrittlement cracking of steel sheets is more likely to occur as the strength of the steel sheet increases. This is thought to be because the higher the strength of the steel sheet, the greater the residual stress in the steel sheet after part formation.

[0004] In particular, in spot welding, gaps are formed between steel sheets at the welding position due to the forming accuracy of the parts, and welding while compressing the gap with a pair of electrodes can generate residual stress in the peel direction in the welded joint after welding. Therefore, spot welding using such high-strength steel sheets makes the welded joint more susceptible to hydrogen embrittlement cracking.

[0005] To combat this type of hydrogen embrittlement cracking, various countermeasures have been proposed to improve the hydrogen embrittlement resistance around the weld.

[0006] For example, Patent Document 1 discloses a spot welding method for spot welding high-strength steel plates, each having a tensile strength of 1200 MPa or more, overlapping each other and having a gap formed between the high-strength steel plates at the welded portions, the spot welding method comprising: a pressurizing step of sandwiching the high-strength steel plates between a pair of spot electrodes at the welded portions while the high-strength steel plates are in a state of electrical conduction with each other, and pressing the high-strength steel plates together with a pressing force of 3000 N or less using the pair of spot electrodes; and a welding step of applying an electric current intermittently to the high-strength steel plates in the pressed state using the pair of spot electrodes multiple times with an intervening current rest period in between, thereby softening the welded portions by heat generated by the current passing through the welded portions and plastically deforming the softened welded portions by the pressing force, thereby bringing the welded portions of the high-strength steel plates into contact with each other and welding the contacting welded portions together.

[0007] According to the spot welding method disclosed in Patent Document 1, in the welding process, the welded portions of high-strength steel plates are softened by heat generated by passing current through them, and the softened welded portions are plastically deformed by pressure. This makes it less likely that tensile stress will remain around the weld where the welded portions of high-strength steel plates are welded together, thereby preventing delayed fracture around the weld. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-134665 Summary of the Invention [Problem to be solved by the invention]

[0009] Here, FIG. 2 shows the results of the inventors' measurements of the two spot welds S of a test piece having the shape shown in FIG. P Evaluation point position P EThe results show the results of an investigation into the occurrence of hydrogen embrittlement cracking when spot welding was performed with different nugget diameters. The test specimen shown in Figure 3 was spot-welded at two points to a plate assembly with a 1.5 mm gap to facilitate the generation of residual stress. In Figure 2, (a) is a cross-sectional photograph of a spot-welded joint welded to a smaller nugget diameter, showing the interfacial fracture caused by hydrogen embrittlement cracking. Meanwhile, (b) is a cross-sectional photograph of a spot-welded joint welded to a larger nugget diameter than the welded joint in (a), showing the appearance of a crack penetrating into the nugget due to hydrogen embrittlement cracking. (c) is an enlarged photograph of the area where a crack has penetrated into the nugget in the cross-sectional photograph shown in (b).

[0010] The inventors investigated the occurrence of hydrogen embrittlement cracking when the nugget diameter was changed using test specimens with the shape shown in Fig. 3, and found that the smaller the nugget, the more likely it is that hydrogen embrittlement cracking will occur, as shown in (a) to (c) of Fig. 2. Conventional countermeasures against hydrogen embrittlement cracking do not address the hydrogen embrittlement cracking that occurs when the nugget diameter is reduced, so there is a need for new countermeasures against hydrogen embrittlement cracking that can also suppress hydrogen embrittlement cracking when the nugget diameter is reduced.

[0011] The present invention has been made in view of the above circumstances, and has an object to provide a spot welded joint in which hydrogen embrittlement cracking is unlikely to occur even when the nugget diameter is small. [Means for solving the problem]

[0012] The present invention includes the following aspects.

[0013] (Aspect 1) A spot welded joint having a nugget on the overlapping surfaces of two or more steel plates, The nugget diameter D of the above nugget N is 4.0√t or less with respect to the thickness t of the steel plate, 0.5D from the edge of the nugget toward the overlapping surface of the steel plate NA spot welded joint characterized in that the sheet separation at the separated position is 0.60 mm or more.

