Spot welded joint manufacturing method and spot welded joint

Abstract

To provide a method of manufacturing a novel and improved spot welded joint capable of suppressing outer cracking of a shoulder part and an outer side of the shoulder part, and to provide the spot welded joint.SOLUTION: In some viewpoint of the present invention, there is provided a method of manufacturing a spot welded joint that is a method of manufacturing a resistance spot joint having a plurality of steel plates spot-welded one over another, wherein one or more of the plurality of steel plates are galvanized high-strength steel plates galvanized to a tensile strength of 980 MPa or more; the galvanized high-strength steel plates are located on outermost surfaces of the plurality of steel plates; and surface flashing is caused in a period in which an integral value S obtained by integrating current values applied to the plurality of steel plates with an electrification period is 20 to 50% of the total integral value in the whole electrification period.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a spot welded joint and a spot welded joint. [Background technology]

[0002] In general, in manufacturing a spot-welded joint, the steel sheets are joined by sandwiching the overlapping surfaces of the steel sheets between a pair of opposing electrodes, heating and melting the overlapping surfaces by applying electrical current using the electrodes, and forming a weld metal (nugget) (see Figure 1). The joint between the steel sheets is also called a spot weld. In addition to the nugget described above, a spot weld includes a pressure weld formed on the outside of the nugget. For example, Patent Document 1 discloses a resistance spot welding method in which two or more steel plates, at least one of which has a tensile strength of 980 MPa or more, are overlapped to form welded steel plates, and the welded steel plates are clamped between a pair of electrodes and joined by passing an electric current while applying pressure, the method comprising an initial current passing step in which, where t1 is the total plate thickness (mm) of the welded steel plates, a current I1 (kA) is passed that satisfies the following equation (2) while applying pressure with a pressure F1 (kN) that satisfies the following equation (1): and a main current passing step in which a nugget having a predetermined nugget diameter is formed, and expulsion is generated during the initial current passing step. 0.2×√t1 <F1≦4×√t1 ···(1) 2×√F1 <I1≦10×√F1 ···(2) [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2021 / 059720 Summary of the Invention [Problem to be solved by the invention]

[0004] When high-strength steel sheets with zinc-based plating on the surface are spot-welded, LME cracking (liquid metal embrittlement cracking) may occur at the spot weld. The conditions for cracking to occur are that the molten zinc comes into contact with the solid steel sheet during spot welding, and that tensile stress (strain) acts at that point. The higher the strength of the steel sheet, the greater the susceptibility to LME cracking tends to be.

[0005] When spot welding multiple steel sheets (including zinc-based plated high-strength steel sheets) together to produce a spot-welded joint, increasing disturbances such as the impact angle or the gap between the sheets (sheet gap) can cause LME cracking (also referred to as external cracking) to occur inside or just outside the pressure weld formed on the outside of the weld metal (nugget), or on the surface of the electrode. As mentioned above, LME cracking (external cracking) occurs when molten zinc comes into contact with solid steel sheets during spot welding and tensile stress (strain) is exerted at that location. The magnitude of this tensile stress (here, stress in a direction parallel to the sheet surface) is affected by many factors, including the pressure applied by the electrode, expansion and contraction of the weld, and springback when the electrode is released.

