JP6451463B2 - Stack spot welding electrode of steel plates with different thickness and spot welding method using the electrode - Google Patents

Description

  The present invention relates to a superposition resistance spot welding electrode for steel plates, and in particular, in assembling welding in the automotive field, when two or more steel plates having a large thickness difference are superposed and resistance spot welding is performed, the welding strength is ensured. The present invention relates to a spot welding electrode that can easily form a necessary nugget at a joint, and a spot welding method using the same.

  Resistance spot welding is mainly used in the joining of stacked steel sheets in the assembly of automobile bodies and the mounting of parts. In spot welding, two or more steel plates are overlapped and energized while pressing the electrodes so that a plurality of steel plates are sandwiched from both sides to form a molten metal. The molten metal is cooled and solidified by heat conduction to form a welded portion (nugget) having an elliptical cross section between the steel plates.

  In such spot welding, the nugget grows from the center of the total plate thickness. For example, in the case of two plate assemblies with a small plate thickness difference, the overlapping surface of the steel plate and the growth starting point of the nugget are almost the same position, Nuggets grow on the overlapped surface, and high joint strength is obtained.

  On the other hand, in an automobile body, a thin steel plate with a thickness of 1 mm or less is used as an outer panel, and a thick steel plate with a thickness of more than 1 mm is used as a reinforcement, member, or inner panel, and the panel, reinforcement, member, or inner plate. In some cases, three or more steel plates (plate assemblies) on which a panel is superimposed are spot-welded.

  When the plate thickness of the steel plate placed on the outermost layer (on the spot electrode side) is thinner than the plate thickness of other steel plates (when the plate thickness difference is large), such as a plate assembly in such a car body, the spot The nugget formed by welding grows from the center of the total plate thickness, and it is difficult for the nugget to grow on the overlapping surface of the thin steel plate and the thick steel plate, which are important for securing the bonding strength. That is, a nugget is formed between the reinforcement and the member or the inner plate panel, but the nugget is hardly formed on the outer plate panel, and the reinforcement and the inner plate and the outer plate panel cannot be welded. .

  The reason why this nugget is difficult to grow will be described with reference to FIG. FIG. 1 shows a situation where spot welding is performed on a plurality of steel plates having a large plate thickness difference using a conventional spot welding electrode. The conventional spot welding electrode 1 includes, for example, a pair of spot welding electrodes 1a and 1b. For convenience, the spot welding electrode 1a on the side in contact with the thin steel plate may be referred to as an upper spot welding electrode, and the other spot welding electrode 1b may be referred to as a lower spot welding electrode. FIG. 1 shows a state in which a nugget 5 is formed by sandwiching a plate assembly 4 in which thin steel plates 2 and thick steel plates 3a and 3b are overlapped and energized and heated. In addition, the arrow has shown the flow of electricity.

  The spot welding electrodes 1a and 1b are provided with cooling means 6a and 6b inside. In the welding of a plate assembly having a large difference in plate thickness, when spot welding electrodes 1a and 1b having the same shape and the same material are used for welding the plate assembly having the same thickness, they are received from the electrodes 1a and 1b. Since the cooling effect is the same for the upper and lower sides, heat transfer from the welded spot to the spot welding electrodes 1a and 1b, that is, heat removal occurs, and the welded spot close to the spot welding electrode 1a (superposed surface of the thin steel plate 2 and the thick steel plate 3a). Nugget 5 is difficult to grow. As a result, the nugget 5 is formed by generating heat from the center between the electrodes.

  Therefore, in spot welding of two or more steel plates with a large difference in plate thickness, a technique has been reported in which the shape of the spot welding electrode is devised in order to form a desired nugget on the overlapping surface of the thin steel plate and the thick steel plate. Yes.

  In Patent Document 1, an electrode having a convex curved tip is brought into contact with a thin plate arranged on the outermost layer, and an electrode having a depressed tip is brought into contact with a thick plate arranged on the other outermost layer in an annular shape. By making resistance spot welding by reducing the contact area with the electrode on the thin plate side compared to the thick plate side, the current density is increased on the thin plate side and a large diameter nugget reaching the thin plate side is formed. Technology is disclosed.