[0014] (Aspect 2) The spot-welded joint according to the above-mentioned aspect 1, wherein the depth of the indentation formed on the surface of the steel plate is 15 to 50% of the plate thickness t of the steel plate.

[0015] (Aspect 3) The spot welded joint according to the first or second aspect, wherein the opening angle of the joining end portion of the nugget located in the direction of the overlapping surfaces is 75° or more.

[0016] (Aspect 4) The spot-welded joint according to any one of the above-mentioned embodiments 1 to 3, wherein at least one steel plate of the two or more steel plates has a Vickers hardness of 350 HV or more. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a spot welded joint that is less susceptible to hydrogen embrittlement cracking even when the nugget diameter is small. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a cross-sectional schematic view of a spot-welded joint 1 according to one embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional photograph of a spot-welded joint, showing the results of an investigation conducted by the inventors into the occurrence of hydrogen embrittlement cracking when the nugget diameter was changed using a test piece having the shape shown in FIG. 3. [Figure 3] FIG. 3 is a perspective view that schematically shows a test piece used when investigating whether or not hydrogen embrittlement cracking occurs. [Figure 4] FIG. 4 is a cross-sectional view that schematically shows an electrode used when examining the welding conditions that result in the spot-welded joint of the present invention. [Figure 5]FIG. 5 shows cross-sectional photographs of various spot-welded joints welded using the electrode shown in FIG. 4 after immersion in hydrochloric acid. [Figure 6] Figure 6 shows cross-sectional photographs to explain the causes of residual stress at the joint edge of a spot-welded joint and the stress reduction effect of specific welding conditions using a cone-flat (CF) electrode. [Figure 7] FIG. 7 is a cross-sectional photograph for explaining the depth of indentation in a spot-welded joint. [Figure 8] FIG. 8 is a cross-sectional photograph for explaining the opening angle of the joint end portion in the spot-welded joint. [Figure 9] FIG. 9 is a cross-sectional photograph of each spot-welded joint of the invention example and the comparative example produced in the working examples after immersion in hydrochloric acid. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, preferred embodiments of the spot welded joint of the present invention will be described in detail with reference to the drawings.

[0020] In order to achieve the above object, the present inventors have conducted extensive research focusing on the shape of the joint end after spot welding. Specifically, the present inventors first used two types of electrodes with the shapes shown in Fig. 4 to weld two spot welds S at two points on a test piece with the shape shown in Fig. 3. P Evaluation point position P E The specimens were spot-welded under specified welding conditions (particularly, conditions that resulted in a specific plunge depth) to produce various spot-welded joints. In Figure 4, (a) shows a dome radius (DR) electrode with a tip diameter of 6 mm, a base diameter of 16 mm, and an R40, while (b) shows a cone flat (CF) electrode with a tip diameter of 3 mm, a base diameter of 16 mm, and a tip angle of 120°.

[0021] Next, to allow hydrogen to penetrate the spot-welded joints, the spot-welded joints were immersed in hydrochloric acid with a pH of 3 at room temperature for 100 hours. After immersion in hydrochloric acid, the welded joints were cut and their longitudinal cross sections were observed to confirm the shape of the joint edge and the presence or absence of hydrogen embrittlement cracking. A cross-sectional photograph of the welded joint after immersion in hydrochloric acid is shown in Figure 5. In Fig. 5, (a-1) is a welded joint welded using the DR-type electrode shown in Fig. 4(a), which is a welded joint with the largest nugget diameter and interfacial fracture. Meanwhile, (a-2) is a welded joint welded using the DR-type electrode, which is a welded joint with the smallest nugget diameter and no delamination of the pressure-welded portion. Furthermore, (b-1) is a welded joint welded using the CF-type electrode shown in Fig. 4(b), which is a welded joint with the smallest nugget diameter and no delamination of the pressure-welded portion, but no cracks have progressed to the nugget. Furthermore, (b-2) is a welded joint welded using the CF-type electrode, which is a welded joint with the smallest nugget diameter and no delamination of the pressure-welded portion.