[0006] External cracks are also related to the occurrence of expulsion (spattering of molten metal; see Figure 2), and the number and length of external cracks tend to increase as the amount of molten metal increases and the thickness of the weld becomes thinner. Figure 6 is a cross-sectional photograph showing specific examples of central cracks and shoulder cracks, which are types of external cracks. Central cracks refer to cracks that occur in the central part of the indentation (the bottom of the depression), which will be described later. Shoulder cracks refer to external cracks in the "shoulder" = part with a small radius of curvature at the end of the indentation (the part formed by being pressed into the part with a small radius of curvature of the electrode), which will be described later. FIG. 7 is a graph showing the correlation between Hw / Hb, which is the minimum weld thickness Hw (mm) divided by the total thickness Hb (mm) of multiple steel plates, and the value obtained by dividing the crack length L (mm) of a shoulder crack by the total thickness Hb (mm) of multiple steel plates and multiplying the result by 100. Each point P1 in FIG. 7 indicates the correlation between Hw / Hb (dimensionless) and L / Hb × 100 (%). The minimum weld thickness Hb (mm) is the thickness of the thinnest part of the cross section obtained when the spot weld is cut along a cross section passing through the center of the nugget and perpendicular to the surface of the steel plate. The total thickness Hb (mm) is the total thickness of multiple steel plates (total plate thickness), and the crack length is the longest length of the observed crack. The inventors carefully examined the graph in FIG. 7 and found that the smaller the minimum weld thickness Hw, the longer the crack tends to be. Furthermore, it was found that when Hw / Hb is 0.75 or less, very large expulsion (large expulsion) occurs. Explosion is particularly likely to occur when the current is increased to ensure a sufficient nugget diameter or when there is a gap between the sheets. Since it is difficult to predict and prevent expulsion, it is also difficult to prevent external cracking.

[0007] Indentations are depressions in the surface layer of a steel sheet caused by electrode pressure or expulsion. Cracks that occur in the center of an indentation (the bottom of the depression) (center cracks) have little effect on joint strength and are often not considered a problem. However, the inventors believe that outer cracks occurring in the "shoulder" (the portion with a small radius of curvature at the end of the indentation, formed by being pressed into the small radius of curvature of the electrode) and slightly outside of it (outside the shoulder) can reduce joint strength depending on their length, and therefore need to be suppressed.

[0008] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a new and improved method for manufacturing a spot-welded joint, and a spot-welded joint, which are capable of suppressing external cracking in the shoulder portion or outside the shoulder. [Means for solving the problem]

[0009] In order to solve the above-mentioned problems, according to one aspect of the present invention, there is provided a method for manufacturing a resistance spot joint by spot welding a sheet assembly including a plurality of steel sheets, characterized in that a zinc-based plated high-strength steel sheet having a tensile strength of 980 MPa or more and having a zinc-based plating is located on at least one surface of the sheet assembly, and when the integral S obtained by integrating the current applied to the sheet assembly over a current-flow period is taken as S, expulsion occurs during a period in which the integral S is 20 to 50% of the total integral over the entire current-flow period.

[0010] Here, the method may include a second current application process in which current is applied to the plate assembly at I2 (kA), and a first current application process which is carried out before the second current application process and in which current is applied to the plate assembly at I1 (kA) which is greater than I2, and the end of the first current application process may be a period in which the integral value S is 20 to 50% of the total integral value.

[0011] Furthermore, the pressure P1 (kN) applied to the sheet pair in the first current application process may be smaller than the pressure P2 (kN) applied to the sheet pair in the second current application process.

[0012] Furthermore, before the actual welding of the sheet assembly, a preliminary test of a test sheet assembly simulating the sheet assembly is conducted, and in the preliminary test, the current value I (kA) at each current flow time tw (sec) is changed to identify the expulsion occurrence current flow time tw1 at which expulsion occurs and the expulsion occurrence current waveform Ie1 at which expulsion occurs, and in the first current flow process, a current with the expulsion occurrence current waveform Ie1 may be applied to the multiple steel sheets for a time equal to or longer than the expulsion occurrence current flow time tw1.

[0013] Furthermore, when the average thickness t (mm) of the plurality of steel plates is the total thickness Hb (mm) divided by 2, the current value I2 (kA) in the second current application process may be set so that the nugget diameter is 5√t (mm) or more.

[0014] Furthermore, the amount of electrode displacement when expulsion occurs may be 0.25×Hb to 0.1×Hb (mm), where Hb (mm) is the total thickness of the plurality of steel plates.

