JP2901765B2 - Spot welding of galvanized steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spot welding method, and more particularly to a spot welding method for improving the weldability and workability of a galvanized steel sheet.

[0002]

2. Description of the Related Art Galvanized steel sheets in which at least one surface of a steel sheet is plated mainly with zinc are widely used as materials for bodies of automobiles and washing machines. This galvanized steel sheet is usually joined by lap resistance welding such as spot welding, projection welding, seam welding, etc., but the galvanized steel sheet is more difficult to weld than a bare steel sheet without a plating layer. There is. This is because the electrical resistance of zinc is relatively small and soft, so that the galvanized steel sheets to be joined to each other have good conformability when pressed against each other, so that the contact electrical resistance of the mating surfaces is small. And sufficient heat generation cannot be obtained. Therefore, for example, in spot welding of a galvanized steel sheet, the welding current is usually set to be 25 to 50% larger and the welding time is set to be 50 to 100% larger than that of a normal steel sheet, and the power consumption is inevitably increased. I do. Moreover, the size of the nugget formed by welding is not constant, and the welding strength becomes unstable,
The heat generated at the contact surface between the electrode and the steel plate increases, causing severe damage to the electrode.Furthermore, elimination of zinc adhering to the electrode must be carried out frequently, thereby avoiding a decrease in productivity and an increase in cost. It is hard.

[0003] Since the corrosion resistance of galvanized steel sheet is mainly due to the sacrificial corrosion action of zinc, it is desirable to increase the thickness of the plating layer in order to enhance the corrosion resistance. In this case, the above tendency is promoted. The weldability is further deteriorated. Therefore, when it comes to automotive body materials,
Except for North America and Northern Europe, where snow melting agent is scattered on the road surface, demand for steel sheets with as little zinc coverage as possible is sacrificed, even at the expense of corrosion resistance. Alloyed galvanized steel sheets, zinc-based alloy-coated steel sheets, and multi-layer galvanized steel sheets have also been used that have been improved to provide good corrosion resistance in the layers.

As described above, in a galvanized steel sheet having a relatively thin plating layer, although the weldability tends to be somewhat improved, it is still insufficient compared with a bare steel sheet having no plating layer. At present, improvements are desired, and there is a strong demand for improved weldability, especially for steel sheets with a large amount of zinc coating used in automobile bodies for North America and Northern Europe, where high corrosion resistance is required. .

[0005] On the other hand, galvanized high-strength steel sheets have recently been used frequently, and for example, they are extremely useful for automobiles and the like from the viewpoint of improving safety and reducing the weight of a vehicle body. However, in spot welding, regardless of whether the base material is high-strength steel, the strength of the weld remains only as low as mild steel, and if high-strength steel is used, the joint stiffness decreases if the plate thickness is reduced. Problems arise.

[0006]

In view of the above-mentioned problems, the present inventors have proposed a spot welding method in which a group of particles is previously interposed in a joint boundary area of a joint mating surface, and zinc welding is performed by the method. The weldability of the plated steel sheet was significantly improved. However, it is very difficult to accurately arrange the particles in the joint area where a predetermined amount of the particles is required.Therefore, particles that protrude extremely beyond the forming nugget remain around the welding line after welding or a part thereof is welded. There is also a problem such that the welding force scatters from the end of the joint.

In order to improve this point, the present inventors have developed a technical means of stacking a group of particles interposed in a region corresponding to a nugget for forming a joint mating surface and a conductive thin film holding the same. Further suggested. However, this means
Occasionally, a part of the conductive thin film or its alteration due to heat may remain around the nugget.Also, most of the conductive thin film is vaporized during welding and is removed from the nugget part. An extra amount of heat is required as compared with the case where the conductive thin film is not used.

According to the present invention, a group of grains for increasing inter-plate resistance is accurately interposed in a region corresponding to a nugget for forming a joint mating surface, while the residue around the nugget is reduced as much as possible to further increase joint strength. It is a technical problem to be solved to achieve the improvement.

[0009]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, according to the present invention, prior to spot welding of a galvanized steel sheet, a resistance increasing particle (hereinafter simply referred to as a particle) is formed in a region corresponding to a nugget for forming a joint mating surface. Technical measures are taken such that an appropriate number of fibers are dispersed and adhered. The present invention further provides a technical means in which a non-conductive sheet having an opening surrounding the group of particles is coexistent, and the fiber end is adhered to the opening edge. The present invention further provides a technical means that the non-conductive sheet is integrally extended along a welding line and has a function of bonding a joint mating surface. Ceramic particles can also be used as the particles, but ferromanganese or a magnesite that evaporates during welding, which has a melting point enough to fuse with the nugget after welding, is used as appropriate, and the mating surface in the joint interface area is used. In order to prevent voids from interfering with each other and secure a void that functions to increase and decrease the resistance, it is desirable that the particle size of the particles is 20 μm or more. The particles are preferably made of a poorly conductive material. However, as long as they can secure a gap between the plates and contribute to an increase in resistance, a conductive material is not excluded.

