Spot-welded joint

Abstract

The present invention provides a spot-welded joint in which, due to a novel configuration, hydrogen embrittlement cracking is unlikely to occur.　This spot-welded joint (1) comprises two or more superposed steel sheets (2, 3), a nugget (N) that joins the steel sheets, and a HAZ-hardened portion that is formed around the nugget (N), the spot-welded joint being characterized in that at least one steel sheet (2) among two of the steel sheets that were in contact with an electrode during spot welding satisfies the relationship in formula (1) where, in a cross section along the thickness direction passing through the center of the nugget (N), the width of the HAZ-hardened portion sd measured at each of the following positions are defined as D1, D2, and Dc respectively: a position at a depth of one-quarter of the sheet thickness t from an electrode contact surface (S1); a position at a depth of one-quarter of the sheet thickness t from a superposed surface (S2); and a sheet thickness center position.　Formula (1):|D1-D2| / Dc≦0.14

Description

Spot welded joints

[0001] This invention relates to spot welding joints.

[0002] In recent years, there has been a trend towards increasing the strength of steel sheets used in various fields such as automobiles, home appliances, and building materials. For example, in the automotive sector, the use of thin, high-strength steel sheets is increasing in order to lighten vehicle bodies and components and improve fuel efficiency.

[0003] On the other hand, the assembly of automobile bodies and the attachment of parts are mainly carried out by spot welding from the standpoint of cost and manufacturing efficiency. However, when spot welding is performed using high-strength steel plates as described above, hydrogen embrittlement cracking is likely to occur in the welded joints. Hydrogen embrittlement cracking is a phenomenon in which a steel member subjected to high stress under operating conditions suddenly fractures due to hydrogen entering the steel from the surrounding environment. This phenomenon is also called delayed fracture due to the manner in which the fracture occurs. In general, it is known that hydrogen embrittlement cracking of steel plates is more likely to occur as the strength of the steel plate increases. This is thought to be because the higher the strength of the steel plate, the greater the stress remaining in the steel plate after part formation.

[0004] In particular, in spot welding, gaps may form between steel plates at the welding location due to the forming accuracy of the parts. When welding while compressing these gaps with a pair of electrodes, a force acts to return the area around the weld to its original shape when the electrodes are released, generating residual stress in the direction of delamination around the weld. Therefore, in spot welding using high-strength steel plates that are highly susceptible to hydrogen embrittlement as described above, hydrogen embrittlement cracking of the weld is a particular concern.

[0005] Various measures have been proposed to improve the hydrogen embrittlement resistance around the welded area in response to this type of hydrogen embrittlement cracking.

[0006] For example, Patent Document 1 proposes a spot welding method that includes: a pressurizing step in which, in a state where two high-strength steel plates with a tensile strength of 1200 MPa or more are electrically connected, the two high-strength steel plates are sandwiched between a pair of spot electrodes and pressurized with a pressure of 3000 N or less in the welded portion where a gap has been formed; and a welding step in which current is intermittently passed through the pressurized high-strength steel plates multiple times with a period of current pause in between, softening the welded portion with heat generated by the current, and then plastically deforming the softened welded portion with the above-mentioned pressure, thereby bringing the welded portions of the two high-strength steel plates into contact and welding the contacted welded portions together.

[0007] According to the spot welding method described in Patent Document 1, tensile stress is less likely to remain around the welded portion when welding high-strength steel plates together, and delayed fracture around the weld can be suppressed.

[0008] Although not relating to hydrogen embrittlement cracking, Patent Document 2 describes a steel plate comprising a first steel plate, a second steel plate, and a spot welded joint joining the first and second steel plates, wherein the first and second steel plates are directly overlapped, the tensile strength of the first steel plate is 1500 MPa or more, the tensile strength of the second steel plate is less than or equal to the tensile strength of the first steel plate, the Vickers hardness of the first steel plate from the surface to a depth of 20 μm is 95% or less of the Vickers hardness at a position 1 / 4 of the plate thickness of the base material of the first steel plate, the carbon equivalent Ceq of the base material of the first steel plate is 0.22% or more, and HC = Ceq × TS ２ × (1 - (TS ２ ×t ２ ３ ) / (TS １ ×t １ ３ )) / √t ２ A spot welded joint is disclosed in which the Vickers hardness is 170 or more, the spot welded joint comprises a nugget and a pressure-welded portion that joins a first steel plate and a second steel plate around the nugget, and the Vickers hardness of the outer peripheral end of the pressure-welded portion at 20 μm on the first steel plate side is 50 or more Hv lower than the Vickers hardness at a position where the base material of the first steel plate is 1 / 4 of the plate thickness.

[0009] Japanese Patent Publication No. 2018-134665, International Publication No. 2023 / 234391

[0010] In the spot welding method described in Patent Document 1, plastic deformation in the weld occurs concentrated in areas that become hot and softened during welding. At this time, because the heat dissipation to the electrode is large and the heat-generating center during welding is at the overlapping surface of the steel plates, as shown in Figure 1(a), the temperature does not rise easily near the electrode contact area of ​​the steel plate where the electrode is in contact, and a relatively lower temperature area (low temperature area) may occur. If such a low temperature area occurs near the electrode contact area of ​​the steel plate, it may hinder the plastic deformation of the steel plate, and thus may not be able to sufficiently reduce residual stress after welding.

[0011] This invention has been made in view of these circumstances, and aims to provide a spot-welded joint that is less prone to hydrogen embrittlement cracking through a novel configuration.