[0015] According to another aspect of the present invention, there is provided a spot-welded joint in which a high-strength zinc-plated steel sheet having a tensile strength of 980 MPa or more and having a zinc-based coating is located on at least one surface, wherein the thinnest thickness Hw (mm) of the weld is related to the total thickness Hb (mm) of the plurality of steel sheets by the following relationship: 0.75 <Hw / Hb<0.9 and a nugget diameter of 5√t (t=Hb / 2) (mm) or more. [Effects of the Invention]

[0016] According to the above-described aspects of the present invention, it is possible to suppress external cracks at or outside the shoulder. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 2 is a side cross-sectional view showing the structure of the nugget and its surroundings. [Figure 2] FIG. 10 is a side cross-sectional view showing an example of scattering. [Figure 3] 10 is a graph schematically showing the transition of the pressure and current value applied by the electrodes. [Figure 4] 10 is a graph schematically showing the transition of the pressure and current value applied by the electrodes. [Figure 5] 10 is a graph showing a specific example of changes in electrode displacement and current value. [Figure 6] FIG. 10 is a side cross-sectional view showing an example of a center crack and a shoulder crack. [Figure 7] 1 is a graph showing the correlation between Hw / Hb, which is the thinnest thickness Hw (mm) of the weld divided by the total thickness Hb (mm) of multiple steel plates, and the value obtained by dividing the crack length L (mm) of a shoulder crack by the total thickness Hb (mm) of multiple steel plates and multiplying the result by 100. DETAILED DESCRIPTION OF THE INVENTION

[0018] <1. Manufacturing method of spot welded joints> Next, an embodiment of the present invention will be described with reference to the drawings. As mentioned above, it is difficult to completely suppress expulsion. Therefore, in the manufacturing method of a spot-welded joint according to this embodiment, the current is set to a relatively high value from the early to middle stages of the current application period to rapidly expand the contact zone, the pressure-welded zone, and the fusion zone. Because the weld is pressurized, internal pressure is generated in the fusion zone. If the growth rate of the fusion zone is sufficiently increased, part of the fusion zone will break the pressure-welded zone or open the contact zone and explode during the current application period. This is the "expulsion" phenomenon. Explosion occurring in the later stages of current application, i.e., after the fusion zone has sufficiently grown in the thickness direction or in directions parallel to the plate surface, results in large-volume fusion zone scattering, which is referred to as "large expulsion." On the other hand, if expulsion occurs from the early to middle stages of current application, part of the fusion zone that has grown to a medium volume will explode, which is referred to as "medium expulsion." In the present invention, this medium expulsion is actively generated to reduce the thickness of the weld within a certain range. At this time, the contact area between the electrode and the steel sheet is thought to expand rapidly, which results in a reduction in the current density in the current-carrying area, which is thought to suppress large-scale expulsion and metal loss in the latter half of welding.Furthermore, it is thought that external cracking can be suppressed in the shoulder and outside the shoulder.

[0019] More specifically, the method for manufacturing a spot welded joint according to this embodiment is a method for manufacturing a resistance spot joint in which a sheet assembly including a plurality of steel sheets is spot welded, and is characterized in that a zinc-based plated high-strength steel sheet having a tensile strength of 980 MPa or more and having a zinc-based plating is located on at least one surface of the sheet assembly, and when the integral S obtained by integrating the current applied to the sheet assembly over the current application period is taken as S, expulsion occurs during a period in which the integral S is 20 to 50% of the total integral over the entire current application period.

[0020] The integral value S can be determined using the current waveform (= a diagram showing the relationship between the current application period and the current value) determined before welding. Explosion may occur multiple times. The expulsion occurring in this embodiment is medium-level (medium expulsion), which differs from small and large expulsion. Here, medium expulsion refers to expulsion resulting in a ratio of the thinnest thickness of the welded portion after welding (Hw) to the total thickness (total plate thickness) (Hb) of multiple steel plates (Hw / Hb) greater than 0.75 and less than 0.9. The definition of the thinnest thickness of the welded portion is as described above. Large expulsion refers to expulsion where Hw / Hb is 0.75 or less, and small expulsion refers to expulsion where Hw / Hb is 0.9 or greater. It is preferable that expulsion not occur during a period in which the integral value S is greater than 50% of the total integral value, but expulsion may occur. In this embodiment, medium expulsion has already occurred, so large expulsion can be prevented during a period in which the integral value S is greater than 50% of the total integral value.

