Resistance spot welding method and welded structure

Abstract

An overlapped body 10 includes at least three steel plates 1a, 1b and 1c and in which at least one steel plate interface 2a has a contact resistance different from the contact resistance of another steel plate interface 2b. A molten pool is formed at the steel plate interfaces so as to join the steel plates 1a, 1b and 1c, and an energization point 5 is formed at the steel plate interface 2b having the largest contact resistance. Initial spot welding is performed under a condition in which a branch current is generated in the energization point 5 so as to form the molten pool. By doing so, it makes it difficult to generate the expulsion and surface flash at the steel plate interface where resistive heating is large and form the molten pool having a sufficiently large size at the steel plate interface where resistive heating is small.

Claims

1. A resistance spot welding method in which in an overlapped body that includes three or more steel plates and in which at least one steel plate interface has a contact resistance larger than a contact resistance of another steel plate interface, a first molten pool is formed at the steel plate interfaces so as to join the steel plates, the method comprising: a preliminary welding step of forming an energization point at the steel plate interface having the larger contact resistance; and a main welding step of performing initial spot welding under a first condition in which a first branch current is generated in the energization point, wherein the initial spot welding is performed such that the first molten pool is formed at a position at a horizontal distance of 30 mm or less from the energization point; and wherein in the preliminary welding step, the energization point is formed at the steel plate interface having the larger contact resistance by energizing the overlapped body while sandwiching and pressing the overlapped body by a pair of electrodes.

2. The resistance spot welding method according to claim 1, wherein in the main welding step, spot welding is further performed repeatedly under a second condition in which a second branch current is generated in the energization point or the first molten pool.

3. The resistance spot welding method according to claim 1, wherein in the main welding step, further spot welding is performed repeatedly such that a second molten pool is formed at a second position at a second horizontal distance of 30 mm or less from the energization point or the first molten pool.

4. The resistance spot welding method according to claim 1, wherein the overlapped body consists of one mild steel plate and two high-tensile strength steel plates.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a conceptual diagram showing, as an example, a resistance spot welding method according to a conventional technique.

(2) FIG. 2 is a conceptual diagram showing, as an example, a resistance spot welding method according to the present invention.

(3) FIG. 3 is a conceptual diagram showing another example of a resistance spot welding method according to the present invention.

(4) FIG. 4 is a diagram showing the structural members of a door opening portion of an automobile.

MODE FOR CARRYING OUT THE INVENTION

(5) As shown in FIG. 2(a), a resistance spot welding method according to the present invention is a resistance spot welding method in which an overlapped body 10 that includes, for example, at least three steel plates 1a, 1b and 1c and in which at least one steel plate interface 2a has a contact resistance different from the contact resistance of another steel plate interface 2b is energized (see a solid white arrow C) while being sandwiched and pressed by a pair of dome-shaped electrodes 3a and 3b so as to form molten pools at the steel plate interfaces and thereby to join the steel plates 1a, 1b and 1c. And, it is important to, in a preliminary step, form an energization point 5 at the steel plate interface 2b having the highest contact resistance. Hereinafter, an example will be described in which the energization point 5 has a circular or generally circular shape on the steel plate interface. Also, the shape of the contact region between a steel plate against which the tip of a dome-shaped electrode is pressed and the electrode is substantially circular or generally circular.

(6) In a main welding step, when initial spot welding is performed under a condition in which a branch current is generated in the energization point 5, as shown in FIG. 2(b), a welding current C is branched (C2=CC1), and thus it is possible to delay the formation of the molten pool 4b at the steel plate interface 2b where resistive heating is large without impeding the formation of the molten pool 4a at the steel plate interface 2a where resistive heating is small. Since the present invention has a configuration as described above, as shown in FIG. 2(c), it is possible to form a sufficiently large molten pool 4 at the steel plate interface 2a where resistive heating is small and the steel plate interface 2b where resistive heating is large.