[0012] The present invention includes the following embodiments.

[0013] (Aspect 1) A spot-welded joint comprising two or more overlapping steel plates, a nugget for joining the steel plates, and a HAZ hardened portion formed around the nugget, characterized in that, at least one of the two steel plates that were in contact with the electrode during spot welding satisfies the following relationship (1) when the widths of the HAZ hardened portion measured at a position 1 t / 4 depth of plate thickness t from the electrode contact surface, a position 1 t / 4 depth of plate thickness t from the overlapping surface, and the center of the plate thickness, respectively, are denoted as D1, D2, and Dc. |D2 - D1| / Dc ≤ 0.14 ... (1)

[0014] (Aspect 2) The spot welded joint according to Aspect 1, characterized in that at least one of the two steel plates in contact with the electrode has an indentation on the electrode contact surface, and when the indentation diameter of the indentation is De in a cross section along the thickness direction passing through the center of the nugget, the following equation (2) is satisfied: D2 / De ≥ 1.16 ... (2)

[0015] (Aspect 3) Among the two steel plates in contact with the electrode, at least one steel plate has a Vickers hardness of 300 HV or more in the base material part, and the spot welding joint according to Aspect 1 or 2 above is characterized in that.

[0016] According to the present invention, a novel configuration can provide a spot welding joint in which hydrogen embrittlement cracking is unlikely to occur.

[0017] Fig. 1 is a cross-sectional schematic view for explaining the ease of deformation due to the temperature distribution of the steel plate during spot welding. Fig. 2 is a cross-sectional schematic view of a test piece prepared to verify the effect of suppressing hydrogen embrittlement cracking by controlling the shape of the HAZ hardened part. Fig. 3 is a cross-sectional photograph after conducting an evaluation test of hydrogen embrittlement cracking using the test piece of Fig. 2. Fig. 4 is a cross-sectional schematic view for explaining the influence of shunting during welding on the shape of the HAZ hardened part. Fig. 5 is a cross-sectional schematic view showing a spot welding joint according to an embodiment of the present invention and a conventional spot welding joint. Fig. 6 is a cross-sectional photograph for explaining the indentation diameter of the indentation part in the spot welding joint. Fig. 7 is a schematic view for explaining the end of the indentation part in the spot welding joint. Fig. 8 is a cross-sectional schematic view showing the test piece shape of the test piece prepared in the example.

[0018] In order to achieve the above object, the present inventors have earnestly studied by paying attention to the temperature distribution during spot welding. First, the present inventors considered that if the temperature of the steel plate could be raised over the entire thickness of the steel plate around the welded part, a low-temperature part that would hinder the plastic deformation of the steel plate would not occur, and when crushing the gap between the steel plates, the steel plate could be efficiently plastically deformed and the residual stress could be reduced. That is, as shown in Fig. 1(b), the present inventors considered that by raising the temperature of the steel plate 2 over the entire thickness of the steel plate 2 in contact with the electrode E around the welded part, a low-temperature part would not occur near the electrode contact part of the steel plate 2, and "shear deformation" in the plate thickness direction would be facilitated. Ｒ By raising the temperature of the steel plate 2 over the entire thickness of the steel plate 2 in contact with the electrode, it was considered that a low-temperature part would not occur near the electrode contact part of the steel plate 2, and "shear deformation" in the plate thickness direction would be facilitated.

[0019] Therefore, the inventors focused on the shape of the heat-affected zone (HAZ), which is the region in the HAZ that hardens due to a temperature rise above the Ac1 point, where the metal structure, metallurgical properties, and mechanical properties are altered by the effects of welding heat. They investigated the relationship between controlling the shape of the HAZ hardened region, specifically by making the width of the HAZ hardened region substantially uniform in the plate thickness direction, and the effect of suppressing hydrogen embrittlement cracking. Specifically, two types of test specimens were prepared by changing only the shape of the HAZ hardened region while keeping the nugget diameter the same, as described below, and the presence or absence of hydrogen embrittlement cracking was evaluated for these two types of test specimens.

[0020] In preparing the test specimens, two types of plate assemblies were first prepared. The first type of plate assembly consisted of two overlapping steel plates, with a 0.8 mm thick shim plate inserted in the gap between the steel plates on either side of the welded area where the nugget would be formed, and fixed with a clamp.

[0021] Similarly, the second type of plate assembly, as shown in the test specimen in Figure 2, consisted of two overlapping steel plates. A shim plate wrapped in insulating tape was placed between the steel plates on either side of the welded area where the nugget was formed, and secured with a clamp. The two steel plates used in each of the two types of plate assemblies were 1470 MPa class cold-rolled steel plates with a thickness of 1.6 mm.

[0022] As described above, by inserting a shim plate or a shim plate wrapped in insulating tape between steel plates, it is possible to control whether or not current shunting occurs when current is applied. In other words, in a plate assembly with a shim plate inserted between steel plates, current shunting to the shim plate is likely to occur when current is applied. On the other hand, in a plate assembly with a shim plate wrapped in insulating tape inserted between steel plates, current shunting to the shim plate is less likely to occur when current is applied. Furthermore, by appropriately setting the welding current according to the means of causing current shunting and the location where the shunting occurs, it is possible to control the amount of current shunted.

[0023] Then, by clamping the welded portions of the two types of plate assemblies prepared as described above with a pair of electrodes and applying current, test specimens of spot-welded joints were fabricated in which a nugget joining two steel plates was formed. Hereinafter, test specimens of spot-welded joints formed from plate assemblies with shim plates sandwiched between steel plates will be referred to as "test specimens with current division." On the other hand, test specimens of spot-welded joints formed from plate assemblies with shim plates wrapped in insulating tape sandwiched between steel plates will be referred to as "test specimens without current division."