  In Patent Document 2, the contact area between one electrode chip located on the plate material side having the lowest rigidity and the work is made smaller than the contact area between the other electrode chip and the work, and the plate material having the lowest rigidity and the other work piece are connected to each other. A technique for improving the welding strength of a workpiece by increasing the current density in the energization path of the contact portion between the plate members and flowing a large current locally to the contact portion to increase the heat generation amount is disclosed. Yes.

Japanese Patent No. 5187806 JP 2003-251468 A

  The techniques disclosed in Patent Documents 1 and 2 are effective techniques in that a desired nugget can be formed on the overlapping surface of a thin steel plate and a thick steel plate in spot welding of two or more steel plates having a large thickness difference. . However, in the technique disclosed in Patent Document 1, when many spots are continuously spot-welded with the same electrode, the recess formed at the tip of the electrode is worn, and the electrode life is shorter than that of a conventional spot-welded electrode. There is a concern that the number of spots until the predetermined welding strength cannot be obtained when the spot welding is performed, that is, when spot welding is performed continuously by spot welding.

  Further, in the technique disclosed in Patent Document 2, when spot welding is performed continuously, the tip of the electrode brought into contact with the thin steel plate is worn and the area becomes large, and the difference between the area of the tip of the electrode brought into contact with the thick steel plate is small. In such a case, there is a concern that nuggets are not formed on the thin steel plate and the thick steel plate.

  Therefore, in view of such a situation, the present invention can ensure the welding strength of the welded joint when two or more steel plates having a large thickness difference are overlapped and spot-welded, and the electrode life is not reduced. An object is to provide a spot welding electrode.

  Therefore, the present inventors diligently studied a method for solving the above problems. First, the shape of the spot welding electrode is changed to a shape that can suppress the heat removal by the spot welding electrode and the spread of the energization path in the steel plate (suppressing the decrease in the energization density). Tried to form a nugget.

  The present inventors performed grooving in the vicinity of the tip of the spot welding electrode to be brought into contact with the thin steel plate, and formed a constricted portion. As a result, heat removal by the electrode can be suppressed, and the current path in the electrode can be narrowed (current) As a result, it was found that the spread of the current path on the overlap surface of the thin steel plate and the thick steel plate is suppressed, and the nugget is formed on the overlap surface of the thin steel plate and the thick steel plate. did.

  Furthermore, when the shape of the constricted part of the spot welding electrode was examined, the contact area of the spot welding electrode with the thin steel plate, the length from the tip of the spot welding electrode to the constricted part, and the minimum cross-sectional area of the constricted part were specified. It was found that the nugget can be stably formed on the overlapping surface of the thin steel plate and the thick steel plate by making the shape satisfying the above relationship.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) In a spot welding electrode used for welding a plurality of steel plates in which a steel plate having a thickness smaller than that of the other steel plate is superimposed on at least one outermost layer of the plurality of stacked steel plates, at least the steel plate having a small thickness One spot welding electrode that contacts the constricted portion has a constricted portion, the minimum cross-sectional area of the constricted portion is B [mm ² ], and the contact area of the spot welding electrode having the constricted portion with the thin steel plate is S [ mm ² ], and the length from the tip of the spot welding electrode having the constricted portion to the constricted portion is C [mm], the spot welding electrode satisfying the relationship of the following formula (1).
(B ÷ S) ≦ 6.2 × exp (−0.5 × C) (1)
(2) A spot welding method using the spot welding electrode according to (1), wherein at least the plate thickness is thin when the plurality of metal plates are sandwiched between the spot welding electrodes and energized to perform welding. A spot welding method comprising contacting a spot welding electrode having the constricted portion with a steel plate.

  According to the present invention, the nugget is stably formed on the overlapping surface of the thin steel plate and the thick steel plate in a plurality of the steel plates stacked two or more including the thin steel plate in the outermost layer without reducing the electrode life. be able to.

