JP6160581B2 - Resistance spot welding method - Google Patents

Description

  The present invention relates to a resistance spot welding method which is a kind of lap resistance welding method, and in particular, when a high-strength galvanized steel sheet having a tensile strength of 780 MPa or more is included in the material to be treated, and further, an unintended gap (plate) between the steel sheets Even when there is a gap), an attempt is made to form a melted portion (nugget) of an appropriate size without occurrence of scattering.

  In recent years, steel sheets have been increased in strength in order to achieve the improvement of the reliability of the vehicle body and the reduction of the vehicle body weight for reducing air pollutants. By adopting such a high-strength steel sheet, the same level of vehicle body rigidity can be obtained even if the vehicle body is made thinner and lighter. However, some issues have been pointed out. One of them is that the quality of the welded part in the vehicle body assembly decreases as the strength increases.

  As shown in FIG. 1, resistance spot welding is performed by attaching a plate set 3 of two or more stacked steel plates (here, a set of two plates of a lower steel plate 1 and an upper steel plate 2) to a pair of upper and lower electrodes (lower Between the electrode 4 and the upper electrode 5 (hereinafter referred to as an electrode pair), and pressurizing and energizing to melt the contact portion to form a nugget 6 of a necessary size to obtain a welded joint.

  The quality of the joint obtained in this way is the size of the nugget diameter, or the shear tensile strength (strength when the tensile test is performed in the shear direction of the joint) and the cross tensile strength (tensile test in the peeling direction of the joint). Strength) and fatigue strength.

As means for securing the joint strength when using a high-strength steel sheet, it is conceivable to increase the number of hit points or increase the nugget diameter from the viewpoint of the welding method. However, if the number of hit points is increased, the influence of the diversion increases. Furthermore, it leads to an increase in work time and deteriorates productivity. Further, in order to enlarge the nugget diameter, it is necessary to increase the applied pressure in order to enlarge the electrode or prevent the weld metal from scattering (scattering). This is not only a large restriction on the apparatus, but also the heat affected zone is enlarged, so that the properties of the base material are impaired.
In particular, when applied to automobiles, the surface of the steel sheet is subjected to galvanization treatment mainly containing zinc for the purpose of rust prevention. And when it has such a galvanization layer, since it becomes easy to generate | occur | produce further, it is known that it has a bad influence on formation of a nugget.

  As a conventional technique, Patent Document 1 discloses a method of forming a nugget in a three-layer steel sheet. According to this method, after the first stage of welding, the second and subsequent stages are cooled and energized in a pulsation form, so that sufficient nuggets can be obtained even in a three-layered board set of thin, thick and thick plates. The diameter can be formed.

  Moreover, in patent document 2, when welding the steel plate which has an alloying aluminum plating layer containing 50% or more and 80% or less of Fe by atomic ratio, time to maintain with an upslope and a fixed electric current is prescribed | regulated by plate | board thickness. In this way, a stable nugget can be formed.

  Furthermore, Patent Document 3 states that a nugget of a certain size can be secured by limiting the time ratio between preliminary energization and nugget formation in a zinc-based plated steel sheet.

  In Patent Document 4, it is assumed that a nugget of a certain size can be secured by repeating cooling and energization at a current value higher than the current value after performing preliminary energization in a zinc-based plated steel sheet. Yes.

Japanese Patent No. 4728926 JP 2011-167742 A Japanese Patent No. 3894539 Japanese Patent No. 3922263

  However, at actual welding sites in automobile assembly, the member positions are not always stable, and the pressed state of the steel sheet is not always constant. In this respect, the methods described in Patent Documents 1 to 4 are based on the premise that the plate assembly is sufficiently pressed and contacted by the electrodes, and therefore there is an unintended gap (plate gap) between the steel plates. In some cases, scattering occurred, and it was difficult to ensure a satisfactory nugget diameter.

