JP6828404B2 - Spot welding method - Google Patents

Description

The present invention relates to a spot welding method in which a plurality of steel plates are superposed and spot welded. More specifically, the present invention relates to a spot welding method for a galvanized steel sheet having an organic film on its surface.

Conventionally, galvanized steel sheets have been widely used in the fields of home appliances, automobiles, building materials, and the like. The zinc-based plated steel sheet used for these applications is coated with an organic film on the surface of the plating layer in order to improve workability, post-coating property, antiglare property and the like.

Since the organic film has an insulating property, when a zinc-based plated steel sheet having an organic film on the surface is spot-welded, it is difficult to weld due to poor energization under general welding conditions.
Therefore, the tip of the electrode has a spherical shape with a radius of curvature of 50 mm or less, the area where the pressing force of the electrode acts on the welded portion is limited, and the pressing force per unit body area is increased, so that an organic film is formed at the tip of the electrode. A method of breaking to secure an energization path and spot welding has been proposed (see Patent Document 1).

JP-A-2002-219578

However, there is a demand for a technique for more preferably spot welding in a zinc-based plated steel sheet having an organic film.

Therefore, an object of the present invention is to provide a spot welding method having more appropriate welding conditions in order to enable spot welding on a galvanized steel sheet having such a highly insulating organic film on its surface. ..

The present invention is a spot welding method in which a plurality of steel plates are superposed and spot welded. At least one of the plurality of steel plates is an organic material having a thickness of 0.1 μm or more and 3 μm or less on one side or both sides. It is a zinc-based plated steel plate having a film, and the shape of the electrode in contact with the zinc-based plated steel plate is a flat shape having a circular flat portion at the tip or a radius shape having a spherical tip at the tip. When the shape of the electrode is the flat shape, it is the following equation (1), and when it is the radius shape, it is the tip circumference of the electrode calculated by the following equation (1)', and the pressing force of the electrode is divided. The present invention relates to a spot welding method for welding under the condition that the value of the linear load at the tip of the electrode defined by the equation (2) is 400 N / mm or more.
L = πD ・ ・ ・ ・ ・ (1)
L = 2π (2Rh-h ² ) ^1/2 ... (1)'
p = F / L (2)
Here, L: the peripheral length of the tip of the electrode (mm), D: the diameter of the circular flat portion when the tip of the electrode is flat, and R: the curvature of the spherical surface when the tip of the electrode is a radius shape. Radius (mm), h: Constant for obtaining the equivalent tip diameter at the tip of the electrode when the tip of the electrode has a radius shape (0.05 mm or more and 0.2 mm or less depending on the strength of the base material and the amount of plating adhesion), p : Wire load (N / mm), F: Electrode pressing force (N)

Further, it is preferable to perform welding under the condition that the value of the linear load is 550 N / mm or less.

Further, it is preferable to perform welding under the condition that the value of the linear load is 500 N / mm or more.

Further, it is preferable that the collision speed of the electrode with the galvanized steel sheet is 500 mm / sec or more.

Further, the plurality of steel sheets are preferably two galvanized steel sheets having an organic film having a thickness of 0.1 μm or more and 3 μm or less on both sides.

According to the present invention, even in a galvanized steel sheet having an organic film on the surface, spot welding can be performed by setting a linear load obtained by dividing the electrode pressing force by the peripheral length of the tip of the electrode as an appropriate condition. ..

It is a schematic diagram which shows the arrangement relation of the flat electrode and the steel plate to be welded by the spot welding method of this invention. It is a schematic diagram which shows the arrangement relation of the radius-shaped electrode and the steel plate to be welded by the spot welding method of this invention. It is explanatory drawing for calculating the equivalent tip circumference in a radius-shaped electrode. It is a figure which shows the relationship between a wire load and an energization rate. It is a figure which shows the relationship between a linear load and a tensile shear load.