[0022] As shown in Figure 5, cross-sectional observation of welded joints after immersion in hydrochloric acid revealed that hydrogen embrittlement cracking occurred during the process of separation of the pressure welded joint (see Figure 5(b-1)) and the process of the crack propagating to the nugget (see Figure 5(a-1)). Regarding the initiation of the initial crack, a comparison of the nugget diameter at the narrowest point of the joint edge (see Figure 5(a-2) and (b-2)) revealed that no cracks occurred in welded joints welded with CF-type electrodes, even in welded joints with small nugget diameters (i.e., more susceptible to hydrogen embrittlement cracking). Furthermore, regarding crack propagation, compared with the welded joint (a-1) welded with a DR-type electrode, which fractured at the interface, in the welded joint (b-1) welded with a CF-type electrode, which had a small nugget diameter (i.e., more susceptible to hydrogen embrittlement cracking), crack propagation stopped within the pressure welded joint and did not propagate to the nugget. In this way, the inventors discovered that by welding under specified welding conditions (particularly conditions resulting in a specific plunge depth) using a CF-type electrode, it is possible to suppress hydrogen embrittlement cracking of welded joints even when the nugget diameter is small.

[0023] The present invention was completed based on these findings, and includes aspects of each embodiment described below.

[0024] <Spot welded joints> Fig. 1 is a cross-sectional schematic diagram of a spot-welded joint 1 according to one embodiment of the present invention. The spot-welded joint 1 according to this embodiment shown in Fig. 1 has a nugget 3 at the overlapping surface between a steel sheet 21 serving as an upper plate and a steel sheet 22 serving as a lower plate. Furthermore, the nugget diameter D N However, the nugget diameter is 4.0√t or less where t is the plate thickness of the steel plate 21 and the steel plate 22.

[0025] 1, the spot welded joint 1 of this embodiment is formed by welding the steel plates 21 and 22 in a direction D from the end 3E of the nugget 3 to the overlapping surface direction D of the steel plates 21 and 22. L 0.5D N It has a unique configuration in which the size of the sheet separation h at the separated position is 0.60 mm or more.

[0026] Here, in this specification, the thickness direction D of the spot welded joint 1 T is a direction perpendicular to the horizontal plane when the spot welded joint 1 is placed on a horizontal plane so that the surface of the steel plate (steel plate 22 in FIG. 1) that constitutes the spot welded joint 1 faces up and down, and is the direction perpendicular to the horizontal plane when the spot welded joint 1 is placed on a horizontal plane so that the surface of the steel plate (steel plate 22 in FIG. 1) that constitutes the spot welded joint 1 faces up and down. b The direction parallel to the thickness direction in the film. In addition, the overlapping direction D of the steel plate at the spot welded joint 1 L is the in-plane direction of the plane P0 corresponding to the overlapping surface (interface) of the steel sheets before welding, passes through the nugget center and is in the thickness direction D T In the cross section including T Here, the sheet separation h is the direction perpendicular to the overlapping surface direction D from the edge of the nugget, as will be described later. L 0.5D NThe sheet separation h is determined by measuring the size of the sheet separation h at a position away from the nugget. In this case, if the size of the sheet separation h varies depending on the cross section, the cross section where the size of the sheet separation h is the largest is adopted. Then, the size of the sheet separation h measured from that cross section is defined as "the distance from the edge of the nugget to the overlapping surface direction D of the steel sheets" in the present invention. L 0.5D N The size of the sheet separation h at the distance.

[0027] The spot welded joint 1 of this embodiment shown in FIG. 1 is formed so as to have the above-described unique configuration, that is, in the overlapping surface direction D of the steel plates 21 and 22 from the end 3E of the nugget 3. L 0.5D N By welding so that the sheet separation h at the distance is 0.60 mm or more, the nugget diameter D N Even with a small nugget diameter where t is 4.0√t or less, hydrogen embrittlement cracking is unlikely to occur.

[0028] The mechanism by which such an effect is obtained is not clear, and it is not intended to be bound by any particular theory. However, by welding so as to form the above-mentioned specific configuration, the joint end P be The main reason is thought to be the reduction in residual stress.

