SPOT WELDING METHOD

Abstract

The spot welding method of the present invention has steps of preliminary conduction, first conduction, second conduction, and third conduction: Preliminary conduction: Conduction method aimed at improving closeness of contact surfaces of steel sheets and reducing sheet gaps by gradually increasing the welding current (for example, upslope conduction). If rapidly applying current, current would be locally carried and that part would melt resulting in expulsion, so this is a conduction method gradually running current (for example upslope conduction) to avoid local heating. First conduction: Conduction method running a constant welding current and using the heat generated by the electrical contact resistance between steel sheets to cause the formation of a nugget when preliminary conduction results in close contact surfaces between the steel sheets. Second conduction: Conduction method making the current lower than the first conduction to suppress inside expulsion while making the nugget grow in the diametrical direction. Third conduction: Conduction method making the current higher than the second conduction and making the nugget grow not only in the diametrical direction, but also mainly in the sheet thickness direction when the nugget size becomes a certain extent of size at the second conduction step.

Claims

1. A spot welding method joining a plurality of superposed steel sheets, the spot welding method comprising a preliminary conduction step of gradually applying current, a first conduction step of running a constant current at a current value I1, a second conduction step of next running current at a current value I2, and further a third conduction step of running current at a current I3, wherein I1>I2 and I2<I3, where the units of I1, I2, and I3 are kA, and a pressing force by spot welding electrodes decreases in the third conduction step.

2. The spot welding method according to claim 1 wherein a sheet thickness ratio comprised of a ratio of a total of the sheet thicknesses of said plurality of steel sheets and a sheet thickness of the steel sheet with the thinnest sheet thickness among said plurality of steel sheets is 4.5 or more.

3. The spot welding method according to claim 1 wherein said plurality of steel sheets are superposed so that the steel sheet with the thinnest sheet thickness becomes the outermost side.

4. The spot welding method according to claim 1 wherein said preliminary conduction step makes a welding current increase by upslope conduction.

5. The spot welding method according to claim 1 wherein said preliminary conduction step is pulsation conduction.

6. The spot welding method according to claim 4 wherein a conduction time of the upslope conduction of said preliminary conduction step is 1 to 30 cycles.

7. The spot welding method according to claim 5 wherein a conduction time of the pulsation conduction of said preliminary conduction step is 1 to 10 cycles and an idling time is 1 to 5 cycles.

8. The spot welding method according to claim 1 wherein said third conduction step includes increasing a weld current by upslope conduction.

9. The spot welding method according to claim 1 wherein when designating a pressing force after said decrease as P2 and designating a pressing force before said decrease as P1,
0.5P1P2<P1.

10. The spot welding method according to claim 1 further comprising a cooling step where no welding current is run at least at one period between said first conduction step and said second conduction step and between said second conduction step and said third conduction step.

11. The spot welding method according to claim 10 wherein the cooling time of said cooling step is 10 cycles or less.

12. The spot welding method according to claim 2 wherein said plurality of steel sheets are superposed so that the steel sheet with the thinnest sheet thickness becomes the outermost side.

13. The spot welding method according to claim 2 wherein said preliminary conduction step is a welding current increase by upslope conduction.

14. The spot welding method according to claim 3 wherein said preliminary conduction step is a welding current increase by upslope conduction.

15. The spot welding method according to claim 12 wherein said preliminary conduction step is a welding current increase by upslope conduction.

16. The spot welding method according to claim 2 wherein said preliminary conduction step is pulsation conduction.

17. The spot welding method according to claim 3 wherein said preliminary conduction step is pulsation conduction.

18. The spot welding method according to claim 12 wherein said preliminary conduction step is pulsation conduction.

19. The spot welding method according to claim 13 wherein a conduction time of the upslope conduction of said preliminary conduction step is 1 to 30 cycles.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0041] FIG. 1 is a view showing one example of the mode of conduction of the present invention.

[0042] FIG. 2 is a view schematically showing one example of a spot welded joint according to the present invention.

[0043] FIG. 3 is a view showing a cross-sectional structure obtained by observing one mode of a welded joint formed in the present invention by an optical microscope.