Hereinafter, an embodiment of the spot welding method of the present invention will be described with reference to FIGS. 1 to 3.
Generally, in resistance spot welding, the welded portion is held at the tip of the weld electrode for the purpose of appropriately controlling the cooling of the welded portion and the growth of the nugget. Therefore, a pressing force is applied to the weld electrode.

Further, in a galvanized steel sheet having an organic film on its surface, it is necessary to destroy the organic film having high insulating properties and secure an energization path by applying a load to the welded portion at the tip of the weld electrode.

When a cone flat type (CF type) electrode having a flat portion at the tip, which is widely used for resistance spot welding, is pressed against a galvanized steel sheet, the peripheral portion of the flat portion of the electrode tip is zinc-based. We paid attention to the strong contact with the plated steel sheet. Then, they have found that a high contact load generated in a narrow range around the flat portion at the tip of the electrode can destroy an organic film having high insulating properties and secure an energization path. That is, the energization path is secured by dividing the pressing force of the electrode by the outer peripheral length of the flat portion of the electrode tip (the peripheral length of the tip of the electrode) instead of the area of the flat portion of the electrode tip. The present invention was reached by finding that it is possible.

FIG. 1 is a schematic view showing the arrangement relationship between a flat-shaped weld electrode and a steel plate to be welded by the spot welding method of the present invention. In FIG. 1A, two galvanized steel sheets 10 having an organic film 110 on both sides are superposed. A state in which two laminated zinc-based plated steel sheets 10 are sandwiched between a pair of CF type electrodes 20A from above and below is shown. FIG. 1B shows a state in which a pressing force F is applied to the CF type electrode 20A so as to sandwich the welded portion of the two galvanized steel sheets 10. The pair of CF type electrodes 20A are energized in a state where the proper pressing force F of the electrodes is applied in this way.

As described above, the proper pressing force of the CF type electrode 20A is proportional to the length of the contour of the electrode tip, that is, the tip peripheral length of the CF type electrode 20A, not the area of the flat portion of the electrode tip. Therefore, assuming that the pressing force applied to the CF type electrode 20A is F (N) and the diameter of the circular flat portion at the tip of the electrode is D (mm), the tip peripheral length L of the CF type electrode 20A is as follows ( Calculated by Eq. 1), the linear load p at the tip of the CF type electrode 20A is defined by Eq. (2) below, and the value of the linear load p is calculated.
L = πD (mm) ・ ・ ・ ・ ・ (1)
p = F / L (N / mm) ・ ・ ・ ・ ・ (2)

Next, a radius-shaped electrode having a spherical tip at the tip will be described with reference to FIGS. 2 and 3. In spot welding, a radius type (R type) electrode having a radius-shaped tip is also widely used.

FIG. 2 is a schematic view showing the arrangement relationship between the radius-shaped weld electrode and the steel plate to be welded by the spot welding method of the present invention. As shown in FIGS. 2A and 2B, the same as in the case of FIG. 1 except that the R-shaped electrode 20B is used as the welding electrode, and thus the description thereof will be omitted.

As shown in FIG. 3A, in the radius-shaped R-shaped electrode 20B, since there is no flat portion at the tip, the outer peripheral length of the spherical surface in contact with the welded portion of the electrode tip is approximately calculated. The equivalent tip circumference L of the electrode is used. Here, as shown in FIG. 3 (b), the diameter of the cut surface obtained by cutting a sphere having a radius R (mm) on a plane passing through a portion h (mm) from the tip of the electrode is the equivalent tip diameter D (mm) of the electrode. ). The equivalent tip diameter D is calculated by the following formula (3), and the equivalent tip circumference L of the electrode is calculated by the following formula (1)'. Therefore, the value of the linear load p at the tip of the R-type electrode 20B is calculated using the following equation (1)'and the above equation (2).
Even when the tip of the electrode has a radius shape as described above, it can be treated in the same manner as the CF type electrode by using the equivalent tip diameter D (mm) of the electrode as the tip diameter of the electrode.
D = (2Rh-h ² ) ^1/2 (mm) ... (3)
L = 2π (2Rh−h ² ) ^1/2 (mm) ・ ・ ・ ・ ・ (1) ´

The above-mentioned h is a constant defined in the range of 0.05 mm or more and 0.2 mm or less depending on the strength of the base material of the steel sheet and the amount of plating adhesion. The greater the strength of the base metal and the smaller the amount of plating adhered, the smaller the sinking of the tip of the R-type electrode 20B. Therefore, the constant h may be set smaller. Further, the larger the amount of plating adhered, the larger the constant h may be set.