[0024] The specific spot welding conditions are as follows. Note that the spot welding conditions for the specimens with and without current diversion are the same, except for the welding current. Electrode: DR40Φ6 Pressing force: 600 kgf Energization time: 0.6 seconds Welding current: (Specimen with current diversion) 6.2 kA, (Specimen without current diversion) 5.0 kA Holding time: 1.0 second

[0025] In each test specimen, rust-preventive oil was applied to the steel plate before spot welding, and hydrogen was introduced during spot welding.

[0026] The two types of test specimens prepared as described above were subjected to the "hydrogen embrittlement cracking evaluation test" described later to assess whether or not hydrogen embrittlement cracking occurred. The evaluation results are shown in Figure 3.

[0027] As shown in Figure 3(a), the specimen with flow diversion had a nugget diameter of 4.5 mm, and no cracks occurred in the hydrogen embrittlement cracking evaluation test. On the other hand, as shown in Figure 3(b), the specimen without flow diversion had a nugget diameter of 4.9 mm, and interfacial fracture occurred in the hydrogen embrittlement cracking evaluation test. Despite having a smaller nugget diameter, which is more prone to hydrogen embrittlement cracking, the specimen with flow diversion did not experience hydrogen embrittlement cracking. This suggests that the residual stress near the electrode contact area of ​​the steel plate was sufficiently reduced.

[0028] Furthermore, focusing on the shape of the HAZ hardened portion in these two types of test specimens, in the test specimen with flow division, as shown in Figure 3(a), the widths D1, D2, and Dc of the HAZ hardened portion measured at a depth of 1 t / 4 of the plate thickness t from the electrode contact surface of the steel plate, a depth of 1 t / 4 of the plate thickness t from the overlapping surface, and the center of the plate thickness were D1 = 7.2 mm, D2 = 7.4 mm, and Dc = 7.6 mm, respectively. In other words, in the test specimen with flow division, the width D1 of the HAZ hardened portion on the electrode contact surface side was 97% of the width Dc of the HAZ hardened portion at the center of the plate thickness, and the width D2 of the HAZ hardened portion on the overlapping surface side was 103%.

[0029] On the other hand, in the specimen without current division, as shown in Figure 3(b), the widths D1, D2, and Dc of the HAZ hardened area measured at each of the above positions were D1 = 4.4 mm, D2 = 6.3 mm, and Dc = 6.7 mm, respectively. In other words, in the specimen without current division, the width D1 of the HAZ hardened area on the electrode contact surface side was 70% of the width Dc of the HAZ hardened area at the center of the plate thickness, and the width D2 of the HAZ hardened area on the overlapping surface side was 106%.

[0030] As described above, it was found that in the test specimen with flow separation, the width of the HAZ hardened area was formed almost uniformly in the thickness direction.

[0031] Here, Figure 4 is a schematic cross-sectional diagram illustrating the effect of flow splitting during welding on the shape of the HAZ hardened area. As shown in Figure 4, in the test specimen with flow splitting, electrode E Ｒ The active current from the shim causes the overlapping surface S2 of steel plates 2 and 3, and its vicinity, to melt due to Joule heating caused by electrical resistance, forming a nugget N. Furthermore, the area around the nugget N is heated by the active current. On the other hand, reactive current diversion to the shim plate also occurs, and the heat generated by reactive current diversion is particularly large near the edges of the electrode contact area. As a result, the temperature rises near the electrode contact area of ​​the steel plate, that is, from the electrode contact surface S1 to a depth of approximately 1 t / 4 of the plate thickness t. Consequently, in the test specimen with current diversion, the width of the HAZ hardened area is thought to be formed to be approximately uniform in the plate thickness direction.

[0032] On the other hand, in the test specimen without current division, electrode E during spot welding ＲThe active current from the electrode forms a nugget N, and the area around the nugget N generates heat due to the active current. However, because reactive current shunting is less likely to occur near the edges of the electrode contact area, the temperature does not rise easily to the vicinity of the electrode contact area of ​​the steel plate. As a result, it is thought that in the test specimen without current shunting, the width of the HAZ hardened area was not formed uniformly in the thickness direction of the plate.

[0033] In this specification, "active current" refers to the current that contributes to welding. "Reactive current" refers to the current that does not contribute to welding.

[0034] From the above verification results, the inventors have found that by generating an ineffective flow diversion of a predetermined flow rate at the edge of the electrode contact area during spot welding, the temperature around the weld can be raised to the vicinity of the electrode contact area of ​​the steel plate, thereby reducing residual stress near the electrode contact area of ​​the steel plate. In the following description, ineffective flow diversion may be simply referred to as "flow diversion." Furthermore, regarding the means for generating an ineffective flow diversion of a predetermined flow rate at the edge of the electrode contact area during spot welding as described above, in the above verification, a shim plate was used to generate the flow diversion, but the use of a shim plate is not essential. For example, the steel plates surrounding the area to be welded may be joined in advance by spot welding to generate the flow diversion during spot welding of the area to be welded.

[0035] The present invention has been completed based on these findings and includes the embodiments described below.