  The present invention advantageously solves the above problems, and includes a high-strength galvanized steel sheet that is prone to scatter in part of the plate assembly, and further scatter occurs due to the presence of sheet gaps between the steel sheets. An object of the present invention is to provide a resistance spot welding method capable of stably forming a nugget having a sufficient diameter without occurrence of scattering even in an easy state.

Now, in order to solve the above-mentioned problems, the inventors have repeatedly studied the influence of the plate gap in resistance spot welding of a plate set including a high-tensile galvanized steel plate.
That is, the gap was simulated, and as shown in FIG. 2, the insulator 7 was sandwiched between one side of the steel plates 1 and 2, and the welding test at each current value and the numerical analysis at that time were performed.

As a result, in a state where the plate gap is large and not sufficiently pressed and contacted, even if a high current value is applied at the initial stage of energization, if the energization time is short, no scattering occurs, and abruptly occurs immediately below the electrode. A phenomenon was observed in which the steel sheet softened due to heat generation. This is considered to be because, in the initial stage of energization, the steel sheet temperature is room temperature or sufficiently low even if it is affected by other welding points, so that the specific resistance is low and the heat generation due to the current density is the main factor. .
On the other hand, when a high current value is applied at the initial stage of energization in a state where there is no gap and the pressure is sufficiently applied, scattering may occur on the contrary. This is considered to be caused by the fact that the current is concentrated on the contact portion by the electrode, so that it melts rapidly and is scattered as a scatter.

Therefore, the inventors next utilized the above phenomenon and applied a specific current (initial energization) prior to the main energization to form the nugget, thereby making contact between the steel plates without occurrence of scattering. We examined whether it could be secured. At that time, considering the on-site construction, the effect of the sheet gap was replaced with the distance between the electrodes, and the ratio to the sheet thickness of the steel sheet was examined.
As the test steel plate, a high-tensile galvanized steel plate having a tensile strength of 780 to 1180 MPa and a plate thickness of 1.0 to 1.6 mm was used. Further, in order to adjust the distance between the steel plates or the distance between the electrodes, a rectangular parallelepiped insulator 7 having a predetermined thickness was sandwiched between the steel plates 1 and 2 as shown in FIG.

Figure 3 shows the result of investigation for the ratio of the current value I _s of the initial energization of the current value I _m in this energized to form a nugget. Here, I _s denotes an average value of the current value of the initial energization in the time zone in which the current value is above than the current value I _m of the current. In the figure, L is the distance between the centers of the electrode pair at the start of initial energization, t is the total thickness of the steel sheet, ○ when no scattering occurs, × when scattering occurs, and scattering Even if it occurred, the small ones were arranged as △. The current time in initial energization was fixed at 60 ms, and the current time in main energization was constant at 280 ms.

As shown in FIG. 3, it has been found that by adjusting the I _s / I _m ratio appropriately after adjusting the L / t ratio, a remarkable effect can be obtained in terms of preventing the occurrence of scattering.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. Including at least one galvanized high-tensile steel sheet having a base metal tensile strength of 780 MPa or more having a plating layer mainly composed of zinc on the surface, sandwiched by a pair of electrodes, a pair of electrodes, In resistance spot welding that is energized and joined while applying pressure, it consists of main energization and initial energization preceding it,
(1) When the total thickness of the stacked steel plates is t and the distance between the centers of the electrode pairs is L, the t and L are expressed by the following relational expression: 0.9 × t ≦ L ≦ 1.1 × t
Starting initial energization in a condition that satisfies
(2) A resistance spot welding method, wherein a higher current value is applied in the initial energization than in the main energization.

2. When T _s is the time during which the initial energization current value exceeds the main energization current value, this T _s is expressed by the following equation: 10 ms ≦ T _s ≦ 100 ms
2. The resistance spot welding method according to 1 above, wherein the following range is satisfied.

3. When the average of the current value of the initial conduction in a time zone wherein exceeds than the current value of the current was set to I _s, the relationship between the current value I _m this I _s is in the energized following formula I _m × 1 .1 ≦ I _s ≦ 15.0kA
3. The resistance spot welding method according to 1 or 2 above, wherein the range is satisfied.