When the appropriate electrode pressurization conditions were examined using such an index, it was found that a good energization state can be obtained by setting the linear load p at the tip of the electrode to 400 N / mm or more. In order to secure a more stable energized state, the linear load is preferably 500 N / mm or more.
If the linear load p is made too high, the dent of the welded portion becomes remarkable, and the joint strength may rather decrease. Therefore, the maximum value of the linear load p is preferably 550 N / mm or less.

Further, since the impact when the surface of the galvanized steel sheet 10 comes into contact also contributes to the destruction of the organic film 110, it is preferable that the collision speed with the galvanized steel sheet 10 is 500 mm / sec or more. It is more preferably 000 mm / sec or more.

Further, the present invention relates to a method for enabling energization in spot welding, but in actual spot welding, the nugget diameter formed when the tip peripheral length L of the electrode is too small with respect to the plate thickness t of the steel plate. Becomes smaller, and sufficient strength cannot be obtained by design. Therefore, it is necessary to have a tip diameter of an electrode of a certain value or more.
According to Welding News No. 2614 (published on April 12, 2005), the conditions for spot welding unplated bare steel sheets are to use CF type electrodes for steel sheets with a thickness of t (mm). When welding using, it is recommended that the diameter of the electrode be 5√t (mm) and the pressing force of the electrode be 2.5√t (kN). Further, in the case of a plated steel sheet, it is common to increase the pressing force of the electrode as compared with the bare steel sheet to increase both the current and the energizing time. In the present embodiment, the pressing force F (N) of the electrode is set within a range of a diameter that can have sufficient strength from the tip diameter of the CF type electrode of 5√t (mm), which is a general condition. Welding conditions are set so that the linear load p is 400 N / mm or more.

An example is shown below.
Two laminated Zn-Al-Mg plated steel sheets (plating adhesion amount 60 g / mm ² ) having a thickness of 1.6 mm and having an organic film with a thickness of 1.6 μm on both sides were prepared as a test material and placed. Resistance spot welding was performed using a pressure-type single-phase AC welding machine directly above the formula, and the energization rate was measured.
In this example, the welding electrode was made of chrome copper, and the CF type (cone flat type) electrode and the R type (radius type) electrode described above were used. Welding was performed by setting the initial pressurization time to 35/60 seconds, the energizing time to 12/60 seconds, and the cooling water flow rate to 3 L / min.

For the welding current, the appropriate welding current range was investigated in advance for each electrode shape, the lower limit current for which the nugget diameter ≥ 4√t (t: plate thickness) was obtained, and the lower limit current was set to + 0.5 kA. In addition, the energization rate was measured as follows, and those with 80% or more were evaluated as ◯. The energization rate was expressed as a percentage of the number of welding trials, which was the number of times of energization out of at least 10 or more welding trials for each electrode shape. Since it is shown that the energization rate does not depend on the surface load q obtained by dividing the pressing force of the electrode by the area of the electrode tip, the value of the surface load q is also shown for reference.
The results are shown in Table 1, and the relationship between the energization rate and the linear load is shown in FIG. In addition, the sample No. of Examples. Sample Nos. 1 to 4 of Comparative Example. Regarding No. 11, the relationship between the linear load and the tensile shear load is shown in FIG.

As shown in Table 1, the sample No. that satisfies the conditions of the present invention. In the examples 1 to 6, a good energized state was obtained under any of the conditions. On the other hand, the sample No. in which the linear load p is less than 400 N / mm. In the comparative examples of 7 to 14, the energization rate was not improved. The notation of 3CF in the types of electrodes in Table 1 indicates that it is a CF type electrode having a tip diameter of 3 mm, and the notation of 6R indicates that it is an R type electrode having a radius of curvature of 6 mm. The same applies to the notation of other numerical values.