The fiber is preferably, for example, a copper fine wire or cellulose. The above-mentioned granules are dispersed and applied to a required portion of the fiber by an adhesive, and the fiber is applied to both ends as necessary. Is attached to a required portion of the joint mating surface. In the case where a non-conductive sheet provided with an opening surrounding the particle group is provided, the current is concentrated by using the non-conductive sheet as a resistor. Can also.

Further, if the hardly conductive sheet provided with double-sided adhesiveness is integrally extended along the welding line and an optional opening or notch is provided in the sheet, a margin can be provided in terms of the bonding effect and the accuracy of hitting. Can be granted. The present invention can be applied to all galvanized steel sheets. That is,
In addition to ordinary double-sided or single-sided galvanized steel sheets, the purpose is to improve the corrosion resistance of galvanized steel sheets and galvanized steel sheets in which the galvanized layer is treated with an Fe-Zn alloy to improve the adhesion with paint and resistance weldability. Alloy-plated steel sheets such as Zn-Ni, Zn-Co-Cr, Zn-Al-Sn based alloys to which alloying elements are added, Zn / Mn based, Zn improved not only in corrosion resistance but also in weldability and press formability, etc. / N
The present invention can be applied to i-type and other multi-layer galvanized steel sheets.

[0012]

The spot welding method according to the present invention is carried out by precisely interposing particles which hinder the adhesion between the mating surfaces of two galvanized steel sheets to be joined. Since the electric resistance of the region increases, welding can be performed at a low current and in a short time. Therefore, the temperature rise at the contact surface between the electrode and the steel plate is small, and the consumption of the electrode is reduced, so that a stable welding operation can be performed. For the same reason, the disappearance of the zinc layer in the indented portion generated on the surface of the spot weld is small, and the deterioration of the rust prevention performance can be avoided.

In addition, heat is concentrated on a limited portion of the joint mating surface. As a result, the thickness of the nugget and the heat affected zone are reduced, and the occurrence of dents and deformation due to welding is reduced.
A decrease in the strength of the weld is suppressed. For the same reason, it is difficult to form a nugget in a portion other than the mating surface. When a relatively thin plate and a thick plate are spot-welded, a nugget is formed at an intermediate portion in the thickness direction of the thick plate and may not be formed on a required mating surface. As a result of the increased electrical resistance of the surface, the occurrence of such a condition can be neglected.

Moreover, the above-mentioned reduction of the electric current not only has the effect of reducing power consumption, but also shortens the welding time and improves the efficiency of the welding work, and further avoids the occurrence of dust and explosion. In addition, since the proper current range in which a proper nugget can be formed is widened, the effect of easily managing the welding conditions and reducing the defect rate is also obtained. In particular, in the method of the present invention, the particles that hinder the adhesion of the joint are stably retained on the fiber by being confined to the region corresponding to the forming nugget, and the fiber may directly or coexist at the end thereof. Using a sheet, the particles can be easily adhered to the required portions without disturbing the particles, so that the troubles caused by the surplus particles remaining around the nugget and scattering can be eliminated, and furthermore, the above-mentioned method can be applied to the joint mating surface. This can also be done with a completely arbitrary working posture when applying the fibers.

Further, if an organic substance such as cellulose is used as the fiber, and magnesite which evaporates during welding is used as the granule, these are vaporized and expanded at a relatively low temperature, so that air flows into the molten nugget. In addition, the melted and vaporized zinc is skillfully discharged to the outside, which more effectively acts to increase the inter-plate resistance.
The use of titanium oxide particles has the effect of ionizing during welding to maintain a stable current. If the above ferromanganese, ferrosilicon, etc. having an appropriate melting point are adopted as the particle group, the particle group dissolves during welding to obtain an extremely stable nugget without any residue of the particle group. Manganese and silicon melted in the nugget
Both act as alloying elements and play an effective role in deoxidizing nuggets and the like. Further, in the case where a non-conductive sheet having an opening surrounding the above-mentioned particle group is coexistent, the non-conductive sheet acts as a resistor, and the current is applied to the central portion of the electrode where the pressure is high, that is, Concentrate on the nugget formation region and assist in the early formation of the nugget.