It is a figure which shows the condition which spot welds several steel plates with a big plate | board thickness difference using the conventional spot welding electrode. It is a figure which shows the condition which carries out the spot welding of the some steel plate with a big plate | board thickness difference using the spot welding electrode which has a constriction part. It is a figure which shows the outline of the spot welding electrode which has a constriction part. It is a figure which shows the relationship of (constriction part cross-sectional area B [mm < ² >]) / (tip contact area S [mm < ² >]) with respect to front-end | tip part length C [mm].

  The spot welding electrode of the present invention (hereinafter also referred to as “welding electrode of the present invention”) is sometimes referred to as another steel plate (hereinafter referred to as “thick steel plate”) in at least one outermost layer of a plurality of stacked steel plates. ) Out of a plurality of steel plates (hereinafter also referred to as “plate assemblies”) each having a thinner steel plate (hereinafter also referred to as “thin steel plates”), they are brought into contact with each other during welding. A spot welding electrode having a constricted portion.

In the welding electrode of the present invention, the minimum cross-sectional area of the constricted portion of the spot welding electrode is B [mm ² ], the contact area of the spot welding electrode with the thin steel plate is S [mm ² ], and the spot having the constricted portion When the length from the tip of the welding electrode to the constricted portion is C [mm], the following relationship (1) is satisfied.
(B ÷ S) ≦ 6.2 × exp (−0.5 × C) (1)

  Since the welding electrode of the present invention has the same shape and area of the contact portion with the steel plate as that used in the conventional spot welding electrode, it is possible to overlap the thin steel plate and the thick steel plate without reducing the electrode life. The nugget can be stably formed on the surface.

  Hereinafter, the background of the study that led to the welding electrode of the present invention will be described, and the welding electrode of the present invention will be described.

  In spot welding of two or more steel plates having a large difference in plate thickness, a technique capable of stably forming a desired nugget on the overlapping surface of thin steel plates and thick steel plates is desired. A technique for making the shape of the tip portion an appropriate structure has been reported. The shape of the tip portion of the spot welding electrode is related to the electrode life, and it was necessary to consider that the electrode life is not reduced.

  Therefore, the present inventors examined the shape of a spot welding electrode that can form a desired nugget on the overlapping surface of a thin steel plate and a thick steel plate, and formed a constricted portion by performing groove processing near the tip of the spot welding electrode. Inspired to do. Therefore, the following test was performed.

  First, a plate assembly in which one thin steel plate with a thickness of 0.6 mm and two steel plates with a thickness of 1.4 mm (thick steel plates) are used and the three steel plates are overlapped so that the thin steel plate is the outermost layer. Prepared. And the said board assembly was spot-welded using the spot welding electrode which has a constriction part. The prepared test piece was cut in the thickness direction so that the nugget was included, and the presence or absence of the nugget on the overlap surface of the thin steel plate and the thick steel plate was observed with a magnifier. As a result, a desired nugget was formed on the overlapping surface of the thin steel plate and the thick steel plate.

  FIG. 2 shows a situation in which spot welding is performed on a plurality of steel plates having a large difference in plate thickness using a spot welding electrode having a constricted portion. The spot welding electrode 11 having a constricted portion is composed of a pair of spot welding electrodes 11a and 11b, similarly to the conventional spot welding electrode 1. For convenience, the spot welding electrode 11a on the side in contact with the thin steel plate may be referred to as an upper spot welding electrode, and the other spot welding electrode 11b may be referred to as a lower spot welding electrode.

  The spot welding electrodes 11a and 11b are respectively provided with cooling means 16c and 16d, and arrows indicate the flow of electricity. The spot welding electrode 11 a includes an electrode main body portion 17, a constricted portion 18, and a tip end portion 19. When the nugget 15 is formed by energizing and heating the plate assembly 14 using the spot welding electrode 11a having the constricted portion 18, the nugget 15 grows from the center of the total plate thickness, but the constricted portion 18 is formed. The amount of heat transferred from the welding location to the spot welding electrode 11a is suppressed. As a result, the nugget 15 grows on the overlapping surface of the outermost thin steel plate 12 and the thick steel plate 13a.