Looking at the relationship between the surface load q and the energization rate, the sample No. of the comparative example. In No. 13, the value of the surface load q is 604 N / mm ^2, and the sample No. Despite the value as large as the surface load q of 1 to 6, the energization rate was 50% and the evaluation was x. That is, it was shown that it is difficult to break the organic film and secure the energization path simply by increasing the surface load q.

As shown in FIG. 4, the energization rate tends to increase as the linear load p increases. Further, when the linear load p was 400 N / mm or more, the energization rate was 80% or more, indicating a good energization state.

As shown in FIG. 5, when the linear load p was around 550 N / mm, the tensile shear load, which is an index of the joint strength, showed a peak. It is considered that this is because when the linear load q is made too high as described above, the dent of the welded portion becomes remarkable. Therefore, the maximum value of the linear load p is preferably 550 N / mm or less.

Although one embodiment of the spot welding method of the present invention has been described above, the present invention is not limited to the above-described embodiment and can be appropriately modified.
For example, as the electrode having a flat shape in the embodiment, a cone flat type (CF type) electrode is shown, but the present invention is not limited to this. A flat type (F type) electrode may be used. Further, when a dome radius type (DR type) electrode having a curved surface having a large radius of curvature at the tip of the electrode is used, the curved surface of the tip is substantially flat, so the tip diameter may be used as the tip diameter D.

Further, in the above-described embodiments and examples, an example is shown in which two zinc-based plated steel sheets having an organic film on both sides are superposed and spot welded, but the present invention is not limited to this. The present invention can be applied even when at least one of the plurality of steel sheets is a galvanized steel sheet having an organic film on one side.

10 Galvanized steel sheet 20A Cone flat type electrode 20B Radius type electrode 110 Organic film

Claims (5)

It is a spot welding method in which a plurality of steel plates are overlapped and spot welded.
At least one of the plurality of steel sheets is a galvanized steel sheet having an organic film having a thickness of 0.1 μm or more and 3 μm or less on one side or both sides.
The shape of the electrode in contact with the galvanized steel sheet is a flat shape having a circular flat portion at the tip or a radius shape having a spherical tip at the tip.
When the shape of the electrode is the flat shape, the pressing force of the electrode is divided by the following equation (1), and when the shape of the electrode is the radius shape, the pressing force of the electrode is divided by the peripheral length of the tip of the electrode calculated by the following equation (1)'. A spot welding method for welding under the condition that the value of the linear load at the tip of the electrode defined by the following equation (2) is 400 N / mm or more.
L = πD ・ ・ ・ ・ ・ (1)
L = 2π (2Rh-h ² ) ^1/2 ... (1)'
p = F / L (2)
Here, L: the peripheral length of the tip of the electrode (mm), D: the diameter of the circular flat portion when the tip of the electrode is flat, and R: the curvature of the spherical surface when the tip of the electrode is a radius shape. Radius (mm), h: Constant (0.05 mm to 0.2 mm depending on base metal strength and plating adhesion amount) for obtaining the equivalent tip diameter at the tip of the electrode when the tip of the electrode has a radius shape, p: Wire load (N / mm), F: Electrode pressing force (N) The spot welding method according to claim 1, wherein welding is performed under a condition that the value of the linear load is 550 N / mm or less. The spot welding method according to claim 1 or 2, wherein welding is performed under a condition that the value of the linear load is 500 N / mm or more. The spot welding method according to any one of claims 1 to 3, wherein the collision speed of the electrode with the galvanized steel sheet is 500 mm / sec or more. The spot welding method according to any one of claims 1 to 4, wherein the plurality of steel sheets are two galvanized steel sheets having an organic film having a thickness of 0.1 μm or more and 3 μm or less on both sides.

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