[0029] Here, Fig. 6 is a cross-sectional photograph to explain the cause of residual stress generation at the joint end of a spot-welded joint and the effect of reducing residual stress under specific welding conditions using a cone-flat (CF) electrode. In Fig. 6, (a) shows the evaluation welding point P of the test piece in Fig. 3. E (b) is a cross-sectional photograph of a welded joint in which spot welding was performed under normal welding conditions using a DR-type electrode. E In contrast, this is a cross-sectional photograph of a welded joint spot-welded under welding conditions with a specific plunge depth using a CF-type electrode.

[0030] The evaluation point position P of the test piece in Figure 3 E In contrast, when welding under normal conditions using DR electrodes, the 1.5 mm gap is crushed by the pressure of the pair of electrodes during welding, and as a result, residual stress in the peeling direction (i.e., peeling stress) is generated at the joint edge due to the reaction force that causes the crushed gap to return to its original shape after welding, as shown in Figure 6(a).It is thought that this peeling stress makes the joint edge more susceptible to hydrogen embrittlement cracking.

[0031] On the other hand, the evaluation point position P of the test piece in Fig. 3 E On the other hand, when welding is performed using a CF-type electrode under welding conditions that result in a specific plunge depth so that the sheet separation h is 0.60 mm or more, as shown in Figure 6(b), the greater the plunge depth by the electrode, the greater the plastic flow toward the joint edge, which reduces the amount of bending of the steel sheets after welding and increases the sheet separation, thereby reducing the effective amount of crushing of the sheet gap and reducing the residual stress in the peel direction (peel stress).And, due to this effect of reducing the residual stress at the joint edge, the spot welded joint 1 of this embodiment has a nugget diameter D N It is believed that hydrogen embrittlement cracking is less likely to occur even in small nugget diameters where the t is 4.0√t or less.

[0032] The configuration of the spot welded joint 1 of this embodiment will be described in further detail below.

[0033] [Nugget diameter D N is 4.0√t or less with respect to the thickness t of the steel plate] As described above, the spot welded joint 1 of this embodiment has a nugget diameter D NThe steel plates 21 and 22 have a small nugget diameter of 4.0√t or less, where t is the sheet thickness (mm) of the steel plates 21 and 22. A spot-welded joint with a nugget diameter of 4.0√t or less is particularly susceptible to residual stress at the joint edge, i.e., is particularly susceptible to hydrogen embrittlement cracking. However, the spot-welded joint 1 of this embodiment is welded so that the sheet separation (h) is 0.60 mm or more, as described above, and therefore is less susceptible to hydrogen embrittlement cracking even with such a small nugget diameter.

[0034] In the spot welded joint 1 of this embodiment, the nugget diameter D N may be 3.9√t or less, 3.8√t or less, 3.6√t or less, 3.4√t or less, or 3.2√t or less. N The lower limit of is not particularly limited, but may be, for example, 0.5√t or more or 1.0√t or more in terms of bonding strength and the like.

[0035] The nugget diameter is measured from a cross-sectional photograph of the spot-welded joint in accordance with "6.1.2 Measurement method for nugget diameter, nugget width and penetration" of JIS Z 3139:2009.

[0036] Furthermore, the plate thickness t of the steel plate means the plate thickness t of the steel plate with the highest strength among the two or more steel plates that constitute the spot-welded joint. In the spot-welded joint 1 shown in Fig. 1 described above, the steel plate 21 and the steel plate 22 have the same plate thickness t.

[0037] The thickness t of the steel plate is measured with a micrometer from a relatively smooth position on the part (formed body) to which the weld joint is applied. Note that the measurement does not include areas where the plate thickness has locally decreased due to thinning during forming, etc.

[0038] [Sheet separation size: 0.60mm or more] As described above, the spot welded joint 1 of this embodiment is formed by welding the steel plates 21 and 22 from the end 3E of the nugget 3 in the overlapping surface direction D L 0.5D NThe size of the sheet separation h at the distance is 0.60 mm or more.