  In addition, due to the formation of the constricted portion 18, the energization path of the electrode main body portion 17 is narrowed by the constricted portion 18, whereby the spread of the energization path is suppressed on the overlapping surface of the thin steel plate 12 and the thick steel plate 13 a. That is, the current density on the overlapping surface of the thin steel plate 12 and the thick steel plate 13a is higher than that in the case where a conventional spot welding electrode is used. Therefore, formation of the nugget 15 is promoted by the overlapping surface of the thin steel plate 12 and the thick steel plate 13a.

  Next, the relationship between the shape of the constricted portion formed on the spot welding electrode and the formation of the nugget on the overlapping surface of the thin steel plate and the thick steel plate was investigated. First, FIG. 3 shows an outline of a spot welding electrode having a constricted portion.

Spot welding electrode 11a having a constricted portion 18, was prepared by grooving a DR-type electrode of the electrode diameter L _E. The spot welding electrode 11a is configured to be connected in the order of the electrode main body portion 17, the constricted portion 18, and the distal end portion 19 in the axial direction. Constriction 18 has a cylindrical shape with a diameter L _B. The tip of the tip 19 is provided with a contact portion 20 that is formed by a gently curved surface and comes into contact with a thin steel plate. The contact portion 20 has a curved surface diameter L _s .

The cross-sectional area obtained than the diameter L _B, the minimum cross-sectional area B of the constricted portion 18 [mm ^2] (hereinafter, sometimes referred to as "constricted cross-sectional area") was set. The contact portion 20 is viewed in plan view from the electrode axis direction (direction from the electrode main body portion 17 toward the tip portion 19), and the area of the circle having the curved surface diameter L _s is defined as the contact area S [mm ² ] (hereinafter, “tip contact area”). Sometimes). The length from the tip of the tip 19 to the end of the constricted portion 18 was the length C [mm] of the tip 19 (hereinafter sometimes referred to as “tip length”).

Details will be shown in the examples described later, but one sheet steel with a thickness of 0.6 mm and two steel sheets (thick steel sheets) with a thickness of 1.4 mm are used so that the sheet steel is the outermost layer. A plate assembly in which a plurality of steel plates were overlapped was prepared. And the spot welding electrode which variously changed the constriction part cross-sectional area B [mm < ² >] and front-end | tip part length C [mm] was created, and the said board assembly was spot-welded. The prepared test piece was cut in the plate thickness direction at the center of the spot hitting point, and the presence or absence of the nugget on the overlap surface of the thin steel plate and the thick steel plate and the length (nugget diameter) were observed with a magnifier.

FIG. 4 shows a relationship of (constriction section sectional area B [mm ² ]) / (tip contact area S [mm ² ]) with respect to the tip length C [mm]. In FIG. 4, data when a nugget is formed is displayed as a square, and data when a nugget is not formed is displayed as a cross. The curve equation shown in FIG. 4 is the following equation (2).
(B ÷ S) = 6.2 × exp (−0.5 × C) (2)

From FIG. 4, it was found that a nugget is formed if C / B is equal to or less than a specific value. When this is expressed by a mathematical expression of (neck portion cross-sectional area B [mm ² ]) / (tip contact area S [mm ² ]) with respect to the tip length C [mm] where the nugget is formed, the following equation (1) Become.
(B ÷ S) ≦ 6.2 × exp (−0.5 × C) (1)

  The present invention has reached the invention described in the above (1) through the examination process as described above, and the necessary and preferred requirements will be further described in order.

The welding electrode of the present invention is attached to the tip of a spot welding gun mounted on a welding robot of a spot welding apparatus, for example, and is used for spot welding of a plurality of steel plates in which a thin steel plate is superimposed on at least one outermost layer. is there. And the spot welding electrode which contacts a thin steel plate has a constriction part, tip part length C [mm], constriction part cross-sectional area B [mm < ² >], and tip contact area S [mm < ² >] of said (1) type | formula. First, the spot welding electrode will be described.