[0036] A spot-welded joint according to one embodiment of the present invention is a spot-welded joint having two or more overlapping steel plates, a nugget for joining the steel plates, and a HAZ hardened portion formed around the nugget. In such a spot-welded joint, at least one of the two steel plates that were in contact with the electrode during spot welding satisfies the following relationship (1) when the widths of the HAZ hardened portion measured at a position 1 t / 4 depth of plate thickness t from the electrode contact surface, a position 1 t / 4 depth of plate thickness t from the overlapping surface, and the center of the plate thickness, respectively, are denoted as D1, D2, and Dc: |D2 - D1| / Dc ≤ 0.14 ... (1)

[0037] In this specification, the "welded part" is a general term for the part including the nugget and the heat affected zone (HAZ) in a spot weld joint. Also, the "nugget" is the metal that has melted and solidified during spot welding. And the "HAZ hardened part" is the region in the heat affected zone that has hardened due to a temperature rise above the Ac1 point.

[0038] Further, in this specification, the "electrode contact surface" of a steel plate is the surface on the side where the electrode was in contact during spot welding among the two surfaces facing each other in the plate thickness direction of the steel plate, and means the outermost surface in the plate stack. In relation to this, in this specification, the portion of the electrode contact surface of the steel plate where the electrode was actually in contact is referred to as the "electrode contact part". On the other hand, in this specification, the "overlap surface" of a steel plate means the surface on the opposite side of the electrode contact surface among the two surfaces facing each other in the plate thickness direction of the steel plate, and is the surface that is overlapped with another steel plate.

[0039] Also, in this specification, the "two steel plates with the electrode in contact during spot welding" means the two outermost steel plates among two or more overlapped steel plates.

[0040] Hereinafter, the spot weld joint according to an embodiment of the present invention will be described in detail with reference to the drawings.

[0041] <Spot Weld Joint> FIG. 5 is a cross-sectional schematic view showing a spot weld joint 1 according to an embodiment of the present invention and a conventional spot weld joint 1'. The spot weld joint 1 of the present embodiment shown in (a) of FIG. 5 has two overlapped steel plates, namely a steel plate 2 serving as the upper plate and a steel plate 3 serving as the lower plate, a nugget N joining these steel plates, and a HAZ hardened part formed around the nugget N.

[0042] And in the spot weld joint 1 of the present embodiment, at least one of the two steel plates 2 that the electrode contacted during spot welding, as shown in (a) of FIG. 5, in the cross section along the plate thickness direction passing through the center of the nugget N, when the widths of the HAZ hardened portions measured at the positions of 1t / 4 depth from the electrode contact surface S1, 1t / 4 depth from the overlapping surface S2, and the plate thickness center position of the plate thickness t are D1, D2, and Dc, respectively, it satisfies the following relationship of formula (1). |D2 - D1| / Dc ≤ 0.14 ··· (1)

[0043] In the above formula (1), the absolute value of the difference between the width D2 of the HAZ hardened portion on the overlapping surface side and the width D1 of the HAZ hardened portion on the electrode contact surface side is 0.14 times or less of the width Dc of the HAZ hardened portion at the plate thickness center position, which means that the width of the HAZ hardened portion of the spot weld joint is formed substantially uniformly in the plate thickness direction.

[0044] Such a HAZ hardened portion is formed when, during spot welding, the heat generation due to ineffective shunting becomes large at the end of the electrode contact portion, and the temperature rises from the electrode contact portion of the steel plate, that is, from the electrode contact surface to the vicinity of the 1t / 4 depth position of the plate thickness t. Therefore, when the spot weld joint 1 having such a HAZ hardened portion performs spot welding while crushing the gap between the steel plates with a pair of electrodes, the steel plate 2 is in a state where it can be easily plastically deformed, so the residual stress is sufficiently reduced and hydrogen embrittlement cracking is less likely to occur.

[0045] On the other hand, in the conventional spot weld joint 1' as shown in (b) of FIG. 5, generally, the widths D1, D2, and Dc of the HAZ hardened portions measured at the positions of 1t / 4 depth from the electrode contact surface S1, 1t / 4 depth from the overlapping surface S2, and the plate thickness center position of the plate thickness t do not satisfy the relationship of the above formula (1), and often the relationship D1 < Dc < D2 holds. In such a conventional spot weld joint 1', during spot welding, a low-temperature portion occurs near the electrode contact portion of the steel plate where the electrode is in contact (that is, near the 1t / 4 depth position from the electrode contact surface S1), which hinders the plastic deformation of the steel plate, so there is a possibility that the residual stress cannot be sufficiently reduced.

[0046] The configuration of the spot-welded joint 1 of this embodiment will be described in more detail below.

[0047] [Width of the HAZ hardened portion measured at a position 1 t / 4 depth of plate thickness t from the electrode contact surface, a position 1 t / 4 depth of plate thickness t from the overlapping surface, and the center position of the plate thickness: |D2-D1| / Dc ≤ 0.14] In the spot-welded joint 1 of this embodiment, as described above, in at least one of the two steel plates 2 that the electrodes were in contact with during spot welding, the widths D1, D2, and Dc of the HAZ hardened portion measured at a position 1 t / 4 depth of plate thickness t from the electrode contact surface S1, a position 1 t / 4 depth of plate thickness t from the overlapping surface S2, and the center position of the plate thickness satisfy the relationship of formula (1) above. In formula (1) above, |D2-D1| / Dc on the left side is preferably 0.13 or less, and preferably 0.12 or less. The lower limit of |D2-D1| / Dc is 0, but it may be 0.01 or more, 0.02 or more, or 0.03 or more.