[0039] Here, as shown in FIG. 1, the size of the sheet separation h is determined by the direction D from the end 3E of the nugget 3 to the overlapping surface of the steel sheets 21 and 22. L 0.5D N Thickness direction D of spot welded joint 1 at a distance T The sheet separation h is the distance (mm) between the steel sheets. The size of the sheet separation h is the distance between the steel sheets that passes through the nugget center of the spot welded joint and is measured in the thickness direction D. T As mentioned above, the size of the sheet separation h is measured from a photograph of a cross section including the nugget center and the thickness direction D. T If the sheet separation h varies depending on the cross section including the cross section, the sheet separation h measured from the cross section where the size of the sheet separation h is the largest shall be used.

[0040] The size of the sheet separation h may be 0.65 mm or more, 0.70 mm or more, 0.75 mm or more, or 0.80 mm or more. There is no particular upper limit to the size of the sheet separation h, and it can be any size depending on the size of the part to which the weld joint is applied, etc. The size of the sheet separation h may be, for example, 3.00 mm or less or 2.00 mm or less.

[0041] In this embodiment, the means for welding to achieve a sheet separation h of 0.60 mm or more is not particularly limited, but can be achieved by appropriately selecting welding conditions such as the type of electrode (electrode tip shape), electrode pressure, current, current application time, pre-current, etc. These welding conditions will be specifically described in the method for manufacturing a spot-welded joint described below.

[0042] [Indentation depth: 15 to 50% of the steel plate thickness t] In the spot welded joint 1 of this embodiment shown in FIG. 1, the indentation P formed on the surface of the steel plate 21 or the steel plate 22 el Depth d elHowever, it is preferable that the depth is 15 to 50% of the thickness t of the steel plate.

[0043] Indentation P el is a depression formed on the surface of the steel sheet by pressing the pair of electrodes, and is also called an indentation. el Depth d el is measured in accordance with "6.1.4 Measurement method of indentation" of JIS Z 3139:2009. Here, Figure 7 is a cross-sectional photograph to explain the indentation depth in a spot welded joint. As shown in Figure 7, the indentation P el Depth d el To determine the indentation diameter (mm), first determine the indentation diameter D from a cross-sectional photograph of the weld joint. el is calculated, and the distance between the center of the weld and the point separated by the electrode indentation diameter (i.e., 2D e1 Draw a line connecting the two points (indentation P) and measure the indentation perpendicular to this line. el ) and calculate the maximum depth of the indentation P el Depth d el (mm). In the welded joint shown in Figure 7, the indentation P el Depth d el is 0.49 mm.

[0044] Indentation P el When welding is performed under welding conditions that result in a specific push-in amount so that the depth of the gap is 15 to 50% of the thickness t of the steel plate, the specific push-in amount can more reliably increase the plastic flow toward the joint edge, thereby more reliably reducing the effective crushing amount of the above-mentioned gap, and as a result, more reliably reducing the residual stress in the peeling direction (peeling stress).

[0045] In addition, indentation P el Depth d el is more preferably 18% to 45% of the thickness t of the steel plate, and further preferably 20% to 40%.

[0046] Indentation P el Depth d el The means for controlling the welding temperature to within the above range is not particularly limited, but can be achieved by appropriately selecting welding conditions such as the type of electrode (electrode tip shape), electrode pressure, current, current application time, and pre-current application.

[0047] [Opening angle of joint end: 75° or more] In the spot welded joint 1 of this embodiment shown in FIG. 1, the overlapping surface direction D of the end 3E of the nugget 3 L The joint end P located at be It is preferable that the opening angle is 75° or more.

[0048] Here, Fig. 8 is a cross-sectional photograph for explaining the open angle of the joint ends in a spot-welded joint. In Fig. 8, (a) is an example of a welded joint where the open angle of the joint ends is an acute angle (57° in the cross-sectional photograph of Fig. 8), and (b) is an example of a welded joint where the open angle of the joint ends is an obtuse angle (200° in the cross-sectional photograph of Fig. 8). The open angle of the joint ends can be determined from a cross-sectional photograph of a welded joint such as that shown in Fig. 8.