<Spot welding electrode>
A spot welding electrode that sandwiches and welds a plate assembly will be described with reference to FIGS.
The spot welding electrode 11 has an upper spot welding electrode 11a and a lower spot welding electrode 11b. The welding electrode according to the present invention is characterized by the structure of the spot welding electrode 11a that comes into contact with the thin steel plates superimposed on the outermost layer. The upper spot welding electrode 11 a has an electrode main body portion 17, a constricted portion 18, and a tip end portion 19. Of the spot welding electrodes, one of the spot welding electrodes is provided with a constricted portion, and the outermost thin steel plate and the electrode are brought into contact with each other as described below. However, the thin steel plates are overlapped on both surfaces of the outermost layer. It is also possible to provide spot welding on both spot welding electrodes and spot welding.

(Electrode body of spot welding electrode)
Electrode main body portion 17 is not intended to particularly limited like shape, for example, electrode diameter L _E is the cylindrical shape of 12～16Mm. The electrode main body 17 is connected at one end to the constricted portion 18 in the electrode axial direction (the direction from the electrode main body 17 toward the constricted portion 18).

(Neck part of spot welding electrode)
Constriction 18 which connects the end portion of the electrode main body portion 17, toward the electrode main body 17 to the tip 19, consisting of a portion diameter L _E of the electrode main body 17 is reduced, the equivalent diameter is the diameter L _E It is a range to a part (one end part of the front-end | tip part 19). Constriction 18 are those having a diameter L _E smaller diameter L _B of the electrode main body 17, in a cross section perpendicular to the electrode axis, the cross-sectional area of the constricted portion 18, the electrode main body portion 17 and the tip 19 It is smaller than the cross-sectional area. Further, the cross-sectional area ratio between the electrode main body 17 and the constricted portion 18 is preferably 0.06 or more and 0.80 or less from the durability strength at the time of pressurization of spot welding. 0.3 or more is more preferable.

  In addition, in the cross-sectional area of the constricted portion 18, if the minimum cross-sectional area B satisfies the relationship of the above formula (1), the cross-sectional area of the constricted portion 18 is the same over the electrode axis direction of the constricted portion 18. It may be different or different. The portion of the constricted portion 18 having the smallest cross-sectional area may be any portion of the constricted portion. Further, the cross-sectional shape of the constricted portion 18 is not particularly limited, and may be the same shape as the electrode main body portion 17, for example, a circular shape.

(The tip of the spot welding electrode)
The tip portion 19 connected to the constricted portion 18 has a cylindrical portion having a shape equivalent to that of the electrode main body portion 17 and a contact portion 20. The constricted portion 18 is connected to the cylindrical portion of the tip portion 19. The diameter of the cylindrical portion is, for example, 12 to 16 mm, similarly to the electrode main body portion 17. The length C from the tip of the tip 19 to the end of the constricted part 18 is not particularly limited as long as it satisfies the relationship of the above formula (1), and is exemplified by 1 to 6 mm. 2-5 mm is preferable. If the tip length C exceeds 6 mm, the spread of the energization path may become large, and the nugget may not be formed on the overlapping surface of the thin steel plate and the thick steel plate. If it is less than 1 mm, the durability is low and the electrode life may be reduced.

The shape of the contact portion 20 of the tip portion 19 is not particularly limited, and can be the same shape as the tip portion of a DR type, CF type, D type electrode or the like. The contact area of the contact part 19 with the thin steel sheet (the area of the contact part 20) shall satisfy the relationship of the above formula (1). The area of the contact portion 20 can be obtained by using a pressure sensitive paper and applying a predetermined pressure and measuring the indentation area obtained with the pressure sensitive paper. Furthermore, when the contact part 20 is a flat surface, it is good also as an area of the flat surface. When the contact portion 20 is a curved surface, the area of the contact portion 20 may be obtained using the curved surface diameter L _s with the curved surface as a part of a spherical body having a predetermined radius. However, when the shape of the contact portion 20 is DR type and R40 or more, the contact surface of the contact portion 20 is a circle having a curved surface diameter Ls when viewed in plan from the electrode axis direction, and the area of the circle is defined as the contact area. It is good.