[0021] Intermediate expulsion rapidly advances contact between the electrode and steel sheet. As a result, the current density in the current-carrying area can be reduced, suppressing large expulsion and thinning in the latter half of the weld. As a result, a large nugget can be formed while suppressing external cracking at the shoulder and outside the shoulder. Furthermore, since thinning of the weld can be suppressed, joint strength can be easily ensured. In addition, water and oil, which are the cause of hydrogen embrittlement cracking (hydrogen sources), can sometimes be evaporated and dispersed when expulsion occurs. Hydrogen embrittlement cracking is cracking caused by hydrogen in the steel sheet embrittling the steel sheet. The hydrogen that causes hydrogen embrittlement cracking can be contained in the steel sheet beforehand, but it can also penetrate into the steel sheet from, for example, oil applied to the surface of the steel sheet.

[0022] One specific method for causing expulsion is as follows. That is, the current application process is divided into a first current application process and a second current application process. Here, the second current application process is a process in which a current is applied to multiple steel sheets at I2 (kA). The first current application process is a process carried out before the second current application process, and is a process in which a current is applied to multiple steel sheets at I1 (kA), which is greater than I2. The end of the first current application process is a period in which the integral value S is 20 to 50% of the total integral value, and preferably coincides with the time when the integral value S is 50% of the total integral value. This makes it easier for expulsion to occur in the first current application process.

[0023] In the case of constant current conduction, a period during which the current value is constant can be considered as one current conduction step. In the case of upslope / downslope conduction, a period during which the rate of change of the current value is constant can be considered as one current conduction step. I1 and I2 may be the maximum current values ​​of each current conduction step.

[0024] The magnitude relationship between I1 and I2 is preferably I1>I2×1.2. It is not essential that the second current application step be performed immediately after the first current application step; a rest step (cool, no current application) may be inserted between them. In particular, there may be multiple first current application steps, and the first current application step may be pulsation with a rest step (cool, no current application) inserted therebetween, or gradually increasing pulsation. In particular, the first current application step may be upslope current application. Schematic examples of the first current application step and the second current application step are shown in FIGS. 3(a) to 3(d). FIGS. 3(a) to 3(d) also show the applied pressure by the electrodes. The horizontal axis represents time, with current application starting at time t0. At time t1, the integral value S reaches 50%. The vertical axis represents applied pressure or current value. The period from t0 to t1 is the first current application step, and the period after time t1 is the second current application step.

[0025] Furthermore, the pressure P1 (kN) applied to the multiple steel sheets in the first current application process may be smaller than the pressure P2 (kN) applied to the multiple steel sheets in the second current application process. In this case, expulsion can be more reliably generated during the period in which the integral value S is 20 to 50%. A schematic example is shown in Figure 4. The horizontal axis represents time, with current application starting at time t0. The integral value S reaches 50% at time t1. The vertical axis represents the pressure or current value. The period from time t0 to t1 is the first current application process, and the process after time t1 is the second current application process.

[0026] The specific current value during the current-flow period is preferably determined in a preliminary test. Specifically, in the preliminary test, a test piece (a test plate assembly) simulating the steel plates (plate assembly) to be actually joined is prepared, and spot welding is performed. The electrode pressure is applied in a preset pressure pattern (e.g., the pressure pattern shown in Figure 3 or Figure 4). The current value I (kA) for each current-flow time tw (sec) is varied to determine the expulsion-occurrence current time tw1 and the expulsion-occurrence current waveform Ie1. The expulsion-occurrence current time tw1 and the expulsion-occurrence current waveform Ie1 are adjusted so that expulsion occurs during a period in which the integral value S described above is 20 to 50% of the total integral value. The occurrence of expulsion can be determined, for example, by the amount of electrode displacement. The expulsion-occurrence current waveform Ie1 may be a waveform exhibiting a constant current value or an upslope / downslope waveform. The expulsion generating current waveform Ie1 may be a pulsation with a rest period (cool, no current flow) or a gradually increasing pulsation. For reference, Ie1 and tw1 are shown in Figures 3 and 4.