[0048] The widths D1, D2, and Dc of the HAZ hardened portion at the positions 1 / 4 of the plate thickness t from the electrode contact surface S1, 1 / 4 of the plate thickness t from the overlapping surface S2, and at the center of the plate thickness are measured by the following cross-sectional observation.

[0049] (Method for measuring the width of the HAZ hardened area) The spot welded joint to be measured is cut along the thickness direction so as to pass through the center of the nugget, exposing the cross-section of the welded joint. A cold-curing resin is used to fill this cross-section. Next, the sample observation surface after resin filling is roughly polished using waterproof abrasive paper with grits of 80, 400, 800, and 1500, and then finely polished using a 3 μm diamond spray. Furthermore, the sample observation surface after polishing is corroded to the extent that the molten boundary can be identified, making the nugget visible. The corrosive solution used to corrode the sample observation surface is, for example, Nital. The corroded sample observation surface is then photographed using a microscope at a magnification of 5 to 50 times to obtain a magnified cross-sectional photograph. From this magnified cross-sectional photograph, the widths D1, D2, and Dc of the HAZ hardened area at each depth position are measured. The HAZ hardened area can be visually identified as an area where the contrast changes due to differences in microstructure. Furthermore, the nugget diameter and the indentation diameter of the indented area (described later) can be measured from this magnified cross-sectional photograph. However, with Nital corrosion, the nugget boundary may be difficult to see. In such cases, the nugget diameter can be measured by repolishing and then etching with picric acid.

[0050] For the widths D1, D2, and Dc of the HAZ hardened area at each of the above positions to satisfy the relationship in equation (1), as described above, it is necessary to generate a predetermined flow division near the electrode contact area, that is, at a depth of 1 t / 4 of the plate thickness t from the electrode contact surface, during spot welding. As a method for generating such flow division, first, before spot welding, a shim plate such as a metal plate is inserted into the gap between the steel plates, or the steel plates around the area to be welded are joined in advance by spot welding to create a condition in which flow division occurs during spot welding of the area to be welded. Next, when spot welding the area to be welded, the flow division rate can be controlled by appropriately setting the welding current according to the means for generating flow division (i.e., shim plate or prior spot welding) and the location where the flow division occurs. The location where the flow division occurs can be adjusted by the arrangement of the means for generating flow division. Furthermore, the placement of the means for generating current division and the setting of the welding current should be determined by conducting several prototypes within the usual range, and then selecting and setting conditions that satisfy the relationship in equation (1) above for the widths D1, D2, and Dc of the HAZ hardened area at each of the above positions based on the results.

[0051] Furthermore, in the spot-welded joint 1 of this embodiment, as shown in Figure 5(a), at least one of the two steel plates 2 that were in contact with the electrode during spot welding has an indentation on the electrode contact surface S1.

[0052] Furthermore, in this embodiment, when the indentation diameter of the indented portion is De in a cross-section along the thickness direction passing through the center of the nugget N, it is preferable that the following relationship (2) is satisfied: D2 / De ≥ 1.16 ... (2)

[0053] [Indentation diameter De of the indented area: D2 / De ≥ 1.16] Equation (2) above means that the width D2 of the HAZ hardened area at a depth of 1 t / 4 of the plate thickness t from the overlapping surface of the steel plates in contact with the electrodes is 1.16 times or more the indentation diameter De. Specifically, it means that during spot welding, the width of the area where the temperature rises due to heat generated by the active current and reactive current shunt (i.e., the high-temperature area) is 1.16 times or more greater than the area where two or more overlapping steel plates are sandwiched between a pair of electrodes (i.e., the electrode contact diameter).

[0054] Normally, during spot welding, the shear deformation of two or more overlapping steel plates is constrained in the thickness direction of the steel plates within the area where they are sandwiched between a pair of electrodes. However, if the relationship between D2 and the indentation diameter De of the steel plate in contact with the electrodes satisfies the relationship in equation (2) above, the high-temperature area extends over a wider area than the electrode contact diameter. Therefore, shear deformation in the thickness direction of the steel plates can be more reliably achieved during spot welding. This makes it possible to more reliably obtain the above-mentioned effect, namely, the effect of sufficiently reducing residual stress and making hydrogen embrittlement cracking less likely to occur.

[0055] Furthermore, these effects are particularly advantageous for spot-welded joints with small nugget diameters. Spot-welded joints with small nugget diameters, i.e., nugget diameters of 4.0√t or less, are particularly prone to residual stress at the joint ends, i.e., hydrogen embrittlement cracking. However, as described above, when D1, D2, and Dc satisfy the relationship in equation (1), and D2 and De satisfy the relationship in equation (2), the temperature rises during spot welding to the vicinity of the electrode contact area of ​​the steel plate and to a wider area than the electrode contact diameter, allowing the steel plate to deform plastically more reliably and easily. As a result, even with such small nugget diameters, residual stress is sufficiently reduced, and hydrogen embrittlement cracking becomes less likely to occur.

[0056] In the above formula (2), D2 / De on the left side is preferably 1.17 or greater, and more preferably 1.18 or greater. The upper limit of D2 / De is not particularly limited, but may be, for example, 1.25 or less, 1.24 or less, or 1.23 or less.

[0057] Here, indentation is defined in JIS Z 3001-6:2013 "Welding Terminology - Part 6: Resistance Welding" as "a depression on the base material surface caused by the electrode tip and disc electrode as a result of welding in lap resistance welding."