(B / S relationship to C)
The spot welding electrode satisfies the relationship of the following formula (1) with respect to the tip length C [mm]: (neck portion cross-sectional area B [mm ² ] / tip contact area S [mm ² ]).
(B ÷ S) ≦ 6.2 × exp (−0.5 × C) (1)

  As described with reference to FIG. 4, a nugget is formed on the overlapping surface of the thin steel plate and the thick steel plate by using the spot welding electrode that satisfies the relationship of the expression (1). Further, the lower limit of B / S is not particularly limited, but is preferably 0.1 or more in order to increase the current density on the overlapping surface of the thin steel plate and the thick steel plate. More preferably, it is 0.5 or more.

(Material for spot welding electrode)
The material of the spot welding electrode is not particularly limited, and examples thereof include Cu, Cu—Cr alloy, and alumina dispersed Cu.

<Multiple steel plates>
Next, a plurality of steel plates (plate assemblies) that are spot-welded will be described.
The plate assembly is not particularly limited as long as it includes a thin steel plate superimposed on at least one outermost layer. The plate thickness of each steel plate spot-welded can be 0.5 to 2.6 mm, and the total plate thickness of the plurality of steel plates can be 1.0 to 6.0 mm. The thicknesses of the thick steel plates and the thin steel plates may be different or the same. The plate thickness ratio (total plate thickness / thick plate thickness) can be 3-8.

  In FIG. 1 and FIG. 2, three steel plates are described as the plurality of spot-welded steel plates, but two or more steel plates can be used depending on the form of the structural parts to be joined. For example, as a plate set used for the body of an automobile, a thick steel plate is a reinforcement, a member or an inner plate, and a thin steel plate is an outer plate panel or the like.

  The steel plate to be spot-welded may have a plate-like portion at least partially, and the plate-like portion may have a portion where the plate-like portions are stacked on each other, and the whole may not be a plate. Further, the plurality of steel plates are not limited to those composed of separate steel plates, and may be a superposition of a single steel plate formed into a predetermined shape such as a tubular shape.

  Further, the plurality of spot-welded steel plates are not particularly limited in terms of component composition, metal structure, and the like. For example, the strength level of the steel plate can be from mild steel to 1500 MPa steel plate. Each steel plate may be coated or not coated.

(Spot welding method)
Next, a spot welding method using the welding electrode of the present invention (hereinafter also referred to as “the welding method of the present invention”) will be described.
First, in the welding method of the present invention, a plurality of steel plates (plate assemblies) are prepared by superimposing thin steel plates on at least one outermost layer. For example, one thin steel plate having a tensile strength of 270 MPa and a thickness of 0.6 mm and two thick steel plates having a tensile strength of 590 MPa and a thickness of 1.4 mm are prepared.

  And a spot welding electrode is pressed so that three steel plates may be inserted | pinched from both sides. In that case, the spot welding electrode which has a constriction part is made to contact at least a thin steel plate. And, between the spot welding electrodes is energized to form a molten metal, and the molten metal is rapidly cooled and solidified by heat removal by the water-cooled spot welding electrode and heat conduction to the steel plate itself after the end of energization, A nugget having an elliptical cross section is formed at a location including the overlapping surface of the steel plates.

  The conditions for spot welding at this time are not particularly limited. For example, the electrode has a DR tip having a diameter of 6 to 8 mm, a pressure of 300 to 500 kgf, an energization time of 0.2 to 0.4 s, and an energization. The current can be 5 to 10 kA. The current can be either a direct current or an alternating current, but is preferably a direct current. In addition, as the current waveform, either single energization or multistage energization can be adopted.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

First, a conventional spot-welded electrode made of Cu-2% Cr alloy, electrode diameter L _E 16 mm, DR type, curved surface diameter L _s 6 mm, R40 as shown in FIG. 1 was prepared. Next, the spot welding electrode shown in Table 1 was grooved so as to have a tip length C [mm] and a constriction depth D [mm], thereby creating a spot welding electrode having a constricted portion. Table 1 shows the constriction portion cross-sectional area B [mm ² ], the tip contact area S [mm ² ], and the electrode main body cross-sectional area A [mm ² ]. The tip contact area S was defined as the area of a circle having a curved surface diameter Ls when the contact portion was viewed in plan from the electrode axis direction.