[0027] In the actual first current application step, a current with the expulsion generation current waveform Ie1 is applied to multiple steel sheets for a time equal to or longer than the expulsion generation current application time tw1. This allows expulsion (intermediate expulsion) to occur when the integral value S reaches 20 to 50% of the total integral value. For reference, FIG. 5(a) shows an example of large expulsion, and FIG. 5(b) shows an example of intermediate expulsion. FIG. 5(b) is an example of this embodiment. Explosion occurs at time t2, when the electrode displacement amount drops sharply. As shown in FIG. 5, the magnitude of the electrode displacement amount when large expulsion occurs is greater than the electrode displacement amount when intermediate expulsion occurs. Furthermore, as shown in FIG. 5(b), intermediate expulsion occurs at the end of the first current application step (the period when the current value is approximately 9 kA).

[0028] As described above, the contact area, pressure weld area, and fusion zone expand rapidly around the time of the occurrence of internal expulsion, and the contact between the electrode and the steel sheet progresses. As a result, the current density in the current application area can be reduced, suppressing large expulsion and metal loss in the latter half of the welding process. As a result, a large nugget can be formed while suppressing external cracking in the shoulder and outside the shoulder. For this reason, the current value I2 (kA) in the second current application process may be set so that the nugget diameter is 5√t or more, where t is the average thickness of the multiple steel sheets, i.e., Hb / 2 (mm), the total thickness of the multiple steel sheets. If large expulsion occurs, the nugget diameter varies greatly, making it impossible to consistently produce nuggets with the above-mentioned diameter. Note that the nugget diameter is the length of the nugget parallel to the surface of the steel sheet, obtained by cutting the nugget along a cross section passing through the center of the nugget and perpendicular to the surface of the steel sheet.

[0029] Furthermore, the electrode displacement amount when expulsion occurs may be 0.25×Hb to 0.1×Hb (mm), where Hb (mm) is the total thickness of the steel plates. As described above, in this embodiment, expulsion occurs, so the electrode displacement amount when expulsion occurs can be as small as 0.25×Hb to 0.1×Hb (mm). This makes it possible to form a large nugget while suppressing indentation. Note that when expulsion occurs, the electrode displacement amount changes abruptly, so it is sufficient to measure the displacement amount at that time.

[0030] Furthermore, the voltage between the electrodes when expulsion occurs may be Vs ≥ 0.7 × Va (Vs: voltage when expulsion occurs, Va: voltage 5 ms before expulsion occurs). This allows for the formation of a large nugget while suppressing external cracking at the shoulder or outside the shoulder. The occurrence of expulsion can be identified by a sudden change in voltage or electrode displacement.

[0031] Furthermore, in the manufacturing method of a spot-welded joint according to this embodiment, it is preferable to optimize the responsiveness of the pressing force of the spot welder, i.e., the ability of the electrode position to follow the expansion and contraction of the weld. Specifically, it is preferable to set the time change in pressing force during normal welding (welding when no expulsion occurs) to a range of -10 kN / s or more (when the weld contracts) or +10 kN / s or less (when the weld expands). Here, when multiple steel sheets are sandwiched between a pair of electrodes, the direction in which the electrodes face the weld is defined as the negative direction. This allows for the formation of a large nugget while suppressing external cracking at the shoulder and outside the shoulder. In order to eliminate the gap between the sheets, the electrode pressure may be set to more than 3.5 kN during the initial electrode squeeze period (the period when the electrodes press the steel sheets together without applying current), which ensures a current path and, ultimately, allows for stable spot welds and expulsion.

[0032] Furthermore, it is preferable to set the electrode speed (change in electrode position over time) during normal welding to -0.4 mm / s or more (when the weld contracts) and +0.4 mm / s or less (when the weld expands). This allows for the formation of a large nugget while suppressing external cracking at the shoulder or outside the shoulder. In particular, it is believed that suppressing the electrode's tracking (lowering speed) when the weld contracts can reduce stress (strain) at the shoulder.