(7) It is preferable that the initial spot welding is pertained such that a molten pool is formed at a position at a horizontal distance W1 of 30 mm or less from the energization point 5 (the distance between the center of the energization point and the center of the weld portion). In particular, when it is particularly difficult to obtain heat generation in the steel plate of the smallest thickness such as when the ratio between the smallest thickness in the thicknesses of a plurality of steel plates overlapped and the total thickness of all of the steel plates (hereinafter referred to as thickness ratio) is large, the horizontal distance W1 from the energization point 5 is preferably set to be 25 mm or less, more preferably 20 mm or less, and even more preferably 15 mm or less. However, if the distance is too short, the amount of branch current will be too large, causing a problem in that the amount of resistive heating at the steel plate interface 2b where resistive heating is large will be smaller than that at the steel plate interface 2a. Accordingly, the distance is preferably set to 10 mm or more. The center of the energization point and the center of the weld portion refer to the center of the contact region of the steel plate surface pressed by the electrode.

(8) In the example shown in FIG. 2, a description has been given by taking an overlapped body composed of three steel plates as an example, but the resistance spot welding method according to the present invention is applicable to an overlapped body that is composed of four or more steel plates and in which at least one steel plate interface has a contact resistance different from that of the other steel plate interfaces. This is because in the case of the overlapped body composed of four or more steel plates as well, the molten pools at the steel plate interfaces where resistive heating is large are more easily formed than the molten pools at the steel plate interface where resistive heating is small, and thus by providing energization points as described above at the steel plate interfaces having a large contact resistance, the formation of molten pools at the steel plate interfaces can be delayed.

(9) The size of the energization point 5 may be smaller or larger than a circle having a diameter required for the molten pool 4 from the design point of view (for example, 4t, where t is the thickness (mm) of the thinner one of the steel plates forming the steel plate interface). However, if the energization point 5 is too small, the influence of so-called constriction resistance will be large, and a branch current flowing through the energization point 4 may not be obtained sufficiently. Accordingly, the size of the energization point 5 is preferably a size corresponding to a circle having a diameter of 1 mm or more.

(10) In the embodiment described above, a description has been given of an example of forming the energization point 5 at which the interface is fusion joined, but it is also possible to form an energization point at which the interface is not fused and is thus in a press contact state. In the case of forming the energization point in a press contact state, it is preferable that the size of a press contact portion between the steel plate 1b and the steel plate 1c (the portion serving as an energization point at the steel plate interface) is a size corresponding to a circle having a diameter of 1 mm or more. Furthermore, in the embodiment described above, an example has been described in which the shape of the weld portions and energization point at the steel plate interfaces is circular or generally circular, but the shape may be polygonal such as triangular or rectangular other than circular or generally circular as long as constriction resistance is not generated. At this time, the horizontal distance W1 and a horizontal distance W2 described later can be measured assuming the centroids of the weld portion and the energization point as the centers thereof.

(11) In the example shown in FIG. 2, both preliminary welding and main welding may be performed in a state in which three steel plates are overlapped, but it is also possible to, for example, perform preliminary welding in a state in which two steel plates constituting a steel plate interface having the largest contact resistance are overlapped in advance, and thereafter having another steel plate overlapped to the two steel plates to perform main welding. From the viewpoint of production efficiency, it is preferable to use a method in which both preliminary welding and main welding are performed in a state in which three steel plates are overlapped.

(12) In the case of welding four or more steel plates (or in other words, in the case where there are three or more steel plate interfaces) as well, both preliminary welding and main welding may be performed in a state in which all of the steel plates are overlapped, but it is also possible to form an energization point and a molten pool (it may be an energization point) in an overlapped body made of three steel plates by any one of the methods described above, and thereafter having another steel plate overlapped to the three steel plates to perform main welding. The same applies to an overlapped body in which five steel plates are overlapped. That is, in the case where N (where N3) steel plates are overlapped, or in other words, in the case where the number of steel plate interfaces is N1, preliminary welding is performed N2 times.