[0049] To determine the opening angle of the weld edge, first, a 0.5D angle is measured from the edge of the nugget toward the overlapping surface of the steel sheets. N The size of the sheet separation h at a distance is measured. Next, as shown in FIG. 8, the midpoint of the sheet separation h is set as the center, and the thickness direction D T Similarly, draw a horizontal line passing through both ends of the line segment whose length is 0.5h. Similarly, draw a horizontal line passing through the midpoint of the sheet separation h in the thickness direction D T Draw horizontal lines passing through both ends of the line segment with a length of 0.25h. Furthermore, find four intersections between these four horizontal lines and the outline of the joint end. Then, draw a line passing through the two upper intersections of these four intersections and a line passing through the two lower intersections, and measure the angle between these two lines that is located relatively outward from the joint end, and use this as the opening angle (°) of the joint end.

[0050] Joint end P be When welding is performed under welding conditions that result in a specific push-in amount so that the opening angle is 75° or more, the specific push-in amount can more reliably increase the plastic flow toward the joint end, thereby more reliably reducing the effective crushing amount of the above-mentioned plate gap, and as a result, more reliably reducing the residual stress in the peeling direction (peel stress).

[0051] In addition, the joint end P be The opening angle may be 80° or more, 100° or more, 120° or more, 150° or more, or 180° or more.

[0052] Joint end P be The means for controlling the opening angle within the above range is not particularly limited, but can be achieved by appropriately selecting welding conditions such as the type of electrode (electrode tip shape), electrode pressure, current, current application time, and pre-current.

[0053] (steel plate) The spot welded joint 1 of the embodiment shown in Figure 1 is composed of two steel plates: steel plate 21 which serves as the upper plate and steel plate 22 which serves as the lower plate. However, in this embodiment, the number of steel plates which make up the spot welded joint 1 is not limited to two, and may be any number of two or more depending on the strength required of the part to which the welded joint is applied, etc.

[0054] In this embodiment, the types of the two or more steel sheets used for spot welding are not particularly limited, and may be unplated steel sheets or steel sheets plated with zinc, aluminum, etc. Furthermore, the two or more steel sheets used for spot welding may all be the same type of steel sheet (for example, unplated steel sheet, zinc-plated steel sheet, etc.), all be different types of steel sheets, or only some of the two or more steel sheets may be different types of steel sheets.

[0055] In this embodiment, the strength of the steel plates used for spot welding is not particularly limited, but it is preferable that at least one of the two or more steel plates has a Vickers hardness of 350 HV or more. When such a high-strength steel plate is used, the risk of hydrogen embrittlement cracking increases significantly, so the present invention is particularly advantageous when such a high-strength steel plate is used.

[0056] In this embodiment, the Vickers hardness of the steel sheet may be 360 ​​HV or more, 400 HV or more, 450 HV or more, 500 HV or more, 550 HV or more, or 600 HV or more. The upper limit of the Vickers hardness of the steel sheet is not particularly limited, but is, for example, 800 HV from the viewpoint of workability, etc.

[0057] The Vickers hardness of steel plate can be measured using a method that conforms to JIS Z 2244:2009 "Vickers hardness test - Test method." When measuring the Vickers hardness of steel plate, the measurement is carried out at a depth of 1 / 4 of the plate thickness of the steel plate under a test load of 500 g.

[0058] The two or more steel plates used for spot welding may all be steel plates of the same strength, all steel plates of different strengths, or only some of the two or more steel plates may be steel plates of different strengths.

[0059] The thickness of the steel plates is not particularly limited, and any thickness can be adopted depending on the strength required of the parts to which the welded joint is applied, etc. The thickness of the steel plates may be, for example, 0.5 mm or more, 0.8 mm or more, or 1.0 mm or more. The thickness of the steel plates may be, for example, 3.5 mm or less, 3.2 mm or less, or 3.0 mm or less. The two or more steel plates used for spot welding may all be steel plates with the same thickness, all be steel plates with different thicknesses, or only some of the two or more steel plates may be steel plates with different thicknesses.

[0060] (Manufacturing method) The spot welded joint 1 of this embodiment has a nugget diameter DN is 4.0√t or less with respect to the plate thickness t of the steel plate 21 and the steel plate 22, and the overlapping surface direction D from the end 3E of the nugget 3 L 0.5D N The sheet can be manufactured by spot welding under welding conditions that provide a specific push-in amount so that the sheet separation h at the separated position is 0.60 mm or more.