4). It is assumed that preliminary energization is further performed before the initial energization, and the current value I _p and energization time T _p in the preliminary energization are respectively expressed by the following expressions: 10 ms ≦ T _p ≦ 100 ms
I _m × 0.6 ≦ I _p ≦ I _m × 0.95
The resistance spot welding method according to any one of the above items 1 to 3, wherein the range is satisfied.

  According to the present invention, when a resistance spot welding method is applied to a plate assembly including a plurality of steel plates, including at least one high-tensile galvanized steel plate, even if a plate gap is generated between the plate assemblies. Even so, it is possible to form a nugget with a sufficient diameter without occurrence of scattering, which is extremely useful in the industry.

It is a figure which shows the structure of resistance spot welding. It is a figure which shows the state which pinched | interposed the insulator on the one side between steel plates. L / t ratio and I _{s /} I _m ratio is a diagram showing the effect on the occurrence of expulsion. It is explanatory drawing of the distance L between the total thickness t of a steel plate, and the center of an electrode pair. It is a figure which shows the relationship between the electric current value in this invention, energization time, and cooling time. It is the figure which showed the electric current waveform at the time of implementing preliminary electricity supply just before initial electricity supply. FIG. 6 is a diagram illustrating a cutting portion for evaluating a nugget diameter in Example 2.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
In the resistance spot welding method of the present invention, as shown in FIG. 1, a plurality of steel plates including a galvanized steel plate and a high-tensile steel plate are overlapped (here, two steel plates, a lower steel plate 1 and an upper steel plate 2). Sandwiched between a pair of upper and lower electrodes 4 and 5 (electrode pairs), and energized while applying pressure to form a nugget 6 of the required size and weld it. A joint is obtained.

  In the present invention, at least one of the plate sets is made of a high-strength galvanized steel sheet. The high-tensile galvanized steel sheet is more likely to scatter compared to a normal steel sheet, and even more if there is a gap between the sheets. This is because the steel plate is prone to occur.

  A welding apparatus suitable for carrying out such spot welding is provided with a pair of upper and lower electrodes, and can be pressurized and energized with a portion to be welded between the pair of electrodes, and any pressure and welding current can be arbitrarily set during welding. Pressurization mechanism (air cylinder, servo motor, etc.), current control mechanism (AC, DC, etc.), type (stationary, robot gun, etc.) ) Etc. are not particularly limited.

In the present invention, at the start of energization, the total thickness t of the superposed steel plates shown in FIG. 4 and the center-to-center distance L between the upper and lower electrodes are as follows: 0.9 × t ≦ L ≦ 1.1 × t
It is important to keep the relationship satisfied.
When L / t is less than 0.9, the electrode contact portion is in a wide state, and the heat generation effect due to energization is reduced. On the other hand, when L / t is greater than 1.1, the contact portion between the plate assemblies is reduced even if softening due to heat generation occurs. The inconvenience that it cannot be secured occurs. Preferably, the range is 0.9 × t ≦ L ≦ 1.0 × t.
The above setting assumes a case in which a gap exists between the steel plates. As a cause of such a gap, for example, a gap in the flange due to shape mismatch in the vehicle body can be considered.
However, the range of 1.1 ≧ L / t> 1.0 is based on the premise that there is some current path in the vicinity of the welding point, such as the contact portion of the steel plate or the existing welding point, or the entire member being a conductor. It is said. In the absence of a current path, current cannot be applied, so that it is difficult to carry out welding according to the present invention.

On the other hand, there is no particular limitation on L / t after the start of energization, but normally L / t gradually decreases with the softening of the steel sheet by energization, and the melted portion (nugget) formed in the latter half of energization. Due to the expansion of L / t, L / t goes through a process of increasing somewhat. In addition, the suitable range of L / t during such an energization period is 0.9 * t <= L <= 1.0 * t.
Further, the pressure applied at the start of energization is usually about 2.0 to 7.0 kN.