[0058] (Method for measuring the indentation diameter of the indented area) The indentation diameter De of the indented area can be determined by using the cross-sectional enlarged photograph obtained by the method for measuring the width of the HAZ hardened area described above, determining the extent of the indented area, measuring the lengths of both ends of the area, and determining the indentation diameter.

[0059] If it is difficult to determine the end of the indentation, the position determined by the following method shall be considered the end of the indentation. Here, Figure 6 is a cross-sectional photograph illustrating the indentation diameter of the indentation in a spot welded joint. Figure 7 is a schematic diagram illustrating the end of the indentation in a spot welded joint.

[0060] (1) As shown in Figure 6, in a cross section along the thickness direction passing through the center of the nugget, let P1 be a point on the steel plate surface that is horizontally separated from the nugget center axis by nugget diameter ND and toward the base material. (2) Let P2 be a point on the steel plate surface that is horizontally separated from P1 by a distance of 1 mm. (3) Let X1 be the line obtained by extending the line segment passing through P1 and P2 toward the P1 side (towards the nugget center). (4) Let P3 be a point on the steel plate surface that is 30 μm vertically separated from P1 in the region of X1 toward the nugget center, and let P3 be the end of the indentation. If the steel plate extends above X1 between P1 and P3, as shown in Figure 7, let P3' be the most convex part perpendicular to X1, and let P3' be the end of the indentation. (5) Perform the above steps (1) to (4) on both the left and right sides of the nugget center axis, and let the distance between P3 or P3' be the indentation diameter De.

[0061] For the relationship between the electrode contact diameter D2 of the steel plate and the indentation diameter De to satisfy the above equation (2), it is necessary to generate a current split over an area at least 1.16 times wider than the electrode contact diameter during spot welding. As a method for generating a current split over such a wide area, the arrangement position of the means for generating the current split (i.e., shim plate or pre-spot welding) is adjusted, and the welding current is appropriately set according to the arrangement position, thereby controlling the amount of current split so that the current split occurs at a position further away from the welded part or extends to a position further away.

[0062] Furthermore, in the spot-welded joint 1 of this embodiment, the nugget diameter of the nugget N is not particularly limited, and examples include nugget diameters of 1.0√t to 6.0√t. Here, t is the thickness of the steel plate.

[0063] The nugget diameter may be 5.5√t or less, 5.0√t or less, 4.5√t or less, or 4.0√t or less. Alternatively, the nugget diameter may be 1.5√t or more, 2.0√t or more, or 2.5√t or more, from the viewpoint of joint strength, etc.

[0064] The nugget diameter is measured from a magnified cross-sectional photograph of the spot welded joint using a method compliant with JIS Z 3139:2009, "6.1.2 Method for measuring nugget diameter, nugget width, and penetration."

[0065] Furthermore, the plate thickness t of the steel plate refers to 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 of the above embodiment, steel plates 2 and 3 have the same plate thickness t.

[0066] The thickness t of the steel plate is measured using a micrometer from a relatively smooth location on the part (formed body) to which the welded joint is applied. Note that areas where the plate thickness has decreased locally due to thinning during the forming process are not included in the measurement.

[0067] (Steel Plates) The spot welded joint 1 of this embodiment shown in Figure 5(a) is composed of two steel plates, an upper steel plate 2 and a lower steel plate 3. However, in this embodiment, the number of steel plates constituting the spot welded joint 1 is not limited to two. The number of steel plates constituting the spot welded joint 1 may be any number of two or more, depending on the strength required of the part to which the welded joint is applied.

[0068] In this embodiment, the types of two or more steel plates used for spot welding are not particularly limited and may be unplated steel plates or plated steel plates with zinc, aluminum, etc. Furthermore, the two or more steel plates used for spot welding may all be of the same type (for example, unplated steel plates, galvanized steel plates, etc.), all be of different types, or only some of the two or more steel plates may be of different types.

[0069] Furthermore, 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 steel plates in contact with the electrodes has a Vickers hardness of 300 HV or more in the base material. When such high-strength steel plates are used, the risk of hydrogen embrittlement cracking increases significantly, and therefore the present invention is particularly advantageous when such high-strength steel plates are used.

[0070] In this embodiment, the Vickers hardness of the steel sheet may be 320 HV or higher, 340 HV or higher, 360 HV or higher, 400 HV or higher, 440 HV or higher, 500 HV or higher, or 550 HV or higher. There is no particular upper limit to the Vickers hardness of the steel sheet, but from the viewpoint of workability, for example, it is 800 HV or 650 HV.

[0071] The Vickers hardness of steel plates can be measured using a method compliant with JIS Z 2244-1:2024 "Vickers hardness test - Test method". To measure the Vickers hardness of a steel plate, ten measurements are taken at a depth of 1 / 4 of the plate thickness of the base material, under a test load of 500 g, and the arithmetic mean of these ten measurements is obtained. At this time, the distance between measurement positions should be at least three times the distance between indentations.

[0072] Furthermore, the two or more steel plates used for spot welding may be all steel plates of the same strength, all steel plates of different strengths, or only some of the two or more steel plates of different strengths.

[0073] Furthermore, the thickness of the steel plate is not particularly limited, and any thickness can be adopted according to the strength required for the part to which the welded joint is applied. The thickness of the steel plate may be, for example, 0.5 mm or more, 0.8 mm or more, or 1.0 mm or more. Alternatively, the thickness of the steel plate 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 of the same thickness, all be of different thicknesses, or only some of the two or more steel plates may be of different thicknesses.