(13) The resistance spot welding method according to the present invention is suitable to perform in welding on, in particular, an overlapped body composed of one mild steel plate and two high-tensile strength steel plates. Such a plate combination is widely used in automotive components as described above. In the case of the pillar shown in FIG. 4, a plate combination composed of a 0.5 to 0.8 mm mild steel plate (having a tensile strength of, for example, 270 MPa grade) as the outer steel plate, a 1.2 to 2.0 mm high-tensile strength steel plate having a tensile strength of 980 to 1500 MPa grade as the reinforced steel plate, and a 1.2 to 2.0 mm high-tensile strength steel having a tensile strength of 340 to 780 grade is used. As used herein, mild steel plate refers to a steel plate typically having a tensile strength of 270 MPa or more and less than 340 MPa, and can be, for example, JAC270D (galvannealed steel plate having a tensile strength of 270 MPa grade) or the like. High-tensile strength steel plate refers to a steel plate typically having a tensile strength of 340 MPa or more, and can be, for example, JSC590R, JSC980DP or the like.

(14) At this time, the steel plate 1a in the diagram is a mild steel plate, the steel plates 1b and 1c are high-tensile strength steel plates, and the interface 2a between the mild the steel plate 1a and the high-tensile strength steel plate 1b is a steel plate interface having a small contact resistance, and the interface 2b between the high-tensile strength the steel plates 1b and 1c is a steel plate interface having a large contact resistance.

(15) In a preliminary welding step, the energization point 5 can be formed by resistance spot welding under a condition of a smaller amount of resistive heating than regular welding when the overlapped body 10 is energized while being sandwiched and pressed by the pair of electrodes 3a and 3b so as to form molten pools at the steel plate interfaces as with regular welding. The resistance spot welding is usually performed by setting the amount of energization and the pressing force according to the material to be welded, but the condition of smaller amount of resistive heating than regular welding means to perform resistance spot welding under a condition in which the amount of energization is set to be smaller or the pressing force is set to be larger than regular welding. It is sufficient that the energization point has a resistance small enough to obtain sufficient branch currents, and the energization point may be an energization point at which the interface is fusion joined or an energization point at which the interface is not fused and is thus in a press contact state.

(16) For example, in the case of an overlapped body composed of a 0.6 mm thick mild steel plate (JSC270F), a 1.6 mm thick high-tensile strength steel plate (JSC980Y) and a 1.6 mm thick high-tensile strength steel plate (JSC980Y), the main welding step is performed by setting the pressing force to 3.43 kN, the current to 6.0 kA and the energization time to 18 cycles (300 ms). However, if energization (main welding) is immediately performed under the above-described conditions in a state in which there is no energization point in the vicinity thereof, the expulsion and surface flash is very highly likely to be generated. To address this, prior to the main welding step, a preliminary welding step is performed in which energization is performed by setting the pressing force to 3.43 kN, the current to 5.0 kA and the energization time to 6 cycles (100 ms), it is thereby possible to form an energization point at the interface between the high-tensile strength steel plates.

(17) In the main welding step, as shown in FIG. 2(d), furthermore, by performing the initial spot welding such that a molten pool 6 is formed under a condition in which a branch current is generated in the energization point 5 or the already-formed molten pool 4, the welding current C is branched (C2, C3) in the same manner as described above. Here, in FIG. 2(d), a branch current indicated by C3 is generated in the steel plate 1a. As the amounts of energization flowing through the interfaces, as in FIG. 2(c), the amount of energization at the interface 2a is C1+C2, but the amount of energization at the interface 2b is C1 only, and thus the interface 2a has a larger amount of energization than the interface 2b. Accordingly, the formation of a molten pool at the steel plate interface 2b where resistive heating is large can be delayed without impeding the formation of a molten pool at the steel plate interface 2a where resistive heating is small, and thus it is possible to form the molten pool 6 having a sufficiently large size at the steel plate interface 2a where resistive heating is small and the steel plate interface 2b where resistive heating is large.