Then, initial energization is started. In the initial energization, a sufficient contact area is ensured prior to the main energization. Subsequent energization forms a nugget with an appropriate diameter.
Here, in the initial energization, it is important to apply a higher current value than in the main energization.
This is because when a current value relatively higher than the current value necessary for forming the nugget is applied for a short time, heat generation due to the current density occurs immediately below the electrodes, so the steel sheet softens and the distance between the electrodes This is because it is possible to significantly reduce

The current waveform in this embodiment is shown in FIG.
As shown in the figure, in the present invention, the energization time T _m, electric current prior to the energization consisting I _m, the energizing time exceeds than the current value of the current T _s, the average current value in the time period performing initial energization consisting of I _s. Here, the average value is obtained by dividing the current value at each of N + 1 time points from the starting point (0th) to the Nth when the T _s is equally divided into N, and then adding up and dividing by N + 1 to calculate the square root. Means the value taken.
The current control according to the present invention is, for example, configured so that the welding machine is set to be higher than the set current, in other words, overshoot before reaching the set current, or multistage current control is possible. It can be realized by using it.

Here, the energization time T _s in the initial energization is 10 ms ≦ T _s ≦ 100 ms.
It is preferable to set it as the range.
If T _s is less than 10 ms, sufficient heat generation due to current density cannot be obtained. On the other hand, if T _s exceeds 100 ms, the risk of occurrence of scattering increases in the galvanized steel sheet.
More preferred range of T _s is 10 ms ≦ _{T s} ≦ 60 ms.

Further, the average current value _{I s} at energization time _{T s} of the initial energization, in relation to the current value _{I m} in this current, the following formula _{I m × 1.1 ≦ I s ≦} 15.0kA
It is preferable to satisfy this range.
Average current value I _s in the initial energization of the energization time T _s is not sufficient exotherm obtained with respect to current density when less than I _m × 1.1, whereas when it exceeds 15.0KA, with high-tensile galvanized steel sheet Scattering is inevitable. From the viewpoint of suppressing excessive heat input, the range is preferably I _m ≦ I ₁ ≦ 12.0 kA. In the case where the energization performs current value control for two or more stages, the current value I _m in this energization and their average value.

Furthermore, in the present invention, preliminary energization can be performed as means for adjusting the L / t ratio before the above-described initial energization. The current value I _p and energization time T _p in the preliminary energization are respectively expressed by the following expressions: 10 ms ≦ T _p ≦ 100 ms
I _m × 0.6 ≦ I _p ≦ I _m × 0.95
It is preferable to satisfy this range.

  In addition, in this invention, the plating layer which has zinc as a main component means all the conventionally well-known zinc plating layers, including a hot dip galvanization layer and an electrogalvanization layer, Zn-Al plating layer, Zn- A Ni plating layer and the like are included. In addition, any plating layer having Al as a main component can be applied.

Example 1
As an embodiment of the present invention, as shown in FIG. 1 described above, a servo motor pressurization type attached to a C gun for a plate set 3 in which two steel plates (lower steel plate 1 and upper steel plate 2) are stacked. Resistance spot welding was performed using a resistance welding machine having a DC power source to produce a resistance spot welded joint.
The energization at this time was carried out under the conditions shown in Table 1 with a current waveform as shown in FIG. The applied pressure was 4.5 kN, and the main energization time Tm was constant at 14 cycles (280 ms). In the experiment, a rectangular parallelepiped insulator was inserted at a position 25 mm away from the welding point, and the distance between the electrodes was adjusted to a predetermined distance. There is only one place where the insulator is inserted.
As the electrode, a DR-type electrode of alumina-dispersed copper having a radius of curvature R40 at the tip and a tip diameter of 6 mm was used. Further, as the test piece, using high-tensile galvanized steel sheet 2 mm from 793 MP a or al 1530 MP a or 1 in mm.
Table 1 shows the results of examining the occurrence of scattering and the nugget shape when welding is performed. In addition, the nugget diameter was evaluated by the etching structure of the cut section. The evaluation of the nugget diameter should be ○ if it is the same or larger than the nugget diameter when only the main current is applied to a plate assembly without inserting an insulator and having no gap. X.