[0074] (Manufacturing Method) The spot-welded joint 1 of this embodiment can be obtained by generating a predetermined flow division during spot welding such that the widths D1, D2, and Dc of the HAZ hardened portions at each of the above positions in the steel plate 2 that was in contact with the electrode during spot welding satisfy the relationship of formula (1) above. The method for generating such flow division is as described above.

[0075] Below, we will describe in detail the spot welding conditions other than the method for generating the predetermined flow rate described above, which can be used in the manufacturing method of the spot welded joint 1 of this embodiment.

[0076] In the method for manufacturing the spot-welded joint 1 of this embodiment, the pair of spot-welding electrodes used during spot welding are not particularly limited, but examples include DR-type electrodes or CF-type electrodes made of chromium copper with a tip diameter of 9 mm or less. Among these, it is preferable to use DR-type electrodes with a tip diameter of 6 mm or less.

[0077] Furthermore, the pressure applied to the plate assembly by this pair of electrodes is not particularly limited, but examples include a pressure of 400 kgf to 800 kgf (approximately 3.923 kN to approximately 7.845 kN). Among these, a pressure of 450 kgf to 750 kgf is preferred.

[0078] The current, i.e., the welding current, must be set within a range that generates a predetermined current division so that the widths D1, D2, and Dc of the HAZ hardened areas at each of the above-mentioned positions in the steel plate that the electrodes contact during spot welding satisfy the relationship in equation (1). For example, a range of 5.5 kA to 7.0 kA can be given as a setting range for such a welding current. From within this range, a current value that generates the predetermined current division can be selected depending on the means for generating the current division (i.e., shim plates or pre-spot welding) and the position where the current division occurs.

[0079] Furthermore, the energizing time is not particularly limited, but for example, it can be between 0.1 seconds and 1.5 seconds. Among these, an energizing time of 0.5 seconds to 1.0 second is preferred. In addition, the holding time is not particularly limited, but for example, it can be between 0.05 seconds and 1.5 seconds. Among these, a holding time of 0.1 seconds to 1.0 second is preferred.

[0080] The number of times current is applied during spot welding is not particularly limited as long as it does not hinder the effects of the present invention, and may be applied only once or two or more times. Specifically, spot welding may be performed with pre-current before main current, post-current after main current, or only main current. In particular, when pre-current is applied, depending on the conditions, plastic deformation of the steel plate can be promoted by this pre-current, thereby reducing the effective amount of crushing of the plate gap and consequently reducing the residual stress (peeling stress) in the peeling direction.

[0081] The welding machine used for spot welding is not particularly limited; for example, various power sources such as inverter DC power supplies, inverter AC power supplies, and single-phase AC power supplies can be used for the spot welding machine. The welding machine can be a spot welding robot system combining a welding gun and an industrial robot, or a stationary type.

[0082] In the method for manufacturing the spot-welded joint 1 of this embodiment, any process that is performed before or after the spot welding process, as is done in normal spot welding, may be carried out. Examples of such optional processes include a plate assembly process, a cooling process, and a surface treatment process.

[0083] (Application Examples) As described above, the spot-welded joint of the present invention is less susceptible to hydrogen embrittlement cracking, and therefore can be applied to various structural components of transportation machinery such as automobiles and industrial machinery where excellent joint strength is required. In particular, because the present invention can efficiently suppress hydrogen embrittlement cracking, it can be used especially suitably in the manufacture of automobile bodies and parts where high production efficiency and excellent joint strength are required.

[0084] The spot-welded joints of the present invention are not limited to the embodiments described above or the examples described later, and can be appropriately combined, substituted, or modified without departing from the purpose and spirit of the present invention.

[0085] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to such examples.

[0086] (Fabrication of Spot Welded Joints) To verify the effectiveness of the present invention, two 1470 MPa class cold-rolled steel plates were first prepared as test materials. The dimensions of the steel plates were 100 mm in length, 30 mm in width, and 1.6 mm in thickness. The two steel plates were stacked on top of each other, and a steel plate measuring 40 mm in length, 30 mm in width, and 0.8 mm in thickness was inserted as a shim plate between the steel plates on either side of the area to be welded, with the shim plate in the center. At this time, the shim plate was fixed with a clamp, and the distance between the shim plates centered on the area to be welded was set to 20 mm. Two types of plate assemblies were formed: one with insulating tape wrapped around the shim plate and one with insulating tape wrapped around the shim plate.

[0087] Then, by performing spot welding on the welded portions of these plate assemblies under the welding conditions shown in Table 1 below, test specimens of spot-welded joints No. 1 to 6, having the shapes A to D shown in Figure 8, were prepared.

[0088] Specifically, test specimen No. 2, which has the shape of test specimen shape A, was fabricated by spot welding the central welded portion using a plate assembly without insulating tape wrapped around the shim plate.

[0089] Test specimens No. 1 and No. 5, which have the shape of test specimen shape B, were prepared by using a plate assembly made of shim plates wrapped with insulating tape, spot welding (pre-welded points) at two locations on both sides of the central welded area with a 32 mm gap between them, and then spot welding the central welded area. Test specimens with the shape of test specimen shape B have a larger flow rate than test specimens with the shape of test specimen shape A.

[0090] Test specimen No. 3, which has the shape of test specimen C, was prepared in the same manner as the test specimen with the shape of test specimen B described above, except that spot welds (pre-welded spots) were made at two locations on either side of the central welded area with a 90 mm gap between them. Test specimens with the shape of test specimen C have a smaller flow rate than test specimens with the shape of test specimen A.