(18) It is preferable that the molten pool 6 is formed at a position at a horizontal distance W2 of 30 mm or less from the molten pool 4 (the distance between the center of the molten pool 4 and the center of the weld portion 6). Furthermore, spot welding may be repeatedly performed at positions close to the weld portions 4 and 6 that have already been formed so as to perform spot welding consecutively at a plurality of spots. For example, the automotive structural member shown in FIG. 4 can be produced by repeating spot welding in an area within 30 mm from the molten pools 4 and 6 that were formed earlier.

(19) It is also possible to, as shown in FIG. 3, form a molten pool 4 near two or more energization points 5 by performing main welding under a condition in which a branch current is generated. At this time, it is preferable to set horizontal distances W3 and W4 from the energization points to be 30 mm or less. A part or all of the energization points may be made as another weld portion (not shown).

Example 1

(20) In order to verify the effects of the present invention, spot welding was performed on one JAC270D steel plate (thickness: 0.7 mm, tensile strength: 270 MPa) and two JSC590DP steel plates (thickness: 2.0 mm, tensile strength: 590 MPa) under conditions shown in Table 1 (tests Nos. 1 to 5). In each example, main welding was performed at a current value of 4.0 kA or more with an increment of 0.25 kA until the expulsion and surface flash was generated, so as to investigate the minimum current value at which all molten pools formed at each interface had a diameter of 4t (t=0.7 mm and 2.0 mm) or more and the maximum current value at which the expulsion and surface flash was not generated. The results are shown in Table 1, with a circle () indicating that the difference between the maximum current value and the minimum current value was 1.0 kA or more, and a cross (x) indicating that the difference between the maximum current value and the minimum current value was less than 1.0 kA. In the examples in which preliminary welding was carried out, an energization point was formed at the interface between two JSC590DP steel plates (the steel plate interface having the largest contact resistance) in a state in which three steel plates were overlapped.

(21) TABLE-US-00001 TABLE 1 Preliminary welding step Main welding step Test Pressing Current Energization Retention time Interval of spot Pressing Energization Retention time Proper current No. force (kN) (kA) time (cycle) (cycle) positions (mm) force (kN) time (cycle) (cycle) range (kA) 1 3.43 7.8 20 10 20 3.43 20 10 2 3.43 7.8 20 10 25 3.43 20 10 3 3.43 7.8 20 10 30 3.43 20 10 4 3.43 20 10 x 5 3.43 7.8 20 10 35 3.43 20 10 x Note: 1 cycle = 1/60 second

(22) As shown in Table 1, in tests Nos. 1 to 3 in which preliminary welding was carried out to form an energization point in advance and thereafter main welding was carried out, a branch current was generated at the energization point during main welding, and thus a proper current range of 1.0 kA or more was attained. It can be seen from this that by using these conditions, it is possible to make it difficult to generate the expulsion and surface flash at the steel plate interface where resistive heating is large and easily form a molten pool having a sufficiently large size at the steel plate interface where resistive heating is small.

(23) In test No. 4 in which main welding was carried out without carrying out preliminary welding, the proper current range was small. In test No. 5, although preliminary welding was carried out, the position at which the molten pool was formed by main welding was too far from the energization point, and thus a branch current was not generated, as a result of which the proper current range was as small as less than 1.0 kA. Accordingly, if a molten pool having a sufficiently large size is attempted to be formed at the steel plate interface where resistive heating is small by using these conditions, the expulsion and surface flash will be easily generated at the steel plate interface where resistive heating is large, which makes it difficult to perform management in the actual operation.

Example 2

(24) Next, spot welding was performed on one JAC270D steel plate (thickness: 0.7 mm, tensile strength: 270 MPa) and two high-tensile strength steel plates of 1180 MPa grade (thickness: 1.6 mm, tensile strength: 1180 MPa) under various conditions (tests Nos. 6 to 8). In each example, main welding was performed at a current value of 4.0 kA or more with an increment of 0.25 kA until the expulsion and surface flash was generated, so as to investigate the minimum current value at which all molten pools formed at each interface had a diameter of 4t (t=0.7 mm and 1.6 mm) or more and the maximum current value at which the expulsion and surface flash was not generated.