In particular, in the case where the above-mentioned non-conductive sheet is provided with double-sided adhesiveness, the bonding effect of the joint mating surface can be obtained. Along with the nugget in which the opening or notch of the non-conductive sheet is formed, it becomes possible to temporarily assemble the structure, and the steel plates required for ordinary spot welding can be bonded together. A jig for maintaining the positional relationship and an operation thereof are not required. Further, if the adhesive has a structural bonding function, it can not only contribute to an increase in the strength of the welded joint, but also improve the joint rigidity. If the sheet is made to have a length corresponding to the welding line, vibration and sound insulation of the joint,
It is effective in preventing corrosion and improving water tightness.

Further, if a soft and hard multi-layer adhesive is used as the sheet, the outer soft portion is suitably deformed and adapted to the misalignment caused when the steel sheets to be welded are overlapped. It is also effective as a countermeasure for problems. Non-conductive sheath
If a hot-melt type adhesive is used, the adhesive around the nugget will soften during welding, helping to relieve the stress in the plate thickness direction generated inside the nugget, and also familiar with the base metal around the nugget, The nugget is wrapped around, contributing to further improvement of the anticorrosion effect, and furthermore, the welding heat is used effectively, and the bonding is completed at the same time as the welding is completed. Can be omitted.

On the other hand, the opening of the non-conductive sheet is usually rectangular or circular, but may be an elongated hole along the welding line if necessary. As a result, a plurality of hit points can be formed in the same opening.

[0019]

Embodiment (Part 1) As shown in FIG.
A particle size of about 300 μm is applied to the 0 μm copper wire fiber 14 using an adhesive.
Four alumina granules 12 of m are adhered at an interval of 2 mm, and three strips of the copper wire fiber 14 are arranged in a nugget formation area of a joint fitting surface, and the end is adhered to a plate surface.

FIG. 1 (B) shows the copper wire fiber 14 extending along the welding line to form a plurality of hit points. FIG. 1 (C) shows a 0.15 mm thick, 5 mm square double-sided The short small copper wire fibers 14 are relayed and held by the adhesive polyethylene film 30. Further, FIG.
Is a belt-like shape, and each short small copper wire fiber 14 is held in a cantilever shape.

(Part 2) As shown in FIG. 2 (A), a double-sided adhesive epoxy resin sheet 40 having a thickness of 0.3 mm is used.
An opening 42 of 5 mm square is provided, and each end of the copper wire fiber 14 (four lines) shown in FIG. 1A is bonded to the edge of the opening 42. FIG. 2B shows the epoxy resin sheet 40 integrally extending along a welding line. FIG.
(C) forms the opening 42 in a round hole 42a or a long hole 42b, while FIG. 2 (D) shows the opening 42 for a lip joint.
Are formed in the notches 42c.

The particles 14 made of copper wire or cellulosic material are not limited to the welding wire or the fibers arranged in parallel to the direction perpendicular to the welding wire, but may be meshed as shown in FIG. They may be arranged obliquely in a shape, and different kinds of grains can be positively mixed in one nugget formation region. Further, the parallel or obliquely attached granular adhered fibers 14
Is extended in the longitudinal direction of the elongated hole 42b, and it is more advantageous since a margin can be given to the hitting accuracy. Note that the elongated hole 42b is made of a non-conductive sheet (epoxy resin sheet).
G) If it spreads over the whole area in the longitudinal direction of 40,
It is also possible to make the non-conductive sheet 40 leap to a parallel rail facing each other.

(Part 3) A galvanized layer of 60 g / m ² was formed on both sides of a mild steel sheet having a thickness of 0.8 mm by the dough dipping method. Prior to performing the spot welding by superposing hot-dip galvanized steel sheets, four strips of the granular coated fibers 14 shown in the above-mentioned embodiment (part 1) are arranged in parallel at 2 mm intervals as resistance increasing elements, and the ends thereof are Arabic. The glue was adhered to the joint boundary area of the joint mating surface so that the particle group 12 almost coincided with the center of the formed nugget, and then a spot welding test was performed under the following conditions.

Working electrode diameter: 16 mm (tip diameter: 6 mm) Pressure: 220 kgf Welding current set value: 11 KA Electric conduction time: 2 cycles The experimental results are shown in FIG.

In the graph of FIG. 3, the abscissa represents the energizing time, the upper part of the ordinate represents the change in resistance between the electrodes during welding (four-cycle conducting specimen), and the lower part the diameter of the formed nugget. In the figure, the white circles are the results obtained by using the resistance increasing element, and the black circles are the results of the ordinary welding method without using the resistance increasing element, which is also shown for comparison. As is clear from the drawing, when a resistance increasing element is interposed in the joint boundary area of the joint mating surface, a nugget is formed in a shorter energization time than in normal welding. The reason for this is that the amount of heat generated in the bonding interface region significantly increases at the beginning of energization, specifically, due to a sharp increase in resistance value due to the melting and vaporization of the zinc layer.