[0091] Furthermore, test specimens No. 4 and No. 6, which have the shape of test specimen shape D, were prepared in the same manner as the test specimens with the shape of test specimen shape A described above, except that a plate assembly with insulating tape wrapped around a shim plate was used. Test specimens with the shape of test specimen shape D are test specimens in which current separation itself is less likely to occur.

[0092] The test specimens No. 1 to 6 above had rust-preventive oil applied to the steel plates before spot welding, and hydrogen was introduced during spot welding.

[0093]

[0094] For each of the test specimens No. 1 to 6 prepared as described above, the following hydrogen embrittlement cracking evaluation test was conducted, with the evaluation point being the central spot weld point, to investigate the presence or absence of hydrogen embrittlement cracking. The results of the hydrogen embrittlement cracking evaluation test are shown in Table 2 below.

[0095] (Hydrogen Embrittlement Cracking Evaluation Test) To promote hydrogen penetration into the spot-welded joint specimen to be evaluated, rust-preventive oil is applied to the steel plate before spot welding as described above, and hydrogen is introduced during spot welding. Then, the specimen after spot welding is cut along the thickness direction, passing through the center of the nugget. The cross-section of the cut specimen is then photographed using a microscope to obtain a magnified cross-sectional image. The presence or absence of hydrogen embrittlement cracking is checked from the magnified cross-sectional image.

[0096] Furthermore, for each of the test pieces No. 1 to 6, the nugget diameter, the widths D1, D2, and Dc of the HAZ hardened portion at a depth of 1 t / 4 of the plate thickness t from the electrode contact surface of the steel plate, a depth of 1 t / 4 of the plate thickness t from the overlapping surface, and the center of the plate thickness, and the indentation diameter De of the indented portion were measured from magnified cross-sectional photographs of each test piece, according to the measurement methods described above. These measurement results are shown in Table 2 below. In Table 2, an underlined value in the |D2-D1| / Dc column indicates that it is outside the scope of the present invention, and an underlined value in the D2 / De column indicates that it is an undesirable characteristic.

[0097]

[0098] As shown in Table 2, none of the test specimens of the present invention examples No. 1 to 3, in which |D2-D1| / Dc was 0.14 or less, showed any hydrogen embrittlement cracking. It is believed that in these test specimens, the width of the HAZ hardened area was formed almost uniformly in the thickness direction, meaning that the temperature rose to the vicinity of the electrode contact area of ​​the steel plate during welding, and residual stress was sufficiently reduced, thus preventing hydrogen embrittlement cracking.

[0099] In particular, the test specimens of the present invention examples No. 2 and 3, which have small nugget diameters, showed no hydrogen embrittlement cracking even with small nugget diameters, because |D2-D1| / Dc was 0.14 or less, and D2 / De was 1.16 or more. The nugget diameter of No. 2 was 4.5 mm (3.6√t), and the nugget diameter of No. 3 was 4.3 mm (3.4√t), both of which are nugget diameters of 4.0√t or less, which are particularly prone to hydrogen embrittlement cracking.

[0100] On the other hand, all of the comparative example specimens No. 4 to 6, in which |D2-D1| / Dc was greater than 0.14, showed interfacial fracture. It is thought that in these specimens, the width of the HAZ hardened area was not formed substantially uniformly in the thickness direction, meaning that the temperature did not rise to the vicinity of the electrode contact area of ​​the steel plate during welding, and therefore the residual stress could not be sufficiently reduced, resulting in interfacial fracture.

[0101] In particular, although the test piece of comparative example No. 4 had a nugget diameter similar to that of test piece No. 1, its |D2-D1| / Dc was large at 0.37. This suggests that the temperature did not rise sufficiently to the vicinity of the electrode contact area of ​​the steel plate during welding, resulting in insufficient reduction of residual stress and subsequent interfacial fracture.

[0102] Furthermore, although specimen No. 5, like specimen No. 1, is specimen shape B with a relatively large flow rate, |D2-D1| / Dc is large at 0.13, indicating interfacial fracture. This is thought to be because, despite specimen No. 5 being a specimen with a small nugget diameter, which is prone to hydrogen embrittlement cracking, the welding current was not set appropriately, and sufficient flow rate distribution could not be generated during welding, resulting in the temperature not rising to the vicinity of the electrode contact area of ​​the steel plate.

[0103] 1 Spot welded joint 2 Steel plate (upper plate) 3 Steel plate (lower plate) N Nugget S1 Electrode contact surface S2 Overlap surface

Claims

1. A spot-welded joint comprising two or more overlapping steel plates, a nugget for joining the steel plates, and a HAZ hardened portion formed around the nugget, characterized in that, at least one of the two steel plates that were in contact with the electrode during spot welding satisfies the following relationship (1), when the widths of the HAZ hardened portion measured at a position 1 t / 4 depth of plate thickness t from the electrode contact surface, a position 1 t / 4 depth of plate thickness t from the overlapping surface, and the center of the plate thickness, respectively, are denoted as D1, D2, and Dc. |D2 - D1| / Dc ≤ 0.14 ... (1) 2. The spot welded joint according to claim 1, characterized in that at least one of the two steel plates in contact with the electrode has an indentation on the electrode contact surface, and when the indentation diameter of the indentation is De in a cross section along the thickness direction passing through the center of the nugget, the following relationship (2) is satisfied: D2 / De ≥ 1.16 ... (2) 3. The spot welded joint according to claim 1 or 2, characterized in that at least one of the two steel plates in contact with the electrode has a base material Vickers hardness of 300 HV or more.

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