[0044] FIG. 4 is a view showing one example of the relationship between a conduction mode and pressing force of the present invention.

[0045] FIG. 5 is a view showing another example of the relationship between a conduction mode and pressing force of the present invention.

[0046] FIG. 6 is a view showing another example of the relationship between a conduction mode and pressing force of the present invention.

[0047] FIG. 7 is a view showing another example of the relationship between a conduction mode and pressing force of the present invention.

[0048] FIG. 8 is a view showing an example having a cooling step in the conduction mode of the present invention.

[0049] FIG. 9 is a view showing another example having a cooling step in the conduction mode of the present invention.

DESCRIPTION OF EMBODIMENTS

[0050] The spot welding method of the present invention (below, sometimes referred to as the present invention welding method) is a spot welding method comprising clamping a set of sheets of a plurality of superposed steel sheets by a pair of spot welding electrodes (in this Description, simply referred to as electrodes), pressing it by the electrodes while running current, and melting and joining the contact parts of the steel sheets.

[0051] In the present invention welding method, the steel sheets applied are not particularly limited in steel type, form, mechanical properties, etc. The present invention welding method can be applied to any steel type, form, and/or mechanical properties of steel sheets so long as conduction is possible. Regarding the form of steel sheets, for example, the present invention welding method can be applied to electroplated steel sheets, hot dip coated steel sheets, and alloyed hot dip coated steel sheets.

[0052] Further, in the method of the present invention, the welding power supply is not limited to a specific power supply so long as it is a power supply enabling conduction of the steel sheets at the required number of cycles. In addition to a single-phase alternating current and three-phase rectified current, a direct current inverter can also be used as a welding power supply. When using a direct current inverter, the effect of the present invention welding method can be achieved.

[0053] FIG. 1 shows one example of a mode of conduction of the present invention welding method.

[0054] The spot welding method according to the present invention is comprised of a preliminary conduction step, first conduction step, second conduction step, and third conduction step. The notations used in the present invention will be explained below.

[0055] Current

[0056] The current value at the first conduction step is designated as I1, the current value at the second conduction step is designated as I2, the current value at the third conduction step is designated as I3, and the units are made kA in each case.

[0057] Conduction Time

[0058] The conduction time at the preliminary conduction step is designated as tu, the conduction time at the first conduction step is designated as t1, the conduction time at the second conduction step is designated as t2, the conduction time at the third conduction step is designated as t3, and the units are made the number of cycles in each case. Here, the number of cycles is the number of cycles of the alternating current power supply waveform relating to the input. The input heat amount is proportional to the integral of the input current value. This is because these are dependent on the alternating current power supply frequency. For example, if a 50 Hz alternating current power supply, 1 cycle= 1/50 seconds. If the welding power supply is a direct current inverter, the number of cycles is one derived from the frequency of the alternating current power supply input to the inverter.

[0059] Sheet Thickness and Sheet Gap

[0060] The average sheet thickness of the steel sheets is designated as Ta, the maximum value of the gap between steel sheets is designated as Tg (in this Description, sometimes called the sheet gap), and the units are all made mm. As explained above, when superposing three or more sheets, the average sheet thickness Ta is defined by the value of the sum of the sheet thicknesses divided by 2.

[0061] The sheet gap Tg is the gap between steel sheets at the portion being spot welded. The gaps between the steel sheets at portions not being spot welded are not considered.

[0062] Sheet Thickness Ratio

[0063] The sheet thickness ratio is the ratio of the total of the sheet thicknesses of the plurality of steel sheets and the sheet thickness of the steel sheet with the thinnest sheet thickness among the plurality of steel sheets.

Sheet thickness ratio=(total of sheet thicknesses of plurality of steel sheets)/(sheet thickness of steel sheet with thinnest sheet thickness)

If a thin steel sheet is arranged at the plurality of steel sheets and a thin steel sheet is arranged in particular at the outermost side, due to the cooling effect by the electrodes, the temperature falls and the nugget becomes harder to form the closer to the electrodes. The present invention solves this problem. The effect is large if the sheet thickness ratio is 4.5 or more. In particular, the upper limit is not set, but if arranging the thinnest steel sheet at the outermost side and the sheet thickness ratio exceeds 10, the spot weldability deteriorates, so the upper limit may also be made 10.