Next, the characteristics of the method of the present invention will be described in more detail with reference to FIG. 4. Numeral 10 denotes a joint made of a galvanized steel sheet having zinc 10a adhered to both surfaces thereof. The particle group 12 to be interposed in the joint boundary area is previously applied to the fiber 14 in advance, and the fiber 14 is manually or separately applied so that the fiber 14 is disposed only in an area corresponding to the nugget to be formed. By simply laying it on the mating surface of one of the joints 10 by using a mechanical method, a gap 16 is secured in the joint area, which functions properly to increase the resistance. When the joint 10 is pressurized and welded with the electrode 18, the group of granules 12 bites into the joint 10 and the joint 10 bends, leaving a gap 20 around the granule 12 as shown in FIG. Partial contact occurs and current flows.

The fibers 14 on which the intergranular bodies 12 are adhered and held are fine and do not cause any trouble. Since the electric resistance is the largest at the mating surface and the melting point of the zinc 10a is low, the zinc 10a at the mating surface is first melted and vaporized. The melted and vaporized zinc 10a is skillfully discharged to the outside through the gap 20. When the zinc 10a is removed in this manner, a gap is newly formed in the zinc 10a layer between the plates, and the gap is filled with zinc vapor to allow current to flow, but the resistance is high. Therefore, the electrical resistance in the junction region increases rapidly. Intense heat generation is induced in the part. As a result, the melting of the base material along the mating surface of the joint 10 proceeds to form a thin nugget. The resistance value decreases as the nugget formation progresses.

On the other hand, in the case of ordinary joints, the electrical resistance is small due to the good conformability of zinc when the joints are overlapped, so that the heat generation is small at the beginning of energization and no nugget is formed. As the energization time increases, the temperature rises over a wide range of the joint due to so-called volume resistance. As the temperature of the joint increases, the resistance value also increases, which increases the amount of heat generated, and eventually a part of the joint melts to form a nugget. When the nugget is formed in this manner, the joint is at a high temperature over a wide range, so that dust is easily generated even by a slight change in the current value, and a relatively thick nugget is formed. You.

As mentioned above, the nuggets formed by the method of the present invention, in contrast, are thin, and thus their total volume is small, indicating that less heat energy is required for bonding. Furthermore, a narrow heating range reduces deformation during welding, residual stress, and a deteriorated portion of the joint, and can be said to be an excellent weld from this point as well.

As is apparent from FIG. 3, the resistance between the electrodes decreases rapidly as the nugget is formed in the case of the method of the present invention. Therefore, by utilizing this stable feature, it is possible to realize the result verification at the same time as the welding, which has been impossible in the past, that is, the welding quality monitor. On the other hand, the electrical resistance and the thermal conductivity at the contact surface between the electrode 18 and the joint 10 are not particularly changed between the case according to the ordinary spot welding method and the case according to the method of the present invention. Since the welding is completed in a short time even with the same current, the temperature of the plating layer in contact with the electrode 18 is much lower than in a normal case, and therefore the electrode 18
The loss of zinc in the joint 10 at the portion in contact with the metal is small, the corrosion resistance is improved, and the wear of the electrode 18 is also reduced.

The present invention can be embodied in variously modified and improved forms without departing from the gist thereof.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing each aspect of a granular adherend fiber according to an embodiment of the present invention.

FIG. 2 is an explanatory view showing each bonding mode of a non-conductive sheet and a granular adhered fiber.

FIG. 3 is a graph showing the results of a welding experiment according to an example of the present invention.

FIG. 4 is an explanatory view schematically showing a joint joint before pressurization by an electrode in the method of the present invention.

FIG. 5 is an explanatory diagram showing an enlarged view of the joint portion after pressurization.

[Explanation of symbols]

10 is a joint, 12 is a particle group, 14 is a fiber, 18 is an electrode,
20 is a gap, 40 is a non-conductive sheet, and 42 is an opening.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B23K 11/11 B23K 11/16

Claims (3)

(57) [Claims] 1. A spot welding method for a galvanized steel sheet, wherein a suitable number of fibers having dispersion-enhanced granules dispersed therein are interposed in a region corresponding to a nugget for forming a joint mating surface. 2. The method according to claim 1, wherein a non-conductive sheet provided with an opening surrounding said group of resistance increasing particles is coexistent, and said fiber end is bonded to an edge of said opening. Spot welding method. 3. The spot welding method according to claim 2, wherein said non-conductive sheet is integrally extended along a welding line and has a function of bonding a joint mating surface.