[0061] More specifically, the nugget diameter D N By appropriately selecting the welding conditions such as the type of electrode (electrode tip shape), electrode pressure, current, current duration, and pre-current so that the thickness is 4.0√t or less and the sheet separation h is 0.60 mm or more, a spot welded joint 1 having the above-mentioned unique configuration can be manufactured.

[0062] Various welding conditions that can be used in the method for manufacturing the spot-welded joint 1 of this embodiment will be described in detail below.

[0063] The pair of spot welding electrodes that can be used in the manufacturing method of the spot welded joint 1 of this embodiment are not particularly limited, but examples include CF-type electrodes and DR-type electrodes made of chromium copper and having a tip diameter of 6 mm or less. Among these, it is preferable to use electrodes with a small tip diameter (for example, a CF-type electrode with a tip diameter of 3 mm) because this makes it easier to obtain the specific plunge depth.

[0064] Furthermore, the pressure applied by the pair of electrodes to the sheet assembly consisting of two or more steel sheets is not particularly limited, but may be, for example, a pressure of 250 kgf to 700 kgf (2.451 kN to 6.864 kN). Of these, a pressure of 300 kgf to 600 kgf is preferable because it makes it easier to obtain the above-mentioned specific pressing amount.

[0065] The applied current is not particularly limited, but may be, for example, 1.0 kA to 12.0 kA. In particular, the applied current is preferably 2.0 kA to 6.0 kA, as this makes it easier to obtain the above-mentioned specific depression amount.

[0066] The energization time is not particularly limited, but may be, for example, 12 to 60 cycles. In particular, a power-on time of 15 to 25 cycles is preferable, as this makes it easier to obtain the specific depression amount. When the power supply frequency is 50 Hz, 1 cycle (1 cycle) is 1 / 50 seconds.

[0067] Furthermore, the number of times current is applied during spot welding is not particularly limited as long as it does not impair the effects of the present invention, and may be, for example, only one current or two or more currents. More specifically, spot welding may involve pre-current application before main current application, post-current application after main current application, or main current application alone. In particular, when pre-current application is performed, this pre-current can also promote plastic deformation of the steel sheet depending on the conditions, thereby reducing the effective crushing amount of the above-mentioned sheet gap and, as a result, making it possible to reduce residual stress (peel stress) in the peel direction.

[0068] The welding machine used for spot welding is not particularly limited, and various power sources can be used for the spot welding machine, such as an inverter DC power supply, an inverter AC power supply, a single-phase AC power supply, etc. The welding machine can be of a spot welding robot type that combines a welding gun with an industrial robot, or a stationary type.

[0069] Furthermore, in the method for manufacturing the spot-welded joint 1 of this embodiment, any optional process that is performed in normal spot welding may be performed before or after the spot welding process. Such optional processes include, for example, a plate assembly forming process, a cooling process, and various surface treatment processes.

[0070] (Application example) As described above, the spot-welded joint of the present invention is resistant to hydrogen embrittlement cracking even when the nugget diameter is small, and therefore can be applied to various structural parts that require excellent joint strength, such as transportation machinery such as automobiles, industrial machinery, buildings, etc. In particular, because the present invention can significantly suppress hydrogen embrittlement cracking in welded joints with small nugget diameters, it can be particularly suitably used in the manufacture of automobile bodies and parts, which require high production efficiency and excellent joint strength.

[0071] The spot welded joint of the present invention is not limited to the above-described embodiments or the examples described below, and can be appropriately combined, substituted, modified, etc. within the scope that does not deviate from the object and intent of the present invention. [Example]

[0072] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0073] (Fabrication of spot welded joints) To verify the effects of the present invention, first, two unplated steel sheets (base material Vickers hardness 700 HV) with a thickness of 1.6 mm and a tensile strength of 2500 MPa were prepared as test materials. These two steel sheets were used to form a plate assembly with a gap of 1.5 mm. Furthermore, two points on this plate assembly were spot-welded to produce the test specimen shown in Figure 3. Then, using two types of electrodes (CF120Φ3 and DR40Φ6) shown in Figure 4, the two spot welds S of the above test piece were P Evaluation point position P E By spot welding these specimens under the welding conditions shown in Table 1 below, various spot-welded joints of invention examples 1 to 3 and comparative examples 1 to 3 were produced.