(25) (Test No. 6)

(26) Main welding was carried out, without carrying out preliminary welding, under the following conditions so as to form a molten pool:

(27) Pressing force: 3.43 kN;

(28) Current: 4.0 kA or more (with an increment of 0.25 kA);

(29) Energization time: 20 cycles; and

(30) Retention time: 10 cycles.

(31) In test No. 6, the minimum current value was 6.0 kA, the maximum current value was about 6.8 kA, and the proper current range was about 0.8 kA.

(32) (Test No. 7)

(33) Preliminary welding was carried out in a state in which three steel plates were overlapped under the following conditions so as to form an energization point at the interface between two high-tensile strength steel plates of 1180 MPa grade (the steel plate interface having the largest contact resistance):

(34) Pressing force: 3.43 kN;

(35) Current: 5.0 kA;

(36) Energization time: 20 cycles; and

(37) Retention time: 10 cycles.

(38) After that, main welding was carried out under the same conditions as those used in test No. 6 so as to form a molten pool at a position corresponding to a horizontal direction from the energization point of 15 mm. The conditions for main welding were the same as those used in test No. 6.

(39) In test No. 7, the minimum current value was 6.5 kA, the maximum current value was 8.0 kA, and the proper current range was increased to 1.5 kA. Even in the case of a plate combination including higher tensile strength steel plates, by performing preliminary welding prior to main welding so as to form an energization point and then performing the initial spot welding under a condition in which a branch current is generated in the formed energization point, it is possible to make it difficult to generate the expulsion and surface flash at the steel plate interface where resistive heating is large and easily form a molten pool having a sufficiently large size at the steel plate interface where resistive heating is small.

(40) (Test No. 8)

(41) Preliminary welding was carried out in a state in which three steel plates were overlapped under the following conditions so as to form an energization point:

(42) Pressing force: 3.43 kN;

(43) Current: 5.0 kA;

(44) Energization time: 20 cycles; and

(45) Retention time: 10 cycles.

(46) After that, main welding was carried out under the following conditions so as to form a first molten pool at a position corresponding to a horizontal distance from the energization point of 15 mm:

(47) Pressing force: 3.43 kN;

(48) Current: 7.5 kA;

(49) Energization time: 20 cycles; and

(50) Retention time: 10 cycles.

(51) After that, furthermore, main welding was carried out under the following conditions so as to form a second molten pool at a position corresponding to a horizontal distance from the first molten pool of 15 mm:

(52) Pressing force: 3.43 kN;

(53) Current: 4.0 kA or more (with an increment of 0.25 kA);

(54) Energization time: 20 cycles; and

(55) Retention time: 10 cycles.

(56) In test No. 8, the minimum current value was 7.0 kA, the maximum current value was about 8.8 kA, and the proper current range was about 1.8 kA. In this way, by performing spot welding under a condition in which a branch current is generated not only in the energization point but also in the molten pool that has already been formed, it is possible to make it difficult to generate the expulsion and surface flash at the steel plate interface where resistive heating is large and easily form a molten pool having a sufficiently large size at the steel plate interface where resistive heating is small.

INDUSTRIAL APPLICABILITY

(57) According to the present invention, even when spot welding is performed on an overlapped body that is composed of three or more steel plates and in which at least one steel plate interface has a contact resistance different from that of another steel plate interface, it is possible to make it difficult to generate the expulsion and surface flash at the steel plate interface where resistive heating is large and easily form a molten pool having a sufficiently large size at the steel plate interface where resistive heating is small. Accordingly, the present invention is optimal in spot welding for producing, for example, a plate combination or the like in which one mild steel plate and two high-tensile strength steel plates are overlapped, in particular, an automotive structural member.

DESCRIPTION OF REFERENCE SIGNS

(58) 1a, 1b, 1c Steel plate 2a, 2b Steel plate interface 3a, 3b Electrode 4, 4a, 4b Molten pool 5 Energization point 6 Molten pool 10 Overlapped body 20 Pillar 21 Overlapped body 22 Flange 23 Weld portion C Current C1, C2, C3 Current (branch current)