[0064] Next, the conduction steps will be explained.

Preliminary Conduction Step

[0065] The preliminary conduction step is a step aimed at gradually enlarging the contact area between steel sheets and gradually increases the amount of the welding current applied (for example, upslope conduction). If rapidly applying current, the current concentrates at a locally contacted part. This part rapidly melts and results in expulsion, so this conduction method runs current so that the amount of current gradually increases (for example upslope conduction) to avoid local heating.

[0066] The method of making the amount of application of welding current gradually increase is the method of making the current increase in upslope conduction (FIG. 1, FIG. 4, FIG. 5, FIG. 6, and FIG. 7). Further, in this case, the current value may be increased from 0, but also may be started from a certain specific current value.

[0067] In addition, for example, pulsation conduction also can be applied. If pulsation conduction, it is possible to alternately perform conduction and cooling and adjust the conduction time and cooling time to enable adjustment of the amount of input heat.

[0068] The current I0 of the pulsation conduction is made less than the current I1 of the first conduction step and continues for the time t0. I0=(0.5 to 0.8).Math.I1 is preferable, but the invention is not limited to this. It is possible to set the current I0 high and make the continuation time t0 shorter or to set the current I0 low and make the continuation time t0 longer.

[0069] For example, by making the conduction time 1 to 10 cycles and the idling time 1 to 5 cycles and repeating conduction and idling, it is possible to obtain effects similar to upslope conduction. Note that, the conduction time and idling time should be suitably set considering the extent by which the required effect is exhibited.

[0070] The conduction time tu should be 1 cycle or more. If the conduction time tu is less than 1 cycle, the above effect of the upslope conduction cannot be obtained, so the time is made 1 cycle or more.

[0071] Preferably, tu2Ta.sup.2Tg. The inventors conducted various tests and discovered that there is a correlation between the conduction time and average sheet thickness and maximum sheet gap and thereby derived the above relationship.

[0072] First Conduction Step

[0073] The first conduction step is a step for suppressing expulsion while using the heat generated due to electrical contact resistance between steel sheets to melt together a thin sheet and thick sheet by running a high and constant current under conditions where preliminary conduction secures a certain contact area between the steel sheets and contact resistance remains. After the current reaches the current I1 due to the upslope conduction or other preliminary conduction, the current I1 is then run at the time t1. By conduction at the current I1 and time t1, the contact resistance between steel sheets is utilized to promote the generation of heat between a thin sheet and thick sheet (temperature rise) and enlarge the melted part. The current I1 is set considering the sheet thicknesses of the set of sheets covered.

[0074] The conduction time t1 is made a time of a range securing the required amount of heat generation and free from the occurrence of expulsion and should be made at least 1 cycle or more. This is because if less than 1 cycle, the input heat is insufficient and sometimes no nugget will be formed.

[0075] Preferably, 1t17Ta. The inventors engaged in various tests and discovered that there is a correlation between the conduction time t1 and the average sheet thickness Ta and thereby derived the above relationship. If T1 is larger than 7Ta, sometimes expulsion occurs during the conduction time t1.

[0076] Further, the conduction current I1 is not particularly limited. However, the inventors discovered that there is a correlation between I1 and the average sheet thickness Ta. That is, I1 is preferably made 10(Ta)+2 or less. If I1 exceeds this value, the nugget rapidly grows and expulsion easily occurs. The lower limit of I1 is also not particularly limited. However, it is preferably made 10/(Ta)4 or more. If I1 is too small, growth of the nugget is not promoted and a nugget of a sufficient size cannot be obtained.

[0077] Second Conduction Step

[0078] This is a step of decreasing the welding current, suppressing the inside expulsion, and making the nugget grow mainly in the diametrical direction when a nugget is formed and grows to a certain extent at the first conduction step. Therefore, it is necessary to make 12<I1. The current I2 may be a current sufficient for promoting melting of the steel sheets, but should be set considering the sheet thicknesses of the set of sheets covered. From this viewpoint, the inventors looked for the relationship with the average sheet thickness whereupon they discovered that preferably I210(Ta). The lower limit is not particularly limited so long as an extent where a nugget grows, but is preferably made 10(Ta)6 or more.