  Next, three steel plates were used, with one thin steel plate having a tensile strength of 270 MPa and a thickness of 0.6 mm and two thick steel plates having a tensile strength of 590 MPa and a thickness of 1.4 mm, and the thin steel plate being the outermost layer. A plate assembly was prepared by superimposing the plates. The plate assembly was sandwiched so that the spot welding electrode having the constricted portion was in contact with at least the thin steel plate. Then, spot welding was performed by varying the welding current with a direct current, an energization time of 330 ms, and a pressurizing force of 350 kgf, and a current value (dust generation current) generated by dust was determined.

  In a spot welded joint produced with a current 0.5 kA lower than the dust generation current, the plate was cut in the plate thickness direction at the center of the spot hitting point, the cut surface was polished, and after observation with a magnifier, it was observed with a magnifier. In the evaluation of nugget formation, the nugget diameter of the portion formed on the overlap surface of the thin steel plate and the thick steel plate was determined to be “◯” when the thickness of the thin steel plate was t and 3√t or more. Table 2 shows the evaluation of B / S, 6.2 × exp (−0.5 × C), A / B, and nugget formation.

  In Invention Examples 1 to 10, since the structure of the spot welding electrode having a constricted portion satisfies the above-described expression (1), a nugget was formed on the overlapping surface of the thin steel plate and the thick steel plate. On the other hand, in Comparative Examples 1 to 9, since the structure of the spot welding electrode having the constricted portion does not satisfy the above-described formula (1), a predetermined nugget was not formed on the overlapping surface of the thin steel plate and the thick steel plate. Moreover, since the ratio (A / B) of the constriction part cross-sectional area B with respect to the electrode main-body part cross-sectional area A is 0.3 or more and 0.8 or less, the invention examples 1-4 are further at the time of pressurization of spot welding. It was a spot welding electrode with high durability.

  According to the present invention, the nugget is stably formed on the overlapping surface of the thin steel plate and the thick steel plate in a plurality of the steel plates stacked two or more including the thin steel plate in the outermost layer without reducing the electrode life. be able to. Therefore, the present invention has high industrial applicability.

DESCRIPTION OF SYMBOLS 1 Spot welding electrode 1a Spot welding electrode 1b Spot welding electrode 2 Thin steel plate 3a Thick steel plate 3b Thick steel plate 4 Plate assembly 5 Nugget 6a Cooling means 6b Cooling means 11 Spot welding electrode 11a Spot welding electrode 11b Spot welding electrode 12 Thin steel plate 13a Thick steel plate 13b steel plate 14 plate assembly 15 nugget 16a cooling unit 16b cooling unit 17 the electrode main body portion 18 constricted portion 19 curved diameter C tip of the distal end portion 20 contact portion L _E electrode diameter L _B constriction diameter L _s tip depth constriction length D The

Claims (2)

In the spot welding electrode used for welding the plurality of steel plates, a steel plate having a thickness smaller than that of the other steel plate is stacked on at least one outermost layer of the plurality of stacked steel plates. One spot welding electrode has a constricted portion, the minimum cross-sectional area of the constricted portion is B [mm ² ], and the contact area of the spot welding electrode having the constricted portion with the thin steel plate is S [mm ² ]. And the spot welding electrode which satisfy | fills the relationship of following (1) Formula, when the length from the front-end | tip of a spot welding electrode which has the said constriction part to the constriction part is set to C [mm].
(B ÷ S) ≦ 6.2 × exp (−0.5 × C) (1)   2. The spot welding method using the spot welding electrode according to claim 1, wherein when the plurality of metal plates are sandwiched between the spot welding electrodes and energized to perform welding, the constriction is caused at least in the thin steel plate. A spot welding method comprising contacting a spot welding electrode having a portion.

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