[0033] Furthermore, if the electrode compliance is set to a value within the range of this embodiment, there is a tendency for the particles to scatter multiple times during current flow, which is also presumed to be effective in reducing shoulder stress (strain) when the particles scatter.

[0034] On the other hand, if the electrode compliance is set to a value outside the range of this embodiment, the decrease in the applied pressure when the spot weld contracts is suppressed, i.e., the effect of maintaining a constant applied pressure is achieved. However, when expulsion occurs, the electrode will impact the weld, compressing it, which may cause high stress (strain) in the shoulder area and lead to LME cracking. Furthermore, if the electrode compliance is set outside the range of this embodiment, a large amount of expulsion will scatter all at once, and the weld thickness will tend to become thinner. This phenomenon is also thought to be a factor in increasing stress (strain) in the shoulder area.

[0035] As explained above, according to the manufacturing method of a spot-welded joint according to this embodiment, expulsion occurs when the integral value S reaches 20 to 50%. Around the time of the occurrence of expulsion, the contact area, pressure weld area, and fusion zone expand rapidly, and contact between the electrode and the steel sheet progresses. As a result, the current density in the current-carrying area can be reduced, suppressing large expulsion and metal loss in the latter half of welding. As a result, external cracking can be suppressed in the shoulder and outside the shoulder.

[0036] The type of zinc-based coating in this embodiment is not particularly limited, and may be, for example, hot-dip galvanizing (GI), galvannealed hot-dip galvanizing (GA), or electrogalvanizing (EG). The composition of the zinc-based coating is also not particularly limited, and various zinc-based coatings, such as Zn-Mg-Al coating, can be used. The number of steel sheets is also not particularly limited, and may be about 2 to 3 sheets, or 4 or more sheets.

[0037] <2. Structure of spot welded joints> Next, an example of a spot-welded joint manufactured by the above-mentioned method for manufacturing a spot-welded joint will be described. The spot-welded joint according to this embodiment is a spot-welded joint in which a high-strength zinc-plated steel sheet having a tensile strength of 980 MPa or more and a zinc-plated coating is located on at least one surface, and in which the thinnest thickness Hw (mm) of the weld is related to the total thickness Hb (mm) of the plurality of steel sheets by the following relationship: 0.75 <Hw / Hb<0.9 and the nugget diameter is 5√t (t=Hb / 2) (mm) or more.

[0038] The spot welded joint according to this embodiment has the above-described structure, i.e., central expulsion, which suppresses external cracking at the shoulder and outside the shoulder. Furthermore, a nugget with a large diameter of 5√t or more is formed. Since the thinnest thickness of the weld satisfies the above relational expression, it is understood that expulsion occurs in the manufacturing method of the spot-welded joint according to this embodiment, but in the spot-welded joint according to this embodiment, traces of expulsion that has scattered and solidified are often found in the sheet separation portion. The sheet separation portion refers to the portion of the gap (sheet gap) between multiple steel sheets adjacent to the spot weld. [Example]

[0039] Next, an example of this embodiment will be described. In this example, two 1.6 mm thick GA980 TRIP steel sheets (galvanized high-strength steel sheets) were prepared and spot-welded. The tensile strength of the steel sheets was measured using the following method and found to be 1012 MPa. Specifically, tensile strength is determined by preparing a JIS-specified tensile test piece, for example, a JIS No. 5 test piece, and subjecting it to a tensile test. The maximum load obtained during the test is divided by the initial cross-sectional area of ​​the test piece. Ten spot welding runs were performed per test. The spot welding conditions are shown in Table 1. The current values ​​in the first and second current-flow processes were constant, and the integral value S (and the value obtained by dividing the integral value S by the total integral value during the entire current-flow period) was adjusted by adjusting the current-flow period for each process. The electrode displacement / total sheet thickness value at the time of expulsion was calculated as the arithmetic mean value of the corresponding values ​​obtained over 10 tests. The minimum thickness of the spot-welded portion / total sheet thickness value was also calculated as the arithmetic mean value of the corresponding values ​​obtained over 10 tests. The average nugget diameter was also taken as the arithmetic mean of the values ​​obtained in 10 tests. The number of shoulder cracks was taken as the number of tests in which shoulder cracks occurred out of the 10 tests. The number of shoulder cracks was measured by cutting the spot welded joint obtained in each test on a plane passing through the center of the nugget and perpendicular to the surface of the steel plate, adjusting the cut surface appropriately, and then observing the presence or absence of cracks with an optical microscope. A shoulder crack count of 3 or less was considered to be acceptable. In Table 1, underlined values ​​do not satisfy the requirements of this embodiment.