[0079] The conduction time t2 at the second conduction step should be 1 cycle or more. This is so that a nugget is formed at the first conduction step and a certain extent of nugget size can be secured. The upper limit is also not particularly limited, but is preferably set to match the next third conduction step.

[0080] Third Conduction Step

[0081] This is a step where the current is made higher than the second conduction and the nugget is made to grow not only in the diametrical direction, but also the sheet thickness direction when the nugget size becomes a certain extent of size in the second conduction step. For this reason, in the third conduction step, a current I3 higher than the current I2 (>I2) is run during the time t3. Melting of the steel sheets is further promoted by conduction by the current I3 and time t3, that is, enlargement of the nugget (enlargement in both of lateral direction and sheet thickness direction) is promoted. Contact and melting of the steel sheets proceed until the end of the second conduction step, that is, the conduction area sufficiently increases (current density falls), so the level of current I3 where expulsion occurs rises. Therefore, I3 can be made larger than I2.

[0082] Furthermore, the inventors studied I3. That is, the current I3 should be set in the range where the nugget shape can be enlarged to the desired shape without causing expulsion. The inventors discovered that the current I3 should be set considering the average sheet thickness since there is an effect of the total sheet thickness. As a result, it was learned that preferably I3>10.Math.(Ta). The upper limit of I3 does not have to be particularly limited. It is sufficient to set it so that no expulsion occurs in spot welding.

[0083] The sum of the conduction time t2 and the conduction time t3 (t2+t3) is an important indicator in terms of the contact between the steel sheets and growth of the nugget. Usually, in spot welding two sheets, it is known that at about 10Ta (cycles) or 10Ta+2 (cycles), the temperature rise tends to become saturated. The inventors sought the relationship between the average sheet thickness and the conduction time from this viewpoint. As a result, they discovered that it is sufficient to make the total conduction time of t2 and t3 5Ta to 15Ta. If the total conduction time of t2 and t3 is shorter than 5Ta, since the nugget does not sufficiently grow, suitable spot welding cannot be obtained. On the other hand, if the total conduction time between t2 and t3 is longer than 15Ta, the temperature distribution becomes substantially steady, the nugget size becomes saturated, and the productivity ends up falling.

[0084] On the other hand, the second conduction step may be made longer, but the speed of growth of the nugget slows by the relatively small amount of the welding current. For this reason, there is a possibility that the welding time for obtain suitable spot welding will become longer. For this reason, t3 may be made longer than t2.

[0085] At the third conduction step, the object is to make the nugget grow. It is particularly important to make it grow in the sheet thickness direction. Therefore, by not rapid heating, but by making the amount of heat input gradually increase in the same way as the preliminary heating, it is possible to suppress the occurrence of expulsion. The method of gradually increasing the input heat amount is not particularly limited, but it is preferable to make the welding current increase by upslope conduction. For example, the third conduction current I3 may be made a function of the conduction time. For example, by making it a primary function of the conduction time, a monotonously increasing upslope conduction pattern is obtained (FIG. 6 and FIG. 7).

[0086] By making the third conduction step upslope conduction, it is possible to gradually grow a nugget, so this can also serve as the second conduction step. That is, in this case, it is possible to make the second conduction step a short time. For example, it is possible to make t2=1 (cycle).

[0087] Furthermore, the inventors discovered that when making the nugget grow in the sheet thickness direction, it is sufficient to reduce the cooling effect by the electrodes. That is, the electrode itself is water-cooled, so the temperature in the sheet thickness direction becomes lowest at the parts in contact with the electrode. For this reason, the nugget is difficult to grow in the sheet thickness direction.