[0074] For the various spot-welded joints thus produced in Invention Examples 1 to 3 and Comparative Examples 1 to 3, the nugget diameter, sheet separation, indentation depth, and joint edge opening angle were measured as described below, and the presence or absence of hydrogen embrittlement cracking was also confirmed. The measurement results and evaluation results are shown in Table 2 below. In Table 2, underlined values ​​indicate values ​​outside the range of the present invention.

[0075] (Measurement of nugget diameter, sheet separation, indentation depth and opening angle of welded joint edge) The spot-welded joint to be measured is cut in the thickness direction through the center of the nugget to reveal a longitudinal cross section of the welded joint. This cross section is then embedded in a cold-setting resin. Next, the specimen observation surface after resin embedding is roughly polished using waterproof abrasive paper with grit sizes of 80, 400, 800, and 1500, followed by precision polishing using a 3 μm diamond spray. The polished specimen observation surface is then corroded to the extent that the fusion boundary can be distinguished, making the nugget visible. The corroding solution used to corrode the specimen observation surface is, for example, picric acid. The corroded specimen observation surface is then photographed using a magnifying observation tool such as a microscope, and the nugget diameter, sheet separation, indentation depth, and opening angle of the joint edge are determined from the enlarged cross-sectional photograph according to the measurement methods described above.

[0076] (Evaluation of hydrogen embrittlement cracking) To allow hydrogen to penetrate the spot-welded joint to be evaluated, the spot-welded joint to be evaluated is immersed in hydrochloric acid with a pH of 3 at room temperature for 100 hours. After immersion in hydrochloric acid, the welded joint is then cut in the thickness direction through the center of the nugget, and the cross section is photographed. The presence or absence of hydrogen embrittlement cracking is confirmed from the cross-sectional photograph. The criteria for determining the presence or absence of hydrogen embrittlement cracking are as follows: if the crack length, measured from the intersection of the crack and the nugget boundary to the end point of the crack, exceeds 100 μm, it is judged as "crack present," and if the crack length is 100 μm or less, it is judged as "crack absent." FIG. 9 shows cross-sectional photographs of the spot-welded joints of Inventive Examples 1 to 3 and Comparative Examples 1 to 3 taken during this evaluation.

[0077] [Table 1]

[0078] [Table 2]

[0079] In the spot welded joints of Examples 1 to 3 of the present invention, the sheet separation size was 0.60 mm or more, and the nugget diameter D N Even with a small nugget diameter of 4.0√t or less, no hydrogen embrittlement cracking occurred. On the other hand, the spot-welded joints of Comparative Examples 1 and 3 had sheet separations of less than 0.60 mm, and therefore hydrogen embrittlement cracking occurred. The spot-welded joint of Comparative Example 2 did not have hydrogen embrittlement cracking, but the nugget diameter D N exceeded 4.0√t, and it was not possible to form a nugget with a small diameter. [Explanation of symbols]

[0080] 1 Spot welded joints 21 (Top) Steel Plate 22 (Lower) Steel Plate 3. Nuggets

Claims

1. A spot welded joint having a nugget on the overlapping surfaces of two or more steel plates, The nugget diameter D of the nugget Ｎ is 4.0√t or less with respect to the plate thickness t of the steel plate, 0.5D from the end of the nugget toward the overlapping surface of the steel plate Ｎ A spot welded joint, characterized in that the size of the sheet separation at the separated position is 0.60 mm or more.

2. 2. The spot welded joint according to claim 1, wherein the depth of the indentation formed on the surface of the steel plate is 15 to 50% of the plate thickness t of the steel plate.

3. 3. The spot welded joint according to claim 1, wherein an opening angle of a joint end portion of the nugget located in the direction of the overlapping surfaces is 75° or more.

4. 3. The spot welded joint according to claim 1, wherein at least one steel plate of the two or more steel plates has a Vickers hardness of 350 HV or more.

5. The spot welded joint according to claim 3, wherein at least one of the two or more steel plates has a Vickers hardness of 350 HV or more.

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