  As shown in Table 1, when resistance spot welding is performed according to the present invention, it can be seen that there is no occurrence of scattering and a nugget with an appropriate diameter is formed as compared with the comparative example.

(Example 2)
Resistance spot welding was performed in the same manner as in Example 1 to produce a resistance spot welded joint.
However, the energization at this time was performed under the conditions shown in Table 2 with a current waveform with preliminary energization added before the initial energization, as shown in FIG. The applied pressure was 4.5 kN, and the main energization time _Tm was constant at 14 cycles (280 ms). And the insulator was inserted between the steel plates, and it adjusted so that the distance between electrodes might become predetermined distance.
Here, the inter-electrode distance L was measured on the spot using a laser displacement meter so that L before the pre-energization and after the pre-energization (before the initial energization) could be measured. Then, L / t before the pre-energization was R1, and L / t after the pre-energization was R2. The electrodes and steel plates used were the same as in Example 1.

  Table 2 shows the results of examining the occurrence of scattering and the nugget diameter when welding was performed. In addition, as shown in FIG. 7, the nugget diameter has a cross section (A cross section) when the steel sheet is cut so as to cross the insulator vertically, and a cross section (B cross section) when cut in a direction perpendicular to the surface. Each (cross section) was evaluated with an etching structure. The evaluation of the nugget diameter is ○ if the diameter of the A cross section is the same or exceeds the nugget diameter when only main energization is applied to a plate assembly without a gap, and × It was. Further, among the circles, those having a diameter of the B cross section / diameter of the A cross section in the range of 0.9 to 1.0 are marked with.

As shown in Table 2, when resistance spot welding is performed according to the present invention, it can be seen that there is no occurrence of scattering and a nugget having an appropriate diameter is formed as compared with the comparative example.
Further, compared to Example 1 in which the preliminary energization was not performed, even if a larger gap exists before the preliminary energization, not only the welding is possible without occurrence of scattering, but the main energization is performed by the preliminary energization. The front contact state is improved, and it is excellent in that a nugget closer to a perfect circle can be formed.

1, 2 steel plate 3 plate assembly 4, 5 electrode 6 nugget 7 insulator

Claims (4)

Matrix tensile strength having a plating layer composed mainly of zinc on the front surface comprises at least one or more of the galvanized high tensile steel 780 MPa, the plate group obtained by superimposing a plurality of steel plates, sandwiched by a pair of electrodes In resistance spot welding that is energized and joined while applying pressure, it consists of main energization and initial energization preceding it,
(1) When the total thickness of the stacked steel plates is t and the distance between the centers of the electrode pairs is L, the t and L are expressed by the following relational expression: 0.9 × t ≦ L ≦ 1.1 × t
Starting initial energization in a condition that satisfies
(2) A resistance spot welding method, wherein a higher current value is applied in the initial energization than in the main energization. When T _s is the time during which the initial energization current value exceeds the main energization current value, this T _s is expressed by the following expression: 10 ms ≦ T _s ≦ 100 ms
The resistance spot welding method according to claim 1, wherein the following range is satisfied. When the average of the current value of the initial conduction in a time zone wherein exceeds than the current value of the current was set to I _s, the relationship between the current value I _m this I _s is in the energized following formula I _m × 1 .1 ≦ I _s ≦ 15.0 kA
The resistance spot welding method according to claim 1 or 2, wherein the following range is satisfied. It is assumed that preliminary energization is further performed before the initial energization, and the current value I _p and energization time T _p in the preliminary energization are respectively expressed by the following expressions: 10 ms ≦ T _p ≦ 100 ms
I _m × 0.6 ≦ I _p ≦ I _m × 0.95
Here, I _m is the resistance spot welding method according to any one of claims 1 to 3, characterized by satisfying the range of the current value <br/> in the energization.

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