[0088] Therefore, the inventors discovered that when making the nugget grow in the sheet thickness direction in the third conduction step, by decreasing the pressing force by the electrodes and decreasing the contact area between the electrodes and steel sheets to raise the current density and simultaneously weaken the cooling effect by the electrodes, the nugget grows more effectively in the sheet thickness direction as well (FIG. 5 and FIG. 7). The lower limit value of the pressing force along with the decrease in the pressing force is not particularly limited. However, if making the pressing force decrease too much, expulsion occurs, so the pressing force is preferably made half () of before the decrease or more. That is, if designating the pressing force before decreasing the pressing force, that is, at the first conduction step and the second conduction step, as P1 and the pressing force after decreasing it P2, the pressing force should be P1P2<P1. Further, preferably P1P2<P1. The upper limit of the pressing force P2 after decrease should be smaller than P1, but to reliably obtain this effect, 0.9P1 should be made the upper limit.

[0089] The timing of decreasing the pressing force is not particularly limited so long as during the third conduction. However, if entering the stage of growth of the nugget in the sheet thickness direction, that is, the third conduction, it is preferable to make the pressing force decrease as fast as possible. For example, the pressing force should be made to decrease within 3 cycles after the start of the third conduction. It is more preferably performed within 1 cycle.

[0090] FIGS. 4 to 7 show the case of combination of the conduction pattern and pattern of the pressing force. FIG. 4 and FIG. 5 are cases where the third conduction step is constant current conduction, while FIG. 6 and FIG. 7 are cases where the third conduction pattern is upslope conduction. Further, FIG. 4 and FIG. 6 show the case where the pressing force is constant, while FIG. 5 and FIG. 7 are cases of reducing the pressing force in the third conduction step.

[0091] Cooling Step

[0092] Between the first conduction step and second conduction step and/or between the second conduction step and third conduction step, a cooling step where no welding current is run may also be provided (FIG. 8 and FIG. 9).

[0093] By providing the cooling step, the nugget growth is eased, the occurrence of expulsion is suppressed, and the nugget formation becomes stable.

[0094] If providing the cooling step, compared with spot welding performing the first conduction, second conduction, and third conduction consecutively, the welding completion time becomes longer by the amount of the cooling time and the productivity appears to fall, but by providing the cooling step, it is possible to set the current I2 and/or current I3 high and shorten the conduction time t2 and/or conduction time t3 at the second conduction step and/or third conduction step, so the productivity does not fall. FIG. 8 shows an example of adding a cooling step to the conduction pattern shown in FIG. 1. FIG. 9 shows one example of the case where the preliminary conduction step of FIG. 8 is pulsation conduction.

[0095] FIG. 2 schematically shows the form of a welded joint formed by the present invention welding method from a set of sheets of three superposed steel sheets (thin steel sheet at outside).

[0096] In a set of sheets comprised of three superposed steel sheets, if an outside steel sheet is the thinnest in sheet thickness, sometimes this thin steel sheet and the adjoining steel sheet are not sufficiently joined at the contact interface, but in the present invention welding method, even if the sheet thickness of the outside steel sheet is the thinnest, the contact surfaces of the outside thin steel sheet and adjoining steel sheet can be strongly joined.

[0097] According to the present invention, even if there is a sheet gap between steel sheets, a nugget is reliably formed between a thin sheet and thick sheet. The size of the nugget can be found by cutting the set of sheets along the line passing through the center of the spot welding after the spot welding, polishing the cross-sectional surface, etching it, then observing it by an optical microscope.

[0098] According to the present invention welding method, as shown in FIG. 2, the nugget 4 is formed across the thin steel sheet and two relatively thick steel sheets. FIG. 3 shows the cross-sectional structure of the welded joint formed by the present invention welding method observed by an optical microscope. It can be confirmed that the nugget is formed passing through the three steel sheets and the interfaces between all steel sheets are sufficiently melted. In particular, the thinnest steel sheet is at the outside, but it is understood that the nugget is also formed including this steel sheet.

EXAMPLES

[0099] Next, examples of the present invention will be explained, but the conditions in the examples are illustrations of the conditions employed for confirming the workability and effects of the present invention. The present invention is not limited to these illustrations. The present invention can utilize various conditions so long as not departing from the gist of the present invention and achieving the object of the present invention.