[0040] [Table 1]

[0041] Test Nos. 2, 3, and 6 met the requirements of this embodiment, and therefore the number of shoulder cracks was at an acceptable level despite the average nugget diameter being as large as 5√t (t=Hb / 2) (Hb: total plate thickness) or more.

[0042] Test Nos. 1, 4, and 5 did not satisfy the requirements of this embodiment, with the ratio of the thinnest spot weld thickness to the total plate thickness being 0.75 or less, resulting in large spattering. As a result, the number of shoulder cracks was at a failing level.

[0043] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

Claims

1. A manufacturing method of a resistance spot joint by spot welding a plate assembly including a plurality of steel plates, A zinc-based plated high-strength steel plate having a tensile strength of 980 MPa or more and having a zinc-based plating is located on at least one surface of the sheet assembly, and A method for manufacturing a spot welded joint, characterized in that, when an integral value S obtained by integrating a current value applied to the plate assembly over a current-flow period is defined as a current value S, expulsion is generated during a period in which the integral value S is 20 to 50% of the total integral value over the entire current-flow period.

2. a second current-carrying step of current-carrying the plate set at I2 (kA); and a first current-carrying step that is carried out before the second current-carrying step and that current-carries the plate set at I1 (kA) that is greater than I2, 2. The method for manufacturing a spot welded joint according to claim 1, wherein the end of the first current application step is a period during which the integral value S is 20 to 50% of the total integral value.

3. 3. The method for manufacturing a spot welded joint according to claim 2, wherein a pressure P1 (kN) applied to the plate combination in the first current application process is smaller than a pressure P2 (kN) applied to the plate combination in the second current application process.

4. Before the actual welding of the plate assembly, a preliminary test of a test plate assembly simulating the plate assembly is performed; In the preliminary test, the current value I (kA) at each current conduction time tw (sec) is changed to identify the scattering occurrence current conduction time tw1 at which scattering occurs and the scattering occurrence current waveform Ie1 at which scattering occurs, 4. The method for manufacturing a spot welded joint according to claim 2, wherein in the first current application step, the current of the expulsion generation current waveform Ie1 is applied to the plurality of steel sheets for a time period equal to or longer than the expulsion generation current application time tw1.

5. 5. The method for manufacturing a spot welded joint according to claim 2, wherein a current value I2 (kA) in the second current application step is set so that a nugget diameter is 5√t (mm) or more, where t (mm) is an average thickness of the steel plates and Hb (mm) is a total thickness of the steel plates divided by 2.

6. 6. The method for manufacturing a spot welded joint according to claim 1, wherein the electrode displacement amount when the expulsion occurs is 0.25 × Hb to 0.1 × Hb (mm), where Hb (mm) is a total thickness of the plurality of steel plates.

7. A spot welded joint having a zinc-based plated high-strength steel plate with a tensile strength of 980 MPa or more and having a zinc-based plating located on at least one surface, The thinnest thickness Hw (mm) of the welded portion and the total thickness Hb (mm) of the plurality of steel plates are related by the following formula: 0.75<Hw / Hb<0.9 Satisfied, A spot welded joint, characterized in that the nugget diameter is 5√t (t=Hb / 2) (mm) or more.

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