Example 1

[0100] Using the two types of sets of sheets shown in Table 1, spot welding was performed. The steel sheets were all galvannealed steel sheets. The amount of deposition per side was 45 g/m.sup.2. The spot welding conditions are shown in Table 2. For the welding power source, a single phase alternating current was used.

[0101] For the electrodes, a CrCu DR type having a nominal size of 16 mm, a tip diameter of 6 mm, and a tip R of 40 mm was used. The electrode holding time after three stages of conduction was made 5 cycles in all cases. After welding, the set of sheets was cut along a line passing through the center of the spot welding. The cut surface was polished and etched, then the size of the nugget was measured by an optical microscope. The measurement results are shown in Table 3 together with the occurrence of expulsion. A nugget of (Nugget size between Steel Sheets 1 and 2)4(sheet thickness of Steel Sheet 1) was judged as passing.

Example 2

[0102] Under the same conditions as Example 1, spot welding was performed by the conduction pattern and pressing force pattern of FIG. 5. The spot welding conditions and results of evaluation are shown in Table 4. For the welding power source, single-phase alternating current was used.

[0103] As a result, no expulsion occurred and a good nugget was formed. Note that, a nugget of a nugget size between steel sheets of 4(sheet thickness of Steel Sheet 1 (thinnest steel sheet)) was judged as passing. Below, the same criteria were used for judgment in Examples 3, 4, and 5 as well.

Example 3

[0104] Under the same conditions as Example 1, spot welding was performed by the conduction pattern and pressing force pattern of FIG. 6. The spot welding conditions and results of evaluation are shown in Table 5. For the welding power source, single-phase alternating current was used.

[0105] As a result, no expulsion occurred and a good nugget was formed.

Example 4

[0106] Under the same conditions as Example 1, spot welding was performed by the conduction pattern and pressing force pattern of FIG. 7. The spot welding conditions and results of evaluation are shown in Table 6. For the welding power source, single-phase alternating current was used.

[0107] As a result, no expulsion occurred and a good nugget was formed.

Example 5

[0108] Using the two types of sets of sheets shown in Table 7, under the same conditions as Example 1, spot welding was performed provided with a cooling step. The cooling in the table shows the cooling step. The steel sheets were all galvannealed steel sheets. The amount of deposition per side was 45 g/m.sup.2.

[0109] The spot welding conditions are shown in Table 8 and the results of evaluation are shown in Table 9. For the welding power source, single-phase alternating current was used.

[0110] As a result, no expulsion occurred and a good nugget was formed.

TABLE-US-00001 TABLE 1 Steel Sheet 1 Steel Sheet 2 Steel Sheet 3 Sheet Tensile Sheet Tensile Sheet Tensile Set of thickness strength thickness strength thickness strength sheets (mm) (MPa) (mm) (MPa) (mm) (MPa) A 0.7 305 1.6 1513 1.6 1024 B 0.6 310 2.0 1026 1.6 610

TABLE-US-00002 TABLE 2 Gap Steel between Electrode Upslope Treat- Sheet 1 steel pressing conduction Conduction Current Conduction Current Conduction Current ment Set of thickness sheets force time tu time t1 I1 time t2 I2 time t3 I3 no. sheets (mm) (mm) (kgf) (cycles) (cycles) (kA) (cycles) (kA) (cycles) (kA) Remarks 1 A 0.7 1.5 400 3.0 4 10 10 8.5 7 9.5 Inv. ex. 2 A 0.7 1.5 400 3.0 4 10 10 10 7 9.5 Comp. ex. 3 A 0.7 1.5 400 3.0 4 10 10 8.5 7 8.5 Comp. ex. 4 A 0.7 1.5 400 0.0 4 10 10 8.5 7 9.5 Comp. ex. 5 A 0.7 2 400 10.0 4 10 10 8.5 7 9.5 Inv. ex. 10 B 0.6 1.5 450 5.0 4 11 12 9.0 8 10.0 Inv. ex. 11 B 0.6 1.5 450 5.0 4 11 12 11.0 8 10.0 Comp. ex. 12 B 0.6 1.5 450 5.0 4 11 12 9.0 8 9.0 Comp. ex. 13 B 0.6 1.5 450 15.0 4 11 12 9.0 8 10.0 Inv. ex.

TABLE-US-00003 TABLE 3 Nugget size Nugget Nugget size size Occur- 4 between between rence (thickness Steel Steel Treat- of of Steel Sheets Sheets ment Set of expul- Sheet 1) 1 and 2 2 and 3 no. sheets sion (mm) (mm) (mm) Remarks 1 A No 3.35 4.7 8.1 Inv. ex. 2 A Yes 3.35 3.1 6.5 Comp. ex. 3 A No 3.35 3.2 7.3 Comp. ex. 4 A Yes 3.35 2.8 6.4 Comp. ex. 5 A No 3.35 4.6 8.0 Inv. ex. 10 B No 3.10 4.5 8.3 Inv. ex. 11 B Yes 3.10 2.9 6.9 Comp. ex. 12 B No 3.10 3.0 7.2 Comp. ex. 13 B No 3.10 4.5 8.1 Inv. ex.

TABLE-US-00004 TABLE 4 Steel Gap Electrode Electrode Upslope Sheet 1 between pressing pressing conduction Conduction Conduction Treatment Set of thickness steel sheets force P1 force P2 time tu time t1 Current time t2 no. sheets (mm) (mm) (kgf) (kgf) (cycles) (cycles) I1 (kA) (cycles) 1 A 0.7 2 400 300 10 4 10 10 13 B 0.6 1.5 450 350 15 4 11 12 Nugget size Nugget size 4 between between (thickness Steel Steel Conduction Generation of Steel Sheets Sheets Treatment Current time t3 Current of Sheet 1) 1 and 2 2 and 3 no. I2 (kA) (cycles) I3 (kA) expulsion (mm) (mm) (mm) Remarks 1 8.5 5 9.5 No 3.3 4.3 8.1 Inv. ex. 13 9.0 6 10.0 No 3.1 4.5 8.2 Inv. ex.

TABLE-US-00005 TABLE 5 Steel Gap Electrode Electrode Upslope Sheet 1 between pressing pressing conduction Conduction Conduction Treatment Set of thickness steel sheets force P1 force P2 time tu time t1 Current time t2 no. sheets (mm) (mm) (kgf) (kgf) (cycles) (cycles) I1 (kA) (cycles) 1 A 0.7 2 400 10 4 10 3 (P1 constant) 13 B 0.6 1.5 450 15 4 11 3 (P1 constant) Nugget size Nugget size 4 between between (thickness Steel Steel Conduction Generation of Steel Sheets Sheets Treatment Current time t3 Current of Sheet 1) 1 and 2 2 and 3 no. I2 (kA) (cycles) I3 (kA) expulsion (mm) (mm) (mm) Remarks 1 8.5 12 12.0 No 3.3 4.2 8.0 Inv. ex. 13 9.0 12 12.0 No 3.1 4.4 8.1 Inv. ex.

TABLE-US-00006 TABLE 6 Steel Gap Electrode Electrode Upslope Sheet 1 between pressing pressing conduction Conduction Conduction Treatment Set of thickness steel sheets force P1 force P2 time tu time t1 Current time t2 no. sheets (mm) (mm) (kgf) (kgf) (cycles) (cycles) I1 (kA) (cycles 1 A 0.7 2 400 300 10 4 10 3 13 B 0.6 1.5 450 350 15 4 11 3 Nugget size Nugget size 4 between between (thickness Steel Steel Conduction Occurrence of Steel Sheets Sheets Treatment Current time t3 Current of Sheet 1) 1 and 2 2 and 3 no. I2 (kA) (cycles) I3 (kA) expulsion (mm) (mm) (mm) Remarks 1 8.5 10 12.0 No 3.3 4.3 8.1 Inv. ex. 13 9.0 10 12.0 No 3.1 4.5 8.3 Inv. ex.

TABLE-US-00007 TABLE 7 Steel Sheet 1 Steel Sheet 2 Steel Sheet 3 Sheet Tensile Sheet Tensile Sheet Tensile Set of thickness strength thickness strength thickness strength sheets (mm) (MPa) (mm) (MPa) (mm) (MPa) A 0.7 317 1.8 1515 1.8 1024 B 0.6 313 2.1 1025 1.8 610

TABLE-US-00008 TABLE 8 Gap Steel between Electrode Upslope Sheet 1 steel pressing conduction Conduction Treatment Set of thickness sheets force time tu time t1 Current no. sheets (mm) (mm) (kgf) (cycles) (cycles) I1 (kA) 1 A 0.7 1.5 450 5 3 10.5 2 A 0.7 1.5 450 5 3 10.5 3 A 0.7 1.5 450 5 3 10.5 4 A 0.7 1.5 450 5 3 10.5 5 A 0.7 1.5 450 5 3 10.5 6 A 0.7 1.5 450 0 3 10.5 7 A 0.7 2 450 10 3 10.5 10 B 0.6 1.5 500 5 4 11.5 11 B 0.6 1.5 500 5 4 11.5 12 B 0.6 1.5 500 5 4 11.5 13 B 0.6 1.5 500 15 4 11.5 20 A 0.7 1.5 450 (7kAx3 4 10.5 cycles + no conduction 2 cycles) x3 21 A 0.7 1.5 450 (8kAx2 4 10.5 cycles + no conduction 1 cycle) x3 Conduction Conduction Treatment Cooling time t2 Current Cooling time t3 Current no. (cycles) (cycles) I2 (kA) (cycles) (cycles) I3 (kA) Remarks 1 2 9 9.0 0 7 9.5 Inv. ex. 2 0 10 8.5 2 6 10.5 Inv. ex. 3 2 9 9.0 2 6 10.5 Inv. ex. 4 2 9 10.5 2 6 10.5 Comp. ex. 5 2 9 8.5 2 6 8.5 Comp. ex. 6 2 9 8.5 2 6 10.5 Comp. ex. 7 2 9 8.5 2 6 10.5 Inv. ex. 10 2 11 9.5 2 7 11.0 Inv. ex. 11 2 11 11.5 2 7 11.0 Comp. ex. 12 2 11 9.5 2 7 9.5 Comp. ex. 13 2 11 9.5 2 7 10.0 Inv. ex. 20 2 9 8.5 2 6 10.5 Inv. ex. 21 2 9 8.5 2 6 10.5 Inv. ex.

TABLE-US-00009 TABLE 9 Nugget Nugget size size Occur- 4 between between rence (thickness Steel Steel Treat- of of Steel Sheets Sheets ment Set of expul- Sheet 1) 1 and 2 2 and 3 no. sheets sion (mm) (mm) (mm) Remarks 1 A No 3.35 4.8 8 Inv. ex. 2 A No 3.35 5.0 8.1 Inv. ex. 3 A No 3.35 4.7 8 Inv. ex. 4 A Yes 3.35 3.3 6.4 Comp. ex. 5 A No 3.35 3.2 7.3 Comp. ex. 6 A Yes 3.35 3.0 6.3 Comp. ex. 7 A No 3.35 4.8 8.1 Inv. ex. 10 B No 3.10 4.7 8.2 Inv. ex. 11 B Yes 3.10 2.9 6.4 Comp. ex. 12 B No 3.10 2.7 7.1 Comp. ex. 13 B No 3.10 4.7 7.8 Inv. ex. 20 A No 3.35 5.0 8 Inv. ex. 21 A No 3.35 4.9 7.9 Inv. ex.

INDUSTRIAL APPLICABILITY

[0111] As explained above, according to the present invention, in a set of three sheets with a high sheet thickness ratio including a high strength thick steel sheet, even if there is gap in the set of sheets, it is possible to suppress the occurrence of expulsion, secure the required nugget size to stably form a required shape of nugget, and secure the desired joint strength. Accordingly, the present invention has high applicability in welded structure manufacturing industries.

REFERENCE SIGNS LIST

[0112] 1. thin steel sheet
2, 3. steel sheet
4. nugget